Abstract
As was shown in a previous paper, a definite relation exists between aging and relaxation rates for rubber vulcanizates. Gee has shown that the modulus of stocks with the same “compound” viscosity determines the average molecular weight between bonded units. We shall, therefore, expect to find a constant sulfur-bond distribution in related vulcanizates with the same niveau of moduli and Mooney plasticity and, hence, the same free energy for the process of oxidation. If various retarding agents such as antioxidants are added, measurement of the rate of relaxation of the stress at constant elongation will conform with the rate of aging expressed in terms of tensile strengths. For vulcanizates with the same system of accelerators but containing different fillers, the moduli will vary widely, and interpretation of the stress relaxation measurements (abbrev. SR) will become difficult. The same will hold for systems with boosted accelerator complexes. Plain accelerator/sulfur stocks seem, on the contrary, to give approximately the same range of stability whether tested in aging or relaxation. Thus the merit of the relaxation method is the rapid evaluation of the catalytic effect of compounds which does not interfere with the state of cure. By a proper choice of the temperature, the SR measurement can be completed in minutes, and it is possible to select the active compounds for the more tedious control in aging. Especially for work with the antioxidant effect in thiuram vulcanizates without addition of sulfur where the shelf aging in air at 95° C lasts for 50–60 days, the use of the stress relaxations measurement at temperatures of 130–150° C represents a very convenient method of information. The copper effect described in the following was actually stated with the SR method. Since the method easily might be conducted half or fully automatically, it is very suited for research work or even for control.