TUT Plastics and Elastomer Technology

MOL-5906 Elastomers (Autumn 2008)

Introduction to elastomeric materials

The natives of South America alighted on exploiting the latex of Hevea Brasiliensis rubber tree to produce waterproof footwear, among others, by soaking their feet to liquid, latex, tapped from tree. From the Indian word “caa-o-chu” (a weeping tree), inherit the words like caoutchouc in English and French, Kautschuk in German language, caucho in Spanish, 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 to be waterproof by rubber. However, the performance of the rubber articles was quite poor, because rubber was at that time still gummy and the fluctuation of temperatures caused great changes in products. It was only at year 1839 when Charles Goodyear conceived nearly by accident the vulcanization of rubber, which made rubber as an elastic material capable of preserving its characters in large temperature range.

The idea of this part of “Professional Development Learning program for Rubber Industries” course is to give students an extensive view of the elastomeric materials. The structure and characters of most typical rubber and thermoplastic elastomers will be examined during this course. In addition, familiarizing with applications and testing of different elastomers, design and construction, as well as recycling of elastomeric products are to be handled.

Definitions of elastomeric materials and rubbers

Monomer Low molar mass molecules, which can react with same or different kind of monomers, thus composing a polymer.

Polymer Macromolecules built up by the repetition of primary monomer units in a way that properties of the material do not alter significantly in consequence of insertion or removing of some primary units.

Homopolymer Polymer, which has been made up of only one kind of monomer.

Copolymer Polymer, which has been made up 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, free of solvent elastomer, which contracts to its 1,5 -fold original length in one minute, after the tension which has stretched the rubber to its double length in the room temperature, has been released.

Natural rubber Cis-1,4-polyisoprene achieved 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

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 at wide temperature range. The term vulcanization is connected to the use of sulphur and its derivatives, whereas the term cross-linking is usually connected with sulfur-free processes.

Thermoplastic elastomer

Thermoplastic elastomers are in many respects rubber like material, which need not to be vulcanized. Rubbery character disappears at the processing temperature but revert when material has reached the operating temperature.

Rubber type Group of rubber elastomers, which produce same kind of characters and applications to products made of that group of elastomers.

Rubber quality Vulcanized mixture of rubber fulfilling certain set of quality requirements.

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Manufacturing process of rubbers

The manufacturing process of synthetic rubber starts by manufacturing raw rubber. The first step in this process is polymerization. This is a chemical reaction, where small molecules (monomers) are joined together to form large molecules (polymers). Natural rubber is collected ready polymerized form. So the manufacturing process of natural rubber starts by mastication. Mastication is a process where molecules are physically or chemically shredded so that mixing and processing would be easier. This makes the rubber softer. Most synthetic rubbers do not need the mastication because they are made of shorter molecules. Peptizing agent prevents reactions between the broken chains. Rubbers consist of elastomer and additives. Additives may be for instance fillers and vulcanisation agents. The purpose of additives is e.g. to improve properties or processability. Rubbers can be processed in many ways (e.g. compression moulding, injection moulding and extrusion). During the process or after that the rubber is vulcanised (cross-linked) when rubber elasticity and dimensional stability will appear. After the process and vulcanisation rubber product often has to finish by e.g. cutting.

Behaviour of elastomers

The predominant property of elastomers is the elastic recovery after deformation in compression or tension. Even after stretching of an elastomer many times its original length, it will return after removal of the tension under ideal circumstances to its original shape and length. In addition to this, elastomers are characterized by a great toughness under static or dynamic stresses, a better abrasion resistance than that of

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steel, by an impermeability to air and water, and in many cases, by a high resistance to swelling in solvents, and to attack by chemicals. Elastomers, like many other polymers, show viscoelastic properties, which nowadays can be tailored for numerous special applications, like e.g. tyres, vibration and shock isolation and damping. These properties are exhibited at 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 fibers, such as rayon, polyamide, polyester, glass, or steel-cord, the tensile strength is increased considerably with a reduction in extendibility. By joining elastomers with 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 com- pound, vulcanizates with considerably different properties with respect to hardness, elasticity, or strength are yielded.

The viscoelasticity of elastomers and rubbers is easy to detect in practice. When stretching a cross-linked elastomeric band, rubber band, a temperature rise of 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 rubber material.

The structure of elastomers at strain and dependence of elastic force on temperature T and entropy S (Gedde).

The temperature range of elastic behavior of elastomers is limited by glass transition temperature. At the 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 under the glass transition temperature. The physical background of elastomeric behavior is described in more detail in the PDLRI section rubber physics.

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 character of loading e.g. whether it is static or dynamic,

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influences strongly in elastomer properties. Satisfactory properties can only be obtained by proper compounding of elastomers with chemicals and additives, and subsequent vulcanisation at adequate circumstances. 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.

In the following chapters, the most frequently specified properties of rubbers will be handled, with reference to ASTM 2000, SFS 3552 and SIS 162602 standards. Many of the properties are presented in more detail in the PDLRI section testing and properties. The comparison of the properties of different rubber types and also thermoplastic elastomers is shown in the attached table, in context with handling of rubber types.

Thermal expansion

The amounts of thermal expansion of different rubbers vary remarkably, depending e.g. on elastomers, fillers and their shares 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 molded elastomer products can be several percent.

Hardness

The hardness of rubber is determined and measured based on a protrusion depth of a standardized body under well-defined conditions. Hardness is one of the most frequently measured property of rubbers. Hardness is commonly quantified with IRHD or Shore 0 ... 100 scale. The hardness of 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 to certain deformation, strain (see figure). Frequently, the stress values corresponding 100 or 300 % deformation are chosen to describe tensile stiffness ( s 100 or s 300 module). The modulus at the early stage of tensile test is called Young's modulus. The stress at the break of the sample is defined as tensile strength (MPa) and the breaking deformation compared to the original length as elongation at break (%). The values of tensile strength of rubbers and thermoplastic elastomers are taken as satisfactory/good on the range 7 ... 15 MPa and excellent when the values are over 15 MPa. The values of elongation at break vary on the range 300 – 1000 %.

Typical tensile curves of different plastics (A, B and C) and rubber (D).

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Modulus of elasticity

As described earlier, aside from 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 for the stiffness of a rubber sample and one of the most important measures for the evaluation of vulcanizates. The stress value is often called a modulus. The use of the word modulus is incorrect, however, since the stress value is always taken in 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 absolute temperature, Mc is the number average molar mass of the chain segments between the cross-links of rubber, λ is relative elongation (L/Lo) and ρ is the density. The equation contains in the form of “modulus”

the facts that tension increases with raising temperature and increasing degree of cross-linking (decreasing Mc ). In addition, the stress depends on deformation speed and the form of 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 cut ting or slitting, offers to the propagation of the tear. Multitude of test specimen configurations has been presented for tear test.

The force (kN) 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 tear strength of elastomeric materials with good tear properties are on 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 for the viscous behaviour of the elastomers. It can be either compression or tension type of set. Permanent set is a measure for the viscous behaviour of the elastomers. The compression set CS, and also tensile set, is given at constant deformation by the relation:

where h0 is the initial height of the sample before deformation, h1 is the height during deformation and h2 the height after a certain amount of time after deformation, for

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example 50%. 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, 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 materials durability under wearing conditions. Most rubbers have exceptionally good abrasion resistance, which is a consequence of rubbers' ability to creep over the irregularities of wearing counterpart in sliding. A good wearing resistance is typically achieved with vulcanized general-purpose rubbers, NR, IR, SBR and BR. In an environment with oil exposure, 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 and the dissipated energy is described with the term resilience. The resilience can be easily evaluated by using modern dynamic mechanical analyzers. Since the dissipated mechanical energy during the dynamic loading is transformed into heat because of molecular friction, the viscous component may be measured directly by monitoring the increase in heat of the sample (heat build- up).

The energy dissipation property of rubbers is often called also internal dampening or hysteresis loss. The hysteresis losses of rubbers depend quite strongly on temperatures and loading amplitudes, and are typically of the order of 5 ... 40 %.

Electrical properties

Most general-purpose elastomers, like natural rubber (NR) and variety of synthetic elastomers (e.g. SBR, IR, BR, EPDM, IIR, MQ ) exhibit a 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 electrical conductivity of all elastomers can be affected over a wide range by the composition of the compound or by the addition of insulating (e.g. light) fillers or of conducting substances, especially carbon blacks or antistatic plasticizers.

Rubber articles with such a high electric conductivity can be produced, too, e.g. for the prevention of static electricity build-up.

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Chemical endurance

Some fluids can cause big volume changes of rubbers, which derive from the impregnation of the fluid among the macromolecules (swell), or from the solution of rubber ingredients into the fluid (shrinkage). Water, acids and bases may also bring about some hydrolysis for certain rubbers, leading to impair of tensile properties. Nitric acid and concentrated hydrochloric acid react with most rubbers and deteriorate them.

Ozone and weathering resistance A deformed rubber with strained parts, gets often cracks outdoors, because the double bonds of macromolecules are broken by the oxygen, ozone or electromagnetic radiation. Adding anti-aging agents, like waxes, antioxidants and antiozonants, can at least partially, prevent the damages. Dynamic properties

Elastomers are viscoelastic materials. It means that part of the deformation is recovered after the load is removed and 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:

• 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'

Classification of elastomers

Elastomers have been classified to groups according to similar 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).

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Rubber type 61 (rubbers for general use)

Type 61 rubbers are used when the product does not require special properties, like 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 rubber type that has not standard. Butyl rubber (IIR), chlorobutyl rubbers (CIIR) and bromobutyl rubbers (BIIR) are elastomers in 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 mineral oils.

Rubber type 63

Rubbers in this group have good oil resistance, but ozone and weather resistance are weak. Applications are products that are touched with oils. Nitrile rubber (NBR) is 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 poly-vinyl-chloride (NBR/PVC). It has better oil, ozone and weather resistance than NBR.

Rubber type 64

Chloroprene rubber (CR) is representative of rubber type 64. It has good resistance to vegetable oils and pretty good aliphatic and naphtenic oils. Disadvantage is poor aromatic oil resistance.

Rubber type 65

Rubbers in this group have good weather and heat resistance and quite good oil resistance. Poly-acrylic 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 they have good weather and oil resistance. Heat resistance is poor.

Rubber type 67

In this group, rubbers (fluorocarbon rubbers (FPM)) have good weather, heat, oil and chemical resistance.

Rubber type 68

Silicone rubbers (Q) are in this group. They have good weather, cold and heat resistance. Mechanical properties are weak.

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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, ethylene-vinyl acetate copolymer (resistant to aliphatic oils) BR, butadiene rubber (good elasticity) XNBR, carboksylated nitrile-butadiene rubber (tough and oil resistant)

Natural Rubber (NR)

Natural rubber can be isolated from more than 200 different species of plants. Commercially significant natural rubber source is Hevea Brasiliensis. The natural rubber is obtained from latex, which is the emulsion of cis-1,4-polyisoprene and water. The latex is obtained from the tree by tapping into inner bark and collecting the latex in cups. Increasing stabilizing agent such as ammonia can prevent too early coagulation.

Cis-1,4-polyisoprene.

Collation of the latex from rubber tree.

The latex can be concentrated by centrifuging or creaming and sold as concentrated latex. The latex can be coagulated with hydrogen carboxylic acid or acetic acid, form in sheets or granulate and then dry it to a solid raw rubber. The raw rubber types are for example ribbed smoked sheets (RSS), air-dried sheets (ADS) and pale crepes.

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The natural rubber also contains few percent s non-rubber constituents such as resins, proteins, sugars and fatty acids that may function as weak antioxidants and accelerators in the natural rubber. The natural rubber is usually vulcanized by using sulphur, but also peroxides and isocyanates can be used.

The biggest producer countries of the natural rubber are Thailand, Indonesia, Malaysia and India. Some classification systems that define the maximum content of dirt, cinder, nitrogen and volatile elements, has been developed in these countries. One well-known system is Standard Malaysian Rubber, which has been used since 1965.

There are numerous methods of processing latex into commercial grades of dry natural rubber and latex, as shown in the enclosed picture below (Rubber Engineering, Indian Rubber Institute, McGraw-Hill, 2000).

Methods of processing latex into commercial grades of dry natural rubber and into concentrated latex.

Operating temperature range of NR is -55...+70 °C.

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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)

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 the

vulcanisation remarkably improves the 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/

Natural rubber has also many modified types. 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 the modifying has been the improvement of NR properties to meet the special needs of rubber manufacturers.

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Isoprene Rubber, Polyisoprene (IR)

Poly-isoprene rubber has the same basic chemical formula as the natural rubber (NR), so it is a synthetic version of NR. The study of materials, which would be comparable with NR started in the beginning of 20th century, but because of the high price of raw materials and the weak quality of polymers, industrial production was not started. Significant production was started in 1970s, when cheaper monomers and catalysts, which produce stereo-specific polymers in solution polymerization, were available. It is possible to create different kinds of isomeric structures by using different catalysts and polymerization conditions in 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. The commercial synthetic isoprene rubbers can be divided in different groups according to the catalyst used.

• Li-IR group, whose catalyst in polymerisation, is alkyl lithium. The amount of cis-1,4-units in Li-IR is about 90 % (10% 1,2- type IR).

• 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 the recent years, which approximates very well natural rubber. The share of 1,4-cis-units in lanthanide IR can be 99.5 %.

The amount of cis-1,4-units influences on crystallization and regularity of molecule structure. When cis-1,4-content arises, crystallization will facilitate, glass transition temperature will decrease and strength properties will get better. Consequently, the strength properties like modules, tensile strength and tear resistance of synthetic

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polyisoprenes are slightly worse than those of NR, whose cis-1,4- content is almost 100 %. Also, the building tack of IR is somewhat inferior to NR, and the green strength is poorer. Otherwise, the properties of synthetic isoprene rubbers are similar to NR. The most significant advantages of synthetic polyisoprenes compared to natural rubber are their purity, good processibility and homogeneity of polymer structure.

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 resistant to many inorganic chemicals

Disadvantages of IR: restricted life time in high temperatures and oxidative circumstance poor 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 improve.

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 pruning of special trees, which are native in Malaysia.

Butadiene Rubber, Polybutadiene (BR)

The forerunner of poly-butadiene rubbers was Buna, which was prepared first time in Germany in 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. In consequence of that, American rubber scientists polymerized poly-butadiene (BR) in 1954. The BR rubbers have much better weather resistance than Buna.

Using solution polymerization in hydrocarbon solvent typically performs the polymerization of the BR. Suitable catalysts are Ziegler-Natta -combinations, lithium and its compounds. The elastomer is often named according to its catalyst or according to metal inside it. Abbreviations used are among others Li-BR (lithium), Co-BR (cobalt) and Ni-BR (nickel).

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Three different kinds of basic construction units can be formed in polymerization. The catalyst and polymerization conditions affect on development of these units.

Cis-1,4-form Trans-1,4-form

1,2-form

The isomeric structures of BR.

The isomeric construction structures, which appear most, determine the properties of polymer. The butadiene rubbers can be divided in three groups according to the amount of cis-units.

Polybutadiene rubbers according to 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 temperature T , °C

g -108 -106 -93

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

Molar mass distribution medium board thin very thin

Branching degree medium low very low

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

Advantages of BR: excellent cold resistance and heat resistance elasticity excellent low temperature flexibility and resilience abrasion resistance

Disadvantages of BR:

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poor processability weak mechanical properties

The processing of BR is really difficult. That's 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

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 was 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. The “cold” emulsion polymerization, at about 5°C, is the most widely used polymerization technique, even though the solution method has steadily conquered the market share.

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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. Another type contains segmented styrene and butadiene blocks (TPE), the other type is rubber elastomer with random distribution of co-monomers in polymer.

The styrene-butadiene rubber can be vulcanized by the use of sulphur, sulphur donor systems and peroxides.

Processing method can affect on SBRs properties considerably. Molar mass, styrene content and the amount of units vary depending on 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 ageing resistance good elasticity low price

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

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Comparison between the properties of SBR and NR. 5 = excellent, 4 = very good, 3 = good, 2 = fair, l = 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 to achieve good tensile and tear strength and durability than natural rubber. 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

Tyre /10/

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The use of SBR in tyres

Varying SBR content in copolymer like in tyre tread blends can modify the properties of tyres.

Monomer contents that are typical of tyre tread blends.

The properties of polymer Low road resistance Good wet grip General use

Vinyl content (%) 10 50 35

Styrene content (%) 15 23 20

The effect of monomer content and Tg 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

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 to the isobutylene chain as trans-1,4 form. Adjusting the polymerization temperature and the share 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 saturation degree. When the amount of the double bonds is low, rubber has good oxygen and ozone resistance. The bigger amount of double bonds accelerates the vulcanisation process and increases the amount of cross-links.

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Isobutylene and isoprene units.

The properties of butyl rubbers can be improved by increasing 1 ... 2 weight percent halogens, forming chlorobutyl (CIIR) and bromobutyl (BIIR) rubbers. Halogens are mostly joined to the double bonded carbon without methyl group in the isoprene unit. 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 does need accelerator. Dioxime compounds together with the oxidizing agent can also be used. In that case cross-links stand heat better than sulphur bonds. CIIR and BIIR have more reactive points to the crosslinking, if the 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 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 agent, vegetable and animal fats and polar solvents heat stability

Disadvantages: poor wear resistance not resistant to hydrocarbon solvent and oil relatively low elasticity

The properties of the halogenated butyl rubbers are similar with basic butyl rubber. However, they have lower gas permeability and better thermal, ozone, weather and chemical resistance. The halogenated butyl rubbers are used in applications that demand 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

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Diving suit /11/

Nitrile Rubber, Nitrile-Butadiene Rubber, Acrylonitrile Rubber (NBR)

Poly-acrylonitrile-butadiene rubber is a copolymer of butadiene and acrylonitrile. It was synthesized first time in 1930. It is used because of good oil, fuel and fat resistance. Acrylonitrile rubbers are also called just nitrile rubber. The NBR is produced by emulsion polymerization. Polymerization rates of acrylonitrile and butadiene are different. Because of that, the content of monomers in copolymer is not same than the content of monomers in reaction mixture. The polymer formed is random copolymer, where the acrylonitrile content varies between 18 ... 50 %. Changing temperature or a feeding of monomers can modify the composition of the polymer.

Butadiene and acrylonitrile units.

Increasing of 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 in room temperature or in high temperatures to accelerate the reactions.

The nitrile rubbers are used in applications, which demand good mechanical properties and oil and fuel resistance. The NBR can be used blended with other rubbers. For

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instance, the increasing of IIR to NBR improves weather properties and thermal stability and decreases 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

Boots /6/ Gloves /9/

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 (partial) saturating the double bonds in main chain butadiene by catalytic hydrogenation. This kind of NBR, HNBR, has been developed to stand better ageing in oil and hot air.

Properties of HNBR: oil- and gasoline swelling as NBR application temperature up to 150°C high tensile strength, weather resistant peroxide curable types (double bond content < 1 %) and

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sulphur curable (double bond content < 4 – 6 %) Main applications: vehicle tubing, seals, cables and profiles

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, ethyleneoxide and some other monomer (typically diene).

The structures of CO and ECO.

In polymerization of epochlorohydrin coordinate catalyst is used. 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 group react with di-functional curing agent, such as diamine, ethylenethiourea 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 in wide temperature range whereas CO is elastic only in elevated temperatures. That's why the epichlorohydrine copolymers are used more than the homopolymers.

Properties of epichlorohydrin rubbers: • resistant 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 • risk of causing corrosion with metal • very good dynamic properties

The use of epichlorohydrin rubbers is similar with 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

Ethylene-Propylene Rubber (EPM), Ethylene-Propylene-Diene Rubber (EPDM)

Ethylene-propylene rubbers can be divided in 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 the high ethylene content material. Especially the 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 ethylene-propylene rubbers are produced mostly by solution polymerization with Ziegler-Natta type catalysts. The EPM rubbers cannot be vulcanized with sulphur because of the absence of unsaturation in the main chain. The EPM can be cured with peroxides or radiation. The 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 application and processing needs

Typical Properties

Ethylene-propylene rubbers are valuable for their excellent resistance to heat, oxidation, ozone and weathering resistance due to their stable, saturated polymer backbone structure. Properly pigmented black and non-black compounds are colouring 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.

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

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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).

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.

Applications: • products of automotive industry: seals and hoses, isolators • gaskets and hosepipes, liners in building industry • roll covers • agricultural equipment: hoses, seed tubes, cushioning, silos • wire and cable

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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 easier and safer route. Chloroprene is polymerized by emulsion polymerization by using potassium persulphate as free radical initiator. The main component of the polymer usually is trans-1,4-units. In 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.

The chloroprene rubbers can be divided in G and W types according to their mechanism to control the molecular weight of the polymer during the 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 ageing resistance, but resilience and tack are better than the W types. The W types chloroprene rubbers require accelerator. The vulcanization doesn't manage with sulphur. Suitable accelerators are metal oxides. The W type rubbers have better ageing properties and thermal resistance than G type rubbers. Different grades of CR rubber and their typical properties have been presented in the figure below.

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Different grades of CR / http://www.dupont-dow.com/ /

Polychloroprene is a versatile elastomer. It is used especially in demanding circumstances.

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

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Mask /2/ Gloves /9/ Rescue suit /7/

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

butyl acrylate C4H9

methoxy ethyl acrylate C2H4OCH3

ethoxy ethyl acrylate C2H4OC2H5

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

The preparing of the polyacrylate rubbers is based on polymerization of acrylate and metacrylate acids. Polymerization technique can be emulsion or precipitation polymerization. In emulsion polymerization, 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-isobutyro-nitrile, 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 kaliumstearate. 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 acids 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

Polyurethane rubbers (AU, EU, PUR)

Polyurethanes are named after urethane group, which forms 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 thermosets or thermoplastics.

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Forming of urethane group.

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, fibers 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 in polyesterurethane rubber (AU) and polyetherurethane rubber (EU) according to polyol used.

The polyurethane rubbers can be divided in castable and kneaded (millable) polyurethanes according to their process method.

Castable polyurethane rubbers are obtained in one-step process or 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 second step the molar mass and the length of the chains of the prepolymer is raised and the structure is cross-linked with chain extenders. The second step is often carried out in mould in elevated temperatures. Extenders may be diols or triols. The two-step casting is more used than the one-step casting.

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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 structure of their chains. Polyester based polyurethane rubbers have usually better mechanical properties and chemical resistance than polyether based polyurethane rubbers. Polyether 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 molded boots /7/

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 vinyl diene fluoride and hexafluoropropylene HFP monomers (see table below), which have been denominated as FKM in ASTM standards and FPM in ISO standards. There are also fluorocarbon rubbers containing chlorine in vinylidene monomers (e.g. CFCl = CF2), denominated as CFM rubbers. The fluorocarbon rubbers are usually produced by emulsion radical polymerization. Peroxy compounds act as initiators.

Monomers used in fluorocarbon rubbers.

Monomer Structure

vinylidene fluoride VF2

tetrafluoroethylene TFE

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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

PFMVE + TFE + X

FKM ECD 006

Most commonly used FKM rubbers can be vulcanized with diamines, polyhydroxide compounds and bisphenols. The vulcanization system has a metal oxide as acid acceptors.

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Advantages of fluorocarbon rubbers: • excellent heat resistance (even 200°C, temporarily 315°C) • good chemicals 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 in low temperatures • the tensile strength decreases substantially at elevated temperatures • high price

The fluorocarbon rubbers are used to special applications that require good heat, oxygen or corrosion resistance and hot solvent and oil resistance.

Applications: • car and airplane seals and hoses • fire resistance coverings • heat resistance insulators • o-rings, shaft seals • gaskets, fuel hose, valve-stem seals

Silicone Rubbers (Q)

The silicone rubbers are inorganic polymers, because their main chain structure does not include carbon atoms. As shown in the picture, 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 at wide range, and consequently there are liquid materials as well as traditionally resinous rubbers available.

The structure of silicone.

The silicone rubbers are usually polymerized from cyclic oligomers to linear macromolecules. The vulcanization can be carried out at room temperature or elevated temperature. Vulcanisation at room temperature occurs with crosslinking agent (e.g. orthosilicon acid ether) or air. For the high temperature vulcanization peroxides are used. The molar mass of silicon 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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The 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, a part of the methyl groups (< 0.5 %) is 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 in 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 the worst of rubbers. 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

vulcanisation) • elasticity • non-toxic, odourless, tasteless • good release properties • good electrical insulation • good ageing resistance in 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 • high shrinkage in moulded articles • the vulcanisation to obtain good mechanical properties has to be carried out

with peroxides • price

Applications: • electrical equipment and technical products at high temperatures • medical devices and hospital supplies • roll coverings • cable coverings and insulators

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• lining compounds • moulds • o-rings • seals for the aeronautics industry

Polysulfide Rubbers (T)

Polysulfide rubbers build up when dihalide react with sodiumpolysulfide. Polysulfide rubbers have only one manufacturer Morton International. Polysulfide rubbers can be divided to four different groups: Thiokol A, FA, ST and LP rubbers. A-types polysulfide rubbers have ethylene-dichloride as a dihalide, FA-rubbers are produced from the blend of ethylene dichloride and dichloroethylene formal. ST-rubbers are produced from dichloroethyenel formal and trichloropropane. LP-types are liquid polymers. They are built up by breaking down a high molecular weight polymer in a controlled manner. The sulphur content of A-type is high (84 %), FA-types’ sulphur content is 49 %, and ST-types' 37 %.

The polymerization of polysulfide. Reactants are ethylchloride and sodiumsulfide.

The A and FA types are usually vulcanized with the addition of zinc oxide. The ST and LP types are vulcanized with oxidizing agent, like metal oxides or metal peroxides.

Properties of polysulfide rubbers: • excellent oil and solvent resistance • good weather and ozone resistance • bad smell • difficult to machine • narrow operating temperature range • corrode copper • very good low-temperature properties

Applications: • paint, oil and fuel hoses • seals • paint and varnish rolls • roller coverings

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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%.

Ethyl-vinyl acetate rubber.

Preparing method of ethyl-vinyl acetate depends on desired vinyl-acetate content. Mass polymerization gives 45 weight percent content at most; emulsion polymerization gives over 50 weight percent content and solution polymerization 30 ... 90 weight percent content. EVA can be vulcanized with peroxides or ionizing radiation. Sulphur cannot be used, because of the saturated main chain.

EVA is often blended with NR and SBR to improve the ozone resistance.

Properties of EVA: • excellent oxygen, ozone and light resistance • extremely good water and oil resistance • good heat resistance • do not resist 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

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 %. Polymerization method is solution polymerization in hydrocarbon. Vulcanisation can be made with sulphur.

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Polypropylene-oxide rubber.

Properties of polypropylene-oxide rubbers: • good properties in 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

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. When 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 %. The chlorosulphonated polyethylene is similar to the chlorinated polyethylene, but it is easier to cure because of the chlorosulphone group. That is why the chlorosulphonated polyethylene is more used than the chlorinated polyethylene. Typical chlorosulphone content in elastomer is less than 1.5 %.

Chlorinated polyethylene.

Chlorosulphonated polyethylene.

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The chlorinated polyethylene can be cured with peroxides or radiation. The 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: • really good UV light resistance • good oil resistance • really good oxygen, ozone and light resistance • good tensile and breaking strength • low compression set (up to 150 °C ) • very good dynamic fatigue • excellent ageing 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 type of oils • high compression set at 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

Rubber blends

Rubber materials in applications are always rubber blends. They contain basic elastomer or masterbatch and additives. This way the properties of material are improved or changed.

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 inform the materials and amounts used in rubber blend. The amounts of constituents are usually represented parts per hundred parts of rubber (phr).

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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-

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 during a couple of last decades.

Thermoplastic elastomers are a wide material group. These materials have many advantages of which the most important ones are

• good properties in low temperatures • excellent abrasion resistance • damping properties • good chemical resistance • easy processability (compared to rubber) • recyclable material

A 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 in vast temperature range is required. The main applications are in automotive industry and sport accessories.

Thermoplastics elastomers can be divided in the following groups:

1. Styrene-diene block copolymer

2. Elastomeric alloys

3. Thermoplastic urethane elastomers

4. Thermoplastic ester-ether copolymers, TPE-E

5. Thermoplastic amide copolymer, TPE-A

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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 that base on styrene are block copolymers, where polydiene unit divide polystyrene blocks. Polydiene may be for example butadiene (SBS), isoprene (SIS), ethylene-butylene (SEBS) or ethylene-propylene (SEPS). Styrene content varies with different materials, but usually it is 20-40 %.

The linear and the radial structure of styrenic TPEs.

Advantages of styrenic TPEs: • high tensile strength and modulus • good miscibility • good abrasion resistance • good electrical properties • large variety in hardness • high friction coefficient (correspond to 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

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Applications: • rubber products in car industry • cables and wires • shoe soles • adhesives • with thermoplastics in multi-component injection moulding and co-extrusion

Elastomeric alloys

Elastomeric alloys are blends of elastomers and thermoplastics that can be processed by processing methods of thermoplastics. Elastomeric alloys are:

• Thermoplastic Olefin Elastomers (TPO) • Thermoplastic vulcanizates (TPV) • Melt Processible Rubbers (MPR)

Thermoplastic Olefin Elastomers (TPO, TOE)

Thermoplastic olefin elastomers are most commonly blends of PP and EPM or PP and EPDM. Natural rubber and butyl rubber have also been used. Blend can be made in mechanical mixing unit like twin-screw extruder or in polymerization reactors.

Properties of thermoplastic olefin elastomers vary according to components, mixture ratio and conditions of alloying. Properties typical of thermoplastic olefin elastomers:

• 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

Thermoplastic Vulcanizates (TPE-V, TPV, DVR)

Thermoplastic vulcanizates are blends of thermoplastics and elastomers that have been dynamically vulcanized during their mixing (see picture). That kinds of materials are for example dynamically vulcanized blends of PP and EPDM and PP and NBR. Properties of material depend powerfully on structure and content of elastomer.

The properties of thermoplastic vulcanizates:

• small permanent deformation • good mechanical properties • good properties in low temperatures • fatigue durability • good liquid and oil resistance

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Applications: • car components • tubes • electrical insulators

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 two-phase structure.

Properties of melt processible rubbers: • excellent elasticity • stress-tensile behaviour correspond to vulcanised rubbers • softness and flexibility

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 in the low temperature properties of TPE-U especially, but also to many other characteristics. Depending whether the soft segment is formed of polyester or polyether, the properties can be compared according to the table below. 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

Microbes resistance -/0 ++

Water absorption 0 +

Impact resistance in low temperatures 0 ++/0

++ excellent, +good, 0 fair, -poor

Advantages of thermoplastic urethane elastomers: • good abrasion resistance • good tear strength • good strength and stiffness properties • low friction coefficient (depends on hardness)

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• 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

Applications: • conveyor belts • footwear • cable and wire coatings • hoses • components of car industry

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

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

Thermoplastic Polyamide Elastomers (TPE-A)

Soft segments of polyesters or polyethers and rigid block of polyamide form thermoplastic polyamide elastomers. Polyamide can be for example polyesteramide (PEA), polyetheresteramide (PEEA), polycarbonate-esteramide (PCEA) or polyether-block-amide (PE-b-A). Properties of thermoplastic polyamide elastomers depend strongly on type of polyamide block, type of polyol block, length and amount of blocks.

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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, ski boots • films that penetrate water vapour

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 use temperature [oC] -70 -60 -50 -65 -40 Highest use temperature [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 New development trends occurring 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

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• 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

Processing

Processing of rubbers

The processing of rubbers starts by blending elastomers and additives. After that rubbers are processed with different kinds of processing methods. The possible methods are calandering, extrusion, moulding techniques (e.g. compression moulding and injection moulding) and dipping. After shaping the rubber product is vulcanized so that the mechanical properties and the dimensional stability will appear. Vulcanisation may occur during the processing or after that in many techniques.

The processing of rubbers is quite difficult. Rubber has high viscosity and that's why high shear forces are needed in the processing. Vulcanisation poses restrictions too.

Shear rates in rubber processing.

Processing of thermoplastic elastomers

The general characteristics of the processing methods of thermoplastic elastomer and thermoplastic elastomers are much the same. The most significant differences between TPs and TPEs lie in the values of processing temperatures and viscosity. In the case of thermoplastics the processing temperatures are usually higher and viscosity values are slightly lower than those of TPEs. But as the first approximation, the processing equipment of thermoplastics is mostly suitable also for processing thermoplastic elastomers. The most common processing methods of thermoplastic elastomers are injection molding, extrusion and blow molding techniques. The viscosity of TPEs is significantly lower than the viscosities of traditional rubber elastomers, which offers many processing advantage for TPEs compared with rubbers.

More detailed handling, see processing of elastomers.

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Benefits of TPE processing (comparison with rubbers): • No compounding • No vulcanisation • Faster processing properties (short cycle times) • Standard thermoplastic processing equipment • Thermally stable • Recyclable • Colourable in a broad range of intensities • Clear grades available • Paintable • Printable • Weldable • Overmouldable onto a variety of different substrates • Foamable

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 outlook or shape (see also orienting studies product development and design)

The purpose of design is also to influence the attitude of consumer towards the product, at various stage of the product life. Advertising, test results, outlook, company image and price are factors that are used for attracting customers.

The important factors influencing the consumer:

• Before buying decision: shop display, total interior of the shop, colours, materials, functions, fashion content and price

• When product is in use: products functions, ease of use and care

Impact of design for 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-sale service Service, repairs

Requirements for the development project may be defined on 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 are based on history rather than the expectation of the market will be. 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 at customer's specific request or develop new product and supply it for several customers. Technical rubber

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products are often developed accordingly the request of the customer. Tyre is a good example of the product that is designed by manufacturer and then marketed for customers. The product design is often made in co-ordination with customer. Elastomer component is often part of bigger unit, which may include metal mountings, restrictions for size and form.

It is important that product meets requirements of customers better than products of competitors. It is also useful if product is suited to further development.

Aspects cater to design • Elastomer type • Dimensioning • Shaping • Economic efficiency of materials and processing • Processing method • Reinforcement

Design process

Theoretically the phases of design process are:

1. Defining the problem and needs for product development and outlining the project

2. Product development phase: Search for ideas and pulling them together to a concentrated solution, developing a prototype and freezing the design

3. Before deliveries, product sketches and data, modification of operating system, testing, full-scale production

And after product launching: • Customer research (consumer research) • After sale service • Define problems and study them

Elastomer selection

The most important criteria of choosing elastomer: • Flexibility • Vibration damping • Heat insulation • Oil and chemical resistance • Mechanical resistance (including abrasion resistance) • Functionality in 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

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basic choices. One way is to feed the criteria to computer program and get a recommendation of suitable material.

Selection tree of rubbers

Rubber factories have several basic rubber blends for different uses. When new blend is needed, a suitable basic blend can be selected and modified to conform to the requirements of the new product. Thus new rubber blend is obtained relatively easy, because good basic data about the properties and processing already exist.

Dimensioning of elastomer products

In dimensioning the most important starting points to be taken into account in early stage of design are

• Functionality • Prediction and ensuring the risks of damage • Prediction of lifetime

Product may be damaged in many ways. It can for example: • Creep • Fracture • Change stiffness because of chemical changes caused by temperature

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• Change stiffness because the chemicals from environment diffuse to the material

To predict the damages, it is important to know models of behaviour and parameters of materials. Models of planning are not very advanced for elastomers. Exploitation of those is complicated because of non-linear loading-deformation phenomena that make the theoretical prediction of practical structures difficult.

Mechanical dimensioning

The purpose of product design is to assure the functionality of product. One of the most important stages is to ensure that load capacity correspond to demands.

The influence of hardness

The most important property that characterizes rubber is hardness (stiffness). Hardness has an approximate connection to compression modulus and shear deformation modulus. The approximate relations can be presented according the equation

E ≈ 1,045 h (MPa) = 2(1 + ν ) ≈ 3 G,

where h = hardness, E = compression module, G = module for shear deformation, ν = Poisson number (indicates the cross-section change in compression or tension). Elastomers are practically incompressible (E = 1.0…3.5 GPa) and therefore their ν ~ 0.5. The equation above is best valid when the hardness of rubber is 30…80 ShA (corresponding G = 0.3…3 MPa).

Shape factor

The deformation of almost incompressible elastomeric materials is drastically influenced by the shape of the loaded piece. The shape of rubber piece is described with shape factor S (see figure below).

Stiffness in different loading situations

Taking the shape factor S into account, dependences between compression stress σ , compression modulus E , c and shear modulus G (see picture below) can be written in the form

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

c2

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 C= Constant (form factor) S = Shape factor

The equations above are best 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). Principal dependence of Ec on shape factor is shown in the picture below.

Stiffness of rubber constructions can be controlled with the help of equations above. The construction is shared with rubber plates that have shape factor greater than single-layer structure. The method is, for example, used with bridge bearings where the load capacity has to be considerable.

Compression modulus of natural rubber as a function of shape factor and hardness.

Allowed loadings for different rubbers

Stiffness dependencies on deformation were described by the equations on preceding page. Using the equations the deformations caused by real loadings situations can be

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estimated. Highest allowed values for different kinds of loading situations are often presented. The values are empirically confirmed.

As an example, a series graphs are presented below (next page). It gives information about the allowed compression loadings. In each graph the transversal lines outline the following areas:

• Under the lower line is the area of allowed loadings • Under the upper line is the area of allowed loadings but to be regarded with

reservation It can be seen in the graph that in practice, allowed deformations 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 factor.

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, if there is an application where the long-term creep is a critical factor, the dynamical loadings have to be estimated separately. That is because they can generate greater permanent deformations than static loadings.

Creeping values to filled and non-filled natural 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 ageing reactions caused by radiation, ozone and oxygen.

Creeping depends on the composition of elastomer. So it is not possible to draw far-reaching conclusions on the grounds of theoretical modeling that base on typical rubber type properties.

Rubber blends can contain 5 ... 20 components, so it is obvious that the properties vary significantly. Thus the properties of new rubber blend are to be measured first and only then the behaviour of the rubber can be evaluated.

Product shaping

As a result of dimensioning, certain limitations for products' dimensions have been achieved. Before the mould design the structure of the elastomer product has to be shaped so that the local stresses are avoided in loading situations. The weather sensitive surfaces should not be exposed to stretch loadings.

The examples of designing for even loading are shown in the figures below.

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Examples of applications used to get even distribution of loading stress and avoiding tension in rubber product.

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. According to this patented method rubber waste was grinded and used as filler.

The main problem with vulcanized rubber products is their use 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 rubbers and legislative restriction demands search for economical and ecological methods of recycling.

Why reclaim or recycle rubber?

Rubber recovery can be a difficult process. However, there are many reasons, why rubber should be reclaimed or recovered;

• The price can be reduced to half compared to synthetic material. • Recovered rubber has some properties that are better than those of virgin

rubber. • Producing rubber from reclaim requires less energy in the total production

process than virgin material. • It is an excellent way to dispose 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.

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• 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.

Recycling methods

Basically waste rubber can be recycled in three ways: It can be used for energy by combustion, in its original form through devulcanization and as ground powder.

Recovery Alternatives

Kind of recovery 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

Incineration

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Incineration is a good and economic method of disposing of rubber. The energy content of rubber is about 32,6 MJ/kg, which is about 10 % less than heavy oil (37,7 MJ/kg) and 1,3 times the energy content of coal (25.1 MJ/kg) Rubber is burned in special incinerator. The purpose is to recover energy as much and as ecological as possible.

Incineration produces oxygen, carbon dioxide, water and some toxic gases. Using sufficiently high temperatures can prevent formation of toxic components, such as dioxin.

Pyrolysis

Pyrolysis involves heating the rubber waste in the absence of oxygen. The temperatures used in this process are typically 400-800 ° C. 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 fibers from tire cords. Each product and by-product is marketable:

• The gas has a high heat value.

• The light oils can be sold for 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.

Grinding of vulcanized rubber waste

Sometimes it is good to reduce the size of the rubber. For example landfill of whole tyres may be prohibited, while it is permissible to dump granulated tyre chips. The size reduction of rubber waste enables 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 in 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 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 processes steps:

1. milling and grinding of dry material at ambient temperature (ambient grinding)

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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).

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 that are:

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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 the carbon – carbon backbones of the network, which are broken down, and preferably the longer chains will be broken. This leads to a more narrow molar mass distribution. The thermo-chemical breakdown step is random. As a result, the percentage of low molar mass-polymer, 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 the different reclaims.

Recycled rubber powder obtained from ambient or cryogenically ground tyres can be utilized as filler in rubber and other polymeric compound. In cryogenically and wet ground rubbers, smaller particle size allows recycled rubber to be used at moderately high levels and still maintain the 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 find applications with low performances. 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

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 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 achieve 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

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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 can be obtained.

At the initial stage of devulcanization reaction, the polysulphide and disulphide bonds are changed to monosulphide bonds by heat. Furthermore, the monosulphide bond is broken by addition of shear stress and finally recycled uncured rubber is achieved.

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 like extruder or two-roll mill and partially decrosslinked or to chemical action to achieve reclaimed material. In addition e.g. devulcanization by microorganism has been 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(benzanidopheny)-disulfide. It enables the polysulfide bond from polymer chain to be destroyed. The use of the treated crumb rubber enhanced the mechanical performance of the rubber compounds produced.

Utilization of unvulcanized rubber waste

Unvulcanized rubber waste is mainly generated from the manufacturing process. It is also very useful to recycle. Reinforced rubbers that contain a steel wire and textile fiber 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 the 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 a vulcanization of an unvulcanized rubber waste. After that the grinding is possible.

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Processing of recycled rubber

Unvulcanized rubber waste

Temperature has a big role in processing of unvulcanized rubber. In order to maintain the temperature rise within an acceptable limit, the mixing equipment has to be coolable. The temperature must be controlled to ensure that excessive plasticization or 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 internal mixer, the mixers have 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 the discharge from the internal mixer, the compound is in the form of lumps and has to be homogenized, cooled and sheeted out on follow-up equipment such as sheeting mill or forming extruder.

In mill mixing a temperature control is easier. The rolls are cooled down to remove excessive heat, which is built up during mixing. That is why the processing of unvulcanized rubber waste with two roll mill is the most safe process, concerning 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.

Vulcanised rubber waste

One of the most effective ways of reusing rubber waste is to load it into new rubber products in the form of fine ground powder (rubber scrap). Rubber scrap is easy to apply with simple equipment and has positive influences on the processing behaviour of a compound.

The main advantages derived from the use of reclaim concern the processing behaviour of the compound. These 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

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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 are 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: the 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.

Devulcanized rubber waste

The devulcanized rubber can be processed, shaped and vulcanized in the same way as the virgin rubber. There are also many benefits derived from the use of devulcanized rubber in rubber compounds:

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 ageing performance on natural rubber Lower raw material costs

One shortcoming of reclaim is that it lowers the green strength of the compounds.

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 only just as such mixed with sand.

Whole tyres can be used for artificial reefs, breakwaters, erosion control, playground equipment and highway crash barriers.

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Recycling of tyres

The recycling methods of rubbers are: product reuse, material reuse and energy recovery. Most tyres of cars and vans can be retreaded. Tyre of cat can be retreated 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) and careful inspection is required prior to start buffing and afterwards during the process. 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 product can not be reused, it can be used in secondary purposes. The biggest secondary reuse applications of tyres are road building, noise barriers and landfills. In those applications the tyre powder may be act as insulator or lightening material between different bearing courses. Blasting mat and buffers in piers are used too.

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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