Carbon fiber and method of producing carbon fiber

Abstract

To provide carbon fibers in which fusion between the fibers is suppressed, generation of fluff during unwinding of the carbon fibers is suppressed, and the carbon fibers have high tensile strength and tensile modulus, and a method for producing the carbon fibers.SOLUTION: There is provided a carbon fiber in which a phosphorus content determined by elemental analysis is from 0.6 mass% to 10 mass%, a nitrogen content is 5 mass% or less, an oxygen content is 0.3 mass% or less, and a tensile modulus of a single carbon fiber is 150 GPa or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to carbon fibers and methods for making carbon fibers. [Background technology]

[0002] Carbon fiber is lightweight and has excellent mechanical properties, so carbon fiber composite materials are being developed for a variety of applications, including aerospace, automotive, and building materials. Known methods for producing carbon fibers include subjecting a precursor fiber bundle obtained by spinning polyacrylonitrile or pitch to a flame retardant treatment and then subjecting the precursor to a carbonization treatment (for example, Patent Documents 1, 2, and 3). Polyacrylonitrile-based carbon fibers, made from fibers obtained by spinning polyacrylonitrile, are the most widely used due to their high mechanical properties. However, because they are spun using wet spinning or dry-wet spinning methods using organic solvents such as dimethyl sulfoxide or dimethylformamide, there is a risk that energy will be required to recycle the organic solvent, which could increase production costs. On the other hand, when pitch is used as a raw material, melt spinning can be performed without using organic solvents, and hydrogen cyanide is not generated during the flame-retardant and carbonization processes. However, melt spinning of pitch-based fibers generally requires high temperatures of 250°C or higher, and the fibers themselves are relatively brittle, which can lead to fibers fusing together or breaking during the flame-retardant process.

[0003] On the other hand, acrylamide polymers containing acrylamide monomers as precursors of carbon fibers, etc. are water-soluble polymers, and water, which is inexpensive and has a small environmental impact, can be used as a solvent when carrying out polymerization, spinning, etc. Therefore, reductions in the production costs of carbon materials are expected (for example, Patent Documents 4 and 5). However, carbon material precursors containing an acrylamide-based polymer as disclosed in Patent Documents 4 and 5 may undergo inter-fiber fusion due to softening during heat treatment, particularly during flame-proofing treatment. For this reason, there is concern that inter-fiber fusion may also occur in the resulting carbon fiber, and if inter-fiber fusion occurs, there is a risk that strength may decrease at the fused portion of the carbon fiber or that fuzz may be generated due to breakage at the fused portion. Furthermore, if fluff is formed due to thermal decomposition of the fibers during carbonization, when the carbon fibers are wound around a winding core or the like after carbonization and then unwound from the winding core or the like for use in carbon fiber composite materials or the like, this may lead to further fluff generation, which may result in reduced handleability. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-183159 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-202208 [Patent Document 3] Japanese Patent Application Publication No. 06-10215 [Patent Document 4] Japanese Patent Publication No. 2022-85514 [Patent Document 5] Japanese Patent Publication No. 2022-143757 Summary of the Invention [Problem to be solved by the invention]

[0005] The above-mentioned Patent Document 5 describes that high-strength carbon fibers having specific physical properties can be obtained by heat-treating flame-retardant treated fibers under an inert gas atmosphere while applying tension. However, depending on the tension conditions in the drawing treatment, yarn breakage and generation of fluff may occur during preliminary carbonization.

[0006] The problem to be solved by the embodiments of the present disclosure is to provide a carbon fiber in which fusion between fibers is suppressed, the generation of fuzz during unwinding of the carbon fiber is suppressed, and the carbon fiber has high tensile strength and tensile modulus, and a method for producing the carbon fiber. [Means for solving the problem]

[0007] The present disclosure includes the following aspects. <1> Carbon fiber having a phosphorus content of 0.6% by mass to 10% by mass, a nitrogen content of 5% by mass or less, an oxygen content of 0.3% by mass or less, and a tensile modulus of elasticity of a single fiber of 150 GPa or more, as determined by elemental analysis of the carbon fiber. <2> In a carbon fiber bundle consisting of 800 fibers, the fusion rate of the carbon fibers contained in the carbon fiber bundle is 15% or less. <1> The carbon fiber according to claim 1. <3> The carbon fiber is a carbon fiber derived from an acrylamide polymer fiber. <1> or <2> The carbon fiber according to claim 1. <4> The tensile strength of the single fiber is 1.4 GPa or more. <1> or <2> The carbon fiber according to claim 1. <5> A method for producing carbon fibers, comprising a step of performing flame retardant treatment on a spread fiber bundle obtained by spreading acrylamide-based polymer fibers, wherein the phosphorus content of the acrylamide-based polymer fibers is 0.1% by mass to 10% by mass as determined by elemental analysis. <6> The method further comprises a step of subjecting the flame-retardant treated acrylamide polymer fiber to a carbonization treatment. <5> The method for producing the carbon fiber according to claim 1.

[0008] <7> The phosphorus content of the flame-retardant treated acrylamide polymer fiber is 0.1% by mass to 10% by mass as determined by elemental analysis, the maximum temperature in the carbonization treatment step is in the range of 1350°C to 1650°C, and the fusion rate of the carbon fibers in the carbonized fiber bundle is 20% or less. <6> The method for producing the carbon fiber according to claim 1. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide a carbon fiber and a method for producing a carbon fiber in which fusion between fibers is suppressed, the generation of fluff when the carbon fiber is unwound is suppressed, and the carbon fiber has high tensile strength and tensile modulus. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram showing an embodiment of a fiber-spreading device that can be used in the carbon fiber manufacturing method of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011] In the present disclosure, numerical ranges indicated using "to" include the numerical values before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in the present disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in the present disclosure, the upper or lower limit value of the numerical range may be replaced with the value shown in the synthesis examples.

[0012] In the present disclosure, each component may contain multiple corresponding substances. When multiple substances corresponding to each component are present in the carbon fiber precursor, the content or amount of each component means the total content or amount of the multiple substances present in the carbon fiber precursor, unless otherwise specified.

[0013] In the present disclosure, the term "carbon fiber" is used to encompass "a single carbon fiber" and "a carbon fiber bundle consisting of a plurality of single carbon fiber fibers." In the present disclosure, the term "carbon fiber precursor" refers to a fiber that can be subjected to a carbonization treatment, or a flame-proofing treatment and a carbonization treatment, to obtain a carbon fiber.

[0014] In the present disclosure, the term "acrylamide polymer" refers to a homopolymer of an acrylamide monomer or a copolymer of an acrylamide monomer and a monomer other than an acrylamide monomer (hereinafter referred to as other polymerizable monomer).

[0015] <Carbon fiber> The carbon fiber of the present disclosure has a phosphorus content of 0.6% by mass to 10% by mass, a nitrogen content of 5% by mass or less, and an oxygen content of 0.3% by mass or less, as determined by elemental analysis of the carbon fiber, and a single fiber tensile modulus of elasticity of 150 GPa or more.

[0016] The carbon fiber of the present disclosure has a phosphorus content of 0.6% by mass to 10% by mass as determined by elemental analysis, and therefore exhibits good suppression of inter-fiber fusion and fuzz generation during unwinding of the carbon fiber, and also has high tensile strength and tensile modulus. The reason for the above effect is presumed to be as follows, but is not limited to this. The carbon fiber of the present disclosure contains a predetermined amount of phosphorus and has low contents of nitrogen and oxygen, which effectively suppresses fusion of adjacent fibers on the fiber surface due to nitrogen and oxygen, and as a result, it is thought that the generation of fluff is suppressed and the decrease in strength and tensile modulus caused by fusion of fibers is suppressed. In particular, even when acrylamide-based polymer fibers, which can be synthesized in an aqueous system and have a good environmental impact, are used as the raw material for carbon fibers, it is estimated that the softening during the flame-retardant treatment can be suppressed and the fusion of single fibers can be suppressed because the phosphorus content is a predetermined amount and the nitrogen and oxygen content is low. For this reason, the present disclosure estimates that the effect is significant when acrylamide-based polymer fibers are used as the raw material for carbon fibers.

[0017] <Elemental content in carbon fiber> The carbon fiber of the present disclosure has a phosphorus content of 0.6% to 10% by mass, a nitrogen content of 5% or less by mass, and an oxygen content of 0.3% or less by mass, as determined by elemental analysis.

[0018] The carbon fiber of the present disclosure has a phosphorus content determined by elemental analysis of 0.6% by mass to 10% by mass. From the viewpoint of further improving the ability to suppress fusion between fibers and the ability to suppress the generation of fluff when the carbon fiber is unwound after being wound around a winding core (paper tube, core, winding rod, bobbin, etc.) or winding plate, etc. after carbonization, the phosphorus content is preferably 0.7% by mass to 5.0% by mass, more preferably 1.0% by mass to 2.0% by mass, and even more preferably 1.1% by mass to 1.8% by mass.

[0019] Elemental analysis of the phosphorus content in carbon fiber can be performed using a sample of carbon fiber dried at 120°C under atmospheric pressure for 1 hour, using the following method. The carbon fiber is heated to ashed, then dissolved in acid, and the resulting solution is subjected to inductively coupled plasma mass spectrometry (ICP-MS) to quantify phosphorus. A NexION2000C (manufactured by PerkinElmer) can be used as the measuring device for ICP-MS. The acid that can be used to dissolve the carbonized carbon fiber can be appropriately selected, and examples of the acid that can be used for dissolution include, but are not limited to, nitric acid, aqua regia, and hydrofluoric acid.

[0020] The means for controlling the phosphorus content in the carbon fiber to fall within the above range may include, for example, adding a phosphorus-based compound, such as phosphoric acid, polyphosphoric acid, and their metal salts (sodium salt, potassium salt, calcium salt, etc.), ammonium salt, amine salt, guanidine salt, urea salt, melamine salt, imidazole salt, etc., to the acrylamide-based polymer during the production of the acrylamide-based polymer, during the production of the acrylamide-based polymer fiber which is a carbon fiber precursor, during the production of the flame-resistant fiber, etc.; an aromatic phosphate ester such as triphenyl phosphate, cresyl diphenyl phosphate, tricresyl phosphate, trixylenyl phosphate, tris(i-propylated phenyl) phosphate, tris(t-butylated phenyl) phosphate, 2-ethylhexyl diphenyl phosphate; 1,3-phenylene bis(diphenyl phosphate), 1,3-phenylene bis(dixylenyl) phosphate, Examples of methods include adding aromatic condensed phosphate esters such as resorcinol bis(diphenyl)phosphate and bisphenol A bis(diphenyl phosphate); halogen-containing phosphate esters such as tris(dichloropropyl)phosphate, tris(β-chloropropyl)phosphate and tris(chloroethyl)phosphate; halogen-containing condensed phosphate esters such as 2,2-bis(chloromethyl)trimethylenebis(bis(2-chloroethyl)phosphate) and polyoxyalkylenebisdichloroalkylphosphate; phosphate esters, cyclic phosphazene compounds, phosphorus pentoxide, phosphorus oxychloride, etc., and from the viewpoint that phosphorus-based compounds also function as catalysts for the cyclization reaction and dehydration reaction during flame retardation and also realize improvements in flame retardation and carbonization yields, a method of adding phosphoric acid, polyphosphoric acid, or an ammonium salt thereof (diammonium hydrogen phosphate, ammonium dihydrogen phosphate, etc.) is preferred. The amount added is preferably 0.3 to 10 parts by mass, more preferably 1 to 5 parts by mass, and even more preferably 2 to 4 parts by mass, relative to 100 parts by mass of the acrylamide polymer. As a means for making the phosphorus content in the carbon fiber 0.6% by mass or more, in addition to adjusting the amount of the phosphorus-based compound to be equal to or greater than the lower limit of the above-mentioned preferred range, there are also mentioned methods for setting the carbonization temperature to 1800°C or less, preferably 1750°C or less, more preferably 1700°C or less, from the viewpoint of suppressing the loss of the phosphorus-based compound and / or products derived therefrom due to thermal decomposition during the carbonization treatment.

[0021] The carbon fiber of the present disclosure has a nitrogen content determined by elemental analysis of 5% by mass or less, preferably 4% by mass or less, more preferably 3% by mass or less, even more preferably 2% by mass or less, and particularly preferably 1.5% by mass or less. There is no particular lower limit for the nitrogen content in the carbon fiber, and it may be 0 mass %, that is, below the detection limit by elemental analysis. If the nitrogen content in the carbon fiber exceeds 5% by mass, the fusion rate in the carbonized fiber bundle increases, which is undesirable.

[0022] As a means for adjusting the nitrogen content in the carbon fiber to 5% by mass or less, a method of adjusting the carbonization temperature to 1300°C or more and 1750°C or less, preferably 1350°C or more and 1650°C or less, can be mentioned.

[0023] The carbon fiber of the present disclosure has an oxygen content determined by elemental analysis of 0.3% by mass or less, preferably 0.2% by mass or less, more preferably 0.15% by mass or less, and even more preferably 0.1% by mass or less. There is no particular lower limit for the oxygen content in the carbon fiber, and it may be 0 mass %, that is, below the detection limit by elemental analysis. If the oxygen content in the carbon fiber exceeds 0.3% by mass, the fusion rate in the fiber bundle subjected to the carbonization treatment increases, which is undesirable.

[0024] Examples of means for reducing the oxygen content in carbon fiber to 0.3% by mass or less include a method in which the carbonization temperature is 1300°C or higher, preferably 1350°C or higher, more preferably 1400°C or higher, and even more preferably 1450°C or higher, and a method in which at least one of pre-carbonization of the flame-resistant fiber and carbonization is performed in an inert gas atmosphere with a low oxygen concentration. The oxygen concentration in the inert gas atmosphere is preferably 30 ppm or lower, more preferably 20 ppm or lower, even more preferably 10 ppm or lower, and particularly preferably 5 ppm or lower.

[0025] The nitrogen and oxygen contents are determined by elemental analysis. The elemental analysis can be carried out by the method described below using a carbon fiber dried at 120°C under atmospheric pressure for 1 hour as a measurement sample. The analytical method used is oxygen circulation combustion and thermal conductivity detector (TCD) detection. As a measuring device, for example, Sumigraph NCH-22F model (manufactured by Sumika Chemical Analysis Center) can be used, and the elemental analysis in this disclosure describes values measured using the above device. Measurement principle: For each measurement sample, a gas chromatograph with TCD is used to circulate O2 gas while burning it for 15 minutes to decompose and completely oxidize it, converting the carbon components into CO2 and the nitrogen components into N2 gas, which can then be detected and quantified.

[0026] The oxygen content in carbon fiber is determined by drying the carbon fiber at 120°C under atmospheric pressure for 1 hour as a sample, and measuring it in an inert gas atmosphere using impulse heating, melting, and non-dispersive infrared (NDIR) detection. The EMGA-920 (manufactured by Horiba, Ltd.) can be used as the device. The measurement principle involves placing a sample in a graphite crucible, passing an electric current through the graphite crucible in He gas (oxygen-free), melting the sample at approximately 2500°C, converting the oxygen component into CO gas, and detecting and quantifying the amount of oxygen.

[0027] To quantify the nitrogen in carbon fiber, carbon fiber is dried at 120°C under atmospheric pressure for 1 hour as a sample, converted into N2 gas, and the detected value is used as a reference, and analysis is performed in the same manner as above.

[0028] <Carbon fiber precursor fiber> The raw material for the precursor fiber of the carbon fiber is not particularly limited, and examples thereof include known raw materials, such as acrylonitrile-based polymers, i.e., homopolymers or copolymers containing acrylonitrile monomer units, acrylamide-based polymers, i.e., homopolymers or copolymers containing acrylamide monomer units, and mixtures thereof. Among these, acrylamide-based polymers are preferred. That is, the carbon fibers of the present disclosure are preferably carbon fibers derived from acrylamide-based polymer fibers.

[0029] (Acrylamide polymer fiber) The acrylamide-based polymer fibers suitable for use in the carbon fibers of the present disclosure contain one or more acrylamide-based polymers. The tensile modulus of a single fiber of the carbon fiber of the present disclosure is preferably 150 GPa or more, more preferably 160 GPa or more, even more preferably 170 GPa or more, and particularly preferably 180 GPa or more. The tensile strength of a single fiber of the disclosed carbon fiber is preferably 1.4 GPa or more, more preferably 1.5 GPa or more, even more preferably 1.8 GPa or more, and particularly preferably 2.0 GPa or more. From the viewpoint of suppressing fusion, the acrylamide polymer fibers are preferably crosslinked acrylamide polymer fibers containing a crosslinked acrylamide polymer. In the present disclosure, crosslinked acrylamide polymer fibers refer to polymer fibers in which acrylamide polymer fibers are crosslinked by actinic radiation (e.g., electron beam radiation, ultraviolet radiation, etc.), heating, etc. The crosslinked acrylamide polymer may be a homopolymer of an acrylamide monomer, or a copolymer of an acrylamide monomer and a monomer other than an acrylamide monomer (hereinafter referred to as other polymerizable monomer).

[0030] The content of acrylamide-based monomer units in the acrylamide-based polymer is preferably 30 mol% or more, more preferably 40 mol% or more, even more preferably 50 mol% or more, particularly preferably 55 mol% or more, and most preferably 60 mol% or more. When the content of the acrylamide-based monomer unit is 30 mol % or more, the solubility of the acrylamide-based polymer in aqueous solvents or aqueous mixed solvents tends to be improved. Furthermore, the upper limit of the content of acrylamide-based monomer units is not particularly limited, but from the viewpoint of suppressing fusion, etc., it is preferably 99.9 mol% or less, more preferably 99 mol% or less, even more preferably 95 mol% or less, particularly preferably 90 mol% or less, and most preferably 85 mol% or less. The content of the acrylamide monomer unit is preferably 30 mol % to 99.9 mol %.

[0031] When the acrylamide polymer is a copolymer of an acrylamide monomer and another polymerizable monomer, the content of the other polymerizable monomer unit in the copolymer is preferably 0.1 mol % or more, more preferably 1 mol % or more, even more preferably 5 mol % or more, particularly preferably 10 mol % or more, and most preferably 15 mol % or more, from the viewpoint of suppressing fusion, etc. Furthermore, from the viewpoint of improving the solubility of the acrylamide polymer in aqueous solvents or aqueous mixed solvents, the upper limit of the content of other polymerizable monomer units is preferably 70 mol% or less, more preferably 60 mol% or less, even more preferably 50 mol%, particularly preferably 45 mol% or less, and most preferably 40 mol% or less. The content of the other polymerizable monomer units in the copolymer is preferably 0.1 mol % to 70 mol %.

[0032] Examples of acrylamide monomers include acrylamide, ethacrylamide, crotonamide, itaconic acid diamide, cinnamic acid amide, maleic acid diamide, N-alkyl acrylamides such as N-methyl acrylamide, N-ethyl acrylamide, Nn-propyl acrylamide, N-isopropyl acrylamide, Nn-butyl acrylamide, and N-tert-butyl acrylamide, N-cycloalkyl acrylamides such as N-cyclohexyl acrylamide, dialkyl acrylamides such as N,N'-dimethyl acrylamide, dialkyl amino alkyl acrylamides such as dimethyl amino ethyl acrylamide and dimethyl amino propyl acrylamide, N-(hydroxymethyl) acrylamide, Hydroxyalkylacrylamides such as N-(hydroxyethyl)acrylamide; N-arylacrylamides such as N-phenylacrylamide; diacetoneacrylamide; N,N'-alkylenebisacrylamides such as N,N'-methylenebisacrylamide; methacrylamide; N-alkylmethacrylamides such as N-methylmethacrylamide, N-ethylmethacrylamide, Nn-propylmethacrylamide, N-isopropylmethacrylamide, N-n-butylmethacrylamide, and N-tert-butylmethacrylamide; N-cycloalkylmethacrylamides such as N-cyclohexylmethacrylamide; dialkylmethacrylamides such as N,N-dimethylmethacrylamide; dimethylaminoethyl Examples of suitable methacrylamides include dialkylaminoalkyl methacrylamides such as N-(hydroxymethyl)methacrylamide and dimethylaminopropyl methacrylamide; hydroxyalkyl methacrylamides such as N-(hydroxymethyl)methacrylamide and N-(hydroxyethyl)methacrylamide; N-aryl methacrylamides such as N-phenylmethacrylamide; diacetone methacrylamide; and N,N'-alkylene bismethacrylamides such as N,N'-methylene bismethacrylamide. Furthermore, from the viewpoint of the solubility of the acrylamide-based polymer in an aqueous solvent or an aqueous mixed solvent, among the above-mentioned acrylamide-based monomers, acrylamide, N-alkylacrylamide, dialkylacrylamide, methacrylamide, N-alkylmethacrylamide, or dialkylmethacrylamide is preferred, and acrylamide is more preferred. The acrylamide monomers may be used alone or in combination of two or more.

[0033] Examples of other polymerizable monomers include vinyl cyanide monomers, unsaturated carboxylic acids and their salts, unsaturated carboxylic acid anhydrides, unsaturated carboxylic acid esters, vinyl alcohol monomers, vinyl carboxylate monomers, and olefin monomers.

[0034] Examples of vinyl cyanide monomers include acrylonitrile, methacrylonitrile, 2-hydroxyethyl acrylonitrile, chloroacrylonitrile, chloromethyl acrylonitrile, ethoxyacrylonitrile, and vinylidene cyanide. Examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, crotonic acid, and isocrotonic acid. Examples of the salts of unsaturated carboxylic acids include metal salts (for example, sodium salts, potassium salts, etc.), ammonium salts, and amine salts of unsaturated carboxylic acids.

[0035] Examples of the unsaturated carboxylic acid anhydride include maleic anhydride and itaconic anhydride. Examples of the unsaturated carboxylic acid ester include methyl acrylate, methyl methacrylate, 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate. Examples of vinyl monomers include aromatic vinyl monomers such as styrene and α-methylstyrene, vinyl carboxylates such as vinyl acetate, vinyl propionate, vinyl butyrate and vinyl pivalate, vinyl chloride, and vinyl alcohol. Examples of the olefin monomer include ethylene, propylene, isopropylene, and butadiene.

[0036] Among the other polymerizable monomers mentioned above, vinyl cyanide monomers are preferred, and acrylonitrile is more preferred, from the viewpoints of spinnability, fusion suppression, spinnability, etc. of the acrylamide polymer. Among the other polymerizable monomers described above, unsaturated carboxylic acids and salts thereof are preferred, and acrylic acid, maleic acid, fumaric acid, or itaconic acid is more preferred, from the viewpoint of the solubility of the copolymer in an aqueous solvent or an aqueous mixed solvent. Among the other polymerizable monomers described above, from the viewpoint of suppressing fusion, unsaturated carboxylic acids or unsaturated carboxylic acid anhydrides are preferred, and acrylic acid, maleic acid, fumaric acid, itaconic acid, or maleic acid anhydride is more preferred. The above-mentioned other polymerizable monomers may be used alone or in combination of two or more.

[0037] From the viewpoints of the solubility of the copolymer in an aqueous solvent or aqueous mixed solvent, spinnability, fusion suppression, etc., the acrylamide-based polymer is preferably a copolymer of an acrylamide-based monomer and an acrylonitrile-based monomer, more preferably a copolymer of an acrylamide-based monomer, an acrylonitrile-based monomer, and an unsaturated carboxylic acid, and even more preferably a copolymer of acrylamide, acrylonitrile, and acrylic acid. From the viewpoint of spinnability, suppression of fusion, etc., the content of acrylonitrile-based monomer units in the copolymer is preferably 1 mol% to 50 mol%, more preferably 5 mol% to 35 mol%, and even more preferably 10 mol% to 25 mol%. From the viewpoint of the solubility of the copolymer in an aqueous solvent or an aqueous mixed solvent, the suppression of fusion, etc., the content of unsaturated carboxylic acid units in the copolymer is preferably 0.1 mol % to 20 mol %, more preferably 1 mol % to 10 mol %, even more preferably 1 mol % to 8 mol %, and particularly preferably 2 mol % to 5 mol %. The preferred range of the content of the acrylamide monomer unit in the copolymer has been described above, and therefore will not be described here. The acrylamide-based polymer of the present disclosure may include water-soluble polymers and / or water-insoluble polymers other than acrylamide-based polymers. Examples of water-soluble polymers other than acrylamide-based polymers include vinyl alcohol-based polymers, alkylene oxide-based polymers (e.g., polyethylene glycol), polyvinylpyrrolidone, polyacrylic acid, alkyl water-soluble polymers (e.g., isobutylene-maleic anhydride copolymers, copolymers in which at least a portion of the maleic anhydride in isobutylene-maleic anhydride copolymers has been ammonia-modified or imidized), water-soluble phenol-based polymers, carboxyvinyl polymers, cellulose derivatives (e.g., carboxymethyl cellulose), etc. The water-soluble polymers may be used alone or in combination of two or more. The water-insoluble polymers are not particularly limited, and examples thereof include polyacrylonitrile-based polymers, diene-based polymers, polyolefin-based polymers, water-insoluble phenol-based polymers, etc., and may be used alone or in combination of two or more.

[0038] When the acrylamide-based polymer fiber contains a polymer other than an acrylamide-based polymer, from the viewpoints of fusion suppression, water resistance, etc., the content of the acrylamide-based polymer relative to the total mass of the acrylamide-based polymer fiber is preferably 50 mass% or more, more preferably 60 mass% or more, and even more preferably 70 mass% or more.

[0039] The weight average molecular weight of the acrylamide polymer is not particularly limited and is usually 10 million or less. From the viewpoint of the molding processability of the carbon fiber precursor, however, it is preferably 5 million or less, more preferably 2 million or less, even more preferably 1 million or less, particularly preferably 500,000 or less, even more particularly preferably 300,000 or less, and most preferably 200,000 or less. The lower limit of the weight-average molecular weight of the acrylamide polymer is not particularly limited, but is usually 10,000 or more. From the viewpoint of the strength of the carbon fiber precursor and the carbon fiber, the lower limit is preferably 20,000 or more, more preferably 30,000 or more, and particularly preferably 40,000 or more. In the present disclosure, the weight average molecular weight is measured by gel permeation chromatography under the following conditions: The measurement device may be an HLC-8220GPC manufactured by Tosoh Corporation or a device equivalent thereto. (Measurement conditions) Column: TSKgel GMPWXL x 2 + TSKgel G2500PWXL x 1 Eluent: 100 mM sodium nitrate aqueous solution / acetonitrile (= 80 / 20 (volume ratio)) ·Eluent flow rate: 1.0ml / min Column temperature: 40℃ Molecular weight standards: Standard polyethylene oxide / standard polyethylene glycol Detector: Differential refractive index detector

[0040] The acrylamide polymer fiber of the present disclosure has excellent anti-fusing properties and therefore does not require the incorporation of an additive component such as an acid, but the acrylamide polymer fiber may contain, in addition to an acrylamide polymer, at least one additive component selected from the group consisting of acids and their salts. By subjecting the acrylamide polymer fiber containing the additive component to a flame retardant treatment, the formation of cyclic structures due to dehydration reactions, deammonia reactions, etc. is accelerated, and the anti-fusing properties tend to be further improved. In addition, at least a part of the additive component and its residue may remain in the flame-resistant fiber. Furthermore, the additive component may be added to the flame-resistant fiber and then carbonized.

[0041] Examples of the acid include inorganic acids such as phosphoric acid, polyphosphoric acid, boric acid, hydrochloric acid, sulfuric acid, nitric acid, and carbonic acid, and organic acids such as oxalic acid, citric acid, and sulfonic acid. Examples of the salts of the above acids include metal salts (sodium salts, potassium salts, etc.), ammonium salts, amine salts, guanidine salts, urea salts, melamine salts, and imidazole salts. Among these, the ammonium salts and amine salts are preferred, and the ammonium salts are more preferred. Among the above-mentioned additive components, from the viewpoints of suppressing fusion, carbonization yield, and shape stability, phosphoric acid, polyphosphoric acid, boric acid, sulfuric acid, or an ammonium salt thereof is preferred, phosphoric acid, polyphosphoric acid, boric acid, or an ammonium salt thereof is more preferred, and phosphoric acid, polyphosphoric acid, an ammonium salt of phosphoric acid, or an ammonium salt of polyphosphoric acid is even more preferred.

[0042] From the viewpoint of suppressing fusion, the content of the additive component relative to 100 parts by mass of the acrylamide-based polymer contained in the acrylamide-based polymer fiber is preferably 0.1 parts by mass to 100 parts by mass, more preferably 0.2 parts by mass to 50 parts by mass, even more preferably 0.5 parts by mass to 30 parts by mass, and particularly preferably 1 part by mass to 20 parts by mass.

[0043] The acrylamide-based polymer fibers of the present disclosure may include other additives. Examples of other additives include antioxidants, release agents, lubricants, plasticizers, colorants, crosslinking aids (e.g., ultraviolet crosslinking aids, etc.), crosslinking catalysts (e.g., acid catalysts, etc.), crosslinking retarders, reinforcing materials (fillers such as carbon nanotubes, graphene, carbon black, biomass fibers (cellulose fibers such as cellulose nanofibers, chitosan nanofibers, chitin nanofibers, etc.), glass fibers, and metal fibers), metal salts (sodium chloride, calcium chloride, magnesium chloride, zinc chloride, manganese chloride, strontium chloride, calcium nitrate, magnesium nitrate, etc.), ultraviolet absorbers, light screening agents, light stabilizers, antistatic agents, and compatibilizers.

[0044] The acrylamide polymer fibers may contain low molecular weight compounds that crosslink when exposed to actinic rays, such as N-vinylacetamide, vinyl acetate monomer, vinylethoxysilane, methacrylic acid, 2-isocyanatoethyl methacrylate, N-vinyl-2-pyrrolidone, N-vinyl-2-caprolactam, triethylene glycol divinyl ether, ethylene dimethacrylate, divinylbenzene, and triallyl isocyanurate, as long as the effects of the acrylamide polymer fibers are not impaired.

[0045] The acrylamide polymer fiber may be a single fiber or a fiber bundle.

[0046] The acrylamide polymer fibers used may be those produced by a conventionally known method.

[0047] The acrylamide-based polymer fibers can be produced by spinning an acrylamide-based polymer or an acrylamide-based polymer composition containing an acrylamide-based polymer and the above-mentioned additive components.

[0048] The spinning method is not particularly limited, and may be, for example, melt-spinning, spunbonding, melt-blown or centrifugal spinning of a molten acrylamide polymer or acrylamide polymer composition. When the acrylamide-based polymer or the acrylamide-based polymer composition is soluble in an aqueous solvent or an aqueous mixed solvent, it is preferable, from the viewpoints of spinnability, reduction in environmental load, cost, and safety, to dissolve the acrylamide-based polymer or the acrylamide-based polymer composition in an aqueous solvent or an aqueous mixed solvent and spin the obtained aqueous solution or aqueous mixed solution to produce an acrylamide-based polymer fiber. Furthermore, when the acrylamide-based polymer is synthesized by solution polymerization, it is preferable to adjust the acrylamide-based polymer solution to a desired polymer concentration (i.e., polymer content) as necessary and then spin the solution to produce an acrylamide-based polymer fiber. When the acrylamide-based polymer composition is produced by wet mixing, it is preferable to adjust the solution of the acrylamide-based polymer composition to a desired polymer concentration (polymer content) as necessary, and then spin the resulting mixture to produce an acrylamide-based polymer fiber.

[0049] The spinning is preferably carried out by dry spinning, wet spinning, dry-wet spinning, gel spinning, flash spinning, or electrospinning, which allows acrylamide polymer fibers having a desired fineness and average fiber diameter to be produced safely and at low cost. From the viewpoint of producing acrylamide polymer fibers safely and at lower cost, it is preferable to use an aqueous solvent as the solvent, and it is more preferable to use water.

[0050] Details of the acrylamide polymer have been described above, and therefore will not be described here. The acrylamide polymer to be used may be a commercially available product, or may be one synthesized by a conventionally known method. The synthesis of an acrylamide polymer can be carried out by utilizing known polymerization reactions such as radical polymerization, cationic polymerization, anionic polymerization, living radical polymerization, etc. Among the above polymerization reactions, radical polymerization is preferred from the viewpoint of reducing synthesis costs. The synthesis of the acrylamide polymer can be carried out by utilizing a polymerization method such as solution polymerization, suspension polymerization, precipitation polymerization, dispersion polymerization, or emulsion polymerization (for example, inverse emulsion polymerization). Furthermore, when synthesizing an acrylamide-based polymer by solution polymerization, it is preferable to use a solvent in which the raw material monomers and the resulting acrylamide-based polymer are soluble. From the viewpoint of low-cost and safe synthesis, it is more preferable to use an aqueous solvent or an aqueous mixed solvent, and it is even more preferable to use an aqueous solvent. Examples of aqueous solvents include water, alcohol, and mixed solvents thereof, with water being particularly preferred. The aqueous mixed solvent means a mixed solvent of the above aqueous solvent and an organic solvent, and examples of the organic solvent include tetrahydrofuran, dimethyl sulfoxide, and dimethylformamide.

[0051] Examples of methods for producing an acrylamide-based polymer composition include a method of directly mixing an additive component with a molten acrylamide-based polymer (melt mixing), a method of dry-blending an acrylamide-based polymer with an additive component (dry mixing), and a method of immersing or passing a fibrous acrylamide-based polymer in an aqueous solution or aqueous mixed solution containing the additive component, or in a solution in which the acrylamide-based polymer is not completely dissolved but the additive component is dissolved. Furthermore, when the acrylamide-based polymer and the additive component are soluble in an aqueous solvent or an aqueous mixed solvent, a method of mixing the acrylamide-based polymer and the additive component in an aqueous solvent or an aqueous mixed solvent (wet mixing) is preferred, from the viewpoint of being able to uniformly mix the acrylamide-based polymer and the additive component. Alternatively, wet mixing may be carried out by mixing the additive components into the aqueous solvent or aqueous mixed solvent in which the acrylamide polymer has been synthesized.

[0052] In wet mixing, it is preferable to use an aqueous solvent as the solvent, and it is more preferable to use water, from the viewpoint of producing an acrylamide polymer composition at lower cost and in safety.

[0053] When the acrylamide polymer composition is produced by wet mixing, the solvent may or may not be removed. The method for removing the solvent is not particularly limited, and at least one of known methods such as distillation under reduced pressure, reprecipitation, hot air drying, vacuum drying, and freeze drying can be used.

[0054] The acrylamide polymer fiber may be a single fiber or a fiber bundle. However, from the viewpoint of exhibiting high mechanical properties, carbon fibers used for structural members in aerospace applications, automotive applications, building materials, and the like are preferably fiber bundles consisting of a plurality of single fibers. When the carbon fiber precursor of the present disclosure is a fiber bundle, the number of filaments per bundle is not particularly limited, but from the viewpoints of productivity and mechanical properties of the flame-resistant fiber and carbon fiber, it is preferably 10 to 360,000, more preferably 50 to 180,000, even more preferably 100 to 72,000, and particularly preferably 800 to 36,000. Furthermore, by setting the number of filaments per bundle to 360,000 or less, it is possible to suppress the occurrence of uneven firing during the flame-resistant treatment or carbonization treatment. In the present disclosure, the fusion rate of carbon fibers is measured based on 800 fiber bundles.

[0055] The fineness of the acrylamide polymer fiber is not particularly limited, but is preferably 1×10 -8 tex / line to 100 tex / line is preferable, and 1×10 -6 tex / line to 60 tex / line is more preferable, and 1×10 -3 It is more preferable that the thickness is 1×10 tex / thread to 40 tex / thread. -2 It is even more preferable that the thickness is 2×10 tex / piece to 10 tex / piece. -2 tex / fiber to 2 tex / fiber is particularly preferable, and 3×10 -2 tex / book~4×10 -1 A tex / book is most preferred. The fineness of the acrylamide polymer fiber is 1 × 10 -8 By making it tex / fiber or more, the occurrence of yarn breakage can be suppressed, which tends to improve the ease of winding the carbon fiber precursor and the stability of the flame retardant treatment. By setting the fineness of the acrylamide polymer fiber to 100 tex / fiber or less, the difference between the structure near the surface layer and the structure near the center of the carbon fiber obtained by the flame retardant treatment can be reduced, and the tensile strength and tensile modulus of the carbon fiber tend to be improved.

[0056] In the present disclosure, the fineness of an acrylamide-based polymer fiber was measured by measuring the mass of an acrylamide-based polymer fiber bundle, calculating the mass per 1000 m as the fineness [tex] of the fiber bundle, and determining the fineness of the single fiber constituting the fiber bundle.

[0057] The average fiber diameter of the acrylamide polymer fibers is not particularly limited, but is preferably 3 nm to 300 μm, more preferably 30 nm to 250 μm, even more preferably 1 μm to 200 μm, particularly preferably 3 μm to 100 μm, even more particularly preferably 4 μm to 40 μm, most preferably 5 μm to 30 μm, and may be 6 μm to 20 μm. By making the average fiber diameter of the acrylamide-based polymer fibers 3 nm or more, the stability of the flame-resistant treatment tends to be improved. Also, by making the average fiber diameter of the acrylamide-based polymer fibers 3 nm or more, the occurrence of fiber breakage can be suppressed, which tends to improve the ease of winding the carbon fiber precursor and the stability of the flame-resistant treatment. By setting the average fiber diameter of the acrylamide-based polymer fibers to 300 μm or less, the difference between the structure near the surface layer and the structure near the center of the carbon fiber obtained by the flame retardant treatment can be reduced, and the tensile strength and tensile modulus of the carbon fiber tend to be improved.

[0058] In the present disclosure, the average fiber diameter is determined by measuring the density of the acrylamide polymer fiber using a dry automatic densimeter to measure the density of the fiber bundle, and then calculating the average fiber diameter of the single fibers constituting the fiber using the following formula: Note that, as the dry automatic densimeter, an Accupyc II 1340 manufactured by Micromeritics or an equivalent device can be used. D={(Dt×4×1000) / (ρ×π×n)} 1 / 2 [During the ceremony, D represents the average fiber diameter (μm) of the single fibers constituting the fiber bundle, Dt represents the fiber bundle fineness (tex), ρ is the density of the fiber bundle (g / cm 3 ) and n represents the number of single fibers that make up the fiber bundle. Note that π is 3.14.

[0059] From the viewpoints of suppressing fusion, water resistance, and suppressing fiber breakage and the generation of fluff during stretching, the acrylamide polymer fiber is preferably a crosslinked acrylamide fiber, and the gel fraction of the crosslinked acrylamide fiber is preferably 5% or more, more preferably 10% or more, even more preferably 30% or more, even more preferably 50% or more, and particularly preferably 70% or more. From the viewpoint of the stretchability of the carbon fiber precursor, the gel fraction of the acrylamide polymer fiber is preferably 98% or less, more preferably 95% or less, and may be 100%. From the viewpoints of suppressing fusion, water resistance, and stretchability of the carbon fiber precursor, the gel fraction of the acrylamide polymer fiber is preferably 5% to 98%. Whether or not an acrylamide-based polymer is crosslinked is confirmed by measuring the gel fraction of the acrylamide-based polymer fiber. If the gel fraction of the acrylamide-based polymer fiber is 3% or more, the acrylamide-based polymer is deemed to be crosslinked.

[0060] <Fusion rate of carbon fibers in carbon fiber bundles> In a carbon fiber bundle consisting of 800 carbon fibers of the present disclosure, the fusion rate of the carbon fibers contained in the carbon fiber bundle is, in one embodiment, preferably 15% or less, more preferably 10% or less, even more preferably 7% or less, and particularly preferably 5% or less. Note that a fusion rate of 10% or less obtained by the above measurement method can be said to be at a level that presents no practical problems. There is no particular restriction on the lower limit of the fusion rate, and it may be, for example, 0%, that is, the carbon fibers are not fused to each other.

[0061] The fusion rate of carbon fibers can be measured as follows. Evaluation fibers (800 fibers / bundle) with a length of 2 cm are cut out from the carbon fiber, and the cross section of this evaluation fiber is observed using a microscope (Keyence Corporation, "Digital Microscope VHX-7000") to count the number of fibers. The ratio of the number of fibers to the number of fibers in the carbon fiber precursor before the flame retardant treatment can be calculated as the fusion rate.

[0062] <Method of manufacturing carbon fiber> The method for producing carbon fibers according to the present disclosure includes a step of subjecting a spread fiber bundle obtained by spreading acrylamide-based polymer fibers, which are carbon fiber precursors, to a flame retardant treatment. The above process will hereinafter also be referred to as the "process for producing flame-resistant fiber."

[0063] (Flame-resistant fiber manufacturing process) The method for producing carbon fibers according to the present disclosure includes a production step of flame-retardant fibers in which the above-described carbon fiber precursor is subjected to a fiber-opening treatment to produce a spread fiber bundle, and the spread fiber bundle is then subjected to a flame-retardant treatment.

[0064] In the present disclosure, the term "spread fiber bundle" refers to a fiber bundle after spreading, regardless of the number of spreading treatments (number of stages). In the present disclosure, the term "spreading treatment" refers to a treatment for separating a fiber bundle consisting of multiple single fibers into individual single fibers. In the present disclosure, the term "carbon fiber precursor fiber" refers to a fiber from which a carbon fiber can be obtained by subjecting it to a carbonization treatment, or a flame-proofing treatment and a carbonization treatment. The method of fiber-spreading applied in the present disclosure is not particularly limited, and preferred methods include fluid, vibration, friction, charging (static electricity), roller (a roller having a mechanism for spreading the fibers in a direction intersecting the longitudinal direction of the fiber bundle), and manual fiber-spreading (spreading the fibers by hand), and from the viewpoint of reducing damage to the fibers, fiber-spreading using a fluid is more preferred. In the present disclosure, the fluid applied to the fiber-spreading process may be a gas or a liquid, and is preferably a gas. In this embodiment, air is used as fluid A. Furthermore, the method of applying the fluid that passes between the fibers of the fiber bundle is more preferably a fluid suction mechanism and / or a fluid blowing mechanism. A particularly preferred fiber-spreading method is one in which a fluid suction mechanism or a fluid blowing mechanism for applying the fluid that passes between the fibers of the fiber bundle and a support member that supports the fiber bundle are installed. The fiber-spreading method may be a combination of any of these methods. There are no particular restrictions on the fiber-spreading conditions, but it is preferable to control one or more of the air volume during fiber-spreading, the width of the airflow, the width of the fibers before fiber-spreading, the tension during transportation, the fiber-spreading speed, the fiber-spreading time, and the number of fiber-spreading stages (number of stages). From the viewpoint of improving productivity, it is preferable to control the fiber-spreading conditions, such as the fiber-spreading speed, fiber-spreading time, and the number of fiber-spreading stages (number of stages).

[0065] The spreading portion of the fiber-spreading device used for the fiber-spreading process may be a single spread portion, but preferably has a plurality of spread portions. A device having a plurality of spread portions is a device in which the respective spread portions are arranged in series and / or parallel, and preferably two successive (two stages), three successive (three stages), four successive (four stages), or more successive spread portions are provided in series to perform the fiber-spreading process continuously. In such a continuous fiber-spreading device, for example, the width of the displacement prevention portion may be gradually increased, which makes it possible to gradually spread the fiber bundle. In addition, in such a fiber-spreading device, In the case of the above, fiber-spreading can be performed by making various changes, such as changing the fiber-spreading conditions by using one of the spread portions, or changing the fiber-spreading conditions by using two or more spread portions. In the case of multi-stage fiber spreading, suction and blowing may be performed alternately. In the method for spreading the fiber bundle, the fibers may be loosened in advance and then spread.

[0066] FIG. 1 shows an example of one embodiment of a fiber-spreading device that uses suction with a fluid and that can be used in the manufacturing method of the present disclosure to spread fiber bundles. Fig. 1 is a schematic configuration diagram of one embodiment of a fiber-spreading device that can be used in the manufacturing method of the present disclosure. A fiber-spreading device 10 for fiber bundles shown in Fig. 1 includes guides 12 as displacement prevention units, support members 14 for the spread portion, and conveying rollers 16 that convey the fiber bundle. The fiber bundle 1 guided by the guides 12 is conveyed in the direction of the arrow by the conveying rollers 16 while being supported by the support members 14. The spread portion also includes a suction device 18 as a suction mechanism that generates an airflow of fluid A so as to suck the fiber bundle 1 during conveyance. However, the device that can be used for spreading is not limited to this.

[0067] The flame-resistant treatment refers to subjecting a carbon fiber precursor to a heat treatment in an oxidizing atmosphere. The carbon fiber precursor may be one produced by the above-mentioned production method. The flame retardation of the carbon fiber precursor is preferably carried out in an oxidizing atmosphere at a temperature of 120°C to 500°C, more preferably at a temperature of 150°C to 480°C, even more preferably at a temperature of 170°C to 470°C, particularly preferably at a temperature of 180°C to 460°C, and most preferably at a temperature of 200°C to 450°C. The maximum temperature in the flame retardant treatment is not particularly limited, but from the viewpoints of improving stability during carbonization (suppressing thermal decomposition at the temperature during pre-carbonization or carbonization) and reducing production costs by shortening the time required for the treatment, the maximum temperature is preferably 290°C or higher, more preferably 300°C or higher, and particularly preferably 330°C or higher.

[0068] Examples of the oxidizing atmosphere during the flame-proofing treatment include oxygen, ozone, air, nitrogen oxides, halogens, sulfur dioxide gas, mixed gases of these, and mixed gases of these with inert gases, etc. Among these, air, mixed gases of oxygen and air, mixed gases of oxygen and inert gas, and mixed gases of air and inert gas are preferred, and air is particularly preferred from the viewpoint of cost reduction.

[0069] The flame retardant treatment time (heating time at the above maximum temperature) is not particularly limited, and heating for a long period of time (for example, more than 4 hours) is possible, but 1 minute to 4 hours is preferred, 2 minutes to 2 hours is more preferred, 3 minutes to 100 minutes is even more preferred, 4 minutes to 90 minutes is particularly preferred, and 4 minutes to 60 minutes is most preferred. By setting the heating time in the flame retardant treatment to the lower limit or more, the carbonization yield can be improved, while by setting it to the upper limit or less, the cost can be reduced.

[0070] In the production process of the flame-resistant fiber, tension may or may not be applied to the carbon fiber precursor during the temperature rise process up to the flame-resistant treatment temperature, etc. In particular, tension is preferably applied during the temperature rise process, etc., from the viewpoint of sufficiently obtaining the effect of applying tension. The tension applied to the carbon fiber precursor may be applied from the initial stage of the temperature increase process or the like, or may be applied at an intermediate stage. In one embodiment, the stretching treatment is carried out at the flameproofing treatment temperature, and the stretching treatment does not need to be carried out at other temperatures. The stretching treatment may be carried out while controlling the moisture absorption rate during the spinning treatment or in a step prior to the flame-proofing treatment.

[0071] During the drawing treatment, the tension applied to the carbon fiber precursor is preferably 0.05 mN / tex to 2000 mN / tex, more preferably 0.1 mN / tex to 500 mN / tex, further preferably 0.1 mN / tex to 200 mN / tex, and particularly preferably 0.2 mN / tex to 100 mN / tex. By setting the tension applied to the carbon fiber precursor during the drawing treatment within the above numerical range, it is possible to improve the fusion suppression property and to suppress the occurrence of breakage and fluff in the flame-resistant fiber. In the present disclosure, the tension (unit: mN / tex) applied to the carbon fiber precursor is a value obtained by dividing the tension (unit: mN) applied to the carbon fiber precursor during the flame-resistant treatment by the fineness (unit: tex) of the carbon fiber precursor in an bone-dry state, that is, the tension per unit fineness of the carbon fiber precursor. The tension can be adjusted by adjusting the speed at the inlet and outlet of a heating device such as a flame-proofing furnace, or by using a load cell, a spring, a weight, an air cylinder, or the like.

[0072] The density of the flame-retardant fiber obtained through the above-described flame-retardant fiber manufacturing process is not particularly limited, but from the viewpoints of carbonization yield, productivity, etc., it is preferably 1.30 g / cm. 3 ~1.75g / cm 3 and preferably 1.35 g / cm 3 ~1.70g / cm 3 More preferably, it is 1.37 g / cm 3 ~1.65g / cm 3 More preferably, it is 1.39 g / cm 3 ~1.60g / cm 3 It is particularly preferable that the 3 ~1.55g / cm 3 It is most preferable that:

[0073] The average fiber diameter of the flame-resistant fiber is not particularly limited, but from the viewpoint of the tensile strength of the resulting carbon fiber, it is preferably 3 nm to 300 μm, more preferably 30 nm to 150 μm, even more preferably 1 μm to 60 μm, particularly preferably 2 μm to 30 μm, most preferably 3 μm to 20 μm, and may be 4 μm to 15 μm. Furthermore, by setting the average fiber diameter of the flame-resistant fiber to the above-mentioned numerical range, it is possible to suppress breakage and fluffing in the flame-resistant fiber.

[0074] From the viewpoint of carbonization yield, the average fiber diameter of the flame-resistant fiber is preferably at least 5% smaller than the average fiber diameter of the acrylamide-based polymer fiber, which is the carbon fiber precursor, more preferably at least 10% smaller, even more preferably at least 15% smaller, particularly preferably at least 20% smaller, most preferably at least 25% smaller, and may be at least 30% smaller.

[0075] The method for producing carbon fiber according to the present disclosure further includes, in addition to the above-described steps for producing flame-resistant fiber, a step of subjecting the flame-resistant treated acrylamide polymer fiber to a carbonization treatment. In one embodiment, the carbonization step is performed such that the phosphorus content of the flame-retardant treated acrylamide polymer fiber is 0.1% by mass to 10% by mass according to elemental analysis, the maximum temperature in the carbonization step is in the range of 1350°C to 1650°C, and the fusion rate of the carbon fibers in the carbonized fiber bundle is preferably 20% or less. The fusion rate of the carbon fibers in the carbonized fiber bundle is more preferably 15% or less, even more preferably 10% or less, particularly preferably 7% or less, and most preferably 5% or less. There is no particular restriction on the lower limit of the fusion rate, and it may be, for example, 0%, that is, the carbon fibers are not fused to each other.

[0076] The carbonization treatment refers to a treatment for carbonizing a carbon fiber precursor, and means that the carbon fiber precursor is subjected to a heat treatment in a low-oxygen environment (preferably an environment in which oxygen is blocked).

[0077] As a method for carbonizing the flame-resistant fiber obtained through the above steps, there can be mentioned a method in which the flame-resistant fiber is subjected to a heat treatment (carbonization treatment) in an inert atmosphere (in an inert gas such as nitrogen, argon, or helium) at a temperature higher than the temperature in the flame-resistant treatment. By carrying out the carbonization treatment step, the flame-resistant fiber is carbonized to obtain the desired carbon fiber. In the carbonization treatment, for example, heat treatment may be first carried out at a temperature below 1000° C., and then heat treatment at a temperature of 1000° C. or higher (carbonization treatment, sometimes referred to as main carbonization treatment). In the carbonization process, the initial heat treatment at a temperature below 1000°C is sometimes referred to as a preliminary carbonization process, and the heat treatment at a temperature of 1000°C or higher that is performed after the preliminary carbonization process is sometimes referred to as a main carbonization process.

[0078] Alternatively, multiple heat treatments may be performed, such as performing a heat treatment at a temperature below 1000°C (i.e., preliminary carbonization treatment), followed by a heat treatment at a temperature of 1000°C or higher (i.e., main carbonization treatment), and then performing a further heat treatment at a temperature of 2000°C or higher (also known as graphitization treatment).

[0079] The maximum temperature in the carbonization treatment of the present disclosure is not particularly limited, but from the viewpoint of easily obtaining carbon fibers that satisfy the physical properties of the present disclosure, it is preferably 1300°C or higher, more preferably 1350°C or higher, even more preferably 1400°C or higher, particularly preferably 1450°C or higher, and most preferably 1500°C or higher. The maximum temperature in the carbonization treatment is preferably 1700°C or lower, more preferably 1675°C or lower, even more preferably 1650°C or lower, particularly preferably 1625°C or lower, and most preferably 1600°C or lower.

[0080] In particular, the maximum temperature during heating in the carbonization treatment is preferably in the range of 1350°C to 1650°C. The maximum temperature refers to the maximum temperature in the heating zone measured by a radiation thermometer installed in the heating zone where the carbonization treatment is performed. By setting the maximum heating temperature in the carbonization treatment to a range of 1350°C to 1650°C, it is easy to achieve the desired contents of phosphorus, nitrogen, and oxygen in the carbon fiber. That is, when the maximum temperature is 1350°C or higher, the removal of nitrogen atoms is good and it is easy to adjust the nitrogen content to 5 mass% or less, and when the maximum temperature is 1650°C or lower, undesired decomposition and removal of phosphorus is suppressed and it is easy to adjust the phosphorus content to the range of 0.6 mass% to 10 mass%.

[0081] The heating temperature in the preliminary carbonization treatment is preferably 400°C or higher, more preferably 500°C or higher, and even more preferably 600°C or higher. The "carbonization step" in the present disclosure may include the graphitization step, which is generally carried out by heating at 2000 to 3000°C in an inert gas atmosphere. The heating time in the carbonization treatment is not particularly limited, but is preferably 30 seconds to 120 minutes, more preferably 30 seconds to 60 minutes, and more preferably 1 to 30 minutes. From the viewpoint of reducing production costs, the upper limit of the heating time is more preferably 20 minutes or less, and particularly preferably 10 minutes or less.

[0082] The average fiber diameter of the carbon fibers obtained by the carbon fiber manufacturing method of the present disclosure is not particularly limited, but is preferably 3 nm to 300 μm, more preferably 30 nm to 150 μm, even more preferably 100 nm to 60 μm, even more preferably 1 to 40 μm, particularly preferably 2 to 30 μm, and most preferably 2.5 to 25 μm. When the carbon fibers have an average fiber diameter of at least the above lower limit, when the carbon fibers are used to produce a composite material with a resin or the like as a matrix, a decrease in the tensile strength of the composite material caused by insufficient impregnation of the resin or the like into the carbon fiber bundles due to a high viscosity of the matrix is suppressed, and when the average fiber diameter is at most the above upper limit, a decrease in the tensile strength of the carbon fibers is suppressed, making it easier to obtain carbon fibers with good tensile strength.

[0083] The carbon fibers obtained by the carbon fiber production method of the present disclosure can also be used, for example, to produce composite fibers having a coating film containing a resin, an oil agent, etc. on the surface of the carbon fiber.

[0084] According to the method for producing carbon fibers of the present disclosure, it is possible to efficiently produce carbon fibers of the present disclosure having a phosphorus content of 0.6% by mass to 10% by mass, a nitrogen content of 5% by mass or less, an oxygen content of 0.3% by mass or less, and a single fiber tensile modulus of 150 GPa or more, as determined by elemental analysis. [Example]

[0085] The above embodiment will be specifically described below using examples, but the above embodiment is not limited to these examples.

[0086] The composition of the monomer contained in the carbon fiber precursor used for the carbon fiber of the present disclosure is shown below. Acrylamide polymer (a-1) A polymer consisting of 75 mol% acrylamide (AM) and 25 mol% acrylonitrile (AN). Acrylamide polymer (a-2) A polymer consisting of 73 mol% acrylamide (AM), 25 mol% acrylonitrile (AN), and 2 mol% acrylic acid (AA). Acrylamide polymer (a-3) A polymer consisting of 65 mol% acrylamide (AM), 33 mol% acrylonitrile (AN), and 2 mol% acrylic acid (AA). Acrylamide polymer (a-4) A polymer consisting of 60 mol% acrylamide (AM), 35 mol% acrylonitrile (AN), and 5 mol% acrylic acid (AA).

[0087] (Production Example 1: Preparation of acrylamide polymer fiber (f-1)) The acrylamide-based polymer (a-1) was dissolved in ion-exchanged water, and 3 parts by mass of phosphoric acid was added to 100 parts by mass of (a-1). Using the obtained aqueous solution, dry spinning was carried out so that the acrylamide-based polymer fiber had a single fiber fineness of 5 dtex, to produce a fiber bundle of acrylamide-based polymer fiber (f-1) consisting of 100 fibers per bundle. The fineness and average fiber diameter of the single fibers constituting the fiber bundle of the obtained acrylamide polymer fiber (f-1) were measured by the following methods, and were found to be 5.2 dtex / fiber and 23 μm in average fiber diameter.

[0088] (Production Example 2: Preparation of acrylamide polymer fiber (f-2)) The above acrylamide-based polymer (a-2) was dissolved in ion-exchanged water, and 3 parts by mass of phosphoric acid was added to 100 parts by mass of (a-2). Using the obtained aqueous solution, dry spinning was carried out so that the acrylamide-based polymer fiber had a single fiber fineness of 7 dtex, to produce acrylamide-based polymer fibers (f-2) consisting of 100 fibers per bundle. The fineness and average fiber diameter of this acrylamide polymer fiber (f-2) were measured by the following methods, and it was found that the fineness was 6.8 dtex / fiber and the average fiber diameter was 26 μm.

[0089] (Production Example 3: Preparation of acrylamide polymer fiber (f-3)) The acrylamide polymer (a-3) was dissolved in ion-exchanged water, and 3 parts by mass of diammonium hydrogen phosphate as a phosphoric acid salt (phosphate salt) was added to 100 parts by mass of (a-3) to obtain an aqueous solution, which was then dry-spun to produce acrylamide polymer fibers (f-3) consisting of 100 fibers per bundle, with a single fiber fineness of 2 dtex. The fineness and average fiber diameter of this acrylamide polymer fiber (f-3) were measured by the following method, and were found to be 2.0 dtex / fiber and 14 μm in average fiber diameter.

[0090] (Production Example 4: Preparation of acrylamide polymer fiber (f-4)) The above acrylamide-based polymer (a-3) was dissolved in ion-exchanged water, and 3 parts by mass of phosphoric acid was added to 100 parts by mass of (a-3). Using the obtained aqueous solution, dry spinning was carried out so that the acrylamide-based polymer fiber had a single fiber fineness of 2 dtex, to produce acrylamide-based polymer fibers (f-4) consisting of 100 fibers per bundle. The fineness and average fiber diameter of the resulting acrylamide polymer fiber (f-4) were measured by the following methods, and it was found that the fineness was 2.3 dtex / fiber and the average fiber diameter was 15 μm.

[0091] (Production Example 5: Preparation of acrylamide polymer fiber (f-5)) The acrylamide-based polymer (a-4) was dissolved in ion-exchanged water, and 3 parts by mass of phosphoric acid was added to 100 parts by mass of the acrylamide-based polymer (a-4). Using the obtained aqueous solution, dry spinning was carried out so that the single fiber fineness of the acrylamide-based polymer fiber became 4 dtex, and a fiber bundle of acrylamide-based polymer fiber (f-5) consisting of 100 fibers per bundle was produced. The fineness and average fiber diameter of the single fibers constituting the fiber bundle of the obtained acrylamide polymer fiber (f-5) were measured by the following methods, and were found to be 3.9 dtex / fiber and 20 μm in average fiber diameter.

[0092] (Production Example 6: Preparation of acrylamide-based polymer fiber (f-6) (without addition of phosphorus-based catalyst)) An aqueous solution of the acrylamide polymer (a-1) dissolved in ion-exchanged water was dry-spun to produce a fiber bundle of acrylamide polymer fiber (f-6) consisting of 100 fibers per bundle, with the acrylamide polymer fiber (f-6) having a single fiber fineness of 5 dtex. The fineness and average fiber diameter of the single fibers constituting the fiber bundle of this acrylamide polymer fiber (f-6) were measured by the following method, and were found to be 5.0 dtex / fiber and 23 μm in average fiber diameter.

[0093] <Fineness of acrylamide polymer fiber> The mass of the resulting fiber bundle of acrylamide-based polymer fibers (100 fibers / bundle) was measured after drying at 120°C for 2 hours, and the fineness of the fiber bundle was calculated using the following formula (1), thereby determining the fineness of the single fiber constituting the fiber bundle (the fineness of the acrylamide-based polymer fiber). Formula (1): Fiber bundle fineness [dtex] = [fiber bundle mass [g] / fiber length [m]] x 10,000 [m]

[0094] <Average fiber diameter of acrylamide polymer fibers> The density of each acrylamide polymer fiber bundle after vacuum drying at 120°C for 1 hour was measured using a dry automatic densitometer (AccuPyc II1340 manufactured by Micromeritics), and the average fiber diameter of the individual fibers constituting the fiber bundle (average fiber diameter of the acrylamide polymer fibers) was calculated using the following formula (2): Formula (2): D={(Dt×4×100) / (ρ×π×n)}1 / 2 [In the above formula (2), D represents the average fiber diameter [μm] of the single fibers constituting the fiber bundle, Dt represents the fineness [dtex] of the fiber bundle, and ρ represents the density [g / cm 3 ] and n represents the number of single fibers that make up the fiber bundle.

[0095] <Preparation of flame-retardant fiber (s-1) by flame-retardantization of acrylamide-based polymer fiber (f-1)> A mixture of 100 parts by mass of acrylamide polymer fiber (f-1) (average fiber diameter 23 μm) fiber bundles (100 fibers / bundle) was mixed with 2 parts by mass of silicone oil (KF-96H-100cs manufactured by Shin-Etsu Chemical Co., Ltd., kinematic viscosity 100 mm at 25°C). 2 After applying a coating of 1000 fibers / min (1000 / min) to the fiber bundle, eight bundles were bundled together to obtain a fiber bundle (800 fibers / bundle, fiber bundle width: 4.5 mm). This fiber bundle was fed onto the support member of the fiber spreader (Fig. 1) at a conveying speed of 150 mm / min, and spread by air suction using air as the fluid (air flow width: 25 mm x 22 mm (length in the fiber axial direction of the fiber bundle)), to obtain a spread fiber bundle with a width of 20 mm. The width of this spread fiber bundle was further expanded to obtain a spread fiber bundle in which the single fibers constituting the fiber bundle were arranged so as not to come into contact with each other (fiber width after spreading: 50 mm, ratio of the fiber width of the spread fiber bundle to the width of the original fiber bundle (spreading ratio): 11.1). Next, this spread fiber bundle was heat-treated (flame-resistant) in a heat treatment device by raising the temperature from room temperature to 350°C over 30 minutes under an air flow while stretching at a draw ratio of 6, and then further held at 350°C for 30 minutes to perform flame-resistant treatment, thereby obtaining a flame-resistant fiber (s-1) (800 fibers / bundle, average single fiber diameter of 8 μm).

[0096] <Preparation of flame-retardant fiber (s-2) by flame-retardantization of acrylamide-based polymer fiber (f-2)> Two parts by mass of a silicone-based oil (KF-96H-100cs, manufactured by Shin-Etsu Chemical Co., Ltd.) was applied to 100 parts by mass of fiber bundles (100 fibers / bundle) of acrylamide polymer fiber (f-2) (average fiber diameter 26 μm), and eight bundles were bundled together to obtain a fiber bundle (800 fibers / bundle, fiber bundle width: 4.5 mm). This fiber bundle was fed onto the support rod of a fiber spreader (Figure 1) at a conveying speed of 150 mm / min and spread by air suction using air as the fluid (airflow width 25 mm × 22 mm (length in the fiber axial direction of the fiber bundle)), resulting in a spread fiber bundle with a width of 20 mm. The width of this spread fiber bundle was further expanded to obtain a spread fiber bundle (fiber bundle width after spreading: 50 mm, fiber spread ratio: 12.5x) in which the individual fibers constituting the fiber bundle were arranged so as to be almost completely free of contact with each other. Next, the fiber bundle was heated from room temperature to 350°C over 30 minutes while being stretched at a draw ratio of 4 under an air flow, and then held at 350°C for 30 minutes to perform flame retardation, thereby obtaining flame retardant fiber (s-2) (800 fibers / bundle, average single fiber diameter 8 μm).

[0097] <Preparation of flame-retardant fiber (s-3) by flame-retardantization of acrylamide-based polymer fiber (f-3)> Two parts by mass of a silicone-based oil (KF-96H-100cs, manufactured by Shin-Etsu Chemical Co., Ltd.) was applied to 100 parts by mass of fiber bundles (100 fibers / bundle) of acrylamide polymer fiber (f-3) (average fiber diameter: 14 μm), and eight bundles were bundled together to obtain a fiber bundle (800 fibers / bundle, fiber bundle width: 3 mm). This fiber bundle was fed onto the support rod of a fiber spreader (Figure 1) at a conveying speed of 150 mm / min and spread using air suction (airflow width: 25 mm × 22 mm (length of the fiber bundle in the fiber axial direction)), resulting in a spread fiber bundle with a width of 20 mm. The width of this spread fiber bundle was further expanded to obtain a spread fiber bundle (fiber width after spreading: 50 mm, fiber spread ratio: 16.7 times) in which the individual fibers constituting the fiber bundle were arranged so that they were barely touching each other. Next, the fiber bundle was heated from room temperature to 350°C over 30 minutes while being stretched at a draw ratio of 4 under an air flow, and then held at 350°C for 30 minutes to perform flame retardation, thereby obtaining flame retardant fiber (s-3) (800 fibers / bundle, average fiber diameter of single fiber: 6 μm).

[0098] <Preparation of flame-retardant fiber (s-4) by flame-retardantization of acrylamide-based polymer fiber (f-4)> Acrylamide polymer fiber (f-4) (average fiber diameter 15 μm) fiber bundle (100 fibers / bundle) 100 mass parts per 2 mass parts of silicone After applying a corn-based oil (KF-96H-100cs manufactured by Shin-Etsu Chemical Co., Ltd.), eight bundles were bundled together to obtain a fiber bundle (800 fibers / bundle, fiber bundle width: 3 mm). This fiber bundle was sent onto the support rod of the fiber spreader (Fig. 1) at a conveying speed of 150 mm / min, and then spread by air suction using air as the fluid (air flow width 25 mm x 22 mm (fiber bundle width)). A spread fiber bundle with a length in the fiber axial direction and a fiber bundle width of 20 mm was obtained. The width of this spread fiber bundle was further expanded to obtain a spread fiber bundle (fiber width after spreading: 50 mm, spread ratio: 16.7 times) in which the individual fibers constituting the fiber bundle were arranged so as not to come into contact with each other. Next, the fiber bundle was heated from room temperature to 350°C over 30 minutes in an air flow while being stretched at a draw ratio of 4 times, and then held at 350°C for 30 minutes to perform flame retardation, thereby obtaining a flame retarded fiber (s-4) (800 fibers / bundle, average fiber diameter of individual fibers: 6 μm).

[0099] <Preparation of flame-retardant fiber (s-5) by flame-retardantization of acrylamide-based polymer fiber (f-5)> Two parts by mass of a silicone-based oil (KF-96H-100cs, manufactured by Shin-Etsu Chemical Co., Ltd.) was applied to 100 parts by mass of fiber bundles (100 fibers / bundle) of acrylamide-based polymer fiber (f-5) (average fiber diameter 20 μm), and then eight bundles were bundled together to obtain a fiber bundle (800 fibers / bundle, fiber bundle width: 4 mm). This fiber bundle was fed onto the support rod of a fiber spreader (Figure 1) at a conveying speed of 150 mm / min and spread using air suction (airflow width 25 mm × 22 mm (length in the fiber axial direction of the fiber bundle)), obtaining a spread fiber bundle with a width of 20 mm. The width of this spread fiber bundle was further expanded to obtain a spread fiber bundle (fiber width after spreading: 50 mm). Next, the fiber bundle was heated from room temperature to 350°C over 30 minutes while being stretched at a draw ratio of 6 under an air flow, and then held at 350°C for 30 minutes to perform flame retardation, thereby obtaining flame retardant fiber (s-5) (800 fibers / bundle, average fiber diameter of single fiber: 9 μm).

[0100] <Production of flame-retardant fiber (s-6) by flame-retardantizing acrylamide polymer fiber (f-1): No opening process required> Two parts by mass of a silicone-based oil (KF-96H-100cs manufactured by Shin-Etsu Chemical Co., Ltd., kinematic viscosity 30 mm / s at 25°C) was applied to 100 parts by mass of fiber bundles (100 fibers / bundle) of acrylamide polymer fiber (f-1) (average fiber diameter 23 μm), and eight bundles were bundled to obtain a fiber bundle (800 fibers / bundle, fiber bundle width: 4.5 mm). The fiber bundle was heated from room temperature to 350°C over 30 minutes while being stretched at a draw ratio of 6 times under air flow, and then held at 350°C for 30 minutes to obtain flame-resistant fiber (s-6) (800 fibers / bundle, average fiber diameter of single fibers 8 μm). The fusion rate of the flame-resistant fiber was 75%.

[0101] <Preparation of flame-retardant fiber (s-7) by flame-retardantization of acrylamide-based polymer fiber (f-6) (without phosphorus-based compounds)> Two parts by mass of a silicone-based oil (KF-96H-100cs manufactured by Shin-Etsu Chemical Co., Ltd., kinematic viscosity 30 mm2 / s at 25°C) was applied to 100 parts by mass of fiber bundles (100 fibers / bundle) of acrylamide polymer fiber (f-6) (average fiber diameter 23 μm), and eight bundles were bundled to obtain a fiber bundle (800 fibers / bundle, fiber bundle width: 4.5 mm). The fiber bundle was heated from room temperature to 350°C over 30 minutes while being stretched at a draw ratio of 6 times under air flow, and then held at 350°C for 30 minutes to obtain flame-resistant fiber (s-7) (800 fibers / bundle, average fiber diameter of single fibers 8 μm). The fusion rate of the flame-resistant fiber was 80%.

[0102] [Preparation of carbon fiber] Example 1 Flame-resistant fibers (s-1) (800 fibers / bundle) were bundled to form 12,000 fibers / bundle, and while maintaining a tension of 70 cN, the bundle was moved through a heat treatment furnace with a temperature gradient of 300°C to 800°C under a nitrogen stream over the course of 3 minutes to perform a preliminary carbonization treatment (3 minutes) to produce pre-carbonized fibers. Next, the pre-carbonized fiber, still under tension of 70 cN, was moved through the heating zone of a heat treatment furnace with a heating zone having a temperature gradient of 1050°C to 1350°C (maximum temperature in the heating zone measured with a radiation thermometer: 1350°C) under a nitrogen gas flow for 3 minutes to undergo a heat treatment (carbonization treatment), and was then wound around a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) under tension of 200 gf to obtain carbon fiber (average fiber diameter 5 μm). The maximum temperature in the heat treatment furnace, 1350°C, was the maximum temperature measured with a radiation thermometer in the heat treatment furnace. In the following examples, the maximum temperature in the heat treatment furnace refers to the upper limit of the set temperature, i.e., the maximum temperature measured with a radiation thermometer in the heat treatment furnace. The maximum temperatures for heat treatment are shown in Table 1 below.

[0103] <Example 2> Flame-resistant fibers (s-5) (800 fibers / bundle) were bundled to form 12,000 fibers / bundle, and while maintaining a tension of 70 cN, the bundle was moved through a heat treatment furnace with a temperature gradient of 300°C to 800°C under a nitrogen stream over the course of 3 minutes to perform a preliminary carbonization treatment (3 minutes) to produce pre-carbonized fibers. Next, while maintaining a tension of 70 cN, the fiber was moved through the heating zone of a heat treatment furnace with a heating zone having a temperature gradient of 1100°C to 1400°C (maximum temperature in the heating zone measured with a radiation thermometer: 1350°C) over a period of 3 minutes under a nitrogen stream to perform a heat treatment (carbonization treatment), and then wound onto a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) while applying a tension of 200 gf to obtain carbon fiber (average fiber diameter 5 μm).

[0104] Example 3 Flame-resistant fibers (s-5) (800 fibers / bundle) were bundled to form 12,000 fibers / bundle, and while maintaining a tension of 70 cN, the bundle was moved through a heat treatment furnace with a temperature gradient of 300°C to 800°C under a nitrogen stream over the course of 3 minutes to perform a preliminary carbonization treatment (3 minutes) to produce pre-carbonized fibers. Next, the pre-carbonized fiber, still under tension of 70 cN, was moved through a heat treatment furnace with a temperature gradient of 1200°C to 1500°C under a nitrogen stream for 3 minutes to perform a heat treatment (carbonization treatment), and was then wound around a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) under tension of 200 gf to obtain carbon fiber (average fiber diameter 5 μm).

[0105] Example 4 Flame-resistant fibers (s-3) (800 fibers / bundle) were bundled to form 12,000 fibers / bundle, and while maintaining a tension of 70 cN, the bundle was moved through a heat treatment furnace with a temperature gradient of 300°C to 800°C under a nitrogen stream over the course of 3 minutes to perform a preliminary carbonization treatment (3 minutes) to produce pre-carbonized fibers. Next, the pre-carbonized fiber, still under tension of 70 cN, was moved through a heat treatment furnace with a temperature gradient of 1300°C to 1600°C under a nitrogen stream for 3 minutes to perform a heat treatment (carbonization treatment), and then wound around a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) under tension of 200 gf to obtain carbon fiber (average fiber diameter 4 μm).

[0106] <Example 5> Flame-resistant fibers (s-4) (800 fibers / bundle) were bundled to form 12,000 fibers / bundle, and while maintaining a tension of 70 cN, the bundle was moved through a heat treatment furnace with a temperature gradient of 300°C to 800°C under a nitrogen stream over the course of 3 minutes to perform a preliminary carbonization treatment (3 minutes) to produce pre-carbonized fibers. Next, the pre-carbonized fiber, still under tension of 70 cN, was moved through a heat treatment furnace with a temperature gradient of 1300°C to 1600°C under a nitrogen stream for 3 minutes to perform a heat treatment (carbonization treatment), and then wound around a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) under tension of 200 gf to obtain carbon fiber (average fiber diameter 4 μm).

[0107] Example 6 Flame-resistant fibers (s-4) (800 fibers / bundle) were bundled to form 12,000 fibers / bundle, and while maintaining a tension of 70 cN, the bundle was moved through a heat treatment furnace with a temperature gradient of 300°C to 800°C under a nitrogen stream over the course of 3 minutes to perform a preliminary carbonization treatment (3 minutes) to produce pre-carbonized fibers. Next, the pre-carbonized fiber, still under tension of 70 cN, was moved through a heat treatment furnace with a temperature gradient of 1400°C to 1700°C under a nitrogen stream for 3 minutes to perform a heat treatment (carbonization treatment), and then wound around a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) under tension of 200 gf to obtain carbon fiber (average fiber diameter 4 μm).

[0108] Example 7 Flame-resistant fibers (s-2) (800 fibers / bundle) were bundled to form 12,000 fibers / bundle, and while maintaining a tension of 70 cN, the bundle was moved through a heat treatment furnace with a temperature gradient of 300°C to 800°C under a nitrogen stream over the course of 3 minutes to perform a preliminary carbonization treatment (3 minutes) to produce pre-carbonized fibers. Next, the pre-carbonized fiber, still under tension of 70 cN, was moved through a heat treatment furnace with a temperature gradient of 1400°C to 1700°C under a nitrogen stream for 3 minutes to perform a heat treatment (carbonization treatment), and then wound around a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) under tension of 200 gf to obtain carbon fiber (average fiber diameter 6 μm).

[0109] <Comparative Example 1> Flame-resistant fibers (s-1) (800 fibers / bundle) were bundled to form 12,000 fibers / bundle, and while maintaining a tension of 70 cN, the bundle was moved through a heat treatment furnace with a temperature gradient of 300°C to 800°C under a nitrogen stream over the course of 3 minutes to perform a preliminary carbonization treatment (3 minutes) to produce pre-carbonized fibers. Next, the pre-carbonized fiber, still under tension of 70 cN, was moved through a heat treatment furnace set at 1000°C under a nitrogen stream for 3 minutes to perform a heat treatment (carbonization treatment), and then wound around a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) under tension of 200 gf to obtain carbon fiber (average fiber diameter 6 μm). <Comparative Example 2> Flame-resistant fibers (s-2) (800 fibers / bundle) were bundled to form 12,000 fibers / bundle, and while maintaining a tension of 70 cN, the bundle was moved through a heat treatment furnace with a temperature gradient of 300°C to 800°C under a nitrogen stream over the course of 3 minutes to perform a preliminary carbonization treatment (3 minutes) to produce pre-carbonized fibers. Next, the pre-carbonized fiber, still under tension of 70 cN, was moved through a heat treatment furnace with a temperature gradient of 1500°C to 1800°C under a nitrogen stream for 3 minutes to perform a heat treatment (carbonization treatment), and was then wound around a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) under tension of 200 gf to obtain carbon fiber (average fiber diameter 6 μm).

[0110] <Comparative Example 3> Flame-resistant fibers (s-4) (800 fibers / bundle) were bundled to form 12,000 fibers / bundle, and while maintaining a tension of 70 cN, the bundle was moved through a heat treatment furnace with a temperature gradient of 300°C to 800°C under a nitrogen stream over the course of 3 minutes to perform a preliminary carbonization treatment (3 minutes) to produce pre-carbonized fibers. Next, the pre-carbonized fiber, still under tension of 70 cN, was moved through a heat treatment furnace with a temperature gradient of 1500°C to 1800°C under a nitrogen stream for 3 minutes to perform a heat treatment (carbonization treatment), and was then wound around a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) under tension of 200 gf to obtain carbon fiber (average fiber diameter 4 μm). <Comparative Example 4> Flame-resistant fibers (s-6) (800 fibers / bundle) were bundled to form 12,000 fibers / bundle, and while maintaining a tension of 70 cN, the bundle was moved through a heat treatment furnace with a temperature gradient of 300°C to 800°C under a nitrogen stream over the course of 3 minutes to perform a preliminary carbonization treatment (3 minutes) to produce pre-carbonized fibers. Next, the pre-carbonized fiber, still under tension of 70 cN, was moved through a heat treatment furnace set at 1350°C under a nitrogen stream for 3 minutes to perform a heat treatment (carbonization treatment), and then wound around a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) under tension of 200 gf to obtain carbon fiber (average fiber diameter 5 μm). <Comparative Example 5> Flame-resistant fibers (s-7) (800 fibers / bundle) were bundled to form 12,000 fibers / bundle, and while maintaining a tension of 70 cN, the bundle was moved through a heat treatment furnace with a temperature gradient of 300°C to 800°C under a nitrogen stream over the course of 3 minutes to perform a preliminary carbonization treatment (3 minutes) to produce pre-carbonized fibers. Next, the pre-carbonized fiber, still under tension of 70 cN, was moved through a heat treatment furnace set at 1350°C under a nitrogen stream for 3 minutes to perform a heat treatment (carbonization treatment), and then wound around a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) under tension of 200 gf to obtain carbon fiber (average fiber diameter 5 μm).

[0111] <Reference example 1> As the raw fiber, polyacrylonitrile fibers (number of fibers: 3000 / bundle, fiber bundle fineness: 360 tex / bundle, single fiber fineness: 0.12 tex / fiber, single fiber diameter: approximately 11 μm) were prepared. This raw fiber was moved under an air flow in a heating furnace with a temperature gradient (temperature increase) from 200°C to 300°C for 60 minutes (flame-resistant fiber manufacturing process). In this way, flame-resistant fiber (number of fibers: 3000 / bundle, fiber bundle fineness: 330 tex / bundle, single fiber fineness: 0.11 tex / fiber, single fiber diameter: approximately 10 μm) was produced. This flame-retardant fiber was transferred under a nitrogen stream for 3 minutes in a heating furnace with a temperature gradient ranging from 300°C to 800°C (maximum temperature 800°C) (pre-carbonization process). This resulted in pre-carbonized fiber (number of fibers: 3000 / bundle, fiber bundle fineness: 180 tex / bundle, single fiber fineness: 0.06 tex / bundle, single fiber diameter: approximately 7 μm). This pre-carbonized fiber was transferred under a tension of 70 cN in a heat treatment furnace with a temperature gradient of 1100°C to 1400°C under a nitrogen stream for 3 minutes for heat treatment (carbonization). The fiber was then wound around a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) under a tension of 200 gf to obtain polyacrylonitrile-based carbon fiber (average fiber diameter 7 μm).

[0112] <Reference example 2> In Reference Example 1, the carbonization temperature was changed to a heat treatment furnace with a temperature gradient of 1200°C to 1500°C (maximum temperature in the heating zone measured with a radiation thermometer: 1500°C), but the process was the same as in Reference Example 1. The fiber was wound around a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) while applying a tension of 200 gf to obtain polyacrylonitrile-based carbon fiber (average fiber diameter 7 μm).

[0113] <Reference example 3> Commercially available polyacrylonitrile carbon fiber (c-1): 12,000 fibers / bundle, fineness: 800 tex, density: 1.76 g / cm 3 , average fiber diameter 7 μm) <Reference example 4> Commercially available polyacrylonitrile carbon fiber (c-2): 12,000 fibers / bundle, fineness: 800 tex, density: 1.8 g / cm 3 , average fiber diameter 7 μm)

[0114] <Evaluation of fusion rate> A fiber bundle having a length of 3 cm for evaluation was cut out from each of the flame-resistant fibers and carbon fibers in the above Examples, Comparative Examples, and Reference Examples. The fiber bundle was based on 800 fibers. The cross section of this evaluation fiber bundle was observed using a microscope (manufactured by Keyence Corporation, "Digital Microscope VHX-7000"), and the number of fibers was counted. At this time, the number of fused fibers and the number of all fibers constituting the evaluation fiber bundle were counted. For example, the number of fused fibers was counted as two when two fibers were fused to each other. The number of all fibers was counted by separating the fused fibers into their pre-fusion state. The fusion rate was calculated based on the following formula (3). The calculation results are shown in Table 1 as "fusion rate." Formula (3): Fusion rate (%) = (number of fused fibers / total number of fibers) x 100

[0115] <Fiber size of flame-retardant fiber> The mass of the obtained flame-resistant fiber was measured when bone dry or after drying at 120°C for 2 hours, and the fineness of the fiber bundle was calculated using the following formula (1), and the fineness of the single fiber constituting the flame-resistant fiber (the fineness of the flame-resistant fiber) was determined. Formula (1): Fiber bundle fineness [dtex] = [fiber bundle mass [g] / fiber length [m]] x 10,000 [m]

[0116] <Average fiber diameter of flame-retardant fiber> Each side of the flame-retardant fiber was observed using a microscope (Keyence Corporation's "Digital Microscope VHX-1000"), and the measurement points for the fiber diameter of each of 10 randomly extracted single fibers were randomly selected to measure the fiber diameters of the single fibers constituting the flame-retardant fiber, and the average value (average fiber diameter of the flame-retardant fibers) was calculated.

[0117] <Average fiber diameter of carbon fibers> Each side of the carbon fiber was observed using a microscope (Keyence Corporation's "Digital Microscope VHX-1000"), and the measurement points for the fiber diameter of each of 10 randomly extracted single fibers were randomly selected to measure the fiber diameters of the single fibers constituting the carbon fiber, and the average value (average fiber diameter of the carbon fibers) was calculated.

[0118] <Quantitative determination of carbon (C) and nitrogen (N) elements in carbon fiber> The carbon fibers were dried at 120°C under atmospheric pressure for 1 hour and analyzed using the following method. Analysis method: Oxygen circulation combustion and thermal conductivity detector (TCD) detection method Apparatus: Sumigraph NCH-22F model (manufactured by Sumika Chemical Analysis Center) Each sample was decomposed and completely oxidized by burning it for 15 minutes while circulating O2 gas using a gas chromatograph with TCD, and the carbon components were converted to CO2 and the nitrogen components to N2 gas, which were then detected and quantified.

[0119] <Quantitative determination of oxygen (O) in carbon fiber> The carbon fibers were dried at 120°C under atmospheric pressure for 1 hour and analyzed using the following method. Analysis method: Impulse heating and melting in an inert gas atmosphere, non-dispersive infrared (NDIR) detection method Device: EMGA-920 (Horiba) The sample was placed in a graphite crucible, and an electric current was passed through the graphite crucible in He gas (oxygen-free). The sample was melted at approximately 2500°C, and the oxygen component was converted into CO gas, which was then detected and quantified.

[0120] <Quantitative determination of phosphorus (P) in carbon fiber> The carbon fibers were dried at 120°C under atmospheric pressure for 1 hour and analyzed using the following method. Analysis method: Ashing / acid dissolution / inductively coupled plasma mass spectrometry (ICP-MS method) Device: NexION2000C (PerkinElmer) The carbon fiber was heated and incinerated, and then dissolved in acid. The resulting solution was analyzed for phosphorus using ICP-MS (ion source: argon gas plasma).

[0121] <Prevention of fluffing during unwinding of carbon fiber> The resulting carbon fiber (length of carbon fiber wound on the paper tube: approximately 10 m) on a paper tube (inner diameter 76.5 mm, thickness 3 mm, length 280 mm) was unwound and paid out from the paper tube at a speed of 0.3 m / min while applying a tension of 70 cN, and a fiber bundle (length 5 cm) for evaluation was cut out. The fiber bundle for evaluation was observed visually and using a microscope (Keyence Corporation, "Digital Microscope VHX-7000") for the presence or absence of fuzz in the carbon fiber caused by catching on other carbon fibers or breakage due to friction during unwinding, and was evaluated according to the following evaluation criteria. Ranks A and B in the evaluation criteria represent levels that are acceptable for practical use. [Evaluation criteria] A: There is no fluff caused by cutting the fibers when they are unwound. B: One fluff is present due to breakage of the fiber when it was unwound. C: There are two or more fluffs due to breakage of the fiber during unwinding.

[0122] <Average fiber diameter of carbon fibers> Each side of the obtained carbon fiber was observed using a microscope ("Digital Microscope VHX-1000" manufactured by Keyence Corporation), and the measurement points for the fiber diameter of each of 10 randomly extracted single fibers were randomly selected to measure the fiber diameters of the single fibers constituting the carbon fiber, and the average value (average fiber diameter of the carbon fibers) was calculated.

[0123] <Tensile modulus and tensile strength of carbon fiber single fiber> Five single fibers were taken from the obtained carbon fiber, and a tensile test (gauge length: 25 mm, tensile speed: 1 mm / min) was performed on each single fiber at room temperature (25°C) in accordance with JIS R7606:2000 using a micro strength evaluation tester (Shimadzu Corporation's "Micro Autograph MST-I") to measure the tensile modulus and tensile strength. Measurements were performed on five samples, and the average values were taken as the tensile modulus and tensile strength. The results are shown in Table 1.

[0124] The precursor (polymer composition), production conditions, content of each element, and evaluation results (carbon fiber fusion rate, suppression of fuzz generation during carbon fiber unwinding, tensile modulus of single carbon fiber, and tensile strength of single carbon fiber) of each of the carbon fibers are shown in Table 1. In the evaluation item names in Table 1, the tensile modulus of single carbon fiber is described as "tensile modulus," and the tensile strength of single carbon fiber is described as "tensile strength." "0.0" for the element content means that the atom is not contained, and also means that the atom is below the detection limit by the elemental analysis. In the evaluation items, "-" indicates that evaluation was not possible.

[0125] [Table 1]

[0126] As can be seen from Table 1, the carbon fibers of each example containing phosphorus, oxygen, and nitrogen in the amounts specified in the present disclosure had a low fusion rate, suppressed the generation of fluff during unwinding, and were excellent in the tensile modulus and tensile strength of the single fiber. On the other hand, the carbon fiber of Comparative Example 1, which had a high nitrogen content, had a low tensile modulus, and the carbon fibers of Comparative Examples 2 and 3, which had a low phosphorus content, and Comparative Example 5, which did not contain phosphorus, had a high fusion rate and produced a lot of fluff when unwound. Although the phosphorus content was within the appropriate range, the carbon fiber of Comparative Example 4, which had not been subjected to fiber-opening and had a high oxygen content, had a high fusion rate and a lot of fluffing during unwinding, and was inferior to the evaluation results of each Example in all evaluation items. [Explanation of symbols]

[0127] 10 Opening device 12 Guide 14 Support member 16 Conveyor roller 18 Suction device

[0128] The present disclosure includes the following aspects. <1> Carbon fiber having a phosphorus content of 0.6% by mass to 10% by mass, a nitrogen content of 5% by mass or less, an oxygen content of 0.3% by mass or less, and a tensile modulus of elasticity of a single fiber of 150 GPa or more, as determined by elemental analysis of the carbon fiber. <2> In a carbon fiber bundle consisting of 800 fibers, the fusion rate of the carbon fibers contained in the carbon fiber bundle is 15% or less. <1> The carbon fiber according to claim 1. <3> The carbon fiber is a carbon fiber derived from an acrylamide polymer fiber. <1> or <2> The carbon fiber according to claim 1. <4> The tensile strength of the single fiber is 1.4 GPa or more. <1> ~ <3> 1. The carbon fiber according to any one of the above. <5> A method for producing carbon fibers, comprising a step of subjecting a spread fiber bundle obtained by spreading acrylamide-based polymer fibers to a flame-resistant treatment, wherein the phosphorus content of the acrylamide-based polymer fibers as determined by elemental analysis is 0.1% by mass to 10% by mass. <6> The method further comprises a step of subjecting the flame-retardant treated acrylamide polymer fiber to a carbonization treatment. <5> The method for producing the carbon fiber according to claim 1.

[0129] <7> The phosphorus content of the flame-retardant treated acrylamide polymer fiber is 0.1% by mass to 10% by mass as determined by elemental analysis, the maximum temperature in the carbonization treatment step is in the range of 1350°C to 1650°C, and the fusion rate of the carbon fibers in the carbonized fiber bundle is 20% or less. <6> The method for producing the carbon fiber according to claim 1.

Claims

1. The carbon fiber has a phosphorus content of 0.6% by mass to 10% by mass, a nitrogen content of 5% by mass or less, and an oxygen content of 0.3% by mass or less, as determined by elemental analysis; A carbon fiber having a single fiber tensile modulus of elasticity of 150 GPa or more.

2. 2. The carbon fiber according to claim 1, wherein the fusion rate of the carbon fibers contained in the carbon fiber bundle is 15% or less in a carbon fiber bundle consisting of 800 fibers.

3. The carbon fiber according to claim 1 or 2, wherein the carbon fiber is derived from an acrylamide-based polymer fiber.

4. 3. The carbon fiber according to claim 1, wherein the tensile strength of a single fiber is 1.4 GPa or more.

5. A method for producing carbon fibers, comprising a step of subjecting a spread fiber bundle obtained by spreading acrylamide-based polymer fibers to a flame-retardant treatment, The method for producing carbon fibers, wherein the phosphorus content of the acrylamide-based polymer fibers as determined by elemental analysis is 0.1% by mass to 10% by mass.

6. The method for producing carbon fibers according to claim 5 , further comprising a step of subjecting the flame-retardant-treated acrylamide-based polymer fibers to a carbonization treatment.

7. the phosphorus content of the flame-resistant treated acrylamide polymer fiber is 0.1% by mass to 10% by mass as determined by elemental analysis, The maximum temperature in the carbonization treatment step is in the range of 1350°C to 1650°C, 7. The method for producing carbon fibers according to claim 6, wherein the fusion rate of the carbon fibers in the fiber bundle that has been carbonized is 20% or less.

Images