Studies of Hard Rubber Reactions

As shown in Fig. 1, all the compounds containing 5 to 150 per cent of sulfur (based on the rubber) indicated a gradual loss of total sulfur, with a relatively rapid loss after about 90 minutes, which corresponds to the period of the hard rubber reaction. Some investigators hold the view that the loss of total sulfur is the result of liberation of hydrogen sulfide by the interaction of sulfur with rubber, resins, and proteins, and sublimation of sulfur. In the present experiments, the cause of the comparatively marked loss of total sulfur seems to be related to the hardening or polymerization reaction in hard rubber. No good mixture could be obtained from which the maximum loss of total sulfur was reached, with reference to the content of rubber, i. e., rubber resins, proteins, and that of sulfur. The loss of sulfur by sublimation was restricted as far as possible by enveloping the samples with cellophane. It appears from these facts that the depolymerization phenomena of soft or semi-hard rubber products may occur during the initial stage of vulcanization and those of the hard rubber after 90 minutes. Considering again the vulcanization curves in the previous report (Part IV), there is no loss of sulfur in the initial stage of vulcanization but considerable losses of sulfur are seen in the present curves of total sulfur; in other words, during the whole course of vulcanization the combined sulfur increases to a certain extent, accompanying the loss of sulfur on one side. In Fig. 2, the relationship between the loss of total sulfur and the tensile strength of the hard rubber product is shown. Of all the hard rubber compounds, the samples containing 70–80 per cent of sulfur showed the maximum loss of sulfur, coinciding with the maximum tensile strength after vulcanization for 90 minutes. It may be assumed that the polymerization and the depolymerization reactions occur simultaneously and are closely related.