A Study of Thiol Based Bound Antioxidants in Acrylonitrile-Butadiene Rubbers

Abstract

Thiol containing antioxidants, 3,5-di-tertiary-buty1-4-hydroxybenzyl mercaptan (BHBM), 4-anilinophenylthioglycollamide (PAPT) and 4-anilinophenylthiopropionamide (PPPT) have been successfully bound to acrylonitrile-butadiene rubber (NBR) by a free radical process in the latex and during processing. Under optimum conditions, latex masterbatches and solid masterbatches of the bound thiol antioxidants were prepared and were used as additives for unstabilised NBR latex and normal solid NBR. The concentrations of bound antioxidants have been estimated by infra-red spectrophotometry. The effect of bound antioxidants on the behaviour of vulcanisates has been investigated using the Monsanto oscillating disc rheometer. PAPT and PPPT increased the rate of cure and cross-link density while BHBM reduced the latter. Conventional oxygen absorption, oven ageing and stress relaxation measurements and in addition a cyclic oil/air test specifically designed for nitrile rubber, in which the mechanical properties were measured, have been used to evaluate the effectiveness of the bound antioxidants before and after extraction with an organic azeotrope. For comparison the effectiveness of commercially available copolymerised antioxidant, N-(4-anilinophenyl) methacrylamide and a high molecular weight partially polymerised antioxidant, 2,2,4-trimethy1-1, 2-dihydroquinoline, were also investigated. The result showed that, although the bound antioxidants retained much of their activity after extraction, the antioxidants behaved differently in different tests. The performance of the thiol based bound antioxidants particularly PAPT, is better in the cyclic oil/air test. Mechanistic studies showed that the antioxidant activity of thiolamides, PAPT and PPPT, involves synergistic chain breaking catalytic peroxide decomposition mechanisms