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Abstract

Nanofibrous media have a low basis weight, high permeability and small pore size that make them appropriate for a wide range of filtration applications, particularly for smaller particles. In contrast to electrostatic filter media, nanofibers' filtration efficiencies depend on the mechanical filtration mechanism and do not degrade with time. In this study, the National Institute for Occupational Safety and Health (NIOSH) requirements for the N95 particulate filtering half mask were achieved using electrospun cellulose acetate (CA) and polyvinylidene fluoride (PVDF) nanofiber coated polypropylene spunbond layers. Specifically, 16 and 15% (w/v), and 14, 12 and 10% (w/w) polymer concentrations were selected for CA nanofibers and PVDF nanofibers, respectively, to adjust the nanofiber diameters. The diameters of CA and PVDF nanofibers were decreased with decreasing polymer concentration for both CA (319.02 to 264.02 nm) and PVDF (236.50 to 142.59 nm) nanofibers. The thickness of the electrospun 16CA, 15CA and 14PVDF, 12PVDF, 10PVDF mats was adjusted by varying the collection period (15 min, 30 min, 60 min). The effects of electrospun CA and PVDF nanofiber diameter on the pore size and the thickness of the mats were compared in terms of filtration performance. 16CA with a nanofiber diameter of 319.02 nm had the largest first bubble point of 26.5 µm and mean flow pore size of 5.71 µm at 15 min with a thickness of 0.019 mm. The smallest first bubble point and mean flow pore values and the smallest pore size were achieved with the finest nanofibers of 10PVDF. Filtration performances were given as initial penetration and air flow resistance (ΔP). 16CA-60 min and 15CA-30 min mats both met the NIOSH requirements with their bulky structure. For PVDF, thinner nanofibers with smaller pores were produced compared to the CA nanofibers, and the NIOSH requirements were only achieved using double-layered, face-to-face 10PVDF-15 min nanofiber mats with the penetration of 1.85% and ΔP of 33.87 mmH₂O.

Keywords

Face masks filtration N95 nanofibers respirators

Introduction

Filtering facepiece particulate respirators and facemasks are widely utilized for reducing the inhalation of airborne particles that cause various health effects [1 –3]. A simple definition of a facemask is “a loose-fitting, disposable mask that composes a physical set between the mouth and nose of the wearer and potential pollutants in the immediate environment” [4]. A conventional facemask usually includes one, two or three layers of three-ply material attached to the head with ear loops [5,6]. On the other hand, a respirator (filtering device) refers to a facepiece with higher filtering efficiency and provides a better fit than a conventional mask [5]. It is generally pre-molded, fits tightly, adheres to the head with a single elastic band and uses filters to remove harmful substances from the air that is being breathed in [5,7]. Respirators help to reduce the wearer's respiratory exposure to airborne contaminants such as particles that are small enough to be inhaled with the size of less than 100 microns (μm) [8] and are essential safety equipment in many dust-generating industrial applications [9].

In the United States, the National Institute for Occupational Safety and Health (NIOSH) tests the filtration efficiency of particulate filtering, air-purifying respirators for certification purposes. NIOSH approves N- (not oil resistant), R- (resistant to oil) and P- (oil proof) series nonpowered air-purifying respirators each at 95, 99 and 99.97% filtration efficiency levels under 42 CFR Part 84 as shown in Table 1 [10]. Among the respirators certified under 42 CFR Part 84, the N95 respirators are the most commonly used [8,11]. The penetration, P, of the particles through a certified N95 respirator cannot exceed 5%, and the efficiency, E, of the respirator is calculated as E = 100% − P and must be at least 95% [12,13] with a maximum 35.0 mmH₂O air flow resistance. Most N95s sold commercially in the last decade generally use electrostatic filter media [14 –16]. This type of media imparts static electric charge on polypropylene fibers to enhance their capture efficiency via Coulombic attraction and polarizing forces [15]. Corona-charged, split polypropylene fiber filters, or electret filters retained their electrostatic charge and capture efficiencies for a year [16]. Chen et al. investigated the filtration characteristics and loading effects of filtering facepiece respirators made from electrically charged filter media [17]. Tsai et al. reported three charging techniques (corona charging, tribocharging and electrostatic fiber spinning) for different polymer types. They also reported that electrospun nanofibers exhibited high efficiency due to mechanical filtration mechanisms and observed that little charge was retained in electrospun polyethylene oxide fibers, but that a high amount of charge was retained in polycarbonate and polyurethane nanofibers [18]. The main drawback of electret filter media is that if the electrostatic charge is lost or removed, the filtration efficiency decreases significantly. To address this drawback, a layer of filter material with true mechanical filtration could be entirely or substantially used.

Table 1.

NIOSH 42 CFR Part 84 particle filter classifications [8].

Minimum efficiency	N-series	R-series	P-series
95%	N95	R95	P95
99%	N99	R99	P99
99.97%	N100	R100	P100

This filter material can provide high protection to a face mask or a respirator. As an important part of this emerging technology, nanofibers and nanofiber webs in filter medias are the key elements for improving the filtration efficiency [6,19,20]. Because of its relative ease of use and ability to fabricate nanofibers, electrospinning is one of the most commonly used methods for the production of nanofibers. In this approach, a high voltage power source, a nozzle and a collector are used [21]. The produced nanofiber media have a low basis weight, high permeability and small pore size that make them appropriate for use in a wide range of filtration applications, particularly for smaller particles. Nanofibers offer unique properties such as high specific surface area (ranging from 1 to 100 m²/g depending on the diameter of the fibers and intrafiber porosity) and good interconnectivity of pores. Nanofibers are fragile, are usually not used alone in filtration, and generally are produced onto a nonwoven fabric [6,20]. Some supports showing good conductivity, permeability and adhesion to the cellulose acetate (CA) nanofiber to create a multilayer were tested to examine the feasibility of an efficient air filter based on CA nanofibers [22]. The results showed that the support has a major influence on the final product and the air filtration performance and that the pore size distribution is closer for the membrane electrospun on a nylon grid because of its more pronounced two-dimensional structure. According to the test results, the substrate must have a relatively even surface and a low thickness. A composite material alone could show a quality factor of 0.055 ± 0.040 Pa⁻¹ at 100 nm and 0.080 ± 0.050 Pa⁻¹ at 300 nm, when tested at the filtration velocity of 5.3 cm s⁻¹ with neutralized NaCl aerosol particles [22].

Nanofiber filter media have enabled new levels of filtration performance in several diverse applications within a broad range of environments. There are many applications of nanofibrous filters that are already commercialized in addition to the applications that are still under development [23]. One of these is the use of nanofibers in facemasks and respirators. Studies and some patents for nanofibers in various facemask applications have been reported [24 –32]. Li et al. produced electrospun polysulfone nanofibers for mask filtration and compared the produced nanofibrous mats for PM_2.5 (particulate matter with a diameter of 2.5 μm or less) with disposable nonwoven face mask, nonwoven mask for use in the operating room, Ito PM_2.5, and N95 and R95 respirators. The thickness of the electrospun fiber mats was varied with the collecting period (15 min < 30 min < 60 min). It was reported that the produced nanofiber mask materials could efficiently filter out the PM_2.5 particles and simultaneously preserve a good breathability [26]. Skaria and Smaldone compared a prototype-fitted facemask with a nanofiber filter media (PT) with an N95 filtering facepiece respirator. It was observed that PT with a nanofiber filter showed more efficient filtration than the commercial masks, and in contrast to the commercial face masks, the nanofiber prototype retained its low resistance. In addition, the nanofiber media significantly reduced the mask air flow resistance (ΔP) [27]. Wang et al. prepared an ultralight weight (2.94 g/m²) PA6 and PAN nanofiber filter medium by the accumulation of nanofibers/nets via multi-jet spinning. Their N6-PAN nanofiber media showed a filtration efficiency of 99.99% for PM_2.5 [24]. Ramaseshan et al. studied a functionalized polymer nanofiber membrane for capturing chemical warfare agents. A catalyst synthesized from β-cyclodextrin and o-iodosobenzoic acid to detoxify nerve agents was used, and PVC nanofiber membranes were produced [28]. It was found that when tested against the decontamination of a nerve agent stimulant (paraoxon), the stimulant became hydrolyzed. It was reported that functionalized nanofibers with different catalysts can also degrade other types of chemical and biological warfare agents and that various nanofiber layers can be combined together with a substrate to form protective garments [28,32,33]. Faccini et al. also developed an efficient protective clothing using PA6 nanofibers against nanoparticulate aerosols [34]. Yang et al. designed a multifunctional face mask with 99.6% filtration efficiency for PM_2.5 and with effective radiative cooling [31]. In US 2016/0015098 A1, methods for utilizing nanofibers in facemasks are described [30], and US 2008/0264259 A1 defines a composite filter media including a nanofiber layer [29]. Thus, the survey of the abovementioned literature demonstrates that a nanofiber filter media for an N95 respirator has not been developed. Only Skaria and Smaldone compared a nanofiber media with a commercial N95, but the structure of the nanofiber mask used in their study was not described. The requirement of at least 95% efficiency along with maximum 35 mmH₂O of ΔP at 0.3–0.4 µm is difficult to achieve even with nanofibers. This study clearly defines the diameters and the amount of nanofibers that are necessary for achieving the N95 requirements.

In this study, cellulose acetate (CA) and a polyvinylidene fluoride (PVDF) nanofiber layer for a facemask or a respirator were developed with 100% mechanical filtration that could meet the requirements of N95 respirators. There are many reports in the literature related to the use of nanofibers in filtration [24,26 –28,31,32]; however, the requirements for their use in facemasks and in particulate filtering differ [12,13]. There are very few studies on the nanofibers in facemasks, and most of these are focused on the capture of chemical vapors [28,32,33]; to the best of author's knowledge, the CA and PVDF nanofibers are investigated in this work as a layer for N95 respirators for the first time. The proposed nanofiber layers show high mechanical filtration efficiency compared to the conventional facemask layers. PVDF was chosen because of its hydrophobicity, high thermal and chemical resistance, and excellent mechanical properties [35 –38]. It is also widely used in ultrafiltration, microfiltration, membrane distillation and some other membrane processes [37 –39] such as affinity membranes. Nanopressure sensors [40,41], polymer electrolytes or separators [42], proton exchange membranes in fuel cells [43] and thin film composite membranes for forward osmosis [44] are some other applications of PVDF nanofibers.

On the other hand, CA is a derivative of the naturally occurring raw material and is an alternative to synthetic polymers produced from petroleum [22]. CA nanofibers could be utilized for various applications ranging from affinity membranes [45] to tissue engineering [46] and sensors [47]. CA was chosen because of its filtration efficiency [20], high hydrophilicity, good liquid permeability, biodegradability and water absorption capacity [22,48]. CA is stable to water and has good solubility in organic solvents [20,49] and has also been applied in filtration, particularly in cigarette smoke filters [20,50] because of its significant filtration selectivity to low-level organic compounds [51]. In addition, it is possible to obtain smooth and fine nanofibers from both PVDF and CA via electrospinning that could lead to high filtration efficiency.

PVDF and CA nanofiber layers were produced onto a spunbond polypropylene media, and the effects of the nanofiber diameter, pore size and the thickness of the nanofiber mats on their filtration performances were compared. The diameters of the produced CA and PVDF nanofibers were decreased by decreasing the polymer concentration, and the thickness of the electrospun CA and PVDF mats was adjusted by changing the collection period (15 min, 30 min, 60 min). Both the nanofiber diameter and the collection period significantly affect the pore size of the nanofiber layers that finally determine the filtration efficiency of the nanofiber layers.

Experımental section

Materials

CA with the average molecular weight (Mn) of ∼30,000 g/mol and acetylation degree of 39.8 %wt, acetone and dimethylacetamide (DMAc) were purchased from Sigma-Aldrich Chemical Company. PVDF (Kynar 761A) was provided from Abalıoglu Teknoloji.

Electrospinning

Polymer concentrations of 16 and 15% for CA and 14, 12 and 10% for PVDF were selected after screening studies. Homogeneous CA solutions were prepared by dissolving 16 and 15% (w/v) of CA powder in acetone/DMAc (2:1 v/v), and electrospun CA solutions were coded with 16CA and 15CA and with their collection periods. Homogeneous PVDF solutions were prepared by dissolving 14, 12 and 10% (w/w) PVDF powder in acetone/DMAc (1:4 v/v), and tetraethylammonium bromide (TEAB 0.015 g) was added to each PVDF solution (30 g) to improve the electrospinnability and the fiber uniformity [37,52 –54]. Electrospun PVDF solutions were coded as 14PVDF, 12PVDF and 10PVDF with their collection periods.

CA and PVDF polymer solutions were electrospun by a setup consisting of a ground electrode and a high voltage supply (Simco, MP Series CM5 30 P, charging generator output 30 kV DC) and a syringe (10 mL) with a stainless steel needle (22 gauges, and flat tip), and CA and PVDF solutions were electrospun at the voltages of 15 and 18 kV, respectively, and a tip-to-collector distance of 15 cm with a feeding rate of 0.5 mL/h. The polymer concentrations, fiber diameters of CA and PVDF nanofibers and their electrospinning conditions are specified in Table 2. A rotating metal drum collector at 200 r/min was used for deposition. It was covered by a 20 g/m² spunbond polypropylene (PP) nonwoven fabric. Each polymer solution was electrospun for 15, 30 and 60 min. Electrospinning experiments were performed at room temperature (22 ± 2℃) and at the relative humidity of 22–40%.

Table 2.

The polymer concentrations, fiber diameters for CA and PVDF nanofibers and their electrospinning conditions.

Codes	Polymer concentrations	Fiber diameter (nm) ± SD	Applied voltage (kV)	Tip-to-collector distance (cm)	Feeding rate (mL/h)
16CA	16% (w/v)	319.02 ± 114.039	15	15	0.5
15CA	15% (w/v)	264.02 ± 118.021	15	15	0.5
14PVDF	14% (w/w)	236.50 ± 67.409	18	15	0.5
12PVDF	12% (w/w)	177.55 ± 42.609	18	15	0.5
10PVDF	10% (w/w)	142.59 ± 32.518	18	15	0.5

Scanning electron microscope analysis

The scanning electron microscope (SEM) images of CA and PVDF nanofibers were obtained using a Phenom G2pro instrument at an acceleration voltage of 5 kV. Prior to SEM observation, electrospun nanofibers were sputtered with a thin layer of gold for 30 s using a Quorum Q150R S ion sputtering device.

Measurement of nanofiber diameters, weight and thickness

The mean diameters of the electrospun PVDF and CA nanofibers were calculated from 50 measurements using the ImageJ program. The weight of the nanofibers was calculated from an area of 10 cm². The thicknesses of the nanofiber mats were measured using a digital micrometer (Mitutoyo) with a precision of 0.01 mm. The values of the mean fiber diameter, weight and thickness measurements are expressed as the mean ± SD. Data were analyzed by one-way analysis of variance (ANOVA) followed by the Tukey HSD post hoc test. Differences with p < 0.05 were considered statistically significant.

Pore size and pore size distribution measurements

The first bubble point (FBP, maximum pore size), the mean flow pore size (MFP, average pore size), the smallest pore size (SPS) and the pore size distribution of the CA and PVDF nanofibers were measured using a Porolux 1000 instrument (Germany) carrying out capillary flow porometry measurements. The mean pore sizes of the nanofiber layers were calculated from wet, dry and half dry conditions and were measured by the wet-up/dry-up method and analysis. All the samples were wetted by Galpore 16 (a wetting liquid with a low surface tension of 16 dyne/cm) and tested. The Automated Capillary Flow Porometer system software calculated the pore sizes according to the ASTM F316-03 (2011) [55].

Evaluation of the filtration performance

According to NIOSH 42 CFR 84, the efficiency level of 95% was tested at a flow rate of 85 L/min at the most penetrating particle size (MPPS) (generally approximately 0.1–0.3 μm) [2,3,10,56]. A TSI Model 8130 Automated Filter Tester (TSI, Inc., St Paul, MN, Figure 1) was used to measure the filter flow resistance (pressure drop in mmH₂O column height pressure) and percent filter penetration. The TSI 8130 delivers a solid polydisperse sodium chloride (NaCl) aerosol that meets the particle size distribution criteria set forth in 42 CFR 84 for NIOSH certification. The NaCl aerosol has a count median diameter of 0.075 ± 0.020 µm and a geometric standard deviation not exceeding 1.86. The TSI 8130 uses a photometer to measure the flux of light scattering from aerosol particles. All the tests were conducted at room temperature with a continuous airflow of 85 ± 2 L/min as a short initial efficiency test [57,58]. According to NIOSH requirements, to determine the filtration efficiency of an N95 filtering facepiece, NaCl aerosol loading tests were performed [15]. In our case, the initial penetration is the maximum value of a loading test. This is the level close to the efficiency range limit, and thus, filtration performances were given as initial penetration and resistance. The tests were performed in the filter test mode. Particle penetration P is defined as

P (%) = \frac{C_{down}}{C_{up}} \times 100

(1) where C_down and C_up are the particle number concentration at the filter downstream and upstream, respectively. Penetration is related to the filtration efficiency E

E = 100 - P

(2)

Figure 1.

(a) Schematic representation of TSI 8130 and (b) filter holder [59].

Results and discussion

The surface morphologies of the electrospun nanofibers of the CA and PVDF nanofibers were investigated by SEM imaging. The representative SEM images of CA and PVDF nanofibers are given in Figures 2 and 3, respectively, according to their polymer concentration at two magnifications. In all cases, uniform CA and PVDF nanofibers were produced. While PVDF nanofibers were more uniform with small polymer splashes, some thicker regions were observed in the SEM images of the CA nanofibers with 1000 × magnification, which also led to a bulkier structure of the CA nanofibers. Fiber distribution histograms are shown in Figure 4. The mean fiber diameters of the produced CA nanofibers were in the range of 319.02 and 264.02 nm, and PVDF nanofibers were produced in the range of 236.50 and 142.50 nm. Differences in the fiber diameters were analyzed by one-way ANOVA followed by a Tukey test for pairwise comparison, and p < 0.05 was considered statistically significant. It was observed that the difference between CA and PVDF nanofiber diameters was statistically important. PVDF nanofibers were thinner than CA nanofibers. It was observed that for both CA and PVDF nanofibers, polymer concentration had a significant effect on fiber diameters. The mean fiber diameter of 16CA nanofibers was significantly larger than that of 15CA nanofibers. The Tukey HSD post hoc test revealed that the mean fiber diameters of 14PVDF, 12PVDF and 10PVDF were significantly different from each other (p < 0.05).

Figure 2.

SEM images at 1000 × and 10,000 × magnification of (a) 16CA and (b) 15CA.

Figure 3.

SEM images at 1000 × and 10,000 × magnification of (a) 14PVDF, (b) 12PVDF and (c) 10PVDF nanofibers.

Figure 4.

Fiber distribution histograms for CA and PVDF nanofibers.

Table 3 lists the thickness and weight values of the CA and PVDF nanofibers. As expected, the thicknesses of the nanofibers were significantly affected by the collection period and were in the order of 15 min < 30 min < 60 min. However, the thicknesses of the 15 min, 30 min and 60 min collected 16CA, 15CA nanofibers and those of the 15 min, 30 min and 60 min collected 14PVDF, 12PVDF and 10 PVDF nanofibers were not significantly different from each other. Polymer concentration did not significantly change the thickness values for each collection period. Thus, the mean thickness and weight of 16CA and 15CA, and 14PVDF, 12PVDF and 10PVDF nanofibers were given based on the collection period. CA nanofibers were produced with a thickness in the range of 0.0191–0.056 mm, and PVDF nanofibers were produced in the thicknesses range of 0.003–0.038 mm. It was observed that CA nanofibers were thicker and heavier than PVDF nanofiber mats. This was related to the weight of the CA and the thicker diameter of the CA nanofibers.

Table 3.

Thickness and weight of CA and PVDF nanofibers.

Polymers and collection periods	Thickness (mm) ± SD	Weight (g/m²) ± SD
CA-15 min	0.019 ± 0.013	5.76 ± 4.12
CA-30 min	0.038 ± 0.013	7.12 ± 1.69
CA-60 min	0.056 ± 0.011	9.87 ± 1.45
PVDF-15 min	0.013 ± 0.004	1.36 ± 0.25
PVDF-30 min	0.031 ± 0.005	2.21 ± 1.77
PVDF-60 min	0.039 ± 0.001	6.99 ± 0.07

FBP, MFP and pore size distribution are important parameters for determining the arrestment capability of the nanofiber layer [60,61]. FBP, MFP and SPS are given for 16CA and 15CA, 14PVDF, 12PVDF and 10PVDF nanofiber layers in Table 4. Liquid (Galpore 16) flow and air flow values at the relevant pressures (at the closest point of MFP pressure) are also included in Table 4. During the porometry run, the wet and dry curves meet at higher pressures, indicating that all the pores within the membranes have been completely “opened” [62]. Pore size distributions are shown in Figure 2(a) and (b), according to their collection period. It is observed that the pore size of the nanofibers was directly related to the nanofiber diameter and the collection period. 16CA with a nanofiber diameter of 319.02 nm had the highest FBP of 26.5 and MFP of 5.71 µm at 15 min with the thickness of 0.019 mm. FBP and MFP decreased to 7.12 and 6.93, and 3.57 and 3.32 µm, respectively, with increasing thickness. When the nanofiber diameter of 15CA decreased to 264.02 nm, at 60 min with the thickness of 0.056 mm, FBP and MFP decreased to 4.59 and 2.38 µm, respectively. With the increasing collection period, 16CA and 15CA nanofiber mats showed greater resistance to both Galpore 16 and air at the given pressures. Compared to 16CA-60 min and 15CA-60 min mats, the required pressure increased to 0.274 bar for 66.48 L/min of permeability. For PVDF nanofiber mats, 14PVDF-60 min showed similar permeability at 0.246 bar to that of 15CA-60 min. However, with a decreasing polymer concentration, for 12PVDF and 10PVDF nanofiber mats, a dramatic decrease in the permeability was observed. Due to the smaller pore sizes, even at a higher pressure such as 0.623 bar, 0.032 L/min air permeability was observed for the dry curve of 10PVDF-60 min. The smallest FBP, MFP and SPS values were achieved with the finest nanofibers of 10PVDF. The mean nanofiber diameter of the 10PVDF nanofibers was 142.59 nm, and at 30 and 60 min collection periods with the thickness of 0.031 and 0.039 mm, it was possible to achieve FBP and MFP values of approximately 1 µm. Figure 5(a) shows the pore size distributions, and it was observed that the thicknesses of the nanofibers due to the collection period had a significant effect. The picks of the pore size distribution curves were close to each other at each collection period. In Figure 5(b), the pore size distribution curves of 14 PVDF were farther than the pore size distribution curves of 12PVDF and 10 PVDF due to the significantly thicker nanofiber diameter (236 nm). The results also showed that 10PVDF-60 min, 10PVDF-30 min, 12PVDF-60 min and 12PVDF-30 min had the SPSs and narrower pore size distribution curves.

Figure 5.

Pore size distributions of (a) 16CA and 15CA and (b) 14PVDF, 12PVDF and 10PVDF nanofibers according to the collection period.

Table 4.

Pore size measurements of 16CA, 15CA, 14PVDF, 12PVDF and 10PVDF nanofiber mats according to the collection period.

Polymers and collection periods	First bubble point (FBP) µm	Mean flow pore size (MFP) µm	Smallest pore size (SPS) µm	Pressure (bar)	Wet curve (L/min)	Dry curve (L/min)
16CA-15 min	26.5	5.71	1.44	0.162	83.49	84.84
16CA-30 min	7.12	3.57	2.08	0.205	76.10	77.76
16CA-60 min	6.93	3.32	2.27	0.244	76.94	78.15
15CA-15 min	13.5	4.52	1.99	0.149	76.15	77.9
15CA-30 min	6.88	3.40	1.88	0.193	76.02	78.64
15CA-60 min	4.59	2.38	1.90	0.274	64.78	66.48
14PVDF-15 min	16.13	3.11	1.36	0.217	81.73	83.98
14PVDF-30 min	14.28	2.75	2.03	0.240	77.19	86.73
14PVDF-60 min	4.93	2.69	1.39	0.246	64.39	66.31
12PVDF-15 min	4.71	1.83	1.57	0.350	0.90	1.80
12PVDF-30 min	1.8	1.74	1.03	0.373	0.014	0.025
12PVDF-60 min	1.77	1.68	0.95	0.384	0.017	0.030
10PVDF-15 min	3.83	1.76	1.43	0.363	2.03	4.188
10PVDF-30 min	1.98	1.14	1.10	0.563	0.662	1.278
10PVDF-60 min	1.05	1.02	0.56	0.623	0.017	0.032

Table 5 provides the values of the filtration efficiencies (initial penetration, %) and resistance at initial penetration (differential pressure, mmH₂O) according to the polymer concentration and collection period. In surface filtration, the use of nanofibers and smaller pore sizes is aimed at capturing the particles at the surface of the medium. The surface pore openings of filter media could be precisely controlled in relation to the particulate size to be filtered. However, particles that are smaller than the pore openings of the nanofibers will penetrate into the thickness of the nanofiber mats. When the filter media is relatively thick compared to the surface filter media, the pores are variable in the flow path length and more than one mechanism of particle capture (inertial impact, interception and diffusion) plays a role in the arrestment of the particles [6]. Thick nanofibers act as a thin depth filter. However, when the particles are very small and their size is in the MPPS range, they do not have sufficient momentum for inertial effects and the particles are captured by diffusion and interception. Another reason that was explained by Grafe and Graham [63] and Chase [64] is that when the ratio of the gas mean free path to the radius of the fiber (K_n: Knudsen number) is larger than 0.1, slip flow will occur at the fiber surfaces, letting more air travel close to the fiber surfaces. This leads to more particles moving close to the fiber surface, increasing the probability of capture [6].

Table 5.

Filtration performance of blank polypropylene spunbond media and CA and PVDF nanofıbers at 85 L/min.

Polymers and collection periods	Resistance at initial penetration (mmH₂O) ± SD	Initial penetration (%) ± SD
Polypropylene spunbond blank media	0.57 ± 0.06	97.30 ± 0.60
16CA-15 min	11.51 ± 2.69	22.30 ± 7.22
16CA-30 min	22.01 ± 5.17	8.88 ± 5.00
16CA-60 min	28.48 ± 3.04	4.9 ± 1.65
15CA-15 min	16.39 ± 2.34	17.10 ± 1.97
15CA-30 min	30.42 ± 5.47	4.67 ± 0.49
15CA-60 min	59.15 ± 6.08	0.70 ± 0.14
14PVDF-15 min	21.48 ± 3.76	17.3 ± 0.28
14PVDF-30 min	35.29 ± 15.99	9.59 ± 4.54
14PVDF-60 min	60.43 ± 7.46	2.08 ± 0.58
12PVDF-15 min	23.28 ± 1.44	9.41 ± 2.25
12PVDF-30 min	44.30 ± 1.99	2.16 ± 0.85
12PVDF-60 min	79.57 ± 2.38	0.31 ± 0.14
10PVDF-15 min	29.44 ± 5.11	8.12 ± 0.86
10PVDF-30 min	75.23 ± 15.36	0.15 ± 0.18
10PVDF-60 min	114.95 ± 6.99	0.04 ± 0.03

Bold values met the requirement of at least 95% efficiency with maximum 35 mmH₂O of ΔP.

When the collection period was increased, the thickness of the mat increased and penetration decreased. Particles were captured by diffusion, interception and by slip-flow effect. However, it was also observed that an increase in the thickness of the nanofiber mats resulted in an undesired increase in the resistance. The initial penetration of 16CA nanofibers was 22.30, 8.88 and 4.9% for 15, 30 and 60 min, respectively. The NIOSH certification requirements of 95% efficiency and ≤35 mm water column height pressure (mmH₂O) were achieved with 16CA-60 min.

It is known that the filter efficiency is improved when the fiber diameter of the filter media decreases [65]. The efficiency curves have a “V” shape, and the deepest point of the “V” indicates MPPS. The MPPS decreases and the capture efficiency of the MPPS increases with decreasing fiber diameter [6,24,65,66]. Finer nanofibers generally enable greater filtration efficiency, particularly for the MPPS. When the polymer concentration of CA decreased to 15%, the mean diameter of CA nanofibers decreased from 319.02 nm to 264.02 nm. It was observed that the NIOSH requirements were achieved with a shorter collection period (30 min) due to the finer fibers. 15CA-30 min nanofiber mats showed a penetration of 4.67% with ΔP of 30.42 mmH₂O. In nanofiber production, the nanofiber thickness is controlled by the production period or speed, so that a smaller thickness implies higher production speeds. By using finer nanofibers, it is possible to use higher production speeds to achieve sufficient filtration efficiency.

In the second part of the study, three concentrations of PVDF were used to produce nanofibers. The mean diameter of the electrospun 14PVDF nanofibers was 236.50 nm, and these nanofibers were significantly thinner than both the 16CA and 15CA nanofibers. However, none of the 14PVDF nanofiber mats met the NIOSH requirements. While the 14PVDF-60 min nanofiber mat showed 2.08% penetration, as a result of long collection and finer pores of approximately 2.6 µm, ΔP of 60.43 mmH₂O was observed, which was far higher than the requirement. When the polymer concentration decreased to 12% and 10%, fiber diameters decreased to 177.55 and 142.59 nm, respectively. Penetration of 9.41% with ΔP of 23.28 mmH₂O for 12PVDF-15 min and penetration of 8.12% with ΔP of 29.44 mmH₂O for 10PVDF-15 min were achieved.

The resistance at initial penetration (differential pressure, ΔP, mmH₂O), initial penetration (%) and MFP (µm) of 16CA, 15CA, 14PVDF, 12PVDF and 10PVDF nanofibers along with their collection period are shown in Figure 6 to examine the correlation between the pore size, ΔP and penetration (%). It was observed that ΔP increased with decreasing pore size and that decreasing pore size significantly affected the penetration (%). When the collection period was increased, the penetration percentages decreased with the pore sizes, providing better filtration efficiencies.

Figure 6.

Resistance at initial penetration (mmH₂O), initial penetration (%) and mean flow pore size (MFP) µm of 16CA, 15CA, 14PVDF, 12PVDF and 10PVDF nanofibers together with their collection period.

Penetrations of the 12PVDF-30 min nanofibers met the requirement, but ΔPs were still higher than required. On the other hand, when the mean diameter was approximately 142.59 nm for the 10PVDF nanofibers, the fibers were thinner than the 12PVDF nanofibers. Higher filtration efficiencies were achieved for all collection periods, but due to the smaller pores that were caused by thinner fibers, it led to higher initial ΔP values. It was also observed from the filtration efficiencies of 10PVDF nanofibers that 0.013 mm was too thin (at 15 min) and 0.031 mm was too thick (at 30 min) for meeting the requirements. Thus, 10PVDF nanofibers were collected for 20 min. In this case, the initial penetration of 2.74% with ΔP of 42.44 mmH₂O was achieved (Table 6). However, ΔP of the mat was higher than the requirement. Thus, to simulate a depth filtration, two 10PVDF-15 min nanofiber mats were put face-to-face together and measured. Penetration of 1.85% and ΔP of 33.87 mmH₂O were achieved, finally meeting the requirement. First, 10PVDF-15 min captured most of the particles and also decreased the velocity of the smaller particles. Then, the second 10PVDF-15 min captured the rest of the particles resulting in smaller differential pressure.

Table 6.

Filtration performance of PVDF-20 min and double layer of 16CA-15 min+30 min, 15CA-15 min+30 min and 15CA-15 min+15 min, 10PVDF-15 min+15 min nanofıbers.

Polymers and collection periods	Resistance at initial penetration) ± SD (mmH₂O)	Initial penetration (%) ± SD
16CA-15 min+30 min double layer	32.34 ± 0.22	2.36 ± 0.09
15CA-15 min+30 min double layer	41.54 ± 1.42	1.36 ± 0.12
15CA-15 min+15 min double layer	36.13 ± 1.52	2.77 ± 0.73
10PVDF-20 min	42.44 ± 2.61	2.74 ± 2.17
10PVDF-15 min double layer	33.87 ± 1.36	1.85 ± 0.27

Bold values met the requirement of at least 95% efficiency with maximum 35 mmH₂O of ΔP.

For CA nanofibers, measurements of the face-to-face configuration were performed for 16CA-15 min+30 min, 15CA-15 min+30 min and 15CA-15 min+15 min in order to examine whether it was possible to achieve better results (Table 6). Penetration of 2.36% and ΔP of 32.34 mmH₂O were measured for 16CA-15 min+30 min, which are better than the results obtained for 16CA-60 min and 15CA-30 min. However, due to the smaller pore size and thicker and bulkier nanofiber mats, 15CA-15 min+30 min and 15CA-15 min+15 min showed ΔP values higher than 35 mmH₂O.

Although PVDF mats had thinner fibers, CA nanofiber mats were bulkier and were more efficient than PVDF nanofiber mats, showing not only that the fiber diameter has a significant effect but also that the thickness of the nanofiber mats has a significant effect on filtration performance. Smaller pore sizes of 12PVDF and 10 PVDF nanofibers compared to that of the CA nanofibers led to higher ΔP values. The results also showed that the weight of approximately 7.12 g/m² CA nanofiber mat with a mean diameter of 264 nm was sufficient to achieve a penetration of approximately 5% with resistance of approximately 30 mmH₂O at a flow rate of 85 L/min. For PVDF nanofibers, two face-to-face PVDF nanofiber mats with a weight of approximately 1.36 g/m² and a mean diameter of approximately 142 nm can be used to achieve the required penetration and ΔP values.

Conclusion

Most of the commercial filtering facepiece respirators generally use electrostatic filter media that may degrade with time or due to several different factors, whereas an unused nanofiber supported mask will not lose its efficiency with time or due to several different factors because of its mechanical filtration efficiency protection by the layers of the mask. In this study, CA and PVDF nanofiber layers were developed for N95 respirators for the first time. Mean fiber diameters of CA nanofibers were obtained in the range of 319.02–264.02 nm, and PVDF nanofibers were produced with diameters in the range of 236.50–142.50 nm. It was observed that PVDF nanofibers were statistically thinner than CA nanofibers. Polymer concentration had a significant effect on fiber diameter, the mean fiber diameter of 16CA nanofibers was significantly larger than that of the 15CA nanofibers, and the mean fiber diameters of 14PVDF, 12PVDF and 10PVDF were significantly different from each other.

CA and PVDF nanofibers were produced on spunbond polypropylene media. As expected, the thicknesses of the CA and PVDF nanofibers were significantly affected by the collection period and the thicknesses were in the order of 15 min < 30 min < 60 min. However, the polymer concentration did not significantly change the thickness values of the CA and PVDF nanofibers for each collection period. CA and PVDF nanofibers were produced with thicknesses in the range of 0.0191–0.056 mm and of 0.003–0.038 mm, respectively. Since pore size and pore size distribution are important parameters for determining the arrestment capability of a nanofiber layer, FBP, MFP and SPS were measured for CA and PVDF nanofibers. It was observed that 16CA-15 min with a nanofiber diameter of 319.02 nm had the highest FBP and MFP values of 26.5 and 5.71 µm, respectively, with a thickness of 0.019 mm. It was possible to decrease the MFP to 2.38 µm by decreasing the polymer concentration to 15% (15CA) and increasing the collection period to 60 min. According to liquid and air permeability results obtained from wet and dry curves of pore size measurements, the required pressure was increased to 0.274 bar for 66.48 L/min of air permeability for 15CA-60 min mats. On the other hand, due to smaller nanofiber diameters, the liquid and air permeability of 12PVDF and 10PVDF nanofiber mats showed a dramatic decrease. The mean nanofiber diameter of 10PVDF nanofibers was 142.59 nm, and at 30 and 60 min collection periods with mat thicknesses of 0.031 and 0.039 mm, respectively, it was possible to achieve FBP and MFP of approximately 1 µm. Even at a higher pressure such as 0.623 bar, 0.032 L/min of air permeability was observed for the dry curve of 10PVDF-60 min.

The filtration efficiencies (initial penetration, %) and resistance at initial penetration (differential pressure, ΔP, mmH₂O) of CA and PVDF nanofibers were compared for different polymer concentrations and collection periods. The initial penetration of 16CA nanofibers was 22.30, 8.88 and 4.9% for 15, 30 and 60 min, respectively. The NIOSH certification requirements of 95% efficiency and ≤35 mm water column height pressure (mmH₂O) were achieved only with 16CA-60 min. 15CA-30 min nanofiber mats showed a penetration of 4.67% with ΔP of 30.42 mmH₂O. Because of the finer fibers, the NIOSH requirements were achieved with a shorter collection period for 15CA-30 min nanofiber mats.

The mean diameter of 14PVDF nanofibers was 236.50 nm and was significantly smaller than the diameters of both 16CA and 15CA nanofibers. However, none of the 14PVDF nanofiber mats met the NIOSH requirements. Penetration of 9.41% with ΔP of 23.28 mmH₂O for 12PVDF-15 min and penetration of 8.12% with ΔP of 29.44 mmH₂O for 10PVDF-15 min were achieved with finer nanofibers, but these values were still unsatisfactory. Smaller pores resulted in lower penetration percentages with undesired higher resistances for 30 and 60 min collected 10PVDF and 12PVDF nanofibers. To simulate depth filtration and increase the filtration efficiency, two 10PVDF-15 min nanofiber mats were placed in a face-to-face configuration and measured. Penetration of 1.85% and ΔP of 33.87 mmH₂O were achieved. This approach was also attempted for CA nanofibers, but due to the smaller pore size and thicker and bulkier nanofiber mats, 15CA-15 min+30 min and 15CA-15 min+15 min showed ΔP values higher than 35 mmH₂O.

Although PVDF mats had thinner fibers compared to CA nanofibers, CA nanofiber mats were bulkier and were more efficient than PVDF nanofiber mats, showing not only that the fiber diameter has a significant effect but also that the thickness of the nanofiber mats has a significant effect on filtration performance.

This study demonstrates that the NIOSH requirements for the N95 particulate filtering half mask of at least 5% of penetration and ΔP of 35 mmH2O can be achieved with CA and PVDF nanofibers with 100% mechanical filtration. A single layer of 16CA-60 min or 15CA-30 min can meet the NIOSH requirements because of their bulky structure using not only surface filtration but also interception and diffusion effects. For PVDF, thinner nanofibers were produced compared to the CA nanofibers, but the N95 requirements could only be achieved with double-layered face-to-face 10PVDF-15 min nanofiber mats. For single layer, CA nanofibers showed better filtration efficiency without increasing the differential pressure at initial penetration.

Footnotes

Acknowledgement

The author would like to thank HIFYBER-Abalıoǧlu Teknoloji for the support given.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Cellulose acetate and polyvinylidene fluoride nanofiber mats for N95 respirators

Abstract

Keywords

Introduction

Experımental section

Materials

Electrospinning

Scanning electron microscope analysis

Measurement of nanofiber diameters, weight and thickness

Pore size and pore size distribution measurements

Evaluation of the filtration performance

Results and discussion

Conclusion

Footnotes

Acknowledgement

Declaration of conflicting interests

Funding

References