Vulcanization. Part III. Rapid Methods for Characterizing Rubber Networks

Equilibrium swelling determinations have long beea used as a means of estimating crosslink density, usually through the Flory-Rehner equation. Most of the reported methods for determining equilibrium swelling in rubber vulcanizates have required long periods of swelling before equilibrium was reached, due to the fact that thick specimens were used. The method presented here makes use of thin films as specimens for swelling, and equilibrium can be reached in periods of one to five minutes. In addition, the use of thin films makes possible the semiquantitative determination of the types of chemical bonds responsible for the crosslinks in rubber vulcanizates. In thin films the reaction of polysulfide crosslinks with lithium aluminum hydride (in benzene-tetrahydrofuran) is rapid. The determination of crosslink density after such a reduction is indicative of only those crosslinks which are not reducible by the hydride and the difference between crosslink densities before and after the reduction is a measure of the number of crosslinks of the polysulfidic type. The work described here has been limited to natural rubber gum stocks. The methods used are undoubtedly applicable to other types of stocks, though in some instances a certain amount of modification will be required.