Even before the automobile was invented in the late 1800s, a self-made chemist, Charles Goodyear, paved the way for the modern tire by adding chemicals to alter the properties of natural rubber from the rubber tree. Natural rubber becomes soft and sticky in the heat of the summer, yet stiff and brittle in the cold of winter. In 1839, Goodyear added sulfur and heated the gummy material to produce a flexible yet durable form of rubber.
During World War II, supplies of natural rubber from Asia became difficult to come by, so chemical engineers in the U.S. and Russia came up with synthetic rubber made from crude oil. Their products fulfilled wartime needs for rubber until rubber-tree supplies resumed after the war. During the 1950s, chemists improved the formulas for synthetic rubber, and by the mid-1960s synthetic materials outpaced natural rubber in tires.
Today’s tubeless tires are a composite of up to 200 different chemicals. Each part of the tire — casing, sidewalls and tread — is manufactured to specific performance criteria, using a variety of natural and synthetic rubber compounds. Racing tires are very different from passenger tires, which are very different from truck tires. Various chemical formulations allow for tradeoffs among tire properties, such as resistance to aging, heat and wear, as well as the critical aspect of traction.
During the 1950s, chemical engineers discovered that they could extend the life of tires by adding chemicals to slow the aging process. Ozone, a highly reactive form of oxygen, is created as a byproduct of automobile exhaust. When ozone contacts rubber, it creates tiny cracks by breaking chemical bonds in the long-chain molecules that give rubber its flexibility and resilience. The cracks grow larger and larger with more ozone exposure.
Compounds that capture ozone before it can break rubber’s chemical bonds proved to be a godsend in saving rubber from deterioration. A closely related group of chemicals, the para-phenylene-diamines, proved especially effective at grabbing ozone when added to rubber. Such chemicals include 6PPD, the most commonly used in tires today, as well as IPPD and DPPD.
Small amounts of these additives (0.5 to 2 percent) in rubber can inhibit ozone degradation by migrating to the surface of the tire, where they form a microscopic film. Experts still debate whether the tire is better protected by the micro-film layer or by the rapid reaction between ozone and PPD, but both come into play.
In any case, tiny bits of rubber eventually fly off onto the roadway and wash into streams. These bits of rubber carry with them a new chemical never added to tires and apparently overlooked when studying the toxicity of tire-related compounds. This chemical is the mysterious, highly toxic compound believed responsible for the deaths of many coho salmon. It’s called 6PPD-quinone, as quinone is a chemical group bearing oxygen atoms picked up during the rapid reaction between 6PPD and ozone.