ABSTRACT
For solutes soluble in scCO2, the fluid is used as a transport solvent, either pure or with the addition of cosolvents, chelating agents, etc. For solutes insoluble in scCO2, antisolvent and particles from gas-saturated solutions (PGSS®) processes are mainly used. Surface modification protocols can be applied to either the external surface of particles, normally denoted as coating processes, or the internal surface of porous materials, labeled as impregnation processes.
Figure 16.1 Process design as a function of substrate and solute behavior in scCO2. scCO2 dyeing of textile is a clear example of surface modification technology developed at an industrial scale. In conventional textile dyeing large quantities of wastewater are produced. This environmental and economical burden is avoided when scCO2is used as the dyeing medium instead of water [3, 4]. Separating residual dye from the CO2 and recycling of CO2 are relatively easy processes. Energy is saved because textiles do not need to be dried after the dyeing process. An additional advantage of scCO2 is the high diffusivity and low viscosity that allow the dye to diffuse faster toward and into the textile fibers. This results in a faster dyeing process. Textiles can be classified into nonpolar, synthetic polymers (e.g., polyester) and polar, natural textiles. The second category can be divided into polymers built from amino acids (e.g., silk and wool) or cellulose (e.g., cotton). In polyester dyeing, scCO2 penetrates and swells the fibers, thereby making them accessible for dye molecules [5]. Upon depressurization, the dye molecules are trapped inside
