Brass adhesion silica

Alpha brass coating is used on steel to improve the adhesion of rubber to the steel. It reacts both with sulfur, producing zinc sulfide, and with the rubber. Organic cobalt compounds catalyze the reaction and necessitate high sulfur dosing. Resor-cin-formaldehyde-silica systems are an alternative. The brass layer is not needed if isocyanates are used as the adhesion promoter, although solvents are then required. Aqueous dispersions of chlorinated or sulfochlorinated polyethylenes cross-linked with polynitroso compounds offer an alternative [32]. 

In the first experiment, TomUnson sealed fused silica fibers in a brass cylinder, heated them to about 1000°C, and observed their adhesion as he manipulated them inside the chamber. The adhesion was substantial. To prove that dry air had no effect on the adhesion, he circulated air through sulfuric add, calcium chloride towers, phosphorus pentoxides and roasted pumice to remove all moisture. He then found that the adhesion remained high for sfac weeks. But when the fibers were exposed to damp air, the adhesion was reduced to almost nothing in a few hours. His conclusion was that smooth silica adheres when dry but not much when wet. The reason for Stone s misleading observations, that the surfaces of glass beads are rough and become fiUed with water, will be eiqilained later in Chapter 7, as illustrated in Fig. 6.2. 

Silicas, which are in competition with carbon blacks as functional fillers for plastics and rubbers, have one significant advantage their white color [62]. The most important role of silicas is as elastomer reinforcements, inducing an increase in the mechanical properties. Other functions, in addition to their use as antiblocks for PE, PP, and other films, are (a) to promote adhesion of rubber to brass-coated wires and textiles, (b) to enhance the thermal and electrical properties of plastics, (c) in accumulator separators, and (d) as rubber chemical carriers. 

Many rubber composites require that the rubber adheres permanently to a substrate such as steel, brass, textiles or other media. There are a number of established proprietary additives, which may be added to the rubber compound, which function in this capacity. The resorcinol-formaldehyde-silica (RFS) system is commonly used to achieve adhesion to metals. This system relies on the interaction of resorcinol (or a resorcinol donor), a methylene donor snch as hexamethylene tetramine and precipitated silica. Other systems that may rely on cobalt complexes or blocked isocyanates may also be used. 

Precipitated silica has wide applications in integral bonding systems based on resorcinol and methylene donor (the RES system) or for systems based on cobalt complexes [55]. High levels of adhesion to steel, zinc plate, brass and a variety of textile substrates can be achieved with these systems. 

Despite this success of silanes, which are easy to use and apply, their use as metal pre-treatments for bonding metals to rubber has been lacking. The only rubber-related use is as a treatment of rubber-grade silica [32, 33]. This process improves the adhesion of the silica to the rubber and therefore the mechanical rubber properties. There is only one patent that describes the use of a silane to bond bare steel cord (in other words not plated with brass) to sulphur-vulcanised tyre cord skim stocks, published by Sharma in 1982 [54]. Good adhesion was reported but only if the compound contained a certain resin. Apparently, the silane reacted more with the resin than with the rubber compound. 

Despite their widespread use in rubber formulations, few mechanistic studies have been performed to determine the role of silica and resin in the adhesion process. The effect that resorcinol/formaldehyde resin had upon the adhesion interface between rubber and a brass film has been investigated [57, 58] using AES in an attempt to correlate spectroscopic data with actual adhesion values. In the unaged state, the structure of the interface was essentially the same for compounds containing resin as those without. Differences became apparent after ageing at high humidity when the... 

The role of silica-only systems on adhesion has been studied using model compounds with squalene [59]. It was shown that the mechanism for increased adhesion to brass-coated wire-to-rubber was not just a simple improvement of the physical properties of the rubber, but that silica moderated the thickness and composition of the interfacial layer by a chemical interaction. SEM-EDX (scanning electron microscopy with energy dispersive analysis of X-rays), XPS, AES and PIXE (proton induced X-ray emission spectroscopy) revealed that silica affected the relative concentrations of compounds present in the interfacial layer, promoting zinc oxide formation in particular. 

While it is possible to get some adhesion using HRH without the hydrated silica, many times insufficient adhesion is imparted. Thus hydrated precipitated silica is a very important component of the HRH system if it is used to achieve adequate rubber-to-metal adhesion. This is particularly true with rubber-to-brass-coated steel tire cord adhesion where there are very few practical alternatives. 

HRH Systems. A review of currently available bonding agents such as HRH and modifications thereof, is given by Weaver." HRH designates the use of hexamethylenetetramine (hexa), resorcinol, and a hydrated silica as additives to rubber in order to improve adhesion to brass coated wire. Normally about 1.5 phr hexa, 2.5 phr resorcinol, and 15 parts of silica are added to a wire skim stock. Pre-... 

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