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
—
J/cm2)
XPS O:C atomic ratio
0.0
0.5
0.12
Corona (0.17 J/cm2)
0.05
0.07
Flame
0.04
0.12
Remote air plasma*
0.1
0.12
Corona (1.7
Ozone
1800
0.13
UV/air
600
0.08
UV/air plus ozone
600
0.14
*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
39
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
41
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.
43
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,
45
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
300
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
180
Phosphate coating
2nd phosphating
180
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.
47
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.
49
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
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51
The Handbook of Rubber Bonding
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Substrate Preparation Methods
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