Degree of Cure in Filler-Reinforced Vulcanizates by the Swelling Method

Abstract The Flory-Rehner theory of swelling allows an estimate of the number of crosslinks in gum vulcanizates. In reinforced rubbers the apparent degree of crosslinking is raised by some fillers, particularly carbon blacks. It has been shown experimentally that, for vulcanizates of a given polymer at a fixed filler loading and type, most physical properties are, in the first approximation, simple functions of the apparent number of network chains per unit volume, regardless of the possible origin of the linkages. Aside from controlling the number of apparent linkages, sulfur and accelerator levels as well as cure time have relatively minor, secondary effects on vulcanizate properties. Comparison of swelling data of gum and black stocks shows that the apparent contribution of the carbon black to crosslinking increases with the level of cure. Softeners and extender oils cause a reduction in the number of linkages, presumably by reducing the efficiency of sulfur utilization. It has been demonstrated that the swelling of vulcanizates in solvents gives an accurate measure of the degree of vulcanization even in filler-reinforced stocks. The results are apparently not obscured by adsorptive linkages between filler and polymer. When chemically different polymers are compared, the swelling volumes may be used to calculate the density of effective network chains; thus a nearly absolute measure of the degree of vulcanization is obtained. The swelling method is not difficult to apply, requires no special equipment, and imposes no limitation on the size or shape of the test specimen. For instance, it has been used with excellent success in this laboratory to check the state of cure in test-fleet tires. A small sample is cut from the side of the first rib of a tire and tested in the usual manner. The data presented offer some insight into the question of reinforcement. The evidence points to the fact that “contributions” of fillers to crosslinking derived from subtraction of the number of network chains or crosslinks in the filled and gum vulcanizates do not quantitatively represent the number of filler-to-rubber linkages. This filler “contribution” depends on the degree of cure, the vulcanization system, and the filler particle or aggregate shape. It depends only slightly on the particle size or surface area of the filler. The most plausible explanation of the observed behavior lies in a superposition of the effects of the filler on the course of vulcanization, filler-to-rubber linkages, and particle shape or aggregate structure.