The influence of the nonsolvent intrusion through the casting film bottom surface on the macrovoid formation

The influence of nonsolvent intrusion through the casting film bottom surface on macrovoid formation in the immersion precipitation phase inversion process was investigated using two kinds of substrates, i.e., glass plate as a representation of dense plate and non-woven fabrics as a representation of porous materials. The results show that the cross-sectional structure of the membranes prepared on the glass plate varied with the casting film thickness which determined whether the nonsolvent could intrude through the casting film bottom surface. Macrovoids formed because of nonsolvent intrusion through the bottom surface when the casting film thickness range was between 150 and 250 μm. The non-woven fabrics supported membrane M100-n-0 possessed macrovoids at the bottom while M100-g-0 whose substrate was a glass plate showed integrated sponge-like structure though they were prepared under the same conditions, which suggested the nonsolvent intrusion through the pores of the non-woven fabrics induced the macrovoid formation. To obtain non-woven fabrics supported membranes with pressure resistance, solvent N-methyl pyrrolidone (NMP) was added into the coagulation bath to reduce the mass transfer rate of solvent and nonsolvent through the pores, and then suppress the macrovoids. A compaction test was carried out to investigate the pressure resistance of the membranes. With the increase of solvent content in the coagulation bath, the quantity and volume of the macrovoids were reduced, hence the pressure resistance of the membranes increased.