Preparation And Performance Of Novel Polyimide(PI)Solvent Resistance Nanofiltration Membrane

Nanofiltration (NF) is a liquid-phase pressure-driven separation process employing a semipermeable membrane with the pore size in the nanometer range, between that of the reverse osmosis (RO) and ultrafiltration (UF) membranes. The traditional nanofiltration membranes possess the asymmetric structure with the pore size in the nanometer range. The rejection performance is depended on the dense top-layer, and the thickness of the membranes influence the mass transfer rate. Most of traditional nanofiltration membranes possess the asymmetric structure, which is adopted for the phase inversion induced by immersion precipitation method. This process results in a membrane with a dense top-layer and lower porosity, so the permeate flux is low.We provided an easy method to prepare high flux polyimide (PI) nanofiltration membranes with thin top-layer and higher porosity structure. Based on previous work, a crystallization process was taken under the condition of low temperature between coating and phase transformation. This step was to ensure PAA membrane crystallization completely. Lastly, PI membranes were formed via thermal imidization method. The results showed that different solvents and PAA polymer had different crystallization abilities, and which lead to different structure and performance of resultant PI membranes. The PI membranes with NMP which was taken a crystallization process show the granular morphology and the highest porosity. The water permeabilities of all the membranes with different solvents on different preparation processes were illustrated. All the membranes had very little flux on coating/phase transformation. However, the water permeabilities of the membranes on coating/crystallization/phase transformation were higher. Especially, the PI membranes with NMP had the hightest flux (28L/m2h) and rejection (99%).The different morphology structure and properties of PI membranes were also investigated under different influence factors, including the different crystallization temperature, crystallization time, different coagulation bath composition and coagulation bath temperature. Experimental results showed that the lower crystallization temperature and the longer crystallization time were better for crystallization, and the porosity of the PI membrane was higher and pure water flux was higher. The result showed that the best crystallization condition was6h at10℃. Ethanol in the coagulation bath and the coagulation temperature had a great influence to the early formation of the crystalline structure. The experimental results showed that the optimal coagulation bath temperature was5℃, and the best coagulation bath was pure water.Solvent-resistance of PI nanofiltration membrane was studied by immersing the membrane samples in selected solvents. The results showed that the nanofiltration membrane retained most initial permeability and had an increase in rejection after the treatment tests in methanol, n-hexane, ethyl acetate, acetone. The results showed that all the rejections kept at a relatively high level except the NaOH solution (pH=10). The Flux increased from21.6L/m2h to30.3L/m2h. The rejection decreased from98.2%to89.3%.