Carbon-fiber-precursor fiber bundle, carbon fiber bundle, and uses thereof

1. A carbon-fiber-precursor acryl fiber bundle, comprising:

a polyacrylonitrile-based copolymer comprising from 95 to 99 mol % of an acrylonitrile unit and from 1 to 5 mol % of a hydroxyalkyl (meth)acrylate unit,

wherein the fiber bundle has a single-fiber fineness of from 1.5 dtex to 3.0 dtex and a roundness of 0.9 or less in a cross-section shape perpendicular to a fiber axis of the single fiber,

wherein the roundness is a value determined using equation (1): roundness=4πS/L 2 , wherein S is a cross-sectional area of the single fiber and L is a circumferential length of the single fiber, and S and L are obtained by observing, under an SEM, the cross-section of the single fiber perpendicular to the fiber axis of the single fiber and analyzing the obtained image, and

wherein when a constant velocity temperature rising exothermic curve is from 30° C. to 450° C. measured at a temperature rising rate of 10° C./minute in air flow at 100 ml/minute at 30° C. and 0.10 MPa using a heat flux type differential scanning calorimeter;

a heat quantity Ja obtained by integrating an exothermic velocity from 230° C. to 260° C. of the constant velocity temperature rising exothermic curve is from 100 kJ/kg to 250 kJ/kg; and

a heat quantity Jb obtained by integrating an exothermic velocity from 260° C. to 290° C. is from 550 kJ/kg to 1050 kJ/kg.

2. The fiber bundle according to claim 1 , wherein the roundness is from 0.8 to 0.9.

3. The fiber bundle according to claim 1 , wherein a melting point under heat and humidity of the polyacrylonitrile-based copolymer is from 160 to 175° C.

4. The fiber bundle according to claim 1 wherein a water contact angle is from 40° to 70°.

5. The fiber bundle according to claim 4 , wherein an oxidation depth De is from 4.0 μm to 6.0 μm in the polyacrylonitrile-based copolymer, and the oxidation depth De is obtained by a method comprising:

dissolving the polyacrylonitrile-based copolymer at a concentration of 25% by mass in dimethylformamide to prepare a copolymer solution;

applying the copolymer solution onto a glass plate;

drying the glass plate on which the copolymer solution was applied in air at 120° C. for 6 hours to evaporate dimethylformamide and make a film having a constant thickness in the range of from 20 μm to 40 μm;

flame-proof treating by treating the obtained film with heat in air at 240° C. for 60 minutes and further in air at 250° C. for 60 minutes to obtain a flame-proofed film;

embedding the flame-proofed film in a resin followed by polishing to obtain a polished flame-proofed film;

observing a cross-section perpendicular to a surface of the polished flame-proofed film at a magnification of 1500 using a fluorescence microscope; and

observing an oxidation progressing part as a relatively dark layer and observing an oxidation non-progressing part as a relatively light layer in the cross-section, thus a distance from the surface of the polished flame-proofed film to a boundary between the dark layer and the light layer is measured at least at 5 points on one cross-section, the same measurement is further repeated on three cross-sections, and their arithmetic average is used as the oxidation depth De (μm).

6. The fiber bundle according to claim 1 , wherein the heat quantity Ja is from 100 kJ/kg to 160 kJ/kg.

7. The fiber bundle according to claim 1 , wherein a calorific value per unit mass at 215 to 300° C. obtained by a measurement using a heat flux type differential scanning calorimeter is 3200 kJ/kg or more wherein a temperature rising rate in the measurement using the heat flux type differential scanning calorimeter is 2° C./minute and an atmosphere is air and

wherein a half-value width of solid 1 H-NMR spectra at a measurement temperature of 160° C. is from 10.0 kHz to 14.5 kHz.

8. The fiber bundle according to claim 7 , wherein the calorific value at 215 to 300° C. is 3300 kJ/kg or more.

9. The fiber bundle according to claim 7 , wherein the half-value width is from 10.0 kHz to 13.5 kHz.

10. A method for flame-proof treatment, comprising treating the carbon-fiber-precursor acryl fiber bundle according to any one of claims 1 , 4 , 7 , 8 , or 9 under an oxidation atmosphere at temperature of from 220° C. to 300° C. for from 30 minutes to 90 minutes or less to obtain a flame-proofed fiber bundle having a fiber density of from 1.35 g/cm 3 to 1.43 g/cm 3 .

11. A method of producing a carbon fiber bundle having a diameter Di of 8 μm or more and a roundness of a shape of 0.90 or less in a cross-section perpendicular to a fiber axis of a single fiber, the method comprising:

flame-proof treating the carbon-fiber-precursor acryl fiber bundle according to any one of claims 1 , 4 , 7 , 8 , or 9 under an oxidation atmosphere at temperature of from 220° C. to 300° C. for from 30 minutes to 90 minutes to obtain a flame-proofed fiber bundle having a fiber density of from 1.35 g/cm 3 to 1.43 g/cm 3 ; and

further carbonizing the flame-proofed fiber bundle at temperature of from 800° C. to 2000° C. in an inert gas,

wherein the diameter Di is as measured by:

preparing a sample, wherein a carbon fiber bundle cut into a length of 5 cm is embedded in an epoxy resin, wherein epomount base: epomount curing agent=100:9 by mass ratio, and cut into a length of 2 cm to expose a cross-sectional surface, to which a mirror surface treatment is given;

performing an etching treatment to the cross-sectional surface of the sample with a Plasma Etching Apparatus JP-170 manufactured by JEOL Ltd. under an atmosphere of Ar/O 2 =75/25, a plasma output power of 50 W, a vacuum degree of about 120 Pa, and a treatment time period of 5 minutes;

observing under SEM, wherein the cross-sectional surface of the samples obtained by the preparing and the performing is observed using SEM PHILIPS FEI-XL20, and five photographs of 5 or more fiber cross-sections on an image are taken randomly; and

measuring a diameter of a single fiber cross-section in a carbon fiber bundle, wherein for each sample, 20 single fiber cross-sections from the 5 SEM photographs, wherein 3 or more single fiber cross-sections from one photograph, are randomly selected, a contour of the fiber cross-section is traced using an image analysis software Image-ProPLUS, manufactured by Nippon Roper K.K., a major axis d, which is a maximum feret diameter of the cross-section, is measured, and a mean value of the major axes of all single fiber cross-sections selected is the diameter Di of the single fiber in the carbon fiber bundle.