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Ultrafine PVDF Nanofibers for Filtration of Air-Borne Particulate polymers Review Ultrafine PVDF Nanofibers for Filtration of Air-Borne Particulate Matters: A Comprehensive Review Ayishe Sanyal 1 and Sumit Sinha-Ray 1,2,* 1 School of Engineering, Indian Institute of Technology Mandi, Mandi 175005, HP, India; [email protected] 2 Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, IL 60607-7022, USA * Correspondence: [email protected] Abstract: The ongoing global pandemic has bestowed high priority uponthe separation of air-borne particulate matters (PMs), aerosols, etc. using nonwoven fibrous materials, especially for face masks as a means of personal protection. Although spunbond or meltblown nonwoven materials are amongst the forerunners for polymer microfiber-based face mask or air filter development in mass scale, relatively new process of nonwoven manufacturing such as electrospinning is gaining a lot of momentum amongst the filter membrane manufacturers for its scalability of nanofiber-based filter membrane fabrication. There are several nanofiber-based face masks developing industries, which claim a very high efficiency in filtration of particulate matters (PM0.1–10) as well as other aerosols for their products. Polyvinylidene fluoride (PVDF), which is commonly known for its use of tactile sensors and energy harvesters, due to its piezoelectric property, is slowly gaining popularity among researchers and developers as an air filter material. Electrospun PVDF nanofibers can be as fine as 50 nm in mass scale, which allows the membrane to have large surface area compared to its volume, enhancing nanofiber–PM interaction. At the same time, the breathability index can be improved Citation: Sanyal, A.; Sinha-Ray, S. through these PVDF nanofiber membranes due to their architectural uniqueness that promotes slip Ultrafine PVDF Nanofibers for flow around the fibers. The conductive nature of PVDF makes it advantageous as a promising electret Filtration of Air-Borne Particulate filter allowing better capturing of ultrafine particles. This review aims to provide a comprehensive Matters: A Comprehensive Review. overview of such PVDF nanofiber-based filter membranes and their roles in air filtration, especially Polymers 2021, 13, 1864. https:// its application in filtrate of air-borne PMs. doi.org/10.3390/polym13111864 Keywords: PVDF; ultrafine fibers; particulate matter; electret; slip flow Academic Editor: Debora Puglia Received: 10 May 2021 Accepted: 31 May 2021 Published: 3 June 2021 1. Introduction In the era of industrialization, automobile growth and over dependence of fossil fuel Publisher’s Note: MDPI stays neutral has led to a severe scenario of air pollution, one of the most severe threats to human race. with regard to jurisdictional claims in According to a data published by World Health Organization (WHO), about 90% of people published maps and institutional affil- breathe polluted air across the globe and air pollution alone is the sole cause of more than iations. seven million deaths every year, as per the same data [1]. From outdoor air pollution to indoor smoke, the situation has become grimmer every year leading to increased cases of respiratory damages. The extent of air pollution is generally quantified using a parameter called air quality index (AQI), based on primarily five components—sulfur oxides (SOx), nitrogen oxides (NOx), Copyright: © 2021 by the authors. carbon monoxide (CO), particulate matters (PM), and ozone (O3), along with volatile organic Licensee MDPI, Basel, Switzerland. compounds (VOC) and can be measured as stated in Equation (1) [2]. This article is an open access article distributed under the terms and (IHi − ILo) AQI = (CP − BPLo) + ILo, (1) conditions of the Creative Commons (BPHi − BPLo) Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ where P stands for the pollutant, CP is the concentration of pollutant, BPHi and BPLo are 4.0/). breakpoint concentrations greater and smaller compared to CP, respectively, IHi and ILo Polymers 2021, 13, 1864. https://doi.org/10.3390/polym13111864 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 1864 FOR PEER REVIEW 2 of 28 ()II− AQI=−+Hi Lo () C BP I ()− PLoLo, (1) BPHi BP Lo Polymers 2021, 13, 1864 2 of 26 where P stands for the pollutant, CP is the concentration of pollutant, BPHi and BPLo are breakpoint concentrations greater and smaller compared to CP, respectively, IHi and ILo are AQI values corresponding to BPHi and BPLo, respectively. The AQI is broadly divided into are AQI values corresponding to BPHi and BPLo, respectively. The AQI is broadly divided intofive fivecategories: categories: namely, namely, good good (0–50), (0–50), satisfac satisfactorytory (50–100), (50–100), moderate moderate (101–200), (101–200), poor (201– poor (201–300),300), very verypoor poor(301–400), (301–400), and severe and severe (401–500) (401–500) [2]. [2]. OutOut ofof allall thethe pollutants, pollutants, the the particulate particulate matters matters lead lead to moreto more pronounced pronounced and and sustain- sus- ingtaining danger danger to human to human health, health, as PMs as PMs are often are often a mixture a mixture of various of various solids solids and liquids, and liquids, with variouswith various chemical chemical composition, composition, dependent dependent upon the upon area the where area they where are they generated are generated [3]. The crop[3]. The stubbles, crop stubbles, vehicular vehicular emissions, emission and industrials, and industrial exhausts containingexhausts containing NOx, SOx ,NO aromaticx, SOx, hydrocarbons,aromatic hydrocarbons, metals, etc. metals, are the etc. constituents are the constituents of PMs, which of PMs, can which be broadly can be classified broadly intoclassified primary into type, primary i.e., air-borne type, i.e., particulates air-borne arisingparticulates from arising anthropogenic from anthropogenic sources, and secondarysources, and type, secondary i.e., gaseous type, precursor/pollutants i.e., gaseous precursor/pollutants [4]. PMs are mainly [4]. PMs divided are mainly into three di- categoriesvided into basedthree oncategories their sizes—PM based on10 their(≤10 sizes—PMµm), PM2.510 ((≤≤102.5 μµm)m),, PM and2.5 ≤(≤PM2.51.0 μm),(1 µ andm), especially≤PM1.0 (1 PMμm),0.1 especially(≈0.1 µm), PM are,0.1respectively, (≈0.1 μm), are, called respectively, thoracic, fine, called and thoracic, ultrafine fine, particles and [ul-5]. Althoughtrafine particles the AQI [5]. values Although are routinely the AQI reportedvalues are in routinely terms of PMreported10, sensitive in terms measurements of PM10, sen- cansitive include measurements PM2.5 measurements. can include PM As2 an.5 measurements. example: On December As an example: 30, 2019, On the December AQI values 30, across2019, the Delhi, AQI India, values varied across between Delhi, India, 430 and varied 490 asbetween an average, 430 and rendering 490 as an the average, atmosphere ren- severelydering the polluted atmosphere [6], where severely the primary polluted pollutant [6], where was the PM primary2.5. As per pollutant the WHO was guidelines, PM2.5. As 3 theper 24the h WHO average guidelines, limit of PM the10 24and h average PM2.5 should limit of be PM <5010 and and 25 PMµg/m2.5 should, respectively. be <50 and The 25 largerμg/m3 particulate, respectively. matters The tendlarger to particulate settle across matters the respiratory tend to settle tracts, across whereas the the respiratory ultrafine onestracts, tend whereas to penetrate the ultrafine the alveoli ones [ 7tend]. A schematicto penetrate is shownthe alveoli in Figure [7]. A1 toschematic demonstrate is shown the levelsin Figure of PM 1 to deposition demonstrate across the various levels depthof PM of deposition human respiratory across various tract. The depth alarming of human rise ofrespiratory PMs has beentract. found The alarming to cause respiratoryrise of PMs troubles,has been acutefound coronary to cause disorder respiratory (ACD), troubles, lung inflammation,acute coronary pulmonary disorder (ACD), disorders, lung cancer, inflammation, and even pulmonary death amidst disorders, children andcancer, elderly and peopleeven death [8]. amidst children and elderly people [8]. FigureFigure 1.1. DepositionDeposition ofof PMsPMs ofof differentdifferent sizessizes throughthrough aa humanhuman respiratoryrespiratory tract.tract. ReproducedReproduced withwith permissionpermission fromfrom Ref.Ref. [[3].3]. CopyrightCopyright Elsevier,Elsevier, 2015.2015. AlthoughAlthough thisthis reviewreview focusesfocuses uponupon thethe removalremoval ofof PMs,PMs, whichwhich werewere soso farfar classifiedclassified mostlymostly basedbased onon thethe effectseffects ofof industrializationindustrialization oror urbanization,urbanization, along with crop burning, etc.,etc., thethe effectseffects ofof bio-aerosols,bio-aerosols, originatingoriginating fromfrom coughs,coughs, sneezes,sneezes, oror regularregular breathing,breathing, cannotcannot bebe ruledruled out,out, especiallyespecially inin thethe backdropbackdrop ofof thethe recentrecent pandemic.pandemic. TheThe lifetimelifetime ofof aerosols/droplets may not be as long as PM generated from the earlier mentioned sources, but their effects can be lethal, which has been already established without an iota of doubt. These aerosols can easily be modelled similarly as PMs and hence the filtering mechanisms are expected to be similar to other anthropogenically created PMs. While the world is fighting with this pandemic, the growth of nonwoven fiber-based face mask has completely
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