| Polyethersulfone(PES)has become an excellent material for preparing ultrafiltration membranes due to its advantages of heat resistance,acid resistance,and radiation resistance.It has been widely used in many fields such as food processing,biochemical pharmaceuticals,and environmental protection.However,in the production process of PES UF membrane,there are problems such as the difficulty in controlling the membrane cross-section structure and poor reproducibility of the membrane production results.In this thesis,PES UF membranes were prepared by phase inversion method to investigate the film formation rules under different filming parameters.Combining the phase diagram of a ternary system and based on the synergy of thermodynamics and kinetics in the film formation process,this thesis explains theoretically the effects on membrane structure and its separation performance of several key filming parameters(PES concentration in the casting solution,composition of the coagulation bath,use of additives,air bath conditions)during film formation,providing references for the effective regulation of the membrane structure during the production of PES UF membranes.The experimental results indicate that the increase of the concentration of PES in the casting solution will inhibit the formation of finger-like pores.However,as the concentration of PES in the casting solution increases,the pure water flux drops sharply.When the concentration reaches 30%,the pure water flux is 0 L·m-2·h-1 under the operating pressure of 0.2 MPa,and the BSA rejection rate is maintained at about 95%.The addition of solvent to the coagulation bath can effectively reduce the concentration difference between the casting solution and the coagulation bath,reduce the mass transfer driving force during the film formation process and prolong the film formation time to promote delayed phase separation.As the solvent content in the coagulation bath increases,the pure water flux increases and the BSA rejection rate decreases.An excessively high solvent concentration in the coagulation bath would cause the ternary system to remain in the homogeneous region,making the film formation process of phase inversion impossible.The addition of small-molecule alcohol additives to the casting solution can effectively suppress the development of the finger-like pores,making it easier for the casting solution to form a non-dense membrane on the surface in the air bath.After entering the coagulation bath,this non-dense membrane can improve the solvent-non-solvent mass transfer resistance,bringing about the adjustment from finger-like structure to sponge-like structure.And at the same time the pure water flux of UF membrane increased from 224.3 L·m-2·h-1 to 475.6 L m-2·h-1,while the BSA rejection rate decreased from 81.4%to 51.8%.The prolongation of air bath time or the increase of air humidity will lead to an increase in the amount of moisture absorbed by the casting solution,and a film will be formed on its surface,which greatly reduces the mass transfer rate between the solvent and the coagulation bath.The large pores gradually disappeared as a consequence and a symmetrical sponge-like structure was prepared.When the air bath time is long enough,a looser mesh-like membrane would appear,and the BSA rejection rate would be reduced to 10.7%.The finger-like pores structure of PES UF membrane has a higher rejection rate in the UF process,and its removal effect to organic macromolecules is much higher than that of an UF membrane with a uniform sponge-like pore structure.However,the PES UF membrane with sponge-like pore structure exhibits good resistance to compaction under high pressure,with its thickness decreased by only 3%,and better stability of its cross-section structure.Under the condition that the ion rejection rate does not change significantly,the NF membranes prepared by using the uniform sponge-like UF membrane as the base membrane has its flux of pure water increased by 22.8%. |
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