The rivers and streams around Washington State’s Puget Sound had long held a mystery.
In the late 1990s, ecologists began reporting cases of “urban runoff mortality syndrome” in the local coho salmon population. “Salmon are born in fresh water, go out to the salt water, spend most of their lives in the ocean, and then they return to the fresh water to spawn,” explains Alexei Calambokidis, Washington policy director for Trout Unlimited, a nonprofit organization that monitors fish populations. “When they’d return to the fresh water as adults, a lot of them would flip upside down, start sort of gasping at the surface, and die before spawning.”
Officials and environmentalists suspected a chemical culprit. “There was an obvious conclusion that it has something to do with water quality,” Calambokidis says. An intriguing clue came from the timing of the salmon die-offs: salmon spawn in autumn, a season when heavy rains run off from highly trafficked roads in Seattle, Olympia, and Tacoma into local waterways.
Perhaps something in the tires, some people speculated, was killing the salmon, which are vital prey for many species in the Pacific Northwest—and a favorite seafood. When scientists exposed juvenile coho salmon to leachate, a solution they prepared from tire-tread particles, in the lab, nearly all the fish died within 24 h. For more than 2 years, the group did the painstaking work of fractioning chemical mixtures from the tires until they narrowed the search for the toxin down to 6PPD-quinone, a transformation product of the antiozone additive N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine, known as 6PPD. They published their results in the journal Science in 2020.
Subsequent testing has uncovered just how acutely toxic 6PPD-quinone is to coho salmon. Less than 0.1 μg/L is enough to kill 50% of a salmon population within 24 h. The amount is equivalent to a few drops in an Olympic-sized pool.
“We don’t deal with a lot of chemicals in the fish world that are just outright killing fish on the spot,” Calambokidis says. And scientists keep turning up other species that are also sensitive to the chemical. These include white-spotted char, brook trout, and rainbow trout, which studies show can have 50% mortality at 0.51, 0.59, and 1.96 μg/L, respectively, also within a day.
Within months of that initial study, regulators and even the tire industry agreed that 6PPD had to go. The Washington legislature proposed a deadline of 2035 to deploy a suitable replacement for tires sold in that state. Tire makers as well as rubber additive companies such as Lanxess, Flexsys, and Sennics are working to find one.
6PPD is great at its job, which is why it has been blended into tire rubber for half a century. Ozone attacks the unsaturated bonds in the rubber, but it will preferentially react with 6PPD if the additive is present, forming 6PPD-quinone in the process. Without 6PPD, tires crack, disintegrate, and may fail more quickly.
If a substitute doesn’t work as well as 6PPD, using the substitute might trade tire safety and durability for nontoxicity. And even if chemical companies find a molecule that matches 6PPD’s ozone-scavenging performance, it will need to have a better toxicological profile.
The eastbound lanes of Highway 520 in Redmond, Washington. Government officials want to replace 6PPD as the antiozonant in tires to make the runoff from roads safer for local fish. Credit:
AP Photo/Ted S. Warren
The tire industry mobilizes to replace 6PPD
In 2023 the California Department of Toxic Substances Control (DTSC) listed tires containing 6PPD as a “priority product” under that state’s Safer Consumer Products Regulations. Because 6PPD can’t be readily removed from tires, the regulations require tire makers to issue an alternative analysis report. Thirty-six tire makers joined together on the report in an effort organized by the US Tire Manufacturers Association (USTMA).
“USTMA encouraged DTSC to list tires containing 6PPD as Priority Products almost immediately after 6PPDQ [6PPD-quinone] was identified,” Tracey Norberg, the USTMA’s executive vice president and general counsel, says in an email to C&EN.
At the onset of the program, the tire makers scoured technical journals, trade publications, books, patents, and chemical company websites for references to potentially useful antiozonants. They found 70 candidates that merited further review.
The consortium then evaluated any studies it could find about the ozone activity of these substances. It also analyzed their chemical structures for likely ozone activity. These steps narrowed the list to 19 molecules that, like 6PPD, are in the p-phenylenediamine (PPD) class and 24 non-PPD alternatives.
The USTMA consortium evaluated the hazards of these molecules, consulting sources such as European Chemicals Agency dossiers. It commissioned the US Geological Survey to conduct toxicity tests on some of the promising PPD-based candidates, and the rubber chemical maker Flexsys evaluated their performance.
In 2024, when the consortium submitted its preliminary alternatives analysis report, the consortium had whittled its list down to seven promising alternatives that warranted further study: N-(5-methyl-2-hexyl)-N‘-phenyl-p-phenylenediamine (7PPD), N-isopropyl-N‘-phenyl-p-phenylenediamine (IPPD), N,N‘-bis(1,4-dimethylpentyl)-4-phenylenediamine (77PD), N,N‘-dicyclohexyl-p-phenylenediamine (CCPD), specialized graphene, octyl gallate, and 2-methyl-4,6-bis(octylsulfanylmethyl)phenol, known commercially as Irganox 1520.
Since then, following the identification of additional potential alternatives, the consortium has expanded the number of compounds it is evaluating from 7 to 24. Its next report is due to the state of California in October.
The distinction between PPDs and non-PPDs has become significant in the search for alternatives. The USTMA consortium said in its report that PPDs, being chemically similar to 6PPD, “are the most logical and possibly easiest to implement alternatives to 6PPD.”
On the other hand, the chemical similarity of other PPDs to 6PPD and the known toxicity of some members of that molecule class have made officials in Washington and California wary of them. After the USTMA issued its preliminary report, Laura Johnson, director of Washington’s Office of Environmental Public Health Sciences, and Katrina Lassiter, manager of a toxics reduction program at the state’s Department of Ecology, warned against PPD-based alternatives in a letter (PDF) to California authorities. “Although compounds outside of PPDs are as yet unproven in protecting tires, they may represent the best chance of finding a chemical that is truly safer and can provide product performance and safety for years to come,” they write.
Top 6PPD replacement candidates emerge
Some of the molecules on the USTMA’s shortlist, particularly the PPDs, are well known to the rubber industry and already in commercial production. For example, IPPD was widely used as a tire antiozonant before 6PPD replaced it in the 1970s. IPPD’s drawback was its excessive migration to the tire surface. One 1971 paper describes “Volkswagen dermatitis,” a rash that one man received whenever he washed his car.
“We don’t deal with a lot of chemicals in the fish world that are just outright killing fish on the spot.”
Not every chemical on the list has companies eager to supply it to the tire industry. For example, when asked whether the manufacturer BASF sees potential in its antioxidant Irganox 1520 as a 6PPD replacement, spokesperson Annette Engeroff responds that it “is not a suitable or perfect replacement for 6PPD in its specific function as an antiozonant.” It isn’t the company’s “standard practice” to recommend it as a replacement, she says.
But chemical maker Lanxess is keenly interested in supplying another molecule on the USTMA’s shortlist: CCPD.
Lanxess is a major producer of rubber additives, including 6PPD. Still, the company recognized the need for a substitute following the 2020 6PPD-quinone paper, according to Michael Essers, the firm’s ecology lead and director of global regulatory affairs. The study was “a valuable piece of detective work,” Essers says. “This started it all rolling and triggered activities for seeking suitable alternatives with a more beneficial hazard profile.”
CCPD has been known for many years to have good antiozone potential, but it was never able to break into the tire market because 6PPD was already established, according to Tobias Lauterbach, a global marketing manager at Lanxess.
The molecule is being assessed now. As for its physical performance in tire rubber, CCPD is “in some parts better and some parts slightly worse than 6PPD,” Lauterbach says. “On average, it’s on the same level as 6PPD, I would say. And that is also the feedback that we received so far from our customers: that antiozone performance is very good.”
But Lauterbach cautions that CCPD isn’t a drop-in substitute for 6PPD. “You will have some minor compounding work to do,” he says, referring to the mixing process. “I think that will be something that most, if not all, solutions to this problem will have.” CCPD, notably, prompts rubber to start curing faster compared to 6PPD after the ingredients are mixed. Lauterbach says the problem can be fixed by different measures, such as by adding retarders that delay vulcanization.
Companies evaluate alternatives to 6PPD
Lanxess has been offering samples of CCPD to customers, some of whom are already testing it in prototype tires. To ramp up this effort, Lanxess is expanding from pilot to larger-scale production of CCPD this year. “This will lead to several tons of material that we can sample to the market,” Lauterbach says.
As for the most important hurdle, environmental performance, Lanxess emphasizes that CCPD has cyclohexylamine terminal groups rather than the phenyl terminal group in other PPDs such as 6PPD and IPPD. According to Essers, this difference makes the molecule more prone to quick degradation than other PPDs. “It hydrolyzes considerably faster compared to 6PPD, which means if we consider heavy stormwater events, that it degrades faster,” he says.
The different structure could also explain a reduced reproductive hazard profile compared with other PPDs, the company says. For example, in testing, unlike the PPDs with a terminal phenyl group, CCPD showed no link to dystocia, a labor complication in people.
In testing on coho salmon, CCPD-quinone, the transformation product of CCPD analogous to 6PPD-quinone, exhibited no toxicity up to the solubility limit of 149 μg /L. But CCPD was more toxic than 6PPD to Daphnia magna, a crustacean used in toxicity testing, Essers says.
Lanxess isn’t unique in developing a 6PPD substitute. Flexsys, which also makes 6PPD, announced last November that it developed its own candidate, one that the USTMA hadn’t considered. Flexsys hasn’t named the compound—or, possibly, compounds—yet, but the company did say it in its announcement that it relies “on chemistry outside the ‘PPD’ family that does not form ‘quinone’ during use, thus eliminating the impact of quinone on the environment entirely.”
In an online forum organized by the Washington State Department of Ecology last December, Neil Smith, Flexsys’s chief technology and sustainability officer, shed some light on the development process.
“The company set out in 2021 with a set of criteria for a 6PPD replacement,” Smith said in his presentation. It wanted the chemical to have an improved environmental and health footprint over 6PPD and, ideally, to be non-PPD and non-quinone forming. It also couldn’t compromise on tire performance. “Tires are not going to be less safe,” he said. “Tires are not going to have shorter service lives.”
In addition, Smith noted, the solution had to be scalable. About 350,000 metric tons of 6PPD are produced each year globally, so Flexsys wanted to employ 4-aminodiphenylamine, a raw material it uses to make 6PPD, to synthesize the new molecules.
Smith said Flexsys scientists “ideated” and filed patents for “thousands” of chemical structures. “At the start of the program, Flexsys chose to focus on aromatic amines as the functionality of choice” because, in addition to the potential manufacturing advantages, they were likely to react rapidly with ozone, Smith tells C&EN in an email. Altogether, it synthesized more than 350 new molecules and tested them at its lab in Akron, Ohio; at independent, third-party laboratories; and with tire industry partners.
About 10 to 12 had an antiozone performance close to or exceeding that of 6PPD, Smith told the forum. A few showed toxicity in subsequent testing and were eliminated, but what remained was “at least one chemical, probably a set of chemicals, that can meet all of our internal targets simultaneously.”
The company plans to unveil the molecules later this year. Meeting the proposed 2035 deadline, Smith tells C&EN, is “reasonable and achievable.” Indeed, Flexsys plans to have the first plant to make the new antiozonant up and running in 2030 or 2031.
Another 6PPD producer, Sennics, a subsidiary of the Chinese chemical maker Sinochem International, unveiled its own alternative, called SA6000, last December. Like Flexsys, Sennics is keeping the molecule’s identity under wraps. But the company does say it is “eco-friendly,” is different from PPDs, and shows a performance comparable to 6PPD.
Sennics’ announcement says the firm chose the final molecule after evaluating 100 new chemical structures. Sennics also notes it had secured patents in the US and China by the end of 2024. One patent (PDF) the firm filed around that time iterates a family of polyalkyl p-phenylenediamine antidegradants.
Not all proposed solutions to 6PPDs are small molecules. On the USTMA’s own shortlist is specialized graphene, under development by firms such as Perpetuus Advanced Materials. Its material is graphene functionalized with amine groups via a plasma-based process.
According to CEO John M. Buckland, “many years” before 6PPD-quinone was fingered as the culprit in salmon die-offs, Perpetuus recognized the potential of its specialized graphene as an antiozonant while it was developing applications such as elastomer reinforcement. “During this work, we observed that amine functional groups distributed across high surface area carbon structures exhibit reactivity toward oxidizing species such as ozone,” he writes in an email to C&EN. “This behavior is conceptually aligned with the mechanism of traditional amine-based antiozonants.”
The company says graphene could replace 6PPD entirely or be used synergistically with other antiozonants to reduce the amount of those molecules needed in the formulation. Perpetuus is now testing compounds to determine whether the reactive sites can be adequately distributed in the rubber matrix.
Will any of these substitutes for 6PPD work? It will be years before we know for sure. In the meantime, the tire consortium’s October report should reveal the remaining candidates and shed light on the tire industry’s options for a product that is safe for both people and fish.
