Apparatus for the Continuous Measurement of Stress Relaxation in Vulcanized Rubber

The measurement of stress relaxation in vulcanized rubber is based on the principle that, when kept at constant elongation, a stretched sample of rubber shows a progressive decrease of stress with time and that this decrease depends both on the nature of the compound and on the temperature of testing. The resultant curve is a decreasing potential, which is expressed approximately by the equation: σt=σ0⋅e−kt, where σt is the instantaneous stress at time t, σ0 is the initial stress, and k is a reaction rate constant which depends on the free activation energy of the rubber chain molecules. Tobolsky was the first to apply the relaxation theory to the study of the oxidation behavior of compounds and to the approximate determination of the nature of vulcanization structures. Certain types of apparatus are available for the discontinuous measurement of this stress relaxation, but we have designed, with the aid of the French Rubber Institute technical service, a relaxometer for the continuous recording of this phenomenon.