Thermal Stress Relaxation of Vulcanizates of Various Structures

Abstract

Abstract Heating stressed vulcanizates in the absence of oxygen results in purely thermal degradation of the three dimensional network accompanied by reduction of the stress; this process is called thermal relaxation. Thermal relaxation of various rubbers (natural, polybutadiene, butadiene styrene, isoprene and carboxylated rubber) vulcanized with various vulcanizing mixtures and methods, including vulcanization by irradiation, is first order. The rate constants of thermal stress relaxation decrease with network density. The product of the rate constant and the initial number of crosslinks (q=N·K) which yields the number of bonds broken per unit time, is independent of the network density if degradation proceeds by rupture of the molecular chains. This product increases with the network density if relaxation is due to rupture of crosslinks. Hence, examination of the relationship between the relaxation rate and the network density is a way to find the points where the network is ruptured. Thermal relaxation of stress in natural rubbers vulcanized by irradiation is mainly due to rupture of the molecular chains. Here, relaxation is accompanied by a decrease of the network density because the polymer radicals formed are localized by reaction with the compounds of low molecular weight, by formation of ring structures and by disproportionation. Thermal stress relaxation in rubbers vulcanized with metal oxides is due to rearrangement of the salt crosslinks by exchange. Here, the molecular chains remain intact, and the number of crosslinks is not altered. Thermal relaxation in rubbers vulcanized with sulfur is due to preferential rupture and rearrangement of the weak polysulfide crosslinks. The new crosslinks contain a smaller number of sulfur atoms. In this case the number of crosslinks may either rise or fall, depending on the secondary reactions of the polymer radicals formed.