In essence, the effect on rubber is one of degradation, as a result of exposure to ultraviolet (UV) rays. With the substance, the degradation comes about as a result of chemical reactions and molecular deformations, both stimulated by the UV ray exposure. There are mainly the following situations:
Degradation of Carbon Black In Tires
There’s a reason why car tires always come in black, though few stop to consider why. In order to protect against the effect of sunlight on cross-linking rubber polymers, tire type is infused with a protective material known as carbon black(CAS NO:1333-86-4), also referred to by those in the auto industry as a “competitive absorber.” The purpose of the carbon black is to absorb sunlight on the surface of the tire, and deflect it from the inner polymers of the tire.
Loss of Elasticity
The reason rubber tends to become less elastic and more cracked after long-term exposure to sunlight is a matter of molecular bonding (specifically, cross-linking bonds). UV rays from the sun have the effect of increasing the cross-linking between polymer molecules within the rubber, increasing the density of these cross-links. At a certain point, the plethora of cross-links formed–due to chemical stimulation from the rays of the sun–strengthen the rubber item, and lessen the original elasticity, making it more brittle and harder to bend or stretch. This process is also considered an oxidation of the rubber polymers.
Dry rot is generally referred to as the breakdown or decay in wooden materials. However, the term has been used colloquially in other fields as well, including tire deterioration, although rubber on tires does not actually rot. When cracks begin to occur within a tire’s rubber, it is said to have dry rot.
Dry rot can be caused by a number of factors. One of the major causes is excessive exposure to ultraviolet rays. One general type of UV stabilizer is called a “competitive absorber.” Competitive absorbers absorb the UV light, instead of the tire sidewall, and convert the UV rays into heat so it can dissipate. The least expensive type of competitive absorber comes in “carbon black.” This is why tires generally come in black, rather than unique colors. Eventually, though, UV stabilizers no longer offer protection to a tire, causing the tire to look gray.
While the deflection and protection abilities of carbon black extend the life of a car tire, they do not do so indefinitely. Eventually, UV rays from the sun destroy carbon black molecules, depleting the shield on the car tire surface, and giving access to the inner tire polymers. The degradation of the chemical in rubber tires explains why car tires become gray and crack as they age.