Some Aspects of the Oxidative Stability of Cis-1, 4-Polyisoprene Vulcanizates

Abstract

The oxygen absorption technique has been used to determine the thermal oxidative behaviour of vulcanizates prepared from unstabilized cis-1,4- polyisoprene. Initially the effect of specimen thickness was investigated to ensure that diffusion controlled oxidation did not occur during the course of the oxygen uptake measurements. A comparison of several types of sulphur vulcanizates indicated that an efficiently cured system, other than that based on a sulphur donor-sulphurless system, possessed outstanding oxidative stability. This was found to be largely due to an extractable antioxidant formed in situ during the curing process. The antioxidant was identified and its modes of action were deduced by model oxygen absorption studies. The latter involved the oxidation of cumene, in the presence of the antioxidant, initiated by free radicals generated from αα'-azodi-isobutyronitrile and benzoyl peroxide and also by the addition of cumene hydroperoxide. Additional data obtained from analysis of the hydroperoxide decomposition products served to define the action of the antioxidant more precisely. A comparison of the oxidative characteristics of solvent extracted vulcanizates showed that the efficiently cured network retained a superior degree of stability. The sequential pro- and antioxidant effects displayed by the more conventionally cured sulphur vulcanizates were absent and an induction period towards oxidation was noted, The feature responsible for this stability was examined,