Multi-Scale Microstructure Investigation for a PM2.5 Air-Filter Efficiency Study of Non-Woven Polypropylene

Article Multi-Scale Microstructure Investigation for a PM2.5 Air-Filter Efficiency Study of Non-Woven Polypropylene

Tu-Ngoc Lam 1,2, Chen-Hsien Wu 3, Sheng-Hsiu Huang 4, Wen-Ching Ko 1,3, Yu-Lih Huang 1, Chia-Yin Ma 1, Chun-Chieh Wang 5,* and E-Wen Huang 1,* 1 Department of Materials Science and Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 30010, Taiwan 2 Department of Physics, College of Education, Can Tho University, Can Tho City 900100, Vietnam 3 Institute of Applied Mechanics, National Taiwan University, 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan 4 Institute of Occupational Medicine and Industrial Hygiene, National Taiwan University, No. 17, Xu-Zhou Rd., Taipei 10055, Taiwan 5 National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan * Correspondence: [email protected] (C.-C.W.); [email protected] (E.W.H.)



Received: 29 August 2019; Accepted: 24 October 2019; Published: 28 October 2019


Abstract: A N95 face-piece respirator and a 3M air filter composed of non-woven polypropylene filter material were investigated for their multi-scale microstructure and resulting filtration performance. Filtration mechanisms of each system are found and quantified. Both media showed a gradually decrease of the most penetrating particle size with respect to an increase in face velocity or surface charge density. Increasing the face velocity and porosity dramatically degraded the collection efficiency in the 3M filter rather than in the N95 system. We exploited three-dimensional X-ray tomography to characterize the morphological and geometrical properties of the fiber arrangement and deposition of aerosol on the fiber surface. Tuning the most predominant material parameters to achieve a precedence in lower pressure drop or higher collection efficiency in a specifically captured particle size range is of great requisite to a peculiar application of the filter media.

Keywords: fibrous filter; electret filter; aerosol; porosity; X-ray tomography

1. Introduction Modern technology has improved the quality of life in many aspects, however, the other side of such a rapid development of industry is severe air pollution, causing serious human health hazards due to the deep penetration of particular matter with fine particle sizes below 2.5 µm (PM2.5) into human lungs [1–3]. Air filtration is one of the most urgent and prioritized tasks for the protection of the global environment and human health by removing aerosol particles through air filter media. Air filter media is commonly made from fibrous filters comprising randomly arranged fibers whose distances are larger than the sizes of aerosol particles [4]. Fibrous filters basically capture aerosol particles via four physical mechanisms, including gravity settling, inertia impact, interception, and Brownian diffusion in which each of them plays a crucial role for a particular size range of aerosol particles [4,5]. The particle size at the lowest filtration efficiency, named as the most penetrating particle size (MPPS), typically around 0.3 µm or smaller, is used to determine the dominant capture mechanism of the air filter [3,4,6]. An ideal filter media is expected to perform both high filtration efficiency and low pressure drop that is hard to acquire from traditional fibrous filters. To satisfy high collection efficiency of fibrous filters for further applications, electret filters have been widely applied in the manufacture of commercial air filter media. Electret filters are dielectric materials-based fibrous filters that possess quasi-permanent

Quantum Beam Sci. 2019, 3, 20; doi:10.3390/qubs3040020 www.mdpi.com/journal/qubs Quantum Beam Sci. 2019, 3, 20 2 of 10 static electric charge on the fiber surfaces [7] in which the electrostatic force dramatically enhances filtration efficiency and is responsible for the main capture of fine aerosol particles, resulting in a significant shift towards a smaller particle size of MPPS [8–11]. One of the most serious challenges of general air filter media is a tradeoff between collection efficiency and pressure drop because the fibers collect aerosol particles and affect the drag force on the gas stream simultaneously [4]. Those two important factors of the electret filters involve a variety of parameters such as the microstructural characteristics of fibrous material (fiber diameter, thickness, porosity, fiber arrangement), surface charge density, and environmental conditions (face velocity, aerosol particle diameter, temperature, humidity). In spite of using the same fibrous material, its own microstructure is possibly tuned to optimize either high collection efficiency or low pressure drop for a specific application of the air filter media that can be engineered through the fabrication process. There are two major application types of the air filter media, wearable respirators and air filter equipment (vacuum cleaner bags, indoor air cleaning, indoor air purifiers), possessing different priority criteria of fibrous properties. To unravel the behavior of structural properties to collection efficiencies and to acquire comparable results from theoretical predictions and experimental data, considerable effort has been devoted to the development of filtration modeling using three-dimensional (3D) filter structures obtained by different approaches as more and more microstructural properties of filter media have been taken into account [12–15]. Moreover, the filtration performance is strongly influenced by the porosity of the fibrous material but no reliable methods in determining porosity is well known. In this study, we have attempted to specify the porosity through different kinds of methods including theoretical analysis, gravimetric measurement, GrabCut analysis from scanning electron microscope (SEM) images, and 3D tomography analysis from transmission X-ray microscopy (TXM). The 3D TXM tomography was especially utilized to provide useful morphological and geometrical properties of solid or hollow fibers, fiber arrangement, fiber entanglement, and fiber-particle contact for two majorly different non-woven polypropylene (PP)-based PM2.5 air filter media, namely the N95 respirator and the 3M filter used in air purifiers. Furthermore, the filtration performances of these two media were experimentally investigated and compared with the theoretical results in order to elucidate the major characterization parameters of microstructural fibers in governing the dominant filtration mechanism of N95 respirators and 3M filters in the presence and absence of electrostatic attraction. Understanding the impact of fiber characterization on filtration performance plays a vital role in the design of efficient fibrous filters for distinct application purposes.

2. Materials and Methods

2.1. Specimen Preparation Two distinct commercial air filter media, the N95 face-piece respirator 8210 Plus® and the 3M filter 9809-R® used in air purifiers were chosen in this work because they are widely used and have a great market share. Both electret filter media are composed of melt-blown non-woven PP material. The N95 respirator, a disposable dust mask, is worn to protect people from hazardous substances. It consists of three layers of which the middle one acts as a filter layer, the outer and inner are support and comfort layers, respectively. The 3M air filter has only one filter layer. The N95 mask should not be worn for more than 8 h while the 3M filter can be used up to 2000 h. The different use times of the N95 mask and the 3M filter will be extensively clarified and associated with their collection efficiency and pressure drop.

2.2. Filtration Test System The two filters were tested as origin specimens. Moreover, we modified these two systems by dipping them in isopropanol (IPA) for 5 min and then drying them naturally to remove the surface charge density [5,8,16] for studying the impact of electrostatic charges on collection efficiency. All the filtration tests were performed at ambient temperate and humidity. Sodium chloride (NaCl) Quantum Beam Sci. 2019, 3, 20 3 of 10 monodisperse aerosols in the particle size range of 10 to 750 nm were used as the emission source of particles. Experimental filtration performances were compared with the filtration efficiencies of single fiber theory to clarify the main capture mechanism of the two filter media.

2.3. Grabcut Analysis SEM images were processed by the GrabCut image segmentation algorithm [17] to analyze the surface topography of the fibers. The image was pre-processed by the GrabCut method; firstly to define the fiber boundary and the energy function, and then to employ the algorithm for separating the image into foreground and background segmentation; thus, the surface morphology of the initially processed surface was obtained. Based on the calculation and porosity analysis method of Martin III et al. [18], the concept of representative elemental area (REA) was utilized to calculate the convergence value of the porosity by calculating a plurality of pore distribution. The value of average porosity is defined within the maximum of porosity curve region and within 5% of the minimum region. ± 2.4. 3D Tomography Transmission X-ray microscopy is a non-destructive image microscopy technique which allows 3D tomography analysis of fiber reconstruction in fibrous materials. TXM tomography was carried out at beamline (BL) 01B1, National Synchrotron Radiation Research Center (NSRRC) in Taiwan with the wavelength of 1.54 Å (8 keV) and spatial resolution of 60 nm. The protocol is archived [19,20]. TXM can construct 3D geometric configuration of materials by capturing a series of 2D X-ray radiographies from different viewing angles. After the auto-alignment processing using the Faproma image registration algorithm [21,22] and filtered back-projection (FBP) reconstruction algorithm, the radiographies can be reconstructed to a 3D tomography. Amira software is then applied to classify, segment the size, shape, or other measurable parameters in the post-processing analysis of the 3D reconstructed tomography, and to filter them into different categories for extracting the structure of interest. The 3D tomography of fiber reconstruction provides useful microstructure characterization not only on the geometrical arrangement of fiber–fiber, fiber–particle but also on the porosity distribution of the two filter media. More detail information of TXM can be found elsewhere [23].

3. Results

3.1. Filter Morphology Characterization The surface morphologies of the N95 mask and 3M filter before capturing NaCl particles were obtained by SEM images at the same magnification. Fibers with different sizes were randomly arranged, shown in Figure1a,b. In Figure1c,d, the capture of di fferent sizes of NaCl aerosols on the fiber surfaces was clearly observed. However, in order to clarify a particular distribution of fibers and NaCl particles, we employed ImageJ software to perform subsequent image processing and analysis from the SEM images. Figure1e,f display the fiber distribution in which the average fiber diameters in the N95 and 3M filter media were 2.9 and 24.6 µm, respectively. The average fiber diameter of the 3M filter was eight times bigger and the fiber diameter distribution in the 3M filter was more homogeneous than that in the N95 mask. Our SEM results were found to agree well with the fiber diameter distribution provided in the Patent [24–26]. As we can observe in Figure1g,h, NaCl aerosols with smaller sizes were preferably collected by the N95 mask rather than by the 3M filter, in which the highest proportion was detected at the favorably captured NaCl particle sizes of 460 and 590 nm in the N95 mask and the 3M filter, respectively. In addition, the thicknesses of the N95 and 3M filters were measured by immersing them in liquid nitrogen following the direct freeze fracture pretreatment and taken by cross-section SEM, found to be 1.53 and 1.23 mm, respectively. Quantum Beam Sci. 2019, 3, x FOR PEER REVIEW 4 of 11 Quantum Beam Sci. 2019, 3, 20 4 of 10 Quantum Beam Sci. 2019, 3, x FOR PEER REVIEW 4 of 11

Figure 1. SEM images of (a) N95 mask and (b) 3M filter before capturing NaCl; those of (c) N95 mask Figure 1. SEM images of (a) N95 mask and (b) 3M filter before capturing NaCl; those of (c) N95 mask andFigure (d) 3M 1. SEM filter images after capturing of (a) N95 NaCl mask aerosols. and (b) 3MFiber filter distribution before capturing in (e) N95 NaCl; mask those and of ( f(c) )3M N95 filter. mask and (d) 3M filter after capturing NaCl aerosols. Fiber distribution in (e) N95 mask and (f) 3M filter. Theand distribution (d) 3M filter of afterNaCl capturing aerosols in NaCl (g) N95 aerosols. mask Fiberand ( hdistribution) 3M filter. in (e) N95 mask and (f) 3M filter. The distribution of NaCl aerosols in (g) N95 mask and (h) 3M filter. The distribution of NaCl aerosols in (g) N95 mask and (h) 3M filter. 3.2.3.2. Filter Filter Performance Performance 3.2. Filter Performance TheThe various various PP PP material-based material-based applications applications depend depend on on filter filter performance. performance. Figure Figure 2a2a depicts depicts the the collectioncollectionThe efficiencyvarious efficiency PP of ofmaterial-based the the N95 N95 mask mask andapplications and the the 3M 3M filter filterdepend as as a afunctionon function filter performance.of of face face velocity. velocity. Figure All All the 2a the collectiondepicts collection the efficienciescollectionefficiencies efficiencyshowed showed the theof typicalthe typical N95 curve mask curve shape and shape the in in 3Mwh which ichfilter the the as minimum a minimum function removal of removal face velocity. efficiency efficiency All is is theobtained obtained collection at at MPPS.efficienciesMPPS. The The collection collectionshowed efficiencythe effi typicalciency of curve of the the N95 N95shape mask mask in waswh wasich around around the minimum 97%, 97%, which which removal revealed revealed efficiency a more a more pronouncedis pronouncedobtained at efficiencyMPPS.efficiency The compared compared collection toto efficiency 45%45% by by the ofthe 3Mthe 3M filter.N95 filter. mask Furthermore, Furtwas hermore,around the 97%, collection the which collection effi revealedciencies efficiencies a gradually more pronouncedgradually increased increasedwithefficiency decreasing with compared decreasing face velocityto 45% face for byvelocity both the the3M for N95 filter. both and theFurt 3M N95hermore, filters, and which 3Mthe filters, arecollection similar which toefficiencies theare resultssimilar gradually obtained to the resultsincreasedin previous obtained with studies in decreasing previous [3,27,28 studies].face The velocity filtration [3,27,28]. for e ffiThe bothciencies filtration the atN95 MPPS efficiencies and in 3M the filters,at N95 MPPS mask which in werethe are N95 97.52% similar mask (53 wereto nm), the 97.52%results96.37% (53 obtained (53 nm), nm), 96.37% in and previous 93.81 (53 nm), % studies (43 and nm) 93.81 [3,27,28]. and % those (43 The nm) in filtration the and 3M thosefilter efficiencies in werethe 3M 49.89% at filter MPPS (346were in nm),the 49.89% N95 41% mask(346 (300 nm), were nm), 41%97.52%and (300 35.93% (53nm), nm), (202 and 96.37% nm)35.93% at (53 the (202 nm), face nm) velocityand at the93.81 face of % 5, (43velocity 10, nm) and andof 15 5, cm those 10,/s, and respectively. in the15 cm/s, 3M filter respectively. The were particle 49.89% The sizes (346particle around nm), sizes41%50 and around (300 300 nm), nm50 and were 35.93%300 the nm most (202 were penetrating nm) the at most the facepenetratin for thevelocity filtrationg for of 5,the process10, filtration and in15 thecm/s, process N95 respectively. maskin the and N95 theThe mask 3M particle and filter, thesizesrespectively. 3M around filter, respectively. An50 and increase 300 nm ofAn facewere increase velocity the most of resultsface penetratin velocity in an obviousg results for the in trend filtration an obvious of MPPS process trend shifting in ofthe towardsMPPS N95 mask shifting a lower and towardsvalue,the 3M in afilter, lower agreement respectively. value, with in agreement previous An increase reports with of previous face [29]. velocity reports results [29]. in an obvious trend of MPPS shifting towards a lower value, in agreement with previous reports [29].

Figure 2. (a) Collection efficiency of the N95 and 3M filters at different face velocities. (b) Collection Figure 2. (a) Collection efficiency of the N95 and 3M filters at different face velocities. (b) Collection efficiency versus pressure drop of the N95 and 3M filters as a function of face velocity. (c) Collection efficiencyFigure 2. versus (a) Collection pressure efficiency drop of the of theN95 N95 and and 3M 3Mfilters filters as a at function different of faceface velocities.velocity. ( c()b Collection) Collection efficiency of the N95 and 3M filters before and after treatment with IPA. efficiencyefficiency of versus the N95 pressure and 3M drop filters of beforethe N95 and and after 3M treatment filters as awith function IPA. of face velocity. (c) Collection efficiencyThe effect of of the face N95 velocity and 3M filters on the before pressure and after drop treatment is described with IPA. in Figure 2b. Compared with the The effect of face velocity on the pressure drop is described in Figure 2b. Compared with the variation of collection efficiency, the change of pressure drop indicates an opposite tendency in which variationThe of effect collection of face efficiency, velocity theon changethe pressure of pres dropsure isdrop described indicates in an Figure opposite 2b. Comparedtendency in with which the the pressure drop decreased linearly with decreasing face velocity for both the N95 and 3M filters. thevariation pressure of dropcollection decreased efficiency, linearly the changewith decreasing of pressure face drop velocity indicates for anboth opposite the N95 tendency and 3M in filters. which Decreasing the face velocity had a more significantly positive impact on the collection efficiency for Decreasingthe pressure the drop face decreasedvelocity had linearly a more with significantly decreasing positive face velocity impact for on boththe collection the N95 andefficiency 3M filters. for the 3M filter while on the pressure drop for the N95 mask. It implies that the collection efficiency theDecreasing 3M filter whilethe face on velocity the pressure had a drop more for significantly the N95 mask. positive It implies impact that on the collection efficiency isfor is the priority factor in the N95 mask while the pressure drop is the preferable element in the 3M thethe priority 3M filter factor while in onthe the N95 pressure mask while drop the for pressure the N95 dropmask. is It the implies preferable that theelement collection in the efficiency 3M filter. is the priority factor in the N95 mask while the pressure drop is the preferable element in the 3M filter.

Quantum Beam Sci. 2019, 3, 20 5 of 10

filter. This result allows a definitely appropriate explanation for the use times of the two filter media. The low face velocity has a positive effect on both collection efficiency and pressure drop and it is one of the tunable parameters in enhancing filter performance due to the shorter interaction time between particles and fibers [5,30]. However, low face velocity is not feasible for practical applications of the filter media as a face-piece respiratory protector. The optimal face velocity of 10 cm/s is commonly used for the filtration performance test to attain a good balance between collection efficiency and pressure drop and it was also chosen for further filtration tests, as follows. Based on the measured MPPS, the dominant capture mechanisms for the N95 mask were predicted to be diffusional deposition and electrostatic attraction while the major ones for the 3M filter were forecasted to be diffusion, interception, and electrostatic attraction. To further investigate the role of electrostatic attraction in filter performance, the two media treated with IPA (discharged specimens) were compared with the original ones. In Figure2c, the two originally charged filters performed at a much higher collection efficiency, with a shift of MPPS towards lower values, as compared with the discharged filters, which agrees with previous works [8,9]. The filtration efficiencies of the N95 and 3M charged filters were two and twelve times higher than those of the N95 and 3M discharged ones, respectively. The absence of electrostatic force leads to the shift of MPPS from 200 to 53 nm and from 400 to 300 nm in the N95 and 3M filters, respectively. The results demonstrate that the electrostatic filters are of great important in capturing fine aerosol particles with extremely high filtration efficiency.

3.3. Filtration Mechanism The four main mechanisms of aerosol filtration were determined based on theoretical single fiber efficiency and the theoretical analyses were compared with the experimental results to examine the dominant filtration mechanism. The microscopic property of single fiber efficiency EΣ is the sum of four individual collection efficiencies, including interception ER [31], impaction EI [31], diffusion ED [31], and electrostatic EE [32], shown as

EΣ = ER + EI + ED + EE (1)

The macroscopic property of collection efficiency [33] is defined as ! 4αE t E = 1 exp − Σ (2) − πd f in which α is the solidity or packing density of the filter, α = 1 porosity, t is the filter thickness, and d − f is the fiber diameter. Figure3 makes a comparison between the theoretical analysis and experimental observation of filtration efficiencies for the N95 and 3M filters. The theoretical single fiber efficiencies were plotted for the N95 and 3M owning fiber diameters of 2.88 and 24.61 µm, solidities of 0.083 and 0.037 (derived from the gravimetric measurement shown in the next section), respectively. The face velocity was 10 cm/s. Among individual mechanisms, the diffusion deposition is more predominant in capturing smaller particle sizes below 100 nm while the electrostatic attraction discloses a key role in capturing a wider range of aerosol sizes with extremely high collection efficiency. The theoretically total efficiency with the presence of electrostatic (Ew E-theo) exhibits a striking enhancement of filtration efficiency in the size distribution of fine particles compared to that without the presence of electrostatic (Ew/o E-theo), which entirely agrees with the experimentally total efficiency with and without the presence of electrostatic (Ew E-exp and Ew/o E-exp, respectively). Similarly measured and predicted collection efficiency curves were achieved in the N95 mask rather than in the 3M filter, which is explained in the following part. Quantum Beam Sci. 2019, 3, 20 6 of 10 Quantum Beam Sci. 2019, 3, x FOR PEER REVIEW 6 of 11

Figure 3. Interception, impaction, diffusion, electrostatic, theoretical single fiber efficiencies, Figure 3. Interception, impaction, diffusion, electrostatic, theoretical single fiber efficiencies, and and experimentalexperimental collection collection e ffiefficienciesciencies with with and and with withoutout the presence the presence of electrostatic of electrostaticfor the N95 (a) and for the N95 (a) and 3M3M (b) ( filters.b) filters.

3.4. Dependence3.4. Dependence of Filter Performanceof Filter Performance on Porosity on Porosity The collectionThe ecollectionfficiency efficiency is greatly is influencedgreatly influenced by electrostatic by electrostatic force force while while both both collection collection e fficiency efficiency and pressure drop are strongly governed by the porosity of the fibrous filters. We herein and pressureexplored drop arethe stronglydependence governed of collection by theefficiency porosity and ofpressure the fibrous drop on filters. porosity We based herein on exploredthe the dependencetheoretical of collection analysis e ffiwhichciency derives and from pressure the experi dropmentally on porosity measured basedaverage on fiber the diameter theoretical at the analysis which derivesface fromvelocity the of experimentally10 cm/s. Since the fiber measured sizes of the average N95 and fiber 3M filters diameter were atlarger the than face 1 velocityμm, we can of 10 cm/s. Since the fiberuse the sizes equation of the below N95 to and calculate 3M filters the pressure were drop larger ∆𝒑 than [33]. 1 µm, we can use the equation below to p η𝑡𝑈𝑓𝜶 calculate the pressure drop ∆ [33]. ∆𝒑 = (3) 𝑑 ηtU0 f (α) ∆p = (3) where ƞ is the air viscosity, Uo is the face velocity, f(αd)2 is the theoretical fiber bulk density, and 𝑓𝛼 = 64𝛼.156𝛼 for 0.006 < α < 0.3 (or 70% < porosityf < 99.4%). Figure 4 describes the pressure drop and collection efficiency variation as a function of porosity where η is the air viscosity, Uo is the face velocity, f (α) is the theoretical fiber bulk density, and f (α) =  which was assumed in the range of 70% to 99.4%. Both the pressure drop and collection efficiency 1.5 + 3 64α 1 56decreasedα for with 0.006 increasing< α < 0.3 porosity. (or 70% However,< porosity the pressure< 99.4%). drop of the N95 mask suffers a much Figuremore4 describes pronounced the decline pressure than dropthe 3M and filter collection while the collection e fficiency efficiency variation of the 3M as afilter function endures of a porosity which wasmore assumed serious in reduction the range than ofthe 70%N95 mask. to 99.4%. It can be Both suggested the pressurethat an optimal drop porosity and collection of 95% in the e fficiency decreased withN95 mask increasing and that porosity. of 70% in the However, 3M filter theenable pressure a fairly high drop filtration of the N95efficiency mask with su aff relativelyers a much more low pressure drop. This optimized porosity of the N95 mask approximates to the porosity used in pronouncedthe decline theoretical than single the fiber 3M filterefficiency, while which the collectionwas supposed effi tociency have caused of the a 3M small filter discrepancy endures a more serious reductionbetween thanthe measured the N95 and mask. predicted It can collection be suggested efficiency that curves an in optimal Figure 3a. porosity On the contrary, of 95% ina the N95 mask and thatconsiderable of 70% disparity in the 3Mbetween filter the enable optimal aporosity fairly highand the filtration porosity used effi ciencyin the theoretical with a relativelysingle low pressure drop.fiber efficiency This optimized of the 3M porosity filter wasof attributed the N95 to maska large approximates difference between to thethe measured porosity and used in the predicted collection efficiency curves in Figure 3b. theoretical single fiber efficiency, which was supposed to have caused a small discrepancy between the measured and predicted collection efficiency curves in Figure3a. On the contrary, a considerable disparity between the optimal porosity and the porosity used in the theoretical single fiber efficiency of the 3M filter was attributed to a large difference between the measured and predicted collection Quantum Beam Sci. 2019, 3, x FOR PEER REVIEW 7 of 11 efficiency curves in Figure3b.

Figure 4. TheoreticalFigure 4. Theoretical analysis analysis of collection of collection effi efficiencyciency and and pressure pressure drop drop of the of N95 the and N95 3M andfilters 3M as a filters as a function offunction filter porosity. of filter porosity.

4. Discussion Since porosity of the fibrous filter is one of the most crucial parameters strongly affecting both collection efficiency and pressure drop, we experimentally specify the porosity based on GrabCut analysis from SEM images and 3D tomography analysis from TXM in comparison with reference values obtained from theoretical analysis and gravimetric measurement. Based on Equation (3), we calculated the porosity from the experimentally measured pressure drop at the face velocity of 10 cm/s. The porosity values of the N95 and 3M filters were 98.1% and 93.7%, respectively. Another method to identify porosity is the gravimetric measurement in which porosity is defined as

𝜌 Porosity =1− (4) 𝜌

where 𝜌 is the density of the filter and 𝜌 is the density of PP material. The porosity values of the N95 and 3M filters based on the gravimetric measurement were determined to be 91.7% and 96.3%, respectively. The porosity can also be obtained based on imaging processing and analysis. We employed the GrabCut analysis from SEM images in which the porosity values of the N95 and 3M filters were found to be 78% and 96%, respectively, shown in Figure 5.

Figure 5. The original SEM image, GrabCut processed image, and REA result of the N95 mask in (a), (b), and (c); those of the 3M filter in (d), (e), and (f), respectively.

Quantum Beam Sci. 2019, 3, x FOR PEER REVIEW 7 of 11

Figure 4. Theoretical analysis of collection efficiency and pressure drop of the N95 and 3M filters as a Quantumfunction Beam Sci.of filter2019, porosity.3, 20 7 of 10

4. Discussion 4. Discussion Since porosity of the fibrous filter is one of the most crucial parameters strongly affecting both Since porosity of the fibrous filter is one of the most crucial parameters strongly affecting both collection efficiency and pressure drop, we experimentally specify the porosity based on GrabCut collection e ciency and pressure drop, we experimentally specify the porosity based on GrabCut analysis fromffi SEM images and 3D tomography analysis from TXM in comparison with reference valuesanalysis obtained from SEM from images theoretical and 3Danalysis tomography and grav analysisimetric measurement. from TXM in comparisonBased on Equation with reference (3), we calculatedvalues obtained the porosity from theoreticalfrom the experimentally analysis and gravimetricmeasured pressure measurement. drop at Basedthe face on velocity Equation of (3),10 cm/s.we calculated The porosity the porosityvalues of from the N95 the and experimentally 3M filters were measured 98.1% and pressure 93.7%, drop respectively. at the face velocity of 10 cmAnother/s. The porositymethod valuesto identify of the porosity N95 and is 3M the filters gravimetric were 98.1% measurement and 93.7%, respectively.in which porosity is definedAnother as method to identify porosity is the gravimetric measurement in which porosity is defined as

ρ𝜌filter PorositPorosityy =1−= 1 (4)(4) − ρ𝜌PP where 𝜌 is the density of the filter and 𝜌 is the density of PP material. The porosity values of where ρfilteris the density of the filter and ρPP is the density of PP material. The porosity values of the theN95 N95 and and 3M filters3M filters based based on the on gravimetric the gravimetric measurement measurement were we determinedre determined to be 91.7%to be 91.7% and 96.3%, and 96.3%,respectively. respectively. TheThe porosity porosity can can also also be be obtained obtained based based on on imag imaginging processing processing and and analysis. analysis. We We employed employed the the GrabCutGrabCut analysis analysis from from SEM SEM images images in in which which the the poro porositysity values values of of the the N95 N95 and and 3M 3M filters filters were were found found toto be be 78% 78% and and 96%, 96%, respectively, respectively, shown in Figure5 5..

FigureFigure 5. 5. TheThe original original SEM SEM image, image, GrabCut GrabCut processed processed im image,age, and and REA REA result result of of the the N95 N95 mask mask in in (a (a),), ((bb),), and and ( (cc);); those those of of the the 3M 3M filter filter in in ( (d),), ( (e),), and and ( (ff),), respectively. respectively.

In addition, we attempted to determine the filter porosity which was calculated by the volume of the selected fiber over the total volume of the 3D TXM images reconstructed using Amira software analysis, seen in Figure6a,c. Fiber volume Porosity = 1 (5) − Total volume The 3D tomography analysis-based porosity values of the N95 and 3M filters were averaged from three different measured areas of 15 x 15 µm, found to be 83.5% and 88.7%, respectively. The porosity values obtained by theoretical analysis and gravimetric measurement are indeed in the same order and exhibit negligible discrepancies. The porosity obtained by GrabCut analysis from SEM images discloses the largest error value, which is attributed to the observation of only surface morphology of the samples. Taking the internal microstructure of filters into account, the porosity values obtained by 3D TXM are closer to those calculated by theoretical analysis and gravimetric method. Such a small difference is ascribed to a limited number of mapping areas. It can be suggested that the more expanded the area mapped by 3D TXM tomography is, the more accurate the porosity value is. Quantum Beam Sci. 2019, 3, 20 8 of 10

Therefore, the 3D tomography analysis from TXM may be considered as a promisingly alternative approach to determine the porosity of fibrous filters. In addition, although much attention has been given to theoretical filtration models of aligned arrangements of fiber networks, further analyses of complicated fibrous networks such as fiber–fiber contact interactions, fiber–particle direct attachments, fiber orientation, fiber morphology, fiber diameter, diameter distribution, and porosity should be fully taken into consideration and much more effort is needed to completely elucidate the significant role of fibrous networks in governing the dominant capture mechanism of the filters. Such intricate fibrous networks may be gained through the reconstruction of 3D TXM images which clearly enables a visualization of geometrical arrangement of fiber entanglement positions at different viewing angles.Quantum Furthermore,Beam Sci. 2019, 3, thex FOR reconstruction PEER REVIEW images along the z-axis allows the identification of structural 8 of 11 morphology properties in which N95 and 3M fibrous filters were characterized to be solid fibers due In addition, we attempted to determine the filter porosity which was calculated by the volume to an unobservable inner boundary, as shown in Figure6b,d, respectively. We conducted 3D TXM of the selected fiber over the total volume of the 3D TXM images reconstructed using Amira software tomography of the N95 and 3M filters after capturing NaCl; however, the direct attachments between analysis, seen in Figure 6a,c. fibers and particles cannot be clearly distinguished with the naked eye, as expected. Because there Fiber volume is not much difference in the absorptionPorosit coey =1−fficients between the PP material and NaCl, the image(5) contrast is not good enough for further image processing.Total volume Further efforts to attain a better contrast betweenThe fiber3D tomography materials and analysis-based aerosols are necessaryporosity values since theof the directly N95 and adhesive 3M filters contact were behaviors averaged of aerosolsfrom three deposited different on measured the fibersurfaces areas of during 15 x 15 the μm, filtration found processto be 83.5% are expected and 88.7%, to berespectively. obtained by The 3D tomography,3D TXM tomography playing aof critical the 3M role and in N95 determining filters can the be predominantseen in the supplementary capture mechanism material. of the filters.

Figure 6. 3D TXMTXM reconstructedreconstructed image image and and the the slice slice plane plane along alongz-direction z-direction of theof the N95 N95 mask mask in (a in) and (a) (andb); those(b); those of the of 3M the filter 3M filter in (c )in and (c) (andd), respectively.(d), respectively. 5. Conclusions The porosity values obtained by theoretical analysis and gravimetric measurement are indeed in theThe same fiber order diameter and exhibit and porosity negligible strongly discrepancies. affect filtration The performance.porosity obtained Despite by beingGrabCut made analysis of the samefrom fiberSEM material,images discloses the N95 mask the larg revealsest error a higher value, collection which eisffi ciencyattributed while to thethe 3M observation air filter exhibits of only a lowersurface pressure morphology drop. Toof satisfythe samples. the vital Taking demand the on intern the specifical microstructure aspects of fibrous of filters filtration, into account, the priority the ofporosity air filter values media obtained is given towardsby 3D TXM higher are collection closer to effi thoseciency calculated or lower pressure by theoretical drop. For analysis the design and ofgravimetric the filter to method. effectively Such capture a small the difference ultrafine is particles, ascribed smaller to a limited fiber diameter,number of stronger mapping surface areas. charge, It can andbe suggested optimal porosity that the shouldmore expanded be involved. the area Our mappe findingsd by demonstrate 3D TXM tomography the promising is, the capabilities more accurate of 3D TXMthe porosity tomography value in is. determining Therefore, thethe porosity 3D tomogr of fibrousaphy filteranalysis as well from as TXM in proposing may be practical considered filtration as a modelspromisingly of real alternative geometrical approach arrangements to determine of fiber–fiber the po androsity fiber–particle of fibrous forfilters. further In addition, simulation although studies. Themuch simulated attention results has been of the given deposition to theoretical of NaCl filtration aerosols onmodels the real of geometryaligned arrangements of fibrous filters of fiber will benetworks, persuasive further in providing analyses of great complicated insight into fibrous fundamental networks breakthroughsuch as fiber–fiber and primarycontact interactions, parameters governingfiber–particle the filtrationdirect attachments, mechanism. fiber orientation, fiber morphology, fiber diameter, diameter distribution, and porosity should be fully taken into consideration and much more effort is needed to completely elucidate the significant role of fibrous networks in governing the dominant capture mechanism of the filters. Such intricate fibrous networks may be gained through the reconstruction of 3D TXM images which clearly enables a visualization of geometrical arrangement of fiber entanglement positions at different viewing angles. Furthermore, the reconstruction images along the z-axis allows the identification of structural morphology properties in which N95 and 3M fibrous filters were characterized to be solid fibers due to an unobservable inner boundary, as shown in Figure 6b and d, respectively. We conducted 3D TXM tomography of the N95 and 3M filters after capturing NaCl, as shown in the supplementary material; however, the direct attachments between fibers and particles cannot be clearly distinguished with the naked eye, as expected. Because there is not much difference in the absorption coefficients between the PP material and NaCl, the image contrast is not good enough for further image processing. Further efforts to attain a better contrast

Quantum Beam Sci. 2019, 3, 20 9 of 10

Author Contributions: Conceptualization, E.W.H.; software, C.-H.W.; formal analysis, T.-N.L., C.-Y.M. and Y-L.H.; investigation, C.-Y.M.; resources, S.-H.H., W.-C.K. and C.-C.W.; writing—original draft preparation, T.-N.L.; writing—review and editing, C.-C.W. and E.W.H.; supervision, E.W.H.; project administration, E.W.H. Funding: The authors are grateful to the support of the Ministry of Science and Technology (MOST) Programs MOST-108-3017-F-009-003, 107-2218-E-007-012, 107-3017-F-009-002, 107-3017-F-007-003, 107-3017-F-009-002, 107-3011-F-002-002, 104-2628-E-009-003-MY3, 106-2811-E-009-032, 106-2218-E-007-019, 106-2514-S-007-004, 106-3011-F-002-002, and 108-2221-E-009-131-MY4. This work was financially supported by the “Center for the Semiconductor Technology Research” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. Also supported in part by the MOST, Taiwan, under Grant MOST-108-3017-F-009-003. EWH and TNL are supported by MOST 107-2628-E-009-001-MY3. Acknowledgments: We especially thank Yu-Hsiang Hsu of the National Taiwan University (NTU) for sample preparations, Chih-Chieh Chen of the NTU for filtration test, and Yao-Hsien Liu of the National Chiao Tung University for simulation. Our sincere thanks go to Yen-Fang Song, who is the BL01B1 team of the National Synchrotron Radiation Research Center (NSRRC) Experimental Facility Division. We greatly appreciate the beam time and help from the NSRRC. Conflicts of Interest: The authors declare no conflict of interest.

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