Eco-friendly solvents and their mixture for the fabrication of polysulfone ultrafiltration membranes: An investigation of doctor blade and slot die casting methods

Nonsolvent induced phase separation has been widely used to fabricate polymeric membranes. Common solvents, such as dimethylacetamide (DMAc) and N-Methyl-2-pyrrolidone (NMP), are toxic and not environmentally friendly; therefore, eco-friendly alternatives are needed. Eco-friendly solvents, such as Rhodiasolv® PolarClean (PolarClean) and γ-valerolactone (GVL), have been investigated to replace DMAc and NMP at the laboratory scale; however, not extensively at the production scale. In this work, the feasibility of fabricating polysulfone (PSf) ultrafiltration membranes using eco-friendly solvents at laboratory and production scales is investigated. First, it was determined that the addition of GVL to dope solutions of PolarClean could reduce the original preparation time at a rotation speed of 450 RPM by two-thirds, and by three-quarters at 600 RPM. Furthermore, dope solutions exhibit Newtonian fluid behavior. Doctor blade extrusion and a roll-to-roll (R2R) system integrated with slot die casting are used to fabricate the PSf ultrafiltration membranes at laboratory and production scales, respectively, and the resulting membranes are compared structurally, morphologically and operationally. The chemical structure of membranes is not affected by the use of different solvents or by the differences in fabrication scale. On the other hand, cross-sectional images show that the structures of the membranes are different, most likely due to differences in diffusion rates between the different solvents/co-solvents into the nonsolvent, water. Furthermore, it has been observed that the roughness values of the membranes relative to the fabrication process is different, possibly due to differences in the required evaporation time. All membranes display similar operational parameters, i.e., flux decline, permeability and recovery, during bovine serum albumin (BSA) filtration. Therefore, this study shows that PSf membranes fabricated using eco-friendly solvent mixtures are comparable to traditional membranes, cast using petroleum-derived solvents, and are scalable using slot die casting on a R2R.