Study On Polyvinylidene Fluoride Fiber Homogeneous Reinforced Hollow Fiber Membran

With the development of membrane separation technology,the hollow fiber membrane is widely used in the field water treatment because of its excellent properties.However,with the continuous expansion of the applications,the hollow fiber membrane prepared by traditional methods can no longer meet the needs of practical application.For example,in the membrane bioreactor system,the homogeneous hollow fiber membrane is prone to breakage due to the fluid disturbance or impact during aeration and backwashing,which affects the reliability of the system operation.Therefore,the heterogeneous fiber usually was as reinforcement.For example,using polyester（PET）braided tube as reinforcement,composite poly（vinylidene fluoride）（PVDF）functional layer on braided tube surface,can effectively improve the membrane tensile strength.However,when it runs for a long time in sewage,interface slippage or stripping occurs easily between the surface functional layer and the heterogeneous reinforcement,which would affect the service life of the membrane.Meanwhile,the recycling process of the abandoned heterogeneous reinforced hollow fiber membrane is also complicated.Therefore,how to have good mechanical properties,interface bonding properties,and conducive to the utilization of waste membrane,while maintaining the original properties of hollow fiber membrane,has attracted much attention.In this research,PVDF was used as the raw material to prepare PVDF fiber,and the fibers with different integration modes were used as reinforcements to prepare homogeneous reinforced PVDF hollow fiber membranes through compounding PVDF functional layers on the surface of the reinforcements.The fiber structure and properties,fiber reinforced mechanism,pore structure regulation and membrane properties were analyzed and discussed,and the applications of membrane in water treatment and oil-water separation were also discussed.In order to prepare homogenous reinforced PVDF hollow fiber membrane with PVDF fiber,PVDF monofilament and multifilament that can be used as reinforcement must be obtained.Therefore,the PVDF fibers with different spin-stretching ratios and post-stretching ratios were fabricated via melt-spinning method.The changes of the structure and properties of PVDF fibers during different stretching processes were studied to provide experience for the subsequent preparation of PVDF monofilament and multifilament with spinning-stretching one-step method.The results showed that the macromolecular chain orientation,crystal orientation and crystallinity of PVDF as-spun fibers all increased nonlinearly with the increase of spin-stretching ratio.When the spin-stretching ratio was 330,the orientation factor of sound velocity was 0.59,the crystal orientation of PVDF as-spun fibers reached 96.01%and the crystallinity reached56.38%.When the post-stretching ratios increased,the crystallinity of PVDF fiber changed slightly,but the crystal phase changed significantly.Compared with the spin-stretching process,the post-stretching process is more beneficial to promote the transformation ofαphase toβphase.When the post-stretching ratio was 11,the PVDF fiber with highβcrystal content（82.85%）could be obtained.In order to improve the mechanical properties and interfacial bonding property of PVDF hollow fiber membrane,the PVDF monofilaments with different structures were firstly prepared by spinning-stretching one-step method as reinforcement.Then,based on the principle of chemical fiber island composite spinning,the embedded homogeneous（three）monofilaments reinforced PVDF hollow fiber membrane was prepared by dry-wet spinning technology.The PVDF monofilaments parallel to the axial direction of the membrane were evenly spaced in the membrane wall along the cross section of the membrane.The results revealed that the tensile properties and the interfacial bonding property of PVDF hollow fiber membrane were improved by embedding three PVDF monofilaments（I_β/I_αis about 4000%）into the membrane wall.The tensile strength can reach 8.3 MPa,which was about 4 times that of the homogeneous PVDF hollow fiber membrane.The pull-out strength reached 4.48 N,which was about 16 times that of heterogeneous（PET）monofilament reinforced PVDF hollow fiber membrane.The separation performance of PVDF hollow fiber membrane was basically the same as that of homogeneous PVDF hollow fiber membrane.The bovine serum albumin permeation flux was 54.09 L·m^-2·h^-1,and the rejection rate was98.04%.In addition,the structure and properties of the membrane can be adjusted by selecting PVDF monofilament with different crystal structures as reinforcement according to practical requirements.In order to solve the problem that the embedded monofilament reinforced PVDF hollow fiber membrane is easily damaged by bending or shaking for a long time,the PVDF multifilament was prepared by spinning-stretching one-step method,and then make PVDF braided tube as reinforcement.Based on the principle of chemical fiber skin/core composite spinning,PVDF functional layer was compounded on the surface of braided tube by dry-wet spinning technology to prepare lined homogeneous braid-reinforced PVDF hollow fiber membrane.During membrane preparation process,the dioctyl phthalate（DOP）was added into PVDF/N,N-dimethylacetamide（DMAc）casting solution as a pore structure regulator.The results showed that when the content of DOP was 13 wt.%,the pore structure formed by double diffusion of DMAc and water interlaced with the pore structure formed by DOP movement and occupation,forming a porous structure with good connectivity,high permeability(kerosene flux>800 L·m^-2·h^-1).The kerosene flux was about 1.7 times of the commercial oleophilic and hydrophobic membrane（Tianjin Xinqiyuan Environmental Materials Co.,LTD.,YR06-140）.The prepared membrane also had good mechanical properties（breaking strength>100 MPa）and interfacial bonding property.In the separation test of continuous water-in-oil（kerosene）emulsion,it showed high separation efficiency（>99%）and good reuse performance.In order to meet the requirements of continuous and efficient water-in-oil emulsion separation,PVDF nanofibers and PVDF/Si O₂ micro/nanospheres were simultaneously deposited on the surface of the prepared PVDF braided tube by electrospinning technique to prepare lined homogeneous braid-reinforced tubular PVDF nanofiber membrane.The effect of Si O₂ content in electrospray solution on membrane structure and properties was studied.It was found that the nanofibers deposited on the surface of the braided tube had a binding effect on the microspheres/nanospheres deposited at the same time,and a multistage micro-nano structure was constructed on the cross section of the membrane.At the same time,the gaps between the nanofibers are increased due to the deposition of micro/nanospheres between them,so as to the permeability of the membrane were improved.With 2 wt.%Si O₂ was introduced,the prepared membrane not only had superoleophilicity（θ_o=0°）and underoil superhydrophobicity(θ_uow=154°),but also have high kerosene flux(>8500 L·m^-2·h^-1)and excellent water-in-oil（kerosene）emulsion separation performance（separation efficiency>99%）.At the same time,it also had good tensile properties（breaking strength>79 MPa）and interfacial bonding property.