Types of Membrane and Their Formation

Symmetric membranes and asymmetric membranes are two basic types of membrane based on their structure. Symmetric membranes include non-porous (dense) symmetric membranes and porous symmetric membranes, while asymmetric membranes include integrally skinned asymmetric membranes, coated asymmetric membranes, and composite membranes. A number of different methods are used to prepare these membranes. The most important techniques are sintering, stretching, track-etching, template leaching, phase inversion, and coating (13,33). 

Generally, in a phase-inversion technique, a polymer transforms from its liquid phase to solid phase in a well-controlled manner. Usually, the process is carried out by starting the transition from one liquid state to two liquid states. One of them has a high polymer 

MF = microfiltration UF = ultrafiltration NF = nanofiltration D = dialysis PV = pervaporation GS = gas separation RO = reverse osmosis. 

There are a number of different techniques belonging to the category of phase inversion solvent evaporation, precipitation by controlled evaporation, precipitation from the vapor phase, thermal precipitation, and immersion precipitation (13,34—36). The most commercially available membranes are prepared by the last method. 

Coagulation bath Fig. 6. Schematic diagrams of the wet spinning apparatus. 

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