Premature vulcanization inhibitor

The effect of the addition of small concentrations of the premature vulcanization inhibitor (PVI), N-(cyclohexylthio)phthalimide, is also given by Figure 7.11. This retarder (Coran and Kerwood, 1970) is frequently used to independently control scorch resistance with little effect on the rate of crosslinking (Coran, 1978). Before the development of N-(cyclohexylthio)phthalimide as a PVI, acidic retarders like salicylic acid, acetylsaUcylic acid, phthaUc anhydride, and benzoic acid were used. These additives improved scorch resistance but also gave greatly reduced rates of crosslink formation after the delay. Another retarder of the past was N-nitrosodiphenylamine, which is less active and not now used because of toxicological concerns. 

If MET could be taken out of the system as fast as it forms, substantial increases in processing safety would result. Such is the case when the premature vulcanization inhibitor, N-(cyclohexylthio)phthalimide (CTP), is present. This compound (Coran and Kerwood, 1970) and others Uke it react rapidly with MET to form 2-(alkyldithio)benzothiazoles, R-S-S-ET, which are active accelerators but which do not interact rapidly with the sulfenamide accelerator ... 

Accelerated vulcanization by this mechanism gives greater crosslinking efficiencies and rates than unaccelerated vulcanization. Here sulfur becomes attached to the rubber hydrocarbon exclusively at aUyHc positions [99 ]. The presence of fatty acids or zinc ions causes an increased early reaction and leads to the formation of rubber-Sj -Ac [100, 101]. This is because the chelated form of the accelerator is more reactive than the free accelerator during the early reactions. The introduction of a premature vulcanization inhibitor such as N-(cyclohexylthi o)phthalamide(CTP) made it possible to control the rate of crosslink formation [ 102]. 

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