Macrovoid evolution 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. We have discovered, for the first time, that 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 from a two-component (one-polymer and one-solvent) system. This observation has been confirmed for hollow fibers spun from different materials such as polysulfone, P84 and cellulose acetate, and may be 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 proposed and a quantitative relationship was observed to relate it with the number of macrovoids per unit area at the critical velocity and critical air gap distance.