Elastomers and Rubbers

Rubber and Elastomers Rubber and elastomers are widely used as lining materials. To meet the demands of the chemical indus-tiy, rubber processors are continually improving their products. A number of synthetic rubbers have been developed, and while none has all the properties of natural rubber, they are superior in one or more ways. The isoprene and polybutadiene synthetic rubbers are duphcates of natural. 

The most economically important materials with respect to ozone damage are paint, elastomers (rubbers), and textile fiber-dye systems. Damage to polyethylene by ozone is considered to be negligible. The 1970 ozone damage to materials has been estimated as follows paint, 540 million elastomers, 569 million and textile fibers and dyes, 84 million—for a total of over 1 billion. Thus, the total combined material and crop damage falls between 1.5 and 2 billion per year. Estimates of damage to natural ecosystems are not available. 

Silicone elastomers, silicone elastomer rubbers and sealants... 

The term plastic is also used in classifying polymers and includes all polymers which are not considered to be elastomers (rubbers) and fibers, i.e., which show neither the elastic properties of elcistomers nor the high crystallinity and strength of fibers, but rather fall in between them in these respects. 

Rubbery materials, and compositions based upon them, are commonly referred to by a number of letters which refer to the monomers on which the polymer is based. The standard recommended practice (D1418-72A) issued by the American Society for Testing Materials (ASTM) is the most widely used for the nomenclature of elastomers (rubbers) and lattices. This practice recommends that the elastomers be grouped and coded into a number of classes according to-the chemical composition of the polymer chain. Of the various types, the R and M classes are the most commercially important. 

The most important considerations when specifying the pipeline are that the pipe material should be able to withstand the applied pressure and that the pipe material should be wear-resistant. Erosive wear is likely to be a problem for transporting abrasive particles at higher velocities (>3m/s). Based on these considerations, pipe materials generally fall into the broad categories of hardened metals, elastomers (rubbers and urethanes), and ceramics. 

The chemical diversity of PHA translates into a wide spectrum of physical properties, ranging from stiff and brittle plastics to softer plastics, elastomers, rubbers and glues. The major diversity is in the length and presence of functional groups in the side chain of the polymer. Table 1 summarizes selected important PHAs. 

Polyolefins. In these thermoplastic elastomers the hard component is a crystalline polyolefin, such as polyethylene or polypropylene, and the soft portion is composed of ethylene-propylene rubber. Attractive forces between the rubber and resin phases serve as labile cross-links. Some contain a chemically cross-linked rubber phase that imparts a higher degree of elasticity. 

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. 

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. 

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). 

Poly(butadiene- (9-acrylonitrile) [9008-18-3] NBR (64), is another commercially significant random copolymer. This mbber is manufactured by free-radical emulsion polymerization. Important producers include Copolymer Rubber and Chemical (Nysyn), B. F. Goodrich (Hycar), Goodyear (Chemigum), and Uninoyal (Paracdl). The total U.S. production of nitrile mbber (NBR) in 1990 was 95.6 t (65). The most important property of NBR mbber is its oil resistance. It is used in oil well parts, fuels, oil, and solvents (64) (see Elastomers, synthetic— nitrile rubber). 

Resilient materials such as rubber and some plastics may be useful in certain applications, especially under conditions of low cavitation intensities. However, such materials are subject to disbondment at the metal and elastomer interface at high cavitation intensities, even if the exposure is brief. 

Such rubbery and thermoplastic polymers may be blended in any proportion, so that on one hand the product may be considered as a thermoplastic elastomer, and on the other as an elastomer-modified thermoplastic. There is, furthermore, a spectrum of intermediate materials, including those which might be considered as leather-like. In this area the distinction between rubber and plastics material becomes very blurred. 

In attempts to further improve the stability of fluorine-containing elastomers Du Pont developed a polymer with no C—H groups. This material is a terpolymer of tetrafluoroethylene, perfluoro(methyl vinyl ether) and, in small amounts, a cure site monomer of undisclosed composition. Marketed as Kalrez in 1975 the polymer withstands air oxidation up to 290-315°C and has an extremely low volume swell in a wide range of solvents, properties unmatched by any other commercial fluoroelastomer. This rubber is, however, very expensive, about 20 times the cost of the FKM rubbers and quoted at 1500/kg in 1990, and production is only of the order of 1 t.p.a. In 1992 Du Pont offered a material costing about 75% as much as Kalrez and marketed as Zalak. Structurally, it differs mainly from Kalrez in the choice of cure-site monomer. 

The market is dominated by flexible foam applications (43% in the United States) and rigid and semi-rigid foam (29%). Cast elastomers (4%) and RIM elastomers (3%) have only specialised outlets. The remaining sizeable 21% of the market cover such diverse uses as thermoplastic rubbers, surface coatings, adhesives, sealants and synthetic leathers. 

FRISCH, K. c.. Recent Developments in Urethane Elastomers and Reaction Injection Moulded (RIM) Elastomers, Rubber Chem. Technol., 53, 126 (1980)... 

In general, the thermoplastic elastomers have yet to achieve the aim of replacing general purpose vulcanised rubbers. They have replaced rubbers in some specialised oil-resistant applications but their greatest growth has been in developing materials of consistency somewhat between conventional rubbers and hard thermoplastics. A number of uses have also been developed outside the field of conventional rubber and plastics technology. 

Class and Chu demonstrated that if a tackifier is chosen that is largely incompatible with the elastomer, a modulus increase due to the filler effect is observed and little change in Ta results, and once again a PSA would not be obtained. This was observed for mixtures of low molecular weight polystyrene resin and natural rubber. The same polystyrene resin did tackify SBR, a more polar elastomer that is compatible with the resin. Hydrogenating the polystyrene to the cycloaliphatic polyvinylcyclohexane changed the resin to one now compatible with the less polar natural rubber and no longer compatible with SBR. These authors also provide... 

Most rubbers used in adhesives are not resistant to oxidation. Because the degree of unsaturation present in the polymer backbone of natural rubber, styrene-butadiene rubber, nitrile rubber and polychloroprene rubber, they can easily react with oxygen. Butyl rubber, however, possesses small degree of unsaturation and is quite resistant to oxidation. The effects of oxidation in rubber base adhesives after some years of service life can be assessed using FTIR spectroscopy. The ratio of the intensities of the absorption bands at 1740 cm" (carbonyl group) and at 2900 cm" (carbon-hydrogen bonds) significantly increases when the elastomer has been oxidized [50]. 

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