Thermal aging of polychloroprene

Abstract

AbstractThe aging of polychloroprene has been studied at normal and elevated temperatures in air, in oxygen, and in nitrogen. A comparison of the molecular weight distribution curves for fresh specimens and for specimens aged under various conditions showed that on aging the polymer becomes heterogeneous. A large increase in the high molecular fractions shows that reactions leading to crosslinking prevail over those leading to degradation. Crosslinking (at least in the beginning) is caused mainly by peroxide bonds, as indicated by the spontaneous and ultrasonic degradation of the first fractions of the aged polymer. This conclusion has also received support in a comparison of the ultrasonic degradation of unfractionated fresh and aged polymer. It can be seen from the infrared spectra that on aging in air there is an increase in the carboxyl, aldehyde, CO, and OH group content, as well as in the number of conjugated double bonds. The number of CCl==CH, CH2, and CR1R2==CHR3 groups diminishes. The formation of conjugated double bonds was also found to take place during crystallization. The ultimate degree of crystallization greatly fell during the aging process (from 34 to 0%); the rate of crystallization, obeying well the Avrami equation for n = 2, changed only to an insignificant extent. On the basis of the measurements made it has been concluded that on lowering the crystallization capacity to 3.5% the degree of crosslinking is larger than in the case of vulcanized natural rubber with a 0.05% sulfur content. The chemical reactions leading to loss in crystallizability proceed with an activation energy of 8.5 kcal./mole, which approaches the value for the energy of activation of hydrogen chloride evolution (9.5 kcal./mole). The evolution of hydrogen chloride is initiated by oxidation reactions and is related to the polymer structure. Products of the decomposition of tetraethylthiuram disulfide or polysulfides were detected in considerable amounts among the gaseous aging products.