Preparation Method of Composite Ultrafiltration Membrane for Ultrapure Water Equipment

Composite ultrafiltration membranes, also known as asymmetric composite membranes, are fabricated by depositing an extremely thin, dense layer onto a microporous support layer through physical casting or chemical polymerization. Early composite membrane preparation involved casting a polymer dissolved in solution onto a porous substrate, allowing the solvent to evaporate and form a very thin surface layer. The membrane thickness was controlled by the concentration of the casting solution. This method suffers from drawbacks: the polymer solution can damage the support layer structure, and the cast dense layer is relatively thick. In recent years, the emergence of new membrane fabrication processes such as interfacial polycondensation, monomer-catalyzed polymerization, and plasma polymerization has greatly advanced composite membrane fabrication technology. Li Yueming et al. used inorganic salts and ammonia as raw materials to prepare a stable and transparent boehmite sol using the sol-gel method. They then used an impregnation process to uniformly coat the boehmite sol onto an inorganic salt Al2(NO)3 support, preparing a defect-free Al2O3 ultrafiltration membrane with uniform pore size. Wang Xinping et al. designed and prepared a chitosan composite membrane with a special structure. The membrane consists of a thin chitosan membrane on top, a porous polyacrylonitrile (PAN) ultrafiltration membrane on the bottom, and a cross-linked layer (sodium alginate) connecting the dense layer and the support layer. Results showed that the presence of the intermediate layer simultaneously improves the membrane's separation performance and durability.

Compared to asymmetric membranes prepared by phase inversion, the composite membrane has the following advantages: 1. Different materials can be selected to fabricate the dense layer and support layer, optimizing the separation performance; 2. Highly cross-linked or ionic ultrafiltration layers with a thickness of 0.01-0.1 micrometers can be prepared using different methods, giving them both good stability and durability, as well as good separation performance.

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