Generation of microcellular foams of PVDF and its blends using supercritical carbon dioxide in a continuous process

Use of supercritical carbon dioxide (scCO₂) as a blowing agent to generate microcellular polymer foams (MPFs) has recently received considerable attention due to environmental concerns associated with conventional organic blowing agents. While such foams derived from amorphous thermoplastics have been previously realized, semicrystalline MPFs have not yet been produced in a continuous scCO₂ process. This work describes the foaming of highly crystalline poly(vinylidene fluoride) (PVDF) and its blends with amorphous polymers during extrusion. Foams composed of neat PVDF and immiscible blends of PVDF with polystyrene exhibit poor cell characteristics, whereas miscible blends of PVDF with poly(methyl methacrylate) (PMMA) yield foams possessing vastly improved morphologies. The results reported herein illustrate the effects of blend composition and scCO₂ solubility on PVDF/PMMA melt viscosity, which decreases markedly with increasing PMMA content and scCO₂ concentration. Morphological characterization of microcellular PVDF/PMMA foams reveals that the cell density increases as the PMMA fraction is increased and the foaming temperature is decreased. This study confirms that novel MPFs derived continuously from semicrystalline polymers in the presence of scCO₂ can be achieved through judicious polymer blending.