Compounding for Brass Adhesion

Very few recent compound optimisation studies can be found in literature, as they are normally kept proprietary. It has been published that the type of sulphur has little effect on adhesion (pull-out) value. On the other hand, the adhesion increased with ZnO level and with the high-structure carbon black, the former because of the brass activation, the latter most likely because of the increased rubber modulus. Other factors that were identified as affecting the pull-out force were the mixing method and the cure time [30]. 

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Other types of coupling agents include 1,2-diketones for steel,27 nitrogen heterocyclic compounds such as benzotriazole for copper,28,29 and some cobalt compounds for the adhesion of brass-plated tire cords to rubber.30... 

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. 

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. 

Only very few recent papers have been published in which new development of substrate treatments were reported. Krone reported on the optimum brass copper content and plating weight for rubber adhesion [44] and confirmed data that were known previously. A more detailed study was published by Goryaev [45]. Several steel cords were prepared and their overall thickness, homogeneity and surface composition altered. The adhesion to a standard NR compound was determined. Among the findings were that ... 

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. 

After removal from the press, the mold or picture frame sandwich is allowed to cool before opening (using brass implements). It is, therefore, useful to have several picture frame molds of comparable dimensions. The best precision in physical property measurement is obtained by allowing the molded sheet to relax at room temperature for a set period of time, often 24 hours. After opening the mold, the sample should be observed for evidence of surface bleed from excess external lubricant. (With compounds intended for metal adhesion, it may be necessary to use an external mold release coating. Those based on FIFE are effective with vinyl compounds and tend not to interfere with subsequent testing. Silicone-based mold releases, if used, should be of the type that is heat-cured to the mold, rather than migratory.)... 

Tire Cord. Melamine resins are also used to improve the adhesion of mbber to reinforcing cord in tires. Textile cord is normally coated with a latex dip solution composed of a vinylpyridine—styrene—butadiene latex mbber containing resorcinol—formaldehyde resin.. The dip coat is cured prior to use. The dip coat improves the adhesion of the textile cord to mbber. Further improvement in adhesion is provided by adding resorcinol and hexa(methoxymethyl) melamine [3089-11 -0] (HMMM) to the mbber compound which is in contact with the textile cord. The HMMM resin and resorcinol cross-link during mbber vulcanization and cure to form an interpenetrating polymer within the mbber matrix which strengthens or reinforces the mbber and increases adhesion to the textile cord. Brass-coated steel cord is also widely used in tires for reinforcement. Steel belts and bead wire are common apphcations. Again, HMMM resins and resorcinol [108-46-3] are used in the mbber compound which is in contact with the steel cord to reinforce the mbber and increase the adhesion of the mbber to the steel cord. This use of melamine resins is described in the patent Hterature (49). 

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. 

The initial polishing is done to get better adhesion. Decorative articles are polished with a buffing wheel with an abrasive compound such as jeweler s rouge (FejOj) on it. For industrial applications where good adhesion is needed, the surface must be roughened much more. This is done by using a compressed air blast of sand or aluminum oxide on iron and steel or ground walnut shells on brass and bronze. 

Phenolic novolak resins and high styrene SBR resins are used for reinforcing and increasing the hardness and modulus of rubber compounds. Resorcinol novolak resins are used as a part of the adhesion system between rubber and brass plated steel cord or organic fibers. Both phenolic novolak and resorcinol novolak resins require the addition of a methylene donor such as hexamethoxymethylmelamine (HMMM) or hexamethylenetetramine (HMTA) to fully crosslink and become a thermoset. Phenol, alkyl phenols, and resorcinol can be reacted in bulk or in a polymeric formulation with methylene donors. Typical donors are 2-nitro-2methylpropanol (NMP), HMTA, and HMMM, used to produce a thermoset resin network in the... 

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. 

The bonding characteristics of the silane films are not strongly dependent on the compound formulation, as distinct from the performance of brass. Even low sulphur and semi-EV systems work well. Hence, with this system, the compound of the skim stock can be optimised for mechanical, fatigue and tear resistance properties rather than for adhesion, as is currently done. This reformulation can, therefore, lead to improved tyre performance. 

Steel cord has been the main reinforcing material for tyres, hoses and conveyor belts for many decades, indeed the first steel reinforced tyres appeared over ninety years ago. However, it was not until the emergence of radial tyres that steel cord became a common form of reinforcement and understandably the adhesion between brass-coated steel cord and rubber compound became a significant factor governing the performance and durability of car and truck tyres. Therefore, it is necessary to achieve a high level of adhesion and sustain this level throughout the service history of the tyre. 

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 generation of strong adhesive forces can be considered to result from chemical interaction between the metal and NBR. This generally results from the reaction of the metal with sulphur compounds and carboxyl compounds to give their metal salts, which have a high bond energy. Polysulphide reactivity is high for copper and brass plates. 

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. 

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... 

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