Ethylene propylene is a copolymer of ethylene and propylene (EPM), or, in some cases, a terpolymer due to the addition of a diene monomer (EPDM). This additional diene monomer can be important because it includes unsaturation to facilitate sulfur crosslinking.
In use since 1961, ethylene propylene is primarily valued for its outstanding resistance to Skydrol® and other phosphate ester type hydraulic fluids (including Pydraul® and Fyrquel®), as well as for its typical temperature range (-65° to +300° F, -54° to +149° C).