Formulating Adhesives and Sealants 9783748602279

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Bodo Müller | Walter Rath

Formulating Adhesives and Sealants Chemistry, Physics and Applications

Cover: BASF SE, Ludwigshafen, Germany

Bibliographische Information der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie ; detaillierte bibliografische Daten sind im Internet über http://dnb.ddb.de abrufbar.

Müller, Bodo and Rath, Walter Formulating Adhesives and Sealants Hanover: Vincentz Network, 2010 European Coatings Tech Files ISBN 978-3-7486-0227-9 © 2010 Vincentz Network GmbH & Co. KG, Hanover Vincentz Network, P.O. Box 6247, 30062 Hanover, Germany This work is copyrighted, including the individual contributions and figures. Any usage outside the strict limits of copyright law without the consent of the publisher is prohibited and punish-able by law. This especially pertains to reproduction, translation, microfilming and the storage and processing in electronic systems. The information on formulations is based on testing performed to the best of our knowledge. Please ask for our book catalogue Vincentz Network, Plathnerstr. 4c, 30175 Hanover, Germany T +49 511 9910-033, F +49 511 9910-029 [email protected] www.european-coatings.com Layout: Vincentz Network, Hanover, Germany ISBN 978-3-7486-0227-9

European Coatings Tech Files

Bodo Müller | Walter Rath

Formulating Adhesives and Sealants Chemistry, Physics and Applications

Bodo Müller/Walter Rath: Formulating Adhesives and Sealants © Copyright 2010 by Vincentz Network, Hanover, Germany

Preface Formulations of adhesives and sealants are listed in very few publications because they are closely guarded industrial secrets. Although guide formulations and patent examples are available, they cannot be used for a textbook without careful selection and revision beforehand. This situation is similar to that for paint and coatings formulations. The recent textbook entitled “Coatings Formulations” shed light on the formulations used in coatings. It seems reasonable, then, that adhesives and sealants might benefit from similar treatment, with the aim of emphasizing the similarities between coatings and adhesives technology. This book teaches adhesive and sealant formulation in two steps. Each section first describes the application and chemical basis of the type of adhesive or sealant concerned. This is followed by formulation advice and – if possible – an analysis of existing recipes (e.g. guide formulations and patent examples.) This analysis includes a calculation of the important characteristic values of the formulations. All calculations based on recipes and formulations are worked through step by step and should therefore be intelligible to beginners, too. Of the many adhesive and sealant systems available, the selection provided in this textbook is restricted to the premium types. In a textbook, fewer, specific examples can prove to be more informative. The formulations have mostly been developed from starting formulations or patent examples and cannot be used to produce adhesives and sealants without further ado. Moreover, starting formulations and patent examples are to some extent no longer state of the art. Patent restrictions or registered trade marks (™ or ®) are not mentioned explicitly. Furthermore, it should be noted that product and trade names change as a result of mergers and acquisitions. It has proved to be advantageous to classify adhesives (Part II) and sealants (Part III) by different criteria. In Part II, adhesive systems are classified by setting mechanism into physically setting, chemically reactive and pressure sensitive adhesives; applications are described as well. This type of classification is closely related to classification by the form of application. As sealants and adhesive-sealants, with the exception of the aqueous polyacrylates, are mostly applied in solventless form, it is beneficial to classify them by their applications in Part III. This textbook seeks to familiarise laboratory assistants, graduates, engineers, skilled workers and chemists with the practice of adhesives and sealants formulation. It presupposes a basic knowledge of chemistry. It will also serve as a reference work for all readers interested in adhesives and sealants. We would like to thank Ray Brown for polishing the text. Esslingen, Germany, and Aachen, Germany January 2010 Bodo Müller & Walter Rath [email protected] | [email protected] Bodo Müller/Walter Rath: Formulating Adhesives and Sealants © Copyright 2010 by Vincentz Network, Hanover, Germany

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6

Contents

Contents

Part I

General basics........................................................................................... 15

1

Introduction............................................................................................... 15

1.1 1.2 1.3 1.4

Definitions......................................................................................................................... 15 Setting of adhesives and sealants................................................................................ 18 Commercial importance................................................................................................. 20 References......................................................................................................................... 22

2

Adhesion.................................................................................................... 23

2.1 Wetting of substrates....................................................................................................... 23 2.2 Adhesion forces and mechanisms................................................................................ 27 2.3 Adhesion promoters/primers........................................................................................ 31 2.3.1 Silane adhesion promoters............................................................................................. 31 2.3.2 Thin, adhesive polymer layers...................................................................................... 32 2.3.2.1 Polyacrylic acids............................................................................................................... 32 2.3.2.2 Phenolic resins................................................................................................................. 32 2.4 References......................................................................................................................... 33 3 Classification of adhesives and sealants.................................................. 34 Part II

Adhesives................................................................................................... 37

1

Physically setting adhesives..................................................................... 37

1.1 1.1.1 1.1.1.1 1.1.1.2 1.1.1.3 1.1.2 1.1.2.1 1.1.2.2 1.1.2.3 1.1.2.4 1.1.2.5 1.1.2.6 1.2 1.2.1

Solvent-based adhesives................................................................................................. 37 Basic principles................................................................................................................ 37 Polymers in solution........................................................................................................ 37 Manufacturing and formulation.................................................................................... 40 Application........................................................................................................................ 41 Chemistry of solvent-based adhesives........................................................................ 44 Polyurethanes (PUR or PU)........................................................................................... 44 Polychloroprenes ............................................................................................................ 48 Nitrile rubbers ................................................................................................................. 51 Polyacrylates .................................................................................................................... 53 Polyvinyl chloride ........................................................................................................... 55 Polyvinyl acetate ............................................................................................................. 56 Water-based adhesives.................................................................................................... 58 Basic principles sensitive............................................................................................... 58

Bodo Müller/Walter Rath: Formulating Adhesives and Sealants © Copyright 2010 by Vincentz Network, Hanover, Germany

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8

Contents

1.2.1.1 1.2.1.2 1.2.1.3 1.2.2 1.2.2.1 1.2.2.2 1.2.2.3 1.2.2.4 1.3 1.3.1 1.3.1.1 1.3.1.2 1.3.2 1.3.2.1 1.3.2.2 1.3.2.3 1.3.2.4 1.3.2.5 1.3.2.6 1.4

Dispersions - theory and stabilisation......................................................................... 58 Formulation of water-based dispersion adhesives .................................................. 64 Application of water-based dispersion adhesives ................................................... 64 Chemistry of water-based dispersion adhesives....................................................... 67 Polyurethanes (PUR or PU)........................................................................................... 67 Polychloroprenes ............................................................................................................ 71 Polyacrylates..................................................................................................................... 73 Polyvinyl acetate and derivatives . .............................................................................. 74 Hot-melt adhesives.......................................................................................................... 76 Basic principles................................................................................................................ 76 Composition and setting process.................................................................................. 76 Application ....................................................................................................................... 79 Chemistry and formulation of hot-melts . .................................................................. 84 Polyolefins and copolymers . ....................................................................................... 85 Polyesters........................................................................................................................... 90 Polyamides........................................................................................................................ 94 Block copolymers based on polystyrene . .................................................................. 98 Acrylates .......................................................................................................................... 98 Specialty products........................................................................................................... 98 References......................................................................................................................... 99

2

Reactive adhesives.................................................................................... 100

2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.2 2.2.1 2.2.1.1 2.2.1.2 2.2.1.3 2.2.1.4 2.2.1.4.1 2.2.1.4.2 2.2.1.4.3 2.2.2 2.2.2.1 2.2.2.2 2.2.2.3 2.2.2.4 2.2.2.4.1 2.2.2.4.2 2.2.2.4.3 2.2.2.5 2.3 2.3.1 2.3.1.1 2.3.1.2 2.3.1.3

Basic principles................................................................................................................ 100 Resins................................................................................................................................. 100 Fields of application......................................................................................................... 100 Components and methods of curing ........................................................................... 101 Toxicology.......................................................................................................................... 104 Polyaddition adhesives .................................................................................................. 105 Polyurethanes................................................................................................................... 105 Basic principles ............................................................................................................... 105 Structure/property relationships ................................................................................ 113 Stoichiometric considerations . .................................................................................... 116 Formulation of reactive PUR adhesives . ................................................................... 124 2-component PUR adhesives......................................................................................... 124 Moisture-curing, 1-component PUR adhesives ....................................................... 128 Heat-curing, 1-component PUR adhesives................................................................. 133 Epoxies............................................................................................................................... 137 Basic principles ............................................................................................................... 137 Structure/property relationships.................................................................................. 151 Stoichiometric considerations . .................................................................................... 153 Formulation of reactive epoxy adhesives................................................................... 156 2-component epoxy adhesives . ................................................................................... 156 Heat-curing, 1-component epoxy adhesives ............................................................. 159 UV curing, 1-component epoxy adhesives................................................................. 161 Trends................................................................................................................................. 162 Polymerisation adhesives ............................................................................................. 163 Acrylates............................................................................................................................ 163 Basic principles................................................................................................................ 163 Structure/property relationships ................................................................................ 166 Formulations .................................................................................................................... 167

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10

Contents

2.3.1.3.1 2.3.1.3.2 2.3.1.3.3 2.3.2 2.3.2.1 2.3.2.2 2.3.2.3 2.4 2.4.1 2.4.1.1 2.4.1.2 2.4.1.3 2.5 2.5.1 2.5.2 2.5.2.1 2.5.2.2 2.5.3 2.5.3.1 2.5.3.2 2.5.4 2.5.4.1 2.5.4.2 2.5.5 2.6

2-component acrylic adhesives ................................................................................... 167 Anaerobic acrylic adhesives ......................................................................................... 172 Radiation-curing acrylic adhesives ............................................................................. 177 Cyanoacrylates ................................................................................................................ 180 Basic principles................................................................................................................ 180 Structure/property relationships ................................................................................ 182 Formulation....................................................................................................................... 183 Polycondensation adhesives.......................................................................................... 185 Phenolic resins................................................................................................................. 185 Basic principles ............................................................................................................... 185 Structure/property relationships.................................................................................. 191 Formulations..................................................................................................................... 193 Curing of hot-melts.......................................................................................................... 195 Concepts............................................................................................................................. 195 Moisture-curing PUR hot-melts.................................................................................... 197 Basic principles ............................................................................................................... 197 Synthesis and formulation............................................................................................. 198 Moisture-curing POR hot-melts (polyolefin reactive hot-melts)............................ 206 Basic principles and structure ..................................................................................... 206 Formulation....................................................................................................................... 207 Heat-curing epoxy hot-melts......................................................................................... 208 Basic principles and structure...................................................................................... 208 Formulations..................................................................................................................... 210 Radiation-curing acrylic hot-melts . ............................................................................ 211 References ........................................................................................................................ 211

3

Pressure sensitive adhesives.................................................................... 214

3.1 3.1.1 3.1.2 3.1.3 3.2 3.2.1 3.2.2 3.3 3.4 3.4.1 3.4.2 3.4.3 3.5 3.6

Basic principles................................................................................................................ 214 Characterisation............................................................................................................... 214 Structure/property relationships.................................................................................. 214 Application........................................................................................................................ 216 Solvent-borne pressure sensitive adhesives.............................................................. 216 Natural rubber ................................................................................................................. 216 Acrylics............................................................................................................................... 217 Latexes (aqueous polymer dispersions)..................................................................... 218 Hot-melt pressure sensitive adhesives....................................................................... 220 Styrene block copolymers.............................................................................................. 220 Ethylene vinyl acetates (EVA)....................................................................................... 222 Atactic polypropylene..................................................................................................... 222 UV curing systems.......................................................................................................... 223 References......................................................................................................................... 226

Part III

Sealants and adhesive-sealants................................................................ 227

1 Adhesive-sealants in the automotive industry........................................ 227 1.1 Basic principles................................................................................................................ 227 1.2 Thermosetting adhesive-sealants in automotive body-in-white construction.... 230 1.2.1 Plastisols............................................................................................................................ 231 1.2.2 Rubbers.............................................................................................................................. 233

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Contents

1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.4

Windscreen bonding with adhesive-sealants............................................................ 236 Moisture-curing, 1-component polyurethanes.......................................................... 237 Reaction-curing, 2-component polyurethanes........................................................... 241 Hot-applied, 1-component polyurethanes.................................................................. 242 Primers............................................................................................................................... 244 References......................................................................................................................... 244

2

Sealants for multi-pane insulating glass................................................. 246

2.1 Basic principles................................................................................................................ 246 2.2 Insulating glass sealants................................................................................................ 247 2.2.1 Primary seal (butyl rubber)........................................................................................... 248 2.2.2 Secondary seal.................................................................................................................. 249 2.2.2.1 2-component polyurethanes.......................................................................................... 249 2.2.2.2 2-component polysulphides........................................................................................... 251 2.2.2.3 Hot-applied thermoplastic elastomers (1-component)............................................. 252 2.2.2.4 System overview.............................................................................................................. 255 2.3 References......................................................................................................................... 256

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Contents

13

3

Construction sealants............................................................................... 257

3.1 3.2 3.2.1 3.2.1.1 3.2.1.2 3.2.2 3.2.3 3.2.4 3.2.5 3.2.6 3.3

Basic principles................................................................................................................ 257 Classification of construction sealants........................................................................ 259 Polyurethanes................................................................................................................... 261 Moisture-curing, 1-component polyurethanes.......................................................... 261 Reaction-curing 2-component polyurethanes............................................................ 262 Polysulphides.................................................................................................................... 264 Silicones............................................................................................................................. 266 Silane-modified sealants................................................................................................. 268 Polyacrylate latexes . ...................................................................................................... 271 Primers............................................................................................................................... 274 References......................................................................................................................... 274

Part IV Design and testing of adhesive joints....................................................... 275 1

Basic principles......................................................................................... 275

1.1 1.2 1.3

System view ..................................................................................................................... 275 Cohesion of adhesives and viscoelasticity ................................................................ 276 Thermomechanical properties...................................................................................... 278

2

Stress analysis of adhesive joints............................................................. 281

2.1 Preliminary note............................................................................................................... 281 2.2 Mechanical loads ............................................................................................................ 281 2.2.1 Static loads......................................................................................................................... 281 2.2.1.1 Analysis of shear deformation....................................................................................... 282 2.2.1.2 Analysis of peel deformation......................................................................................... 284 2.2.2 Dynamic loads................................................................................................................... 286 2.2.2.1 Influence of the rate of deformation ........................................................................... 286 2.2.2.2 Periodic loads.................................................................................................................... 286 2.3 Analysis of ageing influences ...................................................................................... 287 2.3.1 Thermal ageing................................................................................................................. 287 2.3.1.1 Influence of temperature on shear and peel strength ............................................ 287 2.3.1.2 Coefficients of thermal expansion ............................................................................. 287 2.3.1.3 Phase transitions.............................................................................................................. 287 2.3.1.4 Degradation of organic polymers ................................................................................ 287 2.3.2 Analysis of exposure to different fluids ..................................................................... 288 2.3.2.1 Swelling.............................................................................................................................. 288 2.3.2.2 Hydrolysis.......................................................................................................................... 289 2.3.2.3 Sub-surface migration..................................................................................................... 289 2.4 Anisotropic effects .......................................................................................................... 290 3

Rules.......................................................................................................... 290

4

Testing of adhesive joints......................................................................... 292

4.1 4.1.1 4.1.2 4.1.2.1

Basic principles................................................................................................................ 292 Preliminary notes............................................................................................................. 292 Preliminary tests.............................................................................................................. 293 Hand strength................................................................................................................... 293

14

Contents

4.1.2.2 Bead test............................................................................................................................. 293 4.1.2.3 Impact strength test........................................................................................................ 294 4.1.2.4 Water resistance test....................................................................................................... 294 Evaluation of failure mode ............................................................................................ 294 4.1.3 4.1.4 Standardised and defined application tests .............................................................. 295 4.2 Mechanical tests............................................................................................................... 296 4.2.1 Static tests......................................................................................................................... 296 4.2.1.1 Tensile shear strength (TSS)......................................................................................... 296 4.2.1.2 Tensile strength .............................................................................................................. 297 4.2.1.3 Peel strength .................................................................................................................... 297 4.2.2 Tests of static long-term load......................................................................................... 298 4.2.2.1 Creep resistance............................................................................................................... 298 4.2.2.2 Wedge test......................................................................................................................... 299 4.2.3 Dynamic tests................................................................................................................... 299 4.2.3.1 Impact strength tests...................................................................................................... 299 4.2.3.2 Fatigue resistance test.................................................................................................... 300 4.3 Durability and ageing tests............................................................................................ 300 4.3.1 Testing the influence of temperature ......................................................................... 300 4.3.1.1 Heat-resistance tests....................................................................................................... 300 4.3.1.2 Temperature dependence of bond strength .............................................................. 300 4.3.2 Test of humidity resistance .......................................................................................... 300 4.3.2.1 Water immersion test...................................................................................................... 300 4.3.2.2 Boiling water test............................................................................................................. 301 4.3.2.3 Condensed water test...................................................................................................... 301 4.3.2.4 Cataplasma test................................................................................................................ 301 4.3.3 Combined and variable influences . ............................................................................ 301 4.3.3.1 Cyclic tests........................................................................................................................ 301 4.3.3.2 VDA alternating cycles test........................................................................................... 301 4.3.3.3 Specific environments.................................................................................................... 302 4.3.3.4 Salt spray tests................................................................................................................. 302 4.3.3.5 Stress corrosion................................................................................................................ 302 4.3.3.6 Light.................................................................................................................................... 302 4.3.3.7 Testing for volatile components.................................................................................... 303 4.4 Pressure sensitive adhesives........................................................................................ 303 4.4.1 Tack..................................................................................................................................... 303 4.4.2 Peel strength..................................................................................................................... 305 4.4.3 Creep resistance............................................................................................................... 305 4.5 Non-destructive test methods ...................................................................................... 305 5 References......................................................................................................................... 306 General literature...................................................................................... 307 Authors...................................................................................................... 309 Index.......................................................................................................... 310

Definitions

15

Part I General basics 1 Introduction Nowadays, adhesive bonding is an indispensable technology for permanently joining two or more substrates in both industrial and private use. The resultant composite materials often facilitate the manufacture of innovative products [1]. In Germany alone, nearly 800,000 tons of adhesives (worth nearly 1.5 billion EUR) were consumed in 2007, and the trend is growing. Adhesives manufacturers offer more than 25,000 different products for all kinds of applications – and tailored to nearly every purpose [1]. In terms of chemical and applications technology, the field of adhesives and sealants is extremely diversified. This book can only describe the most important types; needless to say, the selection presented here is subjective. From its title “Formulating Adhesives and Sealants” it is clear that the main focus is on the materials (chemistry) and not on the joining technology (adhesive bonding and sealing). The underlying technological aspects are both multidisciplinary and interdisciplinary by nature. Figure I-1.1 illustrates this interdisciplinarity from the point of view of adhesion (see Chapter I-2). Figure I-1.2 illustrates this interdisciplinarity or complexity based on the factors influencing the strength of an adhesive bond.

Figure I-1.1: Interdisciplinarity

1.1

Figure I-1.2: Factors influencing the strength of an adhesive bond

Definitions

Adhesives An adhesive is a non-metallic material which can join substrates by adhesion and cohesion [3]. Adhesives are applied in a liquid-like state, wet the substrates and then set physically or chemically (i.e. solidify). Bodo Müller/Walter Rath: Formulating Adhesives and Sealants © Copyright 2010 by Vincentz Network, Hanover, Germany

16

Part I

Introduction

Adhesives transmit loads between the bonded substrates (adherends); force-fit bonds of the kind yielded by other joining technologies, e.g. welding, brazing, and riveting, are obtained. Adhesive bonds distribute loads more uniformly over the entire joint area. This more uniform distribution of forces allows the material thickness to be used up to its maximum load-bearing capacity, facilitating the use of thinner and lighter components.

Figure I-1.3: Simplified classification of some adhesives with respect to adhesion and cohesion

Adhesion is the attraction between a solid interface and a second phase (two different substances) whereas cohesion is the force that holds materials together. Thus, cohesion is a special form of adhesion, in which homogeneous particles adhere to each other.

Adhesive bonding denotes the joining of the same or different materials by means of an adhesive [3]. Structural adhesive bonding is the use of adhesive bonding to create a durable construction of high stability and rigidity; such adhesive bonds are characterised by high adhesion and cohesion (Figure I-1.3). An adhesive joint is the gap between two glued surfaces which is filled by a layer of adhesive.

Sealants A sealant is a substance for filling joints and gaps (this includes solid materials, such as rubber profiles). A joint sealant is a sealant which is introduced into a joint in a liquid-like state. A joint is a gap between structural elements that is either deliberate or required by tolerances; as a rule, a sealed joint is generally much larger than an adhesive joint; sealants are therefore gap-filling. In practice, though, sealant is the predominant term employed. For the purposes of this book, sealants are paste materials which solidify in joints to yield more or less solid materials. The function of sealants is to fill gaps created by the assembly of structural elements made of similar or different materials, and to seal these joints so as to exclude: • • • • •

gases liquids solids (e.g. dust) energy loss noise.

Further requirements are to provide a flexible support (e.g. for glass panes on metal) or to prevent corrosion where metals are in direct contact. Moreover, a sealant must possess adequate adhesion (like an adhesive) and cohesion for the job at hand. Solid sealants transmit only small loads between the sealed structural elements (mostly less than 1 MPa). The functionality of a solid sealant is greatly affected by its resilience (elastic recovery)[6]. In the ideal case, the resilience should be 100 %; in practice, though, it is somewhat lower. Thus, a solid seal exhibits a degree of plasticity in addition to elasticity. In particular cases, plasticity may even be desirable.

Definitions

17

Adhesive-sealants The transition from high to low load transfer is smooth. The technologically very important transition region between pure adhesives and pure sealants is bridged by so-called adhesivesealants. Figure I-1.4 schematically illustrates the differences between adhesives, adhesive-sealants and sealants on the basis of stress-strain characteristics. A second classification criterion is that of gap-filling ability. Sealants can fill gaps (up to several cm), whereas adhesives cannot (adhesive joints ≤ 1 mm). Adhesive-sealants occupy an intermediate position again, and can fill gaps up to about 5 mm wide. Figure I-1.5 illustrates the differences between adhesives, adhesive-sealants and sealants. More information on classification schemes for adhesives and sealants is presented in Chapter I-3.

Figure I-1.4: Qualitative stress-strain characteristics of adhesives, adhesive-sealants and sealants

Advantages of adhesive bonding [5]: + uniform  stress distribution over the entire bonded surface, + no thermal influence on the microstructure of metal alloys (as is the case for welding), + no thermally induced distortion of structural parts, + bonding of different materials (e.g. glass-metal), Figure I-1.5: Qualitative classification of adhesives, adhesive+ no contact corrosion (when joining sealants and sealants electrochemically different metals), + joining of very thin parts (e.g. sheets), + joining of thermosensitive materials, + weight savings relative to other joining technologies, + combinable with screwing, riveting and spot-welding. Disadvantages of adhesive bonding [5]: -

limited scope for repair and recycling, limited thermal resistance of bonded joints, tendency to creep, extensive quality control and quality assurance measures needed, ageing of organic adhesive layers (e.g. autoxidation), process parameters must be closely observed.

18

Part I

Introduction

1.2 Setting of adhesives and sealants For application, adhesives and sealants must be flowable so that they can wet the substrates; this is a requirement for adhesion (Chapter I-2). Setting by adhesives and sealants is also called solidification and represents the transition from the flowable to the solid state. This transition can occur purely physically or by chemical reaction. Figure I-1.6 schematically shows the most important setting mechanisms. An exception here are pressure sensitive adhesives, which are permanently tacky (e.g. for labels or adhesive tapes) and which, strictly speaking, do not solidify. Pressure sensitive adhesives are applied as solutions, aqueous dispersions or melts (all of which set physically) and even as reactive UV-curing systems (see Chapter II-3.5) thus, they extend beyond the scope of Figure I-1.6.

Physical setting The binder polymers already exist prior to physical setting [3]. They are rendered flowable by dissolving or dispersing them in solvents (including water). Setting occurs by solvent evaporation; compare the physical drying of paints [4]. Another possibility is to melt thermoplastic polymers; this is a reversible process. Contact adhesives set by partial crystallisation of the polymers. The special setting process undergone by plastisols is described in detail in Chapter III-1.2.1. All physically setting adhesives and sealants are plastomers (see Figures I-1.7 and I-1.8) and have disadvantages, such as poor solvent resistance and heat distortion.

Chemical setting The binders of chemically setting (curing) adhesives and sealants are flowable monomers or prepolymers (oligomers). Like chemically curing coatings [4], chemical setting in adhesives and sealants is marked by an increase in molecular mass and more or less pronounced crosslinking. The crosslink density dictates whether thermosets or chemically crosslinked elastomers are formed (see following section).

Figure I-1.6: Classification of adhesives by (the most important) setting mechanisms

Figure I-1.7: Classification of polymeric plastic materials

Organic binders for adhesives and sealants as polymeric materials Here, adhesives and sealants will be considered from the point of view of plastics technology. Basically, organic binders in solid adhesives and sealants are nothing other than polymeric plastics and could be classified in the same way (Figures I-1.7 and I-1.8) [4]. Physical drying of polymer solutions and dispersions or cooling of polymer melts yields plastomers. Chemical reaction (curing) leads to either thermosets or chemically crosslinked elastomers, in accordance with the crosslink density.

Setting of adhesives and sealants

19

One exception is thermoplastic elastomers, which are applied as a melt but exhibit elasticity at room temperature because of secondary valence bonds (see Chapter II-1.3 and III-2.2.2.3). All the different types of polymer materials presented in Figures I-1.7 and I-1.8 are used in adhesives and sealants and will be described as specific examples in the course of this book. The term elasticity is very important with respect to adhesives and sealants. Elasticity is a property by which solid matter returns to its original state after deformation (recovery). Figure I-1.9 schematically shows entropy elasticity (rubber-like elasticity), which is observed especially in polymers. Entropy elasticity is caused by the tendency of polymers to assume a disordered conformation (of high entropy). Figure I-1.10 clearly shows the elasticity, and Figure I-1.8: Simplified diagram of polymeric plastic especially the recovery, of a cured polymaterials sulphide construction sealant (Chapter III-3.2.2). Application of force causes distortion of the sealant (Figure I-1.10, right); when the force is removed, the sealant returns to its original state (Figure I-1.10, left).

Figure I-1.9: Simplified diagram of entropy elasticity or rubber-like elasticity

Figure I-1.10: Test specimen with cured polysulphide construction sealant, with (right) and without (left) application of force

20

Part I

Figure I-1.11: Formulation of adhesives and sealants

Introduction

Paint formulating [4] often entails the mixing of commercially available raw materials, whereas the formulation of adhesives and sealants may also include the synthesis of polymers (Figure I-1.11). Examples are adhesive-sealants for bonding windscreens to car bodies (Chapter III-1.3) where so-called “one-shot processes” are employed. These consist in simultaneous mixing of the ingredients, dispersion of the fillers and production of an isocyanate-terminated prepolymer.

As in coatings, an emerging trend in adhesives is the incorporation of nanoscale, surface-modified (and therefore reinforcing) fillers to improve mechanical properties [13].

1.3 Commercial importance Adhesives Global demand for adhesives in 2007 was estimated at 11 million tons. A regional breakdown of consumption is presented in Figure I-1.12. The various application areas for adhesives are presented in Figure I-1.13 [8]. The bulk of adhesives is consumed by the paper and packaging industry (Figure I-1.13); most packaging is a shortlived mass-produced article [8]. As in paints and coatings [4], construction applications are very important. The various application forms for adhesives are presented in Figure I-1.14 [9]. While aqueous systems predominate in adhesives, just as in paints and coatings [4], emissions-free and recyclable hot-melt adhesives (Chapter II-1.3) are of great economic importance (Figure I-1.14). Solventbased systems are declining in importance for the familiar ecological reasons.

Figure I-1.12: Regional breakdown of demand for adhesives and sealants in 2007 (100 % equates to 11 million tons) [7]

Commercial importance

Figure I-1.13: European adhesives market

21

Figure I-1.14: Application forms for adhesives (renewable means based on renewable raw materials)

Sealants A regional breakdown of the various application areas for sealants is presented in Figure I-1.15. The various applications for sealants are presented in Figure I-1.16 [11], and are predominantly in the area of construction [including glazing, insulating glass (IGS) and do-it-yourself (DIY)]. The various base materials for the most important sealant systems are shown in Figure I-1.17 (regional breakdown) [10]. Silicones predominate here because they are mainly used in construction (see Figure I-1.16). All the various types of sealants presented in Figure I-1.17 will be described in the course of this book.

Base materials The different base materials for adhesives and sealants are recorded in Figure I-1.18 [12]; this shows how multifaceted the chemistry of adhesives and sealants is. It should be emphasized that not all types of adhesives and sealants in Figure I-1.18 can be described in this

Figure I-1.15: Regional breakdown of sealants consumption

Figure I-1.16: End-user market for elastic sealants in Europe

22

Part I

Adhesion

Figure I-1.17: Base materials for sealants (regional breakdown)

Figure I-1.18: Consumption of adhesives and sealants by base raw material in the USA (1998)

book. As this book focuses on premium adhesives and sealants, those binders which are based on amino resins and, to some extent, phenolic resins are not discussed, as they are used in chipboard (high quality phenolic resins, see Chapter II-2.4). Nor will low-quality, bituminous sealants be described.

1.4 References [1] Information series 27 “Adhesives” (in German), Fonds der Chemischen Industrie (2001) [2] E. M. Petrie, Handbook of Adhesives and Sealants, McGraw-Hill, New York (2000) [3] G. Habenicht, Kleben – Grundlagen, Technologie, Anwendung, 4th ed., SpringerVerlag (2002) [4] B. Müller, U. Poth, Coatings Formulation, Vincentz Network (2006) [5] R. Domanski, Basiswissen Haftkleben (Lohmann Gruppe, 2003) [6] J. Krobb, E. Wistuba, Adhäsion (1992) No. 5, p. 14–20 [7] www.feica.com [8] H. Onusseit, Adhäsion, 47 (2003) No. 4, p. 20–24 [9] H. Onusseit, Phänomen Farbe, No. 9/10 (2003) p. 14–19 [10] H. Mack, Selecting the Right Aminosilane Adhesion Promoter for Hybrid Sealants, 2nd Organosilicon Days, Munich (2003) [11] Adhäsion, 40 (1996) No. 10, p. 13 [12] U.S. Adhesives and Sealants Market to Reach $ 13.5 Billion (2000) in www.adhesivesmag.com [13] S. Sprenger, A. J. Kinloch, J. H. Lee, C. Taylor, D. Egan, FARBE UND LACK, 112, No. 7, p. 37–40 (2006)

Wetting of substrates

23

2 Adhesion Adhesive strength is a measure of the resistance of an adhesive bond to mechanical removal from a substrate (for tensile load it is given by force/area: MPa or N/mm2; but for peel strength it is given by force/length: N/mm). In the following, we talk about adhesives; however, similar considerations apply to sealants and – as described earlier [1] – to coatings.

2.1 Wetting of substrates A prerequisite for good adhesion (albeit insufficient on its own) is adequate wetting of the solid substrate by the liquid adhesive during application. The substrate/air interface (surface) is converted to a substrate/liquid interface. Subsequent setting of the adhesive yields a substrate/solid interface. It should be noted that it is difficult to define the solidification state of adhesives. For example, permanently tacky pressure sensitive adhesives on labels are always in the state of a high-viscosity liquid.

Surface and interfacial tension In a liquid, such as water, all molecules in the bulk phase are uniformly surrounded by their neighbouring molecules. Thus, the attractive forces acting on these molecules extend equally in every direction in space and cancel each other out (Figure I-2.1). At the water/air interface, things change dramatically because a water molecule there is surrounded only in the interface itself and in the direction of the bulk phase. Thus, the forces of attraction do not cancel each other out; a force acts on the water molecule towards the inner phase of the liquid (Figure I-2.1). This force causes the liquid’s surface to become as small as possible. That is why a droplet in outer space is spherical – the sphere has minimal surface area combined with maximum volume. This force has to be quantified. The interfacial tension (γ) is a force acting along 1 m of an imaginary boundary between two phases [force/length = N/m; mostly quoted in mN/m, formerly in dyn/cm]. If one of the phases is air, the tension is called the surface tension. For the surface area of a liquid to increase, molecules must move from the bulk to the surface. This means that forces of attraction must be overcome, work done and energy expended. The following mathematical derivation shows how energy/area is equal to force/ length. In other words, surface tension is also a measure of surface energy: Energy J N · m N Force = = = = 2 2 Area m m m Length interfacial or surface tension Typical surface tensions of liquids are preFigure I-2.1: Force diagram to explain surface tension Bodo Müller/Walter Rath: Formulating Adhesives and Sealants © Copyright 2010 by Vincentz Network, Hanover, Germany ISBN 978-3-86630-858-9

sented in Table I-2.1.

24

Part I

Table I-2.1: Surface tension [mN/m] of liquids (these values may vary slightly from one literature reference to another) Mercury

500 1)

Water

73

Epoxy resins

45 to 60

Melamine resins

42 to 58

Alkyd resins

33 to 60

Acrylic resins

32 to 40

Butyl glycol

32

Xylene

29 to 30

White spirit

26 to 27

Butyl acetate

25

Butanol

23

White spirit

18 to 22 2)

Hexane

18

1) liquid metal 2) free of aromatic hydrocarbons

Adhesion

The greater the surface tension, the greater is the cohesion in the respective liquid phase and the stronger are the forces of interaction between the atoms or molecules in that phase. Water molecules associate strongly with each other by means of hydrogen bonds, which generate a high level of cohesion and a high surface tension. The water surface behaves like a skin. The less polar a liquid is, the lower is its surface tension.

Wetting A simplified diagram of the wetting of a solid surface (substrate) by a liquid is presented in Figure I-2.2. A measure of wetting is provided by the contact angle Θ formed between the solid substrate and the liquid drop (Figure I-2.2). The smaller the contact angle Θ, the better is the wetting.

Mathematically, wetting is described by Young’s equation:

γS = γSL + γL · cosΘ Complete wetting of a substrate by a liquid is called spreading: Θ = 0° and cosΘ = 1. For spreading, Young’s equation becomes γ S = γ SL + γ L and γ L = γ S - γ SL. A liquid, such as an adhesive, cannot wet the surface of a solid substrate unless its surface tension is lower than that of the substrate (γ L < γ S). Thus, if a liquid has a higher surface tension than the substrate, there will be no wetting.

γ S

is the surface tension of the solid substrate

γ L

is the surface tension of the liquid

γ SL is the interfacial tension between solid substrate and liquid Θ

is the contact angle of the liquid on the substrate

Figure I-2.2: Wetting of a solid substrate by a liquid

Wetting of substrates

The surface tensions of metals that are quoted in the literature range from several hundred to several thousand mN/m (see mercury in Table I-2.1); these values refer to liquid metals at their melting temperatures [17]. Where wetting is concerned, it is the surface tension (at ambient temperature) of solid metal surfaces covered by oxide and layers of adsorbed substances which is important (Table I-2.2). The critical surface tension of solids (glass, metal, plastics in Table I-2.2) can be measured indirectly by wetting experiments (measurement of contact angles, Zisman method) [14, 16, 17]. Allowance must be made for possible changes in the surfaces of substrates (e.g. oxide layers, mouldrelease agents) as these may greatly affect the surface tension (see Table I-2.3).

Metal surfaces

25 Table I-2.2: Critical surface tension of solid substrates [mN/m] Glass

73

Phosphated steel *

43 to 46

Poly(vinyl chloride)

39 to 42

Tin-plated steel *

approx. 35

Aluminium *

33 to 35

Polyethylene

32 to 39

Polypropylene

28 to 29

Steel (untreated) *

29

Polydimethylsiloxane

19 **

Polytetrafluorethylene

19

* Solid metal (oxide) surfaces! Melted (liquid) metals have much higher surface tensions (see mercury in Table I-2.1). These values may vary slightly in the literature. ** silicones up to 24

Table I-2.3: Contact angle for water on various aluminium surfaces Aluminium surface rolled

Contact angle Θ (± 5°) 63

pickled with a commercial caustic 22 If the prerequisite for wetting is γL 0°).

Figure I-2.3 is a schematic diagram of an aluminium surface [2] that is better described as an aluminium oxide surface; similar considerations apply to the surfaces of all commonly employed metals. If the model presented in Figure I-2.3 reflects reality, the hydrated aluminium oxide surface would be completely wetted by water; but this is not observed (Table I-2.3). Figure I-2.3 is a simplification of the real situation. ESCA/XPS measurements show that variously pretreated aluminium surfaces contain a significant amount of carbon in addition to aluminium and oxygen. Presumably, carbon is adsorbed from the air (e.g. carbon dioxide or hydrocarbons) [3, 6]; see also Chapter I-2.3.2.2 (Figure I-2.21). Thus, while there is still no universal model that describes the structure of metal surfaces, metal oxides are certainly present.

Surfaces of plastics Even more complicated are the surface structures of engineering plastics [2]. The problem here is that the bulk properties of the polymer are different from those of the surface. These differences may be due to the composition of the plastic or to the production or processing conditions. Mostly, their surfaces have low energies (low surface tension), which leads to poor wetting.

Composition of plastics Many plastics contain low-molecular components, such as additives (e.g. stabilisers), solvent residues and sometimes plasticisers. All these components can impair adhesion if they are

26

Part I

Adhesion

Figure I-2.3: Hydrated aluminium (oxide) surface (simplified model) Similar considerations apply to the Fe/Fe2O3 system

present on the surface. Many low-molecular components tend to migrate to the surface and accumulate there. Thus, plastics may have an anti-adhesive layer on the surface (Figure I-2.4).

Production and processing conditions of plastics a) Mould-release agents When injection-moulded or compression-moulded plastic parts have to be released from the mould, internal and external mould-release agents are used. Internal mould-release agents are mixed into the plastic pellets and are distributed throughout the plastic material; therefore, sanding the plastic surface is useless. Internal mould-release agents generate plastic surfaces that are barely if at all wettable. External mould-release agents, in contrast, are sprayed into the open injection mould; they are based on paraffins, soaps and oils (including silicone oils). Because of the processing conditions, external mould-release agents are subsequently found not only in the surface layer, but also in the underlying layers.

b) Surface properties caused by processing conditions

Figure I-2.4: Surfaces of engineering plastics

Injection and compression moulding create surface properties that differ from those of the bulk polymer. The dense surface layers of moulded plas­ tic materials are very smooth (orientated layers; Figure I-2.4).

Wetting of substrates

27

Improvement in wetting In the case of water-based adhesives especially γL > γS leads to poor wetting. There are two ways to improve wetting: • Lower the surface tension of the waterbased adhesive (γL) by adding wetting agents (see [1]). • Increase the surface tension of the substrate (γS). For example, the surface tension of metals is increased by phosphating (see Table I-2.2). Figure I-2.5: Adhesive bond Oxidation of plastic surfaces (e.g. by flame treatment) generates polar functional groups (e.g. -OH, -COOH) on the surface and increases γS. A detailed description of pretreatment processes for various plastics is provided in [2, 10, 15]. Sometimes the wettability of plastic surfaces can be improved by sanding or rubbing with emery paper or cleaning with organic solvents or water-based cleaners. Furthermore, primers may be applied first to improve bonding. For example, polyolefins (low surface tension, Table I-2.2) can be coated with chlorinated polymers, which increase the surface tension [2].

2.2 Adhesion forces and mechanisms So far, no universal theory of adhesion exists, and those theories which do exist explain only aspects of it. Nonetheless, a partial theoretical explanation of phenomena related to adhesion is better than none.

Adhesion/cohesion Adhesives and sealants must exhibit sufficient adhesion to the joined substrates (as well as sufficient cohesion). Adhesion is the attraction which exists at the interface of two different solid phases. Adhesion is expressed in units of energy/area (compare surface tension); in contrast, the units of adhesive strength are force/area. The counterpart to adhesion is cohesion. Cohesion is the attraction which exists within a single phase (in the solidified adhesive or sealant phase). Cohesion is a state in which particles (molecules) of a single substance are held together; it is a special instance of adhesion in which only molecules of the same kind adhere to each other. Figure I-2.5 explains adhesion and cohesion in an adhesive bond.

Modes of adhesive bond failure Loss of adhesion by a bond yields the following failure modes, the extent of which varies with the level of adhesion and cohesion. Adhesive failure:

adhesion < cohesion

Cohesive failure:

adhesion > cohesion

Both types of failure:

adhesion ≈ cohesion

Failure of the substrate:

adhesion and cohesion > strength of the substrate

(desired)

(rare)

28

Part I

Figure I-2.6: Adhesive bond between a plastic and a metal part (simplified)

Adhesion

It should be pointed out that much adhesive failure is often unrecog­ nized cohesive failure within a weak layer of adhesive close to the interface (weak boundary layer; Figure I-2.6). The chemical composition of the adhesive or the arrangement of the polymer molecules in the boundary layer often differs from that in the bulk materials. For example, a zone of reduced strength can be formed between the chemisorbed polymer on the substrate (monolayer) and the bulk polymer. For simplicity, this unwanted failure mode is usually called adhesive failure, too.

Weak layers of adhesive close to an interface are also observed in plastic substrates (polymer materials; see Figure I-2.4) and on metals (certain oxide layers; see Figure I-2.6) [2]. Figure I-2.7 shows an example of adhesive failure; clearly, loss of adhesion causes the solidified sealant in an expansion joint to lose its sealing ability.

Theories of adhesion

Figure I-2.7: Adhesive failure (left) of a sealant in an expansion joint

In general, a distinction is made between specific adhesion (interaction of interfaces irrespective of the geometrical form of the surface) and mechanical adhesion (Figure I-2.8). Mechanical adhesion (Figure I-2.9) takes place when liquid adhesive enters into cavities (voids, roughness) of the substrate and the cured adhesive is anchored mechanically therein. Prerequisites for efficient mechanical adhesion are adequate wetting of the substrate by the adhesive and a low adhesive viscosity. A consequence of Table I-2.4 is that optimal adhesion is the result of primary valence bonds between adhesive and substrate. Primary valence bonds will therefore be discussed first.

Ionic bonds

Figure I-2.8: Mechanisms of adhesion

Ionic bonds (e.g. salt formation) are formed especially on mineral substrates, such as metal oxide layers, phosphated metal surfaces and so on (Figures I-2.10 and I-2.11).

Adhesion forces and mechanisms

29

Figure I-2.10 shows an acid-base reaction between a hydroxyl group on a surface and a carboxyl group from an adhesive. In contrast, Figure I-2.11 shows a metal salt or complex. The disadvantage of ionic bonds is that they can be broken by infiltrating water and this can lead to loss of adhesion in wet conditions.

Chelate complexes on surfaces In contrast, chelate complexes on metal Figure I-2.9: Mechanical adhesion (oxide) surfaces ought to be largely stable to hydrolysis and lead to improved adhesion in wet conditions. Experience shows that both epoxy resins cured with amines and thermosetting phenolic resins (resols) exhibit very good adhesion on metals, even in wet conditions; both observations can be explained by assuming the formation of chelates (Figure I-2.12 and 2.13) [5, 6]. Chelates which form 6-member rings are known to be stable (Figure I-2.13) [6]. Recent molecular calculations [7] suggest that a second resol-metal chelate (8-member ring in Figure I-2.13) may be formed. As a rule, chelates forming 8-member rings are less stable than their 6-member counterparts but their formation ought to be considered in this special case (see Chapter I-2.3.2.2).

Covalent bonds A further possibility is the formation of covalent bonds on polymer substrates (plastics) by chemical reaction between the adhesive and suitable functional groups on the polymer substrate. Oxidation of plastic surfaces (e.g. by flame treatment) yields reactive functional groups (e.g. -OH, -COOH) which can react chemically with an adhesive.

Table I-2.4: Types of chemical bonds and bond energies [13] Type of chemical bond

Bond energy [kJ/mol]

Primary valence bonds · ionic linkage (salt links) · covalent bonds

600 to 1000 60 to 700

Secondary valence bonds · permanent dipoles (Keesom) · induced dipoles (London)

< 20 acrylates) and the alcohol component of the corresponding ester. Within certain limits, the glass transition temperature of the polymer decreases in proportion to the increase in length of the hydrocarbon chain of the alcohol in the ester. Thus, it Table II-1.22: Water-based acrylic adhesive for PVC flooring is possible to make acrylates that Item Raw material Parts by exhibit pressure-sensitive properweight ties by virtue of their low glass transition temperature, and acry1 acrylate dispersion 29 lates whose high glass transition 2 rheological additive 9.8 temperature confers high cohesive 3 resin mixture 12 strength to the point of brittleness. 4 defoaming agent 0.2 For applications in water-based dis5 surfactant, anionic 0.5 persions, there are certain upper limits imposed by the minimum 6 surfactant, non-ionic 0.5 film-forming temperature. 7 dispersing agent 0.5 Formulation proceeds from the choice of suitable base dispersion (20 to 40 %). In accordance with the intended application, this can be modified with resins (10 to 30 %) and fillers, usually chalk (up to 60 %). Additives (only a few percent) are typically surfactants, defoamers, and wetting agents [11].

Guide formulations Table II-1.22 presents a low-emissions, water-based acrylic adhesive for PVC flooring [30]. The product is formulated with a high quantity of filler (to reduce costs and for ease of processing). Table II-1.23 presents a starting formulation for a water-based acrylic contact adhesive [31]. The product is modified with a phenolic resin dispersion.

8

wetting agent

9

filler

0.2 47.3

Item 1: “Acronal A 380”, solids content: 62 %, pH = 5.5-6.5, Tg = -22 °C, BASF Item 2: 2 % “Latekoll D” solution, BASF Item 3: melt from colophony resin (Terhell) and “Novares LS 500”, hydrocarbon resin, (Rütgers Chemicals AG), Resins are homogenised at 100 °C in a ratio 1/1 and hot stirred into the dispersion Item 4: “Agitan 262”, defoaming agent, Münzing Item 5: “Disponil FES 77”, anionic surfactant, Cognis Item 6: “Lumiten N-OG”, non-ionic surfactant Tensid, BASF Item 7: “Pigmentverteiler NL”, polyacrylic acid, BASF Item 8: “Lumiten I-DS 3525”, wetting agent, BASF Item 9: “Ulmer Weiß XM”, CaCO3, Eduard Merkle GmbH & Co KG

Table II-1.23: Water-based acrylic contact adhesive Item

Raw material

Parts by weight

1

water-based acrylate persion

69.8

2

phenolic resin dispersion

29.9

3

additiv

0.3

Item 1: “UCAR Latex 154 S”, DOW Item 2: “BKUA 2370”, Georgia Pacific Resins Inc. Item 3: triethanolamine to adjust pH value of the acrylate dispersion in the range of 6 to 6.5 before addition of the phenolic resin dispersion

74

Part II

Table II-1.24: Water-based acrylic adhesive for ceramic tiles Item

Raw material

Parts by weight 40.9

Adhesives

Table II-1.24 presents a starting formulation for a water-based acrylic adhesive for ceramic tiles [32]. Besides the polymer and the resins, the formulation contains a number of additives typically employed in water-based dispersions.

1

water-based acrylic dispersion

2

defoaming agent

0.7

3

fungicide

0.2

4

surfactant, non-ionic

0.7

5

thickening agent, acrylate basis

4.1

Evaluation The numerous building blocks available for acrylates means that they offer more variety of properties than most other classes of polymer. Acrylates can be made in strengths ranging from “soft as butter” to high strength – whatever is feasibly is done. Acrylates are inexpensive, are highly tolerant of modification and can easily be tailored to the requirements of any application. However, copolymerisation is usually a random process. Consequently, segmentation yielding properties akin to a multiphase polymer of the kind found in polyurethanes, is not possible.

6

plasticiser

1.9

7

ammonia solution

0.5

8

coupling agent

0.1

9

film-forming agent

10

dispersing agent

0.6

11

rheological additive

0.4

12

filler, chalk

31.5

13

water

16.7

Item 1: “UCAR Latex 145”, DOW Item 2: “NOPCO NXZ”, Henkel Item 3: “Fungitrol 234”, Evonik Item 4: “Triton X-405”, DOW Item 5: “UCAR Thickener 146”, DOW Item 6: “Benzoflex 9-88”, Vesicol Item 7: 28 % water-based solution

2

Item 8: “Silane A-187”, (epoxysilane), OSI Item 9: propylene glycol Item 10: “Disperse-Ayd W-28”, Daniel Item 11: “Attagel 50”, Engelhard Item 12: “Drikalite”, ECC Item 13: water, deionised

Advantages

Disadvantages

+ flexible formulation due to diversity of available copolymers, + good resistance to hydrolysis, + good resistance to light and weathering, + transparency (without fillers).

– limited heat resistance.

Field of application Pressure sensitive adhesives represent a very large field of application for water-based acrylic dispersions (see also Chapter II-3). Only polymers of low glass transition temperature are used here. Applications in the packaging industry include glossy film lamination. Major applications in the construction industry are flooring adhesives and tile adhesives. In the latter case, polymers of high glass transition temperatures are also in use.

1.2.2.4 Polyvinyl acetate and derivatives Manufacture and formulation Polyvinyl acetate (PVAc) is manufactured by free-radical emulsion polymerisation of vinyl acetate (see Chapter II-1.1.2.6). As with the acrylates, commercial dispersions (or powders that can be dispersed in water) are available. The polymerisation can be performed in the absence of pressure. However, copolymerisation with ethylene to yield the more flexible ethylene-vinyl acetate copolymers (EVA) requires pressurised conditions. Copolymerisation with maleic esters or acrylates can be used to modify properties. The solids content of commercial products ranges from 50 to 60 %. The dispersions are stabilised by means of protective colloids (i.e. polyvinyl

Water-based adhesives

75

alcohol), surfactants or a combination of both. Where the dispersions have been stabilised with high quantities of protective colloids only, such films are water-sensitive because the polar stabilising system remains within the dry adhesive. On the other hand, they are usually redispersible. This redispersibility may be desirable if a powder-like adhesive (i.e. ceramic tile adhesive) can be formulated with water immediately before use. Conversely, dispersions stabilised only with ionic surfactants form films that are highly resistant to water, because they are not redispersible after coagulation. These dispersions are better suited to thermal reactivation, because no high-melting protective colloids are present. Some commercial dispersions already contain film-forming agents. For many applications, modification is unnecessary, but may be effected with fillers (chalk or silicates), resins (colophony, coumarone-indene or phenol based), plasticisers (phthalates or other esters) and additives, such as rheological additives, wetting agents, defoamers and biocides.

Guide formulation “Mowilith LDL 2555 W” (Celanese) can be used directly as a wood adhesive (wood glue) [33]. The dispersion already contains a film-forming agent. For bonded joints at risk of hydrolysis or exposure to heat (wood for exterior applications), dispersible isocyanates (“Desmodur DN”, Bayer) can serve as crosslinkers.

Evaluation PVAc dispersions are low-cost products. They adhere strongly to cellulosic substrates, such as wood, cardboard and paper. Emulsion polymerisation affords high molecular weight polymers which, in combination with a high glass transition temperature, guarantee high mechanical strength. High solid dispersion adhesives dry and set rapidly. Copolymerisation with ethylene yields highly flexible products, which can serve in flexible laminated packaging.

Advantages

Disadvantages

+ + + + +

– moderate adhesion to many plastics, – moderate peel strength, – moderate impact resistance.

low cost, good adhesion to wood and paper, high solids content (up to 60 %), rapid setting, good water resistance and optional redispersibility, + good mechanical properties due to high molecular weight, + can be used with crosslinkers as 2-component system, + non-toxic.

Fields of application The woodworking and furniture industry are key application areas for PVAc dispersions due to their high strength and good adhesion to wood. Various adhesive applications are also found in the paper and packaging industry. EVA copolymers are easy to thermally reactivate. PVAc dispersions are also used in the construction industry for flooring and mortars.

Polyvinyl alcohol Monomeric vinyl alcohol (PVAl) does not exist in the pure form. As shown in Figure II-1.36, it tautomerises to ethanal. Polyvinyl alcohol is therefore made from polyvinyl acetate. The vinyl acetate function is chemically an ester, which in the presence of base catalysts, can be transesterified with methanol to

76

Part II

Adhesives

methyl acetate and polyvinyl alcohol. However, like polyacrylates, polyvinyl acetate is not ranked as a polyester, as the ester is in the side group only and is not a part of the polymer chain. To an extent depending on the degree of saponification, polyvinyl acetate yields variously Figure II-1.36: Tautomerism of ethanal saponified products, extending as far as pure polyvinyl alcohol. The polarity of the polymer increases with increase in saponification. Consequently, highly saponified polymers (>70 %) are readily soluble in water. However, pure polyvinyl alcohol is less soluble in water than the highly saponified grades due to its high crystallinity. Polyvinyl alcohol has a glass transition temperature of 85 °C and a crystalline melting point of 230 °C. It is therefore very brittle in the pure state [11, 34].

Guide formulation A water-reactivatable adhesive can be obtained from a 30 % solution of a highly saponified polyvinyl acetate, such as “Mowiol 4-88” (Kuraray) [34].

Fields of application Polyvinyl alcohol can serve as adhesive but it can also be used as an additive for other adhesives, such as polyvinyl acetates. Due to its polar structure, PVAl adheres strongly to such substrates as wood and paper. It has various uses as an additive. The effect of partly saponified polyvinyl acetates as protective colloid has already been described. The pronounced polarity can extend the open time of water-based dispersions by keeping the water in the polar environment for longer. PVAl can also act as a rheological additive. For paper bonding or water-reactivatable adhesives (stamps, labels, envelopes), PVAl is used as an adhesive directly.

1.3 Hot-melt adhesives 1.3.1 Basic principles 1.3.1.1 Composition and setting process Hot-melts or hot-melt adhesives (the terms are equivalent) are solventless and water-free adhesives based on thermoplastic polymers. These polymers are solid at room temperature and soften when heated to yield a viscous liquid. They can therefore be applied as a melt. Upon cooling to room temperature, they solidify again and develop cohesive strength (setting). Figure II-1.37 shows the principle behind hot-melts.

Classification by setting mechanism Hot-melts can be classified by their setting mechanism. This mechanism is a consequence of the different compositions of solid polymers.

Class A Amorphous polymers with a glass transition temperature above room temperature possess high strength (high elastic modulus) at room temperature. However, they are quite brittle at room temperature and exhibit very slight elongation. For formulation as hot-melts, either they are compounded with another polymer of low glass transition temperature, or block copolymers (i.e. S-I-S,

Hot-melt adhesives

see Chapter II-1.3.2.5 and II-3) are used. On being heated, these polymers lose their strength as soon as the glass transition temperature is reached. Above the glass transition temperature, the polymer gradually liquefies. Close to the glass transition temperature, the viscosities of such polymers are quite high (characteristic of the WLF equation [1, 2]). Application, therefore, calls for high temperatures, significantly above the glass transition temperature.

77

Figure II-1.37: Principle behind hot-melts

Class B Semi-crystalline polymers having a crystalline melting point Tm above room temperature are not brittle if the molecular weight is sufficiently high and if the glass transition temperature is significantly below room temperature. Where these polymers are used for hot-melts, the scope for formulation is limited because numerous additional ingredients interfere with the crystallising process. Examples of such crystalline or semi-crystalline polymers are EVA copolymers, and many polyesters and polyamides (see also Chapter II-1.3.2.2 to 4). When heated, these products retain their mechanical strength virtually unchanged until the crystalline melting point is reached. Above the melting point, the products liquefy. The viscosities of these polymers above the melting point are usually significantly lower, because the distance from the glass transition temperature is much greater.

Class C High molecular weight polymers having a glass transition temperature below room temperature at room temperature are, strictly speaking, very high-viscosity liquids, but they exhibit many signs of solids, such as a defined shape (one example from everyday life is chewing gum). Due to their low cohesion, hot-melts based on these polymers can normally only serve as sealants and, more precisely, only where really large forces do not have to be transferred (see Chapter III2.2.1). Under load and particularly under the influence of heat, these hot-melts have a tendency to creep (cold flow, viscous behaviour, plasticity). When heated, they gradually lose their strength, which is already low, and behave more and more like liquids. The application temperature has to be quite high as they consist of very high molecular weight polymers, which is necessary for minimum cohesive strength at room temperature. The change in viscosity with change in temperature is shown schematically in the following diagrams. Figure II-1.38 shows the situation for Class A polymers. The glass transition temperature is above room temperature. For easy application, temperatures significantly above the glass transition temperature (approx. 50 to 100 °C) are needed in order that the application viscosity may be sufficiently low. Figure II-1.39 shows the viscosity characteristics of a Class B hot-melt. The crystalline melting point is above room temperature. The viscosity of the melt (besides the mole-

Figure II-1.38: Viscosity characteristics of a Class A hot-melt

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cular weight) is mainly determined by the distance from the glass transition temperature. Since the glass transition temperature is far below the crystalline melting point, temperatures which are only moderately above the melting point (approx. 50 °C) are enough to produce low-viscosity, easily applied melts.

Figure II-1.39: Viscosity characteristics of a Class B hot-melt

Figure II-1.40 shows the situation for a Class C hot-melt. The amorphous polymer has a glass transition temperature far below room temperature. In order to be “solid” at room temperature, it must behave like a very high-viscosity liquid. High molecular weight polymers are therefore essential (see Figure II-1.41). Due to their viscosity/temperature dependence, these polymers need a comparatively high application temperature.

Formulating principles Hot-melts are 100 % adhesives, i.e. no processing auxiliary, such as water or solvent, is added which would have to be removed after application. The following components Figure II-1.40: Viscosity characteristics of a Class C with the functions listed are used for forhot-melt mulation (in the percentages given). The base polymer (30 to 100 %) defines the main properties of the adhesive, such as its application conditions, adhesion, chemical and hydrolytic stability, softening characteristics, setting mechanism and, of course, price. In special cases, polymer blends can be used, too. Resins (up to 40 %) are used to boost tack, adhesion and heat resistance, as well as to lower application viscosity. Consideration must also be given to compatibility with the polymer and any influence on the setting characteristics (Tg or Figure II-1.41: Viscosity as a function of molecular Tm). Plasticisers and waxes (up to 50 %) weight for Class C polymers are added to lower the application viscosity and costs. Sometimes solid plasticisers are used, but the borderline between these and resins can be hard to define. The addition of plasticisers adversely affects the glass transition temperature of amorphous products and the crystallisation characteristics of crystalline polymers. In hot-melt sealants, plasticisers provide some of the mechanical characteristics by virtue of their swelling properties. The purpose of fillers (up to 50 %) employed in large quantities is to lower costs. Many adhesives are formulated without added fillers. Large quantities of fillers are typical of hot-melt sealants. Pigments can be used for colouring. Additives (usually RT) is cooled to below the glass transition temperature, solidification occurs immediately, without any delay. Such adhesives have only a short open time but very quickly develop high green strength. Solidification at the glass transition temperature occurs spontaneously. Semi-crystalline Class B adhesives (Tm > RT) show similar cooling characteristics under similar conditions; however, the solidification process, namely crystallisation, is exothermic and requires some time, because the polymer chains have to orient themselves in order to build up the ordered crystalline structure. Crystalline hot-melts therefore need some time to solidify. The crystallisation process is not always exactly reproducible, and so industrial production lines must provide crystalline hot-melts with some tolerance time as regards setting. On the other hand, compared to a Class A product, a crystalline adhesive of low melting point has a longer open time, because, as a result of retarded crystallisation, it can still be a liquid if it has already cooled to below the crystalline melting point. On cooling, amorphous Class C adhesives (Tg approx. 70 %) consist of aliphatic epoxy resins. To keep costs down and to flexibilise polyols (low-molecular polyols, polyethers, polyesters or castor oil) and up to 15 % copolymerisable vinyl ethers (e.g. triethylene glycol divinyl ether) or high-boiling solvents (e.g. propylene carbonate) are added. When irradiated with UV light, the photoinitiator liberates electrophiles, Lewis acids or proton acids which trigger the polymerisation. The corresponding counter-ions are only of very low basicity and nucleophilicity, and so the acids are sometimes called “super-acids” [10, 29, 41]. Evaluation UV-curing epoxies are notable for their very fast cure. They provide good adhesion and good ageing resistance.

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Advantages

Disadvantages

+ + + + + + +

– – – –

1-component system, fast cure, good adhesion to metal, good adhesion to many plastics, low shrinkage on curing, no impairment by oxygen, thermal postcuring possible.

radiation exposure needs corresponding equipment, expensive, often brittle, reactive adhesive system cause sensitisation.

Fields of application UV-curing epoxies are used to a high extent in the electronics industry for fixturing electronic parts (surface mounted devices) and as chip-potting compounds [41, 99]. They also serve as laminating adhesives [29]. A major advantage of UV curing epoxies is the fact that they can also be used for non-transparent substrates, because curing can be initiated by brief exposure to the radiation and the cationic polymerisation then continues without the need for further irradiation of the adhesive joint. Example Table II-2.21 presents a laminating adhesive which can also be used for substrates which are impervious to UV radiation. For bonding, brief exposure to light is sufficient. Once the protons have been liberated, the ensuing cationic polymerisation continues without the need for further light once the substrates have been mated. The photoinitiator is a sulphonium salt [29]. 2.2.2.5 Trends Nanotechnology is also entering the world of adhesives. New product property profiles can be created by using nanoparticles to reinforce epoxy resins. The nanoparticles consist of silica and are synthesised in a special process. They have a diameter of about 20 nm and are synthesised and dispersed in the epoxy resin without any formation of agglomerates. Unlike fumed silica, these particles have hardly any rheological effect, even up to a solids content of about 30 %; consequently, viscosities stay low and processing remains easy. On account of the small diameter, corresponding resins stay transparent even at high filler loads [35, 42]. The surfaces of the particles have been modified such that they form covalent bonds with the epoxy matrix on curing. Epoxy adhesives containing such particles exhibit improved toughening properties and, owing to the reinforcing fillers, higher modulus, too. Both epoxies and methacrylates can enter into addition reactions with amines [34, 43, 45]. Nucleophilic addition of amines to acrylates proceeds quite rapidly, on the lines of the Michael addition [16]. Epoxy acrylates can therefore be cured with amines in 2-component systems. Compared to 2-component epoxies, curing is quite rapid, even at low temperatures. The resulting adhesives consist of epoxy acrylate networks. Table II-2.21: UV-curing laminating adhesive Item

Raw material

Parts by weight

1

epoxy resin (aliphatic )

2

polyol

9.7

3

photoinitiator

3

Item 1: “Uvacure 1534”, UCB-Group Item 2: Tripopylenglycol, DOW Item 3: “Uvacure 1590”, UCB-Group

87.3

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2.3 Polymerisation adhesives 2.3.1 Acrylates 2.3.1.1 Basic principles Raw materials and functional group Reactive monomers Reactive acrylic adhesives (sometimes called acrylates or reactive acrylics) are based on monomeric or oligomeric mixtures of the esters of acrylic or methacrylic acid. For simplicity, adhesives based on acrylates as well as those based on methacrylates are called acrylic adhesives. Figure II-2.68 shows the most important monomers. For the synthesis of methacrylic acid, see also Chapter II-1.1.2.4. The monomers are usually low-viscosity, colourless liquids of characteristic to pungent odour. Esters of higher alcohols and esters of diols do not have an appreciable odour. As shown in Table II-2.22, the boiling point of the esters increases with increase in the number of carbons in the ester alcohol [48, 49].

Figure II-2.68: Monomers for acrylic adhesives

Chemically, acrylates are α,βunsaturated carbonyl compounds. As shown in Figure II-2.69, they can react with nucleophiles (at the β-carbon or directly at the carbonyl function) and with electrophiles (at the C=C double bond or at the non-bonding electrons on the oxygen). The C=C double bond (conjugated with the acid or ester function) is the most frequently used function for synthesising polymers. As shown in Figure II-2.70, it can be readily converted into the polymer by a free-radical mechanism. In contrast to polyaddition, this reaction does not require stoichiometric ratios of reactants.

Figure II-2.69: Reactions of α,β-unsaturated carbonyl compounds

164

Table II-2.22: Boiling points of monomeric acrylates Monomer

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Part II

bp [°C]

Tg polymer [°C]

As shown in Figure II-2.71, stoichiometric vinylogous nucleophilic addition at the β-position of the carbonyl group can serve as an alternative curing reaction. This addition reaction is also used for curing epoxy acrylates.

methylacrylate

80.5

22

ethylacrylate

99

-8

methyl methacrylate

100

105

ethyl methacrylate

119

67

acrylic acid

141

130

Crosslinker

butylacrylate

145

-43

isobutyl methacrylate

155

64

methacrylic acid

163

162

butyl methacrylate

164

32

Acrylate esters of diols (diacrylates, dimethacrylates; see Figure II-2.69) are polyfunctional in respect of a free-radical reaction. They are added in judicious amounts to regulate the crosslink density in the cured adhesive.

tetrahydrofuryl methacrylate

178

75

cyclohexyl methacrylate

210

104

benzyl methacrylate

>200

54

isobornyl methacrylate

>200

110

ethyleneglycol dimethacrylate

>200

diethyleneglycol dimethacrylat

>200

butanediol methacrylate

>200

Toughening components Adhesives expected to have good impact resistance and good lowtemperature properties require tougheners. These are primarily

Figure II-2.70: Polymerisation of acrylates

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modified polyolefins (e.g. chlorosulphonated polyethylene [50, 53, 55], Figure II-2.72) which have limited compatibility with the cured polymer. Curing agents (initiators, starters) The curing agents are typically substances based on organic peroxides, which are able to form free-radicals. The free-radicals for initiating the polymerisation can be generated by heat, amine catalysis or UV radiation. Important peroxide‑curing agents are shown in Figure II-2.73.

Figure II-2.71: Vinylogous nucleophilic addition to acrylates

Stabilisers Adding small amounts of antioxidants, such as hindered phenols, hydroquinone and quinones (see Figure II-2.74), to the reactive resin prevents premature initiation of the free-radical chain reaction and improves storage stability [58].

Figure II-2.72: Chlorosulphonated polyethylene

Accelerators and activators Many amines catalyse the decomposition of peroxides to free-radicals. Tertiary amines, such as the products shown in Figure II-2.75 (e.g. N,N-dimethyl-p-toluidine, and capped amines, e.g. the imine formed from butanal and aniline), can cause an adhesive formulation to cure at room temperature. Additionally, salts of transition metals, such as copper octanoate (many transition metals can readily change oxidation number and so are able to catalyse peroxide decomposition), which are soluble in organic milieu, make effective accelerators. Increasing use has recently been made of dihydropyridine derivatives [50, 51], as shown in Figure II-2.76, because they are reputed to be less toxic.

Figure II-2.73: Peroxide-curing agents

Figure II-2.74: Stabilisers

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Technologies of curing The low-molecular reactive monomers in the adhesive joint can be converted into a polymer (or copolymer) at a particular time in different ways [10–12, 52, 54, 56].

Figure II-2.75: Accelerators

Figure II-2.76: Dihydropyridine derivative

In the case of 2-component acrylic adhesives, a curing agent (peroxide) and sometimes an accelerator (amine) is mixed into the reactive adhesive formulation, which is based on the monomer system, immediately prior to the joining process. Different types of mixing are possible. In the case of anaerobic acrylic adhesives, the 1-component reactive monomer/oligomer system can be stored in the presence of oxygen, which acts as inhibitor. During the joining process, access of oxygen is excluded by the substrates and the adhesive polymerises spontaneously in the presence of metal ions on the substrate surface.

Radiation-curing acrylates are 1-component reactive monomer/oligomer systems containing photoinitiators. For the bonding process, the adhesive is applied, the substrates are joined and the adhesive layer is then exposed to UV radiation. Therefore, at least one of the substrates has to be transparent to UV radiation. The thick layers typically employed for sealants cannot be cured in this way. 2.3.1.2 Structure/property relationships Polymeric acrylates are amorphous substances, whose glass transition temperature can vary over a broad range and depends on the structure of the monomers. Structural adhesives applications require polymers whose glass transition temperature far exceeds room temperature (see Chapter II-1.1.2.4, Table II-1.13 and 14). The most important monomer for reactive adhesives is methyl methacrylate. The corresponding polymer has a glass transition temperature of 105 °C. Adhesives based on polymethyl methacrylate (PMMA) have high modulus but are hard and brittle. Where they are mainly linear polymers, they are thermoplastic, meltable (and will decompose) and soluble in polar solvents. The chemical and thermal resistance required by structural adhesives applications can be improved by controlled crosslinking with higher-functional acrylates (dimethacrylates; see Figure II-2.69). However, thermal depolymerisation limits their heat resistance to 170 to 200 °C, the exact temperature depending on the formulation. Chemically, polymeric acrylates are polyolefins with pendant ester groups. Esters are relatively stable to chemical and hydrolytic influences, but may be hydrolysed under vigorous conditions to the corresponding acid. As the ester group is located in the side-chain and

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is not part of the polymer chain, hydrolytic degradation of the polymers is hardly ever encountered. To improve the mechanical properties, it is common to add polymers to the adhesive formulation which are soluble in the low-molecular monomeric components, but which have only limited solubility in the corresponding polymeric acrylates. For this reason, phase Figure II-2.77: 2-component acrylic adhesives, 1st generation separation occurs during curing. If the added polymers have a low glass transition temperature, they can boost toughness, low-temperature properties and peel strength, as already discussed for epoxy adhesives in Chapter II-2.2.2. 2.3.1.3 Formulations 2.3.1.3.1 2-component acrylic adhesives Methods of application There are various ways to initiate free-radical polymerisation of acrylic monomers with the aid of peroxides. These methods are inconsistently referred to in the literature as “generations” of acrylic adhesives. 1st generation The adhesive formulation is based on the monomer (typically methyl methacrylate) and already contains an accelerator. Immediately before the bonding process, the second component, a powdered or paste peroxide (e.g. dibenzoyl peroxide), is added. The reaction starts immediately. The pot life of these systems is limited to just a few minutes. Unlike the polyaddition reactions undergone by polyurethanes and epoxies, an exact mixing ratio is not essential. However, major mixing errors will affect the properties. In any case the mixing ratio is unfavourable, because the amount of curing agent is relatively small. Figure II-2.77 schematically shows the application of 1st generation, 2-component acrylic adhesives. 2nd generation (A/B process) The adhesive formulation is based on the monomer (typically methyl methacrylate) and the mixture is divided into two parts. To the first part (A) is added just the accelerator (amine) while, in the case of the second part (B), the curing agent (peroxide) is added either during the adhesive manufacturing process or immediately before the bonding process. The system of monomer and accelerator (part A) has a shelf life of several months, whereas part B containing the peroxide is stable for several weeks to some months, the length of time depending on the type of peroxide. Bonding can be performed in two ways. Prior to Figure II-2.78: 2-component acrylic adhesives, 2nd generation

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application, the two parts (A/B) are mixed homogeneously. This yields a reliably homogeneous mixture, because, if properly formulated, a 50/50 vol% mixture of components of similar viscosities is processed. The mixed adhesive cures uniformly, even in thick layers, and has gap-filling properties. Alternatively, for thin adhesive joints, component A can be applied to just Figure II-2.79: 2-component acrylic adhesives, 3rd generation one of the substrates, and component B to the other (Figure II-2.78). When the two adhesive layers are mated, polymerisation proceeds because both components intermingle along the contact surfaces. 3rd generation (“no-mix adhesives”) One of the substrates to be bonded is brushed with an activator, which is usually solventbased. The activator consists of components which progressively liberate amines to act as accelerators. The solvent evaporates soon after and the substrates have virtually unlimited shelf life, yet are already activated for subsequent bonding. The acrylic adhesive already contains the peroxide needed for the polymerisation. In the presence of certain types of peroxide, e.g. cumene hydroperoxide, the monomer system exhibits storage stability (Figure II-2.73). When the substrates are mated, polymerisation is triggered by contact with the amine released from the activator. Consequently, during the bonding process, there is no need for mixing and metering, because at this point the adhesive can be applied like a 1-component system. As polymerisation proceeds from the interface of the substrate treated with the activator, there are certain limitations regarding the thickness of the gap (0.5 mm max.). The no-mix process is currently the most widely employed for acrylic adhesives because it is easy to perform and it is highly tolerant of errors. Figure II-2.79 schematically shows the application of 2-component acrylic adhesives in the no-mix process [53]. The properties of these adhesives derive from the monomer and the toughening elastomers. They are therefore also called toughened acrylic adhesives. Guide formulation Table II-2.23 presents the starting formulation for a 3rd generation, reactive no-mix acrylic adhesive [53–55]. The reactive monomer system mainly consists of methyl methacrylate. Other esters of acrylic or methacrylic acid may be copolymerised. The use of cyclohexyl methacrylate or tetrahydrofurfuryl methacrylate reduces the odour problem, because both have a higher boiling point and a lower vapour pressure. The composition of the mixture influences the glass transition temperature of the resulting copolymer in the usual way. Adding about 10 % methacrylic acid improves the setting characteristics, adhesion to a large number of substrates and the heat resistance of the bond. The use of bifunctional monomers, such as ethylene glycol dimethacrylate in a proportion of about 10 %, provides a further significant boost to heat resistance. Elastomers can be added to toughen the resulting adhesives. Chlorosulphonated poly­ ethylene (“Hypalon”, DuPont, see Figure II-2.72) has proved especially effective for reactive acrylic adhesives. Chlorosulphonated polyethylene is a low-crystallinity elastomer, synthesised by chlorinating and chlorosulphonating polyethylene (approx. 30 % chlorine content;

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Table II-2.23: 2-Component acrylic adhesive, starting formulation Resin part Item

Raw material

Function

Parts by weight

1

methacrylate

reactive monomers

40 to 75

2

chlorosulfonated polyethylene

toughening

30 to 50

3

peroxide

initiator

0.5 to 3

4

phenole

stabilizer

5

imine from butanal and aniline

accelerator

6

copper octaneoate

accelerator

7

opt. solvent

application aid

300 °C), good hydrolytic stability, good chemical resistance.

polycondensation needs time and the application of heat and pressure, heat-sensitive substrates cannot be bonded, liberated molecules can lead to bubbling in the adhesive layer, liberated products, such as formaldehyde, are of toxicological concern, brittleness of the bonds.

Fields of application Binders Substantial quantities of phenolic resins (often in the form of alkaline water-based solutions of resols) serve as binders in adhesives-related areas. Curing occurs at 100 to 180 °C. In the manufacture of particle board, phenolic resins are used to bind the wood chips. Although this is a bonding process in the broader sense, these high-volume applications of reactive

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195

phenolic resins are not counted Table II-2.30: Phenolic resin adhesive modified with nitrile rubber as adhesives applications. Item Raw material Parts by weight These products are called bind1 phenolic resin 15 ers or binding agents, because 2 nitrile rubber 15 an almost infinite number of 3 solvent 69.7 small, undefined substrates are bonded instantaneously to a new 4 coupling agent 0.3 raw material. The same is true Item 1: novolak/urotropine-mixture of the manufacture of moulded Item 3: acetone parts based on timber-derived Item 4: mercaptosilane, “A189”, Evonik materials for automotive interior applications and the manufacture of furniture. Other applications of low-cost water-based phenolic resins (resols or novolak/urotropine) as binders are the bonding of sand in casting moulds for the manufacture of engine blocks etc. Insulation materials, such as glass and rock-wool, are sprayed with phenolic resins (often water-based resols) and thus made into insulating mats. Phenolic resins are the basis of grinding materials, such as sand-paper, and of friction linings for brakes and clutches in the automotive industry. Adhesives Phenolic resins can serve as 2-component adhesives for bonding wood [68]. Reactive phenolics made from resorcinol-based resins are stable in aqueous alkaline solution. As is typical of a 2-component adhesive, excess formaldehyde is added immediately before application. Even at low temperatures, crosslinked polymers with good bonding properties are obtained. Toughened reactive phenolic resin adhesives are the only types employed for reliably bonding friction linings, such as clutch and brake liners, to the metal base in cars. They exhibit excellent adhesion to both the metal surface and the friction material. They possess high mechanical strength and very good thermal resistance. For bonding brake liners, they act as “structural adhesives” in the truest sense of the phrase. In the aircraft industry phenolic resins are often used in the form of heat-reactive films to bond aluminium and steel. As production quantities in the aircraft industry are very small compared to those in the automotive industry, complex manufacturing procedures, such as the application of pressure and heat, are acceptable. Again, what is important is the ability to bond substrates reliably and durably. Examples Phenolic resin adhesive modified with nitrile rubber Table II-2.30 lists a reactive solvent-based, phenolic resin adhesive modified with nitrile rubber [10].

2.5 Curing hot-melts 2.5.1 Concepts For a variety of reasons (see also Chapter II-1.3), hot-melts have captured a sizable share of the market. Because they set rapidly, they lend themselves to high-speed, high-volume industrial manufacturing. Hot-melts do not require any application auxiliaries, such as solvents or water. Drawbacks include their sometimes moderate adhesion to many substrates and, especially, their limited heat resistance, which stems from their thermoplastic properties. The first reactive hot-melts were developed and commercialised in the early 1980s. These polyurethane-based products were traditional hot-melts in terms of processing and setting

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characteristics. Apart from possessing the typical physical setting mechanism of hot-melts, they can enter into the chemical curing reaction that typifies reactive adhesives. Thus, they ideally combine the advantages of both types of adhesives [71]. There are all sorts of concepts for formulating reactive hot-melts. Basically, the different principles behind hot-melts and reactive adhesive systems can be combined almost arbitrarily, although not all combinations are suitable for practical requirements. Many thermoplastic polymers have hot-melt properties (see Chapter II-1.3). Classification is either by chemical composition (polyolefins, polyesters, polyamides, etc.) or setting mechanism, with a distinction being drawn between polymers with a glass transition temperature (Class A) or a crystalline melting point (Class B) above room temperature and polymers of high mole-

Figure II-2.110: Concepts for reactive hot-melts

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197

cular weight but having a low glass transition temperature (Class C). If these polymers can be linked to the typical functional groups of reactive adhesives, the result is functionalised polymers which can combine both properties. Such functional groups are those already encountered in the field of reactive 1-component adhesives and sealants. They include isocyanates (see Chapter II-2.5.2), silanes (Chapter II-2.5.3), epoxies (Chapter II-2.5.4) and even double-bond systems, such as acrylics (Chapter II-5.5). Curing usually takes place after application, by a mechanism which varies with the functional group, i.e. with ambient moisture or oxygen, temperature increase or high-energy radiation. An overview is provided in Figure II-2.110. By far the most important class of reactive hot-melts are moisture-curing isocyanate-functionalised polyurethanes based on polyesters or polyethers on account of their structural versatility. They will therefore be described in more detail.

2.5.2 Moisture-curing PUR hot-melts 2.5.2.1 Basic principles Development of strength PUR hot-melts are applied hot in the manner of traditional hot-melts. Due to rapid cooling and physical setting after mating, high green strength is quickly achieved. Consequently, bonded parts can be handled within seconds or minutes, without the need for mechanical fixturing. Once the adhesive has been applied, moisture from the environment or from the substrates triggers a chemical reaction with the isocyanate groups to afford a high-molecular, crosslinked polymer within hours or days, which is associated with a further increase in strength. Figure II-2.111 schematically shows the change in bond strength over time [72]. Raw materials PUR hot-melts mainly consist of isocyanate-terminated prepolymers, which can be obtained by making polyols react with excess isocyanate (see Chapter II-2.2.1). The chosen polyol mainly determines the setting characteristics while the system of isocyanate and catalyst is responsible for subsequent curing with ambient moisture. The desired hot-melt properties can be realised

Figure II-2.111: Development of bond strength

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in various ways. The prepolymers can be synthesised from crystalline (e.g. hexanediol adipates) or amorphous polyesters (mixed polyesters based on aromatic dicarboxylic acids) as well as polyethers (polypropylene oxide or poly-THF). These polyols are treated with excess isocyanate to yield functionalised reactive prepolymers. Aromatic isocyanates are preferred, e.g. MDI or its derivatives. These are cheaper than the aliphatic types and have a relatively low vapour pressure which minimises exposure of the user to harmful isocyanate monomer vapours. Additionally, aromatic isocyanates are much more reactive with moisture than are aliphatics, a fact which can be important in the subsequent curing reaction. Chemical curing of the isocyanate groups occurs in the same way as described for 1-component PUR adhesives (see Chapter II-2.2.1), via the formation of urea linkages and the release of carbon dioxide. Side-reactions may lead to the formation of allophanate or biuret linkages, which contribute additional crosslinking. PUR hot-melts can be optimised from various aspects. For users, the most important properties (besides price) include the processing conditions (temperature, viscosity, stability, setting characteristics, curing characteristics, handling safety and toxicology) and the ultimate bonding properties in a given application (adhesion to substrates, transfer of static and dynamic loads, resistance to heat, moisture and other influences). 2.5.2.2 Synthesis and formulation Over the years, various technologies for synthesising reactive hot-melts emerged to accompany the various generations. 1st generation High-molecular thermoplastic polymers, e.g. polyester-polyurethanes, already used in the synthesis of traditional hot-melts, can be mixed with isocyanate-terminated polyurethane prepolymers and other resins. The polymers mainly dictate the st Table II-2.31: PUR hot-melt, 1 generation processing conditions and the Adhesive components (reactive prepolymer, item 7) setting characteristics of the hotItem Raw material Parts by weight melt, while the isocyanate groups 1 diisocyanate 41 in the prepolymer are responsible for curing with ambient moisture. 2 polyesterpolyol 51.6 Due to the use of high molecular 3 chain extender 7.4 weight polymers, products of Polyurethane polymer (item 8) this generation exhibit very fast setting, and bonded adherends 4 diisocyanate 20.6 quickly develop excellent green 5 polyesterpolyol 75.7 strength, a fact which facilitates 6 chain extender 3.7 further handling in industrial Adhesive formulation manufacturing processes. Drawbacks include the high applicaitem Raw material Parts by weight tion temperatures (160 to 180 °C) 7 reactive prepolymer 50 to 65 typical of hot-melts. This limits 8 polyurethane-polymer 5 to 30 the stability of the adhesive under 9 resin 15 to 30 processing conditions and in the Item 1: “Desmodur PF” (MDI-derivate), Bayer, NCO = 23 % ≙ 5476 mval/kg worst case can cause emissions Item 2 and 5: polyesterpolyol from butanediol, ethyleneglycol and adipic acid, of monomeric isocyanates and M = 2000 g/mol, f = 2 val/mol ≙ 1000 mval/kg Item 3: t ripropyleneglycol, M = 192 g/mol, f = 2 val/mol ≙ 10416 mval/kg even damage to heat-sensitive Item 4: “Desmodur 44 MS” (MDI), Bayer, M = 250 g/mol, f = 2 val/mol ≙ substrates. Products of this first 8000 mval/kg generation failed to establish Item 6: 1,4-butanediol, M = 90 g/mol, f = 2 val/mol ≙ 22220 mval/kg Item 9: ketone-aldehyd-condensationproduct themselves on the market.

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199

Example Table II-2.31 shows a corresponding starting formulation. The polyurethane, consisting of crystalline polyesters, dictates the hot-melt properties, while the isocyanate groups of the prepolymer are responsible for the curing properties after application. The resin and the prepolymer serve to lower the viscosity and improve tack. The application temperature of about 180 °C ensures that the high polymer is sufficiently liquid [73]. The NCO/OH ratio = Σ equivalents NCO/Σ equivalents for the reactive prepolymer is calculated as follows (see Chapter II-2.1.3): 41 g “Desmodur PF” corresponds to 41 ⋅ 5476/1000 = 225 mval NCO 51.6 g polyester polyol corresponds to 51.6 ⋅ 1000/1000 = 52 mval OH 7.4 g tripropylene glycol corresponds to 7.4 ⋅ 10416/1000 = 77 mval → NCO/OH = 225/(52 + 77) = 1.7. Similarly, the figures for the polyurethane polymer are: 20.6 g “Desmodur 44 MS” corresponds to 20.6 ⋅ 8000/1000 = 165 mval NCO 75.7 g polyester corresponds to 75.7 ⋅ 1000/1000 = 76 mval OH 3.7 g 1,4-butanediol corresponds to 3.7 ⋅ 22220/1000 = 82 mval OH → NCO/OH = 165/(76 + 82) = 1.03, in other words, the reaction is performed close to the equivalence point in order that a high molecular polymer may be obtained. 2nd generation Low-molecular (M = 1000 to 6000 g/mol), crystalline polyesters (see Chapter II-1.3.2), e.g. hexanediol adipates or butanediol adipates, are allowed to react with excess isocyanate (NCO/OH = 1.5 bis 2.5) to yield prepolymers. These crystalline prepolymers possess hotmelt properties because they are solids at room temperature and can be liquefied to lowviscosity products at temperatures of around 100 °C. Hexanediol adipates (m.p. 50 to 60 °C) are more resistant to hydrolysis than are butanediol adipates (m.p. 40 to 50 °C). Higher molecular polyesters (M = 3000 bis 6000 g/mol) crystallise faster than lower molecular polyesters (M = 1000 to 2000 g/mol). The viscosity of the hot-melt is dictated by the molecular weight of the polyester used and the NCO/OH ratio. The viscosity falls with increase in NCO/OH ratio. Addition of plasticisers is not recommended, as that might impair crystallisation and therefore the setting process. Mixed polyesters can also be used. These can reduce the degree of crystallinity in the prepolymers and so enable the open time and the setting characteristics of the hot-melt to be modified. Adhesion to certain substrates may be additionally improved due to reduced crystallinity and therefore reduced shrinkage upon solidification. The addition of higher-functionalised isocyanates yields a higher crosslink density and therefore the products feature greater heat resistance and improved hydrolytic stability. Overly excessive amounts of isocyanate have to be avoided, because the high quantities of monomeric isocyanate would remain in the product, requiring them to be classified as dangerous goods and creating the problem of environmental pollution when processed under heat. The cure rate can be varied within limits via the catalyst system. Suitable catalysts are those which are suitable for 1-component polyurethanes, such as DBTL. Provided that excessive quantities of catalysts are not used (< 200 to 1000 ppm) and that moisture is excluded, the corresponding melts have excellent stability, even at the application temperature. Cooling as part of the bonding process leads to solidification by crystallisation. Due to the low molecular weight of the prepolymers, products of this second generation exhibit quite low viscosities under mild application conditions (80 to 120 °C). This means that thermally sensitive substrates can also be bonded. According to the current state of knowledge, a combination of low application temperatures and a suitable formulation does

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not cause the release of relevant quantities of monomeric isocyanates. A disadvantage is the poor reproducibility of crystallisation which is part of the setting process. Consequently, certain leeway as regards fixturing times is needed in industrial manufacturing processes. Not only that, but the cooled prepolymers are somewhat brittle (waxy), because they do not have a high molecular weight yet and therefore the bonds in this stage are sensitive to peel and impact. Crystalline products always show a certain tendency to shrink during crystallisation. This can affect adhesion to certain substrates. Thick-layer application and application to moisture-impermeable substrates are not recommended [74, 79]. Example A typical formulation (according to [79, Ex. 3]) is shown in Table II-2.32. The polyol is a crystalline polyester based on hexanediol adipate. The NCO/OH ratio = Σ equivalents NCO/Σ equivalents OH, according to Chapter II-1.3, is calculated as follows: 12 g “Desmodur 44 MS” corresponds to 96 mval NCO, 87.68 g “Dynacoll 7360” corresponds to 47 mval OH → NCO/OH = 96/47 = 2.0. This leads to a low molecular weight for the prepolymers and therefore to a low application viscosity. The content of free monomeric MDI, which is of relevance for declaration as dangerous goods and possible vapour exposure during manufacture and application, can be calculated in accordance with equation (4) in Chapter II-2.2.1.3, as follows:

mres [%] = [(NCO/OH - 1)/(NCO/OH)]2 ⋅ morig [%] = [(2-1)/2]2 ⋅ 12 = 3 [%]

3rd generation When low-molecular amorphous polyesters (M = 2000 to 6000 g/mol) with a glass transition temperature above room temperature react with excess isocyanate to yield prepolymers, the products have a glass transition temperature above room temperature [75, 76, 80]. These products are brittle, of course. So, a second prepolymer based on a polyester or polyether with a low glass transition temperature is added. Where possible, the two prepolymers should not be wholly compatible. The viscosities are adjusted via the NCO/OH ratio (typically 1.5 to 3). In addition, limited amounts of (mainly solid) Table II-2.32: PUR-Hot-melt, 2nd generation phthalate plasticisers may be added. The cure reaction is again Item Raw material Parts by weight regulated via the quantity of cata1 diisocyanate 12 lyst (DBTL, 200 to 1000 ppm). The 2 crystallising polyester 87.68 resultant adhesive can be readily 3 stabilizer 0.23 applied at 100 to 150 °C. Solidifica4 catalyst 0.09 tion of amorphous systems on cooling occurs spontaneously in the Manufacturing: Item 1 to 3 are recated at approx. 80 °C for 1 h under stirring and exclusion mated substrates as soon as the of moisture. Item 4 is added and the reaction is finished over another h at temperature within the adhesive appr. 100 °C. joint falls beneath the glass tranOptionally before filling vacuum is applied in order to minimize gass bubbles. sition temperature of the higherItem 1: “Desmodur 44 MS”, Bayer, M = 250 g/mol, f = 2 val/mol ≙ 125 g/val, melting prepolymer. Besides fast NCO = 33.6 % ≙ 8000 mval/kg setting, third-generation products Item 2: “Dynacoll 7360”, Evonik, M = 3500 g/mol, f = 2 val/mol (manufacturer information) ≙ ca.1750 g/val, OH = 30 g KOH/kg ≙ 0.91 % ≙ possess high flexibility, even in 536 mval/kg ≙ 1865 g/val the uncured state [81]. Compared Item 3: “Additiv TI”, Bayer with crystalline products, hot-melt Item 4: DBTL, Brenntag

Curing hot-melts

adhesives offer improved adhesion to many substrates, because they do not shrink due to crystallisation as they cool. However, they have the disadvantage of tending to creep under load before curing. Using mixtures with prepolymers of crystalline polyesters reduces the creep tendency. Thick layers of amorphous PUR hot-melts can undergo undesirable bubbling during curing. Example Table II-2.33 presents a typical formulation, according to [75]. The polyols are two amorphous polyesters, one of which has a glass transition temperature above room temperature and is responsible for setting, the other having a low glass transition temperature and being responsible for flexibility.

201 Table II-2.33: PUR hot-melt, 3rd generation Item

Raw material

Parts by weight

1

diisocyanate

12.3

2

polyester 1

65.5

3

stabilizer

0.2

4

polyester 2

21.8

5

catalyst

0.05

6

plasticizer

0.15

Manufacturing: 9.1 parts by weight of item 1 (≙ 73 %) are reacted with item 2 and 3 for 30 min at 120°C, the the rest of item 1 (3,2 weight parts (≙ 27 %)) and pos. 4 are added and reacted for another 30 min at 120 °C. Finally item 5 and 6 are added and reacted for 30 min at 120 °C. Optionally before filling vacuum is applied in order to minimize bubbles. Item 1: “Desmodur 44 MS”, Bayer, M = 250 g/mol, f = 2 val/mol ≙ 125 g/val, NCO = 33.6 % ≙ 8000 mval/kg Item 2: “Dynacoll 7140”, Evonik, Tg = 30 °C, M = 5500 g/mol, f = 2 val/mol (manufacturer information) ca. 2250 g/val, OH = 21 g KOH/kg ≙ 0.64 % ≙ 375 mval/kg ≙ 2667 g/val Item 3: “Additiv TI”, Bayer Item 4: “Dynacoll 7231”, Evonik, Tg = -30 °C, M = 3500 g/mol, f = 2 val/mol, (manufacturer information) ≙ ca. 1750 g/val, OH = 30 g KOH/kg ≙ 0.91 % ≙ 536 mval/kg ≙ 1865 g/val Item 5: DBTL, Brenntag Item 6.: “Mesamoll”, Bayer

The adhesive is synthesised in two steps. First, a prepolymer is made with the high Tg polyester. For this prepolymer, the NCO/OH ratio can be calculated as follows: 9.1 g “Desmodur 44 MS” corresponds to 72.8 mval NCO. 65.5 g polyester item 1 corresponds to 24.6 mval. → NCO/OH = 72.8/24.6 = 3.0. The NCO/OH ratio for the second prepolymer, which is made not separately but rather within the first prepolymer and is a flexible, low-Tg prepolymer, is calculated as follows: 3.2 g “Desmodur 44 MS” corresponds to 25.6 mval NCO 21.8 g polyester item 4 corresponds to 11.7 mval OH → NCO/OH = 25.6/11.7 = 2.2. For the overall adhesive formulation, the NCO/OH ratio is: (72.8 + 25.6) / (24.6 + 11.7) = 2.7. A high isocyanate excess makes for good processability and adequate post-crosslinking through the formation of allophanates and biurets. Bubbling during the curing reaction is a disadvantage. The content of free MDI can be estimated with the help of equation (4) from Chapter II-2.2.1.3 if it is assumed that the reaction has been performed in just one step, in accordance with the stoichiometry of the formulation: Content of free MDI in [%] = [(2.7 - 1) / 2.7 ]2 ⋅ 12.3 = 4.85 [%] 4th generation When polypropylene oxides with molecular weights of about 100 to 8000 g/mol react with excess isocyanate to afford prepolymers, the result is the familiar prepolymers whose pro-

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perties, such as the glass transition temperature, are very similar to those of the polyethers used (for polypropylene oxide: about -60 °C). However, when low-molecular polyethers react with just a moderate excess of isocyanate (NCO/OH ratio close to 1), isocyanate-functionalised prepolymers are obtained. These prepolymers have a much higher glass transition temperature than the polyols employed. The glass transition temperature of the prepolymers can thus be readily adjusted via the urethane concentration (as a result of the molecular weight of the polyethers) [77, 78].

Figure II-2.112: Glass transition temperature of prepolymers as a function of the polyether molecular weight for an NCO/OH ratio of 1.2

Figure II-2.113: Glass transition temperature of prepolymers as a function of the NCO/OH ratio for polyethers (PPO with 400 g/mol)

Figure II-2.114: Green strength as a function of glass transition temperature

Figure II-2.112 shows the influence of the molecular weight of polyethers on the glass transition temperature of the resulting prepolymer for an NCO/OH ratio of 1.2. Only polyethers of sufficiently low molecular weight yield prepolymers whose concentration of urethane groups is such that the glass transition temperature is increased to the room temperature range. Polyethers with a molecular weight of about 400 g/mol are ideal [78]. The NCO/OH ratio exerts a significant influence on the glass transition temperature. Only if it is quite close to 1 is there a sufficient quantity of urethane groups in the prepolymer. These urethane groups are responsible for the higher glass transition temperatures. Consequently, an NCO/OH ratio of about 1.2 provides the necessary urethane concentration in the prepolymer, but there are also still enough reactive isocyanate groups for the subsequent curing reaction. Figure II-2.113 shows the influence of the NCO/OH ratio on the glass transition temperature for a polyether of molecular weight around 400 g/mol [78]. Adding a second prepolymer consisting of higher-molecular polypropylene oxide (M = 2000 to 4000 g/mol, diol or triol) or other less compatible polyols, such as hydroxyl-functionalised polybutadienes (see Chapter II-2.2.1 and III-2.2.2.1)

Curing hot-melts

further flexibilises and toughens the adhesive formulation. To improve heat resistance, a small amount of triol or triisocyanate may be used, because it affords a way of increasing the crosslink density in the cured polymer. Stabilisation of the prepolymers should be effected in any case during synthesis by adding the standard stabilisers (Chapter II-2.1.4.2) in a proportion of about 0.1 %. The application viscosity can be controlled via the NCO/OH ratio and the quantity of highermolecular polyether. The cure rate can be adjusted via the catalyst concentration (DBTL, 200 to 500 ppm). Reactive hot-melts made in this way are relatively cheap, because they mainly consist of polypropylene oxide. They set rapidly and have high initial flexibility. Figure II-2.114 shows the green strength of such reactive hot-melts as a function of glass transition temperature. The green strength rises significantly with increase in glass transition temperature [78].

203 Table II-2.34: PUR hot-melt, 4th generation Adhesive components Prepolymer 1 (item 9) Item

Raw material

Parts by weight

1

diisocyanate

41

2

polyether, low molecular mass

58.4

3

stabilizer

0.5

4

catalyst

0.1

Item 1 to 4 are reacted for 1 h at 120 °C

Prepolymer 2 (item 10) 5

diisocyanate

11.7

6

polyether, high molecular mass

87.6

7

stabilizer

0.5

8

catalyst

0.2

Item 5 to 8 are reacted for 15 min at 110 °C

Formulation of the adhesive: 9

prepolymer 1

90.1

10

prepolymer 2

9.9

Item 9 and 10 are mixed under vacuum for 1 h at 120 °C Item 1 and 5: “Desmodur 44 MS”, Bayer, M = 250 g/mol, f = 2 val/mol ≙ 125 g/val, NCO = 33,6 %, 8000 mval/kg Item 2: “Arcol PPG 425”, Bayer, M = 426 g/mol, f = 2 val/mol (manufacturer information), OH = 263 g KOH/kg ≙ 7,98 % ≙ 4696 mval/kg ≙ 213 g/val Item 3 and 7: “Additiv TI”, Bayer Item 4 and 8: DBTL, Brenntag Item 5: “Arcol PPG 4000”, Bayer, M = 4000 g/mol, f = 2 val/mol, OH = 28 gKOH/kg ≙ 0,85 % ≙ 500 mval/kg ≙ 2000 g/val

The disadvantages of polyetherbased reactive hot-melts are their frequently moderate adhesion to important plastics, such as PVC, and, if applied in thick layers, a tendency to foam during curing. Examples Table II-2.34 presents a typical formulation, according to [77, ex. 1]. The adhesive is formulated from two separately synthesised prepolymers. For prepolymer 1, the hot-melt properties are adjusted by allowing a low polyether to react at an NCO/OH ratio close to 1: 41 g “Desmodur 44 MS” corresponds to 328 mval NCO, 58.5 g “Arcol PPG 425” corresponds to 274 mval OH → NCO/OH = 328 / 274 = 1.2. For prepolymer 2, a higher polyether is used. The NCO/OH ratio is adjusted close to 2 in order that a sufficiently low viscosity may be obtained. It would even be possible to use polyols of higher functionality without having the problem of gelling.

11.7 g “Desmodur 44 MS” corresponds to 94 mval NCO, 87.6 g “Arcol PPG 4000” corresponds to 44 mval OH. → NCO/OH = 94 / 44 = 2.1.

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Table II-2.35: Pseudoplastic PUR hot-melt Item

Raw material

Parts by weight

1

diisocyanate

11.4

2

crystallizing polyester

84.5

3

rheological additive

4

4

catalyst

0.1

Item 1, 2 and 4 are mixed and reacted for 1 h at 100 °C. Then item 3 is added an vacuum is applied still in the heat.

The prepolymers are mixed to form an adhesive composed of approx. 10 % of the flexible prepolymer and 90 % of the solid prepolymer. Hybrid types and modifications

The flexibility of the end products, along with their setting and adhesion characteristics, can be adjusted by blending polyethers and polyesters [82–85]. Compatibility then becomes a major consideration. If the polymers are fully compatible, the glass transition temperature of the mixture will be a function of the individual weight fractions (see Chapter II-1.1.2.3, Fox equation). Where they are partially incompatible, the reaction must be suitably controlled to ensure that no macroscopic phase separation occurs during manufacture or application. If, for example, polyethers are co-condensed during the manufacture of polyesters, no macroscopic phase separation can occur.

Item 1: “Desmodur 44 MS”, Bayer, M = 250 g/mol, f = 2 val/mol ≙ 125 g/val, NCO = 33,6 % ≙ 8000 mval/kg Item 2: “Dynacoll 7360”, Evonik, M = 3500 g/mol, f = 2 val/mol, (manufacturer information) ≙ ca.1750 g/val, OH = 30 g KOH/kg ≙ 0,91 % ≙ 536 mval/kg ≙ 1865 g/val Item 3: fumed silica, “Aerosil R 202”, Evonik Item 4: DBTL, Brenntag

Crystalline polyols (like many polyesters or poly-THF) reduce the tendency of the uncured adhesive to creep along with the tendency to bubble during curing. However, they help to increase the brittleness of the prepolymer if they are used in high concentrations. Particularly-rapid-crystallising systems can be made with polyesters based on higher aliphatic polycarboxylic acids, such as dodecane dicarboxylates [86]. The rheological properties can be modified by formulation with 3 to 5 % hydrophobic fumed silica. The resultant products show sag resistance, i.e. will not flow down vertical surfaces [87]. Example Table II-2.35 presents a PUR hot-melt of pronounced pseudoplasticity. The formulation concept in this case is that of a 2nd generation adhesive. However, the principle of rheological modification can also be applied to other generations.

11.4 g “Desmodur 44 MS” corresponds to 91 mval NCO, 84.5 g “Dynacoll 7360” corresponds to 45.3 mval OH. → NCO/OH = 91 / 45.3 = 2.0.

Trends Recent developments are aimed at lowering the content of free monomer. Isocyanates bearing various reactive functional groups can be used to further reduce the NCO/OH ratio for prepolymer synthesis and so to further lower the monomer content (see also Chapter II-2.2.1.3). Reputable producers of isocyanates also supply prepolymers whose content of monomer has already been minimised by thin-layer evaporation. The use of silanes to replace isocyanate groups as the reactive function for moisture curing has failed to catch on. As with MS polymers or silicones (see Chapter III-3), the technology for using silanol groups instead of isocyanate groups in prepolymers exists. However, numerous disadvantages (price, setting process, adhesion) have so far ham-

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205

pered large-scale market introduction. The newly introduced POR hot-melts (see Chapter II-5.3) employ the curing technology of silicones, but not the polymer structure of polyurethanes. Delivery form and application Moisture-curing reactive hot-melts have to be stored away from moisture after manufacture. Packaging varies with the intended application, but includes 300 ml cartridges, 1.5 l tins, 20 l pails and 200 l drums. For cost reasons, packaging is gradually evolving in the direction of laminated aluminium foil bags. They are applied by machines similar to those employed for traditional hot-melts. For small quantities, cartridge pistols are available, while large quantities are applied from pails or drums with the aid of drum melters. In any event, the applicators must be designed to minimise access of moisture to the product before application (e.g. drum punch pumps). Adhesive beads (optionally in foam form) [79] are applied through round nozzles. For two-dimensional application, slot dies nozzles, roll coating applicators or spin-sprayers are used. For laminar, two dimensional applications, as is the case for traditional hot-melts, thermal reactivation is possible within a certain time window before the curing reaction has proceeded too far. Products Products currently offered on the market differ mostly in the following parameters: • • • • • • • • •

application temperature (100 to 180 °C), application viscosity (5000 to 100,000 mPa.s), reactivity/curing time (hours to days), stability under application conditions (varies with supplier), open time (seconds to minutes), setting time (seconds to minutes), adhesion properties on certain substrates (varies with supplier), content of free monomeric isocyanate (varies with supplier), price (polyester or polyether polyols).

One of the most underestimated risk factors in industrial applications is the products’ reactivity with moisture. Even tiny quantities of moisture are sufficient to cause rapid skinning at the elevated application temperatures. Such skins can prove very troublesome for processing through nozzles. And yet, rapid curing to reach the end properties is desirable after application. Safe and straight-forward application are key to the further success of reactive hot-melt technology. The free-MDI content of most commercial products is not critical, according to current knowledge, provided that application temperatures are not too high and sufficient ventilation is guaranteed. In any event, the best-possible safety measures should be adopted, because long-term exposure to even tiny amounts of isocyanates causes sensitisation. For a few years now, MDI has been suspected of being carcinogenic. The worst-case scenario in spin-spray application is that reactive aerosols may be formed. Accordingly, safety measures are imperative. Evaluation Moisture-curing reactive PUR hot-melts have a very interesting product profile. They combine many of the advantages of hot-melts with those of reactive adhesives. They boast the rapid setting of a hot-melt, a fact which is very important for industrial applications, but also exhibit many of the advantages of reactive adhesive systems.

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Advantages

Disadvantages

+ + + + + + + + +

– – – – – –

100 % system, good storage stability, can be used for automated processes, mild application conditions prevent damage to substrates, rapid setting, good adhesion, good heat resistance, good low-temperature properties, good chemical and hydrolytic stability.

price, special packaging required, need for an applicator, moderate green strength compared to traditional hot-melts, curing reaction may vary with weather conditions and substrates, isocyanate is a harmful substance.

Fields of application The highly attractive product properties and scope for automated application are ideal for applications in the automotive industry, especially where non-metallic surfaces have to be joined. However, the success of applications usually hinges not so much on the adhesive properties as on process stability, which has not always been adequate in the years following the introduction of the adhesives. Successful applications include the bonding of headliners to the coated roof, lamination of door panels, dashboards and rear parcel shelves, lamination of car seats, and attachment of ABS hooks to interior door panels. Utility vehicles boast various bonded injection-moulded parts. For reactive hot-melts, wood and timber-derived materials usually make good substrates because they already contain a great deal of humidity and are also moisture-permeable. Thus, applications are to be found in the woodworking and furniture industry for demanding edge-veneer bondings that must be resistant to heat and humidity. The combination of hot-melt with good adhesion properties and improved heat resistance have paved the way for reactive hot-melts in book-binding applications. They are the products of choice when papers which are difficult to bond must satisfy high quality requirements. Use on construction sites is usually too problematic due to the need for an applicator. However, vendor parts, such as flooring elements for computer rooms (metal-laminated particle board) and special roofing parts have been manufactured successfully with the aid of reactive hot-melts. The attachment of shoe soles to shafts, a very demanding adhesive process, which is mainly still done with solvent-based polyurethanes, has been successfully executed in various line trials, but the shoe industry, is fairly conservative and has been unwilling so far to make the necessary changes in products and processes. Even now, it prefers solvent-based products, with all their toxicological and ecological consequences (see Chapter II-1.1.2.1). In the textiles industry reactive hot-melts are being used increasingly to laminate high-quality fabrics. They make ideal products for the manufacture of durable laminates because of their low temperatures, and rapid, automated processing.

2.5.3 Moisture-curing POR hot-melts (polyolefin reactive hotmelts) 2.5.3.1 Basic principles and structure Innovative reactive hot-melts based on silane-grafted amorphous poly-α-olefins have become commercially available in recent years. Amorphous α-olefins (copolymers of ethylene, propylene, 1-butene etc; see Chapter II-1.3.2.2) have good hot-melt properties. They are rapid-setting and for most applications have adequate heat resistance. Silanol groups can be grafted onto them to create reactive functional groups. After application, these silanol

Curing hot-melts

207

Figure II-2.115: Chemical structure and curing reaction of POR hot-melts

groups can be used for crosslinking reactions by condensation with moisture. POR hotmelts have an interesting property profile that sets them apart from the widely accepted PUR hot-melts. Figure II-2.115 shows their chemical structure and curing reaction (see also Chapter III-2.3) [88–90]. 2.5.3.2 Formulation Commercial silane grafted α-polyolefins (e.g. “Vestoplast 206”, Evonik [91]) can be formulated with resins, waxes and additives in the usual manner (see also Chapter II-1.3). Resins

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increase tack and lower melt viscosity, while waxes reduce cost and viscosity. Furthermore, non-functionalised α-polyolefins, fillers and specific additives can be used. The resulting products can be applied at temperatures of 140 to 180 °C. The packaging and applicators are similar to those for PUR hot-melts. Table II-2.36 presents a starting formulation [92]. The product described here is not a typical hot-melt. It contains a large quantity of molecular sieve specifically intended for Table II-2.36: POR hot-melt use as drying agent in insulating glass (see Chapter III-2). Item Raw material Parts by weight 1

silane functionalized poly-α-olefine

20 to 70

Evaluation

POR hot-melts complement the already widely accepted PUR hot3 poly-α-olefine 5 to 30 melts. The amorphous structure of 4 carbon black 5 to 30 the base polymers, combined with 5 molecular sieve, zeolithe, 3 Å 20 to 30 the low glass transition tempera6 catalyst 0.1 to 2 ture, bestows long open times for processing and application. Due 7 organosilane 0.1 to 2 to the relatively high-molecular base polymers, they possess good hot-melt properties, set rapidly and develop high initial flexibility. In contrast to the PUR hot-melts, they do not suffer from bubbling during curing, as the small quantities of liberated methanol slowly are able to diffuse out of the polymer. POR hot-melts are not classified as dangerous goods and are highly stable under application conditions. Furthermore, the combination of silane functions and low-polarity polymer means that POR hot-melts adhere strongly to glass, ceramics and even polypropylene. However, their slow curing reaction is a disadvantage. It can take days or even weeks for the water needed for the curing reaction to diffuse through the low-polarity polymer. 2

butyl rubber

5 to 30

Advantages

Disadvantages

+ + + + + +

– slow curing, – application temperatures higher than for PUR hot-melts.

not classified as dangerous goods, good stability under application conditions, long open time, rapid setting, high green strength, good adhesion to glass, ceramics and PP.

Fields of application In the manufacture of utility vehicles, POR hot-melts are used to bond sandwich elements because they set faster than 2-component PUR adhesives. The slower cure reaction is not critical to further processing of the elements, because the hot-melt property alone provides sufficient strength. In the automotive industry, they can often serve as an alternative to PUR hot-melts, which require a dangerous goods label [93]. Like PUR hot-melts, they are successfully employed in the book-binding industry. They are also used in the insulating glass industry on account of their low vapour-transmission rate [92].

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209

2.5.4 Heat-curing epoxy hot-melts 2.5.4.1 Basic principles and structure The term “heat-curing hot-melt” sounds somewhat like a contradiction. A traditional hotmelt which has solidified on cooling will always soften reversibly again on being heated if the polymer has not been crosslinked. If heat curing of such a system is intended, the

Figure II-2.116: Epoxy prepolymer

Figure II-2.117: Curing of epoxy prepolymers

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Part II

Reactive adhesives

adherends have to be fixtured in position after adhesive application and the subsequent heat-cure temperature must be far higher than the application temperature. Otherwise, a premature reaction, with increase in viscosity, would occur during application. The hotmelt advantage over the curing period would be lost. However, for applications in which fixturing with adhesive is supplemented by mechanical fixturing, a heat-curing hot-melt can be applied. The hot-melt advantage can then consist in offering a degree of resistance to mechanical loads prior to curing. The synthesis of thermally reactive epoxy hot-melts utilises the different rates at which epoxies react with nucleophiles (see Chapter II-2.2.2). On account of their high nucleophilicity, amines react much faster than alcohols [94]. A secondary amine reacts with excess bifunctional epoxy resin under mild conditions to yield a linear, epoxy-terminated prepolymer which still bears OH groups. The molecular weight of such a prepolymer can be adjusted via the epoxy/amine ratio. Figure II-2.116 schematically shows the reaction leading to an epoxy prepolymer. As shown in Figure II-2.117, curing takes place at elevated temperatures by reaction with latent hardeners and reaction between epoxy functions and OH groups to yield a crosslinked epoxy polymer. During curing, each existing OH group creates another OH [94]. Hot-melt properties can also be obtained by using solid epoxy resins (oligomers of bisphenol A diglycidyl ether or epoxy novolaks; see also Chapter II-2.2.2) [95]. A heat-curing system can be realized by formulation with latent hardeners. 2.5.4.2 Formulations Epoxy hot-melts can be formulated by making a secondary diamine (functionality f = 2 val/ mol) react with excess epoxy resin to yield a solid, epoxy-terminated prepolymer. Primary diamines (functionality f = 4 val/mol) can be converted into higher-melting prepolymers, too, if a sufficient excess of epoxy is guaranteed (see Chapter II-2.2.1.3). The viscosity can be adjusted with liquid epoxy resins. The easiest way to formulate hot-melts is to use commercial solid resins, perhaps already toughened. Modified epoxy oligomers (“Bakelite EPR 881”, epoxy content = 11.5 % ≙ 2667 mval/kg ≙ 375 g/val, and “Bakelite EPR 880”, epoxy content = 14.1 % ≙ 3280 mval/kg ≙ 305 g/val [96]) specifically for formulating reactive hot-melts are available on the market. These resins are often modified with liquid grades. The fillers and additives typically employed for 1-component epoxies can be used (see Chapter II-2.2.2). Curing is effected with dicyanodiamide, which can also be modified with accelerators. Example Table II-2.37 lists a starting formulation for an epoxy hot-melt, according to[95]. A solid epoxy resin based on oligomeric bisphenol A diglycidyl ether contributes the hot-melt property. To this end, a liquid epoxy resin (standard type) is added. The mixture is toughened by heating for 3 hours at 120 °C with an amine-terminated polypropylene oxide to afford a solid epoxy prepolymer. This is cooled to 70 °C and further formulated with the usual ingredients, such as curing agents, accelerators and fillers. The epoxy/amine ratio for the epoxy-terminated prepolymer can be calculated as follows: 23.7 g of the solid epoxy resin corresponds to 23.7 ⋅ 2105 / 1000 = 50 mval epoxy 17.3 g of the liquid epoxy resin corresponds to 17.3 ⋅5290 / 1000 = 91.5 mval epoxy 19 g of the oligomeric diamine corresponds to 19 ⋅ 2000 / 1000 = 38 mval

→ epoxy/amine = (50 + 91.5) / 38 = 3.7.

The critical epoxy/amine ratio can be calculated by using a modified form of equation (3) (see Chapter II-2.2.1.3). The gel point for full conversion, excess epoxy and an amine functionality of 4 val/mol is given by:

Curing hot-melts



(3)

211

→

epoxy/amine = famine - 1 = 3

In other words, the reaction is performed at a high epoxy excess to avoid gelling during prepolymer synthesis. The quantity of unreacted epoxy groups is therefore: 191 - 38 = 153 mval. 2.5 g curing agent corresponds to 179 to 208 mval, i.e., a slight overdose of curing agent is present. Evaluation Suitably formulated epoxy hot-melts can attain the tensile shear strengths of standard hotmelts (6 to 10 MPa). For many applications, only fractional amounts are sufficient, e.g. where just wash resistance is required. The products have good storage stability and can be cured thermally, just like the standard 1-component epoxies. As the products are solids at room temperature, they can also serve in the production of readily curable films. Advantages

Disadvantages

+ + + + +

– – – –

rapid setting, no paste consistency after application, good resistance to wash-out, adjustable green strength, 1-component system.

Fields of application Epoxy hot-melts are widely used as hem-flange adhesives in bodyin-white shops in the automotive industry (see also Chapter III-2). In this case, they can be used in the manner of traditional 1-component paste epoxies, but have the advantage of not being washed out in the cleaning baths prior to application of the first paint coat. There is therefore no need for the pre-gelling step required for paste products. The metal sheets are already fixtured by the hem-flanging and the spot-welding, and so the brief liquefaction at the start of the curing reaction is of no consequence.

need for an applicator, no load-transfer during curing, overheating during application and manufacture must be avoided, heat curing limits choice of suitable substrates.

Table II-2.37: Heat-curing epoxy hot-melt Item

Raw material

Parts by weight

1

epoxy resin, solid

23.7

2

epoxy resin, liquid

17.3

3

oligomeric aliphatic diamine

19

4

filler

32

5

rheological additive

6

hardener

2.5

7

accelerator

0.5

5

Item 1: epoxy resin, mp = 60 °C, similar to “Bakelite EPR 191”, Bakelite, M = 475 g/mol, f = 2 val/mol, EEW = 238 g/val ≙ 2105 mval/kg ≙ 9.1 % Item 2: epoxy resin, liquid, similar to “Bakelite EPR 174”, Bakelite, M = 378 g/mol, f = 2 val/mol, EEW = 189 g/val ≙ 5290 mval/kg ≙ 22.8 % Item 3: “Jeffamine D 2000”, Huntsman, M = 2000 g/mol, f = 4 val/mol, AEW = 500 g/val, ≙ 2000 mval/kg amine Item 4: silicate, “Sillitin V 85”, Hoffmann Mineral Item 5: fumed silica, z.B. “Aerosil 200”, Evonik Item 6: dicyanodiamide, “Dyhard 100s”, Evonik, AEW = 12-14 g/val Item 7: Fenuron, “Dyhard AR 300”, Evonik

2.5.5 Radiation-curing acrylic hot-melts See also Chapter II-3, Pressure sensitive adhesives, and Chapter II-1.3 Hot-melts. Polymers which are based on thermoplastic acrylics and have the cohesion of a hot-melt due to their high molecular weight but which are still meltable are applied (along with a photoinitiator) to carriers for adhesive tapes; they are subsequently cured by radiation. The products do not develop the load-bearing capacity of a hot-melt through physical setting. Their primary advantage lies in offering scope for solventless coating and subsequent radiation-induced curing.

212

Part II

Reactive adhesives

2.6 References [1] G. W. Becker, D. Braun, Kunststoffhandbuch 7, Polyurethane, 3rd edition. Hanser (1993) [2] M. Szycher, Szycher’s Handbook of Polyurethanes, CRC Press (1999) [3] H. J. Saunders, K. C. Frisch, Polyurethanes, Chemistry and Technology, Vol. 1 and 2, Interscience (1962) [4] K. Uhlig, Polyurethan Taschenbuch, Hanser (1998) [5] P. J. Flory, Principles of Polymer Chemistry, Cornell University Press (1953) [6] A. W. Fogiel, Macromolecules, Vol. 2, No. 6, Nov - Dec (1969), 581 ff [7] R. Bonart et al., Colloid and Polymer Sci. 260 (1982), 518 ff [8] http://www.polymers-usa.bayer.com/polyurethanes/distributors/pdf/21.pdf [9] R. Schmidheiny, in www.prochem.ch/html/forum/PURchem.pdf [10] S. R Hartshorn, Structural Adhesives, Plenum Press (1986) [11] I. Skeist, Handbook of Adhesives, 3rd edition, Chapman & Hall (1989) [12] G. Habenicht, Kleben, 3rd edition, Springer (1997) [13] Bayer, Baycoll, Brochure, dated 11/99 [14] Sartomer Application Bulletin, Atofina, Polybd® Resins in Adhesive Applications 095/02 [15] R. T. Morrison, R. N. Boyd, Lehrbuch der Organischen Chemie, 3rd edition, Verlag Chemie (1973) [16] J. March, Advanced Organic Chemistry, 4th edition, J. Wiley&Sons (1992) [17] Bayer Material Science, One Pack PUR adhesive, MMX 0336, edition 2004-02-04 [18] J. Kozakiewicz, Adhesion, 13 (1989), 114 ff [19] N. G. Carter, in www.chemsoc.org/pdf/gcn/Industrial.pdf [20] Jowat AG, Power PUR, Techn. Information JN D044 06/2003, D008 05/2003 [21] Z. W. Wicks, Progress in Org. Coatings, 3 (1975), 73-99 [22] Baxenden Chemicals Limited, Blocked Isocyanates, info brochure [23] Bayer, Crelan-Produkte, www.bayer-ls.de/ ls/lswebcms.nsf/0/ae1d6d9f6c2-component76abc1256a96004c6 2ee?OpenDocument - 38 [24] P. Müller et al., Angew. Makromol. Chem., 65 (1977), 23-39 [25] DE 3 940 273 [26] DuPont, Adiprene, Caytur 21, information brochure E-37761 [27] EP 0 311 852 [28] Bakelite, Binders for the Adhesives Industry, company brochure, 2004 [29] www.chemicals.ucb group.com/b_units/b2indust/ radcure/uvacure/uvacure.html - 80k [30] J. Barwich et al., Adhäsion 1989, 33 (5), 27-30 [31] Degussa Fine Chemicals, Dyhard, company brochure, March 2003 [32] Frihart et al., Cognis (Henkel) in www.adhesivesmag.com [33] Dow, Liquid Epoxy Resins, company brochure, 2004 [34] Resolution Products, Starting formulations under www.resins.com/resins/am/product_html [35] www.Hanse-Chemie.com [36] www.schillseilacher.de [37] Air Products,Epoxy Additives for Adhesive Applicartions, company brochure, 2001 [38] Henkel, Epoxy Curing Agents, company brochure, 1998 [39] www.huntsman.com [40] UPPC, Technical Information, 2002 und under www.uppc.de [41] Delo Industrieklebstoffe, Mikroelektronik, company brochure and www.delo.de [42] S. Sprenger et al., Adhäsion 3 ,(2003), 24-30 [43] ATO-FINA, under www.sartomer.com [44] G. Zeppenfeld et al., Angew. Makromol. Chem., 172 (1989), 185-94 [45] P. Cullen et al., FARBE&LACK, 109. Jahrgang, 11 (2003), 20-23 [46] DuPont, Techn. Information (3/02), W-400030 [47] H. Dodiuk et al., J. Adhesion, Vol 22 (1987), 227-251 [48] www.gifte.de [49] www.roehm.de and www.roehm.de/de/monomere/produkte/ produktuebersicht/vernetzer.html - 23k - 6. Juni 2004 [50] D. Dunn, Reactive toughened Acrylic Adhesives Poised for Growth, http://www.bondch.com/bzzt/030521. html [51] Reilly Industries, http://www.reillyind.com [52] W. Wuich, GAK 7/1990, 43, 389-92 [53] Reactive Acrylic Bonding System based on Hypalon, Dupont, Technical Information, 2003 [54] K. W. Allen, Acrylates as reactive Adhesives, Int.J. Adhesion and Adhesives, Vol. 9, No.2, 1989, 103-5 [55] Hypalon, Technical Information, DuPont, 2003 [56] W. Endlich, Zeitgemäße Acrylatklebstoffe, Uta Groebel, Limeshain, 1985

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[57] S. Wellmann et al., Int. J. Adhesion, Vol. 14, No. 1, January 1994, 47-55 [58] US 3.043.820 [59] company brochure Loctite, DELO, Strahlungshärtung, KAT/PI/10/00 [60] www.theochem.uni-duisburg.de/DC/material/ exarbeiten/pmma/pmma3.pdf [61] K. L. Shanta, Developments and Applications of Cyanoacrylate Adhesives, J Adhes. Sci. Technol. 3 (1989), 4, 237 – 60 [62] EP 0 323 720 [63] US 4 845 151 [64] D. Kotsev, V Kabaivanov, Adhesion, 12 (1988), 82 - 105 [65] www.teli.de/jcd/mdm-ca.html Chemie und Kleben [66] Bakelite AG, Bakelite Phenolharze, Leitfaden 2003 [67] R. Jordan, R. Hinterwaldner, Klebharze, R. Hinterwaldner Verlag, München [68] Ullmann’s Encyclopedia of Industrial Chemistry, release 2001, 6th edition [69] Houben-Weyl, 4th ed., XIV/2, 272–291 [70] W. Rath, Dissertation Universität Stuttgart, 1984 [71] H. v. Voithenberg, Europ. Adh.and Sealants, 1 (1984), 4, 28-32 [72] BOSTIK Supergrip 2000, Emhart Chemical Group, Bostik Technical Centre, company brochure (1985) [73] DOS 3 236 313 [74] DOS 2 609 266 [75] EP 0340 906 [76] H. F. Huber, H. Müller, Adhesives Age, 11, 1987, 32–35 [77] EP 0 369 607 [78] H. Gilch, W. Rath, UTECH 90, Conference, The Hague (1990) [79] EP 0 405 721 [80] company brochure, Evonik, Degussa, Dynacoll 7000, 2003 [81] BOSTIK Tucker GmbH, technical data sheet Supergrip 2000-9810 (1989) [82] DOS 199 61 941 [83] DE 4 418 177 [84] DOS 195 04 007 [85] DOS 44 19 449 [86] EP 0 421 154 [87] EP 0 386 897 [88] M. Schindler, Coating, 2003, 1, 22–24 [89] F. Stark, Adhäsion 9, 2002, 16–20 [90] company brochure JOWAT Klebstoffe, JOWAT News, D035, 05, 2003 [91] company brochure Degussa AG, Vestoplast, [92] DOS 196 24 236 [93] H. U. Hürther, under www.automotivetechnology.net/editorials/Dupont.htm [94] A. Groß et al., Adhäsion 1988, 11, 16–24 [95] DE 3 827 626 [96] Bakelite, techn. Data sheets (2003) [97] M. Haufe et. al., Int. J. Adhes. 2000, 20, 333-340 [98] U. Meier-Westhues: Polyurethane, Vincentz Network 2009 [99] www.dymax.de

214

Part II

Pressure sensitive adhesives

3 Pressure sensitive adhesives 3.1 Basic principles 3.1.1 Characterisation Pressure sensitive adhesives (PSAs) are permanently tacky products which adhere to substrates under the application of slight pressure (this may also called tack). The substrates are not bonded at the time of adhesive application. Commonly, the adhesive may also preapplied to one of the substrates, which acts as a carrier (e.g. adhesive tapes, labels). The adhesive strength describes the later load-bearing capacity of an adhesive bond and depends on both the adhesion and the cohesion of the adhesive. The load-bearing capacity of a pressure sensitive adhesive bond is mostly characterised by its peel resistance under a specified peel rate, e.g. the peeling force needed to effect separation (at an angle 90° or 180°). It is also characterised by the shear strength or creep resistance, i.e. the length of time or the temperature at which the adhesive bond can withstand a constant load.

3.1.2 Structure/property relationships A pressure sensitive adhesive is able to wet substrates upon contact, without the help of organic solvents or water. This ability is similar to that of viscous liquids. After adhesive bonding, the pressure sensitive adhesive is able to transfer a certain amount of load. This ability is similar to that of solid elastic bodies. Typically, pressure sensitive adhesives exhibit a combination of the properties of liquids and solids, called visco-elasticity [1]. The plastic flow of a pressure sensitive adhesive during wetting and developTable II-3.1: Parameters benefiting the properties of pressure ment of adhesion to the substrate sensitive adhesives is promoted by a low glass transition temperature (Tg) and low Property Positive parameter molecular weight. Pressure sensiwetting and tack low molecular mass tive adhesives usually have glass low glass temperature transition temperatures which cohesion high molecular mass are 40 to 70 °C below the application temperature (mostly ambient high glass temperature temperature) [6]. Advantageous, crosslinking but not mandatory for good wetting and therefore for the develTable II-3.2: Classification of pressure sensitive adhesives by peel opment of adhesion, are a long resistance (180°) [3] contact time, high contact pressure and high temperature. Pressure sensitive adhesive Peel resistance [N/25 mm] extremely durable durable

>14 10 to 14

partly removable

6 to 8

removable and repositionable

2 to 4

easily removable