Oxidative Breakdown of Rubber

Abstract

Abstract Scope of Survey.—The autoöxidation of vegetable and animal drying oils causes solidification, hardening, and toughening of the original olefinic material, whereas absorption of oxygen by natural rubber normally causes softening, with loss of strength and elasticity. Various synthetic rubbers appear to take an intermediate position between drying oils and natural rubber, but with none of the rubbers does autoöxidation represent a simple phenomenon, leading uniformly to softening as opposed to toughening. The peroxidation process which forms the basis of autoöxidation leads in general to two opposite effects, viz., to oxidative chain-scission (degradation) and to molecule-linking (polymerization or oxygen-vulcanization), and it seems logical to associate the former with the softening of olefinic materials and the latter with their toughening or oxygen-vulcanization. What, however, is far from clear is the cause of onset under given conditions of softening on the one hand or oxygen-vulcanization on the other, and the precise nature of the molecular changes which underlie the two phenomena. At present even qualitative information concerning the intricate course of autoöxidative reactions among olefinic substances is far from abundant, and the necessary kinetic evidence which must both set the final seal of credibility on those chemical explanations which careful experimentation indicates to be sound, and in difficult cases discriminate between alternative possible explanations, is only beginning to become available. The most useful contribution, therefore, which can be made at the moment is to clarify in terms of organic structure and reactions the basic issues involved, and to show how far existing chemical information provides certain or possible explanations of the broad lines of autoöxidative molecular breakdown (and its converse) as it is encountered among rubbers.