Handbook of Rubber Bonding
Bryan Crowther (Editor)
The Handbook of Rubber Bonding (Revised Edition)
Editor: Bryan Cr...
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Handbook of Rubber Bonding
Bryan Crowther (Editor)
The Handbook of Rubber Bonding (Revised Edition)
Editor: Bryan Crowther
rapra TECHNOLOGY
Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net
First Published 2001 by
Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK
©2001, Rapra Technology Limited Revised and Reprinted 2003
All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder. A catalogue record for this book is available from the British Library. Cover photograph reproduced with permission from Rubber Chemistry and Technology, 1994, 67, 4582. Copyright 1994, Rubber Division, American Chemical Society, Inc.
ISBN: 1-85957-394-0
Typeset by Rapra Technology Limited Printed and bound by Rapra Technology Limited
Contents
Introduction .......................................................................................................... 1 1
Substrate Preparation Methods ....................................................................... 3 1.1
1.2
1.3
1.4
Metal Preparation - General Techniques ................................................ 3 1.1.1
Structure of Metal Substrates - Metallography .......................... 3
1.1.2
Bonding ..................................................................................... 5
1.1.3
Rubber Component with Metal Support ................................... 5
1.1.4
Metal Pre-treatments ................................................................. 6
Pre-treatments of Plastics and Rubbers ................................................ 12 1.2.1
Introduction ............................................................................. 12
1.2.2
Studies of Pre-treatments for Plastics ....................................... 13
1.2.3
Hydrocarbon Rubbers with Little or No Unsaturation ............ 19
1.2.4
Unsaturated Hydrocarbon Rubbers ......................................... 20
1.2.5
Halogenated Rubbers .............................................................. 25
1.2.6
Miscellaneous Rubbers ............................................................ 26
1.2.7
Discussion ................................................................................ 27
1.2.8
Summary ................................................................................. 29
Bonding Rubbers to Plastic Substrates ................................................. 29 1.3.1
Introduction ............................................................................. 29
1.3.2
Plastics Substrate Preparation .................................................. 31
1.3.3
Degreasing and Solvent Cleaning ............................................. 35
1.3.4
Adhesive/Bonding Agent Choice .............................................. 36
Substrate Preparation for Bonding Using the Wet Blast Process ........... 42 1.4.1
Summary ................................................................................. 42
1.4.2
The Wet Blast Phosphating Plant ............................................. 42
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The Handbook of Rubber Bonding
2
3
1.4.3
Comparison Between Conventional and Wet Blast Phosphating .. 45
1.4.4
The Wet Blast Phosphating Plant ............................................. 46
1.4.5
Advantages of the Wet Blast Phosphating Plant ....................... 47
Rubber to Metal Bonding ............................................................................. 57 2.1
History................................................................................................. 57
2.2
Bond System Characteristics ................................................................ 62 2.2.1
Adhesive Characteristics .......................................................... 62
2.2.2
Compound Characteristics....................................................... 64
2.3
Adhesion .............................................................................................. 66
2.4
Effective Bond Formation .................................................................... 71
2.5
Post Vulcanisation Bonding ................................................................. 73
2.6
Factors Affecting Bond Integrity .......................................................... 73
2.7
Bond Failure Types .............................................................................. 74
2.8
Bond Test Procedures ........................................................................... 76
2.9
Summary .............................................................................................. 77
Rubber to Metal and Other Substrate Bonding ............................................. 81 3.1
3.2
Introduction ......................................................................................... 81 3.1.1
Foreword ................................................................................. 81
3.1.2
History .................................................................................... 81
3.1.3
Types of Bonding ..................................................................... 82
3.1.4
The Bonding Process - An Overview ........................................ 83
3.1.5
Development of Bonding ......................................................... 84
3.1.6
Bonding Agent Reliability ........................................................ 84
3.1.7
The Environment and Solvent Use ........................................... 86
3.1.8
Methods of Reduction in Solvent Emissions ............................ 87
Substrates and their Preparation .......................................................... 87 3.2.1
ii
Mechanical Treatment of Metals ............................................. 88
Contents
3.3
3.2.2
The Abrasion Process ............................................................... 90
3.2.3
Levels of metal cleanliness ....................................................... 92
3.2.4
Time Window .......................................................................... 93
3.2.5
Chemical Preparation of Surfaces ............................................ 94
3.2.6
Future Developments ............................................................... 96
Bonding Agent Preparation .................................................................. 97 3.3.1
3.4
3.5
3.6
3.7
3.8
Solvent-borne Bonding Systems ............................................... 97
Bonding Agent Application and Use .................................................... 98 3.4.1
Application Methods ............................................................... 98
3.4.2
Waterborne Bonding Systems ................................................... 98
3.4.3
Bonding Agent Thickness......................................................... 99
Post Vulcanisation Bonding ............................................................... 100 3.5.1
Post Vulcanisation Bonding Applications............................... 100
3.5.2
Choice of Bonding Agent for Post Vulcanisation Bonding ..... 100
3.5.3
Rubber Substrate Preparation for PV Bonding....................... 101
3.5.4
Metal Substrate Preparation .................................................. 101
3.5.5
Methods of Application ......................................................... 101
Waterborne Bonding Systems ............................................................. 103 3.6.1
History .................................................................................. 103
3.6.2
Differences Between Solvent and Waterborne Bonding Agents .. 103
3.6.3
Suggested Spraying Equipment and Conditions ..................... 105
3.6.4
Application and Substrate Temperatures ............................... 105
3.6.5
Film Thickness ....................................................................... 106
3.6.6
Layover .................................................................................. 106
3.6.7
Progress in Performance......................................................... 106
Health and Safety in the Workplace ................................................... 109 3.7.1
The Safety Data Sheet ............................................................ 109
3.7.2
Perspective ............................................................................. 110
Bonding Agent Testing ....................................................................... 110
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The Handbook of Rubber Bonding
3.9
Shelf Life Considerations ................................................................... 112 3.9.1
Shelf Life Categories .............................................................. 113
3.9.2
Procedures for Re-certification of Bonding Agents ................ 113
3.10 Troubleshooting ................................................................................. 115 3.11 Summary ............................................................................................ 120 4
Bonding Rubber to Metals with Waterborne Adhesive Systems .................. 125 4.1
4.2
4.3
5
4.1.1
Solvent Elimination by the Rubber Industry .......................... 126
4.1.2
Techniques Necessary in Bonding of Rubber to Meet Local Environmental Pollution Limits ................................... 127
Waterborne Bonding Systems ............................................................. 127 4.2.1
Structure of Organic Solvent-based Bonding Systems ............ 127
4.2.2
Structure of Waterborne Bonding Systems ............................. 127
4.2.3
Fundamentals of Waterborne Bonding Agent Application ..... 128
4.2.4
Waterborne Bonding Systems in Factory Usage ..................... 128
4.2.5
Metal Preparation - For Waterborne Bonding Systems .......... 129
4.2.6
Waterborne Bonding Agent Application ................................ 129
4.2.7
Waterborne Bonding Agent Storage Stability ......................... 130
4.2.8
Non Bond Advantages of Waterborne Bonding Systems ........ 130
4.2.9
General Comments - Waterborne Bonding Agents ................. 130
Waterborne Bonding Agents - A Factory Experience ......................... 131 4.3.1
Thickness Effects ................................................................... 131
4.3.2
Pre-bake Resistance ............................................................... 133
4.3.3
Primers .................................................................................. 134
4.3.4
Polymer Range ....................................................................... 134
4.3.5
Product Range ....................................................................... 134
4.3.6
Current Disadvantages of Waterborne Bonding Agents ......... 134
Rubber to Rubber Bonding ......................................................................... 137 5.1
iv
Introduction ....................................................................................... 125
Bonding of Unvulcanised Rubbers ..................................................... 137
Contents
5.1.1
Tack/Autohesion .................................................................... 137
5.1.2
Influence of Vulcanisation System .......................................... 139
5.1.3
Influence of Filler Type .......................................................... 140
5.1.4
Effects of Plasticisers/Process Oils .......................................... 141
5.1.5
Effects of Tackifiers ............................................................... 141
5.1.6
Effects of Other Ingredients ................................................... 142
5.1.7
Effects of Surface Modification .............................................. 142
5.1.8
Effects of Surface Roughness ................................................. 144
5.1.9
Influence of Contact Time/Pressure/Temperature ................... 144
5.1.10 Effects of Blooming ................................................................ 145 5.1.11 Effects of Ageing .................................................................... 146 5.1.12 Testing of Tack/Autohesion Levels ......................................... 147 5.1.13 Adhesion Theories ................................................................. 148 5.2
Bonding of Vulcanised Rubbers to Unvulcanised Rubbers ................. 150
5.3
Bonding of Vulcanised Rubbers ......................................................... 152 5.3.1
Strip Bonding of Tyre Retreading Components ...................... 152
5.3.2
Effects of Strip Thickness ....................................................... 155
5.3.3
Effects of Surface Roughness ................................................. 156
5.3.4
Effects of Temperature on Bonding ........................................ 156
5.3.5
Effects of the Chemical Nature of Polymers/ Polymeric Additives/Surface Roughness ................................. 156
5.3.6
Urethane Adhesive Systems .................................................... 158
5.3.7
Surface Treatments to Improve Bonding ................................ 158
5.3.8
Effects of Contact Time/Surface Bloom .................................. 159
5.4. The Mechanism of Adhesion of Fully Cured Rubbers........................ 159 6
Rubber-Brass Bonding ................................................................................. 163 6.1
Introduction ....................................................................................... 163
6.2
Mechanism of Rubber-Brass Bonding ................................................ 165 6.2.1
Reviews ................................................................................. 165
6.2.2
Recent Mechanistic Studies .................................................... 165
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The Handbook of Rubber Bonding
6.2.3
Updated Rubber-Brass Adhesion Model ................................ 170
6.2.4
New Evidence for Ageing of the Interfacial Sulphide Film ..... 177
6.2.5
Compounding for Brass Adhesion ......................................... 180
6.2.6
Additives to Compounds for Brass Adhesion ......................... 181
6.2.7
Developments in Metal Pre-treatments .................................. 184
6.2.8
Developments of Novel Alloys for Bonding to Rubber .......... 189
6.2.9
Miscellaneous ........................................................................ 190
6.2.10 Summary ............................................................................... 190 7
8
Review of Tyre Cord Adhesion ................................................................... 197 7.1
Introduction ....................................................................................... 197
7.2
Accepted Mechanisms of Rubber-Brass Bonding ............................... 198
7.3
Ageing of the Rubber-Brass Bond ...................................................... 200
7.4
Metal Organic Cobalt Salts ................................................................ 201
7.5
The Role of Resins and Silica/Resin Systems ...................................... 205
7.6
Summary ............................................................................................ 208
Rubber to Metal Bonding Using Metallic Coagents .................................... 213 8.1
Introduction ....................................................................................... 214
8.2
Metallic Coagents .............................................................................. 215
8.3
8.2.1
Scorch Safety ......................................................................... 217
8.2.2
Tensile Properties ................................................................... 219
8.2.3
Tear Strength ......................................................................... 220
Experimental ..................................................................................... 221 8.3.1
8.4
8.5
vi
Materials ............................................................................... 221
Results and Discussion ....................................................................... 229 8.4.1
Adhesion to Metals ................................................................ 229
8.4.2
Adhesion to Fibres and Fabrics .............................................. 235
Summary ............................................................................................ 238
Contents
9
Rubber to Fabric Bonding ........................................................................... 241 9.1
Introduction ....................................................................................... 241
9.2
Adhesive Systems ............................................................................... 241
9.3
9.4
9.5
9.2.1
Aqueous Fabric Treatments ................................................... 241
9.2.2
Solvent-Based Adhesive Systems ............................................ 248
9.2.3
In Situ Bonding Systems......................................................... 249
Mechanisms of Adhesion ................................................................... 250 9.3.1
Dip/rubber Interface .............................................................. 250
9.3.2
Dip/textile Interface ............................................................... 252
Other Factors Affecting Adhesion ...................................................... 253 9.4.1
Storage of Treated Textiles ..................................................... 253
9.4.2
Adhesion in Service ................................................................ 254
Environmental Aspects ...................................................................... 254 9.5.1
Storage and Handling ............................................................ 254
9.5.2
In Process ............................................................................... 255
9.5.3
Wastes and Disposal .............................................................. 255
10 Bonding Rubber with Cyanoacrylates ......................................................... 259 10.1 Introduction ....................................................................................... 259 10.2 Liquid Cyanoacrylates ....................................................................... 259 10.3 Curing of Cyanoacrylates .................................................................. 260 10.3.1 Factors Affecting Cure ........................................................... 261 10.3.2 Cure Speed ............................................................................. 263 10.4 Types of Cyanoacrylate ...................................................................... 263 10.4.1 Bonding to Acidic and Porous Substrates............................... 264 10.4.2 Toughened Cyanoacrylates .................................................... 265 10.4.3 Flexible Cyanoacrylates ......................................................... 266 10.4.4 UV Curing Systems ................................................................ 266 10.5 Design Considerations ....................................................................... 266
vii
The Handbook of Rubber Bonding
10.5.1
Minimise Peel and Deavage Loads .................................... 267
10.5.2
Bond Line Thickness ......................................................... 268
10.5.3
Special Requirements for Bonding with Cyanoacrylates .... 269
10.5.4
Internal and External Mould Release Agents .................... 269
10.5.5
Successful Joint Design ...................................................... 269
10.6
Bonding to Silicone Rubber ............................................................. 270
10.7
Environmental Resistance ............................................................... 270 10.7.1
Glass Bonding ................................................................... 272
10.7.2
Hot Strength ..................................................................... 272
10.8
Activators ........................................................................................ 274
10.9
Application Methods for Cyanoacrylates ........................................ 275 10.9.1
Pressure/Time Systems ....................................................... 275
10.9.2
Syringe Systems ................................................................. 276
10.10 Health and Safety and Handling Precautions .................................. 276 10.11 Typical Applications........................................................................ 277 10.11.1 Bonding Nitrile, Polychloroprene and Natural Rubbers .... 277 10.11.2 Bonding EPDM ................................................................. 277 10.11.3 Bonding Santoprene and Silicone Rubbers ........................ 279 10.11.4 Bonding Medical Devices .................................................. 279 10.12 Troubleshooting .............................................................................. 280 10.12.1 Blooming of Cyanoacrylates ............................................. 280 11 Bonding Silicone Rubber to Various Substrate ............................................ 285
viii
11.1
Introduction .................................................................................... 285
11.2
Why Bond Silicone Rubber? ............................................................ 286
11.3
Material Combinations of Interest - Examples ................................ 287 11.3.1
Silicone to Silicone Bonding (Soft and Soft) ...................... 287
11.3.2
Silicone to Plastic Bonding (Soft and Hard) ...................... 288
11.3.3
Silicone to Metal Bonding (Soft and Hard) ....................... 288
11.3.4
Why Use Silicone Rubber for Such Composites? ............... 288
Contents
11.4
Some Applications of Silicone Rubber Composites ......................... 290
11.5
Bonding Concepts ........................................................................... 291
11.6
11.7
11.8
11.9
11.5.1
Undercuts .......................................................................... 291
11.5.2
Primers .............................................................................. 292
11.5.3
Self-adhesive Silicone Rubbers .......................................... 292
11.5.4
The Build-up of Adhesion ................................................. 292
Bonding of Liquid Rubber (LR) ...................................................... 293 11.6.1
Properties of Self-adhesive LR ........................................... 297
11.6.2
Limitations of Self-adhesive LR ......................................... 298
Bonding of Solid Rubber (HTV) ..................................................... 299 11.7.1
Self-adhesive HTV Silicone Rubber Applications .............. 299
11.7.2
Applications for Self-adhesive HTV .................................. 301
11.7.3
HTV Used in Other Bonding Applications ........................ 303
Processing Techniques ..................................................................... 303 11.8.1
Liquid Rubbers in Inserted Parts Technology .................... 303
11.8.2
LR in Two-component Injection Moulding Technology (Two Colour Mould) ......................................................... 306
Silicone to Silicone Bonding (Soft and Soft) .................................... 308
11.10 Cable Industry ................................................................................ 309 11.11 Duration of Bonding Properties ...................................................... 309 11.11.1 Duration of Bonding - Chemically Bonded Composites .... 311 11.12 Alternatives to Injection Moulding ................................................. 313 11.12.1 Adhesives .......................................................................... 313 11.12.2 Welding ............................................................................. 313 11.12.3 Mechanical Bonding Techniques After Moulding .............. 314 11.13 Summary ......................................................................................... 314 12 Failures in Rubber Bonding to Substrates ................................................... 319 12.1.1
Introduction ...................................................................... 319
12.1.2
Incorrect Moulding Procedures ......................................... 328
ix
The Handbook of Rubber Bonding
12.1.3
Incorrect Production Quality Testing Procedures .............. 329
12.1.4
Corrosion in Service .......................................................... 330
12.1.5
Product Abuse ................................................................... 333
12.1.6
Other Failure Modes ......................................................... 333
12.1.7
Factors Affecting Adhesion of Rubbers ............................. 334
12.1.8
Topography of Substrate ................................................... 335
12.1.9
Surface Conditions of Adherend ....................................... 335
12.1.10 Classification of Rubber According to their Wettabilities .. 336 12.1.11 Bonding - Interphase or Interface Considerations ............. 337 12.1.12 Problems in Adhesion........................................................ 339 12.2
12.3
Rubber Bonding in Power Transmission Belting ............................. 339 12.2.1
Introduction ...................................................................... 339
12.2.2
Power Transmission Belt Failure Modes ............................ 340
12.2.3
Adhesion Systems in Power Transmission Belts ................. 346
12.2.4
Adhesion Testing in Power Transmission Belts .................. 347
Undesirable Adhesion Occuring Under Service Conditions (Fixing) .. 349 12.3.1
Factors Affecting ‘Fixing’ .................................................. 349
12.3.2
Prevention of ‘Fixing’ ........................................................ 351
12.3.3
Other Methods of Preventing ‘Fixing’ Examined Experimentally ................................................. 351
Abbreviations and Acronyms............................................................................. 357 Author Index ..................................................................................................... 363 Company Index ................................................................................................. 371 Main Index ........................................................................................................ 373
x
Contributors Derek Brewis Loughborough University, Institute of Surface Science and Technology, Department of Physics, Loughborough, Leicestershire, LE11 3TU, UK. Richard Costin The Sartomer Company, 502 Thomas Jones Way, Exton, PA 19341, USA. Bryan Crowther 49 The Avenue, Bengeo, Hertford, Hertfordshire, SG14 3DS, UK. Kenneth Dalgarno School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UK. Steve Fulton OMG Limited, Ashton New Road, Clayton, Manchester, M11 4AT, UK. Robert Goss Henkel Loctite Adhesives Limited, Watchmead, Welwyn Garden City, Hertfordshire, AL7 1JB, UK. Jim Halladay Lord Corporation, Chemical Products Division, 2000 West Grandview Boulevard, PO Box 10038, Erie, PA 16514-0038, USA. Richard Holcroft 5 Brooklands Drive, Birmingham, West Midlands, B14 6EJ, UK. Peter Jerschow Wacker-Chemie GmbH, Johannes-Hess Strasse 24, D-84489 Burghausen, Germany. Rani Joseph Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin 682022, Kerala, India. Mike Rooke Henkel Loctite Adhesives Limited, Watchmead, Welwyn Garden City, Hertfordshire, AL7 1JB, UK.
Commercial rubbers
The Handbook of Rubber Bonding
Berndt Stadelmann Wacker-Chemie GmbH, Johannes-Hess Strasse 24, D-84489 Burghausen, Germany. Walter Strassberger Wacker-Chemie GmbH, Johannes-Hess Strasse 24, D-84489 Burghausen, Germany. Wim van Ooij Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA. Patrick Warren Lord Corporation, Chemical Products Division, 2000 West Grandview Boulevard, PO Box 10038, Erie, PA 16514-0038, USA. Ron Woodcock 5 Lower Leicester Road, Lutterworth, Leicester, LE17 4NF, UK. David Wootton 95 Greenhill Road, Bury, Lancashire, BL8 2LL, UK Keith Worthington Chemical Innovations Limited (CIL), 217 Walton Summit Centre, Bamber Bridge, Preston, PR5 8AL, UK.
Introduction
Although many volumes of information have been published about the subject of adhesion of materials in general, it is some forty years since a publication has been devoted solely to the subject of the bonding of rubbers to various substrates. Three very successful Rapra Technology, conferences on the subject of the bonding of rubber have shown that there is clearly a need for such a publication to be devoted to this topic of wide industrial significance. Although from time to time manufacturers of bonding agent systems publish papers in trade journals there is generally a dearth of available information for the factory practitioner to consult. The subject matter for this present volume has been selected to cover a wide range of interests, both in terms of products and applications. Rubbers in many applications need the support of, or reinforcement by, a variety of materials ranging from fibres to metals. To ensure optimisation of the properties from these composites it is necessary to ensure that the optimum adhesion levels are achieved, both initially and to be maintained throughout the service life of the products. Rubbers are bonded to a variety of substrates in many products, in numerous applications, to meet the needs of the modern world. The Rubber Bonding Handbook draws together the expertise of a number of world authorities engaged in improving the bonded product to meet the ever increasing demands placed on composites and components manufactured from rubbers bonded to metals, fabrics, fibres and plastic substrates. The papers included in this volume have been written by experts in their fields, many of whom have world-renowned reputations. Thus the information they include in their chapters can be considered to be the most up-to-date, state-of-the-art discussions of their respective areas of research and knowledge. The topics range from in depth discussions of such fundamental topics as the mechanisms of bonding of rubbers to brass, bonding techniques for adhesion to fabrics through to methods of preparation of substrates and the development of bonding agent systems for adhesion to metals and plastic substrates. Bonding with silicone rubbers and cyanoacrylate adhesives for post vulcanisation bonding are also included. A section dealing with information related to adhesion, failure and other adhesion related topics such as ‘fixing’ and practical reasons for a variety of bond failures, either during production or service are also covered.
1
The Handbook of Rubber Bonding Although there is some discussion of relevant theory in various sections of text, the emphasis in this volume has been to concentrate on the practicalities of bonding of rubbers, to themselves and substrates. It is considered that this type of information is of immediate interest to the practising technologist dealing with shop floor problems on a daily basis. It is hoped that the publication of definitive papers on the subject of adhesion of rubbers will be of considerable value to the practitioner in factories engaged in the previously seldom discussed variety of bonding applications being carried out by the rubber industry. Because of the legislation now in progress of being implemented by the rubber industry to eliminate sources of environmentally hazardous chemicals, there is information on the development and applications of waterborne bonding systems.
Acknowledgements I would like to express my appreciation of the help and assistance given to me in the editing of this publication. To Claire Griffiths (Editorial Assistant), Sandra Hall for typesetting, to Steve Barnfield for the cover design, Rebecca Dolbey for editorial advice and particularly to Frances Powers (Commissioning Editor), for her support, patience and guidance on general editorial matters. Bryan Crowther November 2000
2
1
Substrate Preparation Methods B. Crowther (Section 1.1) D. Brewis (Section 1.2) K. Worthington (Section 1.3) R. Holcroft (Section 1.4)
1.1 Metal Preparation – General Techniques
1.1.1 Structure of Metal Substrates – Metallography There is little written about the subject of metallography with respect to the bonding characteristics of the various metals used within the hot bonding process carried out by the general goods, rubber to metal bonding profession. Some work has been carried out in the field of adhesives for aeronautical applications [1]. In general only a few of the metals or adhesives described for this type of bonding have much application in the rubber to metal bonding factory, except perhaps if one is post vulcanisation bonding. The lack of fundamental metallography studies in the hot bonding of rubbers to metals is mainly due, no doubt, to the lack of influence which the bonding technologist has in these matters. He is usually told the grade of metal to be used and proceeds to find the best way, according to current factory processes, equipment, practices and experience, to deal with the problem. He can of course discuss the nature of his problem with his bonding agent supplier, who can in turn consult his research department if the problem is really abstruse. Perhaps a better understanding of metallography would enable the factory technologist to choose the best way to pre-treat his customer-dictated metal for his factory processes, or to discuss his customer’s ‘real’ metal requirements. To understand some of the problems associated with the achievement of good rubber to metal bonds it is worth considering some of the scenarios involving the atomic structure of metals at their surfaces. A metal, or an alloy of metals, naturally assumes a crystalline structure and it is likely that it will have a regular shape and lattice structure, with some voids in the interstices. As with rubber compounds, metals are formed by mixing a number of components together which disperse relative to each other, but never, except maybe in the case of pure metals, become one totally uniform uninterrupted phase. Most metals are used as some type of alloy, i.e.,
3
Commercial rubbers
The Handbook of Rubber Bonding steel consists of iron mixed with carbon in varying proportions to produce the different grades of commercial product. Also minor proportions of other metals are added to give different processing and end use characteristics to the steel, e.g., chromium, manganese, molybdenum, nickel, tungsten. The finishing processes of steels can also seriously alter the ability of adhesive to bond to them, due to the altered surface microstructure. With a pure metal its strength will depend on the size of the crystals making up its structure. In general small crystals make strong metals, whilst metals with large crystals, such as zinc, are weaker. The strength of a metal is also affected by the amount of impurity which may be present, as the impurities tend to arrange themselves at the interfaces between the crystals, thus preventing perfect crystal contact. In most metal alloys, as with rubber alloys or blends, the individual metals remain in discrete, but dispersed domains within the metal alloy structure. In an alloy the metal crystals involved, during cooling, have different shrinkage values and thus tend to move apart, allowing either voids to occur or when chemically hardened, other metals to infiltrate into these voids or interstices, at or near the surface. The individual crystals of the metals during cooling and shrinkage can join together to form chain structures, giving interlocking of the various metallic crystals. In some metal mixtures there is a mutual solubility and in these cases all crystals of the metal are the same. Although as a rubber to metal bonder one is not very interested in the metals structure within the mass of the metal, one must consider what is happening in and on its surface layers. Most metals form oxide layers on their surfaces, some of which, like iron are porous and thus continual oxygen ingress enables the oxide layer continually to increase whilst in aerobic conditions. Other metals such as aluminium form a dense oxide film which does not permit oxygen ingress and thus protects the metal underneath from further oxidation. Both metals types are being oxidised, albeit at different rates and this oxidation can be termed as a form of corrosion. Although the rubber to metal bonder must take the precautions necessary to prevent this type of corrosion continuing under his processing conditions, once the bonding agent has been applied, the condition at the metal interface becomes anaerobic and thus further oxidative corrosion is prevented (see Sections 12.1.2.2 and 12.1.5). There are a great variety and complexity of steel microstructures available to the component specifier, which complicate any cleaning procedure carried out prior to bonding. Incorrect chemical cleaning of low carbon and stainless steels, for example, can result in iron oxide ‘smutting’ of the surface leaving a deposit difficult to remove entirely [1] during metal cleaning. These deposits may subsequently give an extremely weak bonding surface and, as a result a bonded product which fails easily under low working stresses in service.
4
Substrate Preparation Methods However, as far the rubber to metal bonder is concerned he must avoid situations which can cause galvanic corrosion, a far more serious condition, which can propagate under the bonding system to cause eventual degradation of the bond and inevitable failure. Galvanic corrosion [2] is caused by the formation of an electrolytic cell between the different metal crystals within a structure in the presence of such agents as acids and salt water. Acids can be generated from degeneration of compounding materials or cleaning and degreasing fluids (see also Section 12.1.4). Certain metals are manufactured for their ability to prevent corrosion, e.g., stainless steel, and they contain chromium to enhance their corrosion resistance. If the level of chromium used in the metal’s makeup is high, then the very tough layer of chromium oxide, which forms on the metal’s surface as the anti-corrosion layer, is also exceedingly difficult to bond to rubbers. Wherever possible therefore stainless steels to be used in conjunction with bonding systems for rubbers should contain as low a chromium level as possible.
1.1.2 Bonding The bonding mechanisms of the multiphase systems involved in making a rubber to metal component are complex and the chemistry of the reactions involved not totally disclosed or understood. In the region of the metal contact the interactions are deemed to be a combination of mechanical and chemisorption processes. From the patent literature and some of the more recent reviews of rubber to metal bonding [3, 4], it can be seen that the primers contain a variety of halogenated rubbers and resins, which are known to have a high ability to wet out metal surfaces, thus ensuring the greatest degree of interface contact. In addition these rubbers and resins act as barriers to the migration of external corrosion catalysts of the metal surface. The resins and rubbers probably form an interpenetrating network of polymer chains within the adhesive system, thus giving strength and structure to the primer and rubber bonding coats. Bond quality depends to a large extent on the ability of all interfaces to freely exchange chemical entities. Any contamination of surfaces will upset the surface chemistry at that point and will reduce the bond strength.
1.1.3 Rubber Component with Metal Support Engineering products for a wide range of applications are made by the use of rubbers bonded to metals during the vulcanisation of the rubber. The quality of bond achieved during the manufacture of this type of component must be of sufficient integrity, not only to be stronger than the rubber itself, but also to outlast the active life of the rubber constituent of the components. To this end, the design of the component and
5
The Handbook of Rubber Bonding metal part must be carefully considered to ensure that no undue stress concentrations are created in the area of the bond between the rubber and the metal. Components consisting of moulded rubber bonded to metal, carried out during high temperature vulcanisation, can have inherent stresses simply due to shrinkage of the rubber when cooling from the vulcanisation temperature and the coefficient of thermal expansion relationship of the rubber/metal combination. The ‘shrinkage’ of the rubber in the system will be different for each type of rubber being used and is dependent also upon the compound hardness, or degree of filler present. Allowances for the rubber shrinkage must be made in determining the shape of the mould cavity and hence the component’s final shape. The environment in which the component is to work will also affect the stresses to which the rubber-metal bond will be subjected. Some oil and solvent environments will penetrate a bond at the interface and thus may weaken or destroy the integrity of the bond until the stress becomes relieved by failure. Corrosion of the metal component of the bonded unit by salt environments can also be a major problem and thus due concern and allowance must be made for the service conditions in which the rubber to metal component will be resident. Corrosion of the bonded metal under the bonding system can also occur if the metal pre-preparation is carried out with acidic degreasing fluids. Care must be taken that degreasing fluids are and remain, neutral in pH throughout their use in the application. Recovery of used solvents and redistillation can significantly change the pH of a solvent. This can be a particular problem with chlorinated solvents, where after redistillation the distillate can be acidic in nature. To effect good long-lasting bonds between rubber and metals it is essential that both materials presented to the interface be clean and free from detritus. The presence of oils and the possibility that compounding ingredients can exude or bloom from the rubber surface, before or after moulding, or during the service life of the component must also be taken into consideration and remedied.
1.1.4 Metal Pre-treatments Metals must be suitably pre-treated for satisfactory bonds to be achieved with rubbers. Two basic methods of preparation are used: • Mechanical, • Chemical.
6
Substrate Preparation Methods
1.1.4.1 Mechanical Methods Metals, especially the more common iron and steel types, come from the foundry and metal plate stamping shop, coated with oil, grease and most often with a generous layer of oxide and rolling mill scale formed on the exposed surfaces. Oxide films can also develop further during storage prior to use by the bonding shop. All these materials must be removed from the surfaces and from the voids in the metal, to ensure that the oils and greases which otherwise may be trapped unseen cannot exude under the increased temperature of vulcanisation, when they become more mobile or volatile. Surface oxides must be removed for they are often only loosely structured in their attachment at the metal substrate and will rupture and detach themselves under duress, causing the metal/ adhesive bond to fail. Once the original oxide layer has been removed, the freshly exposed metal will immediately start to build a new oxide film which must be minimised by rapid degreasing and application of primer/adhesive coat.
• Initial degreasing Metals must be degreased as the first step in any metal preparation process, otherwise oil and grease contamination of blasting media, chemical treatments and machinery can result in severe factory quality problems and unreliable and variable bonding. Traditionally the most usual method of grease and oil removal from the metal surface has been by degreasing in the vapour of a chlorinated solvent such as trichloroethylene or 1,1,1-trichloroethane or perchloroethylene. The chlorinated solvent used must have a neutral pH, otherwise the acidic condition can cause the initiation of underbond corrosion. Re-distilled chlorinated solvents, especially if recovery is carried out in-house, must be adequately checked for neutrality. The metal parts must dwell in the solvent vapour until such time as the metal reaches the temperature of the vapour and condensation has ceased. The solvent will have had the best opportunity to work at its most efficient in grease removal under these conditions. Direct contact with the degreasing solvent is not an efficient way of removing greases from metal surfaces, always leaving a molecular layer at least, still lying on the ‘cleaned’ surface. This cleaning method should not be used for metals to be used in bonding. All air lines in the bonding shop must have oil/water filters connected to them to remove the possibility of oil/water emulsion being sprayed onto the metal surfaces before, after or during bonding agent application. Air compressors are notorious for allowing oil seepage into the pressure vessel, together with an amount of water, which then usually causes an oil/water emulsion to be formed. This emulsion in contact with cleaned metal surfaces will give corrosion or reduce bond formation to a minimum level through the deposit of a film of oil.
7
The Handbook of Rubber Bonding The current legislation trend and environmental pressure for the industry is to move towards the use of alternative means of removal of contaminants from the metal surfaces (see Section 1.4). Equipment is available which uses water and detergents to remove these oils and thus present a more environmentally favourable working atmosphere. The action of the detergent can be supplemented by the use of ultrasonic agitation to remove oxide flakes. These systems being water-based require efficient drying of the metals, especially in the areas between contacting metals, otherwise further oxidation of the cleaned metal will rapidly take place. Careful choice of the detergent is also necessary otherwise its residues can detract from the bond strength achievable. The water quality being used in the degreasing system final wash process will have to be determined to prevent deposit of any salts or metallic ions. The ideal final wash is with de-ionised water. Alternative solvents, if used in a vapour degreasing system must have a similar evaporation rate to that of the presently used chlorinated solvents. Otherwise too rapid evaporation of the condensed solvent on withdrawal of the metals from the solvent vapour will result in rapid surface cooling of the metal, with resultant condensation of water, especially in conditions of high humidity.
• Alkaline removal of oils and greases An alternative method of removal of the metal preparation oils and greases is to use an alkaline cleaning method. The alkaline solution is used either in dip tanks or tumbler spray units (see Section 4.1). The strength of alkaline, the temperature used and the necessary dwell time in the solution to remove the amount of grease encountered will be determined in individual factories. The length of time required for oil and grease removal can be anything up to two hours. The alkaline tanks have to be followed by water rinse tanks to remove the alkaline dip from the metals, followed by drying.
• Solvent dip methods for large scale removal of greases Solvent dip methods are generally expensive to run and do not usually, unless a number of dip tanks are used, completely remove oils and greases from the metal surfaces. Contaminants are easily carried from tank to tank and it is difficult to ascertain whether the metal surface is completely cleaned after its passage through the tank series. This method would not normally be used for anything other than small scale operations. Fast drying solvents such as methylene chloride and acetone evaporate so quickly that they lower the temperature of the metal surface and water condenses.
8
Substrate Preparation Methods
• Removal of surface oxides Metals, after degreasing, have to be blasted with a sufficiently abrasive material to remove the surface oxidation layer. The usual medium used for ferrous substrates is steel or chilled-iron grit to BS EN ISO 11124-4 grades G12 to G24 [5] (see also Section 4.2.2). Alumina or other non-ferrous grits such as quartz sand and carborundum may be used on ferrous metals, but their use on non-ferrous metals is essential to prevent the possible formation of galvanic cells. Initially impingement of the metal surface with abrasive grit has the effect of gouging the surface of the steel to give a larger surface area for bonding, but with use the grit wears and its efficiency decreases. The type of grit used must be coupled to the type of metal being treated. Incorrect grit/ metal combinations can lead to formation of galvanic cells remaining on the surface of the blasted metals and the commencement of underbond corrosion. Grits larger than about 30-50 mesh diameter soon lose their irregularities and grittiness, effectively turning into shot at which stage they must be discarded. The hardness of the steel grits should be a Rockwell C hardness of 60 – 65. Iron or steel shot should not be used as these tend to give cavitation of the blasted metal surfaces, followed by peaning over of the sharp metal pinnacles, often trapping loose shot, blasted material, etc., in the peaned over cavities. These cavitations and their contents cause weaknesses and possible underbond corrosion sites, resulting in ultimate failure in service. The service life of the blasting media should be established for efficiency and quality of surface finish. Grit in use should be cleaned of dust resulting from removed oxide scale and its own degradation products and be downgraded or discarded if it becomes too worn. Revolving drum blast machines give the best production efficiency for metals which are stout enough to resist damage from the tumbling action involved. The metal parts are tumbled on a rubber belt inside a revolving drum whilst being bombarded with the abrasive medium. Once the metal surface has been adequately cleaned of oxide contamination, dusted off and once more degreased, it is vital that the application of a bonding agent primer coat be carried out as quickly as possible to ensure that the re-oxidation of the metal surface is kept to a minimum. Ambient temperature, humidity and dust must all be controlled if the optimum bond strength is to be achieved. To consistently ensure optimum bond quality, metal components, whether unprimed or primed, should be kept in enclosed cabinets. At no time should cleaned and degreased metals be handled with bare hands. Human skin, however clean it may appear, always carries a surface layer of oils and fats, which are bond killers. Neither should metals, whether in the ‘just cleaned’ state, or
9
The Handbook of Rubber Bonding treated with bonding agent, be handled with ‘press gloves’. Press gloves are usually heavily contaminated with a variety of materials, from oil, to mould release agents and sweat. Clean, frequently discarded cotton gloves are the best protection for handling metals. They should not be allowed to become dirty and sweat ridden.
1.1.4.2 Chemical Methods The alternative metal pre-treatment processes to grit blasting use a variety of different chemical routes. It is sufficient to say here that these can be very efficient, but do occupy rather large factory floor areas and can, if not controlled correctly give variable quality of prepared surface. The usual chemical pre-treatment systems consist of acid etching of the surface followed by several water dips and subsequent phosphate or in some circumstances cadmium plating and passivating (render inert). Many of these treatments will have been carried out by the metal processor and are not the rubber bonder’s processes.
• Treatments for stainless steels (see also Section 3.3) There are various systems for the pre-treatment of stainless steels which consist of treating the metal surface with strong acids to attack crystal grain boundaries in the alloys and chromium poor regions around chromium carbide particles. All the methods give surface roughness to the stainless steel which enhances the bond to the adhesive. Mixtures of nitric, hydrofluoric, sulphuric or chromic acid are suggested as most suitable. However, the nature of the substrate alloy and the heat treatment experienced all have a bearing on the bondability of the metal.
• Phosphate coating (see also Sections 1.2, 3.3 and 4.2.5) Steel is often phosphate coated for use within the engineering and decorative laminate industries to reduce corrosion. Iron or zinc phosphate can be used. However, although used for some years as a corrosion protection technique for rubber to steel bonding, it can be difficult to control the process, with a resultant variable thickness of phosphate deposit of varying crystalline structure. If too thick a phosphate layer is obtained it becomes too friable and lacking in the cohesive integrity required to maintain a rubber to metal bond under load during service. If only a moderate phosphate coat is produced it is often necessary to ‘passivate’ the areas of steel, only minimally covered or lacking in a coating of phosphate, by treating with chromic acid to form chromium oxide to prevent corrosion. However, chromium oxide does not readily react with a bonding agent (see Section 3.1). Chromic acid is a restricted material and alternative materials can be recommended by bonding agent suppliers for the passivation or sealing of the phosphate coating.
10
Substrate Preparation Methods The nature of the phosphate deposited on the surface of the steel depends to a large extent upon the nature of the microstructure of the steels and the orientation of its underlying crystal lattice. Hardened steels having a martensite structured surface (consisting of interlacing rectilinear fibrous elements arranged in a triangular shape) support a fine flake phosphate structure. Cold-rolled steel can, having acquired a different surface orientation structure during the rolling process, acquire a lumpy large flake phosphate structure, which is easily broken apart under service stress. Any water going to drain from these processes is a potential pollution hazard and must be tested for zinc content, as this is a hazardous material. Any zinc present must be removed or limited to 1 – 2 parts per million.
• Zinc coating or galvanising To be effective the zinc coating must be hot dipped onto the freshly cleaned metal, to give a ‘galvanised’ finish. Bonding to this finish is not easy, but sometimes demanded by the component specifier. The crystalline structure of the galvanised zinc and its dipped coating thickness, can result in the flaking off, under stress, of some of the coating, resulting in bond failure (see also Section 1.1.1). The recommended treatment [6] for cleaning a galvanised finish is a) degrease metal part b) abrade the galvanised surface with grit c) degrease then apply adhesive as soon as possible or a) immerse in a solution of 20 parts by weight concentrated hydrochloric acid with 80 parts by weight de-ionised water, for 2 – 4 minutes at 25 °C b) rinse thoroughly in cold, running de-ionised water c) dry for 20 – 30 minutes in 70 °C oven d) apply adhesive as soon as possible The second method of zinc coating is more widely used.
• Zinc sheradising A method used to give what is in effect a fused zinc surface to a steel component can be specified which gives very good environmental protection for the steel component.
11
The Handbook of Rubber Bonding The steel part to be bonded is baked whilst being tumbled in zinc dust. The process is not generally suitable for delicate metal parts and causes problems with zinc build-up in screw threaded components (the latter must be protected by a sleeve or require a die running down the thread to clear it). After treatment exposed zinc surfaces do of course oxidise if stored incorrectly, but this is not usually a problem. The oxide forms after both methods of zinc coating.
• Aluminium - anodising Aluminium is usually electrolytically anodised, in the presence of an acid, either sulphuric, chromic or phosphoric, to give a tough resistant oxide film, which usually forms good bonds with the usual bonding systems. The anodising must be carried out with care and with a mind to the type of crystalline structure being formed on the aluminium surface. A uniform reticulated structure is desired, not a microscopically fragmented rippled surface, sometimes called ‘ice flows’ [7], which are unstable, easily fractured, and therefore too unstable to maintain good adhesive quality. If anodising is to be carried out by a custom plater he will need to be informed of the type of anodised structure desired. N.B. The final stages of any ‘wet’ metal preparation process for metals to be bonded to rubber is to ensure that all chemicals used in the processes have been removed in the final water rinse tank, and then to ensure that all faces of the metal parts are fully dried prior to bonding agent application. All warehouse metal storage areas must be held at least 5 – 10 °C above the dew-point and ideally as near to ambient temperature in the bonding agent application shop which should be in the region of 18 – 22 °C minimum.
• Metal preparation - for waterborne bonding systems Although the general principles used for solvent-based adhesives apply, the cleaning of metals for the application of waterborne bonding systems becomes much more critical. Scrupulously clean metals are vital, to ensure maximum wettability of the prepared metal bonding surface. Lord Corporation [8] suggest that calcium modified phosphating of metals is preferable to conventional grit blasting with its potential for ‘re-infecting’ the metal surface after initial degreasing by using contaminated grit. Proper housekeeping should eliminate such problems.
1.2 Pre-treatments of Plastics and Rubbers 1.2.1 Introduction In many cases, rubbers are joined to other materials during the process of vulcanisation. However, in other cases, rubbers are joined to other materials after vulcanisation. With this
12
Substrate Preparation Methods second group, it is often necessary to pre-treat the rubbers before bonding. Pre-treatments range from physical methods such as a solvent wipe or abrasion to chemical methods such as treatment with trichloroisocyanuric acid (TCICA). Physical methods may remove cohesively weak layers from the polymer. This is essential to good bonding unless these layers can be absorbed by the adhesive. However, physical methods will only be effective if the underlying rubber possesses suitable groups which can interact strongly with the adhesive. Chemical methods may also remove weak layers or chemically modify them so that they are more compatible with the adhesive; in addition chemical methods may roughen a surface. However, an effective chemical method will also modify the chemistry of the rubber so that the interaction with the adhesive is increased. In general, rubbers contain a greater variety and quantity of additives than plastics; fifteen components in a particular formulation is quite common. These additives or compounding ingredients as they are often called, may well create a cohesively weak layer on the rubber surface. On the other hand, plastics usually contain a small number of additives and usually in relatively low concentration. Over the last 50 years many methods have been developed to pre-treat plastics and rubbers. Partly because of the much simpler formulations, pre-treatments for plastics have been the subject of much greater scientific interest. Our understanding of pretreatments for plastics is therefore much greater than that for rubbers. Some of the key studies on pre-treatments of plastics will therefore be outlined in Section 1.2.2. Pre-treatments for rubbers have been developed on an empirical basis but some scientific studies of successful pre-treatments have been undertaken. Methods for different rubbers will be reviewed in Section 1.2.3. Rubbers will be considered in groups, namely hydrocarbons that possess little unsaturation, unsaturated hydrocarbons, halogenated rubbers and miscellaneous materials.
1.2.2 Studies of Pre-treatments for Plastics These studies may seem out of context in a book concerned with bonding of rubber but the great deal of work carried out with plastics can be used to understand the problems of rubbers. Some of the most important pre-treatments for plastics were developed in the 1950s. These include the corona and flame treatments for polyolefins [9, 10, 11, 12, 13] and the use of sodium complexes for fluorinated polymers [14 – 17]. The plasma treatment was developed somewhat later [18, 19], as was halogenation [20, 21]. It was suspected that these treatments were chemically modifying the surfaces of the plastics but there was little direct evidence as the analytical methods available at the time were not 13
The Handbook of Rubber Bonding sufficiently surface-sensitive. However, in the 1970s a new method for studying surface chemistry became available, namely X-ray photoelectron spectroscopy (XPS) which is also known as electron spectroscopy for chemical analysis (ESCA). This method is able to characterise and quantify the chemical changes caused by pre-treatments. XPS analyses the first few atomic layers of a material. This is important as some pre-treatments only modify a few nanometers of a polymer. Reflection infrared techniques in the 1970s were often unable to detect changes to the surface chemistry of polymers caused by the pre-treatments. Three of the earliest pre-treatment studies were by Dwight [15], Collins [22] and Briggs [23]. Dwight treated polytetrafluoroethylene (PTFE) and fluorinated ethylene-propylene copolymer (FEP) with sodium in liquid ammonia and sodium naphthalenide in tetrahydrofuran (THF). X-ray photoelectron spectroscopy showed extensive defluorination of the polymers together with formation of carbon–carbon double bonds and various oxygen-containing groups. Collins treated PTFE with ammonia and air plasmas. Again, XPS showed extensive defluorination and in the case of the ammonia plasma, nitrogen containing groups were introduced. Briggs [23] was the first to quantify the chemical modification caused by a pre-treatment. Briggs studied the changes caused by chromic acid etching of low density polyethylene and polypropylene. Some of the results are given in Table 1.1. Angular variation studies, i.e., the angle of incidence of the X-ray beam was varied, showed that in the case of polypropylene, the depth of the chemically modified layer was only a few nanometers.
Table 1.1 XPS data for polyolefins treated with chromic acid [23] Polymer
Treatment
LDPE
PP
Surface composition (atom %) C
O
S
None 1 min/20 °C 6 h/70 °C
99.8 94.4 85.8
0.2 5.2 13.1
0.4 1.1
None 1 min/20 °C 6 h/70 °C
99.8 93.4 94.0
0.2 6.3 5.7
0.3 0.3
NB: The sulphur originates from the attack of the polyolefin by the sulphuric acid present in the chromic acid LDPE: low density polyethylene PP: polypropylene C: carbon O: oxygen S: sulphur Reproduced with permission from D. Briggs, D. M. Brewis and M. B. Konieczko, Journal of Materials Science, 1976, 11, 7, 1270. ©1976, Kluwer Academic Publishers
14
Substrate Preparation Methods A given pre-treatment may result in the introduction of several different chemical groups. There are two methods by which these groups may be quantified and both involve XPS. The first method involves derivatisation reactions and the second method the use of high resolution spectra. The basic idea behind the derivatisation method is to use several reagents each of which will react with only one of the groups introduced by the pre-treatment. There are two other requirements. Each reagent should introduce an atom, e.g., fluorine, that is not already present in the surface and each reaction should proceed to 100% conversion. The method is illustrated by the work of Gerenser [24] where some corona treated polyethylene was derivatised. The reagents and derivatisation reactions are shown in Figure 1.1 and the results of the experiments are shown in Table 1.2.
Figure 1.1 Derivatisation reactions to identify functional groups introduced by pretreatments; a) peroxide groups reacting with sulphur dioxide, b) alcohol group reacting with hexafluoroacetic anhydride, c) carbonyl group reacting with hydrazine, d) epoxide group reacting with hydrogen chloride, e) carboxylic acid group reacting with tertiary amine. (Reprinted from L. J. Gerenser, J. F. Elman, M. G. Mason and J. M. Pochan, Polymer, 1985, 26, 8, 1162. ©1985, with permission from Elsevier Science)
15
The Handbook of Rubber Bonding
Table 1.2 Quantification of surface functionalities after corona treatment using derivatisation Functional group
Group conc. X 102 * Initial
Washed
C
OOH
1.2
0.9
C
OH
1.7
1.1
C
O
1.8
0.9
C
2.3
1.1
1.6
0.8
NO3
0.8
0.4
Total [O] tagged
13.8
7.7
Actual [O] incorporated
~18
~10
O C
O C
OH
Footnote: Allowing for the fact that some of the groups contain more than one oxygen atom, it can readily be calculated that the concentration of oxygen atoms involved in the derivatisation reactions was 13.8%; this is the amount of oxygen tagged. The actual amount of oxygen incorporated the corona treated surface was found by XPS to be 18%. This means that other oxygen-containing groups were present and/or the reactions with the above groups did not go to completion. *Moles of functional species per unreacted initial carbon atom Reprinted from L. J. Gerenser, J. F. Elman, M. G. Mason and J. M. Pochan, Polymer, 1985, 26, 8, 1162. ©1985, with permission from Elsevier Science
The second method to quantify the chemical groups introduced by a pre-treatment involves obtaining a high resolution spectrum of the photoelectrons from the C1s core level and resolving this into the various contributions. This approach is illustrated by Beamson [25] who examined a rubber-modified polypropylene which had been subjected to a corona discharge treatment. The high resolution C1s spectrum is given in Figure 1.2 and
16
Substrate Preparation Methods
Corona treatment after derivatisation
the information on the groups introduced is given in Table 1.3. This method is much quicker than the derivatisation approach but requires an instrument with very good energy resolution and great care in attribution of the various peaks.
Figure 1.2 High resolution C1s spectrum of corona treated polypropylene [25]
Table 1.3 Assignment of peaks for corona treated polypropylene [25] Peak no.
Position (eV)
Area (%)
Assignment
1
285.0
91.7
C-C, C-H
2
286.5
1.2
C-O
3
287.1
2. 3
C-O-O
4
288.1
2.3
C=O
5
288.9
1.2
COOH
6
289.5
1.3
O=C-O-C=O *
* The assignment at 289.5 eV is tentative
17
The Handbook of Rubber Bonding Strobel [12] compared the effectiveness of various gas-phase reactions for polypropylene, by determining how much oxygen was introduced into the polymer surface (the O:C atomic ratio) in a given time. These results are summarised in Table 1.4. It can be seen that to achieve a given level of chemical modification, flame, corona and plasma require much shorter treatment times than ozone or UV or a combination of UV plus ozone. The pre-treatments described above represent just a few of the many studies relating to the mechanisms of pre-treatments for plastics. However, it is clear that much is known about pre-treatments of plastics relating to: • Quantification of the chemical changes caused by pre-treatments, • The depth of the chemical modification, • Identification and quantification of chemical groups, • The rate of chemical modification. In contrast, much less work has been done relating to the mechanisms of pre-treatments for rubbers.
Table 1.4 Surface analysis of treated polypropylene films [12] Treatment
Exposure time (s)
None
—
XPS O:C atomic ratio 0.0
Corona (1.7 J/cm2)
0.5
0.1 2
Corona (0.17 J/cm2)
0.05
0.0 7
Flame
0.04
0.1 2
Remote air plasma*
0.1
0.1 2
Ozone
1800
0.1 3
UV/air
600
0.0 8
UV/air plus ozone
600
0.1 4
*The plasma was produced by a microwave generator and passed 100 mm down a tube onto the polymer surface Reproduced with permission from M. Strobel, M. J. Walzak, J. M. Hill, A. Lin, E. Karbashewski and C. S. Lyons, in Polymer Surface Modification, Ed., K. L. Mittal, VSP, Utrecht, 1996, 233. ©1996, VSP BV
18
Surface analysis
Substrate Preparation Methods
1.2.3 Hydrocarbon Rubbers with Little or No Unsaturation 1.2.3.1 Ethylene-Propylene Rubbers Ethylene-propylene rubbers (EP) have low total surface energies with small polar components. As would be expected, the adhesion of paints and adhesives to untreated EP is poor. To achieve good adhesion to EP, the introduction of suitable functional groups is necessary unless a diffusion mechanism can operate. Bragole [26] found that UV treatment of EPDM coated with a thin layer of benzophenone resulted in large increases in the adhesion of acrylic, epoxy and urethane paints to the polymer. Ellul [27] subjected EPDM/polypropylene and natural rubber/polypropylene blends to various halogenation treatments, namely fluorine/carbon dioxide, sodium hypochlorite/ acetic acid and bromine water. With the natural rubber blend, there was a substantial uptake of fluorine, chlorine and bromine in the surface regions as indicated by energy dispersive X-ray analysis and with all three pre-treatments the adhesion to an acrylic tape was greatly enhanced. In contrast, with the EPDM blend, fluorine was the only reagent which reacted with the rubbers and only this treatment resulted in a significant increase in adhesion to the acrylic tape. The above results can be explained in terms of the different concentrations of carbon–carbon double bonds in the two blends. Substantial incorporation of chlorine and bromine could occur with the natural rubber-polypropylene blend but not with the EPDM blend. However, fluorine gas will react readily with saturated hydrocarbons [28, 29] and therefore the incorporation of fluorine into the EPDM blend is not surprising. Lawson [30] using X-ray photoelectron spectroscopy (XPS) found that trichloroisocyanuric acid (TCICA) in ethyl acetate did not chemically modify EPDM. Lawson [31] also found that a corona treatment improved the wettability of EPDM as indicated by glycerol contact angles and the use of a series of formamide/2-ethoxyethanol mixtures (ASTM D2578 [32]). However, the contact angles increased significantly over a period of one hour, indicating molecular rearrangement with the polar groups introduced by the pre-treatment tending to move to the bulk of the rubber. No improvement in a peel test involving a polyurethane coating was observed. Minagawa [33] treated an EP rubber with UV and sputter etching. Large increases in adhesion were reported. However, the treatment times were long, being 10 minutes for ion etching and one hour for the UV treatment. Scanning electron microscopy (SEM) indicated the two methods caused considerable roughening of the surface. XPS and Fourier transform infrared analysis (FTIR) indicated the introduction of substantial quantities of oxygen-containing functional groups. Kondyurin [34] noted only modest improvements, at best, after treating EPDM with UV, despite clear infrared evidence for the formation of hydroxyl and carbonyl groups after treatment.
19
The Handbook of Rubber Bonding
1.2.3.2 Butyl Rubber Butyl rubber consists of ≥95% of isobutylene units with a small quantity of isoprene which permits crosslinking via sulphur vulcanisation. Butyl rubber has a low surface energy and in addition organic components with a low cohesive strength may exist on the surface. In one study [35] butyl rubber was subjected to several treatments which normally cause substantial chemical modification to polymer surfaces. The treatments included chromic acid etching, corona discharges, flames, bromination, UV radiation and potassium permanganate. Most of the treatments had little effect on the adhesion to an epoxide. It was concluded that much chain scission occurred with the result that suitable functional groups were not introduced in sufficient quantity into long polymer chains. Such chemical modification is necessary for good adhesion unless a diffusion mechanism is operating.
1.2.4 Unsaturated Hydrocarbon Rubbers 1.2.4.1 Natural Rubber Natural rubber (NR), being essentially a hydrocarbon, has a low surface energy. Some of the components in a formulated rubber, such as zinc oxide and carbon black, may substantially increase the surface energy, whereas organic additives such as extender oil and antioxidants may migrate to the surface and create a potentially weak boundary layer. Pettit and Carter [36] found that chlorine gas, acidic sodium hypochlorite and an organic chlorine donor in a organic solvent all much improved the peel strengths of joints involving NR and a polyurethane adhesive. Oldfield and Symes [37, 38] found that aqueous or organic-based chlorination gave much higher joint strengths than a solvent-wipe, abrasion or cyclisation (see Table 1.5). Oldfield and Symes used X-ray fluorescence, infrared analysis and contact angle measurement to study the TCICA treatment. X-ray fluorescence showed the amount of chlorine introduced into the polymer increased with the TCICA concentration; with a 3% TCICA solution, they estimated the chlorine content in the treated NR was 16.7% w/w. Reflection infrared analysis indicated that chlorine substituted at the allylic position in the polymer backbone. Substantial improvements in wettability were achieved especially if the concentration of TCICA was at least 0.8%. Lawson [30] pre-treated various rubbers, including NR, with a 3% w/v solution of TCICA in ethyl acetate and used XPS to study the chemical changes caused by the pre-treatment. In agreement with Oldfield, they concluded that the chemical modification was mainly substitution rather than addition at the carbon–carbon double bond.
20
Substrate Preparation Methods Table 1.5 Effect of pre-treatment on the peel strengths (N mm-1) of NR-epoxide-NR [37] Pre-treatment
Peel strength
Locus of failure
Toluene wipe
0.1
I
Abrasion on grinding wheel
1
I
Acidified hypochlorite
10
R
Cyclisation
1
I
TCICA in ethyl acetate
18
R
I - apparent interfacial ; R - cohesive in rubber Reproduced with permission from D. Oldfield and T. E. F. Symes, Journal of Adhesion, 1983, 16, 2, 77. ©1983, Gordon and Breach Publishers
Extrand [39] treated NR surfaces in an acidified sodium hypochlorite solution and used contact angle measurements and reflection FTIR to study the changes caused by the chlorination. They studied ‘pure’ NR, a peroxide cured formulation and a conventionally cured formulation. Contact angles of glycerol on the rubber surfaces reduced after chlorination as shown in Table 1.6.
Table 1.6 Effect of chlorination on the contact angles between glycerol and various rubber surfaces [39] Contact angle (°)
Substrate
Before treatment
After treatment
‘Pure’ rubber
64
11
Peroxide cured
46
30
Conventionally cured
82
30
Reproduced with permission from C. W. Extrand and A. N. Gent, Rubber Chemistry and Technology, 1988, 61, 4, 688. ©1988, Rubber Division, American Chemical Society
Peel strengths 21
The Handbook of Rubber Bonding With regard to the infrared study, bands at 660, 750 and 1260 cm-1 were assigned to the effects of chlorination. In addition, bands at 780, 916 and 1410 cm-1 were almost certainly due to chlorination. Kusano [40] found that neither corona nor plasma treatments improved peel strength with a polyurethane adhesive despite improved wettability as indicated by water contact angles. FTIR indicated substantial oxidation after the corona treatment but only minor oxidation after the plasma treatment.
1.2.4.2 Styrene-Butadiene Copolymers Styrene-butadiene rubber has a low surface energy, but this may be considerably increased by the incorporation of various components. Organic additives such as antioxidants will tend to migrate to the surface thus creating a potential weak boundary layer. Pettit [36] found that treatment of SBR with chlorine gas, acidified sodium hypochlorite or an organic chlorine donor in an organic solvent resulted in large increases in peel strength for SBR-polyurethane-SBR joints. Oldfield [37] found that physical treatments were inferior to three chemical pre-treatments (see Table 1.7).
Table 1.7 Effect of pre-treatments on the peel strengths (N mm-1) of SBR-epoxide-SBR joints [37] Pre-treatment
Peel strength
Locus of failure
Toluene wipe
0.2
I
Abrasion on grinding wheel
1
I
Acidified hypochlorite
12
R
Cyclisation
12
R
TCICA in ethyl acetate
11
R
I - apparent interfacial; R - cohesive within rubber Reproduced with permission from D. Oldfield and T. E. F. Symes, Journal of Adhesion, 1983, 16, 2, 77. ©1983, Gordon and Breach Publishers.
22
Substrate Preparation Methods Using X-ray fluorescence, they estimated the chlorine concentration in the first few microns of the SBR after treatment with TCICA at various concentrations. With a 3% solution, the resulting chlorine concentration was 16.1% w/w. Pastor-Blas [41] found that physical treatments such as abrasion did not result in significant increases in the peel strengths obtained with a polyurethane adhesive. On the other hand, treatment with TCICA in ethyl acetate resulted in large increases in peel strength. On the basis of the relative amounts of chlorine and nitrogen introduced into SBR, Lawson [30] concluded that both substitution and addition reactions were significant when this rubber was treated with TCICA in ethyl acetate. Similar results were obtained with polybutadiene. Pastor-Blas [42] studied the effect of TCICA concentration in ethyl acetate. For solutions up to 2% w/w mainly chlorinated hydrocarbon and C–O species were reported. At between 2 and 5% w/w an excess of unreacted TCICA was indicated while above 5% w/w there was a detrimental effect on adhesion due to a weak boundary layer consisting of isocyanuric acid. Pastor-Blas [43] treated an SBR formulation with TCICA solutions in ethyl acetate having concentrations ranging from 0.5 – 7% by weight. The chemical changes caused by the pre-treatments are shown in Table 1.8. Rubber strips were bonded with a solvent-based polyurethane (PU) and the joint strengths determined in a T-peel test. After peeling, the test pieces were examined using a variety of techniques; XPS and FTIR confirmed that the treatment introduced various chemical groups. The peel strengths were obtained after treatments with 0.5, 2 and 7% w/w. The highest peel strength was obtained with the 2% solution.
Table 1.8 XPS studies of SBR treated with solutions of TCICA in ethyl acetate [43] Surface analysis (atom %)
Wt% concentration of TCICA
C
O
Si
N
Cl
S
0
92.27
2.8
1.5
-
-
-
2
92.7
4.3
1. 0
1.0
0.8
0.2
7
91.5
4.6
0.7
1.9
0.9
0.4
Reproduced with permission from M.M. Pastor-Blas, J.M. Martín-Martínez and J.G. Dillard, Journal of Adhesion, 1997, 62, 1/4, 23. ©1997, Gordon and Breach Publishers.
Peel strengths
23
The Handbook of Rubber Bonding In a related publication, treatments with fumaric acid in a butan-2-ol/ethanol mixture and TCICA in butan-2-ol were compared [44]. In general, the TCICA was more effective at enhancing the peel strength achieved with a solvent-based PU adhesive. Infrared analysis indicated the treatments were probably effective by removing zinc stearate (reduction in peak at 1540 cm-1) and the introduction of carbon-oxide functionalities (1704 cm-1 and 1670 cm-1 for the TCICA and fumaric acid, respectively). With TCICA, C–Cl bonds were also observed. Pastor-Sempere [45] treated two styrene-butadiene rubbers with fumaric acid in a butan2-ol/ethanol mixture. This resulted in improved adhesion in both cases, but the improvement with one formulation was significantly greater than the other. The lower peel strength was attributed to the presence of paraffin wax and zinc stearate. Roughening prior to treatment with fumaric acid resulted in additional improvements with both rubbers. Infrared analysis indicated that the fumaric acid was effective by introducing C=O bonds and by reducing the concentration of zinc stearate. In addition, the fumaric acid caused a roughening of both rubbers. Later Pastor-Blas [46] demonstrated that high concentrations of TCICA could lead to the formation of weak boundary layers. Treatment of two SBR materials with a 7 wt% solution of TCICA in ethyl acetate resulted in poor peel strengths unless the treated surfaces were vacuum dried for one hour at 1.34 Pa. Other methods have been shown to considerably improve the bondability of SBR materials. Aqueous solutions of an organic chlorine donor or the use of an electrochemical method resulted in large increases in peel strength with a water-based PU adhesive [47]. Kusano [40] found that corona and plasma treatments resulted in large increases in peel strength with a PU adhesive. Lawson [31] reported that a 10 second corona treatment improved the water wettability of an SBR. He also reported cracking of the rubber which he ascribed to the ozone generated in the discharge. Styrene-butadiene block copolymers SBS thermoplastic rubbers have a low surface energy. Therefore, to achieve good adhesion to SBS a chemical pre-treatment may be necessary. A complicating factor is that migratory organic additives may lead to a weak layer. Pettit [36] found that treatment of SBS with chlorine gas, acidified sodium hypochlorite or an organic donor in an organic solvent resulted in large increases in peel strength with a polyurethane adhesive. As with SBR, aqueous solutions of an organic chlorine donor and an electrochemical method were also effective with SBS [47]. Pastor-Blas [48] treated SBS with TCICA solutions (0.5, 2 or 7 wt%) in ethyl acetate. The SBS was bonded with a PU and the joint strengths determined in a T-peel test. The failed surfaces, after peeling, were examined by a variety of techniques including XPS and FTIR.
24
Substrate Preparation Methods It was concluded that the highest strength (3.3 N mm-1) was obtained with the 0.5% solution. It was concluded that the stronger solutions weakened the surface regions. FTIR and XPS showed that the treatment introduced chlorine and oxygen functionalities.
1.2.5 Halogenated Rubbers Introduction of bromine and chlorine atoms in hydrocarbon polymers will enhance adhesion. In the case of PE, introduction of bromine to a Br:C ratio of 0.05:1 resulted in high adhesion to an epoxide adhesive [49]. However, the quantity of halogen in bromoand chloro-butyl rubbers is low and poor adhesion to these polymers is not unexpected especially if organic additives are present on the surfaces. Oldfield [37] only obtained modest adhesion to untreated bromobutyl rubber (see Table 1.9). Of the treatments they investigated only TCICA in ethyl acetate resulted in very high peel strengths, although aqueous chlorination gave a substantial improvement. Using X-ray fluorescence, Oldfield and Symes found that the uptake of chlorine into bromobutyl rubber was very much less than that observed with NR, SBR and nitrile rubber, as would be expected from the relative number of carbon–carbon double bonds using XPS. Lawson [30] found chlorobutyl rubber did not take up any measurable amount of chlorine in treatment with TCICA in ethyl acetate. The reason for the large improvement in bondability with bromobutyl observed by Oldfield is unclear. It may be that the TCICA was acting as an oxidising agent rather than a chlorinating agent. However, Lawson did not observe any introduction of oxygen-containing groups with chlorobutyl rubber.
Table 1.9 Effect of pre-treatments on the peel strengths (N mm-1) of bromobutyl rubber–epoxide–bromobutyl rubber joints [37] Pre-treatment
Peel strength
Locus of failure
Toluene wipe
1
I
Abrasion on grinding wheel
1
I
Acidified hypochlorite
3
I
Cyclisation
0.1
I
TCICA in ethyl acetate
20
R
I - apparent interfacial; R - cohesive in rubber Reproduced with permission from D. Oldfield and T. E. F. Symes, Journal of Adhesion, 1983, 16, 2, 77. ©1983, Gordon and Breach Publishers.
25
The Handbook of Rubber Bonding Polychloroprene (CR) has much more chlorine than the chlorobutyl rubber examined by Lawson and good adhesion to untreated CR would be expected provided there was no weak layer was on the surface. If such layers exist a suitable solvent treatment or abrasion should result in good adhesion. Cyclisation has been recommended as a pre-treatment [50, 51]. Lawson noted a large uptake of chlorine, nitrogen and oxygen on treatment of polychloroprene with TCICA, indicating addition across the carbon–carbon double bond. Lawson [31] reported that a corona discharge treatment of CR increased its surface energy, but did not improve the peel strength with a polyurethane coating. Minagawa [32] reported large increases in adhesion with CR after UV irradiation or sputter ion etching. However, the treatment times were long, being 10 minutes with ion sputtering and one hour with the UV treatment. SEM indicates that the two methods caused considerable roughening of the surface. XPS and FTIR indicated the introduction of substantial quantities of oxygen-containing groups.
1.2.6 Miscellaneous Rubbers 1.2.6.1 Silicone Rubber (see also Chapter 12) Adhesion to untreated silicone rubber is difficult. The poor adhesion may be due to a low surface energy (approximately 24 mJ m-2) or a layer of low cohesive strength or a combination of these two factors. Plasma treatment has been shown to substantially improve the wettability of silicone rubber [50-57]. Peel strengths were measured in one study and found to be much increased by plasma treatment [53]. Swanson [58] found that coating a silicone rubber with photoactive reagents and then exposing the surface to UV resulted in a large increase in joint strengths obtained with a cyanoacrylate adhesive. Combette [59] reported that microwave or radio frequency plasma treatment of silicone rubber with a gas rich in oxygen gave high peel strengths with an epoxide adhesive.
1.2.6.2 Nitrile Rubber Nitrile rubber is moderately polar and good adhesion would be expected between a polar adhesive like an epoxide and the untreated polymer provided no weak boundary layers were present. This was found to be the case by Oldfield [37] as can be seen in Table 1.10. High adhesion values were obtained with a solvent wipe. Cyclisation and TCICA treatments resulted in large increases in adhesion. X-ray fluorescence indicated substantial uptake of chlorine in the latter case [37]. Peel strengths 26
Substrate Preparation Methods Table 1.10 Effect of pre-treatments on the peel strengths (N mm-1) of nitrile rubber-epoxide-nitrile rubber joints [37] Pre-treatment
Initial strength
Locus of failure
Toluene wipe
8
R
Abrasion on grinding wheel
5
I
Acidified hypochlorite
8
R
Cyclisation
18
R
TCICA in ethyl acetate
21
R
I - apparent interfacial; R - cohesive in rubber Reproduced with permission from D. Oldfield and T. E. F. Symes, Journal of Adhesion, 1983, 16, 2, 77. ©1983, Gordon and Breach Publishers.
1.2.6.3 Polyurethanes Polyurethanes (PU) have relatively high surface energies. Adhesion problems with PU substrates are, therefore, likely to be due to cohesively weak material, such as mould release agents on the surface. Abrasion is one of the main methods recommended as a pre-treatment [50]; such a pre-treatment can remove cohesively weak material and expose strong material of relatively high surface energy. Cryoblasting, in which carbon dioxide particles are fired at a substrate, has been shown to be capable of removing silicone release agents from PUs and thus giving large improvements in adhesion [47].
1.2.7 Discussion As noted in Section 1.2.1, there have been many detailed studies relating to the pretreatment of plastics. Much is now known about these pre-treatments including the chemical groups introduced, their concentrations and the depth of chemical modification. In contrast, the number of studies involving rubbers is much lower and in general the studies have been much less informative. One of the reasons for this is that rubbers usually contain several additives, often in relatively high concentrations. These additives make an understanding of the pre-treatments much more difficult. Because of the wide range of formulations for a particular rubber, it is also more difficult to generalise about pre-treatments than it is with plastics. For example, it is known that some formulations of SBR are considerably easier to pre-treat than others.
27
Peel strengths The Handbook of Rubber Bonding The four groups of rubbers considered above will now be discussed. Conclusions about pre-treatments for rubbers will then be presented. Hydrocarbon materials with few carbon–carbon double bonds will be considered first. The most important examples in this group are ethylene-propylene rubbers which may be crosslinked with peroxides or sulphur systems in which case small quantities of dimers are polymerised with ethylene and propylene (EPDM). As EP rubbers contain no polar groups it will normally be necessary to chemically modify the polymers to enable them to interact strongly with polar adhesives such as epoxides and polyurethanes. In the case of plastics such as polyethylene and polypropylene, large increases in adhesion can be achieved by treating with a flame [9, 10], corona [11, 13], plasma [18, 19], or etching solution [23]. It would be expected that EP rubbers would respond in the same way to these pre-treatments. However, this is not always the case. Thus, Lawson [31], found that a corona treatment of an EPDM did not improve the peel strength to a polyurethane coating. It is probable that the reason for the poor adhesion is a layer of low molecular weight material on the EPDM. During corona treatment this layer, rather than the underlying polymer, would be oxidised. Hence, the polyurethane coating would not be able to interact strongly with the EPDM. Even if the EPDM was oxidised by the corona treatment, there would still be a cohesively weak layer on its surface. Many rubbers possess carbon–carbon double bonds. In such cases there is the possibility that pre-treatment may be effective by addition or substitution reactions. Thus some reagents may be effective with unsaturated hydrocarbons such as SBR and SBS but not with EP rubbers. This is demonstrated by the work of Lawson [30] who found that treatment with TCICA in ethyl acetate resulted in the introduction of substantial quantities of chlorine into SBR, polybutadiene and NR, but not into EPDM. Several methods have been shown to be effective at pre-treating unsaturated hydrocarbon rubbers. These include treatment with concentrated sulphuric acid, acidified sodium hypochlorite and TCICA in ethyl acetate. The last method is the most commonly used commercially but in many countries legislation is being introduced to reduce the use of organic solvents. Promising results have been obtained with new solvent-free methods, namely an electrochemical method involving a highly reactive complex ion, and a method involving a water-soluble organic chlorine donor [47]. Like hydrocarbon rubbers, silicones have low surface energies and interactions with polar adhesives will be low unless the surface chemistry is modified. Plasma treatments improve the wettability [52, 53, 54, 55, 56, 57] or bondability [58, 59] of silicones. It is generally accepted that the introduction of a wide range of functional groups makes a polymer much more bondable. The effect of introducing individual chemical groups into polyethylene was demonstrated by Chew [60]. Thus, bromine, carbonyl, hydroxyl
28
Substrate Preparation Methods and carboxylic acid groups were all shown to greatly increase the bondability of polyethylene to an epoxide adhesive. This is in line with the general experience that polymers possessing halogens or oxygen-containing groups are much easier to bond than polyolefins. Whether rubbers containing such groups are easy to bond depends very much on whether the bonding surface is covered by low molecular weight (MW) additives or contaminants. On the one hand, Oldfield [37] achieved high peel strengths with chemically unmodified nitrile rubber whereas Brewis [47] obtained low peel strengths with an as-received polyurethane. However, after the removal of a silicone release agent by cryoblasting, much higher peel strengths were obtained [47].
1.2.8 Summary • Methods are available to pre-treat all rubbers but additives or processing aids may make successful pre-treatment much more difficult. • TCICA in various organic solvents is very effective with those rubbers possessing carbon-carbon double bonds. However, legislation restricting the use of organic solvents is being introduced in many countries. Promising new pre-treatments include the use of water-soluble organic chlorine donors and an electrochemical method in which a highly active complex ion is generated. • With some polymers containing suitable chemical groups, e.g., PU, simply removing cohesively weak material from the surface may be all that is necessary to achieve good adhesion.
1.3 Bonding Rubbers to Plastic Substrates 1.3.1 Introduction This section is based mainly on first hand personal experience and is not intended to be an overview of bonding. It covers the basic practical principles of bonding rubbers to a variety of plastics materials. It is typical to find that those who are skilled in the art of moulding and bonding rubbers have little affinity to plastics materials and vice versa. As for polyurethanes; these are something else altogether. This chapter will concentrate on those plastics and rubbers which are likely to have uses in the manufacture of composite materials (see Appendix 1.1).
29
The Handbook of Rubber Bonding
1.3.1.1 Why Use Plastics? • Cost, • Weight saving, • Technically superior, • Environmentally more acceptable, • Fashion/style.
1.3.1.2 What Form Does the Plastics Material Come In? • Moulded components, • Cast components, • Sheet or film, • Tube/pipe or rod. Fabric, fibres and filaments are obviously important forms and uses of plastics materials. Although the basic principles of bonding plastics apply to fibres and fabrics, the other factors involved in bonding them are a subject in themselves and will not be discussed further. In the bonding of rubbers it is assumed that the plastics component is an item which has been preformed and it is this which will be treated with a bonding agent. In most cases the rubber will be moulded onto the primed surface, by techniques including the following: • Injection moulding, • Reaction injection moulding (RIM), • Compression moulding, • Transfer moulding, • Extrusion blow moulding, • Lamination, which could involve post vulcanisation bonding, • Autoclave vulcanisation - rollers, pipes, hoses, stators, • Casting at zero or low pressure - casting of PU. The basic principles should apply to any form of plastics material and to any method of moulding.
30
Substrate Preparation Methods Of course there is always the potential to mould the plastics material onto the vulcanised rubber, but this is rare. In practice, this type of moulding is an example of post vulcanisation bonding.
1.3.2 Plastics Substrate Preparation In preparing metals for bonding, steel in particular, the idea is to produce a surface which is free of contamination, is easy to wet, has a ‘sharp’ irregular surface to promote a mechanical key and controlled oxidation (see Figure 1.3).
Figure 1.3 Metal surface sites for bond
Fortunately for the commonly used metals this ‘controlled’ oxidation occurs naturally after grit blasting or acid etching. In the case of plastics, no such convenient oxidation process takes place. However, each material will have a unique surface layer containing potential sites for bonding:
• Polyamides
The polar group NH-C=O is capable of hydrogen bonding through the activated C=O group and via the N-H group. The N-H leaves a reactive site for chemical reaction with silanes, epoxies, isocyanates and any chemical adducts, which can release such species or any other species, which can react with an active hydrogen. Of course the amide group needs to be on the surface to be able to undergo hydrogen bonding or chemical reaction and steric hindrance will reduce the capability of such groups to partake in bonding, which is especially so in the case of aromatic polyamides.
31
The Handbook of Rubber Bonding
• Polyesters
The C(=O)O, ester group will partake in hydrogen bonding through both oxygen atoms, especially the activated carbonyl group. Some polyesters will be less easy to bond if steric hindrance is likely. Even PBT proves difficult to bond and often requires further treatment.
• Polyurethanes
In theory PU should be very active towards bonding, with an activated N-H and a carbonyl group, as described for polyamides. However PUs are never that easy to bond and could be due to surface oxidation and/or surface hydrolysis, it is normal to remove the surface, degrease and prime before the surface is too old.
• Polyureas
The sites for hydrogen bonding and chemical reaction are significant and polyureas are generally easy to bond. Being more oxidation resistant and hydrolysis resistant than the urethane group is significant.
• Polycarbonates
A regular repeating stable carbonyl group is available for polar attraction and hydrogen bonding.
• PPS (and PPO)
32
Substrate Preparation Methods As for polycarbonates, a regular repeating stable polar sulphur (oxygen) atom allows for polar attraction and hydrogen bonding. However, in the case of the polyolefines, there are no obvious adhesion sites:
• Polyethylene
• Polypropylene
For the bonding of these an oxidation process is essential. When one looks at the surface of metals and plastics under an electron microscope the disruption in that surface explains why bonding is never straightforward. The surface is often described as a weak surface layer and in the case of plastics one can include the surface stresses, general contamination, the presence of abhesive ingredients, i.e., process aids which have migrated to the surface. Some high temperature moulding processes may lead to variable and unwanted oxidation and/or reversion (crosslink degradation) at the surface. Therefore, one can accept the general opinion that the surfaces of plastics do need some form of abrasive or chemical treatment to remove the weak surface layer, or at least reduce it to an adequate level, as shown by the number of publications on the subject [61-69]. Putting it in simple terms the level of surface preparation depends on the performance requirements of the bond. To apply a pressure sensitive decal, no surface treatment is a feasible option, but to make a suspension mount then the plastics surface will require controlled treatments. Most engineering plastics can be treated with alumina or steel grit as for metals. However, in the real world it is quite normal to find that grit blasting is impractical for many reasons, including: • Loss of shape, especially in thin sections, • The reduction in dimensions is not reproducible,
33
The Handbook of Rubber Bonding • Surface damage, such as fibrillation and plastic flow, • Trapped (embedded) grinding media and other contaminants. The harder and the thicker the surface to be bonded the better it is for grit blasting. Similarly, the more highly filled plastics respond much better to blasting than unfilled plastics, and thermosets, especially glass-filled thermosets, are usually very successfully prepared by blasting. If a standard grit blasting process gives problems then the use of a finer grit in any standard grit blasting machine should be thoroughly tested to determine if there is an effective optimum grit size. Abrasive and chemical techniques include the following: • Treatment with abrasive belts, • Hydrosonic/ultrasonic cleaning, • High pressure water/detergent cleaning, • Acid etching, but effluent control means that this is not feasible for anything other than high priced specialities and for long running applications, • The satinisation process for POM is an example of acid etching and involves a slurry containing p-toluene sulphonic acid, • Phenol treatments of polyamides. This includes RFL treatments, • Alkali etching. As for acid etching, the action is mild surface hydrolysis and loosening of ‘debris’ on the surface, • Oxidation with relatively mild oxidising agents. Hydrogen peroxide and sodium hypochlorite are often cited, but a low hazard system worthy of testing out is ammonium persulphate, • Powerful oxidising agents, such as sulphuric dichromate etching, • Abrasion in an aqueous abrasive slurry. Since this involves effluent waste, it is seldom used on a large scale, but is an effective laboratory method, especially when combined with a mild acid, alkali or oxidising agent, • Direct oxidation by flame, or hot air. Normally only applicable to simple shapes, like extruded film, tube and rod,
34
Substrate Preparation Methods • UV treatments. Again this has restricted use, mainly films, • Plasma treatments. Yet to become a mainstream treatment for rubber to plastics bonding, • Corona discharge, • Chlorination.
1.3.3 Degreasing and Solvent Cleaning Degreasing has always been considered an integral part of ideal surface preparation, but under current environmental pressures, it is quite normal to find it has been partly eliminated or even totally eliminated. The need for thorough degreasing becomes more relevant where the environmental resistance of the bond is important and especially where an abrasive technique has left a contaminated surface. Degreasing of plastics with solvents can cause problems: • Stress cracking of the surface, where the effect can remain undetected, • Absorption and even adsorption of a solvent of a similar solubility parameter to the plastics material. This can be a very serious problem, since retained solvent within the bond line could well act as a release agent. If solvent degreasing/cleaning is going to be employed, then a fast drying solvent which has a relatively low solvating power towards the plastics being degreased needs to be used. Aqueous degreasing can be effective, especially when fully automated. However, any aqueous process can leave a surface which requires desorption of water, which adds another process. Unfortunately, for low pressure moulding and casting in particular, the ultimate bonds are often only achieved if desorption of the adsorbed water and gases is specified. This is most evident with polyamides, some polyesters, PU, melamine and urea resins and some epoxy resins. However, in the majority of high pressure moulding processes adsorbed water and gases do not appear to affect bonding, but long term environmental tests may show up a problem. A general guide to reduce the effects of water adsorption is to dry the plastic’s surface, prime with the bonding agent, dry the primed surface and give the component a prebake (the coated dried surface is heated, prior to the moulding process). Pre-bakes can
35
The Handbook of Rubber Bonding
Table 1.11 A brief summary of the preferred treatments Plastics group
Chemical treatments
Degrease
Grit blasting
Other treatments
A1, A2
Yes Take care with acrylics if in doubt use alcohol
Yes Check for optimum grit size
POM-satinisation polyesters difficult surfaces respond to alkali or ammonium persulphate treatments
Nylon 6 and 66 desorb at >100 °C, especially for low pressure moulding and casting of PU other forms of abrasion work generally for these materials
B
Yes
No
Yes Strong oxidising agents
TPOs flame treat, UV, corona discharge treat. may be followed by chlorination
C
Yes Yes Avoid ketones Unless <50° alcohols are safer Shore D
D
Yes No Solvent attack on Unless > 50° PVC and PVDC. Shore D Avoid aqueous degreasing of PU
PTFE, PVF treat with sodium naphthalene
PTFE, PVF prime with a thin coat of Cilbond 30/31, dry and fuse at >200 °C
be as little as 10 minutes at 70 – 90 °C, which could be part of the drying process, up to 30 minutes at 150 °C, which would be an additional process (see Table 1.11). For plastics group definitions see Appendix 1.1
1.3.4 Adhesive/Bonding Agent Choice 1.3.4.1 Post Vulcanisation Bonding This includes adhesive bonding and bonding with vulcanising bonding agents under the influence of heat and pressure, in those cases where the plastics component needs to be adhered to the preformed vulcanised rubber.
36
Degrease
Grit blasting
Chemical treatments Substrate Preparation Methods
This may be the only method of manufacture for some products and there is a host of adhesives available for plastics, some of which are described in the literature [63 – 69] for example. The main adhesives for bonding plastics to rubbers include cyanoacrylates, two-part urethanes, two-part epoxies, hot melt reactive urethane prepolymers, heat reactive contact cements and silane treatments. Many adhesive bonding applications require a unique answer and it is difficult to make generalised recommendations, as you can within limits, with vulcanisation bonding.
1.3.4.2 Vulcanisation Bonding This is bonding the rubber during the vulcanisation process. The ideal situation is where no bonding agent is required, but in the real world it is rare to find situations where no bonding agent, whether an internal bonding agent (added to the rubber) or a conventional (external) agent is necessary.
• Primers for the Plastics Substrate for Vulcanisation Bonding In theory the primer should match the polarity of the plastics substrate, but this could infer the need for a range of primers depending on the polarity of the plastics to be bonded. In practice, bonding agent primers contain curable polar resins and less polar rubbery polymers, which may or may not be crosslinkable. This gives some versatility in the bonding of a range of polar plastics. An ideal primer would contain highly polar curable resins and speciality polymers. The speciality polymers would vary in their polarity along the polymer chains, giving it variable polarity, a positive attribute in the bonding of a range of plastics. The polymer could be produced by grafting a polar monomer (or monomers) onto an unsaturated polymer such as NR, IR, BR or even NBR, which leads to a polymer which has certain properties: 1. It still contains unsaturation and segments of the original main chain polymer. This means it can crosslink and intermix with the rubber being moulded. 2. Any polar groups on the ungrafted polymer (for example C–N groups in NBR) take part in polar bonding to the plastics substrate. 3. The grafted monomer(s), being polar, can also partake in polar bonding.
37
The Handbook of Rubber Bonding 4. If the grafted monomer retains reactivity it can take part in chemical bonding. Such reactivity could include isocyanates, silanes, epoxides, or even heat reactive adducts, such as blocked isocyanates. 5. If the grafted monomer results in a large and highly polar site, it is possible for this moiety to behave in a way which appears similar to solvent welding (surface softening), but in this case the ‘solvent’ is the polar moiety. This phenomenon is a particular feature of one type of speciality one coat technology, because this ‘welding’ not only applies to the plastics surface, but also to the rubber surface, whether the rubber is in an uncured state or cured state. Though it has been compared to solvent welding the phenomenon described above shows no thermoplasticity, in fact heat and solvent resistance are the big features of this type of technology, along with the capability of post vulcanisation bonding. 6. The ability of the polymer and resin in the primer to react with each other generally improves the environmental resistance of both the bond and the bonding agent. 7. For improved heat resistance, aliphatic chlorine should be avoided in the polymers. For general purpose vulcanisation bonding, conventional primers are available from the established suppliers of bonding agents and all such suppliers can cite many examples of rubber to plastics bonding (see Table 1.12). For improved adhesion and improved environmental resistance the more reactive primers can exhibit advantages, such that in some tough applications, they are the only choice.
• Cover coat/top coat If one is required it should be chosen only with regard to the rubber/rubber being moulded, just as for rubber to metal bonding. (See Table 1.12.)
Summary With attention to detail, most plastics can be bonded to rubbers, provided one accepts the limitations of the rubbers, the plastics and the adhesive system chosen to bond them. It is the aim of those who recommend the adhesives/bonding agents to ensure the bonds are fit for purpose, but it is normal to find that the component manufacturer wants to see no failure attributable to the adhesive.
38
Substrate Preparation Methods
Table 1.12 Rubbers, vulcanisation bonded to plastics - systems and techniques Plastics Materials
Rubber
Environmental Resistance of the Bonds
Bonding System
Special Treatments
PPO
VAMAC (Dupont)
Heat to ≥180 °C
Cilbond 22 Cilbond 60W
Grit blast PPO (200 – 400 μm grit) and degrease with acetone or use alcohol for large or awkward shapes
PPS
NR SBR
Glycol resistant to ≥160 °C
Cilbond 21T Cilbond 22
As above
POM
VMQ
Heat to >>160 °C
Cilbond 65W
Satinse POM with p-TSA
Cilbond 89 Cilbond 22
Abrade or grit blast with fine grit. Degrease with MEK Prebake first thin coat of primer at ≥100 °C use Cilbond 22 for PV bonding
ARAMID XNBR HNBR Heat and fluids to
>170 °C
PTFE
FKM
Hot oils to >>160 °C Cilbond 65W
Sodium treat PTFE and prime as soon as possible
PP FILLED
EPDM
Water to 100 °C
Cilbond 89
Oxidise/flame treat PP. Prebake a first thin coat of primer
GRP
PU rotation cast
Bonds outperforms the PU for roller and pipe coatings
Cilbond 41+B Cilbond 49SF+B
Belt abrade and water or hydrocarbon degrease. Allow first coat to dry >2 h and second coat for >4 h, but <30 h
PET
TPU
Heat to 140 °C
Cilbond 49SF
Alkali or ammonium persulphate etch PET
p-TSA: para-toluene sulphonic acid
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The Handbook of Rubber Bonding
APPENDIX 1.1 Plastics are divided into groups, loosely based on factors such as surface preparation and bonding characteristics:
Group A1 Plastics Engineering Thermoplastics Acrylics POM - Acetals Polyesters - PET, PBT Polycarbonate, PC PES PPO PPS Polyamides - Nylon 6, 66, 11, 12 Aramids PEEK
Group A2 Plastics - Thermosets Epoxies Unsaturated polyesters, FRP/GRP Phenolics, including RF resins Polyamides
Group B Plastics - Other Thermoplastics TPOs
Group C Plastics - Miscellaneous ABS SEBS
40
Substrate Preparation Methods Polystyrene Cellulosics UF, MF
Group D Plastics - Miscellaneous PU PVC, PVDC PTFE, PVF Rubbers
Conventional Rubbers NR SBR IR BR CR CSM ACM, VAMAC NBR, XNBR, HNBR EPDM IIR ECO EVM CPE Millable PU
Others PUs - TPU and castable PU Thermoplastic rubbers VMQ FKM Polyolefins
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The Handbook of Rubber Bonding
1.4 Substrate Preparation for Bonding Using the Wet Blast Process 1.4.1 Summary Abrasive Developments have, in conjunction with their Japanese licensee, developed a wet blast phosphating plant that raises the quality standard within the industry. The solution achieved delivers high quality components from an automatic machine that combines both the cleaning and phosphating processes. The cleaning section benefits from the unique degreasing and surface treatment properties of the VAQUA process. Wet blast phosphating was first developed some 15 years ago in co-operation with the Yamashita Rubber Company, who make anti-vibration rubber and bond it to supporting metal parts exclusively for the Honda Motor Company. Yamashita had two main objectives to achieve from the development of a wet blast phosphating plant: • To increase the strength of adhesive bonding between the anti-vibration rubber and the metal parts, • To improve the corrosion resistance of the metal parts and hence their useful life under any weather conditions. In addition to these objectives, the demand from the automotive industry as a whole for this type of component was increasing, and the requirement was for it to be phosphated prior to bonding whilst still keeping the cost at an acceptable level. To achieve the improved quality and reduced cost requirements the wet blast phosphating plant had to operate continuously and automatically process the metal parts for phosphating.
1.4.2 The Wet Blast Phosphating Plant The plant has two major processing sections, the wet blast section and the phosphate treatment section.
1.4.2.1 The Wet Blast Process The wet blast process is one of the world’s most versatile, efficient and economical processes for metal cleaning and finishing, replacing costly chemicals and the need to sandblast. It saves hours of messy cleaning and eliminates health and environmental hazards associated with strong chemicals and dust from conventional blasting methods. 42
Substrate Preparation Methods
1.4.2.2 How is Wet Blasting Done? The component surfaces are bombarded by a recirculating high volume flow of water borne particles (normally abrasive or glass beads) contained within the cabinet. The specially developed VAQUA pump pulls the concentrated slurry of media and water, inhibitors and degreasing agents, from the cabinet sump and pushes it at constant high volume to the process gun. The VAQUA pumps have been developed to minimise the friction wear from the blast media as it is accelerated round the system by the blast pump itself. Before reaching the process gun a proportion of the water and media is diverted down the bypass to provide agitation in the sump, this ensures a stable concentration of media and water. To accelerate the flow of media particles onto the surface of the work piece and therefore achieve the cleaning and surface finishing effects, a controlled flow of compressed air is introduced into the blast gun. The water within the system lubricates, washes, carries mild inhibitors/degreasers and eliminates dust formation. At the rinse stage, elements of the blasting media will be carried over and need to be removed and recirculated, this is done by cyclones. There is a two-stage cyclone system, with the first stage separating the media and water by centrifugation which removes high concentration slurry, returning it to blast tank from the pipe arrangements located at the bottom of the cyclone. Low concentration slurry is transferred to the second cyclone where the process is repeated and the media further separated from the water. The water separated here is used for subsequent rinse stages and the separated broken down media is transferred to the klarti separator. This is a form of oil and grease settling tank where the broken down abrasive is separated from the water to allow for subsequent removal of the used media.
1.4.2.3 The Wet Blast Section It is important that a certain type of surface finish is produced on the metal components to enable effective phosphating and bonding. The optimal surface roughness for bonding is 5 – 10 μm, this can be best achieved by wet blasting. The machine is equipped with a specially designed barrel in which the components are held, and large capacity process guns, through which the media, water and air combination is delivered. This set up allows metal parts with complicated shapes to be effectively and thoroughly processed giving a uniform and fine satin finish on all of the metal parts. By adding a degreasing agent to the blasting slurry, any oil and grease on the metal part’s surface is completely removed thus providing a clean component for presentation to the phosphating section.
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The Handbook of Rubber Bonding
Figure 1.4 The VAQUA pump
Wet blasting removes the oxide film covering the metal parts and exposes the pure metal under the film offering an ideal condition for the phosphate treatment which follows. The process is so efficient that even cast components, if wet blasted, can be treated with phosphate which was impossible using traditional methods.
44
Substrate Preparation Methods
1.4.2.4 The Phosphate Treatment Section In the phosphate treatment section, metal parts go through multiple vessels containing phosphate, rinse water, and a specially designed barrel in each vessel oscillates to keep the metal parts in continuous motion thus preventing bubbles forming or liquid staying inside the parts that have openings within them. The barrel oscillation also ensures that the metal parts are always exposed to fresh phosphate which is essential for a uniform and stable phosphate film to be created. To avoid cross contamination of the phosphating chemicals, the barrel containing the metal parts does not travel through the individual vessels but stays in a particular vessel. When the processing of the metal parts in the barrel is complete they are automatically dumped into the next barrel for the subsequent process. During their transfer from one vessel to the next, the metal parts are only exposed to the air for a short time which avoids the possibility of them rusting in the future. By automatically transferring products from one vessel to another this also minimises the contamination of chemicals from one vessel to the next.
1.4.3 Comparison Between Conventional and Wet Blast Phosphating The conventional process stages are: 1. Dip in trichloroethane for degreasing, 2. Dry shot blasting, 3. Treat with triethane vapour for degreasing, 4. Water rinse, 5. Phosphate treatment, 6. Water rinse, 7. Hot water rinse, 8. Drying. By comparison the wet blast phosphating stages are: 1. Wet blast, 2. Degrease – detergent system, 3. Water rinse,
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The Handbook of Rubber Bonding 4. Phosphate treatment, 5. Water rinse, 6. Hot water rinse, 7. Drying.
1.4.4 The Wet Blast Phosphating Plant Typical processing time and performance at each process stage:
Process
Time (s)
Performance
Wet blasting
30 0
Wet blasting with satin finish and degreasing
1st rinsing
30
Rinsing work to remove remaining media
Degreasing (non-solvent type)
300
Degreasing areas of the work piece that could not be degreased by wet blasting
2nd rinsing
60
Removing degrease chemicals
3rd rinsing
60
Removing degrease chemicals
1st phosphating
18 0
Phosphate coating
2nd phosphating
18 0
Phosphate coating
Dipping for rinse
60
Removing phosphate
Dipping for hot rinse
60
Removing phosphate and warming up work for drying
Drying
120
Drying work completely
Total process time
1350
46
Substrate Preparation Methods
1.4.5 Advantages of the Wet Blast Phosphating Plant 1.4.5.1 Product Quality High quality phosphate film. The plant produces high quality phosphate film for a number of reasons as listed below: • The quality of the phosphating achieved is very dependent upon the surface finish of the component prior to phosphating. The surface finish achieved through wet blasting is ideal for the phosphating process, hence the high quality film. • The wet blast section connects with the phosphate treatment plant and therefore the metal work pieces are treated with phosphate immediately after blasting. • During transit from the wet blast section to the phosphating plant the work pieces are covered with water so eliminating the possibility of oxidation of the components. A rust inhibitor in the blast system also prevents the oxidation of the components.
1.4.5.2 Clean Components Prior to Phosphating The powerful wash available from the wet blast process removes any kind of oil and grease without any adjustment of the system. The wet blast media physically removes oil and grease from the surface of the components and prevents it from sticking to the surface. This is achieved through the repeated blasting of the water media slurry, in a 7:1 ratio, against the work piece. The media particles in a slurry can reach speeds in excess of sonic speeds so imparting large energy to the component and assisting in the cleaning.
1.4.5.3 High and Uniform Quality Products Made Continuously The wet blast phosphating plant is fully automated which means that once the initial set up is complete, the repeatability of the process ensures a consistent quality of component after phosphating. In an alternative system where a barrel transfers components from one process to the next the inconsistency of time in each stage means that there could be some inconsistency in the end result, which is not the case with the wet blast phosphating plant. In chemical processes involving pre-treatment and processing, the concentration of chemicals may alter depending upon the condition of the work pieces such as the type of oil or grease on them. With wet blast phosphating the type of oil or grease is irrelevant, the process continues to give the same high quality cleaning and phosphating of the work pieces regardless of the type of oil and grease.
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The Handbook of Rubber Bonding
1.4.5.4 The Wet Blast Phosphating Plant Can Process Any Type of Material The wet blast process physically removes the surface oxide film and does not rely on a chemical reaction, therefore the range of materials that can be processed can be anything from common steel and steel alloy to special steels. This physical ‘scraping off’ of the oxide layer means the process is consistent for each work piece and also does not require change of chemicals between different types of component metals. The universal nature of the wet blast process can significantly reduce process times if alternative processes require chemical or other changes between different types of components.
1.4.5.5 Ease of Machine Operation Two factors assist in the efficient operation of this machine, these are: • Automatic operation, • Footprint of the machine. The automatic nature of the wet blast phosphating plant means that once loaded the machine will complete the process automatically allowing the operator to carry out other tasks at the same time. This may even be operating more than one wet blast phosphating plant because the small footprint of this machine will allow two of these machines to be located in the area normally allocated to a conventional process. The reason it is possible to obtain such a small footprint is that there is no requirement for conveyors between the cleaning and phosphating plants as they are incorporated in the same machine. In addition to these operational benefits, the plant is of a single floor type, thus making it simple to install and easy to locate related machinery nearby. It is worth noting that only three days are required for installation before the plant is ready for operation. There is also no ancillary pipe work required for the machine apart from the primary supply piping. The ability to locate related machinery nearby has enabled some users to incorporate automatic load and unload facilities to their plants thus increasing the automation of the machine and hence reducing the labour costs further.
1.4.5.6 Work Pieces of Any Shape Can be Processed Any shape of component can be processed through the wet blast phosphating plant without the possibility of any liquid remaining inside the component. The barrel in the machine has
48
Substrate Preparation Methods been designed to prevent the components slipping inside the barrel whilst the barrel oscillates. Components such as tubes, struts and flat washers are all being successfully processed with the wet blast phosphating plant.
1.4.5.7 Environmental Issues Each country or region has its own laws relating to the safe and environmentally friendly use of chemicals. Cleaning and degreasing is possible without using chemicals which damage the environment. Wet blasting is a physical cleaning method that does not rely upon chemicals. The reduction in the use of chemicals can also reduce the taxes or disposal charges required for some chemicals.
1.4.5.8 The Work Environment Traditional blasting processes have in some cases been associated with high dust levels and therefore a poor work environment, this is not true for the wet blast process. Dust generated by the physical cleaning is absorbed into the liquid supporting the cleaning media and subsequently extracted through the filtration and/or the oil separation system. The wet blast process is a completely dust free cleaning system. As the equipment is essentially self-contained, the clean work environment also benefits from the absence of piping on the floor thus making it easy to clean the area around the machine.
1.4.5.9 Enclosed Phosphate Treatment Plant The design of the phosphating plant is such that any vapour generated is not allowed to escape. The specially designed transfer system allows the phosphate treatment section to be fully enclosed. The transfer system of a conventional machine is such that the barrels themselves have to travel through each process, meaning the enclosure has to be large enough to enclose the whole machine. On the contrary with the wet blast phosphating plant, just each vessel is enclosed and the mechanical devices are outside the enclosure.
1.4.5.10 Additional Benefits With the wet blast phosphating plant, only the work pieces and not the barrels transfer, thus reducing the amount of rinse water consumed. The volume of the rinse water consumed is proportional to the amount of chemical liquid brought into the rinse water vessel. For the same reason, that only the work pieces are transferred, the amount of chemical liquid consumed is relatively small.
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The Handbook of Rubber Bonding
1.4.5.11 Maintenance Maintenance is significantly reduced, with the wet blast phosphating machine not requiring all barrels to be regularly maintained but only the phosphating vessels and the following water rinse vessel. If the maintenance is not sufficient in conventional machines, sludge can be transported into the drying section giving poor quality components with sludge sticking to them.
1.4.5.12 The Wet Blast Phosphating Process Stage 1
Hoist loader
This hoist raises the components to the hopper located at the upper side of the blast section
Stage 2
Blast inlet hopper
This works as a transfer accumulator and shortens the transfer time for loading components into the blast barrel
Stage 3
Wet blast section
Simultaneous degreasing and matt surface finishing through barrel processing
Stage 4
Blast unload bucket
Water collected within the components is removed here before transfer into the next stage
Stage 5
Remove tank
The degreasing process is completed here
Stage 6
2nd and 3rd rinse tanks Removed grease and media are rinsed off with water
Stage 7
Surface adjustment tank Pre-processing is done so that a stable phosphate film can be made in the phosphating process
Stage 8
Phosphate tanks
Two tanks are used to form the phosphate film
Stage 9
Water rinse
Two water rinse tanks are used to remove residual chemicals from the components
Stage 10 Hot water rinse tanks
Two water rinse tanks are used to remove residual chemicals from the components
Stage 11 Dryer
Water is cut off and the components dried by hot air
Stage 12 Unload conveyor
Processed components are unloaded
50
Substrate Preparation Methods
References 1.
H. M. Clearfield, J. Thomas, D. K. McNamarra and G. D. Davis in Adhesive Bonding, Ed., L-H. Lee, Plenum Press, New York, 1991.
2.
J. S. Thornton, R. E. Montgomery and J. F. Cartier, Presented at the ACS Division of Polymeric Materials: Science and Engineering, Chicago, IL, Fall 1985, 53, 465.
3.
T. Symes and D. Oldfield, in Treatise on Adhesion and Adhesives, Volume 7, Ed., J. D. Minford, Marcel Dekker, Inc., New York, 1991, Chapter 2.
4.
F. H. Sexsmith, in Rubber Products Manufacturing Technology, Eds., A. K. Bhowmick, M. M. Hall and H. A. Benarey, Marcel Dekker, Inc., New York, 1994, Chapter 11.
5.
BS EN ISO 11124-4 Preparation of steel substrates before application of paints and related products specifications for metallic blast-cleaning abrasives - low-carbon cast-steel shot, 1997.
6.
C. L. Mahoney, in Handbook of Adhesives, 3rd Edition, Ed., I. Skeist, Van Nostrand Rheinhold, New York, 1990, p.74-93.
7.
L. Setiawan, D. Schoenherr and J. Weihe, International Polymer Science and Technology, 1993, 20, 9, T13.
8.
K. M. Bond and D. H. Mowrey, Presented at the 141st ACS Rubber Division Meeting, Louisville, KY, Spring 1992, Paper No.57.
9.
R. L. Ayres and D. L. Shofner, SPE Journal, 1972, 28, 51.
10. D. Briggs, D. M. Brewis and M. B. Konieczko, Journal of Materials Science, 1979, 14, 6 ,1344. 11. D. Briggs, C. R. Kendall, A. R. Blythe and A. B. Wootton, Polymer, 1983, 24, 1, 47. 12. M. Strobel, M. J. Walzak, J. M. Hill, A. Lin, E. Karbashewski and C. S. Lyons, in Polymer Surface Modification, Ed., K. L. Mittal, VSP, Utrecht, 1996, 233. 13. R. Krüger and H. Potente, Journal of Adhesion, 1980, 11, 2, 113. 14. H. Brecht, F. Mayer and H. Binder, Die Angewandte Makromolekulare Chemie, 1973, 33, 89. 51
The Handbook of Rubber Bonding 15. D. W. Dwight and W. M. Riggs, Journal of Colloid Interface and Science, 1974, 47, 3, 650. 16. I. Mathieson, D. M. Brewis, I. Sutherland and R. A. Cayless, Journal of Adhesion, 1994, 46, 1/4, 49. 17. E. R. Nelson, T. J. Kilduff and A. A. Benderly, Industrial Engineering Chemistry 1958, 6, 221. 18. H. Schonhorn and R. H. Hansen, Journal of Applied Polymer Science, 1967, 11, 8, 1461. 19. C. A. L. Westerdahl, J. R. Hall, E. C. Schramm and D. W. Levi, Journal of Colloid and Interface and Science, 1974, 47, 3, 610. 20. H. Schonhorn and R. H. Hansen, Journal of Applied Polymer Science, 1968, 12, 5, 1231. 21. A. Chew, R. H. Dahm, D. M. Brewis, D. Briggs and D. G. Rance, Journal of Colloid and Interface Science, 1986, 110, 88. 22. G. C. S. Collins, A. C. Lowe and D. Nicholas, European Polymer Journal, 1973, 9, 11, 1173. 23. D. Briggs, D. M. Brewis and M. B. Konieczko, Journal of Materials Science, 1976, 11, 7, 1270. 24. L. J. Gerenser, J. F. Elman, M. G. Mason and J. M. Pochan, Polymer, 1985, 26, 8, 1162. 25. G. Beamson, D. M. Brewis and J. F. Watts, unpublished work. 26. R. A. Bragole, The Journal of Rubbers and Plastics, 1974, 6, 3, 213. 27. M. D. Ellul and D. R. Hazleton, Rubber Chemistry and Technology, 1994, 67, 4, 582. 28. I. Brass, D. M. Brewis, I. Sutherland and R. Wiktorowicz, International Journal of Adhesion and Adhesives, 1991, 11, 5, 150. 29. G. Kranz, R. Lüschen, T. Gesang, V. Schlett, O. D. Hennemann and W. D. Stohrer, International Journal of Adhesion and Adhesives, 1994, 14, 4, 243. 30. D. F. Lawson, K. J. Kim and T. L. Fritz, Rubber Chemistry and Technology, 1996, 69, 2, 245.
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Substrate Preparation Methods 31. D. F. Lawson, Rubber Chemistry and Technology, 1987, 60, 1, 102. 32. ASTM D2578-99a Standard Test Method for Wetting Tension of Polyethylene and Polypropylene Films 33. M. Minagawa, T. Saito, Y. Fujikura, T. Watanabe, H. Iwabuchi, F. Yoshi and T. Sasaki, Journal of Applied Polymer Science, 1997, 63, 12, 1625. 34. A. Kondyurin and Y. Klyachtin, Journal of Applied Polymer Science, 1996, 62, 1, 1. 35. B. C. Cope, D. M. Brewis, J. Comyn, K. R. Nangreave and R. J. P. Carne, Adhesion 10, Ed., K. W. Allen, Elsevier Applied Science Publishers, London, 1986, 178. 36. D. Pettit and A. R. Carter, Journal of Adhesion, 1973, 5, 4, 333. 37. D. Oldfield and T. E. F. Symes, Journal of Adhesion, 1983, 16, 2, 77. 38. D. Oldfield and T. E. F. Symes, Journal of Adhesion, 1992, 39, 2-3, 91. 39. C. W. Extrand and A. N. Gent, Rubber Chemistry and Technology, 1988, 61, 4, 688. 40. Y. Kusano, T. Noguchi, M. Yoshikawa, N. Kato and K. Naito, Presented at Kobe International Rubber Conference (IRC 95), 1995, Kobe, Japan, 432. 41. M. M. Pastor-Blas, M. S. Sánchez-Adsuar and J. M. Martín-Martínez in Polymer Surface Modification Ed., K. L. Mittal, VSP, Utrecht, 1996, 379. 42. M. M. Pastor-Blas, J. M. Martín-Martínez and J. G. Dillard, Journal of Adhesion, 1997, 63, 1/3, 121. 43. M. M. Pastor-Blas, J. M. Martín-Martínez and J. G. Dillard, Journal of Adhesion, 1997, 62, 1/4, 23. 44. A. Torró-Palau, J. C. Fernández-García, A. C. Orgilés-Barceló and J. M. MartínMartínez, Journal of Adhesion, 1996, 57, 1/4, 203. 45. N. Pastor-Sempere, J. C. Fernández-García, A. C. Orgilés-Barceló, R. TorregrosaMaciá and J. M. Martín-Martínez, Journal of Adhesion, 1995, 50, 1, 25. 46. M. M. Pastor-Blas, J. M. Martín-Martínez and J. G. Dillard, Journal of Adhesion Science and Technology, 1997, 11, 4, 447.
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The Handbook of Rubber Bonding 47. D. M. Brewis and I. Mathieson, Presented at the 22nd Annual Meeting of the Adhesion Society, Panama City Beach, USA, 1999, 4. 48. M. M. Pastor-Blas, R. Torregrosa-Maciá, J. M. Martín-Martínez and J. G. Dillard, International Journal of Adhesion and Adhesives, 1997, 17, 2, 133. 49. A. Chew, R. H. Dahm, D. M. Brewis, D. Briggs and D. G. Rance, Journal of Colloid and Interface Science, 1986, 110, 1, 88. 50. J. Shields, Adhesives Handbook, 3rd Edition, Butterworth, London, 1984, 87. 51. R. F. Wegman, Surface Preparation Techniques for Adhesive Bonding, Noyes Publications, New Jersey, 1989, 127. 52. R. R. Sowell, N. J. DeLollis, H. J. Gregory and O. Montoya, Journal of Adhesion, 1972, 4, 1, 15. 53. J. Y. Lai, Y. Y. Lin, Y. L. Denq, S. S. Shyu and J. K. Chen, Journal of Adhesion Science and Technology, 1996, 10, 3, 231. 54. E. P. Everaert, H. C. van der Mei, J. de Vries and H. J. Busscher, Polymer Surface Modification, Ed., K. L. Mittal, VSP, Utrecht, 1996, 33. 55. M. Hudis and L. E. Prescott, Journal of Applied Polymer Science, 1975, 19, 2, 451. 56. J. R. Hollahan and G. L. Carlson, Journal of Applied Polymer Science, 1970, 14, 10, 2499. 57. J. L. Fritz and M. J. Owen, Journal of Adhesion, 1995, 54, 1/4, 33. 58. M. J. Swanson and G. W. Opperman, Journal of Adhesion Science and Technology, 1995, 9, 3, 385. 59. C. Combette, D. Hivert, J. Maucourt, W. Brunat, T. M. Duc, G. Michel, P. Le Prince and G. Legeay, Presented at Euradh 94, Mulhouse, France, 1994, 416. 60. A. Chew, D. M. Brewis, D. Briggs and R. H. Dahm in Adhesion 8, Ed., K. W. Allen, Elsevier Applied Science Publishers, London, 1984, 97. 61. Handbook of Plastics Joining, A Practical Guide, Plastics Design Library, Norwich, NY, 1997. 62. K. W. Allen, Joining of Plastics, Rapra Review Report No.57, Rapra, Shrewsbury, UK, 1992.
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Substrate Preparation Methods 63. D. G. Brandon and W. D. Kaplan, Joining Processes, An Introduction, Wiley, Chichester, 1997. 64. F. Garbassi, M. Morra and E. Occhiello, Polymer Surfaces from Physics to Technology, Wiley, Chichester, 1998. 65. J. Shields, Adhesives Handbook, 3rd Edition (Revised), Butterworths, London, 1985. 66. Handbook of Adhesives, 3rd Edition, Ed., I. Skeist, Van Nostrand Reinhold, New York, 1990. 67. Treatise on Adhesion and Adhesives, Volume 5, Ed., R. L. Patrick, Marcel Dekker Inc., New York, 1981. 68. A. H. Landrock, Adhesives Technology Handbook, Noyes Publications, Parkridge, NJ, 1985. 69. R. C. Snogren, Handbook of Surface Preparation, Palmerton, New York, 1974.
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56
2
Rubber to Metal Bonding J. R. Halladay and P. A. Warren
2.1 History Rubber finds use in many applications as a means of isolating vibration and reducing shock or as a way to seal in solids, liquids and gases. For many of these applications, it is desirable or even imperative that the rubber be attached to a metal substrate in a reliable manner. There is a fundamental difference between bonding of rubber to metal involving crosslinking mechanisms and the physical ‘sticking’ of rubber to metal using a non-vulcanising adhesive. The former involves a chemical reaction (generally during cure) while the latter generally relates to a physical surface tension phenomenon. Bonded rubber parts have found use in a myriad of dynamic applications such as engine mounts, suspension bushings, body mounts, torsional dampers, helicopter rotor bearings, seismic bearings, transmission and axle seals, and as flexible couplings. These parts are usually made by vulcanising the rubber and bonding it to the metal component in a single-stage press operation. Fluid engine mounts (Figure 2.1) for the aerospace industry and automotive hydromounts require the bonded rubber to act as both a seal for the fluid and as a spring component
Figure 2.1 Cross section of a typical Fluidlastic® mount Courtesy of Lord Corporation
57
The Handbook of Rubber Bonding in the isolation system. Many of the applications such as a helicopter rotor bearing (Figure 2.2) comprised of alternating layers of rubber and metal in special geometric configurations, would be impractical or even impossible without the ability to obtain a rubber to metal bond with a high degree of integrity and reliability. Other rubber parts may be bonded to metal simply for ease of assembly or to provide a tolerance for misalignment. Just within an automobile, bonded rubber to metal assemblies are used for engine mounts, oil seals, couplings and bushings for the engine, transmission and drive train, fan hub couplings, body mounts, exhaust hangers, weather strips and window channelling, and tuned vibration absorbers within the frame and suspension. This illustrates just how important bonded rubber to metal assemblies are to everyday life. The increased use of rubber in automotive, aerospace, and industrial applications has driven the requirement for strong and robust bonds between rubber and metal. Much literature has been published on the history and technology of bonding rubber to metal [1, 2, 3, 4, 5, 6, 7, 8]. The earliest historical methods of attaching rubber to metal involved attaching the rubber by mechanical means or by the use of ebonite. Mechanical
Figure 2.2 Cross section of a helicopter rotor bearing (Photograph by James Halladay. Courtesy of Lord Corporation)
58
Rubber to Metal Bonding attachment, which is still used today in some cases, creates an insecure union. Ebonite is made by mixing approximately 30 to 40 phr (parts per hundred rubber) of elemental sulphur with natural rubber (NR). It forms a true bond to the softer sulphur curable rubbers. It also adheres rather strongly to metal. Soft rubber compounds normally contain less than 4 phr sulphur which creates sulphur crosslinks between the rubber molecules. At sulphur levels between 25 and 45 phr, hard rubber or ebonite is formed. In ebonite, a large proportion of the sulphur is believed to be in the form of intramolecular addition since it is noticeably thermoplastic [9]. Bonding with ebonite has several disadvantages. One significant drawback is that the ebonite is thermoplastic and becomes quite weak with moderate temperature exposure. Depending on the amount of sulphur, ebonite based on NR shows a thermoplastic transition temperature, i.e., softening, between 70 and 80 °C. At sulphur levels between 4 phr and 25 phr, NR goes through a transition where it becomes rather leathery and is of little use. Because there will be a gradient of sulphur between the ebonite adhesive and the soft rubber compound, at some point, the sulphur content of the compound must pass through this transition zone. This transition zone weakens the softer rubber in the interfacial region and it reduces the flexibility in that region. As a further drawback, bonding with ebonite limits the chemistry of rubber formulations that can be successfully bonded using this technique. Another method of bonding involved the use of special metal alloys which were capable of reacting with and combining with sulphur. The earliest patent for the use of alloys was in Germany in 1904 [10]. Daft patented alloys containing antimony in the US between 1912 and 1913 [11, 12, 13, 14]. He also claimed the use of alloys of copper and zinc with bismuth and arsenic. These alloys were electrically deposited on the metal and the bonds to rubber were formed during the vulcanisation process. In 1862, Sanderson submitted a British patent application for the use of electrodeposited brass as an intermediary for bonding rubber to iron or steel [15]. It was not until between 1920 and 1930 that the process of bonding to a galvanic layer of brass (brass plating) was commercialised. The bond is obtained by virtue of the chemical reaction that occurs between the brass and the sulphur curative in the rubber and it has the advantage over the ebonite process of not being heat sensitive. This process requires a large investment in processing machinery and it is difficult to keep all the variables in the galvanic bath constant. It is somewhat unpredictable and shows a high sensitivity to processing conditions. As with bonding to ebonite, it limits the chemistry of the formulations that can be successfully bonded to only those compounds with a high sulphur cure (2 to 4 phr). As a further complication, not all types of brass will bond to rubber and it appears that the best ratio of copper to zinc is somewhat compound dependent. In a production environment, consistent results are often hard to obtain without considerable experience and the factors which must be carefully controlled include bath and anode composition, temperature, voltage, current density, time of deposition and hydrogen-ion concentration.
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The Handbook of Rubber Bonding However, the brass plating process has proven quite successful for certain applications such as steel cords for automotive tyres. Another method of rubber to metal bonding was discovered in Germany around the end of World War II and involves the use of isocyanates, in particular triphenylmethane triisocyanate [16, 17]. Polyisocyanates applied to a clean metal surface give good primary adhesion between many types of NR and synthetic rubber formulations and a wide variety of metal substrates without the brass layer. Isocyanates are very sensitive to moisture and steam and because they are extremely reactive, there is the potential for undesirable side reactions with the compounding ingredients in the rubber formulation. Because of their extreme reactivity with moisture, exposure to even moderately humid conditions during the time between application to the metal and the moulding process results in loss of much of the adhesive strength of the bond. During the 1950s, another area of research was based around self-bonding compounds. Patents issued in the late 1950s and early 1960s covered the addition of cobalt salts to the rubber formulation [15]. The use of resorcinol and a formaldehyde donor (RFL system) had been developed in 1935 as a dip for rubber to textile adhesion (see also Chapter 9). In the 1960s, these components were added directly into the rubber along with a reinforcing silica filler by Bayer and Degussa independently to make self-bonding compounds (RFK system). Hexamethylolmelamine ethyl ethers or hexamethylene tetramine were used as the formaldehyde donors to form an in situ curable resin capable of bonding the rubber to textiles or metal, particularly steel tyre cord. More recently, techniques for making self-bonding compounds by incorporating metallic coagents have been proposed [18] (see also Chapter 11). The coagents most often used are zinc diacrylate (ZDA) and zinc dimethacrylate (ZDMA) although other metals such as calcium and magnesium have been noted as somewhat less effective. These metallic monomers derive some unique physical and mechanical properties from the ionic bonds that are formed between the metal cation and the carboxylate anion. The same ionic crosslink mechanism is believed to occur with rubbers that are cured with ZDA or ZDMA. These coagents are only effective with peroxide (or free radical) cure systems. There are several drawbacks to the use of self-bonding compounds which are worthy of mention. To make a self-bonding compound, one must modify the properties of the bulk rubber compound in order to affect the chemistry of the reactions which are required to take place only at the rubber to metal interface. Processing is adversely affected since the compounds want to bond to the mould and metals alike, and the environmental resistance of the rubber to metal bond is generally poor compared to that which is obtained with the use of a conventional rubber to metal adhesive system.
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Rubber to Metal Bonding The dearth of versatile and highly satisfactory bonding approaches prompted the Lord Corporation to investigate improvements in the rubber to metal bonding process shortly after World War II. The efforts led to the development of general purpose chemical adhesives containing polymers and crossbridging agents in solution. The first general purpose adhesive system was introduced in 1956 and comprised a primer and an adhesive top coat or cover-coat which produced rubber tearing bonds over a spectrum of different rubbers which were commercially available at that time [2]. Rubber tearing bonds refers to bonds which show cohesive failure (failure within the rubber) when subjected to destructive testing. Migration of crosslinking agents from the adhesive layer into the rubber compound produce a tougher layer adjacent to the metal and increase the tendency to have cohesive failure in the rubber during destructive testing. The primer/cover-coat system gave bonds with better environmental resistance than any other system available at the time and rapidly became the standard for rubber to metal bonding practice. Perhaps more importantly, the new adhesives were broadly compatible with all the important high diene rubbers over a broad range of curing conditions. They paved the way for bonding newer rubbers such as the polychloroprenes and for compounds containing a wider range of compounding ingredients than was previously possible. They also helped to eliminate the restrictions on the design engineer’s choice of metal for the substrate. The last four decades have seen the introduction of many new rubber to metal adhesives designed to cover the ever increasing range of synthetic rubbers currently available for use in dynamic applications. These include one coat adhesives, adhesives for postvulcanisation bonding, specialty rubber adhesives for silicones, fluorosilicones, fluororubbers, acrylics, and hydrogenated nitrile rubbers, along with the recent introductions of water-based adhesives. Today, many companies make adhesives for chemically bonding rubber to metal. The following companies supply general purpose primers and adhesives: Company
Tradename
Lord Chemical Products Division of Lord Corporation
Chemlok
Henkel KGaA (Lord licensee)
Chemosil
Rohm and Haas
Thixon; Megum
Par Chemie
Parlok
Chemical Innovations Limited
Cilbond
Metalok
Metalok
Proquitec
Adetec
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The Handbook of Rubber Bonding
2.2 Bond System Characteristics 2.2.1 Adhesive Characteristics General purpose primers and adhesives used for bonding rubber to metal are highly proprietary, specially formulated products. They usually contain a mixture of polymers, resins, curatives, pigments, extenders, and other ingredients, e.g., corrosion inhibitors or viscosity stabilisers. These materials are either dissolved or suspended in a liquid media. Up until the early 1990s, rubber to metal adhesives were almost exclusively formulated in organic solvents. Due to the need to reduce emissions of volatile organic compounds (VOCs), a growing number of aqueous rubber to metal adhesives are being commercialised. Rubber to metal primers contain organic resins which react with most metal (steel, aluminum, stainless steel, copper, brass) surfaces during the vulcanisation process to form a chemical bond to the metal. They also contain polymers which allow for better film formation and act as an anchor for the subsequent application of the adhesive. Rubber to metal adhesives contain polymeric materials that are compatible with the ingredients in the primer, as well as the rubber compound to be bonded. Many are based on halogenated polymers. Halogenated polymers or resins are known to wet metals efficiently and can be used in both the primer and adhesive formulation. They provide effective barriers to chemicals that can undermine the adhesive bond. The adhesive also contains very powerful curatives that react with both the polymers in the rubber and the polymers in the adhesive [19]. Difunctional and polyfunctional chemicals are capable of making the film forming polymer a thermoset as well as reacting across the interface of the film to link into the rubber. The rubber to metal bonding mechanism is very complex as there are several reactions occurring simultaneously. All these reactions must take place in a very short period of time (i.e., during the press cure time of the rubber) in order for a strong bond to form. The different reactions taking place are shown in Figure 2.3. Each of the three organic layers in a rubber to metal bond (primer, adhesive, rubber) crosslink or cure during the moulding step. The source of this crosslinking is the presence of either a heat reactive resin or externally added crosslinking agents. This internal curing increases the molecular weight and cohesive strength of each layer. In addition, each layer undergoes reactions with the layers immediately above and below it. These interlayer reactions are caused by the same chemical ingredients that allow for internal crosslinking to occur. The first ‘link’ in a rubber to metal bond is the primer to metal interface. As mentioned previously, in addition to the internal crosslinking that takes place in the primer, organic
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Rubber to Metal Bonding
Figure 2.3 Schematic of vulcanisation bonding process (Courtesy of Lord Corporation)
resins in the primer react with metal oxides on the surface of the metal part to form very strong covalent chemical bonds. This type of reaction is called chemisorption [8]. It is differentiated from normal adsorption or physical bonding in that bonds formed by chemisorption are very resistant to attack from water, heat, and chemicals. In contrast, bonds formed by adsorption are easily destroyed by the application of environmental forces such as heat or chemical exposure. The next link in the rubber to metal bond is between the primer and adhesive interface. The curative present in the adhesive layer migrates or diffuses into the primer layer during vulcanisation and forms a chemical bond between the primer and adhesive. The polymeric film former present in the primer diffuses and knits with the adhesive layer and further strengthens the bond between primer and adhesive because of its compatibility with polymers present in the adhesive layer. The final link in the rubber to metal bond is the adhesive to rubber interface. The curative present in the adhesive layer also diffuses into the rubber during vulcanisation and forms a chemical bond between the adhesive and the rubber. These bonds which span across the layers in the assembly are called ‘crossbridges’ to differentiate them from the crosslinks which occur within the rubber itself. In addition, sulphur from the rubber compound diffuses into the adhesive layer and helps to form additional crossbridges between the rubber and the adhesive. Rubber to metal adhesive systems generally occur in two broad classes. These are primer/ cover-coat systems and one coat (or single coat) systems. In the primer/cover-coat systems,
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The Handbook of Rubber Bonding the primer primarily contains materials which will form strong and enduring bonds with the metal surface. The modulus of the cured primer is intermediate between that of the rubber and that of the metal, but it is closer to that of the metal. The cover coat, on the other hand, primarily contains materials that form bonds with the rubber and the resulting modulus of the cured adhesive is closer to that of the cured rubber. The complete system provides a gradation in the modulus between the rubber and metal and creates a better stress distribution. One coat adhesives, by necessity, contain both materials which react with the metal surface and materials which react with the rubber. These materials, in many cases, are not stable together, so long term shelf stability of one coat adhesives is more difficult to achieve. Each system has its advantages and disadvantages. Typically, primer/cover-coat systems are more resistant to extreme environmental conditions such as hot oil or extended salt spray exposure. However, primer/cover coat systems are more expensive to apply because of the need to have two sets of application equipment, one for primer and another for cover-coat. One coat systems only require one set of application equipment and require only one application step instead of two, and hence, are less costly to process. Inventory issues are significantly simplified with the use of one coat adhesives.
2.2.2 Compound Characteristics Although the development and successful commercialisation of organic adhesives for bonding rubber to metal has freed rubber chemists to use a wider variety of materials while still achieving excellent bonds to metal, there are some general ‘rules of thumb’ that should be followed where possible to improve the probability of bonding to the rubber. The first rule is to use the easiest-to-bond type of rubber that will provide the required service performance of the part. In general, there is a hierarchy among rubber types which ranks them according to their ability to be bonded with adhesives. This hierarchy is called the ‘bondability index’ [4]. What causes differences in bondability is still a matter of debate. It has been attributed to differences in polarity, chemical reactivity, solubility and molecular symmetry between the different available classes of rubbers [4]. Regardless of the cause, the bondability of general purpose rubbers is ranked as follows: Easiest to bond
Nitrile (acrylonitrile-butadiene) rubber (NBR) Polychloroprene (CR) Styrene butadiene rubber (SBR) Natural rubber or polyisoprene (NR or IR) Ethylene propylene diene rubber (EPDM)
Most difficult to bond
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Isobutylene-isoprene (butyl) rubber (IIR)
Rubber to Metal Bonding Many times, the choice of rubber is dictated by the service requirements of the part. Even within classes of rubbers, there are different degrees of bondability based on the specific polymer chosen and depending on the rest of the ingredients in the formulation. Nevertheless, the bondability index shows the relative ease of bonding certain classes of rubbers. The choice of vulcanisation system for the rubber can have a dramatic effect on adhesion. Typically sulphur cured rubbers are easier to bond to than sulphur-free or peroxide cured rubbers. This is believed to be due to the interaction of sulphur with key curative materials in the adhesive. The more sulphur that is present, the more interactions that are available, and hence the better the chance of getting good adhesion. SEV (semiefficient vulcanisation) and EV (efficient vulcanisation) cure packages are typically more difficult to bond because of their lower free sulphur contents. EV refers to cure systems which give predominantly monosulphidic or disulphidic crosslinks whereas conventional sulphur cure systems produce mostly polysulphidic crosslinks. SEV systems fall somewhere between EV and conventional systems in the type of crosslinks produced. Vulcanisation proceeds at different rates and with different efficiencies in different types of polymers, so the amount of sulphur needed to produce an EV cure system will also vary. For example, in NR, an EV system will generally contain between 0.4 and 0.8 phr of sulphur, while in NBR the sulphur level will generally be less than 0.3 phr of free elemental sulphur. In sulphur cured rubbers, accelerators are generally used to reduce the dependency on sulphur in order to achieve more efficient vulcanisation, to improve heat and flex resistance due to the presence of more monosulphidic crosslinks, and to increase the cure rate of the rubber and improve production capacity. Two accelerators which have been shown to enhance bondability of rubbers are 2-mercaptobenzothiazole (MBT) and mercaptobenzothiazole disulphide (MBTS). An accelerator which is known to negatively impact on adhesion is tetramethyl thiuram disulphide (TMTD). Peroxide cured rubbers are the most difficult to bond to metal with conventional adhesives. This is because the free radical peroxide cure mechanism competes with the curative in the adhesive for reactive sites on the rubber backbone. Fillers in the rubber play an important part in adhesion. When used in ‘normal amounts’ (20 – 50 phr), the more highly filled the rubber, the easier it is to bond. This is because increasing the filler loading of the rubber increases the modulus of the rubber to more closely resemble the modulus of the adhesive, thus reducing differential stress at the interface. Of course, at excessive filler loadings, a point is reached where the adhesive film contacts more filler than rubber. At this point adhesion drops off significantly. Carbon black and silica are the preferred fillers for improved adhesion and channel blacks are preferred over thermal or furnace blacks [4].
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The Handbook of Rubber Bonding Processing oils are often necessary to ensure good flow and proper filling of moulds. Unfortunately, the use of these oils can seriously hamper adhesion due to their ability to migrate to the adhesive/rubber interface during vulcanisation and interfere with crossbridging reactions. Lower levels of processing oils are always preferred for best adhesion. Naphthenic oils have the least deleterious effect on adhesion, while aromatic and ester based oils should be avoided if at all possible. Antidegradants including waxes, antiozonants, antioxidants, and prevulcanisation inhibitors are also necessary for good processability and performance of rubbery parts. Unfortunately, these materials are also bad for adhesion because they can also migrate to the rubber surface and interfere with crossbridging reactions. The lowest possible amount of these materials necessary to get acceptable part performance without causing blooming to the surface of the rubber is preferred.
2.3 Adhesion The selection of the best adhesive to use in a particular bonding situation depends on several factors. They include: • The rubber being bonded, • Government regulations in force in a particular area, • The moulding process employed (compression, injection or transfer), • Level of environmental resistance required, • Adaptability to existing adhesive application equipment, • Cost. Of course, one needs to use an adhesive that is designed for use with the particular rubber being bonded. Some adhesives are designed for bonding a broad range of NR and general purpose synthetic rubbers. Others are designed to bond specific hard-to-bond general purpose rubbers such as EPDM or IIR. Some adhesive systems are designed specifically for bonding specialty rubbers such as urethanes, hydrogenated nitrile rubber (HNBR), silicones and fluororubbers. It should be obvious that these are not interchangeable simply from a chemistry standpoint. Adhesives for specialty rubbers are often based on organofunctional silanes or on reactive phenolic resins. Silane-based adhesives chemisorb strongly onto clean glass or metals such as aluminum, brass, iron, steel, titanium, and other alloys. Government regulations regarding solvent emissions in the user’s locality are also of importance. In the US, some customers must use aqueous adhesives or adhesives containing
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Rubber to Metal Bonding certain permissible solvent systems in order to stay in compliance with government regulations on volatile solvent emissions. The type of moulding process being employed is important to consider. Some adhesives perform better at low cure temperatures (less than 155 °C) with long cure times, e.g., compression moulding. Some adhesives are designed to work in high temperature (greater than 170 °C) and short cure time applications, such as injection moulding. The level of environmental resistance required and equipment available may determine whether to evaluate primer/cover coat systems versus one coat adhesive systems. And last, but not least, cost of both the adhesive and the application process is an important factor to consider in any industrial application, rubber to metal bonding being no exception. An important requirement for good bonds is proper preparation of the metal substrate. Untreated metal surfaces are generally subject to corrosion and other changes over time. Corrosion is often described as destruction of metal by chemical or electrochemical reaction. Most environments are corrosive, but by no means to the same degree. Simply bonding a rubber layer over the top of the metal surface does not stop corrosion from occurring underneath the bond. While a strong primary (initial) bond may sometimes be obtained with poorly prepared substrates, the bond will degrade over time, especially with exposure to harsh environments, leading to premature failure or a shortened operating life. Environments which foster corrosion include: air and moisture, fresh or salt water, steam, chlorine, ammonia, hydrogen sulphide, sulphur dioxide, mineral acids, organic acids, alkalis, solvents and fuel gases. Often, the underbond corrosion is aggravated by bonding to surfaces where corrosion has already begun. The substrate and type of contaminant dictate the proper cleaning procedure. In most cases, metal preparation involves two main steps. The first is a cleaning step to remove dirt, oil, and other surface contaminants. The cleaning step can be accomplished by either solvent degreasing (typically with perchloroethylene) or by alkaline cleaning (aqueous). Next comes a step designed to activate the metal. The functions of the metal activation step are: 1. to remove any contaminants such as rust, scale, and pre-existing corrosion byproducts which are bound to the metal surface, 2. to increase the surface area of the metal, 3. to provide an active surface for bonding. The metal activation step can be accomplished by use of mechanical or chemical means. Mechanical methods for steel include blasting with 40 mesh steel grit, sand or aluminium oxide grit. Other metals may be blasted using clean sharp aluminium oxide grit or sand.
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The Handbook of Rubber Bonding Steel grit should not be used on non-ferrous metals as it will leave a ferrous smudge that can later oxidise and degrade the bond. After blasting, it is recommended to use either solvent degreasing or alkaline cleaning on the metal a second time to remove dust and any other particulate byproducts from the blasting process. While it is possible to bond to a freshly abraded or cleaned metal surface, chemical treatments are preferred for rendering the metal surface inactive to corrosion over time. For low carbon steel, phosphatising is the recommended pre-bond surface preparation treatment. Stainless steel should be passivated or acid etched, while titanium is usually treated with a hydrofluoric acid pickle. Aluminium or magnesium are best treated with a chromate conversion coating. Zinc and cadmium are generally prepared mechanically but a phosphate or chromic acid treatment may be used. Brass and copper may be treated with an ammonium persulphate etch or an acid-ferric chloride etch. The type of surface preparation employed depends on several factors. Cost is one important factor. Chemical pretreatments are usually less expensive than mechanical treatments, especially from the standpoint of labour efficiency. But versatility of the process is also important. Mechanical treatments can be useful for many different metal surfaces. However, chemical treatments are usually metal specific. So several chemical treatments may be necessary if different metals are to be bonded in the same plant or operation. It is important to use metal treatments that can be adapted to the user’s manufacturing environment. Available floor space, ventilation, and equipment capacities must be taken into account. The environmental resistance requirements of the bonded part are the key to determining what type of metal treatment may be used. For parts which have very demanding corrosion resistance requirements, i.e., automotive or aerospace applications, chemical treatments are highly recommended. For parts being used in non-demanding environments, for example, indoor applications, mechanical pretreatments may suffice. Finally, local government regulations regarding hazardous waste disposal must be considered. Chemical treatments, such as zinc phosphatising, usually involve the generation of hazardous sludge as a byproduct of the process. If local regulations in force for a given area make disposing of this sludge expensive or inconvenient, then mechanical treatment methods may be a less troublesome option. Regardless of the metal pretreatment used, it is important to maintain as much control over the process as possible in order to assure consistently good results. Lubricants and anticorrosive oils that contain chlorinated paraffins or silicone should be avoided, as they can interfere with adhesion. All degreasing solvents and cleaning solutions must be kept clean and free of contamination. Grits and abrasives must be kept clean and periodically changed. Rinse water and drying air must be checked frequently for purity and kept free from oils. A simple test to measure the effectiveness of the metal pretreatment is called the ‘water-break’ test. In this test, the prepared metal part is dipped briefly in
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Rubber to Metal Bonding de-ionised water and then removed. The surface of the dipped part is examined for signs of poor wetting. If the part can be wet by water with no breaks or ‘fisheyes’, then the metal is considered to be clean. The water break test is described in ASTM (American Society for Testing and Materials) test method F22 [20]. A typical metal pretreatment process for steel might involve the following steps: alkaline cleaning, water rinse, phosphoric acid pickle, water rinse, zinc phosphate treatment, rust inhibitive treatment (seal), water rinse, followed by drying. The zinc phosphate coating must be controlled for maximum effectiveness. While a thin coating of zinc phosphate crystals reduces underbond corrosion and improves environmental resistance, a thick coating will reduce the bond strength. This is because the cohesive strength of the zinc phosphate crystal is less than the adhesive strength of the crystal to the adhesive or to the metal substrate. As a result, too thick a coating will simply fracture within the zinc phosphate crystal causing lower than expected bond values. A typical treatment for stainless steel is a vapour degrease or alkaline clean followed by immersion for 15 to 20 minutes at 50 to 55 °C in a solution consisting of 20 – 25% nitric acid (by weight), 2 – 4% sodium dichromate (by weight), and 71 – 78% water (by weight). There is increasing use of rigid plastics as substrates in place of metals. Plastic surfaces may be prepared by chemical cleaning and/or surface roughening with a mechanical blast. Flame treatment and corona treatment are also viable options. Once an optimum surface condition has been obtained, care must be taken to preserve it until the primer or adhesive has been applied. It is important to prevent exposure to dust, moisture, mould sprays, or oils from handling. Even exposure to air over a period of time can create an oxide layer so thin it cannot be seen, but one which can cause adhesion problems later. One of the best methods of preserving the surface is to apply primer as soon as possible. Primer and adhesive application are generally accomplished by spraying, brushing, tumbling or dipping. Each different application method has its own strengths and weaknesses. The choice of the application method is dependent on the size and shape of the parts, the number of parts to be coated, and whether the part is to be wholly or only partially coated. Spraying assures excellent application where selective or spot coating is required and may be easily automated. Spraying can be accomplished by several methods (conventional air assisted, airless spray or electrostatic spray). Sprayed films usually give the best aesthetic appearance to a coated part (no runs, sags or tears). Dry film thickness is easier to control with spraying because adjustments can be made in adhesive dilution, fluid flow,
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The Handbook of Rubber Bonding and air pressure to give different levels of film buildup. In applications where only specific areas of the part are to be coated with adhesive, spraying allows the best chance to coat only certain areas of the part by the use of spray masks. The weakness of spraying is that adhesive is lost due to the need for overspray to ensure proper coverage of the part. The use of high volume low pressure (HVLP) and electrostatic spray options has improved the efficiency of spraying as an application process. There are many manufacturers of spray equipment who can recommend the proper spray guns for a particular application. The air pressure to the gun should be adjustable, as should the atomisation pressure and the tip opening. Adjustment of these variables will allow for proper wetting and coverage of the metal. Liquid lines should have a 100 mesh screen to prevent dried or agglomerated material from getting to the gun and clogging it. Liquid lines should be short with diameters of 13 mm or less. This will provide good mixing of the product in the line from turbulent flow. Short liquid lines will prevent settling of dispersed particles in the line. Air lines should have water traps to prevent oil or water from contaminating the adhesive and causing bond problems. The adhesive being sprayed should be under continuous agitation to keep the product homogeneous and to prevent settling of active solid materials. Proper maintenance of spray equipment is important to ensure continuous problem-free operation. Gun tips should be checked periodically for adhesive build up. If any is found, it should be removed to keep the spray pattern consistent. Fluid lines should be flushed periodically. Fluid pressure regulators should be cleaned periodically to keep them working properly. Spray guns should be flushed and rebuilt periodically to replace worn parts and to remove any adhesive build up. Dipping is a convenient and economical method for adhesive application for small runs and it can be automated for larger runs using a conveyor belt. Dip application of rubberto-metal adhesives allows for better transfer efficiency because once the metal part comes out of the dip tank, much of the adhesive which drains off the part goes back into the bath thus conserving adhesive. The transfer efficiency of dipping is better than that of spraying and because dipping is a simpler process than spraying, there are less variables to control. However, there are not many things one can do to control dry film thickness in dipping due to its lack of control variables, e.g., adhesive solids and viscosity are fixed unless dilution can be used. Also, in dip applications, the dry film thickness of adhesive increases going from the top to the bottom of the part. This is due to gravity pulling the wet adhesive down the part surface as it dries. Dipped parts are susceptible to formation of tears and drips on the parts. These tears can cause problems with mould fouling during the moulding step. Also, dipping is useful only for those parts where no masking is necessary, since the whole metal part is dipped. Adhesive is applied even to areas where there will be no rubber present for bonding, so some adhesive is wasted.
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Rubber to Metal Bonding Adhesive used in dip applications should be kept under continuous agitation. If a dip tank is used, double diaphragm type circulating pumps are recommended and impeller type agitators should be used. Brushing is useful for prototypes and small or discontinuous production runs. However, brush application of rubber to metal adhesives, while being the simplest method, is not recommended. This is because control of dry film thickness is very difficult to achieve over the part surface. Also, imperfections, such as brush marks or loose bristles in the adhesive film can interfere with performance. Both primers and adhesives should be kept well agitated prior to, and during, application and the coatings should be uniform in thickness across the part. Primers are required for maximum environmental resistance and the primer should be completely dry before the adhesive cover coat is applied. When more than one coat of adhesive is applied, adequate time and temperature must be allowed to ensure complete solvent evaporation between coats. Primers are usually applied to a dry film thickness of 5 to 10 μm. Cover coat adhesives are applied at dry film thicknesses of 10 to 15 μm. One coat adhesives work best at applied dry film thicknesses of 20 to 25 μm. Measuring instruments to check dry film thickness should be used to insure adequate adhesive application and as a quality control check. Beta backscatter machines work well for all types of inorganic surfaces. However, they are not suitable for adhesives that contain radiation absorbers such as lead. Magnetic induction current instruments work well on steel, but not on non-magnetic substrates such as aluminum or glass.
2.4 Effective Bond Formation Even after the metal parts are coated with adhesive, care must be taken to ensure that the surface of the adhesive film does not become contaminated prior to moulding. Any material (dirt, oil, etc.) which can get in between the adhesive and the rubber will prevent the formation of a robust chemical bond and failure will be likely to result. Operators who handle coated parts should wear clean cotton gloves to prevent oils from their hands from contaminating the adhesive. Coated parts should not be stored in areas where they can be exposed to mould releases (either splashed or airborne droplets), dust, and moisture. If coated parts are to be stored for any extended period of time, the container should be covered with either cardboard or untreated Kraft paper. Coated parts should also be kept in areas where they will not be exposed to sunlight or UV radiation for extended periods of time.
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The Handbook of Rubber Bonding The moulding step is arguably the most important step in the process of making rubber to metal bonded parts. It is during this step, with heat and pressure applied for a prescribed amount of time, that the rubber is vulcanised and the actual bond between the rubber and the adhesive-coated part is formed. Each step of the moulding process must be carefully controlled to maintain consistently good quality in the bonded parts. Three of the most important factors are moulding pressure, moulding temperature, and mould design. For the best adhesion, it is important to maintain maximum mould pressure while the rubber is at minimum viscosity. This ensures the best wetting of the rubber over the adhesive surface. Adequate pressure must be maintained throughout the rubber cure cycle. If the pressure is insufficient, the rubber may become porous during the cure and the bond to the adhesive will be poor. The temperature throughout the mould must be maintained at a consistent level. Low temperature zones in the mould can cause undercure of the rubber and this will lead to poor adhesion. High temperature areas in the mould can cause overcure or reversion (crosslink degradation) of the rubber and possible pre-cure of the adhesive before rubber can come into intimate contact with it. When designing moulds, loading of coated metals and removal of bonded parts should be made as easy as possible. The time required to load the mould with adhesive coated metals should be kept to a minimum. The longer the coated metals sit in the hot mould without being exposed to rubber, the greater the chance that premature curing of the adhesive will take place, with a subsequent loss of adhesion. Effort should be made when designing both the part and the mould to place areas of high stress concentration as far away from the rubber/metal interface as possible. Mould parting lines should be avoided in critical bond areas. Sprues and gates into the cavity should be placed if possible in such a manner that the flow of rubber does not cause sweeping of the adhesive from the metal surface. Moulds can create problems for the bonding process if they are either too tight or too loose. If they are too tight, volatile gases cannot escape, but if they are too loose, the rubber compound can continue to seep out under pressure before and during the vulcanisation stage. This continued seepage can cause the adhesive to be swept from the metal surface, reducing the integrity of the bond. It can also cause a loss of pressure inside the cavity, and adequate pressure is one of the factors necessary for good bonding. Moulds should be vented if possible to allow for escape of volatile components from both the adhesive film and from the rubber during the moulding process. If the volatile components from the adhesive are not allowed to escape, they may react with the rubber being introduced into the mould cavity and cause pre-cure of the rubber before it has a chance to fill the cavity. When this happens, bonded parts have small ‘knit lines’ or ‘splits’ formed in the body of the rubber. These knit lines often dramatically shorten the useful service life of a bonded part by developing into premature fatigue cracks during service. If volatile compounds from the rapidly heating rubber are not vented, they may contaminate the adhesive surface, thus reducing the quality of the bond.
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2.5 Post Vulcanisation Bonding Post vulcanisation bonding (also referred to as PV bonding) is a specialised variation of the rubber to metal bonding process. For PV bonding, adhesive is applied to metal, just as for vulcanisation bonding. However, in this process, the rubber has already been moulded and is fully vulcanised. Not all adhesives give good results in PV bonding and the selection of satisfactory adhesives for a given application will be more limited than for vulcanisation bonding. Usually the surface of the vulcanised rubber to be bonded is given a treatment to remove any materials which can interfere with bonding (i.e., surface bloom, mould release agents) and to provide a fresh surface to bond to. Chemical or mechanical methods can be used. Chemical methods involve applying a chlorination treatment to the rubber surface. The chlorination treatment creates reactive sites on the surface of the rubber where the adhesive can interact. It also changes the surface energy and makes it easier for the adhesive to wet the rubber surface completely. A common mechanical method is buffing the rubber surface with an abrasive material, such as sandpaper. The treated rubber is then put in contact with the adhesive coated metal using moulds or tooling fixtures to position the rubber and hold it in place. A compressive deflection of 5 – 10% is maintained to keep the two surfaces in intimate contact while heat is applied until a bond is formed. An oven, autoclave, or induction heating unit can be used, depending on the size and number of parts to be bonded. PV bonding has both advantages and disadvantages. Advantages include the elimination of the need for expensive moulds to make parts and the ability to make bonded parts at lower temperatures (125 – 150 °C). One disadvantage is that extra steps are needed in the process of making the parts (separately moulding the rubber sections followed by treatment of rubber surface and finally bringing the rubber and metal parts into intimate contact at elevated temperatures). Another disadvantage is that since the rubber is already cured, limited interfacial mixing between the rubber and the adhesive may lessen the quality of the final bond.
2.6 Factors Affecting Bond Integrity After bonding, subsequent manufacturing procedures are generally required to finish the part and prepare it for shipment. These procedures can have an effect on the integrity of the bond if caution is not exercised. First, care must be taken during demoulding to avoid over-stressing the bond while the part is at curing temperature. Deflashing procedures such as wire brushing and grinding as well as post-bond machining operations must be controlled to avoid exposing the part to excessive heat and stress. Electroplating can reduce the bond strength if the current densities are not controlled or if the plating solution attacks the adhesive. Applying rust preventatives or high solvent paints to the
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2.7 Bond Failure Types A rubber to metal bonded part can be thought of as a ‘chain’ which holds rubber and metal together. Any chain is only as strong as its weakest link. So it follows that when a rubber to metal part fails, it will fail in the weakest section of the part. A test method exists which specifically covers adhesion of rubber to metal, ASTM D429-2002 [21]. The vast majority of bond failures can be attributed to one or a combination of the following ASTM designations: R
- Rubber failure: failure in the body of the rubber.
RC
- Rubber/Cement failure: failure at the rubber to cement (adhesive) interface.
CP
- Cement/Primer failure: failure at the cement (adhesive) to primer interface.
CM - Cement/Metal failure: failure at the primer to metal interface. For one coat adhesives, this is failure at the adhesive to metal interface. Rubber failure (R) is the type of failure to strive for. It indicates cohesive failure of the rubber. This means that the bond between the rubber and the adhesive is stronger than the tear strength of the rubber. Rubber/cement (RC) failures indicate that the weakest point in the bonded part is at the interface between the rubber and adhesive. These failures are characterised by a relatively glossy and hard bonded surface with little or no rubber present. Common causes of RC failure are the incorrect choice of adhesive, insufficient dry film thickness of adhesive, failure to properly agitate the adhesive to achieve a uniform dispersion prior to application, pre-cure of adhesive caused by excessive dwell time in the mould cavity before introducing rubber, low mould pressure, undercure of the part, migration of plasticisers and other ingredients from the body of the rubber to the rubber/adhesive interface or contamination of the surface of the adhesive coated part. Cement/metal (CM) failures usually indicate a problem with metal preparation or application of adhesive. They are characterised by the appearance of bare metal in the bonded area of the part. Common causes of CM failure include poor metal preparation, i.e., the presence of contamination on the metal before application of adhesive, insufficient dry film thickness of primer, failure to properly agitate the adhesive to achieve a uniform dispersion prior to application, environmental attack (salt, water) on the primer/metal interface, dry spray of primer on to the metal (which does not allow the primer to
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Rubber to Metal Bonding adequately wet out the metal surface), or sweepage (when the flow of rubber strips some or all of the adhesive or primer film from the metal surface) of the primer and adhesive off the metal during injection or transfer moulding. Cement/primer (CP) failures are characterised by the appearance of the primer on the bonded surface. Usually the adhesive and primer are different colours to allow for easier identification of both layers. CP failure is usually caused by contamination of the primer surface before application of adhesive, migration of plasticiser from the rubber into the adhesive/primer interface, insufficient drying of the primer film before application of the adhesive or by incompatibility between primer and adhesive. In many cases, combinations of the above failure modes appear on the same part. For instance, a failed part may show some R failure, as well as RC and CM failure. Steps must be taken to increase the proportion of R failure while reducing the other, unwanted failure modes. Bond failures are usually given in terms of the percent of the bonded area that contains a certain failure mode. For example, a failed part with the designation ‘65R, 15RC, 20CM’ means that in the bonded area, 65% of the bond surface shows failure in the rubber, 15% shows rubber/cement failure, and 20% shows cement/metal failure. Whatever the particular failure mode seen in a part, the goal of the process engineer is generally to work towards maximising the amount of rubber failure and minimise the amount of failure at the other interfaces. Often questions are raised by engineers and/or quality control personnel about how to best evaluate the strength or quality of a rubber-to-metal bond. Discussions and occasional conflicts occur in connection with the writing, enforcing, and interpreting specifications for such bonds. Differing points of view may be held even within groups of engineers, rubber technologists, and quality professionals, and a variety of types and levels of specification requirements can be found scattered throughout industry. From an engineering point of view the best known and perhaps simplest criterion for good bonding is readily demonstrated in welding practice. As long as any forced breaks of a welded assembly always occur away from the weld and in one of the parent substrates, the weld can be considered fully acceptable. Clearly the resistance of any assembly to applied stress cannot exceed that of the principal materials in the load path, so failure in the parent metal is the best that can be expected. This demonstrates the weld itself is stronger than the main mass of metal; and although the true strength of the weld itself is not readily determined, neither is it of particular importance to the engineer. This same criterion has often been applied to rubber-to-metal bonding starting in the early part of this century.
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The Handbook of Rubber Bonding This means that for many years it was reasonably valid to judge rubber bonds by their appearance. A totally rubber covered metal piece after bond rupture confirmed the original bond quality, and a clean metal surface strongly implied a serious problem with the bonding process. The practice of writing specifications calling for destructive bond testing with 100% rubber coverage of the metal surface became common. However, bond appearance after destructive testing is not the only element of a bond specification. Nondestructive proof testing of bonded parts and minimum failure loads are also used routinely.
2.8 Bond Test Procedures ASTM D429-99 [21] evolved as an official bond testing methodology over the years. Method A uses simple flat bond surface geometry to put the bond into tensile stress, and Method B was developed as a peel type test in order to better serve in those instances where rubber-metal bonds were more likely to fail in that mode rather than due to simple tension. After some years of use, D429-99 had Method C added to it, which uses a conical specimen shape precisely in order to create maximum stress along the rubber-metal bond line when a tensile force is exerted on the assembly. More recently Method D was added for testing PV bonds and the part configuration resembles Method A except that the rubber has been vulcanised prior to bonding. Method E is a special test used exclusively to measure adhesion in rubber tank lining applications. Method F is the newest bond test method for vulcanised bonds and it uses a smaller, convex specimen sometimes referred to as the buffer specimen (see Figure 2.4). The part configuration gives a better representation of typical force distributions encountered in actual mount applications. The different bond test methods yield results that do not always correlate closely in terms of bond strength or mode of failure [22]. Some of these contrasts in results reflect substantial differences in the manner in which the rubber to metal bond is stressed. This is likely to be more meaningful for some actual applications of bonded parts and less for others, depending on how the bond is stressed in such applications, e.g., shearing, tension or peeling. The test specimen geometry impacts the ability to discriminate between the bond strength of different formulation and adhesive systems. Of the methods used, Method A seems to be the least discriminating and the other tests should probably be used preferentially, depending on what kind of application is being evaluated. Method F appears to have the most consistent ability to discriminate differences in response [22].
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test procedures Rubber to Metal Bonding
Figure 2.4 Cross section of adhesion test specimens (Courtesy of Lord Corporation)
2.9 Summary Bonding rubber to metal is a complex and multifaceted combination of metallurgy, surface science, adhesion science, rubber chemistry, and process engineering, with a multitude of interactions. In all aspects of bonding, scrupulous cleanliness, adherence to process controls and meticulous attention to detail are essential if good adhesion is to be attained on a consistent production basis. In general, there are a number of factors that should be considered when selecting an adhesive system. The adhesive system should wet the substrate and spread evenly over the surface under varying conditions. The adhesive system must be compatible with both the polymer type and the vulcanisation chemicals used in the rubber formulation. It should have sufficient cohesive strength to avoid sweeping at moulding temperatures. It should be capable of interfacial mixing with the rubber without destroying the integrity of the adhesive film. It should maintain bond integrity both through all post-bond finishing operations and when exposed to the environmental conditions which the part will see during service. Finally, the adhesive system should accommodate variations in cure and processing conditions used in making the parts.
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References 1.
D. M. Alstadt, Rubber World, 1955, 133, 2, 221.
2.
D. M. Alstadt and E. W. Coleman, Jr., inventors; Lord Corporation, assignee; US Patent 2,905,585, 1959.
3.
S. Buchan, Rubber to Metal Bonding, Palmerton, New York, 1959.
4.
W. M. DeCrease, Rubber Age, 1960, 87, 1013.
5.
P. J. Jazenski and L. G. Manino, inventors; Lord Corporation, assignee; US Patent 4,119,587, 1978.
6.
D. J. Elliot in Developments in Rubber Technology -1, Eds., A. Whelan and K. S. Lee, Applied Science Publishers, London, 1979, p.1-44.
7.
M. A. Weih, C. E. Siverling and F. H. Sexsmith, Rubber World, 1986, 195, 5, 29.
8.
A. K. Bhowmick, M. M. Hall and H. A. Benarey, Rubber Products Manufacturing Technology, Marcel Dekker, New York, 1994, p.776-778.
9.
Vulcanisation of Rubbers, Eds., G. Alliger and I. J. Sjothun, Reinhold Publishing Corporation, New York, 1964, p.117-118.
10. German Patent 170361. 11. L. Daft, inventor; Electro-Chemical Rubber & Manufacturing Company, assignee; US Patent 1036576, 1912. 12. L. Daft, inventor; Electro-Chemical Rubber & Manufacturing Company, assignee; US Patent 1057333, 1913. 13. L. Daft, inventor; Electro-Chemical Rubber & Manufacturing Company, assignee; US Patent 1057334, 1913. 14. L. Daft, inventor; Electro-Chemical Rubber & Manufacturing Company, assignee; US Patent 1120795, 1914. 15. Rubber Technology and Manufacture, Ed., C. M. Blow, Newnes-Butterworth, London, 1977, p.296-298. 16. Rubber Technology and Manufacture, Ed., C. M. Blow, Newnes-Butterworth, London, 1977, p.480.
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Rubber to Metal Bonding 17. DT 928.252, 1942, Bayer. 18. R. Costin and W. Nagel, Rubber World, 1998, 219, 2, 18. 19. W. Hofmann, Rubber Technology Handbook, Hanser Publishers, New York, 1980, p.315. 20. ASTM F22 Standard Test Method for Hydrophobic Surface Films by the Water-Break Test, 2002. 21. ASTM D429 Standard Test Methods for Rubber Property - Adhesion to Rigid Substrates, 2002. 22. R. J. Del Vecchio and J. R. Halladay, Presented at the 152nd ACS Rubber Division Meeting, Cleveland, Ohio, Fall 1997, Paper No.25.
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3
Rubber to Metal and Other Substrate Bonding M. Rooke
3.1 Introduction 3.1.1 Foreword This chapter on bonding describes the use of modern bonding agents and some of the requirements necessary to produce a reliable bond. It is intended to serve as an introduction and perspective for students in the field of bonding technology as well as providing a reference for plant engineers and others who are concerned with the implementation of processes related to bonding.
3.1.2 History Rubber to metal bonding was invented, perhaps by accident, in the middle of the 19th century [1] when natural rubber was bonded to brass during the vulcanisation of the rubber. This process of bonding rubber to metals and other substrates during the vulcanisation of the rubber is still the basis of most rubber bonding today. Applications for bonded items such as antivibration components quickly became established in the growing automotive industry of the 20th century particularly from the late 1930s, using brass plated steel as the substrate. The restrictions of using high sulphur compounds that were necessary for bonding to brass, but had inherently poor dynamic properties, were overcome by use of multiple tie layers of rubber with decreasing sulphur levels in each layer to create a ‘modulus bridge’ between the ebonite bonding layer and the rubber used in the component for antivibration. The layers of rubber were applied from a solvent solution. Other approaches for bonding included using polyisocyanates in the 1940s and bonding proceeded in this way until the 1950s. The breakthrough was made in the middle of the 1950s at the laboratories of Hughson Chemicals (which later became the Chemical Products Division of Lord Corporation, Erie, PA, USA) when the first of the modern bonding agents, Chemlok 220, was invented. Chemlok 220 was introduced into the United States market place in 1956, and into the European market in 1959. It was marketed and subsequently manufactured by Henkel KGaA, Düsseldorf Germany under the ‘Chemosil’ trade name. The trade name Chemosil was chosen at that time because the ‘lok’ of Chemlok was not
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The Handbook of Rubber Bonding perceived as having any meaning in French or German, the main European markets at that time and the ‘sil’ of Chemosil was derived from the detergent ‘Persil’, a Henkel invention in 1905. The irony of this however was that while ‘sil’ had no meaning in connection with bonding, ‘Persil’ was actually a scientific derivation as ‘Per’ was from perborate and ‘sil’ from silicate the main ingredients of the world’s first synthetic detergent. This new technology used heat activated polymers applied from a solvent solution. These bonding agents dried to a non-tacky surface, and reacted chemically with the rubber and the metal during the vulcanisation process. They were designed to be stable in the can and had a long chemical shelf life (of years in many cases) when stored at ambient temperatures. Unlike isocyanate bonding systems they were not susceptible to humidity. In the eastern bloc countries where isocyanate systems lingered towards the end of the 20th century, bonding could only be reliably carried out in the winter because of summer humidity. Brass plating as a means of bonding continued in Europe and America into the mid 1960s when it was replaced by modern bonding agents. The process of replacing tie layers with modern bonding agents was often a slow process and there was an element of disbelief that many thick and viscous layers of tie cement could be replaced by a few microns of bonding agent and primer. Around 1970, in one instance, it was found that 7 layers of tie cement were being covercoated with Chemosil 220 making 8 coats for bonding.
3.1.3 Types of Bonding Types of bonding can be divided into three categories and the principles outlined in this section apply in each case. Rubber to substrate bonding is used for three main applications. Antivibration, the chemical or physical properties of the rubber, and a combination of antivibration with the chemical physical properties of the rubber. This is shown in Table 3.1.
Table 3.1 Types of bonding Purpose
Example
Antivibration
Antivibration mounts, seismic mounts, shock absorbers, rollers, solid tyres
Physical and/or chemical properties of the rubber
Chemical tank lining, abrasion resistant linings
Both antivibration and sealing properties
Dynamic seals, hydromounts
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3.1.4 The Bonding Process - An Overview Bonding to substrates requires that a clean stable substrate is prepared for the bonding agent. In the case of metals the surface preparation can be chemical or mechanical. Chemical preparation involves the removal of oxide and usually the deposition of a stable chemical layer, typically phosphate, on to the metal surface. Mechanical preparation involves the abrasion of the metal usually by grit blasting to remove oxide layers from the metal. The bonding agent or bonding agent primer should be applied to the grit blasted surface within a time window for the particular metal so that the surface is not re-oxidised to such an extent that the surface will again be an unstable substrate for bonding. Some metals such as stainless steel re-oxidise fairly rapidly despite the fact that they may have a deceptive glossy lustre and a time window of just 30 minutes is recommended. As stainless steels have many different chemical compositions it is better to err on the side of caution until it is established that other periods between metal preparation and application of bonding agent primer can be used. Polymeric substrates may have surfaces contaminated with mould release agent or lubricants and so may have to be degreased or cleaned in some way. Most grades of bonding agents contain solids that must be fully homogenised and brought into suspension. More bond failures have occurred due to inadequate mixing than almost any other cause. Technologists should always believe in the consistency of science and always look for scientific explanations. The author was called to a problem with a bonding agent that was thick at the bottom of the container but thin at the top despite vigorous stirring with an electric stirrer. Investigation revealed that the blade had dropped off the bottom of the stirrer shaft and only the shaft was rotating. Components coated with bonding agent should be kept free of contamination during the period prior to bonding (layover period). Components should be loaded into the mould and moulded quickly to avoid pre-crosslinking of the bonding agent before rubber contact. The bonding agent during the vulcanisation process reacts chemically with both the rubber and the substrate. A properly made component will have a bond that will endure and not fail in service. These points may be summarised as follows: • Produce a clean substrate and keep it clean and avoid contamination, • Stir the bonding agent and keep it fully homogenous, • Work within the time window for the substrate, • Apply an even coat of bonding agent, • Avoid prebake of bonding agent. A schematic picture of a bonding agent mechanism is shown in Figure 3.1.
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Elastomer Adsorptio n &/or interdiffusion & chemical reaction
Bonding Agent
Crossbridging, interdiffusion & chemical reaction
Primer Metal
Adsorption & chemical reactio n with metal
Figure 3.1 Two coat rubber bonding
3.1.5 Development of Bonding Almost from the very beginning there were changing demands upon rubber bonding agents where the variables were technology, quality and the environment. The demands from technological progress were in the form of newer more difficult to bond to polymers, and this led to the creation in the 1970s of improved bonding agents, the so called ‘Supercovers’ typified by Chemosil 411. The quality revolution, improved reliability from failure mode and effect analysis (FMEA) [2], and finally the social audit of the environment, first questioning what products ought to be used, and then legislating on substances that produced or depleted ozone in the atmosphere, have all had an impact on bonding agent development.
3.1.6 Bonding Agent Reliability Reliability of the bond was a key feature in bonding agent design from the very beginning. In order to produce reliable components the chemical composition of the bonding agents were and are tailored to meet the in-service requirements of the component. This is achieved during the development of the bonding agent through a process described as controlled product design (CPD). The CPD methodology has similar objectives to design FMEA (FMEA is an important concept used in the engineering industries to predict what may go wrong in a controlled system) in that it seeks to eliminate the possibility of future failures by product design. To give one example, layover (time between the
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Rubber to Metal and Other Substrate Bonding application of a bonding agent and the actual bonding) experiments up to two years were carried out in the late 1950s. The use of CPD in the evaluation of how a product could fail enables products made according to this design philosophy (a corollary of this is that ‘equivalent’ products can only really be equivalent if they have the same CPD background) to have some or all of these physical characteristics: • Stable in can chemistry, • Good layover, • Prebake resistance, • Bond strengths greater than the polymer, • Brake fluid resistance, • Dynamic fatigue resistance, • Gasoline resistance, • Heat resistance of bond, • Hot glycol resistance, • Hot water resistance, • Oil resistance, • Salt spray resistance. Engineers are rightly much concerned about process FMEA. In the case of bonding agents using a liquid as a carrier for bonding agents the reactive constituents function as a single phase or single component system. This is due to Brownian movement (Robert Brown, 1827), first explained by Wiener in 1863. Kinetic theory proof was developed by Perrin (1906) using Maxwell-Boltzmann distribution and the chemical diffusion - diffusion theory developed by Einstein in 1905 and 1908. The reliability of the bonding system stems from the fact that not only does production methodology use ISO 9001 [3] procedures for the testing of raw materials and product manufacture but also the final product is both analysed and performance tested according to criteria devised during the CPD programme. That the testing programme is relevant in relation to the original R&D design of the product can be seen from the following example. The number of molecules of bonding agent used in a test is very large. For example using a bonding polymer oligomer with a MW of 50,000 and a test sample of 1 g, some 2.4 x 1018 molecules will be tested. For a MW of 50,000 and solids content of say 20% and using 6.023 x 1023 as the Avogadro number then 6.023 x 1023 x 0.2/50000 = 2.4 x 1018. The FMEA concept was developed in the mid 1960s in the aerospace industry. It was then taken up by sectors of the North American automotive industry and soon
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The Handbook of Rubber Bonding afterwards became a standard procedure used for automobile component design. FMEA asks: 1) What might go wrong, 2) What effect would this have, 3) What might cause it to go wrong. The FMEA process is quantified by ascribing on a scale of 1 – 10 each of the following. The likelihood of a failure occurring, the severity of the consequences of failure, the likelihood of detection. A Risk Priority Number (RPN) is then obtained from the product of these, i.e., multiplying the Occurrence by the Severity by the Detection. Thus the RPN value which equals O x S x D can have a minimum value of 1 and a maximum value of 1000. Although FMEA is an engineering concept and as such does not directly relate to the methodology of single phase systems that are tested, nevertheless calculations can be made to illustrate the type of RPN values that would be achieved if FMEA was used. FMEA in the true engineering sense would only apply if products were not tested, but it was only predicted that they would bond because of design and manufacturing procedures. An example of an FMEA calculation on bonding agent production is: Occurrence of failure: Because of product design and the ISO 9001 manufacturing procedures this is seen as being remote hence a rating of 1. Severity of failure: If the product did not perform as designed then the failure could cause serious injury or death, i.e., catastrophic failure, hence a rating of 10. Detection of failure: Because of the physical properties of a liquid coupled with product testing of a large number of molecular components the probability of a defect reaching the customer is remote, hence a rating of 1. RPN: Thus the RPN is 1 x 10 x 1 = 10.
3.1.7 The Environment and Solvent Use Solvents such as trichloroethane that historically were used for degreasing and to a lesser extent as solvents for bonding agents were banned for being ozone depleting chemicals (ODC). They are known to destroy ozone in the stratosphere where ozone functions as a UV absorber protecting the earth from harmful radiation. Organic solvents on the other hand as volatile organic compounds (VOC) act as photochemical oxidants creating ozone in the lower atmosphere (troposphere). Here ozone is considered harmful to both flora and fauna and is also implicated in the formation of acid rain. The Montreal Protocol imposed restrictions on the use of substances that deplete the ozone layer and ODCs have been largely phased out. On VOCs however, the United Nations Economic Commission for Europe set targets for reducing emissions in Europe by approximately 30% by 1999 compared to a 1988 base line.
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Rubber to Metal and Other Substrate Bonding The EC response to this was to issue EC Directive 84/360 [4] on air pollution from industrial plant and this was implemented in the UK as the Environmental Protection Act 1990, Part I [5] and TA Luft in Germany [6]. In the UK a series of Process Guidance Notes were published by the Government for regional government (Local Authorities) to assist in the implementation of this law. The time frame for compliance of the Environmental Protection Act 1990, Part I as outlined in PG 6/32(97) [7] require that fugitive organic solvent emissions from existing processes should be reduced by 25% by 1 June 1998 and 50% by 1 April 1999, based on the annual organic solvent use detailed in the application for authorisation. The requirements of this clause should not apply where solvent emissions are less than 5% of total solvent use. These limits apply to companies using over 5 tonnes per year of solvents.
3.1.8 Methods of Reduction in Solvent Emissions In order to achieve the objective set down in Section 7(2)(a) of the Environmental Protection Act 1990, Local Authorities are obliged to ensure that, in carrying on a prescribed process, the Best Available Technology Not Entailing Excessive Cost (BATNEEC) [8] will be used: i) for preventing the release of substances and where it is not practicable, reducing the release of such substances to a minimum and rendering harmless any substances which are so released. ii) for rendering harmless any other substances which might cause harm if released into any environmental medium. Thus solvents have to be reduced in line with targets, eliminated, recovered or incinerated. In order to assist customers for whom solvent recovery or combustion is not economically viable, Henkel decided to support the new EEC VOC Directives using a philosophy similar to the BATNEEC philosophy stipulated by the UK Department of the Environment and other national requirements, by developing waterborne products to eliminate solvent emissions.
3.2 Substrates and their Preparation The foundation of a good bond is a clean and stable substrate. The preparation of a surface for bonding depends on the nature of the material which forms the substrate. Substrates can be divided into the following categories: metals and polymeric. Polymeric substrates can be further subdivided into organic and inorganic.
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3.2.1 Mechanical Treatment of Metals • Solvent degreasing The traditional method for the mechanical preparation of metals used a sequence of degreasing, grit blasting and degreasing. The blasting removes friable surface oxide, which if not removed would fracture and flake off when the bonded component was stressed in use. The second degrease removes any dust that is on the gritted components, and also removes oil that may have been present in the airline if the grit was air driven. The solvent that was used historically was trichloroethane but concerns over ozone depletion have led to its replacement with trichloroethylene. New degreasing equipment minimises the emissions of solvents and so of VOCs. The pH of the solvent should be monitored and adjusted to prevent the build up of acidic residues. Particular caution should be taken if metallic dust being removed includes metals such as aluminium (it should also be noted that aluminium containers should not be used for bonding agents containing chlorinated solvents as the aluminium will react with the chlorinated solvents), zinc or cadmium that could react with acidic solvents, forming Friedel-Crafts catalysts. Although Friedel-Crafts (named after Charles Friedel 1832-1899 and James Craft 1839-1917) catalysts are usually associated with acylation of aromatics, the same group of compounds seem to act as catalytic degradants of halo compounds and polymers such as PVC.
• Aqueous degreasing Aqueous degreasing is a highly effective method of surface preparation prior to grit blasting. In experiments Henkel has shown that organic residues from aqueous degreasing can be lower than by solvent degreasing. To compare the cleaning effect of vapour degreasing and alkaline cleaning methods the remaining organic carbon on the cleaned metal surface was determined. On the surface of vapour phase degreased metals 12 – 14 mg/m2 of organic carbon was detected, while on the alkaline cleaned metal surface 6 – 8 mg/m2 of organic carbon was detected. For water-based cleaning three types of systems are known: Acid phosphate systems pH 3.5 – 5.5 Neutral systems
pH 7.5 – 8.0
Alkaline systems
pH 8.0 – 13.0
The acid phosphate systems will be considered in Section 3.2.5 on the chemical conversion of surfaces.
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• Neutral systems These are mainly liquid products containing surfactants and corrosion inhibitors, with an apparent advantage that parts treated with such combinations do not need rinsing afterwards. A thin protective film of oil or organic media is sometimes present on the metal surface after machining to protect metal parts post manufacture. This temporary oil protection coating of metals has to be removed before bonding. It is also important to ensure that the protective oil is actually removed in the single stage degreasing process. Neutral cleaning products do not require rinsing afterwards but the protective film of corrosion inhibitor and residual surfactant remains on the metal surface. Although this would be removed during blasting, the grit would be contaminated with these products and after a while the surfactants will be ‘applied’ onto the clean oxide free metal surface. If the surfactant is not heat stable enough against high temperatures it will decompose and the residues can cause corrosion under the bond during vulcanisation. Superficially the single stage process seems attractive but for the reasons outlined it is not recommended.
• Alkaline cleaning This cleaning method is commonly used in metal industries. The process consists of cleaning parts in a mixture of surfactant, sodium or potassium hydroxide, and combinations with sodium carbonate, sodium silicate, borates and in special cases sodium gluconate. Components may be treated by dipping or spraying methods. There are four main variables in this type of process (see Figure 3.2), and these are: • Chemicals (concentration), • Temperature, • Time, • Mechanical action. These variables are interrelated and if one variable is changed then others must be altered to compensate. For example if a lower temperature is used then the time must be extended, or it must be compensated for by increased mechanical action from ultrasonic treatment to achieve the same time cycle. The greatest efficiency is obtained by immersion, but spray systems give more mechanical action and the cleaning cycle can be reduced. As oil and grease are emulsified by the surfactant, after a time the risk of re-contamination of the parts being cleaned is very high, particularly if a dipping method is used. For this reason it is recommended that a two-stage alkaline cleaning is used. With this cleaning method a certain amount of electrolytes remain on the metal surface and a final rinsing in a three-step cascade-shower is advisable. Difficulties with
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Figure 3.2 Variables in the cleaning process
alkaline cleaning may occur with small oily parts that stick together, where the cleaning solution cannot reach the whole surface. Other problems may arise when components have small holes, flanges or notches, where cleaning solutions either cannot enter or remain after the process. Nevertheless alkaline cleaning is an effective alternative to vapour degreasing, and particularly the spray system which is already used in the field of rubber-to-metal bonding production.
3.2.2 The Abrasion Process Chilled iron grit is recommended for mild steel. Chilled iron grit should not be used on stainless or non-ferrous metals, where particles can become embedded in the substrate, forming galvanic cells that can cause underbond corrosion. In these cases the blast media should be alumina or a non-metallic grit. Metal shots such as steel shot should not be used. They are soft (malleable) and spread on the surface, giving the impression of producing a clean surface, but in reality forming a layer of metal on the substrate surface, that will peel off when the bond is stressed, giving ‘bond failure’. Also steel shots impact the surface, creating craters like a lunar landscape. This action tends to leave the oxide layer intact. Schematic diagrams of this are shown in Figures 3.3 and 3.4. Electron micrographs have shown how steel shot produces impacted craters, whereas chilled iron grit and aluminium oxide produces a mechanically torn surface. Both aluminium oxide and chilled iron grit tear into the surface producing a cleaned substrate.
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Figure 3.3 Diagram showing soft (malleable) grit covering oxide layer
Figure 3.4 Diagram showing fracture of steel coated layer
Research into types of abrasives suitable for bonding was carried out because of field failure of some bonded components. Grit recommendations based on research are given in Table 3.2. Particular care should be taken to avoid grits that apparently clean the surface but whose inherent deficiencies are masked to some degree by the ability of the primer to bond residual particles to the surface. Because there is no guarantee that all of the oxide has been removed by such grits there can be no guarantee of the reliability of such parts in service. The use of such grits that ‘plate’ or incompletely clean the surface of the substrate are not recommended in FMEA terms. The use of such would increase the occurrence of higher R values in the RPN system.
• Cleaning by conditioning Conditioning is a third way to clean metal surfaces. This involves leaving heavy metal castings in the open air for some weeks to rust. Heavy metal castings frequently retain surface mill scale from the casting process. The rusting of the castings enables oxidation to occur back to the metal below mill scale and oil or cutting fluids on the surface. Parts are then grit blasted.
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Table 3.2 Grit recommendations for achieving specific hardnesses Grit
HV
HB
HRc
Recommended
Hard chilled iron grit
650 – 750
555 – 600
57 – 61.8
Yes
Tempered iron grit
455 – 475
430 – 447
44.5 – 46.8
No
Steel shot
470 – 530
442 – 493
46.4 – 50.3
No
Wire drawn steel shot Alumina Grades chilled iron available
Hardness not listed in literature Mohr Scale 9
No Yes
700-1000
60-80
HV – Vickers hardness; HB – Brinell hardness; HRc – Rockwell hardness
3.2.3 Levels of metal cleanliness It is very common to prepare a surface of a large component to Swedish Standard [9, 10, 11]. The original Swedish Standard is now incorporated into many national and international specifications. Surfaces are defined as A-D according to the amount of rust. An ‘A’ surface has no rust or only slight rust whereas a ‘D’ surface is heavily rusted and shows pitting (craters or holes). The degree of blasting is classed from Sa 1 to Sa 3 and this is shown together with recommendations in Table 3.3.
Table 3.3 Surface
Sa 1 (light blast)
Sa 2 (thorough blast)
A (no rust or slight rust)
Sa 21/2 Sa 3 (very thorough (clean to metal) blast) Yes
Yes
B (some rust)
No
No
Yes
Yes
C (heavy rust but no pitting)
No
No
Yes
Yes
D (considerable pitting)
No
No
No
Yes
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Rubber to Metal and Other Substrate Bonding In general it is better to blast to Sa 21/2 or Sa 3 for surfaces used for bonding. Sometimes a surface is ‘stained’ with scale but is free from rust. It may be possible to grit to Sa 2 on A and B surfaces but it is preferable to blast to Sa 21/2.
3.2.4 Time Window The rate of re-oxidation of mechanically treated metals depends on the metal, the presence of oxygen and the level of humidity. Guidelines for common metal substrates under ambient conditions without dehumidification in a northern European climate are: 2 h for mild steel but 30 minutes for stainless steel, aluminium, brass, bronze and zinc and only 2 minutes (approximately) for magnesium It is very important that the primer is applied within the time window for the metal. Solvent-borne coatings should be applied above the dew point. The start of the time window starts at the end of the gritting and not at the end of the degreasing cycle. This is shown in Figure 3.5. Other aspects referred to in the flow diagram will be dealt with in subsequent sections.
Time Sequence for Bonding Agent Application
Henkel
Bonding Agent Prep
Metal Prep
Temperature condition the bonding agent
S tir the p roduct
Degrease
Gritblast
Apply Pri mer
Dilute with correct diluent
Apply Top Coat Dry
Dry
Time Window Degrease
Handling Do not handle Wear Gloves
Do not handle Wear Gloves
Do not handle Wear Gloves
Direction of time Legend Criti cal time path Time window
Figure 3.5 Time sequence for bonding agent applications
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3.2.5 Chemical Preparation of Surfaces It is possible to clean and phosphate in a single operation. This process of acid phosphate cleaning combines the degreasing with a transformation of the metal surface by removal of oxide and the phosphating of the metal. The process can be used for metals such as steel, aluminium and zinc. This process produces an excellent surface for the adhesion of lacquers but it is not suitable for rubber-to-metal bonding. Tests have shown that the corrosion resistance of the components are not satisfactory. Excellent bonding can be achieved with zinc phosphate and mixed metal phosphates but the particle size and quantity of phosphate applied are very important. There are two main physical forms of phosphate that are used in commerce. One is an amorphous structure applied at a level typically below 4 g/m2 and the other an acicular (needle shaped) structure typically applied at a level of 15 g/m2. The acicular form is used as an absorbent substrate for coatings and oils used to enhance the phosphate layer as protective coating. The acicular form is unsuitable for bonding as the crystal structure can fracture under the bonding agent primer. Phosphating should only be carried out on a flat surface, as a profiled surface, that has been grit blasted for example, may not be able to accommodate even packing of phosphate deposits and bare metal patches may be exposed [12]. The phosphating process usually involves cleaning of the metal, rinsing, surface activation, and phosphate conversion producing an amorphous layer before a final rinse and seal. This sequence is: i)
Alkaline degrease,
ii)
Rinse,
iii) (Surface activation), iv) Phosphate, v)
Rinse,
vi) Seal (Passivation), vii) Rinse, viii) Dry. The surface activation step iii) is optional but is designed to produce a finer particle structure. The sealing process (passivation) in step vi) historically used chromate but environmental pressures have made this an optional step. Some alternative chemical seals must be proved to be suitable for bonding. Sealing is done to improve the water resistance of the layer.
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Rubber to Metal and Other Substrate Bonding The phosphate treatment must be carefully controlled to ensure the quality and uniformity of the coating. The bonding primer must be applied within 24 hours of the phosphating process. For bonding, levels of application of zinc phosphate, should not exceed 4 – 4.5 g/m2 for flat surfaces. Higher levels may lead to bond failure due to fracture of the phosphate coating. If the phosphate layer on the metal component of the bonded part is to be subjected to exceptional mechanical stresses, as in a stress relieving process shown in Figure 3.6, then the level of zinc phosphate should be within the range of 1.3 – 1.9 g/m2 to avoid fracture of the phosphate layer. In the 1960s levels of phosphate were recommended at below 2 g/m2 for bonding. With improvements in technology this increased to around 4 g/m3 for increased protection. However the higher levels proved unsuitable for bush bonding and below 2 g is recommended for this application. Technological improvements could advance this figure. With advancing technology higher levels of phosphate for greater protection of the metal may in the future lead to modification of these recommendations. Where high levels of phosphate are required for non-bonded areas as an anti-corrosion layer the process is as follows: i)
Phosphate to a level technically suitable for the bonded component,
ii)
Bond the component,
iii) Pickle to remove phosphate from non bonded areas,
Figure 3.6 Axial forces applied in stress relieving process
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Figure 3.7 Diagram showing component with bonded layer and oil soaked crystalline phosphate protective coating
iv) Re-phosphate to a level of 10 – 20 g/m2, v)
Oil dip or apply a protective surface finishing coating.
This is illustrated in Figure 3.7.
3.2.6 Future Developments In recent years there has been much interest in coatings that can reduce the cost of the final components, and also improve this performance in terms of environmental resistance. Electrophoretic deposition of polymers on metal surfaces would be one example. Coatings
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Rubber to Metal and Other Substrate Bonding applied to substrates for bonding should not be thermoplastic and should provide an even micron thickness. This technology is likely to be important in the future.
3.3 Bonding Agent Preparation 3.3.1 Solvent-borne Bonding Systems Bonding agents can be clear solutions but they are frequently suspensions of solids in polymeric solutions. In order to use the products full homogenisation is required. Where solvent-borne products are exposed to lower temperatures the viscosity characteristics of primer and top coats tend to differ (see Figure 3.8). The temperature viscosity/graphs in Figure 3.8 show the importance of temperature conditioning during the cold weather to retain the same solids content for a given spray viscosity. It is important not to dilute top coats to a particular viscosity if the bonding agent is at a low temperature. Always bring the bonding agent up to room temperature before diluting. The best method is to always add an exact amount of diluent to an exact amount of bonding agent.
Figure 3.8 Variation of viscosity with temperature
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3.4 Bonding Agent Application and Use 3.4.1 Application Methods Bonding agents may be applied in a number of ways. To make an effective bond an even coat of the bonding system is required of the correct thickness. The method of application is not important provided that an even coat of correct thickness is applied. Methods of application are: • Brush, • Dip, • Electrostatic, • Flowcoat, • Reverse roller coat, • Roller, • Sponge, • Spray. Typically 7 μm of primer and 10 μm of top coat are required for bonding. The thickness depends on the profile of the surface. Smoother surfaces require less bonding agent and heavily profiled surfaces require more bonding agent to ensure that the peaks are covered. In a two-coat process it is important that the primer coat dries before the topcoat is applied to prevent mixing and loss of the individual properties of each coat. For most applications electrostatic spraying may not be suitable as not all components have a suitable electrostatic profile for even coating. However where it is possible to use electrostatic spraying it is very effective in reducing overspray.
3.4.2 Waterborne Bonding Systems Storage temperatures of 5 – 30 °C are recommended. Freezing and also long exposure to direct sunlight must be avoided. The material should be temperature conditioned to 25 – 30 °C prior to use. As with solvent-borne products sedimented solids should be brought into suspension by careful agitation for at least 30 minutes before use. When stirring avoid vortex formation that will suck in air and cause foaming. For the same reason waterborne products should only be stirred and never shaken, as this will produce a stable foam. Impeller stirrers with blades 20 – 30 cm long can be used at up to 60 rpm. Shear discs 8 cm in diameter can be used at speeds up to 700 – 1000 rpm. After the solids are brought into suspension the products should be continually stirred at a moderate speed. Dried bonding agent on the walls of containers will not re-dissolve and loose particles could block spray equipment. In-line 500 μm filters are recommended to prevent this.
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3.4.3 Bonding Agent Thickness As some surfaces to be coated may give an uneven coating thickness, care should be taken not to exceed maximum coating thickness values (see Table 3.4). The correct coating thickness should be selected, because layers of less than 10 μm may cause bond failure, and too thick a layer may cause mould fouling or film delamination and cracking during the filling of the mould. However prebake resistance of bonding agent is extended with thicker layers. Tests should be carried out to verify suggested values in each case, as the bondability of compounds will vary according to formulation.
Table 3.4 Bonding agent coating thickness Chemosil (Henkel)
Vulcanisation method
Vulcanising temperature
Prebake resistance
Recommended thickness of the dried film Minimum 5 μm Recommended 8 – 10 μm Stress corrosion resistance 10 – 15 μm
Primer 211 or X2138
C, T, I
130 – 180 °C Optimum up to 170 °C
Normal
Recommended 10 – 15 μm In some circumstances up to 30 μm
XV5524 XV4250
C, T, I
Optimum 130 – 180 °C
Greater than Chemosil 222 and less than Chemosil 411
Recommended 10 – 20 μm up to 30 μm possible
411
C, T, I
130 – 200 °C 180 °C preferred
Very good up to 180 °C
Minimum 10 μm Recommended 15 – 20 μm up to 30 μm possible
222 or 220
231G
C, T, I
130 – 180 °C
Normal
Minimum 10 μm Recommended 15 – 20 μm Apply two coats if necessary
X6070
C, T, I
130 – 170 °C
Normal
Recommended 15 – 20 μm
X6025
C
90 – 170 °C
Less than Chemosil 222
Recommended 15 – 20 μm
C = Compression moulding
I = Injection moulding
T = Transfer moulding
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3.5 Post Vulcanisation Bonding Cured rubbers may be bonded to metals or to cured or uncured dissimilar rubbers with the aid of Chemosil bonding agents. Bonding takes place using heat and pressure, and bond line temperatures in excess of 100 °C are usually required.
3.5.1 Post Vulcanisation Bonding Applications Post vulcanisation (PV) bonding gives enhanced opportunities in the design of bonded components, and examples of bonded components by this method include the following: • Automotive concentric suspension bushes, where the rubber is bonded to the inner metal tube by conventional in-mould bonding, then bonded to the outer tube by a PV bonding technique, • Automotive engine mounts, where vulcanised rubber is PV bonded between metal plates, • Bridge bearing pads, where vulcanised rubber blocks are assembled in a sandwich construction between a series of metal pieces and then the PV bonding technique is applied.
3.5.2 Choice of Bonding Agent for Post Vulcanisation Bonding Chemosil products suitable for use as PV bonding agents are Chemosils X6025, X6070 and 411. The choice of bonding agent is dependent on the rubber type, the nature of the substrate bonding surface, the bonding temperature and the bonding pressure applied. Chemosil PV bonding agents will bond the following rubber/substrate combinations: Rubber range - vulcanised butyl rubber, CP, EPDM, CSM, NR, NBR and SBR rubbers. Substrate range - using Chemosil 211 primer, the PV bonding agents will bond ferrous and non-ferrous metals and plastics. They can be used as one-coat systems to bond vulcanised rubbers to themselves or to other vulcanised rubbers. Chemosil X6025 Recommended for situations using low bonding temperatures (down to 100 °C) and low bonding pressures. Chemosil X6070 used with Chemosil 211 for bonding to metal substrates. Bonding temperatures above 130 °C are recommended.
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Rubber to Metal and Other Substrate Bonding Chemosil 411 Bond cure temperatures above 120 °C are recommended. To bond to metal Chemosil 211 is recommended as a metal primer and compression of 5 to 10% during the curing cycle is recommended. Certain waterborne systems are available which are capable of PV bonding.
3.5.3 Rubber Substrate Preparation for PV Bonding Vulcanised rubber surfaces must be thoroughly cleaned and degreased prior to Chemosil application. A light abrasion using a wire brush should be followed by degreasing to give a good bonding surface. PV bonding of non-polar rubbers such as butyl, EPDM, NR and SBR may be improved if the surfaces are chlorinated or subjected to acid cyclisation treatment to activate the bonding surface. Chemlok 7701 primer will improve bond performance if applied to the cured polymer surface immediately before bonding agent application.
3.5.4 Metal Substrate Preparation All oil, grease and other soluble contamination should be removed by solvent degreasing or alkaline cleaning. Rust, scale and other non-soluble contaminants should be removed by mechanical or chemical methods. Grit blasting is the most commonly used mechanical method, but wheel abrasion, grinding, wire brushing, emery cloth or steel wool can be used. Chemosil 211 primer should be applied as soon as possible after the surface preparation to reduce the risk of contamination or oxidation of the substrate.
3.5.5 Methods of Application Chemosil 211, X6025, X6070 and 411 contain dispersed solids, consequently they must be thoroughly stirred before and at frequent intervals during use. The most consistent bonding performance is obtained when Chemosil is applied to both bonding surfaces. Therefore when bonding to a metal substrate, apply Chemosil 211 primer, allow to dry and apply recommended Chemosil covercoat to both the primed metal and the vulcanised rubber bonding surface.
• Film thickness A dry Chemosil covercoat film thickness of between 15 – 30 μm is recommended. A Chemosil 211 primer dry film thickness of between 5 – 10 μm is recommended.
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• Drying At ambient temperatures allow at least 60 minutes drying time between coating and bonding. At elevated temperatures up to 90 °C drying time can be considerably reduced. High air flow will give the most efficient drying conditions.
• Assembly of coated components Once the Chemosil coated components have been dried they can be assembled and PV bonded. Care should be taken to avoid damaging or contaminating the Chemosil coating. Clean cotton gloves should be worn when handling coated parts. The assembly of concentric bush components which involve an interference fit between the inner rubber core and the inner and outer metal cylinders, can be lubricated by the application of a small amount of a rubber process oil. This will reduce the risk of damaging the Chemosil film during component assembly. Process oils such as Circolight Oil (Sun Oil Company) or Petrofina 2059 (Petrofina UK Ltd.) are suitable. Components such as engine mounts and bearing pads, must be clamped or otherwise compressed during bonding. A compression ratio of 5 to 10% based on the initial dimensions of the rubber component will give optimum bond performance. Compression must be maintained throughout the PV bonding cure cycle and until the bonded component assembly has cooled to below 50 °C, see Table 3.5.
• PV bonding cure cycle The component assembly should be heated to give a bond-line temperature of between 100 °C and 160 °C for 15 to 60 minutes. Individual cure cycles are determined by the component size and the method of heating. Experimentation is often required to identify the most effective bonding conditions.
• Handling bonded components Cool down period: it is very important to allow the PV bonded components to cool down under compression to obtain optimum bond performance. Once the component has cooled to below 50 °C then the compression can be relieved. Post bond treatments: treatments such as machining, painting, anodisation or electroplating should not affect bond performance.
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Table 3.5 Effect of cure cycle/temperature on PV bond performance Effect of compression on PV bonded performance Compression %
Bond Strength N
0
800
2
2100
4
3000
6
3200
8
3300
10
3300
Notes: 1. Test method: ASTM D429-02C [13] 2. Bonding agent: Chemosil 211 primer/X6025 covercoat 3. Substrate: grit blasted mild steel 4. Rubber: vulcanised NR, vapour degreased 5. PV bonding cycle: 45 minutes at 150 °C, air circulating oven
3.6 Waterborne Bonding Systems 3.6.1 History Work on waterborne bonding systems was first begun in the early 1970s and during this time the main requirement was in-can stability. By the 1980s stable products were made that could be used on phosphated surfaces only. By the early 1990s bonding agents for grit blasted surfaces were produced and by the late 1990s products that were superior in some instances to solvent-based products were being made.
3.6.2 Differences Between Solvent and Waterborne Bonding Agents Bonding agents, both solvent and waterborne, are based on especially designed crosslinkable polymers that will bond to both rubbers and substrates. In solvent-borne bonding agents molecules are entangled and give a continuous elastic layer that has good mechanical strength in the uncured state. This is shown schematically in Figure 3.9.
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Figure 3.9 Schematic diagram of polymer entanglement in solvent
When products using a solvent vehicle are deposited on a surface they form a continuous elastic layer that has good mechanical strength and film properties in the uncured state. Waterborne bonding agents are dispersed in water as micelles (small particles). Uncured the micelles are separate and discrete having only surface cohesion between particles. When dried they coalesce and form a continuum in the bonding layer. This is shown schematically in Figures 3.10 and 3.11. When applied in conventional ways to cold surfaces they remain as discrete particles which although individually elastic do not form such a dense and continuous elastic film as do solvent-borne products. However spraying on to pre-warmed components causes the micelles to start to coalesce and good film properties can be achieved.
Figure 3.10 A schematic diagram of polymer micelles in aqueous suspension
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Figure 3.11 A schematic diagram of polymer micelles starting to coalesce
Although waterborne bonding agents can be air dried and successfully bonded the mechanical properties of film is greatly enhanced by heat application which causes the micelles to coalesce.
3.6.3 Suggested Spraying Equipment and Conditions High velocity, low pressure (HVLP) equipment is recommended. Spray guns can be obtained from Walther, Binks, De Vilbiss, Graco or Sprimag are suggested. Line pumping pressure
0.05 – 0.08 MPa [0.5 – 0.8 bar]
Nozzle
0.8 mm is preferred
Gun inlet pressure (HVLP)
0.3 – 0.4 MPa [3 – 4 bar]
Nominal atomising pressures
0.05 – 0.08 MPa (with fan open) [0.5 – 0.8 bar]
Dilution
Typically undiluted or diluted with up to 10% of de-ionised water
3.6.4 Application and Substrate Temperatures Spraying pre-warmed surfaces enables excellent bonding films to be produced. Spraying at high pressures should be avoided, as this will cause powdering of the bonding film.
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3.6.5 Film Thickness Primer should be applied at a thickness of 10 ± 3 μm and the top coat should be applied at a thickness of 12 ± 5 μm. Coating thicknesses should not exceed 15 μm for primer, 20 μm for top coat.
3.6.6 Layover Coated parts may be stored for up to four weeks prior to bonding provided the components are kept clean and dry.
3.6.7 Progress in Performance Over the years there have been significant advances in the performance of waterborne products. These include not only in-can stability and assured performance but also meeting the needs of more advanced specifications for heat and environmental resistance. One of the deficiencies of waterborne products was prebake resistance, and improvements in technology here have lead to the production of waterborne products that are superior to the best of solvent-based products. In these charts the waterborne primer numbers increase chronologically in the development sequence XW1160/1180/1190 and the top coats XW7480/7484/7500 over an approximately five year period. The reference products are the solvent-based primer Chemosil 211 and top coated with solvent-based Chemosil 411. Figure 3.12 shows adhesion values and Figure 3.13 shows rubber retention using ASTM D429-02 [2]. For further details on rubber bond mechanisms see Section 3.10. The bondability table (Figure 3.14) shows a bonding agent and rubber matrix. The ‘Supercovers’ such as Chemosil 411 will bond difficult rubbers such as EPDM. Waterborne products such as Chemosil XW7484 or XW7500 have ‘Supercover’ capability.
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Rubber Retention (%R)
Figure 3.12 Adhesion at a prebake resistance of semi-EV NR at 175 °C
100 90 80 70 60 50 40 30 20 10 0
XW7480/XW1160 XW7484/XW1180 XW7500/XW1190 411/211 4 min
6 min
8 min
Prebak e Tim e
Figure 3.13 Rubber retention at a prebake resistance of semi-EV NR at 175 °C
Figures 3.15, 3.16 and 3.17 show two NR compounds, an SBR and an NBR bonded with solvent-borne Chemosils 211/220, a hybrid solvent-borne primer Chemosil 211 with waterborne top coat Chemosil XW7484 and a waterborne system of Chemosil XW1180 primer and XW7484 top coat. Boiling water tests (95 – 98 °C for 2 hours) show 100% rubber retention in all cases (see Figure 3.19).
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220
411
7484
NR IR BR SBR NBR CR IIR BIIR CIIR CSM ECO ACM HNBR EPDM Figure 3.14 Chemosil rubber bondability
kN
7 6 5 1180/7484 211/7484 211/222
4 3 2 1 0 NR1
NR2
SBR
NBR
Figure 3.15 Tensile strengths using ASTM D429-02A, Method F (P25 buffer test)
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%RF 100 90 80 70 60 50 40 30 20 10 0
1180/7484 211/7484 211/222
NR1
NR2
SBR
NBR
Figure 3.16 Rubber retention after hot water test
kN
3.5 3 2.5 2
1180/7484 211/7484 211/222
1.5 1 0.5 0 NR1
NR2
SBR
NBR
Figure 3.17 Tensile strengths using ASTM D429-02, Test C [13]
3.7 Health and Safety in the Workplace 3.7.1 The Safety Data Sheet The function of a Health and Safety Data Sheet (SDS) [14] is to advise, protect and provide guidelines on the use of a product. The degree of risks and handling procedures in Europe are codified by using a standard list of R risk phrases and S safety phrases in a safety data sheet. However information in Section 2 Composition/Information on
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The Handbook of Rubber Bonding Ingredients and Section 3 Hazards Identification in a SDS is commonly confused with Section 15 Regulatory Information. Sections 2 and 3 describe listable constituents that are part of the manufacture of a product and the hazards associated with them, but these products may or may not be present as such in the final product. Section 15 is concerned with the final product. Technologists and works engineers should be aware of this, as operatives using SDSs frequently misread them on this account.
3.7.2 Perspective The philosophy of science and technology is to obtain utility by harnessing the forces of nature and its elements. In the chemical industry useful products are made from reactive and sometimes dangerous substances. Bonding agents along with rubber chemicals are by their very nature reactive products, and could not be used to make the end product if they were not reactive. The duty of the chemical engineer is to provide a safe working environment for the manufacture and use of reactive and possibly hazardous intermediates. It is the duty of those who use and handle such products to do so with care and to follow safety guidelines and procedures. It is also the duty of the industrial toxicologist on the one hand to look for unforeseen hazards, while on the other it is that of the industrial chemist to produce less dangerous products wherever possible. As part of the Henkel Responsible Care Programme products were developed to be lead free, and solvent free. Chemosil bonding agents are composed of high molecular weight heat reactive polymers in a liquid carrier. This approach enables minimum hazards at the point of use. Typically the main hazards are solvents but some grades contain inorganic lead salts. In all instances lead free versions are available and waterborne products were designed to be free of heavy metals such as lead. Nevertheless there should always be positive ventilation in manual spray operation areas to carry fume particles away from operatives, and to ensure that solvent concentrations do not exceed the threshold limit values (TLV) for solvents which are stated in the Exposure Controls/Personal Protection section of the SDS. Face masks should always be worn to ensure that spray particulates are not inhaled, and polychloroprene or NBR gloves should be worn so that solvents are not absorbed through the skin. As ignition of solvent is the main immediate risk, care should always be taken to ensure that equipment is earthed so that fire is not caused by static. Also if a site does not have a policy requiring safety shoes with non-spark soles for all employees, then areas in which solvents are should be restricted zones to those wearing nonspark safety shoes. Safety Officers should carry out regular audits, and new employees always given training in safety and personal protection procedures.
3.8 Bonding Agent Testing Bonding agents after manufacture are screened with a series of tests involving peel tests and exposure to hot water. These involve the use of the ASTM D429-99B Peel (modified
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Rubber to Metal and Other Substrate Bonding at 45 degrees). The moulding is typically compression moulding but it could also be transfer or injection. This bond peel test has the virtue of being simple and the same component design can also be used for sweep tests if injection or transfer is used. The bond peel test is shown in Figure 3.18. A metal coupon 60 x 25 x 1.5 mm is masked and has bonding agent coated in the middle 25 mm. The coupon after moulding is inserted in a tensile machine. The mode of bond failure should be noted. This should be recorded as % of each type of failure. RF denotes rubber failure, CM denotes failure between the primer and the metal, RC denotes failure between the rubber and the bonding agent. Supercover bonding agents create a highly crosslinked layer of rubber in contact with the bonding agent. This layer is usually harder than the bulk of the rubber and the tear may be along this line of harder rubber. This is referred to as TR which denotes thin rubber. This should not be confused with RC failure which is most commonly due to inadequate mixing of the top coat. CP would denote failure between primer and covercoat. As part of the QC testing bonding agents are also subjected to a boiling water test. The boiling water test is an accelerated test that will determine inadequate substrate preparation, unsuitable bonding agent or an inadequate bond. This can be done by tying back the rubber so that the bond line is stressed but a more scientific method is to use the Henkel test as shown in Figure 3.19. Other ASTM test methods are outlined in Appendix 3.1.
Figure 3.18 ASTM D429-02, Method B, peel (modified at 45 degrees) [2]
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Figure 3.19 Schematic diagram for hot water test
3.9 Shelf Life Considerations All reactive products will chemically combine in the fullness of time. An aspect which is important is the rate of reaction. For example reactive rubber solutions mixed with isocyanates will have a pot life of only a few hours. Rubber bonding agents however have long shelf lives of many years in some instances but nevertheless it is important to qualify that by defining a temperature. Storage at ambient north European temperatures much below 20 °C is very different from hot climates where temperatures are constantly greater than 30 °C. Also storage in temperate climates in stores that are heated can be as demanding as a hot climate. Chemosil bonding agents are heat activated products that, when stored at ambient temperatures, give for the most part, very long shelf lives of many years. A period of 12 months has traditionally been used, in most instances, as a nominal period of shelf life. This not only served as convenient means of identification for stock rotation but also gives customers a frame work of guarantee for the products that they are purchasing. With the advent of quality systems and the need for more precise documentation, defined details of shelf life are required particularly where there may be only occasional use of a particular grade. For this reason it is useful to consider categories of shelf life and recertification procedures.
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3.9.1 Shelf Life Categories SL0
Chemical substances stored in suitable containers that remain unchanged indefinitely for example, solvents and water.
SL1
Products that have a very long chemical shelf life of many years. Some increase in viscosity with age may be exhibited but this does not necessarily affect the bonding capability of the product.
SL2
Products that have a very long shelf life as in SL1 category but are to some degree thixotropic in nature. The thixotropy should not be confused with an SL1 product that has self reacted.
SL3
Products that are new and have been ascribed a tentative shelf life because time data on the true shelf life is not yet possible.
SL4
Products that have a genuine shelf life that is limited, which require very close monitoring and stock control.
SL5
Products whose ultimate shelf life is affected when containers are opened and require particular handling to achieve the same shelf life as in unopened containers.
It is quite possible to subdivide these categories further if required. For example SL3 might be combined with SL5.
3.9.2 Procedures for Re-certification of Bonding Agents It is prudent to have bonding agent test procedures so that products are not needlessly discarded and also that bonds are not made with products that are below an accepted level or standard.
3.9.2.1 Procedures for Re-certification of Bonding Agents in Classes SL1 and SL2 • Test interval The bonding agents should be examined at six monthly intervals or other intervals as defined by a customer to suit its requirements, to determine that the products are in a liquid state and have not chemically reacted and solidified.
• Record A record should be kept of the finding.
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• Viscometry Where viscometers are available the viscosity should be recorded. Viscometers such as the Brookfield LVT are recommended. Flow cups may also be used although they are difficult to calibrate. Bubble tubes such as Gardiner Tubes can be used for clear products.
• Viscosity sensitivity If a process is viscosity sensitive this should be noted and a standard drawn up so products are always used within the prescribed limits.
• Re-certification period Products that have not gelled or solidified and are covered by points test interval, record, viscosity and viscosity sensitivity may be used by a customer for a further period of 6 months. Other pre-defined periods may be used for test intervals.
3.9.2.2 Procedures for Re-certification of Bonding Agents in Class SL3 Bonding agents of class SL3 are by definition new products that require monitoring beyond the initial shelf life indication. It is suggested that products in this category are carefully monitored using procedures as for products of categories SL1 and SL2. If there is any doubt about the suitability of a product within this category technical service assistance should be sought.
3.9.2.3 Procedures for Monitoring of Bonding Agents in Class SL4 Bonding agents in this category have a restricted shelf life and should be monitored as outlined for bonding agents of categories SL1 and SL2 but at weekly or monthly intervals.
3.9.2.4 Procedures for Re-certification of Bonding Agents in Class SL5 • Moisture-sensitive bonding agents When a product is used on an intermittent basis and is likely to be affected by exposure to atmospheric moisture it is recommended that the product should be packed off into smaller containers to maximise its utilisation. The following information includes general advice on handling products of this category as well as testing procedures.
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• Viscosity Large drums should be fitted with desiccant traps to prevent moisture ingress to part used drums. Smaller sized containers should have lids replaced immediately if not being completely used. Viscosity tests and procedures should be in accordance with testing for SL1 and SL2 category products.
• Clarity Cloudiness of clear products is not necessarily a sign that the products are unsuitable. A slight haze does not affect the product unless otherwise advised. A haze standard should be drawn up for products that are likely to be used on an intermittent basis.
• Photosensitive sensitive bonding agents Products that are light-sensitive are supplied in opaque containers. Re-packing of such products to prevent repeated exposure to light should be into suitable opaque or dark brown glass containers. Used material should not be put back into the bulk container. Any photochemical reaction will exhibit itself by an increase in viscosity or reduced clarity. Tests should be carried out as indicated above in the appropriate sections.
3.10 Troubleshooting In the event of bond failure it is important to determine the location of the failure. This can be between the bonding agent primer and the metal or between the rubber and the bonding agent. For historical reasons (this dates back to rubber tie layers before modern bonding agents) the failure between the primer and the substrate was known as CM failure (Cement/Metal) and the failure between the rubber and the bonding agent is known as RC failure (Rubber/Cement). There is also the theoretical possibility of failure between the primer and the top coat which could arise through contamination, or other reasons such as using a primer and top coat that were not designed for each other. A schematic diagram illustrating CM and RC failure is shown in Figure 3.20.
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Figure 3.20 A schematic diagram showing CM and RC failure
CM failure may be due to:
Poor primer preparation • Inadequate stirring, • Incorrect dilution, • Out of shelf life of product. Inadequate stirring can cause bond failure where a non homogenous mixture is applied. Use of incorrect diluent, such as xylene in a ketone based primer, can cause instability in the bonding agent primer, resulting in selective precipitation or disruption of the surface morphology. Most Chemosil primers have long shelf lives and this is not usually regarded as a problem.
Poor primer application • Dry spraying, • Inadequate drying, • No primer or incorrect product.
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Rubber to Metal and Other Substrate Bonding In automated systems it is important to ensure that primer is actually applied. One solution to the problem is to use a product such as Chemosil 211Y primer which is designed to be used in processes with optical sensometery to differentiate the primer from grey phosphate surfaces. Dry spraying occurs where the solvent has evaporated before the bonding agent has wetted out the metal component. The answer is to use a slower diluent, for example MIBK in place of MEK, or to alter the spray gun conditions. No primer at all can arise if a line is stopped on an automated line. Inadequate drying will occur if the top coat is applied to a wet primer surface, so that the layers were diffused, giving reduced properties of both layers.
Incorrect metal preparation • Contaminated blast media, • Worn blast media, • Contaminated degreaser, • Inadequate conversion coating (A coating produced by chemical or electrochemical treatment of a metallic surface that gives a superficial layer containing a compound of the metal), • Outside of the time window. Worn blast media is the most likely cause of insufficient metal preparation. Time windows will be reduced in humid climates. RC failure may be due to:
Poor top coat preparation • Inadequate stirring, • Incorrect dilution, • Out of shelf life of product. Inadequate stirring is probably the greatest single cause of bond failure.
Poor application • Dry spraying, • Substrate too hot,
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The Handbook of Rubber Bonding • Inadequate film thickness, • Inadequate drying, • Incorrect bonding agent, • Contaminated spray air lines. This is an important section. Dry spraying has already been dealt with under CM failure but a too hot substrate also causes bond failure due to failure to wet out. Linked to this is production line stoppage where parts either in a drying oven after primer application or after the final coat of bonding agent is applied may be overcooked and so produce visually perfect parts that in reality are parts that will fail in service. Where a production spray line stops, parts that were in the heating tunnel should be removed for recycling. Adequate film thickness is more important where EV and SEV systems are being used. Frequently ‘Supercovers’ are used to minimise this possibility of failure because they are more reactive at lower micron thicknesses. Spotty spraying is usually associated with water in a spray line. This tends to show up more with top coats because ketones in the primers have some capacity to absorb water. The solution is to empty or even fit traps to the air line.
Moulding operation problems • Deflashing Cryogenic deflashing should be controlled so that only the flash is embrittled. If the polymer in the component becomes brittle the bond could fracture. • Contamination Contamination with mould release agents or substances that will prevent bonding must be avoided. • Leaking moulds To achieve a bond the rubber must be stationary in the mould. Leaking moulds keep the rubber moving and lead to bond failure. • Prebake of bonding agent Bonding agents react with the rubber at moulding temperatures. Excessive time in hot moulds before moulding can lead to pre-cure of the bonding agent, thus giving rise to bond failure.
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Rubber to Metal and Other Substrate Bonding • Platen temperatures Uneven temperatures will cause uneven bonding. • Layover of coated parts Long layovers can lead to contamination unless in a sealed container. • Demoulding Hot tear strengths of bonds immediately after moulding may be less than the final bond strengths achieved by a post cure. • Rubber Ensure specified rubber was used and within shelf life. A troubleshooting flow chart may be used to aid in diagnosis of bond failure, see Figure 3.21.
Figure 3.21 Troubleshooting flow chart to determine type of bond failure
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3.11 Summary In conclusion it may be said that it is possible today, almost a century and a half after the first rubber to metal bond was created, to bond virtually any rubber to any substrate. This is a testament to the creativity of chemists and the energy and enterprise of those in the rubber bonding industry. The real message however to each generation of technologists is not only to rise to the challenge of achieving ever improved quality by careful control of variables, or producing products to meet more demanding specifications, but also a personal one of quo vadis. In a world where plate tectonics will never be a thing of the past, the need to guarantee a universal freedom from fear of the natural world in homes, as well as in the transportation infrastructure of roads and bridges, which can be subjected to the forces of nature is a challenge that technologists and design engineers must meet. However, by building on the experience of the past the future can be viewed with confidence.
Acknowledgements The authors wish to thank Henkel KGaA for permission to publish this information and are particularly indebted to Dr Klaus Marten of Henkel AIA for encouraging the milieu of excellence.
Information supplied by Henkel AIA-SR Team (Europe) Heinz Alberts, Dieter Beiersdorf, Tom Carey, Richard Prince, Klaus Schürmann, Rainer Wefringhaus.
References 1.
C. Sanderson, inventor; no assignee; UK Patent 3288, 1862.
2.
ASTM D429-02 A Standard Test Methods for Rubber Property - Adhesion to Rigid Substrates. Method B - 90° Stripping Test - Rubber Part Assembled to One Metal Plate, 2002.
3.
ISO 9001 Quality Management Systems - Requirements, 2000.
4.
EC Directive 84/360.
5.
Environmental Protection Act, 1990.
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Rubber to Metal and Other Substrate Bonding 6.
T. A. Luft.
7.
PG6/32, 1997.
8.
Introduction to Part 1 of the Act. Guidance Note 4 (GC4) - Secretary of State’s Guidance on the Interpretation of Terms Used in the Process. Guidance Notes, HMSO.
9.
SIS 05 59 00 Preparation of Steel Substrates Before Application of Paints and Related Products - Visual Assessment of Surface Cleanliness, 1989.
10. BS 7079 Preparation of Steel Substrates Before Application of Paints and Related Products, 1990. 11. ASTM D2200-95 (2001)el Standard Pictoral Surface Preparation Standards for Painting Steel Surfaces, 2001. 12. K. Schürmann, Study of Phosphate Surfaces, unpublished work by Henkel KGaA. 13. ASTM D429-02A Standard Test Methods for Rubber Property - Adhesion to Rigid Substrates Method C - Measuring Adhesion of Rubber to Metal with a Conical Specimen, 2002. 14. Safety Data Sheets for Substances and Preparations Dangerous for Supply Approved Code of Practice. HMSO ISBN 071760859X. 15. ASTM D429-02A Standard Test Methods for Rubber Property - Adhesion to Rigid Substrates, Method A - Rubber Part Assembled Between Two Parallel Metal Plates, 2002. 16. ASTM D429-02A Standard Test Methods for Rubber Property - Adhesion to Rigid Substrates, Method D - Adhesion Test - Post Vulcanisation (PV) Bonding of Rubber to Metal, 2002. 17. ASTM D429-99E Standard Test Methods for Rubber Property - Adhesion to Rigid Substrates, Method E - 90° Stripping Test - Rubber Tank Lining - Assembled to One Metal Plate, 2002.
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Appendix 3.1
ASTM Test Methods ASTM D429-02, Method A [15] Butt, tensile test using compression moulding (see Figure 3.22). This is not widely used in rubber to metal bonding. ASTM D429-02, Method B, Peel (Modified at 45deg) [2] is described in Section 3.8. ASTM D429-02, Method C, Conical peel test [13] This has the objective of evaluating bonding agents at a high stress point at the tip of the insert. Moulding is typically transfer. The disadvantages of the test are the expense of the inserts, and ensuring that the points are equivalent in all specimens as this affects results (see Figure 3.23). ASTM D429-02, Method D, Post Bond Vulcanisation butt test [16]. Parts are clamped in a jig and put into an autoclave or oven. ASTM D429-02, Method F. This test was originally devised by Henkel (and is known in Europe as P25 Buffer test) because of the cost of inserts in ASTM D429-99C. This subsequently became ASTM D429-02, Method F. This has proved extremely valuable and reproducible and also has the advantage of using low cost inserts (see Figure 3.24).
Figure 3.22
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Rubber to Metal and Other Substrate Bonding
Figure 3.23
Figure 3.24
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4
Bonding Rubber to Metals with Waterborne Adhesive Systems B. Crowther (Sections 4.1 and 4.2) and R. Woodcock (Section 4.3)
4.1 Introduction Traditionally, since their development in the 1940s, bonding agent systems for bonding rubbers to metals have used organic solvents as the dispersing and carrying medium for the system of polymers, resins, curatives and fillers in their makeup. The solvents may be single systems or mixtures, aliphatic or aromatic in nature. Dilution of the bonding agent systems on the factory site has also necessitated the use of organic solvents. Organic solvents have the ability to easily ‘wet out’ most surfaces to which bonding agents are commonly applied. They also ‘lift’ any small residual contamination spots left after degreasing, by solubilisation, thus avoiding any likely service problem resulting from such minor contamination. Long term experience of the use of organic solvent-based bonding systems by both rubber bonded product manufacturers and their customers, with the accumulated knowledge of the performance of their products in adverse conditions, is a further factor mitigating against swift acceptance of bonding systems using other than organic carriers. Environmentally the use of the organic solvent systems is not acceptable. There are various reasons for this resistance to the continued use of organic solvents: • they create a health hazard in the factory atmosphere, • organic solvents are generally inflammable and present an explosion hazard, • organic solvents are expensive, • there are disposal problems associated with solvents, • operatives require protection from fumes and addictions, • organic solvents in the presence of direct sunlight can form low-atmosphere ozone, which can be extremely detrimental to human health. Costs to industry in controlling the use of solvents in industrial and urban environments, current environmental legislation, coupled with high insurance costs for factories using
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The Handbook of Rubber Bonding solvent systems, all add to the forces pressurising suppliers for changes to non organic solvent-based bonding systems.
4.1.1 Solvent Elimination by the Rubber Industry The use of waterborne bonding systems has been driven forward by the worldwide concern about environmental pollution emanating from the use of solvents by industry in general. A number of rubber manufacturing processes involve the use of solvents, which in the past were often vented to atmospheres after fulfilling their purpose in the manufacturing process. Processes involving solvents include: • Manufacture of adhesives, • Rubber doughs and solutions for spreading to substrates, • Degreasing of metallic and plastic substrates for rubber bonding, • Bonding adhesive solutions. The latter two processes are of immediate concern to the reader in the context of this book. Worldwide limits for solvents in the atmosphere were discussed and set (and subsequently updated) at a conference in Montreal in 1988. The protocol issued after the conference, the Montreal Protocol, laid down a timetable for solvent use reduction for chlorofluorocarbon solvents and specifically for carbon tetrachloride and methyl chloroform. The use of the former to be eliminated by 1997, and the latter to be reduced by 70% by the year 2000, and then to be totally banned by 2005. As expected, these limits have not been met in the UK. The pertinent act for the UK is the Environmental Protection Act 2990 Pt 1, with specific guidance notes for the Rubber Industry, ‘Process Guidance Notes 6/28 (92)’ which applies to companies using over 5 tonnes of solvent per year. All participants at the Montreal Conference drew up their own legislation, which can be determined by the reader from their own country’s controlling bodies for environmental control. The modifications found necessary for UK compliance were incorporated into the Act in 1997, with a 50% reduction to be achieved by 1999 and a 67% reduction by the year 2007. Beyond 2007, new legislation will inevitably be introduced. Participating countries in the Montreal Protocol have their own reduction targets and no doubt are amending their legislation in a similar manner to the UK (see also Section 3.1.7).
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4.1.2 Techniques Necessary in Bonding of Rubber to Meet Local Environmental Pollution Limits By changing from solvent degreasing to aqueous degreasing systems a 25% reduction in pollution can be achieved. To meet the next target reduction of 67% solvent emission it will be necessary to use only waterborne bonding systems for rubber to substrate bonding. Readers must make themselves aware of their own country’s legislation and solvent limits, and ensure that the manufacturing processes comply with the current requirements.
4.2 Waterborne Bonding Systems (see also Sections 3.4.2 and 3.6) Use of water as the carrier for bonding systems seems to be the obvious answer to most of the problems encountered with solvent-based adhesives. However, use of water for the application of bonding systems has a number of shortcomings, which have made the development of the new systems difficult and slow to be presented to and accepted by, the marketplace. Waterborne bonding systems do not have the same ability as organic solvents to ‘wet out’ the metal surface or solubilise any greasy contamination during bonding agent application to the metal surface. Another factor is quick evaporation of the alternative carrier from the applied adhesive film. Use of water requires higher temperatures for its removal from a deposited film of adhesive. Incomplete removal will result in porous adhesive layers, or even in underbond corrosion at the metal surface.
4.2.1 Structure of Organic Solvent-based Bonding Systems The conventional bonding systems used for many years by the bonding industry are two phase systems. The resin and polymer phase is a solution, whilst the second phase, comprising the accelerators and other additives of organic or inorganic origin, is in the form of finely divided suspensions within the first phase.
4.2.2 Structure of Waterborne Bonding Systems Bonding agents with water as the carrier are multiphase systems. The polymer phase consists of an emulsion (or latex) within a continuous phase of water and the second phase of organic and inorganic materials is dispersed within the liquid phase. The fact that the polymer system is based upon latex technology reflects upon the stability of the latices being used and their protective stabilisation against flocculation. Obviously bonding agent suppliers will compound their systems to achieve maximum stability, but outside 127
The Handbook of Rubber Bonding forces, such as water evaporation, vigorous agitation, low or too high temperatures, addition of solvents or diluent water containing salts of polyvalent heavy metals, may well result in irreversible flocculation.
4.2.3 Fundamentals of Waterborne Bonding Agent Application Moore [1] discusses the surface energy requirements of the application of bonding agent to metal surfaces and reviews the thermodynamics of wetting. Critical aspects of the process of application are concerned with the initial scrupulous cleaning of the metal and its surface topography, to ensure that no traces of oil are trapped after grit blasting. Cleaning with chlorinated solvents generally eliminates any problem in this area, but as legislation prohibits their use and resort has to be made to other cleaning methods, then problems are envisaged. Moore concludes that adhesive wetting is ultimately a function of the surface tension of the adhesive and the level of oily residue on the metal surface. A method of determining surface tension, which has been derived from a technique used by the printing industry [2] for determining the wettability of corona treated polyethylene or polypropylene using surface tension pens, has been adapted by Morton International, to determine the effectiveness of metal cleaning methods.
4.2.4 Waterborne Bonding Systems in Factory Usage In use, care must be taken to ensure that the possibility of ‘drying out’ of the bonding system at the edges of dip tanks, paint pots and on and in spray-guns, is not allowed to occur, for contrary to the experience of such deposits with solvent-based systems, water will not ‘dissolve’ them back into the system. Attempts to reincorporate hardened bonding agent, in the hope that it will redissolve, will result in the bonding agent in use being contaminated with hard ‘bits’. Evaporation of water from the bonding agent during use will also result in flocculation if the water loss is too great. Forces come into effect due to the closer proximity of the latex particles to each other as evaporation proceeds. Collision between particles results in flocculation. Similar effects occur as the result of extreme temperatures. Too low a temperature allows the formation of ice crystals, which results in a lowering of the volume of the water phase and thus a closer proximity of the latex particles and hence more likelihood of collision and flocculation. Conversely, at high temperatures the influence of the temperature is to considerably speed up the movement of the particles, and thus the opportunity for collision and flocculation rises rapidly. Dilution of the relatively high solids material can be carried out using deionised or distilled water, definitely not tap water. The agitation normally required to prepare bonding agents for factory use after transport or storage, to lift any dispersed ingredients into complete suspension, must be carried out carefully
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Bonding Rubber to Metals with Waterborne Adhesive Systems with waterborne systems. Too violent an agitation by high-speed or planetary type mixers must be avoided. Drawing in of air by this type of mixer will cause frothing, due to the surfactants necessary to stabilise the latex system. Stirring at less than 60 rpm is recommended. The stirring should be carried out for a minimum of 8 hours after long storage, or for 2 – 3 hours if the bonding agents have been allowed to settle for a couple of days.
4.2.5 Metal Preparation - For Waterborne Bonding Systems The cleaning of metals for the application of waterborne bonding systems is much more critical than for solvent-based systems. Scrupulously clean metals are vital, to ensure maximum wettability of the bonding surface. Bond [3] suggests that calcium modified phosphating of metals is preferable to conventional grit blasting with its potential for ‘re-infecting’ the metal surface after degreasing by using contaminated grit. Proper housekeeping should eliminate such problems. With the advantages and disadvantages of these systems it is still going to be difficult to convince customers that the reliability of the bonded products using waterborne adhesives will be as good as for products produced with solvent-based systems.
4.2.6 Waterborne Bonding Agent Application Application of the waterborne systems is similar to solvent-based system with some fundamental differences. The drying time necessary for the applied coat of bonding agent is longer, but can be speeded up to some extent by applying the waterborne adhesive to metals which have been preheated to 60 ºC. Drying oven temperatures should be increased to 104 ºC and air speed increased in drying tunnels to encourage evaporation. Coated metals can be baked at up to 170 ºC for 5 minutes to prevent compound flow distortion or removal, without problems. Applied film thickness is critical for use of these systems. If the film is too thin, then ‘spotty’ bonding will result. Too much bonding agent applied will result in delamination and will cause flaking. Best film thicknesses have been determined by Bond [4] to be: primer
5.0 – 10.2 μm
topcoat
12.7 – 15.4 μm
giving a total film thickness of
17.7 – 25.6 μm
These levels of primer and adhesive cover were found to be resistant to salt cabinet attack after 72 hours, even if metals had only previously been grit blasted, without any chemical treatment of the metal surfaces.
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4.2.7 Waterborne Bonding Agent Storage Stability In general, the storage stability of waterborne systems is good. However, the viscosity of one grade of Chemlok 855 is known to increase after storage periods in temperatures of approximately 40 °C, until it appears to be ‘gelled’. With high shear stirring, this ‘gel’ quickly breaks down to its working viscosity, without any impairment of performance. Room temperature storage of this grade is 3 months, with little viscosity change.
4.2.8 Non Bond Advantages of Waterborne Bonding Systems There are some advantages from using waterborne bonding agents: • The removal of atmospheric pollution from solvents. • Elimination of fire hazards and thus decreased insurance premiums. • Ease of spillage removal - simply wipe up using warm water containing detergent. Disadvantages include the problems of careful handling to prevent flocculation, and the higher heating costs from metal warm-up prior to and after bonding agent application.
4.2.9 General Comments - Waterborne Bonding Agents In general terms the primer coats developed by the various manufacturers’ have a greater affinity for the metal surface, being designed to fully wet out the metal surface. They also tend to be somewhat rigid in nature, having a high crosslink density. The rigidity of the primer coat acts as a mechanical, as well as a chemical, key to the metal surface. The bonding agent with the higher affinity for the rubber is much more flexible in nature, thus acting as a stress distributor between the layers of materials. Different rubbers require different mechanisms to achieve crosslinks between the system of the bonding agent and the bulk rubber. With the large variety of rubbers available, of both thermosetting and thermoplastic types and a wide variety of substrates to which they have to be bonded, it is very evident that the dream of a single universal bonding system for all rubbers and metals, is likely to remain just that - a dream.
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4.3 Waterborne Bonding Agents - A Factory Experience • The initial objective has been the replacement of the solvent-based top coat. This has been highly successful being less prone to mould fouling or lamination within the rubber matrix than the Chemosil 411/Thixon 530 type solvent systems, due to the fact that the waterborne top coat (Chemosil XW 7484 now Chemosil 7500) is intermediate in its reactivity between Chemosil 220 and Chemosil 411 solventbased materials. • The main problem with waterborne materials is that they need to be applied to warm metals which helps to break down the micelle structure in the system and so produce a continuous film, otherwise the material will flake off. • By using thinner films the waterborne material competes in economic terms with the solvent-based Thixon 530/Chemosil 411 type, bearing in mind the advantage of higher solids content in the waterborne material, thus concentrating the active ingredients in any given volume of dried adhesive film. • Because of the need for preheated metals it is necessary to modify some equipment, but this should not be a major financial factor. There will however be some increase in energy consumption but this will be less than the costs involved for solvent entrapment. Because of the need to preheat metals, especially for primer application, it has been simpler, initially, to introduce waterborne top coats on solvent-based primer. • Waterborne bonding agents can also replace the Chemosil 220/Chemosil 520 solventbased types due to the reduced level of fouling, and this replacement has already commenced even though the economics dictate that this is marginally more expensive. If only waterborne systems are being used in spray machines then less downtime is necessary to clean out the spray system between adhesive types. With mixed systems rigorous cleaning is necessary to prevent solvent/waterborne systems contaminating each other.
4.3.1 Thickness Effects The application of thin films assists rapid drying and prevents crazing or flaking of the dry film. Table 4.1 shows that surprisingly thin coats produce an effective bond. However in practice it would not be advisable to aim below 5 μm on any metals other than flat plates. Any flow of bonding agent when applied to a shaped metal could result in excessively thin spots.
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Table 4.1 Effect of changing the ratio of Chemosil XW7484 (now XW7500) applied to a 9 μ m coat of Chemosil 211 primer Ratio of Chemosil XW7500:water
% Solids
Chemosil XW7500 thickness, μm
Average peel bond strength N/mm 40 IRHD NR
55 IRHD NR
100:0
30.6
21.1
8.5
20.3
75:25
23.0
12.7
8.5
19.3
67:33
20.4
9.6
8.8
16.5
60:40
18.0
8.8
8.5
18.1
50:50
14.7
5.5
8.2
16.5
40:60
12.1
3.6
8.5
15.8
In all cases the rupture was rubber/rubber IRHD: International Rubber Hardness Degree
Also other factors play an important role regarding thickness: • As a general rule the top coat should be twice the thickness of the primer. Otherwise the primer may consume too high a proportion of the crosslinking agent from the top coat, leaving insufficient agent for the satisfactory reaction between top coat and rubber curing system. This applies to solvent and waterborne materials. • Thicker top coats entrap the crosslinking agent resulting in improved pre-bake resistance (see below) with a thicker coat. • Thicker coats are required for soft compounds (below 45 IRHD). The work to date, therefore does indicate good tolerance to thickness variation but when all factors are considered the preferred thickness should be a primer coat of 5 – 7 μm and a top coat of 10 – 14 μm. Limits determined on a factory basis differ from those found under laboratory testing (see Section 4.2.6). It is not advisable to change the spraying technique to achieve the required coating thickness but rather to modify the level of dilution. There is no significant effect on drying time on warm metals when these materials are diluted. Dilution levels above 15% are not recommended.
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4.3.2 Pre-bake Resistance When bonding agent treated metals are introduced to a mould cavity they must be capable of withstanding the mould temperature for sufficient time to allow compound to enter the mould to form the product, before the adhesive has lost its ability to effect a bond to the rubber. Pre-bake resistance is an important aspect. This is especially so with interleaf bushes which have three relatively thin metals per bush, moulded in a 28 cavity mould. The loading time for 84 metals for such a mould charge is quite long and experience to date has shown that a minimum of 7 minutes pre-bake resistance at 170 °C is preferable. There is also a thickness effect on pre-bake resistance. In the above evaluation the thin coats had 4 μm primer and 7 – 14 μm top coat, whilst thick coats were 8 – 14 μm primer and 10 – 18 μm top coat (see Figure 4.1).
Figure 4.1 Effect of application thickness of bonding agent Chemosil 211 Chemosil 211 (primer: solvent-based); 411: Chemosil 411 (topcoat: solvent-based); XW7500: Chemosil XW7500 (topcoat: waterborne); AP10: Thixon AP10 (primer: waterborne); 5005: Thixon 5005 (topcoat: waterborne); XW1190: Chemosil XW1190 (primer: waterborne)
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The Handbook of Rubber Bonding With some waterborne bonding agents, pre-bake resistance was reduced with thinner films of some adhesive products but not with other types. Whilst the Chemosil XW7484 (now Chemosil XW7500) gave the best results with solventbased primer, the pre-bake resistance was reduced when used with a waterborne primer. Similarly the Thixon 5005 was marginally better with solvent-based than with waterborne primer.
4.3.3 Primers Application of primers to warm metals is not a problem with most operating systems. The limiting factor with all waterborne systems is the retention of sufficient heat in the metal to be able to spray both primer and top coat without reheating the metal after primer application.
4.3.4 Polymer Range To date Chemosil XW7484 has been evaluated against a range of polymers and bonding has been successful with NR, IIR, NBR and peroxide vulcanised EPDM and unsuccessful with CR and Vamac. Chemosil XW7484 has now been replaced by Chemosil XW7500 which will also bond CR components. Several other materials on the market such as Thixon 5005 are also claimed to be satisfactory with CR. The results are interesting in that in all cases there is a 7.5 – 9.0% reduction in bond strength with a waterborne top coat, but the rupture still occurs in the rubber matrix (see Table 4.2).
4.3.5 Product Range The range of waterborne materials available has proliferated recently and some of the current materials are listed in Table 4.3. The list includes the Chemlok (USA) range which were reported in Rubber Technology International [6]. However these materials are not manufactured by Henkel who appear to have their own XW range of materials (see Section 3.6).
4.3.6 Current Disadvantages of Waterborne Bonding Agents • Aqueous bonding agents contain polymer and resin emulsions and pigment dispersions, both stabilised with surfactants. The inherent polarity of water makes it much more
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Table 4.2 Polymer type (bond strength in MPa) Bonding system
NR
NR
NR
10.8
7.25
6.05
6.55
Waterborne
9.9
5.7
5.5
6.05
% Reduction in bond strength
8.3
7.6
9.0
7.6
Solvent-based
NR
Table 4.3 Current waterborne bonding agents Company product name Thixon
Megum
Chemosil
Chemlok (USA)
Cilbond
Designation
Application
AP10
Primer
5005
General purpose coat
5100
Improved environmental resistance
2500
One coat system
23501
Primer
23126
General purpose coat
XW 1190
Primer
XW 17864
Increased flexibility primer
XW 7500
General purpose coat
805
Primer
8003
Primer
8006
Primer
8007
Primer
8282
General purpose coat
60W
Primer
R-6100
General purpose coat
R-6100
General purpose coat
R-6180
PU bonding
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The Handbook of Rubber Bonding difficult to keep aqueous adhesives stable, and destabilisation can occur with changes in pH and temperature extremes. The material must be kept frost free. • It is always necessary to preheat the metals. • They are more sensitive to pressure loss resulting in edge defects with old leaky moulds. The action in this case is to refurbish the mould and not seek an alternative bonding agent. • Surfactants tend to foam with high speed mixing. When hand painting only brush in one direction. • To date many waterborne bonding agents have proved unsuitable for high bulk metals, such as metal castings, unless a level of preheating is given to the prepared metals prior to loading in the mould. Thixon 5100, however, is suitable for this application being more reactive. It also has a shorter pre-bake time which does not present a problem with thick metals that are slower in achieving activation temperatures.
References 1.
M. J. Moore, Presented at the 145th ACS Rubber Division Meeting, Chicago, IL, USA, Spring 1994, Paper No.50.
2.
Tappi Method T 698 cm-91, Determination of Wetting Tension of Polyethylene and Polypropylene Films and Coatings (Modified Visking Analytical Technique), The Pulp and Paper Institute Atlanta, Georgia, 1983.
3.
K. Bond, Adhesives Age, 1990, 33, 2, 22.
4.
T. Plasczynski and K. M. Bond, Presented at the 146th ACS Rubber Division Meeting, Pittsburgh, PA, USA, Fall 1994, Paper No.92.
5.
M. J. Moore, Presented at the 145th ACS Rubber Division Meeting, Chicago, IL, USA, Spring 1994, Paper No.43.
6
T. Plasczynski and D. Mowrey, Rubber Technology International, 1996, 128.
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5
Rubber to Rubber Bonding
R. Joseph
Rubber to rubber bonding is unique in that both the substrates are flexible and of comparatively low strength. Bonding of raw rubbers/rubber compounds and their covulcanisation is important in industrial operations such as tyre building, while bonding of vulcanised rubbers is important in operations such as precured (cold) retreading. Bonding of vulcanised rubbers to unvulcanised rubbers is also important in conventional (hot) retreading processes. There are a wide variety of variables affecting rubber to rubber bonding, such as the chemical composition of the rubbers, their compatibility/ incompatibility, their molecular weights and their distributions, additives, crystalline and amorphous contents, surface nature and chemistry, crosslink densities, etc.
5.1 Bonding of Unvulcanised Rubbers 5.1.1 Tack/Autohesion Uncured rubbers at a temperature above their Tg can easily flow into intimate contact resulting in firm adhesion because of their molecular mobility. The ability of two uncured rubber surfaces to adhere together upon contact after the application of moderate pressure for a relatively short time is known as absolute tack (tack) [1]. Interdiffusion of chain segments across the interface is considered to be a major factor for developing tack. Tackiness between surfaces of the same rubber or rubber compounds having the same composition is referred to as autohesion or autohesive tack. An optimum tack is necessary for operations such as tyre building where the various parts, the impermeable lining, the reinforced carcass and the tread, are all usually made from different rubbers and have to hold together until the tyre is vulcanised in the press. Removal or repositioning during the building process becomes difficult with very tacky compounds in addition to promoting air entrapment. Further, uniform distribution of accelerators and sulphur in very tacky compounds at the final stage of mixing is also difficult. On the other hand, less tacky components may tend to fall apart before curing. The green strength (strength of an unvulcanised rubber compound) of an rubber is the upper limit of the tack [2]. The ratio of the absolute tack divided by the green strength is sometimes referred to as relative tack. When the relative tack is close to one it may be
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Commercial rubbers
The Handbook of Rubber Bonding assumed that almost complete contact and interdiffusion has taken place between the two surfaces and that the tack is green strength limited. But, a low value of relative tack shows that very little contact and interdiffusion has taken place and that the tack is interdiffusion limited. Amorphous polymers which do not crystallise under strain rely completely on chain entanglements to provide green strength and hence significant chain interdiffusion is required in order to develop high tack. Polymeric molecules from the two contacting surfaces must diffuse a sufficient distance across the interface to become entangled so as to develop a strong tack, which can resist stress. After sufficient interdiffusion, if the rubber is capable of strain crystallisation (the tendency of structurally regular rubbers to crystallise upon deformation) it can boost the tack to much higher levels. Hence, a strain crystallising rubber like natural rubber (NR) can develop very high tack. The advantage in this case stems from the fact that strain crystallisation does not occur during the interdiffusion or bond forming step and hence it does not interfere with this step. Further, in the case of strain crystallising rubbers less interdiffusion may be sufficient to develop good tack. Polybutadiene rubber (BR) can be modified by reaction with isopropyl azodicarboxylate (IAD) to improve its bonding. The Tg of the modified polymer increases with its IAD content. Eventually at high levels of modification two Tgs are observed indicating the presence of two phases. The tack and the green strength of BR are found to increase with IAD content. However tack reaches a maximum at 28% IAD content and decreases thereafter while the green strength continues to rise [3]. This is because at low levels of IAD, the green strength is enhanced without much loss in molecular mobility. But at high IAD levels the tack decreases due to a severe reduction in chain mobility, thereby prohibiting the formation of an extensive tack bond. Although crystallinity may improve green strength, it severely limits the chain mobility required for bond formation. Hence partially crystalline ethylene propylene rubbers (EPR) or ethylene propylene diene rubbers (EPDM) have very poor tack [4]. The tack or autohesion of raw rubber/rubber compounds depends on a number of factors, such as polymer characteristics like structure and molecular weight, type and amount of compounding ingredients like fillers and plasticisers, rate of crosslinking, presence of tackifying resins and surface conditions such as oxidation or bloom which may occur during processing or storage. In the case of NR, tack or autohesion passes through a broad maximum with increasing molecular weight and thereafter decreases [5] (see Figure 5.1). This broad maximum occurs due to a proper balance between contact diffusion and green strength/intermolecular attraction at intermediate molecular weight. The diffusion of the rubber molecules across the interface increases as the molecular weight decreases,
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Figure 5.1 Effect of NR molecular weight on autohesion (Reproduced with permission from C. K. Rhee and J. C. Andries, Rubber Chemistry and Technology, 1981, 54, 1, 101, Figure 8, p.111. ©1981, Rubber Division, American Chemical Society Inc.)
but the green strength/intermolecular attraction per molecule decreases. Hence rubbers in the liquid form having very low molecular weight cannot develop good tack due to their poor cohesive strength even though they can easily diffuse across the interface. The effect of contact time on autohesion is also shown in Figure 5.1. The autohesive bond is found to get stronger as the contact time increases, probably due to higher interdiffusion. Even though NR has superior tack compared to styrene butadiene rubber (SBR) the influence of the molecular weight can detract from this advantage enjoyed by NR. Since NR undergoes chain scission upon mastication, moderate amounts of mixing will improve its tack as the molecular weight is reduced. However, upon prolonged mixing the tack of NR can become quite low. SBR, on the other hand, is less sensitive to shear degradation and hence its tack will be less altered by mastication. However, the broad maximum in tack exhibited by NR is practically advantageous since it can accommodate considerable variation in mixing history of NR-containing stock without much loss in the tack.
5.1.2 Influence of Vulcanisation System The composition of the vulcanisation systems and the dynamics of vulcanisation can affect the bond strength significantly [6]. The flow behaviour of the stocks is controlled by the vulcanisation rate. During vulcanisation gradual transition from plastic flow
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The Handbook of Rubber Bonding state to elastic state takes place. After vulcanisation the rubber loses its capacity for cohesion. Thus the fusion of separate layers can only take place in the initial stage of vulcanisation. In general, a system which vulcanises more slowly promotes favourable conditions for autohesion. In the bonding of different rubbery compounds the unvulcanised boundary layer does not promote bond strength. The process of covulcanisation is possible if the two phases vulcanise at an identical rate. Mechanical non-uniformity and sharp alteration in the modulus and other properties lead to setting up of zones of over stressing and conditions of deformation in the boundary layer, which may lead to intensive fatigue and destruction. As the concentration of oxygen and oxygen compounds will be high in the boundary layer, fatigue takes place more intensively in the surface layer than in the bulk of the rubber. In the case of rubbers like SBR and polychloroprene (CR), the bond strength depends upon the interface crosslinking as they differ in the cure rate as well as the curing system. Even when two partially crosslinked sheets made from SBR and CR are bonded together by further vulcanisation the bond strength is found to be highly dependent upon the degree of interface crosslinking [7].
5.1.3 Influence of Filler Type The level and type of filler is another significant factor, which controls the tack or autohesion. In the case of reinforcing fillers the major parameters affecting the tack are the extent of reinforcement of the rubbers by the filler and the extent of decrease in surface mobility of rubber molecules due to the presence of the filler. It is interesting to note how these two parameters affect the tack of rubbers like NR and SBR. NR which has a higher chain mobility compared to SBR is more influenced by the reinforcement effect while SBR by the chain mobility effect. In the case of NR, addition of high abrasion furnace (HAF) black up to 40 phr substantially enhances the tack compared to the unfilled compound, and this remains unchanged with contact pressure since the effect of reinforcement predominates over the effect of decrease in mobility in this range. However, addition of carbon black above 40 phr marginally reduces the tack since the effect of the decreased chain mobility predominates over this level. When the rubber is mixed with carbon black, bound rubber is formed on the surface of the filler. The amount of bound rubber increases with increase in surface area and structure. Hence the tack is reduced with decrease in the particle size or increase in the structure of carbon black. In the case of SBR, which cannot easily achieve bond formation, when 40 phr of easy processing channel (EPC) black is added the tack is reduced and then remains almost the same during mastication. The filler in this case further restricts chain mobility, reducing interfacial interactions and thus leading to lower tack. If an SBR can be synthesised with
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Rubber to Rubber Bonding a sufficiently low MW the relative tack can be made close to unity. However, in this case tack would be low due to the poor cohesive strength, but addition of carbon black to such a polymer would be expected to increase tack. Therefore it may be concluded that the effect of carbon black addition on tack depends more on the rubber’s ability to achieve bond formation than its chemical nature. The bond strength between two different rubbers like NR and BR is found to depend on the strength of the weaker substrate [8], as the two rubbers have similar cure rate, and types of curing system. The reinforcement of the weaker substrate, BR, by carbon black or silica improves NR/BR bonding.
5.1.4 Effects of Plasticisers/Process Oils The autohesion of rubber compounds is found to marginally decrease with increasing amounts of plasticiser/processing oils such as aromatic, naphthenic or paraffinic [5]. Since plasticiser/processing oil improves the chain mobility of the rubbers this behaviour shows that interdiffusion alone cannot explain all the factors associated with tack or autohesion. But if the modulus is maintained at a constant by the addition of carbon black, oil essentially has no effect on adhesion [9].
5.1.5 Effects of Tackifiers The bonding of rubbers can be improved by addition of different tackifying resins like phenol-formaldehyde, coumarone-indene, wood rosin and maleated liquid BR. Tackifiers generally have MW in the range 500 – 2,000 with broad MW distributions. The amount of tackifying resins that are added to rubbers varies widely depending upon the end use. When formulating a pressure sensitive adhesive, large amounts of tackifier, as high as 50 – 100 phr, can be added to an rubber of relatively high MW since the adhesive does not need a very high cohesive strength. When preparing a tyre stock much lower amounts of tackifier, in the 1 – 10 phr range can be tolerated. The resin concentration is always found to be higher on the surface of the compound compared to the bulk due to the solubility/insolubility of the resin in the rubber at mixing and ambient temperatures. Hence the presence of the resin can assist tack formation even when its total concentration is low. Tackifying resins such as maleated liquid BR have an edge over conventionally used resins, as they can undergo vulcanisation under identical conditions and using the same chemical curatives common to the rubber industry, and they are compatible with most rubbers [10]. The tackifying resin can also prevent tack degradation during ageing. The initial adhesion is not changed significantly by adding up to 5 phr phenolic resin. However after ageing at 80% relative humidity, retention of tack was improved when the tackifier had been added. The phenolic resin acts as an antioxidant to prevent surface crosslinking, as oxidation of rubber compounds can lead either to crosslinking or chain
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The Handbook of Rubber Bonding scission depending on the rubber type and compounding ingredients. The solubility of a resin in rubber is also an important factor in deciding the properties of the resulting compound. If the resin is insoluble it can act only as filler, and its effect on the compound depends upon how finely it is dispersed.
5.1.6 Effects of Other Ingredients The direct bonding between nitrile rubber (NBR) and fluorocarbon rubber (FKM) during vulcanisation can be improved by addition of calcium hydroxide to the NBR compound, increasing the dosage of vulcanisation accelerator (organic phosphonium salt) in the FKM compound, and introducing a diethyl amino group as a functional group in NBR [11]. This makes it possible to obtain a low cost fuel hose having a thin inner tube made of FKM and an outer tube of conventional NBR. The inner tube made of FKM provides excellent resistance to fuels like oxidised gasoline (sour gasoline) and to alcohol-containing gasoline (gasohol).
5.1.7 Effects of Surface Modification The adhesion of non-polar materials such as NR based thermoplastic rubbers (NRTPE) and EPDM/PP (polypropylene) based thermoplastic rubbers (EPTPE) to polar substrates is normally difficult and the strength of the resulting bond is rather weak unless a suitable primer is used. The surface halogenation of NRTPEs such as NR/PP blends is found to improve their ability to bond to acrylic and other substrates. Halogenation improves the surface energy and surface roughness of the thermoplastic rubber which facilitates its bonding to substrates such as acrylic without a primer [12]. Figure 5.2 shows the variation in the surface roughness of an NRTPE with chlorination. Indeed, all NRTPEs whose surfaces are either chlorinated or brominated have up to a ten times increase in bond strength. In contrast, EPTPEs such as EPDM/PP blends show no improvement in bond strength, since such materials do not have the necessary residual unsaturation to permit chemical addition of chlorine or bromine. The bond strength of EPTPEs can however be improved by gaseous fluorination of the surface. Some efforts have also been made to improve the tack of synthetic rubbers by other surface treatments such as ozone exposure [13] and irradiation of stocks containing photosensitisers and phenolic resins [14]. If the surface treatment can result in the reduction in MW of the surface molecules, it can contribute to the increased mobility of these molecules resulting in improved tack. However, this layer should remain as a thin boundary layer, otherwise it can result in reduced cohesive strength. Another way of improving the strength of tack
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Rubber to Rubber Bonding
a) NRTPE untreated control
b) NRTPE after chlorination Figures 5.2a and 5.2b Scanning electron micrographs of NRTPE surface at a magnification of x 206 Reproduced with permission from M. D. Ellul and D. R. Hazelton, Rubber Chemistry and Technology, 1994, 67, 4, 582, Figures 12a and 12c. ©1994, Rubber Division, American Chemical Society, Inc
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The Handbook of Rubber Bonding bond by irradiation is by grafting phenolic resin onto the rubber chain, and subsequent interaction of these polar moieties across the interface. Yet another method of improving the tack is by applying a thin layer of a very tacky compound over the non-tacky compound.
5.1.8 Effects of Surface Roughness Another important factor, which determines the bond strength, is the surface roughness of the substrates. Even though surface roughness can increase the contact area, good contact between the substrates can be attained only by squashing the surface asperities (unevenness or protrusions). Elastic strain energy tends to force the rough surfaces apart, but there can be an improvement in bonding due to the enhanced surface area. When an uncrosslinked layer of SBR compound is crosslinked in contact with a fully crosslinked SBR layer a higher bond strength is obtained when the fully crosslinked sheet has a rough surface, thus creating an increased bonded area [15]. In this case since one of the substrates is uncrosslinked it could easily flow into the surface pores. However when a lightly crosslinked layer of SBR compound was cured in contact with a fully crosslinked SBR sheet having a rough surface it spontaneously debonded, whereas that bonded to a sheet having a smooth surface had appreciable autohesion.
5.1.9 Influence of Contact Time/Pressure/Temperature Three variables not related to the rubbers or their additives that may affect tack/autohesion are the conditions of bond formation, namely contact time, contact pressure and contact temperature. Of these three variables, the first two, contact time and pressure, are interdependent. On increasing the contact pressure or time, enlargement of the effective contact surface occurs by the flow process and an increase in the adhesive strength results. Effective contact area is considered to be the sum of the domains of the contacting surfaces in which the adhesive force operates. Optimum contact is reached when the contact pressure exceeds the saturation pressure of the particular system. Saturation pressure is defined as the pressure above which, at constant time, no further increase in pressure produces an increase in the adhesive force. If the surface roughness of the samples is smoothed out by suitable preparation, the optimum contact on the corresponding pressure can be reached in a very short time. After a certain contact time, the contact zone disappears and a homogeneous system is obtained. The autohesive strength is then equal to the green strength of the uncrosslinked rubber. In general, tack increases and reaches a plateau after sufficient contact time or pressure. When the plateau is reached, complete contact and interdiffusion have occurred, and tack is identical to green strength. Even for very short contact times the tack of NR is
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Rubber to Rubber Bonding sufficiently high whereas SBR with a similar MW to the NR takes a much longer time to attain good bonding. This shows that NR is able to make contact and/or interdiffuse more rapidly than SBR. For SBR, the strength of autohesion continues to increase even as the time of contact is increased to as high as a thousand seconds, even though the increase in strength is more rapid in the early stages [16]. Almost similar behaviour was observed in the case of BR, EPR and epoxidised natural rubber (ENR). Interfacial failure occurred when ENR adhered to itself at short contact times. But the peel energy was found to be directly proportional to the duration of contact up to a contact time of about an hour when cohesive failure occurred. In contrast a contact time of less than a minute was sufficient for two NR test pieces to fail cohesively. When NR and ENR-50 (NR with 50% epoxidation) are bonded together the upper limit of adhesion was found to be the strength of the weak adherend (NR) [17]. In the case of NR, when contacting was carried out at room temperature, with subsequent bond strength measurement at another temperature, tack decreased with increase in test temperature. This is probably due to the decrease in green strength at elevated temperatures. However, when the samples are pressed together at elevated temperature before testing the bond at room temperature, tack increased with increase in the bonding temperature due to the improvement in bond formation probably resulting from the increase in the rate of interdiffusion with increase in temperature. The combined effect of increasing the contacting as well as testing temperature will be the net effect of the increase in tack due to better bond formation and the decrease in tack due to lower green strength. Since the relative tack of NR is close to one at room temperature, use of an elevated contacting temperature may not improve the tack significantly, and hence it is likely that tack will decrease at elevated temperature due to a lower cohesive strength. The temperature dependence of the tack of SBR is quite different. Since the extent of bond formation is quite poor in this case, having a low relative tack at room temperature, the improvement in bond formation at an elevated temperature can compensate for the corresponding loss of green strength, resulting in a tack which remains more or less the same with increase in temperature. The strength of autohesion of SBR, where both the contact and detachment were made at the same temperature, were almost identical at 3 °C and 25 °C [16].
5.1.10 Effects of Blooming In the bonding of unvulcanised rubbers blooming of ingredients such as sulphur and antioxidants is a serious problem. It is found that some of the bloomed materials melt at about 90 °C while others remain as large flakes (see Figure 5.3) [18].
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Figure 5.3 Surface bloom of sulphur cured natural rubber (Reprinted from A. D. Roberts, Wear, 1997, 42, 119, Figure 7, with permission from Elsevier Science)
While the former creates little problem during the bonding of unvulcanised pieces via crosslinking, the latter leads to severe loss of bonding strength. Sulphur blooming may be prevented by the use of insoluble sulphur, and controlling the mixing temperature as insoluble sulphur gets converted to the soluble form at about 90 °C. In conventional retreading compounds, the amount of soluble sulphur used is below its solubility limit (the balance being insoluble sulphur) to control the sulphur blooming during storage. Ingredients like wax are usually not added to such compounds, to avoid their blooming, even though they can give protection against ozone and UV degradation. In some applications such as tyre building, a solvent wiping is given over the partially built assembly before fixing the next component. A fast evaporating solvent like naphtha is commonly used for this purpose. The solvent partially dissolves the substrates and plasticises the surfaces. This gives rise to considerable increase in the free surface due to the change in the surface texture and the mobility of the polymer chains in the interface in addition to removing any bloomed ingredient.
5.1.11 Effects of Ageing Uncured rubber compounds, aged in the laboratory or external atmosphere, exposed to ozone or UV radiation including sunlight, suffer a reduction in their subsequent cured
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Rubber to Rubber Bonding interfacial adhesion [19]. This reduction in bond strength is dependent upon the rubber used. It has been shown that exposure of uncured rubber surfaces causes an appreciable reduction in the subsequent vulcanised adhesion properties. In the case of UV irradiation, compounds containing higher proportions of NR are the most susceptible to adhesion failure. Compounds based on other rubbers such as BR, CR, NBR, synthetic polyisoprene, hypalon, etc., also show poor resistance to UV radiation, judged from their respective cured interfacial adhesion strengths. It appears that when the rubber compound is exposed to ozone or sunlight/UV radiation, the reduction in tack is due to the formation of an insoluble surface skin. The reduction in tack is found to be greater for rubbers having higher unsaturation. The deteriorating effect is found to be increased for compounds containing carbon black of higher particle size. A 5% HAF black dispersion in naphtha applied at the interface promoted good bonding in an NR based compound exposed to higher dosage of UV radiation or ozone due to improved resistance to these degradants. Solvent scrubbing can restore the interfacial adhesion to some extent. The addition of fast blooming waxes is also claimed to give some protection against UV radiation and ozone exposure, by blooming to the surface and forming a protective layer over the surface. The effect of ageing time and environment on the tack of rubber compounds is of great practical importance. Tyre retreading compounds must maintain good tack several days after being processed. Reduction in tack was found to be less for butyl rubber compounds than SBR or NBR. A rubber with a higher degree of unsaturation undergoes more surface crosslinking and this prevents the good contact and interdiffusion required to maintain tack. The effect of humidity on tack depends upon the hygroscopic nature of the rubber, and the compounding ingredients used in a specific stock. Unlike oxygen that reacts chemically with rubber, moisture is physically adsorbed on the surface. The autohesion of SBR compounds decreases more rapidly on high humidity ageing compared to that of NR, due to the more hygroscopic nature of the former. When the amount of water absorbed is high it will form a barrier to bond formation, even though minor amounts of water absorbed on the surface will have practically no effect on bond formation.
5.1.12 Testing of Tack/Autohesion Levels Two types of test geometry have been commonly used for testing tack/autohesion [20]: 1) a butt joint in which the test surfaces are contacted and then pulled apart in tension, 2) a peeling joint in which joined samples are stripped apart by peeling, usually at an angle of 90° or 180°.
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The Handbook of Rubber Bonding The 180° peel test at a constant strain rate is the most commonly used test method for determining the bond strength, since it gives a direct measure of the energy per unit area required to separate the surfaces. Further, whereas the tensile test gives only a single estimate of the bond strength, peel test yields a trace, which shows how the force varies along the whole distance peeled. In either case, before testing, two test specimens are brought together under a light pressure (few kPa) for a short time (usually a few seconds or minutes). It is well known that the green (cohesive) strength of an rubber increases with testing rate. Even though little data is available on the effect of testing rate on tack/autohesion, tack is found to increase with test rate, as expected due to the close relationship between tack and green strength. However, this statement should be very cautiously applied, because for many rubbers there is a certain rate of testing above which tack decreases very sharply. This sudden drop in tack as test rate is increased is much larger for NR than many other rubbers. It is interesting to compare the tack of NR and that of SBR as a function of test rates. At low peel rates, NR has substantially higher tack than SBR. However, as the rate is increased the difference narrows and at still higher test speed, tack of NR declines abruptly and hence the tack of SBR exceeds that of NR [21]. This shows that the conditions of bond separation can markedly alter the comparison between the tack of different rubbers. The abrupt decrease in tack at high peel rates is attributed to a change in the viscoelastic response of the rubbers from viscous flow to more elastic behaviour. The effect of variation of peel rate and test temperature on the peel fracture energy of plasticised poly(vinyl chloride) (PVC) which is rubbery in nature and hydrogenated NBR has been reported [22]. As the peel rate is reduced from 50 to 0.5 mm/ min, there is a substantial decrease in peel fracture energy. A similar decrease is also reported when the test temperature is increased from 25 °C to 100 °C.
5.1.13 Adhesion Theories The most widely used and successful theory which explains tack or autohesion is the diffusion theory of adhesion of Voyutskii [23]. He suggested that if two rubber surfaces were in sufficiently close contact, part of the long chain molecules on the surface would diffuse across the interface. The surface molecules will interpenetrate and eventually the interface will disappear and the two parts will have become one. For such an interdiffusion to take place, the molecules must be relatively mobile, which further requires that the rubbers must be above their glass transition temperature and that there should not be any appreciable degree of crosslinking in either of them. There is some experimental evidence that such diffusion takes place across the interface during a relatively short time of contact [24]. However, the diffusion theory does not explain the variation of autohesion with the temperature of measurement, or the increase in autohesion with increase in contact pressure.
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Rubber to Rubber Bonding The thermodynamic requirement of the interdiffusion is the compatibility or miscibility of the contacting rubbers, which is a measure of the ability of the two substances to mix. The condition for compatibility is that the Gibbs free energy for mixing is negative. The Gibbs free energy of mixing (Gm) is related to the enthalpy (heat) of mixing (Hm) and the entropy of mixing (Sm) by the equation Gm = Hm - T Sm Where T is the absolute temperature. As mixing always increases disorder, Sm is positive, so the TSm term is always negative and therefore favours mixing. Hence it is the enthalpy of mixing term, which actually decides whether mixing might occur. The most favourable situation is for the enthalpy of mixing to be large and negative or for a large amount of heat to be evolved on mixing. In molecular terms this means that there should be specific stabilising interactions between the components, such as hydrogen bonding and dipoledipole interactions. While such interactions can be advantageously used in fairly low molecular weight rubbers, their applicability is limited in commercial rubbers since such specific interactions lead to increased viscosity. Another group of miscible blends involves polymers of great similarity. The cause for miscibility in this case is the similarity in the physical parameters of the components. An example is NR and 1, 2 polybutadiene (1,2 BR) which are miscible even at higher molecular weights [25]. As a result when NR is brought into contact with 1,2 BR, they interdiffuse spontaneously. In the case of regular rubbers (those in which component 1 and component 2 do not form specific interactions) the enthalpy of mixing can be written as Hm = V ø1 ø2 (δ1 - δ2)2 where V is the molar volume, ø1 and ø2 the volume fractions and δ1 and δ2 the solubility parameters. This equation shows the significance of the solubility parameter: the closer the solubility values for the substrates are, the more likely they are to be compatible. Compositions of copolymer rubbers, e.g., EPR and SBR, can usually be adjusted to match the solubility parameters. How fast the process of interdiffusion takes place may be measured from the diffusion coefficients (D) of rubbers measured as a function of their MW. For typical industrial polymers with a MW of 200 – 300 x 103, D is 10 – 13 cm2/s. With this value of D it is estimated that within one second after contact is achieved an rubber chain would interdiffuse about 45 Å - enough for substantial interpenetration. The mechanical theory of adhesion predicts mechanical interlocking as the driving force for all adhesion, and is especially useful for materials with rough surfaces, and/or porous
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The Handbook of Rubber Bonding materials. While mechanical interlocking cannot explain all adhesion there are instances where such interlocking does play a major role, such as adhesion between textile cords and rubber casing in automobile tyres where the only significant factor contributing to adhesion is the penetration of the fibre ends into the rubber [26, 27]. The bond strength between rubber and cord depends solely on the number of fibre ends involved, and the depth to which they are embedded. In the case of tack or autohesion, the significant role of mechanical interlocking is evidenced by the increase in tack with the roughness of the rubber surface, even though other factors such as removal of a thin surface layer, and improved interfacial contacts, are also involved in this case. For example, in precured retreading, proper bonding between the casing and the precured tread is established only when both the surfaces are roughened by buffing. Rubber surfaces experience varying degrees of adhesion when brought into contact. The physical forces of adhesion are generally thought to arise from two main sources: one electrostatic and the other van der Waals type short range attraction. The adsorption theory attributes bond strength to the formation of intermolecular forces of attraction or van der Waals forces between the surface molecules. According to this theory tack or autohesion is purely a surface phenomenon and there is a direct correlation between the energy of adsorption and the autohesive bond strength and as polarity increases there should be a proportional increase in the bond strength. However, in practice, such a correlation does not exist [28] and highly polar rubbers tend to have less autohesion [29]. No two surfaces are absolutely identical and there will be some contact electrification. The electrostatic theory considers the two surfaces to be bonded as the two plates of an electrostatic condenser, and is due to Deryaguin [30]. According to this theory adhesion occurs due to the electrostatic forces formed by interaction between the substrates. This theory explains the pressure dependence of tack/autohesion very well but it does not explain why raw and compounded rubbers lose most tack/autohesion as they are cured and brought into molecular contact under pressure. Further this theory is also not successful in explaining the time and temperature dependence of the tack/autohesion. By using potential contrast scanning electron microscopy the existence of an electric double layer at the polymer interface has been demonstrated [31].
5.2 Bonding of Vulcanised Rubbers to Unvulcanised Rubbers In the conventional retreading process an unvulcanised tread compound is extruded as a thick strip and a thin layer of a calendered cushion compound is bonded to this strip by passing between a pair of rollers. Then the compound strip with cushion is fixed on to the buffed casing of a tyre, after applying a solution of the cushion compound in naphtha, and vulcanised in a tyre mould. Since the assembly has to be kept in the
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Rubber to Rubber Bonding mould for a long time, some deterioration in the quality of the rubber/reinforcement is inevitable and hence precured retreading is becoming more popular these days. Further, the actual retreading process in the case of the conventional retreading may take place only after a few days of preparing the tread compound and hence the compounding ingredients have to be properly selected. Chances of blooming are high if any ingredient is added above its solubility limit at room temperature. Here the bonding between the vulcanised casing and the unvulcanised tread takes place in the tyre mould. Roughening the casing surface or wiping the surfaces by solvent naphtha improves bonding. A typical formulation for the tread compound and the cushion compound are given in Table 5.1a and Table 5.1b.
Table 5.1a Formulation for the tread compound Ingredients
Parts by weight
NR
60
BR
40
Zinc oxide
4.5
Stearic acid
2.0
HAF (N 330)
30
ISAF (N 220)
25
Aromatic oil
12.5
MOR
1.0
Accinox ZC
1.0
Vulkanox HS
0.5
Soluble sulphur
1.0
Insoluble sulphur
1.25
Accitard RE
0.15
HAF = High abrasion furnace ISAF = Intermediate super abrasion furnace MOR = 2-(4-morpholinyl mercapto)-benzthiazole Accinox ZC = N-(1,3 Dimethyl butyl)-N-phenyl-p-phenylenediamine Vulkanox HS = Polymerised 1,2 dihydro-2,2,4-trimethylquinoline Accitard RE = N-cyclohexyl thiophthalimide
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Table 5.1b Formulation for the cushion compound Ingredients
Parts by weight
NR
100
Pepton 44
0.2
Zinc oxide
6.0
Stearic acid
1.0
GPF (N 660)
40
Pine tar
4.0
CI resin
1.25
Aromatic oil
4.0
Wood rosin
1.5
Vulkanox HS
1.0
MOR
0.95
Insoluble sulphur
2.5
Pepton 44: Activated di(o-benzamidophenyl) disulphide CI resin: Coumarone-indene resin GPF: General purpose furnace
5.3 Bonding of Vulcanised Rubbers The bonding of vulcanised rubber to vulcanised rubber is important in the fabrication of large structures such as hovercraft loop and finger assemblies (skirt components) and inflatable objects. It is also important in the precured retreading process, where a vulcanised tread is bonded to the buffed casing of a tyre. The bond must possess adequate strength, and must be capable of withstanding harsh environmental conditions such as extended periods of water immersion, severe tyre running conditions, etc.
5.3.1 Strip Bonding of Tyre Retreading Components The most common method of bonding vulcanised rubber components is by applying a rubber compound strip in between them after applying a rubber compound solution to the vulcanised surfaces. The compound solution and the strip compound usually contain a high dosage of tackifying resins for improving the bonding. NR and polychloroprene
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Rubber to Rubber Bonding rubber are commonly used for preparing both the strip and the solution due to their high degree of inherent tack. In certain cases, especially for bonding thin pieces, the rubber strip may be avoided and the bond may be developed solely from the rubber solution. A two component NR-based adhesive solution, one containing sulphur and the other containing accelerator, may be used as 10% solution in toluene or solvent naphtha. They are mixed in equal proportions immediately before use. Table 5.2 shows a typical formulation of a NR-based adhesive solution. Neoprene-based adhesive is also found to be effective in bonding vulcanised pieces and a typical formulation is given in Table 5.3. It is used as a 15 – 20% solution in toluene.
Table 5.2 Two component NR-based solutions for bonding thin vulcanised rubber pieces Components NR
Parts by weight 10 0
100
Zinc oxide
5.0
5.0
Stearic acid
0.5
0.5
Vulkanox HS
1.0
1.0
Wood rosin
2.0
2.0
CI resin
2.0
2.0
Sulphur
5.0
–
Zinc isopropyl xanthate
–
5.0
Table 5.3 Neoprene-based adhesive solution for bonding thin vulcanised rubber pieces Components Neoprene AC
Parts by weight 100
Light magnesium oxide
4.0
Zinc oxide
5.0
Vulkanox HS
2.0
Phenol formaldehyde resin
30.0
AC: adhesive grade
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The Handbook of Rubber Bonding In this case the solution can induce bonding even without an accelerator and thus has only a moderate shelf life. However by using a very low concentration of zinc oxide and magnesium oxide, the shelf life can be enhanced. A few drops of water also improve the shelf life. Grades of CR that have a high crystalline rate have been developed for adhesion applications requiring quick and strong bonding. Mixing with an appropriate amount of polyisocyanate, such as Desmodur R, prior to application can increase the bonding of neoprene adhesive. Before applying the rubber solution the surfaces of the vulcanised sheets are roughened, then the adhesive solution is applied over the surfaces and allowed to dry until tacky and finally the sheets are held together under light pressure to expel entrapped air and for bonding. In precured retreading, as the thickness of the components to be held together are high, the strip bonding process is used. In this process the bonding is provided by vulcanising a rubber strip compound kept in between the tread and casing. A typical formulation of the strip compound is given in Table 5.4. The accelerator dosage can be adjusted according to the temperature and time given for retreading.
Table 5.4 Formulation of a NR-based bonding strip compound for bonding vulcanised rubber pieces Components NR
Parts by weight 100
Pepton 44
0.2
Zinc oxide
5.0
Stearic acid
1.5
GPF (N 660)
40.0
Pine tar
4.0
Wood rosin
2.0
Aromatic oil
3.0
MOR
0.8
TMTM
0.7
Vulkanox HS
1.0
Insoluble sulphur
2.5
Accitard RE
0.15
TMTM: Tetramethyl thiuram monosulphide
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Rubber to Rubber Bonding In strip bonding the properties of the compound strip are significant for attaining good bond strength. The cure rate of the strip compound also plays an important role as the vulcanisation of the adhesive strip takes place by the heat conducted through the adherends, which are already vulcanised. The strip compound should not contain any ingredient which may bloom to the surface during storage.
5.3.2 Effects of Strip Thickness The thickness of the rubber compound strip has a profound effect on the vulcanised rubber – vulcanised rubber bonding. In the case of NR/NR and NR-BR/NR-BR blends bonding the peel strength increases with the increase in the thickness of the strip, reaches a maximum and thereafter decreases [32] (see Figure 5.4) when the bonding time and bonding temperature are kept constant. This shows that there is an optimum thickness of the compound strip below which it cannot keep the two thick substrates intact.
Figure 5.4 Variation of the peel strenth with the thickness of adhesive strip at 140 °C for NR/NR (•) and NR-BR/NR-BR blend (o) test pieces (Reproduced with permission from L. Job and R. Joseph, Journal of Adhesion Science and Technology, 1995, 9, 11, 1427, Figure 1, p.1431. ©1995 VSP BV)
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The Handbook of Rubber Bonding However, at higher thickness the strip remains under-cured as the time of bonding is kept constant resulting in a reduction in bond strength. The adhesive strip can also be made from thermoplastic rubbers. The effect of adhesive strip thickness was found to be quite different for two thermoplastic rubber strips used for bonding Mylar film to a flat rubber layer from that of the rubber strip. In the case of a stiff material like Kraton 1101, the peel strength was found to be virtually independent of thickness over a range of 0.1 to 6.0 mm. For a softer, extensible material like Kraton 3202, the peel strength depended strongly upon the thickness, increasing by a factor of about 20 over the range of thickness studied [33]. This behaviour is attributed to the rheological behaviour of the adhesive strips. In the case of Kraton 1101, the effective thickness of the adhesive layer remains the same even when its actual thickness is varied, because only the surface layer gets deformed plastically on peeling.
5.3.3 Effects of Surface Roughness The roughness of the substrates is also a significant factor in the bonding of vulcanised rubbers. In the bonding of vulcanised SBR to SBR using polyurethane adhesives, a mechanical roughening of the rubber surfaces using sandpaper was found to improve bonding. It is well known that the roughening creates heterogenities on the surface, which will favour its interaction with the adhesive and will eliminate some mould release products present on the rubber surface. Also surface roughening improves mechanical interlocking.
5.3.4 Effects of Temperature on Bonding Another important variable, which determines the bond strength, is the temperature of bonding. NR based sheets had maximum peel strength at a bonding temperature of 130 °C whereas NR-BR blend based sheets had a maximum peel strength at a bonding temperature of 140 °C (see Figure 5.5) [32].
5.3.5 Effects of the Chemical Nature of Polymers/Polymeric Additives/ Surface Roughness Material properties such as the chemical nature of the polymers, their polarity, and MW, are also important in determining the bond strength. A non-polar rubber like a polybutadiene has poor adhesion to plasticised PVC, whereas introduction of a polar group like a nitrile group into BR (as in NBR) enhances the joint strength. This may be attributed to the strong dipolar interactions between the two polymers. In the case of
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Rubber to Rubber Bonding
Figure 5.5 Peel strength as a function of the bonding temperature for NR/NR (•) and NR-BR/NR-BR blend (o) test pieces using adhesive strip (Reproduced with permission from L. Job and R. Joseph, Journal of Adhesion Science and Technology, 1995, 9, 11, 1427, Figure 2, p.1432. 1995, VSP BV)
carboxylated nitrile rubber the bond strength was found to be still higher [22]. Plasticised PVC shows higher bonding compared to rigid PVC since the former can easily deform at low temperatures to the surface irregularities of the adherend. Samples bonded at 30 °C failed at the interface irrespective of the contact time. A joint made at 100 °C failed at the interface at short contact times whereas all the samples failed cohesively when bonded at 150 °C. As the contact temperature is increased, the degree of wetting also increases and interdiffusion is faster, leading to the formation of a strong interface within a short contact time.
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The Handbook of Rubber Bonding The addition of thermoplastics like high density polyethylene (HDPE) to NR can improve its self-adhesion [34]. The improvement in bonding reached a maximum when the HDPE content was 30 phr. The surface roughness also plays an important role and enhanced the bond strength by a factor of seven when the surfaces are buffed. The temperature of bonding was kept at 150 °C and pressure of bonding 45 kg/cm2. At 30 phr HDPE the adhesion strength was very high and instead of interfacial failure, cohesive failure occurred. But at an increased concentration of 50 phr, interfacial failure occurred and bond strength was found to be very low.
5.3.6 Urethane Adhesive Systems Polyurethane and CR adhesives are widely used for bonding vulcanised rubber. Urethane adhesives perform better in some respects compared to neoprene adhesives on greasy leather as excessive grease and fatty acid present can affect neoprene adhesives more adversely [35]. Vulcanised SBR can be bonded satisfactorily using neoprene adhesive if the surface is freshly prepared. Urethane adhesives used are of three types: 1) single part - which contains no free isocyanate 2) two part - this is single part urethane to which free isocyanate is added immediately prior to use 3) pre-reacted - this is a mixed urethane/isocyanate system (the isocyanate is not sufficiently quick reacting to cause gelling). Single part urethane is not efficient in the bonding of rubber. The addition of isocyanate improves bonding as it can act as a bridge between rubber and urethane. However, it is necessary to use polyfunctional isocyanate, as it has to react with polyurethane and rubber.
5.3.7 Surface Treatments to Improve Bonding (see also Section 1.2.4.1) The chlorination of the rubber surface can improve bonding to urethanes [36]. It was found that in the case of NR, chlorination produced an eight-fold increase in bonding while for SBR it produced a four-fold increase in peel strength when two vulcanised NR pieces or SBR pieces were bonded together using urethane adhesive. The chlorination of rubber can be brought about by sodium hypochlorite, chlorine in carbon tetrachloride, and hydrochloric acid in organic solvents. However the toxicity of these agents has restricted their use and less hazardous trichloroisocyanuric acid (TIC) solution has been found to be successful in the chlorination. Chlorination produces surface cracks which favour mechanical interlocking of the adhesive in the pores, a surface oxidation and the
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Rubber to Rubber Bonding reaction of chlorine at the double bond of butadiene both favouring chemical adhesion. The surface chlorination also prevents blooming of other chemicals to the surface. The contact angle of ethylene glycol on a rubber surface was found to be lowered after chlorination resulting in increased wettability of the rubber surface.
5.3.8 Effects of Contact Time/Surface Bloom In the case of sulphur vulcanised NR, the contact time markedly influences the level of adhesion. It is commonly observed that an increase in contact time improves bonding. The surface condition of the vulcanisate has direct bearing on the contact time effect. When sulphur vulcanised NR is boiled in isopropyl alcohol for 3.5 hours there is a considerable reduction in the range of adhesion values with the contact time compared with the original sample. It is observed that after three days of storage bloom can reappear and the sample shows contact time effect. Alcohol washing restored shine. This shows that bloom, which is continually re-established by migration of free ingredients from the rubber interior, is an important component of surface contamination, and isopropyl alcohol is unable to extract free compounding ingredients from the bulk of the rubber. Acetone is found to be more effective than isopropyl alcohol. Bloom on a rubber surface consists of a mixture of materials, antioxidants being the major component. Heavily bloomed surfaces show no adhesion on initial contact for about 20 seconds.
5.4. The Mechanism of Adhesion of Fully Cured Rubbers The mechanism of adhesion of fully cured rubbers may be different from that of uncured rubbers. The contribution from interdiffusion will be less significant. However, the application of the compound solution and the compound strip may provide some interdiffusion. The contribution to adhesion may arise principally from factors of interfacial surface energy and rubber hysteresis (the percentage energy loss for cycle of deformation). Bonding arises from several kinds of interactions that may be physical/or chemical in nature.
References 1.
W. C. Wake, Adhesion and Formulation of Adhesives, 2nd Edition, Applied Science Publishers Ltd., Barking, Essex, UK ,1976.
2.
G. R. Hamed, Rubber Chemistry and Technology, 1981, 54, 3, 577.
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The Handbook of Rubber Bonding 3.
G. R. Hamed and C-H. Shieh, Rubber Chemistry and Technology, 1984, 57, 1, 227.
4.
B. G. Crowther and R. E. Melley, Journal of the Institution of the Rubber Industry, 1974, 8, 5, 197.
5.
C. K. Rhee and J. C. Andries, Rubber Chemistry and Technology, 1981, 54, 1, 101.
6.
B. A. Dogadkin, Rubber Chemistry and Technology, 1960, 33, 2, p.545.
7.
A. K. Bhowmick and A. N. Gent, Rubber Chemistry and Technology, 1984, 57, 2, 216.
8.
A. K. Bhowmick, P. Loha and S. N. Chakravarty, International Journal of Adhesion and Adhesives, 1989, 9, 2, 95.
9.
J. R. Beatty, Presented at the 104th ACS Rubber Division Meeting, Denver, Colorado, Fall 1973, Paper No.14.
10. R. E. Drake and J. M. Labriola, Presented at the 139th Meeting of the ACS Rubber Division, Toronto, Canada, Spring 1991, Paper No.48. 11. M. Sugimoto, T. Okumoto, T. Kurosaki, M. Ichikawa and K. Terashima, presented at the International Rubber Conference (IRC 85), 1985, Kyoto, Japan, Paper No.16B16. 12. M. D. Ellul and D. R. Hazelton, Rubber Chemistry and Technology, 1994, 67, 4, 583. 13. R. E. Tarney and J. J. Verbanc, inventors; E.I. Du Pont de Nemours and Company, assignee; US Patent 3,657,203, 1972. 14. A. C. Soldatos, inventor; Union Carbide Corporation, assignee; US Patent, 3,616,362, 1971. 15. A. N. Gent and S. M. Lai, Rubber Chemistry and Technology, 1995, 68, 1, 13. 16. A. N. Gent and H. J. Kim, Rubber Chemistry and Technology, 1990, 63, 4, 613. 17. K. N. G. Fuller and G. J. Lake, Poster presented at the 3rd International Conference on Adhesion, Adhesion ’87, York, UK, 1987. 18. A. D. Roberts and A. B. Othman, Wear, 1977, 42, 1, 119. 19. P. J. Corish, Presented at the PRI Conference, Polymers in Extreme Environments, Nottingham, UK, 1991, Paper No.3.
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Rubber to Rubber Bonding 20. ASTM D413-98 Standard Test Methods for Rubber Property - Adhesion to Flexible Substrate, 2002. 21. G. R. Hamed and C. H. Shieh, Rubber Chemistry and Technology, 1986, 59, 5, 883. 22. N. R. Manoj and P. P. De, Journal of Elastomers and Plastics, 1994, 26, 3, 265. 23. S. S. Voyutskii and V. L. Vakula, Journal of Applied Polymer Science, 1963, 7, 2, 475. 24. J. D. Skewis, Rubber Chemistry and Technology, 1966, 39, 2, 217. 25. C. M. Roland, Rubber Chemistry and Technology, 1988, 61, 5, 866. 26. E. M. Borroff and W. C. Wake, Transactions of the Institution of the Rubber Industry, 1949, 25, 3, 199. 27. E. M. Borroff and W. C. Wake, Transactions of the Institute of the Rubber Industry, 1949, 25, 3, 210. 28. B. V. Deryaguin, Vestnik Akadnauk, 1963, 7, 10. 29. S. S. Voyutskii, Autohesion and Adhesion of High Polymers, Inter Science, New York, 1963, Chapter V. 30. B. V. Deryaguin and V. P. Smilga, Minutes of Technology, Adhesion: Fundamentals and Practice, Elsevier, Amsterdam, 1969, p.152. 31. W. Possart, International Journal of Adhesion and Adhesives, 1988, 8, 2, 77. 32. L. Job and R. Joseph, Journal of Adhesion Science and Technology, 1995, 9, 11, 1427. 33. A. N. Gent and G. R. Hamed, Rubber Chemistry and Technology, 1982, 55, 2, 483. 34. J. Kurian, S. K. De and G. B. Nando, Journal of Adhesion Science and Technology, 1991, 5, 2, 109. 35. D. Pettit and A. R. Carter, Journal of Adhesion, 1973, 5, 4, 339. 36. J. C. Fernandez-Garcia, A. C. Orgiles-Barcelo and J. M. Martin-Martinez, Journal of Adhesion Science and Technology, 1991, 5, 12, 1065.
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6
Rubber–Brass Bonding W. J. van Ooij
This chapter will discuss the state-of-the-art of bonding rubber compounds to brass, a technology primarily used on steel tyre cords. The literature is reviewed since 1991 when the previous review was published [1]. An updated mechanism for the rubber adhesion mechanism of brass is presented. Some new developments, such as proposed alternatives to brass, are also discussed.
6.1 Introduction One of the oldest and most widely practised methods for bonding rubber to metals is by coating steel with a layer of brass of nominally 70% copper. Alternatively, solid brass can also be used as a substrate. It has been established that a strong bond is formed between the brass surface and the rubber during the vulcanisation process of the rubber [2-4]. This bond is very durable, and resistant to high temperature and dynamic loading. For these reasons, this bonding process is used to promote adhesion between steel tyre cords and rubber in radial tyres. The brass plating process is not used much in other areas of technology where rubber–metal bonding is used, such as engine mounts, suspension bushings, transmission and axle seals, and many others. For these areas, discussed in different chapters in this book, the required rubber properties vary widely and the brass plating technique has some limitations that make it unsuitable. In these critical applications brass has been replaced with solvent-based or waterborne adhesives which can cover a much wider range of metal–rubber combinations. The limitations of the brass plating technique are: 1. Brass only bonds by a unique, self-catalysed process (details are discussed later in this chapter), if the rubber has a high degree of unsaturation; in practice, this limits its use to natural rubber (NR), synthetic isoprene rubber (IR), styrene-butadiene-rubber (SBR) and butadiene-rubber (BR). 2. A strong bond is only formed with sulphur-vulcanised compounds. Additionally these compounds need to have a high sulphur level (4 phr) and a certain type of accelerator, i.e., delayed-action sulphenamides. Systems containing lower sulphur levels, semi-
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The Handbook of Rubber Bonding efficient vulcanisation systems (semi-EV) or EV systems and peroxide-cured systems, all fail to bond to brass directly, i.e., without using an adhesive. 3. The brass composition and also its surface composition are important, and adhesion is only obtained within rather narrow limits of copper percentages (roughly 65 – 75%) and oxide film thickness. In other words, the surface preparation of the brass is very important. 4. An important additive to NR compounds used for bonding rubber to brass is a cobalt salt, such as cobalt naphthenate. While this additive improves the stability of the rubber-brass bond, especially in a corrosive environment, it exerts a negative effect on the stability of the rubber network, in that it accelerates reversion phenomena especially in the presence of oxygen and at elevated temperatures. 5. Since brass is not a highly corrosion-resistant metal, the rubber–brass bond is sensitive to attack in a corrosive environment, e.g., by water, steam, high humidity, salt, and so forth. In general, brass containing copper and zinc only, i.e., electrodeposited brass, is prone to uniform corrosion, dezincification (in chloride and/or acid), stress corrosion cracking (especially by ammonia and amines) and intergranular corrosion. These effects are aggravated by the fact that brass in electrical contact with steel forms a galvanic couple, with brass being cathodic and steel the anode. The situation results in accelerated corrosion of the steel if the brass is porous, e.g., on tyre cords, where corrosion can eventually result in wire breakage. However, the advantages of the rubber–brass bonding process still outweigh these limitations and the important properties, namely thermal stability, dynamic loading resistance, crack growth resistance of NR and others, cannot be matched by adhesives. Thus, the NR-brass bond is still exclusively used in tyres. It should be pointed out, though, that the NR used in tyre belt and carcass areas is optimised for bonding properties to the brass-plated tyre cord rather than for the thermal ageing oxidation and crack growth resistance. If a new bonding agent could be developed that would be less sensitive to NR compound formulations, tyre durability could be significantly improved by reformulating the NR skim stock compound. It has taken more than forty years before such new agents are slowly becoming available. They will briefly be summarised in this chapter, but the emphasis will be on the properties of the rubber– brass bond and new developments in terms of brass coatings, compounding and advances in understanding of the bonding mechanisms. Although this mechanism has been studied and published [3, 5-11], there is still controversy over some aspects of it. Some recent studies have shed more light on the nature of the brass-rubber interaction and will, therefore, be discussed.
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6.2 Mechanism of Rubber-Brass Bonding 6.2.1 Reviews This topic was last reviewed by Van Ooij in 1991 [1]. Prior to that date, review articles were published in 1979 and 1984 [3, 4]. All published papers on the mechanism of bonding, brass requirements, compounding for optimum bonding and some patents were reviewed. A paper describing some recent developments in rubber–metal bonding for tyre applications was recently published by Van Ooij [12]. An updated adhesion model was described based on the use of time-of-flight secondary ion mass spectrometry (TOFSIMS) analysis of brass treated in squalene containing rubber chemicals. Some details are given in the following sections. Other novelties are the use of organic plasmapolymerised films as primers for bonding steel to rubber and, for the first time, bonding of rubber to steel by using organofunctional silanes [12]. The rubber compounds were peroxide-vulcanised only and silane processes for bonding to sulphur-vulcanised compounds were announced but have not yet been shown.
6.2.2 Recent Mechanistic Studies The mechanism of rubber–brass adhesion is described in several papers, cited earlier. Although there still is a debate in the literature over the presence of covalent Cu–S bonding across the rubber–brass interface, it is now generally accepted that during the initial stage of the cure, a layer of interfacial sulphides are formed with predominantly nonstoichiometric copper sulphide and smaller amounts of zinc sulphide. The copper sulphide grows as dendrites into the liquid rubber which is still viscoelastic at that stage. When the sulphide growth levels off, crosslinking through the delayed action sulphenamide accelerator occurs and the result is a strong interlocking of the cured network and the rubber compound. This mechanism is summarised in Figure 6.1 [4]. In this model, the adhesive strength and interfacial strength between the sulphide layers are obviously of paramount importance. The type of sulphide, dendrite size, growth rate, defect structure, etc., are dependent on the rubber formulation, such as accelerators, sulphur/accelerator ratio, certain additives, e.g., cobalt or others, and also on the brass parameters (surface cleanliness, oxide thickness, etc.), and thus, each compound must be carefully optimised for bonding characteristics to a particular type of cord. Covalent bonds between the metal and the rubber or between the copper sulphide and the crosslinked rubber have never been demonstrated. However, it has been shown that the only reaction occurring between copper and organic sulphides is the formation of copper sulphides, i.e., that is the stable product [13]. Ageing of the rubber-brass bond in a corrosive environment has been demonstrated to be the result of corrosion of the remaining
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The Handbook of Rubber Bonding a)
b)
Figure 6.1 Mechanism of rubber-brass bonding after van Ooij [4]. Shown are a) the original brass surface composition and, b) the interfacial copper sulphide dendrites interlocked with the crosslinked rubber compound (Reprinted with permission, from W. J. van Ooij, Rubber Chemistry and Technology, 1984, 57, 3, 426, Figure 1 and 445, Figure 11. ©1984, Rubber Division, American Chemical Society, Inc.)
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Rubber–Brass Bonding brass layer under the sulphide film, primarily by dezincification. This process results in the formation of copious amounts of zinc oxide and hydroxide and a weakening of the interfacial strength. Thus, aged adhesion retention depends on how well the existing sulphide film protects the underlying metal against corrosive attack and how stable the sulphide film itself is against the attack by moisture and salt. Important variables are the type of accelerator, the sulphur/accelerator ratio and the presence of cobalt-organic complexes. These factors all have an effect on the type, structure, composition and coarseness of the copper sulphide film, as is summarised in Figure 6.2.
Figure 6.2 Schematic of effects of rubber compound variations on rubber–brass interface, and effects during ageing of the interface in conditions of high humidity (Reprinted with permission, from W. J. van Ooij and M. E. F. Biemond, Rubber Chemistry and Technology, 1984, 57, 4, 686. ©1984, Rubber Division, American Chemical Society, Inc.)
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The Handbook of Rubber Bonding Recent studies aimed at unravelling the bonding mechanism all support this summarised model and provide some detail on aspects such as film composition or thickness. The work summarised next was all of a fundamental nature, using either model brass samples (polished coupons), or liquid squalene systems to simulate rubber compounds, [14] or both. Ball [15] compared sulphide films formed on brass coupons from squalene baths containing different cobalt adhesion promoters. The films were analysed by surface analytical tools including secondary ion mass spectrometry (SIMS) which has a greater sensitivity for cobalt. The model in Figure 6.1b based on dendritic copper sulphide growth interlocked with organic polymer was confirmed. The films contained cobalt, which increased the resistance to steam ageing, especially boroacylate salt which deposited more cobalt in the film. The form of cobalt in the film was postulated to be cobaltsulphur-rubber rather than cobalt sulphide. Similar studies using transmission electron microscopy (TEM) and secondary neutral mass spectrometry (SNMS) were performed by Pieroth [16], who added cobalt naphthenate with and without a boric acid ester to rubber which was contacted with sputtered brass films. A pronounced effect of the boric acid ester on the retention of adhesion was found. Without cobalt, a strong growth of ZnO/Zn(OH)2 occurred during steam ageing, resulting in a weakening of the interface. This effect was effectively suppressed by the combination of the cobalt salt and the boric acid ester, but not by the cobalt salt alone. Different types of commercial bonding agents were compared, again in a squalene liquid model system approach, by Hamed [17]. They investigated the rate of sulphide formation on brass-plated tyre cords and their morphology using scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX). The commercial bonding agents (including a cobalt salt) all initially formed a copper sulphide film with a high surface area prior to the scorch of the squalene. The high-surface area sulphide was considered favourable for penetrating into the viscoelastic rubber, thus resulting in tight interlocking after rubber cure had occurred. The bonding agents also slowed down the growth of the sulphide film during ageing in high humidity at 70% for 10 days. This copper sulphide growth is normally a consequence of the dezincification (corrosion) reaction. These results, as well as the previous ones, indicate that good adhesion promoters are primarily corrosion inhibitors for the dezincification reaction. Prokof’ev [18] studied the effects of oligomeric polysulphides on the rate of sulphidation of copper foil in m-xylene. These oligomers contained linear polysulphide chains. It was reported that the polysulphides themselves did not sulphidise the copper at an appreciable rate. However, when used together with elemental sulphur, sulphidation occurred and the sulphide was more homogeneous and finer than by sulphur alone. These results were used to explain the more stable bonding to rubber containing polysulphides. Apparently, linear sulphides are not reactive but they insert sulphide and then become active for sulphidation.
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Rubber–Brass Bonding Chandra [19] evaluated the effect of an adhesion promoter by investigating the failed interfaces between NR skim stocks and tyre cords to determine the effect of a cobalt adhesion promoter on the mode of failure. The main conclusions were that the model by Van Ooij was confirmed. The cobalt strengthened and stabilised the interface, but had a negative effect on the compound properties in the interface region, as expected. As a result, the failure mode became more cohesive. The following research deals with a detailed description of the sulphide film and the dependence between the sulphide make-up and compound formulation. Potapov [20] reported detailed analyses of the sulphide film and interfacial zones, their degree of crosslinking and oxidation, by spectral and analytical procedures on thin sections. Many parameters were measured, including the cohesive strength of the various copper sulphide films. Oxidation of the compounds was explained on the basis of a catalytic effect exerted by copper or cobalt in the compound. The overall conclusion of this work was that a high degree of non-stoichiometry of the CuxS film results in better adhesion to the metal because of a better match of the lattice parameters. Further, this degree of nonstoichiometry is attained at a specific Cu+/S8 ratio which is specific for each accelerator (three accelerators were compared). Cobalt strengthens the film mechanically and forms cobalt sulphide. Optimised compound formulations were reported which gave the highest mechanical strength of the sulphide film and the interfacial region. Researchers from Graz, Austria [21-26], developed procedures for determining the thickness, structure and composition of the interface formed between thin brass foils reacted with simple rubber compounds. Analytical TEM, EDX, wavelength dispersive x-ray analysis (WDS), and electron energy loss spectrometry (EELS) were used combined with cryomicrotomy. Their results uniquely identified that copper sulphide is the bonding agent. Its thickness should be no more than 250 nm and increases with sulphur content in the compound. Thicker films were more brittle. The locus of adhesion failure is frequently in the interface between the copper sulphide and substrate. Of several brass pre-treatments that were compared, an alkaline treatment optimally activated the brass for bonding. As an overall conclusion, these detailed studies by this group, using highly sophisticated equipment, all confirmed the validity of the model described earlier in this section. The important issue is that all these studies confirmed a tight interlocking between the polymer (rubber) and the sulphide film, i.e., organic material was actually occluded inside the film and no sharp boundary was detected. Giridhar, in a development of a novel tyre coating that could possibly replace brass (to be discussed later) performed a detailed study of the interaction between brass-plated and other alloy-plated tyre cords and squalene mixtures formulated as rubber for the study of the organic chemistry at the metal surface [27]. This technique was instrumental in
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The Handbook of Rubber Bonding detecting the absorption of the cobalt complex on the metal. Other rubber compounding ingredients that were found to be absorbed were the accelerator N, Ndicylohexyl benzothiazole sulphenamide (DCBS) and stearic acid. The implications of these findings for the overall mechanism are given in an update of the model of Figures 6.1 and 6.2 (see Section 6.2.3).
6.2.3 Updated Rubber–Brass Adhesion Model Taking the various recent studies into consideration and with the model of Figures 6.1 and 6.2 as a starting point we can now refine the model as follows. It is important to refer to the recent study by Chaler [28] who revisited the mechanism of rubber vulcanisation using the liquid model system approach [14]. These studies shed new light on the mechanism of sulphur vulcanisation and some of the conclusions by Chaler are used in the model, which is shown in Figure 6.3. Five stages are distinguished, which cover the initial periods of the cure, i.e., until the compound begins to crosslink. For the sake of simplicity, cyclohexyl benzothiazole sulphenamide (CBS) is assumed to be the accelerator here, but other sulphenamides would react similarly, albeit at a different rate. Stage I: Formation of active intermediate products from the accelerator takes place here. Because of the electronegativity of sulphur and the tendency of nitrogen to form positive ions, the ions shown are preferred over radicals. Further, there is strong evidence that the double bonds in NR (or squalene) accelerate the formation of these products. An effect of the double bonds via the formation of a charge transfer π complex as shown in Figure 6.3 is proposed. Such a complex polarises the bond because of the high π electron density of the C=C bond. Stage I: Formation of active intermediate products from the accelerator:
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Rubber–Brass Bonding Evidence for this interaction are the observations that a) the accelerator fragments seen in the TOFSIMS spectrum of the metal surface hung in squalene are not seen if the squalene is replaced with squalane, its saturated counterpart [27]. b) the sulphidation rate of brass is much lower when squalane is used as a model system for rubber than when squalene is the vehicle [4, 5, 12]. As is shown below, the sulphidation rate is a function of the concentration of the intermediate.
c) it has recently been found that in sulphur vulcanisation of squalene, the formation of mercaptobenzothiazole sulphenamide (MBTS) from CBS is decisive for the onset of the crosslinking reaction [28]. Mercaptobenzothiazole (MBT) catalyses the decomposition of CBS, with the formation of MBTS (Figure 6.3 1-c). This compound then forms the active sulphurating species by reacting with ZnO/S8 (ring opening):
The rate-determining step in this sequence is reaction 1a, the slow scission of CBS. It was found that in squalene, the induction period was as follows: 1. mixture of squalene, ZnO, S8, CBS:
25 min
2. as 1, but without ZnO:
35 min
3. as 1, but without squalene:
85 min
4. as 1, but without squalene and ZnO:
infinite
The data indicate that the CBS decomposition is catalysed by the double bonds of squalene thus confirming the observations in a).
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Figure 6.3 Schematic of updated mechanism for bonding rubber to brass
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Rubber–Brass Bonding
Figure 6.3 Continued
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The Handbook of Rubber Bonding Stage II: The MBT or MBTS (both form the same complex shown in stage II) is absorbed on the metal surface where it forms complexes of the type:
Prior to this absorption, the brass surface has been activated by stearic acid which dissolves some of the surface oxide (mainly ZnO). Stage III: The metal-sulphur (Me-S) bonds formed in stage II are active ring openers for S8 molecules and sulphur insertion occurs. It has been postulated that rubber-soluble zinc plays a role in these insertion reactions by forming a chelate with the nitrogen and sulphur atoms of the MBT [28]. The net result is the formation of surface molecules of the type (see Figure 6.3, stage III):
ISy- is inductively ionised sulphur If the compound contains an active cobalt-containing adhesion promoter, metallic cobalt precipitates onto the brass surface and also forms the Me-S bond, in addition to copper. Zinc can also form this bond, but does not bond to rubber, as the zinc sulphide growth rate is low (see stage IV). The role of cobalt in NR skim stocks is thus to activate (or
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Rubber–Brass Bonding depassivate) the brass surface. The result is a higher growth rate and less dependence on oxide thickness and copper content of the brass. Stage IV: The absorption and insertion reactions of stage III occur very early on in the vulcanisation process as the temperature is still low (140 °C). At higher temperatures, the absorbed Me-Sy-X complexes decompose and form Me-S and Sy-1—X, an active radical for NR crosslinking, where X is an accelerator fragment. Meanwhile, copper has diffused to the surface (it is a p-type semiconductor) and has absorbed more accelerator fragments. By this mechanism, the brass surface is sulphidised rapidly with the formation of CuxS and not ZnS. This mechanism, based on sulphidation by the accelerator and not by elemental sulphur, explains, for the first time, why brass sulphidises more rapidly in squalene than squalane, as has been observed [4, 5, 12].
Stage V: This process will continue until fresh accelerator or MBT is no longer available. Sulphidation then stops and the viscoelastic rubber, entrapped in the CuxS dendrites, will crosslink. This process occurs after the sulphidation reaction, as the decomposition of the rubber-bound intermediate (accelerator fragment dangling on a rubber molecule) to form crosslinks, is a much slower reaction. The result is a tight interlocking of the rubber in the copper sulphide dendrites. An important aspect of this mechanism is that the concentration of Sy– and X-Sy– species in the immediate vicinity of the interface is much higher than in the bulk of the compound. Both species are active components in the reactions that result in rubber crosslinking. Hence, the crosslink density of the rubber adjacent to the metal surface will be increased. This factor contributes to the high adhesion attainable to brass. Other bonding agents (including organofunctional silanes, to be discussed later) do not increase the compound’s mechanical properties in the interface, hence their adhesion (pull-out or pull-off force) tends to be lower.
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In summary, in this updated model a covalent bond of the type Cu-Sx-accelerator is formed initially but decomposes at a later stage with the formation of sulphide and a higher modulus of the interface region. This bond is formed with the accelerator and not with the rubber. The net result is that only inorganic sulphides are formed that become entangled in the crosslinked rubber. This model predicts that all metals that can react with the accelerator as in Stage II, should, in principle, bond to rubber. These metals include the transition metals: cobalt, copper, iron, nickel and zinc. Of these metals, only copper and cobalt are very active and indeed, form strong bonds. The other metals do not bond in practice because the sulphide growth (see Stage IV) is slow (in the case of iron or zinc), or the metal is passive (nickel), or the sulphide does not form the dendrites that copper and cobalt do. It is obvious from this model that, in addition to the metal surface conditions, the accelerator, sulphur and S8/accelerator ratio have a strong effect, as is observed experimentally. The following additives affect the bond formation: • Stearic acid: some stearic acid is required for the formation of zinc ions which appear to assist the sulphur insertion reaction. It also activates the brass surface in Stage I. If too much stearic acid is present, the brass surface is actually attacked. Some zinc is dissolved from the brass and an excess of copper sulphide is formed. The adhesion level will drop. A higher Zn2+ concentration will accelerate the S8 insertion (Stage III). Acceleration of the S8 insertion is good but the excess of copper sulphide is undesirable. • Type of accelerator: the decomposition rate of the accelerator depends on its structure. DCBS, which is often used, is less reactive than CBS shown in Figure 6.3, possibly
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Rubber–Brass Bonding because stearic effects will slow down the formation of the charge transfer complex. The necessity to use a delayed-action sulphenamide accelerator is now obvious. In the induction period the accelerator absorption and sulphide formation need to develop prior to the rubber crosslinking process. Only this sequence results in a tight interlocking of high surface-area copper sulphide dendrites and the crosslinked rubber. The amine fragment released by the accelerator is an important factor as well, as it is also absorbed at the metal surface [27]. There it can block the absorption of accelerator and inhibit sulphide growth, and it can also accelerate brass or steel corrosion, for instance if N-oxydiethylene benzothiazole sulphenamide (OBTS) is used. This morpholinothio molecule appears to accelerate corrosion [29]. • Accelerator-to-sulphur level: according to our model, if the number of MBT or MBTS molecules that is absorbed is high, then there will be many nuclei for sulphide formation, and hence a fine-grained film is formed. If the S 8 concentration is also high, the rate of insertion and decomposition is high, and a dense, compact film is formed which is highly non-stoichiometric. If the accelerator concentration is low and the sulphur/accelerator is low as well, a coarse film with larger dendrites and higher stoichiometry is formed. Such a film is of poorer quality, as described before. As a conclusion to this section, it can be stated that optimal adhesion requires a careful optimisation of rubber formulation in terms of choice of components and their levels, in order to balance the rates of brass activation, accelerator activation, accelerator absorption (nucleation), and sulphur insertion into the absorbed accelerator fragment.
6.2.4 New Evidence for Ageing of the Interfacial Sulphide Film The ageing of the rubber-brass bond in a corrosive environment or under thermal ageing conditions has been well documented. It is generally accepted that loss of adhesion is primarily caused by corrosion of the part of the brass layer that has not been converted to a sulphide film [1, 3, 4]. The corrosion process is dezincification, although in some cases stress corrosion cracking cannot be excluded. Van Ooij has shown that in intact tyres the adhesion degradation in the belt of the truck tyres is not the result of corrosive attack, but of initiation and propagation of cracks in the rubber between the individual tyre cord filaments [29]. These aspects are clearly caused by rubber properties (flexural fatigue) and by the type of cord construction used. They will not be discussed further in this chapter. Apart from these two mechanisms, a new one has recently been proposed [13]. An investigation was made of the rubber-brass interface in a cured tyre sample. The rubber was separated
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The Handbook of Rubber Bonding from the tyre cord by cryogenic techniques [29], as described earlier. The cord, still covered with the sulphide film, was analysed and depth profiled by TOFSIMS. An identical sample was kept for several months at elevated temperature. The results are shown in Figure 6.4. The TOFSIMS results show the following phenomena: • The film in the fresh samples consists of mainly of copper sulphide; the exact composition of this sulphide cannot be determined by this technique. • The profile of the aged interface shows that the film has changed; the copper sulphide is no longer the dominant sulphide, but more ZnS has been formed. This is not only concluded from the profiles, but from the reconstructed spectra as well. • Cobalt is clearly detected in the film and it seems to be more associated with the ZnS than with the CuxS component of the film. A new observation is that some cobalt is actually detected in the metal under the sulphide film. Since ternary CuZnCo alloys are more corrosion resistant than pure brass [1, 3, 4], this finding is an alternative explanation for the improved stability of the interface using certain cobalt compounds; the cobalt is reduced to metallic cobalt by the brass, and diffuses into the metal, thus reducing its dezincification sensitivity.
Figure 6.4a Depth profiles by TOFSIMS of the interfacial film between NR and brassplated tyre cords in tyres; a) new tyre
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Rubber–Brass Bonding
Figure 6.4b Depth profiles by TOFSIMS of the interfacial film between NR and brassplated tyre cords in tyres; b) after ageing for 6 months at 70 °C in air
Figure 6.4c Depth profiles by TOFSIMS of the interfacial film between NR and brassplated tyre cords in tyres; c) reconstructed TOFSIMS spectra. The rubber was separated from the cord by the method described by Ahn [29]
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The Handbook of Rubber Bonding • The carbon depth profile illustrates clearly the interlocking effect of the organic material in the sulphide film. This has not changed much in the aged film, so the conversion from CuxS to ZnS does not necessarily destroy the bond, at least initially. The overall conclusion from this work is that the CuxS sulphide film on brass is inherently unstable, as has been concluded before [4]. It is formed because it has a higher growth rate (higher than by reacting with free sulphur) in the presence of the sulphurating agents. However, when heated in a environment without sulphurating species, the underlying brass reduces the copper sulphide to zinc sulphide, which is more stable than copper sulphide. This conclusion has been reached earlier by Van Ooij based on model work with squalene model systems [9], but it is now reported for the first time in actual tyre cords. In the older work, the rate of overgrowth of CuxS by ZnS was observed to be strongly dependent on the accelerator level and the sulphur/accelerator level which, in turn, affected the coarseness of the CuxS film. In the next section the recent literature (1990 to date) will be reviewed and discussed in light of the updated model presented above.
6.2.5 Compounding for Brass Adhesion Very few recent compound optimisation studies can be found in literature, as they are normally kept proprietary. It has been published that the type of sulphur has little effect on adhesion (pull-out) value. On the other hand, the adhesion increased with ZnO level and with the high-structure carbon black, the former because of the brass activation, the latter most likely because of the increased rubber modulus. Other factors that were identified as affecting the pull-out force were the mixing method and the cure time [30]. The effect of silica in compounds for brass adhesion was studied recently [31]. Incorporation of small amounts of precipitated silica can itself improve adhesion of rubber to metals [32], including brass. However, a loss in adhesion after ageing in humidity is then observed. The mechanism is not well understood. It is possible to offset this effect by replacing part of the carbon black with silica and also by adding a cobalt complex containing boron. If compounded to the same physical characteristics, i.e., modulus level, the adhesion retention after salt water ageing was improved. By choosing certain low specific surface area silicas it was also possible to improve the tear resistance and keep the hysteresis low. It should be pointed out that the silica was treated with the silane coupling agent bis-[triethoxysilylpropyl]tetrasulphide. In general, this sulphur silane had a positive effect on rubber properties (through an increased bonding effect) but a somewhat negative effect on humid and heat aged adhesion retention.
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6.2.6 Additives to Compounds for Brass Adhesion It has been demonstrated that many additives can be used to improve initial and/or aged adhesion of rubber to brass. Most of the recent studies involve the use of cobalt salts. Other additives that have been studied include organic resin formers, metal salts other than cobalt, silica, organic sulphide and miscellaneous chemicals. The most widely used and studied additive is a metal-organic cobalt salt, such as cobalt-naphthenate, neodecanoate, stearate, boroacylate, and others. Inorganic cobalt salts consisting of cobalt metaborate absorbed and calcined on precipitated silica have been proposed by Van Ooij [33], but they were never commercialised. The idea behind this ‘cobalt-borosilicate’ approach was to separate the corrosion-inhibiting effect that inorganic cobalt ions have on brass dezincification [10, 33] from the effect that metal-organic rubber-soluble cobalt salts exert on rubber cure (acceleration), crosslink density, crosslink types, reversion, and increased sensitivity to oxidative ageing [33]. In addition to these generally negative effects, cobalt salts are expensive and not always readily available. Thus, it is understandable that research efforts have been aimed at reducing the negative effect of cobalt, either by optimising the anion, or combining salts in which the anion is optimised so that it decomposes readily during the stage of the cure process where the interfacial sulphide layer is formed. The cobalt salt stability and rubber stability are critical parameters, as was demonstrated in the above-mentioned experiments with totally inorganic cobalt. If the stability (the reactivity to produce inorganic cobalt ions), is optimised and tuned to the rubber cure and sulphide formation, the required cobalt level can be reduced, thus alleviating the negative effects on rubber properties. A reduced cobalt level, using optimised metal-organic cobalt salts, was also proposed by Labarre, who introduced new adhesion promoter systems [34]. A systematic comparison of metal-organic salts on the cord-rubber adhesion and rubber vulcanisation kinetics was published by Chandra [35]. Both the cation (cobalt, nickel, zinc) and anion (stearate, naphthenate, boroacylate, neodecanoate) were varied. Among the major results obtained were those concerning the cations. Cobalt always exerted the maximum effect on the cure and the rubber adhesion, followed by nickel and zinc. The effect of the anion was considerably less and could be explained on the basis of the stability of the metal-organic complex, but also, to a certain extent, their direct involvement in the cure reaction. The same group also reported on the adhesion performance of the cobalt, nickel or zinc salts of boroacylates [36]. Addition of hexamethylenemelamine-resorcinol-hydrated silica (HRH) resin former systems to a standard rubber compound for brass adhesion was also considered. It was reported that the cord pull-out force and the ageing resistance in a variety of hostile environments was enhanced by the HRH system, the metal organic salt and, especially, the combination of the two. The cobalt salt improved the adhesion energy more than other salts. It was again found that the performance of the promoter was enhanced by the ease of
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The Handbook of Rubber Bonding dissociation of the interfacial film. The supremacy of cobalt among all cations was confirmed, considering its overall performance and resistance against various ageing conditions. Cobalt was especially effective against salt and steam ageing in this study. A low cobalt and boron-containing additive was also described by Bobrov [37], although their materials were not specified. It was shown that the effect of a resorcinol derivative (aminolysis of the brass coating), could be negated by the cobalt-boron additive, resulting in a high-modulus belt compound with a high corrosion resistance of the steel cord. The merits of combining cobalt salts and resin agents (for example hexamethylene melamine and resorcinol-formaldehyde resin, RFR) to promote bonding and enhance durability were investigated by Hamed [38]. Several black-filled vulcanisates were bonded to a standard brass-plated steel cord using the tyre cord adhesion test (TCAT) pullout geometry [38]. The highest pull-out forces, both initially and especially after ageing, were obtained when the cobalt and the HR additives were used together. Attenuated Total Reflection (ATR) infrared analysis showed that the RFR migrated to the brass surface, providing a barrier to attack by moisture of the interfacial region. Moisture uptake was, indeed, reduced for compositions containing the HR system. In addition to this effect, sulphidation of brass in the presence of cobalt naphthenate gave a surface topology of substantially higher surface area than without this agent. The high surface area will promote interlocking between the rubber and growing sulphide film. These effects were studied using a squalene model system. A new phenomenon reported in this paper was that the RFR resin was found to be able to remove both cobalt and zinc from the squalene mixtures by an as yet unknown action, possibly by complexing. This occurs after the sulphide formation stage. This subsequent removal of cobalt from the rubber phase by the resin may prevent cobalt from promoting degradation of the rubber during ageing, as is indeed observed in compounds containing these combined bonding agents. Hoff reported on the use of certain melamine resins that were able to function as standalone or one-component systems when used in steel wire belt applications [39]. Onecomponent melamine resins were described that were capable of forming a network in the rubber without the need for a co-reactant such as resorcinol. The performance of these new resins was compared to that of some classic two-component methylene donormethylene acceptor systems. The new, one-component systems gave good initial and aged adhesion, equivalent tensile and dynamic mechanical properties and superior crack growth resistance than the currently used two component systems. Whereas the observations described in Section 6.2.6 can easily be accommodated by the adhesion model described earlier, this poses more problems with a series of papers published on the effect of precipitated silica [31, 40, 41]. Partial replacement of carbon black with precipitated fine particle size silica resulted in improved bond fatigue life of
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Rubber–Brass Bonding brass-coated wire to sulphur-cured NR. The bond fatigue life of the wire bonded to rubber was greatly improved by reducing the concentration of zinc oxide. This research, therefore, clearly identified the relationship between silica and zinc oxide. The zinc oxide, after initial solubilisation, leads to the formation of excess zinc sulphide or zinc oxide at the interface. The mechanism of adhesion enhancement by silica may thus involve a reduction of the amount of interfacial zinc oxide when soluble zinc is removed from the system by attachment to silica surface silanol groups. The positive effect of replacement of some carbon black by precipitated silica was further evaluated in a compound for wire adhesion that also contained an organocobalt adhesion promoter. Up to 20 phr of different types of silicas with widely varying surface areas was added. It was found that the energy of adhesion, which increased linearly with the phr of silica, did not correlate with any of the silica properties. Thus, the observed improvement of wire adhesion was not a physical effect due to increased compound tear strength, but was again related to changes in the interfacial layer formed on the wire [40]. Waddell did some work on the changes in the interfacial layers [41]. A wide range of surface-analytical tools were used: Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), SEM, EDX, and particle induced x-ray emission (PIXE), which indicated that silica use reduced the thickness of the interfacial film, and particularly promoted zinc oxide formation and lowered the amount of sulphide formed. Prokof’ev described the use of polysulphides, oligomers and polymers containing polysulphide fragments of the following structure in the main chain [42]: HS-(R-S-S-)n-R-SH with R = CH2-CH2-O-CH2-O-CH2-CH2 It has been demonstrated previously that such additions can increase the strength of rubber-brass adhesion considerably. In this research it was established that polysulphides are only weak crosslinking agents for unsaturated rubber by themselves. In the presence of sulphenamide accelerators, such as OBTS, polysulphides, in amounts of 0.5 – 1 phr, activate the sulphur vulcanisation. However, the reversion process (crosslink breakdown) is not accelerated. A favourable effect on the physicomechanical properties of the vulcanisate was also reported. Although no mechanistic details are given, it can be assumed that the polysulphides, in the presence of soluble zinc, result in –Sx– formation, similar to the reaction with MBTS shown in Figure 6.3. These persulphurated polysulphides will then activate rubber cure and possibly also sulphidise the brass surface effectively.
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The Handbook of Rubber Bonding The final rubber additive recently used for brass adhesion is tetrachlorobenzoquinone (TCBQ) [43]. Low loading into a cobalt-free but otherwise standard compound increased initial adhesion and rubber coverage of the pulled wire markedly. However, higher loading resulted in a decline of the adhesion. Also, aged adhesion was negatively affected by the addition. It was demonstrated that the copper diffusion into the rubber occurred as a result of the presence of TCBQ. These results indicated that a strong activation of the brass can result in improved initial unaged adhesion, but aged adhesion becomes poor as active cobalt metal ions catalyse oxidation and thus degradation. These results are important as they confirm that the bonding to brass is in fact a controlled corrosion process. TCBQ is not a corrosion inhibitor but a corrosion accelerator for brass, especially for the copper component of it. Such additions will not be able to replace the ubiquitous cobalt salt in the compound, as cobalt promotes the formation of sulphides but at the same time inhibits the dezincification reaction of the brass. This is the reason why cobalt is so effective. The search for cobalt replacement still goes on, as the problem with this metal (oxidative ageing catalysis) has not yet been completely solved. However, it has now been shown that the cobalt ions, after having performed their task, can be effectively removed by the addition of certain resin systems and/or silica, thus largely alleviating cobalt’s negative effects. However, it is strongly recommended to search for rubber additives that promote the formation of fine-grained, high-surface area copper sulphides and at the same time inhibit the dezincification effect, while exerting no influence on rubber crosslinking. Model studies using squalene could be useful here.
6.2.7 Developments in Metal Pre-treatments Only very few recent papers have been published in which new development of substrate treatments were reported. Krone reported on the optimum brass copper content and plating weight for rubber adhesion [44] and confirmed data that were known previously. A more detailed study was published by Goryaev [45]. Several steel cords were prepared and their overall thickness, homogeneity and surface composition altered. The adhesion to a standard NR compound was determined. Among the findings were that: 1. There should be no area on the cord with a low brass thickness, in other words less than 100 nm, because these areas will affect the thickness of the ZnO layer, which controls the brass reactivity. 2. A barrier layer of non-stoichiometric ZnO is necessary having certain zinc and oxygen concentration limits. 3. The overall thickness of the ZnO layer must be 40 – 50 nm.
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Rubber–Brass Bonding 4. The concentration of Cu atoms within the ZnO layer must exceed the concentration of Zn and O atoms. Although these requirements may seem difficult to control in an industrial environment, they do indicate that the reactivity of the brass is a decisive factor and further, this reactivity depends strongly on the properties of the oxide film. In this sense, this work confirms the model proposed by Van Ooij in 1984 [4].
6.2.7.1 Organofunctional Silanes in Rubber–Brass Bonding A new development in rubber-brass bonding appears to be in organofunctional silanes. Such molecules are commonly of the type Y-CH2CH2CH2-Si(OX)3 where Y is an organofunctional group and OX is an ethoxy or methoxy group. Such silanes are all stable, colourless liquids. They can be hydrolysed in water or water/alcohol mixtures to yield Y-CH2CH2CH2-Si(OH)3. The silanol groups are active to hydroxyl groups on glass, silica and metal oxides and can form MeOSi bonds of the type: Me-O-Si-CH2CH2CH2Y [46]
(6.1)
The remaining silanol groups in the film can react with each other and form siloxane units:
Since the Me-O-Si bonds are relatively stable, and because the Si-O-Si bonds are highly hydrophobic, the interphase polymer-silane-substrate has become highly resistant to moisture, or in other words, to corrosive attack. The functional group X can be selected from a wide range of available molecules, so that it interacts with functional groups in the polymer. In this way, an extremely stable interface can be obtained that is completely covalently bonded and resistant to water. Such silane technology has been very successful
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The Handbook of Rubber Bonding and is widely used in polymer, paint, adhesives and other industries [46]. Van Ooij and others have recently demonstrated that silanes, in addition to acting as coupling agents, can also be used very effectively as corrosion inhibiting metal treatments [47-52]. In their work the use of so-called bis-silanes of the following types were particularly featured: (XO)3Si-CH2CH2-Si-(OX)3
(6.2)
(XO)3Si-CH2CH2CH2-Sx-CH2CH2CH2-Si-(OX)3
(6.3)
(XO)3Si-CH2CH2CH2-NH-CH2CH2CH2-Si-(OX)3
(6.4)
where OX= methoxy or ethoxy and Sx = tetrasulphide (average x = 3.8 [53]). It was demonstrated that such silanes, alone or in combination with the standard single silanes, can prevent corrosion of the metals of stainless steel, carbon steel, zinc, aluminium, brass, aluminium alloys and magnesium alloys from various forms of corrosion, even without paint coating. Adhesion to paint systems and adhesives was also improved. Several silane-based metal pre-treatment systems are now commercially used successfully in metal finishing processes where they replace chromate metal pre-treatments. Despite this success of silanes, which are easy to use and apply, their use as metal pre-treatments for bonding metals to rubber has been lacking. The only rubber-related use is as a treatment of rubber-grade silica [32, 33]. This process improves the adhesion of the silica to the rubber and therefore the mechanical rubber properties. There is only one patent that describes the use of a silane to bond bare steel cord (in other words not plated with brass) to sulphur-vulcanised tyre cord skim stocks, published by Sharma in 1982 [54]. Good adhesion was reported but only if the compound contained a certain resin. Apparently, the silane reacted more with the resin than with the rubber compound. Van Ooij recently published a series of papers in which it was shown that excellent bonding could be obtained between stainless steel, carbon steel, aluminium or brass and peroxide-cured silica-filled ethylene-propylene diene monomer (EPDM), silicone rubber, fluorosilicone rubber and fluorocarbon rubber [55, 56]. The rubbers were transfermoulded and the silane treatment was a two-step treatment consisting of a brief rinse in bis-[triethoxysilyl] ethane (BTSE) followed by a brief rinse in a vinyl silane (VS) solution. The treatments were identical for the four metals. In addition to 100% cohesive failure in the rubber in initial adhesion tests, very good adhesion, in other words 100% cohesive retention of the bond was reported after ageing in synthetic fuel, boiling acid, boiling alkali, thermal ageing, salt spray exposure and other ageing conditions [57]. This process outperformed all commercially available products, especially in ageing performance. Since peroxide-cured rubber is not used in tyres, the Van Ooij group reported separately on silane processes for bonding metals to sulphur-vulcanised rubber compounds [58]. 186
Rubber–Brass Bonding It was demonstrated that the BTSE/VS process developed for peroxide-cured rubbers did not work for sulphur compounds with a high or low sulphur level. The tetrasulphide silane that is used for modifying silica did not work either. However, a powerful new silane bonding agent was reported that can bond several metals to a wide range of compounds [58]. The treatment consisted of applying a mixture of silanes to the surface of steel, galvanised steel, tin, aluminium, stainless steel or brass. The compounds were based on NR, EPDM, NBR and SBR. The vulcanisation systems used were high sulphur with cobalt, high sulphur without cobalt, low sulphur, peroxide and also mixed peroxide/ sulphur systems. Even cured vulcanisates could be bonded to these metals. The bonds were extremely resistant to humidity, for example, in pressure cookers at 150 °C. The silanes used were formulae 6.3 and 6.4. A mixture of formulae 6.3:6.4 = 3:1 performed best, whereas the two individual silanes did not bond at all. The mechanism of this phenomenal system is still under investigation, but it has been shown that the bis-amino silanes have the capability to form a dry film upon heating, even without hydrolysing the silane. The polysulphur silane does not dry upon heating by itself, but does so if it is heated in a mixture with the bis-amino silane. Thus, the bisamino silane may form a mechanically strong film, with the polysulphur silane providing the reactivity of the film required to bond to the rubber compound. Overall this new process is very attractive and has several environmental advantages, if it could replace brass on steel tyre cords. Tests with silane-treated tyre cords are in progress. The authors proposed use of a new tyre cord without brass coating but with a zinc coating instead, as tyre cords without brass coating are difficult to manufacture (the brass lubricates the die in the final wire drawing process). The final zinc-plated cord is then passed through a silane bath and dried. Quite remarkable in this system is that the silane-based film does not impair the adhesion of brass to sulphur-cured compound. If the adhesion of a brass plated cord is mediocre, the silane process actually improves its performance, as shown in Table 6.1 [58]. In summary, this silane-based adhesion process is a major development due to its simplicity, broad applicability and performance. Among the advantages are: 1. One silane system can bond a wide range of metals to sulphur-cured compounds, including steel, which would not bond by itself, and brass which would bond by itself. The metals used in tyre cords are, thus, no longer limited to brass on steel. 2. The process can be used on regular existing steel tyre cords and would then improve the bonding of brass and would also bond the cut edges of cords, which are frequently the locus of crack initiation leading to belt edge separation. 3. The ubiquitous cobalt can be eliminated from tyre cord skim stocks. In fact, the silane systems work slightly better with compounds not containing cobalt. 187
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Table 6.1 Adhesion performance of silane-treated metals to high and low sulphur NR compounds Compound
Initial adhesion1 (rubber coverage) N/mm
I2
0
70/30 brass
I
10.2 ± 3.6 (50)
Alloy 360 brass
I
0
CRS + silane4
I
11. 7 ± 1.5 (100)
70/30 brass + silane
I
11. 1 ± 1.4 (100)
Alloy 360 brass + silane
I
10.5 ± 1.6 (90)
CRS
II 3
0
CRS + silane
II
10. 2 ± 2.3 (80)
Metal CRS
1 ASTM D429-02 Method B + metal coupons 2 High sulphur, no-cobalt tyre cord skim stock 3 Low sulphur engine mount compound 4 All silanes treatments are by 1:3 volume mixtures
of bis-[trimethoxysilylpropyl]amine
and bis-[triethoxysilylpropyl]tetrasulphide
4. The bonding characteristics of the silane films are not strongly dependent on the compound formulation, as distinct from the performance of brass. Even low sulphur and semi-EV systems work well. Hence, with this system, the compound of the skim stock can be optimised for mechanical, fatigue and tear resistance properties rather than for adhesion, as is currently done. This reformulation can, therefore, lead to improved tyre performance. 5. The silane film provides excellent corrosion protection to the steel, in addition to adhesion performance. Thus, rusting of tyre cords during the lifetime of the tyre cord, especially if there are cuts and punctures, will be reduced. 6. The same (or a very similar) silane process can be used for many other automotive and non-automotive bonding applications, such as engine mounts. Obviously, much developmental work will have to be done on this new system, such as the evaluation of dynamic adhesion properties, cost analysis, and so on. Important factors in favour of the new systems are their ease of application, simple bath make-up and their
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Rubber–Brass Bonding environmental compliance. Both silanes used have no toxicity problems, and they can replace environmentally problematic processes such as brass plating and the use of solventbased adhesives in various automotive non-tyre applications.
6.2.8 Developments of Novel Alloys for Bonding to Rubber There have only been a very few attempts at research aimed at developing modified brass or alternative alloys for bonding to rubber compounds. After the failure of the ternary alloys of the type CuZnX (X = Co or Ni) around 1980, the focus of the tyre cord and rubber industries has been on the development of better compounds or tyre cord constructions. Only two developments are worth mentioning here. Giridhar developed a dual layer coating on steel wires consisting of a ZnCo base layer (1% Co) followed by a NiZn (30% Zn) top layer [27, 59-62]. Both alloys were deposited from simple acid sulphate baths. The advantages of this system are: • superior adhesion to standard NR compounds is obtained, • the cobalt content of the compound can be reduced by 50-75%, but not completely eliminated, • the aged adhesion retention is much improved over brass, • the plating baths are simple and contain no toxic chemicals, and they can be used at high plating speed. A drawback was that these cords were more difficult to draw than brass because the αNiZn phase is harder than α-brass. The α form is a crystalline, single phase state with up to 37% zinc present. The mechanism of adhesion of this cord has not been studied in great detail [27]. However, the limited work that was done indicated that the amount of the interfacial reaction products formed was considerably less than in the case of brass, and it cannot simply be assumed that NixSy would replace the function of CuxS. The corrosion resistance of this cord is much higher than of brass because Zn1%Co has much better resistance than brass or even pure zinc. In a recent publication, the Pirelli group reported that the NiZn top layer had been dropped from the system and a new tyre had successfully been developed with the Zn1%Co coating only [63]. By modifying the crystal orientation of the deposit and by developing an optimised lubricant, a cord could be obtained with equal drawing behaviour,
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The Handbook of Rubber Bonding adhesion performance on par with brass but much superior aged adhesion. Actual tyres built with this cord showed less rust. What was remarkable though, was that the fatigue of the rubber in the cord region, in other words, inter-belt separation, was worse for the new cord, as compared with brass. It is expected that the problem can be solved by modifying the skim stock formulation. For this cord, too, the adhesion mechanism has not been unravelled uniquely. It was theorised that the cobalt in the compound (a required additive) activates the zinc surface. More work in this field is anticipated. A Japanese group has developed a plating alloy consisting of Ni/Cu/P deposit [64]. Good adhesion of NR and SBR was reported. With no more than 20% Cu, cohesive rubber failure was observed. At 50% Cu or higher, failure became largely adhesive. The adhesion seems to depend on the formation of a thin interfacial sulphide film, as can be expected.
6.2.9 Miscellaneous Some miscellaneous publications are summarised here. It was reported by Matyukhin that the bond between brass-plated tyre cord and rubber can be broken electrochemically [65]. The process results in a growth in the interfacial layer of zinc hydroxide which dislodges the sulphide film. The method appears useful for recycling purposes, but it was also reported to be useful for assessing the strength of the cord-rubber bond. Mori reported on a method to increase the cord-rubber bond strength [66]. By intermittently ageing the samples in water at a temperature of 95 °C, the pull-out strengths increased. Their data suggested that strengthening occurred in both the cord and compound sides of the interface. Intermittent ageing gave better results than continuous ageing. A mechanism for this effect was not given. Panchuk proposed a new test for measuring the bond strength between the cord and the rubber under dynamic loading, which was especially useful for large size tyres and thick cords [67]. The method is based on the idea of pulsed cyclic loading of small test specimens. Results can be obtained within 48 hours.
6.2.10 Summary The main emphasis of the work on rubber-brass bonding published in the last 10 years has been on elucidating mechanisms further. The adhesion model accepted since 1984 in which the formation of a film of non-stoichiometric copper sulphide has been confirmed in all studies. It has now become clearer how to obtain a thin copper sulphide film with 190
Rubber–Brass Bonding a high surface area. The ageing effect occurring in the film in the absence of corrosive attack, was found to be due to the decomposition of the copper sulphide with the formation of zinc sulphide. This effect was predicted in 1984 and has now been confirmed. No real alternatives for cobalt have been announced. The effect of the type of cobalt is now better understood, but no alternatives exist. However, the negative effects of these additives can be controlled by using certain additives. In terms of materials, one new tyre cord coating other than brass has been proposed that has actually made it to the tyre testing stage. This cord has shown some unexpected deficiencies, but if they can be overcome, this cord may have a bright future in view of its ease of manufacture and, especially, its much better corrosion resistance and environmental advantages. The other major development that needs to be recapped here is that of silane systems for bonding a wide range of metals to a wide range of compound types and formulations. This process may become a panacea for bonding in many industries, but much more work will have to be done before the process can replace the brass-coating process for bonding rubber steel to tyre cords.
References 1.
W. J. van Ooij, J. Giridhar and J. H. Ahn, Kautschuk und Gummi Kunststoffe, 1991, 44, 4, 348.
2.
S. Buchan, Rubber to Metal Bonding, Crosby Lockwood, London, 1948.
3.
W. J. van Ooij, Rubber Chemistry and Technology, 1979, 52, 3, 605.
4.
W. J. van Ooij, Rubber Chemistry and Technology, 1984, 57, 3, 421.
5.
W. J. van Ooij, Kautschuk und Gummi Kunststoffe, 1977, 30, 10, 739.
6.
G. Haemers, Rubber World, 1980, 182, 6, 26.
7.
W. J. van Ooij and V. Rangarajan, presented at the Materials Research Society Spring 1988 Meeting, Reno, NV, 1988.
8.
W. J. van Ooij, Presented at the International Rubber Conference, Moscow, USSR, 1984.
9.
W. J. van Ooij, W. E. Weening and P. F. Murray, Rubber Chemistry and Technology, 1981, 54, 2, 227.
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The Handbook of Rubber Bonding 10. W. J. van Ooij, Wire Journal, 1978, 11, 40. 11. W. J. van Ooij, Rubber Chemistry and Technology, 1978, 51, 1, 52. 12. W. J. van Ooij and S. K. Jayaseelan, Presented at the Elastomer Service Life Prediction Symposium ’98, ARDL, Akron, OH, 1998. 13. J. F. Beecher, Surface and Interface Analysis, 1991, 17, 245. 14. W. J. van Ooij and M. E. F. Biemond, Presented at the International Rubber Conference, Paris, France, 1982, Paper No.I-15. 15. J. J. Ball, H. W. Gibbs and P. E. R. Tate, Journal of Adhesion, 1990, 32, 1, 29. 16. M. Pieroth, D. Holtkamp and A. Elschner, Kautschuk und Gummi Kunststoffe, 1993, 46, 2, 112. 17. G. R. Hamed and R. Paul, Rubber Chemistry and Technology, 1997, 70, 4, 541. 18. Y. A. Prokof’ev, E. E. Potapov, E. V. Sakharova and E. Y. Khavina, International Polymer Science and Technology, 1997, 24, 7, T/45. 19. A. K. Chandra, A. Biswas, R. Mukhopadhyay and A. K. Bhowmick, Journal of Adhesion Science and Technology, 1996, 10, 5, 431. 20. E. E. Potapov, G. G. Salych and E. V. Sakharova, International Polymer Science Technology, 1990, 17, 6, T/6. 21. T. Kretzschmar, F. Hofer and K. Hummel, Kautschuk und Gummi Kunststoffe, 1992, 45, 12, 1038. 22. T. Kretzschmar, F. Hofer, K. Hummel, and F. Sommer, Kautschuk und Gummi Kunststoffe, 1993, 46, 9, 710. 23. T. Kretzschmar, K. Hummel and F. Hofer, Rubber Chemistry and Technology, 1993, 66, 5, 837. 24. T. Kretzschmar, K. Hummel, F. Hofer, W. Grogger and G. Grubbauer, Fresenius’ Journal of Analytical Chemistry, 1994, 349, 235. 25. K. Hummel, F. Hofer and T. Kretzschmar, Journal of Adhesion Science and Technology, 1996, 10, 5, 461. 26. F. Hofer, G. Grubbauer, K. Hummel and T. Kretzschmar, Journal of Adhesion Science and Technology, 1996, 10, 5, 473.
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Rubber–Brass Bonding 27. J. Giridhar, Ph. D. Thesis, Colorado School of Mines, Department of Chemistry, Golden, Colorado, 1991. 28. N. A. Chaler, Ph. D. Thesis, Ramon Llull University, Barcelona, Spain, 1998. 29. J. H. Ahn, M.S. Thesis, Colorado School of Mines, Department of Chemistry, Golden, Colorado, 1989. 30. C. Lingjun, Luntai Gongye, 1997, 17, 662. 31. N. L. Hewitt, Rubber World, 1991, 205, 3, 30. 32. P. Cochet, D. Butcher and Y. Bomal, Kautschuk und Gummi Kunststoffe, 1995, 48, 5, 353. 33. W. J. van Ooij and M. E. F. Biemond, Rubber Chemistry and Technology, 1984, 57, 4, 686. 34. D. Labarre, T. Duffour, I. Hawkins, L. Tessier, A. Sartre, Y. Bomal, H. W. Gibbs and C. J. Wilson, Presented at Tyretech ’98 London, UK, 1998, Paper No.2. 35. A. K. Chandra, R. Mukhopadhyay and A. K. Bhowmick, Journal of Adhesion, 1997, 60, 1/4, 71. 36. A. K. Chandra, A. S. Deuri, R. Mukhopadhyay and A. K. Bhowmick, Kautschuk und Gummi Kunststoffe, 1997, 50, 2, 106. 37. A. P. Bobrov, A. G. Shvarts, E. A. Ershov and V. E. Shekhter, International Polymer Science and Technology, 1996, 23, 4, T/46. 38. G. R. Hamed and J. Huang, Rubber Chemistry and Technology, 1991, 64, 2, 285. 39. C. M. Hoff, Presented at the 152nd ACS Rubber Division Meeting, Cleveland, OH, Fall 1997, Paper No.21. 40. L. R. Evans, J. C. Hope, T. A. Okel and W. H. Waddell, Rubber World, 1996, 214, 3, 21. 41. W. H. Waddell, L. G. Evans, E. G. Goralski and L. J. Snodgrass, Rubber Chemistry and Technology, 1996, 69, 1, 48. 42. Y. A. Prokof’ev, E. V. Sakharova, E. E. Potapov and G. G. Salych, International Polymer Science and Technology, 1998, 25, 1, T/77.
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The Handbook of Rubber Bonding 43. G. S. Jeon, M. H. Han and G. Seo, Journal of Adhesion, 1999, 69, 39. 44. R. Krone, Kautschuk und Gummi Kunststoffe, 1993, 46, 3, 233. 45. V. Goryaev, A. P. Bobrov, V. I. Zaporozhchenko and M. F. Grigorev, International Polymer Science and Technology, 1995, 22, 7, T/13. 46. E. P. Plueddemann, Silane Coupling Agents, 2nd Edition, Plenum Press, New York, 1991. 47. W. J. van Ooij and T. F. Child, Chemtec, 1998, 28, 2, 26. 48. V. Subramanian and W. J. van Ooij, Surface Engineering, 1999, 15, 2. 49. T. F. Child and W. J. van Ooij, Transactions of the Institute for Metal Finishing, 1999, 77, 64. 50. W. J. van Ooij, V. Subramanian and T. F. Child, presented at the Workshop on Advanced Metal Finishing Techniques for Aerospace Applications, Keystone, Colorado, USA, 1998, published in the Proceedings on CD. 51. S-E. Hörnström, U. Bexell, W. J. van Ooij and J. Q. Zhang in ECASIA 97: 7th European Conference on Applications of Surface and Interface Analysis, Eds., I. Olefjord, L. Nyborg and D. Briggs, John Wiley & Sons, Chichester, 1997, p.987. 52. W. J. van Ooij, J. Song and V. Subramanian, ATB Metallurgie, 1997, 37, 137. 53. H-D. Luginsland, Presented at the 155th ACS Rubber Division Spring Meeting, Chicago, IL, Spring, 1999, Paper No.74. 54. S. C. Sharma, inventor; The General Tyre & Rubber Company, assignee; US Patent 4,441,946, 1984. 55. W. J. van Ooij, C. P. J. van der Aar and A. Bantjes, Presented at the International Conference on Rubbers, Calcutta, India, 1997. 56. W. J. van Ooij, C. P. J. van der Aar, F. Roseboom and A. Bantjes, Presented at Euradh ’98WCARP-1, Garmisch Partenkirchen, Germany, 1998. 57. C. P. J. van der Aar, Ph.D. Thesis, Department of Chemical Engineering, University of Twente, The Netherlands, 1998. 58. W. J. van Ooij and S. K. Jayaseelan, Presented at the 155th ACS Rubber Division Spring Meeting, Chicago, IL, Spring 1999, Paper No.60.
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Rubber–Brass Bonding 59. J. Giridhar and W. J. van Ooij, Surface Coatings Technology, 1992, 53, 243. 60. J. Giridhar and W. J. van Ooij, Surface Coatings Technology, 1992, 53, 35. 61. J. Giridhar and W. J. van Ooij, Surface Coatings Technology, 1992, 52, 17. 62. J. Giridhar and W. J. van Ooij, Wire Journal International, 1993, 26, 30. 63. G. Orjela, S. J. Harris, M. Vincent and F. Tommasini, Kautschuk und Gummi Kunststoffe, 1997, 50, 11, 778. 64. Y. Ikeda, H. Nawafuna, S. Mizumoto, K. Ikeda and K. Yamaguchi, Nippon Gomu Kyokaishi, 1994, 67, 5, 376. 65. S.A. Matyukhin, Kauchuk i Rezina, 1994, 5, 28. 66. K. Mori, S. Kimu, H. Hirahara, Y. Oishi, H. Horie, M. Nakamura and S. Hiratsuka, Nippon Gomu Kyokaishi, 1994, 67, 5, 369. 67. F. O. Panchuk, B. D. Semak, I. F. Grabov’ and N. A. Oplochenko, International Polymer Science and Technology, 1990, 17, 6, T/12.
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7
Review of Tyre Cord Adhesion W. S. Fulton and J. C. Wilson
7.1 Introduction Steel cord has been the main reinforcing material for tyres, hoses and conveyor belts for many decades, indeed the first steel reinforced tyres appeared over ninety years ago. However, it was not until the emergence of radial tyres that steel cord became a common form of reinforcement and understandably the adhesion between brass-coated steel cord and rubber compound became a significant factor governing the performance and durability of car and truck tyres. Therefore, it is necessary to achieve a high level of adhesion and sustain this level throughout the service history of the tyre. Brass-coated steel cords are widely used for tyre reinforcement because they allow the manufacturer to optimise many physical parameters of the tyre, such as strength, stiffness, modulus, durability, stability and uniformity [1]. Currently in Europe, there is great environmental pressure to reduce car emissions, particularly levels of carbon dioxide, and so address certain commitments agreed at Kyoto in the context of the United Nations Framework on Climate Change [2]. The innovation of the so-called ‘green tyre’ [3], with substantially lower rolling resistance, has in some way met this need. Further developments of steel cord with higher tensile strength and higher stiffness/ weight ratio allow the construction of lighter tyres that will also contribute to the reduction of fuel consumption. It is well known that good adhesion between brass-coated steel cord and rubber compound is essential for the long-term performance of tyres. However, the rubbermetal interface is prone to deterioration, particularly under conditions of high humidity and salt, and so reinforcement by the steel cord is reduced, with a concomitant reduction in the life of the tyre. Consequently, various organic cobalt salts are used, alone or in combination with resin systems, to improve and maintain good bonding at the rubbermetal interface. At present, organic cobalt salts appear to be the most efficient adhesion promoter and could be considered as a bench-mark by which the tyre industry assesses rubber-metal bond strength.
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7.2 Accepted Mechanisms of Rubber–Brass Bonding Over the last 20 years the mechanism by which brass-coated steel cord and rubber bond has been the subject of much investigation [4, 5, 6, 7, 8, 9] and review articles [10, 11]. Despite such extensive efforts the absolute mechanism of adhesion has not yet been clarified. As the bonding layer between rubber and brass-coated steel cord is of the order of 20-50 nm thick, specialised techniques must be adopted for analysis, and the use of these techniques is well-documented [12, 13, 14, 15]. Accordingly, the use of surface analytical tools such as X-ray photoelectron spectroscopy (XPS or ESCA), Auger electron spectroscopy (AES), energy-dispersive X-rays (EDX) and secondary ion mass spectroscopy (SIMS) have helped to elucidate chemical structure and morphology at the rubber-metal interface. Analytical electron microscopy (AEM), coupled with electron diffraction has been used to determine the different crystallographic forms of copper sulphide, present in the bonding layers [16]. When forming a tyre cord, copper and zinc are electro-deposited sequentially on to drawn steel wire and treated by a thermal diffusion process to produce a brass alloy coating. Further drawing of the wire eventually produces a steel cord filament coated with a brass layer, approximately 0.2 μm thick. The method of drawing the wire through the forming die creates a surface texture as well as influencing the composition, distribution and thickness of the brass layer [17]. When the metal filaments are drawn, it is possible that organic residues from the lubricant baths are deposited on the brass surface and these may affect the adhesion of the rubber to metal after vulcanisation. Typically, a brass layer is composed of 63.5% copper and it has been shown [18, 19] that the adhesion force tends to a maximum value with a copper content between 67% and 72%, but better retention of adhesion after ageing under humid conditions is achieved at a lower copper content. Adhesion can also be affected by differences in the concentration of zinc and copper in the surface compared to the composition of bulk brass. If the brass coating is insufficient then corrosion resistance is lowered and poor adhesion can occur after ageing. This is due to delamination of the brass by dissolution of iron in exposed areas that can eventually result in cord breakage. When brass-coated wire is drawn during the forming process, Zn2+ ions diffuse to the surface and are oxidised to form a ZnO layer. A thin CuO skin can form, but it is usually present in very small amounts. The enriched ZnO layer also contains metallic copper inclusions formed as a result of the internal oxidation mechanism of zinc (see Figure 7.1). Wire drawing conditions may vary and these can affect the thickness and distribution of ZnO, as well as the number of copper inclusions. During vulcanisation, the brass surface is exposed to active sulphur-containing molecules present in the rubber compound and a strong bond is formed between the rubber compound and the tyre cord by the action of an interfacial, non-stoichiometric CuxS layer (x ≈ 1.8)
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Figure 7.1 Diagram of brass-coated steel cord surface (Reproduced with permission from W. J. Van Ooij, Rubber Chemistry and Technology, 1984, 57, 421, Figure 10. ©1984, Rubber Division, American Chemical Society, Inc.)
that grows before the rubber is fully cross-linked. At an early stage of vulcanisation, Cu+ and Zn+ ions and free electrons move to the surface of the brass wire via cationic diffusion and a CuxS layer, with some ZnS inclusions, is formed by a process called sulphidation. ZnS is formed initially, but is overgrown by CuxS at a later stage and the rate at which the critical thickness of CuxS is reached depends upon several factors, including copper content of the ZnO layer and curing conditions. The different growth rates have been attributed [19] to the much faster rate of oxide formation of a P-type semiconductor (CuxS) to that of a N-type semiconductor (ZnS). During the first stages of sulphidation, ZnS forms slowly and diffusion of copper ions through this layer is hindered because ions migrate by interstitial diffusion and copper ions migrate more slowly than zinc ions because of the different ionic radii. The CuxS thickens as copper ions diffuse into this layer via lattice defects and the diffusion rate is relatively high because of the non-stoichiometric nature of the CuxS. The rate of CuxS formation is sufficient to allow growth to a thickness that is essential for good bond formation and the amount of CuxS formed depends primarily on the number of copper inclusions in the ZnO layer. Sulphidation will cease when all the inclusions are depleted (see Chapter 6.2.2, Figure 6.1b). A previous mechanistic model [19] proposed a synchronisation of sulphidation and network formation. However, this is not necessarily the case and it is now generally accepted that it is essential to delay the cross-linking process long enough to build a CuxS
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The Handbook of Rubber Bonding layer of critical thickness. The amount of CuxS present in the layer is directly related to the degree of sulphidation. It is still not well demonstrated how the CuxS layer interacts with the rubber and it is thought that because the CuxS layer is dendritic in form, a high bond strength is obtained primarily by a tight, physical interlocking between this layer and the vulcanised rubber. If chemical crosslinking of CuxS to the rubber by CuxS-Sy-NR bonds occurs then it is very much a minor factor. Such a physical bonding mechanism has been supported by the fact that a reduction in pressure during vulcanisation results in a loss of adhesion at the CuxS-NR interface [5]. However, in recent studies by Persoone [20] in which AES was combined with Maximum Likelihood Common Factor Analysis (MLCFA), a ‘—C—S—’ component was shown to exist at the CuxS-NR interface, a fact that was also noted previously by Van Ooij [21]. For unaged systems, adhesion between rubber and brass usually exceeds the tear strength of the rubber [4] and therefore bond failure never occurs at the CuxS-rubber interface [6]. The initial chemical composition and surface structure of the brass influence the bonding mechanism by virtue of limiting the amount of CuxS that is formed during vulcanisation [22, 23]. There is an optimum thickness for the CuxS layer that imparts maximum adhesion and if the layer becomes too thick, then it will readily detach from the brass resulting in low bond strength. According to Van Ooij [5], one of the major prerequisites is that growth of the CuxS layer has been completed before cross-linking begins.
7.3 Ageing of the Rubber-Brass Bond Rubber-brass bond degradation may be due to several processes, including thermal ageing and electrochemical corrosion. During heat ageing, degradation of the rubber as well as deterioration of the interface may occur [24]. Copper migrates, by cationic diffusion, through the ZnS and CuxS to thicken the existing CuxS layer [4, 5] which eventually cracks and weakens the rubber-metal bond. When the CuxS layer stops growing, Zn2+ ions diffuse through the entire interfacial layer to create ZnO/Zn(OH)2. The diffusion of Zn2+ ions can be a slow process under dry conditions, but eventually, ZnO/Zn(OH)2 will develop at the metal surface to weaken the bond. In humid conditions, an increase of moisture and oxygen content will accelerate this process by altering the rate of diffusion of the Zn2+ ions. According to Van Ooij [9], the ratedetermining step is the formation of Zn2+ ions at the anode which then diffuse through to the surface layer and thus the CuxS layer is overgrown by ZnS and Zn(OH)2. Moreover, the conductivity of the ZnO and CuxS layers plays an important role in governing the rate of formation of Zn(OH)2 layer. In due course, the formation of ZnO/Zn(OH)2 from the precipitation of Zn2+ destroys the integrity of the CuxS layer and debonding occurs [25]. Such processes are generally
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Review of Tyre Cord Adhesion called dezincification [5] and are assumed to be the major, underlying effects that cause bond failure (see Figure 7.2). Brass plated steel cord that contains low levels of copper is not so sensitive to decohesion caused by moisture. Initial adhesion may be lower because of fewer copper inclusions, but a more coherent ZnO is formed which reduces the amount of Zn2+ ions diffusing through the CuxS layer and so less ZnO/Zn(OH)2 is formed at the interface. Compounding with high levels of ZnO also helps to inhibit the dezincification process by reducing the diffusion rate of Zn2+ ions to the cord surface. Tyre cords are liable to corrosion under wet and saline conditions by various routes, including dezincification and electrochemical reactions [26, 27]. The major reactions, determined by the effect on brass of a 3% NaCl solution, are the preferential diffusion of Zn2+ ions from the bulk of the brass to the ZnO/Zn(OH)2 layer and the partial dissolution of copper ions with some ZnO. As the concentration of ZnO increases, a surface layer of Zn(OH)2 forms which is then slowly dissolved.
7.4 Metal Organic Cobalt Salts There are two main advantages to be gained by the addition of cobalt adhesion promoters. Firstly, is the effect that cobalt has upon sulphidation during the scorch period to ameliorate CuxS growth and dramatically increase the subsequent rubber-
Figure 7.2 A schematic illustration of the interfacial structures formed after ageing of the rubber-metal bond (see also Chapter 6.2.2, Figure 6.2) (Reproduced with permission from W. J. Van Ooij and M. E. F. Biemond, Rubber Chemistry and Technology, 1984, 57, 4, 686, Figure 2 ‘Aged’. ©1984, Rubber Division, American Chemical Society, Inc.)
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The Handbook of Rubber Bonding metal bond strength. Secondly, is the advantage of protection and increased durability of the bond afforded by cobalt during ageing, particularly at high temperature and high humidity. Moreover, under humid conditions, cobalt boron complexes have been shown to improve the levels of adhesion more than simple di-soaps [28]. During the curing process, cobalt inhibits the formation of ZnS at the cord surface by acting as a selective diffusion barrier [29, 30] to allow the rapid formation of CuxS. For systems containing a thin layer of cobalt, copper migrates via grain boundary diffusion to the rubber-metal interface to react to form CuxS. However, zinc cannot diffuse by the same mechanism because cobalt acts as a selective diffusion barrier, and so promotes the growth of CuxS at the expense of surface ZnS. Scanning electron microscopic studies [31] on brass cords immersed at elevated temperatures in squalene mixtures containing a cure system and various bonding agents have revealed the extent of sulphidation. Figure 7.3a shows the micrograph obtained from a wire immersed in a control mixture, containing curatives but no bonding agents and Figure 7.3b shows the micrograph obtained from a wire immersed in a mixture containing cobalt naphthenate. It would appear that sulphidation is preferentially nucleated along the drawing lines of the wire surface. The control reveals a small amount of surface nodules, whereas the ‘cobalt’ wire gives a more dense and irregular nodules of CuxS. These nodules could be interpreted as the dendritic morphology described earlier and thus it can be envisaged how unvulcanised rubber compound will intermesh with the growing Cu xS layer, eventually being locked in place after vulcanisation. Further analysis by EDX of the cobalt naphthenate wire system has revealed a sulphur content approximately three times greater than that without any cobalt, corroborating the fact that the presence of cobalt promotes sulphidation. As well as mitigating the formation of CuxS, cobalt salts will significantly accelerate the cure rate and the cross-link density (or state of cure) is also increased in compounds containing high levels of sulphur [4, 5]. An example of this effect is illustrated by comparing the cure characteristics of three compounds (see Figure 7.4); one without cobalt (blank), one with 0.1 phr cobalt boroacylate (Manobond 680C) and one with 0.1 phr cobalt stearate (Manobond CS95) concentration based on cobalt metal content. The effect of cobalt is evident, whereas the anion has comparatively less effect, indeed the stearate has slightly more influence than the boroacylate and this has been interpreted [32] as an accelerator activating effect in the presence of ZnO. Studies by Chandra [33], revealed the effect of different anions on the cure characteristics and he was able to rank them, with stearate showing the largest effect followed by naphthenate, neodecanoate and boroacylate. Such an effect may be related to the ease by which cobalt salts dissociate to form their constituent ions [10].
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Figure 7.3a Scanning electron micrograph obtained from a brass-coated tyre cord immersed in a squalene mixture without cobalt (magnification x300 left; x400 right) (Reproduced with permission from G. R. Hamed and R. Paul, Rubber Chemistry and Technology, 1997, 70, 4, 541, Figure 4. ©1997, Rubber Division, American Chemical Society, Inc.)
Figure 7.3b Scanning electron micrograph obtained from a brass-coated tyre cord immersed in a squalene mixture with cobalt naphthenate (magnification x300 left; x400 right) (Reproduced with permission from G. R. Hamed and R. Paul, Rubber Chemistry and Technology, 1997, 70, 541, Figure 7. ©1997, Rubber Division, American Chemical Society, Inc.)
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Figure 7.4 Cure characteristics of belt compounds that contain cobalt adhesion promoters
In studies of model systems based on squalene [34], it was shown that cobalt salts undergo a dissociation reaction to form Co2+ ions at the interface and the relative rate of formation and amount of cobalt deposited on the surface depends on the nature of the organic anion. It was also shown how the type of anion affects ageing properties by influencing cure and complex stability and hence the amount of cobalt available to maintain bond strength during ageing. Therefore by judicial choice of the anionic complex, optimum performance was achieved. One of the other benefits of adding cobalt salts is the improved retention of adhesion after ageing at high humidity [35]. Cobalt salts dissociate to form cobalt ions that are incorporated into the ZnO layer and during subsequent vulcanisation, the formation of ZnS is suppressed and CuxS growth is stimulated. Cobalt accumulates at the interface (see Figure 7.5) but not in the ZnO layer. Interestingly, when cobalt boroacylates are used as adhesion promoters, significantly higher concentrations of cobalt were detected than other cobalt salts [36]. As stated previously, decohesion is caused by the build-up of ZnO and Zn(OH)2 during the process of dezincification and that, according to Van Ooij [9], the formation of Zn2+ is the rate determining step. Cobalt affects dezincification by reducing the conductivity of the ZnO layer to slow down the migration of zinc ions and so reduce the rate of dezincification. It has been reported [37] that a combination of cobalt and boron improve the retention of adhesion after steam-ageing and that boric esters increase the mobility of cobalt so that increased concentrations of cobalt were detected at the rubber-metal interface.
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Figure 7.5 Diagrammatic representation of cobalt promoted adhesion (Reproduced with permission from W. J. Van Ooij, Rubber Chemistry and Technology, 1984, 57, 3, 421, Figure 13 (middle diagram). ©1984, Rubber Division, American Chemical Society, Inc.)
However, boron also accumulated at the interface and borates are known to act also as corrosion inhibitors, particularly with steel and so there is a possible dual effect. It has also been postulated that borates may act by buffering the environment at the rubber– metal interface and so help to inhibit corrosion mechanisms. It is important to have an optimum concentration of cobalt at the interface and if the concentration is too high, then metallic cobalt can be precipitated and this will accelerate dezincification and thus degrade interfacial adhesion.
7.5 The Role of Resins and Silica/Resin Systems (see also Chapter 9.2.3) Resin systems have been employed extensively for many years in applications such as the bonding of textiles to rubber, in hoses and belts where they have been used as tackifiers, reinforcers, curing agents and adhesion promoters. The addition of resin and/or cobalt depends upon the application and performance requirements of the rubber article that is to be bonded. Traditionally, resorcinol in combination with a methylene donor such as hexamethylene tetramine (HMT; Figure 7.6) was utilised either alone or in compounds containing cobalt salts. On heating, HMT decomposes to produce ammonia and formaldehyde that reacts with the resorcinol to produce a stable, highly crosslinked polymeric network. At present, the tyre industry generally uses alternatives to resorcinol and HMT because of health and safety concerns over their toxicity. Resorcinol has a tendency to fume at elevated temperatures, whilst formaldehyde generated from HMT is toxic and may cause irreversible effects. A second generation of resins includes resorcinol/
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Figure 7.6 Structure of resorcinol and hexamethylene tetramine
formaldehyde condensation products such as Penacolite B-18S [38] or phenol/ resorcinol resins such as the Ribenol range [39]. These resins have low free resorcinol contents but still require a methylene donor. The use of HMT was superceded in the tyre industry by modified melamine compounds such as hexamethoxymethylmelamine (HMMM) on silica carriers, e.g., Cyrez 964LF [40]. The latest generation of resins includes modified resorcinol/formaldehyde resins, such as Penacolite B-20S [41], cresol-cashew modified phenol/formaldehyde resins (Ribetak 14588A) [39] and urethane modified novolak resins [42]. As stated previously, precipitated silica is being used to replace the carbon black filler in so-called ‘green’ tyres where it has been shown to reduce rolling resistance [43-46], and so help to reduce fuel consumption. Since the late 1960s, silica has been used in combination with resin systems and at that time PPG introduced a system that consisted of HI-SIL® hydrated silica, resorcinol and hexamethylene tetramine (HRH). Initially it was used chiefly for bonding various rubbers to textiles, but such systems have found increasing use in wire coat and belt compounds. Much work has been done describing the benefits to tyre steel cord adhesion by using resins and precipitated silica in conjunction with cobalt adhesion. Tate [28] compared various adhesion promoter systems in a belt compound and concluded that at low addition levels, a combination of cobalt salt, resorcinol, formaldehyde donor and silica gave good physical properties and good adhesion even under the most severe ageing conditions. Wagner [46-50] showed that the addition of silica improved rubber tear strength, abrasion resistance, hardness, modulus and adhesion. More recently, Cochet [51] examined the use of low specific surface area silicas in conjunction with a coupling agent and cobalt salt and found improved retention of adhesion after heat, humidity and salt-water ageing. Evans [52] showed that both compound tear strength and
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Review of Tyre Cord Adhesion adhesion increased when silica was used with a cobalt salt. It has also been shown that the energy of adhesion values [53] increased linearly with increasing silica content [52] which suggested a direct effect that silica had on the rubber/wire interfacial layer. Adhesion data for two methylene/formaldehyde donors, HMT and HMMM, were compared with resorcinol and subsequent analysis revealed that initial adhesion levels were generally higher for the HMT systems [54]. Whilst both donors performed well after thermal ageing, it was shown that addition of HMMM to be advantageous over HMT after humidity and steam ageing. Recent studies [55] by Hoff compared traditional HMT/resin/cobalt systems and HMMM/resin/cobalt systems against a new generation of one component resin systems that did not require a resorcinol acceptor. Hoff claimed that compounds containing new one component resins and no cobalt salt had better adhesion retention after salt ageing than the traditional resin/cobalt systems. Similar single component resins were found to give good rubber-to-steel cord adhesion when used in combination with a triazine and a cobalt salt [56], with the added advantage of easier processing than resin/formaldehyde systems. As stated previously, cobalt salts can be used with or without a resin/silica system and for the latter a combination of all three is required to obtain the desired adhesion and rubber physical properties under all ageing conditions. The concentration of resin is variable but it generally falls in the range of one to four parts per hundred rubber and the methylene donor level is approximately in the same range. For optimum compound properties, it is usual for the resin/donor ratio to be greater than one. Cobalt metal levels generally fall in the range 0.1 to 0.3 parts, whilst silica may used at levels from 0 to 55 parts. When using resin systems, the resin can be mixed in the initial stage with rubber, carbon black and/or silica, cobalt salt and other ingredients, whilst the methylene donor is mixed in the final stage with the curatives at a lower temperature. This ensures that no crosslinking occurs before vulcanisation to cause processing difficulties and also diminish the mode of action by which the resin improves rubber to metal bonding. With the correct choice of curing system, the resin and silica have no discernible effect on cure, and by careful selection of batch ingredients and concentrations, compound properties are not adversely affected. Despite their widespread use in rubber formulations, few mechanistic studies have been performed to determine the role of silica and resin in the adhesion process. The effect that resorcinol/formaldehyde resin had upon the adhesion interface between rubber and a brass film has been investigated [57, 58] using AES in an attempt to correlate spectroscopic data with actual adhesion values. In the unaged state, the structure of the interface was essentially the same for compounds containing resin as those without. Differences became apparent after ageing at high humidity when the
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The Handbook of Rubber Bonding interface remained relatively unchanged in the case of the resin-containing sample (at 3 phr) and it was thought that the resorcinol/formaldehyde resin reacted with HMMM to produce a protective barrier at the interface. Hamed and Huang [57] proposed such a barrier that would prevent attack by moisture and also prevent migration of copper and zinc towards the bulk rubber. Equally, the migration of sulphur and oxygen to the interface would be hindered under humid conditions and consequently degradation of the interface would be suppressed. Alternative theories [10] suggest that HMMM may complex with cobalt in the rubber and thus prevent degradation by retarding the prooxidative effects of cobalt. The role of ‘silica-only’ systems on adhesion has been studied using model compounds with squalene [59]. It was shown that the mechanism for increased adhesion to brasscoated wire-to-rubber was not just a simple improvement of the physical properties of the rubber, but that silica moderated the thickness and composition of the interfacial layer by a chemical interaction. SEM-EDX (scanning electron microscopy with energy dispersive analysis of X-rays), XPS, AES and PIXE (proton induced X-ray emission spectroscopy) revealed that silica affected the relative concentrations of compounds present in the interfacial layer, promoting zinc oxide formation in particular.
7.6 Summary The main purpose of this chapter has been to describe the predominant bonding systems that are presently used in tyres. These are cobalt salts used either alone or in combination with resin/silica systems. They have become very well established and are used by many, if not all, the major tyre companies. Nevertheless, other experimental bonding systems are being investigated and tested. These include metal complexes other than cobalt and other non-metal bonding systems. One of the most recent innovations for bonding tyre cord to rubber has been the use of silanes to promote adhesion and to protect the interface [60, 61]. Other non-metallic systems, such as tetrachlorobenzoquinone [62] and chloropyrimidines and chlorotriazines [63] have also been studied. Regarding copper free coating of steel, alloy systems containing Zn/Ni/Co [64], Zn/Co [65, 66] and other zinc alloys [67] have been proposed. Traditionally, the tyre industry has been conservative when considering changes to the belt region of a tyre because this is such an important and critical part of the tyre. Currently, cobalt systems perform well and failures in the breaker are rare, in fact the tyre is more likely to fail by catastrophic damage, e.g., puncture or just wearing away. Trends towards tyres with longer life and with the capability to be retreaded will mean that tyre cord adhesion systems will have to be designed to meet this need.
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References 1.
M. Basaran, Kautschuk Gummi Kunststoffe, 1993, 46, 7, 550.
2.
Climate Change – Towards an EU Post-Kyoto Strategy, Communication from the Commission to the Council and the European Parliament, Commission of the European Communities, COM(98)353
3.
R. Rauline, inventor; Michelin & Cie, assignee; EP 501227A1, 1992.
4.
W. J. Van Ooij, Rubber Chemistry and Technology, 1979, 52, 3, 605.
5.
W. J. Van Ooij, Rubber Chemistry and Technology, 1984, 57, 3, 421.
6.
W. J. Van Ooij, O. H. Middletown, J. Giridhar and J. H. Ahn, Kautschuk Gummi Kunststoffe, 1991, 44, 4, 348.
7.
W. J. Van Ooij, Presented at the 124th ACS Rubber Division Meeting, Houston, TX, Fall 1983, Paper No.59.
8.
W. J. Van Ooij, W. E. Weening and P. F. Murray, Presented at the 118th ACS Rubber Division Meeting, Detroit, MI, Fall 1980, Paper No.36.
9.
W. J. Van Ooij and M. E. F. Biemond, Rubber Chemistry and Technology, 1984, 57, 4, 686.
10. R. F. Seibert, Presented at the 144th ACS Rubber Division Meeting (IRC ’93), Orlando, FL, Fall 1993, Paper No.52. 11. Y. Ishikawa, Nippon Gomu Kyokaishi, 1992, 2, 86. 12. D. Briggs, in Industrial Adhesion Problems, Eds., D. M. Brewis and D. Briggs, Orbital Press, Oxford, 1985, Chapter 2. 13. D. Briggs, in Polymer Surfaces and Interfaces, Eds., W. J. Feast and H. S. Munro, Wiley, Chichester, 1987, Chapter 2. 14. G. E. Hammer, Presented at the 147th ACS Rubber Division Meeting, Philadelphia, PA, Spring 1995, Paper No.21 15. A. K. Chandra, R. Mukhopadhyay, J. Konar, T. B. Ghosh and A. K. Bhowmick, Journal of Materials Science, 1996, 31, 10, 2667. 16. F. Hofer, G. Grubbauer, K. Hummel and T. Kretzschmar, Journal of Adhesion Science and Technology, 1996, 10, 5, 473.
209
The Handbook of Rubber Bonding 17. R. S. Lehrle and K. J. Niderost, Progress in Rubber and Plastics Technology, 1992, 8, 3, 221. 18. A. Maeseele and E. Debruyne, Rubber Chemistry and Technology, 1969, 42, 2, 613. 19. G. Haemers, Rubber World, 1980, 182, 6, 26. 20. P. Persoone, R. De Gryse and P. De Volder, Journal of Electron Spectroscopy and Related Phenomena, 1995, 71, 225. 21. W. J. Van Ooij, Surface Science, 1977, 68, 1. 22. M. P. Bourrain in Tire Reinforcement and Tire Performance, ASTM STP 694, Ed., R. A. Flemming and W. J. Hannell, ASTM 87, 1979. 23. G. E. Hammer, Metallurgy, Process and Applications of Metal Wires, 1996, 155. 24. W. J. Van Ooij, W. E. Weening and P. F. Murray, Rubber Chemistry and Technology, 1981, 54, 2, 227. 25. P. Bourrain and J. C. Morawski, Presented at the 126th ACS Rubber Division Meeting, Denver, CO, Fall 1984, Paper No.16. 26. T. L. Barr, Surface Interface Analysis, 1982, 4, 185. 27. Y. Ishikawa and S. Kawakami, Rubber Chemistry and Technology, 1986, 59, 1, 1. 28. P. E. R. Tate, Rubber World, 1985, 192, 1, 37. 29. P. Persoone, P. De Volder and R. De Gryse, Solid State Communications, 1994, 92, 8, 675. 30. J. B. Pelletier and S. Toesca and J. C. Colson, Journal of Applied Surface Science, 1983, 14, 375. 31. G. R. Hamed and R. Paul, Rubber Chemistry and Technology, 1997, 70, 4, 541. 32. A. K. Chandra, A. Biswas, R. Mukhopadhyay and A. K. Bhowmick, Journal of Adhesion, 1994, 44, 3, 177. 33. A. K. Chandra, A. S. Deuri, R. Mukhopadhyay and A. K. Bhowmick, Kautschuk Gummi Kunststoffe, 1997, 50, 2, 106. 34. D. Labarre, T. Duffour, I. M. Hawkins, L. Tessier, A. Sartre, Y. Bomal, H. W. Gibbs and J. C. Wilson, Presented at Tyre Tech ‘98, London, UK, 1998, Paper No.2.
210
Review of Tyre Cord Adhesion 35. L. R. Barker, NR Technology, 1981, 12, 4, 77. 36. J. J. Ball, H. W. Gibbs and P. E. R. Tate, Journal of Adhesion, 1990, 32, 1, 29. 37. M. Pieroth, D. Holtkamp and A. Elschner, Kautschuk Gummi Kunststoffe, 1993, 46, 2, 112. 38. A. Peterson and M. I. Dietrick, Rubber World, 1984, 190, 5, 24. 39. B. Stuck, J. C. Souchet and C. Morel-Fourrier, Tire Technology International, 1998, 57. 40. C. M. Hoff, L. R. Evans and W. H. Waddell, Presented at the 148th ACS Rubber Division Meeting, Cleveland, OH, Fall 1995, Paper No.116. 41. A. Peterson, Presented at the 151st ACS Rubber Division Meeting, Anaheim, CA, Spring 1997, Paper No.L. 42. T. Burkhart, S. Wallerswein, G. Brindoepke and G. Walz, inventors; Vianora Resins GmbH, assignee; US Patent 5,859,169, 1999. 43. R. H. Hess, H. H. Hoekje, J. R. Creasey and F. Strain, inventors; PPG Industries Inc., assignee; US Patent 3,768,537, 1973. 44. S. Ahmad and R. J. Schaefer, inventors; The BF Goodrich Company, assignee; US Patent 4,519,430, 1985. 45. S. Wolff, Tire Science Technology, 15, 4, 276. 46. R. Rauline, inventor; Compagnie Generale des Etablissments Michelin - Michelin & Cie, assignee; US Patent 5,227,425, 1993. 47. M. P. Wagner, Rubber Chemistry and Technology, 1976, 49, 3, 703. 48. M. P. Wagner, Elastomerics,1981, 113, 8, 40. 49. M. P. Wagner and N. L. Hewitt, Rubber Chemistry and Technology, 1979, 52, 4, 805. 50. M. P. Wagner and N. L. Hewitt, Rubber and Plastics News, 1984, 13, 23, 20. 51. P. Cochet, D. Butcher and Y. Bomal, Kautschuk Gummi Kunststoffe, 1995, 48, 5, 353. 52. L. R. Evans, J. C. Hope, T. A. Okel and W. H. Waddell, Presented at the 147th ACS Rubber Division Meeting, Philadelphia, PA, Spring 1995, Paper No.16.
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The Handbook of Rubber Bonding 53. R. A. Ridha, J. F. Roach, D. E. Erickson and T. F. Reed, Rubber Chemistry and Technology, 1981, 54, 4, 835. 54. P. Combette and F. Alarcon-Lorca, Revue Generale des Caoutchoucs et Plastiques, 1988, 65, 683, 103. 55. C. M. Hoff, Presented at the 152nd ACS Rubber Division Meeting, Cleveland, OH, Fall 1997, Paper No.21. 56. M. L. Engelhardt and K-K. Kang, inventors; Hanook Tire Manufacturing Co., Ltd., assignee; US Patent 5,723,523, 1998. 57. G. R. Hamed and J. Huang, Rubber Chemistry and Technology, 1991, 64, 2, 285. 58. G. Seo, Journal of Adhesion Science and Technology, 1997, 11, 11, 1433. 59. W. H. Waddell, L. R. Evans, E.G. Goralski and L. J. Snodgrass, Presented at the 148th ACS Rubber Division Meeting, Cleveland, OH, Fall 1995, Paper No.65. 60. W. J. Van Ooij and T. Child, Chemtech, 1998, February, 26. 61. W. J. Van Ooij, L. Shijian and S. Jayaseelan, Presented at IRC’99, Seoul, Korea, 1999, 482. 62. G. S. Jeon, M. H. Han and G. Seo, Journal of Adhesion, 1999, 69, 39. 63. R. F. Seibert, E. L. Wheeler, F. H. Burrows and W. R. True, inventors; Uniroyal Chemical Company Inc., assignee; US Patent 5,126,385, 1992. 64. W. J. Van Ooij, Presented at the Kobe International Rubber Conference (IRC ’95), Kobe, Japan, 1995, 107. 65. G. Orjela, S. J. Harris, M. Vincent and F. Tommasini, Kautschuk Gummi Kunststoffe, 1997, 50, 11, 778. 66. H. Yan, J. Downes, P. J. Bowden and S. J. Harris, Transactions of IMF, 1999, 77, 2, 71. 67. R. N. Beers, T. W. Starinshak and D. A. Benko, inventors; Goodyear Tire and Rubbers (US), assignee; EP Patent 901914A1, 1999.
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8
Rubber to Metal Bonding Using Metallic Coagents R. Costin
Editors Note Metallic Coagents for Peroxide Vulcanisation Metallic coagents are reactive substances, which improve the effectiveness of peroxide crosslinking. Most of them belong to the group of methacrylates or derivatives containing allyls, but polymeric materials with a high content of vinyl groups are also known to react in a similar way. The crosslinking efficiency of many peroxide-initiated free radicals is low. These labile radicals can be converted to more stable radicals by contact in situ in the rubber mix with polyfunctional monomers to form a three-dimensional network. Crosslinking efficiency is thus increased by some 20%. In addition these materials act as plasticisers during processing and in some cases also act as hardening agents. The following chapter discusses the use of such agents in bonding applications for a variety of rubbers.
Titanate and Zirconate Coupling Agents Coupling agents are molecular bridges at the interface between two dissimilar substrates, usually but not limited to, an organic filler and an organic polymer matrix. Organometallic titanates and zirconate coupling agents form monomolecular layers on the substrate of most materials, such as metals, metal oxides, carbonates, sulphides, sulphates, siliceous materials, carbon black, some synthetic fibres such as Kevlar, dispersed dyes and organic pigments to name a few. They render the substrate hydrophobic (moisture free), organophilic (rubber compatible and reactive), organofunctional (such as phosphato flame retardant functionality to provide controlled intumesence) and catalytically reactive with the polymer phase. They may also act as reactive super plasticisers to increase rubber flow while increasing the mechanical properties of the rubber - even above the Tg. Viscosity reduction or polymer 213
Commercial rubbers
The Handbook of Rubber Bonding solvation and higher filler loading can be accomplished with less plasticiser. Flow is achieved through molecular rearrangement and not average molecular weight reduction of the rubber. Neoalkoxy zirconates also provide novel opportunities for the adhesion of fluorinated polymers to metal substrates because the introduction of a zirconate at the interface results in a metal oxygen zirconium (VI) organofluoride.
8.1 Introduction The use of coagents in conjunction with peroxides to cure rubbers has been common practice in the rubber industry for many years. Coagents are typically multifunctional monomers that are highly reactive in the presence of free radicals and readily graft to rubber chains to form a polymeric crosslink network. They are used in peroxide-cured systems to increase the crosslinking efficiency of the vulcanisation process and to increase the crosslink density of the rubber. The increase in crosslink density is directly related to the coagent concentration and has a major effect on the mechanical and physical properties of the cured rubber. Some of the most common coagents in use today are esters of methacrylic acid [1, 2]. Trimethylolpropane trimethacrylate (TRIM) and 1,3-butyleneglycol dimethacrylate (BGDMA) are typical examples of the methacrylate ester class of coagents. Metallic salts of acrylic acid and methacrylic acid also function as coagents in the same way as the methacrylate esters do. They are also highly reactive in the presence of free radicals and readily form a polymeric crosslink network with rubbers. Several examples are reported where they have been used as coagents in peroxide-cure formulations for both saturated and unsaturated rubbers. In a patent issued to the US Army in the late 1980s, the zinc salts of acrylic acid and methacrylic acid were used to upgrade the properties of hydrogenated nitrile rubber (HNBR) [3]. The goal of this work was to develop abrasion-resistant materials that could be used to extend the service life of tank treads. Zinc dimethacrylate (ZDMA) proved to be the best material in the study for improving the tear strength, abrasion resistance, and high temperature performance of HNBR. Also, during this period, Zeon Chemicals discovered that curing HNBR which contained a mixture of zinc oxide and methacrylic acid, resulted in rubber products possessing very high tensile strength and excellent abrasion resistance [4, 5 and 6]. In this case, the zinc oxide and methacrylic acid reacted in situ to form ZDMA prior to curing. This approach requires careful control of the methacrylic acid-zinc oxide stoichiometry, to minimise residual methacrylic acid that could cause corrosion of the mixing equipment.
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Rubber to Metal Bonding Using Metallic Coagents Metallic salts of acrylic acid and methacrylic acid have also been used for many years with polybutadiene to form the rubber cores for two-piece golf balls [7, 8 and 9]. Crosslinking polybutadiene with metal salts of either acrylic acid or methacrylic acid results in a very strong rubber compound with high compression and a high coefficient of restitution (COR). The COR is a measurement of velocity for golf balls. In the test, the ball is propelled against a backboard, and the inlet and exit velocities are measured. The COR is the exit velocity/inlet velocity. Zinc salts have proved to be the most effective of the metal salts and are used extensively in the manufacture of golf ball cores today. Several papers have been published that describe the use of zinc diacrylate (ZDA) and ZDMA as coagents to improve the physical properties of rubbers [10, 11, 12, 13 and 14]. In this context, they are referred to as metallic coagents. They differ from conventional rubber coagents, like triallyl cyanurate (TAC) and TRIM, in that they have ionic bonds that become part of the crosslink network. The ionic bonds allow for a flexible and more forgiving network, particularly at high crosslink levels. This can lead to rubber products with good dynamic properties and a good combination of physical properties. It has been found that, in addition to improving the mechanical properties of rubber, metallic coagents also increase the adhesion of rubber to untreated metals and synthetic fibres during vulcanisation [15, 16]. They offer a way of bonding rubber to metal that is far less intensive and time consuming than the procedure required for conventional adhesives [17, 18]. There are several ways metallic coagents can be used with rubber compounds to improve adhesion. The most straightforward way is to simply mix them with the rubber compound, where they function as internal adhesion promoters upon curing. They can also be mixed with a rubber compound to form an adhesive strip that can be used as a tie layer between various rubber compounds and metals. This technique can be applied to either sulphur- or peroxide-cured stock. And finally, they may be used to form reactive dispersions, which can be applied as pastes to either the rubber or metal surface prior to curing. This chapter reviews these techniques along with some of the physical properties that can be obtained using metallic coagents.
8.2 Metallic Coagents Metallic coagents are defined as the metal salts of acrylic and methacrylic acids. The two products discussed, Saret 633 and Saret 634, are the difunctional zinc salts of acrylic acid and methacrylic acid, as shown in Figure 8.1. They are free flowing, 100% reactive solids, and can be readily compounded with a variety of rubbers. Additional characteristics include low odour, scorch protection
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Figure 8.1 Saret metallic coagents
with a non-nitroso scorch retarder, and improved physical properties. They also provide excellent high temperature stability and good dynamic properties, as well as outstanding fluid resistance. Many of the unique properties associated with the metallic coagents come from the ionic bonds that are formed between the zinc cations and the corresponding carboxylate anions. Metal cations, particularly zinc, are known to increase the strength and the mechanical properties of polymers containing metal-neutralised ionic crosslinks. These products are known as ionomers and their properties are welldocumented [18]. The same ionic crosslink mechanism is believed to occur with rubbers that are cured with Saret 633 and Saret 634. Crosslinking with peroxide results in the formation of a covalent bond. This carbon-carbon bond is quite rigid and stable and accounts for the lower tensile and tear strength of peroxide-cured stocks compared with sulphur vulcanisates. The good heat stability of this covalent bond also explains the superior heat aged characteristics of peroxide cured systems. In contrast, polysulphide crosslinks formed with sulphur curing are thermally weak, but are mobile under stress and can slip along the hydrocarbon chain. This mobility has been used to explain the superior tensile and tear strength in sulphur-cured stocks. The crosslinks formed with metallic coagent in peroxide cured rubbers are ionic due to the zinc carboxylate bonds (see Figure 8.2). These ionic bonds exhibit both good heat aged stability and the ability to slip along, characteristics of both the peroxide and sulphur crosslink systems, giving high tensile and tear strength and excellent heat aged properties. When metallic coagents are used in rubber compounds to increase bonding to metals and reinforcing materials, they also change the physical properties of the rubber compound. Therefore, their use as coagents and their effect on the properties of the rubber compound will be discussed in Sections 8.2.2 and 8.2.3.
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Figure 8.2 Ionic crosslinks formed with metallic coagents
8.2.1 Scorch Safety ZDA and ZDMA increase both the cure state and the cure rate of the rubber compound and, therefore, are classified as Type I coagents. Type I coagents are highly reactive in free radical reactions and tend to produce both rubber crosslinks and homopolymerisation [19]. This will lead to high hardness and high tensile strength, but may also accelerate crosslinking that could reduce scorch safety during mixing. An example of this is shown in Figure 8.3 for ZDA in nitrile rubber. The rheometer curves show the cure characteristics of ZDA with peroxide relative to peroxide alone. It can be seen that the scorch time is significantly less for ZDA with peroxide than
Figure 8.3 Torque versus time for various coagents
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The Handbook of Rubber Bonding for peroxide alone. Also, the cure rate is significantly faster with the ZDA/peroxide combination, which allows for more rapid processing where scorch is not a concern. It should also be noted that the maximum torque is much higher with ZDA and peroxide, which indicates a greater crosslink density and, therefore, higher mechanical properties. To overcome scorch behaviour during processing, scorch retarded versions of ZDA and ZDMA were developed and are recommended for rubber applications [11, 12, 13]. Both Saret 633 and Saret 634 contain non-nitroso scorch retarders, which are safe and effective for rubber compounding. The effect of the scorch retarder on ZDA is also shown in Figure 8.3. In this example, Saret 633 (ZDA with scorch retarder) is compared to ZDA and to peroxide alone. The scorch time of Saret 633 has increased significantly relative to ZDA and is about the same as that obtained with the peroxide alone. This indicates that mixing can be achieved more safely with less chance of premature vulcanisation. Also, the maximum torque obtained with Saret 633 is essentially the same as that obtained with ZDA, indicating that no significant change in crosslink density occurred. The same behaviour occurs with the scorch-retarded version of ZDMA and is illustrated in Table 8.1 with EPDM rubber. In this example, a 40% increase in TS2 is obtained with Saret 634, the scorch-retarded version of ZDMA, without any significant loss in maximum torque (MHF). MHF is the difference between the minimum torque (ML) and the maximum torque (MH) obtained for a rubber compound by oscillating disk rheometer (ODR). It is a measure of the crosslink density of a compound and is part of the curing characteristics of a compound. Thus, with the Saret metallic coagents it is possible to gain the benefit of increased crosslink density and faster cure rates, while maintaining good scorch safety.
Table 8.1 Effect of Saret retarder on scorch safety No Coagent
ZDMA
Saret 634
TS2, min
—
1.5
2. 1
TS5, min
5.5
3.5
4. 0
MHF, N-m
1.24
2.15
2.30
TS2 is the time taken for a 2 unit increase in torque TS5 is the time taken for a 5 unit increase in torque
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scorch safety Rubber to Metal Bonding Using Metallic Coagents
8.2.2 Tensile Properties The increase in modulus obtained with coagents in the peroxide curing of rubber compounds is always accompanied by a corresponding decrease in elongation. This behaviour is noted for all coagents in commercial use today. One surprising characteristic of metallic coagents is that they increase the hardness and modulus of rubbers like other coagents do, but with much greater retention of elongation. This characteristic is compared in Figure 8.4, with Saret 633 and Saret 634 versus TRIM, all in EPDM. In all cases, as the coagent concentration was increased from 0 to 20 phr, the modulus increased and the elongation decreased. However, the metallic coagents, Saret 633 and Saret 634, gave much higher elongation than TRIM for a specific modulus value. Saret 634 gave a slightly better balance of modulus and elongation at low modulus values and Saret 633 gave the highest modulus for a given concentration and the highest modulus overall. Peroxide and coagents normally have very little beneficial effect on the tensile strength of rubbers. Sulphur vulcanisation, on the other hand, normally increases the tensile strength of rubbers. However, it has been found that metallic coagents increase the tensile strength of rubbers such as EPDM and NBR in a manner similar to that obtained with sulphur vulcanisation. This effect is shown in Table 8.2 for Saret 633 and Saret 634 versus TRIM, all in NBR. Saret 633 gave the highest tensile strength at each concentration. It is interesting to note that, with Saret 633, the increase in tensile strength is linear, suggesting that higher tensile strengths could be obtained at
Figure 8.4 Modulus versus elongation values for various coagents
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Table 8.2 Tensile strength versus coagent concentration phr
TRIM (MPa)
Saret 633 (MPa)
Saret 634 (MPa)
0
15.8
15.8
15.8
10
15.2
18.8
16.0
20
15.4
21.4
18. 2
higher coagent concentrations. The tensile strength also increased with Saret 634, but not as dramatically. And, as expected with TRIM, the tensile strength remained essentially unchanged. Similar results were obtained with EPDM.
8.2.3 Tear Strength The effect of metallic coagents on the tear strength of EPDM is displayed graphically in Figure 8.5. With both Saret 633 and Saret 634, the tear strength increased as the coagent
Figure 8.5 Tear strength obtained with metallic coagents
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Rubber to Metal Bonding Using Metallic Coagents was increased. However, tear strength decreased when TRIM was used as the coagent. This is typical behaviour for both Type I and Type II coagents. Type I coagents are more reactive and increase both the cure rate and the cure state of a rubber compound. Type II coagents are less reactive and increase only the cure state. With both Saret 633 and Saret 634 the tear strength increased as the concentration was increased, reaching maximum strength at concentrations near 10 to 15 phr. This is in contrast to the behaviour of conventional coagents, where tear strength normally declines as the crosslink density is increased. An example of normal coagents is shown with TRIM in Figure 8.5. In this case, the tear strength decreased sharply with 5 phr TRIM and then gradually levelled-off at about 10 phr.
8.3 Experimental 8.3.1 Materials 8.3.1.1 Solid Rubbers A nitrile masterbatch, see Table 8.3, was used in all experiments pertaining to nitrile rubbers. Hycar 1042 (Zeon Chemicals) is a general purpose NBR (acrylonitrile content is 33%) with good processing characteristics and high physical properties. An EPDM masterbatch, see Table 8.4, was used in all experiments relating to EPDM rubbers.
Table 8.3 Formulation for a nitrile masterbatch Material Hycar 1042
ph r 100
Table 8.4 Formulation for an EPDM masterbatch Material
phr
Nordel 1040
100 100
Carbon black N365
65
Carbon black N762
Dioctyl phthalate
15
Sunpar oil 2280
50
Zinc oxide
5
Zinc oxide
5
Stearic acid
1
Stearic acid
1
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The Handbook of Rubber Bonding The chlorosulphonated PE compound, see Table 8.5, was used with or without (control) Saret 633. Saret 633 is ZDA containing a non-nitroso scorch retarder to provide scorch safety during processing. The ethylene vinyl acetate copolymer (EVA) compound is given in Table 8.6. The silicone rubber formulation is given in Table 8.7. This formulation was used with or without (control), Saret 633. The natural rubber formulation is given in Table 8.8 and was used with or without (control), Saret 633.
8.3.1.2 Formulation Preparation The rubbers were compounded according to the formulations described in Section 8.3.1.1, using a laboratory 15.2 cm two-roll mill. All masterbatches were masticated on a two-
Table 8.5 Formulation for a Hypalon compound Material Hypalon 40 Poly-Dispersion T (HRL) D-90
ph r 100
Table 8.6 Formulation for an EVA compound Material
phr
44
Elvax 240
100
5
HI SIL 230
25
Whitex clay
40
Zinc oxide
3
Kerflex A
20
Stearic acid
3
Sundex oil 790
15
Trioctyl trimellitate (TOTM)
3
Perkadox 1440
2
Saret 633
3
Carbon black N550
Paraffin Wax
6
DiCup 40KE
5
Saret 633
5
Hypalon 40 is made by DuPont Poly-Dispersion T (HRL) D-90 is made by Rhein Chemie HRL: heat resistant lithage
222
Elvax 240 (DuPont) is 28% vinyl acetate with a melt index of 43
Rubber to Metal Bonding Using Metallic Coagents
Table 8.8 Formulation for natural rubber Material
Table 8.7 Formulation for silicone rubber Material Silicone 6140
phr
SMR CV 60
100
Carbon black
22
ph r
HI SIL 230
5
100
Zinc oxide
4
6916 HA
1
TOTM
3
Triganox 10145
2
Resin D
2
Stearic acid
1
DiCup 40KE
5
Saret 633
10
Silicone 6140 and 6916 HA (stabiliser) are made by General Electric
Saret 633
10
SMR: Standard Malaysian Rubber (Malaysian Rubber Bureau)
roll mill until a flux was created at the nip of the rollers. At this point, the Agerite Resin D, DiCup and coagent were slowly added to the flux roll. The band was then sheeted and folded and then rebanded for mixing. This process was repeated many times to ensure thorough mixing. The coagent concentration was varied as noted in the individual formulations. The compounded rubbers were then cured in a compression mould for twenty minutes at 160 – 166 °C.
8.3.1.3 Compounding and Curing The Saret 633 and 634 coagents can be mixed with most rubbers using a two-roll mill or a Banbury internal mixer. Curing the resulting rubber compound against the metal substrate creates the adhesive bond as well as the mechanical properties of the rubber. The rubber formulations used in most adhesive tests were mixed on a two-roll mill and sheeted out to a thickness of 0.08 mm. Strips of the rubber sheeting were then placed between the appropriate metal panels and cured in a compression mould for 20 minutes at 165 °C. The shear adhesion values and the mechanical properties were determined according to ASTM test methods D816-82 [20] and D412-98a [21], respectively.
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8.3.1.4 Measurement Physical tests were conducted for all moulded compounds after moulding, and again after heat ageing at 100 °C for 70 hours. Tensile strength, modulus and elongation were determined according to ASTM method D412 using a Thwing Albert model 1451-42 tensile tester at a crosshead speed of 51 cm/minute. Shore A hardness tests were determined for samples after moulding using a hand-held Shore A durometer. Cure characteristics, which include scorch time, cure rate and torque values, were measured over a 20 minute period at 160 °C using a Monsanto oscillating-disk rheometer according to ASTM method D1084 [22]. Compression set was determined by compressing a 2.5 cm diameter specimen built up with four plies to 50% original thickness for 22 hours at 100 °C. The specimen was then removed and the permanent set measured as a percentage of original thickness, using the method in ASTM D395 [23]. Rubber-to-metal adhesion was determined by tensile testing a rubber specimen cured between two metal coupons as illustrated in Figure 8.6. The tensile test was run at 2.5 cm/minute crosshead speed and the force in kg to break the lap joint in shear was measured. The rubber specimen, approximately 0.08 cm thick, was cured at 160 – 166 °C for 30 minutes between two 2.5 x 7.6 x 0.08 cm metal coupons overlapped 2.5 cm in a plaque mould under 207 MPa pressure (Figure 8.6). The metal coupons were methanol washed and dried before curing. Adhesion peel tests were run by curing a 0.64 cm thick rubber specimen against a metal coupon and then tensile testing at 180° angle of pull with 2.5 cm/minute crosshead speed. The rubber specimen was hand pressed against the metal coupon.
Figure 8.6 Test specimen - side view
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Rubber to Metal Bonding Using Metallic Coagents
8.3.2.1 Liquid Rubbers for Sealants The liquid rubbers that were used in the sealant applications are listed in Table 8.9. Test formulations for pumpable sealants based on liquid natural rubber, liquid polybutadiene and liquid nitrile rubber are shown in Tables 8.10, 8.11, and 8.12, respectively.
Table 8.9 Liquid rubbers used for sealant applications and their suppliers Trade name
Company
Formulation
Trilene 56 EPDM
Uniroyal
13% DCPD in 49% ethylene 51% propylene MW 5,200
DPR 40 NR
Hardman
36,000-55000 Pa-s viscosity at 38 °C MW 40,000
R45 HT PB
Elf Atochem
Hydroxyl terminated PB MW 2,800
NBR 1312LV
Zeon
33% ACN low MW liquid rubber
ACN: acrylonitrile; DCPD: dicyclopentadiene; MW: molecular weight; PB: polybutadiene
Table 8.10 NR sealant formulation Material DPR 40 NR Calcium carbonate
phr 10 0 50
Table 8.11 PB sealant formulation Material R45 HT PB Calcium carbonate
phr 100 50
Zinc oxide
7.5
Zinc oxide
7.5
Stearic acid
1.0
Stearic acid
1.0
Maglite D
2.0
Maglite D
2.0
Agente resin D
0.5
Agerite resin D
0.5
t-Butyl perbenzoate
7.7
t-Butyl perbenzoate
7.7
Saret 633
30.0
Saret 633
30.0
DPR 40 NR is supplied by Hardman
PB NR
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Table 8.12 Nitrile sealant formulation Material NBR 1312LV Calcium carbonate
phr 100 50
Zinc oxide
7.5
Stearic acid
1.0
Maglite D
2.0
Agerite resin D
0.5
t-Butyl perbenzoate
7.7
Saret 633
30.0
NBR 1312 LV is produced by Zeon Chemicals
The pumpable sealants were prepared according to the above formulations and were cured at 160 °C for 25 minutes. Saret 633 and Saret 634 (Sartomer Company) are the scorch-retarded versions of zinc diacrylate and zinc dimethacrylate, respectively. The Saret metallic coagents are used in rubber compounding to avoid premature curing during mixing. The reactive dispersions (CD-627, CD-628, PRO 1825) are experimental materials developed by Sartomer Company for evaluation in this study. They are proprietary products that consist of a metallic coagent dispersed at various concentrations in different acrylic monomers. A viscous solution or paste is formed which can be used in place of an adhesive to form adhesive bonds during the curing step of the rubber compound (peroxide cure only). They contain no solvents and are 100% reactive. The metallic coagent is a small particle-size powder that is not soluble in monomers, but does form a stable dispersion. A practical use of Saret 633 as an internal adhesion promoter is shown in Table 8.13 which gives a formulation for a pumpable sealant based on liquid EPDM [24]. This formulation not only gives excellent adhesion to steel, but bonds strongly to oily steel as well. In this example, lap shear adhesion was determined using steel coupons, overlapped 2.5 cm, with a spacer to form a uniform sealant thickness of 0.18 cm. Light pressure was applied so that the upper coupon would be pressed against the spacer. The sealant was then cured at 160 °C for 25 minutes. To test for bonding to an oily surface, the steel
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EPDM Rubber to Metal Bonding Using Metallic Coagents
Table 8.13 A typical formulation of an EPDM sealant Formulation Trilene 56 EPDM CaCO3
Saret 633
Control
100
100
50
50
Zinc oxide
7.5
7.5
Stearic acid
1.0
1.0
Maglite D
2.0
2.0
Agerite resin D
0.5
0.5
t-Butyl perbenzoate
7.7
7.7
Saret 633
20
0
Lap shear adhesion To steel, MPa
4.88 (C)
0.14 (C)
To oily steel, MPa
4.26 (C)
0.21 (50% C)
C = Cohesive Failure
coupons were coated with ASTM oil No. 3, the excess oil was removed, and then the sealant was applied and cured as above. This type of performance is particularly attractive for automotive applications where oily surfaces and oil contamination are prevalent. Normally, extensive cleaning is required to prepare the metal surface for adhesion with conventional adhesives and sealants. Table 8.14 shows the benefit of adding Saret 633 to several liquid rubberic sealant formulations. Formulations and cure conditions are described in Section 8.3.2. In each case, a control without Saret 633 was also tested for comparison and found to have adhesive values ranging from 0 to 0.27 MPa. Lap shear adhesion was measured using steel coupons overlapped 2.5 cm. Spacers were used between the coupons to obtain a uniform sealant thickness of 0.18 cm. Light pressure was applied so the upper coupon would bear evenly on the spacers. The sealant compounds were then cured at 160 °C for 25 minutes and then tested at room temperature. When liquid natural rubber is used as the sealant base, shear adhesion increased from 0 for the control to 4.55 MPa with 30 phr Saret 633. Heat ageing at 130 °C for 14 days increased the shear adhesion to 4.90 MPa.
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pumpable The Handbook of Rubber Bonding
Table 8.14 Pumpable sealants based on liquid rubbers Liquid rubber
Saret 633 (phr)
Adhesion* (MPa)
Trilene 56 EPDM
20
4.90 (C)
DPR natural rubber
30
4.55 (C)
NBR 1312LV
30
3.45 (C)
R45HT polybutadiene
30
5.24 (A/C)
* Shear Adhesion to Steel A = Adhesive Failure C = Cohesive Failure
Saret 633 also was evaluated in a liquid polybutadiene sealant as shown in Table 8.15. Again, shear adhesion increased from 0.28 MPa for the control to 5.24 MPa for Saret 633 at a concentration of 30 phr. Shear adhesion decreased to 3.51 MPa when the sealant was exposed to 5% sodium hydroxide solution for 70 hours at 70 °C. A separate test specimen heat aged at 130 °C for 14 days increased the shear strength to over 6.90 MPa. The liquid nitrile rubber was evaluated at 30 phr Saret 633 as shown in Table 8.12. Shear adhesion to steel was 3.45 MPa for the Saret 633 formulation versus 0 MPa for the control. Heat ageing at 130 °C for 14 days also showed an increase with this formulation, with the value increasing to 6.55 MPa. In all cases, Saret 633 increased adhesion to oily and clean steel relative to the control. The failure mode was predominantly cohesive with the Saret 633 in all sealant formulations. The sealant viscosity for each formulation was approximately 1500 MPa. No attempt was made to optimise the cure conditions or the formulations for performance. Liquid polybutadiene formulated with Saret 633 was also evaluated as a glass sealant as shown in Table 8.15. As the data show, adhesive values of 1.03 MPa and 1.72 MPa were obtained for bonding glass to steel and glass to E-Coat (electrostatic phosphate coating applied to metal to aid corrosion resistance and surface uniformity), respectively. Cohesive failure occurred in all cases, suggesting higher values could be obtained with higher strength formulations. The controls, without Saret 633, did not adhere to the glass substrate.
228
polybutadiene glass
Rubber to Metal Bonding Using Metallic Coagents
Table 8.15 Polybutadiene glass sealant Formulation (Cure at 160 °C) R45HT polybutadiene HiSil 230
phr 100 50
Zinc oxide
7.5
Stearic acid
1.0
Maglite D
2.0
Resin D
0.5
Saret 633
30
CADOX BS
10
Shear adhesion, MPa Glass to steel
1.03 (C)
Glass to E-coat steel
1.72 (C)
C = Cohesive Failure CADOX = 50% Benzoyl Peroxide on a silicone carrier
8.4 Results and Discussion 8.4.1 Adhesion to Metals Conventional metal reinforced rubber products require both an adhesive to bond the metal to the rubber and a separate curing system to increase the mechanical properties of the rubber. This entails an intensive, time-consuming series of procedures: first the metal surface has to be prepared, then the adhesive has to be applied and dried. After this step, the rubber compound has to be prepared, moulded and then cured in contact with adhesive treated metal. In most cases a post-cure step also is required. The metallic coagents offer several alternative ways of bonding rubber to metal that are far less intensive and time consuming. This is due in part to the fact that they form strong rubber-to-metal bonds without the use of external adhesives or a separate curing step. Any procedure that places the metallic coagents and peroxide between the metal surface and rubber with applied pressure may be used.
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The Handbook of Rubber Bonding Three techniques to bond rubber to metal and synthetic fibres using metallic coagents have been developed [25]. They are: 1) in the uncured rubber compound as an internal adhesion promoter, 2) in a thin adhesive strip that functions as a tie layer upon curing, 3) in a reactive dispersion that can be applied as a viscous liquid or paste to either the metal or rubber prior to curing. Although the metallic coagents are not adhesives by themselves, there are applications where these techniques may be applied to improve the adhesion of rubber to metals and synthetic fibres.
8.4.1.1 Internal Adhesion Promoter When using Saret 633 and Saret 634 as internal adhesion promoters, it is important to remember that they do crosslink with the rubber during curing and, therefore, change the physical properties of the rubber. An example of this is shown in Table 8.16 for the Nordel EPDM formulation containing 2 phr of the metallic coagents. In this example, both the modulus and tensile strength increased significantly, even at a concentration of 2 phr. Shear adhesion also increased significantly at the 2 phr concentration, especially with Saret 633. Saret 634, while not as effective as Saret 633, also increased adhesion in the example described in Table 8.16. In applications where high tear strength, abrasion resistance, exceptional scorch safety and slow cure are needed, Saret 634 may be the best choice. Metallic coagents based on other metal salts, such as calcium and magnesium, also increase rubber-to-metal adhesion, but not as effectively as Saret 633 and Saret 634.
Table 8.16 Tensile properties obtained with metallic coagents in EPDM Coagent (2 phr)
Modulus (MPa)
Tensile strength (MPa)
Adhesion to steel (MPa)
None
0.55
3.83
0.5 0
Saret 633
1.10
9.14
3.4 4
Saret 634
1.03
8.66
2.1 4
230
Rubber to Metal Bonding Using Metallic Coagents The strength of the adhesive bond increases as the concentration of the metallic coagent is increased. The effect of Saret 633 concentration on the adhesion of EPDM to steel is shown in Table 8.17. Shear adhesion increased from 0.5 MPa to 11.34 MPa as the Saret 633 concentration was increased from 2 phr to 20 phr. In all cases, cohesive failure, i.e., failure within the rubber, is the primary mode of failure for the adhesive bond. It should also be noted that as the Saret 633 concentration is increased from 2 phr to 20 phr, the modulus and hardness of the rubber also increases. As a result, Saret 633 cannot be simply added to a currently used rubber compound to replace an adhesive without modifying the formulation. It would be necessary to adjust the Saret 633 and/or filler concentrations to get the best balance of adhesion and physical properties. As shown in Figure 8.7, strong rubber-to-metal bonding can be obtained with Saret 633 as an internal adhesion promoter for a variety of rubbers such as silicone rubber, EVA, EPDM, NR and Hypalon. All of the compounds contained 5 phr of Saret 633 and were cured with peroxide. In each case, shear adhesion was significantly increased with the Saret 633 cure system over the peroxide control.
Table 8.17 Adhesion to steel obtained with Saret 633 in EPDM Saret 633
2 phr
Adhesion, MPa
0.50
11.34
Modulus, MPa
1.1
2.13
Shore A hardness
55
20 phr
80
Figure 8.7 Adhesion of various rubber to steel
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The Handbook of Rubber Bonding Cohesive failure was found to be the predominant mode of failure for each rubber compound containing Saret 633 (Figure 8.7). Therefore, it would be expected that as the Saret 633 concentration is increased, the rubber compound would become stronger due to additional crosslinking, which would result in an increase in adhesive strength at the interface between rubber and substrate. This proved to be the case and is shown in Figure 8.8 for EPDM bonded to untreated steel. As the Saret 633 concentration was increased from 0 to 20 phr, the shear adhesion increased from approximately 0.55 MPa for the control to over 11.0 MPa. Cohesive failure was the predominant mode of failure at each concentration. Similar performance was observed for other rubbers, such as nitrile, natural, polybutadiene, silicone and hydrogenated nitrile. The Saret 633/peroxide-cure system also promotes good adhesion to other untreated metals, including aluminium, zinc, brass, and stainless steel. This is illustrated in Figure 8.9 for EPDM containing 10 phr Saret 633. In each case, shear adhesion increased with the addition of Saret 633, compared with peroxide alone.
8.4.1.2 Adhesive Tie Layer A further technique is to form an adhesive strip containing the metallic coagent. The adhesive strip may then be used to bond another rubber to a metal surface. When the adhesive strip approach is used, the metallic coagents are milled into the rubber compound. The uncured compound is then formed into sheets by calendaring and then cut into stable, thin strips that contain the metallic coagent, peroxide and other rubber additives. The rubber strip can then be applied between the uncured rubber compound and the
Figure 8.8 The effect of Saret 633 concentration on the shear adhesion of EPDM to steel
232
Rubber to Metal Bonding Using Metallic Coagents
Figure 8.9 EPDM adhesion to various metals
metal surface and cured to form an adhesive bond. In this way, the rubber strip is functioning as a tie layer between the metal and rubber. Figure 8.10 illustrates the test method that was used to evaluate this approach for various rubber compounds. Although other rubbers could have been used, the adhesive strips used in this study were prepared using the EPDM masterbatch containing 10 phr Saret 633 as described in Section 8.3.1.1. The EPDM compound was calendared to form thin strips approximately 0.8 mm thick. The strips were placed between the uncured rubber and the metal coupons as shown in Figure 8.10. After curing, the test specimen was cooled and tested for lap shear adhesion. This technique works for bonding both peroxide and sulphur cured stock to various metals. An example of this technique is illustrated in Table 8.18 with a strip containing Saret 633 in EPDM that is bonding sulphur cured EPDM to steel.
Figure 8.10 Adhesive strip - test specimen side view
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Table 8.18 Bonding sulphur cured EPDM to steel Shear adhesion (MPa)
Table 8.19 Bonding peroxide cured elastomers to steel with the EPDM Saret 633 adhesive strip Rubber
Shear adhesion (MPa)
EPDM
8.53 (C)
No adhesive
0.58
NBR
5.98 (C)
Commercial adhesive
4.68
CPE
7.22 (C)
EPDM Saret 633 strip
5.78
NR
8.53 (C)
C = cohesive failure CPE = Chlorinated polyethylene
The EPDM Saret 633 strip also may be used to bond other rubbers to various metals. Several examples of peroxide cured rubbers are shown in Table 8.19. In each case, 0.8 mm of the EPDM Saret 633 strip was placed between the rubber compound before curing. After curing at 160 °C for 20 minutes, the test specimen was cooled and tested for shear adhesion.
8.4.1.3 Reactive Adhesive Dispersions The third approach involves applying a dispersion of the metallic coagent as a reactive adhesive between the metal and the rubber stock. The reactive dispersions, as described in Section 8.3.1.1, are made by mixing the metallic coagent in a liquid coagent to form a viscous liquid or paste. They may then be applied in place of an adhesive to the surface of the metal or the uncured rubber stock. The only requirement is that the rubber being bonded must contain peroxide as the curative. When the rubber is cured, peroxide in the rubber stock activates the reactive dispersion, forming adhesive bonds between the rubber and the metal surface. Results obtained using this technique are summarised in Table 8.20 for several reactive dispersions, versus a conventional solventbased adhesive, for bonding peroxide-cured EPDM to steel.
sulphur 234
peroxide reactive dispersions Rubber to Metal Bonding Using Metallic Coagents
Table 8.20 Bonding peroxide-cured EPDM to steel with reactive dispersions Shear adhesion (MPa) No adhesive
0.4 8
Solvent adhesive
5.5 0
Reactive dispersion
3.44 – 5.5 0
8.4.2 Adhesion to Fibres and Fabrics Many rubber products are reinforced with metallic or synthetic fibres to increase the strength of the product. Effective adhesion of the rubber to the reinforcing material is critical for good service life and safe performance. If the adhesive bond fails, a weak point will result, causing premature failure. In belts, fabric reinforcement is used to support the wear area for improved abrasion resistance. Again, good adhesion is necessary for the fabric to function effectively in this capacity. Saret 633 and Saret 634 were found to increase the adhesion of rubber to most polar surfaces, including synthetic fibres and fabrics [26]. Examples of this are shown in the next section.
8.4.2.1 Monofilament/Braided Wire Adhesion Saret 633 was evaluated in a ‘T-pull’ adhesion test using the EPDM formulation described in the materials section (see Section 8.3.1.1). The T-pull test measures the adhesion between rubber and a fibre. The fibre is placed in a mould and rubber is then added and cured. The fibre is then pulled in a perpendicular direction from the rubber and the force to break the adhesive bond is measured. The T-pull test is accomplished by using a special mould with a grid of trenches into which can be laid rubber and perpendicular to the rubber a filament. By filling the trench half way with the rubber the filament is embedded. After curing, the filament and rubber resemble a cross. The tensile tester is used to strip the rubber from the filament. Data for Saret 633 and several filaments and braid are shown in Figure 8.11 together with data for the control without the Saret 633. In each case, the Saret 633 significantly improves the T-pull adhesion over the control, especially the metallic monofilaments, copper, aluminium and steel. The adhesive failure
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The Handbook of Rubber Bonding
Figure 8.11 T-pull adhesion of EPDM to fibres and wires
in each case was cohesive, including the polyester and the Nylon, wherein rubber remained on the monofilament after the test.
8.4.2.2. Nylon Braid A comparison of Saret 633 versus conventional resorcinol formaldehyde (RFL) treatment in a T-pull adhesion test with Nylon braid and EPDM is shown in Figure 8.12. The control, which consists of Nylon braid without RFL treatment and EPDM rubber without Saret 633, tested at 1.3 N force. The RFL sample, which is RFL treated braid that is used commercially today, tested at 5.8 N with EPDM without Saret 633. With EPDM containing 10 phr Saret 633, a value of 15.5 N was obtained with untreated Nylon braid. This is more than double the value obtained with the commercially available Nylon braid. Saret 633 clearly increases adhesion to synthetic fibres like Nylon and polyesters. Thus, with Saret 633, it may not be necessary to RFL treat fibres and fabric for good adhesion. This could simplify applications where rubber reinforcement is required.
8.4.2.3 Fabric Adhesion The EPDM formulation described in Section 8.3.1.1, with and without 10 phr of metallic using coagent, was cured in contact with Nylon/polyester warp fabric for 20 minutes at 160 °C. RFL treated fabric was used in the control sample. Peel adhesion (180° angle)
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Rubber to Metal Bonding Using Metallic Coagents
Figure 8.12 T-pull adhesion of EPDM bonded to Nylon braid
was measured on a 1.27 cm wide specimen at a test speed of 2.5 cm/minute. The results obtained with Saret 633 and Saret 634 versus the RFL control are shown in Figure 8.13. Results shown in Figure 8.13 show the superior adhesion characteristics obtained with the Saret 633 when cured against the fabric at 160 °C for 20 minutes. Fabric was peeled from the rubber at an angle of 180. Failure mode was cohesive with rubber remaining on the fabric after the peel test.
Figure 8.13 The effect of metallic coagents on the peel strength of Nylon/polyester fabric bonded to EPDM
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The Handbook of Rubber Bonding In a separate but similar test, Saret 634 was compared with Saret 633. For situations where either water sensitivity or abrasion resistance is an issue, Saret 634 can be substituted for the Saret 633. Saret 634 is less sensitive to moisture then Saret 633.
8.5 Summary Saret 633 and Saret 634 can be used to create strong adhesive bonds between a variety of rubber compounds and untreated metals, synthetic fibres and fabrics. They also function as coagents by increasing the crosslink density of the rubber compound. Thus, they function as adhesion promoters as well as crosslinkers to enhance both the adhesive and mechanical properties of the cured rubber. One requirement is that they are used in peroxide cure compounds for optimum results. Metallic coagents have been used with sulphur to improve adhesion but they do adversely affect the cure state of a sulphur compound. The most straightforward way of using the metallic coagents is to mix them directly into the rubber compound before curing. This approach may be a good fit for rubber rollers, sealants, adhesive tapes, extruded products and certain moulded goods. It would have limited utility for moulded products where mould sticking or mould fouling is a problem. However, using external mould release agents will minimise mould sticking and mould fouling problems. Two other techniques have been described for bonding rubber to metals, fibres and fabrics with the metallic coagents. One involves using the metallic coagent in an adhesive strip to bond another rubber compound to the metal, fibre or fabric upon curing. This approach can be applied to both sulphur and peroxide cured compounds. Another technique is to apply the metallic coagent, in the form of a dispersion, between the rubber stock and the metal prior to curing. In this case, the metallic coagent functions as a reactive adhesive, which is activated by peroxides. Saret 633 is the best coagent for adhesion, but Saret 634 is a good alternative when abrasion resistance, tear strength or moisture resistance are needed in addition to adhesion.
Acknowledgements The author would like to acknowledge the many contributions of Walt Nagel, Gary Ceska and Al Tuccio in developing this technology.
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References 1.
R. Eirich, Science and Technology of Rubber, Academic Press, Inc., New York, 1978.
2.
T. Howath, J. A. Cornell and L. R. Olson, Rubber World, 1963, 148, 5, 69.
3.
P. Touchet, G. Rodriquez, P. E. Gatza, D. P. Butler, D. Crawford, A. R. Teets, H. O. Feuer and P. Flanagan, inventors; USA, Secretary of the Army, assignee; US Patent 4,843,114, 1989.
4.
R. C. Klingender, M. Oyama and Y. Saito, Rubber World, 1990, 202, 3, 26.
5.
R. C. Klingender, M. Oyama and Y. Saito, Presented at the 135th Meeting of the ACS Rubber Division, Mexico City, Mexico, Spring 1989, Paper No.62.
6.
A. Saito, A. Fujino and A. Ikeda, Elastomer Developments: Materials, Applications, Processing and Performance, Society of Automotive Engineers, Special Publication, USA, 1989, 890359.
7.
S. Martin, T. Melvin and J. K. Pieroni, inventors; Uniroyal, Inc., assignee; US Patent 4,266,772, 1981.
8.
C. M. Roland, inventor; The Firestone Tire & Rubber Company, assignee; US Patent 4,720,526, 1988.
9.
P. Molitor, inventor; Spalding and Evenflo Companies, Inc., assignee; US Patent 4,726,590, 1988.
10. R. Costin, W. Nagel and R. Ekwall, Rubber Chemistry and Technology, 1991, 64, 2, 152. 11. R. Costin, R. Ekwall and W. Nagel, Presented at the International Rubber Conference (IKT91), Deutsch Kautschuk Gesellschaft EV, Essen, Germany, 1991, p.205. 12. R. Costin, W. Nagel and R. Ekwall, Kautschuk und Gummi Kunststoffe, 1992, 45, 8, 648. 13. R. Costin, W. Nagel and R. Ekwall, Rubber World, 1992, 206, 5, 27. 14. R. Costin and W. R. Nagel, Rubber & Plastics News, 1995, 24, 19, 19. 15. R. Costin and W. Nagel, Gummi Fasern Kunststoffe, 1995, 48, 5, 796.
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The Handbook of Rubber Bonding 16. R. Costin and W. Nagel, Rubber World, 1995, 212, 6, 18. 17. R. Costin and W. R. Nagel, Rubber & Plastics News, 1996, 25, 17, 14. 18. H. Xie and Y. Feng, Polymer, 1988, 29, 7, 1216. 19. C. Keller, Presented at the 132nd Meeting of the ACS Rubber Division, Fall, 1987, Paper No.74. 20. ASTM D816-82 Standard Test Methods for Rubber Cements, 2001. 21. ASTM D412-98a Standard Test Methods for Vulcanised Rubber and Thermoplastic Elastomers Tension, 2002. 22. ASTM D1084-97 Standard Test Methods for Viscosity of Adhesives, 1997. 23. ASTM D395-03 Standard Test Methods for Rubber Property - Compression Set, 2003. 24. W. R. Nagel, Adhesives & Sealants Industry, 1997, 4, 4, 36. 25. R. Costin and W. Nagel, Rubber World, 1998, 219, 2, 18. 26. W. R. Nagel, Cure Concepts, 4 Volumes, Sartomer, Exton, PA, USA, 1999.
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9
Rubber to Fabric Bonding D. Wootton
9.1 Introduction Many of the frequently encountered ‘rubber’ articles are in fact rubber/textile composites. In order to obtain the optimum performance from such products, it is essential that the various components are adequately bonded together. Generally, this adhesion is obtained by prior treatment of the fabric, although there are other ways by which adequate adhesion can be obtained. Needless to say, the methods used are largely dictated by the type of fibre being used, the polymer to which it is to be bonded and final application of the composite product. The various methods of adhesion are discussed in this chapter, together with other factors which can affect adhesion. Finally, the environmental aspects of the various treatments are considered.
9.2 Adhesive Systems 9.2.1 Aqueous Fabric Treatments The standard aqueous treatment for textile adhesion is based on resorcinol/formaldehyde resin/latex (RFL) dip systems. A typical formulation for such an adhesive dip is given in Table 9.1. The actual ratios of resorcinol to formaldehyde and of resin to rubber solids may be varied within reasonable limits, the generally accepted optimum ratios being between 1:1.5 and 1:2 for the resorcinol to formaldehyde and between 1:5 and 1:7 for the resin to rubber ratio. Within these limits, it is possible to fine tune the RFL formulation to suit the particular rubber compound with which it is to be used. Generally, however, it is more convenient to use a standard dip formulation for most applications, to avoid excessive complexity, while still maintaining perfectly adequate adhesion levels to a wide range of rubber compounds. For most applications, the preferred latex used is a styrene/ butadiene/vinyl pyridine (VP) terpolymer latex. The RFL adhesive dip is prepared by dissolving the resorcinol in water, adding the formaldehyde and then the alkali as condensation catalyst, to produce the aqueous solution of the resin condensate. This generally takes place at room temperature in about 6 hours.
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Commercial rubbers
The Handbook of Rubber Bonding RFL system formulation
Table 9.1 Formulation of RFL system for rayon cord Component
Parts (wet)
Parts (dry)
Resorcinol
9.4
9. 4
13.8
5. 1
7.0
0. 7
Formalin (37%) Sodium hydroxide (10%) Water
157.8
VP latex (40%)
212.0
Water
100.0
TOTAL
500.0
– 84. 8 – 100. 0
This resin solution is then added to the latex, with gentle stirring to ensure good mixing. The completed dip is then allowed to mature for a further 24 hours before use. Although this procedure has been used for many years, alternative methods can offer certain advantages. In an alternative method, instead of allowing the resin precondensation to occur prior to its addition to the latex, a ‘single shot’ method may be used. The resorcinol solution and formaldehyde are added to the latex before the addition of the alkali; the addition of the alkali to the mixture then allows the resin to condense in the presence of the latex. In so doing the resin partially replaces the stabilising surfactants on the latex particles and gives a much more intimate dispersion of the resin throughout the rubber phase, in effect forming an interpenetrating network of resin in rubber, as the two materials are not greatly compatible [1]. With this method of preparation, there are two significant advantages. Firstly, there is greater control over the condensation reaction of the resorcinol and formaldehyde (when the condensation is carried out separately, the exothermic reaction can proceed too rapidly, and it is not unknown for a resin batch to turn solid before the normal condensation time has elapsed). Secondly, the completed dip has a much improved shelf life compared with the two-stage mixing method. With the two-stage mix, optimum adhesion is only obtained within 24 to 72 hours of mixing, while the single stage mix will maintain its adhesion levels for two to three weeks after mixing. Another variation to the standard formulation is to use a precondensed novolak resin. These are high solids (70%) solutions of acid-catalysed resins in water. They are of high viscosity and strongly acidic (pH of around 2.0). Accordingly, the resin requires diluting to a lower solids concentration and neutralising to a pH of greater than 8.5, so as not to coagulate the latex; this is usually achieved by the use of ammonia. A typical formulation for a novolak-based system is given in Table 9.2.
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novolak resin Rubber to Fabric Bonding
Table 9.2 RFL adhesive dip, using a novolak resin Component Water
Parts (wet) 163.0
Parts (dry) –
Novolak resin (70%)
19.1
13.4
Ammonia (SG 0.88)
3.5
1.2
Water Formaldehyde (37%) VP latex (40%) Ammonia TOTAL
100.0
–
9.4
3.5
202.4
81.0
2.6
0.9
500.0
100.0
This formulation is prepared by dissolving the resin in the aqueous ammonia solution and then adding this to the ammoniated latex. The formaldehyde is then diluted and added, slowly and with stirring, to the latex/resin solution. This completed dip is then allowed to mature for at least 18 hours before use. In common with the single shot mixture previously described, this formulation also has a very good shelf life after mixing. In the formulations given in Tables 9.1 and 9.2, both sodium hydroxide and ammonia have been used as the alkali catalyst for the resin condensation reaction. Both are suitable for this, but work by Rijpkema and Weening [2] has shown that in the final dried and cured RFL film, the two alkalis give somewhat different properties to the resultant film. In particular, the use of ammonia gives a film with a much higher elongation than that obtained by using sodium hydroxide; this gives greater flexibility to the film, resulting in improved dynamic fatigue of the bonded composite. The application of the RFL adhesive dips to the textile substrate is generally carried out by immersion in a bath of the dip, followed by removal of excess dip by squeezing, suction, air-knife, etc. The impregnated fabric is then dried, and finally baked to fully cure the RFL dip film. The baking, to complete the crosslinking of the resin component of the adhesive, can take place at temperatures between around 130 °C and 225 °C, with the time varying from 15 to 20 seconds up to 2 minutes, depending on the nature and weight of the substrate fabric being treated. The selection of curing conditions is mainly dictated by the type of yarn used in the textile substrate, and certain modifications to the formulations and processes may be needed for optimum adhesion with some yarn types, as discussed in the following sections.
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• Cotton Generally cotton fabrics do not require any adhesive treatment, as adhesion is largely obtained by mechanical methods, due to the embedding of the free fibre ends into the polymer matrix to which the fabric is being bonded. This mechanism has been studied [3] and a relationship found between the number of protruding fibre ends and the level of adhesion obtained. If, for particular reasons, additional treatments are used, particularly aqueous based ones, there can be difficulties in obtaining satisfactory wetting of the fabric, on account of the naturally occurring waxes on the fibres.
• Rayon (regenerated cellulose) The standard system used for adhesion to these artificial continuous filament fibres is an RFL; however, with the lower tenacity rayon yarns, it is possible to use a mixture of latices, substituting up to 70% of the styrene/butadiene/VP latex with a standard styrene/ butadiene (SBR) latex. With the higher tenacity polynosic yarns (high tenacity, high wet strength viscose rayon), it is preferable to use a higher proportion of VP latex (with the necessary adjustment of water addition to maintain the required solids content), increasing to between 60 and 80% VP latex content. The rayon textile is then dipped through the RFL dip and the impregnated fabric dried and baked, to cure the deposited dip solids. With these fabrics, exposures of 1 – 2 minutes at between 130 ºC and 150 oC are sufficient to achieve good adhesion levels, the time and temperatures being to some extent controlled by the weight of the fabric.
• Polyamide (Nylon 66 and Nylon 6) Again the standard RFL system is ideal for use with polyamide yarns and fabrics. With these fibres, however, it is necessary to use higher levels of VP latex than with rayon, a minimum of 80% VP latex being required to obtain the optimum levels of adhesion. This is illustrated in Table 9.3. Generally with polyamides, higher curing temperatures are required, compared with the corresponding rayon treatments. Minimum temperatures of around 160 oC are needed. It is also suggested that higher temperatures and lower exposure times can be used with advantage with the single shot type dips [5], employing temperatures of 190 – 200 oC but with exposure times of only 20 to 30 seconds instead of 100 to 120 seconds. Again, 244
Rubber to Fabric Bonding
Table 9.3 Effect of VP:SBR ratio on Nylon 66 adhesion Latex % SBR
Solids % VP
Peel Adhesion kN/m
100
0
5.8
80
20
11.9
60
40
16.8
40
60
19.6
20
80
21.5
0
100
22.0
Test: 2-ply peel test using NR/carbon black compound see ASTM D4393-02 [4]
the practical drying and baking times tend to be dictated by the structure, weight and final application of the substrate, rather than those shown to be possible in theory.
• Polyester (polyethylene terephthalate) Polyester yarns are generally much less reactive than the cellulosic or polyamide yarns, and so require additional treatments to achieve adequate levels of adhesion. While solvent pre-treatment with an isocyanate solution has been used, more commonly, aqueous systems have been developed for the first stage of a two dip system, using standard RFL systems for the second stage. One of the earlier but very successful systems was the DuPont D417 or Shoaf system [6], using an epoxy product with a blocked isocyanate as a first stage, to react with the polyester surface, giving improved adhesion when followed with the RFL second stage dip. The basic formulation for this is given in Table 9.4. The polyester fabric is passed through this dip, to give a solids pick-up of approximately 0.5%, and is then treated at a temperature of approximately 225 to 230 °C, to activate the blocked isocyanate and form the reactive film on the substrate. After this pre-treatment, the fabric is then dipped again with a standard RFL dip and cured, again using higher temperatures of at least 200 °C. This system gives very good levels of adhesion, and is quite widely used in the treatment of polyester tyre cord. Other systems have been developed to achieve a single stage treatment, overcoming the adhesion challenges of polyester. One of the more successful of these is the Vulcabond
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Table 9.4 Formulation for polyester pre-dip, DuPont D 417 Component
Parts (wet)
Parts (dry)
Blocked isocyanate (40% dispersion) a
180.0
72. 0
Epoxy resin b
27.2
27. 2
Gum tragacanth solution (2%)
25.0
0. 5
Dispersant (50%)
0.6
0. 3
Water
1767.2
–
TOTAL
2000.0
100. 0
a Such
as Grilbond IL6, a caprolactam blocked methylene bis(phenyl di-isocyanate). Grilbond is a registered trade name owned by EMS - American Grilon b Such as Denacol NER 010a, a liquid epoxy derived from epichlorohydrin and glycerol. Denacol is a registered trade name, owned by Nagase Co
E system, in which the RF resin component of the RFL system is modified by cocondensation with chlorinated phenol [7]. The recommended formulation for this system is given in Table 9. 5. Polyester Emserwerke Nagase Co. RFL adhesive system Vulnax International Ltd.
Table 9.5 Vulcabond* E RFL adhesive system Component
Parts (wet)
Parts (dry)
165.8
–
Resorcinol
9.8
9.8
Formaldehyde (37%)
5.9
2.2
VP latex (40%)
121.5
48.6
Vulcabond E (20% solution)
197.0
39.4
TOTAL
500.0
100.0
Water
* Vulcabond
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is a registered trade mark owned by Vulnax International Ltd
Rubber to Fabric Bonding This system requires just a single pass through this modified RFL dip, again with the need for high temperature curing, around 215 °C. Another approach to improve the adhesion of polyester has been developed, by many of the yarn producers, whereby an activation pre-treatment is applied to the yarn during spinning. This pre-activation replaces the need for special dip formulations and allows good adhesion levels to be achieved using standard RFL dips. Again, higher temperature curing is required than is generally necessary for polyamide yarns. The pre-treated polyester yarns are now very widely used, particularly for general mechanical rubber applications, such as conveyor belting, hose, and V-belt cord.
• Aramid Although chemically closely related to the standard nylon yarns (aliphatic polyamides), the aramids (aromatic polyamides) do not give satisfactory adhesion levels if treated with the standard RFL systems, as the aromatic groups partially sterically shield the reactive polyamide groups in the polymer chain. Again, a two-stage dipping process is required, but in this case, the pre-dip can be somewhat simpler than with polyester, and a solution of the epoxy-based product on its own is generally sufficient to give the necessary adhesion levels, when followed by a standard RFL dip. Also, fibres pre-treated at the spinning stage are now available, requiring only RFL treatment, although as with polyester, high treatment temperatures are required to achieve the full levels of adhesion. One disadvantage of the high temperature processing of aramid and polyester (at least 200 °C) is that the achieved adhesion levels are much more affected by relatively slight variations in the adherend rubber compound [8].
• Adhesion to other polymers The various systems described above have been designed for adhesion between the various textile fibres and the basic hydrocarbon polymers, natural rubber (NR), styrene/butadiene rubbers (SBR), polybutadiene (BR) and polyisoprene (IR). With slight modification to these systems, good adhesion levels can be achieved with other polymers. Generally, for those polymers for which a corresponding latex is available, such as polychloroprene (CR) and nitrile rubber (NBR), the VP latex component can be partly (from around 50%) or completely substituted by the corresponding polymer latex. Apart
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The Handbook of Rubber Bonding from this modification to the formulation, the processing and performance are very similar to that already described. For other polymers alternative methods must be used. For example, for butyl rubber (IIR) it has been found that treatment of the textile with a resorcinol/formaldehyde resin solution will give significant improvement to the adhesion levels. This is often increased still further by the use of an interply of a halogenated butyl polymer (particularly chlorobutyl). Another similar polymer, chlorosulphonated polyethylene (Hypalon), also requires special modifications to achieve adequate adhesion, but in this case it is the basic polymer formulation that is modified, by the addition of from 20 to 40% of NR; this gives a good increase in achieved adhesion without too great a loss of the outstanding weathering and ageing properties of the polymer.
9.2.2 Solvent-Based Adhesive Systems Solvent-based systems have been used for the rubber coating of textiles for many years [9], and are still widely used for many applications. Initially solvent spreading or coating, using a solvent solution or dough of the rubber compound, was primarily a simple technique to achieve a thin coating of rubber on the surface of the fabric. Nowadays, there is much greater attention given to achieving high levels of adhesion between the textile. The use of a solvent solution of an isocyanate as the predip for polyester textiles, followed by an aqueous RFL dip has already been described. The main solvent systems employed today involve the use of rubber cements or solutions, generally including adhesion promoters such as isocyanates, to give the required levels of adhesion between the applied coating and the textile substrate. In a typical application, the rubber compound is dissolved in a mixture of suitable solvents (usually a mixture of aliphatic, aromatic and chlorinated solvents) to give the desired solids content and viscosity for spreading. This dough is then applied to the fabric by spreading, using a knife system to control the thickness and evenness of the applied coating. In this way the required final thickness of coating is achieved by a number of passes, with solvent evaporation between coats. In order to achieve the required adhesion levels, the chosen adhesion promoter is usually only necessary in the first one or two applied coats, with the required final coating thickness being built up with only the base compound dough.
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Rubber to Fabric Bonding Another area where solvent treatment is generally required is in the treatment of heavy cabled cords for cut-edge V-belts. Here complete penetration of the cord is required, in order that on cutting the belt sections from the cured sleeve the integrity of the cord is maintained and no fraying of the cord occurs. This requires cutting across the thickness of the cord at a very low angle, possibly over a length of around 20 cm. The major disadvantages associated with these solvent systems relate to the environmental aspects, which will be considered later.
9.2.3 In Situ Bonding Systems All the systems described in Sections 9.2.1 and 9.2.2 have been treatments applied to the textile before its incorporation into the rubber matrix. However, systems also exist in which certain additives can be included in the rubber formulation to promote adhesion directly with the fabric, without the need for any textile pre-treatment. These are the in situ or direct bonding systems. These systems rely on the inclusion into the rubber matrix of resorcinol (or a resorcinol/ formaldehyde precondensate) together with a methylene donor. The addition of these two components will give a moderate level of adhesion, this can be greatly enhanced by the use of a fine particle size hydrated silica. The original methylene donor used for this system was hexamethylene tetramine. While this was perfectly adequate as a methylene donor from the adhesion standpoint, it also significantly affected the cure system, and thus alternative materials have been used, especially hexamethoxy methyl melamine. These systems give good levels of adhesion with loomstate (untreated) rayon and polyamide textiles, and can improve adhesion levels with cotton fabrics. However, in order to achieve satisfactory levels with polyester, it is necessary to use a pre-treated polyester yarn. Also with polyester, the amine residues from the methylene donor may cause significant degradation of the polymer, giving loss of strength. Great care is therefore needed in selecting the most appropriate methylene donor, if such systems are to be used with polyester fabrics. These systems act by the formation, within the rubber matrix, of a resorcinol/formaldehyde resin; this migrates to the rubber/textile interface and there reacts with the active groups in the textile to give a chemical bond. On account of this migration to the surface of the rubber, it is essential that there is sufficient of the precursors in the rubber to achieve the necessary concentrations at the interface to achieve adequate adhesion. As the resin will also migrate ‘backwards’, into the bulk of the rubber as well as to the surface, it is considered
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The Handbook of Rubber Bonding necessary that the layer of compound, containing the adhesion promoters, be at least 0.25 mm in thickness, to achieve the required levels of adhesion. This consideration therefore rules out the use of the direct bonding systems in spreading applications, as it would not be possible to achieve the necessary concentration of adhesion promoters by this technique. These systems can be used alone, with untreated fabrics. However, if they are used in conjunction with adhesive dipped fabrics, both systems will contribute to the adhesion, and achieved levels can be significantly higher than with either system alone.
9.3 Mechanisms of Adhesion The mechanisms whereby adhesion is obtained between textiles and rubber can be considered in two separate areas, namely the interfaces between the textile and the dip layer, and between the dip and the rubber matrix. With cotton-based fabrics, the basic adhesion is achieved purely by mechanical means, due to the embedding of the individual fibre ends within the rubber matrix [3]. On peeling the bond, it is necessary either to pull these fibre ends out of the rubber or, if this force is greater than the tensile strength of the fibres, to break the fibres. This mechanical adhesion applies basically to all staple fibre-based fabrics, where there is no additional adhesive treatment. However, in the case of the synthetic staple fibres, the individual fibres are smooth and cylindrical, compared with the rougher surface produced by the ‘scales’ on the cotton fibre surface. The adhesion levels obtained with these yarns are significantly lower than with cotton.
9.3.1 Dip/rubber Interface Considering the treated fabrics, the adhesion between the dip film and the rubber matrix is derived mainly from the direct crosslinking of the latex polymer component with the rubber in the matrix compound. There is also some slight contribution from purely mechanical action, due to the penetration of the rubber matrix into the weave structure of the fabric, but this is minimal with all but the most open weave structures. Similarly, there is a small contribution from interaction between the resin component of the dip and the rubber in the matrix compound. In order to achieve the direct crosslinking between the dip latex rubber and the matrix polymer, it is, of course, necessary for the curatives, sulphur and accelerator (or active groupings derived from them), to migrate from the matrix into the dip film. Consequently, the actual formulation of the curing system can have significant effects on the levels of
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Rubber to Fabric Bonding adhesion achieved. In order for adequate proportions of the required substances to migrate, it is desirable that the curing system has a long scorch time and relatively slow cure, to allow time for the formation of the active species and migration into the dip layer. With fast curing systems, or with short, high temperature cures, the final levels of adhesion achieved are generally lower than with slower systems or longer, lower temperature cures. In the extreme case of sulphurless cures, adhesion levels are very low, due to the lack of suitable intermediates to migrate. These effects are shown in Table 9.6. As can be seen from these results, the curing system exerts a significant effect on the adhesion levels. As the speed of cure increases (as shown by decreasing cure time), so the levels of adhesion fall; although the scorch time of CBS systems are greater than with MBTS alone, the cure is faster and the adhesion achieved is reduced. With the activated
Table 9.6 Effect of curing system on adhesion Adhesion (Peel) (kN/m)
Accelerator
21.0
MBTS
19.1
Accelerator dosage (phr)
Sulphur level (phr)
Cure (min at 153 °C)
0.6
2.5
15.0
NOBS
0.5
2.5
15.0
18.7
CBS
0. 5
2.5
12.5
18.0
MBTS DPG
0.4 0.2
2.5
12.5
13.7
MBTS TMTD
0.4 0.1
2.5
10.0
12.0
CBS
2.0
1.0
12.0
10.5
CB S
4.0
0.5
15.0
2.1
BDTPTS TMTD
1.0 3.0
Nil
12.0
1.7
CBS BDTPTS
2.0 3.0
Nil
15.0
BDTPTS: bis(di-ethyl thiophosphoryl) trisulphide CBS: N-cyclo hexyl benzthiazyl sulphenamide MBTS: mercaptobenzothiazole disulphide NOBS: N-oxy di-ethylene benzthiazyl sulphenamide TMTD: tetra-methyl thiuram disulphide DPG: N, N′ diphenyl guanidine
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The Handbook of Rubber Bonding systems, MBTS/DPG and MBTS/TMTD, the greater the activation and the faster the cure, so the levels of adhesion decrease and, with the sulphurless cure, TMTD alone, there is virtually no adhesion other than the purely mechanical contribution.
9.3.2 Dip/textile Interface In considering the other side of the adhesion equation, that is between the textile and the dip, there is no simple explanation of the mechanisms involved. It is generally accepted for rayon and nylon fibres, that the main contribution arises from chemical reaction between the resin component of the dip and the textile polymer, but it is suggested that mechanical and physical interactions also contribute significantly to the total adhesion [10]. The contributions ascribed to these various effects are summarised in Table 9.7. Such reactions between the resin and the fibre will also account for the effectiveness of the in-situ bonding systems for nylon and rayon fabrics. With polyester however the mechanisms are even less clearly understood. In the case of the isocyanate-containing systems it has been suggested that there is a direct chemical reaction between the isocyanate and the polyester groups. However, considering the general low reactivity of the polyester linkages, as shown by the high stability of the polymer generally, and the low level of hydroxyl groups in the polymer chain terminating groups, it is unlikely that such reactions would contribute greatly to the overall levels of adhesion achieved. Work with such systems [11] has suggested that the isocyanates react
Table 9.7 Mechanisms contributing to adhesion (After Schoon & Zierler) [10] Mechanism
Contribution to Adhesion %
Direct mechanical: penetration of dip/rubber into the structure of the fabric
15
Diffusion: microscopic or molecular diffusion into the yarn filaments
5
Primary chemical bonds: direct covalent chemical linkages between the resin and textile fibre
60
Secondary chemical bonds: mainly hydrogen bonding
20
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Rubber to Fabric Bonding to give a polyurethane, which is much more thermodynamically compatible with the surface of the polyester, and therefore in effect provides a different and more reactive surface to the adhesive dip.
9.4 Other Factors Affecting Adhesion In the ideal situation, a treated textile would immediately be incorporated into the polymer matrix and the final product cured immediately. In practice this is rarely, if ever the case, and therefore the treated textile must be stored (and often transported) before it is incorporated into the polymer.
9.4.1 Storage of Treated Textiles After treatment, the fabric or cord should be stored in a cool, dry and dark environment. When stored under such conditions, the adhesion properties will be maintained for a considerable period; often, a period of six months is recommended as the shelf life of such products, but it has been found that even after periods of eighteen months to two years, the adhesion levels have not been significantly degraded. It is always recommended, however, that after a prolonged storage, the adhesion properties of the treated textile be evaluated before committing it to production. In the event of some loss of adhesion, it is usually possible to recover the required levels by redipping the fabric or cord before use and regaining the original adhesion levels. Of the three parameters, heat, moisture and light, the most critical of these is light. Studies have shown that light, ozone and humidity all cause degradation of adhesion [12], with the most significant effects occurring within the first few hours of exposure. Other work [13] has shown that over 50% of the original adhesion can be lost by exposure to daylight for just 48 hours, while storage open, (i.e., opened but not necessarily exposed to direct light) will result in some 40% adhesion loss. Other studies of these effects [14] have indicated that some protection against this degradation can be obtained by incorporation of a wax emulsion into the RFL dip, which would give rise to a protective film on the surface of the treated textile. This work also showed that conventional anti-ozonants or UV absorbers did not give any significant protection, thereby indicating that the loss of adhesion arose from the attack on the surface butadiene double bonds in the dip film, reducing the co-curing of the dip rubber with the matrix polymer. Moisture is generally less of a threat to adhesion; with the relatively low moisture regain of Nylon (4.5% maximum and typically around only 2% in dipped fabrics) and of
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The Handbook of Rubber Bonding polyester (0.5% maximum) the moisture content of the textile is not usually sufficient to interfere with the adhesion or cure of the final composite. In the case of rayon, the high regain of around 11% can give rise to problems, due to the creation of porosity in the final product as the moisture is boiled off during cure. With such fabrics predrying before calendering or spreading is usually necessary.
9.4.2 Adhesion in Service Frequently, the final composites are subjected to various severe environments during service. These can include heat, flexing and moisture. On exposure to heat, there is a progressive loss of adhesion, with levels dropping to around 30 to 40% of the original values after 7 days ageing at 100 °C. Although this may seem rather drastic, the retained adhesion is usually perfectly adequate for the application: the original values obtained are generally significantly above the specified levels for the application, and these specified levels usually include a built in safety margin. Looking at the effects of moisture, or more drastically immersion in water, there is some loss of adhesion with both nylon and polyester fabrics. These losses are usually of the order of 10 – 20% of the original values, but on drying out the composite, the original levels are regained, to within less than 5% loss. These effects are related to the direct contact of the textile component with water, as is the case with cut edge conveyor belting. In many other applications, there is no direct contact of the textile with water, as the edges and joints are sealed to prevent such ingress. Without the direct contact it is most unlikely that there is any affect on the adhesion levels.
9.5 Environmental Aspects All of the systems described present certain environmental aspects, which require consideration during the operation of the processes. These can conveniently be considered for the different stages of production.
9.5.1 Storage and Handling All liquid materials must be in secure vessels, which must be bunded to contain any spillage or leakage from the primary storage vessel. With certain of the materials formaldehyde, ammonia and the various solvents - additional local exhaust ventilation is required to restrict the possibility of exceeding the imposed safe exposure levels for the
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Rubber to Fabric Bonding product. Additionally, standard personal protective equipment - gloves, goggles and breathing mask - should always be worn when handling such materials. In handling isocyanates, additional personal protective equipment is mandatory, as the authorised exposure limits are exceedingly tight (long term exposure limit 0.02 mg/m3).
9.5.2 In Process The different systems each present different aspects for attention. In the case of the aqueous systems, the major concerns are again local exhaust ventilation, to prevent buildup of fumes in the work place. From the process itself, the exhaust from the curing ovens requires monitoring, for the presence of formaldehyde vapour, ammonia, volatile organic compounds (VOC) and particulates. With many of the more recent installations, conformance with the regulating limits should not present much problem, but on occasion, some form of abatement system may have to be installed. With solvent systems (in the UK), these will probably fall within the regulation of the Integrated Pollution Control regulations (soon to become the Integrated Pollution Prevention and Control regulations). Here it is necessary that some form of abatement treatment be applied to the exhaust systems, to control the VOC emissions. This may be achieved either by a solvent recovery system or by an incineration process, in which case, suitable precautions must be taken to ensure that the temperatures achieved during incineration are high enough to eliminate all solvents and to prevent the formation of other noxious by-products of oxidation. Usually temperatures in excess of 700 °C are required to ensure this; the heat generated by such processes can be recovered for process heating, etc. In the case of the use of in-situ bonding systems, the main problems here arise from an increase of rubber fume on mixing and on opening the curing presses, but much work is being done [15, 16] to reduce the effects of resorcinol and the resins on increased fume.
9.5.3 Wastes and Disposal From the aqueous processes, the main process wastes are the residues and washings arising at the end of processing runs. These generally contain latex and resin, but are usually much less concentrated than the normal dips. Such residues are sometimes accepted by the Water Authorities for treatment, provided that the pH, total solids and chemical oxygen demand fall within the limits of Consent to Discharge. Frequently, it is necessary
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The Handbook of Rubber Bonding to install some form of pre-treatment system to remove the bulk of solid materials, by coagulation, flocculation and filtration, before the clarified liquor can be discharged to drain. This then presents the problem of the disposal of the resultant solid matter, which requires disposal as ‘special wastes’, either to special landfill sites or by incineration. With the solvent processes, again the disposal of waste solvents and spreading compound doughs has to be handled as ‘special wastes’, although often, recovery of the solvents themselves is feasible. From all of these systems, there remains the disposal of the process waste treated fabric, from seams, ends, etc. These can be used for certain applications, by shredding the treated textile and using it as a semi-reinforcing and bulking filler in rubber products such as dustbin lids, traffic cones, and so on.
References 1.
D. D. Eley and L. B. J. Pawlowski, Journal of Colloid and Interface Science, 1969, 29, 222.
2.
B. Rijpkema and W. E. Weening, Proceedings of the 144th ACS Rubber Division Meeting (IRC ‘93), Orlando, FL, USA, 1993, Paper No.142.
3.
E. M. Borroff and W. C. Wake, Transactions of the Institution of the Rubber Industry, 1949, 25, 1, 39.
4.
ASTM D4393-02 Test Method for Strap Peel Adhesion of Reinforcing Cords or Fabrics to Rubber Compounds, 2002.
5.
B. Mitchell, Journal of the Institution of the Rubber Industry, 1975, 5, 151.
6.
C. J. Shoaf, inventor; E. I. du Pont de Nemours and Company, assignee; US Patent 3,307,966, 1967.
7.
R. A. Edington and D. H. Aldred, inventors; no assignee; GB Patent 1, 140, 528, 1969.
8.
W. Weening and W. H. Hupje, Kautchuk und Gummi Kunststoffe, 1972, 7, 25, 324.
9.
C. Macintosh, inventor; no assignee; GB Patent 4,804, 1823.
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Rubber to Fabric Bonding 10. Th. G. F. Schoon and L. Zierler, Kautschuk und Gummi Kunststoffe, 1970, 23, 12, 615. 11. R. E. Hartz, Journal of Applied Polymer Science, 1975, 19, 3, 735. 12. H. M. Wenghoeffer, Rubber Chemistry and Technology, 1974, 47, 5, 1066. 13. M. Fahrig, Rubber Technology International ‘98, 1998, 139. 14. Y. Iyengar, Journal of Applied Polymer Science, 1975, 19, 3, 855. 15. K. Meier, U. Goerl and S. Wolff, inventors; Degussa AG, assignee; US Patent 5,321,070, 1994. 16. K. Meier, U. Goerl and S. Wolff, inventors; Degussa AG, assignee; US Patent 5,470,905, 1995.
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Bonding Rubber with Cyanoacrylates R. Goss
10.1 Introduction Cyanoacrylates were first introduced commercially in the late 1960s and into the consumer market in the 1970s. Since then ‘super glues’ have been used and are continuing to be used in both industry and around the home for a huge variety of applications. There is now a bewildering range of cyanoacrylates available to both the home user and particularly the industrial user. Cyanoacrylates offer an unbeatable combination of speed, simplicity and strength particularly when faced with the need to join widely different materials. They will bond metals, plastics, ceramics, wood, leather, paper, cork and rubber in any combination without heat treatment and for most applications without the need for surface pre-treatment. There are many types of rubber available but most can be bonded with cyanoacrylates. Polychloroprene, nitrile, natural rubber (polyisoprene), styrene butadiene rubber (SBR) and butyl are amongst the types of rubber that can be readily bonded with cyanoacrylates. Ethylene propylene diene monomer (EPDM) and fluroelastomers (Viton, registered trade mark of DuPont) can also be bonded, although only with specific grades of cyanoacrylate. Silicone rubber and thermoplastic rubber (Santoprene, registered trade mark of Advanced Elastomer Systems) can be bonded with the aid of a primer. Typical applications and techniques for bonding different grades of rubber are discussed in Section 10.11.
10.2 Liquid Cyanoacrylates In liquid form cyanoacrylates are soluble in most organic solvents and insoluble in water. Most grades are of high volatility and thus are irritating to the eyes and nose; low odour grades are discussed in Section 10.12.1.3. Cyanoacrylates are generally available as colourless liquids in a wide viscosity range from 3 mPa.s (i.e., a thin liquid) to a thixotropic gel consistency in a wide variety of pack sizes. They should be stored in a cool dry place, preferably at a temperature between 5 ºC and 8 ºC for optimum shelf life. Toughened, coloured, flexible, UV curable and low-odour versions are also available.
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10.3 Curing of Cyanoacrylates Cyanoacrylate adhesives contain an acidic stabiliser, which prevents the adhesive from polymerising. The acid stabiliser is neutralised when the product is in contact with the surface moisture. In general, moisture, which is normally found on all surfaces exposed to the atmosphere, is sufficient to initiate curing within a few seconds. The moisture neutralises the stabiliser and thus initiates the cure. Figure 10.1 represents the cyanoacrylate in liquid form: • The large spheres represent the adhesive monomer, • The smaller spheres represent the acidic stabiliser, • The dark spheres represent the surface moisture. During polymerisation (solidification of the adhesive) chains of adhesive molecules build up on the surfaces and interweave to bind the surfaces together, see Figure 10.2. The cure speed of a cyanoacrylate, if left open on a surface, will be quite sluggish, because there is insufficient moisture and the adhesive will remain liquid for several hours (although the cyanoacrylate will cure at the surface interface). When the adhesive is between two close fitting surfaces, where there is moisture on both surfaces, the cyanoacrylate will cure rapidly. The gap between parts should ideally be less than 0.1 mm (see Figure 10.3).
Figure 10.1 A cyanoacrylate in liquid form
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Figure 10.2 The initial chains form rapidly, giving cyanoacrylate adhesives their characteristic fast bonding (cure speed) times
Figure 10.3 When the adhesive is between two close fitting surfaces the cyanoacrylate cures rapidly due to the surface moisture on both surfaces
10.3.1 Factors Affecting Cure The curing of cyanoacrylate adhesives is affected by: i) The relative humidity in the local area, ii) The gap between the parts, iii) Volume of the adhesive applied, iv) The ambient temperature, v) Acidic residues, e.g., residues from surface treatment.
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10.3.1.1 Relative humidity The optimum cure condition for cyanoacrylates is when the relative humidity (RH) is between 40% RH and 60% RH. Lower RH, i.e., 20% RH, will result in a slower cure, and high RH (80% RH) result in a faster cure. A dry environment (low RH) often occurs when the work area heating system is switched on in the autumn months, thus drying out the local air adjacent to the work station. High RH can also be detrimental as the cyanoacrylate can sometimes cure so fast that the adhesive cures before it has properly adhered to the surface and the resulting bond is poor. Figure 10.4 shows how RH can affect the cure speed for an ethyl cyanoacrylate.
Figure 10.4 Graph showing how tensile strength develops with time on nitrile rubber at different levels of humidity. At 100% strength the rubber fails before the adhesive bond
10.3.1.2 Gap Between the Parts Cyanoacrylates are best suited to parts with small gaps (<0.1 mm). Some grades will cure through gaps up to 0.5 mm or even 1 mm using activators, but as a good rule of thumb the thinner the bond line the stronger the joint and the faster the cure.
10.3.1.3 Volume of the Adhesive Applied If excessive adhesive is applied, there is insufficient surface moisture and the cyanoacrylate will be slow to cure and may bloom (see Section 10.12.1). The adhesive should be applied 262
Bonding Rubber with Cyanoacrylates such that there is just sufficient to cover the joint. Dispensing equipment (see Section 10.9) is recommended for production applications.
10.3.1.4 Ambient Temperature The ambient temperature does not contribute significantly to the cure speed of the cyanoacrylate but as with all chemical reactions if the ambient temperature is low, i.e., less than 5 °C, then the cure speed will be longer. This may occur for example in a maintenance application where a cyanoacrylate is used outdoors in winter. An activator may be required to ensure a reliable cure (see also Section 10.8).
10.3.1.5 Acidic Deposits Cyanoacrylates contain an acidic stabiliser, so if they come into contact with an acidic surface the stabiliser is not neutralised and the cyanoacrylate remains liquid. Some materials are naturally slightly acidic, e.g., paper, cardboard, cork and leather, and a standard cyanoacrylate will either be slow to cure or be completely inhibited from curing. Modified cyanoacrylates are now available which overcome the surface acidity and cure well on these surfaces (see Section 10.4.1). Acidic deposits may also be present as a result of an incorrect cleaning or surface preparation process.
10.3.2 Cure Speed Under the right conditions (see Section 10.3.1), cyanoacrylates will gain handling strength within the first minute or so. The two major factors affecting cure speed are the RH and the gap. The definition of ‘handling strength’ will depend on the actual application but is generally regarded as the time when parts can be picked up gently without the adhesive joint failing. The cyanoacrylate should not be disturbed during the critical time when it is changing from a liquid to a solid as the product may never gain its full strength.
10.4 Types of Cyanoacrylate There are three major chemical types of cyanoacrylate adhesives. The major distinguishing feature between these three is the size of the molecule.
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Figure 10.5 Full cure is achieved after 12–24 h (rubber to steel). Approximately 60% strength is achieved within the first minute
Methyl cyanoacrylate, the original cyanoacrylate ester, has the smallest molecule of the three adhesives. Thus a larger number of molecules can be applied over a given area. Consequently, a larger number of polymer chains can be formed - resulting in bonds with a high tensile strength. Methyl cyanoacrylates are thus particularly suited to bonding metals. The ethyl cyanoacrylate ester molecule is slightly larger than the methyl ester molecule although adhesive properties are very similar. However because of their larger size, the number of molecules per given area is less than that for methyl adhesives. This results in a less rigid adhesive and is more suited to bonding rubbers and plastics. The alcoxethyl cyanoacrylate ester molecule is considerably larger than that of both the methyl and ethyl esters and this results in a slower curing adhesive. Whilst a slower cure speed has advantages when assembling intricate parts which have to be aligned after contact, it is the low volatility which gives a butyl cyanoacrylate adhesive its major advantage. Its larger heavier molecule is less volatile than those of the other two esters, hence odour is less and the possibility of blooming (a white deposit adjacent to the bond line) is eliminated.
10.4.1 Bonding to Acidic and Porous Substrates Some substrates, for example paper, cork, cardboard, leather, dichromated metals and some fabrics can be slightly acidic, and since the cyanoacrylate contains an acidic stabiliser the adhesive will be very slow curing or may not cure at all. For these substrates, a
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Bonding Rubber with Cyanoacrylates surface activator or surface insensitive formula must be used to obtain short handling times. The surface sensitive grade includes a special additive which can convert surface contamination into an activating species towards cyanoacrylates and thus allow the adhesive to continue curing in the usual way. This additive is now used in some consumer cyanoacrylates because it reduces the dependence on surface moisture to cure and ultimately leads to good bond strength with many substrates. Similarly because these ‘surface insensitive’ cyanoacrylates are good general purpose bonders, they are particularly suited to the maintenance sector where the condition of the substrate is not well defined. This family of cyanoacrylates are fast curing. However they may cure too quickly in some applications, and fail to adhere properly to the surface.
10.4.2 Toughened Cyanoacrylates Cyanoacrylates can be toughened by the addition of rubber particles. These modified ethyl cyanoacrylates show superior peel strength and often high humidity resistance. The principle of toughening the adhesive is that the rubber particles minimise the propagation of cracks as the adhesive is subjected to high peel or cleavage loads (see Figure 10.6). The rubber toughened cyanoacrylates are particularly well suited for rubber to metal bonding applications, although they are slightly slower curing than the standard ethyl cyanoacrylates.
Figure 10.6 The crack only gets as far as the rubber particle and the stress is dissipated. The crack then has to reform, only to meet another ‘crack arrester’. The resulting adhesive bond line is therefore considerably more resistant to peel and cleavage loads than a conventional cyanoacrylate
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10.4.3 Flexible Cyanoacrylates Recent introductions in cyanoacrylate technology include a modified ethyl monomer with considerably improved flexibility over the standard product. These cyanoacrylates were developed for the loudspeaker industry where a degree of flexibility in the bonded joint is desirable.
10.4.4 UV Curing Systems Another recent introduction are UV curable cyanoacrylates. Light curing cyanoacrylate adhesive technology combines the processing benefits of cyanoacrylate and light-curing adhesives. Light-curing cyanoacrylate adhesives offer the ability to rapidly cure exposed adhesive through exposure to light and the reassurance of the standard cyanoacrylate cure mechanism to cure adhesive in areas shadowed from light.
10.5 Design Considerations To obtain the best results, joints should be designed for adhesive bonding. Cyanoacrylates can offer significant benefits over other methods of joining rubber. There are, of course some additional factors to be assessed when contemplating the use of adhesives. The cyanoacrylate must suit the substrates, be compatible with production methods, transmit anticipated service loads and endure operational environments. Consideration must also be given to surface preparation, application methods, curing time and the cost of this method of bonding. The loads acting on a bonded joint will result in various types of stresses. Stresses are normally expressed in N/mm2. In the case of pure tensile and pure compressive loads the stress distribution over the bond line is very even, so that every part of the bond line carries the same load, and to calculate the stresses, the acting forces are simply divided by the bond area. In reality pure tensile and pure compressive loads are very rare and we are confronted more with shear, cleavage and peel loads. The joint stress distribution, i.e., the location of the stresses across the bond line, is less uniform and more complicated to calculate. As a general rule, increase the joint width rather than the overlap area. Note from the stress distribution curve (Figures 10.7 and 10.8), that the ends of the bond resist a greater amount of stress than does the middle of the bond. If the width of the bond is increased, the bond area at each end also increases; the overall result is a stronger joint. Rubbers and elastomers are often flexible materials and although cyanoacrylates are by nature relatively rigid adhesives (although see Section 10.4.3), good results can be obtained for a wide range of applications. The designer should consider especially the effect of cleavage and peel stresses on the joint.
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Figure 10.7 Highest stresses are at the outside edge of the bonded area
Figure 10.8 Increasing the bond width is more beneficial than increasing the overlap length
10.5.1 Minimise Peel and Deavage Loads Joints subjected to cleavage or peel loads are usually the weakest joints and have the lowest strength. Cleavage loading will concentrate the stress at one end (or side) of the joint, and this can result in uneven loading on the adhesive and possibly premature failure. Peel and cleavage loads should therefore be eliminated as far as possible. See Figure 10.9 for some proposals of how to convert peel or cleavage into more favourable loads.
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Figure 10.9 Peel loads and their conversion by means of design measures
10.5.2 Bond Line Thickness The gap between the parts, and therefore the thickness of the cyanoacrylate film, has an important bearing on the characteristics of the joint. A thick bond line (>0.5 mm) will generally be a weakening feature as the mechanical strength of the cured cyanoacrylate film is likely to be less than that of the rubber or the other substrate. For rubber bonding applications, thin films (<0.05 mm) often give the best results. For example, cyanoacrylates bond exceptionally well to nitrile rubber, and substrate failure of the rubber is the typical failure mode where the bond line is 0.05 mm or less. Adhesive bonds will always be susceptible to environmental attack and a thick bond line offers a ready path for access by moisture or other solvents. However, if dissimilar materials are being bonded and there is a differential change in the dimensions of the two surfaces, due for example to
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10.5.3 Special Requirements for Bonding with Cyanoacrylates Cyanoacrylates cure quickly and care should be taken during the selection process to ensure that sufficient time is made available to position parts prior to the adhesive curing. The fast cure of cyanoacrylates can in many applications offer benefits to the production engineer, as process times will be fast and clamp times are usually short. The thinner the bond line the faster the cure. Note however that the joint must not be disturbed during the cure cycle as the cyanoacrylate will not properly re-configure itself if the molecular chains are broken at the start of the polymerisation. Cyanoacrylates are best suited to the smaller bond areas in rubber bonding applications, as larger areas often require more time to position the parts, and the fast cure of cyanoacrylates make this difficult. Typical applications are illustrated in Section 10.11.
10.5.4 Internal and External Mould Release Agents Internal and external mould release agents are used to guarantee easy release of moulded or pressed plastic or rubber parts. They often produce surfaces that can be difficult to bond. Mould release agents are described as internal if they are already mixed into the granules and take effect during the processing of the plastic or rubber. These mould release agents may be distributed throughout the whole material, so that even mechanical abrasion may not be effective. External mould release agents, on the other hand, are sprayed into the open mould. They are often based on paraffins, soaps and oils, e.g., silicone oil. Because of the manner of treatment, these mould release agents may be found not only on the surface but also in layers near the surface. Proprietary non-transfer mould release agents such as ‘Frekote’ are recommended to ensure optimum bonding surfaces. If mould release agents are present, cyanoacrylates can sometimes be used successfully in conjunction with a primer to give excellent bond strengths.
10.5.5 Successful Joint Design Bond stresses, materials, type of cyanoacrylate, surface preparation, method of application and production requirements should all be considered in relation to each other at the
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10.6 Bonding to Silicone Rubber [see also Chapter 12] Silicone rubber is difficult to bond with cyanoacrylates. The adhesive does not wet the surface properly without special surface treatment, due to the very low surface energy, and therefore will not bond. Polypropylene (PP), polyethylene (PE), polytetrafluoro ethylene (PTFE) and acetal plastics and Santoprene rubber also fall into this category and cannot be bonded without prior surface preparation. Surface primers are now available which can act as an adhesion promoter. Loctite 770 (Henkel Loctite Adhesives Ltd) is a heptane-based primer which can be brushed or sprayed onto the surface of the rubber and then used with an ethyl-based cyanoacrylate. This primer is recommended for PP, PE, Santoprene and other similar rubbers as well as PTFE. It frequently enables cyanoacrylates to successfully bond polyacetals as well. The primer should be applied once only to the surface; over-application will nullify the effect of the primer. The on-part life of the primer is typically eight hours, but best results are achieved if the surfaces are bonded immediately. The Loctite 770 primer contains a fluorescent additive to reveal where the primer has been applied. Treated parts can be inspected simply by passing them under a low intensity, ‘blue’ UV lamp. Flame treatment, corona discharge, plasma etching or chromic acid are also acceptable methods for pre-treating these types of rubbers and plastics. In some applications, surface roughening can be a simple, low cost method of increasing the bondability of the rubber. Surface roughening increases the bondability by dramatically increasing the number of mechanical interlocking sites.
10.7 Environmental Resistance There is such a wide range of substrates, joint configurations, bond areas, gaps, and applications that cyanoacrylates can be used for, that it can be difficult for the supplier to give accurate information about how a particular grade of cyanoacrylate will perform for individual applications. Hot, wet environments should in general be avoided,
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Bonding Rubber with Cyanoacrylates particularly constant immersion in water. Conversely, however, nitrile rubber bonded with an ethyl cyanoacrylate shows excellent retention of strength after 1,000 hours at 40 °C/95%RH. If the same cyanoacrylate were to be used for steel, the joint would normally be expected to degrade after less than 100 hours at 40 °C/95%RH. Excellent retention of strength is also achieved for the bonding of nitrile rubber with cyanoacrylates in oil or glycol. The substrate and the joint configuration will determine the success of the joint. A useful test is to subject the parts to six weeks in an environmental chamber where the conditions are 40 °C/95%RH. It is often the case that a bonded part will show signs of failure within the first 1,000 hours (six weeks) in this environment. Figure 10.10 demonstrates the interpretation of the various outputs of an environmental test. In selecting a cyanoacrylate for a particular application, one of the most important considerations is the environment or surroundings to which the adhesive will be subjected.
Figure 10.10 The graph shows three possible results for an environmental test. Result A is clearly ideal as there has been no degradation in strength. Result B shows an initial fall in strength but no further degradation, and provided the strength remaining still adequately meets the specification, this result may be acceptable. Result C is clearly unacceptable as the strength has continued to fall during the test
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10.7.1 Glass Bonding Cyanoacrylates are not recommended for permanent bonding of rubber to glass or glazed ceramics as the cyanoacrylate will de-bond from the glass after a relatively short time (typically 1–6 weeks).
10.7.2 Hot Strength Cyanoacrylates cure to become thermoplastics and have a glass transition temperature of 120 °C. The products therefore have a typical operating temperature range of –50 °C to +80 °C although many grades will survive higher temperatures. Figures 10.11, 10.12, 10.13 and 10.14 show the hot strength for an ethyl cyanoacrylate and a modified cyanoacrylate with high temperature additives. For the cyanoacrylate with the high temperature additives, the data are presented slightly differently as the adhesive gains hot strength as it is aged at temperature. However, it can be seen that there is still a 50% improvement in the performance.
Figure 10.11 Hot strength for a standard cyanoacrylate tested at temperature indicated and given as a percentage of room temperature strength. Test Procedure: Shear Strength ASTM D1002/DIN EN 1465 [1, 2]; Substrate: Grit blasted mild steel lap shears; Cure procedure: 1 week at 22 °C
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Figure 10.12 Heat ageing for a standard cyanoacrylate aged at temperature indicated and tested at 22 °C and given as a percentage of the initial room temperature strength Test Procedure: Shear Strength ASTM D1002/DIN EN 1465 [1, 2]; Substrate: Grit blasted mild steel lap shears; Cure procedure: 3 days at 22 ºC
Figure 10.13 Heat ageing for a cyanoacrylate with high temperature additives, aged at temperature indicated and tested at 22 °C and given as a percentage of initial room temperature strength
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Figure 10.14 Heat ageing/hot strength curve for a cyanoacrylate with high temperature additives, aged under conditions indicated and tested at temperature
10.8 Activators Activators are often required where the cyanoacrylate needs to be cured through a large volume or where there is adhesive outside the joint line. Unlike primers, activators do not improve the adhesion, but improve the cohesion of the cyanoacrylate by guaranteeing proper curing. A typical example of this is where a cyanoacrylate is used to tack wires onto a printed circuit board (PCB). If an electrical modification is made to the PCB, a wire is taken across the PCB and must be held securely down onto the PCB. A high viscosity cyanoacrylate is applied to the PCB and the wire is placed into this drop of adhesive. The activator is applied by spray over the area and the cyanoacrylate cures in seconds. Activators may also be pre-applied to a surface. Note however that the active ingredient in the activator is an amine which may oxidise fairly quickly, so the on-part life of some of these activators may be less than a minute. Different concentrations of activators are available for individual applications. Cyanoacrylates are widely used for the manufacture of loudspeakers and one application is the bonding of the rubber surround to the chassis where large gaps often exist and an activator is used to ensure complete cure. The activator is sprayed over the bond line and the cyanoacrylate is cured within a few seconds. Some activators contain a fluorescent additive to allow easy inspection. The activators are solvent-based and compatibility tests are recommended before use.
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Bonding Rubber with Cyanoacrylates UV curing cyanoacrylates are often an excellent alternative option if the use of activators is undesirable (see also Section 10.4.4).
10.9 Application Methods for Cyanoacrylates Cyanoacrylates can be dispensed precisely, cleanly and reliably using a range of dispensing equipment, ranging from simple hand-held dispensers to fully automatic systems incorporated into purpose-built special production equipment. The most common principle of dispensing is to use the pressure/time system as described in Section 10.9.1.
10.9.1 Pressure/Time Systems The adhesive is put under pressure and the dispense valve is opened for a certain time to provide the correct quantity of adhesive (see Figure 10.15). Since the cyanoacrylate contacts the pressurised air, the dryness of the air is important and an air filter on the mains air line is recommended. A pressure/time system is best suited for cyanoacrylates up to a viscosity of 5,000 mPa-s, i.e., self levelling products.
Figure 10.15 A pressure/time dispenser
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The Handbook of Rubber Bonding A pressure/time system usually consists of a reservoir, a dispense valve and a controller. The type of dispense valve depends on whether it needs to be hand-held, stationary or advancing, and if there are standard or high precision requirements. The type of application and workplace conditions, such as production rate, dispense quantity and cure speed will determine whether manual, semi-automatic or fully automatic dispensers will be required. A number of dispense valves are available for dispensing cyanoacrylates including pinch valves, diaphragm valves and seat valves. The wetted parts are typically manufactured from PTFE or acetal to prevent premature curing and often incorporate some form of stroke adjustment for fine-tuning of the dispense characteristic. One of the major benefits of the pinch valve is that the fast curing cyanoacrylate is not in contact with any moving parts. These valves have been used in a wide range of production applications for many years.
10.9.2 Syringe Systems Cyanoacrylates can be dispensed via a syringe system although care is required to ensure that the follower bung does not bond itself to the side walls of the syringe as the product is dispensed. Higher viscosity or gel products are generally more suited for syringe systems.
10.10 Health and Safety and Handling Precautions Cyanoacrylates have been used in industry for many years with an excellent safety record. Vapour problems will not normally be of concern in a well ventilated working area, and can be eliminated with the use of low odour/low bloom cyanoacrylates. Cyanoacrylates will bond skin tissue very rapidly, but bonded skin can usually be relatively easily prised apart by subjecting the adhesive to a peel action rather than direct tension. Hot soapy water will help as well as proprietary hand cleansers. It should never normally be necessary to have to resort to drastic measures such as surgery. Dispensing equipment for production applications is available to ensure the correct quantity of adhesive is accurately applied (see Section 10.9.1). For batch production runs or development work the use of small containers fitted with fine bore needles is recommended. This will minimise the quantity of adhesive applied, thus giving thinner
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Bonding Rubber with Cyanoacrylates bond lines and minimal excess adhesive. The thinner the bond line, the faster the cure and the stronger the joint. This in turn means that there is less risk of fumes and less chance of adhesive outside the joint line.
10.11 Typical Applications This section shows some typical applications where cyanoacrylates have been used to bond all types of rubber to an assortment of other substrates. It is important to note that each application has its own unique features, and that an adhesive which works well in one circumstance, may not necessarily be suitable in another.
10.11.1 Bonding Nitrile, Polychloroprene and Natural Rubbers Cyanoacrylates bond well to nitrile, polychloroprene and natural rubbers and fast bonding times can be achieved with ethyl cyanoacrylates (see also Section 10.1). Mould release agents however, can sometimes affect the adhesion and further practical guidelines are given in Section 10.12. In the automotive industry, cyanoacrylates are used to bond a nitrile rubber sleeve to an acrylonitrile–butadiene–styrene (ABS) adjustment arm of an automotive mirror. The adhesive supplements a push fit and tests showed that the cyanoacrylate fully exceeded the strength requirements. Sporting goods and toys are also important application areas. Items such as trophies, rubber foam recoil pads for shot guns and assembling rubber strip inserts into wooden cricket bat handles all benefit from cyanoacrylates’ unique bonding capabilities (see Figures 10.16 and 10.17).
10.11.2 Bonding EPDM Standard cyanoacrylates do not bond particularly well to EPDM but certain grades of ethyl-based cyanoacrylates have been specially developed for bonding to EPDM. One such grade is Loctite 406, an ethyl cyanoacrylate and this is used to attach an EPDM rubber edge strip to an EPDM conveyor belt to prevent potash from falling off the production line. The same product is used for mitre bonding of EPDM rubber seals for automotive doors and windows. The cyanoacrylate produces high strength invisible bonds in seconds.
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Figure 10.16 A toughened cyanoacrylate is used to bond a polychloroprene anti-slip pad onto a cast iron machine foot
Figure 10.17 A fast curing ethyl cyanoacrylate is used to bond a nitrile rubber seal onto an ABS cam belt cover
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Bonding Rubber with Cyanoacrylates For fixing an EPDM rubber seal to a polyvinyl chloride (PVC) beer pump mechanism the cyanoacrylate must not only produce a reliable bond but also be approved for use with potable water. The cyanoacrylate, once fully cured, is essentially an inert plastic and many grades are approved for potable water as well as for medical applications (see Section 10.11.4).
10.11.3 Bonding Santoprene and Silicone Rubbers These grades of rubber require priming prior to bonding (see also Section 10.6). Although this makes it a two-stage operation cyanoacrylates are used for bonding Santoprene and silicone rubbers. Examples in industry include attaching a silicone rubber seal to an aluminium parachute container. A primer is brushed on to the silicone rubber and the cyanoacrylate is applied to the aluminium. Once fully cured the bonded seal assembly prevents dust or moisture entering the safety critical box. In another application a gel cyanoacrylate used with a primer eliminates unsightly mechanical fixings and bonds a Santoprene finishing strip to melamine ceiling panels of railway carriages.
10.11.4 Bonding Medical Devices Adhesives used in the medical industry are tested to the ISO 10993 [3] toxicological classification requirements which essentially means that the physical properties of the cured adhesive must be biologically inert, and corrosion and temperature resistant. Medical grade cyanoacrylates are used by the medical industry for bonding sterile disposable devices. Typical applications include: fluid and drug delivery sets, catheters, hypodermics, hearing aid sub-assemblies and cardiopulmonary devices. Cyanoacrylates (which are solventfree) can replace solvent welding techniques particularly where two different plastics (or rubbers) require to be bonded. Some solvents are highly flammable and some are considered to be potential carcinogens. In addition, the cyanoacrylate has the ability to fill larger gaps than a solvent and so offers the manufacturer wider scope for his manufacturing process. Some sterile reusable devices may be used a number of times, e.g., endoscopes, surgical instruments, and as such will require to be sterilised by autoclaving or exposure to one or two or more cycles of ethylene oxide (EtO). Cyanoacrylates are not generally suitable for applications where the device will be subjected to repeated autoclaving. Medical grades of cyanoacrylates will generally survive single EtO or gamma radiation sterilisation processes. However sterilisation resistance is always design and substrate dependent and must be tested by the device manufacturer.
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The Handbook of Rubber Bonding Silicone rubber is often used in medical devices as it offers good flexibility, a degree of softness (for face masks, etc.) and good resistance to both EtO and gamma radiation sterilisation processes. Silicone rubber can be bonded with the aid of a primer (see Section 10.6).
10.12 Troubleshooting Sometimes cyanoacrylates will form a white powder adjacent to the bond line. This is known as blooming. This section explains the reason for blooming and suggests various options to eliminate it from the production line. Also included are general guidelines for troubleshooting an application.
10.12.1 Blooming of Cyanoacrylates Blooming occurs when cyanoacrylate molecules escape from the main body of the adhesive and react with water vapour in the surrounding air. The molecules of cyanoacrylate cure and then fall to the adjacent surface as a white powder. Even with rubber toughened cyanoacrylates, (which are often black in colour) it is the monomer which escapes and the deposit will still be white (see Figure 10.18).
Figure 10.18 The monomer escapes from the bulk of the adhesive and falls onto the adjacent surface as a white powder
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Bonding Rubber with Cyanoacrylates There are three primary causes of blooming: • Excess adhesive • Slow cure • Low relative humidity
10.12.1.1 Excess Adhesive When excess adhesive has been applied, the surface to volume-of-adhesive ratio is too low and the moisture on the surface will be insufficient to neutralise the stabiliser in the adhesive. The cyanoacrylate vapour will escape and fuming will occur. Reduce the quantity of the adhesive by using fine bore application nozzles and/or dispensing equipment.
10.12.1.2 Slow Cure In the same way as excess adhesive can cause blooming, a slow cure may give a similar result. The cyanoacrylate at the periphery of the joint will search for available moisture from the surrounding air and may then cure as a white powder on the adjacent surface. A slow cure may be the result of excess adhesive, but is also likely to be caused by acidic deposits on the substrate. These acidic deposits can cancel out the neutralising effect of the initiators (moisture) and result in very slow polymerisation or in some cases inhibition of cure completely (see also Section 10.4.1). Slow cure can also be overcome by using an activator (or accelerator). There are a number of different activators available to the production engineer and these increase the level of initiators on the surface to negate the stabiliser and thus increase the speed of polymerisation. Section 10.8 discusses the uses and application methods of activators in more detail. Slow cure may also be due to a thick bond line (adhesive >0.2 mm). Cyanoacrylates are most suited to applications where the bond line is less than 0.1 mm thick although a cure through a volume up to several mm is possible using UV curing cyanoacrylates.
10.12.1.3 Low Relative Humidity Low RH (less than 20%) will also encourage the formation of blooming and irritant fumes because there is less moisture on the surface to initiate the cure. The best results are obtained when the RH is between 40% and 60%. Higher humidities will accelerate the cure process but could affect the final bond strength and in some cases increase
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The Handbook of Rubber Bonding the risk of blooming. The use of activators can considerably assist the cure but placing a container of water adjacent to the work station has been known to increase the local RH. Blooming does not always occur during the first few seconds, indeed it is more likely that parts will bloom some hours after assembly (up to 24 – 48 hours later). A bonding application is often one of the last operations in a production cycle and care is required to ensure that parts are not put straight into a sealed (or semi-sealed) container immediately after cyanoacrylate bonding as there is a risk that the cyanoacrylate will bloom in the box resulting in a poor aesthetic appearance to the end product. Trials should be conducted accordingly. Blooming does not affect the strength of the bonded assembly and is usually only aesthetic. However in some applications, e.g., the bonding of a rubber seal near an infra-red sensor housing, blooming may occur on the surface of the sensor, thus rendering it inoperative. In this case the use of a low bloom product will eliminate the possibility of blooming. In summary, therefore, there are three methods of overcoming blooming:
• Avoid excess adhesive The use of dispensing equipment and ensuring that the minimum quantity of adhesive required to fill the joint is applied will give best results.
• Ensure fast cure The longer the cyanoacrylate remains liquid the greater the risk of blooming. Designing assemblies with close fitting parts and therefore achieving thin bond lines will increase the speed of polymerisation and will decrease the risk of blooming. The use of activators or a UV curing grade is recommended for the fast cure of exposed fillets of cyanoacrylates after assembly.
• Use a heavy molecular weight cyanoacrylate (low bloom product) The heavy molecular weight cyanoacrylates are ideal for applications where bonds must be cosmetically perfect, or for delicate electrical and electronic assemblies. The additional low odour characteristic of these cyanoacrylates is ideal where operators are required to work in confined unventilated spaces. The low bloom cyanoacrylates are also slower curing than standard ethyl or methyl grades which means that more time is available to assemble parts, where careful alignment is required. See Table 10.1 for problems, causes and corrective actions when working with cyanoacrylate adhesives. 282
No cure of the adhesive.
The acidic residue may have originated from a surface treatment process. Large gaps may exist between the two parts or there may be insufficient surface moisture thus inhibiting cure. Check the cure time - cyanoacrylates require between 4 – 24 hours to achieve full cure.
Large gaps. Slow cure.
Use low bloom grade of cyanoacrylate (see 10.12.1).
Slow cure speed. Acidic residues.
Reduce volume applied, e.g., use dispensing equipment. Consider activators to speed up cure.
Increase volume applied.
Insufficient adhesive. Excessive quantity of adhesive.
Check the production process. Cyanoacrylates should not be moved just as they are polymerising from a liquid to a solid.
Disturbance of partially cured adhesive.
Low strengths.
Blooming.
This suggests that the cure was too fast. The Try a slower curing grade of cyanoacrylate, or wash the surface may be alkaline which causes the surfaces with water, or clean with a solvent to remove the cyanoacrylate to cure too quickly and it alkalinity. cures before it sticks.
Can be difficult to remove. Good results can be achieved by washing the parts in a warm water wash (40 ºC) with 5% sodium silicate. Isopropyl alcohol (IPA) can also help.
Mould release agents.
Cured adhesive has a ‘glazed’ appearance.
Apply primer to surface and re-bond. Try writing on the surface with a felt pen. If the ink globules, it is likely that the adhesive will do the same and a poor bond will result. In this case prepare the surface with a primer or try flame etching.
Low energy surface.
Poor adhesion.
Corrective action
Possible cause
Symptom
Table 10.1 Troubleshooting chart
Bonding Rubber with Cyanoacrylates
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References 1.
ASTM D1002-01 Standard Test Method for Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal).
2.
DIN EN 1465 Adhesives – Determination of Tensile Lap-Shear Strength of Rigid– to Rigid Bonded Assemblies, 1995.
3.
ISO 10993 Biological Evaluation of Medical Devices, 2003.
Bibliography 1.
Manufacturing Engineer’s Reference Book, Ed., D. Koshal, Butterworth Heinmann, 1993.
2.
Industrial Adhesion Problems, Eds., D. M. Brewis and D. Briggs, Orbital Press, Oxford, 1985.
3.
H. W. Coover, D. W. Dreifus, and J. T. O’Connor in Handbook of Adhesives, 3rd Edition, Ed., I. Skeist, Van Nostrand Reinhold, New York, 1990, 463-477.
4.
Loctite Worldwide Design Handbook, 2nd Edition, Loctite, Rocky Hill, USA, 1998.
5.
Loctite Technical Data Sheet, Product 770, October1995.
6.
Loctite Technical Data Sheet, Product 406, February 1996.
7.
Loctite Technical Data Sheet, Product 480, February 1996.
8.
Loctite Technical Data Sheet, Product 4204, August 1996.
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poor adhesion low strength blooming no cure glazed appearance
Commercial rubbers
11
Bonding Silicone Rubber to Various Substrate P. Jerschow, B. Stadelmann and W. Strassberger
11.1 Introduction The bonding of silicone rubbers to other materials or to themselves is a very wide field. Most commonly they are classified as: • High temperature vulcanising, solid silicone rubber (HTV), • Liquid rubber, liquid silicone rubber which is also curing at high temperatures (LR), • Room temperature vulcanising, one component (RTV1), • Room temperature vulcanising, two component (RTV2). The intention of this chapter is to outline the advantages of using HTV and LR silicone rubbers. These materials are most commonly used by the silicone rubber industry. It would however be wrong to think that silicone rubbers and their bonding were limited to just these two groups of materials and this chapter will also consider other silicone specialities. HTV and liquid silicone rubber LR, are used in moulding at elevated temperatures (press, transfer or injection moulding). Other silicone rubbers, RTV 1 and RTV 2 (also mouldable), are vulcanised at room temperature. They can be used in a vast number of applications, many of which would also contribute to the topic of rubber bonding. The applications range from baking tray coatings to seals in electronics, encapsulation of electronic modules and devices and cured in place gaskets to medium and high voltage insulators and pourable or spreadable systems for mould making (prototyping). The intention of this chapter is to concentrate on the processes used in both classical and modern rubber companies and applications that are serviced by this fascinating industry. The reader is encouraged to refer to publications on silicone rubbers used in the applications mentioned above, and to trade literature from the industries and applications [1, 2, 3, 4, 5, 6, 7, 8, 9].
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11.2 Why Bond Silicone Rubber? In many applications, moulded parts, soft and hard, soft and soft or hard and hard are put together in order to perform a specific task which a part made of one single material could never perform. Very often it happens that at least one of the parts consists of silicone rubber. Before assembly both parts are produced separately, and then combined in a separate production step, without any bond to each other. They are fixed mechanically. Often, a soft elastic part is used, for example, as a gasket, valve, damper, etc. In order to attach it somewhere, a hard component is required to act as a mechanical support. The bonding is achieved by different methods. The hard component can contain undercuts where the rubber is anchored during its vulcanisation, a primer on the hard component or even using self-adhesive silicone rubber will give sufficient anchorage. The formulation of the latter contains certain adhesion promoters which allow for enhanced affinity between rubber and the other substrate. The chemical nature of selfadhesive materials in most cases is proprietary information which cannot be disclosed. But, several models for bonding are described in the literature [10]. The combination of two soft components such as silicone rubbers of different hardness, colour, electrical conductivity, etc., is also commonly used in a wide range of applications. Typical applications are electronic keypads (as for calculators, control panels, etc.), where electrically conductive silicone rubber (so-called conductive pills required for establishing an electrical contact when a key is pressed), is overmoulded with an insulating silicone rubber pad, and cable accessories, containing an insulator body with a conductive silicone core (so-called stress cone) which allows better control of the shape of the electric field. In electronics transmission and distribution at medium and high voltage, automotive, medical and many other applications silicone rubber is a preferred material for various reasons: its outstanding chemical resistance, heat ageing, dielectric (insulating) properties, unlimited pigmentability, flexibility in the cold and a wide spectrum of further advantageous properties make it the ideal material. In many cases, there is no alternative to silicone rubber. This chapter will focus on LR, because it is the most sophisticated material among silicone rubbers used in moulded parts. It shows very productive processability (short cycle times, no post-treatment like deflashing is needed), it allows use of complex geometry and, moreover, waste free production of two component applications (soft and hard or soft and soft) in only one machine without the need for a primer. In order to combine silicone
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Bonding Silicone Rubber to Various Substrates with silicone, standard LRs or even any combination between LR, HTV and RTV may be used. To make composites out of materials of a different chemical nature and silicone rubbers a so-called self-adhesive LR would preferably be used. This is used in particular for composites of thermoplastics with silicone rubber. In most cases using a primer is undesirable. This requires the application of the primer by spraying, dipping, painting, then drying and curing the primer at elevated temperatures. If a primer has to be used it is not possible to produce a substrate, e.g., a thermoplastic part, and a silicone rubber part, i.e., the whole composite, on one machine. Usually the processor carries out a feasibility study to determine whether a production step using a primer or twin shot technology is more favourable. This chapter will also describe these numerous viable applications and technical solutions using other silicone rubbers. Textile coatings will not be mentioned here. Such materials also require the bonding of the rubber onto the fabric which serves as a substrate. The bonding principles are considered to be similar and thus will not be explained in detail in this chapter.
11.3 Material Combinations of Interest - Examples
11.3.1 Silicone to Silicone Bonding (Soft and Soft) In a number of applications, silicone rubber bonded to silicone rubber is used. This can be achieved either using a primer or without a primer. In most cases, the processor uses a pair of materials with the same curing system. For example, addition curing silicone rubber can be combined with another addition curing silicone, or a peroxide curing rubber can only be used with other peroxide curing materials, e.g., an addition curing silicone might not cure in the presence of peroxides. Typically, one silicone part is cured to an ‘intermediate’ degree of vulcanisation prior to the introduction of the second silicone rubber portion. In the second step, when the second rubber component is injection or press moulded onto the first component, vulcanisation is completed. The incomplete degree of curing of the first material will ensure proper bonding between the two materials as the two layers are fully cured while in intimate contact with each other in the final step of vulcanisation or even as late as during the post-curing step. The most recent development for bonding of silicone rubber to metal is a one component, self-adhesive, solid silicone rubber which is addition curing. It is described in Section 11.6, Table 11.2.
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11.3.2 Silicone to Plastic Bonding (Soft and Hard) Typically, silicone rubber used in such cases bonds to a thermoplastic material which exhibits a very high melting or glass transition temperature. This is necessary, as silicone rubber will vulcanise at elevated temperatures such as 140 °C upwards, whereas the thermoplast should solidify in the first moulding step without cooling down too much, providing some surplus heat for the curing of the silicone. In the case of insert parts which are overmoulded with the silicone, the melting temperature is also of importance. If too high a mould temperature is used for the silicone covering the thermoplastic part, there is a possibility of warping, and loss of shape or mechanical properties of the hard component. Thermoplastic materials which could be used in conjunction with silicone rubbers include nylons (polyamides), polybutylene terephthalate (PBT), polyphenylene sulphide (PPS) and polyethylene terephthalate (PET). Section 11.5. will explain the three alternatives for bonding, where mechanical anchorage (undercuts) or self-adhesive silicone rubber allow for two colour injection moulding.
11.3.3 Silicone to Metal Bonding (Soft and Hard) It is also possible to bond silicone rubbers to various substrates which are not injection moulded. Such materials range from steel and aluminium to ceramics, glass or any other solid. In such cases the processor basically has the choice between undercuts, primers or primerless self-adhesive silicone rubber. The major difference is that the hard component cannot be produced in the same machine, it is inserted into the mould in which it is finally overmoulded with the silicone rubber part. Metals, ceramics, glass, etc., are not too sensitive to high temperatures, which allows the process to run at a higher productivity than in the previous case, see Section 11.3.2, where cycle times are longer due to lower moulding temperatures for the silicone part.
11.3.4 Why Use Silicone Rubber for Such Composites? The entire group of rubbery silicones exhibits a vast spectrum of unique properties rarely superceded by any other rubber. In most cases silicone rubber provides the best choice in terms of processability, mechanical properties, high performance and material cost.
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Table 11.1 Qualitative comparison between a number of commonly used rubbers Fully automatic production
Waste free production
Chemical resistance
Cold flexibility < –40 °C
Heat Stable > 200 °C
Pigmentable, any colour
TPE
++
++
+/–
–
–
+
EPDM
+
–
+/–
–
–
–
Natural Rubber
–
–
–
–
–
–
Silicone LR
++
++
+
++
++
++
TPE: thermoplastic elastomer
++ = very good
+ = good
– = negative
Self-adhesive LR is one of the only rubbers which can be processed fully automatically, producing no waste and at a maximum productivity. Parts with a complex geometry can be realised using quite reasonable injection moulding machines with a low clamping force. Table 11.1 summarises the properties of silicone rubbers and the advantages and disadvantages of some groups of other rubbers that potentially could be used in rubber bonding applications. Please note that this table is intended as a rough guide and is not intended to be a complete and a final statement on the rubbers mentioned therein. As for silicone rubbers other than LR, most of the above mentioned points apply, too. Silicone HTV rubbers can be delivered in a wide variety of preforms which allows the processor to optimise the production process. Such preforms range from blocks and strips to coils and even pellets, use of the latter allowing a more or less continuous operation. Another, useful property of silicone rubbers is their unique electrical behaviour. Silicone rubbers, if processed correctly exhibit extremely high volume resistivity, high dielectric strength and excellent tracking and arcing resistance. This is also the reason why substantial quantities of silicone rubbers are used by the cable, cable accessory and insulator and electronics industries.
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11.4 Some Applications of Silicone Rubber Composites Silicone rubbers, both, peroxide and addition curing, are used in the manufacture of composite parts. Known applications focus on automotive, medical, sanitary, household, transmission and distribution (T&D), electronics and food appliances. Typical applications for silicone HTV and LR rubber composite parts are: Automotive
Connector seals Crank shaft seals Exhaust pipe hangers Ignition cables Membranes Multi-functional switches Radiator seals Rain sensors Spark plug boots
Medical
Anaesthetic tubings (composite) Body contact electrodes Catheters Respiration masks Various pads Various valves, e.g., dialysis apparatus X-ray opaque shunts
Sanitary and household
Gaskets for WCs Gaskets in tap water equipment O-rings (composite) Various valves
Transmission and distribution (T&D)
Medium and high voltage cable accessories Medium and high voltage insulators
Electronics
Anode caps and cables Key pads (composite) Various gaskets
Food appliances
Baby care articles Food dispensing valves Various gaskets
This list of applications gives just a brief idea of items currently produced using twocomponent injection moulding techniques or insert parts.
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Figure 11.1 Alternatives for bonding silicone rubber
11.5 Bonding Concepts Figure 11.1 shows schematically the construction of the three most important concepts of bonding.
11.5.1 Undercuts This is the most traditional bonding concept. It allows the thermoplastic substrate to be injection moulded in the same machine as the silicone rubber. Undercuts in the substrate can be applied by special mould design, holes or other shapes, e.g., in steel sheet inserts. The possibilities for using undercuts are numerous. However, the use of undercuts has the disadvantage that the mould design for injection moulding of both the thermoplastic substrate and silicone rubber becomes more complicated. In many cases this is due to the fact that the design should allow proper venting during the filling stage of the silicone rubber part of the mould. Undercuts always imply a high proportion of ‘dead corners’ where air is entrapped, leading to incomplete filling of the mould and finally to insufficient anchorage.
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The Handbook of Rubber Bonding Another disadvantage of this concept is that the bonding contact between silicone rubber and substrate is achieved only by mechanical ties. In other words, the interface between substrate and silicone is a source for leakage, e.g., in a gasket application. In some cases more silicone rubber is used to fill undercuts than would be required for the rubbery function of the part.
11.5.2 Primers The application of primers does not allow for the injection moulding of the substrate on the same machine as the silicone rubber part. A primer is usually applied onto degreased substrates by dipping or spraying. The primer solution can be used as delivered or diluted in hydrocarbons. After application and drying it is necessary to pre-cure the primer at an elevated temperature. In most cases 10 minutes at 130 °C is sufficient.
11.5.3 Self-adhesive Silicone Rubbers Self-adhesive silicone rubbers do not require a primer and, therefore, the plastic substrate can be injection moulded on the same machine as the silicone rubber. Self-adhesive silicone rubbers for injection moulding are restricted to LR mainly. The most peculiar property of these rubbers is the fact that the mould does not have to be treated with a special release agent (which is expensive and complicated to use) when working with these silicone rubbers. However, at the same time it is possible to achieve sufficient bonding to steel without having problems with the self-adhesive silicone sticking to the mould, as is the case with conventional bonding agents for rubber to metal components. Most recently self-adhesive HTVs have been developed. However, they require special release agents or a specially structured mould suface. They are mainly intended for bonding silicone rubber to steel.
11.5.4 The Build-up of Adhesion In general, maximum adhesion is not achieved as soon as the part is moulded, rather it increases after storage of the material for some time. At room temperature this time
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Bonding Silicone Rubber to Various Substrates could be as long as a few weeks. After that, adhesion can become stronger than the mechanical properties of the rubber. In other words, when trying to separate the rubber from the substrate, the rubber will break. The storage period can be shortened significantly, if it is carried out at an elevated temperature. Approximately 0.5 to 1 hour at 80 °C to 130 °C will provide sufficient adhesion development. The storage or ‘post cure’ temperature is determined by the properties of the substrate, as, for example in thermoplastics, warping might be the result of too high a temperature. It is also essential, that demoulding should be stress free, so that a disruption of the contact surface between silicone rubber and its substrate cannot take place. Once this interface is broken, it is unlikely that proper adhesion will be possible later. Adhesion depends strongly on the surface properties. It is built up faster on smooth surfaces than on rough, sand blasted or electrically eroded substrates. Among other reasons this can be explained by the fact that smooth surfaces are intimately covered to 100% before the cure of the rubber. Uneven surfaces can be interpreted as surfaces with very tiny undercuts, and hence much less than 100% will be covered by uncured rubber, as curing usually happens before these micro-undercuts are filled. As mentioned in Section 11.5.1, undercuts that are not entirely filled might lead to loss of anchorage. When working with standard silicone rubbers stronger adhesion to smooth surfaces is observed. However, cohesive failure is never found when separating the rubber from a substrate which has not previously been primered.
11.6 Bonding of Liquid Rubber (LR) The curing characteristics of LR will be described first as they differ from those for HTV. LR consists of two parts, each of which contains reactive components. Once the components are mixed, they start to cure. This curing speed is virtually zero at room temperature. It rises dramatically when the mixture is heated in the mould. This allows the processor to produce moulded parts by injection moulding. LR has viscosities between 300 and 8000 Pa-s and exhibits extreme shear thinning properties. In other words, once sheared, its viscosity drops and its consistency is very much like honey. HTV rubbers have viscosities far above those of LR. The appearance of an HTV is more solid than liquid (see Figure 11.2). The curable composition of LR (component A mixed with component B) consists of two sorts of polydimethylsiloxane (PDMS) molecules. One contains vinyl groups on the ends
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Figure 11.2 Chemical structures and mechanism for addition curing LR and HTV
of the chain and the other contains at least three hydrogen atoms across the chain. In order to provide sufficient mechanical strength and rheological properties to allow proper processing, this mixture contains a filler, usually fumed silica. A platinum catalyst and a set of inhibitors control the curing characteristics. This mixture is called addition curing, because silicon-vinyl groups and silicon-hydrogen groups add to each other in the presence of platinum and heat. An advantage of this curing mechanism is the absence of peroxides and their by-products and, moreover, the fast speed of curing. HTV rubber is available as peroxide or addition curing. In some applications LR is not capable of ‘out performing’ HTV rubber. LR is perfect for a vast number of applications, but if very high mechanical properties are required, HTV will be the rubber of choice. The recently developed self-adhesive HTV silicone rubbers are addition curing, one component materials which remain processable for several weeks at room temperature. Self-adhesive liquid silicone rubbers are always delivered as two components. This is due to the fact that once components A and B are mixed the processing time is approximately 3 – 10 days at room temperature. Table 11.2 shows a rough comparison between the curing behaviour of a peroxide curing HTV, an addition curing HTV and an addition curing LR.
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Curing time Wacker Chemie GmbH Bonding Silicone Rubber to Various Substrates
Table 11.2 Curing time for HT and self adhesive LR Type of rubber/cure
Curing time (s)
Addition Cure LR
25
Addition Cure HTV
50
Peroxide Cure HTV
120
Injection moulding of a test slab of 6 mm thickness Moulding temperature 180 °C Cold runner mould Materials used: Addition Cure LR: ELASTOSIL LR 3070/40 Peroxide Cure HTV: ELASTOSIL R 401/60 (standard HTV) Addition Cure HTV: ELASTOSIL R 4070/60, (self-adhesive HTV, one component) all produced by Wacker Chemie GmbH
Table 11.2 shows clearly that LR is the material that by having the fastest cure rate, will achieve the highest productivity among the group of silicone rubbers and most of all the other rubbers. LR can be bought ready to use and then processed fully automatically without any waste, providing parts with the highest precision and quality which do not require any mechanical secondary treatment such as for example cryogenic deflashing. Self-adhesive LR can be processed to form a composite part using: • Two machines and a robot, or • One machine and two-component (often called two-colour) equipment. In the first case the substrate is produced in the first machine. It is either injection moulded, punched from a metal sheet, etc. A robot then transfers and inserts it into the LR-mould on the second machine. Self-adhesive LR is then injection moulded onto the part. After the curing time the composite is carefully removed. The second case, using only one machine, involves injection moulding the plastic part in a thermoplastic cavity of the two-component mould. On solidification the moulding is extracted by rotating the indexing platen or using a robot and then inserted into the cavity for the LR which is then injection moulded onto it. Demoulding takes place as in the first example.
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The Handbook of Rubber Bonding It is quite clear that it is essential to take into account the limits of the part to be coated and the requirement of a perfect ‘thermal household’ in the mould during the production of composite moulded parts. The mould temperature of the LR cavity should be as high as possible and the inserted part should be at the highest possible temperature when placed into the silicone cavity, to allow for utmost productivity, which depends very much on the speed of curing. Some proposed conditions are shown in Table 11.3. It is again clear that in injection moulding, LR will be the most viable option for such applications as it allows for a wide processing ‘window’ for adhesives with low curing temperatures.
Table 11.3 Curing times and minimum moulding temperatures for LR and HTV Minimum moulding temperature
Curing time (s)
Addition Cure LR
140 °C
25
Peroxide Cure HTV
170 °C
120
Addition Cure HTV
150 °C
50
Low moulding temperatures also favour the formation of flash. This can be avoided by using state-of-the-art mould technology. The result of a composite moulding was checked with test specimens of a well known ‘dog bone’ shape, as shown in Figure 11.3. These test pieces were produced by injection moulding. They break at a maximum load of 4 kg or even above, if good adhesion has been built up. In many cases the silicone breaks. In other words, its mechanical properties are weaker than the force of adhesion between plastic and rubber. Another similar test is the peel test [11]. A strip of self-adhesive material is peeled off the substrate and the force needed for peeling is recorded. In order not to tear the rubber during the peeling test it is reinforced with a glass fabric. Duration of adhesion (in particular under service conditions) is possibly one of the trickiest questions in connection with self-adhesive LR. In fact, it is unpredictable at present.
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Self bonding LR
Figure 11.3 Test specimen for testing self-adhesive LR as a composite with polyamide (PA)
Surprisingly this is not only because of the lack of extended history but also because of the infinite number of possible different service conditions. The number of factors that could influence the duration of adhesion is vast. Some examples are: • • • • • • • • •
Constant mechanical stress, Dynamic mechanical stress, High temperatures, Low temperatures, Varying temperatures, Migration of additives in the substrate, Surface of the substrate, Ageing of the substrate, Immersion in oil, solvents, chemicals in general.
11.6.1 Properties of Self-adhesive LR Practically, all properties known for LR can be incorporated into a self-adhesive material. Such modifications include LR with enhanced oil, tear and/or heat resistance. In the case of the widely used self-lubricating LR, self-adhesive behaviour can be achieved as well. Even conductive self-adhesive LR is theoretically possible.
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Table 11.4 Self-adhesive silicone rubber ELASTOSIL LR 3070/40 Test results of peeling test according to DIN 53531 [11] Chemical composition
Manufacturer
Tear force [N/mm]
PA 6
Leuna-Miramid GmbH
15,0
R
Ultramid B3EG6
PA 6.6
BASF
16.0
R
Ultramid A3EG6
PA 6.6
BASF
19.9
R
Miramid SE 30 CW
PA 6.6
Leuna-Miramid GmbH
18.3
R
Grilamid TR 55
PA 12
EMS-Chemie
11.9
RM
Grilamid LV 3H
PA 1 2
EMS-Chemie
17.3
R
Crastin SK 605
PBT
Du Pont
15.5
RM
Thermoplast Miramid VE 30 CW
M = adhesion tear (adhesion failure) R = rubber tear (cohesion failure) RM = combines R&M
Table 11.4 gives a summary of results of peel strength achieved with different substrates. The number of substrates is small in order to allow a very quick overview. Slabs 60 x 25 x 2 mm were injection moulded and in a 2K injection moulding process overmoulded by 6 mm of LR. The peeling force was determined after storage for 48 hours at room temperature. No post-cure or heat treatment was applied. None of the substrates mentioned in the table was primered. The table shows various types of plastics (including PA 6, PA 66 and others).
11.6.2 Limitations of Self-adhesive LR The use of self-adhesive LR is limited. Firstly, such materials cannot necessarily be used in applications with food contact, as post curing is required and the formulations often do not comply with respective legislation such as recommendations by FDA / 177.2600 [12] and BgVV XV ‘Silicones’ [13]. Post curing takes place over several hours at 200 °C. Most thermoplastics will not withstand this thermal treatment. But, even much lower post curing temperatures are possible resulting in longer post curing cycles.
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Bonding Silicone Rubber to Various Substrates Secondly, if an application implying self-adhesive LR requires reliable bonding over the whole service period, extensive testing will be necessary prior to its use. At the moment self-adhesive LR cannot be used in medical device technology as no testing according to protocols like ISO 10993-1 [14] or USP Class VI [15] has been carried out. Medical devices should be using post-cured silicone rubbers only. One of the latest developments is a self-adhesive liquid silicone rubber which is compliant to BgVV and FDA, as post cured. It adheres to PBT and PA only.
11.7 Bonding of Solid Rubber (HTV) The bonding of HTV to various substrates works analogously to the bonding of LR. HTV has much lower curing speeds at any given temperature and this enables it to be used in many different applications, which for the most part are described in Section11.4. In addition, processing of HTV differs in most cases from that of LR. In moulding, compression moulding techniques are used rather than fully automatic waste free injection moulding. Consequently a deflashing procedure is required after the moulding step. In many cases this is done by cooling the parts with liquid nitrogen. The rubber becomes brittle and the flash can then be removed (this is also called cryogenic deflashing). In the case of two-component composites such deflashing steps may become critical, if the components have different coefficients of thermal expansion and because of a strong increase of brittleness in case of thermoplastic substrates. In most cases HTV rubbers are bonded to solid substrates using undercuts or primers. Self-adhesive HTV rubbers work in the same way as their LR counterparts. However, moulding is more critical because of substantially longer curing times which can lead to demoulding problems, as mould stickiness increases dramatically with curing time. For such purposes the processor might need to prime his moulds with a special PTFE primer. It should be noted that self-adhesive HTV sticks to steel when moulded!
11.7.1 Self-adhesive HTV Silicone Rubber Applications Self-adhesive HTV silicone rubber is formulated in a similar way to self-adhesive LR. The ready-to-use formulation contains all adhesion promoters. However, other than LR, which is two component and addition curing only, self-adhesive HTV can be purchased both, as peroxide and addition curing. Because of fundamental differences in curing speed between self-adhesive peroxide cured HTV and addition cured HTV the applications differ quite substantially.
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The Handbook of Rubber Bonding Furthermore, HTV silicone rubbers exhibit better mechanical properties than LR. As a consequence applications might differ for the self-adhesive versions of these two family of products, apart from much better adhesion to steel in case of self-adhesive HCR.
• Properties Table 11.5 gives a general overview of the properties of HTV silicones and LR. It is clear that HTV will never be 100% replaced by LR. So far the mechanical properties and heat resistance of HTV rubbers are more favourable than those of LR. However, this summary should emphasise the characteristics of each family of materials and give an idea of how to select the most suitable material for an application.
Table 11.5 Summary of properties of LR versus HTV silicone rubber Liquid rubber
HTV silicone rubber
+ self-adhesive LR available
+ self-adhesive HTV available
+ production highly automated
+ more reasonable moulds
+ very complex geometry of parts possible
+ lower cost for small scale production
+ lower cost for large scale production
+ heat stability higher than LR
+ no post treatment of parts
+ chemical resistance higher than LR
+ no yellowing such as with peroxide HTV
+ better mechanical properties than LR
+ weight reduction by lower density
+ available in pellets – easy handling
+ lower curing temperature possible
+ calendering possible
– relatively high investment costs
+ extrudable
+ addition curing + low toxicity + no peroxides therefore often no post cure + much shorter cycle times + less flashes than HTV
– lower transparency than LR, yellowing
+ available as addition curing + low toxicity + no peroxides therefore often no post cure + shorter cycle times than peroxide HTV + no yellowing for addition cured HTV + positive
300
– negative
Bonding Silicone Rubber to Various Substrates Table 11.5 quite clearly shows why some applications are exclusively restricted to either HTV or LR.
11.7.2 Applications for Self-adhesive HTV 11.7.2.1 Peroxide Curing Self-adhesive HTV The application of self-adhesive peroxide curing HTV is restricted to rollers or similar applications. They work much more effectively than primers. Table 11.6 shows a comparison between a roller using a traditional primer and one using self-adhesive peroxide cured HTV (see Figures 11.4 and 11.5).
Table 11.6 Comparison of rollers produced with primers and with self-adhesive HTV ELASTOSIL R Adhesive Base 90, hardness 90 Shore A Silicone roller with primer Test results at roller function tester Line pressure N/mm
rpm
°C
Diameter mm
Abrasion
Structural break
Notice (test time, surface)
30
200
200
179
No
No
240 min, OK
40
200
200
179
No
Yes
96 min, whole rubber layer was peeled off
Result
5% adhesion to the metal core
Figure 11.4 Silicone roller with primer – rubber peeled off entirely
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Table 11.7 Silicone roller with ELASTOSIL R Adhesive Base 90 (red colour) Test results at roller function tester Line pressure N/mm
rpm
°C
Diameter mm
Abrasion
Structural break
Notice (test time, surface)
30
200
200
181
No
No
240 min, OK
40
200
200
181
No
No
240 min, OK
50
200
200
181
No
Yes
188 min, silicone layer broken
Result
100% adhesion to the metal core
Figure 11.5 Silicone roller with primer – cohesive failure in adhesive base
In this case self-adhesive HTV is used as a substitute to the primer. As it is chemically different from primers, which are not silicone rubbers, it is considered in this section.
11.7.2.2 Self-adhesive Addition Curing HTV This material cures much faster than the peroxide curing offset, which results in much shorter curing times. This makes it suitable for moulding applications. It needs not to be processed in a PTFE treated mould, if the surface of the mould has the right electroeroded structure. The reader may understand that this development is very recent and therefore no adhesion results can be provided in this review. However, one can predict that it adheres stronger to any substrate than self-adhesive LR.
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11.7.3 HTV Used in Other Bonding Applications Using primers or undercuts the field of applications for HTV is increased dramatically. The primers can be applied to ‘normal’ solid substrates as well as to fabrics and single fibres. Depending on the scale of production, HTV can be used for all applications as described in Section 11.4. The only limitation is the speed of curing and the respective moulding temperatures. In all cases HTV cures slower and at higher temperatures than LR. In most cases this moulding temperature and curing time is responsible for the fact that it is hard to combine plastics with HTV. Most typically, metals, steel, iron, etc., will be the substrates.
• Other applications A very popular technique using HTV is the combination of moulded silicone parts together with extruded HTV profiles, tubing or mouldings. Examples of such composites are catheters used as medical devices, electrical cables, long rod insulators and composite insulators.
11.8 Processing Techniques This section will summarise the processing techniques applied in the manufacture of composite parts from LR and HTV silicone rubber.
11.8.1 Liquid Rubbers in Inserted Parts Technology The two alternative concepts for the production of composite products are the thermoplast machine and the LR machine. The prerequisite for a successful product is the choice of suitable materials. The advantages of using the LR machine are: • A higher mould temperature can be used for the LR cavity, resulting in shorter cycles, • A shorter cycle can be used, • Parts of complex geometry can be produced. The main disadvantage is investment cost and that the machine occupies a large space. As mentioned earlier, the insert parts can be produced by any apparatus. This can be extrusion with subsequent cutting, injection moulding, two or multiple colour (also two or multiple component) injection moulding, press moulding, punched and bent metal sheets, cast iron, sintered metal, ceramics in any shape, cut glass, etc., or moulded or extruded silicone rubber (as in catheters and insulators).
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The Handbook of Rubber Bonding In order to achieve short production cycles it is advantageous to use pre-heated inserts. In some cases, primers are applied to the degreased insert in addition to undercuts even when using a self-adhesive LR. The application of such primers takes place by spraying, dipping or printing with subsequent drying and (only after drying) curing for 10 minutes at approximately 130 °C (lower for more sensitive insert materials) or higher. Hence, primered insert parts always are pre-heated. Using an unprimed LR, pre-heated inserts are advisable. This allows substantial heat transfer from the insert into the silicone, which allows faster curing. Another very important property is the thermal stability of the insert. The processor never faces problems using steel or other metals, as their thermal stability is determined by their melting point. However, phase transitions in the plastic or metallic part, e.g., from one crystal structure to another might be dangerous for the subsequent performance of the composite part. For glass or ceramics, brittleness and eventual consequences of thermal shock treatment have to be considered. The trickiest inserts are thermoplastics. They exhibit a relatively low melting or softening temperature that ranges from approximately 60 °C up to 200 °C or higher. In many cases, injection moulded, extruded or press moulded plastic inserts have incorporated ‘residual stresses’ from their method of manufacture. Such structures, often caused by process induced molecular and/or crystal orientation can relax at elevated temperatures which often leads to warping of the inserts. Table 11.8 shows some examples of melting temperatures of thermoplastic materials. These temperatures can be taken as a guide only, as in most cases softening takes place approximately 30 °C to 50 °C below the actual melting point which is quoted in the literature. It is also clear that a melt temperature will be reached only after sufficient contact time. For plastic materials, an insert should enter the hot mould at the optimum temperature that assures no or negligible change in quality when it leaves the mould after being coated with a silicone moulding. One of the most important prerequisites for successful production of composites is a very short heating time in the LR injection stage. Short heating times will be insufficient to thermally harm the plastic but long enough to allow curing of the silicone rubber. For such purposes, so-called faster curing LR have proven to be advantageous. At a given temperature and geometry, savings in cycle times (not heating time) have reached between 25 to 70% in comparison to ‘standard LR’, as indicated in Table 11.9 which shows a compilation of t90 values, the time needed to reach 90% vulcanisation in a Goettfert rheometer.
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Temperatures Curing times Bonding Silicone Rubber to Various Substrates
Table 11.8 Melting transition temperatures of various thermoplastic materials Start of melting (°C)
Polymer LDPE
105
HDPE
132
PB
124
PP
160
PA
170
PBT
230
Table 11.9 Curing times as average t90-values for standard and fast curing LR Liquid Rubber type
ELASTOSIL LR
t90-values at 130 °C, (s)
t90-values at 150 °C, (s)
t90-values at 170 °C, (s)
Standard, general purpose (GP)
3003
50
38
26
Fast curing, GP
3004
38
27
24
Fast curing, NPC
3005
39
27
24
Standard, oil bleeding, NPC
3089
40
38
26
Fast curing, oil bleeding, NPC
3080
39
31
28
Fast curing, oil resistant, NPC
3013
33
25
23
Fast curing, low inflammability
3001
30
25
22
italic: fast curing bold: corresponding figures between two temperatures underlined: corresponding figures bold and underlined: show the efficiency of fast curing grades NPC = no post cure
The cycle time essentially consists of heating time and time for demoulding, closing, opening of the mould and metering of fresh material. Therefore it strongly depends on the shape of the individual product which is manufactured.
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The Handbook of Rubber Bonding Table 11.9 demonstrates quite well that new fast curing grades achieve the same or a similar curing result at a certain time but at much lower temperature as the standard curing LR. Such grades are highly suitable and their use advantageous for two-component injection moulding applications. Bold and/or underlined figures in this table indicate the time-temperature relationships. It goes without saying that such fast curing technology also exhibits highly advantageous curing characteristics in ‘simple’ one-component injection moulding. In order to obtain more reliable predictions of curing times for the injection moulding process itself, the processor must conduct preliminary trials in a mould of related geometry or even a real prototype mould. Consequently, relatively short cycles at lower temperatures allow the use of polymer substrates with lower melting temperatures and less sensitivity to heat treatment as the contact times (curing times) decrease much more than the cycle times.
11.8.2 LR in Two-component Injection Moulding Technology (Two Colour Mould) This technology uses moulds with special indexing plates or handling robots that allow the transfer of the thermoplastic substrate part from the plastic cavity into the cavity for LR moulding. A specific problem in such mould design is the thermal separation – ‘cold’ in the thermoplastic cavity and ‘hot’ in the silicone rubber cavity. This is quite important and thus needs to be optimised. A peculiar fact about this moulding technique is that the plastic cavity requires a ‘hot runner’ and that the LR mould requires a ‘cold runner’. Needless to say, here we look at further two challenges for the mould designer with respect to thermal household.
11.8.2.1 Injection Moulding of Plastic Substrates In the two-component injection moulding processes the slower of the two steps is cycle determining. The thermal properties of both silicone and plastic materials require ‘compromise’ with respect to temperature in both cavities which deviates from the ideal conditions for simple injection moulding. In ordinary thermoplastic moulding, to achieve optimum cycle times for the thermoplastic part a temperature is needed that is as low as possible, or that is optimum for the injection moulding of that specific material. In two-component injection moulding for the production of thermoplastic/silicone composites, the moulding temperature for the thermoplastic should be as high as possible for two reasons. Firstly, when the thermoplastic solidifies, residual stresses are frozen in. This can lead to
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Bonding Silicone Rubber to Various Substrates relaxation processes once the polymer is in contact with hot silicone rubber and consequently warping of the plastic substrate later. If the moulding temperature is high enough, residual stresses will generally be lower. Secondly, a higher moulding temperature for the plastic will allow its residual heat to contribute to the curing of the silicone rubber. For the injection moulding of the plastic parts, hot runners allow a waste free production. The plastic is injection moulded as in a conventional process. When sufficient solidification is reached, the mould opens, and an index plate (this is a rotating platen which takes the plastic parts from the first cavity and puts them into the liquid rubber cavity by rotating them through 180 degrees), robot, etc., extracts the plastic moulding and transfers it into the silicone cavity.
11.8.2.2 Finishing of the Composite - The Moulding of LR onto the Plastic Substrate The plastic is inserted into the hot silicone cavity. The contact surface between plastic and the walls of the cavity should be kept as small as possible, in order to minimise an excess flow of heat into the plastic. Liquid rubber is injected via a cold runner into the mould. The rubber is cured at as low a temperature as possible and the composite is extracted. The advantage of this set up is that less space needed but the disadvantage is that the cycle maybe slightly longer. It is of utmost importance to operate the liquid silicone cavities at the lowest possible temperature that allows sufficient curing times for the rubber but which will not harm the plastic substrate. A qualitative description is shown in Figure 11.6. Figure 11.6 shows the strong dependence of solidification time for the thermoplast and/ or curing time for the liquid silicone rubber from moulding temperature. It is not intended to be symmetric as it may differ for various pairs of materials. The term ‘optimum’ indicates that at a given temperature the time for solidification equals the time needed for curing the silicone. Shorter curing times in the silicone cavity lead to shorter contact times for the thermoplastic. The shorter the contact time, the higher the temperature of the silicone cavity that can be used without it being critical for the plastic substrate. In other words, the temperature of the silicone rubber mould strongly depends on the speed of curing of the liquid rubber used in the processing.
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Figure 11.6 Simplified choice of processing conditions
State-of-the-art technology allows the manufacture of such composites in 32 cavity moulds with modern cold runner technology using needle shut off valves. Materials suitable for two-component injection moulding are both normal and self-adhesive LR. At the time of the release of the component from the mould, adhesion is at its weakest level. Therefore, in the case of self-adhesive or primer technology, it is essential to demould cautiously as, once silicone and substrate are separated, no adhesion will form again. In the case of self-adhesive LR, upon cooling and after sufficient storage and/or further heat treatment, adhesion will develop to its utmost extent. This will be explained further in Section 11.11.1. It is essential that the plastic material used for the moulding of the hard substrate solidifies quickly at a high mould temperature.
11.9 Silicone to Silicone Bonding (Soft and Soft) Such applications are widely used across all fields which use silicone rubbers. The technique is quite simple. In the first step a tubing or a moulding is cured to an incomplete degree of curing. This is achieved by curing at a lower temperature or too short a curing time for complete vulcanisation. These parts are then inserted into another silicone cavity (HTV or LR) and coated with the silicone rubber. It is advisable to make silicone-silicone composites from silicone rubbers with only one curing system. In other words, a peroxide
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Bonding Silicone Rubber to Various Substrates curing HTV silicone rubber should be combined with other peroxide curing HTV silicone rubbers. Addition curing HTV can be combined with addition curing HTV or with LR, the latter invariably being addition curing. This technique is used to produce medical catheters, components which require various combinations of two hardnesses, and technical parts, e.g., electronic keypads. As an example consider silicone keypads. The first silicone components are conductive pills. They can be incorporated using inserted part technology as well as in two-component injection moulding, the latter preferably using LR. In inserted part technology, these pills can be produced after moulding or extrusion by punching. The second component, a LR or HTV, is injection moulded onto the conductive pellets. Again, it is highly advisable to use the same curing system for both components conductive pills and electrically insulating keypad. It is quite clear that this moulding technique can be utilised to produce simple two or multiple coloured parts.
11.10 Cable Industry In some cable applications a bond between insulating silicone and conductive wire or conductive silicone is necessary for various reasons. For cables used in measuring devices and applications used under similar circumstances, adhesion of the insulator to the wire is necessary when the cable is pulled out of a plug. If there is no adhesion the insulating silicone will just come off the wire, if it is pulled strongly. A typical application is ignition cables where some automobile manufacturers specify an adhesion force of 70 N per 5 cm of ignition cable. This type of specification has been setup to assure proper adhesion when the ignition cable is disconnected from the spark plug. If a conductive silicone rubber is used instead of a copper wire, adhesion is built up during the curing process, when the outer insulating layer cures onto the inner conductive silicone lead.
11.11 Duration of Bonding Properties It is technically possible to produce very strong bonding between silicone rubber and another material. However, the silicone-plastic, silicone-metal or silicone composite obtained usually
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The Handbook of Rubber Bonding undergoes ageing in a severe environment typical for the proposed application. Examples are oil or coolant resistant gaskets, spark plug boots, electronic housings, anode caps, etc. After receiving a set of different stresses a chemical and moreover a physical bond can weaken or even be disrupted. Earlier in Section 11.6 a number of examples of influencing factors are given to indicate what could affect the bonding process. Under normal conditions, chemical bonding will not change over time provided the article is not deformed, in particular not deformed dynamically, and provided thermal, mechanical and chemical stresses cannot influence it negatively. In such a case this bonding will persist for years, maybe decades. For mechanical bonding using undercuts or even clamping (clamping in fact is related to undercuts but it is a result of a secondary operation), the situation changes, as the material faces a stress relaxation process (also called ‘set’ of a certain mechanical parameter). This means that each composite has an initial stress distribution as moulded. This stress decays over time. As this stress is also responsible for the strength of the mechanical bond or anchorage the decay should be as slow as possible. A closely related and prominent example for that is the compression set which is analogous to the ‘tension’ set and related to stress relaxation. If an article is compressed over a certain period of time, the elastic force acting against the compression usually decays. In other words, once a material cures at a high temperature around undercuts it will be under tension upon cooling down to room temperature - either slight compression or elongation. The resultant set starts to grow from this very first moment. If a moulded part is under a certain load (stress) during its use and moreover it has some residual stresses built in during the production, such a set can become critical for the performance of the composite. In order to achieve low mechanical setting properties, the silicone rubber has to be post cured. A post cure of composites is not possible in many cases as hardly any thermoplastics will withstand a heat treatment of several hours or more at 200 °C. As an example Table 11.10 shows the compression set of various rubbers as post cured and NPC. It is quite remarkable that special NPC materials exhibit a low compression set (and hence a lower tendency to the decay of internal stresses) without the need for post-curing. This group of materials contains certain additives which result in a low compression set as moulded and after post cure. However, as NPC grades they are suitable for technical applications only. Table 11.10 contains some ranges for the compression set. Such variations originate with batch to batch fluctuation and the accuracy of determination of the compression set.
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Table 11.10 Comparison between compression sets (CS) (Compression set; 25% deformation for 22 h at 175 °C) post cure and NPC. Duration of post cure is 6 h at 200 °C ELASTOSIL type
CS NPC [%]
CS post cure [%]
LR standard
LR 3003/60
60 – 70
10 – 20; max. 35
LR fast curing
LR 3004/60
60 – 70
10 – 20; max. 35
LR self-adhesive
LR 3070/40
60 – 70
10 – 20; max. 35
LR fast curing NPC
LR 3005/60
15 – 25; max. 30
10 – 20; max. 35
LR oil resistant NPC
LR 3013/60
20 – 30; max. 30
10 – 20; max. 35
HTV standard
R 401/60
15 – 25; max. 30
10 – 20; max. 35
HTV oil resistant NPC
R 701/60
20 – 30; max. 30
10 – 20; max. 35
All materials from Wacker Chemie GmbH
The compression set is an interesting parameter and not only relevant for mechanical bonding but also for the overall performance of a gasket or any other related application – irrespective of whether it is a simple moulding or a composite structure.
11.11.1 Duration of Bonding - Chemically Bonded Composites From investigations carried out by Wacker-Chemie GmbH it was found that bonding quality increases over time. Nonetheless, a bond cannot be guaranteed to last over a certain time under all possible circumstances. It is imperative for processors to apply sufficient testing to the bonding. Once a composite part is used under certain conditions where the function of the part essentially depends on the bonding force, sufficient testing of a prototype or a trial series is inevitable prior to use in the field. In many cases an enhanced ageing test under laboratory conditions will provide the solution. A viable solution could be the combination of primer and self-adhesive material, or even a combination between self-adhesive and mechanical anchorage. Mechanical anchorage will ensure a residual stability of the composite, and the self-adhesive material will guarantee hermetic sealing.
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The Handbook of Rubber Bonding In many other cases adhesion is only required until the assembly of a part. In this case a growth or loss of adhesion over time becomes irrelevant. Having mentioned the factors influencing the bond between the silicone rubber and the substrate and bearing in mind the vast number of possible material combinations the necessity of preliminary extensive tests for adhesion is obvious. Figure 11.7 shows the development of adhesion of self-adhesive LR over time. During injection moulding, i.e., on a very short time scale, adhesion is quite weak. Once the part is carefully extracted from the mould and stored for a certain period of time (weeks or months) the adhesive force increases significantly - sometimes to an extent where silicone and substrate cannot be separated and the silicone rubber will break before the hard and soft components can separate. This quite long storage time leading to the desired adhesion can be shortened substantially by using a heat treatment. Figure 11.7 shows that adhesion increases with the course of time. This property is used as evidence to assume that composites made of self-adhesive materials or materials and primers will not fall apart after any time under normal circumstances.
Figure 11.7 Formation of bonding of self-adhesive ELASTOSIL LR 3070/40 as a function of time
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11.12 Alternatives to Injection Moulding Alternatives to multi-component injection moulding are worth considering too. These techniques have not yet been referred to in the present chapter. If the available mould technology is outdated (there are only a few good mould makers capable of providing the appropriate mould design for LR and/or HTV), it is also possible to choose adhesives or even welding (the latter may be considered a special form of twocomponent press curing) for bonding. A further alternative is to use clamping technology which is related to undercuts. Some brief remarks on applications and their required properties will follow.
11.12.1 Adhesives RTV1 (room temperature vulcanising) adhesives are available for joining silicone parts to one another, or to join silicone rubber mouldings or extrusions to a number of different substrates. Adhesive joining also achieves a chemical and/or physical bond between materials which cannot be combined in one of the processes mentioned previously. As RTV1 systems cure at room temperature, the processor faces less temperature restrictions. However, curing takes much longer at room temperature than at high temperatures. It can last as long as 24 hours, depending on the geometry of the adhesive layer. In some cases, it is advantageous to change to an HTV adhesive. This will be the case if the contact surface between the joined parts is too large to provide an unhindered path for the moisture in the air - a reactant in the crosslinking process of RTV1 materials. HTV adhesives require heated parts and will only work with materials suitable in terms of thermal stability and bonding properties. PTFE and polyvinylidene fluoride (PVDF) cause problems because no suitable bonding agents exist. A number of RTV1, RTV2 and HTV self-adhesive materials are available which will not only work as an adhesive at first sight but also in ‘cured in place gasket’, a technology of growing importance in the automotive industry and many other areas. A silicone adhesive can also be used for many types of general purpose bonding of materials other than silicone rubbers.
11.12.2 Welding Welding is widely used for bonding silicone rubbers, and is closely related to injection or press moulding. However, usually one does not apply a complicated mould technology
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The Handbook of Rubber Bonding or a process similar to HTV and LR processing. It is also related to the adhesive bonding technique previously described. The method is quite simple. Extrusions and/or mouldings are mated in a heated ‘cavity’ and an HTV material is pressed into the gap between the parts. The HTV material cures and a chemical bond remains, as the HTV crosslinks internally to the same extent as with the surfaces of the parts to be joined. Welding is a technique widely used in plastics processing, however, welding of silicone rubbers is different. In this case a cold liquid is heated to solidify by crosslinking. In other words, no melting process with subsequent solidification upon cooling is involved at all, which is a contradiction to common experiences with thermoplastics.
11.12.3 Mechanical Bonding Techniques After Moulding A silicone rubber can be bonded to a solid substrate if it is clamped into or onto it. A very good example is the baby soother. The silicone nipple is injection moulded by a common LR process and then post cured. After that the nipples are clamped into a plastic construction or even overmoulded by thermoplastic. In clamping it is essential to protect the silicone part from any mechanical damage such as cuts or scratches. These ruptures of the silicone moulding will lead to potential sources for defects during its use. Mechanical damage after clamping can also be a risk. Another example is insulators for cable connectors which have been previously injection moulded from LR or HTV. Then, they are expanded and held in that state by a plastic spiral which is removed when it is applied onto the cable (thus retaining the original shape of the insulator). A large number of similar applications can be found in the automotive, medical fields, etc. To minimise risks of mechanical rupture a range of silicone rubbers, both LR and HTV grades are available in different levels of mechanical strength and Shore hardness. Table 11.11 gives a brief survey and comparison with ‘standard’ properties. This table is simplified as it refers to tear resistance only.
11.13 Summary For additional information on general aspects of silicone rubber bonding and latest developments in silicone rubber technology, the reader is encouraged to refer to literature such as [17, 18].
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Table 11.11 Comparison of tear resistance between standard silicone rubbers and high tear grades Tear resistance Hardness (Shore A) ASTM D624-00 Remark [16] [N/mm]
Material
ELASTOSIL Grade
LR standard
LR 3003/40
40
25
Addition cure standard grade
LR high tear
LR 3043/40
40
40
Addition cure high tear grade
LR standard
LR 3003/50
50
28
Addition cure standard grade
LR high tear
LR 3043/50
50
47
Addition cure high tear grade
HTV standard
R 401/40
40; Elongation at break ca. 600%
20
Peroxide cure standard grade
HTV high tear
R 420/40
40
50
Peroxide cure high tear grade
HTV high tear
R 4105/40
40; Elongation at break up to 1000%
50
Addition cure high tear grade low modulus
HTV standard
R 401/70
70; Elongation at break ca. 600%
20
Peroxide cure standard grade
HTV high tear
R 420/70
70
50
Peroxide cure high tear grade
HTV high tear
R 4105/70
70; Elongation at break up to 1000%
50
Addition cure high tear grade low modulus
There is much more that could be written on the subject of bonding silicone rubber. The main intention has been to provide a brief overview of what it is currently possible to achieve using silicone rubbers in the rubber industry. Fabric reinforced tubing and related applications have been left out, as there are too many processing techniques to be covered in a chapter of this size. This latter field is closely related to coextrusion. The remarks in Section 11.10 on cables, give some idea of what can be done with ‘general purpose’ coextrusions. This processing technique opens up a much wider set of possible material combinations. This is due to the large differences in the process of extrusion compared to injection or press moulding.
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The Handbook of Rubber Bonding A further topic not mentioned here is the bonding of silicone on plastic inserts or moulded preforms, the surface of which has been plasma treated. Another less important technology is the necessity of priming the mould with a release coating, e.g., Teflon. These two techniques imply additional production steps and are therefore less relevant for the future of silicone bonding to plastics. As mentioned, existing technology allows the use of primerless moulds (not to mix up: primering mould = improvement of mould release properties and primer to achieve bonding). Summing up, composite technology is capable of providing simple but very elegant technical solutions. Composites have been known for decades or maybe even centuries. Composites comprising soft and hard materials are quite new and the extraordinary speed of development of their scope of applications is not only incredible but also unforeseeable. We would like to predict that such technology and in particular technology focussing on silicone composites is going to be underestimated at any time - too many substrates, an indefinite number of ideas and the creativity of material scientists account for that.
References 1.
K. Pohmer, Kunststoffe, 2000, 90, 2, 94.
2.
K. Pohmer, Kunststoffe Plast Europe, 2000, 90, 2, 34.
3.
P. Jerschow, Presented at the Processing of Liquid Silicone Rubber, Seminar, IKV Aachen, Germany, December 1998.
4.
P. Jerschow, Presented at the Injection Moulding of Composites of Hard and Soft Materials Seminar, SKZ Stuttgart, Germany, December 1998.
5.
K. Pohmer, Presented at the Injection Moulding of Composites of Hard and Soft Materials Seminar, SKZ Stuttgart, Germany, June 1999.
6.
P. Jerschow, Kautschuk Gummi Kunststoffe, 1998, 51, 6, 410.
7.
P. Jerschow, Presented at the IRE conference, Manchester, UK, June 1999.
8.
VCI, Umwelt und Chemie von A - Z, Herder Verlag, Freiburg, 1990, Germany, 8th Ed., p.136.
9.
A. Tomanek, Silicone und Technik, Hanser Publishing, Munich, 1990, p.42.
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Bonding Silicone Rubber to Various Substrates 10. J. Comyn, Presented at the Rapra, Rubber Bonding Conference, Frankfurt, Germany, 1998, Paper No.1. 11. DIN 53531-1 Determination of the Adhesives of Rubber to Rigid Materials by the One-plate Method, 1990. 12. FDA, Code of Federal Regulations, 177.2600 13. BgVV, German Health and Veterinary Authorities, Ed., Franck Kunststoffe, 1995, Chapter 15. 14. ISO 10993-1 Biological Evaluation of Medical Devices - Part 1: Evaluation and Testing, 2002. 15. United States Pharmacopeia, current XXIII USP Class VI protocol, Washington DC. 16. ASTM D624-00e1 Standard Test Method for Tear Strength of Conventional Vulcanized Rubber and Thermoplastic Elastomers, 2000. 17. P. Jerschow, Presented at the Rapra, Rubber Bonding Conference, Amsterdam, The Netherlands, 2000, Paper No.14. 18. K. Wieczorek, Presented at the Rapra, High Performance Rubbers Conference, Berlin, Germany, 2000, Paper No.10.
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12
Failures in Rubber Bonding to Substrates B. Crowther (Sections 12.1 and 12.3) and K. Dalgarno (Section 12.2)
Bonds between rubber and substrates can fail for a number of reasons. Section 12.1 deals with some of the causes of rubber to metal bond failures. Section 12.2 examines the type of failures which are adhesion related, in fabric or cord reinforced power transmission belts. Section 12.3 discusses a phenomenon which causes service failures of rubber components, mainly in sealing applications. This phenomenon arises through a ‘bond’ which is formed between the rubber (nitrile) and the metal mating surface of a valve or similar, which is of sufficient strength to rupture the rubber surface when the valve is opened.
12.1.1 Introduction Bond failures in rubber to metal products are fortunately of relatively rare occurrence. When failures do occur they can stem from a number of fundamental areas and the faults are generally very characteristic of those problem areas. The main areas of bond inconsistencies and failures are: • faulty product design, • faulty metal preparation, • incorrect moulding procedures, • incorrect production quality testing procedures, • corrosion in service, • product abuse, • other failure modes.
12.1.1.1 Rubber to Metal Bonded Components Rubber to metal bonded components have been designed and manufactured since the early days of the rubber industry and their technology and manufacture has been discussed elsewhere [1]. However, the design of the rubber to metal component in the modern car engine mount has provided other problems for the rubber manufacturer not related to
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The Handbook of Rubber Bonding its engineering properties. Simple sandwich mountings cause no problem to the rubber to metal bonder, neither do the multi-plate products such as bridge bearings, but modern engine mounts of complex fail-safe nature can often be totally impossible to mould in one operation. This is due to the simple fact that complete bonding of rubber to the complex interlocking metals would create a product which, after bonding, would be totally locked into place in the mould, with no means of removal. This type of product can of course be made by a different bonding procedure known as partial post vulcanisation bonding. The technique is to partially bond within the mould and then to add the final metal plate externally to the mould. These bondings are more expensive because of the additional operations to complete the metal application necessary for the satisfactory operation of the unit.
12.1.2.1 Product Design and its Effects on Bond Failure All rubber to metal product designers must consider the effects of stresses exerted between the metal and rubber at the interface. This is particularly important at the edges of a component. Correct shaping of this part of the moulding will remove the stresses away from vulnerable conjunction points of metal and rubber. It is very important that rubber section corners should be radiused at the point of contact with metal components which exceed the area of the bonded rubber portion. This small modification of the rubber portion design can increase fatigue life by a factor of 7. In components which have the metal and rubber of the same dimension, then it is vital that the metal component be chamfered back to allow rubber to be moulded over a larger area than the end of the metal presents. The ideal shape for this removal of metal should be a wedge shaped chamfer which will give maximum bonding area and stress relief. This type of chamfer, which is simple to machine and does not detract from the component design or function can increase fatigue life by a factor of 6. It is particularly important in the finishing of rubber components that feather edges of rubber are not mutilated by brushes, grinding wheels or other excess rubber removing devices. Too enthusiastic an application of a buffing or grinding wheel against a metal to remove excess rubber will very quickly generate sufficient heat to successfully debond most rubber products. Excessive heating from any external source to the metal to which the rubber is adhered will result in debonding taking place. Likewise ‘nicking’ of the rubber surface can result in the tearing of the rubber back into the mass during flexing in service. If the body of the flange of rubber becomes pierced then this will allow atmospheric pollutants and corrosion creating materials to penetrate to the region of the bond and to commence the process of corrosion, usually electrochemical in nature, which will gradually undermine the bonding agent primer and destroy the bond.
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12.1.2.2 Faulty Metal Preparation Many bonding failures can be traced back to faulty metal preparation, poor application of bonding agents or careless handling techniques on the factory floor.
• Metal Cleaning Poor metal cleaning, or the use of contaminated cleaning medium, can be spasmodic in appearance unless good housekeeping is practised. Metals must be thoroughly cleaned of any grease or oil protective coating applied to prevent corrosion, before grit blasting to prevent oil/grease contamination of metal cleaning grits or chemical solutions used for the removal of corrosion and foundry products. Abrasives used for the cleaning of the metals, after the removal of greases, must be monitored to ensure that the correct type and grade is being used for the application (see Section 3.2.2, Tables 3.2 and 3.3). Incorrect metals used as abrasives can leave metal particles on the cleaned surface, which will set up galvanic cells under the applied primer. Grease contaminated abrasive will apply grease to already cleaned surfaces. Round ‘shot’ must not be used as a cleaning media only grits with the correct properties (see Sections 1.1.4.1 and 3.2.2), otherwise unwanted detritus can be caught into cavities formed by the shot and then the lips of the craters ‘peaned’ over and the contaminant trapped. Finally, worn abrasive will not efficiently clean the surface of the metal and will also create considerable dust which may be left on the surface of the metal. Improperly or insufficiently treated metal being prepared for bonding, may mean that scale and corrosion are not completely removed. Most of this type of surface lying material is only loosely attached to the metal surface and can, under load in service detach itself. The result, of course, is a partial delamination of the contact area for the rubber with the metal. Once this detachment has taken place there will a gradual widening of the detached area usually accompanied by some corrosion. Application of the primer coat to freshly prepared metal must be as quick as possible to prevent atmospheric agents causing corrosion. Oxide films (corrosion) are not usually securely adhered to the surface of the parent metal and thus can be easily pulled away. If the bonding primer has adhered to the corrosion layer it too will be pulled away from the desired contact between the primer and metal. This corrosion may not be visible to the naked eye, but can result in underbond corrosion continuing after vulcanisation. Obviously ambient conditions in the metal preparation area dictate the timing and speed of primer application. Dust from the metal cleaning operation must be removed from the surface of the metal before the application of the primer. Observation of the application of the primer should also show that there has been a complete wetting out of the metal surface by the primer.
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The Handbook of Rubber Bonding This is particularly necessary with water-based primers and bonding agents. Solvent borne bonding agents ‘wet out’ a clean metal surface with far greater ease than do the water-based products. The water-based bonding systems are much more sensitive to the presence of greases on the metal surface, and even though the metal degreasing has been carried out efficiently it will be necessary to ensure that no finger marks are found on the areas to be bonded. Solvent-based systems are capable of absorbing and dispersing small amounts of fats without causing problem to the bond. Water-based systems cannot absorb and disperse fats and therefore greater care has to be taken in the handling of metals.
• Metal cleaning using non-solvent systems The trend for metal cleaning is to move away from using solvents in general and chlorinated systems in particular. A number of alternative cleaning systems exist, but those which use water must be used in such a way that the metals are totally dry and corrosion free when being presented for bonding agent application. N.B. Beware of cross contamination of abrasive and metal treatment plant by personnel other than correct operatives. It has been known for engineering departments and individuals to not clean extremely greasy components and car engine decarbonisation parts prior to using an abrasive plant and thus to completely foul the abrasive supply. The result has been observed to cause extensive bonding problems until the contaminated abrasive was discarded and the plant cleaned of traces of grease and flakes of carbon.
• Preparation of metal surface by chemical modification (anodising, plating, sheradising) The alternative metal pre-treatment process to grit blasting uses a variety of different chemical routes. It is sufficient to say here that these can be very efficient, but do occupy rather large factory floor areas and can, if not controlled correctly, give prepared surfaces of variable quality. The usual chemical pre-treatment systems consist of acid etching of the surface followed by several water dips and subsequent phosphate or in some circumstances cadmium plating and passivating.
• Treatments for stainless steels There are various suggested systems for the pre-treatment of stainless steels which consist of treating the metal surface with strong acids to attack crystal grain boundaries in the
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Failures in Rubber Bonding to Substrates alloys and chromium poor regions around chromium carbide particles. All the methods give surface roughness to the stainless steel, which enhances the bond to the adhesive. Mixtures of nitric and hydrofluoric acid, sulphuric or chromic acid are suggested as most suitable for this. However, the nature of the substrate alloy and the steel’s heat treatment all have a bearing on the bondability of the metal.
• Phosphate coating (see also Section 1.4) Steel is often phosphate coated for use within the engineering and decorative laminate industries to reduce corrosion. Iron or zinc phosphate can also be used. However, although for some years used as a corrosion protection technique for rubber to steel bonding, it can be difficult to control the process, resulting in a variable thickness of phosphate deposit of varying crystalline structures. If too thick a phosphate layer is obtained it becomes too friable and lacking in cohesive integrity to produce the adhesive strength required to sustain a rubber to metal bond under load during service. If only a moderate phosphate coat is produced it is often necessary to ‘passivate’ the areas of steel, only minimally covered or lacking in a coating of phosphate, with chromium oxide to prevent corrosion of the areas of minimal phosphate cover. Chromium oxide, does not readily react with a bonding agent. Chromic acid is an hazardous chemical and alternative materials can be recommended by bonding agent suppliers for the passivation or ‘sealing’ of the phosphate coating. The nature of the phosphate deposited on the surface of the steel depends to a large extent upon the nature of the steel’s microstructure and the orientation of its underlying crystal lattice. Hardened steels having a martensite structured surface (consisting of interlacing rectilinear fibrous elements arranged in a triangular shape) support a fine flake phosphate structure, whereas cold-rolled steel, having acquired a different surface orientation structure, can acquire a lumpy large flake phosphate structure, which is easily broken apart under stress. Any waste water draining from these processes is a potential pollution hazard and must be tested for zinc content, as this is a hazardous material. Any zinc present must be removed or limited to about 1 – 2 parts per million.
• Zinc coating or ‘galvanising’ Metals treated in this way are supplied to the rubber bonder already in a treated form. To be effective the zinc coating must be hot dipped to the cleaned metal, to give a ‘galvanised’ finish. Bonding to this finish is not easy. The crystalline structure of the galvanised zinc and its dipped coating thickness, can result in the flaking off, under
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The Handbook of Rubber Bonding stress, of some of the coating, resulting in bond failure. The recommended treatment [2] for cleaning a galvanised finish is: i)
degrease metal part,
ii)
abrade the galvanised surface with grit,
iii)
degrease then apply adhesive as soon as possible.
or i)
immerse in a solution of 20 pbw concentrated HCl with 80 pbw deionised water, for 2 – 4 minutes at 25 °C,
ii)
rinse thoroughly in cold, running deionised water,
iii)
dry for 20 – 30 minutes in a 70 °C oven,
iv)
apply adhesive as soon as possible.
The second method is more widely used.
• Zinc sheradising A method can be specified which has to be carried out by specialist processors to give what is in effect a fused zinc surface to a steel component, which gives very good environmental protection for the steel component. The steel part to be bonded is baked whilst being tumbled in zinc dust. The process is not generally suitable for delicate metal parts and does cause problems with zinc build-up in screw threaded components (the latter would have to be protected by a sleeve or would require a die running down the thread to clear it). After treatment exposed zinc surfaces do of course oxidise if stored incorrectly, but this is not usually a problem.
• Aluminium - anodising Aluminium is usually anodised electrolytically, in the presence of an acid, either sulphuric, chromic or phosphoric, to give a tough resistant oxide film, which generally forms good bonds with the usual bonding systems. The anodising must be carried out with care and the type of crystalline structure being formed on the aluminium surface must be considered. A uniform reticulated structure is desired, not a
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Failures in Rubber Bonding to Substrates microscopically fragmented rippled surface, sometimes called ‘ice flows’ [3], which is unstable, easily fractured, and therefore unable to maintain good adhesive quality. If anodising is carried out by a custom plater he will need to be informed of the type of anodised structure desired. N.B. The final stage of any ‘wet’ metal preparation process for metals to be bonded to rubber is to ensure that all chemicals used in the processes have been removed in the final water rinse tank and then to ensure that all faces of the metal parts are fully dried prior to bonding agent application. All metal storage areas must be held at least 5 – 10 °C above the dewpoint and ideally be as near to the ambient temperature in the bonding agent application shop, which itself should be in the region of 18 – 22 °C minimum.
12.1.2.3 Application of bonding agent to metals Metals must be completely grease and dust free when the primer is applied. The primer must be properly prepared, especially if new supplies are being prepared from bulk containers, to ensure that all the constituents of the bonding primer system is in suspension when being applied to the metal surface. The application of the primer is the most critical part of the metal pre-treatment process, for if not correct, bonding will either be patchy or non-existent. If spray application is being used, especially hand spraying, then care must be taken to ensure that the primer spray hitting the metal surface is capable of wetting out the complete surface and not ‘dry’ (loss of all solvent) at the time and point of contact with the metal. If ‘dry’ then ‘cobwebbing’ (the condition of the bonding agent resulting from drying out before reaching the substrate surface and is in the form of fine filaments) will occur and although the metal may appear to be covered, poor and patchy bonding will result. Application of the primer to the metals must be carried out in such a way that there is no possibility of entrapping air between the primer and the metal. Any such trapping will act as a buffer between the primer and the metal and no bond will be achieved. The bond between the primer and the metal is essentially a mechanical one taking place around the asperities of the cleaned metal surface; although some degree of chemisorption also takes place. Intimate contact with the metal is therefore essential for good bonding to take place.
• Addition of primer and rubber bond coat Bonding agents, especially the primer systems, are very prone to settlement during storage. The result of the settlement of the suspended materials to the bottom of the drum is the formation of a layer of quite hard material which is very reluctant to
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The Handbook of Rubber Bonding return to the suspended state. It is therefore important that as soon as possible after delivery, and certainly some 24 hours prior to use, the drums contents are agitated continuously to achieve re-suspension of the deposited materials. Drum rolling on ball mill rollers for 24 hours prior to 24 hours stirring with a motor with suitable paddles to lift the suspended material is the only way to ensure that satisfactory reproducible bondings are continuously achieved. At the end of the stirring period the critical visual quality control must be carried out together with a cone bonding test to ensure quality of the bonding agent. Viscosity and flow cup checks do not fully indicate the state of the dispersion of the vital ingredients of the bonding system and cannot readily be used in any case with the water-based systems. Hydrometer measurements are suggested by bonding manufacturers for measurement of the degree of dispersed materials with these emulsions. If multiphase waterborne systems are being used then special considerations concerned with storage conditions and stirring must be observed. Having a surfactant system incorporated to facilitate the preparation and stability of the product, it is necessary to strictly observe the manufacturers instructions, as otherwise flocculation will occur. Once an adequate suspension has been achieved it is then essential to keep that suspension operational whilst treating the metal components. The ideal method is to use a continuously agitated system which does not allow settlement but is not so agitated as to cause bubbling, especially with water-based materials. Before the application of the rubber bonding adhesive layer to the primer it is necessary to ensure that contamination has not occurred to the primer surface in the drying and storage period. Dust should be removed and any parts with embedded contamination in the primer layer must be recycled, otherwise failure will result. Good housekeeping is essential to achieve a well run bonding shop. The rubber bonding coat, which is usually the second layer of bonding agent to be applied to the metal, must only be applied when the solvent from the application of the primer has all evaporated. Failure to allow adequate drying time for the primer coat will result in either bubbling of the surface of the primer or a separation of the two bonding layers at the interface. This is usually seen as polished bonding agent surfaces being evident on both rubber and metal. Application of tie cements to the surface of the rubber bonding coat to assist with the bonding of difficult compounds also need care, to ensure that the solvent is totally removed from the various bonding agent layers before passing the prepared metals to the moulding presses.
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Failures in Rubber Bonding to Substrates The addition of the primer and bonding coat to the metal should be strictly within the specified thickness parameters laid down by the supplier of the bonding agent system. Too little bonding agent or over-diluted systems being used for spraying application will often be insufficient to give an even coating of the bonding agent to cover the hills and valleys of the asperities created during the cleaning process. It is vital that all ‘peaks’ are covered adequately and evenly by the bonding agent otherwise bond failure by tearing can be initiated from such peaks. Conversely the addition of too much bonding agent is unwise not only economically but also from a bonding integrity point of view. The primer is a rigid coat when vulcanised and the bonding coat only semi-flexible and thus too much of either system coating will tend towards a weakness line being formed at the juncture of the bonding agent systems. This fault is usually characterised by bonding agent attached to the surface of the rubber in the failure area being visible. Incomplete drying of the layers of bonding agent prior to the application of the next coat is visible in a similar manner to that described for too much bonding agent being used. Often the detached bond will show that there have been bubbles formed in the bonding agent layer with the result that the outer bubble layers adhered to the rubber are brittle and easily fracture, with resultant bond failure. This problem is much more prevalent with the water-based systems, although heated metals help. N.B. Use of waterborne bonding systems has some major requirements which differ from the way solvent-based systems are handled. Failure to follow the following rules may well result in significant bond failure problems: • Initial storage requires precautions against low temperatures which will result in possible freezing or flocculation. Similarly too high a storage temperature will have similar effects on flocculation. • Too violent agitation of adhesive will result in flocculation. • ‘Dried’ waterborne bonding agent solidified on the edges of a drum, and on equipment, cannot be redissolved back into solution (differs from solvent-based system). Consequently dried agent MUST NOT BE ADDED BACK INTO A SOLUTION, as otherwise failures will result from ‘bits’ being deposited with the bonding agent onto the metal. These ‘bits’ will not bond and will inevitably result in localised bond failures. • Cleaned metals need to be heated to help water to evaporate. It may also be necessary to provide some degree of forced drying to ensure economic drying times.
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The Handbook of Rubber Bonding • Dilution of the waterborne systems requires deionised or distilled water, and definitely not tap water. • Cleanliness of metals is more critical when using waterborne systems. Solvent-based systems can absorb very small quantities of oily contamination without affecting bonding quality. Waterborne systems cannot absorb oils but are repelled by them, and if small traces of oils are present they will prevent the bonding agent ‘wetting out’ the surface and will result in ‘patchy’ cover of the metal by the bonding system.
12.1.2 Incorrect Moulding Procedures It is unlikely that incorrect vulcanisation time will result in poor bond strength, because of normal factory control systems. An exception can occur when an incorrect batch of compound has reached the presses. If correct control limit gates for compound viscosity and Mooney scorch are applied for the compound then the cure-rate should have been designed to allow for adequate compound flow, and consolidation in the mould cavity prior to onset of any cure, to ensure that totally unvulcanised compound comes into contact with the bonding agent surface. If this condition is met then the ability to generate the maximum bond between the bonding agent and the rubber will be maximised. Any compound condition other than this will result in variable bonding values and could lead to field failures in service. Quality control procedures should be such that this cannot happen. Incorrect metal handling techniques or improper storage of prepared metals can be a serious problem at the moulding press and can be the cause of subsequent failures. This type of fault is the responsibility of the manufacturing and quality control departments and should be quickly corrected. If control is lax, dust layers on prepared metals, or contamination by sweaty and greasy moulders gloves, can give bond failures. Greasy thumbprints on metals or rubber blanks can have similar effects. Some organisations prefer to heat treat (bake) metals which have been treated with bonding agents, to ensure that the degree of plasticity still present in the bonding agent does not enable it to be swept off the surface of the metal whilst the rubber compound is being injected or transferred into the mould cavity. Careful mould design can usually eliminate most bonding agent sweeping problems by careful direction of the flow of the injected rubber compound. If it is necessary to prebake the bonding agent to the metal surface, care must be taken to ensure that the prebake does not fully crosslink the bonding agent and thus leave no crosslinking possibilities between the bonding agent and the rubber. Overbaked bonding agent systems usually give bond failures which leave the rubber/bonding agent system faces quite shiny and polished (see also Section 4.3.2).
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Failures in Rubber Bonding to Substrates It is far more likely that incorrect component removal techniques from the mould cavities, causing overstressing of a very vulnerable bond, will affect the integrity of the component. One problem is the over-enthusiastic application of bonding agent causing overspill onto metal edges which come into contact with flat areas of the mould. Bonding agent will bond to mould surfaces just as well as to component metals and can cause severe product removal problems which will result in stressing of the components. Levering out of mouldings with metal tools, instead of hydraulic ejectors, will also give uneven stresses to the bond. The other very significant danger in moulding of rubber to metal components is the prescribed or accidental application of release agent to the mould cavities to ease release of the finished product. Release agents can be trapped between the rubber and the bonding agent faces and prevent an adequate bond being achieved. Ideally no release agent should be used in a mould producing bonded components, neither should it be used in the vicinity of such presses. Moulding shop air contamination by minute droplets of release agents is very easily achieved and these will condense on any exposed metal surface. Ideally the press shop producing bonded components should be at a positive pressure to neighbouring shops to prevent cross contamination taking place.
12.1.3 Incorrect Production Quality Testing Procedures The forming of the required shape and the vulcanisation of the rubber in a rubber to metal bonding process or the production of any rubber component, does not complete the crosslinking and full attainment of the ultimate network structure. Cure times for products are calculated using curemeter information to usually 90 or 95% cure. The 5 to 10% additional cure comes from retained heat in the product after demoulding has taken place. Rubber to metal bonded components are required to mature after vulcanisation, to enable the full crosslinking, structural networking and chemisorption linkage processes to take place between the various layers of the metal/primer/bonding agent/rubber complex. Some structural strength within the bond is achieved immediately after full vulcanisation time has been reached, sufficient to allow removal from the mould. However, it is important to ensure that full stressing and testing of the bond be carried out only after a period of 24 hours has elapsed. Rubber has a poor thermal conductivity and thus rubber to metal bondings which often have a low surface to volume ratio, i.e., large rubber volume, take a considerable time to cool to ambient temperature. This heat retention is often compounded by the close location to the vulcanising press of the skip into which the product is placed at the end of the cure cycle, the moulding shop ambient temperature, and the piling up of hot product as the work shift continues.
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The Handbook of Rubber Bonding Vulcanisation of the rubber does not cease completely at the end of the press/cure cycle. As the temperature of the moulded product reduces, the rate of vulcanisation decreases, but it does still continue, even into service life. Many maturation reactions take place as the product returns to ambient temperatures and even in service some crosslinking reactions will still take place over an extended period of time. The filler, usually carbon black for most components, will form its own reinforcement structure over a period of time, thus contributing to the required product properties. Testing of the product too early can result in overstressing of the rubber in critical stress zones and this in itself can lead to premature failure of the product in service. Particularly prone areas of bond stress in the early part of the components existence are concentrated by protrusions of metal components, such as bolt heads, into the rubber mass. An extensive load created in the area of such protrusions concentrates in their vicinity and localised bond failure can occur. This may not be evident to the observer but cavitation may have been created which can expand under stressing of the product in service, with resultant complete failure through rubber tear. Proof testing of product after manufacture using a calculated limited stress can, if incorrectly carried out, be a source of subsequent product failure. It is necessary to calculate a suitable set of stress parameters to use for testing the product, i.e., a load and extension, such that the product does not become overstressed and does not retain permanent evidence of damage as a result of proof testing. This can appear as distortion or incorrect dimensions.
12.1.4 Corrosion in Service Corrosion in service can occur from a number of different sources: • electrochemical attack - salt or similar, • galvanic sources - underbond, • overheating, • chemical attack.
12.1.4.1 Electrochemical Electrochemical corrosion of a metal beneath a rubber coating necessitates the presence of an aqueous phase. There will also need to be anions and cations to provide conductivity in the aqueous phase and the presence of oxygen to support the cathodic
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Failures in Rubber Bonding to Substrates reaction. To be present at the site of potential corrosion the materials have to find passage through the rubber layer, usually by migration. If an electrical potential is present the migration of water through the rubber is facilitated [3]. A similar effect occurs influencing the migration rate for cations by chemical type, i.e., Ca+ migrates faster than Na+. Thus the rate of cationic delamination occurs at a faster rate in calcium chloride solution than in sodium chloride.
• Anodic Residual stress in the metal component gives rise to an anodic condition which is largely eliminated when the metal is fully annealed. Cold working of a metal by plastic deformation will result in the electrical potential of the metal being moved in the direction of the areas of greatest activity and will be anodic in character. Incompletely cleaned metal surfaces can also be the source of potential differences. Presence of dissolved gases and the formation of protective layers during the corrosion processes are all factors affecting the generation and degree of the corrosion and its spread over the metal surface. Anodic undermining can occur beneath a rubber coating or between the components of a rubber to metal bond. Anodic undermining and delamination occur very slowly in comparison with cathodic delamination.
• Cathodic Alkali can be generated by the cathodic half of a corrosion reaction or the cathodic reaction may be driven by means of an electrical potential. When the cathodic reaction occurs between the rubber and metal surface the pH of the solution under the rubber may be as high as 14. Many factors (summarised by Leidheiser [3]) concerned with cathodic delamination are detailed. No definitive mechanism for this type of delamination has been determined although a number of suggestions have been put forward [3]. These include alkaline attack on the polymer, surface energy considerations and attack of the oxide at the interface. Moisture absorption into textile and fibre reinforcements in specialised seal compounds is often possible where exposed fibre ends are exposed to the surface of the rubber compound, either during moulding or subsequently during trimming operations. As the amount of moisture absorbed alters with atmospheric conditions of humidity and temperature there will be a variation in the activity of the electrolyte formed in the textile/rubber and thus the corrosivity of the compound to contacted metals.
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12.1.4.2 Galvanic Attack Galvanic attack usually occurs as the result of an incorrect choice of abrasive metal cleaning media. Incompatible metals used in assembly of components in service can also result in galvanic cells being generated and the resulting corrosion will, with time, cause bond decay.
12.1.4.3 Overheating The bond between rubber and the metal surface is essentially a mechanical one achieved by the use of various resins and chlorinated rubbers, which when heated and crosslinked within their mass form a rigid network. Subsequent application of a high temperature directly to the area of the bond will result in the loosening of the grip of the primer material, and under the application of a load the bond will fail. This method is sometimes used to reprocess metals from components which have been rejected by quality tests.
12.1.4.4 Corrosion by Chemicals - Not Electrochemical This type of attack can be from a variety of sources, some of which can be deliberately applied to the component. Application of unsuitable metal finishing paints or metal preservatives to the moulded component can soften the primer and/or bonding agent layer with the subsequent loss of product integrity and failure of the component. This type of corrosion can also occur, fortunately infrequently, in rubber to metal bonded components which have been designed to meet specific, often onerous service conditions. High levels of plasticisers necessary to meet very low temperature operating conditions have been known, after long periods of service, to migrate through the layers of bonding agent and effectively debond the component. The mechanism of this type of debonding is not fully understood. However a possible explanation which can be given is as follows. Primer to metal adhesion is fundamentally a combination of a mechanical keying, with a degree of chemisorption taking place. Bonding primers can be based on a complex system of materials with high resin, high chlorinated rubber combinations which probably give an interpenetrating type of structure. Obviously the penetration into and through such a primer layer by a plasticiser from the rubber layer will have the effect of softening and loosening the grip of the primers contact with the metal surface irregularities. If the bond adhesion failure occurs at the primer/bonding agent interface, a very similar explanation can be put forward. At this interface the reactions taking place are a combination of interdiffusion, adsorption and chemical crossbridging and migration of the plasticiser
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Failures in Rubber Bonding to Substrates can cause problems of debonding by either softening of the crossbridging or by chemical interference. In addition of course there may be a chemical attack at the metal surface of the bonded component by the migrating plasticiser which will cause pitting and erosion.
12.1.5 Product Abuse It is difficult to be precise about details of product abuse, as the form this takes can be wide-ranging. Some of the abuse to which rubber to metal bonded products are submitted is totally preventable whilst others are purely accidental. The most frequent cause of failure are: • exposure to high temperatures, • exposure to oils and solvents, • painting to match machinery to which bonding is fitted, • serious misalignment of flexible couplings, • service loading too high compared to original specification, • corrosive ambient service conditions not originally specified, • mechanical interference, e.g., grinding of outer metal surface generating high heat. These are a selection of the problems that can occur to cause either failure or unsatisfactory operation, through ignorance of the properties and susceptibilities of rubber to metal bonded components.
12.1.6 Other Failure Modes Evidence of metallic particles clinging to the rubber surface can be an indication of a number of factors: • poor cleaning and removal of foundry materials from the surface of the metals prior to the coating with bonding agent, • failure to remove all ‘dust’ from metal cleaning from metal surface prior to addition of first bonding agent coat, • poor adhesion of plating applied to metals as decorative or anticorrosion finishes, • inadequate removal of zinc dust if zinc ‘sheradising’ has been used to give metal corrosion resistance in service without painting,
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The Handbook of Rubber Bonding • incorrect coating being laid down during phosphating process of metal treatment, resulting in flaky, porous layer being deposited on the metal. The strength of this flaky coating is insufficient to resist workload stressing of rubber to metal bond, • incorrect anodised coating on aluminium metals which resists bonding to the primer. Some decorative anodised finishes have this problem.
12.1.7 Factors Affecting Adhesion of Rubbers The adhesion of rubbers is further complicated by the rheology of rubber and its added compounding constituents. Blending two or more rubbers will give more complexity to the flow and adhesion behaviour of the system. The ability of the rubber compound to flow and establish intimate contact with the substrate is of paramount importance, to enable the forces discussed previously to establish the conditions required for optimal adhesion. Any material which interferes with the establishment of the interface must be avoided. The establishment of a stable, strong interface between the two materials is the foremost requirement for successful adhesion. The processes which are likely to determine the interface formation are: • the ability of the adhesive/rubber mass to flow uniformly, • complete wetting of the adherend by the flowing adhesive/rubber mass, • the stabilisation of the rubber/adhesive interface.
12.1.7.1 Rubber Flow The ability of the rubber to flow must be a priority in the compound design. Without the ability to flow readily the rubber compound will not be capable of wetting out the surface of the adhesive film and thus the necessary interfacial contact will not be completely achieved. Reduction of the molecular weight of the rubber is beneficial, but other means of promoting flow may be problematical, i.e., addition of oils and plasticisers. Oils, plasticisers and waxy materials will almost certainly exude to the rubber surface either immediately after compound preparation or during the establishment of the interface. Similarly some plasticisers are known to travel to and into the interface layers with time, breaking down the adhesion and resulting in complete failure of the bond.
12.1.7.2 Complete Wetting of the Adherend Complete wetting of the adherend surface is essential if the best interfacial bond strength is to be achieved. However, because of incomplete wetting not all of the surface transforms
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Failures in Rubber Bonding to Substrates into an interface. The surface energies of the adhesive mass and the adherend rarely coincide and thus ready and complete wetting is rarely achieved. The areas of incomplete interfacial bonding impairs bond strength which in turn leads to bonding defects or failure.
12.1.7.3 Stabilisation of the Rubber/Adhesive Interface It is necessary to establish the greatest area of interface between the adherend and adhesive mass. A high contact pressure between the adherend and adhesive mass will ensure the formation of an interface despite the adherend being resistant to wetting purely on grounds of interfacial chemistry. The yield of the interface is proportional to the contact pressure, though naturally the interfacial area will decrease and the surface area increase when the contact pressure is removed. For a stable interface to be retained an area of interface must exist in a stable condition after removal of the contact pressure. Stabilisation of the interface requires that the excess energy in interface formation is minimised by binding between the dissimilar surfaces (for example rubber and metal). Thus the bond in rubber adhesion results from the formation and stabilisation of the interface via the three processes outlined previously.
12.1.8 Topography of Substrate The topography of a substrate is considered to be a direct influence on the occurrence and degree of adhesion. The adhesion is created by the adhesive being locked around protrusions and into cavities of the adherend surface. The rougher the surface, the greater the adhesion strength. The cleanliness of the surface, the ability of the adhesive to wet out the total surface area in contact and the viscosity of the adhesive being critical factors in the achievement of good bond strength. The viscosity of the adhesive must be sufficiently low to enable it to flow into the ‘valleys’ and to ensure that a minimum of air, or solvent vapour, is trapped in the depths of a cavity, but be sufficiently high to ensure that asperities in the surface to be bonded are adequately covered. Failure to achieve the correct adhesive viscosity and a maximum adhesive/substrate coverage will diminish the overall bond strength.
12.1.9 Surface Conditions of Adherend The effecting of intimate contact of the surface of the adherend to be bonded and the adhesive depends on two major factors: • the surface roughness of the adherend, • the flow characteristics of the adhesive.
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12.1.9.1 Surface Roughness of the Adherend Rowe [4] argued that many substances are polycrystalline in nature, as, for example, the oxide layers on metals and in roughening metal surfaces the crystalline form of structure is likely to persist, but with a different size of crystal. Wake [5] points out that the empirical fact remains that for metals, the apparently rougher surface given, as the particle size of grit used in grit blasting is increased, the poorer the adhesion obtained. This is probably because although visually the surface of the adherend appears rougher, in fact the irregularities themselves have smoother walls when viewed under an electron microscope. Even polished, lapped, metal surfaces have irregularities of about 10-7 m and such pits can trap air. If no air is trapped in the pits of the metal then one can see that the surface wetting and adhesive coverage would be complete. Any entrapment of air will lead to incomplete adhesive/metal coverage and much will depend upon the contact angle of the adhesive and capillary attraction. The critical angle for perfect contact between adhesive and substrate approaches zero.
12.1.9.2 Flow Characteristics of the Adhesive The viscosity of the adhesive is obviously a significant factor in the correct application of the correct and complete contact layer on the adherend. A number of adhesive factors will need to be addressed: • The optimum viscosity of the adhesive will need to be determined by experimentation to achieve optimum bond strength. • The application temperature of the adhesive should be consistent and not vary widely due to ambient fluctuations. • Similar controls are desirable for the substrate to ensure consistent adhesive flow. Too low a temperature for the substrate would reduce adhesive flow rate, while too high a temperature would result in too much flow, causing adhesive to drain away from asperities (peaks). A further complication would be encountered when using solvent-based adhesives in possible solvent evaporation causing increased viscosity. Solvent loss can occur during application through temperature fluctuations, or simply by evaporation from open adhesive containers in the case of the more volatile solvents.
12.1.10 Classification of Rubber According to their Wettabilities Lee [6] classifies rubbers by wettability, ranking them according to Zisman’s critical surface tension γc (mJ/m2), see Table 12.1. 336
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Table 12.1 Classification of rubbers by wettability Rubber
Critical surface tension γc
Isobutylene - isoprene
27
cis-Polyisoprene
31
cis-Polybutadiene
32
Styrene butadiene
33
Epichlorohydrin
35
Chlorosulphonated polyethylene
37
Acrylonitrile butadiene
37
Polychloroprene
38
12.1.11 Bonding – Interphase or Interface Considerations The interphase is a thin region which exists between the bulk adherend and the bulk adhesive [7]. A surface oxide, either native or one produced by pre-treatment, is present on most metal adherends. These oxide layers will frequently be contaminated, even after cleaning. The net effect of absorbed layers is: • that the adsorbed layer dominates over the bulk material for separation less than the thickness of the adsorbed layer, • that the bulk dominates where the separation is large compared with the adsorbed thickness, • that the adsorbed layer not only acts as a spacer but causes additional screening of the bonding reaction [8]. A primer is often applied in a production process after pre-treatment and before the application of the adhesive. Typical thickness for the oxide are 0.003 – 0.4 μm, for the primer 5 – 10 μm and for the adhesive 10 – 15 μm. The interphase region is expected to have mechanical properties different from either the adherend or the adhesive. Filbey and Wightman [9] comment that measurement of these properties is important in understanding adhesion, for example, poorly durable bonds are often a consequence of
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The Handbook of Rubber Bonding poor interphase properties. Thus, one of the frontier areas in adhesion science today is determining interphase properties. Interfacial imperfections, such as trapped air bubbles [7, 10] can be the site for high localised stresses. The air responsible for the bubbles may have been trapped into the adhesive during mixing or occur when bridging of high viscosity adhesive occurs over cavities in the substrate surface. Some degree of interfacial imperfection may arise from even normal curing conditions, because volume changes can occur within adhesives en route to full cure. Gaseous by-products and solvent residues can also be the source of problems. Plueddemann [11] has detailed the stresses which can occur due to adhesive shrinkage and coefficient of shrinkage difference between adherend and rubber in the interfacial area. Minford [12] discusses at some length cohesive versus adhesive failure. On the topic of failure due to water desorption of the adhesive Laird [13] has shown that water can progress by diffusion along the interface as much as 450 times faster than by permeation. Adhesive systems can also be sensitive to certain of the strong polar solvents as they contain polar elements themselves. Dilution of an adhesive can be achieved by the use of this principle but after the adhesive has been cured in the bondline the same solvent can attack the adhesive and destroy the adhesion at the interface. A further interfacial factor can be the presence of non adsorbable or non desorbable contaminating films (as previously mentioned above) at the interface. Such materials can be oils, fatty acids, plasticisers from the rubber and metal processing oils from inadequately cleaned metal components. Some of these lubricants can be absorbed by the adhesive if it is solvent-based but in the case of the new waterborne rubber to metal systems this absorption cannot take place, for the systems are neither miscible or compatible with oils. These new waterborne systems have a critical tolerance level for surface contamination of the metal and if this is exceeded then wetting out of the metal by the adhesive will not, at the worst be possible, or at the best complete. When examining the surface of a failed bonding it is extremely difficult to determine whether the failure has taken place at the original interface or whether a new interface has been opened, either in the adhesive or in the rubber, for the distance from the old to the new interface can be extremely small. Thus it may not be possible to determine whether contamination of the interface was the cause of the failure or not. Examination of the surface of the failed bond using very specialised equipment such as Secondary Ion Mass Spectroscopy (SIMS), Ion Scattering Spectroscopy (ISS) and Auger Electron Spectroscopy (AES) can greatly assist with determination of some of the causes of this type of failure.
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12.1.12 Problems in Adhesion Most polymeric adhesive systems undergo some degree of shrinkage during the process of crosslinking and this will create stresses within the adhesive/substrate interface. Adhesives with filler content have a lower shrinkage and thus lower stresses. The presence of water at the interface will cause interference with the establishment of a good bond. Some materials occurring as the substrate are hydrophilic in nature and will always attempt to achieve a monomolecular water film on their surfaces. This can occur, after the substrate has been cleaned and before the adhesive has been applied, from the atmosphere direct to the substrate surface, or later after the bond has been established, by diffusion. The presence of a water film at the interface can result in leaching of materials from the substrate which then can cause corrosion of the substrate, with resultant progressive loss of adhesion as corrosion spreads under the adhesive. Removal of solvent from the adhesive layer can present problems especially if evaporation from the adhesive surface is prevented in some way. If the bond is made before most of the solvent has been removed it will result in significant internal stresses in the adhesive and also lower adhesion strength. If the layer of adhesive is thick and solvent loss from the outer surfaces is rapid, viscosity changes in the outer adhesive layers will also hinder solvent loss. If the adhesive also is of the crosslinking type there can be some solvent left indefinitely internally. Crosslinking of the adhesive restricts molecular chain movement and thus prevents the movements necessary to allow the solvent molecules to travel through the mass. Solvent remaining trapped in the adhesive, usually close to the interface, results in a softer than expected adhesive/substrate interface and thus allows easier breakdown of the adhesion by stripping, etc. Choice of the solvent for the adhesive and its molecular configuration and thus size plays a considerable part in the establishment of a satisfactory bond.
12.2 Rubber Bonding in Power Transmission Belting 12.2.1 Introduction Power transmission belts are largely rubber composites which transmit mechanical power between shafts through either friction or a combination of friction and the engagement of formed teeth in pulley grooves. All power transmission belts have fibre reinforcement of some description, and so they all rely to a great extent on the quality of the bonding between fibres and rubber compounds in order to function correctly. However, there is relatively little literature concerning the role of the adhesion system in ensuring that belts retain structural integrity throughout their working life. This chapter reviews the role of
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The Handbook of Rubber Bonding the adhesion system in belting applications, considering synchronous belts, V and Vribbed belts and conveyor belts. Whilst conveyor belts are not strictly power transmission belts some consideration of the failure of these belts has been taken into account in characterising power transmission belt failure modes. The section begins with an assessment of which belt failure modes are adhesion related, before examining the adhesion systems used in belting applications, and the tests which can be used to examine adhesion strength in belts. As the chemistry of rubber bonding is covered elsewhere in this volume the emphasis within this chapter is on mechanical and applications-based issues rather than on the chemistry of the adhesion systems.
12.2.2 Power Transmission Belt Failure Modes Four different types of belt are considered: synchronous belts, V-belts, V-ribbed belts and conveyor belts. Each type of belt has a distinct set of failure modes and so each is considered in turn below. Only failures which can be considered to be belt failures have been considered, rather than belt/pulley system failures, so that failures as a result of pulley misalignment, for instance, have not been included. For adhesion related failure modes, methods of predicting belt failure where they have been developed, are outlined.
12.2.2.1 Synchronous Belts Most of the literature pertaining to synchronous belts considers the automotive application of these belts and so this section inevitably has an automotive bias. Figure 12.1 shows the structure of a synchronous belt. As Figure 12.1 shows the belt is made up from an rubber compound with two reinforcements, the first being a tension bearing cord which runs through the middle of the belt, and the second a facing fabric which covers the belt
Figure 12.1 Synchronous belt construction
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Failures in Rubber Bonding to Substrates teeth, strengthening them and protecting them from wear. The tension bearing cord is normally glassfibre or aramid, with the facing fabric normally polyamide. The belts transmit power through the engagement of the moulded teeth in pulley grooves. The belts are manufactured by first putting a fabric sleeve around a mandrel which has the belt teeth formed in it. The cord is then helically wound along the mandrel (this means that in a finished belt the cords do not lie along the axis of the belt but at a slight angle to it). The rubber compound (mixed but uncured) is then placed around the cord before an outer casing applies pressure to force the rubber compound into position and heat is applied to cure the rubber compound once it is in position. Once the curing process is complete individual belts are cut to width from the stock. Similar methods are used to manufacture most types of power transmission belt. Test results from experimental studies on belt life suggest the following as the major belt failure modes: tooth root cracking, wear, cord failure and fabric separation [14, 15, 16, 17, 18], and this classification has support from field data [19, 20]. Figure 12.2 shows examples of tooth root cracking, cord delamination and fabric separation failures. Tooth root cracking is the prevalent failure mode for belts and the literature suggests two mechanisms for its generation. The most commonly reported mechanism is fatigue and eventual failure of the facing fabric in the tooth root, followed by rapid crack propagation through the tooth rubber compound, usually across the rubber compound/
Figure 12.2 Synchronous belt failure modes. a) tooth root cracking, b) cord delamination, c) fabric separation
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The Handbook of Rubber Bonding cord interface leaving the tooth (if it remains on the belt) unable to support load and the belt unable to transmit power. Kido [21] point out that this mechanism is often accelerated by wear of the fabric, leaving less material to support the loads. Iizuka [18] reports on both the mechanism outlined above and on a second mechanism to explain tooth root cracking, in which the failure initiation site is at yarn interfaces in the cord; cracks originating in this region propagate first into the tooth rubber compound and then out to the facing fabric in the tooth root. Iizuka observed this second mechanism in belt life tests at torques of around 5 Nm, with facing fabric fatigue observed in tests carried out at a higher torque, and relates the initiation of cracks at the yarn interfaces to local maxima in the belt curvature whilst engaged on the pulley [22]. There are also a number of reported mechanisms for cord failure in the belts. The most obvious of these is the belt tension being so high that the cords simply cannot support the load, as identified by Koyama [14], while Murakami and Watanabe [23] suggest that localised bending in the cord causes debonding within the cord, resulting in interfilament abrasion and a reduction in the tensile strength of the cords leading to cord failure. Dalgarno [17] observed belt failure through debonding of the cord at the sides of the belt, leading to failure when the cord at the side of the belt becomes trapped between belt and pulley. One further source of belt tensile failure is back cracking [21, 24]. In this case failure is initiated through cracking in the back cover of the belt, generally associated with significant ageing of the rubber compound. The cracks have been observed to run across the back of the belt, and once the cracks have propagated to such an extent that the belt cords are exposed, belt tensile failure follows. Fabric separation failure occurs when the belt teeth and fabric land become detached from the belt cords [17] and is essentially seen as purely an adhesion failure, although there may be links between this failure mode and the tooth root cracking failures observed by Iizuka [18], originating from cracks developed in the cord itself through internal delamination. Wear causes belt failure through changing the tooth profile to such an extent that the belt teeth can no longer support the required load [25]. Overall it is interesting that adhesion within the belt can be the root cause of most types of failure, with fabric fatigue (often accelerated by wear), wear itself, and rubber compound cracking, the root causes of those failures not related to adhesion. The observations made by various researchers suggest that both the cord/rubber interface and the fabric/rubber interface are potential failure initiation sites, with interyarn and interfilament adhesion within the cord providing further possible failure initiation sites. In attempting to identify parameters which allow the belt life to be predicted within the adhesion related failure modes identified above, the most common approach has been to use measures of belt distortion. Dalgarno [17] examined belt life data from belt failures within the tooth root cracking, fabric delamination and cord separation failure modes,
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Failures in Rubber Bonding to Substrates and showed that the belt tooth deflection correlated well with belt life regardless of the failure mode. Childs [26] developed a failure analysis based on a combined tooth deflection and bending distortion measure, and Iizuka [27], in examining the same data as Childs [26], used the curvature of the belt as the parameter to which belt life was related. Both Childs and Iizuka show that there is good correlation between belt life and their distortion measures, and Iizuka concludes that the two measures may in themselves be related.
12.2.2.2 V and V-ribbed Belts • V-belts Recent literature on the failure of V-belts has concentrated on raw edge V-belts of the type shown in cross section in Figure 12.3, this type of V-belt being the most commonly implemented belt in automotive applications. As Figure 12.3 shows, the V-belt consists of three distinct zones, a rubber compound which forms the bulk of the V, the tension cord (normally polyester) embedded in a softer cushion rubber, and a rubber impregnated fabric. In addition the rubber compound may be axially reinforced by chopped fibre to give the rib a greater resistance to deformation in the axial direction. To improve the flexibility of the belts the belt may be ‘cogged’ as shown in Figure 12.4.
Figure 12.3 Typical raw edge V-belt cross section
Figure 12.4 Side view of cogged V-belt
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The Handbook of Rubber Bonding The failure of these belts has been studied in detail by Gerbert and Fritzson [28, 29]. With support from field data and laboratory tests they identified four common failure modes for the belts: tensile failure, cord separation, radial cracking, and wear. Belt tensile failures were associated with short belt lives and may be assumed to occur where the belt has been underspecified, or the drive is ill-conditioned. Cord separation was defined as crack propagation along or across the belt, close to the cord layer. Debonding between cord and cushion rubber, and between cushion layer and the V rubber compound were both observed, together with crack propagation in the V rubber compound and in the cushion rubber. Edge cord separation was also observed, and this is thought to be a similar failure mode to cord delamination in synchronous belts. Radial cracking involves the growth of cracks from the bottom of the V towards the belt cord, with the cracks eventually leading to the disintegration of the belt. Belt wear is obviously an ongoing process for the belts, and becomes a failure mode when the shape of the belt has changed so much as a result of wear that the belt is no longer able to transmit power at the required level. The cord separation failure mode is obviously adhesion related, and Gerbert and Fritzson [30] present a systematic method for recording the development of delamination failures. A 100% cord separation failure is defined as when the delamination occurs either across the entire belt width or along the belt for a distance equal to the belt width. Delamination which has not reached this level is then classified through a percentage value of the length of the crack to the width of the belt, with 50%, 10% and 1% the classifications used in this case. The 1% damage level represents the first recorded observation of damage through the failure mechanism of interest. One attribute of this classification system is that it allows the development of damage through more than one failure mechanism to be recorded through the life of the belt, so that failure modes which are developing simultaneously can be monitored, and situations where one failure mode is initiated by another clearly understood. Gerbert and Fritzson also developed a procedure for predicting belt life within the cord separation failure mode, based on the shear stress in the cushion region. The overall shear stress arises from four individual components of shear stress: i) the shear stress due to friction between belt and pulley, ii) a shear stress arising from the fact that the outer cords in the belt carry a higher tension than central belt cords (as a result of the belt section bending axially), iii) the cord layer and the rubber compound having a different resistance to longitudinal bending, iv)
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shear stresses around the cords as a result of radial pressure on the cord layer.
Failures in Rubber Bonding to Substrates Of these shear stress components i), ii) and iii) all act along the length of the belt, with iv) acting across the belt width. The combined effect of these four shear stresses is shown to have a good correlation with belt life where cord separation is the failure mode, and is used as part of a belt life prediction procedure (based on finite element analysis) which encompasses all the failure modes identified by Gerbert and Fritzson.
• V-ribbed belts Figure 12.5 shows a schematic of the structure of a V-ribbed belt. The most commonly reported mechanical failure mode for this type of belt is wear, with delamination not generally perceived as a problem. This is of interest as most flexible composite elements will have a delamination failure mode of some description, and so the most obvious question to ask is why the V-ribbed belt does not. The probable answer is that the belt cord in a V-ribbed belt is isolated from the major distortions of the belt through its position above the belt/pulley interface. The large majority of the distortion of the belt takes place in the belt ribs, away from the cord. If the four shear stresses identified previously by Gerbert and Fritzson for a V-belt are considered, it can be seen that of the four i) and ii) do not apply to the cord layer in a V-ribbed belt. The cord layer in a Vribbed belt therefore does not incur shear stress to the same extent as that in a V-belt. Thus the V-ribbed belt may be considered a better design in composite terms, with the individual elements of the composite more effectively employed and protected.
Figure 12.5 V-ribbed belt construction
12.2.2.3 Conveyor Belts Kozhushko and Kopnov [30] identify a number of possible failure modes for fabric conveyor belts. Abrasive wear, fabric breakage or joint failure are all possible failure modes but the failure mode of most interest here is that of fatigue delamination, and it is
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The Handbook of Rubber Bonding this mode that Kozhushko and Kopnov investigate. Their approach is to consider the delamination as a shear fatigue mechanism with delamination occurring when the shear strain magnitude reaches the permanent shear fracture strain, and with the permanent shear fracture strain seen as a variable which reduces with the number and size of loading cycles a belt has undergone. Kozhushko and Kopnov test this theory through fatigue testing and show reasonable correlation between theory and experiment, and conclude that the approach has value in belting applications but that further work is required to fully validate the approach.
12.2.2.4 Prediction of Belt Life in Delamination Failure Modes If we take an overview of how the belt delamination failure modes have been analysed by researchers to develop belt life prediction routines it is clear that the different methodologies have a lot in common, and it may be of value to highlight how these belt life prediction routines have been developed. The first step is to identify the belt life determining parameter, and for both V-belts and conveyor belts this has been identified as the shear deformation, as would be expected for delamination failures. For synchronous belts the belt life determining parameter has generally been identified as a measure of belt distortion, the success of these belt distortion measures in correlating with belt life would suggest that the belt distortion measures and the shear deformation are related. With this key parameter identified the precise nature of the relationship between the parameter and belt life must be identified. The approaches for synchronous belts and conveyor belts outlined above both sought this relationship through correlation with experimental data, whilst the approach outlined for V-belts was rather more fundamental, using crack growth analysis to follow the progression of the failure (but still using experimental data to validate the approach). All that then remains is for a model of belt behaviour to be developed which can relate the belt operational parameters to the belt life determining parameter, and then an analytical structure exists which allows the belt life to be predicted for a given set of operating parameters.
12.2.3 Adhesion Systems in Power Transmission Belts As with most rubber composites the adhesion system used in power transmission belts is based on an resorcinol/formaldehyde/latex (RFL) type system. General overviews of RFL adhesion systems from a tyre cord perspective have been previously published by Takeyama and Matsui [31], and more recently by Solomon [32]. Bonding with RFL systems is achieved through applying an RFL coating to the fibre structure prior to the
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Failures in Rubber Bonding to Substrates belt being manufactured. The cords and rubber compound are then positioned and cured, with the RFL also curing at the same time. RFL systems work through using the resin part of the RFL to adhere to the fibre, with the latex part used to provide adhesion to the rubber compound through crosslinking. For some fibrous materials a two-stage RFL system is required. This involves precoating the fibre structure with an adhesion promoter of some description prior to the RFL coating being added. Polyester, glass and aramid fibres normally require a two-stage adhesion system. Broadly the strength of an RFL adhesion system will depend on the relative reactivity of all of the components of the composite, on how completely the cord is treated (wetting and penetration), and on the effectiveness of any diffusion mechanism. As the basic RFL system and its application is well documented elsewhere, the RFL system will be considered here from a power transmission belting perspective (see also Chapter 9). The introduction of new materials to belting applications is the most common reason for re-examining the RFL system, and Kubo [33] states that a re-examination of the adhesion system is essential if the perceived benefits of switching to a new material are to be obtained in practice. Kubo specifically examined adaptations to the RFL system required to ensure good adhesion to hydrogenated nitrile rubber (HNBR) for belting applications. Several authors have examined which specific RFL system is of most value where aramid fibres are being utilised in rubber composite applications [34, 35, 36]. Aramid fibres have excellent mechanical properties but are difficult to adhere to other materials, and as such much effort has been expended to develop adhesion systems which allow their excellent mechanical properties to be exploited in belting applications. Often linked to new materials introduction is the development of belts for high temperature applications, and the implications of this on conveyor belt adhesion systems has been examined by Sarkar [37].
12.2.4 Adhesion Testing in Power Transmission Belts Tests to assess the level of adhesion of fibre structures to rubber compounds can be divided into three classifications: • pull out tests to test the adhesion of individual cords to rubber compounds, • peel tests to test the adhesion of fabrics or a row of cord to rubber compounds, • belt tests. Cord pull out tests are commonly used to assess the adhesion between a single cord and a block of rubber compound. As the name suggests the tests concentrate on measuring the load required to pull a cord from a block of rubber compound. There are various
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The Handbook of Rubber Bonding configurations which the pull out tests can take [31] with the T configuration [38], and in particular the H configuration [38, 39, 40, 41] most common where belting applications are considered. For the H configuration ASTM D4776 [42] is the most commonly quoted standard. However, Brodsky [43], in presenting a three cord adhesion test, questions the value of the H pull out test as it does not allow the stress state at the point of failure to be known. The three cord adhesion test presented by Brodsky deliberately generates a known stress state in order to overcome this limitation. Both Kubo [33] and Klingender and Bradford [40] observe that temperature and ageing are important parameters in defining the pull out strength in cord/rubber adhesion systems, with the pull out strength generally declining with an increase in either temperature or amount of ageing. Kubo also presents a methodology for assessing whether or not a cord material has been affected chemically by the adhesion system or the rubber compound. Cords are bonded into rubber compound blocks using the adhesion system of interest, before being dissolved out of the rubber compound blocks using toluene. The strength of cords treated in this way can then be compared to the strength of cords as new. Peel tests are generally used to assess the adhesion between rubber compounds and rows of reinforcing cords or fabrics. The most common format for these tests is a T-peel, where separation between one-half of the rubber compound and the reinforcing layer will be induced or manufactured in. The separated halves of the specimen are then attached to the jaws of a tensile tester and the force required to continue to separate rubber compound from reinforcement layer recorded. Such tests have been used by a number of authors with respect to belting applications [33, 34, 36, 37, 38], with the most commonly quoted standard being ASTM D4393 [44]. The importance of temperature and ageing are again both highlighted in the literature to ensure that the tests are as representative of application as possible. Both the pull out tests and the peel tests are rather idealised methods of assessing adhesion strength and may be considered as most appropriate for comparative testing of different adhesion systems rather than an indicator of potential belt performance. The relationship between the results of the standard pull out and peel tests and more representative tests resulting in belt failures through delamination has not been investigated in the literature. It is possible however to carry out a peel test on a synchronous belt which has been shown to be an indicator of the potential for a belt to fail through fabric separation [45]. The test procedure was to initially cut and peel back the facing fabric and belt teeth from the belt until three belt teeth were separated from the belt. The cut was then carefully extended under the fourth tooth into the tooth root area, taking care not to damage the fabric in any way. The cut section of belt was then T-peel tested as described above, with the fabric being peeled from the rest of the belt as the test progressed. The results of this test together with the results from belt life tests showed that adhesion systems with low peel strength values were more likely to fail through fabric separation.
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12.3 Undesirable Adhesion Occuring Under Service Conditions (Fixing) This topic does not strictly comply with the title of the chapter, but represents a problem that can be very serious in nature and cause total failure of a seal system under service conditions. When nitrile rubber vulcanisates and metals are kept in contact, under pressure, for long periods of time an adhesion force develops between their surfaces; this force has become known as ‘fixing’. The strength of the bond between the two materials can be termed the fixing strength. ‘Fixing’ when it occurs in a component in the field can be very dangerous as it often occurs within the sensor mechanisms for such applications as gas control units and motor fuel systems for vehicles. Less critical occurrences can be seen when attempting to remove a hose from a metal coupling after a period of service; apart from requiring a cleanup of the metal surface before reassembly with a new hose this case causes little problem. Mori [46] examined a number of factors concerned with the phenomenon and developed tests to determine the problem and suggested four indicators of preventative measures to eliminate or control ‘fixing’ (see Section 12.3.2). Their compounds were moulded using a mould surface with an average surface roughness of 0.41 μm. Samples were compressed, in a similar manner to a conventional compression set test between sheets of the desired test metal. The adhesion between the metal and rubber were tested after periods of time and the load required to separate the metal and rubber sheets were plotted. The strength of the ‘fixing’ was found to increase with the time of compression contact between the two materials. Initially the increase was rapid and then followed by a slow steady increase with time. The two rates were considered to be quite different and were attributed to two different mechanisms, i.e., physical and chemical (see Section 12.3.1). The initial physical bond is defined as FSo and the final fixing strength including the chemical bond is defined as FS.
12.3.1 Factors Affecting ‘Fixing’ • Environmental factors It is known that the effects of fixing are more prevalent under conditions of high humidity and high ambient temperatures. The rate of the chemical force generation is strongly influenced by the increase in ambient temperature which causes generation of chemical bonds from the nitrile surface of the rubber to the metal surface. The effects of humidity and the presence of bloomed materials on the vulcanisate surface were also investigated [46]. These blooms were created artificially and wiped from solution
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The Handbook of Rubber Bonding onto the surface of the rubber before the fixing test was carried out. Dependent upon the type of metal being tested there is also a corrosion factor to be considered. This usually proceeds from the surface of the metal plate, travelling towards the centre as corrosion proceeds. The corrosion forms an amorphous oxide layer on the metal surface which acts as a weak bonding layer. This type of corrosion occurs rather easily and is widely known as crevice corrosion [47, 48]. Crevice corrosion results from a galvanic cell forming in areas of stagnant water formed in small recesses in the metal surface. The interface of both the materials in contact must have a significant effect on the ‘fixing’ strength. Their topography caused by moulding of the vulcanisate and the metal finishing and/or plating has a significant influence on the fixing strength. The rougher the surfaces of both materials, the less will be the points of surface contact and this will reduce the strength of a bond developing. The hardness of the compound will also become a significant factor; the harder the compound the lower will be its ability to deform and follow the contours of the metal surface; this will affect the level of FS0. If however the increase in hardness of the rubber compound is produced by the addition of sulphur then there will be an increase in the chemically active units on the nitrile rubber surface and thus bond strength because of copper/sulphur bonds being formed gives an increase in FS.
• Material factors The ‘softer’ the metal the higher will be the ‘fixing’ force, FS0. This relationship is also reflected in the properties and thickness of the oxide films which are generated on the surface of the metals. Metal surface tensions and their ability to attract and hold water affect the level of FS. The surface of metals with low adsorbed moisture will generally give a high FS. The lower FS found in nickel and iron could well be attributable to their lower surface tension. Crystal size within the metal surface in contact with the rubber could well also be a significant factor in determining areas of contact. Metals which develop only a thin oxide film and having a chemically active surface will obviously generate a higher bond strength. However, copper will have a thick oxide film but this and the metal itself are very chemically active to sulphur and polysulphide chains. Aluminium and zinc plates give a low FS, since their oxide films are generally thick and chemically inactive to sulphur and polysulphide chains. The surface treatment of copper therefore has a significant effect on the ‘fixing’ properties. Organic metal treatments [47] give low FS when used in contact with NBR because the plates have a low surface tension below 0.3 mN/cm. The polarity of the NBR has an influence on the level of FS0 which increases with the acrylonitrile (ACN) level. FS also increases with the polarity of the NBR, but to a far less extent than does FS0. 350
Failures in Rubber Bonding to Substrates Carbon black fillers generally do not have much effect on ‘fixing’ except in the differences associated with general hardness increase or decrease. However the high ‘fixing’ strength of NBR may have an association with the included carbon black as it is widely known that carbon black provides active radicals in NBR during mixing and blending. These radicals can easily change to peroxides and give carboxyl groups at the surface of the NBR. The groups will be physically absorbed into the metal surface or react chemically with them. Silica however is generally inert and in fact with increasing dosage the value of FS decreases as the hardness increases. The generation of strong adhesive forces can be considered to result from chemical interaction between the metal and NBR. This generally results from the reaction of the metal with sulphur compounds and carboxyl compounds to give their metal salts, which have a high bond energy. Polysulphide reactivity is high for copper and brass plates.
12.3.2 Prevention of ‘Fixing’ Mori [46] summarises the fixing phenomenon by suggesting that first the segmented molecules of the vulcanisate diffuse into the metal surface and then polar groups such as a nitrile groups are adsorbed onto the surface. At this time, secondary order bond forces such as van der Waals forces and hydrogen bond forces are generated between both materials. Following this chemical reactions occur at the contact points between the two materials, and then both materials are combined by the first order bonds formed. As a result, strong adhesional forces are generated between the two materials. They suggest that that four preventative measure may be used: • control of contact area, • suppression of molecular motion of the NBR segments, • inhibition of interfacial reactions, • introduction of inactive crosslinks and side chains.
12.3.3 Other Methods of Preventing ‘Fixing’ - Examined Experimentally • Surface irradiation with UV light [46] Vulcanisates pre-treated by exposure to UV irradiation are influenced by the atmosphere in which the treatment is carried out. NBR treated in this way in an oxygen atmosphere did not significantly reduce in FS0, but there was remarkable increase in the FS value. This was deemed to show that the active groups such as -
351
The Handbook of Rubber Bonding OH, -CO, and -COOH are present near the surface of the metal and are also formed on the surface of the NBR vulcanisates during the UV treatment in air. If the UV treatment is carried out in an argon atmosphere then the NBR vulcanisates had a greatly decreased value of FS. This however would not be considered an industrial feasibility.
• Prevention of ‘Fixing’ by Bloom Generation Wax films, especially paraffin wax were not found to be effective as a preventative for ‘fixing’, but in fact accelerated it. If the wax is close to melting or melted then a ‘sucking’ effect will be encountered between the two materials. Mori found that amide type materials in blooms with long alkyl or alkenyl groups, provided excellent bloom films with low surface tension and high melting points on the surface of the NBR vulcanisates. Provided that the temperatures did not exceed 60 °C the amide bloom inhibited the chemical reactions occurring during ‘fixing’. The use of blooming agents is very effective for preventing the fixing between metals and NBR vulcanisates. The materials used were stearamide and methylene bis-erucamide.
References 1.
B. G. Crowther, Rubber to Metal Bonding, Rapra Review Report, 1996, Vol. 8, No 3, Rapra Technology Ltd., Shrewsbury 1996.
2.
C. L. Mahoney in Handbook of Adhesives, 3rd Edn., Ed., I. Skeist, Van Nostrand Reinhold, 1990, p.74-93.
3.
H. Leidheiser, Presented at the ACS, Division of Polymeric Materials: Science and Engineering, Fall 1986, Paper No.12
4.
D. Rowe, Private communication to W. C. Wake, 1969.
5.
W. C. Wake, Adhesion and the Formulation of Adhesives, 2nd Edn., Applied Science Publishers, London, 1982.
6.
L-H. Lee, Journal of Polymer Science, 1967, A2, 5, 1103.
7.
A. de Bruyne, The Extent of Contact Between Glue and Adherend, Aero Research Technical Notes, Bulletin No. 168, Duxford, UK, 1956.
8.
A. N. Gent and R. P. Petrich, Proceedings of the Royal Society, 1969, A310, 433.
352
Failures in Rubber Bonding to Substrates 9.
A. Filbey and J. P. Wightman in Adhesive Bonding, Ed., L. H. Lee, Plenum Press, New York 1991, Chapter 7.
10. D. Bascom, The Origin and Removal of Microvoids in Filament Wound Composites, NRL Report 6268, May 24, 1965. 11. E. P. Pleuddemann, Presented at the 25th Annual Technical Conference of the SPI Reinforced Plastics/Composites Institute, Washington, DC, 1970, Section 13-D, 1. 12. J. D. Minford in Adhesive Bonding, Ed., L. H. Lee, Plenum Press, New York, 1991, Chapter 9. 13. A. Laird, Glass Surface Chemistry for Glass Reinforced Plastics, Final Report, Navy Contract W-0679-C (FBM) 1963. 14. T. Koyama, M. Kagatoni, T. Shibata, T. Sato and T. Hoshiro, Journal of the Japanese Society of Mechanical Engineers, 1979, 22, 169, 1988. 15. T. Koyama, M. Kagatoni and T. Hoshiro, Presented at the Design Engineering Technology Conference, Cambridge, MA, USA, 7-12 Oct. 1984, ASME Paper No.84-DET-217. 16. T. H. C. Childs, A. Coutsoucos, K. W. Dalgarno, A. J. Day and I. K. Parker, Presented at the International Conference on Belt Transmissions, Hiroshima, Japan, 1991. 17. K. W. Dalgarno, A. J. Day and T. H. C. Childs, Proceedings of the Institution of Mechanical Engineers, Part D, 1994, 208, 1, 37. 18. H. Iizuka, K. Watanabe and S. Mashimo, Fatigue and Fracture of Engineering Materials and Structures, 1994, 17, 7, 783. 19. K. W. Dalgarno, Presented at the 152nd Meeting of the ACS Rubber Division, Cleveland, Ohio, Fall 1997, Paper No.47. 20. J. A. Stubbs, Industrial Lubrication and Technology, 1994, 46, 6, 7. 21. R. Kido, T. Kusano and T. Fujii, SAE Paper No.960712, 1996. 22. H. Iizuka, S. Tsutsumi, K. Watanabe, S. Mashimo and N. Ohsako, Presented at the International Rubber Conference, Kobe, Japan, 1995, Paper No.25B-17. 23. Y. Murakami and M. Watanabe, SAE Paper No.880415, 1988.
353
The Handbook of Rubber Bonding 24. K. Hashimoto, M. Oyama, N. Watanabe and Y. Todani, Presented at the 128th Meeting of the ACS Rubber Division, Cleveland, Ohio, Fall 1985, Paper No.5. 25. K. W. Dalgarno, T. H. C. Childs, A. J. Day, M. H. Hojjati, D. Q. Yu and R. B. Moore, Kautschuk Gummi Kunststoffe, 1997, 50, 4, 299. 26. T. H. C. Childs, K. W. Dalgarno, M. H. Hojjati, M. J. Tutt and A. J. Day, Proceedings of the Institution of Mechanical Engineers, Part D, 1997, 211, 3, 205. 27. H. Iizuka, G. Gerbert and T. H. C. Childs, Journal of Mechanical Design, 1999, 121, 2, 180. 28. G. Gerbert and D. Fritzson, Presented at the Power Transmission and Gearing Conference, Chicago, USA, 1989, 59. 29. D. Fritzson, Journal of Mechanisms, Transmissions, and Automation in Design, 1989, 111, 3, 424. 30. G. G. Kozhushko and V. A. Kopnov, International Journal of Fatigue, 1995, 17, 8, 539. 31. T. Takeyama and J. Matsui, Rubber Chemistry and Technology, 1969, 42, 1, 159. 32. T. S. Solomon, Rubber Chemistry and Technology, 1985, 58, 3, 561. 33. Y. Kubo, O. Mori, K. Ohura and H. Hisaki, Rubber Chemistry and Technology, 1991, 64, 1, 8. 34. Y. Iyengar, Journal of Applied Polymer Science, 1978, 22, 3, 801. 35. W. E. Weening, Presented at the 124th Meeting of the ACS Rubber Division, Houston, Texas, Fall 1983, Paper No.100. 36. H. Janssen, Journal of Coated Fabrics, 1996, 25, April, 276. 37. P. P. Sarkar, S. K. Ghosh, B. R. Gupta, A. K. Bhowmick, S. Chakraborty and A. Sen, Presented at the 132nd Meeting of the ACS Rubber Division, Cleveland, Ohio, Fall 1987, Paper No.92. 38. H. A. Daan, Kautschuk und Gummi Kunststoffe, 1985, 38, 10, 904. 39. N. A. Darwish, G. Samay and A. Boros, Polymer Plastics Technology and Engineering, 1994, 33, 4, 465.
354
Failures in Rubber Bonding to Substrates 40. R. C. Klingender and W. G. Bradford, Elastomerics, 1991, 123, 8, 10. 41. G. I. Brodsky, Presented at the 128th Meeting of the ACS Rubber Division, Cleveland, Ohio, Fall 1985, Paper No.40. 42. ASTM D4776-02 Standard Test Method for Adhesion of Tire Cords and other Reinforcing Cords to Rubber Compounds by H-Test Procedure, 2002. 43. G. I. Brodsky, Rubber World, 1984, 190, 5, 29. 44. ASTM D4393-02 Standard Test Methods for Strap Peel Adhesion of Reinforcing Cords or Fabrics to Rubber Compounds, 2002. 45. K. W. Dalgarno, Synchronous Belt Materials: Durability and Performance, University of Bradford, UK, 1991, Ph.D. Thesis. 46. K. Mori, A. Watanabe and M. Saito, Rubber Chemistry and Technology, 1988, 62, 2, 195. 47. K. Mori and Y. Nakamura, Nippon Kagaku Kaishi, 1987, 725. 48. A. Kondo, Polymer Digest (Tokyo), 1980, 32, 86.
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356
Abbreviations and Acronyms
ABS
Acrylonitrile-butadiene-styrene
ACM
Ethyl acrylate copolymer
ACN
Acrylonitrile
AEM
Analytical electron microscopy
AES
Auger electron spectroscopy
ASTM
American Society for Testing and Materials
ATR
Attenuated total reflectance
BATNEEC
Best available technology not entailing excessive cost
BDTPTS
Bis (di-ethyl thiophosphoryl) trisulphide
BGDMA
1,3-Butyleneglycol dimethacrylate
BIIR
Bromo butyl rubber
BR
Polybutadiene rubber
BTSE
Bis-[triethoxysilyl]ethane
CBS
N-cyclohexyl-2-benzothiazole sulphenamide
CIIR
Chloro butyl rubber
CM
Cement/metal failure
COR
Coefficient of restitution
CPD
Controlled product design
CP
Cement/primer failure
CPE
Chlorinated polyethylene
CR
Polychloroprene
CS
Compression set
CSM
Chlorosulphonated polyethylene
DCBS
N,N-dicyclohexyl benzothiazole sulphenamide
DCPD
Dicyclopentadiene
DIOP
Diisooctyl phthalate
DOA
Dioctyl adipate
357
Commercial rubbers
The Handbook of Rubber Bonding DOS
Dioctyl sebacate
DPG
N, N´-diphenyl guanidine
DPTT
Dipentamethylene thiuram tetrasulphide
ECO
Epichlorohydrin
EDX
Energy-dispersive X-ray (analysis)
EELS
Electron energy loss spectroscopy
ENR
Epoxidised NR
EP
Ethylene-propylene rubber
EPC
Easy processing channel black
EPDM
Ethylene propylene diene monomer
EPR
Ethylene propylene rubber
EPTPE
EPDM/PP-based thermoplastic rubbers
ESCA
Electron spectroscopy for chemical analysis
ETO
Ethylene oxide
EV
Efficient vulcanisation
EVA
Ethylene vinyl acetate copolymer
EVM
Ethylene-vinyl-acetate rubber
FDA
Food and Drug Administration (USA)
FEP
Fluorinated ethylene propylene copolymer
FKM
Fluorocarbon rubbers
FMEA
Failure mode and effect analysis
FMQ
Silicone rubbers
FPM
Fluoropolymer
FRP
Fibre-reinforced plastic
FTIR
Fourier transform infrared analysis
GP
General purpose
GPF
General purpose furnace black
GRP
Glass-reinforced plastic
HAF
High abrasion furnace black
HB
Brinell hardness
HDPE
High density PE
358
Abbreviations and Acronyms HMMM
Hexamethoxymethylmelamine
HMT
Hexamethylene tetramine
HNBR
Hydrogenated acrylonitrile butadiene rubber
HR
Hexamethylenemelamine-resorcinol
HRC
Rockwell hardness C
HRH
Hexamethylenemelamine-resorcinol-hydrated silica
HRL
Heat resistant litharge
HTV
High temperature vulcanising
HV
Vickers hardness
HVLP
High velocity low pressure
IAD
Isopropyl azodicarboxylate
IASF
Intermediate superior abrasion furnace black
IIR
Butyl rubber
IPA
Isopropyl alcohol
IR
Isoprene rubber
IRHD
International rubber hardness degree
ISO
International Organisation for Standardisation
ISS
Ion scattering spectroscopy
LDPE
Low density polyethylene
LR
Liquid rubber
MBT
Mercaptobenzothiazole
MBTS
Mercaptobenzothiazole disulphide
MEK
Methyl ether ketone
MF
Melamine-formaldehyde
MHF
Maximum torque
MIBK
Methyl isobutyl ketone
MOR
2-(4-Morpholinyl mercapto)-benzthiazole
MW
Molecular weight
NBR
Acrylonitrile butadiene rubber
NOBS
N-oxy di-ethylene benzthiazyl sulphenamide
359
The Handbook of Rubber Bonding NPC
No post cure
NR
Natural rubber
NRTPE
NR-based thermoplastic rubbers
OBTS
N-oxydiethylene benzothiazole sulphenamide
ODC
Ozone depleting chemicals
ODR
Oscillating disk rheometer
PA
Polyamide
PB
Polybutadiene
PBT
Polybutylene terephthalate
PC
Polycarbonate
PCB
Printed circuit board
PDMS
Polydimethylsiloxane
PE
Polyethylene
PEEK
Polyetherether ketone
PES
Polyethersulphone
PET
Polyethylene terephthalate
PG
Propylene glycol
PHR
Parts per hundred rubber
PIXIE
Particle-induced X-ray emission
PNR
Polynorbornene
POM
Acetal (polyoxymethylene)
PP
Polypropylene
PPO
Polyphenylene oxide
PPS
Polyphenylene sulphide
Pt
Platinum
PTFE
Polytetrafluoroethylene
PU
Polyurethane
PV
Post vulcanisation
PVC
Polyvinyl chloride
PVDC
Polyvinylidine chloride
PVDF
Polyvinylidene fluoride
PVF
Polyvinyl fluoride
360
Abbreviations and Acronyms R&D
Research and Development
RC
Rubber/cement failure
RF
Rubber failure
RFL
Resorcinol/formaldehyde/latex
RFR
Resorcinol/formaldehyde/resin
RH
Relative humidity
RIM
Reaction injection moulding
RPN
Risk priority number
RT
Room temperature
RTV1
Room temperature vulcanising, one component
RTV2
Room temperature vulcanising, two component
SBR
Styrene butadiene rubber
SBS
Styrene-butadiene-styrene
SDS
Safety data sheet
SEBS
Hydrogenated SBS
SEM
Scanning electron microscopy
SEV
Semi-efficient vulcanisation
SG
Specific gravity
SIMS
Secondary ion mass spectroscopy
SMR
Standard Malaysian rubber
SNMS
Secondary neutral mass spectrometry
TAC
Triallyl cyanurate
T&D
Transmission and distribution
TCAT
Tyre cord adhesion test
TCBQ
Tetrachlorobenzoquinone
TCE
Trichloroethane
TCICA
Trichloroisocyanuric acid
TEM
Transmission electron microscopy
TETD
Tetraethyl thiuram disulphide
Tg
Glass transition temperature
THF
Tetrahydrofuran
361
The Handbook of Rubber Bonding TIC
Trichloroisocyanuric acid
TLV
Threshold limit value
TMTD
Tetramethyl thiuram disulphide
TMTM
Tetramethyl thiuram monosulphide
TOFSIMS
Time-of-flight secondary ion mass spectrometry
TOTM
Trioctyl trimellitate
TPE
Thermoplastic elastomer
TPO
Thermoplastic olefinic elastomer
TPU
Thermoplastic PU
TR
Thin rubber
TRIM
Trimethylolpropane trimethacrylate
TSA
Toluene sulphonic acid
UF
Urea-formaldehyde
USP
United States Pharmacopoeia
UV
Ultra violet
VAMAC
Ethylene acrylic terpolymer
VMQ
Silicone rubber with methyl and vinyl substituents
VOC
Volatile organic compounds
VP
Vinyl pyridine
VS
Vinyl silane
WC
Water closet
WDS
Wavelength dispersive X-ray (analysis)
XPS
X-ray photoelectron spectroscopy
XNBR
Carboxylated NBR
ZDA
Zinc diacrylate
ZDMA
Zinc dimethacrylate
362
Author Index
A Ahmad S. 211 Ahn J. H. 179, 191, 193, 209 Alarcon-Lorca F. 212 Aldred D. H. 256 Allen K. W. 54 Alliger G. 78 Alstadt D. M. 78 Andries J. C. 160 Ayres R. L. 51
B Ball J. J. 168, 192, 211 Bantjes A. 194 Barker L. R. 211 Barr T. L. 210 Basaran M. 209 Bascom D. 353 Beamson G. 16, 52 Beatty J. R. 160 Beecher J. F. 192 Beers R. N. 212 Benarey H. A. 51, 78 Benderly A. A. 52 Benko D. A. 212 Bhowmick A. K. 51, 78, 160, 192, 193, 209, 210, 354 Biemond M. E. F. 167, 192, 193, 201, 209 Binder H. 51 Biswas A. 192, 210 Blow C. M. 78 Blythe A. R. 51
Bobrov A. P. 182, 193, 194 Bomal Y. 193, 210, 211 Bond K. M. 51, 136 Boros A. 354 Borroff E. M. 161, 256 Bourrain M. P. 210 Bowden P. J. 212 Bradford W. G. 348, 355 Bragole R. A. 19, 52 Brandon D. G. 55 Brass I. 52 Brecht H. 51 Brewis D. M. 29, 51, 52, 53, 54, 283 Briggs D. 14, 51, 52, 54, 209, 283 Brindoepke G. 211 Brodsky G. I. 348, 355 Brunat W. 54 Buchan S. 78, 191 Burkhart T. 211 Burrows F. H. 212 Busscher H. J. 54 Butcher D. 193, 211 Butler D. P. 239
C Carlson G. L. 54 Carne R. J. P. 53 Carter A. R. 20, 53, 161 Cartier J. F. 51 Cayless R. A. 52 Chakraborty S. 354 Chakravarty S. N. 160 Chaler N. A. 170, 193
363
The Handbook of Rubber Bonding Chandra A. K. 169, 181, 192, 193, 202, 209, 210 Chen J. K. 54 Chew A. 28, 52, 54 Child T. F. 194, 212 Childs T. H. C. 343, 353, 354 Clearfield H. M. 51 Cochet P. 193, 206, 211 Coleman Jr., E. W. 78 Collins G. C. S. 14, 52 Colson J. C. 210 Combette C. 26, 54 Combette P. 212 Comyn J. 53, 317 Coover H. W. 283 Cope B. C. 53 Corish P. J. 160 Cornell J. A. 239 Costin R. 79, 239, 240 Coutsoucos A. 353 Craft J. 88 Crawford D. 239 Creasey J. R. 211 Crowther B. G. 160, 352
D Daan H. A. 354 Daft L. 59, 78 Dahm R. H. 52, 54 Dalgarno K. W. 342, 353, 354, 355 Darwish N. A. 354 Davis G. D. 51 Day A. J. 353, 354 De Bruyne A. 352 De Gryse R. 210 De P. P. 161 De S. K. 161 De Volder P. 210 De Vries J. 54 Debruyne E. 210
364
Del Vecchio R. J. 79 De Lollis N. J. 54 Denq Y. L. 54 Deryaguin B. V. 150, 161 Deuri A. S. 193, 210 Dietrick M. I. 211 Dillard J. G. 23, 53, 54 Dogadkin B. A. 160 Downes J. 212 Drake R. E. 160 Dreifus D. W. 283 Duc T. M. 54 Duffour T. 193, 210 Dwight D. W. 14, 52
E Edington R. A. 256 Eirich R. 239 Ekwall R. 239 Eley D. D. 256 Elliot D. J. 78 Ellul M. D. 52, 143, 160 Elman J. F. 15, 52 Elschner A. 192, 211 Engelhardt M. L. 212 Erickson D. E. 212 Ershov E. A. 193 Evans L. R. 193, 206, 211, 212 Everaert E. P. 54 Extrand C. W. 21, 53
F Fahrig M. 257 Feast W. J. 209 Feng Y. 240 Fernández-García J. C. 53, 161 Feuer H. O. 239 Filbey A. 337, 353 Flanagan P. 239
Author Index Flemming R. A. 210 Friedel C. 88 Fritz J. L. 54 Fritz T. L. 52 Fritzson D. 344, 345, 354 Fujii T. 353 Fujikura Y. 53 Fujino A. 239 Fuller K. N. G. 160
G Garbassi F. 55 Gatza P. E. 239 Gent A. N. 21, 53, 160, 161, 352 Gerbert G. 344, 345, 354 Gerenser L. J. 15, 52 Gesang T. 52 Ghosh S. K. 354 Ghosh T. B. 209 Gibbs H. W. 192, 193, 210, 211 Giridhar J. 169, 189191, 193, 195, 209 Goerl U. 257 Goralski E. G. 193, 212 Goryaev V. 184, 194 Grabov’ I. F. 195 Gregory H. J. 54 Grigorev M. F. 194 Grogger W. 192 Grubbauer G. 192, 209 Gupta B. R. 354
H Haemers G. 191, 210 Hall J. R. 52 Hall M. M. 51, 78 Halladay J. R. 58, 79 Hamed G. R. 159, 161, 168, 182, 192, 193, 203, 208, 210, 212 Hammer G. E. 209, 210
Han M. H. 194, 212 Hannell W. J. 210 Hansen R. H. 52 Harris S. J. 195, 212 Hartz R. E. 257 Hashimoto K. 354 Hawkins I. M. 193, 210 Hazelton D. R. 52, 143, 160 Hennemann O. D. 52 Hess R. H. 211 Hewitt N. L. 193, 211 Hill J. M. 18, 51 Hirahara H. 195 Hiratsuka S. 195 Hisaki H. 354 Hivert D. 54 Hoekje H. H. 211 Hofer F. 192, 209 Hoff C. M. 182 193, 211, 212 Hofmann W. 79 Hojjati M. H. 354 Hollahan J. R. 54 Holtkamp D. 192, 211 Hope J. C. 193, 211 Horie H. 195 Hörnström S-E. 194 Hoshiro T. 353 Howath T. 239 Huang J. 193, 208, 212 Hudis M. 54 Hummel K. 192, 209 Hupje W. H. 256
I Ichikawa M. 160 Iizuka H. 342, 343, 353, 354 Ikeda A. 239 Ikeda K. 195 Ikeda Y. 195
365
The Handbook of Rubber Bonding Ishikawa Y. 209, 210 Iwabuchi H. 53 Iyengar Y. 257, 354
J Janssen H. 354 Jayaseelan S. K. 192, 194, 212 Jazenski P. J. 78 Jeon G. S. 194, 212 Jerschow P. 316 Job L. 161 Joseph R. 161
K Kagatoni M. 353 Kang K-K. 212 Kaplan W. D. 55 Karbashewski E. 18, 51 Kato N. 53 Kawakami S. 210 Keller C. 240 Kendall C. R. 51 Khavina E. Y. 192 Kido R. 342, 353 Kilduff T. J. 52 Kim H. J. 160 Kim K. J. 52 Kimu S. 195 Klingender R. C. 239, 348, 355 Klyachtin Y. 53 Konar J. 209 Kondo A. 355 Kondyurin A. 19, 53 Konieczko M. B. 51, 52 Kopnov V. A. 345, 354 Koshal D. 283 Koyama T. 342, 353 Kozhushko G. G. 345, 354
366
Kranz G. 52 Kretzschmar T. 192, 209 Krone R. 184, 194 Krüger R. 51 Kubo Y. 347, 348, 354 Kurian J. 161 Kurosaki T. 160 Kusano T. 353 Kusano Y. 22, 24, 53
L Labarre D. 181, 193, 210 Labriola J. M. 160 Lai J. Y. 54 Lai S. M. 160 Laird A. 338, 353 Lake G. J. 160 Landrock A. H. 55 Lawson D. F. 19, 20, 23, 24, 25, 52, 53 Le Prince P. 54 Lee K. S. 78 Lee L. H. 352 Legeay G. 54 Lehrle R. S. 210 Leidheiser H. 331, 352 Levi D. W. 52 Lin A. 51 Lin Y. Y. 54 Lingjun C. 193 Loha P. 160 Lowe A. C. 52 Luginsland H-D. 194 Lüschen R. 52 Lyons C. S. 51
M Macintosh C. 256 Maeseele A. 210
Author Index Mahoney C. L. 51, 352 Manino L. G. 78 Manoj N. R. 161 Martin S. 239 Martín-Martínez J. M. 23, 53, 54, 161 Mashimo S. 353 Mason M. G. 15, 16, 52 Mathieson I. 52, 54 Matsui J. 346, 354 Matyukhin S.A. 190, 195 Maucourt J. 54 Mayer F. 51 McNamarra D. K. 51 Meier K. 257 Melley R. E. 160 Melvin T. 239 Michel G. 54 Middletown O. H. 209 Minagawa M. 19, 26, 53 Minford J. D. 51, 338, 353 Mitchell B. 256 Mittal K. 18 Mizumoto S. 195 Molitor P. 239 Montgomery R. E. 51 Montoya O. 54 Moore M. J. 136 Moore R. B. 354 Morawski J. C. 210 Morel-Fourrier C. 211 Mori K. 190, 195, 349, 351, 355 Mori O. 354 Morra M. 55 Mowrey D. H. 51, 136 Mukhopadhyay R. 192, 193, 209, 210 Munro H. S. 209 Murakami Y. 353 Murray P. F. 191, 209, 210
N Nagel W. 79, 239, 240 Naito K. 53 Nakamura M. 195 Nakamura Y. 355 Nando G. B. 161 Nangreave K. R. 53 Nawafuna H. 195 Nelson E. R. 52 Nicholas D. 52 Niderost K. J. 210 Noguchi T. 53
O Occhiello E. 55 O’Connor J. T. 283 Ohsako N. 353 Ohura K. 354 Oishi Y. 195 Okel T. A. 193, 211 Okumoto T. 160 Oldfield D. 20, 21, 22, 25, 26, 29, 51, 53 Olson L. R. 239 Oplochenko N. A. 195 Opperman G. W. 54 Orgilés-Barceló A. C. 53, 161 Orjela G. 195, 212 Othman A. B. 160 Owen M. J. 54 Oyama M. 239, 354
P Panchuk F. O. 190, 195 Parker I. K. 353 Pastor-Blas M. M. 23, 24, 53, 54 Pastor-Sempere N. 24, 53 Patrick R. L. 55 Paul R. 192, 203, 210
367
The Handbook of Rubber Bonding Pawlowski L. B. J. 256 Pelletier J. B. 210 Persoone P. 210 Peterson A. 211 Petrich R. P. 352 Pettit D. 20, 22, 53, 161 Pieroni J. K. 239 Pieroth M. 168, 192, 211 Plasczynski T. 136 Pleuddemann E. P. 194, 338, 353 Pochan J. M. 15, 52 Pohmer K. 316 Possart W. 161 Potapov E. E. 169, 192, 193 Potente H. 51 Prescott L. E. 54 Prokof’ev Y. A. 168, 183, 192, 193
R Rance D. G. 52, 54 Rangarajan V. 191 Rauline R. 209, 211 Reed T. F. 212 Rhee C. K. 160 Ridha R. A. 212 Riggs W. M. 52 Rijpkema B. 243, 256 Roach J. F. 212 Roberts A. D. 146, 160 Rodriquez G. 239 Roland C. M. 161, 239 Roseboom F. 194 Rowe D. 352
S Saito M. 355 Saito T. 53 Saito Y. 239 Sakharova E. V. 192, 193
368
Salych G. G. 192, 193 Samay G. 354 Sánchez-Adsuar M. S. 53 Sanderson C. 59, 120 Sarkar P. P. 354 Sartre A. 193, 210 Sasaki T. 53 Sato T. 353 Schaefer R. J. 211 Schlett V. 52 Schoenherr D. 51 Schonhorn H. 52 Schoon Th. G. F. 257 Schramm E. C. 52 Schürmann K. 121 Seibert R. F. 209, 212 Semak B. D. 195 Sen A. 354 Seo G. 194, 212 Setiawan L. 51 Sexsmith F. H. 51, 78 Sharma S. C. 186, 194 Shekhter V. E. 193 Shibata T. 353 Shieh C. H. 160, 161 Shields J. 54, 55 Shijian L. 212 Shoaf C. J. 256 Shofner D. L. 51 Shvarts A. G. 193 Shyu S. S. 54 Siverling C. E. 78 Sjothun I. J. 78 Skeist I. 51, 55, 283, 352 Skewis J. D. 161 Smilga V. P. 161 Snodgrass L. J. 193, 212 Snogren R. C. 55 Soldatos A. C. 160 Solomon T. S. 346, 354
Author Index Sommer F. 192 Song J. 194 Souchet J. C. 211 Sowell R. R. 54 Starinshak T. W. 212 Stohrer W. D. 52 Strain F. 211 Strobel M. 18, 51 Stubbs J. A. 353 Stuck B. 211 Subramanian V. 194 Sugimoto M. 160 Sutherland I. 52 Swanson M. J. 26, 54 Symes T. E. F. 20, 21, 22, 25, 27, 51, 53
T Takeyama T. 346, 354 Tarney R. E. 160 Tate P. E. R. 192, 206, 210, 211 Teets A. R. 239 Terashima K. 160 Tessier L. 193, 210 Thomas J. 51 Thornton J. S. 51 Todani Y. 354 Toesca S. 210 Tomanek A. 316 Tommasini F. 195, 212 Torregrosa-Maciá R. 53, 54 Torró-Palau A. 53 Touchet P. 239 True W. R. 212 Tsutsumi S. 353 Tutt M. J. 354
V Vakula V. L. 161 van der Aar C. P. J. 194
van der Mei H. C. 54 van Ooij W. J. 165, 166, 167, 169, 177, 180, 181, 185, 186, 191, 192, 193, 194, 195, 199, 200, 201, 204, 205, 209, 210, 212 Verbanc J. J. 160 Vincent M. 195, 212 Voyutskii S. S. 148, 161
W Waddell W. H. 183, 193, 211, 212 Wagner M. P. 206, 211 Wake W. C. 159, 161, 256, 352 Wallerswein S. 211 Walz G. 211 Walzak M. J. 18, 51 Watanabe 342 Watanabe A. 355 Watanabe K. 353 Watanabe M. 353 Watanabe N. 354 Watanabe T. 53 Watts J. F. 52 Weening W. E. 191, 209, 210, 243, 256, 354 Wegman R. F. 54 Weih M. A. 78 Weihe J. 51 Wenghoeffer H. M. 257 Westerdahl C. A. L. 52 Wheeler E. L. 212 Whelan A. 78 Wieczorek K. 317 Wightman J. P. 337, 353 Wiktorowicz R. 52 Wilson J. C. 193, 210 Wolff S. 211, 257 Wootton A. B. 51
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The Handbook of Rubber Bonding
X Xie H. 240
Y Yamaguchi K. 195 Yan H. 212 Yoshi F. 53 Yoshikawa M. 53 Yu D. Q. 354
Z Zaporozhchenko V. I. 194 Zhang J. Q. 194 Zierler L. 257
370
Company Index
A Abrasive Developments 42 Advanced Elastomer Systems 259
B Bayer 60
C
Lord Corporation 12, 57, 58, 61, 63, 77, 81
M Metallgesellscahft 61 Metalok 61 Michelin & Cie 209 Morton International 128
Chemical Inovations Limited (CIL) 61
N
D
Nagase Co. 246
Degussa AG 60, 257
E E. I. du Pont de Nemours and Company 160, 256, 259 Electro-Chemical Rubber & Manufacturing Company 78 EMS American Grilon 246
G
P Par Chemie 61 Pirelli 189 Proquitec 61
U Union Carbide Corporation 160 Uniroyal Chemical Company Inc. 212
Goodyear Tire and Rubbers (US) 212
V
H
Vulnax International Ltd. 246
Henkel KGaA 61, 81, 88 Honda Motor Company 42 Hughson Chemicals 81
W Wacker-Chemie GmbH 295, 311
L
Y
Loctite 283
Yamashita Rubber Company 42
371
The Handbook of Rubber Bonding
372
Main Index
A Abrasive wear 345 Accelerator 175, 176, 177 Accelerators 2-mercaptobenzothiazole 65 mercaptobenzothiazole disulphide 65 sulphenamide 183 tetramethyl thiuram disulphide 65 Acrylic acid 215 butyleneglycol dimethacrylate 214 metallic salts 215 Addition curing 294 Additive 164 Adherend surface conditions 335 surface roughness 336 Adhesion 76, 148, 312 braided wire 235 build-up 292 corrosion 350 curing system 251 duration of 296 environmental factors 349 fabric 235, 236 factors affecting 253, 352, 349 hardness 350 humidity 349 in service 254 interfacial 205 mechanisms 250, 252 monofilament 235 prevention 351 problems 339
retention 189 rubber-to-metal 224 surface irradiation 351 T-pull 237 temperature 349 test methods 76 theories 148 tyre cord 197 Adhesion of rubbers 334 rubber flow 334 stabilisation 335 wetting 334 Adhesion peel tests 224 Adhesion promoter 226, 230 internal 226, 230, 231 Adhesion test T-pull 236 three cord 348 Adhesive 25, 62 characteristics 62 cyanoacrylate 37 dispersions 234 epoxide 25 flow characteristics 336 heat reactive contact cements 37 hot melt reactive urethane prepolymers 37 polyurethane 156 primers 71 rubber to metal 62, 63 selection 66 silane treatments 37 tie layer 232
373
The Handbook of Rubber Bonding two-part epoxies 37 two-part urethanes 37 wetting 128 Adhesive application 69 brushing 69, 71 dipping 69, 70 spraying 69 tumbling 69 Adhesive bond strength 231 Adhesive systems 241 solvent-based 248 urethane 158 Alloys metal 59 ternary 189 Aluminium 326 anodising 12 Analysis 19, 20, 24 Fourier transform infrared 19, 23 infrared 20, 24 X-ray fluorescence 20, 25 Antidegradants antioxidants 66 antiozonants 66 prevulcanisation inhibitors 66 waxes 66 Application methods 98, 101, 117 brush 98 dip 98 electrostatic 98 flowcoat 98 reverse roller coat 98 roller 98 sponge 98 spray 98 Aramid 247 ASTM test methods 122 Autohesion 137, 139, 141, 144, 148, 150
374
B Bearing pads 102 Belt failure 341 abrasive wear 345 back cracking 342 cord failure 341 cord separation 344 fabric breakage 345 fabric separation 341, 342 joint failure 345 radial cracking 344 tensile failure 344 tooth root cracking 341 wear 341, 344 Belt life prediction 344 shear stress 344 Blooming 145, 146, 147, 151, 280, 282, 283, 349 Boiling water tests 107 Bond failure 74, 319 corrosion 319 metal preparation 319 moulding procedures 319 product abuse 319 product design 319 Bond formation 71, 144 Bond integrity 73 Bond strength 190 Bondability 64 index 64 Bonded parts 72 knit lines 72 splits 72 Bonding 29, 73, 152, 337 concepts 291 duration 311 interface 337 interphase 337
Main Index mechanical ties 292 post vulcanisation 36, 73, 100, 101 properties 309 rubber to brass 163 rubber to fabric 241 rubber to metal 319 rubber to plastic 29 silicone to metal 288 silicone to plastic 288 silicone to silicone 287, 308 solid rubber 299 strip 152 test procedures 76, 77 test specimen geometry 76 vulcanisation 37 Bonding agent 93, 98, 99 application methods 98 application to metals 325 applications 93 moisture-sensitive 114 photosensitive 115 prebake 118 preparation 97 re-certification 113, 114 solvent 103 testing 110 thickness 99 waterborne 103 Bonding concepts 291 primers 291, 292 self bonding silicone LR 291 self-bonding silicone rubbers 292 undercuts 291 Bonding mechanism rubber to metal 62 Bonding systems 63, 97, 98 application 129 in situ 249 metal preparation 129 one coat 63
organic solvent-based 127 primer/cover-coat 63 waterborne 98, 103, 127, 128, 129, 130, 131 disadvantages 134 factory usage 128 polymer 134 pre-bake resistance 133 primers 134 product range 134 thickness effects 131 Bonding techniques mechanical 314 Braid Nylon 236 Brass 59, 163 composition 164 electrodeposited 59 Brass adhesion 181 additives 181 boroacylate 181 cobalt-naphthenate 181 compounding 180 neodecanoate 181 silica 180 stearate 181 Brass plating 163 Bridge bearing pads 100 Bush components 102
C Cable connectors 314 Cable industry 309 Calcium hydroxide 142 Car emissions 197 reduction 197 Carbon black 141, 351 easy processing channel 140 high abrasion furnace black 147 Chain scission 141
375
The Handbook of Rubber Bonding Chemical treatments 37, 68 phosphatising 68 Chemisorption 63 Chloropyrimidines 208 Chlorotriazines 208 Cleaning abrasives 321 metal 321, 322 Coagents metallic 60 methacrylate ester 214 triallyl cyanurate 215 trimethylolpropane trimethacrylate 214, 215 Coated components assembly 102 Coatings primer 325, 326 rubber bond 325, 326 Cobalt salts 60, 174, 187, 189, 202, 207 boroacylate 202 metal organic 197, 201 naphthenate 164, 168, 202, 203 stearate 202 Coefficient of restitution 215 Cohesive failure 232 Compounding 59, 60, 223 characteristics 64 isopropyl azodicarboxylate 138 self-bonding 60 soft rubber 59 Compression moulding 299 Compression set 224, 310 Contact pressure 144 Contact temperature 144 Contact time 144 Contamination 118 Copper sulphide 178 Cords brass 202 heavy cabled 249
376
Corona discharge 26 Corona treatment 17, 19 after derivatisation 17 Corrosion 5, 6, 62, 68, 164, 177, 186, 321, 330 anodic 331 by overheating 332 cathodic 331 chemical 332 electrochemical 330 galvanic 5, 74, 332 inhibitors 62, 205 resistance 68 underbond 69 Cotton 244 Coumarone-indene 141 Coupling agents 186, 206, 213 titanate 213 zirconate 213 Cover coat 38 Covulcanisation 140 Crosslinking 62, 141, 329 agents 61 peroxide 213, 216 polysulphide 216 Cryogenic techniques 178 Cryoblasting 27 deflashing 118, 299 Curative 234 peroxide 234 sulphur 234 Cure characteristics 224 Cure rate 224 Curing 223 peroxide 235 Curing times 295, 305 Cyanoacrylate 37, 260 acidic substrates 264 activators 274 adhesives 263
Main Index application methods 275, 277 applications blooming 280, 282, 284 bond line thickness 268 bonding EPDM 277 bonding medical devices 279 bonding natural rubbers 277 bonding nitrile 277 bonding polychloroprene 277 bonding Santoprene 279 bonding silicone rubbers 279 bonding to silicone rubber 270 cleavage loads 267 design considerations 266 environmental resistance 270 excess adhesive 281 external release agents 269 glass bonding 272 glazed appearance 284 health and safety 276 heavy molecular weight 282 hot strength 272 internal release agents 269 joint design 269 liquid 259 low strength 284 no cure 284 peel loads 267 poor adhesion 284 porous substrates 264 pressure systems 275 relative humidity 262, 281 slow cure 281 special requirements 269 syringe systems 276 time systems 275 toughened 265 Cyanoacrylate, curing 260, 261 acidic deposits 263 adhesive 262
cure speed 263 relative humidity 262 temperature 263 volume 262 Cyclohexyl benzothiazole sulphenamide 170
D Debonding 200 Deflashing 73, 118 cryogenic 118, 299 non-cryogenic 73 Degreasing 35, 37 aqueous 35 Delamination 344 Demoulding 119 Dezincification 164, 167, 184, 201, 204 Drying 102, 128, 129
E Ebonite 58, 59 Electrochemical corrosion 200 Electroplating 73 Engine mounts 57, 100, 102 car 319 fluid 57 Environmental aspects 254 of processing 255 pollution limits 127 storage and handling 254 wastes and disposal 255 Ethylene propylene diene rubber 64
F Fabric breakage 345 Fabric separation 348 Fabric treatments 241 aqueous 241
377
The Handbook of Rubber Bonding Fabrics cotton-based 250 Failure 61, 74, 145, 319 bond 235 cement/metal 74 cement/primer 75 cohesive 61, 158, 236 cord separation 344 interfacial 145, 158 rubber 74 rubber/cement 74 Fatigue life 320 Fibre reinforcement 339 Fillers 65, 140 carbon black 65 channel blacks 65 silica 65 Film interfacial 178, 179, 182 Film thickness 101, 106 Fixing. See Adhesion Flocculation 127, 128, 327 Formaldehyde 346 Formulation preparation 222
G Galvanic cell 350 galvanising. See also Zinc coating 11 Gibbs free energy 149 Green strength 137, 138, 148 Green tyres 197, 206 Grit blasting 9 34, 37, 321
H Hardness 224 Shore A 224 Health and safety 109 inorganic lead salts 110 solvents 110
378
Hexamethoxymethylmelamine 206, 207 Hexamethylene tetramine 205, 207 High abrasion furnace black 140 HTV silicones 300 properties 300 Humidity 147 Hydrogenated nitrile rubber 214
I Ignition cables 309 Infra red analysis attenuated total reflection 182 Injection moulding 313 hot runners 307 plastic substrates 306 two colour mould 306 Instruments surface tension pens 128 Instruments, measuring 71 beta backscatter 71 dry film 71 magnetic induction current 71 Interdiffusion 148, 149 Interface 62, 205 dip/rubber 250 dip/textile 252 primer to adhesive 63 primer to metal 62 rubber–metal 205 Internal mixer 223 Banbury 223 Ionic bonds 216 Ionomers 216 Isobutylene-isoprene (butyl) rubber 64 Isocyanate 248, 252
J Joint 147 butt 147
Main Index peeling 147 Joint failure 345
K Keypads silicone 309
L Latex 346 Layover 106, 119 Leaking moulds 118 Liquid rubber 227 bonding 293 silicones 300 Liquid squalene 168
M Maintenance spray equipment 70 Martensite 11 surface 323 Mass spectrometry 165 secondary ion 168 secondary neutral 168 time-of-flight secondary ion 165 Mechanical bonding clamping 310 undercuts 310 Metal 4, 6, 8, 10, 67, 129 activation 67 adhesion 229 alloys 3 carbon black 351 chemical pre-treatment 6, 10 cleaning 129 complexes 208 crystal size 350 degreasing 7
heat treatment 328 mechanical pre-treatment 6, 7 oxide layers 4 pre-treatments 6 preparation 3 removal of grease 8 removal of oil 8 silica 351 smutting 4 substrates 3 surface tension 350 Metal preparation 117 anodising 322 chemical modification 322 faulty 321 plating 322 sheradising 324 Metallic coagents 213, 215, 216 Saret 216 Metallography 3 Methacrylic acid 215 metallic salts 215 Microscopy 19, 198 analytical electron 198 electron 33 scanning electron 19, 168, 202, 208 transmission electron 168, 169 Mill 223 two-roll 223 Modulus 224 Montreal Protocol 126 Mould design 328 Mould release agents 269 external 269 internal 269 Moulding 30, 33, 35, 72 compression 30 extrusion blow 30 high pressure 35 high temperature 33
379
The Handbook of Rubber Bonding incorrect procedures 328 injection 30 operation 118 reaction injection 30 transfer 30 Moulding operation 118 Moulds 72 bonding process 72 designing 72
N Natural rubber 64, 138 Natural rubber or polyisoprene Nitrile rubber 64 Novolak resin 242, 243 Nylons 288
O Organic resins 62 Oscillating-disk rheometer 224 Oxide films 7 Oxides surface 9 Ozone exposure 142
P Paints high solvent 73 Peaning 9 Peel energy 145 Peel strengths 21, 23, 24, 26, 28, 157, 237 Peel test 110, 148, 237, 296, 298, 300, 348 T-peel 348 Phenol-formaldehyde 141 Phosphate treatment 68, 94, 95 coating 10, 323
380
Physical tests 224 Plasticisers 141 reactive super 213 Plastics 12, 13, 31, 40, 41 pre-treatment 12, 13 primers 37 substrate preparation 31 Platen temperatures 119 Polyamide 244 Polybutadiene rubber 138 maleated liquid 141 Polybutylene terephthalate 288 Polycarbonates 32 Polychloroprene 64 Polydimethylsiloxane 293 Polyester 245, 246, 248 textiles 248 Polyethylene 33 Polyethylene terephthalate 245, 288 Polyisocyanate 60, 154 Polyisocyanates Polymer micelles 104 Polymers 62 halogenated 62 polyphenylene oxide 32 solvation 214 Polyphenylene sulphide 32, 288 Polypropylene 33 Polysulphides oligomeric 168 Polyureas 32 Polyurethanes 27, 32 Post vulcanisation bonding 102 cure cycle 102 partial 320 Power transmission belts 339 adhesion systems 346 adhesion testing 347 belt life 341, 346 bonding 339 conveyor belts 345
Main Index failure 340 synchronous 340 V-belts 343, 345 Pre-treatments abrasion 13 chemical 68 chlorination 73 corona 13, 28 etching 28 flame treatment 13, 28 metal 69 phosphatising 68 plasma 28 sodium complexes 13 solvent wipe 13 trichloroisocyanuric acid 13 Prebake resistance 99, 106 Preparation methods substrate 3 Primers 62, 321 application 116 rubber to metal 62 Processing oils 66, 141 aromatic-based 66 ester-based 66 naphthenic 66 Processing techniques 303 inserted parts 303 Product abuse 333 high temperatures 333 mechanical interference 333 oils and solvents 333 service loading 333 Product design effects on bond failure 320 Profile carbon depth 180 Properties interphase 338 Pull out tests 347, 348
Q Quality testing 329
R Rayon 244 Re-certification period 114 Reactive dispersion 230, 235 Recycling 190 Reinforcing material 197 Relative tack 141 Release agent 329 accidental application 329 Resins 205 one component 207 Resorcinol 205, 346 RFL adhesive system See also Resorcinol 60, 241, 244, 246 dip 248 formulation 242 novolak resin 243 preparation 242 Rubber 6, 12, 20, 21, 25, 26, 27, 28, 29, 138 butyl 20 conventional 41 ethylene-propylene 19, 28, 138 ethylene propylene diene 138 fully cured 159 halogenated 25 hydrocarbon 19 liquid 225 natural 20 nitrile 26 peroxide cured 65 pre-treatments 12 shrinkage 6 silicone 26 solid 221 strain crystallising 138
381
The Handbook of Rubber Bonding sulphur cured 65 thermoplastic 156 unsaturated hydrocarbon 20, 28 unvulcanised 150 vulcanized 150, 152 Rubber classification wettability 336 Rubber retention 106, 107, 109 Rubber to brass bonding 198 ageing 177, 200 degradation 200 mechanisms 198 organofunctional silanes 185 Rubber to fabric bonding 247 conveyor belting 247 hose 247 V-belt cord 247 Rubber to metal bonding 205 interface 205 Rubber-to-rubber bonding ageing 146 blooming 145 contact time 144, 159 effects of surface modification 142 filler 140 ozone exposure 142 plasticisers 141 polymeric additives 156 polymers 156 pressure 144 process oils 141 strip thickness 155 surface bloom 159 surface roughness 144, 156 surface treatments 158 tackifiers 141 temperature 144, 156 vulcanisation 139 Rust preventation 73
382
S Safety data sheet 109 Scorch safety 217, 219 Scorch time 224 Sealants 225 EPDM 227 formulation 225, 226 natural rubber 225 polybutadiene rubber 225 polybutadiene glass 229 pumpable 225, 226, 228 Self-bonding HTV applications 301 peroxide curing 301 Self-bonding LR 289 limitations 298 properties 297 Shear adhesion 227 Shelf life 112, 113 Sheradising zinc 324 Shoaf system 245 Shrinkage 339 Silanes 186, 188, 208 polysulphur 187 Silica 141, 206, 207, 351 Silica/resin systems 205 Silicone HTV applications 290 Silicone LR applications 290 Silicone roller 301 Silicone rubber composites 290 applications 290 Silicone rubbers 285 bonding 286 high temperature vulcanising 285 liquid rubber 285 primer 287 processing conditions 308
Main Index undercuts 286 Solvent 126 cleaning 35 dip 8 elimination 126 Spectroscopy 14, 19, 183, 198 auger electron 198, 200, 207, 208, 338 electron energy loss 169 electron for chemical analysis 14 ion scattering 338 proton induced X-ray emission 208 reflection infrared 14 Secondary Ion 338 secondary ion mass 198 x-ray photoelectron 14, 19, 23, 183, 198, 208 Spraying equipment 70, 105 Sputter etching 19 Squalane 171, 175 Squalene 170, 171, 175, 184 Stabilizers 62 viscosity 62 Stainless steel 10 pre-treatment 10 treatments 322 Stearic acid 176 Steel cords 182, 184, 197 brass plated 201 brass-coated 197 Storage stability 130 Strength adhesive 165 interfacial 165, 167 Stress cracking 35 Stresses at interface 320 Styrene butadiene rubber 64, 139 Substrate 3, 69 preparation 101
rigid plastics 69 topography 335 Sulphidation 175, 199 Sulphide copper 165 interfacial 165 interfacial film 177 zinc 165, 174 Sulphurating species 171 Surface analysis 19 Surface primers 270 Suspension bushes 100 Sweep tests 111
T T-peel test 23 Tack 137, 141, 144, 145, 147, 148, 150 relative 137 Tackifiers 141 Tear resistance 220, 221, 315 Temperatures melting transition 305 Tensile elongation 224 modulus 224 properties 219 strength 108, 219, 220, 224 testing 148, 224 Testing 69, 147 autohesion levels 147 dog bone shape 296 tack 147 Test specimens 296 water break 69 Textiles 248 treated 253 Thermal ageing 177, 200 Threshold limit values 110
383
The Handbook of Rubber Bonding Tie coat layer 230, 233 cements 326 Top coat 38 preparation 117 Torque values 224 Treatment 34 abrasive belts 34 acid etching 34 alkali etching 34 chlorination 35 corona discharge 35 high pressure water 34 hydrosonic/ultrasonic cleaning 34 mechanical 68 oxidation 34 phenol 34 plasma treatments 35 satinisation 34 UV treatments 35 Tyre cords 163, 178, 179, 187, 189, 198 coating 191 Tyres 150 precured retreading 150 radial 163 reinforcement 197 retreading 152, 154
U Unvulcanised rubbers bonding 137
V V-belts 249 cut-edge 249 Viscosity 97, 115, 130 reduction 213 sensitivity 114 viscometry 114 Volatile organic compounds 62
384
Vulcanisation 30, 139, 213, 329, 330 autoclave 30 bonding 30 efficient 65 peroxide 213 semi-efficient 65 sulphur 219 system 65
W Waterborne bonding 12 metal preparation 12 Welding 313, 314 Wet blast process 42, 43, 47 degreasing agent 43 phosphate treatment 45 phosphating plant 42 VAQUA pump 43, 44 Wood rosin 141
X X-ray analysis energy dispersive 19, 168, 169, 183 wavelength dispersive 169 X-ray emission particle induced 183
Z ZDA scorch retarded 218 ZDMA scorch retarded 218 Zinc coating 11, 323 Zinc diacrylate 60 Zinc dimethacrylate 60, 214 Zinc sheradising 11 Zirconates 214 neoalkoxy 214
