The fundamental aspects of adhesion of brass plated steel cord to rubber compounds. (Tech Service).

Since the advent of steel belted radial tires in the U.S. in the early 1970s, the importance of adhesion of rubber to steel cord cannot be overestimated. It was my good fortune to be in the middle of the research and development effort at B.F. Goodrich at that time. My main project was to develop a rubber compound that would adhere to the brass plated steel cord during tire performance. After numerous laboratory and experimental tire programs, we finally found a rubber adhesion compound that would do the job. Pilot plant production of radial truck and passenger tires was initiated. This was the beginning of the production of millions of steel belted tires.

In the following years much fine tuning was done to optimize the performance characteristics of the steel coat compound. Slowly the mechanisms of steel cord adhesion were becoming clear. Many people were involved in this effort, including raw material suppliers, academicians and others. I will address some of the fundamental chemical details of the adhesion process.

It had been known for many years that sulfur was an effective method of adhering brass to natural rubber, as well as being used as the vulcanizing agent for the rubber. However, the adhesion was sometimes spotty and degraded easily under certain conditions. The exact mechanism causing this adhesion loss was unknown. Examination of failed laboratory specimens of brass plated steel cord embedded in rubber occasionally exhibited bare steel cord with a brassy or steely color and the rubber compound stripped from that cord exhibited a glossy multicolored film on the failed rubber surface. This indicated a film of varying thickness. Analysis of this film demonstrated that sulfur and copper were present during the adhesion reaction. The film was essentially copper sulfide. This copper sulfide formed a weak, brittle bond with the underlying brass plated steel cord and was deleterious to good adhesion. The key to good adhesion was the formation of the rubber, sulfur, copper bond with little or no formation of copper sulfide.

The desired chemical reaction taking place at the rubber/ brass plated steel cord interface during the tire vulcanization may be considered a mixed ionic, covalent chemical reaction with the rubber carbon sulfur linkage being a relatively slow covalent type reaction and the sulfur copper linkage being a fast ionic type reaction. To balance these two reaction rates to give the desired carbon-sulfur-copper linkage proved to be a formidable challenge. It was extremely important to have the proper concentrations of ingredients and catalyst present at the rubber/brass interface; the ingredients being rubber, sulfur and copper and the catalyst being water. The control of the water concentration was critical. Moisture catalyzes the copper sulfide reaction.

Water concentration

Raw materials present in the rubber compound used to coat the steel cord can introduce moisture. During tire manufacture, this normally doesn't occur during the mixing or the calendering operations since the processing temperatures encountered generally exceed the boiling point of water. These high temperatures evaporate most free moisture present. However, after calendering, the calendered steel cord can be exposed to high humidity conditions and absorb moisture. This is especially true if some of the raw materials in the rubber compounds are hygroscopic. Bonding agents and silica filters used to promote adhesion are among these materials. Storage of the calendered and cut steel cord fabric in rolls using air and moisture impermeable plastic liners can retard the moisture absorption to a large extent. Many raw materials in the rubber compound can be both moisture scavengers and moisture suppliers. Before incorporation into the compound they can scavenge moisture from the air. As indicated above, the rubber processing steps evaporate this moisture. However, during storage of the unvulcanized steel cord fabric, moisture scavenging can again occur. Moisture content can increase to the point that, at vulcanization temperatures, these materials actually supply moisture to catalyze the undesirable copper sulfide formation. Armed with this knowledge, steps can be taken to control the moisture concentration at the rubber/ steel cord interface. Moisture may also cause the loss of zinc (dezincification) from the surface or the brass plated steel cord. This can lead to exposure of an undesirably high concentration of copper. (See the section on copper concentration). Rapid turnover of calendered steel cord during tire assembly and vulcanization allows the moisture scavengers to inactivate the moisture during vulcanization. After vulcanization and during tire performance, moisture absorption into the rubber can also occur. This may result in dezincification and copper sulfide formation with consequent adhesion loss.

Sulfur concentration

Relatively high levels of sulfur were used in steel cord coat compounds, with high reinforcing filler levels, to obtain high compound modulus as well as to bring about the rubber/steel cord adhesion reaction. Since the sulfur will tend to migrate to the rubber/steel cord interface both during vulcanization and subsequent tire usage causing the sulfur to form copper sulfide, other metal ions such as cobalt, lead, nickel and zinc, which also react rapidly (ionic reaction) with sulfur, can be incorporated in the coat compound to preferentially react with some of the sulfur near the rubber/steel cord interface. The presence of cobalt sulfide can be demonstrated by the addition of cobaltous stearate to a sulfur containing white sidewall compound. After vulcanization, the normally white stock turned gray with black specks. This indicated the presence of black cobaltic sulfide. After aging in hot air, the exterior of the same sample became a light pink color while the interior remained gray. This demonstrated that the cobaltic sulfide near the surface of the sample had been changed to a cobaltous compound and/or cobaltic hexahydrate, both of which are pink. These observations indicate that the fast ionic reaction to form cobaltic sulfide will effectively reduce the sulfur concentration near the rubber/brass interface, thus diminishing the opportunity for copper sulfide formation. In addition, these cobalt ions may also displace some of the copper ions, thereby also disrupting the formation of the copper sulfide film.

Copper concentration

Steel cord manufacturers knew how to control the copper concentration in the brass plating on the steel cord. This is a complicated process requiring considerable expertise and control during the steel rod electroplating and wire drawing operations. Fortunately, brass, being a copper/zinc alloy, has the excellent dual characteristics of copper carrier and drawing lubricant. Brass is first electroplated on a small diameter steel rod that is then reduced to filament size by multiple drawing operations. Electroplating requires a bath containing the proper concentrations of zinc and copper ions. Since at a given voltage, copper will plate out preferentially before the zinc, the copper concentration is controlled by a complexing ion which temporarily deactivates some of the copper. This allows the copper and zinc to plate out simultaneously, but not equally, to form the brass coating. The thickness of the brass coating is carefully controlled by the voltage, the steel rod residence time in the bath, concentration of the above mentioned ions, the bath temperature, etc.

During the drawing operations, both the steel filament diameter and brass coating thickness are reduced. The brass coating reduction to a specified thickness is very important to obtain the proper amount of copper per unit of surface area of steel cord. The brass thickness is controlled to a very thin layer measuring in the submicron range. The bulk composition of the brass is now approximately two parts copper to one part zinc. However, this composition is not uniform within the brass. At the brass steel interface there is a preponderance of copper, and on the exterior surface of the brass coating the zinc dominates. Since the copper concentration in the brass at the steel cord surface is diluted by the zinc, the tendency for copper sulfide formation at the brass surface is diminished.

Although there are many other important factors to be considered in obtaining good adhesion that are beyond the scope of this article, it is reasonable to believe that these other factors all help maintain the proper concentrations of moisture, sulfur and copper at the rubber brass interface.

The above reactions may not be as simple as outlined, however, they do serve as a workable hypothesis to explain many of the phenomena encountered in achieving satisfactory and durable rubber/steel cord adhesion during steel belted radial tire performance.