TPO compounds include fillers, reinforcements, lubricants, heat stabilizers, antioxidants, UV stabilizers, colorants, and processing aids. Like most thermoplastic elastomers, TPO products are composed of hard and soft segments. In the TPO example described above, TPO-(EPDM+PP) could be the nomenclature: a blend of ethylene–propylene–diene terpolymer with PP, with no or little crosslinking of the EPDM phase, the amount of EPDM present being greater than that of PP. The thermoplastic and the rubber type shall be listed in decreasing order of abundance in the TPO. The ISO 18064 nomenclature of TPOs is identified by a bracketed term comprising the standard abbreviation for the rubber type (see ISO 1629), a “+” sign, and the standard abbreviation for the thermoplastic type (per ISO 1043-1). The diene monomer leaves a small amount of unsaturation in the polymer chain that can be used for sulfur crosslinking. 11 A common rubber of this type is called EPDM rubber, which has a small amount of a third monomer, a diene (two carbon–carbon double bonds in it). Most TPOs are composed of polypropylene (PP) and a copolymer of ethylene and propylene called EPR. Polyolefin thermoplastic elastomer (TPO) materials are defined as compounds (mixtures) of various polyolefin polymers, semicrystalline thermoplastics, and amorphous elastomers. Laurence McKeen, in The Effect of Sterilization Methods on Plastics and Elastomers (Fourth Edition), 2018 10.2 Olefinic Thermoplastic Elastomers (TPO)
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