Tires are an indispensable part of daily life. Without them, our vehicles would just be a bunch of assembled parts — convenient to sit in, but not effective for getting where you are going.

While their usefulness is undisputed, tires do come with some problems. A 2016 Federal Highway Administration report found 280 million tires are discarded annually in the United States. Globally, this number is much higher — over a billion, according to a report by the World Business Council for Sustainable Development. 

What to do with these end-of-life tires is an ongoing and often complicated conversation. 

Tires are composite materials that have a lot of components in them, including a molecule known as 6PPD, which provides UV protection to help the rubber found in tires last longer. The 6PPD accomplishes this by absorbing the sun’s rays and preventing the material from breaking down due to reactions with ozone and other reactive oxygen species in the air. 

As tires wear down through contact with road surfaces, however, they release particles of 6PPD into the environment. Stormwater runoff carries these toxic particles into freshwater systems and other bodies of water, where the chemical can quickly kill fish, even in small doses. Recently, tribes in the Pacific Northwest filed a petition asking the Environmental Protection Agency to consider establishing regulations prohibiting the use of this chemical. 

University of Delaware researchers from the Center for Plastics Innovation and the Department of Chemical and Biomolecular Engineering, led by Dion Vlachos, Unidel Dan Rich Chair in Energy, have developed a method to tackle end-of-life tire decontamination from 6PPD. 

The researchers recently published their approach in Nature Chemical Engineering, demonstrating a way to upgrade 6PPD into safe chemicals and to turn the leftover crumb rubber into aromatics and carbon black, a soot-like material found in everything from pigments to cosmetics to electronics.

Safely repurposing tire materials

According to Vlachos, tires are responsible for about one-third of the microplastics in the environment. This is because nearly 25% of the components in a tire are made of synthetic rubber, which is a plastic.

Under sun radiation exposure, 6PPD converts to 6PPD-quinone, what is called a diketone, or a molecule made up of two ketone groups. One major source for these diketone molecules is the tires themselves. And it’s not just the microplastics that result from tire wear and tear when in use. These molecules also can be released into the environment from tires left in landfills and exposed to the elements, such as rainfall. 

“You can’t put a filter on the environment the way you might have a filter on your household dryer to capture these fibers,” said Vlachos, who also directs the Delaware Energy Institute.

While others in the field have attempted to break tire materials down using high heat, through a process known as pyrolysis, 6PPD is stubborn and the diketone molecules remain in the oil left behind. If the oil is used in fuel or other materials, the diketone molecules go along for the ride, which is a problem.

So, the Vlachos team decided to try and remove the 6PPD through a process known as chemical extraction. This involved placing millimeter-sized pieces of tire, or crumb rubber, into a classic microwave reactor, heating the materials up and using a chemical solvent to quickly separate the 6PPD from the other molecules present. 

Once the 6PPD molecules are removed, they can be chemically converted into safe chemicals that can be used or sold for a small price. The rest of the tire, meanwhile, can be recycled using classic plastic recycling methods — a plus, given there currently are no alternatives for tires in general. This would enable remediated tire materials to be used in practical applications, say, on soccer fields, playgrounds or in asphalt for roads, without worry. The crumb rubber also could be used in aromatics, which are starting materials for a wide range of consumer products, or as carbon black, a soot-like material found in many pigments, conductive/insulating elements and reinforcing agents, among other things. 

The UD research team has protected the novel approach through the University’s Office of Economic Innovation and Partnerships.

To date, the research team has proven this approach at the lab scale, according to Vlachos, and a technoeconomic analysis showed the cost looks to be very reasonable. It’s a positive step, but more work is needed — and time is of the essence.

Worldwide, the number of end-of-life tires continues to grow, with some reports estimating there could be up to five billion tires in need of disposal worldwide by 2030. Meanwhile, scrap tire use in the United States declined by 25% between 2013 and 2021.

“I think actual recycling of the tire itself is important, so there are truly circular solutions that are doing upcycling,” he said. “We must make things at a large enough scale and at a reasonable cost outside of the laboratory. This has to be demonstrated with pilot-scale facilities. We haven't done that.”

Taking solutions from the lab to the real world will require further engineering effort and time. Having a dedicated Center for Plastics Innovation at UD is a definite advantage, Vlachos said, because it brings a critical mass of people talking, thinking and working on these issues. Startups and other minds, along with the automotive industry, will be key to driving solutions toward adoption.

“We need to educate the community. We need social sensitivity, awareness. It's not a problem that will solve itself,” Vlachos said.

Co-authors on the paper include Sean Najmi, Pooja Bhalode, Mongtomery Baker-Fales, Brandon Vance, Esun Selvam, Kewei Yu and Weiqing Zheng.