Mechanical Activation of the Oxidation of Vulcanizates by Static and Dynamic Deformation

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Abstract
The deformation conditions to which rubber parts are subjected have a considerable effect, and in many cases the decisive effect, on their service life. One of us made a qualitative study which established the direct influence which the mechanical stresses created in a vulcanizate by dynamic deformation have on the rate of oxidative reactions. In the present work, a comparison was made between the effects of static and of dynamic deformation on the oxidation and breakdown of vulcanizates with differing structures. Dogadkin and Tarasova established that vulcanizates with tetramethylthiuram disulfide form mainly monosulfide bonds. In vulcanizates containing diphenylguanidine, the presence of a large number of polysulfide bonds was noted. In a thermal vulcanizate, a spatial network with —C—C— bonds was formed. All of the above vulcanizates were found to have approximately the same network densities, as determined from their equilibrium moduli. In order to remove the free ingredients remaining after vulcanization (free sulfur, accelerators, and phenyl-2-naphthylamine), all the vulcanizates before testing were extracted with acetone for 50 hours, in a stream of nitrogen, and were then dried in a vacuum. In experiments involving an antioxidant, the latter was introduced into the purified vulcanizate in a benzene solution. The thickness of the vulcanizate samples studied ranged from 0.2 to 0.25 mm. The work on dynamic deformation was carried out in a specially constructed apparatus, with a compensating mechanism for regulating the permanent set created under fatigue. It should be noted that in all the vulcanizates this accumulation of permanent set through fatigue was observed even in the first few cycles of the testing. The total amount of accumulated permanent set in a vulcanizate, from the time the testing began until the break, did not exceed 20–25 per cent of the original length of the sample. For the determination of the fatigue lives of vulcanizates, parallel tests were made on twenty-five samples. A detailed investigation was made of the influence that the type of cross-links in the vulcanizates has on the fatigue lives of the latter.

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