This is a closed cell foam rubber composed of EPDM (ethylene propylene diene monomer). Foam rubber, otherwise known as rubber sponge, is named for the many air pockets located throughout the rubber material. These particular foam rubber sheets are closed cell sponge, meaning that the air pockets within it are sealed. These sealed air pockets prevent closed cell EPDM from absorbing moisture while also slowing the rate at which the rubber material transfers energy and heat. Since EPDM is already water resistant and relatively heat tolerant for a material, closed cell EPDM is great for aquatic and heat insulation applications. Overall moisture resistance also allows closed cell EPDM rubber to form an effective seal against water, such as for gasketing. EPDM is an especially suitable elastomer for outside applications because it is durable and does not degrade easily. Unlike various other polymers, EPDM resists cracking or discoloration from many common outdoor sources of wear and tear, including UV rays, ozone, and the weather. These properties allow closed cell EPDM rubber to last a long time outdoors, even under prolonged exposure to direct sunlight.