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Transport Title

Stronger, longer-lasting tyres

   
 
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This project combined the industrial know-how of a leading tyre manufacturer, a major chemical company and the specialised expertise and technical capacity of two scientific institutes.
Together, they were able to develop an innovative coating for the filaments of the steel cord used as reinforcement in car and truck tyres. Road testing of prototype car tyres has demonstrates superior steel to rubber adhesion and associated improvements of tyre durability.
In time, these will produce a new generation of safer, longer-lasting tyres. Pirelli is currently engaged in the controlled market testing of truck tyres which incorporate the new technology.

Beneath the rubber tread of a car tyre is buried 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 per cent of overall tyre weight.
On the hot, rough roads of Eastern Turkey, tyres equipped with a new kind of steel cord are being tested by truck fleets in some of the harshest conditions anywhere in the world. Pirelli, Rhône Poulenc Chimie and its two scientific partners in the BRITE-EURAM project EUROTYRE are confident that the tests will demonstrate the superiority of the new wire/rubber adhesion system. Prototypes have already shown improved strength, durability and corrosion resistance.

New coating for permanent bond

But the particular focus of the project was in fact a critical characteristic of the steel cord, its adhesion to the vulcanised rubber of the tyre tread.
Adhesion has traditionally 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. 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.
Pirelli, which coordinated the project, had in the late 1980s developed together with Dr. Van Ooij, the world's leading expert in wire/rubber adhesion, 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. The new cord produced enhanced wire-rubber adhesion performance, but proved to be impossible to manufacture on an industrial scale.
The current project aimed to build on Pirelli's earlier work by mobilising the complementary expertise of a 'hand-picked' development team. The University of Nottingham brought specialist knowledge in the field of corrosion and plating, INFM advanced surface analysis techniques, and Rhône Poulenc Chimie (the world's largest wire lubricant supplier) the capacity to design and develop new lubricants.

Scientific breakthrough

Overturning the earlier results, which had been accepted as the state-of-the-art, detailed scientific investigation showed that the nickel in the outer layer of Pirelli's coating contributed neither to the drawing process nor to the adhesion mechanism. Surface analysis using scanning auger electron spectroscopy and scanning electron microscopy revealed that the nickel is largely stripped away during drawing, with what remains driven into the softer zinc cobalt phase.
Nor was nickel found to be necessary for adhesion. A single zinc cobalt layer, easier to draw than nickel, 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.
That work was not without its difficulties, even after the project's original plan, based upon modification and industrialisation of the two layer concept, had been abandoned.
Drawability at industrial speeds and efficiencies was particularly hard to achieve. The partners found that industrial conditions could not be satisfactorily reproduced in the laboratory, which meant that the evaluation of drawing lubricants all had to be transferred to Pirelli's pilot plant.
Despite these early setbacks, the project was eventually completed on schedule. Three tonnes of the new steel cord had been produced, and 110,000 kilometre road tests of prototype car tyres had confirmed the excellence of the new wire-rubber adhesion bond, although they also showed that further work was needed to improve the rubber compound to match the improved fatigue performance of the new steel cord/rubber bond.

Patented product

Compared with the benchmark provided by the conventional brass coating, the new system offers significant cost and performance gains. Initial adhesion is at least equal to that of brass, and resistance to 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. In addition, the new coating process uses 20 per cent less energy, and is 30 per cent cheaper, than current methods.
Pirelli has patented the coating, which it considers to be world-leading. Confident that it is ahead in the race for a more durable product, the company is happy to share the project's scientific results with its competitors, and has already published prominent articles in several technical and scientific journals.
Despite its success, however, the new technology is unlikely to be used in car tyres for some years. Like the car industry itself, tyre manufacturers have adopted the 'platform' concept. Instead of making piecemeal improvements to individual product lines, new methods, materials and designs are introduced with an all-new platform, and are then adopted right across a product range. Pirelli will use the new steel cord in the next generation of top-end car tyres, which will eventually replace the current P6000 and PZero series.
A full market launch may come later in the truck tyre market, where the cost savings can be expected to give a bigger 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 by Turkish truck drivers.

Industrial spin-offs

The project's other partners are also reaping rewards for their work. Rhône 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. Applications for this simple, corrosion-resistant and highly deformable coating are also expected in the domestic 'white goods' market.
INFM, which had never before taken part in an industrial collaboration, has learned a lot from its participation in the project, and will continue to work closely with Pirelli, to whom it subsequently sold the surface analysis equipment used in the project.

 

Project Title:  
Improved tyre safety and life by a new wire/rubber adhesion system

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: BE-4215

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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