The investigation of irregular inner skin morphology of hollow fiber membranes at high-speed spinning and the solutions to overcome it

In the fabrication of hollow fiber membranes, a high-speed spinning is important to maximize the productivity and minimize the production cost. However, the high-speed spinning process has been shown to influence the morphology of the resultant fibers, especially inner layer of the fibers. For the first time, we report that the increase in the take-up speed results in the deformation of inner shapes of the fibers. This phenomenon is not desirable as it results in the formation of low mechanical strength fibers. It is hypothesized that the phenomenon occurs due to the formation of vacuum condition in the lumen side of the fiber, which may be due to rapid formation of dense skin inner layer. With increasing take-up speed the inner surface becomes denser leading to insufficient bore fluid supply which promotes more vacuum condition in the lumen side. Four possible solutions were proposed to eliminate the inner layer deformation phenomena, i.e. (1) by increasing the bore fluid flow rate, (2) by chemical modification of the bore fluid, (3) by simultaneously increasing dope and bore flow rates while maintaining a constant ratio between them and (4) by simultaneously increasing bore and dope flow rates in the same order with the increase in take-up speed. The increase in bore fluid flow rate can effectively eliminate the deformation. The addition of the solvent to the bore fluid can also eliminate the deformation when 70 wt.% NMP/water was used as bore fluid. No inner layer deformation was observed when dope and bore flow rates were increased up to 4 and 3 ml/min, respectively. However, the first three approaches may have negative side effects. The fourth approach gives us the most promising results. Even after increasing take-up speed up to 470% from its original take-up speed, the resultant fibers show no irregular formation at inner layer morphology and no changes in their inner and outer diameter. Thus, it can be said that by utilizing this approach, more flexible and easy control over fibers can be achieved. (c) 2006 Elsevier B.V. All rights reserved.