Postformation treatment

Immediately following the Sol 2 gel transition in the dry process, a primary gel structure is obtained. This gel is seldom Isolated because continued evaporation produces the completely consolidated membrane, known as the secondary gel. The secondary gel is ordinarily the only product of interest. However, this is not usually the case for the wet process. After the viscous solution has been gelled by Immersion and the solvent system removed, a stable primary gel is obtained. Such a membrane is easily distinguished from the secondary gel which results after the primary structure has been subjected to various postformation treatments. 

Casting-solution and environmental variables permit far greater control over the ultimate structure and performance of phase inversion membranes than does the modification of a primary gel into a secondary gel by postformation treatments. Because the properties of the primary gel determine to a large extent those of its secondary counterpart, the former should be considered as more fundamental and important in discussing the effects of fabrication parameters such as casting-solution composition, upon performance. Once a primary gel has been formed, it may be utilized as such (particularly for low-pressure applications) or it may be subjected to various physical and/or chemical treatments for conversion into a more pressure-resistant secondary gel. 

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