Carbon fiber residue, method for producing carbon fiber residue, and method for producing carbon fiber

Abstract

A carbon fiber precursor is provided that can suppress the fusion of fibers during a continuous stretching and flame-retardant treatment. [Solution] The carbon fiber precursor of this disclosure has acrylamide polymer fibers. The acrylamide polymer fibers contain an acrylamide polymer and a metal element.

Description

[Technical Field]

[0001] This disclosure relates to carbon fiber precursors, methods for producing carbon fiber precursors, and methods for producing carbon fibers. [Background technology]

[0002] Carbon fiber is lightweight and possesses excellent mechanical properties. Therefore, carbon fiber composite materials are attracting attention as alternative materials to metals.

[0003] A known method for producing carbon fibers involves spinning polyacrylonitrile to obtain a fiber bundle, then subjecting it to a flame-retardant treatment, and finally to a carbonization treatment (Patent Document 1 or Patent Document 2).

[0004] However, polyacrylonitrile is poorly soluble in inexpensive, common solvents. Therefore, polymerization and spinning require the use of expensive organic solvents (e.g., dimethyl sulfoxide or N,N-dimethylacetamide), which increases the cost of carbon fiber production.

[0005] On the other hand, carbon fiber precursors consisting of acrylamide polymers containing acrylamide monomers are known (Patent Document 3 or Patent Document 4). Acrylamide polymers are water-soluble polymers. When polymerizing and spinning acrylamide polymers, water, which is inexpensive and has a low environmental impact, can be used as a solvent. Therefore, a reduction in the manufacturing cost of carbon fibers is expected.

[0006] It is known that flame-resistant fibers obtained by flame-retardant treatment of acrylamide polymer fibers are subjected to pre-carbonization treatment under an inert gas atmosphere while applying a predetermined tension, and then carbonized at 1300°C to 1700°C to obtain carbon fibers with excellent tensile strength (Patent Document 5). The average fiber diameter of the single fibers of the carbon fibers is 3 μm to 10 μm. The average intensity ratio (D / G) of the Raman spectrum of the carbon fibers is 0.90 or less in the region within a circle with a diameter of 1 μm centered on the centroid of the cross-section of the single fiber, and also 0.90 or less in the region from the outer circumference to the inner 1 μm of the cross-section of the single fiber. The intensity ratio (D / G) of the Raman spectrum is measured at 1590 cm⁻¹ of the Raman spectrum. -1 1360 cm relative to G peak, which originates from the nearby graphite structure. -1 This shows the intensity ratio of D peaks originating from defects in the nearby graphite structure.

[0007] However, when pre-stretching or flame-retardant treatment is performed on acrylamide polymer fibers, the fibers soften due to heat. Therefore, a high degree of suppression of fiber fusion has been required. As a method for suppressing fiber fusion, a method is known in which a self-crosslinking silicone oil is applied to the acrylamide polymer fiber and a crosslinking treatment is performed (Patent Document 6 or Patent Document 7). [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Patent No. 4543922 [Patent Document 2] Japanese Patent Publication No. 2008-202208 [Patent Document 3] Japanese Patent Publication No. 2019-26827 [Patent Document 4] Japanese Patent Publication No. 2019-167516 [Patent Document 5] Japanese Patent Publication No. 2022-143757 [Patent Document 6] Japanese Patent Publication No. 2023-64697 [Patent Document 7] Japanese Patent Publication No. 2023-174468 [Overview of the project] [Problems that the invention aims to solve]

[0009] To efficiently produce carbon fibers, it is preferable to perform heat treatment (e.g., pre-stretching, stretching flame-retardant treatment, or carbonization treatment) in a continuous heat treatment furnace. "Pre-stretching" refers to stretching the carbon fiber precursor while heating it at 300°C or below before performing the flame-retardant treatment. "Stretching flame-retardant treatment" refers to applying flame-retardant treatment to the carbon fiber precursor while stretching it. The heating rate of continuous heat treatment is faster than that of batch heat treatment. The fusion rate of flame-retardant fibers obtained by continuous heat treatment is higher than that of flame-retardant fibers obtained by batch heat treatment. In carbonization treatment, heat does not easily penetrate sufficiently to the center of the fused flame-retardant fibers, making it difficult for the center of the flame-retardant fibers to be sufficiently carbonized. Therefore, the tensile strength and heat resistance are reduced, and the threads tend to break (fracture) due to the tension applied during the carbonization process, or to decompose at high temperatures, resulting in a lower tensile strength for the carbon fibers.

[0010] Furthermore, from the viewpoint of improving the tensile strength of carbon fibers, a higher stretch ratio during heat treatment is preferable. However, a problem arises when the stretch ratio during heat treatment is high (stretch ratio: 5.0 times or higher), as the oil layer becomes thinner, reducing the anti-fusion effect of the oil and increasing the fusion rate of the flame-resistant fibers.

[0011] Patent Document 5 does not evaluate the fusion rate of flame-resistant fibers. Furthermore, the flame-retardant treatment is carried out at 350°C for a long period of time (i.e., 60 minutes), and the carbonization temperature is high at over 1600°C. As a result, the manufacturing cost is high and CO2 emissions are also high.

[0012] In Patent Documents 6 and 7, flame resistance is achieved by a batch process with a low heating rate. However, when performing a drawing flame resistance treatment while rapidly heating in a continuous heating furnace, the fusion rate increases. Further, when high drawing (drawing ratio: 5.0 times or more) is performed by a preliminary drawing treatment and a drawing flame resistance treatment, the fusion rate further increases. Therefore, the techniques disclosed in Patent Documents 6 and 7 alone were insufficient for suppressing fusion during heat treatment.

[0013] The problem to be solved by one embodiment of the present disclosure is to provide a carbon fiber precursor, a method for producing a carbon fiber precursor, and a method for producing carbon fibers that can suppress fusion of fibers by a continuous drawing flame resistance treatment.

[0014] As a result of intensive research to achieve the above object, the present inventors have found that by including a metal element in an acrylamide-based polymer, the heat resistance of the carbon fiber precursor is improved (that is, the carbon fiber precursor is less likely to soften by heat), and fusion of fibers by a drawing flame resistance treatment can be suppressed.

Means for Solving the Problem

[0015] Specific means for achieving the problem are as follows. <1> A carbon fiber precursor having an acrylamide-based polymer fiber containing an acrylamide-based polymer and a metal element. <2> The carbon fiber precursor according to <1>, wherein the content of the metal element is 0.05 parts by mass to 5.0 parts by mass with respect to 100 parts by mass of the acrylamide-based polymer. <3> The carbon fiber precursor according to <1> or <2>, wherein the metal element is an alkaline earth metal. <4> The carbon fiber precursor according to any one of <1> to <3>, wherein the acrylamide-based polymer has an anionic functional group. <5> A method for producing a carbon fiber precursor, comprising spinning using a solution containing an acrylamide-based polymer, a metal salt, and water. <6> Performing a flame retardant treatment on the carbon fiber precursor according to any one of <1> to <4> to obtain a flame retardant fiber; Performing a carbonization treatment on the flame retardant fiber to obtain carbon fiber; including A method for producing carbon fiber, wherein the flame retardant treatment includes a stretching flame retardant treatment. <7> The method for producing carbon fiber according to <6>, further including performing a preliminary stretching treatment on the carbon fiber precursor before performing the step of obtaining the flame retardant fiber.

Advantages of the Invention

[0016] According to one embodiment of the present disclosure, there are provided a carbon fiber precursor, a method for producing a carbon fiber precursor, and a method for producing a carbon fiber that can suppress fusion of fibers by a continuous stretching flame retardant treatment.

Brief Description of the Drawings

[0017] [Figure 1] FIG. 1 is a diagram for explaining a method for measuring the load resistance of an example.

Embodiments for Carrying Out the Invention

[0018] In the present disclosure, in the numerical range indicated by using "~", the numerical values described before and after "~" are included as the minimum value and the maximum value, respectively. In the numerical ranges described step by step in the present disclosure, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerically described range. Further, in the numerical ranges described in the present disclosure, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the synthesis example, the production example, or the example.

[0019] In the present disclosure, each component may include a plurality of corresponding substances. When there are a plurality of substances corresponding to each component in the carbon fiber precursor, the content or content rate of each component means the total content or content rate of the plurality of substances present in the carbon fiber precursor, unless otherwise specified.

[0020] (1) Carbon fiber precursor The carbon fiber precursor of this disclosure comprises an acrylamide polymer fiber. The acrylamide polymer fiber comprises an acrylamide polymer and a metal element.

[0021] "Carbon fiber precursor" refers to a raw material for carbon fiber production that has not undergone either flame-retardant treatment or carbonization treatment. "Acrylamide polymer" refers to a homopolymer of acrylamide monomers, or a copolymer of acrylamide monomers and monomers other than acrylamide monomers. "Acrylamide polymer fiber" refers to a fiber containing an acrylamide polymer. Specifically, it means that the proportion of the acrylamide polymer to the total amount of acrylamide polymer fiber is 30% by mass or more, and may be 50% to 99.95% by mass, 80% to 99.9% by mass, or 90% to 99% by mass.

[0022] Because the carbon fiber precursor of this disclosure has the above-described structure, it can suppress the fusion of fibers during continuous stretching and flame-retardant treatment. This effect is presumed to be due to, but is not limited to, the following reasons. In this disclosure, the acrylamide polymer fiber contains an acrylamide polymer and a metal element. It is presumed that the amide groups in the acrylamide polymer and the metal element form coordination bonds, etc. Therefore, the polymer chains of the acrylamide polymer are more likely to come into close proximity, and it is thought that the formation of cyclic structures and crosslinked structures by flame-retardant treatment is promoted. Furthermore, because the acrylamide polymer fiber has coordination bonds, etc. between the polymer chains (i.e., has a crosslinked structure), it is thought that it will be less likely to melt even when heated at a temperature exceeding the glass transition temperature Tg (approximately 160°C) of the acrylamide polymer. As a result, it is presumed that the fusion of fibers by continuous stretching flame-retardant treatment can be suppressed.

[0023] Furthermore, in conventional technology, when high stretching (stretch ratio: 5.0 times or more) is performed by pre-stretching and stretching flame-retardant treatment using continuous heat treatment, the fusion rate of flame-retardant fibers tends to be high (e.g., fusion rate: 50% or more). On the other hand, in the carbon fiber precursor of this disclosure, the fusion rate of flame-retardant fibers is kept low (e.g., fusion rate: 10% or less). In addition, in the carbon fiber precursor of this disclosure, the fusion rate of flame-retardant fibers is kept low even without electron beam treatment (e.g., fusion rate: 10% or less).

[0024] The carbon fiber precursor is in the form of a fiber. The carbon fiber precursor may be used as a single fiber, but it is preferable to use it as a fiber bundle. When the carbon fiber precursor is used as a fiber bundle, the number of single fibers per bundle (hereinafter also referred to as "number of filaments") is not particularly limited. From the viewpoint of improving the productivity and mechanical properties of the flame-resistant fiber and carbon fiber, the number of filaments is preferably 10 to 240,000, more preferably 20 to 144,000, and even more preferably 30 to 72,000. If the number of filaments is within the above range, uneven firing is less likely to occur during the flame-resistant treatment. To increase the number of filaments, it is preferable to bundle multiple bundles consisting of one or more carbon fiber precursors.

[0025] (1.1) Acrylamide polymer fibers The carbon fiber precursor has acrylamide polymer fibers. The carbon fiber precursor may have known fibers other than the acrylamide polymer fibers, or it may consist only of a plurality of acrylamide polymer fibers.

[0026] The average fiber diameter of the acrylamide polymer fibers is not particularly limited, but is preferably 14 μm to 40 μm, more preferably 15 μm to 35 μm, and even more preferably 16 μm to 30 μm. When the average fiber diameter of the acrylamide polymer fibers is within the above range, thread breakage is less likely to occur, and stable winding and flame-retardant treatment can be performed. In addition, the structural difference between the surface and core of the flame-retardant fibers does not become too large, and the tensile strength and tensile modulus of the resulting carbon fibers tend not to decrease.

[0027] (1.1.1) Acrylamide polymers Acrylamide polymer fibers contain acrylamide polymers.

[0028] The acrylamide polymer may be a homopolymer of acrylamide monomers, or a copolymer of an acrylamide monomer and a monomer other than an acrylamide monomer (hereinafter also referred to as "other polymerizable monomers"). Furthermore, it may be a mixture of a homopolymer of acrylamide monomers and a copolymer of an acrylamide monomer and another polymerizable monomer.

[0029] It is presumed that in acrylamide polymer fibers, the amide groups contained in the acrylamide polymer and the metal elements form coordination bonds, etc. Therefore, the polymer chains of the acrylamide polymer tend to come into closer proximity, which is thought to promote the formation of cyclic and crosslinked structures during flame-retardant treatment. Furthermore, because the acrylamide polymer fibers have coordination bonds, etc. between the polymer chains (i.e., they have a crosslinked structure), it is thought that they are less likely to melt even when heated at temperatures exceeding the glass transition temperature Tg (approximately 160°C) of the acrylamide polymer. As a result, fusion of fibers during continuous stretching and flame-retardant treatment is suppressed.

[0030] The acrylamide polymer is preferably an acrylamide polymer having anionic functional groups. It is presumed that when acrylamide polymer fibers contain an acrylamide polymer having anionic functional groups and a metal element, the ionized metal element and the anionic functional groups of the acrylamide polymer form ionic bonds. Therefore, the polymer chains of the acrylamide polymer come into closer proximity to each other, which is thought to promote the formation of cyclic and crosslinked structures by flame-retardant treatment. Furthermore, because the acrylamide polymer fibers have ionic bonds between polymer chains (i.e., have a crosslinked structure), they are less likely to melt even when heated at temperatures exceeding the glass transition temperature Tg (approximately 160°C) of the acrylamide polymer. As a result, fusion of fibers by continuous stretching flame-retardant treatment can be further suppressed.

[0031] An "anionic functional group" refers to a functional group that can form an anion (negative ion) in the presence of water. There are no particular restrictions on the temperature at which anions are formed in the presence of water; a functional group that can form anions in the presence of water at high temperatures is also acceptable.

[0032] The content of constituent units derived from acrylamide monomers (hereinafter also referred to as "acrylamide monomer units") 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, relative to the total amount of constituent units of the acrylamide polymer. A content of 30 mol% or more of acrylamide monomer units tends to improve the solubility of acrylamide polymers in aqueous solvents or aqueous mixed solvents. The content of acrylamide monomer units may be 100 mol%, but from the viewpoint of fusion suppression and the like, 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, relative to the total amount of constituent units of the acrylamide polymer. The content of acrylamide monomer units is preferably 30 mol% to 99.9 mol%.

[0033] From the viewpoint of fusion suppression and other factors, the content of constituent units derived from other polymerizable monomers (hereinafter also referred to as "other polymerizable monomer units") 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, relative to the total amount of constituent units of the acrylamide polymer. From the viewpoint of improving the solubility of the acrylamide polymer in aqueous solvents or aqueous mixed solvents, 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, relative to the total amount of constituent units of the acrylamide polymer. The content of other polymerizable monomer units is preferably 0.1 mol% to 70 mol%.

[0034] The composition ratio of the acrylamide polymer is, 13 It can be measured using methods such as 13C-NMR and infrared absorption spectroscopy (IR).

[0035] The weight-average molecular weight of the acrylamide polymer is not particularly limited, but is usually 5 million or less. From the viewpoint of processability in the production of carbon fiber precursors, the weight-average molecular weight of the acrylamide polymer is preferably 2 million or less, more preferably 1 million or less, even more preferably 500,000 or less, particularly preferably 200,000 or less, and even more preferably 150,000 or less. 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 carbon fiber precursor and the strength of the carbon fiber, the weight-average molecular weight of the acrylamide polymer is preferably 20,000 or more, more preferably 30,000 or more, and even more preferably 40,000 or more.

[0036] In this disclosure, the weight-average molecular weight is measured by gel permeation chromatography under the following conditions. A measuring instrument such as the "HLC-8220GPC" manufactured by Tosoh Corporation or an instrument of comparable quality can be used. (Measurement conditions) • Columns: 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°C • Molecular weight standard substance: Standard polyethylene oxide / Standard polyethylene glycol • Detector: Differential refractive index detector

[0037] (1.1.1.1) Acrylamide monomers Examples of acrylamide monomers include N-alkylacrylamides such as acrylamide, ethacrylamide, crotonamide, itaconic acid diamide, cinnamic acid amide, maleic acid diamide, N-methylacrylamide, N-ethylacrylamide, Nn-propylacrylamide, N-isopropylacrylamide, Nn-butylacrylamide, or N-tert-butylacrylamide, N-cycloalkylacrylamides such as N-cyclohexylacrylamide, dialkylacrylamides such as N,N'-dimethylacrylamide, dialkylaminoalkylacrylamides such as dimethylaminoethylacrylamide or dimethylaminopropylacrylamide, hydroxyalkylacrylamides such as N-(hydroxymethyl)acrylamide or N-(hydroxyethyl)acrylamide, N-arylacrylamides such as N-phenylacrylamide, diacetoneacrylamide, and N,N'-methylenebisacrylamide. Examples include alkylene bisacrylamide, methacrylamide, N-methyl methacrylamide, N-ethyl methacrylamide, Nn-propyl methacrylamide, N-isopropyl methacrylamide, N-n-butyl methacrylamide, or N-tert-butyl methacrylamide, N-alkyl methacrylamide such as N-cyclohexyl methacrylamide, N-cycloalkyl methacrylamide such as N,N-dimethyl methacrylamide, dialkyl methacrylamide such as dimethylaminoethyl methacrylamide, or dialkylaminoalkyl methacrylamide such as dimethylaminopropyl methacrylamide, hydroxyalkyl methacrylamide such as N-(hydroxymethyl) methacrylamide, or N-(hydroxyethyl) methacrylamide, N-aryl methacrylamide such as N-phenyl methacrylamide, diacetone methacrylamide, or N,N'-alkylene bismethacrylamide such as N,N'-methylenebismethacrylamide. From the viewpoint of the solubility of acrylamide polymers in aqueous solvents or aqueous mixed solvents, among the above acrylamide monomers, acrylamide, N-alkylacrylamide, dialkylacrylamide, methacrylamide, N-alkylmethacrylamide, or dialkylmethacrylamide are preferred, and acrylamide is more preferred. Acrylamide monomers may be used individually or in combination of two or more.

[0038] (1.1.1.2) Other polymerizable monomers Other polymerizable monomers include vinyl monomers having anionic functional groups.

[0039] (1.1.1.2.1) Anionic vinyl monomers Anionic vinyl monomers have anionic functional groups. Examples of anionic functional groups include carboxyl groups, sulfo groups, phosphate groups (phosphate groups include phosphonic acid groups, phosphite groups, phosphinic acid groups, or phosphinitic acid groups, etc.), nitro groups, or phenolic hydroxyl groups. Examples of anionic vinyl monomers include unsaturated carboxylic acids and their salts, sulfonic acid vinyl monomers and their salts, phosphoric acid vinyl monomers and their salts, nitrate vinyl monomers and their salts, or phenolic vinyl monomers and their salts. The unsaturated carboxylic acids include unsaturated carboxylic acid anhydrides that form an unsaturated carboxylic acid in the presence of water, and unsaturated carboxylic acid precursors (e.g., unsaturated carboxylic acid esters) that form an unsaturated carboxylic acid in the presence of high temperature and at least one of an acid. Examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and its derivatives (e.g., maleic acid monoester), fumaric acid and its derivatives (e.g., fumaric acid monoester), itaconic acid and its derivatives (e.g., itaconic acid monoester), citraconic acid and its derivatives (e.g., citraconic acid monoester), mesaconic acid and its derivatives (e.g., mesaconic acid monoester), crotonic acid, or isocrotonic acid. Examples of salts of unsaturated carboxylic acids include metal salts of unsaturated carboxylic acids (e.g., sodium salts, potassium salts, magnesium salts, or calcium salts), ammonium salts, or amine salts. Examples of unsaturated carboxylic acid precursors include maleic anhydride, itaconic anhydride, citraconic anhydride, acrylic acid esters (e.g., tert-butyl acrylate), or methacrylic acid esters (e.g., tert-butyl methacrylate). Examples of sulfonic acid vinyl monomers include acrylamide tert-butyl sulfonic acid, vinyl sulfonic acid, vinylbenzene sulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid, styrene sulfonic acid, isoprene sulfonic acid, allyl sulfonic acid, or methacrylic sulfonic acid. Examples of salts of sulfonic acid vinyl monomers include metal salts (e.g., sodium salt, potassium salt, magnesium salt, or calcium salt), ammonium salts, or amine salts of sulfonic acid vinyl monomers. Examples of phosphate-based vinyl monomers include vinyl phosphoric acid, 2-acryloyloxyethyl acid phosphate, 2-methachloroyloxyethyl acid phosphate, 2-methachloroyloxyethyl acid phosphate, vinylphosphonic acid, vinyl phosphorous acid, vinylphosphinic acid, phenylvinylphosphinic acid, or vinylphosphinic acid. Examples of salts of phosphate-based vinyl monomers include metal salts (e.g., sodium salt, potassium salt, magnesium salt, or calcium salt), ammonium salts, or amine salts of phosphate-based vinyl monomers. Examples of nitrate-based vinyl monomers include vinyl nitrate and 2-nitrovinylbenzene. Examples of salts of nitrate-based vinyl monomers include metal salts (e.g., sodium salts, potassium salts, magnesium salts, or calcium salts), ammonium salts, or amine salts of nitrate-based vinyl monomers. Examples of phenolic vinyl monomers include 2-vinylphenol and 4-vinylphenol. Examples of salts of phenolic vinyl monomers include metal salts (e.g., sodium salts, potassium salts, magnesium salts, or calcium salts), ammonium salts, or amine salts of phenolic vinyl monomers. Anionic vinyl monomers may be used individually or in combination of two or more.

[0040] Among the anionic vinyl monomers mentioned above, unsaturated carboxylic acids and their salts are preferred from the viewpoint of polymerizability with acrylamide polymers, and acrylic acid, maleic acid, fumaric acid, or itaconic acid are more preferred. Among the above-mentioned anionic vinyl monomers, unsaturated carboxylic acids are preferred from the viewpoint of fusion suppression, and acrylic acid, maleic acid, fumaric acid, or itaconic acid are more preferred.

[0041] The content of constituent units derived from anionic vinyl monomers (hereinafter also referred to as "anionic vinyl monomer units") is preferably 0.1 mol% to 70 mol% relative to the total amount of constituent units of the acrylamide polymer. This allows for the formation of ionic crosslinks and improvement of heat resistance while maintaining the solubility of the acrylamide polymer in aqueous solvents or aqueous mixed solvents. From the viewpoint of ionic crosslink formation, fusion suppression, and promotion of flame-retardant reactions, the content of anionic vinyl monomer units is more preferably 0.5 mol% or more, even more preferably 1 mol% or more, particularly preferably 2 mol% or more, and most preferably 3 mol% or more. From the viewpoint of improving the solubility of the acrylamide polymer in aqueous solvents or aqueous mixed solvents, the content of anionic vinyl monomer units is more preferably 50 mol% or less, even more preferably 35 mol% or less, particularly preferably 20 mol% or less, and most preferably 10 mol% or less.

[0042] (1.1.1.2.2) Vinyl monomers that do not have anionic functional groups Other polymerizable monomers may include, in addition to anionic vinyl monomers, vinyl monomers that do not have anionic functional groups (hereinafter also referred to as "non-anionic vinyl monomers").

[0043] Examples of non-anionic vinyl monomers include vinyl cyanide monomers, aromatic vinyl monomers, halogenated vinyl monomers, vinyl alcohol monomers, vinyl carboxylic acid monomers, or olefin monomers.

[0044] Examples of vinyl cyanide monomers include acrylonitrile, methacrylonitrile, 2-hydroxyethylacrylonitrile, chloroacrylonitrile, chloromethylacrylonitrile, ethoxyacrylonitrile, or vinylidene cyanide.

[0045] Examples of aromatic vinyl monomers include styrene or α-methylstyrene. Examples of vinyl halogenated monomers include vinyl chloride. Examples of vinyl alcohol monomers include vinyl alcohol. Examples of vinyl carboxylate monomers include vinyl acetate or vinyl propionate. Examples of olefin monomers include ethylene, propylene, isopropylene, or butadiene. Non-anionic vinyl monomers may be used individually or in combination of two or more.

[0046] Among the non-anionic vinyl monomers mentioned above, vinyl cyanide monomers are preferred from the viewpoint of inhibiting fusion and promoting flame retardation reactions, acrylonitrile, methacrylonitrile, chloroacrylonitrile, or chloromethylacrylonitrile are more preferred, acrylonitrile, chloroacrylonitrile, or methacrylonitrile are even more preferred, acrylonitrile or methacrylonitrile are particularly preferred, and acrylonitrile is the most preferred.

[0047] The content of structural units derived from non-anionic vinyl monomers among other polymerizable monomers may be 0 mol% relative to the total amount of constituent units of the acrylamide polymer. When the acrylamide polymer contains structural units derived from non-anionic vinyl monomers, the content of constituent units derived from non-anionic vinyl monomers is preferably 0.1 mol% to 69.9 mol%, more preferably 1 mol% to 69.5 mol%, even more preferably 2 mol% to 69 mol%, particularly preferably 3 mol% to 49.9 mol%, even more preferably 5 mol% to 49 mol%, and most preferably 15 mol% to 47 mol%, relative to the total amount of constituent units of the acrylamide polymer.

[0048] (1.1.1.2.3) Synthesis of acrylamide polymers The acrylamide polymer may be synthesized by conventionally known methods. Acrylamide polymers can be synthesized using known polymerization reactions (e.g., radical polymerization, cationic polymerization, anionic polymerization, or living radical polymerization). Among these polymerization reactions, radical polymerization is preferred from the viewpoint of reducing synthesis costs. Acrylamide polymers can be synthesized using polymerization methods such as solution polymerization, suspension polymerization, precipitation polymerization, dispersion polymerization, or emulsion polymerization (e.g., reverse-phase emulsion polymerization). When synthesizing acrylamide polymers by solution polymerization, it is preferable to use a solvent that dissolves both the raw material monomers and the resulting acrylamide polymer. From the viewpoint of low cost and safe synthesis, it is more preferable to use an aqueous solvent or an aqueous mixed solvent, and even more preferable to use an aqueous solvent. Examples of aqueous solvents include water, alcohol, and mixtures thereof, with water being particularly preferred. "Aqueous mixed solvent" refers to a mixed solvent of the above-mentioned aqueous solvent and an organic solvent. Examples of organic solvents include tetrahydrofuran, dimethyl sulfoxide, or dimethylformamide.

[0049] In the synthesis of acrylamide polymers by radical polymerization, it is preferable to use a polymerization initiator. Conventional known radical polymerization initiators such as azobisisobutyronitrile, benzoyl peroxide, 4,4'-azobis(4-cyanovaleric acid), ammonium persulfate, or potassium persulfate can be used as polymerization initiators. When using an aqueous solvent or an aqueous mixed solvent as the solvent, a radical polymerization initiator soluble in the aqueous solvent or aqueous mixed solvent (for example, 4,4'-azobis(4-cyanovaleric acid), ammonium persulfate, potassium persulfate, etc.) is preferred. From the viewpoint of controlling the molecular weight of the acrylamide polymer, it is preferable to use at least one of a polymerization accelerator and a molecular weight modifier in place of or in combination with a polymerization initiator, and it is more preferable to use both a polymerization initiator and a polymerization accelerator in combination. Examples of polymerization accelerators include tetramethylethylenediamine. Examples of molecular weight modifiers include alkyl mercaptan compounds such as n-dodecyl mercaptan. It is particularly preferable to use ammonium persulfate, which is a polymerization initiator, and tetramethylethylenediamine, which is a polymerization accelerator, in combination.

[0050] The temperature of the polymerization reaction described above is not particularly limited, but from the viewpoint of controlling the molecular weight of the acrylamide polymer, it is preferably 35°C or higher, more preferably 40°C or higher, even more preferably 50°C or higher, particularly preferably 70°C or higher, and even more preferably 75°C or higher. The polymerization reaction temperature is not particularly limited and may be 200°C or lower.

[0051] (1.1.2) Metallic elements Acrylamide polymer fibers contain metal elements. One metal element may be used alone, or two or more metal elements may be used in combination.

[0052] The valency of a metallic element is not particularly limited; for example, it can be monovalent, divalent, trivalent, tetravalent, or pentavalent.

[0053] Examples of monovalent metallic elements include Li, Na, K, Rb, Cs, Ag, or Fr. Examples of divalent metallic elements include Be, Mg, Ca, Sr, Ba, Ra, Co, Mn, Ni, Pd, Zn, or Cu. Examples of trivalent metallic elements include Al, Ga, In, Ti, Fe, La, Cr, Sb, or Ce. Examples of tetravalent metallic elements include Ti, Zr, Hf, Ge, or Sn. Examples of pentavalent metallic elements include Sb.

[0054] The valency of the metal element is preferably 2 or higher. This makes it easier for the ionized metal element to form coordination bonds or ionic bonds with the amide groups contained in the acrylamide polymer, or with anionic functional groups, compared to when the valency of the metal element is 1. As a result, the carbon fiber precursor can more effectively suppress the fusion of fibers during the stretching and flame-retardant treatment. From the viewpoint of the stretchability of the acrylamide polymer and its solubility in aqueous solvents or aqueous mixed solvents, the valency of the metal element is more preferably 2 to 4, even more preferably 2 to 3, and particularly preferably 2.

[0055] In particular, the metal element is preferably an alkaline earth metal (Be, Mg, Ca, Sr, Ba, or Ra) from the viewpoint of forming at least one of coordination bonds and ionic bonds, more preferably Mg, Ca, Sr, or Ba, even more preferably Mg, Ca, or Sr, particularly preferably Ca or Mg, and most preferably Mg.

[0056] The metal element may be derived from a metal salt (i.e., an inorganic salt or an organic salt). Examples of inorganic salts include chlorides, hydroxides, carbonates, nitrates, sulfates, phosphates, tungstates, vanadates, molybdates, niobates, borates, or hydrates thereof of the metal element. Examples of organic salts include phenolic salts, carboxylates (e.g., acetates, lactates, formates, oxalates, propions, benzoates, succinates, or adipines), benzenesulfons, or hydrates thereof of the metal element.

[0057] The content of the metal element is not particularly limited, but is preferably 0.05 to 5 parts by mass per 100 parts by mass of the acrylamide polymer. This allows for the formation of coordination bonds and ionic bonds, and improvement of heat resistance, while maintaining the solubility of the acrylamide polymer in aqueous solvents or aqueous mixed solvents. From the viewpoint of coordination bond and ionic bond formation and fusion suppression, the content of the metal element is more preferably 0.1 parts by mass or more, even more preferably 0.3 parts by mass or more, and particularly preferably 0.5 parts by mass or more. From the viewpoint of the solubility of the acrylamide polymer in an aqueous solvent or aqueous mixed solvent and the strength of the carbon fiber, the content of the metal element is more preferably 4 parts by mass or less, even more preferably 3 parts by mass or less, particularly preferably 2 parts by mass or less, and even more preferably 1 part by mass or less.

[0058] (1.1.3) Additives The acrylamide polymer fibers of this disclosure may further contain additives in addition to the acrylamide polymer and metal elements.

[0059] Examples of additives include surfactants, crosslinking agents, flame retardation accelerators (e.g., those consisting of acids and their salts), various fillers (e.g., glass fibers, cellulose nanofibers, cellulose microfibers, carbon black, carbon nanotubes, or graphene), dispersants, smoothing agents, hygroscopic agents, viscosity modifiers, plasticizers, mold release agents, spreading agents, antioxidants, antibacterial agents, preservatives, rust inhibitors, or pH adjusters. The content of additives can be adjusted as appropriate.

[0060] When acrylamide polymer fibers contain the flame retardation accelerator, the formation of cyclic structures through various reactions (e.g., dehydration reaction or deammonia reaction, etc.) is accelerated during flame retardation treatment, and the fusion suppression, carbonization yield, carbon fiber strength, and dimensional stability tend to be further improved. The carbon fibers may contain at least a portion of the flame retardation accelerator and its residue.

[0061] Examples of flame retardant accelerators include inorganic acids (e.g., phosphoric acid, polyphosphate, boric acid, sulfuric acid, nitric acid, or carbonic acid) or organic acids (e.g., oxalic acid, citric acid, or sulfonic acid). Ammonium salts or amine salts of these acids are also examples. Among the above, from the viewpoint of fusion suppression, carbonization yield, carbon fiber strength, and dimensional stability, phosphoric acid, polyphosphate, boric acid, sulfuric acid, or ammonium salts thereof are preferred as flame retardant accelerators, phosphoric acid, polyphosphate, boric acid, or ammonium salts thereof are more preferred, and monoammonium phosphate or phosphoric acid is even more preferred.

[0062] The content of the flame retardant accelerator is preferably 0.1 to 100 parts by mass, more preferably 0.2 to 50 parts by mass, even more preferably 0.5 to 30 parts by mass, and particularly preferably 1 to 20 parts by mass, per 100 parts by mass of the acrylamide polymer, from the viewpoint of fusion suppression, carbonization yield, strength and dimensional stability of the carbon fibers.

[0063] (1.1.4) Other polymers The acrylamide polymer fibers of this disclosure may further contain other polymers different from the acrylamide polymer.

[0064] From the viewpoint of fusion suppression, water resistance, etc., the content of acrylamide polymer relative to the total mass of polymers in acrylamide polymer fibers is preferably 30% by mass or more, more preferably 50% by mass or more, even more preferably 80% by mass or more, particularly preferably 90% by mass or more, even more preferably 95% by mass or more, and most preferably 100% by mass.

[0065] Other polymers are not particularly limited, but polymers that dissolve or disperse in aqueous solvents or aqueous mixed solvents are preferred, polymers that dissolve in aqueous solvents or aqueous mixed solvents are more preferred, and water-soluble polymers are even more preferred. Other polymers that can be preferred include, for example, polymers obtained by (co)polymerizing at least one of the group consisting of specific monomers, and hydrophilic polysaccharide polymers (acetylcellulose, xanthan gum, etc.). Examples of specific monomers include unsaturated carboxylic acids (e.g., acrylic acid or methacrylic acid) and their salts, unsaturated carboxylic acid esters (e.g., methyl acrylate), vinyl carboxylates (e.g., vinyl alcohol or vinyl acetate), vinylpyrrolidone, vinyl sulfonic acid and its salts, vinyl sulfonic acid esters (e.g., methyl vinyl sulfonate), vinylbenzenesulfonic acid and its salts, vinylbenzenesulfonic acid esters, carboxyvinyl, polyalkylene glycols (e.g., ethylene glycol or propylene glycol), unsaturated carboxylic acid anhydrides (e.g., maleic anhydride), unsaturated dicarboxylic acids (e.g., maleic acid or fumaric acid) and their salts, unsaturated dicarboxylic acid esters (e.g., monomethyl maleate or dimethyl maleate), 2-acrylamido-2-methylpropanesulfonic acid and its salts, or 2-acrylamido-2-methylpropanesulfonic acid esters.

[0066] (1.2) Oil layer The carbon fiber precursor of this disclosure may further have an oily layer attached to the surface of an acrylamide polymer fiber. The presence of this oily layer in the carbon fiber precursor makes it easier to bundle, suppressing the generation of fuzz and other imperfections. Furthermore, it can further suppress the fusion of fibers due to the stretching and flame-retardant treatment.

[0067] The "oil layer" refers to a layer derived from a textile oil. Known or commercially available textile oils may be used, including, for example, silicone-based oils, polyalkylene glycol-based oils, polyacrylic acid-based oils, polyacrylic acid ester-based oils, polyester-based oils, polyether-based oils, fatty acid ester-based oils, glycerin fatty acid ester-based oils, paraffin oil-based oils, or mineral oil-based oils. From the viewpoint of suppressing the fusion of carbon fiber precursors, it is preferable that the oil layer contains a silicone-based oil. The textile oil may contain multiple oils. The textile oil may further contain, as needed, diluents, antistatic agents, smoothing agents, hygroscopic agents, surfactants, viscosity modifiers, release agents, spreading agents, antibacterial agents, antioxidants, crosslinking agents, or preservatives.

[0068] The silicone-based oil comprises at least one of a crosslinkable silicone oil and a non-crosslinkable silicone oil. "Crosslinkable silicone oil" refers to a silicone-based oil that forms a crosslinked structure and becomes solid when exposed to external stimuli (e.g., electron beams, ultraviolet light, moisture, or heat). "Non-crosslinkable silicone oil" refers to a silicone-based oil that does not form a crosslinked structure when exposed to external stimuli.

[0069] The oil layer is either in a solid state (i.e., a crosslinked material) or a liquid state (i.e., an uncrosslinked material). From the viewpoint of reducing oil adhesion to rollers and the like, suppressing water absorption by carbon fiber precursors, and suppressing fusion, the oil layer is preferably in a solid state, while from the viewpoint of reducing the crosslinking process, it is preferably in a liquid state. Whether the oil layer is in a solid state can be confirmed by immersing it in an organic solvent (e.g., hexane or acetone) and observing whether or not it dissolves. Specifically, if 50% or more by mass of the oil layer does not dissolve in the organic solvent, the oil layer is determined to be in a solid state. If 50% or more by mass of the oil layer dissolves in the organic solvent, the oil layer is determined to be in a liquid state.

[0070] The oil layer only needs to be present on at least a portion of the surface of the acrylamide polymer fiber. However, from the viewpoint of maintaining a higher carbonization yield and further suppressing the fusion of individual fibers during the flame-retardant treatment, it is preferable that the oil layer adheres to the entire surface of the acrylamide polymer fiber.

[0071] The content of the oil layer is not particularly limited, but is preferably 0.1 to 20 parts by mass, more preferably 0.2 to 15 parts by mass, and even more preferably 0.3 to 10 parts by mass, per 100 parts by mass of acrylamide polymer fibers.

[0072] A solid oil layer is also called a "crosslinked material." A liquid oil layer is also called a "non-crosslinked material."

[0073] (1.2.1) Crosslinkable silicone oil Crosslinkable silicone oils include silicones having self-crosslinking groups (hereinafter also referred to as "self-crosslinking silicones") or silicones having reactive functional groups (hereinafter referred to as "reactive silicones"). Crosslinkable silicone oils may contain one or more types of self-crosslinking silicones, two or more types of reactive silicones, or both self-crosslinking silicones and reactive silicones. "Self-crosslinking groups" refer to functional groups that chemically bond with other self-crosslinking groups upon external stimulation. Reactive functional groups refer to functional groups other than self-crosslinking groups that do not react on their own but react with other types of different reactive functional groups or self-crosslinking groups and form chemical bonds. If the crosslinkable silicone oil contains reactive silicones, the combination of reactive silicones should be such that the silicones react with each other and form chemical bonds. Furthermore, the crosslinkable silicone oil may also contain silicones that do not have self-crosslinking groups or reactive functional groups (hereinafter also referred to as "non-reactive silicones"), as long as they do not inhibit the formation of the crosslinked structure.

[0074] Self-crosslinkable silicones and reactive silicones may have a polysiloxane as their basic structure, in which at least some substituents on the side chains and terminals of the polysiloxane are substituted with self-crosslinkable groups. Self-crosslinkable silicones may be used alone or in combination of two or more types. Self-crosslinkable silicones and reactive silicones may be used in combination.

[0075] Examples of polysiloxanes include polydimethylsiloxane, polymethylphenylsiloxane, polydiphenylsiloxane, polymethylhydrogensiloxane, or mixtures thereof. Examples of substituents include alkyl groups (e.g., methyl group, ethyl group, or propyl group) or aryl groups (e.g., phenyl group or methylphenyl group).

[0076] Self-crosslinking groups refer to functional groups that chemically bond with other self-crosslinking groups upon external stimulation. Examples of self-crosslinking groups include ethylene groups, mercapto groups, alkoxysilyl groups, and silanol groups. Examples of ethylene groups include monosubstituted ethylene groups, 1,1-disubstituted ethylene groups, and 1,2-disubstituted ethylene groups. Examples of monosubstituted ethylene groups include vinyl groups, vinylcarbonyl groups, vinyl ester groups, acryloyl groups, acrylamide groups, allyl groups, allyl ether groups, 4-vinylbenzene groups, or 4-allylbenzene groups. Examples of 1,1-disubstituted ethylene groups include isopropenyl groups, methacryloyl groups, methacrylamide groups, or 4-isopropenylbenzene groups. Examples of 1,2-disubstituted ethylene groups include maleimide groups, fumarate ester groups, or fumaamide groups. In particular, the self-crosslinking group is preferably an acryloyl group, methacryloyl group, acrylamide group, or methacrylamide group from the viewpoint of affinity with acrylamide polymers, and more preferably an acryloyl group or methacryloyl group from the viewpoint of reactivity.

[0077] Reactive functional groups are functional groups other than self-crosslinking groups that do not react on their own but react with other types of different reactive functional groups or self-crosslinking groups to form chemical bonds. Examples of reactive functional groups include amino groups, epoxy groups, alicyclic epoxy groups, carboxyl groups, carboxylic acid anhydride groups, or hydroxyl groups (carbinol groups). Combinations of reactive functional groups that can react with each other and form chemical bonds include {amino group and epoxy group}, {amino group and alicyclic epoxy group}, {amino group and carboxyl group}, {amino group and carboxylic acid anhydride group}, {epoxy group and carboxyl group}, {epoxy group and carboxylic acid anhydride group}, {epoxy group and hydroxyl group}, {alicyclic epoxy group and carboxyl group}, {alicyclic epoxy group and carboxylic acid anhydride group}, {alicyclic epoxy group and hydroxyl group}, {carboxyl group and hydroxyl group}, or {carboxylic acid anhydride and hydroxyl group}. Note that "{A and B}" refers to the combination of A and B. Combinations of self-crosslinking groups and reactive functional groups that can react with each other and form chemical bonds include {acryloyl group and amino group}, {methacryloyl group and amino group}, {silanol group and carboxyl group}, and {silanol group and hydroxyl group}.

[0078] The crosslinkable silicone oil may further contain an organic solvent for dilution. The organic solvent is a good solvent for the silicone contained in the crosslinkable silicone oil and a poor solvent for the acrylamide polymer. The crosslinkable silicone oil may further contain, as needed, diluents, antistatic agents, smoothing agents, hygroscopic agents, surfactants, viscosity modifiers, mold release agents, spreading agents, antibacterial agents, antioxidants, photopolymerization initiators, or preservatives.

[0079] The self-crosslinking silicone used in crosslinking silicone oils may be a commercially available product. Examples of commercially available self-crosslinking silicones include those from Shin-Etsu Chemical Co., Ltd., SILTECH Corporation, or Evonik Industries. Examples of Shin-Etsu Chemical Co., Ltd.'s products include "KP-420", "X-22-164C", "X-22-164", "X-22-164AS", "X-22-164A", "X-22-164B", "X-22-164E", "X-22-2445", "X-22-174ASX", "X-22-174BX", "KF-2012", "X-22-2426" or "X-22-2404", "KF-2001", "KF-2004", "X-22-167B", "X-22-167C", "X-21-5841", or "KF-9701". SILTECH Corporation's products include, for example, "Silmer ACR D208," "Silmer ACR D2," "Silmer ACR D4," "Silmer ACR Di-10," "Silmer ACR Di-50," "Silmer ACR Di-400," "Silmer ACR Di-1508," "Silmer ACR Di-2510," "Silmer ACR Di-4515-0", "Silmer OH ACR Di-10", "Silmer OH ACR Di-50", "Silmer OH ACR Di-100", "Silmer OH ACR Di-400", "Silmer OH ACR C50", "Silmer OH ACR C7-F", "Silmer OH ACR D4", "Silmer TMS C50", "Silmer TMS Di-10", "Silmer TMS Di-50", "Silmer VIN C50", "Silmer VIN J10", "Silmer VIN Examples include "70", "Silmer VIN 100", "Silmer VIN 200", "Silmer VIN 1000", "Silmer VIN 5000", "Silmer VIN 10000", "Silmer VIN 20000", or "Silmer VIN 65000".Evonik Industries' products include the "TegoRad 2010," "TegoRad 2500," and "TegoRad 2700," among others. It can be done.

[0080] The reactive silicone used in the crosslinkable silicone oil may be a commercially available product. Examples of commercially available non-crosslinkable silicone oils include those from Shin-Etsu Chemical Co., Ltd., Dow Toray Industries, Inc., Momentive Performance Materials, Inc., or SILTECH Corporation. Examples of Shin-Etsu Chemical Co., Ltd.'s products include "KF-868", "KF-865", "KF-864", "KF-859", "KF-393", "KF-860", "KF-880", "KF-8004", "KF-8002", "KF-8005", "KF-867", "KF-8021", "KF-869", "KF-861", "X-22-3939A", "KF-877", "X-22-343", "KF-101", "KF-1001", "X-22-2000", "X-22-2046", "KF-102", "X-22-4741", "KF-1002", "KF-1005", "X-22-4039", "X-22-401" 5", "X-22-3701E", "KF-99", "KF-9901", "PAM-E", "KF-8010", "X-22-161A", "X-22-161B", "KF-8012", "KF-8008", "X-22-1660B-3" "X-22-9409" "X-22-163" "KF-105" "X-22-163A" "X-22-163B" "X-22-163C" "X-22-169AS" " "X-22-169B", "KF-6000", "KF-6001", "KF-6002", "KF-6003", "X-22-4952", "X-22-4272", "KF-6123", "X-22-162C" Examples include "X-22-168AS", "X-22-168A", "X-22-168B", "X-22-168-P5-B", "X-22-173BX", "X-22-173DX", "X-22-170BX", "X-22-170DX", "X-22-176F", "X-22-176GX-A", "X-22-3710", "KF-857", "KF-862", "KF-858", or "X-22-9002".Examples of products from Dow Toray Industries, Inc. include "BY-16-880", "BY 16-750", "OFX-8417", "BY 16-849", "FZ-3785", "16-853U", "SF 8413", "SF 8411", "BY 16-839", "FZ-3736", "SF 8421 EG", "BY 16-870", "BY 16-876", or "BY 16-760". Examples of products from Asahi Kasei Wacker Silicone Co., Ltd. include "L 652 SILICONE FLUID", "L 653 SILICONE FLUID", "L 655 SILICONE FLUID", "L 656 SILICONE FLUID", "FINISH WR 301 CN", "FINISH WR 1100 CN", "FINISH WR 1200 CN", "FINISH WR 1300 CN", "FINISH WR 1600", "FINISH WT 1270", or "FINISH WT 1650 LV". Examples of products from Momentive Performance Printers, Inc. include the "TSF4702", "TSF4703", "TSF4704", "TSF4705", "TSF4706", "TSF4707", "TSF4708", "TSF4709", "TSF4730", "YF3965", "TSF4700", "TSF4701", or "XF42-B0970".SILTECH Corporation's products include, for example, "Silmer OH AO-UP", "Silmer OH C50", "Silmer OH J10", "Silmer OH Di-10", "Silmer OH Di-50", "Silmer OH C7-F", "Silmer OHT AO", "Silmer OHT Di-10", "Silmer OHT". "Silmer Di-50", "Silmer OHT Di-100", "Silmer OHT Di-400", "Silmer OHT E13", "Silmer H D2", "Silmer H E4", "Silmer H Di-10", "Silmer H Di-E2", "Silmer NH C50", "Silmer NH Di-8", "Silmer NH Di-50", "Silmer EP C50", "Silmer EPC C50", "Silmer EP J10", "Silmer EP Di-50", "Silmer EP Di-100", "Silmer EPC Examples include the "Di-50," "Silmer EP D208," or "Silmer EPC F418-F."

[0081] (1.2.2) Non-crosslinked silicone oils Non-crosslinkable silicone oils contain at least one of a silicone that does not have self-crosslinking groups and reactive functional groups (hereinafter also referred to as "non-reactive silicone") and a reactive silicone. Furthermore, non-crosslinkable silicone oils do not contain self-crosslinking silicone.

[0082] Non-reactive silicones may have a polysiloxane as their basic structure and may not contain self-crosslinking groups or reactive functional groups. Non-reactive silicones may be used alone or in combination of two or more types. Reactive silicones may be used alone or in combination of two or more types. When two or more types of reactive silicones are used in combination, the combination of reactive silicones should not react with each other or form chemical bonds (for example, a combination of reactive silicones having amino groups, or a combination of reactive silicones having epoxy groups). Non-reactive silicones and reactive silicones may be used together.

[0083] Examples of polysiloxanes include polydimethylsiloxane, polymethylphenylsiloxane, polydiphenylsiloxane, polymethylhydrogensiloxane, or mixtures thereof.

[0084] At least some of the substituents on the side chains and terminals of the non-reactive silicone polysiloxane may be substituted with modifying groups. Examples of substituents include alkyl groups (e.g., methyl, ethyl, or propyl groups) or aryl groups (e.g., phenyl or methylphenyl groups). The modifying groups are not self-crosslinking groups. Examples of modifying groups include amino groups, alicyclic epoxy groups, or alkylene oxide groups.

[0085] The non-crosslinked silicone oil may further contain an organic solvent for dilution. The organic solvent is a good solvent for the silicone contained in the non-crosslinked silicone oil and a poor solvent for the acrylamide polymer. The non-crosslinked silicone oil may further contain, as needed, diluents, antistatic agents, smoothing agents, hygroscopic agents, surfactants, viscosity modifiers, mold release agents, spreading agents, antibacterial agents, antioxidants, or preservatives.

[0086] The non-reactive silicone used in non-crosslinked silicone oils may be a commercially available product. Examples of commercially available non-reactive silicones include those from Shin-Etsu Chemical Co., Ltd., Dow Toray Industries, Inc., Asahi Kasei Wacker Silicone Co., Ltd., or Momentive Performance Materials, Inc. Examples of Shin-Etsu Chemical Co., Ltd. products include "KF-96", "KF-69", "KF-99", "KF-965", "KF-968", "HIVAC F-4", and "HIVAC F-5", "KF-56A", "KF-995", "KF-352A", "KF-353", "KF-354L", "KF-355A", "KF-615A", "KF-945", "KF-640", "KF-642", "KF-6 43'', ``KF-644'', ``KF-6020'', ``KF-6204'', ``X-22-4515'', ``KF-6011'', ``KF-6012'', ``KF-6015'', ``KF-6017'', ``X-22-2516'', ``KF-41'' Examples include "0", "FL-5", "X-22-821", "X-22-822", "FL-100", "KF-412", "KF-413", "KF-414", "KF-415", "KF-4003", "KF-4701", "KF-4917", "KF-7235B", "X-22-7322", "X-22-1877", "X-22-715", "KF-3935", "KF-50", "KF-53", "KF-54", or "KF-6004". Examples of products from Dow Toray Corporation include "BY 16-036", "SH 28", "SF 8428", "501W", "L-7001", "FZ-2104", "L-7002", "SF 8427", "SF 8461", "BY 16-846", or "SH 203".Asahi Kasei Wacker Silicone Co., Ltd.'s products include, for example, "AK 0.65 ~ 10 SILICONE FLUID", "AK 20 ~ 5,000 SILICONE FLUID", "AK 10,000 ~ 1,000,000 SILICONE FLUID", "AKF 100 ~ 10,000 SILICONE FLUID", "AKC 6,000 ~ 50,000 SILICONE FLUID", "H-SILOXANE", "AP 100 ~ 1,000 SILICONE FLUID", "APF 125 ~ 130 SILICONE FLUID", "AR 20 ~ 1,000 SILICONE FLUID", "AS 100 SILICONE FLUID", "TN SILICONE FLUID", "23166VP SILICONE FLUID", "GM 196 SILICONE FLUID", and "L 053 Examples include "SILICONE FLUID", "L 060 SILICONE FLUID", "L 066 SILICONE FLUID", "AF 98 / 1000 SILICONE FLUID", or "AF 98 / 10000 SILICONE FLUID". Examples of products from Momentive Performance Manufacturers, Inc. include "TSF451", "TSF404", "TSF405", "TSF4045", "TSF456", "TSF451", "TSF4300", "TSF437", "TSF400", "TSF401", "TSF4300", "TSF451", "TSF484", "THF450", "YF33", "TSF458", "TSF433", "TSF431", "TSF4600", "TSF410", "TSF411", "XF42-334", "X42-B3629", "TSF4421", "XF42-A3161", "TSF4450", "TSF4440", "TSF4452", "TSF4460", "TSF4441", "TSF4445", or "TSF4446".

[0087] The reactive silicone used in non-crosslinked silicone oils may be a commercially available product. Commercially available reactive silicones are the same as those exemplified for the reactive silicone used in crosslinked silicone oils.

[0088] (1.3) Preferred embodiment The carbon fiber precursor of this disclosure preferably satisfies the first condition. The "first condition" indicates that the content of the metal element is 0.05 parts by mass to 5.0 parts by mass per 100 parts by mass of the acrylamide polymer. When the carbon fiber precursor of this disclosure satisfies the first condition, the fusion of fibers due to the stretching and flame-retardant treatment can be suppressed more effectively than when the first condition is not met.

[0089] The carbon fiber precursor of this disclosure preferably satisfies the first and second conditions. The "second condition" indicates that the metal element is an alkaline earth metal. By satisfying the first and second conditions, the carbon fiber precursor satisfies the first and second conditions. Compared to cases where this treatment is not performed, it is possible to further suppress the fusion of fibers due to the stretching and flame-retardant treatment.

[0090] The carbon fiber precursor of this disclosure preferably satisfies the first to third conditions. The "third condition" indicates that the acrylamide polymer has anionic functional groups. By satisfying the first to third conditions, the carbon fiber precursor of this disclosure can suppress the fusion of fibers by stretching and flame-retardant treatment more effectively than when the first to third conditions are not met.

[0091] (2) Method for producing carbon fiber precursor The method for producing a carbon fiber precursor according to this disclosure includes spinning using a solution containing an acrylamide polymer, a metal salt, and water (hereinafter also simply referred to as the "spinning solution") (hereinafter also referred to as the "spinning process").

[0092] Because the method for producing a carbon fiber precursor according to this disclosure has the above configuration, it is possible to produce a carbon fiber precursor that can suppress the fusion of fibers due to the stretching and flame-retardant treatment.

[0093] (2.1) Carbon fiber precursor The carbon fiber precursor obtained by the method for producing the carbon fiber precursor of this disclosure is the same as the carbon fiber precursor exemplified in this disclosure.

[0094] (2.2) Spinning process The method for producing a carbon fiber precursor according to this disclosure includes a spinning process. In the spinning process, a solution containing an acrylamide polymer, a metal salt, and water is used for spinning.

[0095] (2.2.1) Spinning solution The spinning solution contains an acrylamide polymer. Examples of acrylamide polymers include those exemplified in this disclosure. The method for preparing the acrylamide polymer is not particularly limited and any known method may be used.

[0096] The spinning solution contains a metal salt. The metal salt is similar to those exemplified as metal salts in this disclosure. The method for preparing the metal salt is not particularly limited and any known method is acceptable. The amount of metal salt is not particularly limited and should be adjusted appropriately so that the metal element content of the acrylamide polymer fiber falls within the range exemplified above.

[0097] The spinning solution contains water. The type of water is not particularly limited and may include, for example, deionized water, distilled water, RO (Reverse Osmosis) water, or ultrapure water. The method of preparing the water is not particularly limited and any known method is acceptable. The amount of water used is not particularly limited.

[0098] The spinning solution may further contain additives. Examples of additives are similar to those exemplified as additives in this disclosure. The method for preparing the additives is not particularly limited and may be any known method. The amount of additives used is not particularly limited and may be within the range exemplified as the additive content.

[0099] The method for producing the spinning solution is not particularly limited. For example, the spinning solution may be obtained by dissolving and mixing a metal salt in an aqueous solution of an acrylamide polymer dissolved in water. The spinning solution may be obtained by dissolving and mixing an acrylamide polymer in an aqueous solution of a metal salt dissolved in water. The spinning solution may be obtained by dissolving an acrylamide polymer, obtained by copolymerizing an anionic vinyl monomer that is a metal salt, in water.

[0100] (2.2.2) Spinning The spinning method is not particularly limited and includes, for example, dry spinning, wet spinning, wet-dry spinning, gel spinning, flash spinning, or electrospinning. According to the above spinning method, acrylamide polymer fibers having a desired average fiber diameter can be produced safely and at low cost.

[0101] (2.3) Oil application process The method for producing the carbon fiber precursor of this disclosure may further include an oil application step. In the oil application step, a liquid oil layer (i.e., a non-crosslinked material) is applied to the surface of the acrylamide polymer fiber. As a result, the carbon fiber precursor obtained comprises the acrylamide polymer fiber and the liquid oil layer (i.e., a non-crosslinked material) applied to the surface of the acrylamide polymer fiber. As a result, the carbon fiber precursor of this disclosure becomes easier to bundle and the generation of fuzz and other defects can be suppressed. Furthermore, the fusion of fibers due to the stretching flame-retardant treatment can be further suppressed. The oil application step is performed after the spinning process.

[0102] The oil layer is the same as that exemplified as the oil layer in this disclosure.

[0103] For the adhesion of the oil layer in liquid form, a fiber oil is used. From the viewpoint of suppressing the fusion of carbon fiber precursors, it is preferable to use a silicone-based oil. Specifically, when the oil layer of the carbon fiber precursor is in a solid state (i.e., when the crosslinking treatment process described later is performed), a silicone-based oil containing a crosslinkable silicone oil is used. When the oil layer of the carbon fiber precursor is in a liquid state, a silicone-based oil containing at least one of a crosslinkable silicone oil and a non-crosslinkable silicone oil is used. Examples of crosslinkable silicone oils are the same as those exemplified as crosslinkable silicone oils in this disclosure. Examples of non-crosslinkable silicone oils are the same as those exemplified as non-crosslinkable silicone oils in this disclosure.

[0104] The method of applying the oil layer is not particularly limited and may include coating, immersion, spraying, touch-rolling, or guided lubrication.

[0105] (2.4) Crosslinking process The method for producing the carbon fiber precursor of this disclosure may further include a crosslinking step if it includes an oil layer application step. In the crosslinking step, the liquid oil layer attached to the acrylamide polymer fiber is subjected to a crosslinking treatment. As a result, the carbon fiber precursor obtained has acrylamide polymer fiber and a solid oil layer (i.e., a crosslinked material) attached to the surface of the acrylamide polymer fiber. As a result, the carbon fiber precursor of this disclosure can further suppress oil adhesion to rollers and the like, water absorption, and fusion of fibers due to stretching and flame-retardant treatment. The crosslinking step is performed after the oil application step.

[0106] The crosslinking treatment is not particularly limited and includes methods such as irradiating the liquid oil layer with an electron beam (hereinafter referred to as "electron beam treatment"), irradiating the liquid oil layer with ultraviolet light, or heating and drying the liquid oil layer. Among these, electron beam treatment is preferred from the viewpoint of energy efficiency and processing speed.

[0107] (2.4.1) Electron beam treatment From the viewpoint of fusion suppression, carbonization yield, and shape stability, the electron beam dose is preferably 50 kGy to 10,000 kGy, more preferably 100 kGy to 5,000 kGy, and even more preferably 150 kGy to 1,000 kGy. The preferred numerical range of doses described above is the preferred numerical range of dose when an electron beam is irradiated from one direction onto a liquid oil layer. When an electron beam is irradiated from two or more directions, the electron beam dose is not limited to the above and should be adjusted as appropriate. The electron beam dose is measured using a film dosimeter. A film dosimeter such as the FWT-60 model manufactured by Toyo Medic Co., Ltd., or a device of comparable quality, can be used.

[0108] From the viewpoint of fusion suppression, carbonization yield, and shape stability, the acceleration voltage of the electron beam is preferably 50kV to 10MV, more preferably 100kV to 3MV, and even more preferably 150kV to 1MV. The preferred numerical range for the electron beam acceleration voltage is the preferred numerical range for the acceleration voltage when an active light beam is irradiated from one direction onto a liquid oil layer. When irradiating from two or more directions with electron beams, the electron beam acceleration voltage is not limited to the above and is preferably adjusted as appropriate.

[0109] Electron beam irradiation may be performed in batch mode or in continuous mode. The equipment used for activated ray irradiation is not particularly limited. When electron beam irradiation is performed in batch mode, the "EC110 / 15 / 10mA" manufactured by Iwasaki Electric Co., Ltd. or an equivalent device can be used. When electron beam irradiation is performed in continuous mode, the "EPS-800kV" electron beam irradiation device manufactured by NHV Corporation or an equivalent device can be used.

[0110] (3) Method for manufacturing carbon fibers The carbon fiber manufacturing method described herein is The carbon fiber precursor of this disclosure is subjected to a flame-retardant treatment to obtain flame-retardant fibers (hereinafter also referred to as the "flame-retardant treatment process"), The process includes subjecting the flame-resistant fiber to a carbonization treatment to obtain carbon fiber (hereinafter also referred to as the "carbonization treatment process"). The flame-retardant treatment includes stretching flame-retardant treatment. The flame-retardant treatment process and the carbonization treatment process are carried out in this order.

[0111] "Flame-retardant treatment" refers to the process of heat-treating carbon fiber precursors under an oxidizing gas atmosphere. The heating temperature in flame-retardant treatment is within the range of 120°C to 500°C. "Stretching and flame-retardant treatment" refers to a process in which a carbon fiber precursor is subjected to a stretching treatment with a stretching ratio greater than 1.1 while simultaneously undergoing a flame-retardant treatment. "Stretching treatment" refers to a process in which the carbon fiber precursor is heated to plasticize and then stretched. When pre-stretching, flame-retardant treatment, carbonization, etc., are performed in a continuous manner (i.e., when fibers are transported into the heat treatment apparatus by multiple rollers to undergo pre-stretching, flame-retardant treatment, or carbonization treatment), the "stretching ratio" is determined by the ratio of the fiber extraction speed (hereinafter also called "extraction speed") from the heat treatment furnace to the fiber feed speed (hereinafter also called "introduction speed") of the fibers entering the heat treatment furnace (hereinafter also called "introduction speed"). It can also be determined by the ratio of the length of the fibers entering the heat treatment furnace (hereinafter also called "introduced fibers") to the length of the fibers exiting the heat treatment furnace (hereinafter also called "extracted fibers") (length of extracted fibers / length of introduced fibers). Methods for adjusting the stretching ratio include, for example, adjusting the speed of the rollers at the inlet (V1) and outlet (V2) of the heat treatment apparatus, or adjusting the tension applied to the carbon fiber precursor using weights or springs. Note that V2 / V1 is synonymous with the stretching ratio. There are no particular restrictions on the type of roller used; examples include feed rollers and nip rollers. "Carbonization treatment" refers to a process of carbonizing the flame-resistant fibers of the carbon fiber precursor. More specifically, "carbonization treatment" refers to a process of heat-treating the flame-resistant fibers of the carbon fiber precursor under a low-oxygen atmosphere (preferably an environment where oxygen is blocked).

[0112] Because the carbon fiber manufacturing method of this disclosure has the above configuration, it is possible to manufacture carbon fibers with excellent tensile strength and load-bearing capacity.

[0113] (3.1) Preliminary stretching process The carbon fiber manufacturing method of the present disclosure preferably further includes a preliminary stretching treatment (hereinafter also referred to as the "preliminary stretching treatment step") of the carbon fiber precursor before performing the flame-retardant fiber (i.e., the flame-retardant treatment step). The preliminary stretching treatment step, the flame-retardant treatment step, and the carbonization treatment step are performed in this order.

[0114] "Preliminary stretching treatment" refers to a process in which the carbon fiber precursor is stretched while being heated at 300°C or below, prior to the flame-retardant treatment process.

[0115] When carbon fiber precursors undergo stretching and flame-retardant treatment, they become less plasticizable and therefore more difficult to stretch. Furthermore, if the stretching ratio in the stretching and flame-retardant treatment exceeds 5.0 times, there is a risk of the carbon fiber precursors fracturing. By including a preliminary stretching step in the carbon fiber manufacturing method, the stretching ratio of the carbon fiber precursors can be increased to more than 5 times. As a result, the acrylamide polymer contained in the carbon fiber precursors becomes oriented, and the tensile strength of the flame-retardant fibers and carbon fibers tends to improve further. Consequently, the carbon fiber manufacturing method of this disclosure can produce carbon fibers with superior tensile strength and load-bearing capacity.

[0116] A known heat treatment apparatus may be used for the preliminary stretching process. The preliminary stretching process may be carried out in a batch manner or in a continuous manner.

[0117] The stretching ratio during the preliminary stretching treatment is greater than 1.1 times. From the viewpoint of the stability of the preliminary stretching treatment and the suppression of damage to the carbon fiber precursor, flame-resistant fiber bundle, and carbon fiber bundle, the stretching ratio during the preliminary stretching treatment is preferably 5.0 times or less, more preferably 4.5 times or less, and even more preferably 4.0 times or less. From the viewpoint of the orientation of the flame-resistant fibers and carbon fibers, tensile strength, and load-bearing capacity of the carbon fiber bundle, the stretching ratio during the preliminary stretching treatment is preferably 1.5 times or more, more preferably 1.75 times or more, and even more preferably 2.0 times or more.

[0118] The method of stretching is not particularly limited and is appropriately selected depending on the method of performing the preliminary stretching. If the preliminary stretching is performed continuously, the stretching may be performed by adjusting the speed of the roller at the inlet of the heat treatment device (V1) and the speed of the roller at the outlet of the heat treatment device (V2). Alternatively, the carbon fiber precursor may be stretched by applying appropriate tension to it using weights, springs, air cylinders, or hydraulics.

[0119] The preliminary stretching treatment may be carried out in an oxidizing atmosphere. Examples of oxidizing atmospheres include oxygen, ozone, air, nitrogen oxides, halogens, sulfur dioxide, mixtures thereof, or mixtures of these with an inert gas. Among these, air, a mixture of oxygen and air, a mixture of oxygen and an inert gas, and a mixture of air and an inert gas are preferred, and air is particularly preferred from the viewpoint of cost reduction.

[0120] The temperature for the preliminary stretching treatment should be 300°C or lower. The temperature for the preliminary stretching treatment may be 100°C to 300°C, 140°C to 290°C, or 180°C to 280°C. The maximum temperature during the preliminary stretching treatment may be 290°C or lower, or 280°C or lower.

[0121] There are no particular restrictions on the time for the preliminary stretching process; it may be between 0.1 and 60 minutes, between 0.5 and 30 minutes, or between 1 and 20 minutes.

[0122] (3.2) Flame-retardant treatment process In the flame-retardant treatment process, the carbon fiber precursor of this disclosure is subjected to a flame-retardant treatment to obtain flame-retardant fibers.

[0123] (3.2.1) Stretching flame-retardant treatment Flame-retardant treatment includes stretching flame-retardant treatment. In stretching flame-retardant treatment, the carbon fiber precursor is subjected to a stretching treatment with a stretching ratio greater than 1.1 while simultaneously undergoing flame-retardant treatment.

[0124] A known heat treatment apparatus may be used for the stretching flame-retardant treatment. The stretching flame-retardant treatment may be carried out in a continuous manner.

[0125] The stretch ratio during the stretch flame-retardant treatment is greater than 1.1 times. From the viewpoint of the stability of the stretch flame-retardant treatment and the suppression of damage to the flame-retardant fiber bundles and carbon fiber bundles, the stretch ratio during the stretch flame-retardant treatment is preferably 5.0 times or less, more preferably 4.5 times or less, and even more preferably 4.0 times or less. From the viewpoint of the orientation of the flame-retardant fibers and carbon fibers, tensile strength, and load-bearing capacity of the carbon fiber bundles, the stretch ratio during the stretch flame-retardant treatment is preferably 1.5 times or more, more preferably 1.75 times or more, and even more preferably 2.0 times or more.

[0126] The method of stretching is not particularly limited and is appropriately selected depending on the method of performing the stretching flame-retardant treatment. If the method of performing the stretching flame-retardant treatment is continuous, the stretching treatment may be performed by adjusting the speed of the roller at the inlet of the heat treatment device (V1) and the speed of the roller at the outlet of the heat treatment device (V2). Alternatively, the carbon fiber precursor may be stretched by applying appropriate tension to it using weights, springs, air cylinders, or hydraulics.

[0127] The stretching and flame-retardant treatment is carried out in an oxidizing atmosphere. The oxidizing atmosphere is the same as that exemplified for the preliminary stretching treatment. From the viewpoint of cost reduction, the oxidizing atmosphere is particularly preferably air.

[0128] The temperature for the stretching and flame-retardant treatment is not particularly limited, but is preferably 150°C to 500°C, more preferably 175°C to 450°C, and even more preferably 200°C to 420°C. The temperature of the stretch flame-retardant treatment includes not only the highest temperature during the stretch flame-retardant treatment (hereinafter also referred to as the "stretch flame-retardant treatment temperature"), but also the temperature during the heating process leading up to the stretch flame-retardant treatment temperature.

[0129] The maximum temperature during the stretching flame-retardant treatment is preferably 301°C to 500°C, more preferably 303°C to 450°C, even more preferably 305°C to 425°C, particularly preferably 310°C to 425°C, and most preferably 315°C to 400°C. By setting the stretching and flame-retardant treatment temperature to 305°C or higher, it tends to be possible to improve the heat resistance and carbonization yield of flame-retardant fibers. By keeping the maximum temperature during the stretching and flame-retardant treatment below 500°C, it tends to be possible to suppress the thermal decomposition of flame-retardant fibers.

[0130] The stretching flame-retardant treatment time (i.e., the heating time at the stretching flame-retardant treatment temperature) is not particularly limited, but from the viewpoint of carbonization yield and manufacturing cost, it is preferably 1 to 120 minutes, more preferably 2 to 60 minutes, even more preferably 3 to 50 minutes, and particularly preferably 4 to 40 minutes. The flame-retardant treatment time may exceed 2 hours.

[0131] (3.2.2) Non-stretch flame-retardant treatment The flame-retardant treatment may include both stretched flame-retardant treatment and non-stretched flame-retardant treatment. In the non-stretched flame-retardant treatment, the carbon fiber precursor or flame-retardant fiber is treated under conditions where the stretch ratio is 1.1 or less. The non-stretched flame-retardant treatment may be performed after the stretched flame-retardant treatment.

[0132] By applying a non-stretch flame-retardant treatment to the carbon fiber precursor (i.e., flame-retardant fiber) after the stretch flame-retardant treatment, the flame resistance of the carbon fiber precursor can be further improved.

[0133] The non-stretch flame-retardant treatment is the same as the method exemplified for stretch flame-retardant treatment, except that the stretch ratio is different. Therefore, the explanation of the stretch flame-retardant treatment will be used as a reference, and the explanation of the non-stretch flame-retardant treatment will be omitted.

[0134] The stretch ratio during non-stretch flame-retardant treatment is 1.1 times or less. From the viewpoint of the stability of the non-stretch flame-retardant treatment and the suppression of damage to the flame-retardant fiber bundles and carbon fiber bundles, the stretch ratio during non-stretch flame-retardant treatment is preferably 1.08 times or less, more preferably 1.06 times or less, and even more preferably 1.04 times or less. From the viewpoint of the orientation of the flame-retardant fibers and carbon fibers, tensile strength, and load-bearing capacity of the carbon fiber bundles, the stretch ratio during non-stretch flame-retardant treatment is preferably 0.90 times or more, more preferably 0.93 times or more, and even more preferably 0.98 times or more. The stretch ratio during non-stretch flame-retardant treatment may be 1.0 times.

[0135] The temperature of the non-stretch flame-retardant treatment may be the same as the temperature of the stretch flame-retardant treatment, or it may be higher than the temperature of the stretch flame-retardant treatment. The temperature of the non-stretch flame-retardant treatment includes not only the maximum temperature during the non-stretch flame-retardant treatment, but also the temperature during the heating process up to the non-stretch flame-retardant treatment temperature. The maximum temperature of the non-stretch flame-retardant treatment may be the same as the maximum temperature of the stretch flame-retardant treatment, or it may be higher than the maximum temperature of the stretch flame-retardant treatment.

[0136] The non-stretch flame-retardant treatment time (i.e., the heating time at the non-stretch flame-retardant treatment temperature) may be the same as the stretch flame-retardant treatment time, or it may be longer than the stretch flame-retardant treatment time.

[0137] (3.2.3) Flame-resistant fibers The fusion rate of the flame-resistant fibers is preferably 10% or less, more preferably 9% or less, even more preferably 8% or less, particularly preferably 7% or less, and most preferably 6%, from the viewpoint of suppressing fiber breakage during carbonization treatment. The method for measuring the fusion rate is the same as in the example.

[0138] The average fiber diameter of the flame-resistant fiber bundle is not particularly limited, but is preferably 4 μm to 16 μm, more preferably 5 μm to 14 μm, and even more preferably 6 μm to 12 μm. When the average fiber diameter of the flame-resistant fiber is 4 μm or more, the resistance to carbonization treatment is improved. When the average fiber diameter of the flame-resistant fiber bundle is 16 μm or less, the resistance to carbonization treatment, as well as the tensile strength and load-bearing capacity of the carbon fiber bundle, are improved.

[0139] (3.3) Cumulative stretching ratio The cumulative stretching ratio of the carbon fiber precursor is not particularly limited, but is preferably between 2 and 20 times. If the cumulative stretching ratio of the carbon fiber precursor is between 2 and 20 times, the method for producing carbon fiber bundles according to this disclosure can produce carbon fiber bundles that are superior in single-fiber tensile strength and load-bearing capacity. The cumulative stretching ratio of the carbon fiber precursor is more preferably 4.5 times or more, even more preferably 5 times or more, and particularly preferably 6 times or more, from the viewpoint of the orientation of the flame-resistant fibers and carbon fibers, tensile strength, and load-bearing capacity of the carbon fiber bundle. The cumulative stretching ratio of the carbon fiber precursor is more preferably 16 times or less, and even more preferably 14 times or less, from the viewpoint of suppressing damage to the flame-resistant fiber bundle and carbon fiber bundle.

[0140] "Cumulative stretch ratio" refers to the total stretch ratio from the time the carbon fiber precursor is manufactured until the completion of the flame-retardant treatment process. If the preliminary stretching process is not performed and the flame-retardant treatment consists only of the stretching flame-retardant treatment, the cumulative stretching ratio will be the stretching ratio at the time of the stretching flame-retardant treatment. If a preliminary stretching process is not performed, and the flame-retardant treatment includes both stretched flame-retardant treatment and non-stretched flame-retardant treatment, the cumulative stretching ratio is expressed as the product of the stretching ratio during the stretched flame-retardant treatment and the stretching ratio during the non-stretched flame-retardant treatment. If a preliminary stretching process is performed and the flame-retardant treatment includes both stretched flame-retardant treatment and non-stretched flame-retardant treatment, the cumulative stretching ratio is expressed as the product of the stretching ratio during the preliminary stretching process, the stretching ratio during the stretched flame-retardant treatment, and the stretching ratio during the non-stretched flame-retardant treatment.

[0141] (3.4) Carbonization process In the carbonization process, the flame-resistant fibers are subjected to a carbonization treatment. This carbonizes the flame-resistant fibers, yielding carbon fibers.

[0142] (3.4.1) Carbonization treatment One method of carbonization is to heat-treat the flame-resistant fibers at a temperature higher than the temperature used for flame-retardant treatment, under the atmosphere of an inert gas (for example, nitrogen, argon, or helium). The maximum heating temperature in the carbonization treatment is preferably 500°C or higher, more preferably 1000°C or higher, even more preferably 1100°C or higher, particularly preferably 1200°C or higher, and most preferably 1300°C or higher. The maximum heating temperature is preferably 3000°C or lower, more preferably 2500°C or lower. The maximum heating temperature in the carbonization treatment is preferably between 500°C and 3000°C.

[0143] The heating time at the highest temperature during the carbonization process is not particularly limited, but is preferably 30 seconds to 60 minutes, and more preferably 1 minute to 30 minutes.

[0144] In this disclosure, "carbonization treatment" may generally include "graphitization," which is carried out by heating at a temperature of 2000°C to 3000°C under an inert gas atmosphere. The carbonization process may include multiple heat treatments. For example, a heat treatment (preliminary carbonization) can be performed first at a temperature of less than 1000°C, followed by a heat treatment (main carbonization) at a temperature of 1000°C or higher, and then a heat treatment (graphitization) at a temperature of 2000°C or higher.

[0145] (3.4.2) Carbon fiber The average fiber diameter of the carbon fibers is not particularly limited, but is preferably 3 μm to 10 μm, more preferably 3 μm to 9 μm, and even more preferably 4 μm to 8 μm, from the viewpoint of single fiber tensile strength and load-bearing capacity of the carbon fiber bundle. When the average fiber diameter of the carbon fiber is 3 μm or more, when producing a composite material with a resin or the like as a matrix, insufficient impregnation of the resin or the like into the carbon fiber bundle is unlikely to occur, and the tensile strength of the composite material is improved. Also, the influence on the single fiber tensile strength and the load resistance of the carbon fiber bundle due to surface defects or the like tends to be small. When the average fiber diameter of the carbon fiber is 10 μm or less, the single fiber tensile strength and the load resistance of the carbon fiber bundle are unlikely to decrease due to insufficient carbonization in the center of the carbon fiber.

[0146] The tensile strength of the single fiber of the carbon fiber is preferably more than 1000 MPa, more preferably 1300 MPa or more, still more preferably 1500 MPa or more, particularly preferably 1800 MPa or more, and even more preferably 2000 MPa or more. The tensile strength of the single fiber of the carbon fiber may be 10000 MPa or less. The measuring method of the tensile strength of the single fiber of the carbon fiber is the same as the method described in the examples.

[0147] The load resistance of the carbon fiber bundle is preferably 15 kg / mm 2 or more, more preferably 20 kg / mm 2 or more, still more preferably 30 kg / mm 2 or more, particularly preferably 45 kg / mm 2 or more, and even more preferably 48 kg / mm 2 or more. The load resistance of the carbon fiber bundle may be 1000 kg / mm 2 or less. The measuring method of the load resistance of the carbon fiber bundle is the same as the method described in the examples.

Examples

[0148] [[ID=第29]]Hereinafter, the above-described embodiments will be specifically described by way of examples, but the above-described embodiments are not limited to these examples.

[0149] [1] Preparation [1.1] Synthesis Example 1 (Production of Acrylamide-Based Polymer) As an acrylamide polymer, an acrylamide (AM) / acrylonitrile (AN) / acrylic acid (AA) ternary copolymer (AM / AN / AA = 60 mol% / 35 mol% / 5 mol%) was prepared according to the following procedure. Monomers consisting of 60 mol% AM, 35 mol% AN, and 5 mol% AA were obtained. 100 parts by mass of the obtained monomers and 5 parts by mass of tetramethylethylenediamine were dissolved in 400 parts by mass of deionized water to obtain a first aqueous solution. Ammonium persulfate was added to the obtained first aqueous solution while stirring under a nitrogen atmosphere, and polymerization was carried out by heating at 80°C for 150 minutes to obtain a second aqueous solution. The obtained second aqueous solution was added dropwise to methanol to precipitate the copolymer, and the copolymer was recovered and vacuum dried at 80°C for 12 hours. This yielded a water-soluble AM / AN / AA (60 mol% / 35 mol% / 5 mol%) copolymer (i.e., an acrylamide polymer).

[0150] [1.2] Preparation of carbon fiber precursors Carbon fiber precursors (a-1) to (a-5) and (x-6) to (x-8) were prepared according to the following procedure.

[0151] [1.2.1] Manufacturing Example 1 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1, 3.42 parts by mass of calcium chloride (1.23 parts by mass as the metallic element calcium), and 3.52 parts by mass of monoammonium phosphate were dissolved in deionized water to obtain a spinning solution. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 18 μm.

[0152] Self-crosslinking silicone ("KP-420" manufactured by Shin-Etsu Chemical Co., Ltd.) and self-crosslinking silicone ("X-22-164C" manufactured by Shin-Etsu Chemical Co., Ltd.) were mixed in a 50:50 ratio (by weight) to obtain crosslinking silicone oil A. 3 parts by mass of crosslinking silicone oil A were applied to 100 parts by mass of the obtained acrylamide polymer fibers, and the fibers were arranged into bundles of 800 strands each to obtain a carbon fiber precursor. To cure the crosslinking silicone oil A, the obtained carbon fiber precursor was irradiated with an electron beam using an electron beam irradiation device ("EPS-800kV" manufactured by NHV Corporation). During electron beam irradiation, the acceleration voltage was 800kV and the electron beam dose was 600kGy. This yielded carbon fiber precursor (a-1).

[0153] [1.2.2] Manufacturing Example 2 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 3.42 parts by mass of calcium chloride (1.23 parts by mass as the metallic element calcium) were dissolved in deionized water to obtain a spinning solution. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 20 μm.

[0154] Crosslinkable silicone oil A was obtained in the same manner as in Production Example 1. 4 parts by mass of crosslinkable silicone oil A was applied to 100 parts by mass of the obtained acrylamide polymer fibers, and the fibers were aligned to 800 strands / bundle to obtain a carbon fiber precursor. To cure the crosslinkable silicone oil A, the obtained carbon fiber precursor was subjected to electron beam irradiation in the same manner as in Production Example 1. This yielded carbon fiber precursor (a-2).

[0155] [1.2.3] Manufacturing Example 3 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 6.27 parts by mass of magnesium chloride hexahydrate (0.75 parts by mass as magnesium, a metallic element) were dissolved in deionized water to obtain a spinning solution. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 20 μm.

[0156] Crosslinkable silicone oil A was obtained in the same manner as in Production Example 1. 3 parts by mass of crosslinkable silicone oil A was applied to 100 parts by mass of the obtained acrylamide polymer fibers, and the fibers were aligned into bundles of 800 to obtain a carbon fiber precursor. To cure the crosslinkable silicone oil A, the obtained carbon fiber precursor was subjected to electron beam irradiation in the same manner as in Production Example 1. This yielded carbon fiber precursor (a-3).

[0157] [1.2.4] Manufacturing Example 4 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 6.27 parts by mass of magnesium chloride hexahydrate (0.75 parts by mass as magnesium, a metallic element) were dissolved in deionized water to obtain a spinning solution. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 20 μm.

[0158] 100 parts by mass of the obtained acrylamide polymer fibers were coated with 3 parts by mass of silicone oil B and arranged into bundles of 800 fibers each. This yielded carbon fiber precursor (a-4).

[0159] [1.2.5] Manufacturing Example 5 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 6.27 parts by mass of magnesium chloride hexahydrate (0.75 parts by mass as magnesium, a metallic element) were dissolved in deionized water to obtain a spinning solution. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 26 μm.

[0160] 100 parts by mass of the obtained acrylamide polymer fibers were coated with 5 parts by mass of silicone oil B and arranged into bundles of 800 fibers each. This yielded carbon fiber precursor (a-5).

[0161] [1.2.6] Manufacturing Example 6 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 3 parts by mass of phosphoric acid were dissolved in deionized water to obtain a spinning solution. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 20 μm.

[0162] Four parts by mass of crosslinkable silicone oil C (KP-420, manufactured by Shin-Etsu Chemical Co., Ltd.) were applied to 100 parts by mass of the obtained acrylamide polymer fibers, and the fibers were aligned into bundles of 800 strands each to obtain a carbon fiber precursor. To cure the crosslinkable silicone oil C, the obtained carbon fiber precursor was subjected to electron beam irradiation in the same manner as in Production Example 1. This yielded carbon fiber precursor (x-6).

[0163] [1.2.7] Manufacturing Example 7 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 3 parts by mass of phosphoric acid were dissolved in deionized water to obtain a spinning solution. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 23 μm.

[0164] Two parts by mass of crosslinkable silicone oil D (Silmer ACR Di-1508-TF, manufactured by SILTECH Corporation) were applied to 100 parts by mass of the obtained acrylamide polymer fibers, and the fibers were aligned into bundles of 800 strands each to obtain a carbon fiber precursor. To cure the crosslinkable silicone oil D, the obtained carbon fiber precursor was subjected to electron beam irradiation in the same manner as in Production Example 1. This yielded carbon fiber precursor (x-7).

[0165] [1.2.8] Manufacturing Example 8 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 3 parts by mass of phosphoric acid were dissolved in deionized water to obtain a spinning solution. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 20 μm.

[0166] 100 parts by mass of the obtained acrylamide polymer fibers were coated with 3 parts by mass of crosslinkable silicone oil A, and the fibers were arranged into bundles of 800 to obtain a carbon fiber precursor. To cure the crosslinkable silicone A, the obtained carbon fiber body was subjected to electron beam irradiation in the same manner as in Production Example 1. This yielded carbon fiber precursor (x-8).

[0167] [1.2.9] Manufacturing Example 9 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 3 parts by mass of phosphoric acid were dissolved in deionized water to obtain a spinning solution. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 20 μm.

[0168] 100 parts by mass of the obtained acrylamide polymer fibers were coated with 5 parts by mass of silicone oil B, and then arranged into bundles of 800 fibers each to obtain carbon fiber precursor (x-9).

[0169] [2] Examples and Comparative Examples [2.1] Example 1 Using carbon fiber precursor (a-1), a stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. In the stretch flame-retardant treatment, the stretch ratio was 3.0 times, and the treatment time was 10 minutes. Subsequently, a non-stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 340°C under an air atmosphere. In the non-stretch flame-retardant treatment, the stretch ratio was 1.0 times, and the treatment time was 20 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretch ratio was 3.0 times.

[0170] A "continuous heat treatment furnace from A°C to B°C" indicates that as the carbon fiber precursor moves through the furnace, the temperature inside the furnace gradually increases from A°C to B°C. For example, a continuous heat treatment furnace from 250°C to 330°C indicates that as the carbon fiber precursor (a-1) moves through the furnace, the temperature inside the furnace gradually increases from 250°C to 330°C. The same applies to subsequent examples.

[0171] [2.2] Example 2 Flame-resistant fiber bundles were prepared using the same procedure as in Example 1, except that carbon fiber precursor (a-1) was changed to carbon fiber precursor (a-2).

[0172] [2.3] Example 3 Using carbon fiber precursor (a-2), a preliminary stretching treatment was performed in a continuous heat treatment furnace at 230°C under an air atmosphere. In the preliminary stretching treatment, the stretching ratio was 2.0 times, and the treatment temperature was 10 minutes. Subsequently, a stretching and flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. In the stretching and flame-retardant treatment, the stretching ratio was 2.0 times, and the treatment time was 10 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretching ratio was 4.0 times.

[0173] [2.4] Example 4 Flame-resistant fiber bundles were prepared using the same procedure as in Example 1, except that carbon fiber precursor (a-1) was changed to carbon fiber precursor (a-3).

[0174] [2.5] Example 5 Using carbon fiber precursor (a-3), stretching and flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 350°C under an air atmosphere. During the stretching and flame-retardant treatment, the stretching ratio was 3.0 times, and the treatment time was 10 minutes. Subsequently, non-stretching and flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 350°C under an air atmosphere. During the non-stretching and flame-retardant treatment, the stretching ratio was 1.0 times, and the treatment time was 10 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretching ratio was 3.0 times.

[0175] [2.6] Example 6 Using carbon fiber precursor (a-3), a preliminary stretching treatment was performed in a continuous heat treatment furnace at 230°C under an air atmosphere. In the preliminary stretching treatment, the stretching ratio was 2.0 times and the treatment time was 10 minutes. Subsequently, a stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. In the stretch flame-retardant treatment, the stretching ratio was 3.0 times and the treatment time was 10 minutes. Subsequently, a non-stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 340°C under an air atmosphere. In the non-stretch flame-retardant treatment, the stretching ratio was 1.0 times and the treatment time was 20 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretching ratio was 6.0 times.

[0176] [2.7] Example 7 Using carbon fiber precursor (a-3), a preliminary stretching treatment was performed in a continuous heat treatment furnace at 250°C under an air atmosphere. In the preliminary stretching treatment, the stretching ratio was 3.0 times, and the treatment time was 10 minutes. Subsequently, a stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. In the stretch flame-retardant treatment, the stretching ratio was 2.0 times, and the treatment time was 10 minutes. Subsequently, a non-stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 350°C under an air atmosphere. In the non-stretch flame-retardant treatment, the stretching ratio was 1.0 times, and the treatment time was 20 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretching ratio was 6.0 times.

[0177] [2.8] Example 8 Using carbon fiber precursor (a-4), a stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. In the stretch flame-retardant treatment, the stretch ratio was 3.0 times, and the treatment time was 10 minutes. Subsequently, a non-stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 340°C under an air atmosphere. In the non-stretch flame-retardant treatment, the stretch ratio was 1.0 times, and the treatment time was 40 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretch ratio was 3.0 times.

[0178] [2.9] Example 9 Using carbon fiber precursor (a-4), a preliminary stretching treatment was performed in a continuous heat treatment furnace at 250°C under an air atmosphere. In the preliminary stretching treatment, the stretching ratio was 2.0 times and the treatment time was 10 minutes. Subsequently, a stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. In the stretch flame-retardant treatment, the stretching ratio was 2.5 times and the treatment time was 10 minutes. Subsequently, a non-stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 340°C under an air atmosphere. In the non-stretch flame-retardant treatment, the stretching ratio was 1.0 times and the treatment time was 40 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretching ratio was 5.0 times.

[0179] [2.10] Example 10 Using carbon fiber precursor (a-4), a preliminary stretching treatment was performed in a continuous heat treatment furnace at 250°C under an air atmosphere. In the preliminary stretching treatment, the stretching ratio was 2.0 times and the treatment time was 10 minutes. Subsequently, a stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. In the stretch flame-retardant treatment, the stretching ratio was 2.5 times and the treatment time was 10 minutes. Subsequently, a non-stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 350°C under an air atmosphere. In the non-stretch flame-retardant treatment, the stretching ratio was 1.0 times and the treatment time was 20 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretching ratio was 5.0 times.

[0180] [2.11] Example 11 Using carbon fiber precursor (a-4), a preliminary stretching treatment was performed in a continuous heat treatment furnace at 250°C under an air atmosphere. In the preliminary stretching treatment, the stretching ratio was 2.0 times and the treatment time was 10 minutes. Subsequently, a stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 350°C under an air atmosphere. In the stretch flame-retardant treatment, the stretching ratio was 2.5 times and the treatment time was 10 minutes. Subsequently, a non-stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 350°C under an air atmosphere. In the non-stretch flame-retardant treatment, the stretching ratio was 1.0 times and the treatment time was 10 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretching ratio was 5.0 times.

[0181] [2.12] Example 12 Using carbon fiber precursor (a-5), a preliminary stretching treatment was performed in a continuous heat treatment furnace at 250°C under an air atmosphere. In the preliminary stretching treatment, the stretching ratio was 3.0 times and the treatment time was 10 minutes. Subsequently, a stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. In the stretch flame-retardant treatment, the stretching ratio was 2.5 times and the treatment time was 10 minutes. Subsequently, a non-stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 350°C under an air atmosphere. In the non-stretch flame-retardant treatment, the stretching ratio was 1.0 times and the treatment time was 20 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretching ratio was 7.5 times.

[0182] [2.13] Example 13 Flame-resistant fiber bundles were prepared using the same procedure as in Example 12, except that the stretching ratio for the stretching flame-retardant treatment was changed from 2.5 times to 3.0 times. The cumulative stretching ratio was 9.0 times.

[0183] [2.14] Comparative Example 1 Using carbon fiber precursor (x-6), a stretching and flame-retardant treatment was performed in a continuous heat treatment furnace at 230°C to 350°C under an air atmosphere. The stretching ratio was 3.0 times, and the treatment time was 20 minutes. This resulted in the acquisition of flame-retardant fiber bundles. The cumulative stretching ratio was 3.0 times.

[0184] [2.15] Comparative Example 2 Using carbon fiber precursor (x-6), a stretching and flame-retardant treatment was performed in a continuous heat treatment furnace at 230°C to 340°C under an air atmosphere. The stretching ratio was 3.0 times, and the treatment time was 20 minutes. This resulted in the acquisition of flame-retardant fiber bundles. The cumulative stretching ratio was 3.0 times.

[0185] [2.16] Comparative Example 3 Using carbon fiber precursor (x-6), a preliminary stretching treatment was performed in a continuous heat treatment furnace at 230°C under an air atmosphere. In the preliminary stretching treatment, the stretching ratio was 3.0 times, and the treatment time was 10 minutes. Subsequently, a stretching and flame-retardant treatment was performed in a continuous heat treatment furnace at 230°C to 330°C under an air atmosphere. In the stretching and flame-retardant treatment, the stretching ratio was 2.0 times, and the treatment time was 10 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretching ratio was 6.0 times.

[0186] [2.17] Comparative Example 4 Using carbon fiber precursor (x-6), a preliminary stretching treatment was performed in a continuous heat treatment furnace at 250°C under an air atmosphere. In the preliminary stretching treatment, the stretching ratio was 3.0 times and the treatment time was 10 minutes. Subsequently, a stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 230°C to 330°C under an air atmosphere. In the stretch flame-retardant treatment, the stretching ratio was 2.0 times and the treatment time was 10 minutes. Subsequently, a non-stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 230°C to 330°C under an air atmosphere. In the non-stretch flame-retardant treatment, the stretching ratio was 1.0 times and the treatment time was 10 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretching ratio was 6.0 times.

[0187] [2.18] Comparative Example 5 Using carbon fiber precursor (x-7), a stretching and flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. The stretching ratio was 3.0 times, and the treatment time was 10 minutes. This resulted in the acquisition of flame-retardant fiber bundles. The cumulative stretching ratio was 3.0 times.

[0188] [2.19] Comparative Example 6 Using carbon fiber precursor (x-7), a stretching and flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. The stretching ratio was 3.0 times, and the treatment time was 20 minutes. This resulted in the acquisition of flame-retardant fiber bundles. The cumulative stretching ratio was 3.0 times.

[0189] [2.20] Comparative Example 7 Using carbon fiber precursor (x-7), a stretching and flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. The stretching ratio was 4.0 times, and the treatment time was 10 minutes. This resulted in the acquisition of flame-retardant fiber bundles. The cumulative stretching ratio was 4.0 times.

[0190] [2.21] Comparative Example 8 Using carbon fiber precursor (x-7), a stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. In the stretch flame-retardant treatment, the stretch ratio was 4.0 times, and the treatment time was 10 minutes. Subsequently, a non-stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. In the non-stretch flame-retardant treatment, the stretch ratio was 1.0 times, and the treatment time was 20 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretch ratio was 4.0 times.

[0191] [2.22] Comparative Example 9 Flame-resistant fiber bundles were prepared using the same procedure as in Comparative Example 8, except that the processing time for the non-stretched flame-retardant treatment was changed from 20 minutes to 30 minutes.

[0192] [2.23] Comparative Example 10 Using carbon fiber precursor (x-7), a preliminary stretching treatment was performed in a continuous heat treatment furnace at 230°C under an air atmosphere. In the preliminary stretching treatment, the stretching ratio was 3.0 times, and the treatment time was 10 minutes. Subsequently, a stretching and flame-retardant treatment was performed in a continuous heat treatment furnace at 230°C to 330°C under an air atmosphere. In the stretching and flame-retardant treatment, the stretching ratio was 2.0 times, and the treatment time was 10 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretching ratio was 6.0 times.

[0193] [2.24] Comparative Example 11 Using carbon fiber precursor (x-7), a preliminary stretching treatment was performed in a continuous heat treatment furnace at 230°C under an air atmosphere. In the preliminary stretching treatment, the stretching ratio was 3.0 times and the treatment time was 10 minutes. Subsequently, a stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 230°C to 330°C under an air atmosphere. In the stretch flame-retardant treatment, the stretching ratio was 2.0 times and the treatment time was 10 minutes. Subsequently, a non-stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 230°C to 350°C under an air atmosphere. In the non-stretch flame-retardant treatment, the stretching ratio was 1.0 times and the treatment time was 10 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretching ratio was 6.0 times.

[0194] [2.25] Comparative Example 12 Using carbon fiber precursor (x-8), a stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 230°C to 330°C under an air atmosphere, with a stretch ratio of 3 times and a treatment time of 10 minutes. In the stretch flame-retardant treatment, the stretch ratio was 3.0 times and the treatment time was 10 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretch ratio was 3.0 times.

[0195] [2.26] Comparative Example 13 Using carbon fiber precursor (x-9), a stretch flame-retardant treatment was performed in a continuous heat treatment furnace at 230°C to 330°C under an air atmosphere, with a stretch ratio of 3 times and a treatment time of 10 minutes. In the stretch flame-retardant treatment, the stretch ratio was 3.0 times and the treatment time was 10 minutes. This resulted in obtaining flame-retardant fiber bundles. The cumulative stretch ratio was 3.0 times.

[0196] [3]Measurement method [3.1] Fusion rate of flame-resistant fiber bundles The cross-sections of the flame-resistant fiber bundles obtained in Examples 1-13 and Comparative Examples 1-13 were observed using a microscope (Keyence Corporation's "Digital Microscope VHX-7000"). The flame-resistant fiber bundles were measured so that the total number of fused flame-resistant fibers and unfused flame-resistant fibers was 300 or more. The fusion rate was calculated according to the following formula (i). The calculation results are shown in Table 1. The acceptable range for the fusion rate is 10% or less. Formula (i): Fusion rate [%] = 100 × [Number of fused flame-resistant fibers / (Number of fused flame-resistant fibers + Number of unfused flame-resistant fibers)]

[0197] [3.2] Evaluation of carbon fiber bundles Using the flame-retardant fibers obtained in Examples 1, 4, 7-10, 12, 13, and Comparative Example 1, carbon fiber bundles were prepared and evaluated according to the following procedure.

[0198] [3.2.1] Preparation of carbon fiber bundles Flame-resistant fiber bundles were arranged in bundles of 1600 fibers each and pre-carbonized in a continuous heat treatment furnace at 800°C under a nitrogen atmosphere. The pre-carbonization process took 3 minutes. Subsequently, main carbonization was performed in a continuous heat treatment furnace at 1400°C under a nitrogen atmosphere. The main carbonization process took 3 minutes. This process yielded carbon fiber bundles.

[0199] [3.2.2] Evaluation of the tensile strength of carbon fiber monofilaments Single fibers were extracted from the obtained carbon fiber bundles to obtain samples consisting of single fibers. Tensile tests were performed on a single fiber tensile testing machine (Diastron "LEX820") (gauge between gauge marks: 4 mm, tensile speed: 2 mm / min, at room temperature). The average of the tensile strength measurements of the five samples was defined as the "tensile strength of the single fiber". The measurement results are shown in Table 1.

[0200] [3.2.3] Load-bearing capacity evaluation of carbon fiber bundles The load-bearing capacity of the obtained carbon fiber bundle was evaluated according to the following procedure. Figure 1 illustrates the method for measuring load-bearing capacity. Both ends of the mounting wire 30 were attached to weights 20. Next, as shown in Figure 1, a 100g weight 20 was suspended from the carbon fiber bundle 10 via the mounting wire 30. If the carbon fiber bundle 10 did not break, the weights 20 were increased until the carbon fiber bundle 10 broke. Specifically, one 100g weight 20 was added until the total mass of the weights 20 reached 1600g. After the total mass of the weights 20 exceeded 1600g, one 200g weight 20 was added. The mass of the weights 20 immediately before breakage was substituted into equation (ii) to calculate the load-bearing capacity of the carbon fiber bundle. The calculation results are shown in Table 1. Formula (ii): Load-bearing capacity of carbon fiber bundle [kg / mm²] 2 ] = Mass of the weight just before breaking [kg] / Cross-sectional area of ​​the carbon fiber bundle [mm] 2 ] The cross-sectional area of ​​the carbon fiber bundle was calculated using the following procedure: The cross-section of the carbon fiber bundle was observed using a microscope (a "Digital Microscope VHX-7000" manufactured by Keyence Corporation). Ten unfused carbon fibers were randomly selected from the carbon fiber bundle, and the cross-sectional area of ​​these ten selected carbon fibers was measured. The "cross-sectional area of ​​the carbon fiber bundle" was defined as the product of the average value of the measured cross-sectional areas of the ten carbon fibers and the number of fibers in the carbon fiber bundle.

[0201] [4] Results [Table 1]

[0202] In Table 1, "A" under "Textile Lubricants" refers to crosslinkable silicone lubricant A. "B" under "Textile Lubricants" refers to silicone-based lubricant B. "C" under "Textile Lubricants" refers to crosslinkable silicone lubricant C. "D" under "Textile Lubricants" refers to crosslinkable silicone lubricant D.

[0203] The carbon fiber precursors of Comparative Examples 1 to 13 contained acrylamide polymer fibers. The acrylamide polymer fibers of Comparative Examples 1 to 13 did not contain any metal elements. Therefore, the fusion rate of the flame-retardant fibers of Comparative Examples 1 to 13 was not less than 10%. As a result, it was found that the carbon fiber precursors of Comparative Examples 1 to 13 were not "carbon fiber precursors that can suppress the fusion of fibers by continuous stretching flame-retardant treatment."

[0204] The carbon fiber precursors of Examples 1 to 13 contained acrylamide polymer fibers. The acrylamide polymer fibers of Examples 1 to 13 contained an acrylamide polymer and calcium or magnesium (i.e., a metallic element). As a result, the fusion rate of the flame-retardant fibers of Examples 1 to 13 was 10% or less. Consequently, it was found that the carbon fiber precursors of Examples 1 to 13 are "carbon fiber precursors that can suppress the fusion of fibers by continuous stretching and flame-retardant treatment."

[0205] Table 1 shows that in Comparative Examples 1-13, which did not contain metal elements, the fusion rate was higher than 10%. In particular, in Comparative Examples 3, 4, 10, and 11, where the cumulative stretch ratio was 5.0 times or more, the oil layer became thinner, reducing the fusion prevention effect of the oil, resulting in a fusion rate of 50% or more. In Examples 1-13, which contained metal elements, the fusion rate was 10% or less in all cases. In particular, in Examples 6, 7, 9-13, where the cumulative stretch ratio was 5.0 times or more, the fusion rate was also 10% or less.

[0206] The fusion rate of Comparative Example 13, in which electron beam irradiation was not performed, was 90% or higher. The fusion rates of Examples 8 to 13, in which electron beam irradiation was not performed, were 10% or lower. These results show that the carbon fiber precursor of this disclosure can suppress the fusion of fibers due to stretching and flame-retardant treatment, even without electron beam irradiation.

[0207] Table 1 shows that the tensile strength of the carbon fiber in Comparative Example 1 was 1000 MPa or less. The tensile strength of the carbon fibers in Examples 1, 4, 7-10, 12, and 13 were all 1000 MPa or more. Furthermore, the load-bearing capacity of the carbon fiber in Comparative Example 1 was 1.3 kg / mm². 2 That was the case. The load-bearing capacity of the carbon fibers in Examples 1, 4, 7-10, 12, and 13 is 15 kg / mm². 2 That concludes the findings. Furthermore, a tendency was observed for the tensile strength and load-bearing capacity of the carbon fiber to increase as the cumulative stretching ratio increased.

[0208] Examples 1, 4, 7-10, 12, and 13 showed that magnesium resulted in higher load-bearing capacity of carbon fiber bundles and higher tensile strength of individual fibers compared to calcium. The reason for this result is not entirely clear, but we will consider it below. It is thought that because magnesium has a smaller atomic size than calcium, the space generated at the bonding site between the metal element and the graphite crystal is smaller in magnesium.

Claims

1. A carbon fiber precursor having acrylamide polymer fibers containing an acrylamide polymer and a metal element.

2. The carbon fiber precursor according to claim 1, wherein the content of the metal element is 0.05 parts by mass to 5.0 parts by mass per 100 parts by mass of the acrylamide polymer.

3. The carbon fiber precursor according to claim 2, wherein the metal element is an alkaline earth metal.

4. The carbon fiber precursor according to claim 3, wherein the acrylamide polymer has an anionic functional group.

5. A method for producing a carbon fiber precursor, comprising spinning a solution containing an acrylamide polymer, a metal salt, and water.

6. To obtain flame-resistant fibers by subjecting a carbon fiber precursor according to any one of claims 1 to 4 to flame-retardant treatment, The flame-resistant fiber is subjected to a carbonization treatment to obtain carbon fiber, Includes, A method for producing carbon fibers, wherein the flame-retardant treatment includes a stretch flame-retardant treatment.

7. The method for producing carbon fibers according to claim 6, further comprising pre-stretching the carbon fiber precursor before carrying out the process of obtaining the flame-resistant fibers.

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