Reactions of Hindered Phenols with Unsaturated Hydrocarbons in the Presence of Free Radicals

Abstract

The reaction of hindered phenolic antioxidants , notably 2 ,6- -ditert-butyl-4-methyl-phenol (TBC) and 3,5-ditert-butyl-4-hydroxy-benzyl mercaptan (BHEM) with rubber has been investigated and the percentage of antioxidant bound under optimum conditions has been determined. The byproducts formed during the adduct formation and their effect on the binding reaction was also studied. The reaction mechanism involved in both cases has been studied. It has been found that the BHBM adduct was formed by the addition of thiol to the rubber double bond and that of TEC by the attachment of the benzylic carbon atom to the allylic carbon atom of natural rubber, Tensile measurements, oxygen absorption tests and stress relaxation measurements have been used to evaluate the antioxidant activity of the bound antioxidants. Comparison of their performances with commercial antioxidants have shown that the latter are completely removed under severe environmental conditions, whereas rubber-bound antioxidants exhibit exceptional stability. A major advantage of the rubber-bound systems, which has emerged during the evaluation was their suitability for the preparation of bound antioxidant concentrates and the superior performances of rubbers prepared using concentrated masterbatches have been demonstrated under severe testing conditions. Similar studies were carried out with nitrile-butadiene (NER) latices and it has been found that NER containing bound antioxidants, particularly, the BHEM adducts exhibit superior stability to conventionally added antioxidants, when used in applications where loss of antioxidants by volatilisation or leaching or by extraction may occur.