Brass adhesion

Sulfidation of the brass surface is not due to its interaction with elemental sulfur, but it is the result of the interaction between the brass surface and accelerator-sulfur reaction products, which can be represented by the general structure, Ac-S -Ac and Ac-Sy-H, where Ac is an accelerator-derived moiety (e.g., benzothiazolyl group). The value of the subscript, y, increases with the ratio of the concentration of sulfur to the concentration of accelerator used in the curing system. Generally, high sulfur levels and high ratios of sulfur concentration to accelerator concentration favor good rubber-to-brass adhesion. 

Diffusion of metallic copper domains to the surface following oxidation by R—S c is not affected, as Cu + ions migrate along grain boundaries of the ZnO layer. Thus if a cobalt salt is used, formation of copper sulfide at the cord surface will be accelerated, whereas ZnS generation will be hindered (Figure 14.18). This review is necessarily brief, and the reader is encouraged to consult additional references for further detail on the chemistry of rubber-brass adhesion (Bekaert Corporation, 2004 van Ooij, 1984). 

Reviews on the subject of brass-plated steel cord-natural rubber adhesion have been written by van Ooij who has done much of the work in the field. Van Ooij [46] has given a model for rubber-brass adhesion, in which a copper sulfide layer forms on the brass before the onset of crosslink formation. The thin film of copper sulfide has good adhesion and cohesion. In addition, the film is so porous that rubber molecules can become entangled with it. It is not required that the film forms simultaneously with the formation of crosslinks during vulcanization but, rather, it is required that the copper sulfide film be completely formed before crosshnking starts. Indeed, adhesion between brass-plated steel and natural rubber can frequently be improved by the use of the retarder, CTP [4] or by using a more delayed action accelerator such as N-dicyclohexylbenzothiazole-2-sulfenamide (DCBS) [47]. 

The choice of accelerator also has an effect on the quality of adhesion between cord and rubber. The accelerator should not form a stable copper complex which dissolves in the rubber. This would be quite corrosive to the brass plating. In this respect, benzothiazoles and their sulfenamides are much better than dithiocarbamates. DCBS is a particularly good sulfenamide accelerator for rubber-to-brass adhesion. 

Ziegler, E Macher, J Gruber, D Polt, P Kern W. Investigation of the Influence of Stearic Acid on Rubber-Brass Adhesion. Rubber Chem. Technol, 2012, vol.85, >fo2, 264 - 276. 

It has been demonstrated previously that such additions can increase the strength of rubber-brass adhesion considerably. In this research it was established that polysulphides are only weak crosslinking agents for unsaturated rubber by themselves. In the presence of sulphenamide accelerators, such as OBTS, polysulphides, in amounts of 0.5 - 1 phr, activate the sulphur vulcanisation. However, the reversion process (crosslink breakdown) is not accelerated. A favourable effect on the physicomechanical properties of the vulcanisate was also reported. 

Many times DCBS is selected as the primary accelerator when a slow cure rate is needed to match the slower adhesion chemistry of rubber-to-brass adhesion for steel tire cords. Other commonly used sulfenamide accelerators cure faster than DCBS. 

Antioxidants used in the rubber industry do not exert any great effects on rubber-brass adhesion. 

Figure 4. Schematic representation of rubber-to-brass adhesion, based on XPS analyses, from the work of van Ooij [221. (a) The situation prior to vulcanization indicating the formation of the active sulfide ion from elemental sulfur at the brass surface (b) the formation of Cu S following vulcanization.

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