The durability of the modern pneumatic tire, which makes it effective on vehicles, creates a massive waste problem when the tire reaches the end of its useful life. Globally, an estimated 1.5 billion end-of-life tires (ELTs) are discarded each year, with the United States alone contributing approximately 280 to 317 million waste tires annually. This immense volume necessitates comprehensive recycling strategies that convert the complex rubber, steel, and fiber composite into diverse materials. Tire recycling transforms this challenging waste stream into practical applications across large-scale infrastructure, consumer products, energy generation, and advanced manufacturing.
Large-Scale Infrastructure Applications
Civil engineering projects represent a high-volume application for processed tire material, often utilizing shredded tires or fine crumb rubber to replace traditional aggregates. One of the most common applications is rubberized asphalt, where fine crumb rubber is mixed with the asphalt binder to modify its properties. This addition improves the pavement’s elasticity, making the road surface more resistant to defects like cracking and rutting caused by temperature fluctuations and heavy traffic. Incorporating crumb rubber also acts as a sound absorber, reducing traffic noise in urban highway corridors.
Larger pieces of shredded rubber, known as Tire-Derived Aggregate (TDA), are used as a lightweight fill material in construction, particularly for building road embankments or backfilling retaining walls over soft soils. The material is significantly lighter than conventional soil, typically about one-third the unit weight, which reduces the load on the underlying foundation. TDA is also free-draining and offers exceptional thermal resistivity, helping to insulate road subgrades against frost penetration and freeze-thaw damage.
Consumer and Recreational Surface Materials
The elasticity and shock-absorbing characteristics of recycled rubber make it an ideal material for surfaces focused on safety and comfort. Fine crumb rubber is widely used as the infill material in synthetic turf athletic fields, where the granules are spread between the artificial grass blades. This infill provides cushioning, surface stability, and rotational traction, helping to reduce the risk of non-contact injuries common on older turf systems. The material also helps manage field temperature by providing a layer of insulation.
For playgrounds, recycled rubber is processed into poured-in-place (PIP) surfacing or pre-formed tiles that offer superior impact attenuation. These surfaces are designed to meet stringent safety standards, effectively cushioning a child’s fall from a certain height. The rubber is also molded or re-vulcanized into durable consumer and commercial goods. These products include anti-fatigue mats, landscaping mulch, speed bumps, traffic cones, and dock bumpers, leveraging the rubber’s inherent durability and weather resistance.
Energy Recovery Through Thermal Conversion
A significant portion of end-of-life tires globally is converted into Tire-Derived Fuel (TDF), a process involving the combustion of shredded tires for energy. TDF is a highly calorific energy source, boasting a heat value of approximately 14,000 to 15,500 British Thermal Units (BTU) per pound, which is higher than most grades of coal. This high energy content makes TDF an attractive supplemental fuel source for energy-intensive industries.
The primary industrial users of TDF are cement kilns, pulp and paper mills, and utility boilers. For cement manufacturing, the steel wire content in the tires provides iron, which acts as a raw material flux in the clinker production process. The use of TDF displaces fossil fuels, offering a consistent and cost-effective energy alternative for heat and steam generation.
Advanced Material Feedstock Production
Beyond simple combustion, advanced recycling methods focus on breaking the tire down into its original chemical components to create new feedstocks. Pyrolysis is a thermal decomposition process that heats whole or shredded tires in an oxygen-free environment. This process prevents combustion and breaks the complex polymer chains in the rubber into smaller, reusable molecules.
The pyrolysis process yields four main products: pyrolysis oil, recovered carbon black (rCB), steel wire, and non-condensable gas. Pyrolysis oil constitutes 40 to 50 percent of the output and can be used as industrial fuel or refined into transport-grade diesel. Recovered carbon black (30 to 35 percent of the yield) is a valuable material that replaces virgin carbon black as a reinforcing filler in new tires, rubber goods, and plastics. Steel wire (10 to 15 percent) is recovered and sent back into the steel recycling market.