Electrospinning of nanofibers for filtration media

Since particulate impurity is regarded as the primary cause of lung diseases, purification of air has been a crucial issue. Filtration is the most conventional method to obtain clean air, whereby particulate matter is collected on a fibrous media. The use of fibrous filters is prevalent because of their high filtration efficiency and low pressure drop.;Fibrous filters were fabricated via the electrospinning process which can be used to produce continuous submicron-diameter sized fibers. Polyacrylonitrile (PAN) nanofibers with a mean fiber diameter of 224 nm were electrospun to form fibermats. Filtration tests on fibermats of PAN were conducted to confirm that filters of thinner fibers result in higher collection efficiencies and lower pressure drops than that of thicker fibers as predicted by the theoretical filtration mechanism. Results showed that electrospun PAN nanofibermats had a superior quality factor of 0.067+/-0 compared to 0.031+/-0.001 by the current state-of-the-art microfiber-based high particulate air (HEPA) filtration media. The verified theory implies that nanofibermats of other types of materials could also be considered as promising filtration media since filtration performance is independent of the material used.;As materials for advanced next-generation filtration media, ceramics are favored over polymeric materials due to their robustness against environmental factors such as ultraviolet rays, abrasive particles, and high temperature all of which degrade and damage the fibrous structure. Amidst various ceramic materials, the anatase phase of TiO2 was selected due to its mechanical property and versatility as a photocatalyst and microwave-absorbing material.;Anatase TiO2 fibers were fabricated by electrospinning followed by heat treatment at 500°C for 3 hours. However, early precipitation or gelation of the organic solvent-based TiO2 sol posed a practical challenge in the sample preparation. In order to enhance stability of the precursor sol, a novel aqueous sol with titanium alkoxide was developed. As the result, the time taken for gelation or precipitation was elongated from 4 hours for the organic solvent-based sol to 4 months with the novel aqueous sol. In seeking the proper chemical composition to attain electrospinnability and maximize the period for storage before gelation, the reaction paths of hydrolysis and condensation for one of the components of the aqueous sol were investigated by nuclear magnetic resonance (NMR) spectroscopy. After hydrolysis and condensation reactions, Si-O-Ti bonds were validated to be formed by the reaction mechanism. TiO2-SiO2 composite fibers were successfully electrospun from the aqueous sol system by addition of a spinning agent followed by heat treatment. In contrast to TiO2 fibers in which anatase phase was observed after heat treatment at 500°C, anatase phase was formed at 1100°C in TiO2-SiO2 composite fibers. The formation of Ti-O-Ti bonds was retarded due to the formation of Si-O-Ti bonds, as evidenced by the NMR results.;In regard to the microstructure of TiO2 fibers and TiO 2-SiO2 composite fibers with anatase phases, the TiO 2-SiO2 composite fibers were observed to have no voids or cleavages on the surface than TiO2 fibers which have coarse structures created upon crystallization at magnification of x330,000 by transmission electron microscopy. The coarse structure of TiO2 fibers characterized as having cleavages at exposed surface grain boundaries is anticipated to adversely affect the mechanical stability by enhancing crack formation and propagation which will lead to failure of the fiber. In contrast, amorphous SiO2 fills in the spaces that have been created by the development of anatase phase for TiO2-SiO2 composite fibers. Smoother surfaces were observed as well in contrast with TiO2 fibers due to the amorphous SiO2 in the continuous phase of the composite material. Based on the observations, TiO2-SiO2 composite fibers are expected to have better mechanical stability by reducing the possibility of crack formation and blockage of crack propagation.;The mean fiber diameter of TiO2-SiO2 composite fibers achieved was 243 nm, which is 8% thicker than the PAN fibers achieved and 54% thinner than fibers of the HEPA filter. Hence, the quality factor of the TiO2-SiO2 composite fibers is predicted to be between those of PAN fibermats and the HEPA filter by filtration theory; however it would be closer to that of PAN fibers. Moreover based on transmission electron microscopy (TEM) observation, the mechanical stability was improved as well by achieving denser structures in the fiber than in pure TiO2 fibers.