Sulphur Vulcanized Grades

Due to its advanced technology and low cost the rubber industry often prefers the use of sulphur-vulcanized polyurethane elastomers even though some of their technical properties, e.g. resistance to thermal degradation, are inferior to the peroxide- and diisocyanate-cured grades which often have short processing times (i.e. they are scorchy) and whose cure may be adversely affected by the presence of moisture in the unvulcanized rubber mix water is present in rubber fillers, e.g. carbon blacks usually contain about 0-5-1% and some non-black fillers such as silicas and clays 2-10%. Also to maximize scorch time it is common practice to quench-cool the rubber after internal mixing by immersion in cold-water tanks or by cold-water spray application to the surface of the hot-milled sheet. 

SOME COMMERCIALLY AVAILABLE MILLABLE POLYURETHANE ELASTOMERS 

Sulphur or peroxide Isocyanate (600 series) Peroxide (640 series) Peroxide (5000 series) Sulphur 

Some established commercially available grades are given in Table 7.1. The relatively low level of unsaturation dictates that relatively high proportions of organic accelerators are required if a reasonably fast state of cure is to be obtained, and sometimes this can produce poor hydrolytic instability. 

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Sulphur-curing grades are usually preferred by the rubber industry as they make use of established technology, but two other vulcanization techniques are also well established as they produce elastomers with certain superior properties these are crosslinking with peroxide or diisocyanate. Peroxide curing can be universally used as it has a free radical-type... 

Vulcanization of the epichlorhydrin rubbers is normally brought about by reaction of the chloromethyl group with diamines and polyamines or their derivatives such as hexamethylene diamine carbamate, with 2-mercaptobenzimidazoline, with lead oxide and lead phosphite, and with ammonium salts. The processes are analogous to those used with the polychloroprenes and with those acrylic rubbers containing a halogenated cure site. A sulphur-curable grade was announced in late 1977. 

Two main grades of factice exist, namely brown , which is usually vulcanized by elemental sulphur, and white , which is usually vulcanized by sulphur chloride. 

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