Preparation Of Fibrous Nonwovens With High Porosity And Their Application In Air Filtration

Rapid industrialization and urbanization are always accompanied with terrible particulate matter（PM）in air.In history,the extreme environmental pollution incidents caused by air pollution events have caused millions of premature deaths.Unfortunately,PM has still been one of the leading killers of human nowadays.A report from the World Health Organization（WHO）in 2016 indicated that up to 92%of the world’s population is still exposed to polluted air.The governance of PM issue involves a wide range of social dimensions,therefore it is a high-difficulty and long-term project.Currently,using air filtration materials to filtering the particles in air has become the most convenient and direct measure that can efficiently protect human health from severe particle pollution.Nonwoven fibers show more advantages in air filtration,compared with other forms of air filters,due to their wide adjustable range of fiber diameter and abundant tortuous pore channels.Among the various methods for fabrication of nonwoven fibers involving spunbonding,blowing,melt blowing,and electrospinning,blowing and melt blowing are wildly used to fabricate glass fibrous filters and polypropylene fibrous filters.Glass fibers show structural advantages of small fiber diameter and pore size,as well as the property advantages of thermal stability and anti-corrosion,enabling them to capture fine particles efficiently at high-temperature condition.However,Glass fibers generally show compact assembling behavior,which causes a undesired high energy consumption.In addition,the intrinsic features of brittleness and carcinogenicity of glass fibers limit their application.Electret melt-blown fibers show electrostatic effect,which is beneficial to capture particles while generates negligible effect on airflow.Therefore,electret melt-blown fibers can easily achieve the high air filtration performance.However,electret melt-blown fibers are faced the challenges of easy decay of electrostatic effect and tedious procedures due to the mutual independence of fiber formation and electret treatment.More recently,nonwovens composed of micro-and nanofibers fabricated from electrospinning technology attract extensive attention of researchers,which is owing to the small fiber diameters,interconnected porous channels,and controllable packing structures,as well as the one-step fabrication of electret fibers due to the impose of high-voltage electric field.Although electrospun micro-and nanofibers exhibit promising application potential in the field of air filtration,there still exist some bottlenecks to be solved.First,although achieving excellent filtration efficiency,electrospun fibers generally exhibit high pressure drop due to the relatively low porosity（<85%）caused by their ultrafine diameter.Meanwhile,the dense structure of the electrospun membranes also decreased contribution of electrostatic effect on the filtration efficiency.Second,the current study of electret fibers commonly focuses on polymer/nanoparticle systems,nanotoxicity of which possesses tremendous safety risks and the electret mechanism of electrospun fibers is not revealed.Finally,electrospun fibrous materials often exhibit two-dimensional membrane structure,which resulted in low dust capacity and dramaticlly increased pressure drop during the air filtration.However,the current study always focused on the initial filtration efficiency and pressure drop of electrospun fibrous filters,while ignoring their long-term performance.To solve the bottlenecks of air filtration materials at different application occasions,this paper reports various nonwovens composed of electrospun micro-and nanofibers with high porosity.This paper puts emphasis on the fabrication of electrospun fibrous filters with high porosity,the effect of fibrous aggregates and electrostatic effect on the filtration performance of the fibrous filters,the electret mechanim of electrospun fibers with polymers/nanoparticles system and bi-component polymer system.Finally,we study the comprehensive properties of electrospun fibrous filters from the aspect of practical application.The main conclusions are as follows:（1）High-porosity nanofibrous nonwovens with a wool-like self-crimp structure was prepared,which we call"nano wool".Firstly,based on the existing electrospinning theory of non-solvent induced phase separation,the impact of environmental humidity on the formation of crimp fibers during electrospinning was clarified,namely,the high-humidity condition would promote the rapid solidify of charged polymer jets,leading to the insufficient stretching of molecular chains.Then,the residual stress could cause the molecular chains to retract,and then deposited as the crimping fibers.Based on this theory,polyvinylidene fluoride（PVDF）was selected to study the influence of spinning humidity on the morphology and structure of single fibers and fibrous assemblies.Electrospun PVDF fibers were obtained at the relative humidities of 30%,60%,and 90%,and the single fiber showed a bead-on-string structure,a slender and smooth cylindrical structure,and a crimped structure,respectively.The average fiber diameter increased from 307 to 757 nm,and the porosity of fibrous assemblies improved from 70.7%to 98.7%.The increase in porosity is due to the“spatial support effect”brought by the crimped structure,namely,the crimped structure makes the occupied volume of the single nanofiber expand from a two-dimensional plane to a three-dimensional space.The pressure drop of the crimped PVDF nanofibrous material was reduced by more than one third than the non-crimped fiber material when they had the same filtration efficiency of 85%.In addition,the crimped structure makes the fiber exhibit a phenomenon of coil lasso,which enhanced the tensile strength and elongation of the material,providing a new insight for fabricating mechanically robust high-porosity fiber materials.（2）Based on the preparation of wool-like crimped PVDF nanofibrous material,a high-porosity crimped PVDF/hydroxyapatite（HAP）electret was further prepared by introducing HAP nanoparticles into the electrospinning solutions as an electret effect enhancer.The incorporation of HAP nanoparticles was to further improve the air filtration performance.Firstly,the influence of the content of HAP nanoparticles on the morphology and assembling structure was investigated.It was found that when the content of HAP nanoparticles was increased from 0 to 0.5 wt%,the crimped structure of PVDF/HAP fibers could be well retained,meanwhile,the diameter decreased from 757to 578 nm and the high porosity feature was almost unchange.The diameter of PVDF/HAP nanofibers was two orders of magnitude lower than that of natural wool fibers.More importantly,PVDF/HAP nano wool showed a unique electret effect with a high surface potential of 13 k V.In this process,the electric field intensity around the single fiber was calculated,based on which,the mutual exclusion among electret fibers driven by the electrostatic force was proposed.This is the first time that the relationship between electret effect and porosity of the material was established.The introduction of 0.5 wt%HAP nanoparticles increased the filtration efficiency of PVDF nano wool from 84.32%to 99.952%,and the pressure drop remained almost unchanged.By regulating the basis weight,PVDF/HAP nano wool could achieve high PM_0.3 removal efficiency of>99.5%,>99.95%,and>99.995%,the corresponding pressure drop were 33 Pa,50 Pa,and 55Pa,respectively.The pressure drops of PVDF/HAP nano wool were lower than the commercial and the already reported electrospun filter media,implying its advantages of low energy consumption.（3）On the basis of the high-porosity PVDF/HAP electret fibers,a new idea of coupled polarization between two polymers was proposed,based on which,high-porosity electrospun fibers showing strong electret effect were obtainin by taking advantage of the coupling effect of the polarization behaviors of polymers with different dielectric properties for the first time.The preparation of all-polymer electret fibers overcomes the health risk of the existing polymer/nanoparticle electrospun electret fiber with nano-toxicity.First,six easy-spinning polymers with different dielectric properties were selected,including polystyrene（PS）,bisphenol A polysulfone（PSU）,polyvinyl butyral（PVB）,polyacrylonitrile（PAN）,PAI and PVDF.The electret mechanism dependent of polymers’dielectric properties was investigated.The experimental results implied that the space charges play a dominant role in weakly polar polymers,typical for PS,and the electret effect came from the injection of charges;the dipole charges play a dominant role in the strong polar polymers,typical for PVDF,the electret effect came from the orientation of the dipoles.Furthermore,PVDF/PS composite fibers with a high porosity of 94.4%were prepared by introducing PVDF into the PS solution as a charge enhancer.It was found that PVDF/PS composite fibers showed enhanced electret effect owing to the coupling of polarization behaviors between PS and PVDF,and PVDF was evenly distributed in the composite fibers.Compared to the pure PS fibers,when the weight ratio of PVDF/PS was 14/2,the surface potential and the filtration efficiency of the fiber assemblies increased by 36%and 12%,respectively.The pressure drop was only 0.01%of the standard atmospheric pressure（<10Pa）.By changing the basis weight of the PVDF/PS composite fibers,filter media with a filtration efficiency of 99.983%and a respiratory resistance of32 Pa were obtained.The resulting materials have met the protective requirement of N95 masks,and showed the advantage of structural continuity over the existing electrospun electret fibers composed of polymer/nanoparticles.Thus,the fabricated materials eliminate the health risk of nano toxicity.（4）based on the fabrication of high-porosity two-dimensional membranes,high-porosity three-dimensional nanofibrous aerogel（NFA）with superelasticity was fabricated by introducing the semi-interpenetrating polymer networks（semi-IPNs）into the preparing process of three-dimensional nanofibrous architecture.First,polyamide-imide（PAI）was selected as the fiber matrix,and bismaleimide（BMI）monomers with the different stiffness were selected as the crosslinking agents,including N,N’-1,4-phenylenedimaleimide（PDM）,N’-4,4’-diphenylmethane-bismaleimide（BDM）,and 2,2-bis4-（4-maleimido-henoxy）phenylpropane（BMP）.Different semi-IPNs were in situ constructed within fibers by triggering the self-polymerization of different BMIs.The compressible property of aerogels was tested,indicating that PAI/PDM NFAs showed the highest Young’s modulus（12 k Pa）,the highest compressive stress（7.9 k Pa）,and the lowest plastic deformation（1.4%）,and the greatest thermal stability.The mechanical and thermal performances of NFAs were associated with the multi-level structures involving the intramolecular rotation of monomers,crosslinking density of macromolecular networks,and the mechanical responses of single fiber and fibrous assemblies.Based on the enhancement of mechanical properties,the assembling structure of PAI/PDM NFAs was further regulated.By constructing a gradient porous structure in the filtration direction,PAI/PDM NFAs were endowed with a cascade filtration behavior.This behavior meant that the selective and stepwise removal of polydisperse PM_2.5,so that the materials could have a prolonged service life.In this process,the empirical formula of pressure drop suitable to the three-dimensional fibrous aerogels was revised,which enriched the existing theories suitable to the two-dimensional fibrous membranes.The resulting gradient structured semi-IPN NFAs had excellent comprehensive filtration performance,including high PM_0.3 removing efficiently（99.97%）,low pressure drop（only 50%of the membrane material）,good thermal stability,and high dust-holding capacity(114 g m^-2).In a simulated high-temperature polluted gas emission experiment,NFAs can rapidly reduce the concentrations of PM_2.5 and PM₁₀ by 83.6%and 93.5%within 2 minutes.The size and shape of NFAs could also be controlled easily,showing promising application prospects for dealing with the high-temperature industrial waste gas.