Permanent Set in Vulcanized Rubber

Abstract

Abstract The residual extension which remains after a sample of rubber has been stretched for some period, then released and allowed to recover, is popularly called permanent set. This set, however, is far from being permanent since it continuously decreases with the period of recovery; furthermore, after the rate of recovery has become exceedingly slow and is no longer readily observable, an increase in temperature will usually result in a sharp increase in the rate of recovery. It has been usual to identify this set with irreversible plastic flow, but it will be immediately evident that this can rarely be justified for, owing to incomplete high-elastic recovery, the measured value of set is a combination of both plastic flow and high-elastic deformation which has not completely recovered. Thus before any attempt is made to discuss the interpretation of the results of set tests, a study must be made of the significance of set. Treloar has investigated this phenomenon in raw natural rubber and has shown that entanglements or cohesional linkages may form while the rubber is stretched, and these oppose recovery; further, although van der Waals forces between the long-chain molecules largely control the rate and the amount of recovery, the crystallization of rubber produced by stretching may profoundly influence the set. On the other hand Tobolsky has studied the set which results from stretching rubber vulcanizates at high temperatures ; in such cases the amount of set is controlled by two processes which take place while the rubber is stretched; one of these involves the oxidative breaking of network chains, the other the oxidative cross-linking of network chains. Although these ideas are well founded, they do not provide a completely satisfactory basis for the understanding of set, and the purpose of this work is to extend these ideas and to explain the significance of the results of normal set tests ; in these tests rubber samples were extended at room temperatures to moderate elongations for relatively short periods of time. Most of the tests performed in this investigation were made on dumbbell shaped samples, which were extended by 200 per cent of their initial length for fifteen minutes at room temperature and then allowed to recover for one hour at room temperature; the residual extension was then noted and expressed as a percentage of the initial length. These tests will be referred to as normal set tests. In some tests various periods and temperatures of extension and recovery were used.