Tire Abrasion as a Source of Microplastics

The presence of tire particles resulting from the abrasion of tires on road surfaces has previously been documented in surface water, sediments, and biota. In the European Union, several countries (Germany, Norway, Denmark, and the Netherlands) recognize these tire particles, commonly referred to as tire wear particles, as a major source of microplastics. While tires may not traditionally be thought of as a type of plastic, 60% of rubber used in the tire industry is composed of synthetic elastomers [58]. Synthetic elastomer is a type of polymer derived from petroleum hydrocarbons with viscoelasticity (having both viscosity and elasticity).

In the United States, it is estimated that 500 x 10⁶ kg of tire wear particles are released into the environment each year. About 10-20% of a tire’s weight is worn off during its use due to wear and abrasion. The wear rate of tire treads has been shown to be several times higher during urban driving than on highways due to increased acceleration, braking, and cornering. The composition of automobile tire rubber can vary considerably, but generally speaking, tire tread consists of a blend of synthetic and natural rubber polymers (40-60%), reinforcing agents such as carbon black or silica (20-35%), mineral oils (15-20%) and vulcanizing chemicals (25%).

Typical synthetic polymers used in tire treads include styrene butadiene rubber (SBR) and polybutadiene (PBD). During the vulcanization process, adjacent polymer chains are cross-linked, following the addition of activators (e.g. zinc oxide) and accelerators (e.g. benzothiozole), producing elastomers that give the tire its desired elastic properties. Once emitted into the environment, smaller tire wear particles (<5 µm) become airborne, while larger particles (>20 µm) are deposited on or near the road surface. Stormwater runoff has the potential to transport the particles to stormwater drainage systems and surface waters.

Another potential source of tire particles in the aquatic environment is crumb rubber, which is produced from grinding automotive scrap tires. One application of crumb rubber is infill to provide cushioning on artificial turf. These loose particles have the potential to be washed into stormwater drains and surface waters during a heavy rain or flooding event. Kreider et al. investigated the morphological differences between road-generated tire particles and manufactured crumb rubber. Tire wear particles are typically elongated, while crumb rubber is more jagged in shape. It has been suggested that morphology be used as one of the characteristics for their identification in environmental samples.