Quite the opposite, sponge rubber often starts with expandable rubber sorts like EPDM or neoprene. The rubber compound is then supplemented with a chemical blowing agent, which, on heating, decomposes and releases gases. This process brings about the expansion with the rubber along with the formation of a sponge-like structure. https://rubber-foam-sheets15936.mybuzzblog.com/13514052/the-2-minute-rule-for-rubber-foam-versus-sponge-rubber