Process design for the removal of residual monomer from latex products using supercritical carbon dioxide

Abstract

One of the most important environmental incentives for the polymer industry is to reduce the residual monomer in polymer products. Although several techniques are available for the removal of residual monomer, most methods are rather energy intensive and time consuming. Since supercritical carbon dioxide (scCO 2) is known to have a plasticizing effect on many polymers and is a good solvent for most commonly used monomers, a scCO 2-based process forms an interesting alternative. This paper focuses on the development of a post-emulsion polymerization process, which decreases the amount of methylmethacrylate (MMA) in polyMMA lattices using scCO 2. Typically, the method comprises a counterflow process, in which part of the residual monomer is converted by the increased diffusion inside the polymer particles due to the swelling by scCO 2. In addition, the amount of residual monomer is further reduced by the extraction capacity of scCO 2. Experimental studies have shown that the scCO 2 extraction of MMA is the predominant effect as compared to the enhanced polymerization due to plasticization. From a mass transfer model, the largest resistance in the extraction process appears to be situated in the water phase near the scCO 2 interface due to a relatively small surface area as compared to the overall polymer–water interfacial area. A viability study, including equipment sizing and economic evaluation, has shown that the removal of residual monomer from latex products using scCO 2 in principle yields a process which is both technically and economically feasible, capable to meet the future requirements.