Charged PVDF multi-layer filters with enhanced filtration performance for filtering nano-aerosols

摘要

Filtration of neutrally charged nano- and sub-micron aerosols (50-500 nm) using PVDF electret nanofiber filters based on dielectrophoretic effect was investigated experimentally for the first time. Dielectrophoretic effect is refers to inducing dipole charges on neutrally charged aerosols when they are in close proximity to the charged nanofibers; subsequently, the polarized aerosols are captured electrostatically by the charged fibers. Given the nanofiber diameter is less than 500 nm, the electrostatic attraction force in the proximity of the fiber was very strong, resulting in good aerosol capture without incurring higher pressure drop across the filter. We were able to charge a PVDF nanofiber filter using corona discharge and the performance of the filter remained relatively unchanged within 90 days of repeated testing with quality factor well reaching 0.14 Pa-1 or higher. Optimization of charging parameters was carried out and charge density on filters indicated by surface potential (SP) was found to increase with increasing charging voltage and decreasing charging distance. Correspondingly, filtration efficiency increased due to the enhanced electrostatic attraction between charged fibers and polarized aerosols. The influence of filter basis weight (W) on the filtration performance of electret filters was studied, which showed that the improvement in filtration performance was insignificant with limited increase in filtration efficiency and higher pressure drop. Although higher fiber amount led to higher surface potential, it also caused unsatisfactory charge density increment and higher electrical interference between adjacent fibers due to higher packing density. To enhance the performance of PVDF electret filters, a 'multi-layering' approach was proposed for the first time on electret nanofiber filter whereby the total charged fibers were distributed in thinner stack-up layers, with each layer being isolated by a porous permeable substrate, to reduce the electrostatic interference among fibers of adjacent layers. The latter has been the key culprit in reducing the performance when charged fibers were all stacked into a single layer, resulting in overlapping of intense electrostatic fields from different nanofibers. Moreover, through distributing fibers to multilayers, the filter porosity was increased, leading to lower air flow resistance and pressure drop. Therefore, compared with a filter with only one layer, a multi-layer filter with similar total basis weight of fibers had much better performance, which was further verified by halving the basis weight of each individual layer (W-L) while maintaining the overall value. The single-fiber efficiency based on dielectrophoretic effect confirms larger aerosols got better charge induction resulting in stronger dipoles and better capture. The face velocity dictates the retention time and increasing retention time proves beneficial for dielectrophoretic capture. Multilayering could reduce electrostatic interference, thereby facilitating increasing basis weight of fibers to be used in a filter to achieve high capture efficiency. Moreover, filter durability/stability was investigated, which indicates the multi-layer PVDF electret filters could maintain high performance and are promising for long-term use. It was concluded that multilayer PVDF electret filters have a great performance in aerosol removal and a potential filtration enhancement mechanism was elaborated.