Information on the relative rates of cure of GR-S stocks and similar Hevea stocks in thick sections is of interest to many rubber manufacturers. Since curing conditions for thick articles from Hevea stocks have been established, they would like to know how these conditions must be altered when GR-S stocks are used in the same applications. They could develop a GR-S stock with the same rate of cure as the Hevea stock which it replaces according to laboratory tests on comparatively thin sheets, but this agreement does not mean necessarily that thick sections cure at the same rate. The respective rates of heat flow through the rubbers must be considered. Only if the rates of heat flow as well as the curing rates of thin sections are in agreement, will the curing rates of the thick sections be equal. Juve and Garvey found that GR-S tread stocks cure faster in the center of thick sections than similar Hevea tread stocks. They were unable to explain this behavior because, according to their measurements, the thermal conductivity of the GR-S tread stock was less, and its specific heat greater than, the corresponding values for the Hevea tread stock. They concluded that the difference mav be due to an exothermic reaction.

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The behavior of certain large synthetic rubber gaskets on naval vessels during the past winter points to the necessity for a cold compression-set test in the specifications for these gaskets. It has been found, for example, that Neoprene gaskets on large valves, which perform satisfactorily at temperatures of 60° F and above, are not usable at temperatures of 40° F and below. They take a cold compression-set, while the valves are closed, so that when the valves are opened and then again closed, the leading edge or surface of the valve does not seat properly. This cold compression-set is not permanent; when the gaskets which exhibit cold compression-set are removed to a warm atmosphere (about 80° F), they slowly regain their original shape. A survey of the literature dealing with the effects of low temperatures on rubber compounds yielded no information on cold compression set. The set test proposed by Morris, James, and Evans in connection with their low temperature stiffness test is not directly applicable to the present problem because it is conducted in tension. Therefore to investigate cold compression-set, it was necessary to devise a new testing procedure.

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