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.

Vulcanization, cross-linking

An irreversible process in which the rubber compound is transformed in a chemical reaction (e.g. cross-linking) to a three-dimensional network which preserves its elastic characteristics over a wide temperature range. The term vulcanization is connected with the use of sulphur and its derivatives, whereas the term cross-linking is usually connected with sulphur-free processes.

Thermoplastic elastomer

Thermoplastic elastomers are in many respects a rubber-like material which need not be vulcanized. The rubbery character disappears at the processing temperature but returns when the 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

Operating temperature, °C

- 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

Resistance

- 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

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 cross-sectional 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 general-purpose 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).

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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)

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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.

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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. Cis-1,4 -polyisoprene is a synthetic substitute for natural rubber and trans-1,4-polyisoprene 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 Ti-IR 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-1,4- content 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.

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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.

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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

Glass transition temperatureT , °C

-108 g

-106 -93

Melting temperature T , °C m -11 -22 amorphous

Molar mass distribution medium board thin very thin

Branching degree medium low very low

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Comparision the properties of BR and NR.

5 = excellent, 4 = very good, 3 = good, 2 = fair, 1 = poor

Butadiene rubber BR

Natural rubber NR

Hardness, °IRH 40-80 30-90 Tensile strength at break, N/mm2 7-21 7-28 Elongation at break, % 100-600 100-700 Operating temperature range: - maximum, °C 80 80 - minimum, °C -70 -55 Elasticity 5 5 Electrical properties 4 4 Resistance: - weather and ozone 1-2 1-2 - acids 2-3 2-3 - alkalis 2-3 2-3 - water 3 5 - abrasion 4-5 4-5 - flame 1 1 - radiation 2-3 2-3 Gas permeability 3 3 Adherence 4 4 Tack to the metal 5 5 Residual compression, % (°C) - 20-60 (70)

Polybutadiene rubbers can be vulcanized with sulphur, sulphur compounds and peroxides. The peroxide vulcanization is very effective and produces highly cross-linked 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

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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

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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 Solvent-SBR Molar mass Mn, g/mol 145000 200000 Molar mass Mw, g/mol 651000 420000 Mw / Mn 4.5 2.1 Styrene content, % 23.5 18 Cis-1,4-content, % 18 35 Trans-1,4-content, % 65 54 1,2-content, % 17 11 Glass transition temperature Tg, °C - 50.6 - 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

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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

Styrenebutadiene- rubber SBR

Natural rubber NR

Hardness, °IRH 40....90 30...90 Tensile strength at break, N/mm2 7....25 7...28

Elongation at break, % 100...600 100...700

Operating temperature range

- maximum, °C 100 80 - minimum, °C - 45 - 55 Elasticity 5 5 Resistance: - weather and ozone 1...2 1...2 - abrasion 4 4...5 - radiation 2...3 2...3

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

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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

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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 trans-1,4 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, cross-links 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

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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/

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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

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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

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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 di-functional 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.

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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

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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.

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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 peroxide-cured 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°C

5-200+

Ethylene Content, wt. % 45 to 80 wt. %

Diene Content, wt. % 0 to 15 wt. %

Specific Gravity, gm/ml 0.855-0.88 (depending on polymer 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

* 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.

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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

Cis-1,4-form

1,2- 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.

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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/ Gloves /9/ Rescue suit /7/

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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 Structure, X ethyl acrylate C2H5

buthyl acrylate C4H9

methoxi ethyl acrylate C2H4OCH3

ethoxi ethyl acrylate C2H4OC2H5

Examples of the structure of acrylate rubbers. Monomers are ethyl acrylate and chloroethyl vinyl ether or carboxylic group.

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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.

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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.

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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-

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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.

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Monomers used in fluorocarbon rubbers.

Monomer Structure

vinylidene fluorideVF2

tetrafluoroethylene TFE

chlorotrifluoroethylene CTFE

hexafluoropropylene HFP

1-hydropentafluoropropylene HPTFP

perfluoromethylvinylether FMVE

Structures of fluorocarbon rubbers

Monomers Structural unit Type designation

Commercial types

VF 2 + HFP

FKM Viton A, AHV, A-35, E-60, Fluorel 2140, 2141, 2143, 2146 SFF-26

VF 2 + HPFP

FKM Tecnoflon SL, 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

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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

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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).

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Silicone rubbers can be divided according to their pendant group structure.

Pendant group Rubber type methyl CH3 MQ phenyl C6H5 PMQ vinyl CH2 = CH VMQ vinyl phenyl

CH2 = CH C6H5

PVMQ

trifluoropropyl CF3CH2CH2 FMQ vinyl trifluoropropyl

CH2 = CH CF3CH2CH2

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

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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.

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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.

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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.

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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 (CM, CPE), Chlorosulphonated Polyethylene (CSM, CSPE)

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.

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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

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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

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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/Ethylene-Propylene 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

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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 co-

extrusion

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:

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• 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)

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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. Melt-processible 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.

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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

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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

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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

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4.6 Comparison of different TPEs

Some values for the comparison of different TPEs are given in the table below.

TPE-S TPE-V TPE-U TPE-E TPE-A Density [g/cm3] 0.9-1.1 0.89-1 1.1-1.3 1.1-1.2 Hardness Shore A/D 30A-75D 60A-75D 60A-55D 40-72D 75-63A Lowest util T. [oC] -70 -60 -50 -65 -40 Highest util. T. [oC] 70, 135 135 140 150 170

Compression set at 100oC P(SBS) F/G(SEBS) P F/G F/G F/G

Hydrocarbon resistance F/E G/E F/E G/E G/E Hydrolysis resistance G/E G/E F/G P/G F/G Price order [€/kg] 2...5 3...6 4...7 6...8 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

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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

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between TPs and TPEs lie in the values of processing temperatures and viscosity. In the case of thermoplastics, the processing temperatures are usually higher (150-250oC) 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 Impact of Design Price Production cost

Running cost Running and service costs Quality Durability and quality level

Company image Package, display, promotion Delivery performance On-time deliveries

After sales service 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 (Mpa) = 2(1 + n ) ≈ 3 G, h

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 = E 1 + 2 k S ) x/h and E = G (3 + CS ), c2

c2

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 of Elastomer Properties. Data sources

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 Chemical Group Generally

Resistant to Generally

Attacked by

NR, IR Natural rubber, Isoprene Polyisoprene

Most moderate wet or dry chemicals, organic acids, alcohols, ketones, aldehydes

Ozone, strong acids, fats, oils, greases, most hydrocarbons

SBR, BR

Butadiene, Styrene Butadiene

Styrene, Butadiene Copolymer, Polybutadiene

Similar to natural rubber

Similar to natural rubber

IIR Butyl Isobutylene, Isoprene, polymer Water and steam

Petroleum solvents, coal, tar, solvents, aromatic hydrocarbons

EPM, EPDM Ethylene Propylene

Ethylene Propylene copolymer and terpolymer

Water, steam and brake fluids

Mineral oils and solvents, aromatic hydrocarbons

NBR Nitrile Butadiene, Acrylonitrile copolymer

Many hydrocarbons, fats, oils, greases, hydraulic fluids, chemicals

Ozone, ketones, esters, aldehydes, chlorinated and nitro hydrocarbons

HNBR Hydrogenated nitrile Butadiene, Acrylonitrile copolymer

Similar to NBR but with improved chemical resistance and higher service temperature

Ozone, ketones, esters, aldehydes, chlorinated and nitro hydrocarbons

CO 1 ECO Epichlorohydrin

Epichlorohydrin polymer and copolymer

Similar to nitrile with ozone resistance

Ketones, esters, aldehydes, chlorinated and nitro hydrocarbons

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CR Neoprene Chloroprene polymer

Moderate chemicals and acids, ozone, oils, fats, greases, many oils, and solvents

Strong oxidizing acids, esters, ketones, chlorinated, aromatic and nitro hydrocarbons

CSM Hypalon®

Chlorosulfonated polyethylene with improved acid and ozone resistance

Similar to Neoprene

Concentrated oxidizing acids, esters, ketones, chlorinated, aromatic and nitro hydrocarbons

CM, CPE Tyrin® Chlorinated

polyethylene

Similar to Neoprene with improved acid and ozone resistance

Concentrated oxidizing acids, esters, ketones, chlorinated, aromatic and nitro hydrocarbons

AU, EU Urethane Urethane polymer

Ozone, hydrocarbons, moderate chemicals, fats, oils, greases

Concentrated acids, ketones, esters, chlorinated and nitro hydrocarbons

T Polysulfide Organic polysulfide polymer

Ozone, oils, solvents, thinners, ketones, esters, aromatic hydrocarbons

Mercaptons, chlorinated hydrocarbons, nitro hydrocarbons, ethers, amines, hetercocyclics

Si, VMQ Silicone Organic silicone

polymer

Moderate or oxidizing chemicals, ozone, concentrated sodium hydroxide

Many solvents, oils, concentrated acids, dilute sodium hydroxide

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FSI, FVMQ Fluorosilicone

Fluorinated organic silicone polymer

Moderate or oxidizing chemicals, ozone, aromatic chlorinated solvents, bases

Brake fluids, hydrazine, ketones

TFE/P Tetrafluoroethylene/ Propylene

Fluorinated copolymer

Steam, amines and amine corrosion inhibitors, caustics, high pHmedia, wet sour gas, oil

Aromatic hydrocarbons, chlorinated solvents, ethers, limited in low temperatures

ACM Polyacrylate 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 #1 Fluoroelastomer

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 #2 Fluoroelastomer

Standard or specialty type fluorocarbon. Typically, >66% fluorine

Same as FKM#2. Greater chemical resistance

Ketones, low molecular weight esters and nitro-containing compounds

Zalak® Proprietary fluorocarbon

Greater resistance to acid, base, alcohol, amine and ethers than FKM

Nitrogen-containing compounds

FFKM Perfluoroelastomer Fully fluorinated fluorocarbon

Best fluid resistance of any elastomer

Fluorocarbon-containing refrigerants cause minor effects

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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 Recovery process

Product reuse Repair Retreading Regrooving

Physical reuse Use as weight Use of form Use of properties Use of volume

Material reuse Physical Tearing apart Cutting Processing to crumb

Chemical Reclamation Thermal Pyrolysis

Combustion

Energy reuse 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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