Last week I wrote about the various options for dealing with the world’s waste tyres. We can recycle them, grind them up into asphalt, or burn them for energy. But clearly, the easiest solution is stop generating a mountain of 1.5 billion waste tyres every single year.
In a circular economy, the basic philosophy is that there are two kinds of sustainable materials – technical and biological. Technical materials are metals, polymers and plastics, which should be designed to be recycled. Biological materials, such as wood, paper, or bioplastics, can be returned to the natural environment and composted. Could we make tyres that meet these principles?
First of all, is it possible to make a biodegradeable tyre? Instead of wracking our brains to think of uses for all those waste tyres, we could safely rot them down. Obviously they would need to degrade in the right conditions, and not while in use. But I have a pair of trainers with biodegradeable soles, so it might be possible.
Tyre companies have certainly experimented with organic materials. The Handbook of Biodegradeable Polymers mentions Goodyear’s BioTred, a starch which can be used as tyre filler. Pirelli adds ash from rice husks into their mix. All sorts of alternative synthetic and natural rubbers are being developed, helping to wean the tyre industry off fossil fuels. Continental and others are developing a natural rubber made from dandelions which – seriously – looks likely to be a mainstream source of rubber quite soon. But even if some of these ingredients are biodegradeable, they’re not once they’ve gone into the product itself.
Biodegradeable tyres don’t look possible right now. Let’s look at the technical materials approach instead. Is anyone producing an entirely recyclable tyre that could be melted down and used to make new tyres? I’m not aware of anything on the market, but there is research going into it. Last year scientists at Texas A&M developed a new synthetic rubber made from a refinery by-product called cyclopentene. This is a material that degrades and could thus be reused. Even that is theoretical though. The closest we have in actual use is the rubber powder produced by Lehigh Technologies. They grind up used tyres into very fine powder which can be put back into new tyres, but it can only make up 5-7% of a new tyre.
So that’s a no on both fronts really, but it doesn’t mean a sustainable tyre is completely out of reach. The challenge gets easier if we allow ourselves to think beyond the traditional tyre format. If we can put down the usual pneumatic model of stiff outer layer and softer inner tube, other options emerge.
Around ten years ago Michelin developed a hybrid tyre/wheel called the Tweel. It has no inner tube, so it can’t get punctures. Instead, it relies on strong but flexible polyurethane spokes in triangular arrangements. NASA set Michelin to work providing these for the Mars rover, and you can now get them for fork lift trucks, golf carts and a few other specialty vehicles.
The airless tyre looks like the future to me. You’d never get a puncture, and wouldn’t need to check your pressure. The military is interested because they can’t be shot out. And – at last – they can be made with recyclable materials. In 2011 Bridgestone released a similar tyre to the Tweel and tested it on mobility scooters. It’s made of thermoplastic resin that can be processed and reused, and it’s part of Bridgestone’s long term goal to find a cradle-to-cradle solution for tyres. Earlier this year they released images of an airless bike tyre, and they expect to bring it to market for 2019. If it happens, I’ll be fitting them on mine.
Finally, and most radical of all, we have Michelin’s new concept tyre. It was released in June this year and combines both approaches mentioned earlier, being both recyclable and biodegradeable. It’s airless, its biomimetic structure inspired by coral. It’s durable enough to last the lifetime of the vehicle, and then it can be recycled to make a new tyre. The biodegradeable part is the tread, which will wear off with use. When it does, it can be 3D printed back on without taking off the wheels.
It looks like these new varieties will eventually take the place of pneumatic tyres – eventually. If so, then we might get some way towards a tyre that is circular in both senses of the word.