WO2024232032A1 - Carbon fiber bundle and method for producing carbon fiber bundle - Google Patents

Abstract

Provided is a carbon fiber bundle in which the average diameter of individual carbon fibers is made thick without lowering the strand strength and the strand elastic modulus of the carbon fiber. Also provided is a method for producing the same. Also provided is a carbon fiber suitable for a carbon fiber composite material having excellent compressive strength in the fiber axis direction. The carbon fiber bundle of the present invention has a strand strength of at least 4.5 GPa a strand elastic modulus of at least 320 GPa, is substantially non twisted, and the average diameter of individual carbon fibers is 6.5-8.5 μm.

Description

Carbon fiber bundle and method for producing carbon fiber bundle

The present invention relates to carbon fiber bundles and a method for producing carbon fiber bundles.

In order to improve the mechanical properties of resin-based molded products, it is common to use fibers as a reinforcing material and compound them with resin. Among these, carbon fiber has excellent specific strength and specific elastic modulus, and is lightweight, so it is used as a reinforcing fiber for high-performance resins in a wide range of applications, including aerospace and automotive applications, as well as traditional sports and general industrial applications. In recent years, the advantages of carbon fiber reinforced composite materials, which are 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 carbon fiber reinforced composite materials, especially in automotive and aerospace applications.

These carbon fiber reinforced composite materials are formed, for example, from prepregs, which are intermediate products in which carbon fibers are impregnated with a matrix resin, through molding and processing steps such as heating and pressing.
In order to obtain high tensile strength and compressive strength in the fiber axial direction of a carbon fiber reinforced composite material when combining carbon fibers with a matrix resin, it is important to improve the mechanical properties of the carbon fiber itself, such as strand strength and strand modulus, as well as to increase the impregnation of the carbon fiber bundles with the matrix resin and suppress the generation of voids in the carbon fiber reinforced composite material.

It is known that increasing the diameter of the carbon fiber single fiber improves the impregnation of the carbon fiber bundle with the resin.
In order to increase the impregnation of the carbon fiber bundle with the matrix resin and to increase the compressive strength in the fiber axis direction when a composite material is made of the carbon fibers and the matrix resin, it is possible to consider increasing the diameter of the carbon fiber single fibers.
In order to increase the compressive strength in the fiber axis direction, it is desirable for the single fiber to be rigid, and for this purpose it is necessary to increase the diameter of the single fiber and improve its straightness.
However, when the diameter of the carbon fiber single fiber is increased, the strand strength and the strand elastic modulus decrease.

In view of the above-mentioned background, attempts have been made to obtain a carbon fiber bundle having a large single fiber diameter, high strand strength and strand elastic modulus, and excellent resin impregnation.
Patent Document 1 describes a carbon fiber that is produced by twisting a fiber bundle in a flame-resistant process in the course of producing the carbon fiber bundle, thereby achieving both excellent strand modulus and moldability into a composite material, and that is easy to maintain the fiber length even when used as a discontinuous fiber.
Patent Document 2 describes a carbon fiber bundle that achieves both a high strand modulus and a high compressive strength of the carbon fiber composite material by carbonizing the bundle under a high drawing tension in the carbonization step.
Patent Document 3 describes a technique for increasing the strand strength and adhesive strength of carbon fibers by subjecting the carbon fibers to an electrolytic surface treatment in an electrolyte solution having a nitrate ion concentration within a specific range.
Patent Document 4 describes a technique for obtaining carbon fibers with high strand strength by flame-retarding polyacrylonitrile fibers in a liquid phase.

International Publication No. 2019/244830 International Publication No. 2019/203088 Japanese Patent Application Publication No. 2002-327339 Japanese Patent Application Publication No. 2004-300600

According to detailed studies by the present inventors, it has been found that Patent Documents 1 to 4 have the following problems.
The carbon fiber bundle described in Patent Document 1 is manufactured by adding twisting, which not only reduces productivity due to an increase in the number of steps, but also causes excessive bundling in the fiber bundle, and the carbon fibers remain in a helical state even after untwisting after baking. Therefore, if the long fibers are used as they are, the resin impregnation may be insufficient, and when the carbon fibers are made into a composite material with a matrix resin, the carbon fibers are prone to buckling when compressed in the fiber axis direction, resulting in low compressive strength in the fiber axis direction.
Hereinafter, the term "compression" simply means "compression in the fiber axis direction."

The carbon fiber bundle described in Patent Document 2 has a relatively large single fiber diameter. Among them, the carbon fibers having a smaller single fiber diameter are subjected to a strong entanglement treatment of the carbon fiber precursor fiber bundle in order to perform the carbonization treatment at a high drawing tension. Since the strong entanglement treatment improves the bundle strength of the fiber bundle, the generation of fluff is small even if the drawing tension in the carbonization treatment is increased. However, since the obtained carbon fiber bundle is strongly entangled, the resin impregnation property is insufficient.
In addition, the carbon fiber having a larger single fiber diameter is twisted into a fiber bundle so that it can be carbonized under a high drawing tension, resulting in carbon fiber with high strand strength and strand modulus.
However, twisted carbon fiber bundles have poor impregnation with matrix resin, which results in poor mechanical properties for carbon fiber reinforced composite materials. Even when untwisted, the helical state remains, and the carbon fibers are prone to buckling when the carbon fiber reinforced composite material is compressed, resulting in low compressive strength.

The carbon fiber described in Patent Document 3 has a relatively large single fiber diameter, and although the strand strength is high, the strand modulus is low. Furthermore, because the strand strength and strand modulus were measured using the old JIS method (JIS R-7601), the values are higher than those of the current JIS method (JIS R-7608:2007).

The carbon fiber described in Patent Document 4 has high strand strength but only a low strand modulus. Furthermore, because the flame retardant is applied in a liquid phase, the oxygen concentration is lower than in air, and flame retardant spots are likely to occur, which tends to result in large variations in tensile strength and tensile modulus between single carbon fiber strands.

Furthermore, the market demands carbon fiber bundles with higher strand strength and strand modulus than ever before, but generally, increasing strand strength tends to decrease strand modulus.
When the diameter of the single fiber is increased, it becomes difficult to advance the reaction to the inside of the single fiber in the process of producing the carbon fiber, and it becomes difficult to obtain the carbon fiber stably. In particular, the carbon fiber having a high strand modulus expression is sometimes produced at a high temperature, and the influence becomes significant. Thus, it has been extremely difficult to produce a carbon fiber bundle having improved strand strength and strand modulus while increasing the diameter of the single fiber.
In addition, it is generally believed that the larger the diameter of the single fibers constituting a carbon fiber bundle, the better the resin impregnation properties will be. However, no carbon fiber bundle is known that has a single fiber with a large diameter and that fully satisfies the strand strength and strand elastic modulus.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a carbon fiber bundle in which the average diameter of carbon fiber single fibers is increased without decreasing the strand strength and strand elastic modulus of the carbon fibers, and a method for producing the same.
Another object of the present invention is to provide carbon fibers suitable for carbon fiber reinforced composite materials having excellent compressive strength in the fiber axial direction by using such carbon fiber bundles.

The present invention has the following aspects.
[1] A carbon fiber bundle having a strand strength of 4.5 GPa or more and a strand modulus of 320 GPa or more,
A carbon fiber bundle which is substantially untwisted and has an average diameter of carbon fiber single fibers of 6.5 μm or more and 8.5 μm or less.
[2] The carbon fiber bundle according to [1], wherein the crystallite size Lc of the carbon fiber single fiber is 3.4 nm or more and 4.1 nm or less.
[3] The carbon fiber bundle according to [1] or [2], wherein the average void length of the carbon fiber single fibers is 22.0 nm or less.
[4] The carbon fiber bundle according to any one of [1] to [3], wherein the fracture surface formation energy of a carbon fiber single fiber is 18 N/m or more.
[5] The carbon fiber bundle according to any one of [1] to [4], wherein a coefficient of variation (CV%) of elastic modulus in a single fiber tensile test of a carbon fiber single fiber in the carbon fiber bundle is 17.5% or less.
[6] The carbon fiber bundle according to any one of [1] to [5], which is unentangled.
[7] The carbon fiber bundle according to any one of [1] to [6], wherein the product of the average diameter (unit: μm) of the carbon fiber single fibers and the strand strength (unit: GPa) of the carbon fiber bundle is 31 or more.
[8] The carbon fiber bundle according to any one of [1] to [7], having a strand strength of 4.85 GPa or more and a strand modulus of 365 GPa or more.
[9] The carbon fiber bundle according to any of [1] to [8], wherein the average diameter of the carbon fiber single fibers is 6.8 μm or more, the strand strength of the carbon fiber bundle is 4.65 GPa or more, and the strand modulus of the carbon fiber bundle is 365 GPa or more and 403 GPa or less.
[10] The carbon fiber bundle according to any one of [1] to [9], wherein the average diameter of the carbon fiber single fibers is 7.5 μm or more.
[11] The carbon fiber bundle according to any one of [1] to [10], having a knot strength of 80 N/ ^mm2 or more.
[12] The carbon fiber bundle according to any one of [1] to [11], wherein the density of the carbon fiber single fiber is 1.79 g / cm ³ or more.
[13] A method for producing a carbon fiber bundle, comprising heating a carbon fiber precursor acrylic fiber bundle in an oxidizing atmosphere to form a flame-resistant fiber bundle, and heating the flame-resistant fiber bundle in a non-oxidizing atmosphere to form a carbon fiber bundle,
In the heating in the non-oxidizing atmosphere, the temperature increase rate when increasing the temperature from 1800° C. to 2200° C. is 200 to 500° C./min;
A method for producing a carbon fiber bundle, wherein the average diameter of single carbon fibers contained in the obtained carbon fiber bundle is 6.5 μm or more and 8.5 μm or less.
[14] The method for producing a carbon fiber bundle according to [13], comprising the following steps (1) to (2):
(1) A coagulation step in which an acrylonitrile-based polymer solution is discharged into the air from a discharge hole by using a dry-wet spinning method, and then coagulated in a coagulation bath containing an aqueous solution of an organic solvent having a concentration of 80.0 mass % or more and 81.0 mass % or less at a temperature of 10° C. to obtain a coagulated fiber bundle containing the organic solvent.
(2) A second drawing step in which the coagulated yarn bundle obtained in the coagulation step is drawn at a draw ratio of 2.0 to 3.2 times in a warm aqueous solution having an organic solvent concentration of 40 to 65 mass % at a temperature of 75° C. or higher to obtain the carbon fiber precursor acrylic fiber bundle.
[15] The method for producing a carbon fiber bundle according to [13] or [14], comprising a first drawing step between the coagulation step and the second drawing step, in which the coagulated yarn bundle obtained in the coagulation step is drawn in air at a draw ratio of 1.00 to 1.20 times, and in the second drawing step, the coagulated yarn bundle obtained in the first drawing step is drawn.
[16] The method for producing a carbon fiber bundle according to any of [13] to [15], in the second drawing step, after the coagulated yarn bundle is drawn, the organic solvent is removed, and the bundle is shrunk or drawn in warm water at a temperature of 90° C. or higher at a draw ratio of 0.96 to 1.30, and then drawn in a pressurized water vapor atmosphere at a draw ratio of 3.7 to 4.2 to obtain the carbon fiber precursor acrylic fiber bundle.
[17] The method for producing a carbon fiber bundle according to any one of [13] to [16], wherein the concentration of the organic solvent in the aqueous solution used in the coagulation step is 80.2 mass% or more and 80.6 mass% or less.
[18] The method for producing a carbon fiber bundle according to any one of [13] to [17], wherein the organic solvent is dimethylformamide.
[19] The method for producing a carbon fiber bundle according to any one of [13] to [18], comprising the following steps (3) to (6):
(3) A flame-retarding step of heating the carbon fiber precursor acrylic fiber bundle or the carbon fiber precursor acrylic fiber bundle obtained in the second stretching step in an oxidizing atmosphere having a temperature gradient in a range of an atmospheric temperature of 200° C. or more and 260° C. or less at an elongation rate of 3.0% or more and 8.0% or less to obtain a flame-retardant fiber bundle having a density of 1.33 g/cm3 or ^more and 1.36 g/cm3 or ^less .
(4) A first carbonization step of heating the flame-retardant fiber bundle obtained in the flame-retardant step in a non-oxidizing atmosphere having a temperature gradient in a range of an atmospheric temperature of 300° C. or more and 900° C. or less at an elongation rate of 4.0% or more and 5.0% or less.
(5) A second carbonization step of heating the fiber bundle while applying a tension of 0.15 cN/dtex or more and 0.21 cN/dtex or less to the fiber bundle in a non-oxidizing atmosphere having a temperature gradient in an atmospheric temperature range of 1000° C. or more and 1800° C. or less after the first carbonization step.
(6) A third carbonization step of heating the fiber bundle while applying a tension of 0.15 cN/dtex or more and 0.23 cN/dtex or less to the fiber bundle in a non-oxidizing atmosphere having a temperature gradient in an atmospheric temperature range of 1700°C or more and 2300°C or less after the second carbonization step.
[20] The method for producing a carbon fiber bundle according to [19], wherein in the third carbonization step, the atmospheric temperature is raised from 1800°C to 2200°C at a heating rate of 210°C/min or more and 340°C/min or less.
[21] The method for producing a carbon fiber bundle according to [19], wherein in the third carbonization step, the atmospheric temperature is raised from 1800° C. to 2200° C. at a heating rate of 215° C./min or more and 300° C./min or less.
[22] The method for producing a carbon fiber bundle according to any of [19] to [21], wherein a difference between a maximum atmospheric temperature in the second carbonization step and an inlet atmospheric temperature in the third carbonization step is 500° C. or less.
[23] The method for producing a carbon fiber bundle according to any of [19] to [21], wherein a difference between a maximum atmospheric temperature in the second carbonization step and an inlet atmospheric temperature in the third carbonization step is 300° C. or less.
[24] The method for producing a carbon fiber bundle according to any one of [13] to [23], wherein the maximum heating temperature in the heating in the non-oxidizing atmosphere is 2100 to 2300° C.

According to the present invention, it is possible to provide a carbon fiber bundle having a thickened average diameter of single carbon fibers without reducing the strand strength and strand elastic modulus of the carbon fibers, and a method for producing the same.
According to the present invention, carbon fibers suitable for carbon fiber reinforced composite materials having excellent compressive strength in the fiber axial direction can be obtained.

FIG. 2 is a schematic diagram showing a method for measuring the ultrasonic elastic modulus of a carbon fiber bundle.

[Carbon fiber bundle]
The carbon fiber bundle of the present invention has a strand strength of 4.5 GPa or more, a strand modulus of elasticity of 320 GPa or more, is substantially untwisted, and has an average diameter of single carbon fibers of 6.5 μm or more and 8.5 μm or less.
The term "carbon fiber bundle" refers to a bundle of multiple carbon fiber filaments.

By setting the strand strength of the carbon fiber bundle to 4.5 GPa or more and the strand modulus to 320 GPa or more, a carbon fiber bundle having a good balance between the strand strength and the strand modulus can be obtained, making it easier to obtain a carbon fiber reinforced composite material having excellent mechanical properties.
The conditions for measuring the strand strength and strand modulus are as described in the Examples section below.
The strand strength is the strand strength measured by a tensile test, and the strand modulus is the strand modulus measured by a tensile test.

The carbon fiber bundle of the present invention is substantially untwisted.
In the present invention, "substantially no twist" means that there is no twist in the fiber bundle, or there is localized twist, but there is an equal amount of S twist and Z twist. When there is an equal amount of S twist and Z twist in the fiber bundle, it is preferable that the net twist number in the entire carbonization process is 0.5 turns/m or less.
By the carbon fiber bundles being substantially untwisted, the openability of the carbon fiber bundles is improved, and the performance of the obtained carbon fiber reinforced composite material can be further improved.
It may also include carbon fibers that are sintered in a twisted state and then untwisted. However, when untwisted, the carbon fibers are in a helical state, and when they are made into a composite material with a matrix resin, they tend to buckle when compressed, and the compressive strength in the fiber direction tends to be low. From this point of view, it is preferable that the carbon fibers have straightness.

In the present invention, the compressive strength in the fiber direction of a carbon fiber reinforced composite material can be measured by the following method.

[Method for measuring compressive strength in the fiber axis direction of carbon fiber reinforced composite materials]
(Prepreg Production)
A carbon fiber bundle unwound from a bobbin is placed on release paper coated with epoxy resin #350 (manufactured by Mitsubishi Chemical Corporation) and impregnated with the epoxy resin. A protective film is then laminated on top of the carbon fiber bundle to produce a unidirectionally oriented prepreg (hereinafter referred to as prepreg) with a resin content of approximately 33% by mass and a carbon fiber density of 125 g/ ^m2 .

(Manufacture of unidirectional laminated materials)
Two plies of prepreg are laminated and bagged, and the pressure inside the bag is reduced by a vacuum pump. Then, the resultant is placed in an autoclave, and the temperature inside the autoclave is increased at a rate of 2°C/min, held at 80°C for 1 hour, then increased at a rate of 2°C/min, held at 130°C for 1.5 hours, and cured to obtain a carbon fiber reinforced composite material. At this time, the pressure inside the autoclave is increased to 0.6 MPa after being held at 80°C for 1 hour. The suction by the vacuum pump is stopped when the pressure inside the autoclave is 0.14 MPa, and the bag is opened to the atmosphere.

<Evaluation of compression properties in the fiber direction of carbon fiber resin composite materials>
Six test pieces with a width of 12.7 mm, a length of 80 mm, and a thickness of 1 mm are prepared from the obtained unidirectional laminate. The length direction of the test piece is the 0° direction of the fiber. The obtained test pieces are measured for compressive strength and compressive modulus at a temperature of 23° C., a humidity of 50% RH, and a crosshead speed of 1.27 mm/min using an INSTRON 5882 measuring instrument conforming to SACMA SRM 1R, and the measured values are converted to Vf (fiber volume content) of 56%. The six test pieces are similarly measured, and the average value is calculated. The measurement is performed by adhering tabs cut from the same plate to each test piece.

In the carbon fiber bundle of the present invention, the average diameter of the carbon fiber single fibers, that is, the fiber diameter, is 6.5 μm or more and 8.5 μm or less.
By making the fiber diameter 6.5 μm or more, the gaps between the fibers can be made larger, making it easier to uniformly impregnate the resin, and making it possible to suppress the generation of voids in the carbon fiber reinforced composite material obtained by using the carbon fiber bundle of the present invention. By making the fiber diameter 8.5 μm or less, the cross-sectional double structure is less likely to become noticeable in the flame-proofing step (step (3)) described below, and it is possible to obtain a carbon fiber bundle with high strand strength without decreasing the strand elastic modulus.
In order to achieve both uniformity of resin impregnation and high strand strength, the fiber diameter is preferably 6.8 μm or more, and more preferably 7.5 μm or more. For example, the fiber diameter may be 6.8 μm or more and 8.5 μm or less, or 7.5 μm or more and 8.5 μm or less.
The conditions for measuring the average diameter of the carbon fiber single fibers are as described in the Examples section below.

In the carbon fiber bundle of the present invention, the strand strength of the carbon fiber bundle is preferably 4.65 GPa or more, more preferably 4.85 GPa or more, and even more preferably 5.0 GPa or more. The strand strength of the carbon fiber bundle is preferably higher, and the upper limit is not particularly limited, but is usually 6.0 GPa or less, preferably 5.7 GPa or less, and more preferably 5.5 GPa or less. The strand elastic modulus of the carbon fiber bundle is preferably 365 GPa or more, and more preferably 380 GPa or more. The strand elastic modulus of the carbon fiber bundle is preferably higher, and the upper limit is not particularly limited, but is usually 430 GPa or less, preferably 410 GPa or less, and more preferably 403 GPa or less.

In the carbon fiber bundle of the present invention, the crystallite size Lc of the carbon fiber single fiber is preferably 3.4 nm or more and 4.1 nm or less.
If the crystallite size Lc of the carbon fiber bundle is 3.4 nm or more, the strand modulus of the carbon fiber bundle is easily maintained high. Also, if it is 4.1 nm or less, it is easy to suppress the formation of defects and the decrease in strand strength of the carbon fiber bundle due to the crystallite size becoming too large. From this viewpoint, 3.6 nm or more and 4.1 nm or less are more preferable, and 3.6 nm or more and 3.8 nm or less are even more preferable. The crystallite size Lc can be controlled by adjusting the heating temperature and the temperature rise rate when the flame-resistant fiber bundle is heated and carbonized.
The conditions for measuring the crystal size Lc are as described in the Examples section below.

In the carbon fiber bundle of the present invention, it is preferable that the average void length of the carbon fiber single fibers is 22.0 nm or less.
If the average void length of the carbon fiber bundle is 22.0 nm or less, the strand strength of the carbon fiber bundle is more likely to be maintained high. From this viewpoint, the average void length is more preferably 21.0 nm or less, and further preferably 19.5 nm or less.
Regarding the lower limit of the average void length, if it is 5.0 nm or more, the flexibility of the fiber is easily ensured, and the lower limit of the average void length is more preferably 10 nm or more.
The upper and lower limits can be combined in any manner. For example, the thickness may be 5.0 nm or more and 22.0 nm or less, 5.0 nm or more and 21.0 nm or less, or 10 nm or more and 19.5 nm or less.
The average void length can be controlled by adjusting the heating temperature and the temperature rise rate when the flame-resistant fiber bundle is heated and carbonized.
The conditions for measuring the average void length are as described in the Examples section below.

In the carbon fiber bundle of the present invention, it is preferable that the fracture surface formation energy of the carbon fiber single fiber is 18 N/m or more.
The fracture surface creation energy is determined by forming a hemispherical defect having a predetermined size range on the surface of a single fiber using a laser, breaking the fiber at the site of the hemispherical defect in a tensile test, and then calculating the fracture surface creation energy from the breaking strength of the fiber and the depth of the hemispherical defect using the following Griffith equation (F1).
Fracture surface generation energy = σ ² πC / 2E ... (F1)
Here, σ is the breaking 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 index of the resistance of carbon fibers to breakage and represents the matrix strength. Carbon fibers are materials that exhibit brittle fracture, and their tensile strength is governed by defects. When carbon fibers have the same defects, the higher the matrix strength, the higher the fracture strength. Therefore, by making the fracture surface formation energy of the carbon fiber bundle 18 N/m or more, it becomes easier to increase the strength without decreasing the strand modulus of the carbon fiber bundle, and it becomes easier to improve the performance of the resulting carbon fiber reinforced composite material.
From these viewpoints, the fracture surface generation energy of the carbon fiber bundle is more preferably 19 N/m or more, and further preferably 21.5 N/m or more.
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 fiber constituting the carbon fiber bundle is high and the graphite crystal size is large, and as a result, the compressive strength in the fiber axis direction tends to decrease. In order to obtain a carbon fiber bundle having a good balance between the fracture surface formation energy and the compressive strength, the fracture surface formation energy of the carbon fiber bundle is preferably 35 N/m or less, and more preferably 30 N/m or less.
The upper and lower limits can be combined in any manner. For example, the tension may be 18 N/m or more and 35 N/m or less, 19 N/m or more and 35 N/m or less, or 21.5 N/m or more and 30 N/m or less.
The detailed conditions for measuring the fracture surface generation energy are as described in the Examples section below.

In the carbon fiber bundle of the present invention, it is preferable that the coefficient of variation in elastic modulus (CV %: hereinafter also simply referred to as "coefficient of variation in elastic modulus") in a single fiber tensile test of a single carbon fiber in the carbon fiber bundle is 17.5% or less.
By setting the rate of variation in the elastic modulus to 17.5% or less, the difference in defects between single fibers is reduced, and it is easy to maintain high strand strength of the carbon fiber bundle. From this viewpoint, the rate of variation in the elastic modulus is more preferably 15.0% or less, further preferably 13.0% or less, and particularly preferably 11.0% or less.
The rate of variation of the single fiber elastic modulus can be controlled by adjusting the heating temperature and heating time when the carbon fiber precursor acrylic fiber bundle is heated to obtain a flame-resistant fiber bundle.
The conditions for measuring the rate of variation of the single fiber elastic modulus in the single fiber tensile test are as described in the Examples section below.

The carbon fiber bundle of the present invention is preferably unentangled.
The term "entanglement" used here refers to intentional entanglement using an entanglement device. Carbon fiber that naturally entangles single fibers during the manufacturing process is considered to be non-entangled.
When the carbon fiber bundles are unentangled, the carbon fiber bundles have good openability when made into a carbon fiber reinforced composite material, and the impregnation with the matrix resin is easily improved, making it easier to obtain a material with excellent mechanical properties.
In this case, the hook drop value of the carbon fiber bundle is 500 mm or more, and preferably 1000 mm or more.

The hook drop value is measured in the following manner.
First, a 2000 mm carbon fiber bundle is arranged vertically and the upper end is fixed. Next, a hook with a total weight of 30 g attached with a weight is inserted into the carbon fiber bundle 1. The hook used here is a hook made by molding a metal wire with a diameter of 1 mm, and the radius of the hook part is 5 mm. Next, the hook is allowed to fall freely while being inserted into the carbon fiber bundle. As described above, a carbon fiber bundle is a thread in which a large number of carbon fiber monofilaments are drawn in almost the same direction and integrated with a sizing agent or the like. However, the carbon fiber monofilaments are often entangled with each other at some point. The hook often stops at such a part. Therefore, the distance from the position where the hook is inserted into the carbon fiber bundle to the position where the hook stops can be measured. The falling distance of the hook from the insertion position to the stop position is the hook drop value.

In the carbon fiber bundle of the present invention, it is preferable that the product of the average diameter (unit: μm) of the carbon fiber single fibers and the strand strength (unit: GPa) of the carbon fiber bundle is 31 or more.
By setting the product of the average diameter of the carbon fiber single fibers and the strand strength to 31 or more, high strand strength is likely to be obtained for fiber diameters in which the average diameter of the carbon fiber single fibers is in the range of 6.5 μm or more and 8.5 μm or less.
From this viewpoint, the product of the average diameter of the single fiber and the strand strength is preferably equal to or greater than 33, and more preferably equal to or greater than 35. Moreover, the product of the average diameter of the single fiber and the strand strength is preferably equal to or less than 50, and more preferably equal to or less than 45.
The upper and lower limits can be combined in any manner. For example, the range may be 31 or more and 50 or less, 33 or more and 50 or less, or 35 or more and 45 or less.

The carbon fiber bundle of the present invention preferably has a strand strength of 4.85 GPa or more from the viewpoint of the performance of the resulting carbon fiber reinforced composite material. Also, from the viewpoint of the performance of the resulting carbon fiber reinforced composite material, it is preferable that the strand modulus is 365 GPa or more. The carbon fiber bundle of the present invention may have a strand strength of 4.85 GPa or more and a strand modulus of 365 GPa or more.

In the carbon fiber bundle of the present invention, it is preferred that the average diameter of the carbon fiber single fibers is 6.8 μm or more, the strand strength of the carbon fiber bundle is 4.65 GPa or more, and the strand modulus of elasticity of the carbon fiber bundle is 365 GPa or more and 403 GPa or less.
By satisfying these physical properties, the performance of the resulting carbon fiber reinforced composite material can be improved.
In particular, by setting the average diameter of the carbon fiber single fibers to 6.8 μm or more, the strand strength of the carbon fiber bundle to 4.65 GPa or more, and the strand modulus of the carbon fiber bundle to 365 GPa or more, the performance of the obtained carbon fiber reinforced composite material is likely to be improved. Also, by setting the average diameter of the carbon fiber single fibers to 6.8 μm or more, the strand strength of the carbon fiber bundle to 4.65 GPa or more, and the strand modulus of the carbon fiber bundle to 403 GPa or less, it is easy to prevent the graphite crystal size of the carbon fiber bundle from becoming excessively large and it is easy to prevent a decrease in the compressive strength in the fiber axis direction, so that the performance of the obtained carbon fiber reinforced composite material is likely to be improved.

The carbon fiber bundle of the present invention preferably has a knot strength of 80 N/mm2 or ^more .
The knot strength can be an index that reflects the mechanical performance of the fiber bundle other than the fiber axis direction, and can easily evaluate the performance in the direction perpendicular to the fiber axis. In carbon fiber reinforced composite materials, the material is often formed by pseudo-isotropic lamination, which forms a complex stress field. In addition to tensile and compressive stresses in the fiber axis direction, stresses other than the fiber axis direction are also generated. Furthermore, when a relatively high-speed strain is applied, such as in an impact test, 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 making the knot strength of the carbon fiber bundle 80 N/ ^mm2 or more, the performance of the obtained carbon fiber reinforced composite material can be easily improved. From these points of view, it is more preferable that the knot strength is 90 N/mm2 or ^more .
On the other hand, when the knot strength of the carbon fiber bundle is increased, the compressive strength in directions other than the fiber axis direction also increases, and the graphite crystal size tends to become smaller, resulting in a decrease in strand modulus. In order to obtain a carbon fiber bundle having a good balance between strand modulus and knot strength, the knot strength is preferably 600 N/mm2 or ^less , more preferably 400 N/mm2 or ^less , and even more preferably 200 N/mm2 or ^less .
The upper and lower limits can be combined in any manner. For example, the elastic modulus may be 80 N/mm ² or more and 600 N/mm ² or less, 80 N/mm ² or more and 400 N/mm ² or less, or 90 N/mm ² or more and 200 N/mm ² or less.
The conditions for measuring the knot strength are as described in the Examples section below.

The carbon fiber bundle of the present invention preferably has a density of 1.79 g/ ^cm3 or more. If the density of the carbon fiber bundle is 1.79 g/cm3 or ^more , the strand strength and strand elastic modulus are likely to be increased. From this viewpoint, the density of the carbon fiber bundle is more preferably 1.81 g/cm3 or ^more , and further preferably 1.83 g/ ^cm3 or more.
The density of the carbon fiber bundle is preferably 1.90 g/cm3 ^or less, more preferably 1.88 g/cm3 or ^less , and even more preferably 1.86 g/cm3 or ^less . If the density of the carbon fiber bundle is 1.90 g/cm3 ^or less, it is easy to prevent the graphite crystal size of the carbon fiber bundle from becoming excessively large, and it is easy to prevent a decrease in compressive strength in the fiber axis direction, so that the performance of the obtained carbon fiber reinforced composite material is easily improved.
The upper and lower limits can be combined in any manner. For example, the density may be 1.79 g/cm ³ or more and 1.90 g/cm ³ or less, 1.81 g/cm ³ or more and 1.88 g/cm ³ or less, or 1.83 g/cm ³ or more and 1.86 g/cm ³ or less.
The conditions for measuring the density are as described in the Examples section below.
In one embodiment, the number of filaments in the carbon fiber bundle, i.e., the number of carbon fiber single fibers constituting the carbon fiber bundle, is preferably 8,000 to 20,000, more preferably 10,000 to 18,000, and even more preferably 12,000 to 18,000.
When the number of filaments in the carbon fiber bundle is equal to or greater than the lower limit of the above-mentioned numerical range, the productivity in producing a carbon fiber reinforced composite material is likely to be improved.When the number of filaments in the carbon fiber bundle is equal to or less than the upper limit of the above-mentioned numerical range, a carbon fiber reinforced composite material having good openability and good mechanical properties is likely to be obtained.

[Method of manufacturing carbon fiber bundle]
The method for producing carbon fibers of the present invention is a method for producing carbon fiber bundles, which comprises heating a carbon fiber precursor acrylic fiber bundle in an oxidizing atmosphere to form a flame-retardant fiber bundle, and heating the obtained flame-retardant fiber bundle in a non-oxidizing atmosphere to form a carbon fiber bundle, wherein, at a temperature for heating in the non-oxidizing atmosphere, the temperature rise rate when the atmospheric temperature is raised from 1800° C. to 2200° C. is 200 to 500° C./min, and the average diameter of the carbon fiber single fibers is 6.5 μm or more and 8.5 μm or less.
That is, the method for producing carbon fibers of the present invention is a method for producing carbon fiber bundles, comprising heating a carbon fiber precursor acrylic fiber bundle in an oxidizing atmosphere to convert the carbon fiber precursor acrylic fiber bundle into a flame-resistant fiber bundle, and heating the flame-resistant fiber bundle in a non-oxidizing atmosphere to convert the flame-resistant fiber bundle into a carbon fiber bundle, wherein in the heating in the non-oxidizing atmosphere, the heating rate when heating from 1800° C. to 2200° C. is 200 to 500° C./min, and the average diameter of the carbon fiber single fibers contained in the obtained carbon fiber bundle is 6.5 μm or more and 8.5 μm or less.
The single fiber fineness of the carbon fiber precursor acrylic fiber bundle is preferably in the range of 1.1 to 2.0 dtex, which can be controlled by the amount of the acrylonitrile polymer solution discharged from the discharge hole of the spinning nozzle and the draw ratio.

By setting the temperature rise rate when the atmospheric temperature is raised from 1800° C. to 2200° C. to 200° C./min or more, it becomes possible to manufacture carbon fiber bundles with high productivity. By setting the temperature rise rate when the atmospheric temperature is raised from 1800° C. to 2200° C. to 500° C./min or less, it becomes easy to suppress a violent decomposition reaction accompanying a sudden temperature rise, and it becomes easy to obtain a carbon fiber bundle having high density and high strand strength and knot strength without decreasing the strand modulus of the carbon fiber bundle having an average diameter of single carbon fiber fibers of 6.5 μm or more and 8.5 μm or less.
From these viewpoints, the lower limit of the temperature rise rate when the atmospheric temperature is raised from 1800° C. to 2200° C. is preferably 210° C./min or more, more preferably 215° C./min or more, and most preferably 220° C./min. The upper limit of the temperature rise rate is preferably 480° C./min or less, more preferably 400° C./min or less, and most preferably 340° C./min or less.
The upper and lower limits can be combined in any manner. For example, the heating rate may be 210 to 480° C./min, 215 to 400° C./min, or 220 to 340° C./min.
The heating rate when the atmospheric temperature is raised from 1800°C to 2200°C is the travel time of the fiber bundle at atmospheric temperatures from 1800°C to 2200°C divided by 400°C, which is the difference between 2200°C and 1800°C.

The method for producing carbon fibers of the present invention preferably includes the following steps (1) to (2). The method for producing carbon fibers of the present invention preferably includes the following steps (3) to (6). The method for producing carbon fibers of the present invention preferably includes the following steps (1) to (6).

(1) A coagulation step in which an acrylonitrile-based polymer solution is discharged into the air from a discharge hole by using a dry-wet spinning method, and then coagulated in a coagulation bath containing an aqueous solution of an organic solvent having a concentration of 80.0 mass % or more and 81.0 mass % or less at a temperature of 10° C. to obtain a coagulated fiber bundle containing the organic solvent.
(2) A second drawing step in which the coagulated yarn bundle obtained in the coagulation step (1) is drawn at a draw ratio of 2.0 to 3.2 times in a warm aqueous solution having an organic solvent concentration of 40 to 65 mass % at a temperature of 75° C. or higher to obtain a carbon fiber precursor acrylic fiber bundle.

(3) A flame-retarding step of heating the carbon fiber precursor acrylic fiber bundle or the carbon fiber precursor acrylic fiber bundle obtained in the second drawing step of (2) in an oxidizing atmosphere having a temperature gradient in an atmospheric temperature range of 200° C. or more and 260° C. or less at an elongation rate of 3.0% or more and 8.0% or less to obtain a flame-retardant fiber bundle having a density of 1.33 g/cm3 or ^more and 1.36 g/cm3 or ^less .
(4) A first carbonization step of heating the flame-retardant fiber bundle obtained in the flame-retardant step (3) in a non-oxidizing atmosphere having a temperature gradient in a range of an atmospheric temperature of 300° C. or more and 900° C. or less at an elongation rate of 4.0% or more and 5.0% or less.
(5) After the first carbonization step (4), a second carbonization step of heating the fiber bundle while applying a tension of 0.15 cN/dtex or more and 0.21 cN/dtex or less to the fiber bundle in a non-oxidizing atmosphere having a temperature gradient in an atmospheric temperature range of 1000°C or more and 1800°C or less.
(6) After the second carbonization step (5), a third carbonization step of heating the fiber bundle while applying a tension of 0.15 cN/dtex or more and 0.23 cN/dtex or less to the fiber bundle in a non-oxidizing atmosphere having a temperature gradient in an atmospheric temperature range of 1700°C or more and 2300°C or less.

<Solidification process>
The coagulation step (1) is a step in which an acrylonitrile-based polymer solution is discharged into the air from a discharge hole by using a dry-wet spinning method, and then coagulated in a coagulation bath containing an aqueous solution (A) having an organic solvent concentration of 80.0 mass % or more and 81.0 mass % or less at a temperature of 10° C. or less to obtain a coagulated fiber bundle containing the organic solvent.
The temperature of the coagulation bath, i.e., the aqueous solution (A), is not higher than 10° C. By setting the temperature of the aqueous solution (A) to not higher than 10° C., dense coagulated fibers are easily formed, and in particular the denseness of the fiber surface can be increased, and a carbon fiber bundle having high strand strength and knot strength can be easily obtained without decreasing the strand elastic modulus.
The temperature of the aqueous solution (A) is preferably 4° C. or higher, and more preferably 6° C. or higher. By setting the temperature of the aqueous solution (A) to 4° C. or higher, excessive densification of the coagulated fibers can be suppressed, and stretchability in the subsequent steps can be easily ensured. For example, the temperature of the aqueous solution (A) may be 4 to 10° C., or 6 to 10° C.

The concentration of the organic solvent in the coagulation bath, i.e., the aqueous solution (A), is 80.0% by mass or more and 81.0% by mass or less, and preferably 80.2% by mass or more and 80.6% by mass or less, relative to the total mass of the aqueous solution (A). By setting the organic solvent concentration to 80.0% by mass or more and 81.0% by mass or less, it is possible to obtain coagulated yarns that are dense both on the surface and inside, and as a result, it is easy to increase the strand strength and knot strength of the obtained carbon fiber bundle without decreasing the strand modulus of elasticity.

Examples of the organic solvent contained in the aqueous solution (A) include dimethylformamide, dimethylacetamide, and dimethylsulfoxide. Among these, dimethylformamide is preferred from the viewpoint of forming a denser structure.

<Second stretching step>
The second drawing step (2) is a step of drawing the coagulated yarn bundle obtained in the coagulation step (1) at a temperature of 75° C. or higher in a warm aqueous solution (B) having an organic solvent concentration of 40 mass % or more and 65 mass % or less at a draw ratio of 2.0 times or more and 3.2 times or less to obtain a carbon fiber precursor acrylic fiber bundle.
The temperature of the hot aqueous solution (B) is 75° C. or higher, and 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, and therefore stable stretching can be easily performed.
The temperature of the hot aqueous solution (B) is preferably 98° C. or less, and more preferably 95° C. or less. By setting the temperature of the hot aqueous solution (B) to 98° C. or less, a sudden temperature change of the coagulated yarn bundle can be suppressed, and uniform drawing can be easily performed.
The upper and lower limits may be combined in any manner. For example, the temperature may be 75° C. or higher and 98° C. or lower, or 85° C. or higher and 95° C. or lower.

The concentration of the organic solvent in the hot aqueous solution (B) is from 40% by mass to 65% by mass, and preferably from 50% by mass to 60% by mass, 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 to 65% by mass, a dense structure can be formed both on the surface and inside, and it is easy to obtain a carbon fiber bundle with high strand strength and knot strength without decreasing the strand elastic modulus.

Examples of the organic solvent contained in the hot aqueous solution (B) include dimethylformamide, dimethylacetamide, and dimethylsulfoxide. Among these, dimethylformamide is preferred from the viewpoint of forming a denser structure.

The stretching ratio in the hot aqueous solution (B) is 2.0 times or more and 3.2 times or less, and preferably 2.7 times or more and 3.0 times or less. By setting the stretching ratio in the hot aqueous solution (B) to 2.0 times or more, it is possible to produce a carbon fiber precursor acrylic fiber bundle with sufficient molecular orientation, and it is easy to obtain a carbon fiber bundle 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 less, excessive stretching can be suppressed, making it easy to stretch stably.

In the second drawing step (2), after drawing the coagulated yarn bundle in the hot aqueous solution (B), for example, a step of removing the organic solvent, a step of drawing with hot water, a step of drawing by vaporization with pressurized steam, a step of drawing with dry heat, a step of applying an oil agent, and a step of drying may be appropriately combined to obtain a carbon fiber precursor acrylic fiber.
Specifically, in the second drawing step (2), after the coagulated yarn bundle is drawn, the organic solvent is removed, and the bundle is shrunk or drawn in hot water (C) having a temperature of 90° C. or higher at a draw ratio of 0.96 to 1.30, and then drawn in a pressurized water vapor atmosphere at a draw ratio of 3.7 to 4.2 to obtain a carbon fiber precursor acrylic fiber bundle.
That is, the second drawing step (2) preferably includes, in order, a step (2-1) of drawing the coagulated yarn bundle in a hot water solution (B) at a draw ratio of 2.0 to 3.2 times, a step (2-2) of removing the organic solvent, a step (2-3) of shrinking or drawing the coagulated yarn bundle in hot water (C) at a temperature of 90° C. or higher at a draw ratio of 0.96 to 1.30 times, and a step (2-5) of drawing the coagulated yarn bundle in a pressurized water vapor atmosphere at a draw ratio of 3.7 to 4.2 times. The second drawing step (2) may further include a step (2-4) of applying an oil composition. Step (2-4) can be carried out between steps (2-3) and (2-5).

Step (2-2) is a step of removing the organic solvent from the coagulated yarn bundle (hereinafter also referred to as the "drawn fiber bundle") after drawing in the warm aqueous solution (B). Any method can be used to remove the organic solvent as long as it can remove the solvent. For example, the drawn fiber bundle can be washed and drawn in a multi-stage washing tank set at a temperature in the range of 50°C or higher and lower than 100°C.

In the step (2-3), the drawn fiber bundle after removing the organic solvent is shrunk or drawn at a magnification of 0.96 times or more and 1.30 times or less in hot water (C) having a temperature of 90° C. or more. The step (2-3) can relieve the distortion due to drawing.
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 is possible to uniformly relax the stretching distortion, and it is possible to obtain a carbon fiber bundle having higher strand strength and knot strength without decreasing the strand modulus. The temperature of the hot water (C) is preferably 97° C. or lower. By setting the temperature of the hot water (C) to 97° C. or lower, it is possible to suppress a sudden temperature change in the stretched fiber bundle, it is possible to uniformly relax the stretching distortion, and it is possible to obtain a carbon fiber bundle having higher strand strength and knot strength without decreasing the strand modulus.
The shrinkage or stretching ratio in warm water (C) is 0.96 times or more and 1.30 times or less. By setting the shrinkage or stretching ratio to 0.96 times or more, poor take-up due to loosening of the fiber bundle can be prevented, and stretching distortion can be stably relaxed. By setting the shrinkage or stretching ratio to 1.30 times or less, excessive load can be suppressed, and stretching distortion can be stably relaxed.
In the step (2-3), the stretched fiber bundle after removing the organic solvent is preferably shrunk (relaxed) in warm water (C) to a shrinkage ratio (relaxation ratio) of 0.96 to less than 1.00, or stretched to a stretch ratio of 1.00 to 1.30, more preferably shrunk (relaxed) to a shrinkage ratio (relaxation ratio) of 0.96 to 0.99, or stretched to a stretch ratio of 1.05 to 1.30, and further preferably shrunk (relaxed) to a shrinkage ratio (relaxation ratio) of 0.96 to 0.99.

The step (2-4) is a step of applying an oil composition to the drawn fiber bundle after it has been shrunk or drawn in warm water (C).
The oil composition can be determined in consideration of the functions required for the carbon fiber precursor acrylic fiber bundle. For example, a silicone-based oil composition can be used. The oil composition can further contain additives such as antioxidants, antistatic agents, defoamers, preservatives, antibacterial agents, and penetrants, as necessary.
The oil composition can be applied to the drawn fiber bundle by any known method, such as a roller method, a guide method, a spray method, or a dipping method.
After the oil composition has been applied to the drawn fiber bundle, it may be dried by a conventionally known method, if necessary.

The step (2-5) is a step in which the drawn fiber bundle is shrunk or drawn in warm water (C), preferably after an oil composition is applied thereto, and then dried as necessary, and then drawn at a draw ratio of 3.7 to 4.2 times in a pressurized water vapor atmosphere.
The stretching ratio in the pressurized water vapor atmosphere is 3.7 times or more and 4.2 times or less. By setting the stretching ratio in the pressurized water vapor atmosphere to 3.7 times or more, the molecular orientation of the obtained carbon fiber precursor acrylic fiber bundle is improved, and it is possible to obtain a carbon fiber bundle having higher strand strength and knot strength without decreasing the strand elastic modulus. By setting the stretching ratio in the pressurized water vapor atmosphere to 4.2 times or less, excessive stretching can be suppressed, and stable stretching can be achieved.

<Flameproofing process>
The flame-retardant step (3) is a step of heating the carbon fiber precursor acrylic fiber bundle or the carbon fiber precursor acrylic fiber bundle obtained in the second drawing step (2) in an oxidizing atmosphere having a temperature gradient in an atmospheric temperature range of 200° C. or more and 260° C. or less at an elongation rate of 3.0% or more and 8.0% or less to obtain a flame-retardant fiber bundle having a density of 1.33 g/cm3 ^{or more} and 1.36 g/cm3 ^{or less} .
In the flame-proofing step, the carbon fiber precursor acrylic fiber bundle is preferably heated in an oxidizing atmosphere in a flame-proofing furnace having a linear temperature gradient within the range of 200° C. or more and 260° C. or less.
In the flame-proofing process, a cyclization reaction due to heat and an oxidation reaction due to oxygen occur, and it is important to cause these two reactions to occur in a well-balanced manner in order to obtain a carbon fiber bundle having high strand strength and knot strength without decreasing the strand modulus.
The atmospheric temperature in the flame-proofing step is 200° C. or higher and 260° C. or lower. By setting the temperature of the atmosphere in which the carbon fiber precursor acrylic fiber bundle runs in the flame-proofing step to 200° C. or higher, it is possible to reduce the number of parts in which the oxidation reaction has not sufficiently occurred, and it is possible to suppress the occurrence of large structural irregularities in the cross-sectional direction of the single fiber, so that it is easy to obtain a carbon fiber bundle having high strand strength and knot strength without decreasing the strand elastic modulus. By setting the temperature of the atmosphere in which the carbon fiber precursor acrylic fiber bundle runs in the flame-proofing step to 260° C. or lower, it is possible to suppress the presence of a large amount of oxygen in the part close to the surface of the single fiber, and as a result, it is possible to suppress the reaction that causes excess oxygen to disappear and form defects by the heat treatment in the first carbonization step or later described below, and it is easy to obtain a carbon fiber bundle having high density, strand strength, and knot strength without decreasing the strand elastic modulus.

In the flame-retardant process, the carbon fiber precursor acrylic fiber bundle is heated until the density of the flame-retardant fiber bundle obtained is 1.33 g/cm ³ or more and 1.36 g/cm ³ or less. By making the density of the flame-retardant fiber bundle 1.33 g/cm ³ or more, it is possible to suppress the occurrence of insufficient flame-retardant portions, and as a result, it is possible to suppress the occurrence of decomposition reactions caused by heat treatment in the first carbonization process or later described, which causes the formation of defects, and it is easy to obtain a carbon fiber bundle having a high density, high strand strength, and high knot strength without decreasing the strand elastic modulus. By making the density of the flame-retardant fiber bundle 1.36 g/cm ³ or less, it is possible to suppress the presence of a large amount of oxygen in the flame-retardant fiber bundle, and as a result, it is possible to suppress the disappearance of excess oxygen by heat treatment in the first carbonization process or later described, which causes the formation of defects, and it is easy to obtain a carbon fiber bundle having a high density, high strand strength, and high knot strength without decreasing the strand elastic modulus.

In the flame-resistant process, the carbon fiber precursor acrylic fiber bundle is stretched at an elongation rate of 3.0% to 8.0% to produce a flame-resistant fiber bundle. The elongation rate in the flame-resistant process is preferably 4.0% to 7.0%, and more preferably 5.0% to 6.5%. By setting the elongation rate in the flame-resistant process to 3.0% or more, the molecular orientation of the flame-resistant fiber bundle can be improved, making it easier to obtain a carbon fiber bundle with high strand strength and knot strength without reducing the strand modulus. By setting the elongation rate in the flame-resistant process to 8.0% or less, excessive elongation can be suppressed, making it easier to obtain a stable flame-resistant fiber bundle.

Examples of gases that form an oxidizing atmosphere include air, oxygen, and nitrogen dioxide, with air being preferred from the standpoint of economy.
The treatment time in the flame-proofing furnace (time of flame-proofing treatment) can be, for example, 30 minutes or more and 100 minutes or less.

<First carbonization step>
The first carbonization step (4) is a step of heating the flame-retardant fiber bundle obtained in the flame-retardant step (3) in a non-oxidizing atmosphere having a temperature gradient in the range of an atmospheric temperature of 300° C. or more and 900° C. or less at an elongation rate of 4.0% or more and 5.0% or less.
In the first carbonization step, the flame-resistant fiber bundle is preferably heated in a non-oxidizing atmosphere in a first carbonization furnace having a linear temperature gradient within the range of 300° C. or more and 900° C. or less.
The atmospheric temperature in the first carbonization step is 300° C. or higher and 900° C. or lower. By setting the atmospheric temperature in the first carbonization step to 900° C. or lower, it is possible to prevent the flame-resistant fiber bundle from becoming very brittle, and not only can the bundle be stably passed through the first carbonization step (first carbonization furnace), but also the formation of defects in the heat treatment in the second carbonization step and thereafter described below can be suppressed, and a carbon fiber bundle having high density, high strand strength, and knot strength can be easily obtained without decreasing the strand elastic modulus.

The elongation rate in the first carbonization step is 4.0% or more and 5.0% or less. By making the elongation rate in the first carbonization step 4.0% or more, the molecular orientation of the resulting carbon fiber bundle can be improved, and it is easy to improve the strand strength and knot strength without decreasing the strand elastic modulus. By making the elongation rate in the first carbonization step 5.0% or less, it is possible to suppress excessive elongation, and it is easy to pass through the first carbonization step (first carbonization furnace) stably.

The processing time in the first carbonization furnace (time of the first carbonization process) is preferably 1.0 to 3.0 minutes, more preferably 1.2 to 2.5 minutes. 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 accompanies a sudden rise in temperature, and it is easy to obtain a carbon fiber bundle with high density, strand strength, and knot strength without decreasing the strand elastic 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, and it is easy to obtain a carbon fiber bundle with high strand strength and knot strength without decreasing the strand elastic modulus.

Gases that form a non-oxidizing atmosphere include, for example, nitrogen, argon, and helium, with nitrogen being preferred from an economical standpoint.

<Second carbonization step>
The second carbonization step (5) is a step of heating the fiber bundle while applying a tension of 0.15 cN/dtex or more and 0.21 cN/dtex or less to the fiber bundle in a non-oxidizing atmosphere having a temperature gradient in the range of an atmospheric temperature of 1000° C. or more and 1800° C. or less after the first carbonization step (4). Note that the fiber bundle to be carbonized in the second carbonization step (5) is the flame-resistant fiber bundle that has passed through the first carbonization step.
In the second carbonization process, it is preferable to heat the fiber bundle that has passed through the first carbonization furnace (first carbonization process) in a non-oxidizing atmosphere in a second carbonization furnace having an atmospheric temperature in the range of 1000°C or more and 1800°C or less with a linear temperature gradient.
The atmospheric temperature in the second carbonization step is 1000° C. or higher and 1800° C. or lower. By setting the atmospheric temperature in the second carbonization step to 1800° C. or lower, the formation of defects in the heat treatment in the third carbonization step described below is suppressed, and a carbon fiber bundle having high density, strand strength, and knot strength is easily obtained without decreasing the strand elastic modulus.

Since the fiber bundle passing through the second carbonization process (second carbonization furnace) is significantly contracted, it is important to heat it under tension. In the second carbonization process, a tension of 0.15 cN/dtex or more and 0.21 cN/dtex or less 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), and preferably a tension of 0.17 cN/dtex or more and 0.21 cN/dtex or less is applied. By applying a tension of 0.15 cN/dtex or more to the fiber bundle passing through the second carbonization process (second carbonization furnace), it is possible to maintain a high molecular orientation of the obtained carbon fiber bundle, and it is easy to improve strand strength and knot strength without decreasing the strand elastic modulus. By applying a tension of 0.21 cN/dtex or less to the fiber bundle passing through the second carbonization step (second carbonization furnace), it is possible to prevent single fiber breakage in the carbon fiber bundle due to excessive tension, making it easier to stably obtain carbon fiber reinforced composite materials.

The processing time in the second carbonization furnace (second carbonization processing time) is preferably 1.3 minutes or more and 5.0 minutes or less. 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 that accompanies a sudden rise in temperature, and it is easy to obtain a carbon fiber bundle with high density, strand strength, and knot strength without decreasing the strand elastic modulus. By setting the processing time in the second carbonization furnace to 5.0 minutes or less, it is possible to sufficiently increase the degree of crystal orientation of the carbon fiber bundle while maintaining high productivity, and it is easy to efficiently obtain a carbon fiber bundle with high strand strength and knot strength without decreasing the strand elastic modulus.

<Third carbonization step>
The third carbonization step (6) is a third carbonization step that, after the second carbonization step (5), heats the fiber bundle while applying a tension of 0.15 cN/dtex or more and 0.23 cN/dtex or less to the fiber bundle in a non-oxidizing atmosphere having a temperature gradient in an atmospheric temperature range of 1700° C. or more and 2300° C. or less. The fiber bundle to be carbonized in the third carbonization step (6) is the flame-resistant fiber bundle that has passed through the second carbonization step.
In the third carbonization step, it is preferable to heat the fiber bundle that has passed through the second carbonization furnace (second carbonization step) in a non-oxidizing atmosphere in a third carbonization furnace having an atmospheric temperature in the range of 1700°C or more and 2300°C or less with a linear temperature gradient to obtain a carbon fiber bundle.
The atmospheric temperature in the third carbonization step is 1700° C. or higher and 2300° C. or lower. Considering the temperature in the second carbonization step, it is preferable to make the atmospheric temperature in the third carbonization step higher than the atmospheric temperature in the second carbonization step, and more preferably 1800° C. or higher. By making the atmospheric temperature in the third carbonization step 2300° C. or lower, not only can deterioration of the third carbonization furnace be prevented, but also the formation of defects in the obtained carbon fiber bundle is suppressed, and a carbon fiber bundle having high density and high strand strength and knot strength is easily obtained without decreasing the strand elastic modulus.
The maximum heating temperature is preferably 2100 to 2300°C.
When the maximum temperature is 2100° C. or higher, the strand elastic modulus can be easily increased, and when the maximum temperature is 2300° C. or lower, deterioration of the third carbonization furnace can be prevented.

Since the fiber bundle passing through the third carbonization step (third carbonization furnace) is significantly contracted, it is important to heat it under tension. In the third carbonization step, a tension of 0.15 cN/dtex or more and 0.23 cN/dtex or less is applied to the total fineness of the carbon fiber precursor acrylic fiber bundle immediately before passing through the flame-retardant step (flame-retardant furnace), and preferably a tension of 0.18 cN/dtex or more and 0.22 cN/dtex or less is applied. By applying a tension of 0.15 cN/dtex or more to the fiber bundle passing through the third carbonization step (third carbonization furnace), it is possible to maintain a high molecular orientation of the obtained carbon fiber bundle, and it is easy to improve strand strength and knot strength without decreasing the strand elastic modulus. By applying a tension of 0.23 cN/dtex or less to the fiber bundle passing through the third carbonization step (third carbonization furnace), it is possible to prevent single fiber breakage in the carbon fiber bundle due to excessive tension, making it easier to stably obtain carbon fiber reinforced composite materials.

The processing time in the third carbonization furnace (time of the third carbonization process) is preferably 1.0 minute or more and 3.0 minutes or less. 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 that accompanies a sudden rise in temperature, and it is easy to obtain a carbon fiber bundle with high density, strand strength, and knot strength without decreasing the strand elastic modulus. By setting the processing time in the third carbonization furnace to 3.0 minutes or less, it is possible to sufficiently increase the degree of crystal orientation of the carbon fiber bundle while maintaining high productivity, and it is easy to efficiently obtain a carbon fiber bundle with high strand strength and knot strength without decreasing the strand elastic modulus.

In the third carbonization step, the heating rate when the atmospheric temperature is raised from 1800° C. to 2200° C. is 200° C./min or more and 500° C./min or less, preferably 210° C./min or more and 480° C./min or less, more preferably 215° C./min or more and 400° C./min or less, and most preferably 220° C./min or more and 340° C./min or less.
If the temperature rise rate when the atmospheric temperature in the third carbonization step is raised from 1800° C. to 2200° C. is 200° C./min or more, it becomes possible to produce a carbon fiber bundle with high productivity. If the temperature rise rate when the atmospheric temperature in the third carbonization step is raised from 1800° C. to 2200° C. is 500° C./min or less, it becomes easy to suppress a violent decomposition reaction accompanying a sudden temperature rise.

The difference between the maximum atmospheric temperature in the second carbonization step (5) and the inlet atmospheric temperature in the third carbonization step (6) is preferably not more than 500° C., and more preferably not more than 300° C. By setting the difference between the maximum atmospheric temperature in the second carbonization step (5) and the inlet atmospheric temperature in the third carbonization step (6) to not more than 500° C., it becomes possible to suppress a vigorous decomposition reaction in the initial stage in the third carbonization step (6), and it becomes easy to obtain a carbon fiber bundle having high density and high strand strength and knot strength without decreasing the strand elastic modulus.
The difference between the maximum atmospheric temperature in the second carbonization step (5) and the inlet atmospheric temperature in the third carbonization step (6) is preferably 30° C. or more, and more preferably 50° C. or more.
The upper and lower limits can be combined in any manner. For example, the temperature may be 30° C. or higher and 500° C. or lower, or 50° C. or higher and 300° C. or lower.

<Other processes>
The method for producing carbon fibers of the present invention may include the following (a) acrylonitrile polymer solution preparation step prior to the (1) coagulation step.
The method for producing a carbon fiber of the present invention may have the following first drawing step (b) between the coagulation step (1) and the second drawing step (2).
The method for producing carbon fibers of the present invention may have, after the third carbonization step (6), the following surface oxidation treatment step (c) and sizing step (d).
(a) A step of preparing an acrylonitrile-based polymer solution, in which an acrylonitrile-based polymer solution is prepared.
(b) A first drawing step in which the coagulated yarn bundle obtained in the coagulation step (1) is drawn in air at a draw ratio of 1.00 times or more and 1.20 times or less.
(c) a surface oxidation treatment step of subjecting the carbon fiber bundle obtained in the third carbonization step (6) to a surface oxidation treatment.
(d) a sizing step of subjecting the carbon fiber bundle obtained in the surface oxidation step (c) to a sizing treatment.

(Acrylonitrile polymer solution preparation step)
The acrylonitrile-based polymer solution preparation step (a) is a step of preparing an acrylonitrile-based polymer solution to be used in the coagulation step (1).
The acrylonitrile polymer used in the present invention is a polymer obtained by polymerizing acrylonitrile as a main monomer. The acrylonitrile polymer may be a homopolymer obtained only from acrylonitrile, or a copolymer in which other monomers are copolymerized in addition to acrylonitrile as the main component.

The content of acrylonitrile-derived structural units (hereinafter also referred to as "acrylonitrile units") in the acrylonitrile-based polymer can be determined, for example, taking into consideration the quality required for the resulting carbon fiber bundle, and is preferably 90% by mass or more and 100% by mass or less, more preferably 90% by mass or more and 99.5% by mass or less, and even more preferably 96% by mass or more and 99.5% by mass or less, relative to the total mass of the monomer units constituting the acrylonitrile-based polymer. If the content of acrylonitrile units is 90% by mass or more, fusion between single fibers can be suppressed in each of the flame retardant and carbonization processes for converting the carbon fiber precursor acrylic fiber bundle into a carbon fiber bundle, and a decrease in the strand strength of the carbon fiber bundle can be easily prevented. Furthermore, adhesion between single fibers can be easily suppressed in processes such as stretching with a heated roller or pressurized steam. If the content of acrylonitrile units is 100% by mass or less, preferably 99.5% by mass or less, the solubility in the solvent is less likely to decrease, and precipitation and solidification of the acrylonitrile-based polymer can be prevented, making it easier to stably produce carbon fiber precursor acrylic fiber bundles.

When an acrylonitrile-based polymer has a monomer unit other than an acrylonitrile unit, the monomer unit other than acrylonitrile in the acrylonitrile-based polymer can be appropriately selected from vinyl-based monomers copolymerizable with acrylonitrile, and vinyl-based monomer units that improve the hydrophilicity of the acrylonitrile-based polymer and vinyl-based monomer units that promote the flame retardant reaction are preferred. Examples include acrylic acid derivatives such as acrylic acid, methacrylic acid, itaconic acid, methyl acrylate, and methyl methacrylate; acrylamide derivatives such as acrylamide, methacrylamide, N-methylolacrylamide, and N,N-dimethylacrylamide; and vinyl acetate.

The method for synthesizing the acrylonitrile polymer may be any polymerization method, and the present invention is not limited by the difference in the polymerization method.
Examples of the solvent for the acrylonitrile polymer solution include organic solvents such as dimethylacetamide, dimethylsulfoxide, and dimethylformamide; and aqueous solutions of inorganic compounds such as zinc chloride and sodium thiocyanate. Dimethylformamide is preferred because of its high dissolving power for acrylonitrile polymers.

The polymer concentration of the acrylonitrile-based polymer solution is preferably 20% by mass or more and 25% by mass or less, and more preferably 21% by mass or more and 24% by mass or less, based on the total mass of the acrylonitrile-based polymer solution. By making the polymer concentration 20% by mass or more, the voids inside the coagulated yarn are reduced, making it easier to increase the strand strength of the carbon fiber bundle. By making the polymer concentration 25% by mass or less, the acrylonitrile-based polymer solution can maintain appropriate viscosity and fluidity, making it easier to manufacture carbon fiber precursor acrylic fiber bundles.

The temperature of the acrylonitrile polymer solution when subjected to the solidification step (1) is preferably adjusted to 50°C or higher and 70°C or lower, and more preferably 55°C or higher and 65°C or lower. By adjusting the temperature of the acrylonitrile polymer solution to 50°C or higher and 70°C or lower, the acrylonitrile polymer solution can maintain an appropriate viscosity and fluidity, which makes it easier to manufacture carbon fiber precursor acrylic fiber bundles.

(First stretching step)
The first drawing step (b) is a step of drawing the coagulated yarn bundle in air at a draw ratio of 1.00 times or more and 1.20 times or less.
The first stretching step (b) is preferably carried out between the solidification step (1) and the second stretching step (2).
In the first drawing step (b), the coagulated yarn bundle taken in the coagulation step (1) is drawn in air while still containing a part of the coagulation liquid. The draw ratio in air is 1.00 times or more and 1.20 times or less, and preferably 1.05 times or more and 1.15 times or less. By setting the draw ratio in air to 1.00 times or more, it is possible to suppress uneven shrinkage, and as a result, it is easy to obtain a carbon fiber bundle having high strand strength and knot strength without decreasing the strand elastic modulus. By setting the draw ratio in air to 1.20 times or less, excessive drawing can be suppressed, and stable drawing is easy to achieve.

(Surface oxidation treatment process)
The surface oxidation treatment step (c) is a step of subjecting the carbon fiber bundle obtained in the third carbonization step (6) to a surface oxidation treatment.
The surface oxidation treatment step (c) is preferably carried out after the third carbonization step (6).
The carbon fiber bundle obtained by passing through the third carbonization step (third carbonization furnace) is preferably subjected to a surface oxidation treatment. Examples of the surface treatment method include known methods, i.e., oxidation treatments by electrolytic oxidation, chemical oxidation, air oxidation, etc., and any method may be used, but electrolytic oxidation treatment, which is widely carried out industrially, is preferred in that it is capable of stable surface oxidation treatment.
In the surface oxidation treatment, it is preferable that the ipa, which indicates the surface treatment state, is 0.05 μA/ ^cm2 or more and 0.25 μA/ ^cm2 or less. In order to control it to such a range, a method of adjusting the amount of electricity in the electrolytic oxidation treatment is simple. In the electrolytic oxidation treatment, even with the same amount of electricity, the ipa varies greatly depending on the electrolyte used and its concentration. In an alkaline aqueous solution with a pH of more than 7, it is preferable to perform the oxidation treatment by passing an amount of electricity of 10 coulombs/g or more and 200 coulombs/g or less through the carbon fiber bundle as the anode.
Examples of electrolytes include ammonium carbonate, ammonium bicarbonate, ammonium sulfate, calcium hydroxide, sodium hydroxide, and potassium hydroxide.

(Sizing process)
The sizing step (d) is a step of subjecting the carbon fiber bundles after the surface oxidation treatment step (c) to a sizing treatment.
The sizing step (d) is preferably carried out after the surface oxidation treatment step (c).
The surface-oxidized carbon fiber bundle obtained in the surface oxidation treatment step (c) is preferably subjected to a sizing treatment in succession. The sizing treatment can be carried out by applying a solution in which a sizing agent is dissolved in an organic solvent or an emulsion in which a sizing agent is dispersed in water using an emulsifier to the carbon fiber bundle by, for example, a roller immersion method or a roller contact method, and then drying the bundle.
The amount of sizing agent attached to the surface of the carbon fibers can be adjusted by adjusting the concentration of the sizing agent liquid or the amount of the sizing agent squeezed out.
The drying can be carried out, for example, by using hot air, a hot plate, a heated roller, or various infrared heaters.
As the sizing agent, a known sizing agent can be used, for example, a sizing agent containing an epoxy resin, a polyether resin, an epoxy-modified polyurethane resin, or a polyester resin as a main component.

The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following description as long as it does not depart from the gist of the invention.
The various measurement methods carried out in this example are as follows.

[Method for measuring the average diameter of single carbon fibers]
The cross-sectional area of each carbon fiber single fiber was calculated from the density (g/ ^cm3 ) of the carbon fiber bundle, the mass per meter of the carbon fiber bundle, i.e., the basis weight (g/m), and the number of filaments in the carbon fiber bundle. The diameter of a perfect circle having an area equal to the cross-sectional area was calculated and used as the average diameter of the carbon fiber single fibers.
The density of the carbon fiber bundle was measured in accordance with Method C (gradient density pipe method) described in JIS R 7063:1999.

[Method of measuring strand strength and strand elastic modulus]
The strand strength and strand modulus of the carbon fiber bundle were measured in accordance with JIS R 7608:2007.
The strand modulus was calculated by Method A of the same method.

[Method of measuring knot strength]
Knot strength measurements were performed as follows.
A test specimen was prepared by attaching a 25 mm long grip to both ends of a 150 mm long carbon fiber bundle. When preparing the test specimen, a load of 0.1×10 ⁻³ N/denier was applied to align the carbon fiber bundle. A knot was formed in the test specimen at approximately the center, and the crosshead speed during tension was 100 mm/min. 12 pieces were tested, and the minimum and maximum values were removed, and the average value of the 10 pieces was used as the measured value.

[Method for measuring fracture surface formation energy]
A carbon fiber single fiber was cut to 20 cm, and the center of the single fiber was attached and fixed to a mount for a single fiber tensile test for a sample length of 10 mm as shown in JIS R 7606:2000, and the excess part protruding from the mount was cut and removed to prepare a sample. Next, a laser was irradiated to these samples fixed to the mount to form a hemispherical defect. For the laser interface system, a micropoint (pulse energy 300 uJ) manufactured by Photonic Instruments was used. For the optical microscope required for focusing the laser, an ECLIPSE LV100 manufactured by Nikon Corporation was used. The aperture stop of the optical microscope was set to the minimum, and the objective lens was set to 100 times. Under these conditions, a sample in which a hemispherical defect was formed was obtained by irradiating one pulse of a laser having a wavelength of 435 nm, the laser intensity of which was attenuated by 10% by an attenuator, to the center of the sample in the fiber axis direction and the center in the direction perpendicular to the fiber axis. In order to prevent the carbon fiber sample from shrinkage failure, the sample attached to the mount was sandwiched between films, and the inside of the film was filled with a viscous liquid to perform a tensile test. Specifically, a film with a width of about 5 mm and a length of about 15 mm was prepared, and the film was attached to the upper part of both sides of the mount of the sample with an adhesive, and the mount was sandwiched between the films so as to cover the sample. The space between the films was filled with a glycerin aqueous solution (1 part by mass of glycerin to 2 parts by mass of water), and a tensile test was performed at a tensile speed of 0.5 mm/min to measure the breaking load. Next, the sample pair divided into two in the tensile test was removed from the mount, carefully washed with water, and then naturally dried. Next, the sample was fixed with carbon paste on the SEM sample stage so that the fracture surface of the sample was facing up, and an SEM observation sample was prepared. The fracture surfaces of the obtained SEM observation samples were observed by SEM using a scanning electron microscope (manufactured by JEOL Ltd., product name "JSM6060") under conditions of an acceleration voltage of 10 kV to 15 kV and a magnification of 10,000 times to 15,000 times. The obtained SEM images were imported into a personal computer and analyzed using image analysis software to measure the depth of the hemispherical defect and the cross-sectional area of the fiber. Here, the "depth of the hemispherical defect" was defined as the longest distance when a line was drawn from the circumference of the single fiber to the center.
The breaking load was then divided by the cross-sectional area of the fiber (breaking load/cross-sectional area of the fiber) to calculate the breaking strength (σ).
The fracture surface formation energy was calculated from the following formula (F2): The fracture surface formation energy was calculated for 30 single fibers, and the average value was regarded as the fracture surface formation energy of the carbon fiber bundle.
Fracture surface generation energy = σ 2πC / 2E ... (F2)
Here, σ is the breaking strength, E is the ultrasonic elastic modulus of the carbon fiber bundle, and C is the depth of the hemispherical defect.
The ultrasonic elastic modulus of the carbon fiber bundle was measured in accordance with the measurement method described below.

<Method for measuring ultrasonic elastic modulus of carbon fiber bundle>
The ultrasonic propagation velocity was measured according to the measurement method shown in Fig. 1. The distance L1 between the transmitter and the first receiver was 0.20 m, the distance L2 between the transmitter and the second receiver was 0.25 m, and the tension applied to the carbon fiber bundle during measurement was 0.02 N/tex. A pulse was given from the pulse transmission circuit to the transmitter to drive it and propagate ultrasonic waves to the carbon fiber bundle, and the time from when the ultrasonic waves propagated from the carbon fiber bundle were detected by the first receiver was defined as reception time 1, and the time from when the ultrasonic waves propagated from the carbon fiber bundle were detected by the second receiver was defined as reception time 2.
The ultrasonic elastic modulus of the carbon fiber bundle was calculated from the following formula (F3).
Ultrasonic elastic modulus (GPa)=((0.25 m−0.20 m)/(reception time 2 (sec)−reception time 1 (sec))) ² ×density of carbon fiber bundle (g/cm ³ )×10 ⁻⁶ (F3)

[Variation rate of single fiber elastic modulus in single fiber tensile test]
One single fiber was taken out of the carbon fiber bundle, and the elastic modulus of the single fiber was measured using a universal testing machine (Instron 5500 (trade name) manufactured by Instron Corporation) under test conditions of a test length of 5 mm and a tensile speed of 0.5 mm/min. This was repeated until test results of 100 fibers were obtained from the same carbon fiber bundle sample, and the average value and standard deviation of the elastic modulus obtained from the test results of 100 fibers were used to calculate the elastic modulus variation rate (%) = (standard deviation/average value) x 100, which was used as the elastic modulus variation rate between fibers (the variation rate of the single fiber elastic modulus in the single fiber tensile test). The cross-sectional area of the carbon fiber used to calculate the elastic modulus was calculated from the density of the carbon fiber bundle and the single fiber fineness of the carbon fiber according to the cross-sectional area of the carbon fiber = single fiber fineness of the carbon fiber (mass per unit length) / density of the carbon fiber bundle.

[Crystallite size Lc]
The carbon fiber bundle was cut into a length of 50 mm, and 30 mg of the cut fiber was weighed out and aligned so that the fiber axis of the sample was exactly parallel. The fiber bundle was then adjusted to a uniform thickness of 1 mm using a sample adjustment tool. The fiber bundle was impregnated with a vinyl acetate/methanol solution to prevent the shape from collapsing, and then fixed to a wide-angle X-ray diffraction sample stage. A Rigaku CuKα ray (with Ni filter) X-ray generator was used as the X-ray source, and a Rigaku goniometer was used to detect the diffraction peak near 2θ=25°, which corresponds to the plane index (002) of graphite, by the transmission method using a scintillation counter. The output was measured at 40 kV-100 mA. The crystallite size Lc was calculated from the half-width of the diffraction peak using the following formula (F4).
Lc=Kλ/(β0cosθ) (F4)
(In the formula, K is the Scherrer constant of 0.9, λ is the wavelength of the X-ray used (1.5418 Å since CuKα rays are used here), θ is the Bragg diffraction angle, β0 is the true half-width, and β0=βE-β1 (βE is the apparent half-width, and β1 is an instrument constant, which is 1.05×10 ⁻² rad here).)
The measurements were carried out with n=5.

[Method of calculating average void length]
According to the Ruland method described in Macromolecules, Vol. 33, No. 5, 2000, the average void length was calculated by SAXS (small angle X-ray scattering) as follows.
Using the "SAXSpoint 2.0 system" manufactured by Anton Paar, CuKα (wavelength 1.54 Å) was used as X-rays, the exposure time was set to 30 minutes, the measurement environment was set to vacuum, and the distance from the sample to the detector was set to 610 mm. The carbon fiber bundles were aligned in one direction, and then the fiber axis direction was set vertically on the sample stage, and small-angle X-ray scattering measurement was performed. At this time, scattering originating from voids inside the carbon fibers was observed in a direction perpendicular to the fiber axis.
The analysis by the Ruland method was performed using a graph creation software (Igor Pro 8.0). In this specification, the scattering vector q was defined as 4πsinθ/λ (θ: scattering angle, λ: X-ray wavelength). The two-dimensional scattering profile obtained according to the above measurement conditions was divided into 1000 azimuth angles of 360° and polar coordinate conversion was performed to obtain a scattering intensity map of azimuth angle-scattering vector q. Here, the carbon fiber axial direction is set to an azimuth angle of 0°. The obtained scattering intensity map was analyzed by the Ruland method in the direction perpendicular to the carbon fiber axis (azimuth angle 90°) and in the range not including streaks resulting from total reflection of X-rays on the carbon fiber surface (q = 0.8 to 1.86 nm ^-1 ). Specifically, the scattering intensity map of azimuth angle-scattering vector q was averaged every 5 pixels in the q direction to obtain an azimuth angle vs. scattering intensity profile at each q. The range of azimuth angles of 0 to 180° of this profile was fitted with a Gaussian function to calculate the integral width B. ^q2B2 was plotted against ^q2 , and the intercept and slope of the linear approximation were obtained, and the void length ^L of the voids present in the carbon fiber and the integral width of the void orientation distribution were calculated using the following formula (F5).
(scattering vector q) ² (integral width B of scattering intensity distribution in the azimuth angle direction) ² = (scattering vector q) ² (integral width of void orientation distribution) ² + (2π/L) ² ... (F5)

[Example 1]
<Preparation of carbon fiber precursor acrylic fiber bundle>
An acrylonitrile-based polymer containing 98% by mass of acrylonitrile units and 2% by mass of methacrylic acid units was dissolved in dimethylformamide to prepare an acrylonitrile-based polymer solution having a concentration of 23.5% by mass.
This acrylonitrile polymer solution was spun from a spinneret having several thousand nozzles with a diameter of 0.15 mm, and subjected to dry-wet spinning. That is, the solution was spun into air and passed through a space of about 5 mm, and then coagulated in a coagulation liquid filled with an aqueous solution (A) containing 80.4% by mass of dimethylformamide and adjusted to 8° C., and a coagulated fiber bundle was taken off.
The coagulated yarn bundle was then combined to a filament count of 12,000 and pulled out of the coagulation bath, stretched 1.1 times in air, and then stretched 2.9 times in a stretching tank filled with a hot aqueous solution (B) containing 55% by mass of dimethylformamide and adjusted to 90 ° C. After stretching, the stretched fiber bundle containing the solvent was washed with clean water, and then relaxed 0.98 times in hot water (C) at 96 ° C. Subsequently, an oil agent mainly composed of amino-modified silicone was applied to the stretched fiber bundle to a concentration of 1.1% by mass, and the fiber bundle was dried and densified. The stretched fiber bundle after drying and densification was stretched 4.0 times under a pressurized water vapor atmosphere to further improve the orientation and densify the fiber bundle, and then wound up to obtain a carbon fiber precursor acrylic fiber bundle. The single fiber fineness of this fiber was 1.40 dtex.

<Preparation of carbon fiber bundle>
A plurality of carbon fiber precursor acrylic fiber bundles were aligned in parallel and introduced into a flame-resistant furnace having a linear temperature gradient with an inlet atmosphere temperature of 220° C. and a maximum atmosphere temperature of 245° C. The carbon fiber precursor acrylic fiber bundles were flame-resistant treated by blowing heated air into the flame-resistant furnace onto the carbon fiber precursor acrylic fiber bundles, to obtain a flame-resistant fiber bundle with a density of 1.345 g/cm ^3. The elongation was 6.0%, and the flame-resistant treatment time was 70 minutes.
Next, the flame-retardant fiber bundle was passed through a first carbonization furnace having a linear temperature gradient in nitrogen, with an inlet atmospheric temperature of 300° C. and a maximum atmospheric temperature of 700° C., while being stretched by 4.5%, to perform a first carbonization treatment. The treatment time was 2.0 minutes.
Further, a second carbonization process was carried out using a second carbonization furnace in which a linear temperature gradient was set with an inlet atmospheric temperature of 1100° C. and a maximum atmospheric temperature of 1700° C. in a nitrogen atmosphere. At that time, the elongation rate was −2.0%, and the processing time was 1.6 minutes. At this time, the tension applied to the yarn bundle during the process was 0.20 cN/dtex.
Subsequently, a third carbonization treatment was performed using a third carbonization furnace in which a linear temperature gradient was set with an inlet atmospheric temperature of 1800° C. and a maximum atmospheric temperature of 2300° C. in a nitrogen atmosphere to obtain a carbon fiber bundle. At that time, the elongation rate was −2.0% and the treatment time was 1.9 minutes. At this time, the tension applied to the yarn bundle during the treatment was 0.22 cN/dtex.
In addition, the difference between the maximum atmospheric temperature in the second carbonization furnace and the inlet atmospheric temperature of the third carbonization furnace was set to 100°C, and the temperature increase rate when increasing the atmospheric temperature from 1800°C to 2200°C was set to 350°C/min.
Subsequently, the carbon fiber bundle was run through a 10 mass % aqueous solution of ammonium bicarbonate at a temperature of 30° C., and an electric current was applied between the carbon fiber bundle as the anode and a counter electrode so that the electric quantity was 40 coulombs per 1 g of the treated carbon fiber.
It was then washed with warm water at 90° C. and dried.
Next, 0.5% by mass of a sizing agent (manufactured by DIC Corporation, product name "Hydran N320") was applied (sizing treatment), and the fiber bundle was wound around a bobbin to obtain a carbon fiber bundle.
The carbon fiber bundles after the sizing treatment were measured for the average single fiber diameter, density, basis weight, knot strength, strand strength and strand modulus. The results are shown in Table 3.

[Examples 2 to 3]
Carbon fiber bundles were produced and various measurements were carried out in the same manner as in Example 1, except that the production conditions for the carbon fiber bundles were changed as shown in Tables 1 and 2. The results are shown in Table 3.

[Examples 4 to 7]
Carbon fiber bundles were produced and various measurements were carried out in the same manner as in Example 1, except that the single fiber fineness of the carbon fiber precursor acrylic fiber bundle was set to 1.73 dtex and the production conditions of the carbon fiber bundle were changed as shown in Tables 1 and 2. The results are shown in Table 3.

[Comparative Example 1]
A carbon fiber precursor acrylic fiber bundle was produced in the same manner as in Example 1, except that the production conditions for the carbon fiber precursor acrylic fiber bundle were changed as shown in Table 1 and the single fiber fineness of the carbon fiber precursor acrylic fiber bundle was changed to 1.0 dtex.
Using the obtained carbon fiber precursor acrylic fiber bundle, a carbon fiber bundle was produced in the same manner as in Example 1, except that the production conditions of the carbon fiber bundle were changed as shown in Table 2, and various measurements were carried out. The results are shown in Table 3.

[Comparative Example 2]
A carbon fiber precursor acrylic fiber bundle was produced in the same manner as in Example 1, except that the production conditions for the carbon fiber precursor acrylic fiber bundle were changed as shown in Table 1 and the single fiber fineness of the carbon fiber precursor acrylic fiber bundle was changed to 1.40 dtex.
An attempt was made to produce a carbon fiber bundle in the same manner as in Example 1 using the obtained carbon fiber precursor acrylic fiber bundle, except that the production conditions for the carbon fiber bundle were changed as shown in Table 2. However, the fiber bundle broke during the production of the carbon fiber bundle, and a carbon fiber bundle could not be obtained.

[Comparative Example 3]
A carbon fiber precursor acrylic fiber bundle was produced in the same manner as in Example 1, except that the production conditions for the carbon fiber precursor acrylic fiber bundle were changed as shown in Table 1 and the single fiber fineness of the carbon fiber precursor acrylic fiber bundle was changed to 1.73 dtex.
An attempt was made to produce a carbon fiber bundle in the same manner as in Example 1 using the obtained carbon fiber precursor acrylic fiber bundle, except that the production conditions for the carbon fiber bundle were changed as shown in Table 2. However, the fiber bundle broke during the production of the carbon fiber bundle, and a carbon fiber bundle could not be obtained.

[Reference Example 1]
Various measurements were carried out on a commercially available carbon fiber bundle (manufactured by Toray Industries, Inc., product name "M40JB"). The results are shown in Table 3.

As is clear from the results in Table 3, the carbon fiber bundles obtained in each Example had a large average diameter of single fibers, and were able to exhibit high strand strength without decreasing the strand modulus. The carbon fiber bundles obtained in each Example were substantially untwisted.
On the other hand, the carbon fiber bundle obtained in Comparative Example 1 and the commercially available carbon fiber bundle used in Reference Example 1 had lower strand strength than the carbon fiber bundles obtained in Examples. In addition, since the fiber diameter was also small, there is a concern that in producing a carbon fiber reinforced composite material, insufficient impregnation will occur due to high viscosity of the matrix resin, resulting in a decrease in the tensile strength of the carbon fiber reinforced composite material.

The carbon fiber bundle of the present invention exhibits high strand strength and knot strength without a decrease in strand modulus, and has a large fiber diameter, making it useful in a wide range of applications that require high mechanical properties, 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 (24)

A carbon fiber bundle having a strand strength of 4.5 GPa or more and a strand modulus of 320 GPa or more,
A carbon fiber bundle which is substantially untwisted and has an average diameter of carbon fiber single fibers of 6.5 μm or more and 8.5 μm or less. The carbon fiber bundle according to claim 1, in which the crystallite size Lc of the carbon fiber single fiber is 3.4 nm or more and 4.1 nm or less. The carbon fiber bundle according to claim 1 or 2, in which the average void length of the carbon fiber single fibers is 22.0 nm or less. The carbon fiber bundle according to any one of claims 1 to 3, in which the fracture surface formation energy of the carbon fiber single fiber is 18 N/m or more. The carbon fiber bundle according to any one of claims 1 to 4, in which the coefficient of variation (CV%) of elastic modulus in a single fiber tensile test of the carbon fiber single fiber in the carbon fiber bundle is 17.5% or less. The carbon fiber bundle according to any one of claims 1 to 5, which is unentangled. The carbon fiber bundle according to any one of claims 1 to 6, in which the product of the average diameter (unit: μm) of the carbon fiber single fibers and the strand strength (unit: GPa) of the carbon fiber bundle is 31 or more. A carbon fiber bundle according to any one of claims 1 to 7, having a strand strength of 4.85 GPa or more and a strand modulus of elasticity of 365 GPa or more. The carbon fiber bundle according to any one of claims 1 to 8, in which the average diameter of the carbon fiber single fibers is 6.8 μm or more, the strand strength of the carbon fiber bundle is 4.65 GPa or more, and the strand modulus of elasticity of the carbon fiber bundle is 365 GPa or more and 403 GPa or less. The carbon fiber bundle according to any one of claims 1 to 9, in which the average diameter of the carbon fiber single fibers is 7.5 μm or more. The carbon fiber bundle according to any one of claims 1 to 10, having a knot strength of 80 N / mm2 or ^more . The carbon fiber bundle according to any one of claims 1 to 11, wherein the density of the carbon fiber single fiber is 1.79 g / cm ³ or more. A method for producing a carbon fiber bundle, comprising heating a carbon fiber precursor acrylic fiber bundle in an oxidizing atmosphere to convert the carbon fiber precursor acrylic fiber bundle into a flame-resistant fiber bundle, and heating the flame-resistant fiber bundle in a non-oxidizing atmosphere to convert the flame-resistant fiber bundle into a carbon fiber bundle,
In the heating in the non-oxidizing atmosphere, the temperature increase rate when increasing the temperature from 1800° C. to 2200° C. is 200 to 500° C./min;
A method for producing a carbon fiber bundle, wherein the average diameter of single carbon fibers contained in the obtained carbon fiber bundle is 6.5 μm or more and 8.5 μm or less. The method for producing a carbon fiber bundle according to claim 13, comprising the following steps (1) to (2):
(1) A coagulation step in which an acrylonitrile-based polymer solution is discharged into the air from a discharge hole by using a dry-wet spinning method, and then coagulated in a coagulation bath containing an aqueous solution of an organic solvent having a concentration of 80.0 mass % or more and 81.0 mass % or less at a temperature of 10° C. to obtain a coagulated fiber bundle containing the organic solvent.
(2) A second drawing step in which the coagulated yarn bundle obtained in the coagulation step is drawn at a draw ratio of 2.0 to 3.2 times in a warm aqueous solution having an organic solvent concentration of 40 to 65 mass % at a temperature of 75° C. or higher to obtain the carbon fiber precursor acrylic fiber bundle. The method for producing a carbon fiber bundle according to claim 13 or 14, comprising a first drawing step between the solidification step and the second drawing step, in which the solidified yarn bundle obtained in the solidification step is drawn in air at a draw ratio of 1.00 to 1.20 times, and in the second drawing step, the solidified yarn bundle obtained in the first drawing step is drawn. The method for producing a carbon fiber bundle according to any one of claims 13 to 15, wherein in the second drawing step, after the solidified yarn bundle is drawn, the organic solvent is removed, and the bundle is shrunk or drawn in hot water at a temperature of 90°C or higher at a ratio of 0.96 to 1.30, and then drawn in a pressurized water vapor atmosphere at a ratio of 3.7 to 4.2 to obtain the carbon fiber precursor acrylic fiber bundle. The method for producing a carbon fiber bundle according to any one of claims 13 to 16, wherein the concentration of the organic solvent in the aqueous solution used in the solidification step is 80.2% by mass or more and 80.6% by mass or less. The method for producing a carbon fiber bundle according to any one of claims 13 to 17, wherein the organic solvent is dimethylformamide. The method for producing a carbon fiber bundle according to any one of claims 13 to 18, comprising the following steps (3) to (6):
(3) A flame-retarding step of heating the carbon fiber precursor acrylic fiber bundle or the carbon fiber precursor acrylic fiber bundle obtained in the second stretching step in an oxidizing atmosphere having a temperature gradient in a range of an atmospheric temperature of 200° C. or more and 260° C. or less at an elongation rate of 3.0% or more and 8.0% or less to obtain a flame-retardant fiber bundle having a density of 1.33 g/cm3 or ^more and 1.36 g/cm3 or ^less .
(4) A first carbonization step of heating the flame-retardant fiber bundle obtained in the flame-retardant step in a non-oxidizing atmosphere having a temperature gradient in a range of an atmospheric temperature of 300° C. or more and 900° C. or less at an elongation rate of 4.0% or more and 5.0% or less.
(5) A second carbonization step of heating the fiber bundle while applying a tension of 0.15 cN/dtex or more and 0.21 cN/dtex or less to the fiber bundle in a non-oxidizing atmosphere having a temperature gradient in an atmospheric temperature range of 1000° C. or more and 1800° C. or less after the first carbonization step.
(6) A third carbonization step of heating the fiber bundle while applying a tension of 0.15 cN/dtex or more and 0.23 cN/dtex or less to the fiber bundle in a non-oxidizing atmosphere having a temperature gradient in an atmospheric temperature range of 1700°C or more and 2300°C or less after the second carbonization step. The method for producing a carbon fiber bundle according to claim 19, wherein in the third carbonization step, the temperature rise rate when the ambient temperature is raised from 1800°C to 2200°C is 210°C/min or more and 340°C/min or less. The method for producing a carbon fiber bundle according to claim 19, wherein in the third carbonization step, the temperature rise rate when the ambient temperature is raised from 1800°C to 2200°C is 215°C/min or more and 300°C/min or less. The method for producing a carbon fiber bundle according to any one of claims 19 to 21, wherein the difference between the maximum ambient temperature in the second carbonization step and the inlet ambient temperature in the third carbonization step is 500°C or less. The method for producing a carbon fiber bundle according to any one of claims 19 to 21, wherein the difference between the maximum ambient temperature in the second carbonization step and the inlet ambient temperature in the third carbonization step is 300°C or less. The method for producing a carbon fiber bundle according to any one of claims 13 to 23, wherein the maximum heating temperature in the non-oxidizing atmosphere is 2100 to 2300°C.

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