Wet spinning of integrally skinned hollow fiber membranes by a modified dual-bath coagulation method using a triple orifice spinneret

Three main routes are known to prepare hollow fiber membranes; melt spinning, dry spinning and wet spinning (or dry/wet spinning). The latter is the most important technique for the preparation of industrial hollow fiber membranes. In this process the extruded polymer solution is immersed in a nonsolvent bath where phase separation occurs. Generally there is an air gap between the spinneret and non-solvent and this air gap has a large impact on the ultimate membrane properties. In order to control the processes in this air gap a new type of spinneret has been developed where the extrusion mouth has three openings. The membrane formation process can be controlled by the choice of the nonsolvent which is extruded through the outer opening as a thin film adhering to the polymer solution. In this way both asymmetric microfiltration and ultrafiltration hollow fibers can be prepared as well as integrally skinned hollow fibers with a defect-free top layer suitable for gas separation and pervaporation. The advantage of this spinneret in relation to the phase inversion process will be described and some examples will be given in the field of gas separation and pervaporation.