Scientists in the US have perfected the conversion of rubber tyre waste into high quality graphene.
The method works on both rubber tyres and the black carbon leftovers from tyre processing. ‘You take carbon material and flash it between two electrodes,’ says chemist James Tour at Rice University in Houston, Texas. ‘The high voltage will break every bond and the non-carbon elements will sublime out, even the aluminium and silicon.’ The material reaches around 3000°C in less than half a second.
The broken carbon-carbon bonds rearrange into high-quality graphene. The team then added this to concrete and showed that 0.1% by weight of graphene could increase the compressive strength of concrete by 35%. ‘That means that you could use 35% less concrete, and concrete manufacture is one of the biggest CO2 emitters in the world,’ says Tour.
About 43% of scrap tyres in the US are used as fuel, 25% for ground applications such as concrete additions and 16% are stored in landfills, with the remainder in various civil engineering applications. Meanwhile, cement production swallows 2 to 3% of total energy use.
Rubber is a thermoset, meaning that it is not possible to heat it up and remould it, which makes recycling a challenge. The flash Joule heating process developed in Texas could up-cycle it into graphene sheets and then incorporate it into and replace concrete material. Tour’s lab made 1-5g at a time, but the process has been licensed to Universal Matter in Houston. ‘They hope to be operating at 100t scale within three years,’ says Tour.
The conversion process now reported would require less than $100 of electrical energy per ton of starting carbon. What is produced is ‘turbostratic’ graphene, a high-quality, multi-layered material. ‘There is no order between layers, which means it is easier to disperse in composites,’ says Tour, who contrasts this with graphene produced by graphite exfoliation, where layers are often stuck together quite strongly.
Universal Matter plans to serve concrete, composites, asphalt, lubricants, coatings and other markets with its graphene. Tour is a co-founder and scientific advisor