| abstract |
A polysulfone hollow fiber filter medium having substantially no skin layer in the outer and inner surface areas, the maximum diameter of the pores being from 0.1 to 5 pm., the maximum diameter of the openings of the pores formed in the inner surface being from 0.01 to 10 µm. and the maximum diameter of the openings of the pores formed in the outer surface being from 0.01 to 5 µm., which is prepared by extruding a solvent solution of a polysulfone from a ring nozzle together with an inside coagulating liquid extruded from the inside of the ring nozzle, said solution having a transition temperature at which the viscosity rise changes to the viscosity drop when the temperature of the solution is dropped, and being maintained at a temperature of not less than the transition temperature, and immediately or after passing through a dry space of not more than 50 cm., bringing the extrudate into contact with an outside coagulating liquid, at least one of the inside coagulating liquid, the dry space and the outside coagulating liquid being maintained at a temperature of less than the transition temperature. |