Structure Control Of PVDF Porous Membrane During The Process Of Preparation By Solution Phase Inversion And Study Of PVDF Membrane Performance

Poly(vinylidene fluoride)(PVDF) is one of the widely used membrane materials, due to its good mechanical strength, thermal and chemical stability, radiation resistance. For membrane preparation, the solution phase inversion method has an advantage that the technical conditions are simple. But during the process of membrane preparation, the control of membrane structures is difficult, and membrane reproducibility is poor and membrane mechanical strength is worse. In this paper, we aimed at studying the influences of various technical conditions on phase separation behaviors and membrane structures during PVDF membrane formation by the solution phase inversion method. The PVDF membrane formation mechanism was suggested. The relations of technical conditions, phase separation behaviors and membrane structures were obtained. So the membrane structures can be controlled during preparation process. On the basis of membrane formation mechanism and the rules of membrane preparation mentioned above, the flat homogeneous PVDF microporous membrane and flat composite PVDF membrane with good performances and controllable structures were prepared. And the performances of these membranes were investigated during the process of practical applicationsThe influences of each factor on phase separation behaviors and membrane structures were researched. The thermodynamic properties of membrane-forming systems and precipitation kinetics of membrane formation were characterized by light scattering instrument and light transmission instrument. Combined with characterization results of membrane structures and properties, it was found that compared with temperatures of casting solution, coagulation baths and evaporation time, PVDF concentrations, coagulation compositions and the type and contents of additive had more obvious effect on the phase separation behaviors and membrane structures. During membrane preparation processes above, there were some experiment phenomena and membrane structures that cannot be explained by the classic membrane formation theory. As a result, according to the asymmetric structure of the obtained membranes and two periods of light transmittance curves, different formation mechanism for membrane skin layer and sublayer was proposed.In the rapid demixing systems, the thermodynamics controlled the demixing of top layer. When the polymer concentration was low (<12wt%), the instantaneous liquid-liquid demixing occurred, which led to porous skin layer with pores of 0.1~0.3um diameter. When polymer concentration is higher than 12wt%, the phase separation of the top layer mainly was the solid-liquid demixing. Then the membrane was obtained with a dense skin layer with pores of 10~20nm diameter. The time required by demixing (the delayed time) was controlled by the mutual diffusion of solvent and nonsolvent. The more rapid the mutual diffusion was, the shorter the delayed time was. In the control of kinetics, liquid-liquid demixing happened in the sublayer. The demixing time and sublayer structures were affected by the thermodynamic property and diffusion. The more instable the membrane-forming system and the more rapid the diffusion of solvent and nonsolvent, the shorter of the demixing time. Here the phase separation behavior of sublayer was instantaneous liquid-liquid demixing, resulting in macrovoid structures in membranes. The more stable of the membrane-forming system and the more slow of the diffusion of solvent and nonsolvent, the longer of the demixing time. Then the delayed liquid-liquid demixing dominated the formation of sublayer, which was in favor of the formation of spongerlike structures. And in this slow process, the crystallization was more and more important, so the membrane crystllinity increased. In above series of studies on the rapid demixing systems, it was also found that the crystallization had an obvious influence on the kinetics process. Especially in the systems containing higher PVDF concentrations, the influence of PVDF crystallization on the kinetics process was stronger. The crystallization retarded the demixing speed, made the kinetics curves become flat with time lapsing, and enhanced the membrane crystallinity.In the slow demixing systems, the thermodynamics controlled the demixing of the top layer. The delayed liquid-liquid demixing occurred in the top layer. The mutual diffusion of solvent and nonsolvent dominated the delayed time. The more rapid of the mutual diffusion was, the shorter of the delayed time was. What's more, due to the low PVDF concentration at the interphase of coagulation bath and liquid film, and the better compatibility of nonsolvent and PVDF, the top surface of the resultant membranes was porous and the pores diameter ranged in 0.1~2um. The phase separation of the sublayer was under the control of kinetics, the delayed demixing happened in the sublayer. The demixing time and sublayer structureswere affected by the thermodynamic property and diffusion. The better of the compatibility between nonsolvent and PVDF was, the slower of the diffusion of solvent and nonsolvent, so the slower of the precipitation and the shorter of the demixing. Here the phase separation behavior of sublayer was the delayed liquid-liquid demixing or the delayed solid-liquid demixing, resulting in macrovoid structures reducing and spongerlike structure increasing, or a global particle structure formation. In the process of membrane formation of slow demixing systems, the crystallization had more obvious influence on the kinetics. The crystallization retarded greatly the demixing speed and made the kinetics curves become flat with time lapsing. Concretely speaking, the crystallization prolonged the delayed time and demixing time and improved membrane crystallinity.Based on the above studies, the homogeneous mricroporous PVDF membranes were prepared with a porous top surface, a high porosity, good mechanical strength, and a high water flux and uniform structure. The pore diameter on the top layer and porosity were 0.2~3 \im and approximate 80%, respectively. The max stress and its break stress of membranes was higher 1-2 times than that of membrane without nano-TiC^. The range of the water flux was 423.76-654.9 L/h.m2, which were about 2 times of that membrane containing no nano-TiC?2 were. Finally, The preparation conditions of membrane with excellent performance were summarized as follows: PVDF concentrations varied from 9wt% to 10wt%, the content of PVP was 3-5wt%, the content range of water additive was l~2wt%, the 1, 2-propanediol additive content changed from 3wt% to 5wt%, the content of nano-TiC>2 was from 0.5wt% to 1 wt%, coagulation baths consisted of water and DM Ac and the volume content of DM Ac was 30-50%, the temperatures of casting solutions and coagulation baths were about 30°C, respectively.PVDF/polyester nonwoven composite membranes with outstanding mechanical strength were fabricated by solution inversion method. The range of pore diameter on the obtained membranes surface ranged in 0.05u,m-0.5p.m. Correspondingly, the water fluxes of membrane varied from 400 L/h.m2 to 2000L/h.m2 (0.1 MPa). For membrane preparation with a low water flux (<1000 L/ h.m2), on the one hand, higher PVDF concentration (>12wt%) could be chosen. On the other hand, PEG, LiCl and nonsolvent water was used as additives, and the content of these additives was about 5wt%. For PVP additive, when their contents were lower than 2wt%or higher than 7wt%, the membranes with a small pore diameter and low water flux were also obtained. For membrane preparation with a high water flux (<1000 L/h.m2), on the one hand, PVDF concentration should be lower (>12wt%). On the other hand, PVP was used as additive with the content of about 5wt%. Other process conditions were determined as follows. The temperature of casting solutions and coagulation baths were 25-35° C, evaporation time was 5~30s, and the content of anao-TiO2 additive was 0.3~0.5wt%, respectively.Submerged flat membrane bioreactor (SFMBR) and submerged rotating membrane bioreactor (SRMBR) using PVDF composite membrane separators were applied to wastewater treatment. SFMBR remained stable for a long time continuously running. After one day running, the effluent COD reduced to less than 20 mg/L, although the influent COD value ranged from 312 to 733mg/L. Remove percentage of COD was larger than 95%, and the effluent turbidity values were lower than 0.5NTU. SS can not be detected and the COD of upper clear liquid was less than 30 mg/L, the equilibrium fluxes of the effluent can still remain in the range of 6.47—15.25L/m2h. The optimum operation conditions were as follows: operation pressure was 0.020Mpa, suction time/suspended time was 10min/2min, aeration rate was 0.8m3/L, and membrane pore size is about 0.2um, respectively. Under this condition, the effluent equilibrium flux can reach 15.25L/m2.h.The SRMBR system can be stably operated for a long time. When the influent COD value ranged from 180 to 368mg/L, the COD of the effluent could be reduced to less than 20mg/L after one day running, the remove rate of COD was above 93%, the effluent turbidity values were lower than 0.5NTU, SS could not be detected, the COD in upper clear liquid was less than 30 mg/L, and the effluent equilibrium fluxes could still remain in the range of 42.5~50.5L/m2h, respectively. The best operation parameters were as follows: operation pressure was 0.025 MPa, rotation speed was 25r/min, suction time/ suspended time was 9min/lmin, and aeration rate was 0.4 m /h, respectively. In this condition, the effluent equilibrium flux reached 47.5L/h.m2. Compared with SFMBR, SRMBR revealed superiority with a low energy consume. The effluent equilibrium flux of SRMBR was 4-5 times of that SFMBR was.The membrane extraction of copper ions was carried out using PVDF flat microporous membrane modules and kerosene solutions containing Di-(2-ethylhexyl)phosphoric acid(D2EHPA) as extractant. When the pH of aqueous phase was equal to 4.4, extraction yield reached the max value of 99.3%. The extraction yields were independent of flow rates of two phases. The extraction yield remained above 97% at different flow rates of aqueous phase and organic phase. The concentration of Cu2+ in the organic phase has an effect on the extraction yield. The extraction yield was independent of the concentration of Cu + and remained above 96% before the organic phase saturation, but the extraction yield decreased to zero when the organic phase saturated. The entrainment percentage of the organic phase in the aqueous phase reduced to as low as 10 ppm. The extractant was recovered by NaOH solution, and the recycling extractant can be used as the same as the pristine one. Compared with hollow PP fiber membrane extraction, PVDF membrane extraction needed the longer equilibrium time of about lOOmin. But the instrument of PVDF membrane extraction was simpler. The mass transfer process was different from the traditional one. The total mass transfer resistance was controlled by interfacial complexation reaction between the cu2+ and D2EHPA. The mass transfer resistance was independent of cu2+ concentration at high copper ions concentration;but the mass transfer resistance increased as Cu + concentration reduced at low copper ions concentration.