Preparation And Performance Of Polyamide Nanofiltration Membrane Based On EVOH Nanofiber Coating

Nanofiltration membranes are widely used in the fields of seawater desalination,sewage treatment and industrial material separation.It is an important method to improve the permeable area of nanofiltration membrane and reduce the energy consumption of separation by adjusting and optimizing the structure of porous base membrane.The porous base membrane materials such as polyethersulfone and polysulfone have the problems of low porosity and insufficient hydrophilicity.Nanofiber membrane material has the characteristics of high specific surface area and easy hydrophilic modification,but the existing electrospun nanofiber membranes structure is loose,the apparent pore size is large,which is not good for the formation of ultra-thin interfacial polymer layer with few defects.In this paper,melt blend phase separation is used to prepare nanofiber membranes material,which has the advantages of easy batch production,pore size control,excellent hydrophilicity,easy functionalization,etc.,which provides a possibility for the mass preparation of high-performance nanofiltration membranes.The research content of this article is divided into the following three aspects:The polyvinyl alcohol-ethylene copolymer（EVOH）was uniformly mixed with cellulose acetate butyrate（CAB）,and EVOH nanofibers were prepared by melt blending phase separation.Then EVOH nanofibers were dispersed in a mixed solvent of water and alcohol,and evenly coated on the surface of PET nonwoven fabric to obtain EVOH nanofiber membrane with a weight of 6 g/m².The polyamide nanofiltration membrane（PA/EVOH）was prepared by interfacial polymerization on EVOH surface with piperazine（PIP）as aqueous monomer and 1,3,5-trichloroyl chloride（TMC）as oil monomer.The research results show that the minimum thickness of polyamide selective layer is 25 nm,the time for water droplets to completely infiltrate PA/EVOH surface is 137 s,the rejection rate of 1000 ppm Na₂SO₄ solution at 0.48MPa is about 95.5%,and the penetration flux is about 122.2 L/（m²h）.The rejection rate of Na Cl solution is about 42%,and the penetration flux is about 110.1 L/（m²h）.Permeability of PA/EVOH increased by about 94 L m^-2h^-1MPa^-1 compared to Dow NF270,mainly due to the smaller through pore size of the nanofiber membrane（<200 nm）and larger surface roughness.Polyamide nanofiltration membrane（PA/EVOH-PDA）was prepared by hydrophilic modification of EVOH-PDA using PIP as aqueous monomer and TMC as oil-phase monomer on EVOH/PDA surface.The results showed that the thickness of polyamide selective layer of PA/EVOH-PDA was 23 nm at the minimum,and the time for water droplets to completely infiltrate the surface of PA/EVOH-PDA was 44 s.The rejection rate of 1000 ppm Na₂SO₄solution at 0.48 MPa was about 96.5%.The penetration flux is about 154.5 L/（m²h）,the rejection rate of Na Cl solution is about 13%,and the penetration flux is about 153 L/（m²h）.Compared with PA/EVOH,the permeability increased by about 94 L m^-2h^-1MPa^-1,which was mainly attributed to the large number of amine groups and phenolic hydroxyl groups that improved the hydrophilicity of EVOH-PDA.Many small bubble structures appeared on the surface of the polyamide selective layer of PA/EVOH-PDA,thus effectively increasing the penetration area.Polyamide nanofiltration membrane（PA/EVOH-P）was prepared by interfacial polymerization on EVOH-P surface with PIP as water phase monomer and TMC as oil phase monomer.The surface of EVOH-P was modified by low temperature plasma surface treatment.The research results show that the thickness of the selective layer of PA/EVOH-P polyamine is12.7 nm,the time of water droplets fully soaking PA/EVOH-P is 39 s,the rejection rate of 1000ppm Na₂SO₄ solution is about 95.5%at 0.48 MPa,and the penetration flux is about 202.3L/（m²h）.The rejection rate of Na Cl solution is about 12.38%,and the penetration flux is about216.9 L/（m²h）.The rejection rate of PA/EVOH-P for Na₂SO₄ solution in the concentration range of 500 ppm to 5000 ppm was greater than 95%.At the same time,within the test pressure range of 0.18 MPa to 0.78 MPa,the flux of PA/EVOH-P increased with the increase of the test pressure,and at 0.78 MPa.The rejection rate of 1000 ppm Na₂SO₄ solution is still about 90%.Compared with PA/EVOH,the permeability increased by about 162.1 L m^-2h^-1MPa^-1,which was mainly due to the synergistic effect of the highly wrinkled polyamide selective layer and the etched structure on the EVOH-P surface.The ultrathin polyamide selective layer showed a hierarchical wrinkled surface morphology.Large ridges formed by undulating nanofibers and smaller ridges distributed on the surface of the nanofibers greatly increase the effective penetration area.