Natural rubber, or latex, is a gooey, sticky fluid. But when some chemicals are added it becomes tougher and stronger. It is made of polyisoprene chains, which slip past each other when they are stretched, and the great thing about rubber is its resistance to change. It can return to its original state when the tension is released.
Man's ingenuity has found that the addition of sulphur, which creates cross-links between the chains, turns a useless product into a useful one, but when rubber is exposed to sunlight, the ultraviolet reacts with oxygen and snips the polyisoprene chains into shorter and shorter segments, until it returns to a state similar to its original. Meanwhile, the molecules of short segments form new cross-links and alter the structure of the runner. It becomes brittle and harder.
Rubber may also contain fillers, dyes and . . .