Tunable Fabrication Of Nanofibrous Materials With High-efficiency And Low-resistance And Their Application In Air Filtration

The severe air pollution has been become a great threat to human health and social production in recent years.In terms of the human health,more than 3 million premature deaths are incurred each year wordwide due to the fine particles with an aerodynamic diameter less than 2.5μm,such as sulfates,nitrates,black charcoal and so on,which can penetrate to lungs and cardiovascular system,leading to an increse risk to suffer from cerebral apoplexy,heatr disearse,lung cancer and asthma.Moreover,the risk membranous nephropathy occuring will increas by 14% with each additional 10μg/m3 when the concentration of PM_2.5 higher than 70μg/m3,leading to more deaths result from cardiopulmonary disease.In terms of the production and life,leakage accident resulting from the collection of dust will cause devastation to power system.Moreover,the efficient operation of precise instrument will be hindered by the easy penetration of fine particles.Therefore,the design and fabrication of high efficient filter materials in order to pretect human beings and instruments has become a major issue releated to people’s livelihood and national strategic development.The main air filtration materials currently contain normal nonwoven fibrous materials,electret melt-blown fibrous materials and ultrathin glass fibrous materials.Nonwoven fibrous materials have high pressure drop due to its large fiber diameter and are incapable of capturing fine particles resulting from the large pore size,leading to its not using alone.Electret melt-blown fibrous materials can achieve high efficiency filtration in the case of not increasing pressure drop based on its electrostatic adsoption,however it’s hard to ensure the stability in that electric charge will decay soon under the action of humid and oily environments.Ultrathin glass fibrous materials can achieve efficient filtration towords fine particles on account of fine fiber diameter and small pore size,which produce a dense packing structure leading a high pressure on the contrary.Electrospun fibrous membranes are able to realize not only particle filtration but also gas transportation due to its thin diameter,small pore size,and importantly high porisity.Thus,electrospun fibrous materials have become a focus of research for the air filtration development.In this thesis,we pointed out that the key to solving challenges currently is to clarify the structure controlling principle and the strcture-activity relationship on the basis of summarizing electrospun air filtration materials’ research status.We launched the tunable fabrication of nanofibrous materials for effective air filtration and the research of structure-activity relationship.The air filtration membranes with slip-effect endowing the materials with low air resistance are controllably fabricated via the tuning of fiber diameter and pore size.The optimal regulation approachs to designing air filtration materials with thermal-wet comfort or negative ions releasing are mastered.Futhermore,the long-term electret air filtration materials is fabricated through the coordinate regulation of structure and surface properties.The detailed contents are summarized below.（1）The influence of sigle fiber diameter on air resistance is investigate via designing a series of high-porisity nanofibrous membrane with the same filtration efficiency（about 45%）and different average fiber diameter,aiming to settle the challenge of reducing the air filtration materials’ pressure drop.The pressure drop reduced from 15 Pa to 9 Pa when the fiber diameter refined from 168 nm to 71 nm,however the pressure drop increased by 10% when the fiber diameter refined from 71 nm to 60 nm,furthermore,the pressure drop increased by 20% when the fiber diameter refined from 60 nm to 53 nm.However the pressure drop reduced throughout with the fiber diameter reducing from 168 nm to 50 nm through the simulated data,which proves the pore structure due to the fibers lapping is another factor influcing the pressure drop.The structure-activity relationship between fiber assemblies and slip-effect is revealed through the introduing of an equilibrium factor τ = df/d2,then the optimal range of pore size（> 3.5 μ m）for the slip-effect was determined by means of studying the changing law of pressure drop with different fiber diameter under various filtration efficiency level（44-45%,66-68%,86-88%,95-96%）.Ultimately,the new material displayed high purification efficiency of 99.09%,low air resistance of 29.5 Pa,which also contributes to a short removal time of 15 min for PM_2.5 purified from 500 μg m-3 to 35 μg m-3.This work is expected to provide an efficient approach to designing improved filtration materials.（2）A novel strategy was developed to solving the problem of insufficient thermalwet comfort of air filtration materials.On the one hand,we employ super-hydrophilic PAN/Si O2 fibers to ensure efficiently transferring moisture,on the other hand,we employ hydrophobic PVDF fibers as the repellent components to avoid the formation of capillary water under high humidity which contributes to constant pressure drop.The relation between jet ratio of PVDF/PAN and filtration property is investigated,showing that the filtration efficiency and pressure drop will rise in 60 min with the increse of PAN.The moisture permeable mechanism is established and the optimal ratio（2/4）of jet is obtained via studying the change of water adsorption and diffusion under ratio of jet.The effect of Si O2 nanoparticles with different concentration of 0.5,1,2,5 wt% on moisture vapor transmission rate and filtration property under different humidity is investigate,proving that the moisture vapor transmission rate increase with the increasing of Si O2,and the filtration can achieve the highest of 99.215 when the concentration is 2 wt% under the same pressure drop.More importantly,by constructing a hydrophobic to super-hydrophilic gradient structure,the moisture vapor transmission rate can reach 14 066 g m-2 d-1,moreover the time required to achieve a balance shortened to 30 min when the moisture transmit from hydrophobic region to hydrophilic region compared to the reverse direction of 47 min.A field test in Shanghai shows that the air filter maintains stable PM_2.5 purification efficiency of 99.99% at high MVTR during haze event after 10 h.More importantly,the resultant fibrous membranes presented a low air resistance of 86 Pa,which is recoverable after easily cleaning.The successful fabrication of such material may provide a new approach for the design and development of breathable and cleanable materials for various applications.（3）A noval low-resistance air filter capable of releasing negative ions（NIs）and efficiently capturing PM_2.5 was prepared in order to deal with the limitation of current materials difficule to purify air actively.The polymer type including PSU,PVB,PVDF,fiber diamete,pore size is considered to investigate the changing law of pressure drop and the releasing amounts of negative ions（RANI）,exhibiting that the pressure drop will decrease when the fiber diameter is reduced,while that will increase with the pore size reducing,and the RANI of PVDF is higher than PSU and PVB.In addition,by regulating the concentration of PVDF and the doping contents of NIPs,a membrane with high PANI of 2818cc-1 and optimized filtration efficiency of 99.99% is obtained,which also exhibits a low pressure drop of 40.5 Pa and a short time of 13 min for PM_2.5 reduced from 500 μg m-3 to 35 μg m-3.This innovative material may provide a new insight for the design of function integrated air filtration materials.（4）We present a robust method for creating long-term electret air filtration materials possessing hydrophobic and oleophobic property by using PVB as the matrix polymer,Si₃N₄ nanoparticles as an inspiring charge enhancer and FPU as the hydrophobic and oleophobic modifier through the in situ charging technology of electrospinninig.The effect of different Si₃N₄ nanoparticles doping amount of 0,0.5,1,2 wt% on the surface charge is investigated,showing that the membrane obtained with a doping amount of 1 wt% exhibits the best charge stability whether under different humidity or after oil processing.The electret property is optimized by introducing FPU and regulating the amout of which,resulting a 2.86 times promotion of suface potential under 99% humidity when the FPU is 2 wt% doped compared to the 0 wt%.Ultimately,the new material displayed high filtration efficiency of 99.9408% and 97.2498%,low pressure drop of 63.05 Pa and 64.05 Pa towards Na Cl and DEHS aerogels respectively after a 5 h loading test.More importantly,the materials obtained can achieve the removal of PM_2.5 within 11 min from 500 μg m-3 to 35 μg m-3.This work develops a new approach for the design of electret air filtration materials.