Peroxide-Cured Polyurethane Elastomers

Many organic peroxides can be employed, one of the more widely used ones being dicumyl peroxide. Dicumyl peroxide decomposes either thermally to yield free radicals or in acid media by an ionic cleavage mechanism without the production of free radicals. Since the free-radical mechanism is required for the polymer vulcanization reaction, the ionic cleavage decomposition has to be suppressed by the use of a non-acidic medium. The factor determines what type of filler can be used. 

Crosslinking can sometimes be increased by the use of bifunctional triallylisocyanurate TAIC the level of peroxide required is then lower. TAIC is then called a coagent. 

The rate of decomposition of peroxides into free radicals is dependent upon temperature. The reaction of the free radicals with the polymer is faster and 

Step I. Decomposition of the peroxide by heating into free radicals 

Step 2. Crosslinking by means of the free radicals generated in Step 1 

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Sulphur- and peroxide-curing polyurethane elastomers usually incorporate olefinic groups as sites for cross-linking. A very suitable reactive group is the allyl... 

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. 

Peroxide curing of this polyurethane elastomer can also be practised a typical example is that of dicumyl peroxide whose properties compared with sulphur are shown in Table 7.6. Normally 1 5 phr of the dicumyl peroxide is used with curing temperatures greater than ISC C. It is... 

Polyurethane in the rubber industry can be used in (1) the thermoplastic elastomer form (TPE), discussed earlier, (2) a two-part liquid system in reaction injection molding (RIM), (3) the cast molding of rubber parts, or (4) as a millable gum that can be processed on a two-roll mill and cured with agents such as peroxides or sulfur, just as with conventional rubber. Although different versions of polyurethane elastomers must be tailor-made for each of these four common applications, the basic chemistry used is very similar in all. 

Saturated elastomers are often cured by peroxide, often aided by catalysts. These include chlorinated polyethylene, fluorocarbon, acrylic ester, epichlorohydrin, polysulfide, polyurethane, and silicone. The peroxide radical abstracts an unstable hydrogen from the polymer, leaving a polymer radical, and then polymer radicals couple to produce C-C cross-links. 

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