Operating regimes and mechanism of particle formation during the precipitation of polymers using the PCA process

Abstract

The vapour–liquid equilibrium phase behavior of a solvent and carbon dioxide provides two different regions of operation for the precipitation with a compressed antisolvent (PCA) process. Below the critical pressure of the mixture there is an interface between the liquid- and vapour-phase. Solution droplets are formed by atomisation in the nozzle. Above the critical- (or saturation-) pressure there is no phase boundary and contact between CO 2 and solution takes place by mixing. Additionally, in both operating regions, droplets of a polymer rich-phase are formed as result of a liquid–liquid phase split induced in the polymer solution when it gets in contact with the CO 2. This article provides experimental evidence for the hypothesis that when processing polymer solutions there are two different mechanisms of droplet formation governing the final size of the precipitated polymer particles: hydraulic atomisation and liquid–liquid phase split. The system l-polylactic acid ( l-PLA)–dichloromethane (DCM)–CO 2 was used to demonstrate that particle size can be manipulated by modifying the operating conditions. Working at conditions below the mixture critical pressure of the solvent–CO 2 mixture it was possible to produce polymer particles in the range of 5–50 μm. At conditions where the two fluids are completely miscible, l-PLA particles ranging from 0.1 to 2 μm and microfibers were obtained. The possibility of co-precipitation of cholesterol and l-PLA was also addressed.