(30b) Powderisation of Polymer Solutions by Pgss-Drying Process

Polymers are often manufactured with the help of big quantities of solvents. Usually these solvents have to be separated from the polymer melts in downstream process steps. Afterwards the polymers are granulated and often the granules are ground in mills in order to obtain fine polymer powder. Almost all of the energy expended for milling is transformed into heat. Thus the granules are warmed, they get sticky and a further comminution gets complicated. Therefore the polymer has to be cooled e.g. with the help of liquid nitrogen in order to permit a brittle fracture in the mill. This cooling is complex and expensive.

Aim of the investigation was the micronization of polymer solutions with solvent contents of up to 90 wt.% with the help of the high pressure spray process PGSS-drying (particles from gas saturated solutions). In this process a gas, mostly carbon dioxide, is admixed and partially dissolved into a polymer solution at high pressure in a static mixer. Subsequently the gas containing solution is rapidly expanded through a nozzle into a spray tower, operated at ambient pressure. The solution is disrupted abruptly by the expanding gas into fine droplets. The droplets are directly cooled due to the Joule-Thomson effect of the gas. Fine powders are obtained in the spray tower. The released solvent can be withdrawn with the carbon dioxide by a blower and can be separated from the carbon dioxide afterwards. In the figure a simplified flow sheet of the process is shown.

In the presentation will be shown that particle morphology, particle size as well as the residual solvent content of the powder can be influenced by the choice of the process parameters like pre-expansion pressure, pre-expansion temperature, post-expansion temperature and the specific mass flow ratio (mass flow of the polymer solution in comparison to the carbon dioxide mass flow). As a model substance combination PEG 6000 and water was used. Additionally phase equilibria measurements of PEG 6000, water and carbon dioxide are shown. The phase equilibria was measured in an autoclave at temperatures of up to 120°C and at pressures of up to 30 MPa. These results are presented in addition with measurements of the sorption isotherm of PEG 6000 and water at spray tower conditions. With the knowledge of the phase equlibria and the sorption isotherm it is possible to explain the formation of powder and the experimental observed residual solvent contents with high pressure extraction in the static mixer. Finally some samples of dried polymer solutions are shown.