Fundamental Aspects of Rubber Adhesion to Brass-Plated Steel Tire Cords

Abstract

Abstract During the past several years the fundamental knowledge of the processes which lead to rubber-brass bond formation and to bond degradation has increased considerably. As a result of the application of new instrumental techniques to the study of the rubber-brass interface it has been established that the brass surface is chemically attacked under formation of CuxS, ZnS, and ZnO. These products become an integral part of the rubber-to-metal bond. It has unequivocally been demonstrated that excessive or insufficient formation both lead to a low rubber-brass bond strength. The theories put forward by Buchan in the early forties (culminating in the only monograph on the subject thus far) have thus been proven to be largely correct. The rubber-brass bond can be interpreted as an adhesion of sulfur crosslinks to a sulfidized brass surface. These crosslinks are not necessarily bonded to the surface by fully developed “chemical” bonds, but should rather be interpreted as covalent bonds, comparable to those formed by copper oxide or copper sulfide and various organic complexing molecules containing electron-donating nitrogen or sulfur atoms. In spite of the relatively weak bonds between sulfur crosslinks and sulfidized brass surfaces, the adhesion between rubber and brass exceeds the rubber tear strength, since there is a very intimate contact between rubber and sulfide film which actually grows into the polymer. As a result, the contact area of the two phases is significantly enlarged. There is evidence that only certain types of crosslinks, probably the poly-sulfides, give rise to a high bond strength, possibly because of steric reasons. Best results have been obtained if the brass surface consist completely of cuprous sulfide, although there is evidence that a zinc sulfide surface can also be bonded well, provided the growth of the zinc sulfide layer is high enough to provide a sufficient number of points of attachment to the polysulfide crosslinks.