As is now well known, the use of an organic peroxide as a crosslinking agent for an elastomer was first reported by Ostromislensky in 1915. In this work benzoyl peroxide was used to cure natural rubber; similar acyl peroxides together with more recently discovered, less active, and more convenient dialkyl peroxides have since this time been used to crosslink a very large number of polymers. With the common unsaturated polymers the vulcanizates produced have mechanical properties rather inferior to those obtained with accelerated sulfur cures. They do, however, have the good aging and low compression set properties which are, in general, associated with sulfurless cures. The commercial use of peroxides in unsaturated elastomers is nevertheless small, being employed only where the presence of sulfur would be deleterious. In spite of its technological disadvantages peroxide crosslinking has been extensively studied since it forms a chemically simple means of introducing crosslinks into a wide variety of rubbers and leads to vulcanizates of simple structure with physically and chemically stable, carbon-to-carbon crosslinks. Interest in the industrial use of peroxides as curing agents has increased recently with the introduction of a number of fully saturated rubbers for which the usual accelerated sulfur systems are unsuitable. Such saturated rubbers are, in general, more resistant to aging, are more thermally stable, and will probably be used to increasingly greater extents as operational conditions become more and more severe. The study of curing systems available for use with such rubbers is therefore of considerable importance and organic peroxides, forming one such system, are worthy of detailed examination. Much work has been done on the technological evaluation of peroxide vulcanizates and much of this is summarized in a recent book edited by Alliger and Sjothun which is recommended as a source of references for a more detailed study of this aspect of the subject. The basic mechanism of the peroxide crosslinking of a wide variety of rubbers has, however, been neglected until relatively recently. For this reason, and for reasons of length, it is proposed to limit the present review to this latter aspect of the subject.

The reaction between dicumyl peroxide and butyl rubbers has been followed by means of molecular weight measurements. In all cases except that of the rubber with highest unsaturation the molecular weight falls, and the scission efficiency of the peroxide is found to depend quite markedly upon the unsaturation. For the rubber of highest unsaturation a small overall crosslinking reaction was observed. The results are interpreted as showing that cumyloxy radicals react with isoprene units in the chain about 300 times as fast as with isobutylene units. The molecular weight depen dance upon time during the reaction has been investigated but has not been satisfactorily explained.

The crosslinking efficiencies of dicumyl peroxide in SBR, cis‐polybutadiene, acrylonitrile–butadiene rubber, and polychloroprene have been measured. For the first two rubbers approximately ten crosslinks are formed per molecule of peroxide decomposed, while for the acrylonitrile–butadiene rubber an efficiency close to unity is found. With polychloroprene approximately two molecules of peroxide are used for each crosslink formed. Analysis of the low molecular weight products of the reaction shows that ease of hydrogen abstraction by cumyloxy radicals decreases in the order given above.

Abstract

The crosslinking of ethylene–propylene rubber by means of dicumyl peroxide has been followed in detail. The overall crosslinking efficiency has been found to be about 0.4 crosslinks per peroxide molecule, and this result has been shown by stress relaxation and permanent set experiments to result from a mixture of both scission and crosslinking reactions. The results obtained with two different rubbers are consistent with the view that scission results from abstraction of a tertiary hydrogen atom while crosslinking arises from attack at a secondary hydrogen. Such a scheme allows a relative reactivity of tertiary :secondary of 6:1 to be calculated. The action of sulfur in peroxide cures has been shown to be to introduce labile, presumably sulfur‐containing, crosslinks. Such labile crosslinks explain the increased tensile strength of such vulcanizates. Allyl compounds are effective in increasing the crosslinking efficiency of the peroxide.