Estimation of State of Cure with S-35

The use of S-35 in a vulcanizate can lead to an easy determination of bound sulfur as the ratio of activities of the acetone extracted sample and the same sample in its original state. At present, we do not specify the meaning of the sulfur value determined in this manner otherwise than that it is a gross value ideally free from accelerator sulfur. A number of Russian papers have reported the use of radiosulfur in vulcanization studies—for instance, Blokh's survey. Later Dogadkin described exchange reactions specifying the character of the sulfur bond in combination with an extensive discussion of the chain mechanism for the function especially of thiurams, leading to at least three types of vulcanizates: nonsulfur thiuram cures with —C—C— crosslinks and no exchange at all, DPG stocks having a high exchange rate, and thiazole mixes having intermediate rates. Of course, this is a very general picture explaining only the outstanding differences in aging stabilities imported in the systems mentioned. The quoted papers give very little experimental detail. However, there is a description of the exchange that can be performed by boiling the sample in a benzene solution of the reactant—e.g., sulfur or accelerator. It is of minor importance where the radioactivity is placed; however, the treatment of inactive vulcanizates with solutions of active material is very unpleasant. Therefore the active material is added in every case by compounding and the exchange is carried out by means of an inactive solution in acetone. The present work was undertaken to investigate differences in stability of the sulfur bond in vulcanizates having increasing states of cure, preferentially in a series of vulcanizations at constant temperature. In addition to exchange with elemental sulfur and sulfur donors (tetramethylthiuramdisulfide) we have investigated the Meyer and Hohemenser reaction with methyl iodide, more elaborately developed by the brilliant work of Selker and Kemp. This reaction is thought specifically to indicate the allylic bound sulfur. It is easily performed by boiling the active vulcanizate with a mixture of methyl iodide and acetone. Fletcher and Fogg indicate in their recent work that thiuram-sulfur systems owe their remarkable aging resistance to the content of ZnDC formed by the decomposition of thiuram during cure. Further, it is well known that certain compounds, such as the zinc salt of MBT, give a similar improvement. We therefore stress that stabilities as stated here, based upon the exchange reaction, will not necessarily bear a unique relation to aging stability, but rather show an intrinsic stability of the sulfur bond.