Introduction to Fluoropolymers: Materials, Technology, and Applications (Plastics Design Library) [2 ed.] 0128191236, 9780128191231

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INTRODUCTION TO FLUOROPOLYMERS

PLASTICS DESIGN LIBRARY (PDL) PDL HANDBOOK SERIES Series Editor: Sina Ebnesajjad, PhD ([email protected]) President, FluoroConsultants Group, LLC Chadds Ford, PA, USA http://www.FluoroConsultants.com The PDL Handbook Series is aimed at a wide range of engineers and other professionals working in the plastics industry, and related sectors using plastics and adhesives. PDL is a series of data books, reference works and practical guides covering plastics engineering, applications, processing, and manufacturing, and applied aspects of polymer science, elastomers and adhesives. Recent titles in the series Recycling of Flexible Plastic Packaging 1, Niaounakis, Michael (ISBN: 9780128163351) Plasticizers Derived from Post-Consumer PET 1, Langer, Ewa (ISBN: 9780323462006) Polylactic Acid 2, Sin, Lee Tin (ISBN: 9780128144725) Durability and Reliability of Polymers and Other Materials in Photovoltaic Modules 1, Yang, Hsinjin; French, Roger; Bruckman, Laura (ISBN: 9780128115459) Fluoropolymer Additives 2, Ebnesajjad, Sina; Morgan, Richard (ISBN: 9780128137840) The Effect of UV Light and Weather on Plastics and Elastomers 4, McKeen, Larry (ISBN: 9780128164570) PEEK Biomaterials Handbook 2, Kurtz, Steven (ISBN: 9780128125243) Hydraulic Rubber Dam, Thomas et al. (ISBN: 9780128122105) Electrical Conductivity in Polymer-based Composites, Taherian & Kausar (ISBN: 9780128125410) Plastics to Energy, Al-Salem (ISBN: 9780128131404) Recycling of Polyethylene Terephthalate Bottles, Thomas et al. (ISBN: 9780128113615) Dielectric Polymer Materials for High-Density Energy Storage, Dang (ISBN: 9780128132159) Thermoplastics and Thermoplastic Composites, Biron (ISBN: 9780081025017) Recycling of Polyurethane Foams, Thomas et al. (ISBN: 9780323511339) Introduction to Plastics Engineering, Shrivastava (ISBN: 9780323395007) Chemical Resistance of Thermosets, Baur, Ruhrberg & Woishnis (ISBN: 9780128144800) Phthalonitrile Resins and Composites, Derradji, Jun & Wenbin (ISBN: 9780128129661) The Effect of Sterilization Methods on Plastics and Elastomers, 4e, McKeen (ISBN: 9780128145111) Polymeric Foams Structure-Property-Performance, Obi (ISBN: 9781455777556) Technology and Applications of Polymers Derived from Biomass, Ashter (ISBN: 9780323511155) Fluoropolymer Applications in the Chemical Processing Industries, 2e, Ebnesajjad & Khaladkar (ISBN: 9780323447164) Reactive Polymers, 3e, Fink (ISBN: 9780128145098) Service Life Prediction of Polymers and Plastics Exposed to Outdoor Weathering, White, White & Pickett, (ISBN:9780323497763) Polylactide Foams, Nofar & Park (ISBN: 9780128139912) Designing Successful Products with Plastics, Maclean-Blevins (ISBN: 9780323445016) Waste Management of Marine Plastics Debris, Niaounakis, (ISBN: 9780323443548) Film Properties of Plastics and Elastomers, 4e, McKeen, (ISBN: 9780128132920) Anticorrosive Rubber Lining, Chandrasekaran (ISBN: 9780323443715) Shape-Memory Polymer Device Design Safranski & Griffis, (ISBN: 9780323377973) A Guide to the Manufacture, Performance, and Potential of Plastics in Agriculture, Orzolek, (ISBN: 9780081021705) Plastics in Medical Devices for Cardiovascular Applications, Padsalgikar, (ISBN: 9780323358859) Industrial Applications of Renewable Plastics, Biron (ISBN: 9780323480659) Permeability Properties of Plastics and Elastomers, 4e, McKeen, (ISBN: 9780323508599) Expanded PTFE Applications Handbook, Ebnesajjad (ISBN: 9781437778557) Applied Plastics Engineering Handbook, 2e, Kutz (ISBN: 9780323390408) Modification of Polymer Properties, Jasso-Gastinel & Kenny (ISBN: 9780323443531) The Science and Technology of Flexible Packaging, Morris (ISBN: 9780323242738) Stretch Blow Molding, 3e, Brandau (ISBN: 9780323461771) Chemical Resistance of Engineering Thermoplastics, Baur, Ruhrberg & Woishnis (ISBN: 9780323473576) Chemical Resistance of Commodity Thermoplastics, Baur, Ruhrberg & Woishnis (ISBN: 9780323473583) To submit a new book proposal for the series, or place an order, please contact Edward Payne, Acquisitions Editor at [email protected]

INTRODUCTION TO FLUOROPOLYMERS Materials, Technology, and Applications Second Edition Sina Ebnesajjad President, FluoroConsultants Group, LLC

William Andrew is an imprint of Elsevier The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2021 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/ permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/ or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-819123-1 For Information on all William Andrew publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Edward Payne Editorial Project Manager: Emma Hayes Production Project Manager: Poulouse Joseph Cover Designer: Victoria Pearson Typeset by MPS Limited, Chennai, India

Dedicated to my dear friend Jiri George Drobny

Contents List of Contributors ............................................................................................................................................. xiii Introduction........................................................................................................................................................... xv 1

Fluorine, Fluorocarbons, and Fluoropolymers in Human Life—A Day With the Smiths ...................... 1 Sina Ebnesajjad References......................................................................................................................................................... 5

2

From Fundamentals to Applications ............................................................................................................ 7 Sina Ebnesajjad 2.1 Introduction ............................................................................................................................................... 7 2.2 What Makes Fluorine Unique? ................................................................................................................. 7 2.3 Fluorine Characteristics ............................................................................................................................ 8 2.4 What Are Fluoropolymers?..................................................................................................................... 11 2.5 Fundamental Properties of Fluoropolymers ........................................................................................... 12 2.6 Developmental History of Fluoropolymers ............................................................................................ 13 2.7 Examples of Uses of Fluoropolymers .................................................................................................... 13 References....................................................................................................................................................... 17

3

Fluoropolymers—Discovery, History, Evolution, and Consumption ..................................................... 19 Sina Ebnesajjad 3.1 Roy Plunkett’s Story ............................................................................................................................... 19 3.2 Commercialization of Polytetrafluoroethylene....................................................................................... 21 3.3 Developmental History of Fluoropolymers ............................................................................................ 21 3.4 State of Fluoropolymers.......................................................................................................................... 24 3.5 Summary.................................................................................................................................................. 30 References....................................................................................................................................................... 31 Further Reading .............................................................................................................................................. 31

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History of Expanded Polytetrafluoroethylene and W.L. Gore & Associates ........................................ 33 Sina Ebnesajjad 4.1 Early History of W.L. Gore & Associates ............................................................................................. 34 4.2 Discovery of Expanded Polytetrafluoroethylene.................................................................................... 35 4.3 Who Invented Expanded Polytetrafluoroethylene? ................................................................................ 37 4.4 Other Expanded Polytetrafluoroethylene Players................................................................................... 40 References....................................................................................................................................................... 42

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Introduction to Thermoplastic Fluoropolymers........................................................................................ 43 Sina Ebnesajjad 5.1 Introduction ........................................................................................................................................... 44 5.2 Fluoropolymer Classifications .............................................................................................................. 44

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5.3 Fluoropolymer Products........................................................................................................................ 45 5.4 Monomer Synthesis............................................................................................................................... 45 5.5 Monomer Properties.............................................................................................................................. 48 5.6 Polymerization and Finishing ............................................................................................................... 49 5.7 Structure Property Relationship ............................................................................................................ 52 5.8 Properties of Polytetrafluoroethylene ................................................................................................... 53 5.9 Fabrication Techniques ......................................................................................................................... 56 5.10 Applications........................................................................................................................................... 56 5.11 Safety..................................................................................................................................................... 56 5.12 Polymerization Surfactant..................................................................................................................... 57 5.13 Economy................................................................................................................................................ 58 5.14 Summary................................................................................................................................................ 58 References....................................................................................................................................................... 58

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Manufacturing and Properties of Polytetrafluoroethylene...................................................................... 63 Sina Ebnesajjad 6.1 Introduction ........................................................................................................................................... 63 6.2 Tetrafluoroethylene Polymers............................................................................................................... 64 6.3 Tetrafluoroethylene Polymerization Regimes ...................................................................................... 65 6.4 Tetrafluoroethylene Polymerization Mechanism.................................................................................. 65 6.5 Suspension Polymerization Regimes .................................................................................................... 67 6.6 Polymerizing Tetrafluoroethylene by Suspension Method .................................................................. 69 6.7 Comminution of Suspension Reactor Bead.......................................................................................... 72 6.8 Pelletized (Free Flow) Granular PTFE................................................................................................. 78 6.9 Presintered Granular PTFE ................................................................................................................... 81 6.10 PTFE Filled Compounds....................................................................................................................... 81 6.11 Preparation of Polytetrafluoroethylene by Dispersion Polymerization................................................ 86 6.12 Dispersion Polymerization of TFE with APFO Replacements............................................................ 87 6.13 Dispersion Polymerization Reactor ...................................................................................................... 88 6.14 Preparation of Dispersion Grade PTFE ................................................................................................ 89 6.15 Preparation of Fine Powder PTFE........................................................................................................ 92 6.16 Characterization of Polytetrafluoroethylene by Properties .................................................................. 92 6.17 Commercial PTFE Resins..................................................................................................................... 98 References..................................................................................................................................................... 108 Further Reading ............................................................................................................................................ 110

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Processing and Fabrication of Granular Polytetrafluoroethylene........................................................ 111 Sina Ebnesajjad 7.1 Introduction ........................................................................................................................................... 111 7.2 Resin Selection...................................................................................................................................... 111 7.3 Compression Molding ........................................................................................................................... 112 7.4 Automatic Molding ............................................................................................................................... 119 7.5 Introduction to Isostatic Molding ......................................................................................................... 120 7.6 Ram Extrusion....................................................................................................................................... 122 7.7 Summary................................................................................................................................................ 124 References..................................................................................................................................................... 124 Further Reading ............................................................................................................................................ 124

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Fabrication and Processing of Fine Powder Polytetrafluoroethylene .................................................. 125 Sina Ebnesajjad 8.1 Introduction ........................................................................................................................................... 125 8.2 Resin Handling and Storage ................................................................................................................. 126 8.3 Paste Extrusion Fundamentals .............................................................................................................. 126 8.4 Extrusion Aid or Lubricant ................................................................................................................... 129 8.5 Wire Coating ......................................................................................................................................... 130 8.6 Extrusion of Tubing .............................................................................................................................. 139 8.7 Liner Extrusion...................................................................................................................................... 146 8.8 Fine Powder Resin Selection ................................................................................................................ 146 References..................................................................................................................................................... 147

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Fabrication and Processing of Polytetrafluoroethylene Dispersions .................................................... 149 Sina Ebnesajjad 9.1 Introduction ......................................................................................................................................... 149 9.2 Applications......................................................................................................................................... 150 9.3 Storage and Handling.......................................................................................................................... 151 9.4 Surfactants ........................................................................................................................................... 151 9.5 Principles of Coating Technology ...................................................................................................... 151 9.6 Dispersion Formulation and Characteristics....................................................................................... 155 9.7 Glass Cloth Coating ............................................................................................................................ 158 9.8 Impregnation of Flax and Polyaramide .............................................................................................. 160 9.9 Coating Metal and Hard Surfaces....................................................................................................... 161 9.10 Polytetrafluoroethylene Yarn Manufacturing ..................................................................................... 162 9.11 Film Casting ........................................................................................................................................ 163 9.12 Antidrip Applications.......................................................................................................................... 164 9.13 Filled Bearings .................................................................................................................................... 165 9.14 Dedusting Powders.............................................................................................................................. 166 9.15 Other Applications .............................................................................................................................. 166 References..................................................................................................................................................... 166

10 Manufacturing Melt-Processible Copolymers of Tetrafluoroethylene............................................... 169 Sina Ebnesajjad 10.1 Introduction ....................................................................................................................................... 169 10.2 Molecular and Crystalline Structure ................................................................................................. 169 10.3 Preparation of Perfluoroalkoxy Polymers......................................................................................... 171 10.4 Preparation of Perfluorinated Ethylene Propylene Polymers........................................................... 177 10.5 Preparation of Ethylene Tetrafluoroethylene Polymers ................................................................... 180 References................................................................................................................................................... 183 11 Introduction to Vinylidene Fluoride Polymers ..................................................................................... 185 Averie Palovcak and Sina Ebnesajjad 11.1 11.2 11.3 11.4 11.5 11.6

Synthesis of Vinylidene Fluoride ..................................................................................................... 185 Properties of Vinylidene Fluoride..................................................................................................... 186 Preparation of Vinylidene Fluoride Polymers .................................................................................. 186 Characterization of Polyvinylidene Fluoride.................................................................................... 192 Properties of Polyvinylidene Fluoride .............................................................................................. 193 Processing Polyvinylidene Fluoride.................................................................................................. 201

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11.7 Applications....................................................................................................................................... 202 References................................................................................................................................................... 204 Further Reading .......................................................................................................................................... 205

12 Processing and Fabrication of Parts from Melt-Processible Fluoropolymers................................... 207 Sina Ebnesajjad 12.1 Introduction ....................................................................................................................................... 207 12.2 General Considerations ..................................................................................................................... 207 12.3 Materials of Construction.................................................................................................................. 208 12.4 Rheology of Fluoropolymers ............................................................................................................ 208 12.5 Processing of Fluoropolymers .......................................................................................................... 211 12.6 Injection Molding.............................................................................................................................. 212 12.7 Extrusion............................................................................................................................................ 216 12.8 Fluoropolymer Tube Extrusion......................................................................................................... 221 References................................................................................................................................................... 229 13 Manufacturing and Properties of Polychlorotrifluoroethylene........................................................... 231 Sina Ebnesajjad 13.1 Introduction ....................................................................................................................................... 231 13.2 Chlorotrifluoroethylene Polymers..................................................................................................... 231 13.3 Polymerization of Chlorotrifluoroethylene....................................................................................... 232 13.4 Copolymerization of Chlorotrifluoroethylene .................................................................................. 234 13.5 Properties of Polychlorotrifluoroethylene......................................................................................... 236 13.6 Characterization of Polychlorotrifluoroethylene .............................................................................. 239 13.7 Commercial Polychlorotrifluoroethylene Resins.............................................................................. 239 References................................................................................................................................................... 242 14 Processing and Fabrication of Polychlorotrifluoroethylene ................................................................ 245 Sina Ebnesajjad 14.1 Introduction ....................................................................................................................................... 245 14.2 Processing Considerations................................................................................................................. 245 14.3 Compression Molding ....................................................................................................................... 248 14.4 Injection Molding.............................................................................................................................. 249 14.5 Extrusion............................................................................................................................................ 250 14.6 Machining and Joining...................................................................................................................... 251 References................................................................................................................................................... 252 15 Applications of Fluoropolymers.............................................................................................................. 253 Sina Ebnesajjad 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8

Chemical Processing ....................................................................................................................... 253 Piping............................................................................................................................................... 254 Vessels ............................................................................................................................................. 255 Chemical Process Industry Components ........................................................................................ 256 Self-Supporting Components .......................................................................................................... 259 Trends in Using Fluoropolymers in Chemical Service .................................................................. 259 Semiconductor Processing .............................................................................................................. 260 Trends for the Use of Fluoropolymers in the Semiconductor Industry......................................... 264

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15.9 Electrical Applications .................................................................................................................... 264 15.10 Mechanical Applications................................................................................................................. 266 15.11 Automotive and Aerospace............................................................................................................. 267 15.12 Medical Devices.............................................................................................................................. 268 15.13 Summary.......................................................................................................................................... 269 References................................................................................................................................................... 269

16 Fluoroelastomers....................................................................................................................................... 271 Jiri George Drobny and Sina Ebnesajjad 16.1 Introduction ....................................................................................................................................... 271 16.2 Fluorocarbon Elastomers................................................................................................................... 272 16.3 Fluorosilicone Elastomers ................................................................................................................. 302 16.4 Fluorinated Thermoplastic Elastomers ............................................................................................. 307 16.5 Phosphazenes..................................................................................................................................... 309 16.6 Safety, Hygiene, and Disposal.......................................................................................................... 310 16.7 New Developments and Current Trends .......................................................................................... 311 References................................................................................................................................................... 312 Further Reading .......................................................................................................................................... 315 Acronyms and Abbreviations ..................................................................................................................... 315 Glossary of Terms ...................................................................................................................................... 316 17 Fluoropolymer and Fluorinated Additives ............................................................................................ 321 Sina Ebnesajjad 17.1 Polymeric Fluorinated Additives ...................................................................................................... 321 17.2 Perfluoropolyether Additives ............................................................................................................ 326 17.3 Polytetrafluoroethylene Modified Waxes ......................................................................................... 329 17.4 Fluorinated Graphite ......................................................................................................................... 329 17.5 Fluorination ....................................................................................................................................... 329 17.6 Market Size ....................................................................................................................................... 330 References................................................................................................................................................... 330 Further Reading .......................................................................................................................................... 330 18 Polyvinyl Fluoride: The First Durable Replacement for Paint........................................................... 331 Sina Ebnesajjad 18.1 Introduction ....................................................................................................................................... 331 18.2 Basic Properties................................................................................................................................. 331 18.3 Attribute Application Relationships................................................................................................ 331 18.4 Development and Applications of Polyvinyl Fluoride—A Chronological Treatise ....................... 332 References................................................................................................................................................... 337 19 Fluorinated Coatings; Technology, History, and Applications ........................................................... 339 Laurence W. McKeen 19.1 19.2 19.3 19.4 19.5

Introduction ..................................................................................................................................... 339 Fluoropolymers Used in Coatings .................................................................................................. 340 Fluorocoating Compositions ........................................................................................................... 342 Liquid and Powder Coatings........................................................................................................... 347 Application of Fluorocoatings ........................................................................................................ 348

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19.6 Commercial Fluorocoating Producers ............................................................................................ 352 19.7 A Historical Chronology of Fluoropolymer Finish Technology.................................................... 353 19.8 Food Contact ................................................................................................................................... 357 19.9 Commercial Applications of Fluorocoatings.................................................................................. 358 19.10 Summary.......................................................................................................................................... 366 References................................................................................................................................................... 367 20 Fluorinated Ionomers: History, Properties, and Applications............................................................... 369 Sina Ebnesajjad and Walther Grot 20.1 History ............................................................................................................................................... 369 20.2 Composition ...................................................................................................................................... 369 20.3 Properties........................................................................................................................................... 370 References................................................................................................................................................... 378 21 Functional Fluoropolymers ..................................................................................................................... 379 Claus-Peter Keller and Tomoya Hosoda 21.1 Introduction ....................................................................................................................................... 379 21.2 Functional Groups ............................................................................................................................. 380 21.3 Functional Fluoropolymers—Partly Fluorinated .............................................................................. 380 21.4 Functional Fluoropolymers—Fully Fluorinated ............................................................................... 381 21.5 Processing.......................................................................................................................................... 381 21.6 Applications....................................................................................................................................... 382 21.7 Summary............................................................................................................................................ 388 References................................................................................................................................................... 388 Index ................................................................................................................................................................... 389

List of Contributors Jiri George Drobny Drobny Polymer Associates, Merrimack, NH, United States Sina Ebnesajjad FluoroConsultants Group, LLC, United States Walther Grot C.G. Processing, Inc., Philadelphia, PA, United States Tomoya Hosoda AGC Chemicals, Tokyo, Japan Claus-Peter Keller AGC Chemicals, Tokyo, Japan Laurence W. McKeen Senior Research Associate, DuPont Fluoroproducts, Retired Averie Palovcak Arkema, Inc., Philadelphia, PA, United States

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Introduction In the mid-1990s, the idea for a series of handbooks covering fluoropolymer products science and technology was conceived. There were compelling reasons for the development of the series. First, there were scantly any definitive sources for the study of fluorinated polymers, particularly commercial products. In spite of the importance those polymers have assumed in the industry and indeed the society, there were no “handbooks” for studying commercial fluoropolymers. Certainly, there have been many books covering the chemistry and physics of organic fluorinated chemicals and macromolecules but none devoted to the coverage of commercial fluoropolymer products. That is until this Series was published by Plastics Design Library. Anyone looking for basic properties and characteristics of fluorinated plastics could not consult a definitive source until the year 2000. Commercial manufacturers and processors of fluoropolymers were the primary holders of fluoropolymer preparation, processing, product information, and other technologies. The Greatest Generation had already retired and taken with them most of the knowledge gained during the first decades of the fluoropolymer industry. They left behind no books and few other public recordings of their knowledge, a journal article here and there notwithstanding. Now in the 21st century, the post WW II generations (a.k.a. Baby Boomers), who possessed fluoropolymers knowledge, have begun to retire, many have already left the industry for other reasons. These were among the important reasons behind the inception of fluoropolymer handbooks published by Elsevier. Today, there are a series of books consisting of over a dozen volumes on various types of fluoropolymers. Informally known as “Fluoropolymer Series,” it is housed in the venerated Plastics Design Library (PDL). The mission of the series is broad in scope to allow the inclusion of books about as many fluorinated materials as possible.

Over the years, a new consideration emerged: specification of fluoropolymers by individuals technically trained in diverse fields but not necessarily in fluoropolymer or plastics subjects. This evolution has spurred a requirement for the training, often self-training, a new generation of individuals in plastics including fluoropolymers. For example, biomedical, automotive, or aerospace engineers must select a variety of thermoplastics and elastomers including fluoropolymers. Often without possessing in-depth knowledge of the materials, those and other engineers must select and design fluoropolymers for manufacturing a variety of parts and devices. A beneficial source for nonexperts must not only be reliable but also readily enable engineers to find and apply fluoropolymers information and data. This introductory book was originally conceived and published in 2013 as a quick source for commercial fluoropolymers information. It was developed to further facilitate learning about fluoropolymers for new entrants. That goal remains the same in the second edition. It is likely many readers may find sufficient information to meet their needs in this introductory volume. For those who require more in-depth reading, this book is a jumping off point to finding more detailed references, which have been included at the end of the chapters. The second edition offers a collection of chapters that contain a fair amount of information about various thermoplastic and elastomeric fluoropolymers. The book is intended to serve not only those who wish to begin learning about fluoropolymers but those who are in search of a brief treatise of subject matters. Collected in this book are relatively brief descriptions of fluoropolymer technologies to maximize accessibility to the information for nonexpert readers. In addition to learning about fluoropolymers, the historical sections recount the brilliance of the people who were instrumental in the early discovery, development, and success of fluoropolymers.

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The second edition has been significantly expanded as compared to the original book. Several chapters have been added to the second edition to expand the range of coverage of subject matters. Other chapters have been augmented to include processing and part fabrication methods. Major applications of thermoplastic fluoropolymers have been covered in a separate chapter. Individual chapters of this book may be consulted without reading the preceding chapters as long as the reader has sufficient prerequisite knowledge of the material of interest. Every effort has been made to render this book suitable for beginners and others who wish to learn about the technology, processing, applications, and history of fluoropolymers.

INTRODUCTION

In addition to the two historical chapters, the first chapter describes the role of fluorinated materials, beyond fluoropolymers, in the everyday life of people. This chapter provides relatively light reading to facilitate smooth entry of newly graduated and nonexpert individuals The support that I have received from my friend and colleague Edward Payne, Acquisitions Editor of Plastics Design Library for the publication of the book has been invaluable and is most appreciated. Sina Ebnesajjad November 2020

1 Fluorine, Fluorocarbons, and Fluoropolymers in Human Life—A Day With the Smiths Sina Ebnesajjad FluoroConsultants Group, LLC, United States

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A review of a day in the life of Mr. and Mrs. Smith who live in Tucson, Arizona, will illustrate the essential role of fluorine, fluorocarbons, and fluoropolymers in society. Mr. Smith begins the day shaving using a fancy razor, which has a strip of polytetrafluoroethylene (PTFE) fluoropolymer as a part of the safe design of this razor. Taking eggs out of a refrigerator kept cool with a fluorocarbon refrigerant hidden inside its compressor. Mr. Smith makes an omelet for breakfast. He uses a nonstick frying pan to cook, which is nonstick because of the fluoropolymer coating on its surface. Mr. Smith only needs to use a few drops of olive oil for cooking an omelet (low fat) in the nonstick pan. The pan is also easy to wash, saving on detergent and water. After breakfast, everyone in the family flosses using floss made of PTFE fluoropolymer (e.g., Oral-B Glide made by WL Gore and Associates for Proctor & Gamble) that does not scar gums. The toothpaste contains fluoride to protect the teeth. Fluorine compounds are added to the toothpaste, to help prevent tooth decay. Pain, loss of teeth, gum disease, and disfigurement associated with tooth decay have been reduced since the introduction of fluoridation practice almost 60 years ago. Community water fluoridation and its effect in reducing the burden of tooth decay are considered one of the 10 public health achievements in the 20th century [1]. Numerous studies since 1945 have illustrated the impact of community water fluoridation in the prevention of tooth decay [2].

For example, in 1993, the results of 113 studies in 23 countries were compiled and analyzed [3] (59 out of the 113 studies analyzed were conducted in the United States). This review provided effectiveness data for 66 studies in primary teeth and for 86 studies in permanent teeth. The decay reductions observed were in the range of 40% 60%. Recent evidence continues to indicate that the economic benefit of community water fluoridation exceeds the intervention cost. Further, the benefit cost ratio increases with the community population size [4]. Mrs. Smith is dressing the children. Rain is in the forecast, so to stay dry the children dress in waterrepellent coats that have an expanded PTFE fluoropolymer fabric inside of them, best known as Gore Tex (by WL Gore and Associates). These coats keep the water out but breathe thus keeping the children cool. A similar material is used in hospitals. They pick up their cell phones and laptop computers on their way out of the house. Every one of these devices contains coaxial cables that contain insulation made from fluoropolymers and components, which depend on the unique dielectric properties of PTFE. Modern electronics age has depended on fluoropolymers. The silicon chips are made using fluorine-containing gases for etching and chamber cleaning. Fluoropolymers are used as construction materials for equipment used in factories (“Fab”) that fabricate semicon chips. Mrs. Smith takes her son’s asthma drug along to school. A fluorinated chemical enables safe delivery of metered doses of the asthma medicine out of

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00001-X © 2021 Elsevier Inc. All rights reserved.

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the metal can. A thin layer of fluoropolymer coats the inside of the metal can to prevent the drug from sticking to the surface. Metered Dose Inhaler, as it is called, allows the drug to be administered in a targeted and precise dose. This keeps the exposure of the child limited to the required amount of the asthma drug. Because of its unique chemical properties, fluorine has been instrumental in the development of novel medicines. Approximately, 150 (20%) of all marketed drugs contain fluorine including three of the 10 best-selling drugs (Lipitor, Advair, and Crestor) contain fluorine. Seven out of 35 new drugs approved by FDA in 2011 contained fluorine. Fluorine-enhanced compounds have better pharmaceutical properties. Nearly 70 years ago, Fried replaced 9α-hydrogen in cortisone with fluorine and unexpectedly found the presence of a single fluorine could improve bioactivity by a factor of 11 times [5]. Ever since fluorine has played a multifaceted role in pharmaceuticals. Mr. Smith is the last person to leave the house. He turns off his fluorine-containing displays on his desktop computer and plasma display television. Before leaving, he checks on the thermostat to ensure reduced air conditioning operation while the house is empty. The central air unit in his house contains a fluorocarbon refrigerant called hydrofluoroolefin (HFO) that has no impact on the ozone layer and slight impact on global warming. This material complies with Montreal Protocol by having unique stability and reduced global warming potential. It also helps the air conditioning work efficiently. It would be impossible to envision today’s standards of living without air conditioning. Smith’s house is partially powered by photovoltaic cells that the family installed several years ago. It is a good deal because after they paid for the installation of the cells, they have been enjoying free power for some of the electric needs of the family. A special fluoropolymer called polyvinyl fluoride (PVF) plays an important role in photovoltaic units by protecting them from damage and increasing their useful life. The power plants that supply the rest of the house’s electricity have many fluoropolymer and fluorocarbon-based components, which help reduce carbon dioxide and other emissions. For example, fluoropolymer-coated bag filters remove harmful particles (fly ash) from the smoke discharged by coal burning plants. There would be a huge decrease in particulate emissions if every coal

INTRODUCTION

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burning plant in the world installed fluoropolymercoated bag filters. Mr. Smith looks at the house admiring the landscaping and the crisp looking aluminum siding and steel roof that still looks as bright and clean as it did 10 years ago (Fig. 1.1). What is great is that he has not had to do a thing to keep the exterior looking good! The roof is coated with a paint made with a fluoropolymer called polyvinylidene fluoride (PVDF). This paint endures all the elements of climate, is maintenance free, and is expected to last 30 years. Later, Mr. Smith will travel overseas on a Boeing 787 “Dreamliner” airplane that is equipped with Halon 1301 fluorocarbon fire extinguishers in its cargo compartment to prevent passive fires started by phantom sparks. There are a few hundred miles of wires and cables in this wide body aircraft that allow the plane fly and function. Boeing 777 and Airbus 380 contain 176 and 525 km of wire and cable, respectively. The wires and cables are insulated with fluoropolymers or composite materials that contain fluoropolymers. They allow safe performance of the electrical and signal systems of the aircraft over its lifetime (Fig. 1.2). Mr. Smith may not realize that the interior of the aircraft is surfaced by a composite of fluoropolymer PVF, which is fire safe, stain resistant, durable, and stands up to the harsh cleaning chemicals and disinfectants (Fig. 1.3). Mrs. Smith’s 78-year-old mother is recovering from an operation. A part of one of her arteries was replaced by vascular grafts made of expanded polytetrafluoroethylene (ePTFE) material like the one seen in Fig. 1.4. She suffered no pain during the operation and had a normal anesthetic experience thanks to the use of a fluorocarbon gas. Mrs. Smith’s mother has been given an excellent prognosis for recovering from the surgery and is adapting well to her new ePTFE veins. She will never know that anesthesia from which she recovered was a fluorocarbon gas. She will go on with her active life and enjoy her grandchildren for a long time. Dr. Charles Suckling attempts to prepare the ideal inhalation anesthetic gas that lead to the successful development of Halothane in 1951. It had the desired characteristics including effectiveness, nontoxicity, inflammability, and rapid detoxification. Halothane was the first fluorinated inhaled anesthetic that was extremely successful, rapidly displacing all other potent inhaled anesthetics.

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Figure 1.1 Photograph of a PVDF-painted house roof using Kynar. Courtesy Kynar® is a trademark of Arkema Corp, www.gulfcoastsupply.com, June 20, 2020.

Figure 1.2 Aerospace data cables insulated with PTFE and melt-processible fluoropolymers. Courtesy Harbour Industries, www.harbourind.com.

Efforts to develop other halogenated anesthetics with more of the characteristics of the ideal inhaled anesthetic agent than halothane led to the

introduction of isoflurane, desflurane, sevoflurane, enfluorane, isoflurane, desflurane, sevoflurane, and methoxyflurane [6].

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Figure 1.3 Ceiling and stow bins surfaced with Tedlar PVF Declam. Courtesy Heath Techna Corp, www. HeathTechna.com.

Figure 1.4 Stretch vascular grafts made from Gore-Tex ePTFE. Courtesy Gore Medical, WL Gore and Associates, www.GoreMedical.com.

In 2012, over 232 million surgical procedures were performed in the world. The number of surgeries in the United States in 2016 was over 53 million. A significant number of operations required general inhalation anesthetic. Fluorocarbon compounds such as Sevoflurane and Isoflurane have drastically reduced the long-standing anesthesia risks [7]. During the 1990s, decade estimates for the number of deaths attributed to anesthesia have dropped by greater than 25-fold from 1 in 10,000 anesthetics to 1 in 200,000 to 1 in 300,000 in 2013. Today, surgeons save lives thanks to safe fluorocarbon anesthetics pioneered by Charles Suckling. There are many more similar everyday life examples that save lives and enhance the quality of human life. More exotic examples include parts in airplanes and spacecrafts from the early days of space exploration to today’s International Space Station. Fluorocarbons whether chemicals or

plastics are invisible and often go unnoticed. The reason is that they are inside the systems that have enabled today’s societal human standards. Out of sight leads to out of mind! This metaphor taken to extreme may narrow the focus to the challenges of fluorocarbons without placing due weight on their critical roles in human life. Fluorocarbon and fluoropolymer industries and governments have been working together for decades on reducing the negative impact of these products on the environment, on global warming, and on human health [8]. A good example of success in reduction of the negative impacts of fluorocarbons is the development of ozone safe and low global warming potential (,CO2) refrigerants by the industry. Some of the other detrimental fluorocarbon issues have been corrected and more will be corrected by additional inventions in the future.

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AND

FLUOROPOLYMERS

IN

References [1] Allukian Jr. M, Carter-Pokras OD, et al. Science, politics, and communication: the case of community water fluoridation in the US. Ann Epidemiol 2018;28:401 10. [2] Fluoridation Facts, American Dental Association. ,www.ADA.org.; 2005. [3] Murray JJ. Efficacy of preventive agents for dental caries. Caries Res 1993;27(Suppl. 1):2 8. [4] Ran T, Chattopadhyay SK. Economic evaluation of community water fluoridation. Am J Prev Med 2016;50(6):790 6.

HUMAN LIFE—A DAY WITH

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SMITHS

5

[5] Fried J, Sabo EF. 9α-Fluoro derivatives of cortisone and hydrocortisone. J Am Chem Soc 1954;76:1455 6. [6] Edmond Eger II E. Characteristics of anesthetic agents used for induction and maintenance of general anesthesia. Am J Health Syst Pharm 2004;61(20). [7] ,www.asahq.org.; 2018. [8] Ebnesajjad S. 2nd ed. Fluoroplastics: non-melt processible fluoropolymers, vol. 1. Oxford: Elsevier; 2015.

2 From Fundamentals to Applications Sina Ebnesajjad FluoroConsultants Group, LLC, United States

O U T L I N E 2.1 Introduction

7

2.4 What are fluoropolymers?

11

2.2 What makes fluorine unique?

7

2.5 Fundamental properties of fluoropolymers

12

2.6 Developmental history of fluoropolymers

13

2.7 Examples of Uses of Fluoropolymers

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References

17

2.3 Fluorine Characteristics 2.3.1 Fluorination 2.3.2 Reactivity—an extreme element 2.3.3 Preparation of Fluorine 2.3.4 Organic Chemistry 2.3.5 Fluorine and Nature

8 9 9 10 10 11

2.1 Introduction Fluorine is unique and very different from all other halogens. Halogens have a common characteristic, that is, all possess seven electrons in the outer shell of their atomic structure. Their valence is “
1” in their reactions with hydrogen and metals. The reactivity of halogens has a reverse relationship with their atomic number. The lightest halogen is fluorine and the heaviest is astatine, respectively, at the top and bottom of group 17 in the periodic table of elements. Substituting fluorine for another element, such as hydrogen, in the chemical structure of an organic compound has profound impact on its properties. The reason is fluorine has the highest reactivity of all elements [1]. It has an electronegativity of 4—the maximum value on the Pauling scale (0.7
4, dimensionless). Most chemically resistant elements like platinum are susceptible to fluorine attack. For example, platinum reacts with F2 gas and forms platinum hexafluoride (PtF6) (Fig. 2.1).

2.2 What Makes Fluorine Unique? Other elements such as carbon form strong bonds with fluorine as a result of its highest

electronegativity among all elements. The strength of carbon
fluorine bond (C
F) is the fundamental reason polytetrafluoroethylene (PTFE) is one of the most stable and inert plastics known to man. Yet, its monomer tetrafluoroethylene is flammable and highly explosive thanks to the diversity of fluorine effect. McGraw-Hill Encyclopedia of Chemistry points out about the stability of halogenated compounds: “Organic halogen compounds generally show progressively increased stability in the order iodine, bromine, chlorine, and fluorine” [3]. Fluorine is relatively easy to substitute for hydrogen (and other elements) in organic compounds due to its extreme affinity for grabbing electrons. Substituting fluorine for hydrogen in a chemical compound gives rise to a variety of unique and useful effects. Examples include increased polarity, decreased polarity, chemical activity, chemical neutrality, increased biological activity for pharmaceuticals and agro chemicals, greater thermal and oxidative stability, and increased chemical resistance. An interesting example is fluorination of the surface of a polyolefin (e.g., polypropylene) film [4]. Slight fluorination renders the neutral surface of a polyolefin film polar. Further increases in the fluorine content of the surface result in total neutrality of the film surface.

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00002-1 © 2021 Elsevier Inc. All rights reserved.

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Figure 2.1 Periodic table of elements. Reproduced by permission of International Union of Pure Applied Chemistry. IUPAC periodic table of the elements. r 2017 International Union of Pure Applied Chemistry [2]. Table 2.1 Usual Effect of Increase in Fluorine Content of Polymers. Property

Impact

Chemical resistance

Increases

Melting point

Increases

Coefficient of friction

Decreases

Thermal stability

Increases

Dielectric constant

Decreases

Dissipation factor

Decreases

Volume and surface resistivity

Increase

Mechanical properties

Decrease

Flame resistance

Increases

Resistance to weathering

Increases

In practice, minor fluorination of polyolefin surfaces is used to make them adherable. Inside surfaces of plastic pesticide and herbicide bottles are routinely fluorinated, extensively, to prevent permeation of the agents through the thickness of the container. The fluorination prevents the loss of material, emissions, and human and other exposures. Sometimes fluorine gas is added to the blow molding gas in order to combine the bottle fabrication and fluorination steps. A number of partially and fully fluorinated polymers have been developed because of the unique

effect of fluorine on their properties. Some of the common polymer chemistries include polyolefins, fluorinated elastomers, polymethyl siloxane, acrylic and methacrylic polymers, and perfluoroether polymers. The impact of increasing the fluorine content of olefinic polymers on their properties is listed in Table 2.1. Fluorinated compounds have varied and unusual properties, a number of which are quite useful to the development of commercial materials for a broad range of applications including plastics, electronics, agriculture, pharmaceuticals, and medicine.

2.3 Fluorine Characteristics Fluorine ranks 13th in abundance among the Earth’s rocks, present at an average concentration of 0.1% by weight [1]. Fluorine abundance is 0.08% compared to 0.05% in the Earth lithosphere [5]. Fluorine is considered the most dominant halogen when the whole Earth is considered. The most abundant natural sources of fluorine are fluorspar (CaF2) and cryolith [also called cryolite (Na3AlF6)]. Enamel of teeth is very hard, mechanical strong, and has long-term durability, mainly because of fluoroapatite [Ca5 (PO4)3F or 3Ca3(PO4)2UCaF2], along with hydroxyapatite being its major components. Fluoride is considered a

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trace element because only small amounts are present in the body (about 2.6 g in adults) and because the daily requirement for maintaining dental health is only a few milligrams a day. About 95% of the total body fluoride is found in bones and teeth. Fluoride’s primary function in the human body is to strengthen the bone and it is known to prevent tooth decay. Experts contend that fluoride strengthens the teeth’s enamel by strengthening the mineral composition of the teeth themselves [6,7]. About 40% of fluorspar [1] is used as metallurgical flux in the steel industry, some of which is recovered as synthetic fluorspar. The highest grade of fluorspar ( . 97% CaF2) is reacted with sulfuric acid for the production of HF, which is the starting point of organic fluorinated compounds. Some fluorspar is consumed in uranium processing, petroleum alkylation, and stainless steel pickling [8]. Fluorine is a gas with a green-yellow color, a boiling point of
188.1°C, and a melting point of
219.6°C [9]. Its pungent odor is perceptible at a concentration of 10 parts per million. Fluorine is highly toxic, corrosive, and oxidizes nearly every element, including noble gases xenon and krypton. In contrast to HF, dry fluorine does not etch glassware but reacts with hot platinum and gold. To reduce its reactivity and hazard, fluorine is diluted with nitrogen; a 10% F2 in nitrogen can be stored and transported in passivated steel bottles. Some basic facts about fluorine are presented in Table 2.2.

Table 2.2 Basic Facts about Fluorine. Natural abundance Earth’s crust: 950 ppm; important minerals: Fluorspar CaF2; Apatite Ca5(PO4)3F; cryolite Na3AlF6 (Cl 130 ppm) Ocean: 1.3 ppm (Cl 18,000 ppm) Essential element: 0.3
0.5 mg/day for humans; a human body (70 kg) contains 2. g fluorine Bond distance to C: CH3-F 1.39 A˚ (CH3-Cl 1.77 A˚) Bond dissociation energy from C: CH3-F 116 kcal/mol (CH3-Cl 81 kcal/mol) Fluorine forms the strongest single bond to carbon (and other elements!)

9

2.3.1 Fluorination Fluorinating agents used to introduce fluorine atoms into target molecules. Selectfluor, manufactured by Air Products in the United States [10], is perhaps the best-known electrophilic fluorinating agent. Pharmaceutical researchers use this reagent to fluorinate steroids. Diethylaminosulfur trifluoride transforms hydroxyl and carbonyl groups into CF and CF2 moieties, while triethylamine trishydrofluoride provides a pH neutral, nonvolatile equivalent of hydrogen fluoride, and is a source of fluoride ions for various nucleophilic reactions. In addition, trifluoromethyltrimethylsilane, CF3SiMe3, is a useful CF3 source that reacts with carbonyl systems to yield trifluoromethylated alcohol derivatives. An important technique for commercial preparation of fluorocarbons is an electrochemical fluorination method called the Simons Process. Professor Simons discovered the process in the 1930s while working on a research project sponsored by 3M Corporation. The process is based on the electrolysis of a solution of an organic compound, such as a hydrocarbon, in a solution of hydrofluoric acid [11].

2.3.2 Reactivity—An Extreme Element As has often been stated, fluorine is truly a material of extremes [12]. Fluorine is the most reactive element known to man. It reacts with nearly everything including glass. Nobel gases such as xenon and krypton and precious metals like gold and platinum are not exceptions; all react with fluorine. Moissan has been credited for the first synthesis of fluorine [13]. Moissan conducted an experiment to illustrate the extreme reactivity of fluorine. Oil of turpentine, in the solid state, is attacked by liquid fluorine. To perform this experiment, he placed a small amount of oil of turpentine at the bottom of a glass tube surrounded with boiling liquid air. As soon as a small quantity of fluorine was liquefied on the surface of the solid, a combination reaction took place accompanied by an explosive force. After each explosion, a slow current of fluorine gas was kept up leading to the formation of a fresh quantity of liquid fluorine. Successive detonations occurred at intervals of 6
7 minutes. Finally, after a longer interval of about 9 minutes, the amount of liquid fluorine formed reached the

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INTRODUCTION

sufficient quantity to cause complete destruction of the apparatus. Every time drops of liquid fluorine landed on the floor, accidentally, the wood instantly caught fire.

2.3.3 Preparation of Fluorine Interest in fluorine is literally centuries old even though its successful preparation is relatively recent. A number of unsuccessful efforts to prepare fluorine were made in the past. In 1529, Georigius Agricola described the use of fluorspar (CaF2) as a flux. In 1670, Schwandhard found that glass was etched when exposed to fluorspar treated with acid. Fluorine is one of the last elements to be isolated because of its extreme reactivity. In 1764, Margraff synthesized HF by reacting sulfuric acid and fluorspar. The severe redox potential of fluorine prevented its synthesis because of the lack of a suitable oxidant [9]. The eminent French chemist Henri Moissan first prepared fluorine on June 26, 1886. He used Fremy’s Salt (after the venerable French chemist Edmond Fremy, 1814
1894) technique to make and isolate pure fluorine by electrolysis of KF in anhydrous hydrofluoric acid. Moissan performed electrolysis of a 1
12 mixture of KF and HF in a U-shaped platinum tube cooled at 23°C in a MeCl bath. Henri Moissan was awarded the Nobel Prize in chemistry in 1906. The principle of Moissan’s electrolytic preparation of fluorine is still in use for the industrial manufacture.

2.3.4 Organic Chemistry Carbon forms its strongest bond with fluorine and the credit for demonstrating the stability of C
F bond goes to the French chemists Dumas and Peligot who heated dimethyl sulfate with potassium fluoride and obtained methyl fluoride (Eq. 2.1) [9]. ðCH3 OÞ2 SO2 1 2KF-2 CH3 F1K2 S2 O4

(2.1)

The first nucleophilic replacement of another halogen by fluorine has been attributed to an unfathomable genius, the Russian musician and chemist Alexander Borodin [14]. He was probably composing the Polovtsian Dances from the great opera Prince Igor (later finished by Nikolai Rimsky-Korsakov and Aleksandr Glasunov) while

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synthesizing benzoyl fluoride by replacement of chlorine in benzoyl chloride using Fremy’s Salt (KF 1 HF) [9]. This reaction, known as Halex (short for halogen exchange), remains the most important commercial route to the synthesis of C
F bonds [15]. Fluoroaromatics were better understood in the latter part of the 19th and early part of the 20th centuries. Aliphatic fluorine chemistry lagged behind until the pioneering work of the Belgian Chemist Frederic Swarts. He carried out halogen exchange on polychlorides and polybromides using antimony trifluoride and bromine (SbF3 1 Br2). Swarts demonstrated dehalogenation using Zn and dehydrohalogenation using K2CO3 would preferentially eliminate halogens other than fluorine, forming fluorinated olefins. Swarts has been credited for the first synthesis of CCl2F2 by Midgley and Henne of the Frigidaire Co. (part of General Motors), who pioneered the use of fluorinated hydrocarbons in the refrigeration industry [15]. The years before and during World War II brought important advances to organic fluorine chemistry that facilitated the use of fluorocarbons. The discovery of the catalytic activity of HgF2 by Simons and Block [15] on to allow nonexplosive reaction of carbon and fluorine paved the way for the synthesis of saturated perfluorohydrocarbons. These compounds resisted the highly reactive UF6 and were used as buffer fluids in the gas-diffusion process of the Manhattan Project in the early 1940s. Beginning in the 1950s and 1960s, fluorocarbons were studied and developed for biological activity. For example, a number of fluorocarbons such as Fluroxene (CF3CH2OH 5 CH2) began to revolutionize the field of inhalation anesthetic because of their extreme nonflammability. By the late 1970s, fluorocarbons began to dominate the area of inhalation anesthesiology. Fluorine-containing compounds such as Sevoflurane, Enflurane, and Isoflurane are among the common present-day anesthetics. Other developments of this era include organic fluorine-containing pharmaceuticals, artificial blood, respiratory fluids, and chemical weapons. In the 1980s, the semiconductor fabrication industry began to use fluorinated gases for plasma etching processes and fluorinated chemicals as cleaning fluids. Two important etchants are NF3 and BF3. In the 1980s and 1990s, fluorine-containing compounds have made monumental contributions to the

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agricultural industry, with bioactive compounds used as herbicide, pesticide, fungicide, and plant growth regulators. Today, some 10% of these compounds contain fluorine in some form. Even though the aromatic fluorine chemicals are more expensive than nonfluorinated analogs, their activity and effectiveness leads to the consumption of smaller quantities of fluorinated compounds. Fluorinated liquid crystals were incorporated in the design of active matrix liquid crystal displays in the 1990s. Development work on the 157 nm photolithography technology during the 2000s required fluorinated photoresists for the manufacture of integrated electronic circuits. This section is not a complete history of organofluorine chemistry. Much more can be learned about organic fluorine and its characteristics of fluorine by studying the sources cited in this section elsewhere.

2.3.5 Fluorine and Nature The epic challenge in taming fluorine is to generate C
F bonds by enzyme catalysis. Nature has hardly prepared a biochemistry of fluorine; the plant toxin fluoroacetate described earlier is the most common naturally occurring fluorinecontaining compound. Some exceptionally toxic plants, mostly from Africa, can accumulate fluoroacetate at concentrations up to 8000 ppm (8 mg/g). The bacterium Streptomyces cattleya can mediate the biotransformation of inorganic fluoride to fluoroacetate and also to the amino acid 4fluorothreonine. Researchers at the University of St Andrews and the Queen’s University of Belfast have shown [16] that fluoroacetaldehyde is an intermediate in synthesizing both these metabolites. S. cattleya is apparently capable of generating fluoroacetaldehyde in vivo from metabolic intermediates, presumably by reacting them with inorganic fluoride. Nature certainly seems to find fluoride difficult to manipulate. The reason may be its poor nucleophilicity in aqueous media. Researchers at the University of British Columbia have reported [17] forming C
F bonds from inorganic fluoride by using mutant bacterial trans-glycosidase enzymes. Scientists removed the nucleophilic carboxylate group from a glutamate residue of the natural β-glycosidases by replacing the latter with glycine, alanine, or serine residues.

11

The ability of these mutant enzymes to generate fluoroglycosides is explained by the presentation of fluoride to the reactive center on the sugar. In the case of the serine mutant, a hydrogen bond forms between F and the hydroxyl group. These are some of the first examples of organofluorine compounds produced by purified, albeit engineered, proteins [18
22].

2.4 What Are Fluoropolymers? Traditionally, a fluoropolymer or fluoroplastic is defined as a polymer consisting of carbon (C) and fluorine (F). Sometimes these are referred to as perfluoropolymers to distinguish them from partially fluorinated polymers, fluoroelastomers, and other polymers that contain fluorine in their chemical structure. For example, fluorosilicone and fluoroacrylate polymers are not referred to as fluoropolymers. An example of a linear fluoropolymer is tetrafluoroethylene polymer (PTFE):

A simplistic analogy would be the chemical composition of polyethylene [(
CH2
CH2
)n] where all the hydrogen atoms have been replaced by fluorine atoms. Of course, in practice PTFE and polyethylene are prepared in totally different ways. There are branched fluoropolymers such as fluorinated ethylene propylene polymer (FEP):

Oxygen (O) and chlorine (Cl) are present in the chemical structure of some commercial

12

fluoropolymers. Examples include perfluoroalkoxy and polychlorotrifluoroethylene:

Rf is usually a perfluorinated group consisting of carbon and fluorine. Introduction of nonlinearity, oxygen and side chains, or chlorine invoke a variety of polymer properties which will be dealt with later in this book. There is a second class of fluoropolymers called “partially fluorinated” in contrast to “perfluorinated polymers.” These molecules include hydrogen (H) in addition to fluorine and carbon. Examples include polyvinylfluoride (PVF), polyvinylidene fluoride (PVDF), and ethylene tetrafluoroethylene copolymer:

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2.5 Fundamental Properties of Fluoropolymers The basic properties of fluoropolymers arise from the atomic structure of fluorine and carbon and their covalent bonding in specific chemical structures. These properties become weaker as the chemical structure becomes less “perfluorinated,” as in PVDF. Because PTFE has a linear structure, it is a good subject for discussion of extreme properties. The backbone is formed of carbon
carbon bonds and carbon
fluorine bonds. Both are extremely strong bonds (C
C 5 607 kJ/mol and C
F 5 552 kJ/mol) [23,24]. The basic properties of PTFE stem from these two very strong chemical bonds. The PTFE molecule resembles a carbon rod completely blanketed with a sheath of fluorine atoms [25,26]. The size of the fluorine atom allows the formation of a uniform and continuous sheath around the carbon
carbon bonds and protects them from attack, thus imparting chemical resistance and stability to the molecule. The fluorine sheath is also responsible for the low surface energy (18 dynes/cm) [27] and low coefficient of friction (0.05
0.8, static) of PTFE [25,26]. Another attribute of the uniform fluorine sheath is the electrical inertness (or nonpolarity) of the PTFE molecule. Electrical fields impart only slight polarization in this molecule, so volume and surface resistivity are high. Table 2.3 summarizes the fundamental properties of PTFE. The basic properties of PTFE result in beneficial attributes with high commercial value (Table 2.4). Table 2.3 Fundamental Properties of PTFE.

• High melting point, 342°C (648°F) • High thermal stability • Useful mechanical properties at extremely low and high temperatures

• Insolubility • Chemical inertness • Low coefficient of friction Partially fluorinated fluoropolymers are significantly different from the perfluoropolymers with respect to properties and processing characteristics. For example, perfluoropolymers are more thermally stable but physically less hard than partially fluorinated polymers. The former has much higher “hardness” than the latter.

• Low dielectric constant/dissipation factor • Low water ab/adsorptivity • Excellent weatherability • Flame resistance • Purity

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Table 2.4 Commercially Useful Attributes of PTFE.

• Stability • High continuous use temperature • Excellent weatherability • Excellent chemical resistance • Excellent fire properties • Low surface energy • Good release properties • Biological inertness • Low friction • Cryogenic properties • Retains flexibility • Electrical properties • Low dielectric constant • Low dissipation factor

2.6 Developmental History of Fluoropolymers The development of fluoropolymers began with the invention of PTFE in 1938 and continued to 1992 when a soluble perfluoropolymer (Teflon AF) was introduced. Table 2.5 summarizes the timeline for the development of fluoropolymers that have brought about major changes in properties and/or fabrication processes. The discovery of PTFE was a major leap forward in material science. Yet, the new polymer could not be fabricated by melt processing. The next two forms of PTFE, fine powder and dispersion, were also not melt-processible. The pursuit of a more easily processible polymer led to FEP, which could be melted in an extruder. Compared with PTFE, the major disadvantage of FEP is its reduced mechanical properties at elevated temperatures and thus maximum continuous use temperature (200°C) (Table 2.5). PFA, which was introduced in 1973, offers both melt-processing and the same upper continuous use temperature as PTFE (260°C). Ethylene tetrafluoroethylene (ETFE) addresses the need for a mechanically stronger polymer, albeit at a loss of fluoropolymer properties because of the presence of hydrogen in its molecule. Compared to PTFE, ETFE has lower continuous use temperature (150°C), less chemical resistance, and a higher coefficient of friction. Mechanical properties, including tensile strength, elongation at break, and tensile modulus are increased, leading to cut-through resistance in wire insulation.

13

Teflon AF is an amorphous polymer, which is soluble in certain halogenated solvents. It can be applied as a solution, followed by the removal of the solvent. The remaining coating will be as resistant to almost as many chemicals as PTFE. The thickness of the coating can range upward from less than a micrometer. There are a number of other polymers in this family including polychlorotrifluoroethylene (PCTFE), PVF, PVDF, ethylene chlorotrifluoroethylene (ECTFE), tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride copolymers, perfluoroacrylates, fluorinated polyurethanes, and chlorotrifluoroethylene/vinylether copolymers. Typical properties of commercial fluoropolymers are listed in Table 2.6.

2.7 Examples of Uses of Fluoropolymers The consumption of PTFE has increased over the years as technological advancement has required the properties of fluoropolymers. The applications of PTFE and fluoropolymers in general span all facets of human life, from household uses to the aerospace and electronic industries. Basic properties (Table 2.3) of PTFE lead directly to its applications: chemical resistance, thermal stability, cryogenic properties, low coefficient of friction, low surface energy, low dielectric constant, high volume and surface resistivity, and flame resistance. Applications for fluoropolymers always exploit one or more of the properties (Table 2.7) that set them apart from other materials, particularly other plastics. In the chemical process industry, for example, fluoropolymers are selected for their unmatched resistance to chemical attack. They serve as linings for carbon steel vessels, and for piping and other fluid handling components. They provide durable, low maintenance, and economical alternatives to exotic metal alloys. In these applications, fluoropolymers also offer thermal stability for use at high temperatures. And because they do not react with process streams, they help prevent contamination of products. Fluoropolymers are the materials of choice for many materials integrity management applications within the microelectronics [28], food,

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Table 2.5 Commercialization Timeline of Major Fluoropolymers Versus Key Processing/Application Trade-Offs. Fluoropolymer

Year Commercialized

Monomers

Trade-off 1

Trade-off 2

PTFE

1947

TFE

Continuous use temperature 260°C

Nonmeltprocessible

PCTFE

1953

CTFE

Melt-processible/ Nonmelt-processible

Maximum continuous use temperature 180°C

FEP

1960

TFE, HFPa

Melt-processible

Maximum continuous use temperature 200°C

PVF

1961

VFb

Thin film/weatherable

Maximum continuous use temperature 107°C

PVDF

1961

VDFc

Melt-processible

Maximum continuous use temperature 150°C

ECTFE

1970

CTFE, Ed

Hardness/toughness

Maximum continuous use temperature 150°C

PFA

1972

TFE, PAVEe

Melt-processible, continuous use temperature 260°C

Low molecular weight

ETFE

1973

TFE, E

Hardness/toughness

Maximum continuous use temperature 150°C

Teflon AF

1985

TFE, PDDf

Soluble in special halogenated solvents

High cost

Hexafluoropropylene (CF2 5 CF 2 CF3). Vinyl fluoride (CH2 5 CHF). c Vinylidene fluoride (CH2 5 CF2). d Ethylene (CH2 5 CH2). e Perfluoroalkylvinylether (CF2 5 CF 2 O 2 Rf). f Perfluoro-2,2-dimethyl-1,3,-dioxole. PTFE, Polytetrafluoroethylene; TFE, tetrafluoroethylene; PCTFE, polychlorotrifluoroethylene; CTFE, chlorotrifluoroethylene; FEP, fluorinated ethylene propylene; HFP, hexafluoropropylene; PDD, Perfluoro-2,2-dimethyl-1,3,-dioxole; PVP, polyvinylfluoride; PVDF, polyvinylidene fluoride; ECTFE, ethylene chlorotrifluoroethylene; PFA, perfluoroalkoxy; ETFE, ethylene tetrafluoroethylene; VDF, vinylidene fluoride; E, ethylene; PAVE, perfluoroalkylvinylether; VF, vinyl fluoride; PDD, 2,2-bistrifluoromethyl-4,5 difluoro-1,3-dioxole. a b

beverage, pharmaceutical, and biopharmaceutical industries. Electrical properties of fluoropolymers are highly valuable in electronic and electrical applications. In data communications, for example, FEP is used to insulate cables installed in air-handling spaces (plenums) in office buildings. FEP provides the excellent dielectric properties these cables require to perform well at high data transmission rates as well as longterm stability so performance will not change over the life of the cabling system. Most importantly, FEP helps

these cables meet strict building code requirements for low flame spread and low smoke generation. Fluoropolymers are used to insulate wire for critical aerospace and industrial applications where chemical and thermal resistance is essential. They are also materials of construction for connectors for high-frequency cables and for thermocouple wiring that must resist high temperatures. In the automotive, office equipment, and other industries, mechanical properties of fluoropolymers are beneficial in low-friction bearings and seals that

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15

Table 2.6 Typical Properties of Fluoropolymers (1 5 Best, 5 5 Worst). Property

PTFE

PFA

FEP

ETFE Tefzel

ECTFE Halar

PVF

CTFE Aclar

Specific gravity

2.15

2.16

2.15

1.70

1.68

1.77

2.13

Tensile strength at Brk., RT, %

5000

4500

3000

6500

7000

4500

4000

Elongation at Brk., RT, %

400

300

290

150

200

50

140

Flex strength, psi

No Brk.

NA

3000

7100

7000

9500

8600

Flexural modulus, psi 3 105

0.7 2 1.1

1.0

0.9

2.0

2.4

2.5

1.5

Hardness (Shore, Rockwell)

D50 2 65

D60

D55, R45

D75, R50

D75, R95

R109

R109

Izod impact Ft/ Lbs/In-Notch, RT

3

No Brk.

No Brk.

No Brk.

No Brk.

4

1.2

Melt point, °F

627

575 2 590

500 2 535

520

465

340

394

Maximum operating temperature, continuous, °F

550

500

400

350

340

265

350

Lowtemperature embrittlement, °F

2450

NA

2100

2150

2105

280

2423

Deflection temperature, °F at 66 psi

250

NA

158

220

240

270

258

Deflection temperature, °F at 264 psi

120

NA

NA

160

170

195

NA

Thermal expansion, 1025/In/°C

10.0

12

9.5

7

8

8.5

7.2

Dielectric strength, V/mil (0.001 in.)

4200

4000

6500

7000

2000

1280

3500

Dielectric factor, 103 cycles

2.1

2.1

2.1

2.6

2.6

7.7

2.5

Dielectric constant, 103 cycles

,0.0003

0.0002

,0.0002

0.0008

0.0015

0.018

0.025

(Continued )

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Table 2.6 Typical Properties of Fluoropolymers (1 5 Best, 5 5 Worst).—Cont’d Property

PTFE

PFA

FEP

ETFE Tefzel

ECTFE Halar

PVF

CTFE Aclar

Water vapor permeability (ranked)

5

6

5

4

2

3

1 (best)

Chemical resistance (ranked)

1

1

1

223

2

4

2

Coefficient of friction

1

2

3

4

4

5

5

Brk, Break; RT, room temperature; PTFE, polytetrafluoroethylene; CTFE, chlorotrifluoroethylene; FEP, fluorinated ethylene propylene; PVF, polyvinylfluoride; ECTF3, ethylene chlorotrifluoroethylene; PFA, perfluoroalkoxy; ETFE, ethylene tetrafluoroethylene.

Table 2.7 Major Applications and Some Uses of PTFE and Fluoropolymers. Industry/ Application Area

Key Properties

Typical Uses

Chemical processing

Chemical resistance, good mechanical properties, thermal stability, cryogenic properties

Gaskets, vessel liners; valve, pipe, and fitting liners; T’s, bellows, spacer, highpressure hoses tubing; coatings and fluid handling systems

Electrical and communications

Low dielectric constant, high volume/ surface resistivity, high dielectric breakdown voltage, flame resistance, thermal stability

Connectors, insulation of signal and electric wire and cable, coaxial cable, automotive wiring harness and electronic wiring harness, thermocouple wire, multicore instrumentation cable, jacketing, high-temperature automotive and aerospace wire

Automotive and office equipment

Low coefficient of friction, good mechanical properties, cryogenic properties, chemical resistance

Seals and rings in automotive power steering, transmission, and airconditioning. Copier roller and food processing equipment covering

Housewares

Thermal stability, low surface energy, chemical resistance, purity

Domestic and commercial cookware and bakeware coatings

Medical

Low surface energy, purity, excellent mechanical properties, chemical resistance

Cardiovascular grafts, heart patches, mesh grafts for hernia repair, ligament replacement, vent and intravenous infusion membranes, microporous PTFE tubing for drug dispersion

Architectural fabric

Excellent weatherability, flame resistance, low surface energy

Coated fiberglass fabric for tension and air structures, tents, and stadium and airport roofs, exhibition halls, super shopping malls, entertainment complexes

PTFE, Polytetrafluoroethylene.

2: FROM FUNDAMENTALS

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APPLICATIONS

resist attack by hydrocarbons and other fluids. In food processing, the Food and Drug Administration (FDA)-approved fluoropolymer grades are fabrication material for equipment due to their resistance to oil and cleaning materials, and their antistick and low-friction properties. In houseware, fluoropolymers are applied as nonstick coatings for cookware and appliance surfaces. These applications depend on thermal and chemical resistance as well as antistick performance. PTFE and ETFE are chosen to insulate appliance wiring that must withstand high temperatures. Medical articles such as surgical patches and cardiovascular grafts rely on the long-term stability of fluoropolymers as well as their low surface energy and chemical resistance. For airports, stadiums, and other structures, glass fiber fabric coated with PTFE is fabricated into roofing and enclosures. The architectural fabric is supported either by cables or by air pressure, thus forming a range of innovative structures. PTFE provides excellent resistance to weathering, including exposure to ultraviolet rays in sunlight, flame resistance for safety, and low surface energy for soil resistance and easy cleaning.

References [1] Band RE. Fluorine: the first hundred years. Elsevier; 1986. [2] IUPAC Periodic Table of the Elements. Reproduced by permission of International Union of Pure Applied Chemistry. r 2017 International Union of Pure Applied Chemistry. [3] Parker SP, editor. McGraw-Hill encyclopaedia of chemistry, 2nd ed. New York: McGraw Hill; 1992. [4] Kirk S, Strobel M, Lee C-Y, et al. Fluorine plasma treatments of polypropylene films, 1— surface characterization. Plasma Process Polym 2010;7:107
22. [5] Emsley J. Nature’s building blocks: an A-Z guide to the elements. Oxford University Press; 2001. p. 240
2. [6] Hopps HC. Chemical qualities of water that contribute to human health in a positive way. Sci Total Environ 1986;54:207
16. [7] Community Water Fluoridation. Centers for Disease Control and Prevention, ,www.cdc. gov/fluoridation/index.html.; 2018.

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[8] Miller MM. Fluorspar, US Geological Survey, ,http://minerals.usgs.gov/minerals/pubs/commodity/fluorspar/fluormyb03.pdf.; 2003. [9] Kirsch P. Modern fluoroorganic chemistry: synthesis, reactivity, applications. John Wiley & Sons; 2004. [10] Harta JJ, Syvret RG. Industrial scale production of Selectfluort fluorination agent: from initial concept to full-scale commercial production in a 5 year period. J Fluorine Chem 1999;100(1-2):157
61. [11] Drakesmith FG. Electrofluorination of organic compounds. Topics in current chemistry, vol. 193. Berlin: Springer; 1997. [12] Johns K, Stead G. Fluoroproducts—the extremophiles. J Fluorine Chem 2000;104:5
18. [13] Moissan H, Dewar J. Proc Chem Soc 1897;13:175
86. [14] Festa RR. Alexander Borodin: full-time chemist, part time musician. JChem Educ 1987;64 (4):326. [15] Banks RE, Smart BE, Tatlow JC, editors. Organofluorine chemistry—principles and commercial applications. Plenum Press; 1994. [16] Murphy CD. Isolation of an aldehyde dehydrogenase involved in the oxidation of fluoroacetaldehyde to fluoroacetate in Streptomyces cattleya. Appl Environ Microbiol 2001;67(10): 4919
4921. [17] Zechel DL, Withers SG. Dissection of nucleophilic and acid
base catalysis in glycosidases. Curr Opin Chem Biol 2001;5 (6):643
9. [18] Oyekanm DL, et al. β-Mannosynthase: synthesis of β-mannosides with a mutant β-mannosidase. Angew Chem Int Ed 2001;40:417
20. [19] Sandford G. Organofluorine chemistry. Philos Trans R Soc Lond A 2000;358:455
71. [20] Moss SJ, et al. Chem Commun 2000;2281. [21] O’Hagan D, Harper DB. Fluorine-containing natural products. J Fluorine Chem 1999;100: 127. [22] Zechel DL, et al. Enzymatic synthesis of carbon fluorine bonds. J Am Chem Soc 2001;123: 4350. [23] Cottrell TL. The strength of chemical bonds. 2nd ed. Washington, DC: Butterworths; 1958. [24] Sheppard WA, Sharts CM. Organic fluorine chemistry. New York: W.A. Benjamin, Inc.; 1969.

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[25] Gangal SV. Polytetrafluoroethylene. 4th ed. Encyclopedia of chemical technology, 11. New York: John Wiley & Sons; 1994. p. 621
44. [26] Gangal SV, Brothers PD. On-line encyclopedia of polymer science and technology. John Wiley & Sons; 2010.

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[27] Zisman WA. Surface properties of plastics. Rec Chem Prog 1965;26(1):13. [28] Extrand CW. The use of fluoropolymers to protect semiconductor materials. J Fluorine Chem 2003;122:121
4.

3 Fluoropolymers—Discovery, History, Evolution, and Consumption Sina Ebnesajjad FluoroConsultants Group, LLC, United States

O U T L I N E 3.1 Roy Plunkett’s Story

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3.2 Commercialization of Polytetrafluoroethylene

21

3.3 Developmental History of Fluoropolymers

21

3.4 State of Fluoropolymers 3.4.1 Energy 3.4.2 Economy

24 24 24

Nearly any person directly involved in the creation and production of polymers, and many people around the world not involved whatsoever, are familiar with the origins of the conception of fluoropolymers. After all, Teflon, a trademark of polytetrafluoroethylene (PTFE), is renowned throughout the world. The classic story of discovery of fluoropolymers is replete with the magical combination of brilliance and serendipity. The environment and the context in which the discovery occurred, however, are often overlooked. This section strives to place the invaluable discovery made by Plunkett [1] in the context of its times, events, and personalities. Plunkett’s finding is even more impressive, if at all possible, when viewed through the prism of context of his times. For those who wish to know the outcome of the story, let us begin with the ending. By 1938, Dr. Roy Plunkett had been working for 2 years on developing new fluorinated refrigerants that were safer than old gases: nonflammable, nontoxic, colorless, and odorless. He reacted tetrafluoroethylene (TFE) with hydrochloric acid (HCl) for the synthesis of a refrigerant CClF2-CHF2 [2]. As on many other occasions, on the morning of April 6, 1938, Plunkett checked the pressure on a full cylinder of

3.4.3 Resource Limitation 3.4.4 Market Demand and Growth 3.4.5 State of Technology

25 26 28

3.5 Summary

30

References

31

Further Reading

31

TFE. He found no pressure. The weight of the cylinder was the same as the day before. Plunkett and his technician removed the valve and shook the cylinder upside down. A small amount of a slippery white substance was recovered upon cutting open the gas cylinder. The waxy powder was analyzed and found to be PTFE. The rest is, of course, history.

3.1 Roy Plunkett’s Story Roy came from a poor farm family in New Carlisle, Ohio. He attended Manchester College in North Manchester, Indiana as the Great Depression began. Roy shared a room with an older student named Paul Flory. Roy graduated with a Bachelor of Arts in chemistry in 1932 and followed Paul to graduate school at the Ohio State University. Roy and Paul both earned masters and PhD degrees from the Ohio State University within 2 years of each other. Roy joined the DuPont Central Research in 1936 where Paul had been working since 1934. Roy Plunkett advanced to Kinetic Chemical Co., a joint venture that DuPont and General Motors

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00003-3 © 2021 Elsevier Inc. All rights reserved.

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(GM) had set up to produce safe refrigerants to replace ammonia and sulfur dioxide. Roy was given a laboratory in DuPont’s Jackson Laboratory on the shore of the Delaware River in Deep Water, New Jersey. Roy’s laboratory laid across the hall from Charlie, who was a young chemist and possessed a strong interest in synthesizing new organic compounds. Roy was trying to expand the line of fluorocarbons known as Freon for the explosive growth of automobile production at GM. On April 6, 1938, the day Roy Plunkett found the TFE cylinder without pressure, a small commotion erupted in his laboratory. What was this slippery white powder? He possessed time and cognizance and so paused to ask questions; there was no relentless pressure to meet next month’s deadline because people understood success in research needed a reasonably low-stress work environment. Nevertheless, Charlie came to Roy’s laboratory when he heard the racket that the occupants were making. He later said, “On another occasion, at Jackson laboratory, across the Delaware River in New Jersey where I worked, I noticed commotion in the laboratory of Roy Plunkett, which was across the hall from my own. I investigated and witnessed the sawing open of a cylinder from which was obtained the first sample of Teflon fluoropolymer.” These are the words that have been taken from Charlie Pederson’s 1987 Nobel Lecture. You see Charlie invented new crown ether compounds and was awarded the Nobel Prize for it in 1987. Roy Plunkett lived in heady times. Before long after his 1938 discovery, the world would be engulfed in the bloodiest battles ever fought in human history. World War II was looming in the picture. Of course, the polymer that Roy had found was not useful for much of anything because it melted at over 340°C and when it did melt it just sat there in a ball of clear gel that would not flow. The polymer did not dissolve in anything and did not seem to react with any acid, base, or solvent. Whether anyone would ever find use for this intractable slippery powder was unclear. Nonetheless, Roy Plunkett was rewarded for his curiosity and was promoted out of the refrigerant business and into a management role for the manufacturing of a very successful chemical, tetraethyl lead: the old poisonous octane booster which has been phased out for some time now. Wartime needs rescued Roy Plunkett’s discovery from oblivion. The Manhattan Project was a covert

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program with the aim of developing an atom bomb before Nazi Germany. Lieutenant General Leslie Richard Groves, an extremely competent and dedicated man, leads the project. He made critical decisions on prioritizing the various methods of isotope separation, acquiring raw materials needed by the scientists and engineers. General Groves’ search for new materials to meet the novel needs of the Manhattan Project brought him in touch with PTFE. After hearing about PTFE properties and its resistance to different chemicals, General Grove is purported to have said that the cost, even at $100 a pound, was a bargain! The Project badly needed corrosion-resistant materials for the uranium enrichment process. U-235 had to be separated from U-238 using differential diffusion of UF6. Nowadays, a different technology using thousands of centrifuges is used for separation of the two isotopes. UF6 is highly corrosive, even to most metals while PTFE stands up to it. Upon verification, PTFE was placed under a national “Secrecy Order” by the US Patent Office and was to be referred to as “K-416.” Only one patent with little content was issued to recognize the rights to the invention in 1941. The next time PTFE was heard of was after World War II in 1946 under the now famous trademark of Teflon. DuPont learned a great deal from the intense effort to produce PTFE for the Manhattan Project. In addition, resources formerly reserved for the war effort became available for the unending needs and pursuits of the United States and the world. Favorable was the time to move from the pilot plant to a commercial manufacturing operation. Roy Plunkett began to receive the recognition that his discovery deserved after applications was developed for PTFE and copolymers of TFE in the 1950s and 1960s. A major celebration was held at the 25th anniversary of the discovery of Teflon in 1963. Roy Plunkett received numerous honors and was toasted by the world in 1988 at the 50th anniversary of his discovery. Dr. Plunkett’s own words describing the impact of his discovery are the most fitting tribute that we can pay to him: “The discovery of polytetrafluoroethylene (PTFE) has been variously described as (1) an example of serendipity, (2) a lucky accident and (3) a flash of genius. Perhaps all three were involved. There is complete agreement, however, on the results of that discovery. It revolutionized the

3: FLUOROPOLYMERS—DISCOVERY, HISTORY, EVOLUTION, AND CONSUMPTION

plastics industry and led to vigorous applications not otherwise possible.” (From the speech at the American Chemical Society Meeting, New York, April 15 18, 1986.)

3.2 Commercialization of Polytetrafluoroethylene Efficient monomer synthesis methods, polymerization technologies, and various forms of PTFE had to be developed. Large-scale monomer synthesis and controlled polymerization were technical impediments to commercialization of the new polymer. Intensive studies resolved these problems and small-scale production of Teflon began in Arlington, New Jersey, in 1947. In 1950, commercial production of Teflon in the United States was scaled up with the construction of a new plant in Parkersburg, West Virginia. In 1947, Imperial Chemical Industries built the first PTFE plant outside the United States, in the United Kingdom. Many more plants have been built around the globe. Gujarat Fluorochemicals Limited has built the latest plant in India (c.2007). Over the last eight decades, many forms of PTFE and copolymers of other monomers and TFE have been developed and commercialized. PTFE could not be dissolved in any solvent, acid, or base and when melted formed a stiff clear gel with no flow. Special processing techniques typically used for molding metal powders were modified to mold parts from PTFE. A great deal of

21

effort was devoted to the development of fabrication technologies from the different forms of PTFE known as granular, fine powder, and dispersion. Granular variety is prepared by suspension polymerization without a surfactant. The latter two types of PTFE are produced by dispersion polymerization using a surfactant also called a polymerization aid.

3.3 Developmental History of Fluoropolymers The development of thermoplastic fluoropolymers began with the discovery of PTFE in 1938 and has continued to the recent decades. Fig. 3.1 shows the long sequence of the waves of technological innovation of fluoropolymers mostly because of new products. Several new fluoropolymer products were developed between 1940s and 1980s. In the 1980s, amorphous fluoropolymers and terpolymers like tetrafluoroethylene hexafluoroethylene vinylidene fluoride (THV) were introduced. A significant process innovation has been the use of super critical carbon dioxide (SCC) as polymerization medium [3 5]. The SCC process does not require the use of a polymerization aid/surfactant and generates less waste. An important observation from Fig. 3.1 is the long stretch time during which fluoropolymers research have born new fruits as compared to other plastics such as polyolefins. Throughout the decades, new monomers have been invented and polymerized with the older fluorinated olefinic

Figure 3.1 Technological innovation waves of thermoplastic fluoropolymers during its history.

22

compounds to develop new polymers with unique properties. Examples of monomers include TFE, vinylidene fluoride, hexafluoropropylene (HFP), perfluoropropyl vinyl ether (PPVE), perfluoroethyl vinyl ether, perfluoromethyl vinyl ether, and others. When fluorinated elastomers and novel specialty fluoropolymers are considered, the breadth of fluoropolymers is simply stunning. Examples include fluorosilicones and fluoroacrylates. Successful polymerization of TFE in CO2expanded liquids to synthesize PTFE was demonstrated using both hydrocarbon and hydrofluorocarbon solvents. Yields were satisfactory and samples with thermal properties comparable to commercial polymer were produced. The chain transfer reaction that resulted from the use of hydrogen-containing solvents was not as severe as expected. Acetic acid was identified as a particularly promising expandable solvent. The use of this solvent lowers the pressure required for polymerization of TFE, and acetic acid is less environmentally detrimental than halogenated solvents. A wide range of solvent ratios and pressures were found to be suitable for TFE polymerization in CO2-expanded acetic acid. Colloidal PTFE samples were obtained in an emulsion-like polymerization run in CO2-expanded acetic acid. A commercially available, conventional hydrocarbon surfactant was used in this research; the process did not require fluorinated surfactants such as perfluorooctanoic acid (PFOA) or other the specially designed surfactants for CO2 systems [6]. The most important fluoropolymer process change, in over a half century, has been the

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replacement of the polymerization aid (PFOA) with new polymerization aids. Ammonium perfluorooctanoate has been replaced with shorter chain compounds from 2010 to 2015. An example of the new surfactants is Dyneon ADONA with a chemical formula of:

Fig. 3.2 summarizes the timeline of the development of fluoropolymers that have brought about major changes in properties and/or fabrication processes. Low molecular weight PTFE additives were discovered in 1973 [7] by the application of ionizing radiation to the sintered or unsintered PTFE. The discovery of PTFE was a major leap forward in material science. Yet, the new polymer could not be fabricated by melt processing. The next two forms of PTFE, fine powder and dispersion, were also not melt processible. The pursuit of a more easily processible polymer led to fluorinated ethylene propylene polymer (FEP), which could be melted in an extruder. Compared with PTFE, the major disadvantage of FEP is its reduced thermal stability and lower maximum continuous use temperature (200°C) (Table 3.1). PFA, which was introduced in 1973, offers both melt processing and the same upper continuous use temperature as PTFE (260°C). Ethylene tetrafluoroethylene polymer (ETFE) addresses the need for a mechanically stronger polymer, albeit at a trade-off of some properties because of the presence of hydrogen in its

Figure 3.2 The evolution of fluoropolymers throughout its history.

3: FLUOROPOLYMERS—DISCOVERY, HISTORY, EVOLUTION, AND CONSUMPTION

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Table 3.1 Typical Properties of Fluoropolymers (1 5 Best, 5 5 Worst). Property

PTFE

PFA

FEP

ETFE Tefzel

ECTFE Halar

PVF

CTFE Aclar

Specific gravity

2.15

2.16

2.15

1.70

1.68

1.77

2.13

Tensile strength at Brk., RT, %

5000

4500

3000

6500

7000

4500

4000

Elongation at Brk., RT%

400

300

290

150

200

50

140

Flex strength, psi

No Brk.

NA

3000

7100

7000

9500

8600

Flexural modulus, psi 3 105

0.7 2 1.1

1.0

0.9

2.0

2.4

2.5

1.5

Hardness (Shore, Rockwell)

D50 2 65

D60

D55, R45

D75, R95

D75, R95

R109

R109

Izod impact Ft/ Lbs/In-Notch, RT

3

No Brk.

No Brk.

No Brk.

No Brk.

4

1.2

Melt point, °F

627

575 2 590

500 2 535

520

465

340

394

Max. oper. temp., continuous °F

550

500

400

350

340

265

350

Low temp. embrittlement, °F

2450

NA

2100

2150

2105

280

2423

Deflection temp., °F at 66 psi

250

NA

158

220

240

270

258

Deflection temp., °F at 264 psi

120

NA

NA

160

170

195

NA

Thermal expansion, 105 /In/°C

10.0

12

9.5

7

8

8.5

7.2

Dielectric strength, V/mil (0.001 in.)

4200

4000

6500

7000

2000

1280

3500

Dielectric constant, 103 cycles

,0.0003

0.0002

,0.0002

0.0008

0.0015

0.018

0.025

Water vapor permeability (ranked)

5

6

5

4

2

3

1 (best) (Continued )

24

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Table 3.1 Typical Properties of Fluoropolymers (1 5 Best, 5 5 Worst).—Cont’d Property

PTFE

PFA

FEP

ETFE Tefzel

ECTFE Halar

PVF

CTFE Aclar

Chemical resistance (ranked)

1

1

1

223

2

4

2

Coeff. of friction

1

2

3

4

4

5

5

3.4 State of Fluoropolymers

Figure 3.3 Energy picture in the early 21st century.

Over 80 years after the discovery of PTFE, a large family of fluoropolymers has been developed which continue to grow thanks to the unique properties of these plastics. Fluoropolymers have enabled new technologies and applications because of their extreme characteristics. By all indications, fluoropolymers will continue to be an essential material in the human society for the foreseeable future. Some of the influential factors on the state of fluoropolymers are described in the following paragraphs.

3.4.1 Energy molecule. Compared to PTFE, ETFE has a lower continuous use temperature (150°C), less chemical resistance, and a higher coefficient of friction. Mechanical properties including tensile strength, elongation at break, and tensile modulus are increased, leading to cut-through resistance in wire insulation. Amorphous polymers (AF) are soluble in select aromatic halogenated solvents. AF can be applied as a solution, followed by the removal of the solvent. The remaining coating will be as resistant to almost as many chemicals as PTFE. The thickness of the coating can range upward from less than a micrometer. Other polymers in the fluoropolymer family include polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), ethylene chlorotrifluoroethylene (ECTFE), tetrafluoroethylene/hexafluoropropylene/ vinylidene fluoride (THV) polymers, perfluoroacrylates, fluorinated polyurethanes, fluorosilicones, and chlorotrifluoroethylene/vinyl ether copolymers. Typical properties of commercial fluoropolymers are summarized in Table 3.1.

At present, a small fraction of consumed energy is supplied by renewable sources such as electric, wind, and solar. The output of those energy sources continues to grow as do nuclear and fossil fuel segments. Hydropower is relatively stable. The current breakdown of energy is likely to continue in the years to come (Fig. 3.3). Fluoropolymers are used in many of these energy applications because of their unique properties and are expected to find additional uses.

3.4.2 Economy The world population continues to grow in spite of the reduction rate of the growth rate in all regions (Fig. 3.4). Populations of North America, Europe, and Japan populations amount to 10% of the world. Comparison of populations shows scale of potential markets. The other 90% of the people in the world live in the third world and developing countries (over 6.5 billion, in 2019) vie for better lives. They can learn rapidly about new goods and services available using smartphones and other devices. The populations of the third world and

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25

Figure 3.4 Estimated population of the world between 1950 and 2015 and projection from 2015 to 2100 [8].

developing countries are potential customers of goods and services. Fluoropolymers are consumed by construction, automotive, aerospace, chemical processing, food and beverage, and other industries that enable production of goods demanded by people. Fluoropolymers will continue to grow for the foreseeable future because of the enabling role they play in the industry. Increasing populations will support the future growth of fluoropolymers.

3.4.3 Resource Limitation The critical resource of fluoropolymers is fluorspar primarily consisting of calcium fluoride (CaFe2). All fluorochemicals are derived initially from the manufacture of hydrofluoric acid (HF), itself produced from acid-grade fluorspar (acidspar). The reaction of HF with chloroform is the point of entry of fluorine into hydrocarbons. The largest chemical sector application for HF is in the production of fluorocarbons. In 2017, 1 million ton HF was consumed for fluorocarbons requiring over 2 million tons fluorspar [9]. Figs. 3.5 and 3.6 show the distribution of world reserves and production of fluorspar by country in 2017. Fluorspar reserves are outside United States in countries including China, Mongolia, Mexico, and South Africa. United States imports from

Figure 3.5 Distribution of world reserves of fluorspar by country in 2017 [10].

Mexico have declined over 50% since 2011 down to 200,000 MT/year in 2017. China produced about two-thirds of world consumption of fluorspar. China virtually controls the supply and price of fluorspar, thus prices of fluoropolymers. New fluorspar production capacity in Canada came online in August 2018. It is expected to increase production capacity to 200,000 MT/year acid-grade concentrate. New capacity in Africa is also expected to produce 180,000 MT/year acidgrade in 2019 [9].

26

Figure 3.6 Production of fluorspar by country in 2017 [10].

3.4.4 Market Demand and Growth PTFE is the oldest fluoropolymer and continues to have the highest share of production and consumption. The growth of PVDF and FEP resins has resulted in a slow erosion of PTFE share. For example, PTFE had a share of 60% by weight of consumption in 2008 compared to 53% in 2018. In addition to accounting for the majority of consumption, there is a PTFE production overcapacity globally. China is the biggest user of PTFE at 44% of world consumption in 2017. The country is the world’s largest producer/exporter of PTFE, with a 50% 55% share. In addition to China, PTFE is produced in all regions for at least three reasons: (1) It offers the best properties for the price in most applications. (2) It helps with the cost of TFE intended for higher value products because of enlargement of monomer scale. (3) It stabilizes the TFE manufacturing process by keeping the production rate almost constant. The last two reasons keep Western/Japanese fluoropolymer manufacturers in PTFE business [11]. United States imports increasingly higher portion of PTFE resins consumption because of cost and domestic unavailability. The latter is caused by product rationalization of remaining US suppliers and migration of PTFE production plants to China and other developing countries. United States imports more PTFE resins and products than it exports. In contrast, China and Japan have a positive balance of PTFE import/export (Figs. 3.7 and

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3.8). There is a significant overcapacity of PTFE in the world amounting to tens of thousands of tons which have suppressed the prices of PTFE resins except for modified grades. United States and China each consume about 30% 31% of melt-processible fluoropolymers. Prices of melt processible have been under less competitive pressure because of general lack of production capacity. Technology migration from the developed economies to China has been quite slow. Western/Japanese companies continue to lead this segment. FEP and PVDF resins and products are produced and exported by some Chinese companies. A key reason for FEP production in Chinese is the TFE monomer processes in China produce HFP that has to be consumed. FEP is relatively easy to manufacture and inexpensive. China and India have been slow in mastering PFA, insulation grade FEP, and other specialty meltprocessible products. United States imports significant quantities of ETFE, primarily from Japan [11]. In a study, Acumen Research and Consulting Experts estimated that the global fluoropolymer market is anticipated to witness a significant growth. They estimated growth at a compounded annual growth rate (CAGR) of around 5.9% during the period from 2016 to 2022 and reach the market value of around $9.9 billion in 2023. In terms of volume, the fluoropolymers market is expected to reach over 475 thousand tons in 2022, at a CAGR of 6.45% during the same analysis period [13]. There are numerous diverse studies and forecasts for the growth of fluoropolymers in the future. Even though the reported fluoropolymer growth rates and the market sizes by different studies do not match, they all point to a significant continued future growth of this plastic family. Majority of the future fluoropolymer growth will take place in Asia and Africa where industrial growth is taking place for the purpose of exports in addition to meeting domestic needs. Fig. 3.9 displays the result of averaging the past growth and the forecast of future growth of fluoropolymers over decades. The objective is to assess the arc of fluoropolymers growth (or decline) over it long history. Note the graph shows the rate of growth over the last eight decades not the magnitude of the fluoropolymers. In the 1940s, relatively little commercial activity took place. There was no commercial PTFE manufacturing facility during most of the decade. The first commercial PTFE

3: FLUOROPOLYMERS—DISCOVERY, HISTORY, EVOLUTION, AND CONSUMPTION

27

200 154.69

Value (US$ mn)

150 118.6

107.6

105.79

100

99.26 76.7 54.13

50

50.3

41.49 23.32

0 China

United States

Germany

Netherlands

Italy

Japan

India

Belgium

United Kingdom

28.2

27.18

Mexico

Japan

Russian Federation

Figure 3.7 Top 10 world PTFE exporting countries [12].

200 158.91

Value (US$ mn)

150

99.05

100

84.48

78.38

66.98 41.58

50

39.64

20.89

0 United States

Germany

Italy

China

South Korea

Belgium

Figure 3.8 Top 10 world PTFE importing countries [12].

Figure 3.9 Compounded annual growth rate averaged over decades, %.

France

Brazil

28

manufacturing facility was built by DuPont in Parkersburg, West Virginia with an official launch date of 1950. In the 1950s, fluoropolymers grew at an astounding rate. That decade saw post war reconstruction in Europe and in the rest of world using imported US goods. In the unharmed United States, consumer product industries that had been ignored during the war experienced massive growth. Fluoropolymers grew at the fastest rates in their history during the 1950s through the 1970s. After 1980s, while the growth rate has increased in the developing countries it has declined or remained constant in the United States and other developed countries. Growth rate of fluoropolymers over decades has leveled off at 6 5%, which is quite impressive considering the age of this thermoplastic family. Two important reasons have contributed: (1) the unique properties of these polymers allow their use under extreme chemical and thermal conditions and (2) the continued industrialization of countries in all geographic zones will continue demand for fluoropolymers.

3.4.5 State of Technology The question being addressed in this section is whether fluoropolymers have reached the maturity stage after over 70 years of commercial life. What criterion one should use to assess maturity? This topic has been the subject of numerous studies and publications. One of the approaches to assess the state of a particular technology is to track the arc of occurrence of new inventions. US Patents have been filed for virtually every invention/innovation in the past. The public nature of those patents renders them quite useful for search and discovery of significant developments in product and process areas for most technologies including those in fluoropolymers. An example of this approach is described in this section. Let us first discuss invention versus innovation which is somewhat controversial. There has been a great deal of debate in publications about the differences between the natures and significance of the two events. We choose one of the simpler definitions for this section’s discussions. “In its purest sense, invention can be defined as the creation of a product or introduction of a process for the first time. Innovation, on the other hand, occurs if someone

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improves on or makes a significant contribution to an existing product, process, or service” [14]. For example, light bulb was a clear invention by Thomas Edison. Irving Langmuir’s discovery, that by filling a light bulb with inert gas like nitrogen instead of vacuuming out the air doubled the light bulb’s efficiency, was an innovation. Langmuir’s discovery is considered enabling because of its significant contribution to commercial use of light bulbs. Nick Holonyak, Jr., invented the first visiblespectrum light emitting diode in the form of red diodes. It was an invention rather than an innovation because of the vastly different mechanisms of light generation from Edison’s light bulb [15]. How is one to evaluate patents after finding them? Terry Ludlow has described in an article a simple and effective method to assess the potential value of patents. A subject specialist can search US Patent databases and find a series of inventions and enabling innovations. “The traditional patent valuation techniques, cost, citation, market, and income based, are not particularly relevant to intellectual property and licensing professionals. Since they have no bearing on the true value of patents from a strategic business perspective, they will not be discussed further. Instead, we will focus on the three essential factors that impact patent value: 1. Patent validity. This has to do with how claims were written, their ability to stand up to scrutiny under current legal systems, and whether or not they were truly new or could be subject to prior use assertions. Essentially, a consideration is the patent valid? 2. Technical merit. This looks at the usefulness of a patent’s claimed invention. It includes its unique ability to solve a problem or be used in a particular product/product type. It is important to consider here whether or not the claims accurately reflect the invention. 3. Essentially. This refers to how widely the invention protected by the patent is used in various products and markets. It takes into account its competitive position and the revenue it generates. It also examines whether or not the invention is part of an industry standard” [16]. As an example, the author has applied the above parameters to patents addressing preparation of

3: FLUOROPOLYMERS—DISCOVERY, HISTORY, EVOLUTION, AND CONSUMPTION

PTFE powders by suspension polymerization and part fabrication from those powders. The results are listed in Table 3.2 in which the essential characterizing parameters of each patent have been identified. The same data has been exhibited in Fig. 3.10 which one could call the arc of technology development for the preparation of granular PTFE. The data in Table 3.2 and Fig. 3.10 reflect the experience and views of this author with respect to the overall impact of the inventions on the entire

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commercial granular (moldable) PTFE products. It is understood someone else may choose a different list from this author’s but it is doubtful the differences would be significant. A few words about the structure of Table 3.2 and the patent selection should facilitate the reader’s understanding of the discussion. Most of the format of the table is self-explanatory but for the last column under the heading of impact factor (IF). This factor can be defined by considering the

Table 3.2 Patents (Inventions) That Define the Arc of PTFE Preparation by Suspension Polymerization (Granular). US Patent Number

Invention

Year Granted

Inventor(s)

Impact Factor (1 10, Lowest to Highest)

2,230,654

First polymerization of PTFE

1941

Roy Plunkett

10

2,393,967

High pressure TFE polymerization, peroxy/persulfate initiator

1946

Merlin M. Brubaker

9.5

2,400,099

First billet molding and sintering process

1946

Brubaker and Hanford

9

2,456,621

Presintered PTFE, coating metal surfaces with PTFE

1948

Jack Cheney

8

NA

Low temperature polymerization (work-around the Western technology)

1949

2,936,301

PTFE usable in thin sheet (,0.5 in.) molding

1960

Elliot and Wallace

7.5

3,087,921

First free flow PTFE powder (using water)

1963

Mathews and Roberts

7

3,115,486

First time cutting of PTFE bead in water using Fitzmill

1963

Weisenberger

8

3,245,972

Seeded, two-stage seeded suspension polymerization of TFE

1966

Anderson, Edens, and Larsen

#1

3,265,679

First solvent-based free flow PTFE resin

1966

Black and Foust et al.

6

3,532,782

Solvent-based pelletization of PTFE powders with fine or coarse grind

1970

Hartwimmer

5

3,690,569

High bulk density fine-cut PTFE (Taylor Stiles cutter)

1972

Leverett

3.5

3,766,133

High bulk density filled and filled PTFE

1973

Roberts and Anderson

3.5

3,855,191

PPVE modified PTFE

1974

Doughty, Sperati, and Un

3

5?

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Figure 3.10 Patents (inventions) that define the arc of PTFE preparation by suspension polymerization (granular).

extent of the effect of the patent on the entire granular/moldable PTFE business. For example, the first patent (1941) granted for Roy Plunkett’s discovery impacts 100% of all commercial PTFE resins including the granular variety thus the IF of 10. The second patent (US Patent 2,393,967, 1946) in Table 3.2 is a process invention by Brubaker with impact on nearly all granular PTFE which explains the IF of 9.5. It basically allowed commercial scale manufacture of granular PTFE. US Patent 2,400,099 disclosed techniques to mold PTFE into cylindrical preformed billets and sintering of those billets. This was the first time a commercially useful stock shape of PTFE was developed. At the time of this invention, its impact on granular PTFE was overwhelming. Invention of other molding techniques over time lessened the impact of US Patent 2,400,099 to some extent.

Maturity state of various thermoplastics fluoropolymers are covered in detail in the author’s upcoming book Fluoropolymers in the 21st Century (Elsevier, to be published in 2021).

3.5 Summary A burst of new fluoropolymer products was developed between the 1940s and1970s. In the 1980s, amorphous fluoropolymers and terpolymers like THV (a terpolymer of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride) were introduced. No truly new (inventive) fluoropolymer product has been introduced since the 1980s. PTFE product and technologies have long reached maturity. Ecctreme by Chemours is a significant innovation, though it has found little commercial success

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because of its cost. It is a combination of two existing classes of fluoropolymers as described in US Patent 8,993,695 [17]. Polymerization of fluoropolymers in SCC was patented in the 1990s though its commercial success has been limited. Non-PTFE thermoplastic fluoropolymer products and technologies are for the most part mature. Revolutionary inventions are unlikely to take place while competitive pressures are likely to grow in this industry. Fluoropolymers continue to grow at a healthy rate because of continuing needs for their properties and a lack of viable alternatives.

[7] [8]

[9] [10]

References [1] Plunkett RJ. US Patent 2,230,654, assigned to DuPont Co., 1941. [2] Plunkett RJ. The history of polytetrafluoroethylene: discovery and development. In: Seymour RB, Kirshenbaum GS, editors. High performance polymers: their origin and development, Proceedings of symposium on the history of high performance polymers at the ACS Meeting in New York, April 1986. New York: Elsevier; 1987. [3] DeSimone JM, Guan Z, Elsbernd CS. Synthesis of fluoropolymers in supercritical carbon dioxide. Science 1992;257(5072):945 7. [4] Du L, Kelly JY, Roberts GW, DeSimone JM. Fluoropolymer synthesis in supercritical carbon dioxide. J Supercritic Fluids 2009;47:447 57. [5] Du L, Kelly JY, Roberts GW, DeSimone JM. Fluoropolymer synthesis in supercritical carbon dioxide. J Supercritic Fluids 2009;. [6] Du L, DeSimone JM, Roberts GW. Fluoropolymer synthesis in carbon dioxideexpanded liquids: a practical approach to avoid

[11] [12] [13] [14]

[15] [16]

[17]

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the use of perfluorooctanoic acid. In: Hutchenson KW, Scurto AM, Subramaniam B, editors. Gasexpanded liquids and near-critical media, green chemistry and engineering, Vol. 1006. ACS Publication; 2009. Dillon J. US Patent 3,766,031, assigned to Garlock, 1973. World population prospects: the 2017 revision. Department of Economic and Social Affairs, United Nations; 2017. Roskill. Market report—fluorspar, ,https:// roskill.com/market-report/fluorspar/.; 2019. O’Driscoll M. China supply shortages hit consumers as new sources emerge. Aluminum International Today; 2017. ,https://ihsmarkit.com.; 2019. ,www.plasticsinsight.com.; 2019. Acumen Research and Consulting. Globe Newswire, ,www.globenewswire.com.; 2018. Grasty T. The difference between “invention” and “innovation.” April 3, 2012, Updated December 06, 2017. The history of the light bulb. Department of Energy. ,www.Energy.gov.; 2019. Ludlow T. What is the best way to assess the potential value of a patent portfolio?, ,www. ipwatchdog.com.; 2016. Lahijani J. US Patent 8,993,695, assigned to DuPont Co.; 2015.

Further Reading Ebnesajjad S, Morgan RA. Fluoropolymer additives. 2nd ed Elsevier; 2019. Ebnesajjad S. 2nd ed Non-melt processible fluoroplastics, vol. 1. Elsevier; 2015.

4 History of Expanded Polytetrafluoroethylene and W.L. Gore & Associates Sina Ebnesajjad FluoroConsultants Group, LLC, United States

O U T L I N E 4.3.1 Summary

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4.1 Early History of W.L. Gore & Associates

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4.2 Discovery of Expanded Polytetrafluoroethylene

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4.3 Who Invented Expanded Polytetrafluoroethylene?

4.4 Other Expanded Polytetrafluoroethylene Players 4.4.1 Gore EU and Japan

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References

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In 2018, W.L. Gore & Associates celebrated the 60th anniversary of the founding of their company. Coincidentally, 2018 marked the 80th year since the discovery of polytetrafluoroethylene (PTFE) by Roy Plunkett. Gore is where expanded polytetrafluoroethylene (ePTFE) was discovered and mainly perfected over the years. To be sure, many other companies have contributed to the development of ePTFE. The discovery stories of PTFE and ePTFE are unique and intertwined—similar yet dissimilar. Both discoveries were the result of the technical brilliance, creativity, perseverance, and business savvy of their respective masterminds. Foremost among the common personal qualities is curiosity, when what could have been considered a setback or an odd effect by many was recognized and pursued with vigor. These two developments have resulted in countless new materials that have vitally contributed to the lives of mankind for over a half-acentury, to say nothing of tens of billions of dollars of business generated. There are disparities between the evolution processes of the above discoveries. DuPont was already a large corporation and over a century old when Plunkett came across PTFE. In contrast, W.L. Gore was a successful small company barely a decade old when Robert “Bob” Gore discovered ePTFE, trade-marked Gore-Tex by the company.

ePTFE transformed W.L. Gore into a multibilliondollar giant of creativity that has continued to find new uses for ePTFE. From day one, Gore has engaged in manufacturing fabricated products using ePTFE. W.L. Gore has never sold ePTFE members that at its inception would have been a profitable route to the market. In contrast, DuPont has always sold PTFE and other fluoropolymers as basic materials to processors and fabricators. Without the discovery of PTFE and fluoropolymers, DuPont would have still been a large corporation, albeit somewhat diminished. In the absence of ePTFE, W.L. Gore & Associates, whatever its fate, would not have been the company that it became thanks to Gore-Tex. This is one of the reasons that it is virtually impossible to talk about ePTFE without discussing W.L. Gore & Associates. The indispensability of ePTFE from Gore and vice versa warrants a review of Gore’s company history while describing the discovery and evolution of ePTFE. Studying Gore as a company is a worthwhile effort because of its unique management style and structure that have been credited for its sustained growth through innovation and creativity. This chapter describes the inception and evolution of ePTFE and W.L. Gore as one story. The association of ePTFE (Gore-Tex) and Gore is due to the efforts made by the company to launch this

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00004-5 © 2021 Elsevier Inc. All rights reserved.

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technology into a myriad of applications. The end use products utilizing ePTFE have enhanced the lives of mankind beyond imagination. There is some controversy about which company or individual invented the concept of PTFE expansion first. Regardless of the answer to this question, it is W.L. Gore that continues to propel ePTFE to new frontiers. The question of inventorship of expansion technology is discussed later in this chapter. Over time, competitors to Gore producing ePTFE membranes and products containing them sprouted. The important contributions of many of these companies to the ePTFE technology have been described elsewhere [Ref Ebnesajjad S. Expanded PTFE applications handbook, Elsevier, Oxford; 2017]. A nonexhaustive list of companies participating in ePTFE manufacturing is provided at the end of this chapter.

4.1 Early History of W.L. Gore & Associates Wilbert (“Bill”) L. Gore was born in 1912 in Meridian, Idaho. He spent the majority of his formative years in Salt Lake City, Utah. Bill studied chemistry and engineering and received a Bachelor of Science degree in chemical engineering in 1933 and a Master of Science in chemistry in 1935 from the University of Utah in Salt Lake City. Bill was a quiet and modest man with a passion for innovation and tinkering. In 1935, he married Genevieve Walton who, in addition to being a wife, became Bill’s lifetime business partner. Both Bill and Vieve developed a great love for outdoors, which was bequeathed to their five children. That so many of the Gore-Tex apparel fabrics enhance the outdoors experience of sportsmen, hunters, and others is not surprising! In 1941, Bill Gore joined the DuPont Company where he was assigned to working on advancing the company’s research into polymers, resins, and plastics. During the Second World War, PTFE was placed under a Secrecy Order, which prevented DuPont from developing commercial products. The Secrecy Order was lifted in 1946, opening the way for commercialization of PTFE. A new plant was built in Parkersburg, West Virginia, to produce the new polymer. Bill Gore worked on the

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development of new applications for PTFE during the next 12 years while flurry of research got underway to find new resin manufacturing technology and uses for Teflon PTFE. The focus ranged from solving fundamental problems of polymerization and finishing tetrafluoroethylene polymers to finding end uses and markets for the product. Bill Gore’s interests were focused primarily on finding new uses for Teflon. In fact, the fundamental properties of PTFE rendered the material useful for many applications. There was no material that possesses all the properties of PTFE, including low dielectric constant (good electrical insulator), high thermal resistance, low coefficient of friction, low flammability, resistance to UV light, hydrophobicity and oleophobicity, and chemical inertness. Only imagination could expand the breadth of new applications for this special plastic. In 1958, Bill Gore left the DuPont Company to establish his own business. DuPont in the 1950s was a basic materials supplier and participated in a few fabricated products. Or it did not go down the value chain, as it is said. The Company was over one-and-a-half centuries old, which gave it a well-entrenched culture. Corporate environments were (are) hardly fertile grounds for nonconformers, mavericks, and those who stand out. Bill’s research interests gravitated to new and novel end uses of PTFE while DuPont was more interested in selling the bulk resin. Bill Gore found a need to leave the large corporate environment to pursue his interests more fully. The suggestion to leave the employment of DuPont has been attributed to his wife Vieve. In addition to being the mother of five children, she supported the new fledgling company in many ways. Vieve continued her role until the end of her life in 2005 when W.L. Gore & Associates had approached annual sales of $2 billion. W.L. Gore & Associates’ first commercially viable products were wire and cable products insulated with PTFE. Bill Gore’s oldest child, Bob, played an important role in these innovations. Bob, who at the time was a chemical engineering student at the University of Delaware, is credited with discovering the concept that resulted in Gore’s first patent [Ref US Patent 3,082,292, assigned to Robert W. Gore, September 22, 1964] for PTFE-insulated wire and cable [1]. Some cynics have claimed Bill Gore was the real inventor of the PTFE expansion

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process but his past employment at DuPont made it too risky for him to claim credit for the invention. That cynical view of the story is not supported by the available evidence and testimonials. W.L. Gore & Associates’ first order was for 7.5 miles of insulated ribbon cable (Fig. 4.1) from the City of Denver, Colorado. For the first 10 years, these products comprised the core of the W.L. Gore & Associates’ sales. Multi-Tet cables, as they were trademarked, were recognized for high performance in the defense industries and the nascent field of computers where high signal speed was required which was allowed by PTFE’s low dielectric constant. The cables were even used in the Apollo space program for the first moon landing.

4.2 Discovery of Expanded Polytetrafluoroethylene By the late 1960s, W.L. Gore & Associates was a successful wire and cable supplier. Bob Gore joined the company’s board of directors in 1961. Bob earned a bachelor’s degree from the University of Delaware in 1959, and masters and PhD degrees from the University of Minnesota, all in chemical engineering, before joining the company full time in 1963. Over time, competition to Gore’s cables grew and Bill Gore had to look for ways to reduce cost and develop new products for diversification. One way to cut cost and improve performance, and perhaps to create a new form of PTFE, seemed to be to stretch the PTFE insulation [3]. Bill’s thought was to introduce air into the polymer

Figure 4.1 First W.L. Gore product [2]. Courtesy W. L. Gore & Associates, Inc.

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structure and basically create a foam form of PTFE. Using lower amounts of polymer for the construction of the insulation would reduce the cost of the cables. Note that PTFE is a thermoplastic resin but it is not melt processible because of its ultrahigh melt viscosity. The latter rendered its stretch extremely difficult. Bob placed rods of PTFE in an oven and attempted to stretch the heated rods by hand. But the rods broke disregarding the temperature Bob used and the rate at which he stretched them. Of course, PTFE stretches when it is elongated at very slow rates (,5 cm/min). These rates were not commercially practical and still are not feasible. The story goes that late one night in 1969 Bob became frustrated because of his inability to stretch the PTFE rods. As Gore explained, “We were having really bad luck with that so I started to experiment with it at high temperatures. The more carefully I tried to stretch the material, the more easily it broke. That seemed counterintuitive to me. One evening, I took a piece that had been treated at high temperature and gave it a fast yank, and was surprised to find that it stretched 1,000 percent, rather than the 10 to 20 percent we had been seeing.” [4] (Fig. 4.2). Gore ascertained that ePTFE (trademarked Gore-Tex) was both “very porous and very strong.” The ePTFE gave a unique microstructure of “nodes” and “fibrils” which defined the porous regions (Fig. 4.3).

Figure 4.2 Bob Gore’s depiction of Gore-Tex discovery [2]. Courtesy W. L. Gore & Associates, Inc.

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Figure 4.3 Scanning electron micrograph of an ePTFE membrane. Wikol M, Hartmann B, Brendle J, Crane M, Beuscher U, Brake J, Shickel T. Expanded PTFE membranes and their applications. In: Jornitz MW, Meltzer TH, (editors). Filtration and purification in the biopharmaceutical industry. 2nd ed. Boca Raton, MA: CRC Press; 2007. Informa Healthcare (Chapter 23).

The discovery set the stage for the creation of hundreds of products and fundamentally altered the trajectory of the manufacturing efforts of the company: “I guess that would be my biggest discovery, the basic Gore-Tex material,” Bob Gore once noted. The expanded form possessed the basic properties of PTFE including chemical inertness, low friction constant, wide use temperature range, hydrophobicity, outdoor durability and biocompatibility plus porosity, air permeability, and extreme strength. Given this range of properties, the potential applications of the expanded form were limitless. Today ePTFE is found in thousands of medical, industrial, fiber, and fabric products, as well as in electronic products [4]. A typical example of ePTFE application is filtration fabric produced by Parker Hannifin BHA Company. Bob Gore and his family originally lived in the Rockies, where they would “hike and go backpacking for several weeks, carrying everything on [their] backs.” Given his family’s original habitation, Bob Gore finds the use of ePTFE in outdoor garments and adventure gear personally rewarding. Moreover, ePTFE has filled a large need in the field of medicine because of its inertness and biocompatibility. Millions of people have received

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ePTFE medical implants, which are configured to exclude or accept tissue in-growth depending on the needs of the specific application. Biocompatible ePTFE is used in vascular grafts, stents, cardiovascular and soft tissue patches, facial implants, surgical sutures, and endovascular prostheses. Since first ePTFE application as joint sealants, the number of its products used in the industrial arena has skyrocketed. W.L. Gore & Associates still produces sealants and some of world’s tightest, most chemically resistant gaskets. Sealant products were engineered to produce composite sealants and packing material, such as graphite, oils, and ePTFE for specific uses [Ref Snyder RA. US Patent 4,256,806, assigned to W.L. Gore & Associates; March 17, 1981]. The ePTFE membrane is the key to filtration products for a range of particle sizes, from pollutants found in the energy, mineral, metal, and chemical industries to clean room and computer disk drive microcontaminants [4]. The original Gore product line, which constituted of insulated wires and cables, benefited from the discovery of ePTFE. ePTFE combines the key characteristics of PTFE with the electrical properties of air. It has greater thermal stability, lower loss tangent, higher propagation velocity, more flexibility, and a lower dielectric constant than the solid PTFE. It has been used in printed circuit boards, electromagnetic interference shielding material, and fiber optic assemblies. Its diverse applications are found in the defense industry, industrial automation, computers, telecommunications, and medical technologies. W.L. Gore & Associates has two fundamental core principles that have underpinned the company’s growth [5]. The first being the pursuit of product development through leadership in fluoropolymers, and particularly ePTFE. The second being a commitment to creating a unique and fulfilling work environment: a commitment initiated and articulated in the early days of the company by Bill and Vieve Gore. Bob Gore’s accomplishments have been recognized by numerous awards: the Society of Chemical Industry winner of the 2005 Perkins Medal, election to the National Academy of Engineers, the Society of Plastics Engineers award for benefits to society through the use of plastics, an award for lifetime achievement in fluoropolymers from the DuPont Co., the Winthrop-Sears Award from the Chemical Heritage Foundation and the 2006 induction into the National Inventors Hall of Fame.

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W.L. Gore & Associates has always been heavily committed to research and development, resulting in continual broadening of the range of its products. The Company’s unique corporate culture, termed the “lattice structure,” stresses freedom, fairness, individual commitment, and good judgment in an open and creative work environment. For instance, associates have no titles, communicate directly with one another, and work closely together in teams and task forces. Gore Associates believe this unique culture enables the company to respond quickly to changing market developments and has been a key element in the company’s success and growth [6 9].

4.3 Who Invented Expanded Polytetrafluoroethylene? For decades, W.L. Gore & Associates (hereafter in this section also referred to as “Gore”) had battled disputes over its patent rights to ePTFE and to various products made with the membranes. This section describes some of the events that have occurred since the inception of controversy over the ownership of ePTFE technology in the 1970s. The main sources of available information are legal documents issued by courts as a result of legal actions undertaken by different companies. The proceedings are sufficiently significant that they warrant a review in a historical discussion of ePTFE and Gore. Sources are provided for further reading. The first important action was filed by Gore on November 2, 1979 against Garlock. The suit was filed in the Federal District Court for the Northern District of Ohio. At issue was infringement of process claims 3 and 19 of the US Patent 3,953,566 assigned to W.L. Gore, by Garlock. Gore sought injunctive relief, damages, and attorney fees. Garlock counterclaimed on December 18, 1979, for a judgment of invalidity of the patent, noninfringement, fraudulent solicitation, and entitlement to attorney fees. On February 7, 1980, Gore filed a second suit for infringement of product claims 14, 18, 36, 43, 67, and 77 of the US Patent 4,187,390 assigned to W.L. Gore. The Federal District Court for the Northern District of Ohio consolidated the two suits for trial [10]. The Federal District Court for the Northern District of Ohio ruled that both Gore U.S. Patents 3,953,566 and 4,187,390 were invalid. Gore appealed the ruling to Federal Circuit of US Court

W.L. GORE & ASSOCIATES

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of Appeals. The Appeals Court reversed the District Court’s invalidation ruling of the two patents on November 14, 1983. The Court reinstated some of the claims and remanded the infringement to the District Court for adjudication. Gore’s patent rights were later restored. W.L. Gore & Associates brought action against on April 3, 1984 in the Federal District Court of Arizona. Gore filed a law suit against a company named IMPRA and accused them of infringing its US Patent 4,187,390 (‘390). The patent discloses “a tetrafluoroethylene polymer in a porous form (Fig. 4.4) which has an amorphous content exceeding about 5% and which has a microstructure characterized by nodes interconnected by fibrils. The material has high porosity and high strength. It can be used to produce all kinds of shaped articles such as films, tubes, rods, and filaments.” The final judgment resulted in the abandonment of the final 4 years on the ‘390 patent. This lengthy and rather fascinating case dates back to the early 1970s. It concerns the invention of vascular grafts, artificial blood vessels from ePTFE. The proceedings recounted here do not cover all the details of this complex case. A concise review of this case has been published by Alison Frankel in the November edition of The American Lawyer [12]. Nearly every imaginable action and appeal has been made by the parties throughout the years. Dr. David Goldfarb was trained as a cardiologist at Johns Hopkins and taught at the University of Pittsburgh until 1973. He took a position at the Arizona Heart Institute in Phoenix to teach and research. One of the areas of Dr. Goldfarb’s investigation was artificial blood vessel development. He was approached by W.L. Gore at Flagstaff with a

Figure 4.4 Example of an ePTFE structure [11].

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proposal to work on the new Gore-Tex ePTFE material. He was one of the number of researchers who had been approached by Gore for the purpose of finding new uses for Gore-Tex. Dr. Goldfarb set out to evaluate the new ePTFE material to construct artificial blood vessels. Goldfarb conducted a series of experiments involving 21 grafts made from the tubes Cooper provided. On June 13, 1973, the graft labeled “2-73 RF,” which came from Lot 459-04133-9 provided by Cooper, was determined to be a successful implant in a dog. He was the first of the researchers to create a useful blood vessel from the Gore-Tex, one that could successfully be incorporated in the human body. He believed that intermodal distance, the distance between two adjacent nodes in Fig. 4.3, was an important factor in the success of the blood vessel. He began to order tubes of Gore-Tex with different porous ePTFE structures for evaluation. A successful ePTFE blood vessel would be amenable to cell growth into its pores, which then integrate the artificial blood vessel in a patient’s body. Dr. Goldfarb recognized that artificial vascular prosthesis made from expanded porous PTFE had to have a microstructure consisting of nodes interconnected by fibrils which permitted tissue ingrowth. The fibrils had to have the length in the range of above about 5 µm up to 100 µm. This seemingly simple fact became the core issue that would later determine the fate of this case. In the spring of 1974, Dr. Goldfarb was told by his liaison with Gore (a man named Detton) that W.L. Gore had filed a patent application for a vascular graft. The liaison had been visiting the doctor’s laboratory on a near weekly basis and had been taking orders for different porous structure ePTFE tubes. He was thus aware of the specifications of the candidate tubes with which Dr. Goldfarb was working. The liaison proceeded to tell Dr. Goldfarb that the manager of Gore’s plant in Flagstaff, Peter Cooper, was the inventor. Goldfarb was astonished because neither the liaison nor the plant manager was well educated. Besides, he had not heard anything about the development of vascular grafts at Gore. He could not accept the situation. Dr. Goldfarb joined the board of a start-up company in Tempe called International Medical Prosthesis Research Associates, Inc, abbreviated IMPRA. The former Gore liaison, Detton, who had left the company, was among the partners forming

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IMPRA. In May 1974, IMPRA hired a patent attorney who investigated Goldfarb’s research work and determined an invention had taken place. Dr. Goldfarb, who had resigned from IMPRA to focus on his research filed, for a patent in October 1974. He assigned the rights to his patent to IMPRA in exchange for funding his research work. His partners at IMPRA supported Goldfarb’s patent application by affidavit. Detton’s was quite significant because of his close involvement with both W.L. Gore and Goldfarb during the development. IMPRA failed to provide funding to Goldfarb, resulting in a lawsuit being filed in 1976. Dr. Goldfarb and IMPRA settled their lawsuit in 1979, and in an assignment dated May 21, 1979, IMPRA assigned to Dr. Goldfarb all rights in the Goldfarb Application [13]. In 1983, the patent examiner determined that both Gore and Goldfarb had filed valid applications for patentable invention. The true owner would be determined by the Board of Patent Appeals and Interferences of US Patent and Trademark Office. Twelve years passed before the Board reached a decision. In 1995, the Board of Appeals ruled that Dr. David Goldfarb was the rightful inventor of the vascular grafts. In another turn of events, C. R. BARD acquired IMPRA in August 1996. Gore challenged the Board of Appeals ruling and then the inventorship of the patent by Goldfarb to the Federal Circuit of the Court of Appeals. Neither appeals were successful; the Appeals Court refused to overturn the Board of Appeals ruling. Finally, in 2002, Doctor Goldfarb was awarded the US Patent 6,436,135 (Fig. 4.5). The patent date is an extraordinary 28 years after the application date (October 1974). Bard then approached Gore to negotiate a licensing arrangement. Bard became entitled to royalty on the past and future sale of vascular grafts upon the issuance of the Goldfarb patent. Gore continued with the sale of the products and the parties failed to reach an agreement. Bard (and Goldfarb) sued for infringement and eventually the issue was tried before a jury at the Federal District Court of Arizona in 2007. The jury verdict was in favor of Bard and Goldfarb and a substantial award was granted. The jury awarded Bard 186 million US dollars for lost profit and past royalty in December 2007. Gore challenged the validity of the patent awarded by the jury to Goldfarb, arguing that Goldfarb and his attorneys had withheld evidence from the US Patent and Trademark Office during

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Figure 4.5 Cover page of US Patent 6,436,135.

the application process. In July 2008, the judge ruled against Gore and doubled Bard’s damages and ordered Gore to pay nearly 20 million dollars of attorney fees [13]. In a subsequent appeal, Gore argued invalidity of the Goldfarb patent on technical grounds, which the judge rejected in March 2009 [14]. The district court awarded Bard enhanced damages by a factor of two, doubling Bard’s award from the $185,589,871.02 jury verdict amount to

$371,179,742.04. The court also awarded Bard its attorneys’ fees and nontaxable costs in the amount of $19 million. Additionally, the court denied Bard’s motion for a permanent injunction, but granted Bard’s alternative motion for the imposition of an ongoing royalty. The court awarded Bard an ongoing royalty with a range of royalty rates from 12.5% to 20% for Gore’s various types of infringing grafts. The district court entered an amended final judgment on August 24, 2010, and Gore

40

appealed to the US Court of Appeals for the Federal Circuit on August 25, 2010. From here on the pleadings for legal appeals became even more complex. On February 10, 2012, a three-judge panel of the US Court of Appeals for the Federal Circuit affirmed the March 31, 2009 ruling by Judge Murguia by a vote of 2:1. The majority opinion stated: “Thus, although Gore attempts to recast its argument from inurement in the Interference to joint inventorship in the present case, Gore’s argument remains unchanged and there is still no evidence that Cooper either recognized or appreciated the critical nature of the internodal distance and communicated that key requirement to Goldfarb before Goldfarb reduced the invention to practice. Accordingly, substantial evidence supports the jury’s finding that the ’135 patent (US Patent 6,436,135) is not invalid for improper inventorship, and the district court did not err in denying Gore’s motion for judgment as a matter of law on the issue.” Judge Mary Murguia of the US District Court for Arizona initially ruled the patent was granted rightfully based on the invention by Dr. David Goldfarb who later assigned it to Bard. Her first decision boosted Bard’s $185.6 million jury award to $371.2 million, prompting Gore to appeal. In response to Gore’s further appeal, the US Court of Appeals for the Federal Circuit today reaffirmed its opinion issued on February 10, 2012, with caveats, on June 14, 2012. The Court vacated sections of the ruling related to its prior discussion of willfulness. Gore’s briefs related to the petition for rehearing presented the Court with a new question. The Federal Court remanded the issue of willfulness so that the trial court may reconsider its denial of judgment as a matter of law of no willful infringement in view of this holding. “If the court grants the judgment as a matter of law, it should then reconsider its decisions on enhanced damages and attorneys’ fees.” The Federal Circuit bench tasked Murguia with reviewing her willfulness finding. In October 2013, Murguia upheld that ruling, finding Gore’s infringement willful and declining to revisit her decision on enhanced damages and legal fees and denying Gore’s bid for a new trial. Gore appealed again to the Federal Circuit, which once again affirmed Murguia [Ref Perriello B. Gore asks US Supreme Court to revisit $1B loss to Bard; July 15, 2015, MassDevice, www.MassDevice.com]. Finally, in October 2012 Gore appealed the case to the Supreme Court of the United States

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(W.L. Gore v. C.R. Bard, 12-458). The Supreme Court rejected Gore’s appeal without giving an explanation, as part of a list of orders released on January 14, 2013. In July 2013, the U.S. Patent and Trademark Office upheld the validity of the Bard patent. Bard promptly filed a motion to compel payment, up to $900 million, in the US District Court for Arizona, which was granted by the Court. Gore unsuccessfully appealed the order of the Arizona District Court to the United States Court of Appeals for the Federal Circuit, which denied its petition for rehearing on April 8, 2015. Gore appealed the case to the Supreme Court of the United States on July 7, 2015. W.L. Gore & Associates Petition for certiorari to the US Supreme Court was denied on October 5, 2015.

4.3.1 Summary In summary, it is clear that W.L. Gore & Associates values intellectual property and will vigorously defend its position. New developments are time-consuming, expensive, and a significant investment for Gore. Successful products have been at the core of the phenomenal growth of past decades. Protection of new inventions has continued to be a key component of Gore strategy. A contrasting and undeniable point to the patent controversies is the vital role that W.L. Gore has played in developing and increasing the usage of vascular grafts in medical procedure. Nothing can detract from the contribution of Gore to the medical device and prosthetics fields. W.L. Gore produces a variety of medical products, examples of which can be seen at www.gore. com/en_xx/products/medical/index.html.

4.4 Other Expanded Polytetrafluoroethylene Players Naturally, the lucrative business that W.L. Gore has created attracts competitors. The expiration of Gore’s basic patents accelerated the competition’s growth in all economic regions of the world. The supplier’s size, technological capabilities, and product offerings fall within a broad range anywhere from small independent companies to global powers such as Donaldson Corporations. Nearly every company has adopted, at least to some extent, Gore’s business model of selling designed products primarily

4: HISTORY

OF

EXPANDED POLYTETRAFLUOROETHYLENE

AND

fabricated with ePTFE. This approach, generally, allows capture of a larger value for the ePTFE by the manufacturers. Yet most of these companies continue to sell membranes and laminates.

4.4.1 Gore EU and Japan Japan Gore-Tex Inc. (JGI), a wholly owned Gore subsidiary, functions as the operational base of the Gore group in Japan. JGI is engaged in the development, manufacturing, and sales of diversified Gore-Tex products in the industrial, fabrics, and medical fields. Gore Germany plants are clustered in Putzbrunn and Feldkirchen-Westerham, near Munich, and Pleinfeld, which is close to Nuernberg. The Pleinfeld team focuses on electronic interconnect products. Associates in Putzbrunn and nearby Feldkirchen-Westerham produce or add value to fabrics, membranes, fibers, industrial sealants, filtration products, and vents. Medical product sales are centered in Putzbrunn. Gore facilities in Dundee Scotland produce electronic interconnects. Two plants in Livingston (Scotland or the United Kingdom; specify) are dedicated to fabrics and filtration products and medical sales. Gore manufactures fabrics in a facility in Shenzhen. The Shenzhen facility is also licensed to make certain electronic products. A Gore joint venture company in Shanghai, Shanghai Bag Filtration Equipment Co., Ltd., manufactures filtration products. It is understood that Gore manufactures small quantities of specialty fluoropolymer material at a plant in Shanghai. Donaldson Corporation (www.Donaldson.com) is the largest filtration company in the world. It acquired Tetratec Company in 1994. Acquisition of Tetratec has allowed Donaldson a significant degree of vertical integration in products that contain ePTFE membranes. The company purchases PTFE resin and sells filtration products. Prior to acquisition by Donaldson, Tetratec produced and supplied ePTFE membrane but it did not participate in the manufacturing of fabricated products from its membranes. The company’s membrane business was focused on multiple markets of filtration, apparel, vents, and other markets. Parker Hannifin BHA (www.BHA.com) is a wholly owned subsidiary of Parker Hannifin targeted at reducing particulate emissions. Prior to acquisition by Parker Hannifin BHA was a part of

W.L. GORE & ASSOCIATES

41

GE Membranes. Its principal business is the design, manufacture, and sale of replacement parts and the performance of rehabilitation conversion services for the types of industrial air pollution control equipment known as bag houses, cartridge collectors, and electrostatic precipitators. DeWal Industries (www.DeWal.com) produces ePTFE for filtration and venting applications. DeWAL produces ePTFE for specialized uses such as blood filters, gas sensor membranes, and other applications that require submicron pore sizes. Zeus Industrial Products (www.ZeusInc.com) began producing ePTFE products in 2008. Zeus was founded in 1966 and has developed a broad array of multilumen tubing for medical applications. They have been producing ePTFE tubing for a number of years and have now extended their ePTFE offering a variety of shapes, including ultrathin membranes (,, 10 µm). Since the 1990s, Zeus has established an extensive research and development sector that has resulted in products made from high-performance thermoplastics. C. R. Bard Corporation is involved with the design, manufacture, packaging, distribution, and sale of medical, surgical, diagnostic, and patient care devices worldwide. Bard manufactures its own ePTFE membranes, in contrast to the majority of medical device manufacturers that buy membranes. It offers vascular, urology, oncology, and surgical specialty products. The company’s vascular products include percutaneous transluminal angioplasty catheters, guide-wires, introducers and accessories, peripheral stents, stent grafts, vena cava filters, and biopsy devices; electrophysiology products, such as electrophysiology laboratory systems and diagnostic, therapeutic, and temporary pacing electrode catheters; and fabrics, meshes, and implantable vascular grafts. Phillips Scientific is a small manufacturer of ePTFE products and production systems equipment. The company’s advertised products include tubing, filters, valve stem packing, protective pads, hex rod profiling, joint sealant, thread, tubing, tapes, membranes, and sheeting. It also provides laminating such as ePTFE membrane to felts, spunbonds, mesh, and film. Sumitomo Electric Industries, Japan, produces ePTFE membranes and products from ePTFE membranes. They offer porous separation membranes exclusively, and as membrane modules. Applications include wastewater treatment, clarification, and sterile filtration. The modules are used for gas dissolution and deaeration.

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INTRODUCTION

Some of the other manufacturers of ePTFE membranes and products from ePTFE include

• • • • • • • •

Maquet Cardiovascular

• • • • • •

Shanghai da Gong New Materials

Porex Corporation Phillips Scientific Markel Corp Asian Manufacturers: Daikin Industries Nitto Denko Shanghai Lingqiaq Environmental Protecting Works Anhui Lite Environment Shanghai Linflon Film Technology Yeu Ming Tai Chemical Ind. Mupor, LTD Leetex Technologies

References [1] www.fundinguniverse.com/company-histories/ WL.-Gore-amp;-Associates-Inc-CompanyHistory.html [2] W. L. Gore & Associates, Inc. [3] Manz Charles C, Sims Henry P. Business without bosses: how self-managing teams are building high. John Wiley & Sons; 1993.

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[4] American Institute of Chemical Engineers. Schools salute their engineers, mini profiles, www.aiche.org/About/Centennial/Mini/Profiles/ gore.aspx; 2009. [5] http://dedo.delaware.gov/information/databook/ technology.pdf. [6] The culture of W.L. Gore & Associates, Jim Findlay Product Specialist, Gore-Tex Fabrics Division; 2007. [7] www.gore.com/en_xx/aboutus/culture/index. html; 2009. [8] Innovation management—an overview and some best practices, John P. Riederer, University of Wisconsin, Melanie Baier, Universita¨t Paderborn, Siemens Business Services GmbH & Co. OHG, vol. 4, No. 3, ISSN 1619-7879; 2005. [9] Hamel G, Breen B. The future of management. Boston, MA: Harvard Business School Press; 2007. [10] W L. Gore & Associates, Inc v. Garlock, Inc. United States Court of Appeals, Federal Circuit, 0020721.F.2d.1540; November 14, 1983. [11] Gore RW. US Patent 4,187,390, assigned to W L. Gore & Associates; February 15, 1980. [12] Frankel A. Blood Money—who invented the miraculous artificial blood vessel. The American Lawyer; 2009. [13] Federal District Court of Arizona, ruling by Judge Murguia, dated July 29, 2008. [14] Federal District Court of Arizona, ruling by Judge Murguia, dated March 31, 2009.

5 Introduction to Thermoplastic Fluoropolymers Sina Ebnesajjad FluoroConsultants Group, LLC, United States

O U T L I N E 5.1 Introduction

44

5.2 Fluoropolymer Classifications

44

5.3 Fluoropolymer Products

45

5.4 Monomer Synthesis 5.4.1 Synthesis of Tetrafluoroethylene 5.4.2 Synthesis of Hexafluoropropylene 5.4.3 Synthesis of Perfluoroalkylvinylethers 5.4.4 Synthesis of Chlorotrifluoroethylene 5.4.5 Synthesis of Vinylidene Fluoride 5.4.6 Synthesis of Vinyl Fluoride

45 45 46 46 47 47 48

5.5 Monomer Properties 5.5.1 Properties of Tetrafluoroethylene 5.5.2 Properties of Hexafluoropropylene 5.5.3 Properties of Perfluoroalkylvinylethers 5.5.4 Properties of Chlorotrifluoroethylene 5.5.5 Properties of Vinylidene Fluoride 5.5.6 Properties of Vinyl Fluoride

48 48 48 49 49 49 49

5.6 Polymerization and Finishing 49 5.6.1 Polytetrafluoroethylene (CAS number 9002-84-0) 49 5.6.2 Perfluoroalkoxy Polymer (CAS number 26655-00-5) 50 5.6.3 Perfluorinated Ethylene
Propylene Copolymer (CAS number 25067-11-2) 50 5.6.4 Ethylene-co-tetrafluoroethylene Polymers (CAS number 68258-85-5) 51 5.6.5 Ethylene-co-chlorotrifluoroethylene Polymers (CAS number 25101-45-5) 51

Important thermoplastic fluoropolymers include polytetrafluoroethylene (PTFE), perfluoroalkoxy copolymer (PFA), fluorinated ethylene
propylene copolymer (FEP), ethylene
tetrafluoroethylene

5.6.6 Polychlorotrifluoroethylene (CAS number 9002-83-9) 5.6.7 Polyvinylidene Fluoride (CAS number 24937-79-9) 5.6.8 Polyvinyl Fluoride (CAS number 24981-14-4)

51 51 52

5.7 Structure Property Relationship

52

5.8 Properties of Polytetrafluoroethylene 5.8.1 Polytetrafluoroethylene Properties 5.8.2 Perfluoroalkoxy Copolymer Properties 5.8.3 Fluorinated Ethylene
Propylene Copolymer Properties 5.8.4 Polychlorotrifluoroethylene Properties 5.8.5 Ethylene
Tetrafluoroethylene Copolymer Properties 5.8.6 Ethylene
Chlorotrifluoroethylene Copolymer Properties 5.8.7 Polyvinylidene Fluoride Properties 5.8.8 Polyvinyl Fluoride Properties

53 53 54

5.9 Fabrication Techniques

56

54 54 55 55 55 55

5.10 Applications

56

5.11 Safety

56

5.12 Polymerization Surfactant

57

5.13 Economy

58

5.14 Summary

58

References

58

copolymer (ETFE), ethylene
chlorotrifluoroethylene copolymer (ECTFE), polychlorotrifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF), and polyvinyl fluoride (PVF). This chapter describes polymer

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00005-7 © 2021 Elsevier Inc. All rights reserved.

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preparation, properties, fabrication, applications, safety considerations, and economics of fluoropolymers. Monomer synthesis and properties are also described.

5.1 Introduction In this chapter, a fluoropolymer refers to polymers of olefinic monomers including partially or fully fluorinated monomers such as vinylidene fluoride (VDF) (CH2 5 CF2) and tetrafluoroethylene (TFE) (CF2 5 CF2). These polymers have been discussed in a number of references (see the General References). Specialized fluorinated polymers including perfluoroethers, fluoroacrylates, and fluorosilicones are consumed in significantly smaller volumes than olefinic fluoropolymers. These specialized fluoropolymers and others have been described in detail elsewhere [1
3] and are not covered in the present chapter. Commercial fluoropolymers include homopolymers and copolymers. Homopolymers contain 99% or more by weight of one monomer and 1% or less by weight of another monomer according to the convention of test methods published by the American Society for Testing Materials (ASTM). Copolymers contain more than 1% by weight of one or more comonomers. The major

Figure 5.1 Chronological evolution of fluoropolymer products. PCTFE, Polychlorotrifluoroethylene; PVDF, polyvinylidene fluoride; PVF, polyvinyl fluoride; FEP, fluorinated ethylene propylene polymer; ECTFE, ethylene chlorotrifluoroethylene polymer; ETFE, ethylene tetrafluoroethylene polymer; PFA, perfluoroalkoxy polymer; AF, amorphous fluoropolymer.

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FLUOROPOLYMERS

commercial fluoropolymers are based on three monomers: TFE, vinylidene fluoride (VF2), and to a lesser extent chlorotrifluoroethylene (CTFE). Examples of comonomers include perfluoromethyl vinyl ether (PMVE), perfluoroethyl vinyl ether (PEVE), perfluoropropyl vinyl ether (PPVE), hexafluoropropylene (HFP), CTFE, perfluorobutylethylene (PFBE), and exotic monomers such as 2,2-bistrifluoromethyl-4,5-difluoro1,3-dioxole. A good rule of thumb to remember is increasing the fluorine content of a polymer molecule increases its chemical and solvent resistance, flame resistance, and photostability. Additionally, several polymer properties improve such as a decrease in the dielectric constant and coefficient of friction, an increase of melting point and its thermal stability, and weakening of mechanical properties. Solubility of polymers in solvents usually decreases when fluorine content of the molecule increases.

5.2 Fluoropolymer Classifications The serendipitous discovery of PTFE in 1938 by Roy Plunkett, a DuPont Company chemist [4], began the era of fluoropolymers. PTFE has found thousands of applications because of its unique properties. Numerous fluoroplastics (Fig. 5.1) have been developed since the discovery of PTFE. These plastics are produced by several companies in the United States, Europe, Japan, China, India, and Russia. Fluoropolymers are divided into two classes of perfluorinated and partially fluorinated polymers. Perfluorinated fluoropolymers are homopolymers and copolymers of TFE. Some of the comonomers may contain a small amount of elements other than carbon or fluorine. For example, PFA is a copolymer of TFE and perfluoroalkylvinylether (PAVE) that contains oxygen. Rf is a perfluoroalkyl group containing from one to four carbon atoms (C1
C4).

Partially fluorinated fluoropolymers contain hydrogen (H) or other atoms such as chlorine or

5: INTRODUCTION

THERMOPLASTIC FLUOROPOLYMERS

45

bromine in addition to fluorine and carbon. The most significant are homopolymers and copolymers of VDF. There are also thermoplastic homoand copolymers of CTFE. There are commercial copolymers of ethylene with fluorinated monomers. They include ETFE and ETFE. PVF is only available as a homopolymer from a single supplier.

was obtained by bromination and separation of the dibromide (CF2Br
CF2Br) from the other reaction products. Dehalogenation with zinc was the next step for obtaining pure TFE. Commercially significant techniques for TFE preparation list fluorspar (CaF2), as source for hydrofluoric acid (HF) preparation (Fig. 5.2). HF and chloroform are the starting ingredients for TFE as the reaction sequence in Fig. 5.2 exhibits [7
14]. HFP and a small amount of highly toxic perfluoroisobutylene are among the byproducts of TFE. Edwards et al. [15] demonstrated the impact of adding steam on the conversion of chlorodifluoromethane and the yield of TFE at different residence times. A ratio of 3 mol steam to 1 mol chlorodifluoromethane was maintained constant. The mixture with steam was preheated to 400°C and then held in a tubular reactor for a brief period of time at 700°C. In comparison with the pyrolysis reaction in the absence of steam, far higher yields of TFE were obtained in the presence of steam. Conversion of CHClF2 remained nearly constant. Sherratt [16] and others [17] have provided complete descriptions of preparation of TFE. The overall yield of TFE production depends on the pyrolysis reaction. The products of pyrolysis are cooled, scrubbed with a dilute basic solution to remove HCl, and then dried. The resulting gas is compressed and distilled to separate and recover the unreacted CHClF2 and highly purified TFE. Polymerization of TFE to high molecular weights

TO

5.3 Fluoropolymer Products PTFE cannot be fabricated using conventional melt-processing techniques because of its high viscosity using melt-processing techniques (1010
1012 poise at 380°C). Melt-processible fluoropolymers have been developed by copolymerization of TFE, VDF, and CTFE. For example, FEP, a copolymer of TFE and HFP, has a lower maximum continuous use temperature than PTFE (200°C vs 260°C) because of the deterioration of its mechanical properties. PFA, a copolymer of TFE with PPVE, PEVE, or PMVE, offers thermal stability, melt-processibility, and maximum continuous use temperature of 260°C. Both FEP and PFA are considered prefluoropolymers. Copolymers of ETFE and ECTFE are mechanically stronger than prefluoropolymers but have a higher coefficient of friction than PTFE. They also have reduced chemical resistance and continuous use temperature than PTFE. Amorphous copolymers of TFE with exotic monomers such as 2,2-bistrifluoromethyl-4,5difluoro-1,3-dioxole are soluble in special halogenated solvents. They can be applied to surfaces as a polymer solution to form thin coatings. The dried coating is resistant to almost as many chemicals as PTFE [5].

5.4 Monomer Synthesis 5.4.1 Synthesis of Tetrafluoroethylene The first reliable and complete report of TFE synthesis was published in 1933 by Ruff and Bretschneider [6] in which they prepared TFE (CF2 5 CF2, CAS number 116-14-3) from the decomposition of tetrafluoromethane in an electric arc. TFE

Figure 5.2 Synthesis tetrafluoroethylene.

reactions

of

46

requires extreme purity. Consequently, all traces of telogenic hydrogen or chlorine-bearing impurities must be removed. TFE can auto polymerize, if it is not inhibited with terpenes such as α-pinene, Terpene B, or di-limonene [18]. An extensive description of TFE and other monomers for polymerization of fluoropolymers can be found in the other books [19,20].

5.4.2 Synthesis of Hexafluoropropylene HFP (CF3CF 5 CF2, CAS number 116-15-4) was first prepared by pyrolysis by Benning et al. [21]. Complete synthesis and identification of HFP was conducted by Henne and Woalkes [22]. A six-step reaction scheme starting with the fluorination of 1,2,3-trichloropropane led to 1,2-dichlorohexafluoropropane. The latter compound was dehalogenated with zinc in boiling ethanol to yield HFP. HFP is produced as a coproduct in the synthesis of TFE. HFP yield can be increased by altering the reaction conditions, in lieu of TFE production, by reduction in the pyrolysis temperature and use of steam as diluent of the reactants [23,24]. HFP can be synthesized from hexachloropropylene via a multistep process beginning with fluorination [25]. Later steps convert the initial products to CF3-CFCl-CF3, which is dehalogenated to yield HFP. Other techniques have reported on the synthesis of HFP from mixtures of a variety of linear and

Figure 5.3 Synthesis of perfluoroalkylvinylethers [29
31].

INTRODUCTION

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FLUOROPOLYMERS

cyclic three-carbon hydrocarbons with a partially halogenated three-carbon acyclic hydrocarbon. TFE and HFP can be produced by pyrolyzing one or more of the following compounds: fluoroform, chlorodifluoromethane, chlorotetrafluoroethane, a mixture of chlorodifluoromethane and chlorotetrafluoroethane, and a mixture of chlorodifluoromethane and perfluorocyclobutane [26]. The reaction products include fluoroolefins such as TFE and HFP. The reaction took place in a gold-plated tubular reactor at a temperature in the range of 600°C
1000°C.

5.4.3 Synthesis of Perfluoroalkylvinylethers An important class of TFE comonomers is PAVE such as PPVE (CF2 5 CF
O
C3F7, CAS number 1623-05-8). These monomers are synthesized according to the steps shown in Fig. 5.3 alternative techniques [27]. There are other processes including electrochemical methods for the production of perfluoro-2-alkoxy-propionyl fluoride [28]. PAVEs, in general, can be synthesized from hexafluoropropylene oxide (HFPO) as seen in Fig. 5.3. There are several steps in the preparation method beginning with the conversion of HFP to HFPO by reacting HFP with hydrogen peroxide. In the next step, HFPO is reacted with a perfluorinated acyl fluoride (RfCOF). In the third step, the alkoxy intermediate compound is reacted with an alkaline salt containing oxygen usually a carbonate such as

THERMOPLASTIC FLUOROPOLYMERS

47

sodium carbonate. Different PAVEs are made by the selection of Rf group. PPVE is made by the reaction of perfluoroethyl acyl fluoride CF3
CF2
CFO with HFPO. There are also electrochemical processes for the production of perfluoro-2-alkoxy-propionyl fluoride [28]. Hung and Rozen have described a process for the preparation of PAVEs by fluorination, with elemental fluorine, of selected novel partially fluorinated dichloroethyl ethers, followed by dehalogenation to the corresponding PAVE. PAVEs have found to be useful as comonomers for preparation of thermoplastic resins and elastomers [32].

consisted of a carbon-based support onto which copper was deposited, and at least one Group VIII metal (ruthenium, rhodium, iridium, platinum, and palladium and the mixtures) of the Periodic Table of Elements. The weight content of copper in the catalytic composition ranged from 16% to 19%.

5: INTRODUCTION

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5.4.4 Synthesis of Chlorotrifluoroethylene This monomer is simple to manufacture compared to the perfluorinated monomers [26,33
35]. The commercial process for the synthesis of CTFE (CF2 5 CClF, CAS number 79-38-9) begins with 1,1,2-trichloro-1,2,2-trifluoroethane (TCTFE). It is dechlorinated by pyrolysis at 500°C
600°C in the vapor phase. An alternative method for preparation of CTFE is catalytic dechlorination of TCTFE: CCl3 2 CCl3 1 HF-CCl2 F 2 CClF 2 1 2HCl
catalyst SbClx Fy CCl2 F 2 CClF2 1 Zn-CFCl 5 CF2 1 ZnCl2 ðat 50°C
100°C in methanolÞ The reaction stream is put through a number of purification and distillation steps to remove the gaseous and liquid contaminants. CTFE is further purified by the removal of methyl chloride, dimethyl ether, and water by passing the gas stream through sulfuric acid. Water and hydrochloric acid are removed by passing the CTFE through an alumina column before condensing it to liquid. A 1987 process [36] produced CFTE by dechlorination in the vapor phase of 1,1,2-trichloro-1,2,2trifluoroethane with hydrogen in the presence of an alkali magnesium fluoride catalyst. The reaction took place at 175°C with this catalyst. Reactivation of the catalyst took place by passing oxygen, air, or another gas mixture with oxygen over the catalyst at a temperature in the range of 400°C
600°C. Another process [37
39] for preparing CTFE was by the reaction of 1,1,2-trichloro-1,2,2-trifluoroethane in the presence of hydrogen and a catalyst in the gas phase. The composition of catalyst

5.4.5 Synthesis of Vinylidene Fluoride There are numerous ways to prepare VDF (CF2 5 CH2, CAS number 75-38-7) [40,41]. Two methods, including the popular commercial technique for VDF production, are described. Conversion of 1,1,1-trifluoroethane [42] begins by passing the gas through a platinum-lined Inconel tube, which is heated to 1200°C. Contact time is about 0.01 second. The exit gases are passed through a sodium fluoride bed to remove the HF and are then collected in a liquid nitrogen trap. VDF is separated by lowtemperature distillation. Unreacted trifluoroethane is removed at 247.5°C then recycled. A popular commercial technique begins with hydrofluorination of acetylene followed by chlorination [43], by hydrofluorination of trichloroethane [44], or by hydrofluorination of vinylidene chloride [45]. In each case, the final product, 1-chloro-1,1difluoroethane, is stripped of one molecule of hydrochloric acid to yield VDF. The principal route to VDF has been the dehydrochlorination of 1-chloro-1,1-difluoroethane (HCFC-142b, 75-68-3) [43]. The principal producers are Arkema and Solvay in Europe and the United States and Solvay in the United States. Many patents exist for preparation schemes based upon dehydrohalogenation of various chlorofluorohydrocarbons or related compounds. A wave of new research efforts on the manufacture of VDF was spurred when the curtailment HCFC production was announced in the mid-2000. Companies began to develop alternative methods; for example, Kureha (Japan) produces VDF from 1,1-difluoroethane (HFC-152a). In another method [46], the reaction for pyrolysis of 1,2-dichloro-2,2-difluoroethane in the presence of hydrogen was carried out in the absence of catalyst in an essentially empty reactor at temperatures exceeding 400°C. The term “absence of catalyst” means the absence of a conventional catalyst. A typical catalyst has a specific surface area and is in the form of particles or an extrudate, which may

48

INTRODUCTION

optionally be supported to facilitate the dehydrochlorination reaction by reducing its activation energy. Suitable reactors are constructed from quartz, ceramic (SiC), or metallic reactors. In this case, the reactor material was selected from metals such as nickel, iron, titanium, chromium, molybdenum, cobalt or gold, or their alloys. The metal, chosen more particularly to limit corrosion or catalytic phenomena, may be bulk metal or metal plated onto another metal.

5.4.6 Synthesis of Vinyl Fluoride Vinyl fluoride (CHF 5 CH2, CAS number 75-02-5) [47
51] was first prepared by the reaction of 1,1difluoro-2-bromoethane (CAS number 359-07-9) with zinc. Most approaches to vinyl fluoride synthesis have employed reactions of acetylene (CAS number 74-862) with hydrogen fluoride (HF) either directly or utilizing catalysts. Other routes have involved ethylene (CAS number 74-85-1) and HF; pyrolysis of 1,1difluoroethane (CAS number 624-72-6) and fluorochloroethanes; reaction of 1,1-difluoroethane with acetylene; and halogen exchange of vinyl chloride (CAS number 75-01-4) with HF [52
54]. Addition of HF to acetylene appears a likely commercial route for the preparation of VF, although details of the commercial processes have not been published. A 2003 permit issued to DuPont Company, issued by Jefferson County, Kentucky, USA [55] describes the actual commercial process: Difluoroethane (DFE) is reacted to form vinyl fluoride and hydrogen fluoride. A natural gas fired process heater supplies molten salt used to maintain the reactor temperature. The gaseous reaction products are separated and the hydrogen fluoride and difluoroethane are recycled back to the DFE process. The crude vinyl fluoride is purified and stored until shipped by railcar or truck tanker. Acidic vent gases from this process are controlled by the emergency scrubber of the process. Commercial VF is stabilized with terpenes such as d-limonene to inhibit autopolymerization. Stabilization of VF is required before it can be transported or stored safely. Terpenes are removed by distillation prior to charging VF to the polymerization reactor.

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Additional information about vinyl fluoride including its synthesis has been published elsewhere [56].

5.5 Monomer Properties 5.5.1 Properties of Tetrafluoroethylene TFE [17,57] is a flammable, explosive, colorless, odorless, tasteless, nontoxic gas which boils at 276.3°C and melts at 2142.5°C. Critical temperature and pressure of TFE are 33.3°C and 3.92 MPa. TFE is stored as a liquid; vapor pressure at 220°C is 1 MPa. Its heat of formation is reported to be 2151.9 kcal/mole. Polymerization of TFE is highly exothermic and generates 41.12 kcal/mole heat. TFE undergoes free radical addition reactions typical of other olefins. It readily adds Br2, Cl2, and I2, halogen halides IBr and ICl; and nitrosyl halides, such as NOCl and NOBr. A variety of other compounds such as alcohols, primary amines, and ammonia can be reacted with TFE to prepare tetrafluoroethers (HCF2CF2OR), difluoroacetamide (HCF2CONHR), and substituted triazines. Oxygen can be added to TFE to produce polymeric peroxide or TFE epoxide. In the absence of hydrogen, sodium salts of alcohols will react with TFE to yield trifluorovinylethers (Rf-O-C2F3), which can be homo and copolymerized. Safe storage of TFE requires reduction of its oxygen content to less than 20 ppm. Temperature and pressure should be controlled during TFE storage. Increasing the temperature, particularly at high pressures, can initiate deflagration in the absence of air during which TFE degrades into carbon tetrafluoride and carbon. In the presence of air or oxygen, TFE forms explosive mixtures in the molar percentage range of 14%
43% [58]. Detonation of a mixture of TFE and oxygen can increase the maximum pressure to 100 times the initial pressure.

5.5.2 Properties of Hexafluoropropylene HFP is a colorless, odorless, tasteless, and relatively low toxicity gas, which boils at 229.4°C and freezes at 2156.2°C. In a four-hour exposure, a concentration of 3000 ppm corresponded to LC50

THERMOPLASTIC FLUOROPOLYMERS

49

in rats [49,50,59]. Critical temperature and pressure of HFP are 85°C and 3254 MPa. Unlike TFE, HFP is extremely stable with respect to autopolymerization and may be stored in the liquid state without the addition of a telogen. HFP is thermally stable up to 400°C
500°C. At about 600°C under vacuum, HFP decomposes and produces octafluoro-2-butene (CF3CF 5 CFCF3) and octafluoroisobutylene [60]. HFP readily reacts with hydrogen, chlorine, bromine, but not iodine, by an addition reaction similar to other olefins [61
63]. It reacts with HF, HCl, and HBR, but not HI. By reacting HFP with alcohols, mercaptans, and ammonia, hexafluoro ethers (CF3CFHCF2OR), hexafluoro sulfides and tetrafluoropropionitrile (CF3CFHCF2SR), (CF3CFHCN) are obtained. Diels
Alder adducts have been identified from the reaction of anthracene, butadiene, and cyclopentadiene with HFP [64]. Cyclic dimers of HFP can be prepared at 250° C
400°C under autogenous pressure [53]. Linear dimers and trimers of HFP can be produced catalytically in the presence of alkali metal halides in dimethylacetamide [52].

as chlorodifluoroacetylfluoride [68]. The same reaction can occur photochemically in the vapor phase. CTFE oxide is a byproduct of this reaction. The peroxides act as initiators for the polymerization of CTFE, which can occur violently.

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5.5.3 Properties of Perfluoroalkylvinylethers PAVEs are [28] an important class of monomers used for the “modification” of the properties of PTFE in addition to melt-processible copolymers of TFE. The advantage of PAVEs as modifiers over HFP is their remarkable thermal stability. A commercially significant example is PPVE. It is an odorless colorless liquid at room temperature. It is extremely flammable and burns with a colorless flame. PPVE is less toxic than HFP. Detailed information about the properties of PPVE has been published including by manufacturers [65
67].

5.5.4 Properties of Chlorotrifluoroethylene CTFE is a colorless gas at room temperature and pressure. It is fairly toxic with an LC50 (rat) at 4 hours of 4000 ppm [33] It has a critical temperature and a pressure of 105.8°C and 4.03 MPa. Oxygen and liquid CTFE react and form peroxides at fairly low temperatures. A number of oxygenated products are generated by oxidation of CTFE, such

5.5.5 Properties of Vinylidene Fluoride VDF (CH2 5 CF2) is flammable and is gaseous at room temperature [69,70]. It is colorless and almost odorless and boils at 284°C. VDF can form explosive mixtures with air. Polymerization of this gas is highly exothermic and takes place above its critical temperature and pressure.

5.5.6 Properties of Vinyl Fluoride Vinyl fluoride [75-02-5] (VF) (fluoroethene) is a colorless gas at ambient conditions [50]. Vinyl fluoride is flammable in air between the limits of 2.6% and 22% by volume. Minimum ignition temperature for VF and air mixtures is 400°C. Adding a trace amount (,0.2%) of terpenes is effective to prevent spontaneous polymerization of vinyl fluoride. Inhibited vinyl fluoride has been classified as a flammable gas by the US Department of Transportation.

5.6 Polymerization and Finishing 5.6.1 Polytetrafluoroethylene (CAS number 9002-84-0) PTFE is polymerized [57,71] by free-radical mechanism in an aqueous media via addition polymerization of TFE in a batch process. The initiator for the polymerization is usually a water-soluble peroxide such as ammonium persulfate or disuccinic peroxide. A redox catalyst is used for lowtemperature polymerization. Polymerization temperature and pressure usually range from 0°C to 100°C and 0.7 to 3.5 MPa. PTFE is produced by suspension polymerization without a surfactant to obtain granular resins. PTFE may also be produced in the presence of a fluorinated surfactant by dispersion polymerization. The use of the longstanding surfactant ammonium perfluoro octanoate (APFO) has been discontinued in the developed economies due to its environmental persistence, bioaccumulation, and health characteristics.

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Granular PTFE is produced by polymerizing TFE alone or in the presence of small amounts of a comonomer. A peroxide initiator, little or no surfactant and other additives may be present in the aqueous polymerization medium that is vigorously stirred and sometimes buffered by an alkaline solution. Most of the polymer is formed in the gas phase in the shape of stringy irregular particles. The particles are comminuted to different sizes, depending on the powder properties required by the fabrication process. For example, a smoother surface part requires smaller particle size while powder flow is enhanced by increased particle size. Emulsion grade PTFE is produced by polymerization of TFE in an aqueous medium in the presence of an initiator and a surfactant. The polymerization takes place by dispersion mechanism. To avoid premature coagulation, stability of the dispersion is balanced against the need to break the emulsion to recover the PTFE particles. A low melting point is usually added to the aqueous allowing polymerization to higher solids concentrations. Low shear rate agitation is maintained during the polymerization using a surfactant at below the critical micelle concentration. The rate of polymerization and particle shape and size are affected by the concentration of the surfactant. Majority of the particles are generated in the early part of polymerization and grow as the cycle proceeds. Average molecular weight and composition within the particle can be controlled by the use of polymerization ingredients and conditions. The dispersion polymerization process makes aqueous dispersions of PTFE that are used in coatings and other applications. PTFE dispersion is concentrated and stabilized using ionic and nonionic surfactants. Several concentration methods have been reported including electrodecantation, evaporation, and thermal concentration. Additives are incorporated in PTFE dispersions to modify them for different fabrication processes or to impart specific properties. Filled compounds of PTFE are produced from all three forms of PTFE using fillers such as glass fiber, graphite, metal powder, carbon fiber, and others [71]. Another class of PTFE is called fluoroadditives. These additives have small particle size and relatively low molecular weights compared to other grades of PTFE. They are produced mainly by radiation (electron beam) or thermal degradation of

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high molecular PTFE. They are also produced by direct polymerization as low molecular weight PTFE or as fluoroelastomers. Fluoroadditive powders are commonly used in mixtures with other solid or liquid materials, as minor components, including lubricants, plastics, elastomers, paints, and others. Even as a minor phase fluoroadditives can impart some of the unique properties of PTFE to the host materials [72].

5.6.2 Perfluoroalkoxy Polymer (CAS number 26655-00-5) PFA is a copolymer of TFE and a PAVE like PPVE. Perfluoroalkyl vinyl ethers can be copolymerized with TFE in a halogenated solvent [73,74], in an aqueous phase [75], or water containing some halogenated solvent, usually in the absence of a surfactant [76]. Terpolymers of PFA contain other monomers such as HFP. Commercially, PFA is polymerized by a freeradical mechanism usually in an aqueous media by addition polymerization of TFE and perfluoropropyl vinyl ether. The initiator for the polymerization is usually a water-soluble peroxide such as ammonium persulfate. Chain transfer agents including methanol, acetone, and others are used to control the molecular weight of the resin. Aqueous polymerization regime of PFA resembles the method by which PTFE emulsion is prepared, that is, dispersion polymerization. Polymerization temperature and pressure typically range from 15°C to 95°C and 0.5 to 3.5 MPa. End groups are stabilized by treating the PFA with methanol, ammonia, amines, and elemental fluorine. F treatment generates CF3 end groups [77
80]. The polymer particles are recovered, dried, and melt-extruded into cubes for melt fabrication processes. PFA is also available in bead (i.e., as polymerized), dispersion, and fine powders forms.

5.6.3 Perfluorinated Ethylene
Propylene Copolymer (CAS number 25067-11-2) FEP is a random copolymer of TFE and HFP, polymerized in an aqueous or a nonaqueous media [76,81]. Terpolymers of FEP contain other monomers such as PAVE (e.g., PPVE) intended to improve stress crack resistance.

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Commercially, FEP is polymerized by free-radical polymerization mechanism usually in an aqueous media via addition polymerization of TFE and HFP. It can also be prepared in a nonaqueous medium. The initiator of the aqueous polymerization is a watersoluble peroxide such as potassium persulfate. Chain transfer agents could be used to control the molecular weight of the resin. In general, the polymerization regime and conditions resemble those used to produce PTFE by emulsion polymerization. FEP particles are recovered, dried, and melt-extruded into cubes for melt fabrication processes. FEP is also available in the dispersion form. FEP polymer chains contain unstable end groups. It is important to stabilize those end groups to ensure the polymer has sufficient thermal stability. Stabilized FEP does not produce volatile byproducts that generate bubbles and promote corrosion. Stabilization of FEP end groups can be accomplished by different techniques. Traditionally, FEP is subjected to humid heat treatment. In that method, FEP is treated with water at elevated temperatures, which prevents the formation of carboxylic acid groups. The treatment causes decarboxylation, if the carboxylic acid groups are in ionic form, accompanied by slow formation of very stable
CF2H end groups [82].

5.6.4 Ethylene-cotetrafluoroethylene Polymers (CAS number 68258-85-5) This plastic is a partially fluorinated straight-chain polymer produced in high molecular weights [76]. It is produced by a free-radical polymerization mechanism in a solvent or a hybrid media (a solvent/aqueous mixture), using an organic peroxide initiator. Copolymerization of TFE and ethylene (CH2 5 CH2, CAS 74-85-1) proceeds by addition mechanism. Copolymers of TFE and ethylene are highly crystalline and fragile at elevated temperatures. Mechanical properties of the copolymer are improved by modifying the copolymer with a third monomer. Production of ETFE terpolymers with improved high temperature mechanical (especially tensile) properties has been demonstrated and commercialized [83]. They are typically comprised of 40
60 mole% ethylene, 40%
60% TFE, and a small amount of a polymerizable vinyl tertiary monomer such as perfluoroisobutylene, PPVE, and HFP.

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5.6.5 Ethylene-cochlorotrifluoroethylene Polymers (CAS number 25101-45-5) Ethylene and CTFE have been copolymerized [34] in aqueous and solvent mediums using organic peroxides and oxygen-activated triethylboron. Typical polymerization takes place at a temperature of 60°C
120°C and a pressure of 5 MPa or higher. Polymerization reaction may be initiated by radiation like γ rays. The most effective catalyst is tri-nbutyl boron, which produces an ECTFE with an alternating ethylene to TFE ratio of 1:1. To control the molecular weight of the resin, chain transfer agents such as chlorinated compounds, alcohols, and ketones were required.

5.6.6 Polychlorotrifluoroethylene (CAS number 9002-83-9) CTFE is polymerized by bulk, suspension, and dispersion techniques [84]. Bulk polymerization takes place using halogenated acyl peroxide catalysts or ultraviolet and γ rays. Suspension polymerization is carried out in aqueous medium using inorganic or organic peroxide catalysts. Dispersion polymerization yields a polymer with a normal molecular weight distribution; and a molecular weight-melt viscosity relationship similar to bulk polymerized polymer. Inorganic peroxy catalysts initiate the reaction in the presence of halogenated alkyl acid salt surfactants. Dispersion polymerization produces the most thermally stable PCTFE.

5.6.7 Polyvinylidene Fluoride (CAS number 24937-79-9) The first successful aqueous polymerization of VDF was reported in 1948 [85] using a peroxide initiator in water at 50°C
150°C and 30 MPa. No surfactants or suspending agents were present in the polymerization recipe. PVDF has been polymerized by a number of methods including emulsion, suspension, solution, and bulk. Later, copolymers of VDF with ethylene and halogenated ethylene monomers were also developed and some commercialized [86]. In 1960, a manufacturing process was developed and PVDF was introduced to the market. Polymerization reaction temperature ranges from 10°C to 150°C at a pressure of 1 MPa or higher.

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INTRODUCTION

Similar to TFE, emulsion polymerization of VDF requires a stable fluorinated surfactant and an initiator such as a peroxide or persulfate. Suspension polymerization is conducted in an aqueous medium sometimes in the presence of a colloidal dispersant like hydroxyl cellulose. Solution polymerization of VDF in solvents uses free radical initiators. Commercial PVDF resins are manufactured by aqueous dispersion or suspension polymerization methods [76].

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4. Melting point and specific gravity are more than double PEs. PTFE and PE differences are attributable to the differences of C
F and C
H bonds. The differences in the electronic properties and sizes of F and H lead to the following observations: 1. F is the most electronegative of all elements (4 Paulings); 2. F has unshared pair of electrons; 3. F is more easily ionized to F2;

5.6.8 Polyvinyl Fluoride (CAS number 24981-14-4) PVF is produced by free-radical polymerization [50,56,87]. In the first polymerization of VF, a saturated solution of VF in toluene at 67°C was heated under a pressure of 600 MPa for 16 hours. A wide variety of initiators and polymerization conditions have been explored over time. Examples of bulk and solution polymerizations of VF are possible, though aqueous suspension or dispersion methods are preferred. Copolymers of VF and a wide variety of other monomers have been prepared. Interpolymers of VF with TFE and other highly fluorinated monomers have been reported. Examples of the third monomers include HFP, perfluorobutylethylene, and PEVE. These polymers have been found to possess the typical properties of fluoropolymers such as chemical resistance, thermal stability, and outdoor durability [88,89].

5.7 Structure Property Relationship One way to understand the impact of fluorine is to explore the differences between linear polyethylene (PE) and PTFE [48], which is the ultimate fluoropolymer in terms of properties and characteristics. There are important differences between properties of PE and PTFE: 1. One of the lowest surface energy polymers; 2. Most chemically resistant polymer; 3. One of the most thermally stable polymers;

4. Bond strength of C
F is higher than C
H; 5. The atom of F is larger than H. The electronegativity of carbon at 2.5 Paulings is somewhat higher than that of hydrogen (2.1 Paulings) and significantly lower than the electronegativity of fluorine. Consequently, the polarities of the C
F and C
C bonds are opposites; C
F bond is more highly polarized. In the C
F bond, the fluorine end of the bond is negatively charged compared to the C
H bond in which carbon is negatively charged. The difference in bond polarity of C
H and C
F affects the relative stability of the conformations of the two polymer chains. Crystallization of PE takes place in a planar and trans conformation. PTFE can be forced into such a conformation at extremely high pressure [90]. Below 19°C, PTFE crystallizes as a helix with 1.69 nm per repeat distance: it takes 13 C atoms for completion of a 180 degrees turn. Above 19°C, the repeat distance increases to 1.95 nm which means that 15 carbon atoms are required for a 180 degrees turn. At above 19°C, the chains are capable of angular displacement, which increases above 30°C until reaching melting point (327°C). Substitution of F for H in the C
H bond substantially increases the bond strength from 99.5 kcal/mole for the C
H bond to 116 kcal/mole for the C
F bond. Consequently, thermal stability and chemical resistance of PTFE are higher than PEs because more energy is required to break the C
F bond. The polarity and strength of the C
F bond renders the removal of F atom from the carbon chain, and branching, difficult. In contrast, highly branched PE ( . 8 branches per 100 carbon atoms) can be synthesized. Branching mechanism

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as a tool to adjust crystallinity is not practical for PTFE. Instead, comonomers with pendent groups have to be polymerized with TFE. Crystallinity of never-melted PTFE is in the range of 92%
98% [91] consistent with an unbranched chain structure. FEP, a copolymer of TFE and HFP, has an as-polymerized crystallinity of 40%
50%. In FEP, the pendent CF3 group is bonded to a tertiary carbon rendering it less thermally stable than CF3 bonded to primary or secondary carbon atoms. Degradation curves (Fig. 5.4) indicate degradation onset temperatures of 300°C for FEP (0.02% wt. loss) and 425°C for PTFE (0.03% wt. loss) in air.

5.8 Properties of Polytetrafluoroethylene 5.8.1 Polytetrafluoroethylene Properties PTFE has excellent properties such as chemical inertness, heat resistance (at both high and low temperatures), electrical insulation, low coefficient of friction (static 0.08 and dynamic 0.01), and being nonstick over a wide temperature range (2260°C to 1260°C). PTFE density is in the range of 2.1
2.3 g/cm3 and melt viscosity in the range of 1
10 GPa s [57]. Molecular weight of PTFE cannot be measured by standard methods because of its insolubility in solvents. Instead, an indirect approach is used to assess the molecular weight of PTFE. Standard specific gravity (SSG) is the specific gravity of a chip prepared according to a standardized procedure. The underlying principle is lower molecular weight PTFE crystallizes more readily and extensively, thus yielding higher SSG values [93]. Virgin PTFE (never melted previously) has a crystallinity of 92%
98%, indicating a linear and nonbranched molecular structure. Upon reaching 342°C, PTFE melts and changes from a chalky white color into a transparent amorphous gel. The second melting point of PTFE is 327°C because it does not recrystallize to the same extent as its original state. First-order and second-order transitions have been reported for PTFE. Transition temperatures close to room temperature have practical importance by impact on the processing of PTFE. Below

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Figure 5.4 A comparison of thermal degradation of FEP and PTFE in air [92].

19°C, the crystalline system of PTFE is nearly perfectly triclinic. At above 19°C, the unit cell changes to hexagonal. In the range of 19°C
30°C, PTFE chain segments become increasingly disordered leading to the disappearance of the preferred crystalline phase. One consequence is a large increase in the specific volume of PTFE (1.8%) [94]. That increase must be considered in measurement of the dimensions of PTFE articles. It is best to select an agreed temperature, for example, 25°C, for dimensional measurements. PTFE is by far the most chemically resistant polymer among thermoplastics. The exceptions include molten alkali metals, gaseous fluorine at high temperatures and pressures, and a few organic halogenated compounds such as chlorine trifluoride (ClF3) and oxygen difluoride (OF2). Few other chemicals have been known to attack PTFE at or near its upper service temperature. PTFE reacts with 80% sodium or potassium hydroxide and some strong Lewis bases including metal hydrides. Mechanical properties of PTFE are generally inferior to engineering plastics at the room temperature. Compounding with fillers such as glass and carbon fiber has been the strategy to overcome that deficiency. PTFE has useful mechanical properties in its continuous-use temperature range. PTFE has excellent electrical properties such as high insulation resistance, low dielectric constant (2.1), and low dissipation factor. Dielectric constant and dissipation factor remain virtually unchanged in the range of 240°C to 250°C and from 5 Hz to 10 GHz. Dielectric break down strength (short term) is 47 kV/mm for a 0.25 mm thick PTFE film (ASTM D149). Dielectric break down strength is

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enhanced with a void decrease in PTFE. PTFE properties and fabrication processing have a strong impact on void content of parts. When PTFE is subjected to ionized radiation in air, it begins to degrade at a dose of 0.02 Mrad.

5.8.2 Perfluoroalkoxy Copolymer Properties PFA polymers are fully fluorinated and melt processible [74,76,95]. Their chemical resistance and thermal stability are comparable to those of PTFE. Specific gravity of perfluoroalkoxy resins ranges 2.12
2.17. PFA has an upper continuous use temperature of 260°C. Crystallinity and specific gravity of PFA parts decrease when the cooling rate of the molten polymer is increased. The lowest crystallinity ever obtained by quenching molten PFA in ice was 48%. Specific gravity at 48% crystallinity was 2.123. Similar to PTFE, molecular weight of PFA cannot be measured by conventional techniques. An indirect factor called melt flow rate (MFR) or melt flow index (MFI) is used as a proxy for MW. MFR or MFI measures the amount of polymer melt that would flow in 10 minutes through a capillary rheometer at a given temperature under a defined load. MFR is inversely proportional to viscosity while viscosity is directly proportional to the molecular weight of the polymer. PFA exhibits one first-order transition at 25°C in contrast to two temperatures for PTFE at 19°C and 30°C. It has three second-order transitions at 2100°C, 230°C, and 90°C [76]. PFA has excellent electrical properties such as high insulation resistance, low dielectric constant (2.1), and low dissipation factor. Dielectric constant and dissipation factor remain virtually unchanged in the range of 240°C to 250°C and 102 Hz to 2.4 3 1010 Hz. Dielectric break down strength (short term) is 80 kV/mm for a 0.25 mm thick film (ASTM D149). Chemical properties of PFA are similar to those of PTFE. When PFA is subjected to radiation in air, degradation begins at a somewhat higher dose than that of PTFE.

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5.8.3 Fluorinated Ethylene
Propylene Copolymer Properties Fluorinated ethylene
propylene copolymers are fully fluorinated and melt processible [76,81,96]. They have excellent chemical resistance and thermal stability. Specific gravity of FEP resins is in the range of 2.13
2.15. It has an upper continuous use temperature of 200°C. Similar to PTFE, molecular weight of FEP cannot be measured by conventional techniques. Like PFA, MFR is used to characterize molecular weight of FEP. MFR and the molecular weight are inversely related. Molecular weight distribution is determined by measuring the dynamic module of the polymer melt using rheological analyses. Crystallinity of virgin FEP is 65%
75%. FEP exhibits a single first-order transition—its melting point. Relaxation temperature of FEP increases with HFP content. It has a dielectric transition at 2150°C that is unaffected by the monomer composition or crystallinity. Chemical properties of FEP are similar to those of PTFE and PFA. When FEP is subjected to radiation in air, degradation begins at a dose of 0.2 Mrad (10 times higher than PTFE).

5.8.4 Polychlorotrifluoroethylene Properties PCTFE is a semicrystalline polymer [76,84] with a helical polymer chain and a pseudohexagonal crystal. Crystal growth is spherulitic and consists of folded chains. The large size of chlorine constrains recrystallization when PCTFE is cooled following melting during processing. This resin has good properties at cryogenic temperatures compared to most plastics. Those properties are inferior to commercial fluoropolymers except for PVDF. PCTFE has exceptional barrier properties and superb chemical resistance. It is attacked by a number of organic solvents. Additionally, PCTFE has low thermal stability and degrades upon reaching its melting point thus requiring special care during processing.

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5.8.5 Ethylene
Tetrafluoroethylene Copolymer Properties PVDF and ETFE are isomers but the latter has higher melting point and lower dielectric loss than the former. ETFE crystallizes into unit cells believed to be orthorhombic or monoclinic [76,97]. The molecular conformation of ETFE is an extended zigzag. ETFE has been dissolved in some boiling esters at above 230°C thus allowing determination of molecular weight by light scattering. ETFE has several transitions, α relaxation at 110°C (shifts to 135°C at higher crystallinity), β at 225° C, and γ relaxation at 2120°C. ETFE terpolymers have good mechanical properties including tensile and cut-through resistance and lower creep than prefluoropolymers. ETFE is more resistant to radiation than prefluoropolymers (modestly affected by exposure up to 20 Mrad radiation) and can be cross-linked by radiation such as electron beam. Cross-linking is used to strengthen cutthrough resistance of ETFE wire insulation. ETFE has a dielectric constant of 2.6
3.4 and a dissipation factor of 0.0006
0.010 as frequency increases from 102 to 1010 Hz. ETFE terpolymers are resistant to stress cracking and chemical attack by most compounds. Strong oxidizing acids, concentrated boiling sulfonic acids, and organic bases (amines) attack ETFE in addition to any chemicals that affect PTFE, PFA, and FEP.

5.8.6 Ethylene
Chlorotrifluoroethylene Copolymer Properties ECTFE is semicrystalline (50% 2 60%) and melts at 240°C (commercial grade) [76]. It has an alpha relaxation at 140°C, a beta at 90°C, and a γ relaxation at 265°C. Conformation of ECTFE is an extended zigzag in which ethylene and CTFE alternate. The unit cell of ECTFEs crystal is hexagonal. Similar to ETFE, ECTFE terpolymers have better mechanical, abrasion, and radiation resistance than PTFE and other prefluoropolymers. Dielectric constant of ECTFE is 2.5
2.6 independent of temperature and frequency. It has a dissipation factor of 0.02 that is much larger than ETFEs.

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ECTFE is resistant to most chemicals except hot polar and chlorinated solvents. It does not stress crack or dissolve in any solvents. ECTFE has better barrier properties to SO2, Cl2, HCl, and water than FEP and PVDF.

5.8.7 Polyvinylidene Fluoride Properties PVDF is a semicrystalline polymer (35%
70% crystallinity) with an extended zigzag chain [69,70,76]. Head-to-tail addition of VDF dominates but there are also head-to-head or tail-to-tail adducts (known as “defects”) that affect crystallinity and properties of PVDF. PVDF has a number of transitions and a different density for each polymorph state. There are four known proposed states, named as α, β, γ, and δ. The most common phase is α-PVDF, which exhibits transitions at 270°C (γ), 238°C (β), 50°C (αv), and 100°C (α0 ). PVDF resists most organic and inorganic chemicals including chlorinated solvents. Strong bases, amines, esters, and ketones attack this resin. The impact ranges from swelling to complete dissolution in these solvents depending on the conditions. PVDF exhibits compatibility with a number of polymers. Commercially useful blends with acrylics and methacrylics have been developed and commercialized. PVDF, just as ETFE, cross links readily as a result of exposure to radiation. Radiation (γ rays) has the modest effect on the mechanical properties of PVDF.

5.8.8 Polyvinyl Fluoride Properties PVF is a semicrystalline polymer with a planar, zigzag conformation [29,30,56]. The degree of crystallinity can vary significantly from 20% to 60% and is a function of defect structures. Commercial PVF is atactic, contains approximately 12% head-to-head linkages, and displays a peak melting point at about 190°C [50,51,76]. PVF displays several transitions below its melting temperature. Lower Tg occurs at 215°C to 220°C and upper Tg is in the 40°C
50°C range. Two additional transitions at 280°C and 150°C have been reported. PVF has low solubility in all solvents below about 100°C. Polymers with greater solubility have

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been prepared using 0.1% 2-propanol as a polymerization modifier. The resins made using a modifier were characterized in N,N-dimethylformamide solution containing 0.1 N LiBr. Mn ranged from 76,000 to 234,000 (osmometry) and Ms from 143,000 to 654,000 (sedimentation velocity). High molecular weight PVF is reported to degrade in an inert atmosphere, with concurrent HF loss and backbone cleavage occurring at 450°C. In air, HF loss occurs at about 350°C, followed by backbone cleavage at around 450°C. PVF is transparent to radiation in the ultraviolet, visible, and near infrared light regions. It transmits 90% of the radiation in the wavelength range of 350
2500 nm. PVF embrittles when exposed to electron-beam radiation at a dose of 1000 Mrad, though it resists breakdown at lower doses. It retains strength at 32 Mrad; in comparison, PTFE degrades at 0.2 Mrad. The self-ignition temperature of the PVF film is 390°C. The limiting oxygen index of PVF is 22.6%. Hydrogen fluoride and a mixture of aromatic and aliphatic hydrocarbons are generated from the thermal degradation of PVF.

5.9 Fabrication Techniques With the exception of two fluoropolymers, PVF and PTFE, the rest of the resins described in this chapter can be processed by common meltprocessing techniques including compression, transfer injection, and blow molding, extrusion and rotational molding. Process equipment for fluoropolymers must be capable of high temperatures approaching 500°C. The equipment must be constructed from corrosion-resistant alloys because of the corrosive compounds that are produced when fluoropolymers are heated above their melting points. Higher melt viscosity of these resins requires higher torque and pressure rating equipment [76]. PTFE is processed using processing techniques similar to those of metal powder in which a preform is molded and sintered. Compression molding is used to fabricate PTFE stock shapes and parts with simple geometry. PTFE dispersions are applied by common coating techniques. Paste extrusion is the method by which PTFE is fabricated into tubes, tapes, and wire insulation continuously. PTFE is blended with a hydrocarbon,

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followed by molding a preform that is loaded into a paste extruder and then extruded. The hydrocarbon is vaporized before the part is sintered in an oven. PVF is dispersed in a polar latent solvent such as dimethylacetamide to which additives and pigmented may be added. The resulting mix (slurry) is melt extruded as a plastisol, followed by solvent removal, orientation, and drying [56].

5.10 Applications Fluoropolymers are used in applications requiring chemical resistance, thermal stability, cryogenic properties, low coefficient of friction, low surface energy, low dielectric constant, high volume and surface resistivity, and flame resistance (Table 5.1). Fluoropolymers are used as liners (process surface) because of their resistance to chemical attack. They provide durable, low maintenance, and economical alternatives to exotic metals for use at high temperatures without introducing impurities. Electrical properties make fluoropolymers highly valuable in electronic and electrical applications as insulation, for example, FEP in data communications wire and cable. In automotive and office equipment, mechanical properties of fluoropolymers are beneficial in lowfriction bearings and seals that resist attack by hydrocarbons and other fluids. In food processing, the US Food and Drug Administration-approved grades are fabrication material for equipment. In houseware, fluoropolymers are applied as nonstick coatings on cookware, bakeware, and appliance surfaces. Medical products such as surgical patches and cardiovascular grafts rely on the long-term stability of fluoropolymers as well as their low surface energy and chemical resistance. For airports, stadiums, and other structures, glass fiber fabric coated with PTFE is fabricated into roofing and enclosures. PTFE provides excellent resistance to weathering, including exposure to ultraviolet rays in sunlight, flame resistance for safety, and low surface energy for soil resistance and easy cleaning.

5.11 Safety Fluoropolymers are chemically stable and relatively unreactive. Reactivity, generally, decreases

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Table 5.1 Examples of Applications and Uses of Fluoropolymers [98]. Industries

Functions

Forms

Automotive

Mechanical property, thermal property, chemical property, and friction property

O-rings, gaskets, valve stem seals, shaft seals, linings for fuel hoses, power steering, and transmission

Chemical industry

Chemical resistance, mechanical property, thermal property, and weather stability

Coatings for heat exchangers, pumps, diaphragms, impellers, tanks, reaction vessels, autoclaves, containers, flue duct expansion joints, heavy-wall solid pipe and fittings

Electrical/ electronic

Dielectric constant, flame resistance, and thermal stability

Electrical insulation, flexible printed circuits, ultrapure components for semiconductor manufacture

Architectural and domestic

Weatherability, flame retardancy, friction property, thermal stability

Water-repellent fabric, architectural fabric, nonstick coatings for cookware, and fiberglass composite for constructions

Engineering

Mechanical property, thermal stability, chemical stability, weatherability, and surface energy

Seats and plugs, bearings, nonstick surfaces, coatings for pipes, fittings, valve and pump parts, and gears

Medical

Surface energy, biological stability, mechanical property, chemical resistance

Cardiovascular grafts, ligament replacement, and heart patches

as fluorine content of the polymer increases. Fluorine induces more stability than chlorine. Fluoropolymers can produce toxic gases if overheated. Precautions should be taken to exhaust any degradation gases produced during the processing and fabrication of parts from fluoropolymers [76,99,100]. This family of plastics is reputed to be inert with low toxicity and almost no toxicological activity. Fluoropolymers have not been known to cause skin sensitivity or irritation in humans.

5.12 Polymerization Surfactant For decades, APFO had been an essential processing aid in the manufacture of fluoropolymers. It played a critical role in the polymerization of TFE and fluorinated comonomers used to produce PTFE, PFA (MFA), and FEP. It acted as a potent aid for preparation of the majority of fluoropolymers and was eliminated during the finishing steps. As produced solid-phase fluoropolymers contained small amounts of C8 of the order of a few parts per million. In the case of aqueous dispersion products less than a fraction of percent of APFO was present in the dispersion products.

APFO was found to be persistent in the environment and accumulate in wildlife and humans. Consequently, in mid 2000s, fluoropolymer manufacturers began taking steps to abate environmental emissions and reduce and eliminate the APFO from dispersion products. The industry reduced the usage in its products by 95% by 2010. Fluoropolymer manufacturers committed to the US Environmental Protection Agency (www.EPA.gov) to eliminate the use of C8 completely by finding alternative polymerization aids by 2015. Those steps have been successfully completed and alternative polymerization aids have been developed. Major manufacturers have developed replacement surfactants for APFO and commercialized products based on the replacement compounds [76]. An example of replacement is a compound called GenX [CF3CF2CF2OCF(CF3)COOH.NH3] that was originally developed by DuPont (now Chemours) [101]. Manufacturers in the developing regions have strived to convert away from APFO and for the most part completed the conversion. For more information, readers may contact Fluoropolymers Division of the of Plastics Industry Association (www.Fluoropolymers.org) or fluoropolymer manufacturers directly.

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Other polymerization surfactants include 4,9dioxa-3H-perfluorononanoate, ammonium, 2,3,3,3tetrafluoro-2-(heptafluoropropoxy) propionate, and perfluoro-2,2-methoxypropoxy propanoic acid ammonium. These compounds have been identified based on PTFE patents and publicly available information though no single PTFE type uses all the surfactants. Initiator concentration depends on the rate and degree of polymerization, from 0.01 to 0.5 wt. % of the water [102].

5.13 Economy Fluoropolymers are more costly to produce than polyolefins and many other plastics due to the capital cost and the cost of fluorine. Polymerization and finishing of these resins requires processing of highly flammable hazardous materials, thus mandating the use of expensive construction material and elaborate equipment. Fluoropolymers have a broad price range, from US$ 15
30 per kilogram for PTFE to over US$ 100 per kilogram for specialty grades of PFA. Soluble prefluoropolymers cost in excess of US$ 15
20 per gram and are only used in high-value applications.

5.14 Summary Commercial thermoplastic fluoropolymers are based on TFE, VDF, and to a lesser extent CTFE. Examples of other comonomers include PMVE, PEVE, PPVE, HFP, and PFBE. The consequences of substitution of fluorine for hydrogen in a polymer include increased chemical and solvent resistance, enhanced electrical properties such as lower dielectric constant, lower coefficient of friction, higher melting point, increased photostability and thermal stability, improved flame resistance, and reduced mechanical properties.

References [1] Scheirs John, editor. Modern fluoropolymers: high performance polymers for diverse applications. New York: John Wiley and Sons; 1997.

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[2] Ameduri B, Boutevin B. Well-architectured fluoropolymers: synthesis, properties and applications. Elsevier; 2004. [3] Banerjee S. Handbook of specialty fluorinated polymers: preparation, properties, and applications. 1st ed. Elsevier; 2015. [4] Plunkett RJ. The history of polytetrafluoroethylene: discovery and development. In: Seymour RB, Kirshenbaum GS, editors. High performance polymers: their origin and development, Proceed. Symp. Hist. High Perf. Polymers at the ACS Meeting in New York, April 1986. New York: Elsevier; 1987. [5] Teflon® AF. ,www2.dupont.com/ Teflon_Industrial/en_US/products/product_by_name/teflon_af/.; 2015. [6] Ruff O, Bretschneider O. Z Anorg Chem 1933;210:73. [7] Park JD, et al. Ind Eng Chem 1947;39:354. [8] Hamilton JM. In: Stacey M, editor. Advances in fluorine chemistry, 3. Kent: Butterworth & Co., Ltd.; 1963. [9] Edwards JW, Small PA. Nature 1964;202:1329. [10] Gozzo F, Patrick CR. Nature 1964;202:80. [11] Hisazumi M, Shingu H. Japanese Patent 60 15,353. [12] Scherer O, et al. US Patent 2,994,723, assigned to Farbewerke Hoechst, 1961. [13] Edwards JW, Sherratt S, Small PA. British Patent 960,309, assigned to ICI, 1964. [14] Ukahashi H, Hisasne M. US Patent 3,459,818, assigned to Asahi Glass Co., 1969. [15] Edwards JW, Benning AF, Sheratt S, Small PA. US Patent 3,308,174, assigned to Imperial Chemical Industries, 1967. [16] Sherratt S. In: 2nd ed. Standen A, editor. Kirk-Othmer encyclopedia of chemical technology, vol. 9. New York: Interscience Publishers, John Wiley & Sons; 1966. [17] Gangal SV, Brothers PD. Perfluorinated polymers, polytetrafluoroethylene. Pub Online Ency Polymer Science and Technology; 2010. [18] Dietrich MA, Joyce RM. US Patent 2,407,405, assigned to DuPont, 1946. [19] Ebnesajjad S. 2nd ed. Non-melt processible fluoroplastics, vol. 1. Plastics Design Library, Elsevier; 2015. [20] Ebnesajjad S. 2nd ed. Melt processible fluoroplastics, vol. 2. Plastics Design Library, Elsevier; 2015.

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[21] Downing FB, Benning AF, McHarness RC. US Patent 2,384,821, assigned to DuPont, 1945. [22] Henne AL, Woalkes TP. J Am Chem Soc 1946;68:496. [23] Chinoy PB, Sunavala PD. Thermodynamics and kinetics for the manufacture of tetrafluoroethylene by the pyrolysis of chlorodifluoromethane. Ind Eng Chem Res 1987;26:1340
4. [24] Brayer E, Bekker AY, Ritter AR. Kinetics of the pyrolysis of chlorodifluoromethane. Ind Eng Chem Res 1988;27:211. [25] Webster JL, Trofimenko S, Resnick PR, Bruhnke DW, Lerou JL, Mangue WH, et al. US Patent 5,068,472, assigned to DuPont, 1991. [26] Gelblum PG, Herron N, Noelke CJ, Rao VNM. US Patent 7,271,301, assigned to DuPont Company, 2007. [27] Resnick PR. Patent 6,388,139, assigned to DuPont, 2002. [28] Brice TJ, Pearlson WH. US Patent 2,713,593, assigned to 3M Co., 1955. [29] Carlson DP. US Patent 3,536,733, assigned to DuPont, 1970. [30] Eleuterio HS, Meschke RW. US Patent 3,358,003, assigned to DuPont, 1967. [31] Fritz GG, Selman S. US Patent 3,291,843, assigned to DuPont, 1966. [32] Hung MH, Rozen S. US Patent 5,350,497, assigned to DuPont Company, 1994. [33] Carpenter CP, Smyth HF, Pozzani UC. J Ind Hyg 1949;31:343. [34] Chandrasekaran S. Chlorotrifluoroethylene polymers. 2nd ed. Encyclopedia of polymer science and engineering, 3. New York: John Wiley & Sons; 1989. p. 463
80. [35] Ishihara et al. US Patent 5,124,494, assigned to Central Glass Company, 1992. [36] Cunningham WJ, Piskorz RF, Smith AM. CA 1230131, assigned to Allied Corporation, 1987. [37] Lerot L, Pirotton J, Wilmet V. European Patent EP0747337, assigned to Solvay Company, 2000. [38] Lerot L, Pirotton J, Wilmet V. European Patent EP0496446, assigned to Solvay Company, 1997. [39] Lerot L, Pirotton J, Wilmet V. CA2060036, assigned to Solvay Company, July 26, 1992. [40] Calfee et al. US Patent 2,734,090, 1956.

[41] Nikolaus et al, US Patent 3,830,856, assigned to Bayer Corp, 1974. [42] Hauptschein A, Feinberg AH. US Patent 3,188,356, assigned to Pennsalt Chemicals Corp., 1965. [43] Schultz N, Martens P, Vahlensieck HJ. German Patent 2,659,712, assigned to Dynamit Nobel AG, 1976. [44] McBee ET, et al. Ind Eng Chem 1947;39 (3):409
12. [45] Kaess F, Michaud H. US Patent 3,600,450, assigned to Sueddeutsche KalkstickstoffWerke AG, 1971. [46] Sylvain Perdrieux, Serge Hub. US Patent 8,350,101, assigned to Arkema, France, 2013. [47] Salisbury LF. US Patent 2,519,199, assigned to DuPont Co., 1950. [48] Sianesi et al. US Patent 3,414,627, assigned to Montecatini Edison, 1968. [49] Englander F, Meyer G. US Patent 3,987,117, assigned to Dynamit Nobel, 1976. [50] Ebnesajjad S. 4th ed. Kirk-Othmer encyclopedia of chemical technology, Vol. 11. New York: John Wiley & Sons, Inc.; 2004. [51] Brasure DE, Ebnesajjad S. 2nd ed. Encyclopedia of polymer science and engineering, vol. 17. New York: John Wiley & Sons, Inc.; 1989. p. 468
91. [52] Coffman DD, Cramer R, Rigby GW. J Am Chem Soc 1949;71:979
80. [53] Coffman DD, Raasch MI, Rigby GW, Barrich PL, Hanford WE. J Org Chem 1949;14:747
53. [54] Pajaczkowski A, Spoors JW. Chem Ind London 1964;16:659. [55] Air Pollution Control District, Jefferson County, KY, Title V permit summary, ,www.louisvilleky.gov.; 2003. [56] Ebnesajjad S. Polyvinyl fluoride: technology and applications of PVF. 1st ed. Elsevier; 2012. [57] Gangal SV. Polytetrafluoroethylene, homopolymers of tetrafluoroethylene. 2nd ed. Encyclopedia of polymer science and engineering, 16. New York: John Wiley & Sons; 1989. p. 577
600. [58] Reza A, Christiansen E. A case study of a TFE explosion in a PTFE Manufacturing Facility. Los Angeles, CA: Exponent, Failure Analysis Associates; 2006. [59] Clayton JW. Occup Med 1962;4:262
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[60] Gibbs HH, Warnell JJ. British Patent 931,587, assigned to DuPont, 1963. [61] Knunyants IL, Mysov EI, Krasuskaya MP. Izvezt Akad Nauk SSSR Otdel Khim Nauk 1958;906
7. [62] Haszeldine RN, Steele BR. J Chem Soc 1953;1592
600. [63] Miller Jr. WT, Bergman E, Fainberg AH. J Am Chem Soc 1957;79:4159
64. [64] PCT Int. Appl. WO 89,11,495, M.D. Buckmaster to DuPont Co., 1989. [65] Gangal SV, Brothers PD. Perfluorinated polymers, polytetrafluoroethylene. Encyclopedia of polymer science and technology. New York: John Wiley & Sons; 2010. [66] Gangal SV, Brothers PD. Perfluorinated polymers, tetrafluoroethylene
perfluorovinyl ether copolymers on-line ed Encyclopedia of polymer science and technology. John Wiley & Sons; 2010. [67] Technical information PPVE perfluoropropylvinyl ether, DuPont fluorointermediates, publication no. H-88804-2, 2007. [68] Haszeldine RN, Nyman F. J Chem Soc London 1959;1085. [69] Dohany J. Poly(vinylidene fluoride). 4th ed. Kirk-Othmer encyclopedia chemical technology, vol. 11. New York: John Wiley & Sons; 1994. p. 694
712. [70] Humphrey JS, Amin-Sanayei R. Vinylidene fluoride polymers. 4th ed. Encyclopedia of polymer science and technology, 4. New York: John Wiley & Sons; 2010. p. 510
33. [71] Ebnesajjad S. Non-melt processible fluoroplastics: the definitive user’s guide and data book, plastics design library. New York: William Andrew Publishing (Elsevier); 2014. [72] Ebnesajjad S, Morgan RA. Fluoropolymer additives. Elsevier; 2019. [73] Bro MI. US Patent 2,952,669, assigned to DuPont Co., 1960. [74] Gangal SV, Brothers PD. Perfluorinated polymers, tetrafluoroethylene
perfluorovinyl ether copolymers. Pub Online Ency Polymer Science and Technology; 2010. [75] Berry KL. US Patent 2,559,752, assigned to DuPont Co., 1951. [76] Ebnesajjad S. Melt processible fluoroplastics: the definitive user’s guide and data book, plastics design library. New York: William Andrew Publishing, (Elsevier); 2015.

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[77] Carlson DP. US Patent 3,674,758, assigned to DuPont Co., 1972. [78] Carlson DP. US Patent 4,599,386, assigned to DuPont Co., 1986. [79] Imbalzano JF, Kerbow DL. US Patent 4,743,658, assigned to DuPont Co., 1988. [80] Goodman J, Andrews S. Fluoride contamination from fluoropolymers in semiconductor manufacture. Solid State Technol 1990;33:65
8. [81] Gangal SV, Brothers PD. Perfluorinated polymers, perfluorinated ethylene
propylene copolymers. Pub Online Ency Polymer Science and Technology; 2010. [82] Schreyer RC. US Patent 3085083, assigned to DuPont Co., 1963. [83] Carlson DP. US Patent 3,624,250, assigned to DuPont Co., 1971. [84] Miller WA. Chlorotrifluoroethylene-ethylene copolymers. 2nd ed. Encyclopedia of polymer science and engineering, vol. 3. New York: John Wiley & Sons; 1989. p. 480
91. [85] Ford TA, Hanford WE. US Patent 2,435,537, assigned to DuPont Co., 1948. [86] Ford TA. US Patent 2,468,054, assigned to DuPont Co., 1949. [87] Brasure DE, Ebnesajjad S. Vinyl fluoride polymers. 2nd ed. Encyclopedia of polymer science and engineering, vol. 17. New York: John Wiley & Sons; 1989. p. 468
91. [88] Uschold RE. US Patent 5,229,480, assigned to DuPont Co., 1993. [89] Uschold RE. US Patent 6,403,740, assigned to DuPont Co., 2002. [90] England, D.C., et al., Proc. Robert A. Welch Conf. on Chemical Res. XXVI, R. A. Welch Foundation, pp. 193
243, 1982. [91] Bryant WMD. Free energies of fluorocarbons and their radicalse. Thermodynamics of formation and depolymerization of polytetrafluoroethylene. J Poly Sci 1962;56:277
96. [92] Baker BB, Kasprzak DJ. Thermal degradation of commercial fluoropolymer in air. Polym Degrad Stab 1994;42:181
8. [93] Sperati CA, Starkweather Jr. HW. Adv Polym Sci 1961;2:465. [94] McCrum NG. An internal friction study of polytetrafluoroethylene. J Polym Sci 1959;34:355. [95] Gangal SV. Tetrafluoroethylene-perfluorovinyl ether copolymer. 4th ed. Kirk-Othmer

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encyclopedia of chemical technology, vol. 11. New York: John Wiley & Sons; 2004. p. 671
83. [96] Gangal SV. Perfluorinated ethylenepropylene copolymer. 4th ed. Kirk-Othmer encyclopedia of chemical technology, vol. 11. New York: John Wiley & Sons; 2004. p. 644
56. [97] Gangal SV. Tetrafluoroethylene-ethylene copolymers. 4th ed. Kirk-Othmer encyclopedia of chemical technology, vol. 11. New York: John Wiley & Sons; 2004. p. 657
71. [98] Teng H. Overview of the development of the fluoropolymer industry, ,www. SemanticScholar.org.; 2012.

[99] Guide to safe handling of fluoropolymer resins. 4th ed. The Fluoropolymers Division, The Society of Plastics Industry; 2005. [100] Guide to safe handling of fluoropolymer resins. 5th ed. The Fluoropolymers Division, Plastics Industry Association; 2019. [101] DuPontt GenX® processing aid for making fluoropolymer resins, No. K23743. DuPont; 2010. [102] Henry BJ, Carlin JP, Hammerschmidt JA, Buck RC, Buxton LW, Fiedler H, et al. A critical review of the application of polymers of low concern and regulatory criteria to fluoropolymers. WL Gore & Associates and Society of Toxicology; 2018.

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6 Manufacturing and Properties of Polytetrafluoroethylene Sina Ebnesajjad FluoroConsultants Group, LLC, United States

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6.10.1 6.10.2 6.10.3 6.10.4

6.2 Tetrafluoroethylene Polymers

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6.3 Tetrafluoroethylene Polymerization Regimes

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Granular PTFE Compounds Filled PTFE Production Techniques Fine Powder-Based Compounds Fabrication of Reinforced Gasketing Material

82 82 85

6.4 Tetrafluoroethylene Polymerization Mechanism

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6.11 Preparation of Polytetrafluoroethylene by Dispersion Polymerization

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6.5 Suspension Polymerization Regimes

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6.12 Dispersion Polymerization of TFE with APFO Replacements

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6.13 Dispersion Polymerization Reactor

88

6.14 Preparation of Dispersion Grade PTFE

89

6.15 Preparation of Fine Powder PTFE

92 92 93 93 96 97

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6.6 Polymerizing Tetrafluoroethylene by Suspension Method 6.6.1 High-Temperature Polymerization 6.6.2 Low-Temperature Polymerization

69 69 71

6.7 Comminution of Suspension Reactor Bead 6.7.1 Fluid Energy Milling 6.7.2 Hammer Milling 6.7.3 Comminution of Wet Reactor Bead

73 73 74 76

6.8 Pelletized (Free Flow) Granular PTFE 6.8.1 Processes for Agglomeration (Pelletization) of PTFE

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6.16 Characterization of Polytetrafluoroethylene by Properties 6.16.1 Granular PTFE Resins 6.16.2 Fine Powder PTFE Resins 6.16.3 Dispersions of PTFE

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6.17 Commercial PTFE Resins

6.9 Presintered Granular PTFE

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References

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Further Reading

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6.10 PTFE Filled Compounds

6.1 Introduction There are two distinct types of polytetrafluoroethylene (PTFE) based on two separate polymerization regimes. They are reviewed in this chapter with an emphasis on commercially significant technologies. The manufacturers closely guard the actual manufacturing technologies of PTFE. In spite of the secrecy, publicly available descriptions, journal articles, books, patents, and other publications provide sufficient information for an adequate understanding of the subject.

PTFE is produced by the polymerization of tetrafluoroethylene (TFE) monomer. Sometimes trace amounts of a comonomer are added to modify the properties or processing behavior of homopolymers. Polymerization techniques and the corresponding finishing steps are discussed in this chapter. Characterization methods and typical properties of each PTFE type are described. Polymerization, finishing, and characterization of polychlorotrifluoroethylene (CTFE) are covered in a separate chapter.

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00006-9 © 2021 Elsevier Inc. All rights reserved.

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6.2 Tetrafluoroethylene Polymers TFE polymerizes linearly without branching yielding a virtually perfect chain structure. The chains have minimal interactions and as-polymerized crystallize to form a nearly 100% crystalline structure. Thermoplastics develop good mechanical properties because of the Van der Waals and other forces arising from interchain attractive interactions. How can PTFE polymers with useful properties be produced considering the minimal Van der Waals forces among PTFE chains? The answer lies in the control of the extent of recrystallization of the molten polymer. The only means of controlling the extent of recrystallization of molten homopolymers of TFE (no other comonomer) was by driving up the molecular weight of the polymer. The extremely long chains of PTFE have a significantly higher probability of chain entanglement in the molten phase yet little chance to crystallize back to the premelt extent ( . 90%
95%). This was the reason it was essential to polymerize TFE to a molecular weight of 106
107 for commercial applications. Molecular weight has been speculated to reach values as high as 50 million [1]. Molecular weight of PTFE was controlled by means of polymerization parameters such as initiator content, telogens, and chain transfer agents. One consequence of the high molecular weight of PTFE was reflected in its immense melt viscosity. The melt creep viscosity of PTFE was 10 GA (1011 poise) at 380°C [2]. This was more than a million times too viscous to allow melt processing in extrusion or injection molding requiring unique processing techniques. PTFE

F re e - f lowin g g ra d e s u n fille d a n d fille d ( co m p o u n d s)

Semi finished and finished products Billets Skived films Rods Sheets Tubes

FLUOROPOLYMERS

was a thermoplastic, but it develops no flow upon melting. The elimination of voids in articles made from PTFE does not take place as readily and completely as the other thermoplastics such as polyolefins. A small fraction of void volume was left in parts made from homopolymers of PTFE due to the difficulty and slow rate of void closure in this polymer. Voids affect permeation and mechanical properties of parts such as flex life and stress crack resistance. To meet the demands of extreme mechanical properties and resistance to permeation, the residual voids must be eliminated. Solving this problem required reduction of the viscosity of PTFE and suppression of recrystallization. The remedy has been to polymerize a small amount of a particular type of comonomer with TFE to disrupt the crystalline structure of PTFE. These polymers are called modified PTFE as opposed to copolymers because of small comonomer content (,2 wt.%). Cardinal et al. [3] presented one of the early proposals to modify PTFE in dispersion polymerization whereby the modifier was introduced after some polymerization has taken place. Holmes et al. [4] described utilization of perfluoroalkylvinyl ethers such as perfluoropropylvinyl ether as a modifier. Mueller et al. [5], Doughty et al. [6], and others [7
10] have reported modification (copolymerization) of PTFE with hexafluoropropylene, perfluorobutyl ethylene, and perfluoroalkyvinyl ethers by suspension polymerization regime to prepare granular (industry term) powders. TFE was polymerized by one of two technologies, suspension and dispersion, to produce commercial PTFE resins (Fig. 6.1). The suspension

Suspension polymers N o n - f re e - f lowin g grades u n f i l le d a n d filled ( c o mp o u n d s)

TO

Emulsion polymers

M i c ro p ow d e rs

Additives for plastics, lubricants and printing inks Nonstick and antifriction coatings

D i s p e rs i o n s

Formulation of coating systems Impregnation Coating of glass fibre fabrics

Figure 6.1 Commercial products of suspension and emulsion polymerization [11].

Fine powder extrusion grades unfilled and filled (compounds)

Tubing Unsintered sealing tapes Electrical insulating tapes Wire and cable insulation

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method yields granular polymers fabricated into parts as molding powders. Homopolymers and modified polymers of PTFE are produced by this technique. Dispersion polymerization was the regime by which fine powder and dispersion PTFE products are manufactured. Fine powder resins are fabricated by paste extrusion where a hydrocarbon was added to the powder as an extrusion aid later removed prior to sintering. Dispersion products are primarily consumed as coating and some by filled co-coagulation applications. Homopolymers and modified polymers are produced by dispersion polymerization. This regime of polymerization allows production of particles, which constitute different polymers at different depth inside the particle. All three forms of PTFE are produced by batch polymerization under elevated pressures in specially designed reactors. Polymerization media was high-purity water, which was virtually devoid of inorganic and organic impurities that impact the reaction by inhibition and retardation of the free radical polymerization. The surfactant of choice in dispersion polymerization reactions was an anionic fluorinated carboxylic ammonium salt.

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polymerization aid) accompanied by vigorous agitation. The particles resemble coconut shreds with lengths ranging from a few to several millimeters. Dispersion polymerization was the method by which dispersion and fine powder PTFE products are manufactured. Fine powder resins are also called coagulated dispersion, which was descriptive of their production method. Mild agitation, ample surfactant, and a waxy substance set the dispersion polymerization apart from the suspension method. The paraffin wax was used as a dispersion stabilizer for the emulsion polymerization of TFE. Dispersion and fine powder products are polymerized by the same method. The finishing steps convert the emulsion output of the reactor into each of the two products.

6.4 Tetrafluoroethylene Polymerization Mechanism TFE was polymerized by a typical free radical scheme. The polymerization reaction was initiated by a catalyst or by an initiator based depending on the reaction temperature. If polymerization was carried out at low temperatures (,30°C), a redox catalyst was used. At higher temperatures, peroxides such as a bisulfite or persulfate may be used to initiate the polymerization reaction. Polymerization of TFE proceeds linearly to form entirely unbranched PTFE chains [12]. Commercially TFE was often polymerized using one or more peroxides for the initiation of the reaction. Examples of the initiators include disuccinic peroxide and inorganic persulfates (Fig. 6.2). The most common persulfate was ammonium persulfate (APS) (NH4S2O8). It does not leave a counterion behind because of the degradation of NH4, in contrast to potassium or sodium persulfate. Degradation of APS depends on temperature and pH of the aqueous medium. The types of ions that are produced from the degradation of APS depend

6.3 Tetrafluoroethylene Polymerization Regimes TFE was the main constituent monomer for the preparation of PTFE resins. TFE was polymerized in water in the presence of an initiator, a surfactant, and other additives. Two different regimes of polymerization are practiced commercially for the production of homopolymers of TFE (Table 6.1). Suspension polymerization was the route to production of granular resins. In this regime, TFE was polymerized aqueously in the absence or in the presence of a small amount of surfactant (often called Table 6.1 A Comparison of PTFE Preparation Processes Granular

Fine Powder

Dispersion

Monomers

TFE, PPVE

TFE (HFP, PPVE, PFBE)

TFE

Media

H 2O

H2O

H2O

Regime

Suspension

Dispersion

Dispersion

Reactor

Vertical

Horizontal

Horizontal

Agitation

Vigorous

Mild

Mild

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Figure 6.2 Chemical structure of disuccinic acid peroxide and ammonium persulfate.

chain transfer agent such as methanol, or (3) by being capped by another free radical: 1. Disproportionation COOH 2 CF2 2 ðCF2 2CF2 Þn  1 COOH 2 CF2 2 ðCF2 2CF2 Þm  -COOH 2 ðCF2 2CF2 Þm1n11 2 COOH Figure 6.3 Effect of pH on degradation of ammonium persulfate. Courtesy FMC Corp, www.FMC. com [13,14].

2. Chain transfer COOH 2 CF2 2 ðCF2 2CF2 Þn  1 CH3 OH -COOH 2 CF2 2 ðCF2 2CF2 Þn H1 CH3 O

on the temperature and pH (Fig. 6.3). Every one of the anions can initiate the polymerization reaction. Initiation takes place by formation of new free radicals by the reaction of persulfate free radicals with TFE dissolved in the aqueous phase: 2

SO4  1 CF2 5 CF2 - 2 SO4 CF2 2 CF2 

Propagation was the growth of the free radicals produced in the initiation step by further addition of TFE: 2

SO4 CF2 2 CF2  1 nCF2 5 CF2 - 2 SO4 ðCF2 2CF2 Þn 2 CF2 2 CF2  Free radicals undergo hydrolysis where a hydroxyl end group replaces the sulfate: 2

SO4 ðCF2 2CF2 Þn 2 CF2 2 CF2  1 H2 O -HOðCF2 2CF2 Þn 2 CF2 2 CF2  1 SO4 H2 HOðCF2 2CF2 Þn 2 CF2 2 CF2  1 H2 O -COOHCF2 2 ðCF2 2CF2 Þn  1 2HF

Termination was the final step after the growth of the free radicals. It can take place by (1) disproportionation, (2) chain transfer in the presence of a

New chain was started as a result of chain transfer: CH3 OU 1 CF2 5 CF2 -CH3 OCF2 5 CF2 U 3. Capping by another free radical COOH 2 CF2 2 ðCF2 2CF2 Þn  1 R -COOH 2 CF2 2 ðCF2 2CF2 Þn R Alternative courses of hydrolysis affect the end groups of the polymer chains. There was no sulfur in the polymer structure even though a persulfate was the initiator. Bisulfite initiators form sulfonic acid end groups. Homopolymers of PTFE are completely linear without branches in contrast to polyethylene. Estimates for the number average molecular weight of PTFE range from 0.4 3 106 to 10 3 106. An example of a redox catalyst for lowtemperature polymerization of TFE was potassium permanganate [15]. In spite of being a powerful and versatile oxidant, it was incapable of initiating vinyl polymerization by itself. If, however,

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permanganate was combined with an organic acid, it can catalyze the polymerization of vinyl monomers. A classic example of the permanganate/ organic acid redox system was the KMnO4 oxalic acid (COOH-COOH) pair, which has been widely used as a polymerization catalyst. The oxalic acid was further oxidized to produce the initiating carboxy radicals [16]. 31 Mn41 1 C2 O22 1 CO2 1 COO2 4 -Mn 12 Mn31 1 C2 O22 1 C2 O2 4 -Mn 4

The radicals COO2 and C2O42 initiate radicals for the polymerization reaction similar to SO42. The oxalic-generated radicals have significantly shorter half-lives than those of the persulfates. This allows the reaction to be turned off more quickly than when persulfates are used. Simply the flow of permanganate and oxalic acid was shut off.

6.5 Suspension Polymerization Regimes Numerous variations of the suspension process for polymerizing TFE have been practiced commercially. Difference among them includes the use of different initiators, different pressures, different surfactants, various additives to control reactor wall adhesions, addition of fluorocarbon or chlorofluorocarbon liquids to alter particle physical characteristics, different monomers in minor concentrations in addition to TFE to modify sintering properties, and others. There are variations of suspension polymerization and finishing process. Polymerization reaction was usually conducted either at “low” (,30°C) or at “high” temperatures ( . 30°C). In some suspension polymerization processes temperature was increased during the course of the reaction. The difference between high and low polymerization temperature processes was in the initiator/catalyst used to start the reaction. A basic difference between PTFE products polymerized at low and high temperature was the specific surface area (SSA) defined by nitrogen adsorption of the powder. By an informal industry, convention low-temperature reaction usually produces a lower SSA powder (#3.5 m2/g) than the high-temperature process ($3.5 m2/g).

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Lower SSA of low-temperature polymerized PTFE leads to lower mold shrinkage, which is desirable in some applications. Mold shrinkage versus nominal reaction temperature for suspension polymerization of TFE is given in Fig. 6.4. Another beneficial characteristic of lowtemperature PTFE was softness of its particles. Parts molded and sintered from this type of PTFE have less void (porosity) content than parts made from high-temperature PTFE. Similarly, asrecovered reactor particles have better flowability when TFE was polymerized at lower reaction temperatures. Powder flow can be understood by thinking of the difference between easy “pouring out” of table salt from the shaker versus the difficulty of pouring wheat flour out of a canister. Salt pours smoothly as opposed to flour that does not flow continuously. The challenge of running high-temperature polymerization of TFE was to remove the heat produced by a highly exothermic reaction. Water was the polymerization medium because it allows removal of the exothermic heat of the reaction via heat transfer to an external jacket. The primary role of agitation was heat transfer between the reactor medium and the external jacket. In the end, the heat removal capability of the equipment and the cooling fluid determines the capacity of the polymerization kettle. In a typical polymerization, a batch reactor was charged with high-purity water preferably with volume resistivity of 18 Ω but a minimum of 16 Ω. Sometimes a small amount of a perfluorinated alkyl surfactant was added to the reaction medium. APS initiator was charged to the highpurity water prior to starting the TFE flow. Even though the surfactant was present below its critical micelle concentration (,20 ppm), it regulates the TFE polymerization reaction allowing more predictable reaction kick-off. Consequently, the reaction cycle remains constant resulting in enhanced polymer production yields. At the beginning of each cycle, the polymerization reactor was heated to the reaction temperature. Initially during the incubation period, the TFE polymerization takes place at a slow rate by a mechanism that was emulsion-like even in the absence of an added surfactant. The incubation period was characterized by a low rate of polymerization as seen in Fig. 6.5. The emulsion regime operates for a brief period of time until the emulsion breaks and PTFE

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Figure 6.4 Mold shrinkage as a function of constant TFE polymerization temperature [17].

Figure 6.5 Suspension polymerization rate of TFE.

particles emerge to the surface. The polymerization reaction rate increases exponentially (Fig. 6.5), primarily taking place in the gas phase. Acceleration of polymerization reaction increases the heat generated by the reaction immensely far exceeding the heat required to maintain the reactor temperature constant, cooling begins. From this point on to the end of the cycle, the challenge was to remove sufficient heat from the reaction medium to prevent catastrophic runaway reaction. In general, the important characteristics of commercial suspension polymerization regime include little or no dispersing agent and vigorous agitation at elevated temperature and pressure. The PTFE product has the appearance of fibrous and irregular

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(Fig. 6.6). Disintegrating the particles recovered from the reactor and drying them yields granular PTFE powders that can be molded and sintered. Finishing processes illustrated in Fig. 6.7 are commonly used to produce four different types of powder.

6.6 Polymerizing Tetrafluoroethylene by Suspension Method The polymerization reactor or kettle was usually vertical and was equipped with a cooling/heating jacket and an agitator. Fig. 6.8 shows the basic schematic of a reactor. An actual kettle was far more complex than the drawing in that figure. An important feature of the reactor was a safety

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feature, a rupture disc that breaks open in case of overpressure. The rupture disc must be cleaned regularly to prevent buildup of PTFE. Any malfunction by the rupture disk increases the risk of explosion, if the disc fails to break during an overpressure episode. Temperature (TI) and pressure (PI) are monitored and controlled in the polymerization reactor. The entire interior of the reactor and all accessories must be constructed of stainless steel such as grade 316 (or higher) because of acidic environment of reaction and purity requirements of PTFE. A new reactor or an existing unit that has undergone polishing of its interior walls must be passivated. This can be done by charging an aqueous solution (20%
40%) of nitric acid to the reactor and heated to 70°C for 30 minutes [18]. The acid solution dissolves iron contamination and regenerates the chromium oxides on the surface. To remove contamination, the reactor interior was rinsed with high-purity water followed by boiling a fresh batch of the same water in the reactor.

6.6.1 High-Temperature Polymerization

Figure 6.6 Scanning electron micrograph (at 30 3 magnification) of as-produced PTFE reactor bead.

When polymerization temperature exceeds 30°C, the process was called high temperature. This technology has been well characterized. Variants of this technology are in commercial use by PTFE manufacturers around the world [19]. Typically, a polymerization batch begins with charging high-purity water to fill half to two-thirds of the volume of the kettle. The reactor was evacuated to remove oxygen, pressurized with TFE, and heated to .50°C at a pressure of 1
3 MPa. Sometimes the kettle was purged with

Figure 6.7 Suspension polymerization and finishing processes [2].

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Figure 6.8 Basic schematic of a reactor for suspension polymerization of tetrafluoroethylene.

nitrogen prior to evacuation to aid in the removal of oxygen. Feed rate of TFE was controlled to maintain a constant pressure in the reactor throughout the polymerization. The contents of the reactor were vigorously agitated at 0.0004
0.002 kg m/sec/mL [20]. Temperature of the reaction medium was controlled by running steam or chilled water through the reactor jacket. After exothermic polymerization begins, temperature was controlled by the flow rate of the cooling medium in the jacket. The length of a batch was usually determined by kettle size, polymerization temperature and pressure, and the target cumulative mass of TFE fed to the kettle. Batch length ranged from 1 to 4 hours. To end the polymerization cycle, TFE monomer flow was stopped and the reaction was allowed to continue in order to consume most of the remaining TFE. The agitator was slowed down and finally

stopped after pressure decreased and the reactor contents cool down. Finally, the reactor contents (“batch”) are transferred (dropped) to a cooling tank. After the PTFE slurry reaches room temperature, it was ready for further processing. After completion of polymerization batch, the reactor was prepared for the next batch. Typically, an operator inspects the reactor surface for any apparent problems. The most important action was to clean the interior of the reactor including walls, dome vent lines and the surface of the rupture disk. High-pressure deionized water was used to wash off the residual PTFE or degraded surface adhesions. Automatic wash systems are helpful to reducing the reactor preparation time, thus increasing the productivity of the reactor. Adding a small quantity of citric acid, acetic acid, or formic acid to the reaction mixture reduces adhesion to the reactor wall [21].

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6.6.2 Low-Temperature Polymerization This section describes the low-temperature suspension polymerization of TFE in more detail than the previous section due to the less well-known nature of the subject. Low-temperature polymerization technology has been illustrated by reviewing examples of the lowtemperature process. Polymerization at temperatures as low as 3°C in the presence of a water-insoluble organic liquid was illustrated by Kometani et al. in an old US Patent [22]. One million parts of deoxygenated water and 300,000 parts of trichlorotrifluoroethane (CTFE, CCl3
CF3) were charged to a polymerization vessel equipped with an anchor type agitator. After venting out the air thoroughly, TFE was supplied at 3°C to pressurize the vessel to 6.1 bars while agitation continued at 600 rpm. This was followed by the addition of 10 parts of APS, 5 parts of sodium bisulfite, and 5 parts of ferrous sulfate. A drop in the TFE pressure indicated the commencement of polymerization reaction. The PTFE powder recovered had an average particle size of 750 μm. Practically, all the particles had nearly spherical shape. The powder appeared quite softness and could be flattened between the fingers. Properties of the powder included: SSA 5 3.3 m2/g, break elongation 5 300%, tensile strength 5 26.5 MPa, dielectric breakdown strength (ASTM D149) 5 9 kV/mm and estimated molecular weight (MW) of 8 million (for MW calculation method, see US Patent 3,462,401) [22]. Suspension polymerization of TFE in the presence of an inert gas was reported to take place at a temperature of 15°C by Felix et al. in US Patent 5,153,285. A volume of 100 L of demineralized water containing 6 g of ammonium oxalate to a 200-L capacity enameled reactor. Oxygen was removed by alternate evacuation and flushing with high-purity nitrogen while the temperature was adjusted to 15°C. Kettle pressure was raised to 7 bar and then to 13 bar using 6 bar TFE, charged successively. Polymerization was initiated by addition of a single dose of 150 mg of KMnO4, dissolved in 100 mL of degassed water. TFE was fed continuously during the polymerization at a rate sufficient to maintain a constant concentration of TFE in the gas space. The composition of the gas phase was 54% mole of N2 and 46% mole of TFE. The composition remained essentially constant during the entire duration of polymerization.

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When the amount of TFE consumption reached 40 kg, the total pressure was about 1.2 times the initial pressure. Shutting off the TFE flow and venting out the reactor vapors terminated the polymerization reaction. After flushing the reactor several times with N2 to degassing the polymer, the PTFE batch was removed from the kettle. The particles were ground, while wet, down to about 200 μm size. The particles were then separated from the water by filtration followed by drying at 220°C in an air oven. The PTFE particles had improved grain structure (less fibrous) and better fine grinding efficiency in milling as exhibited by lower energy consumption [9]. A choice has to be made between hightemperature polymerization yielding one set of properties at high space-time yield and lowtemperature polymerization at low space-time yield to obtain different properties. It would be very attractive to have a suspension process yielding such properties at rates approaching those of hightemperature polymerization without the investment in heat transfer capacity necessary to keep temperature low at such rates; Aten has described a process to accomplish this goal, reviewed here [17]. Aten discovered PTFE a suspension process in which the initial temperature started at a low value and increased to higher values as polymerization progressed. Surprisingly the PTFE produced had properties similar to those of polymer made at constant temperature equal to the low starting temperature, even though a substantial fraction of the PTFE was formed at higher reaction temperatures. The process of Aten’s invention resembled typical commercial TFE suspension polymerization processes except for temperature control. TFE pressure ranged 0.3
7 MPa and was either maintained constant or varied during the reaction cycle. Two examples are described that illustrate Aten’s invention. An autoclave was charged with 21.3 L of demineralized water, 1 g of oxalic acid, 0.1 g of potassium meta-bisulfite, 0.7 g of ammonium perfluorooctanoate, 0.0023 g of TritonX-100, and 0.1 g of potassium phosphate. The agitator was turned on at 800 rpm. The autoclave was pressured to 2.17 MPa with approximately 1450 g of TFE and cooled to 15°C. To initiate polymerization, an aqueous solution of potassium permanganate (0.01 g/L) was continuously injected into the autoclave at 25 ml/min for the duration of the reaction. After 15 rain of initiator solution injection, a slight pressure drop was observed indicating start of polymerization (kickoff), the TFE feed

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valve was opened, and additional TFE monomer was continuously fed into the autoclave to maintain the pressure at 2.17 MPa [17]. As the polymerization reaction continued the temperature in the vapor space continuously increased, and the reaction rate also increased continuously. These observations are summarized in Table 6.1, which presents temperature and cumulative amount of TFE fed to the autoclave at various times after kickoff (after opening the TFE feed valve). At 54 minutes after kickoff, 5810 g of TFE had been fed to maintain pressure, temperature, and polymerization reaction rate both increased rapidly, and the monomer feed valve was closed. The pressure was observed to fall to 0.16 MPa in 3 minutes and temperature reached a maximum of 36°C. The autoclave was vented, and the polymer was recovered then cut to average particle size of 17 μm and dried. The particles had a shrinkage of 2.8%.

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The autoclave was charged with 21.3 L of demineralized water, 1 g of oxalic acid, and 0.2 g of potassium meta-bisulfite. The autoclave was pressured to 2.17 MPa with TFE, cooled to 15°C, and agitated at 700 rpm. A solution of potassium permanganate (0.008 g/L) was continuously injected at 25 mL/min into the autoclave for the duration of the batch. TFE monomer was fed to the autoclave to maintain pressure at 2.17 MPa. The maximum temperature observed in the vapor space during polymerization was 18°C. After 100 minutes of reaction starting from kickoff, 5080 g of TFE had been converted into polymer. At this point, the TFE feed valve was closed and the autoclave was vented. The PTFE was recovered, cut to 18 μm average particle size, and dried. A shrinkage of 2.9% was measured [17]. Fig. 6.9 is a replication of Fig. 6.4 except for the addition of two shrinkage data points from this and the previous paragraphs demonstrating Aten’s invention.

Figure 6.9 Mold shrinkage as a function of constant TFE polymerization temperature and Aten’s experiments (blue and red circles) [17].

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Different fluoropolymer manufacturers are known to produce specialty PTFE products by polymerization at low temperatures. The only company purported to use the low-temperature polymerization technology exclusively was the HaloPolymer Company from Russia. The two divisions of the company manufacture a variety of fluorinated products including fluoropolymers at Kirovo-Chepetsk and Perm locations in Siberia. The Company website (www.HaloPolymer.com) describes products made at Kirovo-Chepetsk as fluoroplastics resins and parts along with a long list of fluorinated and nonfluorinated chemicals.

6.7 Comminution of Suspension Reactor Bead The product of suspension polymerization (bead) is usually stringy irregularly shaped powder. They have variable size and shape and are elongated as a result of vigorous agitation; resembling shreds of graded coconut (see Fig. 6.6). The challenge was to convert the bead into a powder, which would ideally have the highest possible bulk density. The latter impacts the size (height) of molded articles. Early in the development of granular PTFE, it was discovered that cutting the bead into smaller size particles could produce articles with useful properties. Smaller cut PTFE particles resulted in lower bulk density powders but stronger unsintered preforms and facilitated their handling. Powders with small particle size also produced parts with superior mechanical properties because of more thorough void elimination during sintering [23]. Mechanical, electrical, permeation resistance, and the ability of PTFE resin to accept fillers improve with reduction of particle size. While comminution renders the PTFE usable as a molding powder, it does not solve the problem of powder flow. The powder did not flow smoothly when filling a mold—resembling the behavior of wheat flour when pouring out of a storage canister. In the most common processes, the reactor bead is rough cut and washed—followed by filtration and drying. After completion of the polymerization, the reactor bead is transferred to another vessel as large as or larger than the reactor and equipped with a high shear agitator. It is

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washed with hot water while the agitator rotates at a high speed. Typically, the size of the reactor bead is reduced down to several hundred microns by the action of the agitator. Water is removed from the rough-cut PTFE by filtration. The wet PTFE is dried for use as final product or as feed for further size reduction by different types of mills. A key consideration in comminution (pulverization) of PTFE is its resilience. Even under cryogenic temperatures PTFE maintains significant elastics properties and does not become brittle. The common techniques for commercial particle size reduction of PTFE include fluid energy milling using air or another gas and impact milling (hammer). PTFE has an important transition temperature of 19°C. At above this temperature, PTFE becomes much softer and pliable as compared to its consistency below 19°C. The size and shape of particles of ground PTFE are greatly influenced by the milling temperature.

6.7.1 Fluid Energy Milling Fig. 6.10 shows an example of a gas mill and its high-pressure gas supply unit. Particle size reduction takes place in the central chamber of the gas mill as the process material is driven at near sonic velocity around the perimeter of the grinding chamber by multiple jets of air (or nitrogen). Size reduction rakes place as a result of high-velocity collisions between particles of the process material itself. The interior of the chamber is designed to allow recirculation of oversized particles, enhancing the incidence and the effect of these collisions (see Fig. 6.11 and accompanying video). As particles’ size is reduced, they lose mass progressively and naturally migrate toward the central discharge port. This movement allows classification to be both automatic and precisely controllable [24]. The micronization process begins with feeding PTFE particles into the feed funnel. Those particles are pulled into the grinding chamber by the vacuum created through the venturi eductor. Grind gas (air/N2) manifold supplies high-pressure gas to the nozzles increasing the gas velocity. Velocity of the movement of the particles increases leading to collisions among the particles.

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Figure 6.10 Example of an air/jet mill and high-pressure gas supply unit/jet mill (Micron-Master Mill) [24]. Courtesy The Jet Pulverizer Co, www.jetpulverizer.com, June 20, 2020.

Figure 6.11 Working principle of air milling [25]. Courtesy Sturtevant, Inc.

Centrifugal force retains the larger PTFE particles in the outer diameter of the grinding chamber. The collisions of particles generate smaller particles that migrate toward the middle of the grinding chamber. The micronized particles exit the grinding chamber in the draft generated in the center of the chamber. By precise metering of the product input and air velocity, predictable and repeatable finished particle sizes can be obtained.

6.7.2 Hammer Milling Hammer mills work mostly based on impact grinding of the hammers (steel bars) driving the material to be ground (PTFE) against a circular screen (Fig. 6.12). In some hammer mills, the screen is replaced with a solid serrated section called a striking plate. PTFE is held in the grinding chamber until it is reduced to the size of the

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Figure 6.12 Schematic diagram of a hammer mill equipped with feedback control system [26].

Figure 6.13 Example of PTFE powder produced by hammer milling at 25°C.

openings in the screen or smaller. The number of hammers on a rotating shaft, their size, arrangement, sharpness of hammer edges, the speed of rotation, clearance at the hammer tip relative to the screen and milling temperature are among the variables affecting grinding capacity and the shape of the product particles. Fig. 6.13 shows a scanning electron micrograph of particles obtained as a result of hammer-milling PTFE at 25°C. If the milling temperature is

increased, PTFE particles will be smaller and will have significantly different shapes. The change in particle shape is significant if the milling temperature is raised to 40°C
50°C. The sketches in Fig. 6.14 illustrate a comparison of the shapes of PTFE particles at two milling temperatures. These differences have a significant impact on the properties of the parts made from the powders: higher temperatures particles yield lower bulk density, higher shrinkage, increased molded tensile strength,

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and better filled-compound properties. Fig. 6.15 shows the effect of milling temperature on some of the properties of three resins milled at different temperatures. Table 6.2 shows examples of commercial products including homopolymer and modified PTFE powders. The latter contain ,1% of another fluorinated or perfluorinated vinylic monomer. Examples of commercial granular PTFE products can be found in Section 6.17.

6.7.3 Comminution of Wet Reactor Bead A less common process for PTFE size reduction entails direct comminution of wet polymer bead

Figure 6.14 Shapes of PTFE powders as a function of milling temperature. ( A) Roundish (B) Elongated.

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into small particles (35 to ,50 μm) and then pelletizing the small particles. There are a few key benefits to this process. Water washing removes of the initiator fragments and any residual surfactant often trapped inside PTFE particles and cause discoloration of parts during sintering. Cutting the PTFE in cold water allows more effective control of mold shrinkage and bulk density. It also can be operated in an almost continuous manner even though TFE polymerization is batch process. Weisenberger devised a method for cutting the PTFE particles into small particles [28]. Slurry of PTFE reactor bead was passed through a cutting system consisting of a water cutter (Fitzmill model K-14 cutter, then commercial) equipped with a screen. The cutter had a series of thick sharpened blades that rotated on a horizontal shaft at 1450 revolutions per minute. PTFE slurry was pumped into the top of the mill and passed through the cutting zone and out of the cutter when it had been reduced enough in size to pass through the screens at the lower end of the mill. The slurry was filtered to remove any impurities in the water originating from the polymer. The drawback to the Fitzmill was its inability to reduce the particle size ,50 μm.

Figure 6.15 Example of the effect of milling temperature of PTFE powder properties.

Table 6.2 Examples of Properties of Commercial Fine Cut PTFE Resins with Low and High Bulk Density [27] Property

Unit

Bulk density

g/L

Std. specific gravity Shrinkage ASTM type/ grade Type of polymer

Test Method

M-12

M-15

M-15X

M-18

M-111

M-112

ASTM D4894

35

465

455

480

360

360

—

ASTM D4894

2.16

2.16

2.16

2.16

2.17

2.15

%

ASTM D4894

3.1

4.4

4.4

3.2

4.4

4.6

ASTM D4894

II

II

II

II

III/1

III/1

Homopolymer

Homopolymer

Homopolymer

Homopolymer

Modified

Modified

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Figure 6.16 A typical Taylor Stiles cutter capable of operating at high speed [32].

The filtered PTFE was remixed with water and agitated at an elevated temperature in the range of 40°C
70°C. The best results with respect to improved powder flow were obtained when water to polymer weight ratio was at least 4:1 but less than 12:1. Weisenberger found agitating the PTFE for at least 4 hours to obtain maximum improvement in powder flow. Afterwards, PTFE pellets were filtered by passing the slurry through a vibrating screen or a centrifuge. The filtered pellets were dried by heated air at 120°C
180°C. A number of post-Weisenburger patents refer to a Taylor Stiles cutters as means of reliably reducing the size of the PTFE particles in water [29
31]. In one example [29], the Taylor Stiles wet cutter was equipped with 6 in. (15.2 cm) long rotor blades operating at a rotor speed of 9600 rpm. There was also an internal screen with 0.17 mm wide openings across the discharge of the cutter. Fig. 6.16 shows a typical Taylor Stiles cutter. For instance, wet PTFE was mixed with water at 8°C and was passed through the cutter at a water flow rate of 1360 L/h and a polymer feed rate of 227 kg/h. Temperature was maintained at 8°C. The final cut polymer had a particle size of 35 μm and an apparent bulk density of 417 g/L.

6.8 Pelletized (Free Flow) Granular PTFE Fine cut PTFE powders have poor flow and relatively low bulk density. These characteristics render them unsuitable for processing by automatic and isostatic molding. These techniques require free flow for complete filling of mold cavities. Another disadvantage of fine cut powder is the requirement of fairly large molds because of low bulk density. To make free flow pellets, fine cut PTFE powders are converted into soft agglomerates to increase their flow and bulk density. Each pellet or agglomerate was comprised of a many fine cut particle. The pellet diameter is of the order of a few hundred microns. Pelletization of PTFE differs from many other materials because it does not involve a binder, which in general refers to a secondary material to “glue” the individual particles together. PTFE particles are mixed with an aqueous or nonaqueous medium and if required surfactants or surfactants are added. The mixture was heated and agitated. PTFE was known to become soft at elevated temperatures, known in the industry as becoming “sticky.” The conventional understanding was that agitation brings about random collision of

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Figure 6.17 Scanning electron micrograph of the surface of PTFE pellets.

Figure 6.18 Scanning electron micrograph of the particles obtained by disintegration of PTFE pellets in Fig. 6.17.

the particles which adhere to one other because softness/stickiness. Mechanical entanglement and van der Waals forces hold the particles together, further enhanced by the drying process. A key characteristic of PTFE pellets is they can be disintegrated into the constituent particles using equipment such as Reitz Mill [33]. Fig. 6.17A
F

shows micrographs of the surface of a pelletized PTFE at 500 3 to 20,000 3 magnification. Higher magnification micrographs indicate physical attachment of individual particles as opposed to “fusion.” Fig. 6.18 shows PTFE particles produced from disintegration of the pellets in Fig. 6.17.

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6.8.1 Processes for Agglomeration (Pelletization) of PTFE Agglomeration of PTFE particles (Fig. 6.17) is achieved by an either dry or wet technique. The dry process makes use of a water-insoluble organic liquid. An early process [34] consisted of mixing the fine cut PTFE powder with a small amount of an organic fluid which had a low surface tension followed by tumbling of the mixture. After formation of the agglomerates, the organic fluid was removed by heat. Toxicity and flammability are two drawbacks of this method. In the past, a variety of chlorinated solvents were commonly used in pelletization processes of PTFE. These solvents have been banned for some time and safer alternatives have been developed. In another dry process, organic solvents such as carbon tetrachloride (banned from use), acetone, p-xylene, or ethanol were the liquid medium for agglomeration of PTFE [35]. Polymer to liquid ratio was 0.6
10 by weight and the process temperature ranged from 20°C to 40°C. The mixture was agitated for up to 2 hours, after which the agglomerates were separated and dried. Typical average particle size was 500
800 μm while apparent density remained between 400 and 600 g/L. Disadvantages of this process include processing of large quantities of organic liquids and the low apparent density of the product. The wet process requires using mixtures of water and a water-insoluble organic liquid. PTFE powder was mixed with this fluid in a stirred vessel. The mixture is heated to a temperature below the boiling point of water and agitated to granulate the PTFE. The next steps involve separation of the agglomerates from the liquid phase and drying. Generally, the wet process is preferable to the dry method because of the easier control of the properties of the agglomerates. There are wet processes in which water is the only agglomeration medium. An advantage of pelletization process is that it allows incorporation of fillers such as glass fiber, carbon powder/fiber, bronze and others without segregation from PTFE. The surface of hydrophilic fillers may be treated with organosilanes or silicone resin to prevent their separation and migration into the aqueous phase. Roberts and Anderson [36] reported agglomerating finely ground PTFE powder in water.

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They added the polymer powder to a tank containing water at 80°C and agitated the mixture for 1 hour. The slurry was filtered, dried, and sieved through a mesh screen having 1000 μm openings. Seventy percent of the product passed through the sieve and had an apparent density of 540 g/L. The ratio of PTFE powder to water was in the range of 88
149 g/L at a temperature above 40°C. More recently, Baron et al. [37] have described a process for preparation of a high bulk density free flow PTFE pellets. The basic steps in producing the pellets consisted of: 1. PTFE and filler powders were wetted with a solvent; 2. The wetted PTFE powder was formed into PTFE pellets; 3. The wet PTFE pellets were dried using in a fluidized bed drier. In examples of this process, agglomerated pellets of PTFE filled with graphite were prepared as follows. A quantity of 41 kg of PTFE G-586 Fluon granular was premixed with 7.2 kg of 80 mesh sifted graphite. The PTFE
graphite mixture was mixed in a 85-L Littleford Day (www.Littleford. com) mixer for 8 minutes dry and then for 2 minutes With 5.2 gallons of a solvent containing 96% of pure water and 4 wt.% of dipropylene glycol n-butyl ether. Afterwards, the PTFE
graphite mixture was passed through a hammer mill and then passed through a 1 mm opening screen onto a vibrating deck that conveyed the mixture to a 75 cm diameter heated tumbler operating at a temperature of 107°C and a tumbling speed of 25 rpm. Particles of a typical commercial free flow PTFE powder are relatively large flow freely and have high bulk density (e.g., 850 g/L). Molds with intricate geometries are easily filled with free flow PTFE resins. A variety of parts and machining stock shapes are molded from PTFE. They are machined and fabricated into parts like small parts such as ball valve seats, seals, discs, and lab ware. Large-diameter rods and tubing made from free flow PTFE are stock shapes that are machined into parts such as electrical insulators, mechanical bushings, and seal rings.

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6.9 Presintered Granular PTFE Presintered resins are free flowing powders made from once-melted [38] PTFE, predominantly intended for ram extrusion for the production of thin wall tubes and small diameter solid rods and to a lesser extent porous parts. These resins are prepared by melting aspolymerized PTFE, cooling the melt, and disintegrating the resin back into small particles. The average particle size of these resins is typically several hundred microns and its melting point is reduced from 342°C to 327°C. The primary advantage of presintered resins is good thus flow easy feeding to a ram extruder. Second, rods, tubes, and other cross sections made by ram extrusion of using presintered PTFE resist fracture at charge marks (Fig. 6.19). Presintered PTFE is produced from fine cut (low flow) PTFE powder. Stainless steel trays are loaded with fine cut resin about 10 cm deep. Multiple trays are placed on carts and placed in a large sintering oven. The oven undergoes a simple sintering cycle consisting of heat-up (3°C/ min) to 370°C, hold at this temperature for about 2
4 hours followed by cooling (3°C/min) to room temperature. Hold-up time must be sufficiently long for the PTFE powder to melt and for a solid slab after cooling. Slabs of PTFE are

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recovered from the trays for size reduction. A downside to size reduction is yield loss caused by generation of very small and very large particles that cannot be consumed in ram extrusion. Small particles cause bridging on feed section of the ram extruder. Large particles leave voids in the part which weakens the extruded rod or tube. A typical PTFE presintering process can easily have a yield loss of 20%. Particle size reduction is conducted in two stages to minimize yield loss. In the first step, the slabs are chopped into small pieces. In the second stage, the chopped pieces are ground into a powder with an average particle size of several hundred microns. One such cutter for the first step is offered by Cumberland Plastics (A Division of ACS Group Company, www.cumberland-plastics.com). The grinding or pulverization of the chapped pieces can be accomplished by hammer mills or jet mills. After grinding, the powder is sieved and very small/large fractions are removed before use in ram extrusion.

6.10 PTFE Filled Compounds Filled compounds of granular PTFE are commercially significant among three PTFE types because of the large volume of consumption. Some companies produce compounds in-house while others purchase from specialty compounders who are skilled in formulation and production of various filled compounds. Compounds of dispersion polymerized PTFE are usually made in-house and consumed to fabricate parts. PTFE dispersions are formulated into coatings and paints by incorporation of a variety of additives [40]. PCTFE compounds are highly specialized and not very common. The incorporation of fillers in PTFE impacts several properties of parts [41]. They include 1. Wear resistance is increased to a very marked extent. 2. Resistance to “creep” or deformation under load is increased by a factor of 2
5.

Figure 6.19 Examples of poor-quality ram extruded PTFE rod (and contaminated) [39].

3. Depending upon the filler used, the thermal conductivity may be increased significantly.

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4. Depending upon the filler used, thermal expansion may be reduced by a factor approaching 5. 5. By suitable choice of fillers, some control over the electrical properties of PTFE can be achieved. In addition to these advantages, filled PTFE generally retains low coefficients of friction, the wide service temperature range and, depending upon the filler, chemical inertness of unfilled PTFE. Filled granular resins have been found desirable for fabricating parts such as gaskets, shaft seals, bearings, bearing pads, and piston rings. Chemical resistance, low friction, and high/low-temperature capability, combined with mechanical requirements, have promoted specification of filled PTFE as material of construction in many demanding applications.

TO

FLUOROPOLYMERS

does not have sufficiently low viscosity to flow upon melting to coat the surfaces of the fillers. Low coefficient of friction reduces mechanical interaction, thus PTFE and filler can easily segregate as a result of motion. Compounding fillers with this polymer and pelletizing the mixture is one way of locking in the uniformity of the mixture. A paramount requirement of a filler to qualify for incorporation in PTFE is it should resist degradation at the sintering temperatures of PTFE, for example, 400° C for several hours, thus excluding many candidates. Characteristics of a filler such as particle size and shape as well as chemical composition of the filler affect the properties of compound. A list of most common fillers and their important effects on compound properties can be found in Table 6.3, which can also be consulted for the selection of compounds. Some of the applications of filled PTFE compounds are listed in Table 6.4.

6.10.1 Granular PTFE Compounds

6.10.1.2 Selection of PTFE Grade

The choice and concentration of the fillers in PTFE depends on the desired properties of the final part. Glass fiber, bronze, steel, carbon, carbon fiber, graphite, and mica are among the common commercial fillers. Up to 40 vol.% of filler can be added to PTFE without complete loss of physical properties of the filled compound. Below 5 vol.% filler content, the impact on the properties of compounds is insignificant. At above 40 vol.% filler content, most mechanical properties of the compound drop sharply. Better than one half of granular PTFE is consumed in the form of filled compounds. Many standard grades are offered by merchant compounders in low flow, free flow (granulated), and presintered forms. Applications of low and free flow powders are similar to those of unfilled PTFE resins. Low flow resin is suitable for compression molding; while free-flow powders are processed by isostatic and automatic molding and ram extrusion. Presintered PTFE and its compounds are strictly converted by ram extrusion process.

Polymer selection for compounding granular PTFE is relatively straightforward. Fine cut resins are used as a starting point to produce filled compounds. These powders have relatively small particle size and form the most uniform compounds. Examples of commercial grades of the resins include Dyneon Hostaflon 1702 and 1750, Daikin PolyflonM-12 and M-15, AGC Inc. Fluon G155 and G170, Solvay Solexis Algoflon F5 and F6, Halopolymer Fluoroplast F-4PN-20 and F-4PN-40, and Gujarat Fluorochemical Company Inoflon 630 and 640. Typically, smaller particle size resins produce compounds with higher physical properties.

6.10.1.1 Fillers PTFE is one of the more difficult polymers to compound. This is due to the extreme charge of particles and functional neutrality of PTFE chains, which precludes any interaction with fillers. PTFE

6.10.2 Filled PTFE Production Techniques The first step in making a compound is blending the filler(s) with PTFE powder. The mixture is usually first tumbled by a drum-tumbler or other equipment to achieve a rough blend of the filler and PTFE. It is then milled to increase the uniformity of the PTFE and the filler(s). Milling can be accomplished in a number of commercial mills. Different procedures and processing conditions are required for specific filled compounds. Commercial mills operate based on two separate principles.

Table 6.3 Effect of a Few Common Fillers on Properties of PTFE Compounds [42] Properties

Bronze

Carbon

Chemical agent resistance

Carbon Fiber

Ceramic

Ceramer

Ekonol

X

X

X

X

Glass Fiber X

Graphite

Mineral

X

X

Molybdenum

Polymide

Stainles Steel

X

X

Compression resistance

X

X

X

X

X

X

X

Deformation under load

X

X

X

X

X

X

X

Electrical insulation Hightemperature resistance

X

X

X

Low coefficient of friction

X

X

X

X

X

X

X

X

X

X

X

Lubrication Thermal conductivity

X

X

Thermal stability

X

X

X

Wear resistance

X

X

X

X X

X

X

X

X

X

X

X

X

X

X

X

X

X X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

Table 6.4 Examples of Applications of PTFE Compounds [42] Applications Compressor rings

Bronze

Carbon

Carbon Fiber

X

X

X

X

X

Gaskets

Ceramic

Ceramer

Ekonol

Glass Fiber

Graphite

Mineral

Molybdenum

Polymide

Stainles Steel

X

X

X

X

X

X

X

X

X

X

Piston rings and seals Plates and bars

X

X

Sealing elements

X

X

Bearings

X

X

Valve seats

X

X

X X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X X

X

X

X

X

X

X

X

X

X X

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First, hammer mills include equipment made by Schutte Buffalo (www.hammermills.com) and Rietz mill supplied by Bepex Corporation (www.Bepex. com). These mills work is by subjecting a blend to a set of small rotating hammers which resides in a mesh screen basket. The particles pass through the mesh screen as they are dispersed and comminuted. The important process parameters include the size of the openings in the mesh screen and the number and speed of the hammers. The second group are small intensity mills like Vshape devices, which function basically as blenders. They consist of two connected cones and are usually equipped with an attrition bar. The mill rotates around a horizontal axis. Material is transferred from one cone to another during each of the rotation, thus mixing the filler and the PTFE powder. The attrition bar sits parallel to the rotation axis and can be operated at different speeds. Patterson Kelly (a Division of Harsco Corp., www.harscopk.com) is a major supplier of V-shape blenders. Low flow blends are also feed for the production of free flow compounds. Production of free flow compounds is based on granulation (agglomeration) of the particles of resin and filler. This is accomplished by mixing the ingredients (the low flow blend) with a water-immiscible organic solvent and/or water and sometimes a surfactant, and then heating the mixture and shearing it. Granules formed in this process are separated from the liquid and dried to remove the liquid residues. A dry process is one in which no water is used.

6.10.3 Fine Powder-Based Compounds Dispersion polymerized PTFE or fine powder is compounded to a much lesser extent than granular PTFE. It is relatively difficult to mix solid fillers with fine powder PTFE to form a uniform blend. This is due to the large average agglomerate size (several hundred microns) of fine powders. Large concentrations of filler particles serve as stress risers, thus deteriorating the physical properties of the compound. Excessive shearing will lead to fibrillation of polymer particles as opposed to deagglomeration. The limitation in the shear that can be applied to the resin limits the filled volume fraction. Size and shape of the filler particles impact the maximum volume fraction; more of the smaller particles can be incorporated. Incorporation of additives including fillers and

85

pigments into fine powder polymer is usually intended to accomplish one of the following objectives:

• Achieve a color; • Increase electrical conductivity; • Increase abrasion resistance. Some applications such as fuel transport hoses require static charge dissipation to avoid igniting the fuel. Conductive grades of carbon black can impart electrostatic charge dissipation to the polymer. A small amount of carbon black (1%
2%) is compounded by adding a dispersion of the carbon black in the lubricant to polymer powder and tumbling them in a V-shape mixer like Patterson-Kelly machine. This compound forms the inner layer of the tubing by co-extrusion of the paste.

6.10.4 Fabrication of Reinforced Gasketing Material Reinforced fine powder PTFE material is primarily used as gaskets, seals, and diaphragms in extreme temperature, pressure, and chemical environments. A gasket in this type of application must be resilient and resistant to corrosive chemicals and also maintain a high tensile strength and dimensional stability at elevated temperature and pressure. PTFE has the necessary corrosion resistance to the majority of industrial chemicals up to its melting point (327°C) but in its neat (without fillers or additives) form, it is not satisfactory in many applications because of the high cold flow (creep) that is inherent to PTFE. After the unfilled PTFE gasket relaxes under the pressure exerted by bolt loads, it begins to leak. An increase in temperature both accelerates and increases creep relaxation. The reinforcement approach deals with the problem of cold flow and dimensional stability by highly filling PTFE with a variety of fillers, as high as 90 wt.%. Examples of fillers are metal powders, ceramic, glass fiber, and carbon black. Proper processing to blend the fillers and the PTFE intimately help retain good physical properties in the reinforced (filled) material. Fabrication of reinforced gasket materials is accomplished by an unusual process that is documented elsewhere [12]. During the processing, PTFE fine powder fibrillates that trap and hold the filler particles.

86

6.11 Preparation of Polytetrafluoroethylene by Dispersion Polymerization Commercially, TFE is polymerized in an aqueous dispersion medium to produce dispersion and fine powder PTFE products. The key characteristics of this polymerization regime include ample surfactant requirement and mild agitation at elevated temperature and pressure. The dispersion/emulsion recovered from the reactor is finished by two distinct processes depending on whether a dispersion or a dry powder (fine powder) is the desired final product, as shown in Fig. 6.20. Examples of dispersion polymerization presented in this section illustrate the technology. In this regime of polymerization, TFE and other vinylic monomers react in an aqueous emulsion medium. The resulting colloidal polymer remains in a stable emulsion. An early report of this process was made by Renfrew in 1950 [43] using disuccinic or diglutaric acid peroxide as polymerization initiator. Gentle agitation was applied to the reactor while holding elevated pressure (0.3
2.4 MPa) at a temperature of 0°C
95°C. The emulsion product was a stable dispersion of small polymer particles containing ,10 vol.% PTFE. The emulsion was easily coagulated after subjecting it to vigorous agitation. The emulsion could also be stabilized by the addition of a second surfactant. Addition of ample quantities of specific surfactants could impart sufficient stability to the emulsion to prevent coagulation during moderate agitation, transportation, and handling. The present

Figure 6.20 Dispersion polymerization of tetrafluoroethylene and finishing processes.

INTRODUCTION

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FLUOROPOLYMERS

commercial dispersion technology closely resembles Renfrew’s invention. Brinker and Bro [44] reported significant improvements by the addition of a small amount of methane, ethane, hydrogen, or hydrofluoroethanes to the reactor prior to the onset of polymerization. The reactor ingredients included a surfactant such as fluoroalkyl carboxylate [45] and an insoluble saturated hydrocarbon as an anticoagulant [46]. A typical reaction contained 0.1%
3% of a dispersing agent like ammonium perfluorocaprylate. The initiator of choice was a water-soluble compound such as APS and disuccinic acid peroxide. Redux initiators, for example, sodium bisulfite with ferric triphosphate, could also be used. The anticoagulant was a saturated hydrocarbon with more than 12 carbons, also known as wax, which was a liquid at the polymerization temperature. The colloidal solids concentration was 36%, well in excess of the Renfrew process. In 1964, Cardinal et al. reported the development of fine powder PTFE with high molecular weight and lower melt creep viscosity, by the introduction of a modifier to the polymerization kettle. The modifier consisted of a nonpolymerizable chain transfer agent such as methane, propane, and perfluoropropyl vinyl ether (PPVE) [3]. The modifier may be introduced at any time during the polymerization. For example, if it is introduced after the consumption of 70% of the monomer, each PTFE particle contains high molecular weight PTFE and a shell containing the low molecular weight modified PTFE. In this example, 30% of the outer shell of the particle, by weight, is modified. Melt creep viscosity was 3
6 3 1010 poise as compared to 10 3 1010 poise for 100% homopolymer. Paste extrusion pressure was decreased by 20%
50% leading to fewer flaws in tubing and wire insulation made from these resins [3]. Cardinal et al.’s [3] invention was another significant improvement in the ability to alter and control dispersion polymerized PTFE properties. The importance of perfluoroalkyl vinyl ether comonomers such as PPVE, perfluoroethyl vinyl ether (PEVE), and other PPVE in dispersion polymerization was explored by Holmes and Fasig [4] Their PTFE was composed of TFE and modifiers (e.g., PPVE) and had excellent mechanical properties. Samples of PPVE-modified PTFE achieved a flex life of 18 million cycles even after being aged for 31 days at 322°C. Standard specific gravity (SSG) of PPVE-modified PTFE was below 2.175 and melt creep viscosity remained below 4 3 1010 poise at 380°C. Polymerization rates

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Table 6.5 An Example of PTFE Dispersion Polymerization Recipe and Properties [4] Reaction Component or Property

Polymer 1

Polymer 2

Modifier type

Perfluoropropyl vinyl ether

Perfluoroethyl vinyl ether

Modifier amount

20.5 ml

3g

Deionized water, g

21,800

3600

TFT, g

10,050

1830

Ammonium sulfate initiator, g

0.33

0.065

Initial

2

4.92

Final

26.7

Wax, g

855

141

Temperature, °C

65
75

75

Pressure, Mpa

2.8

2.8

Agitator speed, rpm

46

105

Solids content of dispersion, wt.% of water

35

33.7

PTFE particle size, μm

0.188

0.1

Standard specific gravity

2.149

2.16

Modifier content of polymer, wt.%

0.102

Melt creep viscosity at 380°C, poise

0.9 3 10

Ammonium perfluorocaprylate surfactant, g

a

0.09 9

2 3 109

a

Determined according to the ASTM Method D4895.

increased to commercially acceptable values by incorporation of purified modifiers and replacement of disuccinic acid peroxide with a persulfate type initiator such as APS. Typical recipe and polymerization data are presented in Table 6.5. Poirier [47] reported preparation of dispersion polymerized PTFE with a composite particle structure. The inner portion of the particle (core) contained a higher concentration of the comonomer than its outer portion (shell). The advantage of this type of PTFE was the possibility of paste extrusion of fine powders at high reduction ratio (RR) without the complication of high extrusion pressure and flaws in parts such as tubing or wire insulation. RR is defined as the ratio of crosssectional areas of PTFE in the extruder barrel to the PTFE in the die. The core constituted 65%
75% of the total weight of the particle. The remaining 25%
35% of the polymer formed the shell at lower comonomer content than the core. Wire insulation was made from these polymers. The number of flaws was minimized in the core-shell polymer made with a lower concentration of comonomer in the shell as compared to homopolymer of TFE alone.

Here is an example of redox polymerization of TFE [48]. A polymerization vessel was charged with ultrapure water, paraffin wax, and ammonium perfluorooctanoate, together with succinic acid and oxalic acid, deaerated by purging with nitrogen, and heated to a temperature of 55°C. When polymerization reaction temperature stabilized, TFE gas was introduced into the vessel to a pressure of 2.7 MPa. An aqueous PTFE dispersion (solids content of 31.4 wt.%) was obtained.

6.12 Dispersion Polymerization of TFE with APFO Replacements The polymerization aid of choice, starting in the 1950s until the early 2000s, had been ammonium perfluorooctanoate, also known as APFO or C8 (CAS 3825-26-1), which is an ammonium salt of perfluorooctanoic acid (PFOA). APFO was also used as a surfactant in a variety of coatings and finishes in the past. The abbreviation PFOA has been used rather loosely in the literature to refer to

88

both acid and salt forms. There are differences in the properties of the acid and salt forms. The acid form is substantially insoluble and is not usually used as a surfactant in the polymerization of fluoropolymers. APFO was designated as bioaccumulative and persistent in the environment by the US Environmental Protection Agency (EPA) in the early 2000s. Consequently, during the decades of 2000s and 2010s fluoropolymer manufacturers took steps to abate environmental emissions by first reducing and then eliminating the use of APFO from dispersion products. The industry committed to reduce the emission of APFO by at least 95% by the year 2010 over the baseline year of 2000. Fluoropolymer manufacturers further committed voluntarily to work toward the elimination of the use of APFO entirely by 2015. The industry now uses replacement surfactants as polymerization aid. Fluoropolymer manufacturers conducted a great deal of research over the decades to find a replacement for APFO. Some of the published research results can be found in patents, reports by Environmental Protection Agency and other publications. A detailed account of the key research and development work has been published elsewhere [12]. Some of the helpful sources have been listed in the References section [49
55]. Two examples of commercial APFO replacements are described next. In 2008, Dyneon announced the introduction of its new surfactant DyneonADONA as a polymerization aid which would eliminate the APFO completely from its production of fluoropolymers. The reported structure of the new surfactant is shown in Fig. 6.21. The toxicity of ADONA has been studied and reported to be superior to APFO. Dyneon reported 2009 was the first full year of APFO-free operation [57]. In June 2010, DuPont (now Chemours Co.) announced the GenX technology including a new processing aid, which is used only for fluoropolymer resin manufacturing. It has the following chemical structure: CF3CF2CF2OCF(CF3)COOH  NH3 [58].

Figure 6.21 ADONA, ammonium 4,8-dioxa-3H-perfluorononanoate, Dyneon’s PFOA replacement [56].

INTRODUCTION

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FLUOROPOLYMERS

6.13 Dispersion Polymerization Reactor PTFE especially modified PTFE tends to adhere to reactor walls and other surfaces during the polymerization. Process variables and equipment conditions affect the adhesion (scale) that may form on the reactor walls. They include surface smoothness of the wall, monomers, surfactant type, initiator, reaction rate temperature, and pressure. This same issue applies to adhesions formation during suspension polymerization of TFE. Degradation of TFE by deflagration reaction generates heat and carbon black, which contaminates the PTFE in the reactor. Adhesions take place in the hard-to-clean areas of the reactor such the rupture disk, vent lines, thermowells, or the inlet of gases. The primary remedies to the adhesions problem include thorough washing of the walls between successive batches. Preventive measures include highly polished internal surfaces and removal of material and heat from the reactor wall. This is accomplished in two ways: first, installation of a wall sweeping agitator that removes both heat and material from the wall. The second measure is temperature uniformity of the heating and cooling jacket to prevent the formation of “hot spots” that are appreciably hotter than the rest of the jacket wall. An important factor that contributes to the buildup of PTFE is surface roughness of the reactor wall. Even though the surface may look smooth to the eyes, it may be rough microscopically. There are numerous opportunities for the buildup of PTFE resin on a rough surface. Abrasive (grit) and electropolishing treatment of rough surfaces are two remedies to decrease reactor surface roughness. The role of the agitator is critical because of the required heat transfer to remove the exothermic heat of TFE polymerization. The preferred types are wall sweeping and cage (Fig. 6.22) agitators. Paddle mixers (Fig. 6.23) actually push the liquid and polymer in the reactor toward the wall, which could add to the problem of adhesion. Baffles are installed on the interior wall of the reactor to break the radial flow toward the wall. The wall-sweeping agitator constantly removes the PTFE from the wall and enhances heat transfer. This action helps in preventing build of the polymer on the interior wall of reactor and other places such as rupture disk and the material inlet and outlet ports.

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Figure 6.22 Example of an electropolished cage agitator.

Figure 6.23 Depiction of paddle mixers in a horizontal reactor equipped with baffle plates.

6.14 Preparation of Dispersion Grade PTFE Raw dispersions produced by dispersion polymerization contain a wide range of PTFE solids, often, in the range of ,10 to 45 wt.% in water. To insure a commercially viable product, the dispersion must have maximum possible PTFE concentration and be sufficiently stable for transportation, storage and handling. That is, it should not form precipitated PTFE particles that cannot be reincorporated in the liquid phase by simple mixing. An obvious way to concentrate dispersions was heating to evaporate the excess water. The major drawback to heating was the irreversible coagulation of the PTFE. Four methods have been used to concentrate fluoropolymer dispersions; these are

• • • •

Thermal concentration; Electrodecantation; Ultrafiltration; Polyacrylic acid concentration.

One approach is to concentrate the PTFE dispersion thermally by the following steps: 1. Removal of the solid wax usually by decantation after cooling the dispersion to a temperature below its melting point; 2. Adding a large amount of hydrocarbon surfactant to the dispersion; 3. “Breaking the emulsion” by thermally deactivating or insolubilizing the surfactant leading to flocculation of the polymer;

90

INTRODUCTION

4. Separating the polymer floc from the bulk of the aqueous phase; 5. Forming a concentrated dispersion by peptizing (redispersing) the polymer floc. This method can produce concentrated dispersions in the range of 35%
75% PTFE content by weight [59]. Marks and Whipple [60] have described a representative technique for thermal concentration of emulsions containing 30%
45% PTFE particles. Ammonium hydroxide was added at 0.01
1.0 wt.% of the dispersion. Next, 6
12 wt.% of dispersion solids of a nonionic surfactant was added with a structure of [R
C6H4
(OCH2CH2)nOH] where R was a monovalent hydrocarbon with 8
10 carbon atoms and n 5 R 1 1 or R 1 2 (R 5 8
10), or [(tertiary octyl)
C6H4
(OCH2CH2)9
10OH]. After stirring, the mixture was heated to a temperature of 50°C
80°C. A cloudy appearance indicated that the nonionic surfactant had begun to insolubilize. PTFE particles settled after a period of time and formed a layer at the bottom of the container. The upper layer, which was relatively clear, was decanted and the lower layer was recovered. The solid content of this dispersion was PTFE particles at 55%
75% content by weight free of coagulated polymer particles. Marks and Whipple’s method does not require peptization which reduces the amount of ionic material in the dispersion. This has a positive impact on the properties of article made from the dispersion, particularly on their electrical properties. The time required for concentration was by one or two orders of magnitude shorter than in Berry’s technique. Table 6.6 provides a number of examples for the process. In each case, the process was followed step by step according to the conditions given in this table. Triton X-100 was the nonionic surfactant for all the examples and an amount equal to 9% of the PTFE content of the dispersion was added to each mixture. The addition of ammonium carbonate electrolyte yielded the shortest concentration time. Some fluoropolymer manufacturers have used the electrodecantation process for concentration of dispersions. PTFE dispersion was placed in an electrolytic cell with electrodes which are charged. The negatively charged PTFE migrates to the positive electrode where it was withdrawn and formulated.

TO

FLUOROPOLYMERS

For example, a PTFE emulsion containing 25 wt.% polymeric solids content was concentrated by electrolysis. The PTFE emulsion was fed to an intermediate storage tank to which 5 wt.% Triton X-100 based on polymeric solids content was added. The pH was adjusted to 7 using ammonia solution and the solution was slowly agitated. The emulsion mixture of the intermediate tank was introduced into the center cell of an electrodialysis apparatus which contained membranes passing only particles having a molecular weight above 1000. Then, 0.1 mol of a volatile electrolyte was introduced to both of the electrode cells. A 10 V voltage generated by electric supplier was charged to the cell, the membranes being spaced at 30 mm intervals, and the emulsion was concentrated for 80 minutes in the batch system. In this case, the initial electric current density was 80 mA/m2. The aqueous PTFE emulsion was concentrated to 70 wt.% of PTFE content [61]. A novel method [62] used a semipermeable membrane to remove water from PTFE by ultrafiltration. In this technique, 0.5%
12% of a surfactant, relative to the weight of the solid polymer, was added to the dispersion. After mixing, the emulsion was circulated over units of semipermeable ultrafiltration membranes at fluid velocity of 2
7 m/s. The dispersion must be kept from contact with components causing frictional forces, lest the colloidal polymer particles coagulate. After removal of a sufficient quantity of water, the concentrated dispersion was removed from circulation. Typically, concentrated products by this technique contained 40%
65% polymer solids by weight. In another procedure for concentration an acrylic polymer containing a large amount of acid groups or its salts was added to PTFE dispersions. The acid content of the acrylic polymer was at least 20% and they had a weight-average molecular weight of 50M
500M; polyacrylic acid was the preferred polymer. Addition of a small amount (0.01%
0.5%) of this type of acrylic polymer to the dispersion caused a phase separation to occur. The lower phase contained 50
70 wt.% of PTFE and was recovered by decantation. To reduce viscosity and increase stability of the concentrated dispersion, either an ionic or nonionic surfactant could be added before the addition of the acrylic polymer [63]. Stability was a key requirement of the final product. Any dispersion must have a reasonable shelf

Table 6.6 Examples of PTFE Emulsion Concentration Process Variable [60] Starting Dispersion Concentration, wt.%

Electrolyte Type

Electrolyte Added, wt.% of Dispersion

Temperature, °C

Concentration Time, min

Final Concentration, wt.%

47

CaCl2

0.04

80

35

60

47

NHCl

0.04

80

30

63

50.7

NH4CO3

0.04

80

12

72

44

NH4OH

0.36

75

60

68

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INTRODUCTION

life of the order of a few weeks to a few months. It should also be able to withstand transportation and handling during processing. The shear rate inherent in these activities must not cause coagulation of the PTFE particles. For a given dispersion, stability was a function of solids content, pH, and viscosity. These properties may be adjusted by trial and error to improve the stability.

6.15 Preparation of Fine Powder PTFE To produce fine powder from the polymerization dispersion, three processing steps have to take place. 1. Coagulation of the colloidal particles; 2. Separation of the agglomerates from the aqueous phase; 3. Drying the agglomerates. Diluting the raw dispersion to a polymer concentration of 10
20 wt.% and possibly adjusting the pH to neutral or basic [64,65] carries out coagulation. A coagulating agent such as a water-soluble organic compound or inorganic salt or acid can be added to the dispersion. Coagulation was helped by adding a water-soluble organic compound (e.g., methanol and acetone), an inorganic salt (e.g., potassium nitrate and ammonium carbonate), and an inorganic acid (e.g., hydrochloric acid, sulfuric acid and nitric acid) as a coagulating agent. The diluted dispersion was then agitated vigorously. The emulsion was agitated in a container equipped with a stirrer more strongly compared with mixing during the reaction. Primary PTFE particles form agglomerates which are isolated by skimming or filtration. An emulsion from polymerization of TFE (modified by 0.23 wt.% CTFE) was coagulated according to the following procedure: 1.15 L of the latex and 30 g of 15 wt.% aqueous solution of ammonium ω-hydroperfluorononanoate were

TO

FLUOROPOLYMERS

placed in a 6-L capacity vessel for coagulation equipped with the anchor type mixing blades and baffle boards. The mixture was diluted by addition of warm water to a liquid specific gravity of 1.074 at a temperature of 48°C. Next, 1.8 mL of nitric acid (60%) was added to the emulsion followed by agitation at a speed of 300 rpm. The polymer was coagulated and separated from water. Agitation was stopped to remove the separated water after which 3 L of pure water was added and the agitation was resumed at 300 rpm. Afterwards, water was removed, and the wet PTFE was poured into a steel tray and dried at 135°C for 18 hours. A fine powder PTFE with an apparent density of 600 g/L and a mean particle diameter of 470 μm was obtained [66]. Drying of the PTFE agglomerates was carried out by vacuum, high frequency, or heated air such that the wet powder was not excessively fluidized [64]. Friction or contact between the particles, especially at a high temperature, adversely affects the fine powder because of easy fibrillation and loss of its particulate structure leading to poor properties of parts made from this resin. Drying temperatures range from 100°C to 180°C and have great influence on the paste extrusion of the resin. High drying temperatures result in high extrusion pressures. Fine powder resins must be protected from fibrillation after drying. PTFE does not fibrillate below its transition point at (19°C for TFE homopolymers) during normal handling and transportation. Storage and transportation of the resin after refrigeration below its transition point was the normal commercial practice for handling fine powder PTFE resins.

6.16 Characterization of Polytetrafluoroethylene by Properties Basic properties of PTFE are characterized by standard test methods (Table 6.7) published by

Table 6.7 Standard Specification Methods for PTFE PTFE Product Type

ASTEM Method

Related ISO Standard

Granular resins

D4894

12086-1 and 12086-2

Fine powder resins

D4895

12086-1 and 12086-2

Dispersion products

D4441

12086-1 and 12086-2

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American Society for Testing Materials (ASTM). Three major methods specify types and define properties for granular, fine powder, and dispersion products. Another set of standards similarly covering the fluoropolymers was published by the International Standards Organization (ISO). This section describes each method and the associated tests.

6.16.1 Granular PTFE Resins ASTM Method D4894 covers specifications for granular PTFE resins and test methods for the as-produced polymer. Methods for processing granular resins into objects are discussed elsewhere of this book. PTFE resins are thermoplastics in that they can be remelted, but they cannot be processed by the normal melt processing technologies due to their extremely high rheology. This polymer does not dissolve in any solvents. These two facts render the direct measurement of PTFE molecular weight virtually impossible. An indirect property named SSG was substituted for molecular weight. SSG is defined as specific gravity of a sample of PTFE molded and fabricated according to the exact procedures prescribed by ASTM D4894. Special

93

emphasis is placed on molding the part identically during each molding. Cooling rate of the molten polymer is closely controlled to maintain a specific rate of crystallization. SSG of samples made with PTFE with the same structure depends on the crystalline, amorphous and the void content of the sample. Density of crystalline PTFE (2.302 g/cm3) [67] was significantly higher than that of amorphous polymer (2.00 g/cm3) [67] due to the closer packing of molecules in the crystalline phase. Properties used in the characterization of granular PTFE are defined in Table 6.8. ASTM Method D4894 classifies the different types of granular powder according to the system summarized in Table 6.9. The specifications that differentiate granular PTFE powders are summarized in Table 6.10. Water content and melting peak temperatures are the other specifications of these resins. Specifications of molded parts are listed in Table 6.11. The actual ASTM Method D4894 must be consulted for a complete description of these procedures.

6.16.2 Fine Powder PTFE Resins ASTM Method D4895 covers specifications for fine powder PTFE resins and test methods for the as-

Table 6.8 Definition of Basic Properties of Granular Polytetrafluoroethylene according to ASTM Method D4894 Bulk densitya

Mass of 1 L of resin measured under the test conditions

D1895

Average particle size and distribution by sieving

E11

Melting characteristicsa

Heat of fusion and melting peak temperature of resin as determined by differential scanning calorimetry

D4591

Water contenta

Water present in the PTFE resin

Standard specific gravity (SSG)b

Specific gravity of a sample of molded and sintered PTFE according to this method

Thermal instability index (TII)b

A measure of decrease in molecular weight of PTFE material determined by the difference between ESG and SSG: TII 5 (ESGSSG) 3 1000

Tensile propertiesb

Elongation and strength at break of a sample made according to the specified method

Shrinkage and growth

The change in the diameter of SSG preform due to sintering

Extended specific gravity (ESG)

The specific gravity of a PTFE specimen molded for SSG after sintering for an extended period of time, compared to the sintering time of SSG

D792, D1505

Electrical properties

Dielectric constant, dissipation factor, dielectric breakdown voltage, dielectric strength

D149

Particle size

a

a

Properties required for resin specification. Properties required for on molded specimen.

b

D792, D1505

D638

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Table 6.9 Granular Polytetrafluoroethylene Resin Classification According to ASTM D4894 ASTM Type

Description

I

General purpose molding and ram extrusion resin

II

Fine cut resin with an average particle size below 100 μm

III

Fine-cut or free-flow comonomer-modified resins

IV

Free-flowing resins

V

Presintered resins

VI

Ram extrusion resins, not presintered

Table 6.10 Granular Polytetrafluoroethylene Resin Property Specifications According to ASTM D4894 Average Particle Size Diameter, µm

Water Content, wt.%

Initial Melting Point, °C

Initial Melting Point, °C

5°C. second melting point

327 6 10

Type

Grade

Bulk Density, g/L

I

1

700 6 100

500 6 150

0.04

2

675 6 50

375 6 75

0.04

,100

0.04

5°C. second melting point

327 6 10

5°C. second melting point

327 6 10

327 6 10

II

327 6 10

1

400 6 125

,100

0.04

2

850 6 100

500 6 150

0.04

1

650 6 150

550 6 225

0.04

2

.800

... ...

0.04

3

580 6 80

200 6 75

0.04

5°C. second melting point

V

635 6 100

500 6 250

0.04

327 6 10

327 6 10

VI

650 6 150

800 6 100

0.04

5°C. second melting point

327 6 10

III

IV

produced polymer. Methods for processing fine powder resins into objects are discussed elsewhere in this book. PTFE resins are thermoplastics in that they can be remelted but they cannot be processed by the normal melt processing technologies due to their extremely high rheology. This polymer does not dissolve in any solvents. These two facts render direct measurement of PTFE molecular weight virtually impossible. An indirect property named SSG was

327 6 10

327 6 10 327 6 10

substituted for molecular weight. Section 5.7.1 contains a discussion of the relationship between molecular weight and SSG. Properties used in the characterization of fine powder PTFE are defined in Table 6.12. Specimen for SSG and tensile properties are prepared by similar techniques to those described for granular PTFE. In this section, the properties specific to fine powder characterization are described.

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Table 6.11 Molded Granular Polytetrafluoroethylene Property Specifications According to ASTM D4894 Type

Grade

Standard Specific Gravity Range

Tensile Strength, MPa

Break Elongation, %

Ia

1

2.13
2.18

13.8

140

2

2.13
2.18

17.2

200

2.13
2.19

27.6

300

1

2.14
2.22

28.0

500

2

2.14
2.18

20.7

300

1

2.13
2.19

25.5

275

2

2.13
2.19

27.6

300

3

2.15
2.18

27.6

200

























II

a

III

a

IV

a

b

V

VI

b

a

Thermal instability index less than 50. No molded property specifications.

b

Table 6.12 Definition of Basic Properties of Fine Powder Polytetrafluoroethylene According to ASTM D4895 Definition

Reference ASTM Method

Mass of 1 L of resin measured under the test conditions

D1895

Average particle size and distribution by sieving

E11

Melting characteristics

Heat of fusion and melting peak temperature of resin as determined by differential scanning calorimetry

D4591

Water contenta

Water present in the PTFE resin

Standard specific gravity (SSG)b

Specific gravity of a sample of molded and sintered PTFE according to this method

Thermal instability index (TII)a,b

A measure of decrease in molecular weight of PTFE material determined by the difference between ESG and SSG: TII 5 (ESG-SSG) x 1000

Tensile propertiesa,b

Elongation and strength at break of a sample made according to the specified method

Extrusion pressurea

The pressure measured while extruding a paste of fine powder PTFE made with an iso-paraffin under specified conditions

Stretch void indexa,b (SVI)

A measure of change in specific gravity of a PTFE specimen as a result of being subjected to tensile strain

Strained specific gravity

Specific gravity of a PTFE specimen after being subjected to tensile strain

Untrained specific gravity

Specific gravity of a PTFE specimen before being subjected to tensile strain

Shrinkage and growth

The change in the diameter of SSG preform due to sintering

Extended specific gravity (ESG)

The specific gravity of a PTFE specimen molded for SSG after sintering for an extended period of time, compared to the sintering time of SSG

Property Bulk densitya Particle size

a

a

a

Properties required for resin specification. Properties required for on molded specimen.

b

D792, D1505

D638

D792,D1505

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Table 6.13 Fine Powder Polytetrafluoroethylene Resin Property Specifications According to ASTM D4895 Type

Bulk Density, g/L

Average Particle Size Diameter, µm

Tensile Strength, MPa

Break Elongation, %

I

550 6 150

500 6 200

19

200

II

550 6 150

1050 6 350

19

200

Extrusion pressure was determined in a paste extruder also called a rheometer. It consists of a vertically positioned breech-loaded tubular barrel with an inside diameter of 32 mm. The barrel was approximately 305 mm long which was not critical as long as it can hold enough resin preform to extrude for 5 minutes. The extruder was equipped with a hydraulic system and a ram with an inside diameter of 32 mm, capable of pushing the lubricated PTFE paste out of a small die. There should also be an appropriate pressure-sensing device. The die orifice size was selected to obtain barrel to orifice cross-sectional ratios of 100:1, 400:1, and 1600:1 called the RR. The choice of RR depends on the resin type. The equipment temperature was maintained at 30°C during the extrusion. The agreed upon rate of extrusion was 19 g/min on a dry resin basis. The lubricant was an iso-paraffin also called extrusion aid and should be blended with the resin at a prescribed ratio. The mixture was placed in a jar and blended by rolling. There are alternative techniques to bottle rolling. After blending, the jar and its content are stored at 30°C for 2 hours or longer to allow the lubricant to diffuse to the inside surface of the polymer particles. The preform was made by molding the blend in a tube with an inside diameter of 32 mm and a length of 610 mm. The lubricated blend was poured into the tube and was pushed down using a plug-in press at a pressure of 0.07 MPa. The preform was loaded in the extruder barrel prior to extrusion during which pressure was recorded. Stretch void index (SVI) was defined below. SVI 5 ðUnstrained specific gravity
Strained specific gravityÞ 3 1000 Unstrained specific gravity was measured on a tensile specimen prior to straining it. The strained specific gravity was measured on a sample of PTFE after it has been strained to break at a strain rate of

5.0 mm/min. The break elongation of the PTFE specimen must be greater than 200% or the experiment was repeated. The two specific gravity values are used to calculate SVI. The specifications that differentiate fine powder PTFE powders are summarized in Table 6.13. Water content and melting peak temperatures are the other specifications of these resins. Specifications of molded parts are listed in Table 6.14. The actual ASTM Method D4895 must be consulted for a complete description of these procedures.

6.16.3 Dispersions of PTFE ASTM Method D4441-96 covers specifications of dispersions of PTFE and test methods for the asproduced polymer dispersion. Methods for processing dispersions into objects are discussed in Chapter 9, Fabrication and Processing of Polytetrafluoroethylene Dispersions, of this book. PTFE resins are thermoplastics in that they can be remelted but they cannot be processed by the normal melt processing technologies due to their extremely high rheology. This polymer does not dissolve in any solvents. These two facts render direct measurement of PTFE molecular weight virtually impossible. An indirect property named SSG was substituted for molecular weight. Section 8.9.1 contains a discussion of the relationship between molecular weight and SSG. Properties that characterize dispersions of PTFE are listed in Table 6.15. ASTM Method D4441 classifies the different types of dispersions according to the system summarized in Table 6.16. SSG and melting characteristics of PTFE in dispersions are measured by the same method as in ASTM D4895. The polymer has to be isolated from the dispersion by coagulation, filtration, and drying. The actual ASTM Method D4441 must be consulted for a complete description of these procedures.

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Table 6.14 Molded Fine Powder Polytetrafluoroethylene Property Specifications According to ASTM D4895 Type

Grade

Class

Standard Specific Gravity Range

I

1

A1

2.14
2.18

B1 C I

2

1

1

A

1

B

C I

3

C D

1 2 2

1

E I

1

II1

4 -

2

B

1

A

Extrusion Pressure, MPa

Reduction Ratio

Maximum SVI

5
15

100:1




2.14
2.18

15
55

400:1




2.14
2.18

15
75

1600:1




2.17
2.25

5
15

100:1




2.17
2.25

15
55

400:1




2.17
2.25

15
75

1600:1




2.15
2.19

15
75

1600:1

200

2.15
2.19

15
65

1600:1

100

2.15
2.19

15
65

1600:1

200

2.14
2.16

15
55

400:1

50

2.14
2.25

5
15

100:1

NA

Table 6.15 Definition of Basic Properties of Dispersions of Polytetrafluoroethylene According to ASTM D4441 Property

Definition

Reference ASTM Method

Solids contenta

The amount of PTFE in the dispersion as wt.%

D4441

Surfactant contenta

Surfactant added to the dispersion plus the remaining polymerization surfactant

D4441

Dispersion particle size

Particle size measured in the presence of added surfactant

D4441

Raw dispersion particle size

Particle size measured in the absence of added surfactant

D4441

Coagulated polymer

PTFE that has coagulated as a result of handling and processing of dispersion

D4441

pH

Acidity/alkalinity of the dispersion

E70

Standard specific gravity (SSG)

Specific gravity of a sample of molded and sintered PTFE isolated from the dispersion according to this method

D4441, D792

Melting characteristics

Heat of fusion and melting peak temperature of resin as determined by differential scanning calorimetry

D4441, D4591

a

Properties required for dispersion specification.

6.17 Commercial PTFE Resins Tables 6.17
6.22 provide data for a number of (not all) commercial granular PTFE grades. Tables 6.23
6.26 provide data for a number of (not all) commercial dispersion PTFE resins.

Tables 6.27
6.30 provide data for a number of (not all) commercial fine powder PTFE grades. Readers are cautioned about the incompleteness of the commercial PTFE products listed in the above tables and the possible obsolescence of some of the grades.

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Table 6.16 Polytetrafluoroethylene, Surfactant, and Tolerance Level Requirements According to ASTM D4441 Type

PTFE, wt.%

Grade

Nominal Added Surfactant, wt.%

Class

Surfactant Tolerance, wt.%

I

# 40

0

.0 ,1

A

6 0.5

1

$ 1 ,2

B

6 1.0

2

$ 2 ,3

C

6 2.0

3

$ 3 ,4

4

$ 4 ,5

5

$ 5 ,6

III

6

$ 6 ,7

III

7

$ 7 ,8

III

8

$ 8 ,9

9

$ 9 ,10

10

$ 10 ,11

IV

11

$ 11 ,12

IV

12

$ 12 ,13

I II

. 40

II III III

IV IV

# 40 (with 0% surfactant added)

. 40 (with 0% surfactant added)

Table 6.17 Properties of AGC Inc. Fluon Granular PTFE Resins [68] Tensile Strength, MPa

Dielectric Breakdown Strength, kV/mm

Fluon

Bulk Density, g/L

Average Particle Size, µm

G-110

225

20

G-155

420

30

33

350

5

80

Filled compounds, molding

G-163

335

25

35

375

5

90

Filled compounds and billets for skived film/ tape

G-201

675

500

22

330

12

50

Ram extrusion

G-204

575

100

Elongation, %

Shrinkage, %

Applications Filled compounds

Molding and additive for coatings

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Table 6.18 Properties of Daikin Polyflon PTFE Molding Powders [69] Bulk Density, g/L

Specific Gravity

Tensile Strength, MPa

Elongation, %

Shrinkage, %

Applications

M-12

350

2.16

43

370

3.1

Billets for thin fil/tape

M-17

420

2.16

43

350

3.1

Filled compounds

M-18

480

2.16

43

360

3.2

Filled compounds

M-531

740

2.18

20

220

3.0

Isostatic and automatic molding and ram extrusion

M-532

870

2.18

46

350

2.9

Isostatic and automatic molding and ram extrusion

M-111

360

2.17

40

500

4.4

Welding, low creep gaskets, liners

M-112

360

2.15

40

425

4.6

Welding, low creep gaskets, diaphragms

Polyflon

Fine cut

Free flow

Modified

Table 6.19 Properties of Dyneon Hostaflon Granular PTFE Resins [70]

Hostaflon

Bulk Density, g/L

Specific Gravity

Average Particle Size, µm

Tensile Strength, MPa

Elongation, %

Shrinkage, %

TF 1750

370

2.16

25

42

350

4.3

Fine cut

TF 1620

850

2.15

220

34

350

3.0

Limited flow

TF 1641

830

2.15

450

33

350

2.8

Free flow

TF 1645

830

2.15

425

32

400

2.6

Free flow

TFM 1700

420

2.16

25

33

450

5.8

Fine cut

TFM 1705

420

2.16

25

33

450

5.8

Fine cut

TFM 1600

830

2.16

450

33

450

3.5

Free flow

2.16

800

21

250

5.5

Free flow

Type

Modified

Ram extrusion TFR 1105

820

Table 6.20 Properties of Solvay Solexis Company Algoflon Granular PTFE Resins [71] Bulk Density, g/L

Specific Gravity

Average Particle Size, µm

Tensile Strength, MPa

Elongation, %

Shrinkage, %

F5

380

2.17

15

40

350

3.0

Fine cut

F5/S

410

2.16

15

44

400

3.1

Fine cut

F6

380

2.17

15

42

370

3.2

Fine cut

F7

410

2.17

15

44

400

3.0

Fine cut

S121

810

2.17

550

37

340

2.7

Free flow

G10

500

2.13
2.18

550

.13.9

150

-

Free flow

Algoflon

Type

Fine Cut

Free flow

Table 6.21 Properties of Halopolymer Fluoroplast Granular PTFE Resins [72] Resin

Type

Standard Specific Gravity

Particle Size D50, urn

F-4(S,P, PN,0,T)

Low flow

2.18
2.21

100
180

F-4PN-90

Low flow

2.19

F-4PN-40

Low flow

F-4PN-20

Low flow

Bulk Density, g/L

Tensile Strength, MPa

Break Elongation, %

26
15

350
150

46
135

25

350

2.19

21
45

25

350

2.19

6
20

25

350

F-4A-1

710

26

310

F-4A-2

690

21

275

F-4A-3

670

21

250

F-4TGGrade 1

Presintered

600
800

600
800

F-4TGGrade 2

Presintered

450
800

F-4TG-2

Presintered

450
600

F-4M

Modified low flow

2.18

28

310

F-4ML

Modified low flow

2.19

24

400

F-4MT

Modified low flow

2.19

24

400

F-4MO

Modified low flow

2.20

15.7

350

F-4MN

Modified low flow

2.22

10

150

Table 6.22 Properties of Gujarat Fluorochemical Company Inoflon Granular PTFE Resins [73] Standard Specific Gravity

Particle Size D5O

Bulk Density, g/i

Mold Shrinkage, %

Tensile Strength, MPa

Break Elongation, %

Resin

Type

610

Homopolymer low flow

2.16

200

500

3

30

300

630

Homopolymer low flow

2.16

33

400

4

33

350

640

Homopolymer low flow

2.16

23

400

4.5

35

400

210

Homopolymer low flow

2.16

600

750

2.5

35

275

220

Homopolymer low flow

2.16

600

825

2

32

275

230

Homopolymer low flow

2.15

300

700

2.25

35

375

510

Presintered

2.16

600

650

NA

20

200

515

Presintered

2.14
2.17

150

600

NA

20

200

M690

Modified low flow

2.16

25

350

5

35

500

M695

Modified low flow

2.165

25

350

5.5

34

550

M290

Modified low flow

2.16

600

750

4

35

500

M295

Modified low flow

2.165

600

750

4.5

35

550

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Table 6.23 Properties of AGC Inc. Fluon PTFE Dispersions [74] Fluon

AD-911E

AD-915E

AD-916E

AD-939E

Solids content, wt.%

60
62

60
62

57
59

60
62

Nonionic surfactant content, wt.%

4
5

2
3

4
5

2
3

Dispersion particle size, μm

0.25

0.25

0.25

0.30

Specific gravity

1.52

1.52

1.49

1.52

pH

9

9 (minimum)

9 (minimum)

Antidrip, for agglomeration purpose

Glass cloth coating (overcoating)

Critical film thickness, µm Applications

General purpose, glass cloth coating

Antidrip, for agglomeration purpose

Table 6.24 Properties of Daikin Polyflon PTFE Dispersions [69] Polyflon

D-210

D-210C

D-210N

D-310

D-610

D-610C

Polymer type

Homopolymer

Homopolymer

Homopolymer

Modified

Homopolymer

Homopolymer

Solids content, wt.%

59
61

59
61

59
61

59
61

59
61

59
61

Nonionic surfactant content, wt.%

6.0
7.2

6.0
7.2

6.0
7.2

6.0
7.2

6.0
7.2

5.5
6.5

Dispersion particle size, μm

0.22
0.25

0.22
0.25

0.22
0.25

0.26
0.30

0.26
0.30

Specific gravity at 25° C

1.50
1.53

1.50
1.53

1.50
1.53

1.50
1.53

1.50
1.53

1.50
1.53

Viscosity at 25°C, cp

,35

,35

35

,35

2 , 35

,35

pH

8.5
10.5

8.5
10.5

11.4
12.4

8.5
9.5

8.5
10.5

9.5
10.5

Critical cracking thickness, μm

14

14

-

12

28

28

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Table 6.25 Properties of Dyneon Hostaflon PTFE Dispersions [70] Hostaflon

TF 5033Z

TF 5035GZ

TF 5041GZ

TF 5050Z

TF 5060GZ

TF 5070GZ

Polymer type

Homopolymer

Homopolymer

Homopolymer

Homopolymer

Homopolymer

Homopolymer

Solids content, wt.%

24

58

58

58

59

50

Nonionic surfactant content, wt.%

5 (ionic)

5

6

5

8

8

Dispersion particle size, nm

195

200

200

205

210

120

Density, g/ cm3

-

Brookfield viscosity at 20°C, mPa S

-

12

130 (D 5 30 S21)

12

25

12

pH

.9

.9

.9

.9.5

.9

.9

1.5

Table 6.26 Properties of Solvay Solexis Company Algoflon PTFE Dispersions [75]

D3511F

D XPH 2530 1N

D XPH 3513 1N

Algoflon

D1610F

D1613F

D1614F

D 2711F

Solids content, wt.%

60

60

60

27.5

59

60

59

Nonionic surfactant content, wt.%

3.5

2.8

3.5

1.7 (anionic)

3.5

3.0

3.5

Dispersion particle size, nm

250

240

240

250

240

.240

240

Specific gravity at 20°C

1.51

1.51

1.51

1.19

1.50

1.50

1.50

24

25

20

21

20

.9

.9

9

.9.0

.9.0

Brookfield viscosity at 20° C, mPa S pH (minimum)

.9

10.3

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Table 6.27 Properties of AGC Inc. Fluon PTFE Fine Powders [74]

Fluon

Polymer Type

Specific Gravity

Average Particle Size, µm

Reduction Ratio Range

Extrusion Pressure, MPa (Reduction Ratio 5 900:1)

CD084

Copolymer

2.15

420

100
700:1

27

470

CD086

Copolymer

2.15

475

100
500:1

30

470

CD090

Copolymer

2.15

580

400
4000:1

38

450

CD097

Copolymer

2.19

510

100
4000:1

38

480

CD126

Copolymer

2.19

520

10
300:1

50

470

CD122

Homopolymer

2.16

500

15
300:1

50

545

CD123

Homopolymer

2.16

475

15
300:1

45

545

CD127

Homopolymer

2.16

475

25
400:1

35

545

CD141

Homopolymer

2.18

475

25
700:1

27

520

Bulk Density, g/L

Table 6.28 Properties of Daikin Polyflon PTFE Fine Powders [69]

Polyflon

Std. Specific Gravity

Average Particle Size, µm

Tensile Strength, MPa

Elongation at Break, %

Bulk Density, g/L

Extrusion Pressure, MPa

F-104

2.16
2.19

400
650

25

300

400
550

120
170

F-104U

2.165
2.175

400
650

25

300

400
550

115
180

F-107

2.150
2.165

400
650

25

300

400
550

130
230

F-131

2.145
2.153

400
650

25

300

400
550

130
250

F-201

2.16
2.18

400
650

25

350

400
550

600
820

F-201L

2.16
2.20

400
650

25

350

400
550

540
650

F-205

2.160
2.180

400
650

25

350

400
600

650
850

F-207

2.163
2.178

400
650

25

300

450
550

310
460

F-208

2.165
2.180

500
700

25
55

300
600

450
550

32
42

F-301

2.150
2.180

400
700

25

350

400
550

140
200

F-303

2.140
2.155

400
650

25

250

400
550

160
270

Table 6.29 Properties of Dyneon Hostaflon Fine Powder PTFE Resins [70]

Reduction Ratio Range

Specific Gravity

Average Particle Size, µm

Tensile Strength, MPa

Elongation at Break,%

Extrusion Pressure, MPa (Reduction Ratio)

Bulk Density, g/L

Hostaflon

Type

TF 2001Z

Modified low flow

20
1000:1

2.15

550

34

400

50 (400:1)

450

TF2033Z

Modified free flow

20
100:1

2.16

520

37

430

32 (400:1)

460

TF207GZ

Modified free flow

20
2000:1

2.16

350

36

400

19 (400:1)

460

TF 2Q21Z

Homopolymer low flow

20
500:1

2.15

500

30

340

30 (400:1)

460

TF 2Q25Z

Homopolymer low flow

10
300:1

NA.

500

34

360

40 (400:1)

480

TF 2Q29Z

Homopolymer low flow

5
100:1

2.15

500

28

340

50 (400:1)

480

TF 2035Z

Homo polymer free flow

15
300:1

2.16

550

37

460

38 (400:1)

480

TF 2053Z

Homopolymer low flow

20
1200:1

2.16

520

24

390

65 (1600:1)

500

TF 2071Z

Homopolymer free flow

20
1600:1

2.16

450

34

410

50(1600:1)

390

TF 2072Z

Homopolymer low flow

50
3000:1

2.17

440

35

470

40 (400:1)

470

TF 2073Z

Homopolymer low flow

50
4200

2.17

430

35

460

30 (400:1)

490

Table 6.30 Properties of Solvay Solexis Company Algoflon PTFE Fine Powder [75]

Algoflon

Reduction Ratio Range

Specific Gravity

Average Particle Size, µm

Tensile Strength, MPa

Elongation at Break, %

Extrusion Pressure, MPa (Reduction Ratio)

Bulk Density, g/L

D120F

10
300:1

2.18

450

30

400

8 (100:1)

500

D130F

30
300:1

2.160

500

30

300

9.5 (100:1)

500

DF 132F

10
300:1

2.15

600

30

350

10 (100:1)

550

DF 261F

10
300:1

2.15

600

35

350

50 (400:1)

475

DF 330F

10
300:1

2.14

600

35

350

10 (400:1)

475

DF 681F

100
2500:1

2.17

550

35

400

48 (1600:1)

500

108

References [1] Fluorocarbon resins from the original PTFE to the latest melt processible copolymers. Technical Paper, Reg. Technical Conf. SPE, Mid Ohio Valley Bicentennial Conf. on Plastics, November 30
December 1; 1976. [2] Gangal SV. Polytetrafluoroethylene, homopolymers of tetrafluoroethylene. 2nd ed. Encyclopedia of polymer science and engineering, 17. New York, NY: John Wiley & Sons; 1989. p. 577
600. [3] Cardinal AJ, Edens WL, Van Dyk JW. US Patent 3,142,665, assigned to DuPont; 1964. [4] Holmes DA, Fasig EW. US Patent 3,819,594, assigned to DuPont; 1974. [5] Mueller MB, Salatiello PP, Kaufman HS. US Patent 3,655,611, assigned to Allied; 1972. [6] Doughty TR, Jr, Sperati CA, Un HW. US Patent 3,855,191, assigned to DuPont; 1974. [7] Kometani Y, Tatemoto M, Takasuki-shi, Fumoto S. US Patent 3,331,822, assigned to Thiokol Chemical Corp.; 1967. [8] Malhotra SC. US Patent 4,908,410, assigned to DuPont Co; 1990. [9] Felix B, Liihr G, Hofmeister W, Henge R. US Patent 5,153,285, assigned to Hoechst Aktiengesellschaft; 1992. [10] Yanagiguchi T, Yano S, Sukegawa M, Yukawa H. US Patent 7,528,221, assigned to Daikin Industries; 2009. [11] Dyneont (a 3M Co) polytetrafluoroethylene product comparison guide. Pub No. 98-05042001-1, www.dyneon.com; 2007. [12] Ebnesajjad S. Fluoroplastics: non-melt processible fluoropolymers, vol. 1. 2nd ed. Elsevier; 2015. [13] FMC Corp. Persulfates technical information. Pub No. FMC9487-2500, www.FMC.com; 2001. [14] Kolthofafn IM, Miller DIK. The chemistry of persulfate. I. The kinetics and mechanism of the decomposition of the persulfate ion in aqueous medium. J Am Chem Soc 1951;73 (7):3055
9. [15] Misra GS, Bajpai UDN. Redox polymerization. Prog Polym Sci, 8. Pergamon Press; 1982. p. 61
131. [16] Launer HF, Yost DM. J Am Chem Soc 1934;56:2571. [17] Aten RM. US Patent 5,405,923, assigned to DuPont Co; 1995.

INTRODUCTION

TO

FLUOROPOLYMERS

[18] Cleaning and descaling stainless steel, a designer’s handbook series, No. 9001. Dist by Nickel Development Institute. American Iron and Steel Institute, Washington, DC, USA; 1988. [19] Brubaker MM. US Patent 2,393,967, assigned to DuPont; 1946. [20] Anderson RF, Edens WL, Larsen HA. US Patent 3,245,972, assigned to DuPont; 1966. [21] Gangal SV. US Patent 4,189,551, assigned to DuPont Company; 1980. [22] Kometani Y, Koizumi S, Fumoto S, Tanigawa S, Nakajima T. US Patent 3,462,401, assigned to Daikin Industries; 1969. [23] Thomas PE, Wallace CC, Jr. US Patent 2,936,301, assigned to DuPont Co; 1960. [24] The Jet Pulverizer Co. www.jetpulverizer.com; 2018. [25] Sturtevant, Inc. www.sturtevantinc.com/micronizer.php; 2018. [26] Takeuchi H, Nakamura H, Iwasaki T, Asai N, Watano S. Development of a novel particle size control system for hammer milling. Adv Powder Technol 2010;21:681
5. [27] Daikin Polyflon® PTFE molding powders, product information, 2.14.250, https://DaikinAmerica.com; 2018. [28] Weisenberger WP. US Patent 3,115,486, assigned to DuPont Co.; 1963. [29] Leverett GF. Patent 3,690,569, assigned to DuPont Co; 1972. [30] Manwiller CH. US Patent 3,981,853, assigned to DuPont Co; 1976. [31] Manwiller CH, Sperati CA. US Patent 3,981,852, assigned to DuPont Co; 1976. [32] LittleFord Day Company. Taylor-Stiles high speed rotary mill, www.Littleford.com. [33] Bepex Corp. Size reduction systems, disintegration, www.bepex.com; 2018. [34] British Patent 1,076,642, assigned to Pennsalt Chemical Corp.; 1963. [35] Kometani Y, et al. US Patent 3,597,405, assigned to Daikin Industries, Ltd.; 1971. [36] Roberts R, Anderson RF. US Patent 3,766,133, assigned to DuPont; 1973. [37] Baron PJ, Eckrote G, James T, Partridge R, Duzick TC. US Patent 6,911,489, assigned to Asahi Glass Fluoropolymers, USA; 2005. [38] Product information, Teflon® 9B, No. E-89762-3, DuPont Co.; 1997.

6: MANUFACTURING

AND

PROPERTIES OF POLYTETRAFLUOROETHYLENE

[39] Fluon® guide for the extrusion of PTFE granular powders. Technical Service Note F2. AGC Inc.; 2002. [40] McKeen LW. Fluorinated coatings and finished. 2nd ed Elsevier; 2005. [41] Physical properties of Fluon® unfilled and filled PTFE. Fluon® Technical Service Notes F12/13. Asahi Glass Company; 2002. [42] PTFE Compounds, Inc. Common properties for selecting compounds, www.ptfecompounds.com; 2018. [43] Renfrew MM. US Patent 2,534,058, assigned to DuPont; 1950. [44] Brinker KC, Bro MI. US Patent 2,965,595, assigned to DuPont; 1960. [45] Berry KL. US Patent 2,559,752, assigned to DuPont; 1951. [46] Bankoff SG. US Patent 2,612,484, assigned to DuPont; 1952. [47] Poirier RV. US Patent 4,036,802, assigned to DuPont; 1977. [48] Kasai S, Ono M, Yamanaka T, Sawada Y. US Patent 7,820,775, assigned to Daikin Industries; 2010. [49] Hintzer K, Yurgens M, Moore GGI, Zipples T, Kaspar H, Koenigsmann H, et al. US Patent 7,671,112, assigned to 3M Innovative Properties Company; 2010. [50] Hintzer K, Yurgens M, Moore GGI, Zipples T, Kaspar H, Koenigsmann H, et al., US Patent 8,222,322, assigned to 3M Innovative Properties Company; 2010. [51] Hintzer K, Kaspar H, Maurer AR, Schwertfeger W, Zipples T. US patent 7,776,946, assigned to 3M Innovative Properties Company, Mar 2, 2010 assigned to 3M Innovative Properties Company; 2010. [52] Higuchi S, Matsuoka Y, Kobayashi S. US Patent 7,973,127, assigned Asahi Glass Company; 2011. [53] Brothers PD, Gangal SV. US Patent US 7705074, assigned to DuPont Co; 2010. [54] Brothers PD, Gangal SV. US Patent US 7932333, assigned to DuPont Co; 2011. [55] Brothers PD, Gangal SV. US Patent US 8519072, assigned to DuPont Co; 2013. [56] 3M/DYNEON Company Progress Reports for 2009 Submission under the EPA 201012015 PFOA Stewardship Program, www.epa.gov/ opptintr/pfoa/pubs/stewardship/preports4. html#2010; 2010.

109

[57] British Patent 1,189,483, assigned to Montecatini Edison S. P. A. of Milan, Italy; 1970. [58] DUPONTt GenX processing aid for making fluoropolymer resins. Pub. No. K23743. DuPont Co; 2010. [59] Berry KL. US Patent 2,478,229, assigned to DuPont; 1949. [60] Marks BM, Whipple GH. US Patent 3,037,953, assigned to DuPont; 1962. [61] Lee JM, Koh JC, Moon SJ, Kim KJ, Jin HK, Kim CU, et al. US Patent 6,045,675, assigned to Korea Research Institute of Chemical Technology; 2000. [62] Kuhls J, Weiss E. US Patent 4,369,266, assigned to Hoechst Aktiengesellschaft; 1983. [63] Jones CW. US Patent 5,272,186, assigned to DuPont; 1993. [64] Shimizu T, Hosokawa K. US Patent 4,840,998, assigned to Daikin Industries Ltd.; 1989. [65] Kawamura T, Ichiba S, Sota T. 5,814,713, assigned to Daikin Industries; 1998. [66] Kitahara T, Hosokawa K, Shimizu T. US Patent 6,503,988, assigned to Daikin Industries; 2003. [67] Sperati CA. Physical constants of fluoropolymers. in: Polymer handbook, polymerization and depolymerization, 3rd ed., vol. 1. John Wiley and Sons, New York, NY, 1989. [68] AGC Inc. Fluon® PTFE environmentally friendly products. Ref FPEFP E10-2016, www.agcce.com; 2016. [69] Daikin America fluoropolymers, https://daikin-america.com; 2018. [70] 3Mt Dyneont fluoropolymers; 2018. [71] Solvay Solexis. Algoflon® PTFE granulars, www.solvay.us/en/markets-and-products/featured-products/Algoflon-PTFE-Granulars. html; 2018. [72] Halopolymer Corp. PTFE and PTFE compounds, www.Halopolymer.com; 2018. [73] Gujarat Fluorochemicals Ltd. www.inoflon. com/ptfe-classification-and-datasheet.php; 2018. [74] AGC Inc. Fluon® PTFE environmentally friendly products, www.agcce.com; 2016. [75] Solvay Solexis. Algoflon® PTFE dispersions, www.solvay.us/en/markets-and-products/featured-products/Algoflon-PTFE-Dispersions. html; 2018.

110

Further Reading Banham J, Browning HE. US Patent 3,882,217, assigned to Imperial Chemical Industries, Ltd.; 1975. Benning AF. US Patent 2,559,749, assigned to DuPont; 1951. Bringer R.P. Chlorotrifluoroethylene polymers. in: Encyclopedia of polymer science and technology, 7:204–219, John Wiley and Sons, New York, NY, 1967. British Patent 729,010, assigned to Farbenfabriken Bayer AG; 1955. British Patent 465,520, assigned to Farbenindustrie, I.G.; 1937. British Patent 805,103; 1958. Bro MI, Sperati CA. End groups in tetrafluoroethylene polymers. J Polymer Sci 1959;38:289
305. Browning HE. US Patent 3,983,200, assigned to Imperial Chemical Industries, Ltd.; 1976. Caird DW. US Patent 2,600,202, assigned to General Electric Co.; 1952. Daikin Industries of Fluoroplastics. Daikin Industries, Ltd., Orangeburg, New York; 1998. Dittman AL, Wrightson JM. US Patent 2,636,908, assigned to M. W. Kellog Co.; 1953. Dittman AL, Passino HJ, Wrightson JM. US Patent 2,689,241, assigned to M. W. Kellog Co.; 1954. Doban RC, Knight AC, Peterson JH, Sperati CA. Paper presented at 130th Meeting Am. Chem. Soc., Atlantic City, September; 1956. Fahnoe F, Landrum BF. British Patent 840,735, assigned to Minnesota Mining and Manufacturing Co.; 1960. French Patent 1,155,143, assigned to Society d’Ugine; 1958. French Patent 1,419,741, assigned to Kureha Chemical Co.; 1965. Gangal SV. US Patent 4,189,551, assigned to DuPont; 1980. Hamilton JM. Ind Eng Chem 1953;45:1347. Hamilton JM. US Patent 2,569,524, assigned to DuPont; 1951. Hanford WF. US Patent 2,820,027, assigned to Minnesota Mining and Manufacturing Co.; 1958. Harvey LW, Martin EN. US Patent 5,502,161, assigned to ICI America’s, Inc.; 1996. Herbst RL, Landrum BF. US Patent 2,842,528, assigned to Minnesota Mining and Manufacturing Co.; 1958. Hoashi J. US Patent 3,301,807, assigned to Thiokol Chemical Corp.; 1967.

INTRODUCTION

TO

FLUOROPOLYMERS

Holmes DA. US Patent 3,704,272, assigned to DuPont; 1972. Honda N, Sawada K, Idemori K, Yukawa H. US Patent 5,189,143, assigned to Daikin Industries, Ltd.; 1993. Hoornaert F, Wauters PAL, Meesters GMH, Pratsinis SE, Scarlett B. Agglomeration behavior of powders in lodige mixer granulator. Powder Technol 1998;96:116
28. Izumo M, Nomura S, Tanigawa S. US Patent 4,241,137, assigned to Daikin Industries, Ltd.; 1980. Kuhls J, Steininger A, Fitz H. US Patent 4,078,134, assigned to Hoechst Aktiengesellschaft; 1978. Lazar M. J Polym Sci 1958;29:573. Miller WT, Dittman AL, and Reed SK. US Patent 2,586,550, assigned to USAEC; 1952. Miller WT. US Patent 2,579,437, assigned to M. W. Kellog Co.; 1951. Miller WT. US Patent 2,792,377, assigned Minnesota Mining and Manufacturing Co.; 1957. Miller WT. US Patent No. 2,564,024, assigned to US Atomic Energy Commission; 1951. Morris PS, Hutzler RH. US Patent 3,778,391, assigned to Allied Chemical Corp.; 1973. Muntell RM, Hoyt JM. US Patent No. 3,043,823, assigned to 3M Co.; 1962. Passino HJ, Dittman AL, Wrightson JM. US Patent No. 2,820,026, assigned to 3M Co.; 1958. Passino HJ, et al. US Patent 2,744,751, assigned to M. W. Kellog Co.; 1956. Plunkett RJ. US Patent 2,230,654, assigned to DuPont; 1941. Product information, Teflon® 7 A, No. E-89757-2. DuPont Co.; 1997. Product information, Teflon® 7 C, No. E-89758-2. DuPont Co.; 1997. Rearich JS. US Patent 2,600,804, assigned to M. W. Kellog Co.; 1952. Roedel GF. US Patent 2,613,202, assigned to General Electric Co.; 1952. Sherratt, S., in Kirk-Othmer encyclopedia of chemical technology 2nd ed., (A. Standen, ed.), vol. 9: 805–831, Interscience Publishers, Div. of John Wiley and Sons, New York, 1966. Sperati CA, Starkweather HW. Fluorine-containing polymers. II polytetrafluoroethylene. Fortschr Hochpoly Forsch 1961;2465
95. Thomas PE, Wallace CC, Jr. US Patent 2,936,301, assigned to DuPont; 1960. Young DM, Thompson B. US Patent 2,700,662, assigned to Union Carbide Co.; 1955.

7 Processing and Fabrication of Granular Polytetrafluoroethylene Sina Ebnesajjad FluoroConsultants Group, LLC, United States

O U T L I N E 7.1 Introduction

111

7.2 Resin Selection

111

7.3 Compression Molding 7.3.1 Mold and Tooling 7.3.2 Presses 7.3.3 Ovens 7.3.4 Densification and Sintering Mechanism 7.3.5 Billet Molding

112 113 114 114 114 115

7.4 Automatic Molding 7.5 Introduction to Isostatic Molding

7.5.1 Description of Isostatic Molding 7.5.2 Wet- and Dry-Bag Isostatic Molding

120 122

7.6 Ram Extrusion 7.6.1 Ram Extrusion Types 7.6.2 Description of Four Steps of Ram Extrusion 7.6.3 Typical Resin

122 122

7.7 Summary

124

119

References

124

120

Further Reading

124

7.1 Introduction This chapter describes the fabrication of suspension polymerized or granular polytetrafluoroethylene (PTFE) into shapes and articles for conversion to parts for end-use applications. This type of PTFE is fabricated by a modified metallurgy technology named compression molding, where the PTFE powder is compressed into a “preform” at ambient temperature. The preform has sufficient green strength to be handled, roughly equivalent to blackboard chalk. After removal from the mold, the preform is heated in an oven above its melting point and is “sintered.” The consolidation of particles during sintering is referred to as coalescence, which produces a homogeneous and strong structure. Varying the cooling rate of a part to below PTFE’s freeze temperature controls the crystallinity of that part which in turn impacts its mechanical properties [1]. There are four basic molding techniques for processing granular PTFE resins. All four rely on the principles of compression molding of PTFE. These

123 124

procedures are applied to convert granular resins into parts ranging in weight from a few grams to several hundred kilograms (Table 7.1). The only continuous process for manufacturing parts from granular PTFE is ram extrusion.

7.2 Resin Selection Selection of resin depends on two factors: (1) the desired properties of the PTFE part in the end use application and (2) the manufacturing method to produce the part. Electrical insulation, reactor liners, and most gaskets are typically made using fine-cut resins to obtain the best possible properties. Mechanical parts such as bridge and heavy equipment bearings do not require the best PTFE properties thus can be made from free-flow (pelletized) resins. Resin flow is a function of the apparent density of the resin. Resins with an apparent density of .500 550 g/L are usually produced by pelletizing

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00007-0 © 2021 Elsevier Inc. All rights reserved.

111

112

INTRODUCTION

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Table 7.1 Selection of Granular Fabrication Process Based on Part Geometry Fabrication Process

Shape

Part Dimension

Part Weight

Billet/Block molding

Rectangular, cylindrical

One centimeter in diameter or height to 5 m in diameter or 1.5 m in height

Ten grams to several hundred kilograms

Sheet molding

Flat sheet

One centimeter to 1 m in width and thickness of 3 75 mm

A few hundred grams to a few tens of kilograms

Automatic molding

Small round

A few millimeters to a few centimeters in diameter

A few grams to a few hundred grams

Isostatic molding

Complex geometry

A few centimeters to 0.5 m in the major dimension

A few tens of grams to a few tens of kilograms

Ram extrusion

Rod or tube

2 400 mm in diameter

Continuous process

Figure 7.1 Schematic diagram of PTFE billet molding and sintering [3].

fine-cut resins and are known as free flow. The consistency of these resins is similar to granulated sugar in contrast to fine-cut resins (with an apparent density of ,500 g/L), which have a consistency resembling that of wheat flour. Resin flow and improvement of many properties are inversely related. A free-flow resin produces a part that has lower elongation, tensile strength, specific gravity, and dielectric breakdown strength than the same part produced with a low flow (fine cut) PTFE powder. Improvement of resin flow raises the efficiency of mold filling. Good resin flow is a requirement for automatic molding, isostatic molding, and ram extrusion processes. Small particles and bimodal or multimodal particle size distribution of resin powder yield the highest packing density [2] and consequently the lowest void content of a molding, leading to improvement in many part properties. The frictional interaction among the particles also affects the degree of compaction, thus the importance of particle shape in addition to its size.

7.3 Compression Molding Compression molding is the method by which small (a few grams) and massive (700 kg) cylindrical (billet), rectangular and sheet shapes of PTFE are molded and sintered (Fig. 7.1). The blocks and cylinders, solid or annular, are the heaviest objects produced from any fluoropolymer. The height of a cylinder can exceed 1.5 m (60 in.). These billets are cut or skived into wide thin films (,0.5 mm thick) or sheets ( . 7 mm thick). Sheets, blocks, and cylinders are utilized as stock shapes for machining more complex shapes. The same principles are applied to molding any other shape. PTFE’s specific gravity is high compared to other plastics. A solid billet with a wall thickness of 130 and 300 mm tall may weigh about 50 kg. Table 7.2 presents the relationship between the dimensions and weight of common size billets. The selection of the size of the billet depends on the properties required in the application. For example, PTFE has low thermal conductivity and

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113

Table 7.2 Approximate OD ID Height Weight Relationships of Typical Billet Moldings OD (mm)

ID (mm)

Wall Thickness (mm)

Weight/Height (kg/m)

500

150

175

386

500

200

150

356

400

100

150

255

480

150

150

305

180

200

140

323

300

50

125

148

300

100

100

136

250

100

75

89

200

50

75

64

150

25

62.5

37

100

20

40

16

75

35

20

7

a thermal gradient forms across the wall thickness during sintering. Dielectric strength (DS) is more influenced than tensile strength by this thermal gradient. This is why thin (0.05 0.125 mm) electric-grade tapes are skived from billets with a wall thickness of 75 100 mm. Mechanical grade sheets are skived from heavier wall billets (125 175 mm). Billet height is determined by the desired width of the film or sheet, itself dictated by the end use. Electrical tapes are commonly made from 300 mm OD billets. Sheets for mechanical applications and for lining chemical processing equipment are made from 1.5 m tall billets. PTFE should be conditioned at a temperature of 21°C 25°C before molding to reduce clumping and enhance the ease of handling. Dew point conditions should be avoided to prevent moisture from condensing on the cold powder. Water expands during sintering and leads to cracking of the molding. Molding at temperatures below 20°C should be avoided because PTFE undergoes a linear thermal expansion of 1% at 19°C. Preforms molded below 20°C can crack during the sintering. Dust, oil, and particles of an organic nature must be prevented from contaminating the resin. The high temperatures to which PTFE is exposed during the sintering cause carbonization of these contaminants into colored spots ranging from light yellow to black. Such a part is considered defective for an overwhelming majority of applications.

7.3.1 Mold and Tooling The equipment used for billet molding consists of stainless steel molds and hydraulic press for fabrication of the preform, and a high-temperature oven for sintering. A lathe and skiving blades are required for preparation of film and sheet. A complete mold consists of a cylindrical or rectangular die and upper and lower end plates and a mandrel for annular parts. The molds are sometimes plated with chromium or nickel to protect them from corrosion. Occasionally, the end plates are made of brass or plastics such as nylon. A small diametrical clearance is designed in the end plates to allow easy assembly and air escape. Fig. 7.2 shows an example of molds and accessories used for production of billet preforms. Molds should be designed carefully to avoid distortion under the preform pressure. The internal dimensions of the mold depend on the properties of the resin. For a given part design, the height of the mold is a function of the apparent density or compression ratio (CR) defined as CR 5

HF HP

(7.1)

where HF 5 Filled height, mm; HP 5 Preform height, mm. PTFE powder is compacted during handling and charging of the mold; therefore, its apparent density increases. Consequently, a given weight of the resin assumes a lower height in the mold than it would if it

114

INTRODUCTION

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pressure is 50 MPa for neat (unfilled) resin and 100 MPa for filled compound PTFE.

7.3.3 Ovens

Figure 7.2 Typical mold assembly for small to medium size billets.

is not compacted under the force of its own weight. Preform density is typically about 1.9 g/cm3 for a fine-cut PTFE powder with an apparent density of 450 g/L (0.45 g/cm3). A typical value for CR for this resin is 3.2. The mold length has to be .3.2 times the height of the tallest billet expected to be made. A billet 1.5 m tall would require a mold with a height of .4.8 m. Mold diameter is determined by resin shrinkage, which means that each mold is usually designed for a narrow range of shrinkage value. The term shrinkage refers to the shrinkage of the part after sintering has been completed.

7.3.2 Presses Hydraulic presses are recommended for preform production. Important elements of a press are smooth pressure application, maximum opening (“daylight”), ram stroke, flatness and levelness of the platens, and tonnage. A programmable press allows smooth application and removal of pressure, which is critical to producing a good part. Jerky and uneven motion of the ram will result in nonuniform application of pressure to the resin resulting in cracking during sintering. The tonnage of the press determines the maximum diameter of the preform. The maximum required preform

PTFE is an excellent thermal insulator. Its thermal conductivity is 0.25 W/m K, which is roughly 2000 times less than copper impacts the time required to sinter a preform. The most common way of delivering heat to a preform is by circulation of hot air. A large volume of air has to be recirculated because of its low thermal capacity. Ideally, the sintering oven is electrically heated for use up to 425°C. Good temperature control is critical to achieving uniform and reproducible part dimensions and properties. The interior of the oven should be designed to maximize air circulation and temperature uniformity in addition to preventing formation of “hot spots.” A highly rated oven insulation will minimize heat loss, which is particularly important during the sintering of a full oven load. Controlled cooling is accomplished through fresh air intake during the cool-down portion of the cycle. The exhaust should be directly from the oven to the atmosphere. A hood should be placed over the oven door, where leaks are most likely to remove PTFE fumes. Adequate ventilation of the sintering area is very important. Fumes and off-gases must not be inhaled because of health hazards.

7.3.4 Densification and Sintering Mechanism PTFE powder is charged to the mold and compressed and held for a dwell period to make a preform. The preform is removed from the mold and allowed to rest for stress relaxation and degassing. Resin powder particles are separated by air which is removed during preforming and sintering. The preform expands due to relaxation and recovery. The pressure placed on the resin during molding exerts changes to the particles of resin. Resin particles undergo plastic deformation and are intermeshed together leading to the development of cohesive or green strength. Particles also deform elastically and experience cold flow under pressure. Removal of pressure allows recovery of elastic deformation, which creates a quick “snap back” of the preform. Over time, stress relaxation partly reverses the cold flow, and the preform expands. The air trapped between PTFE particles is also compressed during preforming, theoretically reaching

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the maximum press pressure. The trapped air requires time to leave the void areas of the preform. Immediate sintering leads to a rise in the already high pressure of the air and leads to catastrophic cracking of the part as the PTFE melts and the mechanical strength declines. The preform should be allowed to undergo degassing during which the internal air pressure equalizes the atmospheric pressure. Sintering of the preform takes place in an oven where massive volumes of heated air are circulated. The initial heating of the preform leads to thermal expansion of the part. After PTFE melts, relaxation of the residual stresses occurs (stored because of the application of pressure to the polymer) where additional recovery takes place and the part grows. The residual air begins to diffuse out of the preform after heating starts. The adjacent molten particles begin to coalesce slowly; usually requiring hours of dwell time because of the massive size of PTFE molecules (molecular weight is 106 107). Fusion of the particles is followed by elimination of the voids, where almost no air is left. It is important to remember that the elimination of all the voids in PTFE is quite difficult because of the size driven limited mobility of the large polymer molecules.

7.3.5 Billet Molding Three processes for producing a billet from granular PTFE are preforming, sintering, and cooling.

7.3.5.1 Preforming Preforming begins with charging the mold with PTFE powder. Filling the mold must be done uniformly because uneven filling leads to nonuniform density in the preform and may even lead to billet cracking. Charging the mold is much simpler using a free-flow PTFE grade than a fine-cut powder. Free-flow resins assume the shape of the mold and require little distribution. The next step is powder compaction by application of pressure to prepare a green part with sufficient strength to allow handling. PTFE manufacturers provide recommended pressure ranges for PTFE grades. Demolding or removal from the mold, degassing, and placement in the oven are the steps that require green strength. Occasionally, a preform may be machined which increases the importance of green strength. PTFE manufacturers provide recommended pressure range for each resin grade. Pressurization rate

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or ram closing speed depends on the size and shape of the billet and the type of resin. The apparent density of the powder determines the air-filled void space, which must be eliminated. The slower the ram speed, the more completely the air will leave the preform, but process productivity suffers at a low closing rate. Very fast ram speeds lead to entrapment of air, resulting in high-porosity and low-density areas, even billet cracking. A ram speed must be selected that offers a compromise between productivity and part quality. The required dwell time at the maximum pressure is almost as long as the time to reach that pressure. Dwell time is necessary to obtain even compaction of the resin in the preform. The rule of thumb is 2 5 minutes of dwell per 10 mm of final height for billets ,100 mm in diameter, and 1 1.5 minutes for 10 mm of height for large billets ( . 100 mm diameter). A key variable is resin temperature during molding. The powder is harder and has better flow below the transition temperature of 19°C, but it does not respond well to pressure. The preforms produced below the transition temperature have low green strength and are more likely to crack during sintering. To avoid these problems, the resin should be conditioned at 21°C 25°C for 24 hours. The temperature in the molding area should ideally be maintained at .21°C. The molding areas should be isolated from the rest of the process such as machining where oil and dust are present. Maximum pressure during the preform molding has a direct bearing on void-closure and the final part properties. DS and shrinkage are also strongly affected by pressure. DS rapidly deteriorates with increasing void content. The effect of preforming pressure on dielectric, tensile strengths at break, ultimate elongation and shrinkage/expansion rate of the sintered part can be seen in Figs. 7.3 7.6. The preform shrinks in the radial (cross direction) during sintering and tends to grow in the height or machine direction, as shown in Fig. 7.6. Raising the preform pressure reduces the shrinkage and increases the growth. Table 7.3 shows the sintering cycle for the part tested in Figs. 7.3 7.6.

7.3.5.2 Degassing Degassing is the last step in the preparation of the preform prior to sintering. Air and residual stress remain entrapped in the preform and should be relieved prior to sintering. All the air does not exit

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Figure 7.3 Relationship between molding pressure and dielectric strength [4].

Figure 7.4 Relationship between molding pressure and tensile properties [4].

the resin during the compression cycle and a small remaining volume is pressurized. This air needs time to escape from the preform; otherwise, it will increase substantially during the heat-up segment of the sintering cycle and crack the billet. Stresses remaining in the preform can be equally potent and lead to billet cracking during the heat-up period.

7.3.5.3 Sintering A preform has limited cohesive strength and is commercially useless; sintering allows coalescence of the resin particles, which provides strength and

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Figure 7.5 Relationship between molding pressure and elongation properties [4].

void reduction. Sintering cycle profiles of time and temperature affect the final properties of the billet. Sintering temperatures exceed the first melting point of PTFE (342°C) and range from 360°C to 380°C. Above 342°C, PTFE is a transparent gel because the crystalline phase has melted. At the sintering temperature, adjacent melted PTFE particles fuse together and coalesce. After two particles have completely coalesced, they would be indistinguishable from a larger particle and voids are eliminated under the driving force of surface tension [5]. Smaller particle resins and higher preform pressures improve coalescence. Coalescence and void elimination require time because of the limited mobility of PTFE molecules. Melt creep viscosity of PTFE is in the range of 1011 1012 poise at 380°C, which severely inhibits any flow similar to that known for thermoplastics. The billet is held at the sintering temperature for a period of time to allow completion of fusion, coalescence, and void elimination, thus maximizing the properties of the PTFE. The graph in Fig. 7.7 is the general form of PTFE sintering cycle. Table 7.4 presents examples of typical sintering cycles as a function of preform size. The rule of thumb for determining the sintering time is 1 hour per centimeter of wall thickness of solid billets and 1.5 hours per centimeter of thickness for billets with a hole in the middle. Small parts need 0.8 hours per centimeter of time sintering temperature. Fig. 7.8 shows the

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Figure 7.6 Effect of molding pressure and rate of change in dimensions [4]. Table 7.3 Sintering Cycle for Figs. 7.3 7.6 [4]

Figure 7.7 General form of a PTFE sintering cycle.

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Table 7.4 Examples of Typical Sintering Cycles as a Function of Preform Size [6] Preform Size

Sintering Cycle

Part Size (mm) (OD/ID) 3 L

Weight (kg)

Heating Rate (°C/h)

Sintering (hours)

Cooling Rate (°C/h)

50 3 50

0.2

50

5 at 370°C

50

100 3 100

1.7

30

10 at 370°C

30

174/52 3 130

6.0

30

12 at 370°C

30

420/150 3 600

150

50°C/h 25°C-150°C

20 at 365°C

10°C/h 365° C-315°C

3 h at 150°C

10 h at 315°C

25°C/h 150°C-250°C

10°C/h 315° C-250°C

3 h at 250°C

25°C/h 250° C-100°C

15°C/h 250°C-315°C 5 h at 315°C 10°C/h 315°C-365°C 420/150 3 1200

300

50°C/h 25°C-150°C

30 at 365°C

10°C/h 365° C-315°C

5 h at 150°C

13 h at 315°C

25°C/h 150°C-250°C

10°C/h 315° C-250°C

5 h at 250°C

25°C/h 250° C-100°C

15°C/h 250°C-315°C 5 h at 315°C 10°C/h 315°C-365°C

effect of sintering temperature on the specific gravity and tensile strength of the billet. Specific gravity increases while tensile strength decreases. Degradation of PTFE at temperatures in excess of 360°C leads to a lowering of the PTFE molecular weight, which crystallizes more easily and has decreased tensile strength.

7.3.5.4 Cooling Cooling cycle begins immediately at the end of the sintering cycle. It plays two important roles: crystallization and annealing of the sintered billet. Many of the properties of PTFE (similar to other semicrystalline polymers) are governed by the crystalline phase content of the part. Crystallinity is determined by the cooling rate. At 320°C 325°C, the molten resin reaches the freeze point and

crystallization begins to take place. Polymer chains that were randomly distributed in the molten state begin to pack in an orderly manner during the crystallization process. The slower the cool down, the higher the number of crystalline structures will be. This means controlling the cooling rate can control the properties of the part. The minimum attainable crystallinity by quenching in ice water is 45%, because of the low thermal conductivity of PTFE. Generally, large billets (150 and 300 kg in Table 7.2) should be cooled at rates between 8°C and 15°C/h down to 250°C. This slow cooling rate allows the middle of the wall of the part to reach the freeze point before faster cooling is commenced. Between 250°C and 100°C, the cooling rate can be increased to 25°C/h and below 100°C the oven doors can be opened. Smaller parts can be cooled at higher rates below 300°C.

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Annealing refers to removal of residual stresses in the billet by holding it for a period of time between 290°C and 325°C during the cooling cycle. It also minimizes thermal gradients in the billet by allowing the wall interior to catch up with the exterior surface. The crystallinity of the part depends on the annealing temperature. A part which is annealed below the crystallization temperature range (,300°C) will only undergo stress relief.

Figure 7.8 Seven-hour sintering: sintering temperature and product quality [4].

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Annealing at a temperature in the crystallization range (300°C 325°C) results in higher crystallinity (higher specific gravity and opacity) in addition to stress relief.

7.4 Automatic Molding Automatic molding is a process for automatic charging of resin into the mold followed by compression. It is usually utilized for mass production of small parts with a fairly simple geometry. The main requirement of the resin is good flow for easy and complete filling of the mold and part-to-part uniformity. Consistent resin shrinkage is mandatory to obtain consistent size parts. The combination of high productivity and low labor requirements of automatic molding render this process highly desirable for production of rings, seals, spacers, valve seats, etc. where large numbers of relatively inexpensive parts are needed. Fig. 7.9 presents a schematic diagram of the four stages of automatic molding. In the first step, the free-flowing resin is charged into the mold cavity formed by the lower ram (punch) and the outer mold. Pressure is actuated during step 2 and the upper ram compresses the resin for a few seconds. In step 3, the upper ram is retrieved. Finally, the lower punch pushes the preform up out of the cavity during step 4, also known as demolding. These

Figure 7.9 The four steps of the automatic compression molding process [4]. The numbers in the graph indicate molding pressure for an article with dimensions of 64 mm OD 3 52 mm ID 3 15 mm length. Courtesy Daikin Industries.

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Figure 7.10 Relationship of molding pressure/dwell period and specific gravity of preform [4].

operations take place automatically according to preset conditions. Fig. 7.10 shows the relationship between molding pressure/dwell period and specific gravity of preform. PTFE powders get increasingly softer and stickier above their transition temperature and tend to form aggregates. This can cause bridging in the feed section of the mold, which leads to uneven filling of the cavity or nonuniformity in each charge. The molding area should be maintained at 23°C 25°C. A higher pressure than ordinary compression molding is required for automatic molding because of the short duration of the compression cycle. Specific gravity of the preform increases with pressure, up to a point, independently of the dwell time. The specific gravity levels off as a function of pressure above the certain value of pressure. Combinations of pressure and dwell period have to be optimized to arrive at the highest productivity with acceptable part properties.

7.5 Introduction to Isostatic Molding This technology was originally invented for ceramic and powder metal processing early in the 20th century. It has been adopted to produce parts from granular PTFE powders. Isostatic molding is another technique for producing PTFE preforms by the application of hydrostatic pressure to the powder. The PTFE powder is loaded in a closed flexible mold. Compaction of the powder into a preform takes place by the pressure applied through the flexible part (bladder) of the mold. The bladder is usually made of an elastomeric material such as polyurethane. This

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Figure 7.11 Isostatically pressed samples of various materials by both wet- and dry-bag methods. Courtesy Berghof Automation GmbH, www.berghof.com.

method allows molding of complex shapes by the placement of mandrels inside the flexible bladder.

7.5.1 Description of Isostatic Molding Isostatic molding is a suitable alternative to compression and automatic molding techniques for the production of PTFE parts with complex shapes in a wide range of sizes. Compression molding produces stock shapes that have to be machined to obtain the desired shape. The drawback to this option is the extensive machining and material cost which can drive up the cost of the object. Isostatic molding requires relatively low-cost tooling and allows significant savings in machining and material costs. Complicated parts in the exact or nearly exact shape and size, requiring some finishing, can be molded and sintered by this method. A bellows is an example of a part that can be directly molded by isostatic molding, while extensive machining is required to achieve the curved contour of the bellows. Isostatic molding is the method by which all shapes of preforms (Fig. 7.11) can be fabricated. Fig. 7.12 shows the principal steps for isostatically molding a simple solid cylinder. The mold cavity is formed inside an elastomeric membrane shaped like a hollow cylinder. In this case, it does not include any mandrels and is completely filled with the powder. The elastomeric bag is closed, sealed, and placed inside a pressure vessel. The vessel containing a fluid is sealed, pressurized, and held for a dwell period during which the powder is

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Figure 7.12 Principle steps of isostatic molding [7].

compacted by the action of a pressurized fluid. At the end of the dwell time, the vessel is depressurized and the mold is removed and disassembled for the removal of the preform. The flexible nature of the bag renders the definition of its volume difficult when compared to a metallic mold. The shape of the bag may also change during the filling step unless it is supported while being charged. The change in the shape of the bag depends on several factors.

• Original mold shape. • Fill uniformity. • Geometry and wall-thickness of the flexible segments.

• Elastic properties of the bag. • Fastening of the rigid and elastic sections of the mold.

• Extent and the rate of powder compaction.

Figure 7.13 Comparison of the direction of force applied to PTFE in compression and isostatic molding techniques [8].

• Residual stress in the bag.

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Exertion of pressure on the bag is multidirectional which conforms the resin powder to all patterns and nonuniformities in the bag (Fig. 7.13). Consequently, the surfaces adjacent to the bag are less smooth than those adjacent to the smooth metallic surfaces. The importance of surface formation is one of the considerations that determine the selection of the type of molding process. Isostatic molding is ideal for manufacturing thin long objects from small tubes (5 mm diameter) or very large diameter thin wall tubes (30 cm diameter). Examples include pipe liners, liners for valves and fittings, flanged parts, closed end articles, and a host of other shapes, which would require extensive machining. Isostatic cycle times as short as 12 seconds are possible for small simple parts. The length of the cycle increases with the complexity and size of the article. In such instances, this technique is often the only available method for the fabrication of those parts. An example is in-situ formation of a PTFE liner inside a fitting such as a T-piece.

7.5.2 Wet- and Dry-Bag Isostatic Molding Wet bag and dry bag are two techniques for isostatic molding which are principally identical but operationally different. The wet bag process is similar to the basic molding procedure in which the mold is submerged in the pressurization fluid. In the dry-bag technique, the mold and the bag are fixed in place and the functions of the mold and the pressurization vessel are combined. The pressurization fluid is introduced through a high-pressure liquid supply system behind the flexible bag. The mold assembly is designed to withstand this pressure. The term dry bag contrasts the absence of submerged mold and wet mold assembly. The dry-bag process has advantages over the wetbag process. The operation of placing the mold in the pressure vessel is eliminated. Sealing and unsealing of the mold and the pressure vessel are reduced to just sealing and unsealing the mold. In addition to cycle time reduction, the risk of contamination of the preform with the pressurization fluid has been eliminated. The dry-bag process can be automated and is an excellent method for the large-scale production of parts. The disadvantage of this process is the large cost differential between the dry over the wet-bag molds. Dry-bag molds must be able to withstand high pressure and in effect act as a pressure vessel. These molds must not be

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modified without reviewing the mold design because of safety considerations. It is important to take into account the need for mold modification in its initial design.

7.6 Ram Extrusion Ram extrusion is the only continuous process for fabrication of parts from suspension polymerized (granular) PTFE powders. All the required steps of granular processing are performed in one machine called a ram extruder. The most common shapes are solid round rods and tubes. Rectangular rods, L-shaped cross-sections, and other ram extrudable profiles are occasionally fabricated. The basic steps for processing granular resins are

• Resin Feed; • Compaction of the powder to make a preform; • Sintering the preform, which consists of heating the preform above its melting point;

• Air quenching or slow cooling the sintered part to allow controlled crystallization of PTFE. These steps are carried out in the ram extruder continuously using a free-flowing resin that is often a special presintered ram extrusion grade or a general-purpose free-flow powder (Fig. 7.14). These resins behave differently during extrusion. The commercial presintered resins have been specially designed for ram extrusion over a wide range of extrusion conditions and can be converted into a wide range of parts such as round rods with a diameter of 2 400 mm. These parts have excellent physical properties and have high resistance to fracture at the interface of charges (or doses) called poker chipping in the industry. Presintered resins can undergo much higher pressures during extrusion than ordinary free-flow granular powders, making them especially suitable for small diameter rods and thin wall tubes. Generalpurpose free-flow granular powders are more suitable for larger rods ( . 2 cm diameter) and thick wall tubes.

7.6.1 Ram Extrusion Types Two types of common commercial equipment are vertical and horizontal extruders where the direction

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Figure 7.14 Schematic of a vertical ram extruder.

of the ram motion and the extrudate are, respectively, vertical and horizontal. The fundamental working principles of the two pieces of equipment are the same. The key difference between them is the method of extrudate support. In horizontal ram extrusion, a tray or other similar means can support the extrudate. In ram extrusion, especially a vertical machine, a mechanical brake may be needed to protect the extrudate and provide backpressure for coalescence. This brake is usually a chuck, collet, or a gland that grips the extrudate and applies a controlled amount of pressure. In both vertical and horizontal machines, a metered quantity of granular powder is charged to the feed section of the die. This end of the die is cooled to allow easy flow of the resin into the die. PTFE particles and agglomerates become sticky and powder flow suffers when heated above approximately 25°C. The next step is compaction of the powder and pushing it into the heated segment of the die by the forward action of the ram. Repetition of these steps advances the compacted resin through the heated length of the die where sintering of PTFE takes place. The adjacent charges of the resin are welded to each other under pressure while sintering is taking place.

7.6.2 Description of Four Steps of Ram Extrusion The main function of the feed section is to provide repeated individual charges of resin with uniform weight during every cycle of the process. The resin must be distributed evenly in the feed port at the cold end (beginning) of the die. The second step in producing a ram-extruded part is compaction. The ram has two modes of motion—advancing and retracting. After the resin has been fed into the feed section, the advancing motion of the ram compresses it. The single charge of resin is squeezed between the bottom surface of the ram and the top surface of the previous charge. This is the mechanism of preform development from the resin inside the die. After compaction, the preform is advanced into the first heated zone of the die for sintering. Sufficient heat must be supplied to the preform to raise its temperature above the melting point of the polymer. There are two considerations in the selection of the die temperature. First, melting temperature of PTFE increases as a function of the polymer pressure. Second, the chosen temperature should be high enough for PTFE to melt and sinter completely during the residence time in the heated length of the die. Adequate pressure should also be applied to the part to

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eliminate the voids and obtain a strong welded bond to the adjacent charges. Finally, the part must be cooled at a rate that will lead to the desired crystallinity content.

7.6.3 Typical Resin Selection of resin for ram extrusion depends on the size of the part. Small diameter tubes and rods (,1 cm in diameter) have to be extruded using presintered grades of polytetrafluoroethylene. Fracturing is avoided by proper fabrication of smaller size parts from these resins. Larger rods and thick wall tubes can be ram-extruded from freeflow grades of PTFE powder.

7.7 Summary PTFE has one of the highest melting points among thermoplastics and by far the lowest coefficient of friction. It is the most inert of all fluoropolymers with a continuous service temperature of 260° C. PTFE molding powders are available in fine-cut and free-flow varieties that can accommodate different molding processes. Granular PTFE powders are available in homopolymer and modified (containing ,2% of a comonomer) grades. Parts molded from granular PTFE have a large number of applications including thin skived film for electronic parts, laboratory equipment, gaskets, diaphragms, O-rings, slide bearings, V-rings, pump parts, seals, sheet lining, mold release films, filled compounds, bellows, impellers, containers, and high-purity bags [9].

References [1] Ebnesajjad S. Fluoroplastics: non-melt processible fluoropolymers, vol. 1, 2nd ed. Elsevier; 2015.

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[2] Mascia L. Thermoplastics. in: Materials Engineering. 2nd ed. Elsevier Applied Science, New York, 1989. [3] Meiko Kogyo Co, Japan. ,www.Meiko.jp.; 2018. [4] Polyflon® PTFE molding powder—product information. Daikin Industries, Osaka, Japan, EGE-11b-1 (0005) AK; 2002. [5] Frenkel J. Viscous flow of crystalline bodies under the action of surface tension. J Phys (USSR) 1945;9:385 91. [6] DaikinPolyflont PTFE molding powders, product information. Daikin America. ,www.daikin-america.com.; 2018. [7] Francis L F, Isostatic pressing, powder processes in materials processing, Elsevier, 2016. [8] Application Note: Fluoropolymers Isostatically molded PTFE of highest quality. Berghof Fluoroplastic Technology GmbH. ,www.berghof-fluoroplastics.com.; 2018. [9] Fluoropolymers. Daikin America. ,https://daikin-america.com/fluoropolymers/.; 2019.

Further Reading American Society of Mechanical Engineers. Boiler and pressure vessel code. In: Perry RH, Green DW, editors. Chemical Engineers’ handbook. 6th ed. New York: McGraw Hill; 1984. Fluon®—isostatic compaction of PTFE powders. Technical Service Note F14. Asahi Glass Co; 2002. Howard EG, Jr., Moss AZ. US Patent 5,420,191, assigned to DuPont, 1995. Howard EG, Jr., Moss AZ. US Patent 5,512,624, assigned to DuPont, 1996.

8 Fabrication and Processing of Fine Powder Polytetrafluoroethylene Sina Ebnesajjad FluoroConsultants Group, LLC, United States

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8.4 Extrusion Aid or Lubricant

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8.5 Wire 8.5.1 8.5.2 8.5.3 8.5.4

130 130 133 133 135

Coating Blending the Resin with Lubricant Pigment Addition Preforming Extrusion Equipment and Process

8.1 Introduction This chapter discusses the fabrication of coagulated dispersion polymerized tetrafluoroethylene, known as fine powder or coagulated dispersion powder, into parts. The most common fabricated commercial forms include rods, tapes, wire insulation, tubes, sheets, and other profiles. Tube diameter ranges from a fraction of a millimeter to almost a meter with a wall thickness of 100 μm to a few millimeters. Rods up to 5 cm outer diameter can be produced and calendared, prior to sintering, to produce tapes. Unsintered tapes are broadly applied as a thread sealant tape in pipe-fittings. Unsintered polytetrafluoroethylene (PTFE) can be fabricated into wire insulations, sheeting, and pipeliner by pate extrusion or by wrapping the wire conductors and mandrels, followed by sintering. This form of PTFE is unique; it is highly crystalline (96%
98%) and has a high molecular weight. The crystalline form of PTFE changes from a triclinic to a hexagonal lattice at 19°C transition temperature. Above this temperature, fine powder PTFE softens and becomes more deformable which

8.5.5 Reduction Ratio 8.5.6 Conductor

139 139

8.6 Extrusion of Tubing 8.6.1 Pressure Hoses 8.6.2 Extrusion, Sintering, and Cooling 8.6.3 Quality Control of Pressure Hoses

139 140 143 144

8.7 Liner Extrusion

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8.8 Fine Powder Resin Selection

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is important to its processing. At 30°C, another transition takes place that further softens the PTFE. Commercial processing of fine powder PTFE is carried out at a temperature above 30°C. Because it does not melt and flow, fine powder PTFE is fabricated by a technology adopted from ceramic processing called paste extrusion. PTFE powder is first blended with a hydrocarbon lubricant (hence the term paste) which acts as an extrusion aid. It is then formed into a cylindrical preform at a fairly low pressure (1
3 MPa) and placed inside the barrel of a ram extruder where it is forced through a die at a constant ram rate. The extrudate is passed through multiple ovens where it is first dried, then sintered, and finally cooled. The lubricant was originally removed by extraction in a hot solvent bath [1]. A major requirement of paste extrusion is that, up to the point of sintering and coalescence, the extrudate must possess sufficient strength to withstand the extensive handling that takes place during the process. The tendency of PTFE fine powders to fibrillate (form a web of strong filaments between particles) when extruded provides the needed

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00008-2 © 2021 Elsevier Inc. All rights reserved.

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strength and the unique characteristics of fine powder articles. Reduction ratio (RR) is defined as the ratio of the cross-sectional surface areas of the preform and the extrudate. RR is an important variable in paste extrusion because of its impact on the pressure during the extrusion. For a given extruder barrel, the smaller the cross section of the final product is, the higher the RR will be. PTFE resins must be able to undergo the necessary reduction during the extrusion. Different fine powder grades have been developed by the resin suppliers to accommodate the wide range of RRs of commercial processes (Fig. 8.1).

8.2 Resin Handling and Storage Fine powder PTFE is susceptible to shear damage, particularly when it is above its transition temperature (19°C). Handling and transportation of the containers could subject the powder to sufficient shear rate to damage it, if the resin temperature is above the transition point. A phenomenon called fibrillation occurs when particles rub against a surface including against another particle. Fibrils are pulled out of the surface of PTFE particles. Uncontrolled fibrillation must be prevented to insure good quality production from the powder. Premature fibrillation also leads to the formation of lumps, which cannot be completely broken up. To ensure the resin does not fibrillate, it should be cooled to below its transition temperature prior to handling and transportation. In practice, drums of resin are stored and transported at ,5°C.

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Individual particles of PTFE form round agglomerates that average several hundred microns in size. Those agglomerate are comprised of many small (,0.25 μm diameter) primary round particles, which should have the same shape as when they were polymerized. This means that the postpolymerization isolation and drying processes should not affect the appearance of the primary particles. Any deformation of the resin particles or fibrillation during finishing operations should be taken as an indication of potential for defects in the fabricated part. Fine powder can be compacted to some extent during transportation and storage, even when refrigerated and handled gently, thereby creating lumps. Sifting the resin through coarse wire mesh will help break up the majority of the lumps. The size of the sieve should not be smaller than 10-mesh; 4-mesh is preferable. The resin should never be scooped out of the container but poured over the sieve to avoid its shearing. The wire mesh should be vibrated gently up and down to avoid shearing as opposed to sideway movement. The remaining lumps of the powder should be removed from the sieve and placed into a separate plastic jar. After the bottle is one-third full, it should be shaken gently to break down the lumps [3]. It would be wise to process this part of the powder separately by making a different preform to minimize the risk of adding damaged powder to the rest of the resin.

8.3 Paste Extrusion Fundamentals The structure of the individual particles shown in Fig. 8.2 is critical to paste extrusion of fine powder

Figure 8.1 Dependency of extrusion pressure on reduction ratio for different types of fine powder [2].

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PTFE. The structure of a single particle is depicted in Fig. 8.3 where almost all the polymer chains are packed in a crystalline lattice. The orderly packing of completely linear polymer chains [5] takes place during the polymerization, monomer by monomer or brick by brick. One of the characteristics of PTFE crystals is that they are loosely packed because of the relatively low van der Waals attraction forces among the PTFE chains in contrast to other polymers such as polyethylene. The transition from a triclinic to a hexagonal cell unit corresponds to 1.3% increase in volume [6]. Chains can thus be removed from the surface of particles by the application of a fairly small force at above the transition temperature. Decreasing the temperature increases the shear force required for chain pull-out. The chains abstracted from the particle are called fibrils. The ease with which chains can be pulled out at higher temperatures is the main reason that fine powder PTFE is handled, stored, and transported below its transition point.

Figure 8.2 Structure of one PTFE fine powder particle.

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The thickness of PTFE fibrils is less than 50 nm and they connect multiple particles (Fig. 8.4). The particles have preserved their round shape and do not appear deformed. The SEM micrographs (Fig. 8.5) taken from a cross section of a paste extruded part at 20,000 times magnification exhibit extensive fibrillation in the machine direction, that is, the direction of extrusion, extensively connecting the particles and conferring strength to the tape in this direction. Some deformation of the round particles can be observed, due to the calendaring that the tape has undergone. The changed melting behavior of the paste material after extrusion is actual proof that a reversible deformation has taken place, as illustrated in Fig. 8.6. The differential scanning calorimetry (DSC) diagram shows a uniform melting peak for

Figure 8.4 SEM micrograph showing the close-up view of individual fibrils extending among multiple particles (magnification 20,000 3 ) [7].

Figure 8.3 Crystalline structure PTFE cooled from 380°C: (A) at 0.12°C/min and (B) at 0.02°C/min, bar 5 1 μm [4].

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Figure 8.5 Changes in PTFE paste as a result of preforming and paste extrusion [7].

Figure 8.6 DSC spectra: left unkneaded (original from the drum) and right kneaded (extruded) PTFE paste; both samples are unsintered [2].

the original and a “bimodal” melting peak for the extruded paste material [2]. The crystalline structure has obviously been changed through the kneading. The original orderly alignment of chains has been partly destroyed. A resin that has been subjected to shear stress prematurely contains fibrillated particles in arbitrary

direction(s). These particles are called abused in the industry parlance. They will not be able to fibrillate properly during the processing of the resin and will appear as defects in the final product. Hose leakage and wire spark-out are typical examples of abused particles. They occur at the point where abused particles are located.

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The following principals for fine powder processing can be summarized as

• Fine powder PTFE is sensitive to mechanical shear, especially above its 19°C transition point.

• Shear stress causes fibrillation of fine powder particles, which is the removal of a group of chains of PTFE from the crystalline phase. The shear force is generated by the pressure that the extruder applies to the PTFE paste. Resin fibrillation increases with increasing extrusion pressure.

• All transportation, storage, and handling of the powder must take place below its 19°C transition temperature.

• Paste extrusion should take place at above the 30°C transition temperature of the polymer.

• A hydrocarbon lubricant is added to PTFE to aid in processing. It is removed prior to sintering the article.

• The extrudate develops strength in the direction of extrusion as a result of fibrillation, permitting its handling during processing.

• Extrusion pressure is a function of the molecular weight of the polymer and lubricant content of the preform under the same processing conditions. The higher the molecular weight, the higher the extrusion pressure will be. The higher the lubricant content, the lower the extrusion pressure will be (Fig. 8.7).

• • • • • • •

129

Reduction ratio; Lubricant content; Lubricant type; Die cone angle length; Die land length; Extrusion speed; Temperature.

Fig. 8.8 shows the unit operations of paste extrusion processes starting from resin through sintering of different shape parts. Those parts include insulated wire, tubes, rods, other profiles, and shapes and unsintered tape.

8.4 Extrusion Aid or Lubricant An extrusion aid is added to fine powder PTFE as a lubricant to enable smooth uniform extrusion. The extrusion aid must easily coat the resin yet be readily removable from the extrudate. It should also not leave a residue, which could alter the color of the product. The volatilization temperature of the lubricant should be lower than the sintering temperature of the polymer. The other requirements of lubricants include high purity, low odor, low polar components, high autoignition temperature, low surface tension, and low skin irritation. Common lubricants are synthetic isoparaffinic hydrocarbons available in a wide boiling range. Some of the

• Extrusion pressure is a function of several variables that are listed below: • Resin type;

Figure 8.7 Influence of extrusion pressure and lubricant content in tube extrusion at different reduction ratios [2].

Figure 8.8 Unit operations of paste extrusion process for manufacturing different parts: tubing, pipe liner, coated wire.

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commercial lubricants include Isopar solvents (available from Exxon Corp.), mineral spirits, and VM&P Naphtha (available from Shell Corp.). Table 8.1 lists the important characteristics of a number of solvents. They have a range of boiling points, which inversely correlates with vapor pressures. The higher the boiling point, the more slowly the solvent will leave the extrudate. Ideally, the lubricant should have a lower surface tension than the critical surface tension of PTFE, which is a very low 18 dynes/cm. While this is not practical for PTFE systems, most solvents have fairly low surface tension, which helps their spreadability on PTFE. Surface tension of Isopar series rises with increasing boiling point, which adversely affects their spreadability. For example, Isopar G has a surface tension of 23.5 dynes/cm and spreads more easily than Isopar V with a surface tension of 30.8 dynes/cm. The amount of lubricant in the compound depends on the type of the product, equipment design, and the desired extrusion pressure. Its concentration should be as low as possible but not so low that the extrusion pressure would become excessively high. A less volatile extrusion aid is often recommended for the manufacture of an unsintered tape [9]. The range of lubricant content is 15%
25% of the total weight of the compound. Petroleum solvents are volatile and their vapors can be ignited causing flash fires or explosion thus requiring caution during handling.

8.5 Wire Coating One of the important applications of fine powder PTFE is wire insulation primarily for automotive, aerospace, and industrial applications where hightemperature rating ( . 250°C) and resistance to chemicals are required. The main use of PTFE insulated wire is for hookup in electronic equipment in the aerospace and military industries. Coaxial cable made by paste extrusion or tape wrapping is the other large volume consumer of fine powder PTFE. PTFE insulated wire is also found in airframe and computer applications. In this section, processing of this resin into a wire coating is described. The important attributes of PTFE insulated wire include

TO

FLUOROPOLYMERS

• Continuous service temperature range of
260°C to 260°C;

• Resistance to all common chemicals, solvents, and moisture;

• Good electrical properties over a frequency range of 102
2 3 1010 Hz;

• High volume (1018 Ω cm) and surface resistivity (1016 Ω per square);

• High dielectric breakdown strength (20
160 kV/ mm);

• High surface arc resistance .300 seconds; • Ability to color by inorganic pigments; • Laser markability of PTFE filled with titanium dioxide.

8.5.1 Blending the Resin with Lubricant Blending the lubricant, PTFE, and pigment must be conducted in a clean enclosed area where the temperature is below resin’s transition temperature (19°C). This area should be controlled at a relative humidity of 50%. Safety features should be designed into the blending room. They include antistatic flooring and clothing, explosion-proof lighting, and grounding for all equipment. All clothing and other fabric should be lint-free to avoid contamination of the extrusion compound. Blending is performed most frequently by two methods: bottle or jar blending and motorized blenders. Neither technique has a clear advantage over the other. The bottle process is suitable for modest scale manufacturing. Large-scale blending is usually done in a V-cone blender such as the units offered by Buflovak, LLC, East Stroudsburg, Pennsylvania, USA. The bottle or jar method requires a wide mouth bottle for easy (low shear) powder loading. The jars must be sealed tightly to prevent the loss of the lubricant by evaporation. The following steps should be taken to prepare the paste extrusion compound: 1. Weigh the powder after screening and carefully load into the bottle.

• Lowest dielectric constant (2.1) and dissipation

2. Create a cavity by giving the bottle a rapid twist.

• Flame resistance and low smoke generation;

3. Pour the lubricant into the cavity in the middle of the powder.

factor (3 3 1024) of any insulation material;

Table 8.1 Properties of Isopar Solvents [8]. Isopar C

Isopar E

Isopar G

Isopar H

Isopar K

Isopar L

Isopar M

Isopar V

Test Method

Kauri-Butanol Value

27

29

28

27

27

27

27

25

ASTM D1133

Aniline point, ° C (°F)

78 (172)

75 (167)

83 (181)

84 (184)

84 (184)

85 (185)

91 (196)

93 (199)

ASTM D611

Solubility parameter

7.2

7.3

7.3

7.3

7.3

7.3

7.2

7.2

Calculated

27 (19)

7 (45)

41 (106)

54 (129)

57 (135)

64 (147)

91 (196)

129 (264)

ASTM D56

Grade Solvency

Volatility Flash point, °C (°F) Distillation, °C (°F)

ASTM D86

IBP

98 (208)

118 (244)

160 (320)

178 (352)

177 (350)

191 (376)

223 (433)

273 (523)

50%

99 (211)

121 (250)

166 (331)

182 (360)

185 (365)

195 (383)

238 (460)

288 (550)

Dry point

104 (219)

137 (279)

174 (345)

188 (370)

197 (386)

207 (405)

252 (487)

311 (592)

Vapor pressure, mm Hg at 38°C (100°F)

98

52

14

6.2

5.7

5.2

3.1

0.3

ASTM D2879

Specific gravity at 15° C/15°C (60°/ 60°F)

0.699

0.722

0.747

0.758

0.760

0.767

0.788

0.817

ASTM D1250

Density, lb/gal

5.82

6.01

6.22

6.31

6.33

6.39

6.56

6.80

Calculated

Color, Saybolt

130

130

130

130

130

130

130

130

ASTM D156

Viscosity, cP at 25°C (77°F)

0.48

0.62

1.00

1.29

1.39

1.61

2.70

7.50

ASTM D445

General

(Continued )

Table 8.1 Properties of Isopar Solvents [8].—Cont’d Grade

Isopar C

Isopar E

Isopar G

Isopar H

Isopar K

Isopar L

Isopar M

Isopar V

Test Method

Autoignition temp., °C (°F)

399 (750)

382 (720)

293 (560)

349 (660)

349 (660)

338 (640)

338 (640)

210 (410)

ASTM D2155

Bromine index

,5

,5

,10

,10

10

,10

5

500

ASTM D2710

Composition, mass % Saturates

100

100

100

100

100

100

99.9

99.5

Mass spectrometer

Aromatics

0.01

0.01

0.01

,0.01

0.01

,0.01

,0.05

,0.5

UV absorbance

Acids

None

None

None

None

None

None

None

None

Exxon method

Chlorides

,3

,2

,1

,3

2

,1

—

7

Exxon method

Nitrogen

—

,2

,1

,1

,1

,1

—

—

Exxon method

Peroxides

0

0

Trace

,1

,1

,1

,1

0

Exxon method

Sulfur

,2

,2

1

,2

,2

,2

,2

1

Exxon method

Surface tension, dynes/cm at 25°C (77°F)

21.2

22.5

23.5

24.9

25.9

25.9

26.6

30.8

duNuoy

Interfacial tension with water, dynes/ cm at 25°C (77°F)

48.9

48.9

51.6

51.4

50.1

49.8

52.2

44.9

ASTM D971

Demulsibility

Excellent

Excellent

Excellent

Excellent

Excellent

Excellent

Excellent

Excellent

Exxon method

Purity, ppm

Surface properties

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4. Close the lid and place the bottle on mixing rollers similar to those used for micing paint (15 rpm) for 20
30 minutes. 5. Let the blend age for at least 12 hours at 35° C to allow complete diffusion of the lubricant into the polymer particles. 6. Any small lumps should be broken by sieving and the lubricated powder rerolled for 3
5 minutes. For large quantities of resin (25
70 kg), a twinshell V-blender (see Fig. 8.9) may be used to incorporate the lubricant. The following steps are taken: 1. Load the powder carefully into the V-blender to avoid shearing the resin. 2. Add the lubricant evenly to the resin. 3. Set the blender to tumble at 24 rpm for 13 minutes for a 25 kg batch of resin. Longer rotation times may be required for larger batches of resin. 4. Screen the compound to break up loose lumps and separate those that do not break easily. 5. Empty the blender and store the lubricated resin in a jar or the original drum and make sure the lid is sealed tightly. Allow the blend to age similarly to step 5 above for jar blending.

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for the quantity of extrusion composition to be mixed. For optimum tumbling, this vessel should be between one-third and two-thirds full. 2. Sieve the pigment, which should be dry, into the polymer through a 60-mesh sieve (nominal aperture 250 μm). 3. Shake the container briefly to disperse the pigment among the polymer particles. 4. Tumble the container end-over-end at 50
60 rev/ minute for 10 minutes. Liquid pigments should be well dispersed prior to the addition to the resin for even dispersion in the polymer. The lubricant and pigment dispersion should be mixed and quickly added to the PTFE powder because the settling of pigment particles occurs rapidly after the addition of the lubricant. The amount of additional lubricant should be adjusted for the hydrocarbon content of the pigment dispersion.

8.5.3 Preforming Preforming is done after lubricant and pigment have been added and the compound aged. This step, which usually takes place at room temperature, shapes the compound into a billet with the same shape as the barrel of the ram extruder. The

8.5.2 Pigment Addition Pigments in dry form or as dispersion can be added to PTFE to color the insulation. Pigment dispersions (liquid pigments) are preferable for critical applications such as thin-wall wire insulation and spaghetti tubing. Dispersions reduce the formation of flaws due to undispersed pigment. Pigments can be dispersed in hydrocarbons using dispersants. Most pigments are commercially available in dispersion form. Pigment loading in the final insulation must be less than 2%
2.5% because of its detrimental effect on the dielectric properties. Inorganic pigments should be selected for coloring PTFE. The four steps for coloring PTFE are listed below: 1. Transfer the required weight of sieved polymer to a clean, dry, wide-necked container with an airtight closure and of ample volume

Figure 8.9 An example of a PK twin-shell liquid solids blender. Courtesy Arnold Equipment Company, www.arnoldeqp.com, January 2019.

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rule of thumb is to compact the resin to one-third of its initial height [10]. Preforming removes the air from the PTFE powder and, by compaction, maximizes the quantity of material available for extrusion. The objective is to extrude the longest possible flawless length of the wire. Preforms are made in a cylinder equipped with mandrel and a pusher similar to the schematic in Fig. 8.10. The surfaces of the barrel wall and mandrel should be smooth and free of scratches and nicks. The mandrel is positioned in the center of the cylinder and the resin is charged in the annular space. The diameter of the preform and the center hole are designed so that 1. The outer diameter is 0.2
1.3 mm less than the inside diameter of the barrel. 2. The core diameter is 0.25 mm larger than the extruder wire guide (mandrel). The aged lubricated resin without lumps is loaded into the preform cylinder and is evenly distributed around the core mandrel to ensure uniform compaction throughout the preform. The pusher is placed on top of the cylinder and compaction started. Resin compression may begin at a fairly

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rapid rate but has to be reduced at the later stages of compaction. This rate reduction is sizedependent and is aimed at the prevention of air entrapment; otherwise, the preform may crack. Low pressure should be used to compact the powder in the cylinder. At the initial stage, 0.5
1 MPa pressure is applied which should be increased to 2 MPa by the end of the compression cycle. The criterion for pressure selection is to compress the resin to sufficiently high pressure to push the air out and to prevent preform cracking. Yet pressure should not be so high that a large portion of the lubricant is squeezed out of the resin by the end of the compaction. A small amount of lubricant tends to be pushed out of the preform even when pressure is adequate and should not raise concern. Fig. 8.11 shows a scanning electron micrograph of a sample of a preform. The particles have been deformed as a result of compaction without evidence of fibrillation. Overly fibrillated preforms do not extrude smoothly and may give rise to defects in the wire insulation. The preform is quite weak and can easily break or deform; therefore, it requires care during removal from the cylinder. The preform may be loaded in the extruder immediately after removal.

Figure 8.10 Preforming equipment for billet molding for paste extrusion.

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ratio of the paste cross-sectional area in the extruder barrel to the cross-sectional area of the extrudate. Wire coating requires resins capable of moderate to high RR, depending on the thickness of the wall of the insulation. The extrusion of the preform is conducted above room temperature (30° C
100°C) to take advantage of the deformability of PTFE at higher temperatures. Electric heating bands equipped with independent controls heat the extruder barrel and the die.

8.5.4.1 Extruder Figure 8.11 Scanning electron micrograph of a preform at 20,000 3 magnification.

It can also be stored at ambient temperature in a plastic tube prior to extrusion to avoid contamination, damage, and lubricant loss.

8.5.4 Extrusion Equipment and Process Fig. 8.12 shows a schematic of a paste extrusion line for wire insulation. The wire is passed through the paste extruder where it is coated with PTFE while moving through the die. It then enters a vaporizing oven where it is stripped of the lubricant by evaporation. Vaporization temperatures depend on the type of the lubricant, heavier hydrocarbons requiring higher drying temperature. Next, the dried coated wire goes into the sintering zone, which usually consists of several individual ovens placed in series. Temperatures of the ovens are set to heat the polymer above its melting point quickly. After leaving the ovens, the wire is cooled and passed through a spark tester where the insulation is subjected to high voltage to “spark out” any flaws, which are counted by the device. The last step is winding the wire on a spool, which is done by a motorized take-up system. The coating that forms the insulation consists of a thin-wall tube, which is paste-extruded onto moving conductors, proceeded by lubricant removal and sintering. The extrusion of the small insulation tube around the wire requires the preform to be reduced by forcing the paste through a small die. This reduction gives rise to an important parameter called reduction ratio, which is a characteristic of fine powder PTFE. Reduction ratio refers to the

The ram extruder for this process is a special unit, which can be either horizontally or vertically oriented. The orientation refers to the direction of the ram movement. The extruder consists of a heated barrel where the preform is loaded, and a hydraulic or screw-driven ram. The conductor is drawn by a power system through a hollow mandrel located at the center of the barrel. The mandrel terminates in a wire guide tube that can be adjusted to alter the position of the tip of guide tube relative to the die. One option is to prepress the preform prior to start of the extrusion. The advisability of prepressing depends on the assembly condition of extrusion equipment. Prepressing could be helpful in driving out the air if the entrapped air cannot escape through the back plate seal and the die seal during the extrusion. The wire payoff system is usually motorized and is equipped with an adjustable tensioning device to keep the wire from slackening or being too tight. The speed of the wire and the ram must be coordinated to produce insulated wire. In commercial extruders, a control system synchronizes the changes in the wire and ram speeds. To extrude the preform is forced through the die by the force of the ram. It is important to be able to control the speed of the ram on a continuous scale and keep it literally constant at a set speed. The uniformity of the thickness of the coating is critically dependent on the constancy of the ram speed. The hydraulic or mechanical drive system must be capable of supplying the force necessary to extrude the preform. Ram pressure capability up to 150 MPa may be required for extrusion at high RRs.

8.5.4.2 Die The preform fibrillates in the die (Fig. 8.13) under ram pressure and forms an extrudate, which

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Figure 8.12 Schematic PTFE wire extruder consisting of an unwind roll and dancer roll, extruder, a drying and sintering oven, deflector roll, a wire puller, electrical breakdown test device, and wire take-up roll [2].

Figure 8.13 Details of the master die set-up for coating wire by paste extrusion [11].

should have the right thickness and smoothness. The design of the die plays a key role not only in the property and quality of the coating but the magnitude of the extrusion pressure. The preform is pressed through the extruder cylinder (barrel) with little pressure development until it reaches the die where the cross-sectional area for the passage of the preform decreases by the angular design of the wall. At this point, the polymer particles are forced to compete for flow through this increasingly smaller cross-sectional area, thereby rubbing past each other and forming fibrils as depicted in Fig. 8.14. The angle of the die wall (cone) affects the surface smoothness of the extrudate. The range of the angle is 15
60 degrees but an angle of 20 degrees for thin coatings on fine conductors and an angle of 30 degrees for thicker coatings have been recommended. Surfaces of the cone and the die land areas should be polished to a mirror finish of 0.1 μm Ra. [10]. The importance of surface finish increases at higher extrusion speeds and pressures.

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Figure 8.14 Depiction of fibrillation of PTFE particles in the paste extrusion die. Courtesy Prof. Savvas Hatzikirikos, Department of Chemical and Biological Engineering, University of British Columbia, 2014.

The conductor (wire) emerges from the guide tube at the tip (Fig. 8.13), which guides it through the segment called the die land. The location of the tip is critical to the insulated wire quality because it affects how the wire and PTFE are brought together. Adjusting the position of the tip can control coating thickness, stripability of the PTFE (tightness), and the number of flaws. The guide tube should be snug around the wire with a maximum clearance of 25
50 μm [10]. It is important to experiment with the tip clearance to obtain the best extrusion condition for a given configuration. The diameter, length, and temperature of the die land area influence the coated wire properties. Allowance should be made for swelling (“blow up”) of the coating when it leaves the die. Relaxation of built-in strains leads to die swelling of the polymer, which is a transient effect. The diameter of the die should accommodate 3%
10% die swell. Die land length of 6
13 mm and die temperature .35°C have been found to produce smooth extrudates. Die design is a fairly complex process and should be tried after an in-depth understanding of the effect of its parameters on the extrusion process and product quality. An iterative process by trial and error can be costly. Benbow and Bridgwate [11] offer an excellent source for the design of paste extrusion dies.

8.5.4.3 Drying The polymer coating containing the lubricant is fairly fragile and susceptible to mechanical damage. The lubricant content of the insulation is nearly 40% by volume, which must be removed prior to

sintering. The coating may crack, if it contains a large amount of the lubricant and reaches the sintering zone. Any remaining hydrocarbons will degrade at the sintering temperatures and leave a colored residue. There are two configurations of drying ovens: internally heated with tubular design and horizontal heated console design; both are vented to remove the vapors. A typical tubular oven is about 3 m long with a diameter of 150
200 mm. One or two 3 m ovens are required for complete drying. Temperature at the oven entrance is 150°C and at the exit is 300°C. In the heated console type oven, several 10 m of the wire are wound around multiple sheaves, which allow longer residence time in this oven than the tubular kind.

8.5.4.4 Sintering and Cooling The wire enters the sintering zone immediately after it leaves the drying oven. It still lacks strength because of its porous unsintered structure. PTFE is heated to temperatures above its melting point undergoing coalescence and void elimination during sintering. After sintering and cooling, PTFE insulation assumes its permanent dimension and ends at 50%
60% crystallinity. Table 8.2 shows examples of manufacturing process variables for coating of wires, which comply with four US Military (MIL) Standards, using two commercial resins. The polymer must be heated to, at least, its melting point of 342°C for a brief period of time before melting occurs. In practice, higher temperatures well above the PTFE melting point are used to reduce melt creep viscosity of the polymer for rapid

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Table 8.2 Examples of Electric Wire Insulation Molded From Daikin PTFE [3]. F-201 Item

I

II

III

Core wire structure (no. of strands/diameter mm)

7/0.320

19/0.127

7/0.127

Wire plating

Silver

Silver

Silver

Wire outside diameter (mm)

0.96

0.64

0.38

Insulation thickness (mm)

0.25

0.25

0.15

MIL standard

E-20

E-24

ET-28

Extruder cylinder diameter (mm)

38

38

38

Mandrel diameter (mm)

16

16

16

Die angle (degree)

20

20

20

Die tip diameter (mm)

1.60

1.321

0.762

Guide tube diameter (mm)

1.067

0.686

0.406

Reduction ratio (RR)

732

899

2751

Amount of extrusion aid blended (weight part)

19.0

21

22

VM&P Naphtha (% by wt.)

15.9

17.3

18.0

Preforming pressure and time (kgf/cm 3 min)

25 3 5

25 3 5

25 3 5

Guide tube/guide tip clearance

0.8

0.6

0.3

Calculated value (mm)

0.78

0.76

0.62

Die temperature (°C)

50

50

50

Extruder ram speed (mm/min)

18.3

19.0

8.0

Haul-off speed (m/min)

8.2

14.0

18.2

615

500

1015

#1 (drying) (°C)

95

95

95

#2 (drying) (°C)

205

205

205

#3 (sintering) (°C)

400

400

400

Outside diameter (mm)

1.52

1.10

0.68

Insulation thickness (mm)

0.28

0.23

0.15

Number of sparks

None

None

None

(Test voltage) (kV)

(3.4)

(3.4)

(1.5)

2

2

Extrusion pressure (kgf/cm ) Oven temperature

Molded product dimensions

void closure. The oven temperatures are typically set at 400°C
600°C, based on the number of variables including the speed of the wire and thickness of the coating. Heat transfer to the polymer accelerates as temperature increases. Care must be taken to prevent the exposure of PTFE to temperatures above 380°C at which degradation begins to accelerate. The sintering ovens should be equipped with

an exhaust system to remove the toxic byproducts of PTFE degradation. Multiple ovens are used for sintering, sometimes as many as eight. The most common ovens are tubular with a typical diameter of 250 mm and about 1 m long. Older ovens have radiant electrical heating elements. The newer ovens are electrically heated with quartz lining, which emits infrared radiation.

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Cooling the coated wire is relatively easy because it is thin compared to granular PTFE parts. The wire insulation exiting ovens is in the molten state, solidifying upon contact with the ambient air. The wire is usually allowed to cool by natural convention in the ambient air. Blowers can be installed to move the warm air away from the area. The crystallinity of the PTFE is about 50%. It is possible to quench the coating by blowing cold air or passing it through a cold water bath. In such cases, crystallinity can be driven below 50%, which has a measurable impact on the properties of thicker coatings. After cooling the wire enters a spark tester, which is a dielectric breakdown tester. It operates in a similar manner to ASTM Method D149 by subjecting the wire insulation to a known voltage continuously. The objective is to measure the number of spots in a length of wire, which are too weak to stand up to the test voltage. The failure makes a dielectric arc accompanied with a buzzing sound, thus called sparks. The number of sparks can be automatically measured and recorded.

8.5.5 Reduction Ratio In this section, we will expand the discussion of this important characteristic of the polymer and paste extrusion process. Reduction ratio (RR) is defined as the ratio of the cross section of the polymer before extrusion to that ratio after extrusion. This ratio can be written as Eq. (8.1). RR 5

AC 2 AG πD2C 2 πD2G 5 AL 2 AW πD2L 2 πD2W

(8.1)

where AC 5 Cross-sectional area of the extruder cylinder (barrel), mm2; AG 5 Cross-sectional area of the guide tube (mandrel), mm2; AL 5 Cross-sectional area of die land, mm2; AW 5 Cross-sectional area of the wire (conductor), mm2. Eq. (8.1) can be simplified, shown in Eq. (8.2). RR 5

D2C 2 D2G D2L 2 D2W

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Practically, the size of the wire is fixed, which leaves the preform as the only practical variable. To make a long length of wire, a reasonable size of preform is required which is why high RRs similar to those in Table 8.2 are encountered. To be sure, smaller barrels are used where possible. Resin manufacturers have developed polymers from very low to very high RRs, some of which are capable of undergoing RRs as high 4000:1. The most important polymer property affecting the operating range of the RR is molecular weight, which can be easily manipulated during polymerization. Higher RR increases the extrusion pressure, which can be reduced by the type of lubricant used and increasing its content. Fig. 8.15 illustrates the effect of RR on extrusion pressure for two commercial resins. The relationship is close to linear, for example, a doubling of RR nearly doubles the extrusion pressure.

8.5.6 Conductor A high-quality wire has a strong impact on the coated product that is produced. A poor quality wire can lead to a larger number of flaws in the PTFE insulation. It often results in a poorly performing process. Stranded wires should be free of loose strands or “high wire”; the latter means extra strands in the stranded bundle. Both lead to snagging and process disruption. One likely solution may be a larger diameter guide tube. This will only add to the sensitivity of the already critical guide tube position. Cleanliness of the wire is another important factor in insuring good quality insulation. The wire should be free of all contaminants such as oil, grease, or dust. An option is to pass the wire through a heated oven on the way to the extruder. Insulation of the wire is assisted by heating the conductor prior to the extruder.

8.6 Extrusion of Tubing (8.2)

where DC 5 Diameter of the extruder cylinder (barrel), mm; DG 5 Diameter of the guide tube (mandrel), mm; DL 5 Diameter of die land, mm; DW 5 Diameter of the wire (conductor), mm. Smaller cylinder, larger mandrel, larger die land, and smaller wire diameter can each reduce the RR.

The majority of tubes made from PTFE by paste extrusion (Fig. 8.16) have fairly thin walls (,8 mm) and are produced in a wide size range from a fraction of millimeter to several centimeters in diameter for applications ranging from fluid transfer in healthcare to fuel and hydraulic transfer in jet engines. Tubing is divided into three categories based on the size and wall thickness, for

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Figure 8.15 Relation of reduction ratio and extrusion pressure [3]. Note: Extrusion aid used: Super VM&P Naphtha (Shell).

various applications. Table 8.3 summarizes the size and applications of each type. Pressure hoses are composite devices made of one or two layers PTFE lining reinforced with over-braiding, usually metal wire, to increase its pressure rating. Each of these tubes requires a somewhat different processing method from the others because of significant disparity of their sizes. Die design for tubing extrusion resembles the design for wire coating except that instead of a guide tube there is a core pin, which extends into the die land area (Fig. 8.17). Wire coating paste extruders can be set up in a modified arrangement to make tubing. In this case, the guide tube should extend beyond the die land exit to prevent its blockage by the polymer. The core pin or the guide tube diameter should be sized to give the correct internal tubing diameter. The ovens should be long enough to allow straight tube production. A horizontal or an upward machine will require motorized take-up to move the tubing through the ovens. Extrusion conditions are quite similar to those used in wire coating. Extrusion pressure can reach very large values (100
150 MPa) because high RRs are necessary to obtain small diameter and wall thickness. The extruder barrel and the die

must be structurally sound to withstand the high extrusion pressure. The lubricant has to be removed prior to sintering as in the wire coating process. Table 8.4 provides an example of extrusion conditions for commercial resins.

8.6.1 Pressure Hoses This class of tubing serves in critical applications in a number of industries. The typical size ranges from 6 to 50 mm in diameter. The tube sizes in the United States have a special designation, called by
(dash) followed by a dash number such as
4 (dash four) or
12 (dash twelve). To obtain the diameter of the tube in inches, the number must be multiplied by 1/16, thus,
4 is equal to 1/4 inch and
12 is equal to 3/4 inch. PTFE tubes form the inner liner component of hoses that come in contact with a fluid. Chemical resistance and durability at extreme temperatures are supplied by PTFE. The liner’s mechanical integrity is fortified by external reinforcement which results in significantly higher operating pressure rating than can be borne by the tube alone. Fig. 8.18 represents one example of braiding by which the tube is fortified. Stainless steel wire (filament) is also braided in

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Figure 8.16 Hydraulic paste extruder for tube fabrication [2].

Table 8.3 Types of Tubing and Applications Made From Fine Powder PTFE. Type of Tubing

Diameter (mm)

Wall Thickness (mm)

Spaghetti tubing

0.2
8

0.1
0.5

Electrical insulation, fluid handling in medical equipment, and chemical applications

Pressure hose

6
50

1
2

Fuel and hydraulic transfer in aerospace, chemical and gas transfer in chemical processing

Pipeliner

12
500

2
8

Lining metal pipes and fitting for chemical processing

double or triple layers including using other fibers such as fiber glass, polyester, polypropylene, or high-performance polyaramid yarns. Some applications require flexibility to bend and curl the hoses. To accomplish this, the extruded and sintered tubes can be convoluted in a separate step. Convoluted tubing uses twice the material of a standard smooth bore tube, creating a tube reinforced within itself to handle higher pressures and offering increased flexibility. The tube is passed through a heated die, which melts the PTFE and

Applications

creates a spiral peak and valley pattern into the tube. A key requirement of the convolution process is to assure the wall thickness remains uniform; in other words, the tube is not stretched. Any thinning of the wall will weaken and reduce the burst pressure of the hose. Another issue in high-velocity transport of hydrocarbons such as jet fuel is the build up of static charge on the interior layer of the PTFE tube. The discharge of static charge in the absence of oxygen can lead to a failure in the form of a

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Table 8.4 Examples of “Spaghetti” Tubes Extruded From Daikin-Polyflon TFE [3]. F-201

Figure 8.17 Details of the master die set-up for extruding tubing by paste extrusion [11].

Item

I

II

III

Die diameter (outside, mm)

1.32

1.60

1.90

Core pin diameter (inside, mm)

1.08

1.27

1.50

Clearance (thickness, mm)

0.12

0.165

0.20

Reduction ratio (RR)

2112

1266

870

Extruder cylinder diameter (mm)

38

38

38

Mandrel diameter (mm)

16

16

16

Die angle (degree)

20

20

20

Die temperature (°C)

50

50

50

Amount of extrusion aid blended

pinhole in the wall of the tube and subsequent leakage. In the presence of oxygen, static discharge could act as an ignition source. To overcome this problem, the inner layer of the tube is made from a 1%
2% carbon-filled PTFE, which would then allow surface drainage of static charge through the metal fittings. The inside part of the preform is made of carbon filled PTFE by essentially partitioning the mold. Pressure hoses find applications where corrosive liquids and gases are transported. Examples include

a

(Weight part)

23

22

21

(wt.%)

18.7

18.0

17.4

Preforming pressure (kgf/cm2)

25

25

25

Extrusion pressure (kgf/cm2)

880

600

540

Extrusion speed (m/ min)

22.0

18.0

16.0

Drying zone temperature (°C)

• Hydraulic fluid and fuel transport in the aero-

#1

100

100

100

#2

250

250

250

400

400

400

Outside diameter (mm)

1.08

1.27

1.54

Inside diameter (mm)

0.94

1.07

1.24

Thickness (mm)

0.07

0.10

0.15

720

720

350

350

Sintering zone temperature (°C)

space industry;

#3

• High-pressure air, fuel and hydraulic transfer

Product dimensions

in the automotive industry;

• Chlorine, steam, acids, and organic compounds in the chemical processing industry;

• High-purity transfer lines in the pharmaceutical industry.

Tensile strength Longitudinal direction (kgf/cm2)

The hose must meet numerous requirements in the various high-pressure applications:

Elongation

• Low permeability; • High flex life; • Good mechanical properties;

Longitudinal direction (%) a

Extrusion aid used: Super VM&P Naphtha (Shell).

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Figure 8.18 Braided smooth and convoluted PTFE tubing using stainless steel wires.

Table 8.5 Major Commercial Grades of PTFE for High-Pressure Hose. Manufacturer

Resin Grade

DYNEON

Hostaflon TFM-2001Z TFM2033Z

Asahi Glass Co.

Fluon CD-086EL and Fluon CD-086EH

Daikin America

Polyflon F-303

Solvay Solexis

Algoflon DF-381

• Chemical resistance; • Service at extreme temperatures; • High purity. Special grades of PTFE have been developed to meet all these stringent requirements. Lower permeability and higher flex life require a polymer with minimal void content after sintering. Lowering the molecular weight normally results in higher crystallinity, therefore reduced flex life. To overcome this dilemma, the resin suppliers have developed modified PTFE by copolymerizing a small amount of another perfluorinated monomer such as perfluorpropylvinylether. Mechanical resilience increases because of the larger content of the amorphous phase. Flex life increases while flex modulus, which is a measure of the ease by which the tube can be bent, decreases. Lower melt viscosity allows improved coalescence of the polymer particles and nearly complete void closure and elimination. The leading commercial PTFE grades specially designed for the fabrication of tubes for pressure hose are listed in Table 8.5.

8.6.2 Extrusion, Sintering, and Cooling Pressure hoses are extruded by both continuous and batch processes, vertically and horizontally. The advantage of a vertical continuous process, for .2 cm diameter tubes, is minimization of the handling of tube preform which is fragile. Any damage to the extrudate usually leads to the formation of defective spot(s) in the sintered tube. This technique has the disadvantage of requiring a tall building (tower). Horizontal extrusion is a batch process that can be operated without much height. It involves handling of the extruded tube preform in order to “curl” it in a pan for drying and sintering in a batch oven. Both processes have to utilize large extruders to accommodate large size tubes with relatively thin walls. The vertical process begins with the downward extrusion of a preform from a large extruder located several stories above the shop floor. The preform is dried and sintered in several in-line ovens. The batch process has the advantage of isolating the cooling step from the extrusion step while in the vertical process the two are combined. Independently controlled ovens are used for drying and sintering in vertical extrusion. Temperature is most closely controlled in quartz infrared ovens each around 1 m long. Half of the zones are usually devoted to dry the extruded tube. The total number of drying and sintering zones depends on the wall thickness and the rate of extrusion. Temperatures are normally set to only partially melt the polymer in the initial sintering zones to avoid subjecting the molten tube to the weight of a long tube length. Majority of the coalescence takes place in the last two or three sintering zones. Hot air is blown counter-currently into the ovens to remove the lubricant vapors and off-gases lest they might

144

ignite. The tube leaving the last zone is in the molten state and must be cooled rapidly to minimize recrystallization. This is necessary due to the importance of flex life and flexibility (low modulus) in pressure hose applications. The process of choice for larger tube diameters ( . 2
5 cm) is continuous downward vertical extrusion without subjecting the tube to any bending prior to the completion of the cooling step. The problem with bending a tube preform or even a partially sintered tube is mechanical damage such as cracking because of bending. That flaw cannot be healed by the sintering process. It is important to remember that, after PTFE melts, its ability to withstand load decreases rather drastically. Molten PTFE virtually lacks melt strength. The fibrillated structure that provides mechanical strength disappears upon reaching the gel state and becomes indistinguishable from nonfibrillated structures. The ideal cooling technique is to quench the tube instantly in very cold water to minimize recrystallization. Practically speaking, modified PTFE can be quenched by hot air or cool water without a sizable increase in its crystallinity. The adequacy of sintering and cooling processes must be determined by testing and data. An effective test method is DSC by which heat of fusion of PTFE is measured. Unsintered polymer is identified by appearance of a peak at or close to the first melting point of PTFE (342°C). The oversintered tube can be detected by an excessively high value of the second heat of fusion (c .28
30 J/g). The intensity of quenching is indicated by the value of first heat of fusion of the sintered (c. 20
23 J/g).

8.6.3 Quality Control of Pressure Hoses Stress cracking is the main reason that highpressure overbraided hoses fail. The mechanical stress at fittings, chemical environment, process variables, and the polymer type are the parameters on which this type of cracking depends. A number of industrial and military aerospace specifications have addressed themselves to the questions of testing and requirements of pressure hoses. An example is MIL-H-25579, which is used by the United States Air Force for military aircraft hose specification. The types of measurements required by such

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specifications are lengthy and time consuming. There are a number of rapid tests, which simplify the task of assessment of hose quality. These tests do not replace those mandated by military or civilian aerospace specifications. A tube that meets the requirements of the simplified tests is highly likely to meet those specifications: 1. Stretch void index (SVI): This test is an indication of the number of voids present in the PTFE tube. 2. Weep test: This method is used to determine the minimum pressure (WP) at the onset of leak of a military fuel through the tube. 3. Orientation index (OI): This index is a measure of the degree of orientation in the machine direction (longitudinal) versus that in the cross direction (transverse). A high-quality aerospace hose should meet the following criteria:

• Total SVI , 2%; • WP . 0.70 (burst pressure); • OI , 0.1. The tests to measure these indices are described in the following sections. 1. Stretch void index (SVI). SVI is a number that is indicative of the number of voids present in a part. It really indicates how well the sintering and coalescence have eliminated the small voids. Voids directly affect the performance of a tube in the end use. For example, a void free of void content part resists flex fatigue longer than a part containing voids. SVI relies on the change in the specific gravity of a specimen, which has been elongated in an extensometer. Application of tensile stress stretches the polymer and enlarges any voids present in the sample. An abundance of small voids alters the appearance of the sample to a dull or blushed look. The procedure for measuring SVI can be found in ASTM Method D4895. It basically consists of preparing a microtensile bar and stretching it to 200% of its initial length in 1
1.5 minutes. Next, specific gravity of both stretched

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and unstretched samples is measured and the SVI is calculated from Eq. (8.3). SVI 5

SGðstretchedÞ-SGðunstretchedÞ 3 100 (8.3) SGðunstretchedÞ

SVI should be measured in both machine and transverse directions to determine the extent of anisotropy in a tube. If the SVI values are very low (,1%) in both directions or equal, the sample is isotropic. This means that the sample has been adequately sintered and stresses have been relieved. If the values are both high, sintering has been poor. If the transverse SVI value is high and the machine direction value is low, then orientation is primarily in the machine direction and it is unbalanced. 2. Weep test. In this test, unbraided samples of the tube are pressurized by filling them with a military fuel (MIL-S-3136, Type III and MIL-H-25579) comprised of 70% isooctane and 30% toluene. A 15 cm length of the tube is filled with fuel to which a small amount of a red dye is added to improve the visibility of the leak. The tube is pressurized to an initial pressure calculated from Eq. (8.4). Initial pressure 5

16:9t d

(8.4)

where t 5 Wall thickness, mm; d 5 Outside diameter, mm. The factor 16.9 is the standard burst strength in the peripheral direction in MPa unit. At this point, pressure is increased in 0.035 MPa increments until seepage is observed. The results are recorded as the Weep Pressure and calculated as a percentage of the peripheral burst pressure. 3. Orientation index. This factor is concerned with orientation of the polymer in the machine and transverse directions, which builds strength in the tube. In the majority of paste-extruded tubes, tensile strength is higher in the machine direction than in the transverse direction. OI provides a numerical value to monitor the disparity of the tube strength in the two directions.

145

If a thin-walled cylinder is subjected to a given pressure P, Eqs. (8.5) and (8.6) can be used to calculate the stress in the longitudinal and peripheral directions [12]. Pl 5 P

d2t 4t

(8.5)

Pt 5 P

d2t 2t

(8.6)

where Pl 5 Stress in the longitudinal direction, MPa; Pt 5 Stress in the transverse direction, MPa; d 5 Outer diameter of the tube, mm; t 5 Wall thickness of the tube, mm. A comparison of the formulas shows that the stress experienced by the tube in the transverse direction is twice as large as that in the longitudinal direction. It is desirable, ideally, for the tube to have twice as much tensile strength in the transverse direction than in the machine or longitudinal direction. The orientation during the paste extrusion, however, takes place predominantly in the machine direction, that is, the direction of extrusion, of the unsintered tube. Sintering the tube properly closes the voids (35%
40% of the volume is occupied by the lubricant) and eliminates the presintering orientation. Molecular orientation is much more random in the molten phase than in the PTFE after sintering. To freeze the orientation, the molten tube must be quenched rapidly in cold water. Slow cooling will allow the crystalline phase to orient itself back in the direction of extrusion, because of the memory of the polymer molecules. Orientation of the tube can be measured by x-ray diffraction. Practically, a comparison of the tensile yield strength of the tube in the two directions provides a measure of the orientation. The OI is, thus, defined as OI 5 1 2

γt γl

(8.7)

where γt 5 Yield strength in the transverse direction, MPa; γ l 5 Yield strength in the longitudinal direction, MPa. It can be seen that an orientation of zero means the tube is randomly oriented, which is ideal. A value of one indicates that all orientation is in the longitudinal direction, which is the worst case.

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Figure 8.19 Horizontal extrusion of a liner along an inner supporting pipe into a supporting half pipe followed by drying and sintering in the oven [2].

8.7 Liner Extrusion The same ram extruders as described for tube extrusion are used for pipeliner extrusion. Due to the heavy weight of the liners, the extruders are generally set up horizontally and require substantially larger barrels as a result of the pipe dimensions. Fig. 8.19 shows how liners are fabricated. The extruded liner is drawn over a supporting interior pipe, if required, and put into a supporting half pipe, considering the low green strength into account. Pipe and half pipe must be corrosion resistant in order to avoid liner discoloration. Unlike tube extrusion, the mandrel diameter may exceed the size of the mandrel rod in order to enable the large liner dimensions. Mechanical stress may be high, which requires the use of large mandrel rods made of high-strength steel. The marked areas at the mandrel of the tool in Fig. 8.19 show the spots exposed to the highest stress. When fabricating thick-walled liners, the phenomenon of orange peeling occasionally occurs. Due to the low RRs, the shear gradient is sometimes reduced to such an extent that a sufficiently homogeneous crackup of the secondary particle is no longer ensured. This problem is obviously not caused by “over-shear” as is often assumed and can therefore be solved by dramatically increasing the shear gradient, for example, through a substantial increase of the extrusion speed.

8.8 Fine Powder Resin Selection A number of PTFE resins are available for paste extrusion. How would one choose one or more

candidates for evaluation? Selection of the resin type is heavily dependent on the part that is going to be made, the fabrication process, and use conditions. Severe chemical environment, particularly at elevated temperatures, is an example of an application where fluoropolymers would be required. The most important feature of the part is the size of its cross section. Paste extrusion, basically, consists of reducing a hollow cylinder of the paste into an extruded preform of the final product by means of pressure. This has led to the definition of RR for a given part. RR is the ratio of the cross section of the paste in the extruder barrel to that of the extrudate. The resin selected to make a particular part must be reducible to the extrudate. This means that it should generate sufficient pressure for fibrillation of the resin, yet the pressure must not exceed the normal range of the available equipment. RR is primarily related to the molecular weight of the polymer; the lower the molecular weight, the higher the RR. Most manufacturers specify the recommended range of RRs for their fine powder grades. The first factor to be determined is the RR of the part. Most extruders allow the use of different size barrels in a fairly limited range, therefore allowing some adjustment of the RR. After the range of RR has been selected, one has to search the product literature of commercial fine powder resins and find those with matching ranges. There are a number of overlapping RRs where one could make more than one resin choice. In these cases, the nature of the application should be used as a guide to select resins. After the appropriate polymer options have been considered, the properties desired in the intended part, processing

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characteristics, end-use conditions, and cost are the final determining factors for the selection of candidate resins. It is quite likely that at the end of this selection sequence, one would arrive at more than one resin supplied by different manufacturers. In such a case, it is best to experiment with each resin to determine which one is more suitable for the specific paste extrusion process and best fits the final application of the part. Other factors such as quality, reliability of supply, availability of support, and technical information from the resin supplier should be taken into consideration in making polymer selections.

References [1] Luntz JF, Jaffe JA, Robb LE. Extrusion properties of lubricated resin from coagulated dispersion. Ind Eng Chem 1952;44(8):1805
10. [2] Proceedings of Dyneon PTFE fine powder. Pub no. PTFEFP201309EN. Dyneon GmbH, 3 M Adv Materials Division; 2013. [3] Fluorocarbon polymers of Daikin Industries. Daikin-Polyflon®TFE fine powder. Daikin Industries, Ltd., Osaka, Japan; 1986.

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[4] Hougham G, Cassidy PE, Johns K, Davidson T, editors. Fluoropolymers 2—properties. Kluwer Academic; 1999. [5] Gangal SV. Polytetrafluoroethylene, homopolymers of tetrafluoroethylene. 2nd ed. Encyclopedia of polymer science and engineering, vol. 16. New York: John Wiley & Sons; 1989. p. 577
600. [6] McCrum NG. An internal friction study of PTFE. J Polym Sci 1959;34:355. [7] Patil PD, Feng JJ, Hatzikiriakos SG. Constitutive modeling and flow simulation of polytetrafluoroethylene (PTFE) paste extrusion. J Non-Newton Fluid Mech 2006;139:44
53. [8] Isopar® solvents, publication DG-1P from Exxon Corp.; 1994. [9] The processing of PTFE coagulated dispersion powder. Technical Service Note F3/4/5, molding powders. Asahi Glass Corp.; 2002. [10] The processing of PTFE coagulated dispersion powder. Fluon® PTFE Resins. Imperial Chemical Industries, Ltd.; 1986. [11] Benbow J, Bridgwater J. Paste flow and extrusion. Oxford: Clarendon Press; 1993. [12] ISO/TC 138 N 1081 and ISO 1167 Standards. International Standards Organization.

9 Fabrication and Processing of Polytetrafluoroethylene Dispersions Sina Ebnesajjad FluoroConsultants Group, LLC, United States

O U T L I N E 9.1 Introduction

149

9.2 Applications

150

9.3 Storage and Handling

151

9.4 Surfactants

151

9.5 Principles of Coating Technology 9.5.1 Coating Processes 9.5.2 Rheology 9.5.3 Surface Energy

151 152 153 154

9.6 Dispersion Formulation and Characteristics 9.6.1 Formulation

155 156

9.7 Glass Cloth Coating 9.7.1 Equipment 9.7.2 Processing

158 159 159

9.8 Impregnation of Flax and Polyaramide 9.8.1 Processing

160 161

9.1 Introduction This chapter discusses the coating of surfaces and fabrication techniques using dispersions of polytetrafluoroethylene (PTFE). It does not cover the topic of PTFE Finishes and Paints. These finishes are usually highly formulated and are often applied as multicoatings, which include special primer and sometimes an intermediate layer. They contain pigments, additives, other resins, and other fluoropolymers besides PTFE. The main applications of PTFE finishes are in cookware, and industrial anticorrosion and high temperature uses. Fluoropolymer coatings have been covered in a separate volume [1]. PTFE dispersions are aqueous milky dispersions consisting of very small particles (,0.25 μm) of

9.8.2 Impregnation of Porous Metals and Graphite 9.9 Coating Metal and Hard Surfaces 9.9.1 Unfilled Polytetrafluoroethylene Coatings 9.9.2 Filled Polytetrafluoroethylene Coatings

161 161 161 162

9.10 Polytetrafluoroethylene Yarn Manufacturing

162

9.11 Film Casting

163

9.12 Antidrip Applications

164

9.13 Filled Bearings

165

9.14 Dedusting Powders

166

9.15 Other Applications

166

References

166

resin suspended in water. This form of PTFE is highly crystalline (96%
98%) and is produced in a wide range of molecular weights. The monomer is polymerized by the dispersion (emulsion) method in which a surfactant is added to the aqueous medium prior to the start of polymerization. As described in Chapter 6, Manufacturing and Properties of Polytetrafluoroethylene, the emulsion product from the reactor is concentrated during which some of the polymerization surfactant is removed. To enhance the stability of the concentrated dispersion, a quantity of a stabilizing surfactant is added. Examples of PTFE-coated surfaces include metal coating, impregnation of fibrous or porous materials such as glass fiber, woven glass cloth, and

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00009-4 © 2021 Elsevier Inc. All rights reserved.

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polyaramide fibers and fabrics. A coated product combines the properties of PTFE and the substrate. PTFE-coated or impregnated products have a number of common attributes as seen in Table 9.1. PTFE dispersions have high utility due to their fluid nature. This is especially important because PTFE does not flow after melting and does not dissolve in conventional solvents, therefore cannot be processed by melt or solution techniques. Among common articles coated using PTFE dispersions are cookware; glass fabrics for architectural; and industrial applications such as stadium roofs and conveyer belts. By formulating the dispersion so that it can be spun through spinnerets produces PTFE fibers are produced. Coatings and fibers of PTFE are usually sintered to improve their mechanical properties. Other examples of applications of

Table 9.1 Attributes of Polytetrafluoroethylene Impregnated/Coated Material. Attribute

Source of Attribute

Good sliding without adding lubricants

PTFE

Nonstick properties

PTFE

High service temperature

PTFE and substrate

Water repulsion

PTFE

Chemical resistance

PTFE

Greater mechanical strength than natural PTFE

Substrate

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FLUOROPOLYMERS

dispersions include cast films, packings, gaskets, bearings, and polymer additives. This chapter reviews some of the basic aspects of coating technology insofar as they are applied to the use of PTFE dispersions. Readers who are unfamiliar with this topic are encouraged to consult the references cited for coating technology for an indepth understanding of the subject.

9.2 Applications The end uses of PTFE dispersions are numerous due to the convenience of coating techniques. They can be classified in different ways from the point of view of product attributes or processing techniques. Table 9.2 is a product type summary of dispersion applications. The focus here is on the shape and form of the part, which influences the process by which they are fabricated. Another advantage of PTFE dispersion is the acceptance of larger amounts of fillers than do PTFE powders. The process to incorporate fillers is called co-coagulation. The main application of these compounds is in the fabrication of special bearings. There are a number of other smaller uses of PTFE dispersions in fuel cells, batteries, dedusting, and chloralkali processing. A different approach to the classification of the applications of PTFE dispersions is the nature of thermal treatment of the fabricated parts. Some articles are sintered, and some are not sintered but heated to remove the water and surfactant. In some applications, the parts are neither sintered nor heated high enough to remove the surfactant.

Table 9.2 PTFE Dispersion Products and Applications. Products

Applications

Coated woven glass cloth and fiber

Architectural fabrics, gaskets and laminates, electrical insulation, release sheets, hoses

Impregnated flax, polyaramides, and PTFE yarn or yarn constructions (asbestos in the past)

Packings, seals, and gaskets

Dispersion cast PTFE films

Diaphragm and dielectric insulation in small capacitors, composite laminates

Coated material surfaces

Low friction and nonstick surfaces

Fabric and fiber finishes

Yarns, industrial fabrics, and filter cloth

Blends with polymeric and nonpolymeric materials

Flame nondrip plastics

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Table 9.3 PTFE Dispersion Application Categories Based on Fabrication Processing. Unsintered, Heated

Unsintered, Unheated

Coated woven glass cloth

Filtration cloth

Packings

PTFE yarn

Batteries

Gaskets

Cast films

Blends with polymeric and nonpolymeric materials

Batteries (sometimes heated)

Sintered

Coated metals

Dedusting

Cocoagulation products

Paint additives

Chloralkali processing Fuel cells

Table 9.3 summarizes the process-based classification of dispersion applications.

9.3 Storage and Handling Most PTFE dispersions should be stored at temperatures between 5°C and 20°C. Freezing the dispersion must be avoided due to its irreversible coagulating effect on the particles. Maximum shelf life for dispersions is 1 year, although some dispersions may have shorter useful lives. Once a month, drums of stored dispersion should be rolled or gently agitated to rejuvenate them. Coagulation of PTFE particles may take place if the storage temperature is too high, it is subjected to vigorous agitation or shearing, shelf life is exceeded, monthly drum rolling is not done, and if chemicals are added to the dispersion. To inspect the PTFE dispersion, it is examined using a microscope which is capable of revealing coagulated PTFE particles. Observation of numerous white lumps (coagulum) under magnification would indicate coagulation or spoilage of the dispersion. Normal PTFE dispersions may contain an occasional coagulum but if they appear uniform, they are considered usable.

151

9.4 Surfactants PTFE dispersions may contain one or more surfactants (and other additives). Dispersions usually contain at least two surfactants: a small amount of polymerization surfactant (,2000 ppm) and a relatively large amount of stabilizing surfactant (2
8 wt.%). For decades, the polymerization surfactant of choice was ammonium perfluorooctanoate (APFO) also known as C8 in the industry. It was found to be biocummulative and persistent in the environment. APFO has now been replaced by other fluorinated surfactants. Fluoropolymer manufacturers have developed replacement surfactants that approach the effectiveness of APFO without its drawbacks. PTFE dispersions are stabilized using nonionic surfactants, which in addition to stabilization of the dispersion also promote wetting of the substrate. Triton X-100 is an example of a legacy stabilizing nonionic surfactant (supplied by Dow Corp.) [2] that was added to PTFE dispersions for decades. This surfactant does not survive the sintering temperatures of PTFE and decomposes. The products of Triton X-100 degradation are mostly gaseous and evolve during the sintering of the coating, leaving a small amount of tarry residue on the surfaces of exhaust ducts. Triton X-100 is from the alkyl phenyl ethoxylate (APE) group of surfactants and has been suspected of endocrine disruption. It has been replaced by surfactants with linear alcohol ethoxylate (LAE) chemical structure that are considered safer than Triton X-100. An example of LAE group is the TergitolTE line of surfactants offered by Dow Corp.

9.5 Principles of Coating Technology The basic objective in coating a surface (substrate) is to adhere a thin layer of a second material to the substrate permanently or temporarily. An everyday example is painting house walls, which is done by applying a relatively thin layer of a wellstirred paint using a roller, sprayer, or a simple brush. Paint must adhere to the wall permanently which takes place while the paint is allowed to dry. PTFE coatings require permanent adhesion. Properties of PTFE require sintering at temperatures well above its melting point ( . 342°C).

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A number of other considerations and requirements apply which are discussed in this section.

9.5.1 Coating Processes Many different processes are used in the coating industry. One way of classifying the processes is by the number of layers that a method is capable of coating, and whether the method coats continuous or discrete units of substrates. PTFE dispersions are usually coated on continuous planar or fibril webs. Single and multiple layers can be coated on continuous webs. A great variety of devices known as rod, dip, doctor blade, knife, gravure, reverse roll, air knife, and forward roll are single layer methods. Slide coating techniques are multilayer techniques. Slot die coating and curtain coating can be tailored to coat single or multiple layers. Discrete methods include curtain, spray, and dip coating techniques. Descriptions of coating methods are found in a number of references including the review by Cohen and Gutoff [3] and Coating Technology Handbook by Satas [4,5]. There are three ways by which a liquid coating is transferred to a substrate (web). First, the substrate is dipped into the liquid coating, thus the term dip-coating. An excess amount of dispersion

Figure 9.1 Two-roll forward roll coater [6].

INTRODUCTION

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FLUOROPOLYMERS

is entrained onto the surface of the substrate as it moves through the coating reservoir (trough). The excess material is removed usually by a blade or a bar and recycled to the reservoir, allowing the metering the amount of liquid laid on the web. The second way to obtain a coating is by means of one or more rolls, which picks up the liquid from the trough and deposits it on the substrate. A doctor blade controls the thickness of the liquid on the roll. The metering action of the blade can be further improved by using rolls, which have engraved channels, with controlled volume. An example of this technique is two-roll coating (Fig. 9.1) and reverse roll gravure coating. The third way to transfer a liquid coating to the web surface is by spraying. This technique atomizes the liquid into small droplets, which form discrete zones immediately after reaching the web surface. Surface tension forces drive the coalescence of these discrete zones toward forming a continuous layer. It is important that a sufficient number of droplets are sprayed on the web so that they would be in sufficiently close proximity to allow surface tension forces to work. Dip coating is the most popular method of coating cloth and fibers with PTFE dispersions. Hard surfaces are coated by roll-coating techniques or spraying.

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Each coating process operates according to a set of common variables with other processes and a few unique variables. It is essential to have a basic understanding of rheology (liquid flow) and surface chemistry (and physics) in order to understand coating processes. Flow of the liquid coating material and the interaction between the liquid and the substrate surface determine the state of the coating. A mostly qualitative review of these two concepts will be made here which should provide the reader with sufficient background to develop a working knowledge of PTFE dispersion coating. More detailed treatment of these subjects can be found in works by Cohen and Gutoff and Satas [3
5].

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references. Plastics and coatings are subdivided into a number of categories based on their flow behavior when they are subjected to an external force. Examples of water, honey, and ketchup which are encountered in everyday life serve to illustrate the differences among the flows. Ketchup requires more force than honey and honey more than water to flow. Viscosity as a measure of flow is defined by the ratio of shear stress and shear rate: Viscosity 5

Shear stress ðdynes  s=cm2 Þ Shear rate Force ðdynes=cm2 Þ Area

(9.2)

Velocity ð1=secondÞ Thickness

(9.3)

Shear stress 5

9.5.2 Rheology Rheology is a general term referring to the science of studying flow and deformation of materials. Viscosity is defined as the resistance of a liquid to flow. For example, ketchup has a higher viscosity than water. An understanding of rheology and viscosity is essential to processing of polymers and coating. PTFE is an exception among thermoplastics for which rheology does not enter the picture in most of its processing techniques. This is simply due to its ultrahigh viscosity even at temperatures exceeding its melting point (1011
1012 Pa s at 380° C) where no flow occurs. This is not true of PTFE dispersions, which are subject to almost all considerations of coatings. The subjects of rheology, viscosity, and behavior of materials have been covered by numerous

Shear rate 5

(9.1)

The reader can learn a great deal by referring to a number of excellent books available about rheology and flow of materials [7
10]. A case applicable to coatings is thixotropic (or shear thinning) liquids, which become thinner when shear rate is increased. Fig. 9.2 shows viscosity of two paints as a function of shear rate representing behavior of a non-Newtonian thixotropic fluid. It appears thinner during rapid stirring because raising the speed of stirring increases the shear rate. Thixotropy is the basis for the so-called dripless house paints. These paints undergo a reduction in viscosity while being brushed. When brushing is slowed, they return to higher viscosity and do not drip or sag. Both

Figure 9.2 Example of thixotropic shear stress versus shear rate of two wall paints [11].

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observations indicate a reduction in viscosity of paints as rapid motion (shear rate) is applied. For the most part, PTFE dispersions are not thixotropic. It is difficult to measure a viscosity decrease for them due to their fairly low values. PTFE dispersions exhibit near Newtonian behavior at room temperature, that is, their viscosity is independent of shear rate. Organic materials, such as acrylic resins, and inorganic thickeners, such as barium nitrate, are added to PTFE dispersions, when a higher viscosity is desired. An example is glass fabric coatings which contain significant quantities of fillers without separation of the fillers. The thickened PTFE dispersions exhibit shear-thinning behavior. Viscosity of PTFE dispersions is strongly affected by temperature. A typical thermoplastic polymer exhibits lower viscosity as temperature is increased above its melting point. Viscosity of PTFE dispersions actually increases at elevated temperatures. When temperature of PTFE dispersions is raised at some point viscosity begins to increase. This is due to the reduced solubility of the stabilizing surfactants in water with temperature. Viscosity increase releases some of the PTFE particles from the micellar structures. Viscosity, then, begins to partly obey the law of slurries in which viscosity has an exponential relationship with the volume fraction of solid at higher concentrations. PTFE particles act as a filler due to their insolubility and inertness. Viscosity of a slurry increases slowly at low concentrations. The increase becomes exponential as solids content increases. Physically, it can be explained that the solid particles set up structures in the liquid phase, which present a resistance to flow. There are a number of equations for the relationship between solids content and viscosity. Einstein’s Eq. (9.4) is used to calculate the viscosity of dilute solids in a Newtonian liquid. Marson and Pierce Eq. (9.5) is a simple expression for calculating viscosity throughout the range of the solids concentration [10]. η 5 ηf ð1 1 2:5φÞ

(9.4)

TO

1 η5 h
2 i 1 2 Aφ

FLUOROPOLYMERS

(9.5)

where φ is the volume fraction of solids, η is the viscosity of the slurry, ηf is the viscosity of the liquid, and A is a constant with a value of 0.64
0.68 for spherical particles and smaller values for other shapes.

9.5.3 Surface Energy This section discusses the interfacial forces between materials. An important concept is surface tension, which is defined as the sum of all forces exerted on a molecule by the surrounding molecules. At the interface of a solid and a liquid, surface tension is used to predict wetting behavior of the liquid. In practice, surface tension between air and a liquid, and air and a solid, are neglected due to their relatively small value compared to the solid
liquid interaction. A liquid is said to wet the surface of a solid when it spreads and flows out forming a thin film. A good example is the wetting of a metal pan surface by water. Nonwetting behavior is the opposite and can be best illustrated by observing a drop of oil on a PTFE-coated frying pan. The oil droplet basically retains its spherical shape and remains on the surface unchanged even after an extended period of time. The angle that the liquid meniscus forms with the solid surface is called contact angle (Fig. 9.3). The angle has a unique value for each pair of liquid
solid combinations at a given temperature. A completely wetting liquid forms a contact angle of 0 degree and a completely nonwetting liquid forms a 180 degrees angle. Any angle between these two extreme values indicates a partial wetting condition. Surface tension and contact angle can be measured for combinations of liquids and solids. PTFE dispersions and solid surfaces have some of the

Figure 9.3 Wetting and nonwetting droplets on a low surface tension substrate.

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Table 9.4 Specific Gravity of PTFE Dispersions.

Figure 9.4 Equilibrium contact angle on an ideal surface [12].

lowest surface tensions of all materials against all liquids, values in the range of 15
18 dynes/cm. Although, there are very few liquids that wet the surface of polytetrafluoroethylene, most surfaces are wetted by PTFE dispersions because of the surfactant present in these dispersions. This is due to the criteria requiring a higher solid surface tension than the liquid’s in order to wet the surface [Eq. (9.6)]. This criterion is derived from Young’s equation [Eq. (9.7) and Fig. 9.4]. This equation is the most simplified form of the wetting criteria in which other small interfacial forces have been neglected. Readers can refer to this source for indepth study of wetting criteria [12]. γ LV , γo SV

(9.6)

γLV cosθ 5 γ o SV 2 γ SL

(9.7)

γ oSV is the critical surface energy of the solid, γLV is the surface energy of the liquid, and γSL is the interfacial energy (or surface tension) of solid liquid. Most metals and inorganic materials have significantly higher specific surface tensions than PTFE dispersions and coatings. That makes wetting their surfaces much easier than low energy materials. Finally, leveling of the coating after application to the surface has to take place for a uniform film thickness to form. Leveling is actually a complex phenomenon and hard to control. Leveling is improved by longer time, lower surface tension of coating, lower viscosity, increased coating thickness, and smaller distance between ridges in the nonlevel areas.

9.6 Dispersion Formulation and Characteristics This section reviews the properties and characteristics of PTFE dispersions, which are important to the formulation and application of dispersions. They include solids content, pH, stability, and critical cracking thickness (CCT). Dispersions of PTFE

Solids Concentration (%)

Specific Gravity

Density of Solids (g/L)

35

1.24

436

40

1.29

515

45

1.34

601

50

1.39

695

60

1.51

906

are colloidal emulsions of small polymer particles (,0.25 μm) in water that are negatively charged. They contain relatively high concentrations of PTFE, which increases their specific gravity. An estimate of solids content is obtained from the specific gravity (Table 9.4). PTFE dispersions are supplied with a basic pH to prevent bacterial growth during storage, particularly when it is hot and humid. Bacteria feed on surfactant in the dispersion. Breakdown of the surfactants generates a rancid odor and brown discoloration in the dispersion. The pH of the dispersion is adjusted by adding acids. It is important to control the amount of acid added to the dispersion because the increase in ionic strength of the dispersion can lead to coagulation of PTFE particles. As a matter of fact, increasing the ionic strength of polymer dispersion is one of the common methods for the coagulation of dispersion to separate and dry the PTFE. Ionic strength of PTFE dispersion affects its conductivity. Conductivity is a very important characteristic of PTFE dispersions and is a good indicator of its shelf life. Conductivity is measured by a conductivity meter quickly and easily. Conductivity also influences the viscosity and shear stability of the dispersion. Very high conductivity destabilizes the dispersion. Stability of dispersion is important to its storage. It will partially settle during extended storage and when it is exposed to elevated temperatures ( . 60° C). A softly settled dispersion can be redispersed by gentle agitation. Freezing of the dispersion will lead to irreversible coagulation. The addition of water-soluble organic solvents or water-soluble inorganic salts and other compounds will also destabilize the PTFE dispersion and polymer coagulation will occur irreversibly.

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The thickness of the wet coating affects the quality of the final sintered coating. An excessively thick layer will result in cracking after the polymer is dried. A CCT is defined as the maximum thickness, which can be coated in a single layer without formation of cracks. Layers up to the thickness of 25 μm can usually be cast without cracking concerns. The exact thickness is dependent on the formulation and type of dispersion, application process parameters, and the geometry of the article being coated. Multiple passes may be used to obtain higher thickness.

9.6.1 Formulation Many applications of dispersions require a number of properties in the applications, which is achieved by the addition of fillers, pigments, leveling enhancement additives, flow improvement additives, and other additives. For example, cold flow (creep) properties of the coating are reduced by the addition of fillers such as fiberglass. Additives must be mixed by mild stirring to avoid coagulation of PTFE. There are applications where the viscosity of the dispersion must be increased to maintain uniform wet thickness in the process. The addition of watersoluble thickeners such as acrylic polymers is one way of increasing the viscosity of the dispersion. For example, the addition of 1% of Carbopol 934

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(supplied by Lubrizol Corp., www.lubrizol.com) increases the viscosity of a dispersion containing 60% solids by 30 times to about 6 poise. Other examples of thickeners include Acrysol ASE acrylic polymers by Dow Corp. (www.dow.com) and Natrosol hydroxyethyl cellulose polymers by Ashland Corporation (www.ashland.com). The addition of nonionic surfactants (Fig. 9.5) does not increase the viscosity to unacceptably high levels. Anionic surfactants are less desirable—the cationic type is unacceptable due to their coagulating or flocculating effects. Thickeners and surfactants degrade and evolve off during the sintering of the coating. Triton X-100 and its analogs had been the surfactant of choice, because of its effectiveness, for stabilizing PTFE dispersions until its replacement in the decade of 2000s. Most commercial PTFE dispersions no longer contain Triton X-100 as stabilizing surfactants. They belong to the APE family of surfactants and have been replaced by linear alkyl ethoxylate (LAE) surfactants that are considered safer and easier to remove. The industry has selected replacement surfactants that have fairly close behavior to Triton X-100 in PTFE dispersions. Some of the important characteristics of LAE surfactants are comparable with those of Triton X-100 (Fig. 9.6). An important difference between the thermal stability of two surfactants is higher stability of Triton X-100 than LAE surfactant. One practical result of

Figure 9.5 Viscosity measurements of a Fluon dispersion at 20°C (A) for various concentrations of Triton X100 at 60% PTFE and (B) various concentrations of PTFE at 6% Triton X-100 [2].

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thermal stability is the color change of PTFE coatings after sintering. Fig. 9.7 shows examples of two coatings applied to glass fabric using PTFE dispersion containing Triton X-100 and a LAE surfactant.

Figure 9.6 Chemical structures of (A) APE Triton X-100 and (B) LAE replacement surfactant.

Figure 9.7 PTFE coatings on glass cloth using dispersions stabilized with Triton X-100 and LAE surfactant.

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The coating sample made with dispersion containing Triton X-100 is appreciably darker indicating more carbonaceous residues left behind after sintering. Thermogravimetric analysis (Fig. 9.8) indicates that LAE surfactant has a 50°C lower onset temperature of degradation than Triton X-100. Other important characteristics of the dispersion include viscosity variation as a function of temperature and the settling tendency of the dispersion. The ideal dispersion would have a flat viscosity in the temperature range of 15°C
45°C. If the viscosity of PTFE dispersion varies with the change in ambient temperature, then the thickness of wet coating layer will be variable. Fig. 9.9 shows the

Figure 9.9 Viscosity of PTFE dispersion containing 6% surfactant versus temperature.

Figure 9.8 Thermogravimetric analysis of Triton X-100 and LAE surfactants in air.

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viscosities of Triton X-100 and LAE surfactants as a function of temperature. It is apparent that LAE surfactant has a more robust viscosity window than Triton X-100. Other important parameters include CCT and shear stability. CCT is important because it impacts the number of passes required to achieve a given thickness on the substrate. If CCT is exceeded, the coating appears “cracked” as seen in Fig. 9.10. Shear stability is measured by subjecting the dispersion to a high level of shear such as in a household food blender. The time it takes for a dispersion to coagulate is defined as a measure of shear stability. The data in Table 9.5 indicate equivalent CCT for two surfactants. Shear stability of dispersion stabilized with LAE surfactant is somewhat lower but sufficiently high for acceptable use. Settling tendency of the two dispersions was virtually identical as indicated by the data in Fig. 9.11.

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A large quantity of PTFE dispersions is consumed in coating glass fabrics. PTFE-coated glass fabric/ cloth is used in many applications including packaging, food processing, PTFE products for expansion

joints, industrial belts, architectural and buildings and photovoltaic modules, and removable insulation covers. In the coating process, glass cloth is coated with PTFE dispersion, which is subsequently sintered in an oven. Typically, the glass fabric is supplied from a payoff roll and is passed through a trough filled with dispersion, followed by a drying and sintering oven. The glass cloth is then collected by a take-up roll. In some instances, the sintering step is omitted on the first few passes. The fabric is calendared to press any broken filaments into the soft PTFE coating and to “heal” mud cracks. The PTFE is then sintered in the remaining passes. Glass cloth is woven from glass fibers. The fibers are coated with a “sizing” agent that acts as a lubricant during the weaving process to prevent the fiber bundles from fraying. The sizing agent degrades and chars during the sintering process and leaves a color ranging from cre`me to brown. Formation of off-white color can be prevented by removal of the sizing chemically or, more economically, by heat. Glass cloth has a smooth surface and is porous. It does not ionize in water or absorb the PTFE dispersion. It picks up a small amount of dispersion per pass, therefore requiring multiple passes (up to a dozen) to obtain a smooth surface, if that is required.

Figure 9.10 Appearance of a mud-cracked fluoropolymer coating under an optical microscope [13].

Figure 9.11 Settling tendency of PTFE dispersions containing 6% surfactant.

9.7 Glass Cloth Coating

Table 9.5 Critical Cracking Thickness and Shear Stability of PTFE Dispersions Containing 6% Surfactant. Parameter

Triton X-100

LAE Surfactant

Critical cracking thickness (μm)

12
15

14
15

Shear stability (ASTM Dxxx) (s)

400
450

300
350

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evaporation of water that would change the resin concentration, thus wet coating thickness. 5. The oven should have three zones consisting of drying, baking, and sintering (Fig. 9.12). 6. Drying zone should be capable of 100°C. 7. Baking zone should reach 315°C. 8. Sintering zone should be capable of 420°C. 9. An annealing chamber is sometimes installed at the exit of the sintering oven to prevent the coated glass from cooling too rapidly to avoid wetting difficulty during the second pass coating. 10. Finally, it is vitally important to exhaust the fumes of the ovens properly, thus avoiding exposure to byproducts of the decomposition of surfactants, additives, and PTFE.

9.7.2 Processing Figure 9.12 Schematic of a three-zone oven.

9.7.1 Equipment The equipment for glass cloth coating is shown in Figs. 9.12 and 9.13, which includes dip tank, oven for drying and sintering (tower), and payoff and take-up rolls. The dip tank should be made of stainless steel and equipped with a submerged roll or slide rod to allow advancing of the glass cloth through the tank. Multiple rolls help improve uniformity of side-to-side dispersion pickup. A partially submerged roll helps increase the pickup by thick fabrics by forcing the dispersion through the cloth. Some considerations for the equipment are listed below: 1. To minimize foaming the tank should be shielded from air current and filled from the bottom, gravitationally. 2. The equipment must be capable of constant speed to insure uniform dispersion pickup. 3. A dip tank water jacket is sometimes necessary to maintain the dispersion temperature at 20°C
25°C. 4. The dip tank should be designed with a minimum exposure to ambient air to avoid

The processing steps include immersion in the dip tank, removal of excess dispersion, drying, baking, calendaring (sometimes), and sintering. PTFE dispersion should be gently stirred by an agitator for several minutes or its drums rolled, then filtered through a fine (5 μm to 20 mesh opening, depending on the application) filter. It should be charged into the dip tank and allowed to reach a constant temperature before beginning to coat the glass cloth. The coating speed is limited by practical parameters such as the rate of return of excess dispersion to the dip tank, foam formation in the dispersion, and oven length and capacity. Excessive coatings may be wiped from the glass cloth by applicators including, in the order of decreasing effectiveness, sharp-edged knives (doctor blade), round-edged knives, wire-wound rods, and fixed gap horizontal metering rolls. Coating thickness after each pass should be less than the CCT. Sometimes multiple unsintered layers are coated on the glass cloth. The coated web must be first dried to remove the water and heated to volatilize and degrade the surfactant. The web is then calendared before sintering to cure the cracks. Calendaring has the benefits of flattening the fabric and tucking in the broken glass filaments in the coating. Broken filaments create defect points and wick moisture into the glass cloth potentially affecting its electrical properties. The thickness and quality of the final coating, the type of glass fabric, and formulation will

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Figure 9.13 Glass cloth coating equipment [2].

Table 9.6 Process Conditions for Glass Cloth Coating [2]. Number of Passes

Dispersion Concentration (% PTFE)

Added Surfactant

Web Speed (m/min)

Web Temperature (°C) Drying

Baking

Sintering

1,2,3

45
50

No

1
2

90
100

200
250

380
400

4,5,6

55
60

Yes

1
2

90
100

200
250

380
400

determine the number of passes that must be made. Typical process conditions for dispersions containing 45%
60% PTFE are shown in Table 9.6. Glass fabric must reach the temperatures shown. Water is removed in the drying zone and the surfactant in the baking zone. It is preferable to extend the baking zone to complete the removal of surfactant as opposed to increasing the sintering temperature due

to the reduction in the mechanical properties of glass fabric.

9.8 Impregnation of Flax and Polyaramide Flax and polyaramide are used to fabricate packings and gaskets. They are available in a number of

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forms that can be coated. The process described below was used in the past to impregnate asbestos and may be used to process flax and polyaramides. Developed economies in North America, European Union, and many other countries have abandoned asbestos use. Impregnated asbestos packings are still used in the developing economies such as India and China. The common forms of asbestos include yarn, cord, braid, and cloth. Spun yarn could contain up to 5% cotton for producing industrial packings. Extruded asbestos yarn has a smoother surface than spun yarn. Packing is produced by braiding and cloth by weaving from spun or extruded yarn. Asbestos packings have two major drawbacks: permeability and high coefficient of friction. Impregnation of asbestos with PTFE reduces permeability and friction coefficient.

9.8.1 Processing Impregnation of asbestos begins by dipping it in the PTFE dispersion followed by drying and baking. In general, each step is similar to the corresponding step for coating glass cloth. Asbestos is partly ionized in water and assumes positive charge, which promotes coagulation of the negatively charged dispersion on the surface of asbestos yarn. This inhibits the penetration of resin into the interior bulk of asbestos article. Equipment for impregnating asbestos is similar to those used for glass cloth coating. Asbestos readily picks up PTFE dispersions and the amount of uptake depends on the concentration of PTFE in the dip tank. The dispersion may be diluted moderately with deionized water or substantially using a dilute aqueous solution of a nonionic surfactant such as Triton X-100 or its replacements, discussed earlier in this chapter. The speed of the asbestos yarn movement through the dip tank has little effect on the amount of uptake. The length and capacity of the oven determines the maximum throughput of the process. After dipping, the asbestos is dried in an oven (80°C
90°C) followed by calendaring to smooth its surface.

9.8.2 Impregnation of Porous Metals and Graphite The applications of impregnated metal and graphite parts are in heat exchangers and bearings where impervious properties are required. The key

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issue in impregnation of graphite and porous metals is penetration of dispersion into the pores, which is done by placing the dipping operation in a vacuum chamber. After pulling vacuum and removing the air from the pores of the article, air is gently allowed back into the chamber. Pressure cycling drives the dispersion into the pores. More than one cycle may be required to increase impregnation. Sintering follows slow drying and baking.

9.9 Coating Metal and Hard Surfaces Metal and ceramic surfaces are coated with PTFE dispersions to protect them from corrosion and create nonstick surfaces. These objectives can be achieved by using filled or unfilled coatings. Examples of applications include household and commercial cookware and industrial equipment. Each coating type has advantages and disadvantages. Unfilled PTFE forms a surface entirely made of the polymer itself, which is smoother and less porous than the coated filled resin. Filled PTFE formulations generate a harder surface, which wears at a slower rate than the unfilled coating. Unfilled PTFE coatings generally adhere to the surface mechanically, while pigmented dispersions adhere by priming the surface chemically. The middle layers of the coating usually contain the pigment.

9.9.1 Unfilled Polytetrafluoroethylene Coatings Unfilled PTFE coatings are only suitable for adhesion to aluminum and few of its alloys such as aluminum/magnesium with a typical sintered coating thickness of ,25 μm. The preparation of the surface of aluminum is of paramount importance to achieve good adhesion with PTFE coatings. The surface of aluminum must be roughened to maximize its mechanical engagement with the coated polymer. This is best achieved by mechanically or chemically etching the surface of aluminum. Using a primer may eliminate the need for special surface preparations. The aluminum surface has to be degreased before it is etched to assure uniform etching. Grease and oil mask the surface of metals and prevent etching. Organic solvents or solutions of phosphates can

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remove grease. Aqueous solutions of phosphates are preferable due to environmental emissions caused by organic solvents. The surface is simply washed with the phosphate solution followed by rinsing with tap water and deionized water. Etching takes place by immersing the part in a dilute solution (20
30 wt.%) of hydrochloric acid. An acid bath can be used to etch the surface of the article. A warm bath (30°C
40°C) requires several minutes for the etching to be completed. The duration of etching should be extended if the bath temperature is decreased. A longer etching time is required as the aluminum content of an alloy is decreased. Hydrogen is a byproduct of the metal’s reaction with acid, which must be removed to avoid explosion. Following the rinsing of the part with water, it is immersed in a dilute solution of nitric acid for a few minutes at room temperature. It is then washed thoroughly with tap water and deionized water and dried. After drying, the part must not be handled or contaminated with any foreign substance until the coating process has been completed. Adhesion of PTFE to contaminated areas of the surface will be weak. The final step is coating the aluminum surface with the PTFE. The coating can be applied by one of the methods described in this chapter. It is helpful to adjust the viscosity of the dispersion to 300
400 centipoise to improve its penetration into the pores of the roughened areas. This can be achieved by adding deionized water for small dilutions or a 3%
5% aqueous solution of an appropriate surfactant, such as Triton X-100 or its replacements, for large dilutions. The added surfactant preserves the stability of the dispersion, which is reduced by water addition. After application to the part, the coating is dried, the surfactant is baked out, and the PTFE is sintered, usually in a single oven comprised of two zones. The initial zone should heat the part and remove the moisture slowly to prevent rapid evaporation. A temperature setting below the boiling point of water around 90°C will remove the water in a few minutes. No water should be left in the coating when it is being sintered. Excessively rapid evaporation of water tends to form defects in the PTFE coating. The coating is sintered in an oven at a temperature well above the melting point of the polymer (at least 380°C). The length of time depends on a number of

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variables such as the heat capacity of the part because the whole part must reach the sintering temperature. Sintering and drying oven must be exhausted properly to remove off gases, which could contain surfactant and PTFE degradation products.

9.9.2 Filled Polytetrafluoroethylene Coatings This type of coating is desirable for applications where both good decorative and wear resistance qualities are important, such as household and commercial cookware and architectural finishes. In addition to improving properties such as wear rate, a pigment filled (usually inorganic) coating has the advantage of completely covering the substrate at a fairly low coating thickness (10
15 μm). These coatings do not adhere directly to as-is metallic substrates and require chemical priming. Typical primers are mixtures of phosphoric and chromic acids. Surface preparation prior to application of a pigmented coating consists of degreasing and sandblasting. Degreasing can be done chemically or by baking the article at a sufficiently high temperature to degrade the organic contaminants. After removal of grease, the article must be protected from handling and contamination to assure good adhesion bonding. Sandblasting is effective when a roughness height of 5
10 μm has been achieved [14]. Filled PTFE dispersions are usually mixed with a priming agent before application. It is usually applied to the article by spray coating techniques. The choice of equipment material should be made to accommodate the highly corrosive priming agent. After spraying, the article is dried and sintered in three steps. Drying or water removal is carried out at 90°C
100°C, rather slowly. In the second step, the article is heated to 250° C
300°C and held to remove the surfactant. Finally, the part is sintered at .380°C which completes the process.

9.10 Polytetrafluoroethylene Yarn Manufacturing This is a process designed to produce yarns of PTFE by overcoming nonmelt processibility of this

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polymer. Fibers of PTFE are used as staple, flock, or multifilament yarn in the fabrication of bearings, filter bags, and packings for valves, agitators, and pumps. Manufacturing PTFE yarn takes advantage of the processes for conversion of cellulosic material, mainly wood pulp, into fiber. The basic technology has been described in US Patent 2,772,444 by Burrows and Jordan [15]. In this process, wood pulp is treated with a basic solution which converts the hydroxyl groups of cellulose to salt. The treated cellulose is blended with carbon disulfide, which converts the alkoxy salt groups to a thiocarboxy group called xanthate. The CS2 converted material is a viscous dope, which is thoroughly mixed, after further processing including filtration, with PTFE dispersion. The mixture of xanthate and PTFE is spun into an acid bath. The acid converts the xanthate to carbon disulfide and cellulose. Carbon disulfide is recovered and recycled. The PTFE fiber is rinsed with water to remove the acid and other accompanying impurities. The fiber is then sintered and drawn to increase its tensile strength. Typical values of the ultimate strength of fiber (280
350 MPa) are about 10 times higher than the strength of molded powders. A specific example of PTFE yarn spinning process is described here [15]. A viscose solution containing 7% cellulose and 6% sodium hydroxide was prepared by using 30 wt.% of carbon disulfide. The viscose solution was filtered and allowed to age before the addition of a 60% PTFE dispersion containing 10% surfactant. The spinning dope contained 40% PTFE and 2.3% cellulose. This mixture was filtered and spun at 20°C. Each spinneret had 60 holes of 125 μm diameter and spun at the rate of 18 m/min. The filaments are entered into an aqueous bath containing 10% sulfuric acid, 16% sodium sulfate, and 10% zinc sulfate. After traveling for over 1 m in the coagulating acid bath, the PTFE yarn is entered into a warm water (79°C) bath. The filament bundles are dried by taking a dozen wraps over a heated roll at 190°C. At this point, the strength of the yarn is about 0.04 g per denier. The cellulose is removed from the fiber during the sintering PTFE using a heated roll at 389°C. The outgoing tension was raised to 0.075 g per denier resulting in seven times stretch over its original length. The resulting yarn had 60 filaments and 375 denier.

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The fiber produced by this process has a black color due to the residual carbon left from the degradation of cellulose during the sintering. A bleaching process is necessary to obtain a white yarn, which is done by heating the yarn at a temperature exceeding 300°C, for a period of 5 days. The drawback of thermal bleaching is that half of the tenacity of the yarn is lost. Chemical bleaching is done by immersing the yarn in boiling sulfuric acid and adding a small amount of nitric acid. The disadvantage of this process is generation of acid waste and its disposal. A number of other sources describe alternative techniques and refinement ideas for spinning PTFE yarns [16
19].

9.11 Film Casting Film casting is a process in which a continuous PTFE film is obtained by coating the dispersion on a carrier web followed by sintering. The cast product is used after it has been stripped, or as a composite with the web. A typical application is wire insulation with the composite of PTFE and Kapton polyimide carrier primarily for aerospace applications [20]. Stripped PTFE films may be laminated to different substrates such as glass fabric. This process is also capable of producing films from thermoplastic fluoropolymers. A cast process has been described in US Patent 2,852,811 [21,22] that involves coating a layer of PTFE dispersion on a metal or polyimide carrier, followed by drying, baking, and sintering the coating. The low CCT of PTFE dispersion limits the thickness that can be applied in a single pass. Multiple coating passes are made until the desired thickness is obtained. The PTFE film is stripped and removed from the carrier in the final step. The nature of the carrier belt is very important. Highly polished corrosion-resistant metal and polyimide belts are used in casting processes [23]. PTFE cast films have highly desirable properties such as an absence of anisotropy (dependence of properties on direction) in their mechanical properties (see Tables 9.7 and 9.8). The drawback of the casting process is its poor economics, which has prevented the widespread use and acceptance of this manufacturing technique. The unfavorable casting economics are caused by the heavy carrier belts, which require special tracking mechanisms. The width of the belt is constant which limits the flexibility of the width of the cast film.

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Table 9.7 Typical Properties of Cast PTFE Films [24]. Properties

Test Method

Value

Tensile strength (MPa)

ASTM D882

29.6

Break elongation (%)

ASTM D882

400

Elastic modulus (MPa)

ASTM D882

413.6

NA

2188 to 260

Dielectric constant, 6
10 range

ASTM D150

2.0

Dissipation factor

ASTM D150

, 0.0001

Surface resistivity (Ω/square)

ASTM D257

9 3 1017

Volume resistivity (Ω cm)

ASTM D257

.1015

Dielectric strength (kV/mm)

ASTM D149

165

Surface arcing resistance

ASTM D495

Does not arc

Continuous use temperature (°C) 8

Table 9.8 A Comparison of Cast and Skived Film Properties [25]. Film Type

Thickness (µm)

Tensile Strength (MPa)

Break Elongation (%)

Elastic Modulus (MPa)

MDa

TDb

MD

TD

MD

TD

Skived

76

52.3

40.4

450

360

469

517

Cast

68

35.8

34.5

530

510

434.5

434.5

a

MD, machine direction. TD, transverse direction.

b

The film casting process has the advantage of allowing composite constructions. For example, PTFE films can be further coated with dispersions of other fluoropolymers such as fluorinated ethylene propylene (FEP) and perfluoroalkoxy (PFA) which are both thermoplastics. Alloys of fluoropolymers containing elastomers or fillers can be cast into the film. Another process [22] involves dip-coating a metal or polyimide carrier with PTFE dispersion, which covers both sides of the belt and doubles the productivity (Fig. 9.14). After dipping in the dispersion, the belt is passed through a metering zone where metering bars remove the excess dispersion from the belt. The belt enters a drying oven for water removal followed by sintering zones to consolidate the PTFE and to form a film. Good thermal conductivity of the metal belt shortens the sintering time, thus maximizing tensile strength and elongation. The film layers are then peeled away from the web.

Multilayer films are manufactured with good peel adhesion properties [26,27]. For example, polyimide films may be coated with a “tie layer” of FEP or PFA before being coated with PTFE dispersion. Other polymers such as ethylene tetrafluoroethylene and polyvinylidene fluoride are also used as the tie layer. These films have good scrape abrasion resistance and are used for insulation of wires for aerospace and similar critical applications.

9.12 Antidrip Applications PTFE is added to other thermoplastic polymers to improve fire performance by suppressing dripping, for a period of time. Fibrillation of PTFE and formation of a structure that retains high viscosity in the molten state, thus preventing dripping of the molten host polymer, have been credited for performance enhancement. Burn time is usually reduced by incorporation of flame-retardants (Table 9.9).

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Both fine powder and dispersion grade PTFE grades are effective for drip suppression. Small quantities (,1 wt.%) of PTFE are added to the host polymer, which must be distributed uniformly for maximum effectiveness. Dispersion is easily added to the host material uniformly. The mixture is dried to remove water, sometimes under vacuum, prior to extrusion compounding, usually in a twin-screw extruder. Examples of common polymers for this application include polycarbonate and polyethylene terphthalate, polybutylene terephthalate (PBT), and acrylonitrile butadiene styrene (ABS).

Figure 9.14 Schematic of the film casting process [22].

165

High molecular weight PTFE is preferred because the fibrillation enhancement brought about by an increase in molecular weight improves drip suppression. Compounding difficulty increases with molecular weight due to premature fibrillation of PTFE during the mixing process. Dispersions of PTFE are easier to uniformly blend with host polymers.

9.13 Filled Bearings A special process has been used for decades to produce filled bearings by coagulation of PTFE in the presence of fillers. The main goal is to reduce wear and cold flow while taking advantage of low friction coefficient of PTFE, without a lubricant. The original process (known as DU Bearing Process) was developed by Glacier Metal Company known as self-lubricating, metal-polymer bearings [28]. This technology is widely practiced today to fabricate filled compositions. This process is also known as co-coagulation. PTFE dispersion is mixed with the filler, and a salt such as aluminum nitrate is added to convert the surfactant to an insoluble species leading to coagulation of PTFE particles. The viscosity of the resulting paste (“mush” in the industry parlance) is adjusted by the addition of an organic solvent such as toluene. This paste is calendared onto the surface of a steel-backed porous bronze strip. The strip is manufactured by sintering bronze powder onto the steel strip. After sintering, the steel strip is rolled into bearings such that the filled PTFE would be its inside surface (Fig. 9.15). Automotive applications are the main end use of these bearings, for example, in shock absorbers. The initial filler of choice was lead but lead-free formulations have been developed over the years. Graphite, bronze, and zinc powders are used to fill PTFE for DU Bearings.

Table 9.9 Effect of PTFE on Drip-Suppression of Polycarbonate (PC).a

a

TFE Content (wt. %)

MFR of PC (g/ 10 min)

Time to Flameout (s)

Number of Drips

UL-94 Rating

0

15

10.3

5/5

V-2

0.3

15

1.2

0/5

V-0

0.3

20

1.0

0/5

V-0

Contained tetrabromobisphenol-A oligomer as flame-retardants.

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9.15 Other Applications

Figure 9.15 Microsection of a DU Bearing.

9.14 Dedusting Powders One of the unusual applications of PTFE dispersion has been dedusting powders. The objective is to prevent dust formation when handling powdery products such as clay from becoming airborne. Although powder particles are not coated by PTFE. Most of the dust is trapped in the webs created by microscopic fibrils that are generated by shearing of the PTFE particles. The rest of the particles adhere to the web through contact. A popular commercial process for dedusting powders is the Harshaw Process for Dustless Powders [29]. The dedusting process works with as little as 0.005 wt.% of PTFE in the powder but a concentration of 0.1% is common. This is equivalent to 1 kg of PTFE in one metric ton of the powder. It is necessary to work the PTFE to initiate fibrillation by working (shearing) the mixture of PTFE and the powder in a blender or a slow turning mixer (like a cement mixer) at an elevated temperature. Room temperature is not effective and a shearing temperature around 100°C is needed. The time required to complete the shearing is dependent on the concentration of PTFE in the mixture and varies between a few seconds to a few minutes. Dedusting cat litter used to be a popular application for PTFE dispersions until detrimental effects of residual polymerization surfactant (APFO) in the dispersion were discovered. Fluoropolymer manufacturers discontinued supplying PTFE dispersion to dedusting applications. After replacement of perfluoroammonium octanoate with safer alternatives, manufacturers have resumed supply of dispersion for dedusting applications [30].

There are many other applications for PTFE dispersions. In automobile gaskets, a thick coating of dispersion is applied to the metal surface. The gasket is heated to remove the surfactant and water but not sintered. The resin undergoes cold flow under load when the gasket is tightened. This is helpful to insure a complete seal in addition to the other beneficial attributes of PTFE. Another interesting and unusual application is the fabrication of insect traps. Surfaces of substrates are coated with PTFE to prevent crawling insects from climbing on these surfaces. Low friction creates a slippery surface, which prevents the insects from leaving the trap.

References [1] McKeen LW. Fluorinated coatings and finishes. 2nd ed Plastics Design Library, Elsevier; 2015. [2] FLUON® polytetrafluoroethylene. Impregnation with PTFE aqueous dispersions. Technical Service Note F6. Asahi Glass Corp.; September 2002. [3] Cohen ED, Gutoff EB. Coating processes. KirkOthmer Encylopedia of Chemical Technology, vol. 6. John Wiley & Sons; 1993. p. 606
35. [4] Satas D, Tracton AA, editors. Coating technology handbook. 2nd edition New York: CRC Press; 2011. [5] Satas D, editor. Web processing and converting technology and equipment. New York: Van Nostrand Reinhold; 1984. [6] Le´cuyer HA, Mmbaga JP, Hayes RE, Bertrand FH, Tanguy PA. Modelling of forward roll coating flows with a deformable roll: application to non-Newtonian industrial coating formulations. Comp Chem Eng 2009;33:1427
37. [7] Bird RB, Stewart WE, Lightfoot EN. Transport phenomena. New York: John Wiley & Sons; 1960. [8] Schlichting H. Boundary layer theory, translated by J Kestin. New York: McGraw-Hill Series in Mechanical Engineering; 1968. [9] Patton TC. Paint flow and pigment dispersion. 2nd ed. New York: John Wiley & Sons; 1979. [10] Dealy JM, Wissbrun KF. Melt rheology and its role in plastics processing: theory and applications. New York: Van Nostrand Reinhold; 1990.

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[11] Paints and coatings, World of rheology, ,www. world-of-rheology.com.; December 2018. [12] Ebnesajjad S, Ebnesajjad CF. Surface treatment of materials for adhesion. 2nd ed Elsevier; 2014. [13] Mckeen LW. Fluorinated coatings and finishes handbook. 2nd ed Elsevier; 2016. [14] Hoechst Plastics—Hostaflon®. Frankfurt, Germany: Hoechst Aktiengesellschaft; 1984. [15] Burrows LA, Jordan WE. US Patent 2,772,444, assigned to DuPont; December 4, 1956. [16] Boyer C. US Patent 3,147,323, assigned to DuPont; September 1, 1964. [17] Steuber W. US Patent 3,242,120, assigned to DuPont; March 22, 1966. [18] Kitagawa H, Kinoshita S, Uchiyama H. US Patent 3,397,944, assigned to Tokyo Rayon Kabushiki Kaisha; August 20, 1968. [19] Gallup AR. US Patent 3,655,853, assigned to DuPont; April 11, 1972. [20] Sahatjian RA, Ribbins RC, Steckel MG. US Patent 4,943,473, assigned to Chemical Fabrics Corporation; September 24, 1990. [21] Petriello JV. US Patent 2,852,811; September 23, 1958. [22] Effenberger JA, Koerber KG, Latorrs MN, Petriello JV. US Patent 4,883,716, assigned to Chemical Fabrics Corporation; November 28, 1989.

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[23] Spohn PD, Keese FM, Sinofsky MW. US Patent 7338574, assigned to Saint-Gobain Performance Plastics Corporation; March 4, 2008. [24] Saint-Gobain Performance Plastics Corp., ,www.ChemFab.com.; 2017. [25] Effenberger JA, Koerber KG, Latorrs MN, Petriello JV. US Patent 5,075,065, assigned to Chemical Fabrics Corporation; December 24, 1991. [26] Effenberger JA, Koerber KG. US Patent 5,106,673, assigned to Chemical Fabrics Corporation; April 21, 1992. [27] Effenberger JA, Koerber KG, Lupton EC. US Patent 5,238,748, assigned to Chemical Fabrics Corporation; August 24, 1993. [28] Tait WH, US Patent 2,689,380, assigned to Glacier Metal Co; September 21, 1954. [29] Owens JE, Vogt JW, US Patent 3,974,089, assigned to Harshaw Chemical Company, August 10, 1976; Owens JE, Vogt JW, US Patent 3,838,064, assigned to Kewanee Oil Company, September 24, 1974; and inventors and assignees Ogura M, Chiba S, Urano T, Miyaji H, Shimoda T, Gocho T, US Patent 5,788,879; August 4, 1998. [30] Fluon® PTFE environementally friendly products, product Information. Asahi Glass Corp., ,www.agcce.com.; 2011.

10 Manufacturing Melt-Processible Copolymers of Tetrafluoroethylene Sina Ebnesajjad FluoroConsultants Group, LLC, United States

O U T L I N E 10.1 Introduction

169

10.2 Molecular and Crystalline Structure

169

10.3 Preparation of Perfluoroalkoxy Polymers 10.3.1 Nonaqueous Polymerization of Perfluoroalkoxy Polymers 10.3.2 Aqueous Polymerization of Perfluoroalkoxy Polymers

171 171

10.4 Preparation of Perfluorinated Ethylene Propylene Polymers 10.4.1 End Group Stabilization

177 178

10.5 Preparation of Ethylene Tetrafluoroethylene Polymers

180

References

183

174

10.1 Introduction This chapter discusses some of the important copolymers of tetrafluoroethylene (TFE) including perfluoroalkoxy (PFA), perfluorinated ethylene propylene (FEP), and ethylene tetrafluoroethylene (ETFE) copolymer. A separate chapter in this book covers homopolymers and copolymers of vinylidene fluoride. A more complete explanation of thermoplastic fluorinated copolymers can be found elsewhere [1]. The rationale of copolymerizing other monomers with TFE is to reduce crystallization of lower MW TFE copolymers. In contrast to polyolefins such as polyethylene, polytetrafluoroethylene cannot be branched as a homopolymer. Linear low-density polyethylene is produced by a low-pressure polymerization reaction similar to resulting in branching. There are relatively few branches though they are long enough to inhibit close packing of the polyethylene molecules thus reduce crystallinity. Similar results can be achieved by incorporating a comonomer in polyethylene during polymerization, for example, C4, C6, and C8 [2]. Comonomers polymerized with TFE play roughly the same role as the branches in linear low-density polyethylene. Some of the common comonomers

used in the industry include hexafluoropropylene (HFP), perfluoromethyl vinyl ether (PMVE), perfluoroethyl vinyl ether (PEVE), perfluoropropyl vinyl ether (PPVE), perfluorobutyl ethylene (PFBE), CTFE, ethylene (Et), and others. Every one of those comonomers has a pendent group with different length.

10.2 Molecular and Crystalline Structure Most of comonomers randomly copolymerize with TFE. A key question is how do these comonomers impact the crystalline structure of the copolymers? Flory model stated random copolymers can potentially crystallize in two extreme ways (Fig. 10.1) [3]. It can form a two-phase system in which the crystalline phase is composed entirely of homogeneous major monomer units in equilibrium with a mixture of amorphous phase of major monomer and noncrystallizable comonomer (minor) units. This is called the exclusion model. Alternatively, the copolymer may form a two-phase system in which the crystalline phase is a solid solution of the major monomer and comonomer units. The comonomer units produce defects in the

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00010-0 © 2021 Elsevier Inc. All rights reserved.

169

170

Figure 10.1 Depiction of (A) exclusion and (B) inclusion models; lines depict the major monomer units and red dots depict the comonomer unit [3].

Figure 10.2 Typical comonomers used in commercial fluoropolymers.

crystalline lattice of the major monomer units. Both crystalline and amorphous phases have the same composition, thus the inclusion model. Real copolymer crystals may exhibit a morphology intermediate to the two extremes in Fig. 10.1. TFE copolymers exhibit inclusion/exclusion crystallization behaviors depending on the size of the pendent groups. For example, HFP, PMVE, and PEVE are less effective in the reduction of recrystallization of TFE copolymers than PPVE and perfluorobutyl vinyl ether. An interesting comonomer is PFBE used to modify the crystallinity of Et and tetraethylene alternating copolymer (Fig. 10.2). The effect and the way the comonomers alter the transition temperature and crystalline structures of copolymers of TFE, VDF, and other monomers have been investigated. The results are briefly reviewed in the rest of this section. Most of the descriptions pertain to the comonomers incorporated in commercial fluoropolymers. The beginning of research on the transition temperatures and crystallization of TFE copolymers dates back to the

INTRODUCTION

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FLUOROPOLYMERS

Figure 10.3 Depiction of lamellar structure (lattice) of tetrafluoroethylene/hexafluoropropylene copolymers; lines: CF2 CF2 ; o: CF3 groups; and molecular end group’[6].

commercial introduction of FEP copolymer. Work by Eby and Wilson [4] illustrated electron microscopy and small angle X-ray diffraction of the copolymers of TFE/HFP to be lamellar with a typical thickness of 20 50 nm. The agreement between the two methods was quite good as seen in Table 8.1 [5]. These dimensions together with the concentration of perfluoromethyl groups indicate the latter are within the lamellae. This idea agrees with the increase of lattice parameter with increasing perfluoromethyl concentration. The perfluoromethyl groups ( CF3) enter the lattice as small, localized, point defects and the regularity of the lattice is preserved except in the immediate area of the defect (Fig. 10.3). The new concept also implies molecular ends may fold within the lattice as defects. For more information, refer [1]. Random copolymers of TFE with minor amounts of fluorinated comonomers (HFP, PMVE, and PPVE) of different compositions are characterized mainly by X-ray diffraction but also by differential scanning calorimetry (DSC) and density measurements in an attempt to establish the possible inclusion of the respective side groups into the crystallites. Structural characterizations are conducted at room temperature (where all the considered copolymer samples are in the less ordered crystalline form I) as well as at #40°C (where all the considered copolymer samples are in the more ordered crystalline form II). Et and TFE form alternating copolymers with useful properties near the molar ratio of 1:1 of the monomers (Fig. 10.4). Molecular conformation of

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171

Figure 10.4 Depiction of ethylene tetrafluoroethylene molecule as obtained by fitting electrostatic potential from the Hartree Fock method [7].

Figure 10.5 Ethylene tetrafluoroethylene has extended zigzag conformation with orthorhombic packing.

ETFE is the extended zigzag. Its molecular packing appears to be orthorhombic where each molecule has four nearest neighbors with the CH2 groups of one chain adjacent to the CF2 groups of the next (Fig. 10.5). A study [8] of miscibility, crystallization behavior, and mechanical properties of blends of PFA, EFA (TFE/PEVE) showed the impact of comonomer selection on processing and properties of parts made from the blends. Melt-processible PFA and EFA (PEVE comonomer: 5.7 wt.%) blends cocrystallize regardless of crystallization speed and blending method. Another study of miscibility and crystallization was reported in 2004 [9]. Two binary blends of PTFE with poly(tetrafluoroethylene-co-10 mol.% hexafluoropropylene) (FEP) and poly(tetrafluoro ethylene-co-2 mol.% perfluoropropyl vinyl ether) (PFA) were studied using differential scanning calorimeter and X-ray diffractometry. In FEP/PTFE blends, FEP and PTFE components were separately crystallized to form their own crystallites. In PFA/ PTFE blends, PFA and PTFE components were cocrystallized to form PTFE-type crystallites incorporating TFE segments of PFA and PTFE.

low-friction properties as PTFE. PFA polymers are prepared by copolymerization of perfluoroalkyl vinyl ethers (PAVEs) (Rf O CF 5 CF2), where Rf is a perfluorinated alkyl group with TFE. Examples of commercially utilized ethers include PMVE (CF3 O CF 5 CF2), PEVE (C2F5 O CF 5 CF2), PPVE (C3F7 O CF 5 CF2), and perfluorobutyl vinyl ether (C4F9 5 O CF 5 CF2). Several percent of ether is usually incorporated in a copolymer. The polymerization of TFE and PAVEs may be learnt by reviewing the technology disclosed in selected patents. Copolymerization of PAVEs with TFE can be accomplished in a halogenated solvent [10], in an aqueous phase [11] that sometimes may contain some halogenated solvent, usually in the absence of a surfactant [1]. An important issue is the stability of the polymer end groups, which depends on its chemical structure. Chemistry of the polymerization and the polymerization conditions determine the nature of the end groups. Unstable end groups degrade and usually produce gases during polymer storage or part fabrication. These gases form bubbles in the part considered defects.

10.3 Preparation of Perfluoroalkoxy Polymers

10.3.1 Nonaqueous Polymerization of Perfluoroalkoxy Polymers

PFA polymer or PFA is one of the most important melt-processible fluoroplastics due to its relative ease of processing and high service temperature equivalent to PTFE (260°C). It also has the same excellent chemical resistance and

In 1960, Bro [10] reported TFE could be polymerized with other fluorinated monomers in halogenated solvent. TFE reacts with organic solvents containing hydrogen, chlorine, and bromine, or unsaturated carbon carbon bonds. A low-MW

172

INTRODUCTION

waxy or brittle solid would form due to the chain transfer effect of these solvents when used as the polymerization medium. Bro reasoned the free radical group (CF2
) withdrew a hydrogen, chlorine, or bromine atom from the solvent leading to chain termination. The only noninterfering solvents were found to be saturated perfluorinated compounds. A major disadvantage of these solvents is they are very expensive. One of the earliest reports of copolymerization of PAVEs and TFE in nonaqueous perfluorinated phase was made by Harris and McCane [12]. They reported on copolymerization of TFE and PAVEs in a perfluorinated solvent perfluorodimethylcyclobutane. The successful PAVE (Rf O CF 5 CF2) in this study usually contained 1 5 carbons in its alkyl (Rf) group. Peroxides and azo compounds were the preferred initiator of the polymerization. In 1970, Carlson [13] reported copolymerization of TFE and PAVEs in halogenated solvents containing hydrogen (one per carbon atom), chlorine, and fluorine. Suitable solvents had to be in the liquid form at polymerization conditions. They included CCl2F2 (Freon 12), CCl3F (Freon 11), CClF2H (Freon 22), CCl2FCCl2F (Freon 112), CCl2FCClF2

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(Freon 113), and CClF2CClF2 (Freon 114); Freon 113 (CCl2FCClF2) was discovered to be the preferred solvent. (All CFCs including these solvents were banned by the Montreal Protocol in 2010 because of their global warming potential.) A lowtemperature initiator soluble in the monomer solvent solution had to be used at temperatures below 85°C, above which the solvent acts as a telomerizing agent. The reaction was conducted in a stirred autoclave at constant pressure maintained by continuous charging of TFE and comonomer to the vessel. B is (perfluoropropionyl) peroxide (3P) was the initiator of choice, which was added in the form of a 1.5 wt.% solution in cyclohexafluoropropene. The polymerization took place in the suspension mode due to the absence of a surfactant. High rates of polymerization were achieved. The product was a tough polymer that could be fabricated into a transparent colorless film by compression molding. Table 10.1 shows examples of combinations of reaction conditions and the resulting polymer properties. It can be concluded the amount of initiator has

Table 10.1 Nonaqueous Polymerization of Perfluoropropyl Vinyl Ether (PPVE)/Tetrafluoroethylene in Solvents [13]. Reaction Examples Case 1

Case 2

Case 3

Case 4

Case 5

Case 6

Reaction ingredients and conditions Solvent type

Freon 113

Freon 113

Freon 113

Freon 113

Freon 113

Freon 113

Solvent, mL

800

800

900

860

860

860

PPVE, g

60

60

59.4

16.5

9

28

Polymerization temperature, °C

40

40

50

90

50

50

Polymerization pressure, kPa

345

345

511

621

173

483

Polymerization time, min

43

42

30

20

16

45

0.30

0.64

0.15

Initial 5 0.06 g 1 0.006 g/min

0.65

0.025

8

8.9

5.2

2.9

2.7

2.1

45.5

2.7

211

2.6

17.9

61.7

Peroxide, g Polymer properties PPVE in polymer, wt.% 4

Melt viscosity, 10 P

Freon 113 5 1,1,2 trichloro, 1,2,2 trifluoroethane, CCI2FCCIF2.

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OF

a strong influence on the melt viscosity, hence the MW, of the polymer [13]. In 2003, Funaki and Sumi [15] reported a process to produce stable end group PFA resins. They described method for producing a TFE/PAVE-type copolymer (PFA), which required polymerization in a medium (such as CHFClCF2 CF2 Cl) in the presence of a chain transfer agent (CTA) of a C1 or C2 hydrofluorocarbon (such as CH3 CF2H) by means of a polymerization initiator such as (CF3 CF2CF2 COO)2. PFA obtained by this method has a small amount of unstable terminal groups contained in its molecule but it is excellent in thermal stability and cracking resistance against liquid reagents. The amount of extractable fluorine ions is also relatively low even when the PFA is used for components such equipment components for the production of semiconductors.

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Studies by Iwasaki and Kino [16] strived to overcome problems of polymerizing so as to generate a more uniform copolymer with a narrower MW distribution for improved flex life. They found out copolymerization of TFE and PAVE in the presence of a terpene in an aqueous polymerization medium produced a melt-fabricable TFE/PAVE copolymer with a well-distributed PAVE. The small amount of terpene added to the polymerization system did not decrease the rate of polymerization but was effective in improving the uniformity of the resin by narrowing the MW distribution. That narrower MW distribution was characterized by a half-width value in its differential scanning calorimeter (DSC) melting peak which was at least 10% less than the half-width value of the copolymer without the presence of the terpene. Table 10.2 summarizes the physical properties of

Table 10.2 Polymerization of Tetrafluoroethylene and Perfluoropropyl Vinyl Ether and Properties of Perfluoroalkoxy Polymer [16]. Concentration of Limonene in Polymerization Kettle, ppm At initiation of polymerization

Case 1

Case 2

Case 3

10.41

32.05

0

At completion of polymerization

4.01

13.17

0

Initiator (APS) (total amount), g

0.356

0.356

0.356

Initial charge of APS, g

0.16

0.16

0.16

APS added later, g

0.196

0.196

0.196

Total PPVE used, g

82.5

Limonene in PPVE, ppm

52.8

Total TFE used, g Ethane, liters at 0°C, 101 kPa Raw dispersion weight, g

888.7 0.77 3341

82.5 1627 829.5 0.77 3283

82.5 0 829.5 0.77 3272

PFA properties MFR, g/10 min

5.8

2.2

7.4

PPVE content, wt.%

5.4

5.5

5.6

DSC half-width value, °C

7.2

5.6

8.9

MWDI

1.22

1.16

1.3

1.6

2.4

1

Flex life ratio

a

a Flex life ratio 5 Flex life ratio is calculated using Eq (10.2). By thus holding the MFR and PPVE content constant, the effect of MWD as reflected in MWDI is seen. ln[FL] 5 11.208 2 1.695
ln[MFR] 2 7.846
ln[MWDI] 1 3.648
ln[PAVE] (10.1) ln[FL] 5 14.70 2 7.846
ln[MWDI] (10.2) Flex life (number of cycles to failure) is [FL]; PAVE content (wt.%) is [PAVE]; MFR (g/10 min) is [MFR]; and molecular weight distribution is represented by the MW distribution index [MWDI], which is defined as MWDI 5 MV10/MV5. MWDI, Molecular weight distribution index; MFR, melt flow rate; DSC, differential scanning calorimetry.

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INTRODUCTION

the PFA resin obtained from the dispersion. The value of the half-width (ΔT1/2) in °C of the first main peak in a melting curve is obtained in a DSC measurement (10°C/min), as seen in Fig. 10.6. The measurement is made on polymer that has not been previously melted, that is, it is a “first heat.” The half-width is the width of the peak at the midpoint between the base and the vertex of the peak. Another representation of MW distribution is MWDI as shown in Table 10.2. The shear rate dependence of MFR is a function of MW distribution [18]. The ratio of MFR determined with different weights is therefore an index of MW distribution. The same melt indexer used for MFR is employed; a 5 g sample is filled into a cylinder, id 9.53 mm, held at 372°C 6 1°C for at least 5 minutes, and extruded through an orifice, 2.1 mm id and 8 mm long, under a 5 kg load (piston plus weight), thereby measuring the viscosity of the resin from the amount of the extrudate per unit time, to be reported as an MV5. Next, a 5 g sample

Figure 10.6 ΔT1/2 (°C) is the full width at half maximum of the first main peak in a melting curve (endothermic) [17].

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FLUOROPOLYMERS

is held at least 5 minutes after having been charged, followed by applying an 833 g load to extrude it through an orifice, 2.1 mm in diameter and 8 mm long, and from the amount of the extrudate per unit time the resin viscosity is obtained, to be reported as MV10. The MW distribution index of a resin is obtained from the ratio between MV5 and MV10. Both PEVE and PPVE are effective in improving the high-temperature mechanical properties of TFE/ HFP polymers. PMVE does not produce such improvements due to the small size of its pendent ether group that cannot efficiently disrupt the crystallization of the polymer chain. The strength of the effectiveness of PPVE is evident even in a polymer of TFE containing another monomer like HFP (see Table 10.3).

10.3.2 Aqueous Polymerization of Perfluoroalkoxy Polymers Solvent polymerization medium has always been objectionable because of safety, environmental emission, purification and recycling problems, and the cost of solvent and processing it. An aqueous medium for copolymerization has always been preferred over solvent. That is, as long the desired polymer properties could be obtained at a reasonable cost. Gresham and Vogelpohl reported the first major development of aqueous polymerization of PFA [20] in which they replaced most of the solvent with water. The first PFA they prepared had a wide MW distribution. Parts made from this type of PFA by extrusion through small dies (orifices) exhibit large swelling during processing. When the copolymer is reheated to near melting point it exhibits high shrinkage. Polymers with high die swelling have a strongly shear stress-dependent viscosity due to the broadness of the MW. Of two polymers with the same melt viscosity, swelling is higher for the polymer with the broader MW.

Table 10.3 Effect of Perfluoroalkyl Vinyl Ether on the Properties of Terpolymers of Tetrafluoroethylene [19]. Polymer Properties Reaction Examples

HFP Content, wt.%

Ether Type

Ether Content, wt.%

Melt Viscosity, 104 P

Melting Point, °C

MIT Flex Life (ASTM D2176)

Case 1

6.3

PPVE

0.8

8.1

294

8500

Case 2

4.5

PEVE

1.2

6.4

291

12,400

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Gresham and Vogelpohl [20] copolymerized TFE and PAVE in a medium consisting of water and only a minor amount of fluorocarbon solvent. Polymer properties were found to be poorer when polymerization took place in purely aqueous medium. The solvent was shown to also increase the polymerization rate. Their method narrowed the range of the MW distribution by conducting the polymerization in the presence of a gaseous CTA such as methane, ethane, and hydrogen. Table 10.4 provides a summary of polymerization conditions and the polymer properties for a number of combinations of variables. Case 1 illustrates the need for a CTA to lower MW. Methanol, hydrogen, and methane, all transfer the chains effectively. The drop in MW can be observed from the reduction in the melt viscosity compared to Case 1, where no CTA was added to the recipe. Methanol is the most convenient practical choice among the three because it can be added as a liquid. It is also a highly potent CTA since the addition of 6 mL to the polymerization medium brought down the melt viscosity from about 20,000,000 to 60,000 P, or nearly three orders of magnitude. The initiator type and the presence/absence of a surfactant determine the phase in which polymerization takes place when a mixture of aqueous and solvent media is present. For example, APS (initiator) is water soluble, therefore indicating that polymerization occurs in the aqueous phase in Table 10.4 systems. In another example [21], TFE and PEVE were polymerized by suspension polymerization without a surfactant. The reaction medium consisted of water and perfluoro-(2-butyltetrahydrofuran) and the CTA was methanol. The initiator was a bis-perfluorobutyryl peroxide as a solution in 1,1,2-trichloro-1,2,2-trifluoroethane (F113) which caused the polymerization to take place in the organic phase of the medium. Hartwimmer and Kuhls [22] reported synthesis of nonelastomeric terpolymers of TFE in aqueous medium. These polymers comprise PAVEs and HFP in addition to TFE. The authors reported increased PAVE, such as PPVE, incorporation due to the presence of HFP. Additional PAVE suppresses the melting point of the copolymer without a loss of mechanical properties. Table 10.5 and Fig. 10.7 illustrate the effect of HFP concentration in the monomer mixture on the

TETRAFLUOROETHYLENE

175

incorporation of PPVE. The incorporation of PPVE is at a low 1.1 wt.% without the addition of HFP. The extent of incorporation of PPVE increases slowly with the addition of HFP. It accelerates significantly beyond 4% HFP in the ternary monomer mixture up to a point (between 14.7% and 23.3% HFP) and then begins to decrease. The melting point decreases with HFP and begins to increase in this same range. True aqueous polymerization of PAVE and TFE, in the absence of any organic solvent, has been reported by Aten and coworkers [23,24]. TFE and PEVE were copolymerized in a medium consisting of water, water-soluble initiator, and a surfactant. The copolymer produced by this technology was melt processible and had high PEVE content, high melt viscosity, and good flex fatigue properties. Table 10.6 shows the effects of varying the PAVE feed rate on the copolymer properties. PEVE can be incorporated into the copolymer at varying concentrations to increase the flex fatigue life. Crystallinity of the copolymer decreased with increasing feed rate, as evidenced by the decline in the heat of fusion at higher PEVE incorporation levels. Tensile properties of the polymer reach a maximum in the range of 10% 14% PEVE. Table 10.7 shows the result of copolymerization at constant PEVE precharge (500 mL) and pumping rate (5 mL/min). The amount of initiator, APS, both precharged and pumped, was varied and no ammonium hydroxide was charged to the reactor. The copolymers of TFE and PEVE appear to retain excellent tensile properties at 150°C [23,24]. In 2007, Aten [25] reported on polymerization of TFE and PAVE using the suspension polymerization method. The technique produced a meltprocessible copolymer TFE and PAVE by suspension polymerization. The process was operated at elevated pressure in an agitated vessel in an aqueous medium, free radical initiator, and a telogen. There were no organic solvents in the polymerization medium. Intensive agitation was run in the reaction vessel so that 90% of the TFE/PAVE copolymer was coagulated, thus allowing the copolymer to be isolated directly from the reaction vessel. Melt viscosity of the copolymer was less than 1 3 106 Pa s. Ideally, the reaction medium was free of surfactants; the telogen was a nonfluorinated organic compound.

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Table 10.4 Aqueous Polymerization of Tetrafluoroethylene and Perfluoroalkyl Vinyl Ethers [20]. Reaction Ingredients and Conditions

Reaction Examples

Water, mL

Freon 113

Ammonium Persulfate, g

Ammonium Carbonate, g

Chain Transfer Agent Type

Chain Transfer Agent Amount, kPaa

Case 1

3440

6

Case 2

3440

300

6

15

Methanol

6 mL

Case 3

3440

200

6

15

Methanol

6 mL

Case 4

3440

300

6

15

Methanol

6 mL

Case 5

3440

300

6

15

Methanol

6 mL

Case 6

3440

300

24

15

Methanol

6 mL

Case 8

4200

300

6

15

H2

276

Case 9

4200

300

2

7.5

H2

690

Case 10

4200

300

10

10

H2

345

Case 11

4200

300

10

10

H2

690

Case 12

4200

300

6

15

CH4

345

Case 13

4200

300

6

15

CH4

Case 14

4200

300

5

15

CH4

173

Case 15

4200

150

5

10

CH4

345

Case 16

4200

150

6

10

CH4

345

Case 17

4200

300

6

15

CH4

290

Reaction Ingredients and Conditions Reaction Examples

a

Polymerization Pressure, kPa

Polymerization Time, min

Comonomer Type/ Amount, g

Polymer Melt Viscosity, 104 P

Case 1

1725

18

PPVE/75

2014

Case 2

1725

18

PPVE/75

6

Case 3

1725

14

PPVE/75

8

Case 4

1725

10

PPVE/75

17

Case 5

1725

9

PPVE/75

27

Case 6

1725

6

PPVE/100

96

Case 8

2000

44

PPVE/75

19

Case 9

2415

20

PPVE/75

74

Case 10

2070

11

PEVE/75

54

Case 11

2415

8

PEVE/75

72

Case 12

2070

45

PPVE/75

31

Case 13

1897

PPVE/75

65

Case 14

1897

47

PPVE/75

102

Case 15

2070

45

PEVE/75

62

Case 16

2070

24

PEVE/75

100

Case 17

2015

49

PPVE/75

7.6

Units are in kPa except for methanol which is expressed in milliliters.

PPVE, Perfluoropropyl vinyl ether; PEVE, perfluoroethyl vinyl ether.

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177

Table 10.5 Effect of Hexafluoropropylene on Perfluoropropyl Vinyl Ether (PPVE) Incorporation in Terpolymers of Tetrafluoroethylene [22]. Starting Monomer Mixture (mol.%)

Terpolymer Composition (wt.%)

HFP

PPVE

PPVE

HFP

Melting Point, °C

0

2.85

1.1

0

325

3.88

2.74

1.4

0.6

320

8.53

2.61

2.3

1.1

311

14.69

2.43

2.7

1.5

309

23.29

2.19

2.15

2.1

312

54.84

1.28

0.9

5.5

311

Figure 10.7 Effect of hexafluoropropylene in the monomer mixture on perfluoropropyl vinyl ether incorporation into the TFE terpolymer.

10.4 Preparation of Perfluorinated Ethylene Propylene Polymers FEP is a copolymer of TFE and HFP. It has the basic properties of PTFE such as chemical resistance, low friction, and good electrical properties. FEP was the first truly melt-processible perfluoropolymer developed in the 1950s and commercialized in 1960. Although HFP could be homopolymerized [26], its commercial applications are in copolymers. The earliest reported copolymerization with TFE was reported in 1952 [27]. In this case, a solution of trichloroacetyl peroxide in trichlorofluoromethane was loaded into a polymerization bomb at 30°C.

Most of the solvent was removed by evacuation of the bomb. The monomers, TFE and HFP, were charged to the bomb in the liquid form after they had been purified. The solids recovered were pressed into a transparent sheet at 300°C 350°C without significant decomposition during the fabrication. Bro and Sandt reported an important development in 1960 [28] in which FEP could be produced economically. The polymerization of TFE and HFP took place at 50°C 150°C C in the presence of an aqueous solution of an inorganic free radical initiator while maintaining a pressure of 2 7 MPa. Polymerization variables were varied with respect to the type and the concentration of initiator, rate of initiator injection, and polymerization pressure. A decrease in the initiator level and an increase in the polymerization pressure resulted in an increased MW of the FEP copolymer. In general, flex life increases with the MW. In 1964, Couture et al. [29] disclosed significant improvements to the polymerization technology of HFP and TFE reported by Bro et al. They addressed the issue of maximum polymer concentration in the dispersion polymerization, which must be kept below the level that would trigger premature coagulation. Productivity is limited due to the solids concentration limit. Adding a surfactant to the reaction mixture usually solves this problem. Temperature is a critical variable in free radical polymerization. Increased temperatures raise the reaction rate. Couture et al. found that, in TFE and HFP polymerization, increased temperature in the range of 95°C 138°C had a strong influence on the polymerization rate. Polymerization rate, polymer composition, and MW could be controlled by the use of a dispersing agent [29].

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Table 10.6 Copolymer Properties in Aqueous Polymerization of Tetrafluoroethylene and Perfluoroethyl Vinyl Ether (PEVE) [23,24]. Reaction Ingredients Reaction Examples

PEVE Precharged, mL

Polymerization Results PEVE Pumping Rate, mL/min

Solids, wt.%

Dispersion Particle Size, nma

Case 1

160

1.5

29.3

112

Case 2

450

4.0

26.3

127

Case 3

530

4.5

31.7

122

Case 4

800

6.5

32.5

107

Case 5

1050

8.0

32.5

92

Polymer Properties

Reaction Examples

PEVE Content, wt.%

Melting Point, °C

Heat of Fusion, J/gb

Melt Viscosity, 103 P

MIT Flex Life, 103 cyclesc

Tensile Strength/ Elongation, MPa %

Case 1

3.0

309

43

16.5

Case 2

9.4

280

27

8.9

440

30.6/190

Case 3

10.1

278

27

7.7

498

31.9/219

Case 4

14.1

265

14

5.4

43.8/263

Case 5

17.4

256

10

3.7

28.0/163

1 nm 5 1029 m. Determined by differential scanning calorimetry per ASTM D459I. c Measured by ASTM Method D2176. a b

A surfactant allows the noncoagulated polymer solids content to be increased in dispersion polymerization. Another advantage of surfactants is in the enhanced efficiency of the monomer utilization that allows a reduction in the HFP content of the monomer mixture without a loss in the HFP incorporation in order to reach the desired polymer composition. FEP as recovered from the coagulation of the reactor dispersion is called fluff or powder. It is treated to eliminate the reactive ends before extrusion and pelletization. In some applications such as rotomolding or rotolining, a free-flowing powder is preferable over pellets. Particle characteristics must be improved to facilitate the flow behavior of the granules. Buckmaster and Morgan [30] have reported a method for producing free-flowing and attritionresistant granules. The granules were produced from aqueously polymerized copolymers of TFE. A terpolymer of TFE, HFP, and PPVE was reported [31] to have superior stress crack resistance than TFE/HFP copolymer. The terpolymer in

this development was prepared by the nonaqueous polymerization process described in US Patent 3,528,954 and 4,029,868 [13,19]. In this procedure, a halogenated solvent, in which PPVE and a CTA had been dissolved, acted as the polymerization medium. Methanol was a common example of an effective CTA. Polymerization was carried out in a stainless steel pressure vessel. The polymer contained 0.2% 2% PPVE and 9% 17% HFP.

10.4.1 End Group Stabilization Instability of fluoropolymers detracts from their utility because of the high melt temperatures required for fabrication processing of these plastics. For example, injection molding and extrusion processes are operated at 300°C 400°C. A common occurrence in the fabrication of copolymers is the formation of gases and bubbles from unstable end groups. Those end groups in thermoplastic fluoropolymers are created by the polymerization process (initiator, transfer agent, solvent, contaminants, etc.)

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179

Table 10.7 Copolymer Properties in Aqueous Polymerization of Tetrafluoroethylene and Perfluoroethyl Vinyl Ether [23,24].

Reaction Examples

Reaction Ingredients

Polymerization Results

APS Precharged, mL

Solids, wt.%

APS Pumping Rate, mL/min

Dispersion Particle Size, nma

Case 1

200

5

30.8

105

Case 2

320

8

29.3

103

Case 3

600

15

37.8

99

Polymer Properties Tensile Strength/ Elongation PEVE Content, wt.%

Melting Point, °C

Heat of Fusion, J/gb

Melt Viscosity, 103 P

Case 1

9.1

279

30

21.7

Case 2

10.0

278

22

15.9

Case 3

36

283

29

3.0

Reaction Examples

MPa/ % at 25°C

MPa/ % at 150° C

28.4/ 254

22.2/ 41.6

. 2400

31.6/ 275

16.2/ 408

190

29.2/ 31.5

14.0/ 457

MIT Flex Life, 103 cyclesc

1 nm 5 1029 m. Determined by differential scanning calorimetry per ASTM D4591. c Measured by ASTM Method D2176. a b

or by the handling methods of the polymer (aging, heating, extrusion, chemical reactions, etc.). Those end groups can be removed by a finishing operation involving heat treatment, ammoniation, or fluorination of the polymer. Schreyer ascribed the principal cause of the instability of fluoropolymer chains, especially FEP, at melt-processing temperatures to the degradation of carboxylate end groups in 1963 [32]. These end groups are formed when the polymerization reaction is initiated by a peroxide initiator or the polymer chain is terminated through the formation of a vinyl bond subsequently oxidized. Initiation with a peroxide gives rise to the formation of an acyl fluoride group ( CFO) which is unstable and hydrolyzes in the presence of water. The resulting end group is a carboxylic acid ( COOH) group (carboxylate) and hydrofluoric acid. The carboxylate is degraded, at fabrication temperatures, into carbon dioxide and a vinyl end group, which can be oxidized to form a carboxylate group again or form a

bond to another chain. Melt viscosity of the polymer increases when chains are bonded together because of end group instability. The latter would lead to an increase in the melt viscosity of the fluoropolymer. The repetition of the degradation of the carboxylate end groups can lead to the buildup of carbon dioxide (CO2), carbonyl fluoride (COF2), and hydrofluoric acid (HF). In peroxide-initiated polymerization, at least one half of the polymer chains will contain carboxylate ( COOH) end groups. The number of these groups can be even greater, depending on the number of vinyl end groups that can be converted into carboxylate groups. It is important to stabilize the end groups of fluoropolymers so that they have sufficient thermal stability and do not produce volatile byproducts that could either generate bubbles or could promote corrosion. Schreyer reported successful achievement of this objective in 1963 [27]. He found that treatment of the polymer with water at elevated

180

INTRODUCTION

temperatures prevented the formation of carboxylic acid groups. The treatment with water (end capping) at elevated temperatures caused decarboxylation, when the carboxylic acid groups were in ionic form, accompanied by slow formation of very stable CF2H end groups. The addition of bases and neutral or basic salts to either the aqueous polymerization medium or the polymer itself enhances the rate of CF2H formation. Fluorination of perfluoropolymers at elevated temperatures is the ultimate solution to replace the unstable end groups with perfluorinated ones (see Table 10.8). In one example, fluorination was conducted in a modified double cone blender equipped with gas inlet and outlet and an electric heating mantle [33]. The fluorinator was rotated at 5 rpm. It was heated to the desired temperature then evacuated to remove the air. A mixture of fluorine and nitrogen (25/75% volume) was fed through the reactor for the desired length of time at the designated temperature. At the end of the fluorination process, the fluorine and heat were shut off. The fluorinator was purged with nitrogen after it had been evacuated to remove the residual fluorine. Cold air was pumped afterward to cool the batch before unloading the pellets. In 1988, Imbalzano and Kerbow [34] reported a technology applicable to PFA polymers to reduce significantly or eliminate all unstable end groups.

TO

FLUOROPOLYMERS

In this technique, PFA is treated with fluorine gas and the unstable end groups are converted to CF3. They devised a method for the measurement of residual fluorine ions, used to measure the effectiveness of fluorination.

10.5 Preparation of Ethylene Tetrafluoroethylene Polymers The most important interpolymers (e.g., copolymers and terpolymer) are TFE (ETFE) and CTFE (ECTFE). These two polymers are generally produced by suspension or dispersion polymerization methods. There is resemblance between the polymerization of ETFE and ECTFE. Indeed, some of the same patent art could be studied to learn about the technology. The properties of the ETFE polymers vary with composition [35]; copolymers containing 40% 90% TFE soften between 200°C and 300°C, depending on the composition. TFE segments of the molecules account for more than 75% of the weight of a 1:1 mole ratio copolymer. TFE and Et monomers readily combine into a nearly 1:1 alternating structure. Joyce and Sauer [36] have made one of the earliest reports of successful copolymerization of TFE

Table 10.8 End Group Analysis of FEP Polymers [33]. Number of End Groups, Per 106 C Atoms FEP Sample Example 1

Example 2

Example 3

Before Fluorination, Per 106 C Atoms

After Fluorination

COF

0

0

COOH (monomeric)

2

5

CF2 5 CF2

51

15

COOH (dimeric)

0

0

COF

24

2

COOH (monomeric)

51

7

CF2 5 CF2

226

12

COOH (dimeric)

19

0

COF

0

0

COOH (monomeric)

0

2

CF2 5 CF2

21

4

COOH (dimeric)

0

0

End Group

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and Et in which a high-MW polymer was obtained. Their work overcame a number of difficulties that had found no solution in the prior attempts to produce interpolymers of fluoroethylenes [37 40]. Joyce and Sauer copolymerized Et and TFE in a medium consisting of water and an organic solvent using a water-soluble salt of an inorganic peracid as the initiator. A number of objections reduced the commercial viability of the past polymerization techniques. Purely aqueous polymerization medium gave rise to polymers, which are not wetted by water and tend to form agglomerates that plug the valves and lines and massively adhere to process equipment surfaces. The use of water-miscible organic solvents along with organic peroxy initiators could solve the problem. The high pressures and temperatures required by these initiators, to obtain high polymer yields, detracted from the viability of such processes. A distinct disadvantage was the formation of explosive mixtures of the peroxy initiators and high concentrations of fluoroethylenes. Typical reaction conditions of the process proposed by Joyce and Sauer employed the following reaction conditions: temperature of 20°C 150°C, a pressure of 2 2.3 MPa, a medium containing water and a solvent such as tert-butyl alcohol, and an initiator such as 0.1 wt.% of APS. The melting point of the ETFE copolymer can be reduced by the inclusion of a CTA in the reaction mixture [36]. The copolymer contained 78.05% TFE which is equivalent to a TFE-to-Et ratio of 1:0.958 with a melting point of 270°C. Melt viscosity was higher than a copolymer made with the same recipe except for the use of tert-butyl alcohol instead of acetone. The use of tert-amyl alcohol produced 480 parts of a copolymer with a similar viscosity to that obtained with tert-butyl alcohol. Borsini and Modena [41] reported a process for the production of ETFE at low temperatures in the presence of a catalytic system comprised of a reducing and an oxidizing agent. Chain-transfer reactions lead to a branching decrease as the temperature was reduced, producing a more linear polymer molecule. Oxidation reduction catalytic (redox) systems have relatively small activation energy required for the production of chaininitiating free radicals. This allows the copolymerization of TFE and Et at relatively low temperatures and pressures. This allowed the polymerization

TETRAFLUOROETHYLENE

181

reaction to be conducted in the range of #60 to 20°C. Examples of the catalytic system include Pb(C2H5)4 and (NH4)2Ce(NO3)6. Researchers have taken advantage of the use of a redox catalyst system for the copolymerization of TFE and Et [42,43] at lower temperatures and pressures. A process was developed at a pressure of less than 180 MPa and a temperature of 0°C 100°C in an acidic aqueous medium using a catalyst selected from the group consisting of an acid of manganese, a manganese salt, and a derivative forming a manganese acid under the reaction conditions. Robinson and Welsh produced a tough, flexible, and high modulus terpolymer composed of TFE, Et, and HFP [44]. The terpolymer of this invention was a nonpolar material with a low dielectric constant and excellent low-temperature properties, retaining complete flexibility at temperatures as low as 278°C. This polymer was prepared by polymerizing the monomer mixture in an aqueous medium containing a reaction accelerator, using a free radical catalyst at a temperature of 35°C 60°C and a pressure of 1.4 3.5 MPa. A terpolymer was obtained that was composed of 50% Et, 44% TFE, and 6% HFP. A comparison of the properties of the polymers made by this invention and other methods is given in Table 10.9 which shows the overall superiority of the Robinson and Welsh terpolymer technology. HFP content of the terpolymer improved its stress crack resistance at elevated temperatures. A microtensile bar of the polymer was punched out of a molded plaque and placed in an air oven at 200°C. This specimen was bent 180°C in a brass channel. The results of the tests are presented in Table 10.10 showing the positive impact of 5% HFP content on the stress crack resistance of the TFE/Et copolymer/ terpolymer. Another terpolymer was reported by Ukihashi and Yamake [45] containing a perfluoroalkyl vinyl compound as the third monomer; the termonomer could be straight chained or branched. This terpolymer had excellent physical properties and improved tensile characteristics, over a copolymer of TFE and Et, at high temperatures. No deterioration of its tensile creep property and heat resistance was detected. Typical composition of the polymer was a TFE to Et ratio of 40:60 to 60:40 and perfluoroalkyl vinyl compound of 0.3 5 mol.%.

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INTRODUCTION

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FLUOROPOLYMERS

Table 10.9 A Comparison of the Properties of Tetrafluoroethylene and Ethylene Terpolymers [44]. Polymer Properties

Technology

Tensile Strength, MPa

Elongation at Break, %

Thermal Stability at 270°C

Melting Point, ° C

TMA Penetration Temperature, °C

US Patent 3,960,825 [144]

63.7

240

Good

285

US Patent 3,960,825 [144]

54.5

330

Good

256

US Patent 3,960,825 [144]

50.4

270

275

US Patent 3,960,825 [144]

48.5

240

271

226

Other methods

55.1

180

291

251

Other methods

56.6

160

285

Other methods

32.7 41.4

100 280

274

Other methods

50.1

325

Other methods

15.2

280

Fair

246

205

276 137

140

TMA, Thermomechanical analysis.

The preferred polymerization medium was a saturated fluorocarbon or chlorofluorocarbon solvent, although an aqueous medium could also be used. The solvent system was used to control the reaction conditions and increase the polymerization reaction rate. The reaction medium could also improve melt processability and increase the thermal stability and chemical resistance of the polymer. The reaction could be carried out by bulk, solution, suspension, emulsion, or vapor phase polymerization regimes. Sulzbach and Hartwimmer [46] reported the development of a stable, aqueous, colloidal dispersion of a copolymer containing, as comonomers in copolymerized form, at most 60 mol.% of TFE, from 60 to 40 mol.% of Et, and from 0 to 15 mol.% of at least one additional olefinic comonomer copolymerizable with TFE and Et. Copolymerization took place in an aqueous medium in which a fluorinated emulsifying agent was dissolved. The catalyst was selected from

the group of acids of manganese, their salts, and manganese compounds capable of being converted into manganese acids or salts, for example, potassium permanganate. A CTA and a dispersion stabilizing agent were present in the reaction medium. ETFE must have high heat resistance in cable covered with the resin to be used in applications requiring exposure to high temperatures. It would thus not crack when the cable is bent. In production process of a semiconductor, it must not affect the purity and other requirements of the semicon process. The Et/TFE copolymer was produced by polymerizing Et with TFE in an organic solvent containing no chlorine atoms as a polymerization medium, in the presence of a CTA containing no chlorine atoms and a polymerization initiator containing no chlorine atoms. No chain transferable compounds with a carbon chlorine atomic bond were present in the reaction system [47].

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OF

Table 10.10 Effect of Hexafluoropropylene on Stress Crack Resistance of Ethylene Tetrafluoroethylene Polymers [44]. Polymer Composition Et

TFE

HFP

Stress Crack Observation, at 200°C

50

50

0

Failed in 5 min

50

48

2

Failed in 5 min

52

45

3

Failed in 2 h

52

43

5

No cracks in 24 h

50

42

8

No cracks in 24 h

Et, Ethylene; TFE, tetrafluoroethylene, HFP, hexafluoropropylene.

The problem of replacement of medium for ETFE polymerization has drawn attention of the researchers. Replacement of this solvent with water has been difficult, in the sense that the aqueous dispersion is unstable such that the ETFE polymer particles tend to coagulate at low solids content during polymerization rather than stay dispersed in the aqueous medium. Aten and Burch reported a process to overcome the problem in 2011 [48]. The process produced particles with core/shell architecture in which the core was PTFE, usually less than 15% wt.% of the entire particle.

References [1] Ebnesajjad S. 2nd edition Fluoroplastics: melt processible fluoropolymers, vol. 2. Elsevier; 2015. [2] Strong A. Brent. Plastics: materials and processing. 3rd ed. Prentice Hall; 2005. [3] Sanchez IC, Eby RK. Crystallization of random copolymers. J Res Natl Bur Stand A Phys Chem MayeJune 1973;77A(3):353 8. [4] Eby RK, Wilson FC. J Appl Phys 1962;33:2951. [5] Eby RK. A copolymer with lamellarmorphology. J Res Natl Bur Stand A Phys ChemMay, 1964; 68A(3). [6] Eby RK. First-order transition temperatures in crystalline polymers. J Appl Phys August 1963;34(8). [7] Chapter 7, Molecular dynamics simulations of fluoropolymers: prediction of glass transition temperatures using united atom force fields,

TETRAFLUOROETHYLENE

[8]

[9]

[10] [11] [12] [13] [14] [15] [16]

[17]

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[19] [20]

[21]

[22]

[23]

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www.wag.caltech.edu/home/gao/thesis/chapter7.pdf; March 2015. Lee JC, Namura S, Kondo S, Abe A. Miscibility and cocrystallization behavior of two melt-processable random copolymers of tetrafluoroethylene and perfluoroalkylvinylether. Polymer 2001;42:5453. Endo M, Ohnishi A, Kutsumizu S, Shimizu T, Yano S. Crystallization in binary blends of polytetrafluoroethylene with perfluorinated tetrafluoroethylene copolymer. Polym J 2004;36(9):716 27. Bro MI. US Patent 2952669, assigned to DuPont Co.; September 13, 1960. Berry KL. US Patent 2559752, assigned to Co.; July 10, 1951. Harris JF, McCane DI. US Patent 3132123, assigned to DuPont Co.; May 5, 1964. Carlson DP. US Patent 3528954, assigned to DuPont Co.; September 15, 1970. Carlson DP. US Patent 3642742, assigned to DuPont Co.; February 15, 1972. Funaki A, Sumi N. US Patent 6528600, assigned to Asahi Glass Company; March 4, 2003. Iwasaki T, Kino M. USPatent 6586546, assigned to DuPont Mitsubishi Fluorochemical; June 1, 2003. Carsote C, Badea E. Micro differential scanning calorimetry and micro hot table method for quantifying deterioration of historical leather. Herit Sci 2019;7:48. Dealey JM, Wissbrun KF. Melt rheology and its role in plastics processing. New York, NY: Van Nostrand Reinhold; 1990. p. 597. Carlson DP. US Patent 4029868, assigned toDuPont Co.; June 14, 1977. Gresham WF, Vogelpohl AF. US Patent 3635926, assigned to DuPont Co.; January 18, 1972. Kurihara S, Murata H, Tatsu H. US Patent 5461129, assigned to Nippon Mektron, Ltd.; October 24, 1995. Hartwimmer R, Kuhls J. US Patent 4262101, assigned to HoechstAktiengesellschaft;April 14, 1981. Aten RM, Jones CW, Olson AH. US Patent 5760151, assigned to DuPont Co.; June 2, 1998. Aten RM, Jones CW, Olson AH. US Patent 5932673, assigned to DuPont Co.; August 3, 1999.

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[25] Aten RM. US Patent 7247690, Melt fabricable tetrafluoroethylene fluorinated vinyl ether copolymer prepared by suspension polymerization, assigned to DuPont Co.; July 24, 2007. [26] Eleuterio HS. US Patent 2958685, assigned to DuPont Co.; November 1, 1960. [27] Miller WT. US Patent 2598283, assigned to US Atomic Energy Commission; May 27, 1952. [28] Bro MI, Sandt BW. US Patent 2946763, assigned to Dupont Co.; July 26, 1960. [29] Couture MJ, Schindler DL, Weiser RB. US Patent 3132124, assigned to DuPont Co.; May 5, 1964. [30] Buckmaster MD, Morgan RA. US Patent 4675380, assigned toDuPont Co.; June 23, 1987. [31] McDermott DE, Piekarski S. US statutory invention registration H130, assigned to DuPont Co.; September 2, 1986. [32] Schreyer RC. US Patent 3085083, assigned to DuPont Co.; April 9, 1963. [33] Morgan RA, Sloan WH. US Patent 4626587, assigned to DuPont Co.; December 2, 1986. [34] Imbalzano JF, Kerbow DL. US Patent 4743658, assigned to DuPont Co.; May 10, 1988. [35] Hanford WE, Roland JR. US Patent 2468664, assigned to DuPont Co.; April 26, 1949. [36] Joyce Jr RM, Sauer JC. US Patent 2479367, assigned to DuPont Co.; August 16, 1949.

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[37] Ford TA. US Patent 2468054, assigned to DuPont Co.; April 26, 1949. [38] Brubaker MM. US Patent 2393967, assigned to DuPont Co.; February 5, 1946. [39] Sargent DE, Hanford WE. US Patent 2467234, assigned to DuPont Co.; April 12, 1949. [40] Hanford WE. US Patent 2392378, assigned to DuPont Co.; January 8, 1946. [41] Borsini G, Modena M, Ragazzini M. US Patent 3401155, assigned to Montecatini Edison; September 10, 1968. [42] Hartwimmer R. British Patent No. 1353535, assigned to Farbwerke Hoechst AG; May 22, 1974. [43] Hartwimmer R. US Patent 3859262, assigned to Farbwerke Hoechst AG; January 7, 1975. [44] Robinson DN, Welsh CB. US Patent 3960825, assigned to Pennwalt Corp.; June 1, 1976. [45] Ukihashi H, Yamake MUS. Patent 4123602, assigned to Asahi glass Co.; October 31, 1978. [46] Sulzbach RA, Hartwimmer R. US Patent 4338237, assigned to Hoechst Aktiengesellschaft; July 6, 1982. [47] Aida S, Sato T, Higuchi Y, Kamiya H. US Patent Application 2007/0232754, Asahi Glass Co.; October 4, 2007. [48] Aten RM, Burch HE. US Patent Application 2011/0092644, assigned to DuPont Co.; April 21.

11 Introduction to Vinylidene Fluoride Polymers Averie Palovcak1 and Sina Ebnesajjad2 1

Arkema, Inc., Philadelphia, PA, United States, 2FluoroConsultants Group, LLC, United States

O U T L I N E 11.1 Synthesis of Vinylidene Fluoride

185

11.2 Properties of Vinylidene Fluoride

186

11.3 Preparation of Vinylidene Fluoride Polymers 11.3.1 Emulsion Polymerization of Vinylidene Fluoride 11.3.2 Suspension Polymerization of Vinylidene Fluoride 11.3.3 Solution Polymerization of Vinylidene Fluoride

11.4 Characterization of Polyvinylidene Fluoride

192 193

186

11.5 Properties of Polyvinylidene Fluoride 11.5.1 Conformations and Transitions of Polyvinylidene Fluoride

188

11.6 Processing Polyvinylidene Fluoride

201

190

11.7 Applications

202

References

204

Further Reading

205

191

There is a class of fluoropolymers called “partially fluorinated” in contrast to “perfluorinated polymers.” The most common element in addition to fluorine and carbon is hydrogen (H). Examples include polyvinyl fluoride, polyvinylidene fluoride (PVDF), ethylene tetrafluoroethylene copolymer, and ethylene-chlorotrifluoroethylene copolymer.

This chapter covers methods for preparation and properties of PVDF beginning with the description of synthesis of vinylidene fluoride (VDF). Common components and applications utilizing PVDF will also be discussed.

11.1 Synthesis of Vinylidene Fluoride VDF is the main monomer for manufacture of PVDF homopolymers and copolymers with monomers

194

such as chlorotrifluoroethylene, tetrafluoroethylene, and hexafluoropropylene (HFP). Several different methods have been developed for the preparation of VDF. A number of these methods are based on dehydrohalogenation of halohydrocarbons. Examples include dehydrobromidation of 1-bromo-1,1-difluoroethane [1,2] or dehydrofluorination of 1,1,1-trifluoroethane [3,4]. The Hauptschein et al. process [3] begins by passing 1,1,1-trifluoroethane through a platinum-lined Inconel tube, which is heated to 1200°C. Contact time is about 0.01 seconds. The exit gases are passed through a sodium fluoride bed to remove the hydrofluoric acid and are then collected in a liquid nitrogen trap. VDF (boiling point: 284°C) is separated by low-temperature distillation. Unreacted trifluoroethane is removed at 247.5°C and is recycled. The effect of temperature and contact time is illustrated in Table 11.1, clearly favoring the higher temperature process. CH3 2 CF3 -CH2 5 CF2 1 HF An industrial route to VDF has been the dehydrochlorination of 1-chloro-1,1-difluoroethane (HCFC-142b) [75-68-3]. The principal producers

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00011-2 © 2021 Elsevier Inc. All rights reserved.

185

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Table 11.1 Effect of Contact Time and Temperature on Vinylidene Fluoride Yield [3]. Variable

Case 1

Case 2

Temperature, °C

1200

800

Contact time, s

0.01

4.4

Space velocity, 1/h

9.700

200

Total conversion, mol.%

75.4

76

Conversion to vinylidene fluoride, mol.%

74

66

Vinylidene fluoride yield, %

98.1

86.5

By-products yield, %

1.9

13.5

are Arkema, Daikin, Kureha, Solvay Solexis, and 3M companies. Many patents exist for the preparation routes based upon dehydrohalogenation of various chlorofluorohydrocarbons or related compounds. A wave of new research efforts on the manufacture of VDF was spurred when the HCFC production was curtailed in mid-2000s. Companies began to develop alternative methods—for example, Kureha (Japan) produces VDF from 1,1difluoroethane (HFC-152a). These processes start with hydrofluorination of acetylene followed by chlorination [5], by hydrofluorination of trichloroethane [6], or by hydrofluorination of vinylidene chloride [7]. In each case, the final product, 1-chloro-1,1-difluoroethane, is stripped of a molecule of hydrochloric acid to yield VDF. The following one-step reaction scheme is shown for vinylidene chloride as a starting ingredient: CH2 5 CCl2 1 2HF-CH3 2 CClF2 1 HCl CH3 2 CClF2 -CH2 5 CF2 1 HCl For example, a mixture of vinylidene chloride and hydrofluoric acid is passed through a heated catalyst bed. The catalyst is prepared by heating CrCl3•6H2O under vacuum to 300°C until it changes color from dark green to a solid violet throughout the porous mass. In this operation, crystallization water is removed (35% weight loss). The cooled mass is comminuted and screened into particles of 2
5 mm diameter that are loaded into a cylindrical reactor and heated to the reaction temperature (250°C
350°C). The resulting gases are

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condensed and VDF (boiling point: 284°C) is separated by low-temperature distillation. In a one-step process [8], a mixture of vinylidene chloride and hydrofluoric acid is heated to 400° C
700°C in the presence of oxygen and a catalyst. Yet another method [9] the reaction for pyrolysis of 1,2-dichloro-2,2-difluoroethane in the presence of hydrogen is carried out in the absence of catalyst in an essentially empty reactor at a temperature of $ 400oC. The term “absence of catalyst” means % the absence of a conventional catalyst. A typical catalyst has a specific surface area and is in the form of particles or of extrudates, which may optionally be supported to facilitate the dehydrochlorination reaction by reducing its activations energy. The reactors that are suitable are quartz, ceramic (SiC), or metallic reactors. In this case, the material constituting the reactor can be chosen from metals such as nickel, iron, titanium, chromium, molybdenum, cobalt or gold, or alloys thereof. The metal, chosen more particularly to limit corrosion or catalytic phenomena, maybe bulk metal or metal plated onto another metal.

11.2 Properties of Vinylidene Fluoride VDF (CH2 5 CF2) is flammable, colorless, and in gas form under ambient conditions. Physical properties of VDF are presented in Table 11.2. It is almost odorless and with a low boiling point at 284°C. VDF can form explosive mixtures with air. Polymerization of this gas is highly exothermic (Table 11.2) and it takes place above VDF’s critical temperature and pressure (3
10 MPa). VDF (HFC1132a) can be stored or shipped in gas cylinders or high-pressure tube trailers without polymerization inhibitors. Terpene and quinone inhibitors, however, maybe used as a safety element to prevent autopolymerization [11].

11.3 Preparation of Vinylidene Fluoride Polymers VDF can be polymerized by a variety of methods such as suspension, dispersion, and solution polymerization. It can be copolymerized with a number of fluorinated and nonfluorinated

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VINYLIDENE FLUORIDE POLYMERS

Table 11.2 Properties of Vinylidene Fluoride [10]. Units/ Conditions

Value

Molecular weight

Da

64,038

Boiling point

°C

284

Freezing point

°C

2144

Vapor pressure

kPa at 21°C

3683

Critical pressure

kPa

4434

Critical temperature

°C

30.1

Critical density

kg/m3

417

Explosive limits

vol.% in air

5.8
20.3

Heat of formation

kJ/mol at 25°C

2345.2

Heat of polymerization

kJ/mol at 25°C

2474.21

Water solubility

cm3/100 g at 25°C/10 kPa

6.3

comonomers. Examples of these two groups include perfluoroolefin monomers and acrylic compounds. VDF polymers consist of homopolymers and copolymers. Copolymers contain at least 50% VDF. The balance is comprised of a monomer from the group of monomers that would readily polymerize with VDF including tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropene, pentafluoropropene, and vinyl fluoride. VDF homopolymer and copolymer parts are fabricated by melt processing as well as coating techniques. The first successful aqueous polymerization of VDF (CF2 5 CH2) was reported in 1948 [12]. VDF was polymerized using a peroxide initiator in water at 50°C
150°C and 30 MPa. No surfactants or suspending agents were present in the polymerization recipe. PVDF has been polymerized by a number of methods including emulsion, suspension, solution, and bulk. Later, copolymers of VDF with ethylene and halogenated ethylene monomers were also produced [13]. In 1960, a manufacturing process was developed and PVDF was first introduced to the market. PVDF is commercially produced by

187

aqueous emulsion or suspension processes. In this section, emulsion, suspension, and solution polymerization techniques have been covered. Ford and Hanford [12] reported the polymerization of VDF in an aqueous medium in a batch process initiated by organic peroxide (0.05
0.5% wt.%) without a surfactant. The reaction temperature and pressure ranges were 50°C
150°C and 50
100 MPa. For example, a stainless-steel reactor was purged with nitrogen and then charged with 50 parts of deaerated water and 0.1 part of benzoyl peroxide. The reactor was evacuated and pressurized with 40 parts of VDF to remove nitrogen. The reactor was then placed in a reciprocating mechanism in order to agitate the contents vigorously. Pressure was maintained by connecting the reactor with a 100 MPa pressure pure water reservoir. After temperature reached 80°C, reactor pressure was raised to 95 MPa by injection of water from the highpressure reservoir. The contents were allowed to react for a total of 10.5 hours while the pressure was maintained in the range of 86
95.5 MPa. A total of 31.5 MPa pressure drop was observed during the reaction time. The recovered powdery PVDF was rinsed with water and dried in a vacuum oven. PVDF made by this process was pressed into film (at 160°C
190°C). It had an elongation of 400%, a tensile strength of 31 MPa, and Young’s modulus of elasticity of 531 MPa. The polymer was insoluble in most solvents and could be pressed into a tough film. Holding the film at 275°C for 5 minutes only caused slight discoloration of PVDF without the loss of other properties, in contrast to films of polyvinyl chloride and polyvinylidene chloride, which are completely degraded into a carbonaceous residue. The specific gravity of the PVDF film was 1.745 g/mL at 25°C. It exhibited stickiness to copper at 145°C at 10 kPa. The polymer was soluble in a few solvents such as toluene, xylene, isooctane, chloroform, and carbon tetrachloride. Promoters are not necessary for the polymerization reaction of VDF. Their use could increase the reaction rate and decrease the reaction time. Suitable promoters include reducing agents such as oxidizable sulfoxy compounds, that is, sulfur compounds which contain a sulfur-oxygen bond. Examples of these compounds include sodium bisulfite, sodium sulfite, sodium hydrosulfite, sodium thiosulfite, and ammonium bisulfite.

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In 1949, Ford [13] reported polymerization of vinyl fluoride with other monomers such as ethylene and halogenated ethylene compounds. Generally, polymers containing 5
95 wt.% VDF could be produced in which the balance was comprised of comonomers. A copolymer of VDF and ethylene was produced in a reactor at 80°C and at 60.5
70 MPa pressure. The polymer contained 8.6% VDF by weight and 91.4% ethylene and could be pressed into a clear film at 140°C.

11.3.1 Emulsion Polymerization of Vinylidene Fluoride Emulsion polymerization of VDF has been carried out [14] by a heterogeneous reaction in the presence of a surfactant, the most suitable of which is a fluorine-containing acid salt of a fluoroalkanoic. Surfactants prevent radical-scavenging reactions that stop the polymerization. Chain transfer agents regulate the molecular weight of the polymer, and buffers regulate the pH of the aqueous polymerization media. The source of free radicals to initiate the reaction is either water-soluble (e.g., persulfate salts) or monomer-soluble (e.g., organic peroxides like di-tert-butyl peroxide). PVDF made by emulsion polymerization, usually in an agitated reactor, is in the form of spheres with a typical diameter of about 0.25 μm. A paraffin wax is added to the reaction medium to stabilize the latex. At the end of the reaction, the product is decanted to remove the wax from the latex, followed by coagulation. After filtration, rinsing, and drying, a fine powder is recovered with an agglomerate size of 2
5 μm in diameter [14]. The powder can be converted into pellets or cubes by melt extrusion. Copolymerization of VDF and HFP in an aqueous medium was reported in 1965 [15]. HFP content of the copolymer ranged from 1 to 13 mol.%, depending on the composition of the reaction mixture and the polymerization conditions. The copolymers obtained at HFP content above 15 mol.% were essentially amorphous and had low torsional modulus and high retention of elastomeric properties over a wide temperature range without embrittlement. The use of activators was recommended in conjunction with peroxy initiators. Examples included sodium bisulfite, sodium metabisulfite, sodium thiosulfate, sodium hydrosulfate, and water-soluble

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reducing agents such as dextrose. The amount of the activator was generally in the range of 0.2
0.8 part by weight per hundred parts of total monomers. Accelerators have sometimes been employed in aqueous polymerization. Water-soluble metal salts of sulfates, nitrates, phosphates, and chlorides were among the effective accelerators. Specific compounds of this class include cuprous sulfate, ferrous sulfate, and silver nitrate. The amount of accelerator used in the reaction mixture was between 0.05 and 0.5 part per hundred parts of total monomers. When an activator such as sodium metabisulfite and an accelerator such as ferrous sulfate are selected, the catalyst/initiator system is called a redox system. The peroxides recommended by Lo [15] worked best in a redox catalyst system. Suitable reducing agents, which act as activators, were combined with the water-soluble initiators to increase their activities. Some specific activators included were alkali metal bisulfites, alkali metal formaldehyde sulfoxylates, or sulfur dioxide. Accelerators such as ferrous sulfate, silver nitrate, ferrous nitrate, and silver nitrate may also be combined with the initiator. The amount of the initiator was varied from 0.02% to 1% of the total weight of the monomer. It was important to control the pH of the polymerization medium between 5 and 8; adjustments were made by adding buffers such as carbonates, bicarbonates, phosphates, and hydrogen phosphates of alkali metals (e.g., sodium and potassium). In 1973, Stallings reported [16] the polymerization of VDF into a polymer suitable for coatings applications. Stallings’ process produced PVDF of colloidal particle size with a monodispersed distribution. The polymer particles were nearly nonporous and well-suited to the preparation of high solids content dispersions. The solvents in these dispersions are known as “latent solvents” and include polar liquids such as gamma-butyrolactone, dimethylformamide (DMF), and dimethylacetamide. The coatings had high gloss, good thermal stability, good chemical resistance, and high postdeformability which means that the coated substrate could be bent, punched, and deformed in other ways without cracking or delaminating the coating. In a 1974 US patent, Dohany [17] disclosed a process by which VDF could be reproducibly polymerized into a high-quality polymer. Both the initiator (diisopropylperoxydicarbonate) and the chain transfer agent (acetone) were fed continuously to

11: INTRODUCTION

VINYLIDENE FLUORIDE POLYMERS

189

the reactor along with VDF. The amount of initiator ranged from 0.1% to 1.0% of the total weight of the monomer. Acetone (1%
20% monomer weight) acted as the means to control molecular weight of PVDF. The presence of acetone in the reaction mixture permitted the addition of substantial amounts of the initiator to increase the reaction rate. Polymerization can be designed to improve specific properties of PVDF. For example, Dohany reported [18] polymerization of VDF to modify the flow behavior to allow extrusion of the polymer at higher speeds. PVDF of this development was produced as a homopolymer or a copolymer containing up to 25% of a comonomer such as tetrafluoroethylene, chlorotrifluoroethylene, and HFP. PVDF homopolymer or copolymer was generally manufactured according to the US Patent 3,193,539 [19] using a fluoroalkyl fluorosurfactant, an organic peroxide, and wax to stabilize the emulsion. Barber [12] reported a process for the production of VDF/HFP copolymer by the emulsion polymerization in a stirred aqueous reaction medium. The aqueous reaction medium consisted of water, VDF, an initiator, and a water-soluble surfactant capable of emulsifying both the initiator and the mass formed during the polymerization reaction. VDF and initiator were fed to the reaction medium to continue polymerization of the VDF until 70%
80% of the total VDF had been consumed. Then, 5
15 wt.% HFP was added to the reaction medium (of total VDF consumption). It would be helpful to add a chain transfer agent like trichlorofluoromethane to the reaction medium. At the end of the polymerization, a copolymer of VDF/HFP was recovered from the reaction medium. The VDF homopolymerization reaction was continued until approximately 61.4 kg (representing 75 wt.% of the total VDF) monomer had been consumed [13]. Afterwards, 9 kg of HFP comprising 10 wt.% of the total weight of combined VDF and HFP monomers consumed was pumped into the reactor at a rate of approximately 45 kg/h while VDF feed was continued. The sudden influx of the relatively slow reacting HFP monomer temporarily suppressed the reaction rate. The rate of initiator addition was increased to restore the polymerization rate to 27.3 kg/h. The reaction continued until a total of 82 kg of VDF had been added to the reaction mass. The resulting resin displayed a melt viscosity of 1.4 kPoise, measured at 232°C and 100 second21 (ASTM D3835), and a differential

scanning calorimetry (DSC) melting point of 163°C
168° C. Reports in 2003 and 2004 described [14] beneficial effects by using ethane or propane as a chain transfer agent in emulsion polymerization of VDF. The benefit of propane was it required a lower concentration (0.5 wt.% of total monomer weight) than ethane at which it was effective [15] The PVDF homopolymers had a significantly reduced tendency to generate cavities at high temperatures and a greater resistance to discoloration at high temperatures. A series of polymerization trials were run with varying levels of ethane. A control was run in which ethyl acetate was used instead of ethane. Table 11.3 shows melt viscosity determined by ASTM D3835 at the temperature and time indicated. Melting points were determined by the differential scanning colorimetry method, using ASTM 3418. The impact of ethane on melting point of PVDF is negligible though its melt viscosity is tunable. In a process [17] to produce heat-stable PVDF, sodium acetate was added after the polymerization had been completed. A potassium alkylsulfonate was optionally added after the polymerization. An aqueous PVDF dispersion was obtained. The PVDF was collected by atomizing the recovered dispersion in air at a temperature of 120°C
220° C. The aqueous dispersion did not require washing prior to atomizing. Evaluation of the heat stability: A plaque was formed from 40 g of PVDF powder by compression molding at 3.0 MPa at 205°C for 6 minutes followed by quenching in water at 20°C [18]. The plaque was reheated in an oven at 265°C for 1 hour. After this heat treatment, the plaque was somewhat

TO

Table 11.3 Effect of Ethane on Melt Viscosity and Melting Point of PVDF Homopolymers [16]. Gram of Ethane/ 2000 g VF2

Melt Viscosity at 230°C and 100 s21

Tm, °C

0

39.6

163.4

5.2

28.3

164.4

10.7

24.3

164.2

19.1

12.1

163.5

25.3

5.8

163.5

16.8

165.4

9.1 (EA)

190

discolored. The color was determined by a yellowing measurement. The plaque was placed on a calibrated white ceramic and the yellowing index was measured using a Minolta CR 200 colorimeter according to the ASTM D1925. Reduction of surfactant content to less than 300 ppm and the addition of sodium acetate and optionally a potassium alkylsulfonate to PVDF reduced the yellowness index by 50%
75%. A PVDF polymer with an ultrahigh molecular weight and unexpected physical properties was reported in 2008. The ultrahigh molecular weight polymer was clear, had a relatively low melting point, reduced crystallinity, excellent impact resistance, and a high elongation at the yield point. The ultrahigh molecular weight PVDF had a solution viscosity of .50 Pa s in 10% n-methyl pyrolidine (NMP) at 20°C. The ultrahigh molecular weight PVDF can be used by itself or blended with other polymers [19]. An invention describes manufacturing stable aqueous PVDF dispersions and using nonionic nonfluorinated emulsifier. Both homopolymers and copolymers of VDF were described. The copolymers contained 70
99 mol.% of VDF monomer units and 1
30 mol.% of one or more other fluoromonomers. It also contained 100 ppm to 0.5%, based on the weight of the fluoropolymer solids, of surfactants containing polyethylene glycol, polypropylene glycol, and/or polytetramethylene glycol blocks. The process was free of fluorinated surfactants. The fluoropolymers of the disclosed process were light colored polymers that resisted discoloration and cavitation at normal temperatures for extrusion, coating, or other fabrication techniques [20,21].

11.3.2 Suspension Polymerization of Vinylidene Fluoride The main objective of batch suspension polymerization of VDF is to limit the formation of polymer scale on the walls of the reactor [22]. A watersoluble polymer, for example, cellulose derivatives or polyvinyl alcohol, is added as a suspending agent to reduce coagulation of the polymer particles during the polymerization. Organic peroxides are the initiators in the polymerization reaction and chain transfer agents control the molecular weight of the PVDF. The product is in slurry form consisting of suspended particles, often spheres of

INTRODUCTION

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FLUOROPOLYMERS

30
100 μm. PVDF powder can be recovered by filtering/separating the particles from water, thorough rinsing, and drying. Suspension polymerization of VDF by similar techniques has been reported by Stallings [23] and by Dohany [24]. Stallings described a process for aqueous polymerization of VDF in a suspension regime. Hydrostatic pressure (i.e., water) was added to that exerted by the monomer at the reaction temperature. Aside from the initiator, a suspending agent such as methyl hydroxyalkyl cellulose and, optionally, a chain transfer agent were added. Preferred polymerization temperature and pressure ranged from 30°C to 110°C and 6.9
21 MPa. Batch reaction was conducted for 0.25
6 hours, depending on the type of the initiator. VDF polymers produced by this process have a predominantly linear structure and exhibit a high degree of crystallinity. A typical polymerization run was conducted in a stirred one-gallon (3.7 L) stainless steel reactor equipped with baffles and a cooling coil, rated at 69 MPa. The vessel was charged with 2470 mL of water, 908 g of VDF, 30 g of an aqueous methyl hydroxypropyl cellulose solution, and 5 g of tertbutyl peroxypivalate. Water pressure was raised to 5.5 MPa at 25°C under which the liquid monomer density is 0.69 g/mL. The reactor was heated to 55° C, increasing the pressure to 13.8 MPa. The reaction was continued for 4 hours during which 800 mL of water was pumped into the vessel to keep the pressure constant. At the end of the polymerization [23], the reactor was cooled and the content was discharged. The polymer was isolated by centrifuging, followed by washing with water, and drying in a vacuum oven. The polymer was composed of spherical particles having an average size of 50
120 μm; the recovered PVDF was 91% of the weight of the monomer. Molecular weight was assessed by measuring the intrinsic viscosity of the PVDF, by ASTM Method D1243, using DMF as the solvent. In this method, viscosity of solutions containing 0.1 g of polymer per 100 g of solution was measured. An intrinsic viscosity value of 1.0
2.0 L/g corresponds to a molecular weight of 50,000
300,000. VDF was polymerized using a number of different initiators. The effect of initiator type on the polymer yield was moderate but it was fairly strong on the molecular weight. The initiator could also be added

VINYLIDENE FLUORIDE POLYMERS

191

during the polymerization with similar polymer yields. Molecular weight could be reduced by the addition of a chain transfer agent such as isopropanol. Dumoulin [25] has published a good discussion of the variables of suspension polymerization. The goal of this work was to obtain chain transfer agents that did not affect the thermal stability of PVDF detrimentally. VDF was polymerized in an aqueous medium containing a suspending agent such as polyvinyl alcohol and a water-soluble cellulose derivative like alkyl and alkylhydroxy celluloses. Free radical initiators included diethyl and diisopropyl peroxydicarbonate and tert-butyl and tert-amyl perpivalates. The most effective chain transfer agents are bis(alkyl) carbonates in which the alkyl group contains 1
3 methylene groups. The amount of suspending agent varied from 0.01 to 0.5 wt.%. The amount of chain transfer agent depended on the desired molecular weight and thus on the intrinsic viscosity of the polymer. Generally, 0.05
2.5 wt.% of agent (based on total monomer) was added which allows production of PVDF from 0.05 to 0.2 L/g. The exact amount of chain transfer agent to obtain a specific molecular weight would have to be determined experimentally. The amount of the initiator ranged from 0.05%
0.5% of the monomer weight. Both the initiator and chain transfer agent could be added at the beginning or during the polymerization. A typical polymerization run was made in a 5-L reactor equipped with a stirrer and a double-walled jacket. After charging 3.3 L of water and 1.1 g of hydroxypropyl methylcellulose, the reactor was evacuated to remove the oxygen. The initiator and the chain transfer agent were next introduced into the reactor and stirring was started. Finally, VDF (1100 g) was loaded and heating started. At the end of the polymerization, stirring was stopped, the reactor was cooled and degassed. The polymer was separated and washed by centrifuging and dried at 60°C in an oven. Saito et al. prepared solutions of 0.1, 0.2, and 0.3 g of the VDF polymers in 50 mL of DMF and measured the specific viscosity of these solutions at 30°C using a Ubbellohde viscometer [26]. A value of 300,000 was determined for the molecular weight of VDF copolymers. This resin had excellent film forming properties as demonstrated by pressing 1 g of the polymer at 180°C for 2 minutes at a pressure of 15 MPa; a uniform film with no

crack was produced. The water repellency improvement was measured by contact angle of water on the film made from the copolymer; typical values were 103°
108°. PVDF resin has been widely known as a resin having a ferroelectric property and has been used for piezoelectric and pyroelectric sensors, acoustic sensors, speakers and so on, taking advantage of the property of the resin. There has been a problem in that it is difficult for the standard PVDF resin to form a uniform thin film. Especially, in cases in which thin films and electrodes are formed and laminated on a semiconductor substrate or a metal substrate, a homogeneous laminated product cannot be obtained unless the thin films are uniform. When high electric field is applied to an uneven thin film, the problems of short-circuit or uneven properties within the thin film arise. Kureha reported [27] development of a PVDF copolymer that was capable of forming a very uniform film, thus overcoming the above shortcomings. The PVDF was soluble in polar solvents such as dimethyl formamide. The copolymer was characterized by the ratio of the scattered-light intensity (I) for a 15% solution of the PVDF copolymer in dimethyl formamide to the scattered-light intensity (Io) for dimethyl formamide, (I/Io), was 10 or lower.

11: INTRODUCTION

TO

11.3.3 Solution Polymerization of Vinylidene Fluoride VDF can be polymerized in saturated fluorinated or fluorochlorinated solvents. These solvents dissolve fluoroalkenes like VDF and organic peroxide catalysts. Polymerization thus takes place in a homogeneous phase and the resulting PVDF is insoluble in the solvent, making the product readily separable from the solvent. In addition to organic peroxides, the reaction can be initiated or induced by radiation. This is helpful in avoiding contamination of the product with other reaction components such as the initiator, surfactant, and the others. Alkyl boron activated by oxygen has also been reported to catalyze VDF and vinyl fluoride polymerization in water and solvents [28]. An early report described [29] polymerization of VDF in fluorinated and fluorochlorinated hydrocarbons using organic peroxide as a catalyst. The solvent had to have a boiling point above room temperature and be capable of dissolving the

192

monomer and catalyst. Suitable compounds included saturated fluorinated and fluorochlorinated hydrocarbons with 10 carbons and less, alone or as a mixture. These compounds have minimal tendency to form free radicals. To minimize the required polymerization pressure, it was important to have solvents with boiling points above room temperature, which was the reason for selecting compounds containing more than one carbon atom. Examples of suitable solvents were fluorotrichloromethane, trifluorochloroethane, and trifluorotrichloroethane. The catalyst (initiator) concentration varied between 0.2% and 2% of the total amount of monomer by weight. Examples of organic peroxide catalysts included di-tert-butylperoxide, tert-butylhydroperoxide, and dibenzoyl peroxide. Polymerization temperature and pressure ranges were 70°C
120°C and 0.6
3.5 MPa. Polymerization experiments were conducted in a 1-L autoclave equipped with a magnetic stirrer. In a typical run, a solution of lauroyl peroxide in 500 g of trifluorotrichloroethane (CF2ClCCl2F) was charged to the autoclave. After the reactor was flushed with nitrogen and evacuated, 160 g of VDF was introduced, generating a pressure of 1.2 MPa at room temperature. The autoclave was heated to 120°C
125°C for 20 hours with agitation, reaching maximum and minimum pressures of 3.5 and 0.6 MPa during polymerization. Monomer conversion to PVDF was 99.1% with a melting point of 169°C. Radiation-induced polymerization of VDF has been explored [30
32] to eliminate contamination of the polymer with the initiator and other ingredients. In general, a reaction vessel was charged [31] with a VDF monomer or a mixture of comonomers and a fluorinated solvent. The mixture was subjected to ionizing radiation such as γ-rays generated by cobalt 60 and cesium 137. Reaction temperature ranged from 260°C to 30°C to keep the reaction sufficiently fast without being uncontrollable. Only a small dosage of irradiation was required for the polymerization consisting of two stages: irradiation polymerization and nonirradiation polymerization. The required dosage was 103
7 3 105 roentgen. A glass ampoule made of borosilicate glass was evacuated and charged with 6.4 g of VDF and 16.1 g of trichlorotrifluoroethane solvent, both in the liquid state. The ampoule was sealed after the removal of air, while it was kept in a mixture of dry ice and methanol. The ampoule was exposed to

INTRODUCTION

TO

FLUOROPOLYMERS

γ-rays generated from cobalt 60 (700 curies) at a dose rate of 4 3 104 roentgen per hour at 240°C. The ampoule was opened after 9 hours, and the unreacted VDF and the solvent were removed, resulting in the recovery of 6.0 g of PVDF (93% weight yield). The yield increased to 100% when the exposure time was increased to 20 hours. The PVDF obtained had a melting point of 175°C compared to 152°C for PVDF obtained by chemical initiation in an aqueous medium. In the absence of the solvent, polymerization yield decreased to 37% after 20 hours exposure.

11.4 Characterization of Polyvinylidene Fluoride ASTM Method D3222 covers specifications for unmodified (homopolymer) PVDF resins and test methods for the as-produced polymer. PVDF resins are thermoplastics in that they can be remelted and can be processed by the normal melt processing technologies. Specific gravity, which is measured on a molded specimen according to ASTM Method D792 or D1505, is used as an indirect measure of molecular weight. Properties used in the characterization of PVDF polymers are defined in Table 11.4. ASTM Method D3222 classifies the different types of PVDF according to the specifications summarized in Table 11.5. Measurement of a number of properties requires a sheet form of PVDF, from which specimens are punched out and conditioned before testing. The sheet is prepared by using a chase in the shape of a picture frame with the adequate depth, between two chromium-plated (150 3 150 3 5 mm) ferrotype plates. The chase is placed over a sheet of aluminum foil. Then, an amount of polymer sufficient to produce the sheet is placed in the center of the chase. A second sheet of aluminum foil is placed on top of the polymer granules and the combination is put between the two plates. The assembly is finally put in a preheated (230°C 6 2°C) compression molding press. The press is closed with a barely noticeable pressure and held for 5 minutes at 230°C. Pressure is increased to 20.7 MPa for 1 minute. At the end of this period, the assembly is removed and quenched in a cold water bath. The sample should be removed from the chase after 1 minute of cooling. The sheet can be used for specimen preparation.

11: INTRODUCTION

TO

VINYLIDENE FLUORIDE POLYMERS

11.5 Properties of Polyvinylidene Fluoride PVDF is mechanically stronger than perfluorinated polymers such as polytetrafluoroethylene. Similarly, it has higher abrasion resistance and resistance to both creep under long-term stress and fatigue during cyclic loading [34,35]. PVDF has good thermal stability, resistance to ultraviolet and higher energy radiation and chemical resistance to most chemicals and solvents. PVDF is not hygroscopic and adsorbs less than 0.05% of water at room temperature. For that reason, drying is not usually a necessary step to take before processing. The properties of PVDF homopolymers (Table 11.6) and copolymers (Table 11.7) are dependent on the chemical composition of the

193

polymer, molecular weight, and molecular architecture [36]. These characteristics are determined by the polymerization process, finishing techniques, and the thermal and to some extent mechanical history caused by postpolymerization processing including fabrication. Some of the important properties of PVDF homopolymers and copolymers are a function of the crystalline content and type of crystalline structure, both which are affected by the processing methods and conditions [37]. The polymer chains of PVDF can have multiple conformations. Amorphous PVDF regions have a density of 1.68 [38]. The density of the homopolymer PVDF rapidly quenched from the melt is 1.76
1.78 g/cm3. The melt density of PVDF homopolymers and VDF/HFP-copolymers is approximately 1.45
1.48 g/cm3 at 230°C and 1 bar [22],

Table 11.4 Definition of Basic Properties of Polyvinylidene Fluoride According to ASTM D3222 [33]. Property

Description

Reference ASTM Method

Melt flow rate

Measured using a 49 N weight at 297°C

D1238

Rheology

Measures flow characteristics of resin using a capillary rheometer

D3835

Melting point, °C

Heat of fusion and melting peak temperature of resin as determined by differential scanning calorimetry

D3418

Specific gravity

Specific gravity of a sample of molded polymer according to tins method

D792 or D1505

Tensile properties

Elongation and strength at break of a sample made according to the specified method

D638

Flex modulus

An indication of the resistance of the sample to being flexed or bent

D790

Impact resistance

An indication of the strength of sample during impact

D256

Limiting oxygen index

A measure of flammability, the minimum amount of atmospheric oxygen required to support combustion of the sample

D2863

Refractive index

Measure at sodium D line at 25°C

D542

D-C resistance

Volume resistivity of the sample

D257

Dielectric strength

Resistance of the sample to break down due to voltage

D149

Dielectric constant

Measured on three specimens (101.6 mm diameter) at 102 and 106 Hz

D150

Dissipation factor

Measured on three specimens (101.6 mm diameter) at 102 and 106 Hz

D150

194

INTRODUCTION

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FLUOROPOLYMERS

Table 11.5 Polyvinylidene Fluoride (PVDF) Resin Specifications According to ASTM D3222 [33]. Type

Type I

Grade

1

2

Description

Emulsion

Emulsion

Suspension







20
150

Particle size, μm Apparent melt viscosity (at 100 s

21

Type II

), Pa s

High viscosity

2500
4000

Medium viscosity

2800
3800

2800
3100

1300
2500

Low viscosity

2300
2800

1300
2800

500
1300

Melting point, °C

156
162

162
170

164
180

Specific gravity

1.75
1.79

1.75
1.79

1.75
1.79

Tensile strength, at 23°C, MPa

.36

.36

.36

Elongation, %

.10

.10

.10

Flex modulus, GPa

.1.38

.1.38

.1.38

Impact resistance, J/m

.133.4

D-C resistance. Ω cm

.1.2 3 10

.1.2 3 10

.1.2 3 1014

Dielectric strength, kV/mm

.57

.57

.57

,11.0

,11.0

,11.0

.7.2

.7.2

.7.2

,0.045

,0.045

,0.045

,0.24

0.24

20.24

.133.4 14

.133.4 14

Dielectric constant (maximum) 102 Hz 6

10 Hz Dissipation factor (maximum), % 102 Hz 6

10 Hz

indicating a volumetric shrinkage of the order of 20% takes place when it transitions from melt to solid.

11.5.1 Conformations and Transitions of Polyvinylidene Fluoride PVDF chains form smaller amorphous molecular dimensions than PTFE because of strong interaction between -CH2- and -CF2- dipoles along the chain [25]. Crystallinity of PVDF is 35%
70% depending on the polymerization method and polymer finishing history. The characteristics of PVDF depend on the molecular weight, molecular weight distribution, the amount of chain irregularity, side chains, and its crystalline regime. In an alternating chain, head (-CF2-) to tail (-CH2) addition dominates. There are occasional reversed head-to-head and tail-to-tail additions resulting in defects, the extent

of which depends on the polymerization conditions, particularly temperature [26]. The amount of the defects is determined by F 19 NMR and other techniques. Emulsion polymerization produces more head-to-head defects that are not followed by tail-to-tail links than suspension polymerization [28,39]. PVDF has several polymorphs including four known chain conformations and a fifth suggested one [40]. The most common crystalline phase is α (density of 1.92 g/cm3). PVDF’s α phase occurs in a trans-gauche-transgauche (TGTG) formation. This formation, as seen in, is not a helical or a planar zigzag but a combination of the two. Either a series of G or TG would represent a purely helical structure [29,30]. It is formed both during polymerization and during cooling of the molten polymer. The β crystal phase of PVDF forms a planar zigzag, or TT where T represents a trans bond that remains in the same plane as the carbon backbone [31].

Table 11.6 Arkema Kynar PVDF Homopolymer [23]. Physical Propertiesa

Standard/ Conditions

Refractive index

460

1000 Seriesb

700 Seriesb

370c

D542/at Sodium D line 77°F (25°C)

1.42

1.42

1.42

—

Specific gravity

D792/73°F (23°C)

1.75
1.77

1.76
1.78

1.77
1.79

1.84 2 1.88

Water absorption

D570/68°F (20°C) Immersion/24 hours

%

0.02
0.04

0.01
0.03

0.01
0.03

0.04
0.06

Mechanical Propertiesa

Standard/ Conditions

Units

460

1000 Seriesb

700 Seriesb

370c

Flexural strength at 5% strain

D790/73°F (23°C)

psi (MPa)

7000
9000 (48
62)

8500
11,000 (58
76)

8500
11,000 (58
76)

20,000
30,000 (138
207)

Flexural modulus

D790/73°F (23°C)

psi (MPa)

200,000
260,000 (1379
1792)

240,000
335,000 (1655
2310)

200,000
335,000 (1380
2310)

800,000
1,000,000 (5515
6895)

Tensile yield elongation

D638/73°F (23°C)

%

10
15

5
10

5
10

0
4

Tensile yield strength

D638/73°F (23°C)

psi (MPa)

5000
7500 (34
52)

6500
8000 (45
55)

6500
8000 (45
55)

5000
8000 (34
55)

Tensile break elongation

D638/73°F (23°C)

%

50
250

20
100

20
100

0
20

Tensile break strength

D638/73°F (23°C)

psi (MPa)

4500
7000 (31
48)

5000
7000 (34
48)

5000
8000 (34
55)

5500
8000 (38
55)

Tensile modulus

D638/73°F (23°C)

psi (MPa)

185,000
220,000 (1275
1520)

200,000
335,000 (1379
2310)

200,000
335,000 (1379
2310)

450,000
750,000 (3102
5171)

Compressive strength

D695/73°F (23°C)

psi (MPa)

8000
10,000 (55
69)

10,000
15,000 (69
103)

10,000
15,000 (69
103)

20,000
25,000 (138
172)

Deflection temperature

D648/at 264 psi (1.82 MPa)

°F (°C)

176
194 (80
90)

220
230 (104
110)

221
239 (105
115)

230
260 (104
127)

Deflection temperature

D648/at 66 psi (0.45 MPa)

°F (°C)

234
284 (112
140)

257
284 (125
140)

257 2284 (125
140)

270
300 (132
149)

Units

(Continued )

Table 11.6 Arkema Kynar PVDF Homopolymer [23].—Cont’d Physical Propertiesa

Standard/ Conditions

Units

460

1000 Seriesb

700 Seriesb

370c

Impact strength notched Izod

D256/73°F (23°C)

Ft-Lb/In

1.8
4

1.8
4

1.8 24.0

0.75
1.50

Impact strength unnotched Izod

D256/73°F (23°C)

Ft-Lb/In

15
40

20
80

20
80

5
10

Hardness

D2240/73°F (23°C)

Shore D

75
80

77
82

76
80

74
79

Tabor abrasion

CS-17 1000 g:pad

mg/1000 cycles

7
9

5
9

5
9

—

Coefficient of friction—Static vs Steel

ASTM D1894 73° F (23°C)

0.23

0.22

0.20

0.18

Coefficient of friction— Dynamic vs Steel

ASTM D1894 73° F (23°C)

0.17

0.15

0.14

0.12

Thermal Propertiesa

Standard/ Conditions

Units

460

1000 Seriesb

700 Seriesa

370c

Melting temperature

D3418

°F (°C)

311
320 (155
160)

337
340 (169
171)

329
342 (165
172)

329
338 (165
170)

Tg (DMA)

at 1 Hz

°F (°C)

241 to 237 (240 to 238)

241 to 237 (240 to 238)

241 to 237 (240 to 238)

241 to 237 (240 to 238)

Coefficient of linear thermal expansion

D696

10E 2 5/°F

5.0
7.0

6.6
8.0

6.6
8.0

2.0
2.5

Thermal conductivity

ASTM D433

BTU-in/ hr ft2 °F

1.18
1.32

1.18
1.32

1.18
1.32

—

Specific heat

DSC

BTU/Lb °F

0.28
0.36

0.28
0.36

0.28
0.36

0.28
0.36

Thermal decomposition TGA

1% wt. loss/in air

°F (°C)

707 (375)

707 (375)

707 (375)

707 (375)

Thermal decomposition TGA

1% wt. loss/in nitrogen

°F (°C)

770 (410)

770 (410)

770 (410)

770 (410)

Electrical Propertiesa

Standard/ Conditions

Units

460

1000 Seriesb

700 Seriesb

370c

Dielectric strength 23°F

D149/73°F (23°C)

KV/Mil

1.6

1.6

1.7

—

Dielectric constant 23°F

D150/ 100 MHz
100 Hz

4.5
9.5

4.5
9.5

4.5
9.5

28.8
33.5

Dissipation factor 23°F

D150/100 Hz

0.01
0.21

0.01
0.25

0.01
0.21

0.06
0.08

Volume resistivity

D257/DC 68°F (20°C)/65% RH

Ω cm

2 3 1014

2 3 1014

2 3 1014

1 3 1011

Flame and Smoke Propertiesa

Standard/ Conditions

Units

460

1000 Seriesb

700 Seriesb

370c

Burning rate

UL/Bulletin 94

V
O

V
O

V
O

V
O

Limiting oxygen index (LOI) a

D2868

% O2

44

60

44
75

d

44

Typical property values. Should not be construed as sales specifications. The Kynar 700 PVDF and Kynar 1000 PVDF series span a wide range of melt viscosities (see page 3). Please contact an Arkema representative for typical values of specific grades. c Filled with graphite powder to reduce mold shrinkage. d Kynar 740-02 is offered with flame retardant package that raises LOI. DMA, Dynamic mechanical analysis; TGA, Thermogravimetric analysis. b

Table 11.7 Arkema Kynar Flex PVDF Copolymers [24]. Physical Propertiesa

Standard/ Conditions

Refractive index

Units

2500

2750/2950

2800/2900

2850

3120

D542/at Sodium D line 77°F (25°C)

1.40

1.41

1.41

1.42

1.41

Specific gravity

D792/73°F (23°C)

1.80
1.82

1.78
1.80

1.77
1.80

1.77
1.80

1.77
1.80

Water absorption

D570/68°F (20° C) Immersion/24 hours

%

—

0.03
0.06

0.03
0.05

0.03
0.05

0.03
0.05

Mechanical Propertiesa

Standard/ Conditions

Units

2500

2750/2950

2800/2900

2850

3120

Flexural strength at 5% strain

D790/73°F (23°C)

psi (MPa)

1500
2500 (10
17)

2000
3500 (14
24)

3000
5000 (20
34)

3000
5000 (20
34)

3000
5000 (20
34)

Flexural modulus

D790/73°F (23°C)

psi (MPa)

28,000
40,000 (192
276)

40,000
60,000 (276
414)

70,000
110,000 (620
827)

150,000 2 180,000 (1034
1241)

90,000
120,000 (620
827)

Tensile yield elongation

D638/73°F (23°C)

%

17
25

15
25

10
20

5
15

10
20

Tensile yield strength

D638/73°F (23°C)

psi (MPa)

1700
2800 (12
19)

2000
3100 (14
21)

2900
5000 (20
34)

4500
6000 (31
41)

3500
5000 (24
34)

Tensile break elongation

D638/73°F (23°C)

%

500
800

200
400

100
300

30
200

300
550

Tensile break strength

D638/73°F (23°C)

psi (MPa)

2000
4500 (14
31)

2900
4000 (20
27)

2500
5000 (17
34)

4000
7000 (27
48)

5000
7000 (34
48)

Tensile modulus

D638/73°F (23°C)

psi (MPa)

35,000
55,000 (241
379)

40,000
65,000 (276
448)

80,000
130,000 (551
896)

150,000
220,000 (1034
1517)

100,000
170,000 (689
1172)

Compressive strength

D695/73°F (23°C)

psi (MPa)

2000
3000 (14
20)

3500
4500 (24
31)

4500
6000 (31
41)

6000
8500 (41
58)

4500
6000 (31
41)

Deflection temperature

D648/at 264 psi (1.82 MPa)

°F (°C)

80
100 (27
38)

95
125 (35
51)

104
131 (40
55)

100
131 (38
55)

110
130 (43
54)

Deflection temperature

D648/at 66 psi (0.45 MPa)

°F (°C)

—

120
150 (49
65)

140
167 (60
75)

140
167 (60
75)

130
170 (54
77)

Impact strength notched Izod

D256/73°F (23°C)

Ft-Lb/In

No break

No break

10
20

2
8

No break

Impact strength unnotched Izod

D256/73°F (23°C)

Ft-Lb/In

No break

No break

No break

No break

No break

Hardness

D2240/73°F (23° C)

Shore D

50
57

57
62

60
70

70
75

65
70

Tabor abrasion

CS-17 1000 g: pad

mg/1000 cycles

28
33

21
25

16
19

6
9

16
19

Coefficient of friction—Static vs Steel

ASTM D 1894/ 73°F (23°C)

0.49

0.55

0.33

0.26

0.31

Coefficient of friction— Dynamic vs Steel

ASTM D 1894/ 73°F (23°C)

0.54

0.54

0.33

0.19

0.30

Thermal Propertiesa

Standard/ Conditions

Units

2500

2750/2950

2800/2900

2850

3120

Melting temperature

D3418

°F (°C)

242
257 (117
125)

266
280 (130
138)

284
293 (140
145)

311
320 (155
160)

322
334 (161
168)

Tg (DMA)

at 1 Hz

°F (°C)

246 to 240 (243 to 240)

244 to 240 (242 to 240)

242 to 239 (241 to 239)

241 to 237 (240 to 238)

242 to 239 (241 to 239)

Coefficient of linear thermal expansion

D696

10E 2 5/°F

8.5
10.8

9.0
12.0

7.0
10.3

7.0
10.3

7.0
10.3

Thermal conductivity

ASTM D433

BTU-in/ h ft 2 °F

1.00
1.25

1.00
1.25

1.00
1.25

1.00
1.25

1.00
1.25

Specific heat

DSC

BTU/Lb °F

0.28
0.36

0.28
0.36

0.28
0.36

0.28
0.36

0.28
0.36

Thermal decomposition TGA

1% wt. loss/in air

°F (°C)

707 (375)

707 (375)

707 (375)

707 (375)

707 (375)

(Continued )

Table 11.7 Arkema Kynar Flex PVDF Copolymers [24].—Cont’d

a b

Physical Propertiesa

Standard/ Conditions

Thermal decomposition TGA

Units

2500

2750/2950

2800/2900

2850

3120

1% wt. loss/in nitrogen

°F (°C)

770 (410)

770 (410)

770 (410)

770 (410)

770 (410)

Electrical Propertiesa

Standard/ Conditions

Units

2500

2750/2950

2800/2900

2850

3120

Dielectric strength 73°F

D149/73°F (23°C)

KV/Mil

0.8
1.1

1.1
1.3

1.3
1.5

1.3
1.6

1.3
1.5

Dielectric constant 73°F

D150/ 100 MHz
100 Hz

4.5
13.5

3.8
12.1

3.5
10.6

3.5
10.2

3.2
10.2

Dissipation factor 73°F

D150/100 Hz

0.05
0.29

0.02
0.24

0.02
0.21

0.01
0.22

0.02
0.19

Volume resistivity

D257/DC 68°F (20°C)/65% RH

Ω cm

2 3 1014

2 3 1014

2 3 1014

2 3 1014

2 3 1014

Flame and Smoke Propertiesa

Standard/ Conditions

Units

2500

2750/2950

2800/2900

2850

3120

Burning rate

UL/Bulletin 94

V
O

V
O

V
O

V
O

V
O

Limiting oxygen index (LOI)

D2868

42/95b

43/95b

42/75b

43/75b

42/95b

% O2

Typical property values. Should not be construed as sales specifications. Optional products available with higher LOI.

11: INTRODUCTION

TO

VINYLIDENE FLUORIDE POLYMERS

201

Table 11.8 Effect of Orientation on Electrical Properties of PVDF Films [46]. Frequency, Hz

60

120

10,000

Film Type

ε0

Tan δ, %

ε0

Tan δ, %

ε0

Tan δ, %

Film Thickness, μm

Unoriented

12.0

4.8

11.8

3.7

11.2

1.8

74

Monoaxially oriented

13.7

1.7

13.7

1.6

13.2

2.4

25

ε0 —Dielectric constant, Tan δ—dielectric loss factor.

The β crystalline form has a density of 1.97 g/cm3 and less favored than α form; it is obtained when PVDF is mechanically deformed, for example, stretched, near its melting point. It has an all-trans chain conformation positioning the fluorine atoms on one side and hydrogen atoms on the other side of the chain. The γ crystals are less usual and are obtained from ultrahigh molecular weight PVDF [32,40]. The δ crystals are generated by the distortion of one of the other crystalline forms [41]. Amorphous PVDF has a density of 1.68 g/cm3, suggesting that a typical part with a density of 1.75
1.78 has a crystallinity of 40% [42]. PVDF has four relaxation temperatures at 100°C (α0 ), 50°C (αv), 238°C (β), and 270°C (γ). The alternating CH2 and CF2 groups are arranged in such a way, in the polymer chain that CF2 groups carry a strong dipole moment. This is why PVDF has a unique polarity, high dielectric constant, polymorphism, and high piezoelectric and pyroelectric activity. The piezo- and pyro-activities only exhibit themselves when a PVDF film is poled (Table 11.8) [43
45]. PVDF has a propensity to liberate HF causing it to be susceptible to attack by nucleophiles such as strong bases. PVDF is soluble in polar solvents such as esters, acetone, and tetrahydrofuran. Solubility of PVDF allows film casting from the solutions.

11.6 Processing Polyvinylidene Fluoride PVDF is offered commercially in a broad range of melt flow rates. It is also compounded with a variety of additives to improve either processing or end-use performance properties. Similar to some other fluoropolymers, PVDF comes in the forms of latex and fine powders from emulsion processes and as granules. PVDF is processed and fabricated into parts and coatings using melt processes. In selecting a grade

of the polymer, it is required to consider the shear rate characteristics of the polymer. The rheological characteristics specified for a given commercial PVDF grade should be a consideration in developing a melt-based process to produce parts [47,48]. PVDF homopolymers and copolymers can be fabricated by melt processing without adding extrusion aids or thermostabilizers. PVDF degrades slightly during processing and generating a small amount of hydrofluoric acid. It is advisable to use special alloys resistant to hydrofluoric acid for construction of process surfaces to prevent corrosion. All the common extrusion and molding techniques can be used to process PVDF into shapes. In injection molding, typical molding temperatures in the cylinder and nozzle are 180°C
240°C and a mold temperature range of 50°C
90°C. PVDF is a semicrystalline polymer and exhibits relatively high mold shrinkage of about 3%, reflecting the high degree of crystallization and the difference between the solid and molten densities. When close part dimensional tolerances are desired, it is important to develop post-forming annealing cycles to stabilize the parts. The entire fabrication process should be taken into account while designing molds to ensure correct final part dimensions and tolerances. If the temperature of PVDF exceeds 300°C during processing, it will degrade thermally and resin will darken. This discoloration arises from the dehydro-fluorination reaction which causes the formation of double bonds in the polymer chain. Darkening of PVDF color during processing is a clear indication of overheating. PVDF parts can be joined by various welding methods. Thermal welding methods are preferred over solvent techniques because of the strength of the bond and the ease of preparation of joint surfaces. Vibration, radio frequency, and induction heating methods have been used to soften or melt PVDF joint areas.

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11.7 Applications There is a variety of components and applications that utilize PVDF resins. Some of the most common industries are Chemical Processing—used for its chemical and thermal resistance, PVDF resins are used in environments to provide solutions to corrosion challenges. PVDF homopolymers are used in environments where the pH is ,1
12. PVDF copolymers have a broader range chemical resistance, due to the higher content of fluorination in the chemical backbone. Because of this, many grades of PVDF copolymer resins can be used in pH , , 1
13.5. Common components in the chemical processing industry include piping systems, both solid thermoplastic and lined piping (metal or fiberglass-reinforced). Depending on the chemical and thermal environment, PVDF piping systems can operate up to 150°C continuously. Due to the low melting temperature of PVDF fluoropolymers, they are easily weldable and joinable for piping systems. Choosing the correct joining method is dependent upon the service conditions and PVDF piping suppliers. Common welding methods include

• • • • • •

socket fusion; butt fusion; infrared fusion; electrofusion; bead and crevice free/smooth inner bore; mechanical.

Additional components utilized in the chemical processing industry include pumps, nozzles, valves, tower packing, and flexible tubing. Tanks can also be fabricated for handling chemicals out of PVDF sheets. Solid plastic tanks as well as lined metal or fiberglass-reinforced are available. To make sheets bondable to metal and fiberglass, they are equipped with fabric or glass backings. An adhesive such as an epoxy would then be used to adhere the sheets to the structural support.

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resistance. Additional properties of PVDF solar components include dirt/mold resistance, extended reflectivity, chemical resistance, and burn/smoke resistance. Longterm moisture barrier properties are proven to be outstanding.

• Lithium Ion Battery—PVDF components are used in battery components. Found in both anode and cathode binders as well as separator coatings, formulations using emulsion PVDF have higher surface areas, making dissolution into NMP or acetone faster.

• Semiconductor—Known for its high-purity specifications, the semiconductor industry has made use of PVDF components. Wet benches fabricated using PVDF sheets as well as highpurity piping for deionized water are a few examples of systems used. PVDF meets FM 4910 specification, which allows for it to be used directly in semiconductor clean rooms.

• Architectural Coatings—PVDF has been used for over 50 years as a base resin for paint coatings for exceptional UV resistance. Such coatings are often used for aluminum, galvanized steel, and aluminized steel. New innovations in architectural coatings using PVDF have water-based, field applied, latex formulations called Kynar Aquatec.

• Water Treatment—PVDF components are used for fluid handling components in the water treatment industry. Fluid handling systems, like those used in the chemical processing industry, are also extensively used in water treatment systems. PVDF is especially recommended for chlorinated services, due to its low permeation with halogens. Furthermore, PVDF is used for membranes, in both hollow fiber and flat sheet formulations. Certain PVDF resins also comply with NSF specifications such as NSF 51 and NSF 61.

• Food and Beverage—PVDF is used in food processing facilities for its resistance to cleaning chemicals like chlorines, bleaches, and peracetic acid. Conveyor belting made of PVDF is increasingly used for its strong mechanical properties and abrasion resistance.

• Photovoltaic/Solar—PVDF is used in various

• Pharmaceutical—Resistance to ozone and

components for the solar industry. Commonly used in solar panel backsheets, PVDF films are specified for their weatherability and UV

steam cleaning make PVDF useful in pharmaceutical and biotechnology applications. From piping systems to single use components like

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fittings, PVDF can be used to uphold purity standards while withstanding aggressive chemistries. Certain grades of PVDF also comply with USP standards.

PVDF also meets specifications for food and pharmaceutical processing industries. PVDF homopolymer resins are strong engineering fluoropolymers. PVDF copolymers, similar to homopolymer resins in purity and chemical resistance, but also have chemical compatibility in high pH solutions, increased impact strength at ambient and low temperatures, and increased clarity. PVDF resins can be fabricated into a wide range of components including Pipes, fittings, and valves; pump assemblies; wire and cable insulation; sheet and stock shapes; films; tubing (flexible and rigid); tanks and vessels; nozzles; membranes and filter housing; powder coatings; foams; FOAM; polymer process aids.

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• Plenum—PVDF in both wire and cable and piping components is used in plenum applications. One of the few plastic materials that can pass the stringent ASTM E84 (25/50) test unmodified, PVDF has inherently low limiting oxygen index (LOI) at around 40 and can even reach up to 95 LOI with special additives.

• Wire and Cable—PVDF copolymers are used as the jacketing materials for many different wire and cable constructions. Some examples include data communication cables, fire alarm cables, fire optic cables, shipboard cables, automotive cables, mining cables, and plenum cables. Certain grades of PVDF copolymer retain the 150°C continuous use rating, and can even be cross-linked to achieve higher continuous use temperatures. There are also grades of PVDF copolymers that can be used in subzero temperatures down to 240°C.

• Oil and Gas—PVDF is used in oil and gas applications for both onshore and offshore components. From flexible flowlines and risers to umbilical, composite pipes, and gas station distribution systems, PVDF can be found for its high-temperature resistance to hydrocarbons.

• Polymer Process Aids—PVDF can be used to improve the processing performance of polyolefins including LLDPE, LDPE, HDPE, and PP. A small percentage of PVDF added as a masterbatch can help eliminate troublesome processing issues such as sharkskin, melt fracture, and die buildup.

• 3D Printing—PVDF is used in filament for 3D printing, used the Fused Filament Fabrication (FFF) 3D printing method. PVDF components are used [49] extensively in

• • • • •

the high-purity semiconductor market, the pulp and paper industry, nuclear waste processing, the general chemical processing industry, and water treatment membranes.

PVDF film can be used for applications requiring long-term protection. The film is produced by monolayer or multilayer technology as thin, thick, wide, or narrow (from 10 to 175 μm), allowing great freedom of design. The commercial range includes both mass-tinted and transparent films, which can be printed with a variety of designs. PVDF film can be laminated onto thermoplastic, thermoset, or coated-metal supports. PVDF homopolymers and copolymers are used in the battery industry as binders for cathodes and anodes in lithium-ion batteries and as battery separators in lithium-ion polymer batteries. PVDF powder coating systems allow formation of a thick spray coating of the resin to be applied to metals for optimum corrosion resistance. PVDF powder coatings can be applied without primer. Some PVDF grades pass the flame spread/smoke developed rating of 25/50 when tested in accordance with ASTM E 84. PVDF films have been developed for use in the protection of backsheet and for front sheet glazing. KYNAR film provides superior solar transmittance and also has excellent dirt shedding and fire resistance properties.

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PVDF resin is a good membrane material for applications ranging from bioprocess separations to water purification because it is extremely chemically resistant and well suited to aggressive chemical environments. PVDF has a high-temperature resistance, which makes it appropriate for applications which require high-temperature cleaning. PVDF tolerates ozone and chlorine (an oxidant increasingly used for water purification) very well. Grades with FDA and NSF listings are compatible with direct food/beverage contact applications. PVDF resin is soluble in n-methyl pyrrolidone, dimethylacetamide, dimethylsulfoxide, and DMF and can be conveniently solution-cast into porous membranes by phase inversion. It is used to manufacture flat sheet, hollow fiber, and membranes. Rotomolding, or rotational molding, is a highly versatile manufacturing option that allows for a wide range of design possibilities at low production costs. PVDF copolymer resins are specifically designed for rotational molding and offer excellent abrasion resistance and toughness. Rotolining allows customers to use PVDF resin to protect complex geometric and/or enclosed shapes that cannot be lined or coated in a conventional manner.

References [1] Swarts F. Bull Clin Sci Acad Roy Belg 1901;7:383. [2] Swarts F. J Chem Soc Abstr 1902;82:129. [3] Hauptschein A, Feinberg AH. US Patent 3,188,356, assigned to Pennsalt Chemicals Corp.; 1965. [4] Trager FC, Mansell JD, Wimer WE. US Patent 4,818,513, assigned to PPG Industries, Inc.; 1989. [5] Schultz N, Martens P, Vahlensieck HJ. German Patent 2,659,712, assigned to Dynamit Nobel AG; 1976. [6] McBee, E.T., et al., Ind Eng Chem, 39(3), 409
412, 1947. [7] Kaess F, Michaud H. US Patent 3,600,450, assigned to Sueddeutsche KalkstickstoffWerke AG; 1971. [8] Elsheikh MY. US Patent 4,827,055, assigned to Pennwalt Corp.; 1989. [9] US 8,350,101, assigned to Arkema, France; 2013.

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[10] Ford TA, Hanford WE. US Patent 2,435,537, assigned to DuPont Co.; 1948. [11] Humphrey JS, Amin-Sanayei R. Vinylidene fluoride polymers. Encyclopedia of polymer science and technology. online edition New York, NY: John Wiley & Sons; 2001. [12] Ford TA. US Patent 2,468,054, assigned to DuPont Co.;1949. [13] Dohany JE. Poly(vinylidene fluoride) under “fluorine compounds organic (polymers)”. 3rd ed. Encyclopedia of Chemical Technology Wiley, vol. 11. 1994. p. 694
712. [14] Lo ES. US Patent 3,178,399, assigned to Minnesota Mining and Manufacturing Co.; 1965. [15] Stallings JP. US Patent 3,708,463, assigned to Diamond Shamrock Corp.; 1973. [16] Dohany JE. US Patent 3,857,827, assigned to Pennwalt Corp.; 1974. [17] Dohany JE, US Patent 4,076,929, assigned to Pennwalt Corp.; 1978. [18] Hauptschein M. US Patent 3,193,539, assigned to Pennwalt Corp.; 1965. [19] Barber LA, US Patent 6,187,885, assigned to Atofina Chem Corp; 2001. [20] US Patent 6,649,720, assigned to Atofina Chemicals, Inc; 2003. [21] Amin-Sanayei R, US Patent 6,734,264, assigned to Atofina Chemicals, Inc; 2004. [22] Stallings JP. US Patent 3,780,007, assigned to Diamond Shamrock Corp.; 1973. [23] Dohany JE. US Patent 3,781,265, assigned to Pennwalt Corp.; 1973. [24] Dumoulin J. US Patent 4,524,194, assigned to Solvay & Cie; 1985. [25] Saito R, Amano T. US Patent 5,925,721, assigned to Shin-Estu Chemical Co.; 1999. [26] Tada M, Katsurao T, Ikeda T, Suzuki K. US Patent 7,208,555, assigned to Kureha Chemical Industry; 2007. [27] Kappler P, Gauthe V. US Patent 7,012,122, assigned to Arkema; 2006. [28] British Patent 1, 057,088, assigned to KaliChemie AG; 1967. [29] British Patent 1, 188,889 assigned to Asahi Glass Co.; 1970. [30] Ichimura M, Shi Y. US Patent 3,616,371, assigned to Asahi Glass Co.; 1967. [31] Doll WW, Lando JB. Radiation-initiated solution polymerization of vinylidene fluoride. J Appl Polym Sci 1970;14(7):1767
73.

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[32] Ebnesajjad S. Fluoroplastics. 2nd ed Melt Processible Fluoropolymers, vol. 2. Plastics Design Library, Elsevier; 2015. [33] Amin Sanayei R. WO Patent 2008005745, assigned to Atofina Chemicals; 2008. [34] Amin Sanayei R. US Patent 8785580, assigned to Atofina Chemicals; 2014. [35] US Patent 8080621, assigned to Arkema, Inc; 2011. [36] US Patent 8,338,518, assigned to Arkema, Inc; 2012. [37] Amin-Sanayei R, Durali M, Kappler P, Burch G. US Patent 8,765,890, assigned to Arkema, Inc;2014. [38] Blaise J. European Patent 215710 B1, assigned to Elf Atochem SA;1987. [39] British Patent 1,004,172, assigned to Deutsche Solvay-Werke GmbH; 1965. [40] Bretz PE, Hertzberg RW, Manson JA. Polymer, 22. 1981. p. 1272
8. [41] Castagnet S, Gacougnolle J-L, Dang P. Mater Sci Eng A 2000;276(1/2):152
9. [42] Kynar® & Kynar® FLEX® PVDF. Performance characteristics & data. Arkema, www.kynar.com; 2012. [43] El Mohajir BE, Heymans N. Changes in structural and mechanical behaviour of PVDF with processing and thermomechanical treatments. 1. Change structure. Polymer 2001;42:5661
7. [44] Nakagawa K, Ishida Y. Koloid Z Z Polym 1973;251:103. [45] Mekhilef N. J Appl Polym Sci 2001;80:230
41. [46] Kynar® and Kynar Flex® Polyvinylidene Fluoride. Performance characteristics & data. Arkema; www.kynar.com; 2019. [47] Lovinger AJ. In: Bassett GC, editor. Developments in crystalline polymers, Vol. 1. Barking: Elsevier Applied Science Publishers, Ltd.; 1982.

[48] Gorlitz M, Minke R, Trautvetter W, Weisgerber G. Agnew Makromol Chem 1973;29/30:137. [49] Lutringer G, Weill G. Polymer 1991;32 (5):877.

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Further Reading Esterly Daniel M. Characterization of electrosprayed poly (vinylidene fluoride)/CNT nanocomposite fluoride)/CNT nanocomposite [Dissertation for Masters of Science Degree]. Virginia Polytechnic Institute and State University; 2006. Bachmann MA, Lando JB. Macromolecules 1981;14:40. Weinhold S, Litt MH, Lando JB. J Polym Sci Polym Lett Ed 1979;17:585. Litt MH, Lando JB. J Polym Sci Polym Phys Ed 1982;20:535
52. Loufakis K, Miller KJ, Wunderlich B. Macromolecules 1986;1271. Dohany JE, Humphrey JS. Vinylidene fluoride polymers. 2nd ed. Encyclopedia of polymer science and engineering, 17. New York, NY: John Wiley & Sons; 1989. p. 532
48. Jungnickel B-J. Polymeric materials encyclopedia, vol. 11. New York, NY: CRC Press; 1996. p. 7115
27. Grego´rio R Jr. and co-workers, Ref. 4, pp. 7128
38. Nalwa HA. Ferroelectric polymers: chemistry, physics, and applications. New York, NY: Marcel Dekker, Inc; 1995. British Patent 1,173,688, assigned to Kureha Kogaku Kogyo Kabushiki; 1969. Ebnesajjad S. Fluoroplastics. Melt processible fluoropolymers: the definitive user’s guide and data book, Vol. 2. New York, NY: Plastics Design Library, William Andrew, Elsevier; 2002.

12 Processing and Fabrication of Parts from Melt-Processible Fluoropolymers Sina Ebnesajjad FluoroConsultants Group, LLC, United States

O U T L I N E 12.1 Introduction

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12.2 General Considerations

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12.3 Materials of Construction

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12.4 Rheology of Fluoropolymers 12.4.1 Characterization of Rheology of Fluoropolymers 12.4.2 Processing of Fluoropolymers

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12.5 Injection Molding 12.5.1 Process Conditions and Operations 12.5.2 Dimensional Stability of Parts

212 213 215

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12.6 Extrusion 12.6.1 Introduction 12.6.2 Extrusion Processes 12.6.3 Fluoropolymer Wire Coating 12.6.4 Processing Equipment

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12.7 Fluoropolymer Tube Extrusion 12.7.1 Sizing of Tubes 12.7.2 Film Extrusion

221 222 223

References

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12.1 Introduction

12.2 General Considerations

Melt-processible fluoropolymers are converted into parts by means of processes such as injection molding, extrusion, blow molding, transfer molding, compression molding, rotational molding, and variations of those processes. These techniques produce simple and complex finished parts in batch or continuous operations. Plastics injection molding is distinguished from most other manufacturing processes because of its capability to produce large numbers of the same part relatively quickly [1]. Generally speaking, a whole series of forming, joining, and finishing operations would be required to replicate an injection, transfer, or blow molded article using other manufacturing methods. It is the efficient nature of the operations of melt processes that renders economically viable despite the high cost of machinery and molds. The capability to produce completely finished parts at high speed balances the equation of high machine and mold costs and makes melt-processed articles cost effective.

Several factors affect any type of melt processing of thermoplastics such as injection molding. They include viscosity, heat, temperature, thermal stability, thermal conductivity, crystallinity, and moisture. Viscosity is a measure of flow of a plastic, that is, how fast a given mass of the plastic will flow as a result of applying a force to it. Flow of plastics is inversely proportional to viscosity. Viscosity decreases with an increase in temperature. Lower viscosity is helpful to reduce the flow time of the plastic, resulting in higher production rates and good geometric definition. Temperature must be increased beyond the melting point of the material to increase the flow of the plastic. Above the melting point, viscosity reduction should be balanced against thermal degradation of the thermoplastic fluoropolymer. In practice, viscosity is represented by the melt flow rate (MFR or melt index) of the polymer. MFR is measured using standard

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00012-4 © 2021 Elsevier Inc. All rights reserved.

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equipment and conditions. The lower the MFR, the higher the viscosity of the polymer. Heat has to be delivered to the fluoropolymer to increase its temperature and melt it. The molding equipment should be able to deliver sufficient heat to melt the polymer being processed. Thermal conductivity, the speed by which heat is transferred through a material, of the plastic is usually the controlling step for the delivery of heat to the material. Crystallinity is an important consideration in melt processing because of the impact of a part’s crystalline content on its properties and appearance. Fluoropolymers are semicrystalline, meaning that there is a relatively large volume increase as the polymer melts and becomes entirely amorphous. After molding, a part has been completed the rate at which it is cooled (how fast the part is cooled) will determine the extent of its recrystallization. Properties such as flex fatigue life and transparency increase with a decrease in crystallinity. Mechanical strength and various moduli increase at higher crystallinity. Finally, the thermoplastic polymer should be free of moisture before molding begins; it should be dried before charging the machine. If the presence of moisture water vapor is released during the heating melting of the polymer usually leaving defects in the parts. Trapped moisture appears as bubbles or voids in the finished part both of which are unacceptable quality defects.

12.3 Materials of Construction When a fluoropolymer melts, it produces highly corrosive compounds that often contain hydrofluoric acid. Corrosion of process surfaces often contaminates the molded products and may be even detrimental to its physical properties. Table 12.1 shows a list of the metals and suppliers recommended for the construction of various parts of the injection-molding machine. For perfluoroalkoxy (PFA), fluorinated ethylene propylene (FEP), polyvinylidene fluoride (PVDF), and ethylene tetrafluoroethylene (ETFE) injectionmolding machines, materials like Xaloy 309 and Bernex C240 are suitable for the construction of barrels. Hastelloy C-276 may be used to construct the screw, adapter, and nozzles. Corrosion rate of the contact surfaces of the mold is typically less than the other parts of the processing equipment

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because mold temperatures are lower than the molten polymer. For short production runs, unplated molds of hardened tool steel, or high-quality chrome-plated or nickel-plated injection mold cavities normally provide adequate protection. For longer runs, more corrosion-resistant materials of construction are required.

12.4 Rheology of Fluoropolymers Rheology of the molten fluoropolymers has critical importance to the processing of fluoropolymers. Those polymers, and generally thermoplastic materials, must be processed below the flow velocity at which melt fracture takes place. This velocity is known as the critical shear rate. Melt fracture in molten plastics occurs when a resin’s flow rate exceeds its critical shear velocity (CSV). That is the point where the melt strength of the polymer is surpassed by internal stresses. CSV of most fluoropolymers is usually much lower than those of other thermoplastics. Parts molded in a process in which CSV is exceeded exhibit typical symptoms of melt fracture. A part may appear to have a frosty or cloudy surface. Or it may have a rough surface also known as shark skinning and orange peel. There are a few ways to eliminate melt fracture, such as reducing the velocity of the melt, increasing the critical velocity, and reducing heat losses (see Table 12.2). These remedies have limitations because of their effect on the design of the mold, fabricated part properties, and degradation of the polymer. MFR or melt flow index (MFI) is a flow characteristic of resins included in their specifications. MFR is defined as the mass of molten polymer (in grams) that flows through the die/orifice of a rheometer in 10 minutes at a specific temperature. MFR is inversely proportional to the melt viscosity of a polymer. MFR is, however, a single data point and does not give any information about the behavior of the molten polymer at other shear rates or temperatures. That is because melt viscosity of polymers varies significantly as a function of shear rate. Unlike a Newtonian fluid such as water, polymers often exhibit different viscosities as extrusion rate is changed. Knowledge of MFR at different temperatures and shear rates is important to the successful fabrication processing of resins.

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Table 12.1 Materials for the Construction of Processing Equipment. Metal

Parts

Hastelloy C

Screw, adapter, and nozzle

Hastelloy C-276

Screw, adapter, and nozzle

Duranickel

Screw, adapter, and nozzle

Monel

Screw, adapter, and nozzle

Xaloy 309

Barrel and barrel lining

Brux

Barrel and barrel lining

Reiloy

Barrel and barrel lining

Bernex C240

Barrel and barrel lining

Table 12.2 Techniques for Elimination of Melt Fracture. Remedies

Preventive Actions

Possible Drawbacks

Reduce the velocity of melt

Enlarge runners, gates, or cavities. Slow ram speed

Polymer degradation. Premature melt freeze

Increase the critical velocity

Increase melt and/or mold temperature

Polymer degradation. Ejectability of the part

Reduce heat losses

Shorten the travel distance of the melt to the gate, i.e., multiple gates

Complication of design

Advance rheological testing to characterize melt versus shear rate behavior of specific fluoropolymers allows rapid selection and optimization of process variables. During processing, fluoropolymers degrade thermally in a time/temperature-dependent manner similar to other polymeric materials. That results in a decrease in the molecular weight of the polymer. Melt viscosity of lower molecular weight polymer is less than the higher molecular weight material, thus affecting the MFR of the molten polymer. Varying degrees of increase in MFR occur during processing of all fluoropolymers. A useful method of monitoring thermal degradation is by measurement of the MFR of the polymer being processed.

12.4.1 Characterization of Rheology of Fluoropolymers There are two approaches to characterizing fluoropolymers for melt processing [2]. The more fundamental methodology centers around the measurement of physical properties such as melt viscosity and thermal diffusivity to generate data for mathematical

modeling (simulation) of injection-molding processes. Careful measurement of these physical properties allows reliable predictions by modeling. The alternative methodology is experimental simulation of injection-molding processes, yielding empirical results that are dependent on the type of apparatus. Thermal and rheological characteristics of fluoropolymer melts are important to their melt processing. The single most important property with respect to filling the mold is viscosity of the melt be it an injection or other type of mold. The reader would need a basic understanding of the subject of rheology to be able to interpret the viscosity data. A number of references can be consulted to gain a working knowledge of rheology. Helpful books include Transport Phenomena by Bird, Stewart, and Lightfoot, Melt Rheology and its Role in Plastics Processing: Theory and Applications by J. M. Dealy and K. F. Wissbrun, and Polymer Melt Rheology: A Guide for Industrial Practice by F. N. Cogswell, Melt Rheology and its Applications in Plastics Industry by Dealy and Wang [3 5]. The data and discussions in the following section assume the reader has a modest knowledge of rheology.

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The flow behavior of perfluoroalkoxy tetrafluoroethylene perfluoromethylvinylether (MFA) and FEP determined by using dynamic rheology in the linear viscoelastic regime is seen in Fig. 12.1. It can be seen MFA 1041 has a constant viscosity as a function of shear rate, thus is Newtonian. In Fig. 12.1, steeper G’ of MFA is indicative of a narrower molecular weight distribution (MWD) than the reference FEP resin. MFA thus allows a better diameter control during the extrusion of wire insulation. Commercial resins are routinely supplied with MFR information that is indicative of the fluoropolymer viscosity at a single temperature, often at a low shear rate. ASTM D1238 is the recognized universal measurement procedure. In this technique, a known weight of resin is placed in the melt reservoir of a melt-indexer apparatus to force the molten polymer out of a nozzle. The amount of melt exiting the nozzle during a predetermined length of time, usually 10 minutes, is measured. The results are reported as the mass of melt that has been discharged through the nozzle per unit time (g/10 min). This number provides a quick means for comparing a number of samples. Viscosity behavior as a function of shear rate depends on the polymer structure (e.g., branching), molecular weight, and MWD. Viscosity of most thermoplastics decreases when shear rate is increased; this effect is known as shear thinning.

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The viscosity of two resins with similar molecular structures and different low stress viscosity values (μa . μb) is likely to have the same order (μa1 . μb2) at higher shear rates (Fig. 12.2). The ratio of viscosity values at high shear rates (μa1/ μb2) is much smaller than the same ratio at low shear stresses (μa/μb). See Figs. 12.2 and 12.3 for actual examples of viscosity versus shear rate for various grades of PVDF. There are different methods for direct measurement of viscosity in contrast to the indirect procedure of ASTM Method D1238. They include two types of devices with different kinematics: capillary and rotational viscometers. In capillary viscometers, melt flow is developed in an annular gap or in a slit flow duct. Pressure drop takes place throughout the length of the capillary or the duct. Flow is initiated by the application of a force (pressure) to the polymer melt. There is close resemblance between the way a capillary rheometer and an injection-molding machine (during filling) work. In contrast, rotational viscometers generate a drag flow. Examples of rotational devices include cone and plate viscometer, parallel plate viscometer, and a Couette viscometer. In a capillary viscometer, a piston of known weight presses the melt through a capillary with a specific diameter and length. The flow of a Newtonian fluid in a capillary obeys the Hagen Poiseuille equation.

Figure 12.1 (from v2, 2e) Rheology profile of MFA 1041 (copolymer of tetrafluoroethylene and perfluoromethyl vinyl ether) [6]. Courtesy Solvay Solexis Corp.

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Figure 12.2 Melt viscosity of PVDF copolymer grades (inset numerical tags) at 220°C [7]. Courtesy Solvay Solexis Corp.

πR 1 V_ 5 ΔP 8L η 4

(12.1)

where V_ is the volumetric flow rate, R is the radius of the capillary, L is its length, η is the melt viscosity, and ΔP is the pressure drop. Of course, polymer melts are non-Newtonian due to their shear-thinning behavior. The value calculated from Eq. (12.1) must be corrected for the shear rate at the wall, which is higher for a polymer melt than that calculated by Eq. (12.1). This method is known as the Rabinowitsch Correction and is based on determination of viscosity at a location inside the flow channel as opposed to the wall. At a representative location, Newtonian and nonNewtonian shear rates coincide. This location is at r 5 πR/4, where the pressure transducer should be located to measure the pressure drop. Another correction would have to be made for other pressure losses; it is called the Bagley Correction [9]. The cone-and-plate viscometer is one of the rotational methods of measuring the polymer viscosity. It consists of a flat horizontal plate and a cone with an obtuse angle. The cone touches the plate at its tip and rotates at a constant speed. The melt is charged into the gap forming between the horizontal plate and the cone. The rotational velocity

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Figure 12.3 Melt viscosity of PVDF at 200°C [8]. Courtesy Solvay Solexis Corp.

determines shear rate and the torque applied gives shear stress. Shear rate is constant across the gap, thus it eliminates the need for non-Newtonian behavior of the melt. In a plate-plate viscometer, the cone is replaced by a second flat plate. The Couette viscometer is comprised of two concentric cylinders where one can be rotated at a constant speed.

12.5 Processing of Fluoropolymers Fluoropolymers are processed by meltprocessing techniques such as injection molding or extrusion. One of the limitations of processing fluoropolymers is the relatively low critical shear rate (Table 12.3). When the processing throughput, such as extrusion rate, is increased the flow becomes unstable when the shear rate exceeds the critical value. The best-known manifestation of this phenomenon is surface melt fracture; the surface of solidified melt resembles sharkskin or washboard. Increasing the productivity, therefore, requires eliminating or postponing the melt fracture phenomena to higher shear rates. There are additives that can delay the onset of melt fracture when incorporated in the polymer at low proportions [11].

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Table 12.3 Critical Shear Rate of Fluoropolymers at Typical Processing Conditions [10]. Resin Type Critical shear rate, s21

FEP 100

FEP 140

PFA 340

PFA 350

ETFE 210

ETFE 200

ETFE 280

20

13

50

10

3000

1000

200

Table 12.4 Effect of Boron Nitride and Polyethylene (PE) on the Melt Fracture Behavior of Teflon FEP 4100 [13]. Polymer/Blend

Critical Shear Rate for the Onset of Surface Deterioration

FEP4100

80 s21

FEP4100 1 0.1 wt.% PE

800 s21

FEP4100 1 0.1 wt.% PE 1 0.1 wt.%BN

1300 s21

FEP, Fluorinated ethylene propylene.

Boron nitride (BN) by itself or combined with other additives such as polyolefins has been demonstrated to act as an effective processing aid for several fluoropolymers including perfluorinated ethylene propylene copolymer [12]. BN can be successfully used as processing aids to eliminate not only sharkskin melt fracture but also substantially postpone gross melt fracture to significantly higher shear rates well within the gross melt fracture region. Conventional fluoroelastomers can only eliminate surface melt fracture [13] (Table 12.4).

12.6 Injection Molding Injection molding is one of the most important processes for mass production of parts from thermoplastics, usually without requirement of additional finishing. Most injection-molding machines are of the universal variety, which can accept all types of molds, within limits. The economics of this process are excellent for parts with complex geometry, an advantage over other techniques. Cost per molded part improves with increasing scale, despite the sizable capital cost of injection-molding machines. Fluoropolymers can be injection molded in plunger or ram type equipment but a screw machine works best. The screw-type injectionmolding machines have a number of advantages (Fig. 12.4). The principle of injection molding is fairly simple. Plastic material is heated until it becomes a viscous melt. It is then forced into a closed mold

that defines the shape of the part to be produced. There the material is cooled until it reverts to a solid, then the mold is opened and the finished part is extracted. Although the principle may be simple, the practice of injection molding is anything but simple. This is a consequence of the complex behavior of plastics melts and the ability of the process to encompass complicated products. The essential mechanisms of injection molding are heat transfer and pressure flow. The essential equipment is an injection-molding machine, sometimes known as a press, and a mold which may also be referred to as a tool or sometimes a die. During injection molding, a thermoplastic polymer is heated above its melting point, resulting in the conversion of the solid polymer to a molten fluid with a reasonably low viscosity. This melt is mechanically forced, that is, injected, into a mold in the shape of the desired final part. The low viscosity of the molten polymer allows complete filling of the mold where the part resides until it is cooled below the freezing point of the polymer. In the case of semicrystalline polymers, the crystallinity of the object (which governs its mechanical properties and appearance) is controlled by in-mold cooling of the part at a defined cooling rate. In the final step, the mold is opened and the part is ejected and recovered (Fig. 12.4). The polymer and additives are mixed and melted in the plastication part of the machine. The polymer and the additives are melted, homogenized, pressurized, and fed to the transfer section of the cylinder through a check valve. Plastication takes place at

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Figure 12.4 A typical reciprocating screw injection machine [14].

elevated temperatures (230°C 400°C) depending on the type of the plastic. PVDF is injectionmolded at the lowest end of the temperature range while PFA is processed at the highest temperature.

12.6.1 Process Conditions and Operations This section covers melt temperature and profile, screw rotation, injection speed and pressure, mold temperature and back pressure, cycle management, clean-out procedure, and shutdown and start-up procedures. Injection-molding conditions for a number of unfilled fluoropolymers have been summarized in Tables 12.5 and 12.6. Table 12.7 lists injectionmolding conditions for a number of compounds of different fluoropolymers. Melt temperature, as measured at the nozzle exit, should be decreased as hold-up time becomes longer. When both hold-up time is long and temperature is high, the rear zone should be set to a lower temperature than the front zone to minimize polymer degradation. Front and rear temperatures should be the same when hold-up time is short. Mechanical work increases the temperature of melt and should be considered. A rear zone temperature that is too high may cause bridging of the feed, while a temperature that is too low results in high torque demand and stalling of the screw. The location of the thermocouples, machine size, screw type and speed, shot size and cycle time should be considered in the selection of melt temperature. PVDF may be molded using a conventional polyolefin screw extruder type. An open nozzle should be used. Hot channels can lead to stagnation at high temperature, which could induce degradation problems.

Even though hot channels are used in the industry, they are not recommended [18]. Shrinkage of injected PVDF parts is 2% 3%. The mold has to be designed to account for that shrinkage. It is best to heat the mold and let the PVDF pieces cool slowly, while applying a sufficient hold pressure. This assures a complete filling and strong weld line (almost 100% of properties achievable on the weld line). The optimum mold temperature is 60°C 90° C although sometimes higher temperature is practiced. The holding pressure should be close to the injection pressure. If lower shrinkage than 2% 3% is required, a compound (reinforced grade) has to be used. In order to release internal stresses, the parts can be annealed at 150°C with slow heating and cooling. A good starting base for annealing time at 150°C is 0.5 hour for 1 cm thickness (1 hour for 2 cm, etc.). ETFE is particularly suitable for molding complex shapes such as pump parts, valves, and electronic and electrical parts using an in-line screw molding process. The injection-molding machine must be capable of operating at 350°C in addition to being corrosion-resistant. The mold gate must be designed based on the thickness of the molded part. For example, a direct or a center gate may be used but the gate should be as large as possible. The runner should be cylindrical and as short as possible. Tables 12.8 and 12.9 show examples of injectionmolding conditions. PFA can be injection molded following the same processes used for normal thermoplastic resins. The low-viscosity grades are particularly designed for injection molding of complex shapes. It is recommended to use three independently controlled heater zones for the barrel and one for the adaptor. The heater controllers must be capable of accurate

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Table 12.5 Injection-Molding Conditions for Fluoropolymers [15]. Process Variable

PVDF

ECTFE

FEP

PFA

ETFE

Rear

193 215

265 277

315 329

315 332

273 302

Center

204 227

271 282

329 343

329 343

302 330

Front

221 232

277 288

371

371

302 330

Nozzle temperature, °C

232 260

288

371

371

343

Mold temperature, °C

Ambient to 93

Ambient to 107

.93

149 260

25 190

282

343 382

343 399

303 329

Cylinder temperature, °C

Stock temperature, °Ca Injection speed, rpm

Slow-fast

Moderately fast

Slow

Slow

Moderately fast

Injection pressure, MPa

6.2

—

21 55

21 55

21 103

Hold pressure, MPa

3.5

—

3.5 4.0

3.5 4.0

2.0 3.5

Back pressure, kPa

b

172

Times, s Injection

3 4

Hold-up

7 8

Cooling

25 30

Mold shrinkage, % (3.2 mm thick test bar)

2.5 3.0

a

Stock temperature refers to the temperature of the resin as it leaves the nozzle. Back pressure is the pressure applied by the hydraulic system which the screw must overcome in its backward motion while recharging the melt. PFA, Perfluoroalkoxy; FEP, fluorinated ethylene propylene; ETFE, ethylene tetrafluoroethylene; PVDF, polyvinylidene fluoride; ECTFE, ethylene chlorotrifluoroethylene polymer. b

temperature control up to 450°C. Reciprocating screw equipment is recommended to assure proper plasticating and reduce polymer stagnation and thermal degradation. The screw should have a short transition section, a constant pitch, and a flight depth ratio from the feed section to metering section of about 3:1 [21]. Table 12.10 provides additional information about the screw specifications for processing PFA. In general, it is important to minimize the rotation speed of the screw. High speeds can be used, in conjunction with the appropriate backpressure, to mold thin and/or long parts. Injection pressure should also be as low as possible. Dimensional stability improves when pressure is lowered because of a reduction in the residual stresses in the molded part. Injection pressure may have to be increased if an improvement in the weld line or a reduction in sink marks is desired. Part design and

equipment capability should always be considered in the selection of injection pressure. Backpressure should be kept at the lowest possible value. Increasing it can sometimes help increase the stock temperature. Injection speed should be selected by consideration of the smallest channel through which the melt has to flow. If the injection speed is too fast, the surface of part will turn out frosty or rough. If the injection speed is too slow, the part surface will be rippled. Selection of the injection speed should take into account all other variables in the process such as the melt temperature, shot size, and mold temperature. Mold temperature should be selected with consideration given to many dependent process parameters, including the design and geometry of the part, surface finish, ejectability, residual stresses, and shrinkage of the part. Mold temperature also

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Table 12.6 Injection-Molding Parameters for PVDF [16]. Grade

Barrel Temperature, °C Rear

Middle

Front

Nozzle

Mold

KYNAR 460

200 230

210 240

220 250

230 255

50 90

KYNAR 710

190 210

200 220

200 240

200 240

50 90

KYNAR 720

190 210

200 220

200 240

200 240

50 90

KYNAR 740

200 220

210 230

210 245

210 245

50 90

KYNAR 1000

200 220

210 230

210 245

210 245

50 90

KYNAR 6000

190 210

200 220

200 240

200 240

50 90

KYNAR 9000

190 210

200 220

200 240

200 240

50 90

KYNAR 370

190 210

200 220

200 240

200 240

50 90

KYNAR SUPERFLEX 2500

170 220

170 230

170 245

170 245

50 90

KYNAR FLEX2750-01

200 220

210 230

210 245

210 245

50 90

KYNAR FLEX 2800-20

200 220

210 230

210 245

210 245

50 90

KYNAR FLEX 2850-04

190 210

200 220

200 240

200 240

50 90

KYNAR FLEX 3120-10

190 210

200 220

200 240

200 240

50 90

influences the cycle time, which is important to the productivity of the machine. Very high mold temperatures should be avoided when the part has thick walls. It can be set higher than normal when the flow path in the mold is long relative to the part’s wall thickness. A technique has been reported for molding thin parts in which the mold temperature was raised above the melting point of the polymer before filling began [23]. After the mold was filled, the mold was cooled down below the freeze point of the polymer. An increase in the mold temperature also reduces the likelihood of part delamination. Thorough cleaning of the injection-molding machines is very important. If the parts are made of noncorrosion-resistant metal, cleaning prevents corrosion of the process surfaces. The residual polymer can be removed by purging, which means running the machine on a material capable of pushing the fluoropolymer out of the machine. A purge can be left in the machine when the equipment is constructed of corrosion-resistant metals. The most effective procedure for cleaning is comprised of taking apart the injection-molding machine before the molten polymer cools and removing the polymer.

12.6.2 Dimensional Stability of Parts Close tolerances can be achieved provided that operating variables of the process are tightly controlled. Mold design is, however, the other critical factor in obtaining high precision molded parts. As tolerances become smaller, cost of manufacturing and complexity of the process grow. Almost every variable in the molding process affects the part dimensions. Shrinkage during the molding process is a critical factor in determining the final dimensions of the part. Shrinkage increases with part thickness and mold temperature because in both cases cooling rate decreases. Consequently, crystallinity increases and internal stresses decrease. A number of plastics exhibit direction-dependent shrinkage. The least amount of shrinkage takes place in the direction of flow due to the highest degree of orientation in this direction. The rule-of-thumb that is useful to the design of molds is that the straighter the path, the lower the shrinkage. Annealing a molded part, that is, heating and holding it above its performance temperature, can prevent further shrinkage during its use [1].

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Table 12.7 Injection-Molding Conditions for Compounds of Fluoropolymers [17]. Process Variable

PFA

PFA

PVDF

FEP

ETFE

ETFE

Filler

Glass fiber

Carbon fiber

Carbon fiber

Carbon or glass fiber

Glass fiber

Carbon fiber

Filler content, wt.%

15 30

20 30

10 30

20

10 25

10 30

Cylinder temperature, °C Rear

316 332

316 332

182 249

316 329

274 302

274 302

Center

329 343

329 343

193 260

329 343

302 329

302 329

Front

338 366

338 366

213 274

366 371

307 335

307 335

Melt temperature, °C

343 385

343 385

210 288

343 385

293 343

293 343

Mold temperature, °C

149 232

149 232

82 104

.93

66 149

66 149

Injection speed fill, mm/s

13 25

13 25

13 25

13 25

25 51

25 51

Screw, rpm

60 90

60 90

60 90

60 90

60 90

60 90

Injection pressure, MPa

55 83

55 83

69 103

21 55

69 103

69 103

Hold pressure, MPa

21 48

21 48

34 69

21 48

34 69

34 69

Back pressure, kPa2

0.34 0.69

0.34 0.69

0.34 0.69

0.34 0.69

0.34 0.69

0.34 0.69

121

121

121

121

121

121

2

2

2

2 4

2

2

Resin drying Temperature, °C Time, h

PFA, Perfluoroalkoxy; FEP, fluorinated ethylene propylene; ETFE, ethylene tetrafluoroethylene; PVDF, polyvinylidene fluoride.

12.7 Extrusion Extrusion is one of the most popular processes for fabricating parts from fluoropolymers. Unlike injection molding, extrusion usually results in a semifinished or intermediate article that needs further processing to arrive at an end product.

12.7.1 Introduction Fiber and filament account for a relatively small share of all fluoropolymer extrusion. The principal end products are wire insulation, tubing, film, and sheet. Fluoropolymer films are widely

used in release, surface protection, and packaging. Sheet and profile extrusions account for a small share of fluoropolymers consumption. The products of degradation of molten fluoropolymers are highly corrosive, often containing hydrofluoric acid. It is important that the surfaces of machines that come in contact with molten fluoropolymers are constructed from special grades of corrosion-resistant metals. These metals are significantly more expensive than lower grades of steel. Corrosion of process surfaces can result in the contamination of the finished product and deterioration of its physical properties [1].

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Table 12.8 Typical Injection-Molding Process Settings for ETFE [19]. Units

Cap nut (OD: 18 mm)

Plate (30L 3 100L 3 2t)

°C (°F)

300 (572)

300 (572)

Rear

325 (617)

330 (626)

Middle

330 (626)

335 (635)

Front

330 (626)

335 (635)

Nozzle

100 (212)

100 (212)

Cylinder temperature

Mold temperature screw speed

rpm

150

150

Injection pressure

MPa (psi)

93.2 (13,518)

93.2 (13,518)

Holding pressure

MPa (psi)

44.1 (6396)

44.1 (6396)

Holding time

s

20

20

1.0

1.0

Injection rate (flow value scale) Cooling time

s

60

60

Molding cycle time

s/cycle

120

120

Table 12.9 Typical Injection-Molding Process Conditions for ETFE [20]. Natural Grade Molding temperature (°C)

Back

260 280

Middle

270 290

Front

280 300

Nozzle

290 320

Mold temperature (°C)

60 120

Injection pressure (MPa)

50 120

Injection speed (ram speed) (mm/s)

1 15

Molding cycle (s)

30 120

12.7.2 Extrusion Processes Extrusion is a continuous process that involves forming a product in two dimensions. These x y dimensions determine the cross-sectional form of the extrudate, and this can be almost unlimited in scope, ranging from a simple tube to a very complex extruded profile. The third (z) dimension is the length of the extrudate. In principle, it can be infinite but in fact it is limited by practical considerations such as those of winding, reeling, storage, and transport. The essential point is extrusion always produces a constant cross-section object. The product cross section is formed in a die; the extrusion process consists of raising a thermoplastic

above its melt temperature and forcing it through the die. The heating and pressurizing device involves one or more screws operating in a heated barrel and is known as an extruder. Downstream of the die, the extrudate is calibrated, cooled, and packaged by an array of ancillary devices including vacuum calibrators, air cooling chamber, water tanks, cooling rolls, haul-offs, cutters, and winders. Upstream of the die, a melt pump may be interposed between the extruder and the die. The exact selection and arrangement of these components of an extrusion system depends on the end product. The principle variants are the single-screw and the twin-screw types. Of these, the single-screw extruder is by far the most popular. The twin-screw

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Table 12.10 Typical Screw Characteristics for Processing PFA [22]. Screw Diameter Ratio Multiply This Number by the Screw Diameter

Characteristics

Description

Length

Length of flighted section of the screw

20

Feed section

Length of feed section

10 12

Transition section

Length of transition section

4 5

Metering section

Length of metering section

4 5

Pitch

Distance between flights

1

Flight width

Width of flight

0.1

FD feed

Flight depth in the feed section

0.16 0.18

FD metering

Flight depth in the metering section

0.06 0.07

Compression

FD feed/FD meter

2.5 2.7

Figure 12.5 (A) Single-screw extruder (B) American Kuhne cutaway extruder as displayed at NPE 2012 [24].

extruder may have parallel or conical screws, and these screws may rotate in the same direction (corotating) or in opposite directions (counterrotating). Extruders with more than two screws are known, for example, the quad-screw extruder, but are not widely

used. Twin-screw extruders are normally used when mixing and homogenization of the melt is very important, and in particular where additives are to be incorporated. Fluoropolymers are normally extruded using a single-screw machine (Fig. 12.5).

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The single-screw extruder consists essentially of a screw that rotates in an axially fixed position within the close-fitting bore of a barrel. The screw is motor-driven through a gear reduction train and is supported by a thrust bearing that opposes the force exerted on the plastic melts. A helical flight on the screw provides the drag-induced conveying motion that develops this force. The barrel is equipped with both heating and cooling means, and its downstream end is provided with an attachment device for a shaping die that determines the cross section of the extruded product, known as an extrudate or extrusion. The upstream or inlet end of the barrel is equipped with a feed throat or an aperture in the barrel wall where a plastics material is input, generally in the form of granules or pellets. During its passage along the helical screw flight, this material is heated by a combination of conducted heat received from the barrel and mechanical shear heat. The latter is derived from the mixing and kneading action of the screw. The output rate of the extruder is a function of screw speed, screw geometry, and melt viscosity. The pressure developed in the extruder system is a function of melt viscosity, screw design, and barrel and die resistance. Extrusion pressures are lower than those encountered in injection molding and are typically less than 35 MPa. The key determinant of extruder performance is the screw. The screw has three functions to perform: feeding and conveying the solid thermoplastic pellets; melting, compressing, and homogenizing the material; and metering or pumping the melt to the die. The typical extruder screw takes the form of a single constant-pitch flight that decreases in depth from the input end to the output end. The pitch is usually equal to the screw diameter. This is sometimes known as a square pitch screw; the resulting helix angle is 17.8°. The screw features three sequential zones, corresponding to the three functions of feeding, compression, and metering.

12.7.3 Fluoropolymer Wire Coating The process of coating wire and cable by extrusion has been around for quite some time. Molten plastic is extruded into a crosshead die through which the wire being coated is passed. After leaving the die, the coated wire is cooled in air and/or

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water bath while it is continuously tested for spark and concentricity (roundness of the coating). In this process, primary insulation is defined as a metal wire directly coated with a plastic to isolate the metal electrically. Jacketing (or sheathing) is referred to covering one wire or a group of wire with a plastic coating or jacket for nonelectrical protection. Jackets are usually put on primary wires. Primary insulation using fluoropolymers date back to the 1960s. A large application of fluoropolymers, particularly FEP resin and ETFE copolymer, is insulation of wire and cable. FEP has a temperature rating of 205°C and retains its chemical resistance, dielectric strength, and low dielectric constant and dissipation factor. Flame resistance of perfluoropolymers such as FEP is an important consideration in plenum applications because these plastics do not increase the combustible fuel loading of the plenum.

12.7.4 Processing Equipment A typical fluoropolymer coating line has several pieces of equipment as summarized in Table 12.11 and illustrated in Fig. 12.5. A standard thermoplastic extruder design built using special materials of construction with a length to diameter (L/D) ratio of 20:1 to 30:1 is typically used to extrude fluoropolymers. High L/D machines are required to obtain sufficient contact heat transfer surface area to melt the resin. Lower L/D results in a lower output, and barrel temperatures would have to be raised risking an increase in polymer degradation. Restriction of flow imposed by the sizing die usually reduces the extruder capacity. Moisture should be avoided during the extrusion because it forms bubbles in the insulation that can become points of dielectric breakdown failure. Fluoropolymers are hydrophobic but can pick up moisture due to condensation of ambient water vapor. Pigments in color concentrates can be hygroscopic and adsorb moisture. The best way to dry the resin is to blow warm air into the feed hopper concurrently to the direction of flow of the pellets. Blowing air at a temperature in the range of 120° C 160°C for 1 2 hours at a velocity of ,1 cm/ second is sufficient for drying the resin. It is also possible to dry the resin in a batch oven prior to charging the hopper.

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Table 12.11 Components of a Typical Wire Insulation Line Equipment. Equipment

Function

Hopper (plastic feed)

Dry and feed resin to the extruder

Unwind

Feed uncoated wire to the crosshead die

Tension control device

Regulates tension in the feed wire

Preheater

Heats up the uncoated wire

Extruder

Melts and heats up the resin and feeds the crosshead die

Crosshead die

Redirects the molten resin 90° to coat the wire

Cooling trough

Quenches the coated wire

Capastan

Drives the wire to line speed

Spark tester

Detects faults in the coated wire

Take-up

Winds up the coated wire

Figure 12.6 Schematic of a typical single-flighted screw [25]. Courtesy Jeff A. Myers of Robert Barr, Inc.

Fig. 12.6 shows a conventional screw design that works for fluoropolymers. A more desirable design would have a longer feed section, a compression ratio of 3:1, and a core progressive profile without exerting excessive shear on the melt. A constant pitch is preferred for a conventional screw. Anytime shear-intensive screws such as a barrier screw or a variable pitch screw are employed, resin could undergo degradation at higher screw speeds. Conveying and melting of the polymer particles (pellets or cubes) occur in the feed section of the screw. A longer feed section (at least eight turns) increases the residence time for heat transfer to the resin from the barrel, thus increasing the melt capacity of the extruder. The metering section of the screw should be sufficiently deep to allow a uniform flow of the polymer melt. It should, however, leave enough root diameters to maintain structural integrity of the screw. The section of the screw that connects the feed section to the metering part is called the transition section. In a core progressive design, compression (pressurization of the melt) takes place over three

or four turns of the screw, ensuring a gradual compression of the melt over the core progressive profile. The greatest advantage of this design is that it reduces damage to shear/pressure-sensitive polymer melts. The metering section acts as a pump generating sufficient pressure to push the melt through the crosshead and sizing dies. The depth of the flight of the metering section determines the pumping capacity; deeper flights have more capacity. The depth of the flights in the feed section should always be more than in the metering section to insure that compression is achieved. The metering section must always be full to allow a uniform flow and pressure melt. In a conventional screw, a metering section with 5 7 flight is capable of producing adequate pressure. A mixing head (Fig. 12.7), to achieve a homogeneous melt when fillers or pigment concentrates are present, should replace a segment of the metering section. Resin stagnation may be avoided by the use of a slightly conical or rounded nose screw tip. The type of the screw tip depends on whether a breaker plate is installed in the extruder. Although fluoropolymer

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extrusion does not require breaker plates, their installation can increase the backpressure for additional mixing. Breaker plates should be streamlined to avoid melt holdup and stagnation. Fluoropolymer screws do not need internal cooling. A screenpack consisting of one 120 mesh and two 80 mesh screens can be installed to filter contaminants out of the melt. Polymer melt travels from the extruder to the head through an adapter. The internal design of the adapter should be streamlined to allow smooth flow. Conical reductions should, thus, be made at a maximum angle of 30°. The adapter should be equipped with a heater band to prevent cooling of the melt and also alleviate the need for overheating the melt in the extruder. Melt viscosity is a strong function of the melt temperature. Extrusion rate (the volume of the resin extruded per unit time) varies with viscosity so any variation in melt viscosity will be reflected in the extrusion rate. It is important to preheat the adapter by using the heater band to prevent solidification of the initial melt on its interior surface, which can result in excessive pressure build up.

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A crosshead die is the transition piece that usually alters the direction of the melt flow by 90° to coat the wire. Design of the crosshead should be streamlined with minimum melt residence time. Fig. 12.7 shows a conventional crosshead design die. In this design, the die is manually adjusted for concentricity while there are other designs in which the die is fixed and concentricity adjusts automatically. The most common type of die for fluoropolymer extrusion is a tube die (Fig. 12.8A) that extrudes a thin wall tube around the wire. The tube is drawn by vacuum onto the wire after it leaves the die. The vacuum is pulled through the clearance between the conductor and its passageway through the crosshead. Another type is pressure die in which the melt comes in contact with the wire prior to leaving the die while the melt is under pressure (Fig. 12.8B). FEP, and in general fluoropolymers, produces toxic fumes as a result of degradation which increases when the melt is exposed to increasing temperatures. Some of the liberated fumes such as PFIB (perfluoroisobutylene) are extremely toxic and must be thoroughly vented. Detailed information about the products of degradation of fluoropolymers have been discussed elsewhere [1]. One effective strategy is to apply local exhaust ventilation. This method removes a small volume of air from a specific location, usually where fumes are emitted. Definitive system design and procedures for exhaust can be found in The Guide to Safe Handling of Fluoropolymers Resins by Plastics Industry Association [26].

12.8 Fluoropolymer Tube Extrusion Figure 12.7 A conventional crosshead design [11].

Tube extrusion is quite similar to the wire insulation process. Processing details depend on the size

Figure 12.8 Schematic diagrams of tubing and pressure dies [24].

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and type of the tube. Tube can be manufactured by both in-line and crosshead dies. FEP tubing can be produced with outside diameters as low as 1 mm to over 20 mm. This range is broken into three processing zones based on the size of the outside diameter: small, medium, and large. The sizing die determines the outside diameter of the extrusion output and the line speed determines the inside diameter. Take-up speed and the die gap, and the difference between the inside diameter of the die and the outside diameter of the tip, set the wall thickness.

12.8.1 Sizing of Tubes Thermoplastics tube size is set by one of four techniques: the vacuum trough, the extended (internal) mandrel, the sizing sleeve, and the sizing plates (Fig. 12.9). Vacuum trough and extended mandrel are the more common techniques for holding the size of fluoropolymer tubes and pipes in a quench bath. These two methods are described in

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detail below. In the sizing sleeve method, the outside diameter of the tube is fixed as it comes in contact with a water-cooled metal sleeve (usually brass). This contact takes place by air pressure inside the tube or by drawing a vacuum through the perforated internal surface of the sleeve. The sizing-plate method predates the vacuum trough technique. In this method, the tube takes its size as it is pulled through a series of brass or stainless steel plates, similar to the way that metal wire is drawn from a metal rod. The tube is forced through the plates by a positive internal air pressure. In the vacuum trough method illustrated in Fig. 12.9A, the tube (or pipe) enters into one end of a closed long trough and is extruded out of the other end. The trough is filled with water, which directly contacts and surrounds the tube, providing efficient cooling. Inside the trough, vacuum is drawn over the water, reducing the pressure in the trough, thus allowing the soft tube to expand against the collars or rings at the entrance and exit. This mechanism prevents tube collapse and ensures well-rounded sizing of the

Figure 12.9 Methods of tube and pipe sizing. (A) Vacuum trough method. (B) Extended mandrel method redraw.

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outside of the tube. The pressure difference between the outside (under vacuum) and the inside of the tube that is open to the atmosphere generates the expansion force. The tube moves through fixed metal collars or rings which give it its size. The first collar at the trough entrance is the most important one. A small stream of water is directed at the pipe just before entrance as a lubricant. The number and positions of the rings and sleeves are usually adjustable. They become slightly smaller as the pipe passes through the trough, taking into account shrinkage that results from cooling and crystallization of the polymer. The vacuum trough method provides excellent cooling with little frictional drag acting against the tube. There are no theoretical limitations to the production rate. The trough has to be longer for higher output rates, which has a bearing on the available space. Large pipes ( . 100 mm in outside diameter) are hard to keep submerged under the water without distorting them or marring their surface. In these occasions, the trough is not filled, but water is sprayed and cascaded all around the pipe while it is passing through. The entire inside space of the trough is maintained under vacuum. The extended mandrel technique, illustrated in Fig. 12.9B, is highly desirable for fluoropolymers because it provides internal cooling and support to the tube. The shrinkage of the plastic as a result of cooling causes a tight contact between the tube and the metal mandrel that can extend up to 30 cm beyond the die. Longer mandrels have been difficult to use due to mechanical difficulties. Surfaces will eventually corrode mandrels made of brass, stainless steel, or aluminum. Their surface is roughened to prevent adhesion of the tube as it passes over the surface. The mandrel is tapered with the larger end closer to the die. The diameter difference between the large and small ends is a function of the fluoropolymer type in addition to the variables related to the tube type and size. The end of the mandrel is slightly larger than the final pipe size to allow additional shrinkage. There is a passage for cooling water inside the mandrel. To ensure a constant temperature, the passage must be uniform. Water cascades accomplish additional cooling over the tube. A small split ring is placed around the tube just as it leaves the die to prevent oscillation of the water, which could otherwise cause water marking. Water cascades further cool the tube after it leaves the mandrel. It finally enters a water trough equipped with sizing plates.

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12.8.2 Film Extrusion Fluoropolymer films can be produced either by extrusion casting or extrusion blowing processes. Each has its advantages and disadvantages. These basic processes result in a film with a molecular orientation predominantly in the machine direction. Superior optical and physical properties can be developed by orienting the film in two orthogonal directions. The process is known as biaxial orientation and it can be applied to both tubular and sheet films. Regardless of the process, film production lines include common downstream equipment such as haul-off, tensioning, and reeling stations. A high-purity melt, free of inclusions, is essential for film production. This is achieved by filtering the melt through a screenpack upstream of the die.

12.8.2.1 Cast Film Cast film is produced by extruding the melt from a slit die and cooling it either by contact with a chill roll or by quenching in a water bath. Both processes are characterized by relatively high melt temperatures and rapid rates of film cooling. This results in films with low haze, good clarity, and high gloss. Cast film grades typically have a MFI in the range of 5.0 12.0 g/10 min. Chill Roll. In the chill roll cast film process, a plastics web is extruded from a slit die (Fig. 12.10) against the surface of a water-cooled chill roll. The die is arranged to extrude vertically or obliquely downwards so that the film web is delivered

Figure 12.10 Vacuum and air knife system to remove air, pinning film to the casting roll [24].

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approximately tangentially to the roll surface. The die is similar in principle to a sheet die but will usually not include a restrictor bar. Film thickness is partially regulated by the gap between the die lips but also by the rotational speed of the chill roll which is arranged so as to drawdown and thin the melt web. Consequently, the die gap is set in excess of the desired film thickness. Typical die gap settings for fluoropolymers are 0.4 mm for films up to 0.25 mm thick and 0.75 mm for film gauges in the range of 0.25 0.6 mm. Die lip adjusters should allow the die gap to be varied at each adjustment point across the die width in order to allow control of the transverse film thickness. Film reel quality will suffer if the transverse thickness tolerance exceeds 6 5% of target thickness. A constant temperature should be maintained across the die so that film drawdown rates and physical properties remain constant across the film web. Attempts to control film thickness by varying the temperature profile across the die will disturb these factors and reduce film quality. When the process is correctly regulated, the thickness uniformity of the chill roll cast film is substantially superior to the blown film. The die is maintained in close proximity (typically 40 80 mm) to the chill roll so that the low-strength melt web remains unsupported for a minimal distance and time (Fig. 12.11). If the die is too close, there is insufficient space for thickness drawdown and widthwise neck-in to take place in a stable manner. The web flows on to the water-cooled chill roll with a wraparound of 240° or more before passing to a second chill roll, and then proceeding to edge trimming, tensioning, and windup stations. The first chill roll is critical to process quality. The cooling capacity must

Rubber nip roll Vacuum

Treatment center Slitter Idler roll

Air knife Rubber nip Stainless nip

Figure 12.11 Typical chill roll cast film line.

FLUOROPOLYMERS

be sufficient to chill the film at high output rates, and the temperature gradient across the width of the roll should not exceed 6 1°C. The actual roll temperature depends on film gauge, line speed, and roll diameter. The chill roll drive speeds must be very precisely regulated to control film drawdown and finished thickness. At line speeds greater than 30 m/min, there is a tendency for a thin cushion of air to become trapped between the film and the chill roll. This results in slow and uneven cooling, affecting the appearance and properties of the film. Two measures are adopted to counteract the problem (Fig. 12.12). An air knife delivers a streamline jet of filtered air that impinges on the film just beyond the point of first contact with the chill roll and presses it against the roll. The air is supplied through a narrow slit of about 1.5 mm gap and controlled at a low differential above atmospheric pressure. An optimum air knife setting improves film clarity and gloss, but excessive pressure induces melt vibration and mars the film surface. The second measure is the provision of a vacuum box, associated with the die, and operating close to the chill roll surface just ahead of the extruded melt web. The vacuum box removes any condensates that have been deposited on the roll by the film and tends to draw air out of the interface between film and roll. The film temperature should be close to ambient at the film windup station, otherwise the reel will continue to shrink after cooling, causing stretch marks and corrugations, and accentuating any variations in thickness. Winding tension should be sufficient to ensure the integrity of the reel but otherwise

Extruder

Highly polished chill rolls

TO

Windup

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Figure 12.12 Detail of the chill roll process.

should be kept low to allow for a small amount of postwinding shrinkage that will tighten the reel. As in other plastic processes, the cast film process depends on many interactive variables so that any defect may have one of several causes. A remedy for one defect may introduce another, so process problem-solving is not straightforward. With these provisos, the troubleshooting chart (Table 12.12) provides a useful guide for problem-solving. Water Quench. The water-quench cast film process (Fig. 12.13) is similar in concept to the chill roll process and uses similar downstream equipment. A water bath takes the place of the chill roll for film cooling, and by cooling both sides of the film equally, it produces a film with slightly differ-

Table 12.12 Chill Roll Film Trouble Shooting Chart. Problem

Possible Cause

Suggested Remedy

Poor clarity

Cooling rate too low

Increase melt temperature Reposition air knife Increase air knife pressure Reduce chill roll temperature

Variable optical properties

Temperature gradient across the chill roll

Adjust chill roll coolant supply Check chill roll cooling circuits

Plateout

Poor chill roll contact

Increase chill roll wrap around Adjust air knife

Flow defects

Melt temperature too low

Increase die and adapter temperatures

Brittle or fibrous heat seals

Melt temperature too high

Decrease die and adapter temperatures

Figure 12.13 Typical water-quench film line [11].

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ent properties compared to the chill roll cast film. The extruder’s slit-die is arranged vertically and extrudes a melt web directly into the water bath at close range. The film passes under a pair of idler rollers in the bath and, for any given rate of extrusion, it is the rate of downstream haul-off that regulates film drawdown and finished thickness. The speed of the process is limited by the tendency of the film to carry over water from the quench bath. Surface defects can also arise from rippling in the water bath. The very rapid quenching induced by the water bath reduces crystallinity and produces a tough film.

12.8.2.2 Blown Film The blown-film process involves extruding a relatively thick tube, which is then expanded or blown by internal air pressure to produce a relatively thin film. The tube can be collapsed to form double-layer layflat film or can be slit to make one or two singlelayer film webs. The air-cooled blown-film process is in very widespread use for polyethylene films. Water Quench. The water-quench blown-film process is also known as the tubular water-quench process and uses a tube die that is arranged to extrude vertically downwards (Fig. 12.14). The process needs a melt strength greater than that required by cast film processes so lower melt temperatures are used. The die should be designed for constant output rates and thickness at every point around the annular die gap. This requires streamlined internal melt flow paths and precise multipoint means of centering the mandrel within the die ring. The die gap is typically about 0.4 mm, with a short land length and a die entry angle of about 10°. Close thickness tolerances are difficult to achieve with the blown film process; variations around the bubble can be limited to perhaps 6 10% of the target film thickness. To distribute these variations evenly, blown film lines may include rotating or oscillating motions applied either to the die, the extruder, or the haul-off. As it emerges from the die, the tube passes through an air ring that lays a large volume of lowvelocity cooling air over the external surface. At the same time, the tube is pressurized internally by air supplied through the die mandrel. The air is confined by downstream nip rollers, so it inflates the still soft tube to form an enlarged bubble. This distends and thins the tube walls to the final film thickness. The

Figure 12.14 Water-quench process for the blown film.

bubble size is limited and calibrated by an annular water cooling weir that covers the outer bubble surface with a stream of cooling water. The water contact is directed and controlled by an annular flexible skirt. The rapid bubble cooling induced by the water stream limits crystallinity and produces a clearer film than is possible by the air-cooled blown process. After cooling, the film passes through a collapsing frame and proceeds through dewatering, hauloff, and winding stations. The process used with fluoroplastics is the aircooled blown-film process. It uses a similar die to the water-quench process but it is normally arranged to extrude vertically upwards. The film passes through a similar, but higher capacity, air cooling ring and is inflated in the same way. The principal difference is that the bubble is much longer in the air cooled process than it is in the water-quench process. This extra length is necessary for bubble cooling to be completed by air contact. Line speed is effectively limited by the bubble length available for cooling. Downstream equipment includes bubble calibration, a collapsing frame, and haul-off, tensioning, and reeling stations. Film thickness and process

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control is a balance between cooling rate, bubble length, blowup ratio, and film tension.

12.8.2.3 Biaxially Oriented Film Fluoropolymers produce a transparent film fabricated in thin gauges ranging typically from 0.012 to 0.040 mm thickness. The film is produced by stretching an extruded sheet or tube in two orthogonal directions—the machine direction and the transverse direction. Stretching is carried out at a temperature below the melting point of the polymer and results in a partial orientation of polymer molecules in the direction of stretch. In principle, biaxially oriented film is isotropic—its properties are the same in both the machine and transverse directions.

Figure 12.15 Blown process for biaxially oriented films.

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In practice, the film produced by the tenter process tends to be more highly oriented in the machine direction whereas the blown process produces a film that is more nearly isotropic. Blown. The blown process, also known as the tubular or bubble process, uses a tubular die to extrude a relatively thick-walled tube in a vertical direction, either upwards or downwards. Downwards extrusion (Fig. 12.15) allows the tube to be quenched rapidly in a water bath after which it is collapsed as a layflat for passage over nip and idler rollers. The film passes through a reheating tunnel where it is raised to a temperature above the softening point but below the melting point. The heated tube is then inflated by internal air pressure that forms a bubble in which the film is stretched

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in all directions. Some machine-direction stretch may take place in the ovens upstream of the bubble; the haul-off rate can be adjusted if necessary to secure an orientation balance. Bubble cooling is provided by an air ring similar to that used in other blown film processes. Subsequent calibration and bubble collapsing operations are also similar. At this point, the film retains shape memory. If it is reheated, it will shrink and revert to dimensions approaching its prestretch shape. If a shrink film is required, the layflat is edge-trimmed, separated, and reeled as two webs at this stage. The nonshrink film is produced by subjecting the layflat to a further heat treatment in which it is annealed or heat set under tension in an oven. The annealing temperature is set slightly higher than the stretching temperature. Tentered. Tentered biaxially oriented film is produced by mechanically stretching the film in a tenter machine. This takes its name from the tenter frame originally used for stretching cloth between grips known as tenterhooks. Simultaneous tentering is possible, involving complex movements of the film edge grips so that the film is stretched in the machine and transverse directions at the same time. However, the process is mechanically complicated and it is difficult to adjust the balance between the stretch directions, so the two-step tenter process (Fig. 12.16) is the one usually adopted for most films. The process starts with the production of a relatively thick chill roll cast film. This is then stretched in the machine direction by passing it around heated rollers rotating at controlled and increasing speeds in excess of the extrusion speed. The degree of stretch is controlled by varying the roll speeds.

Figure 12.16 Tenter process for biaxially oriented films.

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When machine direction stretching is complete, transverse stretching is applied by the tenter machine. This consists essentially of a temperature regulated tunnel in which the film edges are gripped by chain-driven tension clips running on divergent paths. As the film passes through the tunnel, it is progressively stretched in the transverse direction as the clips diverge. The edge grip mechanism must withstand large cross loads and be capable of operating at high line speeds. Transverse stretch is controlled by varying the divergence of the edge clip paths.

12.8.2.4 Fluoropolymer Film Extrusion Fluoropolymer films have found applications where extreme high and low temperatures and aggressive chemicals are involved. For example, these films are used as release sheets in compression molding of high-temperature parts with epoxy and phenolic resins. Another common application is, in general, as liners. One example is as roll cover to protect metal rolls from corrosion in chemical processing industries such as paper manufacturing. Other applications include glazing for solar collectors, gas and liquid sample bag, and drug and food packaging. Partially fluorinated fluoroplastics provide films with especially excellent tensile strength and modulus for applications where mechanical strength is required. Monoaxially and biaxially oriented films of fluoropolymer are made by melt extrusion of the resin into flat webs or tubes. The main function of orientation is to enhance the mechanical properties of the film such as tensile break strength and tear

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Figure 12.17 Schematic design of a coat hanger die.

resistance. The decision to orient is usually made according to the requirements of the end use for mechanical properties. All process surfaces that contact molten fluoropolymers must be corrosionresistant because of the formation of corrosive compounds such as HF and HCl from the hightemperature degradation of these plastics. The most common die to extrude a flat fluoroplastic film is the coat hanger design that has been (Fig. 12.17). It shows a schematic of a coat hanger die. The polymer melt enters the die near its lateral center and is redirected toward the two ends of the die. The melt travels through the channels that are angled toward the exit side of the die, thus distributes across the width of the die. The molten polymer flows through the die gap toward the exit slot, also called die lips. The gap is formed by two flat metal plates and is named the land area, as it is called in other types of dies. The two plates are usually formed by several sections, which can be independently adjusted to increase the control over the thickness of the extruded film/sheet. Today, complex dies are available that are computer-controlled and do not require manual adjustment [21]. The die is designed such that the channels restrict the melt flow in a way that all the melt experiences the same amount of shear. The distance of travel is equalized by the restraints designed in the flow channels. The net effect is equal flow rates at the center and the far ends of the die, thus a uniform web thickness. This means that the flow in the center of the die is most restricted. One advantage of this approach is the uniformity of residence time in the die. This

regime is called plug flow in that no internal shear is generated in the melt while flowing through the die. Dies with widths as high as 3 m have been successfully designed and built, although most commercial dies have widths of less than 1.5 m.

References [1] Ebnesajjad S. 2nd edition Fluoroplastics: melt processible fluoropolymers, vol. 2. Elsevier; 2015. [2] Dealy JM, Wang J. Melt rheology and its applications in plastics industry. 2nd ed. Springer; 2013. [3] Bird RB, Stewart WE, Lightfoot EN. Transport phenomena. 2nd ed. New York, NY: John Wiley & Sons; 2006. [4] Cogswell FW. Polymer melt rheology: a guide for industrial practice. Cambridge: Woodhead Elsevier; 1998. [5] Dealy JM, Wang J. Melt rheology and its applications in plastics industry, 2nd ed., Springer; 2013. [6] Hyflon® MFA design and processing guide. Doc No. Y42E001. Solvay Slexis, www. SolwaySolexis.com; 2008. [7] Polyvinylidene fluoride design and processing guide. BR2001C-B-2-1106. Solvay Solexis Corp., www.SolvaySolexis.com; 2006. [8] PVDF design and processing guide. BR2001CB-2-1106. Solvay Solexis Corp., www. SolvaySolexis.com; 2017.

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[9] Potsch G, Michaeli WR. An introduction to injection molding. 2nd ed. Munich: Hanser; 2007. [10] Achilleos EC, Georgiou G, Hatzikiriakos SG. The role of processing aids in the extrusion of molten polymers. J Vinyl Addit Technol 2002;8:7 24. [11] Maier C, Calafut T, Polypropylene, 1st ed. William Andrew, 1998. [12] Rozenbaoum EE, Randa SK, Hatzikiriakos SG, Stewart CW. Boron nitride as a processing aid for the extrusion of polyolefins and fluoropolymers. Polym Eng Sci 2000;40: ?179 90. [13] Hatzikiriakos SG, Rathod N. Boron nitride based processing aids. Korea-Aust Rheol J 2003;15(4):173 8. [14] Bozzelli JW. Injection molding. Encyclopedia of polymer science and technology, concise. 3rd ed. Wiley Interscience; 2013. [15] Hylar® PVDF fluoropolymer. Technical brochure. Thorofare, NJ: Solvay Solexis; 1998. [16] Kynar® and Kynar Flex® PVDF—performance characteristics and data. Arkema Corp., www.kynar.com; 2009. [17] RTP company, www.rtpcompany.com; 2014.

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[18] Solef® and Hylar® PVDF polyvinylidene fluoride design and processing guide. BR2001C-B-2-1106. Solvay Solexis; 2006. [19] Neoflon® ETFE product information. Pub No. EG-65C. Daikin Corp.; 2014. [20] Technical data, ethylene tetrafluoroethylene copolymer. CA016 ETFE technical brochure. AGC Chemicals, www.Fluon.com; 2006 2007. [21] Hyflon® PFA, perfluoroalkoxy fluorocarbon resins, design and processing guide. BR2001C-B-2-1106. Solvay Solexis, www. SolvaySolexis.com, 2006. [22] Injection molding guide—3M Dyneon® Fluoroplastics PFA, Dyneon. Pub No. 980504-2528-3. 3M Advanced Materials Division, www.Dyneon.com; 2013. [23] Muller F. US Patent 4,963,312; 1990; Muller F. US Patent 5,055,025; 1991. [24] Wagner Jr JR, Mount III EM, Giles Jr HF. Extrusion. 2nd ed. Oxford: Elsevier; 2014. [25] Campbell GA, Spalding MA. Analyzing and troubleshooting single-screw extruders. Hanser; 2013. [26] The guide to safe handling of fluoropolymers resins. 4th ed. The Society of Plastics Industry, Inc.; 2005. BP-101.

13 Manufacturing and Properties of Polychlorotrifluoroethylene Sina Ebnesajjad FluoroConsultants Group, LLC, United States

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13.3 Polymerization of Chlorotrifluoroethylene 13.3.1 Bulk Polymerization of Chlorotrifluoroethylene 13.3.2 Suspension polymerization of Chlorotrifluoroethylene 13.3.3 Emulsion Polymerization of Chlorotrifluoroethylene

232 232 232

13.4 Copolymerization of Chlorotrifluoroethylene

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13.5 Properties of Polychlorotrifluoroethylene

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13.6 Characterization of Polychlorotrifluoroethylene

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13.7 Commercial Polychlorotrifluoroethylene Resins

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13.1 Introduction Polychlorotrifluoroethylene (PCTFE) was discovered in 1934 by Fritz Schloffer and Otto Scherer who worked at IG Farbenindustrie AG. PCTFE was commercialized under the trade name Kel-F 81 by M. K. Kellogg in the early 1950s. It was acquired by 3M in 1957 and manufactured under the brand name Kel-F. By 1996, the 3M Company had discontinued manufacturing the resin and sold the rights to Daikin Industries Ltd, who now produces the resin under the brand name Neoflon PCTFE. After merging with Allied Signal, Honeywell Corp took possession of Aclar which is the trade name of PTFE films and Aclon PCTFE resins. The first documented polymerization of chlorotrifluoroethylene (CTFE) into a solid plastic was reported by Farbenindustrie AG of Main, Germany, in 1937 [1]. These polymers had low molecular weight and lacked sufficient mechanical properties. The initial applications of these polymers were limited to lubricants required to perform under severe chemical and thermal environments. High molecular weight polymers of CTFE were synthesized,

characterized, and utilized during the Manhattan Project [2] in the diffusion process to separate the uranium isotopes. Examples of manufacturing methods of PCTFE are described in this chapter.

13.2 Chlorotrifluoroethylene Polymers PCTFE homopolymers and low copolymers with vinylidene fluoride (VDF) are thermoplastics, which are by far inferior to PTFE with respect to thermal stability and chemical resistance, higher surface energy and coefficient of friction, and less attractive electrical properties. Replacement of one fluorine atom with chlorine imparts some advantages to CTFE plastics such as increased mechanical strength, lower gas permeability, and improved optical clarity over PTFE. Increasing VDF content of CTFE copolymers to 50% produces an elastomeric gum with excellent resistance to oxidizing acids at elevated temperatures. High molecular weight homopolymers and copolymers of CTFE were first commercialized by

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00013-6 © 2021 Elsevier Inc. All rights reserved.

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M. W. Kellog Company, in early 1950s, under the trade name Kel F. Honeywell Corp. manufactures a copolymer of CTFE and its films of which are known as Aclar. Daikin Industries, Ltd. offers PCTFE resins by the trade name Neoflon CTFE.

reaction vessel leads to distinct bimodal molecular weight distribution. After completion of the polymerization, the unreacted monomer is distilled off. The polymer is recovered in the form of a porous plug and subsequently pulverized, washed, and dried prior to fabrication.

13.3 Polymerization of Chlorotrifluoroethylene

13.3.2 Suspension polymerization of Chlorotrifluoroethylene

The Farberindustrie’s pioneering CTFE polymerization was reported to have been conducted under elevated pressure and temperature by solution and aqueous polymerization. Solution polymerization was conducted in two stages, at 40°C
45°C and 60°C
65°C, in alcohol, benzene, or petroleum ether. Peroxides such as benzoyl peroxide were the catalyst and an aldehyde the regulator. Aqueous polymerization was also possible in the presence of an emulsifying agent and a peroxide catalyst. Overall, the polymerization resembled that of vinyl chloride. The synthesis of CTFE has been carried out by bulk [3
7], solution [8
10], suspension [11
17], and emulsion [18
21] polymerization regimes and high molecular weight polymers have been obtained. Free radical initiators and ultraviolet and gamma ray radiation have been used to initiate the reaction. Emulsion and suspension polymers are more thermally stable than bulk polymerized products. Commercial PCTFE plastic is prepared by the former techniques to maximize continuous use temperature of the polymer. Agitated stainless steel or glass lined vessels are two examples of reactors for PCTFE synthesis. No unusual equipment is required. Typical reaction conditions include pressures of 0.34
1.03 MPa at 21°C
53°C.

Suspension polymerization is effected in water with organic or inorganic peroxides. The monomer is present in excess simultaneously with aqueous and solid polymer phases and the mixture is stirred. The polymerization was found to be too slow using a persulfate initiator alone [16]. The addition of a reducing agent such as a bisulfite ion along with lowering the pH to 2.5 increased the polymerization rate. The addition of a silver salt further accelerated the reaction rate and improved reproducibility of the yields of a high molecular weight PCTFE. Polymerization of CTFE by suspension method usually requires a promoting agent to lessen the polymerization time and drive up the molecular weight [15]. These promoting agents fall in three classes: promoters, activators, and accelerators. In most cases, a promoter must be used which consists of an inorganic compound. Examples include water-soluble persulfates, perborates, perphosphates, percarbonates, and hydrogen peroxide. Persulfate and perphosphate salts of sodium, potassium, ammonium, and calcium are especially valuable. The required concentration of promoters is in the range of 0.003
0.1 molarity. The exact concentration is dependent on the specific promoter and the desired molecular weight of the polymer. For example, to produce PCTFE with a softening point of 200°C, a promoter concentration of 0.003
0.07 is required [15]. Activators are preferably added to the system in conjunction with the promoters to increase their activity. Some of the more common compounds include sodium bisulfite, sodium thiosulfate, sodium hydrosulfite, and trimethylamine. Generally, the activators are water-soluble reducing agents. Equal molar concentrations of the activator and the promoter should be present in suspension polymerization of CTFE. Regulating agents are added to the aqueous phase to regulate the pH of the mixture. The desirable pH range is 1
4 and can be maintained by the

13.3.1 Bulk Polymerization of Chlorotrifluoroethylene Bulk polymerization is carried out in a small static vessel and is initiated by a halogenated acyl peroxide [22,23]. Ultraviolet light and gamma rays can also initiate the reaction. There are some disadvantages to this method of PCTFE preparation. Temperature control is difficult resulting in poor product reproducibility. Low conversion (,40%), long reaction time (168 hours), and low temperature (234°C) are the other shortcomings of this technique. The severe temperature gradient in the

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inclusion of a buffering agent. Acetic acid, monosodium phosphate, and propionic acid are examples of an effective buffering agent [15]. PCTFE made by suspension polymerization has generally had an undesirable molecular weight to viscosity relationship, that is, melt viscosity is higher at a given molecular weight [24]. To overcome this deficiency, a small amount of VDF is copolymerized with CTFE. Another issue is the tendency of the molecular weight of suspension polymer to be skewed toward the low end, which can have a deleterious effect on part properties.

13.3.3 Emulsion Polymerization of Chlorotrifluoroethylene Emulsion polymerization is carried out in water using an inorganic peroxy initiator and a surfactant. Nonhalogenated surfactants are not effective in CTFE polymerization. Halogenated hydrocarbon acids and salts are the more effective surfactants. The most active agents are aliphatic perfluorinated and perfluorochlorinated acids and their corresponding salts with the following general formulas: [25] FðCF2 Þn
COOH

n 5 6
12

CCl3 ðCF2
CFClÞn-1
CF2
COOH

n53
6

An example of emulsion polymerization is given in a patent assigned to 3M Corp [26] using the recipe of Table 13.1 in a batch process. The pH of the reaction mixture was adjusted to 7 by the addition of potassium hydroxide. Perfluorooctanoic acid was the surfactant and a persulfate/bisulfite peroxy package initiated the reaction. Polymerization lasted for 20 hours at 30°C while the reaction vessel was tumbled. At the end of the reaction, the vessel was vented and the latex was recovered. Table 13.1 Example of CTFE Emulsion Polymerization Recipe [26]. Reaction Component

Amount, Parts

Deionized water

300

CTFE

100

Potassium persulfate

2.4

Sodium bisulfite

1.1

Perfluorooctanoic acid

2.4

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Freezing helped break the recovered polymer emulsion. PCTFE was filtered out, washed with water, and dried at a yield of 46%. The emulsion polymers are the most difficult to recover but they are superior to the resin obtained by other techniques because of higher molecular weight and thermal stability and a greater reproducibility. US Patent 9,862,8119 disclosed the synthesis of stabilized polymers of CTFE and products manufactured using this polymer. More particularly, the invention is focused on the synthesis of stabilized polymers of CTFE, such as homopolymers and copolymers of CTFE. A neutralizing agent was employed during the polymerization that reduced the acidity. For instance, to maintain a controlled pH, a buffer solution was employed as the neutralizing agent. The addition of the buffer solution improved the thermal stability of CTFE polymers, and products, such as packaging films, manufactured from those polymers [27]. The term “buffer solution” applies to an aqueous solution consisted of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. The buffer solution achieved its resistance to pH change because of the presence of an equilibrium between its components. Buffer solutions may be acidic or basic and have a useful pH range to select from. For the best result, the buffer solutions were an acidic buffer solution with a pH of about 3.0 to about 11.0. Examples of suitable acidic buffer solutions include acetate-, citrate-, lactate-, and phosphate-based buffer solutions. Some neutralizing agents such as ammonium lactate also beneficially terminate the polymerization reaction. Reaction termination and solution neutralization using, for example, an ammonium lactate buffer solution has been surprisingly found to result in a CTFE polymer product that exhibits significantly increased thermal stability over CTFE polymer products that have been neutralized using other solutions [27]. Methods for synthesizing stabilized polymers of CTFE and products manufactured using such polymers are disclosed herein. In one exemplary embodiment, a method for synthesizing CTFEbased polymers includes reacting, in the presence of an initiator and in a reaction medium at a pH of about 1.5 to about 2.5, one or more monomers comprising CTFE and after an amount of polymerization reaction time has passed, adding a neutralizing agent to the reaction medium to increase the

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pH of the reaction medium to within a range of about 1.8 to about 6.0.

13.4 Copolymerization of Chlorotrifluoroethylene Copolymers of CTFE and perfluoropropyl vinyl ether have been manufactured [28]. Typically, a jacketed polymerization reactor equipped with a stirrer, which was charged with water and sodium hydrogencarbonate. After purging the gas atmosphere with pure nitrogen gas, the nitrogen gas was evacuated under reduced pressure. Then, CTFE, perfluoropropyl vinyl ether, and carbon tetrachloride were injected under pressure. After setting the reactor temperature to 20°C, stirring was started. To the stirred mixture, a solution of [Cl (CF2CFCl)2
CF2COO]2 in trichlorotrifluoroethane (0.3 g/mL) was added as a polymerization initiator to kick off the polymerization reaction. After 24 hours polymerization, unreacted CTFE was purged, and a resulting copolymer was recovered, washed with warm water, and dried to obtain a powdery copolymer. Properties of the polymers have been summarized in Table 13.2. CTFE can be copolymerized with acrylic comonomers (Fig. 13.1). An autoclave equipped with baffles and stirrer working at 450 rpm, 0.5 L of demineralized water, 0.66 mL of a hydroxypropylacrylate solution in CFC-113 in 1:1 by volume ratio, and 15 kg of CTFE were introduced. The autoclave was then heated up to the reaction temperature of 20°C corresponding to an initial pressure of 0.6 MPa. Trichloroacetylperoxide, which is an initiator, was pumped to the reactor at 12 mL/h

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FLUOROPOLYMERS

at 217°C as a solution in CFC-113. Further, 0.66 mL of CFC-113 solution of acrylate comonomer was introduced every 30 minutes during the polymerization for 13 hours. The product discharged from the autoclave was dried at 120°C for about 16 hours. Some of the characteristics of the CTFE and different acrylate copolymer products are listed in Table 13.3. Copolymer made according to example 2 in Table 13.3 was extruded into pellets in a single screw extruder. The pellets were processed in an extruder to produce a film of about 100 μm thickness. The mechanical properties of that film were measured and are summarized in Table 13.4. The water-vapor permeability of the copolymer film at a thickness of 89 μm was less than 0.05 g/ (m2 24 hours). Low vapor permeability of CTFE homopolymers and copolymers renders films of this polymer highly attractive for many applications in which water vapor permeability must be kept low. Examples include pharmaceutical pill packs and military packaging. A Honeywell patent [30] describes a process for the polymerization of copolymers using azeotropic mixtures of CTFE and select fluorinated monomers. These azeotropic mixtures had constant compositions,

Figure 13.1 Chemical monomers.

structures

of

acrylic

Table 13.2 Properties of Copolymers of Chlorotrifluoroethylene [28]. Melt Flow

Heat

Rate

Melting

of

Yield

Tensile

Break

Modifier

Amount, mol.%

( 3 1023 cc/s)

Point, °C

Fusion, cal/g

Crystallization Temperature

Strength, MPa

Strength, MPa

Elongation, %

None

0.0

39.4

213.5

4.81

178.5

None

0.0

72.5

PPVE

0.05

27.6

213.5

5.22

174.0

PPVE

0.05

49.5

PPVE

0.5

2.65

201.0

3.04

164.0

PPVE

0.5

61.5

PPVE

1.1

27.6

189.5

2.45

146.0

PPVE

1.1

139.8

VDF

4.0

59.0

193.5

4.61

153.0

VDF

4.0

64.2

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235

Table 13.3 Characteristics of Acrylic Copolymers of Chlorotrifluoroethylene [29]. Second Melting Temperature T2f (°C)

% (by moles)

Permeability O2 (cc mm/ m2 24 h atm)

Examples

Comonomer

1

Hydropropyl acrylate

0.91

203.1

31.5

2

Hydropropyl acrylate

0.48

205.3

3.49

3

Hydropropyl acrylate

0.43

209.1

3.95

4

Butyl acrylate

0.42

207.8

4.21

5

Butyl acrylate

0.37

209.4

4.02

6

Butyl acrylate

1.2

197.0

7.70

7

—

213.3

4.22

Table 13.4 Mechanical Properties of Film Made from Polymer in Example 2 of Table 8.15 [29].

Elastic modulus (MPa) Yield stress (MPa) Yield strain (%) Stress at break (MPa)

MD

TD

1460

1292

41.2

39.1

7

6.5

47

27.3

MD, Machine direction; TD, transverse direction.

were introduced as a single feed stream, and polymerized using an initiator. A more detailed example is described. An azeotrope of CTFE and CF3
CF 5 CH2 monomer combination. A typical polymerization is run in a stirred, stainless steel autoclave in which reactants are added by methods known in the art. The following ingredients were added to a 300 mL autoclave: (NH4)2S2O8 ammonium persulfate: 15 mL of a solution of 0.56 g dissolved in 40 mL of pure water; Na2S2O5 sodium metasulfite: 19 mL of a solution of 1.2 g dissolved in 40 mL of pure water; FeSO4 ferrous sulfate: 0.005 g dissolved buffer solution. Na2HPO4/NaH2PO4 buffer: 1.34/0.68 g dissolved 180 mL. C7F15CO2(NH4) surfactant: 2.44 g dissolved with buffer.

A volume of 180 mL of the emulsion solution [water/Na2HPO4/NaH2PO4/FeSO4/C7F15CO2(NH4)] was loaded in the reactor. The solution was stirred while 40.7 g of a mixture of 60 mol.% CF3
CF 5 CH2 and 40 mol.% of CTFE were added at 10°C. The autogenous pressure was maintained during the polymerization to keep the concentration of the monomer constant. After 7 hours, the polymerization was stopped and the monomers were vented out from the autoclave. After recovery and drying, 5.3 g of a white copolymer was obtained [30]. In another example [31], a polymer was made which consisted of recurring units of VDF and trifluoroethylene (TrFE) with
CF2H and/or
CF2CH3 end groups. A 5-L stirred vertical autoclave equipped with baffles was charged with pure water. After the reactor temperature was raised to 120°C, a sodiumbased microemulsion [32] was introduced. The reactor was charged with 0.735 MPa of VDF and a gaseous mixture of VDF-TrFE (75/25 mol.%) was fed next until the pressure of 3 MPa was reached. The starting gas phase in the reactor has the following composition (mol.%): 82.5% VDF, 111.5% TrFE. The reaction was initiated with 27 mL of pure di-tert-butyl peroxide. The polymerization pressure was maintained by continuously feeding a stream of 75/25 mol.% VDF-TrFE. After feeding 2% of the targeted amount of mixture, the temperature was lowered to 105°C. Gas flow was stopped after 1150 g of the monomer mixture had been fed,

236

INTRODUCTION

followed by pressure reduction to 1.5 MPa. The reactor was then vented, cooled, and the latex discharged, coagulated by freezing, washed with demineralized water, and dried at 100°C. The recovered copolymer had a melt flow index (MFI, at 230°C/5 kg load cell) of 3.5 g/10 min, a second melting temperature of 144.5°C, and a crystallization temperature of 123.7°C. Table 13.5 captures the properties of several examples of the VDF-TrFE copolymer. End groups were determined according to a method described in a 1999 paper. Concentration of relevant end chains is expressed both as mmol/kg of polymer and as mmol/kg of VDF [33]. Poly(CTFE-co-VDC) copolymers were synthesized in solution and in an aqueous process by radical copolymerization initiated by two different systems. A surfactant-free emulsion polymerization process yielded poly(CTFE-co-VDC) copolymers in up to 75% wt yield and exhibiting very good thermal properties. In parallel, a solution polymerization process was used to obtain a range of poly (CTFE-co-VDC) copolymers of lower molecular weight and soluble enough to allow a meticulous characterization by means of NMR spectroscopy. The copolymers were analyzed by 19F and 1H NMR spectroscopy and by elemental analysis. These analyses demonstrated poly(CTFE-co-VDC) copolymers exhibited a statistical microstructure with the VDC incorporation in the copolymer much

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FLUOROPOLYMERS

higher than that in the feed. The poly(CTFE-coVDC) copolymers had satisfactory thermal stability. Decomposition temperature at 10% weight loss ranged from 333°C up to 400°C in air, and a decreasing trend of the thermal stability was observed when increasing the VDC amount in the copolymer. CTFE/VDC copolymers are good candidates for barrier packaging applications [34].

13.5 Properties of Polychlorotrifluoroethylene The difference between PTFE and PCTFE is mainly in the chemical structure. Chlorine group was introduced in place of fluorine and it leads to a massive change in its properties and application. Chlorine atom which is bigger than fluorine atom breaks the structural regularity affecting crystallinity of polymer chains. This leads to a decrease in degree of crystallinity when compared to PTFE. This reduction in structural regularity and crystallinity results in lower melting point and makes PCTFE manufacturable as the transparent product in films and thin sheet markets. The presence of chlorine group instead of fluorine group not only decreases the crystallinity phenomenon but also increases intermolecular forces leading to PCTFE being a harder and stronger polymer than PTFE.

Table 13.5 Properties of Several Examples of VDF-TrFE Copolymer [33]. Run

Ex.1

Ex.2

Ex. 3

Ex.4

Ex. 5

Ex.6C

Ex.7C

Ex.8C

VDF, mol.%

75.1

69.5

75.5

75.6

82.6

70.2

76.9

75.4

TrFE, mol.%

24.9

30.5

24.5

24.4

174

29.8

23.1

24.6

MFI, g/10’

3.5

2.4

5.9

1.7

29.2

2.1

1.5

4.2

Mn/1000

128

136

120

143

106

164

196

125

Chain ends in mmol/kg of polymer
CF2H (a)

45

41

45

42

40

6

9

18


CF2CH3 (b)

26

21

31

31

31

4

3

12

Total (a) 1 (b)

71

62

76

73

71

10

12

30

103

90

15

17

43

Chain ends in mmol/kg of VDF recurring units Total (a) 1 (b)

101

97

108

VDF, Vinylidene fluoride; TrFE, trifluoroethylene; MFI, melt flow index.

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PCTFE is a harder and stronger polymer, with better mechanical properties than PTFE. The crystallinity of this polymer can be altered by controlling its cooling rate in the melt processing operation; this feature is exploited to get wide varieties in properties and applications. With high crystallinity, PCTFE is dense with high mechanical properties and elongation. Alternatively, when quench cooled, PCTFE is lighter, transparent, and more elastic. The quench-cooled PCTFE is widely used in cryogenic engineering applications for handling liquid oxygen and liquid nitrogen. Valve

237

seats made of PCTFE are widely used at cryogenic temperatures [35] (Tables 13.6 and 13.7). PCTFE is compatible with a large number of inorganic and organic chemicals not only at room temperature but also at elevated temperatures, for example, 100°C [37,38]. PCTFE is often used in Aerospace applications due to its extreme low outgassing value. PCTFE is also ideal for cryogenic applications. Products made from PCTFE have excellent physical, mechanical, and electrical properties, heat resistance, chemical resistance, and low moisture absorption [39]. A summary of

Table 13.6 Mechanical and Thermal Properties of PCTFE [36]. Property

Value

Units

Method

Mechanical properties Tensile strength Elongation Flexural strength, 73°F

4860
5710

psi

34
39

MPa

100
250

%

9570
10,300

psi

66
71

MPa

D638 D638

200
243 3 10

psi

1.4
1.7

MPa

2.5
3.5

ft-lb/in

D256

1570
1860

psi

D695

11
13

MPa

2.10
2.17

gm/cu cm

Coefficient of linear expansion

7 3 1025

K21

Melting point

410
414

°F

210
212

°C

1.45

Btu in/h ft2 °F

0.84

W/m K

0.22

Btu/lb/°F

0.92

kJ/kg/°K

259

°F

126

°C

620

°F

327

°C

Flex modulus Impact strength, Izod, 23°C Compressive stress at 1% deformation Density

3

Thermal properties

Thermal conductivity Specific heat Heat distortion temperature, 66 lb/sq.in (0.455 MPa) Processing temperature

ASTMC177

D648

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INTRODUCTION

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Table 13.7 Electrical and Other Properties of PCTFE [36]. Electrical properties Dielectric strength, short time, 0.004v

3000

Arc-resistance

360

Volume resistivity, at 50% RH

2 3 10

Surface resistivity, at 100% RH

1 3 10

Ohm sq

Dielectric constant, 1 kHz

2.6

ε

Dissipation factor, at 1 kHz

0.02

17

V/mil

D149

s

D495

ohm cm

15

D257 21

D257 D150-81 D150-81

Other properties Water absorption

0.00

% increase in weight

Flame rating 1

Nonflammable

D635

Coefficient of friction

D1894

Specific gravity

2.10
2.17

Moisture permeability constant

0.2

O2 Permeability N2 Permeability CO2 Permeability H2 Permeability

1.5 3 10

g/m, 24 h 210

0.18 3 10 2.9 3 10

D792

210

210

56.4 3 10

210

Cc, cm/sq cm, s, atm Cc, cm/sq cm, s, atm Cc, cm/sq cm, s, atm Cc, cm/sq cm, s, atm

Table 13.8 Properties and Attributes of PCTFE Compression Molding [40].

• • • • • • • • • • • • • • • • • • • • •

Near-zero moisture absorption Extremely low gas permeability Outstanding barrier material toward air, water, steam, and fluids, including liquid gases Very low outgassing Nonflammable, even in the presence of a high oxygen concentration Exceptional chemical resistance to all mineral reactants and most organic reactants Excellent corrosion resistance High optical transparency Excellent electrical insulator (even with high humidity and thermal cycling) Resistance to strong UV radiation Retains its properties upon exposure to gamma radiation High compressive strength Low deformation under load, superior rigidity Lower cold flow than other fluoropolymers Useful temperature range of 2240°C to 1205°C Superior cryogenic properties Expensive relative to many other materials PCTFE is attacked by many organic solvents Low coefficient of thermal expansion High dimensional stability Can be machined to precise dimensions

D570-81

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239

Table 13.9 Definition of Basic Properties of Polychlorotrifluoroethylene According to ASTM D1430. Reference ASTM Method

Property

Definition

Specific gravitya

Specific gravity of a sample of PCTFE compression-molded according to this method

D792

Zero strength timea

Time required for a compression-molded specimen of polymer to lose strength at 250°C

D1430

Deformation under loada

Viscoelastic flow of polymer under a constant load

D621b

Melting pointa

D2117

Dielectric constanta

A measure of effectiveness of material as a dielectric

D150

Dissipation factora

A measure of conversion of electrical energy to heat in a dielectric (insulation) material

D150

a

Properties required for resin specification. Obsolete.

b

properties and attributes of PCTFE is given in Table 13.8.

13.6 Characterization of Polychlorotrifluoroethylene ASTM Method D1430 covers the polymers of PCTFE and test methods for the as-produced polymer. Standards ISO 12086-1 and 12086-2 also specify PCTFE resins. PCTFE has very high viscosity and does not dissolve in any solvents. These two facts render direct measurement of PCTFE molecular weight virtually impossible. An indirect factor named zero strength time (ZST) is substituted for the molecular weight. ZST is a physical measurement, which can be correlated with molecular weight, melt viscosity, or melt index. ZST is defined as the time required for a compression-molded specimen (50 mm long,

4.8 mm wide, and 1.58 mm thick) of PCTFE to lose strength under 11.5 g weight at 250°C. One end of the bar is fastened in an oven and the weight is hung from the other end. Properties used in the characterization of PCTFE are defined in Table 13.9. PCTFE materials in powders and pellets are classified into one group. The group is subdivided into classes based on chemical composition. These classes are subdivided into grades as shown in Table 13.10. Type I has three grades and types II and III have two grades as indicated in Table 13.11. Each grade is representative of a different range of apparent molecular weight.

13.7 Commercial Polychlorotrifluoroethylene Resins Tables 13.12 and 13.13 provide data for a number of commercial PCTFE resins.

Table 13.10 Requirements of Polychlorotrifluoroethylene Molded Test Specimens According to ASTM D1430.

Group

Class

01

1

Zero Strength Time, sb

Deformation Under Loadc,d

Melting Point, ° Ce

Dielectric Constantf, kHz

Max MHz

Dissipation Factor,g kHz

Max MHz

Description

Grade

Specific Gravitya, 23/23°C

Homopolymer

1

2.10
2.15

100
199

10

210
220

2.70

2.50

0.030

0.012

2

2.10
2.15

200
299

10

210
220

2.70

2.50

0.030

0.012

3

2.10
2.15

300
450

10

210
220

2.70

2.50

0.030

0.012

1

2.10
2.12

100
199

15

200
210

2.70

2.50

0.030

0.012

2

2.10
2.12

200
299

15

200
210

2.70

2.50

0.030

0.012

1

2.08
2.10

100
199

20

190
200

2.70

2.50

0.035

0.015

2

2.07
2.10

200
299

25

190
200

2.70

2.50

0.035

0.015

0 2

Modified homopolymer

0 3

Copolymer

0 a

See Section 10.1.7 ASTM D1430.

b

See Section 10.1.3 ASTM D1430.

c

See Section 10.1.4 ASTM D1430.

d

Maximum at 1112 N, 24 h,70°C, %.

e

See Section 10.1.5 ASTM D1430.

f

See Section 10.1.6 ASTM D1430.

g

See Section 10.1.6 ASTM D1430.

ASTM, American Society for Testing Materials.

Table 13.11 Requirements of PCTFE Molded Test Specimens ASTM D1430. Type I Homopolymer

Type II Modified Homopolymer

Type III Copolymer

Grade 1

Grade 2

Grade 3

Grade 1

Grade 2

Grade 1

Grade 2

Specific gravity, 23/23°C (73.4/73.4°F)

2.10
2.12

2.10
2.12

2.10
2.12

2.10
2.12

2.10
2.12

2.08
2.10

2.08
2.10

Zero strength times, s

100
199

200
299

300
450

100
199

200
299

100
199

200
299

10

10

10

15

15

20

210
220

210
220

210
220

200
210

200
210

190
200

190
200

kHz

2.70

2.70

2.70

2.70

2.70

2.70

2.70

MHz

2.50

2.50

2.50

2.50

2.50

2.50

2.50

kHz

0.030

0.030

0.030

0.030

0.030

0.035

0.035

MHz

0.012

0.012

0.012

0.012

0.012

0.015

0.015

Deformation under load, max at 1112 N (50 lbf), 24, 70°C, % Melting point, °C

Dielectric constant, max

Dissipation factor, max

PCTFE, polychlorotrifluoroethylene.

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Table 13.12 Properties of Daikin’s Neoflona PCTFE [41
43]. Flow Value 3 103, mL/s

Form

Product

ZST, s

Apparent Density, g/L

M-300

200
300

600

1
3

Powder (10
60 mesh)

M-300H

200
300

950

1
3

Granular powder

M-300P

200
300

1100

1
3

Pellet

M-400H

301
450

950

0.3
0.8

Granular powder

M-400P

301
450

1100

0.3
0.8

Pellet

a

Neoflon is a trademark of Daikin Company.

Table 13.13 Physical Properties of Daikin’s Neoflon PCTFE [41
43]. Property

Test Method (ASTM)

M-300H

M-400H

Melting point (DSC),°C

D1430

210
212

210
212

Zero strength time, s

D1430

200
300

300
450

Specific gravity

D792

2.11
2.16

2.11
2.16

Tensile strength, MPa

D638

32
37

34
39

Elongation, %

D638

50
200

100
250

Tensile modulus of elasticity, MPa

D638

1300
1500

1200
1400

Compressive strength at 1% strain, MPa

D695

40
45

37
42

Compressive modulus of elasticity, MPa

D695

1400
1600

1200
1400

Flexural strength, MPa

D790

69
74

67
72

Flexural modulus of elasticity, MPa

D790

1600
1900

1400
1700

Impact strength, ft-lb/in

D256

2.5
3.5

2.5
3.5

75
85

75
85

, 0.2 1.7
1.9 7.0
9.0

, 0.2 1.4
1.6 4.5
6.5

Hardness, shore D Deformation under load, % 24 h at 7 MPa 25°C 80°C 100°C

D621

References [1] British Patent 465,520, assigned to Farbenindustrie, I. G.; 1937. [2] Miller WT. US Patent 2,564,024, assigned to US Atomic Energy Commission; 1951. [3] Miller WT, Dittman AL, Reed SK. US Patent 2,586,550, assigned to USAEC; 1952. [4] Miller WT. US Patent 2,792,377, assigned to Minnesota Mining and Manufacturing Co.; 19511. [5] Dittman AL, Wrightson JM. US Patent 2,636,908, assigned to M. W. Kellog Co.; 1953.

[6] British Patent 729,010, assigned to Farbenfabriken Bayer AG; 1955. [7] French Patent 1,419,741, assigned to Kureha Chemical Co.; 1965. [8] Young DM, Thompson B. US Patent 2,700,662, assigned to Union Carbide Co.; 1955. [9] Hanford WF. US Patent 2,820,027, assigned to Minnesota Mining and Manufacturing Co.; 1958. [10] Lazar M. J Polymer Sci 1958;29:573. [11] Roedel GF. US Patent 2,613,202, assigned to General Electric Co.; 1952.

13: MANUFACTURING

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POLYCHLOROTRIFLUOROETHYLENE

[12] Caird DW. US Patent 2,600,202, assigned to General Electric Co.; 1952. [13] French Patent 1,155,143, assigned to Society d’Ugine; 1958. [14] Herbst RL, Landrum BF. US Patent 2,842,528, assigned to Minnesota Mining and Manufacturing Co.; 1958. [15] Dittman AL, Passino HJ, Wrightson JM. US Patent 2,689,241, assigned to M. W. Kellog Co.; 1954. [16] Hamilton JM. Ind Eng Chem 1953;45:1347. [17] Passino HJ, Dittman AL, Wrightson JM. US Patent 2,820,026, assigned to 3M Co.; 1958. [18] Hamilton JM. US Patent 2,569,524, assigned to DuPont; 1951. [19] Passino HJ, et al., US Patent 2,744,751, assigned to M. W. Kellog Co.; 1956. [20] Fahnoe F, Landrum BF, British Patent 840,735, assigned to Minnesota Mining and Manufacturing Co.; 1960. [21] Benning AF. US Patent 2,559,749, assigned to DuPont; 1951. [22] Miller WT. US Patent 2,579,437, assigned to M. W. Kellog Co.; 1951. [23] Rearich JS. US Patent 2,600,804, assigned to M. W. Kellog Co.; 1952. [24] Bringer RP. Chlorotrifluoroethylene polymers. 1st ed. Encyclopedia of Polymer Science and Technology, vol. 7. New York, NY: John Wiley and Sons; 1961. p. 204
19. [25] Muntell RM, Hoyt JM. US Patent 3,043,823, assigned to 3M Co.; 1962. [26] British Patent 805,103; 1958. [27] Thenappan A, Rainal E. US Patent 9,862,811, assigned to Honeywell International; 2016. [28] Ihara K, Yamaguchi F. US Patent 5,145,925, assigned to Daikin Industries; 1992. [29] Manzoni C, Abusleme JA, Malavasi M. US Patent 6,342,569, assigned to Ausimont; 2002.

243

[30] Samuels GJ, Shafer HTP. US Patent 8,552,128, assigned to Honeywell International Inc.; 2013. [31] Brinati G, Marrani A, Goffaux B. US Patent 8,575,286, assigned to Solvay Solexis; 2013. [32] Brinati G, Lazzari P, Arcella V. US Patent 7,122,608, assigned to Solvay Solexis; 2006. [33] Pianca E, Barchiesi G, Esposto S, Radice M. End groups in fluoropolymers. J Fluor Chem 1999;95:71
84. [34] Lopez G, Gao C, Ameduri B. Synthesis and microstructural characterization of poly (CTFE-co-vinylidene chloride) copolymers Issue 20 Polymer Chemistry. Royal Society of Chemistry; 2015. [35] Polychlorotrifluoroethylene chemical compound (PCTFE). Fluorocarbon Company, www.fluorocarbon.co.uk; 2016. [36] PCTFE properties. Fluorothermt Company, www.fluorotherm.com; 2018. [37] Ebnesajjad S. 2nd edition Fluoroplastics: nonmelt processible fluoropolymers, vol. 1. Elsevier; 2015. [38] Wypych G. PCTFE polychlorotrifluoroethylene. Handbook of polymers. 2nd ed 2016. p. 330
3. [39] Chemical resistance of PCTFE. Polyfluor Plastics bv, www.polyfluor.nl; 2018. [40] PCTFE/Kel-F®/NEOFLONt. Thermech Eng Corp., www.thermech.com; 2018. [41] Daikin NEOFLONt PCTFE molding powder, product information. Pub No. EG-71j AK, www.DaikinChem.de; 2003. [42] Daikin NEOFLONt PCTFE M-300H, product information. Daikin Chemical Europe GmbH, www.DaikinChem.de; 2015. [43] Daikin NEOFLONt PCTFE M-400H, product information. Daikin Chemical Europe GmbH, www.DaikinChem.de; 2015.

14 Processing and Fabrication of Polychlorotrifluoroethylene Sina Ebnesajjad FluoroConsultants Group, LLC, United States

O U T L I N E 14.1 Introduction

245

14.4 Injection Molding

249

14.2 Processing Considerations 14.2.1 Zero Strength Time 14.2.2 Crystallinity 14.2.3 Stress

245 245 246 247

14.5 Extrusion

250

14.6 Machining and Joining

251

References

252

14.3 Compression Molding

248

14.1 Introduction Polychlorotrifluoroethylene (PCTFE) is a highperformance thermoplastic because of the combination of chlorine and fluorine in its molecule contributes to good properties. PCTFE has high compressive strength and low deformation-underload (cold-flow) characteristics, which are lower than other fluoropolymers. It retains its properties in the temperature range of 2255°C to 1150°C [1]. PCTFE consists of homopolymers and copolymers of chlorotrifluoroethylene. These thermoplastics are utilized due to their chemical resistance and mechanical and electrical properties. PCTFE polymers can be fabricated by conventional melt processing techniques like extrusion and compression and injection molding. PCTFE sheets are produced by the compression molding method. Rods and tubing are fabricated by extrusion while small parts are made by injection molding. Table 14.1 shows the properties of a few different commercial grades of PCTFE and the recommended methods for various part shapes.

14.2 Processing Considerations Properties, particularly molecular weight and mechanical strength, of PCTFE vary depending on the

processing technique. Three factors affect the mechanical properties and performance of PCTFE fabricated by melting the resin. They include molecular weight, crystallinity, and the amount of stress applied to the polymer during the processing. It is not possible to measure the molecular weight of PTCFE. A representive factor for the molecular weight is zero strength time (ZST) which is defined as the time required for breaking a PCTFE specimen under heated conditions specified by ASTM Method D1430. In the ZST test, duplicate measurements are made using two 50 3 4.8 mm strips of film. These strips are V notched on both sides near the center of the strip. A 7.5 g weight is hung from one end of the film strip and the other end is attached to a sample holder inside an oven held at the constant temperature of 250°C. The time required for each specimen to break is recorded and the average is reported as ZST of the polymer sample.

14.2.1 Zero Strength Time ZST is a proxy variable for the molecular weight of PCTFE. High molecular weight is desirable for good physical properties. The generally accepted minimum value is a ZST of 100 seconds. Consequently, very high melt viscosity PCTFE

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Table 14.1 Properties, Processing, and Applications of Daikin Neoflon PCTFE Homopolymer [2]. Product No.

Apparent Density (g/ mL) (Approx.)

Flow Valuea (mL/s) 23

ZSTb (s)

Description

Processing Methods

Uses

M-300

0.60

1 3 3 10

200 300

Powders (10 60 meshes)

Compression

Sheets

M-300Hc

1.00

1 3 3 1023

200 300

Granular powders

Compression Extrusion

Sheets RodsTubing

M-300Pc

1.20

1 3 3 1023

200 300

Pellets

Extrusion Injection

Rods Small parts

1.00

0.5 0.8 3 1023

301 450

Granular powders

Compression Extrusion

Sheets Rods

M-300PL M-400Hc a

Measured by a flow tester at 230°C, underload 100 MPa (nozzle size 1 mm diameter, 1 mm length). ASTM D1430, zero strength time at 250°C. c Recognized by Underwriters’ Laboratories, Inc. b

must be processed to fabricate parts. The only way to reduce the viscosity of an existing polymer is to increase its processing temperature. Degradation temperature of CTFE polymers is about 296°C, which means that processing at higher temperatures results in the degradation of the polymer and a lower molecular weight. A lower molecular weight means lower physical properties and a lower ZST. Table 14.2 provides a comparison of the thermal stability of polytetrafluoroethylene (PTFE) and PCTFE in vacuum and in oxygen. PCTFE is thermally less stable than PTFE. The objective should be to obtain the maximum ZST possible while processing the polymer. Design of a part has a great influence on the ZST because each part requires a unique set of processing variables. Injection molding and extrusion invariably lead to a lower ZST than compression molding, primarily due to higher process temperatures. There is ZST loss in a compression-molded part while the drop can be minimized in injection molding and extrusion. Fig. 14.1 shows the effect of temperature and time on the ZST of PCTFE. Clearly, molecular weight of the polymer decreases rapidly at temperatures above 277°C even when it is held for a short time (B2 minutes).

14.2.2 Crystallinity Crystallinity can be accurately measured by Xray diffraction and less accurately but simply by

Table 14.2 Temperature (°C) for 25% Weight Loss in Two Hours [3]. Polymer

In Vacuum

In Oxygen

PTFE

494

482

PCTFE

349

355

specific gravity. The properties of PCTFE vary with the degree of crystallinity. Table 14.3 contains properties of two resins with different specific gravities. The crystalline resin, one with the higher specific gravity, has higher tensile yield strength and flexural modulus than the amorphous plastic. The amorphous polymer has higher deformationunderload and impact strength than the crystalline resin. How does processing alter the crystallinity of PCTFE? The degree or extent of crystallinity is determined by the rate of crystallization and the thermal history of the polymer. The rate increases with a decrease in ZST or an increase in temperatures due to a reduction in the size of the molecules as a result of degradation. A reduction in the cooling rate leads to an increase in crystallinity at any ZST. The starting temperature for crystallization is 52°C and a maximum is reached in the range of 150°C 190°C. The crystals completely melt upon reaching 204°C 218°C. Specific gravity change as a function of ZST can be seen in Table 14.4.

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Figure 14.1 Zero strength time as a function of PCTFE aging time and temperature [4]. Table 14.3 Effect of Crystallinity on the Properties of PCTFE (ZST 5 250, Compression-Molded Sheet) [4]. Property

Amorphous

Medium Crystallinity

Specific gravity

2.105

2.131

175

105

25°C

1311

1753

70°C

373

1028

125°C

90

22

25°C

0.4

0.2

70°C

7.3

0.4

125°C

. 25

3.6

Impact strength, notched Izod (J/m)

267

80

Tensile yield Strength (MPa) Break elongation (%) Flexural modulus (MPa)

Deformationunderload (%) 24 h at 6.9 MPa

14.2.3 Stress Internal stress develops in all plastics during the processing. The more polymer flow is involved in the operations, the more residual stress is generated

in the part. This means that injection molding and extrusion develop more stress than compression molding. High-viscosity resins like PCTFE develop more stress than the low-viscosity polymers. Fast cooling of a part “locks in” the internal stresses. In injection molding of PCTFE, a high-viscosity polymer is forced through a small opening which subjects the melt to high shear rates, and then it is allowed to cool rapidly. The shrinkage during the solidification also contributes to the trapping of the stresses. This is in contrast to compression molding where the flow is slow and cooling is usually gradual. Excessive residual stresses in a part can manifest themselves in the form of distortion or catastrophic failure, as cracks in the part. Distortion may not be seen until the temperature of the part is elevated. In compression molding, very low shear rates are involved, typically in the range of 1/second [5]. Transition from the rubbery to melt state takes place at low temperatures which means very little rubbery component is present at the maximum process temperature. Consequently, a great relaxation of normal stresses takes place during the compression molding process. This is why parts made by compression molding contain minimal residual stresses and have excellent dimensional stability. There are ways to minimize or overcome the problems arising from the residual stress of parts. Some preventive measures include designing the process equipment to reduce stress buildup in the part by: eliminating many restrictions, reducing shear rates, slowing the cooling rate, and cooling under pressure. These provisions are not always

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Table 14.4 Crystallinity Change as a Function of ZST [4]. Specific Gravity

ZST 5 111 s

ZST 5 225 s

ZST 5 300 s

Initial

2.1240

2.1193

2.1186

After 24 h at 149°C

2.1469

2.1368

2.1344

After 1344 h at 149°C

2.1556

2.1452

2.1422

Figure 14.2 Schematic of the compression molding process: (1) Charge is loaded; (2 and 3) Charge is compressed and cured; and (4) Charge is ejected and removed [6].

practical. Annealing is an efficient method of reducing stress in fabricated parts. This technique consists of slow heating of the part in an oven to about 150°C, followed by slow cool down to room temperature. To combine annealing and an increase in crystallinity, the part can be heated to the maximum crystallization rate of PCTFE. In the case of extruded films, stress is relieved by heating the rolls while the film is being manufactured.

14.3 Compression Molding Compression molding (Fig. 14.2) uses a press to compress either a dough of resin or the layers placed by a hand lay-up method or mechanical means, typically at an elevated cure temperature. With the compressive force, the void content is lower than the ordinary atmospheric pressure processing method. To mold flat sheets of PCTFE, a special press is required. It must be equipped with platens capable of reaching the process heating and cooling temperatures and pressures. The construction material must resist corrosion at elevated temperatures. Mirror-polished

stainless steel plates can be used in conjunction with a thermally stable mold release agent such as silicone. For other shapes, an oven is required. The amount of resin is 2.15 g/mL of the final shape. The resin is placed between the press platens using a polished plate and spacers on the lower platen. The second plate is placed on top before closing the platens. The assembly containing the resin can be alternatively prepared outside the press in which case the platens can be preheated. After closure of the platens and slight pressurization, the resin is allowed to heat to the required temperature. The pressure is slowly increased until it reaches the desired value and is maintained for the specified dwell. After 3 5 minutes have passed, the sheet can be cooled slowly in a press or quenched by removal of the assembly and immersion in water. This is the fabrication technique to obtain a PCTFE part with the best physical properties due to its relatively low temperature. Thick sheets and thick-wall tubes and rods are prepared by this method when the thickness exceeds the capability of the extrusion process. The lengths produced by compression molding are limited to less than 0.5 m. High ZST (300 400 seconds) resins are converted

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by compression molding at pressures of 13.8 20.7 MPa and temperatures of 246°C 274°C [7]. Molding time for a 3-mm-thick sheet at 275°C is 7 14 minutes. Compression molding of large tubes and rods is done in two stages: heating and pressurization [4]. To make a rod, PCTFE pellets are placed in the mold and put in an oven for 48 minutes per centimeter of diameter and allowed to reach a temperature of 245°C 250°C. The assembly is then transferred to a press and pressurized at 1.7 MPa/ cm of mold diameter. Pressure should be increased at a low rate to allow air to escape. Pressure must be maintained during the cooling cycle until the center of the rod is below 90°C to avoid cracking of the part. Compression molding processes for tubes and rods are similar. For a tube, wall thickness is the major dimension in the selection of process conditions versus diameter for a rod. Daikin Neoflon M-300 is used [2] for the molding of transparent sheets. The powder is placed in a pile on the center of a ferro-type plate, and heated to 250°C 300°C between the platens of the press. The appropriate gauge block is placed on the side of the ferro-type plate. When the polymer reaches the desired state, another ferro-type plate is placed on the top of the powder and a pressure of 2.0 9.8 MPa is applied. After holding for a while, the assembly is transferred to cool press platens and quenched under 2.0 9.8 MPa. Both the Daikin Neoflon M-300H and the M400H are used [2] for molding heavy wall parts, such as sheets, rods, and sleeves. M-300 and M300H are used for compression molding of heavy shaped articles. The powder is heated at a temperature of 260°C 300°C in a mold until it reaches the molten state. Then, a pressure of 3.9 9.8 MPa is applied slowly. The assembly is then transferred to a cool press and cooled under a pressure of 9.8 49.0 MPa slowly. Compression-molded parts should be annealed above the maximum service temperature prior to machining. This will increase the part’s dimensional stability in the application.

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the article to be fabricated. The material is allowed to cool to solidification in the mold, which is then opened to eject the part. Although the principle is simple, the actual process is anything but simple. This is the consequence of the complex behavior of molten polymers and the complexity of the parts made by this process. The essential elements of injection molding are heat transfer and forced melt flow. The equipment for injection molding (Fig. 14.3) consists of a machine, sometimes referred to as press, and a mold also called as a tool or die. There are different types of injection molding machines. They all perform the same basic functions including melting the resin, injecting it into the mold, holding the mold closed, and cooling the injected plastic. It is convenient to think of an injection molding machine as consisting of two unit operations. Melting and injection functions are performed in the injection unit while the mold handling is conducted in the clamp unit. The two units are mounted on a common base and are connected by power and control systems. The main advantages of injection molding are high productivity/ low production cost, ability to produce complex parts, and reproducibility of the parts. Injection molding of PCTFE is a controlled reduction in the molecular weight of the resin to decrease the viscosity so that the melt would fill the mold at practical temperatures and pressures. Degradation of PCTFE produces corrosive byproducts such as hydrochloric acid. Consequently, the surfaces in contact with the melted polymer should be made of corrosion-resistant alloys such as Hastelloy C, chromeplated steel, or Xaloy 306 [8]. ZST decrease during injection molding depends on the thinnest section of the part. Minimum ZST with

14.4 Injection Molding In the first step of injection molding, the plastic material is heated until it becomes a melt. It is then forced into a closed mold that defines the shape of

Figure 14.3 Diagram of a typical injection molding machine.

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useful part properties is 100 seconds. The maximum ZST possible must be achieved without leaving voids in the part. Some of the requirements to process PCTFE include

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build up on the equipment surfaces. Their residues should be removed by an organic solvent such as trichloroethylene.

14.5 Extrusion

1. A minimum nozzle pressure of 210 MPa.

Rod stock, tubing, and film shapes of PCTFE can be produced by extrusion. Similar to injection molding, the thickness of the article being manufactured is the determinant of its ZST (molecular weight). Objects made by extrusion are not as strong as those produced by compression molding, but much longer lengths are possible. Wire insulation can also be produced by melt extrusion (Fig. 14.4). PCTFE can be extruded using most common extruders [7]. Sizes from 37.5 to 62.5 mm are adequate and larger extruders can be used to fabricate thick cross sections. The extruder must be capable of reaching a temperature of 340°C. Minimum extruder length to diameter ratio (L/D) is 16:1, but the preferred range is 20:1 to 24:1. Caution must be exercised in the design of the extruder screw because of the capability of highviscosity PCTFE melt to generate extremely high

2. Clamping pressures to counteract the injection pressure. 3. Maximum melt inventory of 15 minutes. 4. Excellent heat and cycle controls. Table 14.5 presents examples of process conditions for injection molding of PCTFE. The independent process variable is the thickness of the thinnest section of the intended part. The actual conditions may vary based on the part design. The best indication of whether a part has been processed properly is its ZST. Specific gravity (measure of crystallinity), physical strength, and appearance are the other measures of the part quality. It is important to consider that PCTFE degrades during the processing and produces corrosive and toxic byproducts requiring good ventilation. Very low molecular weight PCTFE products are oily and

Table 14.5 Process Conditions for Injection Molding of PCTFE [4]. Process Condition

Thickness (mm) ,3

3 6

.6

Rear cylinder

260 280

260 280

260 280

Mid cylinder

270 295

270 295

270 295

Forward cylinder

280 315

280 315

280 315

Nozzle

315 350

315 350

315 350

Mold face (coolant)

90 160

90 160

90 160

Melt leaving nozzle

280 305

280 305

280 305

Melt pressure at nozzle (MPa)

207 414

172 314

138 276

40 90

60 120

70 180

10

15

25

16

64

256

Diameter (round) (mm)

1.6 2.7

2.7 6.2

4.7 9.4

Land length (mm)

1.6

1.6 4.7

3.1 4.7

Temperature (°C)

Timing Total cycle (s) Ram forward time (s) 3

Volume of mold cavity (cm ) (assumed) Recommended gate dimensions (mm)

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pressures, even in excess of barrel strength. The use of a breaker plate is not recommended due to high viscosity of this polymer and its thermal degradation. The die and transition pieces of the extruder should be designed to avoid stagnant areas to prevent degradation. Thin (B25 µm) films of PCTFE are produced by extrusion. ZST values of the extruded films are usually in the range of 90 130 seconds, indicating fairly low molecular weight. These films are not very strong and prone to embrittlement when subjected to heat. Examples of extrusion conditions for a film and a heavy wall shape are given in Table 14.6. Table 14.7 shows the extruder profile for the extrusion of molding rods, tubings, and films.

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14.6 Machining and Joining PTFE has good machining properties for sawing, turning, drilling, milling, and cutting, because of its high melt temperature. Desirable parts may be easily obtained by machining the standard stock, such as sheets, rods, and shaped pieces. The PCTFEmolded parts can be buffed and polished with general paste [2]. PCTFE is easy to machine and behaves similarly to a soft metal while cutting on a lathe. Annealing before precision machining is helpful to insuring dimensional stability. It is recommended to use a coolant to maintain a constant temperature during the machining. High-speed sharp cutters yield the best results.

Figure 14.4 Basic components and peripheral equipment of a typical single screw extruder [9].

Table 14.6 Extrusion Conditions for PCTFE (L/D 5 20:1, rpm 5 10) [4]. Barrel Temperature (°C)

Die Temperature (°C)

Part

Rear

Middle

Front

Body

Tip

Thin film

200

304

321

318

349

Heavy wall shape

177

254

265

260

324

Table 14.7 Extrusion Conditions for Daikin Neoflon PCTFE (L/D 5 20 25:1, rpm 5 10 15, Gradual Transition Metering Type, Compression Ratio 2.5 3.0) [2]. Barrel Temperature (°C)

Die Temperature (°C)

Resin Type

Rear

Middle

Front

Adapter

Die Head

Die Tip

M-300H

230

280

290

295

310

320

M-400H

230

280

295

300

315

325

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PCTFE can be joined by thermal, ultrasonic, and dielectric welding methods [10]. Commercial ultrasonic and thermal impulse sealing equipment can be used. It is easy to heat-seal the films of this polymer by heating them to 200°C 260°C. Adhesive bonding of PCTFE parts is possible using epoxy adhesives after treatment of the adhesion area with a sodium etching agent or another method. Surface treatment of fluoropolymers for adhesion is discussed elsewhere in this book [11]. PCTFE films and sheets can be heat-sealed under certain conditions [2]: 1. Heating temperature: 260°C 280°C. 2. Heating time: approximately 10 minutes for every 2 mm sheet thickness. 3. Operating pressure: approximately 6.9 MPa. 4. Cooling rate: rapid cooling 2250°C/30 minutes.

References [1] Voltalef® PCTFE technical brochure. Arkema Corp., ,www.voltalef.com.; 2004. [2] Daikin NEOFLONtPCTFE molding powder, product information. Pub. No. EG-71j AK. Daikin America; 2003.

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[3] Critchley JP, Knight GJ, Wright WW. Heat resistant polymers. New York: Plenum Press; 1983. [4] Bringer RP, Moreneau GA, Processing of CTFE plastics, In: Plastic design and processing; 1968. p. 17 24. [5] Mascia L. Thermoplastics—material engineering. 2nd ed., Elsevier Applied Science; 1989. [6] Groover MP. Fundamentals of modern manufacturing. 3rd ed., John Wiley & Sons; 2007. [7] Bringer RP. Chlorotrifluoroethylene polymers. Encyclopedia of polymer science and engineering, vol. 7. John Wiley and Sons; 1967. p. 204 10. [8] Chandrasekaran S. Chlorotrifluoroethylene polymers. Encyclopedia of polymer science and engineering, vol. 3. John Wiley and Sons; 1989. p. 465 80. [9] Abeykoon C. Modelling and control of melt temperature in polymer extrusion [PhD thesis]. Belfast: Queen’s University; 2011. [10] Troughton MJ. Handbook of plastics joining: a practical guide. 2nd ed., Plastics Design Library, Elsevier; 2008. [11] Ebnesajjad S, Ebnesajjad CF. Surface treatment of materials for adhesion. 2nd ed., Elsevier; 2013.

15 Applications of Fluoropolymers Sina Ebnesajjad FluoroConsultants Group, LLC, United States

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15.5 Self-Supporting Components

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15.10 Mechanical Applications

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15.11 Automotive and Aerospace

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15.6 Trends in Using Fluoropolymers in Chemical Service

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15.13 Summary

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Polytetrafluoroethylene (PTFE) and other fluoropolymers have found application in thousands of uses in nearly every industry and facet of modern life. The performance nature of fluoropolymers usually keeps them away from the view. Yet, fluoropolymers have enhanced human living standards by playing critical roles in healthcare, aerospace, automotive, consumer, chemical processing, semiconductor, pharmaceutical, biopharmaceutical, electronic, space, defense, food and beverage, construction, energy, and other sectors.

15.1 Chemical Processing In the chemical process industry (CPI), much manufacturing is carried out with chemically aggressive fluids. Fluoropolymers have major applications in chemical processing equipment to prevent corrosion and product contamination. Increasing the resistance of equipment to corrosion is highly beneficial because it extends service life and cuts unscheduled downtime. Even though fluoropolymers may increase the initial equipment cost, lifetime costs can be reduced, allowing

manufacturers to be more competitive; and in especially severe service such as exposure to hydrogen fluoride, carbon steel components lined with PTFE, modified PTFE or perfluoroalkoxy (PFA) have replaced exotic metal alloys at a lower initial cost. PTFE resins modified with perfluoropropyl vinyl ether (PPVE) are weldable, resist permeation, and form smooth surfaces. Modified PTFE is thermoformable although not as easily as melt-processible fluoropolymers. Contamination of process streams by corrosion byproducts or ions from metallic equipment can be detrimental to the processes or products. Problems range from flaws due to corrosion particles in finished products to reduction in yields caused by unexpected reactions promoted by contaminants. Other polymers, such as polyesters, polypropylene, and polyvinyl chloride (PVC), are used to prevent corrosion and contamination. Fluoropolymers are, however, compatible with a much wider range of chemicals and can serve at higher temperatures than other polymers, so they are suitable for demanding environments. Linings and coatings for piping, vessels, fittings, and self-supporting parts are the principal applications

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for fluoropolymers in the CPI. Mechanical support is provided by carbon steel or fiberglass reinforced plastic (FRP) casings and vessels. Fluoropolymers are used in this manner for two reasons: 1. They fall short of the mechanical properties that most CPI structures and equipment required. 2. Even when polymer properties are adequate, lined equipment is often more cost-effective because of the cost of fluoropolymers. Fluoropolymers are also used in accessories including seals, bushings, spacers, bellows, and gaskets and in relatively small self-supporting components that require chemical resistance on all surfaces.

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tube. The resulting structure is called a dual laminate. Typically, the liner tube is a fluoropolymer such as PTFE, PFA, or fluorinated ethylene propylene (FEP). Glass fabric is embedded in the outer wall of the tube. This fabric bonds with the FRP structure so that the lining is held firmly in place. Lined FRP piping can be installed by welding of the linings and then covering the joints with additional FRP. This procedure is also used to form complex manifolds and transition pieces that are difficult or impossible to produce using lined steel. This installation method reduces the number of flanges in a piping system, which is desirable because flanges must be monitored for emissions and maintained by periodically checking the bolts for tightness.

15.2 Piping Flanged steel piping and a variety of fittings (T’s, elbows, reducers, spacers, etc.) are lined with PTFE tubes formed from granular or fine powder resins or melt-processible fluoropolymers (Figs. 15.1 15.3). The ends of the tubes are flared over flanges by thermoforming so that fluids contact only PTFE surfaces when piping sections are joined, usually by bolting. The lining thickness is typically less than 6.4 mm (0.25 in.). FRP piping lined with fluoropolymer (Fig. 15.4) is manufactured by forming FRP over a fluoropolymer

Figure 15.2 PTFE-lined expansion joints. Courtesy SGL Carbon Technic, www.sgl-processtechnology. com.

Figure 15.1 PTFE-lined fittings. Courtesy CRANE ChemPharma Flow Solutions, www.cranechempharma. com.

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Figure 15.3 Examples of PTFE-lined components. Courtesy SGL Carbon Technic, www.sgl-processtechnology.com.

Forming lined piping in the desired configuration can reduce the number of flanges required for lined steel piping. This is accomplished by a proprietary process with pipe manufactured to withstand forming operations.

15.3 Vessels Steel vessels such as scrubbers and tanks are lined with fluoropolymer sheeting joined by welding. PTFE linings can be installed in this way, including those that have a glass fabric backing applied with proprietary technology. This backing allows adhesive bonding to the steel structure for better support of the lining, an important consideration if the process is operated at subatmospheric pressures. Melt-processible fluoropolymers are used similarly to line vessels. They are easier to weld, and unlike PTFE, they can be readily thermoformed to fit vessel heads and provide connections for nozzles, or inlets and outlets. Like piping, lined vessels can be constructed as dual laminates with FRP. In some applications, they are preferred to lined-steel vessels because the FRP vessels often have sufficient chemical resistance to prevent damage by spills, and they do not require periodic painting to prevent corrosion. Also, because of their lower weight they require less support thus installations cost less. Thermoplastic fluoropolymers can also be applied as coatings or linings by powder coating

Figure 15.4 Examples of PTFE-lined FRP pipe and fittings. Courtesy Pureflex, an Andronaco Industries Company, www.pureflex.com.

and rotational lining. Both processes provide relatively thick, void-free fluoropolymer layers compared to coatings obtained with PTFE dispersions. Fluoropolymers are applied to surfaces by powder-coating technique in much the same way as other polymers. The principal differences are higher temperatures required to melt the fluoropolymers, and more stringent surface preparation requirements. In powder coating, powdered resin is applied to a surface using an electrostatic process and then heated in an oven so that the resin melts and forms a continuous layer. Additional layers are applied to reach the desired thickness. Rotational lining, or rotolining, is similar to rotational molding or casting, a process for forming shapes from polymers. The difference is that in rotolining, the product is a liner, while in rotational casting, the product is a formed part which must be removed from the mold. This process produces a seamless lining that can include multiple openings and complex shapes, thus rotolining is well suited for CPI applications. Table 15.1 shows a comparison of rotational lining with alternative technologies. To line a vessel or a component, powdered resin is placed in the component’s cavity and all openings are covered. Then, it is rotated on two axes in

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Table 15.1 A Comparison of Rotolining and Other Technologies.

Courtesy RMB Products, www.rmbproducts.com.

Figure 15.6 A rotolined transition component. Courtesy Plastichem Limited, www.plastichem. co.uk. Figure 15.5 Seamless lining of complex parts— fluoropolymer choices: PFA, ETFE, PVDF, and ECTFE. Courtesy RMB Products, www.rmbproducts.com.

an oven so that the resin melts and flows to cover all surfaces. The amount of resin consumed depends on the desired lining thickness. PFA, ethylene-tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF), and ethylene chlorotrifluoroethylene (ECTFE) have been used with good success in rotolining (Figs. 15.5 and 15.6). Manufacturers have report acceptable adhesion between the resin and the steel shell.

15.4 Chemical Process Industry Components Valves (Fig. 15.7), pumps (Fig. 15.8), and other components can be lined with PTFE through isostatic molding in which the resin is formed within the component’s cavity by pressure from an inflatable elastomeric tooling. After the tooling is deflated and removed, the lining is sintered in place. These components are also lined with meltprocessible fluoropolymers by conventional molding using metal tooling which is removed after molding of the lining is completed. Both lining

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Figure 15.7 A PFA- or FEP-lined plug valve. Courtesy Xomox, CRANE ChemPharma Flow Solutions, www. cranechempharma.com.

Figure 15.8 Cross section of a PTFE-lined (red areas) pump. Courtesy FlowServe Corp., www. FlowServe.com.

methods may require secondary finishing to exact dimensions for mating surfaces such as flanges. Unlined valves made of stainless steel and other metals often have components of PTFE such as

seats, packings, and diaphragms. Along with chemical resistance, PTFE in seats and packings provides conformability, that is, the parts conform to mating surfaces for good sealing, and low friction for ease of operation. Special designs are used, particularly for higher pressures, to prevent PTFE parts from creeping away from contact areas. For diaphragms, PTFE provides good flex life. Pipe threads are sealed using a special unsintered thread-sealant-tape (Fig. 15.9). Seals for rotating or sliding shafts use elements of PTFE compounded with fillers such as graphite. Fillers must have the chemical resistance for the service conditions that will be encountered. Labyrinth seals that do not contact the shaft depend on a tortuous interface, fluid surface tension, and a pump-like design to prevent leakage. Most PTFE seals are supported by metal springs or elastomeric rings to assure good shaft contact (Fig. 15.10). Used by itself, PTFE, even when compounded with fillers, may creep under load and reduce sealing force. PTFE seals may have surface features that, with shaft rotation, produce a pumping action in the direction of the component being sealed.

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Figure 15.9 Thread sealant tapes—yellow indicates special tape for natural gas lines.

Figure 15.10 Metal spring and elastomer supported PTFE gaskets. Courtesy Parker Hannifin Corp., www.Parker.com.

PTFE is a standard material for gasketing (Figs. 15.11 and 15.12) due to its nearly universal chemical compatibility, good performance at temperatures from cryogenic to over 260°C, and conformability. PTFE is used alone, or with fillers or metal supports, or as a dispersion for impregnating fibers such as aramids for gasketing. In addition to excellent sealing performance, gaskets made with PTFE can be used in a broad range of applications so inventories can be reduced and there is less risk of misapplication. Expanded PTFE is fibrous in structure and contains microvoids, which is especially useful for gasketing against uneven or delicate surfaces. With increasing pressure, fillers are added to PTFE to counteract its tendency to creep and cause sealing

Figure 15.11 A Gylon sheet gasketing material. Courtesy Garlock, an ENPRO Industries Co, www. Garlock.com.

to deteriorate. For higher pressures, gaskets incorporate metal rings or flat spirals to support sections of PTFE.

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Figure 15.12 Examples of die-cut filled PTFE gaskets. Courtesy Custom Gasket Manufacturing, www.customgasketmfg.com, January 2019.

15.5 Self-Supporting Components Some of the components for the CPI are made entirely of fluoropolymers. Typically, they do not bear significant loads and require uniform chemical resistance for all surfaces. Examples include dip tubes and distribution manifolds that blend fluids in vessels, and steam spargers that employ steam under pressure to pump aggressive fluids from tanks. Existing or new vessels can be equipped with a variety of internal components constructed from fluoropolymers. Some of the components include [1]

• • • •

Gas and liquid spargers. Dip tubes. Packing support trays (orifice type). Liquid distribution trays (bubble cap, orifice, weir).

• Filtration plates and supports. • Vacuum support cages. Examples of these components can be seen in the photographs in Fig. 15.13. Pressure hoses are versatile components for chemical and gas transfer in CPI. Typically, these hoses must meet some or all of numerous requirements:

• • • • • •

Low permeability. High flex life. Good mechanical properties. Chemical resistance. Service at extreme temperatures. High purity.

Many processes require operation at elevated pressures exceeding the capability of PTFE. To

increase the mechanical strength of hoses, PTFE tubes are overbraided with steel, polyaramide, fiberglass, or other materials. Figs. 15.14 and 15.15 show examples of a variety of hoses with smooth and convoluted PTFE cores. Flexibility of the hoses can be increased by the convolution of the PTFE core. Helical convolution of the PTFE tube takes place in a secondary operation which produces a spiraled convoluted tubing (Fig. 15.16). Both smooth bore (internally smooth) and convoluted bore hoses are manufactured commercially (Fig. 15.15).

15.6 Trends in Using Fluoropolymers in Chemical Service The number of plants handling hazardous chemicals has increased over time as a direct consequence of an ever-increasing variety of products and processes [3]. Consumption of fluoropolymers in the construction of equipment has continued to increase because of their unique properties especially resistance to corrosive chemicals and low/ high-temperature capability. Globalization has resulted in the migration of majority of new factories to developing economies. Fluoropolymers not only provide the ultimate among all plastics but flexibility for construction of complex shapes. PPVE-modified PTFE and PFA are commonly used perfluorinated polymers in chemical service. These polymers allow welding and thermoforming, both of which reduce equipment manufacturing costs. Modified PTFE has other advantages over standard PTFE such as lower creep, formation of smoother surfaces, and higher dielectric breakdown strength. Rotolining has proven to be a reliable and versatile method for lining complex parts with PFA,

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ETFE ECTFE, and PVDF resins. This technique is effective when a few complex parts are required. Rotolining is an economic process for the manufacture of parts in smaller numbers than would be viable for injection molding. In general, the CPI has moved toward extended maintenance intervals to reduce the cost of equipment operation. This consideration has encouraged the use of long-lasting equipment lined or made with fluoropolymers. Pursuit of higher productivity often depends on higher pressures and temperatures and contamination reduction to increase yields, all which favor increasing the use of fluoropolymers. Experience with lined parts such as valves, pipes, vessels, and pumps housings have demonstrated that PFA has certain advantages over other fluoropolymers when high thermal and chemical resistance are the primary requirements. Melt processibility facilitates manufacturing liners and parts free of voids resulting in superiority over PTFE in applications requiring stringent barrier properties. On the other hand, PTFE is still widely consumed for manufacturing chemical processing components and equipment where ultimate flex fatigue resistance is needed. A PFA disadvantage over PTFE is its significantly higher material cost. One of the newer applications of fluoropolymer is in biopharmaceutical manufacturing factories. There are a number of requirements for part qualification in biotechnology and biopharmaceutical plants. PTFE, for example, is chemically compatible, nonleaching, and resistant to biofilm buildup. PTFE process surfaces protect product quality and purity, enhance productivity, facilitate equipment cleaning, enhance durability, and reduce maintenance costs.

15.7 Semiconductor Processing

Figure 15.13 Examples of vessel internals. Courtesy NISSHIN GULF COAST, Inc, www.ngcinc.com.

From its onset, the semiconductor industry has relied on fluoropolymers for wet processing equipment, fluid transport systems, and wafer handling tools (Figs. 15.17 15.19). Semiconductor manufacturing processes are extremely intolerant of particulate and chemical contamination which, even in trace amounts, can cause a severe decrease in

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Figure 15.14 Example of a PTFE-lined braided hose. Courtesy Techna-Fit Fluid Transfer Systems, www.techna-fit.com.

Figure 15.15 Convoluted and smooth PTFE-lined braided hoses for fluid transfer. Courtesy Parker Hannifin Corp., www.Parker.com.

Figure 15.16 Examples of PTFE, FEP, and ETFE convoluted tubing [2]. Courtesy Texloc Division of Parker Hannifin Corp., www.texloc.com.

yields. Fluoropolymers’ purity and resistance to chemical attack are, therefore, important attributes. Carriers are devices that carry silicon wafers through various chemical processes. They were first made from PTFE which is still used for some carriers, usually small ones. The necessary machining to fabricate carriers was PTFE (Fig. 15.20) and material loss has rendered high-purity grades of PFA as the preferred material of construction (Fig. 15.21). PFA can be efficiently fabricated in the required configurations by injection molding technique without costly secondary machining operations. Aggressive ultrapure acids, solvents, and water are required for silicon wafer cleaning/etching. To maintain their purity, these fluids are transported from fluoropolymer-lined storage containers to consumption points through fluoropolymer tubing, usually high-purity PFA, while flow is managed with

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Figure 15.17 High-purity fluid handling components made from PFA, PTFE, and lined metals. Courtesy Entegris, Inc, www.Entegrisfluidhandling.com.

Figure 15.18 Ultrahigh-purity fluoropolymer O-Ringfree Poppet Check Valves using modified PTFEwelded components. Courtesy Swagelok Company, www.Swagelok.com.

valves, pumps, and pressure regulators made from PFA, PTFE, and occasionally other fluoropolymers. These fluid handling components are nearly always solid fluoropolymer constructions. These designs are possible because sizes are relatively small compared with functionally similar but far larger components used in the CPI. Therefore mechanical properties and material cost are not particularly sensitive issues. In addition, semiconductor processes are easily compromised by the presence of tiny amounts of metallic ions, so manufacturers avoid having metal components in the vicinity of process fluids. The designers of these all-fluoropolymer components employ unusual features such as machined PTFE bellows for valves actuated by air pressure and rods incorporating carbon fiber that provides stiffness for wafer carriers.

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Figure 15.19 Example of a PTFE/PFA valve for ultrahigh-purity applications. Courtesy GEMU Corp., www. gemu-group.com.

Figure 15.20 A polytetrafluoroethylene wafer carrier. Courtesy WEIKU, www.weiku.com.

Wet benches, the facilities in semiconductor plants where wet processes are carried out, are typically made of fluoropolymers where surfaces can affect fluid purity. Storage containers and sinks, along with all other plumbing, are constructed from

Figure 15.21 150 mm PFA wafer handling systems. Courtesy Entegris, Inc, www.Entegrisfluidhandling. com.

fluoropolymers. Flame resistance of fluoropolymers is a major advantage in insuring semicon fabs short for fabrication plant (also called foundary).

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15.8 Trends for the Use of Fluoropolymers in the Semiconductor Industry Circuit density has been increased significantly over the decades according to Moore’s law. In 1965, Gordon Moore, a cofounder of Intel Corporation, predicted the number of transistors per unit area of an integrated circuits double every 2 years. That was based on careful observation of an emerging trend in 1965. In practice, the doubling has taken place every 18 months. Along the way the relative chip size and cost have decreased while chip numbers have grown at an exponential pace. That insight, known as Moore’s law, has become the golden rule of the electronics industry. Many experts believe Moore’s law hits its physical and economical limitations in 2017 and has slowed [4]. Each generation of the semiconductor manufacturing process is referred to as technology node. The process’s minimum feature size is the designation given to the technology node. That is the size of the process’s length in nanometers. The common factor to refer to the circuit density is half-pitch defined as half the distance between identical features in an array for a memory cell. As of 2018, 14 nm process chips are commonly in mass production to be followed by 10 nm chips production. Minute defects scan cause failures in the form of loss of entire wafers which could cost tens of millions of dollars for a single batch. Defects can

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originate from chemical and particulate contamination of fluids and particle shedding from solid surfaces. There has been growing demand for fluoropolymers with ever higher purity and resistance to particulate formation. Manufacturers of fluid handling components strive to prevent contamination of fluoropolymer components during processing. Wafer size has been increasing, and more processing steps are required to build more powerful circuits. Fig. 15.22 shows a comparison of 300 and 450 mm silicon chips. These factors greatly increase the value of wafers in work, so upsets such as contamination of process fluids have a severe financial impact. To help guard against contamination, double containment designs are used for fluid transport systems that feature two fluoropolymer barriers between fluids and the external environment.

15.9 Electrical Applications Fluoropolymers are widely used for wire and cable insulation (Figs. 15.23 15.25) but not always for the same reasons. It is true that all applications make use of polymer’s dielectric properties, but not to the same degree. Some uses exploit the ability of fluoropolymers to serve over a wide temperature range, and particularly at high temperatures. Others rely on their resistance to chemicals or their resistance to changes in properties over time.

Figure 15.22 Comparison of 300 and 450 mm silicon wafers.

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Figure 15.23 High-performance aerospace PTFE tape-wrapped wire. Courtesy Thermax Corp., a Carlise Interconnect Technologies, www.carlisleit. com/brands/thermax.

Figure 15.24 Coaxial cables Insulated with PTFE. Courtesy Harbour Industries, www.harbourind.com.

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In electronic connectors for use at high frequencies, PTFE is a standard material due to its low dielectric constant. This dielectric property help minimize the loss of strength of signals being transmitted through the connectors. In thermocouple connectors, ETFE provides resistance to elevated temperatures. A major use of FEP is in insulation for data wire and cables to move signal in computer networks in office buildings. They often are called “plenum cables” because they are installed in air-handling plenum spaces. FEP is used for two reasons: its excellent high-frequency dielectric properties and fire performance. Its low dielectric constant and dissipation factor at high frequencies helps assure good data signal transmission at high frequencies required for computer networks. Its fire performance helps these data cables meet building code safety requirements for low flame spread and low smoke generation (Fig. 15.26). FEP is also used to insulate coaxial cables that must meet the stringent building code requirements. Such cables are used for video transmission in broadcasting studios and other demanding highfrequency applications. In some code jurisdictions, fire alarm cables are insulated with PVDF because of its good fire performance and mechanical toughness. In this use, highfrequency dielectric properties are unimportant. For several decades, virtually all commercial and military aircrafts have used signal, control, and power wire and cables insulated entirely or partly with fluoropolymers. PTFE, FEP, ETFE, ECTFE,

Figure 15.25 Aerospace data cables insulated with PTFE and/or melt-processible fluoropolymers. Courtesy Harbour Industries, www.harbourind.com.

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and PVDF have been used for this insulation. Critical performance characteristics include service at extreme temperatures, good fire performance, and resistance to chemicals such as hydraulic fluids, fuels, and cleaning solutions. The resistance of fluoropolymers to property changes over time is also important for aircraft wiring insulation. PVC and many other common polymers used for insulation must be compounded with plasticizers, fire retardants, and other materials to achieve required levels of performance. Some additives may migrate out of the insulation or undergo changes that will reduce performance to unacceptable levels. This deterioration can occur more rapidly at the high temperatures at which some aircraft wiring operates.

Figure 15.26 Cable for signal transmissions in industrial, video, sound and security and interconnect applications. Courtesy Belden Corp., www. Belden.com.

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Most fluoropolymers do not require additives to enhance performance, and they are chemically stable at relatively high service temperatures, so their performance does not change with aging.

15.10 Mechanical Applications PTFE is used in bearings due to its extremely low friction and inherent lubricity (Fig. 15.27). Compounded with graphite, bronze powder, or other fillers to reduce creep and improve wear resistance, PTFE is molded or machined into bearings for unlubricated service. PTFE offers unusual performance in bearings because its static coefficient of friction is lower than its dynamic coefficient. Therefore PTFE bearings do not exhibit “stick-slip”; the jerking action that occurs in overcoming a higher static coefficient before movement begins. In motion against a PTFE surface, a metal component causes fairly rapid initial wear. After a time, the rate of wear diminishes. That is because PTFE transfers to the metal surface so that the contact is PTFE against PTFE, a combination that produces little wear. Bearings of PTFE usually require the added support of metal structures because PTFE lacks mechanical strength and stiffness. Compared with metal bearings, PTFE bearings are suitable only for relative low loads and velocities. PTFE bearings are used in instruments, aircraft and aerospace vehicles control systems, office machines, and other applications where lubrication is difficult or undesirable. Slide bearings called bearing pads are used in support systems for bridges and some buildings to

Figure 15.27 Example of PTFE bridge bearing. Courtesy Snea Bearings Pvt, Ltd., www.snehabearings.com.

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Figure 15.28 An automotive sensor. Courtesy RB Racing, www.rbracing-rsr.com.

Figure 15.29 Automotive PTFE-lined fuel hose. Courtesy Hot Rod Magazine, www.hotrod.com.

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accommodate thermal and seismic movement without damage to the structures they support. Compared with similar elastomeric supports, they allow greater range of movement, and compared with lubricated metal bearings, PTFE bearing pads require no lubrication and are highly resistant to moisture and chemical attack. Designs often have stainless steel plates riding against a PTFE sheet surface. As for other bearings, the PTFE is usually compounded to improve creep performance. Bearings made with PTFE fiber are used in packaging machinery, pulp and paper processing equipment, and other applications. The PTFE fiber is woven into a fabric with some other fiber that will bond well with adhesives. This fabric is incorporated in a RTP (reinforced thermoplastic) structure to form a spherical bearing that will accommodate misalignment. Low-friction linings for push pull control cables that require no lubrication can be made with PTFE. Such cables are used in automotive and aerospace applications. Another example PTFE coated impregnated glass cloth usually along with a high-temperature silicone adhesive. This product provides a smooth antistick surface. The glass cloth fabric allows for extra strength and dimensional stability. The cloth works well for various mechanical applications like protection of heat-sealing bar, conveyor belt coverings, and gaskets and seals. This product provides excellent insulation characteristics in addition to abrasion and chemical resistance [5].

15.11 Automotive and Aerospace For applications in automobiles and other vehicles, fluoropolymers are selected for their resistance

Figure 15.30 Automotive PTFE-lined fluid transfer hose. Courtesy Titeflex Corp., www.Titeflex.com.

Figure 15.31 A section of an aerospace (R160) braided hose. Courtesy Titeflex Aerospace, a Smith Group Co, www.titeflex.com.

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Figure 15.32 ePTFE vascular graft configured for pediatric shunt. Courtesy WL Gore and Associates, www.Goremedical.com.

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to high temperatures and chemicals. Rising underhood temperatures and the need to prevent atmosphere release of fuel have led to the growth of fluoropolymer applications. Wire insulated with PTFE is used for connections to oxygen sensors (Fig. 15.28) mounted in vehicle exhaust manifolds. In this hot environment, PTFE provides reliable dielectric protection for wiring that is vital to control of exhaust emissions. ETFE is used to insulate wiring for other hightemperature locations near engines, and wiring exposed to hot hydraulic fluid within automatic transmissions. The resistance of ETFE to permeation of fuel components is critical in applications in fuel tank filler necks and fuel and vapor management hoses. These components help automobile manufacturers meet strict emissions regulations. In one successful design, these components are elastomeric structures lined with ETFE. PTFE tubing jacketed with braided stainless steel wire is used for brake lines and coolant circulation in heavy-duty vehicles (Figs. 15.29 15.31). The wire jacket helps prevent damage due to high fluid pressures and abrasion, and the PTFE tubing resists elevated temperatures, aging, and exposure to chemicals in fluids. Fig. 15.32 shows a section of an aerospace hose which has a PTFE core and is braided with stainless steel wires.

15.12 Medical Devices

Figure 15.33 Large diameter ePTFE aortic repair graft. Courtesy Atrium Med, www.atriummed.com.

Medical devices are an important area for both plastics and fluoropolymers. Devices containing PTFE and other fluoropolymers are used in many healthcare areas. They include diagnostics, routine care, prosthetics, implants, and surgical procedures. Graying populations require increasing healthcare for cardiovascular, orthopedic, heart rhythm management, and implantable vascular grafts (Figs. 15.33 and 15.34). There are too many medical devices containing fluoropolymers to list. Some applications of fluoropolymers include introducers, guiding catheter liners, insulation sheaths and

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Figure 15.34 Depiction of an implanted pace maker. Courtesy The London Arythmia Center, London Bridge Hospital, http://www.londonarrhythmiacentre.co.uk.

15.13 Summary This chapter presents only a select number of important applications in which PTFE and fluoropolymers are used. There are many more devices and equipment in which fluoropolymers may be found. It is no exaggeration to state that fluoropolymers have enhanced the standards of living of human beings by contributing to every facet of life in today’s world.

References

Figure 15.35 Example of application of an angioplasty catheter to place a stent in a coronary artery. Courtesy Hackensack University Medical Center, www.hackensackumc.org.

stents, drug-eluding stents lead assembly of pace makers and others (Fig. 15.35).

[1] NISSHIN GULF COAST, Inc, www.ngc-inc. com; 2014. [2] Kruit J. The ins and outs of standard fluoroplastics convoluted tubing. IAPD Magazine; April/ May 2006. [3] Reniers G, Amyotte P. Prevention in the chemical and process industries: future directions. J Loss Prevent Process Ind 2012;25:227 31. [4] Intel Corp., www.intel.com/content/www/us/en/ silicon-innovations/moores-law-technology. html; 2019. [5] Elite tape, PTFE coated glass cloth tape, https://elitetape.com; 2019.

16 Fluoroelastomers Jiri George Drobny1 and Sina Ebnesajjad2 1

Drobny Polymer Associates, Merrimack, NH, United States, 2FluoroConsultants Group, LLC, United States

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16.2 Fluorocarbon Elastomers 16.2.1 Manufacturing Process 16.2.2 Properties Related to the Polymer Structure 16.2.3 Cross-Linking Chemistry 16.2.4 Formulation of Compounds From Fluorocarbon Elastomers 16.2.5 Mixing and Processing of Compounds From Fluorocarbon Elastomers 16.2.6 Solution and Latex Coating 16.2.7 Curing 16.2.8 Physical and Mechanical Properties of Cured Fluorocarbon Elastomers 16.2.9 Applications of Fluorocarbon Elastomers 16.2.10 Applications of Perfluoroelastomers 16.2.11 Applications of Fluorocarbon Elastomers in Coatings and Sealants 16.2.12 Applications of Fluorocarbon Elastomers as Polymeric Processing Additives

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16.3 Fluorosilicone Elastomers 16.3.1 Polymerization 16.3.2 Processing

302 302 303

16.3.3 Properties of Cured Fluorosilicones 16.3.4 Applications of Fluorosilicone Elastomers

304 306

16.4 Fluorinated Thermoplastic Elastomers 16.4.1 Applications of Fluorinated Thermoplastic Elastomers

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280 282 284

16.5 Phosphazenes

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16.6 Safety, Hygiene, and Disposal 16.6.1 Polymerization and Finishing 16.6.2 Compounding, Mixing, and Processing 16.6.3 Hazardous Conditions During Use 16.6.4 Disposal of Used Products

310 310

16.7 New Developments and Current Trends 16.7.1 New Developments in Chemistry and Processing 16.7.2 New Products 16.7.3 Other Development

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291 292 294 295 297 299 300

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16.1 Introduction The introduction of fluorine into the elastomeric macromolecule generally produces materials exhibiting an improved retention of properties at high temperatures, reduced flexibility at low temperatures, and an improved resistance to solvents, fuels, oils, and greases. Essentially, there are two groups of fluoroelastomers: fluorocarbon (or fluorohydrocarbon) elastomers and fluoro-inorganic elastomers.

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311 311 312

Fluorocarbon elastomers are the most common fluoroelastomers and comprise monomeric units with carbon
carbon linkages having fluorinated pending groups with varied amounts of fluorine. Fluoro-inorganic elastomers, such as fluorosilicone [1] and fluorophosphazene [2] elastomers, comprise inorganic monomeric units having fluorinated organic pendant groups. This group of products exhibits a high retention of tensile properties and exceptional low-temperature flexibility [3]. The

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00016-1 © 2021 Elsevier Inc. All rights reserved.

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former group will be discussed as the major subject in this chapter. Short sections on fluorosilicone elastomers and fluorophosphazene elastomers will also be included.

16.2 Fluorocarbon Elastomers Fluorocarbon elastomers are the largest group of fluoroelastomers and have, as pointed out earlier, carbon-to-carbon linkages in the polymer backbone and a varied total amount of fluorine in the molecule. They can be based on several types of monomers: vinylidene fluoride (VDF), tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), perfluoromethyl vinyl ether (PMVE), 1-hydropentafluoropropane (HPFP), ethylene, and propylene. Proper combination of those monomers produces amorphous materials with elastomeric properties. A review of monomer combinations in commercially important fluorocarbon elastomers is given in [4]. VDF-based elastomers have been, and still are, commercially most successful among fluorocarbon elastomers. The first commercially Table 16.1 Major Manufacturers of Fluoroelastomers. Company Chemours Daikin 3M (Dyneon) Solvay AGC Shin-Etsu Dow Corning Momentive Wacker Daikin (China) Dongyue Sichuan Chenguang 3F Zhejiang Juhua Meilan Group Sanhuan NEWERA Guanheng

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available fluoroelastomer was Kel-F, developed by the M. W. Kellog Co. in the late 1950s. Since then, a variety of fluorocarbon elastomers have been developed and made available commercially. Currently, there are seven major manufacturers producing fluorocarbon elastomers and these are listed in Table 16.1. The main commercially available fluorocarbon elastomers are in Table 16.2. In the ASTM D1418, fluorocarbon elastomers have a designation FKM, and in ISO-R1629 their designation is FPM. Worldwide fluoroelastomer demand in 2012 was 9080 metric tons with an average forward annual growth rate of approximately 4.9%. [5]. The fluoroelastomers consumption was 10,900 metric tons in 2014 and is estimated to be 13,800 metric tons in 2019 [6]. Fluoroelastomer market size was valued at USD 1.15 billion in 2016 and is projected to reach USD 1.64 billion by 2022, at a CAGR (compounded annual growth rate) of 6.2% during the forecast period. [7]. Perfluoroelastomers represent a special subgroup of fluorocarbon elastomers. They are essentially rubbery derivatives of PTFE and exhibit exceptional properties, such as unequaled chemical

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Table 16.2 Main Commercially Available Thermoset Fluorocarbon Elastomers. Monomer

HFP

PMVE

CTFE

P

HPFP

VDF

Dai-el 801 (Daikin) Fluorel (Dyneon) Tecnoflon (Solvay) SKF-26 (KCKK)a Elaftor 2000 Series (KCKK) Viton A (Chemours)




Kel-F (Dyneon) SKF-32 (KCKK) Elaftor 2000 Series (KCKK)




Tecnoflon SL (Solvay)

TFE




Kalrez (DuPont) Tecnoflon PFR (Solvay) Dupra (Daikin) Dyneon PFE Series (Dyneon) Perlast (PPE)




Aflas (Asahi Glass) Viton Extreme (Chemours)




VDF 1 TFE

Dai-el 901 (Daikin) Fluorel (Dyneon) Tecnoflon (Solvay) Viton B (Chemours) Fluorel (Dyneon)

Viton GLT (Chemours)




VDF 1 TFE 1 CSM

Viton GH (Chemours) Elaftor 7000 Series (KCKK)










Tecnoflon T (Solvay)

a

Kirovo-Chepetsk Chemical Plant, Russia (www.kckk.ru).

inertness and thermal stability. Currently, there are two types of known commercial perfluoroelastomers, KALREZ and PERLAST. These have ASTM designation FFKM. An alternating copolymer of TFE and propylene (TFE/P) and a terpolymer TFE/P/VDF are

fluorocarbon elastomers commercially available under the trademark AFLAS. They are characterized by improved low-temperature and electrical properties and steam resistance, when compared to FKM and are comparable to FFKM in chemical resistance at lower cost (details below). TFE/P has the ASTM

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D1418 and ISO 1629 designations FEPM and in ASTM D2000/SAE J200 it is classified as Type/ Class HK [8]. As pointed out earlier, the first commercial fluoroelastomer, Kel-F, was developed by M. W. Kellog, is a copolymer of VDF and CTFE. Another fluorocarbon elastomer, Viton A, is a copolymer of VDF and HFP, developed by DuPont ma and made available commercially in 1955. The products developed thereafter can be divided into two classes: VDF-based fluoroelastomers and TFE-based fluoroelastomers (perfluoroelastomers) [9]. Current products are mostly based on copolymers of VDF and HFP, or VDF and PMVE, or terpolymers of VDF with HFP and TFE. In the combination of VDF and HFP, the proportion of HFP has to be in the range from 19 to 20 mol.% or higher to obtain amorphous elastomeric product [10]. The ratio of VDF/HFP/TFE has also to be within a certain region to yield elastomers as shown in a triangular diagram (Fig. 16.1) [9, p. 73]. Fluorocarbon elastomers are classified in ASTM D1418 according to monomer composition as follows: Type 1 (VDF 1 HFP); curable by bisphenol; general purpose, best balance of overall properties. Typically 66 wt.% of fluorine. Type 2 (VDF 1 HFP 1 TFE); curable by bisphenol or peroxide; higher heat resistance, best resistance to aromatic solvents of the VDFcontaining FKMs. Typically 68
69.5 wt.% of fluorine. Type 3 (VDF 1 TFE 1 PMVE); curable by bisphenol or peroxide; improved lowtemperature performance, higher cost. Typically, 62 and 68 wt.% of fluorine.

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Type 4 (TFE 1 P 1 VDF); curable by bisphenol; improved base resistance, higher swelling in hydrocarbons; decreased low-temperature performance. Typically, 67 wt.% of fluorine. Type 5 (TFE 1 HFP 1 VDF 1 E 1 PMVE); curable by peroxide; improved base resistance; low swelling in hydrocarbon; improved lowtemperature performance. Fluorocarbon elastomers are generally prepared by high-pressure, free radical emulsion polymerization [11]. Organic or inorganic peroxy compounds, such as ammonium persulfate, are used as initiators. Inorganic initiators generally produce ionic chain ends, such as aCH2OH and aCF2COOH, which contribute to the colloidal stability of the latex formed during the polymerization [9, p. 77] In this case, suitable emulsifiers, such as ammonium perfluorooctoate, are not strictly required [4, p. 257]. The ionic chains derived from the polymerization initiators also have important effects on the properties of the resulting polymer, such as rheology, mechanical properties, and even sealing properties [9, p. 77]. Chain transfer agents, such as carbon tetrachloride, methanol, and acetone dodecylmercaptane, are used to control the molecular weight of the polymer. Optionally, a cure-site monomer (CSM) may be added. The polymerization may be either a semibatch [12] or a continuous process [13]. The resulting latex is most frequently coagulated into a crumb by adding salt or acid, or a combination of both, or by a freeze-thaw process. The crumb is filtered and washed to remove coagulant and watersoluble residues, washed, and dried. The finished product is supplied as pellets, lumps, or milled sheets [13]. Some fluoroelastomers are also available in latex form.

16.2.1 Manufacturing Process

Figure 16.1 Compositions of VDF/HFP/TFE.

As pointed out earlier, most commercial fluorocarbon elastomers are copolymers of two or more monomers made by free-radical emulsion polymerization. Fig. 16.2 is a schematic of the general process. The polymerization operation may be carried out in continuous or semibatch mode. Numerous process variations are used to produce different products. Molecular structures of fluoroelastomers are determined by polymerization and isolation process conditions, so product and process development are

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Monomers to recovery/recycle Water vapor Reactor Isolation

Compressor Monomers

Degasser/dispersion blender

Polymer to packaging or precompounding

Aqueous solution

Water Initiator Soap Chain transfer agent Cure-site monomer

Figure 16.2 General fluoroelastomer production process.

usually carried out simultaneously in laboratory semiworks units designed to emulate commercial operation [12, Chapter 4]. As indicated in Fig. 16.1, water and other liquid ingredients are added to the polymerization reactor. These include an initiator and soap as aqueous solutions and an optional chain-transfer agent and CSM. Two or three major monomers are fed as gases by a compressor. The reactor is maintained at the temperature, pressure, and hold-up time required for the particular product. Air and other impurities are carefully excluded from the feed and reactor systems. Polymer is formed in the reactor as a dispersion containing 15%
30% solids, with particle size generally in the range of 100
1000 nm diameter. At reactor conditions, much of the monomer present is dissolved in the particles at concentrations of 3%
30%, depending on polymer and monomer compositions, and on prevailing temperature and pressure. The polymer dispersion is discharged from the reactor to a degassing vessel maintained at low pressure to allow removal of residual gaseous monomer. In continuous reactor operation, the reaction vessel is maintained liquid-full and the dispersion is let down through a back-pressure control valve to the degasser. Recovered monomer is recycled continuously to the reactor through the monomer feed compressor. In semibatch reactor operation, the dispersion is let down to the degasser at the end of the polymerization, and recovered monomer is held for subsequent recharging of the reactor for succeeding batches of the same composition. Additional

vessels may be provided for final monomer removal and dispersion blending prior to isolation (Fig. 16.2). Polymer isolation is effected by chemical coagulation of the dispersion, followed by separation of polymer crumb from the aqueous phase, removal of soluble soap and salt residues, and dewatering and drying of the polymer. Usual coagulants are soluble salts of aluminum, calcium, or magnesium. Various means of separating polymer from the coagulated slurry are used commercially, including continuous centrifuges, filters, and dewatering extruders. Methods used for salt removal include washing by repeated reslurrying in fresh water and separation of polymer; washing on a batch filter or continuous filter belt; or expelling most of the aqueous phase in a dewatering extruder. The purified polymer is dried in a batch oven or continuous conveyor dryer, or in a drying extruder. The isolated fluoroelastomer is generally formed into pellets or sheet for packaging and sale as gum polymer. Alternatively, the polymer may be precompounded by adding curatives and processing aids before forming and packaging.

16.2.1.1 Emulsion Polymerization Essentially all fluorocarbon elastomers are produced commercially by emulsion polymerization. As previously described, polymerization occurs in monomerswollen polymer particles some 100
1000 nanometers (nm) in diameter, not in a liquid
liquid emulsion as implied by the name. Particles are stabilized by

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surfactant, either added or made in situ by polymerization in the aqueous phase. A water-soluble initiator system generates free radicals, some of which grow and form or enter particles. In most fluoroelastomer polymerization systems, there is no sizeable reservoir of liquid monomer present. Much of the monomer is dissolved in the polymer particles and is replenished by a continuous feed during the polymerization. Even in semibatch polymerization, the amount of monomer in the reactor vapor space is relatively small. The segregation of growing radicals in small particles under conditions of limited termination by incoming radicals allows attainment of the high molecular weights desired for good elastomeric properties. Especially for VDF-based fluoroelastomers, emulsion systems allow very high productivity in reactors of modest size.

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polymerization system, including monomer recovery and recycle, is shown in Fig. 16.3. Continuous polymerization has the advantage of allowing sustained production at steady state. High rates are attained at moderately high dispersion solids (15%
30%). Most or all of the heat of polymerization is removed by the temperature rise of chilled feed water, so polymerization rates are not limited by relatively low rates of heat removal through a reactor cooling jacket. Continuous polymerization is particularly advantageous for production of a few high-volume types, especially if individual product campaigns are 2 days or more in length. After initial adjustments are made, uniform polymer can be produced at the same conditions for a considerable period. Continuous polymerization is less attractive for a product line comprising many types, requiring short campaigns with frequent reactor startups and shutdowns. Modern control systems allow rapid attainment of goal polymer characteristics and thus good quality even in this situation. However, semibatch systems are better suited to making product lines with many low-volume specialty types. The range of products suitable for a continuous emulsion polymerization process is somewhat restricted. Monomer compositions must allow aqueousphase oligomerization rates high enough so that continuous generation of new particles occurs, and thus steady polymerization rates can be attained.

16.2.1.1.1 Continuous Emulsion Polymerization

DuPont pioneered VDF/HFP/(TFE) polymerization in continuous stirred tank reactors (CSTRs) in the late 1950s. An early version of a continuous fluoroelastomer production process, including isolation, is described by [14]. Recent versions of the continuous emulsion polymerization process as it was run by DuPont Dow Elastomers feature more feed components, monomer recovery with continuous recycle of unreacted monomers, and considerably more monitoring and control systems. A schematic diagram of such a continuous

Monomer hold tank

Reactor system

Recovery compressor Off-gas

Cooler

Recycle monomer FCV Polymer dispersion

PCV

Reactor Degasser

Blend tank

Fresh monomer feeds Feed compressor

LCV

Initiator Monomer recovery system

Water Soap Buffer

CTA CSM

Figure 16.3 Continuous emulsion polymerization system.

Dispersion to isolation

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277

Reasonably high radical generation rates are required, with dispersion stabilization by ionic oligomers and added soap. Suitable compositions include most VDF copolymers, especially the commercially important VDF/HFP/(TFE) and VDF/ PMVE/TFE products. For continuous emulsion polymerization of these VDF copolymers, low levels of highly water-soluble short-chain hydrocarbon alkyl sulfonates (e.g., sodium octyl sulfonate) are effective in place of fluorinated soaps [15]. TFE/PMVE perfluoroelastomers and ethylene/TFE/ PMVE base-resistant elastomers can also be made in continuous reactors, though at much lower rates. Sustained particle nucleation is difficult to attain for TFE/propylene compositions; these do not appear suitable for production in a continuous polymerization. Certain polymer designs that require initial formation of particles with little or no further initiation must be made in semibatch reactors. An example is the Daikin family of polymers with almost all chain ends capped with iodine, made in a living radical polymerization.

systems are not available in the open literature, but smaller scale reactors are described in a number of patents. Fig. 16.4 is a schematic representation of a fluoroelastomer semibatch reactor with associated charging and feed systems and monomer recovery system. Shown are components usually charged initially, and those that may be fed during the course of the polymerization. Semibatch polymerization is suitable for a wide range of compositions, including those having very slow polymerization rates. Semibatch reactors are more versatile than continuous reactors for making specially designed polymers. Feeds of initiator, transfer agents, and CSMs can be varied during the course of a batch to make polymers with different molecular weights and molecular weight distributions, end groups, and cure site distribution along chains. This allows control of rheology, processing, and curing behavior to an extent not attainable in CSTRs. Polymer composition and polymerization rate are readily controlled by setting monomer feeds during the reaction. Commercial semibatch reactors are capable of making a considerable number of low volume specialty products. However, the necessity of keeping different products separate in downstream handling equipment limits the versatility of the reactor system.

16.2.1.1.2 Semibatch Emulsion Polymerization

All fluoroelastomer producers use semibatch emulsion polymerization systems. Detailed descriptions of commercial fluoroelastomer semibatch

Monomer recovery

Monomer charge

Reactor

Cooling jacket

Accumulator

Monomer

Degasserdispersion hold tank

Charge Feed

Compressor

Water charge Soap solution charge Feeds: Initiator Buffer CTA CSM

Figure 16.4 Semibatch emulsion polymerization system.

Dispersion to blending & isolation

278

Semibatch reactors have limitations compared to continuous reactors in the production of highvolume, fast-polymerizing types. Heat of polymerization must be removed by means of a cooling jacket. With this limited cooling capability, polymerization rates must be limited well below those possible in adiabatic CSTRs for many important high-volume products (e.g., VDF copolymers containing 60
80 mol.% VDF). In campaigns of high-volume types, many batches with attendant shutdowns and startups are required, and batchto-batch variability may be significant. For many types, reaction times may be too short to allow monitoring of product characteristics, feedback, and adjustments within each batch. Adjustments can be made on subsequent batches, but large blend tanks may be required to reduce final product variability. Holdup of gaseous monomer mixtures in semibatch reactors and feed systems is greater than that in CSTR systems. Considerable volumes of monomer mixtures under pressure in semibatch reactor vapor spaces and in accumulators after compressors may present potential explosion hazards. The lower operating pressures of semibatch reactors somewhat offset this hazard, compared to the system depicted in Fig. 16.3. However, barricades around semibatch reactors and feed facilities may be necessary to protect personnel. 16.2.1.1.3 Suspension Polymerization

Suspension polymerization is used to make a number of thermoplastic polymers. In suspension polymerization, all reactions are carried out in relatively large droplets or in polymer particles stabilized by a small amount of water-soluble gum. Organic peroxide initiators are used to generate radicals within the droplets. A solvent may be used to dissolve a monomer at a relatively high concentration. The main advantages of suspension polymerization over emulsion systems are that no surfactants, which are difficult to remove from the product, are used, and no ionic end groups are present which may be unstable during processing at high temperatures. What follows is a general introduction of suspension polymerization. In one semibatch suspension process for making VDF homopolymer [16], the reactor is charged with water containing a cellulose gum (about 0.03%) as the suspending agent, an initiator solution, and a VDF monomer. The initiator of choice is diisopropyl peroxydicarbonate, which has a half-

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life of about 2 hours at 50°C. The jacketed reactor is heated with agitation to a temperature in the range of 40°C
60°C, with a pressure in the range of 6.5
7.0 MPa maintained by adding additional water or monomer during the polymerization period of about 3.5 hours. Chain-transfer agents may also be fed. Average particle diameter is typically about 0.1 mm for the dispersion obtained in suspension polymerization. At the end of polymerization, the reactor is cooled, the dispersion is degassed by letting off pressure from the reactor, the polymer is separated by filtering or centrifuging the dispersion, and washed to remove residual dispersion stabilizer. Major features of this process were adapted by Asahi Chemical Industry Co., Ltd. to make VDF/HFP/(TFE) fluoroelastomers. In the initial version of the Asahi Chemical suspension polymerization process [17], a relatively large amount of an inert solvent, trichlorotrifluoroethane (CFC-113, CCl2FaCClF2), is dispersed in water containing 0.01%
0.1% of methyl cellulose suspending agent. The mixture is heated under agitation to the desired polymerization temperature (usually 50°C) and the proper composition of VDF/HFP/ (TFE) monomer mixture to make the desired copolymer is charged in the amount necessary to get the goal concentration in the monomer-solvent droplets. With the solvent used, the pressure is usually relatively low, about 1.2
1.6 MPa. Reaction is started by adding diisopropyl peroxy dicarbonate initiator solution and a monomer mixture, with composition essentially that of the polymer being made, is fed to maintain the reactor pressure constant. Polymerization starts in the monomer-solvent droplets, with initial formation of a low molecular weight fraction. As polymerization proceeds, viscosity of the particles increases, long-lived radicals form, and both polymerization rate and molecular weight increase with reaction time. The resulting polymer has a bimodal molecular weight distribution, with the minor low molecular weight fraction acting as a plasticizer for the bulk high molecular weight polymer. Normally, no chain-transfer agents are used for polymers cured with bisphenol. Polymer viscosity is set from the ratio of total polymer formed to initiator charged. Since reaction times are fairly long (6 hours or more) to attain high dispersion solids (30%
40%), dispersion samples can be taken from the reactor during polymerization to monitor inherent viscosity and predict when to stop polymerization for goal viscosity.

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After polymerization is stopped by turning off the monomer feed, monomers are removed by venting the reactor. Considerable care is necessary during this operation to reduce pressure in stages so that rapid release of monomer from particles does not occur, and carryover of particles into vapor lines is avoided. Particle sizes after degassing are 0.1
1 mm in diameter and are readily separated by filtering or centrifuging the dispersion. Fluoroelastomers made by the suspension process have no ionic end groups and contain a significantly low molecular weight fraction. These copolymers can be made with high inherent viscosities for enhanced vulcanizate properties, while they still retain good processability because their compounds have relatively low viscosity at processing temperatures. Compared to emulsion products of similar composition, bisphenol-curable suspension products exhibit better compression set resistance, faster cure, and better mold release characteristics. Asahi Chemical also developed peroxide-curable VDF/HFP/TFE fluoroelastomers by charging methylene iodide along with the initiator to the suspension polymerization reactor. The resulting chain transfer reactions allow incorporation of iodine on more than half the chain ends. Final polymer molecular weight is determined mainly by the ratio of total monomer fed during the polymerization to iodine incorporated. The suspension process has been adapted to make bimodal VDF/HFP/TFE polymers for extrusion applications, such as automobile fuel hoses, to get smooth extrudates with minimal die swell at high shear rates [18]. These polymers contain 50%
70% very high molecular weight fractions (ηinh about 2.5 dL/g, Mn about 106) and 30%
50% very low molecular weight fraction (ηinh about 0.15 dL/g, Mn about 17,000), with polymer bulk viscosity determined by the relative amounts of the two fractions. The low viscosity fraction has a molecular weight below the critical chain length for entanglement (Me about 20,000
25,000), so it acts as a plasticizer to facilitate extrusion with low die swell. Similar bimodal polymers with low viscosity fractions having molecular weights greater than Me would exhibit very high die swells. Synthesis of these polymers is carried out in two stages of suspension polymerization. A very small amount of initiator is used in the first stage to make the high molecular weight fraction. Then, additional initiator and a relatively large amount of methylene iodide are charged to make

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the low viscosity fraction. The relative amounts of each fraction are estimated from the cumulative monomer feed in each stage. The amount of methylene iodide charged is that required to incorporate 1.5%
2% iodine in the low-viscosity fraction. Polymerization rate in the second stage is very low, so the total reaction time required for the bimodal polymer synthesis is some 40
45 hours. These bimodal polymers are ordinarily cured with bisphenol, but the iodine ends on the low viscosity fraction allow a mixed cure system with both bisphenol and radical components. The radical system links very short chains into longer moieties that can be incorporated into the bisphenol crosslinked network. Similar bimodal polymers made by emulsion polymerization with conventional chaintransfer agents are cured only with bisphenol. The resulting vulcanizates contain sizeable fractions of short chains that are not incorporated into the network and are thus susceptible to extraction when exposed to solvents. The suspension process described above was used by Asahi Chemical for commercial production of Miraflon fluoroelastomers during the early 1990s. However, it was recognized that the use of large amounts of the ozone-depleting solvent CFC113 would need to be phased out. A second version of the suspension process uses a small amount of a hydrogen-containing solvent such as HCFC-141b and CH3-CFCl2. Since only enough solvent is used to dissolve the initiator, the reactor operating pressure must be increased to 1.5
3.0 MPa so that a fraction (10%
30%) of the initial monomer charge condenses to form an adequate volume of droplets to serve as the polymerization medium. In a further improvement, the hydrochlorofluorocarbon solvent is replaced with a small amount of a water-soluble hydrocarbon ester, preferably methyl acetate or t-butyl acetate [19]. These polar hydrocarbon solvents are used mainly to feed the initiator to the reactor. The methyl or tertiary butyl groups are relatively inactive toward transfer, and these solvents are so soluble in water that little is in the polymer phase. After the Asahi Chemicals suspension polymerization technology was acquired by DuPont in 1994, additional development was carried out to extend the technology to VDF/PMVE/TFE fluoroelastomers with CSMs incorporated along the chains [20]. CSMs can be incorporated evenly along chains by careful feed in controlled ratio to polymerization rate of major monomers. In this way, bromine- or

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iodine-containing monomers can be incorporated, in addition to iodine on chain ends from methylene iodide transfer agent, to get polymers with improved characteristics in free radical cures. It should be noted that similar polymers can be made more readily by continuous emulsion polymerization [21]. Of more interest are bisphenol-curable VDF/PMVE/TFE compositions with 2Hpentafluoropropylene, CF2QCHaCF3, as CSM. Bisphenol-cured parts from such polymers have better thermal stability than products made by radical curing. The suspension polymerization process works well for VDF/HFP/TFE and VDF/PMVE/TFE compositions. These monomer mixtures exhibit high propagation rates at relatively low temperatures (45°C
60°C) and low monomer concentrations (less than 15% in monomer/polymer particles). Reasonably high polymerization rates are possible at temperatures below 60°C, so elastomer particle agglomeration is minimized. The amorphous polymers are insoluble in the monomer/solvent mixtures and also the monomer and solvent have low solubility in the polymer-rich phases. The high viscosity of the polymer-rich phase gives hindered termination so that long-lived radicals can grow to high molecular weights. The initial monomer mixtures charged to the reactor can be partially condensed at about 50°C and moderate pressure to form droplets as the initial locus of polymerization, without the need for charging large amounts of solvent or for charging polymer seed particles. Slower propagating compositions like TFE/ PMVE give a lower molecular weight and a less useful polymer when made by suspension polymerization than polymer that can be obtained by emulsion polymerization. For these perfluoroelastomers, monomer solubility in the polymer is high, so

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particle viscosity remains too low for hindered termination and the formation of long-lived radicals. Considerable initiator must be fed during the polymerization to sustain reasonable reaction rates. Several other TFE copolymer compositions give similar results.

16.2.2 Properties Related to the Polymer Structure Essentially, the high thermal and chemical stability of fluorocarbon elastomers, as of any fluoropolymers, is related to the high bond energy of the CaF bond, and to the high bond energy of the CaC and CaH links, caused by the presence of fluorine [22]. Copolymers of VDF and HFP, completely amorphous polymers, are obtained when the amount of HFP is higher than 19%
20% on the molar basis [10]. The elastomeric region of terpolymers based on VDF/HFP/TFE is defined by the monomer ratios. Commercially, VDF-based fluorocarbon elastomers have been, and still are, the most successful among fluoroelastomers. The chemistry involved in the preparation of fluorocarbon elastomers is discussed in some detail in Section 16.2.1. Swelling resistance of fluoroelastomers is directly related to the fluorine content in the molecule. This is demonstrated by data in Table 16.3 [23]. For example, when the fluorine content is increased by mere 6% (from 65% to 71%), the volume swelling in benzene drops from 20% to 3%. Copolymers of VDF and HFP have excellent resistance to oils, fuels, and aliphatic and aromatic hydrocarbons, but they exhibit a relatively high swelling in low-molecular-weight esters, ketones, and amines, which is due to the presence of the VDF in their structure [24]. VDF-based fluoroelastomers (e.g., VITON) have a very good

Table 16.3 Effect of Fluorine Content on Solvent Swell. Percent Swell FKM Polymer

% Fluorine

Benzene/21°C

Skydrola D/21°C

VDF/HFP

65

20

171 (at 100°C)

67

15

127

69

7
8

45

71

3

10

VDF/HFP/TFE VDF/HFP/TFE/CSM c

TFE /PMVE/CSM a

b

Aviation hydraulic fluid. Cure-site monomer. c Perfluoroelastomer. b

b

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281

Table 16.4 General Chemical Resistance of Fluoroelastomers. Outstanding Resistance

Good to Excellent Resistance

Poor Resistance

Hydrocarbon solvents

Low-polarity solvents

Strong caustic

Oxidative environments

(NaOH, KOH)

Dilute alkaline solutions

Ammonia and amines

Apolar chlorinated solvents

Aqueous acids

Polar solvents

Hydraulic fuels

Highly aromatic solvents

(ketones)

Aircraft fuels and oils

Water and salt solutions

(methyl alcohol)

Automotive fuels Engine oils

a

b

a

Unleaded fuels give some problems due to the presence of methyl alcohol. Certain amine additives in engine oils can be detrimental.

b

resistance to strong acids. For example, they remain tough and elastic even after a prolonged exposure to anhydrous hydrofluoric acid or chlorosulfonic acid at 150°C (302°F) [24]. General chemical resistance of different fluorocarbon elastomers is shown in Table 16.4. Perfluoroelastomers, that is, elastomers based on perfluoromethylvinyl ether (PMVE) and TFE, exhibit a virtually unmatched resistance to a broad class of chemicals except fluorinated solvents. On the other hand, they are adversely affected by hydraulic fluid, diethyl amine, and fumed nitric acid resulting in swelling of the elastomer by 41%, 61%, and 90%, respectively [24, p. 29]. Fluoroelastomers based on TFE and propylene (P) (e.g., AFLAS) swell to a high extent in aromatic hydrocarbons because of the relatively low fluorine content (54%). However, because of the absence of VDF in their structure, they exhibit a high resistance to highly polar solvents such as ketones, which swell greatly all fluoroelastomers containing VDF. In addition, elastomers based on copolymers of TFE and propylene exhibit a high resistance to dehydrofluorination and embrittlement by organic amines. This class of fluoroelastomers has a high resistance to steam and hot acids but shows extensive swelling in chlorinated solvents such as carbon tetrachloride, trichloroethylene, and chloroform (86%, 95%, and 112%, respectively, after 7 days at 25°C, 77°F). Surprisingly, they have a high swelling (71%) in acetic acid [24, p. 30]. The low-temperature flexibility of fluoroelastomers depends on their glass transition temperature (Tg), which, in turn, depends on the freedom of motion of segments of the polymeric chain. If the chain segments are flexible and rotate easily, the elastomer will have a correspondingly low Tg and

exhibit good low-temperature properties. Copolymers of VDF and HFP represent the largest segment of the fluorocarbon elastomer industry, but exhibit a Tg of only
20°C (
4°F), which results in very poor low-temperature properties of parts made from them. Terpolymers of VDF, TFE, and perfluoroalkoxy vinyl ethers (e.g., PMVE) have much better low-temperature properties but are considerably more expensive. The importance of flexibility of vulcanizates from fluorocarbon elastomers at low temperatures is demonstrated by the well-known disaster of the space shuttle Challenger. The O-rings on its solid rocket boosters stiffened in the cold and consequently lost their ability to form an effective seal. Useful ranges of service temperature of some commercially available fluoroelastomers are shown in Fig. 16.5 [24, p. 35]. The thermal stability of fluorocarbon elastomers also depends on their molecular structure. Fully fluorinated copolymers, such as copolymer of TFE and PMVE (KALREZ), are thermally stable up to temperatures exceeding 300°C (572°F). Moreover, with heat aging this perfluoroelastomer becomes more elastic rather than embrittled. Fluorocarbon elastomers containing hydrogen in their structures (e.g., FKMs, such as VITON, DYNEON, and DAI-EL) exhibit a considerably lower thermal stability than the perfluorinated elastomer. For example, the long-term maximum service temperature for FKM is 215°C (419°F) as compared with 315°C (599°F) for FFKM. In addition, it was shown that heating VITON A at 150°C (302°F) results in unsaturation and that metal oxides promote this dehydrofluorination at even lower temperatures [24, p. 35]. Copolymers of VDF and CTFE (e.g., Kel-F) with upper long-term use temperature of about 200°C (392°F) are less heat resistant than

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Figure 16.5 Useful service temperatures for commercial fluoroelastomers.

copolymers of VDF and HFP [25]. Fluoroelastomers based on HPFP, such as Tecnoflon SL (copolymer of HPFP and VDF) and TECNOFLON T (terpolymer of VDF
HPFP
TFE), because of a lower fluorine content than that of their analogs with HFP, also exhibit lower thermal stability when compared with them [24, p. 52, p. 53]. Another factor affecting thermal stability of compounds based on fluorocarbon elastomers is the curing (cross-linking) system used. This subject is discussed at some length in the section on compounding. Raw-gum fluorocarbon elastomers are transparent to translucent with molecular weights from approximately 5000 (e.g., VITON LM with waxy consistency) to over 200,000. The most common range of molecular weights for commercial products is 100,000
200,000. Polymers with molecular weights over 200,000 (e.g., Kel-F products) are very tough and difficult to process. Elastomers prepared with VDF as comonomer are soluble in certain ketones and esters, copolymers of TFE and propylene in halogenated solvents; perfluorinated elastomers are practically insoluble [23, p. 431].

16.2.3 Cross-Linking Chemistry Fluorocarbon elastomers based on VDF can be cross-linked by ionic mechanism. However, if the polymer has been prepared in the presence of a CSM it can be cross-linked (cured) by a free radical mechanism. Moreover, many fluoroelastomers can be crosslinked by ionizing radiation (see Section 2.3.3).

16.2.3.1 Cross-Linking by Ionic Mechanism Fluorocarbon elastomers based on VDF/HFP and VDF/HFP/TFE can be cured by bisnucleophiles, such as bisphenols and diamines. The mechanism, proposed [26], is outlined below: 1. Formation of aC(CF3)QCHadouble bond by elimination of “tertiary” fluorine. 2. Double bond shift catalyzed by fluoride ion and formation of aCHQCFadouble bond. 3. Nucleophilic addition of the aCHQCFa double bond with: a. Allylic displacement of fluoride affording the new aC(CF3)QCHadouble bond; b. Addition/fluoride elimination from the same double bond. The detailed description is in Fig. 16.6 [10, p. 79] where the bisnucleophile NuaRaNu represents a bisphenol or diamine cross-linking agent. The general disadvantage of curing fluoroelastomers by ionic mechanism is that dehydrofluorination required for this reaction produces considerably more double bonds than required for the cross-linking itself. This excess of unsaturation represents weak points in the polymeric chain, which can be attacked by basic substances contained in a contact fluid. This has been actually found when parts cured by this method were exposed to new oil and fuels containing basic additives [27,28]. Diamine cure system is used

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283

Figure 16.6 Reaction mechanism for ionic curing (with permission).

very little now. The exception is in latex compounding. Their major deficiency is a tendency to premature onset of cross-linking (“scorch”) typically at processing temperatures in the range 100°C
140°C (212°F
285°F) and relatively slow cure rates at temperatures used at molding, that is, 160°C
180°C (320°F
356°F). Moreover, the retention of physical properties on exposure to temperatures above 200°C (392°F) is relatively poor [12, p. 78]. The bisphenol cure systems gradually displaced the diamine system for curing VDF/HFP and VDF/ HFP/TFE fluoroelastomers. Curing with bisphenols has the advantage of excellent processing safety, fast cures to high states of cure, excellent final properties, especially high resistance to compression set at high temperatures [12, p. 78]. Several bisphenols are suitable for curing these elastomers, but the preferred one is Bisphenol AF, chemically 2,2-bis(4-hydroxyphenyl)hexafluoropropane. An accelerator, such as benzyltriphenylphosphonium chloride (BTPPC) is necessary, along with inorganic bases, such as calcium hydroxide and magnesium hydroxide with small particle sizes [12,

p. 79]. Typical amounts are 2 phr (parts per hundred parts of rubber) of Bisphenol AF, 0.5
0.6 phr of accelerator, 3 phr of MgO, and 6 phr of Ca (OH)2.

16.2.3.2 Cross-Linking by Free Radical Mechanism The reaction is activated by organic peroxides that decompose thermally during the cure. The fluoroelastomer has to contain reaction sites to produce a sufficiently high cross-link density. Bromine-containing fluoroelastomers form a stable network in the presence of peroxide. However, bromine-based fluoroelastomers were found to cause processing problems, mainly mold fouling. Iodine-based fluoroelastomers were found to be much better since they produce much less mold fouling and are suitable for more sophisticated molding techniques, such as injection molding [10, p. 79]. They also exhibit excellent sealing properties; however, their thermal stability is lower than that of bromine-based fluoroelastomers [10, p. 80]. The use of peroxides for cross-linking

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requires the addition of a coagent (radical trap), for example, triallyl isocyanurate (TAIC) or triallyl cyanurate (TAC). Perfluoroelastomers (FFKM) contain a CSM that is essential for their cross-linking. Examples of CSMs are perfluoro(8-cyano-5-methyl-3,6-dioxa-1octene) (8-CNVE), [29] perfluoro(2-phenoxypropyl vinyl ether), VDF, bromine or iodine or nitrile containing monomers, each having specific curing behavior and providing vulcanizates with different characteristics. In order to attain sufficient heat resistance, the compounds require long postcures at high temperatures, such as 288°C (550°F), in some cases under nitrogen [12, p. 93] Details about different systems are in [12, p. 93].

16.2.3.3 Cross-Linking by Ionizing Radiation 16.2.3.3.1 Cross-Linking of Fluorocarbon Elastomer Type of Elastomers

Fluorocarbon elastomers with ASTM designation FKM are predominantly copolymers or terpolymers of different fluorinated or perfluorinated monomers with VDF as pointed out earlier. The presence of VDF in their molecules is responsible for their propensity to cross-link by responding to the ionizing radiation [mainly electron beam (EB) and γ-rays]. Here again, the final result depends on the ratio of cross-linking to chain scission. Radiation promoters (prorads), such as TAC, TAIC, trimethylolpropane trimethacrylate (TMPTM), trimethylolpropane triacrylate (TMPTA), and N,N0 -(m-phenylene) bismaleimide (MPBM), reduce the damage to the elastomeric chain by the radiation [30]. It appears that each fluorocarbon elastomer has the best crosslink yield with a specific prorad. In general, optimized compounds from fluorocarbon elastomers irradiated at optimum conditions attain considerably better thermal stability and mechanical properties than chemical curing systems [31
33]. Typical radiation dose for a sufficient cross-linking of most fluorocarbon elastomers is on the range 10
100 kGy. 16.2.3.3.2 Cross-Linking of Perfluoroelastomer Type of Elastomers

Perfluoroelastomers (ASTM designation FFKM) are essentially copolymers of two perfluorinated monomers, TFE and PMVE with a CSM, which is essential for cross-linking. Perfluoroelastomers can be cured by ionizing radiation without any additives. The advantage of radiation cured FFKM is

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the absence of any additives so that the product is very pure. The disadvantage is the relatively low upper use temperature of the cured material, typically 150°C, which limits the material to special sealing applications only [34].

16.2.4 Formulation of Compounds From Fluorocarbon Elastomers When compounding fluorocarbon elastomers, the basic principles are the same as for other elastomers. The selection of the elastomer grade and of the remaining compounding ingredients depends on required physical and chemical properties of the vulcanizate (cured compound) as well as on the desired behavior of the compound during processing and curing. A typical FKM compound usually contains the following ingredients: one or more fillers, an acid scavenger, and a curing system (cross-linker). Inorganic or organic colorants are used for colored compounds. The development of a compound requires a great deal of experience and understanding of the chemistry involved and of the interactions among the individual ingredients. However, the compounding of fluorocarbon elastomers is relatively simpler than that of other types of elastomers [10, p. 81].

16.2.4.1 Fillers The type and amount of filler affects not only the final properties of the vulcanizate but also the processing behavior of the compound. Since the compounds stiffen very soon after mixing, only relatively small amounts of fillers, typically 10
30 phr, can be used [35]. Various carbon blacks are used for black compounds. Medium thermal black (N990) is the most widely used grade, because it offers the best compromise between physical properties and cost. More reinforcing grades of carbon blacks, such as N774 or N750, produce a higher hardness and better physical properties at the expense of somewhat higher compression set and cost. Lowest compression set values are obtained with Austin Black [35]. White (silica) fillers, often surface treated, are sometimes used to improve flow, moisture resistance, and tensile properties [36
38]. Fillers commonly used in fluorocarbon elastomers are listed in Table 16.5 [35, p. 123].

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285

Table 16.5 Fillers for Fluorocarbon Elastomers. Filler

Comments

MT Black (N908)

Best general-purpose filler; excellent compression set and heat aging

Austin Black (coal fines)

Better high-temperature compression set resistance than MT, but less reinforcing and poorer in processing and tensile strength/ elongation

SRF Black

High strength, high modulus compounds; aggravates mold sticking in peroxide cures

Blanc Fixe (BaSO4)

Best compression set of nonblack fillers; neutral filler good for colors; poorer tensile strength than MT Black

Nyad 400 (fibrous CaSiO3)

General-purpose mineral filler, neutral and good for control stocks; tensile comparable with MT

Ti-Pure R-960 (TiO2)

Good for light-colored compounds; good tensile but poorer heat aging than other fillers

Red Iron Oxide

Used at 5
10 phr with other neutral mineral fillers for red-brown compounds

Graphite Powder or TEFLON Powder

Combined at 10
15 phr with other fillers to improve wear resistance

Celite 350

General purpose neutral filler; good tensile strength

Table 16.6 Acid Acceptors. Acid Acceptor

Usage

Magnesium oxide (MgO)—low activity

General-purpose diamine cures

Magnesium oxide (MgO)—high activity

General-purpose bisphenol cures

Litharge (PbO)

Steam and acid resistance in all cures

Zinc oxide/Basic lead phosphate (ZnO/Dyphos)

Low compound viscosity in bisphenol stocks

Calcium oxide (CaO)

Added to minimize fissuring; can aid metal adhesion

Calcium hydroxide [Ca(OH)2]

General purpose with MgO

16.2.4.2 Acid Acceptor Systems Acid acceptors serve the purpose of neutralizing the hydrogen fluoride generated during the cure or on prolonged aging at high temperatures. The compounds used for that purpose are listed in Table 16.6. Low-activity magnesium oxide is used in diamine cures and not in bisphenol cures. Highactivity magnesium oxide is used in bisphenol cures and not in diamine cures. Lead oxide (PbO) is optimum, where the vulcanizate is exposed to hot acids, and dibasic Pb-phosphite with ZnO for exposure to steam or hot water. However, lead-based curing systems have largely been abandoned due to environmental and health concerns. Superior

performance in dry heat is achieved with CaO and MgO [35, p. 123].

16.2.4.3 Curatives As discussed previously, the mechanism involved in the cross-linking of fluoroelastomers is the removal of hydrogen fluoride to generate a cure site that then reacts with diamine, [38], bisphenol [39], or organic peroxides [40] that promote a radical cure by hydrogen or bromine extraction. Preferred amines have been blocked diamines such as hexamethylene carbamate (Diak No. 1) or bis(cinnamylidene) hexamethylene diamine (Diak No. 3). Preferred phenols are hydroquinone and the bisphenols such as 4,40 -isopropylidene

286

INTRODUCTION

bisphenol or the corresponding hexafluoro-derivative bisphenol AF. The nucleophilic curing system is most common and is used in about 80% of all applications. It is based on the cross-linker (bisphenol AF) and accelerator (phase transfer catalyst, such as phosphonium or amino-phosphonium salt). Both diaminic and bisphenol type cure systems are permitted by Food and Drug Administration regulations governing rubber articles in contact with food. The diaminic curing system is also used in some coating and extrusion applications [10, p. 81]. Peroxidic cure systems are applicable only to fluorocarbon elastomers with cure sites that can generate new stable bonds. Although peroxidecured fluorocarbon elastomers have inferior heat resistance and compression set, compared with bisphenol cured types, they develop excellent physical properties with little or no postcuring. Peroxide cured fluoroelastomers also provide superior resistance to steam, acids, and other aqueous solvents because they do not require metal oxide activators used in bisphenol cure systems. Their difficult processing was an obstacle to their wider use for years, but only recent improvements in chemistry and polymerization are offering more opportunities for this class of elastomers [10, p. 81]. Solid fluorocarbon elastomers are commercially available as pure gum polymers or precompounded grades with bisphenol type curing system included. Some precompounded stocks include processing aids, adhesion promoters, or other applicationspecific additives. The relative strengths and

TO

FLUOROPOLYMERS

weaknesses of commonly used curing systems are listed in Table 16.7. Precompounded grades are optimized by the supplier to provide the best combination of accelerator and cross-linker for a given application [23, p. 418]. Then, the final compounding consists of only the addition of fillers, activators, and other ingredients needed to achieve the required physical properties and processing characteristics. Although development of a formulation for a specific product and process requires a great deal of knowledge and experience, there are some basic rules typical of FKM compounding. The levels of acid acceptor (MgO) and activator [Ca(OH)2] in the bisphenol cure system strongly affect not only the cross-link network as reflected by the physical properties of the material, but also the behavior of the compound during vulcanization. Therefore the curing system must be optimized to achieve the best balance of properties. Examples of formulations for different curing systems are in Table 16.8.

16.2.4.4 Plasticizers and Processing Aids Processing behavior of fluoroelastomers can be improved by the addition of small amounts of plasticizers and processing aids. High-molecular-weight hydrocarbon esters, such as dioctyl phtalate and pentaerythritol stearate, are effective plasticizers in fluoroelastomer compounds. Lower-molecularweight esters also soften such compounds, but they reduce their high-temperature stability because they are

Table 16.7 Curing Systems for Fluorocarbon Elastomers. Characteristics

Diamine a

Bisphenol

Peroxide

G
E

G
E

Scorch safety

P
F

Balance of fast cure and scorch safety

P

E

E

Mold release

G

E

F

Ability to single-pass Banbury mix

No or risky

Yes

Yes

Adhesion to metal

E

G

G

Tensile strength

G
E

F
E

G
E

Compression set resistance

F

E

G

Steam and acid resistance

F

G

E

a E 5 excellent; G 5 good; F 5 fair; P 5 poor. Data from Moran AL, Pattison DB. Rubber World 1971;103:37 and Schmiegel WW, Kautsch Gummi Kunstst 1978;31:137 [40].

16: FLUOROELASTOMERS

287

Table 16.8 Examples of Formulations for Different Curing Systems. Curing System (phr) Ingredient

Diamine

Bisphenol

Peroxide

FKM

100

100




FKM with CSM







100

MgO (low activity)

15







MgO (high activity)




3
6




Inactive fillers

10
50

10
50

10
50

Diamine

1
3










1
3







3
6




Ca(OH)2




3
6




PbO and Pb-phosphite







3
6

TAC or TAIC







1
3







2
6

Bisphenol Triphenylbenzyl phosphonium chloride

a

b

Peroxide a

Accelerator. 2,5-dimethyl-2,5-di(t-butylperoxy)hexane.

b

less stable than fluorocarbons and highly volatile at the usual service temperatures. Carnauba wax, lowmolecular-weight polyethylene (e.g., AC-617), and sulfones act as good processing aids. These additives ensure improved calendering, smoother extrusion, and an improved flow in molds. Low-molecular-weight polyethylene should not be used in compounds with peroxide-curing systems because it aggravates mold sticking. Other commercially available processing aids are low-viscosity fluorocarbon elastomers that improve processing without having an adverse effect on physical properties of the vulcanizate [23, p. 419].

—Cont’d Amounts (parts by weight)

16.2.4.5 Examples of Formulations Compounds for extruded goods Amounts (parts by weight) Ingredient FKM (terpolymer, medium fluorine content)

Compound I 100

Compound II

Compound II

N990 (medium thermal) carbon black

35

15

N326 (high abrasion furnace) carbon black




5

N762 (semireinforcing furnace) carbon black




7

Magnesium oxide

3

9

Calcium hydroxide

6




Carnauba wax

1

1

Bisphenol AF

1.9

1.9

TPBPC (accelerator)

0.45

0.45

Total

147.35

139.35

a

100

(Continued )

Compound I

Ingredient

a

Triphenyl benzyl phosphonium chloride.

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INTRODUCTION

Physical properties—Press-cured 45 minutes at 320°F (160°C) Tensile strength, psi (MPa)

1102 (7.6)

1798 (12.4)

Elongation at break, %

280

330

Hardness, Durometer A

65

73

Elongation at break, %

305

195

Hardness, Durometer A

65

73

Aged in an oven 16 hours at 400°F (204°C)

Amounts (parts by weight) Compound I

Compound II

FKM (terpolymer, medium fluorine)

100

100

N990 (medium thermal) carbon black

15

15

Magnesia

20




Calcium oxide




Tensile strength, psi (MPa)

1500 (3)

1460 (1)

Elongation at break, %

160

170

Hardness, Durometer A

79

80

Aged in an oven 2 days at 600°F (316°C) Tensile strength, psi (MPa)

Brittle

1160 (8.0)

Elongation at break, %







Hardness, Durometer A

98

87

Compression set (ASTM D395, Method B), 22 hours at 450°F (232°C) Set, %

3




HMDA-C




1.2

Total

138.0

136.2

DIAK #3 a

47

Hexamethylenediamine carbamate (curing agent).

Amount (parts by weight)

Ingredient

Scorch Mooney Scorch, MS at 250°F, min

25 1

44

Minimum reading, units

44

50

Physical properties—Press-cured 30 minutes at 300°F (149°C), postcured in an oven 24 hours at 400°F (204°C) Original physical properties

FKM (high fluorine elastomer, containing PMVE)

100

Litharge

3

N990 (medium thermal) carbon black

30

Hard wax

1

Fatty acid amide

0.50

Stearic acid

0.25

TAIC (coagent) 2640 (18.2)

38

Compound for peroxide cured seals

a

Tensile strength, psi (MPa)

FLUOROPOLYMERS

—Cont’d

Compounds for compression-molded seals

Ingredient

TO

50% DBPH (peroxide)

1750 (12.0) (Continued )

Total a

3.00 a

3.00 140.75

2,5-dimethyl-2,5 di(t-butylperoxy)hexane.

16: FLUOROELASTOMERS

289

Physical properties (fully cured) Hardness, Durometer A

—Cont’d Amounts (parts by weight)

75
80

Compound for closed cell sponge Amount (parts by weight)

Ingredient FKM (medium fluorine content)

100

Magnesia (low activity)

15

N990 (medium thermal) carbon black

25

Petrolatum

3

DIAK #1 (curing agent)

a

a

a

5

Diethylene glycol

2

Total

151.25

General purpose

Sorbitan monostearate

1.0

1.0

VAROX 802-40 KEa

4.0

3.0

TAIC (coagent)

5.0

4.0

Total

141.0

144.0

0

α,α -bis(t-butylperoxy) (40% active).

diisopropylbenzene on

Compression set—measured (3.53 mm) cross-section O-rings

1.25

Cellogen AZ (blowing agent)

Low set

Ingredient

After 70 hours at 392°F (200°C), %

0.139

28

in

45

Steam resistant formulationsa

Hexamethylenediamine carbonate.

Properties—cured in beveled compression mold 30 minutes at 325°F (163°C) Density: 22 lb/cu ft. (352 kg/m3) Compression set (ASTM D395, Method B), 50% deflection, 22 h at 158°F (70°C) Set value: 48%. Example of compounds based on TFE/P elastomer Amounts (parts by weight) Ingredient

Low set

General purpose

TFE/P elastomer

100

100

N990 (medium thermal) carbon black

15

35

Austin black

15




Carnauba wax

1.0

1.0 (Continued )

A (phr)

B (phr)

FKM (New Technology, branched polymer)b

100

100

Varox DBPH-50 (peroxide)

2.5

2.5

TAIC DLC-A (coagent)

3

3

Litharge

0

5

N990 MT (medium thermal) carbon black

40

40

Struktol WS 280 paste (processing aid)

0.5

0.5

Total

146.0

151.0

Ingredient

a

Technoflon A Guide to Fluoroelastomers, Solvay Solexis, Inc. 2005. b For example, Tecnoflon P459. Properties—measured on specimens cured 10 minutes at 177°C (350°F), postcured 4 hours at 230°C (496°F). Hardness, Durometer A Tensile strength, MPa

83 85 19.3 19.4 (Continued )

TO

FLUOROPOLYMERS

Elongation at break, % 100% Modulus, psi (MPa) Compression set (measured on Orings), 70 hours at 200°C, %

207 914 (6.2) 24

290

INTRODUCTION

—Cont’d

—Cont’d

Elongation at break, % Modulus at 100% elongation, MPa

223 7.3

225 8.0

Fluid resistance (ASTM D471), water, 70 hours at 200°C (392°F); O-rings Change in Durometer A, points Change in tensile strength, % Change in elongation at break, % Change in modulus at 100% elongation, % Volume change, %

2 12 2 42 7 2 37

2 12 2 45 37 2 44

15

7

Fluid resistance Immersion 70 hours at 200°C (392°F) in reference fuel C 1 15% methanol

Fluid resistance (ASTM D471), steam, 22 hours at 200°C (392°F); O-rings Change in Durometer A, points

29

29

Change in tensile strength, %

2 54

2 53

Change in elongation at break, % Change in modulus at 100% elongation, %

38

71

2 32

2 38

Volume change, %

4

Automotive fuel seals (bisphenol cure) Ingredient

a

FKM (terpolymer, medium fluorine contents)

100

Magnesium oxide, high activity

3

Calcium hydroxide

6

N990 (medium thermal carbon black)

30

BTPPCa (accelerator)

0.5

Bisphenol AF

1.5

Total

141

Tensile change, %

2 23

Elongation at break, change, %

15

Volume change, %

16

Ingredient

Parts by weight

Viton A HV

100.00

Magnesium oxide

15.00

N990 (medium thermal) carbon black

60.00

Copper inhibitor 65a

2.00

DIAK #1 (curing agent)

1.25

Total

178.25

a

Containing disalicylal propylenediamine; this ingredient has a retarding effect at processing temperatures but it activates curing.

Properties—Press-cured 30 minutes at 140°C (284°F), postcured 24 h at 200°C (392°F) in an oven steps from 100°C (212°F)

Benzyltriphenylphosphonium chloride.

Properties—Cured 10 minutes at 177°C (350°F), postcured 8 and 16 hours at 250°C (482°F) Hardness (Shore A), points Tensile strength, psi (MPa)

26

Rotary seal for aircraft

2

Amount (parts by weight)

Hardness change, points

80 2105 (14.5) (Continued )

Tensile strength, psi (MPa)

3000 (20.7)

Elongation at break, %

120

Hardness, Shore A

85
90

From Chandrasekaran VC. Essential Rubber Formulary for practitioners. Norwich, NY: William Andrew Publishing; 2007 [41].

Note: All the formulations in this section and elsewhere in this chapter are based on various literature sources and on the author’s industrial experience and their purpose is merely to show basic principles of formulating fluoroelastomers for practical applications. Since the properties and

16: FLUOROELASTOMERS

performance of the compounds and finished products depend on specific raw materials, mixing equipment, and procedures, neither the author nor the publisher of this book can guarantee the results or any consequences when using these formulations. They are essentially only starting formulations, which can be modified by the user to meet the exact properties and behavior that are required.

16.2.5 Mixing and Processing of Compounds From Fluorocarbon Elastomers 16.2.5.1 Mixing Compounds from solid fluorocarbon elastomers are mixed on the equipment common in the rubber industry. However, the mixing procedures typical for standard types of elastomers are often modified to be suitable for mixing fluorocarbon elastomers. Open-mill mixing is used mainly for special compounds prepared in small volumes. The advantages of mill mixing are its simplicity, the fact that the operator can control the temperature of the material on the rolls, and an easy cleanup. However, mill mixing, especially on production scale, is rather difficult, especially for a number of gum fluoroelastomers. Polymers with narrow molecular weight distribution and low levels of ionic end group levels may not have adequate cohesive strength to form a smooth band without holes on the mill rolls. Very high-molecular weight fluoroelastomers undergo significant breakdown during initial passes through a tight nip of a cold mill, which leads to reduction of physical properties of the resulting vulcanizate. On the other hand, bimodal blends (formed by latex mixing before isolation) have excellent milling characteristics with negligible breakdown of high-molecular weight fraction [12, p. 104]. Highviscosity elastomers with considerable long-chain branching and gel content may also breakdown during milling, possibly improving subsequent processing characteristics [12, p. 105]. Mixing in internal mixers is considerably more productive; however, because of the high intensity of mixing in an enclosed chamber, there is relatively high risk of premature onset of cross-linking

291

(“scorch”). Compounds tending to scorch are most commonly mixed in two steps (“passes”). In the first pass, the elastomer is mixed with processing aids, fillers, pigments, activators, and acid acceptors. The cross-linking agents are almost always added in the second pass.

16.2.5.2 Processing Mixed compounds are almost always transformed into products with required shapes and dimensions. There are several methods to accomplish this. Tubes, solid round profiles and profiles with irregular, often complex shapes, are prepared by extrusion. Sheets, slabs, and rubber-coated fabrics are made mainly by calendering. 16.2.5.2.1 Calendering

Calendering, as mentioned earlier, is used to produce sheets, slabs, and certain types of coated fabrics. The grades most suitable for calendering are those with low viscosity. Processing aids are necessary to improve surface smoothness and a good release of sheets from the rolls. Mixed stocks should be used promptly or stored at temperatures below 18°C (65°F) to prevent scorching, and great care should be taken to exclude moisture. Typical roll temperatures for calendering recommended for VITON E-60C or related DYNEON types are [23, p. 421] Top roll: Middle roll: Bottom roll: Speed:

85°C 6 3.5°C (185°C 6 5°F) 74°C 6 3.5°C (165°F 6 5°F) Cool (ambient temperature) 7
10 m/min or 7.6
11 yd/min

The setting of roll temperatures depends on the cure systems used. Typically, stocks with diamine (e.g., Diak No. 3) are calendered at temperatures of top and middle rolls set 15°C
20°C (27°F
36°F) lower than stocks with bisphenol and peroxide curing systems [12, p. 118]. 16.2.5.2.2 Extrusion

Extrusion of tubes, hose, and profiles is done on standard extruders for rubber. The usual temperature pattern is a gradual increase of temperature from the feed zone to the die. The die temperature is typically 100°C (212°F) and the screw temperature is approximately the same as the temperature of the feed zone [23, p. 419]. Processing aids are almost always

292

required to improve the surface appearance and to increase the extrusion rate. Extrusion represents only a small proportion (about 10%) of the total consumption of fluorocarbon elastomers [10, p. 86]. Cold feed extruders typically used for extrusion of rubber stocks are also suitable for the extrusion of fluorocarbon elastomers. Extrusion should be carried out at temperatures below 120°C (250°F) to avoid scorch. In most cases, a breaker plate and screen pack are used to generate backpressure on the screw and to remove foreign particles from the stock. A straight head is used for the extrusion of profiles or tubing, whereas a cross-head die is used for coating wires or extrusion of veneer on a mandrel as the inner layer of fuel hose. Ram extruders, such as Barwell Precision Preformer [42], are used widely used for production of blanks for compression molding. Typical ram pressures are up to 35 MPa (5070 psi). Various dies, such as dies for rods, tubing, and strips, are available for extrudate diameters up to 190 mm (7.5 in.). Barwell Preformers are useful for processing high-cost specialty fluoroelastomers used in limited volumes of precision-molded parts [12, p. 110]. 16.2.5.2.3 Compression Molding

Compression molding has several advantages for fabrication of fluoroelastomer parts. Loss of expensive material may be minimized by careful control of preform size. The process is advantageous for relatively small production volumes of parts of any size. Compression molding works best with stocks of medium to high viscosity. The disadvantage of compression molding is a high labor cost since the process requires operator attention to loading preforms, closing and opening molds, and removing cured parts. 16.2.5.2.4 Transfer Molding

When compared to compression molding, transfer molding provides better product consistency, shorter cycle times, and better bonding of rubber to metal [43]. However, considerable amount of material is lost as scrap in the transfer pad, sprues, and flash. The basic three-plate multiple cavity mold is more complex and expensive than a comparable compression mold, but is suited better for intricate parts or securing inserts [44].

INTRODUCTION

TO

FLUOROPOLYMERS

equipment is reciprocating screw machine. The compound is usually fed to the screw as a continuous strip or as pellets from a hopper. As the stock accumulates at the front of the screw, the screw is forced backward a specified distance in preparation for the shot. Then, the rotation of the screw is stopped and it is pushed forward to inject the specified amount into closed mold. While the rubber cures, the screw is initially held in the injection position to maintain the predetermined pressure to consolidate the stock. Then after a preset time, the screw rotates again to refill the barrel. Then, the mold is opened for the removal of the cured part and subsequently closed for the next shot. Ram or piston injectionmolding machines are also used for rubber processing. These are somewhat similar to the transfer molding process. Of the all molding processes, injection molding provides the maximum product consistency, shortest cycle times, and minimum flash. The main disadvantage is the highest investment cost of the machine, molds, and auxiliary equipment. The process is most suitable for highvolume production. Fluoroelastomers with low viscosity (less than 30 as measured on the Mooney viscometer at 121°C) are required for injection molding. The use of one or more process aids is essential to enhance mold flow during injection and easy release of parts from the mold after curing. Ram and piston injection-molding units are commonly used in the rubber industry [45]. The rubber stock is fed to a heated cylinder, which is heated to the required temperature and from there; it is forced by a hydraulic ram through a nozzle, mold runners, and restrictive gates to the mold. Ram injectionmolding machines are somewhat lower in cost than the reciprocating screw unit, but are less efficient, especially for high-viscosity stocks. Typical process conditions for injection molding of fluoroelastomers are in Table 16.9. Most molds for injection molding are unique in design, which depends on application, fluoroelastomer compound, and feed system (hot or cold runners). Standard systems are distinguished as two-plate, threeplate, or stack molds [46].

16.2.5.2.5 Injection Molding

16.2.6 Solution and Latex Coating

Injection molding is the most advanced method of molding rubber products. [43]. The widely used

Certain substrates (woven and nonwoven fabrics, foils, and films) are coated by dipping, spreading,

16: FLUOROELASTOMERS

293

Table 16.9 Process Conditions for Injection Molding of Fluoroelastomers. Machine

Ram Type

Screw Type

Feed zone

80
90

25
40

Middle zone

80
90

70
80

Front zone

80
90

80
100

165
170

165
170

205
220

205
220

165
170

165
170

Injection

14
115

14
115

Hold pressure




1

Back pressure




0.3
1

Clamping pressure

Maximum

Maximum

Screw speed (rpm)




40
60

Total cycle

58
75

43
60

Clamp

48
65

33
50

Injection

3
5

3
5

Hold




10
15

Cure (includes hold)

45
60

30
45

Open-ejection of parts

10

10

Temperature settings (°C) Barrel

Nozzle Nozzle extrudate Mold Stock in mold Pressure settings (MPa) /2 injection pressure

Time setting (s) (for thin parts)

Data from Processing guide Viton fluoroelastomer. Technical information bulletin VTE-H90171-00-A0703, DuPont Dow Elastomers; 2003 [45].

or spraying with fluoroelastomers in liquid form. The older method using a solution of fluoroelastomers in volatile solvents (methyl ethyl ketone, toluene, etc.) is gradually being replaced by the use of water-based latexes. Fluoroelastomer latexes can also be used for chemically resistant and heatresistant coatings. Some fluoropolymer producers offer latex in limited quantities to processors skilled in latex applications. Such products are typically based on VDF/HFP/TFE terpolymers with 68% fluorine content [46]. These terpolymers are polymerized into relatively stable dispersions (latexes) containing 30%
40% solids. The dispersions are then

stabilized by pH adjustment and addition of anionic or nonionic hydrocarbon surfactants. A watersoluble gum (e.g., sodium alginate) is then added to increase particle size, allowing creaming (actually settling) to concentrating latex (about 70% solids) [12, p. 119]. The supernatant serum is discarded. Small amounts of biocides are usually added to prevent growth of microorganisms. In most cases, the latexes are formulated by the addition of curing agents (e.g., diamine or polyamine) combined with limited amounts of metal oxide and inert filler. The coatings are always cured at such temperatures, which do not adversely affect the substrates used.

294

INTRODUCTION

Below is an example of an FKM latex formulation and properties (cured and postcured) from it:

Component Latex (100 parts by weight of rubber)

145

Zinc oxidea

10

b

a

Amount, parts by weight

TETA (polyamine curative)

2.5

Nyad 400 (filler)a

20

Sodium lauryl sulfate (surfactant)

1

Cr2O3 (green pigment)a

5

Total

183.5

Aqueous dispersion. Triethylenetetraamine.

b

Properties—cured 1 hour at 90°C, postcure 1 hour at 50°C Modulus at 100%, MPa Tensile strength, MPa Elongation at break, %

5.3 6.1 180

From Moore AL. Fluoroelastomer handbook. Norwich, NY: William Andrew Publishing; 2006. p. 77 [47].

16.2.7 Curing Products made from fluorocarbon elastomers are cured (vulcanized) typically at temperatures from

TO

FLUOROPOLYMERS

170°C to 220°C (338°F
428°F). However, to achieve optimum properties, postcuring in a circulating air oven is required to complete the crosslinking reaction and to remove volatile byproducts, including water. Standard postcure conditions are 18
24 hours at 220°C
250°C (428°F
482°F) [9, p. 87]. Fig. 16.7 illustrates the effects of postcure at different temperatures on tensile strength and compression set of a carbon black-filled fluorocarbon elastomer compound [9, p. 85]. Postcuring is optional for peroxide-cured fluoroelastomer compounds; however, physical properties and, in particular, compression set are improved somewhat by postcuring for 2
4 hours at 200°C. The largest volume of fluoroelastomers (about 60% of total) is processed by compression molding. A blank (preform) is placed into a preheated mold, compressed, and cured at the appropriate temperature (see above) for a time established empirically. A good estimate for the curing time in the mold is the value of t90 from the measurements by an oscillating disk or moving die rheometer. In the mold design, it is necessary to take into consideration that fluoroelastomers shrink considerably more during cure than standard elastomers (3.0%
3.5% vs 1.5%
2.0%) [9, p. 85]. The use of vacuum devices improves quality and reduces scrap. Injection molding is another method to produce parts from fluoroelastomers. It is particularly suitable for small parts such as O-rings, seals, and gaskets produced in large volumes. The nozzle temperature is usually set at 70°C
100°C (158°F
212°F) and the mold

Figure 16.7 Effect of postcuring time and temperature on strength and compression set. Postcuring temperatures: ’ 200°C, ▲ 225°C, K 250°C (by permission).

16: FLUOROELASTOMERS

295

Table 16.10 Elastomer Comparison, ASTM D2000-SAEJ200 Classification. Type

Heat Aging Temperature, °C (70 h)a

Volume Swell, % 10 h/130°C in ASTM No. 3 Oil

Nitrileb

100

10, 40, or 60

Polyacrylicb

130

0 or 60

Silicone

200 or 225

120 or 80

Fluorosilicone

200

10

Fluorocarbon (FKM)

250

10

Tensile change 1 30%, elongation change
50%, hardness change 115 points. Varying acrylonitrile content or acrylate content.

a b

Table 16.11 Service Life Versus Temperature. Limit, Hours of Service

Temperature, °C (°F)

. 3000

230 (356)

1000

260 (410)

240

290 (464)

48

315 (509)

temperature at 180°C
220°C (356°F
428°F). The best results are achieved by applying vacuum during the injection step to avoid air trapping, splitting, and porosity.

16.2.8 Physical and Mechanical Properties of Cured Fluorocarbon Elastomers

deterioration of their mechanical properties. With increasing temperature, the time of service is reduced as illustrated in Table 16.11 [23, p. 427]. An example of heat resistance of two compounds is shown in Table 16.12 [23, p. 428].

16.2.8.2 Compression Set Resistance

As discussed previously, fluorocarbon elastomers are chemically very stable. They exhibit a unique combination of properties, such as resistance to heat, aggressive chemicals, solvents, ozone, and light in which they excel over other elastomeric materials. Moreover, they have a very good high-temperature compression set and flexibility at low temperatures. A comparison of heat aging and oil resistance of typical FKM and several other elastomeric materials is in Table 16.10 [23, p. 423].

The largest volume of fluorocarbon elastomers is used for O-rings and seals. In these applications, compression set is the most important property affecting the performance of the seal. The lowest values of compression set are achieved when using phosphonium chloride accelerator system with bisphenol AF or other phenol cures with certain grades of FKM (e.g., VITON E-60C or DYNEON 2170). Peroxide cures give generally poorer compression set than bisphenol cures. Coagents for the peroxide curing system have an effect on compression set: TAIC gives, for instance, a lower compression set than TAC [48].

16.2.8.1 Heat Resistance

16.2.8.3 Low-Temperature Flexibility

Vulcanizates from fluorocarbon elastomers can be exposed continuously to temperatures up to 200°C (396°F) almost indefinitely without appreciable

Most commercial fluorocarbon elastomers have brittle points between
25°C (
13°F) and
40°C (
40°F). The low-temperature flexibility depends on

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Table 16.12 Heat Resistance of FKM Fluoroelastomers. A-12

B-12

VITON A

100




VITON B




100

Magnesia

15

15

MT carbon black

20

20

Diak No. 3

2

3

Pressure (min/°C)

30/163

30/163

Oven postcure (h/°C)

24/204

24/204

Original

100 days at 232° C (450°F)

20 days at 260° C (500°F)

2 days at 316°C (600°F)

1 day at 343°C (650°F) A-12

A-12

B-12

A-12

B-12

A-12

B-12

A-12

B-12

15.0

15.5

6.90

4.31

8.62

3.79

7.24

3.45

2175

2250

1000

625

1250

550

1050

500

Elongation at break (%)

470

410

160

480

100

400

60

240

Hardness (Duro A)

68

74

87

75

94

83

91

83

99

91

Weight loss (%)



















18

11

36

22

Tensile strength (MPa) psi

B-12 3.97

Brittle

575 15

Table 16.13 Low-Temperature Properties. VITON

A-401C

B-50

B-70

GLT-200S

DYNEON

FC2144

FT2350







TECNOFLON

FOR532

T636




PL455

DAI-EL

G7451

G-551

G-671

LT-304

Fluorine (%)

66

69

66

65

Brittle point (°C)

225 to 230

235 to
40

235 to
40

251

(213 to 222)

(230 to 240)

(230 to 240)

(259)

°C

216

213

219

231

°F

(12)

(19)

(23)

224

218

214

220

230

(°F)

a

Clash-Berg at 69 MPa

TR 10 (°C)

a

a

These values are often difficult to reproduce.

the chemical structure of the polymer and cannot be improved markedly by compounding. The use of plasticizers may help somewhat, but at a cost of reduced heat stability and worsened aging. Peroxidecurable polymers may be blended with fluorosilicones but such blends exhibit considerably lower high-temperature stability and solvent resistance and are considerably more expensive than the pure fluorocarbon polymer.

VITON GLT is a product with a low brittle point of
51°C (
59°F). [12, p. 104] TECHNOFLON FOR containing a stable fluorinated amide plasticizer reportedly exhibits improved low-temperature hardness, brittle point, and compression set without sacrificing physical properties [49]. The most widely test for low-temperature performance is the TR 10 Test (Retraction at low temperatures) of vulcanized rubber (ASTM D1329 and ISO 2921). The sample is

16: FLUOROELASTOMERS

297

Table 16.14 Swelling of Different Elastomers in Fuel Blends. Fuel Gasoline (42% aromatic)

Fuel Composition (vol.%) 100

85

75

85

75

Methanol




15

25







Ethanol










15

25

Rubber

Equilibrium Volume Increase, % at 54°C

VITON AHV

16.2

28.6

34.9

19.7

21.1

VITON B

16.2

22.9

25.6

17.3

18.3

VITON GF

7.5

13.6

14.6

12.6

13.1

Fluorosilicone (FMQ)

16.4

25.3

26.6

23.1

23.0

Nitrile rubber (NBR)

40.8

90.5

95.8

62.4

66.6

Epichlorohydrin (ECO)

42.4

92.6

98.1

75.6

78.5

frozen in the stretched state and then gradually heated until it loses 10% of the stretch. The temperature determined by the test correlates with the brittle point of the material. TR 10 test is considered the most useful indicator of the low-temperature performance of the tested material. Low-temperature characteristics of selected fluorocarbon elastomers are listed in Table 16.13 [23, p. 430].

16.2.8.4 Resistance to Automotive Fuels The use of aromatic compounds in automotive fuels, higher under-the-hood temperatures, combined with automotive regulations presents a challenge for the rubber parts, such as hose, seals, and diaphragms, used in vehicles. Traditional elastomers do not have high enough resistance to meet all these requirements, but fluorocarbon elastomers do. They are being used successfully, for example, in automotive hoses for gasoline/alcohol mixtures and “sour” gasoline (containing peroxides), where epichlorohydrin copolymer depolymerizes and NBR materials embrittle [23, p. 430]. Moreover, studies of permeation have shown that FKM hose has superior resistance to permeation in comparison with other fuelresistant elastomeric materials with permeation rates often over 100 times lower [23, p. 432]. Swelling of selected fuel-resistant elastomeric materials is shown in Table 16.14 [23, p. 431].

solvents, and mineral acids. Vulcanizates from them swell excessively in ketones and in some esters and ethers. They also are attacked by amines, alkali, and some acids, such as hot anhydrous hydrofluoric acid and chlorosulfonic acid [23, p. 430]. Generally, stability and solvent resistance increase with increasing fluorine contents, as shown earlier in Table 16.3. Other than the type of fluorocarbon elastomer used, the main determinant of resistance to acids is the metal oxide used in the compound. Compounds of FKM containing litharge swell markedly less than those containing magnesium oxide or zinc oxide [11]. Compounds based on KALREZ and AFLAS are considerably more resistant to strong alkali and amines than are compounds based on FKM [23, p. 430]. FKM terpolymers cured with peroxides exhibit exceptional resistance to wet acidic exhaust gases in desulfurization systems in coal-fired plants [23, p. 427].

16.2.8.6 Steam Resistance Resistance to steam of FKM-based vulcanizates increases with fluorine content. Peroxide cures are superior to diphenol and diamine cures. Compounds based on AFLAS and particularly on KALREZ surpass FKM in this respect [23, p. 431].

16.2.8.5 Resistance to Solvents and Chemicals

16.2.9 Applications of Fluorocarbon Elastomers

As pointed out earlier, fluorocarbon elastomers are highly resistant to hydrocarbons, chlorinated

As pointed out earlier, current total worldwide market for fluoroelastomers is estimated at about

298

9,080,000 metric tons [5] of which about 60% is automotive, 10% chemical and petrochemical, 10% aerospace, and 20% other markets. Annual growth is estimated at approximately 5%, mainly for new applications or replacement of parts made previously from inferior elastomers [9, p. 88]. O-rings and gaskets consume about 30%
40%, shaft seals and oil seals about 30%, and hoses and profiles 10%
15% [50
54]. Because of their high price, fluorocarbon elastomers are used in special applications with very high demands on high-temperature resistance and resistance to corrosive chemicals and hot oils. There are most widely used in molded and extruded products, mainly gaskets, used in the aircraft, aerospace and automotive

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industries, hoses, membranes, rubber covered rolls, fabrics and in flame-resistant coatings on flammable substrates. Because of their dielectric properties, they are used in electrical insulations for low voltages and frequencies, when resistance to heat and aggressive chemicals is required. Other applications include food industry, binders for solid rocket fuels, and expanded (foamed) rubber. In the latex form, FKM can be used for coated fabrics and as a binder for fibrous materials. Typical current applications are listed in the following sections.

16.2.9.1 Typical Automotive Applications

• • • • • •

Valve stem and valve seals (Fig. 16.8); Shaft seals; Transmission seals; Engine head gaskets; Water pump gaskets; Seals for exhaust gas and pollution control equipment;

• Bellows for turbo-charger lubricating circuits; • Fuel-handling systems including diaphragms

Figure 16.8 Valve stems and valve seals. Courtesy Daikin.

Figure 16.9 Fuel pipe with FKM lining. Courtesy Daikin.

for fuel pumps (see Figs. 16.9 and 10), fuel hose or fuel hose liner, inject or nozzle seals, needle valves, filter casing gaskets, fuel shutoff valves, carburetor parts;

• Speedometer cable seals.

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299

• • • • •

Figure 16.10 FKM diaphragms. r 1998–2020 DiaCom Corporation – All Rights Reserved.

Gaskets for firewalls; Traps for hot engine lubricants; Heat-sealable tubing for wire insulation; Tire valve stem seals; Flares.

16.2.9.3 Chemical and Petrochemical Applications • O-rings (see Fig. 16.11); • Expansion joints; • Diaphragms; • Blow-out preventers; • Valve seats; • Gaskets; • Hose; • Safety clothing and gloves; • Stack and duct coatings; • Tank linings; • Drill bit seals; • V-ring packers. 16.2.9.4 Other Industrial Applications • Valve seals; • Hose (rubber-lined or rubber-covered); • Wire and cable covers (in steel mills and nuclear power plants);

Figure 16.11 O-rings for different applications.

16.2.9.2 Typical Aerospace and Military Applications • Shaft seals; • O-ring seals in jet engines (Fig. 16.11); • Hydraulic hose; • O-ring seals in fuel, lubricant, and hydraulic systems;

• • • •

Fuel tanks and fuel tank bladders; Manifold gaskets; Lubricating systems; Electrical connectors;

• • • •

Diaphragms (see Fig. 16.10); Valve and pump linings; Reed valves; Rubber-covered rolls (100% fluorocarbon elastomer or laminated to other elastomers);

• Electrical connectors; • Pump lining and seals; • Seals in food-handling processes approved by Food and Drug Administration (Fig. 16.12).

16.2.10 Applications of Perfluoroelastomers Perfluoroelastomers (FFKM), such as KALREZ, are particularly suited for extreme service conditions. They are resistant to more than 1500 chemical

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Figure 16.13 O-rings from FFKM. Courtesy DPA. Figure 16.12 Molded parts from FKM. Courtesy Solvay Solexis.

• Mechanical seal of a pump handling a mixture of ethylene oxide and strong acids at 200°C (390°F) and high pressures;

substances, including ethers, ketones, esters, aromatic and chlorinated solvents, oxidizers, oils, fuels, acids, and alkali and are capable of service at temperatures up to 316°C (600°F) [55]. Because of the retention of resilience, low compression set, and good creep resistance, they perform extremely well as static or dynamic seals under conditions where other materials, such as metals, FKM, PTFE, and other elastomers, fail. Parts from FFKM have very low outgassing characteristics and can be made from formulations, which comply with FDA regulations [56]. Primary areas of application of perfluoroelastomers are paint and coating operations, oil and gas recovery, semiconductor manufacture, pharmaceutical industry, chemical process industry, and aircraft and aerospace industry [55]. Examples of FFKM applications are [56,57]

• Static and dynamic seals in a pump for hot asphalt at 293°C
315°C (560°F
600°F);

• O-ring seals in a pump handling 99% propylene at
45°C (
50°F);

• O-ring seals in a pump pumping chromate inhibited water at 196°C (385°F). Since perfluoroelastomer parts are primarily used in fluid sealing environments, it is essential to pay attention to seal design parameters, especially as they relate to the mechanical properties of the elastomeric material being used. The sealing performance depends on the stability of the material in the fluid, its mechanical properties, mechanical design, and installation of the seal [58]. A variety of O-rings made from FFKM are in Fig. 16.13.

• O-ring agitator shaft seals in an oxidation reactor operating at temperatures above 220°C (428°F) and in contact with 70% acetic acid;

• Mechanical seals of a process pump in a chemical plant pumping alternately acetone, dichloromethane, and methyl isocyanate at elevated temperatures;

• Pipeline seal exposed to chloromethyl ether at elevated temperatures;

• Pipeline seal exposed to dichlorophenyl isocyanate at elevated temperatures;

• Seals for outlet valve exposed to a 50/50 mixture of methylene chloride/ethanol at ambient temperature;

16.2.11 Applications of Fluorocarbon Elastomers in Coatings and Sealants Liquid FKM-based systems were developed to satisfy the need for products that combine physical properties of solid fluorocarbon elastomers, such as excellent chemical and heat resistance in a form that is easy to apply and versatile to use. There are two main types of liquid fluorocarbon elastomer systems, namely, solvent-borne and water-borne. Both types are based mainly on VDF-HFP or VDFHFP-TFE and contain only relatively small amounts of filler to obtain soft flexible coatings.

16: FLUOROELASTOMERS

Solvent-borne liquid systems are made by dissolving compounds of low-viscosity FKM elastomers, such as VITON A-35, or DYNEON 2145, in methyl ethyl ketone, ethyl acetate, methyl isobutyl ketone, amyl acetate, or other related ketones [23, p. 432]. These systems are cured with amines, bisphenol A, or peroxides depending mainly on the end use properties required. The products have typical useful storage life of 7 days at 24°C (75°F) and cure within 2 weeks [59]. Daikin is offering solvent-borne DAI-EL DPA-382 system and several DAI-EL GL and GLS water-borne systems. Onepart and two-part water-borne FKM coatings are commercially available [60
63]. They are used in the following applications [64]:

• Flue gas desulfurization units in coal-fired power plants;

• Wind turbine blades; • Fuel cells; • Mesh covering. One-part and two-part adhesives and sealants based on 3M and DuPont FKMs are used for the following applications [65,66]:

• Sealing of flue duct expansion joints in power plants;

• Door gaskets on industrial ovens; • Joint sealants for steel and concrete secondary containment areas;

• Acid-resistant joint sealants for industrial floorings;

• Bonding FKM gaskets to metal; • Adhesives for splicing and bonding fluoroelastomers and O-rings;

• Adhesives used in jet engine maintenance; • Coating of fuel injector hoses in cars; • Coatings of rubber and metal rollers in the printing industry.

16.2.12 Applications of Fluorocarbon Elastomers as Polymeric Processing Additives There has been a long standing interest in fluorinated polymeric processing additives because of

301

the improvements that adding small concentrations of these compounds impart to processing polymers. Technically, the effect is to increase the critical shear rate of the major phase polymer. Generally, thermoplastic materials must be processed below the velocity at which melt fracture occurs, referred to as the critical shear rate. Melt fracture in molten plastics takes place when the velocity of the resin in flow exceeds the critical velocity, the point where the melt strength of the polymer is surpassed by internal stresses. A meltfractured extrudate is no longer smooth, often with a washboard appearance, and has a milky appearance [67]. Fluoroelastomers are known to be effective polymeric processing aids (PPAs). There is multitude of fluoroelastomeric PPAs. The manufacturers hold the most important information proprietary or the manufacturing details are protected by patents. This section covers this subject to a limited extent. The earliest reports of this application are in [68]. A small amount (0.005
2 wt.%) of a copolymer of VDF and HFP (Viton A) was added to poly (ethylene-butene-1) and high-density polyethylene (HDPE). There was a drastic reduction in the tendency of hydrocarbon polymers to undergo melt fracture as a result of adding Viton A. This effect was measured by comparing the transparency of extrudates with and without Viton A. Adding as little as 500 ppm of Viton A increased the transparency by over one order of magnitude from 5.7% to 69%. In [69], the development of free flowing powders of perfluoroelastomers which could be easily added to other polymer powders as a processing aid is reported. An antiblocking agent such as silica, calcium silicate, or calcium carbonate was added before or during the spray-drying of the polymer powder. The free flow powder containing antitack or antiblocking agent is relatively easy to handle during process because it does not cake or bridge easily. Another development reported [70] that extrusion of thermoplastic polyolefins, especially linear low-density polyethylene (LLDPE), was improved by the use of a processing additive comprising a blend of a thermoplastic acrylic polymer and a fluoroelastomer. The processing additive is a homogeneous blend of a thermoplastic styrene/methyl methacrylate polymer and a copolymer of VDF and HFP containing less than 15 wt.% of HFP. The

302

polyolefin extrudate produced according to this invention has reduced levels of melt fracture when extruded in conventional polymer extrusion equipment. DYNEON [71] has suggested use of multimodal fluorinated polymers as an effective PPA. The term multimodal terpolymer means a terpolymer having two or more discrete molecular weight ranges. The polymer processing aid compositions contain a fluoroelastomers that is a copolymer of an ethylenically unsaturated fluoromonomer, TFE, and at least one ethylenically copolymerizable monomer like ethylene or propylene. For example, useful PPAs are made by copolymerizing TFE, HFP, and ethylene or propylene. Examples of preferred subclasses of these polymers include the following: The useful multimodal fluoropolymers can be prepared in a number of ways. For example, the polymer can be produced by means of a suitable polymerization process (“step polymerization”). This process employs the use of specific initiators and chain transfer agents such as short-chain alkanes and halogen alkanes plus hydrogen. In current commercial applications, amounts of fluoropolymer (elastomer or plastic) dispersed in hydrocarbon thermoplastics can greatly improve their extrusion characteristics, reducing melt fracture and die buildup. These improvements are especially important in film extrusion of HDPE and LLDPE resins. To serve this market, DuPont offers Viton FreeFlow additives, Dyneon offers Dynamar, Solvay Solexis offers Technoflon, and AGC Chemicals offers Fluon polymer processing additives (PPAs). These processing aids form a nonstick fluoropolymer coating on the inside of the die, reducing friction so that the resin flows freely and more rapidly through the die to produce an extrudate with smooth surfaces.

16.3 Fluorosilicone Elastomers In this context, by the term fluorosilicone are meant polymers containing CaF bonds and SiaO bonds with hydrocarbon entities between them. Thus the repeating structure may be generally written as [RfX (CH2)n]x (CH3)ySiOz, where Rf is the fluorocarbon group [72]. Commercially available fluorosilicones are based

INTRODUCTION

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on polymethyltrifluoropropyl siloxane (PMTFPS), or more accurately poly[methyl (3,3,3-trifluoropropyl)siloxane]. In some cases, PMTFPS is copolymerized with polydimethyl siloxane (PDMS) for cost/benefit balance [72, p. 360]. The manufacture of monomers for fluorosilicones is discussed in some detail [72, p. 360]. Fluorosilicone elastomers are referred to in ASTM D1418 and ISO 1629 as FMQ or FVMQ. The ASTM name is fluoro-vinyl polysiloxane. Currently, the three major suppliers are Dow Corning Corporation, Momentive Performance Materials, and Shinetsu Chemical Company.

16.3.1 Polymerization The most common method of preparation of PMTFPS is through the base-catalyzed ring-opening polymerization of the corresponding cyclic trimer [72, p. 361, 362]. A specific cure site for peroxide curing is developed by incorporating 0.2 mol.% of methyl vinyl siloxane [73]. Typically, fluorosilicone elastomers are copolymers of 90 mol.% of trifluoropropylsiloxy and 10 mol.% of dimethylsiloxy monomers, but the fluorosilicone content in commercial products ranges from 40 to 90 mol.% [73]. Fluorosilicone polymers are optically clear and are available in a broad range of viscosities, from very low-viscosity fluids to very high-viscosity gums. The physical properties of the raw polymers—such as viscosity, resistance to nonpolar fuels, oils, and solvents; specific gravity; refractive index; lubricity; solubility in polar solvents; the degree of crystallinity; and glass transition temperature (Tg)—depend on the structure, more specifically on the number of trifluoropropyl groups in the molecule. The mechanical properties of the polymer depend on the molecular weight, dispersity, and mol.% of vinyl groups [74]. The presence of fluorine increases the polarity to the level above the standard methylvinyl silicone rubber (MVQ). Consequently, the fluorosilicone elastomers have a considerably greater resistance to oils and many liquids (with the exception of some ketones and esters with only slightly impaired resistance low temperature when compared to MVQ). Still, fluorosilicone elastomers have better low-temperature resistance than FKM. Moreover, when compared to FKM they have lower hardness, higher resilience, and a considerably better bonding to other polymers and to metals.

16: FLUOROELASTOMERS

16.3.2 Processing Fluorosilicones can be compounded by the addition of mineral fillers and pigments. Fillers for such compounds are most commonly silicas (silicon dioxide), because they are compatible with the elastomeric silicon
oxygen backbone and thermally very stable. They range in surface areas from 0.54 to 400 m2/g and average particle size from 100 to 6 nm. Because of these properties, they offer a great deal of flexibility in reinforcement. Thus cured compounds can have Durometer A hardness from 40 to 80. Other fillers commonly used in fluorosilicones are calcium carbonate, titanium dioxide, and zinc oxide. Mill and mold release is improved by the addition of a small percentage of dimethylsilicone oils or gums. Processing aids are mostly proprietary. Plasticizers are generally fluorosilicone oils of various viscosities. The lower the molecular weight, the more effective is the plasticizing action. On the other hand, the higher the molecular weight, the lower the volatility. This is critical when the service temperature is very high. Fluorosilicone compounds can be processed by the same methods used for silicone elastomers based on PDMS. They can be milled, calendered, extruded, and molded. A large proportion of fluorosilicone compounds are used in compression molding. Molded parts produced in large series are made by injection molding, and parts with complex shapes are produced by transfer molding. Calendering is used to produce thin sheets and for coating of textiles and other substrates. Cross-linking of fluorosilicones is done by essentially the same methods as conventional silicones. A comprehensive review of this subject is in [75]. Currently, there are three methods of cross-linking used in industrial practice:

• By peroxides (free radicals); • Condensation reactions; • Hydrosilylation addition. For peroxide cross-linking, organic peroxides, such as dicumyl, di-t-butyl, and benzoyl peroxides, are used in amounts 1
3 phr. Typical cure cycles are 5
10 minutes at temperatures 115°C
170°C (239°F
338°F), depending on the type of peroxide used; each peroxide has a specific use. A postcure is recommended to complete the cross-linking

303

reaction and to remove the residues from the decomposition of peroxide. This improves the longterm heat aging properties [72, p. 362]. An example of a fluorosilicone compound and cured properties of the finished materials from it are below:

Component

Amount, parts by weight

Fluorosilicone (general purpose)

100.0

Silica filler

30.0

Varox DBPH-50 (curing agent)a

1.0

Total

131.0

a

2,5-dimethyl-2,5-di(t-butylperoxy)hexane (50% active on a mineral carrier). Properties (fully cured) Tensile strength, MPa (psi) 100% Modulus, MPa (psi) Elongation at break, % Hardness, Shore A

8.6 (1250) 2.4 (350) 350 62

Condensation reactions are used for crosslinking at ambient temperatures. The acetoxyfunctional condensation system is widely used in fluorosilicone sealants. The cross-linking occurs after exposure to atmospheric moisture [72, p. 362]. The limitation of this system is that it is effective for only relatively thin layers. Moreover, it often requires up to 14 days to cure and the acetic byproduct may corrode certain substances. Thicker sections can be cross-linked by hydrosilylation addition. This is the same chemistry used to produce fluorosilicone monomers with the vinyl functionality present on silicon. The catalyst reaction occurs between a vinyl group and silicone hydride [74, p. 4]. The advantage of this system is that it does not produce volatile byproducts. On the other hand, the disadvantage is that it is available only as a two-part system [72, p. 363]. However, one-part, platinum-catalyzed products have been developed [72, p. 363]. The reaction is very rapid and at room temperature it is completed in 10
30 minutes. It is accelerated with increasing

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Table 16.15 Fluid and Chemical Resistance of Fluorosilicone Elastomers (ASTM D471). Fluid

Immersion Conditions

Hardness Change (points)

Volume Change (%)

ASTM No. 1 oil

3 days/150°C

25

0

Crude oil 7 API

14 days/135°C

2 10

15

JP-4 fuel

3 days/25°C

25

1 10

ASTM Ref. Fuel B

3 days/65°C

25

1 20

Benzene

7 days/25°C

25

1 25

Carbon tetrachloride

7 days 25°C

25

1 20

Methanol

14 days 25°C

2 10

14

Ethanol

7 days 25°C

0

15

Hydrochloric acid (10%)

7 days 25°C

25

0

Nitric acid (70%)

7 days 25°C

0

15

Sodium hydroxide (50%)

7 days 25°C

25

0

Table 16.16 Properties of Typical Commercial PMTFPS Elastomers. Property

Typical Range

Specific gravity (g/cm )

1.35 2 1.65

Hardness (Shore A)

20 2 80

Tensile strength (MPa)

5.5 2 11.7 (22°C) 2.4 2 4.1 (204°C)

Elongation (%)

100 2 600 (22°C) 90 2 300 (204°C)

Modulus at 100% (MPa)

0.5 2 6.2

Compression set (%) (22 h/177°C)

10 2 40

3

Tear strength, die B (kN/m)

16.5 2 46.6

Service temperature (°C)

2 68 2 1 232

a

10 2 40

b

Bashore resilience (%) a

Die B refers to a particular specimen shape in ASTM D624. Bashore resilience is resilience measured by a falling metal plunger according to ASTM D2632.

b

temperature and at 150°C (302°F) it is completed within a few seconds. This makes the compounds ideal for fast automated injection-molding operations [74, p. 5]. One-part systems use the chemical complexing of the catalyst, which is activated at elevated temperatures, or its encapsulation into an impermeable shell, which is solid at room temperature and melts at elevated temperatures [74, p. 5]. Fluorosilicone polymers can be cross-linked by ultraviolet radiation or by EB, but these methods are not commonly used [74, p. 3].

16.3.3 Properties of Cured Fluorosilicones 16.3.3.1 Fluid and Chemical Resistance In general, fluorosilicones exhibit a very good fuel and fluid resistance. The volume swelling in solvents decreases with increasing fluorine content. Cured fluorosilicone elastomers have good resistance to jet fuels, oils, hydrocarbons, and fuels (see Table 16.15 [74, p. 3]).

16: FLUOROELASTOMERS

305

Figure 16.14 Positioning of high-performance elastomers according to ASTM D2000. Courtesy Momentive Performance Materials.

However, higher swelling in ketones and esters are observed [76]. Relatively low swelling is found in alcohol/fuel blends; once the solvents are removed the physical properties return to nearly the original unswollen state [77]. Mechanical properties of PMTFPS elastomers are listed in Table 16.16. A comparison of combined swelling and heat resistance of FMQ with other commercial elastomers is in Fig. 16.14.

16.3.3.2 Heat Resistance Fluorosilicone elastomers have an excellent heat resistance, although they have slightly lower hightemperature stability compared with PDMS [78]. The ultimate temperature stability depends on cure conditions and environment. A typical cured fluorosilicone elastomer (PMTFPS) aged for 1350 hours at 200°C (392°F) will show a two-point reduction in durometer hardness, a 40% reduction in tensile strength, and a 15% reduction in elongation. There are essentially two mechanisms of degradation: reversion (occurs in confinement) or oxidative cross-linking. The latter occurs by radical abstraction of protons, which recombine to form additional cross-linking sites, and this ultimately leads to embrittlement of the vulcanizate [72, p. 364].

16.3.3.3 Low-Temperature Properties The glass transition temperature of PMTFPS is
75°C (
103°F). Moreover, it does not exhibit

low-temperature crystallization at
40°C (
40°F) as PMDS does. Because of this and low Tg, fluorosilicone elastomers remain very flexible at very low temperatures. For example, the brittleness temperature by impact (ASTM D 746B) of a commercial fluorosilicone vulcanizate was found
59°C (
74°F) [53]. This is considerably lower than the values typically measured on fluorocarbon elastomers. Fluorosilicones combine the superior fluid resistance of fluoropolymers with the very good low-temperature flexibility of silicones.

16.3.3.4 Electrical Properties Electrical properties—dielectric constant (ε), representing polarization; dissipation factor (tan δ), representing relaxation phenomena; dielectric strength (EB), representing breakdown phenomena; and resistivity (ρv), an inverse of conductivity—are compared with other polymers in Table 16.17 [72, p. 366]. The low dielectric loss and high electrical resistivity coupled with low water absorption and retention of these properties in harsh environments are major advantages of fluorosilicone elastomers over other polymeric materials [72, p. 366].

16.3.3.5 Surface Properties Fluorosilicones have low surface energy, in fact lower than poly(tetrafluoroethylene) of PDMS [72, p. 364]. Values of selected polymers measured at room temperature are shown in Table 16.18.

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Table 16.17 Electrical Properties of Selected Polymers. Dielectric Strength Eb (60 Hz) V/mil

Dielectric Constant ε (100 Hz)

Dissipation Factor tan δ

Low-density polyethylene

742

2.2

0.0039

2.5 3 1015

Natural rubber

665

2.4

0.0024

1.1 3 1015

PDMS

552

2.9

0.00025

5.3 3 1014

PMTFPS

350

7.0

0.20

1.0 3 1014

Viton

351

8.6

0.040

4.1 3 1011

Polymer

Resistivity ρv (ohm.cm)

Table 16.18 Surface Energy Values from Selected Polymers. Polymer

Solid Surface Energy, σs mJ/m2

PMTFPS

13.6

PDMS

22.8

PTFE

19.1

16.3.4 Applications of Fluorosilicone Elastomers Commercial fluorosilicone elastomer compounds are made from high-molecular-weight PMTFPS (MW is typically 0.8
2.0 million) and are cross-linked by organic peroxides. Such compounds contain some reinforcing filler (usually high-surface-area fuming silica), a small amount of low-molecular-weight fluorosilicone diol processing fluid, and a peroxide catalyst [72, p. 370]. Other additives, such as extending fillers, pigments, and thermal stability enhancers, are often added to meet final product requirements [79]. Frequently, fluorosilicone elastomers are blended with PDMS silicones either to lower compound cost or to enhance properties of the silicone compound. Fluorosilicone elastomers can also be blended with fluoroelastomers to improve their low-temperature flexibility. Properties of cured fluorosilicone elastomers depend on the base polymer and compounding ingredients used. Fluorosilicone elastomers are particularly suited for service where they come in contact with aircraft fuels, lubricants, hydraulic fluids, and solvents. Compared with other fuel-resistant elastomers, fluorosilicones offer the widest hardness range and the widest operating service temperature range of any material [77]. The automotive and aerospace industries are the largest

users of fluorinated elastomers. Typical automotive applications are fuel injector O-rings, fuel line pulsator seals, and fuel line quick-connect seals, gas cap washers, vapor recovery system seals, electrical connector inserts, exhaust gas recirculating diaphragms, fuel tank access gaskets, and engine cover and oil pan gaskets. In the aerospace industry, fluorosilicone O-rings, gaskets, washers, diaphragms, and seals are used in fuel line connections, fuel control devices, electrical connectors, hydraulic line connectors, and fuel system access panels [72, p. 371]. Other uses are aircraft gaskets, seals, hoses, diaphragms, connectors, and general industrial gaskets and seals. Medium-molecular-weight PMTFPS with vinyl or hydroxyl end blocks is used for adhesives and sealants. They are cured either at ambient temperature (RTV-room temperature vulcanization) or at elevated temperature. One-part moisture-activated RTV sealants have been available commercially for many years. Because of their very high resistance to jet engine fuels, excellent flexibility at very low temperatures, and high thermal stability, they have been used in both military and civilian aerospace applications [80]. Two-part, heat-cured fluorosilicone sealants have been used in military aircraft applications and for sealing automotive fuel systems [69, p. 369]. Special class of fluorosilicone sealants is “channel sealants” or “groove injection

16: FLUOROELASTOMERS

sealants,” sticky, puttylike compounds, which do not cure. They are used to seal fuel tanks of military aircraft and missiles [67, p. 370]. Adhesion of fluorosilicone compounds requires surface treatment. For particularly difficult surfaces, plasma treatment is necessary. However, for most common applications, satisfactory bonding is achieved by using a specialized primer [74, p. 12].

16.4 Fluorinated Thermoplastic Elastomers Considering the exceptional commercial success of hydrocarbon thermoplastic elastomers as a frequent replacement of conventional cross-linked (vulcanized) elastomers, it is logical that a similar concept is viable in the field of fluorinated elastomers. This is a particularly attractive concept, considering the rather involved chemistry of cross-linking fluoroelastomers discussed in previous section. Currently, fluorinated thermoplastic elastomers (FTPEs) are produced only in Japan. One is a blockcopolymer type, composed of a central soft fluoroelastomer block and multiple fluoroplastic hard segments (ETFE or PVDF, depending on grade). This type has been available commercially since 1982 and is produced by Daikin under the trade name DAI-EL Thermoplastic [81]. A schematic of this type of FTPE is in Fig. 16.15. The second type is a graft copolymer type comprising main-chain fluoroelastomers and side-chain fluoroplastics. This type was introduced commercially in 1987 by Central Glass Co. under the trade name Cefral Soft [82].

307

There are essentially two methods used for the production of commercial FTPEs. The first is referred to as iodine transfer polymerization, which is similar to the “living” anionic polymerization used to make block copolymers such as styrenebutadiene-styrene (e.g., Kraton). The difference is that this “living” polymerization is based on a free radical mechanism. The products consist of soft segments based on copolymers of VDF with HFP and optionally with TFE and of hard segments that are formed by fluoroplastics such as ETFE or PVDF [83]. The other method is a two-step graft copolymerization using unsaturated peroxides, such as [CH2QCHCH2OC(O)aOaOatert-butyl] and the monomers involved are VDF and CTFE. In the second step, post-polymerizations mainly with VDF to form crystalline segments are repeatedly performed while successively raising the reaction temperature [83, p. 567]. Another method to prepare thermoplastic fluoroelastomers is the extension of the dioiodo technology [84,85]. In the first stage, iodineterminated TFE/VDF/HFP terpolymers are synthesized by emulsion polymerization, using I (CF2)nI as the dioiodo compound. The polymerization of the hard segment component takes place in the presence of the iodine-terminated terpolymer emulsion. One type contains E/TFE/HFP; the other contains poly-VDF as the hard segment. Typically, the product contains 85% soft segment of composition VDF/HFP/TFE and 15% hard segments of composition TFE/E/HFP. According to the basic patent, the molecular weight of the hard segments has to be at least 10,000 and that of the

Figure 16.15 Schematic of a fluorinated thermoplastic elastomer. Courtesy Daikin.

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FLUOROPOLYMERS

Table 16.19 Properties of DAI-EL T-530. Property

Data

Test Method

Color

Light yellow transparent pellet

Visual observation

Specific gravity (23°C)

1.89

JIS K 6268

Melt flow rate

19.7

JIS K 7210

Melting point

Approx. 230°C




100% Modulus

1.6 MPa

JIS K 6251

Tensile strength

11.8 MPa

JIS K 6251

Elongation at break

580

JIS K 6251

Hardness (Shore A)

61

Peak value

Physical properties (original)

Physical properties after 15 Mrad (150 kGy) irradiation 100% Modulus

1.8 MPa

JIS K 6251

Tensile strength

17.7 MPa

JIS K 6251

Elongation at break

500%

JIS K 6251

Compression set

23%

150°C, 70 h, 25% compression

Data Sheet_DAI-EL T-530, Ver. 10 (May 2009), Daikin Industries Ltd.

Table 16.20 Technical Data from DAI-EL Fluoro-TPV, SV Series. Property

SV-1010

SV-1020

SV-1030

SV-1050

Hardness (Shore A)

94

92

90

90

Tensile strength (MPa)

32

24

17

11

Elongation at break (%)

440

410

370

360

1 MHz

2.72

2.99




3.14

10 MHz

2.60

2.75




2.81

100 MHz

2.48

2.56




2.57

1 GHz

2.35

2.42




2.42

1 MHz

0.018

0.037




0.053

10 MHz

0.021

0.037




0.047

100 MHz

0.022

0.030




0.036

1 GHz

0.021

0.024




0.026

4.5

5.3

7.5

20

1.0

1.4

1.5




Physical/mechanical properties

Electrical properties Dielectric constant at

Loss factor at

Fuel permeation and fuel resistance Permeation rate, CE10 2

60°C, g mm/m day Volume change, CE85 60°C, % From SM090131, Daikin America, Inc.; 2012.

16: FLUOROELASTOMERS

central soft block at least 110,000 [12, p. 119]. These FTPEs do not need curatives, metal oxides, fillers, or process aid and this makes them suitable for medical applications. Because of the presence of VDF or E, they can be cross-linked by ionizing radiation if required compression ratios larger than 10% are used [86]. Typical properties of the commercial product DAI-EL T-530 are summarized in Table 16.19. Polyurethane-based FTPEs are produced by reacting fluorinated polyether diols with aromatic diisocyanates. The resulting block copolymers contain fluorinated polyether soft segments [87]. Another possible method of preparation of fluorinated TPE is dynamic vulcanization. Examples are a blend of a perfluoroplastic and a perfluoroelastomer containing curing sites or a combination of VDF-based fluoroelastomers and thermoplastics, such as polyamides, polybutylene terephtalate, and polyphenylene sulfide [88,89]. Daikin offers another grade of thermoplastic fluoroelastomers, namely, DAI-EL Fluoro-TPV that exhibits a very low permeability, high chemical resistance, as well as transparency and high flexibility. It is suitable for the barrier of automotive fuel hose and for wire and cable application. The polymer consists of a vulcanized fluoroelastomer

309

dispersed in a fluorinated thermoplastic matrix. The reported melting point is 220°C and the maximum service temperature is 150°C. Additional technical data are in Table 16.20.

16.4.1 Applications of Fluorinated Thermoplastic Elastomers 16.4.1.1 Chemical and Semiconductor Industries The most common applications for thermoplastic fluorinated elastomers are seals in chemical and semiconductor industries (O-rings, V-rings, gaskets, and diaphragms) (see Fig. 16.16) because of their excellent chemical resistance and high purity [83, p. 563]. These parts are often cross-linked by ionizing (actinic) radiation without adding any other components [90]. Other parts for these industries are tubing and liners of multilayer hoses for corrosive gases or ultrapure water, and liners for vessels for inorganic acids (e.g., HF) [91] (see Fig. 16.17).

16.4.1.2 Electrical and Wire and Cable Because of their flexibility, low flammability, and resistance to oil, fuel, and chemicals, FTPEs find use in electrical and wire and cable industries as wire coating and as sheathing and coating of cables [72,92].

16.4.1.3 Other Applications Other applications include tents and greenhouses, as laminates with polyester fiber-reinforced PVC, and as tubing, bottles, and packaging in food processing and in sanitary goods [83, p. 575].

16.5 Phosphazenes

Figure 16.16 Example of seals and components made from fluorosilicone rubber. Courtesy Dow Corning.

Phosphazene (or phosphonitrilic chloride) elastomers, like silicone elastomers, have a fully inorganic backbone, consisting of nitrogen and phosphorus. The basic building block is aNQPaand the pendant organic groups are attached to the phosphorus. The technology is over 100 years old, [93] but the actual development work leading to commercial products was done only in the 1970s. Two commercial phosphazene elastomers have been developed and marketed in the mid-1980s, namely, poly(fluoroalkoxyphosphazene) elastomer (ASTM designation FZ) and poly(aryloxyphosphazene) elastomer

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INTRODUCTION

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FLUOROPOLYMERS

high cost and relatively small volume market, they are not available commercially other than on special orders.

16.6 Safety, Hygiene, and Disposal

Figure 16.17 O-rings from thermoplastic fluoroelastomer. Courtesy Daikin.

(ASTM designation PZ) [94]. The structure of the fluorinated product is [95]

There are different issues regarding hygiene and safety during the individual stages of life cycle of fluoroelastomers. Their production requires handling of hazardous raw materials under conditions that must be closely controlled. In processing, the compounding, mixing, and curing operations require additional components with reactions at high temperatures commonly used that may generate hazardous byproducts. Fabricated fluoroelastomer products are often used in severe environments where failures may have dangerous consequences.

OCH2CF3

–[P = N]n– OCH2(CF2)xCF2H (x = 1,3,5,7…)

In general, the synthesis of polyphosphazene polymers is unique that in theory, an infinite number of polymers with a variety of properties can be derived from the common polymeric intermediate, poly(dichlorophosphazene) (PNCl2), by replacing the chlorines with different nucleophiles. If the polydichlorophosphazene precursor is reacted with the sodium salts of trifluoroethanol and a mixed fluorotelomer alcohol, a poly(fluoroalkoxyphosphazene) elastomer (FZ elastomer) is obtained. It contains a small amount of an unsaturated substituent as a curing site. The polymer is a soft gum, which can be compounded with carbon blacks and fillers and cured with sulfur, peroxides, or by radiation. FZ elastomer offers a broad service temperature range, namely, from
65°C to 1175°C, or 285°F to 1347°F, [96] excellent flex fatigue resistance, damping properties, and resistance to chemicals and fluids. Main applications have been in aerospace, military, and petrochemical and gas pipeline areas. Phosphazene elastomers were very successful throughout the 1980s being used mainly in military and aerospace industry. However, because of their

16.6.1 Polymerization and Finishing In this stage, the main concern is the safe handling of monomers, since some of them, notably TFE have a high potential for explosions. These are minimized by eliminating possible ignition sources, such as electric arcs, traces of oxygen, and hot spots in equipment.

16.6.2 Compounding, Mixing, and Processing Fluoroelastomers are first compounded into required formulations with a variety of ingredients; some of them may be reactive and/or toxic. In mixing and subsequent processing into finished products, some components of the formulations may generate toxic byproducts. In these production stages, adequate ventilation, particularly in the vicinity of discharge from the internal mixers, near mills, extruders, and openings of hot presses and other processing machinery, is absolutely necessary. During the postcuring of great numbers of products in hot air ovens, there are fire hazards that can produce toxin and/or corrosive products, such as HF, carbon monoxide, carbonyl fluoride, and traces of fluorocarbon monomers.

16: FLUOROELASTOMERS

16.6.3 Hazardous Conditions During Use During actual use, fluoroelastomers may be subjected to elevated temperatures, ionizing radiation, or are in contact with fluids often at elevated temperatures. Here, a proper ventilation and individual protection of personnel are very important.

16.6.4 Disposal of Used Products The end-of-life disposal of products is mostly done by incineration with energy recovery or by burying them in a landfill [12, p. 308]. Incineration is preferred for most materials, including parts contaminated by absorbed fluids. In such a case, the incinerator has to have an effective scrubbing system removing volatile acidic combustion products [12, p. 308]. Disposal into a landfill is acceptable only for solid fluoroelastomers and parts if they are not contaminated by toxic fluids.

16.7 New Developments and Current Trends Fluoroelastomers are attractive materials because of their unique properties as presented in this chapter. As the different technologies and demands develop, the opportunity for fluoroelastomers increases and in many cases new applications develop. Their technology also has to develop to meet the emerging technical and environmental demands, reduce manufacturing costs, and offer new and improved properties. All these issues are discussed in this section.

16.7.1 New Developments in Chemistry and Processing One of the major developments in the polymerization process for the preparation of fluoropolymers in general, but also applicable to fluoroelastomers, is the use of supercritical carbon dioxide as the solvent for monomers, replacing chlorofluorocarbons that are environmentally unacceptable [97]. Originally used for TFE in which case it offers considerably safer process because the mixture of TFE and CO2 is safer to operate than TFE alone, it has been used in the polymerization for fluoroelastomers [98]. A novel method using supercritical carbon dioxide is used for the synthesis of VDF-based FKMs with improved processability [99] and for

311

liquid fluoroelastomers [100]. New types of UVcurable fluoroelastomers have been developed [99,101,102]. Another recently developed technology applicable to fluorocarbon elastomers is the Advanced Polymer Architecture (APA) that produces peroxide-curable FKMs with improved processing, faster cure rates, and improved hot demolding properties. Low-temperature fluoroelastomers made by APA have excellent heat resistance and gain elongation after aging for 5000 hours at 175°C and 200°C [103
105]. In response to growing demands for improved performance from the automotive, oil, and gas recovery industry and chemical processing industry, the manufacturers of FKMs developed new grades with considerably improved low-temperature performance and grades with a high resistance to basic substances and amine-containing automotive fuels. Other new FKM grades do not require postcure [106]. Precision Polymer Engineering Ltd. offers recently developed low-temperature FFKM grade Perlast ICE G 75 LT. The material is designed to perform under extreme conditions, at temperatures 240°C and beyond, depending on specific circumstances and environmental conditions. Perlast ICE can be used in a wide range of industries where extremes of high and low temperature and aggressive conditions exist, such as oil and gas operations, diagnostics, scientific instruments, and analytical equipment, and in biomedical and pharmaceutical sectors [107], www.prepol.com/ice. Wear-resistant material 75 FKM 260 466 has been developed for use in radial shaft seals that interact with aggressive oils in various gearbox sealing applications. The material provides a high level of wear resistance when in contact with synthetic oils. The expected service life is 20,000 hours and more [107], www.simrit.de.

16.7.2 New Products Novel fluorine thermoplastic vulcanizates have been developed by Freudenber NOK-GP offered as FluoroXprene [108]. They are essentially a dynamically vulcanized blend of fluorocarbon elastomers and fluoroplastics, such as PVDF, ETFE, ECTFE, THV, FEP, and MFA prepare in either batch or continuous process. The continuous process using a twin-screw extruder is preferred. The morphology is typical for a TPV that is dispersed cross-linked FKM in the fluoroplastic matrix.

312

INTRODUCTION

The products are available in the hardness range 50
100 Shore A, tensile strength 2.0
25 MPa, elongation at break 10%
350%, and compression set (70 hours at 150°C in air) 27%
55%. Its fluid resistance is considerably better than that of FKM, mainly because the semicrystalline fluoroplastic matrix protects the elastomeric particles. The fluorinated TPV exhibits superior fuel permeation resistance to that of FKM materials. Fluorine TPVs containing VD, ETFE, ECTFE, and THV can be cross-linked by ionizing radiation if desired. Applications for this series of materials are

• Multilayer extruded fuel hose; • Fuel filter neck; • Injection-molded automatic transmission lead cover seal, paint spray tip seal, fuel filler door seal, and injection-molded dynamic shaft seal.

16.7.3 Other Development Enhancement of thermal aging performance and oil resistance of acrylic rubber vulcanizates by adding devulcanized ground fluoroelastomer ultrafine powder as functional filler [109]. The powder is added by solid-state mechanochemical milling at ambient temperature. The tensile strength of thermally aged (72 hours at 150°C) has improved by B65%, the elongation at break increased by 1.5 times. An environmentally friendly approach for recycling of post-vulcanized fluoroelastomer scraps through high-shear mechanical milling [110]. The resulting product exhibited excellent mechanical and thermal properties, indicating a strong potential for future applications. The tensile strength of the product was 6.6 MPa, retaining about 84% of the virging vulcanizate and the elongation at break increased from 337% to 369%. Properties of blends of semicrystalline perfluoropolymers strongly depend on the size of the dispersed phase and are at the best when dispersed phase dimension is well below 0.1 μm, that is, in the nanoscale region. This fine dispersion is obtained with an innovative mixing technology based on microemulsion polymerization. Further improvement of properties can be obtained by generating chemical links between fluoroelastomer and semicrystalline fluoropolymers. Nanoblends combine the performance properties of fluoroelastomers with those of semicrystalline

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FLUOROPOLYMERS

perfluoropolymers. For example, these nanoblends have at the same time the sealing and mechanical properties of fluoroelastomers and the exceptional thermal and chemical resistance of semicrystalline perfluoropolymers. In addition, as dispersed phase size is below visible light wavelength, finished items made with these nanoblends are optically transparent even when they contain as much as 40 wt.% of semicrystalline perfluoropolymer [111]. Effects of EB irradiation were evaluated on the mechanical, thermal, and surface properties of a commercial fluoroelastomer containing carbon black and inorganic filler. The material was irradiated with overall fosse between 10 and 250 kGy. Tensile strength, hardness (Shore A), and compression set were evaluated. Thermal behavior was evaluated by thermogravimetric analysis and differential scanning calorimetry. Surface modifications were inspected by scanning electron microscopy (SEM) and optical microscopy. The experiments have shown that EB irradiation promotes beneficial changes in the fluoroelastomer tensile strength behavior while compression set values remain constant and the glass transition temperature increases. The SEM micrographs have shown compactness in the irradiated samples, although optical observations showed no changes in the surface morphology [112].

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86. [83] Tatemoto M, Shimizu T. In: Scheirs J, editor. Modern fluoropolymers. New York: John Wiley & Sons; 1997. p. 566. [84] Nakagawa T, Tatemoto M. US Patent 4,158,678, to Daikin; 1979. [85] Kawachi S. Gummi Faser Kunststoffe 1986;39:162. [86] van Cleeff A. Chapter 23 In: Scheirs J, editor. Modern fluoropolymers. Chichester: John Wiley & Sons, Ltd.; 1997. p. 359. [87] Tonelli C, Trombetta T, Scicchitano M, et al. J Appl Polym Sci 1996;59:311. [88] Logothetis AL, Stewart CW. US Patent 4,713,418; 1987. [89] Goebel KD, Nam S. Proc. 3rd Int. Conf. Thermopl. Elastomer, Mark. Prod. 55; 1966. [90] Tatemoto M, Tomoda M, Kawachi M, et al. Kokai Tokkyo Koho Japanese Patent 62635; 1984. [91] Kawashima C, Koga S. Jpn Plast 1988;39:98.

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[92] Kawamura K, Kawashima C, Koga S. US Patent 4,749,610; 1988. [93] Stokes NH. Am Chem J 1897;19:782. [94] ASTM D1418, ASTM International, West Conshohocken, PA. [95] Lohr DF, Penton HR. In: Handbook of elastomers, 2nd ed. (Bhowmick AK, Stephens HL eds.). New York: Marcel Dekker; 2001. p. 600. [96] Du L, Kelly JY, Roberts GW, De Simone JM. J Supercrit Fluids 2009;47:447. [97] Logothetis AL, Banks RE, Smart BE, Tatlow JC, editors. Organofluorine chemistry: principles and commercial applications. New York: Plenum Press; 1994. [98] Du L, De Simone JM, Roberts GW. In: Hutschenson KW, Scurto AM, Subramaniam B, editors. ACS Symosium Series 1006, Green chemistry and engineering with gas expanded liquids and near-critical media. Oxford: Oxford University Press; 2008. [99] Rolland JP, et al. J Am Chem Soc. 2005;127:10096. [100] Du L, De Simone JM. Unpublished results. [101] Rolland JP, et al. J Am Chem Soc. 2004;126:2322. [102] Bottino A, Campanelli G, Munari S, Turturro AJ. Polym Sci B Polym Phys 1988;26:785. [103] Stevens RD. J Soc Rubber Industry, Japan 2006;79(5):153. [104] Stevens RD, Rubber World 2006;233(5):23
42. [105] Bowers S. Kautschuk Gummi Kunststoffe 2006;55(6):11. [106] Drobny JG. Technology of fluoropolymers. 2nd ed. Boca Raton, FL: CRC Press; 2009. p. 190. [107] Sealing Technology. ,www.sealingtechnology. info.; 2012. [108] Park EH. Paper presented at TPE TopCon 2010, September 13
15, Akron, OH; 2010. [109] Zhang XX. Mater Res Innovation 2012;16 (1):143. [110] Zhang X, Lu C, Zheng Q, Ling M. Polymers Adv Technol 2011;22(2):2104. [111] Apostolo M, Triulzi F. J. Fluorine Chem 2004;125:303. [112] Giovedi C, Seguro PE, Rabello Rossi M, Brocardo Machado LD. Proceedings from the Seventh International Symposium on

315

Ionizing Radiation and Polymers. Nucl Inst Methods Phys Res B 265(1). 2007. p. 256.

Further Reading Ameduri B, Boutevin B. Well-architectured fluoropolymers: synthesis, properties and applications. Amsterdam: Elsevier B.V.; 2004. Apotheker D, Krustic PJ. US Patent 4,214,060; 1980. Bhowmick AK, Stephens HL, editors. Handbook of elastomers. 2nd ed. New York: Marcel Dekker; 2001. Cheng TC, Kaduk BA, Mehan AK, et al. US Patent 4,935,467; 1988. Ciulio PA, Hewitt N. The rubber formulary. Norwich, NY: William Andrew Publishing; 2007. Drobny JG, Fluoroelastomers Handbook, The Definite User’s Guide, Second Edition, Oxford, UK: Elsevier; 2016. Dyneont high performance fluorolastomers, product comparison guide. Publication No. 98
05041569-8, Dyneon; 2004. Ebnesajjad S, Khaladkar PR. Fluoropolymers applications in chemical processing industries. The definitive user’s guide and databook. 2nd ed. Norwich, NY: Plastics Design Library, Elsevier; 2017. FSRs in extreme applications. Form # 45-1252B-1, Dow Corning, Corporation; 2005. Nagdi K. Rubber as engineering material: guidelines for users. Munich: Hanser Publishers; 1993. Scheirs J, editor. Modern fluoropolymers: high performance polymers for diverse applications. Chichester: John Wiley & Sons, Ltd.; 1997. Scheirs J. Fluoropolymers: technology, markets and trends. Shrewsbury, Shropshire: RAPRA Technology, Ltd., Shawbury; 2001.

Acronyms and Abbreviations ASTM American Society for Testing and Materials (now ASTM International) PFEVE Poly(fluoroethylene vinyl ether) CSM Cure-site monomer CTFE Chlorotrifluoroethylene

316

DIN Deutsches Institut fu¨r Normung eV (German Institute for Standardization) DMI Bismaleimide E Ethylene (monomer) EB Electron beam ECTFE Copolymer of ethylene and chlorotrifluoroethylene FEP Fluorinated ethylene-propylene (copolymer of tetrafluoroethylene and hexafluoropropylene) FEPM Copolymer of tetrafluoroethylene and propylene FFKM Perfluoroelastomer FKM Fluorocarbon elastomer FMQ Fluorosilicone FPM ISO designation for fluorocarbon elastomer of the FKM type (ASTM) FPU Fluorinated polyurethane FTPE Fluorinated thermoplastic elastomer FVE Fluorovinyl ether FVMQ Fluoro-vinyl polysiloxane FTIR Fourier Transform Infrared Spectroscopy Gy SI unit of radiation dose HDPE High-density polyethylene HFIB Hexafluoroisobutylene HFP Hexafluoropropylene HPFP Hydropentafluoropropylene IR Infrared ISO International Organization for Standardization LLDPE Linear low-density polyethylene LOI Limiting oxygen index MA Maleic anhydride Mn Number average molecular weight Mw Weight average molecular weight MFR Melt flow rate MPa Megapascal (SI unit of pressure, stress, tensile strength) MQ Silicone resins MVE Methyl vinyl ether NBS National Bureau of Standards (in 1988 changed to NIST, see below) NIST National Institute of Standards and Technology P Propylene monomer PA Polyamide PAVE Perfluoroalkyl vinyl ether PCTFE Polychlorotrifluoroethylene PDD Perfluoro-2,2-dimethyl dioxole PE Polyethylene PFEVE Poly(fluoroethylene vinyl ether) PFOA Perfluorooctanoic acid PMVE Perfluoromethyl vinyl ether

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PP Polypropylene PPVE Perfluoropropyl vinyl ether PTFE Polytetrafluoroethylene PU, PUR Polyurethane PVC Polyvinyl chloride PVDF Polyvinylidene fluoride PVF Polyvinyl fluoride SBC Styrenic block copolymers SBS Styrene-butadiene-styrene block copolymer SSG Standard specific gravity Tg Glass transition temperature Tm Crystalline melting point TAC Triallyl cyanurate TIAC Triallyl isocyanurate TFE Tetrafluoroethylene monomer TGA Thermogravimetric analysis TMPTA trimethylolprpane triacrylate TMPTM Trimethylolpropane trimethacrylate TPE Thermoplastic elastomer TPV Thermoplastic vulcanizate UL Underwriters Laboratory UV Ultraviolet VDF Vinylidene fluoride VF Vinyl fluoride

Glossary of Terms ASTM American Society for Testing and Materials is a nonprofit organization with the purpose of developing standards on characteristics and performance of materials, products, systems, and services and promoting the related knowledge. Now ASTM International. Average particle size The average diameter of solid particles as determined by various test methods. Block copolymers Copolymers consisting of two or more polymer chains attached at their ends, e.g., polystyrene-polybutadiene-styrene (SBS). Most block copolymers are produced by controlled polymerization methods. Blowing (Foaming) Agent A substance that alone or in combination with other substances can produce a cellular structure in a plastic or elastomeric mass. It can be a compressed gas, a volatile liquid or solid that decomposes into a gas upon heating. Coagent An additive increasing the effectiveness of an organic peroxide used as a cross-linking agent.

16: FLUOROELASTOMERS

Coalesce To combine particles into one body or to grow together. Cold flow (creep) Tendency of a material to flow slowly under load and or/or over time. Comonomer A monomer reacting with a different monomer in a polymerization reaction, the result of which is a copolymer. Contact angle A measure of the ability of a liquid to wet solid surfaces. It expresses the relationship between the surface tension of a liquid and the surface energy of the surface on which the liquid rests. As the surface energy decreases, the contact angle increases. Corona treatment A method to render inert polymers more receptive to wetting by solvents, adhesives, coatings, and inks using high-voltage discharge. The corona discharge oxidizes the surface, thus making it more polar. Cross-linking A reaction during which chemical links are formed between polymeric chains. The process can be carried out by chemical agents (e.g. organic peroxides), reactive sites on the polymeric chains, or by high energy radiation (e.g., electron beam). Cryogenic Refers to very low temperatures, below about 2150°C (2238°F). Crystalline melting point A temperature at which the crystalline portion of the polymer melts. Crystallinity A state of molecular structure attributed to the existence of solid crystals with a definite geometric form. Cure A process of changing the properties of a polymer by a chemical reaction (condensation, polymerization, or addition). In elastomers, it means mainly cross-linking or vulcanization. Dielectric constant The ratio of the capacitance assembly of two electrodes separated by a plastic insulating material to its capacitance when the electrodes are separated by air only. Dielectric heating The heating of polymeric materials by dielectric loss (see below) in a high-frequency electrostatic field. Dielectric loss A loss of energy evidenced by the rise in heat of a dielectric placed in an alternating electric field. It is usually observed as a frequency-dependent conductivity. Dielectric loss factor The product of the dielectric constant and the tangent of the dielectric -loss angle for a material. Dielectric loss tangent The difference between 90° and the dielectric phase angle for a material.

317

Dielectric strength Ability of a material to resist the passage of electric current. Differential scanning calorimetry (DSC) The method to measure the heat flow to a sample as a function of temperature. It is used to measure specific heats, glass transition temperatures, melting points, melting profiles, degree of crystallinity, degree of cure, purity, and more. Dynamic vulcanization A process of preparing thermoplastic vulcanizates (TPVs), most commonly in an internal mixer or twin-screw extruder. An example is a system consisting of EPDM and polypropylene (PP). In the process, the EPDM is vulcanized (crosslinked) and becomes dispersed in the PP matrix. Elasticity The ability of a material to quickly recover its original dimensions after removal of the load that has caused the deformation. Elastomer A polymeric substance with elastic properties. Such material can be stretched repeatedly at room temperature to at least twice its original length and upon immediate release of the stress will return with force to its approximate original length. Electron beam (EB) cure A process using high energy (accelerated) electrons to promote reactions in a polymeric material (cross-linking, polymerization). The reaction is instantaneous. The voltage range used in this process is typically from hundreds of kilovolts to several megavolts. FEPM Elastomeric copolymer of TFE/P or E/TFE/ PMVE FFKM Perfluoroelastomers, copolymers of TFE/ PMVE of TFE/PPVE, that is, perfluorinated monomers (composed only of carbon and fluor atoms); FFKM typically contain a cure-site monomer (CSM) that facilitates curing (crosslinking). FKM Elastomeric copolymer of fluorinated monomers, such as VDF/HFP/TFE or VDF/PMVE/TFE, as defined by ASTM D1418. FPM Elastomeric copolymer of fluorinated monomers as defined by ISO R1629. Film formation A process in which a film is formed after solvent, water, or due to a chemical reaction. Fluorocarbon elastomer In this chapter is a fluorinated copolymer with carbon
carbon chain linkages.

318

Fluoroelastomer Broadly, any elastomer containing an appreciable amount of fluorine. Friction, dynamic Resistance to continued motion between two surfaces, also known as sliding friction. Friction, static Resistance to initial motion between two surfaces. Gamma radiation Ionizing radiation from a radioactive source, such as 60Co (cobalt isotope with atomic weight of 60). Glass transition temperature (Tg) A point below which an amorphous polymer behaves as glass does—it is very strong and rigid, but brittle. Above this temperature, it exhibits leathery or rubbery behavior. Heat buildup Heat generated within a polymeric material due to its viscoelasticity (hysteresis) and friction. It occurs during processing (mainly friction and kneading) and in service (mainly repeated cycling). Hysteresis Incomplete recovery of strain during unloading cycle due to energy consumption. This energy is converted from mechanical to frictional energy (heat). HFP Hexafluoropropylene, a monomer used for the production of fluorocarbon elastomers. Laminate A product made by bonding together one or more layers of material or materials. It is frequently assembled by simultaneous application of heat and pressure. A laminate may consist of coated fabrics; metals, films, or it may be different combinations of these. Latex A stable dispersion of a polymeric substance (most frequently of an elastomer) in an essentially aqueous medium. Melt flow rate (MFR) Measures the rate of extrusion of molten thermoplastic through an orifice at a prescribed temperature and load. Monomer A relatively simple compound usually containing carbon and of low molecular weight, which can react to form a polymer by combination with itself or with other similar molecules or compounds. Oligomer A polymer with a very low molecular weight, usually a liquid substance. Perfluorinated polymer A polymer consisting of monomers where all main chain carbons are combined with fluorine atoms only (e.g., TFE, HFP, PPVE, PMVE).

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PMVE Perfluoro(methylvinyl ether) a monomer used for the production of fluorocarbon elastomers and other fluoropolymers Polymer fume fever An illness characterized by temporary flu-like symptoms caused by inhaling the products released during the decomposition of fluoropolymers, mainly PTFE. Tobacco smoke enhances the severity of this condition. Postcure A second cure at high temperatures enhancing some properties and/or removing decomposition products of the primary reaction. PPVE Perfluoro(propylvinyl ether), monomer for the production of fluorocarbon fluoroelastomers and other fluoropolymers. Prorad Radiation promoter, a compound promoting or enhancing the cross-linking reaction by high energy (ionizing) radiation. Radiation dose Amount of absorbed energy per unit mass during irradiation by electron beam, gamma rays, and X-rays. The SI unit is 1 Gy (1 J/kg), larger and practical unit is 1 kGy, that equals to 1000 kGy. Scorch Premature incipient vulcanization of a rubber compound. Scorch, Mooney The time to incipient of a rubber compound when tested in the Mooney shearing disk viscometer under specific conditions. Substrate Any surface to be coated by a coating or bonded by an adhesive. Surface resistance The surface resistance between two electrodes in contact with a material is the ratio of the voltage applied to the electrodes to that portion of the current between them, which flows through the surface layers. Surfactant A widely used contraction of surface active agent, a compound that alters surface tension of a liquid in which it is dissolved. Telomer Its original meaning is in polymer science to refer to an extremely small polymer—one whose degree of polymerization is between 2 and 5. Tensile strength The stress in tension required to break a given sample; the SI unit is a mega Pascal (MPa). Alternative unit, pounds per square inch (psi) is often used in the United States. Terpolymer The product of simultaneous polymerization of three different monomers or

16: FLUOROELASTOMERS

of grafting of one monomer to the copolymer of two monomers. TFE Tetrafluoroethylene, a perfluorinated monomer used as a feedstock for the production of PTFE and as a comonomer for the production of a variety of other fluoropolymers including FKMs and FFKMs. Thermoforming A process of forming a plastic film or sheet into a three-dimensional shape by clamping it, heating it, and then applying a differential pressure to make the film or sheet conform to the shape of the mold. Thermogravimetric analysis (TGA) A widely used method to determine weight change upon heating, such as decomposition, amount of volatile components, including moisture. Thermoplastic elastomer (TPE) A polymer that can be processed as a plastic and is elastic when cooled to ambient temperature or to other temperature sufficiently lower than its softening temperature. Thermoplastic vulcanizate (TPV) A thermoplastic elastomer consisting of particles of vulcanized (cross-linked) rubber dispersed uniformly in a thermoplastic matrix. An example is an EPDM/ PP system. TPVs are prepared by so-called dynamic vulcanization.

319

TR 10 Test Retraction at low temperatures of vulcanized rubber (ASTM D1329 and ISO 2921). The sample is frozen in the stretched state and then gradually heated until it loses 10% of the stretch. The temperature determined by the test correlates with the brittle point of the material. TR 10 test is considered the most useful indicator of the low-temperature performance of the tested material. Viscoelasticity The tendency of polymers to respond to stress as if they were a combination of elastic solids and viscous fluids. Viscosity The property of resistance of flow exhibited within the body of material. Units of viscosity are Pascal (traditional) and Pa s (SI). Conversion: 10 P 5 1 Pa s or 1 cP 5 1 mPa s. Viscosity Mooney A measure of the viscosity of a rubber or elastomer determined in a Mooney shearing rotation disk viscometer (ASTM D1646). Viscosifying agent A substance used to increase the viscosity of a liquid mainly by swelling. Volume resistivity The electrical resistance between opposite sides of a cube.

17 Fluoropolymer and Fluorinated Additives Sina Ebnesajjad FluoroConsultants Group, LLC, United States

O U T L I N E 17.1 Polymeric Fluorinated Additives 17.1.1 Polytetrafluoroethylene Homopolymer Additives 17.1.2 Fluoroelastomer Additives (Polymer Processing Additives) 17.1.3 Vinylidene Fluoride Polymer Additives

321

17.4 Fluorinated Graphite

329

321

17.5 Fluorination

329

17.6 Market Size

330

References

330

17.2 Perfluoropolyether Additives

326

Further reading

330

17.3 Polytetrafluoroethylene Modified Waxes

329

322 326

Fluoroadditives or micropowders are finely divided low molecular weight polytetrafluoroethylene (PTFE) powders, which are nearly entirely consumed by industries outside fluoropolymers. Micropowders are added to a great number of other products and compounded to enhance the properties or performance of these products. The addition of a fluoroadditive imparts some of the properties of fluoropolymers to a host system. There are few applications, such as dry film lubricants, where a micropowder is used by itself. There are several types of fluorinated additives which are used as additives to plastics, rubbers, paints, oils (lubricants), printing inks, and others. The most commonly used fluorinated additive is PTFE but it has several different forms most of which have finely divided particles down to submicron size. Fluoroelastomers are often used as processing aids to enhance productivity and characteristics of plastic manufacturing and fabrication processes. Fluorinated additives can enhance abrasion resistance, reduce coefficient of friction (COF) and mechanical wear, reduce surface contamination, and modify appearance. The fluoroadditives also provide specific benefits to specialized products. For example, thermoplastic parts, such as gears, benefit from improved wear resistance and reduced friction. Stick-slip behavior can be eliminated.

Elastomeric seals for diverse environments improve in tear and abrasion resistance. Lithographic, flexographic, and gravure inks can be formulated for better image protection and higher productivity. There are many ways to classify fluorinated additives, including by chemical structure, polymeric/nonpolymeric, and molecular weight. In this chapter, we discuss briefly various classes of fluoropolymer additives. These include fluoroplastics and fluoroelastomer additives.

17.1 Polymeric Fluorinated Additives This group of additives is prepared from a precursor which is a product of polymerization of a fluorinated monomer. They include additives based on fluorinated plastics and elastomers prepared using tetrafluoroethylene (TFE) and vinylidene fluoride (VDF) polymers.

17.1.1 Polytetrafluoroethylene Homopolymer Additives Fluoroplastic additives include, for the most part, PTFE polymers. PTFE forms a family of polymers that is manufactured by suspension and emulsion

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00017-3 © 2021 Elsevier Inc. All rights reserved.

321

322

polymerization of a TFE monomer. Sometimes a small amount (,0.5% by weight) of a second fluorinated or nonfluorinated monomer is incorporated in PTFE polymers to modify certain properties. These minor copolymers are still referred to as homopolymers because the overwhelming majority of polymer consists of TFE. Relatively small volumes of high molecular weight PTFE grades are used as additives in applications such as dripsuppression of burning plastics. The majority of PTFE-based additives are consumed as low molecular weight forms. A very small amount of meltprocessible copolymers of TFE is used in highly specialized applications [1]. The discovery of low molecular weight PTFE additives dates back to 1973 [2] via the application of a critical dose of ionizing radiation to sintered or unsintered polytetrafluoroethylene, rendering such material capable of being comminuted to microfineness with no adverse heat side effects, and the resulting particles are readily dispersible in diverse media. Such particles possess the extremely low COF associated with PTFE resin. The dosage level of ionizing radiation in accordance with the process of this invention lies within the range of from about 5 to about 25 Mrads. PTFE is nondestructively degraded so as to be grindable to a powder of an average size of less than 10 µm by a combination of irradiation by electrons or other gamma rays in the presence of oxygen or air and concurrent or subsequent heating at temperatures below the melting point of the material. Fluoroadditives or micropowders are finely divided low molecular weight PTFE material, which are nearly entirely consumed by industries outside fluoropolymers (Tables 17.1 17.6). These additives are available in mostly powder form, and some are available as dispersions. Micropowders are added to a great number of other products and compounded to enhance the properties of these products. The addition of a fluoroadditive imparts some of the properties of fluoropolymers to a host system. There are few applications, such as dry film lubricants, where a micropowder is used by itself. In general, fluoroadditives have a small particle size of the order of a few microns, hence the word micropowders. These powders are either granular (suspension polymerized) or fine powder based (dispersion polymerized), which have

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FLUOROPOLYMERS

different particle morphologies and, therefore, different properties and incorporation manner in the host material. Their molecular weight is in the range of a few ten thousand to a few hundred thousand compared to several millions for the molding resins.

17.1.2 Fluoroelastomer Additives (Polymer Processing Additives) Fluoroelastomers based on copolymers of VDF and hexafluoropropylene and terpolymers containing TFE, introduced commercially in the late 1950s and early 1960s, greatly extended the utility of elastomers. The heat and fluid resistance of fluoroelastomers is superior to that of other elastomers. Fluoroelastomer seals and other components have contributed to reliability, safety, and environmental protection in many areas including the aeronautical, automotive, oil, and chemical industries. Subsequent development of improved cure systems in the 1970s has led to better processing characteristics and enhanced properties of fluoroelastomers. New compositions, including specialty polymers based on perfluoro(methyl vinyl ether) and perfluoroelastomers based on copolymers of TFE and perfluoro(alkyl vinyl ethers) with new cure systems imparting outstanding thermal and fluid resistance, have further extended service limits of elastomers. Various fluoroelastomer families are useful in longterm service in contact with a wide range of fluids up to 200°C 300°C. New products with enhanced performance continue to be developed after more than 45 years. Fluoroelastomer additives are a family of materials that are usually used as polymer processing additives (PPA). When added to other materials such as polymers, they improve the processing of polymers during compounding and fabricating of articles. Some of the improvements include

• Increase in throughput and decrease in power requirement;

• Reduce surface defects also called melt fracture;

• • • •

Reduce and eliminate die build-up; Allow extrusion at lower temperatures; Reduce gel formation; Reduce die cleaning.

Table 17.1 Typical Properties of Asahi Glass Chemical PTFE Fluoroadditive Powders [3].

Grade

PTFE Polymer Type

Production Method

Specific Surface Area, m2/g

Particle Size Distribution, µm

Average Particle Size, µm

Melting Peak Temperature, °C

Bulk Density, g/L

Applications

FL1680

Granular

Irradiation

0.8

13

325 330

450

FL1690

Granular

Irradiation

1.0

21

325 330

480

FL1700

Granular

Irradiation

3.1

325 330

530

FL1710

Granular

Irradiation

3.2

9

325 330

400

L150J

Granular

Irradiation

1.3

9

325

380

Plastics, elastomers, paint, ink, grease

L169J

Granular

Irradiation

2

13

332

370

Plastics, elastomers

L170J

Fine powder

Irradiation

8.2

332

560

Paint, ink, grease

L172J

Fine powder

Irradiation

8.2

330

560

Paint, ink, grease

L173J

Fine powder

Irradiation

8.2

330

300

Paint, ink, grease

Some grades may be obsolete.

7

Table 17.2 Asahi Glass Chemical PTFE Fluoroadditive Powders [4].

Property

Units

Typical Value FL1680

Bulk density

g/L

450

480

530

560

400

450

2.2

2.2

2.2

2.2

2.2

2.2

Relative density

Typical Value FL1690

Typical Value FL1700

Typical Value FL1700H

Typical Value FL1710

Typical Value FL1710H

Particle size (Hegmann gauge)

Microns: AVE

9

17

,2

,2

3

3

Particle size (Malvern laser diffraction)

Microns: AVE 10% , 90% ,

13 5 28

21 8 50

95 25 150

85 20 140

9 4 17

7 4 13

Particle size (Optical SEM)

Microns: AVE

11

16

,1

,1

6

6

Surface area (Krypton absorption)

m2/g

0.8

1.0

3.1

3.1

2.3

2.3

Melting peak temperature (DSC)

°C

328

335

335

335

335

335

Service temperature range

°C

2190

2190

2190

2190

2190

2190

1260

1260

1260

1260

1260

1260

Yes

Yes

Yes

Yes

Yes

Yes

FDA compliance DSC, Differential Scanning Calorimetry.

Table 17.3 Typical Properties of Dyneon PTFE Fluoroadditive Powders [5]. Specific Surface Area, m2/g

Melt Flow (MFR), g/10 min

Mean Particle Diameter, µm (Primary Particle Size)

Bulk Density, g/L

FDA Compliance

Direct polymerization

10

,2

9 (0.2)

350

Yes

Direct polymerization

12

6

4 (0.2)

280

Yes

Thermal polymerization

2

12

8

400

No

17

4

4 (0.12)

260

No

Grade

PTFE Polymer Type

Production Method

TF 9201Z

Fine powder

TF 9202Z

Fine powder

TF 9205 TF 9207Z

Fine powder

Direct polymerization

TF 2021Z

Fine powder

Direct polymerization

500

480

No

TF 2071Z

Fine powder

Direct polymerization

400

490

No

Some grades may be obsolete.

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Table 17.4 Typical Properties of Daikin PTFE Fluoroadditive Powders [6]. No.

Applications/Object

Features

L-2

Coating, grease

Ave particle size: 3 µm Specific surface area: 8 m2/g

L-5

Plastic

Ave particle size: 5 µm Specific surface area: 10m2/g

L-5F

Plastic

Ave particle size: 4 µm Specific surface area: 11 m2/g

LDW-410

Plastic, battery binder

Aqueous dispersion—solids 40% Primary particle size: 0.2 µm

Table 17.5 Typical Properties of Daikin PTFE Aqueous Dispersion Fluoroadditive [6]. Property

Value

Solids content, by wt.%

40

Surfactant content, %

6

Specific gravity at 25°C

1.3

Average dispersion particle size, µm

0.2

Viscosity of dispersion (at 25°C), cP

8

Peak melting temperature, °C

330

pH (minimum)

8 9

These processing aids form a nonstick fluoropolymer coating on the inside of the die, reducing friction so that the resin flows freely and more rapidly through the die to produce an extrudate with smooth surfaces. The die coating forms as the resin containing process aid is fed to the extruder, and removal of the coating by polymer flow is balanced by renewal from process aid dispersed in the continuing feed. Processing aid technology has evolved over the last 20 years, with original fluoroelastomer products replaced with synergistic blends and additives containing optimized polymers and interfacial agents.

17.1.3 Vinylidene Fluoride Polymer Additives An additive has been developed from copolymers of vinylidene fluoride (PVDF) as PPA by Arkema Corporation. The application is as a process aid for the extrusion of polyolefins linear low-density polyethylene (PE), high-density PE, polypropylene, in particular, in the form of film [8]. Kynar Flex PVDF PPA is added in low concentrations, of the order of 0.02% 0.08% by weight, to help increase processing speed while reducing energy consumption, processing temperatures and pressure. It also helps prevent blockages in equipment and, in particular, the formation of drops and knots on converting lines. With respect to finished product, this PVDF additive enhances the film’s

transparency and mechanical properties, eliminates “sharkskin” surface defects, and melts fracture.

17.2 Perfluoropolyether Additives Fluorinated additives and modifiers used in vulcanized elastomers are not fully fluorinated. They are made up of a difunctional perfluoropolyether (PFPE) backbone with hydrogenated end groups. Incompatible additives migrate to the surface too fast and are wiped away easily. Soluble (very compatible) additives do not migrate to the surface well and stay in the bulk material. Mixtures of both incompatible and compatible components are designed to bridge this gap. When elastomers rub against other surfaces like plastic, glass, painted surfaces, and metal, a squeak or an “itching” sound can often be heard due to the high COF that standard elastomeric materials possess. In addition, the lack of lubricity on the surface can cause more serious performance issues like “stick slip,” which is a phenomenon often seen when an elastomer slides across another material with a high COF. The materials “stick” until the force is great enough to break free of the bond between the two surfaces and then the energy releases in the form of a slide between the mating surfaces until the part comes to rest again after the built-up energy is released. The

Table 17.6 Typical Properties of Solvay Solexis PTFE Fluoroadditive Powders [7].

Grade

PTFE Polymer Type

Production Method

Specific Surface Area, m2/g

Particle Size Distribution, µm

Average Particle Size, µm

Melting Peak Temperature, °C

Bulk Density, g/L

Algoflon L100

Fine powder

Direct polymerization

.20

20

323 328

350

Algoflon L101-1

Fine powder

Irradiation

.14

3

323 328

300

Algoflon L101X

Fine powder

Irradiation

.14

7

323 328

300

Algoflon L203

Fine powder

Irradiation

.7.5

,6

328 332

310

Algoflon L206

Fine powder

Irradiation

.7.5

,7

328 333

330

Polymist XPP-512

Granular

Irradiation

2 3

7 9

319 327

350 600

Polymist F5

Granular

Irradiation

3

5.5

326

400

Polymist F5A

Granular

Irradiation

3

4

326

400

Polymist F5A EX

Granular

Irradiation

3

12

328

500

Polymist F284

Granular

Irradiation

3

.20 µm ,7%

9

332

350

Polymist F510

Granular

Irradiation

3

.20 µm 30% .50 µm ,2%

14

331

475

(Continued )

Table 17.6 Typical Properties of Solvay Solexis PTFE Fluoroadditive Powders [7].—Cont’d PTFE Polymer Type

Production Method

Specific Surface Area, m2/g

Particle Size Distribution, µm

Average Particle Size, µm

Melting Peak Temperature, °C

Bulk Density, g/L

Polymist XPA 213

Granular

Irradiation

3

( . 20 µm) ,2%

3.5

326

375

Polymist XPP-510

Granular

Irradiation

2 3

3 4.5

322 326

275 500

Polymist XPP-511

Granular

Irradiation

2 3

15 25

320 330

600 800

Polymist XPP-513

Granular

Irradiation

2 3

11 15

319 327

400 600

Polymist XPP-514

Granular

Irradiation

2 3

10 12

319 327

400 600

Polymist XPP-516

Granular

Irradiation

2 3

2.75 3.25

317 323

325 50

Polymist XPP-517

Granular

Irradiation

2 3

2.75 3.25

323 328

Polymist XPP-518

Granular

Irradiation

2 3

2.75 3.25

323 328

Polymist XPP-519

Granular

Irradiation

2 3

2.75 3.25

319 325

325 525

Polymist XPP-525

Granular

Irradiation

2 3

3 4.5

319 327

275 500

Polymist XPP-529

Granular

Irradiation

2 3

4 5.5

318 328

350 600

Polymist XPP-530

Granular

Irradiation

2 3

4 5.5

318 328

350 600

Polymist XPP-535

Granular

Irradiation

2 3

3.5 4

319 327

350 600

Polymist XPP-538

Granular

Irradiation

2 3

4 5

318 328

300 550

Polymist XPP-539

Granular

Irradiation

2 3

2.5 3

318 328

300 550

Polymist XPP-540

Granular

Irradiation

2 3

12 15

330 334

250 450

Grade

Some grades may be obsolete.

17: FLUOROPOLYMER

AND

FLUORINATED ADDITIVES

surfaces “stick” again and the phenomenon repeats. This is a serious issue for moving parts but also causes assembly difficulties by dramatically increasing the insertion force when parts with an elastomeric component must be inserted into or onto another part. When PFPEs are added to plastics and elastomers, they improve processing of polymers. They provide internal lubrication to improve flow properties, processibility, throughput, and uniform mixing/dispersion of ingredients,

17.3 Polytetrafluoroethylene Modified Waxes Natural waxes, chemically modified natural waxes, and fully synthetic products have been used as additives for printing inks for decades. Synthetic waxes are manufactured by Fischer-Tropsch process. Fischer-Tropsch is a method for the synthesis of hydrocarbons and other aliphatic compounds from synthesis gas, a mixture of hydrogen and carbon monoxide in the presence of a catalyst. The hydrogen-carbon monoxide gas mixture is obtained by coal gasification or natural gas reforming. The process is named after F. Fischer and H. Tropsch, the German coal researchers who discovered it in 1923. Synthesized hydrocarbons are fractionated to different grades of FT waxes with a chain length of up to C100, qualified by their saturated-linear chains, free of aromatics, sulfur, and nitrogen [9]. Any of these waxes can be modified by the addition of PTFE micropowders. The use of modified PE waxes with low molecular PTFE as printing ink additives allows important and useful changes in printing ink properties and print processing characteristics to be achieved. Modification of the rheological properties of the printing ink, especially a reduction in the tack of paste inks that dry by oxidation. The printing process is trouble-free, even on low quality substrates, by the use of “shorter,” lower-tack letterpress, or lithographic inks. The use of wax in printing ink films and as a result of a localized controlled reduction in the adhesive strength, the following significant results are obtained:

• Improvement of slip; • Good antiblocking properties;

329

• Better scratch resistance; • Good rub resistance. The interaction of these properties results in an efficient, high-quality end product [10].

17.4 Fluorinated Graphite This family of fluorinated additives is quite unique. Fluorine reacts with graphite and creates a partially fluorinated carbon fluorinated compound with uses as an additive for lubrication as well as in lithium batteries. Fluorinated graphite has a polymeric structure with the general composition of (CFx)n. Its use on lubricants is based on its surface interactions and the presence of C-F hydrophobic groups. In primary lithium batteries, the high affinity of the graphite fluoride to lithium renders them desirable as cathode materials. The low equivalent weight of the carbon and fluorine elements allows achievement of a high specific energy density [11]. Graphite fluorides with variable properties are made by different processes. The most common commercial process is the direct reaction of fluorine gas with graphite at temperatures in the range of 300°C 600°C. Indeed, pure fluorine gas reacts with graphite only at high temperature to give a compound of general composition (CFx)n. The F/C ratio “x” varies from 0.6 to 1 when the temperature increases from 300°C to 600°C [11,12].

17.5 Fluorination Strictly speaking, fluorination results in the “addition” of fluorine to non or partially fluorinated polymeric substances. Fluorination occurs as a separate step or during the manufacturing of the PE bottles via blow molding using dilute fluorine. Fluorination surface treatment improves the resistance of PE to many organic chemicals. A barrier layer is created by the chemical reaction of the fluorine and the PE, forming a thin (20 40 nm) fluorocarbon layer on the inside surface of the bottle. The addition of fluorine to the surface reduces the solubility of organic liquids in the plastic, thus reducing permeation through the wall of the bottle [13]. Alternatively, fluorine-treated PE bottles can be exposed to fluorine gas in a secondary operation

330

to impart barrier properties against hydrocarbons and aromatic solvents. This surface treatment allows low-cost blow-molded PE bottles to be used for storing aggressive organic liquids. Examples include paint, paint thinner, lighter fluid, polishes, cleaning solvents, cosmetics, and toiletries. Fluorine-treated bottles are excellent for use with insecticides, pesticides, herbicides, photographic chemicals, agricultural chemicals, household and industrial cleaners, electronic chemicals, medical cleaners and solvents, citrus products, flavors, fragrances, essential oils, surfactants, polishes, additives, graffiti cleaning products, stone and tile care products, waxes, paint thinner, gasoline, biodiesel, xylene, acetone, kerosene, and more. For nonbottle applications, fluorination of plastic can provide compliance with state and federal regulations. An example would be fluorination plastic fuel tanks used for lawn and garden equipment and for automobiles. In the blow molding example, fluorination, in effect, creates a fluoropolymer layer at the surface of the interior surface of the bottle. Similar effects are achieved by adding fluoropolymer additives to plastics. Fluorination will not be further pursued here because this book is concerned with fluoropolymer additives that can be physically added to other materials, usually without the occurrence of a chemical reaction.

17.6 Market Size According to “Micronized PTFE Market by Application (inks, thermoplastics, coatings, grease and lubricants, elastomers), by Geography—Global Forecast to 2019,” the micronized PTFE market has a projected value of $870M by 2019, signifying firm growth of 7.1%/year between 2014 and 2019. Asia-Pacific is the biggest market for micronized PTFE and accounted for a share of over 38.00% of the total market size after 2013. China is the key market in Asia-Pacific, consuming more than half of the demand for PTFE micropowders, whereas India is the fastest-growing market [14]. Thermoplastic is projected to be the fastest-growing application segment followed by grease and lubricants between 2014 and 2019. Inks are the biggest application for the micronized PTFE market, followed by thermoplastic having a combined market share of around 66% of the total MP PTFE consumption. The

INTRODUCTION

TO

FLUOROPOLYMERS

United States and China are at forefront of PTFE micropowder consumption, while Germany, Japan, and the United Kingdom are other key markets for PTFE micropowder consumption [14].

References [1] Namura S. US Patent 6649699, assigned to DuPont Mitsui Fuorochemicals; 2003. [2] Dillon J. US Patent 3,766,031, assigned to Garlock; 1973. [3] AGC Chemicals America, www.agcchem.com; 2017. [4] Fluon® Lubricant Powders—a guide to applications, properties and processing. Technical Service Note FTI500. Rev 4. AGC Americas; 2002. [5] 3M Dyneon, www.3m.com; 2017. [6] Daikin Industries, www.daikin.com/chm/products/ additive/additive_01.html; 2017. [7] Solvay Solexis, www.solvay.com/en/marketsand-products/featured-products/Polymist-PTFEAlgoflon-L-Micronized-Powders.html; 2017. [8] Kynar Flex® PVDF, www.arkema-inc.com/ index.cfm?pag 5 683; 2011. [9] Fischer-Tropsch wax. Chemical Plus Corp, www.chemicalplus.com; 2011. [10] Printing inks, Clariant International Ltd, Pigments & Additives Division, Switzerland, www.additives.clariant.com/; 2011. [11] Arda DR, Mackley MR. The effect of die exit curvature, die surface roughness and a fluoropolymer additive on sharkskin extrusion instabilities in polyethylene processing. J Nonnewton Fluid Mech 2005;126:47 61. [12] Kita Y, Watanabe N, Fuji Y. J Am Chem Soc 1979;101:3832. [13] Belcher S. Blow molding. Encyclopedia of polymer science and technology. online edition John Wiley and Sons; 2011. [14] Special Chem Coatings and inks formulation, www.specialchem4coatings.com; 2014.

Further Reading Ebnesajjad S, Morgan RA. Fluoropolymer additives. 2nd Ed Elsevier; 2019. Ebnesajjad S. 2nd Ed Non-melt processible fluoroplastics, vol. 1. Plastics Design Library, Elsevier; 2015.

18 Polyvinyl Fluoride: The First Durable Replacement for Paint Sina Ebnesajjad FluoroConsultants Group, LLC, United States

O U T L I N E 18.1 Introduction

331

18.2 Basic Properties

331

18.3 Attribute Application Relationships

331

18.1 Introduction Polyvinyl fluoride (PVF) is manufactured by polymerizing vinyl fluoride (VF). It is a unique plastic that has found applications in a number of industries for decades. It has prospered as a single polymer without proliferation into copolymers, which has been the norm for nearly all polymer families. The chemical structures of PVF and polyethylene (PE) are depicted in Fig. 18.1. The structures of PE and PVF differ in one fluorine atom substituted for hydrogen. Yet, there are significant property differences brought about by the substitution of a single fluorine atom PE. This large property change illustrates the importance of the impact of fluorine on the polymer structures. An entire book devoted to coverage of PVF may be consulted for in-depth information and data [1].

18.2 Basic Properties VF homopolymers and copolymers have excellent resistance to sunlight degradation, chemical attack, water absorption, and solvents. Additionally, PVF has high solar energy transmittance rate. These properties (Tables 18.1 18.4) have resulted in utilization of PVF films and coatings in outdoor and indoor applications that are both functional and decorative. PVF films and coatings have found

18.4 Development and Applications of Polyvinyl Fluoride—A Chronological Treatise 332 References

337

applications where thermal stability, chemical resistance, outdoor durability, stain resistance, adherence, and release properties are required. PVF is stable at high temperatures, which is important in many of its applications. DuPont was first commercialized a film based on PVF under the trademark Tedlar and is the only known commercial supplier of the polymer and its film products. One of the interesting aspects of PVF is that it has been limited to a single polymer throughout most of its history. Technology developed has allowed manufacturing of PVF into a variety of films including unoriented and biaxially oriented products and coatings for over one-half century. A hallmark of PVF over its history has been continuous renewal by finding new applications. Some of the PVF applications have become obsolete over time due to technological evolution and non-inkind products. Yet that attrition has been more than compensated by the discovery of new applications.

18.3 Attribute Application Relationships PVF provides a long-lasting finish to a wide variety of surfaces exposed to harsh environments while its inert, nonstick properties make it an excellent release film for parts processed under high temperature and pressure. PVF protective films

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00018-5 © 2021 Elsevier Inc. All rights reserved.

331

332

Figure 18.1 Chemical Structures Fluoride and Polyethylene.

INTRODUCTION

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Polyvinyl

Table 18.1 Basic Attributes of Polyvinyl Fluoride.

• UV resistance • Moisture barrier • Resistance to weathering • Mechanical properties • Strength and durability • Electrical insulation • UL recognized • Inertness toward a wide variety of chemicals, solvents, and staining agents Table 18.2 Typical Properties of Polyvinyl Fluoride Film(s) [2]. Density, cc/cm3

1.38 1.72

Tear strength, initial, kJ/m

129 196

Tensile modulus, MPa

44 110

Ultimate elongation, %

115 250

Continuous use temperature, °C

70 107

Water vapor permeability, g/m2 day

24.5

Dielectric strength, short-term dc, kV/µ

0.15 0.19

UL 94 Flame Class

HB

UL 746B RT1, electrical, °C

140

UL 746B RTI, mechanical (Impact Str), °C

120, 125

resist abrasion, scuffs, and stains to preserve the desired esthetics of your graphics and labels for many years. PVF films offer durable gloss retention and an easy-to-clean surface so graffiti and other unwanted stains can easily be removed from your graphics. PVF release films help provide an ideal release for fiber-reinforced polymer and carbon-fiberreinforced polymer composites. High temperature and chemical-resistant properties of PVF make it

TO

FLUOROPOLYMERS

ideal for use in high-voltage electrical power applications. PVF release films offer gas permeation resistance into and out of the sample bags, assuring sample integrity. PVF outperforms competition with optimal balance of high-temperature capability, conformability, toughness, and release from adhesives for printed circuit board (PCB) lamination. For PCB manufacturers who want to increase their press temperatures to reduce production cycles and increase output. PVF has been demonstrated to outperform many of the current release films. PVF films give solar panels the ability to resist weathering, moisture, and UV radiation. PVF films used as a backsheet material give solar panels the ability to resist weathering, moisture, and UV rays. PVF films offer attractive, easy-to-clean, scuff-resistant surface protection to aircraft interiors Fig. 18.2. PVF films give aerospace product manufacturers maximum design flexibility in creating passenger areas that are attractive, easy to clean, and scuffresistant. They are lightweight, conformable, and can be embossed and printed. PVF is an ideal protection material for architectural substrates. PVF films are easily laminated to a wide range of architectural substrates to provide protection against extreme weather, fading, cracking, and corroding. They provide easy-to-clean surface finishes for cars, buses, trains, and marine vessels. PVF film is used as a protective film on interior and exterior of trains, planes, and other forms of transportation. PVF films provide an easy-to-clean surface finish that resists weathering, UV rays, and harmful chemicals for transportation applications. Wind turbine blades protected with PVF films resist weathering, moisture, and UV rays have low drag and exceptional performance. PVF release films are ideal for transfer printing because they offer excellent printability and release cleanly from rubber and metal tubing during steam curing of rubber hoses and fan belts [4].

18.4 Development and Applications of Polyvinyl Fluoride—A Chronological Treatise One of the earlier plastics in the fluoropolymer family, and the simplest one, is PVF. VF was first prepared in 1901 by the reaction of zinc with

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Table 18.3 General Physical and Chemical Properties of Tedlar PVF Films [3]. Physical

Property

Typical Value

Test Method

Test Condition

Bursting strength

29 65 psi

Mullen, ASTMD-774

22°C (72°F)

Coefficient of friction (film/metal)

0.18 0.21

ASTM D-1894

22°C (72°F)

Density

1.37 1.72 g/cc

ASTM D-1505

22°C (72°F)

Impact strength

10 20 lb/mil

Spencer ASTM D-3420

22°C (72°F)

Moisture absorption

,0.5% for most types

Water immersion

22°C (72°F)

Water vapor transmission

9 57 g/m2 day

ASTM E-96

39.5°C, 80% RH

Refractive index

1.46 nD

ASTM D-542 Abbe Refractometer

30°C (86°F)

Tear strength

Chemical

Propagated

15 60 g/mil

Elmendorf-ASTM D-1922

22°C (72°F)

Initial (Graves)

260 500 g/mil

ASTM D-1004

22°C (72°F)

Tensile modulus

3

300 380 3 10 psi

ASTM D-882

22°C (72°F)

Ultimate tensile strength

8 16 3 10 psi

ASTM D-882

22°C (72°F)

3

Ultimate elongation

90% 250%

ASTM D-882

22°C (72°F)

Ultimate yield

6000 4900 psi

ASTM D-882

22°C (72°F)

Chemical resistance

No visible effect

1 year immersion in Acids

25°C (77°F)

Bases

25°C (77°F)

Solvents

25°C (77°F)

2 hours immersion in

Boiling

Acids

Boiling

Eases

Boiling

Solvents Strength and appearance not affected

Soil burial—5 years

—

Carbon dioxide

11.1 cc/(100 in2) (24 h)(atm)(mil)

ASTM D-1434

24°C (75°F)

Helium

150 cc/(100 in2) (24 h)(atm)(mil)

ASTM D-1434

24°C (75°F)

Gas permeability

2

Hydrogen

58.1 cc/(100 in ) (24 h)(atm)(mil)

ASTM D-1434

24°C (75°F)

Nitrogen

0.25 cc/(100 in2)(24 h)(atm)(mil)

ASTM D-1434

24°C (75°F)

3.2 cc/(100 in )(24 h)(atm)(mil)

ASTM D-3985

24°C (75°F)

45 g/(100 m2)(h)(mil)

ASTM E-96, modified

24°C (75°F)

Oxygen

2

Vapor permeability (at part, press, or vapor at given temp.) Acetic acid

2

Acetone

10,000 g/(100 m )(h)(mil)

ASTM E-96, modified

24°C (75°F)

Benzene

2

90 g/(100 m )(h)(mil)

ASTM E-96, modified

24°C (75°F)

Carbon tetrachloride

50 g/(100 m2)(h)(mil)

ASTM E-96, modified

24°C (75°F)

2

Ethyl acetate

1000 g/(100 m )(h)(mil)

ASTM E-96, modified

24°C (75°F)

Ethyl alcohol

35 g/(100 m2)(h)(mil)

ASTM E-96, modified

24°C (75°F)

Hexane

55 g/(100 m2)(h)(mil)

ASTM E-96, modified

24°C (75°F)

Weatherability

Excellent

Florida exposure

Facing south at 45° to horizontal

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Table 18.4 General Thermal and Electrical Properties of Tedlar PVF Films [3]. Thermal

Aging

3000 h

Circulating air oven

Heat sealability

Some varieties—see heat sealability technical bulletin

Linear coefficient of expansion

2.8 3 1025 in/in/F

Shrinkage (Type 2) MD and TD

4% at 130°C (266°F)

Air oven, 30 min

(Type 3) TD only

4% at 170°C (338°F)

Air oven, 30 min

(Type 4) TD only

2.5% at 170°C (338°F)

Air oven, 30 min

Temperature range

72 to 107°C ( 98 to 225°F)

Continuous use

Up to 175°C (350°F)

Short cycles or release (1 2 h)

260°C 300°C (500°F 570°F)

Zero strength Electrical

150°C (302°F)

Ho: Bar TTR20SG4

TWH20BS3

Corona endurance (h)

2.5

6.2

ASTM suggested T method

60 cPs, 1000 V/mil

Dielectric constant

8.5

11.0

ASTM D-150

1 Kc at 22° C (72°F)

Dielectric strength (kV/mil)

3.4

3.5

ASTM D-150

60 cPs, kV/mil

Dissipation factor (%)

1.6

1.4

ASTM D-150

1000 cPs, 22°C (72°F)

2.7

1.7

ASTM D-150

1000 cPs, 70°C (158° F)

42

3.4

ASTM D-150

10 Kc, 22°C (72°F)

2.1

1.6

ASTM D-150

10 Kc, 70°C (158°F)

4 3 1013

7 3 1014

ASTM D-257

22°C (72°F)

2 3 1010

1.5 3 1011

ASTM D-257

100°C (212°F)

Volume resistivity (ohm cm)

MD, Machine direction; TD, Transverse direction.

1,1-difluoro-2-bromoethane [359-07-9] [5]. The first polymerization involved heating a saturated solution of VF in toluene at 67°C under 600 MPa for 16 hours, which yielded brittle and friable products [6]. By 1945, all hydrogen halides such as vinyl chloride had been successfully polymerized to

obtain resins with useful properties, except for VF. None of the processes used for other vinyl halides yielded PVF with useful properties [7]. The resulting PVF polymer had limited solubility and a high softening temperature. These are the very characteristics of the product that eventually made it

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Figure 18.2 PVF films offer attractive, easy-to-clean, scuff-resistant surface protection to aircraft interiors.

commercialized. In the 1940s and 1950s, processes were developed to design commercial polymerization of VF. PVF with sufficiently high molecular weight to have useful properties was found to degrade prior to melting at 190°C [7 9]. Traditional melt processing requires a polymer to melt and remains relatively stable at temperatures above its melting point. Elevating the temperature above a polymer’s melting point reduces its melt viscosity, thus allowing processing at reasonable yields. Melt processing of PVF was ruled out because of severe susceptibility to thermal degradation. PVF required the development of an unusual process in which the polymer was dispersed in a latent solvent which allowed its melt processing without degradation [10]. This process resembles plastisol technology in which polyvinyl chloride (PVC) is mixed with a plasticizer before molding and processing. The difference between PVF and PVC processes is the solvent is removed from the PVF product, while the plasticizer is retained in PVC article. Development of processing technology for PVF allowed manufacturing of biaxially orientable films from this polymer [11]. In 1961, an integrated plant was constructed in Buffalo, New York, which imported VF and exported Tedlar PVF films. The majority of development work for Tedlar had taken place at this site. The same site was originally a production facility of Rayon and later the site of DuPont’s first cellophane manufacturing plant in 1924 [12]. The

heralded Yerkes Laboratory was the birthplace of film products such as Mylar, Kapton, Tedlar, and Clysar. Later, DuPont moved parts of this laboratory to its Experimental Station facility in Wilmington. Other parts were moved to locations such as Circleville, Ohio where a modern Mylar polyester manufacturing plant had been constructed. Some of the useful properties of Tedlar film included the possibility of treatment and functionality of its surface to accept adhesives. Introduction of fluorine reduces the surface energy of PVF films and parts (Table 18.5). Available commercial techniques such as corona and flame treatment were effective with PVF in contrast to perfluorinated polymers. Another desirable characteristic was the possibility of adding color pigment to the PVF, thus producing films with a variety of colors. Heat sealing method could also be applied to Tedlar to produce bags. Weatherability studies were conducted during the 1950s to determine whether PVF would prove to be a suitable replacement for paint on house sidings and roofs. These studies were conducted in South Florida as well as northern locations such as the plant location in Buffalo, New York. The films were laminated to a substrate and allowed to remain outdoors for years. Parallel studies were conducted in machines designed to “age” film samples in an accelerated manner while exposing it to conditions similar to natural settings. Periodically,

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Table 18.5 Surface Energy and Coefficients of Friction of Commercial Olefinic Fluoropolymers [13]. Fluoropolymers

Formula

Coefficient of Friction (Dynamic)

Critical Surface Tension, dyne/cm

Polyethylene

CH2 CH2

0.33

31

Polyvinyl fluoride

CH2 CH2

0.3

28

Polyvinylidene fluoride

CF2 CH2

0.3

25

Polytrifluoroethylene

CF2 CHF

0.3

22

Polytetrafluoroethylene

CF2 CF2

0.04

18

samples were removed from outdoor weathering stations and properties such as color, gloss, thickness, tensile strength, and break elongation were measured. PVF films proved to be weather-resistant by retaining most of their useful properties, including color as long as the pigment was durable. Not only Tedlar was weather resistance but it remained clean and did not collect dirt and grime. The chemical structure of Tedlar made it impermeable to dirt, oil, and grit. So, a good rainstorm will serve to keep it clean. In 1965, DuPont launched a Building Products Venture which included Tedlar [14]. The goal of the Venture was to make use of the company’s plastics, such as nylon, to develop a low maintenance house exterior, including roofing, siding, shutters, trims, windows, and doors [15]. The first major application of PVF film thus became a replacement for paint in the exterior of houses and building. It was available in a variety of colors which made it an attractive laminate for wood shutters, aluminum siding, and many other housing applications. Contrary to paint, Tedlar was said (to) last for decades without requiring maintenance. Successful entry as a finish for sidings and roofs drew a great deal of attention and interest. The April 15, 1965 issue of Popular Mechanic [16] describes aluminum fascia hangers with Tedlar cladding that takes the place of paint. A company by the name of Alsco supplied Tedlar clad aluminum sidings at the advertised price of $80 115 per 100 square feet [17]. In the early 1960s, wooden sidings clad with PVF films were developed by Georgia Pacific and US Plywood [18]. The September, 1965 issue of Popular Science [17] reported on achievement of board and batten effect with Weldwood flat panels and individual battens prefinished with Tedlar.

Weldwood was supplied by US Plywood which guaranteed the requirement of no painting for 15 years. Over time, the warranty for Tedlar on sidings was extended to 25 years. Even the venerable Life Magazine [19] printed an article about the new material in 1964. Ironically, it was in the same issue in which the front cover was graced by a photograph of the Beatles! “Nobody has yet worn Tedlar® out by exposure to weather even after years of testing. This makes the Welwood PF-15 a new way to protect and decorate your home. US Plywood bonds a film of Tedlar® to rugged plywood siding in the factory, so it becomes an inseparable, water proof finish. Ice may coat it, but won’t crack. Sunshine may bake it, but it won’t bubble or craze. However humid or salty or smoky or arid the cilmate may be, it won’t affect Tedlar®. Siding surfaced with Tedlar® resists fading, chalking, crazing, splitting, cracking and bubbling.” [19] Development of weather-resistant and thermally stable vinyl siding and paints led to the substitution of PVF clad sidings with those more economical alternatives. By the early 1970s, however, Tedlar applications had widened to include automobile trim. Ten years later, it was being used as a surface laminate for flexible architectural structures like tents, canopies, outdoor pavilions, and covered sports arenas [20]. In the 1980s, a quantity of PVF was sold as a powder which was converted into a paint for coating of automotive breaklines, and to a minor degree, fuel lines. A coating of PVF protected the lines against salt and other corrosion which could develop leakage with severe risk to the car and its occupants. The advent of PCB opened a large volume of new end-use for PVF as a release film. Low surface energy, mechanical strength, chemical resistance, and thermal stability make Tedlar films an

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excellent candidate as a release ply. PVF films release from epoxies, phenolics, polyesters, copper, and caul plate surfaces. PVF films are used to prevent blocking in the PCB manufacturing processes. Another release application is in the manufacturing of composite parts for aerospace parts. Tedlar was first used in aircraft interiors in 1963 with the launch of the Boeing 727. Tedlar remains the predominant surface on sidewalls, ceiling panels, and stow bin doors in Boeing and Airbus aircraft today. In the early 1990s, Tedlar surfacing appeared on flexible outdoor signs, banners, and awnings. Tedlar continued entry and expansion into new applications such as airplane and train interior surfaces, truck trailer sides, and building panels [19]. In 1986, new stringent flammability and toxicity regulations (FAR 25.853) were promulgated for aircraft cabin interior materials by Federal Aviation Administration. The new rules led to the replacement of PVC with PVF films and laminates. A major product variance is Tedlar SP film which is entirely unoriented and is manufactured in Buffalo, NY and in Japan. SP film is manufactured in several facilities in the USA. The translucent quality, useful for backlight displays, offers an alternative to the choices of transparency or opaqueness with Tedlar. To bond Tedlar requires an adhesive, but Tedlar SP can be applied in multiple layers without any adhesive or heat sealing. Both varieties come in a wide range of colors and glosses. In the 2000s, rapid growing application of PVF films resumed in photovoltaic cells. This is indeed a large resurrection of the same applications only at a greater scale than when it disappeared after expiration of tax benefits in the 1980s. PVF films represent the industry standard for UV and weatherresistant backsheets for photovoltaic modules. Tedlar is offered in standard colors of clear, white, black, and custom colors as a backsheet. The primary functions of a solar backsheet include vapor barrier, physical protection of the wiring, and other sensitive components, electrical insulation, and reduction of cell operating temperatures. The most popular backsheet construction is a trilaminate “sandwich” of polyester films between two layers of Tedlar films. This is commonly referred to as TPT [21]. The backsheet and encapsulant series PV2100, PV2400, PV5200, PV5300, PV5400, PV8600 encapsulant, and PV2111 (a clear-capped white

film) are made by SP technology. As a result of the growth in the solar market, DuPont was expected to triple its capacity of Tedlar products [22]. In 2012, DuPont announced the completion of a $295 million capacity expansion that more than doubled Tedlar production capacity [23].

References [1] Ebnesajjad S. Polyvinyl fluoride-technology and applications of PVF. 1st ed Elsevier; 2013. [2] Ebnesajjad S. Vinyl fluoride polymers. Encylopedia of Polymer Science and Technology. online edition John Wiley & Sons, Inc; 2011. [3] DuPont Tedlar® polyvinyl fluoride (PVF)— general properties. No. H-49725-5. DuPont; 2014. [4] Tedlar® PVF films for industrial applications, www.dupont.com; 2018. [5] Swarts F. Bull Clin Sci Acad Roy Belg 1901;7:383 Swarts F. J Chem Soc Abstr 1902;82:129. [6] Starkweather HW. J Am Chem Soc 1934;56:1870. [7] Coffman DD, Ford TA. US Patent 2,419,008, to E. I. du Pont de Nemours & Co., Inc.; 1947. [8] Johnston FL and Pease DC. Patent 2,510,783, to E. I. du Pont de Nemours & Co., Inc.; 1950. [9] Upton RW. US Patent 2,599,300, to E. I. du Pont de Nemours & Co., Inc.; 1952. [10] Bechteld MF, Bro MI. US Patent 2,810,702, to E. I. du Pont de Nemours & Co., Inc.; 1957. [11] Prengle RS, Richards RL Jr. US Patent 3,139,470, to E. I. du Pont de Nemours & Co., Inc.; 1964. [12] www2.dupont.com/Heritage/en_US/related_topics/buffalo_ny.html; 2011. [13] Zissman WA. Influence of construction on adhesion. Ind Eng Chem 1963;55:18 38. [14] Hounshell DA, Smith JK. Science and corporate strategy: Du Pont R&D, 1902 1980. Cambridge: Cambridge University Press; 1988. [15] Modern Plastics Magazine, 39; 1962. [16] Maher AI. Remodeling made easy. Popular Mechanics 1965;128 33.

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[17] Treves R. Those amazing new sidings with built-in paint. Popular Science 1965;187 (3):130. [18] Ingersoll JH. Big ideas in remodeling materials. Popular Science 1963;183(3):126. [19] Life magazine, 16, 57, 9; 1964. [20] www2.dupont.com/Phoenix_Heritage/en_US/ index.html; 2012. [21] DuPont backsheet offerings. Publication # K23268-1. DuPont Photovoltaic Solutions; 2010.

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[22] Hilde Roekens-Guibert. Next generation Tedlar® PVF films for photovoltaic module backsheets. EnergyAgency.NRW (Energy Agency of North Rhine-Westphalia), www. energieagentur.nrw.de/pv/workshop2007/ 5Roekens_PVF.pdf; 2007. [23] DuPont Press Release. DuPont celebrates completion of $295 million Tedlar® PVF film expansion project to supply growing solar energy industry; 2012.

19 Fluorinated Coatings; Technology, History, and Applications Laurence W. McKeen Senior Research Associate, DuPont Fluoroproducts, Retired

O U T L I N E 19.1 Introduction

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19.2 Fluoropolymers Used in Coatings 19.2.1 Polytetrafluoroethylene 19.2.2 Fluorinated Ethylene Propylene Copolymer 19.2.3 Perfluoroalkoxy Polymers 19.2.4 Ethylene-Tetrafluoroethylene Copolymers 19.2.5 Polyvinylidene Fluoride 19.2.6 Ethylene-Chlorotrifluoroethylene Copolymer

340 340

19.3 Fluorocoating Compositions 19.3.1 Fluoropolymer 19.3.2 Pigments and Fillers 19.3.3 Solvents 19.3.4 Additives 19.3.5 Nonfluoropolymer Binders

342 343 343 344 344 345

19.4 Liquid and Powder Coatings 19.4.1 Liquid Coatings 19.4.2 Powder Coatings

347 347 347

19.5 Application of Fluorocoatings 19.5.1 Substrate 19.5.2 Liquid Coating Application 19.5.3 Powder Coating Application 19.5.4 Baking/Curing

348 348 349 350 351

19.6 Commercial Fluorocoating Producers

352

341 341 341 342 342

19.1 Introduction Fluoropolymer coatings are widely used in many industries, though the consumer and many engineers and scientists are only aware of their use as nonstick coatings for cookware. This chapter will first provide an introduction to fluoropolymer finish technology and market developments. It is a greatly condensed

19.7 A Historical Chronology of Fluoropolymer Finish Technology 353 19.8 Food Contact

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19.9 Commercial Applications of Fluorocoatings 358 19.9.1 Housewares—Cookware, Bakeware, Small Electrical Appliances 358 19.9.2 Commercial or Industrial Bakeware 358 19.9.3 Fuser Rolls 359 19.9.4 Light Bulbs 360 19.9.5 Automotive 361 19.9.6 Chemical Processing Industry 363 19.9.7 Chemical Reactors 363 19.9.8 Ducts for Corrosive Fumes, Fire Resistance 364 19.9.9 Commercial Dryer Drums 364 19.9.10 Industrial Rollers 365 19.9.11 Medical Devices 365 19.9.12 Oil Production and Refining 366 19.9.13 Razor Blade Coatings 366 19.9.14 Architectural Coatings 366 19.10 Summary

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References

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summary of a previous book by this author [1]. Starting with a summary of the fluoropolymers used in finishes and their properties, an overview of liquid and powder technology follows. This will include processing considerations. Next is a summary of the history of fluoropolymers in finishes. Some guidance on food contact end-use requirements follows. The rest of the chapter will show a number of sample

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00019-7 © 2021 Elsevier Inc. All rights reserved.

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end-uses. Each end-use will summarize what type of coatings were used and what properties of fluorofinishes were required.

19.2 Fluoropolymers Used in Coatings Traditionally, a fluoropolymer or fluoroplastic is defined as a polymer consisting of carbon (C) and fluorine (F). Sometimes, these are referred to as perfluoropolymers to distinguish them from partially fluorinated polymers, fluoroelastomers, and other polymers that contain fluorine in their chemical structure. For example, fluorosilicone and fluoroacrylate polymers are not referred to as fluoropolymers. Fluoropolymers have many desirable properties that can be imparted to coatings that use them. The most important properties are

• • • • • •

Release/nonstick; thermal stability; chemical resistance even at high temperature; dry lubrication, slip; abrasion resistance; electrical insulation. It is important to understand the basic properties of fluoropolymers when learning about coating properties. The polymer properties are retained to varying degrees in the formulated finishes. The fundamental properties of fluoropolymers evolve from the atomic structure of fluorine and carbon and their covalent bonding in specific chemical structures. The available types of fluoropolymers available for paint formulation are fortunately varied. The primary fluoropolymers are

• Polytetrafluoroethylene (PTFE); • fluorinated ethylene propylene

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19.2.1 Polytetrafluoroethylene An example of a linear fluoropolymer is the tetrafluoroethylene polymer (PTFE) discovered by Roy Plunkett in 1938 [2]. Formed by the polymerization of tetrafluoroethylene (CH2 5 CH2, TFE), the (CF2 CF2 ) groups repeat many thousands of times. Its structure in simplistic form is shown in Fig. 19.1. The basic properties of PTFE stem from the two very strong chemical bonds, carbon carbon and carbon fluorine. The size of the fluorine atom allows the formation of a uniform and continuous covering around the carbon carbon bonds and protects them from attack, thus imparting chemical resistance and stability to the molecule. PTFE is rated for use up to 500°F. PTFE does not dissolve in any common solvent. The fluorine sheath is also responsible for the low surface energy (18 dynes/cm) and low coefficient of friction (0.05 0.8, static) of PTFE. Another attribute of the uniform fluorine sheath is the electrical inertness (or nonpolarity) of the PTFE molecule. Electrical fields impart only slight polarization in this molecule, so volume and surface resistivity are high. PTFE is called a homopolymer, a polymer made from a single monomer. Recently, many PTFE manufacturers have been adding very small amounts of other monomers to their PTFE polymerizations to produce alternate grades of PTFE designed for specific applications. Generally, polymers made from two monomers are called copolymers, but fluoropolymer manufacturers still call these grades homopolymer. Chemours grades of this type are called Teflon NXT Resins. These modified granular PTFE materials retain the exceptional chemical, thermal, antistick, and low-friction properties of conventional PTFE resin, but offer some improvements: 1. Improved permeation resistance; 2. less creep; 3. smoother, less porous surfaces;

(FEP)

4. better high-voltage insulation.

copolymer;

• perfluoroalkoxy (PFA) polymers; • ethylene-tetrafluoroethylene

(ETFE)

copolymers;

• polyvinylidene fluoride (PVDF); • ethylene-chlorotrifluoroethylene copolymer.

(ECTFE)

Figure 19.1 The structure of polytetrafluoroethylene.

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341

19.2.2 Fluorinated Ethylene Propylene Copolymer If one of the fluorine atoms on tetrafluoroethylene is replaced with a trifluoromethyl group ( CF3), then this monomer is called hexafluoropropylene (HFP). Polymerization of monomers (HFP) and TFE yield a different fluoropolymer called FEP. The number of HFP groups in the polymer chain is typically 5% or less. The effect of addition of the small amount of copolymer is to put a “bump” along the polymer chain as shown in Fig. 19.2. This disrupts the crystallization of the FEP, which has a typical as-polymerized crystallinity of 70% versus 98% for PTFE. It also lowers its melting point. The reduction of the melting point depends on the amount of trifluoromethyl groups added and secondarily on the molecular weight, but most FEP resins melt around 274°C (525°F), though lower melting points are possible. Even high molecular weight FEP will melt and flow. The high chemical resistance, low surface energy, and good electrical insulation properties of PTFE are retained.

19.2.3 Perfluoroalkoxy Polymers Making a more dramatic change in the side group, chemists put a PFA group on the polymer chain. This group is signified as O Rf, where Rf can be any number of totally fluorinated carbons. A typical one is perfluoropropyl ( O CF2 CF2 CF3). These polymers are called PFA and the perfluoroalkylvinylether group is typically added at only a couple mole percent. The structure is shown in Fig. 19.3. Another common a PFA group is perfluoromethylvinylether ( O CF3) making a polymer called MFA. The large side group reduces the crystallinity drastically. The melting point is generally between 305°C and 310°C (581°F 590°F) depending on the molecular weight. The melt viscosity is also dramatically dependent on the molecular weight. Since PFA is still perfluorinated as with FEP, the high chemical resistance, low surface energy, and good electrical insulation properties are retained.

Figure 19.3 The structure of perfluoroalkoxy.

19.2.4 EthyleneTetrafluoroethylene Copolymers ETFE is a copolymer of ethylene (CH2 5 CH2) and tetrafluoroethylene (see Fig. 19.4). This simplistic structure shows alternating units of TFE and ethylene. While this can be readily made, many grades of ETFE vary the ratio of the two monomers to optimize properties for specific end uses. ETFE is a fluoroplastic with excellent electrical and chemical properties. It also has excellent mechanical properties. ETFE is especially suited for uses requiring high mechanical strength, chemical, thermal, and/or electrical properties. The mechanical properties of ETFE are superior to those of PTFE and FEP. ETFE has

• excellent resistance to extremes of temperature, ETFE has a working temperature range of 2200°C to 150°C.

• excellent chemical resistance. • mechanical strength ETFE is good with excellent tensile strength and elongation and has superior physical properties compared to most fluoropolymers.

• with low smoke and flame characteristics, ETFE is rated 94V-0 by the Underwriters Laboratories Inc. It is odorless and nontoxic.

Figure 19.2 The structure of fluorinated ethylene propylene.

• outstanding resistance to weather and aging. • excellent dielectric properties. • nonstick characteristics. One of the trade names of ETFE is Tefzel.

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Figure 19.4 The tetrafluoroethylene.

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structure

of

ethylene-

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Figure 19.5 The structure of polyvinylidene fluoride.

19.2.5 Polyvinylidene Fluoride The polymers made from 1,1-di-fluoro-ethene (CF2 5 CH2, or vinylidene fluoride) are known as PVDF. One of the trade names of PVDF is KYNAR. They are resistant to oils and fats, water and steam, and gas and odors, making them of particular value for the food industry. PVDF is known for its exceptional chemical stability and excellent resistance to ultraviolet radiation. It is used chiefly in the production and coating of equipment used in aggressive environments, and where high levels of mechanical and thermal resistance are required. It has also been used in architectural applications as a coating on metal siding where it provides exceptional resistance to environmental exposure. The chemical structure of PVDF is shown in Fig. 19.5. The alternating CH2 and CF2 groups along the polymer chain provide a unique polarity that influences its solubility and electric properties. At elevated temperatures, PVDF can be dissolved in polar solvents such as organic esters and amines. This selective solubility offers a way to prepare corrosion-resistant coatings for chemical process equipment and long-life architectural finishes on building panels.

19.2.6 EthyleneChlorotrifluoroethylene Copolymer ECTFE is a copolymer of ethylene and chlorotrifluoroethylene (CClF 5 CF2). Fig. 19.6 shows molecular structure of ECTFE. This simplified structure shows the ratio of the monomers being 1to-1 and strictly alternating, but this is not required. Commonly known by the trade name, Halar, ECTFE is an expensive, melt processible, semicrystalline, whitish semiopaque thermoplastic with good chemical resistance and barrier properties. It

Figure 19.6 The structure chlorotrifluoroethylene.

of

ethylene-

also has good tensile and creep properties and good high-frequency electrical characteristics.

19.3 Fluorocoating Compositions While there are relatively few commercial fluoropolymers, there are thousands of fluoropolymer paint formulations. Nearly all fluoropolymer coatings come in two forms. They are either dry powders or liquids. Liquid coatings can be aqueous (water-based) or nonaqueous (solventbased). The coatings can be one-coat or multicoat systems. They may also be thick and thin film ranging from 0.1 mils (2.5 µm) to 80 mils (2000 µm). These coatings may have fluoropolymer content that runs the full range of 0 100%. The coatings are typically baked and the temperatures are high, 350°F 830°F (176°C 443°C). They are applied by many different methods, not just liquid or powder spraying. The bake schedule along with coating thickness, substrate preparation, and production can affect final coating performance. Fluorocoatings are often complex mixtures:

• Fluoropolymer(s): types, molecular weight, blends;

• Pigments: appearance and function (abrasion, porosity, conductivity);

19: FLUORINATED COATINGS; TECHNOLOGY, HISTORY, AND APPLICATIONS

• Solvents: evaporation rates, rheology, surface tension (powder solvents);

coatings

do

not

have

• Additives: film formation, rheology, catalysis, wetting;

• Non-FP binder(s): adhesion, hardness. The paint chemist’s science (though often it is referred to as art) is the selection of the ingredients, their ratios, and how to put them together into a stable product that meets performance goals. It also includes how to apply them and how to bake them. Before that is discussed it is important to know how the various ingredients are used in fluorocoatings. Coatings and paints always have what formulators call a binder. A binder is generally a polymeric material that is solid, or becomes solid, and forms the paint film. The polymeric material generally is classified as thermoset or thermoplastic. Thermoplastic coatings typically melt when reheated, whereas thermosets undergo a chemical reaction during curing that prevents remelting. Unless the coating is a dry powder, the binder is a liquid called the solvent or carrier. Common binders found in household products include materials like acrylics, alkyds, epoxies, or urethanes. Fluoropolymers are usually binders, though they can also be thought of as fillers or additives in some applications. The binder is dissolved, dispersed, or suspended in the solvent. The solvent is usually a mixture. It liquefies the other paint components allowing them to spread out over and wet the substrate being coated. Water is considered an important solvent. Several terms are common in the paint industry. These include medium, vehicle, and carrier. The generally accepted definitions are as follows:

• Vehicle is the liquid portion of paint. The vehicle is composed mainly of solvents, resins, and oils.

• Carrier usually refers to the solvent. • Medium is the continuous phase in which the pigment is dispersed; it is synonymous with vehicle. Liquid coating compositions have properties that are important during application. The viscosity is

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critically important. Viscosity in its simplest definition is the resistance of a liquid to flow or, as the American Heritage Dictionary puts it, “The degree to which a fluid resists flow under applied force.” Viscosity depends upon the shear rate and stress applied during application and the temperature. This means that the viscosity will change depending upon how the coating is applied. Therein lies a key to using and understanding coatings. The viscosity varies with how the coating is used. Actually, viscosity also affects how coatings are manufactured, how they are stored, how they are prepared for use, and how long their shelf life is.

19.3.1 Fluoropolymer Fluoropolymers for use in coatings are available in several forms. PFA, FEP, and PTFE are made in aqueous dispersion form. The aqueous dispersions are typically modified after production. Raw dispersion (as produced) can be unstable and may require stabilization with extra surfactant. Raw dispersions may be too low in solids so they are frequently concentrated. Dispersion can be coagulated, filtered, and dried to make solid fluoropolymer called fluff. The fluff can be processed to bead or cube, which may need heat treatment and grinding to get fluoropolymer powder in the right size for use in coatings. ETFE, FEP, and PFA powders are available in various sizes made by grinding processes. PTFE cannot be ground in its high molecular weight form, it will fibrillate. However, if it is electron beam irradiated to reduce the molecular weight it may be ground producing PTFE micropowders. There are many micropowder grades available that may be used in coatings, inks, and as additives to other polymers. Details on production, modification, concentration, and irradiation may be found in other books in the PDL Series [3,4].

19.3.2 Pigments and Fillers Pigments, fillers, and extenders are essentially insoluble fine particle sized solids that are added to a paint formulation. The pigments are usually added to a fluoropolymer coating for appearance, such as for color or sparkle. Dyes, which are soluble colorants, are used in some coatings applications, but not usually in fluoropolymer coatings.

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Most dyes are organic and would decompose during processing at elevated temperatures. Fillers are typically added to a coating to reduce cost, or for performance enhancement. Performance enhancements might include properties such as permeability, abrasion resistance, and electrical or thermal conductivity. The difference between pigments and fillers is not always clear-cut. One might also consider a powdered fluoropolymer as filler, particularly if the coating is not processed above the melt point of the fluoropolymer. Pigments and fillers usually must be thermally stable because of the processing conditions of fluorocoatings and so are mostly inorganic. They are chosen 1. For appearance—this is the most obvious reason for adding pigment, but appearance factors besides color, include hiding, roughness, or gloss. 2. To alter rheological properties—to produce thixotropy or pseudoplasticity. 3. For economy—to reduce the cost by adding inexpensive ingredients. 4. As carrier for active materials: a. Anticorrosive—to protect the substrate from chemical or environmental attack. b. Antibacterial—to prevent bacteria from growing on the surface. c. Fireproofing—to provide fire resistance. d. Electrical conductivity—to dissipate static electricity or provide electromagnetic shielding. e. Ultraviolet protection—to protect against damage from the sun. 5. Reduce permeability. 6. Reinforcement—to improve physical properties such as strength and abrasion resistance of the coating. Pigments and fillers can be added to a liquid coating directly, or “stirred in” as is commonly referred to in the paint industry. However, pigments as purchased are usually supplied as a filter cake or as a dry powder. The individual particles usually consist of clumps of particles, often called agglomerates. Before adding pigments to paints or coatings, the agglomerates of pigment are separated and sometimes made finer by breaking crystals, by a process called grinding or dispersion. Details of the

INTRODUCTION

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grinding and dispersion process are available in other literature [1].

19.3.3 Solvents Solvent systems are critically important. They are frequently complex and carefully designed by the formulator. They

• Are the carrier of the solids via dispersion or solution for liquid application methods;

• • • •

Are almost always mixtures; Water if used is part of the solvent system; Must wet and flow out over the substrate; Surface tension and viscosity are important properties;

• Evaporate, so their boiling point and vapor pressures are important;

• Flashpoint and safety. One should keep in mind that a mixed solvent system’s composition changes with time after application as more volatile components evaporate first. Because of this, surface tension, viscosity, and solvency change with time. This can cause quality problems or application difficulties.

19.3.4 Additives Additives are chemicals added to paints, usually in small amounts to achieve specific effects or solve specific problems. They are often called “snake oils.” These additives are usually a minor, but important part of a paint formulation. They are typically present at less than 5% by weight, but often at a fraction of 1%. Additives include

• • • • • • • • • •

Surfactants, which help stabilize dispersions; viscosity agents; defoamers; surface modifiers; thermal and UV stabilizers; wetting agents; catalysts; sacrificial film formers; adhesion promoters; dry flow agents in powders.

19: FLUORINATED COATINGS; TECHNOLOGY, HISTORY, AND APPLICATIONS

19.3.5 Nonfluoropolymer Binders The primary function of nonfluoropolymer binders in fluorocoatings is to provide adhesion to the substrate. The nonstick character of fluorocoatings is well known, so the problem with getting adhesion to the substrate is expected. Adhesion is related to absorption of binder molecules to that surface. Chemical bonds between the binders and the substrate will increase adhesive strength. However, to form a good adhesive bond, the molecules of the binder must reach and wet the surface. Wetting the surface means the binder spreads out to cover the surface completely. Many fluoropolymer one-coats and primers are blends of binders. Since the fluoropolymers do not both dissolve in the solvents used in the paints, the fluoropolymers are more like small particles distributed throughout the liquid paint. However, when the paint is baked above the melting point of the binders, the binder molecules are fluid and may intimately mix. If the polymer chains intertwine to a very large extent, the mixture is sometimes referred to as an interpenetrating polymer network. It forms a homogenous mixture of the two binder resins. Depending on the desired paint properties, this may or may not be desirable. Referring to Fig. 19.7, the adhesive resin and fluoropolymer particles within start out well mixed and they are applied as a liquid paint to the

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substrate. When the coating dries it shrinks the thickness as shown in the second part of the figure. When the temperature is raised about the melt points of the adhesive and fluoropolymer, stratification may occur as shown in the bottom part of the figure. The cured coating has a high concentration of fluoropolymer on the surface and a high concentration of the adhesive binder at the substrate. The area in between would be a gradient. This is called stratification and is the basis of many one-coat products and primers. DuPont’s early products using this principle were called Teflon-S. The surface of the finish behaves like a pure fluoropolymer, and the material contacting the substrate behaves like an adhesive. The polymeric binder materials are used for adhesion and pigment dispersion, but they also contribute to

• • • •

Barrier properties; electrical properties; durability; chemical and thermal resistance.

19.3.5.1 Polyamide-imide Polyamide-imide (PAI) is one of the most common and most important binder materials. It is the basis for nearly all the cookware primers. The highly aromatic molecule when cured has very

Figure 19.7 Stratification in fluoropolymer resin binder coatings and primers.

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INTRODUCTION

high thermal stability and can bind strongly to most metal substrates. Its typical structure is shown in Fig. 19.8. PAIs are thermoplastic amorphous polymers that have useful properties:

• • • • • • •

Very stable thermally; outstanding adhesion to metals; insoluble when fully cured; good wear resistance; darkly colored; attacked by strong bases; used in solvent and water-based coatings.

PAI resins are used in liquid forms in most coatings. They are often dissolved in dipolar aprotic solvents such as N-methyl-2-pyrrolidone or Nethyl-2-pyrrolidone. PAI resins are also available in water-based solvent systems.

19.3.5.2 Polyether Sulfone Polyether sulfone (PES) is an amorphous, transparent, and pale amber high-performance thermoplastic and is the most temperature-resistant transparent commercially available thermoplastic resin. It has relatively high water absorption. Stable solutions can be made if solvents are correctly chosen. The most common PES structure is shown in Fig. 19.9. Properties of PES include

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19.3.5.3 Polyphenylene Sulfide Polyphenylene sulfide (PPS) is a semicrystalline high-performance thermoplastic. It has low water absorption and is resistant (and so it is insoluble) to all organic solvents even at elevated temperature. It has a relatively low melting point of 285°C. It has a very low melt viscosity and so flows out well. It is generally strongly colored, so only dark colors can be made with this polymer. It adheres well to most metals. The PPS structure is given in Fig. 19.10. Properties of PPS include

• Semicrystalline; • insoluble in all common solvents; • very chemically stable except for strong oxidizers;

• dark colored at high bakes; • thermally stable but low melting point (465°F); • low melt viscosity. There are many other nonfluoropolymer binders used including

• • • •

Epoxies; chromic acid/phosphoric acid primer; silica (Sol-Gel); polyimide and polyetherimide;

• Excellent thermal resistance; • outstanding mechanical, electrical, flame, and chemical resistance;

• good optical clarity; • only material for light colors; • use up to 220°C.

Figure 19.8 Structure of a typical polyamide-imide.

Figure 19.9 Structure of a typical polyethersulfone.

Figure 19.10 Structure of a typical polyphenylene sulfide.

19: FLUORINATED COATINGS; TECHNOLOGY, HISTORY, AND APPLICATIONS

• polyether ether ketone and polyaryl ether ketone;

• polysulfone and polyphenyl sulfone.

19.4 Liquid and Powder Coatings The previous section covered most of the individual components that go into fluorinated coating formulations, coatings, paints, or products. Putting these components together to make a good product takes experience and experimentation, to say nothing of serendipity, which always helps. Most users of fluorinated coatings rely on someone besides themselves to select the particular product for their end-use. Many applicators of the fluorinated coatings rely on the manufacturers of products for guidance. The selection of a particular coating is based on 1. The desired function or properties that the coating will provide to the end-user; 2. the type of substrate and temperature limits (bake); 3. the application method; 4. environmental, health, and safety issues.

19.4.1 Liquid Coatings Liquid coatings are often blended in a tank with a mixing blade. The loading of the tank with ingredients is not a simple as one might imagine. 1. What ingredients to add and in what order. The order of addition of ingredients can be critically important. Sometimes just swapping the order of two ingredients can be the difference between a good product and scrap. There are numerous chemical reasons for these situations to occur. Often the mixture is more shear-sensitive with one order of addition than another. Surfactants may not wet the particles they are intended to wet if they are not added at the right time. 2. How much of each ingredient to add. The amount of each ingredient is usually defined by weight, but occasionally it is by volume. 3. How to add. Sometimes materials are added by flowing down the side of the mixing tank,

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or through a dip tube to the mixture surface. It could even be pumped in from the bottom of the tank. This is a common practice for aqueous coatings because it minimizes foaming. 4. How fast to add. Each raw material is added to the mill at a specific rate. Some materials being added may be incompatible with the coating when added too quickly. The localized concentration builds up faster than mixing can take place. Adding slowly limits concentration build-up by allowing the mixer to dilute the ingredient that has the potential to cause a problem. Adding powders slowly is necessary to avoid having them clump and become more difficult to break apart and wet. 5. How to mix. There is usually a specific mixing blade in the equipment. The RPM is specified. Mixing too quickly can create foam or can overshear the coating causing the formation of gel particles. If mixed too slowly, the mixture may not be uniform. The mixer RPM is changed after each addition. 6. How long to mix. Overmixing can create too much foam or generate too much grit and gel; undermixing may result in nonuniformity. The liquid paint batch always starts with liquids. Solids can be added after enough liquid has been added to fill the tank above the mixing blade. Some solids are dissolved into the liquids in the tanks. Solids such as pigments or fluoropolymer micropowders are usually made into dispersion with specialized milling equipment to break apart the particles making them smaller. Surfactants and dispersants are added to the dispersion to make them stable enough to store and use at a later time.

19.4.2 Powder Coatings Powder coatings are similar in composition as liquid coatings, they just lack the solvent. They become viscous liquids when they are heated at a high enough temperature enough to melt. Powder particle diameters in powder coatings are typically 30 60 µm (1.2 2.5 mils). Powder coatings are often pure fluoropolymers but formulated powders are also common. The formulated powders are produced in a number of ways.

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The simplest is to dry blend the solid ingredients. This can be more difficult than it sounds, as segregation results when particles separate due to differences in their size, shape, or density. However, equipment, such as ribbon blenders, tumblers, and tube blenders, are well used in industry, and in many cases, blend uniformity is regularly achieved. Unfortunately, segregation can readily occur during blender discharge or during routine handling. A better approach is heat the powder mixture to melt it and blend the melt to make the mixture uniform. The melt is then extruded into thin sheet, ribbon, or rod which are chilled and broken into chips. The chips are then ground into a powder of the appropriate size. The solids ingredients can also be mixed in a liquid. For example, pigments and additives can be mixed with aqueous fluoropolymer dispersion. That mixture can be coagulated, dried, heat treated, and ground to particle size. The liquid mixture can also be spray-dried.

19.5 Application of Fluorocoatings This section will include sections on substrates and their preparation for coating, the application of both liquid and powder coatings, and baking or curing.

19.5.1 Substrate Any substrate, which is dimensionally and thermally stable at the bake temperature required for the bake of a particular product, can be coated with fluoropolymer coatings. Adhesion to that substrate, however, needs to be confirmed. A variety of commercial substrates are coated with fluoropolymer coatings including those listed:

• • • • • • •

Various ferrous alloys; cast iron; steel; stainless steel; treated steel such as tin-plate or galvanized; (zinc); nonferrous metals and alloys;

• • • • • •

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aluminum; cast aluminum; polymeric materials; high-temperature plastics and elastomers; glass, pyroceram, and ceramics; stone.

At bake temperatures greater than 232°C (448°F), certain metallic substrates are unacceptable. The melting points of tin, 232°C (448°F), and lead, 328° C (622°F), are too low to permit bakes required for fluoropolymer coatings. Zinc melts at 419°C (787°F) which is below the processing temperature of many fluoropolymer coatings. Poor adhesion to copper and its alloys is the result of the copper oxide formed when copper is baked in air at high temperatures. Because of reactivity at the high baking temperatures, fluoropolymer coatings have relatively poor adhesion to magnesium and to aluminum/magnesium alloys containing more than 0.5% magnesium. Aluminum permanent mold castings and die castings are successfully coated with fluoropolymer coatings, but may show a high reject rate due to the formation of blisters caused by expansion of air bubbles in the metal during the high-temperature bake. Substrate preparation is aimed at several purposes:

• Cleaning—Normal industrial practices such as chemical washes, or solvent cleaning, and vapor degreasing can be used, but precautions must be taken to remove all residues from the cleaning process. Preheating metal substrates above the bake temperature required for the fluoropolymer coating is a good way to remove traces of oil and other contaminants, especially when the metal is formed by casting and is porous.

• A chemical etch using acids such as phosphoric acid, hydrochloric, or sulfuric acid or bases such as sodium hydroxide solutions can clean and etch the substrate.

• Improving adhesion by increasing surface area—Grit blasting is the method most commonly used to obtain good adhesion of fluoropolymer coatings. The process both cleans and roughens the surface to increase surface area.

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Hard particles such as aluminum oxide, silicon carbide, sand or metal shot are sprayed at the substrate. Grit blasting should precede preheating of ferrous metals to retain the protective oxide formed. With other clean substrates, the order of these two operations is not important.

• Conversion coatings are the modified surfaces of metals resulting from specific chemical treatment. These conversion coatings, which can be applied on steel, aluminum, or most other metals, typically include zinc, manganese, and iron phosphates or chromates. The process usually involves a series of dips or sprays of the item to be coated.

19.5.2 Liquid Coating Application The most common application of fluoropolymer coatings is conventional spray. It is used across many industries and technologies. Liquid paint is atomized by high-pressure air, typically 20 60 lb/ in2 (1.4 4.2 kg/cm2 ), escaping through a narrow orifice. An exploded view of a typical convention spray gun is shown in Fig. 19.11. But as simple as the technique seems to be, the theory of spraying is not well developed, and the entire process is quite complex. A stream of the liquid paint is directed into a fast-moving stream of compressed air. The velocity of the air approaches the speed of sound. The air stream elongates the stream of liquid into thin threads or sheets. The threads break spontaneously into droplets, driven by surface tension. The process of droplet formation is affected by viscosity and by elastic forces if the liquid contains dissolved polymers. The droplet size increases with decreasing air pressure (air velocity) and with increased flow rate of liquid paint. Usually the weight of air pumped through a spray gun is about equal to that of the liquid, but the volume of air is much larger. The paint atomization occurs within a centimeter or so from the spray gun. The shear rates are great at this point and the liquid droplets are quite fine. Solvent composition of the droplets changes rapidly due to evaporation from the high surface area of the finely atomized droplets. The concentrations of low boiling solvents can be dramatically reduced compared to the bulk paint composition. The coating droplets are usually cooled by evaporative cooling. The momentum of air and liquid leaving the

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gun is transferred to the relatively motionless ambient air, creating a turbulent mixture. The mixture continues to travel in the direction the gun is aimed but its forward velocity falls off rapidly and carries more and more air as the distance from the gun increases. The turbulence grows more intense and the velocity of the atomized cloud decreases as the distance from the gun increases. Spray guns often have auxiliary nozzles (called the “fan”) which shape the atomized cloud. The auxiliary nozzles have minimal effect on the dimensions of the atomized liquid droplets. Automatic guns that are used in high volume applications have a similar design except that the handle is eliminated and the trigger is operated automatically. This application method offers many advantages:

• It is very common and very flexible; • it is inexpensive; • it offers “easy” application of thin films. Its primary disadvantage is that overspray (paint that does not deposit on the substrate) is severe, resulting in low transfer efficiency. Typically, less than 40% of the liquid paint deposits on the substrate, leaving 60% as waste. To improve application or transfer efficiency, a system called high-volume, low-pressure (HVLP) atomization is used. Instead of using a small amount of high-pressure air to atomize the paint, large amounts of low-pressure air are used. A “sonic venturi” converts high-pressure compressed air to low pressure. Typically, the air pressure for the atomizing air is 5 10 psi (0.35 0.7 kg/cm2 ). Advantages offered by HVLP application include

• • • • •

Less overspray; less atomization; higher transfer efficiency, but wetter films; meets California Air Quality Standards; easier application of moderately thick films.

Disadvantages include

• Sometimes it is difficult to apply thin films; • less shear during atomization can lead to appearance differences.

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Figure 19.11 Exploded schematic and photo of a conventional air atomization hand-held spray gun (photo by Bersch & Fratscher GmbH, via Wikimedia Commons).

Electrostatic coating is often used to increase the efficiency of liquid coating. When a fine-wire or fine-point high-voltage electrode is placed near a stream of liquid (in a spray gun as described earlier), the liquid is broken down into fine droplets. The droplets are electrically charged with the same polarity as the fine wire or fine point. However, for practical electrostatic spraying of liquid coatings, the liquid coating is usually fed out to the edge of a rotating disk-like or bell-like surface. Fast rotation of the disk or bell will produce a fine spray by shearing the liquid by air, similar to conventional spray guns. A high-voltage electrode is placed near the disk to charge the paint particles. However, in the presence of an electric field, fast rotation is not required and slow rotation is sufficient. The purpose of the rotation is the distribution of the coating at an even thickness that leads to uniformly atomized droplet sizes. The charged liquid paint particles will preferentially be attracted to a substrate that is electrically grounded. That is, the process that leads to higher paint use efficiencies. Liquid coatings can be applied in many ways besides the common spray methods including coil coating, roller coating, dip, dip spin, and curtain coating. Details are available in the literature [1].

19.5.3 Powder Coating Application Powder coating is a method of applying a dry paint to a surface. The dry powder is applied to the

item (substrate) to be painted, then the powder is turned to liquid by melting. The powder in its molten state subsequently flows out to cover the substrate, it coalesces and sometimes it crosslinks. The end result is a painted object. Powders may also be applied by bulk techniques such as fluidized bed coating. The Powder Coating Institute publishes one of the best references [5], although there are numerous others [6]. The process starts with fluidization of the powder. This is commonly done in a hopper or a vibratory feeder. The fluidization serves two purposes. One purpose is to get the powder to flow easily, allowing it to move from the hopper to the spray gun. The second function is to break up any loosely agglomerated powder particles. Moving air separates the powder particles and allows them to flow almost like a liquid through tubing and through a powder spray gun. See Fig. 19.12 for a diagram of typical powder equipment. When using dry blended powders, fluidization segregation results when fine particles in an aerated powder locate toward the top of a container, while the coarse particles deaerate quickly and settle to the bottom of a container. This phenomenon results in a top-to-bottom segregation and is very common with fine powder blends. A charge must be applied to the powder that will make it attract and stick to a grounded part. This is done by two methods called corona charging and tribocharging.

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Control unit Spray gun

Grounded part

Powder pump

Powder pick-up tube

Fluidized hopper

Fluidizing plate

Figure 19.12 Powder coating application equipment [5].

A high-voltage power supply is attached to the spray gun in a corona-charging system. When the high voltage is applied to a charging electrode in the gun, a strong electric field is created between the charging electrode and the grounded attractor electrode, shown in Fig. 19.13. This strong electric field ionizes the air creating what is called a corona. Normally 30 kV will ionize clean dry air, but lower voltages can be used, especially when particles are present as in powder coating. Once the powder particles are charged, they are blown out of the gun toward the substrate to be coated. If the substrate is grounded, the powder particles will be attracted to it and will tend to stick to it because of the electrical charge. Tribocharging is the other process used to charge powder paint particles. This is the process of electricity generation when two different materials rub against each other. Some materials easily give up or accept electrons from other materials under friction. Tribocharging guns are constructed to maximize contact of the powder particles with the side wall of the charging unit. A schematic of a tribocharging powder gun is shown in Fig. 19.14. Most tribo guns are constructed of PTFE which will remove an electron from any powder particle (except a fluoropolymer) giving the paint particle a positive charge. When fluoropolymers are used, special tribo gun constructed of nylon or other plastic is used which will donate the electron to the fluoropolymer particle giving it a negative charge.

Figure 19.13 Corona charging in a powder spray gun. Diagram courtesy The Powder Coating Institute [5].

19.5.4 Baking/Curing The term curing is often used to describe the baking process of fluoropolymer coating systems. According to the free online encyclopedia, Wikipedia (http://en.wikipedia.org), “curing” in polymer chemistry and process engineering refers to the toughening or hardening of a polymer material by the crosslinking of polymer chains, brought about by chemical additives, ultraviolet radiation, or heat. The key point here is “crosslinking,” which is a chemical reaction. Strictly speaking, the common fluoropolymers undergo no crosslinking or significant chemical change during baking. It is a melting process.

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Figure 19.14 Schematic of a triboelectric charging powder spray gun. Diagram courtesy The Powder Coating Institute [5].

Most of the fluoropolymer coatings undergo no curing reaction by this definition unless they are blended with thermosetting resins, which by definition become insoluble and infusible by a chemical reaction. However, the term curing is used so prevalently, it will continue to be used here to describe the physical process of taking the liquid or powder coating to its final film state. If one looks at a simple fluoropolymer coating, such as a dispersion of FEP, as it is baked at different temperatures, the curing process for fluoropolymers becomes clear. The FEP has a melting point of 525°F (274°C). This micrograph shows the applied coating at 500° F (260°C), below the melt point. Severe mudcracks have become volatile. This dispersion would need a film-forming additive that would minimize or eliminate the cracks if this were to be sold. As the temperature is raised to 550°F (288°C), 25°F (14°C) above the melt point, the FEP starts to melt and flow, but just barely. The melt viscosity of the FEP at this temperature is still very high. As the temperature is raised to 600°F (316°C), 75°F (242°C) above the melt point, the FEP now melts and flows well. However, the mud-cracks were very severe and they are just starting to fill in or heal. As the temperature is raised to 650°F (343°C), well above the melt point, the FEP now melts and flows well. This temperature is often the recommended temperature for FEP liquid and powder coats. The mud-cracks were very severe but show significant healing.

At 700°F (371°C), well above the melt point, the FEP now melts and flows very well. Even at this high temperature, the mud-cracks did not completely heal. It is possible that if the coating was held at this temperature for double or triple the time, the mudcrack defects might have disappeared. Most fluoropolymer-based coatings have a specified baking schedule in terms of times at different metal temperatures. These are usually specified by the coating manufacturer and found in their fact sheets. It is very important to follow baking instructions as the bake not only affects the surface smoothness, but is importantin obtaining adhesion to the substrate and between layers. It can also dramatically affect other performance properties such as nonstick and durability.

19.6 Commercial Fluorocoating Producers This section lists some of the major manufacturers of fluorinated coatings. Manufacturers mean those companies that make and sell coatings. It is not a comprehensive list. There are several companies that make and use their own coatings, but do not offer them for sale outside their shop. There are even more companies that buy coatings from major manufacturers but apply their own trade names and trademarks to them in order to shield their process from competitors. Details of their product lines are not included in this chapter.

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Henkel North America (http://www.henkelna. com): Their main interest is corrosion protection, friction reduction, abrasion, and erosion resistance. The coatings are generally applied by spray or dip spin. Their primary trade name for their coatings is Emralon. Whitford Corporation is based in Fraser, Pennsylvania (www.whitfordww.com). Whitford was acquired by PPG in 2019. They offer a comprehensive range of products. Whitford acquired Akzo Nobel’s fluorocoatings business in 2009. Whitford currently uses at least 40 trademarks around the world. Whitford trademarks include Whitford, Xylan, Xylac, Dykor, Xylar, Ultralon, Excalibur, Eclipse, QuanTanium, and Quantum2 Weilburger Coatings (http://www.weilburgercoatings.com) has manufactured nonstick coatings in Germany since about 1990. It is a GREBE GROUP Company. They focus on markets in Europe, India, Korea, and South Africa. Their market focus is cookware, bakeware, and household electrical appliance manufacturers. Their primary trade name is GREBLON. Chemours is based in Wilmington, Delaware. Teflon and SilverStone are their well-known tradenames. Chemours was formed as a spin-off from the DuPont Company in 2015. Mitsui-DuPont Fluorocarbon Co. (MDF) is a joint venture company between Mitsui and Dupont. It produces liquid and powder coatings in Japan. Daikin (http://www.daikin.com/chm/products/ coating/index.html) is a fluorocoatings producer. Dyneon (a 3M company, http://solutions.3m. com/wps/portal/3M/en_US/dyneon_fluoropolymers/ Home/Products_and_Solutions/Industry_Solutions/ Coatings/) manufactures and supplies a broad range of fluoropolymer dispersions and powders used in spray, roller, curtain, and other coating processes. These coatings offer excellent nonstick properties, as well as enhanced corrosion, chemical, and scratch resistance.

19.7 A Historical Chronology of Fluoropolymer Finish Technology The following discussion contains a chronological perspective of the history of fluoropolymer finishes. Cookware, being the largest single market, dominates the discussion and perhaps should be separated from

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the industrial applications, but that is not done here. The discussions of both markets are interspersed in approximate chronological order. The discussion is heavily focused on DuPont for two reasons, the author worked for DuPont and DuPont dominated the markets at that time. The discussion is heavily focused on DuPont for two reasons: the author’s experience and DuPont’s major role in developing the coating applications and markets. DuPont’s Teflon Finishes business first began in 1948 when 126 gallons of coating was sold for $4441. Teflon Finishes was just another product line of the Industrial Finishes Department. The first products were 850-200 Single Package Primer and 851-200 Low Build Topcoat. These finishes found many applications where nonstick or dry lubricity was needed. Analogs of these coatings are still manufactured and used today. These first products were manufactured at the Philadelphia paint plant, which stood on the ground adjacent to the Marshall Laboratory in South Philadelphia, Pennsylvania. Buyers of these first coatings were companies like General Plastics and American Durafilm who were already users of Teflon resins. Sales grew to about $450M to $550M in 4 5 years and then flattened at that level for the next 8 years. While the specific uses for the products during these years are not known, it is generally believed that most of them were used for a variety of industrial applications and some found their way onto cookware. DuPont chemist Verne Osdal at the Marshall Laboratory made the key technical accomplishment that started the Teflon finishes business. His discovery, a method of getting the nonstick Teflon to adhere to metal, came in 1951 [7]. He discovered coating compositions of PTFE and mixed acids that imparted good adhesion to substrates. The first public disclosure of the use of Teflon coated pans came in 1953. However, in Paris, France, in the mid-1950s, Marc Gre´goire saw the possibilities of using a fluoropolymer coating on cookware and began to make coated pans in a very small operation in Paris. His wife, Colette, was the first sales person and she set up a sales operation on the streets of Paris. Her efforts produced an immediate success and soon the coated cookware found its way into Paris retail shops. Gre´goire was granted a patent in 1954 and formed a company to produce en masse in 1956. Gre´goire named his company Tefal. Within several years, pan sales were numbered in the millions.

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In 1958, Thomas Hardie received a Tefal skillet from a friend he had known in Paris during his days as a foreign correspondent for the United Press and International News Service. Hardie became excited about the prospects for the Tefal pan in the United States and he spent much of the next 2 years trying to convince American retailers and manufacturers of its bright future but without success. American retailers and manufacturers were uninterested in marketing this type of cookware unless sanctioned by a government agency. In 1960, after numerous discussions with DuPont and others, US Food and Drug Administration (FDA) disclaimed jurisdiction over home cookware. This meant that no FDA approval was needed to sell Teflon coated cookware in the United States. Within a few months after that announcement, the FDA stated that “FDA scientists believe pans coated with Teflon are safe for conventional kitchen use.” Macy’s Department stores’ entrance into selling nonstick pans sparked public interest and brought Hardie’s quest to fruition. The Tefal process for getting the Teflon coating to adhere to metal was mechanical rather than chemical in nature. The metal was chemically etched to provide a mechanical bond of the nonstick coating to the metal. This was a patented process, so other interested cookware manufacturers had to look for other technology. This logically led them to DuPont because they had an alternative technology in the Osdal primer patent. American cookware manufacturers rushed to get into the marketplace with their nonstick cookware. In 1961, DuPont’s finish sales grew to $1.2 million. However, this surge of new business was shortlived. The inexperience of American cookware manufacturers with the coating process and their lack of quality control led to a flood of poor quality cookware being sold to consumers. This in turn quickly led to consumer complaints, retailer disenchantment, and a drop in DuPont sales of 19% in 1962 rather than the rapid growth that had been expected. DuPont put together a team including manufacturers and advertisers that conducted extensive research with 6000 consumers that revealed no problem with the concept of nonstick cookware, just a problem with the poor quality of nonstick cookware that was then available. Coatings tended to peel because of poor substrate adhesion, caused by inadequate roughening of the metal.

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Cookware made of thin metal to reduce costs contributed to the problems due to hot spots and thermal degradation. In some cases, too much Teflon was applied and this led to cracking of the coated surface. The DuPont team concluded that the business could be revived and had a bright future but fundamental changes in strategy had to be made and DuPont’s role in the marketplace changed dramatically. Instead of merely selling coatings and supplying direct customers with technical support, DuPont recognized an obligation to ensure that their products were used correctly and met the expectations of retailers and consumers. The acceptance of this obligation opened up the opportunity for quality control. These are some of the new strategies for cookware revival:

• Employ a license agreement with all housewares manufacturers who wished to use DuPont technology and products.

• Set and monitor metal thickness and coating standards for all licensed manufacturers.

• Provide licenses with DuPont certification marks to identify their nonstick cookware, samples being submitted, tested, and approved at the Marshall Laboratory in Philadelphia, PA.

• Use the convenience of nonstick cooking with easy clean-up as the unique selling proposition in addition to fat-free cooking.

• Test the use of television advertising to build the nonstick cookware category and the awareness of the Teflon brand. Around 1960, clear coatings were introduced in order to broaden the thrust into markets that required purer Teflon topcoats and that also opened up certain electrical applications that could not tolerate pigments in the finish. DuPont introduced two new coatings in 1961. A topcoat based on FEP was developed for application into nonstick/mold-release areas such as tire molds. This was the first commercial coating based on a melt-processible fluoropolymer. This coating is still sold today under the same product code number. While single package, premixed primers had been successfully used for 10 years, a need for

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more versatility in the primers led to development of the two-package primers. The two packages included an enamel package containing the fluoropolymer and additives and an accelerator package that included chromic acid. Some end-uses required different accelerator-to-enamel ratios to obtain optimum adhesion, so the two-package approach allowed for that flexibility. The two-package system also extended storage life and eliminated the need for refrigerated storage that was required for the premixed primers. In 1965, cookware manufacturers began to spray a material harder than aluminum between the aluminum and the Teflon coating. These materials, aluminum oxide, stainless steel, or ceramic frit were applied in a discontinuous coating that created a peak and valley profile onto which the Teflon coating was applied. The idea was that cooking utensils like spatulas would ride on the peaks of the profile and thus would not scratch the coating. While hard bases were not on the vast majority of coated cookware, they were used to some degree by many manufacturers. Many of the performance claims went well beyond what the product could deliver. At this time, DuPont introduced darker color coatings to mask the staining caused by grease permeation into the relatively porous coating and then carbonizing during continuous cooking, particularly at high cooking temperatures. The consumer’s desire for more durable nonstick coatings drove the technical efforts in the late 1960s. While it was hoped that more durable coatings would be developed for use on cookware, it was also believed that improved durability would open up the possibility of new consumer and industrial applications, thus more opportunities for growth of fluoropolymer coatings. While the DuPont focus of the US business was primarily upon the housewares industry, there was recognition that a small but growing segment of business existed in 1966 (about $1MM) with an array of applicators. DuPont believed that there was also a significant growth potential with these applicators. Hoping to avoid the quality problems, DuPont initially experienced with cookware a few years earlier, they instituted a licensing program for industrial applicators in 1967. These applicators were known as LIAs. In the mid-1960s, DuPont’s research team discovered a new concept in nonstick coatings. It was

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found that hard, adhesion-promoting binder resins could be incorporated with FEP to provide one-coat products that had greater abrasive wear-resistance than prior Teflon coatings while maintaining an acceptable level of dry lubricity and nonstick. These products stratified when baked, leaving the surface mostly comprised of fluoropolymer, and the interface with the metal mostly the tougher resin. DuPont trademarked this new generation of coatings Teflon-S. Additionally, many one-coat finishes could be cured at lower temperatures. The first such coating was commercialized in 1966. It had a low bake requirement (232°C/450°F). The Teflon-S coating had very good release and durability. It was a blend of an epoxy resin and a proprietary low melting FEP resin. Several Teflon-S products were introduced in 1967 and hand-tool manufacturers showed a good deal of interest in their use. DuPont developed a certification program for tools. In 1967, a new Teflon-S was developed. This coating was based on a blend of PAI resin and FEP. It was the hardest and most durable nonstick coating developed until that time. Further, the adhesion of this new coating over smooth metals was excellent. These characteristics coupled with good dry lubricity and fair nonstick properties made this second Teflon-S coating very marketable. End-uses such as bearings, lawn and garden tools, bakeware, and many others covered the markets for this technology. The fluoropolymer coatings industry prior to 1969 was almost entirely about Tefal and DuPont’s Teflon. ICI and Hoechst had a few products using fluoropolymers they manufactured. A new coatings company, one that did not make any fluoropolymer resins, was founded in 1969: Whitford Corporation. Whitford’s first product, Xylan 1010, was developed in March of 1969. Xylan 1010 is a matrix coating, as Whitford calls it. It was designed specifically to solve two problems: 1. Provide a tough, very low-friction film that could withstand the constant wiping of a rubber seal. 2. Capable of being cured at temperatures sufficiently low to avoid blistering or distortion of the casting of the actuator housing. Xylan 1010 remains one of Whitford’s most popular products today. Similarly, Weilburger was an

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old German specialty paint manufacturer who also saw the potential for developing business using the combination of fluoropolymers and other resins. They started manufacturing fluoropolymer-based coatings around 1975. Weilburger Coatings has manufactured nonstick coatings in Germany for over 30 years. These GREBE GROUP companies manufacture and supply a wide comprehensive range of GREBLON nonstick and Senotherm hightemperature decorative coatings to many major cookware, bakeware, and household electrical appliance manufacturers. The products offer flexibility in design, color coordination, performance, and competitive cost. In 1973, a great improvement took place in DuPont’s fry pan coating technology. A new airdry primer, and an improved Teflon enamel topcoat, proved to be significantly superior to Teflon II in terms of adhesion and stain resistance. The improvement in stain resistance utilized new chemistry that reduced the porosity of the topcoat by 90% [8]. So great was the improvement that for the first time DuPont offered a white Teflon to the cookware market. In 1973, DuPont also introduced a new resinbonded coating that could be cured at 350°F (177° C). This low-bake analog, which was a catalyzed epoxy FEP blend, was a relatively inexpensive dry lube coating. These characteristics made it especially attractive for the hardware market. In 1974, DuPont’s first powder coat Teflon finish was introduced; it was an FEP powder coating. Not only was it pollution free, but it enabled the applicators to get thicker FEP coatings than with liquid FEP coatings. In the mid-1970s, IBM developed a new generation of high-speed printers to go with their mainframe computers and DuPont’s technology brought about their use of Teflon-S technology (low-melt FEP plus epoxy) as a coating for the toner beads. One usual property of fluoropolymers was advantageous in this application. That was the ability to tribocharge (develop a static charge by rubbing against another material). In 1976, SilverStone three-coat system for topof-the-range cookware was introduced. A key technical achievement was made by the addition of an acrylic resin to the formula which burned off during the curing bake, but created a film that was less porous and thus less susceptible to staining from carbonized fat during the cooking process [9].

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Additionally, it prevented film cracking, which enabled the thickness of the coating system to be increased by 30% to 1.3 mils and higher and thus provided greater durability. All the pigment was included in the midcoat and the topcoat was clear, thus making it very rich in fluoropolymer and enhancing its nonstick characteristic. It was intended for use on premium quality cookware only, and a new set of quality tests built around it had to be met in order to gain the SilverStone Seal. In 1976, the system called Spectragraphics was developed. It employed a technique that enabled putting images in a Teflon coating. A new series of high-performance Teflon resin-bonded, dry lubrication coatings coded 958-300 was developed. They were specifically designed to meet the needs of a market where high load-bearing conditions are present. Automotive use of this coating was common. In 1979, Teflon-P PFA Powder Coating 5325010 was introduced. This coating has the hightemperature properties of PTFE and the additional advantage of being thermoplastic. During mid to late 1970s, nonstick bakeware coated in flat coils and then stamped into shape began to emerge. This coating process required different finish formulations and the market was supplied by both Whitford and Weilburger. In 1983, SilverStone coatings were introduced on glass ovenware, heavy gauge gourmet cookware, and a line with nonstick outside as well as inside. Another new use of SilverStone that required new technology was launched in 1984 with its use on plastic ovenware for microwave, convection, and conventional oven use. Regal and Northland launched lines that were moderately successful in mid-1980s. SilverStone SUPRA was introduced in 1986. This coating technology was based on blends of PTFE and PFA [10]. Laboratory and in-home testing showed this new system to be 50% tougher than SilverStone, which was based on PTFE only. In 1983, fluorinated coatings companies focused more upon specific industrial market segments. DuPont’s key market segments that accounted for most of their growth were office automation for copier rollers, toner beads, and computer printers; automobile and truck fasteners; food processing; and shoe molds. Significant technical and market development work was

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carried out to penetrate other targeted segments including commercial bakeware and chemical processing equipment. Keys to the companies’ varying degrees of success were their ability to formulate both liquid and powder systems that met specific requirements of the end-use, and it is expected that this ability will continue to be key to future growth. With Mirro executing a strategy focused almost totally upon mass merchandisers and employing a pricing strategy that required a coating material significantly below DuPont’s SilverStone price, Whitford gained access to its first significant cookware account with a one-coat product based upon PPS and PTFE resins. DuPont countered with trade advertising and with press conferences by pointing out and demonstrating the inferior nonstick characteristics of cookware coated with this product. But in fact, consumers generally accepted the poorer performance and equated it with the very low price. The dirty and time-consuming process of grit blasting the metal surface to ensure coating adhesion was always needed for the best performing housewares systems until DuPont developed gritblast-free primer technology [11]. Mastering control of stratification was the key to this technology. The utilization of PFA in varying amounts in the primer, midcoat, and topcoat also made possible durability improvements beyond SilverStone and SilverStone SUPRA. Simultaneously with the introduction of the gritblast-free systems, DuPont also introduced under the AUTOGRAPH trademark a ceramic filled system. In essence, this system had greater durability built into the coating as a result of tough ceramic fillers being included in the three coats at varying ratios [12]. A patent on this technology was granted in 1992. The developments in fluorinated coatings during the late 1990s and early 2000s were formulations for new applications, for new application methods, and for powder coatings. There were a lot of formulations developed for high efficiency application. For cookware, the application techniques were shifting toward roller coating and curtain coating. Powder coatings for cookware have yet to become a commercial success, though powder coatings in industrial applications have expanded. Cookware coatings around 2010 have been developed in a wide range of colors to satisfy

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consumer demands for cookware that not only performs but looks good in the kitchen.

19.8 Food Contact Questions about coatings for food contact are common. The first step toward general understanding is determining what the regulations are in the country where the products will be used; that means the enduser, not where they are applied or produced. This information is best obtained from experts in the field, and this section only provides an overview that applies to the United States. In all cases, one should obtain written documentation from the manufacturers of products and materials being used that state compliance under the appropriate regulations. There are several regulations that govern the use of fluoropolymers as articles or components of articles intended for use in contact with food. These are published in the Code of Federal Regulations commonly called the CFR. The CFR covers just about everything but Section 21 covers food and drugs. For Articles Intended to Contact Food, one of the important sections is 21 CFR 177.1550, “Perfluorocarbon Resins.” Many fluoropolymer resins may be used as articles or components of articles intended to contact food in compliance with this regulation. One should get documentation from the fluoropolymer material manufacturer certifying compliance before use, however. Some fluoropolymer resins are irradiated to facilitate grinding into fine powders for applications needing a very small particle size as is typical for many coating applications. Paragraph (c) of this regulation specifies the allowable dose of radiation and maximum particle size for PTFE resins so processed and restricts their use to components of articles intended for repeated use in contact with food. For coatings, the most important regulation is 21 CFR 175.300, “Resinous and Polymeric Coatings.” Most fluoropolymer resins and fluoroadditives may be used as release agents in compliance with this regulation as long as the finished coating meets the extractives’ limitations of the regulation. Product formulators must take care in ingredient selection, not only for the fluoropolymers used, but also for all the ingredients including pigments, additives, and other polymers.

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Written documentation from each manufacturer needs to be kept on file. Other regulatory agencies may have additional regulations. The United States Department of Agriculture (USDA) has accepted fluoropolymer resins that comply with 21 CFR 177.1550 as components of materials in direct contact with meat or poultry food products prepared under federal inspection. The Dairy and Food Industries Supply Association, Inc. has its “3-A Sanitary Standards for Multiple-Use Plastic Materials Used as Product Contact Surfaces for Dairy Equipment, Number 20-17,” published by the 3-A Secretary, Dairy and Food Industries Supply Association, Inc. US Pharmacopoeia Class V1 (USP) has additional regulations. Representative samples of fluoropolymers have been tested in accordance with USP protocol, and many meet the requirements of a USP Class VI plastic. These tests on representative samples may not reflect results on articles made from these fluoropolymers, especially if other substances are added during fabrication. Testing of the finished article is the responsibility of the manufacturer or seller of the finished product if certification that it meets USP standards is required. USP testing was done to support use of these fluoropolymers in pharmaceutical processing and food processing applications. While USP Class VI certification is not required for pharmaceutical processing, many pharmaceutical customers seeking ISO-9000 certification have requested it.

19.9 Commercial Applications of Fluorocoatings The remainder of this chapter will review just some of the thousands of uses of fluorocoatings. For each application, the discussion will focus on the properties of the coating and how it improves those products or the problems the coating solved will be summarized.

19.9.1 Housewares—Cookware, Bakeware, Small Electrical Appliances Nonstick coated fry pans are the most common and best known application of fluorocoatings. A pan can be coated on smooth metal, over grit-

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blasted metal, or over a flame-sprayed, hard, rough surface. Cast or rolled aluminum is the most common metal, though stainless steel is also common. There have even been coated glass and ceramic cookware. The best coating systems use three different coats: primer, midcoat, topcoat. The primer is almost always based on PAI and fluoropolymer. The primer provides adhesion to substrate and intercoat adhesion to midcoat above it. It can also be formulated to provide cut-through resistance (by cooking utensils) and abrasion resistance. The midcoat is often the thickest layer usually provides color and durability. The midcoat consists of mostly fluoropolymer or a blend of fluoropolymers. The blend is usually PTFE and PFA. The midcoat generally has plenty of pigment to hide the primer/substrate and is usually dark colored to hide the staining of the coating during use. It may contain abrasion resisting particles like aluminum oxide or silicon carbide. The topcoat influences appearance but primarily provides food release. The topcoat usually consists of PTFE or a blend of fluoropolymers, but often has mica in it to provide the common sparkle look. The coating is typically baked, depending on the system, between 700°F and 820°F (371°C 438°C). There are also good quality two-coat systems and cheap one-coats with poor performance. There are many coating systems to choose from and many brands. Recently, more cookware is roller coated or curtain coated. This needs to be done on flat disks that are formed into a pan after completing the coating and curing process. The coating needs to be postformable as the pans are pressed into shape from a coated disk. Small electric appliances (SEAs) include kitchen devices such as breadmakers, waffle irons, and rice cookers. These are often coated with nonstick coatings, but the coatings are usually not the same coatings that are used on fry pans.

19.9.2 Commercial or Industrial Bakeware Commercial bread and bun bakers have used release coatings for a very long time. Those coatings historically were not fluoropolymer but were polysiloxane, commonly called silicones or silicone glazes. A bread or bun pan would be coated with

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this clear or yellow material and it would function with the help of sprayed-on oil for 300 600 baking cycles. At this point, the release would become so poor that the pans were removed, coating stripped, and new coating was applied. Generally, this could be done quickly and relatively cheaply, though there were environmental concerns about emissions with this process. A typical bakery needs about 2000 pans to fill a line and might have many different sets. A different pan is required for different bun sizes, shapes, and configurations. Typical pans are shown in Fig. 19.15. A bun pan is shown on the left and a strap of bread pans is shown on the right. Fluoropolymer coatings are more expensive than the silicone glazes, so the coatings needed to last much longer. The goal would be the life of the pan, but 5000 cycles usually provide economic incentive to the bakers. The pans are usually made from aluminized steel. Aluminized steel is used to minimize

Figure 19.15 Commercial or industrial bakeware coated with fluoropolymer coating.

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corrosion of the bottom of the pans, which see 95% humidity at 105°F (41°C) for up to an hour during the rising of the dough. The coating system is a primer with one or more topcoats. One of the preferred systems is a powdered PFA topcoat. FEP liquid coatings and powder coatings are also used. Extra benefits for the baker include reduced oil consumption which is a cost saving, but also leads to a cleaner and safer bakery.

19.9.3 Fuser Rolls A crucial part of copiers and laser beam printers is a fusing mechanism. This part melts dry powdered toner that has been applied to the paper, melting the toner onto the paper. This process makes the image permanent. The fusing temperature for black and white machines is typically 400° F (200°C). Heat is transferred by a hot metal roller or belt. The fuser must release the molten toner. Fluoropolymer coatings are crucial to this step and have been used since 1966 or earlier. Fig. 19.16 shows a schematic of the fusing unit. A photoimaging drum puts the dry powdered toner on the paper in the proper areas. The paper is fed into the fuser unit. The heated fuser roll and the soft silicone rubber backup roll press together forming an area where the paper is pressed between the two rollers. The toner melts and sticks to the paper. If it stuck to the drum, image quality would be poor. The next time the drum makes a rotation a shadow of the wrong image might print on the paper. Proper release by the fuser drum coating is very important.

Figure 19.16 Schematic of a typical fuser unit of a laser beam printer or photocopier.

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Almost everything that rubs against a fluoropolymer picks up a triboelectric charge. To maintain charge balance, the coated fuser drum picks up an opposite charge. Often, the toner has a residual charge, and if the fuser drum has a charge, the toner may jump off the paper onto the drum. When this happens, the sharpness of the image is lost. This is called electrostatic offset. For this reason, many fuser drum coatings are made conductive so that any charge that forms is lost to ground. The coating systems for these rollers, shown in Fig. 19.17, are complex and many are optimized for a particular machine. Most are applied as liquids. Many rollers of laser beam printers consist of sleeves of PFA that are heat-shrunk onto the metal roller. Those liquid systems usually are twoor three-coat systems. The primers are generally

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PAI based, but the topcoats can range from PTFE to blends of PTFE and PFA to pure PFA. FEP and other fluoropolymers are rarely used on fuser rolls. Paper dust is quite abrasive, so the coatings also need abrasion resistance, which is often improved with additives such as silicon carbide or aluminum oxide. A delicate balance is struck between the release and abrasion resistance properties. There is also a thermistor in contact with the rollers that controls temperature. This is the cause of many roller failures because it can cut through the coating system. Picker fingers make sure the paper comes off the fuser roll. These can also abrade the coating, particularly if they pick up toner and paper dust. They are usually made from engineering plastics such as PES or Vespel. They are frequently coated with a resin-bonded fluoropolymer coating.

19.9.4 Light Bulbs

Figure 19.17 Typical fluoropolymer-coated aluminum photocopier fuser rollers.

Figure 19.18 Perfluoroalkoxy-coated light bulbs.

Coated light bulbs are available in many hardware stores. Bare bulbs with no containing fixtures are often used in factories and in homes. Containment of the glass in a broken bulb is the aim of this fluoropolymer coating end-use. The bulbs can get very hot so thermal resistance and good mechanical properties at high temperature are required. Fluoropolymer coatings are ideal for glass containment, shown in Fig. 19.18. The coating is usually PFA that is applied as a powder coating. An interesting note is that the PFA does not adhere well to the glass bulb. If a primer is used to improve the adhesion, when the glass breaks the PFA no longer contains the broken glass. Therefore

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strong adhesion of the coating to the glass is avoided.

19.9.5 Automotive There are dozens if not hundreds of applications of fluorinated coatings in the automotive industry. They are used for many purposes. Some are important for production processes; some are intended for car performance. The properties of these finishes include combinations of the following: 1. Dry lubrication with a low coefficient of friction; 2. wear resistance; 3. chemical protection-fuels; 4. corrosion protection; 5. nonstick; 6. electrical insulation. A few of these applications are discussed. Automotive air conditioner pistons (Fig. 19.19) are often coated with a resin-bonded fluoropolymer coating. The coating must adhere to the aluminum piston. A low baking temperature is required so that the part retains its hardness and precisely machined dimensions. The coating supplies dry lubrication and abrasion resistance. This is particularly important during the first few hours of use. A small percentage of uncoated pistons fail due to galling, which means metal sticking to metal. The warranty repair is very expensive, and the coated

Figure 19.19 Automotive air conditioner piston.

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piston reduces the failure percentage significantly. The coating also provides some increase in efficiency after break-in. Coated fuel pumps (Fig. 19.20) are common. The coating provides a low coefficient of friction, wear resistance, and chemical resistance to the fuel. This leads to pumps that last significantly longer, with the ultimate goal of lasting the life of the car. Today’s fuels often contain alcohols and other additives that are more corrosive than hydrocarbons. A resin-bonded coating based on PPS and PTFE micropowder is usually used, though the coating systems for fry-pans have also been used in this application. Seat belt D-rings (Fig. 19.21) are usually visible in a car over the left shoulder of the driver or right shoulder of the passenger. These rings are often coated with a resin-bonded coating of epoxy/PTFE or PAI/PTFE. Liquid and powder coatings have also been used. The coating lets the seat belt glide more smoothly over the ring, primarily decreasing the abrasion of the belt, leading to longer life of seat belts. Lots of fasteners are used to assemble an automobile. Many are coated with a fluoropolymer coating. There are two basic reasons to coat the fasteners. One type of fastener is coated to offer improved assembly and corrosion resistance. The improved assembly comes from robotic assembly. Screws are driven by torque sensing wrenches or

Figure 19.20 Automotive fuel pump.

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Figure 19.21 Automotive seat belt D-rings.

Figure 19.22 Automotive weld-nut with fluoropolymer coating on threads.

screwdrivers. A fluoropolymer dry lubrication coating allows the fastener to slip more consistently into or through the materials being fastened. The fasteners are turned to a given torque. Without the dry lube coating, the fastener could snag or gall and might not be properly attached. The fluoropolymer in these resin-bonded coatings allows them to penetrate the materials better, but is designed to still hold the applied torque, keeping the materials attached. The coatings used for this type of fastener are typically resin-bonded PTFE micropowder. They are frequently applied by dip spin. The second type of fastener is called a weld-nut or stud (Fig. 19.22). They are welded to the chassis. The weld-nut accepts a bolt and the stud accepts a nut. Both are welded in place. Often, weld splatter hits the threads of the fasteners and sticks there. When this happens the fasteners become unusable. Before the use of a fluoropolymer coating that resisted the weld splatter, these fasteners had to be masked. The coating on the

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Figure 19.23 Automotive brakeline tubing is often coated with a fluorocoating based on Tedlar PVF.

fastener does more than prevent weld splatter from sticking, however. The chassis is primed in its entirety in the first step of vehicle assembly. The chassis in submerged in a primer paint bath called an electrocoat or electroprime bath. Electrical charge is applied to the chassis and the primer is attracted to it and deposits uniformly. The problem with this process is that the threads of the nut and studs on the chassis that are used later in the assembly are coated with primer. This makes assembly involving these fasteners very difficult. The fluoropolymer coating that prevents the weld splatter problem is not conductive. Therefore the electrocoated primer is not attracted to it and does not coat it, leaving the threads clear. Even if a drop or two of primer happens to remain in the threads, it is easily pushed out because of the nonstick character of the coating. Automotive brake lines, as shown in Fig. 19.23, are made of steel. Salt used to treat roads in the winter time end up coating everything under the car. It has gotten worse with the new road de-icing solutions have been used for the past several years throughout the United States. They are applied up to 48 hours before bad weather is expected and are usually applied as a powder, small granules, or as a liquid spray. The liquid solutions are generally a mix of road salt, or sodium chloride, and magnesium, calcium, or some combination. This ends up increasing the exposure to brakes lines to corrosive salt and water. Uncoated lines are now being replaced in less than 5 years on many cars. A fluorocoating based on a dispersion of Tedlar PVF in

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propylene carbonate [13] is applied to the tubing boosting lifetime up to 30 3 [14]. Other automotive fluid carrying lines such as fuel, transmission, and oil cooler lines are also being coated. Sometimes a fluorocoating is applied to help with production. One such case in the automotive industry is during the manufacture of brake dampers shown in Fig. 19.24. Brake dampers are multilayered constructions that reduce brake noises [15]. One part of the construction involved a stack or multilayers parts with metal on one side and an elastomeric material on the other. Robots were being used to assemble the parts, but they were sticking to each

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other and the robot often would pick up more than one part. Application of a fluorocoating to one side of the metal prevented the sticking and did not otherwise affect the brake function. That coating is applied by coilcoating and the parts are cut from sheet stock afterwards.

19.9.6 Chemical Processing Industry The chemical processing industry (CPI) has many interesting applications for fluoropolymer coatings. These include such items, many of which are very large, as

• • • • • • •

Ducts for corrosive fumes, fire resistance; chemical reactors; impellers; tanks; pipes; fasteners; ducts.

19.9.7 Chemical Reactors

Figure 19.24 Automotive brake dampers.

Impreglon Canada, a DuPont licensed industrial applicator in Edmonton, Alberta, Canada, did the largest items ever coated with a fluoropolymer coating. These vessels, shown in Fig. 19.25, were large polymer reactors being installed at the Nova Chemicals complex in Joffre, Alberta. The largest of three vessels, which were eventually connected

Figure 19.25 Polymer reactor coated by Impreglon Canada. Courtesy Impreglon Canada.

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end-to-end, is shown in this figure. The vessel was fabricated by Dacro Industries in Edmonton. It is 23 feet (7 m) in diameter and 50 feet (15 m) long and weighs in at 126 tons. The DuPont coating system was applied in three coats at a total film build of only 2 mils. The properties that the coating system addressed were thermal resistance, nonstick of molten polymer, and chemical resistance. The coating is expected to last the life of the reactor. Chemical reactions are usually run in a reactor tank such as that shown in Fig. 19.26. The reactors often have pipes (Fig. 19.27) and mixing blades. They are frequently run at high temperatures. These vessels, mixers, and pipes are frequently coated with thick fluoropolymer films. The most chemically resistant and highest temperature rated material is PFA. These films are very thick, typically 40 mils

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(1 mm) or more. They can be applied from liquid coatings or powder coatings. The hot flocking process is often used. Because the topcoats need to be applied in multiple coats with multiple bakes, it can take days or even weeks to coat a reactor. Extra care must be taken to avoid any defects, especially bubbles from resin decomposition. While sometimes these defects can be repaired, often the entire coating must be stripped and the process started over.

19.9.8 Ducts for Corrosive Fumes, Fire Resistance The semiconductor industry uses all sorts of aggressive chemicals in the production of chips. These are produced in a manufacturing site that is called a “fab.” The ductworks in the fab carry corrosive and flammable materials. A resinimpregnated fiberglass material called FRP has historically been used for the ductwork. However, this ductwork is not sufficiently fire resistant. Fluoropolymer-coated metal has replaced FRP in many of these applications (Fig. 19.28). The coatings are ETFE or ECTFE. Both are more chemically resistant and certainly more fire resistant than FRP.

19.9.9 Commercial Dryer Drums Figure 19.26 Polymer reactor tank coated with thick film PFA.

Figure 19.27 Chemical processing pipe coated with thick film PFA.

Commercial dryers like those used in hospitals often end up with plastic materials being unintentionally dried. Plastic bags are particularly a problem. These can melt and adhere to the walls of the dryer basket or drum, and are very difficult to

Figure 19.28 FAB fume ducts coated with ETFE or ECTFE.

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Figure 19.29 Dryer drum panel coated with PFA.

remove. Many of these dryer baskets are coated with a fluoropolymer. In Fig. 19.29, only one panel was coated with a fluoropolymer. As can be seen, the fluoropolymer-coated panel in the middle is essentially melt-free. The nonstick and heatresistant properties are used in this application.

Figure 19.30 Roller used in the paper or fabric industry.

19.9.10 Industrial Rollers Large rollers are used in many industries such as paper making or fabric printing. The rollers are heated to promote drying. These rollers are coated for improved release, easy cleanability, and thermal resistance. One such roller from the paper making industry is shown in Fig. 19.30. The coatings used here can range from chromic acid primer, to PFA, to electroless nickel/PTFE composites.

19.9.11 Medical Devices Fluorinated coatings are used to coat some medical devices. The metered dose inhaler (MDI) is an example of a drug delivery device. It consists of a small spray device that delivers a carefully measured volume of propellant and drug into the lungs. The device, shown in Fig. 19.31, is commonly used by asthmatics, but other drugs can also be delivered in this manner. The device has been marketed for decades. Many drugs do not disperse well in the propellant and adhere to wall of the aluminum container. This causes variation in drug dose delivery and a premature drop off in the dose/actuation profile. A fluoropolymer coating is applied to the inside of the cans to eliminate the adhesion of the drug to the wall permitting the device to deliver the expected number of doses and more complete use of the contents.

Figure 19.31 Metered dose inhaler.

The summarized needs for the coating are 1. Adhesion to smooth aluminum substrate; 2. release characteristics and chemical inertness; 3. optimized application characteristics such as wetting, flow and leveling, and viscosity; 4. regulatory compliance, formulations with FDA listed ingredients and those that use as few ingredients as possible; 5. minimize extractables. Glaxo Smith Kline holds a number of patents [16] on the devices in which they claim a large array of fluorinated coating compositions, most of which are resin bonded fluoropolymers.

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19.9.12 Oil Production and Refining Asphaltene, paraffin, scale, and other forms of deposition are known to cause significant problems in the production tubing on major oil fields around the world and particularly in Latin America. PEMEX, the Mexico state oil company, wanted to increase the productive life of the wells by inhibiting deposition of organic and inorganic materials, particularly asphaltene [17]. The customer also wanted to significantly reduce maintenance cost by reducing cleanups and interventions to these critical wells. Cleaning depositions of these wells requires a day of shutdown for chemical treatment. These cleanups were required every 6 12 months. DuPont (now Chemours) produced over 5000 meters of production tubing coated with a new fluoropolymer based coating system developed for this end-use called StreaMax XF (https://www.teflon.com/en/industriesand-solutions/solutions/productivity-efficiency-flow/ streamax-coatings). When installed in the Yagual 12 well in southern Mexico it performed flawlessly requiring zero cleanups for the four years that well produced oil through the coated tubing. It also slightly improved production, improved flow, due to a lower friction factor with StreaMax coated tubing.

Figure 19.32 Micrograph of a PTFE-coated razor blade.

19.9.13 Razor Blade Coatings Most razor blades are coated with a nonstick material that helps the blade cut more easily and comfortably. There may also be some value to retard oxidation or degradation. The coating is often a dispersion of PTFE having a molecular weight of at least 500,000 to form a coating of the PTFE on the cutting edge. Then subjecting the PTFE coating to ionizing radiation in the presence of an oxygen containing gas to obtain a radiation dose of up to 50 Mrads and then sintering the PTFE coating. The radiation reduces the molecular weight permitting the PTFE to flow out during sintering [18]. A micrograph of a coated razor blade is shown in Fig. 19.32. The thin coating is visible and noticeably smooths out the surface.

19.9.14 Architectural Coatings Fluoroethylene vinyl ether (FEVE)-based coatings such as Lumiflon have been used for decades for architectural coatings for buildings, siding, and

Figure 19.33 The Akashi Bridge in Japan is completely coated with an Asahi Glass FEVE type of fluorocoating. ("Akashi Bridge" by Tysto—Selfpublished work by Tysto. Licensed under CC BY-SA 3.0 via Wikimedia Commons—http://commons.wikimedia.org/wiki/File:Akashi_Bridge.JPG#/media/File: Akashi_Bridge.JPG)

bridges. As an example, the Akashi suspension bridge shown in Fig. 19.33 was completely coated with an FEVE-based fluorocoating, including the cables.

19.10 Summary Fluorocoatings are frequently complex mixtures that provide unique combinations of performance properties. Fluorinated coatings are often a small part of the final product, but are crucial to their function and commercial success.

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References [1] McKeen LW. Fluorinated coatings and finishes handbook—the definitive user’s guide and databook. William Andrew Publishing/ Plastics Design Library; 2006. [2] Plunkett RJ. The history of polytetrafluoroethylene: discovery and development. In: Seymour RB, Kirshenbaum GS, editors. High performance polymers: their origin and development, Proc. Symp. Hist. High Perf. Polymers, at the ACS Meeting in New York, April 1986. New York, NY: Elsevier; 1987. [3] Ebnesajjad S. Fluoroplastics, volume 1: nonmelt processible fluoroplastics, the definitive user’s guide and databook. 2nd ed. Norwich, NY: William Andrew, Inc; 2014. [4] Ebnesajjad S. Fluoroplastics, volume 2: melt processible fluoroplastics, the definitive user’s guide and databook. 2nd ed. Norwich, NY: William Andrew, Inc; 2015. [5] Liberto NP. Powder coating: the complete finishers handbook. 4th ed. Alexandria, VA: The Powder Coating Institute; 2014. [6] Hester CI. Powder coating technology. Norwich, NY: William Andrew Publishing; 1990. [7] Osdal Le Verne, US Patent 2,562,118, assigned to DuPont; 1951.

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[8] Vassiliou E, Concannon TP. US Patent 4,070,525, assigned to DuPont; 1976. [9] Vary EM, Vassiliou E. US Patent 4,118,537, assigned to DuPont; 1978. [10] Vassiliou E, Concannon TP. US Patent 4,070,525, assigned to DuPont; 1978. [11] Tannenbaum HP. US Patent 5,230,961, assigned to DuPont; 1993. [12] Vassiliou E, Concannon TP. US Patent 4,070,525, assigned to DuPont; 1978. [13] Uschold RE. US Patent 5,250,597, assigned to DuPont; 1993. [14] Poly-armour brake lines: fighting the effects of corrosive de-icing processes; 2007. [15] Dunlap KB, Riehle MA, Longhouse RE. An investigative overview of automotive disc brake noise. SAE Technical Paper Series 1999;724:1 8. Available from: https://doi. org/10.4271/1999-01-0142. [16] Ashurst IC, Herman CS, Li-Bovet L, Riebe MT. US Patent 6,131,566, assigned to Glaxo Wellcome Inc. and Glaxo Group Limited; 2000. [17] Translation: Internal coating for production tubing (PT) to inhibit organic and inorganic depositions in the well Yagual 12. PEMEX Magazine; 2007. [18] Causton BE, Glasson, EL. US Patent 6,110,532, assigned to The Gillette Company; 2000.

20 Fluorinated Ionomers: History, Properties, and Applications Sina Ebnesajjad1 and Walther Grot2 1

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20.1 History The first fluorinated ionomer was discovered in the early 1960s at the DuPont Experimental Station near Wilmington, Delaware. It was a perfluorinated ionomer that later became known as Nafion. This polymer in membrane form has revolutionized the process for making chlorine and sodium hydroxide (NaOH). These two chemicals are critical raw materials not only for the chemical industry, but also for producing glass, aluminum, and other products. Polyvinyl chloride, for instance, contains 57% chlorine by weight and poly tetra fluoro ethylene (pTFE), while not containing any chlorine, requires more than 2 kg of chlorine for each kilogram of pTFE produced. Chlorine and NaOH are produced by passing an electrical current through a solution of sodium chloride (NaCl). Chlorine gas is generated at the positive electrode (anode), NaOH and hydrogen gas at the negative (cathode). Contact of chlorine with the other two products must be avoided. The first commercial process (1885) used an asbestos diaphragm as a separator for this purpose. The resulting NaOH was of low purity and concentration. In 1892, H. Castner introduced the mercury process, in which a mercury cathode was used to form sodium amalgam, which then could be reacted with water outside of the electrolytic cell to form hydrogen and very pure and concentrated NaOH. In spite of its much high electric power consumption, this process was widely adopted in Europe and Japan. In the early 1960s, DuPont

introduced the newly discovered NAFION as a selective separator for the electrolysis of NaCl solutions. The acceptance of this new technology was accelerated by two new developments. Also, at about the same time, the Minamata disease in Japan increased the concern about the environmental dangers of mercury, making the elimination of its use in the chloroalkali industry desirable. At about the same time, V. de Nora in Italy introduced titanium-based dimensionally stable anodes (DSAs) to replace the earlier used graphite anodes. Both graphite anodes and asbestos diaphragms had limited lifetimes. Replacing both by longer lasting substitutes would eliminate the need for frequent shutdowns of the cells for maintenance. Thus NAFION and DSA were synergistic.

20.2 Composition The perfluorinated ionomers are copolymers of tetrafluoroethylene (TFE) and perfluorinated vinyl ether terminated by an ionic group, typically a sulfonic acid or, in some cases a carboxylic acid or a sulfon amide group. They are fundamentally different from other fluoropolymers, which merely have to endure a hostile environment in terms of chemical, electrical, mechanical, or thermal stresses, ionomers in their typical application have to actively engage the environment, for instance, by selectively allowing the transport of some species. Other fluoropolymers may be used as insulators to

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00020-3 © 2021 Elsevier Inc. All rights reserved.

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withstand the highest possible electrical potential, an ionomer is frequently used to allow the flow of an ionic current with the minimum potential difference, or as a catalyst, to promote a chemical reaction instead of merely being inert in a corrosive environment. The manufacture of these ionomers starts with the synthesis of a melt-fabricable precursor polymer. This is illustrated in Fig. 20.1. TFE reacts with sulfur trioxide to form a cyclic sultone. This is followed by the addition of 2 mol of hexafluoro propylene epoxide (HFPO) and dehalocarboxylation to yield a vinyl ether monomer. Copolymerization with about 6 mol of TFE results in the formation of the precursor polymer. The HFPO addition and subsequent reactions are similar to those used in the synthesis of other perfluorinated ionomers. The copolymerization with TFE is unique because in order to achieve the required transport rates, the amount of the active comonomer incorporated has to be about 40 wt.%. The composition of these copolymers is usually described in terms of equivalent weight (EW), the weight of polymer in grams

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containing one equivalent of cations. At a typical comonomer ratio of 1:6, the EW would be 1046. As a result of this high comonomer content, the precursor polymers are soft, pliable, and sometimes tacky materials. They can be fabricated in the melt at modest temperature, such as 280°C for melt extrusion. After fabrication into the desired physical shape, possibly including lamination to a reinforcing fabric, the precursor polymer is converted to the ionomer by treatment with a hot solution of KOH or NaOH. In many cases, the resulting potassium or sodium form of the ionomer is converted to the free acid by ion exchange with a strong acid, typically nitric acid. The steps involved in the manufacture of various ionomer products are illustrated in Fig. 20.2.

20.3 Properties The properties of the final ionomers are fundamentally different from other fluoropolymers, including the precursor form of the ionomer. The hydrophobicity, excellent electrical insulating, and nonstick properties have been nullified by the introduction of ionic groups. Compared to the precursor form, the stiffness has been noticeably increased and the tackiness eliminated due to ionic crosslinking. Water and many polar organic solvents are readily absorbed and it is the degree of swelling by these solvents that mostly determine conductivity and other transport properties. Mixtures of polar organics with water will frequently result in more swelling than either solvent alone as illustrated in Fig. 20.3. Similar results are obtained for isopropanol water mixtures. The swelling of an ionomer in a solvent or solvent mixture is a function of several factors: 1. The nature of the solvent or solvent mixture as discussed above. 2. The composition and particularly the EW of the ionomer. 3. The nature of the counterion. 4. In exposure to other than liquid solvents, the vapor pressure (or humidity in case of water). 5. The temperature of exposure. 6. The hydration history.

Figure 20.1 Synthesis of NAFION.

Fluoropolymers are hydrophobic and they would be expected that the presence of water and ions in

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TFE, SO3, HFPO, etc. Monomer synthesis Monomer TEF

Copolymerization Precursor polymer Pellet extrusion Precursor pellets

Hydrolysis, Acid Exch.

H+ pellets

Film extrusion Precursor film pTEF fabric

Hydrolysis, Acid Exch.

H+ film

Lamination Laminate

Hydrolysis

Na+ membrane Acid exchange H+ membrane

Figure 20.2 Manufacture of ionomer products [1].

Figure 20.3 Swelling of NAFION in mixtures of water and an organic.

fluorinated ionomers would lead to a degree of phase separation. In the water-swollen ionomer one could visualize a system of aqueous clusters containing most, if not all, ionic groups imbedded in a fluoropolymer matrix. These clusters must be interconnected to provide the observed ionic conductivity. The matrix,

on the other hand, provides a restraining force against the osmotic forces which are trying to enlarge the clusters. The matrix then determines the mechanical properties of the ionomer. Some of these properties are listed in Table 20.1 for NAFION of a nominal EW of 1100; the actual EW is about 1050.

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Table 20.1 Properties of Nafion Perfluorosulfonic Acid Membranes [1]. Nafion Type

Typical Thickness (µm)

Basis Weight (g/m2)

N 211 (solution cast)

25.4

50

N 112

51

100

N 212 (solution cast)

50.8

100

N 1135

89

190

N 115

127

250

N 117

183

360

N 1110

254

500

Property

N 112, 1135, 115, 117, 1110

N211

N212

Tensile modulus, 50% RH

249

284

258 MPa

In 23°C water

114

In 100°C water

64

Maximum tensile strength, 50% RH (MPa)

43 MD, 32 TD

23 MD, 28 TD

32 MD and TD

In 23°C water

34 MD, 26 TD

In 100°C water

25 MD, 24 TD

Elongation at break, (%) 50(%) RH

225 MD, 310 TD

252 MD, 311 TD

343 MD, 352 TD

In 23°C water

200 MD, 275 TD

In 100°C water

180 MD, 240 TD

Tear resistance, initial (g/mm) 50% RH

6000 MD and TD

In 23°C water

3500 MD and TD

In 100°C water

3000 MD and TD

Tear resistance, propagation (g/mm) 50% RH

. 100 MD, .150 TD

In 23°C water

92 MD, 104 TD

In 100°C water

74 MD, 85 TD

Water uptake at 100°C (%)

38

50

50

Linear expansion in 100°C water, (%)

15

15

15

MD, machine direction; TD transverse direction.

A high degree of swelling is observed if the ionomer in the hydrogen ion form is immersed in a mixture of water and a lower aliphatic alcohol. As the temperature is increased, the osmotic forces trying to swell the ionic clusters increase, while the restraining forces of the fluoropolymer matrix are weakened. Near the boiling point of the solvent mixture, the volume of the polymer has increased about 10-fold and the match of the refractive indices of polymer and solvent is so close that the polymer becomes invisible.

As the temperature is increased further (in an autoclave), at about 225°C, phase inversion occurs where the volume of the ionic clusters has increased so much that the fluoropolymer phase, now near its melting point, can no longer maintain continuity. It forms instead the interior of micelles that have an outer shell of ionic groups. Such “liquid compositions” have fairly high viscosity and can be used to cast films of the polymer. As the temperature is increased further, the size of the micelles decreases

20: FLUORINATED IONOMERS: HISTORY, PROPERTIES, AND APPLICATIONS

until at about 270°C they consist of a single polymer chain. Most applications of perfluorinated ionomers involve the passage of an electric current, in the form of cations, through the ionomer. The current could be supplied by an external power source (electrolysis) or generated internally (fuel cells and batteries). By far the largest application is as a separator in the electrolysis of NaCl. The process is illustrated in Fig. 20.4. The main anode reaction is represented by 2 NaCl-2Na1 1 2e2 1 Cl2. While the sodium ions pass through the membrane, the electrons, driven by the power supply, travel through the external circuit to the cathode. There they react: 2e2 1 2 Na1 1 2 H2O-2 NaOH 1 H2. The current flowing through any cross section of the cell is equal to the sum of negative charges moving to the left and positive charges moving to the right. The distribution between these two modes of transport is quite different in the three ionic conductors in the cell: In the membrane, the transport is almost exclusively by positively charged sodium ions; in the catholyte, it is predominantly by hydroxide ions; and in the anolyte, more than half by chloride ions (in both the cases, the balance is by sodium ions). Membranes used in this application typically employ a barrier layer of a carboxylic ionomer to improve hydroxide ion rejection. They also are coated, usually on the cathode side, sometimes on both sides, with a gas release coating containing zirconium dioxide as a “pigment.” Because of the commercial importance of this application, the three manufacturers of chlor alkali membranes (DuPont, Asahi Glass, and Asahi Kasei)

Figure 20.4 Schematic of the electrolysis of NaCl [1].

373

have made substantial efforts to optimize the performance of these membranes. The most important parameters are as follows: 1. The ability to operate at current densities (CD) of more than 5000 A/m2 (to minimize investment). 2. To operate at the lowest possible voltage (B3 V) at this CD (for minimum power consumption). 3. To give a current efficiency (CE) of more than 96%. 4. To give long service life ( . 3 years) while offering some resistance to brine impurities. At the same time, companies supplying electrolysis equipment (Thyssen-Krupp-Uhde, Asahi Kasei, and Chlorine Engineers) had to develop electrolytic cells suitable for the new membrane technology. These designs are being improved on an ongoing basis, with low operating voltage at a CD of .5000 A/m2 the primary objective in addition to ease of operation, including ease of membrane replacement. As a result, membrane technology today offers not only lower investment than the two older technologies (mercury and asbestos) but also substantially lower power consumption than mercury technology, and purer caustic than asbestos. Further reductions in cell voltage are made possible by the use of oxygen consuming cathodes. The development of production capacity of the three technologies is shown in Fig. 20.5. Large industrial chlor alkali cells are arranged in stacks of single cells in series or parallel electrical

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Figure 20.5 World chlorine capacity in millions of metric tons [2].

Figure 20.6 The Uhde cell stack [2].

connections. Electrodes and membranes are in a vertical orientation to minimize floor space. The Uhde design is used as an example of a cell stack with series connections, also referred to as a “bipolar” design. Fig. 20.6 shows a schematic of a partially assembled stack. A single element consisting of a sealed electrolytic cell with about 2.7 m2 active area is lowered into place. Up to 150 cells may be assembled in a single stack. Hoses to the respective headers then connect the two electrolyte inlets and two outlets. Current of about 15 kA is supplied to the stack through bus bars located at the two opposite ends

of the stack. With an individual cell voltage of about 3 V, a voltage of up to 450 V is required for the stack. Fig. 20.7 shows the cell room of a fairly large chlor alkali plant. About 10 electrolyzers or stacks are visible on the left side of the cell room, more on the right-hand side. Other electrolytic applications include the electrolysis of hydrochloric acid, the regeneration of chromic acid etching and plating solutions, and the manufacture of plating chemicals, such as potassium gold cyanide. The use of perfluorinated ionomers as the electrolyte in fuel cells has received considerable

20: FLUORINATED IONOMERS: HISTORY, PROPERTIES, AND APPLICATIONS

375

Figure 20.8 Exploded view of a PEM fuel cell [1]. Figure 20.7 Cell room of a 1000 t/day chlorine plant [2].

attention in recent years. Fuel cells come in different types, depending on the electrolyte used. Examples are solid oxide, molten carbonate, and phosphoric acid. The fuel cells using ionomers as the electrolyte are referred to as PEM fuel cells (polymer electrolyte membrane). They have the lowest operating temperature of all fuel cells and can self-start at or below room temperature. An exploded view of a PEM fuel cell is shown in Fig. 20.8. The membrane is the ionic conductor that closes the circuit between the two electrodes (in the figure identified as “catalyst layers”). It performs a similar function as the sulfuric acid in a car battery. In both cases, the conducting species is a hydrated hydrogen ion (proton). In addition, the polymer film must prevent mixing of the two gases, which in the presence of active catalysts would result in a fire. The two current collectors/gas flow fields are usually machined out of graphite plates. They allow the bulk flow of gas and electric current. The diffusion media, consisting of a felt made of carbon fibers, then provides the local distribution to the electrodes (catalyst layers). The electrodes consist of an interpenetrating network of three phases: 1. An electronic conductor consisting of carbon black. 2. An ionic conductor consisting of ionomer. 3. A gas (or void) phase. All three phases must be continuous and connect to their respective collectors (one and three to the diffusion media, two to the ionomer membrane). Finely divided platinum is used as a catalyst. In order to be effective, a platinum particle must be in

contact with all three phases, although the gases can diffuse a very short distance through the ionomer phase. The electrodes are prepared from an “ink” consisting of a “pigment” of platinum on carbon black suspended in a solution of the ionomer. A film of this ink is then cast on a release sheet. The volume of ionomer in the ink is selected so as to be insufficient to fill the voids between the closely packed pigment particles as the solvent evaporates. This creates the void spaces needed for gas transport. The two electrode layers are frequently laminated to the two surfaces of the ionomer film (the bulk electrolyte), creating a membrane-electrode assembly. Potassium gold cyanide is used as an example of the use of an open tank cell in industrial electrochemistry. Potassium gold cyanide is the most important gold plating chemical. Almost all the gold used for contacts, etc., in the electronic industry is derived from potassium gold cyanide. The older route to KAu(CN)2 started with the dissolution of metallic gold in aqua regia. It was followed by a lengthy procedure to replace unwanted chloride ions with cyanide ions and to remove excess nitric acid. Some losses of gold compounds were unavoidable and high product purity could not always be assured. Anodic dissolution of gold eliminates the introduction of any unneeded chemicals. The commercial process uses an open tank cell in order to allow easy replenishment of the gold. The tank is filled with a solution of KCN as the anolyte, into which anode baskets are suspended from overhead bus bars. The baskets are filled with granular gold, and fresh metal is added to the baskets as needed. Also, suspended in the anolyte are cathode bags made of Nafion and filled with a solution of KOH catholyte. Stainless steel or nickel can be used as cathode material. Any contact of the membrane with the cathode must be avoided.

376

The regeneration and purification of chromic acid is used as an example of using a completely closed system for handling extremely corrosive and toxic chemicals. In many industrial processes using chromic acid, a spent acid stream containing trivalent chromium and/or cationic contaminants is obtained. Disposal of this stream would require chemical reduction of any remaining hexavalent chromium followed by precipitation of chromium hydroxide and filtration. Electrochemical regeneration is an attractive alternative. It is being used on spent acid streams generated in three applications: 1. Chromic acid used in the oxidation of some organic compounds. 2. Chromic acid used in surface etching of plastic parts prior to metallizing. 3. Chromic acid plating solutions.

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In all the cases, anodic oxidation of trivalent to hexavalent chromium is the main desired reaction. In addition, removal of cationic contaminants is frequently desired. In this case, a design in which gasket failure would spill chromic acid into the work area is out of the question. Also, membrane replacement should involve at most minimum human exposure to hexavalent chromium and cathode removal and cleaning should be accomplished easily. A cylindrical cell based on a welded titanium tank meets these requirements. The tank contains the chromic acid etch solution as the anolyte. The central cathode can be pulled out overhead for removal of deposits of copper, without disturbing the rest of the system. Or the entire membrane/cathode assembly can be pulled with minimum exposure to chromic acid (Fig. 20.9).

Figure 20.9 Schematic of a chromic acid regeneration cell [1].

20: FLUORINATED IONOMERS: HISTORY, PROPERTIES, AND APPLICATIONS

Among applications outside the field of electrochemistry, the drying and humidification of gases are of some importance. Equipment based on capillary tubing made from Nafion sulfonic polymer is

Figure 20.10 Schematic of a tubular humidity exchanger [1].

377

commercially available from PermaPure. A schematic of such a tubular humidity exchanger is shown in Fig. 20.10. The drying of gases may be done for analytical purposes, while the humidification of hydrogen is of interest in fuel cells. The dryer may contain a single tube of Nafion [designated machine direction (MD) followed by the tube diameter in mils] or a tube bundle (designated PD followed by the number of 30 mil o.d. tubes in the bundle). The final number in the code indicates the length of the tubes in inches. The code MD-050-24 therefore indicates a dryer consisting of a single tube of 50 mil (51.27 mm) diameter and 24 in. (5610 mm) length. Any letter after the numbers indicates the material of the housing, for instance, S 5 stainless steel or P 5 polypropylene. The performance of the dryers is indicated by the dew point of the exit gas as a function of gas flow. The incoming sample gas is saturated with water vapor at 20°C. The flow of drying gas is twice the flow of sample gas. It can be seen that the performance is proportional to the number of tubes in the bundle, but not necessarily to the length of the tubes (Fig. 20.11).

Figure 20.11 Performance of tubular humidity exchangers [1].

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Table 20.2 Publications Relating to the Use of NAFION as a Catalyst. Subject

Year

Page

Acylation of aromatics using aroyl chlorides or anhydrides (e.g., benzophenones)

1978

672

Nitration of aromatic compounds using n-butyl nitrate

1978

690

Formation of methoxy methyl ethers using dimethoxy methane

1981

471

Cleavage of these ethers

1983

892

Tetrahydropyranylation of alcohols using dihydro-4H-py ran (and cleavage)

1983

894

O-trimethylsilylation of alcohols, phenols and carboxylic acids

1983

894

Ring closure of diols

1981

474

Dimethyl acetals from aldehydes or ketones using trimethyl orthoformate

1981

282

Hydrolysis of benzophenone dimethyl acetal

1981

283

Ethylene dithioacetals

1981

283

1,1-Diacetates from aldehydes using acetic anhydride (at ambient temperature!)

1982

963

Hydration and methanolysis of epoxides

1981

281

1981

473

1978

671

Rupe rearrangement of alkenyl tertiary alcohols to olefinic ketones 1

Hydration of alkynes (Nafion in the mixed H /Hg

11

Perfluorinated sulfonic acid ionomers have attractive properties for catalytic applications: 1. Very high acid strength (super-acid). 2. Exceptional chemical and thermal stability. 3. The solid state allows easy separation from the products formed; in membrane form, this catalyst may actually prevent mixing of the reactants.

form)

extensively. Olah has published a comprehensive review of this subject, including 181 references [3]. The high catalytic activity of these polymers is a result of the electron withdrawing effect of the perfluoroalkyl group on the sulfonic acid site. The acid strength of NAFION is comparable to that of 96% 100% sulfuric acid. Table 20.2 lists publications relating to the use of NAFION as a catalyst.

Three different types of catalysis may be considered: 1. Catalysis by the hydrogen ion. 2. Catalysis by some other cations acting as a counterion; some hydrogen ions may also be present in the ionomer. 3. Catalysis by some solid particles, such as metal particles. Acid catalysis using perfluorinated ionomers in the sulfonic acid form has been studied most

References [1] Grot W. Fluorinated ionomers. 2nd ed. Elsevier; 2011. [2] 12th Krupp Uhde Chlorine Symposium, Dortmund, 2004. [3] Olah GA, Iyer PS, Prakash S. Perfluorinated resin sulfonic acid (Nafion-H) catalysis in synthesis. Synthesis 1986;7:513 31.

21 Functional Fluoropolymers Claus-Peter Keller and Tomoya Hosoda AGC Chemicals, Tokyo, Japan

O U T L I N E 21.1 Introduction

379

21.2 Functional Groups

380

21.3 Functional Fluoropolymers—Partly Fluorinated 21.3.1 Ethylene/Tetrafluoroethylene 21.3.2 Polyvinylidene Fluoride

380 380 381

21.4 Functional Fluoropolymers—Fully Fluorinated 21.4.1 Perfluoroalkoxy Alkanes

381 381

21.5 Processing

381

21.1 Introduction Functional materials and functional polymers have become key drivers in the development of several high-end technologies. Chemical reactive or otherwise functionalized materials and polymers are often the base for further, specific modifications. This discipline of functional polymers is considered to become of increasing importance for future technology developments. The term “functional polymer” which in the following is adapted to “functional fluoropolymer” is defined as a polymer that (1) bears specific chemical groups or (2) that has specified physical, chemical, biological, pharmacological, or other uses which depend on specific chemical groups [1]. This chapter will focus on functional fluoropolymer grades which are used as construction materials, additives, or multi material components. Fluorinated ionomers and fluorinated coatings are treated in separate chapters of this book. Polyvinylidene fluoride (PVDF) is mentioned briefly here as there is a separate chapter as well.

21.6 Applications 21.6.1 Multilayer Hoses 21.6.2 Surface Lamination 21.6.3 Polymer Modification 21.6.4 Polymer Compatibilizing 21.6.5 Composites 21.6.6 Adhesive Dielectric Interlayer—5G Technology

382 382 383 383 384 385

21.7 Summary

388

References

388

387

Traditional fluoropolymers can be described as “ultranonfunctional” due to their extreme chemical inertness. In the fluoropolymer chain, the smaller carbon atom is surrounded by protecting fluorine atoms. This gives the fluoropolymers a combination of exceptional properties: excellent chemical resistance, high and low temperature resistance, oxidation resistance (incombustibility), and ultraviolet resistance (weather resistance). The polarizability of the fluorine atom and C F bond is small and the refractive index is low. In addition, fluoropolymers have low absorption and high resistance to electromagnetic waves, a low dielectric constant, and dielectric loss tangent. The critical surface energy is very low and they show very low wettability and are finally difficult to adhere to Ref. [2]. Almost logically adhesion was the primary aim when the first functional fluoropolymers have been developed. The desire to modify fluoropolymers is probably as old as the products themselves. In the early years after its discovery in 1938, it was a challenge to put polytetrafluoroethylene (PTFE) to commercial use.

Introduction to Fluoropolymers. DOI: https://doi.org/10.1016/B978-0-12-819123-1.00021-5 © 2021 Elsevier Inc. All rights reserved.

379

380

It was not possible to process this insoluble white powder with a melt viscosity so high that it would hardly flow even at temperatures well above its melting point. Special processing technologies needed to be developed to overcome that hurdle. To use fully fluorinated fluoropolymers in thermoplastic processing technologies, melt-processible grades have been developed in the 1960s with fluorinated ethylene propylene (FEP) and 1970s with perfluoroalkoxy (PFA). Reference [3] is an example from the late 1970s describing the grafting of fluorocarbon copolymers by applying ionizing radiation using a “preirradiation” method or a “simultaneous irradiation method.” The goal was to maintain the unique fluoropolymers properties but to improve characteristics such as adhesion, dyability, reactivity to other compounds, and the like. No proof could be found that such irradiation grafting of PTFE had been put to wider commercial use. Still today, etching of PTFE and gluing it to substrates is a very common technology to adhere PTFE to other substrates—if not done mechanically. In the mid-1990s, Ref. [4] was giving already a wider overview of examples how and where to make potential use of functional fluoro thermoplastics. It describes fluoropolymer powders containing polar functional groups grafted onto the surface of the fluoropolymer particle. Several processing and application examples are given with powders made of Ethylene/tetrafluoroethylene (ETFE), PVF, etc. The fluoropolymer powder surface is described to be irradiation grafted with maleic anhydride or other functional groups. Application examples given are multilayer films, polymer blends, and films made thereof, laminates, metal sandwich constructions and coatings utilizing the adhesive properties of the functional group grafted onto the fluoropolymer powder.

21.2 Functional Groups Functional fluoropolymers are based on more or less linear polymer backbones. Today, commercially available fluoropolymers modified for chemical reactivity or other functionality are typically fluoro thermoplastics such as ETFE, PVDF, or lately PFA. Besides the advantage of using thermoplastic processing technologies, the low melt viscosity is essential for the needed polymer chain movement in the molten stage to allow sufficient chemical interaction during the process of coextrusion, blending, coating, etc.

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Manufacturers typically do not disclose details about their technology and chemistry used for the modification of their fluoropolymer product range to achieve functionality. However, a certain pattern can be found in the patent landscape between the 1990s and today. This is reflecting the traditional chemistry of reactive groups widely used also for other polymer and material modifications. Organic acid anhydrides such as carboxylic anhydrides and, in particular, maleic anhydride are commonly listed in patents. Derivatives thereof are also frequently mentioned. Further functional groups referred to in the patent landscape are hydroxyl-, epoxy-, amino groups, etc. Various methods are described for functionalizing fluoropolymers. A method earlier investigated is the reactive irradiation process. An example is described by Kerbow in Ref. [4]. ETFE powder was mixed with a reagent maleic anhydride grade with a powder/anhydride ratio of 100/1. The blend was then exposed to a total irradiation of 6 Mrad and afterwards heated overnight at 120°C to evaporate any unreacted maleic anhydride. The achieved concentration of grafted maleic anhydride was approximately 0.4 wt.%. Later the method of adding functional groups to the polymer by reactive coextrusion as a posttreatment process after polymerization was established. This radical extrusion is typically done by adding a radical generating agent and typically described as polymer grafting Direct copolymerization of functional groups into the fluoropolymer chain is described as the preferred technology. Direct copolymerization supports the homogeneous distribution of the functional group into the full length of the macromolecules.

21.3 Functional Fluoropolymers— Partly Fluorinated 21.3.1 Ethylene/ Tetrafluoroethylene The widest offering of functional fluoropolymers can be found within the ETFEs. Manufacturers started expanding their commercial offering of functional ETFE in the early years of 2000s to serve the increasing demand for multilayer fuel lines. This also includes carbon filled grades to meet the requirement by the US market for anti-static fuel lines.

21: FUNCTIONAL FLUOROPOLYMERS

381

Therefore, a broader offering of functional fluoropolymers can be found with ETFEs versus other fluoropolymers. Manufacturers of functional ETFE grades are AGC Inc., Chemours, and Daikin Industries. To improve processing compatibility with lower melting polymers in coextrusion processes, for example, AGC Inc. has developed lower melting ETFE copolymers, whereas Daikin Industries is offering an EFEP terpolymer made of ethylene, tetrafluoroethylene, and hexafluoropropylene. Table 21.1 is showing the currently available grades of functional ETFEs.

Various grades are offered by Arkema described as functionalized homopolymer and copolymer. Typical applications are coatings or coextruded liners. As shown in Table 21.2, these ADX grades are available with melting points of 123°C and on the higher end 165°C and 170°C.

21.3.2 Polyvinylidene Fluoride

Partly fluorinated polymers do provide very good chemical or temperature properties and specifically better mechanical performance compared to the fully fluorinated ones. Full fluorination however provides exceptional chemical, as well as higher temperature resistance, and in particular outstanding dielectric properties. Even though often listed in patents, the range of commercially availability fully fluorinated functional grades such as PFA is limited to Fluon 1 PFA EA 2000 offered by AGC Inc. Table 21.3 compares the typical properties of a functional Fluon 1 PFA EA2000 with Fluon PTFE as well as standard PFA. The functionalized PFA shows very similar properties to a standard PFA.

Basic properties and typical uses of PVDF are described in a separate chapter, of this book. Table 21.1 Manufacturers, Grades, and Melting Points of Functional ETFE and EFEP [5 9]. Melting point (°C) DSC

21.4 Functional Fluoropolymers— Fully Fluorinated 21.4.1 Perfluoroalkoxy Alkanes

Manufacturer

Grade Name

AGC Inc.

Fluon 1 LM-ETFE AH-2000

240

Fluon 1 LM-ETFE AH-5000

225

Fluon 1 LM-ETFE LH-8000

190

21.5 Processing

Chemours

Tefzel ETFE HT2202

255 280

Daikin Industries, Ltd.

NEOFLON EFEP RP-4020

155 170

NEOFLON EFEP RP-5000

190 200

ETFE EP-7000

251 259

Basically, identical to fluoro thermoplastics such as PFA, FEP, ETFE, and PVDF, functional fluoropolymers can be processed by thermoplastic processing technologies like extrusion, injection molding, etc. It should be noted that a thermal decomposition of fluoropolymers will generate hydrogen fluoride. Local exhaust ventilation and corrosion-resistant equipment is required for prolonged contact with molten fluororesins.

Table 21.2 List of PVDF Grades Described As Functionalized (Co-)Polymer [10]. Melting Point (°C) ISO 11357-1/-3 10°C/min

Manufacturer

Grade Name

Arkema

Kynar ADX 1285-03 resin

Homopolymer

165

Kynar ADX 1720-03 resin

Homopolymer

170

Kynar Flex ADX 2250-05 resin

Copolymer

123

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Table 21.3 Typical Dataa of Functionalized PFA Versus Standard PTFE and PFA [11]. Fluon 1 PFA EA2000

Fluon PTFE

Fluon PFA P-63P

Item

Unit

Test Method

MFR

g/10 min

ASTM D3307 (327°C, 49N)

10 25

Melting point

°C

DSC

300

327

308

Specific gravity

ASTM D792

2.1

2.1 2.2

2.1

Durometer hardness

Shore D

59

55

59

7 18

Tensile strength at break

MPa

ASTM D638

36

20.6 34.3

40

Tensile elongation at break

%

ASTM D638

460

200 400

450

Flexural modulus

MPa

ASTM D790

640

578

560

Izod impact strength (23°C, notched)

J/m

ASTM D256

Non-break

157

non-break

Coefficient of water absorption

%

ASTM D570

, 0.03

, 0.01

, 0.03

Dielectric Constant (1 GHz)

ASTM D150

2.1

2.1

2.1

Dielectric dissipation factor (1 GHz)

ASTM D150

, 0.002

, 0.002

, 0.002

ASTM D257

6.5 3 1017

. 1.0 3 1018

. 1.0 3 1017

1.05

1.05

Volume resistance

Ω cm

Specific heat

kJ/(kg K)

1.05

cycles

8.0 3 10

MIT folding endurance

ASTM D2176

3

3.4 3 104

a

Not for design or qualification purposes.

Suitable release films made of PI or PTFE should be used in lamination and similar processes to avoid adhesion to metallic or other press components in contact with adhesive fluoropolymer melts.

21.6 Applications The applications described in this chapter are either industrially established or applications where the authors do see potential in the future.

21.6.1 Multilayer Hoses The use in automotive fuel lines can be considered a breakthrough for the use of functional

fluoropolymers. In the early 2000s, new worldwide regulations were launched, including CARB LEVII in the United States, that required significant reductions in hydrocarbon emissions from fuel delivery systems. For such applications, fluoropolymers do provide an excellent barrier for fuels, even those containing low molecular weight alcohols such as methanol. However, a pure fluoropolymer fuel line construction could not meet the physical requirements set forth by the automotive industry. Coextruded fuel lines with an inner barrier layer made of ETFE and an outer layer made of mechanically stronger polyamide (PA) was and is a solution to meet these increasing requirements [12]. Fig. 21.1 is showing a typical fuel line coextrusion process. In the cross-head, the interphase between

21: FUNCTIONAL FLUOROPOLYMERS

383

Figure 21.1 Multilayer coextrusion. Courtesy AGC Inc.

the outer and inner layers is fused together by covalent bonding of the PA’s amino end group and the anhydride functionalized ETFE. The use of functional fluoropolymers in multilayer hoses is still growing and expanding from fuel lines into other applications where harsh media are handled at elevated temperatures.

21.6.2 Surface Lamination Acid anhydride functionalized fluoropolymers are suitable for lamination to selected polymers, metals, or other substrates. Reactive groups such as epoxy-, amino, silane-, hydroxyl groups or similar can provide the required reactivity and adhesion. A fluoropolymer film applied will provide fluoropolymer properties to the laminates surface such as chemical resistance, low friction, and improved flame retardancy. In Ref. [13], Aida is comparing the adhesion strength of a low melting ETFE LH8000 film to various materials. The data in Table 21.4 have been determined from samples pressed at 220°C. The use of such film as a hotmelt between substrates allows higher temperature resistance, and provides electrical insulation to the construction.

21.6.3 Polymer Modification The modification and tuning of polymers is considered a key driver for future innovations. Whereas the number and variety of polymer modifiers seem to be unlimited, the selection of modifiers combining high-temperature resistance with good compatibility is limited. Functional fluoropolymers can be of interest in this field. Other than low MW PTFE additives, the use of functional fluoropolymers as polymer modifiers is still in the

Table 21.4 Adhesion of Low Melting ETFE LH-8000 (Tm 5 190°C) to Other Materials. Adhesion of ETFE LH-8000 to. . .

Adhesion Strength (N/cm)

Hot Press Temp. (°C)

PA12

18

220

PA elastomer

44

220

Polyesterurethane (TPU)

26

220

Epoxide modified polyolefin

70

220

SUS304

10

220

early stage. Reason is certainly the higher costs of functional fluoropolymers versus conventional PTFE additives but also specific requirements for the processing of fluoro thermoplastics. Hosoda and Ozawa have shown how a modified fluoropolymer can improve various properties in a PA [14]. PAs like PA6 are widely used for injection molded or extruded products such as fiber, films, and tubes. It plays an important role in many industries. It is also expected to be of increasing importance as a matrix resin for carbon fiber reinforced thermoplastics (CFRTP). The downside of PA6 and other PAs is the high water absorption due to its polar amide group in the molecule. Water absorption causes dimensional change and strength reduction and can become the cause of hydrolysis during melt processing. Fluoropolymers in general have low water absorption. Fig. 21.2 is comparing the water absorption of a standard PA6 versus modified PA6 at different loading levels. The functional fluoropolymer

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Figure 21.2 Water absorption of injection molded pieces of PA6 versus PA6 modified with a functional fluoropolymer (mPA6-1 and mPA6-2).

Figure 21.3 Impact strength of injection molded test specimen of PA6 versus PA6 modified with a functional ETFE (mPA 6-1 and mPA6-2) according to ASTM D256.

additive is reducing the water absorption by up to 1 wt.% in such blend. At the same time, a functional fluoropolymer added to PA6 is functioning as impact modifier. Fig. 21.3 shows the impact improvement of such blend at 23°C and 240°C. Depending on the loading, the impact strength at room temperature (RT) can be improved by up to three times. At 240°C, the impact strength can be improved by the factor of 2.

21.6.4 Polymer Compatibilizing As per definition, a polymer compatibilizer is an additive that enhances the blendability of two immiscible components [1]. A widely used polymer compatibilizer, for example, is PP-g-AH. Such anhydride grafted polypropylene is added to

achieve a better blending of almost immiscible PP/ PA blends. When compounding fluoropolymers into a base polymer or when compounding additives into fluoropolymers, sometimes compatibility issues can be observed in the injection molding process such as delamination or agglomeration. As an example, low MW PTFE additives (so-called micropowders or lubricants) are used in polymers to improve wear and friction performance. PTFE has a low surface energy of about 20.0 Nm/m. Accordingly, its miscibility into most other polymers is limited. Applied irradiation or thermal treatment to produce a low MW PTFE additive is increasing the surface energy and surface area of the particles, but still during processing these can have a tendency to agglomerate during processing of the compound.

21: FUNCTIONAL FLUOROPOLYMERS

Modified fluoropolymers can function as typical polymer compatibilizers. They can enhance the compatibility of fluoropolymers with other blend components. Adding a small amount of functional fluoropolymer can modify the interfaces and provide a higher stability to the blend. The authors have measured such compatibilizing effect in a PPS grade blended with 20% PTFE to enhance wear and friction properties. Adding a small amount of a functional PFA as a polymer compatibilizer has shown a positive impact on the dynamic friction coefficient as well as the abrasion resistance. In the sweet spot, a reduction of the dynamic coefficient of friction by 50% and an improvement in abrasion resistance down to 30% versus the benchmark of PPS:PTFE (80:20) were measured [15]. Adding a low MW PTFE into the base polymer to improve tribological performance leads to a loss of mechanical properties. Therefore, a fluoropolymer compatibilizer can also be used to reduce the amount of low MW PTFE additives needed to achieve the desired wear and friction performance but maintain a higher level of mechanical performance.

21.6.5 Composites Weight and energy saving are strong drivers for the use of fiber reinforced composites in order to replace metals. Whereas resin-based composites are well established, thermoplastic composites are expected to continue to grow at double digit rates. Compared to resin systems, thermoplastic composites do allow higher production units by using thermoplastic mass production technologies. Other advantages of thermoplastic matrix materials are the higher ductility compared to resins as well as the possibility to use thermoplastic welding technologies. A key criterion for the composites’ performance is the fiber-matrix adhesion. Whereas

385

resin systems are typically designed to create a covalent bonding with the fiber sizing, often thermoplastic matrix materials just shrink onto the fiber and form a mechanical bonding without providing significant covalent bonding. Reference [14] describes how a functional fluoropolymer with introduced reactive groups into the polymer chain can provide such covalent bonding to fibers, sizing (e.g., epoxy or silane based), epoxy resins, selected polymers like PA with carboxyl end groups or metals (hydroxyl groups). Fig. 21.4 shows the adhesion comparison of a standard PFA (PFA fluoropolymer) versus an adhesion modified PFA to a carbon fiber with standard sizing after cracking of the specimen in a flexural strength test. As expected, there is no covalent fiber matrix adhesion with the standard PFA. The flexural strength of this composite is determined by mechanical fiber-matrix adhesion only. After cracking, the PFA matrix was easily removed from the fibers (left). In contrast, the SEM picture (right) with the adhesion modified PFA shows a high level of covalent fiber matrix adhesion leading to a high level of cohesive cracking. The flexural strength can be improved by about 70% in such a composition by utilizing an adhesion modified PFA. Where composite applications benefit from fluoropolymer characteristics such as high/low temperature resistance, excellent chemical resistance, ductility, low coefficient of friction, damping, and galvanic corrosion barrier, reactive fluoropolymers can be used in multiple ways. Reactive fluoropolymers enhance composites’ properties by either introduced as matrix, matrix polymer additive, applied to the surface or interlayer, as film or prepreg or using it as hot melt in sandwich constructions in combination with metal outer surfaces. Hosoda shows in Ref. [14] examples of the use of reactive fluoropolymer film or the use of a surface layer sandwich construction in combination

Figure 21.4 Covalent fiber-matrix adhesion of adhesive Fluon plus PFA versus standard PFA after flexural test. Courtesy AGC Inc.

386

INTRODUCTION

with an epoxy-based carbon fiber reinforced polymer (CFRP) and a PA6-based CFRTP. Fig. 21.5 illustrates such design examples of a sandwich construction using a functional fluoropolymer film on the surface (left) or an outer prepreg layer made with a functional fluoropolymer matrix (right). Table 21.5 shows the results of flame retardancy tests according to UL94HB. The CFRTP with PA6 matrix (upper) was measured with a burn rate of 12 mm/min. Protecting the PA6 construction with a 100 μm adhesive ETFE films leads to an extinguishing of the sample. It is found that the high flame retardancy of an ETFE can be applied to the surface of a PA6 CFRTP. With the same sandwich construction, it is possible to improve the wear resistance and to lower friction versus a pure PA6 CFRTP composite. Table 21.6 is comparing abrasion loss and dynamic coefficient of friction of a

TO

FLUOROPOLYMERS

PA6 composite construction versus a sandwich construction. The polymer modifying characteristics of a functional fluoropolymer as described in Section 21.6.3 can also be transferred to composite matrix polymers. CFRTP prepregs are made by impregnating a polymer into a carbon fiber fabric or a UD (unidirectional) tape. Lamination is used with a number of prepreg layers at temperatures above Tm and under high pressure to produce the CFRTP part. There are various methods for making prepregs. Major impregnation methods are powder and film impregnation. Table 21.7 compares powder versus film impregnation and the impact on physical properties of a reactive fluoropolymer-modified PA6 (mPA6) in a CFRTP. A hot press was used to produce molded test specimens. Due to the reduced water absorption

Figure 21.5 CFRP or CFRTP sandwich constructions with fluoropolymer surface. Table 21.5 Effect of ETFE Surface to Burning Rate in PA6 CFRTP. Total Thickness (mm)

Burning Rate

Layer structrure

PA6 CFRTP

1.5

12 mm/min

3 layer sandwich structure

adh. ETFE film (100 μm) PA6 CFRTP adh. ETFE fim (100 μm)

1.5

Extinguished

Table 21.6 Effect of ETFE Surface to Tribological Performance of a PA6 CFRTP. Total Thickness (mm)

Abrasion Loss (loss/cm3 )

Dynamic Coefficient of Friction

Layer structrure

PA6 CFRTP

1.5

0.015

0.27

3 layer sandwich structure

adh. ETFE fim (100 μm) PA6 CFRTP adh. ETFE fim (100 μm)

1.5

0.005

0.19

21: FUNCTIONAL FLUOROPOLYMERS

387

Table 21.7 Impact on Mechanical Properties of a Modified PA6 (mPA6) by Impregnation Method. Powder Impregnation

Film Impregnation

Matrix resin

m-PA6

m-PA6

Impact resistance

Improved

Improved

Tensile strength

Maintain

Maintain

Flexural strength

Maintain

Maintain

Water absorption

Lowered

Lowered

of the matrix resin, the hydrolysis of PA6 CFRTP during hot pressing should be prevented. For composite prepreg or UD-tape impregnation using the film impregnation technology, a low melt viscosity is desired to secure homogeneous impregnation at moderate press forces. However, film extrusion with low-viscosity thermoplastics (here PA6) can be difficult. Therefore higher viscosity grades with higher melt strength are typically used. The modification of PA6 with a functional fluoropolymer does not only improve the impact resistance, but also allows the use of lower viscosity PA6 for film manufacturing as laid out in Ref. [14]. Fig. 21.6 compares the impact resistance of a high viscosity/low flow PA6 film extrusion grade (left) with a functional fluoropolymer-modified low (middle) and high flow PA6 (m-PA6 and high flow PA6). Hosoda is explaining that such solution allows the use of higher MFR PA6 grades for faster film extrusion as well as an improved impact resistance of the blend.

21.6.6 Adhesive Dielectric Interlayer—5G Technology Fluoropolymers provide excellent dielectric properties over a wide temperature range such as low relative dielectric constant and dielectric loss tangent. However, the new era of “Internet of Things” (IoT) to be realized with the fifth generation of mobile communication system (5G) requires improved hardware to securely control autonomous

Figure 21.6 Izod impact strength PA6 and modified PA-6 versus high flow modified PA-6 according to ASTM D256.

driving, sensitive industrial processes, and so forth. This is, in particular, the case in the short distance FR2 (frequency range 2) operating between 30 and 100 GHz (in practice likely 25 GHz plus) where conventional printed circuit boards (PCBs) are expected to compromise on transmission loss and insertion loss. The dilemma for flexible or rigid PCB manufacturers is that the best dielectric materials typically have poor adhesion and therefore the copper film needs a certain surface roughness. At frequencies .1 GHz, transmission losses start to increase with higher surface roughness due to the skin effect. The authors have investigated the advantages of a functional fluoropolymer coating in highfrequency PCB use. A functional PFA with adhesion characteristics to copper allows the use of smoothest copper films and providing the needed combination of adhesion and dielectric properties. As shown in Section 21.4, the dielectric constant of a Fluon 1 PFA EA2000 at 1 GHz is 2.1, the dielectric dissipation factor ,0.002, and the volume resistivity 6.5 3 1017 Ω cm. The continuous use temperature is typically well above 200°C. Table 21.8 is showing that an adhesion strength of 12 N/cm can be achieved at the lowest copper foil surface roughness (Rz 0.9 μm, Ra 0.13 μm). The standard copper foil used in PCBs can have an about 10 times higher surface roughness (Rz 7.2 μm, Ra 1.45 μm). In particular, in future 5G application between 30 and 80 GHz, the use of smoothest copper surfaces is crucial for the highspeed data transfer.

388

INTRODUCTION

Table 21.8 Adhesion Strength of a Functional Fluon 1 PFA EA2000 to Copper Foils with Different Surface Roughness. Surface Roughness

[3]

Rz (µm)

Ra (µm)

Adhesion Strength (N/cm)

[4]

Regular copper foil

7.2

1.45

.18

[5]

Low profile

3.0

0.25

18

Profile free

1.2

0.19

17

Nonroughened

0.9

0.13

12

Copper Foil

21.7 Summary Whereas fluoropolymers with added functionality are established and widely used as fluorinated ionomers in membranes or as electrode binders or separator coating in batteries, the use of these functional fluoropolymers for more construction-related applications is still in the early stages. The authors believe that functional fluoropolymers will in the future be of increasing importance in high-performance composites, compounds, coatings, and the like. This chapter gives an overview of commercially available grades and an idea of the functionality of the same. It shows established applications as well as design examples for future applications. It should give the reader an idea of technical possibilities and inspire him to consider a functional fluoropolymer as solution provider. Functional fluoropolymers provide chemical reactivity as well as improved compatibility by maintaining the unique combination of characteristics of a fluorinated polymer.

References [1] Jones RG, et al. Compendium of polymer terminology and nomenclature IUPAC recommendations;. online edition 2008. Available from: https://www.iupac.org/cms/wp-content/ uploads/2016/01/Compendium-of-PolymerTerminology-and-Nomenclature-IUPACRecommendations-2008.pdf. [2] Nishi E. Recent trends in synthesis and applications in fluoroorganic materials, chapter 4 6.

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

TO

FLUOROPOLYMERS

CMC Publishing; 2018. Available from: https://www.cmcbooks.co.jp/products/detail. php?product_id 5 5455. Machi S, et al. US Patent 4,129,617, assigned to Japan Atomic Energy Research Institute and Maruzen Oil Company Limited; 1978. Kerbow DL. US Patent 5,576,106, assigned to E. I. Du Pont de Nemours and Company; 1996. Fluon® melt processable fluoropolymers, https://www.agcce.com/brochurespdfs/sales/ MeltProcessableFluoropolymers.pdf; 2018. Fluon® LM-ETFE AH series, grade list, https://www.agc-chemicals.com/jp/en/fluorine/ products/detail/index.html?pCode 5 JP-ENF010; 2019. Tefzelt ETFE HT-2202 fluoroplastic resin, product information, https://www.chemours. com/Teflon_Industrial/en_US/assets/downloads/tefzel-etfe-ht-2202-product-information. pdf; 2015. Adhesive fluoropolymer Neoflon EFEP, https://www.daikinchem.de/sites/default/files/ pdf/EFEP/Product%20Brochure% 20NEOFLON%20EFEP.pdf; 2003. Technical data sheet ETFE EP-7000, https:// daikin-america.com/wp-content/uploads/2013/ 07/TDS-ETFE-EP-009-EP-7000.pdf. Material Data Center, Kynar PVDF Arkema, https://www.materialdatacenter.com/ms/de/ Kynar/Arkema/248. Fluon technical information. Fluon perfluoro https://www.agcchem.com/wpadhesive, admin/admin-ajax.php?juwpfisadmin 5 false& action 5 wpfd&task 5 file.download&wpfd_ category_id 5 30&wpfd_file_id 5 1992& preview 5 1&embedded 5 true. Press release 2003 Ube Industries and Asahi Glass Company Announce development of novel two-layer fuel tube system. Sunbestat http:// www.agc.com/en/news/detail/20030519.html. Aida S. Development of new adhesive fluoro polymer for multi layer fuel tube. J Adhes Soc Jpn 2005;41:67 71. Hosoda T, Ozawa N. Physical property improvement of resin composites by fluorination technology. Shasai-Technology, Technical Information Institute Co., LTD; 2018. Keller C.-P., Paper novel fluoropolymer additives for wear and friction improvement, AMI conference, Du¨sseldorf, September 19 20; 2018.

Index Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

A Abused particles, 128 Accelerators, 188, 232 Acetic acid, 22 Acetoxyfunctional condensation system, 303 Acetylene, hydrofluorination of, 186 Acid acceptor systems, 285t for fluorocarbon elastomers, 285 Acid anhydride functionalized fluoropolymers, 383 Acrylic compounds, 186
187 Acrylic copolymers, of chlorotrifluoroethylene, 235t Acrylic monomers, chemical structures of, 234f Acrylic polymers, 90, 156 Activators, 188, 232 Actual kettle, 69 Additives, 344 Adhesion, 345 Adhesive bonding of PCTFE, 252 Adhesive dielectric interlayer, 387 ADONA, 88, 88f Advanced Polymer Architecture (APA), 311 Aerospace industries, 306 data cables, 265f fluoropolymers applications, 267
268, 267f and military applications, 299 AF. See Amorphous polymers (AF) AFLAS, 297 AGC Inc., 103t Agglomeration process, 344, 384 for PTFE pelletization, 80, 92 Agitation, 67, 71
72, 88, 92 Air-cooled blown-film process, 226
227 Aircraft wiring operates, 266 Air/jet mill, 74f working principle of, 74f Algoflon PTFE dispersions, 104t fine powder, 107t Alkyl boron, 191 Aluminum/magnesium, 348 Aluminum photocopier fuser rollers, 360f American Society for Testing Materials (ASTM), 92
93

Ammonium hydroxide, 90 Ammonium perfluorooctanoate (APFO), 22, 49, 57, 87
88, 151, 274 Ammonium persulfate, 66f, 274 pH effect on degradation of, 66f Ammonium ω 2 hydroperfluorononanoate, 92 Amorphous polymers (AF), 24, 280 Anhui Lite Environment, ePTFE production, 42 Anionic surfactants, 156 Annealing, 119, 247
248 Annealing temperature, 228 Antiblocking agent, 301 Anticoagulant, 86 APFO. See Ammonium perfluorooctanoate (APFO) Applications, of fluoropolymers, 56 Aqueous polymerization, 232 of perfluoroalkoxy polymers, 174
176 of perfluoroalkyl vinyl ethers, 176t of perfluoroethyl vinyl ether, 178t of tetrafluoroethylene, 176t, 178t, 179t of vinylidene fluoride, 51 Architectural coatings, 202, 366 Architectural fabric, 17 Arkema Kynar Flex PVDF copolymers, 198t Arkema Kynar PVDF homopolymer, 195t Asahi Chemical Industry Co., Ltd., 278
279 Asbestos packings, 160
161 Asian Manufacturers, ePTFE production, 42 ASTM. See American Society for Testing Materials (ASTM) ASTM D1418, 272 ASTM D2000-SAEJ200, 295t ASTM Method D1238, 210 ASTM Method D4441-96, 96 dispersion, 96, 97t, 98t ASTM Method D4894, 93 granular PTFE, 93t, 94t ASTM Method D4895, 93
94, 144
145 fine powder polytetrafluoroethylene, 93
94, 95t, 96t, 97t ASTM Method D1430, 239, 240t, 245 ASTM Method D3222, 192, 193t, 194t ASTM Method D1243, 190
191

Attribute
application relationships, 331
332 Autoclave, 71
72 Automatic compression molding process, 119f Automatic guns, 349 Automatic molding, 119
120 Automatic wash systems, 70 Automotive fuels resistance to, 297 seals, 290 Automotive industry, 306 air conditioner piston, 361f applications, 298 brake dampers, 363f fluorinated coatings in, 361
363 fluoropolymers applications, 267
268 fuel pump, 361f seat belt D-rings, 361, 362f weld-nut with fluoropolymer coating on threads, 362f Automotive sensor, 267f Azeotrope of CTFE, 235

B Bagley Correction, 211 Bakeware, 358 Baking/curing, 351
352 Bare bulbs, 360 Batch/continuous process, 311 Bearing pads, 266
267 Benefit
cost ratio, 1 Benzoyl chloride, 10 Benzoyl peroxide, 232 Benzyltriphenylphosphonium chloride (BTPPC), 283 Bepex Corporation, 85 BHA Corporation, ePTFE production, 41 Biaxially oriented film, 223, 227
228, 227f blown process for, 227f tenter process for, 228f Billet molding, 113, 115
119 cooling, 118
119 degassing, 115
116 preforming, 115 preforming equipment for, 134f sintering, 116
118 Bimodal polymer synthesis, 279

389

390

Bioactive compounds, 10
11 Biocompatible ePTFE, 36 2,2-Bis(4-hydroxyphenyl) hexafluoropropane, 283 Bis-perfluorobutyryl peroxide, 175 Bisphenol A, 301 Bisphenol AF, 283 Bisphenol cure systems, 283 Bisulfite, 65
66 Blown-film process, 226
227 Blown process, 227
228 for biaxially oriented films, 227f Boron nitride (BN), 212, 212t Bottle or jar method, 130
133 Brake dampers, automotive, 363, 363f Brakeline tubing, 362f Branched fluoropolymers, 11 Bromination, 45 Bromine-based fluoroelastomers, 283
284 1-Bromo-1,1-difluoroethane, 185 BTPPC. See Benzyltriphenylphosphonium chloride (BTPPC) Buffer solution, 233 Bulk polymerization, 51 of chlorotrifluoroethylene, 232

C Calcium carbonate, 301 Calcium fluoride (CaFe2), 25 Calendering, 291 Carbon fiber reinforced polymer (CFRP), 385
386 Carbon fiber reinforced thermoplastics (CFRTP), 383 PA6 burning rate in, 386t PA6 tribological performance of, 386t sandwich constructions, 386f Carbon
fluorine bond (C
F), 7, 10
12 Carboxylate, 179 Carboxylic acid, 179 Cast film, 223
226, 223f Cathode materials, 329 CCT. See Critical cracking thickness (CCT) Ceramic filled system, 357 C
F bond, 52
53 CFRP. See Carbon fiber reinforced polymer (CFRP) CFRTP. See Carbon fiber reinforced thermoplastics (CFRTP) Chain transfer agent (CTA), 22, 173, 175, 188, 190
191, 274, 278 and CSM, 275 C
H bond, 52
53 Chemical bleaching, 163 Chemical bonds, 345 Chemical processing industry (CPI), 13, 202, 253
254 components, 256
258

INDEX

fluorinated coatings, 363, 364f Chemical reactors, fluorocoatings, 363
364 Chemours, 353 Chill roll cast film, 223
224, 224f, 225f, 225t Chlorine, 369 capacity in millions of metric tons, 374f Chlorine atom, 236 Chlorine gas, 369 Chlorine trifluoride (ClF3), 53 Chlorodifluoroacetylfluoride, 49 1-Chloro-1,1-difluoroethane, 47, 185
186 Chlorodifluoromethane, 45
46 Chloroform, 25, 45 Chlorosulfonic acid, 280
281, 297 Chlorotrifluoroethylene (CTFE), 44, 51, 231
232 acrylic copolymers of, 235t azeotrope of, 235 bulk polymerization of, 232 copolymerization of, 234
236, 234t, 245 emulsion polymerization of, 233
234, 233t homopolymers, 245 plastics, 231 polychlorotrifluoroethylene. See Polychlorotrifluoroethylene (PCTFE) polymerization of, 232
234 Farberindustrie’s pioneering, 232 properties of, 49 suspension polymerization, 232
233 synthesis of, 47, 232 Chromic acid, 376, 376f Circuit density, 264 Coagulated dispersion powder, 65, 125 Coagulating agent, 92 Coalescence, 111 Coat hanger die, 229, 229f Coating metal and hard surfaces, 161
162 Coating systems, 358
359 fluorocarbon elastomers in, 300
301 midcoat, 358 primer, 358 processes, 152
153 topcoat, 358 Coating technology, principles of, 151
155 coating processes, 152
153 rheology, 153
154 surface energy, 154
155 Coaxial cables, 130, 265f Co-coagulation, 150, 165 Coefficient of friction (COF), 321
322, 326 CO2-expanded acetic acid, 22 COF. See Coefficient of friction (COF)

Commercial fluorocoating producers, 352
353 Commercial fluoropolymers, 44 comonomers used in, 170f Commercial/industrial bakeware, 358
359, 359f Commercial PCTFE resins, 239
241 Commercial perfluoroelastomers, 272
273, 282f Commercial PTFE resins, 97
107 Commercial semibatch reactors, 277 Comminution, of suspension polymerization, 73
78 fluid energy milling, 73
74 hammer milling, 74
76, 75f of wet reactor bead, 76
78 Community water fluoridation, 1 Comonomers, 189 units, 170f used in commercial fluoropolymers, 170f Compounded annual growth rate (CAGR), 26 Compression-molded seals, 288 Compression molding, 111
119, 207, 228, 247
249, 248f billet molding, 115
119 cooling, 118
119 degassing, 115
116 preforming, 115 sintering, 116
118 densification and sintering mechanism, 114
115 fluorocarbon elastomers, 292 of large tubes and rods, 249 ovens, 114 presses, 114 and tooling, 113
114 Compression ratio (CR), 113 Compression set resistance, 295 Condensation reactions, 303 Conductor, 139 Cone-and-plate viscometer, 211 Contact angle, 154
155, 155f Continuous emulsion polymerization, 276
277, 276f Continuous stirred tank reactors (CSTRs), 276 Controlled cooling, 114 Conventional fluoroelastomers, 212 Convoluted tubing, 141 Cookware manufacturers, 353
355, 357 Cooling, 137
139, 143
144 billet molding, 118
119 Copolymerization, 44, 186
187, 340
341 of chlorotrifluoroethylene, 234
236, 245 of ECTFE, 45 of ETFE, 45, 341 fluorinated ethylene propylene, 341 polyvinylidene fluoride, 185, 191

INDEX

Corona-charging system, 350
351, 351f Couette viscometer, 211 Covalent fiber-matrix adhesion, 385f C. R. Bard Corporation, ePTFE production, 41 Critical cracking thickness (CCT), 155
156, 158 Critical shear rate, 208, 301 Critical shear velocity (CSV), 208 Crosshead die, 221, 221f Cross-linking fluorocarbon elastomers, 282
284 by free radical mechanism, 283
284 by ionic mechanism, 282
283 by ionizing radiation, 284 of fluorosilicones, 303 peroxide, 303 Cryolite (Na3AlF6), 8
9 Cryolith, 8
9 Crystalline system, of polytetrafluoroethylene, 53, 93, 169
170 Crystallinity, 208, 236, 246 function of ZST, 248t of polymer, 237 of polytetrafluoroethylene, 139 of polyvinylidene fluoride, 194 Crystallization, 118 miscibility and, 171 of TFE copolymers, 170 water, 186 CSTRs. See Continuous stirred tank reactors (CSTRs) CSV. See Critical shear velocity (CSV) CTA. See Chain transfer agent (CTA) Curatives, for fluorocarbon elastomers, 285
286 Cured fluorocarbon elastomers, physical and mechanical properties, 295
297 compression set resistance, 295 heat resistance, 295 low-temperature flexibility, 295
297 resistance to automotive fuels, 297 resistance to solvents and chemicals, 297 steam resistance, 297 Cured fluorosilicone elastomers electrical properties, 305, 306t fluid and chemical resistance, 304
305 heat resistance, 305 low-temperature properties, 305 surface properties, 305 Cure-site monomer (CSM), 274, 279
280 Curing systems for fluorocarbon elastomers, 286t, 294
295 formulations for, 287t peroxide, 295 Cyclohexafluoropropene, 172

391

D DAI-EL T-530, properties of, 308t Daikin Industries, Ltd., 231
232, 353, 380
381 ePTFE production, 42 Daikin Neoflon M-300, 249 Daikin Polyflon PTFE, 138t aqueous dispersion fluoroadditive, 326t dispersions, 103t fine powders, 105t fluoroadditive powders, 326t molding powders, 99t TFE, 142t Daikin’s Neoflon PCTFE, 242t, 251t properties, processing, and applications, 246t Decarboxylation, 51 Dedusting powders, 166 Degassing, 115
116 Delicate balance, 360 Demolding, 119
120 Desflurane, 2
3 Developmental history, of fluoropolymers, 13 DeWal Industries, ePTFE production, 41 Dextrose, 188 Diamine cure system, 282
283 1,2-Dichloro-2,2-difluoroethane, 47
48, 186 Die, 135
137, 136f, 221 for cast films, 223f for fluoropolymer extrusion, 221 Die-cut filled PTFE gaskets, 259f Die design, 137 for tubing extrusion, 140 Die land, 137 Dielectric strength (DS), 112
113 Die lips, 229 Die orifice size, 96 Diethylaminosulfur trifluoride, 9 Differential scanning calorimetry (DSC), 127
128, 144, 170, 173
174 1,1-Difluoro-2-bromoethane, 48, 332
335 Diisopropylperoxydicarbonate, 188
189, 278 di-Limonene, 45
46 Diluted dispersion, 92 Dimensionally stable anodes (DSAs), 369 Dimethylacetamide, 56, 188 Dimethylformamide (DMF), 188, 191 Dip coating, 152 Dip tank, 159 Direct copolymerization, 380 Dispersion polymerization, 51, 64
65, 85, 178 modify PTFE in, 64 polytetrafluoroethylene by, 86
87 recipe and properties, 87t

preparation of, 89
92 reactor, 88 tetrafluoroethylene and finishing processes, 86f of TFE with APFO replacements, 87
88 Dispersions of polytetrafluoroethylene, 50, 56, 96, 155
158 ASTM Method D4441-96, 96, 97t Dyneon Hostaflon, 104t Disposal of used products, 311 Disuccinic acid peroxide, 86
87 chemical structure of, 66f Donaldson Corporation, ePTFE production, 41 Downwards extrusion, 227
228 Dry-bag isostatic molding, 122 Dryer drums, 364
365, 365f Dry film lubricants, 322 Dry lubrication coating, 361
362 Dry powder, 350 Dry process, for agglomeration of PTFE, 80 DS. See Dielectric strength (DS) DSAs. See Dimensionally stable anodes (DSAs) DSC. See Differential scanning calorimetry (DSC) Dual laminate, 254 DU bearing process, 165 DuPont Company, 19
20, 26
28, 33
34, 44, 48, 331, 337, 353
356 coating system, 363
364 fry pan coating technology, 356 spectrographic fry pan, 356 DuPont Dow Elastomers, 276 Dwell time, 115 Dyes, 343
344 Dyneon, 302, 353 PTFE fluoroadditive powders, 325t types, 291 DYNEON 2145, 301 Dyneon ADONA, 88 Dyneon Hostaflon PTFE dispersions, 104t fine powder, 106t granular resins, 99t

E Economy, of fluoropolymers, 24
25, 58 ECTFE. See Ethylene
chlorotrifluoroethylene copolymer (ECTFE) Effective test method, 144 Elastomeric bag, 120
121 Elastomeric material, 120 Elastomeric seals, 321 Elastomers in fuel blends, 297t PMTFPS, 304t

392

Electrical applications, fluoropolymers, 264
266 Electrical properties, cured fluorosilicone elastomers, 305, 306t Electrical/wire/cable industries, 309 Electric-grade tapes, 112
113 Electric heating bands, 135 Electric wire insulation molded, 138t Electrochemical regeneration, 376 Electrodecantation process, 90 Electropolished cage agitator, 89f Electrostatic coating, 350 Electrostatic offset, 360 Emulsion concentration process variable, 91t Emulsion polymerization, 194 of chlorotrifluoroethylene, 233
234, 233t continuous, 276
277, 276f fluorocarbon elastomers, 275
280 of polytetrafluoroethylene, 51 semibatch, 277
278, 277f suspension polymerization, 278
280 of vinylidene fluoride, 51
52, 188
190 End group stabilization, 178
180 Energy, of fluoropolymers, 24 Enfluorane, 2
3 Enflurane, 10 Environmental emissions, 161
162 ePTFE. See Expanded polytetrafluoroethylene (ePTFE) ETFE. See Ethylene tetrafluoroethylene polymer (ETFE) Ethane, on melt viscosity and point, 189t Ethylene, 188, 341 Ethylene
chlorotrifluoroethylene copolymer (ECTFE), 256, 342 dielectric constant of, 55 finishing of, 51 polymerization of, 51 properties, 55 Ethylene
cotetrafluoroethylene polymers finishing of, 51 polymerization of, 51 Ethylene terpolymers, properties of, 182t Ethylene tetrafluoroethylene polymer (ETFE), 13, 22
24, 45, 164, 170, 171f, 183t, 213, 268, 343 adhesion of, 383t copolymers, 341 functional fluoropolymers, 380
381, 381t preparation, 180
183 properties, 55 structure, 342f Et/TFE copolymer, 182 Excessive shearing, 85 Exclusion model, 169
170, 170f Exothermic polymerization, 70

INDEX

Exotic monomers, 44 Expanded polytetrafluoroethylene (ePTFE), 2, 4f, 258 aortic repair graft, 268f vascular graft, 268f Extended mandrel technique, 222
223, 222f Extended polytetrafluoroethylene (ePTFE) application, 36 association, 33
34 biocompatible, 36 discovery of, 35
37, 35f history of, 33 invention, 37
40. See also Polytetrafluoroethylene (PTFE) (Teflon®) membranes, 36, 36f, 42 players, 40
42 structure, example of, 37f Extensometer, 144
145 Extractable fluorine ions, 173 Extruder, 96, 217 Extrusion, 135, 143
144, 216
221, 291
292 equipment and process, 135
139 conductor, 139 die, 135
137 drying, 137 extruder, 135 reduction ratio, 139 sintering and cooling, 137
139 of tubing, 139
145 liner, 146 of polychlorotrifluoroethylene, 250
251, 251t pressure, 96, 129, 129f processes, 217
219 of tubing. See Tube/Tubing extrusion Extrusion aid/lubricant, 129
130

F Fab carry corrosive, 364 FAB fume ducts coated, 364f Fabricated fluoroelastomer products, 310 Fabrication techniques of fluoropolymers, 56 of reinforced gasket materials, 85 Fabric printing, 365 Federal Aviation Administration, 337 Federal District Court, 37
38 FEP. See Fluorinated ethylene propylene (FEP) Ferroelectric property, 191 Ferro-type plate, 249 Fiberglass reinforced plastic (FRP), 253
254 pipe and fittings, 255f Fiber-matrix adhesion, 385 Fibrillation, 126 Fibrils, 126
127

Filled bearings, 165 Filled compounds, of granular PTFE, 81
82 fabrication of reinforced gasketing material, 85 fillers, 82, 83t production techniques, 82
85 selection for, 82 Filled polytetrafluoroethylene coatings, 161
162 Fillers, 82, 156, 257, 343
344 fluorosilicones, 303 for fluorocarbon elastomers, 284, 285t on properties of PTFE compounds, 83t Film casting, 163
164, 165f Film extrusion, fluoropolymer, 223
229 Filtered PTFE, 78 Fine-cut resins, 111, 113
114 Fine powder, 165, 188, 322 based compounds, 85 resin selection, 146
147 types of, 126f Fine powder PTFE, 64
65, 78, 86, 125 ASTM Method D4895, 93
94, 95t, 96t, 97t basic properties of, 95t commercial processing of, 125 extrusion aid or lubricant, 129
130 extrusion equipment and process, 135
139 handling, 92 paste extrusion fundamentals, 126
129 preparation of, 92 principals for, 129 resin handling and storage, 126 resins, 93
96 structure of, 127f tubing and applications, 141t wire coating. See Wire coating Finishing, 310 of ethylene-cochlorotrifluoroethylene polymers, 51 of ethylene-cotetrafluoroethylene polymers, 51 of perfluorinated ethylene
propylene copolymer, 50
51 of perfluoroalkoxy polymer, 50 of polychlorotrifluoroethylene, 51 of polytetrafluoroethylene, 49
50 of polyvinyl fluoride, 52 of polyvinylidene fluoride, 51
52 suspension polymerization and, 68
69, 69f Fire resistance, 364 Fischer-Tropsch process, 329 Fitzmill model K-14 cutter, 76 Flanged steel piping, 254 Flax and polyaramide, impregnation of, 160
161

INDEX

porous metals and graphite, 161 processing, 161 Fluff, 343 Fluid energy milling, 73
74 Fluon granular, 80, 98t Fluon PTFE dispersions, 103t fine powders, 105t Fluoride, 201 Fluorinated additives fluorinated graphite, 329 fluorination, 329
330 market size, 330 perfluoropolyether additives, 326
329 polymeric, 321
326 fluoroelastomer additives, 322
326 polytetrafluoroethylene homopolymer additives, 321
322 vinylidene fluoride polymer additives, 326 polytetrafluoroethylene modified waxes, 329 Fluorinated ethylene propylene (FEP), 11, 14, 50, 53, 164, 254, 257f, 379
380 chemical properties, 54 convoluted tubing, 261f copolymer, 341 crystallization of, 341 disadvantage of, 22 dispersion of, 352 end group analysis of, 180t liquid coatings, 359 production in China, 26 PVDF and, 26 stabilization, 51 thermal degradation of, 53f Fluorinated ethylene
propylene copolymers, 54 Fluorinated graphite, 329 Fluorinated ionomers, 369 composition, 369
370 history, 369 manufacture, 371f properties of, 370
378 Fluorinated liquid crystals, 11 Fluorinated thermoplastic elastomers (FTPEs), 307
309, 309f applications of, 309 chemical and semiconductor industries, 309 electrical and wire and cable, 309 other, 309 polyurethane-based, 309 Fluorination, 9, 329
330 of perfluoropolymers, 180 Fluorinator, 180 Fluorine, 1
2, 7, 340 abundance, 8 basic facts, 9t characteristics, 8
11

393

fluorination, 9 nature and, 11 organic chemistry, 10
11 preparation, 10 reactive element, 9
10 compounds, 1 content of polymers, 8t content on solvent swell, 280t electrolytic preparation of, 10 halogen by, 10 for hydrogen, 7 Fluorine-containing compound, 10
11 Fluorine gas, 7
10 Fluorine sheath, 12, 340 Fluoroacetaldehyde, 11 Fluoroacrylate polymers, 11 Fluoroadditives, 50, 321
322 Fluoroalkyl carboxylate, 86 Fluoroaromatics, 10 Fluorocarbon (FKM), 1
2, 4, 10 commercial preparation of, 9 compounds, 4 diaphragms, 299f fluoroelastomers, 296t fuel pipe with, 298f negative impacts of, 4 terpolymers, 297 whether chemicals or plastics, 4 Fluorocarbon elastomers, 271
302 applications of, 297
299 aerospace and military, 299 chemical and petrochemical, 299 coatings and sealants, 300
301 other industrial, 299 polymeric processing additives, 301
302 typical automotive, 298 in ASTM D1418, 274 compounding, 284
291 acid acceptor systems, 285, 285t for compression-molded seals, 288 curatives, 285
286 for extruded goods, 287
288 fillers, 284 formulations, 287
291, 294 for peroxide cured seals, 288
289 plasticizers and processing aids, 286
287 on TFE/P elastome, 289 cross-linking chemistry. See Crosslinking cured fluorocarbon elastomers, physical and mechanical properties of, 295
297 compression set resistance, 295 heat resistance, 295, 296t low-temperature flexibility, 295
297, 296t resistance to automotive fuels, 297

resistance to solvents and chemicals, 297 steam resistance, 297 curing systems for, 286t, 294
295 emulsion polymerization. See Emulsion polymerization fillers for, 285t manufacturing process, 274
280 mixing, 291 polymer structure, properties, 280
282 processing, 291
292 calendering, 291 compression molding, 292 extrusion, 291
292 injection molding, 292 transfer molding, 292 solution and latex coating, 292
294 thermoset, 273t Fluorochlorinated hydrocarbons, 191
192 Fluorocoatings application of, 348
352 baking/curing, 351
352 liquid coating, 349
350 powder coating, 350
351, 351f substrate, 348
349 commercial application, 358
366 architectural coatings, 366 automotive industry, 361
363, 361f bakeware, 358 chemical processing industry, 363 chemical reactors, 363
364 commercial dryer drums, 364
365 commercial/industrial bakeware, 358
359 ducts for corrosive fumes, 364 fire resistance, 364 fuser rolls, 359
360, 360f housewares—cookware, 358 industrial rollers, 365 light bulbs, 360
361, 360f medical devices, 365 oil production and refining, 366 razor blade coatings, 366 small electric appliances, 358 commercial fluorocoating producers, 352
353 compositions, 342
347 additives, 344 fluoropolymer, 343 nonfluoropolymer binders, 345
347 pigments and fillers, 343
344 solvent systems, 344 fluoropolymer coatings. See Fluoropolymer coatings food contact, 357
358 liquid and powder coatings, 347
348 Fluoroelastomers, 50, 271, 321 additives, 322
326 chemical resistance of, 281t consumption, 272

394

Fluoroelastomers (Continued) fluorocarbon elastomers, 272
302 emulsion polymerization. See Emulsion polymerization manufacturing process, 274
280 polymer structure, properties, 280
282 fluorosilicone elastomers. See Fluorosilicone elastomers FTPEs. See Fluorinated thermoplastic elastomers (FTPEs) molecular structures of, 274
275 new developments and current trends, 311
312 phosphazenes, 309
310 production process, 275f safety, hygiene, and disposal, 310
311 compounding, mixing, and processing, 310 disposal of used products, 311 hazardous conditions, 311 polymerization and finishing, 310 swelling resistance of, 280
281 on TFE and propylene, 281 VDF/HFP/TFE, 278
280 Fluoroethene. See Vinyl fluoride (VF) Fluoroethylene vinyl ether (FEVE)-based coatings, 366 Fluoroglycosides, 11 Fluoro-inorganic elastomers, 271
272 Fluoroolefins, 46 Fluoroplastics, 307, 340. See also Fluoropolymer(s) Fluoropolymer(s), 1
2, 4, 11
14, 33, 44, 185 applications of, 13, 56, 57t, 253 automotive and aerospace, 267
268 chemical processing industry, 253
254, 256
258 in chemical service, using trends, 259
260 electrical applications, 264
266 FRP piping lined with, 254 mechanical applications, 266
267 medical devices, 268
269 piping, 254
255 self-supporting components, 259 semiconductor industry, trends for use, 264 semiconductor processing, 260
263 vessels, 255
256 branched, 11 characteristics, 385 chronological evolution of, 44f classifications, 44
45 commercialization of, 14t consumption, 259 developmental history of, 13, 21
24 of disclosed process, 190 dispersions, 89

INDEX

dry lubrication coating, 361
362 economy, 58 electrical properties of, 14 energy picture in early 21st century, 24f evolution of, 22f extrusion, 216
221 fabrication techniques of, 56 film extrusion, 223
229 finish technology, chronology of, 353
357 flame resistance of, 263 and fluorinated additives, 321 and fluorocarbon-based components, 2 friction of commercial olefinic, 336t functional, 379 fundamental properties of, 12 future growth of, 26
28, 27f growth rate of, 28 injection molding, 212
215 dimensional stability of parts, 215 process conditions and operations, 213
215 injection-molding conditions for, 214t instability of, 178
179 vs. key processing/application tradeoffs, 14t market, 26 melt-processible, 255, 354 O-ringfree poppet check valves, 262f partially fluorinated, 12 polymerization of, 30
31 polytetrafluoroethylene commercialization, 21 powder surface, 380 process equipment for, 56, 219
221 processing of, 211
212 products, 45 resin binder coatings and primers, 345f rheology of, 208
211 characterization, 209
211 Roy Plunkett’s story of, 19
21 safety, 56
57 seamless lining of, 256f shear rate of, 212t solid-phase, 57 state of, 24
30 economy, 24
25 energy, 24 market demand and growth, 26
28 resource limitation, 25 technology, 28
30 thermoplastic, 255 tube extrusion, 221
229 typical properties of, 15t, 23t uses of, 13
17, 16t, 57t wire coating, 219 Fluoropolymer coatings, 149, 339
343, 348 commercial/industrial bakeware with, 359f

ethylene chlorotrifluoroethylene, 342 ethylene tetrafluoroethylene copolymers, 341 fluorocoating compositions, 342
347 perfluoroalkoxy polymers, 341 polytetrafluoroethylene, 340 polyvinylidene fluoride, 342 Fluorosilicone elastomers, 302
307 applications of, 306
307 compounds, 303 cross-linking of, 303 cured. See Cured fluorosilicone elastomers fluid and chemical resistance of, 304t polymerization, 302 processing, 303
304 Fluorosilicone polymers, 302 Fluoro thermoplastics, 380 Fluoro-vinyl polysiloxane, 302 FluoroXprene, 311 Fluorspar, 25, 45 Fluorspar (CaF2), 8
10 production of, 26f world reserves distribution of, 25f Fluroxene, 10 Food and beverage, 202 Food contact, 357
358 Formulations, of PTFE dispersions, 156
158 Fracturing, 123
124 Free-flow (pelletized) resins, 111
112, 115. See also Pelletized granular PTFE Free-radical emulsion of polymerization operation, 274
275 Free radical mechanism, fluoroelastomers by, 283
284 Freeze point, 118 Freeze-thaw process, 274 Fremy’s Salt, 10 Freon, 19
20 FRP. See Fiberglass reinforced plastic (FRP) FTPEs. See Fluorinated thermoplastic elastomers (FTPEs) Fuel-handling systems, 298 Fully fluorinated copolymers, 281 Fully fluorinated functional grades, 381 Functional fluoropolymers, 379 adhesion strength of, 388t applications, 382
387 adhesive dielectric interlayer, 387 composites, 385
387 multilayer hoses, 382
383 polymer compatibilizing, 384
385 polymer modification, 383
384 surface lamination, 383 fully fluorinated perfluoroalkoxy alkanes, 381 functional groups, 380 methods for, 380

INDEX

partly fluorinated ethylene/tetrafluoroethylene, 380
381 polyvinylidene fluoride, 381 PA6 vs. PA6 modified with, 384f PFA vs. standard PTFE, 382t processing, 381
382 Functional polymer, 379 Fuser rolls, 359
360 Fusion of particles, 115 FZ elastomer. See Poly (fluoroalkoxyphosphazene) elastomer (FZ elastomer)

G Gamma-butyrolactone, 188 Glass cloth coating, 158
160, 160f equipment for, 159 process conditions for, 160t processing, 159
160 Glass fabric, 254 Gore manufactures fabrics, 41 Gore-Tex®. See Extended polytetrafluoroethylene (ePTFE) Granular PTFE, 28
30, 29t, 30f to ASTM Method D4894, 93, 93t, 94t automatic molding, 119
120 compression molding, 112
119 billet molding, 115
119 densification and sintering mechanism, 114
115 ovens, 114 presses, 114 and tooling, 113
114 development, 73 Dyneon hostaflon, 99t fabrication process, 111 based on part geometry, 112t filled compounds of, 81
82 fillers, 82, 83t selection for, 82 Gujarat Fluorochemical Company Inoflon, 101t halopolymer fluoroplast, 101t isostatic molding, 120
122 description of, 120
122 principle steps of, 121f wet- and dry-bag, 122 low flow resin, 82 molding techniques for, 111 pelletized (free flow), 78
80 presintered, 81 processing, 111 property specifications, 95t ram extrusion, 122
124 types, 122
123 resins, 93, 94t selection of resin, 111
112 Solvay Solexis Company Algoflon, 100t

395

steps for processing, 122 Graphite fluorides, 329 GREBE GROUP Company, 353, 355
356 GREBLON, 353, 355
356 Grit blasting, 348
349 Guide tube, 137 Gujarat Fluorochemical Company, 102t Gylon sheet gasketing material, 258f

H Hagen
Poiseuille equation, 210
211 Halogenated hydrocarbon acids, 233 Halogens, 7, 10 Halohydrocarbons, dehydrohalogenation of, 185 HaloPolymer Company, 73 Halopolymer fluoroplast granular PTFE resins, 101t Halothane, 2
3 Hammer milling, 74
76, 75f HCFC production, 47 HDPE. See High-density polyethylene (HDPE) Heat, 208 Heat resistance, 295 FKM fluoroelastomers, 296t fluorosilicone elastomers, 305 Henkel North America, 353 Hexafluoropropylene (HFP), 45
46, 183t, 185
187, 341 copolymerization of, 188 copolymers, 170f in monomer mixture, 177f on perfluoroalkyl vinyl ethers, 177t polymerization of, 177 properties of, 48
49 synthesis of, 46 Hexafluoropropylene oxide (HFPO), 46
47, 370 HFPO. See Hexafluoropropylene oxide (HFPO) High-density polyethylene (HDPE), 301 High-molecular-weight fluoroelastomers, 291 hydrocarbon esters, 286
287 polytetrafluoroethylene, 165 High-pressure gas supply unit/jet mill, 74f High-temperature polymerization, 69
71 High-voltage electrode, 350 High-volume, low-pressure (HVLP), 349 Homopolymers, 44
45, 64
65, 340
341 of chlorotrifluoroethylene, 245 Daikin Neoflon PCTFE, 246t polychlorotrifluoroethylene, 231 of polytetrafluoroethylene, 66 polyvinylidene fluoride, 185, 193, 201 Honeywell Corp., 231
232 Horizontal extrusion, 143 of inner supporting pipe, 146f

Horizontal ram extrusion, 122
123 Hot flocking process, 364 Human body, fluoride’s primary function in, 8
9 HVLP. See High-volume, low-pressure (HVLP) Hydraulic fluid, 268 Hydraulic paste extruder, 141f Hydraulic presses, 114 Hydrocarbons, 9, 56, 141
142 Hydrochloric acid (HCl), 19, 186 Hydrofluoric acid (HF), 10, 25, 45, 185
186, 216, 297 Hydrofluorination of acetylene, 186 of trichloroethane, 47 of vinylidene fluoride, 47 Hydrofluoroolefin (HFO), 2 Hydrogen, 162, 185 Hydrogen-carbon monoxide gas mixture, 329 Hydrogen-containing solvent, 279 Hydrogen fluoride (HF), 9, 48 Hydrophobicity, 370 Hydropower, 24 Hydrosilylation addition, 303 Hydrostatic pressure, 190 Hydroxypropyl methylcellulose, 191

I Ideal cooling technique, 144 Impact factor (IF), 29
30 IMPRA. See International Medical Prosthesis Research Associates, Inc. (IMPRA) Impreglon Canada, polymer reactor coated by, 363
364, 363f Impregnation method, modified PA6 by, 387t Inclusion model, 170, 170f Incubation period, 67
68 Independent process variable, 250 Industrial rollers, 365 Injection molding, 212
215, 294
295 conditions for ETFE, 217t dimensional stability of parts, 215 fluorocarbon elastomers, 292, 293t fluoropolymers compounds of, 216t conditions for, 214t parameters for PVDF, 215t of PCTFE, 247, 249
250, 249f process conditions for, 250t principle of, 212 process, 384 conditions and operations, 213
215 settings for ETFE, 217t water absorption of, 384f Innovation, invention vs., 28 Inorganic fluoride, 11

396

Inorganic pigments, 133 Insoluble saturated hydrocarbon, 86 Internal stress, 247 International Medical Prosthesis Research Associates, Inc. (IMPRA), 38 International Standards Organization, fluoropolymers by, 92
93 Interpenetrating polymer network, 345 Invention vs. innovation, 28 Iodine transfer polymerization, 307 Ionic mechanism, fluoroelastomers by, 282
283 Ionic strength of PTFE dispersion, 155 Isoflurane, 2
4, 10 Isolated fluoroelastomer, 275 Isopar Solvents, properties of, 131t Isopropanol, 190
191 4,40 -Isopropylidene bisphenol, 286 Isostatic molding, 120
122 description of, 120
122 principle steps of, 121f wet- and dry-bag, 122

J Jacketing, 219 Japan Gore-Tex Inc. (JGI), ePTFE production, 41 Jar method, 130
133 Joining, PCTFE, 251
252

K K-416, 20 KALREZ, 272
273, 281, 297, 299
300 Kel-F, 231
232, 272, 274 KYNAR film, 203 Kynar Flex® PVDF PPA, 326

L LAE. See Linear alkyl ethoxylate (LAE) Land area, 229 Large-scale blending, 130 Laser beam printer, 359f Latent solvents, 188 Lattice structure, 37 Lead oxide (PbO), 285 Leetex Technologies, ePTFE production, 42 Light bulbs, 28, 360
361 Lightest halogen, 7 Linear alkyl ethoxylate (LAE), 151, 156
158, 157f Linear fluoropolymer, 11, 340 Linear low-density polyethylene (LLDPE), 301
302 Lined FRP piping, 254 Liner extrusion, 146 Liquid coatings, 152, 342, 347 application, 349
350 Liquid FKM-based systems, 300 Liquid fluorine, 9
10

INDEX

Liquid pigments, 133 Lithium ion battery, 202 LLDPE. See Linear low-density polyethylene (LLDPE) Lower molecular weight esters, 286
287 polyethylene, 286
287 polymer, melt viscosity of, 209 Low surface tension substrate, wetting and nonwetting droplets on, 154f Low-temperature flexibility, 295
297, 296t Low-temperature polymerization, 71
73 Low-temperature properties, fluorosilicone elastomers, 305 Low-temperature PTFE, 67 Lubricant, 129
130, 140 blending resin with, 130
133 and pigment dispersion, 133 Lumiflon, 366

M Machining, PCTFE, 251
252 Manhattan Project, 10, 20, 231 Maquet Cardiovascular, ePTFE production, 42 Markel Corp, ePTFE production, 42 MDF. See Mitsui-DuPont Fluorocarbon Co. (MDF) MDI. See Metered dose inhaler (MDI) Mechanical applications, fluoropolymers, 266
267 Mechanical drive system, 135 Mechanical grade sheets, 112
113 Medical devices, 268
269, 365 Medium thermal black (N990), 284 Melt flow index (MFI), 54, 207
208 Melt flow rate (MFR), 54, 207
209 Melt fracture, 208, 211, 301 techniques for elimination, 209t Melt-indexer, 210 Melt-processible copolymers, of tetrafluoroethylene, 169 molecular and crystalline structure, 169
171 Melt-processible fluoropolymers, 26, 207, 255, 354 Melt-processing techniques, 45, 93, 209, 211 of polyvinyl fluoride, 335 Melt temperature, 213 Melt viscosity, 181, 189, 221, 341 Membranes, 373 Metals, 186 Metered dose inhaler (MDI), 365, 365f Methanol, 175, 178 Methoxyflurane, 2
3 Methyl hydroxyalkyl cellulose, 190 Methylvinyl silicone rubber (MVQ), 302 MFI. See Melt flow index (MFI)

MFR. See Melt flow rate (MFR) Micronization process, 73
74 Micropowders, 321
322, 361, 384 Midcoat, coating systems, 358 Miraflon fluoroelastomers, 279 Mitsui-DuPont Fluorocarbon Co. (MDF), 353 Mixed compounds, fluorocarbon elastomers, 291 Modern control systems, 276 Modified fluoropolymers, 385 Modified PA6 (mPA6), 387, 387t Modified PTFE, 64, 76, 144, 253, 259 Moisture water vapor, 208 Molding pressure, 113
114 and dielectric strength, 116f and elongation properties, 116f and rate of change in dimensions, 117f and specific gravity of preform, 120f and tensile properties, 116f Mold temperature, 214
215 Molecular weight distribution (MWD), 54, 190
191, 210 Molten plastic, 219 Molten polymer, 93 Monomer, 232, 275
276 properties, 48
49 synthesis, 45
48 Monomers, 21
22, 272 Moore’s law, 264 Multilayer coextrusion, 383f Multilayer hoses, 382
383 Mupor, LTD, ePTFE production, 42 MVQ. See Methylvinyl silicone rubber (MVQ) MWD. See Molecular weight distribution (MWD) M. W. Kellog Company, 231
232, 272

N Nafion, 369, 370f acid strength of, 378 as catalyst, 378t in mixtures of water and an organic, 371f perfluorosulfonic acid membranes, 372t sulfonic polymer, 377 Narrower MW distribution, 173
174 Nature, fluorine and, 11 Neoflon CTFE, 231
232 N-ethyl-2-pyrrolidone, 346 Neutralizing agent, 233 Never-melted PTFE, 53 New developments in chemistry and processing, 311 other, 312 products, 311
312 Newtonian fluid, 208
209 Nitrogen adsorption, 67 Nitto Denko, ePTFE production, 42

INDEX

N-methyl-2-pyrrolidone, 346 Nobel gases, 9 Nonaqueous polymerization of perfluoroalkoxy polymers, 171
174 of perfluoropropyl vinyl ether, 172t Nonfluoropolymer binders, 345
347 polyamide-imide, 345
346, 346f polyether sulfone, 346, 346f polyphenylene sulfide, 346
347, 346f Nonhalogenated surfactants, 233 Nonstick coated fry pans, 358 Nonstick cookware, 354 Nucleophilic curing system, 286 Nylon, 113

O Octafluoro-2-butene, 49 Octafluoroisobutylene, 49 OI. See Orientation index (OI) Oil production and refining, 366 Olefinic polymers, 8 Open-mill mixing, 291 Orange peeling, 146, 208 Organic acid anhydrides, 380 Organic chemistry, fluorine characteristics, 10
11 Organic/inorganic peroxy compounds, 274 Organic materials, 154 Organic peroxides, 190 Organic solvents, 80, 161
162, 165 Orientation index (OI), 144
145 O-rings, 295, 297
298 agitator shaft seals, 300 for different applications, 299f from FFKM, 300f thermoplastic fluoroelastomer, 310f Ovens, 114 Oxalic acid, 66
67 Oxidation
reduction catalytic (redox) systems, 181 Oxygen, 71 Oxygen difluoride (OF2), 53

P PA6-based CFRTP, 385
386, 387f burning rate in, 386t tribological performance of, 386t Packing, 160
161 PAI. See Polyamide-imide (PAI) Paper dust, 360 Paper/fabric industry, 365f Paper making, 365 Paraffin, 65 Paraffin wax, 188 Partially fluorinated fluoropolymers, 12, 44
45, 185 Partly fluorinated functional grades, 380
381 Paste extrusion processes, 56, 125
126, 146

397

billet molding for, 134f compound, 130
133 die setup for coating wire by, 136f tubing extrusion by, 142f fundamentals, 126
129 unit operations of, 129f Patented process, 354 Patent validity, 28 Patterson-Kelly machine, 85 PAVEs. See Perfluoroalkyl vinyl ethers (PAVEs) PCTFE. See Polychlorotrifluoroethylene (PCTFE) Pelletized granular PTFE, 78
80 agglomeration process for, 80 disintegration of, 79f key characteristic of, 79 line surface of, 79f PEM fuel cells, 374
375 Perfluorinated ethylene
propylene copolymer finishing of, 50
51 polymerization of, 50
51 Perfluorinated ethylene propylene polymers, preparation of, 177
180 Perfluorinated fluoropolymers, 44 Perfluorinated ionomers, 369
370, 374
375 Perfluorinated polymers, 12, 185 Perfluorinated sulfonic acid ionomers, 378 Perfluoro(alkyl vinyl ethers), 322 Perfluoroalkoxy (PFA), 11
12, 44
45, 213
214, 253 alkanes, 381 aqueous polymerization of, 174
176 chemical properties of, 54 finishing of, 50 high-purity grades of, 261 MFA, 210 nonaqueous polymerization of, 171
174 polymerization of, 50 preparation of, 171
176 properties, 54, 173t screw characteristics for, 218t structure of, 341f terpolymers of, 50 wafer handling systems, 263f Perfluoroalkoxy-coated light bulbs, 360f Perfluoro-2-alkoxy-propionyl fluoride, 46
47 Perfluoroalkyl vinyl ethers (PAVEs), 44, 50, 64, 171, 175, 181, 341 aqueous polymerization of, 176t copolymerization of, 171
172, 175 effect of, 174t hexafluoropropylene on, 177t properties, 49 synthesis, 46
47, 46f

tetrafluoroethylene and, 171 Perfluoro-(2-butyltetrahydrofuran), 175 Perfluorobutyl vinyl ether, 170 Perfluorodimethylcyclobutane, 172 Perfluoroelastomers (FFKM), 272
273, 281, 284 applications of, 299
300 cross-linking of, 284 Perfluoroethyl vinyl ether (PEVE), 86
87, 178t, 179t Perfluoroisobutylene (PFIB), 45, 51, 221 Perfluoromethyl groups, 170 Perfluoromethyl vinyl ether (PMVE), 21
22, 281, 341 perfluoroelastomers, 277 terpolymers of, 281 Perfluorooctanoic acid (PFOA), 22, 87
88, 233 Perfluoroolefin monomers, 186
187 Perfluoropolyether additives, 326
329 Perfluoropolymers (Teflon AF), 11
13, 340 fluorination of, 180 Perfluoropropyl, 341 Perfluoropropyl vinyl ether (PPVE), 45
47, 50, 86
87, 143, 175, 234 incorporation, 175, 177f nonaqueous polymerization of, 172t polymerization of, 173t Periodic table of elements, 8f PERLAST, 272
273 Peroxide cross-linking, 303 Peroxide-curable polymers, 295
296 Peroxide curing system, 286, 295 Peroxides, 232 Persulfate, 65
66 PEVE. See Perfluoroethyl vinyl ether (PEVE) PFA. See Perfluoroalkoxy (PFA) PFOA. See Perfluorooctanoic acid (PFOA) Pharmaceutical properties, fluorineenhanced compounds, 2 Phillips Scientific, ePTFE production, 41
42 Phosphazene elastomers, 310 Phosphazenes, 309
310 Photolithography technology, 11 Pigments, 343
344, 373 addition, 133 α-Pinene, 45
46 Pipe threads, 257 Piping, 254
255 Plaque, 189
190 Plastics, flow of, 207
208 Plastics injection molding, 207 Plastisol technology, 335 Plate-plate viscometer, 211 Plenum cables, 265 Plug flow, 229

398

Plunkett, Roy story, of fluoropolymers, 19
21 PMTFPS. See Polymethyltrifluoropropyl siloxane (PMTFPS) PMVE. See Perfluoromethyl vinyl ether (PMVE) Poker chipping, 122 Poly(dichlorophosphazene), 310 Polyacrylic acid, 90 Polyamide (PA), 382
383 Polyamide-imide (PAI), 345
346, 346f Polychlorotrifluoroethylene (PCTFE), 11
12, 44f adhesive bonding of, 252 characterization of, 239 commercial, 239
241 compression molding, properties and attributes of, 238t Daikin’s Neoflon properties, 242t degradation, 249
250 electrical and other properties of, 238t fabrication, 245 films of, 251 finishing of, 51 homopolymers, 231 injection molding of, 247, 249
250, 249f, 250t manufacturing, 231 mechanical and thermal properties of, 237t molecular weight, 245
246 polymerization of, 51 processing, 245 compression molding, 248
249 crystallinity, 246, 247t extrusion, 250
251, 251t machining and joining, 251
252 stress, 247
248 zero strength time, 245
246, 247f properties of, 54, 231, 236
239 quench-cooled, 237 requirements of, 241t Poly(CTFE-co-VDC) copolymers, 236 Polydimethyl siloxane (PDMS), 302 Poly(fluoroalkoxyphosphazene) elastomer (FZ elastomer), 310 Polyesters, 253 Polyether sulfone (PES), 346, 346f Polyethylene (PE), 11, 127, 212t, 331 chemical structures of, 332f crystallization, 52 linear low-density, 169 structure property relationship, 52
53 Polymer(s), 92, 187
188, 190 compatibilizer, 384
385 composition and polymerization rate, 277 dispersion, 275 fluorine content of, effect of increase in, 8t

INDEX

fluoropolymer family, 24 melt viscosity of, 179 modification, 383
384 modifiers, 383 perfluoroalkoxy, 341 tensile properties of, 175 tetrafluoroethylene (TFE), 64
65 tuning of, 383 vinylidene fluoride, 185 viscosity, 278 Polymeric fluorinated additives, 321
326 Polymerization, 171 bulk, 51 of chlorotrifluoroethylene, 231
234 commercial suspension, 68
69 comminution of, 73 dispersion, 50
51. See also Dispersion polymerization of ethylene-cochlorotrifluoroethylene polymers, 51 of ethylene-cotetrafluoroethylene polymers, 51 and finishing, 310 free-radical emulsion, 274
275 heat of, 278 of hexafluoropropylene, 177 high-temperature, 69
70 initiator, 173 low rate of, 67
68 low-temperature, 71
73 media, 65 of perfluorinated ethylene
propylene copolymer, 50
51 of perfluoroalkoxy polymer, 50 of perfluoropropyl vinyl ether, 173t of polychlorotrifluoroethylene, 51 of polytetrafluoroethylene, 49
50 of polyvinyl fluoride, 52 of polyvinylidene fluoride, 51
52 pressure, 177 reaction, 65 surfactant, 57
58 suspension. See Suspension polymerization techniques, 63 of tetrafluoroethylene, 45
46, 48, 50, 57, 92, 173t, 177 by suspension method, 69
73 vessel, 87 of vinyl fluoride, 188 Polymerization aid (PFOA), 22, 65 Polymer processing additives (PPAs), 301
302, 322
326 Polymer reactor coated by Impreglon Canada, 363
364, 363f coated with thick film PFA, 364f Polymethyltrifluoropropyl siloxane (PMTFPS), 302 cured fluorosilicone elastomer, 305

high-molecular-weight, 306 mechanical properties, 305 medium-molecular-weight, 306
307 Polyolefins, 7
8, 21
22, 169 Polyphenylene sulfide (PPS), 346
347, 346f Polyphosphazene polymers, 310 Polypropylene, 253 Polytetrafluoroethylene (PTFE) (Teflon®), 7, 19, 21, 63, 193, 245
246, 253, 257f, 258, 340, 343, 379
380. See also Tetrafluoroethylene (TFE) advantage, 87 aerospace data cables insulated with, 265f applications, 13, 16t, 56 automotive, 267f bearing pads, 266
267 billet molding and sintering, 112f braided smooth and convoluted, 143f bridge bearing, 266f coaxial cables insulated with, 265f commercial grades of, 143t commercialization of, 21 in compression and isostatic molding techniques, 121f convolution of, 259 crystallinity of, 53, 127f, 139 degradation of, 116
118 discovery of, 13, 20
21, 44 by dispersion polymerization, 86
87 recipe and properties, 87t emulsion concentration process variable, 90, 91t emulsion grade, 50 emulsion polymerization of, 51 expanded. See Expanded polytetrafluoroethylene (ePTFE) exporting countries, 27f fabrication, 56 FEP and, 171 fiber, 267 filled compounds, 50, 81
85 applications, 84t fabrication of reinforced gasketing material, 85 fillers on properties of, 83t fine powder-based compounds, 85 granular PTFE compounds, 82 fine powders. See Fine powder PTFE finishing of, 49
50 fluoroadditive powders, 323t, 324t Daikin, 326t Dyneon, 325t Solvay Solexis, 327t and fluoropolymers, 1, 33 and melt-processible fluoropolymers, 3f fundamental properties of, 12, 12t

INDEX

granular. See Granular PTFE homopolymer additives, 321
322 homopolymers of, 66 importing countries, 27f insulated wire, 130 key characteristic of, 79 lined braided hose, 261f convoluted and smooth, 261f lined fittings, 254f and lined metals, 262f low-temperature, 67 manufacturing facility, 26
28 mechanical applications, 266
267 mechanical properties of, 53 modified polymers of, 64
65, 76 modified waxes, 329 molecular weight of, 53, 64 never-melted, 53 pelletization of. See Pelletized granular PTFE and polychlorotrifluoroethylene, 236 polymerization of, 49
50 powder, 113
114 milling temperature of, 76f produced by hammer milling, 75f of preforming and paste extrusion, 128f preparation by suspension polymerization, 29t preparation processes comparison, 65t producer/exporter of, 26 by properties, 53
56 characterization of, 92
96 commercial fine cut, 77t razor blade coatings, 366f reactor bead, 69f resins, 26, 97
107 sintering cycle, 117f standard specification methods for, 92t structure of, 340f structure property relationship, 52
53 tape-wrapped wire, 265f tetrafluoroethylene polymers, 64
65 thermal degradation of, 53f tubing jacketed with braided stainless steel wire, 268 for ultrahigh-purity applications, 263f useful attributes of, 13t virgin PTFE, 53 wafer carrier, 263f wire extruder, 136f Polytetrafluoroethylene (PTFE) dispersions, 50, 56, 64
65, 96, 97t, 149 advantage, 150 antidrip applications, 164
165 applications, 150
151, 150t based on fabrication processing, 151t other, 166 attributes of, 150t coating metal and hard surfaces, 161
162

399

coatings and fibers of, 150 coating technology principles. See Coating technology, principles of cracking thickness and shear stability of, 158t dedusting powders, 166 effect on drip-suppression of polycarbonate, 165t filled bearings, 165 film casting, 163
164, 164t, 165f flax and polyaramide, impregnation of, 160
161 formulation, 156
158 and characteristics, 155
158 glass cloth coating, 158
160 equipment for, 159, 160f processing, 159
160, 160t high molecular weight, 165 ionic strength of, 155 products, 150t settling tendency of, 158f skived film properties, 164t specific gravity of, 155t storage and handling, 151 surfactants, 151 yarn manufacturing, 162
163 Polyurethane, 120 Polyvinyl alcohol, 190
191 Polyvinyl chloride (PVC), 187, 253, 335 Polyvinyl fluoride (PVF), 2, 44
45, 331 attribute
application relationships, 331
332 basic attributes of, 332t basic properties, 331 chemical-resistant properties of, 332 chemical structures of, 332f development and applications of, 332
337 finishing of, 52 high molecular weight, 56 limiting oxygen index of, 56 melt processing of, 335 and polyethylene, 331 polymerization of, 52 processing technology development, 335 properties, 55
56 protective films, 331
332 typical properties of, 332t Polyvinylidene chloride, 187 Polyvinylidene fluoride (PVDF), 2, 3f, 164, 185, 187, 190
192, 193t, 194t, 213, 342, 379, 381t applications, 202
204 characterization of, 192, 194 chemical structure of, 342 components, 203 conformations and transitions of, 194
201 copolymers, 185, 191, 201, 203

crystallinity, 194 electrical properties of, 201t and ethylene
tetrafluoroethylene copolymer, 55 film, 203 finishing of, 51
52 fluoride, 201 and fluorinated ethylene propylene polymer, 26 functional fluoropolymers, 379, 381 homopolymers, 185, 193, 201, 203 melt viscosity, 211f and melting point of, 189, 189t polymer chains, 193
194 polymerization of, 51
52 powder coating systems, 203 processing, 201 properties of, 55, 193
201 resin, 191, 204 Stallings’ process produced, 188 structure of, 342f ultrahigh molecular weight, 190 used in oil and gas, 203 Porex Corporation, ePTFE production, 42 Porous metals and graphite, impregnation of, 161 Postcuring, 294 Potassium alkylsulfonate, 189
190 Potassium gold cyanide, 375 Powder coating application, 350
351, 351f systems, 203, 347
348, 357 technique, 255 Powder metal processing, 120 Powder spray gun, 351f PPAs. See Polymer processing additives (PPAs) PPS. See Polyphenylene sulfide (PPS) PPVE. See Perfluoropropyl vinyl ether (PPVE) Preforming, billet molding, 115
116 Preforming equipment, 133
135, 134f “Preirradiation” method, 380 Premature fibrillation, 126 Presintered granular PTFE, 81 Presses, compression molding, 114 Pressure die, 221, 221f Pressure hoses, 140
143 quality control of, 144
145 Pressurization fluid, 122 Primary fluoropolymers, 340 Primer, coating systems, 358, 362
363 Processing equipment construction of, 209t fluorocarbon elastomers, 291
292 Promoter, 232 Propane, benefit of, 189 PTFE. See Polytetrafluoroethylene (PTFE) (Teflon®) PTFE
graphite mixture, 80

400

Purified polymer, 275 Pusher, 134 Push
pull control cables, 267 PVDF. See Polyvinylidene fluoride (PVDF) PVF films, 335
337

Q Quad-screw extruder, 217
218

R Rabinowitsch Correction, 211 Ram extrusion, 111, 122
124, 135, 146, 292 four steps description of, 123 types, 122
123 typical resin, 123
124 Raw dispersions, 343 Raw-gum fluorocarbon elastomers, 282 Raw polymers, 302 Razor blade coatings, 366 Reactive groups, 383 Real copolymer crystals, 170 Reciprocating screw injection machine, 213f Recrystallization, 64 Redox catalyst, 49 Redox system, 188 Reduction ratio (RR), 87, 96, 126, 135, 146 extrusion pressure, 126f, 140f extrusion process, 139 Reinforced gasketing material fabrication, 85 Residual polymer, 215 Resin-based composites, 385 Resins fine powder PTFE, 93
96 flow, 111
112 granular PTFE. See Granular PTFE Resource limitation, of fluoropolymers, 25 Reverse roll gravure coating, 152 Rheology, 153
154 of fluoropolymers, 208
211 Rheometer, 96 Rotational lining, 255 Rotational molding, PVDF applications in, 204 Rotolined transition component, 256f Rotolining, PVDF applications in, 204, 255, 259
260 and other technologies, comparison of, 256t RR. See Reduction ratio (RR) Rupture disk, 69

S Sandblasting, 162 Saturated fluorinated, 191
192

INDEX

Screw-type injection molding machines, 212 Sealants, fluorocarbon elastomers in, 300
301 Seat belt D-rings, 361, 362f Secrecy Order, 34 Segregation, 348 Selectfluor, 9 Selection, of fine powder resin, 146
147 Self-supporting components, 259 Semibatch emulsion polymerization system, 277
278, 277f Semiconductor industry, 202 fluoropolymers, 264 Semiconductor manufacturing processing, 260
264 Semicrystalline, 208 perfluoropolymers, 312 Service life vs. temperature, 295t Sevoflurane, 2
4, 10 Shanghai da Gong New Materials, ePTFE production, 42 Shanghai Linflon Film Technology, ePTFE production, 42 Shark skinning, 208 Shear-intensive screws, 220 Shear stress vs. shear rate, 153, 153f, 211 Shear thinning, 210 Shenzhen facility, 41 Shrinkage, 113
114, 213, 215, 247 Signal transmissions, cables for, 266f Silicon carbide, 360 Silicones/silicone glazes, 248, 358
359 SilverStone coatings, 356 SilverStone SUPRA, 356
357 Simons process, 9 “Simultaneous irradiation method”, 380 Single-flighted screw, 220f Single-screw extruder, 218f, 219, 251f Sintering, 122, 137
139, 143
145 billet molding, 112f, 116
118 cycle, 117t function of preform size, 118t temperature and product quality, 119f Sizing-plate method, 222 Skived film properties, 164t Slide bearings, 266
267 Slide coating techniques, 152 Small electric appliances (SEAs), 358 “Snake oils”, 344
345 Sodium acetate, 189 Sodium-based microemulsion, 235 Sodium chloride (NaCl), 369, 373f Sodium hydroxide (NaOH), 369 Solid fluorocarbon elastomers, 286 Solid-phase fluoropolymers, 57 Solid plastic tanks, 202
203 Solids, 347 Solution polymerization, 232

of vinylidene fluoride, 51
52, 191
192 Solvay Solexis PTFE fluoroadditive powders, 327t Solvent-borne liquid systems, 301 Solvent systems, 344 “Spaghetti” tubes, 142t Spark tester, 139 Specific surface area (SSA), 67 Spectragraphics, 356 Spray guns, 349 Square pitch screw, 219 SSA. See Specific surface area (SSA) SSG. See Standard specific gravity (SSG) Stainless-steel reactor, 187 Stainless steel wire (filament), 140
141, 143f Standard specification methods, for PTFE, 92t Standard specific gravity (SSG), 86
87, 93
94 of polytetrafluoroethylene, 53 Steam resistance, 297 Steel vessels, 255 Stick-slip behavior, 266, 321, 326 Strained specific gravity, 96 StreaMax XF, 366 Streptomyces cattleya, 11 Stress, 247
248 Stretching, 227 Stretch vascular grafts, 4f Stretch void index (SVI), 96, 144
145 Striking plate, 74
75 Structure property relationship polyethylene, 52
53 polytetrafluoroethylene, 52
53 Substrate preparation, 348
349 Sumitomo Electric Industries, ePTFE production, 41 Super critical carbon dioxide (SCC), 21, 311 fluoropolymers in, 30
31 Supernatant serum, 293 Surface energy, 154
155 Surface lamination, 383 Surface properties, fluorosilicone elastomers, 305 Surfactants, 86, 98t, 178, 188 polytetrafluoroethylene (PTFE) dispersions, 151 Triton X-100 and LAE, 157f Suspension polymerization, 28
29, 51
52, 64
65, 175, 278
280 Asahi Chemical Industry Co., Ltd., 278
279 of chlorotrifluoroethylene, 232
233 comminution of, 73
78 fluid energy milling, 73
74 hammer milling, 74
76, 75f of wet reactor bead, 76
78

INDEX

and finishing processes, 68
69, 69f regimes, 67
69 of tetrafluoroethylene, 67, 70f, 71 low-temperature, 71 method, 69
73 of vinylidene fluoride, 190
191 SVI. See Stretch void index (SVI)

T Taylor Stiles wet cutter, 78, 78f TCTFE. See 1,1,2-Trichloro-1,2,2trifluoroethane (TCTFE) Technical merit, 28 TECHNOFLON FOR, 296
297 Technology node, 264 Tecnoflon SL, 281
282 TECNOFLON T, 281
282 Tedlar PVF films, 331, 336
337 physical and chemical properties, 333t thermal and electrical properties, 334t Teflon®, 20, 34, 353, 355 Teflon FEP 4100, 212t Teflon finishes, 353 Teflon NXT resins, 340
341 Teflon-P, 356 Teflon-S technology, 345, 355
356 Temperature control, 232 Tentered biaxially oriented film, 228 Tenterhooks, 228 Terpene B, 45
46 Terpenes, 48, 186 Terpolymer, 181 Tetrafluoroethylene (TFE), 7, 19, 44, 63, 172t, 321
322, 340, 380
381 aqueous polymerization of, 176t, 178t, 179t commercially, 65
66 comonomers, 46 copolymerization of, 51, 370 copolymers, 170f, 175 degradation, 88 feed rate of, 69
70 homopolymers, 65 melt-processible copolymers of, 49, 169 monomer, 72 monomer flow, 70 and perfluoroalkyl vinyl ethers, 171 perfluoroelastomers, 277 polymerization of, 22, 45
46, 48, 50, 57, 92, 173t, 177 mechanism, 65
67 regimes, 65 by suspension method, 69
73 temperature, 68f, 72f polymers, 11, 64
65 polytetrafluoroethylene. See Polytetrafluoroethylene (PTFE) (Teflon®) properties of, 48, 182t random copolymers, 170

401

redox polymerization of, 87 suspension polymerization of, 67, 70f, 71 synthesis of, 45
46, 45f terpolymers of, 174t, 177t, 178 Tetrafluoroethylene
hexafluoroethylene
vinylidene fluoride (THV), 21 TFE and propylene (TFE/P), 273
274, 289 TFE-based fluoroelastomers, 274 TGTG formation. See Trans-gauchetransgauche (TGTG) formation Thermal behavior, 312 Thermal conductivity, 208 Thermal welding methods, 201 Thermoplastic, 330 Thermoplastic coatings, 343 Thermoplastic fluoropolymers, 21, 255 technological innovation waves of, 21f Thixotropic, 153
154 Thread sealant tapes, 258f Tooling, 113
114 Tool or die, 249 Topcoat, coating systems, 358 TPT, 337 Traditional fluoropolymers, 379 Transfer molding, fluorocarbon elastomers, 292 Trans-gauche-transgauche (TGTG) formation, 194 Transverse stretching, 228 Trapped moisture, 208 Tribocharging guns, 351 Triboelectric charging powder spray gun, 352f Trichlorotrifluoroethane, 71, 192 1,1,2-Trichloro-1,2,2-trifluoroethane (TCTFE), 47, 175 Triethylamine trishydrofluoride, 9 1,1,1-Trifluoroethane, 47, 185 Trifluoroethylene (TrFE), 235 Trifluoromethyltrimethylsilane (CF3SiMe3), 9 Trifluoropropylsiloxy, 302 Triton X-100, 90, 151, 156
158, 157f, 160
162 TR 10 test, 296
297 Tube die, 221 Tube/Tubing extrusion, 139
145, 142f fluoropolymers, 221
229 hydraulic paste extruder, 141f pressure hoses, 140
143 sintering and cooling, 143
144 sizing of, 222
223 Tubular humidity exchanger, 377f Tubular/bubble process, 227
228 Tubular water-quench process, 226 Twin-screw extruder, 217
218 Two-package system, 354
355

Two-phase system, 169
170 Two-roll coating, 152 Two-roll forward roll coater, 152f

U Uhde cell stack, 374f Ultrafiltration membranes, 90 Uncontrolled fibrillation, 126 Unfilled polytetrafluoroethylene coatings, 161
162 United States Department of Agriculture (USDA), fluoropolymer, 358 Unreacted trifluoroethane, 185 Unsintered polymer, 144 Unstrained specific gravity, 96 US Department of Transportation, on vinyl fluoride, 49 US Environmental Protection Agency (EPA), 88 fluoropolymer, 57 US Food and Drug Administration (FDA), 354 USP testing, 358

V Vacuum trough method, 222
223, 222f Valve, stems and seals, 298f V-cone blender, 130 VDF. See Vinylidene fluoride (VDF) VDF-based fluoroelastomers, 274 VDF/HFP/TFE copolymers of, 280
281 fluoroelastomers, 278
280, 282 VDF-TrFE copolymer, 235
236, 236t Velocity, 208 Vertical ram extrusion, 122
123, 123f Vessel internals, 260f Vessels, 255
256 Vinyl chloride, 332
335 Vinyl fluoride (VF), 331 copolymers of, 52 homopolymers and copolymers, 331 interpolymers of, 52 polymerization of, 188 properties of, 49 synthesis of, 48 Vinylidene chloride, 186 Vinylidene fluoride (VDF), 44
45, 169, 185, 231 aqueous polymerization of, 51, 187 contact time and temperature on, 186t copolymerization of, 188 emulsion polymerization of, 188
190 head-to-tail addition of, 55 homopolymer and copolymer, 186
187 and hydrofluoric acid, 186 hydrofluorination of, 47 polymer additives, 326 polyvinylidene fluoride. See Polyvinylidene fluoride (PVDF)

402

Vinylidene fluoride (VDF) (Continued) preparation of, 186
192 properties of, 49, 186, 187t radiation-induced polymerization of, 192 solution polymerization of, 51
52, 191
192 suspension polymerization of, 190
191 synthesis of, 47
48, 185
186 Virgin PTFE, 53 Viscosity, 153, 207
208, 343 behavior, 210 of PTFE dispersions, 154, 157f Viton A, 274, 301 VITON A-35, 301 VITON GLT, 296
297 Volatile electrolyte, 90

W Wafer carrier, 263f Water-quench blown-film process, 226, 226f cast film process, 225
226, 225f Water-soluble compound, 86 Water-soluble initiator system, 275
276

INDEX

Water-soluble organic compound, 92 Water-soluble peroxide, 51 Water-soluble polymer, 190 Water treatment systems, 202 Waxy powder, 19 Wear-resistant material, 311 Weep test method, 144
145 Weilburger Coatings, 353 Welding methods, 202 Weld-nut or stud, 362, 362f Weldwood, 336 Wet-bag isostatic molding, 122 Wet benches, 263 Wet coating, thickness, 156 Wet process, for agglomeration of PTFE, 80 Wet PTFE, 73 Wet reactor bead, comminution of, 76
78 White (silica) fillers, 284 Whitford Corporation, 353 Wire coating, 130
139, 136f blending resin with lubricant, 130
133 fluoropolymer, 219 paste extruders, 140 pigment addition, 133

preforming, 133
135 Wire payoff system, 135 W.L. Gore and Associates, 33, 35f early history of, 34
35 expanded PTFE, discovery of, 35
37 W.L. Gore v. C.R. Bard, 40 Wood pulp, 163

X Xanthate, 163 X-ray diffraction, 170 Xylan 1010, 355
356

Y Yeu Ming Tai Chemical Ind., ePTFE production, 42

Z Zero strength time (ZST), 239, 245
246, 247f crystallinity function of, 248t Zeus Industrial Products, ePTFE production, 41 Zirconium dioxide, 373 ZST. See Zero strength time (ZST)