The following is from an article that David B wrote:
Your bike's tires have several formidable enemies: water, formaldehyde, petroleum distillates, ultraviolet light, and ozone. Water washes away the natural oils in rubber that keep it elastic. Formaldehyde and petroleum distillates act as a solvent, eating rubber on contact. When ozone, an odorless gas which is part of the air we breathe, is combined with ultraviolet (UV) light, a reaction occurs that attacks the tire and its polymers (the agents that bind the rubber).
To protect against ozone and UV damage, a stabilizer molecule called a competitive absorber is blended with the tire polymer. Competitive absorbers work by capturing and absorbing UV radiation and converting it to heat which is dissipated harmlessly. All tire manufacturers use the same competitive absorber, called carbon black. This is why all tires are black.
These absorbers are sacrificial; they expend themselves in performing their function of converting UV light to heat. However, as carbon black loses its ability to perform, it turns gray. This is one reason why black tires discolor as they age.
To protect from further ozone damage, tire manufacturers add a wax compound to their formulas. Tires flex when they are in motion, causing the wax molecules to migrate to the surface. This forms as a protective barrier between the air (ozone and oxygen), water and the tire polymer. In the tire trade this is called blooming. When tires are parked for extended periods, blooming does not occur and ozone quickly attacks the tire polymer. With UV light and ozone working in concert, the degradation is accelerated, resulting in drying, discoloration and cracking.
To combat the negative effects of water, solvents and UV light on tires, the bike care industry has created tire dressings. These dressings condition the tire, restoring essential moisture. Tire dressings fall into two groups:
1. Liquid Silicone Oil Dressings – These penetrating-type silicones form a flexible protective shield on rubber. Liquid silicone seals small openings with a film to prevent penetration of moisture and dirt. Most silicone dressings leave a never-dry gloss film. There are many myths regarding silicone, specifically the negative long-term effects of silicone on rubber and vinyl. The fact is, silicone is an inert material. The benefit of silicone is its ability to easily penetrate the tire’s surface and not evaporate. Some silicone-based dressings contain petroleum distillates as a cleaning agent. Petroleum distillates are harmful to rubber and vinyl, and will cause rubber and vinyl to crack. If you decide to use a silicone oil tire dressing, make sure it does not contain a cleaner.
2. Water-Based Dressings – The water-based dressings do not contain silicone oils, petroleum distillates, waxes, or other dangerous solvents that can harm and dull the surface of rubber and vinyl over time. Most water-based dressings use a water-based silicone, which offers a non-greasy, satin finish. The best products contain UV blocking agents to help keep tires and rubber from cracking, fading and hardening.