Scrap tyres have been a problem for environmentalists for decades. They pile up in landfills, have fed enormous toxic fires, harbor pests and get burned for fuel.
According to the Rubber Manufacturers Association (RMA), nearly 270 million tyres were discarded in the US in 2013 – more than one tyre per adult living in the country – and thousands get stockpiled in landfills. Because tyres are non-degradable, they could potentially stick around indefinitely.
More than half go on to become tyre-derived fuel – shredded scrap tyres that get mixed with coal and other materials to help power cement kilns, pulp and paper mills and other plants. But environmentalists are concerned that the emissions from this practice could be adding harmful pollutants to the air.
Scientists who are trying to counter this problem have developed a new way to make synthetic rubber that can be easily degraded back to its chemical building blocks and reused in new tyres and other products.
A research team led by Hassan S. Bazzi, Ph.D., at the Texas A&M University campus in Qatar (TAMU-Qatar) has been working to make new tyres with degradable materials since 2012.
“The basic idea behind this project was to take a byproduct of the petrochemical industry and turn some of it into recyclable value-added chemicals for use in tyres and other applications,” says Robert Tuba, Ph.D., one of the lead researchers on the project.
Currently, makers of synthetic rubber use butadiene as their base material, but its cost has recently gone up, opening the door to competition. Tuba turned to cyclopentene as a potential alternative.Cyclopentene, a basic molecule, is a low-value major component of the abundant waste from petrochemical refining, in particular its steam-cracking operation C5 fraction.
With colleagues at the California Institute of Technology in the US, they have been experimenting with catalysts to string cyclopentene molecules together to make polypentenamers, which are similar to natural rubber.
Their calculations showed that polymerisingcyclopentene and degrading it under relatively mild reaction conditions, that require minimal energy and expense, should be possible.
“We did theoretical studies to predict the feasibility of the synthesis and recyclability of polypentenamer-based tyre additives using equilibrium ring opening metathesis polymerisation,” explains Antisar Hlil, also at TAMU-Qatar. “Then we did experimental studies and found that the concept works very well.”
Using ruthenium, a transition-metal catalyst, the researchers polymerised cyclopentene at 0 degrees Celsius and decomposed the resulting material at 40 to 50 degrees. For industry, these are low temperatures that do not require a lot of energy.
Additionally, in the lab, they could recover 100% of their starting material from several polypentenamer-based tyre additives they developed.
In progress are new studies that mix the synthetic rubber with other tyre materials, which include metals and fillers. The researchers are also scaling up their lab experiments to see whether the tyre industry could realistically use their processes.
“If the fundamental studies are very promising – which at this point, we believe they are – then our industry partner will come in to continue this project and bring the material to market,” Tuba says.