Thermoplastic Vulcanizates from Isotactic Polypropylene and Ethylene−Propylene−Diene Terpolymer in Supercritical Propane: Synthesis and Morphology

Abstract

New thermoplastic vulcanizates (TPV's) have been synthesized from isotactic polypropylene and ethylene−propylene−diene terpolymer (EPDM) in a supercritical propane solution. The ternary solution of polypropylene 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 enhanced the mutual solubility of the two polymers. EPDM was cross-linked with tert-butyl peroxide while in supercritical solution. On lowering the pressure while remaining above the melting temperature of the polymers, the supercritical TPV gel phase separated in a manner consistent with spinodal decomposition. On isobaric cooling of the supercritical TPV gel, the polypropylene crystallized, freezing the morphology. The cross-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 optical microscopy showed micro-heterogeneous EPDM domains dispersed in the polypropylene matrix for the TPV's. The phase domain sizes were much smaller, by a factor of 5−10 or so, than those of the corresponding melt blends.