Rubber-Thermoplastic Compositions. Part I. EPDM-Polypropylene Thermoplastic Vulcanizates

Thermoplastic elastomers are rubbery materials which can be fabricated by techniques usually associated with thermoplastic resins. Classical elastomers rely upon the crosslinked network, developed during vulcanization, to provide the retractive forces of rubber type elasticity. Thermoplastic elastomers contain rubber domains and resinous thermoplastic domains. The thermoplasticity results from the melting characteristics of the hard thermoplastic phase, while the rubber properties arise from the rubbery domains. Thermoplastic elastomers are, therefore, almost by definition, heterogeneous in their phase morphology. Such materials can be blends or block polymers. In the case of block polymers, the rubbery phase is not crosslinked chemically. However, hard or resinous phase domains occur as the hard segments of the block polymer which separate from the composition by agglomeration during cooling from the molten state. These domains act both as well-bonded reinforcing filler particles and as crosslinks. This is, of course, because the hard blocks are connected to the soft or rubbery segments by primary chemical bonds. In the case of the blend compositions, the hard and soft domains are separate polymeric species. However, there must be some form of interaction between the domains if useful properties are to be realized. Recently, uncured or partially cured EPDM rubber has been blended with polyolefin resin to make thermoplastic elastomer-like compositions. However, these compositions suffer deficiencies in performance as well as in certain aspects of fabricability. Only poor to fair performance at temperatures above 70°C in air or in oil has been achieved with the uncured to partially cured compositions. More recently, it has been found in our laboratories that fully cured EPDM compositions which are fabricable as thermoplastics can be prepared. Such compositions, referred to here as thermoplastic vulcanizates, have superior strength, high-temperature mechanical properties, hot oil and solvent resistance, better compression set, etc. This report outlines critical parameters associated with these unique materials.