Where rubber meets road

C&I Issue 3, 2008

Since Scotsman Robert Thomson first patented his ‘aerial wheel’ in 1845, a year after Charles Goodyear’s invention of vulcanised rubber, the pneumatic tyre has undergone continuous development. In Thomson’s design, a  number of thin inflated tubes were encased in a leather cover in a tyre that could offer different ride performance by varying the inflation of the individual tubes, and took more than one puncture to fully deflate.

Experiments with horse-drawn carriages in Regents Park, London, showed that they were easier and quieter than carriages with standard wheeled carriages, but it took until 1888 for John Dunlop to unveil his rubber pneumatic tyre for bicycles. However, although the final part of the puzzle had to follow in 1890 when the wheel rim and tyre outer with inextensible lip was developed by C K Welsh to overcome the problem of access to the inner tube and tyre replacement as the tyre was stuck to the wheel.

In 1895, Andre Michelin became the first user of pneumatic tyres on automobiles, and in 1911, Philip Strauss developed the first successful tyre with an air-filled inner tube, some eight years after  P W Litchfield of the Goodyear Tire & Rubber Company patented the first tubeless tyre. An innovation before its time, the tubeless tyre was not commercially exploited until much later in 1954. The first synthetic rubber tyres were invented by Goodyear in 1937 using a patented material called Chemigum.

With the move from cross ply to radial tyres, and more recently various low profile, run-flat and other high performance products, tyre producers have increasingly come to rely on their raw material suppliers to help them serve their customers in the original equipment market, and the consumer replacement sector, which is estimated to be twice the size of the original equipment market.

While the consumer generally assumes it is the profile of the tyre that affects its characteristics, every component from the side wall, the tread to the inner liner, plays a part in the tyre’s overall performance, points out Joachim Grub, head of the polybutadiene rubber (PBR) business unit in the performance polymers segment at Lanxess.

The side wall of the tyre must be deformable with heating up, thereby increasing fuel consumption, while the tread must be soft enough to provide the best grip, but sufficently tough to give good service life. The carcass must have good adhesive properties when combined with the commonly used polyamide fibres and steel reinforcement that give the tyre its strength. To eliminate constant pressure checks and pumping up, the inner liner of the tyre must be impermeable to the air that inflates the tyre.

And even the air in the tyre has a role to play. According to the Environmental Transport Association, for every 6psi a tyre is under-inflated, fuel consumption rises by 1%, without accounting for any loss of tyre performance. Nitrogen, for example, is now being used as a replacement for air since  pressure loss through permeation of the air is reduced, the tyre does not heat up so much, and tread life can be increased.

To achieve the best balance of all these requirements, tyre developers focus on three key objectives: wear, wet grip and rolling resistance, known as the ‘magic triangle’. In the past developers wishing to improve one aspect, be it wear or wet grip, would have to compromise on the other aspects, explains Werner Obrecht, rubber expert at Lanxess’ Technical Rubber Products business unit.

‘Boosting tyre service life without losing other key tread properties was a major challenge for the rubber industry. Depite all the positive developments of the past 20 years, after traveling around 40 000km, the tread pattern is normally worn and new tyres are then needed.’

Lanxess’ new nanoparticle rubber additive Nanoprene gets around this problem by via special ‘anchor points’ on the particle surfaces, designed to attach to silica and silanes used in modern tyres to cut rolling resistance and improve grip (see box below left). Adding Nanoprene to a tread mix containing silica therefore improves wear resistance and grip, while keeping roll rsistance at a low level, Obrecht explains: ‘The magic circle is therefore extended.’

The first products containing the additive are already on the market and claim to boost tyre wear resistance by up to 100%.
  In terms of weight, roughly 25% of a tyre today comprises synthetic rubbers, and this percentage is increasing, according to Grub. And these synthetic rubbers can be tailored to their application as they determine tyre performance at the molecular level.

‘Basically, you can’t have an automobile tyre without butyl rubber,’ Grub says, but in terms of performance ‘conventional simple polybutyl rubber [tyres] are at the bottom of the performance pyramid, he adds. The change to tubeless radial ply tyres has driven the demand for halobutyl rubbers, which now represent the biggest, and above average, share of Lanxess’ range of butyl rubbers. Demand for butyl rubbers is growing fastest in Asia-Pacific, due in part to the transfer of tyre production to low-cost countries and rapidly increasing demand in China and India.

In terms of performance, however, emulsion styrene-butadiene rubbers (emulsion-SBRs) and neodymium polybutadiene rubbers (Nd-BRs) offer enhanced properties. And replacing emulsion-SBRs with solution-SBRs gives increased elasticity at low temperatures as well as improvements in terms of wet grip. The linear structure of Nd-BRs also offers improved elasticity, thereby reducing rolling resistance and improving fuel economy.

Silicas lower roll resistance
Performance can also be improved by changing the filler material used in tyres. Silica is replacing carbon black and giving better wear resistance and lower rolling resistance. According to French chemical producer Rhodia, high performance silicas can reduce rolling resistance by up to 25%, generating fuel savings of more than 5%. Silica also strengthens the rubber network. The latest developments involve the use of biodegradable bioplastic to replace the carbon black and silicas. At the recent K2007 plastics trade fair, Italian plastics firm Novamont unveiled a tyre jointly developed with Goodyear using BioTred technology, based on corn starch.

Market movements
The global tyre market has been growing steadily for a number of years at around 3%/year, according to Joachim Grub, head of the polybutadiene rubber (PBR) business unit in the performance polymers segment at Lanxess. ‘However, it is also a market that is very much in flux,’ he adds. ‘Production of standard tyres is increasingly being relocated from traditional manufacturing countries like the US to Mexico, South America and China.’

An example of this trend for restructuring is the recent decision by French tyre manufacturer Michelin to close one of its French plants in Toulouse and reorganise its Spanish plant in Lasarte to produce high performance motorcycle tyres and end car tyre production by end-2008. Michelin did say, however, that it would continue to invest in Western Europe.

In terms of sales, the NAFTA countries (Canada, Mexico and the US), Western Europe, Japan and China are currently similar, although the strongest growth is now being seen in Greater China, including Taiwan and Hong Kong, which is the fastest growing tyre market globally with an average rate of 5.3%/year. India has a higher growth rate, but is starting from a much lower level, says Grub.

Tyre production is also rising in Eastern Europe, contributing to a growth rate of some 2.2%/year for Europe, the Middle East and Africa.

Grub believes that in the medium term, there is unlikely to be any significant change in the dominant market position of the three leading producers: Bridgestone, Michelin and Goodyear. ‘This will be true even if the balance among those in the leading group shifts slightly as Goodyear continues to restructure to increase profitability and Bridgestone initiates an ambitious programme of investments to increase the gap with its competitors,’ notes Grub. There is a considerable gap between these three companies and the next largest producers: Continental and Pirelli, but these producers are all likely to come under pressure from Asian competitors. Korean-based Hankook and Kumho have said that they intend to break into the top five, while significant new production capacities will be built in Greater China, according to Grub. Despite this capacity expansion, he believes there is no apparent risk of falling prices due to overcapacity.

This boost in Chinese capacity is based more on the standard tyre sector, and while high performance tyres are currently still more likely to be produced in Europe, the Americas and Japan, in the medium term the products from China will also have to meet higher quality requirements in terms of fuel economy and optimal safety, says Grub. The higher quality tyre sector is currently growing a 9%/year, and by 2010, high quality and high performance tyres are expected to hold equal shares of the overall market.

This trend towards higher quality tyres is driven by demands for lower fuel consumption, better driving characteristics and greater interest in envionmental protection, which all focus of the development of wear-resistant products.

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