Carbon fiber bundle

Abstract

Provided is a carbon fiber bundle having a filament number of 48,000-60,000 and containing fibrils along the fiber axis direction on the fiber surface, wherein at the fracture starting point of the fiber fracture surface in a single fiber tensile test with a test length of 50 nm according to JIS R7606 (2000), the number percentage of fibers in which fibril-like substances having an aspect ratio of 3.0-10.0 are present on the fracture surface is 1-20%, and the number percentage of fibers in which voids larger than or equal to 100 nm are present on the fracture surface is 1-14%.　The present invention addresses the problem of providing a carbon fiber bundle having high overall fineness and excellent strength, and having excellent workability when subjected to advanced processing.

Description

Carbon fiber bundle

[0001] The present invention relates to a carbon fiber bundle that has a high total fineness and yet is excellent in strength and operability when subjected to advanced processing.

[0002] Carbon fiber bundles have high specific strength and specific modulus, and are therefore used as reinforcing fibers for composite materials in a wide range of applications, including aerospace applications. Recently, they have also been used in industrial applications such as automotive components and wind power generation. In particular, wind power generation requires light weight and rigidity, so carbon fiber bundles with excellent specific modulus are often used, and in recent years, demand for carbon fiber bundles for wind power generation has been expanding.

[0003] In industrial applications, there is a strong demand for cost reduction of final composite material products, and in addition to cost reduction of carbon fiber bundles, high-level processability is considered important when producing carbon fiber composite materials such as intermediate substrates such as prepregs, towpregs, woven fabrics, and sheet molding compounds (SMCs), and pultruded materials from carbon fiber bundles. In order to improve high-level processability, it is particularly important that the carbon fiber bundle has little fuzz and excellent openability, as well as no breakage of the entire carbon fiber bundle or single carbon fiber fibers when unwound from a bobbin and running through the production process, and good operability.

[0004] In industrial applications where cost reduction is strongly required, so-called large tow carbon fiber bundles with a single fiber fineness of 0.6 dtex or more and a filament count of 40,000 or more are often used. Large tow carbon fiber bundles use polyacrylonitrile precursor fibers produced by a highly productive wet spinning method, and cost reduction is achieved by increasing productivity by increasing the processing unit and processing density, and by using simple equipment designed for acrylic fibers for clothing. Large tow carbon fiber bundles are superior in cost to regular tow carbon fiber bundles with a filament count of 12,000 to 24,000, but currently fall short in terms of strand strength, fluff count, and advanced processability of the carbon fiber bundles. Further improvements are needed without sacrificing productivity.

[0005] In response to such problems, Patent Document 1 proposes a technology for producing high-quality large-tow carbon fiber bundles with high productivity by setting the dynamic viscoelastic properties and silicon content of a polyacrylonitrile precursor fiber bundle in a specific range, and then performing heat treatment and drawing under specific conditions.

[0006] Patent Documents 2, 3, and 4 propose techniques for improving the strand strength and quality of the resulting carbon fiber by controlling the composition and amount of oil applied to the polyacrylonitrile precursor fiber bundle to improve the processability in the flame-proofing step.

[0007] Patent Documents 5 and 6 propose techniques for improving the strand strength and quality of the resulting carbon fiber bundle by controlling specific defects that become breakage initiation points in the resulting carbon fiber bundle within a certain range.

[0008] JP 2006-299439 A JP 2016-199824 A JP 2021-50428 A JP 2021-123812 A JP 2019-112730 A JP 2020-153051 A

[0009] However, the prior art has the following problems.

[0010] Although Patent Document 1 discloses the effect of improving the strand strength of large tow carbon fiber bundles and suppressing the generation of fluff during the manufacturing process, it does not disclose or suggest the improvement of operability when subjected to advanced processing, and the kinematic viscosity of the oil, which is important for suppressing voids of 100 nm or more, which is effective in suppressing the generation of fluff during advanced processing, is insufficient (for example, in Example 1, the kinematic viscosity is 450 mm ２ / sec), and there is a problem in that the improvement effect is insufficient.

[0011] Patent Document 2 discloses the effect of improving the strand strength of regular tow carbon fiber bundles and suppressing the generation of fluff during the manufacturing process, but does not disclose or suggest the improvement of operability when subjected to advanced processing. ２ / sec or less, which is insufficient, and there is a problem in that the improvement effect is insufficient.

[0012] Patent Document 3 discloses the effect of improving the strand strength of regular tow carbon fiber bundles and suppressing the generation of fluff during the manufacturing process, but does not disclose or suggest the improvement of operability when subjected to advanced processing. In addition, the kinematic viscosity of the oil agent, which is important for suppressing voids of 100 nm or more, which is effective in suppressing the generation of fluff during advanced processing, is 3,500 to 20,000 mm ２ / sec, there is no specific disclosure about the void state of the polyacrylonitrile precursor fiber to which the oil agent is applied, and the draw ratio in warm water, which is important for reducing voids, is insufficient (for example, 3.5 times in Example 1), and the improvement effect is insufficient.

[0013] Furthermore, the present invention is premised on a polyacrylonitrile precursor fiber bundle having a small number of filaments obtained by a dry / wet spinning method, and when applied to a large tow carbon fiber bundle having fibrils on the fiber surface and a high processing unit and processing density, there is a problem that the amount of silicon is excessive and the effect of suppressing voids of 100 nm or more, which is effective in suppressing the generation of fluff during advanced processing, is insufficient.

[0014] Although Patent Document 4 discloses an improvement in the strand strength of regular tow carbon fiber bundles and an effect of suppressing the generation of fuzz during the manufacturing process, it neither discloses nor suggests an improvement in operability when subjected to advanced processing. Moreover, the kinematic viscosity of the oil agent, which is important for suppressing voids of 100 nm or more and is effective in suppressing the generation of fuzz during advanced processing, is insufficient. In addition, there is no disclosure or suggestion regarding the void state of the polyacrylonitrile precursor fiber to which the oil agent is applied, and the draw ratio in warm water, which is important for void reduction, is insufficient (for example, 3.5 times in Example 1), resulting in a problem that the improvement effect is insufficient.

[0015] Furthermore, the present invention is premised on a polyacrylonitrile precursor fiber bundle having a small number of filaments obtained by a dry / wet spinning method, and when applied to a large tow carbon fiber bundle having fibrils on the fiber surface and a high processing unit and processing density, there is a problem that the amount of silicon is excessive and the effect of suppressing voids of 100 nm or more, which is effective in suppressing the generation of fluff during advanced processing, is insufficient.

[0016] Patent Documents 5 and 6 disclose the effect of improving the strand strength of regular tow carbon fiber bundles and suppressing the generation of fuzz during the manufacturing process by controlling specific defects that appear on the fracture surface when a carbon fiber bundle is subjected to a single fiber tensile test with a test length of 10 mm, but do not disclose or suggest an improvement in operability when subjected to a high-level processing process. Furthermore, although defects that appear on the fracture surface when a carbon fiber bundle is subjected to a single fiber tensile test with a test length of 10 mm have a good correlation with the development of strand strength, the defects are different in type and probability of existence from defects that cause fuzz that occur during high-level processing, and therefore there was a problem in that they did not lead to the identification of the cause or improvement of the defect.

[0017] As described above, in the prior art, techniques for improving strand strength and suppressing the generation of fluff in the carbon fiber bundle manufacturing process have been proposed, but no technique for improving the generation of fluff during advanced processing of carbon fiber bundles has been disclosed, and no defects that cause the generation of fluff during advanced processing of carbon fiber bundles or techniques for identifying these defects have been disclosed. Therefore, it has been difficult to essentially reduce the generation of fluff during advanced processing of carbon fiber bundles.

[0018] Furthermore, with regard to the suppression of voids and fusion, which are effective in suppressing the generation of fuzz during advanced processing, there has been no technology that has comprehensively proposed control of the surface morphology and voids of polyacrylonitrile precursor fibers, assuming large tow carbon fiber bundles, and control of the composition and amount of oil applied that are suitable for this.As a result, there has been insufficient improvement in the generation of fuzz during advanced processing of large tow carbon fiber bundles.

[0019] In order to solve the above problems, the carbon fiber bundle of the present invention has the following configuration: Namely, (1) A carbon fiber bundle having fibrils present on the fiber surface along the fiber axis direction, and in which, at the fracture origin of the fiber fracture surface when subjected to a single fiber tensile test with a test length of 50 mm in accordance with JIS R7606 (2000), the percentage of the number of fibers having a fracture surface on which fibril-like objects with an aspect ratio of 3.0 to 10.0 are present is 1 to 20%, and the percentage of the number of fibers having a fracture surface on which voids of 100 nm or more are present is 1 to 14%, the carbon fiber bundle having a filament count of 48,000 to 60,000. (2) A carbon fiber bundle according to (1), wherein fibrils are present on the fiber surface along the fiber axis direction, and when a single fiber tensile test is performed at a test length of 50 mm in accordance with JIS R7606 (2000), at the fracture origin of the fiber fracture surface, A (%) is the percentage of the number of fibers having a fracture surface where fibril-like objects with an aspect ratio of 3.0 to 10.0 are present, and B (%) is the percentage of the number of fibers having a fracture surface where voids of 100 nm or more are present, A and B satisfy the relationship of the following formula (1):

[0020] B ≦ -0.7A + 20.5 (1) (3) The carbon fiber bundle according to (1), wherein fibrils are present on the fiber surface along the fiber axis direction, and the percentage of fibers having a fracture surface with fibril-like objects having an aspect ratio of 3.0 to 10.0 is 1 to 15% at the fracture origin of the fiber fracture surface when a single fiber tensile test is performed with a test length of 50 mm in accordance with JIS R7606 (2000) and the percentage of fibers having a fracture surface with voids of 100 nm or more is 1 to 10%. (4) The carbon fiber bundle according to any one of (1) to (3), wherein the width of the fibrils is 100 to 600 nm. (5) The carbon fiber bundle according to any one of (1) to (4), wherein the strand strength is 4.5 to 6.0 GPa.

[0021] According to the present invention, a carbon fiber bundle can be obtained which has a high total fineness, yet is excellent in strength and operability when subjected to advanced processing, and which is likely to exhibit mechanical properties when made into a carbon fiber reinforced composite material.

[0022] Figure 1 is a scanning electron microscope (SEM) image of the fracture surface of a carbon fiber. Radial striations converging to a single point are observed. Figure 2 is an enlarged image of the fracture surface of another carbon fiber near the fracture initiation point. Fibrillar matter with an aspect ratio of 3.0 to 10.0 is observed. Figure 3 is an enlarged image of the fracture surface of another carbon fiber near the fracture initiation point. Voids of 100 nm or larger are observed. Figure 4 explains the aspect ratio of the fibril-like matter. Figure 5 explains the aspect ratio of the fibril-like matter. Figure 6 explains the aspect ratio of the fibril-like matter.

[0023] In the carbon fiber bundle of the present invention, fibrils are present on the fiber surface along the fiber axis direction, and when a single fiber tensile test is performed at a test length of 50 mm in accordance with JIS R7606 (2000), at the fracture origin of the fiber fracture surface, the percentage of fibers having a fracture surface on which fibril-like matter with an aspect ratio of 3.0 to 10.0 is present is 1 to 20%, and the percentage of fibers having a fracture surface on which voids of 100 nm or more are present is 1 to 14%, and the number of filaments is 48,000 to 60,000.

[0024] The carbon fiber bundle of the present invention must have fibrils on the fiber surface along the fiber axis. The width of the fibrils is preferably 100 to 600 nm, more preferably 200 to 400 nm. The presence of fibrils on the carbon fiber surface along the fiber axis ensures an appropriate friction coefficient, reduces the generation of fluff when subjected to advanced processing, and improves the spreadability of the carbon fiber bundle. Furthermore, the presence of the fibrils can prevent fusion between fineness fibers, particularly in the early stages of flame retardation, thereby reducing the amount of silicone-containing oil that can lead to void formation and contributing to the reduction of voids. The presence and width of the fibrils can be confirmed by observing the fiber surface with a scanning electron microscope. The fibril width can be determined by observing 10 fibers at a magnification of 25,000 times, measuring the width in the direction perpendicular to the fiber axis at 10 locations per fiber, and taking the arithmetic average of the measurements. The presence and width of fibrils can be controlled by adopting wet spinning as the spinning method for the polyacrylonitrile precursor fiber bundle, coagulation conditions, draw ratio in warm water, etc.

[0025] In the carbon fiber bundle of the present invention, at the fracture origin of the fiber fracture surface when a single fiber tensile test is performed with a test length of 50 mm in accordance with JIS R7606 (2000), the percentage of fibers having a fracture surface on which fibril-like matter with an aspect ratio of 3.0 to 10.0 is present is 1 to 20%, and the percentage of fibers having a fracture surface on which voids of 100 nm or more are present is 1 to 14%.

[0026] The proportion of fibers having a fracture surface on which fibril-like materials with an aspect ratio of 3.0 to 10.0 are present is preferably 1 to 15%, more preferably 1 to 13%, and even more preferably 1 to 10%. The proportion of fibers having a fracture surface on which voids of 100 nm or more are present is preferably 1 to 10%, more preferably 1 to 6%, and even more preferably 1 to 4%. The smaller the proportion of fibers having either type of fracture surface, the easier it is to achieve the effects of the present invention, but on an industrial production scale, it is often sufficient to reduce the proportion to 1%.

[0027] By making the proportion of fibers having a fracture surface on which fibril-like materials with an aspect ratio of 3.0 to 10.0 are present 20% or less and the proportion of fibers having a fracture surface on which voids of 100 nm or more are present 14% or less, the generation of fluff when subjected to advanced processing can be suppressed, and good operability can be obtained.

[0028] In the carbon fiber bundle of the present invention, when the percentage by number of fibers having a fracture surface on which fibril-like materials with an aspect ratio of 3.0 to 10.0 are present is defined as A (%), and the percentage by number of fibers having a fracture surface on which voids of 100 nm or more are present is defined as B (%), it is preferable that A and B satisfy the relationship of the following formula (1):

[0029] B≦−0.7A+20.5 (1) It is more preferable that A and B satisfy the following formula (2), and it is even more preferable that they satisfy the following formula (3).

[0030] B≦−0.7A+17.5 (2) B≦−0.7A+14.5 (3) When A and B satisfy the above-mentioned preferable relationship, the ratio of the number of fibers having a fracture surface on which the fibril-like material is present and the ratio of the number of fibers having a fracture surface on which the voids are present are both small, and defects in the carbon fiber are reduced, so that the generation of fluff when subjected to advanced processing is suppressed and good operability can be obtained.

[0031] The strength of a carbon fiber single fiber is governed by the size, type, and probability of existence of defects, and when the test length is changed, the size and type of defects contained within that test length change, resulting in a change in strength. Strand strength, which is generally used as an index of the strength of a carbon fiber bundle, correlates well with the single fiber strength at a test length of about 10 mm, but the inventors' studies have found that the operability when subjected to advanced processing correlates with the proportion of specific defects at a test length of 50 mm. The reason why test lengths longer than strand strength correlate with operability during advanced processing is not necessarily clear, but it is thought that serious defects, which have a relatively low probability of existence, cause fiber breakage when subjected to tension or abrasion during advanced processing.

[0032] Fibrillar matter with an aspect ratio of 3.0 to 10.0 is a defect that is thought to occur when single fibers fuse together and then peel off during the process of producing a carbon fiber bundle. Polyacrylonitrile precursor fibers are aggregates of fibrils aligned along the fiber axis, and when the above-mentioned fusion and peeling occur, breakage tends to occur at the fibril level. As a result of studies by the present inventors, fibril matter, which refers to defects thought to be caused by breakage at such levels, often has an aspect ratio of 3.0 to 10.0. The proportion of fibers having fracture surfaces on which such fibril matter is present can be calculated according to the method described below.

[0033] In order to control the proportion of fibers having a fracture surface in which fibril-like materials with an aspect ratio of 3.0 to 10.0 are present at the fracture origin of the fiber fracture surface in a single fiber tensile test with a test length of 50 mm in accordance with JIS R7606 (2000), it is important to suppress fusion of single fibers to each other in the process of producing carbon fiber bundles. A commonly known method for suppressing fusion of single fibers to each other is to apply a silicone-based oil to polyacrylonitrile precursor fibers, but the inventors' studies have revealed that simply using a silicone-based oil is insufficient. A preferred control method will be described later as a preferred method for producing carbon fiber bundles.

[0034] Voids of 100 nm or larger are defects that are thought to occur when voids contained in polyacrylonitrile precursor fibers do not disappear during the subsequent flame-proofing process, pre-carbonization process, and carbonization process, but remain. Possible reasons for the voids in polyacrylonitrile precursor fibers not disappearing include the large size of the voids contained in the polyacrylonitrile precursor fibers before the oil agent is applied, and the oil agent penetrating into the voids during the application of the oil agent, inhibiting densification. Research conducted by the present inventors has shown that the size of voids generated by the above-mentioned mechanism is often 100 nm or larger. The percentage of fibers having fracture surfaces with voids of 100 nm or larger can be calculated according to the method described below.

[0035] In order to control the proportion of fibers having a fracture surface in which voids of 100 nm or more exist at the fracture origin on the fiber fracture surface when a single fiber tensile test is performed with a test length of 50 mm in accordance with JIS R7606 (2000), it is important to reduce the size of the voids contained in the polyacrylonitrile precursor fiber, suppress the penetration of an oil into the voids, etc. A preferred control method will be described later as a preferred method for producing a carbon fiber bundle.

[0036] The number of filaments in the carbon fiber bundle of the present invention is 48,000 to 60,000, preferably 50,000 to 55,000. The number of filaments is the number of single fibers constituting the carbon fiber bundle. The higher the number of filaments, the better the productivity of carbon fiber reinforced composite materials. However, if the number is too high, the mechanical properties of the resulting carbon fiber reinforced composite material may be reduced in terms of the spreadability of the carbon fiber bundle and resin impregnation. A filament number of 48,000 to 60,000 provides excellent productivity during composite material molding and is suitable for industrial applications. The number of filaments can be controlled by the number of holes in the spinneret in the spinning process of the polyacrylonitrile precursor fiber bundle, or by dividing or doubling the yarn.

[0037] The strand strength of the carbon fiber bundle of the present invention is preferably 4.5 to 6.0 GPa, more preferably 4.6 to 6.0 GPa, and even more preferably 4.8 to 6.0 GPa. The strand strength can be measured by the method described below, and if it is 4.5 to 6.0 GPa, the carbon fiber bundle can be suitably used for industrial applications such as wind turbine blade materials, pressure vessel reinforcing materials, and automotive structural members.

[0038] Next, a method for producing a carbon fiber bundle that is preferable for obtaining the carbon fiber bundle of the present invention will be described.

[0039] The carbon fiber bundle of the present invention is produced by a process of wet-spinning a polyacrylonitrile polymer, a process of drawing the polymer by 5.0 to 8.0 times in warm water at 30 to 99°C, and a process of drawing a carbon fiber bundle having a kinematic viscosity at 25°C of 6,000 to 20,000 mm ２ A polyacrylonitrile precursor fiber bundle having fibrils on the fiber surface along the fiber axis direction and having a filament number of 48,000 to 60,000, obtained by a method for producing a polyacrylonitrile precursor fiber bundle, the method comprising the step of applying an oil solution containing silicone of 1.21 to 1.23 g / cm 3 in an oxidizing atmosphere at 200 to 300°C. ３ It is preferable to produce the product by carrying out a flame retardant treatment while controlling the silicon content to 0.06 to 0.09 mass % until the silicon content reaches 0.06 mass %, then carrying out a preliminary carbonization treatment in an inert atmosphere at 500 to 1,200°C, and then carrying out a carbonization treatment in an inert atmosphere at 900 to 2,000°C.

[0040] In the above-described method for producing a polyacrylonitrile precursor fiber bundle, the polyacrylonitrile polymer refers to a polymer in which at least acrylonitrile is the main constituent component of the polymer skeleton, and the main constituent component refers to a constituent component that accounts for 90 to 100 mass % of the polymer skeleton.

[0041] The polyacrylonitrile polymer preferably contains a copolymerization component such as itaconic acid, acrylamide, or methacrylic acid from the viewpoints of improving spinnability and efficiently carrying out flame retardant treatment. In producing a polyacrylonitrile precursor fiber bundle, the method for producing the polyacrylonitrile polymer can be selected from known polymerization methods such as solution polymerization and aqueous suspension polymerization. The polyacrylonitrile polymer is dissolved in a solvent to form a spinning solution, which is then used to produce the polyacrylonitrile precursor fiber. The solvent used for the spinning solution can be selected from known solvents capable of dissolving polyacrylonitrile, such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, or an aqueous nitric acid solution, an aqueous zinc chloride solution, or an aqueous sodium rhodanide solution.

[0042] The above-mentioned method for producing a polyacrylonitrile precursor fiber bundle includes a step of wet-spinning a polyacrylonitrile polymer. Here, wet-spinning refers to a spinning method in which a polyacrylonitrile polymer is directly discharged into a coagulation bath through a spinneret. By applying wet-spinning, a fiber surface morphology having fibrils suitable for producing the carbon fiber bundle of the present invention can be obtained.

[0043] In the above-described method for producing a polyacrylonitrile precursor fiber bundle, the number of holes in the spinneret is preferably 3,000 to 200,000 holes in order to achieve the aforementioned number of filaments in the carbon fiber bundle, and a polyacrylonitrile precursor fiber bundle with the desired number of filaments can be obtained by splitting or doubling.

[0044] In the above-described method for producing a polyacrylonitrile precursor fiber bundle, the composition of the coagulation bath preferably contains the solvent used in the spinning dope, such as dimethyl sulfoxide, dimethyl formamide, or dimethyl acetamide, and a so-called coagulation promoter. As the solvent, dimethyl sulfoxide and dimethyl formamide are more preferred from the viewpoint of forming appropriate fibrils on the surface of the polyacrylonitrile precursor fiber without impairing productivity. As the coagulation promoter, a solvent that does not dissolve the polyacrylonitrile polymer and is compatible with the solvent used in the spinning dope can be used, and water is preferred.

[0045] The above-mentioned method for producing a polyacrylonitrile precursor fiber bundle includes a step of drawing the fiber by 5.0 to 8.0 times in warm water at 30 to 99°C. The filament obtained by wet-spinning a polyacrylonitrile polymer is washed to remove the solvent in warm water and then drawn. The washing and drawing may be performed simultaneously or separately, as long as the fiber is drawn by 5.0 to 8.0 times in warm water at 30 to 99°C. When drawing in warm water, it is preferable to draw the fiber stepwise in multiple warm water baths. The warm water temperature is preferably 50 to 99°C, more preferably 70 to 99°C. A higher warm water temperature makes drawing easier, but also increases the likelihood of fusion between fibers. Therefore, it is preferable to use multiple warm water baths and increase the warm water temperature stepwise. The draw ratio in warm water is preferably 5.5 to 8.0 times, more preferably 6.0 to 8.0 times. The higher the draw ratio, the more likely it is that a polyacrylonitrile precursor fiber bundle with fewer voids, which is suitable for producing the carbon fiber bundle of the present invention, can be obtained. If the draw ratio is 8.0 times or less, breakage of the filaments due to drawing can be suppressed, and a polyacrylonitrile precursor fiber bundle with good quality can be stably produced.

[0046] The above-mentioned method for producing a polyacrylonitrile precursor fiber bundle is a method for producing a polyacrylonitrile precursor fiber bundle having a kinematic viscosity of 6,000 to 20,000 mm at 25°C. ２ The method further comprises a step of applying an oil containing a silicone having a kinematic viscosity of 10,000 to 20,000 mm / sec. ２ / sec, more preferably 15,000 to 18,000 mm ２The kinematic viscosity of the silicone at 25°C is 6,000 mm ２ If the kinematic viscosity of the silicone at 25°C is 20,000 mm / sec or more, when an oil is applied to a fiber bundle with few voids that has been stretched 5.0 to 8.0 times in hot water at 30 to 99°C, fusion between the fibers can be suppressed and penetration of the oil into the voids can be effectively suppressed. ２ / sec or less, uneven adhesion can be suppressed, and stable strand strength can be exhibited in the obtained carbon fiber bundle. The kinematic viscosity at 25°C can be measured in accordance with JIS-Z-8803 (2011) or ASTM D 445-46T, for example, using an Ubbelohde viscometer.

[0047] In the above-mentioned method for producing a polyacrylonitrile precursor fiber bundle, the silicone used is preferably an amino-modified silicone from the viewpoint of uniform adhesion. Amino-modified silicones have a basic structure of polydimethylsiloxane, in which some of the methyl groups in the side chains have been modified with amino groups. Those to which another modifying group has been added in addition to the amino group can also be used. The amino group as the modifying group may be either a monoamine type or a polyamine type, but from the viewpoint of promoting crosslinking, a polyamine type is preferred, and among these, a diamine type is even more preferably used.

[0048] The amino group (NH ２ The amino equivalent, which is an indicator of the amount of crosslinking, is preferably 1,000 to 14,000 g / mol, more preferably 1,500 to 6,000 g / mol, and even more preferably 2,000 to 4,000 g / mol. If the amino equivalent is 1,000 g / mol or more, uneven adhesion due to excessive crosslinking can be suppressed, and stable strand strength can be achieved in the obtained carbon fiber bundle. If the amino equivalent is 14,000 g / mol or less, the silicone can be sufficiently crosslinked, and stable strand strength can be achieved in the obtained carbon fiber bundle. The amino equivalent can be measured by a known method such as neutralization titration. The amino equivalent can be controlled by, for example, the amount of amine added when polymerizing the amino-modified silicone.

[0049] In the above-mentioned method for producing a polyacrylonitrile precursor fiber bundle, the oil used has a kinematic viscosity at 25°C of 6,000 to 20,000 mm ２ In addition to the silicone, surfactants, antioxidants, antistatic agents, smoothing agents, etc. may also be included.

[0050] In the above-mentioned method for producing a polyacrylonitrile precursor fiber bundle, it is preferable to carry out a dry heat treatment by a known method after wet spinning, stretching in warm water, and application of an oil agent. By carrying out the dry heat treatment, it is possible to promote densification of voids, which is preferable. The dry heat treatment temperature is preferably 120 to 180°C.

[0051] In the above-described method for producing a polyacrylonitrile precursor fiber bundle, the dry-heat-treated yarn can be further drawn in pressurized steam or under dry heat, but a total draw ratio of 5.0 to 8.0 is preferred. Here, the total draw ratio is the product of the draw ratio in warm water and the draw ratio after dry heat treatment. By setting the draw ratio in warm water to 5.0 or more, a polyacrylonitrile precursor fiber bundle with few voids suitable for producing the carbon fiber bundle of the present invention can be obtained. By setting the total draw ratio after dry heat treatment to 8.0 or less, breakage of the yarn due to drawing can be suppressed, and a polyacrylonitrile precursor fiber bundle with high quality can be stably produced. It is more preferred that the draw ratio in warm water and the total draw ratio are equal, i.e., no drawing is performed after dry heat treatment. When a fiber bundle with 48,000 or more filaments is drawn under pressurized steam or dry heat, temperature unevenness is likely to occur within the bundle, so drawing is preferably performed only in warm water.

[0052] The single fiber fineness of the polyacrylonitrile precursor fiber bundle in the carbon fiber bundle production process is preferably 1.10 to 2.40 dtex, more preferably 1.20 to 2.20 dtex. The single fiber fineness is the mass per unit length of a single fiber. If the single fiber fineness is 1.10 dtex or more, a carbon fiber bundle can be obtained with sufficiently high productivity, and if the single fiber fineness is 2.40 dtex or less, treatment unevenness in the heat treatment after the flame-proofing step is reduced, and a carbon fiber bundle with high mechanical properties can be obtained. The single fiber fineness can be evaluated by measuring the mass per unit length. The single fiber fineness can be controlled by the discharge rate and draw ratio in the spinning step.

[0053] The polyacrylonitrile precursor fiber bundle in the process of producing carbon fiber bundles preferably has a circularity of the cross section of a single fiber of 0.86 to 0.98, more preferably 0.87 to 0.96, and even more preferably 0.87 to 0.93. The circularity of the cross section of a single fiber is defined as follows from the circumferential length L and area A of the cross section of the single fiber: (Circularity) = 4πA / L ２ .

[0054] If the circularity of the single fiber cross section is 0.86 to 0.98, the resulting carbon fiber can have both good bundling ability and abrasion resistance, which is preferable from the viewpoint of improving the operability of the resulting carbon fiber bundle during advanced processing. The circularity of the single fiber cross section of such a polyacrylonitrile-based precursor fiber bundle can be evaluated from an image of a cross section obtained by cutting the single fiber vertically using the method described below. The circularity of the single fiber cross section of such a polyacrylonitrile-based precursor fiber bundle can be controlled by the shape of the nozzle hole of the spinneret in the spinning process and the conditions of the coagulation process.

[0055] In the process of producing carbon fiber bundles, the obtained polyacrylonitrile precursor fiber bundle is oxidized in an oxidizing atmosphere at 200 to 300°C until the density of the flame-resistant fiber bundle becomes 1.21 to 1.23 g / cm ３ It is preferable to carry out the flame retardant treatment while controlling the silicon content to 0.06 to 0.09 mass % until the above-mentioned temperature is reached.

[0056] In the above-mentioned method for producing a carbon fiber bundle, if the temperature at which the polyacrylonitrile precursor fiber bundle is heat-treated in an oxidizing atmosphere is 200°C or higher, a flame-resistant fiber bundle having sufficient flame resistance can be produced, thereby suppressing the generation of fluff due to insufficient flame resistance and providing excellent operability during advanced processing of the obtained carbon fiber bundle. If the temperature for the flame-resistant treatment is 300°C or lower, the heat generation rate does not become excessively high, thereby reducing temperature unevenness in the flame-resistant fiber bundle and providing a carbon fiber bundle having excellent mechanical properties.

[0057] The temperature of such flame-proofing treatment can be determined by inserting a thermometer such as a thermocouple into the flame-proofing furnace and measuring the temperature inside the furnace. If there is a temperature unevenness or temperature distribution when measuring the temperature at several points inside the furnace, a simple average temperature is calculated. The temperature of such flame-proofing treatment can be controlled by the heating output in a heating method used in a known flame-proofing furnace. For example, in the case of a hot air circulation type flame-proofing furnace, the output of the heater used to heat the oxidizing atmosphere can be changed.

[0058] In the above-mentioned method for producing a carbon fiber bundle, the density of the flame-resistant fiber bundle is 1.21 to 1.23 g / cm ３ The silicon content is more preferably 0.07 to 0.08 mass %. The density of the polyacrylonitrile precursor fiber bundle is generally 1.14 to 1.18 g / cm ３ After the flame retardant treatment, the density was 1.30 g / cm ３ The density specified in the present invention is generally 1.21 to 1.23 g / cm ３ means the region at the beginning of the flame-resistant treatment. In the early stage of the flame-resistant treatment, the polyacrylonitrile precursor fiber bundle is treated at a high temperature of 200°C or higher in a state where the flame-resistant structure is not yet developed, so that fusion between the single fibers is likely to occur and the remaining voids are likely to become dense and disappear. In the polyacrylonitrile precursor fiber having fibrils on the fiber surface, the density of the flame-resistant fiber bundle is 1.21 to 1.23 g / cm ３ If the silicon content is 0.06 mass% or more until the density of the flame-resistant fiber bundle reaches 1.21 to 1.23 g / cm, fusion between single fibers at the initial stage of flame-resistant treatment can be suppressed, and fibril-like products due to fusion and peeling of the obtained carbon fibers can be suppressed. ３If the silicon content until the temperature reaches 0.09% by mass or less, the densification of voids in the early stage of flame retardation is less likely to be hindered, and voids in the resulting carbon fiber can be suppressed.

[0059] The density of the fiber bundle is 1.21 to 1.23 g / cm ３ The silicon content until the density reaches 1.22±0.01 g / cm can be measured as follows. A flame-resistant fiber bundle at the initial stage of flame-resistant treatment is sampled from a flame-resistant furnace in which polyacrylonitrile precursor fibers are continuously flame-resistant treated. The flame-resistant fiber bundle is cut every 1 m based on the entrance of the flame-resistant furnace, and the density and silicon content are measured by the method described below. When the density of the flame-resistant fiber bundle reaches 1.22±0.01 g / cm ３ The silicon content of the area is determined so that the density of the flame-resistant fiber bundle is 1.21 to 1.23 g / cm ３ The amount of silicon is defined as the amount of silicon that is added until the

[0060] The density of the flame-resistant fiber bundle is 1.21 to 1.23 g / cm ３ The amount of silicon required to reach this value can be controlled by the amount of oil applied to the polyacrylonitrile precursor fiber bundle and the treatment temperature at the initial stage of flame retardation. The higher the treatment temperature at the initial stage of flame retardation, the higher the density of the flame retarded fiber bundle becomes, and the higher the density of the flame retarded fiber bundle becomes. ３ By applying a sufficient amount of oil to the polyacrylonitrile precursor fiber bundle so that the oil can be uniformly applied, and then appropriately controlling the initial temperature for flame retardation, the density of the flame retarded fiber bundle can be reduced to 1.21 to 1.23 g / cm. ３ It is important to control the amount of silicon within an appropriate range until the silicon content reaches 100%.

[0061] Following the polyacrylonitrile precursor fiber bundle production step and the flame-proofing step, preliminary carbonization is carried out. In the preliminary carbonization step, the obtained flame-proof fiber bundle is carbonized in an inert atmosphere at a maximum temperature of 500 to 1,200°C, preferably to a density of 1.5 to 1.8 g / cm. ３ The draw ratio in the pre-carbonization step is preferably 1.00 to 1.30, and more preferably 1.10 to 1.25.

[0062] Following the pre-carbonization, carbonization is carried out. In the carbonization step, the pre-carbonized fiber bundle is carbonized in an inert atmosphere at a maximum temperature of 900 to 2,000° C. The draw ratio in the carbonization step is preferably 0.94 to 1.05, and more preferably 0.96 to 1.02.

[0063] The carbon fiber bundles obtained as described above are preferably subjected to an oxidation treatment to introduce oxygen-containing functional groups in order to improve adhesion to the matrix resin. Gas-phase oxidation, liquid-phase oxidation, and liquid-phase electrolytic oxidation are used as oxidation treatment methods, but liquid-phase electrolytic oxidation is preferably used from the viewpoints of high productivity and enabling uniform treatment. The method of liquid-phase electrolytic oxidation is not particularly specified, and any known method may be used.

[0064] After the electrolytic treatment, the resulting carbon fiber bundle may be subjected to a sizing treatment to impart bundling properties to the resulting carbon fiber bundle. As the sizing agent, a sizing agent having good compatibility with the matrix resin used in the composite material can be appropriately selected depending on the type of matrix resin used.

[0065] <Tensile test of resin-impregnated strand of carbon fiber bundle> The tensile strength (strand strength) and stress-strain curve of the resin-impregnated strand of carbon fiber bundle are determined in accordance with JIS R7608 (2008) "Test method for resin-impregnated strand." Test specimens are prepared by impregnating a carbon fiber bundle with the following resin composition and subjecting it to curing conditions of heat treatment at a temperature of 130°C for 35 minutes.

[0066] [Resin composition] 3,4-epoxycyclohexylmethyl-3,4-epoxy-cyclohexane-carboxylate (100 parts by mass) Boron trifluoride monoethylamine (3 parts by mass) Acetone (4 parts by mass) The number of strands measured was six, and the arithmetic mean value of the measurement results was taken as the strand strength of the carbon fiber.

[0067] <Measurement of circularity of single fiber cross section of polyacrylonitrile precursor fiber bundle> A polyacrylonitrile precursor fiber bundle is cut perpendicular to the fiber axis direction with a single-edged razor, and the obtained cross section is observed from the perpendicular direction to the fiber cross section using a scanning electron microscope (SEM) "S-4800" manufactured by Hitachi High-Technologies Corporation. The outer periphery of the fiber cross section is selected from the acquired image using image analysis software "ImageJ", and the circularity is calculated from the calculated perimeter and area of ​​the fiber cross section according to the following definition. Furthermore, the circularity is measured for five single fibers randomly in each field of view, and the average of the circularities for a total of 25 single fibers is taken as the circularity of the single fiber cross section of the polyacrylonitrile precursor fiber bundle. The circularity of the single fiber cross section is defined as follows from the perimeter L and area A of the single fiber cross section: (circularity) = 4πA / L ２ .

[0068] <Proportion of Fibers with Fracture Surfaces Present with Fibril-Like Matter and Aspect Ratios of 3.0 to 10.0 and Proportion of Fibers with Fracture Surfaces Present with Voids of 100 nm or Larger> Tensile tests were conducted on carbon fiber single fibers in accordance with JIS R7606 (2000). The test length was 50 mm, and a commercially available cyanoacrylate adhesive was used to secure the carbon fibers to the test specimen mount. The tensile tests were conducted using a special test jig designed for underwater testing, using a tensile tester (in the examples of the present invention, a Tensilon "RTC-1210A" manufactured by A&D). 150 single fibers were randomly selected from the fiber bundle for testing. Tensile tests were conducted on all 150 selected single fibers at a strain rate of 0.4 mm / min, and both the broken and broken single fibers were collected.

[0069] The fracture surface of the recovered single fiber is observed using a field emission scanning electron microscope (in the examples of the present invention, an "S-4800" manufactured by Hitachi High-Tech Corporation). In order to observe minute defects with high precision, no vapor deposition treatment is performed to impart conductivity, which can cause surface irregularities, and observation is performed at an acceleration voltage of 1 keV and a magnification of 25,000 to 50,000 times. In addition, to make it easier to determine the presence or absence of minute defects, the stage is rotated so that the fracture origin faces forward, and the stage is tilted 30° so that the fracture surface is observed from diagonally above. For example, this is the direction shown in Figures 2 and 3. Note that observation is performed on all recovered fibers.

[0070] On the primary fracture surface of carbon fiber due to tensile fracture, radial striations remain as traces of fracture progression from the fracture initiation point. Therefore, as shown in Figure 1, the striations present in the SEM observation image are traced and the point where they converge is taken as the fracture initiation point. If no striations are discernible, or if striations are discernible but are obscured by dirt near the fracture initiation point, the pair of fracture surfaces is excluded from the evaluation. The number of pairs of fracture surfaces that can be finally observed is taken as the total number of fracture surfaces. If the total number of fracture surfaces does not exceed 100 pairs, 150 single fibers are randomly selected from the fiber bundle again, and the tensile test and fracture surface observation are repeated. Once a total number of fracture surfaces exceeding 100 pairs is confirmed, the presence of fibrils or voids at the fracture initiation point is determined.

[0071] The fibril-like material is observed as an elongated deposit as shown in Figure 2. The aspect ratio of the deposit is calculated as follows: The circumscribing circle and the inscribing circle with the largest diameter are determined as shown in Figures 4 to 6. The diameter of each circle is defined as D. Ａ and D Ｂ The aspect ratio is D Ａ D ＢThe calculation is performed by dividing the aspect ratio by the total number of fracture surfaces. Those with an aspect ratio of 3.0 to 10.0 are determined to be fibril-like objects with an aspect ratio of 3.0 to 10.0. When fibril-like objects with an aspect ratio of 3.0 to 10.0 are present on one or both of a pair of fracture surfaces, it is determined that fibril-like objects are present, and the number of such fibril-like objects is divided by the total number of fracture surfaces to determine the proportion of fibers having fracture surfaces with fibril-like objects with an aspect ratio of 3.0 to 10.0.

[0072] Voids are observed as holes as shown in Figure 3. The diameter of the circumscribing circle of the void is taken as the size of the void, and a void with a size of 100 nm or more is judged to be a void of 100 nm or more. If the above-mentioned void exists on one or both of a pair of fracture surfaces, it is considered that a void exists, and the number of voids is divided by the total number of fracture surfaces to determine the percentage of fibers having fracture surfaces with voids of 100 nm or more.

[0073] The aspect ratio of the fibrils and the size of the voids are determined by loading the image into image analysis software and quantifying it using a ruler tool.

[0074] <Density of Flame-Resistant Fiber Bundle> A flame-resistant fiber bundle taken from the flame-resistant furnace at the beginning of the flame-resistant treatment is sampled from the center every 1 m so that the sample weighs 1 to 3 g, and is then bone-dried at 120° C. for 2 hours. Next, the bone-dry mass D (g) is measured, and the bundle is then immersed in ethanol and thoroughly degassed. The mass E (g) of the fiber bundle in the ethanol solvent bath is then measured, and the density is calculated by density = (D × ρ) / (D - E), where ρ is the density of ethanol at the measurement temperature.

[0075] <Silicon content of flame-resistant fiber bundle> A flame-resistant fiber bundle was taken from the flame-resistant furnace every 1 m at the beginning of the flame-resistant treatment, and a sample was taken from the center. The sample was placed on a "Teflon" (registered trademark) plate with a thickness of 1 cm. ２ The silicon content is then quantified using an X-ray fluorescence analyzer. Rather than using the X-ray fluorescence measurement value as is, a calibration curve is created using standard materials with known silicon content, and the X-ray fluorescence measurement value is converted into silicon content.

[0076] <Evaluation of advanced processability> A bobbin of carbon fiber bundle is placed on a creel, unwound at a tension of 1.6 mN / dtex, passed through 10 free rollers, rubbed against 5 fixed guides, taken up by a drive roller at a speed of 10 m / min, and wound on a winder. The number of fluffs generated at this time is counted for 10 minutes just before the drive roller, and evaluated using the following index: A: Less than 10 fluffs / m B: 10 fluffs / m or more but less than 50 fluffs / m C: 50 fluffs / m or more

[0077] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0078] (Example 1) A polyacrylonitrile copolymer consisting of acrylonitrile, itaconic acid, and methyl acrylate was polymerized by solution polymerization using dimethyl sulfoxide as a solvent to obtain a spinning dope. The obtained spinning dope was introduced into a coagulation bath consisting of an aqueous solution of dimethyl sulfoxide through a spinning nozzle having 50,000 holes, and a fiber bundle was coagulated by a wet spinning method. This fiber bundle was subjected to solvent washing in multiple hot water baths at 70 to 99°C and stretched 7.0 times. Subsequently, the fiber bundle after hot water stretching was subjected to a sieving process in which a kinetic viscosity at 25°C of 15,000 mm was obtained. ２ The resulting polyacrylonitrile precursor fiber bundle was heat-treated at 220 to 250°C until the density of the flame-resistant fiber bundle reached 1.21 to 1.23 g / cm. ３ The fiber bundle was subjected to a flame retardation treatment while controlling the silicon content to 0.075% by mass until the temperature reached 100° C., and then to a pre-carbonization treatment at a maximum temperature of 800° C., followed by a carbonization treatment at a maximum temperature of 1,400° C. to obtain a carbon fiber bundle. The properties of the obtained carbon fiber bundle are shown in Table 1.

[0079] (Example 2) The density of the flame-resistant fiber bundle is 1.21 to 1.23 g / cm ３ A carbon fiber bundle was obtained in the same manner as in Example 1, except that the amount of silicon was changed to 0.088 mass % until the carbon fiber bundle reached 0.088 mass %. The properties of the obtained carbon fiber bundle are shown in Table 1.

[0080] Example 3 A carbon fiber bundle was obtained in the same manner as in Example 1, except that the dry heat treatment was carried out and the fiber was further stretched 1.2 times with a heated roll at 180° C., for a total stretch ratio of 8.4 times.

[0081] (Example 4) In the spinning process, the fiber was stretched 6.0 times in multiple hot water baths at 70 to 99°C, and in the flame-proofing process, the density of the flame-proofed fiber bundle was 1.21 to 1.23 g / cm ３ A carbon fiber bundle was obtained in the same manner as in Example 1, except that the amount of silicon was changed to 0.085% by mass until the carbon fiber bundle reached 0.085% by mass. The properties of the obtained carbon fiber bundle are shown in Table 1.

[0082] (Example 5) A carbon fiber bundle was obtained in the same manner as in Example 1, except that in the spinning process, the fiber was drawn 5.0 times in multiple hot water baths at 70 to 99°C and then further drawn 1.6 times with a heated roll at 180°C, for a total draw ratio of 8.0 times. The properties of the obtained carbon fiber bundle are shown in Table 1.

[0083] (Example 6) In the spinning process, the fiber was stretched 6.5 times in multiple hot water baths at 70 to 99°C, and in the flame-proofing process, the density of the flame-proofed fiber bundle was 1.21 to 1.23 g / cm ３ A carbon fiber bundle was obtained in the same manner as in Example 1, except that the amount of silicon was changed to 0.085% by mass until the carbon fiber bundle reached 0.085% by mass. The properties of the obtained carbon fiber bundle are shown in Table 1.

[0084] (Comparative Example 1) A carbon fiber bundle was obtained in the same manner as in Example 1, except that the draw ratio in warm water was set to 2.1 times. The properties of the obtained carbon fiber bundle are shown in Table 1. The proportion of fibers having fracture cross sections with voids of 100 nm or more present was high, and the bundle had poor advanced processability.

[0085] (Comparative Example 2) The same procedure as in Example 1 was carried out except that the draw ratio in warm water was set to 9.0 times. However, winding and yarn breakage frequently occurred during the warm water drawing step, and polyacrylonitrile-based precursor fiber could not be obtained.

[0086] (Comparative Example 3) The kinematic viscosity of the amino-modified silicone at 25°C was 1,500 mm ２ / sec, and the density of the flame-resistant fiber bundle is 1.21 to 1.23 g / cm ３A carbon fiber bundle was obtained in the same manner as in Example 1, except that the amount of silicon until the silicon content reached 0.160% by mass was set to 0.160% by mass. The properties of the obtained carbon fiber bundle are shown in Table 1. The proportion of fracture surfaces on which fibril-like matter having an aspect ratio of 3.0 to 10.0 was present and the proportion of the number of fibers on which voids of 100 nm or more were present on fracture surfaces were high, and the carbon fiber bundle was poor in advanced processability.

[0087] (Comparative Example 4) The density of the flame-resistant fiber bundle is 1.21 to 1.23 g / cm ３ A carbon fiber bundle was obtained in the same manner as in Comparative Example 3, except that the amount of silicon until the silicon content reached 0.081% by mass was set to 0.081% by mass. The properties of the obtained carbon fiber bundle are shown in Table 1. The proportion of fracture surfaces on which fibril-like matter having an aspect ratio of 3.0 to 10.0 was present and the proportion of the number of fibers on which voids of 100 nm or more were present on fracture surfaces were high, and the carbon fiber bundle was poor in advanced processability.

[0088] (Comparative Example 5) The kinematic viscosity of the amino-modified silicone at 25°C was 22,000 mm ２ When the same procedure as in Example 1 was carried out except that the drying time was changed to / sec, winding and yarn breakage frequently occurred during the dry heat treatment step, and polyacrylonitrile precursor fibers could not be obtained.

[0089] (Comparative Example 6) By changing the flameproofing temperature to 230 to 250°C, the density of the flameproofed fiber bundle was 1.21 to 1.23 g / cm ３ A carbon fiber bundle was obtained in the same manner as in Example 1, except that the amount of silicon until the silicon content reached 0.053% by mass was set to 0.053% by mass. The properties of the obtained carbon fiber bundle are shown in Table 1. The proportion of the number of fibers having a fracture surface on which fibril-like matter with an aspect ratio of 3.0 to 10.0 was present was high, and the carbon fiber bundle was poor in advanced processability.

[0090] (Comparative Example 7) The amount of oil applied to the polyacrylonitrile precursor fiber was changed, and the density of the flame-resistant fiber bundle was 1.21 to 1.23 g / cm ３ When the same procedure as in Example 1 was carried out except that the amount of silicon until the carbon fiber bundle reached 0.120 mass % was used, winding and fiber breakage frequently occurred in the flame-proofing step, and a carbon fiber bundle could not be obtained.

[0091] (Comparative Example 8) A polyacrylonitrile copolymer consisting of acrylonitrile, itaconic acid, and methyl acrylate was polymerized by solution polymerization using dimethyl sulfoxide as a solvent to obtain a spinning dope. The obtained spinning dope was passed through a spinning nozzle having 3,000 holes, passed through air once, and then introduced into a coagulation bath consisting of an aqueous solution of dimethyl sulfoxide to obtain a fiber bundle by a dry-wet spinning method. This fiber bundle was subjected to solvent washing in multiple hot water baths at 40 to 70°C and stretched 3.5 times. Subsequently, the fiber bundle after hot water stretching was subjected to a sieve having a kinematic viscosity of 15,000 mm at 25°C. ２ The resulting fiber bundle was then subjected to a drying heat treatment using a heated roller at 150°C, followed by drawing 3.7 times in steam to obtain a polyacrylonitrile precursor fiber bundle having 3,000 filaments and a single fiber fineness of 1.40 dtex. Four of the obtained polyacrylonitrile precursor fiber bundles were then combined to obtain a single fiber count of 12,000. The resulting flame-resistant fiber bundle was then heated at 220 to 250°C to a density of 1.21 to 1.23 g / cm. ３ The carbon fiber bundle was subjected to a flame retardation treatment while controlling the silicon content to 0.075% by mass until the temperature reached 100°C, followed by a pre-carbonization treatment at a maximum temperature of 800°C, and a carbonization treatment at a maximum temperature of 1,400°C to obtain a carbon fiber bundle. The properties of the obtained carbon fiber bundle are shown in Table 1. The proportion of fibers having fracture surfaces on which fibril-like matter with an aspect ratio of 3.0 to 10.0 was present was high, and the carbon fiber bundle had poor high-order processability.

[0092]

Claims

1. There are fibrils along the fiber axis direction on the fiber surface, and at the fracture origin of the fiber fracture surface subjected to a single fiber tensile test with a test length of 50 mm according to JIS R7606 (2000), the proportion of the number of fibers having a fracture surface with fibrillar substances having an aspect ratio of 3.0 to 10.0 is 1 to 20%, and the proportion of the number of fibers having a fracture surface with voids of 100 nm or more is 1 to 14%. A carbon fiber bundle, wherein the number of filaments is 48,000 to 60,000.

2. There are fibrils along the fiber axis direction on the fiber surface, and at the fracture origin of the fiber fracture surface subjected to a single fiber tensile test with a test length of 50 mm according to JIS R7606 (2000), the proportion of the number of fibers having a fracture surface with fibrillar substances having an aspect ratio of 3.0 to 10.0 is A (%), and the proportion of the number of fibers having a fracture surface with voids of 100 nm or more is B (%). The carbon fiber bundle according to Claim 1, wherein A and B satisfy the relationship of the following formula (1). B ≤ -0.7A + 20.5 (1)

3. There are fibrils along the fiber axis direction on the fiber surface, and at the fracture origin of the fiber fracture surface subjected to a single fiber tensile test with a test length of 50 mm according to JIS R7606 (2000), the proportion of the number of fibers having a fracture surface with fibrillar substances having an aspect ratio of 3.0 to 10.0 is 1 to 15%, and the proportion of the number of fibers having a fracture surface with voids of 100 nm or more is 1 to 10%. The carbon fiber bundle according to Claim 1.

4. The carbon fiber bundle according to any one of Claims 1 to 3, wherein the width of the fibril is 100 to 600 nm.

5. The carbon fiber bundle according to any one of Claims 1 to 3, wherein the strand strength is 4.5 to 6.0 GPa.