Can tires turn green?

Rubber crumbs rain down onto more than 150 electric heating plates, raising their temperature to over 600 °C within seconds. In an atmosphere depleted of oxygen, the crumbs rapidly transform into a gas, which is cooled into a synthetic oil and a solid that is mostly carbon black. This process is at the heart of a continuous thermal pyrolysis technology being commercialized by the German start-up Pyrum Innovations to chemically recycle tires. It saves 72% of the carbon dioxide emissions that would have resulted from the existing systems for tire disposal and material recovery, the firm tells C&EN.

Demand for products from Pyrum and other chemical recyclers of tires is increasing and may one day divert millions of used tires from their most common destinations of landfill or incineration. Some 4 billion tires sit in landfills and stockpiles around the world, according to the Tire Industry Project, and they have the potential to catch fire and leach hazardous chemicals into the environment. The incineration of tires generates substantial greenhouse gas emissions.

In a second sustainability shift taking place in the life cycle of tires, manufacturers have started adopting lower-carbon production processes and are using increasing amounts of renewable materials—including those from the chemical recycling of tires.

But just as these activities start to promise substantial environmental benefits, a growing number of scientific studies indicate that tire and road wear particles (TRWPs)—generated when tires rub against the road—could be substantially harming the environment. Scientists at Imperial College London estimate that tires annually release 6 million metric tons (t) of TRWPs—typically linear particles 100 µm long—making it the second-largest source of microplastic pollution, after single-use plastics.

For years, the Tire Industry Project (TIP), set up under the World Business Council for Sustainable Development in 2005 by 11 companies that make about 65% of the world’s tires, has been researching TRWPs’ environmental impact. And while the organization states in a fact sheet that its key takeaway “has been that the presence of TRWP presents no significant risk to humans and the environment,” some scientists say TRWPs’ effect is unclear.

Specifically, their research has linked certain chemicals in tire particles, such as N-(1,3-dimethylbutyl)-N´-phenyl-​​p-​phenylenediamine (6PPD)—an antioxidant that stops tires from degrading—​to mass die-offs of fish.

Greener compounds roll in

The tire industry’s sustainability effort has two focuses, says Sarah Amick, senior vice president of the US Tire Manufacturers Association (USTMA): optimizing tires’ rolling efficiency to minimize vehicles’ fuel consumption and replacing fossil fuel–based raw materials with renewable ones to lower greenhouse gas emissions.

More than 100 compounds—including multiple types of rubber, like styrene-​butadiene rubber—go into tires. The industry is investigating replacing some of them with renewable materials, such as wood lignin for making tire antioxidants and soybean oil for multiple applications.

In the case of tires for combustion-engine vehicles, “more than 80% of the CO₂ emissions that are tire related are linked to vehicle fuel consumption,” Amick says.

Silica is already widely used in tires to reduce rolling resistance and thus fuel consumption. The chemical firm Solvay is both optimizing rolling efficiency and using more renewable materials by making silica from rice husk ash rather than from sand. When produced with renewable energy, rice husk silica has a carbon footprint half that of silica made from sand, the company says.

“The push towards advancing the circular economy is a prime interest for the industry,” Amick says. Using recycled materials instead of virgin feedstocks would lower tires’ emission profile, she says.

Solvay plans to roll out its process commercially in Leghorn, Italy, in 2024 and build a plant in the US after. Goodyear Tire & Rubber is already using silica from rice husk ash at several of its tire factories and aims to introduce tires made entirely of sustainable materials by 2030.

In a similar approach, the German chemical firm Lanxess will soon make the tire antioxidant Vulkanox HS—based on 2,2,4-trimethyl-1,2-dihydroquinoline—from renewable raw materials using a mass-balance approach. In this method, the firm substitutes a portion of its usual fossil fuel raw material with a renewable alternative, such as recovered and processed waste oils, and then attributes a corresponding amount of the final product to the renewable material. “This means that the firm will be able to assign a certain portion of its Vulkanox HS production as low carbon and renewable or recycled,” says Jens Grühn, global marketing manager for functional tire additives at Lanxess.

Tire makers also seek to secure greater volumes of isoprene, the monomer for natural rubber. Currently, over 30% of a truck tire is made of natural rubber, which comes from the milky latex that oozes from the bark of Hevea brasiliensis trees grown in tropical regions of Southeast Asia.

Various terpenes—precursors for isoprene—all with the formula (C₅H₈)_n, can be produced via synthetic or natural processes, says David H. Lamparelli, a polymer chemist at Spain’s Institute of Chemical Research of Catalonia. As well as being renewable, some terpenes could improve tire performance. “Linear terpene-based elastomers are excellent candidates to replace the common types of resins in the formulations of tread compounds,” he says.

Taking a new route to natural rubber, Continental Tire is seeking to begin manufacturing tires that include rubber from dandelion roots within the next 5–10 years. The firm has been researching dandelion rubber in Mecklenburg-Western Pomerania, Germany, since 2018. With the goal of security of supply, Continental aims to get 10% of its natural rubber from sources other than Hevea by 2035.

In a similar move, Bridgestone Group and Nokian Tyres are independently seeking to make rubber from guayule, a woody shrub native to the arid southwestern US. Nokian has begun growing guayule in Spain. “It is a plant that does not exploit areas of any other vegetation or food production; on the contrary, it makes use of wastelands,” says Teemu Soini, head of innovation and development for Nokian. “If guayule succeeds as an alternative source for natural rubber, it will shorten the transporting distance and reduce the CO₂ emissions.”

For now, guayule will be a backup for rubber that Nokian sources from Hevea. “We do not think guayule will replace the majority of natural rubber usage in the short term,” Soini says. Looking more widely, the Finnish firm has a goal of making tires from at least 50% renewable and recycled materials by 2030. “We are taking a holistic approach and looking into all raw materials used in tires,” Soini says.

Bridgestone has spent more than $100 million on guayule. The Japanese firm aims to start using guayule commercially by the end of the decade and to produce tires that are made from 100% renewable materials by 2050.

In April, Bridgestone disclosed that it produced a batch of 200 tires made from 75% renewable materials. Materials include Hevea rubber, plant-based oils and resins, silica from rice husks, recycled steel, mass-balance-produced synthetic rubber, carbon black from recycled tires, and various recycled-rubber chemicals.

Chemical recycling emerges

Widespread adoption of recycling technologies could have a substantial impact on the tire industry. USTMA members collectively generated over 3 million t of CO₂ emissions in 2019, the organization says. The global tire industry produced 2.35 billion tires and consumed 47.7 million t of raw materials in 2021, according to estimates from the market research firm Smithers. About 1 billion tires reach the end of their useful lives annually, according to a report by TIP.

When it comes to the end of a tire’s life, retreading—in which new treads are adhered to the main structure of the tire—is a sustainable, low-energy option that tire companies continue to take, the USTMA’s Amick says. Increasingly, though, tire companies are also interested in buying products recovered from the chemical recycling of tires because they can be made at scale and with fewer greenhouse gas emissions than occur with those made from fossil fuels.

Carbon black and synthetic oils recycled via a pyrolysis-based process developed by the Norwegian firm Wastefront “have a carbon footprint at least 80% lower than that of virgin materials,” CEO Vianney Valès says. Expected to cost $125 million and process 80,000 t of tires per year, the firm’s first tire recycling plant is due to start up in 2025 in Sunderland, England. Earlier this month, Wastefront agreed to sell at least 35% of the recycled carbon black from the plant to the German rubber material firm Weber & Schaer.

In 2021, Bridgestone and Michelin determined that the only way to achieve their tire recycling goals was to mix as much recycled material with their virgin carbon black as they could, Valès says. “Since then, the appetite for recovered carbon black has exploded. It’s now a very, very desired product,” he says. Wastefront plans to introduce 10 tire recycling plants before 2030.

Pyrum’s demonstration plant in Dillingen, Germany, can recycle 7,000 tires per year. The firm plans to triple capacity at the site this year and build 15 other plants by 2030, CEO and cofounder Pascal Klein says. Across the whole industry, “there is a potential for up to 2,000 tire recycling plants around the world,” Klein says. “Europe generates more than 3 million t of waste tires annually. Globally, the market for recycled materials from tires is worth about €20 billion. We are in discussions with companies all around the world, but we try to concentrate on the European market first,” Klein says.

Pyrum says each tire it recycles at the Dillingen plant generates 2.5 L of oil, 5 kg of carbon black, and smaller amounts of textile fibers and steel wire.

The big chemical maker BASF invested in Pyrum in 2020 and agreed to purchase pyrolysis oil from the recycling firm. BASF is using some of the oil to make plastics for Mercedes-Benz cars. Then in a move closing the recycling loop, Pyrum has agreed to recycle several hundred metric tons per year of used tires from Mercedes.

Taking a different route, the technology firm LanzaTech disclosed in April 2022 that it will work with Bridgestone to develop a process for recycling waste tires by applying synthetic biology. As part of the project, the firms will explore using microbial fermentation to make the synthetic rubber ingredient butadiene.

The cloud of tire particles

While the tire industry is already cutting a path to reduce its carbon footprint, it is still at the data-gathering stage when it comes to addressing the impact of tire particles released onto road surfaces and into the air. Scientists estimate that about 6 million t of TRWPs are released by tires every year, but it remains unclear where the particles go and how they interact in the environment.

According to a 2017 report by the International Union for Conservation of Nature and Natural Resources, a coalition of government and civil society organizations, TRWPs make up 28% of all microplastics in the oceans. Another 2017 study, led by Pieter Jan Kole of the Open University of the Netherlands, estimates that 5–10% of all plastics in the oceans are from tires (Int. J. Environ. Res. Public Health, DOI: 10.3390/ijerph14101265). The World Economic Forum flags other findings in the study—namely, that 1.5 million t of tire particles enters the US environment every year. A 2018 study undertaken on behalf of the European Commission concluded that between 50,000 and 140,000 t of TRWPs enters European surface waters each year.

“Some studies have suggested TRWP may have impacts on certain species, while other research has indicated that TRWP are unlikely to significantly impact human health or the environment,” TIP says in an email. The organization has funded 18 peer-reviewed studies into the effects of TRWPs since 2009 and is funding 7 more.

Further studies are required in order to fill multiple knowledge gaps. “Data on degradation is scarce and most studies do not use realistic materials and conditions,” concludes a review of studies on TRWPs published in 2020 by government researchers in Germany (Sci. Total Environ., DOI: 10.1016/j.scitotenv.2020.137823).

Other studies show that TRWPs have a measurable adverse effect on the environment. In 2020, researchers led by Zhenyu Tian at the University of Washington Tacoma and the Center for Urban Waters linked decades of mass deaths of coho salmon in the Seattle area to 6PPD-​quinone (Science, DOI: 10.1126/science.abd6951). This compound is a breakdown product of 6PPD, which tire makers use to protect rubber from degradation caused by ozone, oxygen, mechanical stress, and heat. A study published in 2022 found that 6PPD-quinone was toxic to rainbow trout, brook trout, arctic char, and white sturgeon (Environ. Sci. Technol. Lett., DOI: 10.1021/acs.estlett.2c00050).

Research by the Center for Microbiology and Environmental Systems Science (CMESS) at the University of Vienna led by environmental geoscientist Thilo Hofmann found that five compounds originating from tire particles—one of them 6PPD-quinone—were taken up from soil by plants including a variety of lettuce (Environ. Sci. Technol. 2022, DOI: 10.1021/acs.est.2c05660). The researchers found via a lab-based study that if tire particles are introduced onto farmland by sewage sludge, wind, or wastewater, pollutants from the particles can be taken up by lettuce and so could be eaten by people.

“Tyre wear particles contain a number of organic chemicals, some of which are highly toxic,” Anya Sherman, a PhD student at CMESS and co–first author of the study, says in a press release.

Industry representatives and independent scientists gathered at a December 2022 forum on alternatives to 6PPD. The meeting was convened by the University of Massachusetts Lowell at the request of the Washington State Department of Ecology. “We learned from Dr. Ed Kolodziej at the University of Washington during the December forum that apart from two pesticides, 6PPD-quinone is the most acutely toxic chemical for aquatic species ever evaluated,” says Molly Jacobs, senior research associate for the Lowell Center for Sustainable Production at UMass Lowell.

Removing 6PPD from tires does not appear to be an easy option, though. Industry representatives at the forum stated that without 6PPD, tires could fail after 160–1,600 km of use.

Substituting 6PPD with another compound presents its own set of challenges. As a baseline, an antidegradant cannot interfere with the other chemicals and materials that are mixed into rubber base polymer, Erick Sharp, CEO of Ace Laboratories, told attendees at the forum.

Nokian “is actively following the studies on this topic,” Soini says, though he declines to comment on whether the company is considering replacing 6PPD or other potentially problematic tire compounds. “Reliable field and laboratory tests for understanding the nature, routes of entry, and harmful impacts of the particles are required because many of the current estimates are based on mathematical models and calculations,” he says.

Lanxess, a major producer of 6PPD, has no drop-in replacement for the substance, but it is developing an antioxidant with similar properties to those of 6PPD. “It takes several years to develop a suitable alternative,” says Stephan Meese, global marketing director for functional tire additives at Lanxess.

Substituting 6PPD is “supercomplicated,” acknowledges Joel Tickner, a professor in the Department of Community Health and Sustainability at UMass Lowell. Many issues are involved, including safety requirements, the need for multiple suppliers to be able to produce the alternatives, and the tire industry’s protection of its intellectual property, he says. There are three components in a tire that contain 6PPD, and each contains 0.5–1.5% of the compound, but it is unclear how much is getting into the environment, Tickner says.

“A near-term option could be to design out the part of the molecule that degrades into quinone,” Jacobs says. But such a switch isn’t the long-term solution, she says. “There are reproductive toxicity concerns based on current literature right now with regard to 6PPD itself. This hazard should also be designed out of future alternatives,” she says.

The Washington State Department of Ecology sponsored a toxicity screening of 6PPD along with nine alternatives, most of which are p-phenylenediamines (PPDs). All the PPDs evaluated present a hazard to human or environmental health or both, Jacobs says. Less is known about the toxicity of antidegradants beyond PPDs, she adds.

The industry may have to address 6PPD’s effect on the environment sooner rather than later since Washington is legally obligated to protect the fish for Indigenous peoples. “It’s challenging to change materials in tires, but we are up for that challenge around 6PPD and actively working to assess potential alternatives,” the USTMA’s Amick says.

TIP says in an email that it supports investigating TRWPs’ effects on human and environmental health. The group is currently funding research into the effects of 6PPD and 6PPD-quinone on fish.

Other chemicals in tire particles that pose toxicity concerns include zinc, which is used to vulcanize rubber, and aromatic hydrocarbons. “Many of them are carcinogens when they get released,” Tickner says.

Further studies into tires’ impact are planned. A non-peer-reviewed report that was published in February by a group of scientists at Imperial College London indicates that TRWPs could pose a risk to human health. The Imperial scientists are now seeking funding for an in-depth, multidisciplinary study to determine the extent of tire particle pollution.

Independent scientists and the tire industry agree on one thing: making a tire that does not shed particles while meeting performance requirements—​including stopping and providing comfort, durability, and energy efficiency—may not be possible. But there is “a lot of legroom” for replacing some substances in tires so they are less harmful, says Marc Masen, a mechanical engineer with expertise in tire abrasion and wear and one of the authors of the Imperial study.

The study calls for legislation to reduce the effects of tire particles rather than allow tire companies to develop benign tire compounds at their own pace. “If society doesn’t push to reduce the nasty materials out of rubber, then it won’t happen,” Masen says. “There has not been any legislation driving the use of benign chemicals in tires, and as a result the tire industry’s interests have been limited to grip and energy efficiency.”

The tire industry rejects Masen’s assertions. This is “not at all” the case, the USTMA’s Amick says. “Our industry is fully committed to sustainability, including sustainability of the materials that we’re looking at and using in our products,” she says. When the 2020 coho salmon study was published, within 2 weeks, the USTMA asked the California Department of Toxic Substances Control to add 6PPD in tires to its priority product work plan, which would require the tire industry to assess alternatives, Amick says. But the industry has yet to identify viable alternatives to 6PPD.

In the meantime, the USTMA is advocating for measures to mitigate the effects of 6PPD-quinone in waterways, including the use of porous pavement to trap tire particles and the application of best management practices for stormwater.

Removing potentially problematic compounds such as 6PPD from tires is complex, and manufacturing tires that perform the broad range of tasks required of them without releasing any TRWPs may be impossible. While the substantial environmental footprint of tires at the production and disposal phases could shrink markedly in the coming years, it’s unclear what actions will be taken relating to TRWP pollution.

“It’s great that the industry is moving towards more sustainable materials, but if a sustainable material still has the same toxicity that we have from petroleum products, how is that the ultimate solution?” Jacobs asks. “More sustainable does not mean safer.”