Evaluation and Mechanism of Sulphur-Bound Antioxidants in Rubbers
A quantitative study of the extent of the reaction between the hindered phenolic antioxidant 3,5-di-tertbutyl-4-hydroxyphenyl methane thiol (BHBM) and natural rubber latex, showed that this was low, due to naturally occurring non-rubber constituents. In SBR latex, much higher levels of binding were achieved. The concentration of bound antioxidant was estimated using infra-red spectroscopy after exhaustive extraction. Evaluation of the ageing characteristics of bound BHBM in a conventional CBS vulcanisate showed that the rubber had exceptionally good antifatigue and antioxidant performance before and after solvent extraction. The mechanism of the binding reaction with BHBM was studied using the related model compound, cyclohexene. It was shown that the molar ratio of peroxide and the temperature of the reaction were critical for high yields of adduct formation. MADA was mechanochemically bound into synthetic and natural solid rubber using the RAPRA torgue rheometer and the Buss Ko Kneader. Efficient binding was attained only for rubbers extracted prior to the processing operation. In all cases, the rubbers containing the bound MADA showed effective thermal oxidative and fatigue resistance after extraction. The curing characteristics of the modified rubbers in conventional CBS vulcanisates showed that except for some reduction in scorch time, the vulcanisation characteristics were not affected significantly. The bound antioxidant concentrates have been shown to be suitable for use as conventional additives for unmodified rubbers.
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