What’s green and black? – Advances in rubber for tyres

Materials World magazine
1 Jan 2009

Advances in materials science offer tyre manufacturers the chance to significantly reduce their environmental impact, making a product that is ‘greener’ in terms of manufacture and service. Ian Salusbury reports

The tyre has been called ‘the most complex composite product in mass production today’. Its core component of rubber is reinforced with steel and polymer cord, and particulates, which are protected by chemical additives.

When first developed in the second half of the 19th century, pneumatic tyres used vulcanised natural rubber (polyisoprene). A synthetic alternative was sought, partly to escape the control exerted by rubber producers. This came to fruition during the Second World War when the production of synthetic rubber became imperative.

A number of synthetic rubbers are now used in tyres, most prominently styrene-butadiene and polybutadiene rubber. They are combined with natural rubber to make best use of the different characteristics of these materials. The proportions vary according to the type of vehicle – for passenger cars, approximately 27% of the tyre’s weight is synthetic rubber while 14% is natural rubber. These percentages are reversed for truck tyres.

Natural rubber has also been improved, so tyres can now be made entirely from it and epoxidised natural rubber, which offers more resistance to hydrocarbon oils and improves damping properties. Dr Andy Chapman of the Tun Abdul Razak Research Centre, Hertford, UK, says there are significant environmental benefits of using natural rubber over petroleum-derived synthetic forms. He notes that the rubber tree (Hevea brasiliensis) is very efficient at sequestering carbon dioxide (CO2). A rubber plantation absorbs 16% more CO2 than the same area of rain forest. This tree requires little fertilizer, and plantations support a high level of biodiversity and provide livelihoods for around 20m people.

Chapman believes the tyre industry is changing its attitude to materials, ‘There is a general move towards renewable resources. Apart from the obvious use of natural rubber, there’s not a large amount of production yet, but I guess the tyre companies are thinking that the price of oil isn’t going to go down and, in the long-term, supplies are going to run out. There is also an increased awareness of environmental issues, in particular the increasing desire to use renewable resources’.

Carbon black

Replacing some or all of the carbon black filler in tyres is one option. Carbon black makes up 20-30% of a tyre’s weight and is derived from the heaviest fractions of petroleum. Reducing its use would generate considerable environmental benefits and cost savings.

Dr James Busfield of the Department of Materials at Queen Mary, University of London, UK, says one way to reduce carbon black may lie in adopting different forms of the material. ‘Historically, particles of medium thermal carbon blacks were big spherical blobs. The industry is moving away from those to hollowed out particles, which are dish shaped or cup shaped. You end up with a lot of surface area that the rubber can stick to, and you can make the surface more attractive for the rubber to adhere to, making it more reinforcing. So the stiffness goes up [and] a much smaller amount of material has been incorporated.’

Silica can also be substituted for carbon black. It is already used in the tread portion of tyres to improve wet grip while reducing rolling resistance, which in turn improves fuel economy. Busfield believes its benefits could be even greater, ‘Silica does make tyres a little heavier. But there are hollow silicas that are used for a range of electronics manufacture, such as thin film screens. Tyre companies are clearly going to look at these. You could fabricate a hollow ball of silica, mix it into the rubber and therefore lightweight it’. Reducing tyre weight further improves the fuel economy of the vehicle.

As it is a quarried material, silica’s environmental credentials are mixed. Although it is not a renewable resource, it is not derived from oil. Silica is cheap and available in large quantities.

Such benefits are also offered by nanoclays, such as sepiolite and montmorillonites, which also offer performance improvements. These clays have high shape factors and can provide improved resistance to gas permeation. However, their use presents other challenges. Busfield explains, ‘Their surface activity is nowhere near as good as carbon black, so you have to use a coupling agent, typical of the silane agents used for silica. Significant work will be required to make sure there is a good combination of coupling agents with these new organoclays’.

American company InMat offers a nanocomposite coating made up of nanoclays and a polymer dispersed in water, called Nanolok. Dr Harris Goldberg, President and Chief Executive Officer of the Hillsborough-based company, says its goal is to replace some of the butyl rubber used as inner liner, ‘We get the barrier properties of a millimetre of butyl rubber with somewhere between 10 and 40µm of coating’. This has already been commercialised for use in tennis balls, but has yet to be adopted by the tyre industry.

Food-based alternatives

Reinforcements derived from by-products of food production are being investigated by Dr Sabu Thomas of Mahatma Gandhi University in Kerala, India. ‘We’ve replaced 100% of the carbon black with coir, sisal, banana and oil palm fibres, as well as hybrids of different natural fibres,’ says Thomas. ‘We’ve used them in both natural and synthetic rubber.’ He believes coir fibre has the greatest potential for success, citing its good elasticity.

Another team led by Dr Lei Jong of the United States Department of Agriculture in Peoria, USA, has concentrated on wheat, corn and soy agricultural by-products, while others have used jute, hemp, flax and sisal, as well as more exotic natural materials such as spider silk. Fibre reinforcements offer a lower weight alternative to carbon black and they sequester CO2 as they are grown. Also, as they are derived from by-products of crops, they do not require any diversion of resources from growing food.

Although many natural reinforcements are still at the research stage, some renewable resource technology is already on the market. In 2001, Goodyear, headquartered in the USA, launched a tyre that uses a biopolymeric filler derived from corn starch, which they marketed under the name BioTRED. Goodyear claims the tyre delivers a five per cent performance increase over its predecessor in terms of fuel economy, due to its reduction in rolling resistance and tyre weight.

More recently, Japan’s Sumitomo developed a tyre using only three per cent petroleum-derived materials. The ENASAVE 97 incorporates vegetable oils and fibres, epoxidised natural rubber and silica. The company says its ambition is to develop a petroleum-free product.

Even more exotic materials such as aerogels or a rubber derived from resilin – the elastic protein which endows fleas with their prodigious jumping ability – may one day be used. In the short-term though, there seems to be plenty of materials available to give tyres a ‘greener’ shade.