Role of Carbon in Oxidation of GR-S Vulcanizates
Abstract 1. Carbon black is a catalyst for the oxidation of GR-S vulcanizates. The increased rate of oxygen absorption with increased loading is, however, a function of surface area rather than weight. Different types of carbons show different degrees of activity for a given surface area; the furnace carbons studied were, for example, only 55 per cent as active as channel black in promoting oxidation. 2. The rate constant for the initial rapid absorption of oxygen by GR-S tread stocks is independent of carbon loading. The presence of carbon is, however, essential to activate the centers involved in this initial stage, and the number of activated centers increases with carbon loadings, so that the amount of oxygen absorbed in a given time is also increased. 3. The second or linear stage of oxygen absorption involves two types of reaction with the remaining oxidizable centers in the polymer: a reaction with centers not under the catalytic influence of the carbon black, and a reaction catalyzed by carbon. 4. A theoretical equation was presented which permits the calculation of oxygen absorption by stocks containing either furnace- or channel-type carbons with various surface areas and loadings. The validity of the theory on which the derivation was based is supported by the quantitative correlation of calculated and observed rates of oxygen absorption for stocks containing carbons of both types, with surface areas ranging from 3.7 to 15.7 acres per pound and with loadings of 0 to 75 parts per 100 parts of GR-S. 5. The additional oxygen absorbed at higher loadings of carbon was shown to produce a correspondingly greater deterioration of physical properties.