RUBBER

PRODUCTS
MANUFACTURING
TECHNOLOGY
RUBBER
PRODUCTS
MANUmCTURING
TECHNOLOGY
EDITED BY
ANIL K. BHOWMICK
Indian Institute o f Technology
Kharagpur, India

MALCOLM M. HALL
Consultant
Shrewsbury, England

HENRY A. BENAREY
Industrial Engineering Corporation
Bonita Springs, Florida

MARCEL

Marcel Dekker, Inc . N ew York •Basel

Library of Congress Cataloging-in-Publication Data

Rubber products manufacturing technology I edited by Anil K. Bhowmick, Malcolm M. Hall,

Henry A. Benarey.
p. em .
Includes bibliographical references and index.
ISBN 0-8247-9112-6 (acid-free paper)
1. Rubber industry and trade. I. Bhowmick, Anil K. II. Hall, Malcolm M. III. Benarey,
Henry A.
TS1890.R84 1994
678' .2--dc20 93-43397
CIP

Values, data, opinions, and information contained in this book are not guaranteed for accuracy
or for freedom from errors or omissions. They are not to be used directly for design criteria,
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tion, write to Special Sales/Professional Marketing at the address below.

Copyright © 1994 by Marcel Dekker, Inc. All Rights Reserved.

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 To

A sm it a n d Kutnkum
 Preface

The last few decades have witnessed truly explosive development and growth in rubber
products manufacturing technology. For example, a tire can be made today in a few
minutes using sophisticated machinery without much manpower. Those who are involved
in teaching, research, or business find that there is a dearth o f immediate information
on this subject. This book draws together and systematizes the body o f information
available and presents it in as logical a fashion as possible.
This book covers the compounding, mixing, calendering, extrusion, vulcanization,
and manufacturing technology of a few important rubber products. Since most modem
machinery, whether used in the mixing room or in the forming and shaping o f products,
and other modem manufacturing technology is self-controlling and monitored by com­
puters, the use o f computer-aided design and manufacturing, automation, and micropro­
cessor has also been highlighted. Manufacturing process control is today’s tool to com­
bat excessive costs. Since a very large number o f products are made using mbber, it
was impossible to describe every form o f manufacturing technology. However, the ba­
sic principles o f manufacturing technology may be learned from this book. In a few cas­
es, multiple authors were invited to contribute, to stress the importance o f a subject.
This volume contains material o f interest to both scientists and technologists.
The various developments are described by experts in the field, most o f whom have
extensive industrial or working experience. Their contributions are expected to be o f im­
mediate relevance to those concerned with the applications o f mbber. The editors are
grateful to the contributors for the time and effort they were able to devote to this book.
During the preparation of the book, we received help from a large number o f peo­
ple from both industry and academics. We thank them all. In particular, we are thank­
ful to Prof. K. L. Chopra, Director, IIT Kharagpur, and the faculty members and stu­
dents o f the Rubber Technology Center, IIT Kharagpur; Prof. Takashi Inoue, Tokyo
Institute o f Technology, Tokyo; Dr. E. Maekawa, Bando Chemical Company, Kobe;
VI Preface

Dr. K. Harada, JSR Company, Ltd.; and Dr. D. Banerjee, Mr. A. N. Bhattacharya,
Mr. J. Chatterjee, and Dr. S. N. Chakravarty—all from the Indian Rubber Instimte. We
also thank various companies, authors, editors, and journals for permission to use dia­
grams and photographs from published sources and for other necessary assistance. We
have acknowledged them in appropriate places in this book. If we have missed anyone,
the slip is unintentional. Finally, we thank Mr. and Ms. J. M. Bhowmick and Dr. and
Ms. S. K. Biswas for helping with the manuscript.

Anil K. Bhowmick
Malcolm M. Hall
Henry A. Benarey
 Contents

Preface V
Contributors xi

1 Compound Design 1
A. D, Thom and R. A. Robinson

2 Mixing Technology 103

Peter S. Johnson

3 Mixing Machinery for the Rubber Industry 123

Jürgen W, Pohl and Andreas Limper

4 Calendering Technology 179

Gerd Capelle

5 Extrusion and Extrusion Machinery 267

Michael /. Iddon

6 Vulcanization and Curing Techniques 315

Anil K. Bhowmick and D. Mangaraj

7 Computer-Aided Mold Design 397

M. J. Falconer-Flint

8 Automation and Control in the Rubber Industry 407

D avid M. Ortoli
vii
vin Contents

9 Improving Rubber Testing with Microcomputers 419

Henry Pawlowski

10 Computer-Integrated Manufacturing (CIM) 439

Craig A. Wolf

11 Rubber-to-Metal Bonding 449

F. H. Sexsmith

12 Coated Fabrics 473

B. Dutta

13 Computer-Aided Rubber Product Design 503

Norihiro Shimizu

14 Tire Compound Development 515

W. W. Barbin

15 Developments in Tire Technology 533

Raouf A. Ridha and Walter W. Curtiss

16 Conveyor Belt Technology 565

Gerry Murphy

17 V-Belt and Fan Belt Manufacturing Technology 593

Minoru Fukuda, Tsutomu Shioyama, and Yoshiyuki Mikami

18 Hose Technology 651

John D. Smith

19 Cable Technology 671

Achintya K. Sen

20 Vibration Isolators and Mounts 687

Sadhan Dasgupta

21 Rubber in Automotive Applications 705

R. P. Salisbury

22 Oil-Resistant Rubber Material for Automotive Hoses 711

Tadaoki Okumoto

23 Rubber-Covered Rolls 731

B. Dutta

24 Footwear Technology 757

P. B. Ghosh Dastidar
Contents ix

25 Sealing Technology 771

D. L. Hertz

26 Hard Rubber Compounds 795

Dennis L. Cooper

27 Rubber Sports Goods Manufacturing Technology 801

R. C. Haines and P. J. Corish

28 Latex Product Manufacturing Technology 823

Tony Gorton

29 Foam Products Manufacturing Technology 845

E, V, Thomas

30 Pressure Sensitive Adhesive Tape Technology 855

Richard E. Bennett

31 Just-In-Time and the Rubber Industry 867

D avid C. Barker

32 Waste Disposal 889

Fred W. Barlow

Index 903
 Contributors

W. W. Barbin Goodyear Technical Center, Akron, Ohio

David C. Barker DCB Associates, Shropshire, England

Fred W. Barlow Consultant, Stow, Ohio
Richard E. Bennett 3M Company, St. Paul, Minnesota
Anil K. Bhowmick Indian Institute o f Technology, Kharagpur, India
Gerd Capelle Hermann Berstorff Maschinenbau GmbH, Hanover, Germany
Dennis L. Cooper Tulip Corporation, Niagara Falls, New York
P. J. Corish P. J. Corish & Associates, Snitterfield, England
Walter W. Curtiss Goodyear Technical Center, Akron, Ohio

B. Dutta Bengal Waterproof Limited, Calcutta, India

Sadhan Dasgupta Polybond India Private Limited, Pune, India
M. J. Falconer-Flint Anchor Chemical Australia, Crows Nest, Australia
Minoru Fukuda Bando Chemical Industries, Ltd.,' Kobe, Japan
P. B. Ghosh Dastidar Bata India Ltd., Batanagar, India
Tony Gorton Advanced Latex Consultancy Services Ltd., Hertford, England
R. C. Haines Dunlop Slazenger International, Ltd., Wakefield, England
D. L. Hertz Seals Eastern Inc., Red Bank, New Jersey

XI
XII Contributors

Michael I. Iddon Iddon Brothers Limited, Leylcmd, England

Peter S. Johnson GenCorp Automotive, Wabash, Indiana

Andreas Limper Werner & Pfleiderer Gummitechnik GmbH, Freudenberg, Germany

D. Mangaraj Battelle Memorial Institute, Columbus, Ohio
Yoshiyuki Mikami Bando Chemical Industries, L td., Kobe, Japan
Gerry Murphy BTR Belting Ltd., Lancashire, England
Tadaoki Okumoto Toyoda Gosei Company Ltd., Inazawa-shi, Japan
David M. Ortoli Pirelli Armstrong Tire Corporation, New Haven, Connecticut
Henry Pawlowski Monsanto Instruments & Equipment, Akron, Ohio
Jürgen W. Pohl Werner & I^eiderer Gummitechnik GmbH, Freudenberg, Germany

Raouf A. Ridha Goodyear Technical Center, Akron, Ohio

R. A. Robinson Rapra Technology Limited, Shrewsbury, England

R. P. Salisbury Chrysler Corporation, Detroit, Michigan

Achintya K. Sen Fort Gloster Industries Limited, Howrah, India

F. H. Sexsmith Lord Corporation, Erie, Pennsylvania
Norihiro Shimizu Denki Kagaku Kogyo Company, Ltd., Tokyo, Japan
Tsutomu Shioyama Bando Chemical Industries, Ltd., Kobe, Japan
John D. Smith BTR Hose Limited, Lancashire, England
E. V. Thomas The Rubber Board, Kottayam, India
A. D. Thorn Rapra Technology Limited, Shrewsbury, England
Craig A. Wolf Measurex System Inc., Norcross, Georgia
 1
Compound Design

A. D. Thorn and R. A. Robinson

Rapra Technology Limited
Shrewsbury, England

People are frequently surprised to learn that there are many different types o f rubber,
and that rubber articles are not always black. Confusion sets in when their request for a
typical value for property x is met with the answer that no single value can be given
because the property is dependent on the precise composition o f the rubber compound.
Their confusion is complete when they are shown a typical rubber formulation consist­
ing of up to ten ingredients and told that one, present at a level o f 0.1% by weight, is
absolutely critical to service performance. Rubber technologists are therefore regarded
as practitioners o f a black art—a situation, one suspects, they are not really averse to.
Rubber technology is not a black art and it is hoped that this chapter, by describing
the different types of rubber available, the compounding ingredients used, and how a
rubber compound is designed, will help to dispel the myth. The reader seeking more
information is directed to some basic texts for more details [1-4].

1.1 C O M M E R C IA LLY A V A IL A B L E R U B B E R S
1.1.1 Natural Rubber
Designation in ISO 1629: NR
Repeat Unit

ÇH 3

C H j— C = C H — C H j
 2 Thorn and Robinson

General. Natural rubber can be isolated from more than 200 different species o f
plant; this includes some surprising examples such as dandelions. Only one tree source,
Hevea brasiliensis, is however, commercially significant. Latex, an aqueous colloid o f
the rubber, is obtained from the tree by “tapping” into the inner bark and collecting the
latex in cups. The latex typically contains 30-40% dry rubber by weight, and 10-20%
o f the collected latex is concentrated by creaming, or centrifuging, and used in the latex
form. Historically, such latex has been exported to consumer countries, but it is expen­
sive to ship a product with a high percentage o f water, and consumer companies are
increasingly siting their latex processing plants in the producer countries, where cheaper
labor is an additional incentive. Latex technology is considered to be beyond the scope
o f this chapter and the reader is referred to the “Bible” on this subject by Dr. Blackley
[5].
The remaining latex is processed into dry rubber as sheets, crepes, and bales. There
is an international standard for the quality and packing for natural rubber grades, the
so-called Green Book, published by the Rubber Manufacturers’ Association. The fol­
lowing “grades o f natural rubber listed in the Green Book are sold to visual inspection
standards only:

Ribbed smoke sheets

White and pale crepes
Estate brown crepes
Compo crepes
Thin brown crepes
Thick blanket crepes
Flat bark crepes
Pure smoked crepes

Under each category there are generally up to five divisions (e.g., IRSS, 2RSS, 3RSS,
4RSS, 5RSS for ribbed smoked sheets); the higher the number, the lower the quality.
The Malaysian rubber industry has, however, played a pioneering role in producing
natural rubber grades to technical specifications, and this system is being followed by
other producer countries. Currently the following technically specified grades are sold
by the producing countries:

SMR Standard Malaysian Rubber

SIR Standard Indonesian Rubber
SSR Specified Singapore Rubber
SLR Standard Lanka Rubber
TTR Thai Tested Rubber
NSR Nigerian Standard Rubber

Table 1 lists the SMR technical specifications.

Natural rubber is m -l,4-polyisopren e, o f molecular weight 200,000-500,000, but
it also .contains a small level o f highly important nonrubber constituents. O f these, the
most important are the proteins, sugars, and fatty acids, which are antioxidants and
 O
o
3
“O
o
Table 1 SMR Specification Scheme: Mandatory from October 1, 1991 c
3
a
Latex Sheet Field grade material
o
material: Blend: o
Parameter SMRCV60 SMRCV50 SMRL SMR 5" SMRGP SMR lOCV SMR 10 SMR20CV SMR 20 0>
(5‘
3
Dirt retained on 44 ¡im 0.02 0.02 0.02 0.05 0.08 0.08 0.08 0.16 0.16
aperture (max), % wt
Ash content (max), % wt 0.50 0.50 0.50 0.60 0.75 0.75 0.75 1.00 1.00
Nitrogen (max), % wt 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60
Volatile matter (max). 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80
%wt
Wallace rapid plasticity 35 30 30 30
Po (min)
Plasticity retention 60 60 60 60 50 50 50 40 40
index, (min),
Lovibond color:
individual value (max) 6.0
range (max) 2.0
Mooney viscosity.
60(-h 5 ,-5 ) 5 0 (-f5 ,-5 ) 6 5 (4 -7 ,-7 ) c
M L (l+ 4), 100°C'’
Cure^ R R R R R R
Color coding marker Black Black Light green Light green Blue Magenta Brown Yellow Red
Plastic wrap color Transparent Transparent Transparent Transparent Transparent Transparent Transparent Transparent Transparent
Plastic strip color Orange Orange Transparent Opaque white Opaque white Opaque white Opaque white Opaque white Opaque white
"Two subgrades of SMR 5 are SMR 5RSS and SMR 5ADS, which are prepared by direct baling of ribbed smoked sheet and air-dried sheet (ADS), respectively.
^Special producer limits and related controls are also imposed by the Rubber Research Institute of Malaysia (RRIM) to provide additional safeguard.
^The Mooney viscosities of SMR lOCV and SMR 20CV are, at present, not of specification status. They are, however, controlled at the producer end to 60 ( -H7, - 5 ) for
SMR lOCV and 65 ( -I-7, - 5) for SMR 20CV.
^ Rheograph and cure test data (delta torque, optimum cure time, and scorch) are provided.
Source: MRPRA.

Co
4 Thorn and Robinson

activators of cure. Trace elements present include potassium, manganese, phosphorus,

copper, and iron, which can act as catalysts for oxidation.
Natural rubber is available in a granular form (powdered rubber), and oil-extended
grades are also available.
Two chemically modified types o f natural rubber—graft copolymers o f natural
rubber and polymethyl methacrylate, and epoxidized natural rubber—exhibit useful
properties. The former are used in adhesive systems and for the production o f hard
compounds, while the latter probably has still to find its market niche.
As the name suggests, epoxidized natural rubber is prepared by chemically intro­
ducing epoxide groups at random onto the natural rubber molecule. This chemical
modification leads to increased oil resistance and greater impermeability to gases, as
well as an increase in the glass transition temperature Tg and damping characteristics;
the excellent mechanical properties o f natural rubber are retained.
A 50 mol % epoxidized natural rubber exhibits oil resistance only marginally infe­
rior to that of nitrile rubber.
Natural rubber can strain crystallize, which results in its compounds exhibiting high
tensile strength and good tear strength. Although crystallization can occur at low tem­
peratures, compounding greatly reduces this tendency, which can be effectively
prevented from crystallizing by using sulfur levels greater than 2.5 parts per hundred
rubber (phr) to cure the compound.
Since the main chain o f natural rubber contains unsaturation (residual double
bonds), it, along with other unsaturated rubbers, is susceptible to attack by oxygen,
ozone, and ultraviolet light; therefore, compounds require protection against these agen­
cies.
Natural rubber is not oil resistant and is swollen by aromatic, aliphatic, and halo-
genated hydrocarbons.
Natural rubber can be crosslinked by the use o f sulfur, sulfur donor systems, perox­
ides, isocyanate cures, and radiation, although the use o f sulfur is the most common
method.
The sulfur vulcanization o f natural rubber generally requires higher added amounts
o f sulfur, and lower levels o f accelerators than the synthetic rubbers. Sulfur contents of
2 -3 phr and accelerator levels o f 0 . 2 - l.O phr are considered to be conventional cure
systems.
Natural rubber can yield a hard, rigid thermoplastic with excellent chemical resis­
tance when cured with more than 30 phr o f sulfur. Such a product is termed ebonite.
Natural rubber requires a certain degree o f mastication (reduction in molecular
weight) to facilitate processing, although the advent o f constant viscosity (CV) and oil-
extended grades has substantially reduced the need for mastication.
Peptizers are often used to facilitate breakdown o f the rubber during mixing,
although quantities o f greater than 0.6 phr can cause a reduction in the final level of
physical properties [6].
Uses. The uses o f natural rubber are myriad, and a complete summary is not
really possible. Its unique and excellent properties are utilized in tires, shock mounts,
seals, isolators, couplings, bridge bearings, building bearings, footwear, hose, conveyor
belts, plant linings, and many other molding applications.
Latices and solutions are used to produce adhesives, carpet backings, upholstery
foam, gloves, condoms, and medical devices such as catheters. Natural rubber is also
frequently used in blends with other elastomers.
Compound Design 5

1.1.2 Synthetic Rubbers

Unsaturated Homopolymers

Polyisoprene (Synthetic Natural Rubber).

Designation in ISO 1629: IR
Repeat unit (as for natural rubber)
General. Polyisoprene has the same empirical formula as natural rubber, hence
closely approximates the behavior o f its naturally occurring rival. It has the same cis
structure as natural rubber, good uncured tack, high gum tensile strength, high resili­
ence, and good hot tear strength. Although similar to natural rubber, it does exhibit
some differences:

It is more uniform and lighter in color than natural rubber.

Because o f a narrower molecular weight distribution, it exhibits less tendency to strain
crystallize; hence green strength is inferior, as are both tensile and tear strength.

In general, synthetic polyisoprene behaves like natural rubber during processing,

and it also requires protection against oxygen, ozone, and UV light as a result o f unsa­
turation in the main chain. Oil resistance is poor, and resistance to aromatic, aliphatic,
and halogenated hydrocarbons is lacking.
The absence o f the nonrubber constituents, present in the natural rubber, leads to
some differences in compounding, although, in essence, the principles are the same. An
increased level o f stearic acid is generally required for cure activation, and approxi­
mately 10% extra accelerator is necessary to achieve a cure rate similar to that o f
natural rubber; similar sulfur levels are, however, used. Polyisoprene and natural
rubber can be cured by the same type of system.
Uses. Polyisoprene can be used interchangeably with natural rubber in all but the
most demanding applications, and it is often used in blends with poly butadiene and
styrene-butadiene rubber (SBR) in preference to natural rubber when improved proces-
sibility is required.
It is interesting to note that polyisoprene was more widely used in the USSR,
presumably because o f economy-based supply difficulties with natural rubber, and stra­
tegic considerations. This may, o f course, change in the new political climate.
Polybutadiene
Designation in ISO 1629: BR
Repeat unit

— (c H2--C H =C H --C H 2-)- jj

General. Polybutadiene is produced by solution polymerization, and one impor­

tant feature governing the performance o f the resultant polymer is the c is -\A and 1,2-
6 Thorn and Robinson

vinyl contents. High cisA ,4 polymers ( > 90%) have a Tg around —90°C , hence exhi­
bit excellent low temperature flexibility, exceeded only by the phenyl silicones. They
also exhibit excellent resilience and abrasion resistance. Since, however, the high resili­
ence gives poor wet grip in tire treads, this rubber finds limited use as the sole base for
such compounds.
As the 1,4 content decreases, and 1,2-vinyl content increases, the low temperature
properties, abrasion resistance, and resilience become inferior.
The polymerization o f butadiene results in a polymer with a narrow molecular
weight distribution which can be difficult to process. Indeed, commercially available
grades present a compromise between processibility and performance. Most polybuta­
diene rubbers are inherently difficult to break down during mixing and milling and have
low inherent tack; moreover, the inherent elasticity o f the polymer gives poor extruda-
bility. Peptizers can be used to facilitate breakdown, hence aiding in processing.
The unsaturation present in the main chain necessitates protection against oxygen,
UV light, and ozone. Oil resistance is poor, and the polymer is not resistant to aromatic,
aliphatic, and halogenated hydrocarbons.
Polybutadiene-based compounds can be cured by sulfur, sulfur donor systems, and
peroxides. Less sulfur and a higher level o f accelerators are required than for natural
rubber. The cure of polybutadiene by peroxides is highly “efficient” in that a large
number o f crosslinks are produced per free radical. Thus the resultant highly
crosslinked rubber exhibits high resilience; this factor is utilized in the manufacture of
“superballs.”
Compounds based on polybutadiene give optimum properties only at high filler and
oil loadings.
Uses. Most polybutadiene is used in tire applications, and the majority o f this use
is in blends with other polymers, such as natural rubber and SBR, where polybutadiene
reduces heat buildup and improves the abrasion resistance o f the blend. The coefficient
o f friction on ice o f snow tires is also improved by using higher levels o f polybutadiene
in the tread blend.
When poly butadiene is used in blends for other applications, the improved abrasion
and low temperature flexibility conferred on the blend offer advantage, for example, in
shoes and conveyor and transmission belts.
Polychloroprene
Designation in ISO 1629: CR
Repeat unit

Cl
-^ C H 2 CH2

G eneral This polymer is frequently, but incorrectly, referred to as Neoprene,

which is a trade name.
 Compound Design 7

Polychloroprene is produced by emulsion polymerization, during which the follow ­

ing forms of addition are possible:

«AAAAAAA/'CH;
2. %
aa /w \ aaa/'CH2 h

Ci
> = < Cl
/ \
CH2 »a a /n a a a a a a a /*

cis-\,A addition trans-1,4 addition

Cl Cl- C =CH2

,A A A A A A A rC H 3 ~ ~ ” C «AAAAAAAAAAAA/* *AAA/W\AA/*CH“ “ CHo ^

aaaaaaaaaaat

I
CH = C H 2

1,2 addition 3,4 addition

Since commercial production o f a polymer based on the c is -\A form is impossible,

commercial polymers are based on the trans-\,A form, which has a crystalline melting
point of -f 75°C and a 7g o f —45°C . Pure 1,4-polychloroprene thus crys­
tallizes readily and would normally be considered to be o f limited use for a rubber.
Such a polymer, however, does not crystallize when dissolved in a solvent but will do
so when the solvent evaporates. This feature is used to good effect in the production o f
contact adhesives.
The temperature o f polymerization, however, influences how closely the polymer
attains the tra n s-lA form. Raising the polymerization temperature from —40°C to
+ 40°C increases the percentage o f 1,2 and 3,4 forms, both o f which reduce the stereo­
regularity, hence the tendency to crystallize. Thus chloroprene-based polymers that are
intended to be rubbery are polymerized at higher temperatures. The 1,2 grouping in the
main chain is the site o f crosslinking reactions during cure. The ability to crystallize can
also be controlled by copolymerizing chloroprene with small amounts o f other mono­
mers.
Two different mechanisms are used to control the molecular weight o f the polymer
during polymerization.
In the so-called G types, sulfur is copolymerized with the chloroprene to yield a
product shown schematically as follows:

» A A A A A A A A A A A A A A A A r S X '^ V '^ V \A A A A A A A r S X * 'V ''^ '^ > A A A /W W A r S X 'A '> ^ ^ ^ V W \A A A A r

8 Thom and Robinson

The G types are stabilized with tetraethyl thiuram disulfide (TETD), with the result
that the G types can cure without further acceleration.
In the so-called W types, the molecular weight is controlled by the use o f a mercap­
tan.
The following differences are apparent between the G and W types:
The G type can break down during mixing or milling via cleavage at the group;
this decreases molecular weight, hence reduces the elasticity, or nerve, during
processing. The extent o f breakdown is somewhat dependent on the exact grade.
Neoprene GW being virtually unaffected by milling. Cleavage at the group can also
occur during long-term storage, and the G types therefore have the disadvantage o f a
limited storage life.
The G types do not require further acceleration during cure, but exhibit slightly
inferior aging characteristics. Resilience and tack are generally better than with the W
types.
The W types exhibit superior storage life and aging characteristics but require the
addition o f accelerators to achieve an acceptable rate o f cure. They do not break down
during mxing. During processing they are less prone to scorch, and they will accept
higher loadings o f filler. The cured compound generally exhibits a lower compression
set and a greater ability to resist heat aging.
The chlorine atom in the repeat unit has a tendency to deactivate the double bond in
the main chain. Thus polychloroprene tends to resist oxidation, ozone, and UV light to a
higher degree than the other unsaturated rubbers, although it still requires protection if
the maximum performance is to be obtained. Unfortunately, this deactivation o f the
double bond means that the polymer cannot be crosslinked by sulfur.
The chlorine atom also confers an increased level o f resistance to oils: the oil resis­
tance o f polychloroprene is roughly intermediate between natural rubber and nitrile
rubber, and is often sufficient for many applications. Polychloroprene is also self­
extinguishing in flame tests.
Metal oxides are principally used for curing these materials; peroxides are gen­
erally not used. The most widely used cure system is based on a combination o f the
oxides o f magnesium and zinc, the cured properties achieved being dependent on the
ratio o f the two; the most common MgO/ZnO ratio is 4.0:5.0. Since the zinc oxide
tends to promote scorch, it is added late in the mixing cycle, whereas magnesium oxide
is added early. One drawback o f the MgO/ZnO cure system is that chlorine liberated
during cure reacts with the oxides to yield the chloride, which is hydrophilic, and com ­
pounds containing this cure system can swell in hot water; even in cold water, swell can
be progressive and eventually large.
Lead oxide (PbO or Pb304 ) up to levels o f 20 phr can be used to improve resis­
tance to water because the chloride formed during cure is insoluble.
The W types require additional acceleration, and ethylene thiourea (ETU) gives the
best balance o f all properties. However, the use o f this accelerator is increasingly being
restricted as a result o f fears o f its effects on pregnant women, and now recently men.
Diethylene thiourea, thiurams, and guanidines can also be used. Sulfur is sometimes
used to increase the degree o f cure in the W types, but its presence detracts from the
aging performance o f the vulcanizate.
Uses. As a result o f its balance o f strength, oil resistance, flammability, and
increased resistance to ozone, aging, and weathering, polychloroprene finds widespread
 Compound Design 9

industrial use. Typical uses are V-belts, conveyor belts, wire and cable jacketing,
footwear, wet suit applications, coated fabrics, inflatable products, hoses, extrusions,
and many others goods. Adhesives are also a strong market area.
Polynorbornene
Repeat unit

CHo
:CH
•aaaaaa| w *CH
CH.

General. The large ring structure in the main chain gives polynorbornene a high
Tg o f + 3 5 °C . Thus it is not rubbery at normal ambient temperatures and requires plas­
ticization to achieve elastomeric behavior.
Polynorbornene exhibits some desirable advantages. It can be extended by large
quantities o f oils to give very soft vulcanizates (ca. 20 Shore A) with acceptable
strength, and it gives high damping, which can be useful for vibration and noise reduc­
tion applications. However, since plasticization is required, particular care must be
exercised in the choice o f plasticizer if exposure to higher than ambient temperatures is
anticipated.
Polynorbornene is not oil resistant, and solvents that can extract the plasticizer
obviously will be detrimental to its performance.
The material can be cured by both sulfur and peroxides, but it requires protection
against oxygen, ozone, and UV light.
Uses. Roll covers and elements designed to utilize the high damping properties o f
this material are thought to be the major uses.

Unsaturated Copolym ers

Butyl, Chlorobutyl, and Bromobutyl
Designation in ISO 1629: HR, CIIR, BUR
Repeat units

ÇH3 ÇH3

-C — CH ^ CH2 ^ CH2

CH,

General. Commercial grades o f butyl rubber are prepared by copolymerizing

small amounts o f isoprene with poly isobutylene. The isoprene content o f the copolymer
10 Thorn and Robinson

is normally quoted as the “mole percent unsaturation,“ and it influences the rate o f cure
with sulfur, as well as the resistance o f the copolymer to attack by oxygen, ozone, and
UV light. Being saturated, however, polyisobutylene naturally confers on a polymer an
increased level o f resistance to these agencies when compared to natural rubber. Com­
mercial butyl rubbers typically contain 0 .5 -3 .0 mol % unsaturation.
The close packing o f the isobutylene chain confers on the polymer a high degree o f
impermeability to gases but also results in a very “lossy” rubber. The high hysteresis
loss can be utilized in some circumstances to provide a good coefficient o f friction in
wet conditions.
Chlorobutyl and bromobutyl are modified types containing 1.2 wt % chlorine or
bromine, the isoprene unit being the site o f halogénation. Introduction o f the halogen
gives greater cure flexibility and enhanced cure compatibility in blends with other diene
rubbers. It also confers increased adhesion on other rubbers and metals.
Butyl rubber is not oil resistant.
Butyl and the halogenated butyls can be cured by sulfur, dioxime, and resin cure
systems. In addition, the halogenated types can be crosslinked with zinc oxide and
diamines. Peroxides cannot be used because they tend to depoly merize the poly isobu­
tylene.
Because o f the low level o f unsaturation in the main chain, sulfur cures require the
more active thiuram and dithiocarbamate accelerators to achieve an adequate state of
cure.
Dioxime cures yield vulcanizates with good ozone resistance and moisture imper­
meability and, as such, are frequently used for curing electrical insulating compounds.
Resin cures utilize phenol formaldehyde resins with reactive methylene groups and
a small added amount o f either a chlorinated rubber (e.g., polychloroprene) or stannous
chloride. If halogenated phenolic resins are used, the additional source o f a halogen may
not be required. Resin cures give butyl compounds excellent heat stability and are used
to good effect where this property is required~for example, in tire curing bags, which
must withstand service at 150°C in a steam atmosphere.
Calcium stearate is added to stabilize the chlorobutyl during processing.
Uses. The main applications o f butyl rubber are in wire and cable applications,
inner tubes, inner liners in tubeless tires, tire curing bladders, and pharmaceutical clo­
sures, the latter utilizing butyl’s low impermeability to gases. Other applications include
vibration isolation compounds, caulking and sealants, and sheeting for pond liners and
roofing.
Nitrile Rubber
Designation in ISO 1629: NBR
Repeat units

Ci ;N

- ^ C H j - C H = C H - C H 2^------------ ( - C H j — CH

General. Nitrile rubbers are copolymers o f butadiene and acrylonitrile produced

by emulsion polymerization; “hot” and “cold” polymerized types are available. The
Compound Design 11

“hot” polymerized types generally have higher green strength and are slightly harder to
process than “cold” copolymers.
The introduction o f acrylonitrile (ACN) into the polymer backbone imparts oil
resistance and affects many other properties. Grades that vary in acrylonitrile content
from 18 to 50% are commercially available, the percentage o f acrylonitrile present
forming the basis o f the following grade descriptions:

Low 18-24% ACN

Medium low 26-28% ACN
Medium 34% ACN
Medium high 38-40% ACN
High 50% ACN

Many properties are influenced by the acrylonitrile content:

Acrylonitrile content:
18% - 50%

Oil resistance improvement -*

Fuel resistance improvement —
Tensile strength improvement —
Hardness increase --
Abrasion resistance improvement --
Gas impermeability improvement —
Heat resistance improvement —
Low temperature flexibility improvement
Resilience irnprovement ^
Plasticizer compatibility —

A typical Tg o f an 18% ACN copolymer is —38°C , and that o f a 50% ACN copolymer
-2 °C .
Carboxylated nitriles, hydrogenated nitrile, liquid nitriles, and blends with polyvi­
nyl chloride (PVC) are also commercially available.
The carboxylated types (XNBR) contain one or more acrylic type o f acid as a ter-
polymer, the resultant chain being similar to nitrile except for the presence o f carboxyl
groups, which occur about every 100-200 carbon atoms. This modification gives the
polymer vastly improved abrasion resistance, higher hardness, higher tensile and tear
strength, better low temperature brittleness, and better retention o f physical properties
after hot oil and air aging when compared to ordinary nitrile rubber.
Low molecular weight liquid nitrile grades are available, and these can be used as
compatible plasticizers in the compounding o f nitrile rubber. Such plasticizers can be
partially crosslinked to the main chain during cure; hence they exhibit low extractabil-
ity.
PVC/NBR poly blends can be produced as colloidal or mechanical blends, the
former generally giving superior properties. Commercially available polyblends have
PVC contents ranging from 30 to 55%. The poly blends have reduced elasticity, which
gives improved extrudability, but they also exhibit superior ozone resistance, improved
12 Thorn and Robinson

oil swell resistance, and tensile and tear strength; these properties, however, are
achieved at the expense o f low temperature flexibility and compression set. The ozone
resistance o f such polyblends is improved only if the PVC is adequately distributed and
fluxed. Failure due to ozone attack can occur if proper distribution and fluxing are not
achieved, but this is harder to do in mechanical blends.
Another method by which the ozone resistance o f nitrile rubber can be improved is
the removal o f the double bonds in the main chain o f the copolymer by hydrogenation.
Hydrogenated nitrile rubbers also exhibit much greater resistance to oxidation and
extend the useful service temperature range o f nitriles up to ca. 150°C. Commercially
available grades offer different degrees o f hydrogenation, with residual double bonds
ranging from ca. 0.8 to 6%.
Nitriles have good resistance to oil, aliphatic and aromatic hydrocarbons, and vege­
table oils, but they are swollen by polar solvents such as ketones. The unsaturated main
chain means that protection against oxygen, ozone, and UV light is required.
In compounding, choice o f the correct grade is required if the required balance of
oil resistance and low temperature flexibility is to be achieved.
Nitrile rubber can be cured by sulfur, sulfur donor systems, and peroxides. How­
ever, the solubility o f sulfur in nitrile rubber is much lower than in NR, and a mag­
nesium carbonate coated grade (sulfur MC) is normally used; this is added as early in
the mixing cycle as possible. Nitrile rubber requires less sulfur and more accelerator
than is commonly used for curing natural rubber. A cadmium oxide/magnesium oxide
cure system gives improved heat resistance, but the use o f cadmium, a heavy metal, will
increasingly be restricted.
The hydrogenated nitrile grades that contain the lowest level o f residual double
bonds can be crosslinked only by the use o f peroxides and radiation, while those with a
level o f residual double bonds greater than about 3.5% can be cured by sulfur.
In addition to the normal sulfur cure systems, metal oxides can be used to cure the
carboxylated nitriles.
The low temperature properties o f nitriles can be improved by the use o f suitable
plasticizers (e.g., ester plasticizers).
Uses. Nitrile rubber, because o f its oil resistance, is widely used in sealing appli­
cations, hose liners, roll coverings, conveyor belts, shoe soles, and plant linings. Nitrile
rubber is also available as a latex.
Styrene-Butadiene Copolymers
Designation in ISO 1629: SBR
Repeat units

— ( c H 2 -C H = C H -C H j -)-------------( - C H j CH ---------- )

General. When the United States and Germany were cut off from the supplies of
natural rubber during World War II, both countries sought to produce a synthetic alter­
Compound Design 13

native; SBR was the result, and at one stage it was the most commonly used synthetic
rubber. It can be produced by both emulsion and solution polymerization techniques,
with the emulsion grades being the most widely used. Emulsion polymerization yields a
random copolymer, but the temperature o f the polymerization reaction also controls the
resultant properties. “C old” polymerization yields polymers with properties superior to
those o f the “hot” polymerized types.
Solution polymerization can yield random, diblock, triblock or multiblock copoly­
mers. It is important to note that the triblock, or multiblock copolymers, belong to that
class o f material termed thermoplastic elastomers, and only the random copolymer
types are considered here.
Both random emulsion and solution polymerized SBRs contain about 23% styrene.
SBR continues to be used in many o f the applications for which it earlier replaced
natural rubber, even though it requires greater reinforcement to achieve acceptable ten­
sile and tear strengths and durability. SBR exhibits significantly lower resilience than
NR, so that it has a higher heat buildup on flexing, which restricts its use in truck tires,
with their thicker sections. This inferior resilience (to natural rubber) is an advantage in
passenger car tire treads because the higher hysteresis loss gives increased wet grip and
this, combined with the good abrasion resistance that can be obtained from tire tread
compounds, ensures for SBR a high volume use in tire production.
The oil resistance o f SBR is poor, and the polymer is not resistant to aromatic, ali­
phatic, or halogenated solvents. Because o f unsaturation in the main chain, protection is
required against oxygen, ozone, and UV light.
Oil-extended SBR and SBR carbon black masterbatches are supplied by the poly­
mer producers, and such grades give the advantage o f reducing the necessity o f further
additions o f filler and oil at the mixing stage.
SBR can be cured by the use o f sulfur, sulfur donor systems, and peroxides. Sulfur
cures generally require less sulfur (1 .5 -2 .0 phr) and more accelerator than normally are
required to cure natural rubber.
Uses. The major use o f SBR is in tires, predominantly for car and light trucks; in
the latter use it is frequently blended with NR and BR.
SBR also finds use in conveyor belts, molded rubber goods, shoe soles, hoses, and
roll coverings.
SBR is available as a latex, which is used in carpet backing and other applications.

Polym ers Based on Ethylene

Ethylene Propylene and Ethylene Propylene Diene Methylene Rubbers
Designation in ISO 1629: EPM, EPDM
Repeat units

CHo
r
-CH — j-
\
fc H -—

EPM
14 Thorn and Robinson

CH,

------CH2 ^ ----------------------- CH — ^ CH

CH,
I
CH
II
CH
I
EPDM (terpolymer: 1,4-hexadiene) CH3

G eneral The copolymerization o f ethylene and propylene yields useful copoly­

mers, the crystallization o f both polymers being prevented if the ethylene content is in
the range 45-60% ; grades with higher ethylene contents (70-80% ) can partially crys­
tallize. The lower ethylene types are generally easier to process, while green strength
and extrudability improve as the ethylene content increases. One disadvantage o f the
copolymer is that it cannot be crosslinked with sulfur because there is no unsaturation in
the main chain. To overcome this difficulty, a third monomer with unsaturation is intro­
duced; but to maintain the excellent stability o f the main chain, the unsaturation is made
pendant to it. The three types o f third monomer used commercially are
dicyclopentadiene, ethylidene norbornene, and 1,4-hexadiene. Generally 4-5% o f the
terpolymer will give acceptable cure characteristics, while 10% gives fast cures; dicy­
clopentadiene gives the slowest cure rate and ethylidene norbornene the highest.
Since the main chain o f both EPM and EPDM rubbers is saturated, both co- and
terpolymers exhibit excellent stability to oxygen and UV light, and are ozone resistant.
EPM and EPDM are not oil resistant, and they are swollen by aliphatic and
aromatic hydrocarbons, as well as by halogenated solvents. They have excellent electri­
Compound Design 15

cal properties and stability to radiation. Their densities are the lowest o f the synthetics,
and they are capable o f accepting large quantities o f filler and oil. They exhibit poor
tack; and even if tackifiers are added, this property still is not ideal for building opera­
tions. Adhesion to metal, fabrics, and other materials can be difficult to accomplish.
The copolymers can be cured only by peroxides or radiation, while the terpolymers
can be cured with peroxides, sulfur systems, resin cures, and radiation.
The dicyclopentadiene terpolymer can give higher states o f cure with peroxides
than the copolymer, although in peroxide curing o f both the copolymer and terpolymer
it is common practice to add a coagent, to increase the state o f cure. Triaryl isocyanu-
rate or sulfur is the most common coagent.
Bloom can be a problem in sulfur cures, so selection o f the accelerator system is
important.
Resin cures utilize the same resins that are used for butyl rubber, but more resin
(ca. 10-12 phr) and a halogen donor (10 phr), typically bromobutyl or polychloroprene,
are required. Although heat stability is slightly improved by resin curing when com ­
pared to sulfur cures, the effect is not as marked as in the resin curing o f butyl.
Uses. Wire and cable applications and extrusion profiles (e.g., seals for windows
and car doors) probably form the major applications for EPM and EPDM rubber,
although it is also used in a wide variety o f other extrusion and molding applications.
Washing machine door seals molded from EPDM are starting to replace NR as washer/
dryers are becoming more common.
Chlorinated Polyethylene
Designation in ISO 1629: CM
Repeat units

^ C H j C H -------- ^ ----- CHa— ^

Cl

General. Although polyethylene has a low Tg it is highly crystalline. Hence it is a

thermoplastic and not a rubber at ambient temperatures. If the regularity o f the main
chain could be interrupted, and crystallization effectively prevented, a useful elastomer
might result. The chlorination o f polyethylene is one method by which crystallization
can be prevented, and chlorinated polyethylene is commercially available; the degree o f
chlorination, however, determines how rubbery the modified polymer is. Polymers with
a chlorine content o f ca. 25% are still relatively crystalline, while those with a chlorine
content greater than 40% become increasingly brittle as a result o f interaction between
the now highly polar polymer chains. The most desirable polymers, in terms o f the
absence o f crystallinity and flexibility o f the chains, are obtained when the degree o f
chlorination is around 35 %.
As with other polar polymers, these materials will resist oil, and the absence o f a
double bond in the main chain confers excellent stability to the deleterious effects o f
oxygen, ozone, and light. Because there are no double bonds in the main chains, these
materials can be crosslinked only by the action o f peroxides or radiation. It is recom­
16 Thorn and Robinson

mended that metal oxides (of Mg and Pb) be added to act as acid acceptors during vul­
canization; zinc oxide is not used because it decreases the stability o f the polymer.
Uses. It is fairly true to say that the use o f chlorinated polyethylene has not been
large, possibly as a result o f the greater ease with which the chlorosulfonated
polyethylene (Hypalon), a closely related competitor material, can be cured. When
selected, chlorinated polyethylene finds its major use in the wire and cable industry.
Chlorosulfonated Polyethylene
Designation in ISO 1629: CSM
Repeat units

------^ C H j C H -------- ^ ^CHa----- CHj— ^ - / c H CHj

I
Cl
* y '^ O C I ' z

General. The level o f chlorination in these materials varies, influencing the prop­
erties o f the product in exactly the same manner as the closely related chlorinated
polyethylene. The introduction o f the chlorosulfonyl group in small amounts ( < 1.5%)
gives greater choice in the methods used to crosslink to the polymer. However, in gen­
eral, the properties exhibited by these materials are equivalent to those o f chlorinated
polyethylene.
Dupont recently announced the availability o f a modified chlorosulfonated
polyethylene based polymer trade-named Acsium. In this modified polymer the chlorine
content is reduced, but an additional pendant alkyl group is used to restrict the ability o f
the polymer to crystallize. The result is a polymer with a lower Tg than the conventional
CSM polymer.
In addition to the use o f peroxides for crosslinking, metal oxides, poly functional
alcohols, amines, and epoxide resin cure systems can be used with CSM rubbers. In the
metal oxide based cure systems it is usual to add a weak acid, such as stearic acid, and
accelerators, such as MET, MBTS, or TMTD [see Table 16, below]; magnesium or
lead oxides are generally used.
The most common poly functional alcohol used is pentaerythritol, but a base is also
required to complete the cure system, magnesium and calcium oxide giving more con­
trolled cure rates than stronger bases.
As with chlorinated polyethylene rubber, chlorosulfonated polyethylene exhibits
good resistance to oxygen, ozone, and UV light. The polar nature o f the polymer chain
also confers oil resistance.
Uses. The excellent UV stability o f chlorosulfonated polyethylene has led to a
wide use as a roof sheeting material, and the ability to compound this material to slowly
cure at ambient temperatures is an added advantage. Another sheeting application is
pond liners. Wire and cable applications, coated fabrics and items made from them,
hoses, and molded goods are other areas in which this material finds use.
Acsium is said to have been designed for use in synchronous drive belt applica­
tions.
Compound Design 17

Ethylene Vinyl Acetate Copolymers

Designation in ISO 1629; EAM
Repeat units

-^ C H j------CHz ( cH ----- C H j - ) -
X '
0
1
c=o
I
CH,

General. The copolymerization of ethylene with vinyl acetate (VA) is another

method by which the crystallinity o f polyethylene can be reduced and a rubbery poly­
mer obtained. The final properties o f the copolymer depend on the VA content: at a VA
level o f 50% the copolymer is entirely amorphous; elastomeric grades generally contain
40 -6 0 wt % VA. The oil resistance of the copolymer, which is also dependent on the
VA content, generally lies between that o f SBR and that o f polychloroprene.
The saturated main chain o f the copolymer confers excellent resistance to oxygen,
ozone, and UV light, but it means that these materials cannot be crosslinked by sulfur.
Peroxides and radiation are the only methods by which crosslinking can be accom­
plished, and coagents are often required to achieve the required state o f cure.
Uses. The main use o f EVA is in wire and cable applications, although their
electrical properties are inferior to those of EPDM.
Ethylene/Acrylate Rubbers: Vamac
Repeat units

-^CHa----- CHa— — CHa - ) R-

( - R--------- 1
I
i C=0 C=0
I I
0 OH
1
CH,

General. The terpolymer Vamac, produced by Dupont, exhibits properties gen­

erally comparable to those o f an acrylate. The introduction o f ethylene into the main
chain gives low temperature performance similar to that o f a poly octyl acrylate polymer
and oil resistance similar to that o f a butyl acrylate polymer. In high temperature perfor­
mance, Vamac is slightly inferior to the acrylates, but this deficit is compensated for by
the generally higher physical strength.
 18 Thorn and Robinson

Vamac is generally cured with diamines, with MA^-diphenylguanidine (DPG) as an

accelerator, but it is also possible to crosslink this material with peroxides.
Uses. Vamac finds use as a sealing material in automative applications and marine
motor lead wire insulation. It has also been used as the base polymer for low flammabil­
ity, halogen-free, cable jacketing compounds.

Acrylates
Designation in ISO 1629: ACM
Examples o f repeat units

( c H j ----- CH
CH —
1
- ^ C H — C H j-)-
1
C X o'
'x
1
1 1
1
0 CH.
1 1 ‘
CHj CH.
1 * 1 ‘
1
CHa Cl
Ethyl acrylate Chloroethyl
(95%) vinyl ether (5%)

General. Although the chemical structure shown is used as an example, acrylates are a
class o f materials rather than one single type. These polymers are formed by the copoly­
merization o f an acrylic ester and a cure site monomer, ethyl acrylate and chloroethyl
vinyl ether, respectively, being illustrated here.
The choice o f acrylic ester, hence its polarity, determines the low temperature flexi­
bility and the heat and chemical resistance o f the polymer; both alkyl and alkoxy acrylic
esters are used as monomers. Within the alkyl acrylic esters, ethyl acrylate has the
highest polarity, hence gives the best oil and heat resistance, but the worst low tempera­
ture performance. Low temperature properties improve as the alkyl acrylic ester
changes from ethyl to butyl, and on to higher analogues, but this is at the expense o f
heat stability and oil resistance.
It is, o f course, possible to copolymerize mixed alkyl acrylic esters to achieve a
compromise in these properties. Alkoxy acrylic esters also confer improved low tem­
perature properties.
One factor that has perhaps slowed the use o f these materials is the perceived
difficulties in their processing. The acrylates are soft and thermoplastic, and prone to
scorch if tight process control is not in force in a factory.
The absence o f main chain unsaturation confers good resistance to oxygen, ozone,
and light, while the polarity contributes oil resistance to the copolymer.
The cure site monomer directly controls which cure systems can be used to vulcan­
ize the rubber. Since cure behavior is determined by the cure site monomer, which can
differ among suppliers and, presumably, grades, it is advisable to read the
manufacturer’s recommendation when choosing a cure system.
The first acrylates to be developed were cured by:

1. Amines (e.g., trimene base, triethylene tetramine, Diak No. 1, from Dupont).
2. Red lead and ethylene thiourea.
Compound Design 19

O f the amines, Diak No. 1 is the most efficient, since it does not volatilize during pro­
cessing. Sulfur and sulfur-bearing materials act as retarders in this type o f cure, also as
a form o f antioxidant.
Newly introduced polyacrylate rubbers can be cured with certain amines and are
more responsive to a broad range o f curative systems (e.g., alkali metal stearate/sulfur
or sulfur donor, methyl zimate, and ammonium adipate).
Good, or best, compression set requires a postcure, although at least one manufac­
turer has introduced a new series o f poly acrylates that only require a press cure.
The use o f softeners and plasticizers presents a problem. TP90B, thioethers, and
certain adipates can give low temperature flexibility to —45°C , but these agents are
volatile at postcure temperatures; hence their effect is easily lost.
Uses. The main use for acrylates is in sealing applications, where improved heat sta­
bility over nitrile rubber is the benefit. Automotive transmission seals are probably the
major use.

Fluorocarbon Rubbers
Designation in ISO 1629: FPM
General. The importance of fluorine in polymer chemistry has been known since the
discovery o f Teflon (polytetrafluoroethylene: PTFE) in 1938. Highly fluorinated poly­
mers are very stable and have remarkable resistance to oxidative attack, flame, chemi­
cals, and solvents.
The fluorine-containing compositional units generally used to produce commercial
polymers are as follows:

CHoIZrCFo Vinylidene fluoride

CH, :C F , Chlorotrifluoroethylene
Cl

CF: IC F
I Hexafluoropropylene
CF3

T etrafluoroethy lene

Perfluoro(methyl vinyl ether)

OCF,

Commercial polymers are understood to have the following compositions:

KEL F: a copolymer o f vinylidene fluoride and chlorotrifluoroethylene. This material

generally exhibits better resistance to oxidizing acids and better low temperature
properties than other types.
Types such as Viton A: a copolymer of vinylidene fluoride and hexafluoropropylene.
Types such as Viton B: a terpolymer o f vinylidene fluoride, hexafluoropropylene, and
20 Thorn and Robinson

tetrafluoroethylene. This type has slightly improved thermal stability and fluid
resistance when compared to the Viton A type.
Types such as Viton G: these grades are differentiated from the others by being perox­
ide curable, a condition that is achieved by the addition o f a cure site monomer,
said to be bromotetrafluorobutene. The grades are basically terpolymers, but the
exact composition differs; for example, Viton G is said to be based on vinylidene
fluoride, hexafluoropropylene, tetrafluoroethylene, and the cure site monomer; the
hexafluoropropylene is said to be replaced by perfluoro(methyl vinyl ether) in the
Viton GLT grade. As well as being peroxide curable, these grades exhibit superior
resistance to aqueous media and steam. The GLT grade exhibits superior low tem­
perature properties.
Kalrez: a copolymer o f tetrafluoroethylene and perfluoro(methyl vinyl ether) plus a cure
site monomer. This is the most thermally stable and chemically resistant polymer
currently available, and one o f the most expensive.
Aflas: a copolymer o f tetrafluoroethylene and propylene.

Apologies are due to the other manufacturers o f fluorocarbon rubbers, 3M

(Fluorel), Montedison (Tecnoflon), and Daikin (Dai-EL), for facilitating the presenta­
tion by using the Viton trade name to illustrate part o f the available spectrum o f fluoroe-
lastomers.
The resistance to heat and chemicals o f the fluoropolymers is mainly dependent on
the extent o f fluorination and stability o f the crosslinks. For example, most fluorocar­
bons have fluorine contents o f 50-70% ; more chemically resistant types have 65-69% ;
and Kalrez, which is almost completely fluorinated, has 70%. For comparison, fluoro-
silicones contain about 36% fluorine.
The fluorocarbons have the best heat stability o f all rubbers. Kalrez, which exhibits
the best performance, is capable o f giving extended service life at temperatures exceed­
ing 250°C . As a group, fluorocarbons resist aliphatic, aromatic, and chlorinated hydro­
carbons, as well as most oils and mineral acids. They are also highly resistant to oxy­
gen, ozone, and UV light.
There are several methods by which the fluorocarbons can be cured; the principal
methods are summarized as follows.

Metal oxide
Cure agent acid acceptor Comments

Diamine (e.g., Magnesium oxide or calcium General-purpose cure system,

hexamethylenediamine) oxide fairly resistant to scorch. Best
heat resistance, but not partic­
ularly resistant to acids.
Red lead Best acid and steam resistance,
but exhibits strong tendency
to scorch.
Zinc oxide/dibasic lead phos­ Least tendency to scorch, inter­
phite mediate acid resistance.
Bisphenol A/ Magnesium oxide/calcium Improved compression set,
organophosphonium hydroxide reduced fissuring, and shrink­
salt age. Most common cure sys­
tem for sealing applications.
Compound Design 21

Changes in the acid acceptor

generally give the same
trends as in the use of diam­
ine cures.
Peroxide (not for all Acid acceptor required Inferior compression set to the
grades) bisphenol cure. Improved
resistance to amine-stabilized
oils.
Only the large particle sized blacks, MT or Austin Black, and mineral fillers are
used in the compounding o f these materials.
Uses. The main uses o f the fluorocarbons are in sealing applications.

Silicone Rubber
General. Silicone rubbers contain the following dimethyl siloxane unit:

CH3
I
-S i-0 -
I
CH,

The millable gums, the only type considered here, generally contain 5000-9000 o f
the dimethyl siloxane unit.
Polymers that contain simply the repeat unit above are termed methyl silicones and
are given the ISO designation MQ.
It is possible to replace a few o f the methyl groups ( < 0.5% ) with a vinyl group,
and the resultant vinyl methyl silicones (ISO designation VMQ) exhibit improved vul­
canization characteristics and lower compression set.
The replacement o f 5-10% o f the methyl groups on the silicon atom with phenyl
groups gives polymers that exhibit superior low temperature properties. Brittleness tem­
peratures o f approximately — 117°C can be achieved, compared to the approximately
—70°C for the VMQ types. The ISO designation for the phenyl-modified silicones is
either PMQ or PVMQ depending on whether the grade is vinyl modified.
To improve the solvent resistance of the polymer, a fluoroalkyl group can be substi­
tuted on each silicon atom for one o f the methyl groups, the resultant polymer having
the following repeat unit.

CH3

-S I-0 -
I
CHj
I
CH2
I
CF3
ISO designation: FMQ or FVMQ
 22 Thorn and Robinson

Silicone rubbers exhibit good resistance to heat aging and are considered to be
usable up to 200°C . Although silicones do not exhibit high strength at room tempera­
ture, they do retain their properties at high temperatures to a much greater extent than
other rubbers.
The long-term performance o f silicones is generally excellent, although exposure to
steam at high pressure, as well as aging in closed systems (oxygen essentially excluded),
can lead to degradation via a hydrolysis reaction; this is especially true if acidic perox­
ide remnants have not been driven off during postcuring.
The oil resistance o f silicone is roughly equivalent to that o f polychloroprene, while
the fluorosilicones approach the fluorocarbons in this respect.
Two further interesting points are noted: (1) upon burning, silicones form silica,
which is an insulator, and thus cables insulated with silicone can function after short­
term exposure in a fire situation; and (2) silicones are physiologically inert, and this
property has led to their use in a wide variety o f medical applications, including medical
implants.
Because silicones are saturated, their resistance to oxygen, ozone, and UV light is
excellent, but for this reason peroxides must be used for vulcanization.
Silica fillers are generally used to reinforce these materials, carbon black being less
reinforcing and its use somewhat specialized.
Uses. Silicones are widely used in many applications, such as pharmaceutical, medi­
cal, wire and cable, automotive, and aerospace, which utilize the excellent general inert­
ness o f these materials. They do, however, have a high price.

Other Saturated Rubbers

Epichlorohydrin-Based Rubbers
General. These rubbers are available as a homopolymer:

-CHo-------- CH--------O-

CHjCI ISO Designation: CO

or as a copolymer with ethylene oxide:

-CHo -CH- -CHo -CHo

CH2CI

ISO Designation: ECO

or as a terpolymer with a small amount o f unsaturated allyl glycidyl ether. There is no

ISO designation for the terpolymer, but ETER is used by the American Society for
Testing and Materials (ASTM). As with EPDM, this unsaturation is pendant to the main
chain, which allows vulcanization with sulfur, while preserving the stability o f the main
Compound Design 23

chain. The ability to be cured by sulfur also allows the terpolymer to be used in blends
with other polymers (e.g., nitriles).
As might be expected, the homopolymer having the highest polarity exhibits the
best oil resistance, but this is at the expense o f low temperature flexibility. The homopo­
lymer also has a low permeability to gases. The unsaturated backbone gives these
materials good resistance to oxygen, ozone, and light.
The main method of crosslinking the homopolymer and copolymer is by use o f
thioureas, and, because the cure reaction requires basic conditions, an acid acceptor is
also added. Litharge, red lead, magnesium oxide, and dibasic lead phosphite are com ­
monly used acid acceptors. The most commonly used thiourea is ethylene thiourea, but
this compound has a tendency to promote mold fouling.
The Echo S cure system commercialized by B.F. Goodrich is said to give improved
scorch safety and reduced mold fouling over the ue o f thioureas. Inorganic acid accep­
tors other than those based on lead are recommended for use with the Echo S cure sys­
tem.
The terpolymers can be cured by the use o f sulfur and peroxides as well as by the
use of thioureas.
Uses. The main use o f epichlorohydrin is in the automotive sector, for various
seals and hoses.
Nitrosofluororubbers
Designation in ISO 1629: AFMU
Repeat units

^ „ 0 ^ — ( . C F , - C F , ^ N O ----------------^
' CF- ^ V^ I /
(CFjIaCOOH

General. AFM U is a terpolymer based on trifluoronitrosomethane, tetrafluoro-

ethylene, and perfluoro (nitrosobutyric acid) in which the latter acts as the cure site
monomer. This material is included only for the sake o f completeness, since it is an
extremely expensive elastomer whose uses are restricted to highly specialized applica­
tions. The nitrosofluororubbers exhibit poor heat stability (ca. 150°C) for a fluorinated
material and are difficult to crosslink; the vulcanizates do not exhibit good mechanical
strength. Applications have been in the U.S. space program and areas that have utilized
AFM U ’s reportedly excellent resistance to fire.
Polyphosphazene Rubber
Designation in ISO 1628: FZ and PZ
Repeat units

OCH2CF3
I
-P = N -

OCH2(CF2)xCF2H / „

FZ
 24 Thorn and Robinson

OCfiHs

-P C = N -

OCeH4PC2Hs
PZ

General. The 55% fluorine content o f the FZ type is intermediate between the
fluorosilicones and the fluorocarbons; thus, in general, the chemical resistance also lies
between those two materials. As well as exhibiting a good resistance to chemicals, the
FZ type of polymer gives superior low temperature performance when compared to the
fluorocarbons.
The PZ type is thought to be commercially available as Eypel A (Ethyl Corpora­
tion). This material is not fluorinated but is claimed to give equivalent oil resistance to
chlorosulfonated polyethylene and Vamac. It is self-extinguishing upon the removal o f a
flame and does not contain halogens—advantages indicating that wire and cable applica­
tions could be a potential market.
The FZ materials appear to be precompounded by the supplier, although it is under­
stood that peroxides are the cure agent used.
The PZ materials can be crosslinked by peroxides, sulfur, or radiation.
Polysulfides
Designation in ISO 1629: OT and EOT
General. These materials are formed by the reaction o f a dihalide with sodium
poly sulfide. The main chain o f the polymer formed from this reaction contains the fol­
lowing grouping:

-S -s-
II II
s s

Four types o f polymer are available from Thiokol, the only manufacturer o f this
type o f material.
Thiokol A is produced from ethylene dichloride and sodium poly sulfide. A high
molecular weight polymer is obtained with predominantly hydroxyl end groups on the
polymer chain. This type o f polysulfide rubber was the first commercial grade but has,
to a large extent, been superseded by the later FA type.
Thiokol FA is produced from a mixed dihalide, di-2-chloroethyl formal and
ethylene dichloride, and sodium polysulfide. Here again a high molecular weight poly­
mer (ca. 100,000) is produced with predominantly hydroxyl end groups on the polymer
chain. The sulfur content o f the resultant polymer is 49%.
Thiokol ST is produced from di-2-chloroethyl formal with a small percentage o f
1,2,3-trichloropropane to provide a branch point for improving the cure state obtain­
able, hence the compression set. A much lower molecular weight polymer (ca. 80,000)
is produced, with predominantly mercaptan (SH) end groups. The sulfur content o f the
resultant polymer is 37 %.
Compound Design 25

Thiokol LP grades are liquid polymers used in sealant and mastic applications, and
are formed by breaking down a high molecular weight polymer in a controlled manner.
The liquid polymer again has mercaptan end groups.
The polysulfide grouping in the polymer confers an excellent resistance to solvents,
with the sulfur content of the polymer determining the degree o f swell.
The resistance o f these materials to solvents, especially ketones, is good and is
often the major reason for their use.
The resistance to ozone and UV light is excellent, although the use o f 0.5 phr o f
nickel dibutyl dithiocarbamate (NBC) will improve ozone resistance further at high
ozone concentrations.
Both Thiokol A and FA require peptization to ensure ease o f processing, and this is
normally accomplished by the addition o f MBTS and DPG onto a two-roll mill main­
tained at 160°F prior to the addition o f other ingredients.
The ST types do not require peptization.
The A and FA types can be cured by the addition o f zinc oxide alone at ca. 10 phr.
Further additions o f sulfur, at up to 1 phr, act as accelerators, but these are needed only
if very fast curing compounds are required.
Any one of the following systems may be used to cure the ST polymers.

Component phr

p-Quinonedioxime 1.5
Zinc oxide 0.5
Stearic acid 0.5-3.0
/7-Quinonedioxime 1.0
Zinc chromate 10.0
Stearic acid 1.0
Zinc peroxide 6.0
Stearic acid 1.0

Uses. Polysulfide polymers are used in roller covering applications and hose
liners, as well as in molded goods.
The sealants find use in the construction and aerospace industries.
Propylene Oxide Rubber
Designation in ISO 1629: GPO
Repeat units

-CH------ O -
I
CHo
I
0
1
CHj— ch: ICH,

Propylene oxide Allyl glycidyl ether

26 Thorn and Robinson

G eneral The only commercially available material in this class, Parel, is a copo­
lymer o f propylene oxide and allyl glycidyl ether.
The absence o f any polar grouping gives this material superior low temperature
performance when compared to the epichlorohydrin terpolymers, but this advantage is
secured at the expense o f oil resistance. The unsaturated nature o f the main chain
confers excellent resistance to oxygen, ozone, and UV light.
The only cure systems seen for this material are based on sulfur vulcanization.
Uses. Little comment can be made on the uses o f this material. It is used in some
moldings, where the advantages o f heat resistance, low temperature performance, and
oil resistance, roughly equivalent to the same properties o f polychloroprene, can be util­
ized. It has been investigated for use in engine mounts.
Polyurethane Rubbers
Designation in ISO 1629: A U and EU
G eneral Polyurethanes, as a class o f materials, are one o f the most versatile
available. By varying the reactants, their amounts, and the reaction conditions, one can
obtain millable elastomeric gums, hard rigid plastics, reactive liquids, and foams. The
versatility is such that it is very difficult to provide a brief summary, and readers are
directed to Reference 7 for further information.
The basic reactions in polyurethane chemistry are:

RNCO ROH RNHCOOR' (1)

Isocyanate Alcohol Urethane

RNCO -h R'NH2 R N H -C O N H R ' (2)

Isocyanate Amine Urea

RNCO + R'NHCOOR" —> RlsfCOOR" (3)

1
Isocyanate Urethane CONHR
Allophonate

RNCO -f R'NHCONHR" - > RTSfCONHR" (4)

1
Isocyanate Urea CONHR
Biuret

In reactions (3) and (4) the isocyanate is capable o f reacting with the active hydro­
gen in a urethane or urea group, to give branching or crosslinking by the formation o f
an allophonate or a biuret group.
The most important reactions for the production o f elastomers, however, utilize
diisocyanates and polyols, and the elastomeric products formed can be castable
polyurethanes, millable gums, thermoplastic polyurethanes, and polyurethanes o f other
types.
Castable Polyurethanes. These liquid systems can be produced either in a one-
shot system (i.e., the diisocyanate, polyol, and chain extender reacted in a single stage)
or, more usually, as a prepolymer, which is chain extended and crosslinked at a later
stage.
In the prepolymer system, the diisocyanate and polyol (either a poly ether or a
polyester) are reacted to give a prepolymer, which may be either a liquid or a waxy
 Compound Design 27

solid. The reactant ratios used ensure the prepolymer contains isocyanate groups at the
chain ends.

Isocyanate-terminated prepolymer

OCNRNCO + OH»a a a a w »OH + OCNRNCO

O C N «AAAAAAAAAA/NAAAAAAAATNCO

The prepolymer can, when required, then be chain extended to give a high molecu­
lar weight crosslinked product:

O C N 'A A A A A A A A A A A A A A A A A A A / ^ N C O 0CN«AAAAAAAAAAAAAAAAAAA/*N C0 O C N 'A A A A A A A A A A A A A / V W V W 'N C O

NHRNH NHRNH

Chain extension

O C N »^'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA/VNAAAAAT N C O

Crosslinking and branching can be promoted either by the use o f a triol as a chain
extender or by using less chain extender than is theoretically required; the unreacted
isocyanate end groups then react with urethane groups in the main chain to form allo-
phonate or biuret crosslinks.

0 C N * A ''^ V W \ A A A A A A A A A / V » N H C 0 N H » ^ V \ A A A A A A A A A A A A A A ^ N C 0 + 0CN«AAAAAAAAAA/*NC0

Cross linking

OCN- - N H CONH«a a a a a a a a a a a a a a a a a N C O
C
o
N
H

NCO

Crosslinking by biuret formation

 28 Thorn and Robinson

Typical diisocyanates that are used are:

NCO
I
OCN-f II I
NDI (naphthalene 1,5-diisocyanate)

0CN(CH2)6NC0

NCO
TDI (toluene diisocyanate) HDI (hexamethylene diisocyanate)

Typical polyols include:

H0(CH2)2—^0C0(CH2)4C00(CH2)2— ^ O H
^ ' n
Polyethylene adipate (a polyester)

0 H - ^ (C H 2)40 —

Poly(tetramethylene ether) glycol (a polyether)

Typical chain extenders that are used are MOCA (4,4-methylene bisorthochloroaniline),
butane diol or trimethylolpropane.
Variations can be made in the following compositional categories:

1. Type o f diisisocyanate
2. Type o f polyol and its molecular weight
3. Type o f chain extender
4. Ratio o f prepolymer to chain extender

By altering the foregoing components, it is possible to make products elastomeric

or rigid and, by the inclusion o f water or a blowing agent, cellular products can be
obtained. Clearly, so many possible variations make general comments difficult, but the
following are generally accepted.

1. Polyester polyurethanes generally give superior mechanical properties and chemical

resistance, but inferior hydrolytic stability.
2. Polyether polyurethanes give superior low temperature properties and hydrolytic
stability.
3. Diamine chain extenders give superior properties to diol-cured elastomers.
4. Mechanical properties are generally improved as the hardness increases.
 Compound Design 29

Processing o f liquid systems proceeds as follows:

Prepolymer
mix degas cast > cure (solid)
i
postcure
Chain
extender

The mixing can be done by hand, or in low pressure mixer/dispensers, and in reac­
tion injection molding (RIM) machines. In the latter operation, no degassing is required.
M illable gums. The diisocyanates and polyols are reacted to form high molecular
weight hydroxyl-terminated millable gums. These millable gums are compounded and
processed as conventional elastomers, both sulfur and peroxides being used to cure the
polymers. Here again, polyether and polyester types are available, and the differences
between these two types referred to above also apply here.
Thermoplastic Polyurethanes. Consider the polyurethane:

HO— R-^OCONHR’NHCOOR-^OCONH— R'---- NCO

When R is small, such as the tetramethylene group (CH 2) 4, and R' is diphenyl-
methane, the polymers that result are rigid plastics, similar to polyamides.
If R is a polymeric ester or ether, of molecular weight 1000-3000, a flexible elastic
material will result. By reacting MDI and active hydrogen components (polyether/ester
and a short-chain glycol) in equivalent stoichiometric quantities, a linear polymer with
virtually no crosslinks is obtained.
If U is used for the diisocyanate, G for the short-clain glycol, and a wavy line for
the higher molecular weight flexible polyether or polyester, the resultant polymer can
be represented as follows:

u G UGU u G UGUGUGU ' ' wwwwwv ' U GUG UG U

This is a copolymer o f the (AB)„ type, where the UGU sequences represent the
urethane “hard segment” and the wavy line represents the “soft” flexible segment.
Microphase separation o f the hard segment occurs as shown in Figure 1.
The thicker lines represent the sequences o f “hard” urethane segments, and the
clusters of these effectively act as crosslinks, making the material perform like a con­
ventional elastomer. When the temperature is raised high enough, the clusters disassoci­
ate and the material can be made to flow; when subsequently cooled, the clusters can
re-form and the material again exhibits elastomeric properties. Thus these materials
30 Thorn and Robinson

show elastomeric behavior at room temperature but can be processed as thermoplastics.

Hence the name o f the material class—thermoplastic elastomers (TPEs).
Thermoplastic polyurethanes exhibit the same general properties as the cast and
millable types; unlike the conventional rubbers, they do not require compounding.
Other Types o f Polyurethane. These are listed but not discussed further.

1. One-component blocked, cast polyurethanes. Here reactive isocyanate end groups

are prevented from reacting by a “blocking agent.” Upon heating to cure tempera­
tures, the blocking agent splits off and cure commences.
2 . Cast systems to give rigid plastics.
3. Polyurethane foams and microcellular elastomers.
4. Spandex fibers.

Polyurethane elastomers are exceptionally tough and abrasion resistant; in addition,

they resist attack by oil. The polyester types are susceptible to hydrolytic attack above
ambient temperatures, and certain polyester thermoplastic polyurethanes have been
known to stress-crack in cable jacketing applications when in contact with water at
ambient temperatures; the latter efifect has sometimes, incorrectly, been ascribed to fun­
gal attack. Polyether types are far more resistant to hydrolytic attack. Certain
polyurethanes can be attacked by UV light, and resistance to this agency is determined
primarily by the isocyanate.
The polyurethanes are resistant to ozone attack.
Uses. Polyurethanes are used in a wide variety o f applications: seals, metal form­
ing dies, liners, coupling elements, rollers, wheels, and conveyor belts are examples.
The thermoplastic polyurethanes are used as cable Jacketing materials, conduits, fabric
coatings, in ski boots and other rigid boot soles, in automotive body components, and in
gear wheels and other business machine parts.
Thermoplastic Elastomers
General. This unique class o f materials has formed the basis for whole texts [8,9].
Thermoplastic elastomers exhibit elastomeric behavior at room temperature but can
be processed as thermoplastics. Before one can understand the performance o f these
 Compound Design 31

materials, an understanding o f how they can give their unique properties o f elasticity
and thermoplasticity is required; this is best done by considering such block thermoplas­
tic elastomers as styrene-butadiene-styrene (SBS).
It is possible to produce a block copolymer by the anionic polymerization o f
styrene and butadiene, as depicted in Figure 2: the polystyrene and polybutadiene are
mutually incompatible; hence they phase separate to give the morphology shown. This
simplified representation o f the morphology shows spheres o f polystyrene embedded in
a continuous soft elastomeric polybutadiene phase. Here the polystyrene domains act as

Linear SBS type

Radial (tetrachain) SBS type

Figure 2 Simplified schematic representation of the structure of SBS block copolymers.

 32 Thorn and Robinson

“pseudocrosslinks” and the polybutadiene conveys elasticity to the material. When

heated above the of polystyrene, the domains soften and disassociate, and the
material can be made to flow. When cooled, the polystyrene domains re-form and elas­
tomeric behavior returns.
The following features are therefore required for a material to act as a thermoplas­
tic elastomer:

1. A structural feature that acts as a pseudocrosslink at room temperature but will

disassociate at elevated temperatures
2. A soft elastomeric phase for the development o f elasticity.

The materials listed in Table 2 are the major types o f thermoplastic elastomer available
commercially.
Before briefly discussing each type, it is necessary to consider the performance of
thermoplastic elastomers and the problem o f defining service temperature limits for
them. The structural features that convey the ability to be processed as a thermoplastic
are also responsible for a limiting factor in their use. Since it is the “pseudocrosslinks”
that allow these materials to develop elastomeric behavior, any factor that interferes
with the integrity o f the pseudocrosslinks will weaken the material and allow excessive
creep or stress relaxation to occur under the sustained application o f stress and strain.
Temperature is obviously one such factor.
A method commonly used to derive a maximum service temperature limit is the
Underwriters Laboratories (UL) rating; here, a material is aged at various temperatures
and a property, say tensile strength, is monitored. The maximum service temperature is
then defined as the temperature at which the property being monitored decreases by
50% after 100,000 hours o f aging. Thus for the thermoplastic FEP, the UL rating is
150°C. Another method by which the effect o f temperature can be assessed is the heat
distortion temperature, which has the advantage o f assessing the effect o f temperature
while the material is stressed. For FEP the following limits are found:

Heat distortion temperature at 0.45 MPa, 70°C

Heat distortion temperature at 1.8 MPa, 50°C

Table 2 Major Types of Commercially Available Thermoplastic Elastomer

Examples of
Major type Subtype trade names

Styrenic SBS/SIS Cariflex TR, Kraton

SEBS Kraton G, Elexar
Elastomeric alloys EPDM/PP Levaflex, Santoprene
NR/PP Vynamar
NBR/PP Geolast
Chloro-olefin Alcryn
Polyurethane Ether/ester Desmopan, Estañe
Copolyether esters Hytrel, Arnitel
Polyether amides Pebax
NBR/PVC Chemigum P83"
Used as an additive to PVC to produce a TPE; this type is not discussed further.
Compound Design 33

It can therefore be seen that the UL rating is not an adequate method o f assessing max­
imum service temperature if a material is to support stress or strain in service.
Figure 3 illustrates the dependence of the tensile strength of SBS block copolymers
on temperature. In considering this plot, remember that tensile strength is a short-term
measure of the ability o f a material to resist stress and strain. One might ask the ques­
tion, What would happen to the material if it were strained to 200% extension at 40°C
and held there? The answer is that one would expect a high rate o f stress relaxation, the
rate being an order o f magnitude higher than would be expected o f a conventional elas­
tomer. It is therefore unfortunate that except for Dupont with its Hytrel material, the
manufacturers of TPEs do not include data on the creep and stress relaxation behavior
at elevated temperatures. They tend to rely, instead, on the UL approach to determining
maximum service temperatures; or at best, they furnish data on the variation o f hard­
ness and tensile strength with temperature. The maximum service temperatures quoted
in the following descriptions therefore need to be treated with a high degree o f caution.
It is interesting to note, but not really surprising, that the major uses o f TPEs are in
applications that take advantage o f their general toughness but do not call for the sup­
port of high applied stresses and strains.
Styrenic Block Copolymers
1. SBS/SIS Block Copolymers. In these block copolymers, the center elastomeric
block is either polybutadiene or polyisoprene. The unsaturated nature o f the midblock
renders them susceptible to attack by oxygen, ozone, and light. The morphology o f
these materials was illustrated earlier, as was the dependence o f their strength on tem­
perature. Their ability to function at elevated temperatures is restricted. This drawback
is best illustrated by noting that such block copolymers cannot be used as soling materi­
als for athletic shoes worn in indoor sports arenas; the frictional heat developed by stop­
ping and turning quickly, softens the material and causes excessive wear.

Temp. °C

Figure 3 Dependence of the tensile strength of Cariflex TR grades on temperature. (Courtesy

of Shell Chemicals, U.K.)
 34 Thom and Robinson

The styrenic thermoplastic elastomers are the only type to be fully compounded in
the manner o f conventional elastomers. In this case, however, the addition o f carbon
black or other fillers does not give reinforcement. Additions o f polystyrene, or high
impact polystyrene, and oil are used to vary hardness and tear strength, and fillers can
be used to reduce the cost o f the material. Other added polymers (e.g., EVA) can be
used to increase ozone resistance. These materials also require antioxidants for protec­
tion during processing and service life, and poor UV stability restricts their use in out­
door applications.
While poor resistance to oil and solvents is a drawback in some applications, it is a
positive advantage in solution processing applications (e.g., adhesive production).
Table 3 summarizes the SBS block copolymers. There may be some disquiet over
the apparent use of these materials as glazing strips, given the quoted temperature
range.
2. SEBS Block Copolymers. In this type o f styrenic TPE the poly butadiene mid­
block used in the SBS types is replaced with ethylene butylene, which is saturated:

CoH
2^5

- ^ C H 2 ------ CH2 ^ ----- i c H 2 ----- CH — X

\ 'x ' ' y
n

Ethylene butylene

Table 3 SBS Block Copolymers

Summary
Hardness range, 35 Shore A-40 Shore D
Maximum temperature, 65°C
Minimum temperature, —70°C
Advantages
Fully compoundable, especially for tack
Good wear characteristics in certain shoe sole applications
Ability to be solution processed
Good electrical properties
Wide hardness range
Disadvantages
Low maximum service temperature
Not resistant to oxygen, ozone, and UV light
Not oil or solvent resistant
Uses
Shoe soles, adhesives, plastics modification, bitumen modification, miscellaneous molded items
[e.g., swim fins, black window glazing gaskets (U.K. —30°C -f- 55°C)]; not used in wire and
cable
Compound Design 35

As expected, the materials exhibit the same morphology as the SBS types, but the
saturated midblock confers resistance to oxygen, ozone, and UV light. Although it is
said that improved phase separation gives improved stability to above ambient tempera­
tures, the Tg o f the polystyrene domains still restricts the use at high temperatures o f the
SEES block copolymers.
The SEES types are again fully compoundable, and Table 4 provides a brief sum­
mary. The properties exhibited by the SEES compounds are also representative o f those
of the SES-based compounds.
Elastomeric Alloys. These materials are generally produced by blending an elasto­
mer with a crystalline plastic, polypropylene being the most common. Such blends rely
on the crystal structures o f the plastic to guarantee strength and on the elastomer to pro­
vide a degree o f flexibility. In some blends the elastomeric phase can be partially
crosslinked, and improvements in the final properties result if this is carried out dynam­
ically (i.e., in a mixer).
The available hardness range of the elastomeric alloys is not as great as the styren-
ics, and at higher hardnesses the term “elastoplastic” might be considered to be more
descriptive. This comment also applies to the other nonstyrenic thermoplastic elasto­
mers, whose hardness range is even more limited than the elastomeric alloys.
1. Non-Oil-Resistant Types, EPDM/PP Elends, NR/PP Elends. The EPDM/PP
blends are produced by blending EPDM and polypropylene; in certain types (e.g., San-

Table 4 Properties and Summary of SEES Compounds

Typical properties
Hardness 45A 55A 66A 95A
Tensile strength, MPa 6.5 7.5 10.3 ll.O
Elongation at break, % 800 700 700 425
Tear strength, kN/m 28 21 48 78.8
Compression set, (%)
24 hat 23 °C 45
24 h at 70°C 65

Summary
Hardness range, 35 Shore A-40 Shore D
Maximum temperature, 65-80°C
Minimum temperature, —70°C
Advantages
Fully compoundable for tack
Ability to be solution processed
Good electrical properties
Resistant to UV light, oxygen, and ozone
Disadvantages
Low maximum service temperature
Not oil or solvent resistant
Uses
Wire and cable (U.S. automotive, not U.K.), adhesives, light-colored window glazing strips (U.K.
-3 0 °C to -f5 5 °C )
36 Thorn and Robinson

toprene), the elastomeric phase is dynamically crosslinked. These materials exhibit

excellent electrical properties, and they resist oxygen, UV light and ozone. They are not
however oil resistant.
Table 5 summarizes the strength properties o f Santoprene and lists the major
characteristics o f this type o f thermoplastic elastomer.
Comparison o f the heat-resistance properties o f EPDM and NR indicates that a
lower maximum service temperature should be expected for NR/PP blends, if aging is
the criterion being used. The ozone resistance and UV stability o f NR/PP blends is said
to be greater than those o f natural rubber, although at very high ozone concentrations
some attack would be expected on the softer grades.
These materials are not oil resistant, but they do have good electrical properties.
Table 6 summarizes the NR/PP blends.
2. Oil-Resistant Types, NBR/PP Blends, Chloro-olefinic Types. The blending o f
NBR and polypropylene gives a material that is oil resistant, and the commercially
available example o f this type. Geolast, is dynamically crosslinked. The ozone resis­
tance is again said to be greater than that o f nitrile rubber, but at high concentrations
some attack would be expected on the softer grades.
The commercial example o f the chloro-olefinic type is Alcryn, which is said to be a
blend o f a chloro-olefin with an elastomer. The nature o f the elastomer has not been dis­
closed. Here again the material is oil resistant.

Table 5 Properties of Santoprene and Summary of EPDM/PP Blends

Typical properties
Hardness 55A 73A 80A 40D
Tensile strength, MPa 4.4 8.3 11.0 19
Elongation at break, % 330 375 450 600
Tear strength, kN/m 19 28 34 64
Tension set, % 6 14 20 48
Compression set, %
168 hat 25 °C 23 24 29 32
168 hat 70°C 25 36 41 49
Summary
Hardness range, 55 Shore A-75 Shore D
Maximum temperature, -1- 100°C
Minimum temperature, —50°C
Advantages
Resistant to UV light, oxygen, and ozone
Good electrical properties
Toughness
Disadvantages
Not oil resistant
Hardness range more limited
Uses
Electrical, boots and bellows, weatherstrips, body components, bumpers and sight
shields in the automotive industry; wire and cable, window seals, and other mechani­
cal goods
Compound Design 37

Table 6 Summary of NR/PP Blends

Hardness range, 60 Shore A-50 Shore D

Maximum temperature, 70°C
Minimum temperature, -4 0 °C
Advantages
More ozone resistant than NR
Good electrical properties
Disadvantages
Not oil resistant
Uses
No current examples known

Table 7 presents a summary o f both types.

Thermoplastic Polyurethanes (TPU). The morphology o f this type o f thermoplas­
tic elastomer was given earlier.
The thermoplastic polyurethanes are available in a more limited hardness range
than the sty renies, and they are characterized by excellent strength and toughness, and
oil resistance. There are two major types available, polyester and polyether, the latter
exhibiting superior hydrolytic stability and low temperature performance.
The electrical properties o f the polyurethanes are not adequate for primary insula­
tion applications, but their general toughness leads to their use as cable Jacketing
materials.
Table 8 summarizes these materials.
Copolyether Esters. These materials are segmented copolyether esters formed by
the melt transesterification o f dimethyl terephthalate, poly(tetramethylene ether) glycol,
and 1,4-butane diol. As with the thermoplastic polyurethanes, one can describe a hard
segment and a soft segment, the hard segments forming crystalline areas that act as
“pseudocrosslinks,” as depicted in Figure 4.
These materials are again strong, tough, and oil resistant, but they are available in a
limited hardness range. Their hydrolytic stability is superior to that o f the polyester

Table 7 Summary of Geolast/Alcryn

Hardness range
Geolast, 60 Shore A-50 Shore D
Alcryn, 60-80 Shore A
Maximum temperature, -I- 120°C
Minimum temperature, —40°C
Advantages
Resistant to oxygen, ozone, and UV light
Oil resistance
Toughness
Uses
No current examples known
 38 Thorn and Robinson

Table 8 Properties and Summary of Thermoplastic Polyurethanes

Properties
Hardness 86A 95A(56D) 73D
Tensile strength, MPa 40 40 50
Elongation at break, % 450 400 250
Tear strength, kN/m 70 110 >120
Compression set, %
24 h at 70°C 30 30
70 h at 70°C 55 50
Summary
Hardness range, 75 Shore A-75 Shore D
Maximum temperature, 120°C
Minimum temperature, —40°C
Advantages Disadvantages
Resistant to oxygen and ozone Limited hardness range
Oil resistant Hydrolytic stability
Strong/tough UV stability of some grades
Electrical properties not adequate for primary insulation
Cost
Uses
Cable jackets, conduits, fabric coatings, ski boots, and other boot soles; automotive:
body components, bellows, and lock components; also hose jackets, protective bellows,
mechanical parts, animal ear tags, and other uses

Figure 4 Schematic diagram of proposed 4GT segment (straight lines) and PTMEGT segment
(wavy lines): A, crystalline domain; B, junction of crystalline lamellae; C, noncrystalline 4GT
segment. Only one chain shown for simplicity. (Reproduced by R. J. Celia, 7. Polym. Sci., Poly­
mer Symposia, No. 42, Helsinki, Part 2, 1972. Courtesy of John Wiley & Sons, Inc.)
 Compound Design 39

thermoplastic polyurethanes, but they do require additional protection in applications

calling for a high degree o f such stability. Table 9 gives representative properties o f the
copoly ether esters.
Polyether/Polyamide Thermoplastic Elastomers. These materials exhibit the same
type o f morphology as the copolyether esters, the polyamide providing the hard segment
and the poly ether the soft elastomeric phase. The service temperature is lower than that
o f the copolyether esters, but apart from this difference they exhibit similar properties.
Table 9 summarizes both the copoly ether esters and the poly ether amides.
Uses. Although the tables summarizing the individual types o f thermoplastic elas­
tomer have given indications o f their area o f use, where known, it is interesting to look
at the usage o f TPEs in Europe, as given in Table 10 for the year 1987.
While the thermoplastic elastomers are generally more expensive than the conven­
tional elastomers with which they sometimes compete (see Table 11), the price
differential often can be overcome by reduced processing costs and design
modifications.

Table 9 Summary of the Copolyether Esters and Polyether/Polyamides

Properties
Hardness 40D(92A) 55D 72D
Tensile strength, MPa 25.0 38.0 38.0
Elongation at break, % 450 450 350
Tear strength, kN/m 122 154
Compression set,
70 h at 100°C, % 60 56
Summary
Hardness range, 85 Shore A-75 Shore D
Service temperature
Hytrel - 5 0 to 4-150°C
Pebax - 4 0 to -f 80°C
Advantages
Resistant to oxygen and ozone
Oil resistant
Strong/tough
Disadvantages
Limited hardness range
Hydrolytic stability
UV stability
Electrical properties not adequate for primary insulation
Cost
Uses
Cable jackets, hose Jackets, tubing, seals; automotive bellows; mechanical parts: gear
wheels, business machine parts
 40 Thorn and Robinson

Table 10 Consumption of Thermoplastic Elastomers by End Use in Europe in 1987 ( x 10^ tons)

Elastomers"
End use tyrenics TPOs EAs TPUs COPEs PEBAs Total

Automotive 2 44 1 2 1 2 52
Wire and cable 0 5 1 2 1 0 9
Footwear 54 0 0 10 1 1 66
Polymer modification 8 5 0 N 0 0 13
Hose and tube 0 2 1 2 1 1 7
General mechanical goods 0 5 3 4 1 1 14
Bitumen modification 18 0 0 0 0 0 18
Construction 0 0 N N 0 0 1
Adhesives/coatings 13 0 0 11 0 0 24
Film/sheet 0 0 0 1 N 0 1
Other 5 N N N 2 N 7
Total 98 61 6 32 7 5 212
% of TPE use 46.2 28.7 2.8 15.1 3.3 2.3
"TPO, EPDM/PP blends; EA, elastomeric alloys, apart from TPO; TPU, thermoplastic polyurethane; COPE, copo­
lyether ester; PEBA, polyether/polyamide; N, negligible.

Table 11 Costs per Metric Ton

Material Pounds sterling

SBS ca. 1841

SEBS ca. 2184
Santoprene 2636-2966
Alcryn 3500-4000
Hytrel 3500-6000
TPU 3400-4500
Natural rubber 600
Nitrile rubber 1700-2000
Polychloroprene 2400-2700
Data from 1992; £ = $1..55.

1.2 SUM M ARY O F T H E P R O P E R T IE S O F E L A S T O M E R S

Tables 12 and 13 provide a summary o f the properties o f the different rubbers, apart
from the thermoplastic elastomers. These tables should be read in conjunction with Sec­
tions 1.2.1-1.2.12.

1.2.1 R e sista n ce to Se rvice Tem peratures and the Probiem of

Defining Se rvice Tem perature Lim its
A distinction between the short- and long-term effects o f temperature must be made
when considering the temperatures to be encountered during service. Short-term effects
are generally physical, hence reversible, whereas the long-term effects o f elevated tem­
peratures are mainly chemical. Long-term effects, which are irreversible, are generally
termed “aging.”
Compound Design 41

As the temperature is raised above ambient, mechanical properties (e.g., tensile and
tear strength) decrease, the rate o f decrease being dependent on the particular elastomer
used and the compound formulation.
Long-term exposure to elevated temperatures, or aging, results in a permanent
change in all the properties o f a rubber compound. The rate o f degradation, however, is
also dependent on the environment and the compound formulation. In the absence o f
oxygen, compounds generally can function at higher than their normally accepted ser­
vice temperature limits, whereas exposure to certain chemicals can drastically reduce
the maximum service temperature.
The definition o f a maximum service temperature for a rubber is therefore some­
what problematical, since it is dependent on the stresses and strains involved in an
application, the service life required, the service environment, and the compounding
ingredients.
The long-range elasticity characteristic o f rubbers is due to the flexibility conferred
by cooperative bond movements in the main chain. As the temperature is decreased
from ambient, such cooperative bond movements become restricted and elasticity is
progressively lost until, at the glass transition temperature Tg, they cease altogether and
the chain becomes inflexible. The elastomer then exhibits the characteristics o f a glassy
polymer. The actual Tg of an elastomer is, however, dependent on the method used to
determine it; for instance, if the method utilizes a dynamic property (e.g., tan 6 ), then
the Tg measured is dependent on the frequency o f the dynamic test. This, together with
the increase in the flexibility o f a compound at low temperatures that is possible through
the addition o f plasticizers, means that it is as difficult to define a lower temperature ser­
vice limit as it is the upper.
Long-term exposure to low temperatures has no equivalent among the aging
phenomena at high temperatures, but natural rubber and polychloroprene can exhibit an
increase in “stiffness” with time when exposed to low temperatures above their Tg. This
phenomenon is due to crystallization; with polychloroprene, the rate o f crystallization is
highly dependent on the polymer grade chosen, and with both rubbers the rate is depen­
dent on the exact temperature and compound formulation.
This stiffening, like the stiffening due to the approach to Tg, is physical and reversi­
ble.
The values quoted in Table 12 for service temperatures are thus based on experi­
ence and should be used as guides only.

1.2.2 Environm ental and Chem ical R e sistan ce

Chemical reagents and solvents can impair the structural integrity required for the
retention o f rubbery performance. It should be noted that a high molecular weight
confers no special protection against chemical attack. The chemical reactivity o f a poly­
mer chain is dependent on the repeat unit(s) within it, but there are two conflicting
requirements. Reactivity is required in the chain to allow the crosslinking reaction
necessary to achieve rubbery performance; but ideally, in service, the chain should be
inert. The chemical stability of the crosslinks and the additives in a rubber compound
will obviously contribute further to the reactivity o f a rubber compound.
Diene rubbers, those that contain a double bond in the main chain, are susceptible
to attack by ozone, which can lead to surface cracking and eventually failure. They are
also more easily attacked by oxygen and therefore generally have lower maximum ser­
vice temperatures. Ions of copper, manganese, iron, nickel, and cobalt can further pro­
mote the oxidative breakdown o f diene rubbers.
Table 12 Properties of the Major Rubber Tires “
6
(D
C
(U C
(D c<u tx (/3 2 C/3
<D
X(U
5 ^ C c o <D CL C in
X) c .ii o Cu X ou. <3 <U
X X •o
€ 3 X
23 C (U t-l X OJi
U 3
2 S X
oc o — 3
3 OC
CQ 'o 3 o O O
Z o- c£ on CU W o. CQ a. Z (X CL

EPM/ IIR/CIIR/
ISO Designation NR/IR BR SBR PNR EPDM BUR CR NBR AU/EU TM

Maximum temperature, °C 80 80 80 80 120-f 120 100 1204- 804- 100

Minimum temperature, °C -5 0 -7 0 -4 0 -2 0 -4 0 -2 5 -3 0 -3 0 -3 0 -4 5
-4 0
Physical properties
Hardness range 30-100 45-80 35-95 15-95 30-95 35-95 35-95 30-95 10-95 20-80
(D) (D)
Tensile strength 4000 2500 3000 3500 3000 2500 4000 3000 4000 1500
(approx max), psi
Tear strength E G F G F-G G E F-G E P-F
Resilience E E G P-F G P-F G-E F-G F-G P-F
Abrasion resistance E E G-E G G F G-E G-E E P
Compression set resistance G F G G G F F-G G G-E P-F
Impermeability F F F G-E F E F-G G P-F E IT
O
Electrical resistivity E E E G E E F F F F
Low temperature flexibil­ G-E E G F-G G F F F-G G F-G 0»
3
ity Cl
Environmental resistance D
O
Ozone resistance P P P P E G F-G P E E g;
UV resistance P-F P-F F F E G G P-F F-G G-E 5’
o>
Flame resistance P P P P P P G P P-G P o
3
 o
.’9 o
cd c
o C <y o 3
c2
« O <=i ■e •a
(L> <U oUi cd o
S o o c
(D
>> 2 "2 i2 x: 0c>
o1-1 oW« 3
x: a. o o a
a. o o
•5 c o 'o- o cd
W ■> u 8. w w CLh o
(D
(0
CM/ CO/ MQ/VMQ/ FMQ/ (O'
ISO Designation EAM GPO CSM ECO ACM FZ PVMQ FVMQ FKM FKM 3

Maximum temperature, °C 120 130 120 130 150 160 175 200 + 200 + 225 + 250 +
Minimum temperature, °C -3 0 -5 5 -3 0 -1 0 -3 0 -3 0 -5 5 -5 5 -5 5 -5 + -5
-4 0 -7 5
Physical properties
Hardness range 40-90 40-90 40-95 40-90 40-90 40-90 35-90 25-90 25-90 55-95 55-95
Tensile strength 4000 2500 3000 2500 2500 2200 2000 1250 1250 2500 250
(approx max), psi
Tear strength F-G F-G F-G F-G P-F P-F P-F P-F P-F F F
Resilience F F-G F-G F-G F-G F-G F-G F-G F-G F F
Abrasion resistance G F-G G-E F-G F-G F-G F P-F P-F F-G F-G
Compression set resistance G-E F-G F F-G G G G G-E G G-E G-E
Impermeability G - G E F F-G F P-F P-F G-E G-E
Electrical resistivity E F G F F F F E G G G
Low temperature flexibil- G G-E F F-E P-F P-F E E E F-G F
ity
Environmental resistance
Ozone resistance E E E E E E E E E E E
UV resistance E E E G-E E E E E E E E
Flame resistance P-F P G F-G P-F P G F-G F-G G-E G-E
" The symbol + indicates that choice of polymer, cure system, or other variable can extend the range; D indicates that hardness range can be extended into the Shore D
scale.
Table 13 Summary of the Resistance to Chemicals of the Major Types of Rubber

<D
<D <D
Ut C C
(U ’H- 2^
X) 0) C O
X c
cu. ot-l cx c«
p <D (U
o- c3
’5 O ot-l "O
X X
^ Cu t-l X
a o D C (U o
(L) c X t- 3
X)
oc
o 3 (/3
B OC ’C
■q PQ *0 t | 3 o "o *0
Z a. CU cn Pu w cx CQ (X z cx cu

Water E E G-E E E E P-G F-G P-G F

Steam ( > 120°C) P P P p G G F F P P
ASTM oil no. 1 P P P p P P-F F-G G G G
ASTM oil no. 3 P P P p P P F G G G
Aliphatic hydrocarbons P P P p P P G E G E
Aromatic hydrocarbons P P P p P P P-F F-G P-F G-E
Halogenated hydrocarbons P P P p P P P F-G F G
Alcohols G G G G G G F-G F-G F-G G
Ketones/esters G G G G G-E G-E F P P G-E
Organic acids, dilute G G G G E G E G P-F G
Organic acids, concentrated O
F P P F F F P P P G 3
Inorganic acids, dilute G G G G E G E G F E fi)
Inorganic acids, concentrated F G G F G G G F P P 3
a
Oxidizing acids, dilute P P P P F P F P P P 30
Oxidizing acids, concentrated P P P P P P P P P P o
o;
Alkalies F F F F G-E E E G-E P G 5‘
Animal and vegetable oils P-G F F F G G-E 0)
G G-E G G-E o
3
 "0 <U
0
<U <U 0
"0 =
·;::: -~
·;;: c:.; <U
= <U 3
<U "0 N
0 = <U ;., = = "'C
0
c:.; ..2 = ..::
"&u ..:: 0
<U - <U 0
"'0.. u .0 0
.,z .... <U
"' .... c:
=u <U
= =- 0 "'<U "'0 <U ·;;;
<U "' >.
"'o..c:;.,
<U
= c:.; ..:: 0 N
:::s
e;OJ 0.. =
0 .... Q.
"'u....0
0..
>.>. _;.,
:au >. >.
.... ;., u 0 0 ~
..:: = 0 ..::- ..:: ::::
.... ·s.. u 0 :::: e;; c(!)
£iJ ·s: c.. ug_ l:.tJ Ul < c.. Vi fi: fi: ::..::
ce·en:::s
Water G G G G E P-F F G G E G-E
Steam ( > 120°C) F p P-F p p p p P-F P-F P-F G
ASTM oil no. 1 F-G F F-G G G G G-E F-G G-E G-E G-E
ASTM oil no. 3 F-G F F-G G G G G-E F G-E G-E G-E
Aliphatic hydrocarbons F F G G-E G E E P-F E E E
Aromatic hydrocarbons p p P-F G P-F P-F E P-F G-E E E
Halogenated hydrocarbons p p p G-E p p G F G G G
Alcohols G G G-E G P-F p p G G F-E G
Ketones/ esters F F-G F-G P-F p p p G-G p p G
Organic acids, dilute G F E F p p G G F E
Organic acids, concentrated F p G p p p F F p E
Inorganic acids, dilute G G E G F F E E E E
Inorganic acids, concentrated F F G F p p P-F F E E
Oxidizing acids, dilute F p G p p p F F E
Oxidizing acids, concentrated p p p p p p F F F-G E
Alkalies G F-G E F-G F p F F P-F E
Animal and vegetable oils G F G G-E G-E G-E G-E F-G G-E E E

~
 46 Thorn and Robinson

The ultraviolet part o f the electromagnetic spectrum can cause the degradation o f
the diene rubbers and those that contain carbonyl groups (e.g., polyurethane).
Certain types o f elastomer—namely polyurethanes, silicones, acrylates, and
EVA—are susceptible to hydrolysis, a reaction that leads to chain scission and conse­
quent deterioration. The amine-crosslinked fluorocarbons can be attacked by steam, but
it is the crosslinks that degrade rather than the main chain.
For all practical purposes, liquids that attack crosslinked rubbers either degrade the
rubber or cause swelling through absorption. A swollen elastomeric network is much
weaker and more susceptible to damage, although in certain sealing applications a small
positive swell is beneficial for the retention o f the sealing force. It appears that no
account of the resistance to liquid media is complete without reference to the concept of
solubility parameter ô, which is the square root o f the cohesive energy density:

L-RT
Ô=
MI D

where L = latent heat o f evaporation

R = gas constant
T = absolute temperature
M = molecular weight
D = density

This concept predicts that a solvent with a solubility parameter similar to that o f a
polymer will swell or dissolve the polymer, depending on whether it is or is not
crosslinked. Unfortunately, this concept does not always work. The solubility parameter
o f a fluorocarbon rubber lies in the range o f 6 .3 -8 .3 , while that o f hexane is 7.3. Hex­
ane does not swell a fluorocarbon rubber, contrary to the prediction based on solubility
parameters, because fluorocarbon rubber is polar and hexane is not. As a general rule,
nonpolar solvents will swell nonpolar rubbers, and polar solvents will swell polar
rubbers.
The degree o f swell o f a polar rubber immersed in oil is normally determined by
the level o f aromatics in the oil, the aniline point o f the oil being a good guide to the
level o f aromatics; the lower the aniline point (typically 60-1 3 0 °C ), the higher the level
of aromatics.
The solubility parameter becomes even less useful when compounded rubbers are
considered, since the ingredients in a rubber compound can also reduce, or promote,
swell; increasing the filler content o f a compound will reduce swell in a solvent. The
degree o f crosslinking also affects the degree o f swell; ebonite, a highly crosslinked
natural rubber, exhibits much greater chemical resistance.
Liquid additives in a rubber compound can be extracted by liquids in contact with
the rubber, which leads to the following possibilities:
1. Zero, or a positive swell, when immersed in a solvent, but shrinkage and hardening
o f the compound when the swelling medium is removed.
2. Zero, or a positive, swell but no shrinkage upon drying.
3. Negative swell upon immersion.

If the possibility o f chemical degradation caused by contact with a liquid is included,

other possibilities exist:
Compound Design 47

4. Zero swell, but chemical degradation occurs.

5. Positive swell and chemical degradation.

Since none o f these possibilities is predictable by the use o f solubility parameters,

or any other simple rule, an empirical approach should be adopted when assessing the
resistance o f a rubber compound to liquid media. Reliance on swelling tests alone is in­
advisable. Reference can, of course, be made to the polymer manufacturer’s trade litera­
ture, ISO Technical Report 7620, and References 10 and 11 for guidance with respect
to the likely effects. The chemical resistance values given in Table 13 should be
regarded only as guides to likely performance.
Before leaving this subject, we note that liquid media can extract solid compound­
ing ingredients, the loss o f antioxidants being o f particular concern here. It is rare, how­
ever, for compounded rubbers to be adversely affected by contact with solid reagents.

1.2.3 M echanical Properties

Other terms—static stress-strain, and behavior under the short-term application o f stress
and strain—can also be applied to the mechanical properties o f a given rubber to distin­
guish them from the dynamic (cyclic) tests and from creep and stress relaxation:
Mechanical properties include:

Hardness (a measure of the elastic modulus o f a rubber)

Tensile stress-strain properties
Compression stress-strain properties
Shear stress-strain properties
Torsion stress-strain properties
Flexural stress-strain properties
Tear strength

Although tensile strength is widely measured on rubber compounds, it is generally

only used as an indicator o f the “quality” o f a compound, since strains in this mode are
rarely large enough to be truly o f interest in actual service. Rubbers that crystallize
upon stretching (e.g., natural rubber and polychloroprene) generally exhibit the highest
tensile and tear strengths, but these properties, as with the other mechanical properties,
are greatly influenced by compounding.
Rubber compounds exhibit extremely useful properties under compression, shear,
and torsion, and these are widely exploited in many applications. The complex behavior
of rubber under these deformation modes is dealt with comprehensively in References
12 and 13.

1.2.4 Long-Term Behavior Under S tre ss and Strain

Crosslinking o f a rubber does not totally prevent the occurrence o f flow due to the phys­
ical slippage o f chains under an applied stress or strain. Two phenomena result: creep,
which is an increase in length with time when the elastomer is held under a constant
stress, and stress relaxation, which is a decrease in stress when the elastomer is held
under a constant strain. Aging effects at elevated temperatures can cause chain scission
or further crosslinking to occur, hence can alter the rates o f both phenomena.
The widely used compression set measurement, and the less-used tension set,
represent attempts to quantify the extent to which a rubber compound will exhibit creep
 48 Thorn and Robinson

and stress relaxation, but the former is also widely used as a method for checking the
state o f cure.
The creep and stress relaxation behaviors o f a rubber compound are strongly
influenced by the choice o f cure system and the degree o f crosslinking, among other
factors.

1.2.5 Dynam ic Properties

When a rubber compound is subjected to dynamic stressing, the response is a combina­
tion o f an elastic response and a viscous response, rather than a purely elastic one, and
energy is lost in each cycle. Reference 13 should be consulted for more detailed infor­
mation.
Figure 5 shows the response o f a crosslinked rubber to an applied strain. The resul­
tant stress is considered to precede the strain by the phase angle 5; tan 6 is called the
“loss tangent” and thus is a measure o f the energy lost within the material.
Figures 6 and 7 show the dynamic response (complex modulus and tan 6) o f a
natural rubber compound at different temperatures and frequencies in three-point bend­
ing. It can be seen that these dynamic properties are dependent on both factors, and
these two effects can be related by the W illiams-Landel-Ferry (WLF) equation.
The tan S peak occurs at the Tg o f the compound at the particular frequency o f test,
and its position is primarily dependent on the elastomer base o f the compound. It is pos­
sible, within limits, to vary the position o f the tan S peak (e.g., by incorporation o f low

Figure 5 Sinusoidal chain and stress cycles. I, strain, amplitude a; II, in-phase stress, ampli­
tude b; III, out-of-phase stress, amplitude c; IV, total stress (resultant of II and III), amplitude d
6 is the loss angle.
 LOGE’

Figure 6 Dynamic modulus of a natural rubber compound versus temperature.

Tan 5

49
50 Thorn and Robinson

temperature plasticizers) and the value o f tan 6 at ambient temperatures (e.g., by alter­
ing the filler loading and type). Blends o f different polymers may exhibit more than one
major tan 5 peak or a broadening o f the single peak, depending on the blend composi­
tion; blending is another method by which tan ò can be increased at ambient tempera­
tures.

1.2.6 Fatigue Perform ance

Fatigue, in this context, is taken to mean the growth o f cracks in a rubber compound
undergoing cyclic deformation to moderate strains, although it can also result in a
change o f stiffness and a loss o f mechanical strength.
The Malaysian Rubber Producers Research Association (MRPRA) [14,15] has
undertaken valuable work on the fatigue o f rubber strips in tension. The following
expression was derived for strips cycled from zero strain:
_G_____ i_
N =
(2KW )" Q
where N = fatigue life, in cycles to failure
G = cut growth constant
K = function o f the extension ratio
W = strain energy per unit volume
Cq = initial depth o f cut (or intrinsic flaw)
n = strain exponent, dependent on the nature o f the polymer

Elastomers generally show wide differences in tensile and tear properties, but the
naturally occurring flaw size ( Q ) does not differ widely among elastomers. The rate at
which the crack propagates does, however, differ widely. The crystallizing rubbers
exhibit good resistance to crack propagation, but this resistance is further enhanced by
prestraining the part, since crack propagation is most severe if the part passes through
zero strain. In the noncrystallizing rubbers, fatigue life is increased by increasing the
hysteresis loss or damping (ta n 5 ).
Fatigue resistance can be increased by the addition o f reinforcing fillers, and the
inclusion o f antioxidants and antiozonants, where appropriate. In the sulfur-curable
diene rubbers, resistance to fatigue at high imposed strains is increased by the use o f
conventional sulfur cure systems.

1.2.7 Friction and Wear Perform ance

Friction and wear are both considered here, since friction is a contributory factor in
wear mechanisms; in tire applications, both factors are key performance characteristics.
While the subject is dealt with only briefly here, further discussion o f the subjects can
be found in References 16-18.
The classical laws o f friction do not apply to rubber; at high normal loads the
coefficient o f friction decreases with increasing normal load, and it is also dependent on
velocity and temperature. Under certain conditions, where friction decreases with velo­
city, a “stick/slip” phenomenon can occur, with the coefficient o f friction oscillating
between two values. Aging, wear debris, humidity, and contact with liquids can all
affect the frictional behavior.
In wet or lubricated conditions, the hysteresis loss, tan ô, o f a polymer/compound
assumes the major role in determining the coefficient o f friction, and factors that affect
 Compound Design 51

tan 6, such as the level and type o f carbon black, affect the coefficient. The deliberate
modification o f the surface layers (e.g., by chlorination o f certain diene rubbers) pro­
vides a route by which the coefficient o f friction can be altered; a lowering results in the
example cited.
We have few useful data on friction; hence designers need to do laboratory testing
where this aspect is o f importance. Such tests, however, need to be relevant to the appli­
cation, and the test techniques must be developed with care. In some instances, both
friction and wear can be studied in the same experiment.
Wear, the general term used to describe loss o f material from a body, is most com­
monly caused by rubber moving in contact with another surface. Wear can occur by
three separate mechanisms:

Abrasive wear is caused by hard, sharp projections in the contact surface, which cut into
the rubber.
Fatigue wear is caused by rough surfaces that do not have sharp projections; wear
proceeds by the detachment o f particles fatigued on a localized scale by dynamic
stressing.
Adhesive wear is caused by contact with essentially smooth surfaces, rubber being
detached by roll formation. This mechanism is associated with a high coefficient o f
friction between the surfaces.

In many applications all three wear mechanisms may be involved and which, if any,
predominates will depend critically on the service conditions. Laboratory tests, which
generally measure abrasive wear, are thus notoriously poor predictors o f service life
unless the wear mechanism involved in the application is mimicked as part o f the test.
An example o f this would be the comparison o f a tire tread and the liner o f a shot
blast hose by use o f the abrader o f the German standardization organization (DIN). A
tire tread with a low DIN abrasion loss wears well in service but would wear rapidly if
used as hose liner. A very soft natural rubber compound, with a very high DIN abrasion
loss, resists wear by shot blast media but would be rapidly worn if used as a tire tread.
It is difficult to usefully comment on the wear properties o f polymers, apart from
noting that they are influenced by friction and by the compounding ingredients used.
Poly butadiene, carboxylated nitrile, and polyurethanes are noted for good abrasion
resistance.

1.2.8 Electrical Properties

Electrical resistivity, dielectric constant, tangent o f the dielectric loss angle (tan 6 ), and
dielectric strength are key parameters which determine the usefulness o f elastomers as
potential insulators in wire and cable applications. The electrical properties o f rubbers,
apart from the dielectric constant, differ widely but can be further altered by compound­
ing and as a result o f the susceptibility o f the polymer and/or the compound to picking
up moisture; dielectric constant can be widely varied by the choice o f additives.
Since some elastomers (e.g., nitrile) exhibit deficiencies in one or more o f the key
parameters, they do not find use as primary cable insulations. Silicone rubber is used in
some cable applications where performance in a fire situation is important because when
this material is burned, an insulating char is formed, and insulation, in the short term, is
maintained.
While rubbers are normally considered to be insulating, black-filled vulcanizates
may in fact be sufficiently conductive to be a hazard. Figure 8 shows the resisitivity o f a
natural rubber compound filled with superabrasion furnace (SAP) black.
 52 Thorn and Robinson

Figure 8 Effect of carbon black concentrations on resistivity.

Rubbers are frequently compounded to have antistatic properties by the addition o f

carbon black, and specially conductive blacks and metal powders can be used to pro­
duce truly conductive compounds. Resistivities as low as 5 -1 0 fi -cm can be achieved by
the use o f carbon black, and 0.02 12-cm by using metal powders. The following resis­
tance ranges for products were suggested by Norman [19].

Resistance
Product/characteristic ( 0)
Insulating >10'2
Elimination of static nuisance 10'®-10'2
Antistatic (no lethal currents) 5 X 10^-10®
Conductive >10^

1.2.9 Therm al Properties

Properties considered here are the specific heat, thermal conductivity, and thermal
diffusivity. References 20 and 21 should be consulted for further details on these prop­
erties.
 Compound Design 53

Specific heat is additive, so that this property can be calculated from a knowledge
of the values for each ingredient.
Thermal diffusivity is a property that is becoming o f increasing importance, since it
governs the time-dependent temperature distribution in a rubber compound under non­
steady-state conditions—for example, during processing. It is a parameter that is
required in software packages that predict flow during molding operations and in
software packages designed to calculate cure cycles for very large articles, such as dock
fenders.
Differences in thermal diffusivity among rubbers are generally small and can be
considered to be secondary, since the addition o f fillers significantly alters this property.
The thermal conductivity o f rubber compounds is important in controlling the heat
transfer across the interface with metal during processing. It is o f direct importance,
however, only as a final compound property in heat insulation applications, where a cel­
lular product would be produced, or in “potting” compounds, where heat loss from an
electrical circuit is required. In both product applications the differences in thermal con­
ductivity between rubbers are secondary to that o f the ingredients used. In cellular insu­
lations, the gas used to “blow ” the product is also a significant factor.

1.2.10 Perm eability

Gases, vapors, and liquids will permeate rubbers and their compounds in two stages.
Initially, the gas or liquid dissolves in the polymer, and the dissolved gas or liquid
diffuses through the polymer. The methods used to measure the permeability coefficient
Q are detailed in References 20 and 21. The coefficient is related to the solubility
coefficient S and diffusion constant D by:

Q = DS

For the air gases, 0 is a constant, but for other gases and vapors it varies with pre-
sure and temperature. As with most other properties, the permeability o f a compound is
affected by the quantity and type o f filler added.
Impermeability is an important characteristic in many applications, but in gas seal­
ing applications it can lead to failure by explosive decompression. This phenomenon
results when the external gas pressure is reduced faster than the dissolved gas can per­
meate out from the seal; failure will occur if the rubber compound cannot withstand the
stresses resulting from expansion o f the internally dissolved gas.

1.2.11 A dhesion, Corrosion, and Stain ing

In the many uses to which rubber is put, adhesion to metal, fabrics and fibers, and other
materials is commonly required.
Rubber-to-metal bonds are normally achieved by the use o f proprietary rubber-to-
metal bonding agents applied to the metal, although sulfur-cured diene rubbers can nor­
mally be adhered to brass-coated metals without the use o f this type o f bonding agent.
Achievement o f a good bond may require chemical pretreatment o f the metal, but in
every case stringent in-process control procedures are essential.
Adhesion to fabrics and fibers normally requires the use o f specially treated types
(e.g., fabrics dipped in resorcinol-formaldehyde resin in a latex) and/or the use o f addi­
tives that promote bonding.
In the examples above, the bond is achieved during vulcanization, but many
proprietary adhesive systems are available to bond cured rubber to other materials. The
54 Thorn and Robinson

particular adhesives that are used are dependent on the particular rubber/material com ­
bination, and the polymer manufacturer’s recommendations should be followed. Surface
pretreatments o f either surface may be required, chlorination o f the diene rubbers, and
etching o f PTFE respectively, being examples.
It is not commonly known that compounded rubber can promote the corrosion o f
metals with which it is in contact and that this corrosion may be accompanied by adhe­
sion o f the rubber to the metal. This phenomenon is dependent on the elastomer type
and formulation used, and it should be considered in sealing applications. Acrylates and
nitrile rubber compounds are two types that are known to exhibit this form o f
corrosion/adhesion. Further information can be found in References 22 and 23.
Rubber compounds can cause staining when they are in contact with organic
finishes, but this phenomenon is primarily dependent on the ingredients used, not the
base polymer.

1.2.12 Processib ility

The term “processibility” is not well defined and is commonly used to describe several,
often unconnected, desirable features it is hoped a polymer and compound will exhibit.
Each processing stage must be considered separately.
The molecular weight average and distribution, and the degree and type o f branch­
ing, are the principal factors that govern the processibility o f an elastomer; that is:
viscosity, die swell, quality o f extrudate, milling characteristics, green strength, and
tack. The processibility o f a rubber compound is, however, further influenced by the
level and types o f filler and oil used, the presence o f other process aids, the degree o f
mixing, and the level o f scorch (premature cure).

1.3 COM POUND IN G R E D IE N T S

There are a wide variety o f ingredients from which the rubber compounder can choose
to modify the physical and chemical properties o f an elastomer. The objective in using a
particular ingredient might be one or more o f the following:
1. To improve the physical properties o f the rubber
2. To improve the processing properties o f the rubber
3. To crosslink the rubber compound
4. To control the rate of cure
5. To prolong the service life o f the rubber
6. To extend the service range o f the rubber
7. To reduce the cost o f the rubber compound

Unfortunately, however, any particular additive chosen may have a beneficial effect on
one property but a detrimental effect on another.
Table 14 gives a typical rubber formulation and the function o f the ingredients. In
any one compound, two or more o f the following types o f compounding ingredient may
be present:

A polymer
A vulcanizing agent
A cure activator
A cure accelerator(s)
Compound Design 55

Table 14 A Typical Compound Formulation

Ingredient" Ingredient function Amount (phr)

Natural rubber Rubber 100.00

Whiting Diluent white filler 100.00
HAF N330 Carbon black reinforcing filler 50.00
Naphthenic oil Softener/process aid 20.00
Zinc oxide Cure activator 5.00
Stearic acid Cure activator 1.00
Santoflex 13 Antioxidant/antiozonant 1.00
Heliozone Protective wax for ozone resistance 2.00
MBTS Accelerator (primary) 1.00
DPG Accelerator (secondary) 0.50
Sulfur Vulcanizing agent 2.50
"For carbon black and accelerators, see Tables 16 and 18, respectively.

A filler (reinforcing, semireinforcing, or diluent)

A softener/process aid/tackifier
A plasticizer
A protective agent(s)
Miscellaneous ingredients: pigments, blowing agents, other

The following brief notes discuss each o f the foregoing types and the salient
features o f their use.

1.3.1 V u lcanizing A gents

Various types o f vulcanizing agent can be used to cure natural and synthetic rubbers,
but the most common system for vulcanizing the unsaturated rubbers is sulfur.
The properties that result from a vulcanízate depend on the number and type o f the
crosslinks formed. The number o f crosslinks formed will depend on the amount o f the
cure agent added and the cure time.
Increased amounts o f cure agent will generally increase the number o f crosslinks,
with the following effects on the final properties o f the elastomer:

Modulus/hardness, and resilience will increase.

Permanent set, elongation at break, and the degree o f swell in a solvent will decrease.
Tensile strength and tear strength will generally go through a maximum and then
decrease with an increasing degree o f crosslinking.

With the same amount o f cure agent, but a variation in the cure time, these effects
will again be noticed as the cure time is increased. The magnitude o f the effect will be
reduced, but it is still possible to produce a significant effect on properties by under- or
overcuring. The cure time chosen for any application is therefore a compromise with
regard to the properties required o f the end product.
Some cure systems can revert at the cure stage—that is, the number o f crosslinks
decreases if long cure times or high temperatures are employed at this point—and this
reversion will cause a reduction in most properties.
Other cure systems exhibit an effect termed “marching modulus,” which, not
surprisingly, means that modulus continues to increase with increased cure time.
 56 Thorn and Robinson

Sulfur
The rhombic form o f sulfur, which is the type normally used, has a limited solubility in
rubber at room temperature (ca. 0.8 phr). If the free sulfur level is above this, a surface
bloom may form which can lower surface tack and cause “blotchiness” on cured arti­
cles. Insoluble sulfur, although more expensive and slightly more difficult to disperse,
does not bloom; however, it converts back to the rhombic form if the temperature
exceeds 100°C.
A magnesium carbonate surface treated grade o f sulfur, sulfur MC, is available,
and this is used in nitrile rubber, since the rhombic form is difficult to disperse in this
rubber.
Unaccelerated sulfur cures are lengthy and do not yield a good property spectrum.
Therefore the sulfur vulcanization o f all unsaturated elastomers is modified by the addi­
tion o f accelerators.
The properties that result from a sulfur-cured vulcanízate depend on the number
and type o f crosslinks formed. The effect o f the number o f crosslinks has been dis­
cussed, but the type o f sulfur crosslink formed has an equally important role in deter­
mining the final physical properties.
Figure 9 illustrates the types o f sulfur crosslink that can be formed during vulcani­
zation by sulfur; the type o f crosslink produced depends on the accelerator type and
accelerator/sulfur ratio.
Typical numbers for in polysulfide linkages in natural rubber are as follows:

Cure or system Linkages

Unaccelerated rubber/sulfur cure 40-45

Conventional rubber/accelerator/sulfur 5-10
Efficient vulcanizing (EV) system 4-5
Sulfurless cure systems (sulfur donors) < 4

The differences produced by different cure systems are best illustrated in Figure 10
by the work carried out by Moore on natural rubber.

(a) (b) (c)

Figure 9 Types of sulfur crosslink: (a) monosulfidic, disulfidic, and polysulfidic; (b) pendant
sulfur; (c) intramolecular linkages. Types (b) and (c) are wasteful of sulfur.
 Compound Design 57

S
CM
Total
if) - /
JC
c I
i/) RSS1 100
Monosulfide
/ S 0-^
CBS
Q
■I 6-0
O
u / ZnO
Laune acid
5-0
1-0 J
£
a; P o ly s u lfid e
iz
o
D is u lfid e

Cure time at UO u (h)

(a)

(b)
Figure 10 Variation of crosslink number and type for conventional (a) and EV (b) cure sys­
tems. (From G. C. Moore, Chemistry of Vulcanisation, Proceedings of the Natural Rubber Pro­
ducers Research Association Jubilee Conference, Cambridge 1964, L. Mullins, Ed., London,
McLaren & Sons Ltd., p. 184. Courtesy of MRPRA.)

Polysulfidic crosslinks will give better fatigue properties, but poorer compression
set and aging properties. Monosulfidic crosslinks give better aging properties and
compression set, but poorer fatigue properties. Semi-EV systems give intermediate
properties.
If you have ever wondered why trucks shed their tire treads on highways, then con­
sider the changes to the crosslinks o f the conventional cure system in Figure 10. The
reduced number of crosslinks and the change from polysulfidic to monosulfidic cross­
links drastically reduce the fatigue strength until eventually the tread separates.
 58 Thorn and Robinson

Only unsaturated rubbers, or rubbers with pendant unsaturation, can be vulcanized

with sulfur. For natural rubber, with a high level o f unsaturation, vulcanization takes
place readily, but in polymers where the unsaturation is low (e.g., butyl), vulcanization
is slow, hence generally requires more active accelerators.
A conventional cure system for natural rubber would generally contain 2 .0 -3 .5 phr
o f sulfur and 0 .5 -1 .0 phr o f an accelerator. Table 15 illustrates the different types o f
cure system for natural rubber.
As the level o f accelerator is increased, the amount o f sulfur must be reduced if a
constant crosslink density is to be maintained; hence the number o f atoms in the sulfur
crosslink is reduced. The so-called efficient vulcanizing (EV) systems utilize accelerator
levels of 3.0 phr plus (with low sulfur levels, ca. 0.2 phr), but at least one o f the
accelerators must be a “sulfur donor” ; this feature o f accelerators is discussed later, but
we note here that sulfur crosslinks can be formed without the need for added elemental
sulfur.
The synthetic rubbers require higher proportions o f accelerators than natural
rubber, with lower amounts o f sulfur. It is also possible to use the EV and sulfurless
cure systems.

Peroxides
It is possible to cure most unsaturated rubbers, and some o f the saturated rubbers, by
the use o f covalent peroxides. Polychloroprene and butyl rubber are two unsaturated
rubbers that cannot be cured by peroxides.
Upon heating, a covalent peroxide decomposes to form two radicals:

R -O -O -R 2R-0-
These free radicals are extremely reactive and stabilize themselves by dehydro­
genating the polymer chain, during which process the free radical is transferred to the
polymer chain.

R -0 «AAAAAATCH2

Table 15 Examples of Cure Systems for Natural Rubber

Systems'^
Ingredient A B C

Natural rubber 100.00 100.00 100.00

Zinc oxide 5.00 5.00 5.00
Stearic acid 1.00 1.00 1.00
Sulfur 2.50 0.75 0.20
Insoluble sulfur 1.75
MBT 1.0 1.0
TMTD 3.0
"A, conventional cure system; B, conventional cure system~no bloom; C, EV system.
 Compound Design 59

Two polymer chains, containing a radical, can then react to form a C—C crosslink:

»AAAAAAAAAATC H «AAAAAAAAAACH

«AAAAAAAA/* CH
«^AAAAAAAAA/» CH »AAAAAAAA/*

The C—C crosslink formed is more heat resistant than a C—S crosslink, and perox­
ide cures generally give better compression set properties. However, these advantages
are offset by generally lower strength properties and a tendency for the surface o f the
rubber to become sticky if oxygen is not excluded during cure. Silicone rubber does not
exhibit this latter disadvantage. Peroxide-cured vulcanizates can also exhibit a strong
unpleasant odor.
Other compounding ingredients must be chosen carefully when peroxide cures are
used. Acidic, or strongly alkaline, additives can interfere with the yield o f free radicals
and the rate o f decomposition. Most fillers, and certain plasticizers, reduce the
efficiency o f peroxide curing, and this effect can be countered by an increase in the level
of peroxide used. Process oils used should preferably be straight-chain aliphatics.
Antioxidants can also inhibit the cure by peroxides.
The decomposition temperature o f a peroxide is important in determining the cure
temperature and the process safety; the rate o f cure given by a peroxide is determined
by its half-life at a particular temperature. Half-life is the time taken for half the amount
o f peroxide to decompose.

Number Peroxide decomposed (%) Residual peroxide

of half-lives or cure state (%) remaining (%)

0 0 100
1 50 50
2 75 25
3 87.5 12.5
4 93.75 6.25
5 96.75 3.125
6 98.4375 1.5625
7 99.21875 0.78125

Thus, if the half-life o f a peroxide at 160°C was 7 minutes, a 35-minute cure (5

half-lives) would leave approximately 3% unreacted. Such a cure time would be gen­
erally uneconomical, and peroxide-cured articles usually are given a short press cure,
followed by a much longer postcure in an oven. As well as allowing unreacted peroxide
to decompose, this postcuring serves to remove the decomposition products o f the
peroxides that might cause unnecessary odor and unwanted side reactions; for example,
the decomposition products o f peroxides containing carboxylic acid groups can catalyze
the hydrolytic breakdown o f silicones.
The half-life o f a peroxide is also dependent on the polymer in which it is com­
pounded. The following method can be used to provide a fairly accurate assessment o f
the half-life.
 60 Thorn and Robinson

First a cure trace is obtained on a curemeter at the temperature o f interest and

allowed to proceed until a virtual plateau has been reached. Next a plot o f log (max­
imum torque — torque at time t) versus time t is produced, and the slope m is calcu­
lated. The half-life is then:
- 0 .3 0 1
^'/2 —
m
The amount o f peroxide added to a compound determines the degree o f crosslink­
ing, but additives known as coagents can be used to further increase the degree o f
crosslinking at a constant peroxide level. The best-known example o f a coagent is
triarylcyanurate, but acrylates and sulfur can also be used.
Peroxides do not respond to accelerators, and the only method o f increasing the
cure rate is to increase the temperature o f cure.
The peroxides normally used to cure rubbers are organic and can be divided into
two general types:

1. Peroxides with carboyxlic acid groups (e.g., dibenzoyl peroxide). These exhibit low
sensitivity to acids, low decomposition temperatures, and a high degree o f deactiva­
tion by carbon black.
2. Peroxides without carboxylic acid groups (e.g., dicumyl peroxide, di-rm-butyl
peroxide). These exhibit less sensitivity to acids, aliphatic substitution being prefer­
able to aromatic, higher decomposition temperatures, and less sensitivity to oxygen.

Radiation Curing
The use o f high energy radiation is another method by which crosslinking by free radi­
cals can be initiated.
The use o f electronic generators, such as the Van de Graaif generator, to produce
electron beams o f 0 .5 -1 0 MeV is the most common method o f radiation curing, but col-
bat sources [^^Co] and resonance transformers can also be used.

Metai Oxides
Crosslinking by the use o f metal oxides is used to cure polychloroprene, chlorosul-
fonated polyethylene, polysulfides, and carboxylated nitriles. Metal oxides are also used
as acid acceptors in rubbers containing halogens.
Commonly used metal oxides are magnesium oxide, zinc oxide, lead oxide, and red
lead. The type o f oxide used can influence the water and acid resistance o f the rubber
compound, especially in the case o f polychloroprene. The particle size o f the oxide
influences the cure rate and degree o f cure obtained, the fine particle sizes being pre­
ferred.
Metal oxide cures can be accelerated; the type o f accelerator used and whether it is
necessary depend on the particular polymer and metal oxide being used. These aspects
have been discussed under each particular polymer type.

Resin Cures
Butyl, chlorobutyl, bromobutyl, and EPDM are elastomers that can be crosslinked with
reactive phenol formaldehyde resins o f the type:
 Compound Design 61

OH
OH / 5” \
OHCH. ■GHj CHjOH

It is understood that natural and nitrile rubber have also been crosslinked by this
type o f system, but no great advantages accrued.
In butyl rubber, the resins require activation by a halogen-containing material,
polychloroprene normally being used in preference to SnCl2 . If some o f the hydroxyl
groups on the methylol group of the resin are replaced by bromine, the activation by an
added halogen is not required. Chloro- and bromobutyl do not require the added halo­
gen, but EPDM does.
Normally 5-12 phr o f resin is used, and such cures give butyl rubber excellent heat
resistance, the polymer being capable of service at temperatures some 60°C higher than
temperatures obtainable from other cure systems. In EPDM, the service temperature is
raised by about 3 0 °C compared to the sulfur-cured varieties; this, combined with the
low compression set and the good strength properties exhibited, makes resin cures an
alternative to the peroxide curing o f EPDM.
Typical resin cure systems for butyl rubber and EPDM are as follows:

Polymer phr

Butyl rubber
butyl 95.0
Neoprene W 5.0
zinc oxide l.O
resin 3-6.0
EPDM
EPDM 90.0
bromobutyl lO.O
resin 10-12.0

Diurethane Cure Systems (Novor Cures)

The diurethane cure systems are a fairly recently developed method o f crosslinking
natural and some of the synthetic rubbers, based on chemistry developed by MRPRA.
Generally the Novor cures combine the best features o f conventional and EV sys­
tems, providing thermally stable crosslinks but with superior dynamic fatigue perfor­
mance. The reversion resistance o f the cure system is outstanding, and the physical
properties obtained are also unaffected by cure temperature.
The ultimate heat and reversion resistance is gained by use o f a pure Novor system,
but small additions of sulfur produce a synergistic effect on modulus and a faster cure
rate. The use of sulfur in the “80/20” system does not contribute to inferior aging.
62 Thorn and Robinson

Caloxol is added to the pure Novor cure system to remove water, which could
deactivate the isocyanate cure agent; but steam curing can lead to surface hydrolysis.
Typical cure systems are as follows.
Ingredient Pure Novor (phr) 80/20 system (phr)

Novor 6.7 4.2

ZDMC 2.0
Caloxol 3.0
Sulfur 0.4
TMTM 1.3
Santocure NS 0.08

For the accelerators ZDMC and TMTM, see Table 16.

Table 16 Major Types of Accelerator

Accelerator Common abbreviation

Guanidines
A^,N-Diphenylguanidine DPG
A,A-DiorthotoIyI guanidine DOTG
Orthotolyl biguanidine OTBG
Triphenyl guanidine TPG
Thioureas
A^,A^-Dibutylthiourea DBTU
1,3-Diethylthiourea DETU
MN-Diorthotolyl thiourea DOTU
A^,A-Diphenyl thiourea DPTU
Ethylene thiourea ETU
Tetramethyl thiourea
Trimethyl thiourea
Thiazoles
2-Mercaptobenzothiazole MBT
Zinc-2-mercaptobenzothiazole ZMBT
2 ,2 -Dibenzothiazyl disulfide MBTS
2-(2,4-Dinitrophenyl)mercaptobenzothiazole DMBT
4-Morpholinyl-2-benzothiazydisulfide MBSS
Sulfenamides
2-(4-Morpholinylmercapto)benzothiazoIe MOR (MBS/MOBS)
A^-rerr-Butyl-benzothiazyl sulfenamide TBBS (NS)
N-Cyclohexyl-2-benzothiazyl sulfenamide CBS
A,A-Diisopropyl-2-benzothiazyI sulfenamide DIBS
A-Dimethyl benzothiazyl sulfenamide DMBS
A^-Diethyl benzothiazyl sulfenamide DEBS
MiV-Dicyclohexyl>2-benzothiazyl sulfenamide DCBS
Aldehyde/amine condensates
and related materials
Acetaldehyde/anil ine AA
Butyraldehyde/aniline BA
 Compound Design 63

Table 16 Continued

Accelerator Common abbreviation

Anhydroformaldehyde/anil ine MA
Heptaldehyde/aniline HA
Anhydroformaldehyde//7-toluidine MT
Hexamethylene tetramine HEXA (HMT)
Dibutylamine DBA
Cyclohexylethylamine CEA
Polyethylene/polyamine
Tricrotonylidene tetramine
Thiurams
Dimethyldiphenyl thiuram disulfide MPTD (DDTS)
Dipentamethylene thiuram disulfide DPTD (PTD)
Dipentamethylene thiuram tetrasulfide DPTT
Dipentamethylene thiuram monosulfide DPTM (PTM)
Tetrabutyl thiuram disulfide TBUT (TBT/TBTS)
Tetraethyl thiuram disulfide TETD (TET)
Tetramethyl thiuram monosulfide TMTM
Tetramethyl thiuram disulfide TMTD (TMT)
Dithiocarbamates
2-Benzothiazyl-A,A^,-diethyl dithiocarbamate
Bismuth dimethyl dithiocarbamate BMD (BDMC)
Copper dimethyl dithiocarbamate CuDD (CuDMc)
Lead dimethyl dithiocarbamate LDMC (PbDMC)
Piperidinium pentamethylene dithiocarbamate PPD
Selenium diethyl dithiocarbamate SEDC
Sodium dibutyl dithiocarbamate SBUD
Sodium diethyl dithiocarbamate SDED
Tellurium diethyl dithiocarbamate TEDC
Zinc dibenzyl dithiocarbamate ZBED (ZBEC/ZBD)
Zinc dibutyl dithiocarbamate ZBUD (ZDBC)
Zinc diethyl dithiocarbamate ZDC (ZDEC)
Zinc dimethyl dithiocarbamate ZMD (ZDMC)
Zinc pentamethylene dithiocarbamate ZPD
Xanthates
Sodium isopropyl xanthate SIX
Zinc dibutyl xanthate ZBX
Zinc diethyl xanthate
Zinc diisopropyl xanthate ZIX
Others
Dimorpholinyl disulfide DTDM
2-Mercaptobenzimidazole MB (MBI)
A-Oxydiethylene dithiocarbamyl-A-oxydiethylene OTOS
sulfenamide
Zinc dibutyl dithiophosphate ZDBP
Copper diisopropyl dithiophosphate CuIDP
64 Thorn and Robinson

Amines
Polyfunctional amines are important vulcanizing agents for acrylate and fluorocarbon
rubbers.
For acrylate rubbers, the amines frequently used are a reaction product o f ethylene
chloride, formaldehyde, and ammonia known as Trimene base, and triethylene tetram-
ine. Trimene base is said to give particularly good resistance to aging at high tempera­
tures. Vulcanization with one o f these amines alone gives brittle products after long­
term aging, but by the addition o f small amounts o f sulfur or accelerators (e.g., TMTD
or MBTS: see Table 16), good heat resistance can be obtained. Amine-cured acrylates
tend to stick and to have poor storage stability; skin irritations are another drawback.
In the fluorocarbon rubbers ordinarily diamines or polyamines give poor processing
safety. Thus blocked amines, in which the inert molecule splits into active components
at high temperatures, were developed. Hexamethylene diaminocarbamate and ethylene
diaminocarbamate are examples.
Normal levels o f addition o f amines in fluorocarbon rubbers are 1-1.5 phr, but
these should be increased to 1.5 — 2.0 phr if the compound contains mineral fillers.
Metallic oxides are also required as activators and acid acceptors.

Dioximes
/7-Benzoquinone dioxime can crosslink various elastomers such as natural rubber, SBR,
and EPDM. However, it is o f technical importance only in butyl rubber. The cure rate
increases with the degree o f unsaturation in the butyl, and the addition o f sulfur raises
the modulus and improves scorch safety. However, this advantage is at the expense of
compression set and heat resistance.
To get the most efficient vulcanization, with good heat resistance, MBTS, lead
oxide, and red lead can be added. The addition o f zinc oxide raises the heat stability o f
the vulcanizate but decreases scorch safety.

1.3.2 Cure Activators

The characteristic o f an activator is that relatively small additions to a compound con­

siderably increase the degree o f vulcanization. Often, almost no vulcanization will take
place if the cure activators are omitted.
Organic accelerators almost always require the presence o f organic and/or inor­
ganic activators. Zinc oxide is probably the most important inorganic activator, but
magnesium and lead oxide also find use. The fatty acids (e.g., stearic, palmitic, and lau-
ric acids) are the most important organic activators; polyalcohols (e.g., ethylene glycol)
and amino alcohols (e.g., triethanolamine) also find use and are often used to conteract
the retarding effect o f white fillers.
Activation by zinc oxide is dependent on the particle size used; normal grades
require 3 -5 phr, but grades having very small particle sizes can require as little as 1 phr
for adequate activation. The latter grades would be preferred in transparent articles. The
onset o f vulcanization in sulfenamide-accelerated cures is further delayed if fine particle
sized grades are used.
Basic zinc carbonate which is more soluble than zinc oxide can be used in the pro­
duction o f transparent goods.
 Compound Design 65

1.3.3 A ccelerators
There is such a wide variety o f accelerators, that can be used singly, in combination, at
different levels, and in different polymers, that it is a daunting task to try and present a
brief summary o f these critically important ingredients. The reader will therefore under­
stand why we advise consulting the polymer manufacturers’ trade literature for guidance
in selecting a suitable cure system.
Accelerators are o f most importance in the sulfur cure systems used to crosslink
unsaturated rubbers, but their importance is belied by their low level o f addition to a
compound, typically 1-5 phr. Accelerators strongly influence not only the processing
safety and cure characteristics o f a compound, but also the flnal properties exhibited by
the compound, including long-term service.
Figure 11 summarizes the principal manufacturing routes by which the commer­
cially available accelerators are produced.
Table 16 summarizes the major accelerators available, together with their common
abbreviation(s). In addition to these major types, there are accelerators available whose
composition is undisclosed, as well as proprietary mixtures o f different accelerators.

Guanidines
As a group, the guanidines exhibit very slow curing characteristics in unsaturated
rubbers, and the vulcanízales offer inferior resistance to heat aging and UV light. They
are, however, frequently used as a secondary accelerator with the thiurams, dithiocarba-
mates, and sulfenamides, but most commonly with the thiazoles, because such mixtures
exhibit synergism—that is, there is more activity in the mixture than is represented by
the sum o f the two individual activities, as illustrated in Figure 12. The role o f the
guanidines as secondary accelerators is more restricted in butyl and EPDM rubbers.

Thioureas
The use o f thioureas is restricted to being a secondary accelerator in the unsaturated
rubbers, and even this use is rare except in polychloroprenes, where the thioureas have
enjoyed wide use. Ethylene thiourea has been the most favored, because o f the excellent
balance of properties resulting from its use, but it is now becoming less used as a result
o f doubts over its safety, particularly with women.
Thioureas, and again principally ethylene thiourea, have been widely used as cure
agents, as opposed to accelerators, for the crosslinking o f epichlorohydrin.

Thiazoies
The thiazoles, together with the sulfenamides, can be considrered to be one o f the most
important groups o f accelerators for the sulfur cure o f unsaturated elastomers since, by
correct choice o f type and judicial incorporation with other accelerators, a wide range
o f cure characteristics and vulcanízate properties can be obtained.
The most common accelerators o f the thiazole type are MET and MBTS, whose
structural formulas were shown in Figure 11.
The thiazoles give rates o f cure and processing safety intermediate between those o f
the guanidines and thiurams, MBTS exhibiting a slightly better resistance to scorch than
MBT. Used by themselves, thiazoles give good aging properties, but not a high extent
of cure. For this reason they are commonly used in combination with guanidines, sul­
fenamides, thiuram, and dithiocarbamates; they can be classed as secondary accelera-
 Thorn and Robinson

DINITROPHENYL THIAZYL DISULFIDES (e g. MBTS)

THIAZYL SULFIDES

a>--<30

METAL ALKYL ALKYL DITHIOCARBAMATES

DITHIOCARBAMATES

Figure 11 Principal manufacturing routes for accelerators. The speed of cure in natural rubber
generally increases as one moves clockwise from the substituted thioureas/guanidines to the
xanthates. (Figure compiled by B. Willoughby.)
 Compound Design 67

Modulus

Figure 12 Example of a synergistic effect with accelerators.

tors when used with the latter three types. Only one accelerator, classed here as a thia-
zole, namely 2-(4)-morpholinodithiobenzothiazole (MBSS), is capable o f acting as a sul­
fur donor in sulfurless cures.

Sulfenamides
Compared to the thiazoles, the use o f sulfenamide accelerators in the sulfur vulcaniza­
tion o f unsaturated rubbers results in a longer scorch but a more rapid rate o f cure.
The accelerators TBBS, MOR, and DCBS give greater processing safety than the
most common sulfenamide accelerator, CBS, while TBBS gives the highest state o f
cure. The thiurams and dithiocarbamates can be used as secondary accelerators to
increase the speed o f cure, and sulfenamides can be used as secondary accelerators for
the former types to increase the scorch time and to decrease the rate o f cure.
Sulfenamides are used in semi-EV and EV cure systems, occasionally as the only
accelerator, where levels o f up to 5 phr may be required, or in combination with the thi­
urams and dithiocarbamates, where lower levels 3 phr) are used.
The sulfenamides cannot be used to give sulfurless cures.

Aldehyde/Amine Condensates
The aldehyde/amine condensate accelerators vary from particularly fast accelerators,
with low scorch times, to very slow accelerators. As a class o f accelerators they are
rarely used, and when selected they tend to be secondary accelerators. The curing o f
ebonites is one area o f use.

Thiurams
The thiurams exhibit fast cure rates in unsaturated rubbers, hence are considered to be
ultra-accelerators; they give a longer scorch time than the dithiocarbamates and thus
find a much wider use as accelerators than the latter.
The thiurams have the following structure:

^N— C ----- (S )x — C ------ N <

R II * II
S S ”
68 Thorn and Robinson

Where R is generally an alkyl group and x can be 1, 2, or 4. If jc is greater than 2 it

is possible to use the thiuram as a sulfur donor in a “sulfurless” cure system. If .r is 1,
sulfur must be used as the curing agent. In addition to sulfurless cures, the thiurams are
used in conventional, semi-EV, and EV cure systems, both singly and in combination
with the other accelerator classes.

Dithiocarbamates
A wide variety o f metal and alkyl dithiocarbamates are available, but in solid unsatu­
rated rubbers the zinc-based dithiocarbamates are the most technically important.
The dithiocarbamates are also classed as ultra-accelerators, but since they have
very low scorch times and fast cure rates, they are used as only secondary accelerators
with the slower acting accelerators.
Dithiocarbamates based on zinc, sodium, and ammonium-based alkyls are water
soluble, hence find wide use as accelerators in latex applications, where their high
activity is less problematical because the cure temperatures used are much lower. Simi­
larly, they also find use in solution applications, where cure temperatures tend to be
much lower.
The other metal-based dithiocarbamates listed in Table 16 are rarely used.
Dithiocarbamates based on nickel are not used as accelerators, but as antidegradants in
polychloroprene, chlorosulfonated polyethylene, and epichlorohydrin.

Xanthates
The xanthates, together with the ammonium-based alkyl dithiocarbamates, are the
fastest accelerators known. They are seldom used in curing solid unsaturated rubber. In
solution processing applications, adhesives, and sealants, however, it is possible to
achieve cure at room temperature with these accelerators.

Others
Of the accelerators listed under “Others” in Table 16, dimorpholinyl disulfide and N-
oxydiethylene dithiocarbamyl-A"-oxydiethylene sulfenamide are two types that can be
used as sulfur donors, as well as being used in combination with the other types o f
accelerator.

1.3.4 Fillers
Fillers of many types can be added to rubber compounds to extend the range o f physical
properties, to reduce the cost o f the compound, to modify the processing properties
(e.g., to achieve a reduction in die swell), and to influence the chemical resistance o f the
compound.
In addition, fillers can pigment a compound, impart conductivity to it, and influence
its aging characteristics.
A reinforcing filler can be defined as a filler that improves modulus, tensile
strength, and tear and abrasion resistance. Remember, however, that this improvement
o f properties is not continuous: when the volume percentage o f filler further increases,
the point at which there is insufficient rubber to bind the filler together eventually will
be reached. A noticeable decrease in strength would have become apparent well before
this point.
Fillers can also be classed as semireinforcing or diluent.
The effect o f a particulate filler on a rubber depends on the following factors:
Compound Design 69

1. The surface area o f the filler particles. The surface area o f a filler particle is
directly related to the particle size; the lower the particle size, the higher the surface
area. SAP, a highly reinforcing carbon black, has a particle size o f 11-19 nm and a sur­
face area o f 125-155 m^/g, whereas with SRF black, which is semireinforcing, the
values are 61-100 nm and 17-33 m^/g. Coarse inorganic fillers may have surface areas
of around 1 m^/g.
2. The chemical nature o f the particle surface. The chemical nature o f the parti­
cle can vary among different fillers. On the surface o f carbon blacks there are chemi­
cally active sites that have a profound effect on the reinforcement obtained. If the car­
bon black is exposed to temperatures of 1600-3000°C , these active sites are destroyed,
and reinforcement is lost. Thus both total surface area and surface activity o f a filler are
important requirements for the development o f reinforcement.
The pH o f the filler can influence the cure characteristics. Silica fillers tend to be
acidic and to have —OH groups on the surface which can deactivate accelerators, hence
retard cure. This disadvantage can be overcome by increasing the level o f accelerators
or by inclusion of diethylene glycol, polyethylene glycol, or triethanolamine.
The reinforcing effect o f several white fillers can be increased by the use o f silane
coupling agents, which react with chemical groups on the filler surface.
3. Geometrical characteristics. If carbon black is examined under an electron
microscope, it is seen that the primary particles are fused into larger aggregates. The
size, shape, and number o f voids in this “aggregate” determine the “structure” o f the
carbon black, and this structure can influence the physical, and processing properties
obtained.
Whereas the primary particles o f carbon black are generally spherical, the shapes
of inorganic or mineral filler particles exhibit much greater variation. The anisometry o f
the filler particle is an important characteristic, affecting the viscosity and modulus o f
the resultant rubber compound.
Porosity o f the filler particle is another factor that influences the properties
obtained. Silicas are generally more porous than carbon black, and thus silica fillers
give higher viscosity compounds at equal volume loadings. In carbon blacks, increasing
porosity can decrease the electrical resistivity obtained.
The general effects o f the surface area and geometric factors o f a filler are summar­
ized in Table 17.

Table 17 Summary of the General Effect of Filler Properties on Compound Properties

Increasing surface area Increasing structure

Property (lower particle size) or anisometry

Tensile strength Increases Decreases

Tear strength Increases Minor effect
Modulus Increases Increases
Abrasion resistance Increases Increases
Damping Increases Increases
Viscosity Increases Increases
Extrusion shrinkage Minor effect Decreases
Electrical conductivity Increases Increases
(carbon black only)
 70 Thorn and Robinson

Carbon Blacks
The most important types o f carbon black used in the rubber industry are those pro­
duced by the oil furnace process and thermal blacks; channel blacks have virtually
disappeared.
A wide variety o f carbon blacks are available, and these are summarized in Table
18. At equal volume loadings the tensile, tear, hardness, modulus, abrasion resistance,
and heat buildup properties o f a compound decrease as one moves down the table,
whereas flexibility, resilience, elongation at break, and processibility improve.

Silica and Silicates

Silica fillers are reinforcing white fillers that can be produced by two processes: precipi­
tation from solution and combustion methods (fumed silica). Because the latter method
yields a very fine particle size (ca. 10 nm; surface area ca. 190 m^/g), which produces
very high viscosity, stiff compounds, it is not as widely used as the precipitated type.
The precipitation process yields a filler with a particle size o f 10-40 nm (surface area
ca. 150 m^/g), and this type o f process is the more commonly used.
Silica can give tensile and tear strengths equivalent to those obtainable from the use
o f carbon black, but the resilience, set, and abrasion resistance are generally inferior.
Hot tear strength is reputedly superior to that obtainable with carbon black.
As mentioned earlier, diethylene glycol, polyethylene glycol, triethanolamine, or
silane coupling agents are required to eliminate the cure-retarding effect o f silica fillers.
Both aluminum and calcium silicates can be produced by the precipitation process;
they are classed as semireinforcing white fillers. The calcium silicates, with surface
areas of ca. 80 m^/g, are slightly more reinforcing than the aluminum types, but the
activity o f these fillers is such that the reinforcement achieved is lower than that obtain­
able from a carbon black o f equal surface area.

Table 18 Major Grades of Carbon Black

Unsystematic Particle Surface ASTM D1765

Abbreviation common name size (nm) area (m^/g) designation‘s

SAP Superabrasion furnace 11-19 125-155 NllO

ISAF Intermediate abrasion furnace 20-25 110-140 N220
CF Conductive furnace 20-25 110-140 N293
SCF Superconductive furnace 20-25 110-140 N294
HAF High abrasion furnace 26-30 70-90 N330
HAF-LS High abrasion furnace, low structure 26-30 75-105 N326
HAF-HS High abrasion furnace, high structure 26-30 80-100 N347
EPC Easy processing channel 26-30 95-115 S300
FF Fine furnace 31-39 43-69 N440
FEF Fine extrusion furnace 40-48 36-52 N550
GPF General-purpose furnace 49-60 26-42 N660
SRF Semireinforcing furnace 61-100 17-33 N762
MT Medium thermal 201-500 6-9 N990
"N, normal curing, neutral, or basic blacks; first digit, particle size indication; second and third digits, arbi­
trary designations.
 Compound Design 71

Clays
White clay based fillers are a useful compounding ingredient in rubber. They can be
divided into “hard” and “soft”; the hard clays can be classed as semireinforcing fillers,
their smaller particle size giving a greater effect on modulus than the “soft” types. The
distinction between the two types is, however, somewhat blurred.
Calcination, or removal of combined water from the clay, can modify the proper­
ties imparted to the rubber, the effect on electrical properties being particularly notice­
able.
Chemical treatments of the clay by amines or, more important these days, silane
coupling agents, can result in improved reinforcement.

Calcium Carbonate
Ground calcium carbonate, whiting, has a coarse particle size (0 .5 -3 0 /xm) and is used
only as a diluent filler because of its low cost.
Precipitated calcium carbonate has a smaller particle size ( ^ 0 . 1 /xm), which gives
improved properties compared to the use o f whiting. Stearate-coated grades, which give
improved dispersability, are available.
As with all fillers produced from naturally occurring minerals, traces o f metals
such as Cu and Mn may be present. Thus care is required in the selection o f the grade
o f filler if it is to be used in a rubber where these impurities might catalyze oxidative
breakdown.

Other Types of Filler

Zinc oxide, in amounts larger than the approximately 5 phr used for cure activation, can
act as a semireinforcing filler.
Barium sulfate is available as a ground filler (barytes) or as a precipitated filler
(blanc fixe). Neither form reinforces, but the latter is widely used in the compounding
o f acid-resistant compounds. Barium sulfate is also opaque to X-rays.
Hydrated alumina is used as a flame retardant, but it tends to depress other physical
properties.

1.3.5 Short Fibers

Another method o f reinforcing a rubber compound is the inclusion o f a short fiber, and
the reader is directed to References (24-26) for further information on this subject.
Short fibers (20-30 /xm in diameter, and up to 13 mm long) o f polyester, Kevlar,
nylon, and rayon are commercially available and can provide high reinforcement; the
mechanical properties obtained depend on

Fiber type
Aspect ratio o f the fiber
Fiber concentration
Orientation o f the fiber in the product
Dispersion o f the fiber
Degree of adhesion between the fiber and compound.

Cellulose fibers can also give reinforcement; Santoweb, produced by Monsanto, is

probably the most important commercially. Santoweb is based on unregenerated cellu­
lose treated to promote ease o f dispersion and, in some cases, adhesion. The fibers are
72 Thorn and Robinson

ribbon shaped (major axis diameter 16 ^m, minor axis diameter 8 /xm) and have an
average length o f 1.5 mm.
Certain grades o f PTFE can be added to compounds, the PTFE fibrillating during
processing.
Reinforcement by short fibers can give increased tensile strength and modulus, with
a decrease in the elongation at break and swelling in solvents. Differences in properties
in the longitudinal and transverse directions result if the fibers are highly oriented.

1.3.6 Reinforcing R e sin s

The two major types o f reinforcing resin used in rubber compounds are the high styrene
resins and the phenolic resins.
High styrene resins are copolymers o f styrene and butadiene, with a styrene content
o f 50-85 wt %. These resins are primarily used in the compounding o f natural and SBR
rubber, and the main area o f use is shoe soling applications. The high styrene resins
increase hardness but, since they are thermoplastic, they achieve this without sacrificing
processibility.
Phenolic resins are primarily used as reinforcing resins in nitrile rubber. If the
phenolic resin is crosslinked during vulcanization, the hardness, tensile strength, tear
strength, and abrasion resistance o f the compound are increased. Before vulcanization,
however, the phenolics act as process aids.

1.3.7 Softeners, P ro ce ss A id s, and T ackifyin g R e sin s

The distinction between softeners, process aids, and tackifying resins is blurred, and
many act as dual-purpose ingredients. Plasticizers also act as softeners and process aids
but are dealt with separately here, the term “plasticizer” being reserved for an
ingredient added for the purpose o f lowering the Tg o f a polymer.
The general reasons for adding softeners, process aids, and tackifies are as follows:

1. To improve the processing properties.

2. To modify the final compound properties (hardness and flexibility).
3. To alter the cost o f the compound by allowing further increases in the filler level
while maintaining a certain hardness.
4. To reduce power consumption during processing.

Petroleum Oils
All petroleum oils are a mixture o f paraffinic, naphthenic, and aromatic hydrocarbons,
and the precise composition o f the oil determines its compatibility with a rubber, as well
as the precise effects o f the addition.
The viscosity gravity constant (VGC) and the refractive index (RI) o f an oil are
quoted by the manufacturers, and knowledge o f these two parameters can be used to
assess the composition o f the oil. Table 19 summarizes the VGC and RI for various
compositions o f oil.
The composition o f the oil also controls its compatibility with the various rubbers.
The paraffinic oils are more compatible with EPDM and butyl rubber, while the more
polar aromatic oils are compatible with the polar rubbers, polychloroprene, nitrile, and
chlorosulfonated polyethylene. Most types o f oil are compatible with natural, poly buta­
diene, and SBR rubbers.
 Compound Design 73

Table 19 Viscosity Gravity Constant (VGC) and Refractive Index (RI) of Petroleum
Oils

Type of oil VGC RI

Paraffinic 0.791-0.820 <1.048

Naphthenic 0.851-0.900 1.048-1.065
Aromatic 0.951-1.000 1.053-1.065
Highly aromatic 1.001-1.050 >1.065

The viscosity and volatility of the oil are important aspects in the choice o f the
grade of oil. Viscosity, as well as the type o f oil, has an effect on the low temperature
properties of the oil, hence the low temperature properties o f the compound. Low
molecular weight paraffinic oils are preferred where low temperature properties are
important.
The volatility o f the oil is important for permanence at high temperatures, which is
dependent on the molecular weight and type o f oil.

Process Aids
The major types o f ingredient considered to be process aids are the fatty acids, metal
salts o f the fatty acids, and other fatty acid derivatives. There are numerous additives on
the market that can have an appreciable influence on processibility. In addition to acting
as lubricants for flow during molding, they can improve the dispersion o f additives dur­
ing mixing.
Liquid nitrile and EPDM are also available and can be used to increase processibil­
ity; they have the further advantage o f being crosslinked into the polymer matrix during
cure, hence are not extractable.

Tackifiers
Resins, pine tar, coumarone-indene resins, xylol-formaldehyde, and other hydrocarbon
resins can be used to increase the “tack” o f compounds in the uncured state, hence to
improve building operations during manufacture.
Factices
Factices are vulcanized vegetable oils used as processing aids. Brown factice can give
faster and smoother extrusions and can help to prevent collapse o f extrudates during
vulcanization in open steam. White factice is used in the manufacture o f erasers, where
it reduces the abrasion resistance o f the compound.

1.3.8 P la sticize rs
Synthetic plasticizers based on polyesters are the most important type o f plasticizer
commercially, although ether- and thioether-based types are also available.
The remainder o f Section 1.3.8, on ester plasticizers, is due to Paul Clutterbuck o f
BP Chemicals, who has presented this material to section meetings o f the Plastics and
Rubber Institute. We are indebted to BP for allowing us to publish this in its entirety,
since such excellent summaries are rare in the literature.
 74 Thorn and Robinson

Ester plasticizers play a very important role in rubber technology. They are used to
improve processibility, to improve low temperature properties, and to balance the sw el­
ling effects o f fuels and oils with which rubber products are frequently in contact.
The use o f ester plasticizers enables larger quantities o f filler to be added to the
rubber without the compound becoming too stiff to process. Because they reduce the
compound viscosity, they also reduce the temperatures generated during processing,
which substantially reduces the risk o f premature vulcanization (scorch). The addition
o f ester plasticizers can also reduce compound costs and improve extrusion and
calendering characteristics.
Ester plasticizers are particularly used in nitrile, polychloroprene, and chlorosul-
fonated polyethylene rubbers. However, smaller quantities are also used in other types
o f rubber, including chlorinated polyethylene, epichlorohydrin, polyacrylic, and fluoro­
carbon rubbers.
Selection o f an ester plasticizer for a particular application can often be confusing
because o f the large choice available. However, ester plasticizers can be characterized
by reference to their chemical constitution and end-product application. The considered
use of high performance plasticizers can sometimes lead to cost savings and other techn­
ical advantages.
Nearly all ester plasticizers can be conveniently divided into the following five
groups:

1. Phthalates (general-purpose plasticizers)

2. Low temperature
3. High temperature
4. Flame retardant (mostly phosphate)
5. Permanent (polyester)

Phthalates (General-Purpose Plasticizers)

Phthalates are organic esters o f phthalic acid and alcohols. However, in practice
phthalic anhydride is used instead o f phthalic acid.

COOH

Ct COOH

COOR
—
or + HoO
+ 2R 0H - COOR

CO.

0 = CO /

For general-purpose plasticizers, R ranges from normal butyl (C-4) to the isomeric
branched decyl (C-10) group. The use o f alcohols with mixed chain length is common.
The following list shows phthalates that are currently commercially available or
have been available in recent years.
Compound Design 75

Abbreviation Phthalate

DBP Di-n-butyl phthalate

DIBP Diisobutyl phthalate
DHXP Di-n-hexyl phthalate
DNHP Di-n-heptyl phthalate
DIHP Diisoheptyl phthalate
DOP Di-2-ethylhexyl phthalate
DAP Predominantly branched C-7—C-9 phthalate
DIOP Diisooctyl phthalate
L79P Predominantly linear C-7—C-9 phthalate
610P Linear C-10—C-16
DNP Diisononyl phthalate
810P Linear C-8—C-10 phthalate
L711P Predominantly linear C-7—C-11 phthalate
DIDP Diisodecyl phthalate
L911P Predominantly linear C-9—C-11 phthalate

To explain how users can select the most appropriate material from this group, it is
necessary to consider their chemical structure. Fortunately only two variables need to
be considered:

The number o f carbon atoms in the side chains

The degree o f branching o f the side chains
Carbon Number. The extremes for general-purpose phthalates are DBF and DIDP.

O
II
C — 0-CH 2-CH2— CH2— CH3
C — 0-CH 2-CH2— CH2— CH3
O

Di-«-butyl phthalate (DBP), C-4

O CH3 CH3 pu
II I I ® 1^3
C — O -CHz-CH— CHj— CH— CH—CHj— CHj
C — 0 -C H j-C H — CH,— CH— CH— CHj— CH,
O CH3 CH3 CH3

Diisodecyl phthalate (DIDP), C-10

When phthalates are made from alcohols with a spread o f carbon numbers, perfor­
mance depends on the average carbon number. Thus L79P behaves as a C-8 phthalate,
and L91 IP behaves as a C-10 phthalate.
 76 Thorn and Robinson

Chain Branching. The extremes found in practice are found in 810P and DNP.

C - O - C H j— (CHj)e— CH3
C - O - C H j---- (CHjjj— CHj
O

Linear C -8-C -10 phthalate (810P), behaves as a C-9 phthalate

C —O -C H j-C H j—CH CHj— CiCHJj

C—O -C H j-C H j—CH----CHj— C(CH3)3
° CH,

Highly branched C-9 phthalate, di-3,5,5-trimethylhexyl phthalate (DNP)

From these, the various phthalates can be set out on a grid according to carbon
number and chain branching, as indicated in Table 20.
Increasing the carbon number gives:

1. Reduced compatibility
2. Poorer processibility
3. Higher oil solubility
4. Higher plasticizer viscosity
5. Reduced volatility
6. Reduced water solubility
7. Better low temperature flexibility

Table 20 Classification of Phthalates by Carbon Number and Branching"'

Carbon
number

C-4 DBP DIBP

C-5
C-6 DHXP
C-7 DNHP DIHP
C-8 610P L79P DOP lOP
C-9 810P L711P DINP DNP
C-10 L911P DIDP
"Branching increases from left to right.
 Compound Design 77

Increasing the branching gives:

1. Poorer low temperature performance

2. Increasing volatility
3. Lower stability to oxidation (except DNP)
4. Higher electrical volume resistivity in a compound (poorer conductivity)

Special Considerations. L79P is a phthalate o f predominantly linear C -7-C -9 alcohols,

average C-8. It gives rubber compounds with lower volatility and slightly better low
temperature flexibility than branched C-8 phthalates such as DOP and DIOP.
L911P is the phthalate o f predominantly linear C-9-C-11 alcohols, average C-10.
Its high linearity and relatively high carbon number give it two main technical advan­
tages:

Low volatility and good oxidative stability

Good low temperature performance, particularly when compounded to a fixed hardness

Low Temperature
Low temperature plasticizers are used to give improved flexibility and resistance to
cracking at low temperatures. Compared to general-purpose phthalates, they are gen­
erally more difficult to process and have a higher volatility, more water solubility, and
less compatibility.
Low temperature plasticizers are generally aliphatic diesters. They are made from
linear dibasic acids with the general structural formula:

R 0 0 C ~ (C H 2 )„~ -C 0 0 R
The most popular dibasic acids are adipic (n = 4 ) , azelaic (n = 7 ), and sebacic
(n = S ). Most aliphatic diesters are manufactured from branched-chain alcohols, such
as 2-ethylhexanol or isodecanol. Linear alcohols are generally avoided, since their
esters tend to crystallize at relatively high temperatures, giving storage and handling
problems.
In selecting a low temperature plasticizer for any application, other factors such as
cost and permanence, must also be considered.
DBS (di-«-butyl sebacate) is an excellent low temperature plasticizer for many elas­
tomers. However, it has the disadvantage o f poor volatility and very high cost.
DOS (di-2-ethylhexyl sebacate) has been regarded historically by many technolo­
gists as the rubber industry’s low temperature standard. It is an efficient low tempera­
ture plasticizer that imparts low volatility, heat stability, and water resistance. However,
the price o f DOS has risen dramatically over the last few years, and more cost-effective
replacements are now available.
DOZ (di-2-ethylhexyl azelate) provides a good balance o f low temperature and per­
manence properties. It confers similar low temperature properties on DOS, but has
slightly inferior permanence properties. During the last few years the price o f DOZ has
risen significantly, and more cost-effective replacements are now available.
DOA (di-2-ethylhexyl adipate) confers low temperature properties similar to those
of DOS and DOZ with considerable cost savings. However, it has the disadvantage o f
relatively poor volatility.
DL79A (adipate o f predominantly linear alcohols, average carbon number C-8)
confers low temperature properties similar on DOS and DOZ with considerable cost
78 Thorn and Robinson

savings. Its predominantly linear nature allows it to overcome some o f the volatility
disadvantages o f DO A.
DIDA (diisodecyl adipate) provides a good compromise o f low temperature and
permanence properties but, unfortunately, has comparatively poor compatibility.
Bisoflex 102 [ = Tricap = Harwick SC (triethylene glycol ester o f linear acids,
average C-9)] is an excellent low temperature plasticizer for NBR and CR rubbers. It
confers superior low temperature performance than the adipates and sebacates. The heat
aging properties o f this material are significantly enhanced by the addition o f a suitable
phenolic antioxidant.
Bisoflex 111 [ = TP95 (butyl carbitol adipate)] is an excellent low temperature
plasticizer with good compatibility with a wide range o f rubbers. Because o f its rela­
tively low volatility, it is effective over a wide range o f temperatures.
Bisoflex 120 (aliphatic ester) is an efficient low temperature plasticizer for nitrile
and polychloroprene rubbers. It imparts heat stability, low volatility, and water resis­
tance. Bisoflex 120 is an excellent cost-effective technical replacement for DOS and
DOZ.
Bisoflex 123 [ = TP9B = BCF (butyl carbitol formal)] is an excellent low tempera­
ture plasticizer with good compatibility with a wide range o f elastomers. Unlike most
other low temperature plasticizers, it also provides resistance to fungal attack. How­
ever, it has very poor volatility and low resistance to water extraction.
Figure 13 summarizes the various characteristics o f low temperature plasticizers.

High Temperature
High temperature plasticizers are primarily intended for use in synthetic rubber cable
formulations, where excellent resistance to heat aging is a requirement. They are gen­
erally more difficult to process, less efficient than general-purpose phthalates, and gen­
erally characterized by their low volatility and good resistance to oxidation.
DTDP is the phthalate ester o f tridecanol, which is a mixture o f branched tridecyl
(C-13) alcohols. DTDP is a good high temperature plasticizer that imparts low volatility
and good resistance to extraction by water. However, compared to other high tempera­
ture plasticizers such as the trimellitates, DTDP has poor plasticizing efficiency, poor
processing characteristics, and high viscosity.
Bisflex DUP is the phthalate ester o f predominantly linear undecanol. DUP sur­
passes DTDP in heat aging performance, has greater plasticizing efficiency, and is more
readily compounded with synthetic rubbers. It also imparts good low temperature prop­
erties.
TOT (tri-2-ethylhexyl trimellitate) surpasses DTDP and DUP in heat aging perfor­
mance. TOT demonstrates particularly good resistance to marring (migration into other
polymers) with a wide range o f materials. Compared to other monomeric esters, TOT
has excellent resistance to aqueous extraction.
TL79T is the trimellitate ester o f predominantly linear alcohols, average carbon
number C-8. It is marginally more efficient than TOT and gives slightly superior heat
aging and low temperature performance. In common with other trimellitates, TL79T
has good resistance to marring and excellent resistance to aqueous extraction.
T810T is the trimellitate ester o f linear n-octanol and «-decanol. It is capable o f
withstanding even more extreme heat aging conditions than TOT or TL79T. It also
confers moderate low temperature flexibility and is ideal for use in compounds that call
for a combination o f excellent high temperature and moderate low temperature proper-
Compound Design 79

LOW TEMPERATURE FLEXIBILITY

Bisoflex 102 = Tricap = Harwick SC

Bisoflex n i = TP95
Bisoflex 123 = TP90B = BCF

Figure 13 Characteristics of low temperature plasticizers.

ties. In common with other trimellitates, T810T has good resistance to marring and
excellent resistance to aqueous extraction.
Note that DTDP, DUP, and the trimellitates listed above normally contain phenolic
antioxidants to enhance aging characteristics.
Bisoflex OBC (pentaerythritol-type ester) is an excellent high temperature plasti­
cizer that imparts low volatility and good resistance to oil extraction. The extreme low
volatility, good oxidative stability, and high permanence o f OBC make it a very useful
plasticizer under severe service conditions.
Vulkanol OT (ether thioether) provides a good compromise between low tempera­
ture flexibility and good hot air resistance. It has good compatibility with many elasto­
mers but has the severe disadvantage of being very expensive (more than double the
price of other high temperature plasticizers).