There are many different types of rubber. Most tires are made of blends of synthetic rubbers and natural rubber. Natural rubber is very abrasion resistant but has little resistance to oils, sunlight or ozone. Styrene butydiene rubber is the synthetic rubber that makes up most of the tire. It's properties are similar to natural rubber. Other rubbers used are: EPDM, a synthetic that offers outstanding weather resistance. Butyl; another synthetic that is resistant to passage of air molecules. Butyl is responsible for our not having to air up tires every few weeks as was the case 35 years ago when natural rubber was used for inner tubes. I'm sure some of you guys remember (Dad's car). A variety of materials are added to the base rubbers to create a compound with specific characteristics. Waxes and protectants are added to SBR that slowly bleed to the surface and protect from weathering. Carbon black is added to provide a solid material that is exposed as the rubber is eroded. This essentially creates a series of microscopic "Parasols" to keep the sunlight from shining on the rubber molecules. When chemicals are put on the tire, the waxes and the exposed layer of carbon black may be removed. This exposes the rubber to the elements and can increase the rate of degradation. When a tire is flexed for long periods of time the rubber degrades on the surface exposing the carbon black particlees as planed. The flexed position causes the carbon black particles to be pulled apart slightly, allowing the sun to hit the next "layer" of rubber. This layer erodes, exposes more carbon black which again pulls apart and the cycle continues until huge crack results. Tires that are used become warm internally which speeds up the rate of wax "blooming" or migrating to the surface. They tend not to crack as quickly for that reason. Note: different rubber compounds have different amounts of resistance to the elements. Not all rubbers or tires are the same.
