Both natural and synthetic types of rubber tend to deteriorate over time due to the bonds the rubber molecules have with each other. In order to slow down the process as much as possible, tire manufacturers put a coating on their tires to protect them from the harmful ultraviolet radiation of the sunlight and the corrosive qualities of the air. However, these coatings cannot last forever, and when they wear away the rubber of the tires begins to degrade. This is what causes tire rot.
Rotting tires can be affected by many different factors, most notably climate and weather. A wet climate can wear away at the coatings on a tire with constant exposure to water and dirt, but a hot, dry climate can bake tires and degrade them very quickly with use on hot roads and exposure to sunlight. When tires begin to degrade from these conditions, they develop fine cracks and lines, which grow wider and deeper with time, literally eating into the tire until its structure is compromised.
Tires are always victims of time in the end. The carbon-based protectant that manufacturers use to protect the tires against sunlight has a limited lifespan. In the end, the coating will begin to be used up and slowly wear away into a gray color, leaving the tire open to UV damage. Old tires or tires that have begun to rot usually look grayish because of this effect. After the coating is gone, the radiation from the sunlight pummels energy into the rubber polymers and slowly but surely breaks them apart.
Rubber also needs protection against oxygen and ozone. Oxidation, the same process that causes metal to rust, also affects the rubber of tires and can have very destructive results. In order to slow the process of oxidation, manufacturers usually equip tires with a waxy protectant that seals the rubber away from the oxygen in the air. Unfortunately, through the life of the tire this protectant begins to fade away, too, especially if the tire is not used. Tires that are used regularly "bloom" or have the protectants inside them squeezed out onto the surface where they can do their job. Tires stored away lose their surface protectant and can be damaged more quickly.
Finally, tire rot is quickened by the actions of various degradants that tires are exposed to during their use. This includes most petrochemicals and many different silicone oils, which tires can encounter on the road or in repair shops. These chemical and corrosive liquids like them eat away at the barriers tires have, exposing the rubber polymers even more quickly and hastening the tire rot.
