Generation of Microcellular Polymers using Supercritical CO ₂

Supercritical carbon dioxide is known to be a very good swelling agent and plasticiser for poly(methyl methacrylate), a consequence of an interesting combination of liquid-like and gas-like properties exhibited by supercritical fluids. Making use of this behaviour, we have studied a constant temperature process of generating microcellular polymers which employs a sudden pressure drop to induce phase separation in a solution of supercritical CO ₂ 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 diluent in the polymer rather than heating the polymer to above its normal glass transition temperature. Typically, the process leads to a microcellular core structure encased by non-porous skin. Average cell sizes (0.5 to 15 (μm) and bulk densities (0.4 to 0.9 gm/cm ³ compared with 1.2 gm/cm ³ of the starting material) can be varied by changing the process conditions such as saturation pressure, temperature, crosslinking, 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 approximately 27.59 MPa (4000 psi) (at 40°C). There is a corresponding decrease in the average cell size with increasing saturation pressure. The effect of temperature is rather small and gradual.