Accelerated Aging of Vulcanized Rubber. Factors Determining the Relation between Oven and Bomb Aging

Abstract

Abstract A study has been made of the influence of the following factors on the relative rates of tensile deterioration of natural rubber vulcanizates in the Geer oven and oxygen bomb, both at 70° C : period of vulcanization, sulfur ratio, nature of accelerator. Expressing this relation as the oven/bomb ratio, i.e., the ratio between oven and bomb aging periods that give the same deterioration, the following conclusions are drawn. (1) Varying the period of vulcanization of a given mix over a moderate range (2:3) around the optimum does not noticeably alter the oven/bomb ratio, except in unaccelerated mixes, where overvulcanization gave an abnormally low ratio, owing to the rubber perishing very rapidly in the oven. (2) The effect of varying the sulfur ratio within the usual limits for soft rubbers is small, and appears to result from the quicker vulcanization of the mixes richer in sulfur. (3) The nature of the accelerator profoundly affects the ratio, values ranging from 3 to 27 being found among the present mixes ; this effect is believed to be exerted through the influence of the accelerator both on rate of vulcanization and on the oxidizability of the vulcanizate. Theoretical considerations indicate that a rubber which vulcanizes quickly and (or) oxidizes slowly should give a low oven/bomb ratio, whereas one that vulcanizes slowly and(or) oxidizes quickly should give a higher ratio. The experimental data are generally in accord with this, and there is evidence that both rate of vulcanization and oxidizability can have an important effect, but the data do not enable the separate effect of each to be assessed exactly. The bearing of the results on the use of the oven and bomb tests is briefly discussed. It is shown that they demonstrate the fallacy of using a single factor for converting oven to bomb aging periods or vice versa, and that the relative natural aging resistances of widely different vulcanizates cannot be reliably assessed by either test, a limitation arising from the fact that the relative importance of oxidative decay and aftervulcanization varies from one type of mix to another, and that the temperature coefficients of these processes are not necessarily the same. These facts are very important in relation to the choice and application of accelerated aging tests for specification purposes, since these usually have to be applied to rubbers of unknown compositions, which are likely to vary widely in their relative response to different aging conditions.