Tires

Composition

The materials and composition of tires vary depending on the uses. Natural rubber or polyisoprene is the basic elastomer used in tire making. But rubber is soft and sticky. Vulcanization of rubber is one the main processes of tire manufacturing. It strengthens regular rubber by adding sulfur and complex organic compounds to speed up the process of solidification. Activators like zinc oxide assist the vulcanization. Styrene-butadiene copolymer (SBR) is a synthetic rubber that is often substituted in part for natural rubber based on the comparative raw materials cost. Polybutadiene is also used in combination with other rubbers because of its low heat-buildup properties. Carbon black, one of the original ingredients for tires, forms a high percentage of the rubber compound. It gives reinforcement and abrasion resistance.

Most tires have some amount of halobutyl rubber mixed with additives to decrease air permeability. They also have one or more layers of reinforcing fabric such as rayon, nylon, polyester, and Kevlar. Additives used in sidewall compounds include antioxidants and antiozonants. The beads are bands of high-tensile-strength steel wire encased in a rubber compound. Bead wire is coated with special alloys of bronze or brass. Coatings protect the steel from corrosion. Copper in the alloy and sulfur in the rubber cross-link to produce copper sulfide, which improves bonding of the bead to the rubber. Bead rubber includes additives to maximize strength and toughness. The tread (the thick extruded profile that surrounds the tire body) compounds include additives to impart wear resistance and traction, in addition to environmental resistance.

Production Waste

Producing a tire is normally a controlled process with respect to the production and disposition of its derived waste and levels of toxicity. The International Agency for Research on Cancer (IARC) has found that there is nonconclusive evidence for the carcinogenicity of carbon black to humans. Silica is used together with carbon black in high performance tires as a reinforcement. It has been documented that silica can cause silicosis, a disabling, nonreversible, and sometimes fatal lung disease caused by overexposure to respirable crystalline silica. Silica exposure is currently a serious threat to nearly two million U.S. workers.

Most of the environmental concerns in tire manufacturing and recycling are related to air emissions and solid waste management. The Environmental Protection Agency (EPA) has proposed several recommendations to reduce waste in rubber-related manufacturing companies. Chemicals should be received in closed docks in sealed containers or in bulk rail or truck shipments with a minimal history of spills. Chemical piles should be stored inside the facility to ensure that any fugitive emissions can be contained within the facility.

...