Film Extrusion Manual Third Edition [3, 3 ed.] 9781595102997

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Film Extrusion Manual Third Edition Process, Materials, Properties

Global Editor: James F. Macnamara Jr.

Copyright© 2020

TAPPI PRESS 15 Technology Parkway South Peachtree Corners, GA 30092 U.S.A. www.tappi.org All rights reserved The Association assumes no liability or responsibility in connection with the use of this information or data, including, but not limited to, any liability or responsibility under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published. Within the context of this work, the authors and their employers assume no liability or responsibility in connection with the use of this information or data. The information contained in each of the chapters is believed to be true and accurate, but all statements or suggestions are made without warranty, expressed or implied. The author(s) may use as examples specific manufacturers of equipment. This does not imply that these manufacturers are the only or best sources of the equipment or that TAPPI endorses them in any way. The presentation of such material by TAPPI should not be construed as an endorsement of or suggestion for any agreed upon course of conduct or concerted action. To obtain copyright permission to photocopy pages from the publication for internal or personal use, contact Copyright Clearance Center, Inc. (CCC) via their website at www.copyright.com. If you have questions about the copyright permission request process, please contact CCC by phone at +1-978750-8400. Film Extrusion Manual, Third Edition is an individually licensed product and access is restricted to one user. It is a copyright violation to share information beyond the primary user. ISBN: 978-1-59510-299-7 TAPPI PRESS Item Number: 0101R360EL Printed in the United States of America

Table of Contents

Preface  xv Chapter Editors and Author/Contributor Contact Information   xvii

Chapter 1—Film Extrusion Introduction

Chapter Editor: BRUCE FOSTER, PolyKnows LLC Section 1.1—Film Extrusion Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 BRUCE FOSTER, PolyKnows LLC Introduction  3 History  3 Since 2005  3 The Future  3 References and Additional Resources   4

Chapter 2—PRIMARY EQUIPMENT

Chapter Editor: MARTINE MICHON, Atlantic Coated Papers Ltd. Section 2.1—The Film Extruder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 DONN C. LOUNSBURY, D.C.L. Solutions, Inc. and BILL HELLMUTH, Battenfeld Gloucester Engineering Introduction  7 The Extruder  7 The Barrel and Feed Section   7 Barrel Heating and Cooling Systems   7 Closed-Loop Liquid Cooling   9 Gearcase and Thrust Bearing Assembly   11 Screws  13 Extruder Drives  16 The DC Static Drive   16 AC Variable-Frequency Drive  17 Description  17 Heat Control Panels   18 Extruder Maintenance  18 Preventive Measures  20 Unusual Noise Indicators   20 iii

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Drive Motor Maintenance   21 Stocking Spares and Limiting Downtime   21 Screw Removal: Why, When, and How   22 The Grooved-Feed Extruder   23 History  23 Concept and Description   24 The Process  24 Screw Characteristics  25 References and Additional Resources   27 Section 2.2—Screw Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 ANDREW W. CHRISTIE, SAM North America, LLC Introduction  29 Feedscrew Terminology  29 Extruder Functions and Plasticating Screw Design   30 Feeding/Solids Conveying  30 Grooved Feed-Throat Extruders   32 Plug Flow Compared to Melt Flow   33 Melting  33 Barrier Screw Designs   34 Energy Required for Extrusion   34 Melt Conveying  35 Mixing in an Extruder   35 Mixers  36 Plasticating Screw Design Summary   38 Materials of Construction   38 Extrusion Process Analysis and Operation   39 Objective (Online Measured) Variables   39 Subjective (Offline Measured) Variables   39 Polymer Variables  40 Operating the Extruder   40 Feed Zone  41 Screw Running Tips  42 Summary  42 References and Additional Resources   42 Section 2.3—Die Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 BILL BODE, Battenfeld Gloucester Engineering Introduction and Function of Die   45 Single-Layer Spiral Mandrel Die Components   45 Spiral Mandrel and Body   45 Die Lip Sets   45 Die Gaps  48 Mechanical Considerations  48 Stiffness of Components/Assembly/Disassembly   48 Additional Considerations  48 Materials of Construction and Surface Coatings   49 Materials  49 Plating/Surface Coatings  49 Corrosion  49 Cleaning  49 Coextrusion Dies  50 Rheological (Flow) Considerations   50

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Mechanical Considerations  52 Other Types of Film Dies   52 Spider Dies  52 Wraparound Coathanger Dies   52 Upper Die Geometry   53 Controls  53 Section 2.4—Stacked-Die Technology for Tubular Film Coextrusion. . . . . . . . . . . . . . . . . . . . . . . . . . 55 JOHN PERDIKOULIAS, Compuplast International Inc. Introduction  55 Conventional Coextrusion Dies   55 Stacked Dies  57 References and Additional Resources   61 Section 2.5—Winding and Web Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 R. DUANE SMITH, Davis-Standard LLC Introduction  63 Definition of a Quality Roll   63 How to Achieve Roll Hardness   65 Winder Components  70 The Spreading Operation   72 Winder Drives and Tension Control Systems   74 Proper Shaft Selection   76 Automatic Roll Changing Systems   77 Conclusions  78 References and Additional Resources   78

CHAPTER 3—ANCILLARY EQUIPMENT

Chapter Editor: RORY WOLF, ITW Pillar Technologies Section 3.1- Gear Pumps, Filtration, Static Mixers—Function, Design, Parameters, Examples. . . . . . . . . . . . 81 KEVIN TUTTLE, Nordson Polymer Processing Systems Gear Pumps  81 Filtration  84 Static Mixers  92 Gear Pump Addendum  95 References and Additional Resources   95 Section 3.2—Feedblock Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 CHRISTINE RONAGHEN, Cloeren Incorporated Introduction  97 Primary Functions  97 Coextrusion Behavior: A Brief Introduction to Viscosity   100 Beyond Conventional Coextrusion   102 Coextrusion Performance and Corrective Actions   102 Specifying a Feedblock   104 References and Additional Resources   104 Section 3.3—Film Stabilization, Forming and Collapsing Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 JAMES STOBIE and HARINDER TAMBER, Macro Engineering and Technology Inc. Introduction  105 Film Tube Collapse  108 Bubble Collapsing Improvements   111

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Tube Containment in High Cooling Applications   113 Summary  113 References and Additional Resources   113 Section 3.4—Material Handling Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 CLIFFORD J. WEINPEL and WALTER FOLKL, Foremost Machine Builders Introduction  115 Bulk Unloading and Storage   115 In-Plant Distribution Systems   119 Piping Systems  121 Controls  122 Gravimetric Metering and Blending   122 Accuracy and Resolution   131 Summary  132 Addendum A  132 Addendum B  133 Addendum C  134 Addendum D  137 Section 3.5—Instrumentation and Process Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 TED SCHNACKERTZ, NDC Technologies Introduction  139 Process Controls  142 Summary  143 References and Additional Resources   143 Section 3.6—Blown-Film Cooling Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 HARINDER TAMBER, Macro Engineering and Technology Inc. Introduction  145 Single-Orifice Air Rings   145 Dual-Orifice Air Rings   147 Modified Dual-Orifice Air Rings   148 Automatic Air Rings for Gauge Control   148 Internal Bubble Cooling (IBC)   149 Multiple Air Rings  151 High Cooling Rates   151 Cooling Rate Equations   152 Thermal Analysis of Blown Film Quenching   153 Thermal Load  153 Heat Removal  155 General Considerations for Blown-Film Systems   155 Summary  155 References and Additional Resources   155 Section 3.7—Surface Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 RORY WOLF, ITW Pillar Technologies Introduction: The Need for Surface Treatment   157 Flame Treatment  157 Plasma Treatment  158 Atmospheric Plasma Treatment (APT) Process   159 ASTM Test Methods  161

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Cotton Swab Method   162 Drawdown Test Method  162 Warnings and Cautions   163 Corona Treating  163 Types of Corona—Treatment Systems   164 Bare-Roll System  165 Universal—Roll System  166 Power Supply for Corona Treating   166 Corona—Treating Applications  167 Sizing: Material/Process Parameters   168 Conclusions  170 References and Additional Resources   170 Section 3.8—Atmospheric-Pressure Plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 RORY WOLF, ITW Pillar Technologies Introduction  173 The Ionization Process   176 Surface Effects   176 Influence on Adhesion   178 Surface Modification by Cleaning and Etching   178 Surface Temperature Elevation  179 Surface Sputtering  179 Surface Etching  179 Surface Modification by Functionalization   182 Characterizations of Surface Modification Effects   183

CHAPTER 4—MATERIALS

Chapter Editors: KELLY FREY, Chevron Phillips Chemical Company and DORENE SMITH, Westlake Chemical Corporation Section 4.1—Low-Density Polyethylene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 JOEL PERRITT, Westlake Chemical Corporation Introduction  187 Reactor Technology  187 Polymer Characterization—Properties and Terminology   188 Molecular Weight Distribution  191 Common Polyethylene Additives  193 Film Properties of Polyethylenes   195 General Processing Guidelines for LPDE   196 General Uses of LDPE   197 Section 4.2—Linear Low-Density Polyethylene (LLDPE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 NORMAN AUBEE, NOVA Chemicals Introduction  199 Applications  199 Polymer Characterization  200 Super Hexene LLDPE   202 Typical Film Properties   203 General Processing Guidelines   204 Blends  204

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Section 4.3—Metallocene Polyethylene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 NILESH SAVARGAONKAR, ExxonMobil Chemical Company Introduction  207 Metallocene LLDPE  207 Introduction of Long-chain Branching (LCB)   208 Rheological Behavior and Processing of mLLDPE   208 Physical Properties of mLLDPE   209 Sealing Behavior of mVLDPE and Plastomers   210 Physical Properties of Blends of Plastomers   212 Applications for Metallocene LLDPE   212 Newer Products  212 Summary  212 References and Additional Resources   214 Section 4.4—Polyolefin Plastomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 JACQUELYN DEGROOT and JEFFREY ARIONUS, The Dow Chemical Company Introduction  215 Properties  215 Hot Seal and Hot Tack of Ethylene-Based Polymers   217 Propylene-Based Plastomers  218 Applications  218 Processing Polyolyfin Plastomers   218 Acknowledgements  220 References and Additional Resources   221 Section 4.5—High Molecular Weight—High Density Polyethylene. . . . . . . . . . . . . . . . . . . . . . . . . . . 223 MARK CANRIGHT and AMY M. LAIRD, ExxonMobil Chemical Company Introduction  223 Polymer Characterization  223 Typical Polymer Properties   225 Biaxial Orientation  225 General Processing Guidelines   226 Blown-Film Die  227 Film Cooling  227 Collapsing Parameters  227 Treating  227 Applications and End Uses   227 Summary  228 References and Additional Resources   228 Section 4.6—Polybutylene-1 for Peelable Seals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 STEFANO PASQUALI, LyondellBasell Industries Introduction  229 Polybutene-1 Key Features   229 Easy-Peel Application  229 Hot to Design a Peelable Film   229 Advantages of Using PB-1 in Easy-Peel Applications   232 Application Examples  233 Quality Testing   234 Summary  234 References and Additional Resources   234

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Section 4.7—Polypropylene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 THOMAS J. SCHWAB and MARIO PERRON, LyondellBasell Industries Introduction  235 Types of Polypropylene (PP)   235 Polypropylene Film Manufacture   236 Additives  237 References and Additional Resources   237 Section 4.8—Nylons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 SERGI SALVÀ SÀEZ and JA’NAYSHA HAMILTON, UBE America Inc. Introduction  239 Nomenclature  239 Nylon Synthesis and Chemistry   239 Nylon Extrusion  242 Processing Techniques Used for Nylon Resins   245 Nylon Film Properties   247 Applications  249 Summary  251 References and Additional Resources   252 Section 4.9—Ethylene Vinyl Alcohol Copolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 MARK PUCCI, Soarus, L.L.C. Introduction  253 Synthesis of EVOH Copolymers   253 Oxygen Barrier Properties of EVOH   253 Other Properties of EVOH   254 Multilayer Structures with EVOH   255 Solvent Barrier Prperties of EVOH   256 Processing of EVOH   256 Summary  257 Section 4.10—Polyesters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 JOSE TORRADAS, SALTOR LLC Introduction  259 History  259 Chemistry  259 Pet Versatility  260 Physical State  260 Rheology  261 Properties of Polyester   261 General Processing  263 Specific Film Processes   265 Secondary Operations  265 Applications  266 Summary  266 References and Additional Resources   266 Section 4.11—Tie Polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 BARRY A. MORRIS and I-HWA LEE, The Dow Chemical Company Introduction  269 Tie-Resin Selection  269 Bonding During Coextrusion   270 Bonding in End Use   271 Processing Guidelines  271

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Summary  272 References and Additional Resources   272 Section 4.12—Ethylene Vinyl Acetate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 MATTHEW SONNYCALB, LyondellBasell Industries Introduction  273 Polymer Characterization  273 Applications  275 General Processing Recommendations   276 References and Additional Resources   276 Section 4.13—Acrylate Copolymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 MICHAEL G. BAKER, Westlake Chemical Corporation Introduction  277 Chemistry and Manufacturing   277 Property Review  277 Processing Guidelines  278 Applications  280 Handling and Safety   280 Regulatory  281 Section 4.14—Acid Copolymers and Ionomers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 BARRY A. MORRIS and SCOTT B. MARKS, The Dow Chemical Company Introduction  283 Properties  283 Applications  287 General Processing Considerations   287 References and Additional Resources   288 Section 4.15—Polyvinylidene Chloride (PVDC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289 KUN SUP HYUN, Formerly of the Dow Chemical Company Introduction  289 Characteristics  289 Processing  289 References and Additional Resources   290 Section 4.16—Polymer Processing Additives (PPA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 CLAUDE LAVALLÉE, 3M Company Introduction  291 Interactions  299 Conclusions  301 References and Additional Resources   301 Section 4.17—Additives for Film Products. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 TAD FINNEGAN and R. E. KING, III, BASF Corporation Introduction  303 Acid Scavengers  303 Adhesion Promoters  303 Antiblock Agents  304 Antifogging Agents  305 Anti-Gas Fading (For Color-Critical Applications)   305 Antioxidants (For Long-Term Stability)   306 Antistatic Agents  307 Biocides  308 Cling Agents  308 Colorants  308

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Degradation Accelerators  310 Fillers  310 Flame Retardants  310 Melt-Processing Stabilizers  311 Metal Deactivators  312 Polymer Processing Aids  312 Slip Additives  313 Ultraviolet Stabilizers  313 UV Absorbers  313 Additive Delivery  314 Synergistic and Antagonistic Mixtures of Additives   315 Additive Antagonism  316 Ancillary Properties  316 Performance Testing  317 Coextrusion and its Affect on Additive Use   317 Summary  317 References and Additional Resources   318 Section 4.18—Slip Agents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 ADAM J. MALTBY and RICHARD E. MARQUIS, Croda Universal Inc. Introduction  319 Raw Material Source   319 Manufacture  319 How Amides Function in Polymers   319 Measurement of Friction   320 How Amides are Added to Polymer   320 Summary  323 References and Additional Resources   324

CHAPTER 5—PROCESSING

Chapter Editor: NORMAN AUBEE, NOVA Chemicals Section 5.1—Blown-Film Processing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 HARINDER TAMBER and MIREK PLANETA, Macro Engineering and Technology Inc. Introduction  327 Basics of Blown-Film Process   328 Blown-Film Process: Correlation of Resin, Equipment and Process Conditions   328 Comparison of Air vs. Water Cooled Blown-Film Processes and Properties   337 Start-up and Shut Down of Blown-Film Line   337 References and Additional Resources   339 Section 5.2—Cast Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 CHRISTINE RONAGHAN, Cloeren Incorporated Introduction  341 Cast Unit Configuration   341 Cast Embossed Film   345 Additional Features  345 Section 5.3—Sheet Extrusion Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347 SAM IULIANO, Nordson Extrusion Dies Industries, LLC Introduction  347 Sheet Extrusion Line Equipment   347 Summary  356 References and Additional Materials   357

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Section 5.4—Polymer Rheology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 OLIVIER CATHERINE, Cloeren Incorporated Introduction  359 Polymer Melt Viscosity  359 Shear Viscosity Models  364 Temperature-Dependent Models  365 Some Implications of Shear Rheology for Film Extrusion   366 Viscoelastic Behavior of Polymer Melts   370 Rheology and Coextrusion   372 Measurement Techniques  374 References and Additional Materials   381 Section 5.5—Coating and Laminating Technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 GIANCARLO CAIMMI, Nordmeccanica Group Introduction  383 Preliminary Aspects and Definitions   383 Section 5.6—Metallizing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391 VERONICA ATAYA, Celplast Metallized Products Limited Introduction  391 Metallizing Process  391 Applications  393 Barrier Properties of Metallized Films   393 Processing of Metallized Films   394 Metallized Film Defects   394 Summary  394 References and Additional Materials   395 Section 5.7—Troubleshooting the Extruder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 ANDREW W. CHRISTIE, SAM North America, LLC Introduction  397 Coextrusion Film Systems   397 Troubleshooting Method  398 The Problem Statement   398 The Hypothesis  398 Test the Hypothesis   399 Evaluating the Results   399 Common Problems, Hypothesis, and Tests   399 Section 5.8—Troubleshooting the Blown-Film Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 HARINDER TAMBER and MIREK PLANETA, Macro Engineering and Technology Inc. Introduction  403 Basics of Blown-Film Process   403 Troubleshooting: Blown-Film Equipment and Blown-Film Process   405 Film and Roll Defects   419 Summary  421 References and Additional Resources   421 Section 5.9—Troubleshooting the Cast Film Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 CHRISTINE RONAGHAN, Cloeren Incorporated Introduction  423 Section 5.10—Gel Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427 MARK A. SPALDING, EDDY GARCIA-MEITIN, and STEPHEN L. KODJIE, The Dow Chemical Company Introduction  427 Protocols for Gel Analysis   427

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Examples of Gel Types   429 Locating Stagnate Regions on Screws   435 Poorly Designed Extrusion Processes   436 Nitrogen Inerting on the Hopper   442 The Incumbent Resin Effect   442 Discussion  443 Summary  443 Acknowledgements  443 References and Additional Resources   443 Section 5.11—Extrudable Polymers: Purging and Resin Transactions. . . . . . . . . . . . . . . . . . . . . . . . . 445 SCOTT B. MARKS and BARRY A. MORRIS, The Dow Chemical Company Introduction  445 Theory of Purging/Polymer Transitions   446 Guidelines and Procedures for Purging/Resin Transitions   450 References and Additional Resources   452 SECTION 5.12—Safety in Film Extrusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453 NICOLE E. DOWLING and LAURA K. MERGENHAGEN, The Dow Chemical Company Introduction  453 Preparation  453 Potential Hazards  454 Extrusion  454 Post-Extrusion  455 Maintenance  456 Summary  456 References and Additional Resources   456

CHAPTER 6—STRUCTURE DEVELOPMENT, NOMENCLATURE AND TESTING Chapter Editor: WARREN DURLING, Clorox Services Company

Section 6.1—Film Properties and Performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459 DEAN FERRACANE, SARAH KUHL and KENYATTAH MATHIS, Clorox Services Company Haze  459 Light Transmission  459 Gauge (thickness)  459 Tear Strength  460 Tensile Strength  460 Puncture Resistance  461 Stiffness/Elasticity   461 Density  461 Coefficient of Friction (COF)   462 Surface Roughness  462 Surface Energy  462 Oxygen Transmission Rate (OTR)   462 Water Vapor Transmission Rate (WVTR)   463 Section 6.2—Film Quality and Test Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465 DAVE BOSTIAN, CharterNEX Films Introduction  465 Pre-Fabrication Testing  465 Film Production  465 Visual Properties  466 Physical Properties  467

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Surface Properties  470 Barrier Properties  471 Summary  471 Section 6.3—Introduction to Structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 SCOTT B. MARKS and BARRY A. MORRIS, The Dow Chemical Company Introduction to Packaging and Industrial Structures   473 Functions of a Package   473 Key Questions and Design Criteria for Creating a Package   473 Typical Functional Components in a Packaging Structure   474 Functions of the Layers   474 Surface Layer Choices   475 Bulk Layer Choices   475 Barrier Layer Choices   475 Sealant Layer Choices   476 Adhesive Layer Choices   477 Section 6.4—Critical Requirements for Structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479 THOMAS J. DUNN, Flexpacknology LLC Overall Composite Integrity   479 Packaging  479 Design for Use   480 Design for Manufacture   481 Non-Packaging Applications  481 References and Additional Resources   482 Section 6.5—Quality Control (QC) and Physical Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483 THOMAS J. DUNN, Flexpacknology LLC What to Test: Sampling Production   483 How to Test: Testing Secondary Quality Characteristics   484 Containment Integrity Characteristics   484 Protection/Preservation Characteristics  485 Transportation Integrity Characteristics   487 Communication Integrity Characteristics   488 Extrusion-Coated and Laminated Material Specifications   489 References and Additional Resources   489 Section 6.6—Structure Writing and Nomenclature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 SCOTT B. MARKS, The Dow Chemical Company Writing Guide for Packaging Films and Other Multilayer Structures   491 Guide to Multilayer Structure Writing: Packaging and Industrial Applications   491 Materials (Generic Representations)   492 Structure Examples  494 Trademark References  497 Glossary  499 Conversion Factors for SI (metric Units)  515 Index  517 Chapter Editors and Reviewers Biographies  523

Preface

This manual is a result of almost three years of team effort to update the 2005 Film Extrusion Manual, Second Edition. This project began soon after the update of the Extrusion Coating Manual, Fifth Edition. Many of the same people have been involved in both manuals which helped make the update of this manual go a lot smoother. Thank you to all the editors for soliciting and keeping on top of the authors to ensure the deadlines were met and that we were able to publish the manual in a timely manner. They made my job a lot easier of having to just keep track of the overall picture. This comprehensive publication on the technology and science of polymer film extrusion should be an excellent stand-alone training resource for any professional entering into the industry. Or it can be used as a reference guide for seasoned professionals in the industry looking for specific information. The sources of the material covered in the manual were selected from a combination of sources, such as authors of papers presented at the division conferences, short courses, and subject matter experts across the industry. The authors and editors acknowledge and extend a thank you to TAPPI and its staff for supporting us during this endeavor. They were very helpful and supportive through their guidance and patience in putting this manual together. Special thanks to Kristi Ledbetter and Robert Dawson, our Division Manager(s) for keeping us focused and on task. A special thank you to Mary Anne Cauthen, our Press Publications Lead, for her tireless efforts in managing the content and the continual review, proofreading and publication processes. I have gotten to know Mary Anne much better through this endeavor and have really appreciated her guidance and direction. She is a wealth of knowledge in this area. I would like to thank Warren for his direction and guidance throughout the process, since he did such a great job on the Extrusion Coating Manual and knew what pitfalls to avoid. I would also like to thank my wife, Karen, for being so patient and understanding for all the time I have spent working on the manual. She has been very supportive throughout the process, as well as many other projects I have taken on over the years. The Film Extrusion Manual, Third Edition has been such an enjoyable project to work on because of the people on the Product and Resources team and their continual support throughout the project.

Global Editor First Vice Chairman International Flexible Packaging and Extrusion Division

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Chapter Editors and Author/Contributor Contact Information

CHAPTER EDITORS NORMAN AUBEE NOVA Chemicals [email protected] THOMAS BEZIGIAN PLC Technologies [email protected] THOMAS J. DUNN Flexpacknology LLC [email protected] WARREN E. DURLING Clorox Services Company [email protected] BRUCE FOSTER PolyKnows LLC [email protected] KELLY FREY Chevron Phillips Chemical Company [email protected]

SCOTT B. MARKS The Dow Chemical Company [email protected]

NORMAN AUBEE NOVA Chemicals [email protected]

MARTINE MICHON Atlantic Coated Papers, Ltd. [email protected]

MICHAEL G. BAKER Westlake Chemical Corporation [email protected]

MIKE SHELLENBARGER Oliver-Tolas Healthcare Packaging [email protected]

BILL BODE Battenfeld Gloucester Engineering

DORENE SMITH Westlake Chemical Corporation [email protected] AYSE ALEMDAR-THOMPSON FPInnovations [email protected] RORY WOLF ITW Pillar Technologies [email protected]

DAVE BOSTIAN CharterNEX Films [email protected] GIANCARLO CAIMMI Nordmeccanica Group [email protected] MARK CANRIGHT ExxonMobil Chemical Company [email protected]

AUTHORS/CONTRIBUTORS

OLIVIER CATHERINE Cloeren Incorporated [email protected]

BRAD KRAMER ExxonMobil Chemical Company [email protected]

JEFFREY ARIONUS The Dow Chemical Company [email protected]

ANDREW W. CHRISTIE SAM North America, LLC [email protected]

JAMES F. MACNAMARA JR. Foster Farms [email protected]

VERONICA ATAYA Celplast Metallized Products Limited [email protected]

JACQUELYN DEGROOT The Dow Chemical Company [email protected] xvii

xviii

Chapter Editors and Author/Contributor Contact Information

NICOLE E. DOWLING The Dow Chemical Company THOMAS J. DUNN Flexpacknology LLC [email protected] DEAN FERRACANE Clorox Services Company [email protected] TAD FINNEGAN BASF Corporation [email protected] WALTER FOLKL Foremost Machine Builders Inc. [email protected] BRUCE FOSTER Polyknows LLC [email protected] EDDY GARCIA-MEITIN The Dow Chemical Company [email protected] JA’NAYSHA HAMILTON UBE America Inc. [email protected] BILL HELLMUTH Battenfeld Gloucester Engineering KUN SUP HYUN Formerly of the Dow Chemical Company SAM IULIANO Nordson Extrusion Dies Industries, LLC [email protected] R. E. KING, III BASF Corporation [email protected]

CLAUDE LAVALLÉE 3M Company [email protected]

SERGI SALVÀ SÀEZ UBE America Inc. [email protected]

I-HWA LEE The Dow Chemical Company

NILESH SAVARGAONKAR ExxonMobil Chemical Company [email protected]

DONN C. LOUNSBURY D.C.L. Solutions, Inc. ADAM J. MALTBY Croda Universal Inc. SCOTT B. MARKS The Dow Chemical Company [email protected] RICHARD E. MARQUIS Croda Universal Inc. KENYATTAH MATHIS Clorox Services Company LAURA K. MERGENHAGEN The Dow Chemical Company BARRY A. MORRIS The Dow Chemical Company [email protected] STEFANO PASQUILI LyondellBasell Industries [email protected] JOHN PERDIKOULIAS Compuplast International Inc. [email protected] JOEL PERRITT Westlake Chemical Corporation [email protected] MARIO PERRON LyondellBasell Industries [email protected]

TED SCHNACKERTZ NDC Technologies [email protected] THOMAS J. SCHWAB LyondellBasell Industries [email protected] R. DUANE SMITH Davis-Standard LLC [email protected] MATTHEW SONNYCALB LyondellBasell Industries [email protected] MARK A. SPALDING The Dow Chemical Company [email protected] JAMES STOBIE Macro Engineering and Technology Inc. [email protected] HARINDER TAMBAR Macro Engineering and Technology Inc. [email protected] JOSE TORRADAS SALTOR LLC [email protected] KEVIN TUTTLE Nordson Polymer Processing Systems [email protected]

SARAH KUHL Clorox Services Company [email protected]

CLIFFORD J. WEINPEL Foremost Machine Builders Inc. MIREK PLANETA [email protected] Macro Engineering and Technology Inc. [email protected] RORY WOLF ITW Pillar Technologies MARK PUCCI [email protected] Soarus, L.L.C. [email protected]

AMY M. LAIRD ExxonMobil Chemical Company [email protected]

CHRISTINE RONAGHAN Cloeren Incorporated [email protected]

STEPHEN L. KODJIE The Dow Chemical Company [email protected]

Chapter 1 Film Extrusion Introduction

EDITOR: BRUCE FOSTER, PolyKnows LLC

Section Number

Section Title



Film Extrusion Introduction BRUCE FOSTER, PolyKnows LLC

1.1

Page Number 3

Chapter 1—Section 1

Film Extrusion Introduction BRUCE FOSTER, PolyKnows LLC

INTRODUCTION The film extrusion industry is rich in history and innovation, the result of decades of hard work, R&D, marketing and often serendipitous discoveries. The industry today offers a broad range of very specifically designed polymers and performance-enhancing additives that can be used in sophisticated extrusion equipment technologies, which continually expand the potential for further growth in the industry. A great deal of extrusion innovation occurred after WWII, right up to the present time, and has enabled continuous development of new applications. The purpose of this manual is to provide the reader with the background and most up-to-date information pertaining to film extrusion technologies. HISTORY To recount in detail the history of plastic films would require documenting each discovery by which plastic progressed to the forefront of modern film production, which is worthy of a rather large book. An excellent account of the history of film extrusion was presented by George E. Ealer of Polyfect in the introduction to the 2nd Edition of this manual in 2005 [1]. George’s presentation and the references cited should satisfy even the most inquiring minds interested in the history and progression of this industry.

becoming more common. Similarly, in cast films, there has been a shift from 5–7 layers to 11–14 layers. • Increasing use of nano-layer technology to achieve superior properties while using less of the more expensive resins. • Improvements in bubble quenching on blown film lines, both with air and water-cooling techniques, resulting in better control of the crystalline structure of the final films. • Increased emphasis and advances in sustainable film products. For example, the Sustainable Packaging Coalition, founded in 2004, now has grown to more than 250 member companies and includes film producers, packaging suppliers, resin suppliers, recyclers, brand owners, retailers, academics, and government institutions [2]. • Increased growth in re-sealable packaging. • A more sophisticated workforce. THE FUTURE The authors and editors were also asked to forecast the most significant advances expected in the coming decade, with the following results:

To look at the most significant advances since 2005, the authors and editors of this manual—many of them icons in this industry—were polled to get their take on this question. The poll results were remarkably consistent from author to author:

• Continued emphasis on sustainability in terms of films and packaging that are easier to recycle and growth of bio-based and biodegradable polymers. • Growth of nano-layer films in packaging applications. • Further equipment improvements, resulting in higherquality films and increased throughput. • Resins that are “cleaner” with respect to both gels and organoleptics. • Increased use of automation, robotics, and computer simulation.

• The number of layers in both blown and cast film extrusion has increased significantly. In coextruded blown films, whereas 3–5 layer machines and a few 7-layer machines were running, lines with up to 11 layers are

The future of film extrusion certainly holds many exciting changes and challenges. This book is sure to be a “musthave” reference for those of us fortunate enough to be involved.

SINCE 2005

3

4

CHAPTER 1—FILM EXTRUSION INTRODUCTION

FIGURE 1.1.1.  This top view of an 11-layer blown film line is courtesy of Davis-Standard.

REFERENCES AND ADDITIONAL RESOURCES [1] Ealer, G.E., Film Extrusion Manual, 2nd Edition, pp. 7–14, TAPPI PRESS, Atlanta, 2005. [2] Sustainable Packaging Website: https://sustainablepackaging. org.

Chapter 2 Primary Equipment

EDITOR: MARTINE MICHON, Atlantic Coated Papers Ltd.

Section Number

Section Title

Page Number



2.1

The Film Extruder DONN C. LOUNSBURY, D.C.L. Solutions, Inc. BILL HELLMUTH, Battenfeld Gloucester Engineering

7



2.2

Screw Design ANDREW W. CHRISTIE, SAM North America, LLC

29



2.3

Die Design BILL BODE, Battenfeld Gloucester Engineering

45



2.4

Stacked-Die Technology for Tubular Film Coextrusion JOHN PERDIKOULIAS, Compuplast International Inc.

55



2.5

Winding and Web Handling R. DUANE SMITH, Davis-Standard LLC

63

Chapter 2—Section 1

The Film Extruder DONN C. LOUNSBURY, D.C.L. Solutions, Inc. BILL HELLMUTH, Battenfeld Gloucester Engineering

INTRODUCTION An extruder for a film operation, or any other plastic extrusion operation, can be broken down into four major components: the mechanical extruder, the screw, the drive, and the temperature and drive control panel(s). This chapter will address each of these major components and highlight the features that are critical to a good film operation. Most comments are directed to processing polyethylenes, with brief coverage of processing other materials. THE EXTRUDER The bare mechanical extruder can itself be broken down into four main functional subcomponents: the barrel and feed section, the barrel heating and cooling system, the gearboxthrust section assembly, and probably the key to the entire machine, the screw. THE BARREL AND FEED SECTION The barrel is usually a long, alloy-steel cylinder with flanges on both ends. This cylinder is commonly designed to tolerate internal pressure in excess of 10,000 psi. It is almost always lined with a centrifugally cast, long-wearing, highly crystalline boron alloy. This alloy is frequently an equivalent of Xaloy 101™, although with wide application of tougher LLDPEs, Xaloy 800™ (or equivalent) is used on more abrasive applications. On the grooved barrels that are typical of HMW-HDPE, the cooled feed section is often made of a durable carbide alloy. Otherwise, this section wears significantly in the span of one year. For corrosive applications, higher-nickel alloys such as Xaloy 306™ (or equivalent) are used. Although Xaloy-brand designations have been mentioned for illustration, other barrel liner manufacturers offer similar proprietary alloys. The barrel is divided into zones for heating and cooling purposes with barrel temperature-sensing thermocouples drilled and tapped into the steel of the barrel. The depth of these thermocouples varies from one machine manufacturer to another. Some manufacturers believe that the ther-

mocouple should be right up against the inside hard liner, which comes closest to the melting-polymer skin-layer temperature. Some manufacturers put the thermocouple farther out radially. This trick eases transient temperature control problems with an efficient cooling system, but does so at the expense of controlling to an erroneous temperature that may be quite far from true polymer temperature. The temperature difference between an aluminum heater-cooler in intimate contact with the barrel and a deep-well thermocouple may be as large as 100°F (38°C) under severe water-cooling conditions. Therefore, the depth of thermocouple location in the barrel wall is significant. The feed section of an extruder is merely a pocket in which resin comes into contact with and fills the channels of the screw. It should be gently cooled to avoid formation of any fused polymer because this will cause a blocked screw channel, a condition known as “bridging”. This cooling should not be accomplished with chilled water because airborne moisture will condense on the inside of the feed section, causing water carryover into the melt and possible feed-section rusting, leading to product contamination. This feed section is usually made of cast iron, although in some applications, it is lined with Xaloy (T.M. or equivalent) or nitrided alloy steel for wear resistance. There is usually greater clearance between the feed section and the screw than in the barrel to reduce wear. A seal is commonly provided at the back of the feed section to prevent fines from working back toward the gearbox. Seals for melt-fed extruders are often a “hydrodynamic” seal in which the cooled, highly viscous polymer forms its own seal within a temperature-controlled housing. BARREL HEATING AND COOLING SYSTEMS Barrels are usually electrically heated and cooled either with water or another liquid heat-transfer medium passing through tubes cast into the (usually aluminum) heaters, or by air blown or sucked over the heater. Figure 2.1.1 illustrates a complete air-cooled extruder that incorporates cast-aluminum, deep-finned heater/cooler units as shown in Figure 2.1.2. 7

8

CHAPTER 2—PRIMARY EQUIPMENT

FIGURE 2.1.1.  A complete air-cooled extruder with an insert of the barrel section of a water-cooled one.

Figure 2.1.3 illustrates a typical cast-aluminum watercooled heater-cooler. The individual heater-coolers have coils of Inconel tubing and calrods cast into the aluminum. This is the most efficient of the commonly used heat-extraction systems. The most critical design factors are the surface area of the water-carrying tubes, the machining of the bore, and the bolting or clamping of the two halves to the barrel. Obviously, to extract heat efficiently, the bored ID of

heater-cooler halves must be maintained in contact with the machined OD of the barrel. This is accomplished in one of the following three ways, in order of effectiveness:

FIGURE 2.1.2.  Cast aluminum heater-cooler cutaway for an aircooled extruder (Tempco Electric Heater Corporation).

FIGURE 2.1.3.  Cast-Aluminum Heater-Cooler for a Water-Cooled Extruder (Tempco Electric Heater Corporation).

(1) Bolts of about the same length as the barrel diameter (so that the thermal expansion of the steel of the barrel and the steel of the bolts is approximately matched). (2) Shorter bolts. (3) Straps around the heater-cooler OD.

Section 2.1.  The Film Extruder

9

FIGURE 2.1.4.  Discrete barrel heater. FIGURE 2.1.5.  Discrete heater with extended copper fins.

Figure 2.1.4 illustrates several possible configurations of discrete heaters under a plenum, which may also be an air shroud. These include mica band heaters, ceramic heaters, extruded aluminum channel-type heaters with calrods fitted into the channels, or extruded aluminum channel-type heater-coolers with both calrods and aluminum water tubes side by side in the channels. Inevitably, the contact between these mica band and extruded aluminum channels, which have been bent into a radius approximating the barrel O.D., and the barrel is far less intimate and uniform than the contact between a bored aluminum casting and a turned or ground barrel surface. Furthermore, the contact area of the watercarrying tubing is much less than that of cast-aluminum heater-coolers. Figure 2.1.5 illustrates a discrete heater fitted over copper strip fins to give extended cooling surface area. Radovich (TAPPI 1995 PL&C Conference Proceedings) reported on comparison tests of several available cooling systems. Figure 2.1.6 summarizes the results of those tests and clearly shows the advantage of the cast-aluminum water heatercooler over the air-cooled options studied.

CLOSED-LOOP LIQUID COOLING In any extruder water-cooling system, any minerals put into the system ultimately end up on the inside of the tubes, fouling heat-transfer surfaces and ultimately plugging the tubes. For this reason, distilled water is recommended over demineralized or plant water. It should be free of any stress or crack-causing chloride ions. A closed-loop cooling system with the following components is recommended (see Figure 2.1.7): (1) A stainless-steel tank with a 4 psi pressure cap (similar to an automobile radiator cap). (2) A bronze or stainless-steel fitted pump. (3) A Y-type strainer to prevent fine film flakes from hanging up in the solenoids (optional). (4) Solenoid valves. (5) Manual flow control valves (optional) on each zone if conventional PID controllers are to be used.

FIGURE 2.1.6.  Heat removal vs. barrel temperature for various types of heaters.

10

CHAPTER 2—PRIMARY EQUIPMENT

FIGURE 2.1.7.  Closed-loop cooling system.

(6) A “floating ball” type of water flow indicator (7) A copper-tube, water-water heat exchanger to exchange the barrel heat to city water or, more commonly, a plant tower water system, and then to return the closed-loop distilled water back to the tank. (8) A manifold pressure regulator and a small (1/4-inch) bypass bleed to keep the pump from “deadheading” and to keep the heat exchangers flooded. (9) Corrosion anode pencils installed in both the supply and return manifolds. They corrode preferentially to Inconel, stainless-steel, or brass tubing. This anode is similar to the anode on the top of most domestic hot-water heaters. In the blown-film industry, where directing heat upward from an extruder toward a cooling bubble is necessarily counter-productive, and where extracting the maximum heat possible from the melt is critical, the bulk of new machines installed that are larger than 2-1/2 inches are liquid-cooled. A 2-1/2-inch extruder with an effective air-cooling system can be properly cooled by air, a 3-1/2-inch extruder cools only marginally, and 4-1/2-inch and 6-inch machines are not adequately cooled. The main reason for this reduced ability to remove heat by air cooling is the reduced surface area relative to the volume of melt as the extruder size is increased. This is shown in Figure 2.1.8 and explains why larger ex-

truders tend to be equipped with water-cooling systems that have better heat-transfer capability. A recent innovation is the draw-through air cooling system developed by Battenfeld-Gloucester, with similar designs offered by Egan-Davis Standard and possibly others (Figure 2.1.9). The draw-through system consists of a remote blower with the suction side connected to a plenum in the base of the extruder. The remote blower directs the discharge of hot barrel-cooling air away from the bubble and to the outside. Each zone is fitted with a butterfly valve, which is automatically actuated by the temperature control-

FIGURE 2.1.8.  Air- and water-cooled systems.

Section 2.1.  The Film Extruder

FIGURE 2.1.9.  Draw-through type air-cooled extruder.

ler. When the butterfly valve is actuated, plant air is drawn down through an opening in the barrel covers, down through fins in the cast-aluminum heater, and onto the plenum, thus cooling the barrel. Because it discharges the excess heat away from the bubble, it eliminates one of the objections to air-cooled extruders in blown-film applications.

11

Halving the rated RPM essentially halves the rated HP of the gearbox. Therefore, at the 67 RPM timing of a typical LLDPE, the gearcase can transmit 278 HP at the 1.35 SF mentioned previously, i.e., 415 HP × 0.67. Bearing ratings go down with increased shaft RPM, and therefore when bearings are the limiting factor, increased screw RPM may reduce gearcase capacity. In addition to the mechanical HP rating of a gearbox, there is also a thermal HP rating. Every gearbox is inefficient to some degree, with most helical gearboxes ranging between 3 and 7 percent inefficient. This means that at operating levels, up to seven percent of the power fed into them is dissipated as heat energy. The oil temperature must be maintained below approximately160°F (71°C), or the oil will start to lose lubrication capability. Naturally, as the oil is stirred faster and faster by the gears and any lubricating pumps, it tends to dissipate more energy as heat. Therefore, although the mechanical capacity of a gearcase increases linearly with RPM, the thermal capacity of the gearbox generally decreases as speeds increase. In essence, the thermal rating of the gearbox is that power level at which the inefficiencies result in the inability of the gearbox to keep cool regardless of whether the cooling is natural convection or by blower-forced air. Oil-to-water heat exchangers are added to supplement gearcase cooling. Both the mechanical and thermal capacities of a typical gearcase are illustrated in Figure 2.1.10. Two additional significant characteristics of the gearbox assembly are the dynamic capacity of the radial bearings and the B-10 rating of the thrust bearing. The dynamic capacity of the bearing is its capacity to handle radial load at the speed at which it is turning, i.e., the capacity at which the surfaces of the bearings, rollers, and races begin to crush. Generally speaking, a bearing is

GEARCASE AND THRUST BEARING ASSEMBLY Several ratings characterize the “strength” of a gearbox assembly. Because the AGMA (American Gear Manufacturers’ Association) gearing mechanical HP ratings are for 8 h/day operation and because an extruder is used on a 24 hour per day basis, the gearbox must be rated at 1.25 minimum service factor (SF). This is not a safety factor, but a service factor required by the 24 hour per day operation. As an example, the Davis-Standard 4-1/2-inch “H” gearcase has a 360-HP rating at 1.5 SF, or an actual 1.0 SF rating of 540 HP at 100 RPM. With the heavy loads that LLDPEs apply to the larger (4-1/2-inch and above) extruders, at least a 1.35 SF is needed (curve not shown). Hence, the 4-1/2-inch gearbox can be used to transmit up to 415 HP at 100 RPM. In some cases, the bearings become a limiting factor because they must be sized to accept the side loading resulting from helical gears on the gear shafts. This mechanical HP factor is essentially the torque-carrying capacity of the gear teeth. Therefore, this rating is at a specific RPM and decreases essentially linearly with RPM.

FIGURE 2.1.10.  Mechanical and thermal capacities of a typical gearcase.

12

CHAPTER 2—PRIMARY EQUIPMENT

FIGURE 2.1.11.  Gearbox with bull gear between radial bearings and thrust outboard.

operated under loads at five to twenty percent of its dynamic capacity. When a bearing is operated below about 10 percent of its dynamic capacity, it tends to become inefficient. The second rating of a bearing is the B-10 life in hours. This is the life at which, theoretically, one-tenth of the bearings can be predicted to have failed. The average bearing life is approximately eight times this B-10 figure. Because these B-10 figures are often tens or hundreds of thousands of hours, this implies that the bearing will last under ideal conditions of lubrication, alignment, centering, and load sharing for decades. This rating is affected by several factors, including lubrication, alignment, etc. Consequently, the location of the radial bearings may have an influence of the actual life of the thrust bearing because it controls the amount of deflection of the thrust shaft at the thrust bearing and thus the degree of misalignment. Furthermore, if the bearing is a dual-thrust bearing, as is common on some larger machines, at loads below 10–20%, the bearing does not center properly or spread the load equally between the races and fails prematurely. Figure 2.1.11 illustrates a gearbox in which the bull gear is located between two radial bearings of the gearbox, with the thrust bearing outboard of the radial bearing. The deflection of the thrust shaft due to the reactant load applied to the bull gear is, in this case, 1/2–1 mil or 0.0005–0.001 inch. Figure 2.1.12 illustrates a gearbox in which the bull gear and thrust bearing are both inboard of the radial bearings. The deflection of the thrust shaft due to the reactant load applied to the bull gear in Figure 2.1.12 is around four mils (0.004 inch), or four times as much as in Figure 2.1.11. The Figure 2.1.12 case results in greater cocking of the thrust shaft relative to the thrust bearing. This means that, even with a perfectly aligned extruder, only a few of the balls or rollers of the thrust bearing are actually taking their full load, and the bearing is likely to fail prematurely. The customer inevitably specs out in his next machine a B-10 rating that could properly be measured in eons. The customer’s real problem is not an under-rated thrust bearing, but poor mechanical alignment of the machine. The location of the bearings, the style

of radial bearings (adjustable tapered roller being the best), and the elimination of slip-fit tolerances by shrinking bearings and gears to the thrust shaft all work toward improving the actual life of a bearing, given equal B-10 lives. Specifications of B-10 lives in excess of true requirements only mask the real causes of thrust-bearing failures. These are lubrication inefficiencies, heat buildup, misalignment, and, in the case of the dual-thrust bearing, lack of enough loading to center the bearing. Several other elements of the extruder should be briefly mentioned. Many extruders are vented. If so, those running olefins almost always run with the vent plugged. PVC, styrenes, and some other resins may run vented. The location of the vent is particularly important in a blown-film extruder because blown film must be extruded against a fairly high die pressure. Therefore, the portion of the screw downstream of the vent must be relatively long to pump effectively against that pressure. Vented operation is, without question, an added

FIGURE 2.1.12.  Gearbox with bull gear and thrust bearing inboard of radial bearings.

Section 2.1.  The Film Extruder

difficulty. If it is realistically possible to avoid such an operation, it should be avoided. Every extruder should be fitted with an operating pressure gauge indicating the pressure at the delivery end of the barrel before the breaker plate. Some form of over-pressure relief is also required. There are too many cases of operators starting up with a cold plug of polymer in the adapter or die and consequently blowing the head off the extruder. The overpressure relief is usually a rupture disk fitted at the delivery end of the barrel. Care must be exercised as to where hot polymer will be discharged. The base of an extruder simply ties all the components together and makes it possible to align the machine. It is recommended that any machine above a 4-1/2 inch, 24:1 should be optically aligned with borescoping instruments at installation. SCREWS Before discussing the various types of screws in common use, the basic output estimation and scale-up factors that are applicable to the metering section of conventional or barrier screws will be presented. Equation (2.1.1) indicates the relationship of the outputs of two different-diameter extruders having the same L/D. The output of one extruder is related to the output of the second as their bore area or their diameter squared. From this, it is clear that the output of one 3-1/2inch extruder approximately equals that of two 2-1/2-inch extruders. In scaling from one size extruder to another, however, complications may arise regarding shear rate and its effect on mixing and melt temperature with varying polymer viscosity. If one took a 2-1/2-inch extruder operating at a given RPM and metering depth and proposed to design a 3-1/2inch extruder with the same metering depth and RPM, one would see approximately twice the output predicted in Equation (2.1.1). Melt temperatures would also be severely elevated due to the high shear rates imposed upon the resin. For this reason, it is necessary to scale up at conditions of approximately equal shear and mixing. These relationships are illustrated in Equations (2.1.2) and (2.1.3): Q1 A1 ( D1 ) = = Q2 A2 ( D2 )

(2.1.1)

N1  D2    N 2  D1 

(2.1.2)

N = RPM D = bore diameter SF = scale factor H1 H2

D   2   D1 

SF

(2.1.3)

where: H = meter depth D = bore diameter SF = scale factor For conditions of equal shear, Equation (2.1.2) specifies that RPM varies inversely as the ratio of the diameters raised to an exponential scale factor. Equation (2.1.3) specifies that H, or the metering depth, varies directly with the diameter ratio raised to an exponential scale factor. The value of this exponential scale factor has historically been taken as 0.5. This is known as the square root scaleup and is acceptable for relatively low-viscosity materials. As one moves toward higher-viscosity polymers, scale factors up to 0.9 are used on highly viscous materials. Table 2.1.1 lists a series of scale factors that have been found to be useful in this screw design scale-up. Equation (2.1.4) represents a simplified and frequently used output relationship. This equation estimates the output as a function of a size factor (determined by extruder size and screw channel geometry), the RPM at which the machine is operated, the metering depth of the screw, the polymer density at melt conditions, and an efficiency factor. This efficiency factor can range between 115 percent for the first or metering stage of a compression-relief screw down to 50% for a water-cooled metering screw (see Table 2.1.2). For low-density polyethylene (LDPE), it is approximated at 0.85 for a mixing screw. The size factors for several common-size extruders have been tabulated. This factor is not only a size factor, but also includes the conversion from the inches of depth and grams/cm3 of the density factor into the more useful pounds per hour. Q  FNHE  where:

Q = output A = bore area D = inch extruder diameter



(1) 3-1/2″ extruder approximately = (2) 2-1/2″ extruders (1) 4-1/2″ extruder approximately = (1.75) 3-1/2″ extruders (1) 6″ extruder approximately = (1.75) 3-1/2″ extruders (1) 8″ extruder approximately = (1.75) 3-1/2″ extruders

SF

where:

where:

Thus:

13

Q = output, lb/h N = RPM H = meter depth, in E = efficiency ρ = polymer density (vs. water) F = 17.3 for 2-1/2″ extruder 33.9 for 3-1/2″ extruder 56 for 4-1/2″ extruder 99.9 for 6″ extruder

(2.1.4)

14

CHAPTER 2—PRIMARY EQUIPMENT

TABLE 2.1.1.  Scale Factors. Polymer

Scale Factor

LDPE –0.8–3 Ml

0.65

LDPE –0.1–0.8 MI HDPE PP FVPC

0.75 0.9 0.75 0.7 0.9

RPVC

FIGURE 2.1.13.  Single-stage metering screw.

FIGURE 2.1.14.  Maddock mixing screw.

Efficiency is the key to making the equation work. Careful judgment must be used in selecting the proper efficiency factor. However, in scaling from one size extruder to another (which is operated under essentially the same conditions), the efficiency of the base machine can be calculated and that figure used to estimate the second machine’s output. The expression “metering depth” has been used for all the screws mentioned. This metering depth is the depth of the metering section of a single-stage or barrier screw, or the first meter in a two-stage screw. With conventional, non-barrier screws, an approximate 3:1 compression or depth ratio is generally used. This is the ratio between the feed section depth and the metering section depth. Under certain conditions when extruding fluff or ground film scrap, a deeper feed section is used. One must be careful in cutting the feed section to avoid reducing the torque-carrying capacity of the screw to the point where it shears off from the torque of the drive. This is a very real concern for a 2-1/2-inch and smaller screw, a casual concern on a 3-1/2-inch screw, and seldom a concern for screws larger than 3-1/2 inches. A minimum of a 2:1 SF over the theoretical torsional yield point is recommended. Six of the more common designs of screws used for extrusion of polyolefins and other materials are illustrated in Figures 2.1.13–2.1.18. To avoid confusion, the dimensions of screws are deliberately distorted in the X vs. Y coordinates to clearly point out the distinction between the feed, transition, and metering zones of the various screws. Figure 2.1.13 illustrates a single-stage metering-type screw, which was a basic standard polyethylene screw of the early 1960s. It usually requires water cooling in the metering zone to obtain decent mixing and melt uniformity. Today it is not considered an optimum screw. Today, mixing is a fundamental requirement in the single-screw extruder. In cases where incompatible polymers are being mixed or solid fillers are being incorporated into a polymer blend, dispersive mixing is necessary. This type of mixing is accomplished by imparting shear stress to physically break down the minor phase of the blend, as is done in the Maddock mixer (see below), as the mixture is TABLE 2.1.2.  Efficiency Factors.



E = 0.5–0.7 Water-cooled screw 0.65–0.9 Neutral mixing screw

0.8–1.0 Neutral compression-relief screw

FIGURE 2.1.15.  Pin mixing screw.

FIGURE 2.1.16.  Combination pin-maddock compression relief mixing screw.

FIGURE 2.1.17.  Two-stage vented screw.

FIGURE 2.1.18.  Barrier-type screw.

sheared over the undercut land section. In other cases such as incorporation of color masterbatch, distributive mixing is required. In fact, distributive mixing is also necessary to achieve temperature and viscosity homogenization of the polymer extrudate. This type of mixing occurs with an increase in interfacial area between the two components (or a single material with a non-uniform temperature distribution) until the striation thickness is reduced to an acceptable level. The process occurs to some extent as a result of the basic circulatory flow in the metering channel of a conventional flighted screw. However, additional mixing sections can be used to drastically increase the distributive maxing capacity of the screw design. These include Maddock, pin, Dulmage, and Saxon mixing sections, among others. Figure 2.1.14 illustrates what was probably the standard screw for an olefin application in the early 1980s in a 20:1, 24:1, or 30:1 configuration. It is a screw with a Union Carbide “Maddock” mixing section (named for the inventor, Bruce Maddock). It is covered by a Union Carbide patent. The industry owes a debt of gratitude to Union Carbide for their dedication of this patent to the industry. The Maddock

Section 2.1.  The Film Extruder

mixing section is a series of inlet / outlet channel pairs separated by barriers and undercut lands over which the polymer must pass and be sheared. There are several mixer designs that are similar in principle to the Maddock section, with spiral channels and lands. Figure 2.1.15 illustrates the pin mixing screw. This screw was a standard of the PVC industry in the 1970s and was one of the early mixing screws. This screw is likewise useful in polyethylene. The basic principle of the screw involves splitting and recombining of flow streams as the polymer passes through the several rings of pins. This is a low-shear mixing device that can improve homogenization of the polymer extrudate. Both these latter two mixing screws deliver well-mixed polymer melt with fair output uniformity at moderate temperatures and high outputs. All mixing screws use the basic principle of allowing the entire screw to be cut deeper, meaning that each revolution tends to pump more and the shear rate between barrel and screw channel is lower. This leads to higher output at lower stock temperature compared to a metering screw, with good output uniformity. In both cases, the screws should be run neutral or water-cooled only through the feed section. Feed-section screw cooling reduces bridging without sacrificing output when flake is reintroduced with virgin resin. Figure 2.1.16 represents a combination pin-Maddock compression relief screw, which was found to be optimum on 34:1 long-barreled LDPE applications in the early 1980s. In these long extruders, a single long metering zone, with or without a Maddock section, would result in too much shear-heat buildup. Such a screw is particularly useful for hard-to-melt materials such as polypropylene. It delivers a homogeneous melt at high rates and moderate stock temperatures. This screw configuration has probably been largely supplanted by the optimized barrier screws discussed below. Figure 2.1.17 illustrates a vented screw. It may be a metering-type screw, as shown, or may be deeper and have pins or a Maddock section for higher-output mixing applications. The polymer is pumped from the first meter to the vent section, where it is devolitized. It then passes into a recompression section and into a pumping or second metering section, which must develop the pressure required to pump against the backpressure of the die. This means that in a typical blown-film application, with a 3000–5000 psi die, die inlet pressure creates a very small operating window between running vented into the die without polymer backing up, and polymer flowing out the vent. Here again, 30:1 or 34:1 ratios are found to be useful extruder lengths because the pump section can be increased to approximately a 10 D length. Melt pumps can be used to boost the melt to die inlet pressure, thus enlarging that small operating window. Figure 2.1.18 illustrates a barrier-type screw. In the latter half of the 1980s, this screw design largely replaced previous screw designs. Barrier-type screws deliver more pounds per hour per revolution with lower melt temperatures, with reduced machine-direction flow (pressure) fluctuations, and with lower cross-stream temperature varia-

15

tions. An explanation of why the barrier screw is superior is appropriate here. Once polymer pellets are fed into the extruder, they are conveyed and compacted into a “solid bed”. Melting begins at the interface between this bed and the barrel wall due to the shear stress generated and the conduction of heat through the barrel wall. Because of the geometry and relative motion of the screw and barrel in the extrusion process, a melt pool subsequently forms on the pushing edge of the flight as melt is scraped from the polymer / barrel wall interface. A thin film remains between the solid bed and the barrel wall, where the most efficient melting occurs. In a conventional metering screw, melting occurs primarily in the compression zone of the screw, where channel depth is decreasing from feed depth to metering depth. In the transition zone, the molten material and the solid bed are forced to compress together, often resulting in integration of the solid and melt phases (often referred to as solid-bed breakup), as shown in Figure 2.1.19. As solid particles become encapsulated with melt, the melting process becomes very inefficient. The molten material cannot effectively transmit shear stress or conduct heat, and therefore melting of the remaining solids becomes difficult or may not occur at all. The result may be unmelt in the extrudate. In addition, the instability of the solid bed can result in pressure and output surging, ultimately affecting gauge uniformity and product quality.

FIGURE 2.1.19.  Metering screw melting (melt flow to right).

16

CHAPTER 2—PRIMARY EQUIPMENT

mix a soft EVA and blend in color will tend to run a stiff LLDPE or HDPE too hot. A screw designed for a 5000-psi head pressure will deliver melt with gels and applesauce when feeding a 2000-psi die. A screw designed for 100 percent LLDPE will not sufficiently mix a 30 percent LLDPE/70 percent 2MI LDPE blend. This is where laboratory facilities and the experience of the screw vendor come into full play. A wide variety of screw and mixer designs exist, but most if not all are protected by various patents, and hence the reader should consult the patent literature for more details. EXTRUDER DRIVES

FIGURE 2.1.20.  Barrier screw melting progression (melt flow to right).

The advantage of the barrier screw is that there is a physical barrier between the solid bed and the melt pool, which allows the depth of each channel to vary independently. The flight between the solids channel and the melt channel, the barrier flight, is undercut to allow molten material to pass from the solids channel to the melt channel. The melt channel progressively increases in cross-sectional area along the barrier section to accommodate molten material (Figure 2.1.20). Similarly, the cross section of the solids channel progressively decreases to keep that channel tightly compacted against the barrel wall and to assist in melt extraction. This geometry makes the melting process more efficient by maintaining an intact solids bed and a thin melt film. The result is improved stability, higher output rates, better temperature control, and superior extrudate quality. A mixing section such as the Maddock mixing section described above is often used in conjunction with a barrier screw to ensure excellent homogenization of the extrudate. Barrier screws are inherently more universal in application, but they can definitely be tailored to do specific jobs well. Barrier screw designs can be made to accommodate wide variations in mixing intensity by adjusting several variables, including the compression ratio of the solids channel as well as the undercut of the barrier flight. Higher-intensity designs have higher compression in the solids channel and a tighter barrier undercut. In optimizing a screw design, these variables are chosen to match the characteristics of the resin or resins being processed, as well as the melt temperature and stability requirements of the process. A screw with channel depths and clearance gaps that are designed to melt and

The drives for film extruders must be variable-speed drives. Today (2000), there are two contenders for the film extruder drive market. They are the “traditional” statically rectified DC drive and the variable-frequency AC drive. Today, the price advantage goes to the DC drive above about 150 HP and to the AC drive below 50 HP. That, however, is not the only criterion. Many older film extruders in the field today are driven with static DC, 1750 RPM drives that offer only armature regulation. As slower-turning barrier screws are pumping stiff LLDPE with increased torque demand at a screw-speed range of 40–60 RPM for 4-1/2-inch and 6-inch extruders, lower base-speed drives with a field-regulated higher speed range are being used. An alternative drive, the solid-state variable-speed AC drive, is beginning to come down in price to the point where it is cost-effective in the mid-HP range. These drives use induction or synchronous AC motors, which offer much lower maintenance than DC drives. THE DC STATIC DRIVE The armature-regulated static DC drive usually has a 1750-RPM base-speed motor. It develops constant torque between zero and 1750 RPM, and because HP equals (a constant times) torque times RPM, the HP varies from zero to nameplate power at a 1750-RPM base speed. These drives are available in the low-HP range (up to 250 HP) with 1750RPM base speed/2000 or 2200-RPM top speed motors. The latter are DC drives with a field regulation range above base speed plus the usual armature voltage range below base speed. They have a constant-torque armature voltage range up to 1750 and a constant HP with a decreasing-torque, field regulation range between 1750 and 2000 or 2200 top RPM. The big shift in DC drives today is the frequent use of 1150-RPM base-speed drives (i.e., they develop name-plate HP at 1150 RPM), field-regulated to 1600 or 1800 RPM. This has evolved partly because many processors must on one day process, for example, a 2 MI LDPE resin, requiring relatively low torque at high speed, and on the next day process a fractional MI LLDPE requiring high torque at low speed. For example, such a 4-1/2-inch, 24:1 extruder could be belt-connected to a 150 HP, 1150-RPM base-speed motor field and regulated so that the 1150-RPM base speed with

Section 2.1.  The Film Extruder

17

over perhaps one-third of the new extruder drive market. They are definitely more cost-effective below 40 HP and are reasonably cost-competitive through probably 150 HP. Although they were available a decade ago, drive regulators were felt to be complex and somewhat trouble-prone. Today’s drive-regulating computers or chips can be programmed to accomplish a wide variety of functions and have become quite reliable and maintenance-free. AC motors have several inherent advantages over DC motors in the usual extruder market:

FIGURE 2.1.21.  Three typical operating points.

high torque occurs at about 65 RPM. The field-regulation range then takes over to process lower and lower torque blends to finally reach a 2 MI LDPE at 110 RPM or 2000 motor RPM. The required alternate drive is 250 HP at 1750 RPM connected for 110 RPM. This is illustrated in Figure 2.1.21. Power companies often impose surcharges for a power factor below 0.85. A side benefit of operating low-basespeed drives in or near the field-regulated range is that these surcharges are largely avoided when operating in the fieldregulation range. All DC drives have about a 0.85 power factor at base speed and vary linearly down to zero at zero speed, which means that operation below base speed will result in power-factor correction costs where power companies insist. Some manufacturers recommend that drive connections up to 250 HP be through belts and sheaves. Although these wear out, they do squeal loudly in the event of a frozen screw, at which time the operator shuts down the machine. Most importantly, they offer a wide range of flexibility in retiming the extruder to meet changing conditions in the field. Above 300 HP, direct-coupling the drive to the gearbox is recommended, although it limits flexibility to the gearing built into the machine (without performing an expensive regearing). Inquiries have been made (usually from plants in thunderstorm-prone areas) regarding timed delay features that enable the drive to keep itself going after a momentary (one-half to several seconds) power flicker. Such a feature is available as either a retrofit or as a special purchase when a new extruder and upper nip drive are ordered. This feature costs a major fraction of the price of the drive. AC VARIABLE-FREQUENCY DRIVE AC variable-frequency drives for extruders have taken

• They have no brushes or commutators, and therefore no brush and commutator wear, and no susceptibility to failure due to commutator coating by the fumes inherent in many extrusion plants. Both these factors contribute to low maintenance costs. • Unlike DC drives with their power factor decreasing linearly below 0.85 as motor speeds decrease from base speed, AC drives rectify the full AC waveform, thus imposing a power factor on the incoming AC line of close to 1.0, and this is independent of drive speed. • In the unusual extrusion applications involving the need for explosion-proof drives, AC motors have a great cost advantage over DC motors because the former lack a commutator and brushes (and therefore do not spark), meaning that they can easily be made explosion-proof. The motors used in AC variable-frequency drives must be “inverter-rated” (NEMA MG1) for the high-voltage spikes that are inherent with these drives, but this is not a major cost burden. DESCRIPTION The first functional “block” in the drive regulator is a three-phase AC-to-DC rectifier that converts the 460 volt, 60 Hz incoming power to DC and puts it on the “DC bus” at approximately 625–650 volts DC. The next functional “block” draws power from the DC bus and directs or discharges it in a very large number of very short-duration “bursts” or “pulses” of energy (timed so that the RMS (root mean square) voltage simulates the sine curve of AC power) into the three phases that the AC motor uses. These pulses must obviously maintain the 120-degree interval between each phase and the next for which these motors are designed. The drive’s computer is timing and “picking off” the pulses, modulating their width, and directing them off to the respective phases. Because AC motors can operate over a wide range of speeds as long as the ratio of RMS voltage to AC line frequency applied to the motor is kept constant, the drive regulator can be set to deliver AC, threephase power anywhere from 60 Hz (or higher in “extended range”, the common U.S. incoming power frequency, down through 30, through 6, and even down to 6/10 (and even to zero) Hz. With the usual four-pole motors, this results in 1800, 900, 18, or 1.8 or zero motor-shaft synchronous RPM respectively, or with the slip designed into the motor, 1750,

18

CHAPTER 2—PRIMARY EQUIPMENT

FIGURE 2.1.22.  Maintaining constant rms voltage-to-hertz ratio.

875, 17.5, and 1.75 motor RPM. The drive computer must “space out” the bursts of energy along the sine wave, all at full DC bus voltage, to maintain that constant RMS voltageto-Hertz ratio. Figure 2.1.22 provides an illustration of this concept (but with far fewer and far broader bursts than the real drive delivers). Drive manufacturers have evolved several variants of this theme. The simplest is “open loop” control, or control of motor speed with no feedback, and therefore no correction for sudden load changes or drift. A second is adding an encoder, which the drive computer reads to improve speed regulation and range. A third is adding dynamic braking capability to the drive just described, so that should the drive speed up on load shedding, energy is soaked up by dynamic braking resistors, and the drive slows down. These several features yield differences in low-speed torque characteristics, speed of response, regulation, and extended-range torque capabilities (equivalent to the DC field-regulated, constant HP range). The author is pleased to gratefully acknowledge the assistance of John Shallbetter of Rockwell Automation in preparing this description. HEAT CONTROL PANELS The heat-control panel houses controls for barrel heating and cooling and for die heating (and frequently houses the drive regulator(s) also). The controls may consist of a number of discrete temperature-only control instruments. A temperature controller with time proportioning on the heating side and on-off controls for the air-cooling blower is generally used on discrete types of controllers for air-cooled extruders. Due to the in-rush currents resulting from turning the blower on and off, a time-proportional controller cannot be used because it would rapidly burn out the blower motor

Water-cooled extruders require time-proportioning controls on both the heating and cooling side. Even with this, balancing may be needed between the capability of the controls to control on the cooling side and the capacity of the heat-extraction system to extract heat. This usually means adjusting the proportional band and reset controls of the controller simultaneously while throttling the water flows to the individual zones to achieve stable control. In the past decade, a number of controllers have been developed that feature tuning that is done automatically by the instrument, with minimal operator intervention and manual adjustment of water flow rates. This tuning is enhanced by the presence of a supply-manifold pressure regulator on the closed-loop water system because this regulator makes sure that the instrument “sees” approximately the same heat extraction, regardless of whether one or more zones are cooling simultaneously. Almost all major extruder manufacturers have developed computer process controls that control temperatures, the extruder, and any melt pump drives and ramp-ups, control line speeds to maintain gauge control, maintain a history of the operation, collect data for SPC and SQC reports, log for retention and display line conditions on a CRT, and interface with auxiliary equipment such as gravimetric feeders, treaters, roll scales, and roll label printers. Most of today’s new film lines and the vast majority of new co-extrusion lines feature these computer-based process controls. Ammeters are very helpful on die zones because the wiring in these zones is usually more exposed and susceptible to mechanical damage and the heaters themselves are far more likely to burn out. It is recommended that die zones be limited to 240 V. The calrods in the aluminum barrel heater casting almost never fail and are hard-wired in concealed wiring conduits and troughs. Ammeters on such zones have limited value. EXTRUDER MAINTENANCE This section is a condensation of thoughts presented in the article, “What Processors Should Know (But May Overlook) About Extruder Maintenance” by Donn C. Lounsbury, which appeared in the December 1981 issue of Plastics Technology (and is still valid in the 21st Century) Preventive maintenance does not eliminate unforeseeable production downtime, which can strike even the best-maintained extrusion operation. Continuously tuning individual extruders and scheduling periodic downtime are strategies that make a processor’s lines less prone to the major mishaps that can cause costly production stops. Paying careful attention to extruder maintenance is a form of quality assurance that is basic to producing a high-quality product. Regardless of the type of extruder used, following similar preventive maintenance guidelines can only result in enhanced productivity and increased profits. The key is constant surveillance, which will help stretch an extruder’s life and ensure maximum output. Tables 2.1.3 and 2.1.4 feature preventive maintenance checklists during both stable runs and “downtime”.

19

Section 2.1.  The Film Extruder

TABLE 2.1.3.  Preventive Maintenance Checklist for Extruders: Stable Running. Component

Period

Gearcase

TABLE 2.1.4.  Preventive Maintenance Checklist for Extruders—Downtime. Component

Period

Gearcase W

Visually check condition of gears

Q

W

Replace oil per manufacturer’s instructions

Q

Observe/record oil pressure

W

Clean oil sump and examine for metallic inclusions

Q

Observe/record oil temperature

M

Remove sludge from oil heat exchanger

A

Feel/record thrust housing temperature

M

Change or clean oil filter

Q

Look for oil leaks at shaft seals

M

A

Listen for unusual bearing and gear noises

M

Examine water side of oil heat exchanger, clean if needed

Detect oil leaks at windows, joint lines, etc

A

Check drive belts and replace (as full set) if required

If instruments are available, record noise level of bearings

Q

Be sure all belts have about 1/2 inch play*

Q

Check/record screw runout between gearcase and feed area

Q

Clean filter on armature cooling blower intake(a)*

M Q

See that screw-cooling rotary-union seals are tight

Q

Observe color and surface imperfections on commutator* Clean brush holder, commutator, and windings(a)*

M

Lube motor bearings exactly as specified

A

Feel/record radial bearing cap temperatures Track oil flow to all bearings

Drive Area

Drive Area Q

Check air flow through DC motor

Q

Be sure terminals inside motor conduit box are tight

A

Look for sparking at motor brushes*

M

Check insulating tape in conduit box*

A

Record temperature of motor housing

Q

See that guards are securely back in place

A

Check/record temperatures of motor bearing caps

Q

Observe condition of motor internal housing for foreign material*

M

Check emergency stop switch and hopper barrier guards

Q

Check for excessive motor vibration

Q

A

Be sure belt guards are securely in place

W

Look for gauges or excess wear in feed section casting Check condition of seal and bushing

Q

Feed Section

Feed Section

Barrel, Heating and Cooling Assembly

Check water flow

Q

Check corrosion pencil anodes (zincs)

A

Look for excessive powder leaks

Q

Clean out sump

A

Check temperature of feed section casting

W

If treated water is used, check condition and level(b)

Q

With municipal water systems, flush lines with scaleremoving compound

A

If distilled water, check chloride ion concentration and change K more than 10 ppm*

Q

Check municipal side of water-to-water heat exchanger for scale buildup problems

A

If air-cooled, clean dust from filter screen and wheel, replacing inlet screen if necessary(a)

Q

Lube blower motors per manufacturer’s recommendations

A

Check seating of all thermocouples and heater terminal connections*

A

Measure ohms across heaters to ground*

A

See that all heater clamping bolts or straps are tight

A

Replace all required screws on barrel covers

A

Barrel, Heating and Cooling Assembly Check amps of barrel heaters*

Q

Identity and isolate leaks in closed-loop systems

A

Check pressure and temperature regulators

A

Inspect pump seal for leaks

A

Observe flow of tower/municipal water source

A

Look for apparent solenoid difficulties

U

Check water condition for possible scale problems

U

Feel temperatures of blower motors

Q

Clamp Area Check for leakage at seals

M

Be sure that pressure indicator is functioning properly

W

Determine screw output data base (see text)

A

Panel Check balance (within 15%) of amp draw on all three phases of AC drive input

Q

W = Weekly M = Monthly Q = Quarterly A = Annually *Procedure should be performed only by trained, skilled mechanics familiar with the electrical/mechanical hazards inherent in checking procedures.

Clamp Area Check that over-pressure devices (shear pins, rupture discs, and pressure controllers) are functional

A

Inspect angles of clamp and flange

A

Check breaker plate recess seal surface

Q

Verify pressure calibration of indicator-controller

A (continued)

20

CHAPTER 2—PRIMARY EQUIPMENT

See that all receptacle terminals are grounded A

A

Check condition of all head/die cords and plugs W

W

With the head separated check to see that all die ground blades have solid continuity to the die Q

Q

Inspect the condition of die thermocouples and thermocouple cords Q

Q

ing the machine oil quarterly or at least every 1500 machine hours. It is important to remember to use the same brand and grade of oil because additives used by different manufacturers are not necessarily compatible Changing the oil should include removal and close inspection of the residual sludge. Bronze metal chips may indicate incipient thrust bearing failure, and steel particles in the spent oil usually indicate a defective radial bearing or even gear-tooth problems. Cast-iron powder suggests abnormal wear of a moving part against the interior of the gearcase itself. It is also important to monitor oil leaks, which can increase contamination through careless refilling and decrease the lubricating ability of the oil. Operators should check around the gearcase windows, covers, and fittings as well as near oil lines, seals, filter, pumps, and plugs. Breather plug holes should be kept clear of dust and oil plugs. Operators should also monitor the temperature of bearing covers on the gearbox, paying particular attention to input, intermediate, and thrust shaft radial bearings as well as the thrust housing. If surface temperatures are too hot to touch, i.e., above about 140°F (60°C), excessive friction may be the reason for high bearing housing temperatures. Nevertheless, potentially troublesome bearings should be identified and carefully inspected or replaced during scheduled downtime. A commonly ignored reason for high oil temperature on water-to-oil cooled systems is poor heat exchange due to scale deposits. Commercially available scale treatments (usually supplied by local refrigeration or boiler cleaning contractors) can do wonders for heat-exchange efficiency.

As a final safety check, be sure that all guards are back in place and fully secured, and that all safety signs are posted and legible W

W

UNUSUAL NOISE INDICATORS

TABLE 2.1.4 (continued).  Preventive Maintenance Checklist for Extruders—Downtime. Component

Period

Control Panels Visually inspect power connections for heat buildup or arching*

Q

Check tightness of heat and drive power connections

A

Verify continuity of all ground connections*

A

Check calibration of all temperature controllers and drive meters*

A

Calibrate pressure indicators/controllers

Q

Check function of door interlock, tightness of fuse clips*

Q

Vacuum or blow out dust from panel, especially at contacts(a)*

Q

Clean air filter at ventilation input*

Q

Check operation of ventilation fan*

A

Verify drive potentiometers have steady, dead-spotfree increase in resistance from zero to full value*

A

Check drive acceleration and set 2 second minimum

A

Be sure safety devices and overloads function properly

Q

Plug, Plate

*Procedure should be performed only by trained. skilled mechanics familiar with electrical and mechanical hazards inherent in checking positions. (a)When running dusty material such as PVC in particular. (b)Some sealed water-cooling systems shouldn’t be tampered with; check maintenance manual. (c)The main breaker will be alternately off and on during these tests. Remember that dangerous voltage exists at the upper terminals or line of the breaker even in the off position.

PREVENTIVE MEASURES One of the most revealing indicators of an extruder’s health is the condition of its lubricating oil. Oil contamination or low lubrication levels can easily damage critical, expensive machine parts. Excessive oil pressures or temperatures are very serious symptoms of impending downtime. Although a simple immersion thermocouple and pressure gauge will silently monitor these important symptoms, some processors have added fail-safe devices for further protection against breakdown by adding a thermocouple and transducer and linking them to a microprocessor to issue a warning or shut the extruder down. Often, an apparent problem may not be gearbox troubles, but only a plugged filter or dirty oil. An often overlooked preventive maintenance practice is chang-

Excessive noise from the input, intermediate, and thrust shaft bearings or abnormal sounds from the gears, oil pump, and drive belts all indicate a worn part that will only cause additional damage if not replaced promptly. Although it is not uncommon, be especially wary of a ticking noise much like that of a high-speed train going over rail joints. If detected in the gearcase area, a broken gear tooth may be a problem. Some processors are making use of a sound-impact pressure instrument to periodically monitor gearbox sounds, although the tried-and-true screwdriver method is still commonly used. However, the sound impact stethoscope devices have the advantage of accurately recording the impact forces as bearings rotate. When used often, such an instrument can generate a normal noise pattern for each bearing. Subsequently, these profiles can then be compared to see increasing sound pressure traces and thereby predict bearing failures when the rate of increase changes significantly. Determining which bearing to replace before tearing down the gearbox is especially important. There is nothing more frustrating than ordering a replacement part and then finding out that the offender is a completely different bearing. If cyclical noise is present, the number of cycles per turn of the thrust shaft should be timed. Knowing the ratios in the

Section 2.1.  The Film Extruder

gearcase enables quick identification of probable causes. For example, if a gear ticks every five times that the thrust shaft revolves, the trouble is probably an intermediate pinion, the high-speed gear, or the bearing on the intermediate shaft. An unusual noise occurring about 17 times per revolution usually indicates something defective on the high-speed shaft bearing or pinion. All these statements apply, of course, to the fairly common, double-reduction, 17:1 nominal gearbox. Another good indicator of bearing troubles is in the screw runout, which should be checked under loaded operating conditions between the gearcase and thrust housing. A dial indicator should be used to check the total runout (TIR) regularly, which usually varies from 0.003–0.010 inch depending upon the extruder design. Gradual deterioration signals bearing wear, which can sometimes be dealt with by adjustments. Rapidly increasing runout indicates imminent bearing failure or a dirty screw shank. Corrective action should be taken before further damage is done. When observing the gears, shallow pitting may be visible (unless the gears are very lightly loaded), ideally across 85% or so of the face of the gears. These small depressions, usually about 1/32 inch in diameter and spaced about 1/8 inch apart, are caused by microscopic high spots on the gear tooth face that are more highly stressed than the bulk of the gear. When a gear displays this pattern, such depressions are not a bad condition, but rather a protective one, acting as oil pockets and actually contributing to longer gear wear. However, more frequently spaced and deeper pockets signal either an overload condition, too little lubrication or of the wrong type, poor gears, or some other adverse condition that usually leads to gear replacement. Gears running under high torque loads under very slow speeds can wear prematurely because the lubricant film gets squeezed out of the contact area as the gears mesh, leaving unlubricated gear surfaces to wear against each other. The service life of gears can be prolonged by avoiding running an extruder at full torque, as well as by eliminating jackrabbit starts whenever possible. The extruder drive should be set for a minimum 25-second acceleration time. DRIVE MOTOR MAINTENANCE DC drive motors are probably the most neglected extruder area. Because they are an integral part of the screw’s drive train, poorly maintained motors can subtly rob processors of output. When checking DC motors for symptoms of potential breakdown, only trained personnel familiar with the electrical and mechanical hazards of such maintenance should be used for this task. The primary symptoms of motor troubles include the following: excessive noise, brush sparking, discolored armature, high temperature, inadequate exhaust air flow, and excessive vibration. Visually inspecting the DC motor brush area can be easily simplified by installing an acrylic or polycarbonate sheet commutator brush cover. Most DC- and some AC-driven extruders are equipped with open, drip-proof, guarded motors that are open to the atmosphere and can easily be fouled by dust. Operators

21

should be particularly vigilant about checking for buildup in the motor housing, especially when running flexible PVC. If the machine is not ventilated with clean air, a black sticky film can foul the motor and dampen motor efficiency or cause an arc-over. Check for evidence of this PVC contamination on the exit part of the forced-air passage and view the port at regular intervals. The brush part of the DC drive motor should be cleaned when necessary, and it may be better to ventilate the motor housing with clean air from outside the processing area. Periodically check and replace worn DC brushes with exactly specified replacements, making sure that the commutator shows a thin bluish film around its circumference. Black or pitted areas indicate brush sparking, which is difficult to detect when the extruder is running unless someone crawls beneath the motor. If new brushes do not solve sparking difficulties, it is likely that the motor is overloaded, controls are malfunctioning, or the motor itself is wired incorrectly. AC and DC motor bearings should be re-lubricated exactly according to the manufacturer’s instructions. Generally, the vent plug opposite the grease entry plug must be removed so that the bearings are flushed without blowing their seals and excess carbonized grease inside the motor housing is minimized. Over-lubrication causes heat buildup. Do not overlook the drive armature-cooling blower motor, making sure that it is rotating in the correct direction and that the filter is kept clean. Inefficient motor cooling ultimately results in reduced service life. While inspecting the motor during scheduled downtime, check to ensure that vibration has not worn away any insulation over connections in the AC or DC motor conduit box. Without this protection, a powerful arc might seriously injure operators and probably cause costly damage to the drive. Drive belts should be checked at the same time to be sure that loose or defective belts are not limiting torque. Although most belt manufacturers’ catalogs contain detailed technical discussions on the engineering factors behind belt tensioning, a few simple guidelines are adequate. If there has not been any squealing or slipping during operation, especially during acceleration, belts are probably tensioned correctly. One way to double-check drive belts is to deflect each one in mid-span using a thumb. About one-half inch of deflection indicates correct tensioning. Be sure to check all belts for signs of fraying, cracking, or twisting and replace damaged ones with full “matched length and tied” (ML&T) belts only. The motor position must be slackened so that replacement belts are eased into the grooves, not pried over the sheaves. Newly installed belts should be adjusted during the first month of operation to compensate for normal elongation. STOCKING SPARES AND LIMITING DOWNTIME The optimum amount of capital investment in spares varies according to the number of similar extruders in each operation, the reliability and lead time of the specific components, the criticality of the part to successful operation, and other factors. Some large processors stock a spare screw

22

CHAPTER 2—PRIMARY EQUIPMENT

for every size of machine, as well as barrel heaters, controllers, and even standby thrust bearings. However, even the small extrusion shop can benefit from stocking spares that are prone to failure without tying up excessive capital. Relatively inexpensive parts can easily shut down an entire line. In the drive area, DC motor brushes and drive belts are essential spares to stock. One relatively small and inexpensive part that processors should not be without is a spare set of drive fuses. These are not off-the-shelf items. Overall, necessary spares depend upon individual extrusion operation requirements. However, concentrating on inexpensive yet critical replacement parts seems to be the best policy. Besides having extra filters for every part of the extruder, it is a good idea to carry spare die heaters, die and barrel thermocouples, and solenoids. Die power-lead plugs, shear pins, and rupture discs fall into the required spares category, as well as all commonly replaced hoses, seals, and gaskets. SCREW REMOVAL: WHY, WHEN, AND HOW Worn screws and barrels probably account for the largest productivity losses in extrusion operations. Regular removal, cleaning, and measurement of screw and barrel dimensions, however, can increase output, limit energy consumption, and reduce motor wear. Cardinal symptoms of trouble in the area of gradual loss of output/ RPM over a period of time include excessive frictional heat buildup or problems with surging. For a more complete examination of the effects of screw wear, see the chapter bibliography for a reference to an early (though still valid) screw wear study, a 1982 ANTEC paper that the author presented. The best way to gauge the actual effect of screw and barrel wear is to obtain standardized profiles of each extruder on an annual basis by measuring output and overall performance. Using the same resin, temperature profile, and die system, comparisons of screw output vs. RPM, pressure level and variations, and stock temperatures over time can accurately predict optimum screw replacement cycles. Although it is probably not routinely done, annual removal and measuring of the barrel and screw is a wise policy. Processors of highly filled or other abrasive compounds should consider even more frequent screw removal to optimize extruder productivity. Paying careful attention to this often-neglected maintenance area can only optimize production conditions. When physically extracting the screw, be especially careful not to nick or damage the breaker plate recess, which is usually irreparable. A trick to avoid this common catastrophe and protect the sealing face is to machine a simple ring that is inserted into the recess before pulling the screw. Veteran processors make this ring 0.0l inch greater than the nominal barrel inside diameter and approximately 0.005 inch less than the breaker plate. Barrel replacement is far more expensive than using this simple device. As the screw is slowly and carefully withdrawn, the flights and channels should be brushed by hand with a wire brush. Badly burned screws should be scrubbed using a cop-

per scouring pad moistened with motor oil and sprinkled with non-chlorinated household scouring powder. To clean the barrel, use a round, steel-bristle flue brush of the same nominal diameter as the barrel. This brush can be attached to a long 1/4- or 1/2-inch pipe and driven by a heavy-duty 1/2-inch drill. Also remember to clean out the thrust pocket in the gearcase assembly into which the butt end of the screw seats. Any contamination in this pocket will cause a screw to run eccentrically and wear down the bushings, feed section, and barrel liner. Lubricate this area with anti-seize compound before reinserting the screw. Remember to use pipe dope on the threads of the screw if screw cooling is used to prevent water seepage and rusting in the thrust pocket. Before measuring the screw and barrel for wear, first examine the feed section for gouges and worn areas. This part of the machine wears very slowly, but when a barrel has been replaced a few times, the feed section must be closely inspected, looking for a step between the worn feed section and the new barrel. A new barrel inadvertently mated with a worn feed section may still cause surging or feed problems because of the step diameter change, so be sure to replace the feed section, if required, when changing the barrel. Many cylinder bore gauges can be used to measure barrel inside diameters, including one rigged with a three- or fourlegged carrier, differential transformer measuring head, and digital readout. Such devices will show actual wear. Record readings every foot of the barrel and at cross axes, and retain the records for future comparison. Larger extruders (4-1/2-inch, 24:1 and larger), particularly those with high L/D ratios, may be prone to uneven barrel wear and lost output if they are not properly aligned. If a machine has been moved (or was not aligned at installation), it is a good idea to borescope it to be sure that the barrel, feed section, and gearcase are all on the same axis. Replacing extrusion screws or barrels depends upon both the rate and extent of wear. Nominal screw-barrel radial clearances also vary according to machine size , with a typical radial clearance (on each side of the screw) for a 2-1/2inch unit of 0.003 inch and a clearance of 0.006 for a six-inch extruder. If 15 mils of radial clearance are found beyond the original screw-barrel tolerance, mild wear is indicated and usually does not dictate replacement. Thirty mils of radial wear is a symptom of more serious troubles, and the screw, the barrel, or both should be upgraded. Forty-five mils of clearance is serious wear. Poor output is almost a certainty, and this may explain the sudden onset of severe instability under some conditions. See the previously cited ANTEC screw wear paper. Immediate complete replacement of the barrel assembly is probably needed. Carefully thought-out maintenance and check procedures and better operator training will provide long-term savings in the following ways: (1) Reduction of unplanned downtime (2) Extended machinery life (3) Higher product quality

Section 2.1.  The Film Extruder

23

FIGURE 2.1.23.  Straight-grooved barrel section.

(4) Greater productivity (5) A more easily marketable product. THE GROOVED-FEED EXTRUDER Like the smooth-bore extruder mentioned earlier in this chapter, the grooved-feed extruder can also be broken down into four major components: the mechanical extruder, the screw, the drive, and the heat control panel. Because the mechanical extruder, the drive, and the heat controls are similar between the two, this section will focus on the history, concept of operation, and performance differences of groovedfeed vs. smooth-bore extrusion. HISTORY The grooved-feed extruder was developed to process materials such as HMW high-density polyethylenes and certain polypropylenes that tend to exhibit limited feeding and solids-conveying properties in a smooth-bore extruder. The benefits of these materials include a lower controlled melt temperature, an increased extrusion rate that remains nearly insensitive to head pressure, and shorter residence times. However, the process is not limited to these materials. LDPEs, LLDPEs, high barrier, and pelletized regrind also benefit from the process. The concept of grooved-feed extrusion is not new. Menges et al. described a mixing screw that took advantage of

the higher rate per revolution from a grooved-feed extrusion process in 1972 (2). From there, development moved forward quickly. By 1982, considerable work had been done to support and to develop grooved-feed performance. Mixing screws could not be retrofitted with deep metering sections to increase output because melt quality would suffer. Instead, they were replaced by designs that would capitalize on the increase in output from the grooved-feed process. One of the complications of generating such high pressure in the extruder was that with certain abrasive materials, especially ones with high pigment loadings incorporating silicates and inorganic metal oxides, the wear rate near the end of the grooved-feed section was exceptionally high. New barrelliner and screw-base materials were therefore developed to help minimize this wear. Through the early 1980s, most European developments were accomplished with straight grooves (Figure 2.1.23). However, an ongoing program was underway to perfect helical grooves as a means of increasing the feed rate (Figure 2.1.24). A force balance analysis of the groove/screw flight geometry supported this as a viable concept, but in the end, machining cost outweighed the performance benefits. Through the midand late 1980s, newer and better screw designs and materials were developed to capitalize on the increased feed rate and to better resist wear. Many were single-flighted screws using two mixing sections, whereas others were barrier design. The latter is more versatile, especially in processing materials other than HMW high-density polyethylene.

FIGURE 2.1.24.  Helical-grooved barrel section.

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CONCEPT AND DESCRIPTION The fundamental difference between smooth-bore and grooved-feed extrusion lies in the feed zone of the extruder. Basically, in a grooved-feed extruder, the first several diameters of the barrel starting just upstream of the feed throat opening are grooved. The number of grooves is a function of machine size: • Small extruders (30–50 mm) are normally fitted with four grooves • Large extruders (80–l10 mm) are normally fitted with eight to ten grooves. The size and shape of the grooves, although somewhat subjective, remains pellet size-dependent. There are many designs that perform successfully. THE PROCESS Friction governs extrusion and is a function of temperature, pressure (radial normal force), and velocity of the pellets sliding against the barrel wall (shear rate). High friction between the pellets and between the pellets and the barrel wall creates the drag flow required to move the plastic pellets along the extruder. In conjunction, the friction between the pellets and the screw must be low, allowing slippage on the screw. The greater the frictional difference, the greater will be the forward conveyance. This presents a problem, however, when extruding materials with low pellet COF in smooth barrels. The pellet-to-barrel friction in the feed section drops, thus limiting the feeding and pressure-generating capability of the machine. As the extruder head pressure increases (higher screw speed), the specific rate in kg/RPM-h (pph/RPM) continues to drop, and melt temperature increases out of control. In grooved-feed extrusion, the friction between barrel and resin in the feed section stays very high, and the friction between the resin and the screw stays comparatively low. In fact, it is almost independent of the resin COF. This is due to the radial locking of un-melted pellets into the grooves.

This positive lock prevents circumferential sliding of the pellets and forces them to be conveyed downstream axially, in straight or helical grooves, by the flighted screw. There are three distinct advantages of grooved-feed over smooth-bore extrusion: • First, the process is feed-controlled. This means that the output rate from the grooves is nearly independent of backpressure. As screw speed is increased over its range, the specific rate changes by only a few percent. Resin passing down the grooves is compressed to very high pressures, typically 690 bar (10,000 psi) or higher at the end of the grooves. With all the pumping force already generated, the balance of the screw can be designed primarily for melting and mixing. With a screw properly designed to accept the high flow from the grooved-feed section (Figure 2.1.25), the specific feed rate is always higher, often up to 1.5 times higher, than a smooth-bore screw of the same diameter. This, of course, has the distinct advantage of enabling polymers to be processed with considerably shorter residence times and, for heatsensitive materials, with less time at elevated temperatures. • Next, the extruder’s performance is not compromised when adding lubricants or low-molecular-weight additives to the resin. As an example, adding a polymer processing aid to HMW-HDPE for reasons of improved processability and elimination of melt fracture has the added advantage of higher specific throughput. In theory, this occurs because the processing aid lowers the COF between the screw and the resin, leaving the mechanical keying action of the resin in the grooves unaffected. Thus, the axial friction is reduced further, but the circumferential friction remains high, promoting increased feed rate. • Finally, contrary to most smooth-bore extrusion processes, there is little change in the melt temperature with increasing screw speed. This benefits film blowing of low-melt-strength materials such as linear low-density polyethylene because melt strength is proportional to melt temperature.

FIGURE 2.1.25.   Barrier screw for grooved-feed extruder. note that the final melt-channel depth is greater than the initial feed-channel depth.

Section 2.1.  The Film Extruder

25

Referring to Figure 2.1.23, note that the area adjacent to the feed throat is enlarged slightly, allowing the resin pellets to freely fill the screw channels. As the screw turns, the flights convey material into the grooves, radially “locking” the pellet flow and causing axial flow down the grooves. As the groove depth reduces and tapers out, normally three to four screw diameters downstream of the feed throat, pressure builds as compaction of the pellets displaces voids. The pressure at the end of the grooves, which is always the highest in the grooved-feed extrusion system, can range from 55–1035 bar (8,000–15,000 psi), depending upon the polymer being extruded Again referring to Figure 2.1.23, notice the outer grooves, used for intensive water cooling, that surround the liner. Cooling water, typically 2.1–25°C at 8–28 liters/min (40– 80°F at 2–7 GPM), is required for the process, but only enough cooling is needed to prevent the compacted pellets from melting. Melting, which would cause pellet slippage, reduces locking of the pellets in the grooves, thereby decreasing efficiency and specific rate. Too much cooling also reduces extruder efficiency by decreasing pellet temperature, thus requiring additional energy to melt.

Controls

SCREW CHARACTERISTICS

It is also important to note the effect on grooved-feed extruders at the extreme ends of pellet behavior:

Figure 2.1.25 details a cross section of a screw designed specifically for use with the grooved-feed extruder. The single feature that characterizes these screws is the low compression ratio, which is often less than one and is needed because compression is provided by the tapered grooves. The balance of the screw, about 20 diameters in the case of a 24:1 screw, is then optimized for melting and mixing. This provides surprising versatility, especially when processing resins that are difficult to feed, have low pellet COF, or have a tendency to develop excessive melt temperature during smooth-bore extrusion. As mentioned earlier, screw designs for grooved-feed extruders are often single-flighted mixing screws. This design typically has two mixing sections: a dispersive (or fluted) mixer slightly forward of the screw midpoint, and a distributive mixer (pin-style) near the end of the screw. These are popular with HMW high-density polyethylene. Other popular screw designs include the barrier screw. These, like the one shown in Figure 2.1.28, have fractional compression ratios, have separate melt and pellet channels in the barrier region, and for blown-film applications, often use only one fluted mixer. Because of the high process pressures, screw wear may be a concern, especially in the area immediately following the grooved-feed zone. This becomes most apparent when processing materials that contain large amounts of silicates or inorganic metal oxides. To counter this, tungsten carbide hard facings may be used on the screw flights, and in more severe cases, tool steel may be used as the base material of construction to combat screw root erosion. Further discussion on the finer details of screw design can be found in the following chapter of this manual.

Grooved-feed extruders typically have air-cooled barrels. The information on air cooling covered in the smooth-bore extruder section is applicable here. Performance Data Figures 2.1.26–2.1.30 illustrate grooved-feed performance versatility. There is little dispute among processors that grooved-feed extrusion is the preferred processing method for high-molecular-weight, high-density polyethylene. This is because: • The COF of HMW-HDPE is too low to allow adequate feeding on smooth-bore extruders, and • High-intensity mixing is required to disentangle long polymer chains. Because the pressure and shear needed for melting are generated early, more time is available for mixing. Therefore, slightly lower mixing intensity is used over a longer time, making melt temperature easier to control.

• Hard, medium-COF pellets such as nylons and EVOH can exhibit such aggressive keying into the grooves of the feed section that, under the right conditions, they can stall the extruder drive. It is common practice to use lubricated versions of these resins to promote some slippage in the grooves, so that the energy needed for forward conveyance is lowered. • Very soft, high-COF pellets such as plastomers and VLDPE resins can soften and partially melt too quickly in the feed section. This reduces the efficiency of grooved feeding, and specific rate drops proportionally. A comparison of specific rate and torque for different polyethylenes, smooth bore vs. grooved feed, is shown in Figure 2.1.29.

FIGURE 2.1.26.  Rate vs. RPM for three HMW-HDPE resins, 60-mm straight grooved-feed extruder barrier screw.

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FIGURE 2.1.27.  Rate vs. rpm for four resins, 45-mm grooved-feed extruder with barrier screw.

Figure 2.1.26 illustrates rate vs. screw RPM curves for three popular bimodal HMW-HDPEs run on a 60 mm (2.36 inch) straight grooved-feed extruder. The numbers along the curves are melt temperatures taken at varying RPMs. Figure 2.1.27 illustrates the point of improved versatility, especially with small grooved-feed extruders. Low residence times and high relative rates with a variety of resins are achievable on a 45-mm grooved-feed extruder with a barrier screw. Rate vs. screw RPM is plotted for HMW-HDPE, a l MI LLDPE, a 0.7 MI LDPE, and a nylon 6 resin. Figure 2.1.28 plots torque vs. RPM for the same materials as in Figure 2.1.30. Grooved-feed extruders require more torque than smooth-bore extruders of the same size because of the higher rate and must be fitted with larger drives.

Figure 2.1.29 illustrates the effect of pellet melting behavior on the specific rate of a grooved-feed extruder. The viscosity of LLDPE remains high during shear, and therefore it conveys better than LDPE. Metallocene LLDPE and plastomers are softer, tackier, and melt more easily under shear. This appears to reduce the conveying effect of the grooved-feed process. The opposite is true for smooth-bore extruders, where higher melt tackiness increases conveyance and specific rate. Figure 2.1.30 shows that if the tackiness of metallocene LLDPE pellets is reduced by adding slip and polymer process aid, the specific rate increases. Note that the required torque increases much less markedly than the specific rate. Additives also have a slightly positive effect on the smoothbore extruder as well.

FIGURE 2.1.28.  Rate vs. torque for four resins, 45-mm grooved-feed extruder with barrier screw.

Section 2.1.  The Film Extruder

FIGURE 2.1.29.  Effect of pellet behavior on specific rate.

REFERENCES AND ADDITIONAL RESOURCES [1] Kruger, G., “Grooved Feed Extrusion,” Proceedings, 1981 SPE ANTEC, p. 676. [2] Menges, G., et al., , “A Novel Concept for Single-Screw Machines,” Proceedings, 1972 SPE ANTEC, p. 784. [3] Radovich, J.L., “An Experimental Comparison of Heat Removal in Water or Air-Cooled Cast-Aluminum Extruder Barrel Coolers”, Proceedings, TAPPI Polymers, Laminations and Coatings Conference (August 1995). [4] Butler, T. I., Spalding, M.A., Mayer, A., Yap, P., “A Processing Comparison of POP and EPE Polymers in Blown Film”, distributed by The Dow Chemical Co. Resources. [5] “Extruder Safety Standards: A Review and Interpretation,” Proceedings, 50th Annual Wire Association Convention, 1980. [6] “New Extruder Safety Standards: What They Mean to You,” Plastics Technology 9:85 (1980). 144 The Film Extruder, Film Extrusion Manual. [7] Malinowski, R.J., “Selecting a Polymer Melt Filter,” Proceedings, 1985 Polymers, Laminations and Coatings Conference , TAPPI PRESS, Atlanta, p. 201. [8] Luker, K., “Energy Consumption in Smooth-Bore vs. Grooved,” Proceedings, 1984 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 265. [9] Geiger, C.H., “Productivity Improvement with DC Drives,” Proceedings, 1984 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 165.

27

FIGURE 2.1.30.  Effect of additives on mlldpe processing.

[10] Von Kraus, R.H., “Challenges for the Extruder Manufacturers,” Proceedings, 1984 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 9S. [11] Mutsakis, M., “Processing Viscous Materials: Motionless Mixers,” Proceedings, 1977 Paper Synthetics Conference, TAPPI PRESS, Atlanta, p. 17. [12] Chauvin, T. J., “Surging in Single-Screw Extruders,” Proceedings, 1989 Coextrusion Conference, TAPPI PRESS, Atlanta, p. 63. [13] Butler, T.I., et al., “Coextrusion Extruder Sizing and Resin Stability,” Proceedings, 1989 Coextrusion Conference, TAPPI PRESS, Atlanta, p. 55. [14] Butler, T.I., “The Influence of Extruder Residence Time Distribution,” Proceedings, 1989 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 401. [15] Steward, E.L., “Grooved-Feed Extruder Screw Performance,” Proceedings, 1989 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 635. [16] Post, S.J., “Machinery Design for Processing High-Temperature Polymer,” Proceedings, 1988 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 453. [17] Lounsbury, D.C., “New Studies on Screw Wear and its Effect on Output, Stock, Temperature, Uniformity, and Processor Economics,” ANTEC Preprint, p. 1–4. [18] Hellmuth, W.N., “Blown and Cast Film Processing of Metallocene LLDPE,” Future-Pak ‘96 Conference, Chicago, IL, November 20–21, 1996.

Film Extrusion Manual, Second Edition, 2005

Chapter 2—Section 2

Screw Design ANDREW W. CHRISTIE, SAM North America, LLC

INTRODUCTION Single-screw extrusion is one of the most common and widely used methods for processing thermoplastics today. Thermoplastic polymer processing starts with a basic polymer in solid form, melts it, shapes the molten plastic into the desired product shape, and then re-solidifies the formed melt to hold the product shape. In the case of plastic films, this occurs in a continuous process that requires a steady flow of solid pellets, the removal of air entrained with the pellets, compaction into a continuous solid to melt, adding heat to the polymer through work or conduction to melt, pumping the molten liquid plastic to the desired rate and pressure to be formed, and mixing to ensure the desired consistency of the final film. To convert a polymer on a continuous basis through two phase-change transitions (solid to melt and then melt back to solid) is a complex thermodynamic problem that depends on the system design, which must be tailored to the polymer’s thermodynamic properties. The physical and thermodynamic properties of polymers are well defined and understood in relationship to the process: pellet bulk density, compressibility, friction coefficients, heat capacity, melting or glass transition temperature, viscosity, elasticity, shear and temperature stability, and others. All must be taken into consideration when designing for this process. A single-screw thread rotating inside a barrel with a relatively close tolerance between the screw thread and the barrel wall has been used to perform work for humanity for more than 2000 years. A simple auger was first described by Archimedes and was used to lift water. This concept was applied for moving and conveying various media and evolved to perform additional work and kneading by varying the shape of the thread and the root of the channel. With the development of modern thermoplastics, this basic auger was applied and further modified to become the precursor of today’s plasticating extruder screw. Continuous extrusion for plastic film is essentially the same as any other polymer extrusion process, but includes some unique requirements that may vary depending on the film forming technique. Most tend to be high-output applications, but all include continuous film forming through a

small gap under high shear stress, which makes the process sensitive to any small polymer defect. For cast films, the polymer must have low viscosity and high elasticity so that it can be drawn and stretched out of the flat-slot die to be rapidly re-solidified as it contacts a rotating cooling drum. For blown films, the polymer must exhibit higher viscosity and higher melt strength so that when extruded out of an annular-slot die, the molten polymer can be expanded and drawn slowly as it is air-cooled and supported by its inherent melt strength. Today, most plastic films are coextruded to provide enhanced finished film properties through the unique properties of the individual monolithic polymer layers that are coextruded. For cast films, these coextruded layers are most commonly combined before final forming in a single-manifold flat-slot die. For blown films, the polymers are commonly extruded into a multi-manifold annular-slot die, where they are combined immediately before forming, drawing, and quenching. Each process is uniquely challenging, especially with the thermally and shear-sensitive barrier properties and bio-based polymers that are common today. This section will discuss the science and principles that guide single-screw design to give the reader a common language and an understanding of the competing requirements of the various polymers and application considerations involved in screw design. Today’s demanding applications require that the screw design be optimized even though the production environment may require a variety of polymers, temperatures, and outputs to be run on one screw. The optimum design in this case is a compromise among multiple optimum designs, and the reader should understand the impact of these competing requirements to support the screw designer in making the best decision when balancing one design to accommodate these differing requirements. FEEDSCREW TERMINOLOGY A common descriptive language must be understood among individuals when discussing feed-screw design. The starting designation refers to the size of the extruder, which is commonly based on the bore size of the extruder barrel into which the screw is fitted. U.S. extruder manufacturers 29

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commonly design to inch sizes, for example 2-1/2″, 3-1/2″, 4-1/2″, and 6″, with common sizes ranging from 1/2″ to 12″. Most international OEMs follow metric designations in mm (e.g., 65 mm, 90 mm, 115 mm, 150 mm, etc.), with common sizes ranging from 15–300 mm. The other primary reference that defines the basic extruder feedscrew size is the L/D ratio. L/D is simply the ratio of the working length (flighted length) of the screw to the diameter. Although for other applications, extruder sizes as short as 4 L/D may be used, typical sizes for film extrusion range from 20–48 L/D. The most common ratios for single-stage (non-devolatizing) extruders are 24–32 L/D. Extruder size is correlated with extruder output rate, and the more common, longer L/D extruders today are a result of improved mixer designs integrated into the screw design, which further enable higher output and more versatility. Referring to the diagrams in Figure 2.2.1 and 2.2.2, some common terms used in feed-screw design are: • Shank—that part of the screw that is seated in the gearbox. This is where the motor applies the torque to drive the screw. • Overall length—the total length of the screw. • Flighted length—the total length of the flighted portion of the screw. As discussed above, this is sometime measured in diameters. For example, a 6″ (152 mm), 30:1 L/D (length over diameter) would be a screw with 180″ (4.57 m) approximate flighted length. Older screws may have smaller L/D, as short as 20:1 L/D. Typical lengths for today’s screws are 24:1 L/D to 32:1 L/D. Devolatizing (or vented) screws are typically longer than 36:1 L/D. • Feed section (zone)—the portion of the screw that carries the solid polymer forward to the melting and mixing sections. • Barrier (melting) zone—also called the transition zone for single-flighted screws. This is the section of the screw that compacts the polymer and, melts it through pressure, shear stress, and viscous energy dissipation (VED). • Metering zone—the section of the screw that conveys the polymer to the downstream process. Importantly, this zone develops the pressure required for the application and provides time to establish the desired polymer temperature. • Mixing—the section that disperses and distributes the melted polymer so that it is homogenized.

FIGURE 2.2.1.  Nomenclature.

• Screw OD—the nominal diameter of the screw • Barrel—the cylindrical chamber that houses the screw • Bimetallic liner—the abrasion-resistant liner in the barrel to lessen wear on it; many screws today have a compatible abrasion-resistant alloy welded over the flight face. • Lead—the distance between the flights of the screw. A screw with the same lead as its diameter is called a square-pitch screw. • Channel depth—the distance from the outside of the flight to the root of the screw. • Flight width—the width of the flight. A working rule is the diameter divided by 10. • Flight clearance—the distance between the flight and the barrel wall. A typical clearance is 0.001″ per inch of diameter (0.001 mm per mm of diameter) • Helix angle—the angle of the screw flight; square-pitch screws have a 17.7° helix angle. EXTRUDER FUNCTION AND PLASTICATING SCREW DESIGN The single-screw extruder is basically a volumetric pump, not a positive displacement device. Variations in feeding result in variations in output. Solid conveying, melting, pumping, and mixing occur simultaneously and are interdependent. The screw is required to feed the pellets forward, transfer mechanical energy to melt them through shear and compression, mix the polymer components, and develop the required pressure, flow rate, and temperature. If there is an operating issue with the extruder, it is important to recognize the appropriate location in the extrusion system for more effective troubleshooting. Output rate and surging are typically upstream (feeding / solids conveying) issues, whereas melt quality is a downstream (metering / mixing) issue. FEEDING/SOLIDS CONVEYING The first section of the extruder screw is where polymer feeding (typically as pellets) occurs. The screw can perform this function only if the material is brought to the entry point

FIGURE 2.2.2.  Nomenclature.

Section 2.2.  Screw Design

31

FIGURE 2.2.3.  Hopper conical section.

(the feed throat) in a consistent and uniform manner. The shape of the pellet can vary depending on how the polymer is made. Common shapes are round, cylindrical, cube, lens, chip, or reground as fluff. The form of the feed material impacts the bulk density and the feed rate and consistency. Whatever the shape of the polymer, it must remain in solid form at the beginning of the screw. If it softens or melts prematurely in the feed throat, a bridge or blockage occurs, and polymer material cannot enter the screw. A bridge is most likely to occur if the bulk density is low or the surface area of the feed material is large relative to its weight. Clearly, the hopper and the entrance (the feed throat) are important for proper feeding of the polymer material into the extruder. Feed material, whatever its form, has a natural angle of repose, which varies according to the shape of the solid pellet, its size and weight, and the relative friction between pellets. The material will support itself up to some incline (the angle of repose). For polymers to flow freely from the hopper into the feed throat, the transition angle from the storage volume into the feed throat must be greater than the polymer’s natural angle of repose. Typically, a 60° angle will exceed the angle of repose for most polymer pellets. Hopper entrance cone angles less than this may lead to bridging or inconsistent feed flows (Figure 2.2.3). Once the polymer material flows into the feed throat, it enters the screw channel in the feed section of the extruder screw. The screw must now convey this solid plastic material forward in this loose granular form. In addition to conveying the polymer forward into the barrel, it must compress the loose pellets together, squeezing out the air until they form a solid plug. The rotating screw must continue to advance the solid plug until it begins to melt. Figure 2.2.4 shows the mechanical principle that provides this forwarding action. The section labeled “Plug Flow” represents the ideal cir-

FIGURE 2.2.4.  Feeding—plug flow.

cumstance, in which the polymer sticks to the barrel wall and slides on the screw root. This gives the maximum rate as the rotating screw flight advances the material, like a nut moving axially along a bolt when the bolt is rotated, but the nut is held firm and not permitted to rotate. The section labeled “Melt Plug” represents the case where the polymer sticks to the screw and slides on the barrel wall. The material does not advance; it stays in the same position and rotates around stuck to the screw root, like the nut on a rusty bolt. This melt plug condition must be avoided. What conditions are required for plug flow? Two conditions control this. The first is the ratio between the area at the screw channel and the area at the barrel wall for a unit of lead of the screw. If the ratio is less than one and the coefficient of friction between the polymer and barrel surface is the same as that between the polymer and the screw channel, then plug flow will occur. This coefficient of friction between the polymer and the barrel and screw surfaces is the other condition impacting plug flow. It has been shown [2–4] that the coefficient of friction (COF) for a polymer, such as low-density polyethylene, against a steel surface varies with the temperature of the steel. It is common for the feed-throat section of the extruder to be made of cast ductile steel with internal channels for circulation of thermal fluid for heating or cooling the feed-throat barrel wall. Typically, the thermal fluid is water. It is also common to supply thermal fluid to the root of the screw, typically to a depth of approximately three turns after the start of the screw thread. A single-pass rotary union is mounted in the low-speed shaft of the gear box and is fitted with nested pipes, with the inner supply pipe circulating the coolant to the end of the drilled hole in the screw (Figure 2.2.5).

FIGURE 2.2.5.  Screw root cooling.

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CHAPTER 2—PRIMARY EQUIPMENT

FIGURE 2.2.6.  Static frictional data.

Figure 2.2.6 shows static COF data for HDPE versus steel at different temperatures. There is a temperature range where the COF is maximized and also a range where it is minimized for each polymer. To promote the slip-stick required for maximum plug flow, the barrel and screw temperatures must ensure a higher COF at the barrel surface and a lower COF at the screw surface. By manipulating the temperature of the feed throat and screw root, it is possible to influence this feeding behavior. It is also common to machine grooves into the inside wall of the feed-throat bore to create a high COF. The plug flow equation is:  ne  Qs   2 D 2 Nh sin  cos  1   t   where, Qs = Volumetric conveying rate, mm3/s (in3/s) D = Screw outside diameter, mm (in) N = Rotational velocity, rps (rps) h = Channel depth mm (in) θ = Flight helix angle n = Number of flight starts e = Flight width mm (in) t = Lead mm (in).

(2.2.1)

FIGURE 2.2.8.  Feed rate—screw root 100°C.

In the complex flow patterns of the extruder feed section, static friction data are a good indicator of the response to expect. Figures 2.2.7 and 2.2.8 indicate the complexity of the response when the extruder variables interact. The experimental setup used a 65-mm screw, but analyzed only five diameters of feed length at 11-mm depth at 80 RPM. In Figure 2.2.7, the screw root temperature is 50°C, whereas in Figure 2.2.8, the screw root is at 100°C. The optimum feeding conditions for each polymer and screw design may require iterative experimentation in practice, and what is optimum at one screw speed may vary as the screw speed changes. To operate a single-screw extruder successfully, the feed rate must balance the melt rate of the extruder. One challenge for film extruders today is the continuous reprocessing of trim or off-spec film. It is desirable to reuse this waste continuously in the process; however, it can present several difficulties. The feed zone of the screw is designed around a volumetric rate per turn, and based on the bulk density of the feed material, this rate is balanced. If film scrap is continuously reground and fed back to the extruder feed, the bulk density of the reground film scrap will be about one-tenth of the pellet bulk density. This will require some form of force feeding of the ground film stock to increase the commingled material bulk density, as well as a modification of the extruder screw in the feed section to convey this lowerbulk-density material. Alternatively, the film scrap may be separately processed and densified and then compressed into compact wafers or repelletized and refed as a blended pellet stream. GROOVED FEED-THROAT EXTRUDERS

FIGURE 2.2.7.  Feed rate—screw root 50°C.

The grooved-bore extruder has a different feeding mechanism than the smooth-bore extruder. In a grooved feedthroat extruder, the feed-throat section has a bushing that is inserted into the feed-throat casting. The bushing is typically either made of a hardened tool-steel type material or has a hardened bimetallic liner. The inside bore of this liner is machined with multiple axial grooves (Figure 2.2.9).

Section 2.2.  Screw Design

 ne  QD   2 D 2 Nh sin  cos  1   F t  

33 (2.2.3)

where, QD = Volumetric melt conveying rate (in3/s) F = Factor to account for curvature and channel shape. The feed channel is deeper than the melt channel. As a general working rule, the feed section is designed to supply a volume of 2.5–6 times the melt capacity of the screw. There are several reasons for this. The first is that the air with the pellets takes up some of the volume. The air must travel back out the feed throat as the pellets are compressed into a solid plug. This air is represented by the bulk density of the polymer, which can be 40–80% of the melt density.

FIGURE 2.2.9.  Feed rate: grooved feed section.

The number of grooves is determined by the bore size of the extruder. For example, a 45-mm extruder has six axial grooves, and a 90-mm extruder hasten grooves. Equation (2.2.2) can be used to calculate the typical number of grooves that a certain size of extruder will have. D NG     2  10 

(2.2.2)

where, NG = number of grooves (*) D = screw diameter (mm). *Note: Round down to the nearest even whole integer. Typically, the grooved-feed bushing is between three and five diameters long. For example, a 50-mm diameter extruder has a grooved-feed bushing between 150 and 250 mm long. The axial grooves in the bushing typically taper in depth. The grooves are deeper at the start under the feedthroat opening and taper to match the inside diameter, creating a smooth bore as they extend down the bushing towards the extruder discharge. The function of the feed section in a smooth-barrel extruder is that the resin must stick to the barrel and slip on the screw, as was previously discussed. The basic function of the grooves is that they force the resin pellets to stick to the barrel wall. The pellets become trapped in the grooves and cannot slide around the barrel wall as the screw rotates. The pellet travels towards the discharge end of the extruder in the axial direction of the groove. As the “trapped pellet” travels axially down the length of the groove, it drags along with it the pellets that are adjacent to it and those pellets adjacent to them, and so on until the pellets at the root of the screw are sliding freely.

MELTING This analysis assumes that the melting rate is in balance with the feeding and metering rate. Frequently, the melting rate is the limiting factor in single-screw extrusion processes. Tadmor et al. [7] provided a basic understanding and model for melting, as illustrated in Figure 2.2.10. As the screw rotates within the barrel, the relative movement of the flight (which is adjacent to (1) in Figure 2.2.10) pushes against the polymer to advance it down the screw channel. The polymer begins to melt at the interface between the solid plug and the barrel wall due to the energy dissipated by the mechanical shear stress at the interface and to thermal conduction. As more polymer melts, the pushing flight wipes the barrel wall, and an increasing volume of melt accumulates on the pushing flight. As melting continues, the width of the solid plug diminishes, and the accumulating melt pool increases. In a single-channel screw, the volume of melt becomes much greater than that of the solid plug, and the solid plug separates and is surrounded by melt. This is referred to as solid-bed breakup, and in high-output, single-flight screws without adequate high-shear (dispersive) mixing, can lead to wide temperature variations.

PLUG FLOW COMPARED TO MELT FLOW The equation for melt flow is very similar to that for plug flow:

FIGURE 2.2.10.  Melting model.

34

CHAPTER 2—PRIMARY EQUIPMENT

The basic Tadmor model states that: (Rate of melting) × (Heat of fusion) = (Heat flux from the melted film (3) into the interface) – (Heat flux from the interface into the solid bed (5)) A full mathematical development and analysis may be found in the reference. This understanding of the melting behavior in the feed screw coincided with the development of the “melt barrier screw” concept (or simply “barrier screw”), which introduces a secondary flight into the singleflight screw channel at the onset of melting. This secondary flight is positioned to contain and control the solid-bed interface with the barrel while allowing the melt to escape into the secondary channel. Figure 2.2.11 illustrates the solidbed breakup that occurs in a single-flight screw channel and the secondary barrier flight capturing the solids. Note that as the barrier flight position moves down the screw, the channel depths and flight position change to match the diminishing volume of solids until melting is complete. The barrier screw concept controls not only solid-bed breakup, but also the area of the solids-melt interface, which enables the screw designer to adjust the melt rate to balance feeding, melting, and pumping. A further and most significant advantage of the barrier design concept is the designer can now use more screw length for melting without concern for solid-bed breakup. Once complete melting is ensured, then the metering and mixing shear input can be reduced, providing fully melted polymers at low temperatures. This capability is critical for many low-temperature extrusion applications, especially with very high-viscosity polymers. The barrier design concept is optimized when the barrier position matches the melt rate of the polymer being processed. The concept will lose effectiveness if a wide range of polymers must be run on one design. The barrier application advantages (maximized melting and low-temperature processing) have led to the widespread use of this concept in many film applications. Other applications may not benefit from the melt barrier concept, such as extrusion coating, which has requirements that include low-viscosity polymers, high processing temperatures, and a wide variety of polymers.

BARRIER SCREW DESIGNS The melt barrier screw design is in wide use today for film applications. There are many design variations, but all are based on the fundamental concept of separating the solid bed from the melt pool to maximize melting efficiency and to eliminate solid-bed breakup and the associated thermal and pressure instabilities. ENERGY REQUIRED FOR EXTRUSION Energy for melting and conveying the polymer comes from two sources: the torque put into the screw, and the heating or cooling of the barrel surface. The energy balance of the extrusion process as found in the literature [7] is: ZT  Z H  Z P  HL

(2.2.4)

where,

ZT = Total energy delivered to the process ZH = Energy required to heat the polymer ZP = Energy required to generate the necessary pressure HL = Heat loss from the process. Furthermore, as previously mentioned, ZT  Z M  q

(2.2.5)

where, ZM = Input mechanical power (in-lb/s) q = Input heat by conduction (in-lb/s). The energy ZH can be expanded to: Z H  CP QT or HQ where, CP = Average specific heat (in-lb.in3/°F) Q = Volumetric extrusion rate (in3/s) ΔT = Temperature rise (°F) ΔH = Enthalpy change in plastic (in-lb/in3).

FIGURE 2.2.11.  Melt flow development single flight/barrier flight.

(2.2.6)

35

Section 2.2.  Screw Design

Finally,

tiplied by a percentage less than 100% to provide an output approximation for a certain screw design. The simple formula to calculate total output is:

(2.2.7)

Z P  QP where,

QT  QD  QP  QL

ΔP = Pressure rise in plastic (lb/in3). Enthalpy curves are available to determine the energy input needed to heat a unit of polymer from ambient to the required melt temperature. This amount of energy must be available from the combination of both energy sources (torque and thermal) at the rate of output required to achieve final melt temperature. The rate of mechanical energy input to the thermoplastic in the melted state can be expressed as [8]:   3 D3 N 2    ne  Z M    1   h t   

where,

QT = Total volumetric output rate (in3/s) QD = Drag flow rate (in3/s) QP = Pressure flow rate (in3/s) QL = Leakage flow rate (in3/s).

Equation (2.2.9) or (2.2.10) provides a way to calculate drag flow. The basic formula for pressure flow is:

(2.2.8)

QP 

This is important because it shows that the mechanical energy input has an exponential relationship with screw speed (N). This demonstrates that at very low screw speeds, the bulk of the energy to heat the polymer must come from the barrel heat (because polymers are poor thermal conductors, this will lead to wide temperature variations at low screw speeds). At high screw speeds, little energy is required from the barrel zones to provide the energy required, and often the barrel zones are in cooling mode to maintain energy input into the polymer without overheating. The bore size or diameter of the extruder and the connected horsepower are directly correlated with the extruder throughput. Table 2.2.1 provides some typical reference points for output and power required. The required horsepower and gear in each scenario are verified by the screw designer in each case.

( Dh3 P sin 2  FP )  ne  12  L 1   t  

(2.2.11)

where, ΔP = Incremental pressure difference (psi) L = Section (metering) length (in.) μ = Apparent viscosity of melt (lb-s/in2) FP = Factor to account for curvature and channel shape. Leakage flow is the flow through the clearance gap between the flight face and the barrel wall; this flow is driven by the differential pressure between the pushing side and the trailing side of the flight. For a new screw with a standard radial clearance, this leakage is minimal. As the screw and barrel wear and this gap increases, this flow becomes significant, and the screw will experience a loss in specific output. For a new screw, the leakage flow is typically neglected, and the formula for total output becomes:

MELT CONVEYING

QP 

For a standard square-pitch screw with only one flight start, which is typical of most screw metering zones, the formula for melt drag flow can be simplified to a close approximation as:

( Dh3 P sin 2  FP )  ne  12  L 1   t  

(2.2.12)

MIXING IN AN EXTRUDER

(2.2.9)

QD = 2.07 hD 2

(2.2.10)

Mixing in single-screw extrusion is described as either dispersive or distributive [1]. Today’s film extrusion applications place increased importance on good mixing. Mixtures

Remember that this is for 100% drag flow of the melt. This formula can be used to estimate output and can be mul-

TABLE 2.2.1.  Typical Extruder Specific Outputs and Torques. Screw Diameter (mm)

LDPE Output Rate (kg/h/RPM)

LLDPE Output Rate (kg/h/RPM)

mLLDPE Output Rate (kg/h/RPM)

LDPE Torque (kW/RPM)

LLDPE Torque (kW/RPM)

mLLDPE Torque (kW/RPM)

65

1.2

1.1

1.1

0.2

0.3

0.3

90 120

2.5 5.0 10.0

2.4 4.8 9.5

2.3 4.5 9.0

0.5 1.2 2.5

0.7 1.7 3.6

0.8 1.9 4.0

150

36

CHAPTER 2—PRIMARY EQUIPMENT

of liquids [polymers at different temperatures, alloys of different polymers, and minor component additives (plasticizers, colorants, lubricants…)] must all be mixed. Mixtures of liquids and solids (polymer melt with unmelted solids, fillers (CaCO3, TiO2, …), minor component additives (flame retardants, UV stabilizers, nucleating agents…), or contaminants) will also be encountered. To obtain a quality melt, these elements must be uniformly mixed. Dispersive mixing is the deformation of the commingled components, resulting in a reduced component size and increased interfacial area. The deformation is accomplished through shear, elongation, or compression or a combination thereof. Distributive mixing is a repetitive random or ordered bulk rearrangement of the commingled components. This does not require continuous deformation of the material. Again from Gregory [1], mixing covers the following typical extrusion functions:

The list is only representative and does not cover the total scope of the term “mixing” in the extrusion process. How can a particle of pigment, for example, be deagglomerated or dispersed? The only reliable way of dispersing something in an extruder requires using shear stress on a major constituent to rupture its agglomerates to produce the desired particle size. It is well known that the level of shear stress is dependent upon viscosity as well as shear rate in the extrusion process [9]: t g (2.2.13)

cosity (low temperature), high applied shear rate (high RPM, small shearing gap), or preferably both. High viscosity is most easily obtained from a low melt temperature at the point of dispersion. High shear rate can be easily accomplished in an extruder by vectorially combining the rotational shear rate with the shear rate caused by pressure flow in tight-clearance orifices. If dispersion is not followed by distributive mixing, the end product might not be satisfactory. The reason for this, using pigment as an example, is that deagglomeration by shear stress will streak the ultimate particle-size pigment along the line of applied stress. The pigment concentration will then vary from spot to spot in the melt. Conversely, if the first mixing operation is not dispersive in nature where dispersion must be accomplished, the process will only create a random distribution of undesirably large agglomerates, leading to unsatisfactory quality in the final product. Recognizing that redistribution of the minor constituents is one of the primary functions of the screw extruder, let us examine those aspects of the process that improve distributive mixing. The literature reveals that the way to reorient spatially two molecules starting in close proximity in the extrusion process is to apply a high shear rate to the mixture or to produce a high ratio of pressure flow to drag flow in the screw. In either case, it is desirable to have a relatively low viscosity to promote distributive mixing. The shear rate in a process is, by definition, the slope of the flow velocity vector profile. Hence, the higher the shear rate, the greater the separation that occurs between adjacent molecules per unit of travel in the extruder, which amounts to improved distributive mixing. Pressure flow in the feed screw has been shown to be inversely proportional to the viscosity of the melt and proportional to the pressure across the screw section and the cube of the channel depth being analyzed. From this relationship, it can be seen that to increase the ratio Qp: Qd, the resin viscosity should be low (i.e., high temperature), the pressure increment must be high, and the screw channel must be substantially deep. A high ratio of pressure flow to drag flow, in fact, increases the apparent shear rate to a value considerably above the level predicted by rotational shear rate alone. In addition, as indicated by past work with glass-barrel extruders, the direction of the velocity vector fans within the screw channel will change. Improved distributive mixing is the result of the summation of these two phenomena.

where,

MIXERS

(1) Dispersion (i.e., de-agglomeration) of a pigment in a polymer, followed obviously by a distributive mixing operation. (2) Dispersion and distribution of two resins together to make a new “alloy”. (3) Dispersion and distribution of a high-molecular-weight constituent in a virgin polymer to reduce the end-product “gel count”. (4) Dispersion and distribution of a low-temperature resin in a matrix of higher-temperature melt to approach a homogeneous melt temperature. (5) Dispersion, if necessary, and distribution of additives such as fillers, reinforcers, stabilizers, plasticizers, and lubricants into a polymer matrix to manufacture a desired compound.

t = Shear stress, lb/in2 µ = Viscosity, lb-sec/in2 g = Shear rate, lb/sec. The logical conclusion of this mathematical relationship is that if dispersion is a function of the shear stress applied to a minor constituent, then a high-shear-stress field is mandatory for good dispersive mixing. In the case of an extruder, the shear stress is increased by either high primary fluid vis-

Many mixer designs with a wide variety of effectiveness have been proposed by designers. Based on the discussion above, mixer designs can be readily understood on the basis of their effectiveness. Do they promote dispersion, creating a high level of stress in the polymer matrix to break down undesired agglomerations? Do they generate distribution, rearranging and randomizing the uniform components within the polymer matrix? In addition, other factors that impact

Section 2.2.  Screw Design

37

FIGURE 2.2.12.  UCC or maddock’s mixer.

overall performance should be considered. Is there a small surface area compared to the overall length of the mixer? Is there a uniform wiping action throughout the mixer? What are the residence time and the residence time distribution? These answers will relate to how well the mixer will purge and transition if running different polymers and mixtures. A few common mixers will be discussed here to generate an understanding of how to evaluate mixer design. Probably the most common mixer in use today is referred to as the UCC or Maddock’s mixer and is shown in Figure 2.2.12. A simple review of this design concept shows that it accomplishes both distribution and dispersion. As the polymer flows into the mixer, it is split into multiple channels, and as it exits, it is recombined and rearranged. At the same time as the polymer flows into the inlet channel, it must flow through the tight gap between the inlet and the exit channel, creating high shear stress and dispersion. The success and widespread use of this mixer design have occurred simply because it accomplishes both dispersion and distribution effectively. Looking further, it is apparent that the simple design has a reasonably small surface area relative to its length. This and a short average residence time are both good characteristics. The axial channels, however, require pressure flow to move the polymer forward in the channels. At low pressures (low outputs), the residence-time distribution is quite long as polymer flow slows at the root of the channel. Figure 2.2.13 shows a twisted Maddock’s mixer, which

is an improvement on the original Maddock’s with its axial channels. The twisted Maddock’s also provides distribution by splitting the flow into multiple channels for spatial rearrangement. Similarly, all flow is forced through a high-shear gap between the inlet and outlet channels. The surface-area relationship is similar. The significant difference is the channels, which by being machined on a helix like the flight, provide a similar pumping action. This reduces the dependence on pressure for forward flow, especially at low speed, reducing the wide residence-time distribution. Another common mixer design, a pineapple mixer, is shown in Figure 2.2.14. This mixer, like the twisted Maddock’s, has multiple channels machined in a helical pattern. All channels, however, are open at both the inlet and outlet. In addition, cross channels are machined in an opposing helix, creating a pattern of raised rectangular interconnected knobs reminiscent of the surface of a pineapple. When polymer flow is introduced from the left, it is split multiple times, and as it advances, it is split again and again. This provides very effective distribution. As the flow advances through the mixer, there is always an open channel for the flow to pass through. The polymer flow is not forced to flow through any tight, high-shear gap, and therefore there is minimal dispersion in this mixer. Due to the opposed helical channels, this mixer design has significantly greater surface area per unit length than either of the Maddock-type designs. If the mixing channels are cut deep, this mixer will have a long mean residence time due to the open flow volume. Because there is

FIGURE 2.2.13.  Twisted maddock’s or gregory mixer.

FIGURE 2.2.14.  Pineapple mixer.

38

CHAPTER 2—PRIMARY EQUIPMENT

no continuous helix to pump, this mixer is also dependent on pressure for flow, which will result in a wide residence-time distribution at low screw speeds. All mixers can be evaluated in a similar qualitative fashion to determine whether they provide the two key requirements of mixing: dispersion and distribution. Estimating relative residence time and residence-time distribution indicates mixer cleanliness and transition performance. Remember also that polymer viscosity also impacts shear stress in mixing, and therefore where the mixer is positioned in the polymer flow field will also impact its dispersion and distribution performance. PLASTICATING SCREW DESIGN SUMMARY The plasticating screw in the single-screw extruder is the single most important factor in extruder performance. The design is required to perform four main functions in balance: (1) feeding or solids conveying, (2) uniform and complete melting of the polymer, (3) pumping at the requisite pressure, and (4) homogeneous mixing of the molten polymer matrix. The design must perform with application-defined polymers throughout the range of operating conditions (target pressures, target temperatures, and throughput rates), all while delivering the expected quality. It is contingent on the processor to understand the limitations of requiring one design to meet all these requirements when the application includes wide ranges of polymer physical and rheological properties, because these material properties and application targets drive the design. With an understanding of screw design principles, the processor can ensure the best and most appropriate design to suit product needs. MATERIALS OF CONSTRUCTION The terminology section of this chapter mentioned that the power to turn the screw connects at the screw shank. This means that the input power passes through the feed section before it is dissipated into the polymer. The root at the feed section is the smallest-diameter component of the entire screw. The screw designer runs calculations to determine the material strength needed to handle the torque. Common screw-base materials are high-alloy steels for strength, e.g., 4140HT or 4340HT steel, or a machinable stainless steel like 17-4PH. For highly corrosive polymers, stainless steel or special alloys (e.g., Hastelloy or Inconel) may be used. The wear characteristic and hardness of the flight outside diameter is critical. This surface may be nitride for hardness

and improved wear, or more typically today, the flight is manufactured with an abrasion-resistant alloy to protect the screw from wearing quickly. Some commonly used alloys are listed in Table 2.2.2. In addition to the hardness and wear resistance (corrosive and abrasive) of these materials, they are typically applied at a thickness that should provide continuous high wear resistance throughout the useful life of the screw. The earliest screws had some sort of case hardening, with flame hardening or nitriding being a common choice. Nitriding is still common; however, the hardened nitriding thickness is only about one-tenth that of the typical welded hardfacing alloy. In addition, the higher temperatures of extrusion may reduce nitride hardness. Today, harder materials are either inlaid or laid over the full face of the flights. These alloys are hard, but due to differences in thermal expansion rates between alloys, will exhibit cracking after application. As long as there are no J-cracks (cracks that propagate through the weld and then turn, like a J-curve, at the interface between the weld and the base steel), these cracks are normal and expected. The selection of barrel material is equally critical. It is important that the barrel be harder than the screw flight so that the more easily replaceable part (the screw) wears out first. Barrels are made with a hardened cast-in liner for this purpose. Compatibility between barrel liner and screw hardfacing is critical. Most screws have a thin base layer of chrome over everything except the hardened flight. This layer of chrome not only protects the base metal of the screw, but also reduces adhesion of the melted polymer to the surface, improving screw purging and cleaning. Keep in mind that some polymers can be abrasive and chemically corrosive. The same polymers that are designed to improve heat seal and adhesion will be prone to want to stick to the screw during processing. Proper care of an extruder by not running the extruder out of resin, not leaving the extruder turning at very slow speeds for long periods, and preventing the polymer from overheating will ensure that the screw and barrel have a long life. The essence of this section is to know what materials you will be running as well as at what temperatures and speeds, so that the screw construction will be appropriate for the application. Just as important as the materials of construction in combating premature wear is proper installation and alignment of the screw and barrel. Before installing a screw into an extruder, the barrel straightness and alignment should be checked. Accepted industry practice recommends less than

TABLE 2.2.2.  Hard-Surface Materials. Hard Surface Product Stellite 6 Stellite 12 Colmonoy 56 Colmonoy 83

Base Material

Typical Hardness

% Carbon

% Chromium

% Boron

% Tungsten

Cobalt Cobalt Nickel Nickel

37–42 41–47 50–55 50–55

1.1 1.4 0.70 2.0

28.0 29.0 12.5 20.0

– – 2.7 1.0

4 8 – 34

Section 2.2.  Screw Design

0.002″/foot TIR or 0.17 mm/meter. For example, this means that for a 4-1/2″ × 32:1 L/D extruder (nominal 12′ long barrel) the barrel center should fall within ± 0.024″ in both the “X” and “Y” planes from a line projected from the centerline of the gearbox low-speed drive quill. Note that when setting up to align the barrel to the gearbox quill, the thrust bearing must be preloaded to center the quill shaft. Check with the OEM to confirm how to preload the thrust bearing. Also note that any consecutive measurements that are axially less than two diameters apart should be less than one-half the recommended tolerance. Any misalignment contributes to greater stress on the wear surfaces as well as on the gearbox bearings. The better the initial barrel and screw alignment, the longer will be the life of these components. It is valuable to recheck barrel alignment after heating to temperature (by monitoring external surface positions as the system is heated up) to ensure that alignment is maintained after thermal expansion of the system components. Minimizing screw and barrel wear is critical to any successful extrusion operation. Consideration should be given to all factors that will impact the wear and life of the system. The factors to be considered and addressed include: • Screw, barrel, and drive alignment (0.002″/ft) • Straightness of screw and barrel (per SPI specifications) • Screw design • Uniformity of barrel heating • Material being processed (including additives) • Compatibility of screw surface and barrel liner • Improper support of barrel • Excessive loads on barrel discharge (heavy dies) • Corrosion due to polymer degradation or additives. EXTRUSION PROCESS ANALYSIS AND OPERATION When determining the proper screw design for an application, there are many objectives to consider to achieve the expected performance. Some are easily measured, and some are not because they are more subjective or cannot be readily measured online. Some key parameters and operating suggestions are discussed in the next section. Most, if not all, need to be considered because they interact with the screw design. OBJECTIVE (ONLINE MEASURED) VARIABLES Output Rate This variable is very easy to determine and is often considered the most important, although high output with poor melt quality suddenly makes that output number not as important. A certain rate is determined from the required film thickness, density, width, and line speed. Most specifications will require a specific output range for a particular polymer. Output is listed in lb/h or kg/h. A common working rule is that mixing and melt uniformity are achieved for a 1:3 out-

39

put ratio, meaning that a screw designed for a maximum output of 546 KGPH (1200 PPH) should provide reasonable quality as low as 182 KGPH (400 PPH). Keep in mind that specifying a wide variety of polymers to run on the same screw can reduce that quality range. Stability (surging) Stability of rate is critical. Variations in rate, which are called surging, can cause machine-direction thickness variations that affect product quality. Surging can be detected by observing drive amps and head pressure. A small pressure variation may be caused by the passing screw flight depending on the pressure transducer location in the barrel, but if the variation is greater than ±3% and the amps are changing during steady-state operation, this indicates a surge and a screw that is not properly feeding. For a well-designed screw with a narrow target specification, pressure stability less than ±1.5% should be common. Sometimes pulsations are observed in the melt as it exits the die; if there is no accompanying pressure or amp variation, this pulsing is not due to output rate, but is likely due to melt strength or polymer elasticity and the rate of extension. This is often referred to as draw resonance. Melt Temperature Film manufacturing is very sensitive to melt temperature. For blown film, a too-high melt temperature directly impacts bubble stability. For cast film, a too-low temperature may result in edge tear or draw resonance. If the polymer is too hot, the polymer may degrade and produce gels. The ideal melt temperature can vary widely depending on the polymer and application requirements. Not only is melt temperature important, but the variation in melt temperature across the stream is important as well. Excessive variation can cause product defects such as applesauce appearance or bubble/melt curtain instability. This variation must be examined in both the machine and crossmachine directions. Typically, less than ±5°F (2.8°C) is desired. Often, when running aunder various range of polymers and operating conditions, a range as wide as ±10°F (5.6°C) may be accepted. Mono-extrusion applications are more tolerant of wide temperature variations than co-extrusion because the layer-to-layer distribution is sensitive to the viscosity shift associated with wide temperature variations. Thinner layers and skin layers are the most sensitive. SUBJECTIVE (OFFLINE MEASURED) VARIABLES Resin Degradation Shear caused by the extruder screw, high melt temperature, and long residence time can all create, individually or together, molecular weight changes due to chain scission or oxidation. It is sometimes not easy to see that degradation

40

CHAPTER 2—PRIMARY EQUIPMENT

has taken place or to determine how much degradation is acceptable. Odor Odor is very important to food packaging and food products as well as other industries. The specification of odor level is more difficult to quantify. Barrier Quality Many products need barrier properties for a variety of reasons. Some of these involve the transmission rate for water (MVTR), CO2, O2 (OTR), odor, grease resistance, etc. The barrier properties required determine coating thickness, number of layers, and type of polymers. Optical Properties Products are often inspected for their haze, gloss, color, streaking, and gels. Any one of these that is not correct can lead to a rejected product. Other Physical Properties Other important variables can be heat sealability, hot tack, printability, strength, toughness, impact resistance, and appearance. POLYMER VARIABLES Polymers are selected for each product for specific purposes. Some of these may be processability, coextrusion

ability, physical properties, barrier properties, end-use requirements, and/or economics. The differences among polymers can be substantial. Some polymers may be thermally unstable above 380°F (193°C) and some may require high temperatures 575°F (302°C) for drawability. Running both products on the same screw can be a difficult or impossible challenge. It is useful to look at the rheology (shear rate versus viscosity curves) for polymers run at the expected processing temperature. Large viscosity differentials can indicate difficulty in processing both polymers on the same screw design due to viscous energy dissipation (VED) under the shear stress imparted by the screw. The shear rate-viscosity curves in Figure 2.2.15 represent a few typical polymers for cast films (LDPE, LLDPE, PP, EVOH, PLA, and an mPE). Each is shown at its required processing temperature. At the typical shear rate of 200/sec in a screw, there is more than a 10:1 viscosity difference from the most to the least viscous polymer. This differential needs to be considered when materials for running on the screw are selected. OPERATING THE EXTRUDER To operate the extruder successfully, all the pieces must come together. This starts by schematically putting the screw in the extruder so that the processor can understand how the control variables interact with the screw design and the system design. A 30:1 L/D extruder with four barrel temperature-control zones operates differently, with the same screw design, from one with six barrel temperature-control zones. The same screw delivering to a high-pressure die will be controlled differently if there is a melt pump to generate the die

FIGURE 2.2.15.  Rheology comparison.

Section 2.2.  Screw Design

41

FIGURE 2.2.16.  Proportional system schematic.

pressure, so that the screw delivers to the low inlet pressure of the pump. Are heater zones water-cooled or air-cooled? If air-cooled, is it by natural convection or a high-volume, high-pressure blower? How is the extruder instrumented? Are thermocouples embedded deep into the barrel so that they reflect a close approximation of the process temperature (this may result in slow response to setpoint changes), or is the thermocouple embedded in the heater itself, where there may be little correlation with process temperatures, but quick response to setpoint changes? Are there multiple pressure sensors to provide indications of system component resistance in the process: at the discharge head of the extruder, after the screen changer/filter, after any melt pump, at the start and/or end of long feed pipes, before the feedblock or die? How and where is melt temperature measured? Is the probe flush-mounted, or does it penetrate into the melt? Is the thermocouple junction shielded (insulated) or exposed? What are the precision and accuracy of these various instruments? Figure 2.2.16 shows a simple schematic of the extruder in the barrel with the relationship of the screw design features to the control zones. By visually seeing the screw design in the system, knowing the different material and process requirements, and understanding the basic design functions of the screw, one can now approach the setup and operation of the extruder more rationally. FEED ZONE Typically, the feed zone of the screw is under the feed throat and barrel zone 1 of the extruder. The feed throat is water-cooled, with the water temperature set between 15°C (59°F) and 50°C (122°F) depending on the softening point of the polymer and its tendency to bridge or melt prematurely. If low temperature settings are required, one must be careful to avoid settings below the dewpoint because con-

densation in the feed throat can lead to moisture entrapment and moisture bubbles in the product. Barrel zone 1 is set to optimize feed rate and stability. Depending on the polymer, this may range from 120°C (248°F) to 230°C (446°F). The melting zone of the design often corresponds with the melt barrier zone. Depending on the screw and extruder design, this may range in length from two to four heater zones. This temperature will typically be set to maximize melting rate in a way that avoids overheating of the polymer. Where the melting zone extends over two or more heating zones, it is not unusual to ramp the temperature up from closer to the barrel zone 1 settings towards the target melt temperature. The metering and mixing zone of the screw is typically set to achieve the target melt temperature while working with the mixer design to achieve good melt quality. Where dispersive mixers are concerned, often lowering the barrel setpoint temperature at or before the mixer will increase the effective stress at the barrier for dispersion. At the same time, distributive mixers can be used to either raise or lower the melt temperature, depending on the target requirements and the viscous energy input by the screw design. Understanding the design and the position of the design features relative to the control zones is important. For a cast-film application processing LLDPE with a target melt temperature of 285°C (545°F) and the design shown, where mixer 1 is a dispersive mixer and mixer 2 is a distributive mixer, a reasonable starting barrel profile may look like this: feed throat 30°C (86°F), barrel zone 1 (BZ1) 160°C (320°F), BZ2 210°C (410°F), BZ3 250°C (482°F), BZ4 270°C (518°F),and BZ5 285°C (545°F). It is most informative to look at the melt temperature just before the melt enters the die with an adjustable melt-temperature probe to understand whether this profile needs tuning. With the downstream temperatures also set at the target melt temperature, if the position-dependent variation across the melt stream appears hot in the center and colder at the wall, then one knows that the melt exiting the screw is too hot (BZ5

42

CHAPTER 2—PRIMARY EQUIPMENT

may need to be lowered). If the position-dependent variation across the melt stream appears colder in the center and higher at the walls, then the melt exiting the screw is too cold (BZ3, 4, and 5 may all need to be increased). Today, most new extruders are supplied with a supervisory computer that includes a data acquisition system. This is very useful for tracking and monitoring extruder performance. Critical parameters to evaluate extruder performance include as a minimum: screw RPM, drive motor amps (calculated power), head pressure, melt temperature, and output rate. The temperature profile setting should also be recorded, with deviations from setpoint noted. Recording and monitoring process data and evaluating trends enables operators and process engineers to recognize and preempt system problems, whether the slow accumulation of wear resulting in a steady reduction in specific output accompanied by wider temperature and pressure variation, or sudden variations in temperature or pressure due to an off-spec material. These tools provide an early warning system to prevent errors and help maintain a highly productive extrusion system. SCREW RUNNING TIPS Starting an Empty Screw and Barrel The screw is very long compared to the diameter, and the clearance between the barrel wall and the screw outside diameter is very close. If the screw and barrel are empty, the screw will lie in partial contact with the barrel wall. Under normal process conditions, the melted thermoplastic polymer acts as a lubricant between the feed-screw flight land and the barrel ID at startup, which causes the screw to lift and essentially re-center itself in the barrel during operation1. When the screw and barrel are empty, turn the screw at slow speed (5–10 RPM) to minimize galling and feed polymer to the screw immediately. Only increase screw speed when polymer is seen at the downstream side. Screw and Barrel Wear A general working rule is that a screw is worn when output losses are 10% or more than when the screw was brand-new. Unfortunately, many times an original output check with a standard polymer and settings was never done, and hence there is no way of telling whether the screw is exhibiting too much wear. Screws wear at differing rates that depend on the materials run, speeds, and processing conditions, and it difficult to determine the number of years of the life of a screw. Sometimes, the best option is to pull and clean the screw and also clean and measure the barrel. Each turn of the screw is measured, and the diameter over the flights, the root diameters, and other ODs such as over a mixer are measured and recorded. Special measurement tools can be used to determine the OD along the axis or length of the barrel. Once those numbers are recorded, how do you know if there is too much wear? Below is a list of typical tolerances for some

basic screw diameters. The first set of numbers is the starting OD over the flights, and the next set is acceptable diameter readings for cast- or blown-film viscosity polymers. If your measurements are lower, you need to consider whether to repair or replace the screw. 2″ Dia.—1.994/1.996″, 1.992/1.990″ acceptable 2.5″ Dia.—2.493/2.495″, 2.491/2.488″ acceptable 3.5″ Dia.—3.491/3.493″, 3.489/3.483″ acceptable 4.5″ Dia.—4.489/4.491″, 4.487/4.480″ acceptable 6″ Dia.—5.986/5.988″, 5.984/5.976″ acceptable 8″ Dia.—7.982/7.984″, 7.980/7.968″ acceptable. Below is the same information for barrel wear. The first set of numbers is the standard OD of a new barrel, and the second set is acceptable values. Again, if your measurements are higher, the barrel needs to be replaced. 2″ Dia.—2.000″/2.002″, 2.000″/2.004″ acceptable 2.5″ Dia.—2.500″/2.502″, 2.500″/2.504″ acceptable 3.5″ Dia.—3.500″/3.502″, 3.500″/3.505″ acceptable 4.5″ Dia.—4.500″/4.502″, 4.500″/4.505″ acceptable 6″ Dia.—6.000″/6.002″, 6.000″/6.005″ acceptable 8″ Dia.—8.000″/8.002″, 8.000″/8.006″ acceptable These listed tolerances are working rules only. It is also important to consider that the tolerance for wear is dependent on polymer viscosity. Low-viscosity polymers are more sensitive to wear than are high-viscosity polymers. SUMMARY Plasticating extruder screw design has evolved since the first polymers and extruders were introduced in the early 1900s. Advances in design features, modeling and simulation tools, and new and improved polymers have not changed the fact that there is only one screw design inside any one extruder. To perform under different operating conditions (target outputs or temperatures) or to run different materials, operators and process engineers must understand the design of their extruders so that they can interact with the system to achieve the best possible performance. This understanding and awareness will result in increased quality, productivity, and profitability. REFERENCES AND ADDITIONAL RESOURCE SUMMARY [1] Gregory, R.B., “Chapter 6”, in Extrusion Coating Manual, TAPPI Press, 1999. [2] Grant, D., Rubber and Plastics Age, 38(4):238 (1957). [3] Gregory, R.B., Friction Coefficients of Plastics and Steel, ANTEC, Society of Plastics Engineers, Stanford, CT, 1969, p. 114. [4] Spalding, M.A., et al., An Experimental Investigation of Solids Conveying in Smooth and Grooved Barrel Single-Screw Plasticating Extruders, ANTEC, Society of Plastics Engineers, Stanford, CT, 1998, p. 136.

Section 2.2.  Screw Design

[5] Womer, T.W., “Chapter 7”, in Film Extrusion Manual, TAPPI Press, 2005. [6] Gregory, R.B., “Chapter 6”, in Extrusion Coating Manual, TAPPI Press, 1999. [7] Tadmor Z., et al., Melting in Plasticating Extruders: Theory and Experiments, ANTEC, Society of Plastics Engineers, Stanford, CT, 1967, p. 813. [8] Jepson, C.H., “Future Extrusion Studies”, Industrial Engineering Chemistry, 1952, p. 45. [9] Paton, J.B., Squires, P.H., et al., “Chapter 4”, in Processing of Thermo-Plastic Materials (E.C. Bernhardt, Ed.) Reinhold, 1959.

Textbooks 1. Bernhardt, E.C., Ed., Processing of Thermoplastic Materials, Van Nostrand Reinhold, New York, 1959. 2. McKelvey, J.M., Polymer Processing, Wiley, New York, 1962. 3. Schenkel, G., Plastics Extrusion Technology and Theory, American Elsevier, New York, 1966. 4. Fenner, R.T., Extruder Screw Design, Iliffe, London, 1970. 5. Tadmor, Z., et al., Engineering Principles of Plasticizing Extrusion, Krieger, Huntington, NY, 1978. 6. Tadmor, Z., et al., Principles of Polymer Processing, Wiley, New York, 1979. 7. Rauwendaal, C., Polymer Extrusion, Carl Hanser Verlag, Munich; Macmillan, New York, 1986. 8. Hensen, F. (Ed.), Plastics Extrusion Technology, Carl Hanser Verlag, Munich, 1988. 9. Chung, Extrusion of Polymers: Theory and Practice, Hanser Gardner, Cincinnati, Ohio, 2000. 10. Tadmor, Z. and Gogos, C.G., Principles of Polymer Processing, John Wiley, New York, 2006.

Additional Resources 1. Kramer, W.A., “Evaluating Extruder Screw Performance, Part 2,” Proceedings, 1988 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 5. 2. Steward, E.L., “Evaluating Extruder Screw Performance, Part 1,” Proceedings, 1988 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 1. 3. Chung, C.I., “Extrusion Technology Related to Laminating and Coating,” Proceedings, 1988 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 157. 4. Gregory, R.B., “Screw Design Considerations for Extrusion Coating, Part 1” Proceedings, 1988 Polymers, Laminations and Coatings Conference , TAPPI PRESS, Atlanta, p. 171-3. 5. McKelvey, J.M., “Internal Screw Heat-Pump-Assisted Extruders,” 1987 Proceedings, Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 67. 6. Steward, E.L., et al., “Barrier Screws for HMW-HDPE Blown Film Extruders,” Proceedings, 1987 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 59. 7. Veazey, E.W., et al., “Barrier Screw Performance at Several

43

Blown Film Die Gaps,” Proceedings, 1986 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 5. 8. Yokana, L.D., et al., “Extruder Screws for LLDPE Processing,” Proceedings, 1985 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 213. 9. Rauwendaal, C., “Throughput-Pressure Relationships for Power,” Proceedings, 1985 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 23. 10. Rauwendaal, C., “Analysis of Barrier-Type Extruder Screws,” Proceedings, 1985 Polymers, Lamination and Coatings Conference, TAPPI PRESS. Atlanta, p. 9. 11. Luker, K., “Fluoropolymer-Nickel Plating on Extruder Screw,” Proceedings, 1983 Paper Synthetics Conference, TAPPI PRESS, Atlanta, p. 459. 12. Miller, J.C., “Extrusion of LLDPE with Decreasing-Pitch Screw,” Proceedings, 1983 Paper Synthetics Conference, TAPPI PRESS, Atlanta, p. 451. 13. Thompson, R., “Screw Wear: Causes and Effects,” Proceedings, 1983 Paper Synthetics Conference, TAPPI PRESS, Atlanta, p. 341. 14. Honstrater, R.A., “Extruder Screw and Barrel Wear Detection,” Proceedings,1981 Paper Synthetics Conference, TAPPI PRESS. Atlanta, p. 87. 15. Chung, C.I., et al., “Screw Design for LLDPE Blown Film Extrusion,” Proceedings, 1981 Paper Synthetics Conference, TAPPI PRESS, Atlanta, p. 81. 16. Alzner, B.G., et al., “A Modular System of Dynamic Melt Mixers,” Proceedings, 1986 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 281. 17. Klein, I., “Theory and Practice of Extruder Screw Design,” Proceedings, 1989 Coextrusion Conference, TAPPI PRESS, Atlanta, p. 51. 18. Christiano, J.P., “Barrier and Conventional Screw Design for LLDPE,” Proceedings, 1989 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 547. 19. Christiano, J.P., “Barrier vs. Conventional Screw Design: Field Performance,” Proceedings, 1990 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 777. 20. Harrah, G.L. and Womer, T.W., “A Mixing Study of Various Single-Screw Mixing Elements Using Inline Melt Analysis (I.M.A.),” 1998 ANTEC, Atlanta GA . 21. Womer, T.W. and Harrah, G.L., “An Empirical Study for the Optimization of the Barrier Flight Clearance for Single-Stage Extrusion Using Design of Experiments,” 1998 ANTEC, Atlanta GA. 22. Womer, T.W., “Optimizing Sheet Extrusion Conditions to Minimize Internal Stresses in Thermoformed Sheet,” 1991 ANTEC, Montreal, Canada. 23. Cho, J.W., Logsdon, J., Omachinski, S., Guogiang, Q., Lan, T., Womer, T.W., and Smith, W.S., “Nanocomposites: A SingleScrew Mixing Study of Nanoclay-Filled Polypropylene,” 2002 ANTEC, San Francisco, CA. 24. Womer, T.W., “Things Your Screw Designer Never Told You about Screws,” 2002 ANTEC, San Francisco, CA. 25. Womer, T.W., Wagner, J.R., Harrah, G.L., and Reber, D., “An Experimental Investigation on the Influence of Barrel Temperature on the Output of a Constant-Depth Screw with GroovedBarrel Feeding,” 2000 ANTEC, Orlando FL, p. 259.

Chapter 2—Section 3

Die Design BILL BODE, Battenfeld Gloucester Engineering

INTRODUCTION AND FUNCTION OF THE DIE The blown-film die, as it has evolved, is an assembly of four or more components that fit together to form an annular gap from which a polymer(s) exits and subsequently forms a tubular film bubble. The sole purpose of the die is to distribute the polymer flow around the annulus as uniformly as possible. The most common design in service today uses spiral grooves to distribute the flow. The spiral grooves can be cut onto a vertical cylindrical surface, hereafter referred to as a spiral mandrel or a concentric spiral mandrel die design, or onto the flat horizontal surface of a disc, or onto a conical surface. The latter two designs are commonly referred to as stacked-die designs. The concentric spiral mandrel design is the subject of this chapter. In this design, one or more (coextrusion) cylindrical mandrels with helically cut grooves, known as spirals, wrap the cylinder’s periphery. Other types of dies, such as spider dies and side-fed coathanger dies, see limited use, yet are important for running certain materials and will be discussed later in this chapter. SINGLE-LAYER SPIRAL MANDREL DIE COMPONENTS Figures 2.3.1(a), 2.3.1(b), and 2.3.1(c) show cross sections of a typical spiral mandrel die. There are four main components: the spiral mandrel, the body, the pin (also called the inner die lip), and the sizing ring (also called the outer die lip). Because dies are normally positioned vertically and the upstream equipment horizontally, a device to turn the flow 90 degrees is also necessary; it may be as simple as a sub-base [Figure 2.3.1(a)], a die block [Figure 2.3.1(b)], or a rotator/oscillator [Figure 2.3.1(c)]. The rotator also serves to randomize the gauge variation, which is inherent in the process, as the extrudate exits the die. SPIRAL MANDREL AND BODY The spiral mandrel and body serve to distribute the melt from a single stream to an annulus. The single stream of

polymer impinges on a splitting cone and into multiple feed holes that terminate in spiral grooves. The grooves or channels, which are milled into the outside surface of the mandrel, gradually taper out (decreasing in depth), allowing the melt flow to leak into the widening annulus bounded by the spiral mandrel OD and the body ID. Figure 2.3.2 shows a solid model of the tapering spiral and widening gap. The spiral distributor is the heart of this style of die and, although it may be designed for a particular resin, must be designed with a broad operating window. This means that it must be capable of accurately distributing resins with varying melt rheologies and molecular weight distributions. The relationship between the melt flow and the geometry of the spiral distribution system is quite complex and requires both knowledge of polymer flow properties (rheology) and empirical knowledge to interpret results. The angle at which the grooves taper out versus the rate at which the mandrel die decreases causing the width of the annulus (also referred to as the plenum gap) to increase, are all design variables. In addition to the spiral mandrel surface geometry, the groove lead (or pitch) size, width, quantity, and length may be varied to optimize a design. All this geometry can be modeled or computer-simulated and subsequently refined to produce the desired leakage distribution. The leakage distribution is defined as the percent melt flow into the annular gap (gap flow) vs. the circumferential position along the spiral (the number of pitches). Empirically, it is known that the spiral design is the most important feature in the die design and has the most influence on melt distribution. Furthermore, of all the spiral features, the spiral length appears to have the most influence on die performance and melt distribution. The sample distribution shown in Figure 2.3.3 shows the percent of process flow into the gap (as the spiral tapers) vs. circumferential position on the spiral mandrel. At the top of the spiral mandrel/body section of the die, the flow is 100% annular, yet will need some improvement in melt flow uniformity. DIE LIP SETS The die lip set, consisting of the pin and sizing ring, serves 45

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FIGURE 2.3.1.  (a) Die

sub-base, (b) Die block and (c) Die rotator/oscillator.

Section 2.3.  Die Design

FIGURE 2.3.1 (continued).  (a) Die

47

sub-base, (b) Die block and (c) Die rotator/oscillator.

two purposes; it creates a geometry that filters the polymer flow to improve or attenuate its uniformity around the annulus, and it allows a die size change depending on whether the lip set flares in or out [again Figures 2.3.1(a), (b), and (c)]. Most commercially available blown-film dies allow for significant lip-set size changes, with ±20% not uncommon. The extrudate exits the spiral into a region of lower shear known universally as a plenum; from here, it passes to a region of higher shear, called a choke. If a second plenum follows the choke, then the second choke would be the final gap. The shear rate and pressure gradient are lower in the plenum, giving the stresses in the extrudate a chance to re-

lax and even out, which ultimately results in more uniform gauge. A plenum followed by a choke can be thought of as a mechanical filter: flow perturbations are attenuated because the material in the lower-pressure-gradient plenum region is allowed to flow circumferentially before passing across the higher-pressure-gradient choke. In addition, the choke, because of its narrow gap, acts as a thermal filter because the extrudate, which may be at different melt temperatures,

FIGURE 2.3.2.  Spiral design.

FIGURE 2.3.3.  Spiral flow.

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is in close contact with the higher-conductivity steel walls of the die. The final die gap along with the blowup ratio (BUR) and the final film gauge control the film’s drawdown and some of its mechanical properties. This is covered in detail in a later chapter. Figure 2.3.1(c) shows the monolayer die sitting on top of a rotator/oscillator. The rotator/oscillator, consisting of bearings, seals, and machined components, allows the die to fully rotate or oscillate to help randomize the film-gauge variation on the wound roll. A slip-ring assembly or oscillating device is required to maintain power and control connections at the die. A die size change is accomplished by changing the sizing ring, pin, and pin insert (if part of the design), but a die gap change is accomplished by changing only the pin or pin insert. DIE GAPS The die gap is generally chosen based on the polymer being processed or sometimes according to the final film thickness. Highly branched low- and medium-density polyethylenes (LDPE and MDPE) and other high-melt-strength (shear stress-resisting) materials are generally processed in narrow gaps, typically in the range from 0.035–0.055 in. Linear PEs like LLDPE require larger gaps to avoid melt fracture (a surface disturbance) because of their low melt strengths. Gaps for these range from about 0.070–0.115 in. Most other materials, blends, and material constructions used in film extrusion are processed on gaps in or between these ranges. In Figure 2.3.1(a), the gap is changed by changing the pin insert. The smaller the die gap, the higher will be the shear stress on the polymer as it exits the die. As mentioned above, an undesired surface effect known as melt fracture may occur if the shear stress rises past a known threshold. One way to help minimize this is with a set of die-lip heaters that selectively raise the surface temperature of the processed polymer that is adjacent to the steel surface, thereby reducing shear stress locally. Figure 2.3.4 provides an illustration.

MECHANICAL CONSIDERATIONS The designer must keep certain considerations in mind when designing spiral mandrel dies. These include, but are not limited to, the following: flexural rigidity or stiffness, locations and quantities of fasteners versus the number of feed ports, and the locations and sizes of utilities such as air for internal bubble cooling and power and control wiring for internal heaters. STIFFNESS OF COMPONENTS/ASSEMBLY/ DISASSEMBLY The die components must be rigid enough to withstand the loads induced from the fasteners and the internal operating pressures with minimum deflection. The components that typically see the highest loads are the die body and the lip set. The body sees operating pressure at its ID and atmospheric pressure at the OD, meaning that it often has a larger cross section than other components and must have minimum expansion under load. Often a customer needing a different die-lip set diameter to run a wider or narrower product will choose to replace the lip set rather than purchase a new die. A significant flare in either direction may require larger section components to resist deflection and additional fastening to prevent loosening (and subsequent leaking) of the lip set. One of the most important considerations in the mechanical design of the die is ensuring that all die components remain concentric at operating pressures. The stiffer the mechanical assembly, the better will be the gauge control. Locking or piloting the components to one another is key in reducing deflection and preventing relative movement under load. A die that experiences random movement of the lipset components during operation due to a loose or poorly designed fit will likely produce poor film gauge. A straight pilot fit is inexpensive to fabricate and needs clearance for assembly, meaning that relative movement between components is possible. A taper fit is more expensive to manufacture, but much more effective in preventing movement because, unlike a straight pilot fit, the components can be clamped line-to-line or with a small interference and still be easy to disassemble. Some manufacturers prefer a fit made from dissimilar materials, the concept being that the material with the higher COE will “lock” the components together. An interesting and workable concept, but expensive to fabricate. Figure 2.3.5 illustrates various fit options. ADDITIONAL CONSIDERATIONS

FIGURE 2.3.4.  Die-lip heaters.

These include juggling the space required to pass IBC passages and power and control wiring through the core of the die. Ideally, the air passageways should be large to carry more air for better cooling and hopefully higher rate. Higher rate, however, requires larger process flow channels, the size of which has been compromised by increasing the IBC cavities. Basically, the designer is faced with technical compromises that must be balanced to provide the most efficient die design.

Section 2.3.  Die Design

49

FIGURE 2.3.5.  Die fit mechanisms.

MATERIALS OF CONSTRUCTION AND SURFACE COATINGS Selection of surface coatings and materials depends on careful consideration of the following; • Corrosiveness of the polymers to be processed • Cleaning method being used • Release characteristics of the polymer • Mechanical loads that the die will experience. MATERIALS Most dies today are constructed of medium-carbon alloy steels. The carbon content provides hardenability (and hence yield strength), and the alloying elements provide strength and corrosion resistance. Two popular choices of machinery manufacturers in North America and Europe are AISI 4140 and 4340 steel; both are readily available clean in large bars and forged shapes. The 4340 steel, with its added Ni content, has slightly higher hardness than the 4140, but that can also be a detriment, as will be seen later. Hardening grades of stainless steels, specifically the 4xx and the PH (precipitation hardening) series, are also sometimes used, but mostly where the die processes corrosive materials or very high hardness is needed. PLATINGS/SURFACE COATINGS Platings are used on blown-film dies and like-service machinery for three primary reasons: (1) Release: cleaning times are reduced because the process releases more easily from plated than from unplated surfaces. (2) Lower Coefficient of Friction (COF): the process adjacent to the plated surface slides more easily, which may reduce pressure loss and the tendency of the process to gel. (3) Corrosion Protection: the plating protects the internal surfaces from process corrosion at elevated temperatures and the external surfaces from atmospheric corrosion. Platings and coatings for dies can be classified in one of three categories (in order of descending use): Electroless

coatings—electroless nickel is (EN) the most popular; opentank plating, no electricity required, very uniform deposit and able to plate all difficult-to-reach areas (like deep holes on radially fed film dies), non-porous, corrosion-resistant, reasonable hardness (RC 60) and auto-catalytic; Electrolytic coatings—industrial hard chrome (IHC) and thin dense chrome (TDC) are the most popular; open-tank plating, electrical current, and anodes (steel or lead wires bent and shaped to the part contour) are required for uniform deposition in hard-to-reach areas like deep holes; hard (RC 70), IHC is a porous coating (micro-cracks at extrusion temperatures and therefore not good for corrosive materials . . . see below), but TDC is not porous; Chemical or physical vapor deposition coatings—(CVD/PVD)—these are coatings applied to the work piece in an enclosed vacuum chamber by sublimating a pure metal in a reactant gas at elevated temperature: line-ofsight (so suitable for flat OD surfaces only), very hard (up to RC 90), very thin (0.0001 in), with a range of properties. CORROSION For most commodity resins such as polyethylene, corrosion due to the process is not an issue. This, however, may not be the case for specialty materials like certain acid-based co-polymers and ionomers. Electroless nickel, because it is an amorphous coating, is generally fine for these materials, as is thin dense chrome. Hard chrome in a micro-cracked state (cracks visible at 100× magnification) may allow the process to contact the steel surface, leading to corrosion, especially at high temperatures. Dies designed to process flexible PVC are generally made of 400 series stainless steel to reduce the possibility of corrosion. Because of its highly corrosive potential when in contact with ferrous-based metals, processing PVdC (Saran) requires more specialized alloys. Very high nickel content alloys like INCo’s Hastalloy or Duranickel are the materials of choice. CLEANING Frequency—this is a very subjective decision, yet is also dependent on factors like startup procedures, product change frequency, and die design. Starting up a film die, especially with less experienced operators, generates some scrap. If the processing temperatures during this period rise too high,

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the material can burn, sometimes requiring a full or partial cleaning. Changing products frequently (i.e., short runs) puts demands on how fast the die will purge (i.e., how long does it take for not just the bulk of the material, but all the material to exit). Look closely at the die design; does some of the material sit in poorly streamlined areas and degrade, forcing an otherwise unneeded cleaning? As a working rule, well-designed coextrusion dies processing high barrier are cleaned at 6–12 month intervals, whereas dies processing commodity resins are cleaned less frequently Methods—there are basically two cleaning methods: by hand, and in an oven or a fluidized bed. Both methods require that the die be disassembled. Cleaning by hand is the most common and requires a lot of labor to get the job done before the process solidifies. Manual cleaning is generally done with brass scrapers and other soft tools to protect the die from damage. A faster, more efficient means is by using a pyrolysis oven. These ovens burn the process material off at a temperature of ~800°F, leaving a powdery residue on the parts that can be cleaned off with a hand pad. Note: the oven must operate below the tempering temperature of the steel, or softening may occur (a deterrent for using higher-hardenability steels). In a fluidized bed, a mild abrasive is pumped around and through the die parts to achieve cleaning. Generally, most processors polish the components with a metal polish and check for plating continuity before reassembling. One thing must be stressed here! Almost all damage to film die components occurs during cleaning, so great care must be taken to protect sensitive sharp edges from damage.

COEXTRUSION DIES Description The function of the blown-film coextrusion die is the same as that of the monolayer die, only in multiples, i.e., to distribute, as accurately as possible, two or more layers of polymers into a multilayer annular film tube. The individual flows may enter the die as a single stream, as is the case in non-rotating dies, or as an annulus or circle of holes, as is the case in a rotator. In both cases, the individual flows remain separated and are distributed to an annular geometry before combining. These dies are more complex rheologically and mechanically than ordinary dies and have more parts. Whereas the mono die had one spiral mandrel, the 3-L coex die has three, and the 5-L die has five. Figure 2.3.6 shows a cross section of a typical non-rotating coextrusion die. RHEOLOGICAL (FLOW) CONSIDERATIONS The sequence of flow geometries for each layer in a coex die is similar to that in a mono die. A single stream is divided into multiple streams terminating in spirals that feather out to annuli followed by chokes. Figure 2.3.6 shows that the thinring spiral mandrels nest together concentrically and “lock” to a stepped piece called the A spiral mandrel or ASM. Note that there is more vertical length on the outboard mandrels than on the inner mandrels. This extra length, which is more than needed for distribution, is used by machining the feed

FIGURE 2.3.6.  Cross section of a non-rotating die.

Section 2.3.  Die Design

51

FIGURE 2.3.7.  Oscillating die.

hole vertically to the spiral start. The reason for this is that it is more efficient (lower pressure drop and higher shear rate for the same flow rate) to move the process material in a hole, or a D-shaped milled channel, than in an annulus, as would be the case if the spiral started at the point where the feed hole meets the spiral surface. As in mono dies, the spirals on each thin-ring mandrel feather into a plenum followed by a choke. At the top of the choke, the layer joins another layer entering from the side to form a multilayer flow. In three-layer dies, three flows combine at the same point, commonly called the combined flow point, but in five-, seven-, and nine-layer dies, the layers combine sequentially on both sides of the combined flow point, finally joining the core layer. The geometry in the multilayer flow region effectively acts as a filter, meaning that depending on its location, an individual layer may see as many as four cascaded filters. This is one of the reasons that coex dies often produce better gauge than monolayer dies of the same size and same number of ports. Coex dies may be designed to rotate/oscillate or to remain stationary. The main difference is the upstream distribution

area just ahead of the spiral. As noted earlier, Figure 2.3.6 shows a non-rotating die, whereas Figure 2.3.9 shows an oscillating die. The oscillating die must be designed with lower shear and pressure gradients than the stationary die because the oscillator consumes pressure drop that would otherwise be allocated to the die in the form of higher shear rates and/ or lower residence times. Expect an oscillating die to purge more slowly than a non-rotating die. Coextrusion dies are generally built with odd numbers of layers, primarily to provide symmetry about the core layer, which would likely process a high-barrier polymer. The exception might be two-layer dies where the flow is split into two spirals and hence two separate streams that join outside the die. This concept enables more reliable processing of offspec resin that might otherwise blow holes in the bubble. Figure 2.3.8 provides an illustration. Polymer residence time (RT) and residence time distribution (RTD) are important considerations in BF die design, but are especially important in coextrusion die design. The RT is the average time required for a particle of fluid in the midstream of all the geometries to pass through the die; basically,

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FIGURE 2.3.8.  Coextrusion die with external joining of flows.

it is the total volume of the flow cavities divided by the flow rate. The RTD is a measure of the residence times of all the process fluid and is very much a function of how well the die is streamlined. If an average particle of flow exits the die in one minute, but it takes 30 min for the last particles to purge from the die, one would think that the die lacks streamlining and that the design could be improved. Short RTs are advantageous because the resulting high shear keeps the die running cleaner longer. This is more difficult with coex dies because each layer gets only a fraction of the flow of the same-size mono die running the same rate. The lower rate means smaller channels for the same shear rate, but the smaller channels result in a nonlinear, significantly higher pressure drop that may be prohibitive… a “catch-22”. All this adds to the compromises required in the channel design of the coex die. MECHANICAL CONSIDERATIONS All the mechanical discussions about monolayer dies hold here, and there are added ones. There are more mandrels that need fasteners, and more projected area that is under load from the process pressure. This means that die components will be stressed more highly and that additional consideration must be given to component sizing. Figures 2.3.6 and 2.3.7 show relatively thin mandrels, which is acceptable because they see only process load variations. The body, however, sees from process pressure to atmosphere (a larger differential) and hence must be thicker.

FIGURE 2.3.9.  Spider die.

dated spiral mandrel dies and were some of the first dies used for blown film. Looking at Figure 2.3.9, one notices that the die is center-fed; the flow impinges on a cone on the upstream end of the center mandrel, then flows along that surface to a vertical annular gap. At this point, the flow splits around the spider islands, recombines downstream, and continues to the exit. The problem is that weld lines often form as the material recombines, rendering the film unacceptable. This design is streamlined and well suited for processing materials like flexible PVC. WRAPAROUND COATHANGER DIES This die, shown in Figure 2.3.10, is effectively a flat coathanger style die wrapped into an annulus. The center of the die is clear, with the process material entering from the

OTHER TYPES OF FILM DIES There are two chapters in this book covering blown-film dies: this chapter, covering spiral mandrel dies, and Chapter 2.4, covering stacked-plate designs. There are, however, other types of dies that are used less frequently and for specialty applications. These dies include spider dies and wraparound coathanger dies; the applications include processing resins that are potentially caustic and/or corrosive. SPIDER DIES Figure 2.3.9 shows a spider die; the name arises from the spider islands that support the center pin area. These pre-

FIGURE 2.3.10.  Wraparound coathanger-style die.

Section 2.3.  Die Design

53

side. The flow splits symmetrically as it engages the mandrel surface and follows a path around the mandrel at an incline, reconnecting on the far side. This die is perhaps the most streamlined of all, but may also suffer from a weld line on the far side (180° away from the entry). Downstream of the manifold groove is a distributor that balances the flow. The other downside to this design, which affects the die’s versatility, is the distributor, which must be custom-designed for each resin. These may be fabricated on a cylindrical or a conical surface (cylindrical is shown).

on both skin layers. Such a film can be treated as a monolayer flow. Five-, seven-, and nine-layer films often have different materials in the skin layers. Multicomponent flow in the die geometries that produce these films may be treated by taking the material that makes up the highest percentage of the total flow and again treating this material as a mono layer with the die’s total flow rate. These approaches are sufficiently accurate.

UPPER DIE GEOMETRY

Resistance heating with feedback control (PID) is the standard used on blown-film dies. The number of heating zones may depend on the die style and size. Small dies without IBC may have two zones (sizing ring and lower die); larger dies may have three zones (inside die (pin), sizing ring, and lower die); large multilayer stationary dies may have as many as five zones: the three listed above and two additional zones near the base of the die. Note that if significant differences in component geometries or diameters exist, these components should be zoned separately to ensure even heating. Watt densities for heaters used on a BF die surface are normally in the 20–30 watts/in2 range. Control sensing is normally done with type-K thermocouples.

An earlier discussion touched briefly on multicomponent flow, i.e., sequential joining of flows until as many as nine melt streams are flowing in an annulus. This is viscous flow, and therefore the flow streams are laminar, and only with very dissimilar materials or very long combined flow paths would one experience melt-flow instabilities. Yet the designer still needs to characterize the pressure gradient(s) in multicomponent flow. There are sophisticated mathematical solutions to some of these problems, but they are timeconsuming, awkward, and used less frequently. Three-layer films are generally symmetrical, with the same materials run

CONTROLS

Film Extrusion Manual, Second Edition, 2005

Chapter 2—Section 4

Stacked-Die Technology for Tubular Film Coextrusion JOHN PERDIKOULIAS, Compuplast International Inc.

INTRODUCTION Coextrusion die design indeed confirms the adage that necessity is the mother of invention. Coextrusion started to become more popular among many processors in the late 1970s and early 1980s when OEMs started to include these items in their product list. Before this, advanced processors developed their own coextrusion equipment, but the technology was very closely guarded (and still is today). For the purposes of understanding the development, it helps to divide tubular coextrusion into two categories, barrier and non-barrier. Non-barrier coextrusion includes coextrusion products made primarily of various layers of polyolefins combined for reasons of aesthetics, strength, and cost. Barrier extrusion includes products developed primarily for barrier functions such as food or chemical packaging. In the non-barrier category, almost all OEMs have been providing successful two- and three-layer coextrusion dies since the early 1970s. However, the more limited processing conditions of the materials used in barrier coextrusion brought more difficulties. Barrier-type resins like PA, EVOH, and PVDC have much narrower “processing windows” than the polyolefins to which most equipment manufacturers were accustomed. Barrier-type resins were also more prone to degradation, making them less forgiving of poor die design as well as poor operating practice. As a result, successful coextrusion of barrier films required a combination of good die design, good screw design, and good operating practices. It should also be mentioned that the ongoing and increasing success of coextrusion parallels improvements in resin development. Over the years, resin companies have continued to improve the processing behavior and thermal stability of barrier polymers. There has also been a trend to increase the number of layers in a structure, either to improve film properties or to reduce cost. For example, an excellent five-layer barrier structure could consist of PA/tie/EVOH/tie/Ionomer. However, a corresponding ten-layer structure could be PA/Tie/LLDPE/ Tie/PA/EVOH/PA/Tie/EVA/Ionomer. It can be demonstrated that the ten-layer structure provides improved barrier per-

formance with an equal amount of barrier material, as well as offering the possibility of reducing or replacing some of the more expensive polymer with lower-cost resins that provide adequate performance. CONVENTIONAL COEXTRUSION DIES Probably the most influential initial supplier of coextrusion equipment was Barmag, which is said to have provided more than 150 coextrusion dies by the mid- to late 1980s. These types of dies were based on conventional cylindrical designs with rotating block systems. Figure 2.4.1 shows a cross section of a conventional three-layer coextrusion die with a rotating block system that was provided by Sano in the late 1980s. The rotating die systems (which were required for gauge randomization) were very complex and posed a maintenance problem. Apart from periodic maintenance (often resulting from failure) of the sealing system, the rotating blocks generally provided additional areas for polymer to “hang up” and degrade. Combined with the additional passage length of the rotating block, this resulted in poor purge characteristics and long changeover times. In fact many processors of barrier films had a spare die that enabled them to continue production while another die was being cleaned or serviced. This also limited the number of processors who could attempt to develop coextruded products to those who had the financial strength to cover the intense development costs, production efforts, and maintenance costs. Despite these difficulties, the advantages offered by coextrusion encouraged R&D efforts by processors, material suppliers, and equipment manufacturers alike. The late 1980s and early 1990s witnessed many new materials, die designs, and coextrusion products. One of the most important developments of this era was the widespread introduction (due to patent expiration) of the oscillating haul-off. Although this device is relatively far from the die system, it did remove the burden of gauge randomization from the die, leading to great improvements in die-passage streamlining. Without the need to rotate the die, manufacturers began paying more attention to channel design with the aim of reducing residence times. 55

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FIGURE 2.4.1.  Conventional spiral mandrel-type coextrusion die with rotating block.

However, commercially available coextrusion dies were still essentially made up of three to five single-layer dies that were nested together, as shown in Figure 2.4.2. Each one of these cylinders could be considered as a conventional, single-layer spiral mandrel distribution system, as shown in Figure 2.4.3. The polymer melt was distributed by controlled leakage

of material out of the helical grooves (spirals). The subsequent layering effect also had benefits with respect to mixing and appearance. Although these dies have been successfully used for many years, the basic concentric cylinder design resulted in a dramatic increase in surface area as the number of layers increased. This resulting larger surface-to-volume ratio came with a higher potential for degradation.

FIGURE 2.4.2.  Conventional coextrusion die consisting of five nested cylinders.

FIGURE 2.4.3.  Conventional cylindrical spiral mandrel distribution system.

Section 2.4.  Stacked-Die Technology for Tubular Film Coextrusion

FIGURE 2.4.4.  Stacked coextrusion die.

STACKED DIES In 1989, Brampton Engineering introduced a relatively new concept in coextrusion die design for conventional tubular (blown) film, which used a stacked type of layer distribution system. Unlike conventional systems, the stackedtype dies arrange the layers vertically, as shown in Figure 2.4.4. It has been reported that some processors had developed similar systems for their own use, but it is generally believed that this was the first time that such technology was made available commercially by an OEM for conventional blown film. Note that the idea of stacking the layers is not completely new. In fact, the stacked configuration has been used with blow-molding dies since the late 1950s and 1960s (Colombo, 1958; Schrenk, 1967). However, these designs had some

57

FIGURE 2.4.5.  Typical side-feed distribution system.

disadvantages, which made them unattractive for blown-film applications. In general, these designs used a simple sidefeed distribution system, as shown in Figure 2.4.5. The polymer is essentially split into two flow streams, which are distributed around the die circumference and meet at a point directly opposite the entrance. This resembles a flat die distribution system that has been wrapped around so that the edges would connect. This distribution system, however, suffers from a couple of drawbacks. First, the merging of the streams around the back of the die entrance generally results in a stagnant flow region where the polymer could degrade. Second, there is no overlapping of flow streams, as would happen in a conventional spiral mandrel die. This means that the orientation of the weld line is perpendicular to the axis of the die and is more likely to result in a weak point or a visual defect in the final product. In addition, the overlapping or layering provides some additional mixing of

FIGURE 2.4.6.  Flat-spiral distribution system with two spirals.

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the material. The lack of overlapping makes the die more sensitive to any inhomogeneities in the melt stream from the extruder. For these reasons, it is desirable to incorporate a spiral-type distribution system into the new configuration. Figure 2.4.6 shows a flat spiral distribution system used in a small stacked die. In any spiral-type die, it is highly desirable that the polymer entering the die, be divided equally among the spirals. This is done with a binary primary distribution system before the spirals, as shown in Figures 2.4.6, 2.4.7, and 2.4.8. In the binary distribution system, the polymer entering the

die is first divided into two identical flow streams. Then each of these streams is also divided into two streams, resulting in four streams of equal flow, as shown in Figure 2.4.7. This can be repeated to get 8 or 23, where “3” represents the number of divisions or splits, hence the binary system. The flat-spiral distribution system then distributes the flow in a manner similar to a conventional cylindrical spiral distribution system. The goal is to obtain a circumferentially uniform radial flow stream. This radial stream can then be redirected to flow in the die’s axial direction and ultimately join with other layers before exiting the die. More details on this type of system can be found in publications by Perdikoulias. The primary advantage of the stacked-die system was that it offered the potential to minimize the wetted surface area of each layer in a coextrusion system, thereby minimizing residence time and improving the purge characteristics of the system. The other advantage of the stacked-die system was that it resulted in a modular die design, which had a few more potential advantages. First, it could theoretically allow a die to be more easily modified to conform to various structures by exchanging modules. The other potential advantage is that the individual flow modules could be operated at different temperatures so as not to expose thermally unstable materials to excessive temperatures for a prolonged period. These advantages have been successfully incorporated in tubular (blown) film dies with up to ten discrete layers, as shown in Figure 2.4.9. After the success of dies provided in the early 1990s, many manufacturers followed with their own versions of stacked-die systems. Figures 2.4.10–2.4.17 show the concepts of some of the stacked coextrusion dies offered by various manufacturers. Many of the design details have been

FIGURE 2.4.8.  Brampton Engineering “SCD” die distribution plate with eight spirals.

FIGURE 2.4.9.  Brampton Engineering ten-layer stacked die.

FIGURE 2.4.7.  Flat-spiral distribution system with four spirals, each of which travels 360 degrees.

Section 2.4.  Stacked-Die Technology for Tubular Film Coextrusion

59

FIGURE 2.4.12.  Section view of a W & H 5-layer stacked die.

FIGURE 2.4.10.  Section view of a Brampton Engineering ten-layer stacked die.

omitted or slightly modified for confidentiality reasons but, the basic concept of the design can be seen in the cross-section views provided. Some of the variations involve changes to the orientation of the primary distribution system that directs the flow to the spiral distribution system. Others use spirals on a tapered cone rather than on the surface of a disk, which stack to-

FIGURE 2.4.11.  Section view of a Hosakawa Alpine 5-layer stacked die.

FIGURE 2.4.13.  Section view of an Egan DavisStandard 9-layer stacked die.

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FIGURE 2.4.16.  Cross section of a Labtech 3-layer stacked die.

FIGURE 2.4.14.  Section view of a Macro Engineering and Technology 7-layer stacked die.

gether somewhat like cups. Some even use two distribution systems for each layer in an effort to further improve die performance. Advantages and disadvantages can be claimed for each system. For example, to minimize distortion forces from pressure, the size of the spiral distribution system needs to be kept to a minimum, which makes it difficult to ensure

FIGURE 2.4.15.  Cross section of an Addex 7-layer stacked die.

a uniform distribution of polymer. The tapered-cone style of stacked die can minimize the projected area and therefore the distortion forces, but at the expense of surface area. Using two distribution systems for each melt stream effectively reduces layer variation, but again at the expense of surface area and pressure. Recent advances in CAE and CFD techniques have provided tools that help to overcome many of the design challenges posed by stacked dies. Figure 2.4.18 shows the results of a 3D FEM simulation of a four-spiral, flat-spiral system. The simulation shows the pressure contours and the exit velocity for one-half of the system. Flow simulations help to avoid problems by ensuring that the flat spiral design provides adequate polymer distribution at acceptable values for pressure drop and other flow characteristics. FEM-based stress analysis software is also used to help control distortion of the components under pressure. Although just about any type of system can be made to work, the fundamental flow characteristics of each system, such

FIGURE 2.4.17.  Cross section of a Dual Spiral Systems 7-layer stacked die.

Section 2.4.  Stacked-Die Technology for Tubular Film Coextrusion

61

lenges are being met. To date, many five- and seven-layer dies as well as several eight-, nine-, and ten-layer stackeddie systems are working successfully, to the point where the stacked die is now generally the preferred design (according to the number of installations) for coextrusion systems of more than five layers. REFERENCES AND ADDITIONAL RESOURCES

FIGURE 2.4.18.  Pressure contours and exit velocity profile of a flatspiral distribution system.

as distribution performance, pressure drop, shear rates, and shear stress as well as mechanical issues including the ease of manufacturing and maintenance must all be considered in the design. It is apparent, however, that the design chal-

[1] Perdikoulias, J., “Annular Coextrusion Die Design” Proceedings, TAPPI 1992 Polymers, Laminations and Coatings Conference, TAPPI PRESS, Atlanta, p. 212. [2] Knittel, R., Proceedings, SPE Coextrusion V Conference, p. 84, 1989. [3] Mavridis, H., “Modular Coextrusion Stacked Dies for Blown Film”, ANTEC pp. 101–105, May 1996. Colombo, R., U.S. Patent 2,820,249 (1958). [4] Schrenk, W.J., U.S. Patent 3,308,508 (1967). [5] Perdikoulias, J., Analysis and Design of Annular Dies for Mono and Multi-Layer Annular Flow, Ph.D. Thesis, University of Waterloo, Waterloo, Ontario, Canada, 1997. [6] VEL™ 3D FEM Software V.6.10, Compuplast International, Inc., 2017.

Chapter 2—Section 5

Winding and Web Handling R. DUANE SMITH, Davis-Standard LLC

INTRODUCTION So far in this manual, various formation and conditioning processes have been addressed. After the resulting, hopefully homogeneous combination of various types of thermoplastic materials has been mixed with special additives and made into pellets, it is then remixed with reclaim and heated, blended, sheared, spread thin, combined with other molten layers of other plastic materials, and then forced through a little slot die. The extrudate is then solidified through rapid cooling by casting it onto a chilled iron roll or blowing it into a bubble surrounded by chilled air. It then typically is stretched thin in one or both directions and then treated or mistreated as it proceeds down the process. This material, now called a film, finally ends up at something called a winder. There it is often cut up into narrow strips and forced to be transferred onto a round thing called “a core” and made into a somewhat round package that we affectionately call a “roll of film”. The function of a winder is to continuously transform this single width or multiple widths of thermoplastic material (film) into rolls of material that can be handled, transported, and reprocessed without affecting the properties or dimensional stability of the film. Plastic film webs may still be warm when they are wound. They may contain tack or slip agents that can migrate to the film surface over time. Moreover, plastic film webs have memory. This plastic memory is a history of the previously imposed stresses from the processes mentioned earlier. These stresses will often be relieved or relax during the curing process that will take place anywhere from 12–48 hours after the film web has been formed. This curing takes place inside the roll and causes sometimes wanted, but usually unwanted dimensional changes in the web. If all film webs were perfect, then the ability to produce perfect rolls of film products would not be much of a challenge. Unfortunately, due to the natural variation in resins and additives and the non-uniformities of film formation, there is no such animal as a perfect film. The winding operation’s challenge is to wind film webs with slight imperfections, making sure that these slight imperfections do not stand out in appearance and are not amplified during the

winding process. Then it is the responsibility of the winder operator to make sure that the winding process does not produce additional variations in product quality. The ultimate challenge is to wind plastic film webs with slight imperfections and produce quality rolls that will run on your customer’s process without problems and produce high-quality products for their customers. DEFINITION OF A QUALITY ROLL If the challenge is to wind quality rolls of plastic film webs, then the first task is to define our customers’ definitions of good quality. A baker will tell you that to please their customers, they must produce rolls that are of the right shape, the right size, and the right consistency—not too hard and not too soft, they must look good—no blemishes or visual defects, and they must have a good aroma. Film product customers want rolls of film that are: • The right shape: Round and proper width • The right size: Right diameter or length • The right consistency: Proper roll density (not too hard or soft). Must look good—No blemishes or visual defects. • Aroma: Well, start shipping your customers bad rolls of film and they are going to raise a stink! Importance of Roll Hardness Roll density or in-wound tension is the most important factor in determining the difference between good-quality and poor-quality rolls of film products. Rolls that are wound too soft will go out of round while winding or while being handled or stored. The roundness of rolls is very important in your customer’s operation. When unwinding out-of-round rolls, each revolution will produce a wave of tight followed by slack tension. These tension variations can distort the web and cause register variations in the printing process. The only way to minimize the effect of these tension variations is to run the operation at a much lower speed, which greatly affects production. Rolls that are wound too tight will also cause problems. 63

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Rolls that are wound too tightly will exaggerate web defects. No web is perfectly flat or the same thickness from one side to the other. Typically, webs will have slight high and low areas in the cross-machine profile, where the web is thicker or thinner. When winding hard rolls of film, the highcaliper areas build on each other. As hundreds, even thousands of layers are wrapped into a roll, the high areas form ridges or high spots in the roll. As the film is stretched over these ridges, it is deformed. Then, when the roll is unwound, these areas produce a defect known as bagginess in the film. Hard rolls that have high-gauge bands next to low-gauge bands will produce a roll defect known as corrugations or rope marks in the rolls. Slight defects of these types will not be noticeable in a wound roll if sufficient air is wound into the roll in the low areas and the web is not stretched over the high areas. Still, rolls of material must be wound hard enough so that they will be round and will stay round during handling and storage. FIGURE 2.5.1.  Poor-quality roll of film.

Rolls of some films, when wound too tightly, will “block”. Roll blocking is a defect in which the sheet layers fuse or adhere together. When winding extensible film on thin-wall cores, winding hard rolls can cause the cores to collapse. This can cause problems in removing the wound roll from the winding shaft or inserting a shaft or chucks during the subsequent unwinding operation. Tightly wound rolls contain high residual stresses or high in-wound tension. The film will stretch and deform as these stresses are relieved as the rolls cure during storage.

Randomization of Cross-Machine Variations Some plastic films, either because of their formation process or their coating and laminating process, have crossmachine thickness variations that are too severe to allow the web to be wound without exaggerating these defects. To overcome this, these webs are moved back and forth as they are being slit and wound, or the slitters and winder can be moved back and forth relative to the web. This approach is called oscillation, which randomizes these localized defects across the wound rolls. Oscillation may involve a constant speed, stop, and constant speed back or a sine-type speed curve. What is important is that the oscillation speed is fast enough to randomize defects and slow enough that it does not strain or wrinkle the film and that the rolls, after they have been slit, are wound with straight edges. A working rule for the maximum oscillation speed is 25 mm (1″) per minute per 150 mpm (500 fpm) winding speed. For best results, the oscillation speed should vary proportionally to the winding speed. Profiling Roll Hardness

FIGURE 2.5.2.  Good-quality rolls of film.

As a roll of flexible packaging film material is wound, in-wound tension or residual stresses build inside the roll. If this stress becomes greater during winding, the inner wraps towards the core will be put under high compressive loads. This is what causes a defect known as “buckling” of the web in localized areas in the roll. When winding non-elastic and high-slip films, the inner layers will loosen, which can cause the roll to dish while winding or to telescope while unwinding. To prevent this, the rolls should be wound tight at the core and then wound with less tightness as the roll builds in diameter. This is commonly referred to as roll hardness taper. The larger the finished wound roll’s diameter, the more critical the roll hardness tapering profile becomes. The secret to building a good roll hardness structure is to start out with

Section 2.5.  Winding and Web Handling

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FIGURE 2.5.3.  Roll defects due to soft start and/or improper roll hardness tapering.

a good solid foundation and then to wind with progressively less in-wound tension. This good solid foundation requires starting the winding operation on a high-quality, properly stored core. Most rolls of film materials are wound on paper cores. The paper cores must be of sufficient strength that they can withstand the inwound compressive tension caused by the film being wound tightly on the core. Typically, paper cores are kiln-dried to between 6% and 8% moisture. If these cores are stored in a high-moisture environment, they will absorb this moisture and swell to a larger diameter. Then, after the winding operation, these cores can dry to a lower moisture level and will shrink. When this happens, the solid wound roll’s foundation will be lost! This causes these rolls to have defects such as buckling, telescoping, and/or starring when they are handled or unwound. The next step in obtaining this required good winding foundation is to start winding with as much roll hardness as possible. Then, as the rolls of film material are wound, the roll hardness must be uniformly decreased. The suggested decrease in roll hardness at the finished diameter is generally between 25% and 50% of the starting hardness measured at the core. The amount of starting roll hardness and the amount of taper of in-wound tension is generally a function of the buildup ratio of the wound roll. The buildup ratio is the ratio of the core’s outside diameter (O.D.) to the wound roll’s finished diameter. The larger the roll’s final wound diameter

TABLE 2.5.1.  Working Rule for Hardness Taper. Working Rule for suggested amount of Wound Roll Hardness Taper • 25% on 3–5 to 1 Build-up ratio* • 33% on 6–8 to 1 Build-up ratio* • 50% on 9–12 to 1 Build-up ratio* *Build-up ratio is Wound Roll Dia/Core O.D.

(the taller the structure), the more important starting on a good solid foundation and winding a progressively softer roll becomes! Table 2.5.1 gives a working rule for the suggested amount of hardness taper based on the buildup ratio. Now we know why roll hardness is important. Next, let’s discuss how to achieve and measure roll hardness. HOW TO ACHIEVE ROLL HARDNESS Roll hardness is developed in different ways on different types of winders, but the basic principles for building roll hardness are always the same. To remember these principles, just remember that to consistently wind “dynamite rolls” you need TNT: • Tension: The winding web Tension • Nip: The Nip of the pressure roll or drum • Torque: The Torque from the center drive or Torque drum The following paragraphs describe how each of these tools is used to develop hardness and provide working rules for starting values to produce the required roll hardness for different flexible packaging materials. Web Tension Principle of Winding When winding elastic films, web tension is the dominant principle of winding used to control roll hardness. The more the film is stretched before winding, the harder the wound rolls will be. The challenge is to be sure that the amount of web tension does not induce significant permanent stresses in the film. When winding film on a pure center winder as shown in Figure 2.5.4, the web tension is produced by the winding torque from a center drive. The web tension is set for the desired roll hardness at the start and then tapered as the film winds. The web’s winding tension is produced from the cen-

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The most important tension consideration for laminating flexible film composites is that the individual webs must be tensioned before they are laminated so that the strain (the elongation of the web due to web tension) will be approximately equal for each web. If one web is strained significantly more than the others, curl problems or delamination wrinkling known as “tunneling” can occur in the laminated webs. The amount of tension should be a ratio of the modulus and the web thickness to prevent curl and/or tunneling after lamination. Nip Principle of Winding

FIGURE 2.5.4.  Tension principle on a center winder.

ter drive, which is typically kept stable by a closed-loop control system with feedback by tension-measuring transducers. The amount of starting and finishing web tension for a specific material often needs to be determined empirically. A good working rule for the range of web tension is between 10% and 25% of the film’s elastic limit. Many published papers suggest amounts of web tension to be used for specific web materials. Table 2.5.2 from Guidelines for Web Conveyance and Winding Tensions for Polymer Films, Papers, and Paperboard Web lists suggested tensions for many web materials used in processing flexible packaging materials. For winding on pure center winders, it is suggested that the starting tension be towards the higher value of the suggested tension range. Then the winding tension should be smoothly tapered towards the lower suggested range given in this table. When winding a laminated web of several different materials, to obtain the suggested maximum web tensions for laminated structures, simply add the maximum web tensions for each of the materials that have been laminated together (usually disregarding any coatings or adhesives) and apply the sum of these tensions as the maximum web tension for the laminate. TABLE 2.5.2.  Suggested Tensions for Web Materials. Material Polyester Polypropylene BOPP Polyethylene Polystyrene Vinyl (uncalendared) Aluminum Foil Cellophane Nylon Paper

Tension Range (Metric)

Tension Range (English)

35–105 N/cm/mm 14–35 N/cm/mm 21–70 N/cm/mm 7–21 N/cm/mm 35–70 N/cm/mm 3.5–14 N/cm/mm 35–105 N/cm/mm 35–70 N/cm/mm 7–21 N/cm/mm 24–40 N/cm/m m

0.5–1.5 lb/inch/mil 0.2–0.5 lb/inch/mil 0.3–1.0 lb/inch/mil 0.1–0.3 lb/inch/mil 0.5–1.0 lb/inch/mil 0.05–0.2 lb/inch/mil 0.5–.5 lb/inch/mil 0.5–1.0 lb/inch/mil 0.1–0.3 lb/inch/mil 0.35–0.55 lb/inch/mil

Conversion: kg/cm/mm = Newton/cm/mm × 0.1

When winding inelastic films, nip is the dominant winding principle used to control roll hardness (Figure 2.5.5). Web tension is controlled to optimize the slitting and spreading operations. The nip controls the roll hardness by removing the boundary layer of air following the web into the winding roll. The rolling nip also induces in-wound tension into the roll. The harder the nip, the harder the winding roll will be. The challenge for winding flexible packaging film is to have sufficient nip to remove the air and wind hard straight rolls without winding in so much in-wound tension that it will cause roll blocking or deform the web over the high-caliper area. The important considerations in applying the nip principle of winding are: • The nip must be applied where the web enters the winding roll. • The winding film’s weight and the lay-on roll’s weight, as well as web tension, should not affect the nip loading. • The nip load should be tapered as the roll winds to prevent “starring” and “telescoping”. • The larger the winding roll’s diameter, the more air is introduced to the nip. This produces a nip taper with a constant load. Torque Winding Principle Torque winding is the force induced through the center of the winding roll that is transmitted through the web layers and tightens the inner wraps of film. This torque is used to produce the web tension on center winders. With these types of winder, “tension” and “torque” are the same winding principle. However, when the pressure roll is driven to control the web tension, then the torque induced through the center of the roll can be independently controlled to develop the winding roll’s hardness profile. Gap Winding—Films that are relatively narrow and that can be wound at higher tensions and speeds generally less than 250 mpm (800 fpm) can be pure torque wound in the gap winding mode. Gap winding allows a small amount of air to be wound into the roll to prevent deformation of webs that have high-caliper-band areas. Roll density is controlled through torque, which is the web tension applied through the spindle drive. To successfully control roll hardness when

Section 2.5.  Winding and Web Handling

gap winding, the lay-on roll must move to maintain a small but consistent gap between it and the surface of the winding roll. This meters the air being wound into the roll. The small gap also helps direct the web squarely into the winder rather than trying to move it in at 90 degrees to a slightly coned roll surface. Effect of Film Coefficient of Friction Properties on Winding A film’s layer-to-layer coefficient of friction properties have a major effect on the ability to apply the T.N.T. principles to produce the desired roll hardness without roll defects. In general, films that have a layer-to-layer coefficient of friction (COF) value of 0.2–0.7 will wind well. However, consistently winding defect-free rolls with high-slip or lowslip (low-COF or high-COF) films usually presents major winding challenges. Low Coefficient of Friction Films High-slip films have low layer-to-layer COF (generally COF < 0.2). These films often have inner web slippage or cinching problems when they are winding and/or in subsequent unwinding operations, or have roll handling problems

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in between these operations. This inner web slippage can result in roll defects such as “web scratching”, “dishing”, “telescoping”, and/or “starring”. Low-COF films need to be wound as tightly as possible at the core, usually with high torque, and then winding torque must be tapered to a minimum at three or four times the core OD and desired roll hardness must be built using the nip winding principle. Air is never our friend when winding high-slip films! These films always need to be wound with uniform nip loading to prevent air from entering the roll during winding. High Coefficient of Friction Films Low-slip films have high layer-to-layer COF (generally COF > 0.7). These films will often have blocking and/or wrinkling problems. When surface-winding these high COF films, “out-of-round” rolls can be expected at low winding speeds, and roll bouncing can also be a problem when winding at higher speeds. As explained by well-known consultant and columnist Tim Walker of TJWA Inc., “. . . this is due to the fact that the outer layers of a winding roll require a small amount of sliding action as the layers first enter the winding roll. This sliding action produces in-wound tension as the air following the web is ejected from the nip or out the sides. If the full width slides, this is not a problem. But if one lane or spot sticks and the rest slide, then a local shear stress will develop near the sticking point. This local shear may form a small buckle or soft wrinkle in the top layer. In some products, a small bump or ripple can be wound over and ignored, but in other products (especially optically clear films), the next layer will not smoothly wind over a bump or ripple, but will instead conform over the bump, producing a slightly larger bump or ripple. As additional layers are added, like a rolling snowball, the defect will often get bigger with each turn.”

These defects are commonly called slip knots or slip wrinkles. High-COF films are best gap-wound with a minimum gap between the following roll and the winding roll. Spreading needs to be provided as close to the winding point as possible. A FlexSpreader covering on the following roll has proven to be successful on a number of high-COF winding applications. Film Winding Processes There are three basic winding processes used to wind film webs: center winding, surface winding, and combination center-surface winding. Each process uses one or more of the TNT winding principles to build roll hardness. Center winding

FIGURE 2.5.5.  (a) Tension—nip principles on a center-turret winder and (b) Nip principles on a drum surface winder.

A center winder could be a gap winder where only tension is used to control roll hardness. A center winder could also incorporate a lay-on or pressure roll. This winder uses both tension and nip to control roll hardness. With a center-

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FIGURE 2.5.6.  Center winder on a cast-film line.

winding process, the spindle torque through the center of the roll provides the web tension (Figures 2.5.6 and 2.5.7). An advantage of center winding is that this process can wind softer rolls. This type of turret winder can provide quick indexing and fast cycle times. The disadvantage of center winding of film is the limitation of maximum roll diameter due to the torque applied through the layers of film. Moreover, center winders have a higher probability of generating scrap during roll changes. Turret center winders are: • Best for winding soft rolls (i.e., films with gauge bands). • Best for winding film with high tack. • Best for winding small-diameter rolls. • Easily designed for dual-direction winding. • Able to provide adhesiveless transfers Surface Winding When surface-winding elastic films, web tension is the dominant winding principle. When surface-winding inelastic materials, nip is the dominant winding principle. Surfacetype film winders use a driven winding drum (Figure 2.5.8). The winding rolls are loaded against the drum and are surface-wound.

FIGURE 2.5.7.  Dual-turret center winder on a blown film.

FIGURE 2.5.8.  Drum winder on a cast film line.

The advantage of surface winding is that the web tension is not supplied from applying torque through the layers of film wrapped into the roll. The disadvantage of surface winding of film is that air cannot be wound into the roll to minimize gauge bands and roll blocking problems. Another disadvantage of drum surface winders is that the web tension is not controlled through the wound layers and therefore the winding tension is only known before the drum. A drumtype surface winder provides only single-direction winding (unless on a turntable). Tape or glue on cores is required for automatic transfers. Drum surface winders are: • Best for winding hard rolls (i.e., protective films) • Best utilization of space and horsepower • Best for winding very large-diameter rolls • Best for minimizing waste during transfers • Less expensive • Less equipment • Equipped with a single and smaller winding drive. Center-Surface Winding A center-surface winder uses both center-winding and surface-winding processes (Figure 2.5.9). Center-surface winding uses all three T.N.T. winding principles. The web tension is controlled by the surface drive connected to the lay-on or pressure roll to optimize the slitting and web spreading processes. The feedback from the web tension load cells trims this drive to control web tension. The web tension is held constant during winding. The lay-on roll loading applied to the winding roll controls the nip. Ideally, the web wraps the lay-on roll through 180°, with the resulting tension vector at 90° to the nip. This provides maximum tension isolation between web and winding tension and a configuration where the web tension does not affect the nip loading. The torque from the center drive is programmed to produce the desired in-wound tension for the roll hardness profile desired. The advantage of center-surface winding is that winding tension can be controlled independently of web tension. The disadvantage of center-surface winding is that the winding equipment is more expensive and more complex to operate.

Section 2.5.  Winding and Web Handling

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TABLE 2.5.3.  Material Characteristics vs. Suggested Winding Process. Suggested Winding Process 1. Center Winding Process with Lay-on Roller in Gap or Contact Mode 2. Surface Winding Process 3. Combination Center/Surface Winding Process Material Characteristics

FIGURE 2.5.9.  Tension-nip-torque principles on a center-surface turret winder.

Center-surface winders are: • Best for winding high-slip films to larger diameters • Best for slitting and winding extensible films to larger diameters • Best for their ability to significantly taper in-wound tension without affecting film width • Able to supply in-wound tension without stretching the web over caliper bands. Suggested Winding Processes for Material Characteristics Selecting the type of winding process for the materials to be produced on a film extrusion line is extremely important for consistently producing high-quality rolls of film. Table 2.5.3 lists a few guidelines for selecting the best winding process for different material characteristics and applications. These suggestions, along with an understanding of winding principles and how they are used on the various

Suggested Winding Process

Thin Webs Processed at High Speeds 1 Contact Mode Thin Non-Extensible Webs—Small Dia. 1 Gap or Contact Mode High-Coefficient “Sticky” Materials 1 Gap Mode Extensible “Stretchy” Webs—Small Dia. 1 Gap or Contact Mode Extensible “Stretchy” Webs—Large Dia. 2, 3 Thick Webs Wound to Large Diameters 2, 3 Inline Slitting of Multiple Webs 2, 3 Thin Non-Extensible Webs—Large Dia. 3 Low-Coefficient “Slippery” Materials 3

classes of winders, will hopefully be helpful in selecting the best type of winder to produce consistently high-quality, defect-free rolls of film on your extrusion line. Measuring Roll Hardness Winding of film is often referred to as an “art”. This is because the setting and programming of the Tension, Nip, and Torque will vary depending on: • The type and design of the winder • The type of web material being wound • The width of the rolls being wound • The speed of the winding operation. Not only will the same settings provide different roll hardnesses with the variations above, but also different film products and different end-use applications will vary the roll

FIGURE 2.5.10.  Hardness devices for measuring roll hardness across roll.

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FIGURE 2.5.11.  Hardness devices for measuring roll hardness from core to full roll.

hardness and profile desired. However, after the hardness profile has been established, this hardness must be reproducible on a consistent basis. To ensure that wound rolls of film are produced with consistent roll hardness, hardness measuring devices must be available to the winder operators. With these devices, an operator can check roll hardness and make adjustments accordingly to ensure that the roll hardness is within the acceptable range for that product. To measure the roll hardness across the outer surface of the roll, it is suggested that a Rhometer, a Schmidt hammer, or a PAROtester be used. These are impact-based devices for measuring “relative roll hardness” on a relative scale. The Schmidt hammer was developed for concrete hardness testing and has been borrowed for use on roll hardness testing. The PAROtester is a similar device that was developed specifically to evaluate the hardness of paper, foil, and film rolls. The PAROtester is considerably more sensitive, imparts less impact energy, and is less operator-dependent due to its having a more defined direction of impact than the Schmidt hammer. A new device called a TAPIO RQP (roll quality profiler) is similar to the PAROtester and has the added feature of automatically sampling at fixed distances across the roll as it moves along the surface. This device was specifically developed for flat paper grades, but also works well on many grades of film. A Smith meter is an instrument that can be used to measure the hardness profile from the core to the outer wraps of the roll. The Smith meter measures the penetration of a small needle as it is inserted between the wraps of the web along the roll sides. With computerized data acquisition systems, it is now possible to calculate the roll density factor (RDF) and plot the relative roll density from core to full roll as the roll winds. These systems compare the actual winding roll diameter with the theoretical diameter and plot the ratio as a function of the winding roll diameter. The RDF curve is displayed to the operator on the operator interface terminal (OIT) at the winder console.

WINDER COMPONENTS The first part of this chapter has concentrated on the winding operation and on roll defects due to winding. The discussion will now turn to other components associated with the winder that are important to the overall success of the total winding operation. These components include the: • Slitting operation • Spreading operation • Winder drives and tension control system • Proper shaft selection • Automatic roll changing system. Slitting Operation Slitting is done on practically all film extrusion lines. It is first done after the forming section to separate the collapsed bubble of blown film so that it can be wound as two separate sheets on blown-film lines. On cast-film lines, the edge is trimmed to remove the edge bead or any non-uniform material on the edge of the formed and possibly stretched or “tentered” film. On many film winders, the plastic film is again slit to obtain a smooth “book edge” side of the wound roll in the extrusion operation, and in many cases, the sheet is slit and wound into multiple rolls of varying widths. The challenge in web slitting is to select the best type of slitting for the operation to consistently obtain clean cuts while optimizing ease and safety. The three most common types of slitting used in the film extrusion industry are razor slitting, score slitting, and shear slitting. Razor slitting is predominately used for slitting lightgauge films. These are typically films that are less than 40 microns or 0.0015 inches in thickness and that are formed and wound at speeds less than 300 mpm or 1000 fpm. Razor slitting is a cutting or slicing action that is performed as the web is pulled through a stationary razor blade (Figure 2.5.12). The blade may be oscillated to vary the cut point,

Section 2.5.  Winding and Web Handling

FIGURE 2.5.12.  Razor slitting.

which greatly extends the life of the razor blade. Razor slitting is the simplest and least expensive method of web slitting. It can be easily adapted to fit into almost any location. When slitting very light gauges or at higher line speeds, the film should be supported at the cut point. Score slitting is a crushing action in a nip between a slitter blade and a hardened anvil roller. The best application for score slitting is predominantly harder, low-elongation films such as polyester, which do not extrude out from under the blade nip (Figure 2.5.13). When score-slitting softer, lowelongation films such as polyethylene, the crushing action can form a ridge or “raised edge” along the slit lane.

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Score slitters are easy to adjust. They do not have sharp edges and therefore can be handled more safely than razor blades. However, the slit edges are normally of poorer quality due to the scoring and ripping action of the slitting, and thick and/or dense films require high nip loads, which greatly affect the lifetime of the knife. Moreover, the ability of the blades to absorb high cycle stresses limits the speed of a score-slitting operation. This speed limit is typically 300 mpm (1000 fpm) or less. The profile of the score knife’s cutting edge is extremely important for good cutting action combined with long blade life. The blade profile for plastic films less than 25 microns or 0.001″ in thickness is a knife tip ground to a 0.2 mm (0.008″) radius and with a 30-degree included blade angle. The blade profile for plastic films greater than 25 microns or 0.001″ in thickness is a knife tip ground to a 0.2 mm (0.008″) radius and with a 45-degree included blade angle. Dense and brittle films such as cellophane or acetate should have a blade-tip profile of 0.1 mm (0.004″) radius and with a 30-degree included blade angle. Finally, the score-knife mounting geometry is critical to the success of the slitting operation, as shown in Figure 2.5.8. Shear slitting is the most versatile method of slitting and is used for films formed at high speed and/ or when slitting very difficult-to-cut materials and when edge quality is important. Shear slitting produces a true shearing action between the driven bottom knife blade and the rotating top shear knife. When properly configured, shear slitting produces the highest-quality slit edges of the three types of slitting methods described here. However, it also has the highest initial cost and is the most difficult to set up and to adjust. The life of the top and bottom blades is a function of the bottom knife’s overspeed, the top knife’s profile, the

FIGURE 2.5.13.  Score slitting.

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FIGURE 2.5.14.  Wrap shear slitting.

FIGURE 2.5.15.  Tangential shear slitting.

shear angle, and depth of penetration and side pressure of the bottom knife. The top blade is less hard and therefore is the “sacrificial” member of the pair. Shear slitting is done in two configurations, wrap shear and tangential shear (Figures 2.5.14 and 2.5.15). Generally, wrap shear is used for thin and easy-to-cut films and tangential for heavier and more difficult-to-cut films that do not need sheet support and require significant overspeed for a clean cut.

and the dual-bowed roll spdereading configuration. Another type of spreader not covered in this text is a spreader roll known as a “rubber band” spreader. These are rolls with a number of expanding elements (rubber bands) that make up the outer surface of the roll. These expanding elements are connected to angled end plates that stretch and contract the bands on each revolution. This type of spreading device is common in the wide, low-speed woven industry and has been used on slower blown-film lines. The reverse crown spreader is a conventional roll, normally an idler but may be driven, that has a diameter at the ends slightly larger than the diameter in the center of the roll (Figure 2.5.16). Because the roll has a constant RPM, the surface speed is greater at the ends than the center. The surface speed difference causes an ingoing web tension distribution that is shaped similarly to the speed profile. The roll’s spreading action is a function of its crown and deflection.

THE SPREADING OPERATION Bringing a flat, wrinkle-free web to the winder is extremely important in ensuring that the winding operation can produce quality rolls of film. The web spreader’s operation is to prevent wrinkles during web processing and to separate the slit webs during inline slitting without needing to take bleed trim. Most or all film extrusion lines have spreading devices. Unfortunately, many of these spreaders are misapplied or not properly adjusted. The challenge in web spreading is to install the best spreader for the application and to keep the spreader properly adjusted. The most common spreading systems used to spread plastic films are the reverse crown spreader roll, the FlexSpreader roll covering, the single-bowed spreader roll,

Application: • Wrinkle removal • Most effective on extensible materials • Web wrap should be greater than 90º • Poor man’s reverse crown roll is tape on ends of roll at edges of web.

Section 2.5.  Winding and Web Handling

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FIGURE 2.5.16.  Reverse crown spreader.

Pros: • Simplest to operate and adjust • Least expensive type of web spreader • Easily retrofitted

Cons: • Very product dependent because the amount of reverse crown depends on web extensibility • No control on amount of spread flexibility

The FlexSpreader is a straight roller that has a special grooving cut into a soft outer cover. The grooving is undercut at an angle so that the web tension deflects the lands outward, carrying the web with it to accomplish the spreading action (Figure 2.5.17). The amount of spreading is a function of rubber hardness and web tension. Application: • Wrinkle removal • Prewind web spreading with no slitting • Preslit web spreading with slitting • Most effective on non-extensible materials • Web wrap should be greater than 90º.

Pros: • Relatively low cost, easily retrofitted, easily operated • Self-compensates for tight or loose areas across web, giving more uniform crossmachine web tension • Grooving eliminates slippage due to air entrapment

Cons: • Needs web tension to provide spreading action • Limited control of spread flexibility

FIGURE 2.5.17.  FlexSpreader.

FIGURE 2.5.18.  Single-bowed roll spreader.

The bowed-roll spreader is a curved roll with a stationary axle on which segmented metal rotating sleeves are mounted on numerous bearing sets (Figure 2.5.18). The metal sleeves are typically covered with a flexible piece of tube made of a soft, synthetic rubber. Under high wear conditions, the outer rubber tube can be eliminated, and the metal sleeves are traction-coated. Variable-bow rolls have a split stationary axle where an applied force can change the amount of bow. Application: • Slit web separation • Wrinkle removal • Can be used on both extensible and non-extensible materials • Cover or steel sleeve grooving reduces slippage from air entrainment • Lead-in and lead-out geometry are critical for effective spreading • Spreading action is a function of the amount of wrap, bow, and web tension.

Pros: • Can be used on processes having a wide range of material types • Can be adjusted to tighten center or ends of web • Grooving eliminates slippage due to air entrapment • Readout of amount of bow available as well as automatic bow adjustment from recipe.

Cons: • More complex and costly than other spreaders • Needs to be driven on light tension applications • Bow position and amount of bow on variable-bow rollers may be misadjusted by operators • Bearings cannot be relubricated and may be difficult to replace • Web slitting applications usually limited to four rolls or less.

The dual-bowed roll spreading system is made up of two bowed rolls that can be fixed or variable so that the lead-in and lead-out webs are parallel with the bows pointing 90

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FIGURE 2.5.19.  Single-bowed spreader roll geometry.

degrees to the lead-in and lead-out web paths as shown (Figure 2.5.20). The spreading action takes place between the two bowed rolls, with no spreading effect upstream or downstream. Additional spreading flexibility can be provided by a rotatable table that varies the amount of wrap. Application: • Slit web separation for slitting and spreading multiple rolls (usually five or more) • Can be used for a wide range of material and slitting variations • Roll may be fixed- or variable-bow. • Parallel lead-in and lead-out web leads and sufficient distance between rollers for web separation are critical. • First and second rolls will most likely have different amounts of bow due to deflection from tension vector. • Spreading action is a function of amount of wrap, bow, and web tension.

FIGURE 2.5.21.  Dual-bowed spreader roll geometry.

Pros: • Web path length the same at center and ends • No upstream or downstream spreading action • Excellent control of spreading action • Can be adjusted to tighten center or edges of web by slight bow direction changes.

Cons: • More complex and costly than other spreaders • Needs to be driven on light tension applications • Bearings difficult to lubricate and replace and may be difficult to thread • Bow position and amount of bow of variable rollers can be misadjusted by operators.

WINDER DRIVES AND TENSION CONTROL SYSTEMS

FIGURE 2.5.20.  Dual-bowed roll spreaders.

The ability of the winder to properly control slitting, spreading, and winding web tension depends absolutely on the winder drive or drives and the tension control system. The electric motors and drive-control systems must provide precise speed and torque control throughout the buildup of the winding roll. This control must be programmable to consistently reproduce the speed and tension profiles for all the various products and conditions that the film winder may encounter. A digital drive is a must for any winding operation. Winder drives, especially on center winders that can wind large-diameter rolls, need to have wide torque and speed ranges. Winder drive motors must have sufficient cooling and good filter maintenance for high torque requirements at low motor RPMs when winding at these larger diameters.

Section 2.5.  Winding and Web Handling

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AC vector drives should be supplied on all new winder drive installations. These digital drive systems combined with inverter-duty AC motors provide constant torque over a very wide speed range. Typically, these have a 1000 to 1 speed range versus 100 to 1 for conventional DC drive systems. They are also very efficient to operate because AC vector systems have a much higher power factor and therefore are much more efficient then DC systems when operating at speeds below rated motor speed. The maintenance on AC motors is less than on DC motors because AC motors are brushless and do not require cooling air to be blown through the motor. For constant high-torque applications, especially in warm environments, “air over” motors are suggested, where cooling air is blown over the outside of the motor. Finally, the cost of an AC vector system is now lower than a conventional DC system due to its lower motor cost. This is especially true for the lower-horsepower motors typically used on film winders. This section has already spent considerable time discussing the importance of web tension in winding. Closed-loop tension control is a must for almost all film winders. The winder drive supplies the torque as a function of the winding roll’s or surface drum’s diameter and the tension and taper setpoints. The closed-loop system then measures the actual web tension and trims the motor torque to provide the exact web tension desired. Two tension-sensing systems are commonly used on film winders: dancer control systems and transducer-controlled systems. A dancer tension-control system typically consists of an idler roll with approximately 180 degrees of web wrap supported on a set of pivoting arms that are positioned by one or more air cylinders (Figure 2.5.22). The web tension is a function of the air pressure to the cylinder(s) and the web wrap. A potentiometer indicates the position of the arms and

trims the winder drive to keep the dancer in the proper position. A dancer system does not provide a direct readout of web tension. The web tension can only be calculated using wrap angles, cylinder pressure and area, moment arms, and friction factors. Moreover, dancers, by their inherent design, are slow to respond to tension changes. However, dancers must be used for their “cushioning” properties on winders that have significant web-length changes during the rollchange cycle or on continuous-film web forming lines where the winder must run overspeed for web takeup during the threading process. If a winder dancer is required, it must be properly designed. A dancer system must have:

FIGURE 2.5.22.  “Z” bar dancer.

FIGURE 2.5.23.  Transducer system.

• A center-position control system • Properly sized, low-friction cylinder loading • Roll’s tare weight directed through the arm pivots • Low hysteresis • Inertia compensation for speeds over 300 mpm (1000 fpm). A transducer tension-control system typically consists of a precision-balanced idler roll, which is usually referred to as a tension roll, supported by two electronic load-sensing devices called the load (Figure 2.5.23). A transducer system provides a direct readout of web tension. The output of the load cells is the actual web tension as a function of the web wrap minus the tare weight of the idler roll. For the best control, the web wrap should be as great as possible and the tension roll should be as light as possible, or the roll’s weight vector should be directed through the pivot of the load cell. The winder drive uses this web-tension feedback signal to trim the torque or speed to the winding roll or drum to provide the proper web tension.

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The advantages of using a transducer tension-control system on a film winder are: • Direct tension readout • Faster feedback to the winder drive • No change in web length • Better accuracy and consistency • Easy to thread due to the single roll • Requires less space than dancer systems • Can be located close to the winding roll. When winding highly extensible webs, a surface or center-surface winder with very precise draw control works best. For these webs, tension is obtained by setting the speed or draw between the last driven section and the winder. A closed-loop system is suggested to monitor and control the web tension. However, the correction from the feedback loop needs to be very slow and/or small to avoid getting into a hunting situation. Extensible webs sometimes act like an accordion. The feedback device senses the need for a correction and sends the signal to the winder drive. Because of the stretchiness of the web, the correction is not seen by the feedback device until the winder drive has overcorrected. Then the drive is told to go the other way and again goes too far to correct the overcorrection. This is why stretchy webs are often run under hard speed control. However, a web-tension sensing transducer must be located before the winder. This device will assist the operator to set the proper draw between the last driven section and the winder for the proper winding tension. Often this will be a negative draw to relax previously imposed stresses from upstream processes before slitting and winding. PROPER SHAFT SELECTION Almost all film winders use shafts for the winding operation. The winding shaft supports the roll weight and transmits the torque used in center winding. Most if not all winding shafts are pneumatically actuated, which involves expanding buttons, leaves, lugs, or strips to lock the cores in place, which keeps the winding rolls from shifting sideways and transmits the winding torque. Nominal 75 mm (3″) and 150 mm (6″) shafts are standard sizes for film winding on cores with these inside diameters. For each core size used on the film winder, the type of shaft best suited for the application should be selected. The selection criteria are:

• Steel shafts have: high load capacity, high weight, medium cost. • Aluminum shafts have: lower load capacity, lower weight, medium cost. • Ultra-high-modulus carbon-fiber shafts have: high load capacity, lowest weight, highest cost. Today’s ultra-high-modulus shafts have the same strength as steel and are almost five (5) times stiffer then steel or aluminum shafts, which gives them much higher critical speed and significantly longer wear life than aluminum shafts with expanding buttons or lugs. In recent years, the cost of carbon-fiber shafts has come down dramatically, which has made them the material of choice for many film winder shafts. During inline slitting, loaded shaft deflection must be taken into account when winding larger-diameter rolls of film. Shaft deflection will cause the winding rolls to dish as they wind, which will cause coning, crushing and possibly interweaving when the shaft deflection exceeds the spreading of the slit rolls (Figure 2.5.24). If the maximum amount of roll dish is 1.5 mm (0.06″) at the maximum diameter of the slit roll, then the minimum amount of separation of the slit widths must be 3 mm (0.12″) or greater. The load capacity of the shaft must also be checked to ensure that its loaded deflection does not exceed 1.5 mm (0.06″) at full load. The equation for the maximum loaded deflection is: Allowable Deflection = 1.5 mm (0.06″) × L/D where: D = maximum OD of roll to be slit L = shaft length. Contact your shaft supplier to obtain the loaded shaft deflection for the overall weight of the rolls being slit and wound to a particular diameter.

• Shaft load capacity • Shaft torque capacity • Shaft speed capacity • Shaft weight • Shaft tolerance to poor-quality cores • Shaft cost. These criteria determine the best type of expanding element as well as the best type of body construction and material. Today, the common shaft-body materials are steel, aluminum, and carbon fiber.

FIGURE 2.5.24.  Shaft deflection due to roll weight.

Section 2.5.  Winding and Web Handling

FIGURE 2.5.25.  Bump and cut transfer system.

AUTOMATIC ROLL CHANGING SYSTEMS At the beginning of this chapter, it was stated that, “this material, now called a film, finally ends up at something called a winder. There it is often cut up into narrow strips and forced to be transferred onto this round thing called ‘a core’ and made into this somewhat round package that we affectionately call a ‘roll of film’”. At the end of this chapter on “Winding”, it seems appropriate to end with a discussion on automatic roll changing systems. Film winders use various roll changing systems. These vary from the simplest “bump and cut” (Figure 2.5.25) to much more complex and expensive systems for 100% roll-changing efficiency with transfers straight across the cores regardless of the width and speed of the winding operation (Figure 2.5.28). The selection criteria for a roll-changing system include: • Speed of the winding operation • Width of the winder • Ease of cutting the film • Single- or dual-direction requirements • Core size requirements • Inline slitting requirements • Foldback requirements at the core

FIGURE 2.5.26.  Core enveloper transfer system.

77

FIGURE 2.5.27.  Static transfer system.

• Ability to use adhesive on the cores • Scrap generation at the core • Scrap generation at full roll (transfer cycle time). The following provides additional information on the roll change systems used on film winders (Figures 2.5.25, 2.5.26, 2.5.27 and 2.5.28). For a more in-depth analysis, please consult your preferred winder manufacturer. • Tape on core • Speed-dependent • Large foldback • 300 mpm (1000 fpm) maximum reliable operation • Poor core starts • Lowest cost. • No core tape required • Fold at core start • 400 mpm (1300 fpm) maximum reliable operation • Lengthy transfer cycle times • Large web length changes • Works on films to papers. • No tape required on the core • Slight foldback (one core OD or less) • 500 mpm (1600 fpm) maximum reliable operation • Little top-roll waste with fast turret index • Best for 3 mil or thinner films.

FIGURE 2.5.28.  Stationary knife transfer system.

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CHAPTER 2—PRIMARY EQUIPMENT

CONCLUSIONS Winding good rolls of plastic film is a challenge that every operator faces. Consistently winding good rolls depends on the consistency of bringing good film to the winding operation. A winder operator’s job is not to camouflage poorquality film products into shippable rolls. His or her responsibility is to handle films with slight imperfections and to produce quality rolls that will run without problems on the customer’s process and produce high-quality products for their customers. I hope that the information presented here will help in meeting that challenge. REFERENCES AND ADDITIONAL RESOURCES [1] Smith, R.D. The Challenges of Winding Flexible Packaging Films. Plastic Technology, September 2015, pp. 66–69 [2] Roisum, D.R. Diagnosing Telescoping. 2014 AIMCAL International Coating & Web Handling Conference, June, 8–11, Cascais, Portugal [3] Good, J.K. and Roisum, D.R. Winding Machines, Mechanics and Measurements. Destech Publications and TAPPI Press, 2007. [4] Smith, R.D. Guidelines for Selecting the Best Winding Process. AIMCAL Converting Quarterly, Q2 2013, pp. 77–79. Given at the 2013 TAPPI European PLACE Conference, May 6-8, Dresden, Germany. [5] Smith, R.D., Guidelines for Web Conveyance and Winding Tensions for Polymer Films, Papers, and Paperboard Webs, AIMCAL Converting Quarterly, Q3 2011, pp. 58–62.

[6] Walker, T.W, The Coefficient of Winding Trouble, https:// www.pffc-online.com/web-lines/7116-paper-coefficientwinding-trouble-0509. [7] Smith, R.D., “Developing Roll Hardness on Center and Center-Surface Winders”, AIMCAL Converting Quarterly, Q3, 2014, pp. 68-71. Given at the 2015 TAPPI European PLACE Conference, May 11–13, 2015, Nice, France. [8] Smith,R.D., Developing Roll Hardness on Centre and Centre/Surface Winders. Converter Magazine—United Kingdom, April 2015, pp 30–31. [9] Smith, R.D. The Ultimate Roll and Web Defect Troubleshooting Guide. TAPPI Press, 2014. [10] Schable, R., Four Factors to Consider in Razor Slitting, Converting Magazine, March 2000, pp 68–70 [11] Schable, R., Five Factors to Consider in Choosing Score Slitting, Converting Magazine, November 1999, pp 58–61. [12] Rumson, D.A. The Evolution of Web Slitting and Automatic Positioning Systems, Converting Quarterly, Q3 July-September 2017, pp 32–38 [13] Smith, R.D. The Challenges of Handling Thinner, Wider Webs at Faster Speeds. Converting Quarterly, Q3 July-September 2016, pp36-41. Given at the 2017 AIMCAL R2R conference, October 28-31, Phoenix, AZ. [14] Klassen, C.J., Drive Response Requirements for Web Handling. Given at the 2012 AIMCAL Web Handling Conference, Prague, CZ / Myrtle Beach, SC. [15] Klassen, C.J., Tension Control in a Turret Winder. 2018 AIMCAL Roll 2 Roll Conf. Phoenix, AZ. [16] Koppes, E. and Craig, S. Increasing Web Throughput with Core and Roll Centering Shafts. AIMCAL’s Converting Quarterly, Q2, April-June 2017, pp62-63. Given at the 2017 AIMCAL R2R Conference, October 15–18, Wesley Chapel, Florida.

Chapter 3 Primary Equipment

EDITOR: RORY WOLF, ITW Pillar Technologies

Section Number

Section Title

Page Number

3.1

Gear Pumps, Filtration, Static Mixers—Function, Design, Parameters, Examples KEVIN TUTTLE, Nordson Polymer Processing Systems



3.2

Feedblock Technology CHRISTINE RONAGHEN, Cloeren Incorporated

97



3.3

Film Stabilization, Forming and Collapsing Systems JAMES STOBIE and HARINDER TAMBER, Macro Engineering and Technology Inc.

105



3.4

Material Handling Systems CLIFFORD J. WEINPEL and WALTER FOLKL, Foremost Machine Builders

115



3.5

Instrumentation and Process Control TED SCHNACKERTZ, NDC Technologies

139



3.6

Blown-Film Cooling Systems HARINDER TAMBER, Macro Engineering and Technology Inc.

145



3.7

Surface Treatment RORY WOLF, ITW Pillar Technologies

157

3.8

Atmospheric-Pressure Plasmas RORY WOLF, ITW Pillar Technologies

173

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Chapter 3—Section 1

Gear Pumps, Filtration, Static Mixers— Function, Design, Parameters, Examples KEVIN TUTTLE, Nordson Polymer Processing Systems

GEAR PUMPS

where,

Introduction

Q = volumetric output L = face width OD = outside diameter PD = pitch diameter Ql = back leakage

Gear pumps, Figure 3.1.1, for extrusion were first introduced in the 1970s as a variant of early fiber spinning pump designs. The extrusion gear pump, also commonly referred to as a melt pump, is a positive displacement device that benefits the extrusion process by performing the following basic functions: (1) Accurately meters polymer to the die (2) Builds pressure efficiently for downstream components and die (3) Provides greater flexibility to run higher levels of additives and regrind. Principle of Operation The gear pump at first glance appears to be quite simple. Excluding fasteners, most designs are made up of a housing containing a driver and a driven gear, which is supported by four bearings with two seals for the drive gear shaft (see Figure 3.1.2). The two counter-rotating gears discharge polymer from the precision machined gear pockets. Polymer is supplied to the upstream entrance bore of the pump under enough pressure to fill the gear tooth pockets as they rotate. When these pockets are filled on the inlet side of the pump, they carry polymer to the discharge side of the pump. The gear teeth then mesh precisely, squeezing all the polymer out of the gear pocket while forming a continuous “rotating seal” that isolates the inlet and outlet pressures. The gears are machined to an “involute profile”, which enables them to be in continuous contact along the pitch line of the gears during rotation (Figure 3.1.3). The approximate relation for the volumetric output of a gear pump is as follows: Q

 (OD 2  PD 2 ) L  Ql 2

(3.1.1)

The gear pump delivers the volumetric displacement defined by the tooth geometry according to the previous equation provided that: (1) The teeth are completely filled by the polymer, (2) The gears rotate at a uniform speed, and (3) There is no backflow leakage. The volumetric efficiency, defined as theoretical versus actual output, is usually restricted by the pump efficiency, which is item 3 above. Leakage in the gear pump occurs as follows: • Polymer flow over gear tooth lands • Polymer past sides of gear teeth • Polymer flow past meshing gear teeth • Polymer allowed to flow through bearings for lubrication. Gear pumps have three primary finite mechanical clearances that affect efficiency, as shown in Figure 3.1.4: the gear tip to housing clearance, the gear axial side clearance to bearing face, and the gear shaft OD to bearing ID. Depending on the type of pump design and the application served, these clearances may vary to achieve optimal performance for a given application. For example, a more viscous polymer will normally have wider clearance, but less leakage for a given clearance. A low-viscosity polymer may require a tighter clearance to reduce leakage and improve efficiency. Polymer type, viscosity, differential pressure, gear speed, filler types and filler levels in polymers, and pump metallurgical properties will influence how design clearances are established. 81

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CHAPTER 3—ANCILLARY EQUIPMENT

FIGURE 3.1.1.  Gear pumps.

Pump Gears & Pressure Rating Extrusion film applications involve a wide range of polymer viscosities and pressures. Although there are a number of configurable design options for pumps, the two models commonly used for these applications are defined by maximum operating pressure. Gear designs for both models are made integral to the shaft. Helical gears are typical for standard extrusion pumps operating in pressure ranges up to 5,000 psi. The gear OD and width are equivalent, resulting in a “square” gear. Differential pressures for most standardpressure pumps are mechanically designed to 4,000 psi, but this value may be reduced depending on actual application factors. Higher-viscosity and higher-pressure applications are typically not suitable for standard pumps. As a rule, pressure applications greater than 5,000 psi use spur gears. Unlike standard-pressure pumps, high-pressure pump gears have a narrower gear width than the OD. This non-square gear design can bear higher tooth forces while maintaining the same bearing width as the standard-pressure pump, which

FIGURE 3.1.2.  Gear pump.

results in a higher bearing load rating. Typical high-pressure pumps have operating pressures up to 10,000 psi with 7,500 psi differential rating. Journal Bearings Proper journal bearing design is vital to the function of the gear pump. Bearings are essentially static bushings that support both the driver and driven gear shafts. Journal bearings rely on proper lubrication to support the load and the rotational forces of the gear shaft. The basic journal bearing concept is shown in Figure 3.1.5. A lubrication channel in the bearing allows polymer to flow to lubricate the bearing ID and gear shaft as it rotates. Differential pressure and the meshing of gears allow the polymer to flow through the bearing. The polymer is then recirculated to the low-pressure side of the pump and enters the inlet melt channel.

FIGURE 3.1.3.  Gear profile (compliments of Shigley’s Mechanical Engineering Design, 8th Edition).

Section 3.1  Gear Pumps, Filtration, Static Mixers—Function, Design, Parameters, Examples

83

FIGURE 3.1.5.  Journal bearing & lubrication.

FIGURE 3.1.4.  Pump Clearance. CD: Clearance of gear outer diameter to housing internal diameter, CS: Clearance of gear axial side to side wall of housing and CB: Clearance between bearing internal diameter and gear shaft outer diameter.

The Pump System To function, gear pumps require the following:

Power Consumption Understanding the total power consumption of a gear pump is necessary to size the motor, drive, and mechanical system. The total power consumption of a pump is the sum of the hydraulic and viscous power: Total Power  Hydraulic HP  Viscous HP

(3.1.2)

Hydraulic horsepower is the power needed to increase volumetric rate by the required pressure differential: Hydraulic HP  (P)(Volumetric Flow Rate Q)

(3.1.3)

(1) A mechanical drive system consisting of a gear motor (or a motor and a gear reducer), a universal drive shaft, and appropriate guarding, all mounted on a base plate or stand assembly (see Figure 3.1.6). (2) A controller with operator interface. If an OEM is supplying the gear pump with a new extrusion line, the OEM will typically integrate the gear pump control into their line control. If a gear pump is being retrofitted to an existing extrusion line that does not have a gear pump, a pump controller will be required from the pump manufacturer. (3) Upstream and downstream adapters to connect the pump to adjoining equipment in the extrusion line.

Viscous horsepower is the power required to overcome viscous drag: Viscous HP =

(Viscosity)(Shear Area )(Relative Velocity) Gap (3.1.4)

Hydraulic efficiency is the ratio of power generated to power consumed: Hydraulic Efficiency =

Hydraulic HP Generated (3.1.5) Total HP Consumed

Therefore, hydraulic efficiency is a function of the torque requirements to drive the gear in the journal bearing, the viscous drag of the material, and the required differential pressure: Hydraulic Efficiency 

(P )(Q) Hydraulic HP  Viscous HP (3.1.6)

Once a given pump has been selected for an application and the total power and torque have been calculated, the pump drive system can be sized.

FIGURE 3.1.6.  Gear pump system components.

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Controls & Gear Pump Operation The gear pump control system should meet several basic criteria. In general terms, the system controller should be able to monitor and control the extrusion process with absolute accuracy and safety. It must be equipped with sufficient safety interlocks and control capability to synchronize the extruder and gear pump during startup and operation. It should provide (optional or otherwise) manual control over the gear pump and heaters for servicing the system. Minimal control features should include: • PID pressure loop controller based on pump inlet pressure • Discharge pressure indication • Safety interlocks for high and low pump inlet pressure and high discharge pressure • Amp overload protection for the gear pump drive • Speed and pressure readouts • Reset switch integrated with the pump speed control that will not allow the pump circuit to energize at an RPM greater than zero. • A temperature controller • Cold start protection.

A gear pump in the extrusion process will operate at a continuous fixed speed based on the line production rate. Operating at a fixed speed, the pump will isolate upstream pressure variations and accurately meter polymer to the die. Figure 3.1.7 displays an example of a 33 cm3/rev gear pump operating at 15 rpm, filtering upstream extruder pressure variations and delivering stable downstream pressure to the die. Summary In summary, gear pumps provide consistent, uniform flow rate and uniform pressure stability to the die. By operating at a constant speed, pumps deliver a consistent volume of polymer downstream and filter pressure and flow variations from the extruder. This provides more operating flexibility to the line by enabling the use of higher percentages of regrind and additives that may otherwise present processing challenges. FILTRATION Introduction Filtration systems, Figure 3.1.8, are a common auxiliary component required in the plastic melt stream to eliminate

FIGURE 3.1.7.  Diagram of pump operation.

Section 3.1  Gear Pumps, Filtration, Static Mixers—Function, Design, Parameters, Examples

85

FIGURE 3.1.8.  Filtration system.

and/or reduce contamination. Because this is a requirement for almost every plastic application, many filtration alternatives are commercially available. Plastic film processors are faced with the challenge of determining which design will deliver the right performance to meet product specifications while balancing capital investment, operating cost, operability, and productivity objectives. To make the right choice for a given application, it is first important to understand some basic principles about filtration, the application factors, and the various types of alternatives available. Why Filtration? Contamination in the melt stream is the single largest driving need for filtration. If it were possible to purchase a perfectly clean resin and additives, transport them from the railcar or container through a conveying system to an extruder hopper without introducing any other unwanted materials, and if the extruder was then able to plasticize the polymer compounds, then there would be no need for filtration. However, this is seldom the case. Even with the most stringent prevention practices, contaminants find their way into the melt stream. Reclaiming post-industrial plastic scrap or using postconsumer plastics are common practices within the film industry. Sustainable marketing initiatives are challenging film processing companies to expand their use of used plastics to prevent their entry into landfills. This is not only good environmental stewardship, but it also presents opportunities for cost reduction to processors. At the same time, new plastic compounds and new developments in film end-markets continue to drive the creation of thinner films, which require even finer filtering of the melt. These trends continue to pressure filtration manufacturers to expand their equipment capabilities with new innovative solutions. Today, more than ever, processors have many alternative filtration technolo-

gies to consider, with varying capabilities and price points. To make the right selection for any particular process, it is important first to understand the basic application factors that filtration manufacturers require to properly evaluate filtration sizing and selection. Whatever the driver, contamination must be removed from the melt stream. Selection & Sizing Criteria Although there are many filtration alternatives to consider, selecting the right filtration device is critical to achieving both process and business objectives. A number of application factors must be carefully analyzed for optimal performance, and filtration manufacturers can assist processors in making the right selection. Failure to take the necessary time to collect these data points for the filtration manufacturer can affect the performance outcome. Following is a list of critical application factors that must be considered in the filtration selection process: • Contaminant Particle Size: What particle size is acceptable in the end product, and what size is not? The first step is to define the minimum contaminant particle size that will need to be filtered and removed from the melt stream. Once the minimum particle size is determined, the appropriate screen media can then be selected. • Contamination Level: The amount of contamination in the process is sometimes challenging to define but is critical in the selection of the right screen changer. Virgin polymer, for example, may contain few or no contaminants. Post-consumer recycled materials can have varying degrees of contamination, as high as 5%–15% by weight depending on the types of contaminants and how well they are cleaned before they enter the melt stream. Knowing the amount of contamination in the

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melt stream will help to quickly narrow down the screen changer options that are available for the application. • Selection of Filter Media: The minimum contaminant particle size will define the filter media rating to be used. This media rating is best defined as a micron rating. Once the micron rating is defined, the next step is to define the screen pack assembly. Because there is more to consider about filtration media, this topic will be discussed later in more detail. • Type of Plastic: The plastic consists of the polymer, and/or polymer blends, and any additives that will be processed. Is the polymer thermally stable or easily degradable? Are additives included that are abrasive or corrosive? These questions must be answered to ensure that the right metallurgical and rheological designs are considered. • Polymer Rheology: Polymer rheology is used to analyze velocity and pressure profiles within the filter channels, screen packs, and breaker plate. Polymer curves generated from a capillary rheometer typically consist of a list of shear rates and corresponding viscosity points. Filtration manufacturers can typically generate rheology data from polymer samples if required. • Throughput Rate: Define the nominal, maximum, and minimal flow rate (lbs/h) that is intended for the extrusion process. • Maximum Pressure Limitations: Filtration media, or screens, in the melt stream create resistance to polymer flow, resulting in a pressure increase on the upstream side of the filter media. As contaminants collect over time on the screen, the available area for polymer flow is reduced, and the upstream pressure continues to rise. It is important to define the maximum allowable pressure increase for a given process and ensure that the extruder can generate the needed pressure. In many cases, existing lines of the same size are already in operation with a screen changer, and data can be benchmarked from these lines to assist in determining maximum pressure. Screen micron rating, number of screens in the screen pack, polymer viscosity, flow rate, downstream melt pipes, and die design will determine the pressure requirement from the extruder. The cumulative pressure of these items in the melt stream should be carefully considered to ensure that the line can achieve the desired production rate and product quality. Single-screw extruders are effective at generating pressure, and it is common to see film applications with head pressures ranging from 3,000 psi to 10,000 psi. In single-screw applications, it is most common for the screen changer to connect directly to the extruder because it can generate the necessary pressure. However, a co-rotating twin screw is a very effective mixing device, but has pressure-building limitations. In these cases, it may be necessary to install a gear pump after the twin screw to build the pressure required for the screen changer and downstream components in the melt stream. • Operating Temperature: The melt temperature of the

polymer in the extrusion process can be determined with a melt immersion thermocouple. Barrel or melt pipe surface thermocouple readings are typically not accurate methods of recording actual melt temperature. Actual polymer melt will typically be hotter than barrel surface temperature recordings. It is good to target a minimum and maximum range for operating temperatures. This is helpful when performing rheology tests and looking at equipment design configurability. • Shutdown Frequency: In cast- and blown-film applications, the line may be shut down temporarily to clean dies, change rolls, or for product changeovers. It is possible to change screens when the line is down as long as the contamination blinding on the screen does not require more frequent screen changes than the scheduled shutdown frequency. In this case, a discontinuous screen changer would be an effective solution. In some cases, a larger discontinuous screen changer with more screen area may be a simple solution, or a continuous screen changer may be considered. Although there are more factors to weigh in the decision process, line shutdown frequency is an important decision factor. • Waste, Downtime, Labor, and Screen Cost: The costs of off-spec material and plastic waste are high. With every screen changer, at some point there will be a physical amount of polymer waste to consider in the selection process. For every screen change, there is a small amount of plastic waste in the screen cavity that will require disposal. With some discontinuous models, line shutdown may be required, and waste and/or off-spec material will be generated in both the shutdown and startup of the line. The daily or weekly labor time that an employee contributes to screen changes, cleaning of breaker plates, and general maintenance is a real cost to weigh. Both discontinuous and even continuous screen changers present varying amounts of waste and/or downtime, labor, and screen costs to quantify. Although lower-capital investment alternatives may look attractive, it is recommended to create a total cost of ownership analysis which considers capital acquisition, operating costs, and personnel cost for each screen changer being considered. • Melt Flow and Pressure Stability: If it is determined that screen changes need to occur more frequently than regular line shutdowns (e.g., for die cleaning, product changeovers, maintenance, etc.) where a screen change could be made without affecting productivity, then which type of screen changer should be selected? In most cases this comes down to a process and productivity decision. For example, a screen change with a hydraulic fast-shift slide-plate screen changer could be executed on a 3+ mm thick flat-sheet process without losing the web, but trying to shift a slide plate on a .05 mm cast-film process would likely result in losing the web. In this case, a continuous screen changer would be the best selection to enable screen changes to be executed without interference to quality and productivity.

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Section 3.1  Gear Pumps, Filtration, Static Mixers—Function, Design, Parameters, Examples

• Thermal Stability of Plastic: Thermally stable polymers such as polyethylene can be processed through most filtration devices without degradation defects. However, thermally sensitive polymers such as rigid PVC present process challenges for many screen changer designs and can quickly degrade, ruling out certain designs in the selection process. • Configuration of the Extrusion Line: Information about the film line layout design is important when considering filtration equipment. Single-layer lines may have fewer space limitations than multi-layer lines. In consultation with the filtration manufacturer, line centerline height, space between extruders, aisle access, and operator control locations should all be reviewed to make sure that the desired screen changer is well integrated and can be easily accessed for filter changes, maintenance, and overall line operability. Filtration Designs—An Overview There are a number of filtration designs to consider, of which not all will be covered in this section. Rather, this section is an overview of the more commonly applied screen changers to single and co-extruded film applications. Screen changer designs fall into two basic categories: discontinuous and continuous. These category descriptions reference what happens to polymer flow when a screen change is executed. Discontinuous designs block or momentarily interrupt the flow of polymer during the screen change. The flow interruption may be as short as 1–2 seconds, or may require several minutes to change a screen, depending on design. When screens are changed in a continuous design, polymer flow to the die is not blocked. Flow will continue during the screen change process. There are two subsets in the continuous category: continuous flow and continuous flow constant. Each of these designs can also have a variant that includes automatic filter cleaning during operation. Each of these designs will be reviewed in more detail. Before covering screen changer types, basic information about the screens and breaker plate should not be overlooked. In every screen changer, there is a filter medium. The purpose of a screen changer design is to provide containment, structural support, and efficient exchange of filter media while ensuring safe, effective filtering of contaminants (Figure 3.1.9). Screens and Breaker Plates Although a variety of filter media can be considered, most changer designs use a breaker plate to support the filter. Prevailing breaker-plate designs are round or oval in shape and have a series of holes in the plate, typically in a circular pattern. The breaker plate is designed to maximize flow, minimize pressure drop, and provide structural support for the screen pack. Although a pressure drop is created by the breaker plate, in most cases, most of the pressure drop will occur at the screen mesh.

FIGURE 3.1.9.  Example of a screen, breaker plate, and carrier mechanism (piston).

Two of the more common screens used in film applications are square mesh and Dutch weave. The wire weave pattern for a square mesh screen has the same number of wires in the horizontal and vertical directions, with even spacing between the wires. Square-weave screens, unlike Dutch weave, normally have a higher percentage of open area, lower initial pressure drop, and are less expensive. Under high pressure, square-weave wires may be more prone to shifting, which could compromise the filter objective. Screens have a mesh and micron rating. Determining the mesh is very simple and involves simply counting how many openings there are in one linear inch of screen. A 20-mesh screen has 20 openings across one linear inch of screen. Therefore, as the mesh number increases, the size of the opening decreases. Note that mesh size is not a precise measurement of particle size because of the varying wire diameters that may be used in the screen. When selecting the appropriate screen rating, it is best to rely on the micron rating rather than the mesh rating. The micron rating of a screen defines the average size of the openings between pieces of filter media. When comparing mesh ratings from different screen manufacturers, the actual micron rating can be different from one manufacturer to another, depending on the wire diameter that is used in the screen (Table 3.1.1). Dutch weave screens have a wire pattern that uses larger wire diameter in the horizontal direction and smaller-diameter wire in the vertical direction. Dutch weave, compared to square weave, normally has a lower percentage of open area, a higher initial pressure drop, and is slightly more expensive. TABLE 3.1.1.  Wire Mesh versus Micron. Wire Mesh, % 20 × 20 20 × 20 20 × 20 20 × 20

Wire Dia, (in)

% Open Area

Opening Inches

Micron Retention

0.016 0.020 0.023 0.025

46.2 36.0 29.2 25.0

0.034 0.030 0.027 0.025

863.6 762.0 685.8 635.0

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Discontinuous: Manual Screen Changers

FIGURE 3.1.10.  Screen pack construction.

Dutch-weave screens tend to be more stable at high operating pressures than square-weave screens. Once the screen rating has been determined, efficient use of the screen is highly dependent upon screen pack construction. For example, suppose that a 150 mesh screen has been determined to be the finest screen required for a screen pack. There will not be enough screen rigidity to simply place a 150 mesh screen directly on the breaker plate. Screen dimpling or screen rupture will occur under polymer flow at normal extrusion pressures, and the outer screen diameter will reduce in size under pressure as the screen is pressed into the breaker plate holes. This will allow contaminants to flow around the screen or through the screen in the event of a hole rupture. Figure 3.1.10 is an example of a properly built screen pack. The first screen resting against the breaker plate is typically a 10, 12, or 20 mesh screen. In this example, a 20 mesh screen is used as the base layer. From there, each successive screen layer should be three to five times the mesh value of the screen against which it will nest. A final cover screen in this example is a 20 mesh which will hold the 150 mesh screen securely in place. In addition, Table 3.1.2 provides guidelines for square-weave screen construction for a given micron and mesh rating.

Manual screen changers are one of the simplest and most economical designs available. The screen pack and breaker plate are supported by a carrier mechanism that will enable the screen to be indexed into the online melt stream position and then removed to an offline position for screen exchange. The carrier mechanism is either a sliding plate or a sliding piston that contains one or two breaker plate cavities depending on the configurable design. Most common are two-cavity designs. When a screen is in the online melt stream position, contaminants will be collected on the screen over time, causing the pressure before the screen, also known as head pressure, to increase. Once the head pressure reaches its predefined limit, the extrusion process must be stopped so that the head pressure can be relieved before the online screen position can be shifted offline. In a two-cavity sliding plate, for example, as the dirty screen position indexes offline, the standby screen is indexed online. The extrusion line can then be restarted. The indexing of the carrier mechanism is accomplished by a lever, a mechanical ratcheting device (Figure 3.1.11), or a hand wheel. Manual screen changer size is defined by the breaker plate diameter. Sizes are available from 25–165 mm (1.0″–6.5″). Discontinuous: Hydraulic Screen Changers Hydraulic screen changers (Figure 3.1.12) were first introduced to the market in the 1960s. Similarly to the manual screen changer, the carrier mechanism for the breaker plate and screen is a sliding plate or a piston. The carrier mechanism is indexed using a hydraulic power unit. The hydraulic slide plate design will shift and exchange screen positions in approximately one second. Therefore, the extrusion line does not have to be shut down or stopped as with the manual screen changer. The “fast shift” mechanism enables the dirty screen pack to be removed from service and a clean screen pack indexed online by push-button control during the extru-

TABLE 3.1.2.  Guide for Square-weave Screen Construction. Recommended Screen Pack Construction Asymetrical Build

Micron Rating 860 230 140 10 75 60 43

Filter Screen Mesh

Wire Diameter “inch”

Recommended Screen Pack ← Flow ←

20 60 100 150 200 250 325

0.016 0.0075 0.0045 0.0026 0.0021 0.0016 0.0014

20 20/60/20 20/60/100/20 20/60/150/20 20/60/100/200/20 20/60/150/250/20 20/60/150/325/20

FIGURE 3.1.11.  Manual screen changer.

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Continuous: Piston Screen Changer

FIGURE 3.1.12.  Hydraulic slide plate screen changer.

sion operation. Unlike the manual screen changer, where the interruption of flow can be several minutes due to line shutdown, the hydraulic slide plate design can be shifted under operating pressure during production. The shifting creates a flow interrupt and typically ingests a small amount of air into the melt stream. In most processes, the line can continue to operate with only a minimal waste yield from the screen change and reduced downtime compared to a manual screen changer. A variant of the slide plate is a piston carrier, which also uses a hydraulic power unit. Unlike the slide-plate design, pistons index at a slower speed and may require line shutdown to change the contaminated screen pack. Hydraulic slide-plate screen changers are available in sizes from 63–380 mm (2.5″–15″). Hydraulic piston screen changers sizes range from 30–450 mm in round breaker plate designs and in oval designs up to 410 mm × 580 mm.

Continuous piston screen changer designs commonly use two round pistons as the breaker plate and the screen carrier. The two pistons are mounted in a forged steel block housing, where the pistons can index left or right perpendicular to the polymer flow (Figure 3.1.13). At the upstream entrance to the housing, the polymer flow channel diverges into two channels, advancing polymer to each screen cavity. After flowing through the breaker plates, the polymer channels unite to a single flow bore upon exiting the screen changer. In normal operation, both breaker plates are online in the melt stream. When a screen changer is required, the appropriate screen piston is hydraulically moved out of the housing until the screen cavity is accessible. The contaminated screen pack can be removed and replaced with a clean screen pack. Afterwards, the piston automatically indexes the clean screen cavity into the housing. Before the screen cavity enters the melt stream, the piston stops at the point where a prefill grove in the piston intersects the melt stream. This allows polymer, at a precisely controlled rate, to fill the screen cavity and purge any air. Once the screen cavity has completed the purge step, it indexes back into the online position. The second dirty screen will then be changed in the same manner. Piston screen changers have a number of configurable options offering different degrees of performance capability based on the application need. Depending on the amount of filtration area required, each piston can have either one or two screen cavities. Fourcavity machines provide double the filtration area in a more compact machine size versus a larger two-cavity machine

FIGURE 3.1.13.  Continuous piston with two screen cavities.

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of equivalent area. As well, four-cavity machines offer very stable process pressure because 75% of the filtration area remains online in production during a screen change. Backflushing Options are available for both two- and fourcavity screen changers where moderate to high contamination levels exist in the melt stream. Backflush is a method of automatically purging and removing contaminants from a dirty screen so that the screen pack can be reused multiple times within the machine before being discarded and replaced. The number of times that a screen can be backflushed varies by application, but on average, manufacturers indicate that backflushing can be done 50–100 times before screen replacement. By extending usable screen life, significant operating cost savings can be achieved, as well as reduced labor intervention for screen changes. New screen changer developments continue to expand performance, processing capability, and value. Following are two such examples: • Backflushing developments have continued in recent years. New patented Power Backflush machines can isolate the backflushing process of a screen, use compressed polymer under high pressure to improve filter cleaning, and further extend filter life while providing uniform pressure and flow to the die (Figure 3.1.14). Power backflush options can nearly double the number of times that a screen can be backflushed. The power backflush design delivers stable, consistent process pressure and can be used in applications where there is not enough downstream pressure for conventional backflush screen changers. Although capital investment for this machine is higher, there can be large savings in screen cost, reduced labor, and waste reduction, yielding attractive returns on investment. • Expanding filtration area without expanding overall machine size is a cost and space benefit. New BKG® FlexDiscTM filter designs for piston-activated screen changers substantially enlarge available filtration area without the need to increase machine size, enabling processors of low- to medium-viscosity polymers to achieve finer filtration, higher throughputs, longer filter service life, and less material waste when backflushing. The filters contain a filter stack made up of two to four FlexDiscTM depending on machine size (Figure 3.1.15). Each FlexDiscTM is equipped with two screen packs per piston cavity. As a result, four to eight times more filtration area is available for each cavity than with standard round screens, depending on machine size. Continuous: Rotary Screen Changer The continuous rotary screen changer uses a round wheel as the carrier mechanism for the breaker plates and screens (see Figure 3.1.16). It is essentially a round sliding plate located within a housing. Kidney-shaped breaker plates are housed in the wheel carrier, and the round sliding plate has a geared outer diameter that is used for mechanical rota-

FIGURE 3.1.14.  Continuous piston four-cavity screen.

tion. By using the kidney-shaped breaker plate and screen, one or two screens can be intersecting the melt bore at one time. Therefore, there is minimal mechanical interference in polymer flow during screen-to-screen transitions. A sophisticated controller measures head pressure, and a feedback circuit adjusts the slide-plate indexing speed to maintain a relatively constant head pressure. Effective sealing is dependent on body bolts that are torqued to a precise value for a given melt pressure and polymer viscosity while not limiting the sliding plate’s rotation. Precision machining of the sliding plate and housing sealing surfaces is required for effective sealing. Of the screen changers reviewed in this chapter, the capital investment for the rotary wheel design will typically be the highest for equivalently sized machines. Continuous Belt Screen Changer Continuous belt screen changer designs were introduced in the 1960s. The belt design screen changer uses a long con-

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FIGURE 3.1.15.  FlexDiscTM filter.

tinuous roll of Dutch-weave filter medium that is supported in the melt stream by a breaker plate. The filter roll indexes across the melt stream as contaminants collect on the screen at a rate that provides continuous flow and relatively constant pressure (Figure 3.1.17). Screens index through an entry and exit groove in the screen changer housing, enabling them to move in the transverse direction across the melt stream. Belt indexing is controlled using a series of heating and cooling zones on the entry and exit path of the belt. When the zones are heated, an intentional leak path to atmosphere is created, and the head pressure within the melt chamber will force the screen to index in the direction of polymer plug leakage. As a new

FIGURE 3.1.17.  Belt screen changer (Photo compliments of HighTechnology Corp.)

clean screen moves online, head pressure will decline. A closed-loop controller monitoring head pressure will then eliminate the heat source to the screen entry and exit zones as cooling water is provided to the zone to cool the polymer and prevent screen movement. Screen indexing can also be controlled by means of a time-based indexing cycle. The belt filter can provide a relatively stable pressure during screen changes, but contamination levels must remain moderate to low. Seal adjustment is polymer-dependent, based on a given polymer’s heating and cooling requirements, and heat/cool parameters must be adjusted to ensure the right amount of screen usage. Unlike manual and hydraulic screen changers, the screw cannot be pulled through the belt filter for removal. Summary

FIGURE 3.1.16.  Rotary screen changer (Photo compliments of Gneuss Inc.)

Although there are many filtration alternatives, this overview is not all-inclusive, but intends to target the screen changers that are more commonly used in film applications. The selection process can be complex and must start by taking sufficient time to collect and understand the critical application factors that enable manufacturers to guide processors through the process.

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FIGURE 3.1.18.  Continuous piston with two screen cavities.

STATIC MIXERS Introduction One of the basic performance criteria of the screw is to melt and homogenize, or mix, plastic, with plastic being defined as polymer plus any additives or fillers. The screw works from a basic assumption that when the melt exits the screw, it is adequately mixed and has a stable melt temperature (Figure 3.1.18). However, the machine design downstream of the screw to the die in film applications can vary greatly. Flow channel lengths, diameters, bends, and other equipment in the melt stream can influence the quality of melt that will reach the die. In most applications, the homogeneous state of the melt when exiting the screw has changed to a less-than-optimal condition by the time it reaches the die. Why? It is important to understand the influence of two types of flow properties. Laminar and Turbulent Flow With respect to polymer, laminar flow is best described as layers of flow in the melt stream that do not mix. In general, polymer flow in a melt pipe is normally laminar. If one examines the flow profile of polymer in a pipe, it will show that the fluid acts in layers that slide over one another. The velocity of flow varies from a theoretical state of zero at the walls, due to drag resistance, to a maximum velocity along the cross-sectional center of the pipe. When polymer leaves the screw and flows through a melt pipe unobstructed, laminar flow patterns begin to develop. The various flow layers from the wall to the cross-sectional center have different velocities, as seen in Figure 3.1.19, causing both thermal and viscous variations in the melt stream. Examining the varying flow velocities in the melt channel reveals an associated temperature gradient, as shown in

Figure 3.1.20. Because higher velocities create higher shear stress, there will be viscosity differences across the melt channel. In film and sheet applications, these variations in flow velocity and viscosity pose significant challenges for the die to perform properly. To prevent this scenario, the melt needs further mixing before entering the die. For mixing to occur, turbulent flow is required. An obstruction is needed in the melt stream to produce turbulence. Turbulent flow creates chaotic property changes in the melt flow by producing eddies and swirls that mix laminar layers together. Dispersive and Distributive Mixing There are three kinds of mixing: dispersive, distributive, and extensional. This paper will deal only with dispersive and distributive mixing because extensional mixing occurs predominantly in twin-screw extruders. Dispersive mixing is like putting two materials to be mixed between two plates and rotating one of the plates. The

FIGURE 3.1.19.  Laminar flow.

Section 3.1  Gear Pumps, Filtration, Static Mixers—Function, Design, Parameters, Examples

FIGURE 3.1.20. E   xample of polymer temperature gradients caused by laminar flow in a 2” diameter melt pipe.

shear stress developed in the polymer between the plates is proportional to the distance between the plates and the speed at which the plate is rotated. Distributive mixing is like putting two materials in a bowl and stirring them with a spoon. The number and path of the spoon strokes is proportional to the degree of mixing. Screws inherently provide both dispersive and distributive mixing. Generally, the shallower the screw, the greater is the amount of dispersive mixing that occurs, and the more disruptions in the flow path, the greater is the amount of distributive mixing that occurs. Static Mixer Another approach to use to optimize the melt condition before it enters the die is a static mixer. A static mixer is a motionless obstruction in the melt stream that creates turbulent flow. Static mixers can mix high- and low-viscosity polymers, liquids, additives, and color pigments. A mixer can consist of a single element or a series of elements joined together or stacked in series to distribute flow (Figure 3.1.21). Many static mixer designs are available that

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provide good distributive mixing by constantly dividing and recombining the melt stream so that the plastic exiting the mixer is homogeneous with respect to thermal properties, concentration, and velocity throughout the cross section of the melt pipe. The design of a single element resembles a rectangular plate that has been twisted so that the element entry plane and exit plane are at 90° relative to each other. As depicted in Figure 3.1.21, each mixing element is oriented at 90° to the adjoining adjacent element. As polymer flows into an element, the flow is divided. The flow divisions can be represented as V = 2n, where V is the number of flow divisions and n is the number of elements (Figure 3.1.22). For example, polymer flowing into a six-element mixer would be represented as V = 26 and would have 64 divisions by the time it exited the sixth element. A static mixer can be configured with more or fewer elements depending on the amount of mixing required. In most cases, thermal gradients create the largest challenge for the die; most six-element mixers will adequately resolve thermal variances in the melt stream. One consideration that should be taken into account when adding a mixer is the associated pressure restriction that it will create. Both the diameter of the element and the number of elements will affect the pressure drop. As with any obstruction in the melt stream, a resistance to flow exists. As more elements are added, consideration will have to be given to the flow resistance or the pressure needed to overcome the partial blockage caused by the mixing elements. In these applications, gear pumps can generate sufficient pressure for the mixer and eliminate additional pressure for the screw to overcome. Mixer Location In sheet and film applications, the location of the mixer is critical to achieve its maximum advantage in the melt stream. Having a homogeneous thermal melt condition throughout

FIGURE 3.1.21.  Static mixer element.

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FIGURE 3.1.22.  Example of mixing and flow divisions.

the full cross section of the melt pipe diameter is preferred to optimize die performance. For that reason, static mixers should be located as close as possible to the feed block and die entrance (Figure 3.1.23). In a mono-layer sheet or film line, the die is typically located immediately after the gear pump. A static mixer located between the gear pump and the die with a series of short-coupled adapters minimizes the residence time of plastic. In multilayer film applications, the melt pipes that connect the extruder, screen changer or gear pump to the die feed block can be more complex, with longer pipe lengths and complex angles. Figure 3.1.24 displays a three-layer line arrangement with static mixers optimally located at the feedblock entrance. Another important installation consideration that is easily overlooked is the orientation of the mixing element in the mixer housing. In sheet and film applications, the element should be oriented so that the exit plane of the last element is horizontal and matches the orientation of the film die. If the last mixing element is oriented in a vertical position, it may result in a knit line in the center of the film or sheet.

Static mixers not only improve melt uniformity, but also improve color and additive homogenization so that lower concentrations can be used in the formulation to achieve the same results. When properly installed, a static mixer located before the die will resolve thermal gradients created by laminar flow, thereby enabling the die to perform optimally (Figure 3.1.25). Summary When examining melt stream performance, much emphasis is placed on selecting the right screw and die. Equally important in your equipment selection procedure is the equipment that resides between the screw and the die. This procedure starts by ensuring that you have the right screen changer that can efficiently remove the desired contaminants from the melt, using a gear pump designed to deliver consistent, uniform flow rates, and including a static mixer to homogenize the melt before it enters the die.

FIGURE 3.1.23.  Static mixer location in most mono-layer sheet and film applications.

Section 3.1  Gear Pumps, Filtration, Static Mixers—Function, Design, Parameters, Examples

95

• Locate the pump speed readout and control adjacent to the extruder speed readout and control. • Provide a readout for the pump body temperature and a temperature interlock for the extruder and pump motors. • Provide an ammeter with a setpoint relay for the pump motor drive. DO NOT’s of Pump Use

FIGURE 3.1.24.  Optimal static mixer location is closest to the feedblock and die.

GEAR PUMP ADDENDUM Simple DO’s and DO NOTS of pump use: DO’s of Pump Use • Check the pump shaft for free rotation before startup and after each shut down. • Use nickel anti-seize compound on all bolts when assembling or installing the pump. • Provide a switch interlock that will not allow the pump run circuit to energize with pump speed set to greater than zero RPM. • Provide high pump suction pressure shutdown for extruder and pump. • Provide a low pump suction pressure alarm and shutdown. • Provide high pump discharge pressure shutdown for the extruder and pump.

• Allow inexperienced personnel to run the pump. • Run the pump starved of polymer. • Start the pump cold. • Run the pump when the ammeter reading is too high or increases rapidly as the pump is increased. • Apply overhung loads on the pump drive shaft other than a universal drive shaft or coupling. • Run the pump using chain or belt drives on the pump drive shaft. • Run the pump without D/S equipment mounted to pump. REFERENCES AND ADDITIONAL RESOURCES [1] “What is the Difference between Mesh and Micron? How Do I Know Which Sanitary Screen Gasket to Pick?”, March 28, 2014, Holland Applied Technologies. [2] Jones, Bart, “Screen Changers for Coextruded Film”, Dynisco. [3] Fox, Steve, “The Use of Gear Pumps in Compounding Extrusion Process”, November 30, 1993, SPE RETEC. [4] Wiggins, Stephen and Ottino, Julio M., “Foundations of Chaotic Mixing”, March 11, 2004, The Royal Society. [5] Frankland, Jim, “Single-Screw Mixing 101”, May 1, 2010, Plastics. Technology Textbooks [6] Budynas, Richard G., Nisbett, Keith J., “Shigley’s Mechanical Engineering Design”, 8th Edition. [7] Wagner, John R., Jr., Mount, Eldridge M. III, Giles, Harold F., “Extrusion: The Definitive Processing Guide and Handbook”, 2nd Edition, 2013.

FIGURE 3.1.25.  Static mixer resolves thermal gradients in melt stream.

Chapter 3—Section 2

Feedblock Technology CHRISTINE RONAGHEN, Cloeren Incorporated

KEYWORDS: Coextrusion, feedblock, laminar flow, layer sequencing, layer interfacing, viscosity.

INTRODUCTION Coextrusion can be described as the simultaneous extrusion of multiple polymer layers through a single die to form a composite sheet, film, coating, or filament with aggregate properties above and beyond those of any individual layer. There are several reasons why processors may prefer coextrusion to single-layer extrusion for many applications. Single-layer (commonly referred to as “monolayer”) extrusion may not achieve the desired combination of physical, chemical, thermal, and/or aesthetic properties in the final product. Barrier applications may require several different resins to achieve moisture, oxygen, or light transmission requirements. Skin layers may require unique and differential tackiness (or lack thereof) or seal properties. Operating multiple extrusion stations or subsequent separate processes to apply additional layers becomes complex and inefficient. In addition, coextrusion makes it possible to minimize the consumption of costly functional resins and to bulk up the structure with lower-cost resins while still maintaining the required properties. The extrusion process is a laminar flow process. By definition, there is no turbulence in laminar flow. Laminar vs. turbulent flow is defined by the Reynolds number (Re) of the flow field(s) and is a function of viscosity and shear rate. Turbulent flow in thermoplastics may present itself when the Re approaches 3,000–4,000 Re. In the flow stream converging process for thermoplastics, the Re numbers are typically less than 10 and can commonly approach values less than 1. All thermoplastic extrusion processes are laminar. Free of turbulent flow, laminar flow enables merging two or more flow streams into one common flow stream without intermixing between layers, which is a critical characteristic of successful coextrusion. Optimizing the process of merging multiple flow streams requires coordinating certain fluid flow attributes between the merging layers, such as velocity, shear rate, shear stress, and Re. There are two general methods of coextrusion in flat die processes. In both cases, multiple melt streams are combined into a single composite structure and spread to a desired width and thickness. The differentiation between the two

methods lies in which process occurs first: combining flow streams, or spreading to the full desired width. One method spreads the melt streams individually to the final desired width and then combines them into a composite structure. That process uses a multi-manifold die, as depicted in Figure 3.2.1 and as discussed in more detail later in this manual. Moreover, efficiency may be improved by incorporating regrind or reprocessed resin into a structure while maintaining required quality levels using higher-quality virgin skin layers. In the case of colored or filled structures, incorporating clear skin layers can speed purge times and reduce die-lip buildup and plate-out on the cooling roll. In short, coextrusion finds significant application in many cast-film and blown-film processes. An alternative and more common method involves initially combining all layers into a composite structure and then spreading that composite structure to the final desired width in a single-manifold die, as depicted in Figure 3.2.2. The combining of multiple melt streams before spreading them to final width occurs in a device called the feedblock, which is the main topic of this chapter. PRIMARY FUNCTIONS Feedblocks serve several functions in a co-extrusion film process. Mechanically, the feedblock is the physical interface between multiple extruders and a single flat die. The feedblock must also arrange the incoming melt streams into the desired layer sequence, form these melt streams into a shape suitable for layering, and then combine all the layers together into one composite melt stream free from interfacial defects induced by the feed channel flow passage. These key functions are referred to as “layer sequencing” and “layer interfacing” respectively, and each will be discussed in detail below. It is important to note that it is not the function of a feedblock to influence melt temperature or uniformity, nor to impact the general melt quality of the incoming melt streams. In fact, the feedblock has little or no influence over such process characteristics. Generally, best practice suggests 97

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FIGURE 3.2.3.  Selector PlugTM.

FIGURE 3.2.1.  Multi-manifold die.

setting the feedblock temperature to the nominal operating temperature of the melt streams being coextruded, although supplier recommendations may differ from this standard under certain circumstances. Layer Sequencing Layer sequencing involves arranging the incoming melt streams into the desired sequence to produce a composite structure and can be accomplished in several ways. Melt piping fitted to each extruder could simply deliver the melt streams to their appropriate position at the entrance of the feedblock. However, should a change in sequence be desired, new melt piping would be required. This is a costly,

FIGURE 3.2.2.  Feedblock and single-manifold die.

time-consuming, and labor-intensive endeavor. Modern coextrusion feedblocks use various means of simplifying the process of changing layer sequence. In the mid-1970s, Peter Cloeren Sr. introduced the concept of the Selector PlugTM 1 , in which layer sequencing was achieved by flow channels machined into a cylindrical insert in the feedblock, as pictured in Figure 3.2.3. This insert could easily be exchanged to reconfigure the layers and represented a significant advance in coextrusion technology. Until this point, reconfiguring an extrusion line running an A B C1 structure to one that could run an A B A or C B A structure required planning and capital investment for piping and adapters, time to procure the required components, and possibly one or more full days of lost production to install the new hardware. If subsequent changes in layer sequence were required, the same procedure would be repeated. Implementation of the Selector Plug concept meant that changes in layer sequence could be made in a fraction of the time and at a fraction of the cost of previous methods. Figure 3.2.4 depicts Selector Plugs for a three-layer system representing an A B C structure and a C B A structure. The Selector Plug concept has since been implemented industry-wide and remains the most common method of layer sequencing today. However, other means of layer sequenc-

FIGURE 3.2.4.  Selector PlugTM illustration: “A B C” sequence left; “C B A” sequence right.

Section 3.2.  Feedblock Technology

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FIGURE 3.2.5.  TopHatTM illustration.

ing may be occasionally used. One such method is called the TopHatTM 2 approach and is shown in Figure 3.2.5. In this method, melt streams are fed into the feedblock and then through a series of parallel plates incorporating flow channels which direct each stream to its appropriate position in the final composite structure. In this case, a change in layer sequence would require the plates to be replaced, but the melt piping to the feedblock would remain fixed. Although certainly more involved than the Selector Plug approach, the TopHat approach does enable relatively easy changes in layer sequencing and does find use in some applications. Layer Interfacing Once the polymer melt streams have been directed to the correct sequence within the feedblock, they must then be merged into a single flow stream. Layer interfacing is the convergence of multiple melt streams into one composite structure. Original coextrusion feedblocks generally featured simultaneous convergence of all melt streams in a structure. Figure 3.2.6 shows the Dow-style feedblock (USP 3,557,265 to Chisholm et al.), in which the flow streams are divided into a fixed number of lamellae that are all brought

together at a single point. Inherently, this is a fixed-geometry approach to coextrusion, meaning that no online or instantaneous adjustment of the layer interface is possible. Any flow-channel geometry adjustments are accomplished through removal and replacement of the entire lamella section of the block. Although not common, this style of layer interfacing is still in use today by some suppliers. In another approach, layers in a coextruded structure can be brought together sequentially. Initially introduced by Cloeren in the mid-1980s, these systems are commonly referred to as multiplane feedblocks, and the concept is depicted in Figure 3.2.7. The core component of the coextruded structure flows down the center of the feedblock, and additional layers join the composite stream sequentially thereafter until the complete structure has been formed. Multiplane feedblocks are the most commonly used feedblocks today. Feedblocks designed with a sequential combining approach can be designed with either fixed or variable flow channel geometry. Variable geometry refers to the ability to change the flow path at the convergence point(s) instantaneously while the feedblock is online and in operation, whereas fixed-geometry designs do not afford such flexibility.

FIGURE 3.2.6.  Lamella feedblock concept.

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FIGURE 3.2.7.  Multiplane feedblock concept.

Fixed-geometry designs (Figure 3.2.8) feature a fixed flow channel. They are streamlined, their performance is repeatable, operation is simple, and most fixed-geometry feedblocks do offer the ability to change the flow-path geometry by removing and replacing inserts housed within the body of the block. However, as the name implies, it is not possible to make any instantaneous geometry adjustments while the equipment is in operation. Variable-geometry designs (Figure 3.2.9) offer one or more types of online adjustability to influence the dimen-

FIGURE 3.2.9.  Variable-geometry feedblock.

sions of the flow-path geometry at the convergence point. Adjustable vanes can alter the gap through which each layer passes. Some feedblocks also offer pins or similar adjustable components that can be profiled to influence the distribution of one or more layers in the structure. This concept is discussed further toward the end of this chapter. COEXTRUSION BEHAVIOR: A BRIEF INTRODUCTION TO VISCOSITY

FIGURE 3.2.8.  Fixed-geometry feedblock.

A discussion of coextrusion would not be complete without a short discussion of rheology and its influence on coextrusion behavior. Rheology is the study of the flow and deformation of matter. Specifically, as it relates to the topic of this text, it is the study of the non-Newtonian flow of polymers. Although the broader topic of rheology should be of interest to those in the extrusion field [3], one of the most critical influences in the coextrusion process is viscosity. Simply stated, viscosity is a measurement of a fluid’s resistance to flow. For polymers typically used in extrusion, resistance to flow is a function of both shear rate and temperature. The viscosity of a polymer cannot therefore be described by a single number [4], but instead requires a series of characterizations to reflect accurately the influence of shear rate and temperature on viscous behavior. The viscosity of a given polymer can be measured in a device called a rheometer. Testing should be done by a skilled technician because interpretation of the quantitative data can influence the validity of the results. Ideally, the data obtained from these devices are in the form of discrete data points

Section 3.2.  Feedblock Technology

representing viscosity versus shear rate at several different temperatures close to the typical processing temperature of the polymer in question. A rheologist can use these discrete data points to fit one of several established models, and the resulting relationship of viscosity = ƒ (shear rate, temperature) can then be used to design and optimize various components of an extrusion system. An example of such data is presented in Figure 3.2.10. As shown, there are several areas of interest in the viscosity/shear rate relationship. For most polymers, the dependence of viscosity on temperature is unique. Figure 3.2.10 shows that the entire viscosity versus shear rate curve shifts as temperature changes. Within the context of thermoplastic extrusion, it is safe to say that all polymers exhibit a decrease in viscosity with increasing temperature. However, the magnitude of thermal influence is unique to each polymer. At each temperature tested, there are three distinct regions of the viscosity curve. At low shear rates, most polymers exhibit a Newtonian plateau; that is, as shear rate decreases, viscosity becomes constant, and the polymer behaves more like a Newtonian fluid. At high shear rates, there is a distinct and sometimes rapid drop in viscosity as shear rate increases. This phenomenon is known as shear thinning. Between the Newtonian plateau and the shear-thinning regime lies the transition region, also known as the critical shear rate. The initiation point and sharpness of the transition is also unique to each polymer. The above diversion into viscosity is a necessary baseline

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for understanding coextrusion behavior because the flow resistance of each individual polymer forming a coextruded structure will dictate how that composition behaves when all the polymers are flowing together through a common flow path. Coextruded composite structures formed by polymers of similar viscosities will behave better when flowing together than those made up of resins with more diverse viscosities. Composite streams with layers that differ significantly either in nominal viscosity (given the temperature and effective shear rate) or in the general shape of the viscosity curve will tend to exhibit severe viscous rearrangement as the composite structure flows through a flow channel. The two common forms of viscous rearrangement are viscous encapsulation and viscoelastic rearrangement, both of which are discussed in detail in the Rheology section of this manual. Viscous rearrangement is a time- and shear rate-dependent phenomenon, and although it can be minimized with proper feedblock and die design, it often cannot be eliminated in complex coextruded structures. Where flexibility in resin selection is available, every effort should be made to select materials that are similar in their viscous behavior to minimize rearrangement resulting in non-uniformity of individual layers across the width of the final film. However, when end-product requirements require multiple dissimilar components to achieve the desired composite properties, the feedblock may be used as a tool to enable corrective tuning of individual or selected layer distributions to compensate for viscous rearrangement.

FIGURE 3.2.10.  LDPE viscosity curve.

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BEYOND CONVENTIONAL COEXTRUSION Beyond conventional coextrusion, which can typically involve anywhere from 3 to 15+ layers, the industry is encountering more and more applications that benefit from the use of microlayer or nanolayer coextrusion techniques in their processes. This technology combines two or more of the melt streams that make up a coextruded structure into a large number of very thin alternating layers. Although the general principle of microlayer technology has been present in various forms for decades, two commercially available approaches are commonly used in practice. Nanolayer and microlayer technology has also found niche areas of successful application in non-cast-stretch arenas and continues as a developing technology. COEXTRUSION PERFORMANCE AND CORRECTIVE ACTIONS

FIGURE 3.2.12.  NanolayerTM approach.

Layer Uniformity

Although this section cannot capture every potential coextrusion issue and its resolution, two issues occur frequently enough to warrant mention: (1) poor layer uniformity and (2) interfacial instability. One is offered by Nordson EDI and uses a layer multiplication technique whereby a multilayer coextrusion package is first created and then split and re-stacked to multiply the number of layers. This technique is similar to the original multiplier concept developed by Toller and further modified by Shrenk. The general concept is shown in Figure 3.2.11. The second commercially available system was developed by Cloeren Incorporated in the early 2000s and is similar in nature to a traditional coextrusion feedblock in which the composite structure is created sequentially, as shown in Figure 3.2.12. This approach requires individual flow channels for each adjoining nanolayer. Nanolayer technology is commercially established for cast stretch film, which has seen the industry standard progress from 7–9 layers to 30+ layers and then to the current practice of 50+ layers.

The feedblock in and of itself has very little control over the resulting individual layer uniformity across the width of the extrudate at the die exit. Rather, the geometry of the die flow channel itself and the relative viscosities of the polymers forming the composite structure primarily dictate the resulting layer uniformity. However, the feedblock should be able to address and remedy individual layer distribution defects and to counteract natural time-temperature viscous layer rearrangement through the die if needed. Figure 3.2.13 shows an example of a three-layer B-A-B structure, where the viscosity of B is less than that of A. Layer B appears to be uniformly distributed at the exit of the feedblock. However, viscous rearrangement occurs as the material flows through the die, and the final product has a heavy B layer in the center and a very light B layer on the outer edges. This effect could be addressed by altering the layer viscosities, either by shifting melt temperature or by changing resins. However, if such changes are not possible or not permitted, another solution is to profile the layers in the feedblock inversely to compensate for the viscous rearrangement occurring downstream. Figure 3.2.14 shows the resulting layer distributions at the feedblock and die exits after profiling.

FIGURE 3.2.11.  Multiplier approach.

FIGURE 3.2.13.  Poor layer distribution at die exit.

Section 3.2.  Feedblock Technology

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provement in layer distribution across the full finished width of the film is significant, with the result meeting all specifications for product uniformity. Interfacial Instabilities

FIGURE 3.2.14.  Corrected layer distribution at die exit.

In a variable-geometry feedblock, vane adjustment and/ or pin profiling are used to correct distribution issues. In a fixed-geometry feedblock, one or more fixed inserts must be removed and replaced with a modified design to improve distribution. Although advances in fluid flow modeling provide accurate simulations of complex polymer processes, none of these models has proven to predict viscous layer rearrangement accurately or to provide a solution to correct it. The profile geometry of the pins or inserts required to achieve optimized layer distribution is empirical in nature and requires experience to implement efficiently. Hence, corrective profiling remains much more of an art than a science today. Figure 3.2.15 shows an example of a simple barrier structure suffering from poor distribution of the functional barrier layers and the sealant skin layer at the outer edges of the film. The thickness of each layer in the composite structure is shown before and after profiling in the feedblock. The im-

There are innumerable types of flow instabilities in coextrusion. The challenge with flow instabilities lies in qualifying the type of instability and identifying its cause. Generally, there are three reasons that coextrusion-related flow instabilities may develop; (1) temperature/viscosity non-homogeneity of the melt stream(s), (2) feedblock flow channel geometry, and (3) die flow channel geometry. Within each one of these primary regimes, flow instabilities may present themselves in multiple shapes and forms. Often, these interfacial defects result from drastically differing velocities and/or shear stresses at the point of convergence of the flow streams. Figure 3.2.16 shows an example of a system designed for a 40%/20%/40% structure (left image) running instead a 15%/70%/15% structure without any flow-channel geometry compensation (center image). In this case, velocity and shear stress are not balanced, which could result in a coextrusion instability that could negatively affect the aesthetic and physical properties of the final product. The image to the right reflects a corrected flow path geometry, which was accomplished by re-positioning the rotatable vanes in a variable-geometry feedblock. Although this is easiest in a variable-geometry feedblock, similar flow channel adjustments can be achieved on a fixed-geometry block by replacing the fixed-geometry inserts. Most variable-geometry feedblocks offer the ability to let the vanes “float”, meaning that their position is not fixed and the vanes can automatically adjust to their equilibrium points based on the relative pressure between flow channels.

FIGURE 3.2.15.  Example of effect of feedblock profiling on layer distribution.

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FIGURE 3.2.16.  Variable-geometry feedblock—online geometry modification.

SPECIFYING A FEEDBLOCK As with all extrusion hardware, proper characterization of the extrusion process is essential to achieving best-inclass performance. The performance of any hardware cannot be optimized without comprehensive characterization and proper specification of the process. At a minimum, the following information is required by an equipment supplier to specify and design a feedblock properly. A summary of all anticipated structures to be produced, including the following specifications for each: (1) Number and sequence of layers and volume (or mass) percentage of each layer for each composite structure to be produced. (2) Resin formulation (blend), output rate, and melt temperature for each layer. (3) Resin properties, including (at a minimum) density and viscosity data. (4) Extruder and melt piping arrangement. Note that the feedblock supplier may have valuable input on how to arrange the extruders to optimize overall flow paths for each layer.

All this information will be used to select the correct method for coextrusion (feedblock, multimanifold die, or a combination thereof), the optimal feedblock design (fixed geometry, variable geometry, or a combination thereof), and ultimately to design the flow channels to achieve the desired performance. REFERENCES AND ADDITIONAL RESOURCES [1] The “A B C”, etc., naming convention reflects configuring the respective outputs of extruders “A”, “B”, and “C” in the order listed. A repeated letter (e.g., A B A) indicates that the output of the repeated extruder is initially “split” before rearranging the configured layers. [2] TopHat is a trade name of Cloeren Incorporated. [3] Materials Science of Polymers for Engineers, 2nd Edition, Tim A. Osswald and Georg Menges, © 2003. [4] A polymer’s single “melt index” or “melt flow rate” value represents an empirically defined parameter that is critically influenced by both the physical properties and molecular structure of the polymer and the conditions of measurement (i.e., effective shear rate and temperature). As such, these values are not a fundamental polymer property. (ASTM D1238-13: “Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer”).

Chapter 3—Section 3

Film Stabilization, Forming and Collapsing Systems JAMES STOBIE and HARINDER TAMBER, Macro Engineering and Technology Inc.

INTRODUCTION The blown-film process provides superior mechanical properties compared to cast film. However, compared to cast film, blown film has some inherent limitations related to flatness, surface imperfections, and thickness uniformity, properties that are important in high-speed printing, laminating, and packaging applications. Much effort has been made to address these problems. This section has been written to cover the wide variety of methods used in containing, handling and collapsing the blown film bubble. In the blown-film process, bubble stability and cooling are the rate-determining factors, which are influenced by the external air rings and/or internal bubble cooling (IBC). Although the bubble is initially stabilized by the air ring and/ or IBC, containing the bubble above the frostline and guiding it through its collapse is equally important and requires due attention in blown-film systems. In recent times, “swing lines” have become more common, where processors can run different materials on the same line by dropping lip-sets into the air ring, provided that the die size is the same. The materials can range from those of high melt strength and less stretch like HDPE to low-melt-strength, extensible materials like LLDPE, covering a wide range of materials in between, such as LDPE or blends of LDPE/LLDPE. This variety places more demands on the bubble containment and collapsing equipment, which must be versatile enough to provide film free from scratches and wrinkles. The incorporation of resins such as mLLDPE, plastomers, and elastomers, which can be down-gauged, makes it critical that the appropriate bubble containment and collapsing system be chosen to provide a stress-free film. Bubble containment is done by bubble cages and bubble guides, as described later. Once contained, the bubble must be collapsed to form a layflat tube. A number of means are available to accomplish this task for different film types, which can range from sticky films such as plastomers, elastomers, or high-VA-content EVA to stiff films such as HDPE and PA. To minimize wrinkles, bagginess, and excessive friction, the tacky films are collapsed using low-friction roller surfaces or in some cases air-lubricated surfaces. The sys-

tems that use air lubrication are pressurized chambers where the air provides a lubricating surface to the film as it travels up the frame. With less demanding applications, maple slats can be used for bubble collapsing. However, some films require sophisticated collapsing frames to avoid baggy centers or edges and provide a wrinkle- and stress-free layflat tube for use in high-speed laminating or converting applications. The primary criterion for selecting a collapsing system is accommodating the diverse nature of films fabricated in a production environment. Film Tube Handling and Containment between the Air Ring and the Frostline In the blown-film process, the melt tube extruded from the die is first stabilized by an air ring. Dual-lip air rings are used for low-melt-strength materials with lower frostlines, and single-lip air rings are used for high-melt-strength materials with higher frostlines. Internal bubble cooling systems (IBC) are commonly installed with both kinds of air rings and play an important role in bubble stabilization in addition to their cooling function. In some applications, triple-lip air rings or secondary air rings are used to increase output. In some cases, the melt tube passes the secondary air ring before final expansion to a specific layflat, and enhanced film properties can be obtained. Various devices may be installed on the primary air rings, such as additional stabilizers so that the air moves axially with the bubble to stabilize it. Air shedding from the bubble surface before the frostline may disrupt bubble stability, leading to lower output. Maintaining bubble stability becomes especially critical for low-melt-strength materials at low blowup ratios and at high output. Iris These devices are simple and effective to use on blownfilm lines and are installed above an air ring (Figure 3.3.1). They are versatile because the inner diameter of an iris can be adjusted with the blowup ratio. The purpose of this device is to reinforce the air attachment to the bubble surface by extending the time that air remains in intimate contact with 105

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FIGURE 3.3.1.  Film tube handling above the frostline.

the bubble, prolonging heat removal. Care must be taken to allow the cooling air to pass between the iris and the bubble without creating buffeting. Bubble Guides

FIGURE 3.3.3.  Four-arm bubble guide.

A guide assembly for the blown-film bubble to guide and support the film above the frostline and below the collapser may be required. This support and guidance is necessary particularly at higher outputs. Depending upon the tower height, one or more sets of bubble guides or bubble cages (discussed in the next section) are installed to contain the bubble so that concentricity of the bubble to the die is maintained. A commonly used bubble guide is shown in Figure 3.3.2. This unit consists of four bars, with the ends fitted with spur gears that ride on racks. Each individual rod can be adjusted by a chain and sheave until it makes contact with the bubble and the bubble slides on it. Figure 3.3.3 shows a more advanced bubble guide. This consists of four cylindrical arms or rollers (e.g., of nylon) and is superior to the previous guide because all the arms can be moved by one adjustment and it provides rolling contact with the bubble. This bubble guide can be moved vertically upward or downward

FIGURE 3.3.2.  Bubble guide.

manually, or it can be motorized. In addition, the guide can be used to support IBC control sensors. Bubble Cages Bubble cages are a superior method of film containment and stabilization, as shown in Figure 3.3.4. These devices are also used to mount IBC sensors. The frame should provide the flexibility to adjust the cage height to suit varying frostline heights. Machinery manufacturers may build different bubble cages, but they all have similar features that are necessary to stabilize the bubble, as mentioned here. (1) Bubble stabilizing cages surround the tubular film with radiused arms. (2) The cage height can be adjusted to stabilize the bubble at different frostline heights or in multilayer films to accommodate a varying frostline height. (3) Cage diameter is normally adjustable to provide a BUR adjustment range of 3 or 4:1. (4) Cages are concentric to the die and use low-inertia rollers to minimize friction and drag, improving gauge uniformity and layflat. (5) The bubble is in contact with a rolling surface within the cage. The rollers are low-inertia and made of materials such as carbon fiber or aluminum with felt or silicone sleeves. (6) In bubble cages, larger-diameter rollers turn more easily than smaller-diameter rollers due to their large surface contact area, except if larger-diameter rollers are heavier. In general, shorter rollers conform to the bubble better than longer rollers and also induce less friction. The selection of a bubble cage is important to provide enhanced bubble stability for optimum throughput rates,

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problems is the exposure of the bubble to ambient drafts that can cause gauge variation and also influence bubble stability. A vertical span of 1.5 to 3 meters of bubble after the frostline is the most vulnerable to these ambient drafts, which can be avoided by building enclosures around the bubble. The enclosures can be made of flexible material such as a vinyl curtain (Figure 3.3.5) or of rigid materials like polycarbonate sheet (Figure 3.3.6) encircling the entire bubble above or at the air ring. The rigid structures are supported with steel and have doors to access the bubble. These enclosures are used to maximize rate and stability and to eliminate ambient effects in everyday production. Bubble Stabilizers for High-Stalk Extrusion

FIGURE 3.3.4.  Stabilizing cage.

consistent layflat, and gauge uniformity. Some cages have differential spacing of the cylindrical rolls, with rows closer together at the bottom end of the cage to stabilize the bubble more effectively. Excessive air disturbance near the contact surface with the bubble should be avoided because this undermines the efficiency of the cage by preventing its contact with the bubble. Bubble Enclosures In the blown-film process, a potential cause of quality

FIGURE 3.3.5.  Vinyl enclosure.

In high-stalk extrusion, particularly for HMW-HDPE type materials, the frostline height is 6 to 10 times the die diameter, and therefore this process requires a different type of stabilizer inside the melt tube. A typical stabilizer commonly used for high-neck extrusion is shown in Figure 3.3.7. This unit consists of a support bar and an adjustable stabilizing head. In non-IBC operations, the head often makes contact with the tube and therefore should be made of lowthermal-conductivity materials. The stabilizers are also often wrapped with specialty cloths to avoid impressions inside the tube. On blown-film lines equipped with IBC for high-neck extrusion, a small volume of air exits near the die lip (the rest of the air exits at a second head located where the bubble expands) and follows the melt tube while centering the tube on the stabilizer, as shown in Figure 3.3.8. An iris, as described earlier in this section, can also be used to stabilize the bubble, and these are often placed well above the frostline.

FIGURE 3.3.6.  Rigid enclosure.

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FIGURE 3.3.7.  Internal stabilizer.

FILM TUBE COLLAPSE Theory In the blown-film process, a converging frame is provided for collapsing a moving tube of plastic material from a circular cross section to a flattened form before the flattened tube enters a driven set of nip rolls. In this process, the film at the edges travels at a different speed and over a different distance than the center of the film. As shown in Figure 3.3.9, the edge of the film travels a shorter distance than the center of the film, and consequently the center of the film contacts the collapsing frame for a longer time compared to areas near the edges. This longer contact creates more drag, and therefore the film is subjected to uneven stresses across its width. The higher stress in the center of the film can lead to stretching or creasing of the film, which can become permanently deformed, resulting in center sag. Relatively extensible films such as LDPE, LLDPE, mLLDPE, or plastomers are more forgiving and readily adapt to the changing geometry of the collapsing bubble with minimal adverse effect on the film’s flatness and gauge uniformity. However, stiffer films such as HDPE, nylons, rigid PVC, polystyrenes, cyclic olefin copolymer (COC), and some multilayer films do not possess this flexibility. Therefore, such films should be collapsed while warm, which provides some flexibility depending upon the material. Another approach for stiffer films is to collapse them at a reduced angle. However, this causes a longer contact of the film with the collapsing surface, resulting in more drag or stretch

FIGURE 3.3.8.  HMW-HDPE IBC stabilizing system.

marks. To reduce these issues, a low-friction collapsing surface must be used. Most often, the collapsing-frame length ranges from 1.5–2.5 times the nip-roll face width; however, for high-modulus films, including HDPE, nylon, and stiffer coex structures up to three times the nip-roll face width can be used. The collapsing length can be increased to provide a more uniform travel distance from the side to the center of the film, but at the cost of added friction. Collapsing Devices A number of collapsing surfaces are commonly used in the blown-film industry to optimize the process for different films.

FIGURE 3.3.9.  Collapsing theory.

Section 3.3.  Film Stabilization, Forming and Collapsing Systems

Wooden Slats The most common collapsing surface used for blown film is made of wooden slats (e.g., maple slats). The surface is usually made up of maple slats one to five inches wide, with slightly crowned faces, arranged in an inverted “V” at an angle between 15° and 20°. This style is most commonly found on LDPE, LLDPE, blends of LDPE/LLDPE, and HDPE film lines. The wooden slats also make a good collapsing surface on high-neck extrusion lines to produce materials like HMW-HDPE. To make a gusset in the film, similar wooden side frames are also commonly used. These units have a low heat-transfer coefficient and require little maintenance. A refinement to this collapsing method is the use of plastic covers that snap onto the wooden slats (Figure 3.3.10). Typically made of filled nylon or similar material, these covers offer the benefit of reduced friction over the wooden surface. Ideally these plastic covers are mounted upon aluminum extrusion heat-sink material (instead of wood) for more effective heat dissipation.

109

covered with heat-insulating materials such as cotton, felt, Velcro, or other substances. An improvement on aluminum is the use of lightweight, low-inertia, composite carbon fiber rollers. Additional problems can occur due to variation in the length of the line of contact of each roller with the film, as mentioned above, because different parts of the film are attempting to drive the rollers at different speeds and some scrubbing may occur. Frequently, this will result in undesired wrinkles and center sag. Segmented Roller Collapsers

Modern high line speeds or stiffer co-extruded films require more attention to the method of collapsing. This has led to the development of a variety of collapsing frames that incorporate a series of parallel rollers, each extending the width of the frame (Figure 3.3.11). These rollers collapse the bubble with significantly less friction than wood and are more suitable for IBC applications with higher line speeds. The frame can be trapezoidal, making the rollers at the bottom much shorter and easier to turn. Although an improvement over wood, this full-width collapser does have some disadvantages. If uncoated metal rollers are used, varying contact with the film can cause localized cooling, producing sagging bands and a poorly wound film roll configuration. To prevent differential cooling, these rollers are often

An alternative to the full-width roller is the segmented roller collapsing surface (Figure 3.3.12). This is made up of a series of short, small-diameter rollers rotating on tensioned steel rods within a structural frame. This method provides a substantially continuous surface in contact with the bubble, with the individual rollers free to rotate independently and maintain the same surface speed as the collapsing film. This speed can vary from section to section as required, effectively eliminating friction-induced sag in the film. Typically the width of the segmented roller section increases progressively and symmetrically about the centerline in the film direction so as to contact the increasingly large surface portion of the tube as it flattens between the converging frames. Ideally the rollers range in size from 12.5–25 mm diameter by 1.2 –5 cm long. These short, small-diameter, low-friction rollers have low inertia during startup and correspondingly low momentum when spinning. The preferred material for these rollers is Teflon because of its low friction, high temperature resistance, and low generation of static electricity. However, other materials such as nylon or polycarbonate can also be used. The use of segmented rollers leads to considerable improvement in the appearance of the flattened film, minimizing the wrinkles, center sag, and scratches often associated with a wooden

FIGURE 3.3.10.  Wood slat collapsers.

FIGURE 3.3.11.  Roller collapser—full width.

Roller Collapsers

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FIGURE 3.3.12.  Roller collapser—narrow segmented.

collapsing frame. However these segmented roller systems can become jammed with pieces of film or the buildup of additives that bloom from the film, ultimately stopping the rotation of the rollers and leading to scratching of the film. Spreader Roller Concept in Bubble Collapsing The collapsing frame with parallel rows of rollers perpendicular to the film direction of travel has proven beneficial for minimizing wrinkles during film collapsing. However, some films are still not as wrinkle-free as desired. To address this issue, the concept of spreader rollers has been introduced into the collapsing frame geometry. Figure 3.3.13 illustrates a segmented roller similar to the previous one. Note, however, that the rollers on the left-hand side of the longitudinal centerline have a raised helical contour on the outside diameter surface (comparable to a left-hand thread), which tends to move the tube at an outwardly inclined angle to the centerline as the tube moves upwards over the frame. Similarly, the rollers on the right-hand side of the centerline have a helical contour comparable to a right-hand-side thread, which again directs the film outward. This arrangement reduces the tendency of the collapsing tube to wrinkle, and these rollers can be retrofitted into existing perpendicular segmented roller frames. Figure 3.3.13 also shows the alternative arrangement, where rollers with smooth cylindrical surfaces on each side of the longitudinal center line are mounted on axles downwardly declined to the direction of travel of the film. This configuration has also proven effective in eliminating wrinkles. In some applications, particularly high modulus (stiff) films, brush rollers are used for bubble collapsing. The brush surface of these rollers is flexible and compressible, and they offer the benefit of a spreading action to reduce the incidence of wrinkles. One drawback of these rollers is that they can become contaminated with a buildup of blooming additives from the polymers, and they are also difficult to clean.

FIGURE 3.3.13.  U.S. Patent 5,912, 021, June 15, 1999—Marco Engineering and Technology Inc.

Air Collapsing Surfaces Another method of film tube collapsing is the air collapsing surface (Figure 3.3.14). In this design, the film is floated on a cushion of air with little or no friction in comparison to other collapsing surfaces discussed above. This system is made up of converging rectangular or trapezoidal structures, with the two opposing surfaces facing the film having a series of non-repeating holes through which air is forced. The air is supplied from a suitable blower, and the chambers are baffled to ensure uniform distribution of airflow. An additional benefit of air collapsing is enhanced cooling of the film, which can help alleviate blocking tendencies. Air collapsers are typically employed used on tacky cling films such as PVC and EVA and also on stretch films containing cling additives, which can quickly build up on wood or roller collapsing surfaces. The very low friction associated with this method of collapsing can result in undesired bubble instability because the bubble is essentially unrestrained and prone to wander. This can be partially overcome with the use of air collapsing side guides, which entrap the bubble, but may still allow the bubble to twist. Moving Collapsing Surfaces In the discussions of collapsing surfaces so far in this chapter, the bubble was the moving component, whereas the collapsing surfaces were stationary. Another type of collapsing has been tried experimentally in which two conveyors

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111

lapsers typically have one (or two) position adjustment(s) to make it easy to guide the film to the nip. In addition, two manual adjustments are located right below the nip on both sides to adjust the gap of the top face of the collapser. Similarly, the side stabilizers have two adjustments, both centered, one close to the top end, and the other located at the bottom end to contain the bubble sides. The adjustments of these opposing collapsing surfaces are typically motorized and synchronized to maintain an equal distance from the centerline. BUBBLE COLLAPSING IMPROVEMENTS Collapsing Problems

FIGURE 3.3.14.  Air side collapser.

are used instead of a stationary collapsing frame. The conveyors move and therefore aid in collapsing the film. This system was intended to reduce or eliminate center sag and stretching of the film during collapsing. However, it has not proved to be commercially successful. Collapser and Side Guide Adjustment During film collapsing, a uniform heat history is required to improve film quality, which means that side stabilizers should be used. These side stabilizers are often used with rollers and wooden collapsing surfaces. Figure 3.3.15 shows the points of adjustment for a conventional collapsing framethat is commonly used by film processors. The main col-

FIGURE 3.3.15.  Adjusting points on typical collapsers and side guides.

In the blown-film collapsing process, wrinkles and center sag in the tube can be minimized by choosing the right collapsing surface, modifying the collapsing geometry, and reducing frictional drag coefficients. In some other instances, irregularity in the gauge and melt temperatures arises from upstream components such as the extruder, die, or air ring. These irregularities can be improved by appropriate means such as an automatic air ring or die. The outcome of these modifications is to improve the flatness of the film, particularly for lamination or printing applications. Conventional Collapsing The inherent incompatibility in collapsing a tube of plastic material from a tubular (circular) cross section to a flattened form is shown in Figure 3.3.16, which shows a difference

FIGURE 3.3.16.  Conventional collapsing.

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FIGURE 3.3.17.  Web Path Differential.

in the length element when the bubble is collapsed. Figure 3.3.17 shows a typical example of a web path differential when a 113 cm-diameter bubble was collapsed to a layflat of 178 cm at a 25° collapsing angle. This figure shows that the film collapsing length at point A is longer than at point B. Bubble collapsing can be analyzed using a simple triangulation method from the circular cross section to a layflat at the nip, on the edges, and at the center of the film. Another method uses a graphic representation of collapsing length vs. angular position around the circular tube. The conventional collapsing shown in Figure 3.3.16 can be analyzed again using the triangulation method shown in Figure 3.3.18. This figure compares the collapsing path difference at the center and at the edge of the layflat. Figure 3.3.19 provides a plot of the percentage difference of the crease to the center of the film versus the length of the collapsing frame. As predicted in Figures 3.3.14–19, a 0.5 percent difference of crease versus center length can be obtained by using a collapser length of more than 4.25 times the bubble diameter. The results shown in Figure 3.3.16 can be analyzed in light of the explanation given above. The collapsing paths where the film travels a longer distance make the film stretch and get thinner, whereas the collapsing path where the film travels the shortest distance becomes wavy, or center sag appears in the film. For soft and easily extensible materials like LDPE or LLDPE, the film readily adapts to the changing geometry of the collapsing bubble and is only stretched at the center of the layflat. In the case of tacky materials such as plastomers or film produced from high-molecular-weight materials, the center of the film is stretched, and if relaxation occurs at the winder or during converting processes such as lamination or printing, center sag can appear in the film. Stiffer films or materials with high modulus such as HDPE, nylon, polyester, polycarbonate, polystyrene, rigid

 xD  X  (W  D)  2    D   2  ( xD  2 D)  4  0.2854 D  2   ( H 2  0.25D2 )0.5  ( H 2  0.08145 D2 )0.5  % DIFF .    100 ( H 2  0.08145 D2 )0.5   FIGURE 3.3.18.  Conventional Collapsing. Simple Triangulation Analysis.

FIGURE 3.3.19.  Plot of the percentage difference of the crease to the center of the film versus the length of the collapsing frame.

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113

PVC, EVOH, COC, PVDC, or coextruded films containing these materials are not extensible and hence resist stretching, particularly at the center of the film, which results in wrinkles with a short collapsing path (Figure 3.3.16). Therefore, in these cases, a longer collapsing frame is desirable to contain the bubble and provide a reduced angle of collapse. However, depending on the collapsing surface, this can lead to higher drag or friction, leading to non-uniform stresses and poor layflat quality. High friction can be reduced if lowinertia roller collapsing frames are used. The stiffness of a plastic material such as HDPE, PA, or EVOH can originate from its high crystallinity or because of a high glass transition temperature in materials such as cyclic olefin copolymers (COC) and polystyrene. In singlelayer structures, and particularly in coextruded structures containing EVOH or polyamide, a high frostline is preferred to keep the film warm to enhance flexibility during collapsing and to minimize the risk of wrinkles. Development of new materials and the trend towards multilayer film with combinations of different material properties are increasing. This makes it necessary to choose an appropriate collapsing system designed with the optimal collapsing geometry and contact surface to obtain stress-free uniform film.

tion to center the tube in relation to the heaters for uniform heating. The tube is usually injected with compressed air to make a bubble. An air ring is used to stabilize and cool the bubble. The tube is collapsed to a flat film using a full-width roller collapsing frame similar to the one described in the previous section. This method is used to provide bi-axially oriented single- and multilayer shrink films. In single-layer applications, polyolefins are the main materials, whereas in multilayer structures for long-shelf-life food packaging applications, a barrier material such as EVOH, nylon, or PVDC is coextruded with the polyolefins.

TUBE CONTAINMENT IN HIGH COOLING APPLICATIONS

[1] Stobie, J., “New Developments in Blown Film Bubble Collapsing,” 1995 Macro Letter, Volume 1, Issue 1. [2] Knittel, R.R., “Film Stabilization, Forming, and Collapsing Systems,” 1992 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 311. [3] Bode, W.W., “Current developments in equipment for processing high-molecular-weight high-density polyethylene film,” Proceedings, 1989 TAPPI Polymers, Laminations and Coating Conference, A TAPPI PRESS Anthology of Published Papers 1986–1991, Bentley, D.J. Jr., Ed., p. 312. [4] Krycki, B., “Better bubble management,” Proceedings, 1999 Film Conference, Somerset, NJ, p. 229. [5] Wheeler, A., “Options in haul-off technology,” 1999 Proceedings, 1999 Film Conference, , Somerset, NJ, p. 149. [6] Nunes, D., “Putting the ‘swing’ into blown film lines,” Proceedings, 1999 Film Conference, , Somerset, NJ, p. 33. [7] Knittel, R.R., “Blown film bubble collapsing improvement,” Proceedings, 1986 Polymers, Laminations and Coating Conference, TAPPI PRESS, Atlanta, p. 95 [8] Wright, W.D., “Low-friction collapsing methods for blown film,” Proceedings, 1985 Film Extrusion Conference, TAPPI PRESS, Atlanta, p. 97. [9] Campbell, G.A., et al., “Blown film modeling: Die to frostline,” Proceedings, 1986 Polymers, Laminations and Coating Conference, TAPPI PRESS, Atlanta, p. 247. [10] Sudo, M., et al., “A new cross-oriented polyolefin tubular film,” Proceedings, 1979 Paper Synthetic Conference, TAPPI PRESS, Atlanta, p. 285. [11] Plammer, A., “Monoaxially oriented polyolefin films,” Proceedings, 1979 Paper Synthetic Conference, TAPPI PRESS, Atlanta, p. 167. [12] Knittel, R.R., “HDPE film by blown process,” Proceedings, 1979 Paper Synthetic Conference, TAPPI PRESS, Atlanta, p. 137. [13] Schwarz, E.C.A., “New process for high-strength bags from orient...,” Proceedings, 1977 Paper Synthetics Conference, TAPPI PRESS, Atlanta, p. 90.

Tube Guides and Collapsing Frames In techniques involving biaxial orientation of polymers, as opposed to the conventional blown film process, the films are fabricated with superior tensile, optical, barrier, and/or shrink properties. In this method, the melt tube exiting from the annular die is brought into direct contact with water, either by immersing it in a cold water tank or by spraying cold water to quench the melt. The plastic tube so formed should be contained properly before the tube is nipped. The tube is sometimes filled with a liquid in the form of emulsion or suspension to avoid blocking, and the liquid is circulated to maintain the desired temperature. In other cases, dry powder such as starch is sprayed inside the tube to prevent blocking. In this process, the melt tube is contained with low-friction guide rolls, usually made of Teflon or nylon or some other suitable material. In a number of cases, the melt tube is collapsed with a collapsing frame using segmented or fullwidth coated rollers. In other applications, both a tube guide and collapser are used. Iris, Air Rings, and Collapsing Frames A typical method to provide biaxial orientation is to extrude the melt downwards and quench it to form a primary tube. This collapsed tube is conveyed to a subsequent process where it passes through a ring of heaters to condition the film for subsequent inflation and machine direction stretching (biaxial orientation). Bubble stabilizers, which were discussed in a previous section, are important equipment for this applica-

SUMMARY This chapter has described the different methods of bubble containment and different designs of collapsing frames as supplied by various machinery manufacturers. Some examples are given to provide insight into selecting the right collapser. When selecting a particular method of collapsing, the film manufacturer must address the processing characteristics and end-use requirements of the single- or multilayer film. REFERENCES AND ADDITIONAL RESOURCES

Chapter 3—Section 4

Material Handling Systems CLIFFORD J. WEINPEL and WALTER FOLKL, Foremost Machine Builders

INTRODUCTION Industrial bulk material handling generally centers on moving pelletized, granulated, powdered, or flaked material from storage to process. The methods for accomplishing this task can be broken down into two basic categories, mechanical and pneumatic conveying. Beyond this, the choices become somewhat more numerous and complex. Mechanical conveyors include belt conveyors, screw conveyors, vibrating conveyors, drag conveyors, and other methods. For the purposes of this chapter, it is fair to say that mechanical conveyors are most used outside the plastics industry. There are several reasons why pneumatic conveying systems for pellet, granular, powder, and flake plastic resins are used almost exclusively in the plastics industry, but three of these are key. First, pneumatic conveying systems are relatively economical to install and operate; second, they are relatively clean-running and simple to maintain, and third, they are flexible in terms of rerouting and expansion. These systems can be classified into five basic categories (shown in Table 3.4.1), and of these, the most widely used application solution for conveying plastic pellets is the dilute phase system. The most economical way to accept delivery of plastic resin is in bulk quantities. Bulk shipment is usually by railcar or bulk truck, with railcar generally being the least costly. Many film extruders have access to a rail siding and take advantage of the cost savings associated with railcar delivery of resin. This is true even though the initial investment in a system to unload and store resin delivered by rail is higher than for resin delivered by truck. This chapter will focus on the systems required for pneumatic conveying, storage, and in-plant distribution of plastic pellets when delivered by truck or railcar. BULK UNLOADING AND STORAGE Most bulk trucks today are equipped with self-contained, pressure-type, pneumatic unloaders. For the processor, this means that the only equipment required is a bulk storage silo or multiple silos properly equipped to accept bulk truck de-

liveries, as shown in Figure 3.4.1. The silo should be located as close as possible to both the building and the access area for the bulk trucks. Bulk trucks are usually operated by a driver who has the responsibility for the unloading process after the processor accepts delivery and indicates the storage silo to be loaded. Delivery by railcar requires additional equipment and manpower. A railcar does not come with its own unloader or the personnel necessary to accomplish the material transfer from railcar to storage silo. Bulk unloading of railcars is most often done with one of two basic types of dilute phase material handling systems, known as vacuum (pull) systems (Figure 3.4.2) or combination (pull-push) systems (Figure 3.4.3). The most frequently used of these is the combination (pull-push) system that draws the material out of the railcar by vacuum, passes it through a transfer station, and blows it into the silo by pressure. The pull-push system is used most frequently because, for most applications, it is not possible or practical to attach a pressure system to a railcar, and a vacuum system is not efficient for the distances, elevations, and material transfer rates required for moving material from railcar to storage silo. Based on the equivalent distances involved from railcar to silo and the required transfer rate, a pull-push system may include one positive displacement blower package using both the vacuum and pressure sides of the pump or two blower packages, one for the vacuum side of the system and one for the pressure side. A typical singlepump railcar unloader specification is given in Addendum A. Typical bulk unloading systems with normal accessories are illustrated in Figure 3.4.4 and Figure 3.4.5. Manpower requirements for railcar unloading are not extensive, but an operator is required on at least a part-time basis. The operator must be trained in the proper methods for hooking up the railcar to the unloader and in the operation, troubleshooting, and possibly maintenance of the unloader. Silos are an integral part of the material handling system. Besides storing bulk resin, they are the link between the bulk material delivery system and the material distribution system. As such, they need to be equipped with components that enable both systems to function properly. Stor115

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TABLE 3.4.1.  Classification of Pneumatic Conveyor Systems (Source: Bulk Material Handling Handbook, by Jacob Frutchbaum, 1988). Dense Phase

Dilute Phase Medium-Dense Phase

Dense Phase

Air-Activated Gravity Conveyor

System

± 20 in. H2O

± 7 psi

15–35 psi

30–125 psi

Airslide

Pressure Range

vac: 10–30 pres: 4.5–13

vac: 3–5 pres: 1.0–3.5

0.35–0.75

0.1–0.35

Fan type 0.5 psi (closed) 4–5 psi (open)

45–18

135–45

3–5 cfm/ft2

Saturation Ft3 air/lb mat'l

vac: 1.3–0.45 vac: 4.5–2.5 pres: 4.5–13.0 pres: 13.0–3.8

Mtt'l Loading Lb mat'l/lb air

6000

4000–8000

1500–3000

200–2000

–

Air Velocity (fpm)

100

300

300

400

10 through diaphragm

vac: 100 pres: 200

vac: 200 pres: 500

3000

8000

500

Max Capacity (tph) Practical Distance Limits (ft)

age silos are available in a variety of capacities and types of construction. In the plastics industry, most silos are made from carbon steel, aluminum, or stainless steel and are of welded, bolted, or spiral construction. Arguably, the most prevalent silo supplied to the plastics industry is the 12-foot diameter welded carbon-steel silo in different heights and corresponding capacities. This choice is usually made based on value and availability. Generally, carbon steel, interior epoxy-coated silos are suitable for most plastics. Welded, 12 ft. diameter silos can be manufactured at the provider’s

100 ft 6 ft drop/length 3–10 deg. slope

facility and shipped over-the-road to most locations, making them readily available. The combination yields a better overall cost-to-performance ratio than other options under most circumstances. All silos can be equipped with several accessories, some of which are necessary and others desirable. A typical silo specification with accessories is given in Addendum “B.” Note that all silos are engineered and certified to match the material to be stored and the geographic seismic location in which they are to be installed. A typical seismic location map is illustrated in Figure 3.4.6.

FIGURE3.4.1.  Pressure delivery by bulk truck—Bulk trucks are commonly equipped with a pressure delivery system. A driver or operator will couple to a standard load-line fitting and empty the truck into one or more silos. Conveying air is vented to atmosphere and in many cases filtered through a silo-mounted static or self-cleaning filter assembly.

Section 3.4.  Material Handling Systems

FIGURE 3.4.2.  Vacuum (pull) delivery from railcar—Material is drawn from each compartment of a railcar using a vacuum pump and a material vacuum receiver(s). Receivers can be “pump and dump” style, with gravity-operated discharge valves that periodically open when vacuum is removed and close when vacuum is applied, or “continuous loading” style, which uses a rotary valve for discharge.

FIGURE 3.4.3.  Combination vacuum/pressure (pull/push) delivery from railcar—Material is drawn from the railcar by the vacuum side of a pump assembly and transported by vacuum to a transfer station, where it is dropped out of the conveying air stream, re-entrained in a pressurized air stream, and conveyed by pressure to silos. The transfer station can be configured as a self-cleaning inline filtered receiver or a cyclone receiver with offline air filtration.

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FIGURE 3.4.4.  Typical vacuum system for railcar unloading.

FIGURE 3.4.5.  Typical bulk system with truck delivery (left) and railcar delivery using combination vacuum/pressure unloader (right).

Section 3.4.  Material Handling Systems

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FIGURE 3.4.6.  Typical seismic zone map—Silos are engineered and fabricated for the specific zone in which they are to be installed. Installed silos should not be relocated without first identifying that the destination is in an equal or lesser-rated zone.

IN-PLANT DISTRIBUTION SYSTEMS Material handling assumes different dimensions for inplant distribution systems. Resin stored in outdoor silos must be brought inside the facility and distributed to the extrusion equipment. In some cases, this is a direct transfer from silo(s) to extruder hopper(s). In others, the material must pass through several intermediate pieces of auxiliary equipment before reaching the extruder hopper. A typical system may have material first transferred to inside storage bins, through metal separators and/or dedusters, on to dehumidifying or hot-air hopper dryers, then to blenders, and finally to the extruders. During this typical pre-processing route, the material must be conveyed in a way that all the necessary pre-processing functions are adequately performed. In addition, the amount of mechanical degradation that the resin experiences in the form of fines, stringers, and dust formation must be minimized, and the material must not be contaminated or cause environmental problems. To properly specify an in-plant material distribution system, a complete study needs to be made of the process material requirements. At a minimum, one needs to know the following: (1) Material to be conveyed (2) Pre-processing operations necessary to meet plant production requirements (3) Degree of control required to meet operational requirements.

Most material handling equipment and systems vendors need detailed information about the system configuration and parameters to select the correct type and size of equipment to be used. This is typically communicated through plant layouts, information surveys (see Addendum “C” for a typical survey), and facility visits by systems engineers. Material transfer from silos to in-plant equipment is accomplished with pressure, vacuum, or combination-type systems. In practice, pressure or combination systems are rarely required or used for in-plant plastic pellet distribution systems. Vacuum systems are used almost exclusively, primarily because: (1) Most distribution systems are multiple-supply/multipledestination, which are best handled using vacuum conveying solutions. (2) Vacuum systems generally require less capital investment than pressure or combination-type systems. (3) Vacuum systems are inherently cleaner-operating than pressure systems because any small leaks in the system integrity draw clean air into the system rather than forcing dust-laden conveying air into the plant environment. To remove plastic resin from a silo, a distribution box or vacuum tray adapter (VTA) is installed at the discharge of the silo. This VTA enables one or more distribution lines to be connected to the silo and brought into the facility. In the simplest system, the material supply line is routed past each extruder that will use material from the silo. In practice, a careful analysis of the present and future plant layout is nec-

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essary to determine which of many possible routing schemes will be used for the distribution piping. Storage bins are frequently located inside the facility to provide an in-plant surge capacity for the distribution system. This is especially true if the plant location is such that the outside environment is subject to large seasonal variations in temperature and the extrusion process requires material at a relatively consistent temperature for process stability. Inside storage bins of enough volume, commonly called day bins, allow resin temperatures to stabilize before processing. Inside storage bins are also necessary if the system distances and throughput requirements would require the system conveying lines and pump sizes to be prohibitively large if conveying were direct from silo to destination. The inside bins are located between the silos and the distribution system and are typically loaded by a conveying system separate from the in-plant distribution system. Accessories for inside surge bins are like those for silos. Addendum D gives a specification for a typical indoor storage bin. The pivotal piece of equipment in a resin handling system is the power unit, frequently referred to as the blower package, vacuum loader, fan package, or turbo blower. Regardless of the terminology, this is the motive force producer for the dilute-phase pneumatic conveying system. Two basic types of power units are commonly used for resin conveying systems: the positive displacement blower, and the centrifugal or regenerative blower. Positive displacement blowers are used in both vacuum- and pressure-conveying systems. Centrifugal blowers are used most often in lower-throughput vacuum-conveying systems. In centralized system applications, positive displacement blower packages are used almost exclusively. This is because of their ability to produce the required volume of conveying air, measured in cubic feet per minute (CFM), at high pressures, measured in pounds per square inch (PSI), or vacuum, measured in inches of mercury (IN-HG). These operating characteristics are needed for the relatively high material throughputs and longer total conveying distances common in a centralized system. Manufacturers design systems with positive displacement blower packages sized to handle the conveying parameters of the system. The material to be conveyed, the distance to convey it, pipe sizes, the number of pipe bends, vertical lifts, allowable material velocities, and energy losses are all included in the calculations used to size the blower packages. Because there are no defined sets of equations for mixed gas/solids flow, these factors are used in empirically derived charts and tables that each manufacturer has developed through testing and experience for different materials and system configurations. A typical capacity graph for vacuum conveying system selection is shown in Figure 3.4.7. Processors normally provide the manufacturer with the material specification, the conveying distances, and the throughput requirements as part of the plant survey. The manufacturer performs all the necessary calculations and sizes the power units to meet the required operating parameters. Whenever a vacuum conveying system is used for material transfer, a vacuum receiver is required at each dropoff

FIGURE 3.4.7.  Typical pneumatic conveying capacity vs. distance graph for various size vacuum pumps—Equivalent distance is the actual conveying distance factored to allow for parameters such as vertical rise and changes in direction of piping runs. Graphs of this type are helpful in preliminary sizing of pneumatic conveying systems with common parameters.

or destination point. Figure 3.4.8 illustrates two vacuum receivers configured for different materials and conveying conditions. The basic vacuum receiver for plastic pellets is a simple device that should require little or no periodic maintenance. This is the most prevalent type of receiver used in the plastics industry and is all that is necessary for most systems. Vacuum receivers come in a range of sizes that are used by system suppliers to match the conveying system parameters. When the material being conveyed is heavily dust-laden or is a granular or powdered material, the vacuum receiver is equipped with optional features to minimize the number of particles carried back to the central pump protection filter. At times, equipment and systems vendors provide filters inside vacuum receivers to keep dust, granules, or powder in the process flow. The degree of filtration required is based on the degree of fine particulates being conveyed. Larger amounts of fine particulates require a larger filter area per volume of conveying air. This is expressed as an air-to-cloth ratio in cfm/ft3. Dust-laden and granular materials such as linear low-density polyethylene (LLDPE) rarely require filtration in vacuum receivers, whereas powdered resins such as polyvinyl chloride (PVC) frequently require filters with air-to-cloth ratios of 10:1 or more. The processor should be careful when selecting vacuum conveying equipment that uses vacuum receivers equipped with filters. These are generally much more difficult and costlier both to operate and to

Section 3.4.  Material Handling Systems

121

inch pipe and to add an additional five feet for every inch of pipe diameter increase. A five-inch diameter conveying line would require 5 + 5 + 5 + 5 = 20 feet of horizontal run before each elbow. (3) Angled rises should not be used unless necessary.

FIGURE 3.4.8.  Typical vacuum receivers—A powder receiver is normally equipped with integral, self-cleaning filtration, which is also a good choice for materials laden with dust and fines. A pellet receiver is normally equipped with a coarse pellet screen, which is usually more than adequate for most plastic pellets.

maintain. They should be used only after a thorough investigation of alternatives. Although it is common for material drawn from a silo or an inside storage bin to be delivered directly to a process machine hopper, it is equally common that the material will be subject to pre-processing before delivery to the processing machine. In many cases, several steps are needed through different pieces of auxiliary equipment before the resin is conditioned for processing. In designing a resin handling system, all these actual or potential intermediate steps should be considered in the system design. In every case, a thorough early analysis will produce a system design that both functions better and costs less in the short and long term. PIPING SYSTEMS Piping for dilute-phase pneumatic conveying systems involves two major concerns, design and construction. Piping system design requires careful consideration of factors that can doom a system to failure or marginally acceptable operation. Some basic rules that should be strictly adhered to are: (1) All runs should be direct between material pickup and dropoff with the least possible number of direction and elevation changes. (2) A straight horizontal run of at least five to six feet should be present before every direction change (elbow). This run should be progressively longer for larger pipe sizes. A good working rule is to allow a five-foot run for two-

Construction of a piping system generally involves consideration of both the type and material of pipe to be used. Most pneumatic conveying tubing is made of aluminum because of its light weight and corrosion resistance. Thinwall stainless-steel tubing and schedules 10 and 40 aluminum pipes are used less frequently and at obviously higher cost. A frequently used combination is aluminum thin-wall tubing for low-wear straight runs with thin-wall stainlesssteel elbows for high-wear bends. This combination is both durable and cost-effective. Although not frequently used in contemporary systems, galvanized carbon steel (EMT) is sometimes used in smaller line sizes of 1-1/4 inch and 2 inch (note that these are inside diameters, not outside diameters as with aluminum tubing). EMT is slightly less expensive installed than aluminum thin-wall tubing. It is also heavier and more prone to corrosion, especially when used outdoors. Tubing lengths and elbows are normally connected by galvanized or stainless steel, gasketed, compression-type, three- or four-bolt couplings. These should always have a grounding strap included because the gasket is usually an insulator. Isolated sections of conveying tubing can build up a substantial and potentially dangerous static charge from the conveying process. Most plastic pellets in a dilute-phase system are conveyed at pickup velocities of 4,000–4,500 feet per minute. This is because the dropout or saltation velocity of most plastic pellets is around 3,500 fpm. This is the point where the pellets are no longer conveyed by the air stream and either pile up at the bottom of a vertical rise or fall to the bottom of a horizontal run. When this occurs, the line frequently “plugs” and must be manually cleared before conveying can be resumed. On the other hand, when some plastic pellets such as polypropylene (PP) and low-density polyethylene (LDPE) are conveyed at these velocities, the heat generated from the plastic pellets sliding along the length of the pipes and elbows creates long thin pieces of plastic. These are commonly known as streamers. Streamers can clog parts of the system, especially when they ball up to form a “bird’s nest.” Unfortunately, the nature of dilute-phase conveying is such that conveying velocities increase with the distance conveyed due to the expansion of the compressed conveying air as its pressure drops as it moves to the terminal point in the run. At times, the velocity near the end of a run can be in excess of 7,000 fpm. A considerable amount of research has been done to solve or minimize the problem of stringer formation. Consequently, three types of treated pipe and at least two variations of blind tee, pocketed elbows are presently available as solutions. The treated pipes have their internal surface roughened by sandblasting, directional shot peening, or spiral grooving. Performance tests have shown reductions of up to

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70% in stringer formation with these treatments. Pocketed elbows create a sharp directional change, unlike the gradual sweeping flow of a long-radius elbow. The pocket forms an area for incoming pellets to impinge on pellets rather than the back wall of the elbow. Performance tests have shown reductions of over 90% in stringer formation in systems with these elbows. All these products have significant associated costs. These costs are for both the materials themselves and the need to upsize the conveying equipment to offset the lower conveying efficiency experienced when they are used. CONTROLS Bulk unloading and storage systems and in-plant distribution systems require control systems and wiring that, taken together, can constitute 50% of a system’s capital investment. The degree of system control, integration, and automation, the type of operator interface, and the openness of the control system architecture all play a part in determining the design and costs of a control system, as does the need to conform to local and company electrical standards. At the least, all control systems should comply with the National Electric Code and Fire Protection Safety standards. Selection of control systems should take into consideration the ready availability of replacement parts, the type of training required and available for maintaining the hardware and software, and the availability of service technicians for troubleshooting the system. System controls can be classified as manual, semiautomatic, or fully automatic. In practice, most control systems are semiautomatic. Manual systems, although inherently low-cost, are not justifiable in terms of the ongoing labor cost required. Fully automatic control systems, on the other hand, are not only prohibitively costly, but also remove the ability for necessary operator decision-making. A typical semiautomatic system for railcar unloading would function with the operator connecting the material vacuum line to a railcar compartment and the material pressure fill line to a manual switch station. Once the line is connected properly, the operator would turn on the pull-push pump and initiate an unloading sequenced startup, usually by pushbutton. The control system would then begin a sequenced startup of the system components. If a high-level indicator in the silo signaled a full condition or a low-vacuum sensor signaled an empty compartment, the unit would initiate an automatic controlled shutdown sequence and sound or flash an alarm to alert the operator to reset the material source or destination. Additional automation might include electrically operated diverter valves to select which silo is to be filled, dirty pump inlet filter sensors and warning or shutdown sequences, a graphics display interface to show the operating parameters of system components, and an alphanumeric display interface with operating and fault messages. Systems control integration can range from standalone system logic where each component is controlled discretely to a fully integrated system where all components are controlled by a central system. Discrete controls generally range

from simple electromechanical starters, relays, and switches through proprietary, microprocessor-based, single-board controllers to fully programmable, PLC-based panels. Integrated controls for pneumatic conveying systems are usually programmable logic controller (PLC) or personnel computer (PC) based systems. In addition to controlling the conveying equipment, these integrated systems frequently control all the material pre-processing functions and equipment in the system. A PLC- or PC-based, fully integrated system that does this is usually the most economical control system from both capital and operating standpoints due to the elimination of redundant control components and a level of control that eliminates most unnecessary operator interaction with the equipment. Openness of systems architecture becomes a critical decision factor when specifying system control. Ideally, all control hardware should be off-the-shelf, national brand components. Where software is necessary for systems operation, this should be included as part of the systems documentation. Generally, PLC-based systems will provide the processor with the highest level of “openness.” Programs for PLC- and PC-based systems should be written with well-known, highlevel manufacturer’s or third-party programming software. They should be provided with adequate commenting to permit the processor’s personnel to follow the program flow for troubleshooting or subsequent enhancements and changes. GRAVIMETRIC METERING AND BLENDING Gravimetric metering and blending have become the accepted method for introducing two or more materials into the extrusion process. The transition from volumetric to gravimetric equipment began in the late 1960s. Since then, it has been well established that the advantages of gravimetric technology over volumetric are easily cost-justified. Today, specifying volumetric metering and blending for a film or sheet extrusion line is a rare occurrence. There remains a need to approach metering and blending as part of a decision-making process. Where earlier the choice was to determine whether volumetric or gravimetric equipment was appropriate for the task, now the decision process revolves around selecting the right type, configuration, and features of a gravimetric system to specify for a process line. What is commonly referred to as a gravimetric blender is a combination of two elements. The first is a gravimetric metering element that determines the proportions of different ingredients supplied to the extruder. The second is a blending or mixing element that determines the homogeneity of the mix of those ingredients. The types of gravimetric metering elements that are normally applicable to the film and sheet extrusion process are: gain in weight batch, loss weight-target weight, loss weight-target rate, and additive proportioning feed throat. The types of applicable blending elements normally associated with these types of metering technology are passive (static), active (dynamic), or none. All types of metering and blending elements come in different styles based on the requirements of the application

Section 3.4.  Material Handling Systems

and the suppliers providing the equipment. However, differences in gravimetric metering technology generally drive the choices in blending. There are no hard and fast rules regarding the combination of metering and blending elements. Most suppliers provide standard metering and blending packages that integrate the two elements based on prevalent industry applications with custom packages or individual elements available for unconventional applications. Three basic configurations exist for gravimetric metering and blending systems: extruder throat-mounted, mezzaninemounted, and floor-mounted. Variations exist of the basic configurations that yield several possible arrangements of the metering and blending elements. Gravimetric metering and blending systems that include scrap regrind in the mix of ingredients to be processed require additional consideration. Scrap regrind is typically a combination of online recycle in the form of trims and bleeds and offline recycle in the form of roll and loose scrap. Film or sheet regrind has a bulk density normally one-tenth to one-half that of the other ingredients being fed into the process. Therefore, the blending element of the system must accommodate a broad difference in material properties. Scale Basics Regardless of type, gravimetric metering and proportioning equipment generally used in film or sheet extrusion shares certain common features and elements. All incorporate one or more weighing elements, all use material storage hoppers, and all have feeders or feed control devices. The most common weighing element used on today’s

123

gravimetric metering equipment is the strain gauge load cell. Significantly less common are load-sensing systems based on vibrating wires, linear voltage differential transformers (LVDT), or piezoelectric crystals. These alternative loadsensing technologies have advantages over load cells in different environments and applications. Most weighing applications in film and sheet extrusion, however, do not derive a major benefit from using any of these options. Load cells are commercially available from several manufactures and are designed in several types, styles, accuracy classes, and load capacities. Two of the most common types are the cantilever and the “S” beam (Figure 3.4.9). Depending on the style, a load cell can be used in tension, where the applied load is suspended from the cell, or in compression, where the load is placed on top of the cell. The function of a load cell is the same whether used in tension or compression. As a load is applied to or removed from the cell, strain gauges bonded to the cell, which are “excited” by a regulated DC voltage applied to the cell, change resistance. The gauges are configured as a Whetstone bridge, and their change in resistance causes a change in the voltage measured across the output side of the cell. This output voltage varies proportionately with the applied load. Typically, the output voltage from a load cell ranges from zero at no load to thirty to forty-five millivolts at full load. The motion of the load cell itself between zero and maximum allowable load is only a few thousandths of an inch, which is well within the elastic limit of the cell design. The load cell output, an analog voltage in the range of 0–45 mV, is a low-level voltage signal and as such is susceptible to external electronic “noise”. To limit the effects

FIGURE 3.4.9.  Typical strain gauge load cells—A shear beam load cell (right) is normally used with the weight applied from above on one end and the cell fastened to support from the bottom at the opposite end. An “S” beam load cell is normally used in tension, with the cell fastened to support from the top and the weight applied from the bottom; however, it can also be used in compression with the support and load reversed.

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of noise on the control system, this millivolt signal is either boosted to a higher analog range, typically 0–10 volts DC, converted to an analog milliamp signal, typically 4–20 mA, or digitized. This is done as close to the load cell as possible. If the signal remains in the analog domain during transmission, it is digitized later. During conversion from the analog to the digital domain, the signal is resolved. Resolving the analog signal divides it into digital “bits”. Each of these digital bits represents a portion of the applied load and is proportional to that load. An 8-bit A/D (analog-to-digital) converter can resolve a signal into 28 = 256 divisions. A 12-bit A/D converter provides 212 = 4,096 divisions of resolution. Likewise, a 16-bit A/D yields 216 = 65,536 divisions, and a 20-bit A/D yields 220 = 1,048,576 divisions. It can be seen that a 16-bit A/D converter provides 16 times the resolution of a 12-bit converter. As will be discussed in greater depth later in the presentation, this difference becomes important when considering the sensitivity and responsiveness of gravimetric metering equipment. Metering There are numerous metering devices in use to control the proportions of ingredients in a mix. In the plastics industry, the most common devices in use today for pellet, flake, fluff, and most free-flowing powder are gates and valves, vibratory pan feeders, and auger feeders. Slide Gates and Valves are simple flow control devices. They are generally air- or electrically operated, and most often function in a full open/full closed mode. In some cases, they can be made to function in a staged or dithered mode to provide finer flow control. They are most often used as gravity flow devices, where the material passing through does not change direction from a substantially vertical flow path. Because they rely substantially on the flowability of the material for uniform flow rates, they are used primarily as a low-cost flow control device for gain in weight-batch type metering systems. Vibratory Pan Feeders are also simple flow control devices. They are electrically operated and most often are operated as variable flow rate devices where the amount of material being fed at any point in time is proportional to a variable input signal. In comparison to auger feeders, vibratory feeders are simpler devices with fewer moving parts. They tend to be more durable, cleanable, maintainable, and cost-effective than augers. They generally have a greater turndown ratio than auger feeders. However, they are significantly less linear in their input/output relationship at the lower and upper extremes of their operating range, and they exhibit a hysteresis effect at their lower threshold. For this reason, their normal operating range should be centered and limited to a 10:1 turndown. These feeders are generally used on gain in weight-batch, loss weight-target weight, and loss weight-target rate systems. Because additive proportioningfeed throat systems normally require the additive materials to be fed into a head of gravity-fed materials, vibratory feeders are rarely if ever used in this type of system.

Auger feeders are mechanically the most complex of the three cited flow control devices. They are electrically operated and, like vibratory pan feeders, are most often operated as variable flow rate devices. Auger feeders tend to be less sensitive than vibratory feeders to the effects of the materials being fed on controllability and linearity. They therefore require less complex control algorithms and exhibit a quicker response profile. The turndown range of most auger feeders is in the 10:1 to 15:1 range, but this is usually extendable through a simple change of feed screw. Within the operating range of the weighing element, this capability allows a broader operating range for a metering station equipped with an auger feeder than for a similar station equipped with a vibratory pan feeder. Auger feeders are less susceptible to feed rate variations attributable to supply voltage fluctuations or material head pressure on the feeder. This can be particularly important in loss weight-target rate systems. On a station with a relatively low feed rate, a considerably long time can elapse before a corrective weight-loss signal can be supplied to a metering device, sometimes on the order of minutes. Under these circumstances, it is necessary for the feeder to be a stable volumetric device that can reliably hold a feed rate between corrective feedback signals. Moreover, the more linear the feeder input/output relationship, the less corrective feedback will be required over time. Auger feeders are used in gain in weight-batch, loss weight-target weight, loss weight-target rate and almost exclusively in additive proportioning-feed throat metering systems. Blending/Mixing Blending technology relies on two parameters: intensity and duration. All blenders or mixers, whether passive (static) or active (dynamic), function by subjecting two or more ingredients to a level of agitation for a certain time period. Passive mixing devices accomplish this by controlling the design of the mixing elements to determine intensity and the mixer column height to determine duration. Both parameters are essentially constants for a passive mixer and are not easily changed. Active mixing devices control intensity through mixing element design and the ability to vary speed. Active mixing duration is controlled by gating the mixer discharge to produce intermittent discharge or discharge flow control. Both parameters are variables for an active mixer and are easy to change. Passive mixing is essentially a two-dimensional process, whereas active mixing is three-dimensional. The advantages of active mixing over passive are controllability and versatility. The advantages of passive mixing over active are cost, durability and maintainability. Passive (static) mixers have been commonly used as either liquid or dry bulk blenders in the plastics industry for decades. When used to blend plastic resins, they are usually configured as cascading, baffled, gravity drop chutes for free-falling materials and stream splitting mixers for plug-flowing materials (Figure 3.4.10). These mixers accept multiple ingredients, accurately proportioned by multiple feeders above, and through the action of their mixing baffles

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FIGURE 3.4.11.  Typical dynamic mixer.

FIGURE 3.4.10.  Typical static mixer.

or elements, homogenize these ingredients somewhat. Generally, the mixing action is performed only in the directions perpendicular to the material flow direction. Therefore, variations in the “short-term” proportionality of the ingredients being fed are not averaged through mechanical back blending. Passive mixers of these types are used under the design assumption that the materials being mixed require little if any mixing in the direction of material flow. This assumption works best if: (1) the materials are accurately metered on a relatively short-term basis, (2) once mixed, the materials are not subject to conditions that would cause de-mixing, and (3) the extrusion process provides adequate mixing to smooth out short-term proportionality variations. Both loss weight-target rate and additive proportioning-feed throat metering elements are easily configured to satisfy the first two conditions. In fact, additive proportioning-feed throat metering is often provided without a blending element before the extruder. Active (dynamic) mixers (Figure 3.4.11) have also been used for decades. Generally, gain in weight-batch systems always use active mixers because the batch method of metering is a sequential process that produces a very accurate proportion of ingredients that is stratified before mixing. This requires the intensity of an active mixer to produce

an adequate blend. Loss weight-target weight systems also use active mixers. As with a target rate system, the metering process is simultaneous and proportional. However, one of the main advantages of a target weight system is the combination of highly accurate ingredient proportionality with a homogeneous blend. This combination is achieved through the batching action and the intensity of an active mixer. Centralized, floor-mounted, or mezzanine-mounted gravimetric proportioning units, configured to provide ingredient blends of different constituents and proportions to multiple extruders, frequently use active mixers above the throat of each extruder. This serves the dual purpose of assuring an adequate blending of ingredients after conveying to each extruder and providing an active surge hopper above each machine. Packaged Gravimetric Metering and Blending Systems Packaged metering and blending units are provided by many suppliers for use in film and sheet extrusion. These packages provide frequently combined metering and blending elements as single, integrated units. Except for unusual ingredient blends, custom configuration requirements, or unique operating parameters, these packaged units generally represent the best selection for most processors. Gain in Weight (Batch Weigh) blenders have been used in the plastics industry for over 30 years. Primarily used in the injection and blow molding branches of the industry, they have been frequently and successfully applied to film and sheet extrusion applications. Most batch blenders (Figure 3.4.12) have one weigh hopper and a load cell fed by multiple metering devices. Each

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in which batch-to-batch inaccuracies are smoothed. Because a batching blender relies on a series of weighings to finalize a batch formulation, they are always at least one batch size away from a response to a change in formula. This mixed material in inventory can represent a processing problem if frequent changes in formulation are required to “tune-in” a product and the batch size is relatively large with respect to the processing rate. Loss in Weight (Target Weight) blenders, functioning in a target weight mode, have been used for film and sheet extrusion for over 25 years. Designed for a combination of high-accuracy metering and homogeneous ingredient blending, they provide these features at the higher processing rates typical of the extrusion process. A typical target-weight loss-weight blender (Figure 3.4.13) has each metering feeder mounted to an independent, load cell-supported, weighing hopper. In some designs, the metering feeder is separated (de-coupled) from the weighing hopper to lessen the amount of “dead load” on the load cell. The purpose for this is to improve the resolution of the live load being weighed. This can be important when small amounts of a material are required in a formulation. However, de-coupling adds complexity to the system, is sensitive to poor material flow properties, and can cause errors in blend proportions if the principles of operation are not properly understood and followed. For these reasons, most loss weight feeder stations supplied to the industry use a feeder coupled to the weigh hopper. Each weigh hopper measures the weight of material leaving it. The weighing system is programmed for simultaneous and proportional metering of

FIGURE 3.4.12.  Typical gain in weight (batch blender)—Multiple feeders sequentially fill a weigh hopper to form a batch. The batch is blended in a downstream or integral mixer.

metering device feeds sequentially in turn into the weigh hopper. Normally, this is done using a bulk flow or highspeed method. Either a self-tuning pre-act that adjusts the feeder shutoff or a self-adjusting rampdown to slow (dribble) speed is frequently used to ensure that the precise ingredient setpoint is reached. If one or more ingredients will be less than 5% of the total batch by weight, a second weigh hopper is often used for these minor ingredients to maintain individual ingredient accuracy of +0.5%. When a weighing cycle is complete, the weighed ingredients are released to an active mixer, where they are homogeneously blended. The batch is then discharged to the process. Because sequential ingredient metering has a negative impact on total unit throughput, batch blenders are frequently programmed to perform the metering and mixing functions simultaneously. To maximize homogeneity and overall proportional accuracy of mixed ingredients, batch blenders are at times designed to accept multiple overlapping batches in the mixer. This provides a mechanical averaging technique

FIGURE 3.4.13.  Typical loss weight—Target weight blender—Multiple individually weighed ingredients are fed simultaneously into a batch hopper or mixer to individual proportional weights. The batch is mixed and discharged to the downstream process.

Section 3.4.  Material Handling Systems

each ingredient. Because each ingredient is weighed using an independent load cell, each cell can be sized and spanned to maximize the resolution of the A/D conversions. This yields very precise and accurate weights. Although metering is done proportionally and simultaneously, this type of blender is programmed with a target, proportional weight for each ingredient, with the proportional feed rates set and derived from the weigh proportions. The material that is fed out of the weigh hoppers is delivered to an active mixer. Feeding is paused after the target weights are achieved, and the loss weight hoppers are refilled if necessary, in preparation for the next gravimetric feed cycle. Because, unlike the target rate blenders discussed next, refilling of the weigh hoppers is done while the feed cycle is paused, the feeders always run in gravimetric mode. All material fed is weighed. After the metered materials are mixed, the mixer discharges to the process, and the next weigh cycle is started. Refilling of the weigh hoppers while the feed cycle is paused enables the use of self-loading vacuum weigh hoppers on this type of blender. This feature can be a significant advantage where adequate headroom is not available for separate weigh hoppers with vacuum receivers above. Loss in Weight (Target Rate) blenders, functioning in a target rate mode, have been used in film and sheet extrusion for over 20 years. This configuration of gravimetric blender is arguably the most prominent in film and sheet production and probably for all extrusion processes combined. The popularity of this configuration for extrusion focuses on a few features that add up to widespread acceptance. Targetrate gravimetric blenders are a natural extension of their predecessors, volumetric blenders, which were the earlier industry standard. The similarities are both in form and function. Compact, easily positioned on or above the extruder throat, continuously rate-proportioning with a passive mixer or no mixer, gravimetric target-rate blenders are a natural, self-calibrating, self-correcting replacement for the previous generation of volumetric equipment. As with the target weight blender described previously, a typical target-rate loss-weight blender (Figure 3.4.14) has each metering feeder mounted to an independent, load cellsupported, weighing hopper. Each weigh hopper measures the weight of material leaving it, and the weighing system is programmed for simultaneous and proportional metering of each ingredient. A target weight blender is designed to feed continuously. The proportionality of the individual ingredients is determined by measuring the loss in weight for each ingredient over time (∆W/∆T). The target or setpoint for each ingredient is the rate, typically expressed as lb/h (kg/h). Most often, the operator enters the desired ingredient proportions in percentages, and the controller internally converts these to proportional flow rates. Because, in principle, proportionality in the direction of material flow is the target (rate) being controlled, passive mixing, at most, is all that is required in a target-rate blender package. These principal assumptions lead to several physical and performance features that are common to target rate gravimetric blenders. Some are advantageous, but others are not.

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FIGURE 3.4.14.  Typical loss weight—Target rate blender—Individually weighed ingredients are continuously fed at gravimetrically determined proportional rates directly to the downstream process. Static or occasionally continuous dynamic mixers are used if greater blend homogeneity is required. Feeders are usually locked into volumetric feed during refill of weigh hoppers.

A target-rate blender with an integrated passive mixer or no mixer can function with little mixed material in residence between it and the extruder. Typically, a short drop chute is located under the blender. The drop chute is generally equipped with level sensors, weight sensors, or both. The level of material in the chute is sensed, and the total feed rate of the blender is automatically adjusted to keep the amount of material in the chute at equilibrium. In this way, the feed rate of the blender is kept equal to the extrusion rate. The opposite control scheme, blending with integrated extrusion control, is also available as an option with most target-rate gravimetric blenders. Instead of using the signal from the drop chute to proportionally adjust the output of the ingredient metering devices, the output of the blender is held at a fixed rate, and the output of the extruder is adjusted to match the blender. This feature can be used to control overall product thickness in mono-layer extrusion and layer-to-layer ratio in multi-layer coextrusion. Target-rate gravimetric blenders achieve their highest levels of proportioning accuracy under relatively steady-state operation. Proportionality in the short term is an absolute necessity because, unless equipped with an active mixer, target-rate blenders cannot mechanically average short-term variations. Likewise, mechanical feeding devices such as vi-

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bratory feeders and augers produce material surges and variations when they first start feeding and when they are at the extreme low end of their operating range. Feeder nonlinearity can also be a source of short-term inaccuracy, especially when a feeder operates at the lower or upper end of its total design range. Another potential condition for short-term inaccuracy is a significant change in the selected target rate for an ingredient. This is especially true when the ingredient weigh feeder is operating at the low end of its span and the weight loss signal is accumulating slowly. In this case, there is a significant time lag between a target rate change and a weight feedback. To minimize the negative effects on accuracy of these conditions, most target-rate blenders are designed to run at a total throughput as close as possible to the process rate at all times. To accomplish this, ingredient feeding is rarely paused and restarted. When an ingredient weigh hopper nearly exhausts its supply of material, the station feeder is locked into its present feed rate. The weighing feedback loop is temporarily disabled, and the scale hopper is refilled using the load cell signal to determine when the fill cycle is complete. From an accuracy standpoint, this refill cycle presents a problem. Without the load cell measuring the output from the weigh hopper, the metering process is out of gravimetric control. During this refill time, the proportioning system is essentially at a volumetric level of accuracy. It has been determined that if the total percentage of time that a gravimetric metering system is out of gravimetric mode is 15% or less, the overall long-term accuracy is not significantly affected. For this reason, refill hoppers are normally situated above each weigh hopper. These are normally kept full by the loading system. When a refill signal is generated by a weigh hopper, a valve is opened below the refill hopper and material is rapidly emptied into the weigh hopper, minimizing the time the metering equipment is in volumetric operation. Target-rate gravimetric blenders have major advantages over the other gravimetric technologies in terms of their cleanability, mechanical simplicity, and ability to be easily integrated into a gravimetric extrusion control system. They are particularly well suited for film and sheet extrusion lines that operate with frequent, short product runs and where relatively quick response to changes in ingredient proportions is required. Gravimetric Feed Throat (Additive Proportioning) blenders (Figure 3.4.15) have been used in film and sheet extrusion for over 10 years. Similar in many ways to target weight blenders, they function under the design assumption that the extruder will act as the primary feeder of material in an extrusion/blending system. Under this assumption, a material feeder is not required for one of the ingredients to be fed. This ingredient is usually the major component of the ingredient formulation. Only a weighing hopper is necessary to determine the amount of that material being consumed. All other ingredients are metered proportionally by augers into the flow stream of the major ingredient as it enters the extruder throat. Because the extruder controls the flow of the major ingredient, the additional ingredients are injected

FIGURE 3.4.15.  Typical gravimetric feed throat blender—the primary ingredient is gravity-fed into the extruder throat, with the additive proportioned by individual feeders.

into the plug flow of the major ingredient through an adapter located between the major weigh hopper and the extruder throat. The major weigh hopper is a mass flow device and is decoupled from the extruder. Similarly, the additive weigh hoppers are generally decoupled from their metering augers because the augers are required to feed into a plug-flow stream of material. Without decoupling, the forces acting on the additive augers from this process would prevent accurate weighing of the additive ingredients. When a target formulation is entered into the control for a gravimetric feed throat blender, it assumes that the feed rate for the major ingredient will be the total extrusion rate less the combined total rate of the additive feeders. The proportionality of the ingredient mix is determined by both the extruder screw speed and the actual weight loss of each of the additive feeders. This in turn affects the actual amount of major material used. This nested-loop control scheme can create conflicts between the major and additive ingredients. These conflicts can be especially pronounced when the sum of the additive ingredients is significant compared to the major ingredient. From a practical standpoint, an additive feed throat blender should control extruder screw speed to enhance proportioning control responsiveness. In fact, additive feed throat blenders are well suited for gravimetric extrusion and coextrusion control where all-pellet blends of one major and one or more minor ingredients are used. They can be especially advantageous where pellet materials of substantially different bulk densities or pellet sizes need to be introduced accurately into the extrusion process. Because the materials are combined proportionally in a plug

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flow column, at the extruder throat, there is little chance of subsequent segregation. Gravimetric Film and Sheet Recycling The subject of film and sheet scrap recycling is extensive and beyond the scope of this presentation. This section, then, is a brief overview intended to provide a general understanding of the potential for gravimetric scrap recovery. Film and sheet scrap are usually generated in two forms: • Online scrap is generated during the extrusion process in the form of edge trims and bleeds. These trims are usually a constant portion of the film and sheet production process if trim-to-size is required or if edge product in a cast line is trimmed to ensure product uniformity in the cross-extrusion direction. Trims and bleeds generally constitute 5–15% of an extrusion line’s production. They are generated beginning soon after a product run begins and remain fairly constant throughout the run. • Offline scrap is generated in the form of roll scrap and loose scrap. Roll scrap is most often generated at the start or end of a run when the product is not to specification. Occasionally, it is generated when a problem develops with a line and the product is again out of specification. Loose scrap is frequently generated as peelbacks at the end of a roll or as converted product that was discovered to be out of specification after downstream processing. Offline scrap can be a more significant percentage of a product’s total volume. This is especially true in the case of a short-run product that has experienced line problems during a run. Today’s business and environmental requirements rarely allow film and sheet scrap to be carted off to a landfill, as was the case in decades past. Sale of scrap to dealers remains an option, albeit a costly one. Repelletizing is an effective, but again costly method of dealing with scrap. The alternative is inline scrap recovery. In-line sheet and film scrap recovery has been a standard, cost-effective procedure for dealing with scrap material for almost 50 years. Closed-loop recovery of online scrap is a normal part of most film and sheet lines that generate trim scrap. The addition of offline scrap to the process is handled almost as routinely. The advent of gravimetric blending brought with it a concurrent need to address scrap reprocessing. The responses to this need have resulted in solutions that are like those used with volumetric blending systems, but that accommodate the change in blending technology. When used in combination with gravimetric pellet blending equipment, film and sheet scrap can be recovered using volumetric scrap recovery methods equivalent to those used with earlier volumetric pellet blenders. Extruder-mounted dual-channel feeders that normally introduce scrap material into a pellet stream can be used in essentially the same way. The gravimetric pellet blender simply replaces its volumetric predecessor (Figure 3.4.16).

FIGURE 3.4.16.  Typical volumetric film scrap recovery—the dualchannel scrap re-feeder combines a gravimetrically proportioned pellet blend with volumetric film scrap recovery. A crammer feeder can be substituted for the dual-channel re-feeder for higher levels of film scrap.

Extruder-mounted crammer feeders that normally feed a pellet/scrap mixture can also be used in essentially the same way. The pre-mixed pellet/scrap blend again has the pellet portion of the blend metered gravimetrically, with the scrap added as a volumetric component. Both methods of reintroducing scrap material are effective at eliminating the problems and expense of scrap disposal or re-pelletizing. They are limited in that they assume that the scrap is not a critical component in the ingredient mix that requires careful monitoring to an accuracy level equivalent to the other ingredients in the blend. When gravimetric blending equipment is used as part of a gravimetric extrusion control system, scrap recovery becomes more of a concern. At low levels of scrap (under 15–20%), the scrap can be recovered and the estimated amount of scrap entering the extrusion process entered as a constant for the purpose of gravimetric extrusion control. This assumption produces satisfactory results if the scrap introduction rate is monitored and kept constant. Under several operating conditions, gravimetric scrap recovery is a superior solution. Typical examples are multilayer coextrusion where the multilayered scrap introduced into one layer of a product can be critical to the formulation of that layer, or gravimetric extrusion control with higher than 20% or inconsistent levels of scrap recovery. Gravimetric Scrap Recovery using an extruder-mounted dual-channel feeder (Figure 3.4.17) uses a scrap surge bin and a loss weight-target rate scrap metering station before the fluff feeder. The surge bin accumulates the online and of-

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FIGURE 3.4.17.  Typical gravimetric scrap recovery with dualchannel scrap re-feeder—Both the pellet blend and the regrind are gravimetrically fed on a continuous basis.

fline generated scrap and then supplies the downstream loss weight scrap feeder when it signals for a refill. The scrap metering station gravimetrically adds the scrap material to the dual-channel feeder, where it is combined with the weigh blender’s pellet mixture at the extruder throat. The first scrap storage bin can be equipped with load cells for more precise control of the scrap recovery system. By monitoring the online scrap generation rate with the weighing system in the scrap surge bin and controlling the scrap re-feed with the loss weight feed bin, the system gravimetrically controls scrap recovery. Gravimetric scrap recovery using an extruder-mounted crammer feeder (Figure 3.4.18) for a premixed pellet/scrap blend also uses a scrap surge bin and a loss weight scrap metering station before the crammer feeder. The surge bin acts as the scrap accumulator and refill supply for the gravimetric scrap metering station. The scrap metering station gravimetrically adds the scrap material to the pellets being fed from the other blender stations, and the mixture of pellets and scrap is introduced into the crammer feeder on the extruder throat. Scrap recovery, both volumetrically and gravimetrically, is commonly accomplished with a gain in weight batch and in both target weight and target rate loss in weight blending systems. Additive proportioning feed-throat blenders are not normally used in film or sheet extrusion applications with inline scrap recovery. Gravimetric Extrusion Control

FIGURE 3.4.18.  Gravimetric scrap recovery using a crammer feeder—The crammer feeder is substituted for the dual-channel feeder when higher levels of scrap are to be reprocessed.

Gravimetric extrusion control is an added dimension to gravimetric metering and blending. In its conventional configuration, it is a simple extension of the metering and blending system to include analog inputs and outputs that provide the mechanism for automatic control of extrusion process parameters. By monitoring and controlling extruder throughputs by weight, process parameters such as weight per unit length and (in multilayer systems) layer-to-layer ratios can be monitored and controlled. The basic element of a gravimetric extrusion control system (Figure 3.4.19) is a gravimetric extrusion control hopper located at the throat of each extruder in the system. Like gravimetric blender weigh hoppers, these devices use load cells to determine individual extruder outputs by weight and use these data to report extrusion parameters and, in conjunction with other process I/O such as extruder screw speed and winder speed, to control them. The Gravimetric Extrusion Control Hopper (GECH) is used in several ways, depending on the characteristics of the blending and extrusion systems in the process. A GECH hopper can be used as a continuous zero reference value device. In this application, the material usage data from the upstream gravimetric blending equipment are compared to the deviation from reference value, positive or negative, of the GECH hopper weight value. This provides the actual extrusion rate, by weight, on a real-time basis. In many cases, the GECH hopper is integrated into the gravimetric blender

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ACCURACY AND RESOLUTION

FIGURE 3.4.19.  Typical system for gravimetric blending and extrusion control—The gravimetric extrusion control hopper, which is mounted on the extruder throat, can be integral to the blender or a stand-alone device. Multiextruder, multilayer lines use a GECH hopper on each extruder to provide layer-to-layer ratio control.

as a weighing drop chute (WDC). In either configuration, the function is the same. Another system configuration, which is frequently used when the blending equipment in a system is located off the throat of the extruder, equips the GECH hopper with a demand refill valve. Again, like a gravimetric blender weigh hopper, the GECH monitors its loss in weight to the extruder over time. Periodically, when the material in the hopper falls to a predetermined level, the weight loss signal is ignored, and the refill valve is opened until the GECH is full. This cycle is relatively short, and so the operation of the system is effectively real-time. In all probability, as many configurations exist for gravimetric extrusion control as for gravimetric metering and blending. The important considerations for selecting and operating a gravimetric extrusion control system are essentially the same as for a gravimetric metering and blending system, notably the fundamental concepts of accuracy, resolution, and control.

The topic of accuracy has always been an area of controversy and at times misinformation and misinterpretation when associated with gravimetric blending. This discussion is an attempt to enlighten rather than obfuscate. Accuracy and resolution are frequently confused when discussing weighing systems. The accuracy of a weighing element or station of a gravimetric blender is determined by the least accuracy of the scale components performing the weighing function. Before the advent of electronic scales, the accuracy of a scale was a relatively simple thing to determine. Even today, the simpler the scale, the easier it is to determine its accuracy. As scales become more sophisticated, determination of accuracy becomes more difficult. Determining the accuracy of a digital, electronic loss weight feeder with a de-coupled weigh bin is significantly more difficult than doing so for a mechanical balance scale. Adding to the difficulty of determining accuracy is the difficulty in universally defining it. Load cell accuracy, for instance, is defined by several parameters and measured within a range of ambient and operational conditions. The way that these parameters are measured is carefully prescribed by at least two national agencies and several scale industry standards. If a scale is to be “sold over”, such as the meat scale at the local market or the bag filling scales at the snack food manufacturer, it must be certified to NIST H-44 standards. These standards apply to the entire scale, not just the load cell. Electronic scales have scale meters and mechanical elements that can easily introduce error far in excess of the allowable error in most commercially manufactured load cells of any accuracy class. Moreover, target rate and additive proportioning gravimetric blenders rely on the combined dynamic accuracy of several scales operating in unison, not the absolute weights of each ingredient fed to a mixture. The answer to all this confusion is simplification. An accuracy statement for any weigh blender should not focus on the individual accuracy of one or more components in a system. A poor accuracy statement would be to say that a weigh blender uses load cells with an accuracy of +0.01% of rated output. From the user standpoint, the system accuracy of the blender is the only accuracy of true importance. A good accuracy statement would be to say that a gain in weight-batch blender would produce accurate blends of ingredients to +0.5% of setpoint for each ingredient, over the entire specified range of each ingredient feed rate, and based on a specified range of ingredient densities and particle characteristics. This type of statement specifies the performance accuracy that the user can expect from the weighing system. More importantly, the statement is defined around measurable parameters that are important to the user’s product quality. Accuracy statements for target rate and additive proportioning blenders are slightly more difficult to formulate because “time” becomes an important parameter in the performance accuracy of these units. The addition of a meaningful time-based parameter such as “. . . within 2 sigma based on a

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maximum of XX consecutive YY second samples” generally solves this problem. Resolution of digital scaling equipment is another area of misinformation and misunderstanding in gravimetric metering and blending. As touched on earlier in this presentation, the resolution of a weighing system is a measure of the degree to which an analog signal can be divided up into digital bits. High resolution translates into a large number of fine divisions. For the most part, higher resolution is a good thing. On the other hand, lower resolution is not always bad. The parameters that determine the need or desirability for high resolution are the range and responsiveness necessary in a scale. Generally, target-rate gravimetric blenders benefit from higher resolution more than batch-weight and target-weight blenders because of their time-dependent proportionality requirements. From the point of view of practicality, however, almost every present-day gravimetric weighing system uses 16- or 20bit A/D converters. A 12-bit converter used on a system today should be considered well behind the state of the art and probably substandard. 20-bit A/D converters are usually overkill in a plastics production environment. Weigh systems manufactures however, can use the extra resolution of 20-bit converters to electronically enhance the stability and responsiveness of their scale equipment. Beyond the scope of this discussion and more of a concern for weigh blender manufacturers than for users is the effect of the combination of load cell, dead load or tare weight, live load, and A/D resolution on the weighing capabilities of a metering station. Generally, a weighing station’s capacity should be sized for its normal throughput range. Oversized holding capacities for low-throughput stations or conversely, using high-capacity stations for low throughput usually results in insufficient resolution for accurate weight values. SUMMARY From the details discussed in this chapter, it should be clear that it is critical in today’s industry to spend time choosing and sizing a material handling system that can deliver multiple components to the sophisticated extrusion capabilities being used today. This chapter provides a rather detailed “blueprint” of the considerations involved, but it should be recognized that a plant survey is necessary and that all equipment suppliers should be consulted in detail about what every system is expected to do. Systems that supply materials to plants with multiple lines and complex multilayer extrusion setups require increased flexibility. This increases the possibility for cross contamination or misdirection of feed streams. Attention must be placed on easy identification and sampling of lines, as well as on avoiding buildup of some materials on the walls of material handling systems. The adage “garbage in, garbage out” is applicable. Supplying the “wrong” material or blend of materials to an expensive extrusion line is costly. 

ADDENDUM A Typical Single-Pump Pull-Push Railcar Unloader Specification Parameters: Total Throughput: 40,000 lb/h Material: High-density polyethylene (HDPE) with an average bulk density of 35–40 lb/ft3 Conveying Distance (Vacuum): • 60 ft. horizontal • 5 ft. vertical • (3) 90 deg. elbows • 10 ft. of flex hose Conveying Distance (Pressure): • 80 ft. horizontal • 60 ft. vertical • (3) 90 deg. elbows Description: (1) One (1) Combination Vacuum/Pressure Power Unit Assembly complete with: • Belt driven, positive displacement pump • 40 HP TEFC motor (1.15 SF) • Inlet and outlet silencers • Pump protection cartridge filter with replaceable cartridge on vacuum side of pump • Vacuum and pressure relief valves - adjustable • Vacuum breaker valve, solenoid actuated, air operated • Vacuum and pressure gauges, oil damped • Check valve, pressure side • All mounted on a carbon steel base with carbon steel drive guard and piped as a complete assembly. (2) One (1) Transfer Station, pulse jet filter receiver type, designed to the following operating conditions: • Gas Volume: 680 ACFM • Gas Temperature: 274 deg. F. (max.) • Design Pressure: + 17 in. Hg. • Air-to-Cloth Ratio: 7.5:1 • Anticipated Loss: 3–5 in. W.G. (2.1) Filter Receiver: • 5″ O.D. tube material inlet & vacuum outlet • Carbon steel construction • Twelve (12) polyester filter bags, 0.5 micron with 90 ft2 filter area • NEMA 4 pulse jet solenoid enclosure • Hinged access door • Air header assembly with pressure gauge, FRL, and all necessary piping • Differential pressure gauge, 0–15 in. H2O

Section 3.4.  Material Handling Systems

• High-level rotary switch tied to vacuum breaker valve through controls • Manual maintenance slide gate at discharge, cast aluminum body with stainless-steel slide. (2.2) Rotary Airlock: • Cast iron housing • Carbon steel rotor, welded eight-vane construction • Relieved blade tips and vane edge • Outboard bearings • 1 HP TEFC right-angle gearhead motor • Rotor at 25 RPM (calculated at 1.6 ft3/rev. displacement and 50% pocket fill efficiency) • Sprocket and chain drive with carbon steel drive guard • Carbon steel pellet adapter • Carbon steel discharge adapter with 5″ O.D. tubing connection (3) One (1) Railcar Unloader Control Panel complete with: • NEMA 3R panel, free standing w/12″ high-leg kit • Switch disconnect, through-the-door, w/lockout provision • Main disconnect molded circuit breaker, 100 amps @ 480 VAC • Individual branch circuit breakers for each motor starter • All lights, switches, and external mounted devices rated for outside service • Emergency stop switch, mushroom head, push = stop, pull = reset • Transformer, internally mounted to panel, 15 KVA rating • Starters for power unit drive motor and transfer station rotary valve motor • Programmable Logic Controller, Allen Bradley CompactLogix, to control all railcar unloading/silo loading functions • Panel graphics display to provide operator with equipment/silo designations and system overview for ease of operation.

*

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(4) Exterior finish coat of enamel over copoxy primer with a total of 3 mil minimum DFT. (white) (5) Walk-in door (3′ × 7′) with louvers (6) 12″ diameter bottom discharge opening (7) 20″ diameter center dome with cover plate (8) 2′6″ ground clearance (9) Foundation anchor bolts, anchor saddles, and spacers

*

(10) 2-mil exterior copoxy primer—one (1) coat

1

(11) Exterior finish coat of enamel over copoxy primer with a total of 3 mil minimum DFT. (white)

*

(12) Walk-in door (3′ × 7′) with louvers (13) Low-level switch opening in bin wall (14) Deck perimeter guardrail

*

(15) Outside ladder with safety cage and landing platform per OSHA

*

(16) Crossover walkway with guard and toe board, span between tanks

*

(17) 4″ OD aluminum load line brackets

*

(18) Flanged opening on deck for level transmitter, 5″ ID with 8″ OD flange with cover

1A

(19) Stub nozzle on deck for Donaldson vent filter, 6″ OD

1C

(20) 20″ vacuum/pressure relief manhole combination

*

(21) 4″ OD load line assembly

*

(22) 12″ aluminum slide gate

2

(23) Level transmitter 50′ max.

2A

(24) Level transmitter 100′ max.

2B

(25) Control for level transmitter (1–22) silos

1A

(26) Donaldson filter assembly

*

(27) High-level switch

*

(28) Low-level switch

ADDENDUM B

*

Typical Silo Specification

(29) Aluminum vacuum tray adapter with (4)3″OD outlets

*

(30) NEMA 12 enclosure with high- and lowlevel indicating lights and common alarm for silos

1B

(31) Flange on deck for self-cleaning bin vent filter

1B

(32) Bin vent filter, automatic, self-cleaning

One (1) 12 ft. diameter by 56 ft. overall height welded steel silo, skirted type with a 60-degree cone hopper and a working capacity of 180,495 lbs., based on material bulk density of 35 lbs./cu. ft. Seismic Zone 1. Complete with: (1) Lifting rings (2) 4-mil white epoxy interior - FDA approved (3) 2-mil exterior copoxy primer - one (1) coat

NOTES: *Indicates optional items normally specified. 1 Choice required of item 1, 1A, or 1B. 1C is required with 1A or 1B. 2 Choice required of items 2A or 2B; 2C is additional. Also available is a strain gauge silo inventory weighing system or an ultrasonic continuous level indicator system.

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ADDENDUM C

Section 3.4.  Material Handling Systems

135

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Section 3.4.  Material Handling Systems

ADDENDUM D Typical Indoor Surge Bin Specification One (1) welded carbon steel surge bin, leg type, with a 60-degree hopper bottom and a working capacity of 2,500 lbs. based on high-density polyethylene (HDPE) pellet material with an average bulk density of 35–40 lb/ft3, complete with: • Sight glass, minimum 4-in. dia. • Drain tube, minimum 4-in. dia. with manual slide gate • 60-degree hopper

137

• High-level switch, proximity type, 32 mm threaded, Turck • Low-level switch, proximity type, 32 mm threaded, Turck • Common alarm panel (NEMA 12), including globe light, klaxon, indicating lights, and alarm silence button • 12" manual slide gate, aluminum construction, hand wheel operated • Aluminum vacuum tray adapter (2) outlets, 2″ OD • Vacuum loader receiver mounting extension, cast aluminum with integral, panel-type replaceable filter • High legs for gaylord discharge from drain

Chapter 3—Section 5

Instrumentation and Process Control TED SCHNACKERTZ, NDC Technologies

KEYWORDS: Instrumentation, process control, online gauging, gauging technologies, beta transmission, gamma backscatter, infrared absorption, X-ray transmission, capacitance, profile control, thickness control.

INTRODUCTION

Beta Transmission Sensor

The demand for high-quality films and packaging products, with increasing functionality, continues to rise. Films are becoming thinner, with more complex layer structures, while line speeds are increased to improve production efficiency. The requirements for consistent quality with increased yields, raw material savings, and proof of compliance can no longer be satisfied with periodic (end of run/roll) laboratory sampling because a defect caught in the lab may be hours old! In addition, data collection for new product development and process troubleshooting requires real-time measurement.

The sensor (Figure 3.5.2) assembly consists of an upper and lower head separated by an air gap with the beta source in the upper head and the detector or ionization chamber in the lower. The principle of operation is based on the absorption/attenuation of beta particles (high-speed electrons) by the mass of a material. The ionization chamber is filled with a gas that ionizes when the gas molecules are struck by beta particles. This ionized gas allows a small current to flow, producing an output signal. With only air in the gap, the signal output is at a maximum. As product mass is introduced into the gap, beta particles are absorbed or attenuated, which reduces the output signal in proportion to the mass. Software linearization is used to provide a signal representative of the mass, and calibration factors are used to convert the signal to the required engineering units (mils, g/m2, #/ream, etc.) for display and control.

Benefits of online gauging: • Continuous real-time measurement • Repeatability of measurement • Process control • Reporting capability for product compliance • Data collection for process improvement analysis • Process troubleshooting • Quality improvements and economics. The overall quality improvements and economics are illustrated in Figure 3.5.1. The pre-gauging distribution (without measurement and control) is meeting the lower quality limit with the current target specification. However, once variation is reduced with measurement and control, a potential target reduction becomes available while still not violating the lower quality limit. The result would be material savings along with improved quality. Sensor Technologies The five primary sensor technologies used for film extrusion measurement are: • Beta transmission • Gamma backscatter • Infrared absorption • X-ray transmission • Capacitance.

Beta sensor characteristics: • Total mass measurement (weight/area) • Range 10–1200 g/m2 (krypton-85 isotope) • Highly material-versatile • No layer discrimination capability • Some sensitivity to composition, from minerals or metal additives • Precision/repeatability: ± 0.2% Gamma Backscatter Sensor This sensor (Figure 3.5.3) is single-sided, with the gamma source (photon emitter) and a crystal detector in the same housing. The crystal or scintillator is optically coupled to a photomultiplier tube (PMT). The principle of operation is based on a phenomenon known as Compton photon backscattering. When gamma rays or photons strike a material mass, most pass through without change, but about 15% of the photons are scattered to the side and backward. The number of photons backscattered at 180 degrees undergoes an energy shift that is related to the material mass. The backscattered photons that strike the crystal detector make it emit 139

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FIGURE 3.5.1.  Quality and raw material savings.

light pulses that are amplified by the PMT. The pulse output count of the PMT at the new energy level is now directly proportional to the mass of the material. An increase in mass results in a proportionally higher pulse rate. The pulse rate is converted electronically to an output voltage, which represents the mass (weight/unit area) of the material being measured. This output is linear and only requires conversion to the required engineering units (mils, g/m2, #/ream, etc.) to be used for display and control. Gamma backscatter sensor characteristics: • Wide measurement range (25–25000 g/m2) • Total mass measurement (weight/area) • Composition insensitive • Single-sided measurement • Precision/repeatability: ± 0.5%

Infrared Absorption (IR) The IR sensor for film is a dual-head sensor (Figure 3.5.4) with the transmitter (IR light source) in the upper head and the detector in the lower head. The sensor operates in the near-infrared light wavelength spectrum of 1–5 microns (Figure 3.5.5) to make a measurement. The principal of operation is called selective absorption. When several different wavelengths of infrared light are focused on an organic material, some wavelengths will cause the molecules to vibrate or resonate within the material. This vibration causes energy absorption of the wavelengths responsible for the vibration. The other wavelengths are not absorbed. Different polymers exhibit absorption at different wavelengths. Because all wavelengths are initially at the same energy level leaving the sensor, the unabsorbed wavelengths are the reference,

FIGURE 3.5.2.  Beta sensor.

Section 3.5.  Instrumentation and Process Control

141

FIGURE 3.5.3.  Gamma sensor.

and the ratio between the absorbed and reference wavelengths is used to develop an output signal representative of the material mass. The greater the mass, the more absorption occurs. Measuring specific polymers or coatings is a matter of choosing the correct wavelength(s). The IR sensor uses a quartz halogen lamp as a light source and filters to generate the specified wavelengths of light. The fact that different polymers have different absorption wavelengths gives IR the unique ability to separate polymers in a co-ex structure. For

example, IR can be used to separate EVOH from nylon and olefins in the same co-ex film or to measure coatings directly on film or paper and provide a thickness display output of each. IR sensor characteristics: • Selective absorption mass measurement • Multi-layer discrimination • Clear or pigmented films • Range 0.5–2500 microns • Non-isotope. X-Ray Transmission

FIGURE 3.5.4.  Infrared sensor.

The sensor (Figure 3.5.6) consists of a dual-head assembly, with the upper head containing the X-ray source and the lower head the detector ionization chamber. Operation is similar to the beta sensor, with the exception that the energy source is X-ray photons instead of beta particles (highspeed electrons). The principle of operation is based on the absorption or attenuation of X-ray photons by the mass of a material. The ionization chamber is filled with a gas that ionizes when the gas molecules collide with the photon energy. The ionized gas allows a small current to flow, producing an output signal. With only air in the gap, the signal output is at a maximum. As product mass is introduced into the gap, photons are absorbed or attenuated, which reduces the output signal in proportion to the mass. Software linearization is used to provide a signal representative of the mass, and calibration factors are used to convert the signal to the required engineering units (mils, g/m2, #/ream, etc.) for display and control. One difference between beta and X-ray sensors is that X-rays are more sensitive to composition and metals, requiring more attention to the application materials to be measured.

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FIGURE 3.5.5.  Infrared in the electromagnetic spectrum.

X-ray sensor characteristics: • Mass measurement (weight/area) • Range 5–8,000 g/m2 • Excellent streak resolution • Sensitivity to composition, presence of TiO2 or metal additives • Non-nuclear • Licensing varies. Capacitance The capacitance sensor designed for blown-film lines is a single-head contact sensor (Figure 3.5.7) with two electrodes in the center and ground plate. The measurement is based on the principle that polymer materials are basically insulators with a dielectric coefficient (K). The sensor consists of two electrodes separated by an insulator that forms a capacitor of capacitance value (C). The sensor is part of an electronic “bridge” circuit, which is balanced to zero output volts for the C of the sensor only without web contact. The sensor is then calibrated for the K of each of the materials to

be measured. When web contact is made, the C of the sensor changes in proportion to the thickness of the web with a given K. The sensor C change unbalances the “bridge” circuit, and an output signal is generated that is representative of thickness for a given K value. Some typical K values are: free air = 1.0; PE = 2.25; Teflon = 2.1. The measurement limitation is that its use is restricted to mono films with a single K value, because any co-ex films would have mixed K values, producing an output signal unrelated to the actual thickness. Capacitance sensor characteristics: • Dielectric coefficient mass measurement • Mono film measurement only for accuracy • Typical range 10–300 μm • Temperature sensitivity. PROCESS CONTROLS Two basic automatic gauge control methods are used for film extrusion: target weight/thickness and profile control, and they are usually used together.

FIGURE 3.5.6.  X-ray sensor.

Section 3.5.  Instrumentation and Process Control

143

FIGURE 3.5.7.  Capacitance sensor.

Target Weight/Thickness Control refers to controlling the average weight/thickness to a setpoint target using screw speed or line speed changes. In an extrusion coating application, the control point is usually screw speed, and on a cast-film line, it is usually line speed. An enhancement of this type of control could include automatic target optimization (ATO). This is a software algorithm that uses gauge data to compare the current standard deviation (sigma) to a preset value and automatically reduces the control target in defined increments to a predetermined lower limit if the sigma allows. It will reverse the process if sigma degrades and increase the target to prevent out-of-spec product. ATO can add significant raw material savings without the risk of off-spec product. Auto Profile Control (APC) refers to automatic control of the CD profile using an auto-die fitted with die-bolt heaters that are used to control the movement of each individual die bolt. These heaters are controlled from an interface driven by output from the gauging system. The APC control software is tuned to match the die characteristics: lip stiffness, die-bolt time constant, adjacent bolt coupling, etc., thus enabling the APC algorithm to make profile corrections rapidly either at startup or during process upsets. Automatic control systems make it possible to reduce startup time, deliver consistently uniform product, and save material. Analysis/Diagnostics Software tools are available to assist in these areas: OPC (open-platform communications), SPC (statistical process control), roll defect map, FFT (fast Fourier transform), and streak detection.

Key process diagnostic tools are: • FFT: software performs analysis on cyclic data patterns, breaking these into time and length for comparison to known machine cycles. This is valuable information in the hands of a knowledgeable process engineer. • Streak Detection: software takes scan profile data and filters out normal low-frequency profile variations to detect narrow (1 mm) repetitive web streaks, which would otherwise be lost in the noise of random or periodic process variations. • Roll Defect Map: a useful tool for identifying defect areas within a jumbo roll to the slitter operator. The map indicates the specific area and length of off-spec material within the roll. SUMMARY This section has covered the gauging technologies available for film extrusion and provided some application information. In many cases, more than one sensor type will perform the measurement task in a given application. However, if you look at all the process requirements, i.e., handling the products run today and any future products, balanced with the required measurement and control precision, cost, and implementation considerations, one technology will more than likely stand out. REFERENCES AND ADDITIONAL RESOURCES References at www.ndc.com

Chapter 3—Section 6

Blown-Film Cooling Systems HARINDER TAMBER, Macro Engineering and Technology Inc.

INTRODUCTION In the blown (tubular) film process, the polymer melt from the extruder is passed through an annular die and subsequently drawn and expanded radially while being cooled and stabilized by an air ring. The blown-film cooling device, such as an air ring, plays an important role by directly affecting the output, layflat (blowup ratio), gauge uniformity, and film properties such as MD tear, impact strength, and optics. Depending upon the material processed, single-lip, dual-lip, or stacked air rings are used to cool and stabilize the bubble externally, while internal bubble cooling (IBC) is used to cool the inside. In the blown-film extrusion process, output is usually limited by the rate of cooling. Air rings are designed to cool the melt and stabilize it. This technology involves two important aerodynamic phenomenon, the Venturi and Coanda effects. The Venturi effect is well known and is caused when a fluid flows through a restricted area, resulting in a speed increase and a pressure drop, as shown in Figure 3.6.1. The Coanda effect is less well known and occurs when a free jet emerges close to a surface; the jet tends to bend, “attach” itself, and flow along the surface. The surface may be flat or curved and may be located inclined or offset to the jet, as shown in Figure 3.6.2. The effects of these phenomena will be discussed later in more detail. This chapter covers a wide span of cooling devices such as the single-lip air ring used in the beginning of blown-film technology and its modification to process materials like HMW-HDPE. High-volume, high-velocity, dual-lip air rings were designed for lower melt-strength materials like LLDPE or more generally for polyolefins and their copolymers. In the early 1990s, dual-lip air rings were further modified to cool and stabilize new materials like metallocene-catalyzed linear low-density polyethylene (mLLDPE), single-site constrained geometry polyolefins (SSCGPE), or plastomers that had even lower melt strength than conventional LLDPE. More recently, triple-lip air rings have been developed and Updated by James Stobie, Macro Engineering and Technology Inc.

used to deliver higher throughput rates. Subsequent sections describe the use of IBC, triple-lip air rings, and secondary air rings to further enhance cooling efficiency and increase output. A process to control circumferential gauge tolerance using an automatic air ring is also reported. In the last section, equations to describe the thermal load and heat removal processes are discussed. SINGLE-ORIFICE AIR RINGS In the beginning of blown-film technology, air rings of the single-orifice type were used. Air impinged straight onto, or normal to, the surface of the melt tube as it exited from the die, as shown in Figure 3.6.3. However, the relatively low volume of air that could be applied by direct air impingement to the melt while maintaining bubble stability severely limited throughput. As the technology evolved during the early 1960s, several machinery manufacturers and film extrusion companies such as Egan, MPM, Sterling, and others modified the lip design to turn the air parallel to the extruded melt surface exiting from the annular die, as shown in Figure 3.6.4. This enabled higher volumes of air to move axially along the extruded tube, thereby maximizing the cooling effect. The air near the die creates a low pressure region (Venturi effect), drawing the bubble out close to the air ring, which “locks up” the bubble for higher stability and improved gauge uniformity. Today, with further improvements in lip design, single-lip air rings are used to process high-molecular-weight, highdensity polyethylene (HMW-HDPE, less than 0.1 MI). This material requires a high-neck or long-stalk extrusion (Figure 3.6.5) because neck height has a significant influence on film properties such as MD tear and impact strength. The high molecular weight of HMW-HDPE relates to the long polymer chains that prevent its drawdown to less than 50 microns (2 mil) in a conventional bubble shape, as for LLDPE or LDPE. As reported earlier, a typical drawdown for highly branched molecules (polymer chains) is 16–20 times the melt index and for linear molecules is 160 times. The high-stalk extrusion tube is obtained by reducing the volume of air, which leads the frostline to be 6–10 die diameters 145

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FIGURE 3.6.1.  Venturi Effect: When a fluid flows through a constricted area, its speed increases and the pressure drops.

compared to 1–2 die diameters for LLDPE or LDPE. The high-stalk bubble extends the distance and time available for drawdown to occur, enabling higher drawdown ratios than those achievable with a traditional low-stalk bubble. The relatively slow rate of melt extension in the high stalk area provides time to let the polymer chains relax and randomize.

FIGURE 3.6.3.  Air ring: single-orifice direct impingement design.

The high-stalk neck then expands in the transverse direction and is simultaneously drawn in the machine direction to optimize the film properties. Machinery manufacturers can supply internal bubble cooling (IBC) that delivers good bubble stability with typical throughput rates for high-stalk extrusion of HMW-HDPE in the range of 1.45–1.6 kg/h/mm of die diameter for thin films ( 80 percent of the structure by weight is made up of similar materials such as LLDPE/EVA/ionomer. However, in situations where materials of significantly different thermodynamic properties make up 20 percent of the structure by weight, calculations should be performed for each layer or group of layers using different heat capacities, heat of fusion, and varying frostline heights. An example of this would be structures such as PE/nylon or PE/PVDC; particularly in the latter structure, the density of PVDC is 1.7, and therefore PVDC contributes heavily towards the weight percent of the total structure. Similarly, if a masterbatch containing heavy metals such as TiO2 at greater than 20 percent is used in one of the layers, independent calculations for each separate layer should be performed and then aggregated. It is worth mentioning here that new materials like mLLDPE, SSCGPE, or plastomers have lower crystallization temperatures than conventional LLDPE and are usually run at higher melt temperatures in existing blown-film lines used for conventional LLDPE. Therefore, these materials have higher thermal loads, and hence more efficient cooling is

Section 3.6.  Blown-Film Cooling Systems

required. Modifications to screw and die design have been recommended by machinery manufacturers and resin suppliers to control the melt temperature of these resins. HEAT REMOVAL Once the thermal load has been estimated for a process, the next requirement is to evaluate the method of heat removal from the bubble and establish the heat-transfer coefficients. A model using three estimated heat-transfer coefficients currently exists. Detailed analysis of this model using a typical example has been reported in the literature. In brief, the three heat-transfer coefficients acting on the bubble can be categorized as: the heat transfer on the outside of the bubble from the die face to the frostline (ha), the heat transfer on the outside of the bubble from the frostline to the nip (hf), and the heat transfer inside the bubble (ht). The calculation area is based on a truncated cone of variable area from the top of the air ring to the frostline (or where the actual layflat is obtained), a cylinder from the frostline to the primary nip, and a truncated cone of fixed area inside the air ring. In this chapter only the primary air-ring heat removal component (Ha) is described. The rate of heat transfer from the polymer melt can be approximated with the following equation: H a  ha  area  (Tmelt at die exit  Tair )

(3.6.16)

where: Ha = heat removal rate, Btu/h ha = heat-transfer coefficient, Btu/h ft–2°F–1 area = area of the bubble exposed to cooling air, ft2. Tmelt at die exit – Tair = melt temperature of plastic at die exit – air temperature in the air ring. Based on Equation (3.6.16), the heat removal rate can be increased by increasing the heat-transfer rate (ha) or the surface area or by maximizing the temperature difference (Tmelt at die exit – Tair) using chilled air. The heat-transfer coefficient is maximized by air-ring design. The only factor left in Equation (3.6.16) is the area of bubble exposed to cooling air, and therefore the blowup ratio and film thickness play an important role in heat removal from the bubble. Similarly, in a blown-film installation, use of a secondary air ring and IBC, where permissible, can further increase heat removal rate and therefore increase output. GENERAL CONSIDERATIONS FOR BLOWNFILM SYSTEM In the blown-film system, certain approximations have been found reliable and can be useful during processing projections: (1) In the process, a 1°F increase of air ring or melt tempera-

155

ture usually reduces the rate up to 1% of lb/inch of die circumference. (2) The output rate decreases linearly as the blowup ratio decreases. (3) A decrease in air pressure of one inch WG usually results in a rate decrease of 4–5 lb/h. (4) A decrease in IBC exhaust pressure of one inch WG usually results in a rate decrease of 1–3 lb/h. In the blown-film process, tower height is important in secondary or post-frostline cooling. Machinery manufacturers provide nips with cooling or heating options depending upon the film being fabricated. Options are also available to change the nip height depending upon whether a stiff material like HDPE, cyclic olefin copolymer (COC), polystyrene, or polyamide is being fabricated. These should be collapsed warm to avoid wrinkles, or the nip height should be low. On the other hand, soft materials such as EVA and mPE are collapsed cold to avoid blocking, and therefore the tower height should be increased. SUMMARY To date, the blown-film cooling system has used dual-lip air rings or tandem arrangements and internal bubble cooling (IBC) to increase the cooling efficiency of the bubble both externally and internally. New designs for extruder screws and particularly coextrusion dies as well as new materials make it necessary to modify cooling systems further. Methods covered in proprietary reports show that automatic air rings or even IBC are being used to control circumferential gauge distribution in blown film in certain specific applications.

REFERENCES AND ADDITIONAL RESOURCES [1] Knittel, R.R. and DeJonghe, R.J. Jr., “Blown Film Cooling Systems,ˮ 1992 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 261. [2] Butler, T.I., “Blown Film Bubble Instability Induced by Fabrication Conditions,ˮ Proceedings, 1999 Polymers, Laminations and Coating Conference, TAPPI PRESS, Atlanta, p. 815. [3] Sidiropoulos, V. and Vlachopoulos J., “An Investigation of Venturi and Coanda Effects in Blown Film Cooling,ˮ International Polymer Processing Journal 15(1):40–45 (2000). [4] Bode, W.W., “Current Developments in Equipment for Processing High-Molecular-Weight, High-Density Polyeth ylene Film,ˮ Proceedings, 1989 Polymers, Laminations and Coating Conference, A TAPPI PRESS Anthology of Published Papers 1986–1991, Bentley, D.J., Jr. (Ed.), TAPPI PRESS, Atlanta, p. 312. [5] Perdikoulias, J. and Smith D., “Evaluation of the Performance of a New Tandem Air Ring System,ˮ Proceedings, 1991 Polymers, Laminations and Coating Converence, A TAPPI PRESS Anthology of Published Papers 1986–1991, Bentley, D.J., Jr. (Ed.), TAPPI PRESS, Atlanta, p. 324.

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[6] Stobie, J., “Air Ring Considerations for Optimizing Blown Film Properties,ˮ Proceedings, 1996 Polymers, Laminations and Coating Conference, TAPPI PRESS, Atlanta, p. 231. [7] Jones, D.N., “USC-1 New Cooling System for LLDPE,ˮ Proceedings, 1984 Polymers, Laminations and Coating Conference, TAPPI PRESS, Atlanta, p. 105. [8] Gregory, R.B., “Internal Air Ring for Blown Film,ˮ Proceedings, 1974 Paper Synthetics Conference, TAPPI PRESS, Atlanta, p. 263.

[9] DeJonghe, R.J., Jr., “Thermal Analysis of Blown Film Quenching,ˮ Proceedings, 1986 Coextrusion Conference, TAPPI PRESS, Atlanta, p. 67. [10] Planeta, M., “Test Developments in Blown Film Cooling,ˮ Proceedings, 1983 Paper Synthetics Conference, TAPPI PRESS, Atlanta, p. 359. [11] Paulius, J., “Water Quenching of Blown Film,ˮ Proceedings, 1985 Polymers, Laminations and Coating Conference, TAPPI PRESS, Atlanta, p. 197.

Chapter 3—Section 7

Surface Treatment RORY WOLF, ITW Pillar Technologies

ABSTRACT: With the advent of economically available non-paper substrates (plastics and foils) in the mid-to-late 1950s, the requirement for a reliable, production-speed, surface treatment-process became apparent. Several technologies have been evaluated, but one, corona treatment, has become the preferred primary surface-treatment technology for use throughout the extrusion and converting industries. In this chapter, we will touch on these various technologies, technically describe the need for surface treatment and how it is measured, and describe in detail the current state of the art in corona-treatment equipment, control parameters, and applications. INTRODUCTION: THE NEED FOR SURFACE TREATMENT Generally, plastics have chemically inert and non-porous surfaces with low surface energies, causing them to be nonreceptive to bonding with substrates, printing inks, coatings, and adhesives. The primary purpose of surface treatment is to increase surface energy, thereby increasing the wettability and adhesion of the treated surface. Polyethylene and polypropylene are the lowest in surface energy among the various packaging-related plastics and are therefore the two materials most often subjected to surface treatment to improve their bonding characteristics [1–3]. Surface treatment, however, is not limited to these two materials and can be used to improve the bonding ability of virtually all plastic materials as well as some non-plastic materials. The two non-plastic materials most often subjected to surface treatment are foil and paper. All polymer substrates provide a more bondable surface when they are treated when they are produced. Some polymer substrates are almost impossible to treat if they are not treated at the time they are produced. Treatment at the time of film production is referred to as post-treatment because it is applied after the extrusion or creation process and is used to enhance adhesion for follow-on converting processes such as printing, coating, and adhesive lamination. Methods of Improving Surface Tension The four methods by which surface treatment is accomplished are [4]: (1) Corona treatment [5–7] (2) Flame treatment [8] (3) Plasma treatment [9] (4) Chemical priming [10] Priming is still frequently used alone or in combination with corona discharge treating, but is never done in-line with

the extrusion process. Ozone treatment is another treatment method used, but it is almost exclusively used in the extrusion coating process, not in film extrusion. Flame treatment is frequently used for molded or blow-molded parts. Flame treatment of film and sheet had been almost completely supplanted by corona discharge surface treaters. However, flame treating has seen a comeback recently, especially on paperboard or milk carton stock in extrusion coating applications. Flame is still being used on film extrusion lines, but on a relatively small percentage of applications. Plasma, on the other hand, was not applied to film extrusion lines early on, but has recently evolved to serve this process as well as others in an atmospheric plasma glow discharge design. Both flame and plasma treatments are discussed in some detail below. FLAME TREATMENT Flame treatment typically creates fixed levels of oxidized species on the surface of films, along with forming hydroxyl, carboxyl, and carbonyl functionalities. Treatment (oxidation) depths vary by substrate, as does the generation of lowmolecular-weight organic material at the surface. Flame plasmas have an electron density of approximately 108 cm–1 and electron energies of 0.5 eV (electron volts). Surface exposure to flame treatment directly modifies the electron distributions and densities of polyolefin molecules, resulting in polarization at the polymer surface up to several nanometers deep. The design of existing flame treatment burner technologies has heretofore been driven primarily by the evolution of BOPP film production, paper/paperboard production, and web coating technologies. BOPP processing speeds of 450–500 meters per minute (mpm) are now common, as are coating speeds of 600–800 mpm, and film widths can range up to 10 meters or greater. The improving performance of BOPP co-extrusions, PE, PET, polyamides, and cavitated films has also required improvements in flame treatment performance. Flame system design improvements included better control of flame temperature and air gap at the burner, an increase in thermal 157

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output, burner cooling, better specific power (W/cm²) control, boundary layer penetration, flame stability, flame size, and substrate dwell time within the flame. The predominant burner design used in industrial applications is the ribbontype burner. These are designed for high-heat-release firing and are specified where a continuous flame is essential. They use 0.015–0.020 stainless-steel "ribbons" and "flats" for flame retention and uniform heat distribution. Modification of the respective orientation, widths, and depths of the ribbons and flats can accommodate changes in burner capacities, flame geometries, and sizes. The burner bodies are typically constructed of joined cast-iron segments and are water-cooled with either internal or external channels to prevent burner deflection due to thermal expansion. Control of ribbon-burner energy output is primarily achieved by managing the air/gas mix, system air pressure, and burner design. Recent development objectives of burner technology have included the following: • Increased thermal efficiency • Greater heat transfer (luminous flame radiative-cooling effect) • Increased treatment rate • Low burner deflection/greater gap stability through improved internal recirculative cooling • Port inserts that can be easily removed and cleaned in the field. There are two velocities that determine the shape and treatment efficacy of a burner flame: the velocity of the air/gas flow to/from the burner, and the formed velocity of the flame itself. The focus of control of these velocities is within the design of the combustion system and the burner ports. Burner designs with drilled ports provide high-velocity flames that deliver more surface treatment per unit of time and are effectively leading to treatment productivity improvements. Four variables control the optimum treatment conditions of a flame: (1) Air-to-gas ratio (2) BTU (kJ) output of the burner (3) Distance of the surface from the flame tips (4) Dwell time of the surface in the oxidizing zone.

FIGURE 3.7.2.  Flame treatment burner.

Figure 3.7.1 is an overall flow diagram of the various components of a flame treatment system. A high-volume, low-pressure centrifugal air blower moves a column of air through a venturi mixer. The venturi section is adapted with a needle valve to pull gas according to demand from a pressure-regulated gas line. The resulting air/gas mixture, with 5–15% excess air, is conveyed to the burner face and ignited. The flame's oxidizing zone, which is optimum at 3/8″ to 1/2″ from the flame tip, impinges on the substrate surface, and the excess oxygen activated by the high temperatures combines with carbon molecules to form the polar groupings generally associated with an oxidized surface. The treated surface is approximately one molecular layer thick. The waste heat from this process can be reclaimed by various energy interchange methods and can be returned as usable heat for drying or even for building heat. Some plants with proper insulation have been completely heated during the winter months by such recycling processes. Figure 3.7.2 shows a cross section of a flame-treater burner that features a wide and clean-ported equalizing chamber with relatively large precision orifices drilled along the exit area. The lean gas mixture is combusted in a flame guard zone, which prepares the gaseous envelope for impact with the substrate. This flame guard also protects the burner orifice from blockage by pieces of paper or plastic scrap charring and melting, thus sustaining the flame characteristics in a more pristine state for a longer period of time [8]. This system provides the additional control necessary to overcome the problems inherent in earlier flame-treating systems, such as inconsistent treatment levels and slow response during a line shutdown, while maintaining the required treatment levels. Flame analyzers now compensate for changes in ambient temperature, gas composition, and humidity to maintain a proper oxidizing flame. Burner capacity or firing rate is automatically adjusted to changes in line speed. PLASMA TREATMENT

FIGURE 3.7.1.  Flow diagram of the flame treatment system.

Low-pressure vacuum plasmas have been used for many

Section 3.7.  Surface Treatment

years to surface-treat three-dimensional plastic objects and polymer films. Therefore, the benefits of plasma treatment are well recognized: reduced degradation of surface morphology, higher treatment (dyne) levels, elimination of backside treatment, and extended life of treatment over time. However, the complexity, slow speed, and high cost of these contained plasma systems have made them impractical for all but the most esoteric applications. Now, a system has been developed that enables plasmas to be sustained at atmospheric pressure in a way that permits surface treatment of polymer and other substrates on a continuous web handling system similar to a corona-treating system. The atmospheric plasma treatment (APT) process provides treatment over a broad range of reactive gases and has been successfully tested on various polymer films, including OPP, PE, PET, and PTFE. Furthermore, depending upon the dyne level required and the type of film, line speeds in excess of 400 feet per minute (fpm) are practical, and speeds up to 1500 fpm have been achieved. Specialty film applications with stringent surface morphology specifications or requiring specific surface modification, precise surface coating, or tightly controlled electrical characteristics will find the APT system especially attractive and useful. The APT system is similar to the corona-treatment dual dielectric system in that it can also treat conductive substrates such as foil or metalized film. This technology is also useful on film extrusion lines. Plasma is an ionized form of gas and can be created using a controlled level of AC or DC power and an ionizing gas medium. Plasma, commonly referred to as the fourth state of matter [11], is characterized by an ensemble of randomly moving charged atomic particles with sufficient particle density to remain, on average, electrically neutral. Plasmas are used in a wide range of diverse applications, ranging from manufacturing integrated circuits used in the microelectronics industry, through treating polymer films, to destruction of toxic waste [12–14]. Plasma processes can be grouped into two classes, low-density and high-density, and are often displayed in an electron temperature versus density phasespace plot (Figure 3.7.3) [12]. Low-density, direct-current, and radio-frequency glow discharges are usually non-equilibrium, i.e., the electron and heavy particle (ions, neutrals) temperatures are not equal. Low-density plasmas have hot electrons (Te > 104 K) with cold ions and neutrals. Energetic electrons collide with, dissociate, and ionize low-temperature neutrals and ions creating highly reactive free radicals and ions. These reactive species enable many processes to occur with low-temperature feedstock and substrates. Low-density plasmas are usually associated with low material-throughput processes such as surface modification. Low-density plasmas are used in a variety of processes such as surface treatment, physical sputtering, plasma etching, reactive ion etching, sputter deposition, plasmaenhanced chemical vapor deposition, ashing, ion plating, reactive sputter deposition, and a range of ion beam-based techniques, which all rely on the formation and properties of plasmas. The types of plasmas encountered in surface

159

FIGURE 3.7.3.  Classification of plasmas: electron temperature vs. electron density [12].

treatment processing techniques and systems are typically formed by partially ionizing a gas at a pressure well below atmosphere. For the most part, these plasmas are weakly ionized, with an ionization fraction of 10–5 to 10–1. Electron cyclotron resonance (ECR) plasmas can have higher ionization at high power. Low-density plasmas can be established by AC or DC power input and can have many geometries depending upon the application. Because these systems are run at low pressures, vacuum chambers and pumps are needed to create and contain these plasma processes. ATMOSPHERIC PLASMA TREATMENT (APT) PROCESS The APT process, a new atmospheric plasma treatment system (patented), was developed for treating and functionalizing polymer films and has unique advantages over presently used corona and flame treatment technologies. The APT system enables creation of uniform and homogeneous high-density plasma at atmospheric pressure and at low temperatures using a broad range of inert and reactive gases. The APT process treats and functionalizes polymer films in the same way as the vacuum plasma treatment process. APT production equipment testing has been successfully used to treat various polymer films, including oriented polypropylene (OPP), polyethylene (PE), polyethylene terephthalate (PET), and polytetrafluoroethylene (PTFE) films. The surface energies of the treated films increased substantially (without any backside treatment), thereby enhancing the wettability, printability, and adhesion properties of the films. The prospect of being able to treat and functionalize commercial substrates, inline, at atmospheric pressure, using inexpensive equipment has been very attractive. Substantial value can be added to a product with minimal expense. Typi-

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cal applications include elevation of surface energy, longevity of treatment, printability, and enhancement of hydrophilicity. Plasma treatment provides three key benefits: (1) Longer-life treatments. Substrates that have been plasma-treated hold their treatment levels far longer than corona-treated surfaces. Longer treatment life enables converters to take advantage of economies of scale during production, increase inventory life, and provide enhanced manufacturing flexibility. (2) Higher treatment levels enable treatment of difficult-totreat surfaces. Plasma treatment is a viable alternative for a variety of substrates for which corona-treating is ineffective. For example, all polyolefins as well as fluoropolymer-based materials like Teflon® respond poorly to the corona process, but do respond to plasma treatment. (3) Elimination of corona process limitations. Because APGD is non-filamentary and developed at lower voltage levels, pinholing and backside treatment are eliminated. In the Atmospheric Plasma Treatment (APT) process, plasma is generated at atmospheric pressure and at low temperatures using an AC power source, a proprietary designed electrode, a dielectric layer between the electrodes, and an appropriate gas mixture as the plasma medium. Gases such as helium, argon, and mixtures of inert gas with nitrogen, oxygen, air, carbon dioxide (CO2), methane (CH4), acetylene (C2H2), propane (C3H8), or ammonia (NH3) have been used to sustain a uniform and steady plasma for effective surface functionalization. The electrode is connected to an AC power supply, and the rotating treater roll is grounded and acts as the other electrode. A general overview of the APT treater is shown in Figure 3.7.4. Plasma treatment utilizing the appropriate gas mixture and energy density will effectively increase wettability and adhesion of the surface, thereby improving the ability to bond to inks, adhesives, coatings, and extrusion coating. For a surface to be properly wet by a liquid, the surface energy of the plastic must be higher than the surface tension of the

FIGURE 3.7.4.  Overview of the atmospheric plasma treatment apparatus.

liquid. Surface energy is measured in dynes per centimeter. Ideally, the surface energy of the plastic should be 7 to 10 dynes/cm higher than the surface tension of the ink or coating. For example, a printing ink with a surface tension of 30 dynes/cm would not adequately wet, and therefore not adhere to, a material with a surface energy less than 37 to 40 dynes/cm (Figure 3.7.5). One method for measuring surface energy known as the Wetting Tension Test (ASTM D-2578) was established a number of years ago [15]. Surface energy testing is frequently done on post-treatment surfaces before follow-on converting processes. With this test, a series of mixed liquids with gradually increasing surface tensions are applied to a treated substrate surface until one is found that just wets the surface. The surface energy of the plastic is approximately equal to the surface tension of that particular mixture. Test solutions are available from various manufacturers of corona-treating equipment. Table 3.7.1 details the ratio of formamide and Cellosolve™ for various surface tensions. Cellosolve™ is a registered trademark of Union Carbide for ethylene glycolmonoethylether. The wetting tension test method is by far the most prevalent measurement used to determine the treatment level of post-treated surfaces. Several other methods are in limited use and are detailed in Table 3.7.2. Of these methods, one of the most accurate when properly performed is contact angle measurement. To establish a new equilibrium between substrate surface characteristics, coating material characteristics, and production processes and methods, an accurate method of testing is required. Some years ago, Dr. William A. Zisman, while working with equations developed by Thomas Young, developed the mathematical and practical underpinnings of testing surface tension using accurate contact-angle measurement [16]. Zisman's work cannot be fully discussed here, but Figures 3.7.6 and 3.7.7 provide simple demonstrations of his theory. Much of the experimental work done over the past years has ignored wetting tension solutions as a method of surface tension measurement in favor of contact-angle measurement [6–7] [17–19]. Several companies are currently marketing devices for accurate contact-angle measurement.

FIGURE 3.7.5.  Ink drop on film surface.

Section 3.7.  Surface Treatment

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TABLE 3.7.1.  Concentrations of Formamide: CellosolveTM Mixtures Used in Measuring Surface Energy. Formamide Volume Percent

Cellosolvetm Volume Percent

Surface Tension(a)

DYNES/CM Formamide—CellosolveTM Mixtures 0 2.5 10.5 19.0 26.5 35.0 42.5 48.5 59.0 63.5 67.5 71.5 74.7 78.0 80.3 83.0 87.0 90.7 93.7 96.5 99.0 Formamide Volume

100.00 97.5 89.5 81.0 73.5 65.0 57.5 51.5 41.0 36.5 32.5 28.5 25.3 22.0 19.7 17.0 13.0 9.3 6.3 3.5 1.0

30 31 32 33 34 35 36 37 39 40 41 42 43 44 45 46 48 50 52 54 56

Distilled H20 Volume

Surface Tension

FIGURE 3.7.6.  Substrate surface energy < liquid surface tension.

Equation (3.7.1) is the surface tension formula : YL cos   YSL  YS   

where: γL, γs and γSL are the free energies of the liquid and solid against their saturated vapor and of the interface between liquid and solid respectively, πe is the equilibrium pressure of the adsorbed vapor of the liquid on the solid, and θ is the equilibrium contact angle. ASTM TEST METHODS Dyne Solution Test Methods

Formamide—Distilled Water Mixtures(b) 100.0 81.2 65.0 47.0 30.6 18.2 8.6

0 18.8 35.0 53.0 69.4 81.8 91.4 96.4

3.6 (a)Measured (b)Tentative

(3.7.1)

56 58 60 62 64 66 68 70

Over the years, there have been many methods of measuring the wettability or treatment level of the web surface. All these methods require subjective interpretation by laboratory personnel. In many extrusion and converting establishments, one test method has been chosen as a shop preference. Probably the most accepted method is the wetting tension test using cotton swabs and ASTM standard concentration solutions of formamide and Cellosolve™. This method, although providing an established procedure for determining surface energy, does present several problems regarding the reliability and consistency of the results. Problem areas should be addressed in the following ways: (1) Care must be taken to limit evaporation of solutions. Evaporation changes concentrations and thereby changes dyne level values.

under conditions of 23 ± 2°C and 50 ± 5% relative humidity. mixtures not yet included in ASTM procedure.

TABLE 3.7.2.  Other Treat Level Measurements Used on Post-Treatment Surfaces. Test Method Water Spreading Test Contact Angle Test

Procedure

Measurement

A specified volume of distilled water is dropped on the material before and after corona treating.

The area covered by the water is measured and compared to determine the treat level.

A drop of distilled water is placed on the web droplet and the film. Tilting Platform Test A drop of distilled water is placed on the material, which is secured in a horizontal plane. The plane is gradually tilted. Dye Stain Test Material specimens are dipped into a special dye and dried in a vertical position. Adhesion Ratio Test (based on Pressure-sensitive tape is applied with equal force to treated ASTM tentative D-2141-63R) and untreated material. Ink Retention Test Ink is spread on the surface of the material, allowed to dry, and covered with pressure-sensitive tape.

The relative contact angle between the surface is measured and compared. The angle at which the water begins to move down the plane is measured and compared. The dye stains a treated surface, but not an untreated surface. The degree of force required to peel the tape from the material is measured and compared. The relative area of ink that is lifted from the treated surface when the tape is removed is measured and compared.

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(2) Cotton swabs have been found to have widely different effects on dyne level indications because various manufacturers of swabs use different binders to hold the cotton to the end of the stick. (3) Testing a cotton swab in different dyne solutions contaminates both the results of the tests and the bottle of solution. (4) Although the mixtures of ethyl Cellosolve™ and formamide used in this method are relatively stable, exposure to temperature or humidity extremes should be avoided. (5) Laboratory personnel using cotton swabs have a tendency to vary the amount of liquid picked up by the cotton swab, thus varying the results of the test. Also, the method of rubbing the liquid onto the poly- olefin surface with a cotton-tipped applicator varies from person to person. This rubbing force also tends to give erratic dyne-level results. It is important to maintain an even coating thickness of wetting tension solution, which will result in uniform test results. COTTON SWAB METHOD The wetting tension test using cotton swabs and dynelevel solutions measures the surface tension of polyolefin film surfaces in contact with drops of a specific solution in the presence of air. Solutions are applied to the surface of the film until a solution is found that just wets the surface of the film. A reading of the liquid behavior should be taken in the center of the liquid. Placing too much liquid on the film surface may cause severe peripheral shrinkage of the liquid. The step-by-step test procedure is as follows: (1) Wet the tip of the applicator with a calibrated dyne level solution from a stock container. A minimum amount of solution should be used because excess solution can distort the reading. (2) Spread the test solution lightly over approximately one square inch of the test material. (3) Note the time it takes for the liquid to break into droplets. If the liquid does not break into droplets within two seconds, repeat the test with a surface tension solution of the next higher rating. If the liquid breaks into droplets in less than two seconds, repeat the test with a surface tension solution of the next lower reading. Be sure to use a clean cotton applicator each time the test is repeated to avoid contaminating the solution and distorting the test results. (4) Repeat steps one through three until the solution holds for exactly two seconds before breaking into droplets. The surface tension in dynes/cm of the test solution is the surface energy of the material. This testing procedure has been a standard of the industry for many years, but the results of the testing procedure as

FIGURE 3.7.7.  Enlarged detail of wire-wound drawdown rod.

used by laboratory personnel have been somewhat inconsistent and varied. DRAWDOWN TEST METHOD An alternative method (the results of which are generally easier for laboratory personnel to evaluate) is to use a drawdown rod rather than a cotton-tipped applicator to establish a uniform thickness of wetting tension solution on the polyolefin sample (Figure 3.7.7). The following is a step-by-step procedure for measuring wetting tension solution using the dyne solution drawdown technique: (1) The film to be tested is cut into samples that will fit on a clipboard. The sample is usually about 8 × 11 inches. (2) Select three dropper bottles of dyne solutions, which bracket the desired treatment level. Typically, if a surface treatment specification were for 42 dynes/cm ±2 dynes/cm, one would select the following wet- ting tension solutions: 40 dynes/cm, 42 dynes/cm, and 44 dynes/ cm. (3) Place a film sample on the drawdown clipboard and visually divide it into three equal-sized columns, from left to right, in order of increasing dyne level. Place two to three drops of each dyne solution in the top center of each column. (4) Take a wound-wire-metering rod and place it in contact with the film just above the row of dyne solution drops. With a smooth, continuous movement, draw the rod across the surface of the film so as to simultaneously draw down a thin continuous layer of each of the three dyne solutions (Figure 3.7.8). Interpreting the results of this test is very simple. If after a two-second interval, the three drawdowns appear as in Figure 3.7.8, the wetting energy of the film sample approximately equals the surface tension of the center column. The left column of the dyne solution remains continuous and unbroken after two seconds and therefore has a surface tension lower than the wetting energy of the treated film surface. The center column of the dyne solution just begins to break up into droplets after two seconds and therefore has a surface tension approximately equal to the wetting energy of the

Section 3.7.  Surface Treatment

FIGURE 3.7.8.  Drawdown of three dyne solutions.

treated film surface. Finally, the right column of the dyne solution breaks up into droplets before two seconds and therefore has a surface tension higher than the wetting energy of the treated film surface. If two or more columns remain unbroken after two seconds, then the choice of dyne solution strengths was too low, and the test should be repeated using higher dyne strengths. Of course, the opposite is also true if two or more columns should break up into droplets before two seconds. The metering rod must be cleaned and dried thoroughly between each and every drawdown. The various wetting tension solutions should be kept in stock bottles. The solutions actually used in the testing procedure should be kept in a small dropper bottle and capped to prevent evaporation. Although the wetting tension solutions are very stable, they must be kept in a relatively stable temperature environment. Electronic equipment is available for testing solutions and for accurately determining their surface tensions. Unfortunately, this equipment is very expensive and difficult for a converter's laboratory to justify. It is better either to manufacture or to purchase wetting tension solutions from a reputable supplier. The dye used in the wetting tension solution to make it visible on the polyolefin substrate should not be one that affects the wetting tension of the test solution. Converters seem to prefer a very strongly dyed solution that is easily read by the laboratory technician, but a heavy concentration of dye may affect the integrity of the wetting tension solution. WARNINGS AND CAUTIONS When using wetting tension solutions, the converter should observe all safety precautions on the label. Formamide may cause skin irritation and is particularly dangerous in direct contact with the eyes. Safety goggles should be worn when making up new test mixtures. Ethyl Cellosolve™ is a highly flammable solvent, and precautions should be

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taken when using either ethyl Cellosolve™ or a mixture of ethyl Cellosolve™ and formamide. Both ethyl Cellosolve™ and formamide are toxic and must be handled with due care, as indicated on the label. All tests must be performed in an adequately ventilated area. According to the latest studies, solutions containing formamide or a combination of formamide and Cellosolve™ can have a teratogenic action (tending to cause birth defects). Workers should therefore wear protective gloves when handling wetting tension solutions. Questions about the appropriate precautions for use of these fluids should be directed to the supplier. Users should obtain a material safety data sheet (MSDS) for each of the components of the wetting tension solutions. If the information is not available from the supplier, contact Union Carbide, the manufacturer of Cellosolve™, or a manufacturer of formamide, such as BASF Wyandotte Corp. An MSDS will give complete information about the precautions necessary for using these materials. The wetting tension solution test, although not as objective as one might wish, will serve the converting industry until a more precise test is available. Attempts have been made to develop an inline surface-tension test method to close the loop with a computerized corona-treating system, thus increasing power output in conjunction with increased need for watt density. At this time, no such inline testing system has been developed. The demand for such a test method will undoubtedly cause the industry to continue its search. Table 3.7.3 lists various substrates along with their typical surface tensions. CORONA TREATING What Is Corona Treating? A corona-treating system is designed to increase the surface energy of plastic films, foils, and paper to provide improved wettability and adhesion of inks, coatings, and adhesives. As a result, the treated materials demonstrate improved printing and coating quality and greater lamination strength. The system consists of two major components: (1) The power supply, and (2) The treater station. The power supply accepts standard 50/60 Hz utility electrical power and converts it into single-phase, higherfrequency (nominally 10–40 kHz) power that is supplied to the treater station. The treater station applies this power to the surface of the material through an air gap by means of a pair of electrodes, one at high potential and the other, usually a roll that supports the material, at ground potential. Only the side of the material facing the high-potential electrode should show an increase in surface tension. A corona-treating system in its simplest form can be portrayed as a capacitor (Figure 3.7.9).

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TABLE 3.7.3.  Typical Surface Tensions of Substrates. Surface Tension in Dynes per Centimeter

Polymers HYDROCARBONS Polypropylene-PP, OPP, BOPP Polyethylene-PE Polystyrene PS Polystyrene (Low Ionomer) ABS Polyamide Polymethyl methacrylate-PMMA Polyvinyl acetate/polyethylene copolymer-PVA/PE co0polymer

29–31 30–31 38 33 35–42 300

Medium High Extra High

120–200,000 200–500,000 >500,000

>15 2–15 1–2

Ultra High

>3,000,000

0.5–10 2.0 2.5 50/1250

0.941 84 55 16/12 370/750 40/59 18 0.5 >2.0 2.5 50/1250

0.945 76 43 11/9 400/900 35/55 30 0.5 >2.0 2.5 50/1250

Die gap, mils/microns

LDPE Autoclave

EMA 20% MA Random

EMA 24% MA Random

Section 4.13.  Acrylate Copolymers

279

FIGURE 4.13.2.  Heat seal comparison—EMA versus LDPE.

The properties that make these resins desirable in a finished article must be addressed during film extrusion to avoid problems. For instance, low Vicat and DSC melt points can cause pellets to “bridge” in the extruder feed throat. Water cooling and a lower setpoint of 250°F are recommended to minimize bridging potential. EMA generally processes better at 10–20°F cooler than typical for LDPE of the same melt index. Although the resins are considered thermally stable, the temperature profile should be kept as low as possible to minimize cooling requirements and film blocking. A typical extrusion temperature profile for EMA blown film is shown in Table 4.13.2. Extruder horsepower requirements for processing EMA are the same as for LDPE with the same melt index. Die gaps of 50–80 mils provide optimum drawdown for monolayer

films. Viscoelastic polymers generally will not draw down as thin as their LDPE counterpart grades. A higher melt index may be required if the application demands a film thickness of less than two mils. Coextrusion layer thickness and drawdown are controlled by the same parameters as with LDPE. Tacky polar polymers have an affinity for metal and can cause more polymer buildup on the die lips than with LDPE. Blowing film with copolymer skins requires mitigation of the same resin properties that make the film desirable, i.e., the low Vicat softening point, low modulus, and tackiness. Adequate cooling air capacity and temperature, supplied through air rings and internal bubble cooling (IBC), is essential. EMA film continues to be soft and tacky above the frostline and must be cooled before reaching the collapsing frame. Slip and/or antiblock additives are typically still re-

FIGURE 4.13.3.  EMA—DSC peak melting point vs comonomer content.

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CHAPTER 4—MATERIALS

TABLE 4.13.2.  Typical Blown Film Processing Profile. LDPE, 2 MI, EMA, 2 MI, EMA, 2 MI, 0.923 g/cm3 0% MA 24% MA Extruder Zone 1, °F/°C Extruder Zone 2, °F/°C Extruder Zone 3, °F/°C Extruder Zone 4, °F/°C Extruder Zone 5, °F/°C Adapter, °F/°C Die, °F/°C Melt Temperature, °F/°C Die gap, mils/microns

FIGURE 4.13.4.  Adhesion of various polymers when coextruded.

quired to prevent sticking to the collapsing frame, wrapping the nip rolls, film blocking and to achieve the desired film coefficient of friction and improve roll handling. Likewise, casting these films can result in the molten web sticking to and wrapping the casting roll. This sticking can be managed by maintaining efficient cooling by the casting roll, minimizing extrusion melt temperature, and maximizing contact time with the casting roll. The addition of release aids like slip or antiblock mitigates casting roll sticking and can be necessary for film roll handling, film processing, and desired film properties. Transitioning from LDPE to EMA in an extruder does not require special steps. The usual melt index considerations apply when switching from one grade to another. Because these polar ethylene acrylate copolymers may loosen deposits of degraded polymer from the extruder, adequate time should be allowed for any defects to be flushed from the system. LDPE with an equal or lower melt index will purge out an EMA when switching back to LDPE. Converters commonly blend EMA with LDPE to obtain the desired properties. Due to the tacky nature of these copolymers, it is prudent to purge with LDPE before shutting down the extruder. APPLICATIONS Ethylene acrylate copolymers are used in numerous blown and cast film products. Their favorable heat seal characteristics, flexibility, high coefficient of friction, and compatibility with many polymers make these products possible. Worldwide volume of ethylene acrylate copolymers is small compared to commodity low density polyethylene. Applications

325/162 350/176 340/171 340/171 340/171 360/182 360/182 360/182 50/1250

250/121 325/162 340/171 340/171 340/171 350/176 350/176 350/176 50/1250

250/121 325/162 340/171 340/171 340/171 350/176 350/176 350/176 50/1250

include snack food laminating films, graphics laminating films as the seal layer, peelable lidding stock seal layer, specialty tapes, PET barrier films as the tie or seal layer, modifiers for other seal layers, soft medical films, masterbatch carrier resins, multilayer bags as the tie or seal layer, masking and protective structures as the cling layer, and pouches as the seal and tie layers. HANDLING AND SAFETY Ethylene acrylate copolymers are supplied in pellet form. Shipments can be packaged in bags, boxes, supersacks, bulk trucks, or railcars. Users should refer to the safety data sheet (SDS) provided by the supplier for safety and handling considerations. Like LDPE, these pellets can be air-conveyed with push/pull systems and pose no immediate safety risks due to the polymer itself. Due to their lower softening points, ethylene acrylate copolymers are more prone to form fines, streamers, and pellet clumps during pellet transfers. Pellets with a high percentage of comonomer can potentially tack together or even agglomerate in railcars and be difficult to unload. Soft pellets can deposit polymer on the walls of transfer piping. This thin deposition breaks loose to become streamers or “angel hair” that may clog augers and extruder feed throats. Transfer systems should be designed and operated according to industry guidelines to minimize streamer formation. Particular attention should be paid to the design of piping, elbows, and system layout. Transfer air temperatures above the Vicat softening point or melting point of these copolymers can cause pellet fusing. Clumps of fused pellets can then clog transfer systems or feed augers. These copolymers do not require a hazard warning according to the OSHA Hazard Communication Standard 29 CFR 1910.1200. As with all polyethylene-based polymers, melt extrusion presents the chance for thermal burns. Appropriate

FIGURE 4.13.5.  Random and Blocked Incorporation of Acrylate Comonomer.

Section 4.13.  Acrylate Copolymers

personal protective equipment (PPE) should be worn in and around the extrusion die area. Fumes given off during the melt extrusion of ethylene acrylate copolymers may be irritating to some individuals, but are not generally considered hazardous when the extruder area is properly ventilated. Please consult the safety data sheet (SDS) for any resin before extrusion.

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REGULATORY Statements of regulatory compliance are specific to each resin type and grade and must be provided by the resin manufacturer. Some copolymers have FDA approval for food contact, but others are limited. Please consult each specific resin supplier for detailed regulatory information.

Chapter 4—Section 14

Acid Copolymers and Ionomers BARRY A. MORRIS and SCOTT B. MARKS, The Dow Chemical Company

KEYWORDS: acid copolymer, ionomer, EMAA, EAA, sealant, hot tack, product resistance, oil resistance.

INTRODUCTION Acid copolymers are specialty polymers used as either tie layers or sealants in flexible packaging. They are formed by copolymerization of ethylene with a carboxylic acid in a high-pressure free-radical polymerization process, which is similar to the process for producing LDPE. These polymers have outstanding processability in blown-film, cast-film, and extrusion-coating processes. The carboxylic acid groups incorporated into the polymer provides chemical functionality that enhances both bonding and sealing characteristics, enabling longer shelf life and/or greater seal integrity in flexible packaging applications. Ionomers are distinctive polyolefin copolymers formed by neutralizing acid copolymers with metal salts. The combination of acid functionality (hydrogen bonding) and metal salt complexes (ionic bonding or crosslinking) imparts unusual solid-state and molten characteristics to ionomers. Unlike thermoset polymers, ionic crosslinking is thermally reversible, and therefore the beneficial effects of high molecular weight on melt rheology, toughness, and other end-use properties can be achieved without any sacrifice in melt processing. With acid copolymers, unique product properties are created by the acid content, polymer chain length, and branching. With ionomers, secondary variables such as the type and quantity of metallic ions permit a much wider range of resin properties than can normally be achieved within the production variables of polyolefin polymers. Applications take advantage of the unique combination of low-temperature sealability, wide hot-tack temperature window, high melt strength, oil resistance, toughness, and adhesion to polar substrates. The uses of ionomer resins are spread across a wide range of packaging applications, wherever challenging sealant requirements exist. PROPERTIES General Properties and Characteristics The general performance of acid copolymers is dominated by two factors: the amount of acid functionality, and the mo-

lecular weight (or melt flow rate (MFR), which is inversely related to MW). Table 4.14.1 shows how these factors influence many film and coating properties. There are two commercially important types of ethylene acid copolymers: ethylene-methacrylic acid copolymers (EMAA) and ethylene-acrylic acid copolymers (EAA). These are similar except for the difference in molecular weight of the acid groups, as shown in Figure 4.14 1. The molecular weight of methacrylic acid (86) is slightly higher than that of acrylic acid (72) due to the additional methyl group (CH3). Therefore, there are 20% more functional acid groups in an EAA copolymer than in the same weight percentage of an EMAA copolymer. For example, a 12 wt% EMAA copolymer has the same level of molar acid functionality as a 10 wt% EAA copolymer. The industry generally reports acid level in weight percent. Because most characteristics vary with actual acid group functionality (mole %), this distinction between EMAA and EAA copolymers should be factored into performance comparisons of these sister-like products. Ethylene-acid copolymers are part of a family of polar copolymers based on the polymerization process used for LDPE. This process enables the long-chain branching important for polymer processing as well as the introduction of chemically functional short-chain branches. Other examples of polar copolymers are ethylene vinyl acetate (EVA) and ethylene methyl acrylate (EMA). Other types of copolymers using non-polar short-chain branches (introduced by incorporating butene, hexene, and octene) are based on linear polyethylene technology (containing no or limited long-chain branching) and are non-polar in nature. The latter non-polar linear polymers are typically made in a different polymerization process than LDPE and the long-chain branched polar copolymers such as EMAA, EAA, EVA, and EMA. As with other copolymers, the acid short-chain branches of EMAA and EAA disrupt the formation of crystals as the polymer solidifies. The crystallinity of these resins is reduced proportionately to the number of acid groups (shortchain branches). Crystallinity-dependent properties such as melting point, hardness, and gas barrier are subsequently 283

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TABLE 4.14.1.  Effects of Increasing Acid Content and Increasing MFR. Effects of Increasing Acid Content

Effects of Increasing Melt Flow Rate

• Higher tensile strength • Lower elongation • Lower modulus • Higher tear resistance • Higher abrasion resistance in presence of oil • Lower melting and freezing points • Lower sealing temperature • Greater hot-tack strength • Lower haze • Higher moisture (water) vapor transmission rate (MVTR or WVTR) • Higher aluminum foil adhesion

• Improved drawdown capability • Improved optical properties • Lower melt strength (higher neck-in) • Lower impact resistance • Lower abrasion resistance • Lower flex crack resistance

FIGURE 4.14.2.  lonomer chemistry.

reduced with increasing amounts of acid comonomer. When acid copolymers are compared to LDPE, the lower melting point (e.g., ~95°C at 15% EMAA vs. 111°C LDPE) can be either an advantage or a disadvantage, depending on the application. MAA and AA molecules are much heavier than non-polar short-chain branches. With polyethylene, the density goes down as the number of short-chain branches increases (lower crystallinity). The opposite is true for polar copolymers, including acid copolymers. With these resins, the weight of the branches increases the polymer density. With polyethylene, density is a predictive indicator of properties such as melting point and water-vapor transmission rate. This is not the case for polar copolymers. Acid copolymers are the starting point in the two-step process for making ionomers. In the second step, some of the acid groups are neutralized with a metal ion. As shown in Figure 4.14.2, the neutralized acid groups act like crosslinks

between the polymer chains. Ionomers have some similarity to LDPE and acid copolymers in many aspects that are important for extrusion processing and application end uses. For more commonly available ionomers, melting points range by grade from 82°C (180°F) to 100°C (212°F), and the copolymers can be easily melted from free-flowing pellets. The polymer can then be extruded as thin webs at high elongation rates with very good melt strength. Compared with LDPE, metallic ion crosslinking in ionomers has an important effect on molten resin properties. At low melt temperatures, the ionic structure causes very high resistance to flow (high melt viscosity). At elevated temperatures, crosslinking is reduced, yielding lower viscosity and enabling good melt processing, as illustrated in Figure 4.14.3. The properties of ionomers are highly influenced by their chemistry. The primary factors include the initial amount of acid functionality, the amount of that acid functionality that is neutralized by metal ions, the amount of un-neutralized acid functionality remaining, and the type of metal used. The effects of these variables are summarized in Table 4.14.2. Like acid copolymers, increasing acid functionality reduces crystallinity, which contributes to lower haze, higher transparency, and greater toughness. Metal cations introduce ionic bonds that increase stiffness, toughness, tensile strength, oil resistance, and impact and abrasion resistance. Too much

FIGURE 4.14.1.  Chemical structure of EAA and EMAA.

Section 4.14.  Acid Copolymers and Ionomers

FIGURE 4.14.3.  Melt viscosity vs. temperature.

neutralized acid, however, can lead to low notched tear strength and poor adhesion to PE in coextrusion. Typically, not all acid groups are crosslinked with a metal ion; some level of un-neutralized acid functionality is required for polymer flow. Increasing the amount of un-neutralized acid helps adhesion to polar substrates such as aluminum foil or nylon (polyamide). The two most commonly used metal ions used in ionomers are sodium and zinc; however, ionomers based on other metals are in use, primarily for industrial (non-packaging) applications. Sodium ionomers generally have lower haze and higher oil resistance. Zinc ionomers generally have better adhesion to polar substrates and pick up less moisture than sodium ionomers. Heat-Seal and Hot-Tack Properties The excellent heat-sealing properties of acid copolymers result from the attractive forces of hydrogen bonding between acid groups and from the reduction in melting point that is characteristic of short-chain branching. Hence, heatseal performance is strongly influenced by the acid content of the copolymer. Hydrogen bonding enhances the ability of acid copolymers and ionomers to seal through contaminants because the acid groups lock on to those of the opposing sealant web. Likewise, hot-tack strength is also significantly

285

improved by the molecular attractive forces of hydrogen bonding. In cases where hydrogen bonding between acid groups enhances the hot-tack properties of acid copolymers, the additional attractive energy provided by ionic crosslinking extends the hot-tack strength of ionomers to a higher level and over a broader temperature range. Hence, heat-seal performance is strongly influenced by both the acid content and the ion content of the copolymer. Figure 4.14.4 shows a comparison of 9% MAA acid copolymer (10 MFR) with a low-acid / low-neutralized zinc ionomer (5 MFR) and a high-acid / higher-neutralized zinc ionomer (14 MFR). Compared to linear low-density polyethylene (LLDPE), metallocene LLDPE (mPE) resins, and polyolefin plastomers (POP), ionomer resins provide higher hot-tack strength over a wider heat-sealing temperature range. Polyethylene resins depend on freezing and crystallization to obtain hottack strength because they lack the strong chemical attractive forces that benefit the acid copolymer and ionomer resins. Metallocene linear polyethylene and polyolefin plastomer resins do provide good peak hot-tack strength, particularly in blown-film applications, but the high hot-tack temperature range is usually narrower. Extrusion-coating and cast-film grades of metallocene linear polyethylene resins have less impressive hot-tack curves than their corresponding blownfilm grades, primarily due to the low molecular weight and low melt strength of these polymers. Figure 4.14.5 shows the hot-tack strength for common blown-film grades of ionomer, acid copolymer, and plastomer. Note the wide range of temperatures under which the ionomer can operate and still provide a given level of hot-tack strength. Hence, acid copolymers or ionomers should be selected for applications where hot-tack strength over a broad range of sealing temperatures is required. Keep in mind that on most packaging machines, fluctuations in seal-bar temperature and/or line speeds are normal. When specifying heat sealants, be sure to try out the sealant system over a range of processing conditions on the packaging machine. Although a given sealant may perform well under the controlled conditions of a qualification trial, changing conditions on the packaging machinery may result in seal failures in the future. Both the sealant material and

TABLE 4.14.2.  Properties of Ionomers and Their Relationships. Higher % Acid

Higher % Un-Neutralized Acid

Higher Ion Content

• Stiffer • More oil-resistant • Better optical properties • Higher tensile strength • Higher impact strength • Lower seal initiation temperature

• Higher aluminum adhesion • Higher nylon coextrusion adhesion • Higher flex durability • Higher tear strength • Lower notch sensitivity

• Stiffer • More oil-resistant • Better optical properties • Higher tensile strength • Higher impact strength • Tougher • More abrasion resistant • Lower notch tear strength • Higher / broader hot-tack strength • Less blocking

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FIGURE 4.14.4.  Comparison of 9% MAA acid copolymer with a low-acid/low-neutralized zinc ionomer (1) and a high-acid/higher-neutralized ionomer (2).

its thickness must be able to withstand such variations. Premium sealant materials can often pay for themselves with reduced complaints and claims down the road. Adhesion Adhesion of acid copolymers and ionomers to polar substrates such as aluminum foil and metallized films in extrusion coating is covered in detail in the TAPPI Extrusion Coating Manual [3,4]. In general, high-acid copolymers with 9% AA (10% MAA) or higher provide durable bonds to foil in the presence of aggressive food products. Ionomers also provide durable bonds to foil; for these applications, zinc ionomers with a high level of un-neutralized acid are preferred. For blown- and cast-film coextrusions, acid copolymers generally bond very well to various types of polyethylene,

EVA, and acrylate copolymers. However, for ionomers, adhesion to adjacent materials needs to be considered more closely. In general, most zinc-based ionomers bond well to polyethylene, but for sodium ionomers, the adhesion to polyethylene may not be enough for demanding applications. In these cases, it is often suggested to use a layer of acid copolymer, EVA, or EMA as the adjacent layer in a coextrusion to ensure higher interlayer adhesion to the sodium ionomer and the polyethylene. In some applications, controlled adhesion between an ionomer sealant and a PE adjacent layer can be advantageous. One type of easy-open peel-seal technology that is often used for packaging of dry foods such as crackers or cereal involves coextruding an ionomer with HDPE in a blown film. The ionomer (or blend of ionomers) is selected so that during opening, failure occurs by delamination between the ionomer and HDPE layers. This produces a very controlled

FIGURE 4.14.5.  DTC Hot-Tack Force: Ionomer (1.3 MFR, Sodium), Acid Copolymer (1.5 MFR, 12% Acid), and plastomer (1 MFR, 0.902 g/cm3 Density). Data from J. de Garavilla, 1995, TAPPI Journal, vol. 78, p. 191–203.

Section 4.14.  Acid Copolymers and Ionomers

opening force that is less affected by seal temperature and aging than other peel-seal technologies. Mechanical and Optical Properties For acid copolymers, hydrogen bonding between polymer acid groups provides increased toughness compared to LDPE. Mechanical properties such as tensile strength are influenced by molecular weight, and therefore differences in both acid and melt flow rate (MFR) must be considered. In general, ionomers are tougher, stronger, and stiffer than LDPE and acid copolymers. The toughness, stiffness, and strength properties of ionomers typically age up over a 10 to 30-day period following extrusion. This is due to reorientation or clustering of the ions within the polymer and secondary crystallization. Acid copolymers generally have lower crystallinity than LDPE and therefore have lower haze and higher transparency. Ionomers, especially sodium ionomers, generally have even lower haze and higher gloss in blown films than either LDPE or acid copolymers. Typical Film Properties Typical properties of an acid copolymer (9 wt% MAA, 2.5 g/10 min MFR, equivalent to 7.5 wt% AA grade of same MFR) and an ionomer (sodium neutralized, 1.3 g/10 min MFR) are given in Table 4.14.3. Properties vary across the range of acid level, MFR, neutralization level, and cation available, and therefore these values should be used only as a guide. APPLICATIONS Applications for acid copolymers in flexible packaging fall into two major categories: (1) foil adhesion applications, with the resin often functioning as a tie layer between foil and PE in extrusion coating/lamination, and (2) sealant applications, which take advantage of their low sealing tem-

perature, broad sealing temperature range, very good hot tack, and seal through contamination. Specific applications include juice cartons, toothpaste tubes, meat packaging, spiced oil, medical packaging, condiments, and shampoo sachets. Acid copolymers used for film extrusion typically have 3-12 wt% acid and a melt flow rate (MFR) in the range of 1–5 g/10 min. Extrusion-coating grades are available with higher MFR, typically in the range of 7–10 g/10 min. Ionomers are used as sealants for food and non-food packaging, as skin packaging film, and in medical packaging forming webs. The properties that make them attractive in these applications include low sealing temperature, very broad sealing temperature range, excellent hot tack, and the ability to seal through contamination. Ionomers also have other outstanding properties such as abrasion resistance, clarity, formability, adhesion, and resistance to grease/oil penetration. Examples of the use of ionomers as a sealant include processed meat, powdered products, medical and liquid sachets, canister inner liners, and snack foods. Applications with skin packaging take advantage of ionomers’ excellent clarity, formability, and outstanding adhesion to board stock. Medical packaging forming webs take advantage of the formability and puncture/abrasion resistance of ionomers. Ionomers are also used in many non-packaging applications such as components of golf balls (toughness, resilience), perfume caps (glass-like transparency, chemical resistance, and polymer processing), and protective surfaces (stain and abuse resistance). Ionomers used for film extrusion typically have an MFR of 0.7–14 g/10 min, with melt points of 185–212°F (85–100°C). Higher-MFR grades can be used in blown film than what is typically used for PE due to the additional melt strength provided by the ionic forces. GENERAL PROCESSING CONSIDERATIONS Safety precautions for processing acid copolymers and ionomers are similar to those for other common thermoplas-

TABLE 4.14.3.  Typical Film Properties. Property Melt Point, °C Freeze Point, °C Vicat Softening Point, °C Tensile Strength, psi (103) MD (TD)1 Elongation, % MD (TD)1 Modulus, psi (103), MD (TD)1 Spencer Impact, in lb/mil1 Elmendorf Tear, g/mil MD (TD)1 Gloss, 20°2 Haze, % 12 22

mil blown film, 2.5:1 BUR mil blown film, 3.0:1 BUR

287

Acid Copolymer, 9 wt% MAA, 2.5 MFR

Ionomer, Na, 1.3 MFR

101

96

ASTM D 3418

88 85 4.1 (4.1) 490 (450) 16.5 (16.6) 5.5 102 (137) 50 3.82

68 74 5.3 (4.8) 350 (450) 33 (30) 6–7 18-22 (28-32) 50–90 1–31

ASTM D 3418 ASTM D 1525 ASTM D 882 ASTM D 882 ASTM D 882 ASTM D 3420 ASTM D 1922 ASTM D 523 ASTM D 1003

Method

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tics. Care should be taken to protect hands and other parts of the body when handling molten polymer. At typical processing temperatures, small amounts of fumes may evolve from the resin. Adequate ventilation should be provided to remove fumes from the work area. Extrusion conditions and equipment used for LDPE are generally suitable. Due to the potential corrosive nature of acid copolymers and ionomers, equipment should be duplex-chrome or nickel-plated. However, many equipment manufacturers have been supplying these plated systems for conventional PE resins for years, and therefore this recommendation is more a statement of standard practice than a special requirement for acid copolymers. Normal blown-film processing temperatures for acid copolymers and ionomers are 365–455°F (185–235°C), although in coextrusion, higher melt temperatures may be dictated by the melting point of other polymers in the structure. Because ionomers have a low melting point and a tendency to stick to metal, the rear barrel zone of the extruder should be at 230–275°F (110–135°C) to prevent bridging of pellets in the extrusion feed section. No drying or special storage is required for acid copolymers. Ionomers, however, are hygroscopic and absorb moisture. Although they are produced in a dry state and packaged in foil-lined bags or boxes, opening the packages and exposing the resin to the atmosphere will increase the moisture content, and high levels of moisture will affect film production (although moisture will have no effect on end-use properties such as sealing or thermoforming). In practice, moisture is not a problem if reasonable precautions are taken. Resin that becomes wet may produce die deposits during extrusion or result in film with surface defects or bubbles during extrusion coating. Re-drying of ionomers is quite difficult and requires specialized drying and handling equipment, and therefore using best practices to minimize

moisture pickup of ionomer is important. Consult with your ionomer resin supplier for more information. Equipment running acid copolymers or ionomers should be purged with LDPE before shutdown (See Section 5.11 on extrusion system purging in this manual). Although they have good thermal stability, acid copolymers can crosslink and form gels when exposed to high temperatures over time. © 2019 The Dow Chemical Company REFERENCES AND ADDITIONAL RESOURCES [1] Glick, W. J., 2005, “Acid Copolymers,” in TAPPI Film Extrusion Manual, 2nd ed., TAPPI Press. [2] Glick, W. J., 2005, “Ionomer,” in TAPPI Film Extrusion Manual, 2nd ed., TAPPI Press. [3] Vansant, J. D., Marks, S. B., Morris, B. A., 2017, “Acid Copolymers for Extrusion Lamination and Coating,” TAPPI Extrusion Coating Manual, 5th ed., TAPPI Press. [4] Vansant, J. D., Marks, S. B., Morris, B. A., 2017, “Ionomer Resins for Extrusion Coating,” TAPPI Extrusion Coating Manual, 5th ed., TAPPI Press. [5] Brebner, D. L., “The Role of Acid Copolymers and lonomers in Extrusion Coating,” 1987 Extrusion Coating Conference. [6] Short Course Notes, TAPPI PRESS, Atlanta, p. 19. [7] Brentin, R. P., Modern Plastics Encyclopedia, 1989, p. 72. [8] Daniels, C. A., “Unique Performance from Specialty Resins,” 1985 Film Extrusion Short Course Notes, TAPPI PRESS, Atlanta, p. 55. [9] Green, R. E., Extrusion Coating Manual, TAPPI PRESS, Atlanta, 1990, p. 221. [10] Statz, R. J., Modern Plastics Encyclopedia, 1989, p. 77. [11] Morris, B. A., “Ionomers,” in The Wiley Encyclopedia of Packaging Technology, 3rd ed., Wiley, New York, 2008. [12] Morris, B., Pennias, J., and Walsh, D., “Ethylene Acid Copolymer Metal Salts (Ionomers),” Polymer Data Handbook, 2nd ed., Mark, J.E. (Ed.), Oxford University Press, 2009.

Chapter 4—Section 15

Polyvinylidene Chloride (PVDC) KUN SUP HYUN, Formerly of The Dow Chemical Company

INTRODUCTION Homopolymer of vinylidene chloride (VDC) is not commercially available because it is not practical to fabricate due to the closeness of the decomposition temperature to the melting point. However, copolymers of VDC with greater than 70 percent VDC are available for commercial use in blown monolayer and multilayer films and in cast multilayer flexible and rigid structures. Common comonomers for VDC copolymers are vinyl chloride, methyl acrylate, and butyl acrylate. The copolymer often contains formulants, such as plasticizers and stabilizers, which are added by the manufacturer. These formulated copolymers are commonly referred to as PVDC, but the more precise compositions and formulations remain proprietary. CHARACTERISTICS VDC copolymers are rich in vinylidene chloride and are usually semicrystalline. Common commercial products have 20–50% crystallinity. This results in a thermoplastic polymer that can be melt-processed above its melting point, 266–356°F (130–180°C), but below its degradation temperature. Thermal stability is a function of temperature and time. VDC copolymers have a glass transition temperature near ambient, which usually results in a stiff and brittle material. Plasticizers and film orientation can be used to improve the softness and ductility of the polymer. Weight average molecular weights of VDC copolymers are in the range of 80,000–120,000 Daltons. Pyrolysis and gas chromatography are used to determine compositions. Typical VDC contents for commercial PVDC resins are 80–94%. PVDC films are used in food packaging as barriers to oxygen, moisture, flavorants, odors, and other gases. Monolayer films are widely used as household wrap. Industrial monolayer films are used in laminating; drum and pack liners for moisture-, gas-, and solvent-sensitive products; unit dose packaging of pharmaceuticals, cosmetics, and medical supplies; and food packaging. Multilayer films containing a PVDC barrier layer and laminated shrink films are used to package fresh red meat, processed meat, cheese, and poultry.

A unique characteristic of VDC copolymers is their excellent barrier capability to mass transport over the full range of barrier to oxygen, water vapor, and carbon dioxide. At room temperature, VDC copolymers are available with oxygen permeabilities as low as 1.1 × 1014 cm3 (STP)-cm2/cm3 -sec -cmHg. This is a very important asset in a food packaging application. VDC polymers have also performed as excellent barriers to flavor and aroma. Both d-limonene and trans-2-hexenal have permeabilities in VDC copolymers of 1 × 1018 kg/m2 sec Pa. VDC copolymers melt over a wide range of temperatures, starting as low as 239–374°F (115–190°C). The normally quoted melt temperature is at the peak endotherm. VDC copolymer has low specific heat in both the solid and the molten state. It has a specific gravity of about 1.7. Normal bulk density of powder resin is 800 kg/m3. Melt viscosities of commercial VDC copolymers are a function of molecular weight only within the process temperature range. A typical melt viscosity of 100,000 Mw at 100/sec at 347°F (175°C) is 1000 Pa-s. However, mechanical properties are functions of the comonomer type and level as well as the degree of crystallinity, orientation, and plasticizer content. Typical semicrystalline copolymers tend to have high tensile moduli and low elongations at break. Impact strengths are low unless the polymer is reinforced either by lamination or coextrusion. PROCESSING PVDC resins can be processed in the extruder to make monolayer or multilayer films by the bubble process or the coextrusion cast process. Monolayer films are made by quenching the round tube of extrudate in cold water, reheating and inflating the tube with air, then collapsing the bubble into the takeaway pinch roll to produce a mill roll. Multilayer films are made either by laminating the PVDC bubble onto a blown supporting film or coextruding it with skin layers in a multilayer die, whether a feedblock or a manifold die. VDC copolymers are susceptible to degradation in the presence of metal contaminants such as Fe, Al, Zn, Sn, and Cu. Process equipment that comes into contact with PVDC 289

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melts require nickel-plating or high-nickel alloys such as DuranickelTM (a trademark of Huntington Alloy Products Division, The International Nickel Co., Inc.), or HastalloyTM (a trademark of Union Carbide Corporation). Moreover, because thermal stability is a function of time and temperature, the screw, the transfer line, and the die must be designed to eliminate dead space and minimize residence time. Processing conditions for VDC copolymer resin extrusion depend upon the type of copolymer, its melting point, and its molecular weight. A wide range of screw sizes are used, from 3/4-inch to 4 1/2-inch diameter, with L/D ratios ranging from 9:1 to 24:1. Most screws are single-stage, squarepitch, and single-flighted, with compression ratios ranging from 3.0–5.0. Typical extrusion conditions are available from the PVDC manufacturers.

REFERENCES AND ADDITIONAL RESOURCES [1] Clarke, D. L., “Vinylidene Chloride,” Modern Plastics Encyclopedia, 1988, p. 109. [2] IeLassus, P. T., et al., ACS Symposium Series No. 365, “Food and Packaging Interactions,” Hotchkiss, J. H. (Ed.), 1988, p. 11. PVDC 585. [3] Hyun, K. S., J. Vinyl Tech, 8(3):103 (1986). [4] Park, W. R. R. and Conrad, J., “Biaxial Orientation,” Encyclopedia of Polymer Science and Technology, Vol. 2, p. 344, Wiley, 1965. [5] Wessling, R. A., “Polyvinylidene Chloride,” Polymer Monographs, Vol. S, Gordon and Breach, New York, 1977. [6] Wessling, R. A., et al., “Vinylidene Chloride Polymers,” Encyclopedia of Polymer Science and Technology (2nd ed.), Vol. 17, p. 492, Wiley, 1989.

Film Extrusion Manual, Second Edition, 2005

Chapter 4—Section 16

Polymer Processing Additives (PPA) CLAUDE LAVALLÉE, 3M Company

KEYWORDS: PPA, additive, process aid, fluoropolymer, fluoroelastomer, sharkskin, melt fracture, LLDPE.

INTRODUCTION Processing aid is a general term that refers to several classes of materials used to improve the processability and handling of high-molecular-weight polymers. The name polymer processing additive (PPA) is usually associated with fluoropolymer-based additives. The additives discussed in this chapter are primarily fluoropolymer-based, and use is focused on polyolefin-based polymers. The most commercially relevant example is the use of fluoroelastomer processing additives to eliminate sharkskin melt fracture in LLDPE. However, they can be used in most extrusion processes and in a broad range of resins. A large amount of work has been done to understand how polymer processing additives function within resin systems and how best to capture their benefits. As an example, several papers have been written on the topic of additive interactions [1,2], covering a broad range of additives such as acid neutralizers, antiblocks [3–5], pigments [6], and antioxidants [7]. Studies have also shown additional benefits and applications. For example, although PPAs have been mainly used in linear low-density polyethylene (LLDPE) for melt fracture elimination, they have shown benefits in eliminating die lip buildup [8], gels [9–12], and die swell [13], as well as controlling degradation by enabling lower processing temperatures. In many cases, PPAs could be considered as process stabilizers. A good review of such studies was previously published [14]. PPAs can provide additional benefits by enabling process improvements such as: material reduction through down gauging with higher viscosity resins, energy use reduction through lower extrusion temperatures or lower torque, or waste reduction through faster material transitions. Coating formation is one aspect of PPA technology that has been studied only to a limited extent. The accepted model for the coating process is a dynamic process by which the coating is constantly formed and removed, resulting in an equilibrium layer on the wall. If this is the case, then coating characteristics such as thickness or coated area should depend on process conditions such as concentration, shear rate, and temperature. Examples of

the effect of shear rate on the coating process have been published and have showed a direct correlation between coating time and shear rate [15–17]. Description of Polymer Processing Additive Technologies Fluoropolymer PPA technology was developed in the 1960s to shift the formation of melt fracture to higher extrusion rates [18]. When DuPont Canada first introduced LLDPE to the market, it contained fluoropolymer as a polymer processing additive. The processing additive was added to improve resin processability, enabling the resin to be extruded on LDPE equipment at high shear rates without exhibiting sharkskin melt fracture. When LLDPE was more widely introduced in the 1980s, resin manufacturers continued to add processing additive to match this competitive standard [19]. Typical use conditions are extrusion processes where the shear rate is less than or equal to 2000 sec–1 at temperatures between 170°C and 300°C. PPAs are mostly used in LLDPE and HDPE with melt indices (MI) between 2 and 0.3 g/10 min (190°C, 2.16 kg), but can also be used in resins with other MIs or in polyamide, polyvinyl chloride, polystyrene, polyester, and acrylic. The fluoropolymer most widely used as a processing additive is a copolymer of vinylidene fluoride and hexafluoropropylene (1). This material is commonly referred to as a “fluoroelastomer”, even though the polymer is not crosslinked when used as a processing additive:

(4.16.1)

This polymer received FDA approval as an indirect food additive when used in polyethylene film in 1988. It has been the primary fluoropolymer processing additive for the last 40 years and still enjoys wide usage today. Much has been learned about the fluoropolymer PPA 291

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mechanism in the years since. This understanding has led to improvements in fluoropolymer processing additive technology through blends with other polymers [20,21], formulations with other additives [2,22–25], and material forms. As the technology has evolved, the field of application has widened to include many benefits not attributable to a classical “lubricant”. This evolution has put fluoropolymer processing additives in a class that is separate and distinct from polymer lubricant technology. Other fluoropolymer chemistries are now available [26– 28], and the same principles apply to these additives as to fluoroelastomers. However, FDA status may vary from one additive to another. For sake of simplicity, this discussion will be limited to fluoroelastomers. Several PPA grades were used for the comparisons provided in this chapter. For sake of simplicity, acronyms will be used. Table 4.16.1 provides a reference to the materials used in these examples. Mechanism of Processing Additives Melt Fracture During extrusion, as the extrusion rate increases, LLDPE successively goes through sharkskin melt fracture (SS), cyclic melt fracture (CMF), and gross melt fracture (GMF). PPA will eliminate SS and CMF, but will have little effect on GMF. Under typical blown-film extrusion conditions, only SS will be observed because CMF and GMF are associated with bubble instability and are very difficult to maintain. CMF and GMF can still be observed in other extrusion processes. In summary, as the shear rate is increased, the flow profile inside the die becomes sharper, with higher velocities in the center of the die and still very low velocity at the die wall.

TABLE 4.16.1.  PPA Grades: List of Acronyms. 3M™ Dynamar™ Polymer Processing Additive PPA 1

FX 9613

PPA 2 PPA 3 PPA 4

FX 5920A FX 9614 FX 5929M FX 5911

PPA 5

This leads to higher levels of stored elastic energy in the molten polymer. At the die exit, elastic recovery occurs so that all the polymer layers are moving at the same speed. Depending on the polymer characteristics, temperature, and amount of stored energy, the recoil will stretch the outer layers of the melt. If the stretch rate is high enough, the outer layer of the extrudate will be torn at more or less regular intervals, giving the characteristic sharkskin pattern. Figure 4.16.1 shows some pictures of typical sharkskin extrudate. All the strands of neat LLDPE extruded at more than 50/s show some level of sharkskin, except for the strand extruded at 800/s, which shows CMF. Melt fracture can occur in all forms of extrusion processes. There are a few options for eliminating melt fracture without processing additives, such as raising the melt temperature, widening the die gap, or changing the resin technology. Polymer processing additives usually give a broader processing window than these process changes and can be used effectively in most extrusion processes. Note that PPA efficacy increases with throughput. However, at throughputs nearing CMF conditions, the efficacy starts to decrease again. For this reason, optimized extrusion conditions may be required to reap the benefits of PPA.

FIGURE 4.16.1.  Extrudates from capillary rheometry (increasing shear rates from left to right: 50, 70, 100, 125, 150, 200, 300, 400, 500, 600, 800/s): (a) 0.9-MI LLDPE, 0.918, 200°C; (b) 0.9-MI LLDPE + 500 ppm PPA 3, 200°C (courtesy of 3M).

Section 4.16.  Polymer Processing Additives (PPA)

293

FIGURE 4.16.2.  PPA pattern: (a) coated on a sapphire die surface (500×), (b) imprinted on a polyethylene extrudate (500×), (c) coated on a stainless steel surface (200×) [34,35] (courtesy of 3M).

Coating Mechanism Processing additives work by providing a release surface on the die wall that promotes slippage of the molten polymer against the PPA-coated die wall. Processing additives are usually added at levels between 0.01% and 0.1%. They form a completely immiscible blend with polyethylene. Usually, the PPA will be in the form of a dispersed phase, with a droplet size of a few microns. These droplets will fall onto the die wall in a random fashion, similarly to raindrops on a concrete surface. In the process, the PPA will adhere to the oxides and hydroxides of the metal surface. The high interfacial tension between the PPA coating and the flowing polymer minimizes the removal rate and also favors coating formation [29]. With time, the whole die will be covered, providing a slip surface at the die wall. A large amount of work has been done to understand the slip velocity induced by using a PPA. Studies have been carried out using plate, slit, and capillary rheometers [30,31]. Subsequent work has included a detailed study of the effect of PPA at micro-scale [32]. A complementary study was later published [33], which gave a preliminary evaluation of coating formation on the wall. More extensive studies were published afterward [34,35]. Slip at polymer-polymer interfaces can have large rheological effects [36], as shown by Zhao and Macosko on multilayer structures. This effect of PPAs on extrusion processes was studied in detail through flow visualization to show slippage directly and its effect on the polymer flow profile inside the die [32]. In this case, the study showed release of built-up material from the extruder (cleaning effect), elimination of sharkskin, and formation of a coating on the die wall. In addition, this paper showed an increase in slip velocity and a change in the flow profile induced by the PPA. Finally, these measurements were used to confirm that slippage occurs at the PPA/polyethylene interface and that sharkskin is related to elongational stretch at the die exit and not to a critical shear rate, shear stress, or a slip-stick phenomenon inside the die. These results were confirmed by Schwetz et al. [37], who

also showed a change in the shape of the flow profile by using laser Doppler velocimetry. Again, slip at the die wall was observed when using PPA. Note, however, that the PPA coating process is dynamic, consisting of both PPA deposition and removal, with the end state reaching equilibrium. The PPA concentration in the finished product is the same as the feed composition. If this were not an equilibrium state, the PPA thickness would keep increasing, and eventually the die gap would be affected. Any parameter that affects either deposition or removal rate has an impact on PPA efficiency. The slip surface created by PPA prevents elastic energy storage inside the die by allowing its release through plug flow. It also prevents the molten polymer from sticking to the die, preventing slip-stick. Other benefits of this phenomenon include reduction of die swell, elimination of die buildup, and reduction of gels. Microscopic study of a sapphire die showed that the fluoropolymer coating on the die wall forms a regular pattern [32]. Figure 4.16.2(a) shows a micrograph of the die surface coated with PPA during extrusion. The surface shows a streak pattern on the die surface. The streaks are not static; clearly, they are due to the structure of the fluoropolymer at the surface. The structure of the fluoropolymer coating on the die leaves its imprint on the molten extrudate surface as it exits the die. This is visible in Figure 4.16.2(b). This imprint is not visible on the finished product. Before the polymer crystallizes, elastic recovery (die swell) and surface tension smooth out the extrudate surface. To verify that the streaks were not an artifact of the flow visualization technique, a slit die with metal shims was used. Again, the die was coated by PPA during extrusion. After extrusion, the stainless-steel shim was placed under a reflection microscope equipped with differential interference contrast, and the same streak pattern was observed, as shown in Figure 4.16.2(c). The streaks were not visible when no PPA was used. A detailed view of the pattern is shown in Figure 4.16.3. The streaks are approximately 1 μm in width and

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FIGURE 4.16.3.  Photomicrograph of a PPA 2 coating on a nickel surface (1000×) (courtesy of 3M).

5 μm apart. In some areas, individual PPA droplets are visible. Scanning electron microscopy (SEM) coupled with energy-dispersive X-rays (EDX) was used to confirm the existence of the pattern and determine whether the coating was discontinuous or whether the ridges observed by light microscopy corresponded to thickness variation in a continuous coating. The three pictures in Figure 4.16.4 show clearly that the coating is discontinuous. On the SEM picture, the PPA ridges are visible as four lines marked by shadows. The edge of a fifth line is visible at the top of the picture. The EDX maps corresponding to this area show a good match of the pattern to the SEM. On the carbon map and the fluorine map, the elemental concentration is translated into signal intensity, with higher concentration being indicated by the white areas on the picture. The white patterns on those two maps correspond to the fluoroelastomer on the die. The alignment between the SEM image and the EDX is not perfect, but the pictures can be overlaid to show a perfect match. This

was verified at several locations on the die under a range of magnifications. The formation of a string-like structure under flow conditions is not unique to a fluoropolymer/polyethylene blend. A good description of the phenomenon was given by Migler et al. [38,39], who gave a correlation with material characteristics and shear conditions. Because PPA forms individual strands, the thickness of the PPA strands could be measured by profilometry and was found to be approximately 200 nm [34]. This was afterward confirmed by Kharchenko et al. using frustrated total internal reflection imaging (FTIR) [40]. Previous results showed a much greater layer thickness [41], but the PPA levels used were much higher than any typical level for commercial use. During the coating process, some portions of the die may be preferentially coated, resulting in partially melt-fractured extrudate. At the end of the coating process, if process conditions are adequate, the die will be completely covered, and melt fracture will be completely eliminated. The backpressure in the extruder will also gradually decrease to reach equilibrium at the end of the coating process. Under typical conditions, this coating process will take about 1 hour. This time can be decreased by increasing the PPA concentration. For most commercial applications, a higher PPA concentration is used to obtain a coating within 10–15 minutes. A lower maintenance level will then be used for production. An example of the coating process as indicated by extrusion pressure is shown in Figure 4.16.5. Once the die is coated, lower pressure (or shear stress or viscosity) can be obtained over a range of shear rates. It is then possible to plot either the viscosity or the shear stress vs. shear rate and compare the two curves, as shown in Figure 4.16.6(a). One can also calculate and plot the percent pressure difference between the two samples (pressure reduction), as shown in Figure 4.16.6(b). The polymer flowing through the die slowly abrades the process additive coating. Therefore, an equilibrium state will be reached between the deposition and removal rates of the processing additive coating. Consequently, a minimum level of PPA is required to maintain the coating on the die surface. Like any equilibrium, process conditions can affect the

FIGURE 4.16.4.  SEM picture and EDX analysis of PPA 2 on gold [34,35] (courtesy of 3M).

Section 4.16.  Polymer Processing Additives (PPA)

FIGURE 4.16.5.  Die coating reaching equilibrium as measured by pressure in a capillary rheometer (courtesy of 3M).

coating and removal rates and as a result modify the level required to reach an equilibrium state where the die is fully coated. Another point already mentioned is the affinity of PPA for the oxides and hydroxides on the die metal surface. This part of the coating process can be easily verified by simply comparing the coatings obtained on a nickel surface and a gold surface. The nickel surface, which is covered by oxides and hydroxides, shows a higher coating density than the gold surface, which has no oxides. This is easily observed in Figure 4.16.7. Commercially, most dies are chrome- or nickel-plated. If the plating is worn off, the underlying steel will be exposed. Typically, all these surfaces are covered by metal oxides, and no significant differences will be observed between them. However, if the surface is contaminated by some degraded polymer residue, the oxides and hydroxides

295

may not be available, and PPA coating efficacy may be adversely affected. Several factors have an impact on the equilibrium between deposition and removal of the processing additive and the time to reach equilibrium. For example, several of the additives commonly used in polyethylene will interfere with or provide synergy to PPA. For cost considerations, it is usually advisable to optimize the whole additive package to maximize the benefits. This topic will be discussed later. Several process parameters also affect PPA performance. PPA concentration is the most obvious one. An increase in concentration increases the deposition rate and reduces the coating time. The quality of the processing additive dispersion will also have a large impact on the coating rate. Ideally, the PPA droplets will be well dispersed to ensure even distribution. Uneven distribution and the varying concentration associated with it will lead to variable performance. A well-dispersed PPA is also preferred to ensure that the droplets are not visible as undispersed gels in the finished film. Typical blowup ratio and drawdown ratios (5 to 10) stretch the PPA droplets by approximately the same ratio. Consequently, PPA droplets larger than 30 microns in the melt may be visible in the finished film. PPA droplets smaller than 30 microns are therefore recommended. In a similar fashion, throughput affects coating time. For fixed die geometry, the amount of processing additive reaching the die is directly proportional to throughput [15]. One of the keys to controlling coating efficiency is to increase the shear rate (shear stress) to a maximum while staying below a shear rate that would induce CMF in the base resin. The shear gradient linked to the shape of the flow profile helps migration of PPA toward the die wall. This can be demonstrated by the appearance of the coating pattern obtained at varying shear rates. In Figure 4.16.8, the coating density increases with shear rate until the onset of CMF is reached and the coating pattern becomes erratic. The importance of controlling temperature is often over-

FIGURE 4.16.6.  (a) Apparent shear stress curve, (b) Pressure reduction curve for a 0.9MI LLDPE at 200°C with and without PPA (courtesy of 3M).

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FIGURE 4.16.7.  Photomicrograph of a PPA 2 coating obtained at 300/s on a metal surface (1000×): (a) PPA coating on gold, (b) PPA coating on nickel (courtesy of 3M).

FIGURE 4.16.8.  Photomicrograph of a PPA 2 coating obtained on a gold surface (1000×, 10 µm/Div) over a range of shear rates. At 600 S–1, the extrusion conditions are at the onset of CMF [35] (courtesy of 3M).

Section 4.16.  Polymer Processing Additives (PPA)

297

FIGURE 4.16.9.  Pressure reduction for a 1-MI LLDPE with 0.1% fluoropolymer at 400/s (0.5 mm diameter, L/D = 40) [42] (courtesy of 3M).

FIGURE 4.16.10.  Blown-film melt fracture elimination testing (courtesy of 3M).

looked. An example of this is given in Figure 4.16.9, which illustrates the pressure reduction as measured by capillary rheometry [42]. For a fixed throughput, at high temperature (250°C), the stress is too low, whereas at low temperatures (150°C–170°C), the stress is too high. Note that the optimum temperature depends on the base resin, the output, the equipment, and the die.

Commercially, a fast coating rate is preferred, and higher concentrations are often used in the first few minutes of the process to reach full coating more quickly. The most common way of measuring deposition (or coverage) rate is to start the test by extruding a formulation without PPA and producing a film that is fully covered with sharkskin melt fracture. PPA is then added (marked time 0) and the percentage of the film width that is covered with sharkskin is measured at regular intervals (typically every 10 minutes) until no sharkskin remains. An example of this is given in Figure 4.16.10, which shows an evaluation of the effect of PPA concentration. Similar testing can be carried to evaluate the effect of interacting additives or process conditions.

PPA Melt Fracture Evaluation One of the challenges of evaluating PPA and the optimum additive combination is the sensitivity of PPA performance to material left in extrusion equipment from previous runs. An excess of an interfering additive can show some effect even after it has been removed from the feed. Similarly, residual PPA from previous runs can seriously bias the result of an evaluation. For this reason, it is critical to compare formulations by always beginning with a rigorously cleaned extruder and with the same starting conditions such as temperature and pressure. The system must be thoroughly purged between formulations (usually with a resin containing an abrasive additive). A clean base resin containing no PPA should then follow the purge to ensure that the line is clean. If the line is clean, the same backpressure should always be obtained. In addition, it is advisable to select conditions where the whole surface of the film will be melt-fractured. The production of fully fractured film for a period of at least 30 minutes can then be used as a control and an indication of the cleanliness of the line. Each formulation can then be compared without any bias. PPA performance can be evaluated by comparing the rate of PPA deposition on the die until full coverage is obtained. This can be assessed by selecting conditions where the PPA deposition rate is comparatively slow, usually between 60 and 90 minutes. Selecting conditions that provide a faster coating rate yields only a small differentiation between samples, whereas slower rates result in higher variability.

Other Benefits of Using PPA PPAs are based on high molecular weight fluoropolymers, designed to modify polymer melt/extruder interface. They are only mobile in the melt, do not bloom to the surface after extrusion and are physically entrapped in the host resin. Under normal use conditions, PPAs do not affect the printing, sealing, coating, slip and mechanical properties of the final product [43,44]. Although PPAs were originally developed to circumvent melt fracture, which they do very efficiently, they also have been shown to provide additional benefits to the end user. They usually provide a significant improvement in gloss and surface smoothness. This usually comes with a reduction in die swell, which can provide better gauge control [13]. Because of their lubricating effect, PPAs provide a backpressure reduction for a fixed throughput. This pressure reduction provides a broadening of the processing window that can then enable processing of more viscous resins or resin blends. For example, this approach makes it possible to select a resin with a lower MI or to increase the LLDPE content in an LLDPE/LDPE blend without reaching the ex-

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FIGURE 4.16.11.  Example of pressure reduction and increased throughput [42] (courtesy of 3M).

truder pressure limit. This will enable a reduction in material use by downgauging a film formulation, while still meeting the finished product requirements. Similarly, on pressure-limited extrusion lines, reduced pressure can enable an increase in RPM to achieve increased

throughput, or can enable use of a narrower die to improve the balance of film properties. An example of this procedure is given in Figure 4.16.11. Before the addition of PPA, the screw speed was 50 RPM, and the backpressure was 23.5 MPa. After the addition of PPA, the pressure decreased slowly over a 60-minute time period while the coating on the die was forming. The PPA provided a 24% pressure decrease to 17.8 MPa. At that point, the RPM was increased to return to the original pressure, providing a 91% increase in throughput. The lower pressures afforded by PPAs can also make it possible to decrease extrusion temperature and consequently minimize polymer degradation. Lower extrusion temperatures can also translate into energy savings. Because the PPA forms a release layer inside the extruder and the die, the PPA also provides a reduction in die buildup (die drool, die bearding) and in degradation gels, as well as a significant reduction of the transition time between resins (color change, for example). This can significantly reduce downtime. An example of die buildup elimination is shown in Figure 4.16.12. Here, two pictures of a die are given for the same extrusion conditions and time. The only difference is that the picture on the right was obtained while using a PPA. Similarly, processing additives can help reduce cross-

FIGURE 4.16.12.  PPA for polypropylene (PP) die buildup reduction, 1.5 MFR PP, 3% TiO2: (a) no PPA, (b) 300 ppm PPA 5 (courtesy of 3M).

Section 4.16.  Polymer Processing Additives (PPA)

299

TABLE 4.16.2.  Potential Additive Interference with PPA. Non-Interfering

Interfering

• Primary antioxidants

• HALS

• Secondary antioxidants • Slips

• UV stabilizers • Antistats (higher temperatures)

• Antistats (low temperatures)

• Stearates

• Polyethylene oxides • Carbon blacks

• Hydrotalcite • Antiblocks • Pigments (inorganic)

FIGURE 4.16.13.  Effect of PPA on gel occurrence (wa: weight average) [42] (courtesy of 3M).

linked, oxidized, and un-melted or unmixed gels that may form in the extruder [9,10]. An example of gel reduction is shown in Figure 4.16.13. Without PPA, the weight average gel count is about 100 (dashed line). When PPA is added, the gel count is reduced to about 10 (solid line). If the PPA is removed from the feed resin, the gel count slowly increases back to higher values (dotted line). Figure 4.16.14 shows an example of reduction in time for color changeover. Here, the transition from a carbon blackcontaining film to a clear film is shown. Each fold on the picture represents 5 minutes of extrusion. Without PPA, one can clearly see the tailing of black pigment up to about 60 minutes of extrusion, whereas with PPA the time is reduced to about 20 minutes. In these three examples (DBU, gels, color changeover), the PPA coating prevents stagnation of material on the die, thus reducing degradation, crosslinked gel formation, and color changeover time. This can potentially reduce material waste or off-spec material. Another potential use of PPA is to reduce draw resonance.

This can be accomplished by two routes. Using a PPA should enable a reduction in the die gap without increasing the backpressure. The resulting draw required to obtain the final thickness should be minimized and may help reduce resonance. Alternatively, using a PPA could make it possible to reduce the extrusion temperature without increasing the backpressure. This would increase melt strength and again reduce draw resonance. INTERACTIONS Several studies have described the interactions between PPA and other additives [14]. An interaction can be either positive or negative. Table 4.16.2 lists the main additives used in LLDPE according to their potential for interaction. The negative interactions can be separated into four categories: abrasion, adsorption, chemical reaction, and competition for the metal die wall. Mineral particles such as fillers and pigments can potentially abrade the PPA from the die surface or adsorb it on their surfaces, preventing the coating from forming on the die surface. Alkaline chemicals have the potential to react chemically with the fluoroelastomer, which is slightly acidic. Finally, low-molecular-weight species, especially if they are polar, can interact with the oxides and hydroxides on the metal die surface and compete with PPA in the coating formation process. Main Interacting Additive Classes HALS

FIGURE 4.16.14.  Reduction in time for color change [42] (courtesy of 3M).

There are several hindered-amine light stabilizers (HALS) on the market. HALS interference with PPA performance is in part chemical and in part competition [45]. The chemical interference may be related to the alkalinity of the HALS. Table 4.16.3 ranks a few HALS for their potential chemical interaction with PPA. Higher temperatures as well as high HALS concentration intensify the chemical reaction. In addition, combined HALS/PPA masterbatches can magnify the interference with PPA performance if the additives come into close proximity. Use of separate masterbatches minimizes the potential for interaction and PPA performance interference. HALS are also polar molecules that can compete for the die wall. They potentially exhibit competition or reaction depending on process conditions.

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TABLE 4.16.3.  Additive Interference. Interference Most interfering

HAL

Acid Neutralizers

Antiblocks

TiO2 Pigments

Chimassorb® 944

HT

Synthetic Silicas

Uncoated TiO2

Talc

↓

Chimassorb 119 Chimassorb 2020 Flamestab® NOR 116 Tinuvin® NOR 371 Tinuvin 783

CaSt ZnSt

Coated Talc DE CaCO3

Least interfering

Tinuvin 622

Acid Neutralizers The mode of interference for acid neutralizers (AN) depends on the type of neutralizer used. Stearates compete for the die wall, whereas hydrotalcite (HT) interferes by chemical reaction or adsorption. Interference is increased by low shear rates and high AN concentrations. Various acid neutralizers are ranked in Table 4.16.3. Antiblocks Antiblocks (AB) interfere by either abrasion or adsorption. Of the factors increasing interference, the most important are large particle size and Mohs hardness (abrasion), high surface area (adsorption), and the use of combined masterbatches. Combined masterbatches increase the potential for contact between the AB and the PPA. Table 4.16.3 provides a comparison of a few antiblocks. An example of abrasion is shown in Figure 4.16.15, where the more abrasive silica was able to remove more of the coating than the adsorptive talc. TiO2 Pigments Similarly to antiblocks, TiO2 pigments interfere through abrasion and adsorption. The factors increasing interference

ZnO

Ceramic Spheres

Coated TiO2

are a high tendency towards agglomeration leading to large particle size, a lack of surface treatment, or a basic surface treatment. As stated in Table 4.16.3, uncoated TiO2 is more strongly interfering. Managing Additive Interactions Although interactions cannot be completely eliminated, it is advantageous to manage them as much as possible. Additive interactions and PPA performance optimization can be managed by addressing the formulation or through adjusting the processing parameters. An easy way to get around PPA/additive interaction is to simply increase the ratio of PPA to additive. Selecting a less “interfering” additive is another approach, which could enable PPA usage to be significantly decreased. Because chemical interactions tend to accelerate with higher temperature, decreasing the processing temperature is often recommended when chemical interactions are present. Minimizing the degree of contact by using separate rather than combined concentrates can also provide some improvement. Finally, selecting the optimum PPA from the various materials available can also have an impact on the PPA level required. The additive package should be optimized as a whole for both performance and cost. This can provide lower PPA use levels, faster conditioning/coating time, and less variability.

FIGURE 4.16.15.  Photomicrograph of a PPA 1 coating obtained at 300/s on nickel surfaces (500×, 10 µm/div): (a) no AB, (b) 3000 ppm SiO2, (c) 3000 ppm talc [35] (courtesy of 3M).

Section 4.16.  Polymer Processing Additives (PPA)

301

The kinetics of the coating process are also affected by other additives present in the resin. Inorganic particles abrade the coating and adsorb some of the droplets, slowing down coating formation. Highly basic materials can potentially react with PPA, preventing its deposition. Lowmolecular-weight materials and polar compounds, including resin degradation products, will compete for metal oxides and prevent adhesion of the PPA to the metal. Optimization of the additive package may help circumvent this problem. Once the coating has formed, it induces slip at the polyethylene/PPA interface. This slip results in a change in the flow profile. In return, this reduces the stored elastic energy in the melt and eliminates or postpones the occurrence of sharkskin and CMF to significantly higher shear rates. The slip at the polyethylene/PPA interface also results in lower extrusion pressure. In pressure-limited systems, this decrease in pressure broadens the process window, enabling a reduction in process temperature or an increase in throughput.   FIGURE 4.16.16.  Effect of antioxidants on PPA performance [42] (courtesy of 3M).

Some additives can also have a synergistic effect with PPA. The best-known example is antioxidants (AO). A lack of AO can dramatically reduce the ability of PPA to perform [7]. In this case, the degradation products of the base resin compete for the metal oxide on the die surface and prevent the PPA coating from forming. An example of this is shown in Figure 4.16.16. Here, a resin with no AO was extruded in a capillary rheometer. The same resin was then extruded with 500 ppm PPA, showing a reduction in pressure as indicated by the lower apparent viscosity. When adding 1000 ppm of phenol-based AO for melt processing stability, a significant improvement was obtained. The same kind of benefit can be obtained with other types of stabilizers, such as phosphites. In fact, PPA performance is so sensitive to AO that a lack of stabilization in the resin or purge that was used before introducing a PPA-containing formulation can impact PPA performance. CONCLUSIONS Polymer processing additives (PPA) are a series of commercially available fluoropolymers used to reduce and eliminate flow instabilities such as sharkskin and cyclic melt fracture. They can also be used to eliminate other extrusion problems such as die buildup, formation of crosslinked gels, and slow color changes. They are used either in the neat form or in combination with synergists. Typically, PPAs are present in the base resin at low levels (< 0.1%) in the form of a dispersed immiscible phase. The PPA droplets fall on the die wall and coat the die surface. The concentration, shear field, and interfacial tension all contribute to the kinetics of the coating process. The PPA attaches itself to the oxides and hydroxides of the die through hydrogen bonding.

REFERENCES AND ADDITIONAL RESOURCES [1] Johnson, B.V., Blong, T.J., Kunde, J.M., Duchesne, D., TAPPI Laminations & Coat. Conf., 1988, 249 (1988). [2] Blong, T.J., Fronek, K., Johnson, B.V., Klein, D., and Kunde, J., SPE Polyolefins VII International Conference Proceedings, Houston, Feb. (1991). [3] Blong, T.J., Duchesne, D., SPE ANTEC, 47, 1336 (1989). [4] Blong, T.J., Duchesne, D., J. Plast. Film Sheeting, 5(4), 308– 320 (1989). [5] Blong, T.J., Duchesne, D., Plast. Compd., 13(1), 50-2, 56–57 (1990). [6] Duchesne, D., Schreiber, H.P., Johnson, B.V., Blong, T.J., Polym. Eng. Sci., 30(16), 950–956 (1990). [7] Blong, T.J., Focquet, K., and Lavallée, C., SPE ANTEC, 55, 3011–3018, (1997). [8] Van den Bossche, L., Georjon, O., Donders, T., Focquet, K., Dewitte, G., Briers, J., Maack PolyEthylene ‘97, Milano, Italy (1997). [9] Butler, T.I., Pirtle, S.E., TAPPI Laminations & Coat. Conf., 1996, 601–607 (1996). [10] Woods, S.S., Amos, S.E., TAPPI Laminations & Coat. Conf., 1998, 675–685 (1998). [11] Slusarz, K.R., Christiano, J.P., Amos, S.E., SPE ANTEC, 58, 144–148 (2000). [12] Slusarz, K.R., Amos, S.E., TAPPI Laminations & Coat. Conf. (2001). [13] Amos, S., SPE RETEC Tech. Papers, 133–143 (1997). [14] Amos, S.E., Giacoletto, G.M., Horns, J.H., Lavallée, C., Woods, S.S., “Polymer Processing Aids (PPA)”, in Zweifel, H. (ed.), Plastic Additives Handbook, 5th edition, Hanser Gardner, Cincinnati, pp. 553–584 (2000). [15] Lavallée, C., Woods, S.S., SPE ANTEC, 58, 2857–2861 (2000). [16] Neumann, P., TAPPI Polym. Laminations Coat. Conf., 2005, Paper 6-1, p. 197 (2005). [17] Neumann, P., TAPPI Polym. Laminations Coat. Conf., 2007, p. 1095 (2007). [18] Blatz, P.S., U.S. Patent 3 125 547 (1964).

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[19] Robinson, D.G., “Chemical Engineering in Canada—An Historical Perspective”, in L.W. Shermitt (ed.), Linear Low-Density Polyethylene, Canadian Society for Chemical Engineering (1991). [20] Chapman, R.G., Priester, D.E, U.S. Patent 4 904 735 (1990). [21] Duchesne, D., Johnson, B.V., US Patent 4 855 360 (1989). [22] Kunde, J., Johnson, B.V., US Patent 4 863 983 (1989). [23] Johnson, B.V., Kunde, J.M., SPE ANTEC Tech. Papers, 1425–1429 (1988). [24] Duchesne, D., Blacklock, J.E., Johnson, B.V., Blong, T.J., SPE ANTEC Tech. Papers, 47, 1343–1347 (1989). [25] Radosta, J., Plastic Film & Sheeting, Vol. 7, 181–189 (1991). [26] Amos, S.E., Papp, S., SPO 99, 9th International Business Forum on Specialty Polyolefins, Houston (1999). [27] Amos, S.E., TAPPI Laminations & Coatings Conference Proceedings, pp. 623–632 (1998). [28] Amos, S.E., Dillon, M.P., Woods, S.S., Coggio, W., Kunde, J.M., Lavallée, C., SPE ANTEC Tech. Papers, 59, 2639–2643 (2001). [29] Achilleos, E.C., Georgiou, G., and Hatzikiriakos, S.G., J. Vinyl Add. Tech., 8(1), 7–24 (2002). [30] Hatzikiriakos, S.G. and Dealy, J.M., J. Rheol., 35(4), 497–523 (1991). [31] Hatzikiriakos, S.G. and Dealy, J.M., SPE ANTEC Tech. Papers, 49, 2311–2314 (1991). [32] Migler, K.B., Lavallée, C., Dillon, M.P., Woods, S.S., Gettinger, C.L., J. of Rheol., 45(2), 565–581 (2001).

[33] Migler, K.B., Lavallée, C., Dillon, M.P., Woods, S.S., Gettinger, C.L., SPE ANTEC, 59, 1132–1136 (2001). [34] Lavallée, C., TAPPI Polym. Laminations Coat. Conf., 2003, Paper 28-2 (2003). [35] Lavallée, C., TAPPI Polym. Laminations Coat. Conf., 2005, p. 460 (2005). [36] Zhao, R. and Macosko, C.W., J. Rheol., 46(1), 145–167 (2002). [37] Schwetz, M., Merten, A., and Münstedt, H., Kunststoffe, 91(5), 86–89 (2001). [38] Migler, K.B., Phys. Rev. Lett., 86(6), 1023–1026 (2001). [39] Pathak, J.A., Hudson, S.D., and Migler, K.B., ACS Polym. Prepr., 43(2), 833–834 (2002). [40] Kharchenko, S.B., Migler, K.B., and McGuiggan, P.M., SPE ANTEC Tech. Papers, 61, 2689–2693 (2003). [41] Lo, H.H.-K., Chan, C.-M., and Zhu, S.-H., Polym. Eng. Sci., 39(4), 721–732 (1999). [42] Lavallée, C., Dillon, M.P., Chapter 44: “Polymer Processing Additives (PPA)”, in Butler, T.I. (ed.), Film Extrusion Manual —Process, Materials, Properties, 2nd Edition, TAPPI Press, Atlanta, pp. 539–546 (2005). [43] Blong, T.J., Klein, D.F., Pocius,A.V., Strobel, M.A., SPE ANTEC, 52, 28 (1984). [44] Amos, S.A., Pocius,A.V., Maack Polypropylene ‘00, Basel, Switzerland (2000). [45] Woods, S.S., King, R.E., Lavallée, C., TAPPI Laminations & Coatings Conference Proceedings, pp. 1115–1124 (2000).

Chapter 4—Section 17

Additives for Film Products TAD FINNEGAN and R.E. KING III, BASF Corporation

INTRODUCTION

ACID SCAVENGERS

On any given day, it is possible to manufacture blownand cast-film products without the use of additional additives or modifiers. Furthermore there is no question that these “plain vanilla” film products have intrinsic utility in the marketplace. However, a variety of chemistries are readily available, not only to increase the monetary and aesthetic value of the film product, but also to improve processability. Typically, one or more of these additive chemistries are used in combination to differentiate film products in a very competitive market. The types of additives that can be used in this process can be segmented into “chemical” and “physical” property modifiers, as shown in Table 4.17.1. Table 4.17.1 lists a wide variety of chemical and physical property modifiers, suggesting almost endless possibilities in terms of enhancing the properties of the final film product. Over the last four decades, polymer producers and film manufacturers have been working with these types of additives to develop products that are valued by the market. As new additive technologies become commercially available, they are tested in a variety of applications to determine whether they can be justified with regard to adding another polymer grade to the manufacturer's grade slate. Additive systems that bring real value (vs. stated value) usually make it onto the list, as long as they meet all the requirements demanded of new products. In addition to all these factors, the co-extrusion process to produce multi-layer films is widely used in most markets today. This provides a more precise, cost-effective way to place polymers and additives in film structures. The following sections will briefly review each type of additive listed in Table 4.17.1, followed by short discussions on additive delivery methods, additive synergism and antagonism, and additive ancillary properties. For ease of reading, each of the additive classes will be discussed (arbitrarily) in alphabetical order. The field of additives has been extensively reviewed [1] and is usually updated on an annual basis [2–5]. The author also wishes to acknowledge previous effort on the topics covered in this chapter [6].

Acid scavengers (acid acceptors; anti-acids) are typically part of the “base stabilization” of the polymer. Their primary role is to absorb or react with trace amounts of acidic byproducts (e.g., hydrochloric acid, HCl) resulting from deactivation of the polymerization catalyst or co-catalyst. (The polymerization catalyst is deactivated by the polymer producer; this is not the responsibility of the downstream user.) The trace amounts of HCl generated by the deactivation of the catalyst/co-catalyst are intrinsically incompatible with the organic polymer matrix and, with time, will diffuse out of the polymer. However, the corrosive nature of HCl mandates the use of an acid scavenger to absorb it or render it inactive. It is especially important to deactivate the residual HCl before it can react with polymer additives and stabilizers, rendering them inert or at least significantly transformed. For example, residual HCl can serve as a promoter for the hydrolysis of secondary antioxidants, such as phosphites, or the partial deactivation of hindered amines. Representative acid scavengers include calcium stearate, calcium lactate, zinc stearate, zinc oxide, and dihydrotalcite. Because these additives are included as part of a polymer finishing step/base stabilization system, it is somewhat uncommon to see them used as acid scavengers in a downstream film fabrication process. More often, materials like calcium stearate and zinc stearate are added as internal or external lubricants during processing of higher-molecularweight resins. Note, however, that acid scavengers are useful reagents for buffering the “acidity” or “alkalinity” of a given formulation, which may include somewhat “acidic” or “alkaline” co-additives added downstream as concentrates. Accordingly, it is useful to know that these materials provide this type of buffering functionality. Nevertheless, only in selected instances would they be used in a film operation as acid scavengers. One representative example might be film formulations that include halogenated flame retardants. ADHESION PROMOTERS Extrusion coating and cast-film extrusion for packaging 303

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TABLE 4.17.1.  Additives that Affect Chemical and/or Physical Properties. Chemical Property Modifiers • Phenolic Antioxidants: e.g., usually 2,6-di-t-butyl-4-R*-phenols • Melt Processing Stabilizers: e.g., phosphites, hydroxylamines, lactones • Thiosynergists: e.g., distearylthiodiproprionate (DSTDP) • Ultraviolet Absorbers: e.g., benzotriazoles, benzophenones, triazines • UV Energy Quenchers: e.g., nickel (II) complexes • Hindered Amine Light Stabilizers: e.g., piperidines; piperizinones • Acid Scavengers: e.g., metal stearates, metal lactates, metal oxides, and synthetic dihydrotalcite • Biocides: Agents, e.g., OBPA, isothizolinone, triclosan, silver complexes • Metal Deactivators: e.g., hydrazide functionalized phenols • Anti-Gas Fading Promoters: e.g., "phenol free" stabilization systems

materials are processes that can use coatings. Adhesion promoters enable dissimilar substances to be applied to the normally unreactive film substrate as coatings. Representative examples of technologies used for adhesion promotion are based on organofunctional silanes or chlorinated polyolefins. They can be delivered in solvent-based or water-based forms. Besides facilitating coating application, adhesion promoters can also be used to increase the ability to incorporate insoluble particles or fillers. Using this approach, adhesion promoters are designed with multiple functionalities, with part designed for compatibility with the polymer and part designed to adhere to, adsorb onto, or chemically react with the filler. This means that the filler becomes more an integral part of the product rather than a simple dispersion. ANTIBLOCK AGENTS Blocking is the adherence of adjacent film surfaces to each other. Blocking can be seen during the blown- or castfilm processes or when roll stock or film-coated surfaces are stacked and subjected to pressure, time and heat. The average person encounters a good example of blocking in the produce section of the grocery store when trying to figure out which end of the bag to open. Blocking of film surfaces is usually suppressed with an antiblock agent. These types of materials are added to reduce the intimate contact between the polymer film layers, thereby reducing adherence and easing separation. The two general classifications of antiblocks used are inorganic or organic. The most commonly used anti-blocks are inorganic materials such as diatomaceous earth, synthetic silica, talc, and calcium carbonate. Fatty acid amides are examples of organic antiblocks that can be used effectively in specific applications. However, the organic antiblocks function by migrating to the surface of the film to form a layer that resists blocking. As a result, organic antiblocks can

Physical Property Modifiers • Antifogs: e.g., ethoxylated alkylphenols; fatty acid sorbitan esters • Antistats: e.g., glycerol monostearate; ethoxylated amine • Antiblocking Agents: e.g., diatomaceous earth, talc, synthetic silica • Colorants and Pigments: e.g., organic; inorganic; titanium dioxide; carbon black • Polymer Processing Additives: e.g., fluoropolymers, lubricants • Slip Agents: e.g., erucamide, oleamide • Flame Retardants: e.g., brominated diphenyloxide, phosphate esters; metal hydrates; ammonium polyphosphate • Tackifier or Cling Agents, e.g., polyisobutylene silver complexes • Fillers: e.g., talc, calcium carbonate • Adhesion: e.g., coupling agents

interfere with subsequent film conversion operations that involve that surface, such as treating, printing, and sealing. Their use, therefore, is normally relegated to applications where optical properties are very important and/or where the inorganic particles could cause scratching, marring or cutting of materials packaged in the films. (See Chapter 4.18 in this manual for more discussion about organic antiblocks.) The reduction in optical properties due to light scattering (i.e., increased haze and lower gloss) from adding inorganic antiblock materials is governed by three basic factors; (1) The particle size as compared to the wavelength of light (2) The difference in refractive index between the antiblock and the polymer (3) The wetting of the antiblock particle’s surface by the polymer. Inorganic antiblocks do not migrate and usually have an average particle size between 3 and 5 microns. Therefore, only those particles located near the surface of the film have the major effect of creating a microscopically rough surface to reduce or eliminate blocking. Use levels can vary from a few hundred part per million (ppm) to more than ten thousand ppm, depending on the polymer and the desired level of antiblocking. In terms of trends, the tendency of film surfaces to block, or stick together, is more pronounced with film products that are smooth or glossy than with those with a textured or matte finish. Moreover, films extruded from higher-molecular-weight polymers (i.e., low melt flow) with narrow molecular-weight distributions and higher densities tend to have less blocking. Linear low-density polyethylene (LLDPE) made with longer-chain-length co-monomers tend to block more, and copolymers of high-pressure, low-density polyethylene (LDPE) tend to show the same trend. Finally, in general, as film thickness increases, the film shows less tendency to block.

Section 4.17.  Additives for Film Products

Some of the “properties” that must be taken into consideration when selecting the type and concentration of antiblock are ease of dispersion, optics (clarity, haze), abrasiveness, interactions with other additives in the system, worker exposure issues, and of course, cost. An ancillary concern, based on the relatively high surface area of these materials, is the tendency to absorb other functional additives. In these cases, it may be important to consider a coated antiblock that minimizes adsorption of other, relatively higher-cost film additives. This approach should improve the cost-effectiveness of the enhanced film product. ANTIFOGGING AGENTS Fogging is the tendency of small individual water droplets to form (through condensation) on a film surface. In selected applications, such as food packaging or greenhouse films, this type of fogging is undesirable. Under certain conditions, such as food packaging, light scattering by the individual droplets can obscure the clarity of the film, compromising aesthetics. In applications like greenhouse films, the water droplets can act to focus light, potentially damaging the plants. Antifogging agents were developed to eliminate this problem, primarily by lowering the surface tension between the water droplets and the film, enabling the water to spread out evenly over the film surface. For this to happen, typically the antifogging agent must migrate out of the polymer and concentrate on the film surface, thus lowering the surface tension of the water condensed there. The water droplets then coalesce into is a thin clear film of water, which minimizes light scattering and improves apparent clarity. The additive concentration must be high enough in the matrix so that the additive continues to migrate to the film surface over the lifetime of the item packaged. Representative examples of antifogging agents include (but are not limited to) ethoxylated alkylphenols, polyethoxylated oleic acid esters, and sorbitan esters of fatty acids. Methods of addition include topical treatment (which is shorter-lived) and addition during melt compounding (longer-lasting). ANTI-GAS FADING (FOR COLOR-CRITICAL APPLICATIONS) In some film applications, color changes or discoloration are unacceptable. This may be an issue not only during manufacture of the film product, but more importantly during its storage and transportation. In particular, exposure to oxides of nitrogen (either from gas-fired heaters or exhaust gases from forklifts and transportation equipment) can induce discoloration in the plastic. This discoloration results from the innate ability of oxides of nitrogen to oxidize phenolic antioxidants at room temperature, leading to formation of a chromophore. The chromophore (see below, Antioxidants; Melt Processing Stabilizers) causes a yellow or pink discol-

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oration in the film. The resulting discoloration is not necessarily just a result of having a phenolic antioxidant as part of the formulation, but is also an issue of additive selection and concentration. To avoid this type of situation, both discoloration-resistant antioxidants and more powerful melt-processing stabilizers have been developed. Even if the melt-processing stabilizer is consumed, the discoloration-resistant phenols can provide some level of performance without substantial yellowing or pinking. However, if the phenolic antioxidant is stressed beyond its inherent capabilities, it will be consumed, and most likely, color will develop. Using everyday life as a metaphor, if you write a check for $100 and there is only $90 in the account, the bank will send you a nice little letter describing the overcharge, as well as reducing your balance by another $20. The end result is that you still owe the $100, and now you only have $70. A similar situation exists for the stabilizer system. If you expect more than what the system can deliver, you may end up being “disappointed”. Accordingly, it is important to be judicious when selecting additives (and to know how much money you have in the bank). Use of recycled materials that have already seen one heat exposure, such as edge trim or post-consumer polymer, adds an even more demanding burden to the AO package furnished by the virgin resin supplier. If aggressive melt-processing conditions and/or exposure to oxides of nitrogen cannot be controlled or avoided, other approaches can be used. For example, as an alternative to phenol-based stabilization, new approaches to stabilization have been developed that abandon the use of the phenolic antioxidant. This approach is commonly known as “phenolbased stabilization”. This typically involves a combination of a powerful melt-processing stabilizer system (because the phenol has been removed) and a hindered amine stabilizer (to provide a level of long-term thermal or inventory stability). Binary blends of phosphites and hindered amines have been evaluated for this type of approach; however, meltprocessing stabilization based on phosphites alone cannot usually provide adequate stabilization, especially at high temperatures or under extended processing conditions. Hydroxylamines and lactones have been developed and commercialized as more powerful melt-processing stabilizers. Accordingly, blends of a hydroxylamine or lactone with a phosphite, in combination with a hindered amine, can be used to provide adequate melt-processing stability and some degree of long-term thermal stability. Note: Another potentially unrecognized source of phenol-based gas-fade discoloration is the hydrolysis of certain types of phosphites that are intrinsically reactive with moisture. This can most easily be seen in summer in coastal environments, where both heat and humidity can hydrolyze certain types of reactive phosphites, resulting in the liberation of lower-molecular-weight (LMW) phenols. These LMW phenols follow the same reactive pathways as oxides of nitrogen and are typically more reactive than HMW phenolic AOs due to insufficient steric hindrance. With this in mind, care should be taken when selecting the phosphite portion of the stabilizer system.

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ANTIOXIDANTS (FOR LONG-TERM THERMAL STABILITY) Polymer Auto-Oxidation Free radicals are usually generated at the high temperatures and high shear associated with melt processing. Oxidation (or auto-oxidation) by free radicals is a key mechanism by which a polymer loses its original properties. The autooxidation process is triggered by elevated temperatures and shear, which generate free radicals on exposure to oxygen. The auto-oxidation process then feeds upon itself due to the free-radical byproducts of the process. Polymer impurities also tend to accelerate the process. There are four general steps, where “R” is a macromolecule: chain initiation, chain propagation, catalysis (enhancement), and finally, chain termination. Chain Initiation: (a) RH → R· + H· (heat) (b) RH + O2 → R· + HOO· (heat) (c) RH + Catalysts → Free radicals (heat) Chain Propagation: (d) R· + O2 → RO2· (e) RO2· + RH → ROOH + R· Catalysis: (f) ROOH → RO· + ·OH (heat or UV) (g) ROOH + RH → RO· + R· + H2O (heat) (h) 2 ROOH → RO· + RO2· + H2O (heat) (i) RO· + RH → ROH + R· (j) HO· + RH → H2O + R· Chain Termination: (k) RO2· + RO2· → ROOR + O2 (l) RO2· + RO2· → RO· + RO· + O2 (m) RO2· + RO2· → Inactive Products + O2 (n) RO· + RO· → ROOR (o) R· + R· → R→R These reactions, via free-radical chemistry, eventually lead to changes in the polymer’s molecular weight and molecular weight distribution through chain scission, crosslinking, or combinations of both, ultimately transforming the original properties of the polymer. These changes impact polymer processing characteristics as well as physical properties. Polymer Stabilization Antioxidants (AO) protect plastics against oxidation that can cause deterioration of physical and aesthetic properties. A number of chemistries can be used to interrupt free-radical oxidation. As a general class of additives, antioxidants

are typically divided into two subclasses. One of these is based on chain-breaking or primary antioxidant chemistries (discussed in this section). The other is based on preventive, or secondary, antioxidants that decompose hydroperoxides before they are transformed into free radicals (see also MeltProcessing Stabilizers, discussed in a separate section below). Chain-Breaking or Primary Antioxidants Antioxidants in this general class can interrupt free-radical processes by donating labile hydrogen atoms to cap off the free radical. These hydrogen-donating antioxidants (AH) slow down oxidation by effectively competing with the polymer for free radicals, thereby abbreviating the chain length of the propagation reactions. (a) RO2· + AH → ROOH + A· (b) RO2· + A· → ROOA (a non-radical species) ROOA is not the only product, nor is it the end of the road. Heat, shear, and light can impact this type of molecule to re-initiate free-radical chemistry. However, certain antioxidants, by the nature of their molecular structure, can undergo less damaging transformation chemistries. It should also be recognized that hindered phenols are quite capable of providing melt-processing stability. However, it is the author’s opinion that hindered phenols, as a general class of primary antioxidants, should be used principally to provide long-term thermal stability. Let the meltprocessing stabilizers do their fair share of the work, thereby alleviating the workload on the phenolic AO. This results in more intact phenolic AO after the melt-processing step for long-term thermal stability. In practice, lower-molecularweight phenols are still sometimes used during melt processing; however, they have a tendency to volatilize out of the polymer, thereby limiting their contribution to long-term thermal stability. In addition, lower-MW phenols have a strong tendency to discolor upon prolonged storage. Highermolecular-weight hindered phenols were developed to be more permanent and to have less tendency to discolor during storage. With regard to mechanisms, the phenol group provides stabilization by donating hydrogen atoms to quench oxygencentered free radicals. In a second step, the phenoxy radical can also react with free radicals, thereby facilitating the quenching of the autoxidation cycle. Phenols can provide activity across a wide temperature range, from below freezing to greater than 300°C. Phenols can donate hydrogen atoms, undergo rearrangement reactions, and further react with free radicals until the phenolic moiety is fully consumed (i.e., over-oxidized). This total consumption of the phenolic component is typically undesirable due to generation of color bodies by formation of a conjugated chromophore. Because discoloration is a key issue to be avoided, many practical techniques to avoid total consumption of the antioxidant have been developed. These techniques typically involve us-

Section 4.17.  Additives for Film Products

ing co-additives in combination with the phenolic, such as trivalent phosphorous compounds and scavengers for acidic catalyst residues, once again to alleviate the workload on the phenol. Various high-MW phenols with distinctly different molecular structures have been developed to address specific applications. Another type of chain-breaking antioxidant that is growing in popularity, especially for color-critical applications, is the general class of higher-molecular-weight hindered amines. Hindered amine stabilizers are already well known for their excellent ability to provide ultraviolet (UV) light stability. However, it is important to recognize that one should not be distracted by hindered amines as a compound class, but rather should focus on their mode of activity, which is freeradical scavenging. Hindered amines, which are based on the piperidinyl functional group, have been shown not only to provide remarkable light stability, but also to provide longterm thermal stability (below ~120–135°C). Both modes of stabilization are addressed through free-radical scavenging by hydrogen atom donation followed by prolonged free-radical scavenging by the resulting nitroxyl group. A significant difference between hindered amines and hindered phenols is that hindered amines, as radical scavengers, have a narrower performance “temperature window” than hindered phenols. Accordingly, it is important to recognize that hindered amines are not useful as radical scavengers during melt processing. In addition, hindered amines are not effective during some types of accelerated techniques for gauging long-term thermal stability, such as oven aging at 150°C. Nevertheless, at temperatures from below ~ 120–135°C, hindered amines can provide a significant contribution to long-term thermal stability that is comparable to that of hindered phenols. In the normal temperature range of 20–60°C, their performance is at least equivalent. Hence, in selected color-critical applications, hindered phenols (which have a tendency to discolor once they are used up) are being replaced by hindered amines, which do not discolor. In terms of specific end-use applications, one area of significant interest is gas fade-resistant film products, where “phenol-free” systems have been developed to eliminate the tendency of films to turn yellow or pink after prolonged storage. (See also Anti-Gas Fading above.) Other types of antioxidants include higher-molecularweight aromatic amines (distinctly different from the nonaromatic hindered amines previously discussed), which are commonly used in the rubber industry. They work by a mechanism similar to that of hindered phenols, by donating hydrogen atoms to free radicals, followed by additional radical scavenging by the nitroxyl radical. However, due to their tendency to discolor strongly, hindered aromatic amines are not commonly used in polyolefin film applications. Another type of antioxidant includes sulfur-containing compounds, such as thioesters. When used in combination with a phenolic antioxidant, thioesters can significantly increase the long-term thermal stability of a polymer. The primary mode of activity is decomposition of hydroperoxides and the subsequent conversion of the sulfide group to sulf-

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oxide and sulfone. One drawback of thio compounds is their tendency to generate odors once they are over-oxidized to sulfenic and sulfonic acids. As such, their use in food packaging is limited, and FDA compliance should be checked. As an additive class, antioxidants are typically incorporated into the base polymer by the polymer producer to provide compounding and shelf-life protection. For this reason, antioxidant concentrates are used to a much lesser degree than other types of additive concentrates used by film producers. However, when additional stabilization is required, or for unique applications, antioxidant concentrates are available. (See also Melt-Processing Stabilizers.) ANTISTATIC AGENTS Most plastics are intrinsically nonconductive materials and are therefore quite useful as insulators. However, they are also subject to developing electrostatic charges. The electrostatic charge may develop from friction during the post-extrusion process and with subsequent handling. (Anyone who has walked too close to a large film windup reel that has not been properly grounded learns about this, usually in the first lesson.) The buildup of electrostatic charge is more severe in environments with lower humidity and during the colder months of the year. Presence of a static charge in plastic film causes problems in high-speed windup, converting operations, and automatic packaging lines. The static charge buildup not only represents a potential to deliver a “shocking experience” for operators; a static discharge can cause fires or explosions in dust-filled environments or areas containing volatile chemicals. Static discharges at levels below an individual’s tactile perception damage millions of dollars of sensitive computer components each year. As a less serious issue, the buildup of static charge can also result in the product becoming a “dust magnet”, which can compromise aesthetic appeal. A variety of antistatic agents have been developed over the years to address these problems. One of the first approaches to alleviate the accumulation of electrostatic charge was the incorporation of conductive materials into the polymer matrix, such as carbon black, metallic powders or certain types of conductive fibers. However, this approach usually requires relatively high additive loadings, which not only affect physical properties, but also change the visual appearance as well. An alternative approach to alleviate static charge buildup is to use certain chemicals that are applied topically as a spray or dip. However, this type of topical or external treatment is not commonly used for films due to difficulties associated with the application, the very nature of the film material, and the nature of end-use applications where the additive may be too easily rubbed or washed off. Today, most antistats are organic molecules with surfaceactive properties. These are usually internally incorporated through melt compounding at the polymer producer or by introducing a pelletized concentrate at the film converter. The most common design of antistat is much like a surfactant,

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with a hydrophobic end group that anchors the additive to the polymer surface and a hydrophilic end group that then absorbs moisture / humidity from the atmosphere. Antistats work by having some intrinsic level of incompatibility with the polymer and migrating to the surface. The second part of the antistat mechanism involves forming a coherent conductive microscopic layer of moisture attracted from the atmosphere. Charge is dissipated by transferring the charge built up within the polymer into the water layer. Due to the dependence of charge dissipation on water, humidity in the environment is a key variable. These types of antistats may not work under dry conditions. Representative antistatic agents include ethoxylated amines, glycerides, ammonium salts, phosphates, and sulfonamides. Typical usage levels of antistat concentrate range from 1–10% depending upon the resin, film thickness, and degree of performance required. Test procedures and regulations associated with antistatic film requirements specify certain values for surface resistivity (usually less than 1012 ohms) and static decay time (usually less than 10 seconds, preferably 1 to 2). It is extremely important to have a controlled environment, especially relative humidity (because this is a key variable affecting performance). This environment must include a very precise method for removing and then applying static charge for decay studies. Over the years, it has been recognized that relative humidity is an uncontrollable variable. Accordingly, research and development is ongoing to develop a non-discoloring antistat technology that is independent of moisture in the air. As an alternative to carbon black and metallic powders, new products based on non-discoloring polymers have been developed that do not require atmospheric moisture [6]. The immediate application of this relative humidity-independent technology in the macro- and micro-electronics packaging industry is well underway. BIOCIDES As plastics begin to age, or as they endure a destructive environment, they begin to form microcracks, which continue to grow over time. Microorganisms such as algae, bacteria, and fungi can find their way into these cracks and proliferate. As the polymer degrades to the point where lower-molecular-weight species are developed, the plastic can actually start to serve as a food source, thereby promoting further growth of the organisms. From a biological point of view, this is an interesting observation, but from the viewpoint of a consumer of plastic products, this is “gross”. In our increasingly microbe-aware society, the idea of algae, bacteria, and fungi living in our household plastic items is becoming less acceptable each day. Therefore, a variety of biostatic agents, including antimicrobials, algaecides, bactericides, and fungicides, are being more broadly examined for plastic applications. The preferred approach is for the active ingredient to migrate out of the bulk of the plastic onto the surface. As the surface concentration is depleted, it is replenished by the reservoir that

resides within the bulk of the plastic (as described above; e.g., see Antistats). In principle, biostatic agents should be environmentally safe, non-toxic, easy to handle, reasonably compatible with the polymer matrix (to obtain a controlled rate of blooming or exudation), and yet be unreactive with the other additive chemistries used in the fully formulated product. Although the concept of biostatic agents is becoming more attractive in plastic applications, it should be remembered that the use of these materials is highly regulated (which is appropriate). Most if not all products are considered pesticides and must be registered with the Environmental Protection Agency under the Federal Insecticide, Fungicide, and Rodenticide Act. Many applications already exist in PVC. Work is currently underway to obtain broader regulatory compliance in other polymers, such as polyolefins, but this is a long and difficult process. The most commonly used additives for polymers are 10, 10-oxybisphenoxarsine (OBPA), and 2-n-octyl4-isothizolin-3-one. Both have a good history in selected applications. With regard to the utility of biostatic agents in plastic articles, there are also limitations on the use of some products in plastic articles that must go through at least one meltcompounding step. Representative film applications (for example, with flexible PVC, which uses lower melt-processing temperatures) are products such as wall coverings, pool covers, outdoor upholstery, tarps, fabrics, fabric coatings, and (the ever-ubiquitous) shower curtains. Once the appropriate approvals and regulatory compliances have been obtained for polyolefins, a broader array of products should be commercially available for such kitchen products as trash cans and liners. CLING AGENTS Many types of polymers have an intrinsic nature to block, especially very smooth thin films. However, this level of blocking is typically insufficient for these types of products to serve as cling wrap or stretch wrap-type products. Accordingly, broad molecular weight distribution poly-isobutylene (PIB) as an additive to increase cling, or tack, has been used over the last two decades. The lower-molecular-weight fraction serves as a compatibilizing agent for the higher-molecular-weight fraction. Nevertheless, the higher-molecularweight fraction is intrinsically incompatible and tends to exude to the surface, resulting in an increase in the cling behavior of the film. The use of PIB in the cast-film process is reasonably straightforward. Evidently, in blown-film processes, closer control of processing parameters is necessary, as well as the necessity for a PIB with a different molecular weight and molecular weight distribution. The exact method of addition of these agents is beyond the scope of this chapter. COLORANTS Simply stated, color increases visual appeal. In an increasingly competitive environment, color has been used

Section 4.17.  Additives for Film Products

to make products more attractive or to color-code them. These type of products can be used to change the color and opacity of the final product, in addition to affording a degree of UV stability (e.g., titanium dioxide or carbon black; see below). Special effects, such as fluorescence, phosphorescence, optical brightening, and pearlescence can be achieved with more exotic materials. At higher prices, these special effects can provide significant appeal that can justify the cost. Dyes are used as colorants, but their applicability in polyolefins is limited. Dyes usually need a more polar matrix, such as ABS, polycarbonate, or styrenics to remain compatible. On the other hand, pigments represent a broader class of colorants that are more relevant to the polymer industry, especially with polyolefins. Most pigments are based on relatively simple inorganic compounds or on structurally complex organic compounds. Inorganic pigments, as opposed to organic, provide varying degrees of opacity, are perceived as less costly, and are easier to disperse, but generally provide “duller” colors. Organic pigments have high tinting strength, are fairly transparent, are somewhat more difficult to disperse, and are considered to be more expensive than inorganic pigments. Table 4.17.2 lists some representative inorganic and organic pigments that are commonly used in film products. Pigments can be used in various combinations to achieve different colors or in different combinations that provide the same color. In either case, these combinations will perform differently and may have a wide price range. It is incumbent upon the color matcher to select those pigments that provide optimum cost and performance. Companies that excel in color matching usually have experts who can assess software-based color matches for application suitability and cost. As for the day-to-day variables that affect pigment selection, the following are representative examples of factors to consider: opacity requirements; regulatory compliance for food packaging; heat stability during processing as well as inventory storage; resin dispersion cost; potential reactivity with other co-additives in the system. This brief primer on pigments is certainly far too short, but is provided to illustrate that many pigments are used in color concentrates and yield different degrees of performance. As a representative example of potential complexity, it should be noted that a color concentrate designed for use in low-density polyethylene blown film, but that is switched

to a new application in cast linear low-density polyethylene may result in pigment burnout or plateout on the chill rolls. It should be recognized that expertise in colorants typically involves deep know-how and decades of hands-on experience. Carbon Black: Among the various approaches to black colorants, carbon black most likely has the greatest impact. There are various processes for manufacturing carbon black, each of which has an impact on the level of impurities, particle size and particle size distribution, wettability, and the ability to deliver the darkest shades. Among these criteria, particle size seems to play a dominant role, especially in terms of dispersion and processability, as well as the low degree of reflectance and the ability to generate deeper shades. In practice, it is important to have a good dispersion of fineparticle-size carbon black. The most pragmatic approach is to use concentrates for this purpose because the dispersion of carbon black is quite a challenge even if the appropriate process technology and extrusion equipment are being used. Over the last decade, special grades of carbon black have been developed to improve processability as well as end-use performance. Carbon black can be used an alternative approach to achieve ultraviolet light (UV) stabilization (see also UV Stabilizers below). Although somewhat limited in scope (i.e., you can have any color you want, as long as it is black), carbon black has served the industry well in terms of providing high UV absorption, which among other things helps to maintain the physical properties of the polymer during natural weathering. Titanium Dioxide: TiO2 is the most important of the white colorants and possesses broad utility in the film market. The most prevalent crystal form of TiO2 is rutile, with the anatase crystal form being unsuitable for most plastics applications. There are two major processes for producing the rutile crystal form, known as the chloride process and the sulfate process. After the TiO2 has been manufactured, the next and most important step is to apply some type of coating to pacify the naturally photoreactive surface. There are many types of coatings, some of which are quite simple and add very little cost. Other types of coatings can be quite sophisticated and add a noteworthy level of cost. Nevertheless, the value imparted by these coatings is often worth the cost, especially for more demanding applications. Note that TiO2 can also serve as a type of UV stabilization. The processing of certain

TABLE 4.17.2.  Representative Organic and Inorganic Pigments Inorganic Pigment

Color

Organic Pigment

Color

Titanium dioxide**

White

Copper phthalocyanine

Blue and green

Iron oxides Ultramarine Nickel titanate Chrome titanate

Red, yellow, tan, black Blue, violet Yellow Orange Green

Quinacridone Carbon** Diarylide BONA salts Naphthol

Red and violet Black Yellow, orange Red Red

Cobalt mixed metal oxide

309

** A brief discussion of TiO2 and carbon black is given below due to their overall importance to the film industry.

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types of coated TiO2 at extrusion-coating temperatures can lead to a phenomenon called lacing (hole formation due to water volatization). The value of TiO2 is most recognized in its ability to add brightness, whiteness, opacity and tinting strength. Various grades of TiO2 are differentiated by their processability, purity, particle size distribution, refractive index, crystal form, type of coating, and overall cost versus performance ratio. In practice, processability of TiO2 can play a key role in terms of maximizing dispersion with minimum energy, minimizing die-lip buildup, affording high thermal stability, and the ability to pass through screen packs (without plugging), as well as providing consistent performance on a day-to-day basis. In general, some of the following characteristics have been recognized as providing the best overall performance: chloride process/rutile crystal, narrow particle size distribution, proper inorganic and/or organic coating, and minimal moisture content. Aspects to avoid are grades that lead to visual imperfections in the final film product, reduced impact or tear strength, voids, poor seal strength, lack of brightness/ whiteness, reduced opacity, deterioration upon UV exposure, and accelerated die-lip buildup. DEGRADATION ACCELERATORS A discussion of the pros and cons for degradable plastics is not feasible in this short chapter; however, both photo and biodegradable technologies are used by film producers. Consequently, a brief discussion is worthwhile. Most often, these technologies are implemented by adding degradation concentrates to a base resin at the film producer. Photodegradable-based systems, which are typically used as a concentrate, contain photosensitive materials that initiate a chemical and physical breakdown of a polymer. This happens after some degree of exposure to ultraviolet radiation from the Sun. The rate of breakdown of the polymer obviously varies with the base polymer, weather conditions, time of year, geographic location, and length of exposure. There are also photodegradable polymer products based on ethylene/carbon monoxide copolymers, which are themselves quite photosensitive and break down easily when exposed to the UV portion of the solar spectrum. A number of biodegradable systems for plastics are in different stages of research and development. Note that the term “biodegradable” refers to the entire polymer system or product, not necessarily the polymer matrix. For polyolefin films, starch-based concentrates enjoy a dominant position. Concentrates containing starch and a reagent for promoting oxidation are used at 12–15 percent. When the final product, such as a trash bag in a landfill, is exposed to oxygen and soil, the degradation process for the starch is initiated, which increases the surface area of the substrate. As the starch continues to breaks down, microorganisms of various types consume the starch, further exposing the polymer and hastening the perception of degradation. However, recent attention to the presence of microplastics in the environment may limit this sort of approach to biodegradability. A renewed

focus on compostable and biodegradable polymers such as poly (lactic acid) is attempting to address some of these concerns. Products based on photodegradable and biodegradable product technologies have shelf-life issues and require specific storage precautions. On the upside, especially for products that contribute to litter, this product technology is a plus. On the downside, these types of products must be rigorously excluded from polymer recycling efforts. FILLERS Fillers are polymer-insoluble particulates that are used to extend or reinforce a film product. When fillers that are less expensive than the polymer matrix are used in moderation, they improve the economics of the operation without (completely) compromising product quality. Fillers, used as extenders, are typically used in the 5–20% and even the 30–40% range. Calcium carbonate and talc are two common fillers used in the film industry. Although the economics can be improved by using fillers, every ingredient that is added to the film product gets its fair share of the blame when problems are encountered. If fillers are added at very high levels, they tend to be at the top of the list in terms of potential culprits in the “blame game”. Accordingly, it is important, as with each of the other additive classes, to be selective when choosing products. With the development of higher-quality, fine-particle-size calcium carbonates and talcs, additional benefits can be reaped by using these as functional additives. Suppliers at trade shows and conferences claim improved processing and bubble stability, better heat seal, printability, slitting performance, and improved mechanical properties. Calcium carbonate, talc, and zinc oxide are also being looked at by film companies as extenders to reduce the requirements for titanium dioxide, which can sometimes be in tight supply. Among the various filler chemistries and technologies available, certain criteria (beyond product quality) often must be taken into consideration, most importantly the particle size distribution and its upper limits. If particles are too large, they can certainly lead to processing problems such as film breaks. If particles are too small, then the processability of the product may become an issue due to increased melt viscosity. Most of these issues have been addressed for film-grade fillers. Suppliers are usually quite knowledgeable about what grades are appropriate for specific film applications. FLAME RETARDANTS Flame-retardant additives are used to retard the ignition and burn rate of plastic articles. There are several mechanisms for providing flame-retardant characteristics. Halogenated compounds are most often used in combination with synergists to interfere with the complex chemical reactions in the flame, thereby reducing or suppressing the fire. Phosphorus-based flame retardants are often used as char form-

Section 4.17.  Additives for Film Products

ers, which inhibit progression of the flame front. Metal hydrate flame retardants are heat absorbers in that they release water once heated beyond a certain point. Note, however, that these technologies do not make a plastic fireproof. They work more to limit the risk associated with plastics in an electrical or flame environment. As such, flame retardants enable plastic products to meet the various fire safety and performance standards imposed by a multitude of guidelines governing the flame-retardant polymer industry. Halogenated flame retardants are the most common type of flame retardant in plastics, where the halogen is either chlorine or bromine (bromine is larger among the two). Concentrates are available with either type of additive. Normally, a chlorinated concentrate is used in low-density polyethylene where extrusion temperature is less than 420°F (216°C). Brominated flame retardants are commonly used in linear low-density polyethylene and in high-density polyethylene where extrusion temperatures often exceed 450°F (232°C). Because they can easily be used at these higher processing temperatures, brominated flame retardants are not typically used in low-density polyethylene due to their relatively higher cost. Certain precautions must be taken into consideration when using halogenated flame retardants, especially with chlorinated products. For example, it is very important not to exceed the recommended processing temperatures. The early stages of chlorinated flame retardant degradation will be observed as a brown discoloration. Advanced stages of decomposition yield corrosive byproducts that attack the screw and barrel surfaces of the extruder. For the same reasons, material should not be allowed to sit in the extruder during more than a brief shutdown. To prevent charring and corrosion, it is important to purge these type of materials during extended idle times and especially for equipment shutdown. Guidelines for processing parameters (temperature, shear rates, polymer melt viscosity, etc.) are available from suppliers to ensure product viability and should be carefully considered when using reactive flame retardants. Flame-retardant concentrates can contain up to 40–60 % active ingredient. The supplier will recommend type and letdown depending upon the degree of flame retardancy required, the nature of the base resin, the extrusion temperature, and the film thickness. Typical use levels range from 6–10% for halogenated flame retardants. Even though metal hydrates are well known for their ability to provide flame-retardant performance, their applicability in film products is limited because use levels are often as high as 40–60% of the additive. To say the least, this might be quite a challenge for most film operations. Along with these technical challenges, regulatory pressure is limiting the use of halogenated flame retardants in many applications. Several common flame retardants have been banned from certain applications in the last decade, putting pressure on the industry for alternate solutions. Accordingly, the search is on for non-toxic, non-corrosive, low smoke, environmentally friendly, cost-effective approaches to flame retardancy with broader usability in film applications.

311

MELT-PROCESSING STABILIZERS One of the most damaging species in the auto-oxidation cycle is the hydroperoxide, ROOH. Under elevated temperatures, hydroperoxides decompose by homolytic cleavage to yield two free radicals. This step is representative of the catalytic nature of auto-oxidation. Neutralization of the hydroperoxides that continually build up in the polymer is essential in protecting the polymer. Most commercially available peroxide decomposers are based on trivalent phosphorus compounds used as melt-processing stabilizers. Trivalent Phosphorus Compounds: The chemistry of phosphites and phosphonites has been extensively reviewed in the open literature. In essence, a P(III) compound reacts with a hydroperoxide to convert the hydroperoxide to an alcohol, ROH, with subsequent oxidation of P(III) to P(V), as shown in a representative fashion,. These chemical reactions take place during melt compounding of the polymer in processing equipment. At ambient temperatures, the reaction is relatively slow. (a) ROOH + (R*O)3P → ROH + (R*O)3P = O Phosphorus (III) compounds are also capable of reacting with free radicals; however, the contribution of this chemistry is secondary to hydroperoxide decomposition. (b) RO2· + (R*O)3P → RO· + (R*O)3P = O (c) RO· + (R*O)3P → R· + (R*O)3P = O The groups attached to phosphorus play a key role in the reactivity and hydrolytic stability of the compound. In a very general sense, with less steric hindrance, the reactivity with hydroperoxides increases, and the hydrolytic stability decreases. Sometimes co-additives are used to increase hydrolytic stability, such as tri-isopropanol amine (TIPA), alkaline acid scavengers, or both. In addition, phosphite products with small particle size can be compacted to give larger particles with less surface area. These techniques will improve handling of the material in the open atmosphere. Nevertheless, when the additives are melt-compounded into the polymer, the hydrolysis inhibitors are diluted away into the polymer matrix (in the same way that the phosphorus compound is dispersed). This leaves hydrolytically unstable phosphites at the mercy of trace levels of moisture in the polymer, or when the polymer is held in contact with high humidity for prolonged time periods. Hydrolysis of phosphites, which is a stepwise reaction for each of the RO- groups attached to the phosphorus atom, can be summarized as follows: (d) (R*O)3P + 3H2O → 3ROH + (HO)2HP = O (phosphorous acid). The resulting phosphorous acid has a negative impact on processing and compounding equipment, with its corrosivity leading to a phenomenon known as black specks. In addition, the liberated alcohols, usually low-MW phenols, can be further oxidized, potentially leading to discoloration. (See Anti-Gas Fading above.)

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On the other hand, increasing the steric hindrance of the phosphite can improve hydrolytic stability, which enables easier handling in the open atmosphere. However, the reactivity with hydroperoxides tends to decrease. Typically, the manufacturer must decide what is more important: fast reactivity, or safe handling of the material, as well as stability in the post-production polymer. Some commercial phosphites offer a balance between these two extremes. Efforts during the 1990s focused on designing phosphorus-based compounds with improved hydrolytic stability that also provided excellent performance as melt-processing stabilizers. As an alternative to phosphite chemistry, melt-processing stabilizers have recently been introduced based on hydroxylamine or lactone (benzofuranone) chemistry. Both chemistries are significantly more powerful than phosphites and do more than decompose hydroperoxides. Both product classes can also react with oxygen-centered free radicals, thus functioning as both primary and secondary antioxidants. Transformation products of hydroxylamine, as well as lactone, can react with carbon-centered radicals. It is believed that carbon-centered radical trapping is what gives these products their apparent hyper-reactivity because most conventional primary antioxidants are not known for this capability. METAL DEACTIVATORS Polymers that come into contact with metals with low oxidation potentials, such as copper, are susceptible to oxidation from the metal-catalyzed decomposition of hydroperoxides, as shown below. (a) ROOH + Me+ → RO· + OH– + 2Me+ (b) ROOH + Me2+ → 2RO· + H+ + Me+ Consequently, metal-catalyzed initiation can be described as: (c) ROOH → RO2· + RO· + H2O One way to avoid the negative influence of these types of free-radical activators is to use metal deactivators. Metal deactivators are typically hydrazide or amine functional groups that can complex to the metal, thereby rendering it less reactive. Most products also include a phenolic substituent as part of the molecular structure. Metal deactivators are usually melt-compounded into the polymer (by the polymer producer) and then interact with the metal by migrating to the polymer-metal interface. These products are of particular importance for wire- and cable-coating applications, but less important for blown- and cast-film operations, unless prolonged contact with metals such as copper is considered to be an issue. POLYMER PROCESSING AIDS Linear low-density polyethylene derived from traditional multi-site or single-site catalysts has rheological properties

that typically make it more difficult to extrude than standard LDPE resins. This difference is mainly related to a narrower molecular weight distribution (MWD) and the absence of long-chain branching. In fact, some LLDPE derived from single-site catalysis can be even more challenging. These extrusion requirements are not necessarily problems if the right processing equipment is in place. However, for older extrusion technologies, these higher-melt-viscosity, lower-meltstrength polymers could represent a challenge in terms of the power limitations of the film extruder and limitations on extrusion rates. For example, higher melt viscosities often result in higher extruder-head pressures, higher torque requirements, and reduced extrusion rates. Bubble instability from lower-melt-strength resins (from the narrower molecular-weight distribution with no long-chain branching) can result in less uniform film gauge and loss of productivity. In addition, LLDPE extruded through LDPE dies with narrow die gaps of 30–50 mils (0.76–1.27 mm) may exhibit surface imperfections due to the phenomenon referred to as melt fracture. (Chapter 4.16 of this manual has a more detailed discussion of the melt fracture phenomenon and methods for reducing or eliminating it.) A variety of polymer processing aids (PPA) have been developed to alleviate these types of processing challenges. The most common types used are formulated with fluoropolymers. Co-additives to enhance fluoropolymer performance have also been developed. The active ingredients in a PPA have an affinity for metal (greater than for the polymer) and coat the metal surfaces in the extruder and die, giving the perception of a lubricated surface. The coating process, however, is dynamic, and therefore the additive must be continually replenished. Additives that compete with the processing aid at the polymer-metal interface can reduce its effectiveness. In addition, additives that are abrasive can also reduce the level of PPA at the metal interface. In both cases, relatively higher loadings of PPA are necessary. An alternative approach is to remove or reduce the concentration of these interfering additives if they are not a critical component of the formulation. Sometimes this can be done by co-extrusion by placing the additives in layers other than the outer ones that contact the die metal. In a film-blowing operation, one approach to using these types of products is to “precondition” the extruder and the die by adding higher concentrations of the polymer processing aid for a relatively short period of time. This can most easily be done using an additive concentrate. After melt fracture has been eliminated, the PPA concentration can be quickly reduced to the “maintenance” level, which is the amount of PPA necessary to reap the maximum benefit at the lowest concentration. In practice, color and additive concentrates are often used at the film converter. Depending on the nature of the concentrate, letdown ratios of 30:l to l00:l are used depending upon the resin, extruder die, specific problems, and other factors. Metal stearates, which are commonly present in color concentrates and/or certain resin formulations, in addition to other types of additives, such as titanium dioxide and cal-

Section 4.17.  Additives for Film Products

cium carbonate, may interfere with the processing aid, therefore requiring slightly higher levels. It has recently been recognized that polymer processing aids can be used for purposes beyond eliminating melt fracture, reducing power requirements, and increasing throughput rates. It has been reported in conferences that PPA can also be used to reduce die-lip buildup. The suppression of die-lip buildup can increase the amount of time between maintenance of the die lips as well as improving the surface quality of the film product. Note that the PPA use levels necessary to inhibit die-lip buildup are often reported to be much less than the amounts needed to eliminate melt fracture. It has also been reported that polymer processing aids, in combination with higher concentrations of an antioxidant or melt-processing stabilizer, can reduce gel counts (for those type of gels derived from molecular weight increase in the polymer during melt processing). Again, the use levels for gel-count suppression are typically less than the concentrations needed to eliminate melt fracture. SLIP ADDITIVES Slip additives used in polyolefin film extrusion are generally based on various types of fatty amides. These types of amides are not compatible with the resin, and once the resin exits the film die, the amide migrates to the film surface. This promotes surface lubrication, reducing the coefficient of friction (CoF). The reduced CoF decreases drag across the collapsing frame, thus permitting more rapid extrusion rates and fewer problems with film wrinkles. Levels of CoF are also important in other post-extrusion operations. In addition, some types of slip agents also perform as an antiblock to prevent adhesion of film surfaces. (See Chapter 4.18 for a more detailed discussion.) Slip agents can be added at the polymer producer or can be used as concentrates at the film shop. Concentrates usually contain 1–5% of the slip additive and are typically referred to as slow-blooming or fast-blooming. Oleamide (fast) is generally used for in-line bag-making operations when low CoF is needed quickly. Erucamide (slow) is used in roll stock manufacturing, where too slippery a surface causes winding problems and roll stock “telescoping”. Higher slip-agent concentrations (low CoF) can also interfere with corona treatment, which is sometimes required for good adhesion and print qualities. In this case, lower levels of the slower-migrating erucamide may be desired so that, at the gauge being extruded, the CoF does not fall below 0.4. Table 4.17.3 shows representative slip additive levels. The TABLE 4.17.3.  Typical Slip Additive Levels. Levels of Use (of Erucamide in PE) Low Slip Normal Slip High Slip

CoF

100–200 ppm (0.01–0.02%)

0.50–0.80

500 ppm (0.05%) 1000–2000 ppm (0.10–0.20%)

0.20–0.40 0.05–0.20

313

final use levels are adjusted when the resin type, film thickness, desired CoF, type of slip, and presence of other types of co-additives and the downstream operations that may be performed on the film surface are known. ULTRAVIOLET STABILIZERS Light in the ultraviolet (UV) spectrum reaching the Earth’s surface (with wavelengths between 290 and 400 nanometers) is absorbed by plastics, initiating the destructive process of photodegradation. This degradation causes loss of physical properties such as tensile strength, elongation, and impact strength. Discoloration, chalking, and embrittlement can be visually observed. For example, unstabilized polyolefin films exposed to UV radiation can degrade within weeks to months (of course depending on the substrate and thickness). UV radiation has sufficient energy to break chemical bonds, most notably hydroperoxides, generating free radicals. The free radicals react with the polymer and/or with oxygen to form hydroperoxides, as mentioned above in the sections on thermal auto-oxidation. The photo-oxidation process continues until the polymer is entirely degraded. Processing contaminants, catalyst residues, and chromophores, if present in a plastic material, can also absorb UV radiation, thereby promoting the process. Light stabilizers are incorporated into polymers to inhibit the detrimental effects of UV light. The development of light stabilizer technology has been a critical factor in the growth of the plastics industry. Stabilization of plastic film can be achieved by using one or more UV additive systems. UV ABSORBERS First-generation light stabilizers for plastics came from the class of organic compounds known as UV absorbers. UV absorbers function by preferentially absorbing UV light of wavelengths from < 300 to > 400 nanometers and dissipating the light energy at lower wavelengths in the form of molecular vibrations. The most widely known classes of UV absorbers are benzophenones and benzotriazoles. Other families of UV absorbers based on oxanilide, cyanoacrylate, and hydroxyphenyltriazine chemistries are also suitable for use in plastics. Their activity is related to intramolecular hydrogen bonding between the phenolic –OH group and the neighboring oxygen or nitrogen atom. It has been proposed that electronic distribution in the excited singlet state favors transfer of the hydroxyl proton to the oxygen or nitrogen heteroatom to give a tautomeric excited singlet state. This is followed by a radiationless transition to the tautomeric ground state and finally by transfer of the proton back to the phenolic oxygen. In the process, heat energy is released to the medium. (a) UVA + h → →UVA* (b) UVA* → tautomer* (c) tautomer* → tautomer + heat (d) tautomer → UVA

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The effectiveness of UV absorbers is governed by the Beer-Lambert equation; A = ebC (where A = the absorbance of the polymer containing the UVA; e = the molar extinction coefficient of the UVA; b = the path length of the polymer substrate; and C = the concentration of the UVA). Accordingly, as the part becomes thinner, more UVA is necessary to achieve the same level of protection. As can be expected, there is a limit to how much UVA is appropriate. In the early days, UV absorbers were traditionally used in parts with thicker cross sections and were believed to be relatively ineffective in plastics with high surface areas, like fibers and films. The development of UVA with high molar extinction coefficients has improved the effectiveness of absorbers in thin, high-surface-area applications such as film. Energy Quenchers (a) Polymer + h → → Polymer* (b) Polymer* + Q → Polymer + Q* (c) Q* → h →′ (and/or heat ) Most energy quenchers are based on nickel complexes, which have a characteristic green color. These compounds can also deactivate the excited polyethylene-oxygen complexes that dissociate to produce vinyl groups and hydrogen peroxide. The difference between UV absorbers and energy quenchers is that UVAs protect the polymer by preferentially absorbing incident light, whereas energy quenchers deactivate excited-state polymer molecules. In reality, UVAs may also function as energy quenchers, and energy quenchers may also act by absorbing light. In the past, quenchers have been widely use in agricultural film and pool solar blankets. Some suppliers have withdrawn these types of products due to concerns about nickel in the environment. Radical Scavengers Traditional antioxidants such as hindered phenols usually do not inhibit photo-oxidation effectively enough during long-term polymer exposure to UV radiation. Some benzophenone UV absorbers can trap radicals (because they contain a phenolic moiety), but the UVA may be consumed in the process. Hindered amine light stabilizers (HALS or HAS) are very effective polymer stabilizers due to their ability to scavenge free radicals over a fairly broad temperature range (from < 0°C up to ~120°C and even 135°C). The mechanism of polymer stabilization by hindered amines is complex. Under photo-oxidative conditions, HALS are converted to nitroxyl radicals that can trap carbon-centered radicals. The resulting N-alkoxy derivative then reacts with a peroxy radical to regenerate the nitroxyl radical: (a) >N-H + [O] → >N-O· (b) >N-O· + R· → >N-OR (c) >N-OR + R'OO· → >N-O·

Complexes between hindered amines (or nitroxyl radicals) and hydroperoxides increase the concentration of hindered amine stabilizers in regions of the polymer that are more susceptible to oxidation. In the literature, it has been shown that hindered amines can bind with transition metals, which can facilitate the decomposition of hydroperoxides. Reasonable consideration must be given to the selection of HALS because they are reactive to the presence of other additives and chemicals such as fumigants, pesticides, and flame retardants. Negative reactions may affect the degree of desired UV performance. Selection of an appropriate UV stabilizer or stabilizer combination should consider the following: level of protection required, base resin, film thickness, regulatory compliance, sensitivity to color development, thermal exposure, and as stated with other additive classes, potential reactivity with other co-additives and chemicals. Hindered amines are typically incorporated at levels of 0.1–2%, depending on the variables just discussed. In practice, UV stabilizers can be incorporated by the polymer producer, but are often added as concentrates that typically contain 10–20 percent of a specific additive. These UV concentrates may contain a combination of additives or antioxidants to provide an enhanced effect. Level of concentrate usage depends upon the type used and the specific enduse application. A final letdown concentration in the range of 0.1–2% illustrates the loadings that are necessary for outdoor applications such as greenhouses, mulch film, tarpaulins, silage protection, fertilizer bags, hay bags, pipe wrap, drip irrigation, construction films, tenting, and swimmingpool covers. At times, other additives may bring along with them undesirable pro-degradants such as iron complexes to an application that requires UV additives, which can lead to premature failure of the film. ADDITIVE DELIVERY Raw additives are most commonly available as free-flowing powders, or in some cases as liquids or low-meltingpoint solids. For the polymer producer who manufactures blown- and cast-film grade resins, these pure additives can be taken and melt-compounded into the base resin to give a formulated product. The type of additives used in this type of manufacturing environment are those that are either required for base stabilization, such as antioxidants, melt-processing stabilizers, and acid scavengers or are commonly used filmtype additives such as slip, antiblock, and perhaps a polymer processing aid. Other additives are most often added further downstream at the film producer as an additive masterbatch, mastermix, or concentrate. As stated above, an additive concentrate is a blend of one or more additives and a resin carrier formulated to be mixed (let down) with a specific weight of resin to achieve a desired performance or physical property in the end product. This concentrate approach is helpful to the manufacturer of downstream products because neat additives in liquid and powder forms are not often used by most film extrusion manufacturers (based on the traditional setup

Section 4.17.  Additives for Film Products

of their equipment). Concentrates manufactured for the film industry are most often available in a granular form or as pellets, but also come as liquid dispersions as well. The two most common types of concentrates are additive concentrates and color concentrates. The terms most frequently used for concentrates are letdown (dilution), incorporation (blending or mixing), and dispersion. The term letdown refers to the ratio or percent of base polymer mixed with a given concentrate. For example, a 25:1 letdown equals 25 pounds of resin to 1 pound of concentrate, or in other words, the concentrate is used at approximately 4 percent. The term incorporation (sometimes called distribution) refers to the method of distributing the concentrate in letdown resin to ensure good mixing or blending. Perhaps the most important term is dispersion, which refers to the homogeneity of the given additive and specifically to the wetting and separation of particulate additives in the concentrate. The objective is for that to be done without particles or agglomerates appearing as film imperfections. Additive concentrates range from 1–75% additive loading, depending upon the types of additives that are used. A key factor is the mutual compatibility of the concentrate carrier resin with the letdown resin. For example, there is some latitude in the use of LDPE or LLDPE concentrates in higher-density polyolefin, but a HDPE-based concentrate would not incorporate well into a lower-density polyolefin resin. It is also important that the melt flow rate of the concentrate be at least equal to that of the letdown resin and preferably higher for better incorporation (mixing) and dispersion of the additives. Additive concentrates have a variety of potential advantages. Concentrates can be mixed in central blending systems and conveyed to individual extruders. They can be accurately metered using equipment at the extruder, enabling precise addition of the additive concentrate. The use of concentrates contributes to a more dust-free work environment. Spills are easily vacuumed or swept up. Difficult-to-mix additives can be pre-dispersed in a similar matrix. This pre-dispersion can be of particular importance with color concentrates because the best color is usually achieved when the materials are optimally dispersed. Most concentrate manufacturers use extrusion equipment that can introduce the amount of shear necessary to achieve the level of dispersion required for film products. This is usually not possible in a typical single-screw film extruder. Another attractive strategy with concentrates is to start with a few basic resins in silos and modify those few “base resins” to provide very specific additives for the application and gauge to be produced. On the other hand, additive concentrates have potential disadvantages. Their improper use can lead to off-color film due to miscalculation of the letdown ratio, burnout in the extruder, or contamination. If the additives are not adequately pre-dispersed, aggregate-type gels may form in the film product, and screen pack life may be reduced. If the proper base resin for the concentrate is not used, then the result may be poor incorporation of the additive due to incompatibility of the concentrate base resin with the letdown resin. Inad-

315

equate opacity can be observed when the color concentrate has the wrong pigment content, the use level is off, or the concentrate feed system is out of calibration. Even concentrate pellet geometry can be an issue. Moreover, many color concentrates have a much higher specific gravity than the resins with which they are blended, and if they are delivered to a non-mass flow bin over the extruder feed throat, then resin and concentrate pellet segregation can occur. It should be recognized that the concentrate industry, in its current state, has optimized most of these strengths and minimized most of these potential disadvantages by developing alternative solutions or technical support to get around most problems. As a result, the concentrate industry has been thriving over the last three decades. SYNERGISTIC AND ANTAGONISTIC MIXTURES OF ADDITIVES The word synergy has certainly been overused, but it is still a technical phenomenon, just as much as antagonism is a relevant concept. Using words to define its meaning, synergy is the unexpected improvement in the effect of a set of combined additives compared to the sum of the effects of the individual additives. Antagonism would be the unexpected worsening of the anticipated effect of the combined additives compared to the sum of the effects of the individual additives. However, it should be noted that what might be representative of a synergistic mixture of additives in one polymer does not necessarily translate into what might be the optimum mixture in another polymer system. Another way of mathematically expressing this concept would be as follows: (additive effect is set to “1”): (a) Synergistic Effect: “1” + “1” > “2” (b) Antagonistic Effect: “1” + “1” < “2”. Additive Synergy As a representative example of synergy, blends of a phenolic antioxidant and a phosphite are very useful for melt compounding of polyolefins and engineering polymers. Together, they work well to maintain the molecular weight of the polymer, while at the same time affording low color. The phosphite shares the work load with the phenolic so that neither one is exhausted or over-oxidized. Even if the phosphite is entirely consumed, it has done its share of the work, thereby preserving more of the phenolic antioxidant for long-term thermal stability. Again, blends of a phenolic antioxidant and a divalent sulfur compound are an excellent combination for improving long-term thermal stability of a polymer. Even though the sulfur compound provides no activity during melt processing, it does provide excellent performance as a hydroperoxide decomposer during long-term thermal aging. This contribution along with the free-radical scavenging capability of the phenolic is a good mix (as long as taste or odor is not a key measure of performance).

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These are everyday examples of additive synergism. To reap the maximum benefit of synergistic mixtures, it is useful to determine the optimum ratio of the components. Typically, “synergistic blend titration” experiments are performed (at a set loading of the additives), starting with 100% of component A and 0% of component B. A series of formulations are designed to shift to the other extreme with 0% component A and 100% component B. By measuring a series of performance parameters, the optimum ratio of A to B can be determined. This type of work is time-consuming, but the optimum ratio can be identified. If three or more components are being assessed at the same time, statistically designed experiments are often useful in terms of sorting out the data set. ADDITIVE ANTAGONISM Mixtures of antioxidants can work together synergistically, as described above; they can also work against each other. Chemistries that interfere with each other may not necessarily be obvious until the evidence is presented. By that time, it is too late; the damage is done. Someone is going to want an explanation. For example, to ensure long-term thermal stability and good light stability, one might use a blend of a phenolic antioxidant and a divalent sulfur compound (for thermal stability) in combination with a hindered amine (for light stability). Unfortunately, over prolonged periods of time, the sulfur compound can become over-oxidized, yielding acidic byproducts. These acidic species from the over-oxidized sulfur compound can complex with the hindered amine to form a salt, which inhibits the hindered amine from entering into its free-radical scavenging cycle. This antagonism has been generally known for quite a while and has recently been discussed in the open literature. Less alkaline-hindered amines have been developed in an attempt to circumvent the issue. Other types of antagonistic chemistry often involve relatively strong acids or bases (either Bronsted or Lewis) that can interact with antioxidants in such a way as to divert them into transformation chemistries that have nothing to do with polymer stabilization. These type of interactions are sometimes difficult to find and define, typically time-consuming to quantify, and always a waste of a perfectly good antioxidant. ANCILLARY PROPERTIES In reality, there is more to additives that provide enhanced performance characteristics to the polymer. Other key issues besides rates of reactivity and efficiency include performance parameters such as volatility, compatibility, color stability, physical form, propensity to form transformation products that lead to taste or odor, regulatory issues associated with food contact applications, and polymer performance vs. cost. Volatility As a representative example, most stabilizing additives

are melt-compounded into the polymer after the polymer exits the polymerization reactor. The exiting polymer, as either a molten mass or a free-flowing powder, is converted to pellets that are stored in containers and then shipped to the prospective customer. The customer then transforms these pellets into shaped articles, which are stored and then shipped to the next customer down the line. The two meltcompounding steps represent significant heat histories. Product storage can also be quite warm in certain climates. It is important that the stabilizer, as well as its transformation products that may also provide stability, not volatilize from the polymer. Many commercial antioxidants have been designed with higher molecular weights to address this issue. For those with lower molecular weights, volatility may actually be desired. Compatibility Most additive classes are designed to be compatible with the polymeric matrix, the exceptions being antistats, antifogs, and biostatic materials. If an additive is not designed to be compatible, it should at least migrate or diffuse slowly. This is important because, if the solubility limit of the additive in the polymer is exceeded, there is a driving force for exudation of the additive to the polymer surface. Exudation or blooming involves migration of an additive out of the polymer matrix onto the surface as a very thin film. At the surface, the additive is susceptible to extraction, oxidation, volatilization, or washing away. Blooming of the additive can also diminish surface gloss, create stickiness, or eliminate blocking (cling) of film surfaces to one another. Additives can be modified to improve compatibility, albeit at a cost to the end user. Color Stability It is important that various additive classes provide good performance for the end-use polymer application, but it is also important that they do not provide or develop unwanted color due to transformation chemistries during transfer, storage, or use. As mentioned earlier, some antioxidants are prone to forming color by their very nature, whereas other antioxidants discolor only when they have been over-oxidized. In some pigmented systems, such as those pigmented with carbon black, discoloration is not a key issue. On the other hand, some pigmented systems, particularly white and lighter shades, can emphasize or magnify discoloration effects. Physical Form Additives are most often free-flowing powders, pastes, or liquids. Hazards associated with dusting of fine powders are becoming more of a concern for health and safety-related issues. Many additives are offered commercially in dust-free forms. Liquid and molten additives are another interesting alternative, as long as they are compatible with the polymer

Section 4.17.  Additives for Film Products

matrix. Some polymer producers need powders to mix well with their reactor product. Downstream product manufacturers may prefer to have their additives pre-dispersed in a user-friendly matrix. Taste and Odor For applications or end uses that involve food contact or home or personal use, taste and odor are key issues. This is interesting in light of the fact that the human nose is typically more sensitive than even the most powerful analytical instruments. Regulatory Issues Additives that are used in polymers which come into contact with food require clearance by various regulatory agencies. Chronic and subchronic toxicological testing is performed in different species. Migration studies of the additives from the polymer using different food simulants are also evaluated. Accordingly, concentrations of the material expected in the diet can be assessed. A product should typically have minimal health hazards associated with its use or handling. In addition to studies on the additives themselves, there has been increasing emphasis on examining the byproducts, transformation products, and non-intentionally added substances (NIAS) associated with the additives. Increasing scrutiny on these substances by regulatory agencies is expected to continue in the coming years. Safety is usually assessed by studying the impact during toxicity tests such as oral, inhalation, or eye and skin irritation tests. Mutagenicity tests are also carried out. Performance vs. Cost Additives are not free; however, they are affordable. The point in using them, in a sense, is to choose the appropriate type and level to develop a suitable polymer for a particular end-use application. Some articles are used once, such as bags and food wrap. Conversely, some articles are expected to last decades, such as geomembranes, insulation for wire and cable, or gas and water transmission pipes. For example, if the material is a non-durable good, the type and concentration of antioxidant are chosen to minimize unnecessary costs associated with stabilizing the polymer. The antioxidants should be able to provide stabilization for the initial melt compounding of polymerization reactor granules into pellets and from pellets into a finished article, be it tape, film, fiber, sheet, cups, eating utensils, or whatever. Stabilizing the polymer to last ten years when it is going to be used once or twice does not make much sense. However, it is important to consider the need to maintain quality and the fact that scrap from melt compounding must be recycled. Minimizing costs by reducing antioxidants can sometimes result in inferior products. In addition, a cushion is typically built into the system to avoid unexpected shutdowns and startups or prolonged storage in hot places.

317

On the other hand, if the material is a durable good, that is, a material that is expected to be useful for a long period of time, the types and concentrations of the additives are chosen to meet certain performance criteria. The costs associated with this type or level of additive system are worth the additional value of the final product, such as pipes, insulation bumpers, gears, rotors, covers, housings, membranes, hose, tubing, carpet, furniture, or toys. The value-added aspect of the product pays for the additives that are necessary to ensure high quality and durability. PERFORMANCE TESTING It is outside the scope of this article to discuss performance testing of additives due to the number of different types of additives and the incredible number of polymers that exist. However, in principle, it is important that testing be performed to validate the use of a particular additive system before a product is commercialized. Most often, initial testing involves melt compounding of the polymer with various formulations consisting of different additive systems or mixtures. Retention of molecular weight, color, and appearance are common measurements. Long-term thermal stability testing is usually carried out in ovens set at elevated temperatures to accelerate the aging process. Physical property retention and color development are measured as a function of time at that test temperature. Varying extrusion temperatures and changing shear rates afforded by the processing equipment are also useful measurements. Testing the ancillary properties mentioned above is also important. COEXTRUSION AND ITS AFFECT ON ADDITIVE USE The world of monolayer extrusion is still alive, but the many lines installed in the last 10–15 years have been coextrusion lines, and estimates today indicate that the vast majority of all new lines are coextrusion lines in most or all major film markets for LLDPE/HP-LDPE. This means that additives as well as polymers can be “positioned” within a structure to give maximum benefit with a minimum of negative interactions. One example will suffice. In three-layer heavy-duty sack structures today, the TiO2 pigment, which has a known negative effect on PPA performance, can be placed in the middle layer for opacity, leaving the PPA to be added only to the outer and inner layers that see the die wall. SUMMARY The advent of a wide variety of new polymers, applications, and equipment options has resulted in the use of more and unique types, kinds, and grades of additives, as shown just by the examples in Table 4.17.1. The technology exists to use and manage these additive systems to good benefit in the myriad of applications today. However, there is a need to think carefully and clearly about how to use each of the addi-

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tives required to maximize the performance/cost balance by talking as freely as possible with the suppliers of the resins and additives to be used. This should be followed by some carefully conducted pilot trials. This is the best way to avoid unpleasant surprises. REFERENCES AND ADDITIONAL RESOURCES [1] Gaechter, R. and Mueller, H., Plastics Additives Handbook, 4th ed., Hanser, New York, 1993. [2] Modern Plastics World Encyclopedia, Year 2000 edition, Modern Plastics, New York, 2000.

[3] Plastics Technology Manufacturing Handbook, Year 1999 edition, Plastics Technology, New York, 1999. [4] TAPPI Polymers, Lamination and Coatings Conference, Additive Tutorial Sessions, 1997, 1998, 1999. [5] CRC Plastics Encyclopedia, CRC Publishing, 1996. [6] Turick, J., previous author of the “Concentrates and Additives” chapter in the TAPPI Film Extrusion Manual, and R. E. King III, previous author of the “Additives for Film Products” chapter in the 2005 edition of the TAPPI Film Extrusion Manual. [7] Grob, M. and Minder, E., AddPlast Europe, Wiesbaden, Germany, April 11–14, 2000.

Chapter 4—Section 18

Slip Agents ADAM J. MALTBY and RICHARD E. MARQUIS, Croda Universal Inc.

INTRODUCTION Slip is the common term used to describe both the level of friction (CoF) at a polymer film surface and the chemical additive used to obtain this effect. Most polymer surfaces are fairly high in friction (CoF) and are therefore unsuitable for high-speed processing on modern equipment. Fortunately, this friction can be dramatically lowered by adding a small quantity of a lubricant known as a slip agent. The most commonly used slip additives are a class of compounds known as fatty acid amides, which migrate to the surface of a polymer and reduce the friction between it and another surface. This article describes the manufacture, mode of action, and use of slip additives in a variety of polyolefins.

rape oil is used for C22-based materials and tallow for C18based. MANUFACTURE

The main types of amides used in plastics are those that comply with direct food-contact regulations (EC, FDA, etc.).

Triglyceride oil is hydrolyzed to yield its constituent fatty acids and glycerol. These fatty acids are purified to concentrate the chain type required. Erucic acid in rape oil is around 45–49%; for production of erucamide, this is raised to around 90%. If a saturated species is required, the fatty acid may be hydrogenated, e.g., for manufacturing stearamide or behenamide. The fatty acids are then reacted with (1) ammonia to produce a primary amide, (2) a primary amine to produce a secondary amide, or (3) ethylene diamine to produce a ‘bis’ amide. Depending on the grade required, the amide may be further purified to improve color, odor, and stability. Amides are available in a number of solid physical forms such as fine powders (20–50 µm average), microbeads (100–500 µm), beads (1–2 mm), and pellets (approximately 5 × 3 mm).

RAW MATERIAL SOURCE

HOW AMIDES FUNCTION IN POLYMERS

Amide slip additives are derivatives of naturally occurring fats or oils. These oils are known as triglycerides, which are esters of glycerol with three fatty acids (Figure 4.18.1). The fatty acids can vary by chain length (mostly between 8 and 26 carbons) and number of double bonds (0, 1, 2, or 3). For polymer additives, it is usually desirable to have a narrow chain-length distribution and principally zero or one double bond. It is therefore necessary to choose the source of fatty acids that is closest to the required lipid profile to minimize processing. If the major primary amide additives are considered (erucamide, oleamide, stearamide, and behenamide), it can be seen that a source of 22 or 18 carbon chains with zero or one double bond is needed. There are two principal sources of 22 carbons, rape (e.g., Brassica campestris or napus) and crambe (Crambe abyssinica or hispanica). Eighteen carbon chains can be obtained from tallow (animal fat), palm oil, soya, and low-erucic rape oil, among many other sources. For commercially produced amides, generally high-erucic

Amides are multifunctional additives in polymers and can have a variety of effects. The most important effect is slip, or the lowering of the coefficient of friction (CoF) at the polymer surface. The other major benefit of using amides as polymer additives is assisting an inorganic antiblock or as an organic antiblock to reduce blocking or unwanted adhesion between polymer surfaces. Related effects, which may be considered to be a combination of these two effects, are reducing bottle-cap removal torques and mold release. Amides tend to be compatible with molten polymer, but migrate to the surface of solidifying polymer (Figure 4.18.2). The degree of incompatibility increases with decreasing polarity of the polymer. Hence, oleamide is more compatible with EVA copolymer than polyethylene. However, once the solubility has been exceeded, the amide will migrate to the surface through amorphous regions of the polymer. Once the surface is completely covered, a marked difference in the surface properties of the polymer will be apparent, including a sharp drop in the coefficient of friction.

Types of Fatty Acid Amide Slip Agents

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TABLE 4.18.1.  Main Types of Amides used in Plastics. Amide Oleamide Erucamide Stearamide Behenamide Oleyl palmitamide Stearyl erucamide Ethylene-bis-stearamide Ethylene-bis-oleamide

Type

Chemical Formula

Main Chain*

Primary unsaturated

C17H33C (O)NH2

C18:1

Primary unsaturated Primary saturated Primary saturated Secondary unsaturated Secondary unsaturated Bisamide saturated Bisamide saturated

C21H41C(O)NH2 C17H36C(O)NH2 C21H44C(O)NH2 C18H35NH(O)C16H31 C18H37NH(O)CC21H41 C17H36C(O)NH(CH2)2NH(O)CC17H36 C17H33C(O)NH(CH2)2NH(O)CC17H33

C22:1 C18:0 C22:0 C18:1–C16:0 C18:0–C22:1 C18:0–C18:0 C18:1–C18:1

The number before the colon is the carbon chain length, and that after the colon is the number of double bonds.

How an amide affects the surface properties of the film depends mainly on the amide type, its concentration, and the polymer type. Just because an additive migrates to the surface does not mean that it will act as a slip agent. For example, glycerol mono-oleate migrates to the surface of polyethylene films, but may actually cause friction to increase. Similarly, not all amides perform in the same way. When using amides, it is important to consider anything that might affect migration of the amide to the surface. This includes polymer crystallinity, cooling rate, storage temperature, polymer polarity, and the presence of any other additives that may adsorb the amide or compete with it at the surface. (Inorganic additives with high surface areas (porosity) can have significant effects.) MEASUREMENT OF FRICTION There are many methods of measuring friction on surfaces. For plastics, the most commonly used is dragging a sled of known mass over a test sample. The sled may also be covered with a test sample and dragged over another surface. The second surface may be the same polymer, a different polymer, metal or glass, or something else. A common arrangement is shown in Figure 4.18.3. A number of national and international standards have been developed using variations of the above apparatus to measure friction. Examples include ASTM D1894 and BS 824. ASTM specifies pulling a sled weighing 200 g over the test surface at 150 mm/min over a 150 mm test length. The British Standard, however, uses a larger sled weighing 700 g pulled at 800 mm/min over a 500 mm test length.

HOW AMIDES ARE ADDED TO POLYMER Amides work at very low levels in polymers, and accurate addition is paramount. The various methods of addition can be grouped into four main types: direct, pre-blend, liquid dispersion, and masterbatch. There are many reasons why different methods may be used, including cost, safety, accuracy of addition, historical factors, and availability of equipment. A detailed discussion of the pros and cons of each option is beyond the scope of this article. Direct Addition The amide is added directly to the polymer by dosing into an extruder. This may be achieved by feeding the solid from a loss-in-weight feeder directly into the throat of the main polymer extruder. Alternatively, the polymer may be premelted and pumped into the extruder using a gear pump. Liquid Dispersion In this method, the amide is dissolved or dispersed in a liquid medium (e.g., mineral oil) and pumped into the polymer extruder. Pre-blend The amide is blended, usually in powder form, with a solid carrier (polymer) or with other additives. The blend can be fed directly into the polymer or may be compacted into pellets first to reduce dust and aid feeding. Masterbatch In this method, the amide is added to a polymer at a level one or two orders of magnitude higher than that required in the final application and melt-extruded into pellets. Examples in Polymers

4.18.1.  R1,

FIGURE and are alkyl chains of the same or different lengths and/or degrees of unsaturation. R 2,

R3

In the following examples, CoF = kinetic coefficient of friction. The major variables in the amide structure that affect performance are chain length and the number of double

Section 4.18.  Slip Agents

321

FIGURE 4.18.2.  Slip development in polymers.

bonds. In general, the shorter the chain length, the faster the amide will reach the surface in polyolefins. (Note that slip agents with shorter chain lengths are more volatile at the extrusion temperatures reached with some of the LLDPE resins of today and hence are not useful or recommended in these applications.) Amides with one double bond tend to be good slip additives, whereas those with no double bonds tend to be poor slip agents, but better antiblocking agents. Figure 4.18.4 shows that in LDPE at 500 ppm, stearamide is a poor slip additive, giving a CoF of about 0.7. Oleamide and erucamide, however, give good friction levels around 0.1. Oleamide initially gives a lower CoF, but is surpassed by erucamide. Although oleamide and erucamide are clearly superior at lowering the friction of a polymer surface, stearamide, which is a much harder, more crystalline (higher melting point) amide, is a better antiblocking agent, as shown by the data in Figure 4.18.5. Figure 4.18.6 shows that with time, amide builds up a layer on the polymer surface and that the ultimate slip is generated when a complete layer has been formed. The con-

centration of amide added will affect the final slip (CoF). It is therefore possible to control the final slip (CoF) to some extent by altering the amide concentration. Due to the steep slope of the curve between zero and 500 ppm erucamide in Figure 4.18.2, it is difficult to control the CoF accurately between 0.2 and 0.4. Where medium slip is required, a better alternative strategy would be to use a secondary amide such as oleyl palmitamide or stearyl erucamide (Figure 4.18.7). In this case, the much shallower variation of CoF with concentration enables good control of CoF in the medium-slip region. It is well worth mentioning here that in applications requiring the use of water-based inks, it is necessary to control the amount of slip agent at the surface of the film at the printing press, and hence slip migration control is even more critical in that case. Gauge Effects Note that because the effect of slip agent(s) is felt at the surface, the polymer gauge has a very significant effect on the final slip (CoF) obtained. In other words, much less slip

FIGURE 4.18.3.  Typical slip measurement setup.

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FIGURE 4.18.4.  Effect of different primary amides on LDPE friction (500 ppm Amide).

agent is required in the bulk of the film at 4 mils (100 microns) than at 2 mils because twice the volume of slip agent will be available to migrate to the surface.

FIGURE 4.18.6.  Variation of LDPE friction with erucamide concentration.

So far, the effects of amide structure, time, and gauge have been discussed. Polymer type, however, can have a profound

effect on slip. LDPE requires fairly low levels of slip (400– 750 ppm) and oleamide or erucamide is commonly used. LLDPE requires 600–1500 ppm of slip, with erucamide used for most applications due to the processing temperature and the high-shear environment. Polypropylene requires 1500–2000 ppm of slip, with erucamide being used due to the high processing temperature; however, sometimes oleamide will be used because of its speed of migration. Metallocene plastomers can also require much higher quantities of both slip and antiblock. Some specific applications such as injection molding, soda-bottle top liners, and PVC plastisol consume amides at the 0.5–4% level. Figure 4.18.8 gives an indication of the requirements for producing good slip in commodity polymers. In polypropylene, there are differences in slip performance between homo- and co-polymers. The ethylene random co-polymer shown in Figure 4.18.10 shows lower CoF than the homopolymer in Figure 4.18.9, particularly for the slower-migrating erucamide. Ethylene vinyl acetate (EVA) copolymers present different problems. These are fairly amorphous materials, and therefore crystallinity is not a barrier to migration. However, these materials have a tendency to stick to each other, which gets worse as the vinyl acetate content increases. In fact, the material will often not start to slip until the whole surface has been covered. This is shown in Figure 4.18.11, where

FIGURE 4.18.5.  Effect of primary amides on LDPE blocking.

FIGURE 4.18.7.  Secondary amides for control of medium slip (LDPE 40 µm).

Time Effects It is well known that it takes time for a slip agent to migrate to the polymer surface and come to an equilibrium level of slip (CoF) at that surface. How long this takes is a function of temperature, gauge, slip levels, the molecular weight of the slip agent, the presence of other additives, the type of polymer, and other factors. The final result can be estimated from experience, but the actual result should be checked and verified in every new application. In some cases with LLDPE, as much as 24 hours to 7 days have been required to be sure that equilibrium has been reached. Many extrusion shops that also print in-house wait at least eight hours after extrusion before putting a roll of film onto the printing press. In special cases where excess slip agent could be detrimental to ink adhesion, a secondary treating station is hung just in front of the press to “burn off” excess slip at the surface just before printing. Polymer Effects

Section 4.18.  Slip Agents

FIGURE 4.18.8.  Comparison of the effectiveness of erucamide in commodity polymers.

323

FIGURE 4.18.10.  Slip performance in polypropylene copolymer (1800 ppm amide 1:1 with silica).

there is a lag before significant slip is seen, but this lag can be reduced by increasing the erucamide content.

that may interfere with migration of the slip additive; however, these effects are beyond the scope of this publication.

Coextrusion and Slip Agents (CoF)

REFERENCES AND ADDITIONAL RESOURCES

Today many film extrusion operations are equipped for co-extrusion so that polymers and additives may be delivered more precisely to the exact layer (position) where they are most effective or to keep two additives from contacting each other. Therefore, in some applications, it may be possible to deliver the slip agent to an outside layer to achieve low CoF at the outer surface and a higher level at the inside surface.

There are various papers by this and other authors given at TAPPI Polymer, Lamination and Coatings Conferences. In addition, for information on slip testing, try:

SUMMARY Friction at polymer surfaces can be effectively controlled by using fatty amide slip agents. However, a number of factors should be taken into account before making a decision, including polymer type, process conditions, amide type, coefficient of friction required, and the speed at which this level of friction must be achieved. Many other factors can influence slip, for instance, the presence of other additives

[1] Brown, R., Handbook of Plastics Test Methods, 3rd ed., Ch. 10, Longman Scientific (1988).

General Works on Plastics Additives Gachter, R. and Muller, H., Plastics Additives, Hanser (1985) Edenbaum, J., Plastics Additives and Modifiers Handbook, Volume 107, Chapman &Hall, London (1996). Gordon Simpson, W., Plastics Surface and Finish, 2nd ed., Royal Society of Chemistry (1993). Garbassi, F., Polymer Surfaces: From Physics to Technology, Wiley, 1998. Pearson, B., Sliding Friction: Physical Principles and Applications, Springer-Verlag, 1998.

FIGURE 4.18.11.  Erucamide in EVA (8% VA content).

FIGURE 4.18.9.  Slip performance in polypropylene homo-polymer (1800 ppm amide 1:1 with silica).

Film Extrusion Manual, Second Edition, 2005

Chapter 5 Processing

EDITOR: NORMAN AUBEE, NOVA Chemicals

Section Number

Section Title

Page Number



5.1

Blown-Film Processing HARINDER TAMBER and MIREK PLANETA, Macro Engineering and Technology Inc.

327



5.2

Cast Film CHRISTINE RONAGHAN, Cloeren Incorporated

341



5.3

Sheet Extrusion Process SAM IULIANO, Nordson Extrusion Dies Industries, LLC

347



5.4

Polymer Rheology OLIVIER CATHERINE, Cloeren Incorporated

359



5.5

Coating and Laminating Technology GIANCARLO CAIMMI, Nordmeccanica Group

383

5.6

Metallizing VERONICA ATAYA, Celplast Metallized Products Limited

391



5.7

Troubleshooting the Extruder ANDREW W. CHRISTIE, SAM North America, LLC

397



5.8

Troubleshooting the Blown-Film Process HARINDER TAMBER and MIREK PLANETA, Macro Engineering and Technology Inc.

403



5.9

Troubleshooting the Cast Film Process CHRISTINE RONAGHAN, Cloeren Incorporated

423



5.10

Gel Troubleshooting MARK A. SPALDING, EDDY GARCIA-MEITIN, and STEPHEN L. KODJIE, The Dow Chemical Company

427

Section Number

Section Title

Page Number



5.11

Extrudable Polymers: Purging and Resin Transactions SCOTT B. MARKS and BARRY A. MORRIS, The Dow Chemical Company

445



5.12

Safety in Film Extrusion NICOLE E. DOWLING and LAURA K. MERGENHAGEN, The Dow Chemical Company

453

Chapter 5—Section 1

Blown-Film Processing HARINDER TAMBER and MIREK PLANETA, Macro Engineering and Technology Inc.

INTRODUCTION

BASICS OF THE BLOWN-FILM PROCESS

Blown-film processing is a common method to make flexible, semi-rigid, or rigid films for various packaging applications. Blown-film processes are very efficient and economical for making commodity single-layer packaging films and various specialty multilayer films consisting of various polymers, such as LLDPE, mLLDPE, LDPE, HDPE, plastomers, Nylon, EVOH, PP, PETG, COC, PVC, PVdC, PVdF, tie resins, and PS (to name a few), to provide different optical, mechanical, thermal, and barrier properties. These films are used in a number of applications in the food, medical, construction, agriculture, electronics, chemical, and automotive packaging sectors [1–4]. Blown film is usually fabricated in a thickness range from very thin (6 microns, 0.25 mil) to 250 microns (10 mil) to thick sheets like geomembrane at 1000 microns (1 mm) or even thicker products. In this chapter, a plastic web less than 250 microns (10 mil) thick is classified as film and one greater than 250 microns thick is usually referred to as sheet [1]. The blown-film process is used to fabricate single-layer film for markets ranging from commodity (general packaging) to complex multilayer structures of 2, 3, 5, 7, 9, 11, or more layers, including micro- and nanolayer-based structures. This chapter will focus on the blown-film process. All critical components of the blownfilm process are mentioned, with emphasis on the physical process. The discussion begins with single-layer blown film and slowly adds complexities both in equipment and process parameters to discuss production of multilayer blown film. Therefore, this chapter consists of five main sections:

In the blown (tubular) film process, polymer melt from the extruder(s) is passed through an annular die and subsequently drawn and simultaneously expanded in the machine direction (MD) and the transverse direction (TD), while being cooled by an air ring. The bubble is stabilized by a bubble cage, collapsed into layflat tubing, and wound as a roll (tube or slit sheet). Figure 5.1.1 illustrates the single-layer blownfilm process. A single-layer blown-film line may consists of various components such as a gravimetric system (or volumetric hopper), an extruder, an adapter, a screen changer, a die (stationary, rotating, or oscillating), an air ring (stationary or rotating), internal bubble cooling (IBC), a bubble cage, an air injection system for the blown film (for nonIBC operation), a gauging system, a collapsing frame, a film treatment unit (corona treatment), a secondary nip, a trim takeoff system, a winder, and a roll handling system. For multilayer blown film, depending upon the number of layers, varying numbers of extruders, adapters, screen changers, gravimetric systems, multilayer dies with thermal isolation, micro- and nanolayer blocks, V-guides, an oscillating haul-off, a water bath, and other equipment can form part of a multilayer blown-film line. Figure 5.1.2 shows a schematic of a typical multilayer blown-film process (9-layer). Both processes described above are air-cooled, and thousands of such single- and multilayer blown-film lines are installed worldwide. However, blown-film technology also can use water as a cooling medium. Figure 5.1.3 shows a downward multilayer blown-film process. This water-quench blown-film process is mainly used to make high-clarity single- or multilayer film or sheet. Both air- and water-cooled blown films and sheets have their pros and cons for fabrication and end applications. Some aspects of these are discussed in this chapter. Many OEM’s build blown-film lines, and their equipment designs, layouts, and configurations may be different. Therefore, this chapter mainly aims to provide a basic understanding of the blown-film process. (Please read a separate chapter from the same authors on Troubleshooting the Blown-Film Process and how these issues could be resolved to produce optimum-quality film rolls).

(1) Introduction (2) Basics of the Blown-Film Process (3) Blown-Film Process: Correlation of Resin, Equipment, and Process Conditions (4) Comparison of Air- Versus Water-Cooled Blown-Film Processes and their Properties (5) Startup and Shutdown of a Blown-Film Line.

327

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FIGURE 5.1.1.  Single-layer blown-film process (air-cooled).

The operation of single- or multilayer blown-film lines also requires additional auxiliary equipment such as silos (outside the building), surge bins, and dryers (for drying the resin). Utility equipment such as a compressor (clean, oil-free compressed air) and chillers (to cool the air) are required for the blown-film process. The tower height and the number of levels depend upon the complexity of the blown-film extrusion equipment. BLOWN-FILM PROCESS: CORRELATION OF RESIN, EQUIPMENT, AND PROCESS CONDITIONS In blown-film processing, it is important to understand the correlations of resins, equipment, and process conditions as they affect film fabrication. Therefore, this section starts with basic resin, equipment, and process information. Resins/Additives/Masterbatches Often resin suppliers provide a TDS (technical data sheet) that specifies resin MI (melt index) or MFR (melt flow rate) and some physical (e.g., density), mechanical, thermal, and optical properties of the resin. For blown-film processing, polymer resins from fractional MI (0.3 MI) to 3 MI are often used. However, resins with lower MI or higher viscosity (below 0.3 MI, such as 0.04 MI) or higher MI or lower viscosity, (above 3 MI, such as 6 MI) can also be used under proper process conditions. Resin suppliers also provide SDS (safety data sheets). It is important and necessary to review both TDS and SDS before using a resin for blown-film processing. Similarly, in blown-film operation, many additives (in masterbatch form) are used, such as slip, anti-block, col-

FIGURE 5.1.2.  Multilayer (9-layer) blown-film process (air-cooled)” here.

Section 5.1.  Blown-Film Processing

329

Equipment

FIGURE 5.1.3.  Multilayer blown-film process (water-cooled).

or, PPA, UV stabilizers, and antioxidants. It is important to check the active ingredients, the type of carrier resin (LDPE or LLDPE or EVA), and the MI of the carrier resin used to make the masterbatch. Similarly, the TDS and SDS for the masterbatches or any other ingredients to be used in the blown-film process must also be reviewed. In addition, recycled materials have sometimes been used in blown-film processing for many decades in the form of flakes or re-pelletized granules. However, in the present century, recycling and sustainability have attracted a new focus due to plastic litter problems (in the environment and the oceans). Therefore, many resin and film converters often make single- or multilayer film that can be recycled. The recycled material can come from post-industrial recycling (PIR) or post-consumer resin (PCR), and therefore the quality of the recycled resin should be checked before it is used in blown-film fabrication. Resins can be delivered from railcars and hopper trucks into silos or can be shipped directly in boxes or bags. Additives and recycled material are most often shipped in gaylord boxes or woven sacks. Therefore, appropriate resin handling equipment is required.

Resin and additives are often fed through volumetric hoppers (older systems) or gravimetric hoppers (newer systems), where materials are weighed, blended, and discharged to the hopper to be fed to the extruder. The extruder is the key component of blown-film processing. The blownfilm industry uses three main types of single-screw extruders, which are classified based on the type of extruder feed throat: smooth-bore extruders, light-groove extruders, and deep-groove extruders. In the smooth-bore extruder, the feed throat is smooth, and friction between the barrel and the resin granules aids in feeding the resin. In light-groove extruders, the barrel itself is lightly grooved; friction is created by these light grooves, which aid in feeding the resin granules. In a traditional grooved-feed extruder, a separate grooved feed section consists of different sizes and numbers of grooves, which help to feed hard or slippery pellets. In these extruders, feed throat temperature is controlled by a TCU (thermal control unit) using oil or water. The resin fed from the hopper is compressed by the screw as it moves forward (by turning). The shear forces and external heating melt the polymer granules, and the polymer melt is thermally and compositionally homogenized by the mixing sections of the screw. The melt is metered or pumped into an adapter or die. Extruders are often described by screw diameter and length/diameter ratio (L/D) ratio. The blown-film industry mainly uses extruders from 1.75″ (44.5 mm) to 6″ (152.4 mm) with L/D from 24:1, 26:1 or 30:1. Extruders as small as 1″ (25.4 mm) or as large as 7″ (177.8 mm) or more are used for custom applications. Figure 5.1.4 shows a schematic of an extruder. The screw is important for processing different resins and is often designed depending upon the materials to be processed. Figure 5.1.5 shows a schematic of a barrier screw. Each extruder is divided into three to six or more heat/ cool zones. Depending upon the polymer to be processed, a temperature profile as recommended by the resin supplier is often used to heat different zones. The feed throat tempera-

FIGURE 5.1.4.  Schematic of smooth-bore extruder (courtesy of Macro Eng. & Tech., Inc.).

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FIGURE 5.1.5.  Schematic of barrier screw for smooth-bore extrude (courtesy Macro Eng. & Tech. Inc.).

ture is often kept as low as (15°C) (59°F) for PE resins, but higher (40°C (104°F) or higher) for Nylon [5] or PET-type materials. Please consult the resin supplier for feed throat and barrel temperature profiles for a given resin. Each barrel has a rupture disc rated at 7500, 8500, or 9500 psi, with the barrel rated for over 10,000psi. Please check with the equipment supplier for which rupture disc is appropriate for your equipment. The melt from the extruder is fed through an adapter (often fitted with a screen changer). The adapter has a melt pressure probe before the screen changer. The screen changer is usually a filtration system to keep foreign matter and particles out of the melt. The screens are woven wire mesh. The screen filter is typically made up of multiple layers of different-sized filtration media. The woven pattern of the screen layers can range from 20 to 300 mesh size. For blown-film processing, the typical upper mesh size used is around 120 within a screen pack stack of more open mesh for stiffness. The smaller mesh sizes have larger pore area (20 mesh) and provide lower melt pressure when used in the system. Larger sizes (100 mesh) have smaller pores and provide high melt pressure, but better homogenization. Dutch weave (150 mesh or higher) screens are also used in the blown-film industry. There is an adapter after the screen changer to feed the polymer melt to the die. This adapter is also equipped with a melt temperature probe to check the actual temperature of the polymer melt as well as a melt pressure probe. The melt pressure probes before and after the

screen changer provide an indication of whether the screens are blocked and need to be changed. Depending upon the materials present (pigments, fillers, or recycled material), screens may require more frequent changes than if only conventional natural prime resin is used. Polymer melt from the adapter is fed to the annular die (single-layer or multilayer). Single-layer blown-film dies are often bottom-fed spiral dies. (In a few cases, spiral or side-fed dies are also used). Figure 5.1.6 shows an annular (cylindrical) shape bottomfed die. The polymer melt passes through the vertical-feed cylinder to a distribution chamber, then is redirected through horizontal radial pipes feeding the ports leading to the spirals (inner mandrel or die mandrel). The number of ports (spirals) in the single-layer die mandrel depends upon the size of the die and other rheological and mechanical factors. The die mandrel has an outer jacket called the die body, which has several sets of heaters to keep the polymer melt at appropriate processing temperature. In some custom applications, pipes with hot-oil heating provide the more consistent heat required for some thermally sensitive polymers. The polymer melt flows through the spirals and die body and forms a fairly uniform annular plastic melt tube, which exits from the die lip. At this stage, using compressed air, the thick polymer melt tube is simultaneously oriented in the machine and transverse directions (MD & TD) as a bubble. The bubble is stabilized and collapsed into a layflat film or tube at the primary nip. As shown in Figure 5.1.1, much happens in this section (between the die lip and the primary nip) in terms of melt orientation, and two significant process parameters can be derived: BUR (blowup ratio) and DDR (drawdown ratio). The BUR can be obtained by dividing bubble diameter by die diameter, but it is not always easy to measure bubble diameter while the line is in operation. It is easier to measure the width at the primary nip (or downstream), and therefore the BUR is calculated as: Blowup ratio (BUR) = (layflat width × 2) divided by (die diameter × π) BUR has the most influence on the transverse-direction film properties. Drawdown ratio (DDR) = (die gap opening) divided by (film thickness × BUR)

FIGURE 5.1.6.  Cross section of a bottom-fed spiral blown-film die (Klauber, 1992).

DDR has the most influence on the machine-direction film properties. Melt orientation (BUR and DDR) takes place before the frostline. Many film properties are impacted by the frostline height (from the die), such as MD/TD tear, impact strength,

Section 5.1.  Blown-Film Processing

tensile strength, percent elongation, optical (haze, gloss), and barrier properties. From a theoretical point of view, a blowup ratio (BUR) greater than one means that the polymer melt has a transverse-direction (TD) orientation from blowing the bubble greater than the die diameter. Similarly, a drawdown ratio (DDR) greater than one means that the polymer has a machine-direction orientation (stretching) as a result of pulling the melt away from the die at a faster rate (by primary nip) than the melt exiting the die lip. As shown in Figure 5.1.1, die diameter, die-lip gap, film thickness, and layflat width are required to obtain BUR and DDR. Polymers are viscoelastic materials, and therefore these materials exhibit die swell (the thickness of the extrudate is greater than the die gap), which depends upon resin composition and process conditions. Therefore, the above calculations are only approximate [3]. Air rings are installed on blown-film lines to cool and stabilize the bubble. In the early years of blown-film technology, single-lip air rings were used to cool the LDPE type of material, as shown in Figure 5.1.7. As resin technology developed, LLDPE, mLLDPE, and single-site catalyzed LLDPE were introduced to the blown-film industry. Since then, dual-lip air rings have been used to cool the bubble because they provide high volume and high velocity to cool weaker melt-strength materials (LLDPE) while providing good bubble stability, leading to higher output and gauge uniformity. Figure 5.1.8 shows a typical dual-lip air ring. In blown-film processing, the air ring is a key component because it provides cooling, and high output from blown-film lines depends upon how effectively the expanding melt bubble is stabilized by the air ring. Therefore, this section provides detailed information on the principles of air-ring technology [7,8]. The main concept of the dual-lip air ring is a ring with two main orifices that make cold air impinge on the bubble. The primary orifice near the die exit provides a low volume of air to increase the polymer melt strength, and the secondary orifice, with 1.2 to over 2.5 times the die diameter, pro-

FIGURE 5.1.7.  Single-orifice direct Venturi air-ring lip design (Knittel & DeJonghe, 1992).

331

FIGURE 5.1.8.  Modified dual-orifice air-ring design (courtesy Macro Eng. & Tech. Inc.).

vides a high volume of air to cool the bubble and stabilize it in the air ring. The primary and secondary orifices are separated by a machined mechanical conical surface. The geometry of this piece establishes the bubble shape and guides the air and volume flow in the two orifices. The lower or primary orifice, besides providing a low volume of air to increase melt strength, provides a cushion to the polymer melt to prevent it from touching the cone while the melt travels inside the conical intermediate deflector lip. After the deflector lip, the melt is exposed to the secondary orifice and cooled by a large volume of cold air over a large circumference. At this location, the Venturi effect pulls the melt out and “locks the bubble” to stabilize it and to provide good gauge uniformity. It also provides efficient heat transfer, helping to increase the output of the blown-film line (kg/h of film produced). Cone length and shape determine the diameter of the secondary orifice and therefore the minimum blowup ratio (BUR). In some instances, a number of cylindrical stabilizers are mounted on an adjustable chimney to enhance bubble stability and enable a higher blowup ratio. In these operations, air travels higher axially and closer around the expanding bubble, increasing cooling efficiency and thereby increasing output. Adjustable and non-adjustable dual-lip air rings are both being used in blown-film processing. In adjustable dual-lip air rings, as the name suggests, the primary gap can be adjusted to change the ratio of volume to velocity in the primary and secondary orifices, providing fine tuning of the air ring. This makes it easier to process high-melt-strength materials (LDPE) at low rates and low BURs, and low-meltstrength materials (LLDPE) at high blowup ratios and high outputs. In these air rings, as the lip-set threads wear out, misalignments can occur, resulting in film gauge variations.

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On the other hand, the non-adjustable dual-lip air ring sometimes has enough flexibility to run both low- and high-meltstrength materials with no lip adjustment required. In duallip air rings, because of the balance of the delicate forces on the extrudate, the design of the cone length and its angle, as well as the ratio of velocity to volume in the primary and secondary orifices, are all important factors for optimizing output, bubble stability, and gauge uniformity. Due to the inherent nature of the blown-film process, blown films are fabricated with higher gauge variations than cast films. A tight tolerance of the film in the circumferential direction is required for many downstream applications such as printing and lamination. In blown-film processing, the thick and thin spots in the circumferential direction are distributed across the finished film roll by oscillating extruders, dies, nips, and/or winders. Although these modifications provide good-geometry rolls, they do not eliminate thickness variations in the fabricated film, which is still a concern for downstream applications. To make blown film with tighter gauge variation (flat film), an automatic air ring is used. The automatic air ring is equipped with various features to control fiilm gauge, as shown in Figure 5.1.9. In these methods, the cooling ring is divided into several sections; the amount and/or temperature of the air to be made to impinge on the polymer melt can be controlled within each section by a third small orifice (below the primary orifice). Therefore, a variable amount of cooling air is brought into contact with the molten bubble to control film thickness

FIGURE 5.1.9.  Air-ring dual-orifice automatic thickness control (courtesy of Plast-Control, Inc., USA).

variations. For example, if one section of the film is thin, the amount of air corresponding to that section can be increased, which cools the melt faster and thereby increases film thickness. On the other hand, if the film section is thick, the amount of air corresponding to that section is decreased, or the air temperature can be increased, causing the melt to cool slowly, and hence that section is stretched, and the film thickness decreases. Figure 5.1.9 shows a typical example of this working feature of an automatic air ring. This figure shows that during blown-film processing, the thickness profile is measured (by capacitance or a gamma backscatter sensor or another type of sensor). This thickness profile is divided into several zones (48 zones or a smaller or larger number, depending upon the size of the air ring). Each thickness profile zone corresponds to a particular zone of the air ring (which controls the volume or temperature of air in that section of the air ring). Depending upon thickness variations, the amount of air (or the temperature of the air) directed at the bubble is controlled, and thick or thin spots are thereby reduced to provide better gauge control of the blown film. APC (automatic profile control) gauge measurement sensors are often mounted above the bubble cage (but below the collapsing frame), with the sensor head oscillating around the bubble and sending feedback to the air ring (or die, in the case that APC is in the die). Multiple air rings or triple-lip air rings are being used to further enhance output, but these topics are beyond the scope of this chapter. Air rings are used to cool the bubble externally; however, IBC (internal bubble cooling) can be used to cool the bubble inside, which is specifically good for increasing output and for reducing blocking (anti-block additive may still be required). The IBC system usually consists of an inlet blower connected to a cooling coil or a chiller to control inlet air temperature, with an exhaust blower and a bubble stabilizing cage with ultrasonic sensors mounted to it for bubble diameter control. The plenum connects blowers to the distribution box and in turn to the pipes feeding inside the bubble. IBC pancakes mounted inside the bubble can have various shapes. An inlet pipe through the center of the die exhausts the hot air. Inlet and outlet air pressures are monitored by pressure gauges mounted on plenums. Similarly, inlet air temperature (and, only if required, outlet air temperature) can be monitored. The system controls the bubble diameter by sensing the distance between the bubble and an ultrasonic sensor installed on the cage (the sensor moves with the cage to change the blowup ratio). The sensor feeds an electronic system, which regulates the inlet-outlet flow either by using flow damper(s) or by regulating the speed of the exhaust blower to maintain the bubble diameter or layflat of the film, as shown in Figure 5.1.10. IBC devices of various shapes such as pancake, conical, or cylindrical styles are often installed on blown-film lines for cooling inside the bubble. Their design is critical to eliminating bubble instability by not directing too much air near the die face, which is very important particularly for low-melt-strength materials (LLDPE or mLLDPE). IBC

Section 5.1.  Blown-Film Processing

333

FIGURE 5.1.10.  Internal bubble cooling (IBC) systems (Kinttel & DeJonghe, 1992).

FIGURE 5.1.11.  Stabilizing bubble cage (Kinttel 1992).

control systems are important to regulate the inlet and outlet airflow to maintain a preselected layflat or to change the size of the bubble to a given layflat. Earlier systems were equipped with sonar sensors and had no provision for compensating for changes in the air temperature at the diameter of the bubble or the non-linearity of bubble-cage size changes, and diagnostics were based on simple analog systems. In recent years, systems have become available with ultrasonic sensors to measure the distance between the bubble and the sensor accurately. Temperature sensors are also included to compensate for changes in air temperature. These sensors are often mounted at the bottom of the sizing cage. In blown-film processing, although the bubble is initially stabilized by the air ring and/or IBC, “bubble containment” above the frostline and guiding it through the collapsing frame is equally important and is achieved by a bubble cage, as shown in Figure 5.1.11. Bubble cages move in the vertical direction (up or down) to provide flexibility for height differences to suit varying frostline heights. The common features of bubble cages are.

bubble at different frostline heights or in multilayer films to accommodate multiple frostlines. (3) Cage diameter is normally adjustable to provide a BUR adjustment range of 3:1. (4) Cages are concentric to the die to minimize friction and drag, which improves gauge uniformity. (5) The bubble is in contact with a rolling surface within the cage. The rollers are made of various materials (Teflon®, acetal, Nylon, or fabric) and often have a removable sleeve for easy cleaning. (6) In the bubble cage, large-diameter rollers turn faster than smaller-diameter rollers due to large momentarms, except if the larger-diameter rollers are heavier or have higher friction. In general, shorter segmented rollers conform to the bubble better than longer rollers and also induce less friction. (7) IBC sensors are also mounted on the bubble cage.

(1) The bubble stabilizing cage surrounds the tubular film with radial arms. (2) The cage height can be adjusted axially to stabilize the

The selection of a bubble cage is important to provide enhanced bubble stability for optimum layflat and gauge uniformity. Some cages have differential spacing of the cylindrical rolls, where the rows are closer together at the bottom end of the cage to stabilize the bubble more efficiently.

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FIGURE 5.1.12.  Collapsing theory (Knittel, 1992).

Excessive air disturbances near the bubble/cage contact surface should be eliminated because these undermine the efficiency of the cage by preventing it from contacting the bubble, leading to gauge issues [9–10]. In blown-film processing, a common problem is exposure of the bubble to ambient drafts, which besides causing gauge variations, also influence bubble stability. A span of six to ten feet of bubble after the frostline is the most vulnerable to these ambient drafts, which can be avoided by building an enclosure around the bubble. The enclosure can be made of flexible material such as vinyl or rigid sheets such as polycarbonates. Rigid structures are supported with steel and have doors to access the bubble. Such enclosures are also used to maximize rate and stability besides eliminating or reducing draft impacts on the bubble. Depending upon the tower height, often a V-guide (another small bubble cage) is installed high above the bubble cage (but below the collapsing frame) to contain the bubble further. In blown-film processing, the moving bubble is collapsed into layflat tubing using a collapsing frame. Various styles of collapsing frames are used in industry, including ones based on wooden slats, roller collapsers, segmented roller collapsers, and others. In this process, the film at the edge travels both at a different speed and in a different direction than the center of the film. As shown in Figure 5.1.12, the edge of the film travels a shorter distance than the center of the film, and consequently the center of the film contacts the collapsing frame for a longer time than the part near the edge of the film. This longer contact creates more drag, and therefore the center of the film is stressed to a higher

degree than the edge of the film, which experiences lower drag. The higher stress in the center of the film often leads to stretching or permanently deforming the film, resulting in center sag [11–12]. Relatively extensible films fabricated from LDPE, LLDPE, mLLDPE, or their blends with plastomers or EVA are more forgiving and readily adapt to the changing geometry of the collapsing frame with minimum adverse effect on film flatness and gauge uniformity. However, collapsing stiffer films fabricated from HDPE, Nylons, rigid PVC, polystyrenes, cyclic olefin copolymer (COC), and multilayer film structures containing rigid polymers may cause creases or wrinkle issues while collapsing due to the film’s stiff (less flexible) nature. Therefore, such films should be collapsed while warm, which provides some flexibility depending upon the material. This could be done by installing IR heaters below the collapsing frame or hot-air blowers in the collapsing frame. The other solution for stiffer films is to collapse them at a reduced angle, but this causes longer contact of the film with the collapsing frame, resulting in more drag or stretch marks. To reduce these issues, a low-friction collapsing frame must be used. Most often, a collapsing frame is 1.5 to 2.5 times the nip roll face width. However, for Nylon films, a collapsing-frame length up to three times the face of the nip roll can be used. The collapsing-frame length can be increased for better compatibility of the “side to center” collapsing of the film, but at the cost of added friction. In some applications, gusseted film is required, and in these cases, the collapsing frame is modified to add gusseting boards instead of side stabilizers. In many multilayer lines, a

Section 5.1.  Blown-Film Processing

gauge measurement system is also mounted in the collapsing frame. Because the collapsing frame oscillates with the nip, the gauge sensor scans the thickness of the bubble. The collapsed film (in tube form) is pulled by a primary nip consisting of one steel roll and one rubber roll. In some blown-film lines, an “S-wrap” device consisting of two steel rolls is installed after the nip. The rolls are drilled for water flow, and temperature is controlled by a TCU. The aim is to cool the film to prevent or reduce film blocking. In multilayer blown lines, it is easier to rotate the primary nip than the multilayer die (unlike a single-layer line, where most often the die oscillates or rotates and the nip is stationary). Therefore, the primary nip may be equipped with a horizontal or vertical oscillating nip for film randomization to spread gauge variations across the width of the film to make uniform rolls. In blown-film processing, tower height can vary depending upon the complexity of the blown-film equipment. The tower should be sturdy and free of vibration; otherwise, it can impact bubble stability (in the air ring and during bubble collapsing), resulting in soft creases and safety issues. As the film goes through the oscillating unit and travels down the tower over the tower rolls, film position could go off track. To make a good roll at the winder, an edge guide is critical. The operator must make sure that the web edge sensors are functioning properly to track the film edge. The edge guide is essential to align the film on one edge, which is important for winding and eventually unwinding during downstream converting. In extrusion processing, polyolefin films are often corona-treated to increase surface energy for downstream conversion operations (printing, coating, and lamination). The corona unit consists of two main components: a power supply and electrodes. It is important to ensure that the power

335

requirement or watt density is sufficient to treat materials that require different surface energies, such as polyethylene, polypropylene and other polymers. A secondary nip is installed on most single- and multilayer lines. The nip speed should be synchronized with the primary nip speed, and web tension should be monitored. This nip is used to slit the collapsed tube into two or more sheets. Slitting is an essential step for most or all extrusion operations. In blown-film operation, spreader rolls are used to bring a wrinkle-free flat web to the winder. In blown-film processing, large-diameter rolls (1000– 1500 mm, 40–60 inches) are often wound to reduce changeover time for downstream equipment. These rolls must have optimum hardness for transportation and during unwinding for subsequent operations such as printing, lamination, slitting (multiple-up), and bag making. Different applications in blown-film processes require making a wide range of films and sheets; some may be thin, flexible, tacky, and soft, whereas others may be thick, less extensible, rigid, and stiff. For example, small rolls may be needed for stretch film (tacky, soft and flexible), large rolls for lamination film (flexible), and large rolls for polystyrene or Nylon 6 film (rigid or stiff). In blown-film processes, three main types of winders are used as collecting rolls: surface winders, center (turret) winders, and a combination of center and surface winders [13–15]. Figure 5.1.13 provides a comparison of surface, center and surface/center winders for different films. Process Conditions In blown-film processing (single-layer film), the extruder and die are set at processing temperatures recommended by suppliers for the specific resins. For example, to process an

FIGURE 5.1.13.  Comparison of surface, center, and surface/center winding for different films (courtesy of Macro Engineering).

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LLDPE (1MI)-rich blend with LDPE (1 or 2 MI), the barrel temperature profile could be 165–185°C, (329–365°F) and the die temperature 190–210°C (374–410°F). The resin is fed through volumetric or gravimetric systems as PE pellets (polymer granules), which are fed to the extruder; the resin moves forward (due to screw rotation) and becomes compressed between the screw and the extruder barrel. The barrel heat (thermal energy from heaters) and friction caused by shear (due to the turning screw) generate enough energy to melt the plastic pellets into polymer melt. Usually, barrel heat is required in the beginning (during startup of the line) to heat the extruder/barrel for polymer melting. However, when the extruder is in operation (screw rotation), most of the heat to bring about the phase change, i.e., to convert polymer granules (solid) into polymer melt (liquid), is provided by frictional heat. Polymer melt temperature and its variation across the melt stream is an important process parameter (as measured in the adapter), which is different for different polymers and grades (MI or viscosity). The extruder is mainly a pump and generates melt pressure, which is required to pump the polymer melt through the screen changer or other such melt-filtering devices and into the annular die. Typical single-layer dies are spiral or spider type (center- or side-fed). The polymer melt exits the die as a tube, which is inflated and simultaneously stretched in the machine direction (MD) and the transverse direction (TD). The simultaneous stretching (MD and TD) of the bubble (as it is being pulled away from the die by the primary nip) makes the film thinner and gives it a specific diameter. A high or low melt temperature from the extruder could affect the height of the frostline (as measured from the die) and bubble stability during film fabrication. The bubble is cooled by air ring only or by air ring and internal bubble cooling and contained and stabilized by a bubble cage. The bubble is flattened (from circular to tubular shape) by a collapsing frame, drawn through the nip rollers, surface-treated and slit by a secondary nip into sheeting, and wound as rolls on the winder (or film may be wound as tubing). Multilayer Blown-film Equipment, Resin and Process Conditions In multilayer film structures such as a three-layer blownfilm line for making PE-based film, the die ratio can range from 10/80/10 to 30/40/30. Based on line output and the width and thickness of the final film, extruders and die sizes are selected [16]. For this application (a three-layer blownfilm process), extruder sizes for skin layers are often 2” to 3.5″ (51 mm to 90 mm), and the core extruder could range from 3″–6″ (76–152.4 mm) in diameter. The size of the three-layer conventional or stackable die could range from 10″ (250 mm) to 30″ (750 mm). PE-based structures may contain LLDPE resin blended with LDPE (LLDPE is either butene-LL, hexene-LL, super hexene-LL, or octene-LL), the core layer may be blended with HDPE or mLLDPE, and the sealant layers may contain specialty sealants such as blends of LLDPE with mLLDPE, single-site catalyzed

LLDPE, EVA, VLDPE, plastomers, or elastomers. Lately, some five-layer blown-film lines have been running with only PE-based structures, with the die ratio selected based on the required multilayer PE film properties. In some custom applications (geomembranes), three-layer dies could be 60–80″ (1.5–2.0 m) in diameter, and similarly core extruder sizes could be 6–8″ (152.4–203.8 mm). However, most 5, 7, 9, and 11-layer (or more) blownfilm equipment is used to fabricate multilayer barrier film. The extruder sizes range from 1.75–3.5″ (44.5–90 mm), and multilayer dies range from 12–30″ (300–750 mm) in diameter, with some larger or smaller sizes. For multilayer blown films, dies are often stackable (few conventional-type dies), and in some cases, dies with thermal isolation are provided. Thermal isolation in the die is required when two adjacent polymer melt streams are processed at an approximate melt temperature difference of more than ~40°C (~75°F). Depending upon the application, multilayer blown film may consist of several resins such as EVOH (different grades) and/or Nylon (Nylon 6, Nylon 666, or terpolymers) as internal barrier layers and HDPE, PP, Nylon, or PETG as outer layers, with appropriate tie layers and a sealant layer based on polyolefins (PE, random PP copolymer, plastomers, and ionomers). Multilayer films have appropriate functional or pigmented masterbatches blended with prime resins, and in many cases, bulk layers of multilayer film have recycled material as a part of the film structure. For multilayer blown film, micro- and nanolayer technologies can be used to make film with over 30 layers (micro) or over 100 (nano) layers. This topic is beyond the scope of this chapter. As mentioned above, several different resins are used in multilayer blown film lines; a brief description of a few of these resins is given below. EVOH is a copolymer of ethylene and vinyl alcohol and an excellent oxygen and aroma barrier material [17]. EVOH has several grades such as EVOH-29, EVOH-32, EVOH-38, and EVOH-48; the number (29, 32, 38 & 48) represents mole% ethylene content in the EVOH copolymer. As the ethylene content increases in the copolymer, it becomes easier to process and it is more flexible, but its oxygen barrier properties decline (measured at low RH) because EVOH is a hygroscopic polymer. For example, EVOH-29 has 29 mole% ethylene and therefore has higher oxygen barrier properties than EVOH-48 (48 mole% ethylene content) at low RH; however, because of the higher polar group (VOH) in EVOH-29, its oxygen barrier properties may be dramatically different than those of EVOH-48 at higher RH level (100%). Therefore, it is ideal to discuss EVOH grades and their applications (dry or highmoisture food contents) with the resin supplier before using a specific EVOH grade in multilayer blown film for oxygen and aroma barrier applications. Nylon resins are of many different types. Nylon 6 and Nylon 6/66 are mainly used in the blown-film industry, but amorphous nylons such as Nylon 6/12 or aromatic Nylons are also used [18]. Nylon is a medium-oxygen-barrier material and is often used for its mechanical properties (tensile strength and puncture resistance). Nylon is a polar material

Section 5.1.  Blown-Film Processing

and is sensitive to moisture (hygroscopic); therefore, its oxygen barrier properties decline as the film (or resin) is exposed to higher humidity (RH dependent). Caution! When Nylon resin absorbs moisture, it must be dried before processing (according to the resin supplier’s specification for time and temperature for drying). PETG (glycol-modified polyethylene terephthalate) has a lower melting point than PET and is at times used as a skin layer (high gloss and low haze) in multilayer blown film. Tie resins [19] are often based on maleic anhydride-modified LDPE, LLDPE, or EVA or other base resins and are used to bond polyolefins with Nylon, EVOH, or other dissimilar polymer layers. To understand the processing and chemical characteristics of these resins, please consult the appropriate chapters in this manual. COMPARISON OF AIR- AND WATER-COOLED BLOWN-FILM PROCESSES AND PROPERTIES In many blown-film processes, air is mainly used as the cooling medium (air ring and IBC) to cool the melt; however, in some blown-film processes, water (water ring) is also used as a cooling medium to make single- or multilayer film. The equipment configuration and film properties (optical, thermal, mechanical, and barrier) are different for air- and water-cooled films and are briefly discussed in this section. As shown in Figures 5.1.1 and 5.1.2, when polymer melt from die is cooled by air (slow cooling), randomly oriented polymer chains in the melt start to reconfigure to an “ordered configuration” (crystal lattice) to provide more crystalline phase and some amorphous phase (random phase) in the film. However, when the polymer melt from the die (Figure 5.1.3) is cooled by water (fast cooling), randomly oriented polymer chains in the melt cool much faster (than with air cooling), providing mostly amorphous phase and very little crystalline phase. Therefore, there is a big difference in film morphology between air and water cooling of the melt, which impacts many film properties differently, as discussed below [20]. Optical Properties (haze and gloss) When polymer film is exposed to visible light, the light is reflected, absorbed, transmitted, or scattered. In air-cooled blown film, due to slow air cooling, large crystals are often formed. Polymer crystals are larger than the wavelength of visible light (400–700 nanometer (nm)). Therefore, these crystals scatter or deviate light, and the film looks hazy. However, in a water-cooled blown-film process, due to fast cooling (water temperature 5–20°C (278K to 293K)), a large percentage of amorphous phase and only a small amount of crystalline phase (with very small crystals) are obtained. In water-cooled film, the polymer crystals are smaller than the wavelength of visible light; hence, these crystals do not scatter or deviate light as much as air-cooled film. Therefore, water-cooled film looks very transparent, or at least less hazy. Water-cooled film has fewer surface imperfections; therefore, it has higher gloss than air-cooled film.

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Mechanical, Thermal, and Barrier Properties Water-cooled film has higher softness (lower modulus) than air-cooled film (comparing the same resins for both processes) due to its high amorphous phase. Therefore, this film has high elongation, better impact, and puncture resistance than air-cooled blown film. Water-cooled film can be used for thermoforming and other applications where high clarity and high gloss are required. However, due to their high amorphous phase and low crystalline phase, water-cooled films have lower thermal resistance and lower barrier (comparing the same resins for both processes) than air-cooled blown films. It is essential to check the application before selecting the type of film (air- or water-cooled). Although the component explanation given above is sufficient for both processes, for the water-cooled process, the extruders and die are installed at a specific height above the floor, and the polymer melt flows downward. As water from the water ring falls downward (by gravity) cooling the bubble, it is collected and, after filtration and UV or thermal treatment (to prevent microbial growth), reused in the process. The water ring is used to provide even water flow around the bubble during the water quench process. Many times, the film is annealed to remove waviness to make it more uniform. STARTUP AND SHUTDOWN OF BLOWN-FILM LINE CAUTION! Before starting blown-film processing (operation), professionals (i.e., process engineers, operators, and line attendants) should put on all safety gear (safety glasses, safety shoes, face mask, ear plugs, thermal sleeves, and gloves according to your company’s Health & Safety protocol)! To start a blown-film line (single- or multilayer), it is ideal to use LDPE (1 MI resin), but if this is not available, use 1MI LLDPE blended with 20% LDPE, and the LLDPE grade should have a good melt strength. When starting the line cold (for example shutdown on the weekend, starting on Monday morning), make sure that you start by heating the die, which will take longer to heat up than the extruders and adapters. When the die reaches 80% of setpoint temperature, switch on the heat for the extruders and adapters. Make sure that water is flowing in the extruder feed throat, because otherwise bridging may occur in the extruder feed section. Do not start the extruders until all temperatures reach the required setpoint value. An additional soak time of 30 minutes or more for hot parts (extruder, adapter/screen changer/die) may be required (check with your OEM). Check the barrel or die temperature on the HMI (human-machine interface) or the temperature control panel and/or use a surface temperature probe. The die lip should be full of plastic during heating to prevent oxidation because otherwise air (oxygen) enters the die lips and oxidizes the plastic on the die land, which generates die lines. If scraping the die lips does not help to remove die buildup, one

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may need to remove the die lip (inner and outer) and clean it properly. After cleaning, apply a thin coat of silicon paste or spray on the inner surface and top of the die to prevent plastic from sticking to the die during startup. While the extruders and die are heating up, check the following: (1) Enough correct resin (MI, density) and additives are available and are in the hopper(s). (2) All heaters and blowers (heat/cool units) are functioning properly. (3) Clean air-ring lip set and IBC chimney. These items should be free of oil and dirt. (4) The three blowers, one for the air ring, one for IBC-In, and one for IBC-Out, are functioning properly. For a non-IBC operation, make sure that the compressed air is ready to inflate the bubble and that one blower for the air ring is functional. (5) None of the motors shows a fault (complete line). (6) Primary nips, secondary nip, and winders are working (open/close nips), and tension control is working. (7) All equipment is functioning properly. When the extruder(s) and the die reach the desired temperature: (1) Make sure to change to a new screen pack before purging. (2) Clean (scrape) the die lip with the brass shim. (3) Check with a pin gauge that the die gap is even. It should be within 0.001″ all around the lip gap. (4) Check whether the air ring is in the center of the die; it should be within 1/64″ (use plumb line or laser beam). (5) Check whether the IBC chimney is in the center and mounted properly. There should be no air leakage around the IBC chimney, otherwise it will give bad gauge. The chimney must be fixed and not wiggling from the center. (6) For coextrusion processing, make sure that the extruders are set up for the right layer ratio. When you start the extruders, watch that all the instruments function properly and watch the melt pressure on the extruders, making sure that they do not exceed allowable pressure, usually 6000 to 7500 PSI (check with OEM specification for your equipment). This melt pressure depends on die design and should also be specified by the die manufacturer. If the pressure goes higher than the allowable limit, then shut down the extruder. High pressure could be due to the inner die not yet being up to processing temperature or the adapters not having time to melt all the plastic. Let the heat soak continue for another 30 minutes and try again. (7) Do not soak the die and extruders for an unnecessarily long time. If the die and extruders are soaked too long, the polymer will degrade, creating gels and black specks.

(8) For any reason, if maintenance must be done on the line (before startup), but the extruder and die are at their set temperature, reduce heat to 165–175°C (329–347°F) and set the extruder to operate slowly to extrude polymer (PE), typically 3 to 5 RPM. (9) Starting the line. Make sure that the top nip is open and the nip is turning at 4 to 10 RPM. Place a rope (to string the bubble) by following the web path through the blown-film line (the web path is often provided with the blown film). Start all extruders at low RPM (5 to 10 RPM), depending on size such that polymer exits slowly through the die. Start the air ring at low speed. If the blown line has no IBC, start blowing internal air so that air is entrapped in the melt (to make a bubble and cool it), make a knot with the rope around this melt (bubble) exiting from the die, and start slowly pulling the rope (near the winder) and slowly blowing more compressed air through the die to make a stable bubble. (10) Slowly inflate the bubble and pull the rope through the top nips. When the knot passes through the open nip, close the top nip, which entraps the compressed air in the bubble. Once the bubble is stable, thread the film through the line and onto the winder at slow speed. When the film is on the winder, start increasing extruder speed, air ring speed, and line speed, and blow the bubble to desired width and thickness. (11) If the line has IBC, one can start all three blowers (air ring, IBC-In, and suction blower for IBC). Cut the bubble at an angle, tie the knot to the melt (bubble) using the rope, and make sure that the bubble has passed through the inner IBC chimney (stop cutting the melt/film at this stage). Wait until the knot passes through the top nip and then close the nip. (12) When the bubble is stable with LDPE (or blends with LLDPE) and the coextrusion line is operating, follow the resin supplier’s temperature profile recommendations (for extruder and die mandrels) for tie, Nylon, EVOH, and other resins to be used on each extruder and die mandrel. Ideally, introduce PE and tie layers first and then introduce Nylon and/or EVOH. In addition, make sure that EVOH melt temperature does not exceed 235°C (455°F). Keep EVOH melt temperature and die mandrel temperature as low as possible for processing; at lower EVOH processing temperature, the film will run longer without gels. Watch bubble stability when introducing new resin because some materials have lower bubble stability and therefore extrusion output should be adjusted accordingly. When all the resins or additives have been introduced into their specific extruders and the bubble is stable with no melt fracture or interlayer instability, blow the bubble to proper width and gauge (film thickness). Check the gauge with a micrometer or use an online gauge readout to make sure that the film thickness is on specifi-

Section 5.1.  Blown-Film Processing

cation. If the multilayer die does not have an auto profile system (APC), the die-lip centering bolts can be adjusted manually. If a multilayer line is installed with an automatic gauge profile (APC) system (die or air ring), turn on the APC system to control the gauge profile automatically. Automatic gauge control (APC) systems cannot usually improve gauge by more than 50% of variation (or it may take too long for APC to control the gauge, leading to high scrap). Therefore, make sure the starting gauge is within ±10% (2 sigma) before switching to auto-gauge to obtain effective results. If the single- or multilayer line is operating and providing good gauge and suddenly bad gauge or belly appears on the bubble, this could happen for the following reasons: (1) The die lip has moved, and the die gap needs to be adjusted. For bubble belly, one needs to decrease the die gap because a wider die gap provides thicker melt, which does not cool as fast as the rest of the film and gets stretched to make belly (thinner film). (2) If the belly does not respond to die adjustment, it is caused by higher melt temperature of the polymer melt on the side of the belly. The problem is probably uneven heating on the die, drafts around the bubble, or in a coex die, uneven distribution of high-melt-temperature polymer. Check the temperature around the die (with a temperature probe) if any heater is burned or does not have proper contact with steel. (3) Air ring not in the center of the die. The air ring may be dirty, or the primary orifice of the air ring may have grease or pieces of foreign matter blocking the air. (4) IBC is not centered or has leakage under the pancake or in the die. The bubble will be thicker on the side of the leak. (5) Draft around the bubble. The bubble will be thicker on the side of the draft. (6) If none of these causes is diagnosed, there is a fairly good chance that the die is dirty and needs to be cleaned. Purging and Shutdown of the Blown-Film Line In single- and multilayer blown-film lines, purging is an essential step once a given production campaign is finished. It is ideal to purge low-viscosity material with higher-viscosity material; doing it the other way around will take much more time. Purging pigment is critical and may require more time; make sure that the hopper, additive feeder, and hoses are all clean. If required, a Disco purge can be used for faster cleaning of the extruder/screw and die mandrel containing the pigmented materials. (See the later chapter in this manual on Purging.) On the other hand, purging a multilayer line demands more attention from process engineers and operators. When purging a multilayer blown-film line using Nylon, make sure that the Nylon is purged with a fractional MI LDPE. After purging the Nylon layer(s), make sure that the bubble or film contains no more Nylon. Only then can one lower the temperature profile for the extruder and die

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mandrel and introduce 1MI LDPE or a blend of 1MI LLDPE with LDPE. Similarly, EVOH, tie resins, and skin layers can be purged with 1 MI LDPE or with a blend of 1 MI LLDPE with LDPE. One can use the disco purge or other purging procedures known in the literature. Once all the extruders and the multilayer die contain 1MI LDPE or a blend of 1MI LLDPE with LDPE, there are two options. In option 1, the next production campaign can be started by setting the temperature profile for the extruders and multilayer die (as discussed above) and adding the different materials in each extruder according to the new multilayer film structure requirements. In option 2, the line can be prepared for shutdown following the procedure described below. When all extruders and the die have been purged and LDPE (or a blend of LLDPE with LD) is present in all extruders, one can start lowering the output and the temperature of the extruders and die (Caution! Make sure to watch the melt pressure in all extruders). Once the temperature is below 150°C (302°F or 423.15K), LDPE with antioxidant masterbatch can be introduced to all extruders, and one can simultaneously keep on lowering the temperature of the extruder(s) and die until melt pressure allows. Antioxidants prevent oxidation by air and reduce gel or brown and black specks during the next startup. Once the extruder and die temperatures are low, the line can be stopped. When the line is stopped, extruders are at 0 RPM (no plastic melt is coming out of the die). Make sure to cool the extruder(s) and die further by lowering temperature for all zones according to the specifications provided by your equipment or resin supplier. While equipment is being cooled down, make sure to seal and label all bags, specifically tightly closing leftover bags or boxes of Nylon or EVOH to prevent moisture ingress. Make sure that the production hall is clean, with no pellets on the floor, and that all the gaylord boxes or bags are labeled and back on the shelf. When the line has cooled down, turn off the power, compressed air, and water, and the blown-film equipment is ready for the next startup. Please make sure to follow all safety procedures until the end! REFERENCES AND ADDITIONAL RESOURCES [1] Schwank, D., “Film Applications”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 15. [2] Ealer, G., “Film Extrusion Introduction”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 7. [3] Klauber, M., “Film Extrusion Process”, 1992 Film Extrusion Manual, TAPPI PRESS, Atlanta, pp. 15–30. [4] Koch, K., et al., “Packaging Applications Using Enhanced Polyethylene and Polyolefin Plastomers Produced by Constrained Geometry Single-Site Catalysts”, Metallocene-Based Polyolefins, Volume Two, Wiley, 2000, p. 205. [5] Sergi, S., “Copolyamide Solutions for New Packaging Trends”, UBE Engineering Plastics S. A., AMI Multilayer Packaging Films, Newark, 2011. [6] Butler, T.I., “Blown Film Bubble Instability Induced by Fabrication Conditions”, Proceedings, 1999 Polymers, Laminations and Coating Conference, TAPPI PRESS, Atlanta, p. 815.

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[7] Tamber, H., “Blown-Film Cooling Systems”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta, pp. 185–200. [8] Knittel, R.R. and DeJonghe, R.J. Jr., “Blown-Film Cooling Systems”, 1992 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 261. [9] Krycki, B., “Better Bubble Management”, Proceedings, 1999 Film Conference, sponsored by Plastics Technology and Polymer Process Communications, Somerset, NJ, p. 229. [10] Stobie, J., “Air-Ring Consideration for Optimizing BlownFilm Properties”, Proceedings, 1996 Polymers, Lamination and Coating Conference, TAPPI PRESS, Atlanta, p. 231. [11] Stobie, J. and Tamber, H., “Film Stabilization, Forming and Collapsing Systems”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 201. [12] Knittel, R.R., “Film Stabilization, Forming and Collapsing Systems”, 1992 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 311.

[13] Smith, D., “Winding”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 313. [14] Planeta, M., “Automatic Surface Winder Offers Unique Solutions”, Macro Letter, Volume 4, Issue 2, 1998. [15] Planeta, M., “Automatic Turret Winding Features”, Macro Letter, Volume 5, Issue 1, 1998. [16] Butler, T.I., “Coextrusion”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 249. [17] Pucci, M., “EVOH”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta p. 509. [18] Christ, M., “Nylon Resins”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 495. [19] Tanny, S.R., “Tie Layer Adhesives”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 403. [20] Tamber, H., “Comparing Extrusion Technologies: Blown, Cast and Bi-Ax Processes”, Webinar organized by Plastic Technology USA, April 2014.

Chapter 5—Section 2

Cast Film CHRISTINE RONAGHAN, Cloeren Incorporated

KEYWORDS: Cast film, casting unit, chill roll, cooling, edge pinning, air knife, vacuum box, soft box, stripper roll, gauge, gauging, auto profile control. INTRODUCTION This chapter will introduce the casting unit as it exists in a typical cast-film process. This component of the extrusion line is primarily responsible for effectively and uniformly cooling the melt curtain after it exits the die. Although the casting unit may seem complex at first glance, its main objective is to successfully quench the film (Figure 5.2.1). CAST UNIT CONFIGURATION The general configuration of a flat-cast film line typically includes the die at either a 90-degree or 45-degree angle (Figures 5.2.2 and 5.2.3 respectively) relative to the chill roll. The distance that the melt curtain travels unsupported between the die exit and its first contact with the chill roll (referred to as the “air gap”) is generally minimized. Once in contact with the chill roll, the melt is pinned to the roll by one of several means. The purpose of pinning is to (1) stabilize the edges of the film and (2) maximize contact of the melt with the chill roll to control the quench characteristics of the film. Most casting sections also include a roll (or series of rolls) that form a nip with the chill roll subsequent to melt quenching for the purpose of removing plate-out. A plate-out roll, or plate-out roll assembly, nips the film to the roll after quenching to encourage the plate-out to remain with the film instead of depositing on the roll. The Air Gap Although it is obviously a very small portion of the entire web path in a cast-film extrusion line, the “air gap” is in fact a critical part of the process. Much occurs in what is often on the order of a 25 mm distance between the die and the chill roll. Within this distance, the melt flow is drawn from the lip-gap dimension down to the finished film dimension (established by mass flow rate and line speed); the relationship between the lip gap and the finished film thickness is commonly referred to as the draw ratio. How fast the thickness

changes is a function of line speed and air-gap length and is commonly referred to as the draw rate. The path and length of the melt curtain as it travels from the die-lip exit to its first contact with the chill roll is critical for several reasons. First, the angle at which the melt exits the die affects die-lip buildup (and therefore the time between required line stops for die-lip cleaning), as well as the optical appearance of the film. Ideally, the melt exits the die perpendicular to the lip face, preventing the melt from “dragging” over either the flexible or the fixed lip. If the exit angle is less than 90 degrees, dragging of the melt over the die lip tends to produce more rapid die-lip buildup. Dragging of the melt over the die lip may also create a melt fracture or surface fracture effect, resulting in optical disturbances and potentially increasing haze or diminishing clarity in the film. The path of the melt curtain between the die and the chill roll is influenced by several variables. The physical position of the die relative to the chill roll is a primary factor. Typically, the casting section position is adjustable both in height and in the machine direction. The offset of the die to the chill roll in both planes, as well as the machine-direction length of the air gap, together determine the melt exit angle from the die that is required to achieve tangential contact of the melt to the chill roll (Figure 5.2.4). The vacuum box, discussed in more detail below, can also influence the exit angle of the melt relative to the die-lip face. With the casting unit and chill roll positioned such that the melt is dragged slightly over the lip furthest away from the chill roll, applying a moderate vacuum between the die and the roll will draw the melt back to a perpendicular exit angle, in addition to evacuating air from between the chill roll and the melt. Curtain Path. The length of the air gap, or the linear distance that the melt travels between the die-lip exit and the chill-roll contact point, can influence the finished product in several ways and must be established based on the polymers being processed and the desired properties of the finished film. 341

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FIGURE 5.2.1.  Cast film (rendering).

Both the stability and the quench behavior of the web are primarily influenced by the degree of contact between the web and the chill roll. Without any outside forces, the melt curtain would effectively float around the chill roll. A very thin layer of air, which is inherently present due to the motion of the melt curtain as it travels from the die to the roll and the rotation of the roll itself, would be present between the web and the roll for the entire length of their travel together. Because this air would prevent intimate contact between the melt and the chill roll, and because in fact air is an insulator that does NOT help to achieve the desired result of a quick and controlled quench, it must be removed. Several mechanisms can help remove air from between the web and the roll and thereby improve contact between the two. A vacuum box (Figure 5.2.5) is typically positioned be-

tween the die and the chill roll. It is typically mounted to the die and positioned in such a way that a uniform vacuum can be applied across the width of the die slot to remove air from between the melt and the chill roll. Vacuum boxes can be very simple for thicker films and/or slower processes or more sophisticated multi-chamber designs for thin films produced at very high line speeds. Stability and uniformity of the vacuum is a critical parameter in these processes and can influence film thickness, quench rate, and ultimately the physical properties of the finished product. Another pinning mechanism typically used in addition to the vacuum box is edge pinning (Figure 5.2.6). Edge pinning is, as its name implies, applied at the outboard edges of the film as the melt curtain makes initial contact with the chill roll. It can take the form of pressurized air or static emitters, both of which create impingement of the edge of the curtain to the roll. On many film lines, both methods are used

FIGURE 5.2.2.  Simple cast section line schematic (90°).

FIGURE 5.2.3.  Simple cast section line schematic (45°).

Melt Curtain-to-Chill Roll Contact

Section 5.2.  Cast Film

343

FIGURE 5.2.4.  Simple die/chill roll configurations and resulting path in the air gap.

in unison. Edge pinner assemblies are typically mounted on an adjustable arm to accommodate varying film widths. In addition to stabilizing the edges, air edge pinners can also serve as a secondary cooling mechanism for edge beads that form as a result of neck-in of the melt curtain in the air gap. Subsequent to the edge pinning mechanisms, some line configurations feature devices called air knives (Figure 5.2.7) or air chambers (also known as soft boxes). The purpose of these devices is also to improve contact between the melt and the casting rolls, in this case by directing a uniform impingement of air onto the web. Applying pressure to the melt curtain forces any air traveling on the backside of the film to exhaust, enabling more intimate contact of the web to the roll. The air knife uses a narrow stream of relatively high-volume air to bring about that impingement, whereas the air chamber uses a reduced-pressure impingement, but over a longer footprint. As with the air edge pinners, these devices involve exposing the film surface to a forced air flow at ambient temperature and therefore slightly reduce the film surface temperature. All these web pinning features of course ultimately result in faster and more controlled film cooling because they all act to improve contact (and therefore heat transfer) between the melt and the roll.

FIGURE 5.2.5.  Vacuum box.

Heat-Transfer Rolls Current technology offers several possibilities for construction of heat-transfer rolls. Of critical importance are the mechanical integrity of the roll and its heat-transfer capability. Most cooling rolls are dual-shell, spiral baffle geometries that maximize both the mechanical characteristics and the cooling efficiency of the roll. The internal spiral baffle design enables increased coolant velocity through the roll. Cooling demand is dictated by the thermal properties of the melt curtain being cooled, in addition to its specific output rate, and the desired temperature reduction (that is, the difference between the melt temperature of the extrudate and the desired strip-off temperature of the film). In addition to the geometry of the roll and its cooling channel, the flow rate and incoming temperature of the coolant flowing through the roll can have a significant impact on the quench rate of the polymer and the overall cooling capacity of the roll. The incoming coolant temperature dictates the nominal temperature reduction capability and the rate of temperature reduction, whereas the coolant flow rate dictates the temperature gradient across the width of the extrudate. Regular maintenance of the entire heat-transfer and

FIGURE 5.2.6.  Edge pinners.

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circulation system is recommended to avoid fouling of flow channels and subsequent limitations on the overall cooling capacity of the system. Although most chill rolls have a steel shell, copper-shelled rolls are an available option. With a heat-transfer coefficient nearly 10 times that of steel, the heat-transfer properties of copper make it an ideal choice for applications with very high cooling demand. As shown in Figure 5.2.8, there is a significant temperature differential at the chill-roll surface between copper and steel when an identical heat-transfer medium is run through the system at the same flow rate and temperature. Practical implications are (1) increased cooling capacity without having to upsize the chill roll, and (2) running a higher coolant temperature (thereby reducing the tendency for condensation and improving energy efficiency), but achieving the same total temperature reduction. Although copper roll technology offers appreciable benefits, it cannot influence heat transfer through the thickness of the melt curtain being quenched; it is inherently limited by the thermal properties of the polymers contained in the coextrusion. As previously stated, quench rate and cooling characteristics can have a profound impact on both the physical and the aesthetic properties of films. They can influence clarity in CPP and barrier films, impact the physical properties of machine- and hand-stretch films, and introduce differential shrinkage (leading to curl) in high-barrier asymmetric structures. In addition to the cooling characteristics of a chill roll, its surface finish is also critical and can have significant influence on a process. The roll surface can have a mirror finish, resulting in a glossy surface on the film in contact with the roll. The smooth mirror surface encourages intimate contact

between the melt curtain and the roll, which improves heat transfer, but can sometimes make it a challenge to strip the film from the roll at the end of the casting unit. Alternatively, the chill roll can have a matte finish. The surface roughness inherent in a matte-finish roll creates less contact between the melt curtain and the chill roll, which can (1) slow the quench rate, and (2) improve film release from the roll at the time of strip-off. It is also possible for the surface of the chill roll to be custom-embossed; this type of cast-film application is referred to as cast embossed and is discussed later in this section. Plate-Out Assemblies It is not uncommon in cast film that low-molecular-weight components of the formulation, including degraded polymer or additives, may bloom to the surface of the film and in fact transfer from the film to the chill roll over the duration of their travel together. A plate-out roll, or plate-out roll assembly, nips the film to the roll after quenching to encourage the plate-out to remain with the film instead of depositing on the roll. Polymer formulation, melt temperature, chill-roll temperature, and pinning methodology can all influence plateout. Although it is possible to reduce or minimize plate-out, in some situations, it is not avoidable. Regular cleaning of the chill-roll surface ensures that contact between the melt curtain and the roll surface remains at the maximum. Secondary Cooling Rolls Some casting units have only a primary chill roll, which serves to adequately quench the film down to the desired temperature (and temperature uniformity) without additional tem-

FIGURE 5.2.7.  Air knife.

Section 5.2.  Cast Film

345

FIGURE 5.2.8.  Heat transfer: steel vs. copper chill roll.

FIGURE 5.2.9.  Cast embossed film.

perature-controlled rolls or heat-transfer apparatus. However, some line configurations include additional heat-transfer rolls immediately downstream of the primary chill roll. These additional rolls may serve the purpose of enhanced cooling. On thicker films, temperature gradient through the film thickness may be a concern. Allowing the side of the film previously exposed to air to now contact an intensively cooled roll surface directly ensures that the film achieves a uniform temperature throughout its thickness by the time it exits the casting section. In other cases, the film may need to be annealed; that is, quenched on initial contact with the primary chill roll, reheated on a secondary (and possibly tertiary) heat-transfer roll, and then finally cooled again on a final chill roll before exiting the casting section. Such a process may be used to reduce the impact of secondary crystallization on wound rolls, to influence curl on asymmetric structures, and/or to influence the clarity of a finished film in cases where resins with very different crystallization behaviors have been coextruded. CAST EMBOSSED FILM The foregoing discussion describes a standard flat-cast film process. An alternative cast process is referred to as “cast embossed” film, in which the melt curtain travels from the die-lip exit into a nip typically formed by a rubber nip roll against a patterned or engraved steel chill roll, as depicted in Figure 5.2.9. In this case, the formed film exiting the casting unit already has the desired finished pattern imposed on its surface. In this configuration, the air and static pinning methods are eliminated. In their place is the rubber roll, which forms

a nip with the chill roll, thereby ensuring that the melt curtain makes intimate contact with that chill roll. Any pattern, be it a particular surface roughness or a specific machined geometry, is imparted to the film as it is pressed against the cold roll and quenched. This process is much like extrusion coating, minus the incoming substrate. Unlike extrusion coating, the cast embossed process involves continuous direct exposure of the rubber nip roll to the hot melt. Therefore, line speeds can be limited by the cooling rate of the rubber. As the surface temperature of the rubber increases, so does the likelihood that melt will stick to it. Water-cooled steel rolls can be used to chill the rubber surface. For more direct cooling, an alternative approach is a wet embossed process, whereby the surface of the rubber roll is directly cooled by water. This is referred to as a wet embossed process and can potentially provide higher line speeds and throughput capacity. ADDITIONAL FEATURES The casting unit may sometimes house additional line components subsequent to the cooling roll assembly. In some cases, a trim station is mounted on the casting unit to remove a first (or only) edge trim from the formed film, especially where downstream processes involve stretching or otherwise manipulating the film before winding. This may also be typical in an application with both primary and secondary edge trims; the first trim station is typically located on or near the casting section. Although often located further downstream in the process, the casting unit in some cases also houses the gauging system.

Chapter 5—Section 3

Sheet Extrusion Process SAM IULIANO, Nordson Extrusion Dies Industries, LLC

INTRODUCTION Extruded plastic sheet is often shaped by thermoforming into products that we use every day. Examples of common items that start out as flat sheet include fruit cups, drink cups and lids, refrigerator liners, clamshell containers, hot tub shells, outdoor cabinets, and kayaks—to name just a few. This is an important sector of the plastics industry, complete with its own set of processing challenges and opportunities. Molten plastic extrudate from a sheet extrusion die usually feeds a three-roll stack that cools and polishes the sheet. In contrast, thinner plastic film is normally cooled on a single casting roll because it is thin enough to be cooled from one side. In addition to the cooling method, cast film and sheet products can be differentiated by their product thickness. In theory, the maximum cast-film thickness is 0.25 mm. Products thicker than this are generally classified as sheet. There always can be exceptions. Heavier-gauge film can in some cases be produced on a casting roll, and thinner-gauge sheet can sometimes be produced in a three-roll stack. Sheet from 0.25 mm up to 3.5 mm thick is, when possible, wound into a finished roll. Some sheet extrusion lines, however, have inline thermoforming, in which the plastic parts are formed immediately, without first creating rolls of product. Sheets thicker than 3.5 mm, up to 60 mm thick or more, are generally sheared into slabs and stacked flat. Plastic sheet producers have several common goals for their operations. The sheet produced needs to have a good appearance (showing virtually no lines or streaks, with uniform color, and having an evenly polished surface). It needs to lay flat and be within acceptable thickness tolerances. The sheet also needs to have a limited amount of machine-direction orientation to facilitate thermoforming of dimensionally accurate plastic products. Producers constantly strive for greater production efficiency and versatility, driving a strong demand for extrusion-line features that speed up product changeovers and reduce waste. SHEET EXTRUSION LINE EQUIPMENT The main components in most sheet extrusion lines are,

from left to right in Figure 5.3.1: the extruder, screen changer, gear pump, static mixer, extrusion die, and the three-roll stack. For coextrusion lines, the components are similar, except that there is more than one extruder, and multiple melt streams are combined in a coextrusion feedblock just upstream of the die. A basic overview of most of these components will be provided, along with some troubleshooting advice. Extrusion dies and coextrusion feedblocks will be covered in greater detail because this is where the author has special expertise. Extruder Polymer pellets are fed into the hopper of the extruder and are melted and conveyed by a rotating screw in a heated barrel. The screw turns in a direction that would make it unscrew backwards out of the melt, if not for a thrust bearing that holds it in place. Because of this mechanical arrangement, the net result is that the screw rotation conveys the pellets, and eventually the melt, forward. Most of the heat that is generated to melt the plastic pellets is created by shearing the plastic between the flights of the screw and the barrel wall. Screws of various designs are available, including barrier-type screws, which melt more efficiently by separating, as the polymer mass moves down the screw, the growing volume of melted resin from the diminishing volume of solid pellets. These types of screws, as shown in Figure 5.3.2, help to prevent gels in the sheet resulting from unmelted polymer. An extruder screw has three major sections, which are commonly called the feed zone, the compression zone, and the metering zone (from inlet to outlet). If melt pressure at the extruder exit fluctuates, the resin may be melting too early in the screw. Reducing the feedzone temperature setpoints can often help improve the situation. If the melt temperature is too high, try reducing the screw RPM to lower the shear heating. In addition, reducing the metering-zone temperature setpoints may help reduce the melt temperature. If the melt quality is poor, try increasing the feed, com347

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FIGURE 5.3.1.  Cross-sectional schematic of a sheet extrusion line.

pression, and metering zone temperature setpoints while also reducing the screw RPM (to increase residence time and improve mixing) [1]. Screen Changer Contaminants that enter the extruder hopper, such as dust, wood, and gels or carbonized particles generated in the extrusion process, must be filtered out of the melt to produce high-quality sheet. This can be done with a simple breaker plate that supports a pack of metal screens (a screen pack). Finer-mesh screens provide finer filtration and can improve melt quality by generating more backpressure for improved mixing of the melt in the extruder. When the screen pack becomes overly contaminated, a higher level of backpressure will indicate that it is time to change to a new screen pack. Manual screen changers require the line to be shut down to switch to a clean screen pack, making the operation less efficient. Hydraulic screen changers, such as that shown in Figure 5.3.3, can switch to clean screens during operation with minimal flow disruption [2]. Figure 5.3.3: If leakage occurs, the remedy could be to increase the head pressure. The screen changer seals tend to become tighter as head pressure increases. At lower pressure, the seals will relax and can leak slightly, especially with lower-viscosity resins.

FIGURE 5.3.2.  Barrier screw with dual flights to separate solid pellets from melted resin.

Gear Pump A consistent output rate from an extruder depends on how well many variables are controlled. The output rate can fluctuate because of variations in the feed bulk density or problems with the barrel temperature control setpoints. Adding a gear pump (sometimes called a melt pump) (Figure 5.3.4) to the line can often dramatically improve melt pressure uniformity, which results in more uniform flow from the extruder. The gear pump has two inter-meshing, counter-rotating gears that produce a volumetric displacement of the melt. The resulting discharge flow rate is highly uniform, with little influence from variations in inlet pressure [3]. Improving the melt pressure tolerance delivered to the die system to the ±1.0% range generally results in better sheet quality, especially when coextruding. Gear pumps provide many benefits, such as uniform metering of the melt, reduced melt processing temperature, increased line output capacity, and greater flexibility to process a wider range of polymers. However, in some cases, less mixing of the melt can result from a reduced load on the

FIGURE 5.3.3.  Hydraulic screen changer can switch to a clean breaker/screen pack assembly in less than one second.

Section 5.3.  Sheet Extrusion Process

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FIGURE 5.3.5.  Static mixer blade elements use extruder or gearpump pressure to provide the mixing energy.

A vertically oriented final blade could introduce a weld-line defect at the center of the sheet. Coextrusion Feedblock FIGURE 5.3.4.  Gear pump has inter-meshing gears that precisely meter and pressurize the molten melt.

extruder. Installing a static mixer between the gear pump and the die can remedy this potential problem. Static Mixer The purpose of a static mixer is to homogenize the melt. A common mixer uses stationary flights that divide and reorient the melt stream multiple times as the flow passes through it, as shown in Figure 3.3.5. By repeatedly directing melt from the center of the flow to the walls, and vice versa, improved melt viscosity and melt temperature uniformity can be achieved [4]. Positioning static mixers in the line close to the coextrusion feedblock or extrusion die helps ensure that a homogeneous melt is being combined and spread to the desired sheet width. If a disturbance is noticed at the center of the sheet, check to see whether the static mixer assembly has been installed correctly. The final blade of the mixer should be horizontal.

Multilayer extrusion enables processors to create sheet products with desirable properties in an efficient and economical way. For many applications, it would be prohibitively expensive, and perhaps impossible, to obtain all the required functional and aesthetic properties from just one material. Coextrusion enables, for example, sheet producers to bury off-color regrind in the middle of the sheet and then put virgin colored and glossy layers on the visible outside surfaces. Alternatively, a minimal amount of an expensive oxygen barrier material can be sandwiched inside other adhesive or structural layers, providing the required properties with less cost. A coextrusion feedblock accepts melt streams from multiple extruders and assembles these individual streams into a combined multilayer stream that flows into the extrusion die. Typically, the feedblock exit passage is around 25 mm tall by 100 mm wide, but can be larger when needed, for example, to reduce shear stresses for very high output rates and/or very high-viscosity materials. Figure 5.3.6 shows a typical fixed-geometry feedblock with coextrusion inserts that can be accessed by a cover plate. Also shown in this figure is a cross-sectional view, with five layers from three extruders being combined.

FIGURE 5.3.6.  Fixed-geometry five-layer coextrusion feedblock.

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FIGURE 5.3.7.  Standard feedblock flow insert on left and profiled feedblock flow insert on right.

Satellite extruder melt streams entering the feedblock can be split in two, enabling packages of up to five layers to be produced from three extruders, for example. The melt streams are combined using flow inserts, which are designed like miniature 100-mm-wide extrusion dies that are housed inside the feedblock. If the layers become distorted as they pass through the die due to large viscosity ratios or large layer ratios, then one common remedy is to use profiled flow inserts to reduce distortion. Figure 5.3.7 shows comparative views of an unprofiled flow insert with a straight layer combination plane and a profiled flow insert that is designed to discourage flow of a skin layer from the edges of the sheet.

Using profiled feedblock inserts to help bring final layer uniformity into acceptable tolerance has many advantages. Once the correct flow insert profile has been established, the system becomes geometrically fixed. Shift-to-shift consistency can be a valuable benefit of this fixed-geometry approach for operations that have a relatively small number of coextruded products to produce. Shutdowns are required to change from one product-specific profiled insert to another, but especially in a plant with long production campaigns, this approach is often the best path forward. An alternative coextrusion feedblock is the adjustable version, with the working concepts shown in Figure 5.3.8. This adjustable feedblock has two adjustable components for each skin and sub-skin layer: a pivoting combination plane, and a rotating profile actuator. The pivoting combination plane is the blade-tipped device on the shaft in the images. Imagine that the combination plane section from the fixed insert now is a separate piece that can rotate in a socket located in the adjustable flow insert housing. By pivoting this plane, the processor can change the velocity of the layer at the combination point—larger gaps create a lower combination velocity, whereas smaller gaps create a higher combination velocity. If different sheet products require different layer thickness ratios, for example, then the user can adjust the gaps to be proportional to the thickness ratio without the need to shut down and change any parts.

FIGURE 5.3.8.  Adjustable coextrusion feedblock with pivoting combination planes and rotating profile actuators.

Section 5.3.  Sheet Extrusion Process

The rotating profile actuators are the large-diameter pins that form the combination gap and have a bronze bar installed in them. This bronze bar can have a variety of profile shapes. For example, it can be proud at the edges to form a tighter combination gap in those areas and recessed at the center to form a larger combination gap there. These largediameter pins can be rotated to position the profile bar. When the profile bar is positioned upstream of the combination plane tip (the left views in the figure), the effect of the profile in the bar is minimized. When the profile bar is positioned at the combination point (the right views in the figure), the effect of the profile bar is maximized. If you study the velocity field plots at the bottom of the figure, you will notice that the velocity gradient across the width of the combination layer is less extreme on the left where the profile bar is upstream of the combination point than on the right where the profile bar is at the combination point. In the left view, the velocity is around 100 mm/s on the edges of the combining layer and around 150 mm/s at the center. In the view on the right, the velocity is around 55 mm/s on the edges of the combining layer and around 170 mm/s at the center. Consider a scenario where you want to extrude skin layers that have much lower viscosity than a core layer and that tend to flow more readily to the edges of the die. To compensate for these thicker skin layers towards the edges of the sheet, the processor would adjust the profile actuator to position the profile bar closer to the combination point to help make the final layer profile more uniform. The flow would be starved at the edges in the feedblock to compensate for the fact that these lowerviscosity skin layers will tend to overtake the core layer as they flow towards the ends of the die. Because the profile bars can be positioned at several locations, many different distribution results can be achieved from just one profile bar, and this profiling can be done while the extruders are running—saving many hours of downtime compared with the fixed-geometry feedblock approach. If your process involves many short runs with, for example, many different layer thickness ratios and many different layer viscosity ratios, then an adjustable feedblock with a few different profile bars may be the best solution for your operation. Extrusion Die Flat extrusion dies are typically used to produce thermoplastic products such as cast film, sheet, and oriented film, or they may be used to convert substrates by extrusion coating or laminating methods. Dies for sheet extrusion are optimized for larger operating lip gaps and may include an adjustable restrictor bar to help tune the flow distribution. The primary function of the die is to distribute the molten polymer(s) uniformly along the length of the die slot. This uniform flow distribution is generally achieved by designing the die’s internal flow-channel geometry to provide a nearly identical total pressure drop for each flow path along the length of the die, as shown in Figure 5.3.9. The flow channel, however, is only ideally optimized for one specific

351

FIGURE 5.3.9.  Uniform flow distribution through a flat extrusion die channel (P1≈P2≈P3≈P4).

processing scenario. Die adjustment systems are available to compensate for variations in process variables such as: (1) the type of plastic to be distributed, (2) the output rate, and (3) the sheet thickness. For processing scenarios that are outside the optimized case, the processor needs to make more extensive use of die temperature zone profiling, lip adjustment profiling, and restrictor bar profiling (if available) to make a sheet that is within acceptable thickness tolerance. The primary manifold is the largest chamber in the flow channel. In Figure 5.3.9, the primary manifold has a diminishing-volume design in that the channel is deeper and longer at the center and becomes smaller in cross section towards each end of the die. This diminishing-volume primary manifold design promotes more rapid purging due to its significantly shorter polymer residence time than other primary manifold designs with a constant cross-sectional area. The preland is a smaller-gap channel section that has a longer land length in the center than it does at the ends. This change in preland length is often called a preland drop. When this geometry is properly optimized by the channel designer, it provides a nearly equivalent total pressure drop for each polymer path through the die: the additional resistance at the center of the preland compensates for the shorter path length and forces evenly distributed flow towards the ends of the die. This type of flow channel, which as shown in Figure 5.3.10 does not include a restrictor bar, is commonly used for sheet dies that extrude the product thinner than 2.0 mm. For dies producing sheet that may be thinner or thicker than 2.0 mm, the flow channel should include an additional section for a restrictor bar, as shown in Figure 5.3.11. If the sheet will always be thinner than 2.0 mm, then a flex-lip die without a restrictor bar is fit for use. A restrictor bar is positioned in the die between the preland and the lip. It can be driven into the flow path and can be bent into a smile, for example, to encourage more flow to the ends of the die. Alternatively, the bar can be profiled into a frown shape to promote more flow in the middle of the die. The restrictor bar adjustment spools are typically spaced on 70-mm centerlines, enabling more elaborate and localized profiling of the restrictor bar gap. In general, for products thinner than

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FIGURE 5.3.10.  Flexible-lip die without a restrictor bar, used solely for thinner sheet products.

2.0 mm, the processor should tune the sheet thickness uniformity with flexible lip adjustments while leaving the restrictor bar in the full-up position. For smaller die-lip gaps, adjusting the restrictor bar is unnecessary because the lip pressure is sufficient to make lip-gap profiling highly effective. For products thicker than 2.0 mm, the processor should set a straight lip gap and tune the thickness uniformity by adjusting the restrictor bar. When coextruding, multiple layers of molten polymer are stacked together in the coextrusion feedblock upstream of the die. This coextrusion sandwich is then spread through the die to make the final multilayer sheet. Acceptable coextrusion uniformity can be difficult to achieve, even with dies that provide good overall flow distribution with reasonable polymer residence time. This occurs because certain

channel geometries, which work adequately for mono-layer extrusion, can promote heavy distortion of the layer interfaces when coextruding. There are four main drivers of coextrusion interface distortion: (1) viscous encapsulation, (2) elastic secondary flows, (3) wave instability, and (4) zig-zag instability. When a die is used for coextrusion, the die flowchannel geometry must be designed to resist the development of these four types of coextrusion problem to minimize the need for feedblock profiling. Viscous encapsulation, as shown in Figure 5.3.12, can be described as the tendency for lower-viscosity layers to migrate towards flow channel wall areas of higher shear stress. In doing so, the lower-viscosity layers act as a lubricant for higher-viscosity layers, and individual layer thickness profiles become non-uniform. Viscous encapsulation is more pronounced when high shear stress exists at the layer interfaces, when the viscosity difference between layers is large, and when the die is very wide (because there is more time for encapsulation to develop). Elastic secondary flows are driven by an imbalance of normal forces, as illustrated in Figure 5.3.13. Large recirculation areas in the corners of flow channels can cause major changes in individual layer thicknesses [5]. To reduce this defect, processors should choose resins that are similar in viscoelasticity, or they should adjust melt processing temperatures to reduce the difference in material viscosity. Corner recirculation regions tend to be larger in square channels with sharp corners and tend to be smaller as the channels become more rectangular (longer in the machine direction and shallower in channel height) and as the corners become more rounded. These more rectangular channels are usually described by their aspect ratio, as shown in Figure 5.3.14. This elongated manifold design also reduces shear stresses,

FIGURE 5.3.11.  Mid-range sheet extrusion die with end seal plates removed and with key features labeled.

Section 5.3.  Sheet Extrusion Process

353

FIGURE 5.3.12.  Viscous encapsulation—lower-viscosity layers migrate to high wall shear-stress areas, resulting in interface distortion.

especially at the manifold backline, to help resist viscous encapsulation [6]. Figure 5.3.15 shows a wave coextrusion instability, which looks like a series of parabolas that are oriented in the machine direction. This problem is more prone to occur if there is a large velocity difference among the molten polymer streams being combined in the feedblock, when the layer structure is asymmetric (for example: a one-sided 5% A/95% B structure instead of the symmetric structure 5% A/90% B/5% A), when the skin layer material has higher elongational viscosity than the adjacent layer, when the die channel geometry is asymmetric, and when the spread ratio (die slot width divided by feedblock exit width) is large. Figure 5.3.16 illustrates a zig-zag coextrusion instability, which in mild form results in haze for clear product as the coextrusion interfaces begin to intermingle. This instability, in more advanced form, results in the appearance of arrow heads or chevrons on the surface of the sheet. Zig-zag originates from high shear stress at the layer interfaces. Critical interfacial shear stresses (at which the instability begins) are known to be in the range of 30–80 kPa. Generally, this problem occurs in tight gaps in the die geometry where the highest levels of shear stress occur (usually the lip land, and sometimes in the preland gap). To remedy zig-zag instabil-

FIGURE 5.3.13.  Elastic secondary flows due to imbalanced normal forces. Interfaces are disturbed near the corners of a square channel. Long rectangular channels with generous corner radii work better.

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FIGURE 5.3.14.  Illustration of a diminishing-volume primary manifold with a constant 1.5:1 manifold aspect ratio. Lower shear stresses, especially at the manifold backline, help resist viscous encapsulation problems.

ity, interfacial shear stress must be reduced. Setting larger die gaps, designing flow channels with larger preland gaps, raising melt processing temperatures, and slowing down the extrusion rate are all stabilizing actions. Moving downstream in the die, away from the primary manifold, this discussion will focus on the very important topic of die-lip area. The die-lip gap does not directly determine the final sheet thickness. It is the combination of extrusion rate and takeoff speed that establishes the sheet thickness for a given die-lip length, regardless of the die-lip gap. The average lip-gap setting does influence, however, phenomena such as extrudate swell, melt fracture, and machinedirection orientation. The average lip-gap setting is typically larger than the targeted sheet thickness (to avoid too much lip-face contact and discourage plate-out or buildup that can lead to die lines). The draw ratio can be defined as the lip gap divided by the sheet thickness. High draw ratios enable long run intervals between having to stop and clean away contamination from the lip faces. However, low draw ratios generally result in minimal orientation in the sheet, so that an oven shrinkage test would show low MD shrinkage. Extrudate swell is less of a problem for thicker products because of the larger lip gaps—the reduced velocity in the larger lip

gap results in less velocity rearrangement as the melt leaves the die. Hence, typical draw ratios become smaller as products get thicker. In addition, draw ratios tend to be much higher for materials with lower melt strength because they have less “polymer memory”, making orientation less of an issue. Table 5.3.1 lists typical draw ratios for common resin types (polyolefins, styrenic polymers, and PVC) and for two lower-melt-strength resins (polycarbonate and polyester). Sheet extrusion dies often have a removable lower lip. This enables the processor to exchange the lip for a new one that provides a different range of lip gaps and has a different lip-land length. The lip land is the length of the final flat in the die that creates the lip gap. Longer lip lands are employed for larger lip gaps to generate enough lip pressure for effective flexible-lip control of thickness uniformity. Longer lip lands reduce the amount of extrudate swell. Too much swell can lead to lip-face contamination and eventually a shutdown to address “die lines”, as shown in Figure 5.3.17.

FIGURE 5.3.15.  Wave coextrusion instability.

FIGURE 5.3.16.  Zig-zag coextrusion instability.

Section 5.3.  Sheet Extrusion Process

TABLE 5.3.1.  Typical Ratios of Lip Gap to Sheet Thickness (also known as draw ratios). Sheet Thickness Range (mm) 0.25–0.40 0.50–0.70 0.80–1.30 1.40–2.30 2.40–3.50

Typical Draw Ratios for Olefins, Styrenics, PVC

Typical Draw Ratios for PC, PET

2.00 : 1 1.30 : 1 1.20 : 1 1.10 : 1 1.08 : 1

2.00 : 1 1.30 : 1 1.20 : 1 1.10 : 1 1.08 : 1

Because sheet processors strive for efficient operations with versatile tooling, advanced sheet die technology has been developed. One such development, the SmartGapTM die, enables the processor to change the lip-gap opening from a single adjustment point, saving the time needed to perform several die-lip adjustments. At the same time that the lip gap is being adjusted, the internal mechanism will vary the lipland length as needed to help manage die swell, orientation, tuning sensitivity, and backpressure. This simultaneous land and gap adjusting mechanism, as shown in Figure 5.3.18, broadens the operating window for high-quality sheet production. Proper lip exit geometry can be established quickly without the need to shut down and change lips. The extruded sheet width can be reduced by blocking the lip exit on each edge of the die slot using lip sealing mechanisms, commonly called deckles. This reduces the size of the edge trimmings that are required to get the finished sheet to the final required width. A compact, basic solution, commonly called fixed external deckles, is effective, but requires a shutdown to reposition the parts to vary the extrudate width. When enough space is available due to sufficient polymer melt strength between the die lips and

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the roll stack, then adjustable FastDecklesTM can be used, as shown in Figure 5.3.19. This deckle option can be adjusted while the line is running to increase the productivity of the sheet line. Three-Roll Stack The purpose of the sheet stack is to cool and polish the extruded sheet. Generally, for sheet that is thinner than 1.5 mm, the die feeds the nip formed between the top and center roll (downstack arrangement, Figure 5.3.20). For thicker sheet, the die is positioned between the bottom and center rolls (upstack arrangement) because the weight of the sheet helps keep it in contact with the cooling rolls. The rolls are cooled with chilled water, ideally so that there is less than 2°C difference between the inlet and outlet water temperatures. This ensures that the roll temperature is relatively uniform from side to side. Roll nip gaps should be set close to the desired sheet thickness (the first nip gap should be about 2% greater than the desired sheet thickness, and the second nip should be about 1% greater) [7]. If the sheet curls up on the edges and the web path is in a downstack arrangement, then the remedy is to increase the temperature of the bottom roll. If the sheet curls down on the edges, then raise the temperature of the middle roll. If cross-direction lines are seen in the sheet, it is likely that the web is sticking to the final roll in the sheet stack. The likely remedy is to lower the temperatures of the middle and final roll. You may also need to reduce the melt processing temperature by changing the extruder barrel temperature setpoints, primarily in the metering zone. If the surface of the sheet has an orange-peel appearance, then the surface of the sheet is freezing too quickly, before it can be adequately polished. In this case, you should consider raising the temperature of the middle and final rolls [8].

FIGURE 5.3.17.  Die swell due to insufficient lip-land length, can lead to lip contamination and result in “die lines”.

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FIGURE 5.3.18.  SmartGapTM sheet die with single-point adjustment to quickly change lip gap and land length.

SUMMARY Efficient sheet processing requires that the extrusion line components be properly tuned and that they be designed to minimize downtime. For high-quality sheet production, the processor needs to develop optimized processing parameters that will result in homogeneous melt quality at the proper melt processing temperature without contaminants, melt pressure uniformity to ±1.0% for each melt stream, uniform layers without wave or zig-zag defects when coextruding, and ultimately a blemish-free sheet of uniform thickness that can be thermoformed into acceptable products. In most cases, production efficiency can be

maximized by including certain features in the line components, such as: • Barrier screws that melt more efficiently • Hydraulic screen changers that can switch to a clean screen pack in less than one second • Gear pumps that provide uniform metering of the melt and enable increased line speeds • Static mixers that correct for any melt-temperature stratification just before the feedblock/die • Adjustable coextrusion feedblocks that can accommodate many changes to layer thickness and viscosity ratios • Extrusion dies with single-point adjustments to vary the lip gap and land length and adjustable deckles.

FIGURE 5.3.19.  Fixed external deckles (left image) and adjustable FastDecklesTM (right image).

Section 5.3.  Sheet Extrusion Process

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REFERENCES AND ADDITIONAL RESOURCES

FIGURE 5.3.20.  Three-roll sheet stack with downstack arrangement.

[1] Vlachopoulos, J., The SPE Guide on Extrusion Technology and Troubleshooting, The Society of Plastics Engineers, CT, 2001, pp. 1-28 through 1-29. [2] Calhoun, A., Single- and Twin-Screw Extruders: Plastics Technician’s Toolbox, The Society of Plastics Engineers, CT, 2004, pp. 217–222. [3] Calhoun, A., Single- and Twin-Screw Extruders: Plastics Technician’s Toolbox, The Society of Plastics Engineers, CT, 2004, pp. 229–232. [4] Calhoun, A., Single- and Twin-Screw Extruders: Plastics Technician’s Toolbox, The Society of Plastics Engineers, CT, 2004, pp. 225-226. [5] Dooley, J., SPE-ANTEC Technical Papers, 53, 2383 (2007). [6] Iuliano, S., SPE-ANTEC Technical Papers, 753–759 (2016). [7] Calhoun, A., Extrusion Processes: Plastics Technician’s Toolbox, The Society of Plastics Engineers, CT, 2004, pp. 59–66. [8] Iuliano, S., “Action Plan for Keeping Your Sheet Dies in Line”, Plastics Technology Magazine, November 2010, pp. 19–21.

Chapter 5—Section 4

Polymer Rheology OLIVIER CATHERINE, Cloeren Incorporated

INTRODUCTION The flow behavior of polymer melts is a critical area of the polymer film industry, especially considering the following trends [1]: • Downgauging: Films, in general, are becoming thinner, and thickness distribution is more critical for thinner films. • More layers: both blown-film and cast-film structures have become increasingly complex. Controlling the layer thickness distribution for each layer in the coextruded product requires some consideration of the flow properties of each polymer. • More Polymer Variety: because of requirements for more performance out of thinner films, resin suppliers have developed a variety of polymers with specific molecular structures. This also results in specific flow behavior that must be characterized to take full advantage of the performance provided. • Wider and Faster Machinery: During the past three decades, line speeds have been increasing. Films made by blown-film, cast-film, or oriented-film processes have also increased in width. This, considered with the other trends, demonstrates that to reach these levels, a full understanding of polymer melt flow is a must. POLYMER MELT VISCOSITY Definition of viscosity The notion of viscosity was introduced by Sir Isaac Newton in 1713 and can be best described as the resistance of a fluid to flow. Newton’s experiment consists of placing a thick fluid on a table with a moving plate on top, placed at a specific height, parallel to the base table. The viscosity of the fluid was determined by measuring the force required to pull the top plate at a constant velocity, while maintaining the gap h between the table and the moving plate constant (Figure 5.4.1). Newton found a constant ratio between the moving plate velocity V and the force F required to pull the plate for a given fluid and called it viscosity.

Shear-Thinning and the Macromolecular Nature of Polymer Melts Polymer molecules are macromolecules made of many repeating units called monomers. Polymer molecules in the melt phase can be described as flexible coiled chains with a high degree of entanglement (Figure 5.4.2). Higher entanglement means more intramolecular interaction, and hence higher viscosity. Polymer macromolecules, compared to short molecules, experience an influence of stress or deformation rates on the degree of entanglement. This is shear-thinning behavior: viscosity will decrease because increasing deformation rate decreases entanglement degree (Figure 5.4.3). Shearthinning has many direct consequences in film processing; some key examples will be discussed in the next paragraphs. Shear Flow In the previously described experiment by Newton, the top plate was pulled at constant velocity, and the bottom plate was fixed. Consequently, a linear velocity gradient was created. This is a steady simple shear flow situation, and shear rate is defined as the velocity gradient over the gap [2]:

 

V h

Shear rate is constant through the gap between the plate and the table in this situation. Shear rate is defined as the local velocity gradient over an increment of distance in the direction perpendicular to the flow. Most flows in polymer processing are shear-driven. For example, in the case of a pressure flow in a pipe under a pressure drop ΔP, the velocity field for a non-Newtonian material such as a polymer melt has the shape of a flat parabola v(r) with its maximum in the center, as shown in Figure 5.4.4. Shear rate is defined as:

 

dv(r ) dr 359

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FIGURE 5.4.1.  Newton’s experiment to characterize fluid viscosity, 1713.

Shear rate can be interpreted as the local slope of the velocity profile at each point r. It is interesting to note that in this case, the maximum shear rate occurs at the wall, and the shear rate is zero at the center of the pipe. For shearthinning fluids such as polymer melts, the viscosity profile is such that the minimum is at the wall (maximum shear rate) and the maximum is at the center of the flow. In other words, even when dealing with a given polymer, viscosity is not constant as soon as the polymer melt is flowing into a pipe. By definition, shear stress is   , where η is viscosity. In Newton’s experiment, shear stress is simply the ratio between the force F and the surface area of the top plate. In the particular instance of a non-Newtonian viscosity material flowing in a pipe, the profile τ(r) shows a maximum at the channel wall and is zero at the center of the channel. Due to their macromolecular nature, polymers have a memory. For example, as soon as a load is removed, the macromolecules will relax and, over time, return to their natural, fully coiled state. The relaxation time is a viscoelastic characteristic that depends on molecular weight, molecular structure, and other factors.

FIGURE 5.4.2.  Coiled polymeric macromolecules with a high degree of entanglement.

FIGURE 5.4.3.  Influence of shear flow on macromolecular entanglement. Left: at rest, fully coiled macromolecule. Right: oriented macromolecule in the shear flow, with lower density of entanglement.

Typical Shear Viscosity Curve Figure 5.4.5 shows a typical shear viscosity curve as a function of shear rate, with four distinct regions: (1) The Newtonian plateau at low shear rate: the polymer viscosity, referred to as zero-shear viscosity, is constant because the structure is not affected by the slow deformation rate (compared to the relaxation time of the polymer melt). The value of the zero-shear viscosity depends on entanglement density and therefore on molecular weight. (2) The transition region: the structure starts to shear-thin as the deformation rates move closer to the relaxation time. Molecular structure and molecular weight distribution affect how broad and smooth the transition is. (3) The shear-thinning region occurs when the deformation rates are higher than the relaxation time. (4) Finally, there is a theoretical Newtonian plateau corresponding to a fully disentangled structure. This plateau can be measured in dilute polymer solutions, but very rarely for polymer melts. Therefore, it becomes irrelevant for extrusion applications and will be ignored for the rest of this chapter.

FIGURE 5.4.4.  Velocity, shear rate, viscosity and shear stress profiles for a non-Newtonian polymer melt flowing under a pressure gradient in a pipe.

Section 5.4.  Polymer Rheology

361

FIGURE 5.4.5.  Typical viscosity curve as a function of shear rate for polymer melt.

Effect of Temperature Temperature also has a significant effect on viscosity. As temperature increases, so does molecular mobility, resulting in an overall decrease in melt viscosity. The viscosity curve is also shifted to the right as temperature increases, as shown in Figure 5.4.6. Most polymer melts follow the time-temperature superposition (TTS) [3]: a shift in temperature corresponds to a shift in time scale. This is a key property of polymer melts because most linear viscoelastic parameters follow this superposition principle. In particular, the zero-shear viscosity at a given temperature T can be calculated from the viscosity at a defined reference temperature T0 by a shift factor aT, defined as: aT 

0 (T ) 0 (T0 )

When applicable, the TTS is one of the most powerful principles in melt rheology. For example, it is possible to build a master curve for shear viscosity as a function of

FIGURE 5.4.6.  Effect of temperature on viscosity.

shear rate at a given reference temperature by shifting the data obtained at several different temperatures. The TTS can be written as follows: • For steady-state measurements (e.g., capillary rheometry):

 ( , T )  aT  (  aT , T0 ) where η is the shear viscosity, γ is the shear rate at the wall, T is the temperature, T0 is the reference temperature, and aT is the TTS shift factor at temperature T. • For dynamic measurements (e.g., oscillatory rheometry):

 * ( , T )  aT  * (  aT , T0 ) where η* is the complex viscosity, ω is the angular frequency, T is the temperature, T0 is the reference temperature, and aT is the TTS shift factor at temperature T. There are several benefits from building the master curves. First, it results in an extension of the shear-rate range, which adds precision to shear-dependent viscosity models. It also makes it possible to predict viscoelastic parameters (for example, viscosity or complex viscosity, complex modulus, storage modulus, or loss modulus) at any given temperature within the range of shifted temperatures. A practical example is given in Figure 5.4.7, which illustrates how a TTS viscosity master curve is built from measurements performed at eight different temperatures [4]. The measurements were performed for an LDPE using a rotational rheometer with a cone-plate fixture and in dynamic mode. One known limitation of the rotational rheometer is the frequency range: the maximum frequency attainable is limited to 100 Hz (628 rad/s) by the instrument. In some instances, this is not sufficient to cover the extrusion process. Building a viscosity master curve by applying the TTS,

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Influence of Molecular Weight on Viscosity Figure 5.4.9 compares the complex viscosity master curves at T0 = 280℃ of two grades of LDPE with different molecular weights: • LDPE 1 is a low-density polyethylene with a molecular weight of about Mw = 150,000 g.mol–1 and 7.0 MFI. • LDPE 2 is a low-density polyethylene with a molecular weight of about Mw = 135,000 g.mol–1 and 16.0 MFI. The figure shows both the TTS-shifted (reduced) data as well as the resulting viscosity model curve (details on viscosity models are given in the next section). As expected, the higher the molecular weight, the higher will be the zero-shear viscosity: • η0 = 471.92 Pa⋅s for LDPE 1 (Mw = 150,000). • η0 = 223.34 Pa⋅s for LDPE 2 (Mw = 135,000).

FIGURE 5.4.7.  Example of complex viscosity as a function of frequency measured at eight different temperatures.

as shown in Figure 5.4.8, extends the maximum frequency beyond 20,000 rad.s–1. This range covers most extrusion processes.

Melt elasticity is also a function of molecular weight: Viscosity curve fitting gave the following relaxation times at T0 = 280°C: • LDPE 1: λ = 0.0181 s • LDPE 2: λ = 0.0114 s. This means that the higher-molecular-weight LDPE 1 exhibited higher melt elasticity, as expected. Influence of Molecular Structure on Viscosity

This section presents concrete examples measured in our laboratory that demonstrate some of the concepts and theories discussed in the previous paragraphs.

Both molecular structure and molecular weight distribution influence the shape of the viscosity curve, particularly the width and sharpness of the transition region. Figure 5.4.10 compares LDPE 2, which was introduced earlier, and a 19.0 MFI metallocene catalyzed polyethylene (mPE) with

FIGURE 5.4.8.  Complex viscosity master curve obtained by applying time-temperature superposition (TTS) at a reference temperature of 280°C.

FIGURE 5.4.9.  Comparison of complex viscosity master curves at T0 = 280°C for two LDPE resins: LDPE 1 (purple circular markers), a 7 MFI, 0.918 specific gravity grade, and LDPE 2 (gray diamondshaped markers), a 16 MFI, 0.918 specific gravity grade.

Some Measurement Examples

Section 5.4.  Polymer Rheology

FIGURE 5.4.10.  Comparison of the complex viscosity master curves of LDPE 2 (gray diamond-shaped markers), a 16 MFI, 0.918 specific gravity LDPE grade, and mPE, a 19 MFI, 0.918 specific gravity metallocene LLDPE.

a specific gravity of 0.918 g/cm3. The curves look very different: the mPE exhibits Newtonian behavior over a large portion of the frequency range, whereas LDPE 2 exhibits shear-thinning behavior over most of the frequency range. The transition between the Newtonian and the shear-thinning regions is also sharper and narrower for mPE. The characteristics of the mPE viscosity curves (wide Newtonian region and narrow transition) are a signature of both a narrow molecular-weight distribution and a linear molecular structure, as represented schematically in Figure 5.4.11. The LDPE 2 viscosity curve features an almost nonexistent Newtonian region in the measured frequency range and a broad transition region. These are signatures of a broad molecular-weight distribution and a high degree of longchain branching (LCB).

FIGURE 5.4.11.  Representation of the linear molecular structure of mPE (green) and the highly branched molecular structure of LDPE 2 (dark gray).

363

FIGURE 5.4.12. Temperature-dependent   viscosity data for an HDPE grade.

With shear rheology, it is not a trivial problem to discriminate the effects of molecular weight distribution and LCB. Complementary techniques such as transient extensional viscosity measurements can help identify the effects of LCB. Influence of Temperature on Viscosity Depending on the nature of the given polymer structure, temperature can have effects of various magnitudes on viscosity. This is clearly illustrated by the following graphs presented in Figure 5.4.12 (low effect of temperature) and 5.4.13 (high effect of temperature).

FIGURE 5.4.13. Temperature-dependent   viscosity data for a TPU grade.

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For these particular examples, the HDPE shows very little temperature dependency, with a small viscosity variation over a 90°C range. On the other hand, the particular TPU grade shows an order of magnitude viscosity change for a 20°C temperature variation. This trend is well-known for TPU in general. The processing implications are that this TPU grade will require very close attention to temperature control in the extrusion equipment. SHEAR VISCOSITY MODELS Shear viscosity models can be necessary for in-depth process analysis, including flow simulations. A large variety of viscosity models have been reported in the literature. Depending on the complexity of the material flow behavior, simpler shear-thinning models may be used, or more complex models that describe the Newtonian, transition, and shear-thinning regions may be required. To describe a particular extrusion process accurately, shear-rate-dependent and temperature-dependent models are needed. However, these two models can be separated from each other. Power Law The Power Law is adapted to behaviors that exhibit only shear-thinning over the whole shear-rate range. This can apply to rigid PVC or to highly loaded polymers, for example. The Power Law is the simplest non-Newtonian viscosity model with only two parameters and is written as follows:

 ( , T ) 

0  aT )

1n

  a  1   0 * T     

Note that the time-temperature superposition (TTS) shift factor aT for shear rate and temperature dependency has been added. In the Cross model: • T is the temperature, • η0 is the zero-shear or Newtonian viscosity, • τ* is the characteristic shear stress corresponding to the transition, • n is the pseudo-plastic (shear-thinning) index. Figures 5.4.15 and 5.4.16 show the viscosity modeling approach for a polystyrene sample. First, measured viscosity data sets at temperatures of 190°C, 220°C, and 250°C were used to build the TTS master curve of viscosity at the chosen reference temperature T0 = 220°C. A good superposition was found by applying the following shift factors: • a190 = 5.0 • a250 = 0.35 The obtained TTS master curve was then used in a curvefitting routine, as shown in Table 5.4.1. Finally, the resulting viscosity model at each temperature is plotted and compared with the measured data at each temperature, as shown in Figure 5.4.16.

 ( )  K  n1 where: • η is the viscosity in Pa⋅s, • γ ̇ is the shear rate in s–1, • K is the consistency in Pa⋅sn, • n is the pseudo-plastic index. The value of n represents the degree of shear-thinning and is between 0 and 1. The smaller the value of n, the more shear-thinning occurs. When n = 1, the material viscosity is not shear-dependent, but Newtonian. Figure 5.4.14 shows an example of rigid PVC viscosity showing clear power-law behavior. Figure 5.4.14 depicts an example of a power-law curve fit to rigid PVC viscosity data. The Cross Model In many instances, a power-law model is not enough, for example, because it does not take the Newtonian plateau into consideration. The Cross model is a well-known viscosity model that gives a good description of Newtonian, transition, and shear-thinning regions with only three parameters. The model is often used for flow simulation and polymer process modeling and can be written as follows:

FIGURE 5.4.14.  Shear viscosity data and power-law curve fit for rigid PVC measured in a capillary rheometer at 190°C.

Section 5.4.  Polymer Rheology

365

TABLE 5.4.1.  Cross Parameters from Curve Fitting. Cross Parameter

Value

η0 (Pa.s)

1289.3

τ^ (Pa) n T0 (K)

3.13E+04 0.23439 493.15

The Carreau-Yasuda Model Like the Cross model, the Carreau-Yasuda model is a shear viscosity model that describes the Newtonian plateau at low shear rates, the transition, and the shear-thinning region. This model uses one more parameter than the Cross model and can have a little more physical meaning:

 ( , T ) 0  aT [1  (  aT ) a ]

n1 a

where: • η0 is the Newtonian plateau (or zero-shear viscosity) in Pa.s • λ is the relaxation time at which the transition between the Newtonian and shear-thinning regions occurs (in s) • a is the Yasuda parameter that defines the sharpness of the transition zone. Typically, 0 ≤ a ≤ 2, and for the original Carreau model, a = 2. • n is the pseudo-plastic (shear-thinning) index (with a value between 0 and 1).

FIGURE 5.4.16.  Measured viscosity data and Cross model at each temperature (190°C, 210°C, and 230°C) for a polystyrene sample.

The two materials (LDPE 2 and mPE) that were compared earlier in Figur 5.4.10 were very accurately fitted using the Carreau-Yasuda model (solid lines). The parameter values obtained are shown in Table 5.4.2. TEMPERATURE-DEPENDENT MODELS Shear viscosity and temperature dependency models can be independent of each other, and many models are also used to describe how temperature affects viscosity. The most widely used models are the Arrhenius and the WilliamsLandel-Ferry (WLF) models. The Arrhenius Empirical Model The Arrhenius model is a single-parameter model that typically works well for temperature ranges higher than TABLE 5.4.2.  Carreau-Yasuda Parameters for mPE and LDPE 2. mPE Param.

FIGURE 5.4.15.  Viscosity master curve (TTS applied to measured data) at the reference temperature of 220°C and the Cross viscosity model for a grade of polystyrene.

LDPE 2

Value

Units

Param.

η0

110.50

Pa.s

λ a n

5.2 × 10–4 0.818 0.3404 280

s – – °C

T0

Value

Units

η0

223.34

Pa.s

λ a n

1.14 × 10–2 0.365 0.3389 280

s – – °C

T0

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Tg + 100°C (where Tg is the glass transition temperature). The model can be expressed as: E aT  exp  a  R 

 1 1       T T0  

where: • aT is the time-temperature superposition shift factor, • Ea is the activation energy (J.mol–1), • R is the ideal gas constant = 8.3144621 J.mol–1.K–1, • T0 is the reference temperature (K). The model parameters can be determined by curvilinear regression by plotting ln(aT) as a function of 1/T, as shown in Figure 5.4.17. Figure 5.4.17 shows the regression results obtained for the shift factors determined in Figure 5.4.8 for LDPE 1. The regression coefficient indicates a very high-quality data fit. The value of the activation energy is determined by the value of the slope. The Williams-Landel-Ferry (WLF) Model The WLF model is better suited than the Arrhenius model for lower temperature ranges, typically Tg < T < Tg + 100°C. This model uses two parameters, C1 and C2, that can be determined by curve fitting:  C (T  T0 )  aT  exp  1   C2  (T  T0 ) 

SOME IMPLICATIONS OF SHEAR RHEOLOGY FOR FILM EXTRUSION Implications of Relatively High Polymer Viscosity The relative high viscosity of polymers has direct and critical consequences for film extrusion processes, some being advantages and some being challenges for equipment manufacturers and processors. High Pressure High viscosity means high pressure in most cases. It is not rare to see melt pressure of approximately 10 MPa at the entrance of a cast film die and head pressure of 30 MPa at the extruder barrel flange. Polymer viscosity is typically higher for blown film due to the requirement for high melt strength for bubble stability. Melt pressures can be much higher for blown film than for cast film for this reason. In any case, equipment must be designed accordingly and be mechanically capable of handling these pressures. Viscous Dissipation Deforming any material generates internal friction, which results in heat. This is true when forging a piece of metal with a hammer on an anvil. It is also true for polymer processing, especially at high rates of deformation. The heat generation due to shear flow is often referred to as “viscous dissipation” or “shear heating”. The fact is that heat is created when polymer melts are flowing, and in case of shear flow (dominant in extrusion), the heat generation is proportional to the viscosity and the shear rate squared: W  n 2 Noticeably, shear rate has the greatest influence on viscous dissipation, with a quadratic dependency. Viscosity has also a strong influence, with its linear effect on viscous dissipation. The heat generated by polymer flow is located in the high-shear regions of the flow, and due to the poor ther-

FIGURE 5.4.17.  Arrhenius fit for LDPE 1.

FIGURE 5.4.18.  Temperature elevation in a pipe flow by viscous dissipation.

Section 5.4.  Polymer Rheology

mal conductivity of polymer melts (typically k = 0.2–0.3 W.m–1⋅K–1), this results in localized temperature elevation. The temperature increase can affect flow performance because the polymer stream will have a non-uniform temperature profile. In cases of thermally sensitive polymers, the temperature increase can lead to degradation of the material. Practically, viscous dissipation will happen in many highshear locations of the process:

Ca  T ( z )  T0 

kL

 CVR

5 V 2 6 k

2



aL VR 2

,

z    48 1  exp   5 Ca L    

• the conduction in the thickness of the flow, k ∆T R2

The schematic in Figure 5.4.18 shows several thermal flow regimes [5]: • Adiabatic regime: at the beginning of flow, viscous dissipation is relatively low. The heat flux dissipated by conduction to the wall is therefore negligible, and the thermal regime behaves close to adiabatic (no heat transfer). • Transient regime: further downstream, viscous dissipation gains some amplitude, and now heat transfer by conduction to the wall and center of the flow becomes more significant. • Equilibrium: eventually, viscous dissipation reaches a limit when the amount of heat created balances heat transfer by conduction. This then corresponds to a steady-state situation. In the case of extrusion and film processing, the latter regime is almost never reached. The transient or adiabatic regimes are generally more likely to describe a specific flow. Relatively simple equations can be used to evaluate the temperature elevation in a given pipe flow. For the adiabatic regime, the temperature increase follows the simple formula: T 

p C

In this case, the temperature increase is independent of viscosity or pipe diameter. Considering typical values for polymer melts: ρC = 2 × 106 J⋅m–3℃–1. A pressure drop of 10 MPa (≈ 100 bars), which is the order of magnitude of pressure drop in an extrusion die for example, results in an increase in average temperature of ∆T = 5℃. In the case of a transient regime (most cases), when the flow-channel wall temperature is controlled to T0, some of the viscous dissipation heat will be transferred by conduction to the wall. Temperature estimation is a bit more complex in this case: 5 V 2 6 k

This equation introduces the dimensionless number of Cameron, Ca:

This number is a ratio between:

• Extruder screws • Adapters and melt pipes • Static mixers • Gear pumps • Screen packs • Extrusion dies.

T ( z )  T0 

367

  48 kz  1  exp   2    5  CVR 

• and the convection in the direction of flow: T L

 CV

Going back to the notion of the thermal regime of the flow, the Ca number can help distinguish between the three regimes established earlier: When Ca is small (Ca < 10–2), heat transfer by conduction to the wall is negligible compared to axial convection. This is the adiabatic regime. In this case, the previous equation simplifies to: T ( z )  T0 

8V 2

 CR 2

z

When replacing the average velocity by the expression for a pipe flow (Poiseuille): V

1 p 2 R 8 L

the result is indeed: T 

p C

When Ca > 1, convection in the axial direction z is small compared to transverse conduction. This is the characteristic of the equilibrium regime. In this case: 5 V2 T ( z )  T0    6 k The transient regime encompasses 10–2 < Ca < 1. Figure 5.4.19 results from the calculated average temperature for the following pipe flow scenario: • Melt pipe length up to 20 meters (to see all three regimes) • Melt pipe diameter of 20 mm • Polymer viscosity η = 2,000 Pa·s • Polymer melt density ρ = 740 kg·m–3 • Polymer melt thermal conductivity k = 0.2 W.°C–1·m–1 • Heat capacity C= 1,500 J.kg–1.°C–1 • Diffusivity a = k/ρC = 1.80 × 10–7 m2·s–1.

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sembled, they will not mix. The negative side is that polymer melts do not mix well because turbulence is preferred for mixing in general. This is why dispersing additives or colors or blending two different polymers can be challenging and requires specific equipment and processes. One criterion to determine whether a flow is laminar or turbulent is to calculate the dimensionless Reynolds number. The Reynolds number compares inertial to viscous forces and can be expressed as: Re 

FIGURE 5.4.19.   Calculated temperature elevation due to viscous dissipation in a pipe flow as a function of flow length.

The calculation above is for a longer flow length than the typical extrusion system. This was done for illustration purposes to show all three thermal regimes. In most extrusion cases, the adiabatic regime or the initial part of the transient regime applies. For more complex flow geometries and more complex rheology, it is possible to solve the coupled flow-thermal problem with more advanced computation techniques, such as 3D computational fluid dynamics (CFD). Figure 5.4.20 shows an example of a flow-thermal solution, which is a temperature solution for a complex polymer rheology (nonisothermal, non-Newtonian behavior) and a complex flow geometry (a static mixer) [6].

VD 

where V is a characteristic velocity (for example, the average velocity at a given location in the flow), and D is a characteristic dimension (for example, D can be the diameter of a pipe flow, or the gap in a parallel plate flow). For pipe flow, the critical value at which the flow becomes turbulent is Re = 2,200. This is illustrated by Figure 5.4.21. In most extrusion processes, at any given location in the flow, Re is orders of magnitude lower than the critical value, and therefore flows are always laminar. Even in the case of higher-speed processes such as injection molding, typically Re < 1. The schematic in Figure 5.4.22 shows some typical areas of a flat-film die flow channel, and Table 5.4.3 shows the calculated Re values for the following extrusion scenario: • Die width = 1 m • Polymer (Newtonian) viscosity = 100 Pa⋅s • Polymer density = 1,000 kg.m–3 • Flow rate = 1,000 kg.h–1. All the Reynolds numbers calculated for this particular scenario are orders of magnitude lower than the critical value of 2,200 at which flow becomes turbulent.

Laminar Flow High polymer viscosity combined with relatively low density means that polymer flows in film extrusion processes are laminar. The implications can be positive or negative, depending on what is being targeted. On the positive side, the laminar flow is great for coextrusion: once layers are as-

FIGURE 5.4.20.  3D CFD calculation of a static mixer showing the temperature solution for viscous dissipation.

FIGURE 5.4.21.  Laminar vs. turbulent flow and corresponding Reynolds number.

Section 5.4.  Polymer Rheology

369

FIGURE 5.4.22.  Flat-film die flow channel and characteristic areas.

Implications of Shear Thinning Shear-thinning has many implications for film processing. It can affect velocity profiles, melt pressure, and flow distribution uniformity in extrusion dies. Some examples are described below. Velocity Profiles Shear-thinning affects the shape of the velocity profile in a flow channel. This point is illustrated by Figure 5.4.23, which shows the calculated velocity profile in a Poiseuille pipe flow for power-law shear-thinning fluid. As the figure shows, the velocity profile is a perfect parabola for a Newtonian material (power-law index = 1). As the power-law index decreases (more shear thinning), the parabola becomes flatter at the center of the flow channel. This shape change for the velocity profile is illustrated for a circular cross section (pipe), but is true for any flow-channel shape, included parallel-plate or slit geometries. Head Pressure vs. Extrusion Output A direct consequence of shear-thinning polymer melts is the nonlinear relationship between head pressure or extruder torque for a given film extrusion process and extrusion output. This relationship is linear for Newtonian material: if you double the output, you also double the head pressure and the extruder torque. Thankfully, in the case of shear-thinning materials, the increase of head pressure or torque is less than

FIGURE 5.4.23.  Velocity profiles for power-law fluids with pseudoplastic indices from 1 (Newtonian) to 0 (viscoelastic flow).

double when doubling the output. This enables fast extrusion rates at reasonable pressures (Figure 5.4.24). Flow Distribution in a Die Flow Channel Generally speaking, shear thinning is the most critical parameter affecting flow distribution performance from an extrusion die. Inherently, a polymer flowing in a complex flow-channel geometry such as a cast- or blown-film die is exposed to a broad distribution of shear rates. Figure 5.4.25 shows the typical areas of a cast-film die flow channel, with typical shear-rate values in the caption. Some larger sections of the flow geometry, like a manifold channel for example, will result in lower shear rates on the one hand. On the other hand, tight flow sections such as the preland or lip-land gaps will lead to higher shear rates.

TABLE 5.4.3.  Reynolds Numbers in Different Areas of the Die Flow Channel. Area

Characteristic Dimension (m) 10–2

Velocity (m.s–1)

4.42 × 10–2

(1) Entrance channel

2.00 ×

(2) Manifold channel center (3) Manifold channel 1/3 to end (4) Preland

1.50 × 10–2

1.96 × 10–1

2.95 × 10–2

1.50 × 10–2

6.48 × 10–2

9.73 × 10–3

1.50 × 10–2 6.00 × 10–2

1.85 × 10–1 4.63 × 10–1

2.78 × 10–3 2.78 × 10–3

(5) Lip land

2.21 ×

Re

10–2

FIGURE 5.4.24.  Schematic of head pressure or extruder torque as a function of output for different types of shear-thinning behaviors.

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FIGURE 5.4.25.  Schematic of a cast-film die flow channel. The following areas associated with their respective typical shear-rate ranges are given in Table 5.4.4.

The magnitude of the difference between the viscosity at low shear rate and the viscosity at high shear rate is critical for the flow distribution. Considering the distribution of shear rates in the flow channel, the shape of the whole viscosity curve plays a significant role in the flow distribution performance of the channel. This is, in essence, how much shear thinning the particular polymer melt exhibits. Figure 5.4.26 shows an example of different shear-thinning behaviors for several polyethylene grades. To illustrate this important notion, consider a cast-film die flow channel that was designed and optimized for mPE, a metallocene polyethylene (shown in green in Figure 5.4.28). The flow distribution is represented by the schematic in Figure 5.4.27. Now, the same die is used to extrude LDPE 2 material. Because the material is much more shear-thinning and has higher zero-shear viscosity than mPE (Figure 5.4.28), it will tend to flow more easily than mPE at higher shear rates (the preland). In addition, LDPE 2 will not flow easily at low shear rates (the end of the manifold channel) compared to mPE. In the end, LDPE 2 will flow preferentially towards the center of the channel. The flow distribution for LDPE 2 is represented by the schematic in Figure 5.4.29. Although the previous discussion focused on trends in flow patterns based on shear rheology, more advanced flow simulations can be carried out with modern 3D CFD methods, as discussed earlier. Such flow simulations can take into account intricate flow geometries, complex boundary conditions, and non-isothermal, non-Newtonian rheology.

FIGURE 5.4.26.   Viscosity curves for various grades of polyethylene. Representation of the typical shear-rate range for film extrusion.

Typical results include pressure drop, temperature, velocity, shear rate, and shear stress. In addition, when using a particle tracking technique, evaluating complex residence-time distributions becomes possible, with some limitations. Figure 5.4.30 shows the flow distribution in a flat-film die flow channel as an example [7]. VISCOELASTIC BEHAVIOR OF POLYMER MELTS Due to the macromolecular nature of polymers, the melt phase has an inherently elastic nature that can be observed as a delayed response to a stress or deformation. This elastic component results in many known polymer processing effects such as swelling at the die exit and layer rearrangement in coextrusion flows, to name a few. Viscoelasticity in polymer melts can be very complex in nature, but through rheological measurements, it is possible

TABLE 5.4.4.  Typical Shear-rate Values by Area of the Flow Channel. Description

Shear-rate Range (1/s)

1

Entrance channel

50–200

2 3 4 5 6

Manifold channel near center line Manifold channel near mid-width Manifold channel near ends Preland Plenum Lip land

25–100 10–50 1–10 25–300 10–150 50–1000

Label

7

FIGURE 5.4.27.  Representation of the melt curtain coming out of the die uniformly for mPE because the die flow channel was optimized for that material.

Section 5.4.  Polymer Rheology

371

FIGURE 5.4.28.  Shear viscosity curves for LDPE 2 and mPE showing the difference in shear-thinning.

to evaluate and quantify the viscoelastic nature of a given polymer melt. Viscoelastic measurements are detailed in the “Measurement Techniques” section. To quantify the importance of viscoelasticity in a given area of the process, a dimensional analysis approach is possible with the Deborah number, which compares the polymer relaxation time λ to a given deformation time t, characteristic of the process: De 

 t

This method can help to determine trends or orders of magnitude in a comparative fashion. When De >> 1, the polymer melt is highly elastic. On the contrary, when De 1, the behavior is viscous-dominant, and a fortiori, when tan δ < 1, the behavior is mostly elastic Figure 5.4.48 shows an example of tan δ as a function of angular frequency and temperature for LDPE 2. Overall, the material elasticity is relatively low and is directly measured for lower temperatures only (from 120–180°C). Extensional Viscosity Measurements Although shear flow is the most influential factor in extrusion applications, extensional flows can also be significant. This is especially the case in cast film in the air gap between the die lips and the contact point of the chill roll. In blown film, extensional rheology is also extremely important to understand the flow between the die exit and the frostline, which sees extensional flow in both machine and transverse directions. In both processes, critical extensional flow of the melt occurs outside the die, providing the film with its final properties.

Section 5.4.  Polymer Rheology

379

Melt Strength Measurement Although melt strength measurements do not result in viscoelastic functions, these measurements can give a good indication of the extensional behavior of polymer melts and can be practically used to characterize and compare melts of various natures in a relatively simple test environment. Some variations of the test are available, but essentially it resembles an instrumented fiber-spinning process, using a capillary rheometer for melt preparation and extrusion. The measurement principle is illustrated by the simple sketch in Figure 5.4.49. Examples of melt strength measurements are shown in Figure 5.4.50. The latter figure shows, for example, that LDPE melts have the poorest elongation, but the highest melt strength. On the other hand, metallocene-based LLDPE has excellent elongation, but poor melt strength. Transient Extensional Viscosity Measurements

FIGURE 5.4.48.  tan(δ) as a function of angular frequency and temperature for LDPE 2.

To characterize and compare the extensional behaviors of polymer melts, two measurement techniques described below are commonly used: melt strength measurement, and transient extensional viscosity measurement.

Transient extensional viscosity measurement is an important technique that can contribute valuable information on molecular structure as well as process understanding. Shear rheology, especially viscoelastic measurements, can deliver much valuable information and indications about the macromolecular structure and processability of a given polymer melt. However, it is sometimes difficult to discriminate clearly the influences of molecular weight distribution and long-chain branching. Transient extensional viscosity measurements can provide this information in an unequivocal fashion. The measurements are performed with a special fixture that can be attached to a rotational rheometer. Again,

FIGURE 5.4.49.  Melt strength measurement principle.

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FIGURE 5.4.51.  Principle of transient extensional viscosity measurement with the SER fixture.

FIGURE 5.4.50.  Example of melt strength measurements for various polymer melts.

several variants are available commercially, but here the Sentnamat Extensional Rheometer (SER) [16] will be discussed because it is the pioneering system for these types of measurements. The principle is relatively simple: the fixture is placed in a convection oven like regular shear rheology attachments. The sample, typically in the form of a piece of film of known dimensions (length, thickness, and width) is placed and fixed on a set of stainless-steel drums (Figures 5.4.51 and 5.4.52). The measurement consists of first melting the film sample and then stretching the film by opposite rotation of the drums, creating a uniform, uniaxial elongation. While the film melt is stretching, the necessary torque is measured, and the drum velocity and torque are converted into strain rate and extensional stress respectively. Because the test continues all the way up to sample failure, only one strain test at a time can be performed. The technique can produce a wealth of information and give useful indication about molecular structure. For example, Figure 5.4.53 shows transient extensional viscosity as a function of time for LDPE 2 at 120°C for Hencky strain rates ranging from 0.1 to 25 s–1. The black dotted line is the linear viscoelastic envelope, or LVE. This reference or baseline is calculated from shear rheology using the Trouton ratio Tr = 3 according to continuum mechanics for Newtonian, isotropic, incompressible fluids [17]:

for each series of measurements corresponds to the ultimate strain at the moment of sample failure. Table 5.4.5 can be used to quantify the extensional behavior of particular LDPE grades. It shows the extent of strain hardening because the calculated Trouton ratios are one order of magnitude higher than the theoretical value of three for Newtonian materials. The behavior of mPE was the exact opposite, as shown in Figure 5.4.54. The material exhibited a lack of strain hardening and stretched to the full capacity of the SER-3 fixture without failing. The measurement was extremely difficult due to considerable sagging at the beginning of the test. The point is that the linear nature of the molecular structure can

 E (t )  3   (t )  where  E (t ) is the transient extensional viscosity as a function of time and η+(t) is the transient shear viscosity as a function of time. The measured data show a strong departure from the LVE at all tested Hencky strain rates. This is a direct quantification of melt strength, and this extra growth in tensile strength is often called “strain hardening”. The strain-hardening property seen here is a direct consequence of LCB present in the molecular structure of LDPE 2. The last measured point

FIGURE 5.4.52.  Example of SER attachment in an Anton Paar MCR 502 rheometer equipped with CTD 450 convection oven (not shown in the picture).

Section 5.4.  Polymer Rheology

381

TABLE 5.4.5.  Quantifying the Extensional Behavior of LDPE Grades. Ultimate or Peak Hencky Strain (dimensionless)

Ultimate or Peak Extensional Viscosity (Pa.s)

Ultimate or Peak Trouton Ratio (dimensionless)

0.1

4.45

269,980

31.9

0.3 1 3 10

4.54 4.54 4.53 4.60 4.25

335,380 311280 225,240 119,990 66,500

45.9 53.2 50.4 38.9 31.1

Hencky Strain Rate (1/s)

25

be unequivocally identified and is characterized by the absence of strain hardening. In fact, what was being measured was tensile stress growth below the LVE. REFERENCES AND ADDITIONAL RESOURCES [1] Cloeren, P., Proceedings, 2015 AMI Stretch and Shrink Film USA Conference, Applied Market Information LLC, New Orleans. [2] Shenoy, A.V. and Saini, D.R., Thermoplastic Melt Rheology and Processing, Marcel Dekker, New York, 1996, pp. 54–55. [3] Dealy, J. M., Larson, R. G., Structure and Rheology of Molten Polymers, Hanser, Munich, 2006, pp.120–126. [4] Catherine, O., Proceedings, TAPPI PLACE 2016 Conference, TAPPI PRESS, Fort Worth TX. [5] Agassant, J. F., Avenas, P., Sergent, J. P., Vergnes B., Vincent, M., Mise en Forme des Polymères—4ème Édition, Lavoisier, Paris, 2014, pp. 212–225. [6] Catherine, O., Proceedings, SPE EUROTEC 2013 Conference, Society of Plastics Engineers, Lyon, France, pp. 254–260.

FIGURE 5.4.53.  Tensile stress growth curves for LDPE 2 at 120°C, measured with the SER-3.

[7] Catherine, O., Proceedings, SPE ANTEC 2015 Conference, Society of Plastics Engineers, Orlando, FL. [8] Dooley, J., “Viscoelastic Flow Effects in Multilayer Polymer Coextrusion”, Ph.D. Thesis Technische Universiteit Eindhoven, The Netherlands, 2002. [9] Schrenk, W. J., Bradley, N. L., and Alfrey, T., Polym. Eng. Sci., 18, 620 (1978). [10] Zatloukal, M., Kopytko, W., Vlcek, J., and Sáha, P., Proceedings, SPE ANTEC 2005 Conference, p. 101. [11] Catherine, O., Proceedings, TAPPI PLACE 2013 Conference, TAPPI PRESS, Dresden. [12] Rabinowitsch, B. Z., Physik. Chem., A145, 1 (1929). [13] Bagley, E. B., J. Appl. Phys., 28, 264 (1957). [14] Cox, W. P. and Merz, E. M., J. Polym. Sci., 28, 619 (1958). [15] Mezger, T. G., Applied Rheology, Anton Paar, Austria, 2017, pp. 89–122. [16] Sentmanat, M. L., Rheol. Acta, Vol. 43, p. 657 (2004). [17] Macosko, C. W., Rheology—Principles, Measurements, and Applications, Wiley-VCH, 1994, pp. 79–80.

FIGURE 5.4.54.  Transient extensional viscosity measurements for mPE at 120°C, measured with the SER-3.

Chapter 5—Section 5

Coating and Laminating Technology GIANCARLO CAIMMI, Nordmeccanica Group

INTRODUCTION In general, “adhesive lamination” covers a significant number of technologies used in flexible substrate lamination to produce complex structures made of several layers of different materials. Flexible substrate definition covers those webs, in most cases produced and handled in rolls, which have the characteristic of not being rigid. These webs can consist of a variety of natural or synthetic polymer films, foams, foil, or paper. Lamination for flexible substrates is a technology that has evolved in parallel with rotary presses. Lamination and print share many common aspects, not just on the web handling side, but with regard to coating technology as well. The vast majority of coating technologies have been developed around the same design as certain printing technologies: rotogravure and flexo for low-viscosity coatings, and offset for high-viscosity coatings. The coating and laminating industry was born through the use of printing technologies to “calibrate” the amount of a chemical compound layered onto a substrate. Substrates themselves are undergoing significant evolution as demanded by their final applications. In the past few decades, the range of substrates used in coating and laminating has grown significantly to include paper, cellophane, polymers, foil, foams, and other materials. As well, the formulation and properties of these materials have evolved. The most popular application for lamination in the flexible packaging industry, based on the number of installations, is print protection. This is the primary reason to laminate and is used in applications ranging from the simplest packaging to the most complex multi-layer compounds for a variety of applications in industrial products, pharmaceuticals, and food packaging. Print protection consists of trapping the printed layer between two substrates. The first layer contains the print, and the second, a transparent film, is applied on top of the printed layer to protect it from mechanical abuse such as scratches, as well as environmental (UV) and chemical exposure. By properly combining the specific characteristics of alternative webs, lamination has evolved into a technology

that is suitable for producing multi-layer structures featuring the combined characteristics of each substrate involved: light barrier, gas barrier, mechanical strength, puncture resistance, chemical resistance, or whatever. In typical flexible substrate laminations, a structure is generated that performs, through the properties of each layer, a specific function. In food applications, the goal is to protect the food to maximize shelf life. For industrial applications, a variety of properties such as thermal shielding, impact protection, light reflection, and protection against counterfeiting may be required depending on the end application. This section will present the various facets of lamination by discussing the most used technologies and the technical aspects related to each. PRELIMINARY ASPECTS AND DEFINITIONS Before lamination comes coating. Coating on flexible substrates involves deposition of an adhesive onto a substrate that is not rigid. As briefly discussed in the previous section, coating is closely related to printing, and the same goes for the adhesive. Adhesives are, for most applications, very similar to inks. In printing technology, the target is to layer pigments onto a substrate. In lamination, the objective is to layer a resin onto a substrate. Adhesive technology and formulations will be discussed later in full detail; here, the focus is on how resin is handled and deposited in a proper amount onto a substrate. In printing, there are two main families of inks: (1) Low-viscosity (2) High-viscosity. In low-viscosity inks, the pigments are dispersed or dissolved into a vehicle. The vehicle is a liquid that is temporarily used to lower the viscosity of the ink pigments, making it possible for the pigments to be handled and dosed on a typical low-viscosity ink printing press (flexographic or rotogravure). The vehicle (specifically water or a combination of solvents) is then removed by evaporation in an oven. It is the same for the adhesive; the pigment is simply replaced by a resin. 383

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In high-viscosity inks, the formulations are very high in solids content, with very little vehicle to be evaporated. These inks have the characteristics of being very thick and difficult to dose. The task is achieved on offset presses, where a multi-roller printing head enables proper dosing of the ink. The same applies to adhesives. A high-viscosity adhesive consists of 100% solids or 100% resin, with no vehicle present. The viscosity is very high, and a multi-roller coating head is required to calibrate the coating amount properly. The coated substrate (primary material) and the laminated substrate (secondary material) are finally combined at the lamination nip. The lamination nip is a motorized, temperature-controlled pressure nip where adhesion between the two substrates is promoted by pressing the two webs together, with adhesive in the middle, through a supporting chromed roller and a rubber-covered pressure roller. The essence of this process is dosing a specific amount of glue, the quantity being important for both economic and technical reasons. It may consequently seem elementary to define the word “deposition” and figure out technical solutions. Instead, it took centuries for the technology to evolve. A quick flashback to the early days can focus on the two main technology streams that required some sort of “deposition”: textile manufacturing and the paper industry. In textiles, the concern was mainly coating oils or waxes for waterproofing oilskin. In the paper industry, it was substantially part of the evolution of printing. The target was to print or coat a roll of paper at much greater speed than the initial design that involved sheet printing. Initially, the substances used were 100% solids: oil or wax. These were used for an array of applications, from adhesion to waterproofing. Then, with the development of “vehicle” technology, or the ability to dilute or disperse a resin into a liquid, the technology expanded into other applications. Coating technology was born when a substrate was soaked in an oil or in melted wax. The need to control the amount of coating influenced the rest of coating evolution, which basically advanced side to side with the evolution of rotary presses. New substrates, including cellophane and synthetic polymers, enabled the final expansion of the technology and its end-use applications. The process of coating can be simplified into a few steps: handling a layer of a substrate; presenting it into a coating station; applying a substance onto the substrate; and placing a calibrated amount of coating on it. Coating technology is influenced by the nature of the substance to be coated. It is essentially a function of viscosity at the coating head. A variety of alternative designs enable proper handling, the primary target being to properly dose the applied quantity. Once applying a substance onto a substrate was mastered, the evolution into “lamination” was rather simple. Lamination of flexible substrates is the technology that makes it possible to glue together layers of different substrates with the aim to complete a structure that will feature the combined characteristics of all the substrates involved. The process can be represented as the application of an adhesive onto a “primary” substrate and/or the lamination of this substrate, in a

pressure nip, to a “secondary” substrate, and then repeating until the number of layers required is present. Coating Technologies This section will describe the technologies used to dose the quantity of adhesive that is layered onto a substrate. Dosing introduces one of the most important variables: coat weight. Contrary to printing, the target here is not to achieve a specific color density; it is the amount of adhesive applied that matters. The amount of adhesive influences some specific technical factors in lamination, including bond strength. Bond strength can be defined as the ability of an adhesive to bind together two substrates. Coat weight has a definite influence on bond strength. Nevertheless, too high a coat weight can have a negative influence on optics. Optical properties are a characteristic of lamination that defines the quality and the influence of a laminating process involving at least one transparent substrate, as determined by visual analysis. The target is to achieve a crystal-clear visual result. Any interference, such as an excessive amount of adhesive or a nonuniformly layered adhesive, will negatively influence optics. Coat weight can be expressed in pounds per ream (PPR) or grams per square meter. Coat weight is adjusted as a function of the specific coating-head design and the percentage of solids. This last variable defines the percentage of solids (resin) and vehicle in a low-viscosity adhesive compound; it is particular to a specific formulation, but provides some flexibility. Therefore, by adjusting the percentage of solids, within certain limits, it is possible to influence coat weight. Low-viscosity Adhesives This category of adhesives involves compounds formulated around a vehicle and the adhesive (the resin). Lowviscosity adhesives are liquid at room temperature and at the time of coating. The coating technologies are very similar to the most important printing technologies. A list of the most significant coating technologies to be considered for this category of adhesives includes: Rotogravure coating, which involves a coating station similar to that on a rotogravure printing press, as schematically represented in Figure 5.5.1. With this configuration, the adhesive is contained in two trays: a smaller one in contact with the rotating engraved cylinder, and a larger one used to contain splash and overflows. A positive oscillating doctor blade removes excess adhesive and limits transfer to the actual cell volume. Coat weight is adjusted by the engraving specification of the roller and by fine-tuning the percentage of solids. The adhesive is circulated by means of a pump between an adhesive reservoir and the coating station. Offset Gravure An offset gravure station, also known as “semi-flexo”, is configured with two rollers. The actual coating roller is a

Section 5.5.  Coating and Laminating Technology

385

FIGURE 5.5.1.  Rotogravure coating.

steel, chromed roller that is motorized and turns in the web direction at a surface speed equal to the web speed. The second roller, which is rubber-coated, is also independently motorized and can turn clockwise or counterclockwise and at a surface speed equal, lower, or higher than the coating roll. By adjusting these variables, the coat weight can be calibrated. This is one of the main characteristics of offset gravure: the station offers a very flexible way to vary the coat weight. It features the ability to coat the adhesive as a thin

layer, avoiding the dot pattern usually produced by a rotogravure station. Flexo Coating A flexo coating station has a design that enables a high degree of blending of the adhesive. It is similar to a press station. A flexo coating station for adhesive lamination can be configured with a three-, four-, or five-roller setup. In adhesive technology, a closed-chamber doctor blade is not a good

FIGURE 5.5.2.  Offset gravure coating.

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FIGURE 5.5.3.  Solventless coating station.

solution. Increasing the number of rollers increases the complexity of the mechanical design, but definitely improves the ability to calibrate the adhesive layer. High-viscosity Adhesives This category of adhesives has high viscosity at room temperature, and in the case of solventless adhesives, even at the coating stage. Solventless Coating Station A solventless coating-head design can properly dose adhesives that are 100% solids or have high viscosity at the time of coating. The design is based on multiple rollers turning at an increased surface speed to provide proper dosing. As the adhesive passes through rollers C, B and A, each turning at a higher speed, it will be dosed as a function of the rate of differential speed set between the two motors, as indicated in Figure 5.5.3. Lamination for flexible substrates in general is performed on materials rewound in rolls. Unwinds and rewinds are configured like those on a printing press. The drying oven in most cases uses hot air, but UV, electron beam (EB), and microwave technologies are also in use.

In this process design, one of the substrates must be porous to allow water to evaporate through it. A part of the vehicle is also absorbed by one of the substrates, which consequently must be hydrophilic, paper being a perfect example. A schematic of the wet lamination process is presented in Figure 5.5.5. Coating heads used in wet lamination are primarily rotogravure and offset gravure. Substrates for Wet Lamination As mentioned above, paper is involved in the vast majority of wet-lamination industrial applications. Foil or film, mostly met-PET, is often the second web. Applications for Wet Lamination Applications such as the inside wrap for cigarettes packs are made with a wet lamination of paper to foil, whereas the main pack is a wet lamination of paper to film. Paper-to-foil wet lamination is also used for chewing gum wrappers and other odor-sensitive applications.

Wet Lamination Technology This process implies the use of low-viscosity adhesives. The vehicle is water. Wet lamination is essentially a process in which two substrates are combined in a pressure nip while the adhesive layer coated on one of the two webs is still wet. The combined compound then runs through a drying oven to evaporate the vehicle. A flowchart of this process is presented in Figure 5.5.4.

FIGURE 5.5.4.  Wet-bond lamination.

Section 5.5.  Coating and Laminating Technology

387

FIGURE 5.5.5.  Wet lamination technology.

Dry Lamination Technology Dry lamination uses low-viscosity adhesives. The vehicle can be either water or a combination of solvents. The vehicle must be removed from the coated compound in a drying oven before the lamination nip, as shown in Figure 5.5.6. The machine layout is presented in Figure 5.5.7. Dry-bond lamination is the most common configuration in adhesive laminations. Coating heads in use depend on the actual adhesive and the coat weight. Rotogravure technology has the largest number of installations, followed by offset gravure and flexo. A secondary branch of dry-bond lamination is called thermal lamination. This technology saw some industry interest in the 1950s and 1960s. In thermal lamination, the final act of combining two substrates occurs at the converter shop using one web made or printed in-house and a second web that is pre-coated. The technology simplifies the lamination step and can be implemented on a very simple machine equipped exclusively with a thermally conditioned roller that is used to activate the adhesive pre-coated onto one of the two webs. The technology faded as higher technical performance requirements such as thermal properties were required from the laminated structure, and once the new generation of compact laminators enabled converters to bring the entire process in-house, maximizing the profit related to the conversion process.

proved due to new digital technologies in motors and drives. This has enabled thinner gauges, which have enabled converters to tailor customized solutions targeting energy saving and light-weighting. Applications for Dry Lamination This is one of the most used technologies in flexible substrate lamination. The list of applications covers almost the entire range of flexible packaging, pharmaceuticals, security, and the vast majority of industrial products. Wax-hot Melt Lamination Technology Lamination performed using wax and hot-melt adhesives belongs to the category of thermoplastics. This category involves adhesives that are completely solid at room temperature, meaning that no vehicle needs to be removed. The process is based on melting the adhesive, coating the adhesive onto a substrate, laminating the primary web to a secondary web, and then cooling the laminated compound on a chill roller to bring the adhesive back to a solid state and activate the adhesion. A flowchart is shown in Figure 5.5.8.

Substrates for Dry-bond Lamination Dry-bond lamination is the most widely used conversion configuration in lamination and has seen applications on virtually every material. Plastic films, foil, paper, foams, and other materials are converted daily on thousands of laminators worldwide. The most interesting developments in substrates have been related to thickness. The ability to control web handling at very high levels of accuracy has greatly im-

FIGURE 5.5.6.  Dry-bond lamination.

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FIGURE 5.5.7.  Dry-bond lamination technology..

Adhesive melting can be performed in a reservoir or, for energy savings and process optimization reasons, in continuous melter-dispensers that provide adhesive on demand. A schematic machine layout is presented in Figure 5.5.9. Wax-lamination coating heads have a configuration similar to those used for dry lamination, but must account for the very nature of a thermoplastic adhesive. Therefore, each component of the coating station is heated and temperaturecontrolled: the coating cylinder, the adhesive tray, and the doctor blade. Substrates for Wax Lamination In most cases, wax lamination is performed on substrates such as paper or foil. Applications for Wax Lamination

or with thermal curing. The process can be configured as shown in Figure 5.5.10. Due to this specific characteristic, the process is associated with the lowest energy consumption in adhesive lamination. The simplicity of the process and the low cost of the equipment associated with the no-emission policy make this technology the fastest-growing in the industry. A schematic of the machine layout is shown in Figure 5.5.11. The technology was established in the 1970s, but did not gain commercial acceptance until the 1980s as a result of important equipment developments. Given the peculiar characteristic of the adhesive, the areas of technical advancement required included coating technology, web handling, and temperature handling. Recall the five-roller coating head described in a previous section. It involved a setup patented in the 1980s that provided consistent control of coat weight.

Wax-laminated packaging is used to wrap products such as butter and margarine, as well as certain cheeses and soaps. Wax laminations are also found in packaging for chocolate and biscuits. Lamination is not used to protect the printing, and therefore inks used to print those packages do not come into contact with the wax. SL (Solventless) Lamination Technology Solventless lamination involves high-viscosity adhesives. Adhesive viscosity at the coating station is in the range of 1000 cps and up. Adhesives are a 100% solid formulation and are chemically cured. Based on isocyanate chemistry, adhesive curing is triggered by mixing two components: a resin and a hardener (the hardener in a few cases can be water, and in that case the adhesive is in effect single-component). There is no drying system associated with this process

FIGURE 5.5.8.  Thermoplastic lamination process.

Section 5.5.  Coating and Laminating Technology

389

FIGURE 5.5.9.  Wax/hot melt adhesive process.

Web handling also needed specific attention. Basically, the target was to keep under consistent control the lamination of two substrates, each substrate having its own specific mechanical characteristics, with no help given by the adhesive due to its very low tack at green. Due to the low tack of these adhesives at green (the stage of the process immediately after coating), in the early days of the technology, it was significantly difficult to control the tension of two substrates of different mechanical nature consistently, as required in a lamination process. The solution relied on the ability of the designer to set up a machine that could minimize the side effects of these adhesives. As drive and motor technology evolved, the task became less problematic and the technology more operator-friendly. Now it is technically possible to control web handling and lamination accuracy in very thin and very elastic substrates up to very high production speeds. Design evolution enabled a few additional technical developments and the ability to control a three-ply lamination with solventless adhesives, as well as the ability to consistently control a lamination process involving thin foil. Temperature handling is critical in a process involving adhesives that are thermally sensitive due to their viscosity variation with temperature. Machine design had to be tuned to solutions that enabled consistent temperature control and process stability. As of today, solutions implemented in commercial machinery, are playing a very important role in process handling, although this role is often misunderstood. It is safe to state that most lamination conversion, especially in flexible packaging, is today performed by solventless lamination.

bond lamination for market share because the substrates involved are practically the same. Applications for Solventless Lamination Today, solventless lamination is, as mentioned above, the fastest-growing adhesive lamination technology. The list of packages that can be converted with solventless lamination is quite extensive and covers the same list as most wet- and dry-bond applications, with some exceptions. One limitation to be considered is post-lamination high-temperature exposure. This limitation includes sterilized and pasteurized applications as well as applications in retort packaging. For these applications, dry-bond lamination is still the most used in the industry. Equipment for Coating and Lamination As discussed in an earlier part of this section, equipment for coating and lamination has evolved through the devel-

Substrates for SL Lamination Solventless lamination is competing with dry- and wet-

FIGURE 5.5.10.  Solventless lamination process.

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FIGURE 5.5.11.  Solvent lamination machine.

opment of this industry, based primarily on the application of printing technologies. Consequently, for decades, coaters and laminators have been practically a one-color press with two unwinds. Through the 1970s, the business was exclusively dry-bond and thermoplastics, with some niche market reserved for thermal laminations. The coating and lamination industry was in fact almost exclusively a business for specialty converters, with thermal lamination covering a market niche and enabling non-specialists to have access to a simplified process. On the industrial side, coating and lamination was the main focus of the converting process, which on the packaging side was limited to a minority of the market. Machinery consequently was developed with a focus on the specific application, with almost every machine one being custom-designed and custom-built specifically for a particular application. This remains true to the present day in some industrial applications, but in packaging, pharmaceutical, tamper-evident sealing, and other segments, the specific needs can be handled through integration, making it possible to offer the industry high-productivity equipment at a very favorable price/performance ratio. Two main large families can be identified: (1) Custom-built machinery. Destined primarily for one-ofa-kind applications. Designed and built to the exclusive parameters required by the specific case. (2) Integrated machinery. Made of a custom combination of standard modules that make it possible to offer the industry highly reliable products at competitive costs. It is this second case that opened the coating lamination market to virtually every converter with a potential need. The investment has been resized from the order of magni-

tude of millions down to the order of magnitude of hundreds of thousands of dollars. This was an industry-changing evolution. Simple-to-handle equipment, installed at competitive costs, with a very fast return on investment, opened the way to major evolution in the industry and in the machinery installed at virtually every converter shop. In the 1980s, two main machine setups emerged: the drybond laminators, who held onto a significant market share, especially in the coating segment and the specialty products area, and an emerging family of solventless laminators directed mainly toward the packaging industry. This second family, with its low investment, low energy consumption, and no-emission characteristics, is growing very rapidly in market share. Then in 1992, a new concept of coaters and laminators was invented: a machine setup designed initially for the specific needs of the European market and then rapidly expanding on a global scale. The short-run nature of the European market, which is segmented in areas of small population with different languages and cultures, suggested the development of a new machinery concept re-engineered around these needs. Therefore, machines with a compact footprint, stop-and-go shaftless unwind and rewind for quick order setup, and the ability to switch promptly among production configurations rapidly became available. This was the start of the era of the compact designed machine, intended for high output on short runs, designed for the use of medium-level operators, and as well the birth of the Combi machine design featuring a drying oven and the ability to interchange coating heads quickly, enabling one single piece of equipment to cover as many production needs as possible.

Chapter 5—Section 6

Metallizing VERONICA ATAYA, Celplast Metallized Products Limited

INTRODUCTION Metallization refers to coating the surface of a material with a metal. Metallization for flexible packaging is done as a batch process to produce metallized films, which are key to providing barrier against light and permeation, for conductivity or resistivity purposes, or for aesthetic purposes because the metallized reflective look offers an eye-catching appearance. Aluminum is the most common material used in vacuum metallization because it is inexpensive, corrosion-resistant, readily available, offers good reflective properties, and has a low melting temperature compared to other metals; however, zinc, copper, chromium, silver, and gold are also used in some applications. Several techniques are used to metallize materials, including evaporation, sputtering, plating and spraying, among others. This section will discuss evaporation under vacuum, which is the main technique used to manufacture metallized films. METALLIZING PROCESS Vacuum metallization is a physical vapor deposition technique in which aluminum is melted under vacuum and deposited on the substrate at high speed. The process takes place under vacuum to optimize the transfer of aluminum to the substrate and avoid collision of the aluminum particles with any other molecule; this makes the process very efficient. Metallization takes place in a cylindrical chamber from which the air is pumped out by diffusion pumps to achieve a high vacuum of approximately 10–4 mbar or 10–8 atm, which is similar to the pressure in outer space. As shown in Figure 5.6.1, unwind and rewind stations are positioned in the chamber, allowing the film to pass from unwind to rewind over the active boat-bed area of the chamber, which is heated to 1500°C to melt the aluminum and form a vapor cloud above the boat beds. Note that Figure 5.6.1 shows an unsupported process or free span, which means that cooling takes place after metallization. Supported machines are also avail-

able, referred to as drum metallizers, in which cooling takes place at the same time that metallization occurs. The process operates at high speed (typically 1,200–2,000 fpm), with the substrate being exposed to high heat for a minimal time, around 0.1 s or less. As the substrate passes over the active deposition area, a thin layer of aluminum is deposited onto the film, which is then quickly condensed as the material goes through the chill roll, thus creating a barrier layer. The aluminum coating is very thin because only 20–30 nm of aluminum is applied to the surface of the film; therefore, the coating does not affect the yield or mechanical properties of the material. Besides good vacuum, one of the key components of the metallization process is the evaporators or boats. These are made of ceramic materials that offer good heat conductivity and control the rate at which aluminum is evaporated. As the aluminum wire is fed onto the boat, it creates a pool of aluminum in the boat cavity, which then evaporates and condenses onto the film as it travels a few inches away from the evaporation area. Good heat distribution on the boat is required for optimal results and to prevent aluminum from melting too quickly or too slowly in the cavity, which could lead to issues during processing. Another key component in the process is the aluminum wire. Impurities present in the aluminum can create defects in the finished product and affect boat life.1 Aluminum alloys used in metallization must have minimum 99% aluminum purity, with other components such as iron, copper, magnesium, and titanium making up the remaining 1%. The wire can vary in diameter and hardness; the choice of alloy and wire properties depends on the preference of the metallizer and the requirements of the finished product. Figure 5.6.2 shows the boats and the aluminum wire inside a commercial metallizer. There are several boats spread equally across the width of the machine, with each boat fed by a wire spool. The aluminum wire is fed onto the heated evaporation boat at a predetermined controlled speed. The feed rate affects the size of the molten pool formed on the boats and, if not well controlled, could lead to the creation of defects in the film. There are individual controls for boat heat and wire speeds to ensure that conditions are optimal 391

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FIGURE 5.6.1.  Schematic of a free-span vacuum metallizing chamber.

for metallization and to minimize the risk of defects. The boats are heated by the resistance generated when passing an electrical current through the ceramic blend making up the boats. The coating thickness of the aluminum applied to the film is reported as an optical density, which is a measure of the light transmission of a material. There is a correlation between aluminum coating thickness and optical density, which can vary from polymer to polymer. Figure 5.6.3 shows the correlation between optical density and coating thickness as a reference for polyethylene terephthalate (PET) film [1]. Metallizers are equipped with optical density controls to set parameters for each production run and to control the amount of aluminum applied based on the light transmission measured in-line. Modern metallizers can automatically adjust the boat temperature or wire speed to ensure that optical density is meeting the required specifications.

To optimize metallized film performance, substrates typically must be treated to improve adhesion of aluminum to their surface. Most substrates can be treated or coated to modify their surface energy to enhance metal adhesion. Surface treatment is also used to enhance bond strength in adhesive lamination and ink adhesion in printing processes. Substrate surfaces can contain oligomers or low-molecularweight components that could detrimentally affect metal adhesion. In such cases, the substrate is treated in the metallizer right before the deposition area, with the purpose of cleaning, modifying, and preparing the substrate surface for optimal aluminum deposition. Because films have different surface chemistries, the type and level of treatment will be different for each material. Plasma treatment is the most common method used to treat the substrate surface inside the vacuum chamber and can be applied using a variety of gases and power levels. By

FIGURE 5.6.2.  Boat and aluminum wire inside a commercial metallizing chamber.

FIGURE 5.6.3.  Correlation between aluminum thickness and optical density for PET film [1].

Section 5.6.  Metallizing

treating the film right before the evaporation area, no aging or contamination of the surface takes place, which is always a risk with atmospheric treatments. Care must be taken not to over-treat the material because this will defeat the purpose of the treatment in the first place, which is to improve the performance of the metallized product. APPLICATIONS Metallized films are used for a variety of purposes: to block light; to reduce permeation of oxygen, moisture, nitrogen, or aroma; to provide reflectivity or conductivity; or for aesthetic reasons, among others. The type of substrate used depends on the application requirements. PET and biaxially oriented polypropylene (BOPP) films are the most common metallized substrates; however, it is possible to metallize nylon (PA), polyethylene (PE), cast polypropylene (CPP), polylactic acid (PLA), or cellophane, among others. Films that are temperature-sensitive are more difficult to metallize, and special processing conditions must be used. Metal provides a shiny, silvery appearance that is desired in many applications because it provides an eye-catching look that can help brand owners differentiate their product. Because metallized films are mainly polymers with a very thin layer of aluminum, they provide better tensile strength and heat-seal properties and are lighter that aluminum foil, replacing the latter in many cases with a material with improved performance. The main application for metallized films is for barrier in food packaging. These films are widely used to package potato chips, coffee, granola bars, pet food, snacks, lidding, wine bag-in-box, tomato and fruit pouches, chocolates, and nuts. Other uses are in reflective insulation, vacuum insulation panels, balloons, aerospace, pharmaceuticals, board lamination, electronics, clothing, agricultural films, and nutraceuticals. BARRIER PROPERTIES OF METALLIZED FILMS Barrier is needed in food packaging to protect the integrity of the product and extend shelf life, to reduce the need for preservatives, and to prevent the permeation of water vapor, oxygen, aroma, oil, flavor, and/or light. The most common barrier films are PET, BOPP, PA, PE, and CPP. Metallized

FIGURE 5.6.4.  Barrier properties of metallized PET film at different optical densities.

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TABLE 5.6.1.  Barrier Properties of Several Barrier Films. Film Metallized PET

Metallized BOPP Metallized PE Metallized PA Metallized CPP EVOH Aluminum foil Transparent barrier (AlOx and SiOx) PVDC-coated PET

WVTR (g/100 in2/d)

OTR (cm3/100 in2/d)

0.06 0.02–0.03 (High barrier)

0.07 0.02–0.03 (High barrier)

0.03 3–5 0.01 (High barrier) 0.1 (High barrier) 0.07 15–20 0.2 0.05 0.05 3–5 6.5 0.05 0.005 0.005 0 (>25µm) 0 (>25µm) 0.10 0.15 0.05 (High barrier) 0.02 (High barrier) 0.5 0.5

versions of these films offer good barrier properties, but the level of barrier depends on the substrate in each case. Materials with more uniform surfaces usually provide films with better barrier properties. Table 5.6.1 shows the barrier properties of several metallized films compared to other common barrier films. Metallized PET film is widely used because it offers barrier to both moisture and oxygen, whereas metallized BOPP is used in applications where primarily moisture barrier is required. Compared to aluminum foil, metallized films do not offer the same properties; however, due to their improved tensile strength and flex resistance, they are less prone to scratching and pinholing and easier to process. Metallization conditions, as well as surface chemistry, directly impact the barrier properties of metallized film. The same substrate can provide different barrier properties, depending on how the material is metallized and the optical density; however, increasing the thickness of the aluminum layer does not always translate into higher barrier. A limit is reached at an optical density level around 3.5–4, beyond which point there is no further benefit (Figure 5.6.4).

FIGURE 5.6.5.  Causes of defects in metallized films.

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TABLE 5.6.2.  Advantages and Disadvantages of Several Barrier Films. Film

Advantages

Disadvantages

Medium barrier, clear

Yellowish look, contain chlorine, regulated in Japan and Western Europe

Excellent barrier, block light, puncture-resistant

Can be scratched or damaged during processing, barrier not as high as foil Hydrophilic and moisture-sensitive, barrier will degrade under high moisture Dead fold, prone to wrinkling and cracking, difficult to process More prone to flex-cracking than metallized films (unless top-coated)

PVDC-coated films Metallized films EVOH Aluminum foil Transparent barrier films (AlOx and SiOx)

Good oxygen barrier, clear, oil- and solventresistant, can be combined with polyolefins Excellent barrier, good dead fold, blocks light Good barrier, clear, chlorine-free, microwavable

PROCESSING OF METALLIZED FILMS Metallized films should be kept in moderate moisture and temperature environments to avoid oxidation, especially around the edges of the roll, where it could lead to loss of metal and blocking. The material should be wrapped in polyethylene or metallized film to protect it from the environment. These films are usually laminated to other layers, and the metal layer is buried because exposing it to the environment over time will lead to degradation and oxidation of the aluminum. Table 5.6.2 compares the advantages and disadvantages of metallized film and other barrier products. Metallized films offer excellent puncture resistance and barrier properties, especially compared to some other substrates. However, care must be taken when processing the material because the aluminum layer could be scratched or damaged, negatively impacting barrier. All rollers in contact with the metal layer must be clean and free from debris. Tension control is also important because too much stretching (>4%) could crack the aluminum layer and affect barrier performance. METALLIZED FILM DEFECTS The barrier properties of metallized films can be com-

promised if the material is scratched or if the substrate has oligomers or other particles on its surface that could flake off during processing, as shown in Figure 5.6.5. These defects are referred to as pinholes and are the most common type of defect in metallized films. To help minimize the risk of pinholes, the metallizing chamber must be cleaned thoroughly after each roll is metallized and before the next roll is started. This minimizes any particles or dust from transferring to the film during metallization. At the same time, the cleanliness of the surface to be metallized is key to producing material with minimal pinholes. Plasma treatment and film quality must be carefully controlled to provide the best-performing product. Spitting is another type of defect created as the result of a molten droplet of aluminum being ejected from the puddle in the boat, perforating the film, and leaving a hole. This defect is usually seen with products metallized at optical densities >2.5. Consistent boat temperature control should help minimize this issue. Pictures of these common defects can be found in Figure 5.6.6. SUMMARY Metallization is a batch process by which aluminum is evaporated inside a vacuum chamber and can be applied to different substrates. Metallized films are mostly used in

FIGURE 5.6.6.  Pinholes and Spitting in Metallized Films.

Section 5.6.  Metallizing

food packaging applications, but there are also uses in other industries such as construction, aerospace, and decorative sheets. The properties of metallized films depend on the substrate being metallized, with some materials offering better barrier properties than others. Metallized PET and BOPP are the most commonly produced films because they are lightweight and provide excellent barrier and mechanical properties. Care must be taken

395

when processing these materials to minimize the risk of scratching or damaging the metal layer, which adversely affects the barrier properties of the product. REFERENCES AND ADDITIONAL RESOURCES [1] AIMCAL. Metallizing Technical Reference. 5th Edition. May 2012.

Chapter 5—Section 7

Troubleshooting the Extruder ANDREW W. CHRISTIE, SAM North America, LLC

INTRODUCTION Coextruded film structures push performance and productivity every day. By incorporating a number of functional polymers into a composite structure, this translates into more sophisticated film products with increased numbers of layers that frequently improve overall film performance with less overall material. In addition, novel materials, especially biobased ones, are adding to operating complexity. The systems to produce these films have likewise advanced and further increased complexity. Recent advances include separate extruders for individual layers, multi-component blending feed systems, advanced feedblock and die technologies, higher operating speeds, integrated control systems, and automated winding and roll handling systems. Each of these advances brings a unique set of potential failure modes, which place more demand on operations. This section will discuss typical film-system components, the basic troubleshooting method and will outline several specific problems with a method to verify and correct them. Because few film systems are exactly the same, this will not be a comprehensive troubleshooting guide; however, the approach set forth in this section should be usable for troubleshooting any machine system. COEXTRUSION FILM SYSTEMS When troubleshooting a film problem, a common approach is to reduce the source of potential problems to a set of problems that are known from past experience. Frequently, this approach may never address the root cause of the problem. Often certain adjustments to one subsystem may appear to solve a problem, when in reality the adjustments simply masked the fundamental problem in another part of the system. Although it is important to focus on the elements of the film system within your control, the troubleshooting process should succeed in identifying root-cause problems even if they are outside the troubleshooter’s immediate control. Following is a list of typical elements that make up a co-

extruded film system today. Although the specific designs are significantly different between cast- and blown-film systems, the same fundamental elements exist within each system: (1) Polymeric materials (2) Resin delivery system (3) Resin blending system (4) Material rate control system (5) Film product structure (material layers, sequence, percentage) (6) Extruders (electrical and mechanical drives, screws, heating/cooling system) (7) Filtration device (8) Transport pipes (9) Layer-combining system (feedblock or die) (10) Film-shaping system (flat or annular die) (11) Temperature control system (12) Film forming/quenching system (13) Thickness control system (14) Surface treatment system (15) Trimming/slitting system (16) Trim removal and recycle system (17) Continuous winding system (18) Drive control system (DC or AC vector) (19) Roll removal and shaft reloading system (20) Supervisory control and data acquisition (SCADA) system (21) Packaging and palletizing system. To produce a consistent high-quality coextruded film product, all these systems must work in concert. When a quality defect occurs or the machine system goes down, it is imperative that the troubleshooter be able to quickly identify and correct the root cause problem. For the highly complex coextrusion systems in operation today a reliable method of consistent problem resolution is required. 397

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TROUBLESHOOTING METHOD

  THE PROBLEM STATEMENT

When confronted with a complex coextrusion system today, many operators and technicians may feel overwhelmed when challenged to isolate a problem. A typical seven-layer coextrusion system may have six extruders, three or four component blenders on each extruder, and gravimetric rate control. These blending systems may draw from eight or ten different resin silos or railcars. A cast-film combining station may have a feedblock with the capability for multiple structures and the ability to compensate for throughput changes or viscosity mismatch. The die may have over a hundred thermally actuated die bolts, internal or external deckles, and edge encapsulation. The quenching system may include a variety of pinning and cooling mechanisms, used independently or in combination. Forming systems may have multiple cooled or heated rolls with individual temperature controls and drive controls. Web transport rolls may be individually driven and position-adjustable. Trim removal systems may include multiple choppers, blowers, grinders, and fluff re-feed machines or re-pelletizing. Continuous winders may be speed-, torque-, or tension-controlled, with center or surface drives or a combination thereof. Programmable lay-on roll loading, as well as sophisticated transfer logic, is typical. Automated handling systems may enable handling of multiple core sizes, spacings between cores, adhesive application, roll doffing, pick and place systems, and so forth. How does the troubleshooter start? Whenever confronted with a complex system, a consistent approach and method is our most important ally. If we approach a complex system in a haphazard manner, we are likely to overlook simple solutions. One of the most powerful tools for troubleshooting was taught to each of us in early school years—the scientific method. There are four steps involved in this approach, and if followed rigorously, it will never fail in problem-solving. The four steps may be repeated multiple times in solving any problem, but persistence and consistency in this approach will ensure “root cause” problem-solving every time. The scientific method can be described as follows:

The first benefit of a formal approach in a group environment is that everyone recognizes and is working on the same problem. The first step is to write down the problem statement. This is now a visible guide for everyone working on the problem to focus on the same issue. The problem statement must be unbiased. The problem statement should not include the assumed solution.

(1) State the problem (2) Identify a possible solution (the hypothesis) (3) Test the hypothesis (4) Evaluate the results (and repeat as necessary). Whether we recognize it or not, this is the method we all use in problem-solving. Most of the time we execute this very informally and vaguely. When it is successful, we quickly move on and fail to recognize exactly how we succeeded. When our informal methods fail, we resort to “experts” or senior and more experienced personnel, which expands the number of hypotheses. A consensus on how we test and evaluate the hypotheses may also be a stumbling block if this method is not formalized. A brief discussion of the formal application of the scientific method for troubleshooting may be in order.

For example, Problem Statement: Unmelt gels are causing holes in the film. This problem statement will cause the troubleshooting effort to focus on melting as the problem, and a contamination or crosslinking problem may be overlooked. Problem Statement: Gels are causing holes in the film. This statement assumes no pre-conceived cause for the gel. This will allow a more complete consideration of potential causes for the gels in the hypothesis stage. Taking the effort to formally write down each step in this process not only provides focus and clarity to the troubleshooting effort, but can be maintained as a record for future reference should the problem recur. This will reduce the time to correct future problems and improve the troubleshooting capabilities of the entire group. THE HYPOTHESIS This is often the most important step. Spending a little extra time at this stage in problem-solving often allows considerable time savings in the overall effort. This should be approached very much like a brainstorming exercise. The more comprehensive the list of potential causes of the problem, the more likely it is that one can isolate the root cause of the specific problem that has occurred. In the exercise of developing hypotheses that might have caused our problem, we want to consider the entire coextruded film system, including all elements. Again, in “root cause” problem-solving, ignoring one element as a potential cause may result in a solution that masks the actual root cause. Looking at the film system as outlined and the gel problem statement, the hypotheses generated might include: (1) High gel count/off-spec resin (2) Ingress of contaminants through the resin delivery system (3) Cross contamination in blending components (4) Incomplete melting in extruder (5) Over-shearing/over-heating in extruder (6) Over-heating in downstream equipment (7) Hangup in melt flow path (8) Cross contamination between layers in the combining system

Section 5.7.  Troubleshooting the Extruder

399

(9) Over-heating/hangup in die or feedblock (10) Die-lip buildup (11) Chill roll contamination (12) Inconsistent sizing and re-feed of trim scrap.

plete the test and the gel still exists, can we conclude that it is not unmelted resin? Because these tests are indicators, but will not develop a conclusive answer, the hot-stage microscope is chosen for the evaluation.

The objective at this stage should be to generate as many reasonable potential causes for the problem as possible. The next step requires selecting one possible cause and developing a test that will be able to eliminate that root cause as the source of the specific problem being worked on. Some of the potential problems may have a low probability of being the root cause, but may be easily tested and eliminated. Some of the potential problems may have a much higher probability of being the root cause, but testing and verification may be more difficult. Selecting the hypothesis to pursue and developing a test that can identify that hypothesis without a doubt as the root cause or eliminate it from the potential cause list is the next step.

EVALUATING THE RESULTS

TEST THE HYPOTHESIS Now we are ready to develop some tests to verify whether our hypotheses are true or false. This is a process of elimination until the root cause is identified. It is important to design a test that will truly indicate the validity of the hypothesis. With the gel problem, we have developed twelve potential causes. More than one test will be required, although some tests may eliminate more than one hypothesis from consideration. Some hypotheses may not require a specific test, but may be eliminated due to available data. For example, hypothesis 1 (off-spec resin) may be eliminated as a potential cause if materials from the same source lot are being processed on another film system without the gel problem. Let us assume that due to our experience and the appearance of the gel, we feel that the cause is likely unmelted resin (hypothesis 4). A number of tests can be proposed to check this hypothesis, as follows: (1) Increase barrel temperatures to promote melting (2) Increase system pressure to retard the advance of the solid bed in the screw channel and promote mixing (3) Change the extrusion feed (screw) to increase mechanical work on the melt stream (4) Re-melt a sample gel under observation on a hot-stage microscope (5) Reduce the screw speed and increase all system temperature settings. There are almost an unlimited number of tests that could be proposed that will indicate whether this hypothesis is valid. We would like to proceed with a test that proves conclusively whether the hypothesis is true. Looking at the five proposed tests, we see that only number 4 truly eliminates unmelted resin as the potential problem. If we try the other tests and the gel is eliminated, we have determined that the cause was very likely unmelted resin; however, if we com-

The last step in this troubleshooting process is to evaluate the results of the test and determine whether the source of the problem has been found. If we feel we have found the cause, we can then move to corrective action. If the test is inconclusive or if it indicates that our hypothesis was not the root cause of the problem, then we have to return to our list of hypotheses. Depending on what is learned from the first test, additional hypotheses may be generated. Back to our example, if we find that as we reach the melting point of the film, that the gel melts, we have demonstrated that this gel is unmelted resin. If it does not melt at this temperature, we can continue to heat the sample and see whether it melts at some other temperature, indicating that our system is contaminated with some foreign polymer or mildly crosslinked resin. If it never melts, we may conclude that the gel is severely crosslinked resin or that some other foreign material has entered the system. At the conclusion of this test, we clearly know whether the root cause problem is unmelted resin. If the cause was discovered to be unmelted resin, we may now pursue a strategy to melt the resin more completely. If the cause was clearly not unmelted resin, we can return to our original list of hypotheses and select another one to evaluate. Because we gained additional data in this process, for example, if the gel never melted and we observed a foreign material at the center of the gel, we would now try to pursue one of the hypotheses that assumed that foreign material was contaminating the system (hypotheses 2, 3, and 11). Comment on testing and evaluating; Generally we are troubleshooting in production under extreme pressure to determine the cause and return to production. We often have to rely on machine instrumentation that may not be recently calibrated or located at the proper position relative to the problem being tested. We need to be cautious and to record data on any doubts or compromises taken in the name of efficiency. We may need to go back and repeat a test if the results are then ambiguous. It may make sense to adapt our instrumentation to improve data quality; a 10,000 psi pressure sensor with an accuracy of ± 2.5% used to evaluate the pressure stability of a process running at 3,500 psi may be replaced for troubleshooting with a 5,000 psi sensor with an accuracy of ± 1.0%. Often accurate evaluation of a sample requires outside testing, but we may want to collect multiple samples and proceed with some crude local test results while awaiting the more accurate formal evaluation. COMMON PROBLEMS, HYPOTHESES, AND TESTS This powerful method of diagnosing and solving prob-

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lems may be applied formally or informally. It is indeed applied informally by all problem solvers, although occasionally steps are omitted or a decision is made based on ambiguous results, but progress is made, and the problem is solved. Recognizing the method and formally documenting it will result not only in quicker problem resolution, but a record of the behavior of your particular system and trends that

may enable you to recognize potential problems before they result in lost production and that may guide you towards system design improvements. The remainder of this section consists of a table that lists common coextrusion film system problems, potential causes (hypotheses), and verifying tests. Until the tests are conducted, results cannot be analyzed. See Table 5.7.1 for further details.

TABLE 5.7.1.  Troubleshooting Guidelines. Problem 1. Gels in Film

Potential Cause (Hypothesis) 1. High gel count/off-spec resin 2. Ingress of contaminants in resin delivery 3. Cross contamination in blending system 4. Incomplete melting in the extruder 5. Over-shearing/overheating in extruder 6. Overheating in downstream equipment 7. Hangup in melt flow path 8. Cross contamination in combining system 9. Overheating in die or feedblock 10. Hangup in die or feedblock 11. Die-lip buildup 12. Chill roll contamination 13. Inconsistent sizing and re-feed of trim scrap

Test Run same lot of resin on other film line Feed material direct to extruder manually Bypass blending system Analyze on hot-stage microscope Run at reduced speed Run at significantly reduced downstream temps Disassemble, clean, and rerun Add color to layer, run, disassemble, inspect Run at significantly reduced temps Disassemble, clean, and rerun Clean die lips, run with video observation Clean chill roll, rerun Divert trim scrap

2. Poor Clarity

1. Extrusion temperature too low 2. Coextrusion interface instability 3. Quenching temperature too high 4. Poor finish on chill roll 5. Inappropriate polymer for application

Raise run temperature Adjust relative extruder outputs Reduce quenching temperature Evaluate alternative finishes 1. Evaluate alternative materials 2. Blend in clarifying agent

3. Wrinkling

1. Poor gauge control 2. Non-uniform quenching 3. Non-uniform melting (shear history) 4. Transport rolls misaligned 5. Poor tension control 6. Non-uniform pinning 7. Web not centered on spreading rolls

Measure samples with alternative gauge (offline) Monitor web temperature by CD position Monitor melt temperature by CD position Check level and trim Adjust and observe Clean and align pinning device Adjust deckles to center web in machine

4. Unable To Reach Output

1. Resin supply system unable to keep up

Disconnect feed from extruder and verify maximum rate Inspect feed throat and supply lines for obstruction Conduct rate checks at various speeds and pressures, confer with screw designer Conduct rate checks with and without downstream components connected

2. Restriction in feed 3. Improper feed (screw) design 4. Restriction in downstream system 5. Poor Mixing of Melt

1. Resins incompatible 2. Mismatch of masterbatch rheology 3. Inconsistent dosing from blenders 4. Improper feed (screw) design 5. Operating temperature improper for optimum 6. Specific residence time inadequate for optimum mixing 7. Stratification of melt components in flow path

6. Melt Temperature Too Low

1. Wide melt temperature variation 2. Improper barrel set temperatures 3. Specific residence time inadequate for temperature development 4. Improper feed (screw) design

Confirm compatibility, run in alternative system Compare rheology at processing conditions Monitor dosing size and frequency, check random sample consistency Conduct rate checks at various speeds and pressures, confer with screw designer Adjust setpoints to increase shear input mixing Increase head pressure/specific residence time Add stationary mixing device Check temperature uniformity across flow with exposed junction melt T/C - See item 5 Adjust to higher set temperatures Increase head pressure/specific residence time Conduct rate checks at various speeds and pressures, confer with screw designer (continued)

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Section 5.7.  Troubleshooting the Extruder

TABLE 5.7.1 (continued).  Troubleshooting Guidelines. Problem 7. Melt Temperature Too High

Potential Cause (Hypothesis)

Test

1. Improper barrel set temperatures 2. Specific residence time excessive for temperature development 3. Improper feed (screw) design

Adjust to lower set temperatures Reduce head pressure/specific residence time

8. Extruder Power Insufficient

1. Drive not developing design power 2. Barrel setpoints too low 3. Improper feed (screw) design

Verify drive power inputs Adjust to higher set temperatures Review application with feed (screw) designer

9. Streaks Or Lines In Film

1. Die is dirty

Clean die (shim) in location of streak and rerun, or split and clean die Split die and inspect Check die gap, check bolt powers and backlash Check air-knife gap adjustments Check vacuum box for leaks in seals and high air velocities

2. Imperfections in die 3. Die lip out of adjustment 4. Air knife out of adjustment 5. Vacuum box out of adjustment

Conduct rate checks at various speeds and pressures, confer with screw designer

10. Melt Appearance Defects Applesauce

1. Poor mixing 2. Extrusion temperature too low or too high

See item 5 See items 6 & 7

Sharkskin

3. Poor melt temperature uniformity

See item 5

Orange Peel

4. Mismatched velocities/shear stress at interface

Interface Instability

4. Poor purging technique 5. Resin contamination

Disassemble, clean, and rerun See item 1

11. Thickness Variation - Cd

1. Die lines or gauge bands 2. Improper operation of automatic gauge 3. Interlayer non-uniformity

See item 9 Run in manual mode Introduce color in alternating layers, check uniformity (Note: typical gauges will not compensate for layer density variations)

12. Thickness Variation - Md

1. Unstable extruder outputs 2. Poor tension control 3. Unstable vacuum box or air knife pressure

See item 16 Verify drive speed uniformity and control Monitor frostline position stability during steady state

13. Poor Wound Roll Appearance

1. Non-uniform gauge 2. Poor tension control, improper tension 3. Excessive slip additive in resin 4. Blocking 5. Inadequate cooling before windup 6. Overtreatment 7. Winder or idler alignment

See items 9 & 11 Monitor drive loads, adjust or taper tension Wind film without slip additives Add anti-block Reduce cooling roll temps, reduce line speed Reduce treat levels Check roll level and trim

14. Edge Tear - Unstable Edge

1. Inadequate resin draw strength 2. Improper setting of edge pinners 3. Material too cold 4. Deckles set too narrow (too wide) 5. Leakage (weepage) around deckles

Use alternative material, use edge encapsulation Adjust pinning, die, chill roll relation Adjust to higher melt temperatures Adjust deckles Adjust deckles or die bolts

15. Pinholes

1. Gels 2. Abrasive roll surface 3. Air (or volatiles) entrained in polymer melt

See item 1 Inspect and modify rolls Check drying, check temperature settings, review screw design Clean die (shim) in location of streak and rerun, or split and clean die

4. Die is dirty 16. Extruder Surging

1. Inconsistent material feeding 2. Overheating in feed throat 3. Overheating on screw root 4. Improper feed (screw) design

1. Adjust relative extruder outputs 2. Adjust relative manifold shape at combining point (requires adjustable feedblock or die) 3. Adjust relative viscosities through material selection or temperature adjustment

Monitor resin supply to feed throat Monitor feed jacket temperature, adjust cooling supply Monitor root cooling temperature, adjust cooling supply Conduct rate checks at various speeds and pressures, confer with screw designer (continued)

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TABLE 5.7.1 (continued).  Troubleshooting Guidelines. Problem

Potential Cause (Hypothesis)

Test

17. Draw Resonance

1. Improper air gap 2. Improper melt temperature 3. Inconsistent polymer release from die surface 4. Improper die gap 5. Balance of extensional response

Increase air gap Adjust melt temperature Add polymer processing aid Adjust die gap Adjust air impingement draw resonance eliminator with rate of cooling

18. Discoloration of Film

1. Melt temperature too high 2. Resin contamination 3. Degraded material from improper shut down

Reduce melt temperature, screw speed See item 1 Purge or disassemble and clean

19. Poor Heat Seal

1. Inappropriate resin 2. High melt temperature 3. Excessive corona treatment 4. Improper additive levels 5. Contamination with airborne silicone

Use alternative resin See item 7 Reduce treat levels Use alternative resin or additive Discontinue use of silicone sprays

20. Odor - Flavor Scalping

1. Inappropriate resin 2. High melt temperature

Use alternative resin See item 7

21. Poor Strength

1. Inappropriate processing temperature 2. Poor gauge control 3. Inappropriate resin 4. Excessive pressures or temperatures at nip rolls

Adjust processing temperature See items 11 & 12 Use alternative resin Adjust nip pressures and temperatures

22. Film Blocking

1. Inadequate cooling 2. Winding tension too high 3. Static buildup 4. Overtreatment 5. Inadequate levels of anti-block

Reduce cooling roll temps, reduce line speed Reduce winding tension Add static elimination Reduce treat levels Adjust anti-block levels

23. Poor Printability

1. Non-uniform treatment 2. Inadequate treat levels 3. Non-uniform gauge

Check treater gap, corona appearance Increase treat levels, reduce line speed See items 11 & 12

24. Camber

1. Inadequate quenching 2. Non-uniform stress / thermal history 3. Non-uniform transport forces

Reduce cooling roll temps, reduce line speed Review die design Reduce unsupported web spans, pre-trim edge beads

25. Scratches

1. Idler rolls not turning at web speed 2. Abrasive roll surface

Check all roller speeds Inspedt and modify

Chapter 5—Section 8

Troubleshooting the Blown-Film Process HARINDER TAMBER and MIREK PLANETA, Macro Engineering and Technology Inc.

INTRODUCTION In the blown (tubular)-film process, polymer melt from the extruder/s is passed through an annular die and subsequently drawn and simultaneously expanded in the machine direction (MD) and the transverse direction (TD) while being cooled by an air ring. The film bubble is stabilized by a bubble cage, collapsed into layflat tubing, and wound as a roll (tube or slit sheet). Figure 5.8.1 shows a single-layer blown-film process, and Figure 5.8.2 shows a multilayer blown-film process, both of which are air-cooled. Figure 5.8.3 shows a downward blown-film process, where water is used as the cooling medium. This water-quenched blownfilm process is mainly used to make high-clarity single- or multilayer film or sheet. A plastic web thickness less than 250 microns is classified as film, whereas a plastic web thickness greater than 250 microns is usually referred to as sheet [1]. Many OEMs build blown-film lines; their equipment design, layout, or configuration may vary from one to another. Therefore, this section mainly deals with providing a basic understanding of the troubleshooting issues in a blown-film process and how these issues can be resolved to produce optimum-quality film rolls. As shown in Figure 5.8.1, a singlelayer blown-film line consists of various components such as a gravimetric feeding/blending system, an extruder, an adapter, a screen changer, a die (stationary, rotating, or oscillating), an air ring (stationary or rotating), an internal bubble cooling (IBC) system, a bubble cage, an air injection system for the bubble (for non-IBC operation), a gauging system, a collapsing frame, a film treatment unit (corona treatment), a secondary nip, a trim takeoff system, and a winder and roll handling system. (Note that many of the items mentioned are not actually shown in Figure 5.8.1). Multilayer blown-film lines (2 to 13 layers or more, consisting of micro- or nanolayers) have the basic equipment mentioned above, with the difference being the number of extruders (depending on whether each layer is fed by an individual extruder or one extruder is feeding two or more layers using a feedblock in micro- or nanolayers). The multilayer die could be stackable or of conventional type. Mul-

tilayer blown-film lines are also equipped with a horizontal or vertical oscillating nip for gauge randomization. In some multilayer blown-film lines, a water bath is used after the primary nip for moisture absorption into the film, specifically for polyamide-containing film structures. The operation of single- or multilayer blown-film lines also requires auxiliary equipment such as silos (outside the building), surge bins, and dryers (for drying the resin). Utility equipment such as compressors (with clean, oil-free compressed air) and chillers (for cooling air) is also required for the blown-film process. The tower height and number of levels depend upon the complexity of the blown-film extrusion process. For troubleshooting, it is important to understand the basic function of each extrusion component, the resin(s), and the process, and how each impacts the film or sheet produced by the blown line. Therefore, the topics in this section are covered by seven subsections: (1) Introduction (2) Basics of the Blown-Film Process (3) Troubleshooting: Blown-Film Equipment and Processes (4) Film and Roll Defects (5) Summary (6) Literature Cited BASICS OF THE BLOWN-FILM PROCESS The blown-film extrusion process used for film fabrication consists of various steps. As shown in Figure 5.8.1 (for a single-layer blown-film process), granular resin is fed through a hopper into an extruder feed throat. As plastic pellets (polymer granules), flakes, or powders are fed into the extruder, the resin moves forward (due to screw rotation) and becomes compressed between the screw and extruder barrel. The barrel heat (thermal energy from heaters) and friction caused by shear (due to the turning screw) generate enough energy to melt the plastic pellets into polymer melt. Usually barrel heat is required in the beginning (during line startup) to heat the extruder/barrel for polymer melting. However, when the extruder is in operation (turning or ro403

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FIGURE 5.8.1.  Single-layer blown-film process (air-cooled).

FIGURE 5.8.2.  Multilayer (nine-layer) blown-film process (aircooled).

tating), most of the heat needed for the phase change, i.e., to convert polymer granules (solid) into polymer melt (liquid), is provided by friction. Polymer melt temperature and its variation across the melt stream is an important process parameter (as measured in the adapter), which varies for different polymers and grades (MI or viscosity). A high or low melt temperature could affect the height of the frostline (measured from the die) and bubble stability during film fabrication. The extruder is mainly a pump and generates melt pressure, which is required to pump the polymer melt through a screen changer or other melt-filtering devices and to feed it into an annular die. Typical single-layer dies are spiral- or spider-type (center- or side-fed). The molten polymer exits from the die as a tube, which is inflated and simultaneously stretched in the machine direction (MD) and the transverse direction (TD). The simultaneous stretching (MD and TD) of the bubble as it is being pulled away from the die makes the film thinner and gives it a specific diameter. The bubble is cooled by the air ring only or by the air ring and internal bubble cooling and is contained and stabilized by the bubble cage. The bubble is flattened (going from a circular shape to a tubular shape) by the collapsing frame, drawn through nip rollers, surface-treated and slit by a secondary nip into sheeting, and wound as rolls on the winder (or film is wound as tubing). The troubleshooting and maintenance of various extrusion components of the blown-film extrusion line is important to optimize the process to obtain high-quality film/sheet and roll. This subsection focuses on theory to discuss melt orientation and some basic terms used in blown-film fabrication. As shown in Figure 5.8.1, the polymer melt exits from the die as a thick melt tube, which is simultaneously oriented in the machine and transverse direction (MD and TD) as a

Section 5.8.  Troubleshooting the Blown-Film Process

405

FIGURE 5.8.3.  Multilayer blown-film process (water-cooled).

bubble and then stabilized and collapsed into a layflat film/ tube at the primary nip. Much happens in this section of the machine, and there are many significant process parameters, such as BUR (blowup ratio) and DDR (drawdown ratio). BUR can be obtained by dividing bubble diameter by die diameter, keeping in mind that it is not always easy to measure bubble diameter while the line is in operation. It is easier to measure the width at the primary nip (or downstream), where BUR can be calculated as: Blowup ratio (BUR) 

Drawdown ratio (DDR) =

layflat width  2 die diameter   velocity at hauloff velocity at die exit

Melt orientation (BUR and DDR) takes place before the frostline, and many film properties are impacted by the frostline height, such as MD/TD tear, impact strength, tensile strength, percent elongation, optical (haze and gloss), and barrier properties. From a theoretical point of view, a blowup ratio (BUR) greater than one means that the polymer melt has a transverse direction (TD) orientation from blowing the bubble greater than the die diameter. Similarly, a drawdown ratio (DDR) greater than one means that the polymer melt has a machine direction orientation (stretching) as a result of pulling the melt away from the die at a faster rate (by the primary nip) than the melt exiting the die lip. As shown in Figure 5.8.1, die diameter, die-lip gap, velocity at the die lip, and nip speed are required to calculate BUR and DDR. Polymers are viscoelastic materials, and therefore these materials exhibit die swell (the thickness of the extrudate is greater

than the die gap), which depends upon resin composition and process conditions. Therefore, the above calculations are only approximate [2,3]. The next subsection discusses each component of a blown-film extrusion line and methods to troubleshoot the equipment and the blown-film process. TROUBLESHOOTING: BLOWN-FILM EQUIPMENT AND THE BLOWN-FILM PROCESS Blown-film processes are very efficient and economical for making commodity single-layer packaging films and various specialty multilayer films consisting of different polymers, such as LLDPE, mLLDPE, LPDE, HDPE, plastomers, Nylon, EVOH, PP, PETG, COC, PVC, PVdC, PVdf, PS, and tie layers (to name a few). These resins provide different optical, mechanical, thermal, and barrier properties. These films are used in numerous applications in the food, medical, construction, agricultural, electronics, chemical, and automotive packaging sectors [4]. Resin from Silos, Gaylords, or Bags The blown-film extrusion process requires resin (mainly in granular form; some applications use powdered resin) which is often delivered from silos, in Gaylords, or in bags. If the material is transferred from silos located outside the building, care should be taken that the resin temperature is thermally equilibrated to “production hall” temperature. If resin is transferred from a silo (which is colder in winter), it can be discharged into a surge bin to achieve the ambient conditions of the production hall. Similarly, there should be no moisture condensation on the pellets, because both the lower temperature of the resin (from the silos) and any

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moisture condensation on the resin granules can impact the blown-film process. In cases where the resin is conveyed from Gaylords (boxes) or bags, specifically for moisturesensitive resins (Nylon, EVOH, PETG etc.), it is recommended to cut the foil liner in an area where the wand or hose is to be inserted, keeping the rest of the foil liner intact because it will prevent or reduce moisture uptake [5]. After blown-film production, the Gaylords or bags (for moisturesensitive resins) should be tightly packaged to prevent any moisture ingress until the leftover resin can be reused in the next production schedule. Moisture in the resin can produce fish eyes or other defects (gels) in the film. Therefore, if the resin is used from previously used (but closed) Gaylords or bags, it is recommended to check the moisture content (in parts per million, referred as ppm or wt. %). If the moisture content is higher than the resin supplier’s recommended ppm level (wt. %), dry the resin for the specified time and air temperature. A similar approach may be required for additives and masterbatches with regard to handling and drying for the process. Resin Handling and Volumetric or Gravimetric Systems Always start the blown-film extrusion process with a clean resin handling system, which means that, before starting the blown-film line (single- or multilayer), make sure that the resin loading system, conveying hoses (flexible or rigid), and hoppers are clean (with no old resin). Make sure that the filters of the resin-conveying system above the hopper are clean (no dust, no angel hair, no pellets sticking to the hopper wall due to static, etc.). The feeders (vibratory or screw-type) should be dismantled and cleaned. To start up the blown-film process, PE or other resins are required; make sure to bring labeled Gaylords or bags (with bag content transferred into a labeled bin). If startup resin is delivered from silos, the correct conveying pipe configuration should be connected. All resin Gaylords, surge bins, and bags in use should be covered with lids to prevent cross contamination and ingress of dust into the resin. The greater the number of resins or additives (for a multilayer blown-film process) for a given recipe, the greater will be the chance for cross contamination, which is one of the reasons for poor fabricated film quality or roll defects. Volumetric hoppers were more commonly used in the past (with a few exceptions on modern extrusion lines), where a specific volume of resin is dropped into the extruder/s. Nowadays, most extrusion lines are equipped with gravimetric systems where the amount of resin (with known density) is weighed (by a load cell) before discharging into the extruder. The blown-film industry commonly uses two types of gravimetric systems, batch blenders and loss-in-feed, both of which have merits and demerits, but their discussion is not within the scope of this chapter (please refer to the appropriate section of this manual). However, make sure that the gravimetric system used for the blown-film process provides fairly homogeneous physical blending to the various compo-

nents in the recipe before feeding the blend to the extruder; this blend mainly consists of resins (prime and recycled), additives, masterbatches, and pigments. Physical blending becomes more important, specifically when very low-concentration additives and higher-density masterbatches are blended with the main resin. The first phase of the compositional homogenization of the resin blend is accomplished in the form of physical blending in a gravimetric/blending system. This is an important step to homogenize these additives (later on, main compositional and thermal homogeneous blending is performed by the extruder screw) in fabricated blown film. (Therefore, after a production schedule (or film fabrication), it is important to clean all the hoppers and resin-handling systems for the next different recipe). Recycling systems for trim material and scrap rolls should be monitored to ensure that the flow of trim is smooth, trimming blades (to make appropriate fluff size) are sharp, fluff is not jamming the system, and the correct ratio of fluff to granular resin is being generated. Variation in fluff size or in its blend ratio with prime resin could cause surging in the extruder, which causes gauge variation and thus affects film and roll quality. Therefore, some blown-film companies convert trim into pellets, which are easier to handle than fluff, but add a heat history to the resin that could cause gels or other defects. Extruder: Gearbox, Motor, and Drives For an extruder, the gearbox is the critical component, which is equipped with a variable-speed drive and motor to provide consistent output and depends upon extruder size and motor HP. Gear-box alignment to the barrel and leveling of the extruder are critical during extruder installation and should be checked during regular extruder maintenance. This prevents and minimizes wear of the screw or barrel. An ammeter can be used to check power consumption, and a tachometer can be used to check actual screw RPM. Air filters should be cleaned periodically; dirty air filters can cause overheating of the solid-state drive components, the motor (if equipped with an air filter), and the heater cabinets, resulting in damage to these units. The gearbox (extruder transmission and thrust bearing) should be properly lubricated, and the oil level can be viewed through the small glass window on the gearbox. The water-cooling system for the gearbox should be checked periodically for any flow interruptions causing overheating The blown-film industry uses three main types of singlescrew extruders, which are classified based on the type of feed throat. These are smooth-bore extruders, light-groove extruders, and deep-groove extruders. In the smooth-bore extruder, the feed throat is smooth, and friction between the barrel and the resin granules aids in feeding the resin. In light-groove extruders, the barrel is lightly grooved, and friction is created by these light grooves, which aid in feeding the resin granules. In smooth-bore and light-groove feed extruders, the feedthroat temperature is controlled by a thermal control unit

Section 5.8.  Troubleshooting the Blown-Film Process

(TCU) using oil or water. Make sure that the feed throat temperature is consistent with the type of resin used in the extruder. Most semi-crystalline rubbery polymers (PE-type resins) need the feed throat to be cold or at ambient temperature (10–20°C). However, feed-throat temperature should be adjusted higher for semi-crystalline glassy resins (Nylon, PETG, or EVOH) and for amorphous polymers with high glass transition temperature (polystyrene). Make sure that the flow of cooling medium (oil or water) is uniform and at optimum temperature. Any change in these two parameters (flow and temperature) during blown-film operation can cause torque (percent load) changes in the extruder, which could lead to surging or “inconsistent feed”, resulting in output and thickness variations (a film defect). In deep-groove extruders, a separate feed section is inserted between the barrel and the gearbox, which has a certain number of grooves with a specific width and depth. The groove dimensions depend upon the size and shape of the resin granules (resin pellets) and has higher output/RPM than other single-screw extruders discussed above and usually lower melt temperatures. Therefore, the temperature of the grooved feed section (deep grooves) is controlled by a cooling/heating medium (water or oil) and a TCU, depending upon the type of resin to be processed. Make sure that the fluid flow and temperatures are optimum for the resin (check with resin suppliers). Make sure that there is no condensation around the feed throat. The torque requirement and the output are different for these three types of extruders. The grooved-feed extruder requires higher maintenance due to wear on the grooves. Therefore, a periodic data check on the output of the extruder (Kg/ RPM) for a given recipe may be required. Many extruders are equipped with magnets to trap metal (iron) filings; make sure to remove these magnets to clean them periodically and reinstall these magnets. Extruder: Heaters, Cooling Units, and Controls Depending upon extruder size (25.4 mm to 254.0 mm, or 1.0 inch to 10.0 inch), each barrel is divided into three, four, five, or more barrel zones and each zone is separately controlled by heaters (a different Kw heater, depending upon extruder size). In most extruders, the thermocouple wires and heaters around the barrel can be checked after removing the barrel cover. It is ideal to check periodically for any loose wires, defective thermocouples, and faulty heaters. Follow your company’s safety procedures during troubleshooting and maintenance!. Check whether heater bands are correctly wired and heaters of the same watt density (two halves) are used for a given zone, because mixing heater bands of different watt densities and sizes for a given barrel zone can lead to hot and cold spots, which can be intensified by the location of the thermocouple. It is important to ensure that heater bands are correctly wired and leads are not crossed, for example, whether zone 2 is controlling zone 3 and vice versa. This can lead to improper zone heating and issues with extruder melt in the barrel. Make sure that heaters are in “full contact” with the barrel; if required, a heat-transfer

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compound can be applied to the barrel to provide effective heat transfer. Each zone in the barrel is cooled by air or water. Air cooling is done by blowers; make sure that the air supply is uniform around each barrel zone. If possible, hot air from the barrel is removed by ducts to a remote location within the plant (winter) or outside the plant (summer). During extruder operation for different resins, extruder barrels are heated to a set value as recommended by the resin supplier. Make sure to monitor the temperature controller of each extruder zone for any under- or overriding temperatures. This shows whether heaters or cooling units (air blowers) are working normally. Due to viscous dissipation, the displayed “actual value” of temperature for each zone on the human-machine interface (HMI) can provide useful information about any deviations from the “set value” of the barrel zone. Troubleshooting the barrel temperature profile is important to ensure a homogenized melt, with low melt temperature and a narrow temperature spread within the melt. For example, a polymer melt temperature of 225°C (437°F) can have a temperature spread from 220°C (428°F) to 230°C (446°F) or greater, as measured near the wall and at the middle of the melt stream. A low melt temperature and a narrower melt temperature spread is better in the blown-film process for maintaining melt strength and output and reducing gauge variations. One can also change the “set values” of the barrel profile from a “ramp profile” to a “hump profile” or to a “reverse profile” to observe whether a change in the barrel temperature profile leads to change (optimization) in the “actual value” (reducing overrides or underrides) and to a low melt temperature and a narrower melt temperature spread. The barrel may have two probes: one to measure melt temperature, and a second to measure melt pressure. The barrel is also equipped with a rupture disc to prevent excessive melt-pressure buildup in the barrel. Rupture disc rating is very important and should be determined after discussion with your extruder OEM and resin supplier. Make sure that the rupture disc is pointed towards the ground for safety! For each extruder, amp measurements (percent heating or cooling for each zone) and actual RPM as measured by tachometer are other important process parameters that should be monitored for each recipe. A semi-annual or annual comparison of these process data (melt pressure, melt temperature, RPM, amps, torque, and percent load) for a given recipe can provide important information about process changes or extruder/barrel wear. Due to the configuration of extruders around the die, hot air (if not ducted away) can rise and cause local temperature variations around the bubble, causing gauge variations in the film and impacting film and roll quality. For water-cooled extruders, ensure that the water is clean and that its flow is consistent in the cooling jacket around the barrel zone. Water is an excellent cooling medium, but non-uniform cooling around the heated barrel zone can change the polymer solid/melt temperature (depending upon the zone), leading to large fluctuations in the melt temperature inside the extruder, which can in turn cause changes in viscosity, melt pressure, and/or surging. Some barrels are bored for a gas-

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injection system (nitrogen, carbon dioxide, or hydrocarbon gas) to make foamed products. Make sure that the injection port is functioning properly by recording the flow (volume) of the gas or liquid. These are common causes of extruder malfunctions and can be fixed during periodic maintenance or troubleshooting. Screw: Design and Cooling In an extruder, the screw is an important processing element. The screw can be a general-purpose one to process most materials (specifically in coextrusion), but screws specific to processing a given material are also used for dedicated materials in blown-film equipment. During regular maintenance when the screw is removed, the screw dimensions should be checked and compared to the original screw dimensions, which will reveal wear on the screw. Acidic resins or certain masterbatches containing heavy metals do corrode and wear out the screw more than, say, polyolefin. Some screws are bored (up to the feed throat) for cooling inside the screw; make sure that the cooling medium (flow) is uniform and controlled at return. Some screws are bored for injection of PIB or other liquid additives; ensure that the flow of liquid additive is consistent during film production. Manual, Hydraulic, and Continuous Screen Changer, Clamp/Breaker Plate, and Adaptor In many blown-film extrusion lines, the extruder barrel is connected to a screen changer (manual or hydraulic) and subsequently to an adapter. Many new extrusion lines have two melt-pressure probes, one probe before the screen changer (possibly in the barrel), and a second after the screen changer. If the difference between the melt-pressure readings of these two probes is large, the screens are plugged and should be replaced. In most cases, extruders have only one melt-pressure probe, and care should be taken to record the melt pressure once clean screens have been installed. Melt pressure should be monitored and screens (mesh) replaced according to process requirements. Depending upon the recipe of materials (prime resins, recycled content, additives, and masterbatches), the frequency of screen changes (the number of times that the screens should be replaced per shift or per day) may vary. In addition, the process needs to be interrupted during a screen change by stopping the extruders. During a screen change, make sure that the feed throat of the extruder is cooled; otherwise, plastic can melt in the feed throat, causing feeding issues such as bridging once the extruder is restarted. Look out for any melt leakage from the screen changer, because this indicates wear of the pressureactivated seals in manual screen changers. If this occurs, then the seals should be replaced. In hydraulic screen changers, make sure to maintain oil level with proper seals. In some extrusion lines, the extruder barrel is connected to an adapter through a breaker plate; this assembly is clamped with a three-piece clamp or bolted to the flange. During screen changeover, make sure that the heated three-piece clamp is

held with proper gloves for dismantling. A new breaker plate with screens should be ready to install once the old breaker plate with dirty screens has been removed. Make sure that both faces (barrel and adapter) are clean of any polymer melt before installing the new breaker plate with the screens. The extruder must be stopped during changeover of the breaker plate and screens. Gear or Melt Pumps Gear or melt pumps are not commonly installed on blownfilm lines, but some custom applications do require their use in blown-film operation. A gear pump is an accurate metering unit with a stated maximum operating pressure; therefore, care should be taken to avoid a cold start to prevent damage to the gear pump. Often the gear pump is equipped with a rupture disc or two melt-pressure probes (before and after the pump). Ensure that a rupture disc with a proper rating is used and that the melt-pressure sensors are connected to an alarm system with automatic shutdown of the line to prevent damage to the gear pump. Rotator, Die Cart, Die Block, and Single-Layer Die In many single-layer blown-film operations, die rotators are used, which consist of commutator rings and brushes for full rotation of the die and need to be kept clean. However, during oscillation of the die, the thermocouple wire and power cables should have enough length and freedom for movement. The gears in the oscillator should be well lubricated; any wear on the gear can cause vibrations that could affect film gauge. The air hose used for bubble inflation should be removed before starting die oscillation. In a single-layer blown-film operation where a stationary die with the block (mounted on the die cart) is used, care should be taken to torque the die properly to the block or to the cart and label the wires or leads to prevent cross wiring. Alignment Check at Operating Temperature Before operating a blown-film line, the hot components (extruder, screen changer, adapter, and die) are heated to operating temperatures and soaked at this temperature for a specific period of time as suggested by the OEM and the resin supplier. It is recommended to check the torque of hot parts. As the equipment is brought up to operating temperature, due to differences in mass, the centerline heights of the extruder, screen changer, adapter, and die may change at different rates because the components may thermally expand to different dimensions. In general, production floors are not as level as required for installation of a blown-film line. Therefore, care should be taken in the alignment and adjustment of the equipment at operating temperature. The extruder on casters should be leveled and then connected to the screen changer, adapter, and die. The die-cart casters should also be uniformly touching the floor, and the die should be leveled and aligned with the air ring. It is also important to

Section 5.8.  Troubleshooting the Blown-Film Process

check the equipment alignment of the extruder and die with the primary nip. Any misalignment could cause creases, wrinkles, film wandering (film off course), and tracking issues, resulting in gauge bands and poor-quality film. Blown-Film Process: Die-Lip Adjustment During the heat soak process (before operation of the blown-film line), it is important to check the die-lip gap and to ensure that the gap is even between the inner and outer lips of the die. Make sure that the die-lip is clean, with no previous melt in the die lip, because this could impact the die-lip gap measurement. The die-lip gap adjustment (the gap between the outer and inner lips) is important during blownfilm operation. Due to different resin formulations, viscosities, and processing temperature settings at extruder/die and melt pressures, there could be a gauge variation in the film [6], which could be corrected by adjusting the die bolts. Figure 5.8.4 illustrates a common type of die design with adjusting bolts in the ring above the die body. The number of adjusting bolts depends upon the die diameter; usually, for a 203 mm (8″) die diameter, there are six to eight adjusting bolts. Typical die-lip gaps in the blown-film industry are 1 mm (40 mil), 1.5 mm (60 mil), 2.2 mm (90 mil), and 3 mm (120 mm), although narrower or wider die gaps can be used in specialty applications. For example, for high- to mediumdensity foam processes, the recommended die gap is 20 mil (0.5 mm). With reference to die-lip adjustment, some simple steps should be followed. The first step is to ensure that the die is clean and heated to operating temperature (with no plastic inside) and to check the die gap across several points [3]. If the gap is not uniform, then the second step is for the op-

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erator to loosen all die-lip adjusting bolts. The third step is to adjust the die gap by pushing on the ring (because there are no pull bolts). The fourth step is a small adjustment that should be made each time, because the initial die gap (when the die was put together) was adjusted to uniform thickness (without the plastic). The fifth step is, once the die adjustment is done, to finger-tighten all the bolts using an “Allen key” to make sure the die ring is properly locked. Note that these adjustments (steps two through five) can be performed when the blown-film line is in operation; however make sure that any such adjustment is done in minor steps because during operation, one does not know how much the die gap should be adjusted. In addition, after each small adjustment, the frostline (if visible) should be observed, and the gauge should be measured, before making any further adjustments to the die-lip gap. To understand this concept better, let us consider the case where the blown film produced has half thick and half thin gauge profile. The thick gauge of the blown film originated from the narrower die gap (yes, you read it correctly) because the thinner melt originating from this narrow die gap cooled faster, leading to a thicker film. On the other hand, a wider die gap provides thinner film because the thicker melt (from the wider die gap) took a much longer time to cool down and was stretched more, leading to a thinner film. This concept can be further understood from Figures 5.8.5(a), (b), and (c), which illustrate a non-uniform die-lip gap (or a die gap that was not adjusted (a). This results in an uneven (high and low) frostline on the bubble (b), which leads to thicker and thinner bands on the film gauge profile (c). At point B, the wider die-lip gap of 1.5 mm (60 mil) resulted in thinner film of 50 microns (2.0 mil) and a higher frostline at this point on the bubble. At point D, the narrower die-lip gap of 1.2 mm (48 mil) resulted in a thick-

FIGURE 5.8.4.  Die adjustment bolts in a single-layer die.

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FIGURE 5.8.5.  Blown-film gauge variation caused by uneven die gap.

er film of 65 microns (2.6 mil) and a lower frostline at this point on the bubble. Once the die gap is adjusted, the film will show uniform thickness. Multilayer Die (Conventional and Stackable) In many multilayer blown-film operations, conventional or stackable dies are used. Figures 5.8.6 and 5.8.7 show two different stackable dies. Care should be taken to label all zones and leads because any cross wiring can lead to a wrong temperature setting, specifically when high-temperature-processing polymers such as Nylon and PETG are coextruded. If the die is dismantled, make sure that each die mandrel is properly cleaned (polished to 2–4 RMS as recommended by OEM). Use only 12-grade bolt design for high temperature, like Holochrome or Allen. All bolts should be inspected and any stretched bolts replaced, and all bolts should be torqued to proper OEM specification (the die supplier). Improper torquing can result in interlayer leakage, which can cause extensive downtime to evaluate which layer/s have issues. All heater bands should be checked for proper watt densities. Do not mix heater sizes or watt densities, and make sure to match the two half-heaters for die mandrels. Die mandrels should be cleaned with soft brass or copper mesh only. For auto profile control (APC) dies, the cartridge heaters (or any other heater types) used for the APC function should be inspected. Make sure that the Home position of the APC system matches with the specific heater (e.g., zone #1). Make sure that each heater is labeled, displayed on the control panel (HMI), and functioning properly. Air Ring (External Cooling to the Bubble) The air ring is installed on the blown-film line to cool the bubble. Most often, an insulation pad is installed on the die before the air ring is installed; the purpose is to insulate

the air ring from the thermal effects (heat) of the die. Once the air ring has been installed on the die, make sure that the air-ring opening and the die opening are concentric (even gap). The air ring can be centered by adjusting or mounting screws. Any misalignment of the air ring to the die can cause gauge bands during blown-film operation. A dual-lip air ring (for some extrusion processes, a single-lip air ring is also used) is commonly used to cool the blown-film bubble [7,8]. The dual air-ring lip-set should be clean, free of oil and waxes, and dent-free to allow a uniform flow of air to impinge on the bubble from both the primary and secondary orifices of the air ring. The air ring should be insulated if this operation is performed in a humid environment, because moisture condensation on the cold surface of the air ring can cause moisture marks on the film. The air filter of the air-ring blower should be clean to remove any dirt, impurities, or angel hair, because these can be trapped inside the air-ring body, causing gauge variations. The cooling coil used to cool the air should have a uniform flow of water. The air duct from the cooling coil to the distribution box should be insulated to maintain the cold-air temperature. It is ideal to use hoses that are of the same length, flexible, and nickfree to provide identical air volume and pressure from the distribution box to the air ring. The air distribution box is located under the die or on the mezzanine. The illustration shown in Figure 5.8.8 shows that the gauge variation did not occur from a centered die, but an off-center (tipped) air ring. Dirty die lips or uneven die gaps (as mentioned above) or dirty air rings show similar thickness profiles, with thick and thin gauges in the blown-film. A visible frostline in the blown-film operation and film thickness measurements are good indicators of gauge variation, which may arise for many reasons. Therefore, it is important for the operator or process engineer to carry out a root cause analysis for gauge variation; otherwise, there is a fair chance of making incorrect adjustments without getting the proper results.

Section 5.8.  Troubleshooting the Blown-Film Process

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FIGURE 5.8.6.  Multilayer die (three layers), showing IBC pipes and mandrels (courtesy of Macro Engineering).

FIGURE 5.8.7.  Multilayer flat-pack die (nine layers) (courtesy Macro Engineering).

In many applications, the air ring is also used for auto profile control (APC). Make sure that the APC mechanism for the air ring, consisting of stepper motors, control volume elements, or heaters, is free of wax or dirt. Any contamination could impact the ability of the auto profile control elements of the air ring (APC) to perform at optimum level for film gauge correction. In APC systems, shift angle (twist angle is discussed later) should be checked to make sure that the home positions of the gauge sensor and the APC air ring (zone #1) are the same (this should be checked with the APC die also). Any deviations in shift angle should be checked by stopping the rotation of the die, sensor, or hauloff nip and by activating a specific zone by putting in more air (air ring) or

heat (die). Make sure that the change in gauge is registered at the correct zone in the APC system (die or air ring). If the sensor home position and APC (zone #1) are aligned, the shift angle is zero, and the correction will be in-phase. If the sensor home position and the APC (zone #1) are not aligned, the gauge correction will be off in the APC system. The shift angle should be checked periodically while the line is in operation because this will help to correct the mapping of all the control air valves or stepper motors (for the APC air ring) and all the die heaters (for the APC die). During processing, “twist angle” is another important parameter. Most polymers are viscoelastic, which means that as the primary nip is oscillating (the die is fixed in multilayer blown-film

FIGURE 5.8.8.  Blown-film gauge variation caused by tipped air ring.

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lines), there is a lag (or angle) between the oscillating nip and the bubble due to the elastic component of the polymer melt, and this causes a twist angle. The twist angle depends upon film thickness, blowup ratio (layflat), and film tackiness. It is important to calculate (or approximate closely) the twist angle as a correction factor and to input the value to the APC software for proper gauge correction. If the twist angle changes from one type of film structure to another, the mapping to each zone of the air ring or die (the APC system) will change, which in turn will perform the gauge correction at the incorrect position on the film. The illustration in Figure 5.8.9 shows an automatic gauge control (APC) air-ring system. Many multilayer blown-film extrusion lines are installed with a triple lip or a binary set of air rings specifically for fabricating high-output polyethylene film. Ensure that the centering of these air rings to the die is very precise. In blown-film operations with triple-lip air rings, the air ring is lifted above the die by a four-screw elevating mechanism or center assist mechanism. Ensure that this vertical movement is straight, with no vibrations. Similarly, in cases where a binary set of air rings is used in blown-film operation, the lifting mechanism for the secondary air ring (the primary air ring is installed on the die) must show vibration-free movement, and the primary and secondary air rings should be concentric to the die lip. Any vibration or misalignment could cause gauge bands (a film or roll defect). Internal Bubble Cooling (IBC) Internal bubble cooling (IBC) is used to cool the bubble from the inside. IBC consists of inlet and outlet pipes and of pancakes (two or more stacks). The inlet and outlet pipes are built primarily of light metal (aluminum) and should have proper thermal insulation to prevent heat exchange between the pipes carrying the cold inlet air and the hot exhaust air. IBC pancakes usually consist of two or more stacks; each stack has orifices with certain openings measured in mil (mm) for the flow of cold air towards the bubble. Make sure that these orifices are not plugged with dirt, low-molecular-weight oils, or grease, because these could cause gauge bands in the film. Therefore, periodic cleaning of IBC pancakes and pipes should be performed. During IBC reassembly, the IBC pancakes should be checked to ensure a uniform orifice gap opening for each stack, because any misalignment or non-concentricity could change the air flow volume, leading to thick and thin spots (gauge variations) in the film. The IBC blower filter should be changed periodically because clean air (oil- and dirt-free) is required and should be used for inlet to the IBC. Similarly the exhaust blower for the IBC should be inspected. In many IBC operations, the control valve is on the outlet (exhaust air-supply pipe). Make sure that this valve is not stuck and provides free opening or closing for the exhaust air. IBC increases output by 25–30%. Most blown-film lines do have IBC installed in the line; however, some old lines are being retrofitted by drilling the die to accommodate IBC. In such retro-

fits, make sure that the extruder can pump more output at a reasonable melt pressure and temperature and that the die is designed to accommodate the higher output. Make sure that the web handling system (tension controls) can handle higher line speeds. Fluctuations in film width can happen with IBC operation, and therefore the air intake and exhaust air blowers should be balanced properly with the correct location of the IBC sensor, as shown in Figure 5.8.10. If the sensor is at the wrong location, as shown in the illustration, IBC performance may not be optimum. Bubble Cage, Gauge Measuring System, and Bubble Guide Bubble cages are used for film containment and stabilization after the air ring [9]. The bubble cage should also be aligned very well with the air ring/die and the primary nip using a laser beam or a plumb line. It is important that bubble containment be concentric; any non-concentric configuration of the bubble cage could lead to squeezing the blown-film bubble, which could eventually cause gauge variations or shifting of the bubble to one side, leading to soft creases. The cage should be free of vibration during axial motion (up and down) or when opening and closing (for different blowup ratios). The moving bubble is in contact with a rolling surface within the cage; therefore, make sure that all rollers are free-turning to minimize friction and drag and thereby improve gauge and film uniformity. In

FIGURE 5.8.9.  Air-ring dual-orifice automatic thickness control (courtesy of Plast-Control, Inc., USA).

Section 5.8.  Troubleshooting the Blown-Film Process

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FIGURE 5.8.10.  IBC sensor position in the blown-film process (courtesy of D. R. Joseph Inc., Texas, USA).

some bubble cages, rollers are fitted with removable sleeves because dirty sleeves can make impressions on the bubble. Make sure to remove the dirty sleeves, clean them, and install new sleeves periodically. The mechanical functionality of the bubble cage is important to provide enhanced bubble stability for optimum layflat and gauge uniformity. In the vicinity of the bubble cage (close to the air ring), excessive air disturbances near the contact surface of the bubble with the cage rollers should be eliminated, because these may undermine the efficiency of the cage by preventing its contact with the bubble and thus negatively impacting gauge uniformity. IBC sensors and bubble diameter (layflat control) sensors are mounted on the bubble cage. Make sure that these sensors are cleaned periodically. As well, the cage position (or the position of the IBC sensor) relative to the frostline of the bubble is very important, as illustrated in Figure 5.8.10. The gauge measurement and control system is often mounted above the bubble cage with the sensor head oscillating around the bubble. In many multilayer lines, the gauge system could also be mounted in the collapsing frame because the collapsing frame oscillates with the nip; therefore, the gauge system scans the thickness of the bubble. Make sure that the sensor head is cleaned periodically (contact or non-contact). Due to the difference in the functionality of various sensor types (capacitance, gamma backscatter, X-ray, or infrared), make sure to follow proper maintenance and troubleshooting instructions according to the manufacturer. In addition, capacitance-type sensors do not require any regulation. However, for a gamma backscatter sensor, the main source of measurement is radioactive, and therefore this sensor should be handled with proper licensing through national nuclear regulatory commissions. Similarly, for blown-film lines with tall towers, a bubble guide is installed above the bubble cage and below the collapsing frame. Concentricity of the bubble guide with the die or air ring and primary nip is important because after this containment, the bubble enters into the collapsing frame.

Collapsing Frame, Side Stabilizers, Gussets, Primary Nip, Horizontal and Vertical Oscillating Nips In blown-film lines, after the bubble has been contained by the bubble cage or bubble guide, the next step is collapsing the bubble from a round tube to form a layflat tube. Collapsing frames and side stabilizers are built of different materials and have different configurations, including wooden slats, roller collapsers, segmented roller collapsers, or a collapsing frame with an air cushion [10]. The collapsing frames should be aligned with each other and to the primary nip with side stabilizers. Alignment of these components (primary nip to the center of the die or air ring) must be done by plumb line or laser beam. Any misalignment creates a skewed bubble, leading to hard or soft wrinkles or creases (film or roll defects). The angle and length of the collapsing frame should also be consistent with the nature of the film material to be collapsed (rigid or hard, flexible or stretchy). The spatial arrangement or the gap from the top of the collapsing frame to the primary nip is critical; an excessive gap often leads to slight re-inflation of the bubble, which can cause soft creases on the edge or the middle part of the collapsed film. The wooden collapsing frame (maple slats) can be covered with a low-surface-energy material to reduce frictional heat. Some blown lines are equipped with wooden collapsing frames, where air is pumped through small holes to avoid friction between tacky films and the collapsing-frame surface. Make sure that all these holes are open (not plugged with oil or wax) and that the collapsing surface is clean, because this surface can reduce frictional heat, but requires periodic maintenance. Carbon rollers or other low-friction rollers would be ideal for the collapsing frame, but it is critical to clean the rollers from dust and any oil or waxes from polymers, which can create drag, leading to higher friction and thus deforming the film (bagginess) or can make scratch marks on the film (film and roll defects) [11]. The collapsing frame plays an important role in blown-

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FIGURE 5.8.11.  Web path differential (Knittal, 1992).

film fabrication; this means that if the line is not designed for a given material (web), even if the extruder or the die has been modified, it may require a proper collapsing frame for web handling. Therefore, make sure to check the angle and length required to collapse any new proposed film to be fabricated on an existing blown-film line. During the process of collapsing a moving bubble from a circular cross section to a flattened tube, the film at the edge travels both at a different speed and in a different direction compared to the center of the film. The inherent incompatibility in collapsing a bubble from a tubular (circular) cross section to a flattened tube is illustrated in Figure 5.8.11, which shows a difference in length when the bubble is collapsed. In this specific example, a 113.2 cm (44.5 inch) diameter bubble was collapsed to a layflat of 177.8 cm (70 inches) at a 25° collapsing angle. This figure shows that the film collapsing length at point A is greater than at point B. This difference can be reduced as the collapsing frame length increases, as shown in Figure 5.8.12, ‘% Difference Crease’ vs. ‘Center Length’ [12]. However, there is an issue because the center of the film contacts the collapsing frame for a longer time compared to the edge of the film. This longer contact creates more drag on the film, and the intensity of this drag could further increase if the collapsing frame were not centered, or the rollers were not turning freely, or the wooden slats were not covered with a lowfriction material. This higher drag in the film creates more friction for the center of the film, compared to lower drag for the edge of the film. This higher drag or friction induces more stress in the center of the film, leading to stretching or permanent deformation of the film, resulting in center sag. This center sag could cause light creases or soft wrinkles.

FIGURE 5.8.12.  Percent difference, crease vs. center length (Knittal, 1992).

Films that are relatively extensible, such as LDPE, LLDPE, and mLLDPE or their blends with plastomers, are more forgiving and readily adapt to the changing geometry of the collapsing bubble into the layflat tube with minimal adverse effect on gauge uniformity and film flatness. However, films containing stiffer polymers such as Nylon, PETG, HDPE, PP homopolymer, COC, and PS possess less flexibility (semicrystalline glassy polymers or amorphous polymers with high glass transition temperature). Therefore, such films should be collapsed while warm, which provides some flexibility to the bubble. In many blown-film lines, infrared heaters or hot-air blowers are installed either just below the collapsing frame or inside the collapsing frame to keep the film warm (“warm” means above or close to the glass transition temperature, Tg; above this temperature, a polymer changes from a glassy or rigid phase to a rubber or flexible phase). The number of heaters and the air temperature depend upon the thickness or modulus of the film. The second option for stiffer films is to collapse them at a reduced angle, but making sure to use a low-friction collapsing surface; otherwise, increased contact time of the film with the collapsing frame could lead to more drag and stretch marks. Bubble collapsing can be analyzed using a simple triangulation method from the circular cross section to a layflat at the nip. The other method is graphic representation of collapsing length vs. angular position around the circular tube, but this method is beyond the scope of this section. If gussets are installed on the line, make sure that the inand-out movement is synchronous. The gussets should be centered and aligned with the primary nip and the die or air ring.

Section 5.8.  Troubleshooting the Blown-Film Process

The primary nip may consist of a steel roll and a rubber roll; make sure that there is not much difference in the temperature of the primary nip rolls, because this can induce slight curling. If provisions are there to cool the steel roll, make sure that the water flow is uniform. Ensure that the temperature used does not create condensation, especially in towers where corona surface treatment is performed; otherwise, the treater will short out, damaging the sleeve and pinholing the film. The primary nip drive is often used as the master drive and is installed with an encoder to measure exact speed. Make sure that this drive and other downstream drives are synchronized for speed (RPM); otherwise, excessive web tension could cause web handling issues (blocking or film stretching). In some blown-film lines, an S-wrap consisting of two steel rolls is installed after the nip. The rolls are drilled for water flow and temperature and controlled by TCU to cool the film to prevent or reduce film blocking. In multilayer blown-film lines, it is easier to rotate the primary nip than the multilayer die, unlike a single-layer line, where most often the die is oscillated or rotated and the nip is stationary. Therefore, the primary nip may be equipped with a horizontal or vertical oscillating nip for film randomization to spread gauge variation across the width of the film to make uniform rolls. Various reasons for gauge variations and methods of randomization to make uniform film rolls are discussed later in this section. Gauge Variations (MD & TD) The blown-film process is known to provide film with superior and more balanced physical properties compared to cast film. However, in the blown-film process, even if the extruder/screw, die design, and process conditions are fully optimized, the film produced will have gauge variation from the target thickness. Film thickness variation is reported as standard deviation from a mean value as ± 2 sigma. In statistics, 2-sigma means that 95.47% of measured points across the width are within specification. Gauge variation can happen in the machine direction (MD) or the transverse direction (TD) of the film. There are several reasons for gauge variation as discussed in previous subsections, but in brief, some issues are: (1) Bulk density of the recipe changes due to addition of fluff. (2) Temperature of the feed throat changes due to change in temperature or flow of the cooling medium. (3) Simultaneous cooling of two- or three-barrel sections results in viscosity and minor output change. (4) Extruder motor drive should not have speed variations of more than 1 percent with a base motor speed at 95 percent load. (5) Variation in melt pressure or melt temperature can causes change in output. (6) A non-optimized screw or a wrong barrel temperature profile can cause surging.

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Machine direction (MD) gauge variations can be checked by stopping the rotation or oscillation of the die (single-layer) or the primary nip (horizontal or vertical) for multilayer lines and measuring the gauge in the machine direction. Transverse direction (TD) gauge variation occurs across the web and causes high or low spots in the gauge, leading to gauge bands in the roll. As more film layers are added to build the roll, the thicker sections of the film build upon themselves to create high points, and the thinner sections of the film create low points across the width of the roll, as illustrated in Figure 5.8.13(A). This distorted roll has several issues during downstream converting. However, a uniform roll is obtained when gauge bands are randomized (by rotating/oscillating the die or oscillating the top nip), as shown in Figure 5.8.13(B). Similarly, a slight gauge variation on one end of the roll (one end slightly thicker than the other end) is sufficient to create a tapered roll configuration, as illustrated in Figure 5.8.14. This could happen due to lack of proper randomization or inconsistent gauge from a fixed source, such as an air draft or rising heat from the extruder. This distortion is difficult to detect with a hand-held micrometer and requires a careful analysis of total gauge profile. This will show slightly more hardness on the heavy side, but may not be easy to detect. This phenomenon is often described as camber in the flat film and can easily be identified by unrolling a few meters of roll on a level floor and looking for a slight ripple on one side of the film. Therefore, during fabrication, blown film is either rotated or oscillated to distribute the gauge bands to make a more uniform roll for downstream applications. Gauge Randomization Methods There are several methods to perform oscillation or rotation of blown film. In a single-layer blown-film process, die rotation or oscillation has been proven to be a successful

FIGURE 5.8.13.  (A) Gauge bands on the roll (no randomization); (B) Uniform roll when gauge bands are randomized (Vedder & Roller, 2005).

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FIGURE 5.8.14.  Tapered Roll of Blown Film (Vedder & Roller, 2005).

method of gauge randomization. In recent years, oscillation of the die has supplanted rotation because it eliminates the need for maintenance of collector ring assemblies and simplifies temperature control; this approach is mainly suitable for low-pressure resins (below ~4500 psi). The oscillating or rotating single-layer die will randomize gauge variations occurring in the hot sections (extruder and die), but any gauge bands appearing during collapsing or in the downstream section are not addressed. In a very limited number of blownfilm processes, the extruder and die are mounted on a common platform, which oscillates through 360°. This method is more effective for gauge randomization for all gauge irregularities from the extruder, die, and air ring. However, this method is expensive and impractical for large extruders and dies or for proper space utilization. In addition, this method does not address the gauge irregularities that may arise from the bubble cage, air drafts, or the collapsing frame. In some other rare cases, the winder is mounted with the nip on a rotating platform. This method distributes most gauge irregularities, but is an impractical method for most multilayer machines. A more commonly used method for gauge randomization in multilayer extrusion processes is to use vertical or horizontal oscillating nips, which are installed on blown-film lines. This assembly consists of a collapsing frame, nip rolls, and an oscillating platform. This method eliminates many disadvantages mentioned with other methods. For multilayer blown-film lines installed with a horizontal oscillating nip, the film remains parallel to the floor, the turning bars are horizontal, and the film is directed up to 360° rotation. The fixed turning bar is a hollow tube coated with low-surfacefriction material and having several small holes across the width of the roll. Air is forced through these holes to provide a cushion between the roll and the film sliding over it. The turning bar rollers with small orifices provide an air cushion to the film, and the film slides over the rollers. These orifices should be open to prevent film dragging over the rollers, which could cause scratch marks on the film (film defect). Make sure that the oscillating platform has a steady, smooth movement; otherwise, any shaking or jerking can

result in soft creases or hard wrinkles, depending upon the material. It is equally important to use a proper oscillation speed (often between 120 and 300 seconds for one rotation), depending upon the stiffness and thickness of the multilayer film and also depending on the film gauge and line speed. In this case, the collapsing frame with side stabilizers is important to lock the bubble to prevent bubble movement, which can cause wrinkles. The advantage of this system is that it requires less headspace and is easy to thread, but depending upon the height of the tower and the elastic memory of the film, the collapsing bubble will lag oscillation. This could be a concern to processors who are fabricating relatively largediameter sheeting rolls with in-line slitting of multiple rolls, because it limits the amount of gauge randomization in all the slit rolls (roll defect). In multilayer blown-film lines with a vertical oscillating nip, the unit can be modified to provide 720° of oscillation. This method provides better gauge distribution than the previous method, but has the disadvantage of requiring more headroom. In addition, wide webs should be supported by idler rollers; otherwise, web wandering or sagging could result in wrinkles. A randomized blown film will have better roll uniformity, as illustrated in Figure 5.8.13(B). Water Bath In many multilayer blown-film lines, a water bath is installed on the line with the aim that the outer Nylon layer should absorb moisture to reduce curing in asymmetrical structures. Ensure that if the nip is in the water bath, its speed (RPM) is synchronized with the primary nip, because excessive tension/pulling could have negative effects on curling. The residence time of the film in the water bath and the water temperature should be monitored for optimized moisture absorption. Water should be treated by UV light to prevent microbial growth, should be filtered, and can be reused. Tower and Edge Guide The blown-film tower should be sturdy and free of vibration; otherwise it can impact bubble stability (in the air ring and during bubble collapsing), resulting in soft creases and safety issues. The rollers on the tower should be clean, with low inertia, and properly aligned; otherwise, web tracking could cause downstream wrinkling issues. As the film goes through the oscillating unit and travels down the tower over the tower rolls, film position could go off track. To make a good roll at the winder, an edge guide is critical. Make sure that the web edge sensors are functioning properly for tracking the film edge. An edge guide is essential to align the film on one edge and is important for winding and eventually unwinding during downstream converting. Corona Treatment Although there are several methods to increase the surface energy of plastic films, corona treatment is the most com-

Section 5.8.  Troubleshooting the Blown-Film Process

monly used method to increase the surface energy of blown films. In the blown-film extrusion process, films are often corona-treated to increase surface energy for downstream conversions such as printing, coating, and lamination. The corona unit consists of two main components: the power supply and the electrodes. Make sure that the power requirement or watt density is sufficient to treat materials requiring different surface energies, such as polyethylene, polypropylene, and polyester. Corona treatment is often done on the outer tubing layer (both sides of a collapsed tube). Make sure that the treatment bar is straight, with a proper gap across the full width to the roller. The roller is grounded and covered with a dielectric sleeve unless it is a bare-roll system. The selection of sleeve material is important because depending upon the dyne level (for example, 42 dynes/cm or higher), the sleeve is exposed to high temperatures and ozone. Therefore, the sleeve should have good chemical, thermal, mechanical, and electrical resistance. The roller should be grounded back to the treater to prevent any backside treatment. Make sure that the segmented fingers used for the electrode and the sleeve or roller are clean. The power should be off for any maintenance or troubleshooting on the corona treatment unit. The exhaust duct should be inspected regularly for any kinks or leaks. Dyne pens are used to check treatment levels. These inks have a certain shelf life, and therefore dyne inks should be purchased at regular intervals. Treatment should be checked to make sure that it has covered the full roll width. For lane treatment, the treatment level should be checked for treated and non-treated areas across the full width of the film. Over-treatment on film edges may negatively impact seal integrity. It may be important to check the treatment level 24 hours and again one week after treatment because this shows retention of the treatment and may be helpful in checking whether any further treatment is required during downstream converting processes (printing and lamination), specifically where higher surface energies are required. Secondary Nip, Web Slitting, Spreading, Dancer, and Load Cells A secondary nip is installed on most single- and multilayer lines. The nip speed should be synchronized with the primary nip speed, and web tension should be monitored. This nip is used to slit the collapsed tube into two or more sheets. Slitting is an essential step for all extrusion operations. Make sure to select a proper slitting system (razor, score, or shear slitting) to obtain consistently clean cuts to avoid raised edges or ridges along the cut line. For a trimless operation, the blades should be sharp (make sure that company safety protocols are followed in handling the blades on the line). Often compressed air is used in a trimless system; make sure that this air is clean and free of oils. The suction blowers and pipes used to carry the trim should be monitored. In a blownfilm operation, spreader rolls are used to bring a wrinkle-free flat web to the winder. Make sure that the spreader rolls (reverse crown spreader, flex-spreader, or bow roll spreader) or devices (dual-bowed roll spreading system) are properly ad-

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justed and are used for the correct film or sheet applications. For example, the amount of reverse crown in the spreader roller depends on the extensibility of the film [13]. During web conveying, tension control is of prime importance. Web tension is sensed (not measured) by a dancer; however, web tension is measured by a load cell. Each (dancer or load cell) has its pros and cons. The dancer has more storage space or web length than the load cell, which means that any changes that create a web path difference, such as roll changeover or movement of a linear lay-on roll, do not cause large tension changes. The disadvantage of a dancer is that its inherent inertia and friction make it difficult to respond quickly, and high maintenance requirements or any alignment issues can cause creases or wrinkles. On the other hand, a load cell measures tension and responds quickly to any transients during steady-state conveying (winding). Load cells are low-maintenance, less costly, accurate, and can be installed close to the winder after spreader rollers. Winders In blown-film processes, large-diameter rolls (1000 mm– 1500 mm, 40–60 inch) are often wound to reduce changeover time for downstream equipment. These rolls must be produced with optimum hardness for transportation and during unwinding for subsequent operations such as printing, lamination, slitting (multiple-up), and bag-making. Different applications in blown-film processes require making a wide range of films and sheets; some may be thin, flexible, tacky, and soft, whereas others may be thick, less extensible, rigid, and stiff. For example, small rolls for stretch film are tacky, soft, and flexible, large rolls for lamination film are flexible, and large rolls of polystyrene or Nylon 6 film are rigid or stiff. In blown-film processes, three main types of winders are used to collect rolls: surface winders, center (turret) winders, and combination center/surface winders [14,15]. Figure 5.8.15 shows the variation among the various types of surface winding. Make sure that roll hardness is optimum, because rolls wound too soft may telescope (go out of round) during transportation. Rolls that are wound too hard will exaggerate web defects (high or low spots in the webs) due to deformation of high spots in the film. Roll hardness can be controlled by three methods during winding: web tension, nip pressure (or winding drum pressure), and torque (from the winding shaft). Depending upon the winding method, one or two or all three methods are used to control roll hardness. Web tension is an important parameter and is often calculated empirically to obtain “maximum allowable web tension” (during film fabrication) for a given thickness and width of the film without inducing permanent stress in the film. In surface winders, the measurable (by load cell) web tension is between the lower nip and where the film enters the surface drum. Taper tension is another important parameter during roll buildup. As the roll builds up, the winding tension is tapered (reduced) to obtain good roll density. The tension is tapered up to 50% from the original set value of winding tension. For example, if at the beginning of the roll,

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roll on the winding roll. The motor and drive control system must provide precise speed and torque control throughout the buildup of the winding roll. Any small error in winding speed control results in a much larger error in web tension, affecting roll quality. However, in torque-control winding, torque and tension are directly related. Hence, roll hardness depends upon the type and design of the winder, the thickness, width, and type of material, and the winding speed. Figure 5.8.16 illustrates the types and characteristics of film collected as rolls on these three different winder types (surface, turret, and surface/center winders). Therefore, it is important to measure roll hardness and control it to provide optimum-quality rolls for downstream operations. Necessary Tools and Equipment for Troubleshooting

FIGURE 5.8.15.  Variations in Surface Winding (Courtesy of Macro Engineering).

the set winding tension is at 9 kg (20 lb) with a taper tension of 25%, the winding tension will be tapered or reduced to 6.75 kg (13.5 lb) as the roll attains maximum OD. If the film is wound with high winding tension, the built-in stresses will be relieved (due to the viscoelastic nature of polymers), resulting in compressive forces in the inner layers and in extreme cases leading to a crushed core (roll defect). With surface winders, web tension and nip are the dominant factors for roll hardness. For elastic films, web tension controls the hardness, and for inelastic films, nip pressure is the main influencing factor. In surface winding, large rolls can be produced, but air cannot be wound into the roll because the winding drum is turning the rolls by friction. Therefore, the rolls produced may have blocking issues, and if the film is not fully randomized, the high spots could be distorted, leading to roll defects. To provide softer rolls, gap winding can be added to a surface or center winder. In this method, a small gap is maintained between the lay-on roll and the surface drum to enable a small amount of air entrapment, thereby producing softer rolls. In center (turret) winding, both web tension and nip pressure are used to control roll hardness. In turret winders, torque is applied through the winding shaft (through layers of film). Therefore, roll diameter can be a limitation due to turret size and during indexing, and there is a higher probability of generating more scrap. In center/surface winding all three winding principles (web tension, nip pressure, and torque) apply, and web tension is controlled by the surface drive. The web tension load cell provides the feedback to trim the drive to control web tension. The torque applied through the center drive (through layers of film) provides wound-in tension. The nip pressure is provided by the loading mechanism of the lay-on

The maintenance (electricians and millwrights) and production staff (process engineers, operators, and line attendants) often cooperate to detect and troubleshoot any issues on the blown-film line. They should be given proper tools (calibrated when required) to operate and maintain the line. Thickness gauges (micrometers, gauges, and calipers) should be calibrated periodically. For new or upgraded lines, statistical process control (SPC) and process parameter trends should be used to check for any abnormalities in process conditions, film thickness, and output. For any maintenance work, the area should be clean, with proper tables, tools, and safety equipment such as gloves, glasses, hearing protection, safety shoes, and high-temperature (Kevlar) sleeves. On a regular basis, all components of the line should be visually inspected with all safety guards on! If there is any unusual sound from any part of the line, it should be checked. For example, if a suction blower on the resin conveying system is turning on more frequently, one should check for resin (the Gaylord or surge bin may be empty), a dirty filter, or any other mechanical/electrical issues. Similarly, extruder drives, motors, chillers, cooling coils, the resin-handing system, air-ring/IBC blowers, and other components on the line should be monitored periodically for any unusual noise. Air filters must be changed, and the flow of water or other fluids (oil for the gearbox) should be checked periodically. Thermo-anemometers can be used to check the temperature and speed of air for the air ring and IBC. Temperature probes (calibrated) can be used to check surface temperatures on gearboxes and motors. Equipment that is properly designed, installed, aligned, and maintained can operate smoothly for many years. Blown-film equipment may require quarterly, half-yearly, or yearly maintenance, depending upon the frequency required for components for optimum operation. For example, filters for blowers and cabinets require more frequent changeovers than thermocouple or heater wires (which actually need inspection and testing). Roller bearings may need frequent inspections for smooth turning to avoid drag and in turn scratch marks on the film. Core sizes, cutting blades, and any additional trim removal system components require close attention. Finished-roll handling equipment such as roll extractors and cranes for handling the shaft with

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FIGURE 5.8.16.  Comparison of Surface to Center to Surface/Center Winding for Different Films (Courtesy of Macro Engineering).

the roll should be properly aligned and handled for easy roll removal. Many single-layer and some multilayer lines are also installed with printing, in-line embossing, slit sealing, gusseting, and bag-making equipment. Troubleshooting for this equipment should be carried out according to OEM (or manufacturer) manuals and prescribed procedures. FILM AND ROLL DEFECTS In a blown-film process, ideally film defects should be detected and eliminated before the roll is shipped. In many cases, defects are marked by the operator for easy identification of the faulty region. This helps during downstream roll converting (lamination, printing, or bag-making). Film Defects Film properties (optical, thermal, mechanical, and barrier) can be measured by following ASTM standards. Film appearance (haze, clarity, gels) is often used to describe film quality during visual inspection and is determined by a combination of equipment, resin, and process optimization. Resin quality is controlled by the resin supplier, who provides a technical data sheet and a COA (certificate of analysis) mentioning the melt index, density, additive package, recommended melt temperature, process temperature profile, gels, fish eyes, moisture content, and other properties. Resin handling at every stage in the blown-film manufacturing facility should ensure prevention of cross contamination, angel hair, dust, and core shavings (paper) getting into the resin or additives. Dirty hoses or pipes can create stream-

ers and fines that can create gels. If off-spec resin and higher percentages of recycled materials are used in the process (depending upon the quality of the recycled materials), then gel content in the film can increase. Therefore, make sure to optimize process conditions (extruder and die temperature), change screens frequently (if they are plugged), and count gels on fabricated film. Hoses or wands can fall out from a Gaylord, and the vacuum system can pull debris (dust) from the floor, leading to off-spec film. If resin is delivered from a fiber drum, any contamination of these fibers can cause gels. Dies that are left stagnant at operating temperatures (specifically for multilayer dies processing high-temperature polymers) can lead to polymer degradation into brown and black carbon particles, which may also stick to the die lip and make die lines. If an extruder is operated at high temperature, but with low output, there is a fair chance of finding gels due to long residence time and higher temperature in the extruder. During screen changeover, the extruder needs to be stopped, but how long it is stopped has an impact on film quality when the extruder is restarted. There could be gels in the film, which means that one needs to purge, resulting in scrap and yield loss for a longer time before quality film can be produced. Gels originating from a screen changer or a dirty adapter may appear only in one area of the film. Film defects can also occur if the bubble is not locked in the air ring; the result may be high gauge variations. If the bubble is too close to the air-ring deflector lip, chatter marks on the film may result. High melt temperature and too-high air impingement flow onto the bubble can make the bubble vibrate, touching the metal and causing marks. To fix these issues, the air-ring setting and even the melt tem-

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perature controller must be corrected. The film may have scratch marks, which are typically coarse, whitish, and may be spaced intermittently in the machine direction. Haze, die, and weld lines may appear on the film; these should be corrected by purging the die and/or by die-lip cleaning. In many applications, high-output or very-low-MI PE resins (high-viscosity) suffer from melt fracture or applesauce or orange peel effect. The effect appears on the film as a faint to medium sharkskin pattern or aberrations, causing objects to be distorted when looking through the film. A proper die gap, process conditions, or addition of a process aid can eliminate this. Interlayer instability in coextrusion structures does generate a sharkskin effect, but it is between the two layers. This can be eliminated by adjusting the layer ratio, the viscosity of one of the layers, or the temperature. In coextrusion, a wave type of instability does occur, which can also impact film clarity and can be fixed by changing the viscosity and velocity of the two layers. Incompatible resins or poor mixing of semi-compatible resins also show such effects. During blown-film fabrication, film blocking occurs for many reasons, such as lack of anti-block additive due to high web tension from the primary nip to the winder, or a tacky inner layer, or an inner layer (resins below 0.912 MI) that may have been extruded at higher melt temperature and therefore not properly cooled. In many cases, hot and/ or humid conditions make cooling less effective, inducing roll blocking. Tower height may be low, resulting in blocked film. Surface treatment and high gloss can to some extent induce film blocking. It is important to check the tension and if possible, collect the trends (from load cells). In many instances, films with low BUR are produced, giving a high DDR (drawdown ratio), and can cause splitty film, which means too strong an orientation of polymer chains in the machine direction or when a wide die gap is used. In this case, the die gap can be reduced to fix the issue, but making sure that this does not result in melt fracture in the film. HMW HDPE resins are prone to splitting, and therefore care should be taken to adjust neck height to die diameter. Film defects such as bagginess, gauge variation, flatness, camber, fuzzy or raised slit edges, treatment level, gels, moisture marks, interlayer instability (zig-zag and wave types), melt fracture, gloss, haze, and clarity and uniformity of color should be monitored and eliminated using root cause analysis. If possible, it is important to retain trends for all process conditions (usually this feature is available with new lines) and retain small film samples with the lot numbers of resins and additives used for fabrication. It is important to measure film properties like thickness profile, COF, MD/TD tear, impact, tensile strength, impact strength, sealing, barrier to oxygen, CO2 and aroma, and others. In multilayer barrier films, tie and barrier layers are often used at low percentage (optimum level only) due to cost vs. performance issues. Therefore make sure to check the thickness and distribution of the tie and barrier layers across the full circumference of the bubble or layflat tube by checking thickness at several spots across the multilayer web.

Roll Defects In a blown-film extrusion process, roll defects can be classified as due to film defects, poor web handling (including melt orientation), and /or improper roll handling. Rolls with uneven ends or corrugations can be caused by web wander, poor web tracking, misaligned rolls, or improper edge guide control. Bubble breathing, bubble oscillation, vertical motion of the bubble, and improper locking of the bubble in the air ring can cause a slight width change in the tube or sheet within the roll. Tacky films with no slip or anti-block additives, which are slit without blade oscillation, can cause bumpy edges or even bumps in the roll. Variable tension during slitting or edge trimming can affect sheet width. At low tension, the sheet is not stretched that much, and hence the slit width is slightly wider than in a high-tension operation, where the slit width is slightly narrower. A poorly aligned blade in the perpendicular or parallel (to the web) direction could cause fuzzy edges. Trimless or edge-slit film can also have fuzzy roll ends because the film is not precisely guided to maintain the blade location exactly in the center of the edge fold. Very slight bubble breathing and even a slight imbalance in the IBC blowers can cause changes in the width and the precise location of the blade. A dull blade can also provide a fuzzy-edged roll. Some concave or convex types of deformed rolls can result from gauge variation originating from specific conditions (air drafts or rising heat from extruders). The thicker film at both ends or at the center supports the rolls, which may show normal hardness; however, during unwinding, gauge variation may cause bags or ripples in the film, specifically in high-line-speed operations such as lamination and printing. This type of roll defect can be observed on wide lines where two, three, or more rolls of sheeting are cut from the wide web and wound on a single shaft. As soon as the thinner-gauge film is slit, it can wind with little or no tension. During winding or doffing-off the roll, it may telescope. In wide film lines, lack of proper randomization and issues of alignment of wide-web supporting rollers can cause this effect. Besides, if extruder and die sizes are large, some heat can rise, causing a consistent influence on the bubble (film). New lines have effective heat-removal systems from the extruder; for old lines, the bubble can be shielded. Air drafts from the plant (fans and open doors in summer) and air-ring blowers can cause enough draft to impact the bubble before the frostline (in the melt phase), which can be prevented by putting a shroud around the bubble. An unleveled extruder, die, or tipped air ring makes the bubble travel longer from the tipped side, thus causing a roll defect. A slightly worn primary nip or a nip with inconsistent pressure on one side can cause the tubed film to be pulled more on one side than on the other. Film can also slip on one side where nip pressure is low. As covered above under gauge randomization methods, die, extruder, and air-ring rotation or oscillation cannot correct these defects because the bubble does not rotate with the die and the air ring. These defects occur in the bubble from the exit of the die up to frostline height; because the

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bubble is in the melt phase, it is very sensitive in this section, and it takes only a few seconds for air draft, scratching, or heat to influence bubble gauge. These film defects can be eliminated or reduced when the rotational unit is above the primary nip (horizontal or vertical oscillating nips). In addition, rolls should have no scuff marks, damaged edges, buried or protruded cores, or damaged cores. Roll defects such as out-of-round rolls, inconsistent hardness, roll blocking, gauge bands, crowning, corrugation, buckling, starring, core offset, and poor start should be checked during production. The roll defects discussed above may occur more frequently on wider lines than on narrower lines. The greater the width of the film web, the more attention is needed to roll alignment. Issues in smaller (narrower) lines are exacerbated in larger lines due to the sheer mass and size of the large lines.

crease recycled content to improve sustainability of resources, which helps to change (partially) the linear economy to a circular economy. Make sure that recycled materials are clean; if possible, check viscosity vs. shear rate of recycled materials and compare it with prime resin. As recycled content increases in the formulation, this may require changes in process conditions and equipment, such as screw design and filter system. Make sure to monitor melt temperature, melt pressure, specific output, and general process conditions when higher percentages of recycled content are processed through a blown extrusion line. As well, watch for optical (haze, gloss), mechanical, and thermal properties and monitor gel count to make sure that film and roll quality is not impacted by increasing recycled content in the film.

Yield Improvement: Waste and Scrap Reduction (Purge, Changeover, and Shutdown of the Line)

SUMMARY

In general, single-layer blown-film lines processing natural polymers (no pigment) require less purging or changeover time between recipes, and scrap rates are low. However, as the complexity of the blown-film process increases by the introduction of more layers and different resins, additives, masterbatches, and pigments, the purge time (changeover time) and scrap rates will increase. Therefore, special attention should be given to these processes. For example, suppose that a single-layer blown-film line processing PE film requires only a short changeover time to another resin. However, if the same line is used to make pigmented film, the purge time increases, because the pigment needs to be purged out of the extruder and die. As well, the hopper or additive blender needs to be clean to avoid any colored granules sticking by static charge to the resin handling system. If the multilayer blown-film line is used to process Nylon, PETG, EVOH, and polystyrene with tie, polyolefin, and pigments, then the purge or changeover time to a new recipe increases dramatically. Therefore, it is recommended to control the number of recipes, specifically where a specific layer must be purged out (for example, where Nylon is purged out and replaced by another layer like PETG). Use of Disco purge and selection of correct resin viscosities is required to purge high-temperature-processing polymers (Nylon, PETG, etc.). During shutdown of the blown-film line, if antioxidant purge is added, it will help to reduce gels and degraded particles. During the next startup of the blown line, it is important to check film quality before resuming production. One of the main goals in the blown-film process is to increase the yield of quality film and reduce the scrap rate (startup, trim, and purge). This can be done by proper scheduling of different recipes and correct purge and shutdown procedures. Certainly, during production, a low trim percentage is important to keep overall scrap low. Recycling, Sustainability, and the Circular Economy: In recent years, there has been a significant effort to in-

Single- and multilayer blown films are commonly produced using air or water as the cooling medium using a wide variety of resins and equipment configurations. In all cases, film, sheet, or rolls are shipped to external customers or are further converted in-house (lamination, printing, or bagmaking) before shipment. Therefore, the main focus should be to obtain optimum (best) film and roll quality from the blown-film extrusion process. The troubleshooting information on equipment and the process given in this section is general. Please check with equipment suppliers, resin and additive suppliers, and test film properties for root cause analysis to troubleshoot and reduce defects in the film or roll. Lean and six-sigma methodologies can be used to carry out root cause analysis for troubleshooting. Lean means reducing waste (or increasing yield), and six-sigma means reducing variations (defects). The blown-film industry uses two or three sigma (statistics) in thickness profiles. Aim to use six-sigma methodologies like DMAIC (Define, Measure, Analyze, Improve, and Control) and ask yourself ‘where you are’ and ‘where you want to be’ in terms of improving quality. This is done by many other manufacturing or service industries. Lean and six-sigma methodologies can be used to troubleshoot and analyze equipment, process, and film defects with the aim of shipping optimum-quality film rolls to the customer to increase process yield and reduce variations. REFERENCES AND ADDITIONAL RESOURCES [1] Schwank, D. “Film Applications”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta, p.15. [2] Prall, G. M., “Operation, Maintenance, and Troubleshooting for Blown Film”, 1992 Film Extrusion Manual, TAPPI PRESS, Atlanta, pp. 649–689. [3] Roller, R. and Vedder, D. A., “Blown Film, Maintenance and Troubleshooting”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta, pp. 161–184. [4] Koch, K., et al. “Packaging Applications Using Enhanced Polyethylene and Polyolefin Plastomers Produced by Con-

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strained Geometry Single-Site Catalysts”, Metallocene-Based Polyolefins, Volume Two, John Wiley, 2000, p. 205. [5] Sergi, S., “Copolyamide Solutions for New Packaging Trends”, UBE Engineering Plastics S. A., AMI Multilayer Packaging Films, Newark, 2011. [6] Butler, T. I., “Blown Film Bubble Instability Induced by Fabrication Conditions”, Proceedings, 1999 Polymers, Laminations and Coating Conference TAPPI PRESS, Atlanta p. 815. [7] Tamber, H., “Blown-Film Cooling Systems”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta, pp. 185–200. [8] Knittel, R. R. and DeJonghe, R. J. Jr., “Blown-Film Cooling Systems”, 1992 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 261. [9] Krycki, B., “Better Bubble Management”, 1999 Proceedings of Film Conference, sponsored by Plastics Technology and Polymer Process Communications, Somerset, NJ, p. 229. [10] Stobie, J., “Air-Ring Consideration for Optimizing Blown-

Film Properties”, Proceedings, 1996 Polymers, Lamination and Coating Conference TAPPI PRESS, Atlanta, p. 231. [11] Stobie, J. and Tamber, H., “Film Stabilization, Forming and Collapsing Systems”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 201. [12] Knittel, R. R., “Film Stabilization, Forming, and Collapsing Systems”, 1992 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 311. [13] Smith, R. D., “Winding”, 2005 Film Extrusion Manual, TAPPI PRESS, Atlanta, p. 313. [14] Planeta, M., “Automatic Surface Winder Offers Unique Solutions”, Macro Letters, Volume 4, Issue 2, 1998. [15] Planeta, M., “Automatic Turret Winding Features”, Macro Letters, Volume 5, Issue 1, 1998. [16] Tamber, H., “Comparing Extrusion Technologies: Blown, Cast, and Bi-Ax Processes”, Webinar organized by Plastic Technology USA, April, 2014.

Chapter 5—Section 9

Troubleshooting the Cast Film Process CHRISTINE RONAGHAN, Cloeren Incorporated

INTRODUCTION Film Defects, Table 5.9.1 Problem: Foreign particulate, gels, degraded material, unmelted material, or nonhomogeneous material identified in the extrudate. Action: Before any corrective action is taken, the identity of the film defect should be confirmed. Otherwise, corrective actions may in fact make the problem worse. Coextrusion, Table 5.9.2 Problem: General visual or functional defects related to compatibility or uniformity of individual layers in a coextruded structure. Action: Before corrective actions are taken, determine the overall uniformity of the individual layers. Methods for

determining layer uniformity include optical microscopy or infrared analysis. If detailed analysis of individual layers is not possible, other methods of differentiation are available; consult with your equipment or resin supplier for direction. Gauging, Table 5.9.3 Problem: Poor uniformity in film thickness in the machine and/or transverse directions. Action: Before other corrective actions are taken, confirm whether variation is in the machine direction or transverse direction exclusively. Generally, attempt to stabilize any machine-direction variation before focusing on transversedirection variation. Casting/Film Properties, Table 5.9.4 Problem: As stated in Table 5.4.9.

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TABLE 5.9.1.  Film Defects. Observed Problem Gels / degraded material

Potential Causes Poor temperature control

Potential Solutions Check thermocouples for correct seating and functionality Check that all heaters are in working order and properly controlling

System sat idle too long or at high temperature while idle

Follow recommended purge procedures to eliminate degraded material from system. Disassemble and clean any components effected

Excessive melt temperature or residence time

Review appropriateness of screw design for processing the current formulation. Adjust temperature settings to accommodate resin requirements and limitations Check screw for damage or wear Check barrel for damage or wear

Unmelts

Unbalanced melting capacity of screw design

Optimize screw design for present formulation Adjust barrel temperature settings Adjust pressure and/or screen pack filter mesh

Poor mixing

Adjust formulation components if possible Increase pressure to assist with mixing and homogenization Change screw design to achieve homogenization.

Contamination

Identify source of cross feed Change filters in conveying system components Improve changeover procedures

TABLE 5.9.2.  Coextrusion. Observed Problem Small “zigzag” flow lines

Large parabolic flow lines

Potential Causes

Potential Solutions

Poor homogenization of one or more melt streams

Increase pressure, modify temperature settings, improve screw design to homogenize melt stream

Die flow channel not optimized for present formulation

Reduce flow rate, increase temperature (if possible), consult with manufacturer

Inconsistent air flow from air knife or vacuum box

Clean air knife / vacuum box and ensure uniform air flow and sealing where appropriate

Incompatibility of coextruded resins with one another

Adjust viscosity (by changing resins or by changing temperature) of one or more components in the structure Adjust flow rate of one or more components in the structure

Variation of individual layers across film width in a coextruded structure

Design of flow channel at converging interface

Depending on feedblock configuration, adjust geometry of inserts to balance flow of all layers

Viscous encapsulation / rearrangement

Use available adjustments in the feedblock to compensate for layer non-uniformity (see feedblock chapter) Adjust melt temperatures to remedy layer nonuniformity. Choose viscosity-compatible resins when designing a coextruded structure

Lack of adhesion

Inappropriate resin selection

Check with resin manufacturer

Temperature too low

Adhesion is a function of temperature; ensure that all melt temperatures are uniform and meet resin manufacturer’s recommendations.

Adhesive volume too low

Increase adhesive layer percentage

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TABLE 5.9.3.  Gauging. Observed Problem Machine-direction gauge profile variation

Potential Causes

Potential Solutions

Extruder is surging, as indicated by pressure or torque variation

Adjust barrel temperature profile to eliminate surging Adjust pressure to eliminate surging Check consistency of resin handling and feeding into hopper Attempt to correlate MD profile variation to cycles in temperature control zones, torque, or pressure

Inadequate or unstable pinning

Ensure that the web is being pinned correctly to the chill roll using vacuum box, edge pinning, and/or air-knife assemblies

Excessive draw or draw resonance

Adjust die-lip gap Adjust air-gap height

Transverse-direction gauge profile variation

Tension control

Check uniformity of line tension control, which might be influencing melt or film behavior on the chill roll

Die out of adjustment

Reset lip gap and follow manufacturer’s recommendations for initial calibration Run die in “manual” mode with-out Automatic Profile Control engaged to isolate process issues from control issues

Die is dirty

Stop line and clean die lip and nose according to manufacturer’s recommendations

Process requirements outside design specification

Excessive correction to lip gap may be required. Consult manufacturer for recommended operation

Die-lip adjustment bolts require maintenance

Check lip adjustment system regularly to ensure that all components are functional

TABLE 5.9.4.  Casting/Film Properties. Observed Problem Film width too narrow

Potential Causes

Potential Solutions

Die is too narrow

Adjust deckled slot width if applicable

Air gap too long

Reduce air gap to reduce neck-in

Insufficient pinning

Use or increase vacuum box Optimize edge pinning positions

Melt fracture

Poor melt strength

Use edge encapsulation if available

Shear stress too high

Increase die-lip gap Increase temperature Decrease resin viscosity

Machine-direction flow lines

Thermal non-uniformity

Die lines

Die is dirty or has damage at the lip land

Shim lip according to manufacturer’s recommendations

Poor chill roll release

Poor heat transfer; web is too hot at exit of cast unit

Check / decrease chiller water temperature

Check melt temperature uniformity Check air knife / vacuum box for consistent and adequate air flow Split, clean, and inspect die for damage to flow channel and lip land/face Confirm good operating condition of water flow channels through chill roll Improve contact of web to roll to increase cooling efficiency by means of air knife or vacuum box

Edge instability

Incorrect surface finish on chill roll

Rougher surface finish on the roll will aid in polymer release properties

Poor melt strength

Change one or more resins in the formulation Use edge encapsulation Reduce temperature at edges slightly

Web tear

Increase extrudate edge thickness. Adjust edge zone temperatures Draw limitations

Reduce lip gap

Ramp rate mismatch

If breaks occur on ramp-up, slow down the ramp speed of the line relative to that of the extruders

Chapter 5—Section 10

Gel Troubleshooting MARK A. SPALDING, EDDY GARCIA-MEITIN, and STEPHEN L. KODJIE, The Dow Chemical Company

INTRODUCTION Gels in polyethylene (PE) film products are very common and often very difficult to identify and mitigate. They can be very difficult to identify because of their small size and almost endless sources. With care during processing and properly designed equipment, the level of gels in a product can be kept very low. This section will describe how to identify gel types, their potential sources, and methods to mitigate them in film products. The term “gel” is commonly used to refer to any small defect that distorts a film product, reducing the quality of the film. There are many types of gels [1–3]. The most common gels include: (1) highly oxidized polymeric materials that appear as brittle black specks; (2) polymeric materials that are crosslinked by an oxidative process, resulting in soft and often discolored gels; (3) highly entangled polymeric materials (such as high-molecular-weight species) that are undispersed and not crosslinked; (4) unmelted resin; (5) filler agglomerates from masterbatches; and (6) a different type of resin or a contaminant such as metal, wood, fibers, insects, or dirt. A crosslinked resin gel is typically formed during an oxidation process, resulting in crosslinking of the resin chains. The gels often have a yellow to dark brown color. If the crosslinking levels become extremely high, brittle black specks will occur. Highly entangled gels are typically high-molecular-weight polymer chains that are entangled and therefore difficult to disperse during extrusion. When analyzed using a hot-stage microscope, this gel type will melt as the stage temperature is increased. When the stage temperature is then decreased, the gel will reform, creating the appearance of a gel as a solid polymer fragment. Because these gels are not oxidized, they are not associated with color. They are commonly referred to as undispersed or unmixed gels. Unmelted resin may sometimes exit with the extrudate, especially at high extrusion rates. These gels will melt during heating on a hot-stage microscope, and typically they will not reform during the cooling phase. Numerous sophisticated methods are available for analyzing gels, including epi-fluorescence microscopy, polarized light microscopy, and electron mi-

croscopy with X-ray analysis. These methods will be discussed in the following subsections. Gels can originate from many sources, including: (1) the resin manufacturer, (2) the converting process, (3) pellet blending of resins with significantly different shear viscosities, (4) pellet blending of different resin types, and (5) direct contamination. Modern resin manufacturing processes exclude oxygen from the system and are very streamlined, so that process areas with long residence times do not exist. Therefore, crosslinked and oxidative gels are not typically generated by the manufacturer. Moreover, careful quality control techniques at the manufacturing plant are typically in place to verify that the resin does not contain an unacceptable level of gels. Improperly designed extrusion equipment and processes, however, are common, leading to oxidative degradation of resins and crosslinked gels. PROTOCOLS FOR GEL ANALYSIS Gel and defect analysis in films and fabricated articles requires a disciplined approach and an array of analytical tools. The goal of this subsection is to provide an overview of the analytical protocols used to interrogate and identify gels and defects associated with film manufacturing. A flowchart illustrating the steps associated with gel analysis is shown in Figure 5.10.1. The topics in this figure will be discussed in the following subsections. Gel Classification Most defects in films and fabricated articles can be identified upon visual inspection (>100 µm) if proper illumination is used. A fiber-optic light under incident illumination can be used to accentuate surface defects, or a light box with polarizing film can be used to transmit light through translucent film to help locate gels. Once gels have been identified and marked, they can be further examined using a low-power (7–80×) stereo microscope. The microscope should have a combination of transmitted and reflected bright-field illumination and polarized light capabilities. Gels can then be 427

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are also very useful and can provide additional information about gels, fillers, and surrounding matrix components. Optical Microscopy

FIGURE 5.10.1.  Flowchart of the analysis protocol for interrogating gels and defects in films.

categorized according to shape, size, color, and appearance. Gels can appear round, flat, elongated, or fibrous and can have different colors. Fibrous gels are commonly associated with natural or synthetic fibers such as cotton, cellulose, or polyester. The source of these gels is typically gloves, clothing, or packaging that has been used during handling and processing of raw materials. Gels associated with fiber contamination are common, but can easily be mitigated with proper environmental controls. Sample Preparation Once gels have been classified and grouped based on size, shape, appearance, and color, they can be further analyzed. Representative gels from each grouping are isolated using a razor blade in preparation for cross-sectional analysis. During gel isolation, ensure that unaffected material is present on each side of the gel for comparison. Gels are then placed into microtome clamps and cryogenically sectioned below their glass transition temperature (Tg). A microtome equipped with cryogenic capabilities (–80°C to –160°C) is used with a diamond knife to collect cross sections near gel midpoints. Several sections (two or three) about 5 to 10 µm thick are collected for each gel. Additional thin sections are used for hot-stage and compositional analysis using Fouriertransform infrared micro-spectroscopy (micro-FTIR). For optical microscopy (OM), sections are placed on a glass slide containing a drop of silicon oil, covered with a glass cover slip, and examined under a compound light microscope. The microscope should be equipped with transmitted bright-field (TBF) illumination, polarized light (PL), and epi-fluorescence illumination. Other illumination conditions such as differential interference contrast (DIC) and dark field (DF)

For semi-crystalline polymers such as PE and polypropylene (PP), sections are first examined using polarized light to locate the gel [4]. In most cases, gels display a different birefringence than the surrounding matrix. In addition, they can display fine or coarse spherulitic structures that stand out from the surrounding matrix. When non-crystalline additives are present, very little birefringence is observed, depending on the levels present. Examination at higher magnification using bright-field illumination or DIC can help reveal these particles. Epi-fluorescence microscopy is used to determine the response of a gel when excited by a specific band of shortwavelength light (300–500 nm). Fluorescence occurs when molecules absorb light of a specific wavelength, and the resulting photon interaction causes the molecules to emit energy at a longer wavelength. The emission energy usually occurs in the visible range of the spectrum [5]. Objects like the silicon oil in the background and the film matrix will not respond to short-wavelength excitation and will remain dark. However, gels that display yellowish, reddish, or dark brown coloration will almost always fluoresce. These gels are indicative of polymer degradation and can display a broad range of fluorescence emissions depending on the level of carbonyl defects present. Defects associated with unsaturated carbonyls are the best candidates for luminescence of oxidized PE [6]. Polymer additives and inorganic fillers can also give rise to fluorescence. However, in most polymer gels, emission from epi-fluorescence illumination is associated with oxidation. Hot-Stage Analysis The additional thin sections collected from each gel can be used to determine the melting temperatures associated with the gel and matrix. Gels are examined using a temperaturecontrolled hot stage in combination with a compound optical microscope. The microscope should be equipped with longworking-distance objectives (4 to 20×) to enable placement of a hot stage under the objective lens. Some commercial hot stages accept standard microscope slides, but others require smaller slides or cover slips. The slide examined by optical microscopy can be used, or a new slide can be prepared with the extra gel cross sections that were collected. After the slide is placed into the heating chamber, cross-polarized light is used to locate birefringence associated with the semicrystalline sample. Heating rates between 3 and 6°C/min are used for this analysis. If melting between a gel and matrix occurs at similar temperatures, a slower heating rate will help separate individual thermal events to discriminate between materials. PE-based products can exhibit a wide range of melting temperatures depending on polymer densities and additives. Knowledge of the material being examined

Section 5.10.  Gel Troubleshooting

is important in selecting a starting temperature for hot-stage analysis. In general, a starting temperature of 90°C is used for low-, medium-, and high-density PEs. However, if lower-density components are present, starting temperatures between 50 and 70°C will be required. Once the test is started, melting is observed and recorded when birefringence disappears in the film and gel. Depending on compositions and processing histories, gels can melt at lower or higher temperatures than the surrounding matrix or at similar temperatures. Gels can be a simple unmelt (solid polymer fragment) of the polymer matrix or of more complex composition. Gels can arise from degraded or crosslinked polymer, contaminants, additives, fillers, or a higher-molecular-weight material in the polymer. In some cases, crosslinked or partially crosslinked gels will lose birefringence, as if melted. If crosslinking is present, birefringence will reappear upon stressing the gel. This is done by using a dental tool to press lightly on the cover slip over the gel. However, gels that are highly crosslinked will generally retain their birefringence even when heated to higher temperatures. Electron Microscopy and X-Ray Microanalysis Scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX) microanalysis is used to help identify the composition of inorganic contaminants [7,8]. Many materials contain additives and fillers to impart specific resin properties. Gels can arise from fillers, pigments, poorly mixed additives, or residues from manufacturing. In addition, metallurgy from manufacturing equipment, extruders, or railcars can also be a source of gels. Gels can also develop from trace quantities of inorganic additives associated with catalyst residues or post-reactor components from manufacturing. Oxygen plasma etching can be used to enrich these additives at the surface. Due to density differences between the polymer matrix and inorganic additives, plasma etching preferentially removes the polymer and enriches low concentrations of inorganics to enable detection. For SEM and EDX analysis, a cryo-polished block face is used to collect the thin optical sections is preferred. Analysis of bulk samples provides a greater interaction volume with the electron beam over thin sections for X-ray microanalysis. To analyze the gel, the cryo-polished block face containing the remaining gel is secured face-up onto an SEM sample mount. A carbon evaporator is used to apply a thin carbon coating to the sample to render it conductive for electron microscopy.

429

from organic contaminants like fibers, polymers, and additives, the ability to determine their composition plays a key role in mitigating them. For small particles or cross sections containing gels and defects, micro-FTIR is used (FTIR with a microscope). Cross sections collected through the defects (5–15 µm thick) are mounted on a potassium bromide (KBr) window to allow light transmission and infrared radiation through the material. FTIR spectra are acquired in transmission mode using an infrared spectrometer coupled with a microscope and a liquid nitrogen-cooled mercury-cadmiumtelluride (MCT) detector. The spectral range covered is 4000 to 400 cm–1, with a resolution of 4 cm–1. Each transmission spectrum is recorded by adding 512 scans together while using a 150 × 10 μm aperture. Other experimental parameters include one level of zero filling, Happ¬Genzel apodization, Mertz phase correction, and a mirror velocity of 2.5317 cm/s. EXAMPLES OF GEL TYPES This subsection provides examples of several types of gels and defects commonly encountered in film products. Other examples from some non-film products are also presented because the defects and the root causes are similar. Crosslinked Gels in PE Films Crosslinked gels are oxidized gels, but the level of oxidation is not enough to cause them to fluoresce under ultraviolet (UV) light. The gels can display fine or coarse spherulitic structures or may appear more or less crystalline than the matrix. Differences in birefringence associated with crystalline structures can be observed under polarized light. For example, PE resins can be manufactured with different densities and molecular weight distributions that target specific applications [9]. Figure 5.10.2 shows several gels isolated from a monolayer PE film with low- and high-molecularweight components in the formulation. The gels were clear, not discolored, and very similar in appearance. Cross sections were prepared, and the gels were analyzed using polarized light and hot-stage microscopy, as shown in Figure 5.10.3. Polarized light microscopy showed that the gel had a similar birefringence to the surrounding film matrix before melting, as shown in Figure 5.10.3(a.) Upon

Fourier-Transform Infrared Spectroscopy Fourier-transform infrared (FTIR) spectroscopic analysis is a very useful technique for identifying organic and some inorganic materials in a wide range of applications. When a material is exposed to infrared radiation, the absorbed radiation excites molecules into a higher vibrational state. The wavelengths absorbed by the sample are characteristic of its molecular structure. Because gels and defects can arise

FIGURE 5.10.2.   Transmitted bright-field images showing isolated film gels having a very similar appearance.

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FIGURE 5.10.3.  Hot-stage analysis of a partially crosslinked PE gel under polarized light: (a) gel before melting, with a similar birefringence to the matrix; (b) the gel melts at 129ºC; (c) the matrix melts at 131°C, and (d) a stress test showing birefringence consistent with crosslinking.

further heating, the melting temperature (Tm) of the gel was observed at about 129°C [Figure 5.10.3(b)] and 131ºC for the film matrix [Figure 5.10.3(c)]. Both melting points were consistent with PE resin. However, the lower melting temperature of the gel was consistent with a lower-density, higher-molecular-weight component in the resin. To determine whether the gel was unmixed (highly entangled, but not crosslinked), it was held above its melting temperature (135°C). A dental tool was used to press lightly on the glass cover slip to stress the gel. The gel displayed a level of birefringence under polarized light indicating that it was crosslinked, as shown in Figure 5.10.3(d). An unmixed gel would not show birefringence when melted and stressed. Oxidized Gels in Multilayer Films The most common type of gel is caused by oxidative processes that crosslink the PE chains. The best way to identify this gel type is with a compound light microscope under polarized light and with epi-fluorescence illumination. Figure 5.10.4 shows a cross section of a three-layer, co-extruded film with gels. The gels were brown, displayed sharp edges, and were extremely brittle when sectioned. Examination under polarized light determined that the gels were located in the core layer of the film, as shown in Figure 5.10.4(a). Examination using epi-fluorescence illumination caused an intense fluorescence emission that is characteristic of thermal

oxidation and possible polymer crosslinking. FTIR identified the gels as oxidized PE. This material likely formed on hot metal surfaces associated with the core layer extruder, separated from the surface, and then contaminated the film as gels. Unmixed Gels As stated previously, unmixed gels are highly entangled species that are molten when they are discharged from the die, but solidify first upon cooling to produce a gel that appears as a solid polymer fragment. These types of gels are easily removed from an extrusion process by subjecting all molten resin to a one-time high level of stress near the discharge of the extrusion screw using a properly designed Maddock mixer. The details of this mixer will be described later. A film process was producing a monolayer film that had a low level of gels. The gels were tested using hot-stage microscopy and identified as highly entangled species. These gels melted and then disappeared when heated and stressed by pressure smearing using a dental tool, as shown in Figure 5.10.5. Unmelts in Films and Thin-Walled Pipe Blemishes on films and thin-walled pipe products are typ-

Section 5.10.  Gel Troubleshooting

431

FIGURE 5.10.4.  Transmitted polarized light image of a thermally oxidized and crosslinked gel in a multilayer film: (a) image under polarized light, and (b) the gel fluorescing under epi-fluorescence illumination.

ically caused by resin degradation and solid polymer fragments that did not melt during extrusion. Typical solid polymer fragments appear as “windows” when the pipe wall is cross-sectioned for inspection of unmelts. Optical cross sections of a clear polymer fragment causing a gel are shown in Figure 5.10.6. The thin-wall pipe was produced from a highdensity PE (HDPE) resin and a carbon black masterbatch using a grooved-bore extruder. The clear gel windows are due to solid polymer fragments of the HDPE resin. Because this natural pellet did not melt and disperse in the extruder, it did not contain black pigment from the masterbatch. These solid polymer fragments were the source of pinholes that occurred in the wall of a thin pipe. Solid polymer fragments can also occur in film and sheet products. On numerous occasions, a resin processor will switch from a natural PE resin plus a black masterbatch (or another color) to a pre-compounded black PE resin. The switch is made as a solution to eliminate windows from the pipe wall. No other changes to the process are typically made. The processor typically does not complain about solid polymer fragments with the pre-compounded resin, although they are likely present in the pipe. Because the fragments are black like the surrounding resin, they cannot be visually detected. When the natural resin and the black masterbatch are extruded, natural solid polymer fragments are easily observed in a black-tinted matrix. Solid fragments from the master-

batch would not be visually detected, although they are also likely present. Gels from Pigments and Fillers Poor dispersion of pigments and fillers (from incomplete mixing) or poorly produced masterbatches can cause product quality issues and produce gels. Carbon black is produced by combustion of petroleum feedstocks and is found in many fabricated products such as films, pipe, belts, hoses, and coatings. If properly dispersed, carbon black provides pigmentation and protection against UV exposure. Carbon black can be purchased as a raw powder or as masterbatch concentrates containing 25–40 wt.% carbon black in a carrier resin. The pigment can be added by melt-blending masterbatch pellets with natural pellets before extrusion, which becomes a compounding process. Another method is dry blending (salt-and-pepper effect) the masterbatch pellets with natural resin pellets at the converting step. Extrusion equipment is used to disperse the pigment concentrate with the natural resin during product fabrication. Figure 5.10.7 shows microtomed optical cross sections under transmitted BF illumination. Due to the intense light scattering associated with carbon black, high-intensity BF illumination was used. Figure 5.10.7(a) shows an image of carbon black dispersion in a PE product that used melt blending to in-

FIGURE 5.10.5.  Photographs of an unmixed gel at selected 100°Ctemperatures using a hot-stage microscope. The unmixed gel melted at about 135°C. When the gel was smeared by moving the glass cover slip, the stress was enough to disentangle the polymer chains, so that the gel would not reappear upon cooling.

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FIGURE 5.10.6.  Transmitted bright-field images of a thin-wall HDPE pipe cross section with a clear solid polymer fragment: (a) low-magnification, and (b) high-magnification image. The feedstock was a natural resin and a carbon black masterbatch.

corporate a 2–3 wt.% pigment loading, and Figure 5.10.7(b) shows a dispersion produced from dry blending the black masterbatch during product fabrication. Note the dense stratification of natural resin (bright swirls) and carbon blackrich regions (dark swirls) from dry blending. Homogeneous carbon black dispersion is important for both UV protection and for products such as films, pipes, and fittings. Pipes and fittings are required to pass a carbon black appearance and agglomerate rating based on ISO 18553:2002ISO [10]. In these examples, dry blending [Figure 5.10.7(b)] produced an unacceptable appearance rating compared to images in the ISO standard, whereas melt blending [Figure 5.10.7(a)]

was acceptable. A properly designed Maddock mixer could improve dispersion for both methods. In addition, poor carbon black incorporation can lead to gel-like defects that can occur on exterior surfaces and throughout the resin matrix. Figure 5.10.7(c) shows a large gel-like defect that formed a surface pit. Optical cross sections showed that the defect was caused by a 380-µm carbon black agglomerate, as shown in Figure 5.10.7(d). SEM was used to examine the cryo-polished block face containing the agglomerate in Figure 5.10.7(d). The agglomerate morphology was made up of poorly wetted, “grapelike” clusters containing 50–70 nm particles, consistent with

FIGURE 5.10.7.  Optical images of carbon black pigment dispersed in PE: (a) dispersion after melt blending, (b) dispersion after dry blending, (c) round surface defect on an exterior surface, and (d) cross section of a carbon black surface defect.

Section 5.10.  Gel Troubleshooting

433

FIGURE 5.10.8.   SEM image of a carbon black agglomerate with poor resin wetting of the carbon black particles in the pigment masterbatch.

carbon black, as shown in Figure 5.10.8. The lack of resin “wetting” suggested poor incorporation of the carbon black into the carrier resin of the masterbatch. Poor pigment dispersion and surface defects can be mitigated with better mixing and incorporation of the carbon black masterbatch into the product during compounding. In another example, a 50-µm-thick PE film containing gel-like surface defects was examined. The defects appeared optically similar and ranged from about 80–500 µm in diameter. Figure 5.10.9(a) shows a large defect surrounded by smaller particles on the film surface. An optical cross section showed a dense, spherical mass along the core of the film, as shown by Figure 5.10.9(b). The defect appeared similar in color to finer particles in the surrounding matrix.

FIGURE 5.10.10.  EDX microanalyses of a gel-like defect detected Zn and O, indicating that the defect was an agglomerate of ZnO additive.

SEM photomicrographs of Figures 5.10.9(c) and 5.10.9(d) and the EDX shown in Figure 5.10.10 were used to determine that the defect was an agglomerate of small crystals. The crystals within the defect were similar in morphology and elemental composition (Zn and O) to crystals dispersed throughout the film matrix, indicating that all were from the same source. Proper handling, mixing, and incorporation of pigments, fillers, and additives into base resins during masterbatch production can reduce or eliminate future problems

FIGURE 5.10.9.  Film defect: (a) optical image of film surface showing a gel-like defect under reflected light, (b) image of film cross section showing defect under transmitted DIC, (c) backscatter SEM image of the film blockface with defect, and (d) SEM image of agglomerated crystals in the defect.

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FIGURE 5.10.11.  Photographs of gels in the core layer of a three-layer film: (a) transmitted polarized light and (b) hot stage microscopy were used to determine the melting temperatures of the core resin and the defects.

associated with surface and internal defects from poor dispersion. Gels from Resin Contamination A multilayer film product was experiencing occasional gels. The gels were isolated and the cross sections were collected, as shown in Figure 5.10.11. These gels contained highly birefringent particles that resided in the core layer. The outer film layers appeared amorphous, and the core layer was slightly birefringent. The optical melting temperature of the core layer was determined to be 123°C, whereas the birefringent gels melted at 265°C. The melting temperature of 123°C was consistent with the PE resin used to produce the core layer. Micro-infrared analyses of the defects indicated that the higher-melting-temperature material was a foreign contaminant, which was identified as a polyester resin. The polyester resin was used in another process in the converting plant, and it inadvertently contaminated the PE feedstock. Gels from Fibers Another common source of gels and defects can arise from poor environmental controls during fabrication. The

source of gels from fibers is typically gloves, packaging, or clothing associated with handling and processing of raw materials. Fibrous gels are associated with natural fibers such as cotton or synthetic fibers like polyester or nylon. Cellulosic fibers are birefringent, will fluoresce under UV illumination, and can be identified based on appearance and composition [11–13]. Figure 5.10.12 shows two examples of fibrous defects that resulted in product being rejected for gel-like defects. The product shown in Figure 5.10.12(a) was a PE structure containing large clumps of brown fibrous defects embedded in a container. The defects were large enough that a razor blade could be used to isolate material for analysis. FTIR analysis identified the defect as a cellulosic contaminant. The defects likely originated form a paper product like cardboard that contaminated the resin during shipment or handling before fabrication. Fibrous contamination occurred in thin (50 µm) PE film, as shown in Figure 5.10.1(b). The film was clear and therefore could be examined under a compound optical microscope under transmitted bright-field illumination and polarized light. The fiber displayed a flat, tube-like, twisting structure and was birefringent under polarized light. The appearance and morphological characteristics of the fiber were a match for cotton. Natural fibers such as cotton have unique

FIGURE 5.10.12.  Optical images showing fibrous contamination: (a) brown cellulosic contaminant embedded in a PE container, and (b) cotton fiber contaminant in clear PE film.

Section 5.10.  Gel Troubleshooting

435

FIGURE 5.10.13.  Optical images of a film cross section of a cellulosic defect in a PE seal layer: a) viewed in polarized light, and b) the defect displaying an intense fluorescent emission under epi-fluorescence illumination.

structures that, if clearly visible, can be identified using a compound microscope with proper illumination. A five-layer co-extruded, laminated film structure containing light-brown, gel-like defects is shown in Figure 5.10.13. The defects were embedded in the film and not easily identified. Cross sections were prepared and examined under polarized light [Figure 5.10.13(a)] and epi-fluorescent illumination [Figure 5.10.13(b)]. The images showed that the defects were located at the center of an interior PE layer. The defects exhibited a cellular structure, were birefringent, and had an intense fluorescent emission. The appearance and optical characteristics suggested a cellulosic material. FTIR analysis was used to examine the fibrous material extracted from the film shown in Figure 5.10.13. Figure 5.10.14(b) shows an optical image under cross-polarized light of the extracted defect material, showing that it was a bundle of natural fibers. The fiber-like defects were identified as cellulose by FTIR analysis, as shown by Figure 5.10.14(a). As with the previous examples, gel-like defects

occurring from fibrous contaminants can be mitigated with proper material handling and environmental controls. Metal Contamination The origin of defects causing discoloration in polyolefin pellets can be identified using light and electron microscopy. For example, an in-plant recycle re-pelletizing process produced PE pellets that were off-color and had black specks, as shown in Figure 5.10.15(a). One of these defects was isolated using the cross-sectioning technique, as shown in Figure 5.10.15(b). The cross section revealed an intense reddish particle that caused the discoloration of the pellet. SEM and EDX microanalysis were used to determine that the defects contained primarily iron and oxygen and were likely composed of iron oxide, as shown in Figure 5.10.16. A photograph in the figure shows a backscatter electron image (BEI) of the pellet block-face sample showing the defect causing the discoloration and the elemental spectrum. Metalbased defects can originate from process equipment, railcars used for shipment, pellet transfer lines, and poor housekeeping. The iron oxide likely originated from a storage bin. LOCATING STAGNANT REGIONS ON SCREWS

FIGURE 5.10.14.  Fibrous defect: (a) FTIR spectrum of the defect identified as cellulose, and (b) transmitted cross-polarized light image of a fiber bundle extracted from the film.

If the gel analysis indicates that defects are due to degraded resin, then the best way to locate the region on the screw where degradation is occurring is to remove the screw from the extruder while hot. For this procedure, pellet flow to the hopper is stopped, while screw rotation is continued. The screw is rotated until resin flow out of the die stops. Next, screw rotation is stopped, and the transfer line is removed from the discharge end of the extruder. The hot screw should be pushed out about three diameters and then photographed and studied for indications of resin degradation. The metal surfaces should appear clean, with only mild discoloration. If a stagnant region exists, then dark-colored, degraded material will occupy the space. Once the segment has been studied and photographed, the hot resin should be removed from the screw using brass tools. Another three diameters

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FIGURE 5.10.15.  Photographs of foreign contamination in pellets of a re-pelletized PE reclaim stream: (a) photomicrograph of discolored pellets containing dark defects, and (b) transmitted polarized light micrograph of a pellet cross section containing a defect.

are then pushed out, and the process is repeated. If the process is running a natural resin and a colored masterbatch, the extruder should be first purged with the natural resin until the extrudate is essentially free of colorant. POORLY DESIGNED EXTRUSION PROCESSES Single-screw extruders are the machine of choice for plasticating and pumping molten resins to downstream filmforming operations. Poorly designed screws and processes can create regions where resins degrade. Small process instabilities can cause the degradation to separate from the screw and ultimately create a gel in the film product. This subsection will describe the most common screw design errors that are observed in commercial operations. Barrier Melting Sections Barrier melting sections are used extensively to increase

FIGURE 5.10.16.  EDX microanalysis of an inclusion in PE pellet cross section from Figure 5.10.15(b). The analysis indicated that the particle was likely iron oxide.

the maximum operating rate of an extruder while reducing the temperature of the extrudate [2]. Although screws with barrier melting sections have been very successful, the device has the risk of restricting flow at the section entrance. The device works by adding a second or barrier flight that separates the solids from the molten resin, as shown by the schematic in Figure 5.10.17. The barrier flight is undercut to allow the newly molten material from the solids channel to collect in the melt channel. Moreover, flow over the barrier flight provides a level of dispersive mixing. The restriction occurs in many designs because the cross-sectional area available for flow is higher for the solids conveying section in the feed than it is for the solids channel of the barrier section. The area reduction occurs because of the introduction of the barrier melting flight. The most restricted design is the well-known Maillefer [14] barrier melting section shown in Figure 5.10.18. For this design, the barrier flight starts at the pushing flight and ends at the trailing flight downstream. As shown by this figure, the width of the upstream solids conveying channel (W1) is greater than that of the downstream solids channel of the barrier melting section (W2). This width reduction causes the cross-sectional area perpendicular to the flight to reduce the area available for solids flow, creating the restriction.

FIGURE 5.10.17.  Schematic of the cross section of a barrier melting section [2]. The flow is out of the plane of the page towards the reader.

Section 5.10.  Gel Troubleshooting

437

FIGURE 5.10.18.  EDX microanalysis of an inclusion in PE pellet cross section from Figure 5.10.15(b). The analysis indicated that the particle was likely iron oxide.

The restriction can reduce the specific rate to less than half the specific rate expected; the specific rate is the rate divided by the screw speed. This specific rate reduction will cause some of the downstream channels to operate partially filled and at zero pressure. These partially filled regions in the channels will be void at first and will then allow some resin to adhere to the screw for extended periods of time, eventually leading to degraded resin, as shown in Figure 5.10.19. Small process disturbances will cause some of the degraded resin to separate from the screw, resulting in gels in the film product. As previously mentioned, the pressure in the partially filled channels is zero. This zero pressure can be predicted using very simple calculation techniques. An axial pressure profile for a process that is running properly and a process running partially filled is shown in Figure 5.10.20. The calculation method is beyond the scope of this section, but can be found in [2]. Resin stagnation can also occur at the entry of a Maillefer barrier melting section, as shown in Figure 5.10.18. As indicated by the photograph, the melt conveying channel at the entry is very deep and very narrow, causing flows to be very low and stagnant deep at the root. A similar stagnant region occurs at the exit of the section, where the barrier flight merges with the trailing flight. Many modifications have been made to mitigate this stagnation since the original patent issued in 1962. The design, however, is still used, even though the defect is known to occur. A much improved barrier melting design has the lead

FIGURE 5.10.19.  Screw showing resin flow and degraded resin due to stagnant regions [2].

FIGURE 5.10.20.  Axial pressure profiles [2,15] for a) an extruder that is operating properly (full and pressurized channels), and b) an extruder that is operating improperly, with partially filled channels (zero pressure).

length of the solids conveying and metering sections equal to the barrel diameter and the lead length of the primary and barrier flight in the barrier melting section set to about 1.3 times the barrel diameter, as shown in Figure 5.10.21. For this design, the width of the solids channel upstream of the barrier section (W3) is nearly the same as the width of the solids channel in the early portion of the barrier section (W5). As shown in the photograph, increasing the lead length in the barrier section increases the width (W4) relative to the solids conveying channel (W3). This type of entry almost always results in the metering channel being the rate-limiting section of the extruder as designed, eliminating the restriction at the entry. Therefore, all channels will be completely full and pressurized, as shown in Figure 5.10.20(a). The restriction on an existing screw can be mitigated by selectively removing metal from the entry area of the barrier flight [16]. The details are beyond the scope of this work, but can be found in [2,16]. Barrier-flighted melting sections are typically used on screws that are 70 mm or more in diameter. The barrierflighted section enables higher melting capacities and therefore higher rates while providing a very stable and consistent rate and discharge pressure. Screws less than 70 mm in diameter typically should not be designed with a barrier melting section. For this case, the performance gain for melting is minimal, and the risk of a restriction at the entry to the barrier section is high. Many screws with diameters less than 70 mm, however, have been designed successfully with barrier melting sections.

FIGURE 5.10.21.  A less restricted barrier melting section entry.

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Poor Solids Conveying Poor solids conveying from the feed section can also cause the downstream sections of the screw to operate partially filled in a manner identical to that shown in Figure 5.10.20b. For this case with smooth-bore extruders, the solids conveying channel is controlling the rate, rather than the metering channel. Partially filled channels almost always lead to gels in film products. Poor solids conveying can be caused by a feed channel that is too shallow or too short, a screw that is too hot, or a barrel temperature in the feed section that is not properly set. Flow surging at the die is frequently observed in this situation. Flow surging is defined as an oscillatory change in the extruder rate while maintaining constant setpoint conditions. Small Flight Radii—Moffatt Eddies Small flight radii in channels filled with molten resin can cause resin degradation at the radii. The channels include metering sections, melt-conveying channels of barrier melting sections, and specialized mixing devices. The degraded resin will eventually separate from the screw and contaminate the film product. The separation process occurs slowly yet continuously, causing continuous background gels in the film. The gels are typically small, crosslinked, and with color, as shown in Figure 5.10.22. A photograph of degradation at the flight radii in a metering channel is shown in Figure 5.10.23. The resin degradation is caused by recirculation flows in the channel that are known as Moffat eddies [17]. Moffat eddies are recirculations or vortices that occur at sharp corners, as shown in Figure 5.10.24. When fluid is put in motion with top-driven cavity flow, such as in an extrusion channel, the main circulation is as shown in Figure 5.10.24. A secondary recirculation is set up in the stationary corners of the channel, creating a low-velocity helical eddy that is outside the high-velocity flows of the main part of the channel. The degradation in the eddies occurs because the resin has an extremely long residence time. The Moffat eddies that created the degraded resin occurred because the flight radii were too small for the depth of the channel. If the flight radii had been larger, the Moffat

FIGURE 5.10.22.  Photomicrographs of gels that originated from stagnant regions, such as from Moffat eddies for a PE resin: a) transmitted bright-field image, and b) the same gel photographed using epi¬fluorescence illumination, indicating that the gel is highly oxidized.

FIGURE 5.10.23.  PE resin degradation at the flight radii due to Moffatt eddies. The eddies were caused by small flight radii.

eddies would not have occurred, and hence degradation deposits would not have formed. The Society of the Plastics Industry, Inc. (SPI) guidelines state [18] “unless otherwise specified, the root radius will not be less than 1/2 of the flight depth up to 25 mm radius.” Many screws are often designed, however, with flight radii that are very small, between 10% and 20% of the channel depth. Previous research [19] has indicated that the SPI guideline is appropriate as a minimum for many resins. For PE resins, the radii should be 1.5 times the local depth. However, for thermally sensitive resins, radii up to 2.5 times the depth are optimal. Flight radii sizes are shown in Figure 5.10.25. Maddock Mixer Designs Maddock-style mixers [20–23] are designed into most screws for PE extrusion processes. Their widespread use is due to their low cost to build, simplicity of the design, and their ability to trap, melt, and disperse solid polymer fragments from incomplete melting. Moreover, they are the ideal device for mitigating unmixed gels. A schematic of a Maddock-style mixer is shown in Figure 5.10.26. Maddock mixers work by dividing the flow into two to eight inflow flutes (depending on the diameter of the screw), passing the flow through a narrow restriction, collecting the flow in the outflow flutes, and then recombining the flow for the downstream sections of the screw. The narrow restriction

FIGURE 5.10.24.   Two-dimensional flows in a screw channel with H/W = 1 (channel depth/channel width). The arrows show the recirculation flows. The shaded area in the lower right corner is expanded to show the Moffat eddy [2]. The main flow is out of the plane of the page towards the reader.

Section 5.10.  Gel Troubleshooting

FIGURE 5.10.25.  Schematic of flight radii: (a) small flight radius (R1) that would likely cause a Moffat eddy, and (b) a large flight radius (R2) relative to the channel depth (H). R2 would likely not form a Moffat eddy.

will trap, melt, and disperse solid polymer fragments from the barrier melting section. The flutes are shown parallel to the screw axis in Figure 5.10.26. It is more common to spiral the flutes with a lead length that is about 5.4 times the diameter, providing a helix angle of 60°. Many innovations have occurred to the mixer since it was first described. One of the innovations was to increase the depth of the flute channels to mitigate pressure consumption and energy dissipation. If the flutes are made too deep, however, resin can become stagnant at the root and sides, causing resin degradation and gels [24,25]. Photographs of two spiral Maddock mixer designs are shown in Figure 5.10.27. The mixer in Figure 5.10.27(a) is poorly designed, with the depth of the flutes being too large, likely causing resin to stagnate and degrade. Degraded resin is observed in this mixer as black hard material. The mixer in Figure 5.10.27(b) is much better, with the depth of the flute set at half the width of the flute. The recommended Maddock mixer is of a typical design except for two features [25]. These design features include setting the mixing flight undercut to about 0.5% of the barrel diameter. Most designs set this undercut at 1–1.5% of the diameter. The small undercut specified here will trap and disperse all solid polymer fragments that happen to flow out of the barrier melting section. A larger undercut of 1–1.5% of the diameter can allow solid polymer fragments to discharge from the extruder. Moreover, a mixer with a small undercut will disperse highly entangled or unmixed gels [3]. The unmixed gels are removed by increasing the stress level in the Maddock mixer. The stress level is increased

439

FIGURE 5.10.26.  Schematic of a Maddock-style mixer.

by decreasing the clearance on the mixing flight. The stress level required to disperse unmixed gels depends on the resin and the level of chain entanglement. In past experience, the stress level required to disperse PE unmixed gels was about 100 to 200 kPa.

FIGURE 5.10.27.  Spiral Maddock mixers: (a) mixer with very deep flutes and evidence of resin degradation, and (b) properly designed mixer with the depth of the flute set at half the width of the flute.

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The shear stress that the material experiences during flow across the mixing flight of the Maddock mixer can be estimated using Equations 5.10.1 and 5.10.2. The shear stress is responsible for breaking up the entangled species. This calculation is based on screw rotation physics [2].

M 

 ( Db  2u  2 ) N (u   )  M M

(5.10.1) (5.10.2)

where γM is the average shear rate for flow over the mixing flight in 1/s, N is the screw rotation rate in revolutions/s, η is the shear viscosity at the temperature of the mixing process and at shear rate γM , Db is the barrel diameter, u is the undercut distance on the mixing flight, u is the main flight clearance, and τM is the shear stress that the material will experience for flow over the mixing flight. The second different feature of the Maddock mixer recommended here is that the depth of the inflow and outflow flutes should be half the width of the flute [25]. The flutes are cut into the screw using a ball end mill. The end mill is cut into the steel of the screw up to the equator of the ball. If the flutes are made deeper, resin degradation can occur, creating gels in the film product. Schematics of the recommended flute shape and a flute design that is too deep are provided in Figure 5.10.28. Maddock mixers with extremely deep flutes and large clearances on the mixing flights are common on smooth-bore extruder designs. Poorly Designed Mixing Sections Numerous mixer designs are currently available for grooved-bore extruders. Several of these mixers, however, are prone to resin degradation and gels. Grooved-bore extruders typically operate at higher specific rates than smooth-bore extruders. These high specific rates can exceed the melting capacity of the barrier melting section, allowing solid polymer fragments to flow downstream. If too many solid fragments are generated, only a portion of them will be trapped and dispersed in the downstream mixers. Three common mixers designed into grooved-bore extruder screws are the cavity transfer mixer (CTM), the cross-flow plate mixer, and spiral dams. Photographs of a CTM and a crossflow plate mixer are provided in Figure 5.10.29. The CTM [26,27] is constructed by positioning concave cavities on the screw shaft and into the barrel wall. Various cavity shapes are used. Mixing occurs by forcing material into and out of

FIGURE 5.10.28.  Cross sections of common flute designs for Maddock-style mixers: a) recommended design, and b) a deep flute that will cause resin degradation.

FIGURE 5.10.29.  Commonly used mixers with grooved-bore extruder screws, showing locations for resin degradation and stagnant regions: a) a CTM, b) a cross-flow plate mixer with resin degradation, and c) a side view of a cross-flow plate mixer.

the cavities. The cross-flow plate mixer is constructed by positioning circular plates on the shaft of the screw and cutting holes through the plates that allow resin to flow from near the barrel wall to the root of the screw and also from the root to the barrel wall. The land of the plate is in close proximity to the barrel wall. The CTM performs well to melt and disperse solid polymer fragments. This style of mixer, however, can increase the temperature of the extrudate to unacceptable levels and cause resin degradation deep in the cavities. Because of resin degradation, CTMs are not recommended for extrusion of PE resins. The cross-flow plate mixer has numerous sharp corners where the circular plates attach to the root of the screw. These corners create regions that are stagnant and allow resin to remain for extremely long residence times, causing the resin to degrade. Resin degradation is observable in Figure 5.10.29(b). Small process upsets will cause the degraded resin to separate from the screw and contaminate the film product. Moreover, a large portion of the barrel wall in the section is not wiped by the screw, reducing local heat transfer and allowing some material to remain there for extended residence times. Cross-flow plate mixers are not recommended for extrusion of PE resins. Other mixer types are often used with grooved-bore extruder screws, including spiral dams. Spiral dams are created by positioning a dam (or a secondary flight) that starts at the pushing side of the channel and ends downstream at the trailing side, as shown in Figure 5.10.30. The small pockets at the entry and exit are typically stagnant and are regions where resin can degrade. The design flaw here is essentially identical to that of the entry and exit regions of a Maillefer barrier section. Dispersive mixing occurs as the resin flows across the narrow dam clearance. Spiral dams are not recommended for extrusion of PE resins. Transfer Lines Poorly designed transfer lines can be a source of gels. In the

Section 5.10.  Gel Troubleshooting

441

not a factor because the specific rate is nearly constant with changing discharge pressure. The stress at the wall and the residence time distributions can be obtained using computational fluid flow techniques [29]. A simplistic method for calculating the shear stress and pressure gradient using a Newtonian fluid is presented below [30]: P 8Q  L  R4

FIGURE 5.10.30.  Schematic of a spiral dam [2].

cases described here, the designer had optimized the transfer line for cost and pressure requirements for flow through the pipe. In other words, as the pipe diameter increases, the cost of the pipe increases, and the pressure gradient required to make resin flow from the extruder to the die decreases. If the pipe is too large in diameter, however, the resin will have extremely long residence times near the wall. If the residence time is long enough, resin degradation will occur, and gels will appear in the film product. Shear stress at the wall must also be considered when optimizing transfer lines. The shear stress at the wall should be between 20 and 30 kPa [28] to mitigate gels originating from transfer lines. At this stress level, the resin near the pipe walls will periodically slough off the wall and hence not be in the pipe long enough to degrade to crosslinked gels. At lower stress levels, the resin near the wall will not slough off and has the potential to be in the pipe long enough to degrade and crosslink. As expected, the pressure gradient required to achieve flow at these high stress levels is considerably higher than that for a larger-diameter pipe that allows gels to form. For smooth-bore extruders, the higher discharge pressure needed will cause the specific rate to decrease slightly and the extrudate temperature to increase. For grooved-bore extruders, the temperature effect with increasing discharge pressure is

FIGURE 5.10.31.  Shear stress for transfer pipes of three diameters at a rate of 30 kg/h for LDPE resin [28].

w  w 

4Q

 R3 4Q

 R3

(5.10.3)

(5.10.4)

(5.10.5)

where ∆P is the pressure change for a pipe of length L, Q is the volumetric flow rate in m3/s, η is the Newtonian shear viscosity, R is the pipe radius, γw is the shear rate at the wall, and τw is the shear stress at the wall. As an example, a blown-film line was producing a twolayer coextrusion structure with the outside layer coming from the extruder of interest. The line consistently produced film with a high level of gels. The screw was removed from the extruder and was determined not to be the source of the gels. For convenience, this extruder was positioned to the side of the die. The transfer line coming from the extruder was 150 cm in length and 31.8 mm in diameter. The line was running 30 kg/h of an LLDPE resin with a melt index of 1.5 dg/min (190°C, 2.16 kg). The discharge temperature at the die was 210°C. The pressure change needed to make the resin flow through the pipe was 1.5 MPa. The shear stress for this pipe is shown in Figure 5.10.31. The shear stress at the wall was calculated as 8.1 kPa. Because this stress is less than the minimum stress of 20 kPa to keep the wall renewed, the transfer line was the likely source of the gels. The residence time for flow near the wall was over 4.5 hours, leading to resin degradation. Minor process variations can cause the degraded resin to separate from the transfer line wall and appear in the film as a gel. To mitigate the gel issue, the transfer line should be replaced by one that is smaller in diameter. Nineteen and 15 mm diameter transfer lines were studied. As shown in Figure 5.10.31, the wall shear stress increased to 20.5 kPa for the 19 mm transfer line and to 30.4 kPa for the 15 mm transfer line. Because these shear stresses are in the accepted range for surface renewal at the wall, degradation and gels should not form with either diameter. The residence time for flow through the pipes near the wall decreased to about 30 minutes for 15 mm pipe and to 58 minutes for 19 mm pipe. The pressure changes required to make resin flow through the pipes were calculated at 6.4 and 12.3 MPa for the 19 and 15 mm diameter pipes respectively. Because of the lower pressure requirements, a new transfer line was constructed using the 19 mm diameter pipe.

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FIGURE 5.10.32.  Photograph [2] of a screw that had numerous shutdowns when the extruder was maintained at operating temperature for an extended time. The extruder was purged before removing the screw, yet dark degraded resin covers most areas.

Poor Extruder Shutdowns An extrusion line shutdown occurs for many reasons, including planned shutdowns for maintenance, shift changes, changing filtering screens, product changes, and unplanned events. A shutdown period is defined here as a period when the screw is not rotating and therefore when resin is not discharging from the line. If the shutdown period is relatively short, meaning that very little resin degradation can occur, then the extruder barrel temperatures can be maintained at the operating setpoint temperatures. If the length of the shutdown is long relative to the time required to create a significant amount of degradation products, then the extruder should be either purged with a more thermally stable resin, or the barrel setpoint temperature should be decreased to a considerably lower value. An extruder that is maintained at process conditions long enough to create degradation products can be very difficult to bring back online running prime product. In this case, the surfaces of the screw and all metal surfaces in contact with molten resin may become coated with degradation products, as shown in Figure 5.10.32. The time to purge these out can be extremely long and very expensive. For example, LLDPE resins can form crosslinked gels and black specks after 30 minutes of being offline at process temperatures [31]. If the shutdown is under 30 minutes, the barrel can be held at process temperature. If the shutdown is longer, but the line will be brought back online soon, the screw could be rotated at a low speed of 5 RPM to keep resin flowing, mitigating the formation of degradation products. For longer shutdown periods, the extruder should be purged using an LDPE resin or another suitable purge resin and then cooled to ambient temperature. NITROGEN INERTING ON THE HOPPER Nitrogen inerting on the hopper is a method of reducing resin degradation in the extruder, transfer lines, and die for thermally sensitive resins. For this process, nitrogen gas is added at the base of the feed hopper, so that oxygen from the air is purged from between the resin pellets. For nitrogen inerting of a flood-fed extruder, it is common to install a smalldiameter manifold (e.g., drilled tubing) across the feed hopper as close as practical to the extruder feed throat to help ensure uniform distribution of nitrogen. A single entry point

FIGURE 5.10.33.  Schematic of a nitrogen purge manifold for a small-diameter extruder. The white exit holes here are facing the screw.

through the side of the hopper is not advised because the nitrogen will channel up the side, and the opposite side of the hopper will see little to no nitrogen flow, allowing oxygen to enter the extruder. The manifold can consist of a 9.5 mm diameter tube with two or three uniformly spaced drilled openings, as shown in Figure 5.10.33. For proper nitrogen gas distribution, the cross-sectional area of the manifold should be at least three times the sum of the cross-sectional areas of the openings. When installing the manifold, the drilled holes should be pointed down towards the screw. To ensure adequate inerting, the required nitrogen flow rate should be three times the gas volume incoming with the pellet feed to the extruder. For example, if the pellet feed rate to the extruder is 100 kg/h, the pellet feedstock bulk density is 0.66 g/cm3, and the solid density of the polymer is 0.92 g/cm3, the incoming air rate that must be expelled is 710 cm3/min. Therefore, the minimum nitrogen feed rate is 2130 cm3/min. The calculation is as follows: Va 

1  m 1000  1    60  b  s 

where ρb is the bulk density of the feedstock in the hopper, ρs is the solid density of the resin, m is the mass rate of the extrusion in kg/h, and Va is the rate of the air that is being expelled from the extruder. The units for ρb and ρs are g/cm3. Based on a mass balance, the recommended amount of nitrogen purged (VN) should be three times the volume of air expelled, as follows: VN = 3Va If the extruder feed is in granular or powder form, additional calculations are recommended to ensure that the nitrogen flow does not result in fluidization of the feedstock. Moreover, whenever nitrogen is used, adequate ventilation is required to prevent asphyxiation and death. THE INCUMBENT RESIN EFFECT New and innovative film products are constantly produced on pilot and manufacturing lines using either a castor blown-film process. Often these films are sent to end users for evaluation. Acceptable film quality and properties are keys to the success of the new film product. Most of the

Section 5.10.  Gel Troubleshooting

time, the new film produced has acceptable quality (very few gels) and excellent physical properties. Alternatively, the incumbent resin may be replaced with a new resin or a challenger resin as a method to remove cost from the product [31]. In most cases, the incumbent resin has been running on the film line for an extended period of time, producing a product with acceptable quality. Here, the gel level would be relatively low, with only minor levels of black specks and brown soft gels. The gels would likely originate from regions of the screw that were stagnant, allowing the resin to degrade. When the challenger resin is fed to the extruder, slight differences in the processability of the resin relative to the incumbent resin will cause these degraded resin fragments to separate from the screw and contaminate the challenger extrudate. In many cases, the challenger resin receives the blame for the gels rather than the incumbent resin and a poor screw design. A necessary condition for the incumbent resin effect is minor design flaws in the screw, as discussed previously. These flaws are regions where small amounts of resin stagnate, degrade, and then separate from the screw, causing defects in the film product. The level of gels is manageable under steady-state conditions for the incumbent resin. However, the slight upsets that occur when a challenger resin is introduced can cause the degraded material to separate from the screw at a faster rate. If the challenger resin were processed for an extended period of time, it is likely that the same level of gels would eventually occur. Typically, the challenger resin is extruded for only short trials and is incorrectly blamed for the higher level of gels. The incumbent effect starts off by running a single-screw extruder for extended periods of time with the same resin (incumbent). Here, the extrudate and film appear acceptable, with only a few gels and black specks in the film product. These gels and black specks are generated in stagnant regions of the screw. Most of the gels, however, remain attached to the screw and are stable, i.e., not separating from the screw at a rate high enough to alarm quality specialists in the plant. Next, as a short trial or prototype run, the incumbent resin is switched with the challenger resin. Even though the challenger resin may be similar, it will likely process slightly differently than the incumbent resin. This slight difference in processing is often enough to cause the old degraded material that is adhering to the screw to separate from the screw and contaminate the extrudate. The old degraded material will start to come out of the die typically in about 5 minutes after the resin switch for blown-film lines. The initial discharge of gels is sometimes viewed as a gel shower, and then a high level of gels will continue to be observed for the short duration of the trial run. In many cases, plant personnel will unknowingly blame the high level of gels on the last change, in this case, the switch to the challenger resin. In other words, the challenger resin is incorrectly blamed for the high level of gels. In severe cases, the trial is stopped, and the challenger resin is eliminated as an acceptable resin for the application. The incumbent resin continues as the

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preferred resin. The root cause for the gels, however, is a poorly designed extruder screw and not the challenger resin. This is the incumbent resin effect [31]. DISCUSSION The equipment and techniques required to diagnose many gel types properly are expensive and require highly trained people. Many small converters cannot afford to develop these capabilities. Most resin suppliers, however, have the capabilities and are willing to aid customers to identify and mitigate gels. SUMMARY Gels can originate from many sources, including contamination and poorly designed extrusion equipment. This section has presented the most common types of gels that can contaminate a PE film and the equipment design issues that lead to resin degradation. Poor extruder design and operation can cause a high and unacceptable level of gels. The most common flaws include the design of the entries and exits of barrier-flighted melting sections, poor selection of mixers, poorly designed Maddock mixers, small flight radii in channels, and poorly designed transfer lines. This section provides methods for identifying gel types and guidelines for troubleshooting extruder design. ACKNOWLEDGEMENTS The authors thank Rajesh Paradkar and Praveenkumar Boopalachandran for their assistance with FTIR analysis, and Stephen Kodjie and Stephen Werner for assistance with hot-stage analysis. The authors also thank Rajen Patel, Babli Kapur, and Kalyan Sehanobish for their valuable discussion and insights regarding products investigated in this chapter. © 2019 The Dow Chemical Company REFERENCES AND ADDITIONAL RESOURCES [1] Butler, T.I., “Gel Troubleshooting,” in Film Extrusion Manual, Chapter 19, T.I. Butler (Ed.), TAPPI Press, Atlanta, GA, 2005. [2] Campbell, G.A. and Spalding, M.A., Analyzing and Troubleshooting Single-Screw Extruders, Hanser, Munich, 2013. [3] Spalding, M.A., Garcia-Meitin, E.I., Kodjie, S.L., Campbell, G.A., and Womer, T.W., “Troubleshooting and Mitigating Gels in Polyethylene Film Products,” J. Plastic Film & Sheeting, 34, 300 (2018). [4] McCrone, W.C., McCrone, L.B., and Delly, J.G., Polarized Light Microscopy, Ann Arbor Science Publishers, Ann Arbor, MI (1978). [5] Young, M.R., “Principles and Technique of Fluorescence Microscopy”; J. of Cell Sci.; s3-102, 419 (1961). [6] Roma, G., Bruneval, F., and Martin-Samos, L., “Optical Properties of Saturated and Unsaturated Carbonyl Defects in Polyethylene,” Phys. Chem. B, 122, 2023 (2018). [7] Garcia-Meitin, E., “Failure and Defect Analyses of Polymers

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via Morphological Investigation,” SPE-ANTEC Tech. Papers, 60, 2096 (2014). [8] Garcia-Meitin, E., Kapur, M., and Patel, R., “Troubleshooting Fabricated Products via Microscopic Defect and Failure Analysis,” Microscopy and Microanalysis, 22, 1728 (2016). [9] Spalding, M.A. and Chatterjee, A.M. (Eds.), Handbook of Industrial Polyethylene and Technology, Wiley-Scrivener, Hoboken, NJ, 2017. [10] International Standard ISO 18553:2002, Method for the Assessment of the Degree of Pigment or Carbon Black Dispersion in Polyolefin Pipes, Fittings and Compounds (2002). [11] McCrone, W.C., “Polarized Light Microscopy in Conservation: A Personal Perspective,” J. Amer. Instit. Conservation, 33, 101 (1994). [12] Olmstead, J.A. and Gray, D.G., “Fluorescence Spectroscopy of Cellulose, Lignin and Mechanical Pulps,” J. Pulp and Paper Sci., 23, 571 (1997). [13] Castellan, A., Ruggiero, R., Frollini, E., Ramos, L.A., and Chirat, C., “Studies on Fluorescence of Cellulosics,” Holzforschung, 61, 504 (2007). [14] Maillefer, C., Swiss Patent 363,149 (1962). [15] Spalding, M.A., “Metering Channel Flows and Troubleshooting Single-Screw Extruders,” SPE-ANTEC Tech. Papers, 50, 329 (2004). [16] Hyun, K.S., Spalding, M.A., and Powers, J., “Elimination of a Restriction at the Entrance of Barrier Flighted Extruder Screw Sections,” J. Plastic Film & Sheeting, 11, 179 (1995). [17] Moffat, H.K., “Viscous and Resistive Eddies near a Sharp Corner,” J. Fluid Mech., 18, 1 (1964). [18] Recommended Dimensional Guideline for Single Screws, The Society of the Plastics Industry, Inc.

[19] Spalding, M.A., Dooley, J., and Hyun, K.S., “The Effect of Flight Radii Size on the Performance of Single-Screw Extruders,” SPE-ANTEC Tech. Papers, 45, 190 (1999). [20] LeRoy, G., Apparatus for Extrusion of Thermoplastics, US Patent 3,486,192 (1969). [21] Maddock, B.H., Mixing of Thermoplastic Materials, US Patent 3,730,492 (1973). [22] Gregory, R.B., Mixing Section for Extruder Feed Screw, US Patent 3,788,614 (1974). [23] Andersen, P., Shih, C.-K., Spalding, M.A., Wetzel, M.D., and Womer, T.W., “Breakthrough Inventions in Polymer Extrusion,” SPE-ANTEC Tech. Papers, 55, 668 (2009). [24] Sun, X., Gou, Q., Spalding, M.A., Womer, T.W., and Uzelac, N., “Optimization of Maddock-Style Mixers for Single-Screw Extrusion,” SPE-ANTEC Tech. Papers, 62, 898 (2016). [25] Sun, X., Spalding, M.A., Womer, T.W., and Uzelac, N., “Design Optimization of Maddock-Style Mixers for Single-Screw Extrusion Using Numerical Simulations,” SPE-ANTEC Tech. Papers, 63 (2017). [26] Gale, G.M., Extruder Mixer, US Patent 4,419,014 (1983). [27] Wang, C. and Manus-Zloczower, I., “Flow Field Analysis of a Cavity Transfer Mixer,” Polym. Eng. Sci., 34, 1224 (1994). [28] Perdikoulias, J., Compuplast, personal communication, 2018. [29] Sun, X. Dutta, A., and Spalding, M.A., “Transfer Line Optimization,” SPE-ANTEC Tech. Papers (2019). [30] Tadmor, Z. and Gogos, C.G., Principles of Polymer Processing, Second Edition, Wiley, Hoboken, New Jersey, 2006. [31] Spalding, M.A., Gou, Q., Sun, X., and Shi, Q., “The Incumbent Resin Effect for the Single-Screw Extrusion of Polyethylene Resins,” SPE¬-ANTEC Tech. Papers, 62, 826 (2016).

Chapter 5—Section 11

Extrudable Polymers: Purging and Resin Transactions SCOTT B. MARKS and BARRY A. MORRIS, The Dow Chemical Company

INTRODUCTION There are many myths and folklore regarding the best way to transition from one extrudable polymer to another and how to purge or clean out the extruder between runs to avoid quality issues. Methods used in the industry include: • Letting it run—let the extruder run at typical speeds or even slowing down to a drool; • Blowing it out—running at high extruder speed; • Water purge—adding a very small amount of water to resin pellets (note that if this is not done very carefully, there may be serious safety consequences. A very controlled plan in writing is needed for this, with specific limits on the amount of water to be used for a given extruder size.) • Physical cleaning—pulling the screw and cleaning the screw and barrel manually. Also disassembling the die and adaptors and cleaning them manually. This should be a method of last resort. • Disco Purge—systematically varying the extruder speed (RPM) • Commercial purging compounds—these have their uses for specific needs and should be part of a written plan when needed but are not required or useful for all resins. Some of these methods have better credibility than others. The purpose of this section is to apply engineering principles to the subject that enable operators to be more confident in their purging practices. Why should an extrusion operation worry about polymer transitions and purging? Effective purging is critical for a number of reasons, including: • Ensuring that the new polymer has successfully transitioned: For example, an acid copolymer may be introduced following a polyethylene in a planned production schedule. The acid copolymer is used to provide adhesion to a foil or sometimes a primed substrate. If the polymer is not transitioned in properly, then adhesion may suffer due to little or no acid functionality at the surface of the molten extrudate when it hits the

substrate. In many documented cases, a very thin layer of PE has remained on the flow surface of the die for extended periods, leading to deficient adhesion in startup production rolls. • Cleaning out “gels”: Gels or imperfections come from a number of sources and are discussed elsewhere in this manual. See the section on ‘Gel Troubleshooting” to understand the various sources of gels, as well as references 1, 2, and 3 at the end of this section. Examples include unmelted polymer, contaminants, and degraded polymer. Proper resin transitions and purging techniques can help push out degraded polymers from channel walls, elbows, and other troublesome regions that may produce gels. • Proper shutdown and startup: Shutting a line down on a resin that has poor thermal stability may lead to polymer degradation, gels, black specks, and other issues when the line is restarted. Most lines are shut down on polyethylene (PE) or polypropylene (PP) to avoid startup issues. Note: PP resins usually contain antioxidants, but not all PE resins contain antioxidants. If shutting down on PE, it is preferable to use a grade containing antioxidants or to add an antioxidant masterbatch to the plain PE. • Preventing corrosion: Specialty resins such as acid copolymers, ionomers, ethylene vinyl acetate, ethylene acrylate copolymers, ethylene vinyl alcohol, polyamides, and tie resins contain functional groups that may lead to corrosion of metal surfaces if not properly purged after running. • Coming in quickly on specification: Time is money, not only in material costs, but also in labor, utilities, and depreciation. The ability to transition quickly from one polymer to the next while obtaining proper material performance is important for maximizing earnings. • Avoiding heat-seal problems: During a polymer transition, the incoming polymer essentially becomes blended with the incumbent resin. Blends of different resins can result in loss of heat-seal performance. • Good manufacturing practice: Having a good purging and transition procedure that is always followed helps ensure good manufacturing day in and day out. 445

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THEORY OF PURGING AND POLYMER TRANSITIONS Definitions Shear rate, shear stress, and viscosity are important concepts for understanding polymer flow and transitions. Shear rate is the change in speed or velocity divided by the change in distance perpendicular to the direction of flow. In the simple shear flow illustrated in Figure 5.11.1, a fluid is placed between two parallel plates separated by a distance ‘h’. The bottom plate is held in place, while the upper plate is moved at a velocity ‘U’. The shear rate is simply U/h and has units of reciprocal seconds (1/s). In more complex flows, such as pressure-driven flow through a channel (as in a die), the shear rate varies across the height of the channel and is at a maximum near the channel wall. Shear stress is the shearing force divided by the area parallel to the flow. It has the same units as pressure, typically pounds per square inch (PSI) or mega Pascals (MPa). Shear viscosity is the shear stress divided by the shear rate: the shear stress is equal to the viscosity times the shear rate. Viscosity has units of Pa⋅s or poise (1 g/cm/s). 1 Pa⋅s equals 10 poise. Velocity and Viscosity Effects As polymer throughput (velocity) is increased, shear rate and stress increase. This increase gets the polymer flowing and is important for moving material away from metal surfaces. The shape of the velocity profile may change as the velocity is increased, as illustrated in Figure 5.11.2. For pressure-driven flow through a circular pipe or rectangular channel, the velocity is zero or nearly zero at the wall and maximum at the centerline. At low throughput, the velocity profile is parabolic. This is characteristic of a Newtonian fluid in which the viscosity remains constant with changing shear rate. As the flow is increased, the velocity profile becomes flatter; this plug-flow regime is characteristic of a shear-thinning fluid. Most polymer melts are non-Newtonian. At low shear rates, they behave like a Newtonian fluid, but at high shear rates, they are shear-thinning, as shown in Figure 5.11.3. The transition between the Newtonian region

FIGURE 5.11.1.  Illustration of shear rate in simple shear flow between flat plates. The bottom plate is stationary, and the top plate is moved at velocity U. Increasing U increases the shear rate, as illustrated on the right.

FIGURE 5.11.2.  Velocity profiles for polymer flow in a tube or pipe.

and the shear-thinning region differs among polymers. For example, linear low-density polyethylene (LLDPE) generally has a longer Newtonian plateau and is less shear-thinning than low-density polyethylene (LDPE). LDPE has long chain branches that affect viscosity behavior. Because polymers have complex flow properties, the optimum extruder speed for purging will not be the same for every polymer. Since it is difficult in advance to know what the optimum screw speed should be, varying the extruder speed during purging is often a good strategy. Note that the shear rate is not constant across the flow channel shown in Figure 5.11.2. The shear rate is zero at the centerline and at its maximum at the wall of the channel. The polymer experiences different shear rates depending on where it is located in the channel. Likewise, because viscosity varies with shear rate, the viscosity of the polymer will depend on where it is located in the channel. When the second resin is introduced, its viscosity plays a role in determining how fast it will push out the incumbent resin. If the viscosities of the two resins are significantly different, it will take longer to complete the transition. Two scenarios are illustrated in Figure 5.11.4. In the first (left side

FIGURE 5.11.3.  Effect of branching on viscosity behavior of polyethylenes of similar molecular weight. Long-chain branching can be found in LDPE. Linear polyethylene (LLDPE) contains only shortchain branches. The region to the left of constant viscosity is known as the Newtonian region. The region to the right, where viscosity decreases with increasing shear rate, is called the shear-thinning region. Long-chain branched polymers typically have higher viscosity in the Newtonian region and are more shear-thinning than their linear counterparts.

Section 5.11.  Extrudable Polymers: Purging and Resin Transactions

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FIGURE 5.11.4.  Effect of viscosity of polymers flowing down a channel. FIGURE 5.11.5.  Effect of temperature on viscosity of ethylene polymers.

of Figure 5.11.4), resin A (the resin being purged in) has a lower viscosity than B (the incumbent resin in the extruder). This creates a low-shear zone near the wall, making it harder to push out resin B. If resin A has a higher viscosity than B (right side of Figure 5.11.4), the shear is higher near the wall, creating more favorable conditions to push out resin B. The viscosity of polyethylene is often described in terms of ‘melt flow rate’ (MFR) or ‘melt index’ (MI). In this test, the amount of molten polymer pushed from a tubular orifice at a specified temperature under an applied weight is measured. The results are reported in grams per 10 minutes. MFR (and MI) are inversely related to viscosity; a high-viscosity polymer is indicated by a low MFR. When there is a large difference in MFR between the polymers being transitioned, expect a longer transition time. Sometimes it can be beneficial to introduce a polymer of intermediate MFR to speed up the transition. The melt flow of polyethylene is usually measured at 190°C with a weight of 2160 g. Other polymers may use different conditions. For example, polypropylene is often measured and reported at 230°C with a weight of 2160 g. If the measurements were made under different conditions, this must be taken into account. The melt flow reflects the viscosity at only one shear rate, and because of the nature of the test method, that shear rate is not the same for every polymer. Simply comparing melt flow rates of resins can be insufficient in many cases. The full viscosity vs. shear rate curve at several temperatures can often be obtained from the resin supplier and provides much more insight into flow issues. Temperature The viscosity of all polymer melts decreases with increasing temperature, but the amount of change in viscosity with temperature differs for each polymer family. This is illustrated in Figure 5.11.5 for LDPE, acid copolymers, and ionomers. Here, all three resins have the same viscosity at 190°C, indicating that they have the same melt flow rate as would be reported on a product datasheet. However, the viscosity of the ionomer decreases with increasing temperature faster than that of the acid copolymer or LDPE. This can be used to help improve transitions and purging:

• When purging an ionomer out with an LDPE, the LDPE will have a higher viscosity, which should aid in the transition. • When purging into the ionomer, it will be helpful to drop the temperature first so that the incoming ionomer will have a higher viscosity than it would at typical LDPE processing temperature. As discussed above, different polymers also respond differently to changes in shear rate: linear polymers typically have a lower zero-shear viscosity, a longer Newtonian plateau, and exhibit less shear-thinning behavior than a longchain branched polymer of the same molecular weight. The combination of changes in temperature and shear rate (extruder speed) can be used to speed up transitions: • When purging into a linear PE from LDPE, high extruder RPMs are favorable. Because the LLDPE is less shear-thinning, it will have a higher viscosity than LDPE at high shear rates. • When purging from a linear PE to an LDPE, moderate screw speed is favorable. LDPE typically has a higher low-shear viscosity than LLDPE of the same molecular weight. • Temperature can also be used to advantage, as discussed previously. Difficult Regions Various difficult locations that are challenging to purge exist in the film extrusion system. These include: • Screw tips with hourglass adaptors • Breaker plates • Valved adaptors • Static mixers in adapter pipes • Adapter pipes with 90-degree turns • Internal flow surfaces of coextrusion feedblocks • Nested spiral and stacked-plate blown-film die entry ports • Keyhole-style T-slot cast-film dies.

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Streamlining the geometry of all flow regions through an extrusion system is a goal of every good designer. However, there are sections that are more challenging to streamline than others. In addition, some machinery manufacturers do not bother to optimize flow in various sections due to the cost of manufacture versus what a system buyer is willing to pay for the equipment up front. Unfortunately, sometimes cutting costs on the equipment design in the beginning results in higher costs later in wasted polymers and substrates when trying to create in-specification structures. The best solution is to streamline the geometries as well as possible in the beginning. Some areas of concern are illustrated in Figures 5.11.7–5.11.10. Purging issues that arise in these regions are due to dead spots, regions of flow circulation, or low-flow zones. At high throughput (RPM), secondary circulatory flows or eddies, as shown in Figure 5.11.6, may impede the continuous flow of polymer, causing long residence times and polymer degradation. At low throughput, the flow may not be sufficient to push polymer away from the dead zones because there is not enough shear. The best throughput rate may be somewhere in between, but this is difficult to predict. Figure 5.11.7 provides an illustration of a breaker plate, which is a common name for a screenpack holder. These are typically a round piece of metal through which holes are bored. On one side, there is a recessed cavity to install the screenpack to face the incoming flow of polymer. The screenpack serves two purposes: one is to act as a melt filter to catch particles and contaminants and prevent them from flowing to the die; the other is to create a flow impediment to create backpressure on the extruder screw. The backpressure aids in creating proper melting of polymers in the screw. The passage holes of breaker plates are commonly drilled with no counterboring or countersinking on the exit side. This creates areas of dead flow between the holes. There is a pressure drop on the back side of these devices, and with reduced velocity and pressure at the hole exit, polymer can build up on the flat sections between holes. These areas will allow polymer to stagnate and degrade, creating gels and black specks that will eventually slough off and come through the die.

FIGURE 5.11.6.  Circulatory secondary flow pattern in a convergent flow channel, such as in an adapter pipe or other section where the flow-channel diameter is reduced.

One practice that has been found to be helpful is to countersink the holes on the exit side, eliminating the flat areas and creating a more self-wiping flow stream where the polymer exits the breaker-plate bore holes. Some also prefer to countersink the entry side of the holes where the screenpack is seated to eliminate the flat areas there as well. If only one side is done, it is more important to do the exit side of the bore holes. Figure 5.11.8 illustrates a common type of valved adapter used in extrusion coating, but also is found on some cast film lines. The purpose of the valve is to facilitate changes in backpressure on the screw, in addition to the backpressure created by the screenpack. The valve sits in the adapter that changes the direction of the polymer from the exit of the screw to the entry of the pipe connected to the flat extrusion die, or feedblock upstream of the die. There are other designs of valved adapters, but the purpose of each is similar. The valve pin is an area where the polymer flow is split to flow around it and downward. Stagnation points can occur at the top of the pin (12 o’clock position), as well as at the bottom (6 o’clock position). Some converters rarely if ever change the valve position unless they run a large variety of polymers with differing viscosities. A good practice to work

FIGURE 5.11.7.  Eliminating dead zones in breaker plates by counter-boring the inlet and outlet sides.

Section 5.11.  Extrudable Polymers: Purging and Resin Transactions

FIGURE 5.11.8.  Dead zones and solution for backpressure valves.

into your operations plan is to rotate the valve routinely during polymer changeovers. Rotation only needs to be 90 degrees from normal, and then 90 degrees the other way from normal. The purpose of this is twofold. One is to keep the valve mobile and not allow it to get stuck in one position. From a purging point of view, the purpose is to move the known stagnation points into the sides of the flow, which helps remove the material that has been stuck on the pins in the stagnation zones. Figure 5.11.9 shows one style of adapter found in the industry, in which the flow passage has been shaped to facilitate flow from the screw. However, if the end of the screw is not properly shaped to match the adapter section, a buildup of polymer can occur on the screw tip due to steady-state circulation in that area. The solution in this case is to modify the screw tip so that it extends more into the hourglassshaped section of the adapter. Figure 5.11.10 shows two styles of elbows that are used when creating a 90-degree turn in piping on coextrusion systems. In general, it is better to use 45-degree (or less) turn sections when creating coextrusion adapter piping, but there are cases where available space will not allow this. Coex piping length should always be minimized to reduce the residence time of polymer flow. Therefore, when faced with a situation where a 90-degree turn is required, a better option is to use a machined-radius elbow. This is a more expensive unit to manufacture, but it can save you time and

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FIGURE 5.11.10.  Dead zone in coextrusion elbow and streamlined elbow. The streamlined elbow is difficult to manufacture.

material costs with a more efficient flow channel for purging and resin transitions. Resin Considerations Resins that are more compatible (similar) with each other transition and purge more readily. One way to think about this involves adhesion. Compatible resins adhere well to each other. The adhesion of the purging polymer to the incumbent must be greater than the adhesion of the incumbent resin to the channel wall for the incumbent to be carried away. Gels cannot effectively be removed without adhesion. To accomplish this, short (five-minute) intervals of very low screw speed should be incorporated into the purging process to allow time for the purge polymer to bond to the gel. Some people advocate using a functionalized polyolefin, such as an acid copolymer or a coex tie resin (containing anhydride functionality), to enhance adhesion to gel particles for a more effective purge [4]. A specific procedure for transitioning out of nylon using this concept is described later in this section. Adhesion is also the reason that introducing a functionalized polymer often results in a burst of gels. This is generally not from the resin itself, but from the effect that it has on pushing out degraded polymer from previous runs. Some resins should not be directly mixed in the extruder because they interact too strongly and may create gels. These include EVOH with PA (nylon), EVA and acid copolymers (when the temperature is greater than 235°C (455°F)), and EVOH or PA (nylon) with standard or concentrate tie resins. Instead, polyethylene or another inert polyolefin should be introduced between runs involving these materials. (See later in this section for a suggested procedure for purging nylon using a ‘diluted’ tie resin in a specific way.) Some resins will crosslink and form gel if they are run too hot. These include: • Ethylene vinyl acetate copolymer (EVA) • Ethylene acrylic acid copolymer (EAA) • Ethylene methacrylic acid copolymer (EMAA) • Ethylene vinyl alcohol copolymer (EVOH).

FIGURE 5.11.9.  Extending the screw tip can eliminate dead zones in hourglass adaptors.

Even PE may crosslink because of oxidation. To prevent degradation of these polymers, care should be taken to control temperature below the maximum recommended by the

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resin supplier. Always refer to product and safety data sheets from the resin supplier for a given grade of polymer. Purge Compounds The use of purge compounds may be warranted when polymer has degraded in the extruder or die and when a particularly thorough cleanup is needed. These compounds often contain foaming agents to reach dead zones, inorganic scrubbing additives such as silica, and reactive compounds such as organic acids to help remove degraded polymer deposits. Purging these compounds out afterwards can be challenging and may take more resin than normal purging. Hence, it is recommended to use these materials only when needed to remove heavy gels or when facing other challenges such as colored polymers. GUIDELINES AND PROCEDURES FOR PURGING AND RESIN TRANSITIONS Resin Introduction • Empty the hopper before introducing the new resin, but do not allow the extruder to run empty while running at production speed. The heat from the process is carried away by the polymer. If the polymer feed is reduced while the extruder is still operating at full speed, the energy will be absorbed by a smaller quantity of polymer mass, causing the temperature of the remaining polymer to spike higher and possibly causing degradation. In severe cases, off-gassing may cause pressure to build up inside the extruder, which can be dangerous. • If the new resin is run at a higher temperature setting than the old one, allow the new polymer to be completely introduced before raising the temperature setting. This helps to prevent the previous resin from seeing too high a temperature. (If the new resin has a higher melting point than the operating temperature of the old resin, this will have to be modified. Slowing down the line before raising the temperature is recommended in this case.) This strategy may also need to be modified if temperature is being used to help create a viscosity match for effective purging.

• Likewise, if the new resin is run at a colder temperature than the old resin, lower the temperature first before introducing the new resin. Disco Purge Method—10-Minute Cycles Based on field testing experiences in real-life polymer extrusion operations, as well as laboratory tests to compare various methods of conducting resin transitions and extruder purging, for most cases the “Disco Purge” method has been shown to minimize polymer changeover time [5]. This method varies the speed of the extruder at one-minute intervals over a five-minute period, followed by a five-minute interval at a reduced speed. DuPont called this the Disco Purge procedure. (The name was taken from the late 1970s/early 1980s dance craze during a training class as a way to explain to a group of students that the purpose was to ‘shake up’ the extrusion system. The name stuck and has since been used around the world.) This method is now used by many converters globally. Some machinery manufacturers have incorporated it into their extrusion-control programming to allow an extruder operator to set the extruder into the 10-minute Disco Cycle in single or repeat mode, with safety protocols built in to monitor pressure and extruder torque so that neither is allowed to exceed safety limits. Pressure and torque vary by polymer type, so this is important to monitor. A worksheet for the 10-minute Disco cycle for an extruder with a smooth-bore feed section is given in Figure 5.11.11. For example, if the maximum extruder screw speed is 100 RPM, the cycle would be one-minute intervals of 30, 90, 50, 15, 70, 15, 15, 15, 15, and 15 RPM. These speeds can be adjusted up or down depending on the actual maximum screw speed of the extruder. If amperage or pressure is limiting the output, adjust the maximum screw speed accordingly. The 10-minute cycle may need to be repeated more than once. For a grooved feed section extruder, consult with the polymer supplier’s technical advisor, as pressure limits may be more of an issue in these systems. The Disco procedure works efficiently because it takes advantage of the mechanisms discussed in the Theory section: • Changing screw speed varies the shear rate to change flow patterns, shear rate, and viscosities.

FIGURE 5.11.11.  Disco Purge Worksheet [5].

Section 5.11.  Extrudable Polymers: Purging and Resin Transactions

• Slow periods are incorporated to help with adhesion and to disrupt flow circulation patterns. The Disco resin transition method is often effective without purge compounds. It does not, however, scour tough die deposits and is not always effective in removing gels. Purge compounds may be required for particularly challenging situations. When using purge compounds, follow the instructions provided by the manufacturer for best results. Suggested Purge Procedures for Specific Resin Transitions In the following discussion, the term “polymer” and “PE” refer to the following: • “Polymer” examples include ionomer, acid copolymer, EVA, acrylate copolymer, coex tie resins, and other specialty ethylene copolymers. • “PE” examples include LDPE, HDPE, MDPE, LLDPE, and m-LLDPE. Note that instead of “PE”, sometimes PP (polypropylene) is used. Changeover Procedure from PE to “Polymer” (1) With PE still in the machine, reduce the temperature profile to “polymer” conditions according to the data sheet. (2) When the machine temperature is fully reduced to the “polymer” profile, empty all PE from the hopper and replace it with the “polymer” resin. (3) Fully open the deckles (if applicable on the cast film die). (4) Run extruder speed (RPM) up to about 50% of maximum RPM until the new resin starts to come out of the die, displacing PE. Then start the Disco Purge. (5) Use the Disco Purge procedure to purge out PE for two cycles of 10 minutes each. (If the cast film die has internal deckles, move these in and out to aid in cleaning.) During the five minutes of low RPM, shim the die with soft metal to clean off die lines on the die lips. The metal used must be softer than the chrome or nickel plating on the die lips. (6) Inspect the web. If it is clear and free from gels and other impurities, then start to run production. If not, repeat another 10-minute cycle of Disco Purging. Changeover Procedure from “Polymer” to PE (1) Empty all the “polymer” resin from the hopper and replace it with LDPE resin. Note: Keep temperature profile at “polymer” conditions. (2) Fully open the deckles (if applicable). (3) Run at about 50% of maximum RPM until you see LDPE start to come out of the die.

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(4) Carry out Disco Purging for at least two cycles of 10 minutes each. Usually, three cycles are needed to purge out specialty resins and return to PE. (If the die has internal deckles, move these in and out to aid in cleaning.) During the five minutes of low RPM, shim the die with soft metal to clean off die lines on the die lips. (5) Inspect the web. If impurities still exist in the layer being transitioned, repeat another 10-minute Disco Purge cycle. (6) Once the web looks good and all “polymer” is out, ONLY then raise the temperature profile to PE conditions. If the temperatures are raised too soon before the “polymer” is properly purged, you run the risk of creating crosslinking gels. Note: The web may start to look good after one disco cycle, but there will still be a thin layer of the incumbent polymer on the metal surfaces. You need to go through the disco cycle at least twice at the lower temperatures to clean the thin layer of incumbent polymer off the inner metal surfaces. Precaution: NEVER shut down the extruder with a specialty “polymer” in the machine. Always change over to PE using the Disco Purge Procedure and shut down the extruder with only PE in the machine. (PP can also be used for shutdown when appropriate.) The following is a special procedure specifically for transitioning out of polyamide (nylon). Changeover procedure from polyamide (nylon) 6 or 6/66 to PE The following is a suggested procedure specifically for purging out nylon polymers. This procedure was developed over time in tests in the field at various extrusion operations and is also used in the authors’ laboratory complex on single-screw extruders. (1) Do not reduce temperatures: maintain nylon processing temperature. (2) First resin flush: (a) For a cast film line, introduce low-MFR (e.g., 1.5 to 3.5 g/10min measured at 190°C and 2160 g) lowdensity polyethylene (LDPE). (b) For a blown film line, introduce low-MFR (e.g., 0.6 to 0.9 g/10min measured at 190°C and 2160 g) lowdensity polyethylene (LDPE). (3) Disco Purge with low-MFR LDPE (a) Watch pressure and motor load in particular on a cast film lines where die gaps are typically tighter than on blown film lines. (b) Run one 10-Minute Disco cycle. (4) Introduce a blend of a maleic anhydride containing PEbased coex tie-resin polymer with the low-MFR LDPE used in step 2. Use about 10 kg (22 lb) of the blend in a 65 mm (2.5 inch) extruder. Scale up or down as appropriate: (a) Use a “pre-formulated ready-to-use” tie resin.

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(b) Blend 50% of the tie resin with 50% of the lowMFR LDPE. (c) A low-MFR LDPE-based tie resin is preferred to an LLDPE-based tie resin if available. (d) BYNEL™ 4206 (2.5 g/min) is an example of a lowMFR LDPE tie resin for use in extrusion-coating and cast-film purge blends. (e) BYNEL™ 4208 (0.4 g/min) is an example of a lowMFR LDPE tie resin for use in extrusion-coating and cast-film purge blends. (f) If a low-MFR LLDPE-based tie must be used, the MFR should be in the same range for each process (blown versus cast) as mentioned above. ™Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow. CAUTION: Do not use a maleic anhydride-type polymer that is classified as a concentrate or semi-concentrate for blending to produce a tie-layer polymer. These materials contain too much maleic anhydride, and blending them down to very low levels can yield a less effective purge material. Higher doses in a purge blend conversely can lead to gel creation. It is preferable to work with a “ready-to-use” tie resin for this purging blend process. (5) Disco purge with this blend: (a) Watch pressure and motor load, in particular on an cast film line. (b) Run one 10-minute Disco cycle. (6) Return to pure low-MFR low-density polyethylene and run one 10-minute Disco cycle. (7) Evaluate the web, and if all looks good, reduce temperature settings as appropriate from nylon to PE conditions. If the web does not look appropriate, repeat 10-minute Disco Purge cycles.

(8) Introduce the appropriate LDPE or LLDPE that will be run next. Continue to run Disco cycles occasionally as temperatures normalize to the new PE settings to help improve the transition out of nylon. (9) Once temperatures are normalized at the new conditions, check the film or web to see whether it meets your needs. © 2019 The Dow Chemical Company REFERENCES AND ADDITIONAL RESOURCES [1] Butler, T.I., 2005, “Gel Troubleshooting,” in Butler, T.I. (Ed.), 2005, Film Extrusion Manual, TAPPI Press, Atlanta, pp. 275– 286. [2] Campbell, G.A. and Spalding, M.A., 2010, Troubleshooting & Analysis of Single-Screw Extrusion, Hanser Gardner, New York, pp. 484–490. [3] Spalding, M.A., Garcia-Meitin, E., Kodjie, S.L., Campbell, G.A., 2013, “Troubleshooting and Mitigating Gels in Polyolefin Film Products,” SPE ANTEC Conference. [4] Botros, M.G., 1999, “Purging Extruders with Functionalized Polyolefins,” in Bezigian, T. (Ed.), Extrusion Coating Manual, 4th ed., TAPPI Press, Atlanta, pp. 279–286. [5] Vansant, J.D., 2007, “Theory and Practice of Extruder System Purging,” Proceedings, TAPPI PLACE Conference.

Note: The information herein is intended for guidance only. Although it is believed reliable, Dow neither warrants nor guarantees its completeness, efficacy, or safety in use. It is intended for use only by competent personnel. The user accepts full responsibility for its use and fully indemnifies and holds Dow harmless from any loss, claim, damage, or injury arising out of its use regardless of cause, unless user shows such to have been caused directly by ill will or willful misconduct of Dow. Also, no license is granted or implied, and no recommendation to infringe any patent is made.

Chapter 5—Section 12

Safety in Film Extrusion NICOLE E. DOWLING and LAURA K. MERGENHAGEN, The Dow Chemical Company

INTRODUCTION It is recognized that the extrusion process contains potentially hazardous areas. Companies using this process must provide an adequate safety program for employees. However, even the best safety program does not completely ensure freedom from injury. Defeating interlocks, removing guards, lack of equipment knowledge, and individual carelessness can all lead to incidents that may result in personal injury. Accidents don’t just happen, they are human-related and require continuing individual involvement to prevent. PREPARATION Training Training is an essential part of a good safety program. According to American National Standards Institute (ANSI) 151.7, it is the responsibility of the manufacturer to furnish instructions with equipment and to establish guidelines for use, care, and safe operation of the machine. The employer can reference manufacturer’s instructions as well as other source materials regarding the formal training of operators. Qualified, experienced instructors should be used to indoctrinate new operators. Periodic refresher sessions should be conducted regarding safe operation of equipment. As required by ANSI and under the sponsorship of the Society of the Plastics Industry (SPI), a periodic review of consensus standards led to the development of four safety standards for film, sheet, coating and laminating, and plastic web winding machinery in 1999: • ANSI/SPI B 151.2—machinery for film casting, cast embossing, laminating, and extrusion coating Publisher's Note: This chapter contains safety suggestions only. It is not intended to create a standard of care, to imply that no other safe methods of operation are possible, or to suggest that compliance with the suggestions herein will ensure safe operations.

• ANSI/SPI B 151.4—machinery for blown-film manufacturing • ANSI/SPI B 151.5—winding machinery for plastic film and sheet • ANSI/SPI B 151.20—machinery for sheet manufacturing. Additional information on ANSI standards is available electronically from ANSI at http://www.ansi.org. An operator who is well-trained in processing operations, knowledgeable about the machinery being operated, aware of the purpose and location of the safety devices provided on the machinery, and instructed in good safety procedures is equipped with the most effective safety devices that can be provided. Procedures Safety checklists are an effective tool for identifying hazards in a work area. A good checklist should break the process down into basic steps: (1) Tooling changes and setups (2) Preparation (such as assembling raw materials and training on auxiliary equipment) (3) Startup (4) Operation (5) Shutdown (including emergency shutdown) (6) Cleanup. A more formal analysis of potential safety hazards, known as the EN (European Normal or “standard”) 1050, was developed by the European Community. In this method, potential hazards are identified and then ranked based on their ratings on three criteria. The criteria include the frequency of exposure to the hazard, the severity of the potential injury, and the susceptibility of personnel to injury. The ultimate goal is to eliminate or reduce the potential hazard based on the results of the analysis. Additional information on the EN 1050 is contained in [2]. 453

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POTENTIAL HAZARDS Slipping and Tripping All surrounding floors should be kept in good condition and as clean and dry as possible because resin and water spills can present slip hazards. Adequate clearance around equipment enables operators to work without interfering with one another. Burns Serious burns can result from even limited contact with metal surfaces around the adapter and die due to the high thermal conductivity of the metal. If contacted, the polymer melt can adhere to the skin. To avoid serious burns, proper training and regular practice using heat-resistant gloves is recommended. If the melt does adhere to the skin, it is essential to cool it quickly by plunging into cold water. It is also important to avoid removing burned skin, except under medical supervision. Always clean and clear the area with tools rather than hands. It is dangerous to stand near the die of a cast-film unit because it is not uncommon for the melt to “spit” when the die is being purged. This will also occur if moisture is present in the resin. Also, drafts can blow the melt curtain. When purging, clearing, or adjusting the die, a facemask, long-sleeved jacket, and gloves are recommended safety equipment. Facemasks should be worn when performing screen changes. There have been instances where hot gas buildup near the screen has squirted out hot melt during a screen change. Electricity High-voltage electricity is potentially fatal. Unauthorized or untrained personnel should not be allowed to tamper with electrical equipment. Always isolate equipment before working. Unsafe electrical arrangements should be reported and repaired immediately. The concern for electrical hazard is increased by the presence of water. Good housekeeping is essential. Although static electricity is unlikely to injure, the involuntary reaction it produces can be dangerous. Antistatic equipment can be installed and is strongly recommended when working with polyester and polypropylene. Heat, Noise, and Fumes Noise associated with motors and blowers may be excessive. Adequate hearing protection must be worn for exposure to high noise levels over a sustained period of time. Polyethylene can degrade at high melt temperatures and generate chemicals in the form of fumes. The polymer’s Safety Data Sheet (SDS) can be consulted for precautions recommended during extrusion. In cast film and extrusion

coating, a fume hood should be located above the die position to minimize the exposure of employees to fumes. Cleaning agents and solvents may have possible toxic effects and flammability potential. Solvents should be handled in a well-ventilated area. Avoid sparks or open flames. Pinch Points Nip rolls are potential hazards even when stationary. The mechanical systems that close nip rolls can exert very high forces and can easily trap and crush. When rotating, they are even more dangerous due to the risk of catching limbs, long hair, or loose clothing. Loose clothing and rotating equipment do not mix. Winders can also cause serious injury due to the potential for entrapment between the web and rollers. Emergency stop buttons or bars should be located at appropriate positions along the line, and operators should know how to stop the line quickly. Proper equipment for handling heavy rolls and adequate training in its use are essential. Cuts Severe cuts can result from contact with the trim blades. Ozone The operation of corona discharge treaters generates ozone, which is a toxic element. Ozone is especially dangerous because operators often become accustomed to its smell. If a visitor can smell ozone, levels should be checked. Adequate ventilation is the only solution. EXTRUSION The function of an extruder depends on three basic conditions: temperature, pressure, and torque. The ability to select and maintain proper operating conditions is provided by the controls and indicating devices with which all extruders are equipped. Pre-Heat and Startup Temperature controllers, pressure-indicating devices, speed indicators, and ammeters not only should be used to control processing conditions, but should also be used as an effective source of information for maintaining safe operation. Check that all gauges are in working order and that all guards are in place. The troubleshooting guide featured in Table 5.12.1 can be useful during pre-heat and startup to identify the source of a temperature- or pressure-related problem. Pressure indicators and relief devices serve important safety functions. Never replace a pressure gauge by inserting a plug into the barrel. Keep bleed ports clean, and never plug a port or rupture-disc mounting hole.

Section 5.12.  Safety in Film Extrusion

Transfer Piping Extreme caution must be used during pre-heat and startup of transfer piping. In some cases, the smaller mass of the transfer pipe in proportion to the barrel and die results in a large enough ratio of power input to thermal mass that the transfer pipe can heat up rapidly and come up to temperature long before the extruder and die have reached the melting point of the plastic material, resulting in a solid plug of material at each end of the transfer pipe. At high temperatures and long residence times, the molten material may degrade, or a gas may be generated and produce excessively high pressure that can rupture the pipe and cause injury. The sudden release of this pressure may cause the plug, gas, or molten material to be sprayed out of the die. Recommended safety precautions related to transfer piping include the following: (1) Do not turn on the heat to the transfer piping until the equipment on each end of the pipe is above the polymer melt point and a soak period has elapsed to ensure melting of the polymer. (2) Keep all personnel clear of the transfer piping and die exit until all extruders are running and all resin in the equipment during heat-up has been purged from the die. (3) Always use the same wattage on the heaters as recommended by the equipment manufacturer. (4) Keep all insulation, covers, and safety devices in place and in working order. (5) Use the same safety precautions when heating a pipe offline as when heating it on the extruder. (6) Never heat the pipe with a torch. Additional information on the design of transfer piping is contained in [4]. Purging Shutdown and startup are potentially dangerous periods because the mass of polyethylene produced when purging

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can retain its heat for a long time. Care should be taken not to touch or step in the hot polyethylene. Screen Changers Molten plastic can be forced out of the screen changer under pressure and injure nearby personnel. The operator should stand behind or alongside the hydraulic cylinder when actuating the valve. Do not place hands in the ejection path of the screen. Full face guards are recommended during screen-change operations. If maintenance is to be performed on the changer, the hydraulic power pack should be locked out electrically and the accumulator charge dumped. If the charge is not dumped, the system will be under pressure. POST-EXTRUSION Blown-Film Towers and Nip Rolls Only authorized personnel should be allowed on film cooling towers. Posted warning signs will alert personnel on the ground that they should wear approved protective headgear to avoid injury from falling objects. Extreme caution should be used when in the vicinity of nip rolls. Always be aware of the location of emergency stops. All threading operations should be performed with the nip rolls in the open position and the rolls rotating at low speed. Threading should be done with a leader to guide the web. If two people are available during threading, one should be stationed at the controls until the operation is finished. Never attempt to clear a wrap-up with the nips closed. Never place hands in the open nip. Shutting down the roll drive during removal is an added safety measure. Corona Corona discharge treaters use high voltage, high-frequency electrical discharges, which can shock or burn. Metal electrodes can become hot during treatment, meaning that there is burn potential even when they are electrically isolat-

TABLE 5.12.1.  Extruder Troubleshooting Guide. Mode

Normal

Abnormal Response

Possible Cause

Pre-heat

• All ammeters show power input • Normal heater power, • Normal temperature • Normal heater power, Normal temperature

• No ammeter power reading • Normal heater power, Low temperature • Normal heater power, • High temperature

• Burned-out heater or faulty connection • Faulty thermocouple or short • Faulty cooling system (valve open) • Faulty cooling system (valve closed) • Low coolant volume • Coolant temperature too high

Startup

• Normal motor power, • Normal head pressure

• High pressure and no controller response • Normal motor power, Low pressure • High pressure or pressure surge with power surge

• Faulty controller • High temperature occurring — Pre-heat temperature — Low operating temperature — Obstruction — Dirty screen pack

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ed. Ozone generated during treatment must be removed by an effective ventilation system. In addition, many ventilation systems are equipped with a scrubber to avoid venting ozone into the atmosphere. For more information on corona treatment, refer to Section 2.7 of this manual, Surface Treatment.

ensure that all safeguards are in safe condition and function as originally intended. Periodic operational inspections and maintenance as well as inspections for mechanical defects are recommended. Guidelines for equipment are contained in Section 2.1 of this manual, The Extruder.

Slitting

SUMMARY

Never run hands over the surface of the web. Always be aware of the position of knives, slitters, or other cutting devices. The risk of injury is increased during setup and maintenance procedures when exposure to the slitter blades is increased. Installation of blade guards helps to reduce operator exposure to the blades; however, they do not ensure complete safety. Winding and Cooling The operating area should be clear before initiating any winder function. Always pre-thread the winder with a leader to enable the product to be drawn in through the winder at startup. MAINTENANCE Opening Equipment Removing dies, opening head clamps, and all other operations in the area of the extruder head must be performed with extreme caution. Even when the extruder has been shut off and allowed to cool down, the danger of residual head pressure is ever-present. No one should ever stand in front of an extruder. Inspections It is the employer’s responsibility to establish and follow a program of periodic and regular equipment inspections to

Safe film extrusion involves proper operator training, adherence to safety procedures, continuing maintenance of controls and equipment, and careful observation during operation. Although it is nearly impossible to eliminate safety hazards, it is possible to minimize them through proper education and emphasis on safety. Focusing on safety and adopting it as a way of life are the keys to identifying potential hazards and avoiding injury. REFERENCES AND ADDITIONAL RESOURCES [1] American National Standards Institute 151.7 (Safety Requirements for Blown, Cast, and Extrusion Equipment). [2] Lounsbury, D.C., “ANSI Standards Combined with European Hazard Ranking Methods Provide Plastic Web Processors with Powerful Tools for Enhancing Workplace Safety,ˮ TAPPI Polymers, Laminations, & Coatings Conference, 2000, p. 591. [3] Ladd, C.F., “A Checklist For Safe Extruder Operation,ˮ Plastics Engineering, January, 1978, p. 47. [4] Smith, D.J. and Bayless, R., “Safety Considerations in the Design and Operation of Polymer Transfer Piping,ˮ TAPPI Coextrusion Short Course, 1983, p. 1. [5] Bria, F., “Safety With Extrusion Coating,ˮ Chapter 22, TAPPI Extrusion Coating Manual, 1990, p. 343. [6] Murk, W. and Birch, J., “Extrusion Safety,ˮ Proceedings, TAPPI Paper Synthetics Conference, 1979, p. 25. [7] USI Division of Quantum Chemical, “A Simple Safety Program,ˮ PM&E, September 1990, p. 39.

Film Extrusion Manual, Second Edition, 2005

Chapter 6 Structure Development, Nomenclature and Testing EDITOR: WARREN DURLING, Clorox Services Company

Section Number

Section Title

Page Number



6.1

Film Properties and Performance KENYATTAH MATHIS, SARAH KUHL and DEAN FERRACANE, Clorox Services Company

459



6.2

Film Quality and Test Methods DAVID BOSTIAN, CharterNEX Films

465



6.3

Introduction to Structures SCOTT B. MARKS and BARRY A. MORRIS, The Dow Chemical Company

473



6.4

Critical Requirements for Structures THOMAS J. DUNN, Flexpacknology LLC

479



6.5

Quality Control (QC) and Physical Testing THOMAS J. DUNN, Flexpacknology LLC

483



6.6

Structure Writing and Nomenclature SCOTT B. MARKS, The Dow Chemical Company

491

Chapter 6—Section 1

Film Properties and Performance DEAN FERRACANE, SARAH KUHL and KENYATTAH MATHIS, Clorox Services Company

HAZE Haze describes the degree of clarity or cloudiness of a transparent or translucent plastic film. The cloudy appearance of plastic film results from the scattering of light passing through the film. This can be caused by imperfections or abrasions on the film surfaces as well as the size and concentration of impurities or additives within the film. The clarity of a plastic film can be essential to both the performance and appearance of a product. For instance, in food protection films, such as cling wrap and resealable food bags, the ability for the consumer to see the object encased in film is a critical benefit of the product being used. Although films with some haze can provide sufficient contact clarity for the product to perform, less hazy films are often perceived as premium and desirable. Several key factors can strongly impact the haze of a plastic film, including processing and film formulation. Although additives might be needed for other benefits, such as process aids or antimicrobial efficacy, they can also increase haze. In processing, faster cooling can result in smaller crystal sizes and a less hazy film. In fact, film haze changes at the frost line during extrusion can provide insight into when molten plastic begins to crystallize and help troubleshoot blown film issues. Test Method(s): • ASTM D1003-13—Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics LIGHT TRANSMISSION Light transmission is a measure of the percentage of light that passes through a film. When interpreting the light transmission data associated with a film, it is important to understand what these percentages mean. Zero percent represents very high opacity (no light passes through), and one hundred percent represents high clarity (all light passes through). Light transmission measurements assist manufacturers in determining the level of white, black, or colored masterbatch

and/or reprocessed material that is needed to make film more or less opaque with regard to a consumer’s need for a specific product. Trash bag film tends to be more opaque because the consumer’s need is to avoid seeing the contents of the bag. However, with food storage bag film, the preference is the opposite. A clear bag provides visibility to see the contents of the bag. The following equation mathematically defines light transmission for plastic films and other applications: Light Transmission (%) = Y/Z × 100 Y = light going out Z = light going in To manipulate the light transmission of film, opacifiers such as white, gray, or black colorants can be added. The benefit of adding an opacifier to a film is to prevent light from passing through the film to create the perception that the film is thicker and of higher quality. This method enables the manufacturer to add this value to a consumer at a lower cost. Test Method(s): • ASTM D1003-13—Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics • ASTM D1003–00—Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics GAUGE (THICKNESS) Gauge is defined as the thickness of a material. Gauge can be measured in mils, microns, millimeters, and/or inches using various instruments and methods. Some examples of gauge measurement methods are gauge by weight, point to point, and gauge profiles using capacitance measurement over a longer, continuous section of film. Gauge by weight is the gauge determined by weighing a section of film of known density. To calculate the gauge of the film, it is first necessary to weigh the film. Using the weight and density, the gauge can be calculated as: 459

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CHAPTER 6—STRUCTURE DEVELOPMENT, NOMENCLATURE, AND TESTING

T = M/(L × W × D); where T = thickness of film, M = mass, L = length of film, W = width of film, and D = density of film. The point-to-point method measures the consistency of the actual gauge of the film using a specific number of measurements. The capacitance profile measures a larger number of points to capture gauge variability more accurately. The thickness of a film acts as a barrier and is a tool for improving or reducing oxygen and/or water vapor transmission through the film. This is important in the case of produce freshness. In more extreme cases, such as pet food, barrier films are used to keep food fresh for longer shelf lives in place of more traditional plastic containment. If the film does not have the specified thickness, it can cause a failure due to tearing, splitting, or leaking. Product quality can be compromised if the standard gauge has been set and the gauge falls below the specified thickness. Gauge falling below the set standard implies that the film is now “too thin,” which can translate as subpar quality to a consumer. Process and cost control also play an important role in gauge. Gauge targets are often created by balancing physical properties, process reliability, and cost. If the film is “too thick,” it is likely that unnecessary incremental cost is being incurred due to overuse of resin. Test Method(s): • ASTM D6988-13—Standard Guide for Determination of Thickness of Plastic Film Test Specimens • ASTM D8136-17—Standard Test Method for Determining Plastic Film Thickness and Thickness Variability Using a Non-Contact Capacitance Thickness Gauge TEAR STRENGTH Tear resistance is defined as the force required to propagate or extend a tear in a film after the tear has been initiated. The three key test methods shown to the right are used to test this property in blown and cast films. Similarly to tensile strength, tear strength varies in the machine and transverse directions depending on the machine-direction orientation of the film. In fact, the ratio of machine- to transverse-direction tear strength for a film can provide insight into its level of biaxial orientation. Tear resistance is an important property in products designed for containment—if the film punctures, high tear strength is needed to avoid propagation of a tear and subsequent loss of contents. Strong tear resistance in both the machine and transverse directions is important in this case and can be delivered by choosing the right material, such as polymers with higher-molecular-weight chains, but lower overall density. An example of film with poor tear strength is free grocery store bags, which are typically made from highdensity polyethylene. When a box corner pokes through the film, the entire bag rips quite easily, resulting in groceries falling to the ground.

In other applications, it is critical to have the right balance of tear strength, which can be delivered through process development. For instance, plastic wrap manufacturers look for lower transverse-direction tear strength and higher machine-direction tear strength to ensure good propagation along the cutter bar without propagation of the tear in the wrong direction. Test Method(s): • ASTM D1922—Standard Test Method for Propagation Tear Resistance of Plastic Film and Thin Sheeting By Pendulum Method (Elmendorf Tear) • ASTM D1004—Standard Test Method for Tear Resistance (Graves Tear) of Plastic Film and Sheeting • ASTM D1938—Standard Test Method for Tear-Propagation Resistance (Trouser Tear) of Plastic Film and Thin Sheeting by a Single-Tear Method TENSILE STRENGTH Tensile strength measures the force needed to stretch a material in a particular direction to the point where it fractures or breaks completely. Traditionally, tensile strength is tested in the machine direction (MD) and the transverse or cross direction (TD) to obtain a more comprehensive profile of the film’s strength. MD is defined as the direction in which the film is extruded, and TD is defined as the direction perpendicular to the machine direction. There are three definitions of tensile strength. Yield strength indicates how far a material can stretch before it is permanently deformed. Ultimate strength indicates how much force the material can withstand before failure. Breaking strength is the material’s maximum capability to stretch without breaking. Much like tear strength, tensile strength can be manipulated by adjusting process variables, such as blowup ratio, that control film orientation. The stronger the orientation in a particular direction, the higher the tensile strength in the same direction, because it is much more difficult to break polymer chains than to overcome the weaker intermolecular forces holding these chains together. Stretching a material using specific loads makes it possible to determine how the material will respond to stress in real-world applications. This analysis can help provide insight when evaluating different formulations or processing conditions in an effort to optimize the film’s intended end-use performance. In relation to trash bag film, the key is to determine the correct amount of stretch that enables the consumer to lift the bag successfully without tearing or breaking the seals. Another reason that tensile strength is important in trash bags is that it helps determine how full a bag can be stuffed before it breaks. Similarly, in cling-wrap film, the key is to provide the right amount of stretch that translates to clinging onto an object without tearing as the film stretches.

Section 6.1.  Film Properties and Performance

Test Method(s): • ASTM D638-14—Standard Test Method for Tensile Properties of Plastics • ASTM D882-18—Standard Test Method for Tensile Properties of Thin Plastic Sheeting PUNCTURE RESISTANCE Puncture resistance defines the ability of a material to resist another material from entering it. Unlike a single tensile test, a method used to evaluate resistance to puncture looks at the film strength in all directions in a single test. One way to measure this attribute is with a small force from a rounded object for an extended period of time. This can simulate, for example, the force required to push fingers through a trash bag. Impact resistance measures the resistance to puncture from a larger force over a shorter period of time. Often this is measured with a falling dart, which in the case of a trash bag can simulate the impact of throwing a heavy piece of trash into the bag. High impact and puncture resistance is typically desired by end users for strong films. When processing films, this can be achieved by reducing machine-direction orientation and extruding a film with more balanced properties. In addition to processing, selecting grades of plastic with lower density and higher elasticity can generally help improve the impact strength of plastic films. Test Method(s): • ASTM D1709—Standard Test Methods for Impact Resistance of Plastic Film by the Free-Falling Dart Method • ASTM D3763 / ASTM D7192—Standard Test Method for High Speed Puncture Properties of Plastics / Plastic Films Using Load and Displacement Sensors • ASTM D5628—Standard Test Method for Impact Resistance of Flat, Rigid Plastic Specimens by Means of a Falling Dart • ASTM F1306—Slow Rate Penetration Resistance of Flexible Barrier Films and Laminates • ASTM D7136—Standard Test Method for Measuring the Damage Resistance of a Fiber-Reinforced Polymer Matrix Composite to a Drop-Weight Impact Event STIFFNESS /ELASTICITY Elasticity is a measure of the capability of a film to restore its initial state after it is deformed. The term secant modulus is often used to describe the stiffness of a plastic film in its inelastic region and is a ratio of stress to strain given by the slope of a line from the origin to any other point along this portion of the stress-strain curve. The tangent modulus is the rate of change of stress to strain and is the slope at any given point on a stress-strain diagram. It is the same as the secant modulus in the inelastic region. Materials with high modulus are considered stiffer or more rigid, whereas materials with low modulus are more elastic

461

and require less force to stretch. Stiffer plastics are typically used in products such as pop bottles, where the material needs to maintain its shape and avoid stretching or deforming. More elastic plastics are typically used in products such as food wraps, which require some manipulation to stretch over a bowl. Historically, plasticizers have been added to materials to increase their elasticity at various temperatures and improve processability. Although some plasticizers (i.e., phthalates) have been deemed potentially harmful to human health, there are other, less harmful options available for needed applications. Some materials retain a portion of the stress imparted upon them for a period of time; this is known as memory. This can be evaluated through hysteresis testing, which is cyclic tensile testing below the deformation point with various hold times between pulls. This type of evaluation is critical for elastic films that will experience multiple consumer engagement points throughout the product lifetime. Test Method(s): • ASTM D882—Standard Test Method for Tensile Properties of Thin Plastic Sheeting • ASTM E606 / E606M-12—Standard Test Method for Strain-Controlled Fatigue Testing DENSITY The density of a film is directly dependent on the resin or resin blend used to extrude the film. It cannot be manipulated like other physical properties by processing resin differently. Density is a measure of the compactness of a material and is reported in units of mass per volume. Whereas solid properties are affected by density, changes to melt properties will not be evident because there is no crystallinity in molten plastic. Although density cannot be changed by processing conditions, it is important to identify a starting resin or resin blend to obtain the desired density in the end product. This can dictate some strength properties; for example, higherdensity polyethylene films are typically stiffer, have lower tear resistance, have lower impact strength, and have higher seal initiation temperatures than lower-density polyethylene films. Lower-density polyethylene films also typically have better optical properties. Comparing HDPE with LDPE, the higher density comes from more linear, less branched polymer chains that can pack more tightly into a given space. LDPE, on the other hand, has a highly branched structure, which reduces the number of chains in a given space. It is important to consider the impact of additives such as anti-blocks or colorants (finished pellet density vs. reactor density) when calculating the density of a finished film formulation. Test Method(s): • ASTM D792—Standard Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement

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CHAPTER 6—STRUCTURE DEVELOPMENT, NOMENCLATURE, AND TESTING

COEFFICIENT OF FRICTION (COF)

SURFACE ENERGY

Two types of coefficient of friction (COF) are typically studied on plastic films: static COF and kinetic COF. Static COF is the force required to start one object moving across another, and kinetic COF is the force required to keep one object moving across another. In the case of films, two films are typically tested by moving one across the other. Films with a lower COF are more slippery, and films with a higher COF are rougher. A COF that is too high or too low can cause tracking challenges when processing a film post-extrusion. If the COF is too high, the film web can drag across equipment. If the COF is too low, film such as flexible packaging is more likely to fall off sloped conveyor belts on a packaging line. To modify COF, materials such as slip or anti-block additives can be added during processing, or an adhesive can be added after the film is extruded.

The surface energy of a plastic film is critical for determining its potential for bonding with other materials. In general, it takes energy to create a new surface because the original material must be broken into two parts. Materials with stronger bulk intermolecular forces are more difficult to separate and as a result have higher associated surface energies. For example, polyester films (dipole-dipole forces) have significantly lower surface energies than sodium chloride and other salts (ionic bonds), but do not have as low a surface energy as polyethylene films (dispersion forces). The lower surface energy of some polymers can be problematic when trying to apply functional coatings, whether inks for printing or adhesives for creating seals and/or laminates. For a material to be suitable for such an application, the applied liquid coating must be able to wet the surface sufficiently. A simple term used to capture this concept is spreading, which is defined as:

Test Method(s): • ASTM D1894—Coefficient of Friction Film Testing SURFACE ROUGHNESS Surface roughness is one of the three components of surface texture, along with waviness and lay. This parameter measures the variability in the direction perpendicular to the ideal surface. A high degree of deviation from the ideal surface indicates roughness, whereas minimal to no deviation signals that the surface is smooth. The roughness of a film has an impact on other film properties discussed in this section. For instance, a degree of surface roughness can help promote adhesion of inks or other coatings, while also resulting in a higher coefficient of friction compared to a smoother film. Too much surface roughness, however, can negatively impact other surface-relevant performance attributes, such as sealing or tracking through a film conversion process post-extrusion. With extremely thin films, the roughness of a film surface can have a more pronounced impact on other important metrics, such as dart impact strength. The surface roughness of a film can be measured with a profiler, either through direct contact with the film by a stylus, or indirectly without contacting the film by scattering light or a laser scanning microscope. There are ways to reduce surface roughness. For instance, the presence of melt fracture can be remedied by reducing melt viscosity (i.e., increasing die lip temperatures) or adding a process aid to coat the die. That said, it becomes increasingly expensive and difficult to reduce roughness beyond a certain point with large-scale, high-speed manufacturing processes. Test Method(s): • ISO 4287:1997—Geometric Product Specifications (GPS) Surface Texture: Profile method—Terms, definitions, and surface texture parameters

Spreading = A – B – C where, A = surface energy of film, B = surface energy of liquid, and C = surface energy of the solid-liquid interface. If spreading is negative, the liquid will bead up and not wet appropriately. If spreading is zero to positive, the liquid will be able to wet the surface sufficiently (although a strongly positive number may result in a difficult-to-control coating). This equation shows that higher-surface-energy films are more conducive to printing and applying adhesives. For this reason, polyethylene films often require corona treatment or other discharge technologies to raise the surface energy to acceptable levels. However, it is critical to avoid overtreatment because this can lead to tradeoffs in other areas such as heat-sealing strength. Test Method(s): • ASTM D2578—Standard Test Method for Wetting Tension of Polyethylene and Polypropylene Films OXYGEN TRANSMISSION RATE (OTR) The oxygen transmission rate (OTR) is the amount of oxygen that can permeate through a film at steady state over a given period of time at a specified temperature and humidity. The impact of temperature and humidity on OTR is material-dependent. Although this property of film can be expressed in a variety of units, cm3/100 in2/24h and cm3/100 m2/24h are most commonly used in U.S. standard and metric units respectively. The transport of oxygen, or any molecule, through a film takes place in three discrete steps: (1) the molecule first adsorbs into the film, (2) the molecule then diffuses through the film, and (3) the molecule finally desorbs from the other side of the film surface. The process is driven by a con-

Section 6.1.  Film Properties and Performance

centration (or pressure) gradient, with molecules moving from the side with higher concentration to the side with lower concentration. This process highlights two key components that impact the oxygen transmission rate: thickness and film formulation. For a given film formulation, increasing the film thickness forces the molecule to diffuse through a greater distance, resulting in a lower OTR. In fact, this relationship is proportional, with a 2-mil film having one-half the OTR of a 1-mil film of the same formulation. If significant changes in OTR are required, the film formulation is the right lever to shift because this choice impacts all three steps in the transport process. For instance, ethyl vinyl alcohol (EVOH) has an OTR approximately 2,000 times lower than that of high-density polyethylene (HDPE). Within materials with similar chemical structure, density can also impact OTR to a lesser extent, with higher-density films having lower OTR’s. The OTR of a film is an important characteristic for a variety of different applications, especially for food preservation. For some highly respiring foods such as broccoli and Brussels sprouts, it is critical to have a breathable (high-OTR) film to maintain the right environmental conditions within the storage container. For other applications like preserving meat, it is important to have a barrier film (low-OTR) to prevent oxygen from seeping into the package and causing premature oxidation and spoilage. Odor control products are another area where high-barrier (low-OTR) films are valued for their ability to keep odors completely locked in. Test Method(s): • ASTM D3985—Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using a Coulometric Sensor WATER VAPOR TRANSMISSION RATE (WVTR) The water vapor transmission rate (WVTR) is the amount

463

of water vapor that can permeate through a film at steady state over a given period of time at a specified temperature and humidity. The impact of temperature and humidity on WVTR is material-dependent. Although this property of film can be expressed in a variety of units, g/100 in2/24h and g/m2/24h are most commonly used in U.S. standard and metric units respectively. As with oxygen, the transport of water vapor through a film requires the same three steps: (1) adsorption, (2) diffusion, and (3) desorption, and is similarly driven by a concentration gradient. Thickness is also linearly related to WVTR, making normalized WVTR (WVTR of a 1-mil-thick film) an important term when comparing the inherent moisture barrier properties of films from different material classes. As with OTR, material choice is a significant lever in shifting WVTR; however, the same trends do not necessarily hold. Although HDPE has a significantly higher OTR than EVOH, its WVTR is approximately 10× lower than that of EVOH. WVTR is a critical property for films that are used in food preservation or freshness applications, whether to keep dry goods dry (i.e., chips, cookies, etc.) or moist goods moist (cheese, yogurt, etc.). Without sufficient WVTR, these products would equilibrate with the outside environment, leaving soggy pasta or dried-out granola bars. In these applications where precise WVTR values are critical, it is important to look beyond just material choice and thickness. With PE, for instance, aspects such as chain length, molecular weight, density, extrusion conditions, coextrusion structure, and combinations thereof can have more significant impacts on WVTR than anticipated at first glance. For example, water-vapor barrier properties are increased when extruding PE with a high blowup ratio (BUR), high frost line, and narrow die gap. Test Method(s): • ASTM F1249—Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor

Chapter 6—Section 2

Film Quality and Test Methods DAVID BOSTIAN, CharterNEX Films

INTRODUCTION The quality of films produced on blown- or cast-film lines continues to improve due to advances in production equipment and resin technology. Customer expectations are continuously becoming more demanding, making high-quality products ever more necessary and valuable. The properties that must be tested in manufacturing both cast and blown films have changed little in the last twenty years. What has changed in many cases is the frequency of these tests and the methods used. Numerous improvements and automation have made quality control easier for film manufacturers. Most, if not all, of these improvements can be attributed to Deming’s philosophy of quality control. One of the main tenets is that quality cannot be “inspected in”. Machinery manufacturers have done a good job in automating some critical film attributes, such as gauge, gauge variation, and gel count, to name a few. For those tests that must still be done manually, much improvement remains to be made. PRE-FABRICATION TESTING An extruder’s decision about whether to test incoming raw materials is very much an individual choice. With the advent and acceptance of documented quality systems, if an extruder is not going to conduct tests on incoming resin themselves, then they must rely on a Certificate of Analysis (COA) provided by the resin supplier. At a minimum, most film producers would need to verify or conduct incoming testing on: (1) Resin density (specific gravity), ASTM D-792 (2) Melt index (MI) or melt flow rate (MFR), ASTM D1238 (3) MWD (molecular weight distribution), ASTM D-3593 (4) Film gel rating, Various methods. Resin density can be critical in many applications because density, especially in polyethylene-based materials, has a strong influence on many film attributes, among them stiffness, clarity, and toughness. Melt index is a simple measure-

ment of a resin’s viscosity. Molecular weight distribution (MWD) is a measure of the range of molecule sizes within one polymer as measured by gel permeation chromatography (GPC) or other methods. A simple measurement of the distribution is known as melt flow ratio I10/I (or sometimes I20/I2). FILM PRODUCTION Many film problems at the converter or end user can be traced to gauge control. Issues with COF, tracking, strength, toughness, and sealing can all be caused by gauge variation across the film. Auto gauge control systems that were first developed for cast film have become the norm on both blown- and cast-film lines. Blown-film gauge control is intrinsically more difficult because of the slower cooling rate and the complications that come with the annular die body and oscillating haul-offs. Blown-film systems generally function either through viscosity control (die systems) or frostline control (air-ring systems). Cast and sheet lines maintain gauge control by adjusting die lip gap. In the past, most blown-film extruders were happy to have gauge control variation of ± 10% in the cross direction, but today quality films have less than half that variation. Those running old or new lines built without auto-gauging equipment struggle to handle large gauge variations. Even with a gauge control system, gauge issues can still occur. Problems with gauge control can manifest themselves in many ways, such as baggy edges, film camber, film wrinkling, poor film tracking, blocking, and telescoping. It is probably safe to say that before auto gauge control systems entered the mainstream, many extruders would list gauge control problems as their top manufacturing defect. Even with good gauge control, gauge-related issues may still pop up. Perhaps the most insidious part of this problem is that it is usually not discovered until the customer tries to use the film. Many in-plant schemes have been devised to combat this phenomenon by putting an objective test system in place. Taking circumference measurements across the face of a roll can give an indication of gauge bands. Many plants have 465

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used a circumference difference of 3/16″ as the criterion for a good or bad roll. Circumferential variations greater than this can indicate excessive profile variation or poor rotational randomization. Even with good gauge control, issues can still occur if randomization speeds are too slow. Roll hardness, like circumference readings, provides hope of detecting gauge variation problems. It can be measured by thumping a roll across its face and measuring the “hardness” profile. Early on, instruments such as the concrete industries’ Schmidt hammer were used because other instrumentation did not exist at that time. The Schmidt hammer has a plethora of testing and consistency problems, but if used correctly by the same person, it can give some indication of roll consistency. Newer testing equipment such as the Parotester can give more distinct, accurate, and reproducible data. A common, no-nonsense plant method to test a roll was to find a large area with a straight line accessible. By rolling the film out, in free form, a filmmaker could see the film “track” next to the guide. If the film turned off, or skewed, in one direction or another, they would have reason to believe that gauge variation was significant enough to cause the film to fail to run straight through downstream equipment. This, of course, is a destructive test, but many plants are willing to sacrifice a roll, or a portion of a roll, for the peace of mind of knowing that the film is flat. Finally, some extruders have designed their own equipment to attempt to measure film bagginess. Of greater or lesser complexity, this equipment inevitably included an unwind and a rewind stand and some number of idlers between them, replicating some downstream converting operation. Once the film was strung and running through the idlers, the manufacturer could attempt to measure the displacement from trueness on either side of the film web. This displacement effectively quantifies an otherwise nebulous problem like baggy film. One problem with this method, however, is that bagginess can be fleeting and can move throughout and across a roll. Nonetheless, it is a worthwhile exercise to quantify what “baggy” is and determine its severity. All these methods have their positive and negative properties. Some are difficult, others are easy, some are subjective, and others are hard to replicate. Their success at predicting how well film will run is mixed. All have the fatal flaw that they do nothing to correct the root problem, which is gauge control at the die. Thankfully, with today’s modern extrusion lines, these issues occur less often. Finished film testing can be broken up into the following major areas, each of which will be explored individually. (1) Visual properties (2) Physical properties (3) Surface properties (4) Barrier properties. VISUAL PROPERTIES When a consumer sees a film product, either in a lamination or on its own, visual properties can be an important

component of how that person perceives the quality of the film or the product inside. With today’s emphasis on highend graphics to sell a product, the color or hiding power of the film can also be extremely important in how the printing looks. Haze, clarity, gloss, color, and opacity are all routinely measured on films. Haze is a measure of wide-angle light scattering and results in a film being milky or, in more extreme cases, translucent. HDPE films are a good example of films that have high haze. A large component of haze may be surface haze that “disappears” when the film is laminated or when a product is in contact with the film. This is often referred to as contact clarity. ASTM D-1003 (ISO 13468): Haze is measured on a scale of 0–100, with 0 being perfect, i.e., light shines through the film with no wide-angle scattering, resulting in a film that is “glass” clear. Most clear-cast films have haze values of 1–6, whereas most clear-blown films have haze values of 5–15. These values are very dependent on resin selection, process parameters, and film thickness. Sometimes it is desirable to identify the surface haze component of the total haze. In this case, the surface(s) of the film may be wetted with oil and the haze tested. The surface wetting reduces the surface haze to near zero, giving a result that represents the internal haze of the material and may give a better idea of the material’s contact clarity. Clarity is a measure of narrow-angle light scattering. High clarity results in a “clear” film that has no distortions in it. In the industry, people often refer to films that have poor clarity as being hazy, but these are really two different problems and often have different causes or solutions. A film can have poor clarity, but low haze, i.e., objects behind it appear distorted or the film looks wavy, but the film is not milky. A film can also have low haze, but excellent clarity, i.e., the film is milky, but objects viewed through it have good definition and detail. ASTM D-1746: Clarity is measured on a scale of 0 to 100, with 100 being perfect, with no distortions. ASTM D-2457: Gloss is a measure of a film’s “shine” or sparkle. Films with a very smooth surface reflect light very well, giving them a bright, sparkly look. This sheen is often desirable in packaging, especially for consumer goods, where sheen has a quality connotation. Duller or matte-surface films can also be desirable, often when a package is replacing or replicating a paper structure. Gloss is measured primarily at three angles, depending on the relative gloss of the substrate. These angles correspond to the incident angle at which the light is introduced to the surface. It is very common for blown-film manufacturers to use 45 degrees as the standard; for glossier and duller substrates, 60 degrees and 20 degrees respectively may be used. Haze, clarity, and gloss can all be adversely impacted by poor processing or material choices. Haze and gloss issues are often inherent in the material type used, but can be affected by processing, especially cooling rate. Quicker quenching, as on a cast-film line for example, results in lower crystallinity and smaller crystals, making for a clearer, glossier film.

Section 6.2.  Film Quality and Test Methods

Poor clarity is usually the result of processing or rheological effects. Most extruders are familiar with a defect known as melt fracture. This phenomenon is particularly common in LLDPEs. Melt fracture gives a “sharkskin”-like appearance, which roughens a film’s surface and hurts its clarity and gloss. Objective measurements of melt fracture typically use a surface profiler and can be characterized by roughness (variation in the film surface). Other visual defects in this family include orange peel and mottled or matted surface. In coextrusion, interlayer rheology issues can also result in a melt-fracture appearance, which is sometimes referred to as internal melt fracture. Quality control (QC) techniques used to quantify pigmented films range from the very simple to the more complex task of color matching. White and black films are commonly monitored for how much light passes through the film (opacity). This can be done with equipment that measures optical density, light transmission, or opacity. High opacity can be important to hide package contents or to provide a solid background for printing. Most tests that measure how much light passes through a film correlate well with one another, but the supplier and the customer should agree on which equipment will be used. Once standards have been set, any of these tests can be used to confirm the consistency and level of colorants being dosed by an extruder’s additive hoppers. White films can also be measured for their whiteness; these values typically range between 50 and 100, with 100 being a pure, bright paper white. Lower values indicate a less pure, grayer white. These measurements are often necessary for consumer products, and films that are destined for laminations must generally meet this requirement. Colored films can be compared to standards visually or analytically using a spectrophotometer and laboratory measurements. Perhaps the most notorious of all visual defects is the gel. A description of gels and their troubleshooting can be found in Section 5.10, but for quality control purposes, gels (and related carbons) must be quantified by size and number. The critical nature of gels, like most other problems, depends very much on the end-use application for the film product. Some applications, such as photographic negatives and electronics, require gel-free film. Others, such as can liners, can have a high tolerance for gels. Where on this spectrum the end-use falls can dictate the method used for counting and sizing gels. Newer optical and image-based technologies can make this examination thorough, reliable, and continuous. Inspecting for gels manually is difficult because it relies heavily on random sampling and because obtaining a significant sample from a large roll of film is almost impossible due to the size of the population, i.e., the large square footage of the film. Some of the worst get problems come in short bursts, making them especially difficult to detect without continuous automated monitoring. Once discovered, gels can be quantified by size using a template, such as the one used in the TAPPI test method. The makeup of the gel can be explored with a number of analytical techniques, including hot-stage melting-point determination, FTIR, and DSC.

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PHYSICAL PROPERTIES As is usually the case when dealing with anisotropic materials such as polyethylene, film properties can vary significantly with process and equipment changes. Variables such as blowup ratio, die gap, drawdown ratio, frostline height, output rate, and many others can have a major impact on film physical properties. Because of this, in-plant testing is critical for some applications. Even with good process control and process stability, many attributes must be verified upon production of each lot. Technical data sheets common to the film industry almost always list tensile properties to characterize one film versus another. Tensile properties are measured using a universal tester such as an Instron. The most common tensile properties are elongation at break, tensile stress at yield, tensile stress at break, and secant modulus. The industry has long reported tensile stress at break as a measure of film strength because it is an easy number to obtain, but a more important number for a packaging film is likely to be tensile stress at yield. Secant modulus can be measured at varying strains or elongations, but is typically measured at 1% or 2% strain. Secant modulus is a good indicator of film stiffness for monolayer films, and tensile modulus and flexural modulus correlate fairly well for homogeneous materials. However, secant modulus cannot be used as a proxy for the stiffness of coextruded films because the bending modulus is highly dependent on where in the film the stiff or soft layers are located. A stiff layer on the skin results in a much stiffer film than a stiff material in a core layer. This is often thought of as the “I-beam” effect. All these tensile properties are anisotropic and can vary substantially in one direction versus the other, i.e., machine (MD) vs. transverse (TD) or cross direction (CD). How much a particular property varies is highly dependent on a film’s composition and fabrication conditions. ASTM D-882: The tensile strength of a film is a measure of the stress required to elongate a film. A film specimen of specified dimensions is placed in the grips of a universal tester that is capable of constant crosshead speed and initial grip separation. The crosshead speed is determined based on the polymer being tested. Typically, for LDPE and LLDPE films, the crosshead speed is 20 in/min (500 mm/min) with a grip separation of 2 in (50 mm). HDPE films are typically run at 2 in/min (50 mm/min). The force as a function of time is measured using a load cell. The elongation is determined from the crosshead speed as a function of time. At least five samples are averaged to determine the tensile value for a film. The sample is tested to the point of rupture. The force measured on the specimen to yield and rupture and the length to which it was stretched at these two points are recorded by computer (or by strip chart on older equipment). Using the width and thickness of the film sample, the stress and strain are calculated (see Figure 6.2.1). Typically, film tensile properties are measured in both the MD and CD. Tensile properties include yield tensile, ultimate tensile, elongation, and modulus and are used to determine the relative strength and flexibility of films.

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FIGURE 6.2.1.  Stress-Strain Curve Obtained from a Tensile Test.

The stress-strain curve contains much information about the film. The initial slope of the curve is the region where the film recovers from any deformation when the stress is removed and represents the first part of the elastic region. In this region, only the amorphous regions of the film morphology exhibit any deformation, but they can recover to their original dimension when the stress is removed. The crystalline regions of the film experience no deformation in this region. At the end of the elastic region, some strain softening begins to occur. Here, the amorphous regions have become extensive enough to prevent full recovery when the stress is removed, producing a change in dimension. The crystalline regions have not yet experienced any deformation. The maximum stress measured in this region is referred to as the tensile yield strength. After the maximum stress is reached, more amorphous deformation may occur, as shown at the start of the plastic region. At this point, the film begins to neck-in or narrow as it is stretched further. The necking region of the stress-strain curve is shown as the relatively flat portion of the plastic region. Here, the crystalline regions are beginning to experience deformation by slipping or sliding between folded chain structures. A long necking region usually indicates that there is significant crystalline orientation in the direction of the test. It also implies that the film could be a good candidate for secondary orientation processes. The final part of the stress-strain curve is the strain hardening region. Deformation causes the crystalline regions that are connected to tie molecules to begin to unfold rapidly, increasing the stress levels until the film ruptures at the ultimate tensile strength.

Yield strength measures the highest stress where a film, when deformed, will resume its original dimensions when the force is removed. The yield stress is expressed as force per original area (engineering stress). The tensile yield of a film is strongly correlated with polymer density but is also a function of the crystallization rates that the film encountered during fabrication. Faster cooling lowers the yield stress. Ultimate tensile is a measurement of the force per unit of original area where the film ruptured. The ultimate tensile strength is used to determine the relative strength of the film. Film thickness is included in the calculation of ultimate tensile strength; however, it is strongly influenced by orientation, and therefore the values can vary significantly even at the same film thickness. Increasing the degree of orientation in a film increases its ultimate tensile strength. Higher molecular weight and narrow MWD produce higher tensile strength. Ultimate elongation is a measurement of deformation per original length where the film ruptured, as shown in Figure 6.21. Elongation is strongly influenced by orientation, and therefore the values can vary significantly even at the same film thickness. Increasing the degree of orientation in a film will increase its ultimate tensile strength. The elastic region also relates to the stiffness and flexibility of the film and is referred to as the secant modulus. The secant modulus is determined by the ratio of force to strain at either 1% or 2% elongation and is reported as psi (MPa). The tensile toughness (tensile energy to break) is defined as the area under the stress-strain curve. The results are reported as ft-lbf /in (J/cm3). Many films in use today, particularly blown films, are in use because of their toughness. Toughness is a property that can have many meanings. Sometimes, when an application requires tough film, the application demands film with high tensile properties. In other cases, it may call for film that stretches and deforms, but does not puncture. For purposes of this discussion, toughness refers to any one of, or any combination of, the following properties: abrasion resistance, flex cracking, impact strength, drop strength, tear resistance, and fast and slow puncture strength. Several of these tests are outlined below. ASTM D-1242: Although not a routine manufacturing QC test, abrasion resistance can be vital to meeting the needs of a packaging or industrial application. Abrasion resistance is a measure of a substrate’s ability to withstand rubbing, scraping, and any other frictional contact with surfaces. It is commonly measured as Taber resistance, but can also be somewhat quantified using methods such as Rockwell hardness. Many other non-standard tests have been developed to measure scratch resistance in various applications. Flex Crack: In the same family of tests, certain films are subjected to flex crack resistance testing. In this test, a film or a multilayer laminate is twisted and re-twisted to the point of failure; an example of this test is the Gelbo-Flex test. This test is meant to mimic the abuse that a package sees in shipping and handling.

Section 6.2.  Film Quality and Test Methods

Impact strength and puncture resistance is measured in many ways, using a plethora of impact probes. The method and probe type may be selected depending on the goods to be packaged. For example, packaging pasta may require a completely different test method from packaging coffee beans. Often, for difficult or unusual items, an end user may develop a method related to the end-use application needs. ASTM D-1709 (ISO 7765): The most common impact strength test method in the film industry is dart drop. In the ASTM method, darts are dropped from one of two heights (Method A or B) and the weight varied until an average weight for 50% failure value is obtained. This is a highspeed impact test. Spencer impact is a similar impact test, although the impact speed is somewhat lower. It is quicker and perhaps more reliable than dart drop. Dart/Spencer impact value is dependent on film thickness, and the test is sensitive to orientation effects in the film. The tackiness of the film surface (slip additives) and the condition of the dart head have also been shown to influence the values determined by this test. Because dart impact is not tested in any particular film direction, it is sensitive to orientation effects in both directions. Best results are usually obtained when the orientation is balanced in the MD and CD. If a film is asymmetrical, care should be taken to ensure that the same side is always facing the impact because this can affect the results. Two methods are used in dart-drop impact testing: Method A drops the dart from a height of 26 inches (660 mm) above the film sample. The values typically range between 40 and 1400 grams. This method works well for thin films (< 3 mils or < 75 microns). Method B drops the dart from a height of 60 inches (1524 mm). This method is used for thick films or films with very low crystallinity. The values from either of the two methods (A or B) cannot be converted to the other method. ASTM D-4272: An alternative to dart impact is the TEDD, or total-energy dart drop, in which the energy before and after impact is known, and therefore the energy consumed at impact can be computed. Because the dart-drop test is somewhat time-consuming, many manufacturing plants have designed their own quick impact test. The concept is very much the same, except that an in-plant test may use a ball, or something like it, as the impact device and vary the height of the drop. This is especially common for films that are gusseted or folded and can be highly useful for testing seam strength. Drop tests: Often a drop test is performed using the actual filled package to see whether it can withstand actual dropping. Many test variable are important: product size, orientation of the package, temperature, drop height, number of drops, etc. Many people have developed their own tests to suit their product’s needs. Puncture tests: Impact tests are often done with a largediameter head (>0.25″), which may not indicate how a film will resist puncture by a sharper probe. To try to understand this property better, a fixture can be attached to a tensile tes-

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ter, and probes of various sizes and sharpness can be slowly pushed through the film. The strength needed to puncture the film may give an indication of how it will perform in its real-world application. As with impact tests, the film’s COF and the probe’s surface can significantly impact the results. Probe speed is also an influencing variable. Tear strength is another common test, especially for blown-film extruders. Tear strength can be measured either as uninitiated or as propagated tear. Uninitiated tear-strength methods attempt to quantify the force necessary to begin ripping a substrate. Many of these tests are performed on tensile testers using a film cut like a V-shaped template. An oriented material such as oriented polypropylene (OPP) can have very high uninitiated tear strength, but very low notched or initiated tear. ASTM D-1922—Elmendorf tear strength is the force required to propagate a tear over a specified distance in a particular direction (1.7 inch, 43 mm) with an initial slit across a semicircular specimen. The test is typically reported for both MD and CD. This test, although originally developed for the paper industry, is used extensively in the plastic film industry. Elmendorf tear is dependent upon film orientation, with higher tear values obtained when orientation is increased in the opposite direction. Elmendorf tear is reported in grams and may change with film fabrication conditions at the same film thickness. The force required to tear the specimen fully is measured either manually or electronically. A minimum of ten (10) samples should be used to determine the average tear in each direction. Sealing characteristics: Many packaging applications require a film to be sealed to itself or to another substrate to form a package, and other applications may also require a film to seal or bond. How well a film does this is an important film property and depends not only on the actual material to be sealed, but also on the other layers of the film, any additives, and how the film was processed. Seal strength and hot tack: How strong a seal a film makes can be measured by forming a seal with two strips of film and then trying to pull them apart on a tensile tester. Key sealing variable are time and temperature. Pressure is also a factor, but above a certain threshold, it has a minimal effect on the results in most cases. To measure seal strength, a film is sealed and allowed to cool to room temperature before the seal strength is tested. To measure hot tack, the film is sealed on a special piece of equipment, and then immediately, or after a very short time ( ”—indicates the direction in which some type of surface treatment has been applied, such as flame, corona, ozone, or plasma. • For Example—Paper < flame / LDPE / foil / LDPE MATERIALS (GENERIC REPRESENTATIONS) Webs/Films/Substrates: OPET—biaxially oriented polyester film. (Occasionally called BOPET.) • Writing “OPET” for a film will prevent confusion with extruded PET. vm-OPET—vacuum-metallized OPET film. Metallization on the left. • Also known as “met-PET” or “met-OPET”. OPET-vm—vacuum-metallized OPET film. Metallization on the right. • Also known as “PET-met” or “OPET-met”. OPET-SiOx—OPET film with silicon oxide on the right side of the film. SiOx-OPET—OPET film with silicon oxide on the left side of the film. OPET-PVdC—oriented polyester film with a PVdC coating on the right. PVdC-OPET—oriented polyester film with a PVdC coating on the left. PVdC-OPET-PVdC—oriented polyester film, two-sidecoated with PVdC. OPP—biaxially oriented polypropylene film, also known as BOPP. BOPP—biaxially oriented polypropylene film, also known as OPP. vm-OPP—oriented polypropylene film with metallization on the left side. OPP-vm—oriented polypropylene film with metallization on the right side. OPP-A—oriented polypropylene film with an acrylic coating on the right side. A-OPP—oriented polypropylene film with an acrylic coating on the left side. CPP—cast polypropylene film. vm-CPP—cast polypropylene film with metallization on the left side. CPP-vm—cast polypropylene film with metallization on the right side. C-Nylon—cast nylon film, also written as Cast PA for cast polyamide film. O-Nylon—uniaxially oriented nylon film. BONy—biaxially oriented nylon film. BOPA—biaxially oriented polyamide (nylon) film. K-Nylon—biaxially oriented nylon film, with PVdC on one side. BONy-PVdC—“K-Nylon”, but written to specify PVdC on the right side.

Section 6.6.  Structure Writing and Nomenclature

Alu—aluminum (aluminium); can also be written as “AL of alu”. Foil—typically refers to a thin aluminum sheet. The term can be used for other metals, so if it is not aluminum, please specify. Examples: tin foil, gold foil, ... • In Europe sometimes the word “foil” is used to indicate any type of thin film, which can cause communication issues globally.) Cello—plain, uncoated cellophane. K-Cello—two-side PVdC-coated cellophane. NC-Cello—two-side nitrocellulose-coated cellophane. Paper—use for lightweight paper-based webs. • There are various types of paper, so please specify when possible: Kraft, Bleached Kraft, SBS, clay-coated, etc. Board—use for heavyweight paper-based webs. Various types exist. Please specify when possible. Nonwoven—generic term for nonwoven webs if the type or brand is not known. • Nonwoven HDPE, Nonwoven PP, Nonwoven PET, —please specify • Use brand name if known; such as DuPont™ Tyvek®, Berry Global Reemay®, Berry Global Typar®. Woven Fabric—use for generic fabric weaves with a “tight weave”. • Woven PP, woven HDPE, woven PP. Scrim—use for woven mesh type fabrics with an “open weave”. • Scrim PP, Scrim PET, Scrim HDPE. Thickness or Gauge Representations: #—usually means, “pounds per ream” (a ream typically being 3000 square feet). mils—used for the English measurement of 0.001 or 1/1000 of an inch. inches—English unit for thick layers, such as in bottles and sheet applications. u or μ—abbreviation for micron in the metric system of measurement (0.001 or 1/1000 of a millimeter). • Note: although use of the ‘mu’ character (μ) is more proper, it is commonly acceptable to use the letter ‘u’ for simplicity when typing on a standard keyboard. mm—millimeters, a metric unit used for thick layers, such as in bottle and sheet applications. gsm—grams per square meter. pt.—usually means “point”, which is a unit of thickness for paperboard. 1 point = 1 mil or 0.001”. ga—used to indicate the “gauge” or thickness of a film in some systems. • 100ga = 1 mil = 25.4μ • E.g.—48 ga = 12μ = 0.5 mils. Adhesives and Primers: PUR—polyurethane; PUR adhesive or PUR primer; can be solvent- or water-based.

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PEI—polyethylene-imine; PEI primer, usually a waterbased primer. EAA—ethylene acrylic acid; EAA primer (water-based). EAC—ethylene acrylate; EAC primer (water-based). • There are others in use, though less common to our industry, for example organic titanates) Surface Treatments: Corona—electron discharge excitement of atmospheric air over a material surface. Plasma—electron discharge excitement of a gas other than standard air over the material surface. Examples include nitrogen, argon, helium, neon, and hydrogen as well as various gas mixtures. Flame—surface treatment with burning gases. The gas mixture could be either an oxidizing flame, a reducing flame, or a stoichiometric flame. Ozone—molten web oxidation by exposing the polymer to a dry ozone flow created offline and pumped to the molten web area through a tube with small outlet holes along its length. When used, it is typically seen on an extrusion-coating line rather than in other processes. Extrudable Resins (generic names for when brand and/ or grade are unknown): HDPE—high-density polyethylene. HMW-HDPE—high-molecular-weight HDPE. MDPE—medium-density polyethylene. LDPE—low-density polyethylene. LLDPE—linear low-density polyethylene. VLDPE—very low-density (linear) polyethylene (also known as VLLDPE). ULDPE—ultra low-density polyethylene (also known as ULLDPE). mPE—metallocene polyethylene, generic. mLMDPE—metallocene linear medium-density polyethylene. mLLDPE—metallocene linear low-density polyethylene. mVLDPE—metallocene very low (linear)-density polyethylene (sometimes referred to as a ‘plastomer’). PP—polypropylene (generic indication when minimal information is known). CoPP—copolymer polypropylene. HoPP—homopolymer polypropylene. ACR—acid copolymer resin, which is a generic name for EAA and EMAA resins. EAA—ethylene acrylic acid copolymer, such as SKC Primacor®, Dow NUCREL™, and ExxonMobil Escor®. EMAA—ethylene methacrylic acid copolymer, such as Dow NUCREL™. Ionomer—generic name for ionomeric copolymer resins such as Dow SURLYN™. Acrylate—generic name for various acrylate copolymers such as Westlake EMAC®, ExxonMobil Optema®,

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Dow ELVALOY™ AC, and Arkema Lotryl®. EBA—ethylene butyl acrylate. EEA—ethylene ethyl acrylate. EMA—ethylene methyl acrylate. EMMA—ethylene methyl methacrylate. EiBA—ethylene iso-butyl acrylate. EnBA—ethylene normal-butyl acrylate. EVA—ethylene vinyl acetate, such as Celanese Ateva®, Dow ELVAX™ LyondellBasell Ultrathene®, ExxonMobil Escorene® Ultra. • If % VA is known, please indicate; for example, 9% EVA, 15% EVA, or 28% EVA. PS—polystyrene. EPS—expanded (or foamed) polystyrene. HIPS—high-impact polystyrene. GPPS—general-purpose polystyrene. PVC—polyvinyl chloride. PVdC—polyvinylidene chloride; polyvinyl di-chloride. Most commonly seen as a coating on a film, but there are also extrudable grades, such as SKC Saran® and Solvay Ixan®. PA—polyamide, commonly called Nylon. Nylon—polyamide; when unspecified, quite often is Nylon 6. • Note: other polyamide polymers exist, such as; Nylon 11, Nylon 12, and Nylon 6/ 12. Nylon 6—polymer from caprolactam (also written “PA 6”). Nylon 6,6—polymer from adipic acid plus hexamethylenediamine. • Also written as “PA 6,6” or “PA 6:6” or “PA 66”. Nylon 6/6,6—copolymer of 6 and 6,6 types of polyamide. • Also written as “PA 6/6,6” or “PA 6:66”. Amorphous PA—amorphous nylon, such as DuPont™ Selar® PA or EMS Grivory®. MXD6—crystalline nylon of the MXD6 type, such as Mitsubishi Nylon-MXD6. EVOH—ethylene vinyl alcohol, such as Kuraray EVAL™, Nippon-Gohsei Soarnol™. • If the mole % ethylene is known, it should be indicated, for example: 32% EVOH. PAN—polyacrylontrile, sometimes written as “ACN”. SAN—styrene acrylonitrile copolymer. AN-MA—acrylonitrile methyl acrylate copolymer, such as the now-discontinued Ineos Barex®. ABS—acrylonitrile butadiene styrene copolymer. LCP—liquid crystal polymer. COC—cyclic olefin copolymer, such as TAP Topas® or Mitsui Apel®. PUR—polyurethane, extrudable-type polymer. SBC—styrene butadiene copolymer, such as ChevronPhillips K-Resin®. TPS—thermoplastic starch polymer, such as Kuraray Plantic® TPS resin or Novamont MaterBi®. PET—extruded polyester. Could be monolayer or in a coextrusion. PET ext.ctg.—extrusion coating of polyester. APET—amorphous polyester.

CPET—crystalline polyester. PETG—polyester copolymer with glycol. PEN—polyethylene naphthalate. PTT—polytrimethylene terephthalate, such as DuPont™ Sorona®. PLA—polylactic acid. PGA—polyglycolic acid. PHA—polyhydroxyalkanote. PHB—polyhydroxybutyrate. Terpolymers: EVACO—terpolymer of ethylene, vinyl acetate, and carbon monoxide. EnBACO—terpolymer of ethylene, normal-butyl acrylate, and carbon monoxide. EnBAgMA—terpolymer of ethylene, normal-butyl acrylate, and glacial methacrylate. EiBAMAA—terpolymer of ethylene, isobutyl acrylate, and methacrylic acid. EBAMAh—terpolymer of ethylene, butyl acrylate, and maleic anhydride. EEAMAh—terpolymer of ethylene, ethyl acrylate, and maleic anhydride. EMAMAh—terpolymer of ethylene, methyl acrylate, and maleic anhydride. • Note: there are various other terpolymers in the industry for specialized applications. Grafted Resins: EVA-gMAh—ethylene vinyl acetate with a graft of maleic anhydride. LLDPE-gMAh—LLDPE with a graft of maleic anhydride. HDPE-gMAh—HDPE with a graft of maleic anhydride. PP-gMAh—polypropylene with a graft of maleic anhydride. • Note: there are various other grafted resins in the industry for specialized applications. Others: Peel Seal or Easy Peel—generic name for a peelable sealant resin of unknown brand / grade. • If the brand is known, please indicate, such as Dow APPEEL™, Mitsui-Dow CMPS™, Yasuhara Hirodyne™. H.S. lacquer—generic name for a ‘heat-seal lacquer’, usually applied by gravure. Hot Melt—generic name for a hot-melt adhesive applied as a sealant. STRUCTURE EXAMPLES Structures should be written from “outside to inside”, with regards to the order of layers, starting from the left. OPET / ink / adhesive // CPP Û An adhesive lamination of cast polypropylene film (CPP) to reverse-printed polyester film (OPET). An

Section 6.6.  Structure Writing and Nomenclature

adhesive of unknown type is applied to the printed surface of the polyester film. OPET / primer / CoPP Û An extrusion coating of copolymer polypropylene onto an unprinted polyester film. A primer of unknown type is applied to the OPET before extrusion coating. 48ga OPET / ink / PEI primer / 28# LDPE Û A reverse-printed 48-gauge OPET film that is primed with a PEI primer and then extrusion-coated with 28 pounds per ream of low-density polyethylene. 15μ BONy / ink / PUR adh // 88μ LDPE Û An adhesive lamination of an LDPE film to a reverseprinted biax nylon film. The PUR adhesive is applied to the reverse-printed nylon film. 0.5 mils OPET-PVdC / ink / primer / 0.8 mils LDPE / 1.5 mils Peel Seal Û A 0.5-mil OPET film with PVdC coating is reverseprinted on the PVdC side. The printed side is then primed and extrusion-coated with 0.8 mils LDPE. A 1.5mil extrusion coating of a peelable sealant resin is applied onto the LDPE. This is done either through tandem or two-pass extrusion coating, which is not specified. 12μ OPET / primer / 15μ LDPE / 6μ alu / ( 11μ EMAA – 25μ Peel Seal ) Û A 12-micron OPET film is primed and extrusionlaminated with 15 microns of LDPE to a 6-micron aluminum foil. The bare aluminum side is then coextrusion-coated with a two-layer coating consisting of 11 microns of EMAA and 25 microns of a peelable sealant resin. OPET / primer / LDPE / sealant // TL // leather Û An oriented polyester film that is primed and extrusion-coated with LDPE and then a sealant resin. This structure is then thermally laminated to leather. (The application may be for an emblem or other fabric decoration that requires surface protection.) Ink / 30μ alu < flame / ( EMAA – Peel Seal ) Û A surface-printed aluminum foil that is inline flametreated and then coextrusion-coated with an EMAA copolymer and a peelable sealant resin (a cup- or traytype lidding application). OPET / primer / ( LDPE - [LDPE + white MB] - LDPE ) / foil / EMAA Û An OPET film is primed, and then foil is extrusionlaminated to it using a three-layer coextrusion, with the center layer containing a white masterbatch blended into LDPE. The foil is then extrusion-coated on the other side with EMAA. 20μ OPP / ink / adhesive // 7μ alu / 33 gsm EAA Û A 20-micron OPP film is reverse-printed, then adhesively laminated to 7-micron aluminum foil. The adhesive is applied to the printed surface, not the aluminum. The laminate is then extrusion-coated with 33 grams per square meter of EAA. 20μ OPP-PVdC / low-temp PUR primer / 35μ 18% EVA PVdC-coated OPP is extrusion-coated with an 18% EVA. Û A special low-activation-temperature PUR primer is

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used to enable direct extrusion-coating of an EVA at 235°C onto a primed film. (235°C is the upper limit for EVA extrusion temperature.) BONy / ink / solventless adhesive // ( LLDPE – LDPE – ionomer ) Û A solventless adhesive lamination of a reverse-printed biaxially oriented nylon film to a three-layer coextruded film. The adhesive is applied to the printed surface of the nylon film. OPP < corona / solvent adhesive // ( Nylon 6 – tie – LDPE – ionomer ) Û An OPP film that is corona-treated inline and then has a solvent-based adhesive applied to its surface. This film is then combined with a four-layer coex film, usually with a heated pressure nip. O.L. / ink / 40# Paper < flame / 0.7 mils LDPE / 0.35 mils alu / 1.8 mils ionomer Û A surface-printed and overlacquered 40-pound paper is flame-treated and extrusion-laminated using LDPE to aluminum foil. The other side of the alu is then extrusion-coated with the ionomer. It must be specified whether this is done in a two-pass operation on a single-station extrusion-coating line or in one pass on a tandem extrusion-coating line. O.L. / ink / Paper / EMAA / alu / [ionomer + Slip MB] Û A surface-printed and overlacquered paper is extrusion-laminated to alu foil with EMAA. The other side of the alu is then extrusion-coated with a blend of ionomer and masterbatch. OPET / ink /adh // SiOx-OPET / primer / ( LDPE – ionomer ) Û A reverse-printed OPET is adhesively laminated to a silicon oxide-coated OPET. The adhesive is applied to the printed surface of the outer OPET. Then the plain side of the silicon oxide-coated OPET is primed and coextrusion-coated with a two-layer coating of LDPE and ionomer. OPET / ink / primer / met / adhesive // ( Nylon - tie - LLDPE ) Û A reverse-printed OPET film is primed on the ink surface, then metallized. This film is then adhesively laminated to a three-layer coex film. The adhesive is then applied to the metallized surface. OPET / ink / adhesive // OPET-VM / ( LDPE – ionomer ) Û A reverse-printed OPET is adhesively laminated to a metallized OPET. The adhesive is applied to the ink surface. The metallized layer is facing the inside of the package structure, not the outside. The metallized surface is coextrusion-coated with a two-layer coating of LDPE and ionomer. OPP / ink / primer / LDPE / { primer / VM-CPP } Û A reverse-printed OPP that is primed and extrusionlaminated using LDPE to a metallized cast polypropylene film that also has a primer applied to the metallized side. OPET / ink / primer / LDPE / VM-OPET // adhesive / CPP Û A reverse-printed OPET that is primed and then extrusion-laminated using LDPE to a metallized OPET. This is then adhesively laminated to a cast polypropyl-

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ene film. The adhesive is applied to the CPP film. OPET / ink / primer / LDPE / VM-OPET / adhesive // CPP Û A reverse-printed OPET that is primed and extrusionlaminated using LDPE to a metallized OPET. This is then adhesively laminated to a cast polypropylene film. The adhesive is applied to the OPET film. (Note the subtle difference in placement of “//” markings to define where the adhesive is applied, compared to the previous structure.) 12μ OPET-met / adh // 15μ BONy / adhesive // corona > ( 20μ LLDPE – 10μ LDPE – 45μ EVA ) Û An OPET that has been metallized, then adhesively laminated to BONy film. The other side of the nylon film is then adhesively laminated to a coex film that has been inline corona-treated (and likely re-treated to burn off slip agents in the film). OPET / ink / primer / LDPE / met-OPET < corona / (tie – LDPE) / LLDPE film Û A reverse-printed OPET film that is primed and then extrusion-laminated to metallized PET film using an LDPE. The plain side of the OPET film is then corona-treated and extrusion-laminated to LLDPE film using a special tie resin and LDPE in a coextrusion process. 20μ OPP / ink / primer / 15μ LDPE / 13μ met-OPET < corona / (6μ tie - 6μ LDPE) / 12μ ionomer Û A reverse-printed OPP that is primed and extrusionlaminated to a metallized PET film using LDPE. The plain side of the OPET film is corona-treated and coextrusion-coated with a tie resin and LDPE. The LDPE side is then extrusion-coated with an ionomer. ( LLDPE – W.O. LLDPE – LDPE ) film / EAA / alu / EAA / ( LDPE – LLDPE ) film Û An extrusion lamination with EAA of a three-layer film to one side of the alu. The outer film is white, but the pigment is only in the middle layer. The other side of the alu has a two-layer film extrusion laminated to it with an EAA (a toothpaste-tube laminate-type application). Cello / ink / primer / LDPE / alu / EMAA Û Cellophane that is reverse-printed, primed, and extrusion-laminated to aluminum with LDPE. The other side of the alu is then extrusion-coated with EMAA. K-Cello / ink / primer / LDPE / alu / ionomer Û A PVdC-coated cellophane (two-side-coated) that is reverse-printed, primed, and extrusion-laminated to aluminum foil with LDPE. The other side of the alu foil is then extrusion-coated with an ionomer. O.L. / Ink / 50gsm Paper / LDPE / 7μ Alu / 35μ EAA • Note: (made by tandem extrusion line) Û 50-gsm paper that is surface-printed and overlacquered. It is then extrusion-laminated with LDPE to alu. The other side of the alu is extrusion-coated with EAA. This is specified as being processed by tandem extrusion lamination instead of two-pass coating. LDPE / ink / 18 pt. Board / LDPE / alu / (EAA- LDPE ) Û 18-point printed paperboard that is surface extrusion-

coated with LDPE and then extrusion-laminated with LDPE to alu. The other side of the alu is then coextrusion-coated with a combination of LDPE and EAA. 50μ Blue EAA //TL// alu //TL// 50μ EAA Û A thermal lamination of blue pigmented EAA film to alu on one side. Another film of clear EAA is thermally laminated to the other side of the alu (a cableshielding laminate-type structure). Green EAA / 200μ alu / EAA Û An extrusion coating of green pigmented EAA is applied to one side of the 200μ alu. A clear EAA is extrusion-coated to the other side of the alu (a cableshielding laminate-type structure.) ( LDPE – EAA ) //TL// 150u alu //TL// ( EAA – LDPE ) Û 150μ aluminum that has coex films thermally laminated to each side (a cable-shielding type structure). { ( mLLDPE – LDPE – tie – Nylon – EVOH – Nylon – EVA tie ) //BL// ( EVA tie – Nylon – EVOH – Nylon – tie – LDPE – mLLDPE ) } Û A thirteen-layer structure that is made by intentionally blocking a coex blown film in the main collapsing nip. The film from the die is actually seven layers. In the main nip, the bubble is blocked on purpose. The nip rolls may be heated to help this. The tower height is usually very short, so that the film is still quite warm entering the nips. The tie resin at the blocking interface is known to be an EVA-based tie resin, but the exact grade or brand is not known. ( Nylon 6,6 – Tie A – Nylon 6 – Tie B – ionomer ) Û A five-layer coextruded film. It must be specified whether this is made by a cast-film or blown-film process. In a straight coextrusion, this is helpful to know for physical property reasons. ( PP – tie A – Nylon 6 – EVOH – Nylon 6 – tie B – LLDPE ) Û A seven-layer coextruded film. The tie-layer resins are not known, but it is known that two different grades are being used in the one structure. It must be specified whether the film is made by a cast-film or blownfilm process. ( Nylon – tie – Nylon – tie – LLDPE – mPE – ionomer ) Û A seven-layer coextruded film. The tie layers are believed to be the same and therefore are not labeled differently. An mPE is used to facilitate adhesion of the ionomer to the LLDPE. An upgrade would be to specify the grade or density of the mPE and the grade or ion type of the ionomer. It must be specified whether the film is made by a cast-film or blown-film process. ( 15μ Nylon 6 – 5μ LLDPE tie – 45μ LLDPE ) Û A three-layer coextruded film. The thickness of each layer is known and therefore is specified in the structure. The tie-resin grade is not known, but it is known to be based on LLDPE. It must be specified whether the film is a cast film or a blown film. ( 20μ [Nylon 6 + Amorphous PA] – 4μ tie – 40μ LLDPE ) Û A three-layer coex film. The blend ratio of the outer layer is unknown. The layer thickness for each layer is

Section 6.6.  Structure Writing and Nomenclature

specified. It must be specified whether this is made by a blown- or cast-film process. ( [ 80% Nylon 6 + 20% Amorphous PA ] – tie – EVOH – tie – LLDPE ) Û A five-layer coextruded film. The outer layer is a blend of 80% Nylon 6 plus 20% amorphous nylon. It must be specified whether the film is made by a blown-film or a cast-film process. ( 850μ HDPE – 55μ HDPE tie – 55μ Nylon 6 ) Û A three-layer coextrusion, which from the thickness is likely a coex blow-molded bottle. This type of layer thickness is often used for barrier bottles, such as for agricultural chemicals, where nylon is often used as the inner layer. Please indicate the application when drafting out structures such as this to prevent confusion, because this could also be a sheeting application. ( 725μ HDPE – 12μ regrind – 55μ HDPE tie – 55μ Nylon 6 ) Û Similar to the above structure for an agchem bottle, except with a layer of regrind material between the outer HDPE and the tie layer. ( 125μ PP – PP tie – 35μ EVOH – PP tie – 100μ PP ) Û A coextrusion that from its thickness is likely a sheet material. The type of structure may seem to indicate that it might be thermoformed into some type of cup or tray. A description of the actual application for the structure should be included in any communication. Sample Structures for Practice Descriptions: • K-Nylon < corona / adhesive // ( mLLDPE – 12% EVA – peel seal resin) • BONy-PVdC / adhesive // ( mLLDPE – mLLDPE – EMAA ) • BONy-PVdC / primer / ( LDPE – LDPE – EMAA ) • OPET / ink / adh / alu / adhesive // ( LLDPE – LLDPE – EMAA ) • PET-VM / adhesive // BONy / adhesive // corona > ( LLDPE - LDPE - EVA) • OPP / ink / adh // met-OPET / primer / ( LDPE – ionomer ) • OPET-PVdC /ink / adhesive / ( LDPE – EMAA – ionomer ) • PVdC-OPET-PVdC / ink / adhesive / LLDPE • Woven PP < corona / ( tie – LDPE ) / LLDPE film • Woven PP < corona / ( tie – CoPP ) • OPP / ink < corona / ( tie – [ LDPE + EMAA ] / metOPET / primer / ozone > ( LDPE – EVA ) Û This incorporates some minor editing updates, but is essentially the same chapter which has been reprinted from ‘The Science and Technology of Flexible Packaging’ by Barry A. Morris. ‘Writing Guide for Packaging Films and Other Multilayer Structures’ by Scott B. Marks, pages 685–695, Copyright 2016, with permission from Elsevier. This chapter is also similar to the one of the same name in the TAPPI Extrusion Coating Manual 5th Edition, Copyright 2017 by Scott B. Marks.

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TRADEMARK REFERENCES The following references for companies and trademarks are as of May 2018. Please note that companies change names and businesses are sold and acquired, and that in the future the ownership of a given trademark may shift. Please consult current literature when referencing a trademark of a material. Arkema: Evatane®, Lotader®, Lotryl®, Orevac®, Pebax®, Orgalloy® Berry Global: Reemay®, Typar® Celanese Corp: Ateva® Chemours: Teflon®, Tefzel® ChevronPhillips Chemical: K-Resin®, Marlex®, MarFlex®, mPact™ Curwood Inc.: (Subsidiary of Bemis Company, Inc.) EZ Peel™, Reseal™ Dow: APPEEL™, BIOMAX™, BOOSTER™, BYNEL™, CONPOL™, ELVALOY™, ELVAX™, ENTIRA™, FUSABOND™, NUCREL™, SURLYN™, ADCOTE™, AFFINITY™, AGILITY™, AMPLIFY™, ATTANE™, DOWLEX™, ELITE™, ENGAGE™, FLEXOMER™, INFUSE™, INTUNE™, SEALUTIONS™, VERSIFY™ DuPont-Teijin Films: Mylar®, Melinex® DuPont: Hytrel®, Sorona®, Tensylon®, Tyvek®, Zytel®, Kevlar®, Nomex®, Crastin®, Selar® EMS Group: Grivory®, Grilon® ExxonMobil Chemical: Escor™, Exact™, Exceed™, Escorene™, Optema™, Enable™, Exxco™, VistaMaxx™, Vistalon™ Henkel: Liofol®, Tycel®, Loctite® Ineos: Eltex®, Rigidex® Kuraray: EVAL™ LyondellBasell Chemical: Ultrathene®, Petrothene®, Plexar®, StarFlex®, Moplen®, Clyrell™, Adsyl®, Adstif™, Pro-Fax® Mica Corp: MICA™ Michelman Corp: Michem® Mitsubishi Chemical: Modic™ Mitsui Chemical: Apel®, Admer® Mitsui-Dow Chemical: CMPS™ Nippon Gohsei: Soarnol™ Nova Chemical: Novapol®, Sclair®, Surpass® Reliance Industries Ltd: Relene™, Repol™ SK Chemical: Yuclair™, Primacor™, Saran™ Solvay: Ixan®, Diofan® Topas Advanced Polymers: Topas® Toyo Chemical: Oribain™, Tocryl™ Versalis (Eni SpA): Clearflex®, Flexirene®, Eraclene® Westlake Chemical: EMAC®, EBAC®, Mxsten®, Tymax®, Hifor®, Elevate®, Epolene® Yasuhara Chemical: Hirodine™, Hirotac™ © 2019 The Dow Chemical Company

Glossary of Film Terms COMPILED BY: EARL HERRIMAN and SCOTT B. MARKS UPDATED BY: JAMES F. MACNAMARA JR.

This glossary provides brief definitions of terms and concepts used daily in the industry associated with production and usage of blown and cast films, coatings or laminations. The information was compiled from previously printed sources. Additional terms and descriptions were contributed by industry personnel. While it is fairly comprehensive, it is not all inclusive. ABS or Acrylonitrile Butadiene Styrene—A terpolymer manufactured from acrylonitrile and styrene liquids and butadiene gas. Resin can be produced by polymerization, grafting, physical mixture, or combinations. Abrasion Resistance—Ability of a plastic to withstand mechanical action that tends to wear material from its surface. Absorption—Penetration of one material into the mass of another by molecular or chemical action. Acrylic—A synthetic resin of acrylic or methacrylic esters. Methacrylates are usually methyl, ethyl, butyl, lauryl, or stearyl. Acrylics are widely known for their clarity properties. Actuators—Device that controls the movement or mechanical action of a machine indirectly, usually accomplished by means of pneumatic or hydraulic cylinders. Adapter—A long, heated cylindrical pipe used to convey molten resin from an extruder into an extrusion die. Additive—Any substance compounded into resin to modify its properties. Additives can be antioxidants, colorants, pigments, light stabilizers, etc. Adiabatic—A process condition in which no heat is deliberately added or removed. Used to describe extrusion methods where heat is derived from mechanical action of the screw to an extent that sufficient heat is generated to melt the resin. Adsorption—Adhesion of the molecules of one material to the surface of the solid or liquid with which they are in contact. Air Ring—In blown film extrusion, a circular manifold mounted above the extrusion die used to distribute an even flow of air against a blown film bubble.

Alloy—Terms in plastics used to describe a blend of polymers with other polymers or copolymers, usually where the properties of the alloy exceed those of the constituents (see Synergism). Amorphous—Having no crystallinity. At processing temperatures, most plastics are amorphous. Anisotropy—The tendency of a material to have properties that differ according to the direction of measurement. Anilox—It is a method used to provide a measured amount of adhesive to a coating roller in the lamination process. Annealing—The process in which a plastic material is heated to a predetermined temperature, maintained at this temperature for a predetermined time and slowly cooled at a predetermined rate to relieve stresses. Antiblock Agent—Additive incorporated in film to prevent adhesion (sticking) between touching layers of film during fabrication, storage, or use. For example, these additives can be diatomaceous earth, sihca, and talc. Antimony Oxide—A white, odorless, fine powder widely used as a flame retardant in plastics. Antioxidant—An additive which inhibits the degradation and oxidation of a material when exposed to ambient air during processing and subsequently in the end product form. Antistatic Agent—Additive which imparts a slight degree of electrical conductivity to plastics, permitting the dissipation of static electricity. Artificial Weathering—Process of exposing plastics to environmental conditions developed by laboratory methods designed to simulate actual outdoor exposure. Laboratory conditions are usually intensified in comparison to actual outdoor conditions. Ash Content—Solid residue left after a polymer has been incinerated at high temperatures sufficient to drive off all combustibles. Atactic Polymer—Polymers with a random arrangement of radical groups above and below a molecular chain, usually used in reference to polypropylene resins. Average Molecular Weight—Molecular weight of polymers is determined by viscosity of the material in solution 499

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Glossary of Film Terms

at a specific temperature. This results in an average molecular weight of the molecular chains independent of specific chain length. The value obtained falls between weight and number average molecular weight. Back Pressure—In extrusion, the resistance of molten polymer to flow forward, caused by a pressure difference between two points along the path of flow. Banbury Mixer—A compounding apparatus consisting of two contra rotating spiral shaped blades encased in intersecting cylindrical housings so as to leave a ridge between blades. Band Heater—Electrical heating units fitted to extruder barrels, adapters, dies, nozzles, etc. utilized for heating the polymer to a desired temperature. Barrel—The tubular portion of the extruder in which the extruder screw is placed and rotates. Barrier Resins—Polymers which have very low permeability to gases. Barrier Screw—Typically two start flight screws which are designed to continually separate the melt from unmelted plastic and to subject the un-melted material to high shear forces to induce melting; the two melt streams are combined and passed through a mixing head to produce a uniform melt temperature. Basis Weight—The areal density of a material, that is, its mass per unit of area. Beta Gauge—A thickness measuring device used for film and sheeting. The device operates by beta radiation being emitted on one side of the film and a detector placed on the opposite side. When the film is passed through the beam, some of the beta radiation is absorbed, which is indicative of the film thickness. Biaxial Orientation—The process of stretching a hot plastic film in two directions under conditions resulting in molecular orientation in two directions. Biodegradation—The degradation of plastics by microorganisms when buried in the soil. Some plastics can be modified to become biodegradable by the incorporation of a biodegradable additive such as corn starch. Biopolymer—A polymeric substance (such as a protein or polysaccharide) formed in a biological system. Blanking Die—The cutting of a flat sheet to shape by striking it sharply with a metal punch while supporting it on a mating die. Blend—The mixing of polymers with other polymers or copolymers, usually where the mixture results in the desired physical properties. Blister Packaging—A method of packaging in thermoformed pouches shaped to fit the contours of the article. The product is placed in the pouch and a panel board or plastic backing is sealed to the formed sheet to enclose the product. Blocking—An undesirable adhesion between layers of film or sheeting which may have developed during processing or storage. Blocking can be prevented by adding antiblock agents to the resin.

Bloom—A thin, greasy film on the surface of a plastic film usually caused by the exudation of an additive. Slip additives are designed to migrate or bloom to the surface of films. Blow Hole—Blow outs or loss of internal air from a blown film bubble usually due to a rupture caused by fisheyes, gels, or contamination. Blown Film—A Process that involves extruding a continuous thin walled tube of plastic and inflating it immediately after it leaves the die. The pressure is such that the tube stretches, increasing its diameter and reducing its wall thickness to desired gauge. Air is trapped within the blow tube (bubble) between the die and collapsing rolls which convert it to lay flat film to facilitate winding onto a roll. Stress orientation is in both the machine direction and transverse (cross) direction. Blown Film Tower—Apparatus for handling film in blown film extrusion between the extruder die and take up equipment. The blown film tube passes through the tower where it is cooled, sized, and gauged. Nip rolls are located at the top where the inflated tube is collapsed prior to winding. Blow Up Ratio—The ratio of the final tube diameter to the die diameter in blown film extrusion. In blow molding, it is the ratio between the mold cavity diameter and the parison diameter. Boss—A small projection provided on an article to add strength or facilitate alignment with another part during assembly. May also be used to attach one part to another. Branched Polymer— A polymer in which side chains are attached to the backbone of the molecular chain. Breaker Plate—A perforated plate located at the rear end of an extruder head or die adapter serving to support the screen pack. The breaker plate also helps to generate back pressure in extrusion. Bubble—The inflated film tube during processing. Bubble Sizing Cage—Used to stabilize the bubble between the frost line and the collapsing frame. Bulk Density—The density (mass per unit of volume) of a resin in solid form (granular, nodular, pellet, powder, etc.) expressed in g/cc or lb/ft3. Butene—A class of hydrocarbons having four carbon atoms and a double bond comprising 1-butene, cis-2- butene, trans-2-butene, and iso butylene. Has numerous applications in plastics including acting as monomers with styrene, acrylics, olefins, and vinyls. 1-butene is a comonomer used in the production of linear low density polyethylene. Cadmium Pigments—Inorganic pigments based on cadmium sulphide and cadmium sulphoselenides used widely in polyethylenes. Includes cadmium maroon, orange, red, and yellow. Calcium Carbonate—A filler and extender used in thermoplastics. It occurs naturally in the form of minerals such as calcite, chalk, limestone, marble, and whiting.

Glossary of Film Terms

Calendering—Process in which film and sheet material is produced by squeezing heated, viscous material between two or more counter rotating rolls. The gap between the last pair of heated rollers determines the thickness of the sheet. Subsequent cold rollers cool the sheet. Caliper—It is a device used to measure the distance between two opposite sides of an object. Calorimeter—A device used for measuring the heat transferred during thermal reactions. Capillary Rheometer—An instrument for measuring the shear flow properties of polymer melts. The data obtained is usually presented as graphs of shear stress against shear rate at a constant temperature. Carbon Black—A multifunctional pigment used in plastics as a conductor of electricity, a pigment, a filler extender, and as a UV stabilizer. Cartridge Heater—Cylindrical bodied, electrical heater for providing heat for injection, compression and transfer molds, injection nozzles, runnerless mold systems, hot stamping dies, sealing, etc. Cast Film—Film extruded from a flat die onto chill rolls. Benefits are that line speed can be extremely high (3300+ fpm), and clarity also very high. Drawbacks include high capital costs and film orientation is predominantly in the machine direction. Catalyst—A substance which causes or accelerates a chemical reaction when added to the reactants in minor amount without being permanently affected by the reaction. Chain Length—The number of monomeric or structural units in a linear polymer. Cartridge Heater Channel Depth Ratio—In an extruder screw, the ratio of the depth of the first channel at the hopper end to the depth of the last channel in the metering section. Charge—Precise, weighed amount of material placed in an open mold. Also determined by volumetric measurement. Chill Roll—A cored roll, usually Temperature controlled with circulating water, which cools a molten polymer web on contact before winding. Chromatography—The process used for analysis and separation of mixtures of two or more substances, and for determining many characteristics in research work. Chrome Pigments—Pigments based on basic lead chromate. Included are chrome yellow and orange. They have intense colors, good acid resistance, and heat stability. Coating Weight—Weight of coating per unit area. Coefficient Of Friction—Resistance to movement of sliding or rolling surfaces of solid bodies in contact with each other. Coefficient of Thermal Expansion—Change in dimension of a material per unit change in temperature. Coextrusion—The technique of extruding two or more materials through a single die being fed by separate extruders. Collapsing Frame—Uniformly transforms the inflated film

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tube to a flat tube. Various surfaces are used to reduce film drag, including: • Air Board—Perforated surface with a positive air flow • Plastic Covered Aluminum Slats • Rollers—Solid Segmented • Wood Slats Colorant—Dyes or pigments which impart color to plastics. The dyes are synthetic or natural compounds of submicroscopic size, soluble in common solutions, yielding transparent colors. Pigments are organic and inorganic substances with larger particle sizes and are usually insoluble in common solvents. Color Concentrate—A plastics compound which contains a high percentage of pigment to be blended into base resins. The term masterbatch is sometimes used for color concentrate as well as for concentration of other additives. Colorimeter—An instrument for measuring and matching colors. Compatibility—The ability of two or more substances to mix together without separation. Corona Discharge—A method of rendering the surface of inert plastics, such as polyethylene, more receptive to inks, adhesives, or coatings by subjecting their surfaces to an electrical discharge. Typical method is to pass film over a grounded metal cylinder above which a high voltage electrode is spaced to leave a small air gap. The corona discharge oxidizes the film leading to the formation of polar groups. The surface now becomes receptive to the coatings. Creep—Creep is the permanent deformation resulting from prolonged application of a stress below the material’s yield point. Crosslinking—The establishment of chemical bonds between the molecular chains in polymers. Crosslinking can be accomplished by chemical reaction, vulcanization, degradation, and radiation. Crystallinity—The state of molecular structure in some resins denoting uniformity and compactness of the molecular chain. Cyclohexane—A colorless liquid derived from the catalytic hydrogenation of benzene, used as a solvent in percent extractables testing. Dancer Roll—A roller used as a tension maintenance device in the production of films and sheeting. Dart Impact—A traditional method for evaluating the impact strength or toughness of a plastic film. Deckle—A rod or plate attached to each end of a cast film or extrusion coating die which is used to adjust the length of the die opening. Deflection Temperature—The temperature at which a standard ASTM D 648 test bar deflects 0.010 inch under a load of 264 psi. Deformation—Any change of form or shape in a body; the linear change of a dimension of a body ‘in a given direction produced by the action of external forces. Degradation—A deteriorous change in the chemical struc-

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Glossary of Film Terms

ture, physical properties, or appearance of a plastic caused by exposure to heat, light, oxygen, or weathering. Degree of Polymerization—The average number of monomer units per polymer molecule, a measure of molecular weight between the layers. Delamination—The separation of one or more layers in a laminate caused by the failing of the adhesive bond. Density—Weight per unit volume of a substance usually reported in g/cc or lb/ft3. Die—A steel block containing an orifice through which plastic is extruded, shaping the extrudate to the desired form. Die Adapter—The part of an extrusion die which holds the die block. Die Cutting (Blanking)—The process of cutting shapes from sheets of plastic by pressing a shaped knife edge into one or several layers of sheeting. Die Gap—Distance between the metal faces forming the die opening. Die Land—(See Land). Die Lines—Vertical or horizontal marks on the extrudate and in the finished product caused by damaged die elements or by contamination held up in the die land. Die (Extrudate) Swell—In extrusion, the increase in diameter of the extrudate over that of the die opening through which it is extruded. Die (Extrudate) Swell Ratio—In extrusion, the ratio of the film thickness to the die gap. Differential Scanning Calorimetry (DSC)—Analytical method used to determine thermal histories of polymers such as melting points and glass transition points. Differential Thermal Analysis (DTA)—An analytical method similar to thermo gravimetric analysis, except that the specimen is heated simultaneously with an inert material as a control, each having its own temperature sensing and recording apparatus. The curve shows the weight losses of both materials under the same rates of heating. Dimensional Stability— The ability of a plastic film to retain the precise width and length shape in which it was blown or cast. Dip Coating—Method by which an object is coated by dipping into a plastisol or organosol. Discoloration—Any change from an initial color possessed by a plastic; a lack of uniformity in color where color should be uniform over the whole area of a plastic object. Dispersion—Fine division of particles of a resin or solid in suspension in another material. Doctor Blade (Bar)—A flat bar used for Regulating the amount of liquid material on the rollers of a coating machine, or to control the thickness of a coating after it is applied to a substrate. Dowel (Pin)—A pin used to maintain alignment between two or more parts of a die. Drag Flow—In the metering section of an extruder screw,

drag flow is the component of the total material flow caused by the relative motion between the screw and the cylinder. Drawdown—In extrusion, the process of pulling the extrudate away from the die at a linear speed higher than that at which the melt is emerging from the die, thus reducing the cross sectional dimensions of the extrudate. Drooling—Running an extrusion system at a very low rpm during changeovers, to prevent degradation in the system from excessive time/temp exposure on a polymer. Ductility—Amount of strain a material can withstand before it fractures. Dwell Time—The time in which a heat seal operation transpires, such as the time the heat sealing jaws are closed down on the film. Dyne—A measurement of the level of surface energy of a material. Edge Guide—A steering guide providing web/strip position correction by bending the web through a long entering span. Elastic Deformation—The portion of deformation of an object under load which can be recovered after the load is removed. Elasticity—The property (of plastic materials) of recovering original size and shape after deformation. Elastomer—A material which, at room temperature, can be stretched under low stress to at least twice its original length and, upon immediate release of the stress, will return with force to its approximate original length. Elmendorf Tear—A test for measuring the tearing resistance of a material. Elongation—Deformation caused by stretching; the fractional increase in length of a material stressed in tension. Embossing—Technique providing a textured surface to roll goods. It is used in—line with extruders and calenders or off line in an unwind, emboss, and rewind operation. The focal point of the equipment is the textured roll which imparts the impression. Endothermic Reaction—A reaction which is accompanied by the absorption of heat. Entrance Angle—In extrusion, the maximum angle at which the molten material enters the land area of the die, measured from the center line of the mandrel. Environmental Stress Cracking—The susceptibility of a thermoplastic resin to crack or craze when stressed in the presence of surface active agents or other environments. Epoxy Resins—Thermosetting resins which, in the uncured form, contain one or more reactive epoxide or oxirane groups. These groups serve as crosslinking points in the subsequent curing hardener. Epoxy resins are used in protective coatings, bonding adhesives, in building and construction, and electrical uses. Erucamide—A fatty acid based slip additive used in polyolefin resins.

Glossary of Film Terms

Ethylene—A colorless, flammable gas derived by cracking of petroleum and natural gas fractions. Also serves as a monomer for polyethylene. Ethylene Vinyl Acetate (EVA)—Copolymeric member of the polyolefin family derived from random copolymerization of vinyl acetate and ethylene. Ethylene Vinyl Alcohol (EVOH) Copolymer of ethylene and vinyl alcohol. Because the latter monomer mainly exists as its tautomer acetaldehyde, the copolymer is prepared by polymerization of ethylene and vinyl acetate to give the ethylene vinyl acetate copolymer followed by hydrolysis Exotherm—The temperature versus time curve of a chemical reaction and the amount of heat given off. Maximum temperature occurs at peak exotherm. Exothermic Reaction—A reaction in which heat is given off. Extensibility—The ability of a material to extend or elongate upon application of sufficient force, expressed as a percent of the original length. Extrudate—The film, coating, or other output product of the extrusion process. Extrudate Swell—See Die Swell. Extrudate Swell Ratio—See Die Swell Ratio. Extruder—A machine for producing more or less continuous lengths of plastic sections such as films, rods, sheets, tubes, profiles, and cable coatings by melting and pumping resin through a forming die. Extruder, Compounding—The basic functions of a compounding extruder are to melt the polymer and evenly disperse and distribute additives or fillers to meet the specifications of the end product. Large scale compounding is done on either single or twin screw extruders. Single screws are used for basic operations where little variation in material formulation and viscosity is expected. Twin screw compounders offer better mixing characteristics. Extruder, Single Screw—Basic machine consists of a screw, barrel, drive mechanism, resin feed arrangement and controls. The constantly turning screw augers the resin through the heated barrel where it is heated to proper temperature and blended into a homogeneous melt. Before the melt can leave the barrel, it must pass through a breaker plate and screen pack. The melt is then extruded through the die into the desired shape. Extruder, Twin Screw—Two screws, side by side, are placed within the extruder barrel, either co-rotating or counter rotating. Counter rotating twin screw extruders are used primarily for processing PVC products such as pipe, siding, sheet, pellets. and film. The co-rotating units are used for compounding materials where thorough mixing and high output rates are important. The twin screw unit resembles a positive displacement screw pump. It conveys the material at low speeds with controlled shear. The positive action assures that all portions of the material experience a uniform residence time.

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Extruder, Vented—An extruder provided with a vent hole, usually in the metering zone where the material has attained a molten condition, for the withdrawal of gases and air. Extruder Size—The nominal inside diameter of the extruder barrel. Extrusion—Compacting and melting a plastic material and forcing it through an orifice in a continuous fashion. In the extrusion process, the material is conveyed through the heated machine barrel by a helical screw where it is heated and mixed to a homogeneous state and then forced through a die of the shape required for the finished product. Extrusion Plastometer—A type of viscometer used for determining flow rates of a polymer under specified temperatures and loads, more commonly known as a melt indexer. Fatigue Strength—The maximum cyclic stress a material can withstand for a given number of cycles before failure occurs. Feed Section—First section or zone of an extruder screw which is fed from the hopper and conveys solids. Filler—A material which is added to plastics to make it less costly. Filler can be inert or can alter various properties of the plastic. Film—Sheet material having a nominal thickness not greater than approximately 250 microns or 10 mils. Fisheye—Small globular mass which has not melted/blended completely into the surrounding material resulting as a fault in film or sheet. Flame Retardant—Reactive compounds and additive compounds to render a polymer fire resistant. Reactive compounds become an integral part of the polymer. Flame Treating—A method of rendering the surface of inert thermoplastics or other substitutes, particularly polyolefins, receptive to inks, lacquers, paints, adhesives, and the like by bathing the surface of the article in a highly oxidizing flame. This treatment oxidizes the surface layer of the article, making it receptive to coating. Flammability—The measure of the extent to which a material will support combustion. The test usually used is described in ASTM D 1433, the results being expressed in seconds required for specimen to bum over six inches of its length. Flexographic Printing—It uses flexible photopolymer printing plates wrapped around rotating cylinders on a web press. The inked plates have a slightly raised image and rotate at high speeds to transfer the image to the substrate. Flexural Modulus—Ratio of applied stress to strain in outer fibers of a plastic specimen during flexure. Flexural Strength—Resistance of a plastic material to cracking or breaking during bending. Flight—The outer surface of the helical ridge of metal on an extrusion screw. Flow—The fluidity/molten movement of a polymer during processing.

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Glossary of Film Terms

Flow Lines—Distinctive surface marks caused when two flow fronts meet and weld together during film extrusion. Fluoroplastics—Polyolefin polymers in which fluorine, fluorinated alkyl groups, or other halogens replace hydrogen atoms in the carbon chain. This structure has outstanding electrical properties, excellent resistance to chemical attack, low coefficient of friction, excellent fire resistance, exceptionally good performance at high and low temperatures, low moisture absorption, and outstanding weatherability. Fluorocarbon Elastomers—Polyolefin containing fluorocarbons which are intended to be incorporated as a plastics modifier to aid in the extrusion of film, pipe, sheet, etc. Mostly used in construction with LLDPE, HDPE, HMW HDPE, and UHMW HDPE. Otherwise known as polymer process aid (PPA). Fractionation—A method of determining the molecular weight distribution of polymers based on the fact that polymers of high molecular weight are less soluble than those of low molecular weight. Two basic methods in use are precipitation fractionation and extraction fractionation. Free Radical—An atom or group of atoms having at least one unpaired electron. Most free radicals are short lived intermediates with high reactivity and high energy, difficult to isolate. They play a role in many polymerization processes. Frostline—In the extrusion of blown film, a ring shaped zone of frosty appearance located at the point where the resin solidifies, caused by a reduction in film temperature below the melting point of the resin. Gauge—Thickness of plastic film measured in mils (thousandths of an inch) or microns (thousandths of a millimeter). Gauge Band—An area of gauge imperfection in the film. Gelbo Flex Testing—Measures flex durability of flexible plastic and composite barrier materials. Gel—Small globular mass which has not melted and blended completely into the surrounding material resulting in a fault in the film or sheet. Gel Permeation Chromatography—A developed column chromatography technique employing a stationary phase (gel) in the presence of a diluent which is a nonsolvent for the polymer. The polymer to be analyzed is introduced at the top of the column and then is diluted with a solvent. The polymer molecules diffuse through the gel at rates dependent on their molecular size. As they emerge from the bottom of the column, they are detected by a differential refractometer coupled to a recording chart, on which a molecular size distribution curve is plotted. Glass Transition Temperature—The temperature at which a reversible change occurs in an amorphous polymer when it is heated to a certain temperature and undergoes a rather sudden transition from a hard, glassy, or brittle condition to a flexible or elastomeric condition.

Gloss—Brightness or luster of a plastic resulting from a smooth surface. Gradient Tube Density—A method for measuring densities of very small samples, often used in the plastics industry. A vertical glass tube (the gradient tube) is filled with a heterogeneous mixture of two or more liquids, the density of the mixture varying linearly or in other known fashion with the height. A drop or small particle of the specimen is introduced in the tube and falls to a position of equilibrium which indicates its density by comparison with positions of known standard samples. Granular—Composed of coarse particles. Often used in reference to Granular LLDPE. Gravure Printing—The depressions in an engraved printing cylinder or plate are filled with ink, the excess raised portions being wiped off by a doctor blade. Ink remaining in the depressions is deposited on the plastic film or other substrates as it passes between the gravure roll and resilient back up roll. Gussets—The inward fold in the sides of bags which reduce the width of the bag and allow the bags to assume a rectangular form when opened. Haze—Cloudiness in plastic film. Measured as percent haze, anything below five percent is generally considered high clarity. Heat Distortion Point—The temperature at which a standard test bar deflects a specified amount under a stated load. Heater Bands—Electrical heating units shaped to fit extruder barrels, pipes, adaptors, dies, and the like, for heating the plastic material to the desired temperature. Heat Sealing—The process of joining two or more thermoplastic films or sheets by heating areas in contact with each other to the temperature at which fusion occurs, usually aided by pressure. Helix Angle—The angle of the flights on an extruder screw. Hexene—A comonomer (hexene- 1 or 4 -methyl pentene- 1) used in the production of linear low density and high density polyethylene. High Density Polyethylene—This term is generally considered to include polyethylenes ranging in density from about 0.945–0.960 g/cc and over. Whereas the molecules in low density polyethylene are branched in random fashion, those in the higher density polyethylenes have fewer side branches, resulting in more rigid material with greater strength, hardness, chemical resistance, and higher softening temperature. HMW High Density Polyethylene—High molecular weight high density polyethylene is usually defined as a polyethylene with a density of 0.945 or greater and a flow rate of less than 0.1 dg/min on the standard melt index scale, or 1 to 20 dg/min on the modified test scale which uses a 10× heavier load (190°C/21.6 kg). The weight average molecular weight ranges from 200,000 to 500,000. High Load Melt Index—The ASTM condition of 190°C and a load of 21.6 kg used for determining the flow rate of

Glossary of Film Terms

molten HMW-HDPE through a standard orifice. More often referred to as an I-10 melt flow rate. Homopolymer—The result of the polymerization of a single monomer, a homopolymer consists of a single type of repeating unit. Hopper—In polymer processing, the container holding a supply of extrusion resin material to be fed to the screw. The hopper may be intermittently filled or continuously fed. Hopper Blender—Mixes material such as virgin resin, regrind, slip agents, fillers, and colorants in desired proportions. Materials to be blended are metered in ratio to a mixing chamber and then discharged into the hopper of the processing machine. Hopper Loader—A device for automatically feeding resins to hoppers of extruders, and the like. Hot Tack—The property of an adhesive or seal layer to resist forces that would pull the seal apart while it is still hot. Hydrolysis—Decomposition of a substance by reaction with water. Hydrophilic—Having a strong affinity for water. Hydrophobic—Lacking affinity for water. Hydroscopic—(Hygroscopic) Readily absorbing and retaining environmental moisture. IBC (Internal Bubble Cooling)—A system of cooling the air inside of the bubble, while maintaining the desired bubble diameter. Idler Roller—A cylindrically-shaped material handling components that convey items through a machine, process, or environment. Immiscible—Incapable of intermittently mixing, i.e., oil and water are immiscible. Impact Strength—Ability to withstand shock loading. Initiator—A material such as Peroxide used to start the process of abstracting hydrogen from a polymer backbone. They are used in free radical polymerizations, curing thermo setting resins, as crosslinking agents for elastomers and polyethylenes, and for polymer modification. Ionomer—Thermoplastic that combines transparency with toughness, particularly at low temperatures. The polymer’s main component is an ethylene copolymer, but it contains both inorganic and/or organic materials linked by both covalent and ionic bonds. Film of this material is used in many applications such as skin packaging, meat/food packaging, and healthcare packaging. Isotactic—Pertaining to a type of polymeric molecular structure containing a sequence of regularly spaced asymmetric atoms arranged in like configurations in a polymer chain, usually used in reference to polypropylenes. L/D Ratio—The ratio of the length (L) to diameter (D) of an extruder. This ratio can be expressed based on barrel length or screw flighted length. Laminar Flow—The movement of one layer of fluid past or over another layer without the transfer of matter from one to the other.

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Lamination—The process through which two or more flexible packaging webs are joined together using a bonding agent. Land—(1) The bearing surface along the top of the flights of a screw in a screw extruder. (2) The surface of an extrusion die parallel to the direction of melt flow. Layflat—The width of the film after collapsing the bubble. Lead Chrome Pigments—A series of inorganic pigments including yellows, oranges, and greens, used in polyolefins and other plastics. Lensing— Is a term which refers to a film defect which causes undesirable elongated thin voids in an extruded film. Are sometimes referred to as windows or airpockets. Linear Polymer—A polymer in which the monomeric units are linked together in linear fashion with little or no long chain branching. Examples are linear low density polyethylene and high density polyethylene. Linear Low Density Polyethylene—See Linear Polymer. Includes linear polyethylenes ranging in density from 0.915 to 0.930 g/cc. Low Density Polyethylene—This term is generally considered to include polyethylenes ranging in density from about 0.915 to 0.930. In low density polyethylenes, the ethylene monomeric units are linked in random fashion, with the main chains having long and short side branches. This branching prevents the formation of a closely knit pattern, resulting in material that is relatively soft, flexible and tough, and which will withstand moderate heat. Lubricant—A substance which when interposed between parts or particles tends to make surfaces slippery, reduce friction, and prevent sticking between the lubricated surfaces. Lubricants are added to plastics to assist flow in extrusion, assist in knitting and wetting of the resin in mixing and milling operations, and impart lubricity to finished products. m-PE—Any of a number of polyethylene resins made using a metallocene type catalyst system. Polymers are linear in nature and made at relatively low reactor pressures with comonomers of butene, hexene, or octane. Densities vary in relation to comonomer content. m-LLDPE, m-LMDPE, m-HDPE, m-ULLDPE, m- VLLDPE. Mandrel—The portion of an extrusion die that forms the hollow center in an extruded tube. Mandrel Coextrusion Die—A multiple layer die with each segment placed circumferentially around the previous segment. Manifold—Configuration of piping in a block of metal that takes a single channel flow of resin from an extruder and divides it into various flow channels to feed more than one outlet. Masterbatch—A concentrated blend of pigment, additives, filler, etc. in a base polymer. Masterbatch is added in small amounts to a large volume of material (the same as or compatible with the base polymer) to produce the desired formulation.

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Glossary of Film Terms

Mayer Rod—A Mayer rod is a stainless steel rod that is wound tightly with stainless steel wire of varying diameter. The rod is used to doctor of the excess coating solution and control the coating weight. Medium Density Polyethylene—This term is generally considered to include polyethylenes ranging in density from about 0.930 to 0.945. In medium density polyethylenes, the ethylene monomeric units are linked in random fashion, with the main chains having medium and very short side branches. Melt—A thermoplastic stock which is in a molten state due to temperature. Melt Index (Melt Flow Index)—The number of grams, of a polyethylene thermoplastic resin which can be forced through a 0.0825 inch orifice when subjected to 2160 grams force in 10 minutes at 190°C. Melt Flow Rate—The ASTM condition of 190°C and a load of 2.16 kg used for determining the rate of flow of most molten copolymers of polyethylene, through a standard size orifice (0.0825 inch). For measuring polypropylene, the temperature is raised to 230°C. For measuring EVOH, the temperature is raised to 210°C. Melt Fracture—Is a phenomenon of melt extrudate in which the surface appears rough or wavy upon exit from the die. Melt fracture may appear uniformly or in certain sections only. Melt Instability—An instability in the melt flow through a die that causes irregularities in the finished part. Melt Strength—The elastic strength of a polymer in the molten state. Melt Probe—A probe in contact with the melted polymer inside of the system, measuring the temperature of the melt. Melting Point—The temperature at which a resin changes from a solid to a liquid. Metering Zone—The final zone of an extruder barrel, in which the melt is conveyed at a uniform rate to the breaker’ plate or die. Metallization—It is a general term that refers to the application of a metal coating to another metallic or nonmetallic surface. Depending on the desired result, the coating can consist of metals such as zinc, gold, aluminum or silver. Mettallocene—A compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II. Micron—A unit of length equal to 0.001 millimeters (0.00003937 inches). Migration—The exudation of an ingredient from one material by another material, such as the migration of a plasticizer from one material into an adjacent material with a lower plasticizer content. Modulus Of Elasticity—The ratio of stress to strain below the yield point of the material. Moisture Absorption—The pick-up of water vapor from the atmosphere by a material. It relates only to vapor withdrawn from the atmosphere by a material and must be

distinguished from “water” absorption, which is the take up of water by immersion Moisture Vapor Transmission—The rate of permeation of water through a material at a specific temperature and relative humidity rate. Molecular Weight (MW)—The sum of the atomic weights of all atoms in a molecule. Molecular Weight Distribution (MWD)—The relative amounts of polymers of different molecular weights (MW) that make up a specific polymer. Molecule—It is the smallest particle in a chemical element or compound that has the chemical properties of that element or compound. Monomer—A single molecule which can join with another monomer or molecule to form a polymer or molecular chain. Neck In—In extrusion coating, the difference between the width of the extrusion die opening and the width of the coating on the substrate. Newtonian Flow—A flow characteristic evidenced by viscosity that is independent of shear rate that is the rate of shear is directly proportional to the shearing stress. Water and thin mineral oils are examples of Newtonian fluids. Nip—The V shaped gap between a pair of rollers where incoming material is nipped and drawn between the rolls. Nip Rolls—In film blowing, a pair of rolls situated at the top of the tower which close the blown film envelope, seal air inside of it, and regulate the rate at which the film is pulled away from the extrusion die. One roll is usually covered with a resilient material, the other being bare metal. Notch Sensitivity—Extent to which the sensitivity of a material to fracture is increased by the presence of a surface inhomogeneity, such as a notch. Nuclear Magnetic Resonance (NMR)—Determinations of the number of hydrogen atoms in a complex molecule and the characteristic grouping in which they occur, conducted by placing the specimen in a strong constant magnetic field, then applying a perpendicular r.f. alternating magnetic field. At certain frequencies of the latter field, a hydrogen atom nucleus will absorb and emit energy, the frequency and amount of which are indicative of the characteristic grouping in which the atom is located e.g., a CH3, CH2 or an –OH group. Nucleating Agent—Finely divided solid material added to semi-crystalline polymers to modify the crystalline structure by providing sites for initiation of crystalline growth. A properly nucleated polymer will possess improved clarity, hardness, and tensile strength. Number Average Molecular Weight (MWn)—The average molecular weight of a high polymer expressed as the first moment of a plot of the number of molecules in each molecular weight range against the molecular weight. In effect, this is the total molecular weight of all molecules divided by the number of molecules. Nylon—Generic name for all long chain polymers which

Glossary of Film Terms

have recurring amide groups (CONH) as an integral part of the main polymer chain. Nylons are described in such a manner that the numbers used relate to the number of carbon atoms in the various reactants. i.e., nylon 6, nylon 6:6, nylon 6:12. Octene—A comonomer used in the production of linear low density polyethylenes. Optical Density—It is a measurement of a refractive medium or optical component’s ability to slow or delay the transmission of light. It is commonly used in metallized film as a specified attribute. Offset Printing—A printing process in which the image to be printed is first applied to an intermediate carrier such as a roll or plate, then is transferred to a plastic film or molded article. Oleamide—An ivory colored powder used as a slip additive in polyolefins. Olefins—The group of unsaturated hydrocarbons named after the corresponding paraffins by the addition of “ene” and “ylene” to the stem. Examples are ethylene, propylene, and butenes. Polymers of olefins are sometimes called olefin plastics or polyolefins. Oligomer—A polymer consisting of only a few monomer units such as a dimer, trimer, tetramer, etc. or their mixtures. Other definitions in the literature place the upper limit of repeating units in an oligomer at about ten, or decimer. Opacity—It represents a substrate’s light blocking ability Orange Peel—An uneven surface texture of a plastic article or its finished coating somewhat resembling the surface of an orange. See Melt Fracture. Orientation—The process of stretching a hot plastic article to align the molecules, thus altering mechanical properties. When the stretching force is applied in one direction, the process is called uniaxial orientation. When stretching is in two directions, the term biaxial orientation is used. Upon reheating, an oriented film will shrink in the direction(s) of orientation. This property is useful in applications such as shrink packaging and for improving the strength of molded or extruded articles such as pipe and fibers. Orifice—In extrusion, the opening in the die formed by the orifice bushing (ring) and mandrel. Oscillating Nip—In a typical oscillating nip system, the collapsing frame, nip roll assembly and platform oscillate continuously through 360°, in relation to the stationary film die. This provides gauge randomization of irregularities that occur at the blown film die. Oxygen Transmission Rate—The rate of permeation of oxygen through a material at a specific temperature and relative humidity rate. Oxidation—In respect to polyethylenes, the reaction of air or oxygen in polyethylene causing the formation of hydroxy groups which affects the physical properties adversely. Pancake (Plate) Coextrusion Die—A multiple layer die with the layer segments positioned one on top of the other,

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rather than a mandrel die with each segment placed around the previous segment. Pellets—Globules of uniform size, consisting of resins or mixtures of resins with or without compounding additives which have been prepared for film extrusion operations by shaping in a pelletizing machine or by extrusion and chopping into short segments. Perforating—Processes by which plastic film or sheeting is provided with holes ranging from relatively large diameters for decorative effects (by means of punching or clicking) to very small, even invisible, sizes. The latter are attained by passing the material between rollers or plates, one of which is equipped with closely spaced fine needles or by spark erosion. Permeation—The passage or diffusion of a gas, vapor, liquid, or solid through a barrier without physically or chemically affecting it. Permeability—Permeability is the property of a material, i.e. the degree to which it allows permeation to occur. Phenolics, Hindered—A group of primary antioxidants used to protect polyolefins against oxidation and thermal degradation. Phenolics—These thermosetting resins are credited with being the first commercialized wholly synthetic polymer or plastic. The basic raw material is usually formaldehyde and phenol. In the uncured and semi- cured condition, phenolic resins are used as adhesives, casting resins, potting compounds, and laminating resins. As molding powders, phenolic resins can be found in electrical uses. Phosphite, Organic—A group of antioxidants used to protect polyolefins against oxidation and thermal degradation. Photodegradation—Degradation of plastics due to the action of light. Most plastics tend to absorb high energy radiation in the ultraviolet portion of the spectrum, which results in the formation of free radicals and causes oxidation, cleavage, and other degradative reactions. Pinhole—Tiny hole in cast, extrusion coating, or extruded sheet product. Pitch—With respect to extruder screws, the distance from any point on the flight of a screw to the corresponding point on an adjacent flight, measured parallel to the axis of the screw or threading. Plasma Treatment—A surface modification technique that readily primes any surface for better acceptance of secondary manufacturing applications. Plastic—(adj.) An adjective indicating that the noun modified is made of, consists of, or pertains to plastics. (noun) A material that, in its finished state, contains a synthetic polymer of high molecular weight, is a flexible or rigid solid but not an elastomer in its finished state, and at some stage in its manufacture or processing can be shaped by flow or by in situ polymerization or curing. Plasticate—To impart flexibility in a plastic through the input of heat and mechanical work as in the plasticating of the resin in an extruder machine.

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Glossary of Film Terms

Plastic Deformation—Occurs when an object does not return to its original shape or size after pressure, stress or load is removed. Plastic Memory—The tendency of a thermoplastic material which has been stretched while hot to return to its unstretched shape upon being reheated. Plate Out—The undesirable deposition of additives or pigments on machinery during processing of plastics. Polybutenes—The family of polymers of isobutene, butene 1, and butene 2.—Depending on molecular weight, they range from oils through tacky waxes, crystalline waxes, and rubbery solids. Polycarbonate—Polymers derived from the direct reaction between aromatic and aliphatic dihydroxy compounds with phosgene, or by the ester exchange reaction with phosgene derived precursors. When the aromatic dihydroxy is bisphenol A, the resulting polycarbonate is thermoplastic, the most commonly used form. Such polycarbonates have high impact strength, good heat resistance, low water absorption, and good electrical properties. They are transparent, and may be injection molded, extruded, thermoformed, and blow molded. Polyester Resins—A family of polyesters in which the polyester backbones are saturated and hence unreactive. The most common commercial types are polyethylene terephthalate (PET), a thermoplastic which may be extruded, injection or blow molded. Unsaturated polyesters are thermoset and used in the reinforced plastics industry for applications such as boats, auto components, etc. Polyethylene—A family of resins obtained by polymerizing the gas ethylene. By varying the catalysts and methods of polymerization, properties such as density, melt index, crystallinity, degree of branching and crosslinking, molecular weight, and molecular weight distribution can be regulated over wide ranges. Further modifications are obtained by copolymerization, chlorination, and compounding additives. Polyethylene Terephthalate—A saturated, thermoplastic polyester resin made by condensing ethylene glycol and terephthalic acid, used for fibers and films such as DuPont Mylar® PET film, and, more recently, for injection molded parts. It is extremely hard, wear resistant, dimensionally stable, resistant to chemicals, and has good dielectric properties. Polyisobutylene—See Polybutene. Polylactic Acid—Polylactic acid or polylactide is a thermoplastic aliphatic polyester derived from renewable resources. Polymer—The product of polymerization reaction. See Polymerization. The product of polymerization of one monomer is called a homopolymer, monopolymer, or simply a polymer. When two monomers are copolymerized, the product is called a copolymer. The term terpolymer is used for polymerization products of three monomers.

Polymer Processing Aid (PPA)—Additives incorporated into plastics as a modifier to aid in the extrusion of film, pipe, sheet, etc. Polymerization—A chemical reaction in which the molecules of a simple substance (monomer) are linked together to form large molecules whose molecular weight is a multiple of that of the monomer. Polyolefins—The class of polymers made by polymerizing relatively simple olefins, including ethylene, propylene, butenes, isoprenes, and pentenes. Polypropylene—A tough, lightweight, rigid plastic made by the polymerization of high purity propylene gas in the presence of an organometallic catalyst at relatively low pressures and temperatures. Polyvinyl Acetate—A thermoplastic material composed of polymers of vinyl acetate in the form of a colorless solid. Used extensively in adhesives for paper and fabric coatings. Polyvinyl Alcohol—A thermoplastic material composed of polymers of vinyl alcohol. Polyvinyl Chloride—Thermoplastic compounds formed by polymerization or copolymerization of vinyl or vinylidene chlorides and vinyl esters. Polyvinilidene Chloride (PVdC)—A thermoplastic material produced by the polymerization of vinylindene chloride. It is optically clear with a high degree of gloss and has outstanding oxygen and moisture barrier properties. Porosity—The ratio of the volume of air or void contained within a material to the total volume (solid material plus air or void), expressed as a percentage. Port Lines—Weld lines in film from where two flows join together in an extrusion die, typically in blown film dies. Pressure Roll—In extrusion coating, a roll used to apply pressure to consolidate the substrate and the plastic film with which it has been coated. Pressure Transducer—An instrument used to measure the pressure at a desired point inside a pressurized system. Proportional Limit—The greatest stress which a material is capable of sustaining without deviation from proportionality of stress and strain (Hooke’s Law). It is expressed in force per unit area, usually pounds per square inch. Pseudoplastic Fluid— A pseudoplastic fluid is one whose apparent viscosity or consistency decreases instantaneously with increase in rate of shear i.e., an initial relatively high resistance to stirring decreases abruptly as the rate of stirring is increased. Purging—In extrusion, the cleaning of one color or type of material from the machine by forcing it out with the new color or material to be used in subsequent production, or with another compatible purging material. Pyrolysis—The decomposition of a complex organic substance to one of simpler structure by means of heat in the absence of others. Some polymers will depolymerize in the presence of excessive temperatures,

Glossary of Film Terms

either to polymers of lower molecular weight, or, in some cases, back to the monomers from which they were derived. Pyrometer—An instrument for measuring heat. The type most widely used in plastics processing equipment consists of a thermocouple and a millivoltmeter for measuring the voltage which is proportional to the temperature of the junction. Quench—A process of shock cooling thermoplastic materials from the molten state. Radical—A group of atoms existing in a molecule which is capable of remaining unchanged through many chemical reactions. Random Copolymer—Acopolymer consisting of alternating segments of two monomeric units of random distribution, including single molecules. A random copolymer usually results from the copolymerization of two monomers in the presence of a free radical initiator. Regrind—Waste material such as edge trim, and rejected film, which has been reclaimed by shredding or granulating. Regrind is usually mixed with virgin compound at a predetermined percentage for reprocessing. Residual Monomer—The unpolymerized monomer that remains incorporated in a polymer after the polymerization reaction is completed. Resin—An organic substance of natural or synthetic origin characterized by being polymeric in nature. Most resins, though not all, are of high molecular weight and consist of long chain or network molecular structure. Usually resins are more soluble in their lower molecular weight forms. Rheology—The study of the behavior of materials as they are deformed. Rheometer—An instrument for determining the flow behavior of a thermoplastic material, usually of high viscosity or in the molten condition. Ribbon Blenders—Mixing devices comprising helical ribbon shaped blades rotating close to the edge of a U shaped vessel. They are used for relatively high viscosity fluids and dry blends such as PVC calendering and extrusion compounds. Roll Mill—An apparatus for mixing a plastic material with compounding ingredients, comprising two rolls placed in close proximity to one another. The rolls turn at different speeds to produce a shearing action to the materials being compounded. Saran®—Trade name (Dow Chemical Co.) for thermoplastics consisting of polymers of vinylidene chloride or copolymers of same with lesser amounts of other unsaturated compounds. Scrap—All products of a processing operation which are not suitable for further use in a finished good or as part of a recycle stream for reprocessing. Screen—Woven metal screens are installed across the flow of plastic in an extruder. They are located between the tip of the screw and the die. Supported by a breaker

509

plate, the screens strain out contaminants and increase back pressure. Screen Changer—A method & mechanism for changing the breaker plate melt filter screens without removing the die from the extruder. Screw—In extrusion, the shaft provided with helical grooves which conveys the material from the hopper outlet through the barrel and forces it out through the die. Screw Flight—The helical metal thread of a screw in an extruder machine. Screw Speed—The rate of revolution (in rpm) of an extruder machine screw. Seal Initiation Temperature—It is defined as the heat sealing jaw temperature at which a specific level of seal strength is obtained and may be determined by plotting the seal strength versus seal jaw temperature. Sharkskin—A surface irregularity of a film during extrusion. See Melt Fracture. Shear—Shear is the product of shear rate and resistance time. It is often used to describe the degree of mixing experienced by a material. Shear Heating—Heat generated within the plastic melt as the polymer is sheared. It is caused by viscous dissipation of work. Shear Rate—Rate of change of velocity across the flow channel. Shear Stress—Stress developed in a polymer melt where a material is sheared. Sheeting—Sheets are distinguished from films in the plastics and packaging industry only according to the thickness. A web under 250 microns or 10 mils (0.010 inch) thick is usually called a film, whereas a web 250 microns or 10 mils and over in thickness is usually called a sheet. Sheeting is most commonly made by extrusion, casting, and calendering. Shelf Life—The length of time over which a product will remain fit for use during storage under specific conditions. Shrink Film—A term sometimes used for pre-stretched or oriented film used in shrink packaging. Shrink Packaging—An item or group of items packaged by wrapping in a film or bag which, when heated, fits tightly around the contained article. Silica—Naturally occurring silica occurs in deposits which are 99 percent silicon dioxide. The hardness provides both mechanical strength and abrasion resistance. Silicas are an economical extender filler which is thermally stable, pure, low in ionic impurities, and hard. They are often used as anti-blocking agents in polyolefins. Silk Screen Printing—A printing process widely used on plastic bottles and other articles which employs a taut woven fabric secured in a frame as a stencil. The fabric is coated in selected areas with a masking material which is not affected by the ink being used. The stencil fabric is commonly called a silk screen. Skin Packaging—A variation of the thermoforming process in which the article to be packaged serves as the mold.

510

Glossary of Film Terms

The article is usually placed on a printed card prepared with an adhesive coating or mechanical surface treatment to seal the plastic film to the card. Slip Agent—Provides surface lubrication following the processing of plastics. Compounded into the plastic, the additive gradually migrates to the surface where it reduces the coefficient of friction. Slitting—The conversion of a given width of plastic film, tube, or sheeting to several various widths by means of knives. Solenoid Valve—Magnetically operated needle valves used in water cooled extruders to control the cooling rate. Specific Gravity—The ratio of the weight of a given volume of a substance to that of an equal volume of water at the same temperature. The temperature selected varies among industries, 15°C (60°F) being the usual standard. Specific Heat—The amount of heat required to raise a specified mass by one unit of a specified temperature, usually expressed as Btu/Ib/°F. or cal/g/°C. Spherulites—A rounded aggregate of radiating crystal lamella. Spherulites are present in most semi-crystalline plastics. They originate from a nucleus such as a particle of contaminant, catalyst residue, or a chance fluctuation in density. They may grow through stages: first needles, then bundles and sheaflike aggregates, and finally the spherulites. Spherulites may range in diameter from a few tenths of a micron to several millimeters. Spider—In extrusion, a term used to denote the arms supporting a mandrel within the head and die assembly. Spider Lines—In film extrusion, vertical marks on the film caused by improper welding of several melt flow fronts formed by the legs with which the die is fixed in the extruder head. Spreader Roll—Used to remove film wrinkles and maintain a flat film during processing. Bowed rolls and helically contoured rolls may be used. Stabilizer—Ingredient used in the formulation of some polymers to assist in maintaining the physical and chemical properties of the compounded materials, for example, heat and UV stabilizers. Static Eliminators—Devices for removing electrical static charges from plastics articles. Types of static eliminators include static bars, ionizing blowers, and air guns. Stearamide—A slip additive used in polyolefins. Stiffness—The capacity of a material to resist strain where stressed. Strain—In tensile testing, the ratio of the elongation to the gauge length of the test specimen, that is, the change in length per unit of original length. The term is also used in a broader sense to denote a dimensionless number that characterizes the change in dimensions of an object during a deformation or flow process. Stress—The force producing or tending to produce deformation divided by the area over which the force is applied. As generally defined in tensile testing (engi-

neering stress), stress is the ratio of applied load to the original cross sectional area. True stress (instantaneous stress) is applied load per instantaneous cross sectional area. Stress Concentration—The amplification of the level of an applied stress in the region of a notch, void, or inclusion. Styrene Acrylonitrile Copolymer (SAN)—Copolymers of about 70 percent styrene and 30 percent acrylonitrile, with higher strength, rigidity, and chemical resistance than straight polystyrene. They may be blended with butadiene, either as terpolymer or by grafting onto the butadiene, to make ABS resins. Surface Tension—A fluid in contact with a surface exhibits phenomena, due to molecular attractions, which appears to arise from a tension in the surface of the fluid. It may be expressed as dynes per centimeter or as ergs per square centimeter. Surfactant—A compound that alters surface tension of a liquid in which it is dissolved to improve contact area between two materials. Surging—In extrusion, an unstable pressure buildup leading to variable output and waviness of the surface of the extrudate. In extreme cases, the flow of extrudate may even cease momentarily at intervals. Synergism—A phenomenon wherein the effect of a combination of two additives is greater than the effect that could be expected from the known performance of each additive used singly. Tack—The stickiness of an adhesive, measurable as the force required to separate an adherent from it by viscous or plastic flow of the adhesive. Tackifier—A substance such as a resin ester which is added to synthetic resins or elastomeric adhesives to improve the initial and extended tackiness of the film. Take Off—The mechanism for drawing extruded or calendered material away from the extruder or calender. Talc—A natural hydrous magnesium silicate, used frequently as a filler such as steatite, talcum, or mineral graphite. Tape Test—A Method for evaluating the adhesion of a coating to a substrate. The adhesive tape is adhered to the coating with a uniform force, and then pulled away. The degree (percentage) to which the coating peels away from the substrate with the tape indicates its adhesion. Teflon®—DuPont’s trademark covering all of its fluorocarbon resins, including PTFE, FEP, and various copolymers. Tear Resistance—Resistance of a material to a force acting to initiate and then propagate a failure at the edge of a test specimen. Tear Propagation Resistance—Resistance of a material to a force acting to propagate an initiated tear in the material. Tensile Strength—The maximum tensile stress sustained by the specimen before failure in a tension test. Usually

Glossary of Film Terms

expressed in pounds per square inch or megapascals. The cross sectional area used is that of the original specimen, not at the point of rupture. Terpolymer—The product of copolymerization of three different monomers, or of the grafting of one monomer to the copolymer of two different monomers. An example of a terpolymer is ABS resin, derived from acrylonitrile, butadiene, and styrene. Thermal Conductivity—The rate at which heat is transferred by conduction through a unit cross sectional area of a material when a temperature gradient exists perpendicular to the area. Thermal Decomposition—Decomposition resulting from action by heat. It occurs at a temperature at which some components of the material are separating or reacting together, with a modification of the macro or micro structure. Thermal Stability—Ability of a polymer to maintain its initial physical and chemical properties at elevated temperature. Thermocouple—A device which uses a circuit of two wires of dissimilar metals or alloys, the two junctions of which are at different temperatures. A net electromotive force (emf) occurs as a result of differences in conductivity. The minute electromotive force, or current, is sufficient to drive a galvanometer or potentiometer. Thermoforming—The process of forming a thermoplastic sheet into a three dimensional shape by clamping the sheet in a frame, heating it to render it soft, then applying differential pressure to make the sheet conform to the shape of a mold or die positioned below the frame. Thermogravimetric Analysis (TGA)— The measurement of changes in weight of a specimen as it is heated. Some tests are conducted in air and some in other atmospheres. The resulting data reveals information about the materials thermal stability and polymerization processes. Thermoplastics—Resins or plastic compounds which, in their final state as finished articles, are capable of being repeatedly softened by an increase of temperature and hardened by a decrease of temperature. Thermosets—Resins or plastic compounds which in their final state as finished articles are substantially infusible and insoluble. Thermosetting resins are often liquids at some stage in their manufacture or processing, and are cured by heat, catalysis, or other chemical means. After being fully cured, thermosets cannot be re-softened by heating. Thickness Gauges—Instruments used to measure the film thickness online. Methods used may be Capacitance, Infrared and Nuclear (Beta Gauge). Thinning—This refers to the finished wall of a blow molded container or the comers of a thermoformed part. The wall thickness has thinned out in some areas due to improper blowing or excessive stretching.

511

Tie Layer—Chemically-modified polyolefins used as extrudable adhesives in coextrusion or coating operations to bond two dissimilar materials together that otherwise would have poor adhesion to each other. Titanium Dioxide—A white powder available in two crystalline forms, the anatase and rutile types. Both are widely used as opacifying pigments in thermosets and thermoplastics. i.e.—DuPont’s TiPure®. Toxicity—The degree to which a substance is poisonous. Although most pure resins and polymers are relatively nontoxic, compounding additives such as stabilizers, colorants, and plasticizers must be carefully selected when products are to be used for food packaging or other applications involving body contact. Transfer Piping—Adapter piping used to convey the melt from the extruder to the die. Transition Section—In an extruder, the section of the screw that contains material in both the solid and molten state. Transition Temperature—The temperature at which a polymer changes from (or to) a viscous or rubbery condition to (or from) a hard and relatively brittle one. Treater—Term to refer to equipment and process used to render a surface of inert plastics, such as polyethylene more receptive to inks, adhesives, or coatings. Turning Bar—A stationary diagonal 45° bar used in oscillating nips to redirect the film to move horizontally around several rotating idler rolls. The film is then directed to a second diagonal 45° turning bar, which changes the film direction to vertical. The turning bars have a series of small holes at the film contact area to provide an air cushion between the bar and the film. Ultimate Strength—The maximum stress developed in a tensile compression specimen. Ultra Low Density (Linear) Polyethylene—This term is generally considered to include polyethylenes ranging in density from about 0.885–0.900. In ultra-low density polyethylenes, the ethylene monomeric units are linked in random fashion, with the main chains having some short side branches. This branching prevents the formation of a closely knit pattern, resulting in material that is relatively soft, flexible and tough. Ultraviolet (UV) Stabilizer—Chemical agents which absorb or screen out radiation beyond the violet end of the spectrum of electromagnetic radiation. Such radiation has sufficient energy to initiate reactions leading to the degradation of many plastics. These agents are often combined with other additives, e.g. heat stabilizers and antioxidants, with which they act in synergistic fashion. UV stabilizers can be UV absorbers or radical scavengers. Vacuum Metallizing—A process that allows you to create a layer of metal on a substrate, usually of another material. Van Der Waals Forces—Forces that exist between molecules of a substance after all of the primary valences within covalent molecules are saturated. Also called secondary valence forces or intermolecular forces.

512

Glossary of Film Terms

Vapor—As most frequently used, the term vapor describes a substance which, although present in the gaseous phase, generally exists as a liquid or solid at room temperature. Vapor Barrier—A layer of material through which water vapor will not pass readily or at all. Vent—A small hole or shallow channel in an extruder which allows air or gas to exit at a pressure let down region in an extruder. Vibratory Feeders—Devices for conveying dry materials from storage hoppers to processing machines, comprising a tray or tube vibrated by mechanical or electrical pulses. The frequency or amplitude of the vibrations control the rate of flow. Vicat Softening Point—The temperature at which a flat ended needle of I square millimeter circular or square cross section will penetrate a thermoplastic specimen to a depth of 1 mm under a specified load using a uniform rate of temperature rise. (ASTM D 1525 58T). Vinyl Acetate—A colorless liquid obtained by the reaction of ethylene and acetic anhydride in the presence of a catalyst. It is the monomer for polyvinyl acetate, and a co-monomer and intermediate for many members of the vinyl plastics family. Virgin Material—Any plastic compound or resin that has not been subjected to use or processing other than that required for its original manufacture. Viscoelasticity—The tendency of plastics to respond to stress as if they were a combination of elastic solids and viscous fluids. This property, possessed by all plastics to some degree, dictates that while plastics have solid like characteristics such as elasticity, strength, and form stability, they also have liquid like characteristics such as flow depending upon time, temperature, rate, and amount of loading. Viscosity—The measure of the resistance of a fluid to flow (either through a specific orifice or in a rotational viscometer). The absolute unit of viscosity measurement is the poise (or centipoise). Often expressed as kilopascal (kPa). Viscous Flow—A type of fluid movement in which all particles of the fluid flow in a straight line parallel to the axis of a containing pipe or channel, with little or no mixing or turbidity. Void—(1) An unfilled space in a cellular plastic substantially larger than the characteristic individual cells. (2) An empty space in any material or medium. Volatile—Material capable of being driven off as a vapor at room or slightly elevated temperatures. Volatile Content—The percent of volatiles which are driven off as vapor from a plastic or impregnated reinforcement. Volatile Loss—The loss in weight of a substance caused by vaporization of a constituent. Warp—Dimensional distortion in a plastic object after film forming due to the release of molded in stresses. Water Vapor Transmission—The amount of water vapor

passing through a given area and thickness of a plastic sheet or film in a given time, when the sheet or film is maintained at a constant temperature and when its faces are exposed to certain different relative humidities. The result is usually expressed as grams per 24 hours per square meter. Weatherometer (WOM)—An instrument which is used to subject articles to accelerated weathering conditions, e.g., rich UV source and water spray. Web—(1) A thin sheet in process in a machine. In extrusion coating, the molten web is that which issues from the die, and the substrate web is the material being coated. (2) A continuous length of sheet material handled in roll form as contrasted with the same material cut into sheets. Weight Average Molecular Weight (MWavg) —The first moment of a plot of the weight of the polymer in each molecular weight range against molecular weight. The value of MW can be estimated by light scattering or sedimentation equilibrium measurements. Weld Line—A flaw on a blown or cast film caused by the incomplete fusion of two flow fronts which meet during the extrusion operation. Wrinkle—A surface imperfection in plastic films that has the appearance of a crease or wrinkle. Yellowness Index—A measure of the tendency of plastics to turn yellow upon long term exposure to light or heat. Yield—It is the measure of a film’s coverage per unit weight. Values are expressed as in2/1b in US standard and m2/kg in metric (or SI) units. Yield Point—In tensile testing, yield point is the first point on the stress strain curve at which an increase in strain occurs without an increase in stress. This is the point at which permanent deformation of the stressed specimen begins to take place. Yield Strength—The stress at the yield point of a specimen, usually expressed in pounds per square inch or megapascals. Ziegler Catalysts—A large group of catalysts made by reacting a compound of a transition metal chosen from groups IV through VIII of the periodic table with an alkyl, hydride, or other compound of a metal alkyl, hydride, or other compound of a metal from groups I through III. A typical example is the reaction product of an aluminum alkyl with titanium tetrachloride or titanium trichloride. These catalysts were first developed by the German scientist Karl Ziegler for the polymerization of ethylene. Subsequent work by G. Natta, showed that these and similar catalysts are useful for preparing stereoregular polyolefins. Thus, the family of catalysts is sometimes called Ziegler Natta catalysts. Zinc Oxide—An amorphous white or yellowish powder, used as a pigment in plastics. It is said to have the greatest ultra-violet light absorbing power of all commercially available pigments. Zinc Stearate—A white powder used as a lubricant and antioxidant synergist.

Glossary of Film Terms

ABBREVIATIONS A­—Angstrom Unit AO—Antioxidant ABS—Acrylonitrile butadiene styrene AIMCAL—Association of International Metallizers, Coaters and Laminators ASTM—American Society for Testing and Materials ATIR (ATR)—Attenuated Total Reflectance of Infrared (Spectrometry) BSI—British Standards Institute BTU—British Thermal Unit °C—Degrees Celsius CA—Cellulose Acetate CN—Cellulose Nitrate cp—Centipoise cm—Centimeter COC—Cyclic Olefin Copolymer COF—Coefficient of Friction CPP – Certified Packaging Professional dm—Decimeter DE—Diatomaceous Earth DTA—Differential Thermal Analysis DSC—Differential Scanning Calorimeter EC—Ethyl Cellulose EP—Epoxy Resins EPOM—Ethylene propylene diene Terpolymer EPR—Ethylene propylene Rubber ESCR—Environmental Stress Cracking Resistance EVA—Ethylene Vinyl Acetate °F—Degrees Fahrenheit fl oz—Fluid Ounce ft—Foot FDA—Food and Drug Administration FEP—Fluorinated Ethylene Propylene FI—Flow Index FPA—Flexible Packaging Association FTIR—Fourier Transform Infrared (Spectrometry) FR—Flow Rate FRP—Fiber Reinforced Plastics g—Gram g/cc—Grams per Cubic Centimeter gal—Gallon GC—Gas Chromatograph GPC—Gel Permeation Chromatograph GRP—Glass Reinforced Plastics HALS—Hindered Amine Light Stabilizer hp—Horsepower HDPE—High Density Polyethylene HLMI—High Load Melt Index

513

HPB—Health Protection Branch HT 1—Nylon Resin HMW HDPE—High Molecular Weight High Density Polyethylene in—Inch in H2O—Inches of Water IOPP – Institute of Packaging Professionals IR—Infrared (Spectrometry) IUPAC—International Union of Pure and Applied Chemistry J/mm—Joules per Millimeter kg—Kilogram kgf—Kilogram of Force kg/cm2—Kilograms per Square Centimeter km—Kilometer kPa—Kilopascal kph—Kilometers per Hour kJ—Kilojoule kW—kilowatt kT—Kilotonne l—Liter lb—Pound lbf—Pound Force Lb/in2—Pounds per Square Inch lb/ft2—Pounds per Square Foot lb/ft3—Pounds per Cubic Foot LC—Liquid Chromatography LDPE—Low Density Polyethylene LLDPE—Linear Low Density Polyethylene m—Meter mil—0.001 Inches ml—Milliliter mm—Millimeter mPE—metallocene Polyethylene mpg—Miles per gallon mph—Miles per hour MAN—Methyacrylonitrile MB—Masterbatch MEK—Methyl Ethyl Ketone MEKP—Methyl Ethyl Ketone Peroxide MI—Melt Index MFR—Melt Flow Rate MH—Mega Hertz MPa—Megapascal MWn—Number Average Molecular Weight MWavg—Weight Average Molecular Weight MW—Molecular Weight MWD—Molecular Weight Distribution MVT—Moisture Vapor Transmission NBR—Acrylonitrile butadiene Copolymer NBS—National Bureau of Standards NMR—Nuclear Magnetic Resonance

514 NTC—Novacor Technical Center NVCR—Novacor Chemicals Ltd./Inc OD – Optical denstiy OIT—Oxidative Induction Time OTR – Oxygen Transmission Rate oz—Ounce pH #—Expresses Acidity or Alkalinity pt—Pint Pa—Pascal PAN—Polyacrylonitrile PB—Polybutene. PC—Polycarbonate. PCTFE—Polychlorotrifluoroethylene PE—Polyethylene PET—Polyethylene Terephthalate PHR—Parts per Hundred PIA—Plastics Institute of America PIB—Polyisobutylene PMAN—Polymethacrylonitrile PP—Polypropylene PTFE—Polytetrafluoroethylene PS—Polystyrene PVA—Polyvinyl Alcohol PVAC—Polyvinyl Acetate PVC—Polyvinyl Chloride

Glossary of Film Terms

PVdC—Polyvinylidene chloride PVOH—Polyvinyl Alcohol qt—Quart sec—Second SAN—Styrene Acrylonitrile Copolymer SBR—Styrene Butadiene Rubber SIT – Seal initiation temperature SPE—Society of Plastics Engineers SPI—Society of Plastics Institute t—Tonne TAPPI—Technical Association of Pulp and Papermaking Industry Tg—Glass Transition Temperature TFE—Tetrafluoroethylene TGA—Thermogravimetric Analysis THF—Tetrahydrofuran TPP—Triphenyl Phosphate TEP—Triethyl Phosphate TNPP—Tris Nonylphenyl Phosphite UV—Ultraviolet UHMW HDPE—Ultra High Molecular Weight High Density Polyethylene WVT—Water Vapor Transmission

Conversion Factors for SI (Metric) Units

Conversion Factors for SI (Metric) Units Quantity or Test

Value in Trade or Customary Unit

Breaking Length Burst Index Bursting Strength Caliper Comora Crush Edge Crush Energy Flat Crush Force Length

Mass

Mass per Unit Volume Puncture Resistance Ring Crush Stiffness (Tabor) Tear Strength Tensile Breaking Load Volume, Fluid Volume, Solid

Conversion Factor

=

Value in Si Unit

Symbol

square inches

6.45

square centimeters

cm2

square feet square yards acres

0.0929 0.836 0.405

square meter square meter hectares

m2 m2 ha

lb (17 x 22,500) lb (24 x 36,500) lb (2508 - 500) lb (25 x 40,500) pounds per 1000 sq ft (Paperboard) meters g/cm2 pounds per square inch mils pounds pounds per Inch British thermal units (Btu.) pounds per square inch kilograms pounds angstroms microns mils feet tons (2000 lbs) pounds ounces (avdp) ounces per gallon pounds per cubic foot foot pounds pounds (for a 6 in. length) gram centimeters (Taber Units) grams pounds per inch kilograms per 15 millimeters ounces (US Fluid) gallons cubic inches cubic feet cubic yards

3.760 1.627 1.480 1.406 4.882

grams per square meter grams per square meter grams per square meter grams per square meter grams per square meter

g/m2 g/m2 g/m2 g/m2 g/m2

0.001 0.0981 6.89 0.0254 4.45 0.175 1055 6.89 9.81 4.45 0.1 1.0 0.0254 0.305 0.907 0.454 28.3 7.49 1.60 1.36 0.0292 0.0981 9.81 0.175 0.654 29.6 3.79 16.4 0.0283 0.765

kilometers kPa kilopascals millimeters newtons kilonewtons per meter joules kilopascals newtons newtons nanometers micrometers millimeters meters metric tons kilograms grams kgs per cubic meter kgs per cubic motor joules kilonewtons per motor millinewton meters millinewtons kilonewtons per meter Kilonewtons per meter milliliters liters cubic centimeters cubic meters cubic meters

km

Area

Basis Weight * or Substance (500 sheet ream) or Grammage* when expressed in g/m2

×

kPa mm kN/m J kPa N N nm um mm m t kg 9 kg/m3 kg/m3 J kN mN m mN kN/m kN/m mL L cm3 m3 m3

*See TAPPI Technical Information Sheet 0800 01.

515

Index

abrasion resistance, 239, 284, 287, 468, 475 acid copolymer, 271, 283, 285–287, 445, 447, 449, 451, 476, 493 acid scavenger, 303 acrylate, 188, 265, 277, 278, 280, 281, 283, 286, 289, 445, 451, 493, 494 adapter, 13, 119, 128, 133, 137, 327, 329, 330, 336, 337, 403, 404, 408, 419, 447, 448, 449, 454 additive, 94, 122, 124, 125, 128, 131, 167, 169, 195, 232, 246, 248, 291, 292, 294, 295, 297, 300, 301, 303, 305, 307, 308, 310–317, 319–321, 323, 332, 339, 401, 402, 408, 419, 420, 421, 433, 467, 470 adhesion, 38, 157, 159, 160, 163, 167, 173, 174, 176, 178, 179, 181, 182, 218, 227, 265, 269, 270–275, 277, 278, 283–287, 301, 304, 313, 319, 322, 384, 387, 392, 424, 445, 449, 451, 462, 475–477, 481, 486, 491, 496 adhesive, 77, 157, 182, 247, 256, 265, 269, 271, 272, 275, 349, 383–389, 392, 398, 424, 462, 473–475, 477, 481, 485, 491–497 air entrapment, 73, 418 air gap, 139, 157, 163, 164, 167, 174, 227, 237, 270, 271, 341, 343, 378, 402, 425, 477 air knife, 237, 341, 343, 401, 424, 425 air pinning, 341–345, 398, 400, 401, 425 aluminum foil, 66 amide, 174, 239, 240, 269, 313, 319–323 amorphous PET, see also APET, 260, 261 anhydride, 244, 269–272, 337, 449, 451, 452, 475, 477, 494 anti-block, 187, 188, 194, 196, 220, 328, 332, 401, 402, 420, 462 antifog, 270 antioxidants, 187, 196, 220, 237, 270, 273, 291, 299, 301, 303, 305–307, 312, 314, 316, 317, 329, 445 antistatic agents, 307 APC, see also auto profile control, 143, 332, 339, 410–412 APET, see also amorphous PET, 260, 261 atmospheric pressure plasma, 174 autoclave, 187, 189, 192, 215, 250, 273, 277 automatic profile control, see also APC, 332 auto-oxidation, 306, 311, 313 backscatter, 139, 140, 332, 413, 433, 435 barrel, 7–11, 13, 15, 16, 18, 19, 22–24, 29–36, 38–42, 86, 243, 288, 311, 329, 330, 336, 337, 347, 348, 355, 366, 375–377, 399–401, 403, 406–408, 415, 424, 425, 437–440, 442, 445, 454, 455

barrier, 13–16, 19, 23, 25, 26, 29, 34, 40, 41, 50, 51, 55, 103, 113, 151, 175, 176, 178, 180, 182, 190, 195–197, 206, 218, 223, 225, 227, 228, 231, 236, 237, 239, 242, 244, 246–251, 253–257, 259–266, 269, 270, 272, 275–278, 280, 283, 289, 322, 327, 329–331, 336, 337, 344, 347, 349, 373, 383, 391, 393–395, 405, 419, 420, 436–440, 443, 460, 463, 471, 473–476, 480, 481, 483–487, 489, 490, 497 biaxially oriented polyester films, see also BOPET or OPET, 259 biodegradable, 3, 259, 266, 310 biodegradable polymers, 3, 310 bio-PET, 259 Bi-orientation of Nylon, see also BON and BOPA, 246 blending, 122 bond strength, 183, 270, 272, 384, 392, 469, 470, 479, 481, 483 BOPA, see also bi-orientation of nylon, 239, 244, 246, 247, 262, 492 BOPET, see also biaxially oriented polyester films or OPET, 259, 263, 492 branched polymer, 447, 461 bridging, 7, 15, 31, 219, 220, 276, 279, 288, 337, 408 capacitance, 139, 142, 332, 413, 459, 460 capillary rheometer, 86, 202, 295, 301, 364, 377, 379 carbon black, 299, 304, 307–309, 316, 431–433 cast film, 3, 39, 68, 105, 166, 168, 202, 204, 209, 215, 219, 231, 234, 236, 237, 259, 264, 270, 271, 273, 277, 278, 280, 341, 344, 345, 347, 351, 366, 378, 415, 448, 451, 452, 454, 465, 473, 491, 496 casting unit, 341, 344, 345 catalyst, 194, 199, 200, 202, 207, 208, 212, 215, 223, 225, 229, 235, 273, 303, 307, 313, 429, 487 chain initiation, 306 chain termination, 306 chemical primers, 475 chemical resistance, 190, 225, 287, 383, 475, 477, 481, 486, 488 chill roll, 151, 168, 237, 246, 264, 270, 341–345, 378, 391, 400, 401, 425, 477 clarity, 151, 187, 194–196, 228, 270, 272–275, 278, 287, 305, 327, 337, 341, 344, 345, 398, 403, 419, 420, 459, 465, 466, 467, 475 cling, 110, 197, 215, 218, 280, 308, 316, 459, 460 coating weight, 483

517

518 COC, see also cyclic olefin copolymer, 108, 113, 155, 206, 327, 334, 405, 414, 476, 494 coextrusion, 27, 43, 51, 52, 55, 61, 97, 156, 170, 256, 265, 269, 272, 279, 323, 340, 349, 372, 373, 381, 400, 423, 424, 456 colorants, 36, 309, 459, 461, 467 composite integrity, 479 compostable, 259, 310 contact angle, 160, 161, 178, 179, 182 control systems, 74 converting, 78, 170, 474 conveying, 23, 26, 29–34, 38, 85, 115, 117, 119–122, 125, 219, 224, 406, 417, 418, 424, 436–438 conveying distance, 120 cooling, 3, 7, 9–11, 14, 15, 18–22, 25, 29–35, 48, 63, 74, 75, 91, 97, 105–110, 145–155, 158, 165, 196, 199, 202–204, 209, 227, 231, 236, 237, 240, 245, 246, 249, 264, 265, 273, 276, 278–280, 320, 327, 331–333, 336, 337, 341, 343– 345, 347, 355, 387, 391, 397, 398, 401–404, 406–408, 410, 412, 415, 418, 420, 421, 425, 427, 430, 431, 455, 459, 465, 466, 468 co-polyester, 259, 265 copolymerization, 215, 239, 240, 241, 245, 248–251, 277, 283 corona treatment, 157, 237, 417 CPET, see also crystallized PET, 260 cross-linking, 195 crystallinity, 113, 151, 182, 187, 188, 190, 194–196, 199, 204, 210, 215, 217, 218, 223, 224, 235, 239, 240, 245–250, 259–263, 272, 273–278, 283, 284, 287, 289, 320, 322, 461, 466, 469, 471, 476 crystallized PET, see also CPET, 260 cyclic olefin copolymer, see also COC, 108, 155, 334, 494 DBD, see also dielectric barrier discharges, 175 dead zones, 448–450 degradation, 39, 40, 55, 56, 87, 119, 159, 174, 226, 237, 242, 244, 264, 265, 270, 278, 289, 291, 298, 299, 301, 310, 311, 313, 367, 394, 419, 427, 428, 431, 435, 436, 438–443, 445, 448–450, 471, 486, 487 density, 13, 23–25, 29, 31–33, 39, 48, 63, 66, 70, 104, 113, 120, 121, 123, 132, 133, 137, 145, 147, 154, 157, 159, 160, 163, 166, 168, 169, 173, 174, 176–179, 181, 182, 187–190, 192–196, 199–203, 206–210, 212, 213, 215, 217–219, 223–225, 228, 233, 234, 256–266, 270, 273–275, 285, 289, 291, 295, 304, 309, 311, 312, 315, 328, 335, 338, 348, 360–368, 371, 384, 392, 393, 401, 406–419, 429, 430, 431, 442, 446, 451, 452, 459–461, 463, 465–471, 476, 486, 487, 492, 493, 495, 496 desiccant drying, 240, 264, 487 design for manufacture, 481 design for use, 480 die, 12–16, 18, 20, 22, 29, 39–41, 45, 47–63, 81, 84, 86, 87, 90, 92–95, 97, 98, 101–107, 111, 113, 143, 145–155, 168, 193, 194, 196, 197, 202–204, 218, 220, 225–227, 231, 234, 236, 237, 240, 242, 245–247, 264, 265, 269–271, 275, 276, 279, 281, 288–301, 310, 312, 313, 317, 327, 329–339, 341–343, 345, 347–356, 366–375, 378, 397– 416, 419–421, 425, 430, 435, 438, 441–443, 445–448, 450, 451, 454, 455, 462, 463, 465–467, 477, 496 die bearding, 298 dielectric barrier discharges, see also DBD, 175 die streaks, 143, 293, 347

Index

differential measurement, 22, 35, 40, 52, 81–83, 97, 107, 109, 112, 123, 235, 274, 277, 293, 333, 344, 386, 414, 428 disco purge, 339, 421, 452 Double Bubble®, 246, 247, 265 draw control, 76 draw ratio, 341, 354 draw resonance, 39, 299, 373, 402, 425 drive systems, 75 drying, 158, 241, 252, 253, 259, 264, 265, 272, 278, 288, 328, 337, 386–388, 390, 401, 403, 406 dyne level, 159, 161, 162, 168, 417, 470 EAA, see also ethylene acrylic acid, 188, 195, 219, 283, 284, 449, 492, 493, 495, 496 EAA, see also ethylene acrylic acid copolymer, 188, 195, 219, 283, 284, 449, 492, 493, 495, 496 EBA, see also ethylene butyl acrylate, 197, 277, 278, 494 edge bead, 70, 237 edge pinning, 341–343, 425 electrodes, 142, 160, 163, 165, 166, 168, 180, 335, 417, 455 EMAA, see also ethylene-methacrylic acid copolymers, 283, 284, 449, 492, 493, 495–497 EMA, see also ethylene methyl acrylate, 188, 197, 265, 277–280, 283, 286, 475–477, 494 encapsulation die, 101, 352–354, 372, 398, 401, 424, 425 end-use, 40, 69, 113, 168, 194, 223, 225, 228, 234, 237, 254, 255, 266, 269, 273, 283, 288, 307, 309, 314, 316, 317, 384, 460, 467, 469, 471, 473, 474 engineering polymer, 239, 251 entanglement, 202, 359, 360, 439, 481 environmental stress crack, see also ESCR, 189 ESCR, see also environmental stress crack, 189, 192 ethylene acrylate, 277, 278, 280, 281, 445, 493 ethylene acrylic acid copolymer, see also EAA, 449 ethylene acrylic acid, see also EAA, 188 ethylene butyl acrylate, see also EBA, 277 ethylene-methacrylic acid copolymers, see also EMAA, 283 ethylene methyl acrylate, see also EMA, 188, 283 ethylene vinyl acetate, see also EVA 188, 217, 283 EVA copolymers, 192, 273, 274, 275, 276 EVA, see also ethylene vinyl acetate, 16, 55, 105, 110, 154, 155, 188, 192, 195, 197, 212, 215, 217, 219, 232–244, 262, 265, 269, 270, 272–276, 278, 283, 286, 319, 322, 323, 329, 334, 336, 337, 449, 451, 474–477, 491, 494–497 EVOH, 25, 40, 55, 113, 141, 151, 195, 244, 248, 250, 253–257, 262–265, 269, 271, 272, 278, 327, 336–340, 373, 393, 394, 405–407, 421, 449, 463, 470, 471, 474, 476, 477, 494, 496, 497 extruder, 7, 8, 18, 27, 30, 35, 43, 104, 129, 189, 190, 204, 279, 280, 347, 367, 397, 401, 406, 407, 415, 425, 442, 444, 452, 455, 456 extruder purging, 450 extrusion, 3, 7, 14–18, 22–25, 29, 30, 33–3639, 42, 48, 49, 55, 69, 70, 72, 81–83, 84, 86, 88, 97, 98, 100, 101, 104, 107, 109, 119, 120, 122, 123, 125–132, 139, 142, 143, 145–147, 149, 152, 157, 159–161, 164, 165, 167, 168, 188, 192, 193, 195, 196, 197, 199, 200–202, 204, 205, 208, 212, 215, 218, 219, 223, 224, 227, 228, 231, 232, 234, 235, 237, 239, 240, 242–249, 252, 257, 259, 264–266, 271, 272, 275–281, 283, 284, 286, 287, 288, 290–294, 296– 299, 301, 303, 307, 309–315, 317, 321, 322, 323, 328,

519

Index

335, 338, 341, 345, 347–352, 354, 356, 360–362, 364, 366–370, 373–375, 378, 379, 399, 403, 404, 405, 406, 408, 410, 412, 416, 417, 420, 421, 427, 430, 431, 438, 440, 442, 443, 445, 447, 448, 450–454, 456, 459, 462, 463, 466, 473–477, 479–483, 486, 489, 491–496 extrusion coating, 34, 143, 157, 160, 165, 168, 212, 215, 219, 231, 234, 239, 259, 264, 266, 286–288, 345, 351, 448, 453, 454, 473–477, 491, 494–496 extrusion curtain, 39, 107, 220, 237, 245, 270, 341–345, 370, 454 extrusion dies, 350, 351, 354, 369 extrusion lamination, 288 failure mode, 485 feedblock, 41, 94, 95, 97–104, 289, 347, 349–353, 356–401, 403, 424, 448 feedscrew, 29 fillers, 14, 36, 92, 194, 299, 304, 310, 330, 428, 429, 431, 433 film, 3, 4, 7, 9–12, 14, 15, 16, 18, 21, 24, 25, 29, 30, 32, 34, 35, 39, 41, 42, 45, 47–50, 52, 53, 55, 57, 58, 63–72, 74–78, 82, 85–87, 91–94, 97, 101, 102, 103, 105–113, 115, 122, 123, 125–130, 139–143, 145–155, 157, 159–170, 177, 182, 189–197, 199, 200, 202–205, 207–210, 212, 215, 218–220, 224–232, 234–237, 239–241, 243–250, 252, 255, 256, 259–262, 264–266, 269–273, 275–280, 283, 285–289, 291, 292, 295, 297–299, 303–305, 307–317, 319–323, 327–339, 341–345, 347, 351, 359, 366–370, 372–374, 378, 380, 383, 386, 391–394, 397–401, 403, 404–421, 423–425, 427–431, 433–438, 440–443, 447, 448, 451–456, 459–463, 465–471, 473–477, 486, 487, 488, 490–497 film substrate, 168, 304 flame retardants, 36, 237, 303, 310, 311, 314 fluoropolymer, 43, 291 foil adhesion, 284, 287 food packages, 239, 249, 250 free radicals, 159, 174, 176, 177, 179, 180, 183, 306, 307, 311–314 function, 13, 20, 24, 30, 33, 36, 50, 52, 63, 65, 70–75, 82, 83, 97, 100, 105, 115, 122–124, 127, 128, 131, 152, 154, 165, 194, 203, 212, 213, 224, 229, 230, 232, 244, 254, 289–291, 304, 313, 314, 317, 322, 338, 341, 351, 360– 362, 366, 368, 369, 376–380, 383, 384, 386, 403, 410, 424–465, 467, 468, 473–476, 479, 480, 485, 486, 489

330, 334, 336, 338, 403, 407, 410, 411, 414, 424, 455 heat seal, 38, 215, 218, 223, 277, 280, 310, 474, 484, 502 heat sealing, 221, 504 heat seals, 263, 483–485 heat-seal strength, 485 high-density polyethylene, 23, 25, 113, 137, 145, 256, 264, 311, 460, 463, 471, 493 hot tack, 40, 215, 223, 232, 233, 283, 287, 469 humidity, 147, 158, 161, 162, 174, 194, 239, 243, 244, 249, 254, 255, 256, 269, 305, 307, 308, 311, 337, 462, 463, 471, 486, 487, 506, 507 hydrodynamic, 7, 223 hygroscopic, 239, 240, 243, 244, 249, 250, 252, 264, 277, 278, 288, 336, 337 induction sealing, 16 inertial compensation, 368 infrared absorption, 139 infrared light wavelength spectrum, 140 interfacial instability, 102, 248, 374 internal deckles, 451 intrinsic viscosity, 202, 259, 261 ionomer, 154, 212, 271, 283, 285–288, 447, 451, 476, 492, 495–497

gamma backscatter, 139, 140 gas-tight sealing material, 218 gels, 3, 16, 39, 40, 219, 228, 257, 288, 291, 293, 295, 298, 299, 301, 313, 315, 338, 347, 348, 398, 406, 419–421, 423, 427–431, 434, 437–443, 445, 448–451, 467 gloss, 40, 187, 194–196, 204, 236, 237, 248, 249, 275, 287, 297, 304, 316, 331, 337, 405, 420, 421, 466, 467, 470, 475, 491, 508 glycol modified copolyester, see also PETG, 259 gravimetric blending, 129–131 gravure cylinder, 384–387, 494, 504 grease resistance, 40

laminar flow, 92–94, 97, 368, 372 laminate, 66, 275, 383, 468, 473, 475, 495, 496, 502 layer interfacing, 97, 99 layer multiplication, 102 layer sequencing, 97–99 layer structure, 55, 256, 353, 496 LDPE, see also low-density polyethylene, 14–17, 26, 35, 40, 48, 101, 105, 108, 109, 112, 121, 145, 146, 170, 187–197, 199–205, 208, 209, 212, 219, 220, 224, 225, 228, 230, 232–234, 244, 255, 262, 271–280, 283, 284, 287, 288, 291, 297, 304, 312, 315, 317, 321, 322, 327, 329, 331, 334, 336, 337, 338, 339, 361, 362, 363, 365, 366, 370, 371–374, 378–381, 414, 441, 442, 446, 447, 451, 452, 461, 467, 474–477, 492, 493, 495–497, 513 lidding, 234 linear low-density polyethylene, see also LLDPE, 24, 120, 145, 147, 285, 291, 309, 311, 446, 493 LLDPE, see also linear low-density polyethylene, 11, 16, 26, 27, 35, 40, 41, 43, 48, 55, 105, 108, 109, 112, 120, 145–148, 153, 154, 156, 189, 192, 194–197, 199–210, 212, 214, 215, 217, 219, 220, 225, 227, 228, 233, 244, 262, 270, 273–278, 285, 291, 292, 295, 297, 299, 304, 312, 315, 317, 321, 322, 327, 329, 331, 332, 334, 336–339, 363, 379, 405, 414, 441, 442, 446, 447, 451, 452, 467, 470, 474–476, 491–497, 504, 505, 513 low-density, 13, 24, 31, 120, 121, 145, 147, 159, 174, 179, 187, 199, 202, 207, 208, 215, 273, 285, 291, 304, 309, 311, 312, 362, 446, 451, 452, 470, 492, 493, 495 low-density polyethylene, see also LDPE, 13, 24, 31, 120, 121, 145, 147, 199, 202, 207, 215, 273, 285, 291, 304, 309, 311, 312, 362, 446, 451, 452, 492, 493, 495

haze, 40, 187, 194–196, 204, 235–237, 247, 248, 275, 278, 284, 285, 287, 304, 305, 331, 337, 341, 353, 405, 419–421, 459, 466, 475, 504 heaters, 7, 9, 10, 18, 19, 22, 48, 53, 84, 113, 143, 149, 265, 305,

macromolecules, 359, 360 maintenance planning, 16–18, 20–22, 55, 61, 74, 75, 86, 87, 109, 115, 120, 133, 294, 312, 313, 338, 343, 404, 406, 407, 408, 413, 416–418, 425, 442, 455, 456, 501

520 manifold systems, 10, 18, 29, 53, 97, 98, 147, 289, 351–354, 369, 370, 401, 442, 499 mechanical properties, 48, 105, 147, 208, 209, 212, 225, 229, 239, 243, 245–247, 249–251, 253–255, 260, 263, 289, 297, 310, 336, 391, 395, 507 melt behavior, 7, 10, 13–16, 18, 23–26, 29–36, 39–42, 45, 47, 48, 53, 56, 58, 60, 81, 82, 84–86, 88–95, 97–99, 102– 105, 107, 111, 113, 145–155, 187, 189–193, 195–197, 199, 200, 202–205, 207, 208, 210, 212, 215, 217–220, 223–228, 234–237, 242, 245, 253, 260, 261, 263–266, 270, 272, 273, 275–281, 283–285, 287–289, 291, 292, 294, 295, 297, 299, 301, 304–308, 310–317, 320, 327– 332, 336–339, 341–345, 347–349, 350, 352, 354–356, 359–362, 366, 367, 369–374, 377–380, 387, 389, 391, 398–405, 407–409, 412, 415, 419, 420, 421, 424, 425, 427, 429, 431, 432, 436–441, 447, 448, 454, 455, 461, 462, 465, 467, 476, 481, 494, 499, 500, 502–506, 508–511 melt flow index, 187 melt flow rate, 104, 215, 219, 235, 253, 261, 283, 287, 315, 328, 447, 465, 505 melt flow ratio, see also MFR, 189, 190 melt fracture, 24, 48, 204, 220, 291, 292, 294, 297, 301, 312, 313, 338, 341, 354, 372, 373, 420, 462, 467 melt index, 104, 145, 187, 189, 191–193, 195, 196, 197, 200, 202, 203, 205, 208, 210, 212, 215, 217, 219, 224, 261, 273, 279, 280, 328, 374, 419, 441, 447, 476, 481, 504, 508 melt quality, 23, 30, 39, 41, 97, 347, 348, 356 melt stability, 204 melt strength measurement, 379 melt temperature, 13, 16, 23–25, 35, 36, 39, 41, 42, 86, 92, 97, 102, 104, 148, 153–155, 190, 197, 199, 202, 204, 220, 226, 227, 234, 264, 272, 275, 276, 280, 289, 292, 330, 336, 338, 339, 343, 344, 347, 349, 400– 402, 404, 407, 415, 419, 420, 421, 424, 425, 500 metal deactivators, 312 metallized film, 392–394, 477, 507 metallocene, 26, 27, 207, 208, 212, 214, 221, 285, 322, 339, 422 metering, 13–15, 23, 30, 33–35, 41, 122–132, 162, 163, 197, 242, 264, 347, 348, 355, 356, 408, 437, 438, 501–503 MFR, see also melt flow ratio, 189, 190, 224, 230, 232–235, 253, 254, 255, 261, 283–287, 298, 328, 374, 447, 451, 452, 465, 513 modulus, 66, 76, 108, 110, 112, 190, 204, 210, 215, 218, 235–237, 239, 241, 243, 246, 251, 252, 275, 276, 278, 279, 284, 337, 361, 377, 378, 414, 461, 467, 468, 476, 481, 488 moisture, 7, 41, 65, 97, 188, 190, 196, 207, 223, 225, 227, 243, 244, 249–251, 254–257, 259, 261, 264, 265, 270–272, 277, 278, 284, 285, 288, 289, 305, 308, 310, 311, 336, 337, 339, 393, 394, 403, 405, 406, 410, 416, 419, 420, 454, 463, 471, 474, 475, 480, 485–487, 504, 505, 508 moisture absorption, 243, 244, 250, 403, 416, 504 moisture vapor transmission rate, see also MVTR, 40, 187, 278, 284 multilayer structures, 113, 239, 246, 248, 255, 257, 293, 327, 474, 491 MVTR, see also moisture vapor transmission rate 40, 187, 278, 284 neck-in, 202, 237, 273, 284, 343, 425, 468, 476 nip pressure, 417, 418, 420, 470 nonwovens, 175, 479, 481, 482, 483, 488, 489

Index

nylon, 26, 106, 108, 109, 112, 113, 141, 154, 195, 196, 220, 240–243, 249, 255, 256, 269, 274, 276, 285, 393, 434, 449, 451, 452, 471, 475, 476, 492, 494–497, 507 oil resistance, 283–285 online gauging, 139 OPET, see also biaxially oriented polyester films or BOPET, 259, 262, 266, 271, 474–476, 477, 491, 492, 494–497 optical, 113, 179, 182, 190, 194, 196, 204, 205, 212, 215, 217, 218, 229, 232, 236, 239, 246, 247, 261, 266, 284, 285, 304, 309, 327, 328, 331, 337, 341, 392–394, 405, 419, 421, 423, 428, 429, 431, 433–435, 461, 467, 476, 487, 507 organoleptics, 3, 476 oscillatory rheometry, 361, 376 OTR, see also oxygen transmission rate, 40, 187, 196, 219, 249, 254, 255, 262, 266, 278, 393, 462, 463, 475, 514 oxidation, 39, 157, 175, 178, 194, 237, 243, 249, 278, 306, 310, 311–314, 316, 337, 339, 394, 427–430, 449, 463, 477, 493, 499, 506, 507 oxide layer, 164, 165, 175, 181, 301, 303, 309, 310, 435–437, 491, 492, 495 oxygen, 97, 158, 160, 173, 175, 176, 178, 179, 195, 196, 206, 207, 237, 246–251, 253–255, 261, 265, 269, 275, 289, 306, 310, 312–314, 336, 337, 349, 393, 394, 420, 427, 435, 442, 460, 462, 463, 470, 471, 473–477, 483, 484, 485, 486, 487, 490, 502, 507, 508 Oxygen transmission rate, see also OTR, 40, 187, 196, 219, 249, 254, 255, 262, 266, 278, 393, 462, 463, 475, 514 ozone treatment, 164, 165, 166, 175, 417, 454, 456, 470, 492, 493, 497 PA6, 239–242, 244, 245, 246, 247, 248, 249, 250, 251, 252 package, 63, 77, 102, 115, 120, 127, 194–196, 217, 218, 231, 232, 237, 249, 250, 253, 255, 256, 257, 262, 269, 271, 272, 289, 295, 300, 301, 305, 393, 419, 463, 466–469, 473, 474–477, 480, 484–490, 495 packaging, 3, 4, 78, 214, 228, 233, 262, 266, 272, 276, 288, 290, 339, 397, 421, 422, 473, 474, 479–482, 484, 489, 491, 497, 500, 509, 513 paperboard, 66, 78 papermaking, 473 paper properties, 22, 65, 70, 78, 92, 141, 157, 158, 163, 168, 181, 228, 246, 265, 293, 383, 384, 386–388, 419, 434, 466, 467, 469, 475, 477, 482, 488, 489, 491, 493, 495, 496, 508 PA, see also polyamide, 55, 113, 151, 170, 239, 393 PB-1, see also polybutylene-1, 229–234 peel, 161, 178, 182, 229–234, 269, 270–272, 286, 287, 355, 420, 467, 469, 470, 485, 489, 497 peel strength, 269–272 PEN, see also polyethylene naphthalate, 259, 260, 262, 263, 266, 267, 494 PETG, see also glycol modified copolyester, 259, 260, 262–264, 327, 336, 337, 405–407, 410, 414, 421, 494 PET, see also polyethylene terephthalate, 157, 159, 164, 168, 188, 255–267, 269, 275, 278, 280, 330, 337, 355, 373, 386, 392, 393, 395, 475–477, 492–494, 496, 497, 508, 514 pigments, 93, 270, 291, 299, 300, 309, 330, 383, 406, 421, 429, 431, 433, 487, 488, 492, 499, 500, 501, 505, 508, 511, 512 pinholes, 394, 431, 489 plasma analyzer, 157, 159, 160, 170, 174, 175, 180, 181, 183, 392, 394, 493, 507

Index

plasma treating, 157, 159, 160, 170, 174, 175, 180, 181, 183, 392, 394, 493, 507 polyamides, 157, 262, 263, 264, 272, 278, 445 polyamide, see also PA, 55, 113, 151, 170, 239, 393 polybutylene-1, see also PB-1, 229 poly-condensation, 240 polyester, 71, 112, 132, 246, 251, 259–266, 271, 275, 291, 354, 417, 428, 434, 454, 462, 492, 494, 495, 508 polyethylene, 13–15, 23–25, 31, 49, 71, 87, 113, 120, 121, 132, 137, 145, 147, 159, 164, 168, 182, 187, 188–196, 199–202, 204, 206, 207, 212, 215, 217, 223, 224, 228, 229, 236, 253, 256, 257, 259, 264, 265, 267, 269–274, 276–278, 280, 283–286, 291, 293–295, 301, 304, 309– 314, 319, 320, 335, 337, 362, 370, 373, 392–394, 412, 417, 427, 445–447, 449, 451, 452, 455, 460–463, 465, 467, 470, 471, 477, 492–495, 500, 501, 503–508, 511 polyethylene naphthalate, see also PEN, 259 polyethylene terephthalate, see also PET, 157, 159, 164, 168, 188, 255, 256, 259–267, 269, 275, 278, 280, 330, 337, 355, 373, 386, 392, 393, 395, 475, 476, 477, 492–494, 496, 497, 508, 514 polymer extrusion, 29, 450 polymer properties, 190, 194, 196, 203, 224, 274 polymer substrates, 157 polyolefin elastomers, see also POD, 215 polyolefin plastomer, see also POP, 285 polypropylene, 15, 121, 157, 159, 166, 168, 218, 229, 235, 236, 237, 246, 259, 269, 275, 298, 322, 323, 335, 393, 417, 428, 445, 447, 451, 454, 469, 471, 476, 492–496, 499, 506 polyurethane, 164, 181 polyvinylidene chloride, see also PVDC, 262, 265 pressure, 7, 9, 10, 12, 13, 15, 16, 18–20, 22–25, 29, 30, 34–42, 47–49, 51–53, 60, 61, 65–68, 72, 75, 81–95, 103, 115– 122, 124, 132, 133, 145, 146, 150, 155, 158–161, 168, 173–182, 187, 189, 190, 194, 204, 207, 209, 215, 223, 229, 240, 243, 247, 250, 269, 273, 277, 283, 294–298, 301, 304, 311, 330, 332, 336–339, 343, 347–349, 351, 352–360, 366–372, 375–377, 384, 386, 391, 399–401, 404, 407, 408, 410, 412, 415–418, 420, 421, 424, 425, 430, 437, 439, 441, 446, 448, 450–452, 454–456, 463, 470, 474, 480, 485-488, 491, 492, 495, 500, 504, 508–512 primary web, 387 process aid, 26, 192, 291, 420, 462, 504 processing aids, 301 product resistance, 283, 476 profile control, 139, 142, 332, 341, 410, 411 protecting/preserving, 483 purging, 38, 90, 272, 288, 338, 339, 351, 401, 420, 421, 445, 446, 447, 449, 450, 451, 452, 454, 455, 508 PVDC, see also, polyvinylidene chloride, 262, 265, 269, 275, 494 razor slitting, 70 resin blending, 397 resin handling, 120, 121, 269, 329, 406, 421, 425 resin properties, 104 resin transitions, 445, 449, 450 retort, 233, 250, 251 rheology, 40, 45, 86, 100, 208, 209, 219, 235, 236, 260, 261, 266, 283, 361, 363, 366, 368, 370, 372–374, 378–380, 400, 467 roll blocking, 64 roll covering, 72, 164, 165

521 roll defects, 67, 70, 406, 413, 418, 420, 421 rollers, 11, 12, 73, 74, 106, 109–111, 113, 247, 333, 336, 384, 386, 394, 404, 412–414, 416, 417, 420, 454, 501, 502, 506, 507 roll hardness, 63–65, 69, 78 safety, 11, 20, 70, 84, 133, 163–166, 179, 232, 234, 251, 271, 280, 281, 311, 316, 320, 328, 335, 337, 339, 407, 416–418, 445, 450, 452–456, 480, 481, 485, 488 sampling, 70, 132, 139, 467, 483, 489 SCADA software, 397 screw design, 13–16, 25, 29, 30, 32, 34, 35, 38–42, 55, 148, 204, 242, 264, 401, 421, 424, 436, 443 sealant, 103, 207, 210, 212, 217–220, 237, 262, 263, 265, 269, 273, 275, 283, 285–287, 336, 469, 473–476, 485, 494, 495 seal initiation temperature, see also SIT, 3, 200, 210, 212, 218, 229, 232, 233, 277, 285, 476, 481 seal strength, 212, 232, 234, 310, 469, 479, 480, 485, 489, 509 seal through contamination, 287, 476 semi-crystalline, 190, 196, 199, 224, 229, 239, 240, 260–265, 277, 407, 414, 428, 506, 510 sharkskin, 234, 291–293, 297, 301, 420, 467 shear flow, 359 shear viscosity, 202, 360, 361, 362, 365, 370, 373, 376, 377, 380, 440, 441, 447 shear viscosity curve, 202, 360, 373 single site catalyst, 221 SIT, see also seal initiation temperature, 212, 232, 485 slip, 12, 17, 26, 32, 33, 48, 63, 64, 67, 69, 166, 167, 174, 178, 187, 188, 193, 194, 220, 237, 243, 247, 270, 273, 276, 280, 293, 297, 301, 313–323, 328, 401, 420, 428–431, 454, 462, 469, 470, 475, 492, 496, 502, 505, 507, 510 soft box, 341 solvent resistance, 257 specifications, 12, 462, 489 speed control, 76, 84, 418 stiffness, 48, 113, 143, 190, 200, 205, 206, 210, 225–228, 235, 259, 277, 284, 287, 330, 416, 461, 465, 467, 468, 474, 475 storage modulus, 361, 378 stripper roll, 341 structure writing, 473, 491 surface activation, 182 surface chemistry, 170 surface energy, 157, 160–163, 173–175, 182, 335, 392, 416, 417, 462, 475, 502 surface etching, 173, 175, 178–181 sustainability, 3, 228, 329, 421 swell, 193, 502, 503 tackifier, 304, 510 tear resistance, 224, 460, 510 tension, 63–76, 123, 160–164, 167–170, 173–175, 220, 264, 276, 293, 301, 305, 335, 338, 389, 398, 400–402, 412, 415, 416–418, 420, 425, 470, 489, 501, 502, 510 thermal stability, 87, 511 thermocouple, 7, 20, 41, 86, 407, 408, 418, 455, 509 thermoforming, 206, 239, 244, 250, 288, 337, 347, 476, 509 thickness control, 397 tie layer, 340, 511 tie resin, 269, 270, 271, 272, 449, 451, 452, 496 titanium dioxide, 309, 511 torque winding principle, 66

522 transition, 14, 15, 29–31, 37, 38, 101, 113, 122, 175, 183, 218, 223, 243, 247, 255, 262, 273, 278–299, 313, 314, 360, 362–366, 407, 414, 428, 445–451, 452, 485, 502, 504, 512 transmission, 139, 141, 190, 196, 206, 275, 459, 462, 463, 506, 507, 512, 513, 514 transportation integrity, 487 ultraviolet absorbers, 304 unwinding, 63, 64, 67, 220, 335, 416, 417, 420 vacuum box, 237, 341, 342, 401, 424, 425 vacuum conveying, 119, 120 variable-frequency drive, 17 variable-speed drives, 16 velocity profiles, 369, 446 Vicat Softening Point, 192, 219, 220, 287, 512 viscosity, 13, 14, 26, 29, 34–40, 42, 81, 82, 86, 90–97, 100–104, 187, 192, 193, 202–208, 217, 223, 224, 230–235, 242–50, 259, 261–264, 273, 284, 285, 289, 291, 294, 301, 310– 312, 328, 336, 339, 348–353–368, 370–381, 383, 384, 386–398, 404, 407, 415, 420, 421, 424, 425, 440, 441, 446, 447, 450, 462, 465, 499, 503, 506, 508, 509, 512

Index

viscous encapsulation, 352, 353, 372, 424 waste reduction, 90, 291 water vapor, 173, 175, 176, 237, 239, 243, 248, 275, 289, 393, 460, 463, 471, 483, 484, 487, 506, 512 water vapor transmission rate, see also WVTR, 471 watt density, 163, 166, 168, 335, 407, 417 web handling, 159, 383, 387–389, 412, 414, 415, 420 web slitting, 78, 417 wettability, 157, 159, 160, 161, 163, 174, 178, 179, 182, 183, 237, 309 wetting, 160–163, 167, 304, 315, 433, 466, 470, 505 winding, 63–70, 72, 74–78, 167, 187, 220, 276, 313, 335, 345, 397, 402, 416–418, 420, 453, 470, 500, 501 wrinkles, 67, 72, 105, 109–111, 113, 155, 227, 264, 313, 409, 413, 414, 416, 417, 510 WVTR, see also water vapor transmission rate 196, 219, 224, 262, 266, 284, 393, 463, 471, 475 X-ray transmission, 139 Ziegler-Natta catalyst, 199, 200

Chapter Editors and Peer Reviewers

Norman Aubee graduated from the Plastics Engineering Program at the Northern Alberta Institute of Technology in 1984. Norm joined NOVA Chemicals in 1985 and performed various functions in the technical service laboratories. In 1991, he transferred into NOVA Chemicals flexible packaging technical services group for polyethylene products. In addition to his technical service responsibilities, Norm is involved with NOVA Chemicals field market development and application development for flexible packaging applications.

Tom Bezigian is an adjunct professor of plastics engineering at the University of Massachusetts at Lowell and an industry consultant serving the film, extrusion and converting industries. He received his B.S from UMASS-Lowell before beginning his career at Cryovac Division, W.R. Grace & Co. R&D Center in South Carolina. He continued on to Mobil Chemical, Plastics Division and then Fortifiber Corporation, at which time he earned his MBA at Bryant University in Rhode Island. Tom then became the Technical Director of the Food Packaging Division of James River Corporation in Kalamazoo, Michigan. After the dissolution of James River Corporation, he began consulting full-time, during which time he formulated plans to begin his own business and wrote extensively for Converting Magazine and TAPPI, culminating with TAPPI’s Extrusion Coating Manual, 4th Edition. After a long-term contract as Technical Director at Schoeller Technical Papers, Tom started his own specialty extrusion business in 1997, Great Lakes Technologies LLC, located in Syracuse, New York. After the sale of his business in 2002, Tom returned to full-time consulting. Tom consults, writes, and teaches globally on all aspects of extrusion and converting for his own practice, as well as for UMASS-Lowell, TechnoBiz Group (Bangkok), Routsis Training (San Juan), PFFCOnline (Chicago), and Packaging Films Magazine (Germany). He has written and edited more than 100 technical papers, articles, and blogs during his career, and is the recipient of many awards, including the TAPPI PLACE Division’s Leadership and Service Award and the U.S. Small Business Administration Technology of the Year Award. He resides in the Syracuse, New York area.

Thomas J. Dunn has worked in the flexible packaging Industry since 1979. He worked 30 years in product development and regulatory for Printpack, Inc., and upon retirement there, he continues consulting within the industry. His undergraduate and Master’s degrees are from Yale University, where he was a National Science Foundation Graduate Fellow. He is an active technical and volunteer contributor to TAPPI, the Society of Plastics Engineers, and the Institute of Food Technologists. These three organizations have each awarded him career achievement awards, as has PMMIs “Packaging Hall of Fame”. Dunn lives in Atlanta, where he consults with producers and users of the industry’s products regarding product design and food safety.

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Chapter Editors and Peer Reviewers

Warren E. Durling, Associate Research Fellow for Clorox Services Company (GLAD Division) in Willowbrook, IL, earned his Bachelor of Science Degree in Packaging from Michigan State University in 1979. He spent the first 20 years of his career on the Research & Development side of flexible packaging converting with Specialty Papers/James River, American Packaging, and Reynolds Metals/ Alcoa. For the past 19 years Warren has been employed by the GLAD Division of the Clorox Company working on various consumer food storage and trash bag products, including Glad Zipper bags, GladWare containers, and Press ‘n Seal adhesive wrap. He is currently responsible for packaging design and implementation across the Glad portfolio of products. Warren has expertise in cast and blown film extrusion, thin layer co-extrusion coating and laminating, adhesive lamination, flexo and gravure printing, complex web handling, profile cast extrusion, and thin wall thermoforming. He has a strong background in the design and use of folding cartons, corrugated cases, and various flexible packaging structures and applications. Bruce W. Foster is president of PolyKnows LLC, a polymer science, product development, and customer support consultancy based in Southbury, Conn. Launched in 2014, PolyKnows serves clients in the plastics converting and packaging industries in the U.S. and India. Previously, Foster served as a technical sales manager for Mica Corporation in Shelton, Conn. where he was responsible for all domestic and international technical sales, service, and marketing activities from 2000 to 2014. He began his career at Eastman Chemical in Longview, Texas, where he worked from 1981 to 1999 as a polymer scientist in extrusion coating product development and technical service. Foster holds eight patents in the U.S. and Canada involving polyolefin compositions and/or the processes for using them. A TAPPI member since 1981, Foster has earned several Association awards, including the TAPPI PLACE Division Leadership & Service Award in 2010. He has published several articles in TAPPI Journal and Adhesives Age magazine dealing with new adhesives, methods for studying them, and a rheological method for measuring MWD of polymers. Foster has also authored papers appearing in Biophysical Journal and Biochemistry, dealing with the study of biological macromolecules. Foster holds Master’s and Bachelor’s degrees in chemistry from Texas A&M University. He is also a veteran of the Vietnam War. Kelly Frey is an Extrusion Coating Technical Service Specialist at Chevron Phillips Chemical Company LP. He graduated from the Northern Alberta Institute of Technology in Edmonton, Canada, in 1990 with a degree in plastics engineering technology. Upon graduation, Kelly was employed by Dow Chemical Canada in Sarnia, Ontario, where he worked in polyethylene technical service and development for approximately 10 years. While employed at Dow Chemical, Kelly worked in TS&D, supporting Dow’s blow molding, blown film, and injection molding polyethylene business. Kelly then joined Chevron in Orange, TX in 1999, where he worked in Chevron’s molding technical service group and then transitioned to extrusion coating technical service in 2000. Kelly then joined Chevron Phillips Chemical Company during its formation in 2001 and has since worked in extrusion coating technical service. Kelly has presented technical papers at both ANTEC® and the TAPPI International Flexible Packaging and Extrusion conference (formerly PLACE) and is an active member of the TAPPI International Flexible Packaging and Extrusion division. Kelly is currently married, has two daughters, and currently resides in Owasso, OK. Brad Kramer currently works as a Senior Market Developer in the Primary Packaging Segment for ExxonMobil Chemical Company, based in Spring, TX. He has a Bachelor’s degree in Chemical Engineering from the University of Minnesota and has been active in packaging for more than 30 years. Brad began his career working in product development for a packaging converter and has worked in a variety of both technical and commercial roles for three polymer suppliers, two flexible packaging converters, and a brand owner. Brad has extensive knowledge in multilayer blown films, barrier coextruded films, extrusion coating, and copolymers, and has been engaged globally in the development of packaging solutions. Brad is currently active in mentoring/advising many new engineers both inside and outside ExxonMobil. He speaks frequently at industry conferences, has been active in the TAPPI International Flexible Packaging and Extrusion Conference (formerly PLACE Conference), and has two patents. Brad is single and resides in Houston, TX.

Chapter Editors and Peer Reviewers

525

James (Jim) F. Macnamara Jr. has been active in packaging for more than 27 years, working for both converters and end-users during his career. He started out in converting working for American Packaging Corporation before moving on to Kraft Foods in long-term strategic research. Since then, he has worked at Wells’ Dairy, Cello-Pack Corporation, Leprino Foods, and FTD and is currently at Foster Farms. He has a Bachelor of Science degree in Chemical Engineering and a Master of Science Degree in Food Science from Drexel University. He also has a Master of Science Degree in Packaging from Michigan State University. Jim is a Certified Packaging Professional in IOPP and a Certified Food Scientist in IFT, in addition to being active in TAPPI over the years in various capacities, serving as the Technical Program Chair, Second Vice Chair, and currently the First Vice Chair of the IFPED division. He served as one of the section editors on the fifth edition of the Extrusion Coating Manual and is currently the Global Editor of the third edition of the Film Extrusion Manual. He will serve as the International Flexible Packaging and Extrusion Division chair for 2020–2022. He enjoys traveling and spending time with his wife, Karen, and his dog, Chance. Scott B. Marks holds degrees in Mechanical Engineering and Business Administration from Rutgers University of New Jersey. Upon graduating from Rutgers, Scott worked for the DuPont Company in several positions from June 1982 through March 2019. In April 2019, the business unit he works in was moved from DuPont to Dow and is currently a Dow employee. In November 1983, he joined the Technical Service and Development group. During the period of February 1986 through April 2008, he worked a variety of roles for DuPont’s packaging materials business covering mainly Asia-Pacific; technical troubleshooting, market development, application development, training (both internal and external), and mentoring new sales personnel. The processes that Scott handles are predominantly coextrusion of; blown film, cast film, extrusion coating/laminating, sheet extrusion and tubing extrusion. He also has worked with adhesive laminations, as well as end-use packaging/lidding equipment, as this is integral to the business needs. The market application areas that Scott covers are highly diversified due to his working with a variety of countries and cultures throughout his career. These include; meat, cheese, seafood, oils, toothpaste, spices, snacks, dairy/yoghurt, personal care, pharmaceutical, and most anything that would be considered “hard to hold in a package”. Additionally, he has worked with a variety of non-packaging applications that involve ethylene copolymers and coextrusion adhesive resins. April 2008 through December 2015, Scott worked on several Lab Extrusion Facilities revitalization projects for the DuPont labs in the USA and China, as well as handing technical service and development for North American customers, and general global consulting with Asia, Europe, and Latin America. January 2016 onward brought involvement in the project of moving the DuPont Ethylene Copolymers business to Dow, and during this time Scott has transitioned into being a key member for global technical support of the heritage DuPont materials that have moved to Dow. In this role he consults on technical applications with North American extrusion converters, supports global inquiries as needed from Dow regional colleagues, and conducts global training programs internally and externally for technical proficiency. Scott has been a speaker and session chair in conferences and seminars in the USA for TAPPI and other organizations. In TAPPI he has been co-chair of the International Planning Committee and has held all the Division Council positions in IFPE, and currently is a Chair Emeritus of the Division, and a Council Member At-Large. He has been a recurring instructor in the IFPE Extrusion Coating Short Course and the IFPE Blown & Cast Film Short Course for many years.

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Chapter Editors and Peer Reviewers

Martine Michon, Quality Manager for Atlantic Coated Papers, earned her Master’s degree in Chemical Physics from the University of Sherbrooke, Canada, in 2002. She has been working in the packaging industry since 2007 for different manufacturers as process technician, product development, food safety, and quality manager. She is an active member of TAPPI and Ordre des chimistes du Québec. Martine obtained a Lean Six Sigma Green Belt certification. She has contributed to the TAPPI Extrusion Coating Manual, Fifth Edition as a section editor.

Michael Shellenbarger has worked for the last 18 years at Oliver Healthcare Packaging. He currently works as a Product Engineer with a primary focus on heat seal coatings. Previously, he was employed at American Packaging Corporation for seven years in Quality Assurance. He has a Master’s degree in Food Science from Drexel University and has been attending TAPPI events since 2006.

Dorene Smith is a Market Development Manager at Westlake Chemical Corporation. She holds a Bachelor of Science Degree in Chemical Engineering from the University of Arkansas and has over 35 years of experience in the plastics industry in several areas, including engineering, manufacturing, technical service, product development, and market development. Her career began at Eastman Chemical Company, where she held individual and management positions during her 25 years of service there. For the past 13 years, she has worked at Westlake Chemical supporting their polyethylene business.

Ayse Alemdar Thomson is a Business Intelligence Analyst at FPInnovations and holds a doctoral degree in Physics Engineering from Istanbul Technical University. She has twenty years of multidisciplinary experience in product innovation in bioplastics, nanocellulose, and bio-composites from conceptualization to commercialization in Europe and North America. Her career started as a scientist at Université Grenoble Alpes, where she developed a bio-sourced filtration medium. She is the author of 33 papers published in peer-reviewed journals with over 1500 citations. Ayse is an active member of the TAPPI International Flexible Packaging and Extrusion Technical Program Team. She has also served as Chair of the TAPPI Tissue Conference Technical Program Committee.

Rory Wolf is the Business Unit Manager for ITW Pillar Technologies, Hartland, WI, USA and a recognized industry resource in the field of polymer surface modification. He has 33 years of experience in international positions in the plastics and packaging industries and specific experience in the polymerbased flexible packaging, polymer surface modification systems, and printing industry segments. He has published 35 technical papers, 42 industry articles, and three books on plastic surface modification by atmospheric plasma technology and has three patents.