Purification and Simultaneous Micronization of Polymers by Supercritical Antisolvent Precipitation

One of the most important environmental concerns for the polymer industry is to produce new products with near-zero residual monomer content. A new process based on supercritical CO2 has been successfully tested for the extraction of the residual monomer and the simultaneous micronization of the polymer. The process has been tested on a pilot-scale plant. The reduction of methyl methacrylate (MMA) in PMMA is described as a representative model system. The polymer is dissolved in its monomer until a dilute homogeneous solution is formed; than the resulting solution is sprayed in a vessel where supercritical-CO2 is delivered at constant pressure and temperature. CO2 acts as an antisolvent for the polymer and as a solvent for the monomer that is removed from the polymer. The results show that it is possible to produce micro and nanoparticles with a very low solvent residue in one process step. It is also shown that it is possible to modify the particle dimension in a wide range of dimension by changing the operating pressure and/or temperature. INTRODUCTION One of the major drawbacks of polymerization techniques (bulk, emulsion, dispersion) is the difficulty to obtain complete monomer conversion at standard processing conditions. This difficulties result in the presence of relatively large amounts of residual monomer in the final product of the polymerization process. For health, safety and environmental reasons, it is desirable to reduce the residual monomer content in commercial polymers. Some techniques for residual monomer removal are already used. In general, the available methods are based on two concepts: 1) conversion, or 2) extraction of residual monomer. The first method requires the introduction of extra chemicals that have to be subsequently removed and additional post-polymerization steps. Extraction methods are performed using high temperature steam that can cause problems of polymer stability. Moreover, steam extraction is an energy consuming process. These techniques have specific characteristics that have been discussed by Kemmere et al. [1]. Since legislation on levels of residual monomer in consumer products is expected to become more severe in the near future, there is considerable room for improvement of the existing techniques. Supercritical CO2 (SC-CO2) is an interesting alternative for residual monomer removal because it is an excellent extraction medium for a variety of monomers [2]. Indeed, many monomers have a relatively high solubility in SC-CO2 whereas the solubility of the corrisponding polymers is very reduced or negligible. In addition, small changes in pressure and temperature can tune solubility, diffusion in SC-CO2 and viscosity. The extraction of