Rubber-Brass Bonding

This chapter will discuss the state-of-the-art of bonding rubber compounds to brass, a technology primarily used on steel tyre cords. The literature is reviewed since 1991 when the previous review was published [1]. An updated mechanism for the rubber adhesion mechanism of brass is presented. Some new developments, such as proposed alternatives to brass, are also discussed. 

Brass only bonds by a unique, self-catalysed process (details are discussed later in this chapter), if the rubber has a high degree of unsaturation in practice, this limits its use to natural rubber (NR), synthetic isoprene rubber (IR), styrene-butadiene-rubber (SBR) and butadiene-rubber (BR). 

A strong bond is only formed with sulphur-vulcanised compounds. Additionally these compounds need to have a high sulphur level (4 phr) and a certain type of accelerator, i.e., delayed-action sulphenamides. Systems containing lower sulphur levels, semi- 

The brass composition and also its surface composition are important, and adhesion is only obtained within rather narrow limits of copper percentages (roughly 65 -75%) and oxide film thickness. In other words, the surface preparation of the brass is very important. 

An important additive to NR compounds used for bonding rubber to brass is a cobalt salt, such as cobalt naphthenate. While this additive improves the stability of the rubber-brass bond, especially in a corrosive environment, it exerts a negative effect on the stability of the rubber network, in that it accelerates reversion phenomena especially in the presence of oxygen and at elevated temperatures. 

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FIGURE 17 Interfacial copper sulfide film in rubber-brass bonding. (From Gough [7].)... 

Figure 6.1 Mechanism of rubber-brass bonding after van Ooij [4], Shown are a) the original brass surface composition and, b) the interfacial copper sulphide dendrites interlocked with the crosslinked rubber compound (Reprinted with permission, from W. J. van Ooij, Rubber Chemistry and Technology, 1984, 57, 3, 426, Figure 1 and 445, Figure 11. 1984, Rubber Division, American Chemical Society, Inc.)...

The main emphasis of the work on rubber-brass bonding published in the last 10 years has been on elucidating mechanisms further. The adhesion model accepted since 1984 in which the formation of a film of non-stoichiometric copper sulphide has been confirmed in all studies. It has now become clearer how to obtain a thin copper sulphide film with... 

Analysis of failure surfaces can also lead to information about the reactions which occurred in the interface. Results of an XPS analysis(39) of a vulcanized rubber/brass interface are shown in Figure 34. Products formed at the interface were identified as ZnO, ZnS, and Cu S the latter two were confirmed by X-ray diffraction analysis. The components of the vulcanizing rubber attack the brass leading to the formation of an interfacial film of reaction products, which become an integral part of the rubber-brass bond. Ageing in either steam or oxygen leads to further reaction and the formation of a weak boundary layer and low strength. 

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