CELLULAR FIBER AND METHOD FOR PRODUCTION THEREOF

摘要

[PROBLEMS] To provide a high-quality cellular fiber having a small fiber fineness, having no large air bubble therein, and is uniform in fiber diameter in the lengthwise direction. [MEANS FOR SOLVING PROBLEMS] Disclosed is a cellular fiber comprising a thermoplastic polymer and has an average fiber diameter of 1 to 50 µm inclusive, wherein air bubbles constitute 10 to 90% inclusive of the cross-section of the fiber, the ratio of the maximum diameter of an air bubble to the diameter of the fiber is 0.08 or less, and the fiber has a U% (half) of 2 or less. The cellular fiber can be produced by: kneading a thermoplastic polymer and nitrogen and/or carbon dioxide in a molten state to prepare a mixed polymer, introducing the mixed polymer to a spinning die; applying a shear rate (?_H) of 50,000 to 1,000,000 sec^-1 to the mixed polymer at the spinning die; discharging the mixed polymer into a low-pressure area to decrease the pressure of the mixed polymer, thereby causing to foam the polymer; cooling the foamed polymer; drawing the polymer; and winding the polymer.