Carbon fiber bundle and method for producing carbon fiber bundle

Abstract

Provided a carbon fiber bundle in which the average diameter of carbon fiber single-fibers is increased without reducing the strand strength and strand elastic modulus of the carbon fibers, and a method for producing the same. Also provided is a carbon fiber suitable for a carbon fiber composite material having excellent compressive strength in the fiber axial direction. The carbon fiber bundle of the present invention has a strand strength of 4.5 GPa or more, a strand elastic modulus of 320 GPa or more, substantially no twist, and an average diameter of carbon fiber single-fibers of 6.5 μm or more and 8.5 μm or less.

1 . A carbon fiber bundle having a strand strength of 4.5 GPa or more and a strand elastic modulus of 320 GPa or more,
 having substantially no twist, and   having an average diameter of carbon fiber single-fibers of 6.5 μm or more and 8.5 μm or less.   
     
     
         2 . The carbon fiber bundle according to  claim 1 , wherein the carbon fiber single-fibers each have a crystallite size Lc of 3.4 nm or more and 4.1 nm or less. 
     
     
         3 . The carbon fiber bundle according to  claim 1 , wherein the carbon fiber single-fibers have an average void length of 22.0 nm or less. 
     
     
         4 . The carbon fiber bundle according to  claim 1 , wherein the carbon fiber single-fibers have a fracture surface formation energy of 18 N/m or more. 
     
     
         5 . The carbon fiber bundle according to  claim 1 , wherein the carbon fiber single-fibers in the carbon fiber bundle have a coefficient of variation (CV %) in elastic modulus in a single-fiber tensile test of 17.5% or less. 
     
     
         6 . The carbon fiber bundle according to  claim 1 , wherein the carbon fiber bundle is non-entangled. 
     
     
         7 . The carbon fiber bundle according to  claim 1 , wherein a product of the average diameter (unit: μm) of the carbon fiber single-fibers and the strand strength (unit: GPa) of the carbon fiber bundle is 31 or more. 
     
     
         8 . The carbon fiber bundle according to  claim 1 , wherein the strand strength is 4.85 GPa or more, and the strand elastic modulus is 365 GPa or more. 
     
     
         9 . The carbon fiber bundle according to  claim 1 , wherein the average diameter of the carbon fiber single-fibers is 6.8 μm or more, the strand strength of the carbon fiber bundle is 4.65 GPa or more, and the strand elastic modulus of the carbon fiber bundle is 365 GPa or more and 403 GPa or less. 
     
     
         10 . The carbon fiber bundle according to  claim 1 , wherein the average diameter of the carbon fiber single-fibers is 7.5 μm or more. 
     
     
         11 . The carbon fiber bundle according to  claim 1 , wherein the carbon fiber bundle has a knot strength of 80 N/mm 2  or more. 
     
     
         12 . The carbon fiber bundle according to  claim 1 , wherein the carbon fiber single-fibers have a density of 1.79 g/cm 3  or more. 
     
     
         13 . A method for producing a carbon fiber bundle, the method comprising heating a carbon fiber precursor acrylic fiber bundle in an oxidizing atmosphere to form a stabilized fiber bundle, and heating the stabilized fiber bundle in a non-oxidizing atmosphere to form a carbon fiber bundle, wherein
 in the heating in the non-oxidizing atmosphere, a temperature rising rate when a temperature is raised from 1800° C. to 2200° C. is from 200 to 500° C./min, and   an average diameter of carbon fiber single-fibers contained in the produced carbon fiber bundle is 6.5 μm or more and 8.5 μm or less.   
     
     
         14 . A method for producing a carbon fiber bundle according to  claim 13 , the method comprising the following steps (1) and (2):
 (1) a coagulation step of discharging an acrylonitrile-based polymer solution from a discharge hole into air by using a dry-wet spinning method, and then coagulating the acrylonitrile-based polymer solution in a coagulation bath including an aqueous solution having a temperature of 10° C. or lower and an organic solvent concentration of 80.0 mass % or more and 81.0 mass % or less to produce a coagulated yarn bundle containing the organic solvent; and   (2) a second stretching step of stretching the coagulated yarn bundle produced in the coagulation step at a stretching ratio of 2.0 times or more and 3.2 times or less in a hot aqueous solution having a temperature of 75° C. or higher and an organic solvent concentration of 40 mass % or more and 65 mass % or less to produce the carbon fiber precursor acrylic fiber bundle.   
     
     
         15 . The method for producing a carbon fiber bundle according to  claim 13 , the method further comprising a first stretching step of stretching the coagulated yarn bundle produced in the coagulation step in air at a stretching ratio of 1.00 times or more and 1.20 times or less between the coagulation step and the second stretching step, wherein the coagulated yarn bundle produced in the first stretching step is stretched in the second stretching step. 
     
     
         16 . The method for producing a carbon fiber bundle according to  claim 13 , wherein in the second stretching step, the coagulated yarn bundle is stretched, then the organic solvent is removed, the coagulated yarn bundle is shrunk or stretched at a ratio of 0.96 times or more and 1.30 times or less in hot water at a temperature of 90° C. or higher, and is stretched at a stretching ratio of 3.7 times or more and 4.2 times or less in a pressurized water vapor atmosphere to produce the carbon fiber precursor acrylic fiber bundle. 
     
     
         17 . The method for producing a carbon fiber bundle according to  claim 13 , wherein the concentration of the organic solvent in the aqueous solution used in the coagulation step is 80.2 mass % or more and 80.6 mass % or less. 
     
     
         18 . The method for producing a carbon fiber bundle according to  claim 13 , wherein the organic solvent is dimethylformamide. 
     
     
         19 . The method for producing a carbon fiber bundle according to  claim 13 , the method comprising the following steps (3) to (6):
 (3) a stabilization step of heating a carbon fiber precursor acrylic fiber bundle or the carbon fiber precursor acrylic fiber bundle produced in the second stretching step in an oxidizing atmosphere having a temperature gradient within an ambient temperature range of 200° C. or higher and 260° C. or lower at an elongation percentage of 3.0% or more and 8.0% or less to produce a stabilized fiber bundle having a density of 1.33 g/cm 3  or more and 1.36 g/cm 3  or less;   (4) a first carbonization step of heating the stabilized fiber bundle produced in the stabilization step in a non-oxidizing atmosphere having a temperature gradient within an ambient temperature range of 300° C. or higher and 900° C. or lower at an elongation percentage of 4.0% or more and 5.0% or less;   (5) a second carbonization step of heating the fiber bundle while applying a tension of 0.15 cN/dtex or more and 0.21 cN/dtex or less to the fiber bundle in a non-oxidizing atmosphere having a temperature gradient within an ambient temperature range of 1000° C. or higher and 1800° C. or lower after the first carbonization step; and   (6) a third carbonization step of heating the fiber bundle while applying a tension of 0.15 cN/dtex or more and 0.23 cN/dtex or less to the fiber bundle in a non-oxidizing atmosphere having a temperature gradient within an ambient temperature range of 1700° C. or higher and 2300° C. or lower after the second carbonization step.   
     
     
         20 . The method for producing a carbon fiber bundle according to  claim 19 , wherein in the third carbonization step, a temperature rising rate when the ambient temperature is increased from 1800° C. to 2200° C. is 210° C./min or more and 340° C./min or less. 
     
     
         21 . The method for producing a carbon fiber bundle according to  claim 19 , wherein in the third carbonization step, the temperature rising rate when the ambient temperature is increased from 1800° C. to 2200° C. is 215° C./min or more and 300° C./min or less. 
     
     
         22 . The method for producing a carbon fiber bundle according to  claim 19 , wherein a difference between a maximum ambient temperature in the second carbonization step and an inlet ambient temperature in the third carbonization step is 500° C. or lower. 
     
     
         23 . The method for producing a carbon fiber bundle according to  claim 19 , wherein the difference between the maximum ambient temperature in the second carbonization step and the inlet ambient temperature in the third carbonization step is 300° C. or lower. 
     
     
         24 . The method for producing a carbon fiber bundle according to  claim 13 , wherein a maximum temperature of a heating temperature in the heating in the non-oxidizing atmosphere is from 2100 to 2300° C.