Abstract:

Objective Particulate matter in the air poses a significant health risk to humans. Utilizing fibrous materials to filter fine particles is the most prevalent method for golving the problem. Most filtration media struggle to achieve a balance among filtering efficiency, pressure drop, and service life. In order to create effective air filters, the fiber structures must be precisely designed. High-specific-surface-area nanofibers offer better filtering efficiency but greater air resistance. Beaded fibers provide abundant open spaces between fibers and lower air pressure as well as increasing service life. However, few studies have considered the influence of varied fiber sizes and morphologies, and fiber deposition order on filtering performance.

Method By controlling the mass concentrations and types of polymers with mass concentrations of 20% polyamide 6 (PA6), 20% polystyrene (PS20), and 30% PS (PS30), respectively, the single nozzle electrospinning technique was adopted to produce PA6 nanofibers (referred to as PA6 mono-membrane), PS beaded nanofibers (referred to as PS20 single fiber membrane), and PS microfibers (referred to as PS30 single fiber membrane), as well as composite fiber membranes with varying single fiber membrane deposition sequences. A sequentially deposited PA6/PS20/PS30 membrane, a reverse-deposited PS30/PS20/PA6 membrane, and a three-nozzle electrospun PA6-PS20-PS30 membrane were presented. The produced fibrous membranes were tested for initial filtration, filtration performance under variable face velocity, and dust holding.

Results PA6 nanofibers had greater filtering efficiency (99.18%) and larger pressure drop (85 Pa). PS20 beaded nanofibers could balance the contradiction between filtration efficiency and air resistance, with the highest quality factor (with filtration efficiency of 78.47%, air resistance of 20 Pa, quality factor of 0.079 4 Pa^-1) among the three mono-membranes. The pressure drop of PS30 microfibers was the lowest among the three, which was 10 Pa. None of the three mono-membranes can solve the problem of reduced filtration efficiency at extremely high wind speed. The filtration efficiency and air resistance of the three composite membranes were approximately the same, however the filtration performance was different when dust was loaded. In the 30 min dust loading test, the air resistance of PA6/PS20/PS30 membrane increased faster, whereas that of PS30/PS20/PA6 membrane grew slowest. It is speculated that this is related to the size of the fiber structure on the windward side and the pore structure between the fibers. From SEM images before and after dust collection, it is seen that a large number of coarse fibers and holes existed on the wind side of PS30/PS20/PA6 and PA6-PS20-PS30 membranes, which are conducive to the entry of fine particles into the membranes and delay the formation of "cake-layer filtration". In addition, as the upwind side of the PA6-PS20-PS30 membrane comprised nanofibers, microfibers, and beaded fibers simultaneously, filtration efficiency and air resistance can be maintained at severe wind speeds. The most penetrating particle size (MPPS) of PA6 mono-membrane under the challenge of 30-500 nm monodisperse particles was around 90 nm, with a filtration efficiency <70%. The MPPS of PS20 single fiber membrane was 30 nm, and the minimum filtering efficiency was 80.21%. The size fraction filtering efficiency of PA6/PS20/PS30 composite fiber membrane was more than 94%, and the MPPS was around 90 nm. Its filtering performance was superior to that of PA6 and PS20 single fiber membrane. Thus, owing to the diversity of fiber diameter and shape, the composite fiber membrane may demonstrate higher filtering performance under diverse particle sizes.

Conclusion By depositing PA6, PS20, and PS30 single fiber membranes in various sequences, composite fiber membranes with a beaded structure and nano-to microscale fibers were produced. Due to the complementary of fibers with various diameters and morphologies, the composite fiber membranes' initial filtration efficiency, dust-loading capacity, and filtration efficiency in the presence of high wind speeds are significantly increased. The wind side of the composite fiber membrane with an open pore structure permitted fine particles to enter the filter, hence delaying the increase in air resistance over time and prolonging the service life of the air filters. The PA6-PS20-PS30 membrane has a filtration efficiency of 93.13% and a pressure drop of 30.67 Pa, which is superior to the H10 commercial glass fiber filter. Thus, multi-nozzle electrospinning composite fiber membranes have greater potential for real-field filtration.

Key words: polyamide 6, polystyrene, multiple-nozzle electrospinning, air filtration, multi-layer composite filter, micro-nano fiber, beaded structure