Residual monomer reduction in polymer latex products by extraction with supercritical carbon dioxide

The increased awareness for environmental issues as well as for public health, demands "greener" processes as well as products. In the polymer industry, one of the major concerns relate to residual monomer in polymer (latex) products. Low molecular species are toxic. When monomer units diffuse out of their polymeric matrix, they contaminate the surrounding area. Decreasing the residual monomer level in polymer (latex) products is especially of great importance in food packaging, toys for children, etc. Conventional techniques to reduce residual monomer in polymer (latex) products are energy intensive, time consuming and not able to fulfill the increasingly stringent requirements for further reduction of residual monomer in polymer products, i.e. <100 parts per million (ppm). In this thesis the reduction of residual monomer in polymer (latex) products by extraction with supercritical carbon dioxide (scCO2) is explored. The Peng-Robinson equation of state has been used to describe the binary phase diagram of CO2 and styrene as well as of CO2 and methyl methacrylate. The ternary phase diagram of CO2-styrene-polystyrene as well as of CO2-methyl methacrylate-polymethyl methacrylate has been estimated with the Perturbed Chain - Statistical Associated Fluid Theory. The partition coefficient of the distribution of monomer between CO2 and CO2 swollen polymer has been calculated. In a high pressure autoclave the partition coefficient of the distribution of monomer between CO2 and CO2 swollen polymer has been determined experimentally at a range of temperatures and pressures. The experimental results demonstrate the partition coefficient is approximately independent of pressure and temperature. For the partitioning of styrene between CO2 and CO2 swollen polystyrene a value of 2 [kg PS/kg CO2] has been found, while for the partitioning of methyl methacrylate between CO2 and CO2 swollen polymethyl methacrylate a value of 0.3 [kg PMMA/kg CO2] has been determined. Mass transport of monomer from the polymer phase to the CO2 phase is governed by the shuttle effect. The monomer is hereby directly transferred from the polymer particles to the CO2 phase due to Brownian motion of the particles to and from the CO2/H2O interface. The main resistance against mass transfer has been demonstrated to be located in the polymer particles. The countercurrent extraction process to reduce residual monomer in polymer (latex) products from 104 ppm to 1 ppm in a column has been designed. Latex has been chosen as the continuous phase in which 1 mm CO2 bubbles are dispersed with a hold up of 20 %. The column is operated at 100 bar and 35 °C. A column volume of 4.75 m3 applies for a throughput of 6000 kg PS latex (45% solids)/hr. The extraction process reducing the residual monomer costs 22 €/ton latex for a production capacity of 50 kt PS latex (45% solids)/yr. Continuous steam stripping only costs 12 €/ton latex to reduce the residual monomer level from 104 to 100 ppm, whereas the purification costs of the CO2-based extraction process are similar for a reduction to 100, 10 or 1 ppm. Reduction to lower levels of residual monomer, lower than 100 ppm, by means of steam stripping is hardly possible, mainly due to thermodynamic (and to some extent transport) limitations. The results of calculations for a residual monomer reduction from 104 to 10 ppm with 100 °C steam point to a tremendous increase of the utility costs. The costs of a continuous steam stripping process are then more than 75 €/ton latex. The lowest residual monomer concentration in latex of 1 ppm can certainly only be achieved by extraction with (sc)CO2.