1. business and industrial
2. chemicals industry
3. plastics and polymers

ELASTOMERIC
MATERIALS
TAMPERE UNIVERSITY OF TECHNOLOGY
THE LABORATORY OF PLASTICS AND ELASTOMER
TECHNOLOGY
Kalle Hanhi, Minna Poikelispää, Hanna-Mari Tirilä
Summary
On this course the students will get the basic information on different grades of
rubber and thermoelasts. The chapters focus on the following subjects:
-
Introduction
Rubber types
Rubber blends
Thermoplastic elastomers
Processing
Design of elastomeric products
Recycling and reuse of elastomeric materials
The first chapter introduces shortly the history of rubbers. In addition, it cover
definitions, manufacturing of rubbers and general properties of elastomers. In this
chapter students get grounds to continue the studying.
The second chapter focus on different grades of elastomers. It describes the
structure, properties and application of the most common used rubbers. Some
special rubbers are also covered. The most important rubber type is natural rubber;
other generally used rubbers are polyisoprene rubber, which is synthetic version of
NR, and styrene-butadiene rubber, which is the most important sort of synthetic
rubber.
Rubbers always contain some additives. The following chapter introduces the
additives used in rubbers and some common receipts of rubber.
The important chapter is Thermoplastic elastomers. Thermoplastic elastomers are a
polymer group whose main properties are elasticity and easy processability. This
chapter introduces the groups of thermoplastic elastomers and their properties. It
also compares the properties of different thermoplastic elastomers. The chapter
Processing give a short survey to a processing of rubbers and thermoplastic
elastomers.
The following chapter covers design of elastomeric products. It gives the most
important criteria in choosing an elastomer. In addition, dimensioning and shaping
of elastomeric product are discussed
The last chapter Recycling and reuse of elastomeric materials introduces recycling
methods. It also covers processing of recycled rubber and applications of waste
rubber.
After studying this course, the students have the basic information on different
grades of rubber and thermoplastic elastomers. They will know the recycling
practices of rubbers and they will understand the design practices of elastomeric
materials.
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Table of contents
Summary....................................................................................................................2
Table of contents........................................................................................................3
1. Introduction to elastomeric materials ................................................................5
1.1 Definitions of elastomeric materials and rubbers ..........................................6
1.2 Manufacturing process of rubbers .................................................................7
1.3 Behaviour of elastomers ................................................................................8
1.4 General properties of elastomers ...................................................................9
2. Classification of elastomers.............................................................................14
2.1 Natural Rubber (NR) ...................................................................................16
2.2 Isoprene Rubber, Polyisoprene (IR) ............................................................18
2.3 Butadiene Rubber, Polybutadiene (BR) ......................................................20
2.4 Styrene-Butadiene Rubber (SBR)................................................................23
2.4.1 The use of SBR in tyres ........................................................................26
2.5 Butyl Rubbers ..............................................................................................27
2.6 Nitrile Rubber, Nitrile-Butadiene Rubber, Acrylonitrile Rubber (NBR) ....29
2.6.1 Modified nitrile rubbers........................................................................30
2.7 Epichlorohydrin Rubbers.............................................................................31
2.8 Ethylene-Propylene Rubber (EPM), Ethylene-Propylene-Diene Rubber
(EPDM)................................................................................................................32
2.8.1 Typical Properties.................................................................................33
2.9 Chloroprene Rubber, Polychloroprene (CR) ..............................................35
2.10 Polyacrylate Rubbers (ACM) ....................................................................37
2.11 Polyurethane rubbers (AU, EU, PUR).......................................................38
2.12 Fluorocarbon Rubbers (FKM, FPM) .........................................................41
2.13 Silicone Rubbers (Q) .................................................................................44
2.14 Polysulphide Rubbers (T) ..........................................................................46
2.15 Ethylene-Vinyl Acetate Copolymer (EVA)...............................................47
2.16 Polypropylene Oxide Rubbers, Polypropylene Oxide-Allyl Glycidyl Ether
Copolymer (GPO)................................................................................................48
2.17 Chlorinated Polyethylene (CM, CPE), Chlorosulphonated Polyethylene
(CSM, CSPE).......................................................................................................49
3. Rubber blends ..................................................................................................50
4. Thermoplastic elastomers (TPE) .....................................................................51
4.1 Styrenic thermoplastic elastomers (TPE-S).................................................52
4.2 Elastomeric alloys........................................................................................53
4.2.1 Thermoplastic Olefin Elastomers (TPO, TOE) ....................................53
4.2.2 Thermoplastic Vulcanizates (TPE-V, TPV, DVR)...............................54
4.2.3 Melt-Processible Rubbers (MPR).........................................................55
4.3 Thermoplastic Urethane Elastomers (TPU, TPE-U) ...................................55
4.4 Thermoplastics Polyester-Ether Elastomer (TPE-E) ...................................57
4.5 Thermoplastic Polyamide Elastomers (TPE-A) ..........................................58
4.6 Comparison of different TPEs .....................................................................59
4.7 New development trends occuring in the field of TPEs ..............................59
5. Processing ........................................................................................................60
5.1 Processing of rubbers...................................................................................60
5.2 Processing of thermoplastic elastomers.......................................................60
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6.
Design of elastomeric products .......................................................................61
6.1 Design process ...............................................................................................63
6.2 Elastomer selection......................................................................................63
6.3 Dimensioning of elastomer products ...........................................................65
6.3.1 Mechanical dimensioning.....................................................................65
6.3.2 The influence of hardness.....................................................................65
6.3.3 Shape factor ..........................................................................................65
6.3.4 Stiffness in different loading situations ................................................66
6.3.5 Allowed loadings for different rubbers.................................................67
6.4 Product shaping ...........................................................................................69
7. Comparison of Elastomer Properties. Data sources ........................................70
8. Recycling and reuse of elastomeric materials .................................................74
8.1 Why reclaim or recycle rubber? ..................................................................74
8.2 Recycling methods.......................................................................................75
8.2.1 Incineration ...........................................................................................75
8.2.2 Pyrolysis ...............................................................................................76
8.2.3 Grinding of vulcanized rubber waste....................................................76
8.2.4 Devulcanization ....................................................................................78
8.3 Utilization of unvulcanized rubber waste ....................................................80
8.4 Processing of recycled rubber......................................................................80
8.4.1 Unvulcanized rubber waste...................................................................80
8.4.2 Vulcanized rubber waste.......................................................................80
8.4.3 Devulcanized rubber waste...................................................................81
8.5 Applications of waste rubber .......................................................................82
8.6 Recycling of tyres ........................................................................................82
References................................................................................................................84
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1. Introduction to elastomeric materials
The natives of South America got the idea to exploit the latex of the Hevea
Brasiliensis rubber tree to produce waterproof footwear, among other products
from soaking their feet in the liquid, latex, tapped from the tree. From the Indian
word “caa-o-chu” (a weeping tree) are derived the words caoutchouc in English
and French, Kautschuk in German, caucho in Spanish and caucciù in Italian. The
word rubber originates from the early applications of rubber, i.e. from the property
of caoutchouc to rub out pencil writing.
In the 18th century, when rubber appeared in Europe, it was used for the fabrication
of suspenders and straps. Different kinds of materials were impregnated with
rubber to make them waterproof. However, the performance of the rubber articles
was quite poor, because rubber was at that time still gummy and fluctuation in
temperature caused great changes in products. It was only in the year 1839 that
Charles Goodyear discovered nearly by accident the vulcanization of rubber, which
made rubber as an elastic material capable of preserving its characteristics over a
wide temperature range.
The idea of this part of the “Virtual Education in Rubber Technology” course is to
give students an extensive overview of elastomeric materials. The structure and
characteristics of most typical rubber and thermoplastic elastomers will be
examined during this course. In addition, the applications and testing of different
elastomers, their design and construction and the recycling of elastomeric products
will be treated.
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1.1 Definitions of elastomeric materials and rubbers
Monomer
Low molar mass molecules which can react with the same or a
different kind of monomers, thus composing a polymer.
Polymer
Macromolecules constructed by the repetition of primary
monomer units in such a way that the properties of the material do
not change significantly due to the insertion or removal of some
primary units.
Homopolymer Polymer constructed of only one kind of monomer.
Copolymer
Polymer constructed of two or more monomers.
Elastomer
High molar mass material which when deformed at room
temperature reverts quickly to nearly original size and form when
the load causing the deformation has been removed (ISO
1382:1996)
Rubber
Cross-linked, vulcanized elastomer free of solvent which contracts
to its 1.5 -fold original length in one minute after the tension
which has stretched the rubber to double length at room
temperature has been released.
Natural rubber
Cis-1,4-polyisoprene obtained from the latex of the rubber tree,
most frequently from Hevea Brasiliensis plants.
Synthetic
rubber
Rubber which has been produced by polymerizing one or more
monomers.
An irreversible process in which the rubber compound is
transformed in a chemical reaction (e.g. cross-linking) to a threedimensional network which preserves its elastic characteristics
Vulcanization,
over a wide temperature range. The term vulcanization is
cross-linking
connected with the use of sulphur and its derivatives, whereas the
term cross-linking is usually connected with sulphur-free
processes.
Thermoplastic elastomers are in many respects a rubber-like
Thermoplastic material which need not be vulcanized. The rubbery character
disappears at the processing temperature but returns when the
elastomer
material has reached the operating temperature.
Rubber type
A group of rubber elastomers having the same kind of
characteristics and enabling the same applications for products
made of that group of elastomers.
Rubber quality
a vulcanized mixture of rubber satisfying a certain set of quality
requirements.
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1.2 Manufacturing process of rubbers
The manufacturing process of synthetic rubber starts with the manufacturing raw
rubber. The first step in this process is polymerization. This is a chemical reaction
in which small molecules (monomers) are joined together to form large molecules
(polymers). The basics of polymerization are presented on VERT module organic
chemistry.
Natural rubber is collected in ready polymerized form. Thus, the manufacturing
process of natural rubber starts by mastication. Mastication is a process in which
molecules are physically or chemically shredded to make mixing and processing
easier. Mastication makes the rubber softer. Most synthetic rubbers do not need
mastication because they are made of shorter molecules. A peptizing agent prevents
reactions between the broken chains.
Rubbers consist of elastomer and additives. Additives may be for instance fillers
and vulcanization agents. The purpose of additives is e.g. to improve properties or
processability. Rubbers can be processed in many ways (e.g. by compression
moulding, injection moulding and extrusion). You can learn more about processing
on the VERT module Processing of elastomeric materials. During the process or
after it the rubber is vulcanized (cross-linked), due to which rubber elasticity and
dimensional stability appear. Vulcanization is explaind more deeply on the VERT
modules Rubber chemistry and Raw materials and compounds in rubber industry.
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After processing and vulcanization the rubber product often has to be finished e.g.
by cutting.
1.3 Behaviour of elastomers
The predominant property of elastomers is elastic recovery after deformation in
compression or tension. Even after stretching an elastomer to many times its
original length, under ideal circumstances it will return after removal of the tension
to its original shape and length. In addition, elastomers are characterized by great
toughness under static or dynamic stresses, by better abrasion resistance than that
of steel, by impermeability to air and water and in many cases by high resistance to
swelling in solvents and attack by chemicals. Elastomers, like many other
polymers, show viscoelastic properties, which nowadays can be tailored for
numerous special applications, e.g. tyres, vibration and shock isolation and
damping. These properties are exhibited over a wide temperature range and are
retained under various climatic conditions and in ozone-rich atmospheres.
Rubbers are also capable of adhering to most other materials, enabling different
hybrid constructions. In combination with fibres, such as rayon, polyamide,
polyester, glass or steel-cord, the tensile strength is increased considerably with a
reduction in extendibility. By joining elastomers to metals, components which
combine the elasticity of elastomers with the rigidity of metals can be achieved.
The property profile which can be obtained with elastomers depends mainly on the
choice of the particular rubber, the compound composition, the production process
and the shape and design of the product. Depending on the type and amount of
rubber chemicals and additives in a compound, vulcanizates with considerably
different properties with respect to hardness, elasticity or strength are obtained.
The viscoelasticity of elastomers and rubbers is easy to detect in practice. When
stretching a cross-linked elastomeric band, a rubber band, a temperature rise in the
band can be observed as a consequence of emerging heat due to friction of viscous
deformation. The force that induces the recovery of deformed rubber, is dependent
on the entropy of the rubber material.
The structure of elastomers in strain and the dependence of elastic force on
temperature T and entropy S.
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The temperature range for the elastic behaviour of elastomers is limited by the
glass transition temperature (Tg). At temperatures lower than glass transition
temperature the movement of molecule chains is very restricted and the large
elastic deformations are not possible. Elastomers are rigid and fragile materials
below the glass transition temperature. The physical background of elastomeric
behaviour is described in more detail in the VERT section on rubber physics.
1.4 General properties of elastomers
The property profiles of elastomers depend mainly on the choice of the particular
rubber, the compound composition, the production process and the shape and
design of the product. Moreover the type of loading, e.g. whether it is static or
dynamic, strongly influences elastomer properties. Satisfactory properties can be
obtained only by proper compounding of elastomers with chemicals and additives,
and by subsequent vulcanization in appropriate conditions. Depending on the type
and amount of rubber chemicals and additives in a compound, and depending on
the degree of vulcanization, a given rubber can yield vulcanizates with
considerably different properties with respect to hardness, elasticity or strength.
The following chapters will deal with the most frequently specified properties of
rubbers reference to the standards ASTM 2000, SFS 3552 and SIS 162602. A
comparison of the properties of different rubber types and also thermoplastic
elastomers is given in the next table.
Thermal expansion
The degree of thermal expansion of different rubbers varies considerably
depending e.g. on the elastomers and fillers and their properties in the rubber
compound. Generally speaking, the linear thermal expansion coefficient of
elastomeric materials is five 5 ... 20 -fold compared with e.g. that of steels.
Consequently, the heat shrinkage of moulded elastomer products can be several
percent.
Hardness
The hardness of rubber is determined and measured based on the protrusion depth
of a standardized body under well-defined conditions. Hardness measurement is
one of the most frequently measured properties of rubbers. Hardness is commonly
quantified using the IRHD or Shore 0 ... 100 scale. The hardness of a conventional
elastomeric product is around 50 ... 70 IRHD.
Tensile properties
In order to obtain tensile material properties, it is customary to define the stress
which is required for a certain deformation, strain (see figure below). Frequently,
the stress values corresponding 100 or 300 % deformation are chosen to describe
tensile stiffness ( s 100 or s 300 modulus). The modulus at the early stage of the
tensile test is called Young's modulus. The stress at the breaking point of the
sample is defined as tensile strength (MPa) and the breaking deformation compared
to the original length is defined as elongation at break (%). The values of the tensile
strength of rubbers and thermoplastic elastomers are taken as satisfactory/good on
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Properties of different rubbers
5 = exellent, 4 = really good, 3 = good, 2 = fair, 1 = poor
NR
SBR
IIR
NBR
ECO, CO CR
AU, EU
FPM
Q
EPDM
CSM
Tensile strength , MPa
4-25
4-25
4-15
4-18
4-18
4-20
15-30
7-15
3-10
4-18
4-12
Break elongation , %
100-600
100-500
100-800
100-400
100-500
100-500
100-800
100-200
100-400
100-400
100-500
- long-term
60
70
80
70
80
70
60
175
200
80
80
- short-term
100
100
140
130
150
130
80
250
275
150
150
- cold
-60...-30
-50...-20
-40...-10
-50...-10
-50...-10
-50..-20
-20...0
-40...-20
-80...-50
-60...-30
-40...-20
Compression set, % (°C)
20-60
(70)
20-60
(70)
20-80
(100)
20-60
(100)
20-60
(100)
30-80
(100)
20-60
(70)
30-50
(175)
20-60
(150)
25-60
(100)
60-80
(100)
Elasticity
5
5
2
3-4
3
3-4
5
2
1-3
3
3
Electrical properties
4
4
4-5
1-2
1
3
3
3
4
4
3-4
- weather and ozone
1-2
1-2
3-4
1-3
4-5
4
5
5
4
5
5
- acids
2-3
2-3
4
3
3
3
1
3-4
1-3
3-4
4
- alkalis
2-3
2-3
4
2-3
3
3
1-2
1-3
1-2
3-4
4
- aliphatic oils
1
1
1
4
4
2-3
3-4
4
1-2
1
1-2
- aromatic oils
1
1
1
3
3
1
1-2
4
1-2
1
1
- abrasion
4-5
4
2-3
3-4
3-4
3-4
4-5
3
1-3
3
3
- flame
1
1
1
1-2
3
3-4
1-2
4
2-3
1
3
- radiation
2-3
2-3
1
2-3
1
2-3
3
2-3
2-4
1
2-3
Gas permeability
3
3
5
3
4
3-4
3
4
2
2-3
4
Adherendce
4
4
3-4
3-4
3-4
3-4
3
1-3
2-4
1
2-3
Operating temperature, °C
Resistance
the range 7 ... 15 MPa, and as excellent when the values are over 15 MPa. The
values of elongation at break vary in the range 300 – 1000 %.
Typical tensile curves of different plastics (A, B and C) and rubber (D).
Modulus of elasticity
As mentioned earlier, in assition to the tensile strength, the force is measured which
corresponds to a certain strain and is calculated to correspond to the original crosssectional area. This stress value is a measure of the stiffness of a rubber sample and
one of the most important measures for the evaluation of vulcanizates. The stress
value is often called modulus . The use of the word modulus is incorrect, however,
since the stress value is always taken for an area where Hooke's law (Young's
modulus with low strains) does not apply any more.
The statistical mechanical theory of rubber elasticity gives the following equation
for the force and stress:
Here R is general gas constant , T is the absolute temperature, Mc is the number
average molar mass of the chain segments between the cross-links of rubber, l is
the relative elongation (L/Lo) and r is the density. The modulus
shows that tension increases with rising temperature and an increasing degree of
cross-linking (decreasing Mc ). In addition, the stress depends on deformation
speed and the form of the deformed body. The shape of the body is typically
described by shape factor S.
Tear strength
Tear strength is defined as the resistance force which a rubber sample, modified by
cutting or slitting, offers to the propagation of the tear. A multitude of test
specimen configurations have been presented for tear test.
The force (kN) requires to tear the sample divided by the thickness (m) of the
sample is defined as the value of tear strength. Also the tear energy - which is
largely independent of sample geometry has gained importance in material
evaluation.
The values for the tear strength of elastomeric materials with good tear properties
are in the range 50-100 kN/m, and values over 100 kN/m are excellent. Natural
rubber is one of the best elastomers in this respect.
Permanent set, relaxation and creep
Permanent set is a measure of the viscous behaviour of elastomers. The set can be
either of the compression or tension type.
The compression set CS, and also the tensile set, is given at constant deformation
by the relation:
,
where h o is the initial height of the sample before deformation, h1 is the height
during deformation and h2 the height a certain time after deformation. Frequently,
the samples are stored in the compressed state at an elevated temperature in order
to simulate the requirements of gasket materials where changes due to aging effects
play a role.
Relaxation and creep express the time dependence of the stress or the deformation.
During the relaxation test the strain is kept constant and the change in stress is
monitored, while during the creep test the stress is kept constant and the time
dependent strain is measured. The stronger the viscous component, the more
pronounced relaxation or creep is.
Abrasion resistance
Abrasion resistance describes the durability of materials under wearing conditions.
Most rubbers have exceptionally good abrasion resistance, which is a consequence
of the ability of rubbers to creep over the irregularities of the wearing counterpart
in sliding. Good wearing resistance is typically achieved with vulcanized generalpurpose rubbers, NR, IR, SBR and BR. In an environment exposed to oil,
polychloroprene (CR) and nitrile rubber (NBR) are the rubbers with best abrasion
resistance. Buthyl and ethylene-propylene rubbers, on the other hand, have the best
abrasion resistance at elevated temperatures.
Resilience and hysteresis
The ratio of stored, reversible energy in deformation to dissipated energy is termed
resilience. Resilience can be easily evaluated using modern dynamic mechanical
analyzers. Since the mechanical energy dissipated during dynamic loading is
transformed into heat due to molecular friction, the viscous component may be
measured directly by monitoring the increase in heat in the sample (heat build-up).
The energy dissipation property of rubbers is often called also internal dampening
or hysteresis loss. The hysteresis loss of rubbers depends quite strongly on
temperature and loading amplitude, and is typically of the order of 5 ... 40 %.
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Electrical properties
Most general-purpose elastomers, like natural rubber (NR) and a variety of
synthetic elastomers (e.g. SBR, IR, BR, EPDM, IIR, MQ), exhibit very low
electrical conductivity and are therefore suitable as electrical insulating materials.
However, some other types, like CR and NBR, contain electrically polarizable
groups or dipoles and are therefore less suitable as electrical insulators.
The range of electric conductivity of all elastomers can be affected extensively by
the composition of the compound or by the addition of insulating (e.g. light) fillers
or conducting substances, especially carbon blacks or anti-static plasticizers.
Rubber articles with a high electric conductivity can be produced, too, e.g. for the
prevention of static electricity build-up.
Chemical endurance
Some fluids can cause big volume changes in rubbers which derive from the in
filtration of the fluid into the macromolecules (swell) or from the dissolving of
rubber ingredients in the fluid (shrinkage). Water, acids and bases may also bring
about some hydrolysis in certain rubbers, leading to impairment of tensile
properties. Nitric acid and concentrated hydrochloric acid react with most rubbers
and vause them to deteriorate.
Ozone and weathering resistance
A deformed rubber with strained parts often becomes cracked outdoors because the
double bonds of macromolecules are broken by oxygen, ozone or electromagnetic
radiation. Adding anti-aging agents, such as waxes, antioxidants and anti-ozonants,
can at least partially prevent such damage.
Dynamic properties
Elastomers are viscoelastic materials. It meansthat part of the deformation is
recovered after the load is removedand part of the deformation is permanent.
Dynamic properties of elastomers depend on temperature, type frequency of
loading and amplitude of deformation. Also shape of the product affects on
dynamic properties.
Values describing dynamic properties:
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•
•
•
•
Dynamic modulus E*
o E* = E' + iE''
Elastic modulus = storage modulus E'
o Represents the stiffness of the material
Viscous modulus = loss modulus E''
o Represents the amount of energy dissipated into heat under load
Tan delta, loss factor
o The ratio of loss and storage modulus
o The smaller the value of tan delta, the more elastic the material
o tan delta = E''/E'
2. Classification of elastomers
Elastomers have been classified in groups according to similarity of properties and
applications. Rubber types that have been standardized (ASTM D 2000, SFS 3551,
SIS 162602) are suitable for several industrial applications (e.g. tyres, belts, tubes
and seals).
Rubber type 61 (rubbers for general use)
Type 61 rubbers are used when the product does not require special properties,
such as oil, heat or weather resistance. These rubbers have good mechanical
properties and processability. They also have low price. Elastomers that belong to
this group are natural rubber (NR), polyisoprene rubber (IR) and styrene-butadiene
rubbers (SBR) and the blends of these elastomers.
Rubber type 62
Rubber type 62 is a rubber type that has not been standardized. Butyl rubber (IIR),
chlorobutyl rubbers (CIIR) and bromobutyl rubbers (BIIR) are elastomers which
belong to this group. They have good ozone and weather resistance. In addition, the
gas permeability is low and they are resistant to vegetable oils, but not to mineral
oils.
Rubber type 63
Rubbers in this group have good oil resistance, but their ozone and weather
resistance are weak. Applications are products that come in contact with oils.
Nitrile rubber (NBR) is of rubber type 63.
Rubber type 631 is rubber that has developed from nitrile rubber. It has better
ozone, weather and heat resistance than nitrile rubber. Hydrogenated nitrile rubber
(HNBR) belongs to this group. Rubber type 632 is nitrile rubber blended with
polyvinylchloride (NBR/PVC). It has better oil, ozone and weather resistance than
NBR.
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Rubber type 64
Chloroprene rubber (CR) is representative of rubber type 64. It has good resistance
to vegetable oils and fairly resistance to good aliphatic and naphtenic oils. A
disadvantage is their poor resistance to aromatic oil.
Rubber type 65
Rubbers in this group have good weather and heat resistance and quite good oil
resistance. Polyacrylic rubbers (ACM) are in this group.
Rubber type 66
Rubber type 66 is not standardized. Polyurethane rubbers (AU, EU) belong to this
group. These rubbers are tough and have good weather and oil resistance. Their
heat resistance is poor.
Rubber type 67
Rubbers in this group (fluorocarbon rubbers (FPM)) have good weather, heat, oil
and chemical resistance.
Rubber type 68
Silicone rubbers (Q) belong to this group. They have good weather, cold and heat
resistance. Their mechanical properties are weak.
Rubber type 69
Epichlorohydrin rubbers (CO, ECO, GECO) belong to this group. They have
medium weather, oil and heat resistance.
Rubber type 70
Rubber type 70 comprises ethylene-propylene rubbers (EPDM, EPM). They have
good ozone, weather and heat resistance and poor oil resistance.
Other rubbers
These rubbers are not standardized:
•
•
•
•
•
CM, chlorinated polyethylene (medium weather and heat resistance)
CSM, chlorosulphonated polyethylene (good weather and acid resistance)
EVA, ethylenevinylacetate copolymer (resistant to aliphatic oils)
BR, butadiene rubber (good elasticity)
XNBR, carboxylated nitrile-butadiene rubber (tough and oil resistant)
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2.1 Natural Rubber (NR)
Natural rubber can be isolated from more than 200 different species of plants.
Commercially significant source of natural rubber is Hevea Brasiliensis. Nnatural
rubber is obtained from latex, which is the emulsion of cis-1,4-polyisoprene and
water. Latex is obtained from the tree by tapping the innerbark and collecting the
latex in cups. A stabilizing agent, such as ammoniac, can prevent too early
coagulation.
Cis-1,4-polyisoprene.
Collection of the latex from rubber tree.
Latex can be concentrated by centrifuging or creaming and sold as concentrated
latex. Latex can be coagulated with hydrogen carboxylic acid or acetic acid, formed
in sheets or granulated and then dried to a solid raw rubber. Raw rubber types are
for example ribbed smoked sheets (RSS), air-dried sheets (ADS) and pale crepes.
Natural rubber also contains a few percent of non-rubber constituents such as
resins, proteins, sugars and fatty acids, which can function as weak antioxidants
and accelerators in the natural rubber. Natural rubber is usually vulcanized using
sulphur, but also peroxides and isocyanates can be used.
The biggest producer countries of natural rubber are Thailand, Indonesia, Malaysia
and India. Some classification systems that define the maximum content of dirt,
cinder, nitrogen and volatile elements have been developed in these countries. One
well-known system is the Standard Malesian Rubber system, which has been used
since 1965.
There are numerous methods for processing latex into commercial grades of dry
natural rubber and latex, as shown in the diagram below (Rubber Engineering,
Indian Rubber Institute, McGraw-Hill, 2000).
©TUT 2007
16
Methods of processing latex into commercial grades of dry natural rubber and into
concentrated latex.
The operating temperature range for NR is -55...+70 °C.
Advantages of NR:
•
•
•
•
•
•
•
•
•
•
good processability
excellent elastic properties
good tensile strength
high elongation
good tear resistance
good wear resistance
little dissipation factor - low heat build-up in dynamic stress
excellent cold resistance
good electrical insulator
high resistance to water and acids (not to oxidizing acids)
©TUT 2007
17
Disadvantages of NR:
•
•
•
•
poor weather and ozone resistance
restricted high temperature resistance (short-time maximum temperature
100°C)
swelling in oils and fuels: low oil and fuel resistance
unsuitable for use with organic liquids in general (even though
vulcanization considerably improves swelling resistance), the major
exception being low molecular weight alcohols
Applications:
•
•
•
•
•
•
•
tyres (60 - 70%)
tubes, conveyor belts and V-belts
coatings
gaskets
latex products
footwear
adhesives
Balloons. /1/
Rubber boots /6/
There are also many modified types of natural rubber. There is e.g. oil-extended
natural rubber (OENR), which contains 20-30 % oil, epoxidized NR and
methacrylate grafted NR (Heveaplus MG). The purpose of these modifications is to
improve the properties of NR to meet the special needs of rubber manufacturers.
2.2 Isoprene Rubber, Polyisoprene (IR)
Polyisoprene rubber has the same basic chemical formula as natural rubber (NR)
and thus it is a synthetic version of NR. The study of materials comparable with
NR started at the beginning of 20th century, but because of the high price of raw
materials and the weak quality of polymers, industrial production was not begun.
Significant production was started in the 1970s, when cheaper monomers and
catalysts, which produce stereo-specific polymers in solution polymerization
became available.
©TUT 2007
18
It is possible to create different kinds of isomeric structures using different catalysts
and polymerization conditions in the polymerization of isoprene monomers. The
structures which are exploitable are 3,4-, cis-1,4- and trans-1,4-polyisoprenes. Cis1,4 -polyisoprene is a synthetic substitute for natural rubber and trans-1,4polyisoprene is a hard thermoplastic material (Gutta-percha or Balata).
Cis-1,4-addition
Trans-1,4-addition
1,2-addition
3,4-addition
The isomeric structures of polyisoprene.
The properties of polyisoprene depend on the amount of its cis-1,4-units.
Commercial synthetic isoprene rubbers can be divided in different groups
according to the catalyst used:
•
The Li-IR group, whose catalyst in polymerization is alkyl lithium. The
amount of cis-1,4-units in Li-IR is about 90 % (10% 1,2- type IR). The TiIR group. In these polymerizations, the catalysts are different kinds of
Ziegler-Natta catalysts. The typical content of cis-1,4-cis-units in Ti-IR is at
level 96 - 98 %.
•
Lanthanide polyisoprenes have been developed in recent years. They
approximate very well to natural rubber. The share of 1,4-cis-units in
lanthanide IR can be 99.5 %.
The amount of cis-1,4-units influences crystallization and regularity of the
molecule structure. Whit a increase in cis-1,4-content, crystallization is facilitated,
the glass transition temperature decreases and strength properties improve.
Consequently, strength properties such as modulus, tensile strength and tear
resistance are slightly worse in synthetic polyisoprenes than in NR, whose cis-1,4content is almost 100 %. Also, the building tack of IR is somewhat inferior to that
of NR, and the green strength is poorer. Otherwise, the properties of synthetic
isoprene rubbers are similar to those of NR. The most significant advantages of
synthetic polyisoprenes compared to natural rubber are their purity, good
processibility and homogeneity of polymer structure.
©TUT 2007
19
Advantages of synthetic IR:
•
•
toughness good abrasion resistance cold resistance competitive price
processability and adherence good uncured tack high tensile strength high
resilience good hot tear strength
resistance to many inorganic chemicals
Disadvantages of IR:
•
•
restricted life time at high temperatures and in oxidative conditionspoor oil
resistance needs protection against oxygen, ozone and light is not resistant
to hydrocarbons
unsuitable for use with organic liquids
IR is often used with other rubbers. By blending other rubbers with isoprene,
tensile and tear strength and flexibility are improved. Applications of IR are similar
to natural rubber:
•
•
tyres conveyor lines and transmission straps gaskets, tubes, paddings
footwear, sports equipment protective gloves
sealants and sealing materials
Trans-1,4-polyisoprene (gutta-percha) resembles plastic and is used e.g. in golf
balls, deep sea cables, orthopedic applications and adhesives. Gutta-percha can also
be obtained from the pruning of special trees which are native to Malaysia.
2.3 Butadiene Rubber, Polybutadiene (BR)
The forerunner of polybutadiene rubbers was Buna, which was prepared for the
first time in Germany in the 1920s. Buna was a compound of butadiene and
sodium. During World War I it was noticed that the cold resistance of Buna was
not good enough. For this reason, American rubber scientists polymerized
polybutadiene (BR) in 1954. BR rubbers have much better weather resistance than
Buna.
Using solution polymerization in hydrocarbon solvent typically performs the
polymerization of BR. Suitable catalysts are Ziegler-Natta combinations and
lithium and its compounds. The elastomer is often named according to its catalyst
or according to the metal in it. Abbreviations used are among others Li-BR
(lithium), Co-BR (cobalt) and Ni-BR (nickel).
Three different kinds of basic construction units can be formed in polymerization.
The catalyst and polymerization conditions affect the development of these units.
©TUT 2007
20
Cis-1,4-form
Trans-1,4-form
1,2-form
The isomeric structures of BR.
The properties of polymer are determined by the isomeric structures which appear
most. Butadiene rubbers can be divided in three groups according to the amount of
cis-units.
Polybutadiene rubbers according to the catalyst used.
Co-BR
Ti-BR
Li-BR
Cis-1,4-content, %
96
93
38
Trans-1,4-content, %
3
3
52
1,2-content, %
1
4
10
-106
-93
Glass transition temperature
-108
Tg, °C
Melting temperature Tm, °C
-11
-22
amorphous
Molar mass distribution
medium board
thin
very thin
Branching degree
medium
low
very low
©TUT 2007
21
Comparision the properties of BR and NR.
5 = excellent, 4 = very good, 3 = good, 2 = fair, 1 = poor
Hardness, °IRH
Tensile strength at break, N/mm2
Elongation at break, %
Operating temperature range:
- maximum, °C
- minimum, °C
Elasticity
Electrical properties
Resistance:
- weather and ozone
- acids
- alkalis
- water
- abrasion
- flame
- radiation
Gas permeability
Adherence
Tack to the metal
Residual compression, % (°C)
Butadiene
rubber BR
Natural
rubber NR
40-80
7-21
100-600
30-90
7-28
100-700
80
-70
5
4
80
-55
5
4
1-2
2-3
2-3
3
4-5
1
2-3
3
4
5
-
1-2
2-3
2-3
5
4-5
1
2-3
3
4
5
20-60 (70)
Polybutadiene rubbers can be vulcanized with sulphur, sulphur compounds and
peroxides. The peroxide vulcanization is very effective and produces highly crosslinked polybutadiene rubbers.
Advantages of BR:
•
•
•
•
excellent cold resistance and heat resistance
elasticity
excellent low temperature flexibility and resilience
abrasion resistance
Disadvantages of BR:
•
•
poor processability
weak mechanical properties
©TUT 2007
22
The processing of BR is really difficult. That is why it is usually blended with some
other rubbers, such as NR and SBR. In those blends, the purpose of BR is to reduce
heat build-up and improve the abrasion resistance of the blend. It also improves
flexibility.
Applications:
•
•
•
•
•
•
•
•
tyres (BR content typically 30-50%, blended with SBR and NR)
shoe soles
coatings of cylinders, V belts
gaskets, tubes
coatings
toys
transmission belts
conveyor belts
2.4 Styrene-Butadiene Rubber (SBR)
Styrene-butadiene rubber is the most important sort of synthetic rubber. It was
initially developed to replace natural rubber. The manufacturing method of SBR
co-polymer was developed in Germany in 1929 when the emulsion polymerization
method at about 50 ° C became mastered. In that method, macromolecular
amorphous copolymer is polymerized with styrene and butadiene.
There exist four different basic construction units in SBR. Three of them originate
from butadiene
Cis-1,4-form
Trans-1,4-form
1,2-form
Styrene
Isomeric structures of polybutadiene and the structure of styrene.
At present, styrene-butadiene elastomers can be produced by emulsion or solution
polymerization techniques. “Cold” emulsion polymerization, at about 5°C, is the
©TUT 2007
23
most widely used polymerization technique, even though the solution method has
steadily increased its the market share.
In solution polymerization the polymerization typically occurs in dry hydrocarbon
solvent with anionic methyl-lithium catalyst. Depending on the specific
polymerization process, two different elastomer types can be formed. One type
contains segmented styrene and butadiene blocks (TPE), the other type is rubber
elastomer with random distribution of co-monomers in polymer.
Styrene-butadiene rubber can be vulcanized using sulphur, sulphur donor systems
and peroxides.
The processing method can affect on the properties of SBR considerably. Molar
mass, styrene content and the amount of units vary, depending on the
manufacturing technique. Examples are shown in the attached table.
Properties of emulsion polymerization and solvent polymerization.
Emulsion - SBR
145000
651000
4.5
23.5
18
65
17
Molar mass Mn, g/mol
Molar mass Mw, g/mol
Mw / Mn
Styrene content, %
Cis-1,4-content, %
Trans-1,4-content, %
1,2-content, %
Glass transition temperature Tg,
- 50.6
°C
Solvent-SBR
200000
420000
2.1
18
35
54
11
- 69.7
Commercial products of SBR: Buna EM, Krylene, Cariflex S a.o.
Type designation according to numeric code:
•
•
•
•
•
•
•
•
10xx hot polymer without filler
12xx solution - SBR
15xx cold polymer without filler
16xx cold polymer, carbon black master batch
17xx hot polymer, oil-extended
18xx cold polymer, carbon black/oil master batch
19xx emulsion- resin- master batch
'xx' indicates viscosity, coagulant, content of styrene
Advantages of SBR rubbers:
•
•
•
good abrasion and aging resistance
good elasticity
low price
©TUT 2007
24
Disadvantages:
•
•
•
•
•
•
inferior mechanical properties (require reinforcements)
adhesion properties
poor oil resistance
poor ozone resistance
do not resist aromatic, aliphatic or halogenated solvents
low elongation at break
Comparison between the properties of SBR and NR.
5 = excellent, 4 = very good, 3 = good, 2 = fair, 1 = poor
Hardness, °IRH
Tensile strength at break,
N/mm2
Elongation at break, %
Styrenebutadienerubber SBR
40....90
Natural rubber
NR
30...90
7....25
7...28
100...600
100...700
100
- 45
5
80
- 55
5
1...2
4
2...3
1...2
4...5
2...3
Operating temperature range
- maximum, °C
- minimum, °C
Elasticity
Resistance:
- weather and ozone
- abrasion
- radiation
SBR needs more reinforcement than natural rubber to achieve good tensile and tear
strength and durability. SBR also has lower resilience than NR.
Applications:
•
•
•
•
•
•
•
•
•
•
car tyres (blended with BR, IR and NR)
footwear
conveyor belts
hoses
toys
moulded rubber goods
sponge and foamed products
waterproof materials
belting
adhesives
©TUT 2007
25
Tyre /10/
2.4.1
The use of SBR in tyres
Varying the monomer content in SBR copolymer, used in tyre tread blends, can
modify the properties of tyres.
Monomer contents that are typical of tyre tread blends.
The properties of
polymer
Low
rollingresistance
Good wet grip
General use
Vinyl content (%)
10
50
35
Styrene content (%)
15
23
20
The effect of monomer content and Tg:n on the properties of tyres
The properties of tyres
Higher Tg
Growing styrene
content
Growing vinyl
content
Wet grip
Increasing
Increasing
Increasing
Wet steerability
Increasing
Increasing
Increasing
Dry grip
Increasing
Increasing
Increasing
Dry steerability
Increasing
Increasing
Increasing
Fuel consumption
Increasing
Increasing
Increasing
Ice grip
Decrease
Decrease
Decrease
Snow grip
Decrease
Decrease
Decrease
Life time
Decrease
Decrease
Decrease
©TUT 2007
26
2.5 Butyl Rubbers
Isobutylene-Isoprene Rubber (IIR), Chlorobutyl Rubber (CIIR), Bromobutyl
Rubber (BIIR)
Butyl rubbers are prepared by copolymerizing small amounts of isoprene with
isobutylene. Isoprene units are placed randomly in the isobutylene chain in trans1,4 form. Adjusting the polymerization temperature and the proportion of
monomers can vary the composition of the polymer. A typical butyl rubber
contains 0.5 ... 3 mole percent isoprene.
The properties of butyl rubbers depend on the length of the molecule chains and the
saturation degree. When the amount of double bonds is low, rubber has good
oxygen and ozone resistance. A greater amount of double bonds accelerates the
vulcanization process and increases the amount of cross-links.
Isobutylene and isoprene units.
The properties of butyl rubbers can be improved by adding 1 ... 2 weight percent of
halogens and by forming chlorobutyl (CIIR) and bromobutyl (BIIR) rubbers.
Halogens are mostly joined to the double-bonded carbon without the methyl group
in the isoprene unit. The addition of the halogens increases chain flexibility and
enhances cure compatibility in blends with other diene rubbers.
The butyl rubber can be cured with sulphur, but it needs accelerator. Dioxime
compounds together with an oxidizing agent can also be used. In that case, crosslinks stand heat better than sulphur bonds. CIIR and BIIR have more reactive
points in the cross-linking if a curing agent (sulphur or metal oxides) has been used
in curing. Peroxides cannot be used, because they may break down the elastomer
chains.
Advantages of butyl rubbers:
•
•
•
•
•
•
•
•
stabile in long-term-use and at high temperatures
low gas permeability
good ozone resistance
good weather resistance
elasticity in wide temperature range -73...100°C
low water absorption
resistant to oxidizing agents, vegetable and animal fats and polar solvents
heat stability
©TUT 2007
27
Disadvantages:
•
•
•
poor wear resistance
not resistant to hydrocarbon solvent and oil
relatively low elasticity
The properties of halogenated butyl rubbers are similar to those of basic butyl
rubber. However, they have lower gas permeability and better thermal, ozone,
weather and chemical resistance. Halogenated butyl rubbers are used in
applications that require rubber with a high vulcanization rate.
Applications:
•
•
•
•
•
•
•
•
•
inner tyres of cars and bicycles
steam hoses
coatings of fabrics and cables
base element of chewing gum
waterproof films
gutter gasket
inner tubes
pharmaceutical closures and membranes
vibration isolation
Diving suit /11/
©TUT 2007
28
2.6 Nitrile Rubber, Nitrile-Butadiene Rubber, Acrylonitrile Rubber
(NBR)
Poly-acrylonitrile-butadiene rubber is a copolymer of butadiene and acrylonitrile. It
was synthesized for the first time in 1930. It is used because of its good oil, fuel
and fat resistance. Acrylonitrile rubbers are also called just nitrile rubber.
NBR is produced by emulsion polymerization. The polymerization rates of
acrylonitrile and butadiene are different. Because of that, the content of monomers
in copolymer is not the same as the content of monomers in reaction mixture. The
polymer formed is random copolymer in which the acrylonitrile content varies
between 18 ... 50%. Changing the temperature or feeding with monomers can
modify the composition of the polymer.
Butadiene and acrylonitrile units.
Increasing the acrylonitrile content improves oil resistance, hardness, abrasion
resistance and heat resistance, but raises the glass transition temperature.
Unlike most other synthetic rubbers, nitrile rubbers can be vulcanized with several
cross-linking systems. The vulcanization can take place at room temperature or at
high temperatures to accelerate the reactions.
Nitrile rubbers are used in applications which demand good mechanical properties
and oil and fuel resistance. NBR can be used blended with other rubbers. For
instance, the increasing of IIR to NBR improves weather properties and thermal
stability and decreases the gas permeability of NBR.
Properties of NBR:
•
•
•
•
•
high oil and heat resistance
low ozone resistance
high swelling with some solvents (ketones and esters) and some oils
good resistance to oil, aliphatic and aromatic hydrocarbons and vegetable
oils
good abrasion and water resistance
Applications of NBR:
•
•
•
•
•
seals, hoses, joints
roll coverings
conveyor belts
containers
protective clothes and shoes
©TUT 2007
29
Boots /6/
Gloves /9/
2.6.1 Modified nitrile rubbers
Carboxylated nitrile rubbers (XNBR) and hydrogenated nitrile rubbers (HNBR)
are special modifications of NBR. The XNBR rubbers contain randomly placed
carboxyl groups that are derived from metacrylate acid or acrylate acid. The XNBR
has better abrasion resistance, hardness and tensile strength. It also has better low
temperature brittleness and better retention of physical properties after hot-oil and
air ageing compared to NBR.
Hydrogenated nitrile-butadiene rubber (HNBR)
The nitrile rubber can also be improved by (partially) saturating the double bonds
in main chain butadiene by catalytic hydrogenation. This kind of NBR, HNBR, has
been developed to resist better aging in oil and hot air.
Properties of HNBR:
•
•
•
•
•
oil and gasoline swelling as for NBR
application temperature up to 150°C
high tensile strength, weather resistant
peroxide curable types (double bond content < 1 %) and
sulphur curable (double bond content < 4 – 6 %)
Main applications: vehicle tubing, seals, cables and profiles
©TUT 2007
30
2.7 Epichlorohydrin Rubbers
Epichlorohydrin Homopolymer (CO), Epichlorohydrin/Ethylene Oxide
Copolymer (ECO), Epichlorohydrin Terpolymer (ETER)
There are three different types of epichlorohydrin elastomers: epichlorohydrin
homopolymer (CO), epichlorohydrin/ethylene oxide copolymer (ECO) and
epichlorohydrin terpolymer (ETER), which form from epiclorohydrin, ethylene
oxide and some other monomer (typically diene).
The structures of CO and ECO.
In polymerization of epochlorohydrin, a coordinate catalyst is used. The catalyst
can be for example a compound of aluminium alkyl, water and acetyl acetone. The
polymerization method used is solution polymerization in hydrocarbon solution.
When vulcanizing homo- and copolymer, chloromethyl groups react with a difunctional curing agent, such as diamine, ethylene thiourea or urea. Terpolymers
can be vulcanized with sulphur or peroxide.
The biggest differences between epiclorohydrin homopolymer (CO) and copolymer
(ECO) are in elasticity and cold resistance. ECO is very elastic over a wide
temperature range, whereas CO is elastic only at elevated temperatures. That is
why the epichlorohydrine copolymers are used more than homopolymers.
Properties of epichlorohydrin rubbers:
•
•
•
•
•
•
•
•
•
•
•
resistance to oils, fuels and chemicals
good fire resistance
high cold and heat resistance
good weather, ozone and thermal resistance
good damping properties
good processability
low gas permeability
weak tensile strength (fillers reinforce)
high price
can cause corrosion with metal
very good dynamic properties
The use of epichlorohydrin rubbers is similar to that of nitrile rubbers. However,
ECO offers better oil resistance, elasticity and processability.
©TUT 2007
31
Applications:
•
•
•
•
•
•
•
•
gaskets
oil and petrol tanks and hoses
belts, rolls
coatings of wires and cables
coatings of textiles
vibration isolator
membranes
resilient mountings
2.8
Ethylene-Propylene Rubber (EPM), Ethylene-Propylene-Diene
Rubber (EPDM)
Ethylene-propylene rubbers can be divided into two groups: ethylene-propylene
rubbers (EPM) and ethylene-propylene-diene rubber (EPDM). EPM is a copolymer
of ethylene and propylene and EPDM is a terpolymer of ethylene, propylene and
diene. The most frequently used dienes which offer the cross-linking sites for the
elastomer are dicyclopentadiene, ethyldienenorborne and 1 ,4-hexadiene (see
formulas below).
Rubbers usually contain 45 ... 60 wt.-% of ethylene monomer. Material with low
ethylene content is easier to process than high ethylene content material. Especially
green strength and extrudability improve as the ethylene content increases. Diene
content is usually 4-5 %, but sometimes it can be even 10 %.
The structure of EPM.
The structures of EPDM
Dicyclopentadiene as a terpolymer
©TUT 2007
32
Ethyldienenorborne as a terpolymer
1,4-hexadiene as a terpolymer
Ethylene-propylene rubbers are produced mostly by solution polymerization with
Ziegler-Natta type catalysts. EPM rubbers cannot be vulcanized with sulphur
because of the absence of unsaturation in the main chain. EPM can be cured with
peroxides or radiation. EPDM can be vulcanized with sulphur, peroxide, resin cures
and radiation. Polymerization and catalyst technologies in use today provide the
ability to design polymers to meet specific and demanding applications and
processing needs
2.8.1 Typical Properties
Ethylene-propylene rubbers are valuable for their excellent resistance to heat and
their oxidation, ozone and weathering resistance due to their stable, saturated
polymer backbone structure. Properly pigmented black and non-black compounds
are colour-stable.
As non-polar elastomers, they have good electrical resistivity as well as resistance
to polar solvents such as water, acids, alkalies, phosphate esters and many ketones
and alcohols.
Amorphous or low crystalline grades have excellent low temperature flexibility
with glass transition points of about -60°C.
©TUT 2007
33
Heat aging resistance up to 130°C can be obtained with properly selected sulphur
acceleration systems and heat resistance at 160°C can be obtained with peroxidecured compounds. Compression set resistance is good, particularly at high
temperatures, if sulphur donor or peroxide cure systems are used.
These polymers respond well to high filler and plasticiser loading, providing
economical (obs. low density too), easily processible compounds. They can
develop high tensile and tear properties, excellent abrasion resistance, as well as
improved flame retardance.
As the disadvantages of EP rubbers, bad oil and hydrocarbon resistance and poor
tack can be mentioned.
A general summary of properties (property ranges) is shown in the table below.
Property
Value range
Mooney Viscosity, ML 1+4 @ 125 5-200+
°C
Ethylene Content, wt. %
45 to 80 wt. %
Diene Content, wt. %
0 to 15 wt. %
Specific Gravity, gm/ml
0.855-0.88
composition)
Hardness, Shore A Durometer
30A to 95A
Tensile Strength, MPa
7 to 21
Elongation, %
100 to 600
Compression Set B, %
20 to 60
Useful Temperature Range, °C
-50 ° to +160 °
Tear Resistance
Fair to Good
Abrasion Resistance
Good to Excellent
Resilience
Fair to Good (stable over wide temp. ranges)
Electrical Properties
Excellent
(depending
on
polymer
* Range can be extended by proper compounding. Not all of these properties can
be obtained in one compound.
Source: International Institute of Synthetic Rubber Producers.
©TUT 2007
34
Applications:
•
•
•
•
•
products of automotive industry: seals and hoses, isolators
gaskets and hosepipes, liners in building industry
roll covers
agricultural equipment: hoses, seed tubes, cushioning, silus
wire and cable
2.9 Chloroprene Rubber, Polychloroprene (CR)
Polychloroprene was one of the first synthetic rubbers. The first chloroprene
monomers were prepared from acetylene. Nowadays they are synthesized from
butadiene, because it is an easier and safer route. Chloroprene is polymerized by
emulsion polymerization using potassium persulphate as free radical initiator. The
main component of the polymer usually is trans-1,4-units. In the vulcanizing of
CR, zinc oxide and magnesium oxide blend is usually used.
Trans-1,4- form
1,2- form
Cis-1,4-form
3,4- form
Isomeric structures of CR.
Chloroprene rubbers can be divided into G and W types according to their
mechanism for controlling the molecular weight of the polymer during
polymerization. In G types, sulphur is copolymerized with the chloroprene, when it
does not require acceleration during curing. The G-type rubbers have slightly
inferior aging resistance, but resilience and tack are better than in the W types. The
W types of chloroprene rubbers require an accelerator. The vulcanization cannot be
carried out with sulphur. Suitable accelerators are metal oxides. The W type
rubbers have better ageing properties and thermal resistance than G-type rubbers.
Polychloroprene is a versatile elastomer. It is used especially in demanding
circumstances.
©TUT 2007
35
Advantages of chloroprene rubber:
•
•
•
•
•
•
•
good abrasion resistance
good ozone resistance
good tear strength
good oil and solvent resistance
inflammability
good adhesion to metals
increased hardness in high-temperature environments
Disadvantages of CR:
•
High swelling in some oils, hot water, acids and some organic solvents
Applications of chloroprene rubbers:
•
•
•
•
•
•
•
•
•
conveyor belts and V belts
hoses
wire and cable coverings
vibration isolators
adhesives
gaskets
footwear
coated fabrics
wear suit applications, inflatables
Mask /2/
©TUT 2007
Gloves /9/
Rescue suit /7/
36
2.10 Polyacrylate Rubbers (ACM)
Polyacrylate rubbers are elastomers that are prepared from acrylic esters (typically
ethyl and methyl acrylate) and reactive cure site monomer (carboxylic acid or
chloroethyl vinyl ether).
The basic structure of acrylates.
The basic monomers of acrylate rubbers.
Monomer
ethyl acrylate
buthyl acrylate
methoxi ethyl acrylate
ethoxi ethyl acrylate
Structure, X
C2H5
C4H9
C2H4OCH3
C2H4OC2H5
Examples of the structure of acrylate rubbers. Monomers are ethyl acrylate and
chloroethyl vinyl ether or carboxylic group.
©TUT 2007
37
The preparation of polyacrylate rubbers is based on polymerization of acrylate and
metacrylate acids. The polymerization technique can be emulsion or precipitation
polymerization. In emulsion polymerization, the catalyst can be persulphate salt or
redox system. In precipitation polymerization, the catalyst can be peroxide. The
peroxides are solvents to monomer or atso-bis-isobytyro-nitril, which degrade
easily.
To make reactive sites for vulcanization, polyacrylate elastomers are
copolymerized with 1 ... 5 weight percent reactive component, such as carboxylic
acid or chloroethylene vinyl ether or epoxy compounds. Common vulcanization
agents are methylene dianiline or hexamethylene diamine carbamate, or
metalcarboxyl soaps, such as sodium- or potassium stearate. Sulphur acts as a
catalyst.
Properties of ACM:
•
•
•
•
•
•
•
•
•
•
•
excellent ozone and weathering resistance
very good heat resistance
good oil resistance
good elasticity
excellent flexing properties
resistant to oil and aliphatic solvents
low gas permeability
poor water, alkali and acid resistance
good heat aging resistance
low resistance to hot water
not highly corrosive to steel
Applications:
•
•
•
applications in automotive industry (e.g. boots, grommets and seals)
seals, hoses, wire coverings
adhesive formulations
2.11 Polyurethane rubbers (AU, EU, PUR)
Polyurethanes are named after the urethane group, which forms when the
isocyanate group reacts with the hydroxyl group of the alcohol. Depending on the
type and amount of feeding stocks and additives, polyurethanes can be thermosets
or thermoplastics.
Forming of urethane group.
©TUT 2007
38
Polyurethanes are the single most versatile family of polymers there is.
Polyurethanes can be solid or microcellular elastomers (both cross-linked rubbers
and thermoplastic elastomers), foams, paints, fibres or adhesives. They can also be
processed with most processing methods known at present (see figure).
Polyurethane rubbers (PUR) and also urethane thermoplastic elastomers (TPE-U)
are built up of long, soft segments and short, hard segments. The soft segments are
formed by the reactions between polyesterdiol or polyetherdiol with hydroxyl
group ends. The hard segments are formed by the reactions between isocyanates
and chain extenders. The polyurethane rubbers can be divided into
polyesterurethane rubber (AU) and polyetherurethane rubber (EU) according to the
polyol used.
Polyethene adipate (a polyester)
Poly(tetramethylene ether) glykol (a polyether)
Typical polyols used in polyurethanes.
©TUT 2007
39
MDI (diphenylmethane 4,4'-diisocyanate)
NDI (naphthalene 1,5-diisocyanate)
TDI (toluene diisocyanate)
HDI (hexamethylene diisocyanate)
Typical diisocyanates that are used in polyurethanes.
The polyurethane rubbers can be divided into castable and kneaded (millable)
polyurethanes according to their processing method.
Castable polyurethane rubbers are obtained in a one-step process or a two-step
process. In the one-step casting method polyol, di-isocyanate and chain extender
react and the product is formed in the same step. In the two-step casting method a
prepolymer is prepared first by the reaction between diisocyanate and polyol. In the
second step the molar mass and the length of the chains of the prepolymer are
increased and the structure is cross-linked with chain extenders. The second step is
often carried out in a mould at elevated temperatures. Extenders may be diols or
triols. The two-step casting is more used than one-step casting.
The cross-linking which forms the three-dimensional network in PUR can be
brought out, as described above, by multifunctional chain extenders or isocyanates,
but also with sulphur and peroxides (especially the kneaded PUR grades).
The properties of polyurethane rubbers depend on the structure of their chains.
Polyester-based polyurethane rubbers usually have better mechanical properties
and chemical resistance than polyether-based polyurethane rubbers. Polyether-
©TUT 2007
40
based polyurethane rubbers have better properties in low temperatures and better
hydrolysis resistance.
Properties of polyurethane rubbers:
•
•
•
•
•
•
•
good abrasion and tear resistance
good tensile strength
hardness
good oxygen and ozone resistance
resistant to aliphatic hydrocarbons and oils
low friction coefficient
good insulator
Applications:
•
•
•
•
wearing surfaces of wheels and rollers
power transmission elements
seals
soles
Slit rings /5/
Injection-moulded boots /7/
2.12 Fluorocarbon Rubbers (FKM, FPM)
Fluorocarbon rubbers are very stable materials because of the strength of the bond
between fluorine and carbon. The most typical grades of fluorocarbon rubbers are
based on vinylidene fluoride and hexafluoropropylene HFP monomers (see table
below), which are referred to as FKM in ASTM standards and FPM in ISO
standards. There are also fluorocarbon rubbers containing chlorine in vinylidene
monomers (e.g. CFCl = CF2), referred to as CFM rubbers. Fluorocarbon rubbers
are usually produced by emulsion radical polymerization. Peroxide compounds act
as initiators.
©TUT 2007
41
Monomers used in fluorocarbon rubbers.
Monomer
Structure
vinylidene
VF2
tetrafluoroethylene
TFE
fluoride
chlorotrifluoroethylene
CTFE
hexafluoropropylene
HFP
1-hydropentafluoropropylene
HPTFP
perfluoromethylvinylether
FMVE
Structures of fluorocarbon rubbers
Monomers
Structural unit
Type
Commercial
designation types
VF 2 + HFP
FKM
Viton A,
A-35,
Fluorel
2141,
2146
SFF-26
VF 2
HPFP
+
FKM
Tecnoflon
SH
VF 2 + HFP
+ TFE
FKM
Viton B, B-50
VF 2 +
HPFP + TFE
FKM
Tecnoflon T
VF 2
TFCIE
CFM
KEL-F
3700,
5500, SKF-32
+
©TUT 2007
AHV,
E-60,
2140,
2143,
SL,
42
PFMVE
TFE + X
+
FKM
ECD 006
An example of a structure of the fluorocarbon rubbers, VF2 / HPTFP / TFE copolymer.
The most commonly used FKM rubbers can be vulcanized with diamines,
polyhydroxide compounds and bisphenols. The vulcanization system has a metal
oxide as acid acceptor.
Advantages of fluorocarbon rubbers :
•
•
•
•
•
•
excellent heat resistance (up to 200°C, temporarily 315°C)
good chemical and solvent resistance
excellent oxygen, ozone and weather resistance
incombustible
good abrasion resistance
good high-temperature compression-set resistance
Disadvantages:
•
•
•
•
•
low alkali resistance
relatively poor mechanical properties
limited elasticity at low temperatures
the tensile strength decreases substantially at elevated temperatures
high price
The fluorocarbon rubbers are used for special applications that require good heat,
oxygen or corrosion resistance and hot solvent and oil resistance.
Applications:
•
•
•
•
•
car and airplane seals and hoses
fire-resistant coverings
heat-resistant insulators
o-rings, shaft seals
gaskets, fuel hoses, valve-stem seals
©TUT 2007
43
O rings /3/
V seal /3/
2.13 Silicone Rubbers (Q)
Silicone rubbers are inorganic polymers, since their main chain structure does not
include carbon atoms. As shown in the diagram, silicone and oxygen atoms –
siloxane groups - form the polymer main chain. There are typically also some
pendant groups, usually methyl groups, attached to the polymer chain. The molar
mass of silicone rubbers can vary over a wide range, and consequently there are
liquid materials as well as traditionally resinous rubbers available.
The structure of silicone.
Silicone rubbers are usually polymerized from cyclic oligomers to linear
macromolecules. The vulcanization can be carried out at room temperature or
elevated temperature. Vulcanization at room temperature occurs with crosslinking
agent (e.g. ortho-silicon acid ether) or air. For high temperatures vulcanization
peroxides are used. The molar mass of silicone rubber vulcanized at elevated
temperatures is higher (300 000 - 1 000 000 g/mol) than in room temperature
vulcanization (10 000 - 100 000 g/mol).
©TUT 2007
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Silicone rubbers can be divided according to their pendant group structure.
Pendant group
methyl
phenyl
vinyl
vinyl
phenyl
trifluoropropyl
vinyl
trifluoropropyl
CH3
C6H5
CH2 = CH
CH2 = CH
C6H5
CF3CH2CH2
CH2 = CH
CF3CH2CH2
Rubber type
MQ
PMQ
VMQ
PVMQ
FMQ
FMVQ
In VMQ rubbers, some of the methyl groups (< 0.5 %) are replaced with vinyl
groups. This facilitates vulcanization and reduces deformation set of the rubber.
PMQ and PVMQ rubbers have phenyl groups (5...10 %) instead of methyl groups.
This improves the properties of the silicone rubbers at low temperatures.
Fluorosilicones (FMQ and FMVQ) have better solvent resistance than other
silicone rubbers.
Reinforcement fillers, such as silica, have to be used, because the mechanical
properties of pure silicone rubber are rather weak. For example, the tensile strength
of pure silicone rubber is worse than that of any other ruccer. However, the
mechanical properties of silicone rubber do not weaken at high temperatures as
much as in the case of other rubbers.
Advantages of silicone rubbers:
•
•
•
•
•
•
•
•
high temperature resistance, wide operating temperature range (even -100 ...
+300°C)
UV light, oxygen and ozone resistance (peroxides have to be used for
vulcanization)
elasticity
non-toxic, odourless, tasteless
good release properties
good electrical insulation
good aging resistance at high temperatures
good resistance to low concentrations of acids, bases and salts
Disadvantages of silicones:
•
•
•
•
•
•
weak oil resistance (exception aliphatic oils)
low resistance to steam, acids and alkalis
weak mechanical properties without additives
large shrinkage in moulded articles
vulcanization to obtain good mechanical properties has to be carried out
with peroxides
price
©TUT 2007
45
Applications:
•
•
•
•
•
•
•
•
electrical equipment and technical products in high temperatures
medical devices and hospital supplies
roll coverings
cable coverings and insulators
lining compounds
moulds
o-rings
seals for the aeronautics industry
2.14 Polysulphide Rubbers (T)
Polysulphide rubbers are formed when dihalide reacts with sodium polysulphide.
Polysulphide rubbers have only one manufacturer, Morton International.
Polysulphide rubbers can be divided into four different groups: Thiokol A, FA, ST
and LP rubbers. A-type polysulphide rubbers have ethylene dichloride as a
dihalide, FA rubbers are produced from the blend of ethylene dichloride and
dichloroethylene form. ST-rubbers are produced from dichloroethyenel form and
trichloropropane. LP types are liquid polymers. They are formed by breaking down
a high molecular weight polymer in a controlled manner. The sulphur content of
type A is high (84 %), The sulphur content Fa types is 49 % and that of ST types 37
%.
The polymerization of polysulphide. Reactants are ethyl chloride and sodium
sulphide.
The A and FA types are usually vulcanized by the addition of zinc oxide. The ST
and LP types are vulcanized with an oxidizing agent, e.g. with metal oxides or
metal peroxides.
©TUT 2007
46
Properties of polysulphide rubbers:
•
•
•
•
•
•
•
excellent oil and solvent resistance
good weather and ozone resistance
bad smell
difficult to machine
narrow operating temperature range
they corrode copper
very good low-temperature properties
Applications:
•
•
•
•
paint, oil and fuel hoses
seals
paint and varnish rolls
roller coverings
2.15 Ethylene-Vinyl Acetate Copolymer (EVA)
Ethylene-Vinyl Acetate elastomer is a copolymer of ethylene and vinyl acetate. The
properties of the rubber depend on the vinyl acetate content. EVA polymer has
rubbery properties when the vinyl acetate content is 40...60 wt%.
Ethyenel-vinyl acetate rubber.
The method of preparing ethyl-vinyl acetate depends on the desired vinylacetate
content. Mass polymerization gives 45 weight per cent content at most, emulsion
polymerization gives over 50 weight per cent content and solution polymerization
30 ... 90 weight per cent content. EVA can be vulcanized using peroxides or
ionising radiation. Sulphur cannot be used because of the saturated main chain.
EVA is often blended with NR and SBR to improve the ozone resistance.
©TUT 2007
47
Properties of EVA:
•
•
•
•
•
•
•
•
•
•
•
excellent oxygen, ozone and light resistance
extremely good water and oil resistance
good heat resistance
no resistance to organic solvents
fire resistance
good tack to other materials
low price
poor tear resistance
low abrasion resistance
low elasticity due to the thermoplastic character
with reinforcements, high tensile strength can be obtained
Applications:
•
•
•
•
•
cable and wire coverings
seals
floor materials
some medical extrusions
hoses
2.16
Polypropylene Oxide Rubbers, Polypropylene Oxide-Allyl
Glycidyl Ether Copolymer (GPO)
Polypropylene oxide rubbers are copolymers of propylene oxide and allyl glycidyl
ether. The typical allyl glycidyl ether content is about 5 %. The polymerization
method is solution polymerization in hydrocarbon. Vulcanization can be done with
sulphur.
Polypropylene oxide rubber.
©TUT 2007
48
Properties of polypropylene oxide rubbers:
•
•
•
•
•
•
•
•
good properties at low temperatures
good elasticity
good heat and cold resistance
excellent oxygen, ozone and UV light resistance
weak oil resistance
low internal damping
high price
broad temperature range
Applications:
•
•
•
•
•
vibration absorbers
engine mounts
body mounts
suspension bushing
seals
2.17 Chlorinated Polyethylene
Polyethylene (CSM, CSPE)
(CM,
CPE),
Chlorosulphonated
Polyethylene is normally a semi-crystalline thermoplastic. However, chlorine can
be added to polymer chain to prevent crystallization. The amount of chlorine in
chlorinated PE determines the properties of the polymer. In using small contents
(25 %), the material is still crystalline. Incorporation of higher chlorine content (>
40 %) will make the material too brittle. The best rubbery properties are attained
when chlorine content is about 35 %. Chlorosulphonated polyethylene is similar to
chlorinated polyethylene, but it is easier to cure because of the chlorosulphone
group. That is why chlorosulphonated polyethylene is used more than the
chlorinated polyethylene. The typical chlorosulphone content in elastomer is less
than 1.5 %.
Chlorinated polyethylene .
Chlorosulphonated polyethylene.
©TUT 2007
49
Chlorinated polyethylene can be cured using peroxides or radiation.
Chlorosulphonated polyethylene can be vulcanized with peroxides, metal oxides
and amines. Increasing chlorine content increases oil, fuel and solvent resistance,
but decreases low-temperature flexibility.
Properties of CM:
•
•
•
•
•
•
•
•
•
•
very good UV light resistance
good oil resistance
very good oxygen, ozone and light resistance
good tensile and breaking strength
low compression set (up to 150 °C )
very good dynamic fatigue
excellent aging resistance
very good chemical resistance
good flame resistance
very good colour stability
Properties of CSM:
•
•
•
•
•
•
oxidation and ozone resistance
chemical resistance good
relatively difficult to process
high swelling in some types of oils
high compression set in high temperatures
good cold, heat and flame resistance
Applications:
•
•
•
•
•
•
•
•
•
•
cable and wire coverings
electrical insulator
floor materials
coated fabrics
hoses
pond liners
moulded goods
automotive tubes
boots
dust covers
3. Rubber blends
Rubber materials used in applications are always rubber blends. They contain basic
elastomer or masterbatch and additives. In this way the properties of the material
are improved or changed. Additives and fillers are presented on VERT module
©TUT 2007
50
Raw materials and compounds in rubber industry. VERT module Reinforcing
materials in rubber products presents reinforcements.
The compositions of rubber blends are described in recipes. The basic recipes are
simple and they are standardized. These recipes can be modified when new blends
are developed. Recipes provide information the materials and the amounts used in
rubber blend. The amounts of constituents are usually given in parts per hundred
parts of rubber (phr).
The basic recipe for rubber vulcanized with sulphur.
Material
phr
Raw rubber
100
Sulphur
0-4
Zinc oxide
5
Stearic acid
2
Accelerator
0.5-3
Antioxidant
1-3
Filler
0-150
Plasticizer
0-150
Other additives
0-
4. Thermoplastic elastomers (TPE)
Thermoplastic elastomers are a polymer group whose main properties are elasticity
and easy processability. The use of thermoplastic elastomers has grown noticeably
in recent decades.
Thermoplastic elastomers are a wide group of materials. These materials have
many advantages of which the most important are:
•
•
•
•
•
•
good properties at low temperatures
excellent abrasion resistance
damping properties
good chemical resistance
easy processability (compared to rubber)
recyclability
©TUT 2007
51
Restrictive features of thermoplastic elastomers compared to rubbers are the
relatively low highest operating temperature (< 130 - 160°C), small selection of
soft grades and high price of TPE's.
Thermoplastic elastomers are used in areas where elasticity over a wide
temperature range is required. The main applications are in the automotive industry
and sport accessories.
Thermoplastics elastomers can be divided into the following groups:
•
•
•
•
•
Styrene-diene block copolymer
Elastomeric alloys
Thermoplastic urethane elastomers
Thermoplastic ester-ether copolymers, TPE-E
Thermoplastic amide copolymer, TPE-A
4.1 Styrenic thermoplastic elastomers (TPE-S)
SBS (Styrene/Butadiene Copolymer), SIS (Styrene/Isoprene Copolymer),
SEBS (Styrene/Ethylene-Butylene Copolymer), SEPS (Styrene/EthylenePropylene Copolymer)
Thermoplastic elastomers based on styrene are block copolymers in which a
polydiene unit divides polystyrene blocks. The polydiene may be for example
butadiene (SBS), isoprene (SIS), ethylene-butylene (SEBS) or ethylene-propylene
(SEPS). The styrene content varies with different materials, but usually it is 20-40
%.
The linear and the radial structure of styrene thermoelasts
©TUT 2007
52
Advantages of styrenic TPEs:
•
•
•
•
•
•
•
high tensile strength and modulus
good miscibility
good abrasion resistance
good electrical properties
large variety in hardness
high friction coefficient (corresponds to that for NR)
colourless, good transparency
Disadvantages:
•
•
•
poor high temperature resistance (highest operation temperature, SBS 65°C,
SEBS 135°C)
weak oxygen, ozone and light resistance of SBS (exception SEBS)
poor oil and solvent resistance
Applications:
•
•
•
•
•
rubber products in car industry
cables and wires
shoe soles
adhesives
with thermoplastics in multi-component injection moulding and coextrusion
4.2 Elastomeric alloys
Elastomeric alloys are blends of elastomers and thermoplastics that can be
processed using thermoplastic processing methods. Elastomeric alloys are:
•
•
•
Thermoplastic Olefin Elastomers (TPO)
Thermoplastic vulcanizates (TPV)
Melt Processible Rubbers (MPR)
4.2.1 Thermoplastic Olefin Elastomers (TPO, TOE)
Thermoplastic olefin elastomers are most commonly blends of Polypropylene and
EPM or Polypropylene and EPDM. Natural rubber and butyl rubber have also been
used. A blend can be made in a mechanical mixing unit, e.g. in a twin-screw
extruder or in polymerization reactors.
The properties of thermoplastic olefin elastomers vary according to components,
mixture ratio and conditions of alloying. Properties typical of thermoplastic olefin
elastomers are:
©TUT 2007
53
•
•
•
•
•
good chemical resistance
excellent weathering resistance
low density
good processibility
low price
Applications:
•
•
•
buffers and outside profiles in car industry
wire and cable coatings
hoses
4.2.2 Thermoplastic Vulcanizates (TPE-V, TPV, DVR)
Thermoplastic vulcanizates are blends of thermoplastics and elastomers that have
been dynamically vulcanized during their mixing (see picture). Those kinds of
materials are for example dynamically vulcanized blends of PP and EPDM and PP
and NBR. The properties of the material depend greatly on the structure and
content of the elastomer.
The structure of TPE-V, showing finely dispersed vulcanized rubber particles in
thermoplastics matrix.
The effect of rubber particle size in TPE-V (AES)
©TUT 2007
54
The properties of thermoplastic vulcanizates:
•
•
•
•
•
small permanent deformation
good mechanical properties
good properties at low temperatures
fatigue durability
good liquid and oil resistance
Applications:
•
•
•
car components
tubes
electrical insulators
4.2.3 Melt-Processible Rubbers (MPR)
Melt-processible rubbers are very rubbery materials that look and feel like
traditional rubbers. However, they can be processed like thermoplastics. Meltprocessible rubbers have one phase structure, so they differ from other
thermoplastic elastomers that have a two-phase structure.
Properties of melt-processible rubbers:
•
•
•
excellent elasticity
stress-tensile behaviour corresponds to that of vulcanized rubbers
softness and flexibility
4.3 Thermoplastic Urethane Elastomers (TPU, TPE-U)
Polyurethanes are named after the urethane group, which is formed when
isocyanate group reacts with the hydroxyl group of the alcohol. Depending on the
type and amount of feeding stocks and additives, polyurethanes can be
thermoplastics, rubbers (PUR) or thermoplastic elastomers.
Forming of urethane group.
Thermoplastic polyurethane elastomers form from long (MW around 600 – 3000
g/mol) soft segments of linear polyester (TPE-AU) or polyethers (TPE-EU) and
short, hard urethane segments that are formed of di-isocyanate and small alcohol
molecule chain extender, e.g. butane diol.
©TUT 2007
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The structure of thermoplastic urethane elastomers : long ester or ether diol chains
and hard urethane segments
The properties of thermoplastic urethane elastomers vary strongly according to
feedstocks and the ratio of hard and soft segments in the material. The soft segment
component influences especially the low temperature properties of TPE-U, but also
many other characteristics. Depending on whether the soft segment is formed of
polyester or polyether, the properties can be compared according to the table
below.
Advantages of thermoplastic urethane elastomers:
•
•
•
•
•
good abrasion resistance
good tear strength
good strength and stiffness properties
low friction coefficient (depends on hardness)
good oxygen, ozone and weather resistance
Disadvantages of thermoplastic urethane elastomers:
•
•
•
poor hydrolysis resistance
poor resistance to chlorinated and aromatic solvents
relatively poor UV light resistance
©TUT 2007
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The properties of TPAU and TPEU.
Property
TPAU
TPEU
Tensile strength
++
0
Abrasion resistance
++
0
Tear resistance
++
0
Radiant energy resistance
+
0
Hydrolysis resistance
-/0
+
Low swelling in oil, fat and petrol
+
0
Weather resistance
+
0
Oxidation resistance
+
-/0
Microbies resistance
-/0
++
Water absorption
0
+
Impact resistance at low temperatures
0
++/0
++ excellent, +good, 0 fair, -poor
Applications:
•
•
•
•
•
conveyor belts
footwear
cable and wire coatings
hoses
components of car industry
4.4 Thermoplastics Polyester-Ether Elastomer (TPE-E)
Polyetherglycols, such as polyethylene, polypropylene or polybutylene ether
glycols are soft segments in thermoplastics polyester-ether elastomers. Hard
segments are dimethylterephtalate or 1,4-butanediol.
Advantages:
•
•
•
good oxygen and ozone resistance
good oil resistance
good strength properties
©TUT 2007
57
Disadvantages:
•
•
•
•
•
small variety in hardness
low elongation at break (requires own design principles of products)
poor hydrolysis resistance
poor UV-light resistance
high price
Applications:
•
•
•
cable and wire coatings
gaskets
hoses, tubes
4.5 Thermoplastic Polyamide Elastomers (TPE-A)
Soft segments of polyesters or polyethers and a rigid block of polyamide form
thermoplastic polyamide elastomers. The polyamide can be for example
polyesteramide (PEA), polyetheresteramide (PEEA), polycarbonate-esteramide
(PCEA) or polyether-block-amide (PE-b-A). The properties of thermoplastic
polyamide elastomers depend strongly on the type of polyamide block, the type of
polyol block and the length and amount of blocks.
The structure of thermoplastic polyamide elastomers.
Properties of thermoplastic polyamide elastomers:
•
•
•
good heat resistance (up to 170°C)
good chemical resistance
good abrasion resistance
Applications:
•
•
•
•
•
components in car motors and under the hood
wire and cable coatings
hoses
footballs, skiing boots
films penetrating water vapour
©TUT 2007
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4.6 Comparison of different TPEs
Some values for the comparison of different TPEs are given in the table below.
3
Density [g/cm ]
Hardness Shore A/D
Lowest util T. [oC]
Highest util. T. [oC]
Compression set at 100oC
Hydrocarbon resistance
Hydrolysis resistance
Price order [€/kg]
TPE-S
0.9-1.1
30A-75D
-70
70, 135
P(SBS)
F/G(SEBS)
F/E
G/E
2...5
TPE-V
0.89-1
60A-75D
-60
135
TPE-U
1.1-1.3
60A-55D
-50
140
TPE-E
1.1-1.2
40-72D
-65
150
TPE-A
75-63A
-40
170
P
F/G
F/G
F/G
G/E
G/E
3...6
F/E
F/G
4...7
G/E
P/G
6...8
G/E
F/G
7...10
4.7 New development trends occuring in the field of TPEs
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Material innovations
New polymerization techniques, metallocene techniques
Foamed materials, e.g. supercritical gases
Electrical properties, conductivities
Paintability
Blends including nanofillers
Processing
Coextrusion, coinjection, overmoulding
Adhesion & joining
Milling, thermoforming, extrusion, injection & blow moulding (all
processing alternatives)
Recycling
Product innovations/development, hybrid products
Product design to maximize the benefits of TPEs
Smart products, functionality
Design
Food and health applications, bioapplications
©TUT 2007
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5. Processing
5.1 Processing of rubbers
The processing of rubbers starts by mixing elastomers and additives. After that
rubbers are shaped by using different kinds of processing methods. The possible
methods are calandering, extrusion, moulding techniques (e.g. compression
moulding and injection moulding) and dipping. The methods are presented in the
VERT module "Processing of elastomeric materials". After shaping, the rubber
product is vulcanized so that mechanical properties and dimensional stability
appear. Vulcanization may occur during the processing or after it in many
techniques.
Rubber process
The processing of rubbers is quite difficult. Rubber has high viscosity and that is
why high shear forces are needed in the processing. Vulcanization poses
restrictions too. The processing temperature of rubbers is typically 70-140oC.
Shear rates in rubber processing.
5.2 Processing of thermoplastic elastomers
The general characteristics of the processing methods of thermoplastics and
thermoplastic elastomers are much the same. The most significant differences
©TUT 2007
60
between TPs and TPEs lie in the values of processing temperatures and viscosity.
In the case of thermoplastics, the processing temperatures are usually higher (150250oC) and viscosity values are slightly lower than those for TPEs. However, as the
first approximation, the processing equipment for thermoplastics is mostly suitable
also for processing thermoplastic elastomers. The most common processing
methods for thermoplastic elastomers are injection moulding, extrusion and blow
moulding techniques. The methods are presented in the VERT module "Processing
of elastomeric materials". The viscosity of TPEs is significantly lower than the
viscosities of traditional rubber elastomers, which offers many processing
advantages for TPEs compared with rubbers.
TPE process
Benefits of TPE processing (comparison with rubbers):
•
•
•
•
•
•
•
•
•
•
•
•
•
No compounding
No vulcanization
Faster processing properties (short cycle times)
Standard thermoplastic processing equipment
Thermally stable
Recyclable
Colourable in a broad range of intensites
Clear grades available
Paintable
Printable
Weldable
Overmouldable onto a variety of different substrates
Foamable
6. Design of elastomeric products
The purpose of design is to assist in converting inventions into successful
innovations. The target can also be to prolong the life of a product by giving it a
new appearance or shape.
The purpose of design is also to influence the attitude of the consumer towards the
product at various stages of the product's life. Advertising, test results, appearance,
company image and price are factors that are used for attracting customers.
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The important factors influencing the consumer:
•
•
Before the decision to buy: shop display, total interior of the shop, colors,
materials, functions, fashion content and price
When the product is in use: the product´s functions, ease of use and
required care
Impact of design on consumer.
Marketing mix
Price
Running cost
Quality
Company image
Delivery performance
After sales service
Impact of Design
Production cost
Running and service costs
Durability and quality level
Package, display, promotion
On-time deliveries
Service, repairs
Requirements for the development project may be defined on the basis of customer
interviews. However, it is important to review these requirements, as often
customers do not really know what they want, or their wishes may be based on
history rather than the future market. The feasibility of various solutions is
evaluated by a feasibility study. The impact of each solution is tested in terms of
profitability.
The manufacturer of elastomer products can design the product according to the
customer's specific request or develop a new product and supply it for several
customers. Technical rubber products are often developed in line with the specific
request of the customer. A tyre is a good example of a product that is designed by
the manufacturer and then marketed for customers. The product design is often
made in co-ordination with the customer. An elastomer component is often part of
a bigger unit, which may include metal mountings and restrictions regarding size
and form.
It is important that the product meets the requirements of customers better than
products of competitors. It is also useful if the product is capable of further
development.
Aspects related to design
• Elastomer type
• Dimensioning
• Shaping
• Economic efficiency of materials and processing
• Processing method
• Reinforcement
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6.1 Design process
Theoretically the phases of the design process are:
1. Defining the problem and needs and outlining the product development
project
2. Product development phase: Searching for ideas and combining them into a
unified solution, developing a prototype and freezing the design
3. Before deliveries: product sketches and data, modification of operating
system, testing, full-scale production
After product launching:
•
•
•
Customer research (consumer research)
After sales service
Definition of problems and their study
6.2 Elastomer selection
The most important criteria in choosing an elastomer:
•
•
•
•
Flexibility
Vibration damping
Heat insulation
Oil and chemical resistance
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•
•
•
•
•
•
•
Mechanical resistance (including abrasion resistance)
Functionality at low and elevated temperatures
Weather and ozone resistance
Impermeability for gases and fluids
Elasticity and vibration damping properties
Long-term creep
Processability
Many methods that may facilitate elastomer selection have been developed. One
example is the selecting tree. By means of the selecting tree, it is easy to make
some basic choices. One way is to feed the criteria into a computer program and
obtain a recommendation regarding suitable material.
Selection trees for rubbers
Rubber factories have several basic rubber blends for different uses. When a new
blend is needed, a suitable basic blend can be selected and modified to conform to
the requirements of the new product. Thus, a new rubber blend is obtained
relatively easily, because good basic data on the properties and processing already
exist.
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6.3 Dimensioning of elastomer products
In dimensioning, the most important starting points to be taken into account at the
early stages of design are
•
•
•
Functionality
Predicting and ensuring against the risks of damage
Predicting lifetime
The product may be damaged in many ways. It can for example:
•
•
•
•
Creep
Fracture
Change in stiffness because of chemical changes caused by temperature
Change in stiffness because chemicals from the environment diffuse to the
material
To predict damage, it is important to know models of behaviour and parameters of
materials. Planning models are not very advanced in the case of elastomers. Their
exploitation is complicated because of non-linear loading-deformation phenomena
that make the theoretical prediction of practical structures difficult.
6.3.1 Mechanical dimensioning
The purpose of product design is to ensure the functionality of a product. One of
the most important stages is to ensure that load capacity corresponds to demands.
6.3.2 The influence of hardness
The most important characteristic of rubber is its hardness (stiffness). Hardness is
roughly related to compression modulus and shear deformation modulus. The
approximate relation can be presented by the equation
E ≈ 1.045 h (Mpa) = 2(1 + n ) ≈ 3 G,
where h = hardness, E = compression module, G = module for shear deformation, n
= Poisson number (describes the compressibility of an elastomer). Elastomers are
practically incompressible (E = 1.0…3.5 GPa) and therefore their n = 0.5. The
equation above is most valid when the hardness of rubber is 30…80 ShA
(correspondingly, G = 0.3…3 MPa).
6.3.3 Shape factor
The deformation of almost incompressible elastomeric materials is considerably
influenced by the shape of the loaded piece. The shape of a rubber piece is
described by the shape factor (S, see figure below).
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6.3.4 Stiffness in different loading situations
Taking the shape factor S into account, dependences between compression stress s,
compression modulus Ec and shear modulus G (see picture below) can be written
in the form
s = F/A = Ec 1 + 2 k S2) x/h and Ec = G (3 + CS2),
where
F = Pressing force
A = Cross-sectional area
E = Compression module
G = Shear module
k = Parameter that depends on hardness of rubber (assumption k = 1)
x = Compression deformation
h = Height in stress direction
S = Shape factor
The equations above are most valid with the shape factor values 1 - 10. The factor
C depends also on the form of the sample, being typically between 4 (long stripe)
... 6 (round plate). The principal dependence of E c on shape factor is shown in the
picture below.
The stiffness of rubber constructions can be controlled with the help of the
equations above. The construction is shared with rubber plates whose shape factor
comprises more than single-layer structure. The method is, for example, used in the
case of bridge bearings where the load capacity has to be considerable.
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Compression modulus of natural rubber as a function of shape factor and
hardness.
6.3.5 Allowed loadings for different rubbers
Stiffness dependencies on deformation were described by the equations on the
preceding page. Using the equations, the deformations caused by real loading
situations can be estimated. The highest allowed values for different kinds of
loading situations are often presented. The values are empirically confirmed.
As an example, a series of graphs are presented below. They provide information
on the allowed compression loadings. In each graph the transversal lines outline the
areas as follows:
•
•
Under the lower line is the area of allowed loadings
Under the upper line is the area of allowed loadings but which are to be
regarded with reservation
It can be seen in the graphs that in practice the allowed deformation is for
•
•
Harder rubbers about 15 %
Softer rubbers 20 – 25 %
Again, the loading stress has to be < 1MPa in compression and 0.3 MPa in shear.
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Loading estimation graphs for rubber products having different hardness and
shape factors.
The greatest values allowed in mechanical loading depend on the rubber and also
on other stress factors (including chemical loadings).
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For example, in applications where long-term creep is a critical factor, dynamical
loadings have to be estimated separately. That is because they can generate greater
permanent deformations than static loadings.
Creep values for filled and non-filled natural rubber with static and dynamical
loading.
Loading in stretching has also to be taken into account. Stretching deformations
should be minimized, because rubber molecules are susceptible to aging reactions
caused by radiation, ozone and oxygen.
The creep depends on the composition of the elastomer. Thus, it is not possible to
draw conclusions on the grounds of theoretical modelling that is based on typical
properties of rubber types.
Rubber blends can contain 5 ... 20 components and thus it is obvious that properties
will vary significantly. For this reason the properties of new rubber blend should
first be measured since only then can the behaviour of the rubber be evaluated.
6.4 Product shaping
Dimensioning sets certain limitations. Before mould design, the structure of the
elastomer product has to be shaped so that local stresses are avoided in loading
situations. Weather-sensitive surfaces should not be exposed to stretch loadings.
The examples of designing for even loading are given in the figures below.
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Examples of applications that are used to achieve even distribution of loading
stress and to avoid tension in the rubber product.
7. Comparison
sources
of
Elastomer
Properties.
Data
There are numerous sources of information where data on elastomer properties are
available. Not the least important are the technical information services by the
material deliverers.
In the table below, we have picked up some examples of the general properties of
different elastomers and their chemical resistances.
http://www.timcorubber.com/definitions/Comparison_to_Elastomer_Properties.pdf
Prof. Dr. M. Häberlein, HTML-Lecture Rubber Technology :
http://www.fbv.fh-frankfurt.de/mhwww/KAT/English/indexrubber.htm
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Chemical resistance of rubbers
Material
Natural rubber,
NR, IR
Isoprene
Chemical Group
Polyisoprene
Generally
Resistant to
Generally
Attacked by
Most moderate
wet or dry
chemicals,
organic acids,
alcohols,
ketones,
aldehydes
Ozone, strong
acids, fats, oils,
greases, most
hydrocarbons
Styrene, Butadiene
SBR, Butadiene, Styrene
Similar to
Copolymer,
BR
Butadiene
natural rubber
Polybutadiene
IIR
Butyl
Similar to natural
rubber
Petroleum
solvents, coal,
Isobutylene,
Water and steam tar, solvents,
Isoprene, polymer
aromatic
hydrocarbons
Ethylene
Propylene
EPM,
Ethylene Propylene
copolymer and
EPDM
terpolymer
Mineral oils and
solvents,
Water, steam
and brake fluids aromatic
hydrocarbons
Butadiene,
Acrylonitrile
copolymer
Many
hydrocarbons,
fats, oils,
greases,
hydraulic fluids,
chemicals
Ozone, ketones,
esters, aldehydes,
chlorinated and
nitro
hydrocarbons
Butadiene,
HNBR Hydrogenated nitrile Acrylonitrile
copolymer
Similar to NBR
but with
improved
chemical
resistance and
higher service
temperature
Ozone, ketones,
esters, aldehydes,
chlorinated and
nitro
hydrocarbons
NBR
Nitrile
CO 1
Epichlorohydrin
ECO
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Epichlorohydrin
polymer and
copolymer
Ketones, esters,
Similar to nitrile aldehydes,
chlorinated and
with ozone
nitro
resistance
hydrocarbons
71
CR
Neoprene
CSM Hypalon®
CM,
CPE
AU,
EU
T
Chloroprene
polymer
Strong oxidizing
Moderate
acids, esters,
chemicals and
ketones,
acids, ozone,
chlorinated,
oils, fats,
aromatic and
greases, many
nitro
oils, and solvents
hydrocarbons
Chlorosulfonated
polyethylene with Similar to
improved acid and Neoprene
ozone resistance
Concentrated
oxidizing acids,
esters, ketones,
chlorinated,
aromatic and
nitro
hydrocarbons
Tyrin®
Chlorinated
polyethylene
Similar to
Neoprene with
improved acid
and ozone
resistance
Concentrated
oxidizing acids,
esters, ketones,
chlorinated,
aromatic and
nitro
hydrocarbons
Urethane
Ozone,
hydrocarbons,
Urethane polymer moderate
chemicals, fats,
oils, greases
Concentrated
acids, ketones,
esters,
chlorinated and
nitro
hydrocarbons
Organic
polysulfide
polymer
Ozone, oils,
solvents,
thinners,
ketones, esters,
aromatic
hydrocarbons
Mercaptons,
chlorinated
hydrocarbons,
nitro
hydrocarbons,
ethers, amines,
hetercocyclics
Organic silicone
polymer
Moderate or
oxidizing
chemicals,
ozone,
concentrated
sodium
hydroxide
Many solvents,
oils, concentrated
acids, dilute
sodium
hydroxide
Polysulfide
Si,
Silicone
VMQ
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FSI,
Fluorosilicone
FVMQ
TFE/P
Fluorinated
organic silicone
polymer
Tetrafluoroethylene/ Fluorinated
Propylene
copolymer
Moderate or
oxidizing
Brake fluids,
chemicals,
hydrazine,
ozone, aromatic
ketones
chlorinated
solvents, bases
Steam, amines
Aromatic
and amine
hydrocarbons,
corrosion
chlorinated
inhibitors,
solvents, ethers,
caustics, high pH
limited in low
media, wet sour
temperatures
gas, oil
Copolymer of
acrylic ester and
acrylic halide
Ozone, extreme
pressure,
lubricants, hot
oils, petroleum
solvents, animal
and vegetable
fats
Water, alcohols,
glycols alkali,
esters, aromatic
hydrocarbons,
halogenated
hydrocarbons,
phenol
FKM
Fluoroelastomer
#1
Standard
fluorocarbon
dipolymer 66%
fluorine
All aliphatic,
aromatic and
halogenated
hydrocarbons,
acids, animal
and vegetable
oils
Ketones, low
molecular weight
esters and
alcohols and
nitro-containing
compounds
FKM
Fluoroelastomer
#2
Standard or
specialty type
fluorocarbon.
Typically, >66%
fluorine
Ketones, low
Same as
molecular weight
FKM#2. Greater
esters and nitrochemical
containing
resistance
compounds
Proprietary
fluorocarbon
Greater
resistance to
Nitrogenacid, base,
containing
alcohol, amine
compounds
and ethers than
FKM
Fully fluorinated
fluorocarbon
FluorocarbonBest fluid
containing
resistance of any refrigerants
cause minor
elastomer
effects
ACM Polyacrylate
Zalak®
FFKM Perfluoroelastomer
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8. Recycling and reuse of elastomeric materials
The global consumption of vulcanized elastomers is about 17.2 million tons/year.
Approximately 40 % of that is natural rubber. Goodyear developed the first
recycling method. In this patented method, rubber waste is ground and used as
filler.
The main problem with vulcanized rubber products is what to do with them after
their useful life has expired. Rubber waste is usually generated from both the
products of the manufacturing process and post-consumer products, mainly
consisting of scrap tires.
The environmental problems created by waste rubber and legislative restrictions
make it necessary to search for economical and ecologically sound methods of
recycling.
8.1 Why reclaim or recycle rubber?
Rubber recovery can be a difficult process. However, there are many reasons, why
rubber should be reclaimed or recovered:
•
•
•
•
•
•
•
•
Final price can be half compared with the use of synthetic material.
Recovered rubber has some properties that are better than those of virgin
rubber.
Reclaiming rubber requires less energy in the total production process than
virgin material.
It is an excellent way to dispose of unwanted rubber products, which is
often difficult.
It conserves non-renewable petroleum products that are used to produce
synthetic rubbers.
Recycling activities can generate work in developing countries.
Many useful products are derived from reused tyres and other rubber
products.
If tyres are incinerated to reclaim embodied energy, they can yield
substantial quantities of useful power. In Australia, some cement factories
use waste tyres as a fuel source.
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8.2 Recycling methods
Basically waste rubber can be recycled in three ways: it can be used for energy by
combustion, it can be used in its original form through devulcanization, and it can
be used as ground powder.
Recovery Alternatives
Recovery type
Product reuse
Material reuse
Energy reuse
Recovery process
Repair
Retreading
Regrooving
Physical reuse
Use as weight
Use of form
Use of properties
Use of volume
Physical
Tearing apart
Cutting
Processing to crumb
Chemical
Reclamation
Thermal
Pyrolysis
Combustion
Incineration
Waste management hierarchy
8.2.1 Incineration
Incineration is a good and economical method of disposing of rubber. The energy
content of rubber is about 32.6 MJ/kg, which is about 10 % less than heavy oil
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(37.7 MJ/kg) and 1.3 times the energy content of coal (25.1 MJ/kg). Rubber is
burned in a special incinerator. The purpose is to recover as much energy as
possible in as ecologically sound a manner as possible.
Incineration produces oxygen, carbon dioxide, water and some toxic gases. Using
sufficiently high temperatures can prevent the formation of toxic components, such
as dioxin.
8.2.2 Pyrolysis
Pyrolysis involves heating the rubber waste in the absence of oxygen. The
temperatures used in this process are typically 400-800°C. The pyrolysis process
produces three principal products: pyrolytic gas (10-20%), oil (40-50%) and char
(30-40%). Char is a fine particulate composition of carbon black, ash, and other
inorganic materials, such as zinc oxide, carbonates and silicates. Other by-products
of pyrolysis may include steel, rayon, cotton, or nylon fibres from tyre cords. Each
product and by-product is marketable:
•
•
•
The gas has high calorific value.
The light oils can be sold as gasoline additives to enhance octane and the
heavy oils can be used as a replacement for number six fuel oil.
The char can substitute for carbon black in some applications, although
quality and consistency is a significant impediment.
The quality and quantity of pyrolytic products depend on the reactor temperature
and reactor design. Heating rate, reaction time and pressure are also important
process variables.
Pyrolysis does not pollute air significantly because most of the pyro-gas generated
is burned as fuel in the process. During burning, the organic compounds are
destroyed. The decomposition products are water, carbon dioxide, carbon
monoxide, sulphur dioxide and nitrogen oxides.
8.2.3 Grinding of vulcanized rubber waste
Sometimes it is beneficial to reduce the size of the rubber. For example, landfill
consisting of whole tyres may be prohibited, while it is permissible to dump
granulated tyre chips. The size reduction of rubber waste facilitates the burning
process too. In most other cases the grinding of rubber articles is required to
remove the rubber from reinforcing textiles or metals and prepare the rubber for the
next processing step, such as adding to virgin rubber or other polymeric
compounds, surface activation or devulcanization.
Size reduction can be caused by impact, cutting or tearing, or by degradation of the
rubber. There are three ways in which to break down tyres into crumb rubber. All
three begin by shredding or cutting the tyres into relatively large pieces (average
size 20 x 20 mm). There are three process steps:
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1. Milling and grinding of dry material at ambient temperature (ambient grinding)
Typical ambient grinding system
2. Milling of frozen material cooled to liquid nitrogen temperatures (cryogenic
grinding)
Typical cryogenic grinding system
3. Milling of swollen material with subsequent solvent recovery (wet grinding).
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Crumb rubber is measured by mesh or inch and it is generally defined as rubber
that is reduced to a particle size of 3/8-inch or less. Crumb sizes can be classified
into four groups:
•
•
•
•
large or coarse (9-5 mm or 3/8” and 1/4”) (ambient grinding)
mid-range (10-30 mesh or 2-0.6 mm or 0.079”-0.039”) (ambient grinding)
fine (40-80 mesh or 0.425-0.180 mm or 0.016”-0.007”) (cryogenic and wet
grinding)
superfine (100-200 mesh or 0.149-0.074 mm or 0.006”-0.003”) (cryogenic
and wet grinding)
The chemical composition, the duration of breakdown and the ratio of thermal to
mechanical breakdown influence the physical properties of reclaim . Varying the
duration and ratio of the different breakdown steps allows the production of custom
reclaims differing in viscosity, tensile strength and other related properties. An
explanation of this effect is found in the selectiveness of the mechanical breakdown
step: it is primarily restricted to the carbon, i.e. to the carbon backbones of the
network, which are broken down, and preferably the longer chains. This leads to a
narrower molar mass distribution. The thermo-chemical breakdown step is random.
As a result, the percentage of low molar masspolymer, acting as a peptizer and
having no reinforcing effect on the network, increases and the tensile strength of
the cured reclaim decreases. These differences in reclaim quality influence the
properties of a compound containing different reclaims.
Recycled rubber powder obtained from ambient or cryogenically ground tyres can
be utilized as filler in rubber and other polymeric compounds. In cryogenically and
wet ground rubbers, smaller particle size allows recycled rubber to be used at
moderately high levels and still retain processability. The incorporation of GRP
into polymeric matrix typically impairs the mechanical properties of the resulting
composites. This is because of poor matrix-filler adhesion and the lack of reactive
sites on the particle surface. Thus, the related end products generally are used in
applications with low performance requirements. To overcome this problem
various surface treatments of GRP have been proposed:
•
•
•
•
•
•
•
Coating of the GRP
Interfacial compatibilizing
High-energy radiation such as plasma, corona and electron beam radiation
Reactive gas treatment
Chlorination
Surface grafting
Use of coupling agents
8.2.4 Devulcanization
Devulcanization is one of the new methods of recycling waste rubber products.
Devulcanization means the cleavage of cross-linking sulphur bonds in rubber
vulcanizates, without cleavage of the polymer chain bonds. Devulcanization is a
good way of utilizing rubber waste because it assumes renewal of the original
chemical formula of elastomers and provides a possibility of recovering elastomers
from rubber vulcanizate waste. It can also be incorporated into the compound in a
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considerably larger amount than surface-modified or non-modified rubber scrap. In
general, instead of adding one part of unmodified rubber scrap, about three parts of
surface-modified or about seven parts of devulcanized rubber can be added.
However, in practice a total devulcanization process is very difficult to carry out
since many problems are caused by accompanying chemical transitions such as
depolymerization, thermal destruction and oxidation that worsen the properties of
the recovered elastomers. The main problem is the very low thermal conductivity
of rubber and the extremely difficult selective regulation of the quantity of energy
carried to the cross-linking bonds. In practice, it is virtually impossible to achieve
such levels of energy evenly distributed in all materials. It is necessary to find
experimentally the optimal devulcanization conditions that lead to devulcanized
products with good properties.
In the initial stage of the devulcanization reaction, the polysulphide and disulphide
bonds are converted to monosulphide bonds by heat. Furthermore, the
monosulphide bond is broken by addition of shear stress and finally recycled
uncured rubber is obtained.
Mechanism of cross-linking bond breakage reaction
Both physical and chemical processes are used to carry out the devulcanization or
reclaiming of GRP. The powder is either subjected to shear action in suitable
equipment, e.g. in an extruder or two-roll mill, and partially decrosslinked or to
chemical action to obtain reclaimed material. Devulcanization by microorganism
has been also examined. There is also a commercially produced devulcanization
agent on the market (De-Link R). Physical processes involve applications of
mechanical, thermo-mechanical, microwave or ultrasound energy to partially
devulcanize the rubber. In the chemical reclaiming process, different chemical
reactants like diphenyldisulphide, dibenzyldisulphide, diamyldisulphide,
mercaptan, xylenethiol, iron oxide/phenyl hydrazine mixture, etc. have been used
for the treatment of scrap ground rubber powders at elevated temperature. In
chemical treatment done by Kim and Park, the chemical reagent used was di(cobenzanidopheny)-disulphide. This enables the polysulphide bond from polymer
chain to be destroyed. Using the treated crumb rubber enhanced the mechanical
performance of the rubber compounds produced.
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8.3 Utilization of unvulcanized rubber waste
Unvulcanized rubber waste is mainly generated in the manufacturing process. It is
also very useful to recycle it. Reinforced rubbers that contain a steel wire and
textile fibre are difficult to recycle. However, all these include valuable raw
materials. The utilization of unvulcanized rubber waste provides a variety of
processing advantages. It also has a positive effect on energy consumption because
much less energy is consumed through the production and utilization of products
including waste rubber than through the manufacture of virgin raw materials. One
option of course is vulcanization of unvulcanized rubber waste. After that, the
grinding is possible.
8.4 Processing of recycled rubber
8.4.1 Unvulcanized rubber waste
Temperature has a big role in the processing of unvulcanized rubber. In order to
keep the temperature rise within acceptable limits, the mixing equipment has to be
coolable. The temperature must be controlled to ensure that there is not excessive
plasticization and to prevent early scorching. Every elastomer has an optimum
temperature for efficient heat exchange.
When mixing unvulcanized rubber waste with virgin rubber and ingredient in an
internal mixer, the mixer has to be cooled very well in order to remove the heat
generated during the mixing cycle. Unvulcanized rubber waste has to be filled into
the mixer in the shortest possible time and also the whole mixture has to be
discharged very rapidly. After discharge from the internal mixer, the compound is
in the form of lumps and has to be homogenized, cooled and sheeted out using
follow-up equipment, e.g. a sheeting mill or forming extruder.
In mill mixing, temperature control is easier. The rolls are cooled down to remove
excessive heat built up during mixing. That is why the processing of unvulcanized
rubber waste using two roll mills is the safest process from the point of view of the
risk of scorching.
To prevent the risk of scorching, also some retarders can be used. The advantage of
using unvulcanized crumb rubber is that it can be bonded directly to the elastomer
matrix.
8.4.2 Vulcanized rubber waste
One of the most effective ways of reusing rubber waste is to incorporate it into new
rubber products in the form of fine ground powder (rubber scrap). Rubber scrap is
easy to apply using simple equipment and has a positive effect on the processing
behaviour of a compound.
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The main advantages derived from the use of reclaim concern the processing
behaviour of the compound. These advantages include:
•
•
•
•
Shorter mixing cycles, resulting in reduced processing costs
Lower mixing, calendering and extrusion temperatures, resulting in fast and
uniform calendering and extrusion
Improved penetration of fabric and cord
Lower swelling and shrinking during extrusion and calendering
Other important advantages are:
•
•
•
•
Lower raw material costs
Better air venting properties
Improved stability during curing in hot air or open steam
Improved reversion and aging performance of natural rubber compounds
(ozone, UV)
In the processing of cryogenically ground rubber, certain particle sizes are more
suitable in specific applications:
•
Extrusion: 80-100 mesh cryogenically ground rubber is needed to avoid
fracturing and rough edges. In extrusion of thick section, 50-60 mesh
cryogenically ground rubber can be used, depending on the surface
smoothness of the final product. The optimum level of cryogenically
ground rubber to be added to virgin rubber is 5%.
•
Calendering: for optimum surface smoothness of products, whose thickness
is 1.5 mm or less, the compound requires 80-100 mesh cryogenically
ground rubber. Where smoothness is not so important/critical, 30-60 mesh
can be used. The optimum level of cryogenically ground rubber in
calendering is 10%.
•
Moulding: cryogenically ground rubber in all mesh sizes can be used
because all mesh sizes help in removing trapped air during moulding. The
cured rubber particles provide a path for the air to escape by bleeding air
from the part.
•
Mould flow: cryogenically ground rubber generally improves mould flow.
Shrinkage is usually less for compounds containing cryogenically ground
rubber. The shrinkage reduction is proportional to the amount of
cryogenically ground rubber in the compound. So less mould flashing was
found with increase in the percentage of cryogenically ground rubber.
8.4.3 Devulcanized rubber waste
Devulcanized rubber can be processed, shaped and vulcanized in the same way as
virgin rubber. There are also many benefits deriving from the use of devulcanized
rubber in rubber compounds:
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•
•
•
•
•
•
•
•
•
Shorter mixing cycles – lower power consumption
Low calendering, mixing and extrusion temperatures – greater uniformity
Improved penetration of fabric and cord
Increased tack with minimal effect on temperature variation
Low swelling and shrinkage during extrusion or calendering
Improved stability during curing in hot air or open steam
Better air venting
Improved reversion and aging performance on natural rubber
Lower raw material costs
One shortcoming of reclaim is that it lowers the green strength of compounds.
8.5 Applications of waste rubber
•
•
•
•
•
•
Pavements
Sound barriers
Polymer mortars and concretes
Recycled rubber can be applied to the entire range of rubber products,
including tyres, technical rubber goods, conveyor belts, shoe soles and
industrial coatings.
Rubber powder can be applied to sport surfaces as a rubber mat when
bounded with a polymer binder, e.g. polyurethane, or just mixed with sand.
Whole tyres can be used for artificial reefs, breakwaters, erosion control,
playground equipment and highway crash barriers.
8.6 Recycling of tyres
The methods of reusing rubbers are: product reuse, material reuse and energy
recovery. Most tyres of cars and vans can be retreaded. Tyre of car can be retreaded
once and tyres of vans 2 to 3 times. Retreated tyres should only be mounted on
low-speed rated cars. There must be an age-restriction for the acceptance of worn
tyres (e.g. 6 years); also careful inspection is required prior to starting buffing, and
afterwards during the processing. In addition, lower weight or longer running tyres
are becoming less suitable for retreading operations. Lifetime and driving distance
expectations for truck tyres have increased and retreading is common.
If a product cannot be reused, it can be used in secondary reuse. The biggest
secondary reuse applications of tyres are road building, noise barriers and landfills.
In those applications the tyre powder can act as insulator or lightening material
between different courses of other materials. Blasting mat and buffers in piers are
other uses.
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Examples of the amount of tyres in different secondary reuse applications.
Application
The amount of
tyres [piece]
The largeness of the product
Bitumen asphalt
2500
Per road km
Noise barrier
20000
Per road km (3m high)
Playground
1400
about 500 m2
Playground safety ground
300
about. 50 m2
Sports field
6000
6000 m2
Sport hall
1300
1000 m2
Electricity production
150-675 tons
Per month
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References
/1/ www.ilmapallokeskus.fi
/2/ http://www.scottsafety.com/fin/fsari.htm
/3/ http://www.ramikro.fi/frame.htm
/4/ http://www.ramikro.fi/frame.htm
/5/ http://www.ramikro.fi/frame.htm
/6/ http://www.nokianfootwear.fi/nfi/
/7/ http://www.ursuk.com/ursuit/pdf/fi_pelastus_koko.pdf
/8/ www.fipa-online.com
/9/ http://www.tamrex.fi/k%E4sineet3.pdf
/10/ http://www.nokiantyres.com/
/11/ http://www.ursuk.com/ursuit/pdf/heavy_light_fz_bz_res.pdf
/12/ http://www.itdg.org/docs/technical_information_service/recycling_rubber.pdf
/13/ ASTM 2000
/14/ SFS 3552
/15/ http://www.pslc.ws/macrog/urethane.htm
/16/ http://www.fbv.fh-frankfurt.de/mhwww/KAT/English/indexrubber.htm
/17/ http://www.pslc.ws/macrog/pb.htm
/18/ Simpson R.B (edit), Rubber basics, Rapra tevhnology Limited 2002
/19/ Morton, M. (edit), Rubber technology third edition, Chapman & Hall, 1995
/20/ Andersen, c., (edit), Lifespan of rubber materials and thermoplastic elastomers
in air, water and oil, IFP – The Swedish Insitute for Fibre –and Polymer Research,
1999
/21/ Franta, I., Elastomers and rubber compounding materials: manufacture,
properties and applications, Elsevier 1989
/22/ Lamminmäki, J. Research on the utilization of waste rubbers in polymer
matrices, Licentiate thesis, Tampere University of Technology, Department of
Material Engineering, 2005.
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