(104c) Residual Monomer Reduction In Polymer Latexes Using Supercritical Carbon Dioxide

An important issue in the production of polymeric materials is the amount of residual monomer that remains in the product after polymerization. Existing techniques to reduce the amount of residual monomer are energy-intensive, time consuming and not able to meet future requirements for the level of residual monomer in polymer based products. As an alternative a (sc)CO₂ based extraction process of the monomer from the polymer is evaluated.

Carbon dioxide is chosen as extraction medium for its many advantageous characteristics over other solvents. Carbon dioxide is a so called ?green solvent? due to its environmentally benign nature and its low toxicity and flammability. Moreover, carbon dioxide at supercritical conditions (T_c=31°C and P_c=74 bar) is liquid-like in density and gas-like in viscosity and diffusion coefficient of solutes. As the critical conditions are relatively mild, easy separation of low molecular species is obtained by simply tuning pressure and temperature.

In the (sc)CO₂ based extraction column for the reduction of residual monomer in latex products, latex droplets (~250 mm) are dispersed in (sc)CO₂. The latex droplets contain submicron (~100 nm) polymer paricles. Monomer diffuses from the polymer via the water phase into the (sc)CO₂ phase. A parallel mass transfer mechanism is diffusion of the monomer from the polymer particles directly to the (sc)CO₂ phase. This occurs when a particle resides at the water-(sc)CO₂ interface and is in direct contact with the (sc)CO₂ phase. This mass transfer mechanism is also reffered to as the shuttle effect. Figure 1 illustrates the mass transfer mechanisms in the extraction column.

Figure 1 Mass transfer in series (a) and the shuttle effect (b).

For the shuttle effect to dominate mass transfer, only a small fraction of the (sc)CO₂-water interface has to be occupied with polymer particles due to the plasticizing effect of (sc)CO₂. The plasticizing effect of (sc)CO₂ results in a tremendous increase of the monomer diffusion coefficient in the polymer. Our work demonstrates that (sc)CO₂ based extraction to reduce residual monomer in polymer products is a very promising and a sustainable alternative to existing techniques, e.g. steam stripping.

References

1. M. F. Kemmere, M. H. W. Cleven, M. A. van Schilt, J. T. F. Keurentjes, ?Process design for the removal of residual monomer from latex products using supercritical carbon dioxide?, Chemical Engineering Science, 2002, 57, 3929-3937.

2. M. Gornert, G. Sadowski, ?Phase Equilibrum Measurements of the Polystyrene/Styrene/Carbon Dioxide Ternary System at elevated Pressures using ATR-FTIR Spectroscopy?, Macromolecular Symposia, 2007, 259, 236-242.