Effect of process conditions on the spray characteristics of a PLA + methylene chloride solution in the supercritical antisolvent precipitation process

A high-resolution imaging system was used to study the supercritical antisolvent precipitation process (SAS). The spray of a 1 wt% solution of poly(l-lactic) acid (PLA) in methylene chloride into carbon dioxide was visualized at several distances from the nozzle outlet. Experiments were performed at a fixed density of 0.33 ± 0.02 g/cm3 with select pressure and temperature combinations and at a fixed temperature of 323 K with select pressure and density combinations. Jet break up was observed to occur by either atomization or as a gas-like plume depending on the process conditions. In the case of atomization into distinct droplets, the average droplet diameter was observed to increase and then decrease as distance from the nozzle increased. In both sets of experiments, the shift from atomization to a gas-like plume occurred as the temperature and pressure were increased. Despite large variations in the spray characteristics there was little variation in the properties of the PLA particles produced during these experiments.