Electrostatic charged nanofiber filter for filtering airborne novel coronavirus (COVID-19) and nano-aerosols

Abstract

The World Health Organization declared the novel coronavirus (COVID-19) outbreak as a pandemic on March 12, 2020. Within four months since outbreak in December 2019, over 2.6 million people have been infected across 210 countries around the globe with over 180,000 deaths. COVID-19 has a size of 60-140 nm with mean size of 100 nm (i.e. nano-aerosol). The virus can be airborne by attaching to human secretion (fine particles, nasal/saliva droplets) of infected person or suspended fine particulates in air. While NIOSH has standardized N95, N99 and N100 respirators set at 300-nm aerosol, to-date there is no filter standards, nor special filter technologies, tailored for capturing airborne viruses and 100-nm nano-aerosols. The latter also are present in high number concentration in atmospheric pollutants. This study addresses developing novel charged PVDF nanofiber filter technology to effectively capture the fast-spreading, deadly airborne coronavirus, especially COVID-19, with our target aerosol size set at 100 nm (nano-aerosol), and not 300 nm. The virus and its attached aerosol were simulated by sodium chloride aerosols, 50-500 nm, generated from sub-micron aerosol generator. PVDF nanofibers, which were uniform in diameter, straight and bead-free, were produced with average fiber diameters 84, 191, 349 and 525 nm, respectively, with excellent morphology. The fibers were subsequently electrostatically charged by corona discharge. The amounts of charged fibers in a filter were increased to achieve high efficiency of 90% for the virus filter but the electrical interference between neighbouring fibers resulted in progressively marginal increase in efficiency yet much higher pressure drop across the filter. The quality factor which measured the efficiency-to-pressure-drop kept decreasing. By redistributing the fibers in the filter into several modules with lower fiber packing density, with each module separated by a permeable, electrical-insulator material, the electrical interference between neighboring charged fibers was reduced, if not fully mitigated. Also, the additional scrim materials introduced macropores into the filter together with lower fiber packing density in each module both further reduced the airflow resistance. With this approach, the quality factor can maintain relatively constant with increasing fiber amounts to achieve high filter efficiency. The optimal amounts of fiber in each module depended on the diameter of fibers in the module. Small fiber diameter that has already high performance required small amounts of fibers per module. In contrast, large diameter fiber required larger amounts of fibers per module to compensate for the poorer performance provided it did not incur significantly additional pressure drop. This approach was applied to develop four new nanofiber filters tailored for capturing 100-nm airborne COVID-19 to achieve over 90% efficiency with pressure drop not to exceed 30 Pa (3.1 mm water). One filter developed meeting the 90% efficiency has ultralow pressure drop of only 18 Pa (1.9 mm water) while another filter meeting the 30 Pa limit has high efficiency reaching 94%. These optimized filters based on rigorous engineering approach provide the badly needed technology for protecting the general public from the deadly airborne COVID-19 and other viruses, as well as nano-aerosols from air pollution which lead to undesirable chronic diseases.

Keywords: 100 nm; Air filtration; COVID-19; Charged fibers; Electret; Iso-quality factor; Multilayering/multi-modules; Nano-aerosols; Novel coronavirus; PVDF nanofiber filter.

Fig. 1a

Electrospinning nanofibers using a syringe connected to a high voltage supply.

Fig. 1b

Corona discharge.

Fig. 1c

Aerosol generator and filter tester.

Fig. 1d

Sodium chloride 50–500 nm used to simulate the aerosol size of coronavirus and carrier.

Fig. 2

SEM images of four different electrospun nanofiber diameters in zoom-in view (see 1-μm bar) and zoom-out view (see 5-μm bar). (a) and (e) Correspond to 84 nm, (b) and (f) 191 nm, (c) and (g) 349 nm, and (d) and (h) 525 nm.

Fig. 3a

Comparing electrostatic charged PVDF nanofibers with diameters 84, 191, 349, and 525 nm with 0.191 gsm in a single layer of nanofibers.

Fig. 3b

Breakdown of efficiency due to mechanical diffusion plus interception and electrostatic effect for different nanofiber diameters for a single layer with 0.191 gsm.

Fig. 3c

Comparing single-layer configuration with multilayer configuration using different amounts of nanofibers 0.383, 0.574, and 0.77 gsm filter all with 84-nm diameter fibers.

Fig. 3d

Breakdown of efficiency due to mechanical diffusion plus interception and electrostatic effect for different nanofiber diameters for 4L with 0.764 gsm.

Fig. 4

Optimizing efficiency versus pressure drop and different nanofiber basis weight.

Fig. 5

6 sets of test filters with fiber diameter 84 nm (0.191 gsm), 191 nm (0.191 gsm), 349 nm (0.096 and 0.191 gsm), and 525 nm (0.191 and 0.765 gsm), respectively.

Fig. 6a

Efficiency of nano-aerosols versus pressure drop for multilayer versus single layer for 84-nm diameter filter. Iso-QF curves for 0.018 and 0.03 Pa⁻¹ are shown.

Fig. 6b

Efficiency of 100 nm versus pressure drop for discharged (mechanical) multilayer versus single layer for 525-nm diameter filter. Iso-QF curves for 0.025, 0.03 and 0.035 Pa⁻¹ are shown.

Fig. 7

Efficiency of 100-nm aerosol versus pressure drop for electret multilayer versus single layer for 84 nm, 191 nm, and 525 nm diameter filters. Iso-QF curves for 0.07, 0.1 and 0.15 Pa⁻¹ are shown.

Fig. 8

Efficiency of nano-aerosols versus pressure drop for electret multilayer filters with stack-up module 0.191 gsm versus stack-up module of 0.096 gsm for 349 nm diameter filters. Iso-QF curves for 0.07, 0.1 and 0.15 Pa⁻¹ are shown.

Fig. 9a

Comparing multi-layered filters at 525 nm diameter with modules, respectively, 0.191 gsm and 0.764 gsm.

Fig. 9b

Larger diameter nanofiber (450 nm and 525 nm) with increased basis weight (0.87 and 0.765 gsm) were used as basic module for building multilayer filter. These followed the iso-QF curves initially and have some deviations for the 3rd and 4th modules in the multilayer filter.

Fig. 10

Comparing the optimized multilayer nanofiber electret filter with conventional microfiber electret.

Fig. 11a

Multilayer versus single layer for 525-nm diameter charged filter.

Fig. 11b

Electret/charged versus uncharged (mechanical) filters, and multilayer versus single-layer filters all with 525-nm fiber diameter (3.06–4.6 gsm filter).

Fig. 11c

Charged versus uncharged (mechanical) filters, all with 349-nm fiber diameter (less than 1 gsm filter).

Fig. 12

Efficiency based on 300 nm for 7 multilayer nanofiber electret filters.

Fig. 13

Efficiency based on 50 nm for 7 multilayer nanofiber electret filters.

Fig. 14

a-f. Actual filtration efficiency in multilayer filter versus predicted efficiency from the basic module in stacking up multilayer configuration for electret nanofiber filter made up of 0.349-nm diameter nanofibers. (a)-(c) corresponds to using basic module of 0.191 gsm for building electret filter with total basis weight of fibers 0.38, 0.57 and 0.77 gsm respectively, using 2, 3 and 4 layers of basic modules. (d)-(f) corresponds to using basic module of 0.096 gsm for building electret filter with total basis weight of fibers 0.38, 0.57 and 0.77 gsm respectively, using 4, 6 and 8 layers of basic modules.

Fig. 15

Predicted versus test efficiency for multilayer nanofiber electret filter with 525-nm diameter fiber.

Fig. 16

Pressure drop versus fiber diameter for the multilayer filter.

Fig. B1

Single layer versus multilayer nanofiber for the same gsm for 191 nm diameter nanofiber filter.

Fig. B2

Performance for the 349 nm fiber diameter filter for various configurations.

Fig. B3

525 nm diameter filter.