Polyamide-6 Air Filter

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Highly Integrated Polysulfone/polyacrylonitrile/polyamide-6 Air Filter for Multi-level Physical Sieving Airborne Particles Shichao Zhang, Ning Tang, Leitao Cao, Xia Yin, Jianyong Yu, and Bin Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10094 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 5, 2016

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ACS Applied Materials & Interfaces

Highly Integrated Polysulfone/polyacrylonitrile/polyamide-6 Air Filter for Multi-level Physical Sieving Airborne Particles Shichao Zhang,†,§ Ning Tang,‡,§ Leitao Cao,‡,§ Xia Yin,‡ Jianyong Yu,§ and Bin Ding*,†,‡,§

†

State Key Laboratory for Modiﬁcation of Chemical Fibers and Polymer Materials, College of

Materials Science and Engineering, Donghua University, Shanghai 201620, China. ‡

Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles,

Donghua University, Shanghai 201620, China. §

Nanofibers Research Center, Modern Textile Institute, Donghua University, Shanghai 200051, China.

* Corresponding author. E-mail: [email protected]; Phone: +86-21-62378202; Fax: +86-2162378202

ACS Paragon Plus Environment

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ABSTRACT

Rational structural design involving controlled pore size, high porosity, and particle-targeted function is critical to the realization of highly efficient air filters, and the filter with absolute particle-screen ability has significant technological implications for applications including individual protection, industrial security, and environmental governance; however, it remains an ongoing challenge. In this study, we first

report

a

facile

and

scalable

strategy

to

fabricate

the

highly

integrated

polysulfone/polyacrylonitrile/polyamide-6 (PSU/PAN/PA-6) air filter for multi-level physical sieving airborne particles via sequential electrospinning. Our strategy causes the PSU microfiber (diameter of ~1 µm) layer, PAN nanofiber (diameter of ~200 nm) layer, and PA-6 nanonets (diameter of ~20 nm) layer to orderly assemble into the integrated filter with gradually varied pore structures and high porosity; thus enables the filter to work efficiently by employing different layers to cut off penetration of particles with certain size that exceeds the designed threshold level. By virtue of its elaborate gradient structure, robust hydrophobicity (WCA of ~130o), and superior mechanical property (5.6 MPa), our PSU/PAN/PA-6 filter even can filtrate the 300 nm particles with a high removal efficiency of 99.992% and a low pressure drop of 118 Pa in the way of physical sieving manner, which completely gets rid of the negative impact from high airflow speed, electret failure, and high humidity. It is expected that our highly integrated filter has wider applications for filtration and separation, and design of 3D functional structure in the future. KEYWORDS microfiber, nanofiber, nanonets, integrated filter, air filtration


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1. INTRODUCTION Due to rapid economic growth and industrialization as well as a surge in car usage and urbanization, China is subjected to severe particulate matter (PM) in the atmosphere, which causes serious impacts on populace health, production efficiency, visibility, and is even a significant contributor for climate deterioration and ecosystem damage.1-3 Asian-development Back report indicates that only 70%) that endows the filter with robust air permeability, the fibrous filter has attracted much more attention and is widely used in various air filtration fields. However, the existing fibrous filters has many drawbacks that are unable to overcome, owing to the microsized fiber diameter (from several to tens of microns) and resultant large pore size, including relatively low filtration efficiency, safety hazards from unexpected failure, bulkiness, and low quality factor, especially for filtrating ultrafine airborne particles.7,8 In contrast to the microfiber counterparts, the nanofibers function as air filter can offer an enhanced removal efficiency and a much lighter basis weight, due to their significantly reduced fiber diameter (100-800 nm) and pore size.9,10 And, based on the electrospinning technology that provides the most efficient strategy to manufacture non-woven fabrics with designed dimensions, morphologies, and functional components;11-13 many polymeric nanofibers have been prepared and used as air filter medium, like polyurethane,14 polyacrylonitrile (PAN),6,15 polysulfone (PSU),15,16 poly(lactic acid),17 polycarbonate,18 poly(vinyl alcohol),19 polyamide,20-22 etc. Unfortunately, some bottleneck problems


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still remain for these filters as a result of their limited structural controllability involving thicker fiber diameter (>100 nm) compared with the mean free path of air molecules (~65 nm) and larger pore size that is incapable of trapping ultrafine particles.7,23 Therefore, the nanofiber filters usually achieve a rather high removal efficiency for ultrafine particles at the expense of high air resistance and material consumption. Then, PAN/PSU filter composited of binary fibers was fabricated and gained an improved air permeability by virtue of the cavity structures and enlarged pore size, however, its filtration capacity still suffered from huge potential hazards from particle properties, high airflow speed, and high humidity, since the removal ability is mostly depended on the diffusion and adhesion effect of the particles.15 Considering that the drag force due to nanofibers (88%) (HCOOH, Shanghai Chemical Reagents Co, Ltd, China), its concentration was maintained at 15 wt%. The isopropyl alcohol (>99.7%) for electret elimination was bought from Shanghai Chemical Reagents Co., Ltd., China. Ultrapure water with a resistance of 18.2 MΩ was prepared by the Heal-Force system. The polyethylene terephthalate nonwoven fabric with negligible filtration capacity (filtration efficiency of ~3.5% and pressure drop of ~0.5 Pa for 300 nm particles under the face velocity of 32 L/min) for fiber receiving was supplied by Hainan Xinlong Nonwoven Fabric Co, Ltd, China. 2.2. Fabrication of Fibrous Membranes. The PSU microfiber, PAN nanofiber, PA-6 nanofiber/nets pure membranes, and PSU/PAN/PA-6 composite membrane were all fabricated by using DXES-3 spinning machine (SOF Nanotechnology Co., China). Typically, the same and designed solution was transformed into three syringes that clamped on mechanical slide unit and pumped out at the rate of 1 mL/h. A stable electric potential of 30 kV was supplied to the metal needles to form the charged liquid jets and droplets which would evolve to be the fibers and nanonets finally after rapid solvent evaporation during 20 cm tip-roller distance. All the membranes were deposited on the nonwoven fabric that overlaid on grounded metal roller with rotating speed of 80 rpm. To control the membrane uniformity we allowed the injection pump to horizontally move backwards and forwards at a speed of 150 cm/min via using a power sliding table. Moreover, the simple electric shield parts composed of symmetrically fixed copper wires on each needle was also used to force the charged jets to fly forward, therefore the resultant nanofibers can deposit on the substrate uniformly based on the quadrature motion process resulted from the synchronous movement of the roller and sliding table. The relevant temperature during electrospinning was maintained at 25 ± 2 oC, while the relative humidity was adjusted to 45 ± 2%, 45 ± 2%, and 25 ± 2% for fabricating PSU microfibers, PAN nanofibers, and PA-


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6 nanofiber/nets, respectively. While for the PSU/PAN/PA-6 composite membrane, the PA-6, PAN, and PSU solutions were sequentially electrospun onto the nonwoven substrate under respective humidity (RH of 25% for PA-6, RH of 45% for PAN and PSU) and with designed spinning durations. The depositing time was carefully regulated to control the basis weight of the fibrous membrane. All the membranes were treated by a solvent-immersion and then vacuum-dry process to remove the residual charges according to the method recommended by the latest European standard for air filters (EN779: 2012). The detailed electret elimination of the filtration membranes was presented in the Supporting Information. 2.3. Characterization. The morphology of the fibrous membrane before and after filtration was characterized by using VEGA3 LMU SEM (Tescan Ltd, Czech Republic) with an acceleration voltage of 5 kV after being coated with gold for 1 min. The fiber diameter and distribution in the membrane was measured by Adobe Photoshop CS6 image analysis software, and its thickness was tested by CHYC2 gauge (Labthink Co, Ltd, China). And, at least 50 fibers/wires and 5 positions were measured to guarantee the veracity of the average value for each sample, respectively. The pore size and distribution of the fibrous membrane was measured using a CFP-1100AI capillary flow porometer (Porous Materials Inc., USA) based on capillary flow analysis method. The water contact angle (WCA) data (3 µL) was collected with SL200B contact angle goniometer (Kino Industry Co., Ltd, USA), while the mechanical properties were checked by using XQ-1C tensile tester (Shanghai New Fiber Instrument Co., Ltd, China). The surface potential of the membrane was measured by using TREK-542A-2-CE electrostatic voltmeter equipped with non-contacting electrostatic probe (TREK Inc., USA). Filtration performance evaluation for particles with various diameters and under different airflows was carried out using LZC-H filter tester (Huada Filter Technology Co., Ltd, China) under ambient temperature of


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25 oC and humidity of 50%, and detailed measurement was given in the Supporting Information. A minimum of 3 regions were measured for each sample to accurately acquire its filtration capacity.

3. RESULTS AND DISCUSSION 3.1. Design and construction of PSU microfiber filter layer. To create an air filter that can effectively sieve particles with various diameters under a low air resistance requires the sophisticated structural design of the filtration medium, based on revealing the internal relation between particulate and their pore structures.27-29 We constructed our integrated air filter via a multi-level filtration layer design according to the following criteria: (a) moderate pore size to ensure the physical sieving for particles with certain diameters at minimum sacrifice of air permeation, (b) high porosity and thin thickness (namely low basis weight) to facilitate the airflow, (c) gradient filtration to avoid sharply increment for air pressure due to particle blocking. These three requirements are satisfied by a facile and scalable strategy of multi-level medium integration, which allows the PSU microfiber layer, PAN nanofiber layer, and PA-6 nanofiber/nets layer to assemble into a highly integrated air filter to progressively sieve the airborne particles with >2, >0.5, and >0.3 µm under low air resistance. We selected the PSU microfibers as the building blocks to construct the first filtration layer. The SEM image of PSU microfiber membrane was presented in Figure 1a, indicating a typical nonwoven structure composed of smooth and uniform microfibers. The fiber diameter in PSU membrane was 1.02 ± 0.24 µm (Figure 1d). Since abundant residual charges that exist in the fresh electrospun membranes just obtained from a strong electric field would cause a temporary filtration capacity for the particles, especially for the ultrafine ones;10,18 the PSU filter suffered from a sharply deterioration of filtration efficiency (from 85.162% to 66.219% for >0.3 µm particles, from 99.365% to 92.989% for >2 µm particles) due to the rapid charge dissipation from 1.2 to 0.26 kV just after 12 h in the humid


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environment, as shown in Figure S2. Here we employed the solvent immersion method mentioned in the latest European standard for air filters to eliminate this unstable and uncontrolled influential factor of the residual charges, which were space charges injected by the high potential power during the electrospinning, and usually trapped in shallow traps and easily dissipated due to the changeable external environment. After a careful elimination of the residual charges for PSU microfiber membrane which caused its surface voltage dramatically decreased from 1.2 to 0.03 kV (Figure 1e), 0.3-0.5 million sodium chloride (NaCl) aerosol particles and few atmospheric dust, with electric neutrality and various diameters ranging from 0.3 to 10 µm, were delivered through the membrane to evaluate the filtration performance of this PSU microfiber layer. From Figures 1b and c we can know, abundant particles were captured on the top surface of PSU membrane and their diameters focused in the range of >2 µm, while few particles that trapped on the bottom surface possessed a diameter of 2 µm particles for PSU microfiber layer by varying its basis weight. The PSU membrane with spinning time of 25 min could achieve a rather high removal efficiency level of >90%, and a moderate pressure drop of 6 Pa, while its basis weight was just


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1.25 g/m2, as shown in Figure 2b. Although the removal efficiency of PSU membrane can slightly increase (from 90.5% to 97.6%) with further increasing the spinning time (25-40 min), its pressure drop increased from 6 to 20 Pa, suggesting an unreasonable trade-off design for practical application: slightly enhanced efficiency at the expense of hugely increased air resistance. Here, the QF value was employed to appraise the comprehensive filtration capacity of our resultant membranes, and it can be calculated by the particular formula as presented in the previous works: = − (1 − )/ , in which η was the removal efficiency, △p was the pressure drop of the membrane filters.15,32,33 The PSU microfiber membrane spun for 25 min achieved a robust QF value of 0.392 Pa-1 (Figure 2c), further indicating its promising application especially for energy-efficient purpose. In view of that different airflow requirement was proposed for air filters for various purposes, we further investigated the filtration performance of PSU microfiber layer under 32, 60, 85, and 100 L/min, since they were typical airflow standards for industry equipment, ventilation and air conditioning, personal respirator, and extreme cases, respectively. Benefiting from its physical sieving manner based on controlled pore size, the PSU microfiber membrane exhibited a virtually unchanged removal efficiency (from 90.47% to 90.02%) with increasing the airflow from 32 to 100 L/min, as shown in Figure 2d. While, its pressure drop increased linearly according to the Darcy's theory,34,35 and demonstrated a slope of fitting line of ~0.16, further revealing the contribution of superlight basis weight and designed pore structures on promoting the air permeability of PSU microfiber membrane filter.


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Figure 2. (a) Filtration efficiency curves of PSU microfiber membranes obtained with various spinning durations for airborne particles with various diameters. (b) Pressure drop and basis weight of PSU microfiber membrane obtained with various spinning durations. (c) Quality factor of PSU microfiber membranes obtained with various spinning durations for particles with diameter of >2 µm. (d) Filtration efficiency and pressure drop of PSU microfiber membrane (spinning duration of 25 min) for >2 µm particles under various airflows. The filtration performance in (a), (b), and (c) is tested under air flow of 32 L/min. 3.2. Design and construction of PAN nanofiber filter layer. Generally, a decrease in fiber diameter would cause a decrease in pore size of the fibrous membrane, leading to the enhanced capture ability for particles with smaller diameters.8,24 To further improve the filtering precision, we employed the PAN nanofibers to construct the second filter layer. Figures 3a and d presented that the PAN nanofibers exhibited randomly arranged porous structure with fiber diameter of 220 ± 40 nm, revealing the promising structure for solid interception and gas permeation, since the momentum of particle made it more likely to move along its course and be captured when the airflow slipped sideways past the fibers.


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Similarly, the surface voltage of PAN membrane sharply changed from 0.6 to 0.03 kV after the electret elimination treatment, as displayed in Figure 3e, indicating that the unstable electret effect was successfully eliminated by the solvent-immersion method. Then, we carefully observed the morphology of the top and bottom surface of PAN nanofiber membrane after filtration using 0.3-10 µm NaCl aerosol particle (Figures 3b and c). Obviously, the diameter of particles caught on the top surface was almost >0.5 µm, and they were tightly intercepted by the tortuous pore structures constructed by packing the nanofibers. From Figure 3c we can see that, this super-thin PAN nanofiber membrane acted as a “fishing net” that caught all the particles with diameters larger than net pore size, fully confirming the safest filtration way that can exclude the negative influence from particle properties and ambient conditions. The result of pore structure analysis offered deep explanation for this sieving manner. As shown in Figure 3f, the PAN nanofiber membrane possessed a narrow pore distribution ranging from 0.5-0.7 µm, which was just suitable for intercepting the >0.5 µm particles without obviously sacrificing its air permeability.


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Figure 3. SEM images of (a) original PAN nanofiber membrane. (b) The top surface and (c) bottom surface of PAN membrane after filtration (airflow of 32 L/min, testing for 2 min). (d) Fiber diameter distribution and (f) pore size distribution curve of PAN nanofiber membrane. (e) Surface voltage of PAN nanofiber membrane before and electret elimination. Based on the analysis mentioned above, we employed the particles with diameter of >0.5 µm to systematically investigate the filtration performance of PAN nanofiber membrane, accordingly. Figure 4a offered the removal efficiency curves of PAN layers with various spinning durations ranging from 5 to 20 min, revealing a gradual increase trend versus the increased particle diameter owing to their limited pore size. When the spinning time reached 10 min, PAN nanofiber membrane showed a hugely improved removal efficiency of 91.40% compared with 82.08% of the membrane spun for 7 min, and a rather low pressure drop of 31 Pa in contrast to 56 Pa of the membrane spun for 15 min, as demonstrated in Figure 4b. Meanwhile, this membrane possessed a superlight basis weight of 0.3 g/m2, which was regarded as the major contributing factor for its enhanced air permeability.28 Furthermore, the PAN nanofiber membrane with spinning duration of 10 min gained a robust QF value of 0.79 Pa-1 (Figure 4c), effectively justifying the optimized spinning time of PAN nanofiber layer for constructing our designed integrated air filter. Undoubtedly, with the controlled pore structures, the PAN nanofiber membrane could still remain rather high removal efficiency of 91.30%, 91.16%, and 91.07% under much higher airflows involving 60, 85, and 100 L/min (Figure 4d), respectively. Additionally, the linear fitting of their pressure drops under increased airflow presented that, the slope of the fitting line was only ~0.89, which could be ascribe to the high fluffy and porous networks of the PAN nanofiber membrane.


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Figure 4. (a) Filtration efficiency curves of PAN nanofiber membranes obtained with various spinning durations for airborne particles with various diameters. (b) Pressure drop and basis weight of PAN nanofiber membrane obtained with various spinning durations. (c) Quality factor of PAN nanofiber membranes obtained with various spinning durations for particles with diameter of >0.5 µm. (d) Filtration efficiency and pressure drop of PAN nanofiber membrane (spinning duration of 10 min) for >0.5 µm particles under various airflows. The filtration performance in (a), (b), and (c) is tested under airflow of 32 L/min. 3.3. Design and construction of PA-6 nanonets filter layer. According to the existing research work from industrial fields and/or academic circle in air filtration, ~300 nm is usually considered to be the most penetrating particle size (MPPS) for particles, and the filtration of these airborne particles has become a grand challenge that the researchers are always striving to resolve.16,36 By aid of our proposed novel electrospinning/netting technology,12,37 we fabricated the PA-6 nanofiber/nets membrane to function as the core filtration medium to sieve these MPPS particles. Figure 5a indicated a representative SEM image for PA-6 nanofiber/nets membrane. The PA-6 membrane exhibited a


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bimodal structure including 1D common electrospun nanofibers and 2D nanonets with topological Steiner-tree structures; and the latter was comprised of interlinked 1D ultrathin nanowires and supported by the former fibers, as highlighted in the inset of Figure 5a. Over 80% distribution of PA-6 nanowire diameters located in the range of 20-26 nm with an average value of 24.2 nm (Figure 5d), which was one order of magnitude less than that of conventional electrospun nanofibers. Of particular interest was that PA-6 nanonets exhibited the integrated characters of 2D complanate pore structures, extremely small pore size (50-300 nm), and enhanced interconnectivity, rising as a shining star for essentially promoting the filtration efficiency of the filter.25 Thanks to its small pore size, the PA-6 nanofiber/nets membrane could facilely sieve the particles with diameters of even dozens of nanometers, which was proved by the SEM observation of the top surface of the membrane, as displayed in Figure 5b. And, almost no particles could be seen from the bottom surface of the PA-6 membrane (Figure 5c). Most noteworthy was that, this filtration process was realized by the absolute physical sieving manner, and independent of the electret effect, which could be indicated by its negligible surface voltage of 0.025 kV (Figure 5e). To provide insight into the pore structures, we investigated the pore size and distribution of this novel PA-6 nanofiber/nets membrane: a single and narrow distribution peak located between 0.25 and 0.3 µm, further confirming not only the uniformity and coverage rate of the 2D nanonets, but also their amazing contribution on reducing the pore size of the whole membrane.


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Figure 5. SEM images of (a) original PA-6 nanofiber/nets membrane. (b) The top surface and (c) bottom surface of PA-6 nanofiber/nets membrane after filtration (airflow of 32 L/min, testing for 2 min). (d) Wire diameter distribution and (f) pore size distribution curve of PA-6 nanofiber/nets membrane. (e) Surface voltage of PA-6 nanofiber/nets membrane before and electret elimination. Figures 6 a-d offered the investigation of the filtration capacity of our PA-6 nanofiber/nets layer towards >0.3 µm NaCl aerosol particles. With increasing the spinning duration from 10 to 80 min, the filtration efficiency of PA-6 membrane rapidly increased at first, and then underwent a gradually increased change after the spinning time excessed 40 min, facilely achieving a relatively high level of 83.06%, as demonstrated in Figure 6a. And, its pressure drop and basis weight were 67 Pa and 0.23 g/m2 (Figure 6b), respectively. Further increasing the spinning time to 80 min could effectively improve the filtration efficiency (99.34%) of PA-6 membrane; however, this change would result in the sharply increased air resistance of 220 Pa due to its closely packed porous structures, which was intolerable in many practical air filtration fields like respirator, window screening, to name a few. Balancing the competition between its removal efficiency and air permeability, the PA-6 nanofiber/nets membrane spun for 40 min exhibited a robust QF value of 0.026 Pa-1 against filtrating the MPPS


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particles, as shown in Figure 6c. More significantly, under much higher airflow ranging from 60 and 85 to 100 L/min, this nanofiber/nets membrane layer (spinning duration of 40 min) can still achieve a stable removal efficiency of 83.01%, 82.86%, and 82.79% (Figure 6d), indirectly confirming the structural stability of 2D nanonets that can withstand the deformation pressure from high speed air stream and the friction/strike from airborne particles.38,39 Similarly, with increasing the airflow during filtration process, the pressure drop of PA-6 nanofiber/nets membrane increased obviously and presented the linear fitting line with rather high slope of 2.03, which also could be ascribed to its compacted structures, and inevitably hinder its practical application when functioned alone as solo filter.

Figure 6. (a) Filtration efficiency curves of PA-6 nanofiber/nets membranes obtained with various spinning durations for airborne particles with various diameters. (b) Pressure drop and basis weight of PA-6 nanofiber/nets membrane obtained with various spinning durations. (c) Quality factor of PA-6 nanofiber/nets membranes obtained with various spinning durations for particles with diameter of >0.3 µm. (d) Filtration efficiency and pressure drop of PA-6 nanofiber/nets membrane (spinning duration of


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40 min) for >0.3 µm particles under various airflows. The filtration performance in (a), (b), and (c) is tested under airflow of 32 L/min. 3.4. Structural construction and performance evaluation of PSU/PAN/PA-6 integrated filter. Inspired by the current air filtration system composed of coarse filter, medium filter, high-medium filter, sub-high efficiency particulate air (sub-HEPA) filter, HEPA filter, and ultra-low penetration air (ULPA) filter;2,40 we designed, for the first time, the highly integrated filter via sequential deposition of polymeric fibers with controlled diameters ranging from micrometer and sub-micronmeter to nanometer scale. The inset of Figure 7a presented the cross-section view of our novel PSU/PAN/PA-6 integrated air filter consisting of PSU microfibers, PAN nanofibers, and PA-6 nanonets, revealing a tightly bonded and interconnected structure on the interface boundary of these three layers. As shown in Figure 7a, the PSU/PAN/PA-6 integrated filter with respective spinning duration (25 min for PSU layer, 10 min for PAN layer, and 40 min for PA-6 layer) possessed the robust removal capacity for airborne particles with various sizes: it easily realized a capture efficiency of 100% towards >1 µm particles, and even achieved the efficiency of 99.992% for the 0.3 µm NaCl particles, while its pressure drop remained a low level of 118 Pa, fully indicating the contribution offered by these three respective layers on gradually capturing the particles with low air resistance. Plotting the removal efficiencies, pressure drops, and QF values of PSU/PAN/PA-6 integrated filter versus various airflows, as shown in Figure 7b, revealed different variations upon increasing the airflow in the range of 20-100 L/min: the removal efficiency only decreased slightly from 99.995% to 99.990% owing to its unique sieving manner; the pressure drop exhibited a linear increase obeying the Darcy’s law; and these two changes caused a regular decrease of QF value, it can achieve a robust value of 0.028 Pa-1 even at 100 L/min. Evidence for the sieving manner of our integrated filter also came from pore structure data, as demonstrated in Figure 7c. The pore size of PSU/PAN/PA-6 integrated filter was distributed in an extremely narrow range of 0.32-0.34 µm with an average value of 0.33 µm, suggesting that the


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combination of these three layers caused the pore structure to be more uniform and endowed simultaneously the filter with small pore size, highly fluffy structure, and high porosity (93.2%).

Figure 7. (a) Filtration efficiency of PSU/PAN/PA-6 integrated filter for airborne particles with various diameters. Inset is the corresponding image of its cross-section view. (b) Filtration efficiency, pressure drop, and quality factor of PSU/PAN/PA-6 integrated filter for >0.3 µm particles under various airflows. (c) Pore size distribution curve of PSU/PAN/PA-6 integrated filter. (d) Photographs of dynamic measurement (top) of water adhesion on the surface of PSU/PAN/PA-6 integrated filter, change of filtration performance (bottom) of the filter over 120 h under high humidity of >90%. (e) Comparison of surface voltage and filtration efficiency between PSU/PAN/PA-6 integrated filter and commercial electret filter before and after electret failure. (f) Tensile stress-strain curve of PSU/PAN/PA-6 integrated filter membrane. The spinning duration for PSU microfiber layer, PAN nanofiber layer, and PA-6 nanofiber/nets layer of the PSU/PAN/PA-6 integrated filter in (a)-(f) is 25, 10, and 40 min, respectively. The filtration performance in (a), (d), and (e) is tested under airflow of 32 L/min. As is well known, the hydrophobicity of the filter media is a very important performance parameter for airborne particle filtration, especially for the real industrial application.41 Figure 7d (top) displayed the photographs of a 3 µL water droplet touching and leaving the surface of PSU/PAN/PA-6 integrated


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air filter. We forced the droplet to contact the surface of the top filter layer (PSU microfiber membrane) until an obvious deformation occurred, and then carefully observed the shape change of the droplet during its lifting up process. The resulting photographs indicated almost no droplet deformation when leaving the filter surface, revealing the extremely low water adhesion for this integrated filter due to its high WCA of 130 ± 3o (Figure S3). This robust hydrophobicity can be ascribed to the dense and obvious micropores and wrinkles formed on the surface of PSU microfibers (Figure S4), which would greatly improve the surface roughness of the fiber layer and thus enhance the hydrophobicity of the membrane. To clarify the filtration performance stability of our integrated filter, we tracked its performance change when being exposed to humidity of >90% over 120 h, as demonstrated in Figure 7d (bottom). It was clearly visible that, the filter still remained a rather high efficiency of >99.91% with a slightly increased pressure drop from 118 to 125 Pa, further confirming the hydrophobicity properties resulted from PSU macromolecular structure. Furthermore, the PSU/PAN/PA-6 integrated filter completely got rid of the hidden hazards from electret degradation, which bottleneck the electret meltblown microfibers suffered from and was strictly limited by EN779: 2012 filter standard.42 From Figure 7e we can know that, after the same solvent immersion treatment, both our integrated filter and commercial electret filter exhibited sharply decreased surface voltage: the former one reduced from 0.33 to 0.03 kV, the latter one changed from 0.39 to 0.03 kV. However, the filtration efficiency of the electret filter dramatically deteriorated from 99.998% to 95.980% after electret failure, in comparison with that, the efficiency of PSU/PAN/PA-6 integrated filter reminded almost unchanged (from 99.998% to 99.992%). Additionally, our PSU/PAN/PA-6 integrated filter with optimal filtration capacity (removal efficiency of 99.992% and pressure drop of 118 Pa) possessed superior mechanical property of tensile strength of 5.6 MPa with break elongation of 45% (Figures 7f and S5), suggesting the sufficient capability for their subsequent processing and practical application.


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3.5. 3D simulation of filter structure and filtration process. To provide insight into the relationship between robust filtration performance and unique structural features of the PSU/PAN/PA-6 integrated filter, we carried out the 3D simulation by using FiberGeo and FilterDict procedures based on the SEM observation and pore structure parameters,43,44 as illustrated in Figures 8a-d. FiberGeo produced representative domains for PSU microfiber layer, PAN nanofiber layer, PA-6 nanofiber/nets layer, together with the whole structure of this PSU/PAN/PA-6 integrated filter; and the cross section of each layer was also demonstrated, respectively (Figures 8a and b). These virtual clones offered visual and intuitionistic exhibitions for elaborating their structural characters. Obviously, the apparent pore size gradually reduced with decreasing fiber diameter ranging from PSU microfibers and PAN nanofibers to PA-6 nanofiber/nets, which was consistent with the experimental results of pore structures mentioned above. In order to further reveal the capture efficacy of these three membrane layers for airborne particles with various sizes, the simulation of filtration process was performed by tracking the particles across the filter medium under a designed fluid condition.45 As demonstrated in Figure 8c, after a certain amount of particles with various sizes were fed into the filter layers, the PSU microfiber layer could only sieve the particles with size of ≥2 µm, while the smaller ones (0.3-2 µm) all penetrated through the medium; the PAN nanofiber layer failed to capture the particles with size of 0.3-0.5 µm, and only the PA-6 nanofiber/nets layer could trap almost all the particles by virtue of its extremely small pore size. Figure 8d exhibited the flow pressure drop field during airflow pass process through the three solo layers and the integrated filter. According to the color models, the filter layer with thicker fiber diameter showed a lower pressure drop that represented by blue and green color at the cost of its capture efficacy for airborne particles, as shown in Figure 8d (1-3). It was worthwhile to note that, all particles with various sizes were trapped by the PSU/PAN/PA-6 integrated filter, and the particle deposition exhibited an obvious regulation: >2 µm particles deposited in PSU microfiber layer, >0.5


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µm particles captured in PAN nanofiber layer, and >0.3 µm particles sieved in PA-6 nanonets, as displayed in Figure 8d (4). Moreover, our integrated filter possessed a much lower pressure drop compared with the solo medium, further confirming the synergistic effect of gradient structure for enhancing the filtration performance of air filters.

Figure 8. (a) Illustration of the concept of the three layers (PSU microfibers, PAN nanofibers, and PA6 nanofiber/nets) of the integrated filter. (b) 3D structure model of PSU/PAN/PA-6 integrated filter and the cross section of each layer. (c) Illustration showing the removal process for airborne particles with various diameters of the three different layers of PSU/PAN/PA-6 integrated filter. (d) Illustration showing the flow pressure field during air pass process through (1) PSU layer, (2) PAN layer, (3) PA-6 layer, and (4) integrated filter.


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4. CONCLUSIONS In summary, we have demonstrated a facile and scalable strategy to prepare the highly integrated PSU/PAN/PA-6 membrane for effective air filtration via sequential electrospinning. The PA-6 nanofiber/nets layer, PAN nanofiber layer, and PSU microfiber layer were successively constructed by employing different polymer systems and optimizing their respective spinning durations. The resultant PSU/PAN/PA-6 integrated filter exhibits gradually varied pore structures with size ranging from ~2.2 and ~0.6 to ~0.27 µm, and high porosity; and can cut off penetration of particles with certain size that exceeds the designed threshold level. Benefiting from its elaborate gradient structure, robust hydrophobicity (WCA of ~130o), and high tensile strength (5.6 MPa), our highly integrated filter even can filtrate the 300 nm NaCl particles with a high removal efficiency of 99.992%, a low pressure drop of 118 Pa, and a robust QF value of 0.08 Pa-1 in the way of physical sieving manner, which completely gets rid of the negative impacts from high airflow speed, electret failure, high humidity. The fascinating results establish our construction approach as powerful strategy to fabricate filters for various applications ranging from individual protection and industrial security to environmental governance.


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ASSOCIATED CONTENT Supporting Information. Details of electret elimination of the filtration membranes. Filtration measurement of the fibrous media. Optical photograph and schematic diagram of the air filter tester (Figure S1). Long-term filtration performance and surface voltage of the PSU microfiber filter (Figure S2). Droplets of dyed water on the top surface of PSU/PAN/PA-6 integrated filter (Figure S3). FESEM image of PSU microfibers layer of PSU/PAN/PA-6 integrated air filter (Figure S4). Tensile stress-strain curves of PSU microfiber layer, PAN nanofiber layer, and PA-6 nanofiber/nets layer (Figure S5). AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work is supported by the Key Technologies R&D Program of China (No. 2015BAE01B02), the National Natural Science Foundation of China (Nos. 51273038, 51322304, and 51503030), the ‘DHU Distinguished Young Professor Program’, the Fundamental Research Funds for the Central Universities, and the Shanghai Sailing Program (No. 15YF1400600).


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Table of Contents/Abstract Graphic

Highly integrated polysulfone/polyacrylonitrile/polyamide-6 air filter can multi-level physical sieve airborne particles with high filtration efficiency and low pressure drop.


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Polyamide-6 Air Filter

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