Tires are the biggest source of rubber waste globally. The World Council for Sustainable Development estimates that globally 4 billion waste tires are currently in landfills and stockpiles, and an additional 1 billion are generated annually.1 Rubber products, including tires, consist of a complex mixture of natural and synthetic rubber polymers, fillers, and additives to impart good mechanical properties. During manufacturing, the rubber polymers are vulcanized (permanently chemically crosslinked), increasing their strength but preventing their breakdown for recycling. Unlike plastics, vulcanized rubber cannot be melted and reshaped, nor can the vulcanization process be easily reversed to revert rubber to its virgin state. If conventional recycling isn’t an option, where do waste tires end up?
There are several conventional and cutting-edge pathways for these end-of-life tires, each with its own pros and cons.
Landfilling and Stockpiling
Conventionally, tires were seen as linear products and were simply sent to landfills or stockpiled on-site once worn out. Stockpiling is especially common in the mining industry, where large tires are unwieldy to transport offsite, and large open areas are available for storage. Stockpiling and landfilling have minimal costs and are largely compatible with conventional waste management systems but carry substantial risks. Stockpiled tires present a serious fire risk, and tire fires release toxic chemicals and are extremely difficult to extinguish. Longer-term risks include the leaching of dangerous chemicals into the groundwater supply and the proliferation of pests, such as mosquitos, that easily breed in retained still water.2 Tire stockpiling is discouraged or outright banned in many countries, but existing stockpiles tend to remain in place until government abatement programs are enacted.3
Tires and Rubber Crumb in Downgraded Applications
Whole tires or shredded waste rubber are used in various civil engineering applications. Whole tires can be used as support structures (e.g., in retaining walls), maintaining some, but not all, of their desirable properties. Shredded tires (rubber crumb) can be used as an additive in concrete or in mortar materials. In both cases, rubber crumb replaces virgin materials such as sand, steel and gravel and can impart desirable properties to the final mixture such as vibration dampening.
Rubber crumb is also used in other application types, such as a filler or mix-in for rubber-modified-asphalt, artificial sports fields, playgrounds, and molded rubber products such as car mats. In these applications, many of the desirable properties of vulcanized rubber are lost, and the material ‘downgraded’ compared to a functional tire.
The above applications divert waste rubber from landfills, but result in a loss of desirable properties and therefore a loss in value. Whole tire and crumb rubbers can have hazardous effects similar to landfilling, and civil engineering applications risk becoming de facto landfills at their end-of-life. There may be health risks associated with increased rubber crumb exposure when used in playgrounds and artificial turf. The rubber may be abraded with use and release micro plastics into the environment. From an economic standpoint, there is insufficient demand for rubber crumb in these applications compared to the enormous amounts that are produced annually.
Combustion: Tire Derived Fuel
“Tire-derived fuel” (TDF) comprises shredded tires that are burned to produce energy. TDF combustion results in thermochemical decomposition and the release of energy that can be converted into electricity, and is typically utilized by highly energy consuming industries. Waste rubber has a high carbon content and produces similar amounts of energy to petroleum fuels and coals when combusted, while producing slightly fewer pollutants. Combustion does not retain the value of the rubber material, and less energy is recovered than is utilized in the production and shredding of tires.
Pyrolysis
Decomposition of a material under high heat in an oxygen-free environment is known as pyrolysis. In the absence of oxygen, rubber decomposes into different products than it otherwise would in air. Rubber pyrolysis produces petrochemical products (oils and gasses) and an inorganic filler material called recovered carbon black, recovering some (but not all of) the initial value of the waste rubber. Pyrolysis is energy intensive, potentially polluting, and its products have limited economic demand compared to their high production costs.4
Devulcanization
Devulcanization, or the reversion of chemical crosslinks in vulcanized rubber, is the subject of ongoing research. Devulcanization involves selectively breaking the chemical bonds formed during vulcanization, leaving the bonds in the rubber polymer backbone intact. A high-quality, ‘virgin-like’ rubber material is obtained that can be used to make new rubber products, including tires. Waste rubber is recycled into new rubber products, maintaining the economic value of the material.
Although promising, devulcanization technology is not fully mature. A few organizations use devulcanization on a commercial scale, but the process is often energy and capital-intensive or requires potentially toxic chemicals. The resulting reclaimed rubber can suffer from some quality loss and may not be used as a 1:1 replacement for virgin rubber in all applications. Both process and product are continually improving as a result of research and development efforts by a number of academic and industrial organizations, but both have yet to be adopted industry-wide.
Conclusions
No existing waste tire pathway is truly ideal. On top of existing stockpiles, increasing amounts of rubber waste are produced annually as more vehicles hit the roads. Innovative solutions, developed with input and efforts from all stakeholders, are required to turn rubber from a linear into a truly circular material. One pathway may not be suitable for all waste streams and value chains, but efforts are ongoing to tackle rubber waste in an environmentally friendly and economically viable way.
References
- World Business Council for Sustainable Development. (2008). (rep.). Managing End-of-Life Tires. Retrieved from https://docs.wbcsd.org/2018/02/TIP/End_of_Life_Tires-Full-Report.pdf.
- Formela, K. (2021). Sustainable development of waste tires recycling technologies–recent advances, challenges and future trends. Advanced industrial and engineering polymer research, 4(3), 209.
- Valentini, F., & Pegoretti, A. (2022). End-of-life options of tyres. A review. Advanced Industrial and Engineering Polymer Research, 5(4), 203.
- Han, W., Han, D., & Chen, H. (2023). Pyrolysis of Waste Tires: A Review. Polymers, 15(7), 1604.
