Characterization Of Poly(Vinylidene Fluoride) Ultrafiltration Membranes And Application In Water Reuse

Poly(vinylidene fluoride)(PVDF) ultrafiltration(UF) membranes were prepared by combination of non-solvent induced phase separation(NIPS) and thermal induced phase separation(TIPS) method, using polyethylene glycol400(PEG400) as non-solvent additive. The properties and structure of membranes were systemically characterized by using various test methods (flux, retention rate, porosity, bubble point, mechanical properties, SEM and etc.). Analisys various problems of reverse osmosis(RO) membrane and PVDF hollow fiber ultrafiltration membrane in engineering application using "Daily processing2,000tons of waste water and water reuse project" as an example. The detailed results are listed as follows.Firstly, the mixability parameters(MP) of non-solvents PEG400, PEG200, DEG and Glycerol in PVDF/DMAc, PVDF/DMF and PVDF/NMP solutions were determined at different temperatures by cloud point titration method. They were found vaiying in following order:PEG400>PEG200>DEG>Glycerol. The effects of the PEG400content and three different solvent DMAc, DMF, NMP on membrane properties and structure were discussed. With the increasing of PEG400content, the flux and mechanical properties were improved, the gelation rate was slowed down and the finger-like voids transformed into sponge-like structure. The flux of different solvent membrane was ordered NMP>DMAc>DMF and the retention rate nearly had no difference. The effects of PVDF concentration and coagulation bath temperature on membrane properties and structure were studied. With the increasing of PVDF concentration, the flux decreased and the mechanical properties improved by the dense sttucture. The increasing of coagulation bath temperature made the thick finger-like structure cortex, the flux decreased and retention rate increased. More macroporous structure appeared and the mechanical properties went down.Secondly, effects of different additives were discussed. The flux was improved over100L·m-2·h-1·bar-1by the addition of VAH, Tween80, PEG20000and nucleating agent. Compared the data of flux, retention rate and bubble point. Tween80and PEG20000had the effect of making pores. Nucleating agent changed the crystallization situation, formed a lot of spherulites and weakened the toughness of membrane. VAH. LiCl and PEG20000decreased the contact angle of the membrane upper surface. PVDF hollow fiber membranes were prepared based on flat sheet membrane formulations. As the double cortex of hollow fiber membrane, the flux decreased and the retention rate increased, compared to the flat sheet membrane. For PVDF/DMAc/PEG400/Dibutyltin dilaurate system, the blend of PTFE and nano-SiO2enhanced the breaking strength, decreased the retention rate and elongation. The addition of PTFE decreased the elasticity modulus while the nano-SiO2increased it.At last, some relevant engineering experience of "Daily processing2,000tons of waste water and water reuse project" was introduced through the analysis of water production data, raw water and water production COD data, the UF running monitoring data and RO running monitoring data.