Some Observations on Rubber Testing

The subject of rubber testing and its standardization has received so much attention in recent years, that some apology might seem necessary for discussing it further. A few years ago the present writer had occasion to call attention to several respects in which existing test methods showed unnecessary lack of uniformity (Trans. Inst. Rubber Ind., 5, 139 (1929)), and it must be admitted that this lack of uniformity still persists to a great extent, with a corresponding loss in the value of published test results. It is the object of the present note to call attention more particularly to some unsatisfactory features in the selection of the “best” or “optimum” cure of a mix for purposes of comparison with other mixes. It is scarcely necessary to emphasize the fact that in comparing the properties of different mixings, as in examining the effects of fillers, accelerators, antioxidants, softeners, etc., all the mixings must be vulcanized to comparable states of cure. Nevertheless, an examination of published data shows that often insufficient attention is paid to ensuring strict comparability. For instance, it is not uncommon to find the effect of fillers examined by taking an accelerated base stock, adding the fillers to it, and vulcanizing all the resulting mixings for the same time, it being assumed that the fillers do not affect rate of cure. This assumption is not justifiable, even in the case of seemingly inert materials. An obvious case is that of gas black, which, though chemically inert, retards vulcanization in presence of organic accelerators. Moreover, such “inert” materials as barytes, blanc fixe, whiting, and strontium sulfate may markedly alter the rate of cure. Thus, in some experiments made by the writer, a base mix accelerated with dephenylguanidine (0.75% on the rubber) gave optimum mechanical properties after 90 minutes at 141° C., but mixings containing the fillers mentioned had optimum curing times ranging from 65 to 110 minutes. The writer has, indeed, come across a filler consisting of a very inert compound, which nevertheless completely puts out of action the most powerful organic accelerators. It is thus clearly unsafe to assume that any material will be inert as regards its effect on rate of vulcanization, whether in presence of an organic accelerator or not.