Elastomers synthetic rubbers

Elastomers. Elastomers are polymers or copolymers of hydrocarbons (see Elastomers, synthetic Rubber, natural). Natural mbber is essentially polyisoprene, whereas the most common synthetic mbber is a styrene—butadiene copolymer. Moreover, nearly all synthetic mbber is reinforced with carbon black, itself produced by partial oxidation of heavy hydrocarbons. Table 10 gives U.S. elastomer production for 1991. The two most important elastomers, styrene—butadiene mbber (qv) and polybutadiene mbber, are used primarily in automobile tires. 

See Latbx tbchnology Elastomers, synthetic Rubber chemicals. 

Tellurium dimethylthiocarbamate in combination with mercaptoben2othia2ole, with or without tetramethylthiuram disulfide, is the fastest known accelerator for butyl mbber. It is used extensively in butyl tubes for buses and similar vehicles and in other butyl appUcations (see Elastomers, synthetic Rubber, natural). 

Compatibility and Corrosion. Gas turbine fuels must be compatible with the elastomeric materials and metals used in fuel systems. Elastomers are used for O-rings, seals, and hoses as well as pump parts and tank coatings. Polymers tend to swell and to improve their sealing abiUty when in contact with aromatics, but degree of swell is a function of both elastomer-type and aromatic molecular weight. Rubbers can also be attacked by peroxides that form in fuels that are not properly inhibited (see Elastomers, synthetic Rubber, natural). 

Rubber and Synthetic Elastomers. For many years nondecorative coated fabrics consisted of natural mbber on cotton cloth. Natural mbber is possibly the best all-purpose mbber but some characteristics, such as poor resistance to oxygen and ozone attack, reversion and poor weathering, and low oil and heat resistance, limit its use to special appHcation areas (see Elastomers, synthetic Rubber, natural). 

Conjugated dienes such as 1,3-butadiene very readily polymerize free radically. The important thing to remember here is that there are double bonds still present in the polymer. This is especially important in the case of elastomers (synthetic rubbers) because some cross-linking with disulfide bridges (vulcanization) can occur in the finished polymer at the allylic sites still present to provide elastic properties to the overall polymers. Vulcanization will be discussed in detail in Chapter 18, Section 3. The mechanism shown in Fig. 14.3 demonstrates only the 1,4-addition of butadiene for simplicity. 1,2-Addition also occurs, and the double bonds may be cis or trans in their stereochemistry. Only with the metal complex... 

Fisher C.H, G.S Whitby and E.M.Beavers, Miscellaneous Synthetic Elastomers Synthetic Rubber by G.S. Whitby, John Wiley and Sons 1954. 

Synthetic Rubber A polymer material which acts as an elastomer. Synthetic rubber is used primarily in the production of tires. 

For most engineering polymers a high Tg (above 100 C) is required, whereas elastomers (synthetic rubbers) need low Tg (usually below — 50 C). It is also possible to reduce Tg by external plasticification (addition of plasticizers which generate a solid solution with organic liquids of very low transition temperatures) or by polymerization with other comonomers which leads to lower transitions. 

Thermoplastics Thermosetting plastics Thermoplastic elastomers Thermosetting elastomers synthetic rubbers... 

Typical Use Metals, plastics, rubbers, ceramic, glass. Typical Use Metals, plastics, rubbers and elastomers (synthetic rubbers). ... 

Typical Use Metals, plastics, rubbers and elastomers (synthetic rubbers). 

ELASTOMERS,SYNTHETIC - ETHYLENE-PROPYLENE-DIENE RUBBER] (Vol 8)... 

EPM/EPDM compounding [ELASTOMERS, SYNTHETIC - ETHYLENE-PROPYLENE-DIENE RUBBER] (Vol 8) -as insecticide [INSECT CONTROL TECHNOLOGY] (Vol 14)... 

Nitrile mbber finds broad application in industry because of its excellent resistance to oil and chemicals, its good flexibility at low temperatures, high abrasion and heat resistance (up to 120°C), and good mechanical properties. Nitrile mbber consists of butadiene—acrylonitrile copolymers with an acrylonitrile content ranging from 15 to 45% (see Elastomers, SYNTHETIC, NITRILE RUBBER). In addition to the traditional applications of nitrile mbber for hoses, gaskets, seals, and oil well equipment, new applications have emerged with the development of nitrile mbber blends with poly(vinyl chloride) (PVC). These blends combine the chemical resistance and low temperature flexibility characteristics of nitrile mbber with the stability and ozone resistance of PVC. This has greatly expanded the use of nitrile mbber in outdoor applications for hoses, belts, and cable jackets, where ozone resistance is necessary. 

Acrylonitrile (AN), C H N, first became an important polymeric building block in the 1940s. Although it had been discovered in 1893 (1), its unique properties were not realized until the development of nitrile mbbers during World War II (see Elastomers, synthetic, nitrile rubber) and the discovery of solvents for the homopolymer with resultant fiber appHcations (see Fibers, acrylic) for textiles and carbon fibers. As a comonomer, acrylonitrile (qv) contributes hardness, rigidity, solvent and light resistance, gas impermeabiUty, and the abiUty to orient. These properties have led to many copolymer apphcation developments since 1950. 

Butyl mbber, a copolymer of isobutjiene with 0.5—2.5% isoprene to make vulcanization possible, is the most important commercial polymer made by cationic polymerization (see Elastomers, synthetic-butyl rubber). The polymerization is initiated by water in conjunction with AlCl and carried out at low temperature (—90 to —100° C) to prevent chain transfer that limits the molecular weight (1). Another important commercial appHcation of cationic polymerization is the manufacture of polybutenes, low molecular weight copolymers of isobutylene and a smaller amount of other butenes (1) used in adhesives, sealants, lubricants, viscosity improvers, etc. 

Ethylene—Propylene Rubber. Ethylene and propjiene copolymerize to produce a wide range of elastomeric and thermoplastic products. Often a third monomer such dicyclopentadiene, hexadiene, or ethylene norbomene is incorporated at 2—12% into the polymer backbone and leads to the designation ethylene—propylene—diene monomer (EPDM) mbber (see Elastomers, synthetic-ethylene-propylene-diene rubber). The third monomer introduces sites of unsaturation that allow vulcanization by conventional sulfur cures. At high levels of third monomer it is possible to achieve cure rates that are equivalent to conventional mbbers such as SBR and PBD. Ethylene—propylene mbber (EPR) requires peroxide vulcanization. 

Nitrile Rubber. Nitrile mbbers are made by the emulsion copolymerization of acrylonitrile (9—50%) and butadiene (6) and designated NBR. The ratio of acrylonitrile (ACN) to butadiene has a direct effect on the properties on the nature of the polymers. As the ACN content increases, the oil resistance of the polymer increases (7). As the butadiene content increases, the low temperature properties of the polymer are improved (see Elastomers, SYNTHETIC-NITRILE RUBBER). 

Specialty Elastomers. Polychloroprene and polysulfide mbber were the first synthetic specialty elastomers discovered. Since theh invention in the 1930s the total number of classes of synthetic mbbers has grown to almost 30. The foUowing lists standard acronyms by the International Synthetic Rubber Producers (IISRP) and the American Society for Testing and Materials (ASTM) for several specialty elastomers. 

Butadiene—Acrylonitrile Latices. Nitrile latices are copolymers of butadiene and acrylonitrile ia which those copolymerized monomers are the main constituents (see Elastomers, synthetic-nitrile rubber). The latices differ mainly ia ratio of comonomer and stabilizer type. They can be classified as low and medium acrylonitrile (ACN) types. The latter contain 35—40 wt % nitrile mbber, and low types ca 27—29 wt %. 

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