Rubber was first used to manufacture tires in the mid-nineteenth century. By the turn of the twenty-first century, there were about three billion waste tires stockpiled or clogging leak pollutants intolandfills in the United States alone. Worldwide, about 700 million scrap tires are generated annually. This waste rubber has become a major source of pollution.

Although rubber, made from latex secreted by trees, has been used for thousands of years, it wasn't until 1839, when Charles Goodyear invented vulcanization, that rubber became important for manufacturing. Vulcanization uses sulfur to cross-link the latex fibers into a rubber that is strong, flexible, durable, and resistant to heat and cold. Like natural rubber, modern synthetic rubbers are polymers (long chains of similar molecules) that are cross-linked by vulcanization. Tires consume 60–70% of all rubber produced, twothirds of which is synthetic.

Of the 273 million waste tires generated annually in the United States, about 25% end up...