During the last few years the rubber industry has made use of certain thiols, under the technical name of peptizing agents, which have the property of accelerating the plasticization of raw rubber during mastication. It is now known that this process of plasticization involves oxidation of the rubber, and that it does not take place in an atmosphere of an inert gas. Accordingly the present authors were induced, on the one hand, to follow the transformation of thiols during their participation in the mastication of rubber and, on the other hand, to observe their influence on the tendency of rubber to oxidize. In the first of these objectives, the analytical method utilized was ultra- violet absorption spectrography. To avoid pertubations in the spectra caused by the resins present in rubber, crepe rubber purified by acetone extraction was used in the experiments. The rubber was masticated at 100° C, and the thiol was added soon after the beginning of this mastication in the proportion of 5 per cent of the rubber. Samples were withdrawn at successive intervals of time, and the transformation products of the thiol, which were isolated by acetone extraction, were identified by their ultraviolet absorption spectra. In these experiments, chloroform solutions containing 0.5 gram per liter were employed.

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It was shown by one of us that certain substances, called deactivates, can protect vulcanized rubber against aging by a process which is different from that of antioxidants. The proposed mechanism involved a deactivation of the primary peroxides by transforming them into oxides of rubber without causing any chain scission. This hypothesis, however, did not appear to be completely satisfactory. In fact, deactivators do not protect raw rubber against oxidation, but actually accelerate its degradation in solution, especially in the presence of a peroxide which enhances this deterioration. Moreover, if was shown that the deactivators possess a chemical structure which is very similar to that of certain vulcanization accelerators, and also that they have an effect on the vulcanization. This had led to the suggestion that their effect could be attributed to the initial structure of the vulcanized rubber. We have considered the possibility of obtaining some useful information on this subject by means of stress relaxation measurements which involve the decrease in tensile strength with time of a stretched test piece. The measurements were carried out either by maintaining the test piece at constant elongation (continuous relaxation), or by stretching the relaxed sample to a constant elongation from time to time (discontinuous relaxation). It has been established that the continuous relaxation method accounts solely for the chain scissions which are produced in the vulcanized rubber network, whereas the intermolecular linkages formed during the tests contribute to the discontinuous relaxation picture. The results of some preliminary experiments we have carried out are given in Figure 1. Two rubber compounds, accelerated with diphenylguanidine and mercaptobenzothiazole, respectively, and known to have quite different aging properties, were employed. Vulcanizates containing either an antioxidant A (phenyl-2-naphthylamine) or a deactivator D (zinc mercaptobenzimidazolate) were compared with the control vulcanizates T.

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