Rubber to brass

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). 

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. 

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. 

An important additive to NR compounds used for bonding rubber to brass is a cobalt salt, such as cobalt naphthenate. While this additive improves the stability of the rubber-brass bond, especially in a corrosive environment, it exerts a negative effect on the stability of the rubber network, in that it accelerates reversion phenomena especially in the presence of oxygen and at elevated temperatures. 

Figure 6.3 Schematic of updated mechanism for bonding rubber to brass...

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. 

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. 

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. 

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.

The type of materials to be joined, i.e. wood to wood, steel to steel, brass to brass, rubber to rubber, etc, or wood to steel, rubber to brass, and so on. In cases where dissimilar materials are to be ponded the peculiarities of each Imust be allowed for. 

---