Polyamide fibers having improved properties and their production

Claim 1

A stretched polyamide fiber excellent in strength, which is characterized by having a relative viscosity of not less than 2.3 (measured on a 96% by weight sulfuric acid solution having a polyamide concentration of 10 mg/ml at 20° C.), having an index of birefringence in section which satisfies the following relationship: Δn.sub.A -Δn.sub.B <0 (wherein Δn A is the index of birefringence of fiber at the position of r/R=0.9, Δn B is the index of birefringence of fiber at the position of r/R=0.0, R is the radius of the fiber section and r is the distance from the central axis of the fiber section), and showing the following physical constants: Index of birefringence of fiber (Δn) (measured after 24 hours under the conditions of 30° C. and 80% relative humidity)≧50×10 -3 ; Break strength not less than 12 g/d; Fiber long period spacing value at length by small angle X-ray diffraction≧100 Å; Specific gravity≧1.140; Dry heat shrinkage≦15%; Monofilament denier of less than 35 d said polyamide being selected from the group consisting of polycapramide, polyhexamethylene adipamide, polyhexamethylene sebacamide, copolymers of the monomeric components of the aforestated polyamides, and condensation products of diamines and linear aliphatic dicarboxylic acids.