Mechanisms of antioxidant action: The reaction of hindered phenols with rubber in the presence of free radicals

Simple hindered phenols, containing ortho or para methyl groups, react with natural rubber latex in the presence of oxidizing free radicals (alkoxy or acyloxy), giving up to 20% yields of bound antioxidant. This reaction is most efficient when the concentration of the phenolic antioxidant in the rubber is less than 1%. As the concentration is increased, side reactions involving both the phenol (stilbenequinone formation) and the rubber (cross-linking) supervenes. The efficiency of the bound antioxidants is much higher than conventional antioxidants under aggressive conditions of air oven ageing and solvent or detergent extraction. This effect is due primarily to the non-removal of the bound antioxidants under these conditions whereas the conventional antioxidants are removed completely.