Mechanism and Theories of Rubber Adhesion to Steel Tire Cords—An Overview

Abstract

Abstract An overview is given of the mechanism and theories of adhesion of brass-plated steel cords to rubber. The literature on this topic in the period 1979–1983 is reviewed. Although the actual type of bond between the interfacially formed copper sulfide film and the cured network still has not been identified conclusively, this aspect of rubber-brass adhesion is becoming rather irrelevant since the literature is unanimous that bond failure seldom occurs adhesively, i.e., between CuxS and rubber, but usually cohesively within the sulfide film or adhesively at the interface sulfide-substrate. A model is described in which most of the recent findings are incorporated. In this model, brass develops a high bond strength to NR as a result of the in situ growth of a thin (≤50 nm) copper sulfide film onto the brass during the early stages of the cure before crosslinking sets in. This sulfide film has good adhesive and cohesive properties but is, on the other hand, so porous that rubber molecules become entangled in this film (interlocking model). ZnS or FeS are considered nonbonding because they lack the microporosity required for interlocking. Synchronization of sulfide growth and rubber cure is not required. The major prerequisite is that sulfide film growth has been completed before cross-linking begins. Previous rubber oxidation theories and the effect of CuxS on crosslink density of rubber adjacent to the cord surface are also discarded for lack of experimental evidence. Ultimate bond strength and bond durability are determined by cord and by compound properties. Optimum cord parameters are predicted by the model as a thin brass coating with a high concentration of diffused iron, a low copper content (60–65%) and a thin but very homogeneous ZnO surface layer which is passivating and which contains copper atoms. Copper oxides are undesirable. The homogeneous ZnO film improves the cords' corrosion resistance, the included Cu atoms are required for rapid initial CuxS formation and prevention of non-bonding ZnS formation. The major compound parameter is the sulfur-to-accelerator ratio. There is evidence that brass is sulfidized by intermediates of the type R-S-Sy-NR where R is an accelerator fragment, rather than by free sulfur. The properties of the CuxS film, such as adhesion, cohesion and stability in corrosive environments depend on the type of CuxS formed. High values of y in the intermediate lead to good CuxS films, low values produce poor films, regardless of the quality of the cord.