Bonding brass process

The brass process makes use of the fact that electrodeposited brass of certain composition and crystal structure is capable of directly bonding rubber mixtures, providing their composition is compatible with the adhesion process [77]. Whereas today the brass process is of only minor significance for the production of rubber-metal composites, it is widely used in tire manufacture for bonding the bead wires and for bonding the carcass mixture of steel cord. 

Certain polyisocyanate adhesives allow bonding of rubber to metal with far less technical effort than by the brass process. However, polyisocyanates are extremely sensitive to atmospheric moisture and react with compounds containing OH and NH2 groups, such as antiagers. This reaction leads to a weakening of adhesion. 

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). 

There has been increasing interest in the process of electro-plating plastics. Plating can produce chromelike, brass, silver, gold, or copper surfaces in both smooth and textured forms. There are several systems available commercially for plating plastic materials. In the case of certain plastics such as electroplated ABS, it can be surface-treated chemically to promote bonding of the metals in subsequent steps. 

The bond is not unduly sensitive to influences such as humidity, low mix viscosity and low vulcanizing pressures. The main drawbacks are connected with the plating process, which requires considerable investment and expertise. Brass plating has now been largely superseded by the use of proprietaiy bonding agents, except in the tyre industry, where bonding to brass-coated steel cords is still undertaken. ... 

Rubber to metal primers contain organic resins which react with most metal (steel, aluminum, stainless steel, copper, brass) surfaces during the vulcanisation process to form a chemical bond to the metal. They also contain polymers which allow for better film formation and act as an anchor for the subsequent application of the adhesive. 

Brass only bonds by a unique, self-catalysed process (details are discussed later in this chapter), if the rubber has a high degree of unsaturation in practice, this limits its use to natural rubber (NR), synthetic isoprene rubber (IR), styrene-butadiene-rubber (SBR) and butadiene-rubber (BR). 

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 process can be used on regular existing steel tyre cords and would then improve the bonding of brass and would also bond the cut edges of cords, which are frequently the locus of crack initiation leading to belt edge separation. 

Some miscellaneous publications are summarised here. It was reported by Matyukhin that the bond between brass-plated tyre cord and rnbber can be broken electrochemically [65]. The process results in a growth in the interfacial layer of zinc hydroxide which dislodges the sulphide film. The method appears useful for recycling purposes, but it was also reported to be useful for assessing the strength of the cord-rubber bond. 

The other major development that needs to be recapped here is that of silane systems for bonding a wide range of metals to a wide range of compound types and formulations. This process may become a panacea for bonding in many industries, but much more work will have to be done before the process can replace the brass-coating process for bonding rubber steel to tyre cords. 

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