Research, litigation keep 6ppd top-of-mind for tire makers

The tire additive 6ppd remains a salient sustainability challenge for the world's largest tire manufacturers, as ongoing litigation with Pacific Northwest fisheries, rulemaking processes by the EPA and research into an alternative continue.

The small-molecule ozone and oxygen scavenger has been in use since the late 1960s in tire compounding, "blooming" to the surface of a tire to protect it from cracking and splitting as the tire wears.

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

An injunction, filed by two fisheries in the region for an immediate ban on the use of 6ppd under the Endangered Species Act, remains in litigation.

The chemical is under consideration for rulemaking by the EPA, prohibiting the manufacturing, processing, use and distribution of 6ppd in tires under the Toxic Substances Control Act (petitioned by several Pacific Northwest Native American tribes).

The past year also bore fruits from research, as the U.S. Tire Manufacturer's Association in March whittled possible 6ppd alternatives from about 60 chemicals to about seven.

In August, the California Department of Toxic Substances Control approved this first stage of research and cleared the way for a second, final alternatives analysis stage to begin.

While 2023 was defined by litigation from fisheries and tribes seeking an immediate ban on 6ppd in tire manufacturing, 2024 opened with the completion of the first stage of a rigorous analysis by the USTMA to find an alternative to the antidegradant.

The first phase of the analysis was completed by the USTMA and a consortium of 30 tire manufacturers March 25, along with Gradient L.L.C., a Boston- and Seattle-based environmental and health consultant and testing firm.

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

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

The California Department of Toxic Substance Control in August ultimately approved seven possible alternatives for study in a second stage.

The initial stage looked at the full lifecycle of 6ppd (and the tire itself), from raw materials to the energy and resources required to make the chemical; and further studies that seem to confirm 6ppd-quinone's toxicity on salmonids.

For the USTMA and its members, safety remains the top priority for any alternative.

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

Fast forward to Aug. 26, when the California Department of Toxic Substance Control officially approved the first stage of the alternative analysis, adding two more possibilities to be studied in a second stage for a total of seven chemicals.

USTMA officials have said that they are optimistic that, by the end of the alternatives analysis process, "one or more possible 6ppd alternatives with potential to replace or materially reduce the level of 6ppd in motor vehicle tires" will be identified, subject to performance testing and comparable safety performance.

During Stage 2, USTMA and the 32-company consortium (including the 16 USTMA member companies) will confirm an even shorter list of possible alternatives in light of any new data that emerges, assess the potential impacts of these options and initiate a more detailed review of the potential hazards and exposure-related chemical properties.

The consortium's Stage 2 report is due to DTSC Aug. 19, 2026, with an annual progress report due Aug. 19, 2025.

Other options (outside of an alternative to 6ppd), such as the non-pneumatic tire, infrastructure tire road wear particle collection options and tire-particle catchers on vehicles themselves were mentioned, but purposefully not considered as options as part of the alternatives analysis.

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.

The legal battlefield over 6ppd saw some movement in the last year as well.

6ppd is the subject of at least one major lawsuit—the request for an immediate ban on 6ppd—brought in October 2023 under the Endangered Species Act by two Pacific Northwest fisheries against the tire manufacturers.

Fisheries bringing the lawsuit include the Institute for Fisheries Resources and the Pacific Coast Federation of Fisherman's Associations.

A motion to stay the immediate injunction, filed by 13 of the world's largest tire manufacturers March 8 under the "primary jurisdiction doctrine," was denied by the U.S. Court for the Northern District of California July 10.

And the chemical is under consideration for rulemaking by the EPA, prohibiting the manufacturing, processing, use and distribution of 6ppd in tires under the Toxic Substances Control Act, as petitioned in August 2023 by several Pacific Northwest Native American tribes.

EPA granted the tribes' petition in November 2023, effectively entering 6ppd in "consideration of" the rulemaking process.

However, EPA also stated in granting the petition that the agency "cannot commit to a specific rulemaking timeframe or outcome," aiming to publish a notice of proposed rulemaking by fall 2024 and to finalize a rule before 2025.

The U.S. district court added that regardless of what the EPA decides in regards to the tribes' petition under the TSCA, "there will likely be a lot to fight about over 6ppd and 6ppd-q in this case."

The U.S. EPA in June finalized water quality screening guidance for use in determining 6ppd and acute 6ppd-quinone levels in freshwater ecosystems, including stormwater runoff.

The threshold values and methodology, known as EPA Method 1634 and published in the Federal Register under the Clean Water Act June 10, is intended for measurement of short-term concentrations of 6ppd and 6ppd-q.

The guidance was fast-tracked for use by tribes, states and local governments, EPA said.

"Achieving clean water that protects aquatic life and supports commercial, recreational and cultural fishing practices takes partnership at all levels of government," said Bruno Pigott, acting EPA assistant administrator for water. "6ppd and 6ppd-q can be lethal to fish, so it's important we know where it's polluting waterways. The EPA's screening values provide important information that our partners can use to protect fish populations—especially coho salmon on the Pacific coast."

According to the guidelines, the screening values for acute exposures to 6ppd and 6ppd-q in freshwater are 8,900 nanograms per liter (ng/L); and 11 ng/L, respectively.

EPA admits that major data gaps remain.

Further, much of the available data was developed using aquatic toxicity testing approaches that "do not fully conform" with other EPA ecological and ASTM testing guidelines, EPA states.

The screening values "are not regulations and do not substitute for the CWA or the EPA's regulations."

The prospect of a consumer- and environmentally safe 6ppd alternative remains hopeful entering 2025 on still another front, this time from academia.

Researchers at the University of Delaware have developed a technique to remove the antidegradant from end-of-life tires, whereby 6ppd is "upgraded" into a safer chemical in the process.

According to Dion Vlachos, lead researcher on the project, ELTs are ground into millimeter-scale pieces of crumb rubber and microwaved.

A chemical solvent is used to separate the 6ppd from other molecules, and the 6ppd is converted to a safer form.

The crumb rubber also is converted to aromatics and carbon black during the separation process, according to UD researchers from the Center for Plastics Innovation and the Department of Chemical and Biomolecular Engineering.

According to Vlachos, tire pyrolysis (high heat in the absence of oxygen) has not been successful in separating 6ppd from the tire compound, as diketone molecules remain in the resulting pyrolysis oil.

The remainder of the tire can be used in athletic fields and road surfaces or processed into feedstock materials such as carbon black.

The University of Delaware approach has been proven at lab-scale, and an economic analysis showed "the cost looks to be very reasonable."