Page 253 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.45

Zinc oxide is the usual cross-linking agent. It is used along with magnesium oxide. The
magnesium oxide is used for scorch resistance. The reaction is thought to involve the al-
lylic chlorine atom, which is the result of the small amount of 1,2-polymerization:

A mechanism that has been written for the vulcanization of CR by the action of zinc
oxide and magnesium oxide is shown in Scheme 15.
Most accelerators for accelerated-sulfur vulcanization do not work for the metal oxide
vulcanization of neoprene rubbers. An exception to this is in the use of the so-called mixed
curing system for CR, in which metal oxide vulcanization is combined with accelerated-
sulfur vulcanization. In this case, along with the metal oxides, accelerators such as tetram-
ethylthiuram disulfide(TMTD) or N,N´-di-o-tolylguanidine (DOTG) are used with sulfur.
This may be desirable for high resilience or for good dimensional stability.
The accelerator that has been most widely used with metal oxide cures is ethylenethio-
urea (ETU), N,N´-diphenylthiourea or 2-mercaptoimidazoline. The use of ETU in the vul-

SCHEME 15

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