carbon fiber bundle

Abstract

To provide a carbon fiber bundle having a large fiber diameter and a balanced material property between strand modulus of elasticity and strand strength, and a method for manufacturing a carbon fiber bundle. [Solution] A carbon fiber bundle in which multiple single carbon fibers are bundled together, wherein the diameter of the single fibers is 5.7 μm or more and 6.5 μm or less, the strand strength is 4.7 GPa or more, and the strand modulus is 310 GPa or more. The knot strength of the carbon fiber bundle is 400 N / mm². 2 Preferably, the above conditions are met, and the fracture surface formation energy of the carbon fiber bundle is preferably 19 N / m or higher.

Description

[Technical Field]

[0001] This invention relates to a carbon fiber bundle. [Background technology]

[0002] To improve the mechanical properties of resin-based molded products, it is common practice to composite resins with fibers as reinforcing materials. Among these, carbon fiber is particularly advantageous due to its superior specific strength, specific modulus of elasticity, and lightweight nature. As a reinforcing fiber for high-performance resins, it is used in a wide range of applications, including not only conventional sports and general industrial uses, but also aerospace and automotive applications. In recent years, the advantages of carbon fiber-reinforced composite materials, obtained by integrating carbon fiber as a reinforcing fiber with a matrix resin, have been increasing, and there is a growing demand for improved performance of fiber-reinforced composite materials, especially in automotive and aerospace applications.

[0003] These carbon fiber reinforced composite materials are formed, for example, from prepregs, which are intermediate products in which reinforcing fibers are impregnated with a matrix resin, through molding and processing processes such as heating and pressurizing. In order to pursue high performance in the composite of carbon fibers and matrix resin, it is important not only to improve the mechanical properties such as the strength and elastic modulus of the carbon fibers themselves, but also to increase the impregnation of the matrix resin into the carbon fibers and suppress the formation of voids in the carbon fiber reinforced composite material.

[0004] Given the background described above, attempts have been made to obtain carbon fiber bundles that possess high strength and elastic modulus, as well as excellent resin impregnation properties. Patent Document 1 describes a technique for improving the productivity and mechanical properties of composite materials while maintaining excellent tensile modulus by imparting twist to the fiber bundles during the carbonization process in the manufacturing process of carbon fiber bundles. Patent Document 2 describes a technique for improving the productivity and mechanical properties of composite materials while maintaining excellent tensile modulus by carbonizing them with high tensile tension during the carbonization process. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2019 / 244830 [Patent Document 2] International Publication No. 2019 / 203088 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, the carbon fiber bundles described in Patent Document 1 need to be manufactured by twisting, which not only reduces productivity due to the increased number of steps, but also causes excessive converging of the fiber bundles by twisting, resulting in insufficient resin impregnation even after untwisting after firing. Furthermore, although the carbon fiber bundles described in Patent Document 2 have high strength, they are subjected to a strong entanglement treatment in order to perform carbonization treatment at high tensile strength. Due to the strong entanglement treatment, the bundle strength of the fiber bundles is improved, so less fuzz is generated even when the tensile strength is increased during the carbonization treatment. However, because the resulting carbon fiber bundles are strongly entangled, the impregnation of the resin was insufficient. Generally, it is believed that the thicker the diameter of the individual fibers that make up a carbon fiber bundle, the better the resin impregnation.

[0007] Furthermore, while the market demands carbon fiber bundles with higher strength and elastic modulus than ever before, generally, increasing strand strength tends to decrease strand elastic modulus, and increasing strand elastic modulus tends to decrease strand strength. Thus, improving strand strength and improving strand elastic modulus are contradictory, making it difficult to improve both properties simultaneously.

[0008] The present invention was made to solve the above problems and aims to provide a carbon fiber bundle having a large fiber diameter and a balanced physical property of strand modulus and strand strength, as well as a method for manufacturing a carbon fiber bundle. [Means for solving the problem]

[0009] The present invention has the following aspects. [1] A carbon fiber bundle in which a plurality of single fibers of carbon fiber are bundled, where the diameter of the single fiber is 5.7 μm or more and 6.5 μm or less, the strand strength is 4.7 GPa or more, and the strand elastic modulus is 310 GPa or more. [2] The carbon fiber bundle according to [1], wherein the product of the strand strength and the strand elastic modulus is 1650 or more. [3] The carbon fiber bundle according to [1], wherein the strand strength is 4.9 GPa or more and the strand elastic modulus is 365 GPa or more. [4] The carbon fiber bundle according to [1], wherein the strand elastic modulus is 370 GPa or more and 400 GPa or less. [5] The carbon fiber bundle according to [1], wherein the strand strength is 5.4 GPa or more. [6] The carbon fiber bundle according to any one of [1] to [5], wherein the knot strength is 400 N / mm 2 or more. [7] The carbon fiber bundle according to any one of [1] to [6], wherein the knot strength is 415 N / mm 2 or more. [8] The carbon fiber bundle according to any one of [1] to [7], wherein the fracture surface generation energy is 19 N / m or more. [9] The carbon fiber bundle according to any one of [1] to [8], wherein the fracture surface generation energy is 20.5 N / m or more.

[10] The carbon fiber bundle according to any one of [1] to [9], wherein the density is 1.79 g / cm 3 or more.

[11] The carbon fiber bundle according to any one of [1] to

[10] , which is substantially untwisted.

[0010]

[12] A method for producing a carbon fiber bundle, comprising the following steps (1) to (6). (1) A step of discharging an acrylonitrile-based polymer solution from a discharge hole once into the air using a dry-wet spinning method, and then coagulating it in a coagulation bath composed of an aqueous solution having a temperature of 10 °C or less and an organic solvent concentration of 79.3% by mass or more and 82.0% by mass or less to obtain a coagulated yarn bundle containing the organic solvent. (2) A step of obtaining a carbon fiber precursor acrylic fiber bundle by stretching the solidified yarn bundle in a warm aqueous solution at a temperature of 75°C or higher and with an organic solvent concentration of 40% by mass or more and 65% by mass or less, to a stretch ratio of 2.0 times or more and 3.2 times or less. (3) A flame-retardant step in which the carbon fiber precursor acrylic fiber bundle is heated in an oxidizing atmosphere to obtain a flame-retardant fiber bundle. (4) A first carbonization step in which the flame-resistant fiber bundle is heated in a non-oxidizing atmosphere with a temperature gradient in the range of 300°C to 900°C, with an elongation rate of 4.0% to 5.0%. (5) A second carbonization step, which is performed after the first carbonization step, in which the fiber bundle is heated in a non-oxidizing atmosphere having a temperature gradient within the range of 1000°C to 1800°C, while applying a tension of 0.15 cN / dtex to 0.21 cN / dtex to the fiber bundle. (6) A third carbonization step, which is performed after the second carbonization step, in which the fiber bundle is heated in a non-oxidizing atmosphere having a temperature gradient within the range of 1700°C to 2300°C, while applying a tension of 0.15 cN / dtex to 0.23 cN / dtex to the fiber bundle.

[13] A method for producing the carbon fiber bundle according to

[12] , comprising a step between step (1) and step (2) of stretching the solidified fiber bundle in air to a stretch ratio of 1.00 times or more and 1.20 times or less.

[14] A method for producing a carbon fiber bundle according to

[12] or

[13] , wherein in step (2) above, after stretching the solidified yarn bundle, the organic solvent is removed, the bundle is shrunk or stretched to a ratio of 0.96 to 1.30 in hot water at a temperature of 90°C or higher, and the bundle is stretched to a ratio of 3.7 to 4.2 in a pressurized steam atmosphere to obtain the carbon fiber precursor acrylic fiber bundle.

[15] The step in (3) above involves heating the carbon fiber precursor acrylic fiber bundle in an oxidizing atmosphere with a temperature gradient in the range of 220°C to 280°C, with an elongation rate of 3.0% to 8.0%, and a density of 1.33 g / cm³. 3 More than 1.36g / cm 3 A method for producing a carbon fiber bundle according to any of the above

[12] to

[14] , which is a flame-retardant step for obtaining the following flame-retardant fiber bundle.

[16] A method for producing a carbon fiber bundle according to any of the

[12] to

[15] , wherein the concentration of the organic solvent in the aqueous solution used in step (1) is 79.8% by mass or more and 81.2% by mass or less.

[17] A method for producing a carbon fiber bundle according to any of

[12] to

[16] , wherein the organic solvent is dimethylformamide.

[18] A method for producing a carbon fiber bundle according to any of the

[12] to

[17] , wherein in step (6), the heating rate when raising the ambient temperature from 1800°C to 2200°C is 300°C / min or more and 600°C / min or less.

[19] A method for manufacturing carbon fiber bundles according to any of the

[12] to

[18] , wherein the difference between the maximum ambient temperature in step (5) and the inlet ambient temperature in step (6) is 700°C or less.

[20] A method for manufacturing carbon fiber bundles according to any of the

[12] to

[19] , wherein the difference between the maximum ambient temperature in step (5) and the inlet ambient temperature in step (6) is 500°C or less. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a carbon fiber bundle having a large fiber diameter and physical properties that are well-balanced between strand modulus of elasticity and strand strength, as well as a method for manufacturing a carbon fiber bundle. [Brief explanation of the drawing]

[0012] [Figure 1] This is an explanatory diagram illustrating the method for measuring the ultrasonic modulus of carbon fiber bundles. [Modes for carrying out the invention]

[0013] [Carbon fiber bundle] The carbon fiber bundle of the present invention is a fiber bundle in which multiple individual carbon fibers are bundled together. The diameter of the single fiber, i.e., the fiber diameter, is 5.7 μm or more and 6.5 μm or less. By setting the fiber diameter to 5.7 μm or more, the gaps between fibers can be increased, making it easier to uniformly impregnate the resin and suppressing void formation in the fiber-reinforced composite material obtained using the carbon fiber bundle of the present invention. By setting the fiber diameter to 6.5 μm or less, the double cross-sectional structure is less likely to become prominent in the flame-retardant process (step (3)) described later, making it possible to obtain a carbon fiber bundle with high strand strength without reducing the strand modulus. In order to achieve both uniformity of resin impregnation and high strand strength, the fiber diameter is preferably 5.7 μm or more and less than 6.5 μm, and more preferably 5.8 μm or more and 6.3 μm or less. The measurement conditions for the diameter of the single fiber are as described in the examples below.

[0014] The carbon fiber bundle of the present invention has a strand strength of 4.7 GPa or more and a strand modulus of elasticity of 310 GPa or more. By setting the strand strength of the carbon fiber bundle to 4.7 GPa or higher and the strand modulus of elasticity to 310 GPa or higher, a carbon fiber bundle with a balanced strand strength and modulus of elasticity is obtained, making it easier to obtain carbon fiber reinforced composite materials with excellent mechanical properties. The measurement conditions for strand strength and strand modulus are as described in the examples below.

[0015] In the carbon fiber bundle of the present invention, it is preferable that the product of the strand strength and the strand modulus is 1650 or more. By setting the product of strand strength and strand modulus to 1650 or higher, a carbon fiber bundle with a superior balance of strand strength and modulus is obtained, making it easier to obtain carbon fiber reinforced composite materials with superior mechanical properties. The product of the strand strength and the strand modulus of elasticity is more preferably 1700 or more, more preferably 1750 or more, and even more preferably 2000 or more. Furthermore, the product of the strand strength and the strand modulus of elasticity is preferably 3000 or less, and more preferably 2500 or less.

[0016] The carbon fiber bundle of the present invention preferably has a strand strength of 4.9 GPa or more and a strand modulus of elasticity of 365 GPa or more. By setting the strand strength of the carbon fiber bundle to 4.9 GPa or higher and the strand modulus of elasticity to 365 GPa or higher, it is possible to further improve the performance of the resulting fiber-reinforced composite material.

[0017] The carbon fiber bundle of the present invention preferably has a strand modulus of 370 GPa or more and 400 GPa or less. By setting the strand modulus of the carbon fiber bundle to 370 GPa or higher, it is possible to improve the performance of the resulting fiber-reinforced composite material. Furthermore, by setting the strand modulus to 400 GPa or lower, it is possible to suppress the excessive growth of graphite crystal size in the carbon fiber bundle, thereby suppressing the decrease in compressive strength in the fiber axis direction, and thus further improving the performance of the resulting fiber-reinforced composite material.

[0018] The carbon fiber bundle of the present invention preferably has a strand strength of 5.4 GPa or higher. By increasing the strand strength of the carbon fiber bundle to 5.4 GPa or higher, it becomes possible to further improve the performance of the resulting fiber-reinforced composite material. From these viewpoints, the strand strength of the carbon fiber bundle of the present invention is preferably 4.7 GPa or higher, more preferably 4.9 GPa or higher, more preferably 5.0 GPa or higher, even more preferably 5.4 GPa or higher, and particularly preferably 5.5 GPa or higher. On the other hand, when the strand strength of the carbon fiber bundle is high, graphite crystals develop, and the compressive strength in the fiber axis direction tends to decrease. In order to obtain a carbon fiber bundle with a balanced strand strength and compressive strength, the strand strength of the carbon fiber bundle is preferably 8.0 GPa or lower, more preferably 7.5 GPa or lower, even more preferably 7.0 GPa or lower, particularly preferably 6.5 Pa or lower, and most preferably 6.0 Pa or lower.

[0019] The carbon fiber bundle of the present invention has a knot strength of 400 N / mm². 2 It is preferable that the above conditions are met. The knot strength can be an index reflecting the mechanical performance of the fiber bundle other than in the fiber axis direction, and in particular, the performance in the direction perpendicular to the fiber axis can be simply evaluated. In fiber reinforced composite materials, the material is often formed by pseudo-isotropic lamination, forming a complex stress field. At that time, in addition to the tensile and compressive stresses in the fiber axis direction, stresses other than in the fiber axis direction also occur. Furthermore, when a relatively high-speed strain such as an impact test is applied, the stress state generated inside the material is quite complex, and the strength in a direction different from the fiber axis direction becomes important. Therefore, by setting the knot strength of the carbon fiber bundle to 400 N / mm 2 or more, it becomes possible to further improve the performance of the obtained fiber reinforced composite material. From these viewpoints, it is more preferable that the knot strength of the carbon fiber bundle is 415 N / mm 2 or more, and it is even more preferable that it is 430 N / mm 2 or more. On the other hand, when the knot strength of the carbon fiber bundle increases, the compressive strength other than in the fiber axis direction also increases, and the graphite crystal size becomes small and the elastic modulus tends to decrease. In order to obtain a carbon fiber bundle with a balanced strand elastic modulus and knot strength, the knot strength of the carbon fiber bundle is preferably 600 N / mm 2 or less, more preferably 550 N / mm 2 or less, and even more preferably 500 N / mm 2 or less. The measurement conditions of the knot strength are as described in the examples described later.

[0020] The carbon fiber bundle of the present invention preferably has a fracture surface generation energy of 19 N / m or more. The fracture surface generation energy is obtained from the following Griffith's formula (1) from the fracture strength of the fiber and the depth of the hemispherical defect by forming a hemispherical defect having a predetermined size on the surface of the single fiber with a laser and breaking this fiber at the hemispherical defect site by a tensile test. Fracture surface generation energy = σ 2 πC / 2E ···(1) Here, σ is the fracture strength, E is the ultrasonic elastic modulus of the carbon fiber bundle, and C is the depth of the hemispherical defect. The fracture surface formation energy is an indicator of the resistance of carbon fibers to fracture and represents the matrix strength. Carbon fibers are materials that exhibit brittle fracture, and their tensile strength is governed by the defect points. When carbon fibers have the same defect points, the higher the matrix strength, the higher the fracture strength. Therefore, by setting the fracture surface formation energy of the carbon fiber bundle to 19 N / m or higher, it is possible to increase the strength without reducing the elastic modulus of the carbon fiber bundle, thereby improving the performance of the resulting fiber-reinforced composite material. From these viewpoints, it is more preferable that the fracture surface formation energy of the carbon fiber bundle be 20.5 N / m or higher. On the other hand, when the fracture surface formation energy of the carbon fiber bundle is high, the strength of the surface of the single fibers constituting the carbon fiber bundle increases, the graphite crystal size increases, and as a result, the compressive strength in the fiber axis direction tends to decrease. In order to obtain a carbon fiber bundle with a balanced fracture surface formation energy and compressive strength, it is preferable that the fracture surface formation energy of the carbon fiber bundle be 30 N / m or less, and more preferably 25 N / m or less. The detailed measurement conditions for the fracture surface formation energy are as described in the examples below.

[0021] The carbon fiber bundle of the present invention has a density of 1.79 g / cm³. 3 It is preferable that the above conditions are met. The density of the carbon fiber bundle is 1.79 g / cm³. 3 If the above conditions are met, the strand strength and strand modulus can be increased. From this perspective, the density of the carbon fiber bundle is 1.81 g / cm³. 3 The above is more preferable, 1.83 g / cm³ 3 The above is even more preferable. Furthermore, the density of the carbon fiber bundle is 1.90 g / cm³. 3 The following is preferable: 1.88 g / cm³ 3 The following is more preferable: 1.86 g / cm³ 3 The following is even more preferable: The density of the carbon fiber bundle is 1.90 g / cm³. 3If the following conditions are met, it is possible to suppress the excessive growth of graphite crystal size in the carbon fiber bundles and suppress the decrease in compressive strength in the fiber axis direction, thereby improving the performance of the resulting fiber-reinforced composite material. The conditions for measuring density are as described in the examples below.

[0022] The carbon fiber bundle of the present invention is preferably substantially untwisted. In the present invention, "substantially twistless" means that there is no twist in the fiber bundle, or that there is localized twist, but S-twist and Z-twist are present in equal proportions, and the net number of twists in the entire carbonization process is 0.5 turns / m or less. Because the carbon fiber bundles are virtually untwisted, the fiber-opening properties of the carbon fiber bundles are improved, making it possible to achieve higher performance in the resulting fiber-reinforced composite material.

[0023] [Method for manufacturing carbon fiber bundles] The present invention provides a method for producing carbon fibers, comprising the following steps (1) to (6). (1) A step of obtaining a solidified yarn bundle containing the organic solvent by extruding an acrylonitrile polymer solution into the air from an extrusion hole using a dry-wet spinning method, and then solidifying it in a solidification bath consisting of an aqueous solution at a temperature of 10°C or lower and an organic solvent concentration of 79.3% by mass or more and 82.0% by mass or less. (2) A step of obtaining a carbon fiber precursor acrylic fiber bundle by stretching the solidified yarn bundle in a warm aqueous solution at a temperature of 75°C or higher and with an organic solvent concentration of 40% by mass or more and 65% by mass or less, to a stretch ratio of 2.0 times or more and 3.2 times or less. (3) A flame-retardant step in which the carbon fiber precursor acrylic fiber bundle is heated in an oxidizing atmosphere to obtain a flame-retardant fiber bundle. (4) A first carbonization step in which the flame-resistant fiber bundle is heated in a non-oxidizing atmosphere with a temperature gradient in the range of 300°C to 900°C, with an elongation rate of 4.0% to 5.0%. (5) A second carbonization step, which is performed after the first carbonization step, in which the fiber bundle is heated in a non-oxidizing atmosphere having a temperature gradient within the range of 1000°C to 1800°C, while applying a tension of 0.15 cN / dtex to 0.21 cN / dtex to the fiber bundle. (6) A third carbonization step, which is performed after the second carbonization step, in which the fiber bundle is heated in a non-oxidizing atmosphere having a temperature gradient within the range of 1700°C to 2300°C, while applying a tension of 0.15 cN / dtex to 0.23 cN / dtex to the fiber bundle.

[0024] <(1) Step> Step (1) involves extruding an acrylonitrile polymer solution into the air from an extrusion hole using a wet-dry spinning method, and then solidifying it in a solidification bath consisting of an aqueous solution (A) at a temperature of 10°C or lower and with an organic solvent concentration of 79.3% by mass or more and 82.0% by mass or less to obtain a solidified yarn bundle containing the organic solvent. The temperature of the coagulation bath, i.e., aqueous solution (A), is 10°C or lower. By keeping the temperature of aqueous solution (A) below 10°C, it is easier to form dense coagulated fibers, and in particular, the density of the fiber surface can be increased. This makes it possible to obtain carbon fiber bundles with high strand strength and knot strength without reducing the strand modulus of elasticity. The temperature of aqueous solution (A) is preferably 4°C or higher, and more preferably 6°C or higher. By setting the temperature of aqueous solution (A) to 4°C or higher, excessive densification of the coagulated fibers can be suppressed, and drawability in subsequent processes can be ensured.

[0025] The concentration of the organic solvent in the coagulation bath, i.e., aqueous solution (A), is preferably 79.3% by mass or more and 82.0% by mass or less, relative to the total mass of aqueous solution (A), and more preferably 79.8% by mass or more and 81.2% by mass or less. By setting the concentration of the organic solvent to 79.3% by mass or more and 82.0% by mass or less, it is possible to obtain coagulated yarn that is dense both on the surface and inside, and as a result, it is possible to increase the strand strength and knot strength without reducing the strand modulus of elasticity of the resulting carbon fiber bundle.

[0026] Examples of organic solvents included in the aqueous solution (A) are dimethylformamide, dimethylacetamide, and dimethyl sulfoxide. Among these, dimethylformamide is preferred from the viewpoint of forming a more compact structure.

[0027] <(2) Step> Step (2) is a step in which the solidified yarn bundle is stretched to a stretch ratio of 2.0 to 3.2 times in a warm aqueous solution (B) at a temperature of 75°C or higher and with an organic solvent concentration of 40% to 65% by mass, in order to obtain a carbon fiber precursor acrylic fiber bundle. The temperature of the hot aqueous solution (B) is 75°C or higher, preferably 85°C or higher. By setting the temperature of the hot aqueous solution (B) to 75°C or higher, sufficient stretchability can be ensured, thus enabling stable stretching. The temperature of the hot aqueous solution (B) is preferably 98°C or lower, and more preferably 95°C or lower. By keeping the temperature of the hot aqueous solution (B) at 98°C or lower, rapid temperature changes in the coagulated yarn bundle can be suppressed, enabling uniform stretching.

[0028] The concentration of the organic solvent in the hot aqueous solution (B) is 40% by mass or more and 65% by mass or less, preferably 50% by mass or more and 60% by mass or less, relative to the total mass of the hot aqueous solution (B). By setting the concentration of the organic solvent in the hot aqueous solution (B) to 40% by mass or more and 65% by mass or less, a dense structure can be formed both on the surface and inside, making it possible to obtain a carbon fiber bundle with high strand strength and knot strength without reducing the strand modulus of elasticity.

[0029] Examples of organic solvents included in the hot aqueous solution (B) are dimethylformamide, dimethylacetamide, and dimethyl sulfoxide. Among these, dimethylformamide is preferred from the viewpoint of forming a more dense structure.

[0030] The stretching ratio in the hot aqueous solution (B) is between 2.0 and 3.2 times, and preferably between 2.7 and 3.0 times. By setting the stretching ratio in the hot aqueous solution (B) to 2.0 times or higher, it becomes possible to produce carbon fiber precursor acrylic fiber bundles with sufficient molecular orientation, and it becomes possible to obtain carbon fiber bundles with high strand strength and knot strength without reducing the strand modulus. By setting the stretching ratio in the hot aqueous solution (B) to 3.2 times or lower, excessive stretching can be suppressed, and stable stretching becomes possible.

[0031] In step (2), it is preferable to obtain carbon fiber precursor acrylic fiber bundles by appropriately combining steps such as stretching the solidified yarn bundles in a hot aqueous solution (B), removing the organic solvent, stretching with hot water, stretching by vaporization in a pressurized steam atmosphere, stretching with dry heat, applying an oil, and drying. Specifically, it is preferable to stretch the solidified yarn bundles, remove the organic solvent, shrink or stretch them to a magnification of 0.96 to 1.30 in hot water (C) at a temperature of 90°C or higher, and stretch them to a magnification of 3.7 to 4.2 in a pressurized steam atmosphere to obtain carbon fiber precursor acrylic fiber bundles. In other words, step (2) preferably comprises, in order: a step of stretching the solidified yarn bundle in a warm aqueous solution (B) to a stretch ratio of 2.0 to 3.2 times (2-1); a step of removing the organic solvent (2-2); a step of shrinking or stretching in warm water (C) at a temperature of 90°C or higher to a stretch ratio of 0.96 to 1.30 times (2-3); and a step of stretching in a pressurized steam atmosphere to a stretch ratio of 3.7 to 4.2 times (2-5). Furthermore, step (2) more preferably further comprises a step of applying an oil composition (2-4). It is preferable that step (2-4) is performed between steps (2-3) and (2-5).

[0032] Step (2-2) is a step to remove the organic solvent from the solidified fiber bundles (hereinafter also referred to as "stretched fiber bundles") after stretching in a hot aqueous solution (B). Any method that can remove the organic solvent is acceptable. For example, it is preferable to wash and stretch the stretched fiber bundles in a multi-stage washing tank set to a temperature in the range of 50°C to less than 100°C.

[0033] Step (2-3) is a process in which the stretched fiber bundle, after the organic solvent has been removed, is shrunk or stretched to a magnification of 0.96 to 1.30 times in hot water (C) at a temperature of 90°C or higher. Step (2-3) can alleviate the distortion caused by stretching. The temperature of the hot water (C) is 90°C or higher. By setting the temperature of the hot water (C) to 90°C or higher, it becomes possible to uniformly relieve the stretching strain, making it possible to obtain a carbon fiber bundle with higher strand strength and knot strength without reducing the strand modulus of elasticity. Preferably, the temperature of the hot water (C) is 97°C or lower. By setting the temperature of the hot water (C) to 97°C or lower, it is possible to suppress rapid temperature changes in the stretched fiber bundle, making it possible to uniformly relieve the stretching strain, making it possible to obtain a carbon fiber bundle with higher strand strength and knot strength without reducing the strand modulus of elasticity. The shrinkage or stretch ratio in hot water (C) is between 0.96 and 1.30. By setting the shrinkage or stretch ratio to 0.96 or higher, poor take-up due to fiber bundle separation can be prevented, and the stretching strain can be stably alleviated. By setting the shrinkage or stretch ratio to 1.30 or lower, excessive load can be suppressed, and the stretching strain can be stably alleviated. In step (2-3), it is preferable to shrink (relax) the stretched fiber bundle after removing the organic solvent in warm water (C) to a shrinkage ratio (relaxation ratio) of 0.96 or more and less than 1.00, or to stretch it to a stretching ratio of 1.00 or more and 1.30 or less, more preferably to shrink (relax) it to a shrinkage ratio (relaxation ratio) of 0.96 or more and 0.99 or less, or to stretch it to a stretching ratio of 1.05 or more and 1.30 or less, and even more preferably to shrink (relax) it to a shrinkage ratio (relaxation ratio) of 0.96 or more and 0.99 or less.

[0034] Step (2-4) is a step of applying an oil composition to the stretched fiber bundle after it has shrunk or stretched in hot water (C). The oil composition can be determined considering the functions required for the carbon fiber precursor acrylic fiber bundle, and a silicone-based oil composition is preferred. The oil composition may also contain additives such as antioxidants, antistatic agents, defoamers, preservatives, antibacterial agents, and penetrating agents, as needed. Known methods such as the roller method, guide method, spray method, and dipping method can be used to apply the oil composition to the stretched fiber bundle. After applying the oil composition to the stretched fiber bundle, it is preferable to dry it using a conventionally known method, if necessary.

[0035] Step (2-5) involves shrinking or stretching the fibers in hot water (C), then preferably applying an oil composition, and if necessary, stretching the dried fiber bundles to a stretch ratio of 3.7 to 4.2 times in a pressurized steam atmosphere. The stretching ratio in a pressurized steam atmosphere is between 3.7 and 4.2 times. By setting the stretching ratio in a pressurized steam atmosphere to 3.7 times or higher, the molecular orientation of the resulting carbon fiber precursor acrylic fiber bundle is improved, making it possible to obtain a carbon fiber bundle with higher strand strength and knot strength without reducing the strand modulus. By setting the stretching ratio in a pressurized steam atmosphere to 4.2 times or lower, excessive stretching can be suppressed, and stable stretching becomes possible.

[0036] <(3) Process: Flame-resistant treatment process> Step (3) is a flame-retardant step in which the carbon fiber precursor acrylic fiber bundle is heated in an oxidizing atmosphere to obtain a flame-retardant fiber bundle. In the flame-retardant process, it is preferable to heat the carbon fiber precursor acrylic fiber bundle in an oxidizing atmosphere with a temperature gradient within the range of 220°C to 280°C, and it is more preferable to heat the carbon fiber precursor acrylic fiber bundle in an oxidizing atmosphere in a flame-retardant furnace with a linear temperature gradient within the range of 220°C to 280°C. In the flame-retardant treatment process, a thermal cyclization reaction and an oxygen-induced oxidation reaction occur. Maintaining a good balance between these two reactions is crucial for obtaining carbon fiber bundles with high strand strength and knot strength without reducing the strand modulus. The ambient temperature during the flame-retardant process is preferably between 220°C and 280°C. By setting the ambient temperature through which the carbon fiber precursor acrylic fiber bundle passes during the flame-retardant process to 220°C or higher, the areas where oxidation reactions are not sufficiently occurring can be reduced, and the occurrence of large structural irregularities in the cross-sectional direction of the single fibers can be suppressed. As a result, it is possible to obtain a carbon fiber bundle with high strand strength and knot strength without reducing the strand modulus. By setting the ambient temperature through which the carbon fiber precursor acrylic fiber bundle passes during the flame-retardant process to 280°C or lower, the presence of more oxygen near the surface of the single fibers can be suppressed. As a result, the excess oxygen disappears during the heat treatment in the first carbonization process and subsequent processes described later, suppressing the reaction that forms defect points. As a result, it is possible to obtain a carbon fiber bundle with high density and high strand strength and knot strength without reducing the strand modulus.

[0037] In the flame-retardant treatment process, the density of the flame-retardant fiber bundle obtained in this process is 1.33 g / cm³. 3 More than 1.36g / cm 3 It is preferable to heat the carbon fiber precursor acrylic fiber bundle until the density of the flame-resistant fiber bundle reaches 1.33 g / cm³. 3 By doing so, the occurrence of areas with insufficient flame resistance can be suppressed, and as a result, decomposition reactions that occur during heat treatment in the first carbonization process and beyond, as described later, can be suppressed, making it possible to obtain a carbon fiber bundle with high density and high strand strength and knot strength without reducing the strand modulus. The density of the flame-resistant fiber bundle is 1.36 g / cm³. 3By doing the following, the presence of a large amount of oxygen within the flame-resistant fiber bundle can be suppressed. As a result, the excess oxygen disappears during the heat treatment in the first carbonization step and subsequent steps described later, suppressing the reaction that forms defect points. This makes it possible to obtain a carbon fiber bundle with high density and high strand and knot strength without reducing the strand modulus.

[0038] In the flame-retardant process, it is preferable to stretch the carbon fiber precursor acrylic fiber bundle to a length of 3.0% or more and 8.0% or less to form a flame-retardant fiber bundle. It is more preferable that the length of the fiber in the flame-retardant process be 4.0% or more and 7.0%, and even more preferable that it be 5.0% or more and 6.5% or less. By setting the length of the fiber in the flame-retardant process to 3.0% or more, the molecular orientation of the flame-retardant fiber bundle can be improved, making it possible to obtain a carbon fiber bundle with high strand strength and knot strength without reducing the strand modulus. By setting the length of the fiber in the flame-retardant process to 8.0% or less, excessive stretching can be suppressed, making it possible to stably obtain a flame-retardant fiber bundle.

[0039] Examples of gases that form an oxidizing atmosphere include air, oxygen, and nitrogen dioxide. Of these, air is preferred from an economic standpoint. The processing time in the flame-retardant furnace (flame-retardant treatment time) is preferably, for example, 30 minutes or more and 100 minutes or less.

[0040] <(4) Step: First carbonization step> Step (4) is a first carbonization step in which the flame-resistant fiber bundle is heated in a non-oxidizing atmosphere with a temperature gradient in the range of 300°C to 900°C, with an elongation rate of 4.0% to 5.0%. In the first carbonization step, it is preferable to heat the flame-resistant fiber bundle in a non-oxidizing atmosphere in a first carbonization furnace having a linear temperature gradient within the range of 300°C to 900°C. The ambient temperature in the first carbonization process is between 300°C and 900°C. By keeping the ambient temperature in the first carbonization process below 900°C, it is possible to suppress the extreme brittleness of the flame-resistant fiber bundles, allowing them to pass through the first carbonization process (first carbonization furnace) stably. Furthermore, it is possible to suppress the formation of defects during the heat treatment in the second carbonization process and beyond, as described later, and to obtain carbon fiber bundles with high density, high strand strength, and high knot strength without reducing the strand modulus.

[0041] The elongation rate in the first carbonization process is between 4.0% and 5.0%. By setting the elongation rate in the first carbonization process to 4.0% or higher, the molecular orientation of the resulting carbon fiber bundle can be improved, making it possible to improve strand strength and knot strength without reducing the strand modulus. By setting the elongation rate in the first carbonization process to 5.0% or lower, excessive elongation can be suppressed, allowing the bundle to pass through the first carbonization process (first carbonization furnace) stably.

[0042] The processing time in the first carbonization furnace (first carbonization treatment time) is preferably 1.0 minute or more and 3.0 minutes or less, and more preferably 1.2 minutes or more and 2.5 minutes or less. By setting the processing time in the first carbonization furnace to 1.0 minute or more, it is possible to suppress the violent decomposition reaction that occurs with a rapid rise in temperature, making it possible to obtain a carbon fiber bundle with high density and high strand strength and knot strength without reducing the strand modulus. By setting the processing time in the first carbonization furnace to 3.0 minutes or less, it is possible to suppress the decrease in the degree of crystal orientation of the carbon fiber bundle, making it possible to obtain a carbon fiber bundle with high strand strength and knot strength without reducing the strand modulus.

[0043] Examples of gases that form a non-oxidizing atmosphere include nitrogen, argon, and helium. Among these, nitrogen is preferred from an economic standpoint.

[0044] <(5) Step: Second carbonization step> Step (5) is a second carbonization step performed after the first carbonization step, in which the fiber bundle is heated in a non-oxidizing atmosphere with a temperature gradient in the range of 1000°C to 1800°C, while applying a tension of 0.15 cN / dtex to 0.21 cN / dtex to the fiber bundle. The fiber bundle in step (5) is the flame-resistant fiber bundle that has passed through the first carbonization step. In the second carbonization step, it is preferable to heat the fiber bundle that has passed through the first carbonization furnace in a non-oxidizing atmosphere in a second carbonization furnace having a linear temperature gradient within the range of 1000°C to 1800°C. The ambient temperature in the second carbonization process is between 1000°C and 1800°C. By keeping the ambient temperature in the second carbonization process below 1800°C, it is possible to suppress the formation of defects during the heat treatment in the third carbonization process described later, and to obtain a carbon fiber bundle with high density, high strand strength, and high knot strength without reducing the strand modulus.

[0045] Because the fiber bundles passing through the second carbonization process (second carbonization furnace) undergo significant shrinkage, it is important to heat them under tension. In the second carbonization process, a tension of 0.15 cN / dtex to 0.21 cN / dtex is applied to the total fineness of the carbon fiber precursor acrylic fiber bundle immediately before passing through the flame-retardant process (flame-retardant furnace), preferably 0.17 cN / dtex to 0.20 cN / dtex. By applying a tension of 0.15 cN / dtex or higher to the fiber bundles passing through the second carbonization process (second carbonization furnace), it is possible to maintain a high molecular orientation of the resulting carbon fiber bundles, thereby improving strand strength and knot strength without reducing the strand modulus. By applying a tension of 0.21 cN / dtex or lower to the fiber bundles passing through the second carbonization process (second carbonization furnace), it is possible to suppress single fiber breakage of the carbon fiber bundles due to excessive tension, thereby enabling the stable acquisition of fiber-reinforced composite materials.

[0046] The processing time in the second carbonization furnace (second carbonization treatment time) is preferably between 1.3 minutes and 5.0 minutes. By setting the processing time in the second carbonization furnace to 1.3 minutes or more, it is possible to suppress the violent decomposition reaction associated with a rapid temperature rise, making it possible to obtain carbon fiber bundles with high density and high strand and knot strength without reducing the strand modulus. By setting the processing time in the second carbonization furnace to 5.0 minutes or less, it is possible to maintain high productivity while sufficiently increasing the degree of crystal orientation of the carbon fiber bundles, making it possible to efficiently obtain carbon fiber bundles with high strand and knot strength without reducing the strand modulus.

[0047] <(6) Step: Third carbonization step> Step (6) is a third carbonization step performed after the second carbonization step, in which the fiber bundle is heated in a non-oxidizing atmosphere with a temperature gradient within the range of 1700°C to 2300°C, while applying a tension of 0.15 cN / dtex to 0.23 cN / dtex to the fiber bundle. The fiber bundle in step (6) refers to the flame-resistant fiber bundle that has passed through the first carbonization step and the second carbonization step. In the third carbonization step, it is preferable to obtain carbon fiber bundles by heating the fiber bundles that have passed through the second carbonization furnace in a non-oxidizing atmosphere in a third carbonization furnace having a linear temperature gradient within the range of 1700°C to 2300°C. The ambient temperature in the third carbonization process is between 1700°C and 2300°C. Considering the temperature of the second carbonization process, it is preferable to make the ambient temperature in the third carbonization process higher than that of the second carbonization process, and more preferably 1800°C or higher. By keeping the ambient temperature in the third carbonization process below 2300°C, it is possible not only to prevent deterioration of the third carbonization furnace, but also to suppress the formation of defects in the resulting carbon fiber bundles, and to obtain carbon fiber bundles with high density, high strand strength and knot strength without reducing the strand modulus.

[0048] Because the fiber bundles passing through the third carbonization process (third carbonization furnace) undergo significant shrinkage, it is important to heat them under tension. In the third carbonization process, a tension of 0.15 cN / dtex to 0.23 cN / dtex is applied to the total fineness of the carbon fiber precursor acrylic fiber bundle immediately before passing through the flame-retardant process (flame-retardant furnace), preferably a tension of 0.18 cN / dtex to 0.22 cN / dtex. By applying a tension of 0.15 cN / dtex or higher to the fiber bundles passing through the third carbonization process (third carbonization furnace), it is possible to maintain a high molecular orientation of the resulting carbon fiber bundles, thereby improving strand strength and knot strength without reducing the strand modulus. By applying a tension of 0.23 cN / dtex or lower to the fiber bundles passing through the third carbonization process (third carbonization furnace), it is possible to suppress single fiber breakage of the carbon fiber bundles due to excessive tension, thereby enabling the stable acquisition of fiber-reinforced composite materials.

[0049] The processing time in the third carbonization furnace (third carbonization treatment time) is preferably between 1.0 minute and 3.0 minutes. By setting the processing time in the third carbonization furnace to 1.0 minute or more, it is possible to suppress the violent decomposition reaction associated with a rapid temperature rise, making it possible to obtain carbon fiber bundles with high density and high strand and knot strength without reducing the strand modulus. By setting the processing time in the third carbonization furnace to 3.0 minutes or less, it is possible to maintain high productivity while sufficiently increasing the degree of crystal orientation of the carbon fiber bundles, making it possible to efficiently obtain carbon fiber bundles with high strand and knot strength without reducing the strand modulus.

[0050] In step (6), the heating rate when raising the ambient temperature from 1800°C to 2200°C is preferably 300°C / min or more and 600°C / min or less, more preferably 350°C / min or more and 550°C / min or less, and even more preferably 400°C / min or more and 500°C / min or less. By setting the heating rate when raising the ambient temperature from 1800°C to 2200°C to 300°C / min or more, it is possible to manufacture carbon fiber bundles with high productivity. By setting the heating rate when raising the ambient temperature from 1800°C to 2200°C to 600°C / min or less, it is possible to suppress the violent decomposition reaction associated with a rapid temperature rise, and it is possible to obtain carbon fiber bundles with high density and high strand strength and knot strength without reducing the strand modulus. The aforementioned heating rate is the time it takes for the fiber bundle to travel from an ambient temperature of 1800°C to 2200°C, and is the value obtained by dividing the difference between 2200°C and 1800°C, which is 400°C, by the current value.

[0051] Furthermore, the difference between the maximum ambient temperature in step (5) and the inlet ambient temperature in step (6) is preferably 700°C or less, and more preferably 500°C or less. By setting the difference between the maximum ambient temperature in step (5) and the inlet ambient temperature in step (6) to 700°C or less, it is possible to suppress the violent decomposition reaction in the initial stages of step (6), making it possible to obtain a carbon fiber bundle with high density and high strand strength and knot strength without reducing the strand modulus of elasticity. The difference between the maximum ambient temperature in step (5) and the inlet ambient temperature in step (6) is preferably 30°C or more, and more preferably 50°C or more.

[0052] <Other processes> The carbon fiber manufacturing method of the present invention may include the following step (a) before step (1). It is also preferable to include the following step (b) between step (1) and step (2). Furthermore, the following steps (c) and (d) may be included after step (6). (a) A step of preparing an acrylonitrile polymer solution. (b) A step of stretching the solidified yarn bundle in air to a stretch ratio of 1.00 to 1.20. (c) A step of surface oxidizing the carbon fiber bundle obtained in the third carbonization step. (d) A step of sizing the carbon fiber bundle after the surface oxidation treatment.

[0053] (Step (a)) Step (a) is the step of preparing an acrylonitrile polymer solution. The acrylonitrile-based polymer used in this invention is a polymer obtained by polymerizing acrylonitrile, which is the main monomer. The acrylonitrile-based polymer may be a homopolymer obtained solely from acrylonitrile, or it may be a copolymer obtained by copolymerizing acrylonitrile, the main component, with other monomers.

[0054] The acrylonitrile unit content in the acrylonitrile polymer can be determined considering the desired quality of the resulting carbon fiber bundle. For example, it is preferable that the acrylonitrile unit content be 90% to 99.5% by mass, and more preferably 96% to 99.5% by mass, relative to the total mass of monomer units constituting the acrylonitrile polymer. If the acrylonitrile unit content is 90% by mass or more, fusion between individual fibers can be suppressed in the flame-retardant and carbonization processes for converting the carbon fiber precursor acrylic fiber bundle into carbon fibers, thereby preventing a decrease in the strand strength of the carbon fiber bundle. Furthermore, adhesion between individual fibers can be suppressed in processes such as stretching with heated rollers or pressurized steam. If the acrylonitrile unit content is 99.5% by mass or less, solubility in solvents is less likely to decrease, and precipitation and solidification of the acrylonitrile polymer can be prevented, thus enabling the stable production of carbon fiber precursor acrylic fiber bundles.

[0055] Other monomer units in the acrylonitrile polymer can be appropriately selected from vinyl monomers copolymerizable with acrylonitrile, with vinyl monomer units that improve the hydrophilicity of the acrylonitrile polymer and vinyl monomer units that promote the flame-retardant reaction being preferred. Any polymerization method may be used to synthesize the acrylonitrile polymer, and the present invention is not limited by differences in polymerization methods. Solvents for acrylonitrile polymer solutions include organic solvents such as dimethylacetamide, dimethyl sulfoxide, and dimethylformamide, and aqueous solutions of inorganic compounds such as zinc chloride and sodium thiocyanate. Among these, dimethylformamide is preferred due to its high solubility in acrylonitrile polymers.

[0056] The polymer concentration of the acrylonitrile polymer solution is preferably 20% to 25% by mass, and more preferably 21% to 24% by mass, based on the total mass of the acrylonitrile polymer solution. By setting the polymer concentration to 20% by mass or higher, the number of voids inside the coagulated yarn is reduced, thereby increasing the strand strength of the carbon fiber bundle. By setting the polymer concentration to 25% by mass or lower, the acrylonitrile polymer solution can maintain appropriate viscosity and fluidity, making it easier to manufacture carbon fiber precursor acrylic fiber bundles.

[0057] The temperature of the acrylonitrile polymer solution is preferably adjusted to 50°C to 70°C, and more preferably to 55°C to 65°C. By maintaining the temperature of the acrylonitrile polymer solution at 50°C to 70°C, the solution can maintain appropriate viscosity and fluidity, thus facilitating the production of carbon fiber precursor acrylic fiber bundles.

[0058] (Step (b)) Step (b) is a step of stretching the solidified yarn bundle in air to a stretching ratio of 1.00 times or more and 1.20 times or less. Step (b) is preferably performed between steps (1) and (2). In step (b), the solidified yarn bundle taken up in step (1) is stretched in air while still containing the solidification solution. The stretching ratio in air is 1.00 times or more and 1.20 times or less, preferably 1.05 times or more and 1.15 times or less. By setting the stretching ratio in air to 1.00 times or more, it is possible to suppress uneven shrinkage, and as a result, it is possible to obtain a carbon fiber bundle with high strand strength and knot strength without reducing the strand modulus of elasticity. By setting the stretching ratio in air to 1.20 times or less, excessive stretching can be suppressed and stable stretching is possible.

[0059] (Step (c)) Step (c) is a step of surface oxidizing the carbon fiber bundle obtained in the third carbonization step. Step (c) is preferably performed after step (6) (third carbonization step). The carbon fiber bundles obtained after passing through the third carbonization process (third carbonization furnace) are preferably subjected to surface oxidation treatment. Known surface treatment methods include oxidation treatment by electrolytic oxidation, chemical oxidation, and air oxidation. Any of these methods may be used, but electrolytic oxidation, which is widely practiced industrially, is more preferable because it allows for stable surface oxidation treatment. In surface oxidation treatment, the IPA representing the surface treatment state is set to 0.05 μA / cm². 2 More than 0.25μA / cm 2 The following is preferable. To control 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 amount of electricity of 10 coulombs / g to 200 coulombs / g with a carbon fiber bundle as the anode. Examples of electrolytes include ammonium carbonate, ammonium bicarbonate, ammonium sulfate, calcium hydroxide, sodium hydroxide, and potassium hydroxide.

[0060] (Step (d)) Step (d) is a step of sizing the carbon fiber bundle after the surface oxidation treatment. Step (d) is preferably performed after step (c). It is preferable that the surface-oxidized carbon fiber bundles are subsequently subjected to a sizing treatment. The sizing agent is applied to the carbon fiber bundles by methods such as roller immersion or roller contact, using an emulsion solution dissolved in an organic solvent or dispersed in water with an emulsifier. The sizing treatment can then be performed by drying the bundles. Furthermore, the amount of sizing agent adhering to the surface of the carbon fiber can be adjusted by adjusting the concentration of the sizing agent solution or the amount of sizing applied. Furthermore, drying can be carried out using hot air, hot plates, heating rollers, various infrared heaters, etc. As sizing agents, known substances can be used, such as sizing agents mainly composed of epoxy resin, polyether resin, epoxy-modified polyurethane resin, and polyester resin. [Examples]

[0061] 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.

[0062] [Method for measuring the diameter of a single carbon fiber] Density of carbon fiber bundles (g / cm³) 3 The cross-sectional area of ​​a single carbon fiber was calculated from the mass per meter of carbon fiber bundle, i.e., basis weight (g / m), and the number of filaments in the carbon fiber bundle. 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 density of the carbon fiber bundles was measured in accordance with Method C (density groove piping method) described in JIS R 7063:1999.

[0063] [Method for measuring strand strength and strand modulus of elasticity] 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 method.

[0064] [Method for measuring knot strength] Knot strength was measured as follows. A 150mm long carbon fiber bundle was used as a test specimen, with 25mm long gripping sections attached to both ends. During the preparation of the test specimen, 0.1 × 10 -3 Carbon fiber bundles were aligned by applying a load of N / denier. A single knot was formed near the center of each specimen, and the crosshead speed during tensioning was 100 mm / min. Twelve specimens were tested, and the minimum and maximum values ​​were removed, with the average of the remaining 10 specimens used as the measurement value.

[0065] [Method for measuring fracture surface formation energy] A single carbon fiber was cut to a length of 20 cm. The central portion of this single fiber was attached and fixed to a single fiber tensile test mount for a sample length of 10 mm as specified in JIS R 7606:2000. The excess portion extending beyond the mount was cut off to prepare the sample. Next, hemispherical defects were formed on these samples fixed to the mount by irradiating them with a laser. A Micropoint laser interface system (pulse energy 300 uJ) manufactured by Photonic Instruments was used. A Nikon ECLIPSE LV100 optical microscope was used for focusing the laser. The aperture diaphragm of the optical microscope was set to the minimum, and the objective lens was set to 100x. Under these conditions, a single pulse of a 435 nm laser, with the laser intensity attenuated by 10%, was irradiated to the center of the sample in the direction of the fiber axis and perpendicular to the fiber axis, to obtain a sample with hemispherical defects. To prevent the carbon fiber sample from shrinking and breaking, the sample, which was attached to a backing board, was further sandwiched between films, and the inside of the films was filled with a viscous liquid for a tensile test. Specifically, a film approximately 5 mm wide and 15 mm long was prepared, and the film was attached to the top of both sides of the sample backing board with adhesive, so that the sample and backing board were sandwiched together with the film. The space between these films was filled with a glycerin aqueous solution (2 parts by mass of water to 1 part by mass of glycerin), and a tensile test was performed at a tensile speed of 0.5 mm / min, and the breaking load was measured. Next, the sample pair, which had been divided into two by the tensile test, was removed from the backing board, carefully washed with water, and then air-dried. Then, the sample was fixed to the SEM sample stage with carbon paste so that the fracture surface of the sample was facing upwards, and an SEM observation sample was prepared. The obtained SEM-observed samples were examined using a scanning electron microscope (JEOL Ltd., product name "JSM6060") under conditions of acceleration voltage of 10kV to 15kV and magnification of 10,000x to 15,000x to 15,000x to observe the fracture surface. The obtained SEM images were imported into a computer and analyzed using image analysis software to measure the depth of hemispherical defects and the fiber cross-sectional area. Here, "depth of hemispherical defects" is defined as the longest distance when a line is drawn from the circumference of a single fiber to its center. Next, the breaking load was divided by the fiber cross-sectional area (breaking load / fiber cross-sectional area) to calculate the breaking strength (σ). The fracture surface formation energy was determined using the following equation (1). The fracture surface formation energy was calculated for 30 individual fibers, and the average value was taken as the fracture surface formation energy of the carbon fiber bundle. Fracture surface formation energy = σ 2 πC / 2E ···(1) Here, σ is the fracture strength (GPa), E is the ultrasonic modulus of the carbon fiber bundle (GPa), and C is the depth of the hemispherical defect (m). The ultrasonic modulus of the carbon fiber bundle was measured in accordance with the measurement method described later.

[0066] <Method for measuring the ultrasonic modulus of carbon fiber bundles> The ultrasonic propagation velocity was measured according to the measurement method shown in Figure 1. The distance L1 between the transmitter and the first receiver was 0.20 m, and the distance L2 between the transmitter and the second receiver was 0.25 m. The tension applied to the carbon fiber bundle during measurement was 0.02 N / tex. A pulse was supplied from the pulse generation circuit to the transmitter to drive it and propagate ultrasonic waves through the carbon fiber bundle. The time from when the first receiver detected the ultrasonic waves propagated from the carbon fiber bundle was defined as reception time 1, and the time from when the second receiver detected the ultrasonic waves propagated from the carbon fiber bundle was defined as reception time 2. The ultrasonic modulus of the carbon fiber bundle was determined from the following equation (2). Ultrasonic modulus of elasticity (GPa) = ((0.25m - 0.20m) / (reception time 2 (sec) - reception time 1 (sec))) 2 ×Density of carbon fiber bundles (g / cm³) 3 ) × 10 -6 ...(2)

[0067] [Example 1] <Preparation of carbon fiber precursor acrylic fiber bundles> An acrylonitrile polymer 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 solution of the acrylonitrile polymer. This acrylonitrile polymer solution was spun using a wet-dry spinning method by extruding it through a spinneret with several thousand discharge holes, each 0.15 mm in diameter. Specifically, the solution was spun into air and passed through a space of approximately 5 mm, then coagulated in a coagulation solution filled with an aqueous solution (A) containing 80.4% by mass of dimethylformamide, which was heated to 8°C, and the coagulated yarn bundle was taken up. Next, the coagulated fiber bundles were combined to form 12,000 filaments, withdrawn from the coagulation bath, and stretched 1.1 times in air. Then, they were stretched 2.9 times in a stretching tank filled with a warm aqueous solution (B) containing 55% by mass dimethylformamide, which was heated to 90°C. After stretching, the stretched fiber bundles containing the solvent were washed with clean water, and then relaxed 0.98 times in warm water (C) at 96°C. Subsequently, an oil mainly composed of amino-modified silicone was applied to the stretched fiber bundles to a concentration of 1.1% by mass, and they were dried and densified. The stretched fiber bundles after drying and densification were stretched 4.0 times under a pressurized steam atmosphere to further improve orientation and densification, and then wound up to obtain an acrylonitrile-based precursor fiber bundle. The fineness of this fiber was 1.08 dtex.

[0068] <Fabrication of carbon fiber bundles> Multiple carbon fiber precursor acrylic fiber bundles were aligned in parallel and introduced into a flame-retardant furnace with a linear temperature gradient, with an inlet ambient temperature of 220°C and a maximum ambient ambient temperature of 280°C. The carbon fiber precursor acrylic fiber bundles were flame-retardant treated by blowing heated air from the furnace onto them, resulting in a density of 1.345 g / cm³. 3 Flame-resistant fiber bundles were obtained. The elongation rate was set to 6.0%, and the flame-retardant treatment time was 70 minutes. Next, the flame-resistant fiber bundle was passed through a first carbonization furnace with a linear temperature gradient, set to a nitrogen atmosphere with an inlet temperature of 300°C and a maximum atmosphere temperature of 700°C, while being stretched by 4.5% during the process. The processing time was 2.0 minutes. Furthermore, a second carbonization treatment was performed using a second carbonization furnace with a linear temperature gradient set at an inlet temperature of 1100°C and a maximum ambient temperature of 1200°C in a nitrogen atmosphere. During this treatment, the elongation rate was -2.0%, and the treatment time was 1.6 minutes. The tension on the yarn bundle during treatment was 0.19 cN / dtex. Subsequently, carbon fiber bundles were obtained by third carbonization treatment using a third carbonization furnace with a linear temperature gradient set at an inlet temperature of 1800°C and a maximum ambient temperature of 2300°C in a nitrogen atmosphere. The elongation rate was -2.0%, and the treatment time was 1.9 minutes. The tension applied to the fiber bundle during treatment was 0.21 cN / dtex. Furthermore, the difference between the maximum ambient temperature of the second carbonization furnace and the inlet atmosphere of the third carbonization furnace was set to 600°C, and the heating rate when raising the ambient temperature from 1800°C to 2200°C was set to 450°C / min. Subsequently, the device was run through a 10% by mass aqueous solution of ammonium bicarbonate, and an electric current was applied between the carbon fiber bundle (which served as the anode) and the counter electrode to achieve an electric charge of 40 coulombs per gram of carbon fiber being treated. Next, it was washed with hot water at 90°C and then dried. Next, 0.5% by mass of a sizing agent (manufactured by DIC Corporation, product name "Hydran N320") was applied (sizing treatment), and the material was wound onto a bobbin to obtain a carbon fiber bundle. For carbon fiber bundles after sizing, the diameter, density, basis weight, knot strength, fracture surface formation energy, strand strength, and strand modulus were measured. These results are shown in Table 3. Note that "Strength × Elastic Modulus" in Table 3 refers to Strand Strength × Strand Elastic Modulus.

[0069] [Examples 2-5] Except for changing the concentration of the solidification bath and the maximum ambient temperature of the second carbonization furnace as shown in Tables 1 and 2, carbon fiber bundles were prepared in the same manner as in Example 1, and various measurements were performed. The results are shown in Table 3.

[0070] [Comparative Example 1] Except for changing the conditions for preparing the carbon fiber precursor acrylic fiber bundle as shown in Table 1, and changing the single fiber fineness of the carbon fiber precursor acrylic fiber bundle to 1.0 dtex, the carbon fiber precursor acrylic fiber bundle was prepared in the same manner as in Example 1. Using the obtained carbon fiber precursor acrylic fiber bundles, carbon fiber bundles were prepared in the same manner as in Example 1, except that the carbon fiber bundle preparation conditions were changed as shown in Table 2, and various measurements were performed. The results are shown in Table 3.

[0071] [Comparative Example 2] Except for changing the conditions for preparing the carbon fiber precursor acrylic fiber bundle as shown in Table 1, and changing the single fiber fineness of the carbon fiber precursor acrylic fiber bundle to 0.77 dtex, the carbon fiber precursor acrylic fiber bundle was prepared in the same manner as in Example 1. Using the obtained carbon fiber precursor acrylic fiber bundles, carbon fiber bundles were prepared in the same manner as in Example 1, except that the carbon fiber bundle preparation conditions were changed as shown in Table 2, and various measurements were performed. The results are shown in Table 3.

[0072] [Reference example A] Various measurements were performed on commercially available carbon fiber bundles (manufactured by Toray Industries, Inc., product name "M40JB"). The results are shown in Table 3.

[0073] [Table 1]

[0074] [Table 2]

[0075] [Table 3]

[0076] As is clear from the results in Table 3, the carbon fiber bundles obtained in each example had high strand strength and strand modulus, and were well-balanced. Furthermore, the carbon fiber bundles obtained in each example were essentially untwisted. On the other hand, the carbon fiber bundles obtained in Comparative Examples 1 and 2 had lower fracture surface formation energy, strand strength, and strand modulus compared to the carbon fiber bundles obtained in each example. Furthermore, although the carbon fiber bundle used in Reference Example A, a commercially available product, had a fracture surface formation energy comparable to that of the carbon fiber bundles obtained in each example, it had a low CF density and low strand strength and strand modulus. In addition, because the fiber diameter was small, there is a concern that insufficient impregnation may occur due to the high viscosity of the matrix resin when fabricating fiber-reinforced composite materials, leading to a decrease in the tensile strength of the fiber-reinforced composite material. [Industrial applicability]

[0077] The carbon fiber bundle of the present invention exhibits high strand strength and knot strength without a decrease in elastic modulus, and its large fiber diameter makes it useful in a wide range of applications where high mechanical properties are 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. A carbon fiber bundle in which multiple single carbon fibers are bundled together, A carbon fiber bundle in which the diameter of the single fiber is 5.7 μm or more and 6.5 μm or less, the strand strength is 4.7 GPa or more, and the strand modulus is 310 GPa or more.

2. The carbon fiber bundle according to claim 1, wherein the product of the strand strength and the strand modulus of elasticity is 1650 or more.

3. The carbon fiber bundle according to claim 1, wherein the strand strength is 4.9 GPa or more, and the strand modulus of elasticity is 365 GPa or more.

4. The carbon fiber bundle according to claim 1, wherein the elastic modulus of the strand is 370 GPa or more and 400 GPa or less.

5. The carbon fiber bundle according to claim 1, wherein the strand strength is 5.4 GPa or more.

6. Knot strength of 400 N / mm ２ The carbon fiber bundle according to any one of claims 1 to 5.

7. Knot strength of 415 N / mm ２ The carbon fiber bundle according to any one of claims 1 to 6.

8. A carbon fiber bundle according to any one of claims 1 to 7, wherein the fracture surface formation energy is 19 N / m or more.

9. A carbon fiber bundle according to any one of claims 1 to 8, wherein the fracture surface formation energy is 20.5 N / m or more.

10. Density is 1.79 g / cm³ ３ The carbon fiber bundle according to any one of claims 1 to 9.

11. A carbon fiber bundle according to any one of claims 1 to 10, which is substantially untwisted.

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