The formation of polysulfone hollow fiber membranes by the free fall spinning method

The basic principles of the spinning of polysulfone hollow fiber membranes by the dry-jet wet spinning process, where the polymer solution is extruded through an air gap between the spinneret and coagulation bath by the free fall spinning method, have been discussed. The main distinctive feature of the method is that the deformation of both the extruded polymer solution and the nascent hollow fiber (spinneret drawing) is due to the action of the gravity force alone without applying an external tensile force. Published data on the effect of the shear rate of the spinning solution at the spinneret outlet and nascent fiber drawing in the air gap on the structure and permeability of the hollow fiber membranes have been analyzed. The main factors affecting the spinneret drawing and dimensions of the hollow fiber membranes formed by free fall spinning have been experimentally revealed using polysulfones of different molecular weights. The factors are the dope composition, approaching ratio and viscosity, the air gap length, the temperature and the feed rate of the dope and the bore fluid, coagulation power of the bore fluid with respect to the dope. It has been found that as the spinneret draw value increases, the hydraulic permeability and the rejection coefficient of the resulting fiber generally change in a nonmonotonic manner: the pure water flux of the hollow fibers passes through a maximum and the rejection coefficient, through a maximum or minimum. The behavior is caused by the fact that the pore structure of the hollow fiber membranes is formed during uniaxial drawing and, in some cases (at increasing bore liquid flow rate), during biaxial deformation. When an external mechanical force is applied to the forming hollow fiber, an increase in the fraction of interconnected pores and the transition from cylindrical to slitlike pores are possible, which results in an increase in the hydraulic permeability of the fiber walls. A further increase in the spinneret draw ratio results in the reverse process, a decrease in the membrane matrix porosity due to the orientation and collapse of the pores, yielding a decrease in the flux and an increase in the rejection coefficient. By blocking the process of hollow fiber shrinkage through an increase in the bore fluid flow rate (increasing the internal diameter of the hollow fiber), it is possible to enhance the effective porosity of the fiber walls without substantial change in pore size, i.e., a transition from a system with isolated or partly isolated pores to a system of interconnected pores. A sharp increase in the hydraulic permeability of the hollow fiber membranes without a substantial change in their rejection is supposedly caused by this structural change.