Helical hollow fibers via rope coiling: Effect of spinning conditions on geometry and membrane morphology

Helical hollow fiber membranes from poly(ether sulfone) were spun via dry-wet spinning, making use of the liquid rope coiling phenomenon. The polymer solution composition was changed by varying the coagulation value and adding PEG400 as pore former. The bore liquid composition, outer coagulation bath temperature, air gap, polymer dope and bore liquid flowrates were varied to map the conditions where helical fibers form. It was observed that increasing air gap changed fiber geometry from straight to helical. Increasing the outer coagulation bath temperature caused helical geometries to become irregular or straight under identical spinning conditions, possibly due to the higher water vapour absorption in the air gap. Regularly helical fibers could, however, be spun with a coagulation bath at 50 degrees C under a variety of conditions. Comparing fibers spun under the same conditions with different solutions, it was observed that as the solution's viscosity increased, the geometry shifted towards helical. Increasing the bore liquid solvent strength so as to form the skin on the bore side yielded straight fibers due to the fast solidification of the fibers before curling and all helical fibers fabricated had a denser skin on the shell side. During filtrations of yeast suspensions, the helical fibers experienced less fouling with the feed on either the bore or the shell side of the membrane, which shows that using liquid rope coiling to spin helical fibers is a promising, practical method of alleviating fouling in membrane filtrations.