| Suspended particulate matter（PM）in the air is a great threat to human health and social production.At present,the terminal filtration method using filter materials is considered to be an effective way to control pollution.In the field of air filtration,polytetrafluoroethylene（PTFE）fiber membrane is currently only used in high-temperature-resistant bag filters.The defects such as single preparation method and product,and the control of fiber membrane microstructure are still the bottlenecks restricting the widespread application of fiber membrane.In this thesis,we first used CATIA to establish three-dimensional models of fiber assemblies with different structures,and then used STAR-CCM+software to simulate and analyze their air filtration performances for particulate matters,which provided guidance for the preparation and structural optimization of PTFE fiber membrane air filtration materials with different structures.On this basis,the PTFE fiber membrane materials were prepared by three methods.After this,we systematically studied the relationship between the fiber diameter and pore structure of PTFE fiber membrane and its air filtration performance,and to provide theoretical support for the practical application of PTFE fiber membrane air filtration materials.The specific work is as follows:（1）Numerical simulation of air filtration of fiber assemblies with different structures.we first used CATIA to establish three-dimensional models of fiber assemblies with different structures,and then used STAR-CCM+software to simulate and calculate the pressure distribution and velocity field of the model.The influence of the structure of fiber assemblies on particle filtration performance was studied by CFD-DEM method.The focus of the study was on the influence of arrangement,diameter,spacing,surface roughness structure of fibers in the fiber assembly and the thickness,asymmetric structure of the fiber assembly on its air filtration performance.The results showed that the uniformly arranged fiber assembly had a uniform pore structure,and its internal static pressure and fluid velocity were evenly distributed,which was helpful to improve the filtration performance of fiber assembly.In a uniformly arranged fiber assembly,with the increase of fiber diameter and thickness and the decrease of fiber spacing,the filtration efficiency and resistance pressure drop increased.The asymmetric structure of the fiber assembly could realize the organic combination of high filtration efficiency and low resistance.The rough structure of the surface effectively increased the surface area of the fiber,which made the particles have more chances to collide with the fiber,and the filtration performance of the fiber assembly was enhanced.（2）Preparation of PTFE plate fiber membrane with randomly stacked fibers by centrifugal spinning.The mixed solution of PTFE emulsion and polyvinyl alcohol（PVA）was used as the spinning solution to prepare the membranes with randomly stacked fibers by centrifugal spinning,and the PVA was removed by sintering to obtain the PTFE fiber membrane.The effects of PVA concentration in the spinning solution,spinneret speed,spinneret needle type and sintering temperature on the morphology and pore structure of PTFE fiber membrane were investigated.The internal relationship between morphology,pore structure,base weight of the PTFE fiber membrane and its air filtration performance was analyzed.The results showed that the diameter of the prepared PTFE fiber ranged from 0.8μm to 1.5μm.The fiber diameter and the pore size of the membranes increased with the increase of the PVA concentration in the spinning solution and the inner diameter of the spinneret needle,but decreased with the increase of the spinner’s speed and sintering temperature.The PTFE fiber membranes with smaller fiber diameter and pore size,larger base weight had higher air filtration efficiency,but the correspondingly higher resistance pressure drop.When the PVA concentration was 7.0 wt%,the spinneret speed was 3500 rpm,the spinneret needle type was 30 G,and the sintering temperature was 380℃,the fiber membrane with a base weight of about 15 g/m2 had the best filtration performance.This membrane had a high filtration efficiency（95.3475%）and a low pressure drop（98.34 Pa）for 300 nm sodium chloride aerosol particles at a 32 L·min-1 airflow velocity,and the quality factor reaches0.03 Pa-1.PTFE plate fiber membrane with randomly stacked fibers prepared by centrifugal spinning had potential application value in high temperature resistant bag dust collector.（3）Preparation of PTFE hollow fiber membrane with asymmetric structure by the axial stretching method.Using PTFE resin as raw material,the asymmetric structure PTFE hollow fiber membrane was prepared through a cold pressing method including mixing,pressing,paste extrusion,uniaxial stretching and heating.The effects of extrusion,stretching and sintering processes on the morphology and pore structure of PTFE hollow fiber membrane were investigated,and the effects of morphology and pore structure on the air filtration performance of PTFE hollow fiber membrane were analyzed and the air filtration mechanism was explored.The results showed that the asymmetric PTFE hollow fiber membranes with a fibril diameter in the range of 0.4-0.6μm could be prepared by the axial stretching method.The membrane surface had a microstructure composed of fibrils and nodes.By changing the extrusion cone angle,different apparent densities were obtained on the inner and outer surfaces of the PTFE hollow tube,resulting in shorter and less fibrils on the outer surface of the membrane than on the inner surface,forming an asymmetric structure,ensuring the filtration efficiency and higher flux of the membrane.With the increase of extrusion cone angle,the difference in the length of the fibrils on the inner and outer surfaces of the membrane increased.As the stretching ratio and sintering temperature increase,the fibril length elongated,and the pore size and porosity of the membrane increased gradually.When the pore size and porosity of the membrane increased,the air flow through the membrane would increase,but the penetration of particles would also increase.When the extrusion cone angle was 60o,the stretching ratio was 450%,and the sintering temperature was 380℃,the PTFE hollow fiber membrane had the best filtration performance.Under the negative pressure of 300 Pa,the air volume generated per unit membrane area was 18.5 m3/m2·h-1,the filtration efficiency for particles with a particle size of 2.5μm was 99.9972%,and the filtration efficiency for particles with a particle size of 0.3μm was 97.7326%.The membrane was reusable,and its air filtration performance recovery rate reached 80%after backwash cleaning.PTFE hollow fiber membrane had obvious characteristics of high efficiency and low resistance,and had application prospects in the field of indoor air filter materials.（4）Preparation of PTFE tubular fiber membrane with grid pore by wrapping method.The gridded porous PTFE sheet membranes prepared by vertical and horizontal bidirectional stretching method were selected as the substrate and wrapped on the tubular support to prepare PTFE tubular fiber membrane.Firstly,the influences of the morphology,pore structure,thickness and other parameters of PTFE sheet membrane on its air filtration performance were analyzed,and the relationship between the diameter,length and number of wrapping layers of the prepared tubular fiber membrane and its air filtration performance was further studied.Finally,the effect of the arrangement of PTFE tubular fiber membrane on its anti-fouling performance was investigated by CFD simulation.The results showed that the fibril diameter in the PTFE sheet membrane for wrapping ranged from 0.04 to 0.2μm.The filtration efficiency curve for particles with particle size of 10 nm to 300 nm presented a typical"V"shape.At higher surface filtration rate,the membrane with smaller pore size had smaller most permeable particle size（MPPs）,and the membrane with smaller pore size and larger thickness had higher filtration efficiency.Under the condition of 300 Pa negative pressure,the air flow through the PTFE tubular fiber membrane increased with the increase of diameter,but decreased with the increase of thickness and length.On the contrary,the filtration efficiency of the particles increased with the increase of diameter,but increased with the increase of thickness and length.When the particle size was larger than the pore size of the membrane surface,the fiber membrane behaved as surface filtration,on the contrary,the fiber membrane behaved as deep filtration mechanism.The CFD simulation results showed that the staggered arrangement of tubular fiber membranes had better antifouling effect than the aligned arrangement.The gridded porous PTFE tubular fiber membrane prepared by wrapping method had potential application value in air filtration in the fields of electronics,semiconductor,food,medicine and so on. |
