Aging of Elastomers. Comparison of Creep with Some Conventional Aging Methods

Subjection of elastomers to mechanical stresses results in unusually complicated behavior. Recent theoretical researches have shown that this behavior cannot be described satisfactorily by either of the classical theories of elasticity or viscosity. The general molecular theories which describe the behavior of elastomers have experimental verification manifested by three regions of temperature-stress relationship: (1) a low temperature region in which stiffening is observed, due to the stability of secondary bonds between network chains, (2) an intermediate temperature region in which the secondary bonds are so unstable that complete relaxation occurs before measurements can be obtained; the scission of primary valence bonds is occurring at such a slow rate that no measurable effects are obtained during the course of the usual laboratory experiment; and (3) a high temperature region in which the relaxation of stress with time is associated with a chemical reaction which, through breaking of primary-valence bonds in the network, severs the chains rapidly enough to be measured during the course of usual laboratory experiments. The high temperature region is that in which elastomers soften and (or) harden and finally lose their rubbery characteristics. Oxygen has been shown to be necessary for the chain-scission reaction. Several papers have described this fundamental experimental technique for the stress-relaxation and creep of different elastomers. Well known laboratory methods for artificially aging elastomers in oxygen and air bombs and in circulating air atmosphere have selected conditions somewhat arbitrarily. In exploratory searches for promising compounds to be used as antioxidants in elastomers and in the evaluation of well known antioxidants, it has often been found that the conventional methods of aging do not differentiate among several antioxidants. It is the purpose of this paper to describe an application of the previously described creep technique as a convenient and precise means of studying the relative performance of antioxidants and accelerators in Hevea and GR-S rubbers.