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 Samples were withdrawn at successive intervals of time, and the transformation products of the thiol, which were isolated by acetone extraction, were identified by their ultraviolet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.

Substances which peptize rubber, of which the most active fall within the class of aromatic thiols such as 2-naphthalenethiol and xylenethiol, are also accelerators of plasticization. Their action, which differs from that of “plasticizers”, is of a chemical nature, and it can be modified by certain agents, such as metalloids of the sulfur family. The use of plasticization accelerators requires certain precautions. A study of the oxidizability of rubber in the presence of plasticization accelerators has shown that these substances make rubber more sensitive to oxygen, whose part in the plasticization of rubber is already well known. At the beginning of the reaction, oxygen is absorbed rapidly under the catalytic influence of the thiol, which is simultaneously transformed into the corresponding disulfide. By means of ultraviolet spectrography, this transformation of thiols while playing the part of catalytic agents can be followed.