Tires.

Overview

Over half a billion automobiles throng the world's roads, and keeping them rolling requires some 1.1 billion new pneumatic tires every year. Each of them is a sophisticated bit of transportation technology, and a far cry from the ancestral leather air-filled tire invented by Englishman Robert Thomson in 1845. That line went extinct, but another lives on: the descendants of the rubber bicycle tire crafted a bit later by John Boyd Dunlop, an Irish veterinarian. Dunlop's sire tire, adapted for auto use, led step by step to the dominant member of the modern tire family, the steel-belted radial.

Disposal

In effect, the use of a tire also begins its disposal, as the outer six millimeters of tread wears off. After a tire is worn out and discarded--which happens roughly a billion times a year worldwide--it might end up as sandals, a boat bumper or tire swing, or at the bottom of a river. But most tire carcasses go on to one of three afterlives. Up to half are landfilled or piled up to await some other use. Estimates of the total in U.S. stockpiles alone vary from 300 million to 2 or 3 billion. Sometimes those piles catch fire, which spews pollutants into the air and groundwater.

A surprising number are retreaded, which uses only about 30 percent of the energy required to make a new tire. Retreads are used on school buses and commercial aircraft, and on all U.S. government vehicles. Perhaps 45 million scrap tires are used to make about 25 million retreads every year in the United States. Other reuses include construction-related applications.

The biggest single use of scrap tires (nearly half of U.S. scrap) is as fuel. Tires are burned in cement kilns, waste-to-energy plants, and industrial boilers. It's a tempting solution to the stockpile problem, but burning a tire yields only about one-sixth of the energy it took to make it, and 85 percent of a tire is carbon--making them another source of greenhouse gas emissions. More recent developments have allowed rubber powders to be to be incorporated into carpets, roofing, and molded products.

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Materials and Manufacture

A tire is more than a hollow rubber donut; it's a complex structure built up in stages by skilled workers using expensive machines and incorporating a wide variety of materials, including natural rubber, butadiene rubber, and carbon black (sidewalls); butyl rubber (liner); styrene-butadiene rubber and more carbon black (tread); fiberglass, nylon, and/or polyester (cords); steel in the beads (the wires that help seat the tire on the wheel rims) and in the belts; and various antioxidants and pigments. In all, dozens of chemicals and types of materials are used in tires--30 types of petroleum-based synthetic rubber alone, which together make up about one-quarter of a tire's weight. Cars run on oil in more ways than one.

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Use

For most parts of an automobile, wear is a bad (if unavoidable) outcome of use, and strenuous efforts are made to prevent wear. But tires must wear out in order to work; it's the friction between the tire and the road that allows a tire to do its job, and that friction abrades away the tire's surface. Although tire disposal causes lots of environmental problems (see left), a study from the University of California at Berkeley estimated that 95 percent of a tire's environmental impact derives from its use.