Mechanical properties of natural and synthetic rubbers

Abstract

The tensile creep behavior of natural rubber, high and low temperature GRS, polybutadiene, and a synthetic cis‐polyisoprene have been investigated. By combining measurements over the temperature range −70 to +60°C., it has been possible to obtain a composite creep curve covering over 15 decades of time for each of these materials. In addition, similar curves have been obtained for most of these materials at several degrees of cure and containing various amounts of carbon black. All measurements were confined to elongations below about 5%. One can conclude from these data that the synthetic rubbers, except for the cis‐polyisoprene, do not vulcanize to give a mechanically stable network. That is to say, these materials never reach equilibrium elongation under an applied load. It is concluded that the number‐average molecular weights of GRS is so low that excessive degrees of crosslinking are needed in order to form a stable network. Natural rubber and certain synthetic cis‐polyisoprene, do not suffer from this defect. The excessive heat generation found in GRS upon flexure is attributed to the fact that an equilibrium extension is never achieved in these materials. A theoretical relation for the heat production in flexed rubbers is given. It is showed that the theory predicts the relative behavior of the various rubbers in respect to heat generation under flexure.