The Absorption of Oxygen by Rubber

Abstract

Abstract In order to determine the relation between natural aging and oxygen absorption at 80° C., the effects upon oxygen absorption of a number of factors which are known to affect natural aging have been studied. An apparatus is described with which it is possible to follow the absorption of oxygen at constant pressure, temperature, etc., over long periods of time. It has been found that the absorption of oxygen is autocatalytic in nature and appears to consist of at least two separate reactions—a decomposition and an addition reaction. The rate of absorption of oxygen at 80° C. is dependent upon surface area when the area is small, but when it is increased to as large as 40 square centimeters per gram, small variations in surface area are without great effect, probably because under these conditions rates of diffusion are high enough to keep the rubber particles practically saturated with oxygen. As rubber absorbs oxygen it appears to undergo changes similar to those occurring during vulcanization with sulfur. Surface checking is regarded as a preferential surface oxidation. The mechanism of oxygen absorption appears to be different for raw rubber from that for cured rubber. Curing rubber increases both the rate and amount of oxygen absorption over that for raw rubber, and changes the character of the absorption curve. The rate of absorption increases with the degree of cure. The action of anti-agers is one of negative catalysis rather than one of preferential absorption. The rate of oxygen absorption may be greatly retarded by their presence and their effectiveness varies with their concentration and constitution. The type of absorption of ozonized oxygen differs markedly from that for pure oxygen. The former is taken up very much more rapidly at first, but the rate rapidly decreases and less oxygen is finally taken up than in the case of pure oxygen. The action of ozonized oxygen upon rubber appears to protect it from the action of pure oxygen. The reaction between rubber and ozonized oxygen is not autocatalytic.