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2.2 Membrane Production Processes - Asymmetric Membranes Asymmetric membranes have a change in pore structure with depth. Typically, a thin permselective layer forms on the top of the film that is underlain by an open porous support layer. The dense layer at the top face of the membrane increases rejection rates, and the porous support layer increases water flux rates. This type of membrane is produced through thermal phase inversion. In this process, differential polymer precipitation is caused by a rapid change in temperature at the film surface, or by exposure of the polymer to a precipitant. 2.2.1 Thermal Phase Inversion Membranes. - Phase inversion is a process whereby the polymer is precipitated in phases so that it is present both as a solid and a liquid. Membranes made through phase inversion may be symmetric or asymmetric, depending on the exact procedure used. One way to induce phase inversion with a polymer that is insoluble in a particular solvent at room temperature is to dissolve it in hot solvent. The solution is cast as described above. As the solvent cools, the polymer develops a spongy uniform cell structure. The size of the cells depends on the cooling time. Longer cooling times produce a finer cell structure. As the polymer precipitates, the surface of the membrane is partially protected from atmospheric vapor by the solvent, producing an even, symmetrical structure. Asymmetric phase inversion membranes result when the polymer-solvent mixture is exposed to a precipitant, such as water. As the precipitant is absorbed into the fti the polymer precipitates at a faster rate than it would if only caused by the evaporation of the solvent. The precipitant is introduced either as a vapor or as a wash. If a wash is used, the surface of the film quickly precipitates, causing a skin to form. The skin limits the absorption of precipitant into the interior of the film. After this point, precipitation is controlled by the evaporation of solvent through the skin. When a vapor is used to introduce the precipitant, it is absorbed into the interior of the film without skin formation, but cell structure near the surface will be finer than at depth. The structure of the film interior is governed by the concentration of polymer in the casting solution, the solubility of the polymer in the solvent, and the gelation time or precipitation rate. Generally, the longer it takes to precipitate the polymer completely, the liner the cell structure will be. When a skin is formed on the surface of the membrane and a low concentration of polymer is present in the casting solution (~20% by weight), fingerlike structures tend to form within the film. Because the skin restricts the escape from the surface, the evaporating solvent accumulates in pockets and eventually breaks through the skin. The pocket is lined with a skin of precipitated polymer which further isolates the interstices. There, the polymer precipitates slowly, forming a line cell sponge texture.PDF Image | DESALTING AND WATER TREATMENT MEMBRANE
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