This is a synthetic, vulcanized rubber that is made by kneading together several materials. This includes an organic agent and a crosslinking agent. An additive, often times sulfur, is usually added to make it more durable.
There are many characteristics to the rubber sponge that allow for several variations. You can choose between different foam structures, expansion ratios and hardness. The chemical properites can also be chosen, such as whether it is fire resistant(CR) or chemical resistant(EPDM). EPDM even has resistance to some acids and alkalis.