Preparation And Hydrophilic Modification Of Braided Tube Reinforced Polyvinylidene Fluoride Hollow Fiber Composite Membrane

Since its inception,polyvinylidene fluoride(PVDF)has been widely used as a main membrane material for microfiltration and ultrafiltration in the field of water treatment because of its good chemical stability,excellent UV resistance,acid and alkali resistance,weather resistance,and good membrane formation.However,the PVDF hollow fiber membrane prepared by the traditional solution spinning method has low mechanical strength and poor toughness.In actual use,the phenomenon of membrane filament breakage often occurs,which limits the wider application of the PVDF hollow fiber membrane.On the other hand,due to the hydrophobic nature of the PVDF membrane material,PVDF hollow fiber membrane tend to exhibit lower water fluxes due to poor wettability during water treatment,and a large amount of protein,colloid and other hydrophobic organic substances are easily adsorbed on the surface of the membrane and cause adsorption pollution,which causes membrane pores to be blocked and attenuates membrane flux and is difficult to clean.It seriously affects membrane stability and reusability,shortens the membrane's service life,and results in membrane separation operating costs increased.Therefore,how to improve the strength and hydrophilic properties of PVDF hollow fiber membrane has become the two main issues studied in this subject.(1)First of all,in order to solve the problem of poor mechanical properties of traditional PVDF hollow fiber membrane,by introducing fiber braided tube as a reinforcement and using silane coupling agent to surface modify the braided tube.A self-developed coated immersion gel phase inversion technology was used to prepare a braided tube reinforced hollow fiber composite membrane capable of considering both tensile strength and burst strength.The research results show that the surface structure and performance of the braided tube are improved after the surface modification treatment of the polyester braided tube with the silane coupling agent KH-550.As the surface roughness and specific surface area of the braided tube increase,the wetting performance of the braided tube is greatly improved,the mechanical engagement of the casting membrane fluid and the braided tube layer and the physical adsorption and adhesion are strengthened.After curing,the membrane can form a good interface layer with the braided tube.The burst strength of the composite membrane was increased from 0.12MPa before modification to 0.58MPa after modification,and the interface bonding strength of the composite membrane was greatly improved.At the same time,due to the improvement of the interfacial bonding properties of the composite membrane,the tensile properties of the modified composite membrane have also been improved to a certain extent.(2)Secondly,in order to further optimize the membrane formation process,in the PVDF/PVP/DMAC casting membrane fluid system,the effects of the concentration changes of polymer PVDF and porogen PVP on the structure and properties of composite membrane and the pore-forming mechanism of porogen PVP during membrane formation were investigated,and the optimum input ratio of polymer PVDF and porogen PVP were determined,and the differences in the effects of three different types of porogens(PVP,PEG,PVA)on the structure and performance of composite membrane were investigated and compared.The research results show that under the premise that the pore size of the composite membrane is appropriate,the distribution is uniform,and the membrane formation effect is good,the preferred concentration range of PVDF is between 13%and 19%.When the PVDF concentration is 17%,the membrane formation effect of the composite membrane is the best and the performance parameters are excellent.The addition of porogen PVP significantly improves the separation performance of the composite membrane,but the content of PVP has a suitable concentration range.When the concentration of PVP is 6%,the permeability of the composite membrane is optimal.Different types of porogens(PVP,PEG,PVA)have a large effect on the surface structure and properties of composite membrane.Composite membrane prepared with PVP as porogen have small average pore size,high porosity,narrow pore size distribution,pure water flux and retention rate are high,and the comprehensive performance is optimal.(3)Finally,aiming at the problem of poor hydrophilicity of PVDF membrane material,PVDF/TiO2 hydrophilic modified hollow fiber composite membrane was prepared by blending super-hydrophilic nano-TiO2 particles with PVDF.The chemical composition and structure,surface morphology,separation performance,and anti-pollution performance of the modified composite membrane were tested and characterized.Compared with the unmodified pure PVDF composite membrane,the structure and properties of the PVDF/TiO2 modified composite membrane and the effects of TiO2 concentration on the structure and performance of the modified composite membrane were investigated.The research results show that the addition of TiO2 is beneficial to the increase of the number of hydroxyl groups on the surface of the composite membrane,and can effectively improve the hydrophilicity of the surface of the composite membrane.The addition of an appropriate amount of TiO2 can promote the phase separation and pore formation process,make the pore formation of the membrane more uniform,and the surface rough degree is reduced.When the concentration of TiO2 is 1.5%,the composite membrane has the highest porosity,the best water permeability and retention performance,the flux decay rate and the pollution rate is the smallest in the sewage treatment process,and the flux recovery rate after cleaning is the largest,showing excellent anti-pollution performance.