Carbon fiber bundle and method for manufacturing same

Abstract

The present invention addresses the problem of providing a carbon fiber bundle and a method for manufacturing the same, wherein the interfacial shear strength (IFSS) of the carbon fiber bundle indicating adhesion to resin is high, and the 90° bending strength of a carbon fiber reinforced composite material can be increased without reducing the strand elastic modulus and strand strength. The crystallite size is 2.9 to 3.4 nm and the iPa on the carbon fiber surface is 0.30 μA / cm2 or greater. In a method for manufacturing a carbon fiber bundle including a flame-proofing step for heating a carbon fiber precursor fiber bundle in an oxidative atmosphere and a carbonization step for heating the flame-proof fiber bundle after the flame-proofing step in a non-oxidative atmosphere, a substantially untwisted carbon fiber precursor fiber bundle is used, and the elongation of the fiber bundle in a carbonization temperature region of 1,000°C or greater in the carbonization step is set to −2.7% or greater, and the maximum carbonization temperature is set to 1,800-2,200°C.

Description

Carbon fiber bundle and method for manufacturing the same

[0001] This invention relates to a carbon fiber bundle and a method for producing the same. This application claims priority based on Japanese Patent Application No. 2024-194704, filed in Japan on November 6, 2024, the contents of which are incorporated herein by reference.

[0002] Carbon fibers possess higher specific strength and specific modulus compared to other fibers. Carbon fiber bundles, which are bundles of individual carbon fibers, are being widely used as reinforcing fibers for composite materials in sports and aerospace applications, as well as in general industrial applications such as automobiles, civil engineering and construction, pressure vessels, and wind turbine blades, and there is a high demand for even higher performance.

[0003] Polyacrylonitrile (PAN) carbon fibers, which are widely used among carbon fibers, are generally manufactured industrially as follows: First, a spinning solution containing an acrylonitrile polymer, which serves as a precursor, is wet-spinned, dry-spinned, or wet-dry-spinned to obtain carbon fiber precursor acrylic fiber bundles. The obtained carbon fiber precursor acrylic fiber bundles are then heated in an oxidizing atmosphere, for example at 180-400°C, to convert them into flame-resistant fiber bundles, and then carbonized by heating in an inert atmosphere, for example at 1000°C or higher, to obtain carbon fiber bundles.

[0004] Carbon fibers are brittle materials, and even small defects can cause a decrease in strength. Therefore, various methods have been explored to reduce defects that cause fracture. Specifically, methods have been proposed to control the size and state of defects in carbon fibers so that they fall within a specific range. Until now, carbon fibers have tended to decrease in tensile strength as their tensile modulus increases, and achieving both tensile modulus and tensile strength has been required for lightweight composite materials.

[0005] For example, Patent Document 1 discloses a carbon fiber bundle that simultaneously exhibits high compressive strength and high strand modulus of elasticity of carbon fiber composite materials, as well as a method for manufacturing the same. Specifically, in order to solve the problem, the crystallite size of the carbon fiber bundle, the compressive strength of the single fiber, and the initial modulus of elasticity in the strand test are set within a predetermined range, and the adjustment of flame-retardant treatment conditions, the tension range in the carbonization process, and the relationship between crystallite size and maximum temperature are disclosed.

[0006] International Publication No. 2019 / 203088

[0007] However, Patent Document 1 does not clearly specify the processing conditions in the flame-retardant process, the processing time in the carbonization process, or the temperature gradient, making it unclear whether high-quality carbon fiber bundles can be reliably obtained.

[0008] The main objective of the present invention is to provide a carbon fiber bundle and a method for producing the same that can increase the 90° bending strength of a carbon fiber reinforced composite material by having high interfacial shear strength (IFSS) that indicates adhesion to resin without reducing the strand modulus of elasticity and strand strength.

[0009] The present invention has the following aspects: [1] Crystallite size of 2.90 to 3.40 nm, iPa of the carbon fiber surface of 0.30 μA / cm ２ A carbon fiber bundle having the above characteristics. [2] The carbon fiber bundle according to [1], wherein the O / C ratio of the carbon fiber surface is 0.138 or higher. [3] The carbon fiber bundle according to [1] or [2], wherein the N / C ratio of the carbon fiber surface is 0.050 or higher. [4] A fiber density of 1.70 to 1.85 g / cm³ ３[1] to [3] A carbon fiber bundle according to any one of the following: [5] A carbon fiber bundle according to any one of the following: [1] to [4] The diameter of the single carbon fiber is 4.0 to 7.0 μm. [6] A carbon fiber bundle according to any one of the following: [1] to [5] It is substantially untwisted. [7] A carbon fiber bundle according to any one of the following: [1] to [6] The interfacial shear strength obtained by fragmentation testing is 42 MPa or more. [8] A carbon fiber bundle according to any one of the following: [1] to [7] The strand strength is 4.00 GPa or more. [9] A carbon fiber bundle according to any one of the following: [1] to [8] The strand modulus is 370 GPa or more.

[10] A carbon fiber bundle according to any one of the following: [1] to [9] The carbon fiber bundle includes a single fiber whose single fiber strength measured at a test length of 5 mm is 6.50 GPa or more.

[11] A method for producing a carbon fiber bundle, comprising a flame-retardant step of heating a carbon fiber precursor fiber bundle in an oxidizing atmosphere, and a carbonization step of heating the flame-retardant fiber bundle after the flame-retardant step in a non-oxidizing atmosphere, wherein the carbon fiber precursor fiber bundle is substantially untwisted, the elongation rate of the fiber bundle in the region where the carbonization temperature of the carbonization step is 1000°C or higher is -2.7% or higher, and the maximum carbonization temperature is 1800°C or higher and 2200°C or lower.

[12] The method for producing a carbon fiber bundle according to

[11] , wherein the generated tension of the fiber bundle in the region where the carbonization temperature of the carbonization step is 1800°C or higher is 5 mN / dtex or higher.

[13] The method for producing a carbon fiber bundle according to

[11] or

[12] , wherein the generated tension of the fiber bundle in the region where the carbonization temperature of the carbonization step is 1000 to 1700°C is 5 mN / dtex or higher.

[14] The fiber density of the flame-retardant fiber bundle after the flame-retardant process is 1,320 to 1,400 g / cm³. ３ The method for producing a carbon fiber bundle as described in any of

[11] to

[13] .

[0010] According to the present invention, it is possible to provide a carbon fiber bundle and a method for manufacturing the same that can increase the 90° bending strength of a carbon fiber reinforced composite material, while maintaining high interfacial shear strength (IFSS) that indicates adhesion to resin without reducing the strand modulus of elasticity and strand strength.

[0011] The present invention will now be described in detail. The following embodiments are merely illustrative for illustrating the present invention and are not intended to limit the present invention to these embodiments. The present invention can be implemented in various forms without departing from its spirit.

[0012] [Carbon Fiber Bundle] The carbon fiber bundle according to this embodiment is composed of multiple individual carbon fibers bundled together, and may be in a continuous fiber state or cut to a predetermined length. Examples of carbon fibers include polyacrylonitrile (PAN) carbon fibers, rayon carbon fibers, and pitch carbon fibers. From the viewpoint of excellent productivity and mechanical properties on an industrial scale, PAN carbon fibers are preferred as the carbon fibers.

[0013] The number of filaments in a carbon fiber bundle, that is, the number of individual carbon fibers in the carbon fiber bundle, is preferably 8,000 or more, more preferably 10,000 or more, even more preferably 12,000 or more, preferably 24,000 or less, and even more preferably 18,000 or less. If the number of filaments is above the lower limit, the carbon fiber bundle exhibits excellent processability. If the number of filaments is below the upper limit, the carbon fiber bundle exhibits excellent handling properties. The preferred lower and upper limits for the number of filaments can be arbitrarily combined; for example, 8,000 to 24,000 is preferred, 10,000 to 18,000 is more preferred, and 12,000 to 18,000 is even more preferred.

[0014] The carbon fiber bundle according to this embodiment has a crystallite size of 2.90 to 3.40 nm and an iPa of 0.30 μA / cm² on the carbon fiber surface. ２The above is the case. If the crystallite size of the carbon fiber bundle is 2.90 nm or more, the strand elastic modulus tends to be high. From this viewpoint, the crystallite size of the carbon fiber bundle is preferably 2.95 nm or more, and more preferably 3.00 nm or more. Further, if the crystallite size of the carbon fiber bundle is 3.40 nm or less, surface modification by electrolytic oxidation treatment proceeds easily, and thus high adhesive strength with a resin can be easily obtained. From this viewpoint, the crystallite size of the carbon fiber bundle is preferably 3.30 nm or less, and more preferably 3.20 nm or less. The lower limit and the upper limit of the crystallite size of the carbon fiber bundle can be arbitrarily combined. For example, it may be 2.95 to 3.30 nm, or may be 3.00 to 3.20 nm. For example, the crystallite size of the carbon fiber bundle can be adjusted by adjusting the maximum carbonization temperature. The crystallite size of the carbon fiber bundle can be determined by Scherrer's formula using the measurement data of X-ray diffraction (XRD) as described in the examples below.

[0015] In the carbon fiber bundle according to the embodiment, iPa on the surface of the carbon fiber is 0.30 μA / cm ２ or more, and preferably 0.35 μA / cm ２ or more, and more preferably 0.40 μA / cm ２ or more. If iPa is equal to or greater than the lower limit value, affinity between the carbon fiber bundle and the resin can be obtained. The upper limit of iPa on the carbon fiber surface is not particularly limited, but is substantially 0.70 μA / cm ２ or less, preferably 0.60 μA / cm ２ or less, and more preferably 0.50 μA / cm ２ or less. The lower limit and the upper limit of iPa on the carbon fiber surface can be arbitrarily combined. For example, it may be 0.30 to 0.70 nm μA / cm ２ or may be 0.35 to 0.60 nm μA / cm ２ or may be 0.40 to 0.50 nm μA / cm ２ or may be. iPa on the carbon fiber surface can be controlled, for example, by adjusting the electrolytic treatment conditions in the surface treatment of the carbon fiber. iPa on the carbon fiber surface can be measured by X-ray photoelectron spectroscopy using the carbon fiber as a symmetric electrode.

[0016] The strand strength of the carbon fiber bundle according to the embodiment is preferably 4.00 GPa or higher, more preferably 4.80 GPa or higher, even more preferably 5.10 GPa or higher, particularly preferably 5.30 GPa or higher, and most preferably 5.40 GPa or higher. If the strand strength is above the lower limit, it is easier to obtain sufficient elongation when it is made into a composite. The upper limit of the strand strength of the carbon fiber bundle is not particularly limited, but may substantially be 10 GPa or less. The lower and upper limits of the strand strength of the carbon fiber bundle can be arbitrarily combined, for example, it may be 4.00 to 10 GPa, 4.80 to 10 GPa, 5.10 to 10 GPa, 5.30 to 10 GPa, or 5.40 to 10 GPa. The strand strength of the carbon fiber bundle can be controlled, for example, by adjusting the elongation rate of the fiber bundle in the flame-retardant process and the carbonization process. The strand strength of the carbon fiber bundle is measured in accordance with JIS R 7608:2007.

[0017] The strand modulus of the carbon fiber bundle according to the embodiment is preferably 370 GPa or higher, more preferably 374 GPa or higher, and even more preferably 380 GPa or higher. If the strand modulus is above the lower limit, sufficient rigidity is easily obtained when it is made into a composite. A higher strand modulus of the carbon fiber bundle is preferable, but from the viewpoint of not reducing productivity, it is preferably 430 GPa or lower, more preferably 420 GPa or lower, and even more preferably 400 GPa or lower. The lower and upper limits of the strand modulus of the carbon fiber bundle can be arbitrarily combined, for example, 370 to 430 GPa is preferred, 374 to 420 GPa is more preferred, and 380 to 400 GPa is even more preferred. The strand modulus of the carbon fiber bundle can be controlled by adjusting the heating temperature when heating and carbonizing the flame-resistant fiber bundle. The strand modulus of the carbon fiber bundle is measured in accordance with JIS R 7608:2007A method.

[0018] The fiber density of the carbon fiber bundle according to this embodiment is 1.70 g / cm³. ３ The above is preferable, specifically 1.73 g / cm³. ３ The above is more preferable, 1.76 g / cm³ ３ The above is even more preferable, and also 1.85 g / cm³. ３The following is preferable: 1.83 g / cm ３ The following is more preferable: 1.81 g / cm ３ The following is even more preferable. If the fiber density is at least the lower limit value, the material tends to have excellent specific strength and specific modulus. If the fiber density is at most the upper limit value, the carbon fiber bundle can be manufactured with better productivity. The preferable lower and upper limits of the fiber density of the carbon fiber bundle can be arbitrarily combined. For example, 1.70 to 1.85 g / cm ３ is preferable, 1.73 to 1.83 g / cm ３ is more preferable, 1.76 to 1.81 g / cm ３ is even more preferable.

[0019] For the carbon fiber bundle according to the embodiment, the diameter of the single fiber is preferably 4.0 μm or more, more preferably 4.5 μm or more, even more preferably 5.0 μm or more, most preferably 5.2 or more, and preferably 7.0 μm or less, more preferably 6.5 μm or less, even more preferably 6.1 μm or less. If the diameter of the single fiber is at least the lower limit value, the carbon fiber bundle has excellent fiber opening property. If the diameter of the single fiber is at most the upper limit value, the convergence property of the carbon fiber bundle is maintained and it is easy to handle. The preferable lower and upper limits of the diameter of the single fiber can be arbitrarily combined. For example, 4.0 to 7.0 μm is preferable, 4.5 to 6.5 μm is more preferable, 5.0 to 6.1 μm is even more preferable, and 5.2 to 6.1 μm is particularly preferable. The diameter of the single fiber is determined as follows. From the fiber density (g / cm ３ ) of the carbon fiber bundle, the mass per 1 m of the carbon fiber bundle, that is, the linear density (g / m), and the number of filaments of the carbon fiber bundle, the cross-sectional area of one single carbon fiber is calculated, and the diameter of a perfect circle having the same area as that cross-sectional area is defined as the diameter of the single carbon fiber of the carbon fiber.

[0020] The carbon fiber bundle of the present invention is preferably substantially untwisted. In the present invention, "substantially untwisted" means that the carbon fiber bundle does not contain any twists intentionally imparted during its manufacture. In other words, it means that there are no continuous twists in the fiber bundle, or that while there are localized twists, S-twists and Z-twists are present in equal proportions. Being substantially untwisted results in a carbon fiber bundle with excellent fiber-opening properties in the width direction of the carbon fiber bundle.

[0021] In the embodiment, the carbon fiber bundle preferably has a ratio of oxygen atoms to carbon atoms on the carbon fiber surface (O / C) of 0.138 or more, and a ratio of nitrogen atoms to carbon atoms on the carbon fiber surface (N / C) of 0.050 or more. Carbon fiber bundles that satisfy these conditions tend to have good affinity with resins.

[0022] The O / C ratio of the carbon fiber surface is preferably 0.138 or higher, more preferably 0.139 or higher, and even more preferably 0.140 or higher. The upper limit of the O / C ratio of the carbon fiber surface is not limited, but may be, for example, 0.200 or lower. The lower and upper limits of the O / C ratio can be arbitrarily combined, for example, 0.138 to 0.200, 0.139 to 0.200, or 0.140 to 0.200. The N / C ratio of the carbon fiber surface is preferably 0.050 or higher, more preferably 0.051 or higher, and even more preferably 0.052 or higher. The upper limit of the N / C ratio of the carbon fiber surface is not limited, but may be, for example, 0.100 or lower. The lower and upper limits of the N / C ratio can be arbitrarily combined, for example, 0.050 to 0.100, 0.051 to 0.100, or 0.052 to 0.100. The Si / C ratio on the carbon fiber surface is preferably 0.001 or higher, more preferably 0.002 or higher, and even more preferably 0.003 or higher. The upper limit of the Si / C ratio on the carbon fiber surface is not limited, but may be, for example, 0.010 or lower. The lower and upper limits of Si / C can be arbitrarily combined, and may be, for example, 0.001 to 0.010, 0.002 to 0.010, or 0.003 to 0.010. The O / C, N / C, and Si / C ratios on the carbon fiber surface can be controlled by adjusting the electrolytic treatment conditions in the surface treatment of the carbon fiber. The O / C, N / C, and Si / C ratios on the carbon fiber surface can be measured by X-ray photoelectron spectroscopy of the carbon fiber bundle.

[0023] The interfacial shear strength (IFSS) determined by the fragmentation test of the carbon fiber bundle according to the embodiment is preferably 42 MPa or higher, more preferably 44 MPa or higher, and even more preferably 46 MPa or higher. If the interfacial shear strength is above the lower limit, the adhesion between the carbon fiber bundle and the matrix resin tends to be sufficiently high, and the bending strength and torsional strength of the carbon fiber reinforced composite material can be increased. A higher interfacial shear strength of the carbon fiber bundle is preferable, but it may substantially be 65 MPa or lower. The lower and upper limits of the interfacial shear strength can be arbitrarily combined, for example, they may be 42 to 65 MPa, 44 to 65 MPa, or 46 to 65 MPa. The calculation of the interfacial shear strength by the fragmentation test is carried out by the method described in the example.

[0024] The single fiber strength of the carbon fiber bundle according to the embodiment, measured at a test length of 5 mm, is preferably 6.50 GPa or higher, more preferably 6.60 GPa or higher, and even more preferably 6.70 GPa or higher. If the single fiber strength is above the lower limit, the tensile properties of the composite can be well exhibited. The single fiber strength of the carbon fibers in the carbon fiber bundle is preferable as it is higher, but may substantially be 12 GPa or lower. The lower and upper limits of the single fiber strength can be arbitrarily combined, for example, it may be 6.50 to 12 GPa, 6.60 to 12 GPa, or 6.70 to 12 GPa. The single fiber strength can be controlled, for example, by adjusting the elongation rate of the fiber bundle in the flame-retardant process and the carbonization process. Note that "single fiber strength" refers to the average value measured for 100 single fibers. The single fiber strength is measured by a method compliant with JIS R 7606 Carbon Fiber - Test Method for Tensile Properties of Single Fibers.

[0025] [Method for Manufacturing Carbon Fiber Bundles] The method for manufacturing carbon fiber bundles according to the embodiment includes the following flame-retardant step and carbonization step. Flame-retardant step: A carbon fiber precursor fiber bundle is heated in an oxidizing atmosphere. Carbonization step: The flame-retardant fiber bundle after the flame-retardant step is heated in a non-oxidizing atmosphere. In the method for manufacturing carbon fiber bundles according to the embodiment, the carbon fiber precursor fiber bundle is substantially untwisted, the elongation rate of the fiber bundle in the region where the carbonization temperature of the carbonization step is 1000°C or higher is -2.7% or higher, and the maximum carbonization temperature is 1800°C or higher and 2200°C or lower. The method for manufacturing carbon fiber bundles according to the embodiment may further include at least one of a surface oxidation treatment step and a sizing treatment step, if necessary.

[0026] (Carbon Fiber Precursor Fiber Bundles) Carbon fiber precursor fiber bundles are substantially untwisted. This allows each individual fiber in the bundle to be heated evenly during the flame-retardant and carbonization processes, making it easier to obtain carbon fiber bundles with high strand strength. The statement that carbon fiber precursor fiber bundles are "substantially untwisted" means that they do not contain any intentionally added twist during the manufacturing of the carbon fiber precursor fiber bundles.

[0027] Examples of carbon fiber precursor fiber bundles include those made by bundling single fibers such as PAN fibers, rayon fibers, and pitch fibers. A carbon fiber precursor fiber bundle in which single PAN fibers are bundled is preferred. Hereinafter, a carbon fiber precursor fiber bundle in which single PAN fibers are bundled will also be referred to as a "carbon fiber precursor acrylic fiber bundle."

[0028] The carbon fiber precursor acrylic fiber bundle can be obtained, for example, by spinning a spinning solution containing an acrylonitrile polymer to form a coagulated yarn, and then subjecting it to conventionally known treatments such as washing with water, bath stretching, oil application, drying and densification, and stretching as needed. The acrylonitrile polymer only needs to have acrylonitrile units in its molecular structure, and may be a homopolymer of acrylonitrile or a copolymer of acrylonitrile and other monomers. Examples of the other monomers include methacrylic acid. The ratio of acrylonitrile units to other monomer units in the copolymer can be appropriately set according to the properties of the carbon fiber bundle to be produced.

[0029] There are no particular limitations on the spinning method of a spinning solution containing an acrylonitrile polymer, but examples include wet spinning, in which the spinning solution is spun directly into a coagulation bath; dry spinning, in which the spinning solution is coagulated in air; and wet-dry spinning, in which the spinning solution is spun into air first and then coagulated in a bath. From the viewpoint of reducing surface wrinkles on the sides of the single fibers and easily suppressing the generation of defects due to surface shape, it is preferable that the carbon fiber precursor acrylic fiber bundle is a fiber bundle produced by wet-dry spinning a spinning solution containing an acrylonitrile polymer. The spinning method by wet spinning or wet-dry spinning can be carried out by spinning the spinning solution into a coagulation bath from a nozzle having a circular cross-section hole. From the viewpoint of ease of solvent recovery, it is preferable to use an aqueous solution containing the solvent used in the spinning solution as the coagulation bath.

[0030] The single fiber fineness of the carbon fiber precursor fiber bundle is preferably 0.5 dtex or higher, more preferably 0.7 dtex or higher, and preferably 2.5 dtex or lower, and more preferably 2.0 dtex or lower. If the single fiber fineness is above the lower limit, a carbon fiber bundle with fewer fiber breaks is more likely to be obtained. If the single fiber fineness is below the upper limit, a carbon fiber bundle with less performance variation is more likely to be obtained. The lower and upper limits of the single fiber fineness of the carbon fiber precursor fiber bundle can be arbitrarily combined; for example, 0.5 to 2.5 dtex is preferred, and 0.7 to 2.0 dtex is more preferred.

[0031] The number of filaments in a carbon fiber precursor fiber bundle, that is, the number of single fibers constituting the carbon fiber precursor fiber bundle, is preferably 8,000 or more, more preferably 10,000 or more, even more preferably 12,000 or more, preferably 20,000 or less, and even more preferably 18,000 or less. If the number of filaments in a carbon fiber precursor fiber bundle is above the lower limit, the carbon fiber precursor fiber bundle exhibits excellent processability. If the number of filaments in a carbon fiber precursor fiber bundle is below the upper limit, the carbon fiber precursor fiber bundle exhibits excellent handling properties. The lower and upper limits for the number of filaments in a carbon fiber precursor fiber bundle can be arbitrarily combined; for example, 8,000 to 20,000 is preferred, 10,000 to 18,000 is more preferred, and 12,000 to 18,000 is even more preferred.

[0032] (Flame-retardant treatment) In the flame-retardant treatment process, carbon fiber precursor fiber bundles are heated in an oxidizing atmosphere to convert them into flame-retardant fiber bundles. For example, the flame-retardant treatment is performed in a hot air circulation type flame-retardant furnace at 180 to 280°C, preferably so that the fiber density of the flame-retardant fiber bundles after the flame-retardant treatment is 1,320 to 1,400 g / cm³. ３ One method involves passing a carbon fiber precursor fiber bundle through the material until it reaches a certain state. If the fiber density of the flame-retardant fiber bundle is above the lower limit, adhesion between individual fibers is easily prevented during the carbonization process. If the fiber density of the flame-retardant fiber bundle is below the upper limit, the flame-retardant process does not become too long, making it economical. Examples of gases that form an oxidizing atmosphere include air, oxygen, and nitrogen dioxide. Air is preferred from an economic standpoint. The flame-retardant treatment time is preferably, for example, 30 to 100 minutes.

[0033] In flame-retardant treatment, it is preferable to perform an elongation operation because it is easier to maintain the orientation of the fibril structure. The elongation rate of the carbon fiber precursor fiber bundle in flame-retardant treatment is preferably 1 to 8%. If the elongation rate of the fiber bundle in flame-retardant treatment is above the lower limit, it is easier to maintain and improve the orientation of the fibril structure, and a carbon fiber bundle with excellent mechanical properties is more likely to be obtained. If the elongation rate of the fiber bundle in flame-retardant treatment is below the upper limit, the fibril structure itself is less likely to break, and the subsequent formation of the carbon fiber structure is less likely to be impaired, making it easier to obtain a high-strength carbon fiber bundle. The elongation rate of the fiber bundle is calculated from the surface speed A of the first drive roller and the surface speed B of the last drive roller in each firing furnace (flame-retardant furnace, carbonization furnace) using the following formula: Elongation rate (%) = (Surface speed B - Surface speed A) / Surface speed A × 100

[0034] The fiber density of the flame-retardant fiber bundle after the flame-retardant treatment process is 1,320 g / cm³. ３ The above is preferable, and 1,330 g / cm³ is preferred. ３ The above is more preferable, specifically 1,340 g / cm³. ３ The above is even more preferable, and also 1,400 g / cm³. ３ The following is preferred: 1,380 g / cm³ ３ The following is more preferable: 1,360 g / cm³ ３ The following is even more preferable: If the fiber density of the flame-resistant fiber bundle is above the lower limit, a carbon fiber bundle with a good carbonization yield can be obtained, thus improving productivity. If the fiber density of the flame-resistant fiber bundle is below the upper limit, a carbon fiber bundle with superior performance is more likely to be obtained. The lower and upper limits of the fiber density of the flame-resistant fiber bundle can be arbitrarily combined, for example, 1.320 to 1.400 g / cm³. ３ Preferably, 1,330 to 1,380 g / cm³ ３ More preferably, 1,340 to 1,360 g / cm³ ３ That is even more preferable.

[0035] (Carbonization Treatment) In the carbonization process, flame-resistant fiber bundles are carbonized under an inert atmosphere to obtain carbon fiber bundles. Examples of gases that form the inert atmosphere include nitrogen, argon, and helium. From an economic standpoint, nitrogen is preferred.

[0036] In the carbonization process, the elongation rate of the fiber bundle in the region where the carbonization temperature is 1000°C or higher is -2.7% or higher, preferably -1.9% or higher, and more preferably -1.5% or higher. If the elongation rate is above the lower limit, a carbon fiber bundle with excellent tensile modulus is easily obtained. The upper limit of the elongation rate of the fiber bundle in the region where the carbonization temperature is 1000°C or higher is not particularly limited, but may be, for example, 0% or less. The lower and upper limits of the elongation rate of the fiber bundle in the region where the carbonization temperature is 1000°C or higher can be arbitrarily combined, for example, -2.7% to 0%, -1.9% to 0%, or -1.5% to 0%. Since the fibers shrink in the carbonization process, the elongation rate can be adjusted according to the tension generated in the fiber bundle.

[0037] In the region where the carbonization temperature of the carbonization process is 1800°C or higher, the generated tension of the fiber bundle is preferably 5 mN / dtex or higher, more preferably 7 mN / dtex or higher, and even more preferably 10 mN / dtex or higher. If the generated tension is above the lower limit, it is easier to obtain a carbon fiber bundle with excellent tensile modulus. The upper limit of the generated tension in the region where the carbonization temperature of the carbonization process is 1800°C or higher is not particularly limited, but for example, it is preferably 20 mN / dtex or lower, more preferably 13 mN / dtex or lower, and even more preferably 12 mN / dtex or lower. The lower and upper limits of the generated tension in the region where the carbonization temperature of the carbonization process is 1800°C or higher can be arbitrarily combined, for example, they may be 5 to 20 mN / dtex, 7 to 13 mN / dtex, or 10 to 12 mN / dtex. The tension generated by the fiber bundle is the average value obtained by dividing the average of four measurements taken with a digital tension meter (Nidec-Shimpo Corporation, DTMB-10C) on the fiber bundle at the exit of the process by the total fineness of the carbon fiber precursor fiber bundle (unit: mN / dtex).

[0038] The maximum carbonization temperature in the carbonization process is 2200°C or lower, preferably 2190°C or lower, more preferably 2150°C or lower, and even more preferably 2100°C or lower. If the maximum carbonization temperature is below the upper limit, a carbon fiber bundle with a smaller crystallite size and a surface that is easily electrolytically oxidized can be obtained, and the resulting carbon fiber bundle has excellent adhesion to the resin. The maximum carbonization temperature in the carbonization process is 1800°C or higher, preferably 1850°C or higher, and more preferably 1900°C or higher. If the maximum carbonization temperature is above the lower limit, it is easier to suppress the decrease in strand strength and strand modulus. The preferred lower and upper limits for the maximum carbonization temperature in the carbonization process can be arbitrarily combined, for example, 1800 to 2200°C is preferred, 1800 to 2190°C is more preferred, 1850 to 2150°C is even more preferred, and 1900 to 2100°C is particularly preferred. The term "highest carbonization temperature" refers to the highest ambient temperature measured at five or more locations within each carbonization furnace.

[0039] In the carbon fiber bundle manufacturing method according to the embodiment, it is preferable that the generated tension of the fiber bundle is 5 mN / dtex or more in the carbonization temperature range of 1000 to 1700°C in the carbonization step, and more preferably that the elongation rate of the fiber bundle is -2.7% or more and the generated tension is 5 mN / dtex or more. If the generated tension is 5 mN / dtex or more, it is easier to obtain a carbon fiber bundle with a high degree of crystal orientation, and if the elongation rate in the above range is -2.7% or more, it is easier to obtain a carbon fiber bundle with a high tensile modulus. From these viewpoints, the generated tension of the fiber bundle in the carbonization temperature range of 1000 to 1700°C in the carbonization step is more preferably 7 mN / dtex or more, even more preferably 10 mN / dtex or more, and may be 5 to 20 mN / dtex, 7 to 13 mN / dtex, or 10 to 12 mN / dtex. Furthermore, in the carbonization temperature range of the carbonization process, the elongation rate of the fiber bundle is more preferably -1.9% or higher, and even more preferably -1.5% or higher. In the carbonization temperature range of the carbonization process, for example, the elongation rate of the fiber bundle may be -2.7 to 0% and the generated tension may be 5 to 20 mN / dtex, the elongation rate of the fiber bundle may be -1.9 to 0% and the generated tension may be 7 to 13 mN / dtex, and the elongation rate of the fiber bundle may be -1.5 to 0% and the generated tension may be 10 to 12 mN / dtex.

[0040] The heating temperature (carbonization temperature) when carbonizing flame-resistant fiber bundles is preferably 300 to 2200°C. It is preferable to raise the carbonization temperature during the carbonization process. This can be achieved, for example, by setting up multiple carbonization furnaces, setting the temperature of each furnace so that the temperature increases from the upstream carbonization furnace to the downstream carbonization furnace, and sequentially passing the flame-resistant fiber bundles through the furnaces from upstream to downstream.

[0041] The following describes an example of a carbonization treatment performed by increasing the carbonization temperature. In this embodiment, the flame-resistant fiber bundle is carbonized by sequentially performing a first carbonization treatment in a first carbonization furnace with a temperature gradient of 300 to 800°C in an inert atmosphere, a second carbonization treatment in a second carbonization furnace with a temperature gradient of 1000 to 1750°C in an inert atmosphere, and a third carbonization treatment in a third carbonization furnace with a temperature gradient of 1800 to 2200°C in an inert atmosphere.

[0042] The temperature rise gradient in the first carbonization treatment is preferably a linear gradient of 300 to 800°C, and more preferably a linear gradient of 300 to 700°C. However, the temperature rise gradient in the first carbonization treatment does not have to be linear. The starting temperature (minimum temperature) of the first carbonization treatment is preferably 300°C or higher, and the maximum temperature is preferably 800°C or lower. If the maximum temperature of the first carbonization treatment is 800°C or lower, it is easier to suppress the process yarn from becoming brittle. The processing time for the first carbonization treatment is preferably 1 to 3 minutes. If the processing time for the first carbonization treatment is above the lower limit, decomposition reactions due to rapid temperature increases are less likely to occur, and high-strength carbon fiber bundles are more easily obtained. If the processing time for the first carbonization treatment is below the upper limit, the decrease in the degree of crystal orientation is suppressed, and carbon fiber bundles with excellent mechanical properties are more easily obtained.

[0043] In the first carbonization treatment, it is preferable to perform an elongation operation because it is easier to maintain the orientation of the fibril structure. The elongation rate of the fiber bundle in the first carbonization treatment is preferably 2 to 7%. If the elongation rate in the first carbonization treatment is above the lower limit, it is easier to maintain and improve the orientation of the fibril structure, and a carbon fiber bundle with excellent mechanical properties can be easily obtained. If the elongation rate in the first carbonization treatment is below the upper limit, the fibril structure itself is less likely to break, and the subsequent formation of the carbon fiber structure is less likely to be impaired, so a high-strength carbon fiber bundle can be easily obtained.

[0044] In the second carbonization treatment, a linear temperature rise gradient of 1000 to 1750°C is preferred, and a linear temperature rise gradient of 1100 to 1700°C is more preferred. However, the temperature rise gradient in the second carbonization treatment does not have to be linear. The temperature in the second carbonization treatment can be set according to the desired strand modulus required for the carbon fiber bundle. To obtain a carbon fiber bundle with excellent mechanical properties, it is preferable to have a low maximum temperature in the second carbonization treatment. By lowering the maximum temperature in the second carbonization treatment and increasing the treatment time, it becomes easier to obtain a carbon fiber bundle with a high strand modulus. In addition, since a longer treatment time in the second carbonization treatment allows for a gentler temperature rise gradient, it also has the effect of suppressing the formation of defects. As an example, it is preferable to set the minimum temperature of the second carbonization treatment to 1000°C or higher, and the maximum temperature to 1750°C or lower. The treatment time for the second carbonization treatment is preferably 1 to 3 minutes.

[0045] In the second carbonization treatment, the process fibers undergo significant shrinkage, so it is preferable to perform the treatment while the fiber bundle is under tension. The elongation rate of the fiber bundle at this time is preferably -2.7 to 0%. If the elongation rate in the second carbonization treatment is above the lower limit, the orientation of the crystals in the fiber axis direction is less likely to decrease, and sufficient tensile properties can be obtained. If the elongation rate in the second carbonization treatment is below the upper limit, the structure that has been formed so far is less likely to be destroyed, and the strength can be maintained well.

[0046] The temperature rise gradient in the third carbonization treatment is preferably a linear gradient of 1800 to 2200°C, more preferably a linear gradient of 1800 to 2190°C, and even more preferably a linear gradient of 1800 to 2150°C. However, the temperature gradient in the third carbonization treatment does not have to be linear. The minimum temperature of the third carbonization treatment is preferably 1800°C or higher, the maximum temperature is preferably 2200°C or lower, more preferably 2190°C or lower, and even more preferably 2150°C or lower. If the maximum temperature of the third carbonization treatment is below the above upper limit, the lifespan of the furnace material of the third carbonization furnace can be extended. The processing time for the third carbonization treatment is preferably 1 to 3 minutes. The total processing time for the second and third carbonization treatments is preferably 2 to 6 minutes.

[0047] In the third carbonization treatment, the process fibers undergo significant shrinkage, so it is preferable to perform the treatment while the fiber bundle is under tension. The elongation rate of the fiber bundle at this time is preferably -2.7 to 0%. If the elongation rate in the third carbonization treatment is above the lower limit, the orientation of the crystals in the fiber axis direction is less likely to decrease, and sufficient tensile properties can be obtained. If the elongation rate in the second carbonization treatment is below the upper limit, the structure that has been formed so far is less likely to be destroyed, and the strength can be maintained well. Furthermore, the combined elongation rate of the fiber bundle from the second and third carbonization treatments is preferably -2.7 to 0%.

[0048] In the carbonization process described above, the carbonization temperature is raised using three carbonization furnaces with different temperature gradients. However, two or more carbonization furnaces may be used for the carbonization process. Alternatively, multiple heating zones may be set up within a single carbonization furnace, and the temperature of each heating zone may be set so that the temperature increases from the upstream heating zone to the downstream heating zone, and the flame-resistant fiber bundles may be passed through sequentially from the upstream heating zone to the downstream heating zone. However, in any of these methods, the heating rate when raising the temperature to 1800 to 2200°C during the carbonization process is preferably 300 to 600°C / min, more preferably 350 to 550°C / min, and even more preferably 400 to 500°C / min.

[0049] (Surface Oxidation Treatment) Known methods can be used for surface oxidation treatment of carbon fiber bundles, such as electrolytic oxidation, chemical oxidation, and air oxidation. Among these, electrolytic oxidation, which is widely used industrially, is preferred because it allows for stable surface oxidation treatment.

[0050] In the surface oxidation treatment of carbon fiber bundles, the iPa value representing the surface treatment state of the carbon fibers is set to 0.30 μA / cm². ２It is preferable to keep the values ​​above. To control the values ​​within this range, a simple method is to adjust the amount of electricity using electrolytic oxidation treatment. In electrolytic oxidation treatment, even with the same amount of electricity, the iPa will vary greatly depending on the electrolyte used and its concentration. However, in an alkaline aqueous solution with a pH greater than 7, it is preferable to perform the oxidation treatment by flowing an electric charge of 10 to 200 coulombs / g with carbon fiber as the anode. Examples of electrolytes include sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, ammonium carbonate, ammonium bicarbonate, ammonium sulfate, calcium hydroxide, sodium hydroxide, and potassium hydroxide.

[0051] (Sizing Treatment) Sizing treatment can be performed by applying a sizing agent-containing solution, which consists of a solution in which a sizing agent is dissolved in an organic solvent or an emulsion solution in which a sizing agent is dispersed in water with an emulsifier, to a bundle of carbon fibers using methods such as roller immersion or roller contact, and then drying the solution. Known sizing agents can be used, such as sizing agents mainly composed of epoxy resin, polyether resin, epoxy-modified polyurethane resin, or polyester resin. The amount of sizing agent adhering to the surface of the carbon fibers can be controlled by adjusting the concentration of the sizing agent-containing solution and the amount of squeezing. Drying can be performed using hot air, hot plates, heated rollers, various infrared heaters, etc.

[0052] <Effects> The carbon fiber bundle of the present invention described above has a crystallite size of 2.90 to 3.40 nm and an iPa of 0.30 μA / cm² on the carbon fiber surface. ２ As a result, the interfacial shear strength (IFSS), which indicates adhesion to the resin, can be increased without reducing the strand modulus of elasticity or strand strength, and the 90° bending strength of the carbon fiber reinforced composite material can be increased.

[0053] <Applications> The carbon fiber bundles of the present invention can be used in a variety of applications by, for example, combining them with a matrix resin and molding them as carbon fiber reinforced composite materials. The applications of carbon fiber reinforced composite materials are not particularly limited and include, for example, automotive components, aerospace materials, civil engineering and construction materials, sports and leisure materials, pressure vessels, wind turbine blades, and other industrial materials.

[0054] The matrix resin is not particularly limited, but examples include thermosetting resins such as epoxy resins and phenolic resins; radical polymerization resins such as acrylic resins, vinyl ester resins, and unsaturated polyester resins; and thermoplastic resins such as thermoplastic acrylic resins, polyamide resins, polyimide resins, polycarbonate resins, polypropylene resins, and polyethylene resins. Modified versions of these resins can also be used. Commercially available products may be used as the matrix resin. One type of matrix resin may be used alone, or two or more types may be used in combination.

[0055] The present invention will be specifically described below with reference to examples, but the present invention is not limited by the following description unless it exceeds the gist of the invention. The various measurement methods used in these examples are as follows.

[0056] [Method for measuring the diameter of a single carbon fiber] Fiber density of a carbon fiber bundle (g / cm³) ３ The mass per meter of carbon fiber bundle, i.e., basis weight (g / m), and the number of filaments in the carbon fiber bundle were used to calculate the cross-sectional area of ​​a single carbon fiber. The diameter of a perfect circle with an area equal to that cross-sectional area was calculated and used as the diameter of the single carbon fiber. The mass per meter of carbon fiber bundle was calculated by measuring the mass of 10 carbon fiber bundles, each 1 meter long, and using the average value.

[0057] [Method for measuring the fiber density of carbon fiber bundles] The fiber density of carbon fiber bundles was measured in accordance with Method C (density groove method) described in JIS R 7603:1999.

[0058] [Method for measuring crystallite size] The crystallite size Lc was calculated using Scherrer's formula shown below, based on measurement data from a wide-angle X-ray diffractometer (peaks around 2θ = 25-26°). • Apparatus: Rotating pair cathode X-ray generator, Rigaku Corporation, TTR-III • X-ray source: CuKα rays (tube voltage 50kV, tube current 300mA) • Detector: NaI scintillation counter • Scanning range: 2θ = 10-40° • Scanning mode: Step scan, step unit 0.04°, counting time 1.5 seconds The Kα2 component included in the obtained diffraction profile was removed by the Rachinger method. For the peaks appearing around 2θ = 25-26°, the peak position and integral width were calculated using the profile fitting method, and these were then applied to the diffraction angle θ. Ｂ and half-width β Ｅ The following Scherrer formula was used to calculate the crystallite size from these values: Crystallite size (nm) = Kλ / (β ０ cosθ Ｂ ) However, the symbols in the above formula have the following meanings: K: 0.9 λ: 0.15418 nm (wavelength of X-rays) β ０ :(β Ｅ ２ -β １ ２ ) １／２ β Ｅ : Full width at half maximum (measured value) rad β １ : 1.75 x 10 －３ rad (value specific to the measuring device) θ Ｂ Bragg diffraction angle

[0059] [Measurement Method for Strand Properties] The strand strength and strand modulus of carbon fiber bundles were measured in accordance with JIS R 7608:2007. The strand modulus was calculated using Method A of the same standard.

[0060] [Measurement Method for O / C, N / C, and Si / C on Carbon Fiber Surfaces] X-ray photoelectron spectrometer: Quantera II, a product of ULVAC-FI, was used. Measurement method: Carbon fibers were placed on a sample stage and fixed, and the detector was positioned perpendicular to the fiber axis. XPS was measured under normal conditions. The O1s peak area was integrated from 528 eV to 540 eV, and the C1s peak area was integrated from 281 eV to 291 eV. The C1s peak area and O1s peak area were corrected using the correction coefficients installed in the instrument, and the surface oxygen functional group concentration (O / C) was obtained as a ratio to the carbon concentration. The N1s peak area was integrated from 395 eV to 408 eV, and the C1s peak area was integrated from 281 eV to 291 eV. The C1s peak area and the N1s peak area were corrected using the correction coefficients installed on the instrument, and the surface nitrogen functional group concentration (N / C) was obtained as a ratio to the carbon concentration. The Si2p peak area was integrated in the range of 97 eV to 110 eV, and the C1s peak area in the range of 281 eV to 291 eV. The C1s peak area and the Si2p peak area were corrected using the correction coefficients installed on the instrument, and the surface silicon functional group concentration (Si / C) was obtained as a ratio to the carbon concentration.

[0061] [Method for measuring iPa of carbon fiber bundles] iPa was measured by the following method. A 5% by mass aqueous phosphoric acid solution with pH 3 was used as the electrolyte, and dissolved oxygen was removed by gently bubbling nitrogen into it for 15 minutes. The bubbling process was performed each time the carbon fiber bundle sample was changed. The carbon fiber bundle sample was immersed in the electrolyte as one electrode, a platinum electrode with sufficient surface area was used as the counter electrode, and an Ag / AgCl electrode was used as the reference electrode. The immersion length of the sample was 40 mm. The scanning range of the potential applied between the carbon fiber electrode and the platinum electrode was set to -0.2 V to +0.8 V, and the scanning speed was set to 2.0 mV / sec. A current-voltage curve was drawn and swept at least three times. Using the potential at +0.4 V relative to the Ag / AgCl electrode as the reference potential, the positive current value i in the second and third current-voltage curves was read, and iPa was calculated from the average value of the current value i according to the following formula. iPa (μA / cm) ２ ) = i (μA) / sample length (cm) × (4π × basis weight (g / cm) × number of single fibers / fiber density (g / cm)３ )) １／２ The diameter of a single fiber was calculated from the fiber density, basis weight, and number of fibers of the carbon fiber bundle. The apparent surface area was calculated from the diameter of the single fiber, the number of fibers, and the sample length, and the current value i was divided by the surface area to obtain iPa. This measurement was performed using an electrochemical measurement system (manufactured by Meiden Hokuto, product name: HZ-7000). For sizing-treated carbon fibers, the measurement was performed after removing the sizing agent. Methods for removing the sizing agent include, for example, washing with a solvent in which the sizing agent is soluble, or heat treatment (e.g., heat treatment at 500°C for 1 hour). In the removal of the sizing agent in this invention, the Soxhlet extraction method using methyl ethyl ketone was applied. A 2m carbon fiber bundle was fixed in an extractor, and the sizing agent was removed by refluxing 200mL of methyl ethyl ketone at 90°C for 8 hours. The sample was dried for 12 hours to remove the methyl ethyl ketone, and iPa measurement was performed on the resulting sample.

[0062] [Method for measuring bending strength (FS90°)] (Preparation of prepreg) An uncured curable resin composition consisting of 70 parts by mass of oxazolidone-type epoxy resin (TSR400: manufactured by DIC Corporation), 30 parts by mass of meta-type triglycidylaminophenol (MY0600: manufactured by Huntsman Japan Co., Ltd., product name "MY0600"), and 17 parts by mass of curing agent (1400F: dicyandiamide: manufactured by Air Products Co., Ltd.) is made into a film using a comma coater (manufactured by Hirano Texseed Co., Ltd., "M-500"), with a resin basis weight of 16.7 g / m². ２ A resin film was prepared. This resin film was then used to align carbon fiber bundles, resulting in a fiber weight of 100 g / m². ２ The carbon fiber sheet is laminated to both sides and impregnated using a heated roll, resulting in a fiber weight of 133.4 g / m². ２ An uncured prepreg with a resin content of 25% by mass was obtained.

[0063] (Preparation of Fiber-Reinforced Plastic Sheet) The uncured prepreg with a resin content of 25% by mass obtained above was cut to 300 mm x 300 mm, and 24 sheets were stacked so that the fiber direction when viewed from the stacking direction is [0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0° / 0°], that is, all the fiber directions were aligned, to obtain a laminate. This laminate was heated in an autoclave at a pressure of 0.04 MPa at a rate of 2°C / min, held at 80°C for 60 minutes, and then heated at a pressure of 0.6 MPa at a rate of 2°C / min, held at 130°C for 90 minutes to heat-cur it and obtain a fiber-reinforced plastic sheet with a thickness of 2.1 mm.

[0064] (Measurement of 90° bending strength (FS90°)) The 2.1 mm thick fiber-reinforced plastic sheet obtained in the above [Preparation of fiber-reinforced plastic sheet] was processed into a test specimen measuring 60 mm in length and 12.7 mm in width. The test specimen was subjected to a universal testing machine (INSTRON, "INSTRON 5565") equipped with a three-point bending jig (indenter R = 5.0 mm, support R = 3.2 mm), with the ratio of the distance between supports (L) to the thickness of the test specimen (d) being L / d = 16, and the crosshead speed (meters per minute) = (L ２ The bending strength of the fiber-reinforced plastic sheet was measured at 90° using the formula (x0.01) / (6xd). A higher bending strength indicates better adhesion between the carbon fibers and the matrix resin.

[0065] [Method for Measuring Interfacial Shear Strength (IFSS)] The adhesive strength between carbon fibers and matrix resin was evaluated by measuring the interfacial shear strength using the single-fiber embedding (fragmentation) method. As a single-fiber embedding method, for example, the method described on pages 157-160 of "Development and Evaluation Methods for Carbon Fibers" (Realize Co., Ltd.) can be used.

[0066] First, a single carbon fiber was extracted from a carbon fiber bundle, and the extracted carbon fiber was embedded in a matrix resin to prepare a test specimen. For the preparation of the test specimen, a mixture of 100 parts by mass of jER828, a product of Mitsubishi Chemical Corporation, and 15 parts by mass of metaphenylenediamine was used as the matrix resin. This mixture was poured into a dedicated mold and cured at 20°C for 24 hours and at 60°C for 6 hours. Next, a tensile test was performed on the test specimen. The tensile test was performed at room temperature, and after applying elongation within the range where the test specimen did not break (elongation of 7%), the length of the broken fiber in the matrix resin was read using a polarizing microscope. The above measurement was performed 10 times. The average fiber length Ln of the 10 measurements was calculated from the length of the broken fiber. Then, the interfacial shear strength (IFSS) was calculated from the following equations (1) and (2). Critical fiber length L0 [mm] = 4 × average fiber length Ln [mm] / 3 ... (1) IFSS [MPa] = fiber strength S0 [MPa] × fiber diameter [mm] / (2 × critical fiber length L0 [mm]) ... (2) In equation (2), the fiber strength S0 is the fiber strength at the critical fiber length L0, obtained by extrapolating from the single fiber strength measured in accordance with the test method for tensile properties of carbon fiber single fibers, JIS R 7606:2000.

[0067] [Method for measuring single fiber strength] The single fiber strength of the obtained carbon fiber bundle was measured using a method in accordance with JIS R 7606 Carbon Fiber - Test method for tensile properties of single fibers, with a test length of 5 mm.

[0068] [Overall Evaluation] A: Strand strength of 5.40 GPa or higher, strand modulus of elasticity of 370 GPa or higher, IFSS of 50 MPa or higher, FS90 of 70.0 MPa or higher B: Strand strength of 5.40 GPa or higher, strand modulus of elasticity of 370 GPa or higher, IFSS of 42 MPa or higher and less than 50 MPa, FS90 of 67 MPa or higher and less than 70 MPa C: Strand strength of less than 5.40 GPa, strand modulus of elasticity of 370 GPa or higher, IFSS of less than 42 MPa, FS90 of less than 67 MPa D: Strand strength of less than 5.40 GPa, strand modulus of elasticity of less than 370 GPa, IFSS of less than 42 MPa, FS90 of 65 MPa or lower

[0069] [Example 1] <Preparation of carbon fiber precursor fiber bundles> An acrylonitrile copolymer containing 98% by mass of acrylonitrile units and 2% by mass of methacrylic acid units was dissolved in dimethylformamide to prepare a 23.5% by mass spinning solution. This spinning solution was spun from a spinneret and wet-dry spinning was performed. That is, the fibers were spun into the air and passed through a space of about 5 mm, then solidified in a coagulation solution filled with an aqueous solution containing 79.0% by mass of dimethylformamide, which was heated to 10°C, and the solidified yarn was taken up. Next, the solidified yarn was stretched 1.1 times in the air, and then further stretched 2.5 times in a stretching tank filled with an aqueous solution containing 35% by mass of dimethylformamide, which was heated to 60°C. After stretching, the process fiber bundles containing the solvent were washed with clean water, and then stretched 1.4 times in hot water at 95°C. Next, an oil solution mainly composed of amino-modified silicone was applied to the fiber bundle at a concentration of 1.1% by mass, and the bundle was dried and densified. The dried and densified fiber bundle was stretched 2.6 times between heated rolls to further improve orientation and densification, and then wound up to obtain a carbon fiber precursor fiber bundle (carbon fiber precursor acrylic fiber bundle) consisting of 12,000 fibers. The single fiber fineness of the obtained carbon fiber precursor fiber bundle was 0.77 dtex, and it was substantially untwisted.

[0070] <Preparation of carbon fiber bundles> Multiple carbon fiber precursor bundles obtained are introduced into a flame-retardant furnace in a parallel arrangement, and air heated to 220-280°C is blown onto the carbon fiber precursor bundles to flame-retardant treatment, resulting in a fiber density of 1.345 g / cm³. ３ Flame-resistant fiber bundles were obtained. The elongation rate of the fiber bundles was set to 0%, and the flame-retardant treatment time was 50 minutes.

[0071] Next, the flame-resistant fiber bundle was passed through a first carbonization furnace with a temperature rise gradient of 300 to 700°C in a nitrogen atmosphere, while applying a 4.5% elongation, to perform the first carbonization treatment. The temperature rise gradient was set to be linear. The treatment time was 2.0 minutes. Furthermore, a second carbonization treatment was performed using a second carbonization furnace with a temperature rise gradient of 1000 to 1700°C in a nitrogen atmosphere, and subsequently, a third carbonization treatment was performed using a third carbonization furnace with a temperature rise gradient of 1800 to 2040°C in a nitrogen atmosphere to obtain a carbon fiber bundle. The total elongation rate of the fiber bundle in the second and third carbonization furnaces was -0.7%, and the total treatment time was 3.5 minutes. The tension generated in the fiber bundle in the second carbonization furnace, where the carbonization temperature was in the range of 1000 to 1700°C, was 12 mN / dtex. The minimum temperature in the second carbonization furnace was 1000°C, and the maximum temperature was 1700°C. Furthermore, the minimum temperature in the third carbonization furnace was set to 1800°C, the maximum temperature to 2040°C, the heating rate when increasing the temperature from 1800°C to 2040°C was 453°C / min, and the tension generated in the fiber bundle was 13 mN / dtex. In addition, the temperature rise gradients in the second and third carbonization furnaces were set to be linear.

[0072] Next, the carbon fiber bundle was run through a 5% phosphoric acid aqueous solution, with the carbon fiber bundle as the anode. An electric current was applied between the carbon fiber bundle and the counter electrode to achieve an electric current of 40 coulombs per gram of carbon fiber to be treated. Then, the carbon fiber bundle was washed with 90°C hot water, and then run through a 10% by mass aqueous solution of ammonium bicarbonate. With the carbon fiber bundle as the anode, an electric current was applied between the carbon fiber bundle and the counter electrode to achieve an electric current of 40 coulombs per gram of carbon fiber to be treated. Next, the carbon fiber bundle was washed with 90°C hot water and dried. Then, 0.5% by mass of a sizing agent (DIC Corporation, "Hydran N320") was applied to the carbon fiber bundle (sizing treatment), and it was wound onto a bobbin. For the carbon fiber bundle after sizing treatment, the diameter and number of single fibers, the basis weight and fiber density of the carbon fiber bundle, the strand strength, and the strand modulus were measured. These results are shown in Table 1. Carbon fiber bundles with high strand strength and interfacial shear strength (IFSS) were obtained.

[0073] [Examples 2-3] Carbon fiber bundles were prepared in the same manner as in Example 1, except that the elongation rate and generated tension of the fiber bundles in the second carbonization furnace, where the carbonization temperature was in the range of 1000-1700°C, and the maximum temperature set in the third carbonization furnace were changed as shown in Table 1, and various measurements were performed. The results are shown in Table 1. Good carbon fiber bundles were obtained with a good balance of strand strength, strand modulus, IFSS, and FS90.

[0074] [Comparative Example 1] Carbon fiber bundles were prepared in the same manner as in Example 1, except that the maximum temperature setting of the third carbonization furnace was set to 2320°C, and the elongation rate and generated tension of the fiber bundles in the second carbonization furnace, where the carbonization temperature was in the range of 1000 to 1700°C, were changed as shown in Table 1. Various measurements were then performed. The results are shown in Table 1. The obtained carbon fiber bundles had low strand strength and IFSS.

[0075] [Comparative Example 2] Carbon fiber bundles were prepared in the same manner as in Example 1, except that the maximum temperature set in the third carbonization furnace was 2220°C, the elongation rate and tension generated of the fiber bundles in the carbonization process were changed as shown in Table 1, and the amount of electricity in the 5% aqueous phosphoric acid solution was set to 20 C / g. Various measurements were then performed. The results are shown in Table 1. The obtained carbon fiber bundles resulted in a lower FS90 of the fiber-reinforced plastic plate.

[0076]

[0077] As shown in Table 1, the carbon fiber bundles of Examples 1 to 3 exhibited small crystallite size, high interfacial shear strength, and suppressed decreases in strand strength and strand modulus. On the other hand, the carbon fiber bundle of Comparative Example 1 had lower strand strength relative to its fiber density, a larger crystallite size, and lower interfacial shear strength compared to the carbon fiber bundles of Examples 1 to 3. Furthermore, the carbon fiber bundle of Comparative Example 1 had a lower FS90 of the fiber-reinforced plastic plate compared to the carbon fibers of Examples 1 to 3.

[0078] Because the carbon fiber bundle of the present invention has high strand strength, it is useful in a wide range of applications where high strength is required, such as automotive components, aerospace materials, civil engineering and construction materials, sports and leisure materials, pressure vessels, wind turbine blades and other industrial materials.

Claims

1. Crystallite size of 2.90–3.40 nm, and iPa of the carbon fiber surface of 0.30 μA / cm². ２ The above is a carbon fiber bundle.

2. The carbon fiber bundle according to claim 1, wherein the O / C ratio of the carbon fiber surface is 0.138 or higher.

3. The carbon fiber bundle according to claim 1, wherein the N / C ratio of the carbon fiber surface is 0.050 or higher.

4. Fiber density of 1.70–1.85 g / cm³ ３ The carbon fiber bundle according to claim 1.

5. The carbon fiber bundle according to claim 1, wherein the diameter of the individual carbon fibers is 4.0 to 7.0 μm.

6. The carbon fiber bundle according to claim 1, which is substantially untwisted.

7. The carbon fiber bundle according to claim 1, wherein the interfacial shear strength obtained by fragmentation testing is 42 MPa or more.

8. The carbon fiber bundle according to claim 1, wherein the strand strength is 4.00 GPa or more.

9. The carbon fiber bundle according to claim 1, wherein the strand modulus of elasticity is 370 GPa or more.

10. The carbon fiber bundle according to claim 1, comprising single fibers as carbon fibers, wherein the single fiber strength measured at a test length of 5 mm is 6.50 GPa or more.

11. A method for producing a carbon fiber bundle, comprising a flame-retardant step of heating a carbon fiber precursor fiber bundle in an oxidizing atmosphere, and a carbonization step of heating the flame-retardant fiber bundle after the flame-retardant step in a non-oxidizing atmosphere, wherein the carbon fiber precursor fiber bundle is substantially untwisted, the elongation rate of the fiber bundle in the region where the carbonization temperature of the carbonization step is 1000°C or higher is -2.7% or higher, and the maximum carbonization temperature is 1800°C or higher and 2200°C or lower.

12. The method for producing a carbon fiber bundle according to claim 11, wherein the tension generated in the fiber bundle in the region where the carbonization temperature of the carbonization step is 1800°C or higher is 5 mN / dtex or higher.

13. The method for producing a carbon fiber bundle according to claim 11, wherein the generated tension of the fiber bundle in the carbonization temperature of the carbonization step is in the range of 1000 to 1700°C is 5 mN / dtex or more.

14. The fiber density of the flame-retardant fiber bundle after the flame-retardant treatment process is 1,320 to 1,400 g / cm³. ３ The method for producing a carbon fiber bundle according to claim 11.

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