(243f) Evolution of Macrovoids and Critical Factors to Form Macrovoid-Free Hollow Fiber Membranes

The origins of macrovoid and the ways to eliminate it have received great attention and heavy debates during the last five decades, but no convincing and agreeable comprehension has been achieved. This work will systematically investigate the key factors to form macrovoid-free hollow fiber membranes. We have discovered there should be critical values of polymer concentration, air gap distance and take-up speed, only above all of which the macrovoid-free hollow fibers can be successfully produced. This observation was confirmed for hollow fibers spun from different materials such as polysulfone, P84 and cellulose acetate, and may be even universally applicable for other polymers. The major mechanisms why these critical parameters can effectively suppress macrovoids have been elaborated. The concept of acceleration of stretch was also proposed. A quantitative relationship was observed to relate it with the number of macrovoids at the critical velocity and critical air gap distance: n=C1*a^1/2 + C2(where C1 and C2 are materials and processing related constants which possibly are functions of dope formulation, rheology, precipitation and spinning conditions). Even though this mathematical description is still open and requires further investigation due to the complexity of spinning process, this model does provide some fundamental understanding of material related and process related tendency of macrovoid formation.