Substituting 6PPD in motor vehicle tyres with a within-class PPD replacement could be challenging under US state green chemistry schemes, regulators have suggested.
The uncertainty follows a US Tire Manufacturers Assocation (USTMA) announcement last month that it had identified four PPDs as possible safer substitutes for 6PPD in tyres. The findings occurred in the first part of an alternatives assessment (AA) conducted under California’s Safer Consumer Products (SCP) programme, an effort stemming from 6PPD-containing tyres’ listing as a priority product last autumn.
The substance, which functions as a tyre stabiliser and antidegradant, has increasingly drawn scrutiny in the US and beyond for aquatic harms linked to its breakdown product 6PPD-quinone.
But questions about moving to within-class replacements show how thorny transitioning to an alternative may get as regulators seek to prevent regrettable substitution.
California PPD listing
While the USTMA’s AA report also homed in on one non-PPD option, switching to any of the four recommended PPDs could be complicated by California’s plan to make the whole class of PPDs and their derivatives SCP candidate chemicals.
A candidate chemical listing brings no obligations for businesses, but lets the Department of Toxic Substances Control (DTSC) examine uses and hazard traits in the context of specific products, opening up the door to future regulation. The agency told Chemical Watch News & Insight it expects to publish a proposal for the PPD rulemaking process in the second half of this year.
The SCP scheme "does not preclude substitution of a chemical of concern [like 6PPD] with a candidate chemical" and compels a lifecycle comparison for any possible replacement, it said.
But if the industry consortium does ultimately select a candidate chemical as a replacement, the agency said it will factor that decision into its proposal for any required regulatory responses".
For instance, the DTSC could choose to mitigate potential adverse impacts to human health or the environment by:
- mandating disclosure of product details like the common names of the substituted candidate chemical(s), toxicological hazards and safe handling instructions;
- restricting product use, such as by only permitting purchases by individuals who have satisfied training specifications; or
- imposing additional measures to reduce exposure to the replacement candidate chemical(s), for example by fitting manufacturing sites with air filters.
Washington AA
A search for safer substitutes will likewise happen in Washington state, which just approved a law deeming 6PPD-containing tyres a priority product under cycle 2 of the Safer Products for Washington (SPW) programme. However, the state’s Department of Ecology (Ecology) indicated it may not accept in its own AA certain PPDs industry recommended as possible substitutes in California.
The agency told Chemical Watch News & Insight "it is reviewing USTMA’s stage 1 (preliminary) AA to identify possible alternatives that meet tyre performance requirements," including the 40 options initially picked by the consortium.
"We will review existing hazard data to determine whether alternatives that have the potential to meet our 6PPD AA hazard criteria warrant further investigation," the agency said.
It said some of the identified chemicals, including 7PPD and 77PD, will not fulfill the criteria due to reproductive toxicity or acute aquatic toxicity to Coho salmon.
But it intends to take a closer look at others, "especially those with promising performance evaluations that lacked hazard data".
Ecology noted that "there are no regulatory hurdles to identifying other PPDs as safer alternatives compared to other non-PPD compounds" because it has not designated the PPD class as an SPW priority chemical.
During a subsequent phase of SPW cycle 2, Ecology said, it will utilise its ongoing research to conclude whether a safer, feasible substitute exists for 6PPD-containing tyres. If it finds such an option, the agency can prohibit 6PPD in tyres through a rulemaking procedure, with a proposal slated for June 2027, and final regulations anticipated one year afterwards.
The USTMA said it is engaging with Ecology "to ensure that any possible alternatives identified as a result of our work with DTSC also meet Washington Ecology’s criteria as their evaluation moves forward".
NGO criticises industry approach
Meanwhile, Toxic-Free Future said it was "extremely disappointed" that most of industry’s identified alternatives are in the same class as 6PPD.
"We have serious concerns that some of these chemicals have the same or similar high hazards to 6PPD," it said.
According to the organisation, because such substitutes could exacerbate the negative effects seen with 6PPD, "the entire class of PPDs should be eliminated from consideration."
However, the USTMA told Chemical Watch News & Insight the SCP framework mandates the study of all PPD and non-PPD options. The association said it analysed 19 PPDs and 21 non-PPDs, and only advanced those with a hazard screening that was better than 6PPD’s, as directed by the agency.
"The DTSC requirements are science-based and very rigorous," the group said. "For us to make a blanket assumption about 1,800-plus [PPD] chemicals would not meet that rigour."
For many non-PPD substances, "there aren't data available to move them forward to further evaluation," the USTMA said, noting that it will keep trying to discover more in stage 2. That final phase is not necessarily limited to the five compounds pinpointed in the preliminary AA report, and includes steps to evaluate new information that emerges, it added.