Five finalists: USTMA study identifies potential 6ppd alternatives

WASHINGTON, D.C.—Five possible alternatives to the tire antidegradant 6ppd have been identified, following a comprehensive "preliminary" analysis completed by a consortium of 30 tire manufacturers March 25.

Results of the first stage of this alternatives study under California's rigorous Safety Consumer Product Regulations were released March 29 by the U.S. Tire Manufacturers Association, which led the project along with Gradient L.L.C., a Boston- and Seattle-based environmental and health consultant and testing firm.

"The rigor, collaboration and transparency involved in this preliminary evaluation demonstrates our industry's commitment to this alternatives analysis process," said Tracey Norberg, USTMA executive vice president and general counsel, and project lead on the report. "Investigation of possible alternatives to 6ppd is a dynamic area of research."

At the heart of the study is the additive 6ppd, a small-molecule ozone and oxygen scavenger that migrates to the surface of a tire to protect the tire from cracking and splitting as it wears.

However, as 6ppd is shed, its molecular structure can morph into the salmon-killing 6ppd-quinone, which ultimately finds its way into the streams and waterways of the Pacific Northwest via tire road wear particles.

A lawsuit was filed in November 2023 by the non-profit Earthjustice on behalf of two fisheries against 13 of the world's largest tire manufacturers relative to the environmentally unfriendly impact of 6ppd-q.

A motion to dismiss the lawsuit was filed March 8 by the tire makers. Part of the argument for this motion, the tire manufacturers say, is to allow this alternatives analysis to play out.

Earthjustice declined comment for this report.

"The safety, performance and sustainability of our products remains our uncompromising priority, and there is important work to be done to ensure any potential alternatives meet that high standard," said USTMA President and CEO Anne Forristall Luke. "This preliminary alternatives analysis marks an unprecedented milestone in the tire industry's search for an alternative to 6ppd in tires.

"The depth and breadth of this work represents significant progress toward the identification of potential alternatives, and further demonstrates our industry's commitment as a leading force in the search for solutions."

While 6ppd-q was identified in December 2020 (Tian et al., 2021), 6ppd has been used since the late 1960s in tire compounding.

"Elimination of 6ppd without a replacement is not an option," the March 25 report states.

With about 36.5 million tires shipped into California last year alone, there are no tires, the report states, that are absent of 6ppd. 6ppd also can be found in non-tire rubber products.

The California Department of Substance Control listed tires containing 6ppd as a "priority product" to be studied under the SCP Regulations Oct. 1, 2023.

"This (alternatives analysis) is notable for the large and diverse number of candidate alternatives identified and screened, the robust approach to considering potential impacts of possible alternatives across the entire product lifecycle, and the thoughtfulness, technical knowledge and high level of engagement by industry experts," said Gradient project lead Tom Lewandowski.

The report cautions that there are major information gaps as well, including the biochemical mechanisms by which salmonids are affected by 6ppd-q; the environmental effects of the alternatives that "made the cut" for further study; and the transformational products (like 6ppd exhibits with 6ppd-q) of the identified alternatives.

But the mission is clear, according to the report: "Do potentially safer, functionally acceptable and technically feasible alternatives to the priority product exist that should be given more in-depth consideration to determine if they qualify as acceptable alternatives?"

A total of 60 initial chemical candidates were considered during this initial analysis of alternatives for 6ppd, with 21 non-ppd-derived chemicals and 19 ppd-derived chemicals making an initial cut to 40.

Of those, five made the final cut as "potential possibilities" to replace the embattled 6ppd: 7ppd, Ippd, 77pd, CCpd and a specialized graphene, known as Prophene.

While non-ppds "pose greater challenges in terms of incorporation into tire chemistry," alternative ppds represent "the most logical and possibly easiest to implement" options.

Potential hazards to both human and environmental health, physical chemical properties and initial performance data were considered in the first phase of this study, according to the report.

Antidegradants like 6ppd, which "quench" the ozone and oxygen that can degrade a tire during abrasion, must provide protection against ozone; protection against oxygen; protection against fatigue; optimal migration rate/diffusion; compatibility with the tire manufacturing process; and compatibility with "other aspects of tire safety and performance."

The first phase of the report looks at the full lifecycle of 6ppd (and the tire itself), from raw materials to the energy and resources required to make the chemical; the dizzying amount of regulations, by continent and country, to which any potential alternative must adhere, including ASTM testing thresholds; and further studies that seem to confirm 6ppd-q's toxicity on salmonids.

"Tire composition cannot responsibly be modified without great care, including extensive and rigorous testing," the report states. "Any material identified as a possible alternative must continue to ensure comparable performance in a tire and continued compliance with federal safety regulations."

Completion of this initial stage of the alternatives study establishes a basis for further evaluation in Stage 2, which will result in a final alternatives analysis report, according to Norberg.

During Stage 2, as required by the DTSC process, USTMA and the consortium will confirm a "short list" of possible alternatives (in light of any new data that emerges in this phase), assess the potential impacts of these options and initiate a more detailed review of the chemicals' potential hazards and exposure-related chemical properties.

USTMA and Gradient officials say they are optimistic that the consortium "will have identified one or more possible alternatives (following the second stage) that hold promise to replace or materially reduce 6ppd in motor vehicle tires, subject to future performance testing to ensure comparable tire safety and performance."

Other options, such as the non-pneumatic tire, infrastructure TRWP collection options and tire-particle catchers on vehicles themselves were mentioned but purposefully not considered as options as part of this initial alternatives analysis phase.

Non-pneumatic tires still will require an antidegradant, and tire manufacturers cannot dictate policy for something like a tire-particle catcher, according to the report.