Solution Blow Spinning (SBS) Nanofibers for Composite Air Filter

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Solution Blow Spinning (SBS) Nanofibers for Composite Air Filter Masks Noel Peter Tan, Shierlyn Paclijan, Hanah Nasifa Ali, Carl Michael Jay Hallazgo, Chayl Jhuren Lopez, and Ysabella Ebora ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00207 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 30, 2019

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ACS Applied Nano Materials

Solution Blow Spinning (SBS) Nanofibers for Composite Air Filter Masks Noel Peter B. Tana,b,*, Shierlyn S. Paclijana , Hanah Nasifa M. Alia, Carl Michael Jay S. Hallazgoa, Chayl Jhuren F. Lopeza, Ysabella C. Eboraa aChemical

Engineering Department, Xavier University – Ateneo de Cagayan, Corrales Avenue, Cagayan de Oro City, 9000 Philippines bNano

and Advanced Materials Institute, Ltd. Hong Kong Science Park, Shatin, New Territories, Hong Kong *Corresponding author’s email: [email protected]

Abstract Particulate matter (PM) pollution in the air has become a serious environmental issue that has demanding requirements for the fabrication of new materials. In this study, three different types of nanofiber materials (i.e., Cellulose diacetate (CDA), Poly (acrylonitrile) (PAN) and Poly (vinylidene fluoride) (PVDF)) produced from solution blow spinning were used in composite multi-layered filter mask and evaluated their filtration performance for particulate matter of at least 2.5 micrometers in size (PM2.5). PM2.5 capture efficiency and nanofiber filter pressure drop were tested using the fabricated set-up with simulated burnt cigarette smoke as a source of PM2.5. Filter mask performance using different nanofiber materials was evaluated through their corresponding quality factors (QF). Among the three nanofibers tested, PAN nanofiber has the highest filtration performance with a quality factor of 0.052 Pa-1 and acceptable air permeability. Whereas, PVDF has the least air filter quality among all the nanofibers studied, with a quality factor of 0.02 Pa-1. Insights on the high performance of the nanofiber materials in filter masks are further discussed in this study. Aside from the nanoscale advantage of the nanofibers tested, it is argued that the different molecular functionalities present in the polymeric nanofibers have a significant influence on its filtration performance. In conclusion, all nanofibers used in this study have better filter performances compared to the commercial surgical masks. Keywords: Nanofiber, Particulate Matter (PM2.5), Solution blow spinning (SBS), Air Filtration, Filter mask

Introduction In recent years, air pollution has become a global threat. Such threat is due to its adverse effect on human health resulting from the haze problem caused by particulate matter (PM) or fine particles. Particulate matter pollutants in the atmosphere include mineral powder, coal and carbon powder (i.e., produced by industrial emissions), vehicle exhaust, combustion from daily life, and smoke from forest fires, among other sources [1]. Epidemiological studies concluded that air pollution due to fine air particle has influences on cardiovascular problems such as cerebro-vascular accidents, heart problems, or myocardial infarction. [2,3] Indeed, exposure to air pollution has many substantial effects on human health, responsible for a growing range of health effects. [4] In a study conducted in Austria, France, and Switzerland in 2000, it was found that air pollution caused 6% of total mortality or more than 40,000 attributable cases per year. About half of all death caused by air pollution was attributed to motorized traffic, accounting to different chronic diseases, viz. more than 25,000 new cases of chronic bronchitis for adults; more than 290,000 episodes of bronchitis

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for children; more than 0.5 million asthma attacks; and more than 16 million person-days of restricted activities. [5] Consequently, short and long-term exposure to air pollution has also been linked to reduced life expectancy causing more than 7 million people in the world in 2014. [6] In the Philippines, the primary source of small size particulate matter including PM2.5 stems from public utility vehicles. PM2.5 refers to atmospheric particulate matter (PM) that has a diameter of 2.5 micrometers or less. In the recent estimates, the PM emissions in the Philippines totaled to fifty-six thousand (56,000) tons for the past decade [7]. Among the sources air pollution exposure, cigarette smoke exposure gives the highest concentration of PM2.5 [8]. In developing countries such as the Philippines, the use of filter masks is considered to be the most practical means to prevent inhalation of particulate matter from the atmosphere. In fact, part of the government’s plan to combat air pollution includes the use of reception-based solution by developing composite masks as a medium for effective capture of hazardous particulate matters. Hence, air filtration is still considered the most promising, practical and effective technique for protection from particulate matter in the atmosphere [9]. On the other hand, traditional air filtration media such as commercial masks with microscopic fibers (spun-bonded fibers, glass fibers, and melt-blown fibers)[6] display low filtration efficiencies for fine particles with diameters ranging from 0.1–0.5 μm. Such low capabilities are due to their large pores sizes of filtration fibers. Although some nonwoven filtration materials exhibit satisfactory filtering efficiencies for particulate matters, their performance is still far for sub-micrometer PM. Another disadvantage of using a thicker filter medium is its high-pressure drop which results in high energy costs [6]. With the advent of new discoveries and technologies, the emergence of new materials with excellent physical and chemical properties promote the development of highly efficient filtration technology, along with the fast progress in the field of membrane materials science and nanotechnology. One of these developments is the use of aero-spun nanofibers membranes in high-performance air filters. The use of nanofiber as a potential material in developing a composite air filter media for PM2.5 is an up-and-coming alternative. Nanofibers have an extensive surface area-tovolume ratio, which significantly increases the probability of the particulate matter deposition on the fiber surface and thereby improves the filter efficiency with a relatively low-pressure drop. The high filtration efficiency of nanofibers is believed to effectively prevent the ambient PM2.5 from entering the human body due to the accompanying five catching mechanisms, [6,10,11] viz. interception, inertial impaction, Brownian diffusion, the electrostatic effect and the gravity effect [12]. The complex arrangement of the nanofibers in air filtration membrane results to a convoluted streamline. As the air flows, particulate matters are isolated from the airflow line under the action of inertial impaction [13], and eventually accumulates onto the fiber membrane. In this study, several nanofibers of Cellulose diacetate (CDA), Poly (acrylonitrile) (PAN) and Poly (vinylidene fluoride) (PVDF) produced from solution blow spinning (SBS) method were used in composite air filter mask and compared their capture efficiencies, pressure drop and quality factors. A typical set-up of SBS is discussed in the work of Salva et al., [14]. Cellulose diacetate (CDA) was used since it is the most abundant natural polymer [15] with excellent chemical resistance, thermal stability and biodegradability [16]. Polyacrylonitrile (PAN) nanofibers, on the other hand, are strong and tough polymers [17]. Also, polyvinylidene fluoride (PVDF) is known for its good chemical and thermal resistance and commonly used as commercial membrane material [18]. The use of solution blow spinning method here over electrospinning is of a significant step in this study. Although electrospinning has gained attention in producing nanofibers, drawbacks such as scalability and safety have hindered its further industrial applications. However, the synthesis of nanofiber through solution blow spinning is not discussed in this study. Instead, this study focuses on the essential properties considered for mask filters and factors affecting the filtration performance of different nanofibers. Analyses of its performance are based using statistical analysis. Image J software was used for nanofiber diameter determination, porosity


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and mean pore area estimations. Morphological comparisons were carried out through SEM analyses. Furthermore, this paper contributes to relating the nanofibers’ structures and functionalities to their respective filtration performances. Such relationship provides more understanding in choosing the appropriate polymer nanofiber for the improvement of air filtration mask. The immediate practical application of this study is on the use of composite nanofiber air filter media to effectively capture PM2.5 in major cities in the Philippines. This study aims to develop a reproducible and more effective filter in comparison to the widely known and easily available in the market; leaning towards environmental and health impact. From this data and method, scale-up studies are further considered.

Experimental Materials Cellulose diacetate (12% CDA/Dimethyl sulfoxide:Acetone (2:1 volume ratio)), Poly (acrylonitrile) (9% PAN/Dimethyl formamide) and Poly (vinylidene fluoride) (10% PVDF/Dimethyl formanide) nanofibers were altogether provided from a commercial manufacturer using solution blow spinning method. These nanofibers were used as received from the source. A box of Rosemed brand commercial surgical masks (CSM), composed of poly(propylene) (PP) was obtained from Berovan, Cagayan de Oro City, Philippines was used. Assembly of composite air filter media (CAFM) Composite air filter media (CAFM) were arranged into a 3-layer assembly (Figure 1). The first and last layers were composed of a 5cm x 5cm nonwoven polypropylene (PP) fiber from the CSM. The middle layer is composed of one, two, or three layers of 3cm x 3cm layers of nanofibers. The nanofibers were either CDA, PAN, or PVDF. The PVDF and PAN nanofibers as provided were embedded on a Polyethylene terephthalate (PET) substrate. These kinds of CAFMs were used in comparison to the commercial surgical mask.

Figure 1. Schematic diagram on the assembly of the 3-layer composite air filter media (CAFM) including different nanofibers.



The nanofiber layers correspond with the weight basis in g/cm2 which was obtained by weighing the sample and measuring its area. The weight basis (WB) is in three levels, labeled in ascending order as shown in Table 1. Such table shows the average values of the weight basis of each material with the arrangement of layers of the CAFM. All the arrangements from weight basis 1 to 3 were composed of polypropylene nonwoven fiber in the first and last layer. Weight basis 1 (WB1) consists of one layer of nanofiber. Weight basis 2 (WB2) has two layers of nanofibers, while weight basis 3 (WB3) has three. Table 1 Average Weight Basis of CDA, PAN, PVDF nanofibers and CSM used in the study

Air filtration Performance Test Particulate matter concentration and air pressure drop were tested simultaneously using the fabricated experimental set-up in Figure 2. Figure 2a shows the test setup composed of four (4) chambers; 1) Chamber 1 is a 5-cm cube shaped structure each side with a 3cm diameter hole at the center bottom for the entrance of generated smoke. A 2-cm diameter circular hole at the left side is intended for the air blower and particle counter; 2) Chamber 2 is a 3-cm cube shaped structure with two 1-cm hole at the bottom intended for the tube of the digital manometer and particle counter; 3) Chamber 3 is the same size with chamber 2 with two holes at the bottom for the other tube of the manometer and particle counter; and 4) Chamber 4 is the same size with chamber 1 but has a 2-cm hole on the right side intended for the insertion of the particle counter. The four chambers, as depicted in Figure 2a, are interconnected with 2 conduits having each a regulating valve. Figure 2b shows samples are mounted between the middle chambers (2 and 3) of the test-set-up during testing. The first chamber contains the source of PM2.5 from a burned smoke. A blower is used to induce the flow of the smoke into the proceeding chambers (Figure 2c). Burnt smoke was created by burning a stick of cigarette in a can for 10 seconds using denatured ethanol-fueled burner lamp. Its smoke was isolated first in chamber 1, then, induced for thirty (30) seconds in all succeeding chambers (i.e., 2, 3, 4) during filtration process. The use of the flow regulating valve is to stop the flow of the burnt smoke from chamber 1 into the chamber 2 and from chamber 3 to chamber 4, respectively. They are globe valves which are necessary to isolate the smoke within each chamber to aid measurement of particulate matter concentrations. PM2.5 concentrations were measured at Chamber 1 before filtration and chambers 3, and 4 after filtration. PM measurements were normalized by multiplying the particle concentration (µg/m3) with the corresponding total volume of the chamber. Three (3) trials were carried out per sample. The ambient conditions were kept at constant temperature and relative humidity (RH) at 30oC and 65% respectively.


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Figure 2. Schematic illustration of the set-up for PM capture removal efficiency and pressure drop measurement, a) Overall Set-Up with corresponding labels, b) insertion of the samples between chambers 2 and 3, c)view on the blower side. Air flow ( i.e., 63 ft3/min.) remained constant in all experiments. Capture efficiencies The PM particles used in this study were generated by burning cigarette. The cigarette smoke PM particles had a wide size distribution ranging from 10 μm. However, the majority of particles are less than 1 μm. The PM2.5 concentrations from upstream and downstream were measured using a particle counter (X in Test HT9600) with units µg/m3. The Efficiency (η) was calculated by the following equation (1).

𝜂=1―

𝐶𝑑𝑜𝑤𝑛𝑠𝑡𝑟𝑒𝑎𝑚 𝐶𝑢𝑝𝑠𝑡𝑟𝑒𝑎𝑚

(1)

where, Cupstream is the concentration of particulate matter (i.e., PM2.5) taken before filtration and Cdownstream is the concentration of particulate matter taken after filtration. Pressure drop The pressure drop test was determined by measuring the pressure difference across chamber 2 and 3 during filtration tests. A series of test was conducted for all samples with and without smoke. The installed digital manometer was used to measure the pressure drop (∆p) (Figure 2a). Preliminary pressure readings without any mask samples were obtained to ensure that the working pressure is constant.



Quality factor To determine the overall performance of the air filters, the QF was calculated based on the experimental data of the filtration efficiencies and the pressure drops as: 𝑄𝐹 = ―

𝑙𝑛(1 ― 𝜂) ∆𝑝

(2)

Statistical analysis Spearman’s rank-order correlation (rs) was used to determine the correlation of capture efficiencies versus varying weight basis. The same parameter is used to measure the strength and direction of monotonic association between two variables (capture efficiency and weight basis) without assuming a normal distribution. The value of r ranges from -1 to +1. A value of -1 or +1 indicates perfect association between X and Y. A positive (+) sign occurs for identical rankings and negative (-) sign occurs for reverse rankings. If the calculated value is close to zero, variables are uncorrelated. Spearman’s rank-order correlation is denoted by Equation S1in the Supplementary information provided. Single factor analysis of variance (ANOVA) was performed using Microsoft excel (refer to Figure S2 for summary output sample) to compare filter performance between commercial mask (CSM) and the nanofiber composite masks. Dunnett’s test (also called Dunnett’s Method or Dunnett’s Multiple Comparison) was further used to determine significant filter performance difference between the different composite air filters variables against a control condition. It is noted that when an ANOVA test has significant findings, it doesn’t report which pairs of means are different. Dunnett’s test is necessarily calculated after the ANOVA to further identify which sets of parameters have statistically significant differences. Refer to Equation S2 for the test statistic of Dunnett’s test. Scanning electron microscope Scanning electron microscope (SEM) samples were examined through a desktop SEM Hitachi TM3030 Plus Tabletop Microscope using an accelerating voltage of 15 kV. Dried sample of different nanofibers of CDA, PAN, PVDF, and commercial masks were cut into pieces and pasted on a carbon adhesive on top of a copper substrate. A thin layer of gold film was sputtered on the dried sample under vacuum.

Results and discussion Morphologies of nanofibers Figure 3 shows the morphologies of the different nanofibers used in this study. This included CDA, PAN, PVDF nanofibers, and the commercial PP microfiber. All nanofibers used in the study were not treated and used as received from the source. They were also collected in PET substrate from the SBS method. Commercial PP microfiber, PAN and PVDF nanofiber show uniform and well-defined structures as shown in Fig. 3a, c, and d. However, cellulose diacetate nanofibers (Figure 3b) show partially beaded nanofibers different from the rest as shown in Fig. 2b. Average fiber diameters and average pore sizes were determined using an open source plugin of ImageJ software called Diameter J (https://imagej.net/DiameterJ) estimated from their corresponding SEM images. Refer to Figure S1 for the diameter and pore size image analysis.


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Figure 3. SEM analysis images of for a) commercial propylene (PP) microfiber, b) CDA, c) PAN, d) PVDF. Table 2 summarizes mean fiber diameters and pore sizes. As expected, the commercial surgical masks were in micron size while the nanofiber diameter ranges from 580- 900nm. PAN nanofibers show the smallest average fiber diameter compared to the other filter media which promotes deposition of particles [19]. PVDF nanofibers have lower average fiber d diameter and pore sizes than CDA. The average pore sizes of CDA and PVDF nanofibers were approximately the same in the range 40 – 50 µm2. Table 2 Summary of CDA, PAN, PVDF nanofiber and PP microfiber average diameters and pore sizes.

PM2.5 Efficiency, Pressure Drop and Quality Factor The filtration properties of a commercial surgical mask were investigated to make a valid subsequent comparison (Table 3) with the developed CAFMs using nanofibers. Filtration experimental results for CDA, PAN and PVDF CAFMs are shown in Table 3. Commercial surgical mask (CSM) has an average PM2.5 efficiency of ca. 64%. Such efficacy is comparable to all other nanofiber CAFMs at WB1. However, at higher weight basis (i.e., WB2 and 3), filtration efficiencies are higher than the CSM, ranging from 66% - 90% depending on which type of nanofiber used. In all three WBs of different CAFMs pressure drops are lower than the CSM, ranging from 40 – 63 Pa, except PVDF WB3 (i.e., 70 Pa). Subsequently, the Quality Factors of all CAFM nanofibers have higher values than the CSM. PAN-based CAFMs have the higher Quality Factor values among the CAFMs and in all weight bases. Nonetheless, PAN CAFM exhibits best filtration performance for PM2.5.



Table 3. Average Filtration Properties of different weight basis of CAFMs and CSM.

Capture mechanisms SBS-spun nanofibers used in this study as part of a filter media works similar to any stable filtration. Traditional filtration theory mentions different catching mechanisms at a steady phase. Such mechanisms include interception, inertial impaction and Brownian diffusion [10] as shown in Figure 4. During filtration, aerosol particles move randomly from the gas stream mainly when they are near the fibers. Such stray movements of particles affect the filtration performance of the resulting membrane. Interception occurs when a particle, following the air streamline around the fiber, comes in contact with the fiber. Eventually, the particle sticks to the fiber and is therefore removed from the streamline. The interception capture mechanism is dominant within the particle size range of 0.1-1 μm. The inertial impaction can partly explain why the bigger particles with more significant inertia force would typically result in higher filtration efficiency. Such impaction happens when a PM particle with large mass unable to follow curve paths of the air streamline around the fiber but collides onto the fiber due to its momentum. This mechanism turns into the dominant capture mechanism for particles bigger than one μm, particularly for bigger particles and at higher gas stream velocities. [20] Although the addition of the nanofiber layer may lessen the breathability, but with a low air resistance guarantees its natural breathability. More importantly, aerodynamic behavior of airflow around the periphery of fibers would be changed dramatically when the diameter of fibers is reduced to the nanoscale. Random Brownian motion, on the other hand, could also cause particles to crash into the fiber, prompting deposition. Typically, such movement causes an aerosol particle to deviate from its original flow line randomly. Once the deviation is sufficiently vast to enable impact between particle and fiber, the particle can be captured


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immediately [13] particularly for small particles with sizes below 0.1 μm. Thus, filtering effect resulting from various fiber mechanisms may differ based on the particle size [21].

Figure 4. Filter mechanisms of fiber material with different filtering effect on the particle size. Figure 5 shows the deposition of the particulate matters on the different nanofibers used in this study. In Figure 4a, the particles deposited were hardly seen on the commercial PP microfiber. It is also possible that most particles may have passed through the PP microfiber. However, mixed and dense particle depositions were observed in CDA nanofiber (Figure 5b). PAN, on the other hand, showed clear particle interception within the nanofiber (Figure 5c). The same is true to PVDF nanofiber, wherein particulate matter deposited wrapped around the fibers (Figure 5d).

Figure 5. SEM analysis after filtration of a)PP microfiber, b)CDA, c)PAN and d)PVDF nanofibers. The authors believe that the governing mechanisms of the deposition of the particulate matter in this study can comprise the three types of mechanisms as discussed previously. Specifically, inertial impaction can have more influence in this case since the particles involved are bigger than one μm [14,19,22].



Capture Efficiency Figure 6 shows a plot of different PM2.5 capture efficiencies versus weight basis comparing the commercial PP microfiber with the different nanofibers used in this study. The PM2.5 capture efficiency of CDA significantly (P-value < 0.001, n=9) increases with increasing weight basis. Such behavior means that PM2.5 capture efficiency of the CDA in CAFM have strong positive (rs = 0.9832) monotonic relationship with the weight basis. The same result is observed with PAN in CAFM PM2.5 capture efficiency, wherein a strong positive (rs = 0.9192) monotonic association with the weight basis. PVDF in CAFM also shows a direct relationship of its PM2.5 capture efficiency (P-value < 0.001, n = 9) with its weight basis. However, the PM2.5 capture efficiency of PVDF CAFM is not highly correlated (rs = 0.7980) with its weight basis compared to other CAFMs. It also shows that the results were not significant in this case (P-value = 0.010, n=9).

Figure 6. PM2.5 capture efficiencies vs. weight basis of the different nanofibers used in CAFMs and commercial CSM. Consistent in all nanofibers used, PM2.5 capture efficiencies increases with its weight basis. Differences in PM2.5 capture efficiency can further be explained according to the fiber diameter and fiber pore sizes in addition to its weight basis. The decreasing fiber diameter and fiber pore sizes would increase the PM2.5 capture efficiency of composite air filter media such as in PAN average value of ca. 92 % specifically for WB3. Such high capture efficiency is due to the high available surface area [23]. The weight basis substantially affects its capture efficiencies such as in the case of WB1 PAN, which gained smallest capture efficiency among the CAFM. Hung et al. [24] concluded that large weight basis is helpful in increasing the filtration efficiency. Such study validates the case for WB1 PAN filtration efficiency, which has the lowest average weight basis of 1.00E-04 g/cm2, and increases as its WB increases. However, PVDF has the lowest capture efficiencies in both WB2 and WB3. Moreover, this can be attributed to its larger pore sizes and larger fiber diameter compared to PAN and CDA. Overall, PM2.5 capture efficiency of commercial fiber is around 64%. This is lower than


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CAFMs specifically in WB2 and WB3 of all nanofibers used in this study. Again, this can be due to having way larger fiber diameter and pore sizes compared to the nanofibers used in CAFM. As to the case of WB1 of nanofibers CAFM, the effect of weight basis may have substantially caused a lower PM2.5 capture efficiency. Pressure Drop Pressure drop is the resistance of airflow across an air filter media. This parameter was measured using a digital manometer. The results are shown in Tables 3. It can be deduced from this data that the most influential factor that dominates the change in pressure drop is the different weight basis. Larger basis weights increase pressure drop as illustrated by Hung et al. [24]. The same phenomenon was observed in this study as further validated in Figure 7. The figure shows the relationship between the pressure drop and its weight.. In all trials conducted, CDA CAFM’s pressure drop increased from 50 Pa to 70 Pa. In the same way, PAN CAFM’s pressure drop increased from 40 Pa to 50 Pa. PVDF CAFM, on the other hand, increased from 60 Pa to 70 Pa as weight basis increased respectively. Among the nanofiber CAF, PAN has the lowest pressure drop of 40 to 50 Pa. This pattern is attributed to the fact that it has way smaller weight basis than the others, even with increasing layers of PAN nanofiber mats.

Figure 7. Pressure drop vs. weight basis of the different CAFMs and CSM. Quality Factor Quality factor (QF) is the ratio of capture efficiency and pressure drop. It pertains to air filter performance considering the capture efficiency and the resistance across the filter. High capture efficiency with the low-pressure drop is desired to have a high-quality factor. The filtration QFs were derived from the experimental results of filtration efficiency & pressure drop presented in Tables 3. Figure 8 compares the filtration performance of commercial surgical mask with the different type of CAFMs for PM2.5.The comparison between WBs of each CAFM showed that WB3 of each CAFM consistently has the highest



Quality Factor. Such phenomenon is attributed to the relatively high capture efficiency and low-pressure drop. Among the CAFMs, PAN consistently showed to have the highest filter performance substantially due to its parameters such as fiber diameter, pore sizes and weight basis. It also has a high capture efficiency due to having a smaller fiber diameter and pore sizes. Also, even having a lower weight basis, the pressure drop across the filter is small, which gives a high filtration performance, specifically in WB3 PAN (i.e., 0.05±2.40E-03 Pa1). All of the CAFM containing nanofibers have a higher filter performance (QF) than CSM. CSM’s QF value lower than all the CAFM is due to having larger fiber diameter ca. 3.80 μm and pore size ca. 360.00 µm2. Another factor influencing low QF value is CSM’s more substantial weight basis, compared to CAFM, which caused to having low PM2.5 capture efficiency and higher pressure drop respectively.

Figure 8. Quality factor (QF) comparison of the different CAFMs and CSM.

Statistical analysis results Analysis of variance (ANOVA) from different types of CAFM against a commercial surgical mask (CSM) reveals significant findings. A significant finding is identified if Fcritical ≤ significance F and if the significance value, α = 0.05 ≥ P-value. A summary output of ANOVA is found in Figure S2 Supplementary information. Furthermore, F-critical is a function of the degrees of freedom (df) between the group and within the group to be found in Figure S2 Supplementary information. Generally, since each nanofiber type at different weight basis (i.e., in triplicates) has the same number of data and degrees of freedom which is 3 and 8 respectively, their F-critical values are the same throughout which is equivalent to 4.07 (Figure S4-S6 Supplementary Information). A one-way ANOVA was conducted to compare the effect of type of CAFMs and Commercial Surgical Mask concerning PM2.5 quality factor. An analysis of variance of different weight basis of CDA and CSM showed that the impact of CDA CAFM on quality factor was significant, F(3,8)=20.28, P < 0.001. Consistently, in the case of PAN CAFM in different weight basis vs CSM, also showed a significant difference regarding their effect on quality factor (i.e., F(3,8) = 305.38, P < 0.001). However, in the case of PVDF CAFM and CSM, their effect on PM2.5 quality factor was not significant, F(3,8) = 2.18, P =0.17. Such a conclusion is based on its statistical F value of


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2.18, which is lesser than F-crit = 4.07 and P=0.17, and also higher than significance level α=0.05. Furthermore, Dunnet’s test showed that PAN and CDA CAFM quality factor at different weight basis were significantly different and higher than CSM, while PVDF CAFM is comparable to the CSM. Functionalities of nanofibers in CAFM Aside from the nanoscale effects of nanofibers in composite air filters (i.e., CAFMs), recent studies confirm that surface chemistry (i.e., functional groups) of nanofiber air filters play a significant role in successfully capturing particulate matters due to their surface properties. Nanofibers with a polar chemical functional group have a strong binding affinity towards PM2.5 [25]. PM2.5 in polluted cities is a complex combination of fine organic carbons and majorly atmospheric moisture [26, 27, 28]. In this study, PM generated by burning cigarette acts as similar to PM2.5 atmospheric pollutions [29,30,31]. The three nanofiber CAFMs composed of different polymers contain specific functional groups such as acetyl, nitrile, and vinylidene for CAD, PAN and PVDF respectively. Among the three functional groups, nitrile exhibits the most robust polarity due to the presence of very electronegative nitrogen, thus having also the largest dipole-moment [32]. Therefore, the affinity of nitrile groups towards PM2.5 in PAN CAFM is due to its favorable surface properties that enable strong interaction (i.e., hydrogen bonding and dipole-dipole interactions) to particulate matters [29,30]. Such behavior implies that the high value of capture efficiency of PAN CAFM is supported by its fiber surface properties that have a strong affinity towards PM2.5.With such high surface activities of the nanofibers due to its high surface area to volume ratio (nano-size effect) coupled with high affinities to its functional groups involved, particulate matters can hold on firmly on the nanofiber surface even at high airflow. In effect, the pressure drop on nanofiber composite masks is lower than the commercial surgical mask.

Conclusion The filtration performance was significantly improved by using SBS-spun nanofibers in this study. The trend these nanofiber mats had good filter performance in a single layer configuration is a compensation of the inferior parts of the masks and kept high filter quality. The sandwiched configuration composed of the nonwoven layers and the intermediate nanofibers resulted in a quality factor better than commercial surgical masks. The study demonstrated that these nanofiber-based masks could be highly effective because of its nanoscale fibre diameter, small inter-fiber distribution and surface chemistry. These nanofiber-based masks are a step forward in improving the performance of the existing masks in the market. The results also demonstrate that multiple thin layer structure is a promising method for enhancing the nanofiber filtration performance compared to a single layer with considerable basis weight. The successful fabrication of functional nanofibers from solution blow spinning proves to be a sufficient potential for designing and constructing a composite mask for particulate matter filtration with good air permeability.



Supporting Information Additional supporting figures such as Image J nanofiber diameter locations and pore analysis of the different nanofibers, Table of critical values for Dunnett’s test, Analysis of variance (ANOVA) analyses comparing the different filter masks (i.e., CAFMs, CSM) with different weight basis used before and after filtration, and the Dunnett’s test summarized results are available in a separate file.

Acknowledgements The researchers are grateful to Xavier University - Kinaadman Support for Student Research (XU-KURO) for their financial assistance to this study, to the Balik Scientist Program of the Department of Science and Technology, Philippines (BSP-DOST) through the Philippine Council for Industry, Energy, and Emerging Technology Research and Development (PCIEERD) on the assistance and encouragement for Research and Development in the country, as well as to the Chemical Engineering Department of Xavier University- Ateneo de Cagayan laboratory personnel and its faculty.

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Fabrication of multi-layered nanofiber mask from SBS. 271x155mm (116 x 116 DPI)


Solution Blow Spinning (SBS) Nanofibers for Composite Air Filter

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