Microcellular Polymeric Foams (MPFs) Generated Continuously in Supercritical Carbon Dioxide
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Microcellular Polymeric Foams (MPFs) Generated Continuously in Supercritical Carbon Dioxide Srinivas Siripurapu,1 Yvon J. Gay,1 Joseph R. Royer,1 Joseph M. DeSimone,1,2 Saad A. Khan1 and Richard J. Spontak1 1 Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695 2 Department of Chemistry, University of North Carolina, Chapel Hill, NC 27599 ABSTRACT Microcellular polymeric foams (MPFs) hold tremendous promise for engineering applications as substitutes to their solid analogs in light of reduced manufacturing/materials costs and improved properties. We present a two-part study addressing the generation of such materials in the presence of supercritical carbon dioxide (scCO2). The first part describes the production of polystyrene MPFs in a continuous extrusion process, as well as the effect of operating conditions such as temperature and CO2 concentration on foam morphology. The second part discusses microcellular foaming of poly(vinylidene fluoride) (PVDF), a semicrystalline polymer, via blending with the amorphous polymer poly(methyl methacrylate) PMMA. Foams of pure PVDF possess ill-defined morphologies, whereas those of PVDF-PMMA blends show an improvement with cell sizes on the order of 10 µm or less and cell densities in excess of 109 cells/cm3. INTRODUCTION Foamed microcellular plastics have received increasing attention in recent years due to several reasons. The very idea of solid polymers replaced by microcellular polymeric foams (MPFs) without compromising required functionality is exciting as an alternate process strategy [1]. The banning of chlorofluorocarbons (CFCs) has forced the foam industry to look for environmentally benign blowing agents, such as supercritical carbon dioxide (scCO2). Supercritical carbon dioxide possesses interesting physical properties such as liquid-like densities, which account for its high solvent power, and gas-like viscosities, which permit high rates of molecular diffusion [2]. These considerations have made scCO2 an attractive alternative to traditional solvents in a variety of applications including polymerizations, coatings and foaming [3]. A typical MPF structure has a uniform distribution of closed cells with a mean cell diameter of 10 µm or less and a cell density in excess of 1010 cells/cm3. The basic foaming process is a three-step process [2], as depicted in Fig. 1. The first step is the saturation of a polymer melt with an inert gas, such as CO2, under high pressures, followed by thorough mixing. Thermodynamic instability is then rapidly induced so that numerous vapor nuclei form within the polymer matrix. This can be accomplished either by a pressure quench [2] or a sudden temperature increase [1]. The final step in the process relates to the growth of the large number of stable bubble nuclei and the subsequent development of a desired microcellular morphology under controlled temperature conditions. Most studies reported on the subject of MPFs involve foaming in a batch-wise process [1,2,4]. More recently, a continuous extrusion p
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