PVA-co-PE Nanofibrous Filter Media with Tailored Three-dimensional

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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PVA-co-PE Nanofibrous Filter Media with Tailored ThreeDimensional Structure for High Performance and Safe Aerosol Filtration via Suspension-Drying Procedure Zhibing Yi,† Pan Cheng,† Jiahui Chen,† Ke Liu,*,† Qiongzhen Liu,† Mufang Li,† Weibing Zhong,‡ Wenwen Wang,† Zhentan Lu,† and Dong Wang*,†,‡

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†

Hubei Key Laboratory of Advanced Textile Materials & Application, Hubei International Scientific and Technological Cooperation Base of Intelligent Textile Materials & Application, Wuhan Textile University, Wuhan 430200, China ‡ College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China S Supporting Information *

ABSTRACT: High efficient filtration of air pollutants exerts a great demand for nanofiber-based materials with superior structures. In this work, a mixed solution of tert-butyl alcohol (TBA) and water was employed to stably disperse PVA-co-PE nanofibers. Nanofibrous filter media based on PP nonwoven fabric substrate with various porosity were then prepared via three nanofiber-suspension drying techniques: spray-air-drying, spray-freeze-drying, and container loading-freeze-drying. The prepared nanofiber composite filter media with average nanofiber diameter about 155 nm present controllable threedimensional structure: nanofiber layer porosity increases from 76.6% to 97.3%, various pore sizes increase from 0.493 to 2.268 μm, as well as an increased nanofiber layer thickness from about 10 to 55.3 μm for filter media with 2.667 g/m2 nanofiber coverage density (NFCD). On the basis of the structure, filter media possess a best comprehensive filtration performance with the quality factor 1.110 mmH2O−1 (99.955% and 6.73 mmH2O) at NFCD = 2.677 g/m2 and a best efficient performance with efficiency 99.999% (0.645 mmH2O−1 and 17.86 mmH2O) at NFCD = 6.583 g/m2, respectively. The theoretical analysis shows the excellent properties are mainly derived from the stable threedimensional structure of nanofiber network which can provide superior torturous channels for capturing airborne nanoparticles and facilitating the penetration of air flow indicating a typical deep bed filtration. The result of electret treatment test shows that the filter medium exhibits remarkably stable filtration properties and is basically insusceptible to the electrostatic charges rather than the commercial PP nonwoven fabric substrate. Furthermore, the repetitive-use test showed that present nanofibrous composite filters possess higher dust holding capability (24.3 g/m2) and longer service life (8.5 h) than commercial filter with the similar initial filtration property. This implies the superiority of present nanofiber composite filter media in the application as a high stable, cost-effective, and safer air filter medium.

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INTRODUCTION With the accelerated urbanization and industrialization, air pollution has emerged and captured the attention of governments and individuals throughout the world.1,2 Particulate matter of ≤2.5 μm (PM2.5, U.S. Environmental Protection Agency) pollution, including absorbed hazardous matter rooted from the automobile exhaust fumes, factory waste gases, and agriculture dusts, has caused pernicious effects on citizen health, climate, and ecosystems.3,4 Nowadays, the filtration based on porous fibrous material has been widely acknowledged as the commonly used removal technique of contaminates in air pollution control due to their physical and sized-based separation mechanism without the degradation of air transmission capability.5−7 Nonwoven fabric is one of the most frequently used traditional filter material including melt-blown fibers, spun-bonded fibers, and glass fibers with micrometer© XXXX American Chemical Society

sized diameter and large size pore, which has many drawbacks involving low service life derived from depth loading feature, low filtration efficiency combined with the incapability of capturing the ultrafine particles, low quality factor, and large packing density.8,9 Electret treatment by charging technologies is an effective and common approach to improve the filtration performance of the traditional filter via the physicochemical interaction between harmful particles and polarized fibers.10 However, this type of filter with enhanced filtration efficiency presents two crucial deficiencies: the easy-degraded electret effect and massive deposition particles inside the filter, which Received: Revised: Accepted: Published: A

June June June June

5, 2018 23, 2018 25, 2018 25, 2018 DOI: 10.1021/acs.iecr.8b02523 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research make the filter more bulky, unstable in filter efficiency and hazardous from charge dissipation.11 As a result, some new worldwide or continent-wide standards (such as EN779-2012, ASHRAE 52.2, and ISO16890) have been issued with significant consideration of the electret eliminating procedure. Recently, profiting from the smaller diameter, nanofibrous filter media has gained growing attention in handling actual environmental issues12,13 with the advantages of high specific surface area and admirable interlocking tortuous pore channels,14 whose “slip effect” results in remarkable improvement of the filtration efficiency.15 Except for the high filtration efficiency, nanofibrous filter also overcomes the bottleneck of traditional fabric filters on the dust jams from depth filtering, limited regeneration, and recycling ability.9 At present, approaches containing template synthesis, self-assembly, meltblown, solution blowing process, electrospinning, and phase separation have been applied to prepare nanofibers16 with small diameter to dozens of nanometers. These methods complement each other well and push the nanofibrous filter technology forward to meet the cost-effective air purification. However, the conflict between the low pressure drop and high efficiency remains a great challenge for the filtration with low consumption.17 To solve this problem, diverse structure design methods have been explored to optimize the structure for sieving the particles and transferring the air flow immediately. Surface modification is the first effective method to improve the interaction between airborne particles and nanofibers through hybrid nanofiber with functional nanoparticles such as TiO2 and SiO2,10,18 which is responsible to the rough fiber surface, and utilization of hydrophobic or oleophobic polymers19 resulting in a low surface energy. They further lead to the better removal efficacy compared to their counterparts but still face the challenge of the high air resistance and safety hazards from nanoparticles.6 Additionally, stacking nanofibers with multiscale diameters to various structures have been widely employed. The structures were designed mainly for two filtration patterns: surface filtration pattern advocates thinner nanofiber coverage density and smallest pore size, which can be achieved by nanofiber nets smaller than 50 nm.20,21 Another one is the deep bed filtration pattern which is based on the three-dimensional structure with proper pore size and tortuous pore channel.16,22 They can availably strike a balance between high efficiency and permeability. Arranging filters with multilayers is another useful method. The sandwich structure23,24 and multilayered membrane18,25 are most commonly adopted. They can afford the filter media pressure drop reduction as well as further efficiency enhancement in virtue of the pores with gradient size and intra-/ interlayer void.23 Generally, appropriate proportion of mesoand macropores in nanofibrous filter is the critical factor and presents a potential to offer both superior particle capture efficiency of mesopores and high air permeability of macropores.26 Hence, it is imperative to seek alternative approach for designing the superior three-dimensional structure of nanofibrous filter with consideration of the above techniques. Nanofibrous aerogels offer open, tortuous pores with high porosity (80−95%) and large surface area (300−1000 m2/g), which present a strong potential to efficiently remove airborne nanoparticles in 10−300 nm.26 To date, despite strong potential, air filtration using nanofibrous aerogels has been studied rarely. A hybrid monolithic aerogel of syndiotactic polystyrene (sPS) and polyvinylidene fluoride (PVDF) was prepared by

thermoreversible gelation method, and the open pore structure with 97% porosity created inside generates an efficiency of ≥99.999% and permeability of ∼10−10 m2.27 An oxidized cellulose nanofibril (TOCN) aerogel was fabricated by freezedrying the suspension on the support of glass fibrous HEPAgrade base filter, which helps to improve the efficiency significantly to 99.999% but hardly changes pressure drop.28 Except for the previous method, air-drying can also be employed to adjust the structure of nanoporous networks of nanofibers.29 Our group has explored the melt-phase separation method to prepare thermoplastic polymeric nanofibers and various nanofiber suspensions for the formation of membrane filter with tailored microstructure, which have been applied in the field of air and water filtration.10,17 In this work, PVA-co-PE nanofiber was fabricated and suspended in aqueous solution. The suspension was sprayed on the PP melt-blown fabric nonwoven or directly covered on the support in a container. Nanofibrous membranes with diverse three-dimensional structures were obtained by air-drying or freeze-drying techniques. The airborne particle filtration performance was investigated and the relation between the membrane structure and the filtration properties was also analyzed. Finally, the filtration efficiency of 99.999% to 0.3 μm nanoparticle with only 17 mmH2O pressure drop and 24.3 g/m2 dust holding capacity was achieved implying present membrane is a potential candidate in application of costeffective air filtration.

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EXPERIMENTAL SECTION Materials. Poly(vinyl alcohol-co-ethylene) (PVA-co-PE, 44% ethylene) was obtained from Aldrich Chemica Ltd. Cellulose acetate butyrate (CAB, butyryl, 38%, viscosity, 20 Pa·s) was purchased from Eastman Chemical Co., Ltd. Acetone and tertbutyl alcohol (TBA) were obtained from Sinopharm Group Chemical Reagent Co., Ltd. The melt-blown polypropylene nonwoven fabric substrate support (about 160 μm, 20 g/m2) was provided by Handan Hengyong Protective Cleaning Supplies Co., Ltd. Distilled water was prepared in laboratory with resistivity of 18.2 MΩ·cm after a series of the filtration systems, which was obtained from Wright Rider Pure Water Equipment Technology (Beijing) Co., Ltd. Preparation of Nanofibrous Suspensions. The preparation of PVA-co-PE nanofibers (NFs) has been reported in our previous studies.30 PVA-co-PE and CAB pellets (w/w = 20/80) were fed into a Leistritz corotating twin-screw extruder (model MIC 18/GL 30D, Nuremberg, Germany) at a feed rate of 12 g min−1. Twin screw extruder temperature parameter ranged from 170 to 225 °C. The blends were extruded with a draw ratio (the area of the cross section of the die to that of the extrudates) of 25 and cooled using water. PVA-co-PE NFs were prepared by Soxhlet extraction of CAB with the continuous flow acetone for 36 h at 70 °C. After being dried, the NFs (Figure S1a) were dispersed in a mixed solution of TBA and distilled water (weight ratio of 1:1) (Figure S1b) by a high speed shear mixer to form a suspension with 0.9 wt % NFs (Figure S1c), at room temperature, which is still stable after being kept overnight (Figure S1d). Preparation of Nanofibrous Composite Filters. Airdried nanofibrous composite filter media (ANCF) with nanofibers coverage density from 0 g/m2 to 3.522 g/m2 were obtained by PVA-co-PE spraying nanofiber suspensions onto the surface of 15 cm × 18 cm PP nonwoven fabric substrate with a high pressure air flow device followed by being dried. Another same wet membrane after suspension spraying was also further B

DOI: 10.1021/acs.iecr.8b02523 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Scheme 1. Preparation Process of PVA-co-PE Nanofibrous Air Filter

Figure 1. SEM image of (a) PP nonwoven and (b) cross section of PP nonwoven, (c) optical photo of C-FNCF, (d) ACNF and (e) for enlarged one, (f) cross section of ACNF, (g) C-FNCF and (h) enlarged one, (i) cross section of C-FNCF and (j) S-FNCF, NFCD = 2.667 g/m2.

as that of S-FNCF and finally to obtain a freeze-dried nanofibrous composite filter media via confining suspensions (labeled as C-FNCF) as shown in Scheme 1. Characterization. The nanofibrous filters were characterized by using a field emission scanning electron microscope (SEM) (JEOL, JSM-IT300A) with an acceleration of 20 kV after being coated with Au−Pt of 5 min. The fiber diameter

treated and underwent a freeze-drying process at −30 °C for 12 h under a pressure of 9 Pa and finally formed a freeze-dried nanofibrous composite filter (labeled as S-FNCF). Third, nanofiber suspension was slowly poured into a disc-shaped container with the PP nonwoven fabric substrate with diameter 15 cm on the bottom. Then the container with uniformly distributed suspension was freeze-dried using the same method C


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Industrial & Engineering Chemistry Research

Figure 2. Pore size distribution and contact angle of (a) PP nonwoven fabric, (b)ANCF, (c) S-FNCF, and (d) C-FNCF with NFCD = 2.667 g/m2.

is beneficial for the formation of small pores (about 0.493 μm) compared to PP nonwoven substrate (around 16.95 μm) as shown in Figure 2a and Figure 2b. Figure 1f shows a pronounced fluctuations of nanofibers layer in thickness from about 1 to 5 μm (Figure S3d). Actually, besides the layer structure, the nanofibers have also attached to many microfibers located not only on the surface but also in the interior of nonwoven fabric, which further generates many random distributed nanofiber networks nearly perpendicular to the filter surface (Figure S3a− c). Different from the air-drying method, frozen-drying offers an excellent approach for nanofiber layers to duplicate the spatial structure of the nanofibers in the stable suspension. This inspires us to tailor the three-demissional structure of present nanofiber composite filters through controlling of the structure of nanofiber suspension. As a result, the as-prepared C-FNCF shows large micrometer pores and a 3D network structure as indicated in Figure S1g and Figure S1h, in which the nanofiber randomly stacked upon on the PP nonwoven fabric and interconnected with each other with the pore size of 2.268 μm larger than that of ACNF. Due to the solvent evaporation in the process of spraying, the nanofibers in S-FNCF were frozenframed in smaller spaces of suspension coated on the surface of substrate compared with that of C-FNCF, which leads to a thinner nanofiber layer (about 35 μm for S-FNCF and 55 μm for C-FNCF), a smaller porosity (about 93% for S-FNCF and 95% for C-FNCF), and a smaller pore size (1.068 μm in Figure 2c) though they have the same nanofiber diameter and NFCD (OR nanofiber content) (summarized in Tables 1 and 2). Some nanofibers can be observed implanting into the PP nonwoven fabric and further enhance the network structure (Figure S4). Additionally, water contact angle in Figure 2 shows that present nanofiber based composite filters show better hydrophilicity than PP nonwoven due to the hydroxyl group of PVA-co-PE as presented in Figure 3. The water contact angle difference of the three nanofibrous filters is probably derived from the pore structure distinction of the filters.

distribution was measured by image-Proplus analyzer (Adobe Photoshop CS6). The chemical structures of filters were characterized by FTIR-ATR (Tensor 27, Bruker). The surface potential of the filter was measured by using an electrostatic voltmeter equipped with noncontacting electrostatic probe (ME279, Monroe Inc., USA) at 20 different positions on the sample.31 The filtration performance of nanofibrous filter media was analyzed by an automatic aerosol particle filter test (LZC-H, Suzhou Huada Instrument Equipment Co., Ltd.) (Figure S2) with NaCl aerosol particle of around 0.3 μm. All filtration tests were conducted at peripheral temperature of 25 °C and humidity of 50% with NaCl concentration of 20 g/L. For an efficient comparative analysis, pristine PP nonwoven fabrics before and after static eliminating handling (immersed in TBA for about 30 min) were also tested. Complete dust holding tests were performed through evaluation of the weight change of filters after filtering NaCl aerosol particles (NaCl concentration: 30 g/L) at the terminative condition: pressure drop of 51.02 mmH2O.21 The recycled filtration test was further conducted by mechanical shaking and air blowing the filters to a constant weight. In this course, the filtration efficiency and pressure drop were also monitored.

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RESULTS AND DISCUSSION Morphology and Structure of the Filter Media. Figure 1 shows the morphology of substrate and as-prepared nanofiber composite air filter media with fiber coverage density (NFCD) of 2.667 g/m2. The PP nonwoven fabric substrate is a typical nonwoven matrix of porous structure with the fiber diameter ranged in 1−7 μm (Figure 1a) indicating a common feature of low efficient filter with smooth microfibers. After nanofiber suspensions-coating and air-drying, the nanofibers were uniformly stacked upon the surface of nonwoven fabric as shown in Figure 1d−f. PVA-co-PE nanofibers intertwined tightly with each other and formed a net-like structure with the diameter range from 100 to 300 nm. The reduced fiber diameter D


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E

99.999 17.86

97.3 ± 0.15 92.3 ± 0.52

6.583/22.0 218.8 ± 12.6/179.4 ± 33.7

2.667/22.0 56.2 ± 4.1/179.4 ± 33.7 2.268 95.8 ± 0.31 88.2 ± 1.5 5.797 99.955 6.73 a

NFCD: 2.667 g/m2. bNFCD: 6.583 g/m2. cEstimated by the method in ESI.

2.667/22.0 37.4 ± 3.9/179.4 ± 33.7 1.068 93.7 ± 0.72 86.4 ± 2.1 2.778 99.563 14.08 0/22.0 0/179.4 ± 33.7 16.95 84.7 ± 2.9 84.7 ± 2.9 2.275 16.881 1.53

where η and ΔP are the filtration efficiency and pressure drop across the filter media, respectively. Figure 4b and Figure 4c show the higher filtration efficiency of S-FNCF and C-FNCF with diverse NFCDs compared to that of PP nonwoven fabric and ACNF. When NFCD is no smaller than about 2.6 g/m2, the frozen-dried nanofiber composite filter medium presents a filtration efficiency close to 100%. The higher filtration efficiency can be attributed to the 3D network structure with larger porosity, larger pore size, and corresponding larger layer thickness discussed previously, which result in the higher specific surface area of nanofiber layer (Table 2). This further extends the time spent on passing through the nanofibrous composite filter media as well as enhances the probability of interaction between airborne particle and nanofibers.2,33 On the basis of this, C-FNCF exhibits better capture ability than SFNCF except for the case of NFCD < 1 g/m2. Furthermore, the higher porosity larger than 90% (Table 2) is also beneficial for air flow to pass through the filter media such as S-FNCFs and C-FNCFs as shown in Figure 4b and Figure 4c.

2.667/22.0 7.9 ± 1.3/179.4 ± 33.7 0.493 69.7 ± 4.8 84.1 ± 2.8 1.106 84.723 45.92

C-FNCFa

(1)

basic density (g/m2), nanofiber layer/substrate L (μm), nanofiber layer/substrate average pore size (APS) (μm) ε of nanofiber layer (%)c ε of whole filter(%)c specific surface area (SSA) (m2/g) η (%) ΔP (mmH2O)

ln(1 − η) ΔP

Table 2. Structure Parameters and Filtration Properties of Filter Media in This Work

QF =

S-FNCFa

Filtration Performance Evaluation of Filter Media. The filtration performance of ACNF with various basis density under the velocity of 32 L/min was presented in Figure 4a. ACNFs with NFCD of 0 g/m2, 0.663 g/m2, 1.166 g/m2, 2.667 g/m2, and 3.522 g/m2 present filtration efficiency remarkably increased from 16.881% to 97.475%, while the pressure drop sharply raises up from 1.53 mmH2O−1 to 102.04 mmH2O−1, respectively. It is indicative of the direct dependence of filtration property on nanofiber coverage density. In contrast, PP nonwoven fabric treated by TBA possesses a 16.881% capture capability versus 0.3 μm NaCl aerosol particles due to the larger pores size as shown in Figure 4. The filtration efficiency of 16.881% is mainly because the deposition of nanoparticles smaller than 300 nm obeys Brownian movement mechanism even for the microfibers.5 The filtration efficiency of air filter media considerably promoted by nanofibers coating can be attributed to the smaller pore size (0.439 μm), which can intercept much more particles on the surface of fibers. However, it reduces the channel area for air penetrating and further increases the pressure drop sharply. Herein, the ACNF filter media obtained a weak QF value of 0.06 mmH2O−1 (NFCD = 2.667 g/m2), as shown in Figure 4d, where the quality factor (QF) value (eq 1) is the common estimation parameter for the comprehensive filtration performance of filter media, and the higher the QF is, the higher the cost-effective filter medium is.32

ANCFa

filtration efficiency porosity density of fiber in filter gas mean free path aerosol particle diameter filter thickness nanofiber diameter total single fiber deposition efficiency single fiber deposition efficiency for Brownian diffusion single fiber deposition efficiency for direct interception Kuwabara number Knudsen number

PP

physical implication

η ε ρ0 λg dp L df E Ed Ei Ku Knf

property

symbol

C-FNCFb

Table 1. Parameters Mentioned in Equations 2 and 3


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surface of ANCF (Figure 5c and Figure 5d). Much more particles smaller than pore size also can be observed attached on the surface of nanofibers than substrate control because of the enhanced direct interception by nanofiber net and higher probability of diffusion deposition on the surface of nanofibers. In contrast, FNCF exhibited the filtration characteristic of depth loading. The three-dimensional nanofiber network provides numbers of longer tortuous channel where particles would spend more time to penetrate, and it further increases the interaction chance between particles and fibers in filter media.34,35 As indicated in Figure 5e and Figure 5f, NaCl aersol particles were trapped by nanofibers not only on the surface but also in the interior of filter media (indicated by green arrows). Additionally, the appearance of particle clusters (blue arrows) on the present nanofiber composite filter media rather than on PP substrate is probably derived from the predominant effect of van der Waals force between nanoparticles and nanosized fibers.36 The EDX and FTIR spectra also indicated a mass of captured nanoparticles and no chemical reaction in the filtration process as shown in Figures S5 and S6. Furthermore, after the filtration test, the particles with the size larger (red arrows) and smaller (orange arrows) than 300 nm both can be sieved by the fibers especially nanofibers as shown in Figure 5b,d,f, indicating a wide diameter range of particles that nanofibers can capture. Performance Stability Evaluation of Filter Media. Despite the prominent role of electret effect on the filtration efficiency, its sensitivity to the environment makes electret more futile in improving the filtration performance. Considering the electret feature of PP nonwoven substrate, a rigorous test was employed to evaluate the effect of electret on the filtration properties and further estimate the stability of filters. In present work, we detected the surface potential of various filter media in the following cases: (1) original as-prepared state, (2) electret elimination by TBA (only for PP substrate), (3) electret treatment by corona treatment, and (4) electret elimination in high humidity atmosphere (see Figure 6 and Figure S7) in sequence. After electret treatment, there is distinct change in the morphology and porosity neither for filters in this work nor PP substrate as shown in Figure S8 and Table S1. However, the filtration properties exhibit different behavior for these filter media. The surface potential of filter media was enhanced from 5 times to about 10 times the original ones. Then after the

Figure 3. FTIR spectra of PP nonwoven fabric, ANCF, S-FNCF, and CFNCF with NFCD = 2.667 g/m2.

We can see that the filter media have much smaller pressure drop than ANCF, especially for C-FNCF; when NFCD is about 2.667 g/m2, the pressure drop is just about 8.7 mmH2O, only 14.7% of that for ANCF and 47.7% of that for S-FNCF. This can be assigned to the larger distance among nanofibers which generates a huge number of interconnected tortuous channels with larger channel width across the filter media facilitating air flow penetration. Fortunately, this structure is also significantly important for not weakening but improving the filtration efficiency to about 99.95% (NFCD = 2.667 g/m2).10 These finally result in a much higher QF of C-FNCF than the other two types of filter media (maximum 10.5 mmH2O−1 for C-FNCF = 2.667 g/m2) implying the validity of the design of nanofiber layer via controlling suspension-drying procedure. The detailed property−structure relationship analysis of present nanofibrous filter media will be performed in the mechanism section. SEM images were also offered to characterize the interaction between NaCl aerosol particles and the filter media at the air flow of 32 L/min. In Figure 5a, a small number of NaCl aerosol particles are captured on the surface of PP nonwoven. Interestingly, these particles were adsorbed by PP fibers rather than relying on the intercept of pore size mainly due to the Brownian diffusion deposition mechanism.5,34 However, the nanofibers layer coating on the PP nonwoven fabric support produced a nanoporous net resulting in a surface loading. Herein, a mass of NaCl aersol particles were intercepted in the

Figure 4. Filtration efficiency, pressure drop of (a) ACNF, (b) S-FNCF, (c) C-FNCF with various NFCD and the corresponding quality factor in (d), (e), and (f), respectively, 0.3 μm NaCl aerosol, 32 L/min. F


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Figure 5. SEM images of (a) the air filter media after filtration (32 L/min, 3 min) and (b) PP nonwoven fabric, (c, d) ACNF, (e, f) C-FNCF with NFCD = 2.667 g/m2.

S-FNCF present extreme stability on the filtration performance with efficiency kept near 99% under various electrostatic/ humidity environment highlighting the dominant three-dimensional structure of nanofibers on the enhancement of interception ability of filter. Additionally, the pressure drop is reasonably independent of the electret as indicated in blue lines in Figure 6b,d,f,h, where the pressure drop is almost constant for each filter. It should be noted that the diverse treatments in this work are significantly more severe than any situation present under natural environmental conditions. Therefore, present filter media with stable properties can be applied as a potential filter in various fields. In another words, present nanofibrous filter can be well-designed to avoid the adverse electrostatic effect on the filter with excellent comprehensive filtration properties and become safer for the terminal application such as respirator mask. Mechanism of Filtration Based on the Three-Dimensional Structure of Nanofibers. For aerosol particles filtration, it has been reported that the most penetrating particle size (MPPS) is in the range 100−500 nm, which is dominated by direct interception and Brownian diffusion/electrostatic mech-

following electret elimination process, the electrostatic filter media were restored again to the one with low surface potential (smaller than ±10 V) indicating the validity of the electret incorporation and electret eliminating process in present work. The surface potential (about ±90 V) of PP nonwoven fabric substrate after electret treatment about 2 times higher than that (about ±40 V) of other three nanofibrous filter media is probably due to the hydrophilic nature of PVA-co-PE. The corresponding filtration efficiency and pressure drop have been investigated for each filter treated by previous diverse process (red line in Figure 6). PP nonwoven fabric presents a large fluctuation in filtration efficiency between 20% and 84% as the surface potential varies, suggesting the dependence of filtration efficiency on the electrostatic interaction for the PP nonwoven fabric. In contrast, ANCF with 1.166 g/m2 NFCD shows more stable filtration property with the efficiency changing between 50% and 65%, which is much larger than that of PP nonwoven fabric after electrostatic elimination treatment. This implies that the main effect factor on the filtration efficiency of ANCF is the nanofiber layer structure rather than the electrostatic adsorption. Likewise, C-FNCF and G


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Figure 6. Surface potential of (a) ANCF, (c) S-FNCF, (e) C-FNCF, and (f) PP nonwoven fabric. Filtration efficiency and pressure drop of (b) ANCF, (d) S-FNCF, (g) C-FNCF, and (h) PP nonwoven fabric obtained with different treatment process, NFCD = 1.166 g/m2.

anism. The filter with ultrafine fibers play a key role in promoting the filtration efficiency for the MPP with the pressure drop rising moderately.37 To explain the filtration property differences of present nanofibrous composite filter media with diverse structure, some simple estimates using the commonly known equations formulated in the classical theory of depth filtration were performed as follows:38,39 ij −4Em yz zz = 1 − expjij−D E zyz η = 1 − expjjjj zz z j π d εσρ ε{ k k f 0{

ΔP =

64μU (1 − ε)(1 + 56(1 − ε)3 ) = K (1 − ε) df 2

(1 + 56(1 − ε)3 )

E = 1 − (1 − Ed)(1 − Ei)

(3) (4)

The parameters mentioned are listed in Table 1. Fortunately, for given nanofibers, porosity ε is the only variant, which makes the analysis of eqs 2 and 3 more simple. Moreover, it is easy to know that with ε increasing, η increases (see detailed analysis in

(2) H


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Scheme 2. Schematic of Nanofiber Composite Filter Media with Diverse Scaffold Structure for Particulate Contaminate Interception

Table 3. Physical and Filtration Properties of Filter Media in This Work and Other Reference property

this work

this work

average fiber diameter (nm) thickness (μm) (substrate/nanofiber layer) basis weight (g/m2) (substrate/nanofiber layer) particle/size (nm) air flow (L/min) filtration efficiency (%) pressure drop (mmH2O) quality factor (mmH2O−1)

155 ∼151.5/∼55.3 22/2.677 NaCl/300 32 99.955 6.94 1.110

155 ∼151.5/∼261 22/6.583 NaCl/300 32 99.999 17.86 0.645

ESI) and ΔP decreases. Here, the filtration efficiency contributed by electrostatic mechanism is ignored due to the experimental results shown in Figure 6. The structure parameters of as-prepared nanofiber based filter media were estimated as comparably indicated in Table 2. On the basis of previous discussion, porosity is regarded as the key factors influencing the filtration behavior of nanofiber based filter media. For NFCD = 2.667 g/m2, the porosity of nanofiber layer is about 76.6%, 93.4%, and 95.8% for ANCF, S-FNCF, and C-FNCF, respectively, which causes a similar trend of ε of the whole filter. Due to the same PP nonwoven substrate for each filter media, the porosity of the whole filter media will be not considered in the discussion. The lower porosity of ANCF compared with that of substrate is mainly assigned to the high proportion of two-dimensional nanofiber net in ANCF (Figure S3). Besides, the less-than and greater-than signs are derived from the gradient distribution of nanofibers in ANCF (Figure S3c) and the transition region between nanofiber layer and PP substrate for S-FNCF and C-FNCF (Figure S4b), respectively. As a result, considering the parameter values in Table 2, the above equations can be visualized to plausible curves compared to that of experimental data as shown in Scheme 2. It should be

PS/PVDF26 ∼25 3500 NaCl/300 95 99.999 13.05 0.882

wood pulp27 ∼100 66/0.031 DOP/250 32 99.999 35.5 0.324

PEO24

PA-66/BaCl2/PP19

∼208 166/4.34 29.08/0.7 NaCl/300 32 92.256 16.56 0.154

∼193.25 ∼100/3.96 NaCl/300 32 96.5 9.5 0.352

indicated that the evaluated data exclude the contribution of nonwoven substrate, which has a filtration efficiency of about 17% and a pressure drop of about 1.5 mmH2O. From Scheme 2, we can see that thicker filter results in a longer channel for particle penetrating and more chance for nanofibers to capture particles via Brownian diffusion (corresponding to Ed) and direct interception (corresponding to Ei) mechanism, which finally causes a higher η. In this case, the larger size channel facilitates the air flow penetrating and the filter media possess lower pressure drop (ΔP). Although for thinner filter, much more particles can be sieved by nanofibers net, but the air flow channel is very limited which certainly will increase the pressure drop. Therefore, the three-dimensional nanofiber network with high porosity is superior for the capture of aerosol nanoparticles and air permeability simultaneously. More importantly, the suspension-drying method in the present study is a potential for the mass production of nanofiber based filter with enhanced comprehensive filtration property. In previous reports, the filtration performance of some other compound filter materials has been investigated as listed in Table 3. Compared with these materials, C-FNCF with NFCD = I


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Figure 7. (a) Filtration efficiency, (b) pressure drop, (c) quality factor, (d) dust holding capacity, (e) dust holding capacity per filter volume, and (f) dust holding capacity per filter weight for C-FNCF(24.667 g/m2) and commercial filter (88 g/m2) during a dust holding cycle test. A, B, and C indicate initial state, terminative state, and dust cleaning state.

2.667 g/m2 presents the best comprehensive filtration property with the QF = 1.110 mmH2O−1. Even for NFCD = 6.583 g/m2, the filtration efficiency of 99.999% and 17.86 mmH2O are still quite high values and little lower than that of PS/PVDF filter.27 This is mainly because of its smaller fiber diameter compared with that in the present work, indicating the necessity to control fiber morphology to improve the filtration performance. In addition, as an aerogel based filter, the present C-FNCFs are thinner, more light, and more convenient to be used than the PS/PVDF filter. Filter media with thin nanofiber film (smaller than 10 μm) like wood pulp,28 PEO,25 and PP/PA620 have QF smaller than 0.4 mmH2O−1 indicative of the greater contradiction between the filtration efficiency and pressure drop than present filter media. The main reason is probably that nanofibers film intercepting most of the airborne particles cannot provide enough pore channels for the air flow, which is absolutely different from present filter media with high porosity and plenty of long tortuous channels. This implies the significance of threedimensional structure of nanofiber networks in the contribution to the filtration property of filter material. Dust-Holding Capability and Reusability. The dust holding capacity and the reusability of present filters have been also investigated because they are important for the real

application in industry.21,40,41 Here, the service life of C-FNCF with NFCD = 2.667 g/m2 in the present work and the commercial filter (PP melt-blown nonwoven fabric, 88 g/m2, 717.6 μm thick, supplied by Handan Hengyong Protective Cleaning Supplies Co., Ltd.) was evaluated at the designed pressure drop states including initial state, terminative state (500 Pa),21 and dust cleaning state in the procedure of filtrating NaCl aerosol nanoparticles. The selected commercial filter presents similar initial filtration efficiency and pressure drop but smaller quality factor compared to C-FNCF as shown in Figure 7a−c. It was found that the time for C-FNCF to reach the terminative state is 8.5 h longer than that of commercial filter 5.5 h. Moreover, the dust holding capacity of C-FNCF in the first test round is about 24.3 g/m2 higher than 14.2 g/m2 for commercial filter (Figure 7d) indicating the superior dust-holding capability of C-FNCF. Besides, although possessing the thinner and lighter feature (Table 2), nanosized PVA-co-PE fibers in C-FNCF provided much more sites and smaller pores for capturing NaCl aerosol particles than that of commercial filters as indicated in Figure S9, which further results in the higher dust holding capacity of present filter. During this process, C-FNCF exhibited more sharply increased filtration efficiency and pressure drop than that of commercial filter. But after the dust-cleaning treatment, C-FNCF showed slight lower filtration efficiency and J


Article

Industrial & Engineering Chemistry Research 17.6 Pa lower than the pressure drop of commercial filter. In addition, the above results can also be indirectly revealed by the optical images of the surface morphology of the filter at terminative state and dust-cleaning states of both filters, as demonstrated in Figure S10. After four filtration tests, the dust-holding capacity of CFNCF and commercial filter decreased to about 4.76 and 1.32 g/ m2, respectively. In these cyclic tests, C-FNCF remained as having higher dust-holding capability than the commercial one. It should also be noted that the basic density (24.667 g/m2) and thickness (235.6 μm) of C-FNCF are both smaller than that of commercial filter (88.0 g/m2 and 717.6 μm) as shown in Figure 7d−f. Thus, considering these two factors, C-FCNF exhibits distinctly superior dust holding capacity compared to the commercial filter, implying the higher service performance of present filter in the real application of air purification.

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AUTHOR INFORMATION

Corresponding Authors

*K.L.: e-mail, [email protected]. *D.W.: e-mail, [email protected]. ORCID

Ke Liu: 0000-0002-5090-8186 Dong Wang: 0000-0002-8139-8502 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank “Wuhan Engineering Technology Research Center for Advanced Fibers” providing partial support for materials processing. The authors are thankful for the financial support of National Key Research and Development Program of China (Grants 2016YFC0206101 and 2016YFC0400504), Nature Science Foundation of Hubei Province (Grant 2016CFA076), National Nature Science Foundation of China (Grants 51401149 and 51503160), National Science-Technology Support Program of China (Grant 2015BAE01B01), Science and Technology Innovation Major Projects of Hubei Province (Grant 2016AAA019).

CONCLUSION In summary, three nanofiber-suspension drying techniques (spray-air-drying, spray-freeze-drying, and container loadingfreeze-drying) have been employed to prepare PVA-co-PE nanofibrous filter media on PP nonwoven fabric with various porosity. On the basis of the procedures, the nanofiber layer porosity increases from 76.6% to 97.3%, and pore size varies from 0.493 to 2.268 μm as well as an increased nanofiber layer thickness from about 10 to 55.3 μm for filter media with 2.667 g/ m2 NFCD. In virtue of the structure, present filter media possess excellent comprehensive filtration performance: the quality factor is 1.110 mmH2O−1 (99.955% and 6.73 mmH2O) at NFCD = 2.677 g/m2 and 0.645 mmH2O−1 (99.999% and 17.86 mmH2O) at NFCD = 6.583 g/m2, respectively, which is relatively higher than most of the reference values (Table 3). The results can be attributed to the stable three-dimensional structure of nanofiber network, which provides plenty of torturous channels for capturing airborne nanoparticles, facilitating the penetration of air flow. Additionally, after electret treatment and electret elimination process, the filter media exhibit remarkably stable filtration properties and are basically insusceptible to the electrostatic charges, to which the commercial PP nonwoven fabric substrate is highly sensitive. Additionally, present nanofibrous composite filters possess higher dust holding capability (24.3 g/m2) and longer service life (8.5 h) than commercial filter with the similar initial filtration property. This indicates a superiority of present nanofiber composite filter media in the application as a high cost-effective and safer air filter media with high performance stability.

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image of filter media after electret treatment, Figures S9 and S10 showing the SEM images and photographs of the air filter media under continuous aerosol loading test (PDF)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b02523. Evaluation method of porosity of filter media, electret treatment method, theoretical analysis filtration performance of filter media contributed by nanofibrous layer, Figure S1 showing photographs of PVA-co-PE nanofibers and their suspensions, Figure S2 showing experimental setup for filtration performance evaluation, Figures S3 and S4 showing SEM image of PVA-co-PE nanofiber distribution in ANCF and C-FNCF, Figures S5 and S6 showing EDAX and FTIR spectra of filter media after filtration test, Figures S7 and S8 showing photograph of the electrostatic equipment, treatment process, and SEM K


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PVA-co-PE Nanofibrous Filter Media with Tailored Three-dimensional

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