| As current research and development of filter materials are based on improving efficiency and reducing resistance, the content of the thesis is mainly divided into the following three aspects:(1) Study the existing nonwoven filter materials on the market, further experiments and comparative analyses were conducted based on an imported needled filter made of flat fibers, which has high efficiency and low resistance and is significantly better than the needled filter of circular-cross section fibers.(2) A new filter using flat fiber was developed by using PTFE crack fibers. To solve processing difficulties of PTFE fibers, PP fibers were blended with PTFE fibers before carding and needling reinforcement. The following two aspects were systematically studied: the influence of PTFE and PP blended ratio and needle punching parameters on the performance of filter material.(3) Using through needle punching alone, there is still large pore in filter material. Therefore, the thesis mainly studied pure PP needle-punched filter and PTFE/PP composite filter, the two needle-punched filter materials were finished by PTFE emulsion impregnation forming a micro-porous surface on the filter. The effects of four different parameters on filtering efficiency, including emulsion concentration, impregnation time, curing temperature and time were studied. An orthogonal experiment was designed to optimize the processing parameters.The two types of filter materials were also compared to provide the experimental reference for the research of PTFE emulsion impregnation process.Results from the above research suggested that:(1) The flat fiber was experienced further fibrillation during needle punching. The coarser fibers were found to act as the skeleton to ensure the fabric thickness, while the surface of fibrillation fiber was increased, which was conducive to capture tiny particles. The resistance of filter material showed a slight increased, whereas the filtration performance was increased greatly.(2) Hydrophobic PP fiber could not solve PTFE fiber carding problems, but hydrophilic PP fibers were suitable to be used as the raw material of blended filter material; Under similar fineness, 3.3dtex PP fiber was better than 2.2dtex and 4.4dtex PP fibers regarding comprehensive factors such as filtration efficiency and carding complexity;(3) With the increasing of PP fibers, carding process became easier, but the filtering performance declined. When the PTFE to PTFE ratio was set as 70:30, optimal processability and filtration performance were achieved;(4) In the needling process of PTFE/PP composite filter material, the thickness decreased with the increase of needle depth, number of needle punching and needle density. However a critical value was observed. Beyond the critical value, fibers would be damaged by the excessive needling force. Therefore, the filtration efficiency would be reduced when the needle-punched process parameters were beyond the critical value.(5) Comparative Analysis before and after PTFE emulsion impregnation was performed. After impregnation, the average pore diameter decreased, and the aperture distribution area became concentrated, the filter material was more even and filtration efficiency has improved significantly.(6) Comparative Analysis the two kinds of filter material with PTFE emulsion finishing showed that for PTFE/PP compound needled filter material with PTFE emulsion impregnation, primary and secondary order of the influence factors were as follows: curing time > curing temperature > impregnation time > emulsion concentration; For pure PP needled filter material with PTFE emulsion impregnation, primary and secondary order of the influence factors were as follows: curing time > emulsion concentration > impregnation time > curing temperature; The optimal parameters of the orthogonal experiment were also inconsistent between the two filters. This was true to the fact that the finishing of PTFE/PP composite filter material, less PTFE emulsion but higher baking temperature were needed; After emulsion impregnation, the improvement rate of filtration efficiency of PTFE/PP compound filter was twice than that of pure PP filter, indicating the former as a promising candidate for industrial air-filtration. |
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