Experimental Investigation On Preparing Asymmetric Organic Membrane By Supercritical Fluids Process

A new process has been proposed and developed to prepare organic asymmetrical membranes by phase separation induced by supercritical fluids during recent years. Compared with the conventional immersion precipitation (L-S) process, the new process shows a lot of advantages and promising application prosperity. But the research on this process is just at its beginning and there is not enough experimental research on the process, which results in the lack of theoretical work. It is necessary to investigate experimentally the process of membrane preparation by supercritical fluids so as to provide a basic information for a theoretical study.In this work, polystyrene (PSt) membranes were prepared by the phase separation induced by the supercritical CO2. Effects of the experimental conditions on membrane formation of PSt were investigated. Influences of various factors such as pressure fluctuation, depressurize rate and membrane thickness on the apparent structure of membrane were studied in detaile. The results show that the thickness of casting solution plays a key role, while other factors have a great effect on the membrane structure when the thickness is 300 μm and a little effect when the thickness is 200 μm. The effects of pressure, temperatureand polymer concentration on the micro-structure (presented by it's porosity) of PSt membrane were also investigated.The results show that the porosity increases to a peak and then decreases with the increase of temperature, and the porosity changes in the same way with the increase of pressure and polymer concentration.Then, cellulose acetate (CA) membranes were also prepared. The effects of pressure, temperature and polymer concentration on the micro-structure (presented by it's porosity) of CA membrane were investigated. The results show that the porosity increases with the increase of pressure, and decreases with the increase of polymer concentration, but increases to a peak and then decreases with the increase of temperature. In addition, an orthorhombic experiment of CA membrane preparation was done to find the best operating condition for obtaining the best porosity.The results show that the best operating condition is: temperature of 45℃, pressure of 8MPa and polymer concentration of 15wt.%.Then, polyvinyl alcohol (PVA) membranes and nylon 6 membranes were also prepared. The experimental results show that PVA membranes and Nylon 6 membranes prepared by supercritical fluids process have a significant asymmetric structure.Finally, CA membranes and Nylon 6 membranes were prepared by the conventional L-S process. The membranes were observed with SEM, and compared with CA membranes and Nylon 6 membranes prepared by supercritical CO2 process respectively. The results show that membranes prepared by supercritical CO2 have better porosity and more significant asymmetric structure. And also, the L-S process compared with supercritical CO2 process, advantages of this phase separation process with supercritical CO2 were concluded.