THERMOPLASTIC VULCANIZATES FROM ISOTACTIC POLYPROPYLENE AND ETHYLENE-PROPYLENE-DIENE TERPOLYMER IN SUPERCRITICAL PROPANE - SYNTHESIS AND MORPHOLOGY

New thermoplastic vulcanizates (TPV's) have been synthesized from isot actic polypropylene and ethylene-propylene-diene terpolymer (EPDM) in a supercritical propane solution. The ternary solution of polypropylen e and EPDM in supercritical propane exhibited less solubility than the corresponding binary solutions of polypropylene or EPDM separately in propane. Higher pressure in the supercritical polymer solution enhanc ed the mutual solubility of the two polymers. EPDM was crosslinked wit h tert-butyl peroxide while in supercritical solution. On lowering the pressure while remaining above the melting temperature of the polymer s, the supercritical TPV gel phase separated in a manner consistent wi th spinodal decomposition. On isobaric cooling of the supercritical TP V gel, the polypropylene crystallized, freezing the morphology. The cr oss-linking of EPDM in the homogeneous supercritical propane solutions was found to be nearly complete. The final TPV's were phase separated , exhibiting two melting transitions. The morphology of the TPV's was a microporous, apparently closed-cell polymeric foam. Phase contrast o ptical microscopy showed micro-heterogeneous EPDM domains dispersed in the polypropylene matrix for the TPV's. The phase domain sizes were m uch smaller, by a factor of 5-10 or so, than those of the correspondin g melt blends.