Mass transfer in polymers in a supercritical CO2-atmosphere

Diffusion in poly-(ethylene-terephthalate) (PET), during treatment in a supercritical CO2-atmosphere, was investigated in order to achieve a better understanding of the mass transfer mechanism in the amorphous regions of polymers. Recently, some promising applications have been developed where mass transfer in the polymer phase by means of a supercritical fluid is of major importance. Examples are the dyeing of PET with supercritical CO2, membrane separations of gas–fluid mixtures and the treatment of polymers with supercritical fluids to change their properties (generation of foams, extraction of impurities, etc.). A new experimental method to determine diffusion coefficients has been developed. It is based on the gravimetric measurements of mass transport with a simultaneous investigation of the swelling behavior of PET in supercritical CO2, on a single polymer sample. The swelling of PET was compared with the swelling behavior of another glassy polymer (bisphenol-A polycarbonate) in supercritical CO2. Experimental data of the sorption of CO2 and different kinds of disperse dyes in the polymeric matrix have been measured in order to calculate diffusion coefficients for the different substances. The diffusion coefficients have been calculated by a classical diffusion model for cylindrical solids. It could be shown that diffusion of disperse dyes in PET strongly depends on the dye itself, which is key information for the process development.