Tensile Strengths of Pure Gum Natural Rubber Compounds

The tensile properties of a range of pure-gum natural rubbers have been reviewed, and it has been shown that their principal features can be understood on the assumption that the tensile strength measured in a given test depends directly on the amount of crystallization at break. The most important single factor in determining tensile strength is the degree of cross-linking. Cross-linking is only needed in order to prevent plastic flow, thus making it possible for the molecules to align themselves by stretching, and hence to crystallize. A very highly cross-linked rubber is weak because the load required to stretch it is so high that the rubber is broken before the elongation becomes large enough to produce crystallization. In general, vulcanization also involves reactions, e.g., the combination of sulfur with the rubber, which inhibit crystallization by producing structural modifications of the rubber. These reduce the tensile strength, especially when the degree of cross-linking is large. These ideas readily explain the effects of swelling and of the temperature of test. They are also used in a brief discussion of the phenomena of overcure, reversion, and aging.