Chances are you've encountered tire blooming and you don't even know it. Blooming is what causes a tire to look brown. That new set of tires that you have to scrub over and over again to get them to look black, or the tires on a car you detail less frequently that are closer to the color of chocolate than they are black. Modern rubber compounds are becoming increasingly complex, far more than most people realize. Tire manufactures are continually pushing the envelop with chemistry and design to create tires that can keep up with the demand of todays cars and drivers. Higher mileage, more miles per gallon, better all-weather traction, or high speed and cornering as cars get better, faster, more intense the tires they roll on must change to keep up.
We expect increased performance from our vehicles and tires are an integral part of that, but rarely do we take the time to understand what exactly has changed about tires other than going from bias ply to radial in the late 1960's.
What does this all have to do with your tires turning brown? Read on.
An anti-ozonant is probably something you've never heard of. Its an organic compound added to rubber materials that prevents, or at the very least, slows the deterioration caused by exposure to the elements. Anti-ozonants are used as an additive in most all of the exterior rubber and plastic parts to one degree or another, but they are most prevalent in tire manufacturing. The anti-ozonant additive keeps plastics and rubbers from becoming dry, brittle, oxidized or cracking. It does this by preventing the surface of the material from oxidizing and keeps the material pliable.
Thanks to anti-ozonants in rubber compounds we have have high mileage tires, performance tires, and everything in between. Without it sports cars would shred tires incredibly fast after just a few high speed turns or long track runs where the tires were heated up. Even your daily driven commuter car would need tires far more often as the sun and heat slowly rotted away the rubber compounds.
Tire rubber compounds are designed in a way that allows the anti-ozonant to continually work its way to the outside of the tire and as such, continually keeps the outer surface and sidewall pliable and resistant to oxidation.
Once anti-ozonant reaches the outside of the tire and is exposed to air and moisture it oxidizes, the result being a brownish residue. The term for this ugly brownish tire look is 'tire blooming'. Just like metals left exposed to the outside world will slowly begin to rust (oxidize) as it is exposed to water and air, so does the anti-ozonant component of the tire rubber.
Making matters worse is the use of mold releases in the manufacturing processes. These lubricant type chemicals provide a non-stick surface for the inside of a tire mold. The mold release chemical bonds with the tire and hold anti-ozonants onto the surface of the tire.