JP5251655B2 - Carbon fiber and method for producing the same - Google Patents

Description

  The present invention relates to a carbon fiber excellent in adhesive strength with a matrix resin and a method for producing the same.

  Carbon fibers produced using polyacrylonitrile-based fibers as raw materials are used in a wide range from aircraft and automobile parts to general industrial uses such as sports equipment. In particular, high-strength and high-modulus carbon fibers are widely used for aircraft material applications, and there is a need for further enhancement of performance.

  In such applications, in most cases, carbon fibers are used as a composite material having a resin as a matrix. In order to obtain sufficient mechanical properties as a composite material, it is necessary that the carbon fiber and the matrix resin have sufficient and stable adhesive strength. For this reason, carbon fibers are generally produced by subjecting polyacrylonitrile-based fibers or the like to a flame treatment, followed by a carbonization / graphitization treatment, a so-called firing step, and then a surface treatment. And it is known that this adhesive strength is influenced by the kind of carbon fiber and matrix resin, the surface treatment after firing of the carbon fiber, and the like. In particular, carbon fibers having a high elastic modulus with a strand tensile elastic modulus of 350 GPa or more are often graphitized at a high firing temperature of 2000 ° C. or higher, and generally have a high crystal size and degree of orientation. The modification is also difficult to proceed, and the interfacial adhesion with the matrix resin is generally weak.

  In order to solve this problem, the relationship between the surface oxygen concentration, nitrogen concentration and silicon concentration and the method of surface electrolysis treatment are defined by measuring and analyzing the X-ray photoelectron spectroscopy of the carbon fiber surface. Has been proposed (see Patent Document 1). However, in this proposal, since the definition regarding the chemical state and structure of each element is not clear, it is difficult to say that the surface state is uniquely defined. Therefore, it does not necessarily show excellent adhesion.

  Moreover, it is proposed that the surface state of the carbon fiber excellent in adhesiveness to the matrix resin is shown from the result of the peak splitting of the carbon C1s spectrum of the X-ray photoelectron spectroscopy on the surface of the carbon fiber (see Patent Document 2). . However, this proposal is not an indicator of carbon fibers having excellent adhesiveness because of the high volatility of the peak division itself and the large error of the peak division itself.

  Furthermore, the surface oxygen concentration and nitrogen concentration of X-ray photoelectron spectroscopy, the half width of N1s spectrum and peak splitting results, and the surface area measurement and analysis by AFM, the adhesion to the matrix resin is maintained while maintaining the balance with strength. The carbon fiber which can improve the composite characteristic of CAI (Compression After Impact: Compact After Impact: Compact after impact) of the SACMA method is proposed (refer patent document 3). However, in this proposal, since the definition relating to the chemical state and structure of carbon that is the main component element on the surface of the carbon fiber is not clear, it is not a carbon fiber that is uniquely defined as in the above-mentioned Patent Document 1, The carbon fiber is also defined from the analysis of the peak splitting of the nitrogen N1s spectrum, but this is also not a definition of a sufficient surface state from the viewpoint of the arbitraryness and error of the peak splitting as in the case of the above-mentioned Patent Document 2. Therefore, the carbon fiber was not excellent in properties such as adhesiveness, strength or CAI.

In addition, a surface treatment method for carbon fibers having an elastic modulus of 30 tf / mm ² or more is defined, and the characteristics of the modified surface are derived from the analysis of electrochemical cyclic voltammetry and the relationship with the surface fragile layer and surface area. It has been proposed to try to improve adhesiveness with a matrix resin (see Patent Document 4). In this proposal, the influence of the fragile layer of excessive oxidation is certainly considered, but the influence of the chemical state and structure on the surface of the carbon fiber is much greater than that, and the carbon fiber surface treatment method with an excellent adhesive force is not necessarily used. There wasn't.

  As described above, even with the techniques proposed so far, the carbon fiber excellent in adhesive strength with the matrix resin and the manufacturing method thereof are not always satisfactory.

JP-A 62-276075 JP 05-156522 A JP 2006-183173 A JP-A-01-092470

  An object of the present invention is to define an ideal surface state of a carbon fiber and to provide a carbon fiber excellent in adhesive strength with a matrix resin and a method for producing the carbon fiber.

  In order to achieve the above-mentioned object, the present invention has the following configuration.

That is, the carbon fiber of the present invention has a carbon fiber surface oxygen concentration (O / C ratio: number of oxygen atoms / number of carbon atoms) in the range of 0.20 to 0.35 in the measurement and analysis of X-ray photoelectron spectroscopy. From the binding energy position (284.6 eV) of the carbon-carbon component, which is the main peak of the carbon C1s inner-shell spectrum, within the range of +1.6 to +2.6 eV (binding energy 286.2 to 287.2 eV). A carbon fiber having a sub-peak component at an energy position , wherein the carbon C1s inner-shell spectrum has a sub-peak height that is one third or more of the height of the main peak .

  In addition, the carbon fiber production method of the present invention uses the carbon fiber to be treated as an anode and the electrolytic treatment through a direct current between the cathode and the cathode. Electrolytic treatment is performed using an aqueous solution containing a peroxodisulfate having a peroxodisulfate ion concentration of 1.0 wt% or more as an electrolytic solution at 30 to 300 coulombs / g per unit.

  According to a preferred embodiment of the method for producing carbon fiber of the present invention, the peroxodisulfate is ammonium peroxodisulfate, sodium peroxodisulfate, potassium peroxodisulfate, rubidium peroxodisulfate, barium peroxodisulfate and peroxodisulfate. At least one salt selected from the group consisting of lead is particularly preferably used.

  According to the present invention, carbon fibers excellent in adhesiveness with a matrix resin, that is, interfacial shear strength can be obtained.

  In the present invention, carbon fibers are produced by firing carbon fiber precursor fibers in the production process.

  Examples of the carbon fiber precursor fiber used in the present invention include fibers such as polyacrylonitrile fiber, rayon fiber, pitch fiber, and polyvinyl alcohol fiber. Among them, a fiber obtained from an acrylonitrile polymer or a copolymer thereof, that is, a polyacrylonitrile fiber can be preferably used as a carbon fiber precursor because a carbon fiber exhibiting high tensile strength can be obtained. . The carbon fiber precursor fiber is provided with a silicone-based oil for the purpose of giving high heat resistance, suppressing fusion between single fibers in the firing process, and improving the passability of the firing process. Preferably it is.

  In the present invention, in the carbon fiber production process, the above-mentioned carbon fiber precursor fiber is heat-treated in an oxidizing atmosphere such as air at 200 ° C. to 300 ° C., and then a maximum temperature of 300 to 2000 ° C. A so-called graphitization step in which heat treatment is performed in an inert gas atmosphere such as nitrogen, and more preferably, a heat treatment in an inert gas atmosphere such as nitrogen having a maximum temperature of 2000 ° C. to 3000 ° C. It is desirable to do.

  In the present invention, the carbon fiber precursor fiber is usually an aggregate of a plurality of filaments, that is, a fiber bundle, but the number of filaments constituting one fiber bundle is generally 1,000 to 80,000. The scope of the book. However, if the number of filaments is too large, the unevenness of the surface treatment of carbon fiber, which is a subsequent process, becomes large, and therefore about 1,000 to 50,000 are preferable. The number of filaments of the carbon fiber produced in the present invention is also usually equal to the number of filaments of the carbon fiber precursor fiber.

  In the present invention, it is important to perform a carbon fiber surface treatment after the firing step in order to enhance the adhesion with the matrix resin. The surface treatment method can be performed by electrolytic treatment from the viewpoint of reducing the variation in treatment.

  The electrolytic treatment of the surface of the carbon fiber means that the treated carbon fiber is the anode when the treated carbon fiber is used as an anode and a direct current is passed between the cathode and the electrolyte in an electrolytic solution such as an acid or alkali. This is a treatment for imparting a functional group to the carbon fiber surface by utilizing an oxidation reaction that occurs on the carbon fiber side.

  In the present invention, it is important to perform electrolytic treatment using an aqueous solution containing peroxodisulfate as the electrolytic solution. Further, in the aqueous solution containing peroxodisulfate, at least selected from the group consisting of ammonium peroxodisulfate, sodium peroxodisulfate, potassium peroxodisulfate, rubidium peroxodisulfate, barium peroxodisulfate, and lead peroxodisulfate. It is preferable to use an aqueous solution containing one kind of salt as the electrolytic solution.

  The reason why aqueous solutions containing these peroxodisulfates are used as the electrolyte is that peroxodisulfate ions have a strong oxidizing action in the electrolyte, and at the same time as electrolysis at the anode (carbon fiber surface), the outermost surface of the carbon fiber This is probably because the introduction of oxygen on the surface was promoted while changing the structure of the carbon skeleton. The concentration of the electrolytic solution is preferably in the range of 0.001 to 1 mol / l. Furthermore, the concentration of the electrolytic solution is more preferably in the range of 0.001 to 0.2 mol / l from the viewpoint of safety in handling chemicals.

  In the present invention, it is important that the peroxodisulfate ion concentration in the peroxodisulfate is 1.0 wt% or more. The upper limit of the peroxodisulfate ion concentration is about 15 wt% from the viewpoint of chemical safety.

  Moreover, it is important that the total amount of electrolytic electricity employed in the electrolytic treatment is 30 to 300 coulomb / g per gram of carbon fiber. If the total amount of electrolytic electricity is less than 30 coulombs / g, functional groups may not be sufficiently imparted to the carbon fiber surface, and sufficient adhesive strength cannot be obtained. On the other hand, when the total amount of electrolytic electricity exceeds 300 coulombs / g, the carbon fiber strength is reduced due to electrical breakdown.

  In the present invention, in the above electrolytic treatment, the peroxodisulfate ion concentration in the peroxodisulfate is 1.0 wt% or more, and the total amount of electrolysis used in the electrolytic treatment is 30 to 300 coulomb / g per gram of carbon fiber. The carbon concentration on the surface of the carbon fiber (O / C ratio: number of oxygen atoms / number of carbon atoms) is in the range of 0.20 to 0.35, and carbon that is the main peak of the carbon C1s inner shell spectrum A carbon fiber having a sub-peak component at an energy position in the range of +1.6 to +2.6 eV (binding energy 286.2 to 287.2 eV) is obtained from the binding energy position (284.6 eV) of the intercarbon component.

  The electrolysis method is either a direct energization method in which an electrode is directly in contact with a carbon fiber bundle or an indirect energization method in which an electric current is indirectly passed through an electrolyte between the electrode and the carbon fiber bundle. However, the indirect energization method is preferable in terms of safety.

  In the present invention, it is most important to perform X-ray photoelectron spectroscopy measurement / analysis on the surface of the carbon fiber after the surface treatment, washing with water and drying. Here, the measurement method of X-ray photoelectron spectroscopy is to measure the kinetic energy of photoelectrons emitted from the surface of the carbon fiber with an apparatus called an energy analyzer by irradiating the carbon fiber of the sample with X-rays in an ultra-high vacuum. It is an analysis technique to do. By examining the kinetic energy of the photoelectrons emitted from the carbon fiber surface of the sample, the binding energy converted from the energy value of the X-rays incident on the carbon fiber of the sample is uniquely determined. From the binding energy and the photoelectron intensity The type and concentration of the element present on the outermost surface (-10 nm) of the sample and its chemical state can be analyzed.

  In the present invention, the surface of the carbon fiber after surface treatment, washing and drying is subjected to measurement / analysis by X-ray photoelectron spectroscopy, and the oxygen concentration (O / C ratio: number of oxygen atoms / carbon atom) on the surface of the carbon fiber. Number) is in the range of 0.20 to 0.35, and from the binding energy position (284.6 eV) of the main peak (peak top) of the carbon C1s core spectrum, +1.6 to +2.6 eV (binding energy) 286.2 to 287.2 eV) carbon fiber having a sub-peak component with a height of 1/3 or more of the main peak (cps) is excellent in adhesion to the matrix resin. It will be a thing. If the oxygen concentration is too high, a surface fragile layer may be generated due to excessive oxidation, so a preferable oxygen concentration is 0.20 to 0.28.

  The reason why the result of measurement / analysis of X-ray photoelectron spectroscopy affects the adhesion to the matrix resin is considered as follows.

  That is, the oxygen concentration on the carbon fiber surface is considered to correspond to the total amount of functional groups such as hydroxyl groups and carboxyl groups containing oxygen generated by surface treatment or the like. Normally, the higher the oxygen concentration, the better the adhesion in the literature so far, but simply increasing the oxygen concentration warps the matrix resin due to the formation of a weak surface layer. It is known that the adhesiveness of the resin deteriorates. Therefore, it may be necessary to form a surface having a specific functional group or chemical structure. In the present invention, from the binding energy position (284.6 eV) of the main peak (peak top) of the carbon C1s core spectrum of X-ray photoelectron spectroscopy, +1.6 to +2.6 eV (binding energy 286.2 to 287.2 eV). The effect of the functional group of the sub-peak component having a component at an energy position in the range of Moreover, it discovered that the carbon fiber surface in which a sub peak has the height (cps) of 1/3 or more of a main peak is very effective in improving adhesiveness with a matrix resin. If the sub-peak has a sub-peak intensity equal to or higher than that of the main peak, it may be excessively oxidized and a surface fragile layer may be generated. Therefore, the height of the sub-peak is preferably in the range of one third to less than one.

  Although the functional group component that enhances adhesion is not completely understood, the bond energy is higher than the peak position of the ether or hydroxyl component in the carbon fiber material, and the bond energy is lower than that of the carbonyl group (ketone group) component. This peak component is located in the graphite graphite ring, which is not obtained by normal surface treatment, because the carbon skeleton of the surface changes with electrolysis due to the action of peroxodisulfate ions. This suggests the possibility that an intermediate component between the hydroxyl group and the hydroxyl group is formed.

  Due to the effect, it is considered that the adhesion at the interface with the matrix resin such as epoxy resin is remarkably improved by the influence of the surface structure having a hydrogen bond or a polar group such as a hydroxyl group in a very strong state directly connected to the graphite structure. . Furthermore, in the carbon fiber graphitized at a high temperature exceeding 2000 ° C., the surface thereof has many graphite structures and is a more preferable carbon fiber for the present invention.

  In this invention, a sizing agent is provided as needed and a carbon fiber is obtained.

  Hereinafter, the present invention will be described more specifically with reference to examples. The measuring methods in the examples were as follows.

<Surface oxygen concentration (O / C ratio) of carbon fiber and peak intensity analysis of inner shell C1s spectrum>
The surface oxygen concentration O / C ratio was determined by X-ray photoelectron spectroscopy according to the following procedure. After cutting the surface-treated carbon fiber bundles and spreading them on a stainless steel sample stage, the photoelectron escape angle is set to 45 °, MgKα1,2 wires are used as the X-ray source, and the inside of the sample chamber is 1 × The measurement was performed at a vacuum degree of 10 ⁻⁸ Torr. As correction of the peak accompanying charging during measurement, first, the binding energy value of the main peak (peak top) of C1s is adjusted to 284.6 eV. The C1s peak area was obtained by drawing a straight base line in the range of 282 to 296 eV, and the O1s peak area was obtained by drawing a straight base line in the range of 528 to 540 eV. The surface oxygen concentration O / C was expressed as an atomic ratio calculated by dividing the ratio of the O1s peak area by the sensitivity correction value unique to the apparatus. In the examples, ESCA-1600 manufactured by ULVAC-PHI Co., Ltd. was used, and the sensitivity correction value unique to the apparatus was 2.33.

  Further, a 282 to 296 eV linear base line whose area was obtained at the C1s peak was defined as the origin (zero point) of the photoelectron intensity, and the height of the main peak with a binding energy of 284.6 eV (cps: photoelectron intensity per unit time) ) And the binding energy position and height (cps) of the sub-peak component, and the ratio of the height to the sub-peak with the main peak as a reference was calculated.

<Measurement of strand tensile strength>
Carbon fiber, 1000 g (930 wt%) of “Bakelite” (registered trademark) ERL-4221 manufactured by Union Carbide Co., Ltd., 30 g (3 wt%) of boron trifluoride monoethylamine (BF3 · MEA), and 40 g of acetone The (4 wt%) mixed epoxy resin composition is impregnated, and then this is heated at a temperature of 130 ° C. for 30 minutes and cured to obtain a resin impregnated strand. The tensile strength σ was determined according to the resin impregnated strand test method of JIS R7601 (1986).

<Measurement of interfacial shear strength (IFSS)>
Bisphenol A type epoxy resin compound “jER” (registered trademark) 828 (manufactured by Japan Epoxy Resin Co., Ltd.) and metaphenylenediamine (Sigma Aldrich Japan Co., Ltd.) are mixed well at a weight ratio of 7: 1, and carbon fiber single yarn (fiber A silicone rubber mold having a thickness of 2 mm, a width of 10 mm, and a length of 150 mm was impregnated and pre-cured at a temperature of 100 ° C., and then heated and cured at a temperature of 130 ° C. for 120 minutes. Tensile force was applied to the single yarn (fiber bundle) -embedded resin test piece in the fiber axis direction to cause 10% distortion, and the number of fiber breaks in the range of 22 mm in the center of the test piece was measured with an optical microscope. . Next, the critical fiber length lc was calculated from the average breaking fiber length la by the formula of lc (μm) = 4/3 × la (μm). The strand tensile strength σ and the diameter d of the carbon fiber single yarn (fiber bundle) were measured, and the interfacial shear strength IFSS, which is an index of the bond strength between the carbon fiber and the resin interface, was calculated by the following equation. In the examples, the average of the number of measurements n = 5 was used as the test result.
Interfacial shear strength IFSS (MPa) = σ (MPa) × d (μm) ÷ 2 lc (μm)
Example 1
Acrylic precursor fiber having a single fiber fineness of 0.8 dtex and a single fiber number of 12,000 by dry and wet spinning using an acrylonitrile copolymer consisting of 99.5 mol% of acrylonitrile and 0.5 mol% of itaconic acid Got a bunch. The acrylic precursor fiber bundle was provided with 1.1% by weight of a silicone oil agent made of amino-modified silicone with respect to 100% by weight of the carbon fiber. This oil agent adhesion amount can be confirmed by a measurement method defined in the method defined in Japanese Patent Application Laid-Open No. 2000-146776.

  That is, an oil agent is obtained by decomposing and extracting a silicone-based oil agent in which a resin is formed on or inside the carbon fiber in the presence of a catalyst solution in a solvent, and measuring the infrared absorbance of the extract solution from which the catalyst solution or the catalyst dissolving agent has been removed. The amount of adhesion can be confirmed.

The acrylic precursor fiber bundle thus obtained was heated in an air atmosphere at a temperature of 240 to 280 ° C. to obtain a flame resistant fiber bundle. Next, the obtained flame-resistant fiber bundle was pre-carbonized in a temperature range of 300 ° C. or higher and 800 ° C. or lower in a nitrogen atmosphere, followed by carbonization treatment at a temperature of 1700 ° C. in a nitrogen atmosphere, A carbon fiber bundle to be treated was obtained by graphitization at a temperature. Thereafter, the obtained carbon fiber bundle was immersed in an aqueous solution of ammonium peroxodisulfate (NH ₄ ) ₂ S ₂ O ₈ having a peroxodisulfate ion concentration of 1.8 wt% (aqueous solution concentration 0.09 mol / l) for 5 seconds. The surface treatment was carried out by electrolysis with an electric quantity of 120 coulomb / g, followed by washing with water and drying at a temperature of 150 ° C. Thereafter, when X-ray photoelectron spectroscopy on the carbon fiber surface was measured and analyzed, the surface oxygen concentration O / C ratio was 0.30. The carbon C1s inner-shell spectrum had a sub peak at a position of +2.0 eV (binding energy 286.65 eV) from the main peak, and the intensity ratio with respect to the main peak was 0.5. Thereafter, in the sizing process, 0.3% by weight of a sizing agent mainly composed of an epoxy resin is applied to the carbon fiber, and the sizing agent is further dried by a dryer controlled to a temperature of 180 ° C. Obtained. About the obtained carbon fiber, strand tensile strength and IFSS were measured. The measurement results are shown in Table 1.

(Example 2)
Except that the aqueous solution for electrolytic treatment was changed from an aqueous solution of ammonium peroxodisulfate to an aqueous solution of potassium peroxodisulfate K ₂ S ₂ O ₈ (aqueous solution concentration 0.09 mol / l, peroxodisulfate ion concentration 1.6 wt%). Carbon fibers were obtained by the same production method as in Example 1. The results are shown in Table 1.

(Example 3)
Implementation was performed except that the aqueous solution for electrolytic treatment was changed from an aqueous solution of ammonium peroxodisulfate to an aqueous solution of sodium peroxodisulfate Na ₂ S ₂ O ₈ (aqueous solution concentration 0.09 mol / l, peroxodisulfate ion concentration 1.1 wt%). Carbon fibers were obtained by the same production method as in Example 1. The results are shown in Table 1.

Example 4
The aqueous solution for the electrolytic treatment is an aqueous solution of ammonium peroxodisulfate (aqueous solution concentration 0.09 mol / l, peroxodisulfate ion concentration 1.8 wt%), and the amount of electricity in the electrolytic treatment is changed from 120 coulomb / g to 30 coulomb / g. Except for this, carbon fibers were obtained by the same production method as in Example 1. The results are shown in Table 1.

(Example 5)
The aqueous solution for electrolytic treatment is an aqueous solution of ammonium peroxodisulfate (aqueous solution concentration 0.09 mol / l, peroxodisulfate ion concentration 1.8 wt%), and the amount of electricity in the electrolytic treatment is changed from 120 coulomb / g to 200 coulomb / g. A carbon fiber was obtained by the same production method as in Example 1 except that it was changed. The results are shown in Table 1.

(Comparative Example 1)
The aqueous solution for electrolytic treatment is an aqueous solution of ammonium peroxodisulfate (aqueous solution concentration 0.09 mol / l, peroxodisulfate ion concentration 1.8 wt%), and the amount of electricity in the electrolytic treatment is changed from 120 coulomb / g to 5 coulomb / g. A carbon fiber was obtained by the same production method as in Example 1 except that it was changed. Table 1 shows the measurement and analysis results of the X-ray photoelectron spectroscopy of the carbon fiber, and the results of strand tensile strength and IFSS. Although a sub-peak of the carbon C1s inner-shell spectrum of X-ray photoelectron spectroscopy was detected, IFSS was lower than that of Example 1.

(Comparative Example 2)
The aqueous solution for electrolytic treatment is an aqueous solution of ammonium peroxodisulfate (aqueous solution concentration 0.09 mol / l, peroxodisulfate ion concentration 1.8 wt%), and the amount of electricity in the electrolytic treatment is changed from 120 coulomb / g to 400 coulomb / g. A carbon fiber was obtained by the same production method as in Example 1 except that it was changed. Table 1 shows the measurement and analysis results of the X-ray photoelectron spectroscopy of the carbon fiber, and the results of strand tensile strength and IFSS. Although a sub-peak of the carbon C1s inner-shell spectrum of X-ray photoelectron spectroscopy was detected, IFSS was lower than that of Example 1.

(Comparative Example 3)
The aqueous solution for electrolytic treatment was an aqueous solution of ammonium peroxodisulfate (aqueous solution concentration 0.01 mol / l, peroxodisulfate ion concentration 0.2 wt%), and the same production method as in Example 1 except that the aqueous solution concentration was changed As a result, carbon fibers were obtained. Table 1 shows the measurement and analysis results of the X-ray photoelectron spectroscopy of the carbon fiber, and the results of strand tensile strength and IFSS. Although a sub-peak of the carbon C1s inner-shell spectrum of X-ray photoelectron spectroscopy was detected, IFSS was lower than that of Example 1.

(Comparative Example 4)
A carbon fiber was obtained by the same production method as in Example 1 except that the aqueous solution for the electrolytic treatment was changed from an aqueous peroxodisulfate aqueous solution to an aqueous sulfuric acid solution (aqueous solution concentration 0.09 mol / l, no peroxodisulfate ion). . Table 1 shows the measurement and analysis results of the X-ray photoelectron spectroscopy of the carbon fiber, and the results of strand tensile strength and IFSS. The sub-peak of the carbon C1s inner spectrum of X-ray photoelectron spectroscopy was not detected, and IFSS was lower than that of Example 1.

(Comparative Example 5)
The aqueous solution for electrolytic treatment is changed from an aqueous solution of ammonium peroxodisulfate to an aqueous solution of sulfuric acid (aqueous solution concentration 0.09 mol / l, no peroxodisulfate ion), and the amount of electricity in the electrolytic treatment is changed from 120 coulomb / g to 500 coulomb / g Except for this, carbon fibers were obtained by the same production method as in Example 1. The results are shown in Table 1. The subpeak of the carbon C1s inner shell spectrum of X-ray photoelectron spectroscopy was not detected, and the strand tensile strength and IFSS were lower than those of Example 1.

(Comparative Example 6)
A carbon fiber was obtained by the same production method as in Example 1, except that the aqueous solution for the electrolytic treatment was changed from an aqueous ammonium peroxodisulfate solution to an aqueous ammonium sulfate solution (aqueous solution concentration 0.09 mol / l, no peroxodisulfate ion). . The results are shown in Table 1. The sub-peak of the carbon C1s inner spectrum of X-ray photoelectron spectroscopy was not detected, and IFSS was lower than that of Example 1.

(Comparative Example 7)
A carbon fiber is obtained by the same production method as in Example 1 except that the aqueous solution for electrolytic treatment is changed from an aqueous ammonium peroxodisulfate solution to an aqueous ammonium carbonate solution (aqueous solution concentration 1.0 mol / l, no peroxodisulfate ion). It was. The results are shown in Table 1. The sub-peak of the carbon C1s inner spectrum of X-ray photoelectron spectroscopy was not detected, and IFSS was lower than that of Example 1.

(Comparative Example 8)
The carbon fiber was prepared by the same production method as in Example 1 except that the aqueous solution for electrolytic treatment was changed from an aqueous solution of ammonium peroxodisulfate to an aqueous solution of ammonium hydrogencarbonate (aqueous solution concentration 1.0 mol / l, no peroxodisulfate ion). Obtained. The results are shown in Table 1. The sub-peak of the carbon C1s inner spectrum of X-ray photoelectron spectroscopy was not detected, and IFSS was lower than that of Example 1.

(Comparative Example 9)
Carbon fiber was produced by the same production method as in Example 1 except that the aqueous solution for electrolytic treatment was changed from an aqueous peroxodisulfate aqueous solution to an aqueous triammonium phosphate solution (aqueous solution concentration 0.17 mol / l, no peroxodisulfate ion). Got. The results are shown in Table 1. The sub-peak of the carbon C1s inner spectrum of X-ray photoelectron spectroscopy was not detected, and IFSS was lower than that of Example 1.

(Comparative Example 10)
The carbon fiber was prepared by the same production method as in Example 1 except that the aqueous solution for electrolytic treatment was changed from an aqueous solution of ammonium peroxodisulfate to an aqueous solution of sodium hydroxide (aqueous solution concentration 0.05 mol / l, no peroxodisulfate ion). Obtained. The results are shown in Table 1. The sub-peak of the carbon C1s inner spectrum of X-ray photoelectron spectroscopy was not detected, and IFSS was lower than that of Example 1.

Claims (3)

In the measurement and analysis of X-ray photoelectron spectroscopy, the oxygen concentration (O / C ratio: number of oxygen atoms / number of carbon atoms) on the surface of the carbon fiber is in the range of 0.20 to 0.35, and the carbon C1s inner shell spectrum Carbon having a sub-peak component at the energy position in the range of +1.6 to +2.6 eV (binding energy 286.2 to 287.2 eV) from the binding energy position (284.6 eV) of carbon and the carbon-carbon component which is the main peak. A carbon fiber having a carbon C1s inner spectrum whose sub-peak height is one third or more of the height of the main peak . When the carbon fiber to be treated is used as an anode and the electrolytic treatment is performed between the cathode and the direct current through a direct current, the total amount of electrolytic electricity used in the electrolytic treatment is 30 to 300 coulomb / g per 1 g of carbon fiber, and the peroxodisulfate ion concentration A method for producing carbon fiber, characterized in that an electrolytic treatment is performed using an aqueous solution containing 1.0 wt% or more of peroxodisulfate as an electrolytic solution. The peroxodisulfate is at least one salt selected from the group consisting of ammonium peroxodisulfate, sodium peroxodisulfate, potassium peroxodisulfate, rubidium peroxodisulfate, barium peroxodisulfate and lead peroxodisulfate. 2. The method for producing a carbon fiber according to 2 .