Stress relaxation of a nitrile rubber surrounded by an oil that increases the network density

Abstract

When a rubber item is subjected to a periodically varying deformation, the restoring force can exhibit a much more rapid decay than when the deformation is held constant. This is studied by dynamic mechanical stress relaxation. Based on well known relations from statistical rubber elasticity theory, a theory for the dynamic mechanical stress relaxation is proposed. In this paper it is shown that the special dynamic stress-relaxation behaviour has network degradation as its origin at elevated temperatures, and that data from measurements of continuous stress relaxation and intermittent stress relaxation can be related to results from measurements of dynamic stress relaxation. Results are reported from stress-relaxation measurements on nitrile rubber surrounded by a naphthenic oil with a gear-lubricant additive, which is found to be the origin of a reaction that increases the network density in the nitrile rubber.