Brass adhesion to rubber

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. 

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. 

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. 

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.

Geon and Seo [47] also determined the effect of vulcanization time on the adhesion of natural rubber to brass-plated steel. For relatively short times, there was a peak at the end of the copper profile that corresponded well with a peak in the sulfur profile. Similarly, peaks in the zinc and oxygen profiles corresponded well. These results showed that copper sulfide and zinc oxide mostly formed at short times but some evidence for formation of zinc sulfide was also obtained. For long times, the peak in the sulfur profile no longer corresponded with that in the copper profile. Instead, the peak in the sulfur profile corresponded to the peak in the zinc profile. It was concluded that the formation of zinc sulfide increased substantially at long times. An increase in vulcanization time correlated well with a decrease in the force required to pull brass-plated steel wires out of rubber blocks. 

The adhesion between rubber and brass-plated steel (e.g., steel tire cords for belted radial tires) has been the subject of much study and speculation. Brass plating is the major method of obtaining adhesion between natural rubber and the steel of tire cords. Over the years there has been much speculation about its mechanism, but there is agreement on one aspect of the adhesion of natural rubber to brass-plated steel the actual adhesion between the natural rubber and the brass-plated cord, formed in situ during the vulcanization process, is an interfacial layer of sulfides and oxides of copper (Buchan, 1959 van Ooij, 1979, 1984). 

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. 

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. 

Zinc oxide is essential in rubber technology because it is the most commonly used activator for sulfur cure systems. Just about every rubber compound that uses sulfur as the vulcanizing agent will most likely contain a small amount of zinc oxide to activate the cure. Also zinc is alloyed with copper to form brass. Special brass-plated steel tire cord is a primary reinforcing material for producing steel-belted radial tires. The brass coating of the steel tire cord enables very good rubber-to-metal adhesion. Therefore, zinc metal and zinc oxide are very important to the rubber industry. 

Copper is the principal metal in brass alloy that is commonly used to coat steel substrates for metal-to-rubber adhesion. Also, pure copper can be used to achieve good rubber-to-metal adhesion. Lastly, copper can be alloyed with tin to form bronze. Bronze can also achieve good adhesion to rubber under the right conditions. 

Cobalt is important to the rubber industry to promote rubber-to-metal adhesion. The use of cobalt salts, such as cobalt stearate or cobalt naphthenate as compounding additives, will promote better adhesion between cured rubber and brass-coated steel tire cord. 

Steel tire cord is usually brass plated in order to achieve good rubber-to-metal adhesion. Also, tire bead wire is commonly plated with bronze to achieve rubber-to-metal adhesion. The steel cable used in rubber mining belts may also be brass or bronze plated. 

Resorcinol formaldehyde resin is a vital ingredient in the HRH rubber adhesion compounds (such as wire coat stock or breaker stock) in order to achieve good rubber-to-brass-coated steel tire cord adhesion. 

Resorcinol formaldehyde resin is an extremely important component of the HRH system for achieving good rubber-to-brass-plated steel tire cord adhesion. This RF resin functions as a resorcinol donor in the rubber compound that reacts with the... 

RF resin has been used to achieve good rubber-to-brass-piated steei tire cord adhesion since the 1960s. There are very few aiternatives, and these aiternate substitutes may not reaiiy be viabie in the iong term. 

Cobalt stearate (or other cobalt salts) is sometimes used as rubber compounding ingredients to improve rubber-to-brass steel tire cord adhesion under certain circumstances. Commonly, a careful use of cobalt soap such as cobalt stearate may actually improve certain adhesion characteristics if it is used properly. Since rubber-substrate adhesion is a variable phenomenon, many technologists feel that the contribution of cobalt is to improve the reliability of the adhesion rather than the adhesion per se. Over the past three decades, this reliability of adhesion has been found to be of much importance in the manufacture of steel-wlre-reinforced tires and other rubber products. Thus the end result is a greater consistency of product quality, with fewer production rejects and subsequent failures in actual service. 

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. 

Zinc metal is also used to alloy with copper to form brass. Brass-coated steel tire cord is used to achieve good rubber-to-metal adhesion. 

Jaeger and Korb" described the use of added carboxyl nitrile rubber to modify a standard rubber-to-metal adhesive containing chlorinated rubber and phenolic resin and thereby enhance the rubber-to-metal bonding capability. Cylinders of natural rubber were bonded by vulcanization for 25 min at 145°C and 70 kg/cm. The bond strength for the control adhesive was 59 kg as compared to 74 kg for the modified adhesive. This adhesive was also used successfully for bonding cw-polybutadiene rubber and nitrile rubbers to steel, aluminum, and brass adherends. The Dunlop Rubber Company patent claims the use of a butadiene... 

Other Factors Affecting Adhesion of Rubber to Brass Coated Steel Wire... 

One of the first published investigations into practical adhesion phenomena in which surface analysis techniques were used was that into the adhesion of rubber to brass by van Ooij in the mid-1970s. Subsequently this particular research was extended and refined, and publications by the same author on the topic now-cover more than two decades. The adhesion of vulcanized rubber to brass is of critical importance in the tire industry since the steel wires used for tire reinforcement are brass-plated to ensure good bonding to the rubber of the tire. The need to brass-plate the wires had been known for many years but it was not until van Ooij s benchmark publications of 1977 that the chemistry involved in this particular adhesion process became fully understood. 

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. 

Nitrile rubber adhesives. The main application corresponds to laminating adhesives. PVC, polyvinyl acetate and other polymeric films can be laminated to several metals, including aluminium and brass, by using NBR adhesives. NBR adhesives can also be used to join medium-to-high polarity rubbers to polyamide substrates. The adhesive properties of NBR rubbers can be further improved by chemical modification using polyisocyanate or by grafting with methyl methacrylate. 

Wire coats good adhesion to brass coated steel wire and to adjoining rubber compounds, tear, fatigue, and age resistance... 

Adhesive Studies. The DHA-4VP copolymers had excellent properties as adhesion promoters for rubber-steel (brass-coated) composites (Table VII). The adhesion values of 44.8-47.6 lbs approach the limit of the test, i.e. rubber failure may occur at 45-50 lbs. The adhesion values listed in Table VII are the optima obtained in numerous tests in which pH, type of RF latex resin, curing temperatures, etc. were varied. Adhesion values ranged from the lower forties to the upper thirties, and coverages were in the 3+ range. 

Recently, van Ooij et al. have reviewed adhesion of steel tire cord to rubber (van Ooij et al., 2009). The authors reviewed the literature extensively and provided an updated model for adhesion to brass-plated tire cord, which incorporated observations made by many techniques. They discussed the effects of different compounding ingredients and the possible alternatives to the current brass coatings. They note that the use of cobalt compounds improves the adhesion between rubber and brass-coated cords, but new adhesion promoters have been developed as replacements for Co, or for combined use with Co. They also discussed the use of phenolic-resin adhesion promoters. They describe the various techniques that have been developed to study the rubber-brass interface and its aging mechanism. 

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. 

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). 

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]. 

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