Particle Formation from Gas-Enriched Polymeric Melts and Polymeric Solutions

Abstract

Within the scope of this subproject, a gas-assisted high-pressure spray process (PGSS—Particles from Gas-Saturated Solutions) was investigated. The gas used for the atomization is carbon dioxide (CO₂) which for the most purposes is in supercritical state. In the spray process, the gas is partly dissolved into the liquid before the atomization. The dissolved CO₂ changes the physical properties of the liquid such as viscosity and surface tension and enables high-viscous substances to be atomized. Several authors were able to atomize high-viscous substances such as polymers, fats, and waxes with such a spray process. Nevertheless, till now the fundamental influence of the CO₂ on the atomization phenomena has not been taken into account. For this reason, experiments which shall clarify how the CO₂ affects the spray and particle formation have been carried out systematically within this project. Sprays of water, polymers, and aqueous polymeric solutions were observed and investigated. The liquids were sprayed as pure substances as well as CO₂-enriched substances from an autoclave in a batch wise working process. To get a deeper understanding of the observed effects, optically transparent spray geometries were developed. Observations of the flow regime of the gas-enriched liquids gave explanations of the governing atomization phenomena. The results of the experiments show that the working principle of the PGSS process is based on the phenomena of flash atomization. Classical phenomena such as jet and lamella breakup can be neglected for the continuously working process. In further experiments, powders of polyethyleneglycols (PEG 6000) were generated using the continuously working process. Several spray devices were used such as hollow cone nozzles, capillaries, and orifices. The results show that powders with average particle size distributions in the order of 100 μm can be generated independently from the design of the spray device. Therefore, it is proven, that the flash atomization is the governing working principle in the PGSS process.

In addition to the investigations of the spray phenomena, the effect of the CO₂ on the solidification of droplets was observed. The temperature distributions in sprays gained from the batch wise process as well as from the continuously working process were compared. It was found that the cooling mainly results from the undissolved compressed gas. Compared to the amount of dissolved gas, the amount of undissolved gas is much higher. In the continuous process, there is a huge amount of undissolved gas. Therefore, in the continuous process, lower temperatures can be reached which enhance the solidification of the droplets.