Correlation of Tensile Strength with Brittle Points of Vulcanized Diene Polymers

Abstract

For several years work has been carried on here to evaluate a large number of diene polymers and copolymers as rubberlike materials. The writers have observed that changes in polymer composition which result in improved tensile strength and crack-growth resistance of the vulcanizate cause an increase in low temperature stiffness and a rise in brittle point. This generalization seems to apply to tensile values measured at elevated temperatures as well as to those at room temperature. For example, a butadiene copolymer of dichlorostyrene can be made which, as a tread type of vulcanizate, exhibits a tensile strength of over 1500 pounds per square inch at 93° C, in comparison with 800 to 1000 pounds per square inch for GR-S in the same test tread formula at the same temperature. The brittle point of the butadiene-dichlorostyrene rubber, however, is −35° C or higher. GR-S treads in the same test have brittle points between − 55° and −60° C. Probably of greater practical importance is the fact that the vulcanizate with the higher brittle point is stiffer at temperatures well above the brittle point. The purpose of this investigation was to determine to what extent the maximum tensile strength of tread stocks of several synthetic rubbers varies with the temperature difference between the brittle point and the tensile testing temperature of each rubber. These data can then be used to judge the validity and extent of the general observation that changes in copolymer composition which increase strength also raise the brittle point.