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
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
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
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
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
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.
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.