6PPD is an antioxidant and antiozonant that helps prevent the degradation and cracking of rubber compounds caused by exposure to oxygen, ozone and temperature fluctuation. 6PPD is used industry wide to help tires resist degradation and cracking, which is vital for driver and passenger safety. Antioxidants support increased tire endurance. There are no known alternatives to 6PPD that provide the same safety and performance characteristics in a tire.

The problem for the tire industry as a whole is that tire wear dust containing 6PPD is washed into the waterways, and there is some evidence that 6PPD-Quinone (a derivative of 6PPD) is linked to the death of Coho salmon.

Nick Molden from Emissions Analytics UK spoke at the recent ETRA conference in Brussels about the issues arising from tire wear dust, pointing out that the wear particles are so small that they not only contaminate watercourses, though much is absorbed in roadside land, but the tiniest particles become airborne and can be inhaled by humans and animals.

Tire wear dust is a tire manufacturer's challenge to solve, but it also poses a concern to recyclers shredding tires, and there's the issue of such dust possibly being present in crumb rubber infill (though it should have been classified out as a higher value product).

A 2020 study led by the University of Washington and Washington State University found the toxin was highly deadly, killing some young Coho salmon in as few as four hours after exposure.

As part of the study, the USTMA in Washington, D.C. gave tire samples and information to the Washington researchers. Following the publication of the findings, the association joined forces with other environmental organizations and other agencies to petition the state to examine the chemical.

“We are pleased that there have been many advancements in the body of research on 6PPD-Quinone,” said Sarah Amick, a vice president at the association. “However, many data gaps still remain, so we remain committed to collaborating with researchers, regulators and stakeholders to fill these knowledge gaps and help find a viable alternative to 6PPD that does not compromise tire performance or driver safety and also ensures environmental safety.”

The USTMA also recently announced that it supports the proposed rulemaking released by the California Department of Toxic Substances Control to designate 6PPD in automobile tires as a priority product for review under the state’s Safer Consumer Products green chemistry programme.

“USTMA supports the decision by the California DTSC to list 6PPD as a priority product under the state’s green chemistry program,” said USTMA President and CEO Anne Forristall Luke. “USTMA members are committed to producing the safest, most durable and most efficient tires possible. This is why we made the request to the DTSC to list 6PPD as a priority product a few weeks after the release of the Tian et al. study – and why we have been working with the agency and many other stakeholders ever since. We are fully committed to continuing to work with the DTSC team to find a viable alternative.”

Tire manufacturers have taken seriously the study and more recent scientific findings which indicate that 6PPD-Quinone, a previously unknown transformation product of 6PPD – an essential antioxidant and antiozonant used in tires – has contributed to the mortality of Coho salmon and potentially poses risks to other aquatic species in the Pacific Northwest.

Since the study's release 18 months ago, the USTMA says it has collaborated with researchers, regulators, and legislators to better understand the implications of the findings and find solutions, including filling research gaps around 6PPD-Quinone, advocating for a 6PPD alternatives analysis in California, and supporting stormwater runoff mitigation efforts.

Article by Tyre & Rubber Recycling magazine.