Description of Dry Rot in Tires
The term "dry rot" was originally used to describe certain types of decay in timber and wood construction. Although in construction circles, the "dry" part of dry rot refers to the type of wood (not the conditions under which it happens), the term takes on a bit more literal meaning when applied to tires.
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Tire Composition
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Modern tires are composed of many different elements, not just the rubber once used exclusively. Passenger car tires only contain about 14 percent rubber; the rest is carbon black (a petroleum distillate), silica and silicone that binds the carbon and silica together.
Rubber Structure
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Rubber comes from latex, a naturally occurring polymer. Polymers are chains of molecules tied together with one of several different types of bonds. In its natural state, rubber's molecules sit in a coil; the material's elasticity comes from the polymer's ability to uncoil and spring back into shape when released. Rubber depends on a strong network of connections between its molecules. When those bonds break, the rubber hardens and eventually crumbles into a black dust.
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Oxidation
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Like any organic substance, rubber reacts with oxygen in a sort of slow burn called "oxidation." On a molecular level, the oxygen interacts with the rubber molecules, slowly pulling enough atoms out of the structure to break the bonds. The effect is almost exponential. The smaller the polymer chains get, the more places oxygen has to attack and the quicker it destroys the material.
Appearance
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Dry rot cracks usually are first apparent on the tire's sidewalls, because the sidewalls are wide and flat enough to make the cracks apparent. The cracks start out as tiny slits, but as the material hardens and pulls away, the crack grows in both length and width. Dry rot cracks are rarely solitary creatures; the tire will have thousands of tiny little slits coalescing together as they grow.
Movement
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The quickest way to cause dry rot damage is to park your car for weeks at a time and then drive it. Moving the car causes the rubber to flex, which drastically accelerates crack propagation, once the rubber starts to harden. Take a look at the sidewalls of any motorhome or camper, and you're bound to see some dry rot.
Radiation Damage
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Ultraviolet (UV) radiation breaks down tire polymers in much the same way as it does the cell walls in your skin. Modern tires contain several additives that act as a sort of sunscreen to prevent UV radiation damage, but those chemicals can only negate so much UV before radiation destroys them too. Allowing your tires to sit in direct sunlight will result in the same depolymerization as that caused by oxygen exposure.
Protection
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The simplest way to prevent dry rot is to regularly apply a tire conditioner with UV protectant. Tire conditioners keep the rubber soft and supple, helping to negate polymer breakdown. Many tire protectants also contain silicone, which can help keep oxygen from getting to the surface in the first place. Of course, that doesn't do anything for the tread but it does help protect the sidewalls.
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References
- Photo Credit Tire with a nail image by Scott Griessel from Fotolia.com
