Abstract
An investigation of the effect of varying conditions of high-temperature exposure has shown that, at a constant temperature, the supply of oxygen is the principal factor which causes stiffening of the vulcanizates. The loss of volatile plasticizers, when present, also contributes to the stiffening. The test-tube technique, developed in the course of this investigation, in which dumbbell samples are suspended in stoppered test-tubes (38 by 300-mm.) heated by immersion in an oil bath, appears to give results which are considerably more duplicable than those obtained by the usual aging in circulating air ovens. The test-tube technique also gives somewhat better differentiation between good-heat aging and poor-heat aging compounds. The rate of deterioration as measured by elongation change is doubled for an 18° F increase in the exposure temperature. As measured by hardness change the rate is doubled by an increase of 42° F. Tests run at 250° F by this method are no more reproducible than those run at 300° F.
