Coagulation process, phase transition

We need to understand under which conditions a colloidal system will remain dispersed (and under which it will become unstable). Knowing how colloidal particles interact with one another makes possible an appreciation of the experimental results for phase transitions in such systems as found in various industrial processes. It is also necessary to know under which conditions a given dispersion will become unstable (coagulation). For example, one needs to apply coagulation in wastewater treatment so that most of the solid particles in suspension can be removed. Any two particles coming close to each other, will produce different forces. 

Most of the membranes listed in Table 5.20 are formed through phase separation processes, i.e., melt extrusion or coagulation of a polymer solution by a nonsolvent. In melt extrusion, a polymer melt is extruded into a cooler atmosphere which induces phase transition. The melt extrusion of a single polymer usually gives a dense, isotropic membrane. However, the presence of a compound (latent solvent) that is miscible with the polymer at the extrusion temperature but not at the ambient temperature, may lead to a secondary phase separation upon cooling. Removal of the solvent then yields a porous isotropic membrane. Anisotropic membranes may result from melt extrusion of a dope mixture of polymers containing plasticizers. 

The dry-cast process is marked by complete evaporation of solvent and nonsolvent from an initially homogenous polymer solution. This process is more amenable to experimental as well as modeling studies than other phase-transition processes for membrane formation, since it avoids the complications associated with the use of a coagulation bath, as in the immersion precipitation process. 

The actual formation of the fibers and films in a spinning or extrusion process takes place during coagulation of the lyotropic solution. In this process step a phase transition to the solid state is induced by a non-solvent. The conditions of the coagulation process, e.g. the concentration of the polymer solution, the composition and the temperature of... 

Because the fiber microstructure is established in the spinbath, the coagulation conditions employed are the result of extensive optimization. The critical part of this process is the transition from a liquid to a solid phase within the... 

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