Buried beneath the rubber tread of a car tyre is a reinforcing belt of steel cord, produced from coated steel filaments as thin as a quarter of a millimetre across. In a truck tyre, steel is also used in the tyre walls and in the bead which holds the tyre to the wheel rim, and may constitute as much as 25% of overall tyre weight.
The focus of the Euram project is on a critical characteristic of the steel cord its adhesion to the vulcanised rubber of the tyre tread. Traditionally, adhesion has been achieved by the electro-chemical application of a brass coating prior to the fine drawing of the steel filaments. As well as bonding the steel to the surrounding rubber, the brass acts as a lubricant in the drawing process. However, both brass-steel and brass-rubber interfaces are susceptible to corrosion. In the past, corrosion resistance has only been improved at the expense of performance (due to the non-uniform mechanical structure of steel cord), by twisting the filaments together in such a way as to create a less permeable but mechanically inferior cord.
BETTER WIRE-RUBBER ADHESION
In the late 1980s, Pirelli developed a new wire-rubber adhesion technology based on a coating consisting of an inner layer of zinc cobalt and an outer layer of nickel zinc. It improved wire-rubber adhesion performance, but proved impossible to manufacture on an industrial scale.
The current project built on Pirelli’s earlier work by bringing together the complementary expertise of a ‘hand-picked’ development team. The University of Nottingham brought specialist knowledge in the field of corrosion and plating, INFM brought advanced surface analysis techniques, and Rhone Poulenc Chimie the capacity to design and develop new lubricants.
Detailed scientific investigation showed that the nickel in the outer layer contributed neither to the drawing process nor to the adhesion mechanism. Surface analysis revealed that the nickel is largely stripped away during drawing what remains is driven into the softer zinc cobalt phase.
Nor was nickel found to be necessary for adhesion. A single zinc cobalt layer was shown to have both adhesion and corrosion resistance properties equal to the earlier two-layer coating, and formed the basis of all subsequent development work.
Road tests of prototype car tyres has confirmed the excellence of the new wire-rubber adhesion bond. Compared with the conventional brass coating, the new system offers significant cost and performance gains. Initial adhesion corrosion is 50 times better, reducing the possibility of adhesion loss during ageing, and allowing a return to cord construction methods which give better compression and extension mechanical properties.
Pirelli has patented the coating, and will share the project’s scientific results with its competitors. Pirelli will use the new steel cord in the next generation of top-end car tyres.
In the truck tyre market, the cost savings can be expected to give a big competitive advantage. Truck tyres incorporating the new steel cord in four belts and in the tyre walls have already been built, and are currently undergoing carefully monitored destruction testing.
For its part, Rhone Poulenc will market the newly developed thermally stable and environmentally friendly lubricant, which allows filament drawing without die wear. Nottingham University is applying its work on zinc and zinc-based coatings to car bodies, in collaboration with a major motor manufacturer. INFM, which had not taken part in an industrial collaboration before, will continue to work closely with Pirelli, to whom it subsequently sold the surface analysis equipment used in the project.