Where the rubber really does hit the road
Tyres are a worldwide environmental problem of huge proportions but now a scientific team has discovered a way to change waste rubber into a strengthening agent for concrete, with the potential to eliminate millions of waste tyres from landfill.
Each year, the world discards more than a billion worn-out road tyres, and, because they are almost impossible to recycle economically, most of them are either burned or end up in landfill.
Given that global demand for rubber currently exceeds supply in a world where resources are scarce and increasingly limited, finding a way to recycle these tyres is one of the greatest scientific and ecological challenges of our age.
However, a group of scientists from the US has discovered a process that can turn this once useless rubber refuse into a resource that has continued use and benefit.
The process, detailed in a study published in the scientific journal carbon this week, converts waste rubber into a substance called graphene, which is an allotrope of carbon that forms in a single layer (a monolayer) of carbon atoms, tightly bound in a hexagonal honeycomb lattice.
Most importantly though, graphene when combined with concrete massively strengthens concrete at the molecular level.
The environmental benefits of adding graphene to concrete are clear, says chemist James Tour.
“Concrete is the most-produced material in the world, and simply making it produces as much as 9 per cent of the world’s carbon dioxide emissions,” James says.
While the majority of the billion tires discarded annually are burned for fuel or ground up for other applications, 16% of them wind up in landfills.
“Reclaiming even a fraction of those as graphene will keep millions of tires from reaching landfills,” James Tour says.
The process that changes rubber into graphene was developed in 2020 by James Tour and his colleagues at the William Marsh Rice University in Houston, Texas. It has been used to convert food waste, plastic and other carbon sources by exposing them to a jolt of electricity that removes everything except carbon atoms from the sample.
Those atoms reassemble into valuable turbo-stratic graphene, which has misaligned layers that are more soluble than graphene produced via exfoliation from graphite. That makes it easier to use in composite materials.
Rubber proved more challenging than food or plastic to turn into graphene, but the lab perfected the process by using waste rubber from tyres that had decomposed after prolonged exposure to heat.
After useful oils are extracted from waste tires, this carbon residue has until now had near-zero value, Tour said.
Tire-derived carbon black or a blend of shredded rubber tires and commercial carbon black can be flashed into graphene. Because turbo-stratic graphene is soluble, it can easily be added to cement to make more environmentally friendly concrete.
The Rice lab flashed tire-derived carbon black and found about 70% of the material converted to graphene. When flashing shredded rubber tires mixed with plain carbon black to add conductivity, about 47% converted to graphene. Elements besides carbon were vented out for other uses.
The electrical pulses lasted between 300 milliseconds and 1 second. The lab calculated electricity used in the conversion process would cost about $100 per ton of starting carbon.
The researchers blended minute amounts of tire-derived graphene with Portland cement and used it to produce concrete cylinders. Tested after 28 days, 0.1 per cent graphene produced an increase of at least 30 per cent in strength. And that’s very promising, says James Tour: “If we can use less concrete in our roads, buildings and bridges, we can eliminate some of the emissions at the very start.”