Abstract
Natural rubber and synthetic elastomers deteriorate on aging as a result of the contribution of a number of factors. It has been generally recognized for many years that the changes in properties on aging are due primarily to the deteriorating effects of one or more of the following factors: heat, light, oxygen, and ozone. Some nonoxidative thermal changes are involved but the effects of heat and light are for the most part a result of their effect upon the oxidation reaction. Thus oxygen in the form of O2 or O3 is the primary cause of deterioration of elastomers. Rapid combination with ozone is a characteristic reaction of unsaturated organic compounds. In the case of rubber the reaction results in cleavage of the molecular chains with the development of surface cracks if the rubber is under stress. These cracks grow both in length and depth and soon result in serious deterioration of the material. The ozone problem is quite different from deterioration by ordinary atmospheric oxygen and is of sufficient importance to justify separate treatment in a future issue of Rubber Reviews. Consequently, this review will be concerned primarily with oxidative deterioration other than that due to ozone. Both thermal and light-initiated oxidation appear to proceed by similar free-radical chain mechanisms involving the formation of hydroperoxides. Thus, information derived from one type of oxidation is usually applicable in some measure to the other. This review will be concerned primarily with thermal oxidation and the resultant effect of the oxidation upon physical properties. Studies of the mechanism of oxidation of polymers is complicated by the difficulty of identification of both initial and final products. Small concentrations of oxygenated groups are introduced on the polymer chain and thus cannot be physically separated from the unreacted portions of the same molecular chain. It is for this reason that extensive use has been made of model compounds with similar structures in trying to determine the nature of the oxidation reaction. There is always some question as to the extent to which information obtained with simple olefins can be applied to the more complex systems of natural and synthetic rubber. Nevertheless, much of the present knowledge has been obtained in this way. In this review we will first summarize some of the pertinent information obtained with model compounds and then consider studies carried out directly on the polymers. The problem is further complicated by the effect of various compounding ingredients as well as the effect of changes brought about by vulcanization upon the oxidation reaction and upon the resultant deterioration of properties of the vulcanized product. One question of particular interest is the way in which inhibitors or antioxidants function to reduce the rate of oxidation and retard the deterioration of properties. This review is directed toward the factors and mechanisms involved in the aging of vulcanized elastomers as brought about by thermal oxidation of inhibited stocks. In the process it will be necessary to consider also the oxidation of model compounds and raw polymers as well as vulcanizates, and to compare inhibited and uninhibited oxidation. No attempt has been made to include all the published material on the subject since many other reviews are available. Rather the author has attempted to interpret the present state of knowledge on aging and oxidation of elastomers, as he sees it, based on the information available at this time.