TPO materials are defined as compounds of various polyolefin polymers, semi-crystalline thermoplastics, and amorphous elastomers. The most common types of TPOs are composed of polypropylene and ethylene-propylene rubber (EPR). EPR may be a copolymer of either only ethylene and propylene monomers or a third monomer. The diene monomer provides a small amount of unsaturation in the polymer chain for sulfur cross-linking. This is called EPDM rubber