Changing tyres – natural rubber
Dr Stuart Cook, Director of Research, at the Tun Abdul Razak Research Centre in Brickendonbury, UK, discusses how natural rubber can meet the demands of the modern tyre.
Known to man for over 3,500 years, natural rubber (NR), chemically 1,4-cis-polyisoprene, is perhaps the most well established renewable polymer raw material. It has remained essentially unchanged since its arrival on the industrial scene nearly 200 years ago.
Regarded as one of the world’s four most important industrial raw materials, NR accounted for about 43% of the global elastomeric polymer market last year. With close to 10Mt of NR being tapped from the Hevea brasiliensis tree (the primary source of NR) in 2008, it is the single most common industrial elastomeric polymer. The significance of NR is often a surprise to many who regard it as a polymer in decline, having largely been replaced by synthetic equivalents. However, its global market share has steadily risen to its current level from a low point in the mid 1970s of about 30%. The largest consumer of NR is the tyre industry, which uses about 65% of the material produced.
Often overlooked by the consumer, tyres are high performance products, highly sophisticated in their design and use of materials. Their many components require different characteristics from rubber compounds.
For example, a car tyre tread has a road contact patch roughly equivalent to that of a human footprint and is responsible for keeping a car on the road and stopping it in wet or dry conditions, in remarkably short order when required. The tread may have to perform for a lifetime of nearly 50,000 miles.
The tyre sidewall is only a few millimetres thick yet supports the weight of the vehicle, withstands high speed corning forces and flexes up to 80 million times during its lifetime without cracking, notwithstanding the occasional encounter with a kerb. A tyre’s inner liner, while also needing to flex continuously, has to keep air contained for weeks at a time without the need for topping up.
Natural rubber is a minor, yet significant component in a modern car tyre and is critically important in larger versions for trucks, earthmovers and aircraft. The very high molecular weight of NR and its ability to strain crystallise impart high strength and tear resistance, compared to the standard synthetic tyre rubbers, butadiene rubber (BR) and styrene-butadiene rubber (SBR). These attributes, coupled with low energy losses, or hysteresis, mean that larger tyres, which generate a significant amount of heat in service, rely upon NR to function.
Resistance is futile
The traditionally conservative tyre industry is entering a period of change not seen since the introduction by Michelin, headquartered in Clermont-Ferrand, France, of the so-called ‘Green Tyre’ technology in 1992. Michelin’s innovation resulted in a significant reduction in rolling resistance without loss of either wet grip or wear performance in car tyres.
This step change in performance was achieved by using tyre tread compounds based on blends of BR with the then newly developed solution polymerised SBR. The technology used reinforcing silica filler and silane coupling agent to disperse the silica and then chemically bond it to the rubber. In Europe, this technology has since been adopted by all major tyre manufacturers and now dominates the original equipment passenger tyre market.
This year, new minimum standards of rolling resistance for improved fuel economy, wet grip and noise were set in Europe for tyres. Similar requirements are anticipated in the USA. Furthermore, the recent awakening of the consumer’s consciousness to environmental issues has prompted tyre companies to establish their ‘green credentials’, often with the increased use of materials derived from renewable resources. Although NR is an obvious renewable raw material, on its own it is unable to meet one key requirement of today’s car tyres – good wet traction.
When the use of NR was facing decline, work in the 1970s and early 1980s at the Malaysian Rubber Producers’ Research Association, a forerunner of the Brickendonbury-based Tun Abdul Razak Research Centre (TARRC), sought to develop chemically modified forms of NR. The most successful modification proved to be epoxidation.
Epoxidised natural rubber (ENR), known commercially as Ekoprena, was produced from NR latex under acid catalysed conditions using hydrogen peroxide. The introduction of epoxy groups was found to occur randomly along the polymer chain while retaining NR’s high stereoregularity.
As well as increasing the polarity of the polymer, epoxidation raised its glass transition temperature (Tg) by about one degree Celsius for every mole per cent epoxidation. Crucially, below about 55 mole per cent epoxidation, ENR retained the ability to strain crystallise.
Another key characteristic of ENR was its ability to interact strongly with silica filler, which, unlike synthetic tyre polymers, did not require the action of a coupling agent. The optimal epoxide content for ENR for tyre use was found to be 25 mole per cent, which gave a material with a similar Tg to that of modern day solution SBR polymers. Silica-filled ENR-25 tread compounds provided both significantly reduced rolling resistance and improved wet traction compared with contemporary standard carbon black-filled tread compounds.
There are several reasons why tyres containing silica-filled ENR-25 are not used extensively today. Technologically, the case was proven by 1987, but at that time the price of oil was relatively low, ENR-25 was more expensive than synthetic alternatives and environmental issues did not have the profile they do today. Furthermore, Michelin’s ‘Green Tyre’ technology entered the market five years later and changed the direction of tyre technology.
The car in front
Over the past three years, the tyre industry’s perception of ENR has, however, changed. The material has been shown to be every bit as advanced as the latest synthetic tyre polymers. As a result of recent work at TARRC, the technical advantages of silica-filled ENR-25 tread compounds are now more fully understood.
The strong interaction between ENR and silica, which is responsible for the low hysteresis exhibited by silica-filled ENR-25 compounds, is significantly influenced by hydrogen bonding. Low levels of adventitious hydroxyl functionality (less than one mole per cent), introduced during the commercial latex epoxidation process, are believed to be largely responsible for hydrogen bonding to silica.
Moreover, the bonding can be moderated in a fully reversible manner by water. A theory is that, in wet conditions, the interaction between silica and ENR-25 is disrupted sufficiently in the tyre’s surface layer to increase hysteresis, thereby improving wet grip. However, as soon as the tyre-road interface dries out, hydrogen bonding between the rubber and the filler is restored. This type of behaviour is not possible if the silica filler is permanently bonded to the rubber.
A further consequence of the high level of interaction between ENR-25 and silica is illustrated during the mixing process whereby ingredients, including the filler, process oil, antioxidants and curatives, are distributed and dispersed within the rubber matrix prior to building and curing the tyre. Improved silica microdispersion has been found in ENR-25 compounds compared with synthetic compounds containing silane coupling agents. Moreover, good microdispersion has been invoked as one of the factors necessary to achieve good wear in a tyre compound.
A significant endorsement of ENR has come from the development by Sumitomo Rubber Industries (SRI) in Japan of its ENASAVE range of tyres. The latest of these, the ENASAVE 97, increases the proportion of non-petroleum derived resources used in its construction to 97%, from about 44% for a standard tyre. The use of ENR in the tread, sidewall and inner liner helps achieve a 35% reduction in rolling resistance and to improve wet skid performance compared with a standard SRI tyre.
Other new modified forms of NR have also been developed. Pureprena is a deproteinised version with superior performance in engineering product applications. Natural rubber-based thermoplastic vulcanisates with exceptionally good recovery properties have also been created and similar materials based on ENR show excellent oil resistance and heat ageing behaviour. Although NR has been around as an industrial raw material for almost 200 years in Europe, it still has much to offer.
Further information: Dr Stuart Cook