A team of researchers from the University of Delaware has devised an approach for limiting the end-of-life tire decontamination of 6PPD — a molecule found in the composite materials used to manufacture tires.

The researchers highlighted that 6PPD provides UV protection, prolonging the lifespan of tire rubber by absorbing sunlight and preventing degradation caused by reactions with ozone and other reactive oxygen species in the air.

Heating performance, product distribution and morphological evolution in microwave pyrolysis of EOL tires. Source: Nature Chemical Engineering (2024). DOI: 10.1038/s44286-024-00110-9

The team explained that as tires wear down through repeated contact with the surface of the roads, they release particles of 6PPD into the environment. Meanwhile, stormwater runoff transports those toxic particles into freshwater systems and, subsequently, other bodies of water, where the chemical can, even in small doses, kill fish.

As such, the team devised a method that transforms the 6PPD into safe chemicals and converts the crumb rubber residue into aromatics and carbon black, which is a soot-like material found in products ranging from pigments to cosmetics to electronics.

To accomplish this, the team removed the 6PPD through a process known as chemical extraction wherein millimeter-sized pieces of tire, or crumb rubber, are placed into a microwave reactor that heats the materials up and that uses a chemical solvent to separate the 6PPD from the other molecules present in the mix.

When the 6PPD molecules are removed, the team explained that they can be chemically converted into safe chemicals. The remainder of the tire can be recycled using common plastic recycling methods. Additionally, the crumb rubber could potentially be used in aromatics, which are the starter materials for a range products, or as carbon black.

An article detailing the process, “End-of-life tire decontamination from 6PPD and upcycling,” appears in the journal Nature Chemical Engineering.