Design of high efficiency PVDF-PEG hollow fibers for air filtration of ultrafine particles

Abstract

This study reveals as the first attempt to apply hollow fibers for air filtration of ultrafine particles. Different from symmetric nano-fiber filters and non-woven fabrics, asymmetric polyvinylidene fluoride – polyethylene glycol (PVDF-PEG) hollow fibers with high gas permeances have been developed by the dry-jet wet-spinning process. The addition of high molecular weight PEGs in spinning dopes facilitates the formation of loosely connected cross-section and porous outer skin, thus enhances the gas permeance for air filtration. Under the inside-out testing mode, all PVDF-PEG hollow fibers display excellent filtration efficiency of 99.999% against polydispersed NaCl particles with a geometric mean size of ~30nm. Since permeance increases with an increase in PEG molecular weight in spinning dope, the PVDF-PEG hollow fiber with a PEG molecular weight (MW) of 12,000Da possesses the highest quality factor because it has the highest permeance and lowest transmembrane pressure. However, the PVDF-PEG hollow fiber with a PEG MW of 8000Da has the best mechanical properties. Under the dead-end filtration, the filtration efficiency increases with an increase in air flow rate. This trend is contrary to the findings observed in the flat and symmetric fibrous filtration. The asymmetric structure in the cross-section of the newly developed hollow fibers may enhance aerosol deposition via direct impaction and Brownian motion at high flow rates. The dead-end filtration results also show that the quality factor is higher at a lower flow rate. Similarly, the cross-flow filtration results show that the hollow fiber modules operated at low cross-flow ratios have high quality factors. Therefore, it is preferred to operate the newly developed PVDF-PEG hollow fiber at a low flow rate or low cross-flow ratio. This study may provide useful insights for developing hollow fibers for air filtration with the optimal operation conditions.