On the contrary, sponge rubber usually begins 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 method contributes to the enlargement in the rubber as well as formation of the sponge-like structure. https://augusthwkyn.anchor-blog.com/13968148/rubber-foam-versus-sponge-rubber-an-overview