Stress Relaxation of Natural and Synthetic Rubber Stocks

Abstract

Measurements of stress decay as a function of time made at constant elongation on thin bands of gum and tread type natural rubber (Hevea), Neoprene, Butyl, Buna S, and Butaprene N stocks indicate that both secondary and primary bondrelaxation occur. Practically complete relaxation is observed to take place in the experimental time (about 100 hours) at temperatures at and above 100 degrees C. The manner in which the rate of relaxation depends on temperature and the fact that the rate is independent of elongation and of the presence of carbon black in the vulcanizate indicate that stress decay is caused by a definite chemical reaction which deteriorates the rubberstructure, and oxidative scission is suggested as the mechanism of deterioration of the primary bonds. The stress relaxation data, obtained over a temperature range from −50°C to +150°C, appear to verify modern concepts of the structure of elastomers.Theoretical equations are derived which give very good agreement with the observed relaxation data at high temperatures. The free energy of activation for the oxidative scission is found to be 30.37 kcal. per mole for the Hevea gum stock, and differs from this value by less than ±2.0 kcal. per mole for all other stocks, indicating the same general reaction for all. However, these small differences in free energy of activation correspond to considerable differences in times of decay, which fact might be significant in evaluating the resistance of rubber stocks to deterioration.