CARBON FIBER BUNDLE AND METHOD FOR PRODUCING CARBON FIBER BUNDLE

Description

This application is a continuation application of International Application numbered PCT/JP2023/017517, filed on May 10, 2023, the content of which is incorporated herein by reference.

TECHNICAL FIELD

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

BACKGROUND ART

For the purpose of improving mechanical properties of resin-based molded articles, fibers are generally used as a reinforcing material to form a composite with a resin. Among them, carbon fibers are excellent in specific strength and specific elastic modulus and are light in weight, and thus, are utilized as reinforcing fibers for high-performance resins not only for sports and general industrial applications in the related art but also for a wide range of applications such as aerospace applications and automobile applications. In recent years, carbon fiber reinforced composite materials obtained by using carbon fibers as reinforcing fibers and integrating the carbon fibers with a matrix resin have increasingly become more advantageous, and there is an increasing demand for improvement in performance of carbon fiber reinforced composite materials, particularly in automobile, aerospace, and other applications.

These carbon fiber reinforced composite materials are formed from, for example, a prepreg, which is an intermediate product in which carbon fibers are impregnated with a matrix resin, through molding and processing steps such as heating and pressurization.

In compounding of the carbon fibers and the matrix resin, to obtain high tensile strength and compressive strength in the fiber axial direction of the carbon fiber reinforced composite material, it is important to increase an impregnation property of the matrix resin into the carbon fiber bundle and suppress generation of voids in the carbon fiber reinforced composite material, in addition to improvement of mechanical properties such as strand strength and a strand elastic modulus of the carbon fibers themselves.

It is known that the impregnation property of a resin into a carbon fiber bundle is increased by increasing a diameter of a carbon fiber single-fiber.

To increase the impregnation property of the matrix resin into the carbon fiber bundle and to increase the compressive strength in the fiber axial direction when the carbon fiber bundle is formed into a composite material of the carbon fibers and the matrix resin, it is conceivable to increase the diameter of the carbon fiber single-fiber.

To increase the compressive strength in the fiber axial direction, it is desirable that a single fiber is rigid, and for this purpose, it is necessary to increase the diameter of the single fiber to increase a rectilinear property.

However, when the diameter of the carbon fiber single-fiber is increased, the strand strength and the strand elastic modulus are decreased.

In view of the above background, attempts have been made to obtain a carbon fiber bundle having a large diameter of a single fiber, high strand strength and strand elastic modulus, and an excellent resin impregnation property.

Patent Literature 1 describes a carbon fiber that achieves both an excellent strand elastic modulus and moldability into a composite material by twisting a fiber bundle in a stabilization step in a process of producing a carbon fiber bundle, and that easily maintains a fiber length even in a case of being used as a discontinuous fiber.

Patent Literature 2 describes a carbon fiber bundle that achieves both a strand elastic modulus and compressive strength of a carbon fiber composite material at high levels by performing carbonization treatment at a high stretching tension in a carbonization step.

Patent Literature 3 describes a technique for increasing strand strength and adhesive strength of carbon fibers by subjecting the carbon fibers to electrolytic surface treatment in an electrolyte solution having a nitrate ion concentration within a specific range.

Patent Literature 4 describes a technique for obtaining a carbon fiber having high strand strength by stabilizing a polyacrylonitrile fiber in a liquid phase.

CITATION LIST

Patent Literature

• • Patent Literature 1: WO 2019/244830
  • Patent Literature 2: WO 2019/203088
  • Patent Literature 3: JP 2002-327339 A
  • Patent Literature 4: JP 2004-300600 A

SUMMARY OF INVENTION

Technical Problem

According to the detailed study of the present inventors, it has been found that Patent Literatures 1 to 4 have the following problems.

The carbon fiber bundle described in Patent Literature 1 is produced by adding a twist, and not only the productivity is lowered due to the increase in the number of steps, but also excessive bundling occurs in the fiber bundle by adding a twist to cause carbon fibers to remain in a helical state even when the carbon fiber bundle is untwisted after being fired. Thus, when the carbon fibers are used as they are in the form of long fibers, the carbon fibers are liable to buckle in a case where the impregnation property of the resin becomes insufficient or in a case where the carbon fibers are compressed in the fiber axial direction when formed into a composite material with a matrix resin, and the compressive strength in the fiber axial direction becomes low.

Hereinafter, the term “compression” means “compression in the fiber axial direction”.

As the carbon fiber bundle described in Patent Literature 2, carbon fibers having a relatively large single fiber diameter are described. Among them, for carbon fibers having a smaller single fiber diameter, a carbon fiber precursor fiber bundle is subjected to strong entangling treatment to be carbonized at a high stretching tension. The bundle strength of the fiber bundle is improved by the strong entangling treatment, and thus, generation of fluff is small even when the stretching tension in the carbonization treatment is increased. However, the obtained carbon fiber bundle is strongly entangled, and thus, the resin impregnation property is insufficient.

On the other hand, for carbon fibers having a larger single fiber diameter, a fiber bundle can be subjected to carbonization treatment at a high stretching tension by twisting the fiber bundle, and carbon fibers having high strand strength and a strand elastic modulus are obtained.

However, the twisted carbon fiber bundle has a poor impregnation property with a matrix resin, does not improve the mechanical properties of the carbon fiber reinforced composite material, remains in a helical state even when untwisted, is likely to cause buckling of the carbon fibers when the carbon fiber reinforced composite material is compressed, and thus has low compressive strength.

As the carbon fibers described in Patent Literature 3, carbon fibers having a relatively large single fiber diameter are described, and have high strand strength but a low strand elastic modulus. In addition, the strand strength and the strand elastic modulus are measured by the old JIS method (JIS R 7601), and thus are higher than those measured by the current JIS method (JIS R-7608:2007).

As the carbon fibers described in Patent Literature 4, only carbon fibers having a low strand elastic modulus even with high strand strength are obtained. In addition, the stabilization is performed in the liquid phase, and thus, the oxygen concentration is lower than that in air, and the stabilization unevenness is likely to occur, which makes the variation in tensile strength and tensile elastic modulus between the carbon fiber single-fibers likely to increase.

Moreover, a carbon fiber bundle having higher strand strength and strand elastic modulus than ever is required in the market, but in general, when the strand strength is increased, the strand elastic modulus tends to decrease.

When a single fiber diameter is increased, it becomes difficult to advance the reaction to the inside of the single fiber in the step of producing a carbon fiber, and it is difficult to stably obtain a carbon fiber. In particular, a carbon fiber having a high strand elastic modulus development property is sometimes produced at a high temperature, and the influence thereof becomes remarkable. As described above, it has been extremely difficult to produce a carbon fiber bundle having improved strand strength and strand elastic modulus while increasing the single fiber diameter.

In addition, it is generally said that the larger the diameter of a single fiber constituting a carbon fiber bundle, the better the impregnation property of a resin, but a carbon fiber bundle having a large single fiber diameter and sufficiently satisfying the strand strength and the strand elastic modulus has not been known.

The present invention has been made to solve the above problems, and is directed to providing a carbon fiber bundle in which the average diameter of carbon fiber single-fibers is increased without decreasing the strand strength and the strand elastic modulus of carbon fibers, and a method for producing the same.

The present invention is also directed to providing a carbon fiber suitable for a carbon fiber reinforced composite material having excellent compressive strength in the fiber axial direction by such a carbon fiber bundle.

Solution to Problem

The present invention includes the following aspects.

[1]A carbon fiber bundle having a strand strength of 4.5 GPa or more and a strand elastic modulus of 320 GPa or more,

• • having substantially no twist, and
  • having 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 carbon fiber single-fibers each have a crystallite size Lc of 3.4 nm or more and 4.1 nm or less.

[3] The carbon fiber bundle according to [1] or [2], wherein the carbon fiber single-fibers have an average void length of 22.0 nm or less.

[4] The carbon fiber bundle according to any one of [1] to [3], wherein the carbon fiber single-fibers have a fracture surface formation energy of 18 N/m or more.

[5] The carbon fiber bundle according to any one of [1] to [4], wherein the carbon fiber single-fibers in the carbon fiber bundle have a coefficient of variation (CV %) in elastic modulus in a single-fiber tensile test of 17.5% or less.

[6] The carbon fiber bundle according to any one of [1] to [5], wherein the carbon fiber bundle is non-entangled.

[7] The carbon fiber bundle according to any one of [1] to [6], wherein a 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], wherein the strand strength is 4.85 GPa or more, and the strand elastic modulus is 365 GPa or more.

[9] The carbon fiber bundle according to any one 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 elastic 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], wherein the carbon fiber bundle has a knot strength of 80 N/mm² or more.

[12] The carbon fiber bundle according to any one of [1] to [11], wherein the carbon fiber single-fibers have a density of 1.79 g/cm³ or more.

[13] A method for producing a carbon fiber bundle, the method including heating a carbon fiber precursor acrylic fiber bundle in an oxidizing atmosphere to form a stabilized fiber bundle, and heating the stabilized fiber bundle in a non-oxidizing atmosphere to form a carbon fiber bundle, wherein

• • in the heating in the non-oxidizing atmosphere, a temperature rising rate when a temperature is raised from 1800° C. to 2200° C. is from 200 to 500° C./min, and
  • an average diameter of carbon fiber single-fibers contained in the produced carbon fiber bundle is 6.5 μm or more and 8.5 μm or less.

[14] A method for producing a carbon fiber bundle according to [13], the method including the following steps (1) and (2):

• • (1) a coagulation step of discharging an acrylonitrile-based polymer solution from a discharge hole into air by using a dry-wet spinning method, and then coagulating the acrylonitrile-based polymer solution in a coagulation bath including an aqueous solution having a temperature of 10° C. or lower and an organic solvent concentration of 80.0 mass % or more and 81.0 mass % or less to produce a coagulated yarn bundle containing the organic solvent; and
  • (2) a second stretching step of stretching the coagulated yarn bundle produced in the coagulation step at a stretching ratio of 2.0 times or more and 3.2 times or less in a hot aqueous solution having a temperature of 75° C. or higher and an organic solvent concentration of 40 mass % or more and 65 mass % or less to produce the carbon fiber precursor acrylic fiber bundle.

[15] The method for producing a carbon fiber bundle according to [13] or [14], the method further including a first stretching step of stretching the coagulated yarn bundle produced in the coagulation step in air at a stretching ratio of 1.00 times or more and 1.20 times or less between the coagulation step and the second stretching step, wherein the coagulated yarn bundle produced in the first stretching step is stretched in the second stretching step.

[16] The method for producing a carbon fiber bundle according to any one of [13] to [15], wherein in the second stretching step, the coagulated yarn bundle is stretched, then the organic solvent is removed, the coagulated yarn bundle is shrunk or stretched at a ratio of 0.96 times or more and 1.30 times or less in hot water at a temperature of 90° C. or higher, and is stretched at a stretching ratio of 3.7 times or more and 4.2 times or less in a pressurized water vapor atmosphere to produce 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], the method including the following steps (3) to (6):

• • (3) a stabilization step of heating a carbon fiber precursor acrylic fiber bundle or the carbon fiber precursor acrylic fiber bundle produced in the second stretching step in an oxidizing atmosphere having a temperature gradient within an ambient temperature range of 200° C. or higher and 260° C. or lower at an elongation percentage of 3.0% or more and 8.0% or less to obtain a stabilized 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 stabilized fiber bundle produced in the stabilization step in a non-oxidizing atmosphere having a temperature gradient within an ambient temperature range of 300° C. or higher and 900° C. or lower at an elongation percentage 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 within an ambient temperature range of 1000° C. or higher and 1800° C. or lower after the first carbonization step; and
  • (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 within an ambient temperature range of 1700° C. or higher and 2300° C. or lower after the second carbonization step.

[20] The method for producing a carbon fiber bundle according to [19], wherein in the third carbonization step, a temperature rising rate when the ambient temperature is increased from 1800° C. to 2200° C. is 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 temperature rising rate when the ambient temperature is increased from 1800° C. to 2200° C. is 215° C./min or more and 300° C./min or less.

[22] The method for producing a carbon fiber bundle according to any one of [19] to [21], wherein a difference between a maximum ambient temperature in the second carbonization step and an inlet ambient temperature in the third carbonization step is 500° C. or lower.

[23] The method for producing a carbon fiber bundle according to any one of [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 lower.

[24] The method for producing a carbon fiber bundle according to any one of [13] to [23], wherein a maximum temperature of a heating temperature in the heating in the non-oxidizing atmosphere is from 2100 to 2300° C.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a carbon fiber bundle in which the average diameter of carbon fiber single-fibers is increased without decreasing the strand strength and the strand elastic modulus of carbon fibers, and a method for producing the same.

According to the present invention, it is possible to obtain a carbon fiber suitable for a carbon fiber reinforced composite material having excellent compressive strength in the fiber axial direction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating a method for measuring an ultrasonic elastic modulus of a carbon fiber bundle.

DESCRIPTION OF EMBODIMENTS

Carbon Fiber Bundle

A carbon fiber bundle of the present invention has a strand strength of 4.5 GPa or more, a strand elastic modulus of 320 GPa or more, substantially no twist, and an average diameter of carbon fiber single-fibers of 6.5 μm or more and 8.5 μm or less.

Note that the “carbon fiber bundle” means a bundle of a plurality of single fibers of carbon fibers.

When the strand strength of the carbon fiber bundle is 4.5 GPa or more and the strand elastic modulus is 320 GPa or more, the carbon fiber bundle has a good balance between the strand strength and the strand elastic modulus, and a carbon fiber reinforced composite material having excellent mechanical properties is easily obtained.

Measurement conditions of the strand strength and the strand elastic modulus are as described in Examples described below.

The strand strength is a strand strength by a tensile test, and the strand elastic modulus is a strand elastic modulus by the tensile test.

The carbon fiber bundle of the present invention has substantially no twist.

In the present invention, the wording “having substantially no twist” means that no twist is present in the fiber bundle, or a twist is present locally, but S twists and Z twists are present equally. In a case where the fiber bundle has S twists and Z twists equally, the net twist number is preferably 0.5 turns/m or less in the entire carbonization step.

When the carbon fiber bundle has substantially no twist, openability of the carbon fiber bundle becomes favorable, and performance of the obtained carbon fiber reinforced composite material can be further enhanced.

A carbon fiber obtained by firing in a state where the carbon fiber is twisted to untwist the twisted carbon fiber can be included. However, in a case of being untwisted, the carbon fiber is in a helical state, and thus, when a composite material with a matrix resin is formed, buckling is likely to occur in a case of being compressed, and compressive strength in a fiber axial direction is likely to decrease. From this viewpoint, the carbon fiber preferably has a rectilinear property.

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

Method for Measuring Compressive Strength of Carbon Fiber Reinforced Composite Material in Fiber Axial Direction

Production of Prepreg

A carbon fiber bundle unwound from a bobbin is placed on a release paper coated with an epoxy resin #350 (available from Mitsubishi Chemical Corporation) and impregnated with the epoxy resin. A protective film is layered thereon to prepare a unidirectionally oriented prepreg (hereinafter, referred to as prepreg) having a resin content of about 33 mass % and a carbon fiber density of 125 g/m².

Production of Unidirectionally Layered Material

Two plies of prepreg are layered and bagged, and the inside of the bag is depressurized by a vacuum pump. Thereafter, the resultant is placed in an autoclave, the temperature in the autoclave is raised at a temperature rising rate of 2° C./min, the inside of the autoclave is held at 80° C. for 1 hour, then the temperature is raised at a temperature rising rate of 2° C./min, and the inside of the autoclave is held at 130° C. for 1.5 hours to perform curing, thereby obtaining a carbon fiber reinforced composite material. At this time, the pressure in the autoclave is increased to 0.6 MPa after being kept at 80° C. for 1 hour. Suction by the vacuum pump is stopped when the pressure in the autoclave reaches 0.14 MPa, and the inside of the bag is opened to the atmosphere.

Evaluation of Compressive Physical Properties in Fiber Direction of Carbon Fiber Resin Composite Material

Six test pieces each having a width of 12.7 mm, a length of 80 mm, and a thickness of 1 mm are prepared from the resulting unidirectionally layered material. The length direction of each of the test pieces is a 0° direction of fibers. The compressive strength and compressive elastic modulus of each of the obtained test pieces are measured in accordance with SACMA SRM 1R using an INSTRON 5882 measuring machine equipped with a 100 kN load cell under a condition of a cross head speed of 1.27 mm/min in an environment of a temperature of 23° C. and a relative humidity of 50%, and the measured values are converted to Vf (fiber volume content) of 56%. The six test pieces are measured in the same manner, and the average value is determined. Note that the measurement is performed by bonding a tab cut out 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.

When the fiber diameter is 6.5 μm or more, gaps between fibers can be increased, a resin can be easily impregnated uniformly, and generation of voids in the carbon fiber reinforced composite material obtained using the carbon fiber bundle of the present invention can be suppressed. When the fiber diameter is 8.5 μm or less, a cross-sectional double structure is less likely to be noticeable in a stabilization step (step (3)) described below, and a carbon fiber bundle having high strand strength can be obtained without decreasing a strand elastic modulus.

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.

Measurement conditions of the average diameter of the carbon fiber single-fibers are as described in Examples described 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 still more preferably 5.0 GPa or more. The strand strength of the carbon fiber bundle is preferably larger, and the upper limit thereof 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 larger, and the upper limit thereof 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 carbon fiber single-fibers preferably have a crystallite size Lc of 3.4 nm or more and 4.1 nm or less.

When the crystallite size Lc of the carbon fiber bundle is 3.4 nm or more, the strand elastic modulus of the carbon fiber bundle is easily maintained at a higher level. When the crystallite size Lc is 4.1 nm or less, it is easy to suppress formation of defects and a decrease in strand strength of the carbon fiber bundle due to the excessively large crystallite size. From this viewpoint, the crystallite size Lc is more preferably 3.6 nm or more and 4.1 nm or less, and still more preferably 3.6 nm or more and 3.8 nm or less. A crystallite size Lc can be controlled by adjusting a heating temperature or a temperature rising rate when a stabilized fiber bundle is heated and carbonized.

Measurement conditions of the crystallite size Lc are as described in Examples described below.

In the carbon fiber bundle of the present invention, an average void length of the carbon fiber single-fibers is preferably 22.0 nm or less.

When the average void length of the carbon fiber bundle is 22.0 nm or less, the strand strength of the carbon fiber bundle is easily maintained at a higher level. From this viewpoint, the average void length is more preferably 21.0 nm or less, and still more preferably 19.5 nm or less.

When the lower limit of the average void length is 5.0 nm or more, bendability of the fiber is easily secured, and the lower limit is more preferably 10 nm or more.

The above upper and lower limits can be combined in any manner. For example, the average void length may be 5.0 nm or more and 22.0 nm or less, may be 5.0 nm or more and 21.0 nm or less, or may be 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 rising rate when the stabilized fiber bundle is heated and carbonized.

Measurement conditions of the average void length are as described in Examples described below.

In the carbon fiber bundle of the present invention, a fracture surface formation energy of the carbon fiber single-fibers is preferably 18 N/m or more.

The fracture surface formation energy is determined by forming a hemispherical defect having a size within a predetermined range on a surface of a single fiber by a laser, breaking the fiber at the hemispherical defect site by a tensile test, and calculating the fracture surface formation energy from the breaking strength of the fiber and a depth of the hemispherical defect by means of the following Griffith equation (F1).

Fracture ⁢ surface ⁢ formation ⁢ energy = σ 2 ⁢ π ⁢ C / 2 ⁢ E ( F ⁢ 1 )

Here, σ is the breaking strength, E is an ultrasonic elastic modulus of the carbon fiber bundle, and C is the depth of the hemispherical defect.

A fracture surface formation energy is an index of difficulty of fracture of the carbon fiber and represents a matrix strength. A carbon fiber is a material that exhibits brittle fracture, and the tensile strength thereof is dominated by a defective point. In a case where carbon fibers have the same defective point, a higher matrix strength thereof leads to a higher fracture strength. Thus, when the fracture surface formation energy of the carbon fiber bundle is set to 18 N/m or more, the strength of the carbon fiber bundle is likely to be further increased without decreasing the strand elastic modulus of the carbon fiber bundle, and the performance of the resulting carbon fiber reinforced composite material is likely to be further increased.

From these viewpoints, the fracture surface formation energy of the carbon fiber bundle is more preferably 19 N/m or more, and still more preferably 21.5 N/m or more.

On the other hand, when the fracture surface formation energy of the carbon fiber bundle is increased, the strength of surfaces of single fibers constituting the carbon fiber bundle is increased, a graphite crystal size is increased, and as a result, the compressive strength in the fiber axial direction tends to be decreased. 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 above upper and lower limits can be combined in any manner. For example, the fracture surface formation energy may be 18 N/m or more and 35 N/m or less, may be 19 N/m or more and 35 N/m or less, or may be 21.5 N/m or more and 30 N/m or less.

Detailed measurement conditions of the fracture surface formation energy are as described in Examples described below.

In the carbon fiber bundle of the present invention, the coefficient of variation in elastic modulus in a single-fiber tensile test of the carbon fiber single-fiber in the carbon fiber bundle (CV %: hereinafter, also simply referred to as “coefficient of variation in elastic modulus”) is preferably 17.5% or less.

When the coefficient of variation in elastic modulus is 17.5% or less, the difference in defects among single fibers is reduced, and the strand strength of the carbon fiber bundle is easily maintained at a high level. From this viewpoint, the coefficient of variation in elastic modulus is more preferably 15.0% or less, still more preferably 13.0% or less, and particularly preferably 11.0% or less.

The coefficient of variation in single-fiber elastic modulus can be controlled by adjusting the heating temperature and the heating time when the carbon fiber precursor acrylic fiber bundle is heated to obtain the stabilized fiber bundle.

Measurement conditions of the coefficient of variation in single-fiber elastic modulus in the single-fiber tensile test are as described in Examples described below.

The carbon fiber bundle of the present invention is preferably non-entangled.

The term “entanglement” as used herein means intentional entanglement using an entangling apparatus. Carbon fibers in which single fibers are naturally entangled with each other in the production process of the carbon fibers are regarded as non-entangled.

When the carbon fiber bundle is non-entangled, the openability of the carbon fiber bundle is improved, the impregnation property of the matrix resin is easily improved, and a carbon fiber reinforced composite material having excellent mechanical properties is easily obtained.

In this case, a hook drop value of the carbon fiber bundle is 500 mm or more, and is preferably 1000 mm or more.

The hook drop value is measured by the following method.

First, the carbon fiber bundle of 2000 mm is arranged in the vertical direction, and the upper end is fixed. Then, a hook having a total weight of 30 g with a weight is inserted into a carbon fiber bundle. The hook used here is a hook prepared by molding a metal wire having a diameter of 1 mm, and has a hook portion with a radius of 5 mm. Then, the hook is allowed to fall freely while remaining inserted into the carbon fiber bundle. As described above, the carbon fiber bundle is a yarn in which a large number of carbon fiber monofilaments are aligned in substantially 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 positions. The hook is often stopped at such a portion. Thus, a distance from the position where the hook is inserted into the carbon fiber bundle to the position where the hook is stopped can be measured. The hook drop value is the falling distance of the hook from the inserted position to the stopped position.

In the carbon fiber bundle of the present invention, a 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 preferably 31 or more.

When the product of the average diameter of the carbon fiber single-fibers and the strand strength is 31 or more, high strand strength is easily obtained for the fiber diameter in the range of 6.5 μm or more and 8.5 μm or less of the average diameter of the carbon fiber single-fibers.

From this viewpoint, the product of the average diameter of the single fibers and the strand strength is preferably 33 or more and more preferably 35 or more. The product of the average diameter of the single fibers and the strand strength is preferably 50 or less, and more preferably 45 or less.

The above upper and lower limits can be combined in any manner. For example, the product of the average diameter of the single fibers and the strand strength 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. In addition, from the viewpoint of the performance of the resulting carbon fiber reinforced composite material, the strand elastic modulus is preferably 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 elastic modulus of 365 GPa or more.

In the carbon fiber bundle of the present invention, it is preferable 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 elastic modulus of the carbon fiber bundle is 365 GPa or more and 403 GPa or less.

When these physical properties are satisfied, the performance of the resulting carbon fiber reinforced composite material is likely to be further enhanced.

In particular, when 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 elastic modulus of the carbon fiber bundle is 365 GPa or more, the performance of the resulting carbon fiber reinforced composite material is likely to be further enhanced. In addition, when 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 elastic modulus of the carbon fiber bundle is 403 GPa or less, the graphite crystal size of the carbon fiber bundle is easily prevented from being excessively large, and the decrease in compressive strength in the fiber axial direction is easily prevented, and thus the performance of the resulting carbon fiber reinforced composite material is likely to be further enhanced.

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 reflecting the mechanical performance of the fiber bundle in a direction other than the fiber axial direction, and in particular, the performance in a direction perpendicular to the fiber axis can be simply evaluated. In the carbon fiber reinforced composite material, the material is often formed by quasi-isotropic layering, and a complicated stress field is formed. At this time, in addition to tensile and compressive stresses in the fiber axial direction, stresses in directions other than the fiber axial direction are also generated. Furthermore, in a case where a relatively high speed strain is applied as in an impact test, the state of stress generated inside the material is considerably complicated, and the strength in a direction different from the fiber axial direction is important. Accordingly, when the knot strength of the carbon fiber bundle is set to 80 N/mm² or more, the performance of the resulting carbon fiber reinforced composite material is likely to be further enhanced. From these viewpoints, the knot strength is more preferably 90 N/mmz 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 axial direction is also increased, and the graphite crystal size is decreased, and thus, the strand elastic modulus tends to be decreased. To obtain a carbon fiber bundle having a good balance between the strand elastic modulus and the knot strength, the knot strength is preferably 600 N/mm² or less, more preferably 400 N/mm² or less, and still more preferably 200 N/mm² or less.

The above upper and lower limits can be combined in any manner. For example, the knot strength may be 80 N/mm² or more and 600 N/mm² or less, 80 N/mm² or more and 400 N/mm² or less, or 90 N/mm² or more and 200 N/mm² or less.

Measurement conditions of the knot strength are as described in Examples described below.

The carbon fiber bundle of the present invention preferably has a density of 1.79 g/cm³ or more. When the density of the carbon fiber bundle is 1.79 g/cm³ or more, the strand strength and the strand elastic modulus are likely to be further enhanced. From this viewpoint, the density of the carbon fiber bundle is more preferably 1.81 g/cm³ or more, and still more 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 still more preferably 1.86 g/cm³ or less. When 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 suppress a decrease in compressive strength in the fiber axial direction, and thus it is easy to further enhance the performance of the resulting carbon fiber reinforced composite material.

The above upper and lower limits can be combined in any manner. For example, the density may be 1.79 g/cm³ or more and 1.90 g/cm³ or less, 1.81 g/cm³ or more and 1.88 g/cm³ or less, or 1.83 g/cm³ or more and 1.86 g/cm³ or less.

Measurement conditions of the density are as described in Examples described below.

In one embodiment, the number of filaments of the carbon fiber bundle, that is, the number of carbon fiber single-fibers constituting the carbon fiber bundle is preferably from 8000 to 20000, more preferably from 10000 to 18000, and still more preferably from 12000 to 18000.

When the number of filaments of the carbon fiber bundle is the lower limit value or more within the above numerical range, the productivity at the time of producing the carbon fiber reinforced composite material is likely to be improved. When the number of filaments of the carbon fiber bundle is the upper limit value or less within the above numerical range, a carbon fiber reinforced composite material having good openability and good mechanical properties is likely to be obtained.

Method for Producing Carbon Fiber Bundle

A method for producing a carbon fiber bundle of the present invention is a method for producing a carbon fiber bundle, the method including heating a carbon fiber precursor acrylic fiber bundle in an oxidizing atmosphere to obtain a stabilized fiber bundle, and heating the obtained stabilized fiber bundle in a non-oxidizing atmosphere to obtain a carbon fiber bundle, in which at a temperature for heating in the non-oxidizing atmosphere, a temperature rising rate when the ambient temperature is raised from 1800° C. to 2200° C. is from 200 to 500° C./min, and an average diameter of carbon fiber single-fibers is 6.5 μm or more and 8.5 μm or less.

That is, the method for producing a carbon fiber bundle of the present invention is a method for producing a carbon fiber bundle, the method including heating in an oxidizing atmosphere for forming a carbon fiber precursor acrylic fiber bundle into a stabilized fiber bundle, and heating in a non-oxidizing atmosphere for forming the stabilized fiber bundle into a carbon fiber bundle, in which in the heating in the non-oxidizing atmosphere, the temperature rising rate when the temperature is raised from 1800° C. to 2200° C. is from 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.

Single-fiber fineness of the carbon fiber precursor acrylic fiber bundle is preferably in a range of 1.1 to 2.0 dtex. The single-fiber fineness can be controlled by an amount of an acrylonitrile-based polymer solution discharged from a discharge hole of a spinning nozzle and a stretching ratio.

When the temperature rising rate when the ambient temperature is raised from 1800° C. to 2200° C. is set to 200° C./min or more, it is possible to produce a carbon fiber bundle with high productivity. When the temperature rising rate when the ambient temperature is raised from 1800° C. to 2200° C. is set to 500° C./min or less, a violent decomposition reaction accompanying a rapid temperature rise is easily suppressed, and a carbon fiber bundle having a high density, high strand strength, and high knot strength is easily obtained without lowering the strand elastic modulus of the carbon fiber bundle in which the average diameter of the carbon fiber single-fibers is 6.5 μm or more and 8.5 μm or less.

From these viewpoints, the lower limit of the temperature rising rate when the ambient 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 or more. The upper limit of the temperature rising rate is preferably 480° C./min or less, more preferably 400° C./min or less, and most preferably 340° C./min or less.

The above upper and lower limits can be combined in any manner. For example, the temperature rising rate may be from 210 to 480° C./min, may be from 215 to 400° C./min, or may be from 220 to 340° C./min.

The temperature rising rate when the ambient temperature is raised from 1800° C. to 2200° C. is a value obtained by dividing 400° C., which is a difference between 2200° C. and 1800° C., by a traveling time of the fiber bundle at the ambient temperature of 1800° C. to 2200° C.

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

(1) A coagulation step of discharging an acrylonitrile-based polymer solution from a discharge hole into air by using a dry-wet spinning method, and then coagulating the acrylonitrile-based polymer solution in a coagulation bath including an aqueous solution having a temperature of 10° C. or lower and an organic solvent concentration of 80.0 mass % or more and 81.0 mass % or less to obtain a coagulated yarn bundle containing the organic solvent.

(2) A second stretching step of stretching the coagulated yarn bundle obtained in the coagulation step of (1) at a stretching ratio of 2.0 times or more and 3.2 times or less in a hot aqueous solution having a temperature of 75° C. or higher and an organic solvent concentration of 40 mass % or more and 65 mass % or less to obtain the carbon fiber precursor acrylic fiber bundle.

(3) A stabilization step of heating the carbon fiber precursor acrylic fiber bundle or the carbon fiber precursor acrylic fiber bundle obtained in the second stretching step of (2) in an oxidizing atmosphere having a temperature gradient within an ambient temperature range of 200° C. or higher and 260° C. or lower at an elongation percentage of 3.0% or more and 8.0% or less to obtain a stabilized 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 stabilized fiber bundle obtained in the stabilization step of (3) in a non-oxidizing atmosphere having a temperature gradient within an ambient temperature range of 300° C. or higher and 900° C. or lower at an elongation percentage 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 within an ambient temperature range of 1000° C. or higher and 1800° C. or lower after the first carbonization step of (4).

(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 within an ambient temperature range of 1700° C. or higher and 2300° C. or lower after the second carbonization step of (5).

Coagulation Step

The coagulation step of (1) is a step of discharging an acrylonitrile-based polymer solution from a discharge hole into air by using a dry-wet spinning method, and then coagulating the acrylonitrile-based polymer solution in a coagulation bath including an aqueous solution (A) having a temperature of 10° C. or lower and an organic solvent concentration of 80.0 mass % or more and 81.0 mass % or less to obtain a coagulated yarn bundle containing the organic solvent.

The temperature of the coagulation bath, that is, of the aqueous solution (A), is 10° C. or lower. When the temperature of the aqueous solution (A) is set to 10° C. or lower, dense coagulated fibers are likely to be formed, and in particular, denseness of the fiber surface can be enhanced, which makes a carbon fiber bundle having high strand strength and knot strength likely to be obtained without lowering the strand elastic modulus.

The temperature of the aqueous solution (A) is preferably 4° C. or higher and more preferably 6° C. or higher. When the temperature of the aqueous solution (A) is set to 4° C. or higher, excessive densification of the coagulated fibers can be suppressed, and the stretchability in the subsequent step is likely to be secured. For example, the temperature of the aqueous solution (A) may be from 4 to 10° C., or may be from 6 to 10° C.

The concentration of the organic solvent in the coagulation bath, that is, the aqueous solution (A) is 80.0 mass % or more and 81.0 mass % or less, and preferably 80.2 mass % or more and 80.6 mass % or less, with respect to the total mass of the aqueous solution (A). When the concentration of the organic solvent is set to 80.0 mass % or more and 81.0 mass % or less, a coagulated yarn having a dense surface and inside can be obtained, and as a result, the strand strength and the knot strength of the obtained carbon fiber bundle are likely to be increased without decreasing the strand elastic modulus.

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

Second Stretching Step

The second stretching step of (2) is a step of stretching the coagulated yarn bundle obtained in the coagulation step of (1) at a stretching ratio of 2.0 times or more and 3.2 times or less in a hot aqueous solution (B) having a temperature of 75° C. or higher and an organic solvent concentration of 40 mass % or more and 65 mass % 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. When the temperature of the hot aqueous solution (B) is set to 75° C. or higher, sufficient stretchability can be ensured, and thus, the fiber bundle is easily stretched stably.

The temperature of the hot aqueous solution (B) is preferably 98° C. or lower and more preferably 95° C. or lower. When the temperature of the hot aqueous solution (B) is set to 98° C. or lower, it is possible to suppress a rapid temperature change of the coagulated yarn bundle, and it is easy to uniformly stretch the coagulated yarn bundle.

The above upper and lower limits can 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 40 mass % or more and 65 mass % or less, and preferably 50 mass % or more and 60 mass % or less, with respect to the total mass of the hot aqueous solution (B). When the concentration of the organic solvent in the hot aqueous solution (B) is set to 40 mass % or more and 65 mass % or less, a structure having a dense surface and inside can be formed, and a carbon fiber bundle having high strand strength and knot strength is likely to be obtained without decreasing the strand elastic modulus.

Examples of the organic solvent contained in the hot aqueous solution (B) include dimethylformamide, dimethylacetamide, and dimethyl sulfoxide. Among these, dimethylformamide is preferable 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. When the stretching ratio in the hot aqueous solution (B) is set to 2.0 times or more, it is possible to produce a carbon fiber precursor acrylic fiber bundle having sufficient molecular orientation, and a carbon fiber bundle having high strand strength and knot strength is likely to be obtained without decreasing the strand elastic modulus. When the stretching ratio in the hot aqueous solution (B) is set to 3.2 times or less, excessive stretching can be suppressed, and stable stretching is likely to be performed.

In the second stretching step of (2), the carbon fiber precursor acrylic fiber may be obtained by stretching the coagulated yarn bundle in the hot aqueous solution (B), and then appropriately combining, for example, a step of removing the organic solvent, a step of stretching with hot water, a step of stretching in a pressurized water vapor atmosphere, a step of stretching with dry heat, a step of applying an oil agent, and a step of drying.

Specifically, in the second stretching step of (2), the coagulated yarn bundle may be stretched, then the organic solvent may be removed, and the coagulated yarn bundle may be shrunk or stretched at a ratio of 0.96 times or more and 1.30 times or less in hot water (C) having a temperature of 90° C. or higher, and stretched at a stretching ratio of 3.7 times or more and 4.2 times or less in a pressurized water vapor atmosphere to obtain a carbon fiber precursor acrylic fiber bundle.

That is, the second stretching step of (2) preferably includes a step (2-1) of stretching the coagulated yarn bundle in the hot aqueous solution (B) at a stretching ratio of 2.0 times or more and 3.2 times or less, a step (2-2) of removing the organic solvent, a step (2-3) of shrinking or stretching the coagulated yarn bundle in the hot water (C) having a temperature of 90° C. or higher at a ratio of 0.96 times or more and 1.30 times or less, and a step (2-5) of stretching the coagulated yarn bundle in a pressurized water vapor atmosphere at a stretching ratio of 3.7 times or more and 4.2 times or less, in this order. The second stretching step of (2) may further include a step (2-4) of applying an oil agent composition. The step (2-4) can be performed between the step (2-3) and the step (2-5).

The step (2-2) is a step of removing the organic solvent from the coagulated yarn bundle after being stretched in the hot aqueous solution (B) (hereinafter, also referred to as “stretched fiber bundle”). The method for removing the organic solvent may be any method as long as the solvent can be removed. For example, the stretched fiber bundle can be washed and stretched in a multistage washing tank set to a temperature in a range of 50° C. or higher and lower than 100° C.

The step (2-3) is a step of shrinking or stretching the stretched fiber bundle after removing the organic solvent in the hot water (C) having a temperature of 90° C. or higher at a ratio of 0.96 times or more and 1.30 times or less. The step (2-3) can relax strain of stretching.

The temperature of the hot water (C) is 90° C. or higher. When the temperature of the hot water (C) is set to 90° C. or higher, it is possible to uniformly relax the strain of stretching, and it is possible to obtain a carbon fiber bundle having higher strand strength and knot strength without decreasing the strand elastic modulus. The temperature of the hot water (C) is preferably 97° C. or lower. When the temperature of the hot water (C) is set to 97° C. or lower, it is possible to suppress a rapid temperature change of the stretched fiber bundle, to uniformly relax the strain of stretching, and to obtain a carbon fiber bundle having higher strand strength and knot strength without decreasing the strand elastic modulus.

The shrinkage or stretching ratio in the hot water (C) is 0.96 times or more and 1.30 times or less. When the shrinkage or stretching ratio is set to 0.96 times or more, it is possible to prevent poor take-up due to loosening of the fiber bundle, and it is possible to stably relax the strain of stretching. When the shrinkage or stretching ratio is set to 1.30 times or less, an excessive load can be suppressed, and the strain of stretching can be stably relaxed.

In the step (2-3), the stretched fiber bundle after removing the organic solvent is preferably shrunk (relaxed) in the hot water (C) at a shrinkage ratio (relaxation ratio) of 0.96 times or more and less than 1.00 times or stretched at a stretching ratio of 1.00 times or more and 1.30 times or less, more preferably shrunk (relaxed) at a shrinkage ratio (relaxation ratio) of 0.96 times or more and 0.99 times or less or stretched at a stretching ratio of 1.05 times or more and 1.30 times or less, and still more preferably shrunk (relaxed) at a shrinkage ratio (relaxation ratio) of 0.96 times or more and 0.99 times or less.

The step (2-4) is a step of applying an oil agent composition to the stretched fiber bundle after being shrunk or stretched in the hot water (C).

The oil agent composition can be determined in consideration of a function required for the carbon fiber precursor acrylic fiber bundle. Examples thereof include a silicone-based oil agent composition. The oil agent composition may further contain an additive such as an antioxidant, an antistatic agent, a defoaming agent, a preservative, an antibacterial agent, or a penetrant, as necessary.

As a method for applying the oil agent composition to the stretched fiber bundle, a known method such as a roller method, a guide method, a spray method, or a dip method can be used.

After the oil agent composition is applied to the stretched fiber bundle, the oil agent composition may be dried by a known method in the related art, as necessary.

The step (2-5) is a step of stretching the stretched fiber bundle, which has been shrunk or stretched in the hot water (C), preferably applied with the oil agent composition, and dried as necessary, at a stretching ratio of 3.7 times or more and 4.2 times or less 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. When the stretching ratio in a pressurized water vapor atmosphere is set to 3.7 times or more, the molecular orientation of the obtained carbon fiber precursor acrylic fiber bundle is improved, and a carbon fiber bundle having higher strand strength and knot strength can be obtained without decreasing the strand elastic modulus. When the stretching ratio in the pressurized water vapor atmosphere is set to 4.2 times or less, excessive stretching can be suppressed, and stable stretching can be performed.

Stabilization Step

The stabilization step of (3) is a step of heating a carbon fiber precursor acrylic fiber bundle or the carbon fiber precursor acrylic fiber bundle obtained in the second stretching step of (2) in an oxidizing atmosphere having a temperature gradient within an ambient temperature range of 200° C. or higher and 260° C. or lower at an elongation percentage of 3.0% or more and 8.0% or less to obtain a stabilized fiber bundle having a density of 1.33 g/cm³ or more and 1.36 g/cm³ or less.

In the stabilization step, the carbon fiber precursor acrylic fiber bundle is preferably heated in a stabilization furnace having a linear temperature gradient in an oxidizing atmosphere within an ambient temperature range of 200° C. or higher and 260° C. or lower.

In the stabilization step, a cyclization reaction by heat and an oxidation reaction by oxygen occur, and it is important to cause these two reactions in a balanced manner to obtain a carbon fiber bundle having high strand strength and knot strength without decreasing the strand elastic modulus.

The ambient temperature in the stabilization step is 200° C. or higher and 260° C. or lower. When the temperature of the atmosphere in which the carbon fiber precursor acrylic fiber bundle is caused to travel in the stabilization step is set to 200° C. or higher, a portion where the oxidation reaction has not sufficiently occurred can be reduced, and occurrence of large structural unevenness in the cross-sectional direction of a single fiber can be suppressed, and thus, a carbon fiber bundle having high strand strength and knot strength is likely to be obtained without decreasing the strand elastic modulus. When the temperature of the atmosphere in which the carbon fiber precursor acrylic fiber bundle is caused to travel in the stabilization step is set to 260° C. or lower, it is possible to suppress the presence of a larger amount of oxygen in a portion close to the surface of a single fiber, and as a result, it is possible to suppress disappearance of excess oxygen and the reaction of forming a defective point by the heat treatment in the first carbonization step and the subsequent steps described below, which makes it 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 stabilization step, the carbon fiber precursor acrylic fiber bundle is heated until the resulting stabilized fiber bundle has a density of 1.33 g/cm³ or more and 1.36 g/cm³ or less. When the density of the stabilized fiber bundle is set to 1.33 g/cm³ or more, it is possible to suppress the occurrence of a portion where the stabilization is insufficient, and as a result, it is possible to suppress the formation of a defective point due to the occurrence of a decomposition reaction by the heat treatment in the first carbonization step and subsequent steps described below, which makes it easy to obtain a carbon fiber bundle having a high density, high strand strength, and high knot strength without decreasing the strand elastic modulus. When the density of the stabilized fiber bundle is set to 1.36 g/cm³ or less, it is possible to suppress the presence of a large amount of oxygen in the stabilized fiber bundle, and as a result, it is possible to suppress the disappearance of excess oxygen and the reaction forming a defective point due to the heat treatment in the first carbonization step and subsequent steps described below, which makes it 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 stabilization step, the carbon fiber precursor acrylic fiber bundle is elongated at an elongation percentage of 3.0% or more and 8.0% or less to obtain a stabilized fiber bundle. The elongation percentage in the stabilization step is preferably 4.0% or more and 7.0% or less, and more preferably 5.0% or more and 6.5% or less. When the elongation percentage in the stabilization step is set to 3.0% or more, the molecular orientation of the stabilized fiber bundle can be improved, and thus, a carbon fiber bundle having high strand strength and knot strength is likely to be obtained without decreasing the strand elastic modulus. When the elongation percentage in the stabilization step is set to 8.0% or less, excessive elongation can be suppressed, and a stabilized fiber bundle is likely to be obtained stably.

Examples of the gas forming the oxidizing atmosphere include air, oxygen, and nitrogen dioxide, and air is preferable from the viewpoint of economic efficiency.

The treatment time in the stabilization furnace (time of stabilization treatment) may be, for example, 30 minutes or longer and 100 minutes or shorter.

First Carbonization Step

The first carbonization step of (4) is a step of heating the stabilized fiber bundle obtained in the stabilization step of (3) in a non-oxidizing atmosphere having a temperature gradient within an ambient temperature range of 300° C. or higher and 900° C. or lower at an elongation percentage of 4.0% or more and 5.0% or less.

In the first carbonization step, the stabilized fiber bundle is preferably heated in a first carbonization furnace having a linear temperature gradient in a non-oxidizing atmosphere within an ambient temperature range of 300° C. or higher and 900° C. or lower.

The ambient temperature in the first carbonization step is 300° C. or higher and 900° C. or lower. When the ambient temperature in the first carbonization step is set to 900° C. or lower, it is possible to prevent the stabilized fiber bundle from becoming very brittle, and not only the stabilized fiber bundle can be caused to stably pass through the first carbonization step (first carbonization furnace), but also the formation of a defective point is suppressed in the heat treatment in the second carbonization step and subsequent steps described below, which makes it easy to obtain a carbon fiber bundle having a high density, high strand strength, and high knot strength without decreasing the strand elastic modulus.

The elongation percentage in the first carbonization step is 4.0% or more and 5.0% or less. When the elongation percentage in the first carbonization step is set to 4.0% or more, the molecular orientation of the obtained carbon fiber bundle can be improved, and the strand strength and the knot strength are likely to be improved without decreasing the strand elastic modulus. When the elongation percentage in the first carbonization step is set to 5.0% or less, excessive elongation can be suppressed, and the stabilized fiber bundle is easily caused to stably pass through the first carbonization step (first carbonization furnace).

The treatment time in the first carbonization furnace (time of the first carbonization treatment) is preferably 1.0 minutes or longer and 3.0 minutes or shorter, and more preferably 1.2 minutes or longer and 2.5 minutes or shorter. When the treatment time in the first carbonization furnace is set to 1.0 minutes or longer, it is possible to suppress a violent decomposition reaction accompanying a rapid temperature rise, and a carbon fiber bundle having a high density, high strand strength, and high knot strength is likely to be obtained without decreasing the strand elastic modulus. When the treatment time in the first carbonization furnace is set to 3.0 minutes or shorter, a decrease in a degree of orientation of crystals in the carbon fiber bundle can be suppressed, and a carbon fiber bundle having high strand strength and knot strength is likely to be obtained without decreasing the strand elastic modulus.

Examples of the gas for forming the non-oxidizing atmosphere include nitrogen, argon, and helium, and nitrogen is preferable from the viewpoint of economic efficiency.

Second Carbonization Step

The second carbonization step of (5) is a step of heating the fiber bundle after the first carbonization step of (4) in a non-oxidizing atmosphere having a temperature gradient within an ambient temperature range of 1000° C. or higher and 1800° C. or lower while applying a tension of 0.15 cN/dtex or more and 0.21 cN/dtex or less to the fiber bundle. Note that the fiber bundle to be carbonized in the second carbonization step of (5) is the stabilized fiber bundle that has passed through the first carbonization step.

In the second carbonization step, the fiber bundle that has passed through the first carbonization furnace (first carbonization step) is preferably heated in a non-oxidizing atmosphere in a second carbonization furnace having a linear temperature gradient within an ambient temperature range of 1000° C. or higher and 1800° C. or lower.

The ambient temperature in the second carbonization step is 1000° C. or higher and 1800° C. or lower. When the ambient temperature in the second carbonization step is set to 1800° C. or lower, formation of a defective point is suppressed in the heat treatment in the third carbonization step described below, and a carbon fiber bundle having a high density, high strand strength, and high knot strength is likely to be obtained without decreasing the strand elastic modulus.

The fiber bundle passing through the second carbonization step (second carbonization furnace) is accompanied by large shrinkage, and thus, it is important to heat the fiber bundle under tension. In the second carbonization step, a tension of 0.15 cN/dtex or more and 0.21 cN/dtex or less, preferably a tension of 0.17 cN/dtex or more and 0.21 cN/dtex or less, is applied, with respect to the total fineness of the carbon fiber precursor acrylic fiber bundle immediately before passing through the stabilization step (stabilization furnace). When the tension applied to the fiber bundle passing through the second carbonization step (second carbonization furnace) is set to 0.15 cN/dtex or more, the molecular orientation of the obtained carbon fiber bundle can be maintained at a high level, and the strand strength and the knot strength are likely to be improved without decreasing the strand elastic modulus. When the tension applied to the fiber bundle passing through the second carbonization step (second carbonization furnace) is set to 0.21 cN/dtex or less, it is possible to suppress single-fiber breakage of the carbon fiber bundle due to an excessive tension, and the carbon fiber reinforced composite material is likely to be obtained stably.

The treatment time in the second carbonization furnace (time of the second carbonization treatment) is preferably 1.3 minutes or longer and 5.0 minutes or shorter. When the treatment time in the second carbonization furnace is set to 1.3 minutes or longer, it is possible to suppress a violent decomposition reaction accompanying a rapid temperature rise, and a carbon fiber bundle having a high density, high strand strength, and high knot strength is likely to be obtained without decreasing the strand elastic modulus. When the treatment time in the second carbonization furnace is set to 5.0 minutes or shorter, the degree of orientation of crystals of the carbon fiber bundle can be sufficiently increased while maintaining high productivity, and a carbon fiber bundle having high strand strength and knot strength is likely to be efficiently obtained without decreasing the strand elastic modulus.

Third Carbonization Step

The third carbonization step of (6) is 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 within an ambient temperature range of 1700° C. or higher and 2300° C. or lower after the second carbonization step of (5). Note that the fiber bundle to be carbonized in the third carbonization step of (6) is the stabilized fiber bundle that has passed through the second carbonization step.

In the third carbonization step, the fiber bundle that has passed through the second carbonization furnace (second carbonization step) is preferably heated in a non-oxidizing atmosphere in a third carbonization furnace having a linear temperature gradient within an ambient temperature range of 1700° C. or higher and 2300° C. or lower to obtain a carbon fiber bundle.

The ambient temperature in the third carbonization step is 1700° C. or higher and 2300° C. or lower. In view of the temperature of the second carbonization step, the ambient temperature in the third carbonization step is preferably higher than the ambient temperature in the second carbonization step, and more preferably 1800° C. or higher. When the ambient temperature in the third carbonization step is set to 2300° C. or lower, not only deterioration of the third carbonization furnace can be prevented, but also formation of a defective point in the obtained carbon fiber bundle is suppressed, and a carbon fiber bundle having a high density, high strand strength, and high knot strength is likely to be obtained without decreasing the strand elastic modulus.

The maximum temperature of the heating temperature is preferably from 2100 to 2300° C.

When the maximum temperature is 2100° C. or higher, the strand elastic modulus is easily increased, and when the maximum temperature is 2300° C. or lower, deterioration of the third carbonization furnace can be prevented.

The fiber bundle passing through the third carbonization step (third carbonization furnace) is accompanied by large shrinkage, and thus, it is important to heat the fiber bundle under tension. In the third carbonization step, a tension of 0.15 cN/dtex or more and 0.23 cN/dtex or less, preferably a tensile force of 0.18 cN/dtex or more and 0.22 cN/dtex or less, is applied, with respect to the total fineness of the carbon fiber precursor acrylic fiber bundle immediately before passing through the stabilization step (stabilization furnace). When the tension applied to the fiber bundle passing through the third carbonization step (third carbonization furnace) is set to 0.15 cN/dtex or more, the molecular orientation of the obtained carbon fiber bundle can be maintained at a high level, and the strand strength and the knot strength are likely to be improved without decreasing the strand elastic modulus. When the tension applied to the fiber bundle passing through the third carbonization step (third carbonization furnace) is set to 0.23 cN/dtex or less, it is possible to suppress single-fiber breakage of the carbon fiber bundle due to an excessive tension, and the carbon fiber reinforced composite material is likely to be obtained stably.

The treatment time in the third carbonization furnace (time of the third carbonization treatment) is preferably 1.0 minutes or longer and 3.0 minutes or shorter. When the treatment time in the third carbonization furnace is set to 1.0 minutes or longer, it is possible to suppress a violent decomposition reaction accompanying a rapid temperature rise, and a carbon fiber bundle having a high density, high strand strength, and high knot strength is likely to be obtained without decreasing the strand elastic modulus. When the treatment time in the third carbonization furnace is set to 3.0 minutes or shorter, the degree of orientation of crystals of the carbon fiber bundle can be sufficiently increased while maintaining high productivity, and a carbon fiber bundle having high strand strength and knot strength is likely to be efficiently obtained without decreasing the strand elastic modulus.

In the third carbonization step, the temperature rising rate when the ambient 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.

When the temperature rising rate at the time of raising the ambient temperature from 1800° C. to 2200° C. in the third carbonization step is 200° C./min or more, a carbon fiber bundle can be produced with high productivity. When the temperature rising rate at the time of raising the ambient temperature from 1800° C. to 2200° C. in the third carbonization step is 500° C./min or less, a violent decomposition reaction accompanying a rapid temperature rise is easily suppressed.

The difference between the maximum ambient temperature in the second carbonization step of (5) and the inlet ambient temperature in the third carbonization step of (6) is preferably 500° C. or less, and more preferably 300° C. or less. When the difference between the maximum ambient temperature in the second carbonization step of (5) and the inlet ambient temperature in the third carbonization step of (6) is set to 500° C. or less, it is possible to suppress a violent decomposition reaction at the initial stage in the third carbonization step of (6), and a carbon fiber bundle having a high density, high strand strength, and high knot strength is likely to be obtained without decreasing the strand elastic modulus.

The difference between the maximum ambient temperature in the second carbonization step of (5) and the inlet ambient temperature in the third carbonization step of (6) is preferably 30° C. or more, and more preferably 50° C. or more.

The above upper and lower limits can be combined in any manner. For example, the difference may be 30° C. or more and 500° C. or less, or 50° C. or more and 300° C. or less.

Additional Step

The method for producing a carbon fiber of the present invention may include an acrylonitrile-based polymer solution preparation step of the following (a) before the coagulation step of (1).

The method for producing a carbon fiber of the present invention may include a first stretching step of the following (b) between the coagulation step of (1) and the second stretching step of (2).

The method for producing a carbon fiber of the present invention may include a surface oxidation treatment step of the following (c) and a sizing step of the following (d) after the third carbonization step of (6).

• • (a) An acrylonitrile-based polymer solution preparation step of preparing an acrylonitrile-based polymer solution.
  • (b) A first stretching step of stretching the coagulated yarn bundle obtained in the coagulation step of (1) at a stretching ratio of 1.00 times or more and 1.20 times or less in air.
  • (c) A surface oxidation treatment step of subjecting the carbon fiber bundle obtained in the third carbonization step of (6) to surface oxidation treatment.
  • (d) A sizing step of subjecting the carbon fiber bundle after the surface oxidation treatment obtained in the surface oxidation treatment step of (c) to sizing treatment.

Acrylonitrile-Based Polymer Solution Preparation Step

The acrylonitrile-based polymer solution preparation step of (a) is a step of preparing an acrylonitrile-based polymer solution to be used in the coagulation step of (1).

The acrylonitrile-based polymer to be used in the present invention is a polymer obtained by polymerizing acrylonitrile as a main monomer. The acrylonitrile-based polymer may be a homopolymer obtained from acrylonitrile alone, or may be a copolymer obtained by copolymerizing acrylonitrile as a main component with another monomer.

The content of a constituent unit derived from acrylonitrile (hereinafter, also referred to as “acrylonitrile unit”) in the acrylonitrile-based polymer can be determined in consideration of, for example, quality required for the carbon fiber bundle to be obtained, and is, for example, preferably 90 mass % or more and 100 mass % or less, more preferably 90 mass % or more and 99.5 mass % or less, and still more preferably 96 mass % or more and 99.5 mass % or less, with respect to the total mass of monomer units constituting the acrylonitrile-based polymer. When the content of the acrylonitrile unit is 90 mass % or more, in each of the stabilization and carbonization steps for converting the carbon fiber precursor acrylic fiber bundle into a carbon fiber bundle, fusion between single fibers can be suppressed, and a decrease in strand strength of the carbon fiber bundle is easily prevented. Furthermore, in treatment such as stretching with a heating roller or pressurized water vapor, adhesion between single fibers is easily suppressed. When the content of the acrylonitrile unit is 100 mass % or less, preferably 99.5 mass % or less, the solubility in a solvent is less likely to decrease, and precipitation and coagulation of the acrylonitrile-based polymer can be prevented, and thus, the carbon fiber precursor acrylic fiber bundle is likely to be produced stably.

In a case where the acrylonitrile-based polymer has a monomer unit other than the acrylonitrile unit, the monomer unit other than the acrylonitrile in the acrylonitrile-based polymer can be appropriately selected from vinyl-based monomers copolymerizable with acrylonitrile, and a vinyl-based monomer unit that improves hydrophilicity of the acrylonitrile-based polymer and a vinyl-based monomer unit that promotes a stabilization reaction are preferable. Examples thereof 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.

A method for synthesizing the acrylonitrile-based 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-based polymer solution include: organic solvents such as dimethylacetamide, dimethyl sulfoxide, and dimethylformamide; and aqueous solutions of inorganic compounds such as zinc chloride and sodium thiocyanate. Dimethylformamide is preferred from the viewpoint of high dissolving power for the acrylonitrile-based polymer.

The polymer concentration of the acrylonitrile-based polymer solution is preferably 20 mass % or more and 25 mass % or less, and more preferably 21 mass % or more and 24 mass % or less, with respect to the total mass of the acrylonitrile-based polymer solution. When the polymer concentration is 20 mass % or more, voids inside the coagulated yarn are reduced, and thus, the strand strength of the carbon fiber bundle is likely to be increased. When the polymer concentration is 25 mass % or less, the acrylonitrile-based polymer solution can maintain appropriate viscosity and fluidity, and thus, the carbon fiber precursor acrylic fiber bundle is easily produced.

The temperature of the acrylonitrile-based polymer solution when subjected to the coagulation step of (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. When the temperature of the acrylonitrile-based polymer solution is 50° C. or higher and 70° C. or lower, the acrylonitrile-based polymer solution can maintain appropriate viscosity and fluidity, and thus, the carbon fiber precursor acrylic fiber bundle is easily produced.

First Stretching Step

The first stretching step of (b) is a step of stretching the coagulated yarn bundle in air at a stretching ratio of 1.00 times or more and 1.20 times or less.

The first stretching step of (b) is preferably performed between the coagulation step of (1) and the second stretching step of (2).

In the first stretching step of (b), the coagulated yarn bundle taken up in the coagulation step of (1) is stretched in air while containing a part of the coagulation liquid. The stretching 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. When the stretching ratio in air is set to 1.00 times or more, it becomes possible to suppress uneven shrinkage, and as a result, a carbon fiber bundle having high strand strength and knot strength is likely to be obtained without decreasing the strand elastic modulus. When the stretching ratio in air is set to 1.20 times or less, excessive stretching can be suppressed, and stable stretching is likely to be performed.

Surface Oxidation Treatment Step

The surface oxidation treatment step of (c) is a step of subjecting the carbon fiber bundle obtained in the third carbonization step of (6) to surface oxidation treatment.

The surface oxidation treatment step of (c) is preferably performed after the third carbonization step of (6).

The carbon fiber bundle obtained through the third carbonization step (third carbonization furnace) is preferably subjected to the surface oxidation treatment. Examples of the surface treatment method include known methods, that is, oxidation treatment by electrolytic oxidation, chemical oxidation, air oxidation, or the like, and any method may be used, but electrolytic oxidation treatment which is industrially widely performed is preferable from the viewpoint that stable surface oxidation treatment is possible.

In the surface oxidation treatment, ipa representing a surface treatment state is preferably set to 0.05 μA/cm² or more and 0.25 μA/cm² or less. To control ipa within such a range, a method of adjusting an amount of electricity by electrolytic oxidation treatment is simple. In the electrolytic oxidation treatment, ipa varies greatly depending on an electrolyte used and a concentration thereof even with the same amount of electricity, but in an alkaline aqueous solution having a pH of more than 7, it is preferable to perform the oxidation treatment by flowing an amount of electricity of 10 coulomb/g or more and 200 coulomb/g or less using the carbon fiber bundle as an anode.

Examples of the electrolyte include ammonium carbonate, ammonium bicarbonate, ammonium sulfate, calcium hydroxide, sodium hydroxide, and potassium hydroxide.

Sizing Step

The sizing step of (d) is a step of subjecting the carbon fiber bundle after the surface oxidation treatment step of (c) to sizing treatment.

The sizing step of (d) is preferably performed after the surface oxidation treatment step of (c).

The surface-oxidized carbon fiber bundle obtained in the surface oxidation treatment step of (c) is preferably subjected to sizing treatment subsequently. The sizing treatment can be performed by applying a solution obtained by dissolving a sizing agent in an organic solvent or an emulsion obtained by dispersing a sizing agent 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 resultant.

The amount of the sizing agent attached to surfaces of carbon fibers can be adjusted by adjusting the concentration of the sizing agent solution or adjusting the amount of squeezing.

The drying can be performed by using, for example, hot air, a hot plate, a heating roller, or various infrared heaters.

As the sizing agent, a known sizing agent can be used. Examples of the sizing agent include sizing agents containing an epoxy resin, a polyether resin, an epoxy-modified polyurethane resin, or a polyester resin as a main component.

EXAMPLES

Hereinafter, the present invention will be described specifically with reference to examples, but the present invention is not limited to the following description as long as the gist of the present invention is not deviated.

Various measurement methods performed in the present examples are as follows.

Method for Measuring Average Diameter of Carbon Fiber Single-Fibers

The cross-sectional area per single fiber of carbon fibers was calculated from the density (g/cm³) of the carbon fiber bundle, the mass per 1 m of the carbon fiber bundle, that is, the weight per unit area (g/m), and the number of filaments of the carbon fiber bundle. The diameter of a perfect circle having an area equal to the cross-sectional area was calculated and defined as an average diameter of single fibers of the carbon fibers.

Note that the density of the carbon fiber bundle was measured in accordance with Method C (density gradient tube method) described in JIS R 7063:1999.

Method for Measuring Strand Strength and Strand Elastic Modulus

The strand strength and the strand elastic modulus of the carbon fiber bundle were measured in accordance with JIS R 7608:2007.

Note that the strand elastic modulus was calculated by Method A of the same.

Method for Measuring Knot Strength

The knot strength was measured as follows.

A grip portion having a length of 25 mm was attached to both ends of a carbon fiber bundle having a length of 150 mm to obtain a test piece. In the preparation of the test piece, the carbon fiber bundle was aligned by applying a load of 0.1×10⁻³ N/denier. A knot was formed in the test piece approximately in a central portion, and a cross-head speed during tensioning was 100 mm/min. The number of test pieces was 12, the minimum and maximum values were removed, and an average value of 10 test pieces was used as the measured value.

Method for Measuring Fracture Surface Formation Energy

A sample was prepared by cutting a single fiber of carbon fibers to be 20 cm, attaching and fixing the central portion of the single fiber to a mount for a single-fiber tensile test for a sample having a length of 10 mm shown in JIS R 7606:2000, and cutting and removing an extra portion protruding from the mount. Next, a hemispherical defect was formed by irradiating the samples fixed to the mount with a laser. As a laser interface system, Micropoint available from Photonic Instruments Ltd. (pulse energy: 300 uJ) was used. As an optical microscope necessary for condensing the laser, ECLIPSE LV 100 available from Nikon Corporation was used. An aperture diaphragm of the optical microscope was set to minimum and an objective lens was set to 100×. Under these conditions, a central portion of the sample in the fiber axial direction and in the direction perpendicular to the fiber axis was irradiated with one pulse of a laser beam having a wavelength of 435 nm and having a laser intensity attenuated by 10% by an attenuator to obtain a sample having a hemispherical defect formed therein. To prevent the shrinkage fracture of the carbon fiber as a sample, the sample in a state of being attached to the mount was further sandwiched between films, and the space between the films was filled with a viscous liquid, followed by a tensile test. To be specific, a film having a width of about 5 mm and a length of about 15 mm was prepared, and the film was attached to the upper portion of each of both surfaces of the mount of the sample with an adhesive, and the sample was sandwiched between the films together with the mount to cover the sample. After filling the space between the films with an aqueous glycerin solution (in a ratio of 2 parts by mass of water to 1 part by mass of glycerin), the tensile test was performed at a tensile speed of 0.5 mm/min to measure a breaking load. A sample pair obtained by dividing the sample into two in the tensile test was then taken out from the mount, carefully washed with water, and then naturally dried. Next, the sample was fixed to an SEM sample stage with a carbon paste in such a manner that the broken surface of the sample faced upward, thereby preparing an SEM observation sample. The broken surface of the obtained SEM observation sample was subjected to SEM observation with a scanning electron microscope (available from JEOL Ltd., trade name “JSM6060”) under conditions of an acceleration voltage of 10 kV or more and 15 kV or less and a magnification of 10000 times or more and 15000 times or less. The obtained SEM image was transferred to a personal computer, and subjected to image analysis by an image analysis software to measure a depth of the hemispherical defect and a 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 a point of the circumference of the single fiber to the center.

Next, the breaking load was divided by the fiber cross-sectional area (breaking load/fiber cross-sectional area) to calculate the breaking strength (σ).

The fracture surface formation energy was determined from the following equation (F2). The fracture surface formation energy was calculated for 30 single fibers, and the average value thereof was defined as the fracture surface formation energy of the carbon fiber bundle.

Fracture ⁢ surface ⁢ formation ⁢ energy = σ ⁢ 2 ⁢ π ⁢ C / 2 ⁢ E ( F ⁢ 2 )

Here σ is the breaking strength, E is an ultrasonic elastic modulus of the carbon fiber bundle, and C is the depth of the hemispherical defect.

Note that 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

An ultrasonic propagation velocity was measured in accordance with a measurement method illustrated in FIG. 1. A distance L1 between a transmitter and a first receiver was set to 0.20 m, a distance L2 between the transmitter and a second receiver was set to 0.25 m, and a tension applied to the carbon fiber bundle during the measurement was set to 0.02 N/tex. A time from when a pulse was given to the transmitter from a pulse transmission circuit to drive the transmitter and an ultrasonic wave was propagated to the carbon fiber bundle until the ultrasonic wave propagated from the carbon fiber bundle was detected by the first receiver was defined as a reception time 1, and a time until the ultrasonic wave propagated from the carbon fiber bundle was detected by the second receiver was defined as a reception time 2.

The ultrasonic elastic modulus of the carbon fiber bundle was determined by the following equation (F3).

Ultrasonic ⁢ elastic ⁢ modulus ⁢ ( GPa ) = ( ( 0.25 m - 0.2 m ) / ( reception ⁢ time ⁢ 2 ⁢ ( sec ) - reception ⁢ time ⁢ 1 ⁢ ( sec ) ) ) 2 × density ⁢ of ⁢ carbon ⁢ fiber ⁢ bundle ⁢ ( g / cm 3 ) × 1 ⁢ 0 - 6 ( F3 )

Coefficient of Variation in Single-Fiber Elastic Modulus in Single-Fiber Tensile Test

One single fiber was taken out from the carbon fiber bundle, and an elastic modulus of the single fiber was measured under test conditions of a sample length of 5 mm and a tensile speed of 0.5 mm/min using a universal tester (Instron 5500 (trade name) available from Instron Corporation), and this measurement was repeated until test results of 100 fibers were obtained from the same carbon fiber bundle sample. A coefficient of variation was determined from the average value of the elastic moduli obtained from the test results of 100 fibers and the standard deviation in accordance with coefficient of variation (%)=(standard deviation/average value)×100, and the obtained value was defined as a coefficient of variation in elastic modulus among fibers (coefficient of variation in single-fiber elastic modulus in the single-fiber tensile test). The cross-sectional area of the carbon fiber used for the calculation of the elastic modulus was calculated from the density of the carbon fiber bundle and the single-fiber fineness of the carbon fiber in accordance with cross-sectional area of carbon fiber=single-fiber fineness of carbon fiber (mass per unit length)/density of carbon fiber bundle.

Crystallite Size Lc

The carbon fiber bundle was cut to have a length of 50 mm, 30 mg thereof was accurately weighed and collected, the obtained fibers were aligned in such a manner that the fiber axes of the sample were accurately parallel to each other, and then were arranged into a bundle of fiber samples having a uniform thickness of 1 mm using a sample adjustment jig. The bundle of fiber samples was impregnated with a vinyl acetate/methanol solution to fix the bundle of fiber samples not to lose its shape, and then fixed to a wide-angle X-ray diffraction sample table. As an X-ray source, a CuKα ray (using a Ni filter) X-ray generator available from Rigaku Corporation was used, and a goniometer available from Rigaku Corporation was used to detect a diffraction peak in the vicinity of 20=25° corresponding to the plane index (002) of graphite with a scintillation counter by a transmission method. The output was measured at 40 kV and 100 mA. The crystallite size Lc was determined from the half-value width of the diffraction peak using the following equation (F4).

Lc = K ⁢ λ / ( β0 ⁢ cos ⁢ θ ) ( F4 )

(In the equation, K is a Scherrer constant of 0.9, λ is a wavelength of X-ray used (here, CuKα ray is used, and thus, 1.5418 Å), θ is a Bragg diffraction angle, β0 is a true half-value width, and β0=βE−β1 (βE is an apparent half-value width, and β1 is an apparatus constant which is 1.05×10⁻² rad here)) Note that the measurement was performed at n=5.

Method for Calculating Average Void Length

The average void length was calculated by SAXS (small angle X-ray scattering method) as follows in accordance with the Ruland method described in Macromolecules, Vol. 33, No. 5, 2000.

“SAXSpoint 2.0 system” available from Anton Paar GmbH was used, CuKα (wavelength: 1.54 Å) was used as X-ray, an exposure time was 30 minutes, a measuring environment was vacuum, a distance from the sample to the detector was set to 610 mm, and the carbon fiber bundle was aligned in one direction, and then was placed on the sample table with the fiber axial direction vertically to perform small angle X-ray scattering measurement. At this time, scattering derived 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 graph creation software (Igor Pro 8.0). Note that in the present specification, a scattering vector q was defined as 4π sin θ/λ (θ: scattering angle, λ: X-ray wavelength). The two-dimensional scattering profile obtained under the above measurement conditions was subjected to polar coordinate conversion by dividing the azimuth angle of 360° into 1000 pieces, thereby obtaining a scattering intensity map of the azimuth angle-scattering vector q. Here, the carbon fiber axial direction is defined as an azimuth angle of 0°. The obtained scattering intensity map was analyzed by the Ruland method in a range (q=0.8 to 1.86 nm⁻¹) that was perpendicular to the axes of the carbon fibers (azimuth angle: 90°) and included no streak derived from total X-ray reflection on the surfaces of the carbon fibers. Specifically, the scattering intensity map of the 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 the azimuth angle of 0 to 180° of this profile was fitted with a Gaussian function to calculate an integral width B. q²B² was plotted against q², an intercept and a slope in linear approximation were obtained, and a void length L of voids present in the carbon fibers and an integral width of orientation distribution of the voids were calculated from the following equation (F5).

( Scattering ⁢ vector ⁢ q ) 2 ⁢ ( integral ⁢ width ⁢ B ⁢ of ⁢ scattering ⁢ intensity ⁢ 
 distribution ⁢ in ⁢ azimuth ⁢ angle ⁢ direction ) 2 = ( scattering ⁢ vector ⁢ q ) 2 ⁢ ( integral ⁢ width ⁢ of ⁢ orientation ⁢ distribution ⁢ of ⁢ voids ) 2 + ( 2 ⁢ π / L ) 2 ( F5 )

Example 1

Preparation of Carbon Fiber Precursor Acrylic Fiber Bundle

An acrylonitrile-based polymer containing 98 mass % of acrylonitrile units and 2 mass % of methacrylic acid units was dissolved in dimethylformamide to prepare an acrylonitrile-based polymer solution having a concentration of 23.5 mass %.

The acrylonitrile-based polymer solution was spun from a spinneret provided with 2000 discharge holes each having a diameter of 0.15 mm to perform dry-wet spinning. That is, the acrylonitrile-based polymer solution was spun in air, caused to pass through a space of about 5 mm, and then coagulated in a coagulation bath filled with an aqueous solution (A) containing 80.4 mass % of dimethylformamide adjusted to have a temperature of 8° C., and the coagulated yarn bundle was taken up.

Next, the coagulated yarn bundle was doubled to have a filament number of 12000, pulled out from the coagulation bath, stretched at 1.1 times in air, and then stretched at 2.9 times in a stretching tank filled with a hot aqueous solution (B) containing 55 mass % of dimethylformamide adjusted to have a temperature of 90° C. After stretching, the stretched fiber bundle containing the solvent was washed with clean water, and then relaxed at 0.98 times in hot water (C) at 96° C. Subsequently, an oil agent containing an amino-modified silicone as a main component was applied to the stretched fiber bundle to be 1.1 mass %, and the stretched fiber bundle was dried and densified. The stretched fiber bundle after drying and densification was stretched at 4.0 times under a pressurized water vapor atmosphere to perform further improvement of the orientation and densification, and then wound to obtain a carbon fiber precursor acrylic fiber bundle. The single-fiber fineness of the fiber was 1.40 dtex.

Preparation of Carbon Fiber Bundle

A plurality of carbon fiber precursor acrylic fiber bundles were introduced into a stabilization furnace having a linear temperature gradient with an inlet ambient temperature of 220° C. and a maximum ambient temperature of 245° C. in a state where the plurality of carbon fiber precursor acrylic fiber bundles were aligned in parallel. The carbon fiber precursor acrylic fiber bundles were subjected to stabilization treatment by blowing air heated in the stabilization furnace to the carbon fiber precursor acrylic fiber bundles, thereby obtaining stabilized fiber bundles having a density of 1.345 g/cm³. The elongation percentage was set to 6.0%, and the stabilization treatment time was set to 70 minutes.

Next, the stabilized fiber bundle was caused to pass through a first carbonization furnace having a linear temperature gradient with an inlet ambient temperature of 300° C. and a maximum ambient temperature of 700° C. in nitrogen while being elongated by 4.5%, thereby performing first carbonization treatment. The treatment time was set to 2.0 minutes.

Further, second carbonization treatment was performed in a nitrogen atmosphere using a second carbonization furnace in which a linear temperature gradient was set with an inlet ambient temperature of 1100° C. and a maximum ambient temperature of 1700° C. At that time, the elongation percentage was set to −2.0%, and the treatment time was set to 1.6 minutes. The tension applied to the yarn bundles during the treatment was 0.20 cN/dtex.

Subsequently, third carbonization treatment was performed in a nitrogen atmosphere using a third carbonization furnace in which a linear temperature gradient was set with an inlet ambient temperature of 1800° C. and a maximum ambient temperature of 2300° C. to obtain carbon fiber bundles. At that time, the elongation percentage was set to −2.0%, and the treatment time was set to 1.9 minutes. At this time, the tension applied to the yarn bundles during the treatment was 0.22 cN/dtex.

The difference between the maximum ambient temperature of the second carbonization furnace and the inlet ambient temperature of the third carbonization furnace was set to 100° C., and the temperature rising rate when the ambient temperature was raised from 1800° C. to 2200° C. was set to 350° C./min.

Subsequently, the carbon fiber bundles were caused to travel in a 10 mass % aqueous solution of ammonium bicarbonate at a temperature of 30° C., and energization treatment was performed between the carbon fiber bundles as an anode and a counter electrode in such a manner that an amount of electricity per 1 g of carbon fibers to be treated was 40 coulombs.

Then, the carbon fiber bundles were washed with hot water at 90° C. and dried.

Then, 0.5 mass % of a sizing agent (available from DIC Corporation, trade name “Hydran N320”) was attached (sizing treatment), and the resultant was wound around a bobbin to obtain carbon fiber bundles.

The carbon fiber bundles after the sizing treatment were measured for an average diameter of single fibers, a density, a weight per unit area, knot strength, strand strength, and a strand elastic modulus. The results of these are shown in Table 3.

Examples 2 to 3

Carbon fiber bundles were prepared in the same manner as in Example 1 except that the preparation conditions of the carbon fiber bundles were changed as shown in Tables 1 and 2, and various measurements were performed. The results are shown in Table 3.

Examples 4 to 7

Carbon fiber bundles were prepared in the same manner as in Example 1 except that the single-fiber fineness of the carbon fiber precursor acrylic fiber bundle was changed to 1.73 dtex and the preparation conditions of the carbon fiber bundles were changed as shown in Tables 1 and 2, and various measurements were performed. The results are shown in Table 3.

Comparative Example 1

A carbon fiber precursor acrylic fiber bundle was prepared in the same manner as in Example 1 except that the preparation conditions of 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.

A carbon fiber bundle was prepared in the same manner as in Example 1 except that the obtained carbon fiber precursor acrylic fiber bundle was used and the preparation conditions of the carbon fiber bundle were changed as shown in Table 2, and various measurements were performed. The results are shown in Table 3.

Comparative Example 2

A carbon fiber precursor acrylic fiber bundle was prepared in the same manner as in Example 1 except that the preparation conditions of 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 prepare a carbon fiber bundle in the same manner as in Example 1 except that the obtained carbon fiber precursor acrylic fiber bundle was used and the preparation conditions of the carbon fiber bundle were changed as shown in Table 2, but the fiber bundle was broken in the process of producing a carbon fiber bundle, and it was impossible to obtain a carbon fiber bundle.

Comparative Example 3

A carbon fiber precursor acrylic fiber bundle was prepared in the same manner as in Example 1 except that the preparation conditions of 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 prepare a carbon fiber bundle in the same manner as in Example 1 except that the obtained carbon fiber precursor acrylic fiber bundle was used and the preparation conditions of the carbon fiber bundle were changed as shown in Table 2, but the fiber bundle was broken in the process of producing a carbon fiber bundle, and it was impossible to obtain a carbon fiber bundle.

Reference Example 1

Various measurements were performed on a commercially available carbon fiber bundle (available from Toray Industries, Inc., trade name “M40JB”). The results are shown in Table 3.

	TABLE 1
	Preparation conditions of carbon fiber precursor fiber

Under

	Coagulation bath				pressurized
	(aqueous solution (A))		Hot aqueous solution (B)	Hot water (C)	water vapor

		Concentration	In air		Concentration			Shrinkage or	atmosphere
		of organic	Stretching		of organic	Stretching		stretching	Stretching
	Temperature	solvent	ratio	Temperature	solvent	ratio	Temperature	ratio	ratio
	° C.	wt %	—	° C.	wt %	—	° C.	—	—

Example 1	8	80.4	1.1	90	55	2.9	96	0.98	4.0
Example 2	8	80.4	1.1	90	55	2.9	96	0.98	4.0
Example 3	8	80.4	1.1	90	55	2.9	96	0.98	4.0
Example 4	8	80.4	1.1	90	55	2.9	96	0.98	4.0
Example 5	8	80.4	1.1	90	55	2.9	96	0.98	4.0
Example 6	8	80.4	1.1	90	55	2.9	96	0.98	4.0
Example 7	8	80.4	1.1	90	55	2.9	96	0.98	4.0
Comparative	12	79.2	1.1	60	30	1.67	95	1.6	3.5
Example 1
Comparative	8	80.4	1.1	90	55	2.9	96	0.98	4.0
Example 2
Comparative	8	80.4	1.1	90	55	2.9	96	0.98	4.0
Example 3

	TABLE 2
	Preparation conditions of carbon fiber

Stabilization finance

Second carbonization finance

	Inlet	Maximum		Inlet	Maximum
	ambient	ambient	Elongation	ambient	ambient	Elongation
	temperature	temperature	percentage	temperature	temperature	percentage	Tension
	° C.	° C.	%	° C.	° C.	%	cN/dtex
Example 1	220	245	6.0	1100	1700	−2.0	0.20
Example 2	220	240	6.0	1100	1700	−2.0	0.20
Example 3	210	2 5	6.0	1100	1700	−2.0	0.20
Example 4	230	2 0	6.0	1100	1700	−2.0	0.1
Example 5	220	245	6.0	1100	1700	−2.0	0.1
Example 6	220	240	6.0	1100	1700	−2.0	0.17

Preparation conditions of carbon fiber

Difference

							between
							maximum
							ambient
							temperature
							of second
							carbonization
						Temperature	finance and
						rising	inlet ambient

Third carbonization finance

from ambient

temperature

		Inlet	Maximum			temperature	of third
		ambient	ambient	Elongation		1800° C.	carbonization
		temperature	temperature	percentage	Tension	2200° C.	finance
		° C.	° C.	%	cN/dtex	° C./min	° C.
	Example 1	1800	2300	−2.0	0.22	40	100
	Example 2	1800	2300	−2.0	0.22	290	100
	Example 3	1800	2300	−2.0	0.22	220	100
	Example 4	1800	2300	−2.0	0.19	450	100
	Example 5	1800	2300	−2.0	0.19	350	100
	Example 6	1800	2300	−2.0	0.19	290	100
	indicates data missing or illegible when filed

	TABLE 3
	Carbon fiber bundle

	Average				Fracture
	diameter		Weight		surface	Average	Average
	of single		per unit	Knot	formation	void	void
	fiber	Density	area	strength	energy	length	diameter
	μm	g/cm	g/m	N/mm	N/m	nm	nm
Example 1	7.0	1.82	0.84	90	19	18.0	0.72
Example 2	7.0	1.83	0.84	100	29	18.7	0.69
Example 3	6.9	1.84	0.83	1 0	23	18.9	0.71
Example 4	7.9	1.81	1.06	120	18	21.5	0.73
Example 5	7.8	1.81	1.04	120	20	19.8	0.74
Example 6	7.8	1.83	1.05	120	24	18.9	0.73
Example 7	7.8	1.84	1.05	120	26	19.4	0.71
Comparative	5.9	1.82	0.60	—	17	22.3	0.71
Example 1
Reference	5.2	1.77	0.45	—	20	22.7	0.63
Example 1

Carbon fiber bundle

						Average
			Coefficient			diameter
		Single-	of variation			of single
		fiber	in single-		Strand	fibers ×
	Crystallite	elastic	fiber elastic	Strand	elastic	strand
	size L	modulus	modulus	strength	modulus	strength
	nm	GPa	CV %	GPa	GPa	—
Example 1	3.7	309	11.5	5.1	380	35.7
Example 2	3.7	282	7.2	5.4	385	37.6
Example 3	3.9	342	7.8	5.3	400	36.8
Example 4	3.5	325	17.2	4.5	350	35.6
Example 5	3.7	324	12.2	5.0	360	39.0
Example 6	3.9	318	8.6	5.3	370	41.3
Example 7	4.0	312	9.8	5.3	39	41.3
Comparative	3.9	316	18.0	4.4	375	26.0
Example 1
Reference	3.3	—	—	4.4	375	22.9
Example 1
indicates data missing or illegible when filed

As is clear from the results in Table 3, the carbon fiber bundles obtained in respective examples had a large average diameter of single fibers, and were able to exhibit high strand strength without a decrease in strand elastic modulus. Note that the carbon fiber bundles obtained in respective examples had substantially no twist.

On the other hand, the carbon fiber bundle obtained in Comparative Example 1 and the carbon fiber bundle used in Reference Example 1 which was a commercially available product had lower strand strength than that of each of the carbon fiber bundles obtained in examples. In addition, the fiber diameter is small, and thus, there is a concern that, in a case of preparing a carbon fiber reinforced composite material, impregnation is insufficient due to a high matrix resin viscosity, and the tensile strength of the carbon fiber reinforced composite material decreases.

INDUSTRIAL APPLICABILITY

The carbon fiber bundle of the present invention can exhibit high strand strength and knot strength without decrease in the strand elastic modulus, and has a large fiber diameter, and thus, is useful in a wide range of applications requiring high mechanical properties, for example, industrial materials such as automobile members, aerospace materials, civil engineering and construction materials, sports and leisure materials, pressure vessels, and windmill blades.