Oxidation and Antioxidant Action in Rubber Vulcanizates

The variation of rate of oxygen absorption in the constant rate stage with oxygen concentration involves both a square-root and a first-power term for most rubber vulcanizates, although the latter term may be negligible in some cases. A mechanism of oxidation and antioxidant action has been postulated which is consistent with the observed behavior, and which indicates that the antioxidant may function in at least four ways, some beneficial and some harmful. Direct oxygen attack on the antioxidant may produce radicals capable of initiating oxidation chains. Initiation by this mechanism appears to be more important with amine antioxidants than with phenols. The antioxidant may participate in the propagation stage by a chain-transfer mechanism. While this effect alone for a typical antioxidant would probably have only a small effect on the over-all rate of oxidation, it might also affect in a favorable way the secondary reactions which determine changes in physical properties. Amines also appear to be more inclined to chain transfer than are phenols. Termination by a chain-stopping mechanism is one way in which antioxidants retard oxidation. Phenols appear to be more efficient than amines in this connection. Destruction of peroxide by antioxidant-induced decomposition to stable products reduces initiation by peroxide decomposition, and is thus another important mechanism of antioxidant action. The amines are more efficient than the phenols in this regard, and the presence of carbon black is essential for the amines to exhibit their maximum effectiveness. Carbon black is known to promote the decomposition of peroxide to free radicals, but in the presence of amine antioxidants it also promotes the decomposition to stable products. This dual effect of carbon black results in a reversal of effectiveness of amines and phenols in gum and black stocks. The phenols (which function best as chain stoppers) are more effective in the gum stocks; the amines (which function best by reducing peroxide initiation by directing the decomposition to stable products) are more effective in the black stocks.