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

Abstract

Provided is a carbon fiber bundle for obtaining a fiber-reinforced plastic having high mechanical characteristics. An acrylonitrile swollen fiber for a carbon fiber having openings of 10 nm or more in width in the circumference direction of the swollen fiber at a ratio in the range of 0.3 openings/μm 2 or more and 2 openings/μm 2 or less on the surface of the swollen fiber, and the swollen fiber is not treated with a finishing oil agent. A precursor fiber obtained by treating the swollen fiber with a silicone-based finishing oil agent has a silicon content of 1700 ppm or more and 5000 ppm or less, and 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. The fiber is preferably an acrylonitrile copolymer containing 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.

1 . An acrylonitrile swollen fiber for a carbon fiber having openings of 10 nm or more in width in the circumference direction of the swollen fiber at a ratio in the range of 0.3 openings/μm 2  or more and 2 openings/μm 2  or less on the surface of the swollen fiber, and that is not treated with a finishing oil agent. 
     
     
         2 . The swollen fiber according to  claim 1 , wherein, in a fine pore distribution measured by a mercury press-in method, an average fine pore size is 55 nm or less and a total fine pore volume is 0.55 ml/g or less. 
     
     
         3 . The swollen fiber according to  claim 1  or  2 , wherein a polymer constituting the swollen fiber is an acrylonitrile-based copolymer containing an acrylonitrile unit in an amount of 96.0 mass % or more and 99.7 mass % or less and an unsaturated hydrocarbon unit 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. 
     
     
         4 . A method of producing a swollen fiber, including:
 [1] a step of 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;   [2] a step of 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;   [3] a step of 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   [4] a step of 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.   
     
     
         5 . The method according to  claim 4 , wherein the organic solvent is either dimethyl formamide or dimethyl acetamide. 
     
     
         6 . The method according to  claim 4  or  5 , wherein a draw ratio in the warm aqueous solution is 2.5 times or more and 4.0 times or less. 
     
     
         7 . A precursor fiber bundle for a carbon fiber formed of an acrylonitrile 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, 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. 
     
     
         8 . The precursor fiber bundle according to  claim 7 , wherein a fineness of a single fiber is 0.5 dtex or more and 1.0 dtex or less; the ratio of the major axis and the minor axis (major axis/minor axis) of a cross-section of a single fiber is 1.00 or more and 1.01 or less; no surface uneven structure extending in the fiber-axis direction of a single fiber is present; difference in height (Rp-v) between a highest portion and a lowest portion is 30 nm or more and 100 nm or less; and a center-line average roughness (Ra) is 3 nm or more and 10 nm or less. 
     
     
         9 . A method of producing a precursor fiber bundle for a carbon fiber including applying a finishing oil agent containing silicone compounds as main components to a bundle of the swollen fiber obtained by any of the methods according to  claims 4  to  6 , in an amount of 0.8 mass % or more and 1.6 mass % or less based on 100 mass % of the swollen fiber, followed by drying and then drawing by a heat drawing method or a steam drawing method at a ratio in the range of 1.8 times or more and 6.0 times or less. 
     
     
         10 . The method according to  claim 9 , wherein as the silicone compound, an amino-modified silicone compound satisfying the following conditions (1) and (2) is used:
 (1) kinematic viscosity at 25° C. is 50 cSt or more and 5000 cSt or less, and   (2) amino equivalent mass is 1,700 g/mol or more and 15,000 g/mol or less.   
     
     
         11 . A method of producing a precursor fiber bundle for a carbon fiber by applying a finishing oil agent containing silicone compounds as main components to a bundle of the swollen fiber according to any of  claims 1  to  3 . 
     
     
         12 . A method of producing a stabilized fiber bundle including feeding the precursor fiber bundle obtained by the method according to  claim 11  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/cm 3  or more and 1.360 g/cm 3  or less. 
 
     
     
         13 . A method of producing a stabilized fiber bundle including feeding the precursor fiber bundle according to  claim 7  or  8  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/cm 3  or more and 1.360 g/cm 3  or less. 
 
     
     
         14 . The method of producing the stabilized fiber bundle according to  claim 12  or  13 , wherein extension treatment is separately performed in at least three sets of conditions: an extension rate of 3.0% or more and 8.0% or less at a fiber density in the range of 1.200 g/cm 3  or more and 1.260 g/cm 3  or less; an extension rate at 0.0% or more and 3.0% or less at a fiber density in the range of 1.240 g/cm 3  or more and 1.310 g/cm 3  or less; and an extension rate of −1.0% or more and 2.0% or less at a fiber density in the range of 1.300 g/cm 3  or more and 1.360 g/cm 3  or less. 
     
     
         15 . A carbon fiber bundle, wherein a strength of a strand impregnated with a resin is 6000 MPa or more; a strand elastic modulus measured by an ASTM method is 250 to 380 GPa; the ratio of the major axis and the minor axis (major axis/minor axis) of a cross-section of a single fiber perpendicular to the fiber-axis direction is 1.00 to 1.01; the diameter of a single fiber is 4.0 μm to 6.0 μm; and the number of voids having a diameter of 2 nm or more and 15 nm or less present in the cross-section of a single fiber perpendicular to the fiber-axis direction is 1 or more and 100 or less. 
     
     
         16 . The carbon fiber bundle according to  claim 15 , wherein the average diameter of the voids is 6 nm or less. 
     
     
         17 . The carbon fiber bundle according to  claim 15  or  16 , wherein the sum A (nm 2 ) of areas of the voids is 2,000 nm 2  or less. 
     
     
         18 . The carbon fiber bundle according to  claim 16  or  17 , wherein voids corresponding to 95% or more of the sum A (nm 2 ) of areas of the voids, which are present in the cross-section of a single fiber perpendicular to the fiber axis direction, are present in an area from the surface of the fiber to a depth of 150 nm. 
     
     
         19 . The carbon fiber bundle according to any of  claims 15  to  18 , wherein the carbon fiber has a knot tenacity of 900 N/mm 2  or more. 
     
     
         20 . A method of producing a carbon fiber bundle, including treating the precursor fiber bundle according to  claim 8  with heat under an oxidizing atmosphere to obtain a stabilized fiber bundle having a density of 1.335 g/cm 3  or more and 1.355 g/cm 3  or less; then performing heating in a first carbonization furnace having a temperature gradient of 300° C. or more and 700° C. or less under an inert atmosphere while extending the extension rate to a rate of 2% or more and 7% or less for 1.0 minute or more to 3.0 minutes or less; and subsequently performing heat treatment in at least one carbonization furnace having a temperature gradient from 1000° C. to a desired temperature under an inert atmosphere while extending the extension rate to a rate of −6.0% or more and 2.0% or less for 1.0 minute or more and 5.0 minutes or less. 
     
     
         21 . A method of producing a carbon fiber bundle, including treating the precursor fiber bundle obtained by the method according to  claim 9  or  10  with heat under an oxidizing atmosphere to obtain a stabilized fiber bundle having a density of 1.335 g/cm 3  or more and 1.355 g/cm 3  or less; then performing heating in a first carbonization furnace having a temperature gradient of 300° C. or more and 700° C. or less under an inert atmosphere while extending the extension rate to a rate of 2% or more and 7% or less for 1.0 minute or more to 3.0 minutes or less; and subsequently performing a heat treatment in at least one carbonization furnace having a temperature gradient from 1000° C. to a desired temperature under an inert atmosphere while extending the extension rate to a rate of −6.0% or more and 2.0% or less for 1.0 minute or more and 5.0 minutes or less.