Carbon fiber bundle, method for producing same, and carbon fiber precursor bundle

Abstract

A carbon fiber bundle according to the present disclosure comprises a plurality of carbon fibers containing an alkaline earth metal. The content of the alkaline earth metal is 0.05% to 5.0% by mass relative to the total mass of the carbon fiber bundle.

Description

Carbon fiber bundle, method for producing carbon fiber bundle, and carbon fiber precursor bundle

[0001] The present disclosure relates to a carbon fiber bundle, a method for producing a carbon fiber bundle, and a carbon fiber precursor bundle.

[0002] Carbon fiber is lightweight and has excellent mechanical properties, so carbon fiber composite materials are attracting attention as an alternative to metal materials.

[0003] As a method for producing carbon fibers, a method is known in which a fiber bundle of carbon fiber precursor fibers obtained by spinning polyacrylonitrile is subjected to a flame retardant treatment and then subjected to a carbonization treatment (Patent Documents 1 and 2).

[0004] However, polyacrylonitrile is poorly soluble in inexpensive general-purpose solvents, and therefore requires the use of expensive organic solvents (e.g., dimethyl sulfoxide and N,N-dimethylacetamide) during polymerization, spinning, and the like, resulting in a problem of increased production costs for carbon fibers.

[0005] Meanwhile, carbon fiber precursors made of acrylamide-based polymers containing acrylamide-based monomers are known (Patent Documents 3 and 4). Acrylamide-based polymers are water-soluble polymers. When polymerizing and spinning acrylamide-based polymers, water, which is inexpensive and has a low environmental impact, can be used as a solvent. This is expected to reduce the production costs of carbon fibers.

[0006] It is known that carbon fibers with excellent tensile strength can be obtained by subjecting acrylamide polymer fibers to a flame-retardant treatment, to a preliminary carbonization treatment under an inert gas atmosphere while applying a predetermined tension, and then subjecting the resulting flame-retardant fibers to a carbonization treatment at 1300°C to 1700°C (Patent Document 5). The average fiber diameter of the carbon fibers is 3 μm to 10 μm. The average value of the intensity ratio (D / G) of the Raman spectrum of the carbon fiber is 0.90 or less in a region within a circle having a diameter of 1 μm centered on the center of gravity of the cross section of the single fiber, and is 0.90 or less in a region extending from the outer periphery of the cross section of the single fiber to 1 μm inside. The intensity ratio (D / G) of the Raman spectrum is 0.90 or less in a region of 1 μm from the outer periphery of the cross section of the single fiber. －１1360 cm for the G peak due to the graphite structure in the vicinity －１ The intensity ratio of the D peak originating from defects in the graphite structure in the vicinity is shown.

[0007] However, when acrylamide-based polymer fibers are subjected to a flame-retardant treatment or when flame-retardant acrylamide-based polymer fibers are carbonized, the fibers are softened by heat. As a result, fusion of the fibers to each other has been a problem. As a method for suppressing fusion of the fibers to each other, a method of applying a self-crosslinking silicone oil to acrylamide-based polymer fibers and then performing a crosslinking treatment (Patent Documents 6 and 7) is known.

[0008] Patent Document 1: Japanese Patent No. 4543922 Patent Document 2: Japanese Patent Application Laid-Open No. 2008-202208 Patent Document 3: Japanese Patent Application Laid-Open No. 2019-26827 Patent Document 4: Japanese Patent Application Laid-Open No. 2019-167516 Patent Document 5: Japanese Patent Application Laid-Open No. 2022-143757 Patent Document 6: Japanese Patent Application Laid-Open No. 2023-64697 Patent Document 7: Japanese Patent Application Laid-Open No. 2023-174468

[0009] In fused carbon fibers, heat is not sufficiently transmitted to the center of the cross section perpendicular to the fiber axis direction (hereinafter simply referred to as "center"), and the center of the carbon fiber is not sufficiently carbonized. The tensile strength of carbon fibers (single fibers) is evaluated using unfused carbon fibers (single fibers) extracted from a carbon fiber bundle. Therefore, when a carbon fiber bundle contains a large number of fused carbon fibers, there is a risk that the load-bearing capacity of the carbon fiber bundle will be low even if the tensile strength of the single fibers is high.

[0010] In Patent Document 5, only the tensile strength of the single fiber is evaluated, and the fusion rate and load capacity of the carbon fiber bundle are not evaluated. In addition, the flame retardant treatment is performed at 350°C for a long time (i.e., 60 minutes), and the carbonization temperature is high at 1600°C or higher. Therefore, the production cost is high, and CO ２ There is also a lot of emissions.

[0011] Patent Documents 6 and 7 disclose the fusion rate of the flame-resistant fibers, but do not evaluate the load-bearing capacity of the carbon fiber bundles. Furthermore, the flame-resistant treatment and carbonization treatment are carried out in a batchwise manner with a slow temperature increase. However, if the flame-resistant treatment and carbonization treatment are carried out in a continuous heating furnace with a rapid temperature increase, the fusion rate increases. Therefore, the techniques disclosed in Patent Documents 6 and 7 alone are insufficient to suppress the fusion of carbon fibers.

[0012] An object of one embodiment of the present disclosure is to provide a carbon fiber bundle in which fusion between carbon fibers is suppressed and which has excellent load-bearing capacity, and a method for producing the carbon fiber bundle.An object of another embodiment of the present disclosure is to provide a carbon fiber precursor bundle that can be used to produce a carbon fiber bundle in which fusion between carbon fibers is suppressed and which has excellent load-bearing capacity.

[0013] As a result of extensive research to achieve the above object, the present inventors have found that by incorporating an alkaline earth metal into carbon fibers, fusion between carbon fibers in a carbon fiber bundle is suppressed and load-bearing capacity is improved.

[0014] Specific means for achieving the object are as follows. <1> A carbon fiber bundle having a plurality of carbon fibers containing an alkaline earth metal, wherein the content of the alkaline earth metal is 0.05% by mass to 5.0% by mass with respect to the total amount of the carbon fiber bundle. <2> The carbon fiber bundle according to <1>, wherein the degree of crystal orientation of the carbon fiber bundle is 60% or more. <3> The carbon fiber bundle according to <1> or <2>, wherein the alkaline earth metal contains at least one of magnesium and calcium. <4> The carbon fiber bundle according to any one of <1> to <3>, wherein the fusion rate of the carbon fiber bundle is 15% or less. <5> A method for producing a carbon fiber bundle, comprising: subjecting a carbon fiber precursor bundle to a flame-resistant treatment to obtain a flame-resistant fiber bundle; and subjecting the flame-resistant fiber bundle to a carbonization treatment to obtain a carbon fiber bundle, wherein the carbon fiber precursor bundle contains a plurality of acrylamide-based polymer fibers containing an alkaline earth metal. <6> The method for producing the carbon fiber bundle according to <5>, comprising obtaining the acrylamide-based polymer fiber by spinning a solution containing an acrylamide-based polymer, the alkaline earth metal salt, and water. <7> A carbon fiber precursor bundle having acrylamide-based polymer fibers containing an acrylamide-based polymer and an alkaline earth metal.

[0015] According to one embodiment of the present disclosure, there are provided a carbon fiber bundle in which fusion between carbon fibers is suppressed and which has excellent load-bearing capacity, and a method for manufacturing the carbon fiber bundle. According to another embodiment of the present disclosure, there is provided a carbon fiber precursor bundle which can be made into a carbon fiber bundle in which fusion between carbon fibers is suppressed and which has excellent load-bearing capacity.

[0016] FIG. 1 is a diagram illustrating a method for measuring the withstand load in the embodiment.

[0017] In the present disclosure, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In numerical ranges described in stages in the present disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in numerical ranges described in the present disclosure, the upper or lower limit value of that numerical range may be replaced with the value shown in the synthesis examples, production examples, or working examples.

[0018] In the present disclosure, each component may contain multiple corresponding substances. When multiple substances corresponding to each component are present in the carbon fiber precursor bundle and the carbon fiber bundle, the content or amount of each component means the total content or amount of the multiple substances present in the carbon fiber precursor bundle and the carbon fiber bundle, unless otherwise specified.

[0019] (1) Carbon Fiber Bundle The carbon fiber bundle of the present disclosure includes a plurality of carbon fibers containing an alkaline earth metal. The content of the alkaline earth metal is 0.05 mass % to 5.0 mass % relative to the total amount of the carbon fiber bundle.

[0020] The carbon fiber bundle of the present disclosure has the above-described configuration, thereby suppressing fusion between carbon fibers and exhibiting excellent load-bearing capacity. This effect is presumably due to, but not limited to, the following reasons. Carbon fiber bundles are typically obtained by subjecting a carbon fiber precursor bundle to a flame-resistant treatment to obtain a flame-resistant fiber bundle having a cyclic structure (e.g., a nitrogen-containing heterocycle, an imide ring, an acridone ring, etc.), and then subjecting the resulting flame-resistant fiber bundle to a carbonization treatment. The carbonization treatment results in the formation of graphite crystals through a reaction between the cyclic structures. Because alkaline earth metals interact with acrylamide-based polymers (e.g., by forming coordinate bonds between amide groups and alkaline earth metals), polymer chains of the acrylamide-based polymers are more likely to approach each other. Therefore, the formation of cyclic structures and crosslinked structures is promoted during the flame-resistant treatment. Furthermore, because the alkaline earth metal interacts with the cyclic structure formed during the flame-resistant treatment (for example, due to the formation of a coordinate bond between a nitrogen-containing heterocycle, an imide ring, or an acridone ring and an alkaline earth metal), the distance between the cyclic structures becomes shorter. This promotes graphite crystal formation and the interaction between the graphite crystals during the carbonization treatment. Therefore, the carbon fiber precursor and the flame-resistant fiber are less likely to melt. As a result, a carbon fiber bundle is obtained in which fusion between carbon fibers is suppressed. Furthermore, it is believed that the promotion of graphite crystal formation and the interaction between graphite crystals also improves the tensile strength of the carbon fiber and the load-bearing capacity of the carbon fiber bundle. Furthermore, fused carbon fibers have difficulty in transmitting heat to the center of the carbon fiber, and the center of the carbon fiber is not sufficiently carbonized. The tensile strength of a single fiber is evaluated using unfused carbon fibers extracted from the carbon fiber bundle. Therefore, if a carbon fiber bundle contains a large number of fused carbon fibers, the load-bearing capacity of the carbon fiber bundle will be low even if the tensile strength of the carbon fiber (single fiber) is high. Therefore, it is considered that the load-bearing capacity of the carbon fiber bundle can be improved by reducing the number of fused carbon fibers in the carbon fiber bundle. As a result, it is presumed that the carbon fiber bundle of the present disclosure has suppressed fusion between carbon fibers and is excellent in load-bearing capacity.

[0021] The carbon fiber bundle has a plurality of carbon fibers. The carbon fiber bundle may consist of only a plurality of carbon fibers. The carbon fiber bundle may also contain known fibers other than carbon fibers.

[0022] The number of carbon fibers (single fibers) per bundle of carbon fiber bundles (hereinafter also referred to as the "number of filaments") is not particularly limited. From the viewpoint of high productivity and improved mechanical properties, the number of filaments is preferably 10 to 240,000, more preferably 20 to 144,000, and even more preferably 30 to 72,000.

[0023] The fusion rate of the carbon fiber bundle is preferably 15% or less. When the fusion rate of the carbon fiber bundle is 15% or less, the load-bearing capacity of the carbon fiber bundle is improved. From the viewpoints of the load-bearing capacity of the carbon fiber bundle and adhesion when composited with a resin, the fusion rate of the carbon fiber bundle is more preferably 13% or less, even more preferably 11% or less, and particularly preferably 9% or less. The method for measuring the fusion rate of the carbon fiber bundle is the same as the method described in the Examples.

[0024] The tensile strength of the unfused carbon fibers contained in the carbon fiber bundle (hereinafter also referred to as "single fiber tensile strength") is preferably more than 1000 MPa, more preferably 1300 MPa or more, even more preferably 1500 MPa or more, particularly preferably 1800 MPa or more, and most preferably 2000 MPa or more. The single fiber tensile strength may be 10000 MPa or less. The method for measuring the single fiber tensile strength is the same as the method described in the Examples.

[0025] The load capacity of the carbon fiber bundle (hereinafter also simply referred to as "load capacity") is preferably 15 kg / mm ２ More preferably, 20 kg / mm ２ More preferably, 30 kg / mm ２ More preferably, 45 kg / mm ２ More than 48 kg / mm ２ That's all. The load capacity is 1000 kg / mm ２ The method for measuring the load capacity is the same as that described in the examples.

[0026] (1.1) Carbon Fiber Carbon fiber is a fiber whose main component is carbon. The phrase "mainly composed of carbon" means that the carbon content is 90 mass % or more of the total amount of carbon fiber.

[0027] The carbon fiber contains an alkaline earth metal. The alkaline earth metal may be used alone or in combination of two or more kinds.

[0028] Examples of alkaline earth metals include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Among these, the alkaline earth metal preferably contains at least one of magnesium and calcium. When the alkaline earth metal contains at least one of magnesium and calcium, fusion between carbon fibers is suppressed, and a carbon fiber bundle with excellent load-bearing capacity can be obtained. The alkaline earth metal may be at least one of magnesium and calcium, or may contain magnesium, or may contain calcium.

[0029] The alkaline earth metal content is 0.05% by mass to 5.0% by mass relative to the total amount of the carbon fiber bundle. If the alkaline earth metal content is less than 0.05% by mass, there is a risk of increased fusion of the carbon fiber bundle and a decrease in the load-bearing capacity. If the alkaline earth metal content exceeds 5.0% by mass, there is a risk of decreased single fiber tensile strength and a decrease in the load-bearing capacity of the carbon fiber bundle. From the viewpoints of the single fiber tensile strength and the load-bearing capacity of the carbon fiber bundle, the alkaline earth metal content is preferably 0.06% by mass or more, more preferably 0.08% by mass or more, and even more preferably 0.10% by mass or more. From the viewpoints of the single fiber tensile strength and the load-bearing capacity of the carbon fiber bundle, the alkaline earth metal content is preferably 3.0% by mass or less, more preferably 2.0% by mass or less, even more preferably 1.0% by mass or less, and particularly preferably 0.6% by mass or less. The alkaline earth metal content is measured by the same method as that described in the Examples.

[0030] The degree of crystalline orientation of the carbon fiber bundle is not particularly limited, but is preferably 60% or more. "Crystalline orientation degree" quantitatively indicates the degree to which graphite crystals constituting the carbon fiber are aligned in the fiber axis direction. A higher crystalline orientation degree indicates that the graphite crystals are aligned in the fiber axis direction. A carbon fiber bundle with a crystalline orientation degree of 60% or more has better single fiber tensile strength and load-bearing capacity than a carbon fiber bundle with a crystalline orientation degree of less than 60%. From the viewpoints of single fiber tensile strength, single fiber tensile modulus, and load-bearing capacity of the carbon fiber bundle, the degree of crystalline orientation of the carbon fiber bundle is more preferably 62% or more, even more preferably 63% or more, and particularly preferably 64% or more. From the viewpoints of single fiber tensile strength and load-bearing capacity of the carbon fiber bundle, the degree of crystalline orientation of the carbon fiber bundle may be 99% or less, or may be 95% or less. The method for measuring the crystalline orientation degree of the carbon fiber bundle is the same as that described in the Examples.

[0031] 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 the load-bearing capacity of the carbon fiber bundle. When the average fiber diameter of the carbon fibers is 3 μm or more, insufficient impregnation of the resin or the like into the carbon fiber bundle is unlikely to occur when a composite material is produced using a resin or the like as a matrix, and the tensile strength of the composite material is improved. Furthermore, the influence of surface defects of the carbon fibers and the like on the single fiber tensile strength and the load-bearing capacity of the carbon fiber bundle tends to be reduced. When the average fiber diameter of the carbon fibers is 10 μm or less, a decrease in the single fiber tensile strength and the load-bearing capacity of the carbon fiber bundle due to insufficient carbonization of the center of the carbon fiber tends to be unlikely to occur.

[0032] (1.2) Preferred Aspects The carbon fiber bundle of the present disclosure preferably satisfies the first condition. The "first condition" indicates that the degree of crystalline orientation of the carbon fiber bundle is 60% or more. When the carbon fiber bundle satisfies the first condition, the single fiber tensile strength and load capacity are superior to those in cases where the first condition is not satisfied.

[0033] The carbon fiber bundle of the present disclosure preferably satisfies the first and second conditions. The "second condition" indicates that the alkaline earth metal includes at least one of magnesium and calcium. When the carbon fiber bundle satisfies the first and second conditions, the single fiber tensile strength and load capacity are superior to those in which the first and second conditions are not satisfied.

[0034] The carbon fiber bundle of the present disclosure preferably satisfies the first to third conditions. The "third condition" indicates that the fusion rate of the carbon fiber bundle is 15% or less. When the carbon fiber bundle satisfies the first to third conditions, the load capacity is superior to when the first to third conditions are not satisfied.

[0035] (2) Manufacturing Method of Carbon Fiber Bundle The manufacturing method of the carbon fiber bundle of the present disclosure includes: subjecting a carbon fiber precursor bundle to a flame-resistant treatment to obtain a flame-resistant fiber bundle (hereinafter also referred to as a "flame-resistant treatment step"); and subjecting the flame-resistant fiber bundle to a carbonization treatment to obtain a carbon fiber bundle (hereinafter also referred to as a "carbonization treatment step"). The carbon fiber precursor bundle includes a plurality of acrylamide-based polymer fibers containing alkaline earth metals (hereinafter also simply referred to as "acrylamide-based polymer fibers"). The flame-resistant treatment step and the carbonization treatment step are performed in this order.

[0036] The term "carbon fiber precursor bundle" refers to a raw material for producing a carbon fiber bundle that has not been subjected to either a flame-retardant treatment or a carbonization treatment. The term "acrylamide-based polymer" refers to a homopolymer of an acrylamide-based monomer or a copolymer of an acrylamide-based monomer and a monomer other than an acrylamide-based monomer. The term "acrylamide-based polymer fiber" refers to a fiber containing an acrylamide-based polymer. Specifically, the ratio of the acrylamide-based polymer to the total amount of the acrylamide-based polymer fiber is 30% by mass or more, and may be 50% by mass to 99.95% by mass, 80% by mass to 99.9% by mass, or 90% by mass to 99% by mass. The term "flame-retardant treatment" refers to subjecting a carbon fiber precursor bundle to a heat treatment in an oxidizing gas atmosphere. The heating temperature in the flame-retardant treatment is within a range of 120°C to 500°C. The carbon fiber precursor bundle may be stretched during the flame-proofing treatment, or may be stretched (hereinafter also referred to as a "pre-stretching treatment step") before the flame-proofing treatment step. "Stretching treatment" refers to a treatment in which the carbon fiber precursor bundle is heated to plasticize it and stretch it. When the pre-stretching treatment step, flame-proofing treatment step, carbonization treatment step, or the like is performed continuously (i.e., when the fiber bundle is subjected to the pre-stretching treatment, flame-proofing treatment, carbonization treatment, or the like by transporting the fiber bundle through a heat treatment device using multiple rollers), the "stretch ratio" can be determined by the ratio (pulling speed / introduction speed). The ratio (pulling speed / introduction speed) refers to the ratio of the pulling speed of the fiber bundle emerging from the heat treatment furnace (hereinafter also referred to as the "pulling speed") to the feed speed of the fiber bundle introduced into the heat treatment furnace (hereinafter also referred to as the "introduction speed"). The "stretch ratio" can also be determined by the ratio (length of drawn fiber / length of introduced fiber). The ratio (length of drawn fiber / length of introduced fiber) indicates the ratio of the length of the fiber introduced into the heat treatment furnace (i.e., "introduced fiber") to the length of the fiber emerging from the heat treatment furnace (i.e., "drawn fiber"). Examples of methods for adjusting the draw ratio include a method of adjusting the ratio (V2 / V1) of the speed (V1) of the roller at the inlet of the heat treatment device to the speed (V2) of the roller at the outlet of the heat treatment device, and a method of adjusting the tension applied to the carbon fiber precursor using a weight, a spring, or the like.The ratio (V2 / V1) is synonymous with the draw ratio. The roller is not particularly limited, and examples thereof include a feed roller and a nip roller. The term "carbonization treatment" refers to a treatment for carbonizing the flame-resistant fiber of the carbon fiber precursor bundle. Specifically, the term "carbonization treatment" refers to a heat treatment of the flame-resistant fiber bundle in a low-oxygen atmosphere (preferably an environment in which oxygen is blocked).

[0037] The method for producing a carbon fiber bundle according to the present disclosure has the above-described configuration, and therefore can produce the carbon fiber bundle according to the present disclosure (i.e., carbon fibers in which fusion between carbon fibers is suppressed and which have excellent load-bearing capacity).

[0038] (2.1) Preparation Step The method for producing a carbon fiber bundle according to the present disclosure may further include preparing a carbon fiber precursor bundle (hereinafter also referred to as the "preparation step"). The carbon fiber precursor bundle includes a plurality of acrylamide-based polymer fibers. The preparation step is performed before the flame-resistant treatment step is performed.

[0039] (2.1.1) Carbon fiber precursor bundle (2.1.1.1) Acrylamide-based polymer fiber The carbon fiber precursor bundle includes a plurality of acrylamide-based polymer fibers containing an alkaline earth metal. The carbon fiber precursor bundle may include known fibers other than acrylamide-based polymer fibers. The carbon fiber precursor bundle may be composed only of a plurality of acrylamide-based polymer fibers.

[0040] The number of acrylamide polymer fibers (single fibers) per bundle of carbon fiber precursor bundle (i.e., the number of filaments) is not particularly limited. From the viewpoints of high productivity and improved mechanical properties, the number of filaments is preferably 10 to 240,000, more preferably 20 to 144,000, and even more preferably 30 to 72,000.

[0041] The average fiber diameter of the acrylamide-based polymer fiber is not particularly limited, and from the viewpoints of the single fiber tensile strength and the load-bearing capacity of the carbon fiber precursor bundle, it is preferably 10 μm to 80 μm, more preferably 14 μm to 40 μm, and even more preferably 18 μm to 30 μm.

[0042] (2.1.1.1.1) Alkaline earth metal acrylamide polymer fibers contain alkaline earth metals. The alkaline earth metals may be used alone or in combination of two or more.

[0043] Examples of alkaline earth metals include those exemplified as alkaline earth metals contained in the carbon fiber bundle. Among these, the alkaline earth metal preferably contains at least one of magnesium and calcium, similar to the alkaline earth metal contained in the carbon fiber bundle. The alkaline earth metal content is not particularly limited, and may be appropriately adjusted to an amount such that the alkaline earth metal content of the carbon fiber falls within the range exemplified above. From the viewpoints of suppressing fusion of the carbon fiber precursor bundle and the carbon fiber, tensile strength, and load-bearing property, the alkaline earth metal content of the acrylamide-based polymer fiber is preferably 0.05% by mass to 5.0% by mass, more preferably 0.1% by mass to 3.0% by mass, and even more preferably 0.1% by mass to 1.5% by mass, relative to the total amount of the acrylamide-based polymer fiber.

[0044] (2.1.1.1.2) Acrylamide-Based Polymer Acrylamide-based polymer fibers typically contain an acrylamide-based polymer.

[0045] The acrylamide polymer may be a homopolymer of an acrylamide monomer, a copolymer of an acrylamide monomer and a monomer other than an acrylamide monomer (hereinafter also referred to as "other polymerizable monomer"), or a mixture of a homopolymer of an acrylamide monomer and a copolymer of an acrylamide monomer and another polymerizable monomer.

[0046] It is believed that when the acrylamide-based polymer fiber contains an acrylamide-based polymer and an alkaline earth metal, the amide groups contained in the acrylamide-based polymer and the alkaline earth metal form coordinate bonds or the like. This facilitates the proximity of polymer chains of the acrylamide-based polymer, which is thought to promote the formation of cyclic structures or crosslinked structures due to the flame-retardant treatment. Furthermore, since the acrylamide-based polymer fiber has coordinate bonds or the like between polymer chains (i.e., has a crosslinked structure), it is thought that the acrylamide-based polymer is less likely to melt even when heated at a temperature above the glass transition temperature Tg (approximately 160°C) of the acrylamide-based polymer. As a result, fusion of fibers due to the continuous pre-drawing treatment and flame-retardant treatment can be further suppressed.

[0047] The acrylamide polymer is preferably an acrylamide polymer having an anionic functional group. It is believed that when the acrylamide polymer fiber contains an acrylamide polymer having an anionic functional group and an alkaline earth metal, the ionized alkaline earth metal and the anionic functional group of the acrylamide polymer form an ionic bond. This facilitates the proximity of polymer chains of the acrylamide polymer, which is believed to promote the formation of cyclic structures or crosslinked structures through the flame-retardant treatment. Furthermore, because the acrylamide polymer fiber has ionic bonds between polymer chains (i.e., crosslinked structures), the acrylamide polymer is unlikely to melt even when heated at a temperature above the glass transition temperature Tg (approximately 160°C) of the acrylamide polymer. As a result, fusion of fibers during the continuous pre-stretching and flame-retardant treatment can be further suppressed.

[0048] The term "anionic functional group" refers to a functional group that can form an anion (negative ion) in the presence of water. The temperature at which the anion is formed in the presence of water is not particularly limited. The anionic functional group may be a functional group that can form an anion in the presence of water at a high temperature.

[0049] The content of structural units derived from acrylamide-based monomers (hereinafter also referred to as "acrylamide-based 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 structural units of the acrylamide-based polymer. Having an acrylamide-based monomer unit content of 30 mol% or more tends to improve the solubility of the acrylamide-based polymer in aqueous solvents or aqueous mixed solvents. The acrylamide-based monomer unit content may be 100 mol%, but from the viewpoint of fusion suppression and the like, it is preferably 99.9 mol% or less, more preferably 99.0 mol% or less, even more preferably 95.0 mol% or less, particularly preferably 90 mol% or less, and most preferably 85 mol% or less, relative to the total amount of structural units of the acrylamide-based polymer. The acrylamide-based monomer unit content is preferably 30 mol% to 99.9 mol%.

[0050] The content of structural units derived from other polymerizable monomers (hereinafter also referred to as "other polymerizable monomer units") is preferably 0.1 mol% or more, more preferably 1.0 mol% or more, even more preferably 5.0 mol% or more, particularly preferably 10.0 mol% or more, and most preferably 15.0 mol% or more, based on the total amount of structural units of the acrylamide-based polymer, from the viewpoint of fusion suppression, etc. From the viewpoint of improving the solubility of the acrylamide-based 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, based on the total amount of structural units of the acrylamide-based polymer. The content of other polymerizable monomer units is preferably 0.1 mol% to 70 mol%.

[0051] The composition ratio of the acrylamide polymer is: １３ It can be measured by C-NMR, infrared absorption spectroscopy (IR), or the like.

[0052] The weight-average molecular weight of the acrylamide-based polymer is not particularly limited, and is usually 5,000,000 or less. From the viewpoint of the manufacturing processability of the carbon fiber precursor bundle, the weight-average molecular weight of the acrylamide-based polymer is preferably 2,000,000 or less, more preferably 1,000,000 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-based polymer is not particularly limited, and is usually 10,000 or more. From the viewpoint of the tensile strength of the carbon fiber precursor bundle and the carbon fiber bundle, the weight-average molecular weight of the acrylamide-based polymer is preferably 20,000 or more, more preferably 30,000 or more, and even more preferably 40,000 or more.

[0053] In the present disclosure, the weight average molecular weight is measured by gel permeation chromatography under the following conditions. The measurement device may be an "HLC-8220GPC" manufactured by Tosoh Corporation or a device equivalent thereto. (Measurement conditions) Column: 2 TSKgel GMPWXL columns + 1 TSKgel G2500PWXL column Eluent: 100 mM aqueous sodium nitrate solution / acetonitrile (= 80 / 20 (volume ratio)) Eluent flow rate: 1.0 ml / min Column temperature: 40°C Molecular weight standards: standard polyethylene oxide / standard polyethylene glycol Detector: differential refractive index detector

[0054] (2.1.1.1.2.1) Acrylamide Monomers Examples of acrylamide monomers include acrylamide, ethacrylamide, crotonamide, itaconic acid diamide, cinnamic acid amide, maleic acid diamide, N-alkylacrylamides such as N-methylacrylamide, N-ethylacrylamide, N-n-propylacrylamide, N-isopropylacrylamide, N-n-butylacrylamide, and N-tert-butylacrylamide, N-cycloalkylacrylamides such as N-cyclohexylacrylamide, dialkylacrylamides such as N,N'-dimethylacrylamide, dialkylaminoalkylacrylamides such as dimethylaminoethylacrylamide and dimethylaminopropylacrylamide, hydroxyalkylacrylamides such as N-(hydroxymethyl)acrylamide and N-(hydroxyethyl)acrylamide, N-arylacrylamides such as N-phenylacrylamide, and N,N'-alkylenebisacrylamides such as diacetoneacrylamide and N,N'-methylenebisacrylamide, N-alkyl methacrylamides such as methacrylamide, N-methyl methacrylamide, N-ethyl methacrylamide, N-n-propyl methacrylamide, N-isopropyl methacrylamide, N-n-butyl methacrylamide, and N-tert-butyl methacrylamide; N-cycloalkyl methacrylamides such as N-cyclohexyl methacrylamide; dialkyl methacrylamides such as N,N-dimethyl methacrylamide; dialkylaminoalkyl methacrylamides such as dimethylaminoethyl methacrylamide and dimethylaminopropyl methacrylamide; hydroxyalkyl methacrylamides such as N-(hydroxymethyl) methacrylamide and N-(hydroxyethyl) methacrylamide; N-aryl methacrylamides such as N-phenyl methacrylamide; and N,N'-alkylene bismethacrylamides such as diacetone methacrylamide and N,N'-methylene bismethacrylamide.From the viewpoint of the solubility of the acrylamide polymer in an aqueous solvent or an aqueous mixed solvent, among the above acrylamide monomers, acrylamide, N-alkylacrylamide, dialkylacrylamide, methacrylamide, N-alkylmethacrylamide, or dialkylmethacrylamide is preferred, and acrylamide is more preferred. The acrylamide monomers may be used alone or in combination of two or more.

[0055] (2.1.1.1.2.2) Other Polymerizable Monomers The other polymerizable monomers include vinyl monomers having an anionic functional group.

[0056] (2.1.1.1.2.2.1) Anionic Vinyl Monomers Anionic vinyl monomers have anionic functional groups. Examples of anionic functional groups include carboxyl groups, sulfo groups, phosphate groups (phosphate groups also include phosphonate groups, phosphite groups, phosphinate groups, and phosphinous acid groups), nitro groups, and phenolic hydroxyl groups. Examples of anionic vinyl monomers include unsaturated carboxylic acids and their salts, sulfonic acid vinyl monomers and their salts, phosphate vinyl monomers and their salts, nitric acid vinyl monomers and their salts, and phenolic vinyl monomers and their salts. Examples of the unsaturated carboxylic acids include unsaturated carboxylic acid anhydrides that form unsaturated carboxylic acids in the presence of water, and unsaturated carboxylic acid precursors (e.g., unsaturated carboxylic acid esters) that form unsaturated carboxylic acids in the presence of at least one of high temperature and acid. Examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and its derivatives (e.g., maleic acid monoester, etc.), fumaric acid and its derivatives (e.g., fumaric acid monoester, etc.), itaconic acid and its derivatives (e.g., itaconic acid monoester, etc.), citraconic acid and its derivatives (e.g., citraconic acid monoester, etc.), mesaconic acid and its derivatives (e.g., mesaconic acid monoester, etc.), crotonic acid, and isocrotonic acid. Examples of salts of unsaturated carboxylic acids include metal salts (e.g., sodium salts, potassium salts, magnesium salts, calcium salts, etc.), ammonium salts, and amine salts of unsaturated carboxylic acids. Examples of unsaturated carboxylic acid precursors include maleic acid anhydride, itaconic acid anhydride, citraconic acid anhydride, acrylic acid esters (e.g., tert-butyl acrylate, etc.), and methacrylic acid esters (e.g., tert-butyl methacrylate, etc.). Examples of sulfonic acid vinyl monomers include acrylamido tertiary butyl sulfonic acid, vinyl sulfonic acid, vinylbenzene sulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid, styrene sulfonic acid, isoprene sulfonic acid, allyl sulfonic acid, and methacrylic sulfonic acid.Salts of sulfonic acid vinyl monomers include metal salts of sulfonic acid vinyl monomers (e.g., sodium salts, potassium salts, magnesium salts, calcium salts, etc.), ammonium salts, and amine salts. Phosphate vinyl monomers include vinyl phosphoric acid, 2-acryloyloxyethyl acid phosphate, 2-methacryloyloxyethyl acid phosphate, 2-methacryloyloxyethyl acid phosphate, vinylphosphonic acid, vinyl phosphorous acid, vinylphosphinic acid, phenylvinylphosphinic acid, and vinylphosphinic acid. Salts of phosphoric acid vinyl monomers include metal salts of phosphoric acid vinyl monomers (e.g., sodium salts, potassium salts, magnesium salts, calcium salts, etc.), ammonium salts, and amine salts. Nitric acid vinyl monomers include vinyl nitrate and 2-nitrovinylbenzene. Salts of nitric acid vinyl monomers include metal salts of nitric acid vinyl monomers (e.g., sodium salts, potassium salts, magnesium salts, calcium salts, etc.), ammonium salts, and amine salts. Examples of the phenolic vinyl monomer include 2-vinylphenol and 4-vinylphenol. Examples of the salts of the phenolic vinyl monomer include metal salts of the phenolic vinyl monomer (e.g., sodium salt, potassium salt, magnesium salt, calcium salt, etc.), ammonium salts, and amine salts. The anionic vinyl monomers may be used alone or in combination of two or more.

[0057] Among the above anionic vinyl monomers, from the viewpoint of polymerizability with the acrylamide polymer, unsaturated carboxylic acids and their salts are preferred, and acrylic acid, maleic acid, fumaric acid, or itaconic acid is more preferred. Among the above anionic vinyl monomers, from the viewpoint of fusion suppression, unsaturated carboxylic acids are preferred, and acrylic acid, maleic acid, fumaric acid, or itaconic acid is more preferred.

[0058] The content of structural 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 structural units of the acrylamide polymer. This allows for improved ionic bond formation and heat resistance while maintaining the solubility of the acrylamide polymer in aqueous solvents or aqueous mixed solvents. From the viewpoints of ionic bond formation, fusion suppression, and promotion of the flame retardant reaction, the content of the 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 the 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.

[0059] (2.1.1.1.2.2.2) Vinyl Monomers Not Having Anionic Functional Groups In addition to the anionic vinyl monomers, the other polymerizable monomers may include vinyl monomers not having an anionic functional group (hereinafter also referred to as "non-anionic vinyl monomers").

[0060] Examples of the non-anionic vinyl monomer include vinyl cyanide monomers, aromatic vinyl monomers, vinyl halide monomers, vinyl alcohol monomers, vinyl carboxylate monomers, and olefin monomers.

[0061] Examples of vinyl cyanide monomers include acrylonitrile, methacrylonitrile, 2-hydroxyethyl acrylonitrile, chloroacrylonitrile, chloromethyl acrylonitrile, ethoxyacrylonitrile, and vinylidene cyanide.

[0062] Examples of aromatic vinyl monomers include styrene and α-methylstyrene. Examples of halogenated vinyl monomers include vinyl chloride. Examples of vinyl alcohol monomers include vinyl alcohol. Examples of vinyl carboxylate monomers include vinyl acetate and vinyl propionate. Examples of olefin monomers include ethylene, propylene, isopropylene, and butadiene. The non-anionic vinyl monomers may be used alone or in combination of two or more.

[0063] Among the above-mentioned non-anionic vinyl monomers, from the viewpoint of suppressing fusion and promoting the flame retardation reaction, vinyl cyanide monomers are preferred, acrylonitrile, methacrylonitrile, chloroacrylonitrile, or chloromethylacrylonitrile are more preferred, acrylonitrile, chloroacrylonitrile, or methacrylonitrile are still more preferred, acrylonitrile or methacrylonitrile are particularly preferred, and acrylonitrile is most preferred.

[0064] The content of the structural units derived from non-anionic vinyl monomers among other polymerizable monomers may be 0 mol% relative to the total amount of structural units of the acrylamide polymer. When the acrylamide polymer contains structural units derived from non-anionic vinyl monomers, the content of the structural units derived from non-anionic vinyl monomers relative to the total amount of structural units of the acrylamide polymer 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%.

[0065] (2.1.1.1.3) Additives The acrylamide-based polymer fiber may further contain additives.

[0066] Examples of additives include surfactants, crosslinking agents, flame retardant accelerators (e.g., a group consisting of acids and their salts), various fillers (e.g., glass fiber, cellulose nanofiber, cellulose microfiber, carbon black, carbon nanotubes, graphene, etc.), dispersants, smoothing agents, moisture absorbents, viscosity modifiers, plasticizers, release agents, spreading agents, antioxidants, antibacterial agents, preservatives, rust inhibitors, and pH adjusters. The content of the additives can be adjusted as appropriate.

[0067] When the acrylamide polymer fiber contains the flame retardant accelerator, the formation of cyclic structures due to various reactions (e.g., dehydration reaction, deammonia reaction, etc.) is accelerated during flame retardation treatment, and the fusion suppression, carbonization yield, strength, and shape stability of the carbon fiber tend to be further improved. The carbon fiber may contain at least one of the flame retardant accelerator and its residue.

[0068] Examples of the flame retardant accelerator include inorganic acids (e.g., phosphoric acid, polyphosphoric acid, boric acid, sulfuric acid, nitric acid, and carbonic acid), ammonium salts of inorganic acids, amine salts of inorganic acids, organic acids (e.g., oxalic acid, citric acid, and sulfonic acid), ammonium salts of organic acids, and amine salts of organic acids. Among these, from the viewpoints of fusion suppression, carbonization yield, strength, and shape stability of the carbon fiber, phosphoric acid, polyphosphoric acid, boric acid, sulfuric acid, or ammonium salts thereof is preferred as the flame retardant accelerator, phosphoric acid, polyphosphoric acid, boric acid, or ammonium salts thereof is more preferred, and monoammonium phosphate or phosphoric acid is even more preferred.

[0069] 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, still more preferably 0.5 to 30 parts by mass, and particularly preferably 1 to 20 parts by mass, relative to 100 parts by mass of the acrylamide polymer, from the viewpoints of fusion suppression, carbonization yield, strength and shape stability of the carbon fiber.

[0070] (2.1.1.1.4) Other Polymers The acrylamide-based polymer fibers of the present disclosure may further contain other polymers different from the acrylamide-based polymer.

[0071] From the viewpoints of fusion suppression, water resistance, and the like, the content of the acrylamide-based polymer relative to the total mass of polymers in the acrylamide-based polymer fiber 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.

[0072] The other polymer is not particularly limited, but is preferably a polymer that dissolves or disperses in an aqueous solvent or an aqueous mixed solvent, more preferably a polymer that dissolves in an aqueous solvent or an aqueous mixed solvent, and even more preferably a water-soluble polymer. Examples of the other polymer include a polymer obtained by (co)polymerizing at least one selected from a group consisting of specific monomers, and a hydrophilic polysaccharide polymer (acetyl cellulose, xanthan gum, etc.). Specific examples of the monomer include unsaturated carboxylic acids (e.g., acrylic acid, methacrylic acid, etc.) and their salts, unsaturated carboxylic acid esters (e.g., methyl acrylate, etc.), vinyl carboxylates (e.g., vinyl alcohol, vinyl acetate, etc.), vinylpyrrolidone, vinyl sulfonic acid and its salts, vinyl sulfonate esters (e.g., methyl vinyl sulfonate, etc.), vinylbenzenesulfonic acid and its salts, vinylbenzenesulfonate esters, carboxyvinyl, polyalkylene glycols (e.g., ethylene glycol, propylene glycol, etc.), unsaturated carboxylic acid anhydrides (e.g., maleic anhydride, etc.), unsaturated dicarboxylic acids (e.g., maleic acid, fumaric acid, etc.) and their salts, unsaturated dicarboxylic acid esters (e.g., monomethyl maleate, dimethyl maleate, etc.), 2-acrylamido-2-methylpropanesulfonic acid and its salts, and 2-acrylamido-2-methylpropanesulfonic acid esters.

[0073] (2.1.1.2) Oil layer The carbon fiber precursor bundle may further have an oil layer attached to the surface of each of a plurality of acrylamide-based polymer fibers. When the carbon fiber precursor bundle further has an oil layer, the carbon fiber precursor bundle becomes easier to bundle, and the generation of fluff and the like can be suppressed. In addition, fusion of fibers due to the preliminary drawing treatment and the flame-resistant treatment can be further suppressed.

[0074] The "oil layer" refers to a layer derived from an oil for fibers. The oil for fibers may be a known one. Examples of the oil for fibers include 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, and mineral oil-based oils. From the viewpoint of suppressing fusion of the carbon fiber precursor, it is preferable that the oil layer contains a silicone-based oil. The oil for fibers may contain multiple oils. The oil for fibers may further contain a diluent, an antistatic agent, a smoothing agent, a moisture absorbent, a surfactant, a viscosity modifier, a release agent, a spreading agent, an antibacterial agent, an antioxidant, a crosslinking agent, or a preservative, as necessary.

[0075] The silicone-based oil contains at least one of a reactive silicone and a non-reactive silicone. "Reactive silicone" refers to a silicone having a reactive functional group. "Non-reactive silicone" refers to a silicone not having a reactive functional group. It is preferable that the silicone-based oil contains a reactive silicone.

[0076] The oil layer may be present on at least a portion of the surface of the acrylamide polymer fiber, but from the viewpoint of further suppressing fusion of the acrylamide polymer fibers during the flame-resistant treatment while maintaining a higher carbonization yield, it is preferable that the oil layer be attached to the entire surface of the acrylamide polymer fiber.

[0077] The content of the oil agent layer is not particularly limited, and 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, relative to 100 parts by mass of the acrylamide polymer fibers.

[0078] (2.1.1.2.1) Reactive Silicone Reactive silicone has a reactive functional group. The term "reactive functional group" refers to a functional group that reacts with another functional group to form a bond in response to an external stimulus (e.g., electron beam, ultraviolet light, moisture, heat, etc.).

[0079] The reactive silicone may have a polysiloxane structure as a basic structure, and at least some of the substituents on the side chains and ends of the polysiloxane may be substituted with reactive functional groups. One type of reactive silicone may be used alone, or two or more types may be used in combination.

[0080] Examples of polysiloxanes include polydimethylsiloxane, polymethylphenylsiloxane, polydiphenylsiloxane, polymethylhydrogensiloxane, and mixtures thereof. Examples of substituents include alkyl groups (e.g., methyl, ethyl, and propyl groups) and aryl groups (e.g., phenyl and methylphenyl groups).

[0081] Examples of the reactive functional group include an amino group, an epoxy group, an alicyclic epoxy group, a carboxy group, a carboxylic acid anhydride group, a hydroxy group (carbinol group), a vinyl group, a vinylcarbonyl group, a vinyl ester group, an acryloyl group, an acrylamide group, an allyl group, an allyl ether group, a 4-vinylbenzene group, a 4-allylbenzene group, an isopropenyl group, a methacryloyl group, a methacrylamide group, a 4-isopropenylbenzene group, a maleimide group, a fumaric acid ester group, and a fumaramide group. Among these, from the viewpoint of affinity and reactivity with an acrylamide-based polymer, the reactive functional group is preferably an acryloyl group, a methacryloyl group, an acrylamide group, a methacrylamide group, an amino group, an epoxy group, or an alicyclic epoxy group, more preferably an acryloyl group, a methacryloyl group, an amino group, or an epoxy group, and even more preferably an acryloyl group, a methacryloyl group, or an amino group.

[0082] The silicone-based oil may further contain an organic solvent for dilution. The organic solvent is a good solvent for the reactive silicone contained in the silicone-based oil and a poor solvent for the acrylamide-based polymer. The silicone-based oil may further contain a diluent, an antistatic agent, a smoothing agent, a moisture absorbent, a surfactant, a viscosity modifier, a release agent, a spreading agent, an antibacterial agent, an antioxidant, a photopolymerization initiator, or a preservative, as necessary.

[0083] The reactive silicone may be a commercially available product. Examples of commercially available reactive silicone products include products from Shin-Etsu Chemical Co., Ltd., SILTECH Corporation, Evonik Industries, Dow-Toray Co., Ltd., Wacker Asahi Kasei Silicone Co., Ltd., and Momentive Performance Materials, Inc.Examples of products from Shin-Etsu Chemical Co., Ltd. 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," "X-22-2404," "KF-2001," "KF-2004," "X-22-167B," "X-22-167C," "X-21-5841," "KF-9701," and "KF-8 68'', ``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-22-4 039", "X-22-4015", "X-22-3701E", "KF-99", "KF-9901", "PAM-E", "KF-8 010", "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", Examples include "KF-6003", "X-22-4952", "X-22-4272", "KF-6123", "X-22-162C", "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", and "X-22-9002".Examples of SILTECH Corporation's products include "Silmer ACR D208," "Silmer ACR D2," "Silmer ACR D4," "Silmer ACR Di-10," and "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 70'', ``Silmer VIN 100'', ``Silmer VIN 200'', ``Silmer VIN 1000'', ``Silmer VIN 5000'', ``Silmer VIN 10000'', ``Silmer VIN 20000” and “Silmer VIN 65000”. Examples of products from Evonik Industries include "TegoRad 2010," "TegoRad 2500," and "TegoRad 2700." 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," and "BY 16-760."Examples of products from Wacker Asahi Kasei Silicone Corporation 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," and "FINISH WT 1650 LV." Examples of such products include "TSF4702", "TSF4703", "TSF4704", "TSF4705", "TSF4706", "TSF4707", "TSF4708", "TSF4709", "TSF4730", "YF3965", "TSF4700", "TSF4701", and "XF42-B0970".

[0084] (2.1.1.2.2) Non-reactive silicones Non-reactive silicones do not have reactive functional groups.

[0085] The non-reactive silicone may be any silicone having a polysiloxane structure as a basic structure and not containing a reactive functional group.

[0086] Examples of polysiloxanes include polydimethylsiloxane, polymethylphenylsiloxane, polydiphenylsiloxane, polymethylhydrogensiloxane, and mixtures thereof.

[0087] At least some of the substituents on the side chains and ends of the non-reactive silicone polysiloxane may be substituted with a modifying group. Examples of the substituents include alkyl groups (e.g., methyl, ethyl, and propyl groups) and aryl groups (e.g., phenyl and methylphenyl groups). The modifying group is not a reactive functional group. Examples of the modifying group include ether groups, aralkyl groups, fluoroalkyl groups, ester groups, and amide groups.

[0088] The silicone-based oil may further contain an organic solvent for dilution. The organic solvent is a good solvent for the non-reactive silicone contained in the silicone-based oil and a poor solvent for the acrylamide-based polymer. The silicone-based oil may further contain a diluent, an antistatic agent, a smoothing agent, a moisture absorbent, a surfactant, a viscosity modifier, a release agent, a spreading agent, an antibacterial agent, an antioxidant, a preservative, or the like, as necessary.

[0089] The non-reactive silicone may be a commercially available product. Examples of commercially available non-reactive silicones include products from Shin-Etsu Chemical Co., Ltd., Dow-Toray Co., Ltd., Wacker Asahi Kasei Silicone Co., Ltd., and Momentive Performance Materials, Inc. Examples of products from Shin-Etsu Chemical Co., Ltd. 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 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", and "KF-6004". Examples of products from Dow Toray Industries, Inc. include "BY 16-036," "SH 28," "SF 8428," "501W," "L-7001," "FZ-2104," "L-7002," "SF 8427," "SF 8461," "BY 16-846," and "SH 203."Examples of products from Wacker Asahi Kasei Silicone Co., Ltd. include "AK 0.65 to 10 SILICONE FLUID," "AK 20 to 5,000 SILICONE FLUID," "AK 10,000 to 1,000,000 SILICONE FLUID," "AKF 100 to 10,000 SILICONE FLUID," "AKC 6,000 to 50,000 SILICONE FLUID," "H-SILOXANE," "AP 100 to 1,000 SILICONE FLUID," "APF 125 to 130 SILICONE FLUID," "AR 20 to 1,000 SILICONE FLUID," and "AS 100 Examples of products from Momentive Performance Materials, Inc. include "SILICONE FLUID," "TN SILICONE FLUID," "23166VP SILICONE FLUID," "GM 196 SILICONE FLUID," "L 053 SILICONE FLUID," "L 060 SILICONE FLUID," "L 066 SILICONE FLUID," "AF 98 / 1000 SILICONE FLUID," and "AF 98 / 10000 SILICONE FLUID." "TSF451", "TSF404", "TSF405", "TSF4045", "TSF456", "TSF451", "TSF4300", "TSF437", "TSF4 00", "TSF401", "TSF4300", "TSF451", "TSF484", "THF450", "YF33", "TSF458", "TSF433", "TSF 431", "TSF4600", "TSF410", "TSF411", "XF42-334", "X42-B3629", "TSF4421", "XF42-A3161", "TSF4450", "TSF4440", "TSF4452", "TSF4460", "TSF4441", "TSF4445", and "TSF4446".

[0090] (2.1.2) Spinning Process The preparation process includes a spinning process. In the spinning process, a solution containing an acrylamide polymer, an alkaline earth metal salt, and water (hereinafter also referred to as a "spinning solution") is used for spinning. This produces a plurality of acrylamide polymer fibers containing an alkaline earth metal.

[0091] (2.1.2.1) Spinning Dope The spinning dope contains an acrylamide-based polymer, an alkaline earth metal salt, and water.

[0092] (2.1.2.1.1) The acrylamide-based polymer spinning solution contains an acrylamide-based polymer. The acrylamide-based polymer is preferably a copolymer of an acrylamide-based monomer and another polymerizable monomer.

[0093] The acrylamide polymer may be synthesized by a conventionally known method. The synthesis of an acrylamide polymer can be carried out by utilizing known polymerization reactions (e.g., radical polymerization, cationic polymerization, anionic polymerization, living radical polymerization, etc.). Among the above polymerization reactions, radical polymerization is preferred from the viewpoint of reducing synthesis costs. The synthesis of an acrylamide polymer can be carried out by utilizing polymerization methods such as solution polymerization, suspension polymerization, precipitation polymerization, dispersion polymerization, or emulsion polymerization (e.g., reverse-phase emulsion polymerization). When synthesizing an acrylamide polymer by solution polymerization, the solvent is preferably one that dissolves the raw material monomers and the resulting acrylamide polymer. From the viewpoint of low-cost and safe synthesis, an aqueous solvent or an aqueous mixed solvent is more preferred, and an aqueous solvent is even more preferred. Examples of aqueous solvents include water, alcohol, and mixed solvents thereof, with water being particularly preferred. The term "aqueous mixed solvent" refers to a mixed solvent of the above aqueous solvent and an organic solvent. Examples of organic solvents include tetrahydrofuran, dimethyl sulfoxide, and dimethylformamide.

[0094] In the synthesis of an acrylamide polymer by radical polymerization, it is preferable to use a polymerization initiator. Examples of the polymerization initiator include conventionally known radical polymerization initiators. Examples of radical polymerization initiators include azobisisobutyronitrile, benzoyl peroxide, 4,4'-azobis(4-cyanovaleric acid), ammonium persulfate, and potassium persulfate. When an aqueous solvent or aqueous mixed solvent is used as the solvent, a radical polymerization initiator soluble in the aqueous solvent or aqueous mixed solvent (e.g., 4,4'-azobis(4-cyanovaleric acid), ammonium persulfate, and potassium persulfate) is preferred. From the perspective 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 regulator in place of or together with the polymerization initiator, and it is more preferable to use a polymerization initiator and a polymerization accelerator in combination. Examples of the polymerization accelerator include tetramethylethylenediamine. Examples of the molecular weight regulator include alkyl mercaptan compounds such as n-dodecyl mercaptan. It is particularly preferable to use ammonium persulfate as a polymerization initiator in combination with tetramethylethylenediamine as a polymerization accelerator.

[0095] The temperature of the polymerization reaction is not particularly limited, and 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.

[0096] (2.1.2.1.2) Alkaline Earth Metal Salt The spinning solution contains an alkaline earth metal salt. The alkaline earth metal salt may be used alone or in combination of two or more kinds.

[0097] Examples of alkaline earth metal salts include inorganic salts of alkaline earth metals and organic salts of alkaline earth metals. Examples of inorganic salts of alkaline earth metals include chlorides, hydroxides, carbonates, nitrates, sulfates, phosphates, tungstates, vanadates, molybdates, niobates, borates, and hydrates thereof. Examples of organic salts of alkaline earth metals include phenols, carboxylates (e.g., acetates, lactates, formates, oxalates, propionates, benzoates, succinates, adipates, etc.), benzenesulfonates, and hydrates thereof. Among these, from the viewpoints of solubility in the spinning solution, ease of bonding between the acrylamide polymer and the alkaline earth metal, etc., it is preferable that the alkaline earth metal salt be an alkaline earth metal chloride.

[0098] The method for preparing the alkaline earth metal salt is not particularly limited, and any known method may be used. The amount of the alkaline earth metal salt to be added is not particularly limited, and the amount may be appropriately adjusted so that the alkaline earth metal content of the carbon fiber falls within the range exemplified above.

[0099] (2.1.2.1.3) The aqueous spinning solution contains water. The water is not particularly limited, and examples thereof include ion-exchanged water, distilled water, RO (reverse osmosis) water, and ultrapure water. The method for preparing the water is not particularly limited, and any known method may be used. The amount of water blended is not particularly limited.

[0100] (2.1.2.1.4) Additives The spinning solution may further contain additives. The method for preparing the additives is not particularly limited, and any known method may be used. The amount of the additives to be blended is not particularly limited, and any amount may be used as long as the amount falls within the range exemplified as the content of the additive in the carbon fiber precursor bundle.

[0101] (2.1.2.1.5) Method for producing spinning solution The method for producing the spinning solution is not particularly limited. For example, the spinning solution may be obtained by dissolving and mixing an alkaline earth metal salt in an aqueous solution in which an acrylamide polymer is dissolved in water. The spinning solution may be obtained by dissolving and mixing an acrylamide polymer in an aqueous solution in which an alkaline earth metal salt is dissolved in water. The spinning solution may be obtained by dissolving an acrylamide polymer obtained by copolymerizing an alkaline earth metal salt among anionic vinyl monomers in water.

[0102] (2.1.2.2) Spinning The spinning method is not particularly limited and examples thereof include dry spinning, wet spinning, dry-wet spinning, gel spinning, flash spinning, electrospinning, etc. According to the above spinning methods, acrylamide-based polymer fibers having a desired average fiber diameter can be produced safely at low cost.

[0103] (2.1.3) Oil Application Step The preparation step may further include an oil application step. In the oil application step, an oil layer is applied to the surface of the acrylamide-based polymer fiber. The resulting carbon fiber precursor bundle has a plurality of acrylamide-based polymer fibers and an oil layer applied to the surface of each of the plurality of acrylamide-based polymer fibers. As a result, the carbon fiber precursor bundle is easier to bundle, and the occurrence of fuzz and the like can be suppressed. In addition, fusion of fibers due to the preliminary drawing treatment and the flame-resistant treatment can be further suppressed. The oil application step is performed after the spinning treatment step.

[0104] The oil layer is the same as the oil layer exemplified for the carbon fiber precursor bundle.

[0105] A fiber oil is used to apply the oil layer, and from the viewpoint of suppressing fusion of the carbon fiber precursor bundles, it is preferable to use a silicone-based oil.

[0106] The method for applying the oil agent layer is not particularly limited, and examples thereof include coating, dipping, spraying, touch roll, and guide oiling methods.

[0107] (2.1.4) Crosslinking Treatment Step The preparation step may further include a crosslinking treatment step. In the crosslinking treatment step, a crosslinking treatment is performed on at least one of the acrylamide-based polymer fiber and the oil solution layer attached to the acrylamide-based polymer fiber. As a result, a carbon fiber precursor bundle can be produced that can suppress adhesion of the oil solution to a roller or the like, water absorption by the carbon fiber precursor bundle, and fusion of fibers due to preliminary drawing treatment and fusion of fibers due to flame retardation treatment. The crosslinking treatment step may be performed at least either before or after the oil solution attachment step.

[0108] The crosslinking treatment is not particularly limited, and examples thereof include a method of irradiating the oil agent layer with an electron beam (hereinafter referred to as "electron beam treatment"), a method of irradiating with ultraviolet light, a method of heat treatment, etc. Among these, the crosslinking treatment is preferably an electron beam treatment from the viewpoints of energy efficiency and treatment speed.

[0109] (2.1.4.1) Electron Beam Treatment From the viewpoints of suppressing fusion, carbonization yield, and shape stability, the dose of the electron beam 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 range of the dose described above is the preferred range of the dose when the carbon fiber precursor bundle is irradiated with an electron beam from one direction. When irradiating with an electron beam from two or more directions, the dose of the electron beam is not limited to the above and is adjusted appropriately. The dose of the electron beam is measured using a film dosimeter. As the film dosimeter, an FWT-60 model manufactured by Toyo Medic Co., Ltd. or an equivalent device can be used.

[0110] From the viewpoints of suppressing fusion, carbonization yield, and shape stability, the acceleration voltage of the electron beam is preferably 50 kV to 10 MV, more preferably 100 kV to 3 MV, and even more preferably 150 kV to 1 MV. The preferred numerical range of the acceleration voltage of the electron beam is the preferred numerical range of the acceleration voltage when the carbon fiber precursor bundle is irradiated with actinic rays from one direction. When the electron beam is irradiated from two or more directions, the acceleration voltage of the electron beam is not limited to the above, and it is preferable to adjust it appropriately.

[0111] The electron beam irradiation may be carried out in a batch system or a continuous system. The device used for actinic radiation irradiation is not particularly limited. When electron beam irradiation is carried out in a batch system, an "EC110 / 15 / 10mA" manufactured by Iwasaki Electric Co., Ltd. or a device equivalent thereto can be used. When electron beam irradiation is carried out in a continuous system, an electron beam irradiation device "EPS-800kV" manufactured by NHV Corporation or a device equivalent thereto can be used.

[0112] (2.2) Preliminary Stretching Treatment Step The method for producing a carbon fiber according to the present disclosure preferably further includes a step of stretching the carbon fiber precursor (hereinafter also referred to as a "preliminary stretching treatment step") before obtaining the flame-resistant fiber (i.e., before performing the flame-resistant treatment step). The preliminary stretching treatment may be performed two or more times.

[0113] The term "preliminary stretching treatment" refers to a stretching treatment of the carbon fiber precursor bundle while heating the carbon fiber precursor bundle at 300° C. or less before carrying out the flame-proofing treatment step. The definition of the "stretching ratio" in the preliminary stretching treatment step is the same as the definition of the stretching ratio in the flame-proofing treatment step.

[0114] Once the carbon fiber precursor bundle has been subjected to a flame-resistant treatment, it becomes difficult to plasticize and therefore difficult to draw. Furthermore, if the draw ratio in the flame-resistant treatment is 5.0 times or more, the carbon fiber precursor bundle may break. By further including a preliminary drawing treatment step in the carbon fiber manufacturing method, the cumulative draw ratio of the carbon fiber precursor bundle can be increased to more than 5 times. Therefore, the acrylamide-based polymer contained in the carbon fiber precursor bundle tends to be oriented, and the tensile strength of the flame-resistant fiber and carbon fiber tends to be further improved. As a result, the carbon fiber bundle manufacturing method of the present disclosure can manufacture a carbon fiber bundle with superior tensile strength and load capacity.

[0115] The pre-stretching step may be carried out using a known heat treatment device. The pre-stretching step may be carried out in a batch system or a continuous system.

[0116] The draw ratio during the preliminary stretching treatment is more than 1.1 times. From the viewpoints of the stability of the preliminary stretching treatment and suppressing damage to the carbon fiber precursor bundle, the oxidized fiber bundle, and the carbon fiber bundle, the draw 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 viewpoints of the orientation of the oxidized fiber and the carbon fiber, the tensile strength, and the load-bearing capacity of the carbon fiber bundle, the draw 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.

[0117] The method of the stretching treatment is not particularly limited and is appropriately selected depending on the method of performing the preliminary stretching treatment. When the method of performing the preliminary stretching treatment is continuous, the stretching treatment may be performed by adjusting the speed (V1) of the roller at the inlet of the heat treatment device and the speed (V2) of the roller at the outlet of the heat treatment device. In addition, the carbon fiber precursor bundle may be stretched by applying an appropriate tension to the carbon fiber precursor bundle using a weight, a spring, an air cylinder, hydraulic pressure, or the like.

[0118] The preliminary stretching treatment may be carried out in an oxidizing atmosphere. Examples of the oxidizing atmosphere include oxygen, ozone, air, nitrogen oxides, halogens, sulfur dioxide gas, mixed gases thereof, and mixed gases of these with inert gases. Among these, air, mixed gases of oxygen and air, mixed gases of oxygen and inert gas, and mixed gases of air and inert gas are preferred, and air is particularly preferred from the viewpoint of cost reduction.

[0119] The temperature of the pre-stretching treatment may be 300° C. or less. The temperature of the pre-stretching treatment may be 100° C. to 300° C., 140° C. to 290° C., or 180° C. to 280° C. The maximum temperature in the pre-stretching treatment may be 290° C. or less, or 280° C. or less.

[0120] The time for the preliminary stretching treatment is not particularly limited, and may be 0.1 to 60 minutes, 0.5 to 30 minutes, or 1 to 20 minutes.

[0121] (2.3) Flame-resistant Treatment Step In the flame-resistant treatment step, the carbon fiber precursor bundle of the present disclosure is subjected to flame-resistant treatment to produce a flame-resistant fiber bundle.

[0122] (2.3.1) Flameproofing Treatment Flameproofing treatment may be performed while the carbon fiber precursor bundle is being stretched. The flameproofing treatment may be performed two or more times.

[0123] The flame-proofing treatment may be carried out using a known heat treatment device. The flame-proofing treatment may be carried out continuously.

[0124] The draw ratio during the flame-proofing 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 viewpoints of the stability of the flame-proofing treatment and suppressing damage to the flame-proofed fiber bundle and the carbon fiber bundle. The draw ratio during the flame-proofing treatment is preferably 1.5 times or more, more preferably 1.75 times or more, and even more preferably 2.0 times or more, from the viewpoints of the orientation of the flame-proofed fiber and the carbon fiber, the tensile strength, and the load-bearing capacity of the carbon fiber bundle. In the second or subsequent flame-proofing treatments, the draw ratio is preferably 0.7 times to 3.0 times, more preferably 0.8 times to 2.0 times, and even more preferably 0.9 times to 1.5 times.

[0125] The method of the stretching treatment is not particularly limited and is appropriately selected depending on the method of carrying out the flame-proofing treatment. When the method of carrying out the flame-proofing treatment is continuous, the stretching treatment may be carried out by adjusting the speed (V1) of the roller at the inlet of the heat treatment device and the speed (V2) of the roller at the outlet of the heat treatment device. In addition, the carbon fiber precursor bundle may be stretched by applying an appropriate tension to the carbon fiber precursor bundle using a weight, a spring, an air cylinder, hydraulic pressure, or the like.

[0126] The flame-proofing treatment is carried out in an oxidizing atmosphere. The oxidizing atmosphere is the same as the oxidizing atmosphere exemplified for the preliminary stretching treatment. From the viewpoint of cost reduction, the oxidizing atmosphere is particularly preferably air.

[0127] The temperature of the flame-resistant treatment is not particularly limited, and 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 flame-resistant treatment includes not only the maximum temperature during the flame-resistant treatment (hereinafter also referred to as the "flame-resistant treatment temperature"), but also the temperature during the temperature rise process up to the flame-resistant treatment temperature.

[0128] The maximum temperature during the flame-resistant 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 maximum temperature during the flame-resistant treatment to 305°C or higher, the heat resistance and carbonization yield of the flame-resistant fiber bundle tend to be improved. By setting the maximum temperature during the flame-resistant treatment to 500°C or lower, thermal decomposition of the flame-resistant fiber bundle tends to be suppressed.

[0129] The flame-resistant treatment time (i.e., heating time at the flame-resistant treatment temperature) is not particularly limited, but from the viewpoints of carbonization yield and production costs, 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-resistant treatment time may be more than 2 hours.

[0130] (2.3.3) Flame-resistant Fiber Bundle From the viewpoint of suppressing fiber breakage during carbonization, the fusion rate of the flame-resistant fiber bundle is preferably 15% or less, more preferably 10% or less, even more preferably 8% or less, particularly preferably 7% or less, and most preferably 6% or less. The method for measuring the fusion rate is the same as that described in the examples.

[0131] 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 carbonization treatment resistance is improved. When the average fiber diameter of the flame-resistant fiber bundle is 16 μm or less, the carbonization treatment resistance, tensile strength, and load-bearing capacity of the carbon fiber bundle are improved.

[0132] (2.4) Cumulative Draw Ratio The cumulative draw ratio of the carbon fiber precursor bundle is not particularly limited, and is preferably 2 to 20 times. When the cumulative draw ratio of the carbon fiber precursor bundle is 2 to 20 times, the method for producing a carbon fiber bundle according to the present disclosure can produce a carbon fiber bundle having superior single fiber tensile strength and load-bearing capacity. From the viewpoints of the orientation of the flame-resistant fibers and carbon fibers, the tensile strength, and the load-bearing capacity of the carbon fiber bundle, the cumulative draw ratio of the carbon fiber precursor bundle 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 suppressing damage to the flame-resistant fiber bundle and the carbon fiber bundle, the cumulative draw ratio of the carbon fiber precursor bundle is more preferably 16 times or less, even more preferably 14 times or less.

[0133] The "cumulative draw ratio" refers to the total draw ratio from the time when the carbon fiber precursor bundle is produced to the time when the flame-proofing treatment process is completed. When a preliminary stretching treatment process is not performed, the cumulative draw ratio refers to the draw ratio at the time of the flame-proofing treatment. When a preliminary stretching treatment process is performed, the cumulative draw ratio is expressed as the product of the draw ratio at the time of the preliminary stretching treatment and the draw ratio at the time of the flame-proofing treatment. When the flame-proofing treatment is performed two or more times, the cumulative draw ratio is expressed as the product of the draw ratio at the time of the preliminary stretching treatment and the draw ratio at each of the flame-proofing treatments.

[0134] (2.5) Carbonization Step In the carbonization step, the flame-retardant fiber bundle is carbonized to obtain a carbon fiber bundle.

[0135] Examples of carbonization methods include a method in which the flame-resistant fiber is heated in an atmosphere of an inert gas (e.g., nitrogen, argon, helium, etc.) at a temperature higher than the temperature in the flame-resistant treatment. 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 500°C to 3000°C.

[0136] The heating time at the maximum temperature in the carbonization treatment is not particularly limited, but is preferably 30 seconds to 60 minutes, more preferably 1 minute to 30 minutes.

[0137] In the present disclosure, "carbonization" may generally include "graphitization," which is performed by heating at a temperature of 2000°C to 3000°C in an inert gas atmosphere. The carbonization may include multiple heat treatments. For example, a heat treatment (preliminary carbonization) may be performed first at a temperature below 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.

[0138] (3) Carbon Fiber Precursor Bundle The carbon fiber precursor bundle of the present disclosure includes an acrylamide-based polymer fiber. The acrylamide-based polymer fiber contains an acrylamide-based polymer and an alkaline earth metal.

[0139] The carbon fiber precursor bundle of the present disclosure has the above-described configuration, and therefore can be a carbon fiber bundle in which fusion between carbon fibers is suppressed and which has excellent load-bearing capacity. This effect is presumed to be due to the same reasons as those described above for the carbon fiber bundle of the present disclosure to exhibit the effects of the present disclosure, but is not limited thereto.

[0140] Examples of the carbon fiber precursor bundle include those similar to those exemplified as the carbon fiber precursor bundle in the method for producing a carbon fiber bundle of the present disclosure.

[0141] The above embodiment will be specifically described below using examples, but the above embodiment is not limited to these examples.

[0142] [1] Preparation [1.1] Synthesis Example 1 (Preparation of Acrylamide-Based Polymer) An acrylamide-based polymer, an acrylamide (AM) / acrylonitrile (AN) / acrylic acid (AA) terpolymer (AM / AN / AA = 60 mol% / 35 mol% / 5 mol%), was prepared according to the following procedure. 100 parts by mass of a monomer consisting of 60 mol% AM, 35 mol% AN, and 5 mol% AA, and 5 parts by mass of tetramethylethylenediamine were dissolved in 400 parts by mass of ion-exchanged water to obtain a first aqueous solution. Ammonium persulfate was added to the obtained first aqueous solution while stirring under a nitrogen atmosphere, and the mixture was then heated at 80°C for 150 minutes to polymerize, thereby obtaining a second aqueous solution. The obtained second aqueous solution was added dropwise to methanol to precipitate a copolymer, which was then recovered and vacuum-dried at 80°C for 12 hours. As a result, a water-soluble AM / AN / AA (60 mol % / 35 mol % / 5 mol %) copolymer (i.e., an acrylamide-based polymer) was obtained.

[0143] [1.2] Preparation of carbon fiber precursor bundles

[0144] Carbon fiber precursor bundles (a-1) to (a-3) and (x-1) to (x-5) were produced according to the following procedure.

[0145] [1.2.1] Production Example 1 A spinning solution was obtained by dissolving 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1, 3.42 parts by mass of calcium chloride, and 3.52 parts by mass of monoammonium phosphate in ion-exchanged water. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 18 μm.

[0146] A reactive silicone ("KP-420" manufactured by Shin-Etsu Chemical Co., Ltd., reactive functional group: acryloyl group) and a reactive silicone ("X-22-164C" manufactured by Shin-Etsu Chemical Co., Ltd., reactive functional group: methacryloyl group) were mixed in a 50:50 ratio (mass ratio) to obtain a silicone-based oil agent A. 3 parts by mass of the silicone-based oil agent A was applied to 100 parts by mass of the obtained acrylamide-based polymer fibers, and the fibers were aligned to 800 fibers per bundle to obtain a carbon fiber precursor bundle. In order to cure the silicone-based oil agent A, the obtained carbon fiber precursor bundle was subjected to electron beam irradiation using an electron beam irradiation device ("EPS-800 kV machine" manufactured by NHV Corporation). In the electron beam irradiation, the acceleration voltage was 800 kV, and the electron beam dose was 600 kGy. In this way, a carbon fiber precursor bundle (a-1) was obtained.

[0147] [1.2.2] Production Example 2 A spinning solution was obtained by dissolving 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 6.27 parts by mass of magnesium chloride hexahydrate in ion-exchanged water. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 20 μm.

[0148] Silicone-based oil agent A was obtained in the same manner as in Production Example 1. 3 parts by mass of silicone-based oil agent A was applied to 100 parts by mass of the obtained acrylamide-based polymer fibers, and the fibers were aligned to 800 fibers per bundle to obtain a carbon fiber precursor bundle. In order to cure the silicone-based oil agent A, the obtained carbon fiber precursor bundle was irradiated with an electron beam in the same manner as in Production Example 1. In this way, a carbon fiber precursor bundle (a-2) was obtained.

[0149] [1.2.3] Production Example 3 A spinning solution was obtained by dissolving 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 6.27 parts by mass of magnesium chloride hexahydrate in ion-exchanged water. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 25 μm.

[0150] A silicone-based oil agent B made of reactive silicone ("KF-859" manufactured by Shin-Etsu Chemical Co., Ltd., reactive functional group: amino group) was prepared. 3 parts by mass of the silicone-based oil agent B was applied to 100 parts by mass of the obtained acrylamide-based polymer fibers, and the fibers were aligned to 800 fibers per bundle. This gave a carbon fiber precursor bundle (a-3).

[0151] [1.2.4] Production Example 4 A spinning solution was obtained by dissolving 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 3 parts by mass of phosphoric acid in ion-exchanged water. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 20 μm.

[0152] A silicone-based oil solution C made of a reactive silicone ("KP-420" manufactured by Shin-Etsu Chemical Co., Ltd., containing an acryloyl group) was prepared. 4 parts by mass of the silicone-based oil solution C was applied to the obtained acrylamide-based polymer fibers, and the fibers were aligned to 800 fibers per bundle to obtain a carbon fiber precursor bundle. In order to cure the silicone-based oil solution C, the obtained carbon fiber precursor bundle was irradiated with an electron beam in the same manner as in Production Example 1 to obtain a carbon fiber precursor bundle (x-1).

[0153] [1.2.5] Production Example 5 A spinning solution was obtained by dissolving 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 3 parts by mass of phosphoric acid in ion-exchanged water. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 23 μm.

[0154] A silicone-based oil agent D made of a reactive silicone ("Silmer ACR Di-1508" manufactured by SILTECH Corporation, containing an acryloyl group) was prepared. 2 parts by mass of the silicone-based oil agent D was applied to the obtained acrylamide-based polymer fibers, and the fibers were aligned to 800 fibers per bundle to obtain a carbon fiber precursor bundle. In order to cure the silicone-based oil agent D, the obtained carbon fiber precursor bundle was irradiated with an electron beam in the same manner as in Production Example 1. As a result, a carbon fiber precursor bundle (x-2) was obtained.

[0155] [1.2.6] Production Example 6 A spinning solution was obtained by dissolving 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 3 parts by mass of phosphoric acid in ion-exchanged water. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 20 μm.

[0156] Three parts by mass of silicone-based oil agent A was applied to 100 parts by mass of the obtained acrylamide-based polymer fibers, and the fibers were aligned to 800 fibers per bundle. This resulted in a carbon fiber precursor bundle. To cure the silicone-based oil agent A, the obtained carbon fiber precursor bundle was irradiated with an electron beam in the same manner as in Production Example 1. This resulted in a carbon fiber precursor bundle (x-3).

[0157] [1.2.7] Production Example 7 A spinning solution was obtained by dissolving 100 parts by mass of the acrylamide polymer obtained in Synthesis Example 1 and 3 parts by mass of phosphoric acid in ion-exchanged water. Dry spinning was performed using the obtained spinning solution to obtain acrylamide polymer fibers with an average fiber diameter of 25 μm.

[0158] 100 parts by mass of the obtained acrylamide polymer fibers were coated with 3 parts by mass of silicone oil B, and the fibers were aligned to a bundle of 800 fibers, thereby obtaining a carbon fiber precursor bundle (x-4).

[0159] [1.2.8] Production Example 8 A carbon fiber precursor bundle (x-5) was produced in the same manner as in Production Example 7, except that phosphoric acid was not added to the spinning solution.

[0160] [2] Examples and Comparative Examples [2.1] Example 1 Using a carbon fiber precursor bundle (a-1), a first flame-resistant treatment was carried out in an air atmosphere in a continuous heat treatment furnace at 250°C to 330°C. In the first flame-resistant treatment, the draw ratio was 3.0 times, and the treatment time was 10 minutes. Thereafter, a second flame-resistant treatment was carried out in an air atmosphere in a continuous heat treatment furnace at 250°C to 340°C. In the second flame-resistant treatment, the draw ratio was 1.00 times, and the treatment time was 20 minutes. A flame-resistant fiber bundle was thus obtained. The cumulative draw ratio was 3.0 times.

[0161] "A°C to B°C continuous heat treatment furnace" means that the temperature in the heat treatment furnace increases stepwise from A°C to B°C as the carbon fiber precursor bundle moves through the heat treatment furnace. For example, a 250°C to 330°C continuous heat treatment furnace means that the temperature in the heat treatment furnace increases stepwise from 250°C to 330°C as the carbon fiber precursor bundle (a-1) moves through the heat treatment furnace. The same applies hereinafter.

[0162] The obtained flame-resistant fiber bundles were aligned to 1,600 fibers per bundle and subjected to preliminary carbonization in a continuous heat treatment furnace at 800°C under a nitrogen atmosphere. The preliminary carbonization treatment time was 3 minutes. Thereafter, the fibers were subjected to main carbonization in a continuous heat treatment furnace at 1,400°C under a nitrogen atmosphere. The main carbonization treatment time was 3 minutes. In this way, a carbon fiber bundle was obtained.

[0163] [2.2] Example 2 Using carbon fiber precursor bundle (a-2), a first flame-resistant treatment was performed in an air atmosphere in a continuous heat treatment furnace at 250°C to 330°C. In the first flame-resistant treatment, the draw ratio was 3.0 times, and the treatment time was 10 minutes. Thereafter, a second flame-resistant treatment was performed in an air atmosphere in a continuous heat treatment furnace at 250°C to 340°C. In the second flame-resistant treatment, the draw ratio was 1.10 times, and the treatment time was 20 minutes. A flame-resistant fiber bundle was thus obtained. The cumulative draw ratio was 3.3 times. The obtained flame-resistant fiber bundles were aligned to 1,600 fibers per bundle, and preliminary carbonization and main carbonization were performed in the same procedure as in Example 1 to obtain a carbon fiber bundle.

[0164] [2.3] Example 3 Using the carbon fiber precursor bundle (a-2), a preliminary drawing treatment was performed in a continuous heat treatment furnace at 250°C under an air atmosphere. In the preliminary drawing treatment, the draw ratio was 3.0 times, and the treatment time was 10 minutes. Thereafter, a first flame-resistant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. In the first flame-resistant treatment, the draw ratio was 2.0 times, and the treatment time was 10 minutes. Thereafter, a second flame-resistant treatment was performed in a continuous heat treatment furnace at 250°C to 350°C under an air atmosphere. In the second flame-resistant treatment, the draw ratio was 1.00 times, and the treatment time was 20 minutes. As a result, a flame-resistant fiber bundle was obtained. The cumulative draw ratio was 6.0 times.

[0165] The obtained flame-resistant fiber bundles were aligned to 1600 fibers per bundle and subjected to preliminary carbonization and main carbonization in the same manner as in Example 1 to obtain carbon fiber bundles.

[0166] [2.4] Example 4 Using the carbon fiber precursor bundle (a-3), a preliminary drawing treatment was performed in a continuous heat treatment furnace at 250°C under an air atmosphere. In the preliminary drawing treatment, the draw ratio was 3.0 times, and the treatment time was 10 minutes. Thereafter, a first flame-resistant treatment was performed in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. In the first flame-resistant treatment, the draw ratio was 3.0 times, and the treatment time was 10 minutes. Thereafter, a second flame-resistant treatment was performed in a continuous heat treatment furnace at 250°C to 350°C under an air atmosphere. In the second flame-resistant treatment, the draw ratio was 1.03 times, and the treatment time was 20 minutes. As a result, a flame-resistant fiber bundle was obtained. The cumulative draw ratio was 9.3 times.

[0167] The obtained flame-resistant fiber bundles of 800 fibers / bundle were pre-carbonized in a continuous heat treatment furnace at 800°C under a nitrogen atmosphere. The pre-carbonization treatment time was 2 minutes. Thereafter, the fibers were main-carbonized in a continuous heat treatment furnace at 1400°C under a nitrogen atmosphere. The main-carbonization treatment time was 2 minutes. In this way, a carbon fiber bundle was obtained.

[0168] [2.5] Example 5 Using the carbon fiber precursor bundle (a-3), a preliminary drawing treatment was performed in a continuous heat treatment furnace at 250°C under an air atmosphere. In the preliminary drawing treatment, the draw ratio was 3.0 times, and the treatment time was 10 minutes. Thereafter, a first flame-resistant treatment was performed in a continuous heat treatment furnace at 250°C to 320°C under an air atmosphere. In the first flame-resistant treatment, the draw ratio was 3.0 times, and the treatment time was 20 minutes. Thereafter, a second flame-resistant treatment was performed in a continuous heat treatment furnace at 250°C to 320°C under an air atmosphere. In the second flame-resistant treatment, the draw ratio was 1.13 times, and the treatment time was 60 minutes. As a result, a flame-resistant fiber bundle was obtained. The cumulative draw ratio was 10.2 times. The obtained flame-resistant fiber bundle of 800 fibers / bundle was subjected to preliminary carbonization and main carbonization under the same conditions as in Example 4 to obtain a carbon fiber bundle.

[0169] [2.6] Comparative Example 1 Using the carbon fiber precursor bundle (x-1), a flame-resistant treatment was carried out in a continuous heat treatment furnace at 230°C to 350°C under an air atmosphere. In the flame-resistant treatment, the draw ratio was 3.0 times, and the treatment time was 20 minutes. As a result, a flame-resistant fiber bundle was obtained. The cumulative draw ratio was 3.0 times.

[0170] The obtained flame-resistant fiber bundles were aligned to 1600 fibers per bundle and subjected to preliminary carbonization and main carbonization in the same manner as in Example 1 to obtain carbon fiber bundles.

[0171] [2.7] Comparative Example 2 Using the carbon fiber precursor bundle (x-1), a pre-stretching treatment was performed in a continuous heat treatment furnace at 250°C under an air atmosphere. In the pre-stretching treatment, the draw ratio was 3.0 times, and the treatment time was 10 minutes. Thereafter, a first flame-resistant treatment was performed in a continuous heat treatment furnace at 230°C to 330°C under an air atmosphere. In the first flame-resistant treatment, the draw ratio was 2.0 times, and the treatment time was 10 minutes. Thereafter, a second flame-resistant treatment was performed in a continuous heat treatment furnace at 230°C to 330°C under an air atmosphere. In the second flame-resistant treatment, the draw ratio was 1.00 times, and the treatment time was 10 minutes. As a result, a flame-resistant fiber bundle was obtained. The cumulative draw ratio was 6.0 times.

[0172] The obtained flame-resistant fiber bundles were aligned to 1600 fibers per bundle, and preliminary carbonization and main carbonization were carried out in the same manner as in Example 1. In Comparative Example 2, the carbon fiber bundle broke during the main carbonization.

[0173] [2.8] Comparative Example 3 Using the carbon fiber precursor bundle (x-2), a first flame-resistant treatment was carried out in an air atmosphere in a continuous heat treatment furnace at 230°C to 330°C. In the first flame-resistant treatment, the draw ratio was 4.0 times, and the treatment time was 10 minutes. Thereafter, a second flame-resistant treatment was carried out in an air atmosphere in a continuous heat treatment furnace at 250°C to 330°C. In the second flame-resistant treatment, the draw ratio was 1.0 times, and the treatment time was 30 minutes. As a result, a flame-resistant fiber bundle was obtained. The cumulative draw ratio was 4.0 times.

[0174] The obtained flame-resistant fiber bundles were aligned to 1600 fibers per bundle and subjected to preliminary carbonization and main carbonization in the same manner as in Example 1 to obtain carbon fiber bundles.

[0175] [2.9] Comparative Example 4 A heat treatment was performed in the same procedure as in Comparative Example 3, except that in the second flame retardant treatment, the treatment temperature was changed from "250°C to 330°C" to "250°C to 350°C" and the treatment time was changed from 30 minutes to 20 minutes, to obtain a carbon fiber bundle.

[0176] [2.10] Comparative Example 5 Using the carbon fiber precursor bundle (x-2), a first flame-resistant treatment was carried out in an air atmosphere in a continuous heat treatment furnace at 230°C to 330°C. In the first flame-resistant treatment, the draw ratio was 4.0 times, and the treatment time was 20 minutes. Thereafter, a second flame-resistant treatment was carried out in an air atmosphere in a continuous heat treatment furnace at 250°C to 330°C. In the second flame-resistant treatment, the draw ratio was 1.0 times, and the treatment time was 40 minutes. As a result, a flame-resistant fiber bundle was obtained. The cumulative draw ratio was 4.0 times.

[0177] The obtained flame-resistant fiber bundles were aligned to 1600 fibers per bundle and subjected to preliminary carbonization and main carbonization in the same manner as in Example 1 to obtain carbon fiber bundles.

[0178] [2.11] Comparative Example 6 Using the carbon fiber precursor bundle (x-2), a pre-stretching treatment was performed in a continuous heat treatment furnace at 230°C under an air atmosphere. In the pre-stretching treatment, the draw ratio was 3.0 times, and the treatment time was 10 minutes. Thereafter, a first flame-resistant treatment was performed in a continuous heat treatment furnace at 230°C to 330°C under an air atmosphere. In the first flame-resistant treatment, the draw ratio was 2.0 times, and the treatment time was 10 minutes. Thereafter, a second flame-resistant treatment was performed in a continuous heat treatment furnace at 230°C to 350°C under an air atmosphere. In the second flame-resistant treatment, the draw ratio was 1.00 times, and the treatment time was 10 minutes. As a result, a flame-resistant fiber bundle was obtained. The cumulative draw ratio was 6.0 times.

[0179] The obtained flame-resistant fiber bundles were aligned to 1600 fibers per bundle, and preliminary carbonization and main carbonization were carried out in the same manner as in Example 1. In Comparative Example 6, the carbon fiber bundle broke during the main carbonization.

[0180] [2.12] Comparative Example 7 Carbon fiber precursor (x-3) was used and subjected to a flame-resistant treatment in a continuous heat treatment furnace at 250°C to 330°C under an air atmosphere. In the flame-resistant treatment, the draw ratio was 3.0 times and the treatment time was 10 minutes. As a result, a flame-resistant fiber bundle was obtained. The cumulative draw ratio was 3.0 times. In Comparative Example 7, no carbonization treatment was performed.

[0181] [2.13] Comparative Example 8 A flame-resistant fiber bundle was produced in the same manner as in Comparative Example 7, except that (x-4) was used as the carbon fiber precursor. In Comparative Example 8, no carbonization treatment was performed.

[0182] [2.14] Comparative Example 9 A flame-resistant fiber bundle was produced in the same manner as in Comparative Example 7, except that (x-5) was used as the carbon fiber precursor. In Comparative Example 9, no carbonization treatment was performed.

[0183] [3] Measurement Methods [3.1] Measurement of Alkaline Earth Metal Content in Carbon Fiber Bundles The alkaline earth metal contents of the carbon fiber bundles obtained in Examples 1 to 5 and Comparative Examples 1 and 3 to 5 were measured by inductively coupled plasma mass spectrometry (ICP-MS). Specifically, the carbon fiber bundles were dried at 120°C for 1 hour, then incinerated and dissolved in acid to obtain analytical samples. The obtained analytical samples were analyzed using an ICP mass spectrometer (PerkinElmer's "NexION2000C") to measure the alkaline earth metal contents. The measured values ​​of the alkaline earth metal contents were defined as the "alkaline earth metal contents of the carbon fiber bundles."

[0184] [3.2] Measurement of the degree of crystalline orientation of carbon fiber bundles Wide-angle X-ray scattering measurement was performed using a beamline (the "BL8S3 beamline" of the Aichi Synchrotron Light Center) for the carbon fiber bundles obtained in Examples 1 to 5 and Comparative Example 1. The carbon fiber bundles were placed vertically to the ground, and measurement was performed under the following conditions.

[0185] <Measurement conditions for wide-angle X-ray scattering measurement> Measurement method: transmission method Light source wavelength: 0.092 nm Camera length: 200 mm Exposure time: 30 seconds Temperature: room temperature (23°C) Pressure: atmospheric pressure

[0186] From the obtained X-ray scattering image, the circumferential intensity distribution of the (002) plane was determined, and after Gaussian fitting, the half-widths W1 and W2 [°] of the two scattering peaks were calculated. The obtained values ​​of W1 and W2 were substituted into formula (i) to calculate the crystalline orientation degree [%]. The calculated value of the crystalline orientation degree was defined as the "crystalline orientation degree of the carbon fiber bundle." Formula (i): Crystalline orientation degree [%] = 100 × (360 - (W1 + W2)) / 360

[0187] [3.3] Measurement of fusion rate The cross sections of the carbon fiber bundles obtained in Examples 1 to 5 and Comparative Examples 1, 3 to 5 were observed using a microscope (Keyence Corporation's "Digital Microscope VHX-7000"). The carbon fiber bundles were measured so that the total number of fused carbon fibers and the number of unfused carbon fibers was 300 or more. The fusion rate was calculated according to the following formula (ii). In addition, for Comparative Examples 2 and 6 to 9, the fusion rate of the flame-resistant fiber bundle was measured. The calculation results are shown in Tables 1 and 2. The acceptable range of the fusion rate is 15% or less. Formula (ii): Fusion rate [%] = 100 × [number of fused carbon fibers / (number of fused carbon fibers + number of unfused carbon fibers)]

[0188] [3.4] Load-bearing Capacity Evaluation of Carbon Fiber Bundles The load-bearing capacity evaluation of the carbon fiber bundles obtained in Examples 1 to 5 and Comparative Examples 1, 3 to 5 was performed according to the following procedure. FIG. 1 is a diagram illustrating the load-bearing capacity measurement method. Both ends of an attachment wire 30 were attached to weights 20. Next, as shown in FIG. 1, a 100 g weight 20 was suspended from the carbon fiber bundle 10. If the carbon fiber bundle 10 did not break, the weight 20 was increased until the carbon fiber bundle 10 broke. Specifically, 100 g weights 20 were added one by one until the total mass of the weights 20 reached 1600 g. Once the total mass of the weights 20 reached 1600 g or more, 200 g weights 20 were added one by one. The mass of the weights 20 immediately before the carbon fiber bundle 10 broke was substituted into formula (iii) to calculate the load-bearing capacity of the carbon fiber bundle. The calculation results are shown in Tables 1 and 2. The allowable range of the load capacity of the carbon fiber bundle is 15.0 kg / mm ２ Formula (iii): Load capacity of carbon fiber bundle [kg / mm ２ ] = Mass of the weight before breakage [kg] / Cross-sectional area of ​​the carbon fiber bundle [mm ２The cross-sectional area of ​​the carbon fiber bundle was calculated by the following procedure. The cross section of the carbon fiber bundle was observed using a microscope ("Digital Microscope VHX-7000" manufactured by Keyence Corporation). Ten unfused carbon fibers were randomly selected from the carbon fiber bundle, and the cross-sectional areas of the selected ten carbon fibers were measured. The average value of the measured cross-sectional areas of the ten carbon fibers was multiplied by the number of fibers in the carbon fiber bundle, and the product was defined as the "cross-sectional area of ​​the carbon fiber bundle."

[0189] [3.5] Evaluation of tensile strength of single fibers in carbon fiber bundles The tensile strength of the single fibers in the carbon fiber bundles obtained in Examples 1 to 5 and Comparative Example 1 was evaluated according to the following procedure. Single fibers were taken out from the obtained carbon fiber bundles to obtain samples consisting of single fibers. A tensile test (gauge length: 4 mm, tensile speed: 2 mm / min, room temperature) was performed using a single fiber tensile tester ("LEX820" manufactured by DiaStron). The average value of the measured tensile strengths of five samples was taken as the "single fiber tensile strength." The measurement results are shown in Tables 1 and 2.

[0190] [4] Results

[0191]

[0192] In Table 1, "A" in "Textile oil" indicates silicone-based oil A. "B" in "Textile oil" indicates silicone-based oil B. "C" in "Textile oil" indicates silicone-based oil C. "D" in "Textile oil" indicates silicone-based oil D. Regarding "*1," the fusion rates of Comparative Examples 2 and 6 to 11 indicate the fusion rates of the flame-resistant fiber bundles. "Break" indicates that the carbon fiber bundle to be measured could not be produced.

[0193] [4.1] Comparative Examples 1 to 9 The carbon fiber bundles and carbon fiber precursor bundles of Comparative Examples 1 to 9 did not contain alkaline earth metals. Therefore, the fusion rates of the carbon fiber bundles of Comparative Examples 1 to 9 were not 15% or less. The load-bearing capacity of the carbon fiber bundles of Comparative Examples 1 and 3 to 5 was 15.0 kg / mm ２It was not more than that. In Comparative Examples 2 and 6, breakage occurred during the carbonization. In Comparative Examples 7 to 9, the fusion rate after flame retardation was as high as 49% or more, and there was a high possibility of breakage during the carbonization treatment, so the carbonization treatment was not carried out. As a result, it was found that the carbon fiber bundles of Comparative Examples 1 to 9 were not "carbon fiber bundles in which fusion between carbon fibers is suppressed and which have excellent load-bearing capacity." It was found that the manufacturing methods of the carbon fiber bundles of Comparative Examples 1 to 9 were not "manufacturing methods of carbon fiber bundles which can produce carbon fiber bundles in which fusion between carbon fibers is suppressed and which have excellent load-bearing capacity." It was found that the carbon fiber precursor bundles of Comparative Examples 1 to 9 were not "carbon fiber precursor bundles which can produce carbon fiber bundles in which fusion between carbon fibers is suppressed and which have excellent load-bearing capacity."

[0194] [4.2] Examples 1 to 5 The carbon fiber bundles and carbon fiber precursor bundles of Examples 1 to 5 contained alkaline earth metals. The content of alkaline earth metals was within a range of 0.05 mass% to 5.0 mass% relative to the total amount of the carbon fiber bundles. Therefore, the fusion rates of the carbon fiber bundles of Examples 1 to 5 were 15% or less. The load capacity of the carbon fiber bundles of Examples 1 to 5 was 15.0 kg / mm ２ The above was the result. As a result, it was found that the carbon fiber bundles of Examples 1 to 5 were "carbon fiber bundles in which fusion between carbon fibers is suppressed and which have excellent load-bearing capacity". It was found that the carbon fiber precursor bundles of Examples 1 to 5 were "carbon fiber precursor bundles in which fusion between carbon fibers is suppressed and which can be made into carbon fiber bundles with excellent load-bearing capacity". It was found that the carbon fiber precursor bundles of Examples 1 to 5 were "carbon fiber precursor bundles in which fusion between carbon fibers is suppressed and which can be made into carbon fiber bundles with excellent load-bearing capacity".

[0195] The load capacity of Comparative Examples 1, 3 to 5 was 2 kg / mm ２ Although the load capacity of Examples 1 to 5 is smaller than ２ In particular, Examples 3 to 5, which had a crystal orientation degree of 60% or more and contained magnesium, had a load capacity of 45 kg / mm ２ That was all.

[0196] The tensile strength of the carbon fiber single fiber was 1000 MPa or less in Comparative Example 1, but 1000 MPa or more in Examples 1 to 5. In particular, in Examples 3 to 5, which had a crystal orientation degree of 60% or more and contained magnesium, the tensile strength of the single fiber was 1800 MPa or more.

[0197] From Examples 1 to 5, the load-bearing capacity and tensile strength of a single fiber of the carbon fiber bundle containing magnesium were higher than the load-bearing capacity and tensile strength of a single fiber of the carbon fiber bundle containing calcium. The reason for this result is unclear, but is considered below. The atomic size of magnesium is smaller than the atomic size of calcium. Therefore, it is thought that when the carbon fiber contains magnesium, the space generated at the bonded portion between the alkaline earth metal and the graphite crystal is smaller than the space when the carbon fiber contains calcium.

[0198] The disclosure of Japanese Patent Application No. 2024-150999, filed on September 2, 2024, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A carbon fiber bundle having a plurality of carbon fibers containing an alkaline earth metal, wherein the content of the alkaline earth metal is 0.05% by mass to 5.0% by mass relative to the total amount of the carbon fiber bundle.

2. The carbon fiber bundle according to claim 1, wherein the degree of crystalline orientation of the carbon fiber bundle is 60% or more.

3. The carbon fiber bundle according to claim 2, wherein the alkaline earth metal includes at least one of magnesium and calcium.

4. The carbon fiber bundle according to claim 3, wherein the fusion rate of the carbon fiber bundle is 15% or less.

5. A method for producing a carbon fiber bundle, comprising: subjecting a carbon fiber precursor bundle to a flame-retardant treatment to obtain a flame-retardant fiber bundle; and subjecting the flame-retardant fiber bundle to a carbonization treatment to obtain a carbon fiber bundle, wherein the carbon fiber precursor bundle contains a plurality of acrylamide-based polymer fibers containing an alkaline earth metal.

6. The method for producing a carbon fiber bundle according to claim 5, comprising obtaining the acrylamide polymer fibers by spinning a solution containing an acrylamide polymer, an alkaline earth metal salt, and water.

7. A carbon fiber precursor bundle having an acrylamide-based polymer fiber containing an acrylamide-based polymer and an alkaline earth metal.

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