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

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 CO₂. 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)-CO₂ 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-CO₂ 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.