EPDM (ethylene propylene diene monomer) is a kind of synthetic rubber, not foam. This rubber is made from a blend of ethylene, propylene, and diene monomer, as its name implies. Foam is a cellular material that contains air or other gas bubbles, while rubber is a solid material that doesn't contain any air or gas. Foam is also typically lighter and not as dense as rubber.
Foam is most often found in situations where cushioning or insulation is the goal, while rubber is found more often in cases where elasticity, strength, and weather resistance are essential. When lining a pond, rubber is much more ideal for this purpose, as it is typically water-tight and weather resistant.
