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The effect of heat aging on the physical properties of an elastomer is generally considered the result of oxidation, which produces both chain scission and cross-linking in the polymer. Early in the development of GR-S, a marked difference in the aging of GR-S and natural rubber vulcanizates was observed. From the effect of aging on hardness, tensile strength, modulus, and elongation it appeared that cross-linking occurred more rapidly than chain scission with GR-S, but the reverse was true with natural rubber. The work reported here was undertaken to provide a better understanding of the differences in aging of GR-S and natural rubber and to introduce new experimental methods for studying the mechanism of oxidation and antioxidant action in elastomers.

The marked changes in physical properties of elastomers which occur during aging as a result of the combined effect of heat, light, oxygen, and ozone are well known. Until recently, however, the nature of the chemical changes that occur during aging has been almost unknown. Some insight into the nature of these chemical reactions has been obtained during the last decade. It is the object of this paper to summarize the present status of knowledge concerning the chemical changes that occur in elastomers and antioxidants during aging. Only chemical changes occurring during thermal or photochemical oxidation will be considered.