Preparation Of Reverse Osmosis Composite Membrane By Plasma Modification Of PVDF Base Membrane And Its Performance Research

Because of the hydrophobicity on the PVDF material surface,and the pollution in the using process,this limits its application in water treatment.This paper hopes to start from the hydrophilic modification of the surface of PVDF ultrafiltration base membrane,change the chemical composition of the membrane surface,and finally prepare a high-performance membrane material,which provides a reference for the use of PVDF materials in water treatment.The specific experimental content is going to obtain hydrophilic functional groups on the surface of PVDF base membrane through plasma modification,and composited membranes prepared by interfacial polymerization.Optimizing the properties of the base membrane through modification to improve the performance of the reverse osmosis composite membrane.Finally,the response between the change of the surface properties of the base membrane and the separation performance of the composite membrane was established through the experimental data,and the influence rule of the properties of the base membrane on the performance of the composite membrane was obtained.Firstly,the composite membrane was prepared by plasma liquid phase grafting modified base membrane,and the effect of plasma liquid phase grafting modified base membrane on the separation effect of composite membrane was investigated.Through orthogonal experiments and single factor experiments,the optimal factor levels of the plasma-liquid phase graft-modified base membrane for preparing the composite membrane with the best separation performance were obtained.The composite membrane prepared by the modified base membrane had a desalination rate of 94.324%for 0.05mol/L Na Cl solution and a flux of13.256 L/（m²·h）;the desalination rate of 0.05mol/L Mg SO₄ solution could reach 98.314%,The flux was 15.827 L/（m²·h）.Compared the separation effect of each group of composite membranes,it was found that the retention order of composite membranes for Na Cl and Mg SO₄ with the same molar concentration was Na Cl<Mg SO₄,which was explained by Donnan effect and preferential adsorption-capillary flow theory.The durability test found that after tested the performance of the composite membrane for a long time,the salt rejection rate of the modified composite membrane was still significantly higher than that of the composite membrane directly prepared from the base membrane,but the flux was lower than that of the composite membrane directly prepared from the base membrane.The surface properties of the modified base film at different grafting concentrations were tested,and it was found that the changes of the surface hydrophilicity,surface tension,interfacial free energy and Zeta potential of the base film were related with the changes of the grafting concentration.Appropriate grafting concentration can increase the content of oxygen-containing functional groups on the surface of the base film,which can improve the surface performance of the film.FTIR-ATR analysis showed that acrylic acid was successfully grafted to the surface of the base film.EDS analysis found that after modification the content of F element on the surface of the base film was decreased and the content of O element was increased,further indicated that the graft modification was successful.Through SEM analysis,it was found that the pores of the base film increased after the plasma liquid phase grafted acrylic acid;the structure of the composite film prepared by the unmodified base film was loosed,while the surface structure of the composite film prepared by the modified base film was more dense and uniform,indicated that the grafted acrylic acid The hydrophilicity of the surface of the base film was increased,which improved the performance of the composite film.Secondly,the composite membrane was prepared by plasma gas phase grafting modified base membrane,and the effect of plasma gas phase grafting modified base membrane on the separation effect of composite membrane was investigated.Through orthogonal experiments and single factor experiments,the best separation was obtained.The composite membrane prepared by the modified base membrane had a desalination rate of 95.771%for 0.05mol/L Na Cl solution,a flux of 15.669 L/（m²·h）,and a desalination rate of 97.043%for 0.05mol/L Mg SO₄ solution,the flux was 16.349 L/（m²·h）.The durability test found that after tested the performance of the composite membrane for a long time,the salt rejection rate of the modified composite membrane was still significantly higher than that of the composite membrane directly prepared from the base membrane,but the flux was lower than that of the composite membrane directly prepared from the base membrane.The surface properties of the modified base film under different grafting time were tested,and the changes of the surface hydrophilicity,surface tension,interfacial free energy and Zeta potential of the base film were related with the change of the grafting time.Through SEM analysis,it was found that the pore size of the base membrane increased after the plasma gas phase grafted acrylic acid;the composite membrane prepared by the unmodified base membrane had a loose structure,while the surface structure of the composite membrane prepared by the modified base membrane was more dense and uniform,indicatied that the grafted acrylic acid increased.The surface of the base film was hydrophilic,which improved the performance of the composite film.EDS analysis found that the content of F element on the surface of the base film decreased and the content of O element increased after modification,which also indicated that the graft modification was successful.Next,the composite membrane was prepared by using the plasma polymerization modified base membrane.And the influence of the plasma polymerized modified base membrane on the separation effect of the composite membrane was investigated.The composite membrane prepared by the modified base membrane had a desalination rate of89.912%for 0.05mol/L Na Cl solution and a flux of 24.360L/（m²·h）;the desalination rate of0.05mol/L Mg SO₄ solution could reach 92.741%,the flux was 26.028L/（m²·h）.The durability test found that after tested the performance of the composite membrane for a long time,the salt rejection rate of the modified composite membrane was still significantly higher than that of the composite membrane directly prepared from the base membrane,and the flux was also higher than that of the composite membrane directly prepared from the base membrane.The surface properties of the modified base film under different polymerization currents were tested,and the changes of the surface hydrophilicity,surface tension,interfacial free energy and Zeta potential of the base film were related to the changes of the polymerization current.SEM analysis found that the surface of the unmodified film was smoother and smoother,the surface of the plasma-polymerized modified film showed etching marks,but the pores were significantly reduced and different degrees of bumps appeared.The composite film prepared by the unmodified base film had a loose structure,while the surface structure of the composite film prepared by the modified base film was more dense and uniform.EDS analysis found that the content of F element on the surface of the modified base film was significantly reduced,and the content of O element was increased,because the O element in the acrylic monomer was polymerized to the surface of the base film.The increase of O element increased the hydrophilicity of the surface of the base film,and the performance of the prepared composite film was improved.The content of N element in the composite film prepared by the modified base film increased,indicated that the polyamide film on the film surface was denser.Finally,the influence of base film modification on the performance of composite membranes was explored.The surface properties of the unmodified base film and the three modified base films were compared.This paper Compared the separation performance and durability of the composite membranes prepared by the three modified base membranes and the unmodified membrane,it was found that the composite membranes prepared by the two plasma-grafted modified base membranes had higher salt rejection and lower flux,and in the durability test,it was found that the salt rejection rate and flux of the composite membrane prepared by the two graft-modified base membranes were relatively stable,which was because the grafting reaction can increase the effective path length of the composite membrane for desalination.The skin layer of the composite membrane prepared by plasma polymerization modified base membrane only adhered to the surface of the base membrane,which maked the separation performance lower than that of the graft-modified composite membrane,while the salt rejection rate was higher and the flux was higher,and in the durability test,with the extension of the test time,the salt rejection rate of the composite membrane decreased significantly,and the flux increased significantly.The analysis of correlation between the surface performance of the base membrane and the separation performance of the composite membrane showed that the pure water contact angle,surface tension,interfacial free energy,Zeta potential of the base membrane and the salt rejection and flux of the composite membrane were related to different degrees.That is to say the performance of the base film would affect the performance of the composite film to a certain extent.The better the hydrophilicity of the bottom membrane,the greater the absolute value of surface tension and interfacial free energy,and the higher the negative charge density on the surface,which would lead to a higher possible salt rejection rate and lower flux of the composite membrane.And due to the influence of functional groups,there was also a certain correlation between the performance indicators of each base film.That was to say when the film surface had the better hydrophilicity,it would also have the higher surface tension and the higher interfacial free energy,and the higher the surface negative charge density.The separation performance of the composite membrane which was prepared directly by the PS ultrafiltration membrane and the composite membrane which was prepared after the modification of the PVDF ultrafiltration membrane in this experiment was compared.The amount was higher than the composite membrane prepared by the PS ultrafiltration membrane,It is indicated that using the relatively hydrophobic PVDF-based membrane prepare the composite membrane has research significance.