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Characterisation data for electrocharged facepiece respirator fabrics using common materials

Citation

Bandi, M. M. (2020), Characterisation data for electrocharged facepiece respirator fabrics using common materials, Dryad, Dataset, https://doi.org/10.5061/dryad.ffbg79crq

Abstract

Face masks in general, and N95 filtering facepiece respirators (FRs) that protect against SARS-Cov-2 virion in particular, have become scarce during the ongoing COVID-19 global pandemic. We developed a technique for fabrication of electrocharged filtration layers employed in N95 FRs using commonly available materials and easily replicable methods. Our input polymer was polypropylene or polystyrene, and could include discarded plastic containers of these materials, and the fabrication setup was based on the cotton candy (CC) principle.  We translate the primary parameters underlying the CC principle to simple design rules that allow anyone to construct their own fabrication system from common parts, or employ a commercial cotton candy machine with minimal modifications. Finally, we present basic characterization results for structural and filtration properties of electrocharged fabrics made using the CC principle. The data uploaded here belongs to our characterization studies for use by any interested party.

Methods

Density: The electrocharged fabric densities were measured by weighing electrocharged fabric samples of known dimensions and then dividing the mass by the volume.

Scanning Electron Micrographs: Structural properties of the electrocharged filtration fabrics were studied using scanning electron microscopy (SEM). As a preparatory step, platinum-palladium sputter coating deposition was performed on the fabric sample surfaces for SEM visualization, followed by interrogation under a scanning electron microscope (Quanta 250 FEG, Manufacturer: FEI Thermo Fisher) at 2 kV acceleration voltage.

Filtration Test:

Setup: Filtration tests for face mask certification are usually performed on specialized equipment such as the Portacount Respirator Fit-Tester and MITA 8120, both from TSI Inc. or AccuFIT 9000 from Accutec-IHS Inc. We neither had access to such special testing system nor could we afford their price, and therefore designed our own filtration testing system in-house. We employed a manikin head used in retail store fronts and drilled a hole from its mouth to the back of its head. The face mask under test was then mounted onto the manikin head's face and placed in a confining box. An inexpensive piezeoelectric atomizer (APGTEK Aluminum Mist Maker) usually employed in home decoration was submerged in Sodium Chloride solution (5% by weight NaCl in de-ionized water) to generate aerosol particles. The generated mist was exposed to negative ion air purifier to charge the aerosol particles for some of the tests. The mist could pass through a pipe with a second connecting pipe open to ambient air and both pipes had valves to help control the total aerosol concentration in the air entering the confining box.

 

A portable PM2.5 air quality monitor (Manufacturer: Dienmern) used normally for home and office air quality monitoring was placed in the confining box (Monitor A) to measure the particle concentration within the box. By reading this PM2.5 monitor, we were able to adjust the two values for aerosol mist and ambient air and control aerosol concentration in the confining box. The back of the manikin head was connected to a pipe which exited the confining box and terminated in a box containing a second PM2.5 air quality monitor (Monitor B). This monitor gave reading of particles that had passed through the fabric and manikin mouth and allowed measurement of filtration quality. This box containing the second PM2.5 monitor was, in turn connected to a vacuum pump. When the vacuum pump was turned on, a suction pressure was felt in the confining box and aerosol particles mixed with ambient air were sucked into the confining box and passed through the face mask to enter the drilled hole in the manikin head and exited the confining box. By controlling the vacuum pump valve, we were able to simulate flow rates for normal (30 liters per minute) and high (85 liters per minute) respiration rates.

Design shortcomings: We emphasize that this filtration test setup does not conform to some of the stringent testing specifications employed in face piece respirator certification. For instance, NIOSH 42 CFR Part 84 standard for N95 facepiece respirators (FRs) requires filter performance of greater than or equal to 95% with NaCl test agent at 85 liters per minute flow rate and inhalation resistance (maximum pressure drop across mask) of less than or equal to 343 Pa and exhalation resistance of less than or equal to 245 Pa. We had no means to measure the pressure drop nor could we simulate the oscillating respiratory air flow from inhalation and exhalation. Our scheme could only generate steady suction flow.

 

An important quantity in face mask filtration quality testing is the Most Penetrating Particle Size (MPPS); MPPS for N95 FRs is 300 nm or 0.3 microns. An important shortcoming of our filtration test setup is we do not know the sizes of aerosol particles generated by our relatively inexpensive piezoelectric atomizer. Secondly, commercial respirator testing systems use laser-based particle counter sensors that are sensitive to detection of particles down to at least the MPPS value. The PM2.5 air quality monitors, as their name suggests, are rated for measuring particles as small as 2.5 microns. Be that as it may, PM2.5 monitors also employ the same laser-based particle counter sensors and do hold the capability to detect particles down to 0.3 microns. Though we were unable to verify, it is reasonable to assume the PM2.5 monitors could detect particles down to 0.3 micron diameter.

Test Results: FR filtration efficiency is usually measured in terms of the penetration percentage (P), defined as percentage of particles present in the environment that pass through the FRs and is quoted against the particle diameters. Since we could not measure particle sizes with our PM2.5 monitors, we define the penetration as:

P(t) = C_B(t)/C_A(t) x 100%

where C_A(t) and C_B(t) are the particle concentrations of PM2.5 monitors A and B respectively at time t. We followed the penetration as a function of time to study any deterioration in filtration properties. The PM2.5 monitors A and B were connected to a laptop and programmed to record concentration values at 15 minute intervals over a duration of 12 hours. The results we quote are for this 12 hour time series under various calibration and test conditions.

Usage Notes

FabricDensity.txt: This file contains measured values for density of the electrocharged fabrics reported in Table 1 of the main manuscript. Reported here are measurements from a commercial N95 facepiece respirator (N95 FR), Surgical Mask (SM), Isotactic Polypropylene (PP), and Polypropylene-Polystyrene (PP-PS) blend material. The density measurements were obtained by dividing the measured weight (in grams) by the volume obtained from measured dimensions of the measured samples (in centimeter cubed). This is a tab-limited text file with the following format:

Column 1: N95 FR -- there was only one N95 facepiece respirator sample we could cut open to measure properties of its electrocharged filtration layer. Accordingly, only one measurement value is reported.

Column 2: SM -- 10 independent measurements reported on various commercial surgical masks.

Column 3: Isotactic PP -- 10 independent measurements reported for electrocharged filtration fabrics manufactured using the cotton candy method for isotactic polypropylene as the fabrication material.

Column 4: PP-PS -- 10 independent measurements reported for electrocharged filtration fabrics manufactured using the cotton candy method for a blend of 80% polypropylene and 20% polystyrene as the base material.

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Scanning Electron Micrographs: Full resolution Scanning Electron Micrographs (SEM) for fabrics from a commercial N95 facepiece respirator (N95 FR), Isotactic polypropylene (PP), Low Crystallinity polypropylene (PP), Polystyrene (PS), and polypropylene-polystyrene (PP-PS) blend. The specific SEM image files are:

N95.tif: Scanning Electron Micrograph of the electrocharged filtration layer from a commercial N95 filtering facepiece respirator (N95 FR). This image is the full resolution SEM for figure 6a of the manuscript.

IsotacticPP.tif: Scanning Electron Micrograph of the electrocharged filtration layer from a sample electrocharged filtration fabric manufactured using the cotton candy method with isotactic polypropylene as the fabrication material. This image is the full resolution SEM for figure 6b of the manuscript.

PP-PS.tif: Scanning Electron Micrograph of the electrocharged filtration layer from a sample electrocharged filtration fabric manufactured using the cotton candy method with low crystallinity Polypropylene (80%) and polystyrene (20%) which we call PP-PS blend as the fabrication material. This image is the full resolution SEM for figure 6c of the manuscript.

LowCrystallinePP.tif: Scanning Electron Micrograph of the electrocharged filtration layer from a sample electrocharged filtration fabric manufactured using the cotton candy method with low crystallinity polypropylene as the fabrication material. This image is the full resolution SEM for figure 7a of the manuscript.

PS.tif: Scanning Electron Micrograph of the electrocharged filtration layer from a sample electrocharged filtration fabric manufactured using the cotton candy method with polystyrene as the fabrication material. This image is the full resolution SEM for figure 7b of the manuscript.

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Mask Filtration Efficiency Test Data: These files provide the time series for particle penetration using the method described in the Methods section above and in the manuscript (Section 5b). The tests were performed using a filtration test setup constructed in-house as presented in figure 8 of the manuscript. The tests were conducted on a commercial N95 filtering facepiece respirator (N95 FR) and commercial Surgical Mask (SM) for calibration. These calibrations were followed by filtration tests on Isotactic PP and PP-PS fabrics tested on surgical mask as secondary layer and on Montana Mask 3D printed fabric holder. All data files are provided in tab-limited text format and their structure is described below:

N95FR.txt: Penetration values as measured per Equation 5.1 of the manuscript, over a 12 hour period for a commercial N95 filtering facepiece respirator (N95 FR). This data is presented in Figure 9a of the manuscript.

Column 1: Time (Hours)

Column 2: Penetration values (%) for charged aerosol particles at 30 liters per minute flow rate.

Column 3: Penetration values (%) for neutrally charged aerosol particles at 30 liters per minute flow rate.

Column 4: Penetration values (%) for charged aerosol particles at 85 liters per minute flow rate.

Column 5: Penetration values (%) for neutrally charged aerosol particles at 85 liters per minute flow rate.

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SM.txt: Penetration values over a 12 hour period for a commercial surgical mask (SM). All measurements in this test were performed using only neutrally charged aerosols. This data is presented in figure 9b of the manuscript.

Column 1: Time (Hours)

Column 2: Penetration values (%) without tape on the surgical mask at 30 liters per minute flow rate.

Column 3: Penetration values (%) with tape on the surgical mask at 30 liters per minute flow rate.

Column 4: Penetration values (%) without tape on the surgical mask at 85 liters per minute flow rate.

Column 5: Penetration values (%) with tape on the surgical mask at 85 liters per minute flow rate.

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IsotacticPP.txt: Penetration values as measured per Equation 5.1 of the manuscript, over a 12 hour period for an isotactic polypropylene fabric made using the cotton candy method. This data is presented in Figure 9a (columns 1-5 below) and b (columns 1 and 6-9 below) of the manuscript.

Column 1: Time (Hours)

Column 2: Penetration values (%) for non-electrocharged filtration fabric mounted onto a taped surgical mask at 30 liters per minute flow rate with neutrally charged aerosols.

Column 3: Penetration values (%) for non-electrocharged filtration fabric mounted onto a taped surgical mask at 85 liters per minute flow rate with neutrally charged aerosols.

Column 4: Penetration values (%) for electrocharged filtration fabric mounted onto a taped surgical mask at 30 liters per minute flow rate with neutrally charged aerosols.

Column 5: Penetration values (%) for electrocharged filtration fabric mounted onto a taped surgical mask at 85 liters per minute flow rate with neutrally charged aerosol particles.

Column 6: Penetration values (%) for electrocharged filtration fabric mounted on a 3D printed Montana Mask holder exposed to charged aerosols at 30 liters per minute flow rate.

Column 7: Penetration values (%) for electrocharged filtration fabric mounted on a 3D printed Montana Mask holder exposed to neutrally charged aerosols at 30 liters per minute flow rate.

Column 8: Penetration values (%) for electrocharged filtration fabric mounted on a 3D printed Montana Mask holder exposed to charged aerosols at 85 liters per minute flow rate.

Column 9: Penetration values (%) for electrocharged filtration fabric mounted on a 3D printed Montana Mask holder exposed to neutrally charged aerosols at 85 liters per minute flow rate.

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PP-PS.txt: Penetration values as measured per Equation 5.1 of the manuscript, over a 12 hour period for a polypropylene-polystyrene blend (PP-PS)  fabric made using the cotton candy method. This data is presented in Figure 9c (columns 1-5 below) and c (columns 1 and 6-9 below) of the manuscript.

Column 1: Time (Hours)

Column 2: Penetration values (%) for non-electrocharged filtration fabric mounted onto a taped surgical mask at 30 liters per minute flow rate with neutrally charged aerosols.

Column 3: Penetration values (%) for non-electrocharged filtration fabric mounted onto a taped surgical mask at 85 liters per minute flow rate with neutrally charged aerosols.

Column 4: Penetration values (%) for electrocharged filtration fabric mounted onto a taped surgical mask at 30 liters per minute flow rate with neutrally charged aerosols.

Column 5: Penetration values (%) for electrocharged filtration fabric mounted onto a taped surgical mask at 85 liters per minute flow rate with neutrally charged aerosol particles.

Column 6: Penetration values (%) for electrocharged filtration fabric mounted on a 3D printed Montana Mask holder exposed to charged aerosols at 30 liters per minute flow rate.

Column 7: Penetration values (%) for electrocharged filtration fabric mounted on a 3D printed Montana Mask holder exposed to neutrally charged aerosols at 30 liters per minute flow rate.

Column 8: Penetration values (%) for electrocharged filtration fabric mounted on a 3D printed Montana Mask holder exposed to charged aerosols at 85 liters per minute flow rate.

Column 9: Penetration values (%) for electrocharged filtration fabric mounted on a 3D printed Montana Mask holder exposed to neutrally charged aerosols at 85 liters per minute flow rate.

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