Why are tires black?

Tires didn't start out black

The first car tires were pale. A vintage-tire maker describes them as “off white”³, and according to a carbon black producer, the English “Silvertown” tires of that era had gray tread⁴. A Michelin representative put the lighter shade down to “the natural color of rubber”². That doesn't mean rubber is snow-white: the latex tapped from rubber trees is milky white¹, but solid natural rubber can turn distinctly yellow, and sometimes even darker¹¹.

Later, zinc oxide, a white powder, was added to make tires even brighter³.

Enter carbon black

In the 1910s, tire makers started mixing carbon black into the rubber^{1, 2, 3}. One company history says B.F. Goodrich experimented with “Silvertown” tires, whose tread was tinted gray with a little carbon black, and found they also lasted longer⁴. Goodrich added more, and in 1911 asked for a million pounds of carbon black a year⁴.

The payoff was huge. Michelin says carbon black produces “a tenfold increase in wear resistance”¹. Today it makes up about 25 to 30 percent of the rubber compound, by Michelin's figures¹, and roughly 73 percent of the world's carbon black goes into tires⁵. So the black color comes with the filler: the powder is there mainly for strength, and it turns the whole mix black^{1, 3}.

Why a black powder makes rubber tougher

Carbon black is not ordinary soot. It is made under controlled conditions¹², and most types are more than 97 percent carbon, while soot is typically less than 60 percent carbon⁶. Its basic particles measure roughly 10 to 300 nanometers⁶, far too small to see without an electron microscope².

Cured rubber on its own is fairly weak; reinforcing particles raise its tensile strength, tear resistance and abrasion resistance⁵. Part of the rubber clings to the particle surfaces so tightly that solvents can't wash it off, which chemists call “bound rubber”⁵. When researchers heat-treated carbon black at 3,000 °C, it lost much of its reinforcing effect: far less rubber stayed bound to it, and the rubber wore away two to three times faster⁵. Exactly how rubber and particles bond is still debated⁵.

In the factory, a mixer kneads rubber, carbon black and other ingredients into “a homogenized batch of black material”⁸, so the color runs all the way through, not just on the surface.

Extra jobs, and a white teammate

The black also helps against sunlight: Michelin says it is “very effective in acting against ultraviolet rays,” which keeps the rubber from cracking¹.

Since the 1990s, many tires also contain silica, a white filler made from sand, especially in the tread^{1, 9}. The U.S. tire makers' association says both fillers reinforce rubber and that silica improves rolling resistance⁷; Continental says it also greatly improves wet grip⁹.

Whitewall tires may be a leftover of the switch: according to one vintage-tire maker, a smaller company put carbon black only in the tread to save money, leaving the sidewalls white³.