Application of XPS to the study of polymer-metal interface phenomena

Abstract

XPS has been used to identify the mechanism of adhesion and adhesion failure of metals to rubber. The polymer was cured in contact with a series of metals and alloys. The rubber-metal interface was analyzed by XPS and the results were compared with those of bond strength measurements. High bond strengths are obtained only if an interfacial film of sulfidic reaction products is formed which becomes an integral part of the bond. It is postulated that metal sulfidization affects the rubber chemistry in the interface region leading to an increased crosslink density of the polymer. Several different types of adhesion failure have been observed. CuNi alloys do not bond well because of poor adhesion between the interfacial NiS layer and the metal substrate. The very strong bond formed by CuZn alloys fails during aging in air of high humidity or in steam as a result of partial dezincification of the unreacted brass leading to a Weak Boundary Layer consisting of ZnO and Cu x S. Minor changes in compound formulation may result in drastic differences of aged adhesion retention of brass, however. Good compounds form homogeneous sulfide films containing ZnS and Cu x S which passivate the brass. Poor compounds form irregular sulfide films which contain ZnO as well and which do not passivate the metal. Addition of a few per cent of Ni to brass results in a much improved aged adhesion retention. XPS analyses suggest that the nickel addition inhibits brass dezincification leading to a suppression of interfacial film growth during bond aging.