Precipitation by controlled evaporation

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. 

The concept of phase inversion covers a range of different techniques such as solvent evaporation, precipitation by controlled evaporation, thermal precipitation, precipitation from the vapour phase and immersion precipiution. The majority of the phase inversion membranes are prepared by irrunersion precipitation. 

Precipitation by controlled evaporation was already used in the early years of this century. In this case the polymer is dissolved in a mixture of solvent and nonsolvent (the mixture acts as a solvent for the polymer). Since the solvent is more volatile than the nonsolvent, the composition shifts during evaporation to a higher nonsolvent and polj mer content. This leads eventually to the polymer precipitation leading to the formation of a skinned membrane. 

Phase inversion is known to be an effective way to create porous structures in membranes, where a competitive mutual diffusion between solvent and nonsolvent occurs to yield the porous structure. Phase inversion can be described as a demixing process whereby the initially homogeneous polymer solution is transformed in a controlled manner from a liquid to a solid state [24]. Apart from immersion in a nonsolvent bath, or immersion precipitation (IP), a variety of related techniques, such as precipitation by solvent evaporation, precipitation by absorption of water Irom the vapor phase, and precipitation by air cooling, corresponding to thermally induced phase separation (TIPS), vapor-induced phase separation (VIPS), and air-casting phase separation... 

Solvent Volatilization Successive precipitation of polymer species from a solution of the polymer in a solvent/ nonsolvent mixture by controlled evaporation of the more volatile solvent. The larger molecules precipitate first. 

It is necessary to adjust the Ca, Mg, phosphate, and citrate content of the concentrate to control aggregation and precipitation of the proteins and minerals during sterilization. By controlling protein aggregation, this adjustment provides optimum viscosity to stabilize the protein, mineral, and milk fat emulsion systems during prolonged storage of the sterile product. Some milk concentrates are stabilized by addition of Ca and Mg salts, whereas others are stabilized by addition of phosphate or citrate salts (Parry, 1974). Chemical compounds approved for addition to evaporated milk include calcium chloride, sodium citrate, and disodium phosphate (CFR 1982). 

Noncrystalline solids formation needs to be clarified by reference to the natnre of the final product. Several methods lead to amorphous fine particles, for instance, in solution by controlled precipitation. Through gel formation, poorly crystalline Ti02 can be obtained according to the process Ti(OR)4 -I- H2O Ti(OH)4 gel amorphous Ti02 + H2O. In this case, the compound obtained is thermodynamically stable, as when amorphous films are obtained by chemical vapor deposition, sputtering, flash evaporation, and so on. 

Phase inversion refers to the controlled transformation of a cast polymeric solution from a Hquid into a soHd state. During the phase-inversion process, a thermodynamically stable polymer solution is usually subjected to controlled Hquid-H-quid derabdng. This phase separation of the cast polymer solution into a polymer-rich and a polymer-lean phase can be induced by immersion in a non-solvent bath ( immersion precipitation ), by evaporating the volatile solvent from a polymer that was dissolved in a solvent/non-solvent mixture ( controlled evaporation ), by lowering the temperature ( thermal precipitation ) or by placing the cast film in a vapor phase that consists of a non-solvent saturated with a solvent ( precipitation from vapor phase ) [1]. 

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