Preparation And Property Of Polyimide (PI) Solvent Resistance Nanofiltration Membrane

Nanofiltration (NF) is a promising separation technology for the food, wastewater treatment, refining and pharmaceutical industries due to the advantages of low operating pressure, low investment and low operation and maintenance cost. However, an industrial NF membrane used for aqueous system is often unsuitable for intensive solvent system because of its extensive swelling, which leads to a drastic loss of selectivity. To fit these applications, the NF membranes must be desirably stable in the organic solvents. Many researchers have focused on improving the stability of the NF membranes, especially the solvent resistant nanofiltration (SRNF) membranes.This paper describes the preparation of polyimide solvent resistance nanofiltration membrane by two-step method. The influence of the polymer concentration, thickness of membranes, phase inversion time, temperature of the imidization and thermal imidization procedure were studied. The membranes with the highest rejection rate of Fast Green FCF (MW808.86) were prepared at the following conditions:polymer concentration was 13%, membrane thickness was 150μm, phase inversion time was 1h and the thermal imidization procedure was 200℃for 2.5h,250℃for 2h, and 300℃for 2h in vacuum environment.In this study, the imidization degree of PI membrane prepared at different temperature was investigated by ATR-FTIR and weighing method, and the results showed that the imidization degree was about 92.23%; the surface and cross section of membrane were charactered by SEM image; the porosity of membrane was 72.3%.The properties of PI membranes were also investigated, including solvent resistant property; permeate flux of pure solvents; rejections of salts with different type of charges, molecular weights and valences; and performance of membranes at elevated temperature. It showed that the membrane was stable in n-hexane, cyclohexanone, THF, methanol, NMP, and ethyl acetate. The fluxes of some common solvents were equal to or higher than a number of commercial SRNF membranes and the rejection of several dyes were up to 90% except methyl orange. It showed that the membranes can be used normally at 80℃and the flux of membrane increased about 2 times while the rejection kept a relative high level. It made this membrane possible to apply at high temperature separation.