Acrylonitrile swollen fiber for carbon fiber, precursor fiber bundle, stabilized fiber bundle, carbon fiber bundle and production methods thereof

A method of producing a swollen fiber, comprising:

• (i) preparing a dope at a temperature of 50° C. or more and 70° C. or less by dissolving an acrylonitrile-based copolymer, which is obtained by copolymerizing acrylonitrile in an amount of 96.0 mass % or more and 99.7 mass % or less and an unsaturated hydrocarbon having at least one carboxyl group or ester group in an amount of 0.3 mass % or more and 4.0 mass % or less, as essential components, in an organic solvent in a concentration in the range of 20 mass % or more and 25 mass % or less;
• (ii) obtaining a coagulated fiber bundle containing the organic solvent by ejecting the dope from ejection holes into the air by use of a dry-wet spinning method, followed by coagulating in a coagulation bath constituted of an aqueous solution containing an organic solvent in a concentration of 78.0 mass % or more and 82.0 mass % or less, at a temperature of −5° C. or more and 20° C. or less;
• (iii) drawing the coagulated fiber bundle in the air at a ratio in the range of 1.0 time or more and 1.25 times or less, followed by further drawing in a warm aqueous solution containing an organic solvent, a total draw ratio of both drawing processes being 2.6 times or more and 4.0 times or less;
• and
• (iv) subsequently removing the solvent with warm water and further drawing in hot water at a ratio of 0.98 times or more and 2.0 times or less.

A method of producing a stabilized fiber bundle comprising feeding a precursor fiber bundle to a hot-air circulation type oven for stabilization at a temperature of 220 to 260° C. for 30 minutes or more and 100 minutes or less, thereby applying heat treatment at an extension rate of 0% or more and 10% or less under an oxidizing atmosphere, and the method satisfying the following conditions:

• (1) intensity ratio (B/A) of peak A (2θ=25°) and peak B (2θ=17°) in the equatorial-line direction, which is determined by wide angle x-ray diffraction measurement of the fiber bundle, is 1.3 or more,
• (2) orientation degree of peak B is 80% or more,
• (3) orientation degree of peak A is 79% or more, and
• (4) density is 1.335 g/cm3 or more and 1.360 g/cm3 or less,
• wherein, prior to the feeding, the precursor fiber bundle is obtained by copolymerizing acrylonitrile in an amount of 96.0 mass % or more and 99.7 mass % or less and an unsaturated hydrocarbon having at least one carboxyl group or ester group in an amount of 0.3 mass % or more and 4.0 mass % or less, as essential components, and having a silicon content of 1700 ppm or more and 5000 ppm or less when the fiber bundle is treated with a finishing oil agent containing silicone compounds as main components, wherein the silicon content is 50 ppm or more and 300 ppm or less after the finishing oil agent is washed away with methyl ethyl ketone by using a Soxhlet extraction apparatus for 8 hours.