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With spin casting, the solution is spread by centrifugal force on the inside of a cylinder. As the surface thickens, centrifugal force pushes the higher density solution back against the cylinder, bringing fresh solution to the surface. This cycling of the casting solution prevents a skin from forming on top of the film and produces a uniform cell structure throughout the depth of the film (Strathmann, 1990). 2.1.2 Melt Pressing. - Melt pressing does not involve solvents. The dry, powdered polymer is pressed between sheets of teflon coated foil or cellophane with heated plates at pressures of 13,800 to 35,000 kPa. The temperature should be just high enough to fuse the polymer into a film. Shims or forms are used to control the thickness of the membrane. Porosity depends on the particle size of the powdered polymer or other material used (Strathmann, 1990). 2.1.3 Pore Formation. - Microporous membranes can be formed by irradiating symmetrical membranes with charged particles from a nuclear reactor. The particles pass through the membrane, leaving regularly sized sensitized tracks. The membrane is then passed through an etching bath that dissolves damaged membrane material. The sensitized tracks of the particles are dissolved, producing the pores. The longer the membrane is in the etching bath, the larger the pore diameter will be. Pores produced in this manner are called capillary pores. Nuclepore polycarbonate microfilters are the only commercial microfiltration membranes made by this process. Variation in capillary pore size is minimal because of the uniform size of the charged particles; however, spacing is random. The longer the membrane has been irradiated, the greater the number of pores, and the greater the probability of pore overlap. Stretching is another method used to create pores. Partially crystallized polymer membranes are stretched perpendicular to the direction of extrusion. Pores created in this way are linear lesions in the polymer matrix held together by fine strands. Overall pore distribution is much more regular than with irradiation, which means that greater porosities are possible without increasing the variation in pore diameter. At low porosities however, pores produced by stretching exhibit a higher level of variability than capillary pores. One last method of pore formation is template leaching. If a soluble component is mixed with the casting/polymer solution, the component can be leached out from the finished membrane, leaving pores behind. The size of the pores and their distribution depends on the leached component and the level of mixing of the soluble component in the polymer solution (Porter, 1990).PDF Image | DESALTING AND WATER TREATMENT MEMBRANE
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