Crumb rubber is a recycled material derived from the millions of scrap tires discarded annually, offering a significant solution to end-of-life tire management. This material is a carefully processed granular product that retains the beneficial characteristics of rubber, such as durability and flexibility. Its creation transforms a bulky, non-biodegradable waste product into a valuable secondary raw material. The use of this recycled product diverts a tremendous volume of waste from landfills and stockpiles each year. This size reduction and purification process makes the resulting material suitable for high-performance applications across civil engineering, consumer goods, and athletic facilities.
Defining Crumb Rubber and Its Source
Crumb rubber consists of uniform rubber particles obtained by reducing scrap tires or other rubber products into granules, with reinforcing materials systematically removed. The primary source is the end-of-life tire (ELT), including passenger car, truck, and off-the-road tires. Processing separates the rubber compound from contaminants like steel belts and nylon or polyester fiber, which are also recycled.
The rubber particles are classified by size using a mesh system, which dictates the number of holes per linear inch in the screening equipment. A higher mesh number indicates a smaller, finer particle size, while a lower mesh number corresponds to larger, coarser granules. For example, 10-20 mesh material consists of particles roughly the size of coarse sand, often used for artificial turf infill. Different mesh sizes are necessary because the required texture and ability to blend vary depending on the final product application.
Transforming Scrap Tires into Crumb Rubber
The process of converting whole tires into fine crumb rubber is a multi-stage operation involving size reduction technologies. Two distinct methods dominate the industry: ambient grinding and cryogenic grinding, each producing rubber particles with unique characteristics. Ambient grinding is the most common method and involves mechanically shredding and granulating the tires at or slightly above room temperature.
In the ambient process, whole tires are first shredded into chips. These chips are fed into granulators that cut the rubber into smaller pieces, while liberated steel is continuously removed magnetically and fiber is removed using air separation. The resulting particles are typically irregular and jagged, possessing a rough surface area that allows for physical binding when mixed with other materials. Although effective for large-volume production, the mechanical shearing generates heat that must be managed to prevent rubber degradation.
Cryogenic grinding uses a different approach, subjecting pre-treated tire chips to extremely cold temperatures, often between -80°C and -120°C, using liquid nitrogen. At this temperature, the rubber reaches its glass transition point, becoming brittle and glass-like instead of elastic. The embrittled material is then sent through a hammer mill, where it shatters easily into small, fine particles.
This technique is more energy-intensive due to the nitrogen requirement. However, it yields very fine, smooth particles with a low surface area. Cryogenic processing is effective for achieving high purity and producing ultra-fine mesh sizes, sometimes up to 80-200 mesh, required for specific manufacturing applications.
Major Commercial Applications
The physical properties of recycled rubber—elasticity, durability, and shock absorption—make it a preferred material for high-volume commercial uses. One visible application is its use as infill material for synthetic turf fields, consuming a significant portion of the total crumb rubber produced. The fine rubber granules are poured between the artificial grass blades, holding the synthetic fibers upright and providing a layer of cushioning. This layer improves shock absorption, helping to reduce impact forces on athletes and making the surface more playable than older turf systems.
A second major application is in road construction, where crumb rubber is used to produce Rubber Modified Asphalt (RMA). When blended with asphalt binder, the rubber improves the pavement mixture’s elasticity and resistance to temperature fluctuations, mitigating cracking and rutting. This modification leads to longer-lasting roads that require less maintenance. The rubber’s properties also contribute to reduced traffic noise levels.
A third category involves its incorporation into various molded and extruded products. These applications leverage the material’s durability and weather resistance to create items such as playground safety tiles, which offer fall protection. Crumb rubber is also used in rubber mats for industrial, agricultural, and commercial settings, as well as in molded components like speed bumps, railroad crossings, and sound barriers.
Environmental and Safety Considerations
The widespread use of crumb rubber, particularly in high-contact areas like artificial turf, has generated public concern regarding the potential release of chemical components. Tires contain various additives, including zinc, sulfur, and processing oils that may contain polycyclic aromatic hydrocarbons (PAHs). Safety questions revolve around the potential for these substances to leach into the environment or volatilize into the air.
One environmental concern is leachate, where water runoff carries soluble compounds from the rubber granules into surrounding soil and water bodies. Studies have focused on the release of zinc and PAHs, noting that increased concentrations could affect aquatic organisms. Another exposure pathway is the release of volatile organic compounds (VOCs) into the air, particularly under high temperatures.
Multiple federal agencies in the United States, including the Environmental Protection Agency (EPA) and the Consumer Product Safety Commission (CPSC), have conducted research to assess these risks. The consensus from many health and environmental studies suggests that exposure to chemicals from crumb rubber under normal recreational conditions is very low and unlikely to pose a health risk. The ongoing Federal Research Action Plan (FRAP) continues to investigate potential exposures to provide a balanced understanding of this recycled material.