Mechanisms of antioxidant action: Auto-synergistic antioxidants based on thiol rubber adducts

The mechanism of by-product formation in the reaction of a thiol antioxidant, 3,5-di-tertbutyl-4-hydroxybenzyl mercaptan (BHBM), with rubber latices is shown to involve two processes, viz dimerization of the intermediate thiyl radical to give disulphide (BHBDS) and the reaction of the intermediate quinonemethide with water to give the aldehyde and with BHBM to give the monosulphide (BHBMS). The two sulphides themselves react with rubber in a slower reaction.The bound antioxidants obtained with natural rubber and nitrile-butadiene rubber are very much more effective than the present generation of commercial heat ageing antioxidants under conditions where the antioxidants can be lost from the rubbers by volatilization or solvent or detergent leaching. This is partly due to their resistance to removal from the polymers but there is also evidence that the auto-synergistic antioxidant bound to the rubber is more effective than single function antioxidants, due to the presence of the sulphide linkage.