GENERATION OF MICROCELLULAR POLYMERS USING SUPERCRITICAL CO2

Supercritical carbon dioxide is known to be a very good swelling agent and plasticiser for poly(methyl methacrylate), a consequence of an in teresting combination of liquid-like and gas-like properties exhibited by supercritical fluids. Making use of this behaviour, we have studie d a constant temperature process of generating microcellular polymers which employs a sudden pressure drop to induce phase separation in a s olution of supercritical CO2 and poly(methyl methacrylate). The method is different from commonly used temperature quench methods in that it makes use of the glass transition depression due to the presence of d iluent in the polymer rather than heating the polymer to above its nor mal glass transition temperature. Typically, the process leads to a mi crocellular core structure encased by non-porous skin. Average cell si zes (0.5 to 15 mum) and bulk densities (0.4 to 0.9 gm/cm3 compared wit h 1.2 gm/cm3 of the starting material) can be varied by changing the p rocess conditions such as saturation pressure, temperature, crosslinki ng, and time of saturation. Classical nucleation theory well describes the trends in observed cell density which increases very sharply with increasing pressure above 13.79 MPa (2000 psi), levelling out at appr oximately 27.59 MPa (4000 psi) (at 40-degrees-C). There is a correspon ding decrease in the average cell size with increasing saturation pres sure. The effect of temperature is rather small and gradual.