Mechanism of Rubber Brass Wire Adhesion

The thin coating of brass on the steel cord is the primary adhesive used in steel-to-rubber bonding. The quality of this bonding system built up during vulcanization of, for example, a radial tire will influence the performance of the steel ply or steel belt in the tire and, ultimately, the durability of the product. Though the mechanism of bond formation in rubber-steel cord adhesion is very complex, a brief review of the current understanding of wire to rubber adhesion is presented. 

FIGURE 14.17 Interfacial copper sulfide film in rubber-brass txjnding (from Gough, 1968.) 

Zinc sulfide and iron sulfide do not bond because they do not grow rapidly enough during vulcanization, do not form porous domains, and thus cannot interlock with the polymer. Because the primary requirement is the formation of a copper sulfide domain before the initiation of crosslinking, reduction of compound scorch time, consequently, can adversely affect bond formation. 

Mechanical stability of the copper sulfide domains is essential to retain long-term durability of the rubber-to-wire adhesion. However, corrosion of the wire-rubber adhesive bond is catalyzed by Zn + ions that diffuse through the interfacial CuS layer. This will eventually result in an excess of either ZnS or ZnO/Zn(OH)2. Under dry conditions, this process is slow. Nevertheless, Zn + will migrate to the surface with a consequent drop in mechanical interlocking of the CuS domains and rubber followed by adhesion loss. 

Migration of Zn + ions is a function of the electrical conductivity of the brass coating. Addition of Co or Ni ions will reduce this conductivity. 

---