Special-purpose rubbers

The butadiene-acrylonitrile rubbers were first prepared about 1930 about five years after the initial development of free-radical-initiated emulsion polymerisation. Commercial production commenced in Germany in 1937, with the product being known as Buna N. By the late 1980s there were about 350 grades marketed by some 20 producers and by the early 1990s world production was of the order of 250000 tonnes per annum, thus classifying it as a major special purpose rubber. 

The polychloroprenes have been commercially available for half a century, being first marketed by Du Pont in 1931. Today these materials are amongst the leading special purpose rubbers (which in the language of the rubber technologist effectively means non-tyre rubbers) and are well known under such commercial names as Baypren (Bayer), Butachlor (Distagul) and Neoprene (Du Pont). 

In addition to poly(methyl methacrylate) plastics and polyacrylonitrile fibres, acrylic polymers find widespread use. First introduced in 1946, acrylic rubbers have become established as important special purpose rubbers with a useful combination of oil and heat resistance. Acrylic paints have become widely accepted particularly in the car industry whilst very interesting reactive adhesives, including the well-known super-glues are also made from acrylic polymers. 

Typical of these materials are the poly(vinyl thioethers), the poly(vinyl isocyanates), the poly(vinyl ureas) and the poly(alkyl vinyl ketones). Methyl isopropenyl ketone and certain vinylpyridine derivatives have been copolymerised with butadiene to give special purpose rubbers. 

Rubber blends with cure rate mismatch is a burning issue for elastomer sandwich products. For example, in a conveyor belt composite structure there is always a combination of two to three special purpose rubbers and, depending on the rubber composition, the curatives are different. Hence, those composite rubber formulations need special processing and formulation to avoid a gross dissimilarity in their cure rate. Recent research in this area indicated that the modification of one or more rubbers with the same cure sites would be a possible solution. Thus, chlorosulfonated polyethylene (CSP) rubber was modified in laboratory scale with 10 wt% of 93% active meta-phenylene bismaleimide (BMI) and 0.5 wt% of dimethyl-di-(/ r/-butyl-peroxy) hexane (catalyst). Mixing was carried out in an oil heated Banbury-type mixer at 150-160°C. The addition of a catalyst was very critical. After 2 min high-shear dispersive melt mix-... 

The properties of these elastomers are widely different. All require vulcanization. In general, sulfur is used only for unsaturated polymers peroxides, qui-nones, metallic oxides, or diisocyanates effect vulcanization with saturated types. Many are special-purpose rubbers, some can be used in tires when loaded with carbon black, others have high resistance to attack by heat and hydrocarbon oils and thus are superior to natural mbber for steam hose, gasoline and oil-loading hose. Most are available in latex form. 

Also called Thioplasts or Thiokol. These are special purpose rubbers. General properties include poor mechanical properties excellent resistance to oils and degreasing solvents. The swelling resistance increases as the sulphur content increases. Good oxidative, ozone, and weathering resistance. Approximate working temperature range -50 C to + 90 C. 

Recently various special-purpose rubbers have received wide circulation. Such rubbers should include methylvinylpyridine raw rubbers, cured rubbers made from which are distinguished by great ability to work under repeated deformations, high elasticity at low temperatures, and stability to the action of complex esters within a broad range of temperatures. 

Elastomers can be divided into two general categories, natural rubber and synthetic rubbers. Synthetic elastomers in turn are either termed general purpose rubbers (GPR) or special purpose rubbers. Natural rubber is generally obtained from southeast Asia or Africa. Synthetic rubbers are produced from monomers obtained from the cracking and refining of petroleum. The most common monomers are styrene, butadiene, isoprene, isobutylene, ethylene, propylene, and acrylonitrile. There are monomers for specialty elastomers which include acrylics, chlorosulfonated polyethylene, chlorinated polyethylene, epichlorohy-drin, ethylene-acrylic, ethylene-octene rubber, ethylene-propylene rubber, flu-oroelastomers, polynorbornene, polysulfides, sihcone rubber, thermoplastic elastomers, urethanes, and ethylene-vinyl acetate. 

Polychloroprene was one of the first synthetic elastomers to be developed and still today may be considered to be the most important of the special-purpose rubbers. With world production, excluding the Soviet Union, of the order of 300 000 metric tons per annum in the mid-1970s it accounts for about 5% of the world elastomer market. First introduced by Du Pont under the trade name Duprene in 1931 it became marketed under the name Neoprene which Du Pont have subsequently used ever since. Whilst Du Pont remains the major producer, other closely related materials are available under such names as Baypren (Farbenfabriken Bayer), Sovprene, Nairit (USSR), Butachlor (Distugil), Denka Polychloroprene (Denki Kagaku Kogyo) and Skyprene (Toyo Soda). 

The development of the polyurethane rubbers has enabled alternative approaches to be devised which have no foundation in traditional rubber technology. Some of these methods have been extensively developed and have helped to make the polyurethane rubbers into one of the most important classes of special-purpose rubbers. 

The special purpose rubbers are typically premium priced but cost effective in supplying one or more imique properly required for specific demanding applications. 

There also exists a number of speciality rubbers whose applications are either very limited or well established. These include the chlorinated polyethylenes, the epichlorhydrin rubbers, the fluorosilicones, the ethylene-vinyl acetate rubbers, the polysulphides, the nitroso rubbers and the phosphonitrile fluoroelastomers. Changes in the uses of these materials are likely to have very little impact on the usage of general purpose and major special purpose rubbers. They have also been the subject of a recent review. ... 

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