Tyred out

Here’s my Materials Witness column for the September issue of Nature Materials. It springs from a recent broadcast in which I participated on BBC Radio 4’s Material World – I was there to talk about synthetic biology, but the item before me was concerned with the unexpectedly fascinating, and important, topic of tyre disposal. It seemed to me that the issue highlighted the all too common craziness of our manufacturing systems, in which potentially valuable materials are treated as ‘waste’ simply because we have not worked out the infrastructure sensibly. We can’t afford this profligacy, especially with oil-based products. I know that incineration has a bad press, and I can believe that is sometimes deserved; but surely it is better to recover some of this embodied energy rather than to simply dump it in the nearest ditch?

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In July it became illegal to dump almost any kind of vehicle tyres in landfill sites in Europe. Dumping of whole tyres has been banned since 2003; the new directive forbids such disposal of shredded tyres too. That is going to leave European states with an awful lot of used tyres to dispose of in other ways. What can be done with them?

This is a difficult question for the motor industry, but also raises a broader issue about the life cycle of industrial materials. The strange thing about tyres is that there are many ways in which they could be a valuable resource, and yet somehow they end up being regarded as toxic waste. Reduced to crumbs, tyre rubber can be incorporated into soft surfacing for sports grounds and playgrounds. Added to asphalt for road surfaces, it makes the roads harder-wearing.

And rubber is of course an energy carrier: a potential fuel. Pyrolysis of tyres generates gas and oil, recovering some of the carbon that went into their making. This process can be made relatively clean – certainly more so than combustion of coal in power stations.

Alternatively, tyres can simply be burnt to create heat: they have 10% more calorific content than coal. At present, the main use of old tyres is as fuel for cement kilns. But the image of burning tyres sounds deeply unappealing, and there is opposition to this practice from environmental groups, who dispute the claim that it is cleaner than coal. Such concerns make it hard to secure approval for either cement-kiln firing or pyrolysis. And the emissions regulations are strict – rightfully so, but reducing the economic viability. As a result, these uses tend to be capacity-limited.

Tyre retreads have a bad image too – they are seen as second-rate, whereas the truth is that they can perform very well and the environmental benefits of reuse are considerable. Such recycling is also undermined by cheap imports – why buy a second-hand tyre when a new one costs the same?

Unfortunately, other environmental concerns are going to make the problem of tyre disposal even worse. Another European ruling prohibits the use of polycyclic aromatic hydrocarbon oil components in tyre rubber because of their carcinogenicity. It’s a reasonable enough precaution, given that a Swedish study in 2002 found that tyre wear on roads was responsible for a significant amount of the polycyclic aromatics detected in aquatic organisms around Stockholm. But without these ingredients, a tyre’s lifetime is likely to be cut to perhaps just a quarter of its present value. That means more worn-out tyres: the current 42 million tyres discarded in the UK alone could rise to around 100 million as a consequence.

Whether Europe will avoid a used-tyre mountain remains to be seen. But the prospect of an evidently useful, energy-rich material being massively under-exploited seems to say something salutary about the notion that market economics can guarantee efficient materials use. Perhaps it’s time for some incentives?