Sustainable Tires

Tire manufacturers are adopting greener production processes and more renewable materials, but they have yet to get a grip on tire particle pollution.

Brief:

Tires’ substantial environmental footprint is set to drop as producers increase their use of renewable materials and efficient technologies emerge for chemically recycling them. Currently, most tires are incinerated or landfilled, at a high environmental price. While the sector’s footprint may be shrinking, millions of metric tons of rubber particles get released annually as tires wear down on roads. And independent scientists say this pollution is a cause for concern. The tire industry says it is trying to understand the impacts of these particles. Until issues around them are addressed, however, the sustainable tire will remain out of reach.

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.

Credit: Pyrum Innovations

After being mechanically ground into granules, rubber-rich tire particles are fed into Pyrum Innovations’ vertical pyrolysis reactor to make recycled carbon black and synthetic oil.

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. 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.

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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.

Credit: Nokian Tyres

Nokian Tyres is growing guayule shrubs in Spain as an additional source of natural rubber to go into its tires.

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.

Credit: Continental Tire

Continental Tire hopes to use latex from the roots of a Russian variety of dandelion in its tires.

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.”

We are taking a holistic approach and looking into all raw materials used in tires.

Teemu Soini, head of innovation and development, Nokian Tyres

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.

Related: Podcast: Tires face yellow flags and pit stops on the way to sustainability

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.

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?

Molly Jacobs, senior research associate, Lowell Center for Sustainable Production, University of Massachusetts Lowell

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.

Learn More:

• Tire Pyrolysis
• Recovered Carbon Black
• Pyrolysis Oil
• What Is Pyrolysis
• Feasibility Studies
• Turn-Key Projects

Greener Tyres Start By Recycling The Old Ones

Pyrolysis turns end-of-life tyres into carbon black and synthetic oil — the same recycled feedstocks tiremakers now build greener tires from.

Klean Industries supplies the pyrolysis and rCB capacity that makes the sustainable tire road real.

Why Choose Klean Industries’ Tire Pyrolysis?

✅ Recovers tyres into carbon black and oil
✅ Carbon footprint below virgin feedstocks
✅ Feedstocks the tire industry is buying today
✅ Commercial plants with proven offtake records

Ready to turn waste into a resource?

Contact Klean Industries about sustainable tire recycling » GO.