Microfiltration membranes via electrospinning of polyethersulfone solutions
Nanofibers or nanowebs produced by electrospinning have several prominent properties such as high surface area to volume ratio, high porosity and pore size in nanorange. As the porosity of electrospun nanoweb is more than 90%, they are candidates for air filters and liquid filtration membranes. However, the application of electrospun nanofibers membranes has yet to make breakthrough in other avenues of separation than air filtration, especially in pressure-driven liquid separations, such as ultrafiltration (UF) or nanofiltration (NF). In this project, the preparation of polyethersulfone (PES) membranes by electrospinning was studied. The influence of processing parameters, i.e., polymer concentration, applied voltage, flow rate, spinneret-to-collector distance, relative humidity, were investigated. The treatment of the proto-membrane formed by immersion in an aqueous coagulation bath was also studied. More comprehensive characterizations of the nanofiber membranes, including fiber diameter, pore size, porosity, thickness, basic weight and tensile strength as well as air and water permeability were investigated. Thereby, we expect that this work will open up the avenue toward the use of nanofibers for very important applications of separation technology. Of particular interest are membranes in water purification, e.g., pre-filters to minimize contaminations and fouling prior to ultra- or nano-filtration. PES (Ultrason 6020P) was dissolved in N-methyl-2-pyrrolidone (NMP) at concentrations of 9%, 15%, 22%. The polymer solution was electrospun under processing conditions i.e., a spinneret-to-collector distance of 10 cm, an applied voltage of 30 kV, a flow rate of 20 μL/min, and a spinneret diameter of 0.8 mm, stationary substrate set-up, aluminum foil and PET nonwoven served as the substrate. The first results showed that the 22% PES solution can be electrospun into well-defined nanofiber. The overall morphology of the membranes obtained is changed from a fiber network into spherical particles connected by fibers with the decrease of the polymer concentration in the solution used for electrospinning. The properties of nanofiber can be measured on aluminum foil or PET nonwoven as substrate. Image analyses gave a mean fiber diameter of 489 ± 142 nm but under stationary spinning conditions that leads to a 3 dimensional fiber web on the substrate. When the 22% PES solution was electrospun membrane using a moving substrate under processing conditions i.e., applied voltage of 18 kV, a spinneret-to-collector distance of 10 cm, a flow rate of 20 μL/min, a spinneret diameter of 0.8 mm, a speed of substrates moving of 2.2 cm/min, 65% RH and served the PET nonwoven as substrate, yielded a more planar and homogeneous membrane. The thickness of membrane was 200 μm. The image analyses gave a mean fiber diameter of 800 nm. However, the proto-membrane which had been treated by immersion into the water bath lead to a pronounced porosity on the nanofiber surface which will be useful, for instance, for increasing binding capacity to the fiber surface. In addition, the membranes which had been electrospun under processing condition at high humidity resulted in irregular fiber formation. All test results for membranes showed that the fiber diameter and membrane structure and, consequently, membranes properties were clearly affected by applied voltage and spinneret-to-collector distance. The electrospun membrane was prepared by an applied voltage 18 kV at distance spinneret-to-collector of 10 cm, a flow rate of 20 μL/min, a spinneret diameter of 0.8 mm, a speed of substrates moving of 2.2 cm/min, 65% RH and served the PET nonwoven as substrate exhibited the pore size of 1.8 μm, the porosity of 93% and basic weight of 0.169 mg/cm2. All membranes showed similar and high contact angle. The electrospun membranes prepared by applying higher voltage have lower flux than membranes prepared with lower voltage. The electrospun membranes which were prepared by increasing distance between spinneret-to-collector (kept other conditions) exhibited high water flux and clear correlation between structure and performance that decreasing of mean pore size leads to decreasing water flux of electrospun membranes. However the results suggested that PES electrospun nanofiber membranes are excellent materials for high water flux MF applications. Regarding nitrogen gas flow through electrospun membranes, the membranes which were prepared by increasing applied voltage showed decreasing gas permeability. Moreover with the DEHS aerosol, with particles size of 400 - 1000 nm, filtration performance of electrospun nanofiber web was much greater than that of the 4 layers commercial nonwoven (Novatexx 2429) with pore size of 8 μm. This result clearly demonstrated the potential of electrospun nanofiber in the development of filter material against aerosol nanoparticles. The filtrate fluxes of commercial membrane (Membrana MicroPES; pore size 1 μm) was much smaller than the filtrate fluxes of PES electrospun membrane with pore size ranging between 1.7 - 4.5 μm. Overall, PES electrospun membrane showed greater water flux than commercial membrane both before and after separation of silica nanoparticles (size 35 nm). The water flux before separation of all membrane was higher than after silica nanoparticles separation. The PES electrospun membranes had higher particles rejection than PES commercial membrane. Besides, the rejection of electrospun membranes was well above 90%, while commercial membrane rejected the nanoparticles by only 85% in the beginning of the filtration. Moreover, the PES electrospun membranes exhibited the rejection above 98% at the end of rejection experiment run. Such the results showed, electrospun nanofiber PES membranes can be used in various applications such as removal of nano or microparticles from waste-water, e.g., pre-filters to minimize contaminations and fouling prior to UF or NF.
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