Elastomers in Current Use

The foregoing sections outline the theory of rubber elasticity. This section describes the classes of elastomers in current use. While many of these materials exhibit low modulus, high elongation, and rapid recovery from deformation and obey the theory of rubber elasticity, some materials deviate 

1 Diene Types The diene elastomers are based on polymers prepared from butadiene, isoprene, their derivatives and copolymers. The oldest elastomer, natural rubber (polyisoprene), is in this class (see Section 9.2). Polybutadiene, polychloroprene, styrene-butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR) are also in this class. 

The diene double bond in equation (9.94) may be either cis or trans. The cis products all have lower glass transition temperatures and/or reduced crystallinity, and they make superior elastomers. A random copolymer of butadiene and styrene is polymerized to form SBR (styrene-butadiene rubber). This copolymer forms the basis for tire rubber (see below). The trans materials, such 

Thermoplastic elastomers Poly(styrene-feZocA -butadiene-block-slymne) 

Natural rubber is widely used in truck and aircraft tires, which require heavy duty. They are self-reinforcing because the rubber crystallizes when stretched. 

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Bhowmick, A. K. and H. L. Stephens (eds.), Handbook of Elastomers , Dekker, New York, 1988. Covers the recent developments in the commercially important synthetic and natural elastomers in current use. 

As mentioned in Section 8.1.1, ethylene-propylene elastomers are currently used in compounds for tyre sidewalls. In this application also ternary blends have been used, including halogenated butyl rubber.With regard to EPDM and butyl rubber blends, there are no covulcanisation problems because both have a similar level of unsaturation. 

However, conductive elastomers have only ca <10 of the conductivity of soHd metals. Also, the contact resistance of elastomers changes with time when they are compressed. Therefore, elastomers are not used where significant currents must be carried or when low or stable resistance is required. Typical apphcations, which require a high density of contacts and easy disassembly for servicing, include connection between Hquid crystal display panels (see Liquid crystals) and between printed circuit boards in watches. Another type of elastomeric contact has a nonconducting silicone mbber core around which is wrapped metalized contacts that are separated from each other by insulating areas (25). A newer material has closely spaced strings of small spherical metal particles in contact, or fine soHd wires, which are oriented in the elastomer so that electrical conduction occurs only in the Z direction (26). 

The development and optimisation is described of a new curing system to replace lead-based compounds used in epichlorohydrin elastomers currently used in automotive applications. The system is based on 2,4,6-trimercapto-1,3,5-triazine and dialkyldithiophosphate, which is shown to produce a scorch-safe curing system and which confers excellent physical and ageing properties on epichlorohydrin rubbers. Trials are conducted in formulations for multilayer fuel hoses. 9 refs. 

Thanks to their multiphase constitution, block copolymers have the originality to add advantageously the properties of their constitutive sequences. These very attractive materials can display novel properties for new technological applications. In this respect, thermoplastic elastomers are demonstrated examples (l, 2, 3) they are currently used without any modification as elastic bands, stair treads, solings in the footwear industry, impact resistance or flexibility improvers for polystyrene, polypropylene and polyethylene whereas significant developments as adhesives and adherends are to be noted (5.). 

There is every indication that the next several years will witness a continued rapid increase in the use of petroleum raw materials in the production of elastomers and plastics, and that the petroleum companies will become increasingly active, not only in providing the starting materials, but also in operating the chemical processes of converting them to the required monomers and polymers. The current increase in production of thermoplastic resins such as polystyrene, polyvinyl chloride, polyethylene, and acrylonitrile polymers is based on the development of widespread new applications at the consumer level, and the outlet for plastic materials in many of these uses is presently limited by the capacity to produce and process the resins rather than by consumer demand. 

Physically, vegetable oils and their methyl and ethyl esters are very similar to diesel fuel. There has been no indication that any of the metals currently used in the distribution, storage, dispensing, or onboard fuel systems for diesel fuel would not be compatible with vegetable oil fuels. However, there are reports of some signs of incompatibilities with fuel transfer hoses [3.15], and nitrile and butadiene elastomers [3.16] with methyl esters. Elastomers with high fluorine... 

Currently, there is concern about the use of ammonium perfluorooctanoate (APFO), also known as C8 , which is necessary for the manufacture of fluorinated plastics and elastomers in water. C8 is a perfluorinated anionic surfactant used as a dispersing agent in the polymerization and copolymerization of many fluoropolymers, including poly(tetrafluoroethylene) (PTFE), poly(vinylidene... 

Currently, thermoplastics account for less than 5% of the elastomeric closures for parenterals. Their limited resistance to heat deformation imder stress during autoclave sterilization is the main reason for this limited use. However, thermoplastics have two advantages over thermosets. First, they are chemically less complex and therefore less prone to interact with parenteral medications, and second, they may be manufactured by a simpler and more automated process. Thermoplastic elastomers have found use in baby bottle nipples and dropper bulbs that are not typically heat sterilized under compression. 

Materials Compatibility Einally, any lubricant is required to be compatible with non-metallic components used in the engine, such as plastics, resins and elastomers. In particular, polymeric materials used in seals and plastics need to retain their integrity when in contact with the lubricant. ACEA and most OEMs have material compatibility tests to ensure that the lubricant will not cause undue degradation in key physical parameters of the polymer. These parameters include tensile strength, hardness, volume and crack formation. Any such loss of polymer integrity could be manifest as oil seal leaks or in more extreme cases as a blown gasket. Current engine test examples for American, European and selected OEM specifications are shown in Table 9.5. 

Example 15.7 Currently available thermoplastic elastomers cannot be used in applications with use temperatures above 170°C. Comment on this statement. 

The Polymer Data Handbook offers, in a standardized and readily accessible tabular format, concise information on the syntheses, structures, properties, and applications of the most important polymeric materials. Those included are currently in industrial use or they are under study for potential new applications in industry and in academic laboratories. Considerable thought was given to the criteria for selecting the polymers included in this volume. The first criterion was current commercial importance—the use of the polymer in conunercial materials—for example, as a thermoplastic, a thermoset, or an elastomer. The second criterion was novel applications—a polymer that is promising for one or more purposes but not yet of conunercial importance—for example, because of its electrical conductivities, its nonlinear optical properties, or its suitability as a preceramic polymer. The hope is that some readers wiU become interested enough in these newer materials to contribute to their further development and characterization. Finally, the handbook includes some polymers simply because they are unusually interesting—for example, those utilized in fundamental studies of the effects of chain stiffness, self-assembly, or biochemical processes. 

These are shown in Table 13.12. All these areas are currently growing at the extent of 2-3% per annum and this trend is expected to continue especially relevant is the growing use of electrically powered vehicles for which PU is often superior as a tyre material at the low speeds and relatively high loads involved. Future advances of PU elastomers in this field will depend upon the development of tyres with improved dynamic performance through lower hysteresis and better traction. 

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