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Preparation of Polymeric Membranes 91
membranes (D) and the revised factor (e). The revised factor e should be less than 1, which
reflected the shrinkage degree of nascent hollow fiber membranes due to the exchange of
solvent and nonsolvent in the coagulant bath. Because of the exchange of solvent and
nonsolvent between the dope solution and the coagulant (water and bore fluid), the revised
factor e for P84/NMP/water spinning system was approximately 0.8–0.9 (98). In the spinning
process, the ratio of the membrane thickness to membrane outer diameter can be controlled
by the volume ratio of the dope solution to the bore fluid (o) and the revised factor e.
5.2. Preparation of Flat Sheet Membranes
The original Loeb-Sourirajan membranes were made by casting 22–25% cellulose acetate
solutions on glass plates. After solvent evaporation, the cast film was immersed in a water
bath to precipitate and form the membrane. Rozelle et al. (104) also used the same method to
prepare supported membranes with polysulfone and polycarbonate et al. for composite
membranes. This method has been using for the preparation of flat sheet membranes in the
laboratory till now, but for commercial production, a casting machine is usually used. Figure
2.34 shows a typical schematic diagram of the fabrication of flat sheet membranes. The
nascent films were cast onto a moving nonwoven fabric web by the casting blade, and then
exposed to a gaseous environment for solvent evaporation prior to entering the coagulant
bath, finally precipitated by quenching in the coagulant bath as shown in Fig. 2.34.
The fabrication of flat sheet membranes is also a complicated process, which involves the
dope preparation, the rheology of the casting solution, air gap and immersion precipitation in
the coagulant bath et al. The process mainly concerns three steps: the rheology of the dope
dope solution
shear
non-woven fabric
spinning
velocity
casting
window
dope
shear rate
casting blade
shear stress
shear flow
velocity distribution
air gap
coagulation
formula
approaching ratio
temperature
temperature, humidity
relaxation
evaporation
temperature
approaching coagulation ratio
solidification
post-treatment
Fig. 2.34 Schematic diagram of flat sheet membrane fabrication.
dynamic thermodynamic