Table Of ContentPhysical Testing of Rubber
Physical Testing
of Rubber
Third Edition
R.P. Brown
Technical Manager, RAPRA Technology Ltd,
Shawbury, Shrewsbury, UK
Springer-Science+Business Media, B.V.
© R.P. Brown 1996
Originally published by Chapman and Hall in 1996
Softcover reprint of the hardcover 3rd edition 1996
Typeset in 10/12 Times by Best-set Typesetter Ltd, Hong Kong
ISBN 978-94-010-4235-2 ISBN 978-94-011-0529-3 (eBook)
DOI 10.1007/978-94-011-0529-3
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A catalogue record for this book is available from the British Library
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Contents
Preface ix
1 Introduction 1
2 General considerations 6
2.1 Philosophy 6
2.2 Test conditions 10
2.3 Limitations of results - statistics 11
2.4 Sampling 12
2.5 Quality control 14
2.6 Test equipment 17
2.7 Product testing 19
3 Standards and standards organizations 22
3.1 Standards - test methods and specifications 22
3.2 Organizations producing standards 25
3.3 Units 34
4 Preparation of test pieces 35
4.1 Mixing and moulding 35
4.2 Cutting from sheet 38
4.3 Test pieces from finished products 40
5 Conditioning and test atmospheres 44
5.1 Storage 44
5.2 Conditioning 45
5.3 Test conditions 47
5.4 Apparatus for conditioning 48
5.5 Mechanical conditioning 51
6 Tests on unvulcanized rubbers 55
6.1 Standard methods for particular polymers 56
6.2 Sample preparation 56
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[-vi] CONTENTS
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6.3 Viscoelastic flow behaviour 57
6.4 Scorch and cure rate 71
6.5 Tack 76
6.6 Other tests 77
7 Density and dimensions 82
7.1 Density 82
7.2 Dimensions 85
8 Short-term stress and strain properties 92
8.1 Stress/strain relationships 92
8.2 Poisson's ratio 97
8.3 Data for finite element analysis 97
8.4 Hardness 98
8.S Tensile stress/strain 109
8.6 Compression stress/strain 125
8.7 Shear stress/strain 129
8.8 Flexural (bending) stress/strain 131
8.9 Tear tests 132
9 Dynamic stress and strain 144
9.1 Rebound resilience 150
9.2 Free vibration methods 155
9.3 Forced vibration methods 160
10 Friction and wear 168
10.1 Friction 168
10.2 Wear 174
11 Creep, relaxation and set 187
11.1 Creep 188
11. 2 Stress relaxation 190
11.3 Set 197
12 Fatigue 203
12.1 Flex-cracking and cut-growth tests 204
12.2 Heat build-up 210
13 Electrical tests 214
13.1 Resistance and resistivity 214
13.2 Surface charge 222
13.3 Electric strength 223
13.4 Tracking resistance 224
13.5 Permittivity and power factor 224
14 Thermal properties 228
14.1 Thermal analysis 228
14.2 Specific heat 229
14.3 Thermal conductivity and diffusivity 230
14.4 Surface heat transfer coefficient 233
15 Effect of temperature 235
15.1 Thermal expansion 235
15.2 Transition temperature 237
15.3 Low-temperature tests 238
15.4 Heat ageing 247
16 Environmental resistance 259
16.1 Moist heat and steam tests 259
16.2 Effect of liquids 260
16.3 Effect of gases 267
16.4 Weathering 279
16.5 Biological attack 281
16.6 Fire 282
16.7 Radiation 283
17 Permeability 287
17.1 Gas permeability 288
17.2 Vapour permeability 294
18 Adhesion, corrosion and staining 299
18.1 Adhesion to metals 299
18.2 Adhesion to fabrics 305
18.3 Adhesion to cord 309
18.4 Corrosion of, and adhesion to, metals 310
18.5 Staining 311
Appendix A: National standards bodies (ISO members) 317
Appendix B: Thermal equilibrium times for non-ambient testing 326
Index 339
Preface
Knowledge of the physical properties of materials is essential for design,
specification and quality control. The particular nature of rubbers de
mands that specific test procedures are used for many of the properties
and the large number of national and international standards which have
been produced for rubbers bears witness to the importance of the subject
to industry.
A text devoted to the physical testing of rubbers written by RAPRA
staff first appeared in 1965 with the publication of the work of the late Dr
J.R. Scott. The first edition of my own book came in 1979 and the
second, and now this third, edition reflects the continuing technical devel
opments. There have been many changes in the methods used and
especially in the instrumentation as more modern technologies are adopted
by instrument manufacturers and the requirements of industry become
more sophisticated. The aim of this work is to present a comprehensive,
up-to-date account of the procedures used by suppliers and users to
characterize and investigate the physical properties of rubber materials.
The book collates the many standard methods, comments on their
virtues and defects and considers procedures needed for both quality
control and the generation of design data. The content owes much to the
experience gained due to RAPRA's position over many decades as an
international centre for rubber research, as a test house with a history of
developing test procedures and making a very significant contribution to
national and international standardization.
The book is primarily intended as a reference for those directly con
cerned with testing rubbers, whether it be for quality control, evaluation
of products, production of design data or research, and for students of
rubber technology. However, it is believed that it will also be of value to
those indirectly involved in testing such as design engineers and technical
specifiers.
R.P. Brown
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______In_ t_r_O_d_u_ct_i_o_n _____
The physical properties of materials are of critical importance for the
design, production and quality control of products. Consequently, it is
not surprising that a whole spectrum of test methods have been devised to
measure these properties. Whilst many features of testing materials are
common to all materials, the particular characteristics and uses of each
group of materials, such as metals, ceramics, polymers, etc., have provided
good reasons why each group has developed its own procedures. That
is not to say that there are also bad reasons, such as insularity, and
that there is not room for greater cooperation and hence unification of
methods.
Rubbers can claim a particularly strong case for needing their own test
methods, being complex materials exhibiting a unique combination of
properties, whilst a virtually infinite number of rubber compounds is
possible. They differ very considerably from other engineering materials;
e.g. they are the most deformable materials, exhibiting almost complete
recovery and are virtually incompressible with a bulk modulus some
thousand times greater than shear or Young's modulus. For the design
engineer particularly, it is important that such properties are measured
and understood. The fact that so many variations in compounds, and
hence properties, are possible simply means that standard grades hardly
exist and one must evaluate every rubber compound which is met with.
The basic structure of rubbers and their sensitivity to small compounding
or processing changes means that they are prone to unintended variations
in properties from batch to batch and present the processor with a
difficult quality control problem.
Hence, it is not surprising that with such unusual and complicated
materials the procedures used for measuring their physical properties
often differ markedly from procedures used for other materials. Methods
and philosophies taken from other materials cannot be simply transferred
if meaningful results are to be obtained so that there is a particular
technology of rubber testing. Over the years an enormous effort has been
put into developing satisfactory procedures both for quality control and
~ __2_ _~ 1 I_N_T_R_O_D__V _C_ T_IO_N_
LI_ ___________________
for providing design data, but particularly from the design aspect many
procedures remain painfully inadequate. The difficulty of formulating
meaningful test procedures for rubbers is due to a number of reasons,
some of which are general to testing materials, but most because of the
rubber's intrinsic properties.
The aim of this book is to present an up-to-date account of rubber
testing procedures. It intends to be comprehensive in covering all of the
tests in common, and sometimes not so common, use. Inevitably the bulk
of methods are the standard ones, often somewhat arbitrary and primarily
intended for quality assurance purposes, but in each case the require
ments for obtaining meaningful design data are discussed.
Standard tests have the unfortunate habit of not being standard, in
that different countries and different organizations each have their own
'standards'. Fortunately this tendency has very much diminished in recent
times as more countries adopt the international methods. It is perhaps
appropriate here to make a plea for the adoption of recognized standards
without modification when there is really no strong technical reason for
change. It goes without saying that this makes for efficiency because, if
we all use the same, well-documented method, silly disputes due to the
effects of apparently minor differences will be lessened. The principal
standard methods discussed in this book are those of the International
Standards Organization (ISO). Less emphasis is placed on the various
national bodies than was the case in earlier works, reflecting the increased
importance of ISO or rather the increased tendency for national methods
to be aligned with ISO. However, the equivalent British Standards
Institute (BSI) methods are referenced in each case and also for many
properties those of the American Society for Testing and Materials
(ASTM). Most British standards are now dual numbered so that the
national standard is verbatim the same as the ISO standard. It is a great
pity that more national standards bodies do not follow this practice. In
Europe the intention is to align all the national standards by producing
European (CEN) standards but at the time of writing this process has not
been carried through for rubbers. Hopefully, CEN standards for rubbers
when they are produced will in fact be copies of ISO methods. Generally,
test methods peculiar to particular commercial companies have not been
considered at all. An interesting survey of the use of various national
standards in Europe was undertaken by Gorton, Brown and Cropper
[1,2].
It is inevitable that between writing and publication there will have
been new editions of standards produced. To counteract this as far as
possible the likely trends in test methods have been estimated from
the current drafts in circulation and the known activities of relevant
committees.
The layout of subject matter in a book on testing is inevitably to some
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c
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INTRODUCTION 3
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extent subjective, as is the dividing line between what to include and what
to omit. Firstly, there is bound to be some overlap between rubbers and
plastics and indeed in my own laboratory no real distinction is made, both
groups of polymers being tested side by side. In this context it is useful to
refer to a book on the testing of plastics [3] which serves something of
a complementary function and perhaps one should be considering both
when testing flexible plastics. There is in particular the question of ther
moplastic rubbers. It is believed that in general thermoplastic elastomers
can best be tested by the methods used for vulcanized rubbers, which is in
line with the conclusions drawn in a paper considering the particular
testing requirements of the thermoplastic materials [4]. Most significantly
this agrees with the action taken by ISO Technical Committee TC45 who
have decided to add thermoplastic to the scope of rubber test methods
wherever the method is thought suitable. Although thermoplastic ela
stomers are not specifically mentioned in each chapter, with few excep
tions it is assumed that both vulcanized and thermoplastic are covered.
Cellular rubbers have been deliberately omitted as it is my belief that
this very distinct class of materials should be treated separately, both
rubbers and plastics being considered together. Similarly, tests on latex
have also been omitted although products made directly from latex, by
dipping, for example, will have many properties tested in the same way as
for those formed from solid rubber. Ebonite has not been included as it
has been accepted by ISO TC45 and TC61 that it should be treated as
a thermosetting plastic. Some tests on simple composites have been
included, e.g. rubber/metal and rubber fabric, although the majority
of tests on coated fabrics have not been considered as, once again,
this particular product type can be considered as a special subject in its
own right.
Comment is made, as appropriate, about testing finished products but
a separate chapter on this has not been written, simply because such
procedures are too specialized for general treatment. It is apparent,
however, that increasingly specifications include tests on the complete
product as in many cases this is the best or the only way of being sure that
the product will perform satisfactorily. A short discussion of when to test
products is given in Chapter 2.
I have lost no sleep in debating what is physical - if popular opinion
treats tests as part of the physical spectrum (e.g. ageing tests) then they
are physical. Not surprisingly, chemical analysis is excluded but it can
be noted that the thermal analysis techniques straddle both camps and
they have been included or excluded depending on their purpose. The
intention has been to include every type of physical test and, hopefully,
this has been achieved in the main. However, three areas immediately
come to mind which do not have their own section, acoustic properties,
optical properties and non-destructive testing (NDT).
