Brass, adhesion bonding

Elastomer adhesion mechanisms are discussed first from the viewpoint of vulcanization bonding to metals. The approaches of brass plate bonding, bonding with ebonite, with butadiene-methacrylic acid copolymers, and with proprietary adhesives are cursorily reviewed to invoke theories of chemisorption, diffusion, and directed chemical reaction. The time dependency of peel strength as an adhesion response through the vulcanization cycle is cited as a possible indication of mechanism. 

The performance of steel belted tires, as with tires reinforced with other fibers, is dependent to a large extent on the adhesive bond between the tire cord and the rubber in the skim stock surrounding the cord. The use of a thin brass layer on steel tire cord is the major adhesive... 

Results obtained by Van Ooij in his ESCA studies on the composition of interfaces between rubber and brass" confirm that cobalt salts and HRH form essentially the same inter-facial products with a standard brass surface. Therefore, the mechanism of brass-rubber bonding must be the same for both bonding systems and differences in adhesion values must be explained by a modification of rubber properties (crosslink density, cure rate, modulus, etc.) or the rate of brass attack. Clearly, adhesion of brass to HRH-NR compounds cannot be explained on the basis of hydrogen bonds with the substrate, as in rubber-to-textile bonding. 

Various etchants have been recommended for the remaining metals of engineering practice, but it is doubtful if sufficient work has been reported to differentiate between them or to assess their effect on the durability of the bonds formed with different adhesives. Strong, durable bonds are uncertain with copper because of the ease with which a weak, friable oxide is formed. Even when coated with an adhesive, oxygen can diffuse to the interface and eventually cause failure. Brass has an oxide film almost entirely of zinc oxide and, as with zinc galvanized iron, it can hydrate or form salts with the tackifiers added to some contact adhesives. Cadmium is met with as a plating if a strong, durable adhesive bond is essential, it should be replaced by chromium, the surface of which can be treated as stainless steel. 

There are few practical examples of bonded parts in machine tool structures. One of the few examples, PTFE (tetrafluoroethylene. Teflon) impregnated with brass particles, is bonded on saddles by epoxy adhesive to improve the friction and abrasion characteristics of slide ways. This is popular in normal NC (numerical control) machine tools. However, as improvement of the structural characteristics through application of adhesive bonding to machine tool structures is increasingly sought, the bonding application examples that follow are mainly those of research and development. 

Another market appHcation for naphthenic acid is the tire industry, where cobalt naphthenate is used as an adhesion promoter (see Adhesives Tire cords). Cobalt naphthenate improves the bonding of brass-plated steel cords to mbber, presumably by suppressing the de-zincification of brass (50). Its first reported use was in 1970 and the first patent for its use was issued in 1975 (51). About 900 t of cobalt naphthenate is used worldwide as an adhesion promoter, half of it in North America. The unit value fluctuates between 8.75—13.25 /kg because of the volatility of cobalt prices. Although it is the industry standard, the use of cobalt naphthenate is declining with the advent of more economical high metal-containing substitutes. 

Insoluble Sulfur. In natural mbber compounds, insoluble sulfur is used for adhesion to brass-coated wire, a necessary component in steel-belted radial tires. The adhesion of mbber to the brass-plated steel cord during vulcanization improves with high sulfur levels ( 3.5%). Ordinary rhombic sulfur blooms at this dose level. Crystals of sulfur on the surface to be bonded destroy building tack and lead to premature failure of the tire. Rubber mixtures containing insoluble sulfur must be kept cool (<100°C) or the amorphous polymeric form converts to rhombic crystals. 

Adhesives in the Tire Industry. Cobalt salts are used to improve the adhesion of mbber to steel. The steel cord must be coated with a layer of brass. During the vulcanization of the mbber, sulfur species react with the copper and zinc in the brass and the process of copper sulfide formation helps to bond the steel to the mbber. This adhesion may be further improved by the incorporation of cobalt soaps into the mbber prior to vulcanization (53,54) (see Tire cords). 

For bonding molded parts of polycarbonate to other plastics, glass, wood, aluminum, brass, steel, and other materials, a wide variety of adhesives can be used. Generally, the best results are obtained with solventless materials, such as epoxies and urethanes. 

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. 

An example of a 100% solid, non-flammable, heat-activated hot-melt adhesive recommended for structural bonding of aluminum, steel, copper, brass, titanium, fabric, and some plastics is 3M Company s Scotch -Weld Thermoplastic Adhesive Film 4060. Strength data are shown in Table 5.5. " Bonding using this clear, amber, unsupported film adhesive takes place rapidly. The speed of bonding is hmited only by the heat-up time required to reach the optimum bonding temperature of 149°C at a pressure sufficient to maintain contact between the surfaces to be bonded. The adhesive... 

In the adhesive mechanism of rubber to metal surface, ZnO plays an important role. In the bonding of rubber to brass, ZnO reacts with copper oxide on the brass surface to form a tightly adhering zinc-copper salt. 

Many metals, especially transition metals, are well known for their catalytic activity. Some cases of such chemical influence of a metallic substrate on a cnring reaction of an adhesive have been demonstrated. A well-known example, which has a considerable effect on adhesion, is the influence of brass or copper on the vnlcanization of rubber (see Rubber to metal bonding - basic techniques). 

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