Preparation And Separation Performance Of Asymmetric Structure PTFE Hollow Fiber Membrane

Textile filter materials, as a significant variety of technical textiles, have become one of the important directions of textile technology developments. Low filtration efficiency of the conventional textile filter materials highlights its limitations in separation fields. However, Pore size controllability of new fiber membranes is excellent and such membranes have better separation performance.Polytetrafluoroethylene has great potential in membrane manufacturing due to its temperature and chemical resistance. Micropore structure of PTFE sheet membrane prepared by the biaxially stretching operation can be easily manipulated. Nevertheless, pore size and porosity of PTFE hollow fiber membrane cannot be controlled simultaneously via the uniaxially stretching operation, the internal cause of which was barely researched, together with asymmetric PTFE membrane. In addition, PTFE membrane cannot be used in the liquid-solid separation owing to its intrinsic hydrophobicity. Meanwhile, the intrinsic hydrophobicity is not up to requirements of vacuum membrane distillation(VMD). Therefore, those drawbacks mentioned above severely confined the application of PTFE hollow fiber membrane.In view of the above situation, a novel idea that PTFE hollow fiber membrane was wrapped by the PTFE sheet membrane was put forward to fabricate the asymmetric PTFE hollow fiber membrane and solve the problem about the manipulation between the pore size and porosity in this work. In addition, bifunctional materials containing the hydrophilic and reactive groups were chosen to achieve the surface hydrophilic modification of PTFE hollow fiber membrane, which endowed the stable hydrophilicity, making the membrane be available in liquid-solid separation. Finally, ultra-hydrophobic modification was achieved using heat treatment to meet the requirements of VMD. Relationship between the structures and performances was studied by FESEM, AFM, pore size measurement, gas-solid separation, etc. The main contents and conclusion were as follows,1. To solve the problem about the pore size and porosity manipulation, a novel idea that PTFE hollow fiber membrane(as the support layer) prepared by the biaxially stretching operation was wrapped by the PTFE sheet membrane(as the skin layer) prepared via the uniaxially stretching operation was put forward to fabricate the asymmetric PTFE hollow fiber membrane on the basis of research into the micropore structure evolution in stretching operation(biaxially and uniaxially operations) and Casting-Lyophilization-Calcining(CLC) process.Firstly, micropore structure evolution of PTFE membranes in biaxially and uniaxially stretching operation is comparatively studied from the angle of micropore formation. The findings are as follows:(1) For the PTFE sheet membrane prepared by the biaxially stretching operation, PTFE fibrils and nodes form in the longitudinal stretch. Then the bidirectional split of node takes place in the subsequent transverse stretching operation, leading to the formation of new fibrils and slant of fibrils formed before, which fabricates the cross structures of fibrils. Therefore, pore size and porosity can be simultaneously manipulated by the biaxially stretching operation.(2) For the PTFE hollow fiber membrane prepared by the uniaxially stretching operation, the nodes split unidirectionly and fibers elongate monofonically, which is the internal reason for that pore size and porosity of PTFE hollow fiber membrane cannt be well controlled.Secondly, the advantage of asymmetric structure in micropore structure control was confirmed using the CLC process. Asymmetric structure on the surface of PTFE sheet membrane is fabricated when the calcining temperature is 190 oC, which decreases the maximum pore size and keeps the intrinsic porosity simultaneously. These results provide a scientific reference for preparing the asymmetric PTFE hollow fiber membrane.Thirdly, preparation of asymmetric PTFE hollow fiber membrane by wrapping method was studied. The findings are as follows:(1) To ensure that the support layer can be totally coated by the skin layer(Seamless wrapping), a relationship must be followed: ≥ (w, r and φ refer to the width of the skin layer, outer diameter of the support layer and wrapping angle, respectively); Meanwhile, transverse shrinkage of the skin layer must be controlled by adjusting the transverse stretching ratio, heat setting and sticking parameters to ensure positive lap width.(2) To achieve the non adhesive bonding, it is necessary to control the tension between the support layer and the skin, together with the second fibrillation: the skin layer would suffer the tension due to the different shrinkage, which leads to the second fibrillation at the interface between the support layer and the skin layer, resulting in the non adhesive bonding. Peeling strength increases as the increase of longitudinal stretching ratio, transverse stretching ratio and wrapping tension because of the enhanced fibrillation at the interface.The maximum pore size can be reduced by the wrapping method and the intrinsic high porosity of the support layer is kept at the same time, which can be verified by the gas-solid separation experiment: The skin layer can not only retard the blocking of inner micropore but also increase the collection efficiency, which plays the decisive role in separation.Therefore, asymmetric hollow fiber membrane can be fabricated by the wrapping method and the problem about the pore size and porosity manipulation is solved, which achieves the purpose of high performance of hollow fiber membrane.2. To meet the requirements in liquid-solid separation, surface hydrophilic modification of asymmetric PTFE hollow fiber membrane was carried out by surface coating and in situ polymerization. Effects of such two ways on the hydrophilicity and micropore structures were studied, together with the influence of asymmetric structure on the liquid-solid separation: Water contact angle(WCA) decreases after surface coating using polyacrylic acid(PAA) and increases after a long-time water washing process because the PAA on the surface of membrane falls off. Meanwhile, micropore blocking would be induced owing to the high-viscosity PAA solution, leading to the reduction of porosity. However, low-viscosity acrylic acid would penetrate around the fibrils and nodes. The fibrils and nodes would be coated by the obtained PAA after polymerization, which is similar to the “rattan wrapped around the tree” structure. Stable hydrophilicity can be obtained because PAA is firmly tangled on the surface of fibrils and nodes. Furthermore, the micropore structures remain unchanged. When the membrane modified by the in situ polymerization is applied in the liquid-solid separation, the skin layer of the asymmetric structure can not only avoid the blocking of inner micropore but also reject the solid particles, which further retards the flux decline and increases filtration precision. Therefore, persistent and stable hydrophilicity can be provided by the in situ polymerization and the liquid-solid separation performance could be significantly enhanced by the asymmetric structure.3. To meet the ultra-hydrophobic requirement in VMD, modification of membrane was achieved using heat treatment. Effect of the modification on the hydrophobicity and its internal reason was studied. In addition, the role of ultra-hydrophobicity and asymmetric structure in VMD experiment is discussed. The findings are as follows: WCA reaches at 155 o when the membrane is treated at 380 oC for 2 min, indicating the enhanced hydrophobicity. Surface tension of materials is closely related to the crystalline structure. The surface tension of crystal phase is higher than the amorphous phase. The crystallinity of membrane decreases from 90 % to 65 % after heat treatment, which means that the amorphous phase increase and brings about the low surface tension and enhanced hydrophobicity. VMD separation results indicates that membrane wetting and feed penetration phenomenon are avoided due to the less adsorption of JFC(a fatty alcohol polyoxyethylene ether, surfactant) on the membrane surface, which ensure that the low permeate conductivity. Meanwhile, feed penetration induced by the high vacuum pressure would be avoided owing to the high LEP, resulting in the low conductivity.