Carbon fiber tows, prepregs, and carbon fiber reinforced composites

Abstract

The present invention provides a carbon fiber bundle having excellent strand tensile strength and strand tensile modulus of elasticity and single fiber compressive strength, and a prepreg and a carbon fiber reinforced composite material using the same. The carbon fiber bundle of the present invention is a carbon fiber bundle having a strand tensile strength of 4.5 GPa or more and 6.5 GPa or less, a strand tensile modulus of elasticity of 400 GPa or more, a crystallite size of 4.0 nm or more and 4.7 nm or less, a single fiber diameter of 5.2 μm or more and 6.2 μm or less, and substantially no twist.

Description

Technical Field

[0001] The present invention relates to carbon fiber bundles suitable for use in sports applications such as golf clubs and fishing rods, as well as other general industrial applications, prepreg blanks obtained using the carbon fiber bundles, and carbon fiber reinforced composite materials. Background Technology

[0002] Carbon fiber bundles, due to their extremely high specific strength and specific modulus of elasticity, have been widely used as reinforcing fibers in various industrial fields in recent years as fiber-reinforced composite materials. Especially in areas where lightweighting is emphasized, the replacement of traditional metal materials with carbon fiber composites has accelerated. In sports applications, in addition to golf clubs, fishing rods, and bicycles, rackets and other items also require lightweight construction, and their applications are expanding rapidly.

[0003] Regarding lightweight applications in sports, there is a demand for further improvement in the mechanical properties of carbon fiber bundles, focusing on the tensile modulus of elasticity of resin-impregnated fiber bundles (hereinafter sometimes simply referred to as bundle modulus of elasticity). Furthermore, there is a need for excellent balance in a wide range of physical properties, including improved tensile and compressive strength as a carbon fiber reinforced composite material. For the most widely used polyacrylonitrile-based carbon fiber bundles, industrial manufacturing involves the following processes: a flame-retardant process to convert polyacrylonitrile precursor fiber bundles into flame-retardant fiber bundles under an oxidizing atmosphere at 200–300°C; a pre-carbonization process to pre-carbonize in an inert atmosphere at a maximum temperature of 500–1000°C; and a carbonization process to carbonize in an inert atmosphere at a maximum temperature of 1200–3000°C. Regarding the bundle modulus of elasticity of carbon fiber bundles, it is known that the higher the maximum temperature in the carbonization process, the higher the bundle modulus of elasticity of the carbon fiber bundle. However, it is generally believed that increasing the maximum temperature of the carbonization process results in a higher elastic modulus of carbon fiber bundles, leading to larger crystallite sizes and a decrease in the compressive strength of the carbon fiber reinforced composite material (Patent Document 1). The compressive strength and elastic modulus of the carbon fiber reinforced composite material are generally in a trade-off relationship. Therefore, research has been conducted on increasing the elastic modulus of the carbon fiber bundle without increasing the crystallite size. For example, as a technique to reduce the crystallite size of the carbon fiber bundle, a technique has been proposed to inject ions into the surface of the carbon fiber to reduce the crystallinity of the surface layer and increase the compressive strength of the single fiber (Patent Document 1). Furthermore, as a technique to increase the elastic modulus of the carbon fiber bundle without increasing the crystallite size, techniques to increase the draw ratio in the carbonization process are known. To increase the draw ratio in the carbonization process without reducing processability, techniques have been proposed to increase the tensile strength of the precursor fiber bundle by applying interlacing and twisting (Patent Documents 2-6). Additionally, to increase the compressive strength of the carbon fiber reinforced composite material without relying on stretching, a technique has been proposed to increase the compressive strength of the single filament of the carbon fiber bundle by controlling the flame-retardant structure (Patent Document 7).

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 3-180514

[0007] Patent Document 2: Japanese Patent Application Publication No. 2014-141761

[0008] Patent Document 3: Japanese Patent Application Publication No. 2014-159665

[0009] Patent Document 4: International Publication No. 2019 / 244830

[0010] Patent Document 5: International Publication No. 2008 / 047745

[0011] Patent Document 6: Japanese Patent Application Publication No. 2008-308776

[0012] Patent Document 7: Japanese Patent Application Publication No. 2015-10290 Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] However, as shown in Patent Document 1, by injecting ions into the carbon fibers, the crystallite size is reduced, and the apparent compressive strength measured by the coil method increases to a maximum of 10.0 GPa. However, this is not satisfactory from the viewpoint of balancing with the tensile modulus of elasticity of the bundle (hereinafter sometimes simply referred to as the bundle modulus of elasticity). Furthermore, as shown in Patent Documents 2-6, by applying interlacing and twisting, even under high tension during the carbonization process, breakage does not occur, and the bundle modulus of elasticity is increased. However, this does not meet the requirements for single-fiber compressive strength and a low number of fluff defects during prepreg processing. Additionally, according to the technology in Patent Document 7, the flame-retardant structure can be controlled to improve single-fiber compressive strength, but the bundle modulus of elasticity is low. When simply increasing the maximum temperature of the carbonization process based on the technology in Patent Document 7, the single-fiber compressive strength decreases significantly, making it impossible to simultaneously exhibit high levels of resin-impregnated bundle tensile strength, bundle modulus of elasticity, and single-fiber compressive strength. To solve the above problems, the object of the present invention is to obtain carbon fiber bundles with good quality and simultaneously excellent bundle strength, bundle modulus of elasticity, and compressive strength.

[0015] Methods for solving problems

[0016] The inventors of this application discovered the following method and completed the present invention, namely, while increasing the fineness of the carbon fiber precursor fiber to improve productivity, by controlling the stretching in the pre-carbonization process and controlling the stretching and heating rate and maximum temperature in the carbonization process, the single fiber diameter of the carbon fiber is increased and the microcrystal size is reduced, thereby obtaining a carbon fiber bundle that can be achieved at a level that is impossible to reach with previous carbon fiber bundles, while taking into account both improved mechanical properties and maintained quality.

[0017] To achieve the above objectives, the carbon fiber bundles of the present invention have the following characteristics.

[0018] (1) A carbon fiber bundle, wherein the bundle tensile strength is above 4.5 GPa and below 6.5 GPa, the bundle tensile elastic modulus is above 400 GPa, the crystallite size is above 4.0 nm and below 4.7 nm, the single fiber diameter is above 5.2 μm and below 6.2 μm, and the carbon fiber bundle is substantially untwisted.

[0019] (2) The number of unwound fibers in the carbon fiber bundle according to (1) is less than 0.3 fibers / m.

[0020] (3) The carbon fiber bundle according to (1) or (2) has a number of unwound fibers of 6 or less per 100m.

[0021] (4) The carbon fiber bundle according to any one of (1) to (3) has a density of 1.84 g / cm³. 3 the following.

[0022] (5) The carbon fiber bundle according to any one of (1) to (4) has a single fiber diameter of 5.5 μm or more and 6.2 μm or less.

[0023] (6) The carbon fiber bundle according to (1) or (2) has a tensile strength of 4.8 GPa or more and 6.5 GPa or less.

[0024] (7) The carbon fiber bundle according to (1) or (2) has a tensile strength of 5.0 GPa or more and 6.5 GPa or less.

[0025] (8) The carbon fiber bundle according to (1) or (2) has a tensile strength of 5.5 GPa or more and 6.5 GPa or less.

[0026] (9) A prepreg blank, which is a prepreg blank made by impregnating a carbon fiber bundle as described in any one of (1) to (8) with a thermosetting resin, wherein the number of fluff defects in the prepreg blank is 7 per 100m. 2 the following.

[0027] (10) A prepreg blank, which is a prepreg blank made by impregnating a carbon fiber bundle of any one of (1) to (9) with a thermosetting resin, wherein the elastic modulus of the cured thermosetting resin is 3.0 GPa or more.

[0028] (11) A prepreg blank, which is a prepreg blank made by impregnating a carbon fiber bundle of any one of (1) to (10) with a thermosetting resin, wherein the elastic modulus of the cured thermosetting resin is 3.8 GPa or more and 5.5 GPa or less.

[0029] (12) A carbon fiber reinforced composite material comprising any one of (1) to (11) a carbon fiber bundle and a matrix resin.

[0030] (13) A carbon fiber reinforced composite material comprising any one of (1) to (12) a carbon fiber bundle and a matrix resin,

[0031] Among them, the compressive strength of the composite material at 0° is above 1200MPa and below 1350MPa, and the tensile modulus of elasticity of the composite material at 0° is above 245GPa and below 270GPa.

[0032] Invention Effects

[0033] According to the present invention, it is possible to achieve the effect of maintaining good quality while exhibiting the bundle strength, bundle elastic modulus, and compressive strength of carbon fiber bundles. Detailed Implementation

[0034] The tensile strength of the carbon fiber bundles of the present invention is 4.5 GPa or higher and 6.5 GPa or lower. When the tensile strength of the carbon fiber bundles is 4.5 GPa or higher, a good balance is achieved with the high single-fiber compressive strength of the carbon fiber bundles of the present invention, making it easy to maintain sufficient compressive strength when producing carbon fiber reinforced composites. The higher the tensile strength of the bundles, the better, but from the viewpoint of productivity, 6.5 GPa or lower is preferred. The bundle strength is a value evaluated using the bundle tensile test described in the examples. In order to keep the bundle tensile strength within the above range, it is important to perform flame retardant treatment on the polyacrylonitrile precursor fiber bundles while controlling the temperature, and to perform pre-carbonization and carbonization on the flame retardant fiber bundles while controlling the temperature and the draw ratio.

[0035] Furthermore, the wire harness strength is preferably between 4.8 GPa and 6.5 GPa. If the wire harness strength is 4.8 GPa or higher, the tensile strength of the composite material is further improved. The wire harness strength can be mainly adjusted by controlling the maximum carbonization temperature and the draw ratio of the carbonization process.

[0036] Furthermore, the aforementioned wire harness strength is more preferably 5.0 GPa or higher and 6.5 GPa or lower. If the aforementioned wire harness strength is 5.0 GPa or higher, the tensile strength of the composite material is further improved. The aforementioned wire harness strength can be mainly adjusted by controlling the maximum carbonization temperature, the heating temperature of the carbonization process, and the stretching ratio of the carbonization process.

[0037] Furthermore, the strength of the aforementioned wire bundle is particularly preferably 5.5 GPa or higher and 6.5 GPa or lower. If the wire bundle strength is 5.5 GPa or higher, the tensile strength of the composite material is particularly improved. The aforementioned wire bundle strength can be adjusted by controlling the carbonization process, primarily based on the density of the flame-retardant fiber bundles, and controlling the flame-retardant temperature and processing time according to the structure of the flame-retardant filaments so that the peak ratio in the infrared spectrum of each flame-retardant fiber bundle after the first and second flame-retardant processes is within a specific range. The control of these conditions will be explained in detail in the manufacturing method. The aforementioned wire bundle strength is further preferably 5.8 GPa or higher.

[0038] The tensile modulus of elasticity of the carbon fiber bundle of the present invention is 400 GPa or more, preferably 420 GPa or more, more preferably 430 GPa or more, and particularly preferably 440 GPa or more. A tensile modulus of elasticity of 400 GPa or more is preferred because it increases the tensile modulus of elasticity of the carbon fiber reinforced composite material. Furthermore, a higher tensile modulus is preferred, but from the viewpoint of compressive strength, it is preferable to adjust it to an upper limit of 500 GPa. Generally, when the tensile modulus of elasticity of the bundle increases in a tensile test, the compressive strength of a single fiber decreases; however, in the present invention, this is achieved even under this condition. To ensure that the tensile modulus of elasticity of the bundle is within the above-mentioned range, it is necessary to simultaneously increase the orientation degree of the polyacrylonitrile precursor fibers, increase the pre-carbonization treatment process described later, the draw ratio in the carbonization process, and the maximum temperature of the carbonization process, while simultaneously controlling these parameters.

[0039] The crystallite size of the carbon fiber bundle of the present invention is 4.0 nm or more, preferably 4.1 nm or more, and more preferably 4.2 nm or more. When the crystallite size is less than 4.0 nm, high stretching is required in the carbonization process to improve the orientation in order to meet the elastic modulus requirements, which sometimes degrades the fiber quality. If the crystallite size is 4.0 nm or more, the compressive strength of the carbon fiber bundle, the elastic modulus of the bundle in the tensile test, and the fiber fluff quality can be simultaneously satisfied. Furthermore, when the crystallite size is too large, the compressive strength of a single fiber sometimes decreases; therefore, the crystallite size is adjusted to be 4.7 nm or less, preferably 4.5 nm or less, and more preferably 4.4 nm or less. That is, the crystallite size is 4.0 nm or more and 4.7 nm or less, preferably 4.1 nm or more and 4.5 nm or less, and more preferably 4.2 nm or more and 4.4 nm or less.

[0040] Generally, higher crystallite size in carbon fibers tends to correlate with lower single-fiber compressive strength, but the carbon fiber bundles of this invention exhibit both high crystallite size and high single-fiber compressive strength. The crystallite size in this invention can be evaluated using the wide-angle X-ray diffraction method described in the embodiments. To control the crystallite size within the aforementioned range, it is necessary to increase the maximum temperature of the carbonization process and the draw ratio of the carbonization process, etc.

[0041] The carbon fiber bundles of the present invention have a single fiber diameter of 5.2 μm or more. The single fiber diameter of the carbon fiber bundle is evaluated by the method described in the embodiments. When the cross-sectional shape of the single fiber is not a perfect circle, an equivalent circular diameter is used instead. The equivalent circular diameter refers to the diameter of a circle having a cross-sectional area equal to the measured cross-sectional area of ​​the single fiber.

[0042] When manufacturing prepreg preforms, impregnation depends on the diameter of individual fibers; therefore, larger fiber diameters enable efficient manufacturing of composite materials. Furthermore, the breaking load per fiber depends on the bundle strength and cross-sectional area of ​​the fiber; thus, the fiber diameter affects the breaking load per fiber. Additionally, larger fiber diameters tend to result in less fuzzing due to friction during processing, thus affecting the product's quality.

[0043] If the diameter of a single fiber is 5.2 μm or more, the quality of carbon fiber bundles and carbon fiber composites produced is more likely to be good. The single fiber diameter is preferably 5.3 μm or more, and more preferably 5.5 μm or more. When the single fiber diameter becomes too large, the reaction within the single fiber becomes uneven during the firing process, sometimes resulting in a decrease in bundle strength and bundle elastic modulus. Therefore, the single fiber diameter is adjusted to be 6.2 μm or less. That is, the single fiber diameter is 5.2 μm or more and 6.2 μm or less, preferably 5.3 μm or more and 6.2 μm or less, and more preferably 5.5 μm or more and 6.2 μm or less.

[0044] The diameter of a single fiber can be adjusted by controlling the amount ejected from the spinneret during the spinning of the carbon fiber precursor fiber bundle, the draw ratio in each process, and other factors.

[0045] The carbon fiber bundles of the present invention are substantially untwisted. "Substantially untwisted" means that there is no twist at all, or even if there is twist, it is less than 0.5 twists per meter. When used as reinforcing fibers in carbon fiber reinforced composites, the untwisted carbon fiber bundles exhibit excellent scalability, resulting in superior physical properties and quality of the carbon fiber reinforced composites.

[0046] In this invention, the density of the carbon fiber bundle is preferably 1.84 g / cm³. 3 The lower the density, the higher the specific strength and specific modulus of elasticity, thus enabling the efficient production of carbon fiber composites. For example, if the density is 1.84 g / cm³...3 The following methods allow for the easy and efficient production of carbon fiber composite materials. A more preferred density is 1.83 g / cm³. 3 The following is a further preferred value: 1.82 g / cm³ 3 the following.

[0047] Regarding the carbon fiber bundle of the present invention, the number of unwound fibers is preferably 0.3 fibers / m or less, more preferably 0.2 fibers / m or less, and even more preferably 0.1 fibers / m or less. When the number of unwound fibers in the carbon fiber bundle increases, the processability of the carbon fiber reinforced composite material may decrease, and the compressive strength of the carbon fiber reinforced composite material may also decrease. If the number of unwound fibers is 0.3 fibers / m or less, the processability and compressive strength of the carbon fiber reinforced composite material are more likely to remain at satisfactory levels.

[0048] The number of unwound fibers was determined using the method described in the examples. In order to adjust the number of unwound fibers present in each 1m of carbon fiber bundle to within the above range, it is necessary to control the draw ratio in the pre-carbonization process described later, and at the same time control the draw ratio and carbonization temperature in the carbonization process.

[0049] Regarding the carbon fiber bundle of the present invention, the number of unwound fibers in the carbon fiber bundle is preferably 6 or less per 100m, more preferably 5 or less per 100m, and even more preferably 4 or less per 100m. When the number of unwound fibers in the carbon fiber bundle increases, sometimes: the processability of the carbon fiber reinforced composite material decreases; furthermore, during the manufacturing of the carbon fiber reinforced composite material, the points where fibers break due to compressive stress become the failure initiation points, and the compressive strength of the carbon fiber reinforced composite material decreases. If the number of unwound fibers in the carbon fiber bundle is 6 or less per 100m, it is preferable that the processability and compressive strength of the aforementioned carbon fiber reinforced composite material can be maintained at satisfactory values.

[0050] The number of unraveled fibers was determined using the method described in the examples. In order to adjust the number of unraveled fibers per 100m of carbon fiber bundle to be within the above range, it is necessary to control the draw ratio of the pre-carbonization process described later, and at the same time control the draw ratio and carbonization temperature in the carbonization process.

[0051] The prepreg blank of the present invention is a prepreg blank formed by impregnating the above-mentioned carbon fiber bundles with a thermosetting resin. Preferably, the number of fluff defects in the prepreg blank is 7 per 100m. 2 The following is more preferably 3 per 100m 2 The following is a further preferred option: 1 unit / 100m 2The following applies. When the number of fluff defects in the prepreg increases, the fiber breakage points sometimes become the starting point of failure during the fabrication of carbon fiber reinforced composites, resulting in a decrease in the tensile and compressive strength of the carbon fiber reinforced composites. For example, if the number of fluff defects in the prepreg of the carbon fiber bundle is 7 per 100m... 2 The following tends to maintain the high processability, tensile strength, and compressive strength of the aforementioned carbon fiber reinforced composite materials at satisfactory values.

[0052] The number of prepreg fiber fluff defects was determined using the method described in the examples. To determine the number of defects per 100m³, 2 To adjust the number of prepreg defects in the prepreg blank to within the above range, it is necessary to control the stretching ratio in the pre-carbonization process described later, and at the same time control the stretching ratio and carbonization temperature in the carbonization process.

[0053] The prepreg blank of the present invention is a prepreg blank formed by impregnating the aforementioned carbon fiber bundles with a thermosetting resin. Preferably, the thermosetting resin has a cured elastic modulus of 3.0 GPa or higher, more preferably 3.2 GPa or higher, and even more preferably 3.8 GPa or higher. A higher elastic modulus of the cured thermosetting resin allows for sufficient expression of the single-fiber compressive strength of the carbon fiber bundles in the resulting carbon fiber reinforced composite material, thereby improving the overall compressive strength of the carbon fiber reinforced composite material. If the elastic modulus of the cured thermosetting resin is 3.0 GPa or higher, the required compressive strength value for the carbon fiber reinforced composite material can be met. A higher elastic modulus of the cured thermosetting resin is preferred, but 7.0 GPa, more preferably 6.7 GPa, and even more preferably 5.5 GPa is sufficient. The type of thermosetting resin is not particularly limited, and examples include epoxy resin, vinyl ester resin, phenolic resin, benzoxazine resin, bismaleimide resin, cyanate ester resin, and polyimide resin. From the perspective of excellent workability and curability in the pre-cured state, epoxy resin is preferred.

[0054] Examples of epoxy resins include bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin, biphenyl type epoxy resin, naphthalene type epoxy resin, Novolac type epoxy resin, epoxy resin with a fluorene backbone, epoxy resin made from copolymers of phenolic compounds and dicyclopentadiene, glycidyl ether type epoxy resins such as diglycidylresorcinol, tetra(glycidyloxyphenyl)ethane, and tri(glycidyloxyphenyl)methane, and glycidylamine type epoxy resins such as tetraglycidyldiaminodiphenylmethane, triglycidylaminophenol, triglycidylaminocresol, and tetraglycidylphenyldimethylamine. These epoxy resins can be used alone or in combination.

[0055] As a curing agent for epoxy resin, there are no particular limitations as long as it cures the epoxy resin. Examples include aromatic amines, alicyclic amines, acid anhydrides, polyaminoamides, organic hydrazides, and isocyanates. However, amine curing agents are preferred from the perspective of obtaining resins with excellent mechanical properties and heat resistance. Examples of amine curing agents include diaminodiphenyl sulfone and diaminodiphenylmethane as aromatic amines, and dicyandiamide or its derivatives and hydrazide compounds as aliphatic amines.

[0056] In addition, curing agents can also be used in combination with curing accelerators. Examples of curing accelerators used in combination include ureas, imidazoles, and Lewis acid catalysts. Among these, urea compounds are preferred considering the balance between storage stability and catalytic activity. Examples of such urea compounds include N,N-dimethyl-N'-(3,4-dichlorophenyl)urea, toluenebis(dimethylurea), 4,4'-methylenebis(phenyldimethylurea), and 3-phenyl-1,1-dimethylurea.

[0057] A prepreg is a sheet-like intermediate substrate comprising carbon fiber bundles and thermosetting resin. Such a prepreg is obtained by impregnating the carbon fiber bundles with thermosetting resin. Examples of impregnation methods include wet impregnation and hot-melt impregnation (dry impregnation).

[0058] The wet impregnation method involves immersing a carbon fiber bundle in a solution made by dissolving a thermosetting resin in a solvent such as methyl ethyl ketone or methanol, then lifting the carbon fiber bundle and using an oven or similar device to evaporate the solvent from the carbon fiber bundle, thereby impregnating the epoxy resin composition within the carbon fiber bundle. The hot-melt method involves directly impregnating a carbon fiber bundle with a thermosetting resin that has been reduced in viscosity by heating; or pre-preparing a film made by coating a release paper or similar material with thermosetting resin, then overlapping the film from both sides or one side of the carbon fiber bundle, and subjecting it to heating and pressure, thereby impregnating the carbon fiber bundle with resin. Here, a single carbon fiber bundle can be used, or multiple carbon fiber bundles can be used in parallel.

[0059] The carbon fiber reinforced composite material of the present invention is a composite material comprising the aforementioned carbon fiber bundles and a matrix resin. As the matrix resin, a cured thermosetting resin, a thermoplastic resin, or a mixture thereof can be used. When using a thermosetting resin, it can also be produced via the aforementioned prepreg. In this case, from the viewpoint of improving the compressive strength of the carbon fiber reinforced composite material, it is preferable to use a thermosetting resin with an elastic modulus of 3.0 GPa or higher as described above. The type of thermosetting resin is not particularly limited, and appropriate combinations of the aforementioned thermosetting resins can be used.

[0060] The carbon fiber composite material of the present invention is a carbon fiber composite material manufactured using a prepreg blank made by impregnating the aforementioned carbon fiber bundles with a thermosetting resin. The 0° compressive strength of the composite material is preferably 1200 MPa or more, more preferably 1220 MPa or more, and even more preferably 1240 MPa or more. The 0° tensile modulus of elasticity of the composite material is preferably 245 GPa or more, more preferably 250 GPa or more, and even more preferably 255 GPa or more. When the 0° compressive strength and 0° tensile modulus of elasticity of the carbon fiber composite material are high, the molded body manufactured from the obtained carbon fiber composite material can be made into a lighter and more physically robust molded body. If the 0° compressive strength of the composite material is 1200 MPa or more and the 0° tensile modulus of elasticity is 245 GPa or more, the lightweighting and improved physical properties brought about by the carbon fiber composite material can be achieved. A higher 0° compressive strength is preferred, but 1350 MPa, more preferably 1330 MPa, and even more preferably 1310 MPa is sufficient. In addition, the higher the elastic modulus at 0° of the composite material, the better, but 270 GPa, more preferably 265 GPa, and even more preferably 260 GPa is sufficient.

[0061] The manufacturing method for producing the carbon fiber bundles of the present invention will now be described.

[0062] In the manufacture of carbon fiber bundles, polyacrylonitrile-based precursor fiber bundles are obtained. Polyacrylonitrile copolymers are preferably used as raw materials for manufacturing polyacrylonitrile-based precursor fiber bundles. It should be noted that in the manufacturing method of the carbon fiber bundles of the present invention, the term "polyacrylonitrile copolymer" refers to a polyacrylonitrile copolymer in which at least acrylonitrile is a major component. The term "major component" generally refers to a component that accounts for 90 to 100% by mass of the polymer. From the viewpoint of promoting flame retardancy, monomers containing one or more carboxylic acid groups or amide groups are preferably used as copolymerizing components. Examples of monomers containing carboxylic acid groups include acrylic acid, methacrylic acid, itaconic acid, and their alkali metal salts and ammonium salts. Examples of monomers containing amide groups include acrylamide.

[0063] In the manufacture of polyacrylonitrile precursor fiber bundles, known polymerization methods can be selected as the manufacturing method for polyacrylonitrile copolymers.

[0064] When manufacturing polyacrylonitrile precursor fiber bundles, either dry-wet spinning or wet spinning can be used, with dry-wet spinning being preferred as it yields stronger fiber bundles. The fiber-making process includes a spinning step where spinning solution is ejected from a spinning spinneret into a coagulation bath; a washing step where the fibers obtained in the spinning step are cleaned in a water bath; a water bath stretching step where the fiber bundles obtained in the washing step are stretched in a water bath; and a drying heat treatment step where the fiber bundles obtained in the water bath stretching step are dried. If necessary, a steam stretching step where the fiber bundles obtained in the drying heat treatment step are steam stretched may also be included. It should be noted that the order of these steps can be appropriately varied.

[0065] The spinning solution is a solution made by dissolving the aforementioned polyacrylonitrile copolymer in an aqueous solution of organic solvents such as dimethyl sulfoxide, dimethylformamide, and dimethylacetamide, or in a solvent that can dissolve the polyacrylonitrile copolymer, such as nitric acid, zinc chloride, or sodium rhodanide.

[0066] The aforementioned coagulation bath preferably contains solvents such as dimethyl sulfoxide, dimethylformamide, and dimethylacetamide, which are used as solvents in the spinning solution, and a coagulation-promoting component. As the coagulation-promoting component, a component that does not dissolve the aforementioned polyacrylonitrile copolymer and is compatible with the solvent used in the spinning solution can be used. Specifically, water is preferably used as the coagulation-promoting component.

[0067] As for the water bath in the aforementioned washing process, it is preferable to use a water bath with multiple temperature ranges from 30 to 98°C. Furthermore, the stretching ratio in the water bath stretching process is preferably 2 to 6 times.

[0068] After the water bath stretching process, to prevent the individual fibers from fusing together, it is preferable to apply an oil containing silicone or the like to the fiber bundle. The aforementioned silicone oil preferably uses modified silicone, and more preferably uses a silicone oil containing amino-modified silicone with high heat resistance.

[0069] The drying heat treatment process can utilize known methods. For example, the drying temperature can be 100–200°C.

[0070] Following the aforementioned washing, water bath stretching, oiling, and drying heat treatment processes, steam stretching is performed as needed to obtain polyacrylonitrile precursor fiber bundles suitable for producing the carbon fiber bundles of the present invention. Steam stretching is preferably performed in pressurized steam at a stretching ratio of 2 to 6 times.

[0071] From the viewpoint of improving the bundle strength of carbon fiber bundles, the fineness of a single fiber in a polyacrylonitrile precursor fiber bundle is preferably 0.4 to 1.5 dtex, more preferably 0.5 to 1.4 dtex, and even more preferably 0.6 to 1.3 dtex.

[0072] In a method for manufacturing carbon fiber bundles, carbon fiber bundles are obtained by feeding polyacrylonitrile-based precursor fiber bundles into a flame-retardant process, a pre-carbonization process, and a carbonization process.

[0073] In the manufacturing method of the carbon fiber bundle of the present invention, the so-called flame-retardant process refers to heat-treating the polyacrylonitrile precursor fiber bundle at 200-300°C in an oxygen-containing atmosphere. The processing time of the flame-retardant process can preferably be appropriately selected within the range of 10-100 minutes, but for the purpose of improving the tensile strength of the obtained carbon fiber bundle in the tensile test, the specific gravity of the obtained flame-retardant fiber is preferably set in the range of 1.30-1.36, more preferably 1.31-1.35. A more preferred flame-retardant processing time depends on the flame-retardant temperature. If the specific gravity of the flame-retardant fiber is 1.30 or higher, the elastic modulus and other physical properties of the carbon fiber bundle can be fully exhibited; if the specific gravity is 1.36 or lower, the tensile strength of the bundle can be improved. The specific gravity of the flame-retardant fiber is controlled by the flame-retardant processing time and the flame-retardant temperature.

[0074] In the method for manufacturing carbon fiber bundles of the present invention, in order to improve the tensile strength of the carbon fiber bundle in the tensile test, especially when the polyacrylonitrile precursor fiber bundle is supplied to the flame-retardant process, it is preferable that the infrared spectrum of the obtained flame-retardant fiber bundle has a value of 1453 cm⁻¹. -1 Peak intensity relative to 1370cm -1 The ratio of peak intensities is 0.70 to 0.75, and the peak intensity at 1254 cm⁻¹ in the infrared spectrum is [missing value]. -1 Peak intensity relative to 1370cm -1 The peak intensity ratio was controlled to be in the range of 0.50–0.65. The peak intensity ratio at 1453 cm⁻¹ in the infrared spectrum was [not specified]. -1 The peak originates from olefins and decreases as flame retardancy treatment progresses. 1370cm -1 The peak and 1254cm -1 The peak originates from the flame-retardant structure and increases as the flame-retardant reaction proceeds. Furthermore, a peak of 1254 cm⁻¹ is preferred. -1 Peak intensity relative to 1370cm -1 The flammability treatment conditions are set such that the ratio of peak intensity to flammability is 0.50 to 0.65. The peak intensity decreases as flammability treatment progresses, especially in the initial stage. However, depending on the flammability treatment conditions, sometimes even with increased time, the peak intensity ratio will not fall below 0.65.

[0075] To achieve both peak intensity ratios within the target range simultaneously, the main focus should be on setting the following conditions: a low amount of copolymer component in the polyacrylonitrile polymer constituting the precursor fiber bundle; a high degree of crystal orientation in the precursor fiber bundle; reduced single fiber fineness of the precursor fiber bundle; and increased flame-retardant temperature in the latter half of the process. Preferably, the polyacrylonitrile carbon fiber precursor fiber bundle is flame-retardant treated for 8–25 minutes until reaching 1453 cm⁻¹ in the infrared spectrum. -1 Peak intensity relative to 1370cm -1 The ratio of peak intensity to the peak intensity becomes 0.98 to 1.10 (first flame-retardant process). Then, flame-retardant treatment is carried out at a higher temperature than the first flame-retardant process for 5 to 20 minutes until the peak intensity reaches 1453 cm⁻¹ in the infrared spectrum. -1 Peak intensity relative to 1370cm -1 The ratio of peak intensities is in the range of 0.70 to 0.75, and the peak intensity at 1254 cm⁻¹ in the infrared spectrum is [missing value]. -1 Peak intensity relative to 1370cm -1 The flame-retardant treatment process continues until the peak intensity ratio of the infrared spectrum reaches a range of 0.50 to 0.65 (second flame-retardant treatment step). To shorten the flame-retardant treatment time in the second flame-retardant treatment step, the flame-retardant treatment temperature can be increased; however, the appropriate flame-retardant treatment temperature depends on the characteristics of the precursor fiber bundle. To adjust the peak intensity ratio of the infrared spectrum to the aforementioned range, the flame-retardant treatment temperature is preferably set to 260–290°C. The flame-retardant treatment temperature does not need to be constant and can be set in multiple stages. To improve the bundle strength of the obtained carbon fiber bundle, it is preferable to increase the flame-retardant treatment temperature and shorten the flame-retardant treatment time. In the first flame-retardant treatment step, it is preferable to perform flame-retardant treatment for a preferred time of 10–25 minutes and at a flame-retardant treatment temperature within the aforementioned range.

[0076] The flame-retardant time mentioned here refers to the residence time of the fibers in the flame-retardant furnace, and the flame-retardant fiber bundle refers to the fiber bundle after the flame-retardant process and before the pre-carbonization process. Furthermore, the peak intensity mentioned here is the absorbance at each wavelength after baseline correction of the infrared spectrum obtained by sampling a small amount of the flame-retardant fiber bundle, without any specific peak segmentation. The sample is diluted with KBr to a concentration of 0.67% by mass. This allows for the study of conditions simply by measuring the infrared spectrum each time the flame-retardant conditions are changed. By controlling the conditions to ensure that the peak intensity ratio of the infrared spectrum of the flame-retardant fiber bundle is within an appropriate range, the bundle strength of the obtained carbon fiber bundle can be adjusted.

[0077] In this invention, the so-called flame-retardant treatment process refers to heat treatment of the precursor fiber bundle at 200-300°C in an oxygen-containing atmosphere. The total processing time of the flame-retardant treatment process can preferably be appropriately selected within the range of 15-40 minutes. Furthermore, in order to improve the bundle strength of the obtained carbon fiber bundle, the flame-retardant treatment time is preferably set such that the specific gravity of the obtained flame-retardant fiber is 1.28-1.32. More preferably, the processing time of the flame-retardant treatment process depends on the flame-retardant temperature. If the specific gravity of the flame-retardant fiber bundle is less than 1.28, the bundle strength of the carbon fiber bundle may decrease. If the specific gravity of the flame-retardant fiber bundle is less than 1.32, the bundle strength can be improved. The specific gravity of the flame-retardant fiber bundle can be adjusted by controlling the processing time and flame-retardant temperature of the flame-retardant treatment process. Furthermore, regarding the timing of switching from the first flame-retardant treatment process to the second flame-retardant treatment process, it is preferable that the specific gravity of the fiber bundle is in the range of 1.21-1.23. At this point, the conditions for the flame-retardant process are also preferentially controlled by satisfying the aforementioned range of infrared spectral intensity ratios. The preferred ranges for these flame-retardant processing times and temperatures vary depending on the characteristics of the precursor fiber bundles and the copolymer composition of the polyacrylonitrile polymer.

[0078] In the pre-carbonization process of pre-carbonizing the fiber bundles obtained in the flame-retardant process, the obtained flame-retardant fiber bundles are heat-treated in an inactive atmosphere at a maximum temperature of 500 to 1,000°C. If the maximum pre-carbonization temperature is 500°C or higher, carbonization can be carried out in the subsequent carbonization process without the pre-carbonized fiber bundles breaking due to thermal decomposition. There is no particular upper limit to the maximum pre-carbonization temperature, but it is preferably below 1,000°C to ensure that the carbonization temperature in the subsequent carbonization process is below that. In addition, the draw ratio in the pre-carbonization process is preferably 1.020 to 1.090. The above draw ratio is more preferably 1.030 to 1.050. Generally, the higher the draw ratio in the pre-carbonization process, the higher the bundle strength and bundle elastic modulus of the carbon fibers, but it is more prone to producing fluff. If the above draw ratio is 1.020 or higher, it is a draw ratio sufficient to improve the bundle strength and bundle elastic modulus of the carbon fiber bundles.

[0079] In the carbonization process of carbonizing the pre-carbonized fiber bundle, the obtained pre-carbonized fiber bundle is heat-treated in an inactive atmosphere at a maximum temperature preferably 2,300 to 2,500°C, more preferably 2,310 to 2,400°C, and even more preferably 2,330 to 2,380°C.

[0080] Generally, the higher the maximum temperature of the carbonization process, the larger the crystallite size and the more uniform the orientation, thus increasing the elastic modulus of the fiber bundle and decreasing the compressive strength of the individual fibers. If the carbonization temperature is 2,300°C, carbonization is sufficient and is adequate for increasing the crystallite size. If the carbonization temperature is below 2,500°C, it is adequate for maintaining the compressive strength of the individual fibers of the fiber bundle.

[0081] Furthermore, in the manufacturing method of the carbon fiber bundle of the present invention, it is preferable to control the heating rate (°C / min) of the carbonization process within the range of 1,000°C to 2,400°C to 250 to 500°C / min. When the heating rate of the carbonization process is 250 to 500°C / min, it is easier to control the structure formation during the carbonization process, thus enabling uniform tension to be applied to the fiber bundle and facilitating stretching during the carbonization process without compromising quality. The heating rate (°C / min) referred to here can be defined by the formula {maximum temperature reached in the carbonization furnace (°C) - inlet temperature of the carbonization furnace (°C)} / processing time (minutes).

[0082] The preferred heating rate for the carbonization process is 250–500°C / min, more preferably 300–400°C / min. When the heating rate is less than 250°C / min, it is difficult to improve productivity. When the heating rate is greater than 500°C / min, the heat treatment of the fiber bundle becomes uneven, resulting in uneven tension distribution within the fiber bundle, thus easily leading to a decrease in quality and making it difficult to achieve both quality and grade simultaneously.

[0083] Furthermore, in the manufacturing method of the carbon fiber bundle of the present invention, the processing time at the highest temperature of the carbonization process is preferably controlled within 120 to 300 seconds. When the processing time at the highest temperature of the carbonization process is 120 to 300 seconds, it is easier to control the formation of microcrystalline structures during the carbonization process, thus improving the elastic modulus without compromising quality. The processing time at the highest temperature of the carbonization process is preferably 150 to 280 seconds, more preferably 180 to 250 seconds. When the processing time at the highest temperature of the carbonization process is less than 120 seconds, the growth of microcrystalline size becomes insufficient, making it difficult to maintain quality. When the processing time at the highest temperature of the carbonization process is greater than 300 seconds, the growth of microcrystalline size tends to become excessive, making it difficult to balance quality and grade.

[0084] Furthermore, the draw ratio in the carbonization process of the pre-carbonized fiber bundle is preferably 0.940 to 0.995. More preferably, it is 0.945 to 0.980. Generally, a higher draw ratio in the carbonization process increases the bundle strength and elastic modulus of the carbon fibers, but it also makes the fibers more prone to fluffing. A draw ratio of 0.940 or higher is sufficient for improving the bundle strength and elastic modulus of the carbon fiber bundle. By controlling the heating rate in the carbonization process, a balance can be achieved between improving the bundle strength and elastic modulus and the quality of the carbon fiber bundle.

[0085] The carbon fiber bundles obtained in the above manner are preferably subjected to oxidation treatment to introduce oxygen-containing functional groups. In the manufacturing method of the carbon fiber bundles of the present invention, surface treatment can be performed by gas-phase oxidation, liquid-phase oxidation, and liquid-phase electrolytic oxidation, but from the viewpoint of high productivity and uniform processing, liquid-phase electrolytic oxidation is preferred. In the manufacturing method of the carbon fiber bundles of the present invention, there are no particular limitations on the liquid-phase electrolytic oxidation method, as long as it is performed by a known method.

[0086] Following the electrolytic treatment described above, a sizing process is preferably performed to impart bundled properties to the obtained carbon fiber bundles. The sizing agent can be appropriately selected based on the type of matrix resin used in the composite material, ensuring good compatibility with the matrix resin. Considering both abrasion resistance and resin impregnation, it is preferable that the amount of sizing agent applied is 0.5 to 2.0% by mass when the total carbon fiber bundle containing the sizing agent is set to 100% by mass.

[0087] Example

[0088] The methods for determining various physical properties used in the evaluation of this invention are as follows.

[0089] <Tension test of carbon fiber bundles>

[0090] The tensile modulus and tensile strength of the carbon fiber bundle were determined according to JIS R7608 (2008) "Bundle Test Method". Seven bundles were tested, and the arithmetic mean of the results was used as the bundle strength, modulus of elasticity, and initial modulus of elasticity in the tensile test. The modulus of elasticity was measured within the strain range of 0.1% to 0.6%. Furthermore, the initial modulus of elasticity in the tensile test was defined as a quadratic function y = ax², expressed as x (strain) and y (stress) (GPa), within the range of 0 ≤ y ≤ 3. 2+bx+c represents the coefficient b of the first-order term when fitting the SS curve obtained in the wire bundle tensile test. Strain was measured using an elongation meter. The test pieces were prepared as shown below. The test pieces were prepared by impregnating the following resin composition into a carbon fiber bundle and curing it under heat treatment at 130°C for 35 minutes.

[0091] [Resin Composition]

[0092] ·3,4-Epoxycyclohexylmethyl-3,4-epoxy-cyclohexane-formate (100 parts by weight)

[0093] • Boron trifluoride monoethylamine (3 parts by weight)

[0094] · Acetone (4 parts by weight)

[0095] As the aforementioned 3,4-epoxycyclohexylmethyl-3,4-epoxy-cyclohexane-carboxylate, Celloxide P2021P (manufactured by Celloxide Co., Ltd.) was used.

[0096] <Crystal Size>

[0097] The carbon fiber bundles to be measured were fixed using a collodion-alcohol solution to prepare a 4 cm long, 1 mm long quadrangular prism test specimen. Measurements were then performed on the prepared test specimen using a wide-angle X-ray diffractometer under the following conditions.

[0098] • X-ray source: CuKα rays (tube voltage 40kV, tube current 30mA)

[0099] • Detector: goniometer + monochromator + scintillation counter

[0100] • Scanning range: 2θ = 10°~40°

[0101] • Scanning mode: step scan, step unit 0.01°, scan speed 1° / min.

[0102] In the obtained diffraction pattern, peak fitting was performed using Gaussian for the peaks appearing around 2θ = 25°–26°. The full width at half maximum (FWHM) was calculated, and the crystallite size was then calculated using the following Scherrer equation.

[0103] Crystallite size (nm) = Kλ / β0cosθ B

[0104] in,

[0105] K: 1.00, λ: 0.15418 nm (wavelength of X-rays)

[0106] β0: (βE) 2 -β1 2 ) 1 / 2

[0107] β E Apparent full width at half maximum (measured value) rad, β1: 1.046 × 10⁻⁶ -2 rad

[0108] θ B : Bragg's diffraction angle.

[0109] The measurement was performed 10 times per level, and the average value was taken as the crystallite size. An XRD-6100 (manufactured by Shimadzu Corporation) was used as the wide-angle X-ray diffraction apparatus.

[0110] <Diameter of a single fiber in a carbon fiber bundle>

[0111] For a carbon fiber bundle containing multiple carbon filaments to be measured, calculate the mass A per unit length. f (g / m³) and density B f (g / cm 3 The number of filaments in the measured carbon fiber bundle is set as C. f The single fiber diameter (μm) of a carbon fiber with a cross-section assumed to be a perfect circle is calculated using the following formula.

[0112] The diameter of a single carbon fiber (μm) = ((A) f / B f / C f ) / π) (1 / 2) ×2×10 3 .

[0113] <Intensity ratio of infrared spectrum>

[0114] The flame-retardant fiber to be tested was cryogenically pulverized, and 2 mg was accurately weighed and collected. This mixture was thoroughly mixed with 300 mg of KBr and placed into a molding fixture. A press was used to compress the mixture at 40 MPa for 2 minutes to prepare a tablet for testing. The tablet was then placed in a Fourier transform infrared spectrophotometer and analyzed at 1000–2000 cm⁻¹. -1 The spectrum was measured within the range of 1700–2000 cm⁻¹. It should be noted that background correction was performed using a wavelength of 1700–2000 cm⁻¹. -1The minimum value within the range is obtained by subtracting the minimum value from each intensity in such a way that the minimum value becomes 0. It should be noted that in the embodiments and comparative examples described later, a Perkin Elmer Paragon 1000 was used as the Fourier transform infrared spectrophotometer described above.

[0115] <Density Determination of Flame-Resistant Chemical Fiber Bundles and Carbon Fiber Bundles>

[0116] Density B of flame-retardant chemical fiber bundles and carbon fiber bundles f (g / cm 3 The calculations were performed using the Archimedes method with o-dichlorobenzene as the specific gravity liquid. The number of samples was determined in triplicate.

[0117] <Adhesion amount of sizing agent>

[0118] The carbon fiber bundles coated with 2.0 ± 0.5 g of sizing agent were weighed (W1) (read to 4 decimal places) and then subjected to an electric furnace (capacity 120 cm³) at a temperature of 450 °C in a nitrogen flow of 50 mL / min. 3 The carbon fiber bundles were placed in a container for 15 minutes to allow complete thermal decomposition of the sizing agent. Then, they were transferred to a container with a dry nitrogen gas flow rate of 20 L / min. After cooling for 15 minutes, the carbon fiber bundles were weighed (W2) (read to the fourth decimal place). The heat loss was calculated by subtracting W1 from W2. This heat loss was converted to a mass percentage (rounded to the third decimal place) with the sizing agent-coated carbon fiber bundles representing 100% of the total mass. This converted value was taken as the amount of sizing agent adhered (mass%). Two measurements were performed, and the average value was taken as the amount of sizing agent adhered.

[0119] <Number of unwound fibers>

[0120] The carbon fiber bundle bobbins are placed on a warp beam support and wound with a roller at a speed of 2 m / min under a tension of 1.6 mN / dtex. During this time, the number of fibers generated between the warp beam support and the roller, as defined below, is counted for 5 minutes, and the number of unwound fibers is calculated using the following formula. The number of bobbins used for measuring the number of unwound fibers is set to 10, and the arithmetic mean of the measurement results is taken as the number of unwound fibers.

[0121] Unwinding fluff: Broken carbon fiber monofilaments protruding more than 5mm from the carbon fiber bundle are called fluff, and there are no more than 2 such fluffs within a test length of 10mm.

[0122] <Count of fluff fibers>

[0123] The carbon fiber bundle bobbins are placed on a warp beam support, pulled by a roller at a speed of 2 m / min under a tension of 1.6 mN / dtex, and wound with a winder. During this time, the aggregated fibers and balls generated between the warp beam support and the rollers (as defined below) are counted for 50 minutes, and the number of unpacked fibers is calculated using the following formula. The number of bobbins used for measuring the number of unpacked fibers is set to 10, and the arithmetic mean of the measurement results is taken as the number of unpacked fibers.

[0124] Count of loose fibers (fibers / 100m) = Count of fibers (collected fibers + pom-poms) (fibers) / Measured length (m) × 100

[0125] Clustered fluff: Broken carbon fiber monofilaments protruding more than 5mm from the carbon fiber bundle are called fluff, and there are more than 3 such fluffs within a test length of less than 10mm.

[0126] Pompon: A cluster of carbon fiber monofilaments that are exposed from the carbon fiber bundle and become entangled, forming a clump with a diameter of more than 5 mm.

[0127] Here, the diameter of the pom-pom refers to the length of the longest line segment from end to end of the pom-pom.

[0128] <Test for Single Fiber Compressive Strength of Carbon Fiber>

[0129] The determination of the single-fiber compressive strength of single-fiber composites based on the compression fragmentation method is carried out by the following steps (A) to (E).

[0130] (A) Resin Preparation

[0131] 190 parts by weight of bisphenol A type epoxy resin compound "EPOTOHTO (registered trademark)" YD-128 (manufactured by Nippon Steel Chemical Co., Ltd.) and 20.7 parts by weight of diethylenetriamine (manufactured by Wako Pure Chemical Industries, Ltd.) were added to a container, stirred and mixed using a spatula, and degassed using an automatic vacuum degassing device.

[0132] (B) Sampling of carbon fiber monofibers and fixing them into the mold

[0133] A carbon fiber bundle approximately 20 cm in length was roughly divided into four equal parts, and single fibers were sampled sequentially from each of the four bundles. At this point, sampling was performed as comprehensively as possible from the entire bundle. Next, double-sided tape was applied to both ends of the perforated backing paper, and the sampled single fibers were fixed to the perforated backing paper under a certain tension. Next, a glass plate with a polyester film labeled "LUMIRROR" (registered trademark) (manufactured by Toray Industries, Inc.) was prepared, and a 2 mm thick spacer used to adjust the thickness of the test piece was fixed onto the film. The perforated backing paper with the fixed single fibers was placed on the spacer, and then the glass plate with the film attached was placed on top of it, with the film-attached side facing down. At this point, to control the fiber embedding depth, tape approximately 70 μm thick was applied to both ends of the film.

[0134] (C) From resin casting to curing

[0135] The resin prepared in step (A) above is poured into the mold (the space surrounded by the spacer and membrane) from step (B) above. The mold containing the resin is heated for 5 hours in an oven preheated to 50°C, then cooled to 30°C at a rate of 2.5°C / min. The mold is then demolded and cut to obtain a test piece measuring 2cm × 7.5cm × 0.2cm. The test piece is then cut such that a single fiber is located within 0.5cm of the center of the test piece's width.

[0136] (D) Fiber embedment depth measurement

[0137] For the test piece obtained in step (C) above, the fiber embedment depth was measured using a laser Raman spectrophotometer (NRS-3200, Japan) and a 532nm notch filter. First, the surface of a single fiber was irradiated with a laser, and the stage height was adjusted to minimize the laser beam diameter; this height was set as A (μm). Next, the surface of the test piece was irradiated with a laser, and the stage height was adjusted to minimize the laser beam diameter; this height was set as B (μm). The fiber embedment depth d (μm) was calculated using the refractive index of the resin, 1.732, measured using the laser, and by the following formula (1).

[0138] d=(AB)×1.732 · · · (1).

[0139] (E) 4-point bending test

[0140] For the test specimen obtained in step (C) above, compressive strain was applied at four points using a clamp with outer indenters spaced 50 mm apart and inner indenters spaced 20 mm apart. Strain was applied stepwise at 0.1%, and the test specimen was observed using a polarizing microscope. The number of fractures in the central 5 mm along the length of the test specimen was measured. Twice the measured number of fractures was taken as the fiber fracture count (fractures / 10 mm). The compressive stress calculated based on the average fiber fracture count greater than 1 / 10 mm in test number 30 and the initial elastic modulus was taken as the single-fiber compressive strength. Additionally, the strain ε (%) of the single-fiber composite was measured using a strain gauge attached approximately 5 mm from the center of the test specimen in the width direction. The final compressive strain ε of the carbon fiber single fiber was then determined. c It is calculated by taking into account the strain sensitivity factor κ of the strain gauge, the fiber embedment depth d (μm) measured in step (D) above, the residual strain 0.14 (%), and by the following equation (2).

[0141] ε c =ε×(2 / κ)×(1-d / 1,000)-0.14 · · · (2).

[0142] The measured single-fiber compressive strength was evaluated using the following indicators.

[0143] S: Above 3.1 GPa

[0144] A: Above 3.0 GPa and below 3.1 GPa

[0145] B: Above 2.9 GPa and below 3.0 GPa

[0146] C: Less than 2.9 GPa.

[0147] <Prepreg grade and number of prepreg defects>

[0148] Observe the prepreg blank along its length for 50m, and convert it to 100m. 2 The number of defects in the prepreg blank (number / 100m) was evaluated using the following indicators. 2 Prepreg blank fluff defects refer to fluff balls with a diameter of 10 mm or more. The diameter of the fluff ball here refers to the length of the longest line segment from end to end of the fluff ball.

[0149] S: 3 units / 100m 2 the following

[0150] A: More than 3 per 100m 2 7 units / m 2 the following

[0151] B: More than 7 per 100m 2 30 pieces / m 2 the following

[0152] C: More than 30 per 100m 2 .

[0153] <Determination of the elastic modulus of cured thermosetting resins>

[0154] The resin component of the thermosetting resin was added to a kneader, and the mixture was kneaded at 150°C for 1 hour while simultaneously heating. Then, the temperature was lowered to 60°C while still kneading. A curing agent and a curing accelerator were then added and the mixture was further kneaded to obtain an uncured thermosetting resin. This uncured thermosetting resin was degassed in a vacuum and then cured at 130°C for 2 hours in a mold with a 2mm thick spacer made of Teflon (registered trademark), resulting in a 2mm thick cured thermosetting resin. Test pieces with a width of 10mm and a length of 60mm were cut from this cured piece. The span was set to 32mm, the crosshead speed was set to 2.5mm / min, and a 3-point bending test was performed according to JIS-K7171 (1994) to determine the elastic modulus. The number of samples was n=5, and the average value was taken as the elastic modulus of the cured resin.

[0155] <Determination of 0° compressive strength and 0° tensile modulus of carbon fiber reinforced composites>

[0156] A resin composition is obtained by mixing raw materials of thermosetting resins other than curing agents and curing accelerators using a kneader and stirring for 1 hour. Next, the obtained resin composition is coated onto the silicone side of a release paper coated with silicone, thereby obtaining a resin film. The obtained resin film is wound onto the surface of a steel roller with a circumference of approximately 2.7 m and a temperature adjusted to 60–70°C, with the surface of the resin composition facing outwards. Then, carbon fiber bundles unwound from the bearing frame are arranged on the surface of the resin composition wound onto the steel roller using a traverse device. The aforementioned resin film is then applied to cover the surface with the resin composition facing the carbon fiber bundles. For the outer surface of the resin film, a separately prepared roller is brought into contact with and rotated while applying pressure, causing the resin to impregnate the fiber bundles, thus producing a unidirectional prepreg blank with a width of 300 mm and a length of 2.7 m. Here, the fiber area weight of the prepreg is adjusted to 190–200 g / m² by adjusting the rotational speed of the roller and the feed speed of the traverse device. 2Multiple prepreg blanks were laminated with the fiber direction aligned in one direction. The laminates were then treated at 130°C and 0.3 MPa for 2 hours to cure the resin, resulting in a 1 mm thick laminate (fiber-reinforced composite material). Test pieces with a thickness of 1 ± 0.1 mm, a width of 12.7 ± 0.13 mm, a length of 80 ± 0.013 mm, and a gauge length of 5 ± 0.13 mm were cut from the laminate. It should be noted that the gauge length of 5 ± 0.13 mm was used to secure the reinforcing plate at both ends of the test piece (37.5 mm from each end). According to ASTM D695 (1996), the compressive strength was determined for a number of test pieces at a strain rate of 1.27 mm / min. The obtained compressive strength was converted to a fiber volume fraction of 60%. Measurements were performed with n = 6, and the average value was taken as the 0° compressive strength of the carbon fiber reinforced composite material in this invention. Additionally, as described in JIS K7017 (1999), unidirectional reinforcing material was cut into pieces 12.7 mm wide and 230 mm long, and glass fiber reinforced plastic sheets 1.2 mm wide and 50 mm long were bonded to both ends to obtain test pieces. Tensile tests were performed on these test pieces using an INSTRON universal testing machine at a crosshead speed of 1.27 mm / min to determine the 0° tensile modulus of elasticity. The obtained tensile modulus of elasticity was converted to a fiber volume fraction of 60%. Tests were conducted with n=6, and the average value was taken as the 0° tensile modulus of elasticity of the carbon fiber reinforced composite material in this invention.

[0157] The present invention will be further described in detail below through examples.

[0158] (Example 1)

[0159] Polyacrylonitrile copolymers copolymerized with itaconic acid were polymerized using dimethyl sulfoxide as a solvent via solution polymerization to produce polyacrylonitrile copolymers. Solidified filaments were then produced using a wet-dry spinning method, in which the spinning solution obtained from the manufactured polyacrylonitrile copolymer was temporarily ejected from a spinneret into the air, passing through a space of approximately 4 mm, and then introduced into a solidification bath controlled at 3°C, containing a 35% by mass aqueous solution of dimethyl sulfoxide. After washing the solidified filaments with water using conventional methods, they were stretched 3.5 times in a two-tank warm water bath. Next, the water-bath stretched fiber bundle was treated with an amino-modified silicone oil and dried and densified using a heated roller at 160°C. Two filaments were then combined to produce 12,000 individual fibers. The bundle was then stretched 3.7 times under pressurized steam, setting the total stretch ratio to 13 times. Following this, an interlacing process was performed to obtain a polyacrylonitrile precursor fiber bundle with a crystal orientation of 93%, a fiber fineness of 0.75 dtex, and 12,000 individual fibers. Subsequently, the first flame-retardant process used a flame-retardant temperature of 240°C, and the second flame-retardant process used 244°C, resulting in a flame-retardant filament density of 1.30 g / cm³. 3 The flame-retardant treatment time was adjusted by means of stretching the polyacrylonitrile precursor fiber bundles at a stretch ratio of 1 in an air-atmospheric oven while performing flame-retardant treatment to obtain flame-retardant fiber bundles. Here, the flame-retardant treatment process performed in the "first furnace" is equivalent to the first flame-retardant treatment process, and the flame-retardant treatment process performed in the "second furnace" is equivalent to the second flame-retardant treatment process. It should be noted that in this invention, there is no limitation on the number of flame-retardant treatment furnaces for performing the first and second flame-retardant treatment processes. The infrared spectrum of the fiber after the first flame-retardant treatment process shows a value of 1,453 cm⁻¹. -1 The peak intensity relative to 1,370 cm⁻¹ -1 The peak intensity ratio is 0.68. The infrared spectrum of the fiber after the second flame-retardant treatment at 1453 cm⁻¹... -1 The peak intensity relative to 1,370 cm⁻¹ -1 The peak intensity ratio was 0.49, 1,254 cm⁻¹. -1 The peak intensity relative to 1,370 cm⁻¹ -1 The peak intensity ratio was 0.55. For the obtained flame-retardant fiber bundles, pre-carbonized fiber bundles were obtained in a nitrogen atmosphere at temperatures ranging from 300 to 800 °C. For the obtained pre-carbonized fiber bundles, carbonization treatment was performed in a nitrogen atmosphere with controlled maximum temperature, heating rate, and draw ratio. The flame-retardant conditions, pre-carbonization conditions, and carbonization conditions are summarized in Table 1.

[0160] [Table 1-1]

[0161]

[0162] [Table 1-2]

[0163]

[0164] The obtained carbon fiber bundles were surface-treated and coated with a sizing agent to produce the final carbon fiber bundles. The amount of sizing agent was adjusted to 1.2% by mass. The resulting carbon fiber bundles had a single fiber diameter of 5.3 μm and a density of 1.82 g / cm³. 3 The microcrystal size is 4.2 nm, the tensile strength of the wire bundle is 4.8 GPa, the tensile modulus of elasticity of the wire bundle is 438 GPa, the compressive strength of a single fiber is 3.0 GPa, and the mechanical properties are high. Additionally, the number of unwound fibers is known to be 0.5 per m, the number of unwound fibers is 10.0 per 100 m, and the number of fiber defects in the prepreg blank is 28 per 100 m. 2 The quality is excellent. The results are summarized in Table 2.

[0165] [Table 2-1]

[0166]

[0167] [Table 2-2]

[0168]

[0169] (Example 2)

[0170] The heating rate, processing time at the highest temperature, and stretching ratio in the carbonization process were modified as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2, showing that high-grade carbon fiber bundles with excellent mechanical properties were obtained.

[0171] (Example 3)

[0172] The draw ratio in the pre-carbonization process and the maximum temperature, processing time at the maximum temperature, heating rate, and draw ratio in the carbonization process were changed as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2, showing that carbon fiber bundles with sufficiently high grade and high mechanical properties were obtained.

[0173] (Example 4)

[0174] The draw ratio in the pre-carbonization process and the heating rate, processing time at the highest temperature, and draw ratio in the carbonization process were varied as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2, showing that carbon fiber bundles with very high quality and excellent mechanical properties were obtained.

[0175] (Example 5)

[0176] The draw ratio in the pre-carbonization process and the maximum temperature, processing time at the maximum temperature, heating rate, and draw ratio in the carbonization process were changed as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2, showing that carbon fiber bundles with very high quality and very high mechanical properties were obtained.

[0177] (Example 6)

[0178] The draw ratio in the pre-carbonization process and the maximum temperature, processing time at the maximum temperature, heating rate, and draw ratio in the carbonization process were changed as shown in Table 1. Otherwise, carbon fiber bundles were obtained and various evaluations were performed in the same manner as in Example 1. The results are summarized in Table 2, showing that carbon fiber bundles with sufficiently high grade and sufficiently high mechanical properties were obtained.

[0179] (Example 7)

[0180] The stretching ratio in the pre-carbonization process was modified as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2, showing that carbon fiber bundles with sufficiently high grade and high mechanical properties were obtained.

[0181] (Example 8)

[0182] Regarding the flame-retardant treatment in the flame-retardant process, the flame-retardant temperature and time were controlled to achieve the flame-retardant structure shown in Table 1. The stretching ratio in the pre-carbonization process was modified as shown in Table 1, and the stretching ratio in the carbonization process was also modified as shown in Table 1. Otherwise, carbon fiber bundles were obtained and various evaluations were performed in the same manner as in Example 1. The results are summarized in Table 2, showing that high-grade carbon fiber bundles with excellent mechanical properties were obtained.

[0183] (Example 9)

[0184] The flame-retardant treatment in the flame-retardant process was controlled to achieve the flame-retardant structure shown in Table 1. The stretching ratio in the pre-carbonization process was modified as shown in Table 1. The maximum temperature, treatment time at the maximum temperature, heating rate, and stretching ratio in the carbonization process were also modified as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2, showing that high-quality carbon fiber bundles with excellent mechanical properties were obtained.

[0185] (Example 10)

[0186] Regarding the flame-retardant treatment in the flame-retardant process, the flame-retardant temperature and time were controlled to achieve the flame-retardant structure shown in Table 1. The stretching ratio in the pre-carbonization process was modified as shown in Table 1. The maximum temperature, treatment time at the maximum temperature, heating rate, and stretching ratio in the carbonization process were also modified as shown in Table 1. Otherwise, carbon fiber bundles were obtained and various evaluations were performed in the same manner as in Example 1. The results are summarized in Table 2, showing that high-quality carbon fiber bundles with excellent mechanical properties were obtained.

[0187] (Example 11)

[0188] Using the carbon fiber bundles obtained in Example 4, prepregs and carbon fiber reinforced composites were prepared using the resin composition shown below. The tensile modulus at 0° was measured to be 259 GPa, and the compressive strength at 0° was measured to be 1240 MPa. The physical properties of the resulting carbon fiber reinforced composites are shown in Table 3. It should be noted that the elastic modulus of the cured resin obtained from the same resin composition was measured, and the result was 4.4 GPa.

[0189] Resin composition:

[0190] • Liquid bisphenol A type epoxy resin (“jER (registered trademark)” 828: manufactured by Mitsubishi Chemical Corporation): 20 parts by weight

[0191] • Triglycidyl m-aminophenol (“Araldite (registered trademark)” MY0600: manufactured by Huntsman Advanced Materials Co., Ltd.): 50 parts by weight

[0192] Phenol-Novolac type epoxy resin (“jER (registered trademark)” 154: manufactured by Mitsubishi Chemical Corporation): 30 parts by weight

[0193] Hardener:

[0194] • Dicyandiamide (manufactured by Mitsubishi Chemical Corporation): 6 parts by weight

[0195] Curing accelerator:

[0196] ·3-(3,4-dichlorophenyl)-1,1-dimethylurea (manufactured by Hodogaya Chemical Industry Co., Ltd.): 3 parts by weight.

[0197] (Comparative Example 1)

[0198] The draw ratios in the pre-carbonization and carbonization processes were modified as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2. The obtained carbon fiber bundles exhibited high mechanical properties, but the individual fiber diameters were small and the quality was insufficient.

[0199] (Comparative Example 2)

[0200] The draw ratio in the pre-carbonization process and the maximum temperature, heating rate, and draw ratio in the carbonization process were changed as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2. The obtained carbon fiber bundles have high mechanical properties, but high density, large crystallite size, insufficient bundle strength and single fiber compressive strength, and insufficient grade.

[0201] (Comparative Example 3)

[0202] The draw ratios in the pre-carbonization process and the carbonization process were modified as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2. The obtained carbon fiber bundles have high mechanical properties, but the single fiber diameter is small and the grade is not sufficient.

[0203] (Comparative Example 4)

[0204] The draw ratio in the pre-carbonization process and the maximum temperature, processing time at the maximum temperature, heating rate, and draw ratio in the carbonization process were varied as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2. The obtained carbon fiber bundles were of very high quality, but the elastic modulus of the bundles was insufficient due to the low maximum carbonization temperature.

[0205] (Comparative Example 5)

[0206] The draw ratio in the pre-carbonization process and the heating rate and processing time at the highest temperature in the carbonization process were changed as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2. The obtained carbon fiber bundles had insufficient mechanical properties due to the high carbonization heating rate.

[0207] (Comparative Example 6)

[0208] The draw ratio in the pre-carbonization process and the maximum temperature, processing time at the maximum temperature, and draw ratio in the carbonization process were varied as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2. The obtained carbon fiber bundles have high mechanical properties due to the low carbonization temperature and high draw ratio in the carbonization process, but the grade is not sufficient.

[0209] (Comparative Example 7)

[0210] The draw ratio in the pre-carbonization process and the maximum temperature, heating rate, processing time at the maximum temperature, and draw ratio in the carbonization process were changed as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2. The obtained carbon fiber bundles had insufficient mechanical properties and quality due to the low carbonization temperature and high draw ratio in the carbonization process.

[0211] (Comparative Example 8)

[0212] The draw ratio in the pre-carbonization process and the maximum temperature, heating rate, processing time at the maximum temperature, and draw ratio in the carbonization process were changed as shown in Table 1. Otherwise, carbon fiber bundles were obtained and evaluated in the same manner as in Example 1. The results are summarized in Table 2. The obtained carbon fiber bundles had insufficient mechanical properties and quality due to the low carbonization temperature and high draw ratio in the carbonization process.

[0213] (Comparative Example 9)

[0214] Using carbon fiber bundles “TORAYCA (registered trademark)” M46J (bundle elastic modulus: 434 GPa (manufactured by Toray Industries, Inc.)) and the resin composition shown below, a prepreg blank and a carbon fiber reinforced composite material were prepared. The result of the 0° tensile elastic modulus measurement was 256 GPa, and the result of the 0° compressive strength measurement was 1060 MPa. These values ​​are similar to the 0° tensile elastic modulus of the composite material in Example 10, but the 0° compressive strength is lower.

[0215] It should be noted that the elastic modulus of the cured resins obtained from the same resin composition was measured, and the result was an elastic modulus of 3.3 GPa.

[0216] (Comparative Example 10)

[0217] Using carbon fiber bundles "TORAYCA (registered trademark)" M40S (bundle elastic modulus: 380 GPa (manufactured by Toray Industries, Inc.)) and the resin composition shown below, prepreg blanks and carbon fiber reinforced composite materials were prepared. The 0° tensile elastic modulus was measured to be 223 GPa, and the 0° compressive strength was measured to be 1240 MPa, which is the same as the 0° compressive strength of the composite material in Example 11, but the 0° tensile elastic modulus is lower. It should be noted that the elastic modulus of the cured resin obtained from the same resin composition was measured, and the result was 3.3 GPa.

[0218] Resin composition:

[0219] • Liquid bisphenol A diglycidyl ether resin (“jER (registered trademark)” 1001: manufactured by Mitsubishi Chemical Corporation): 20 parts by weight; 30 parts by weight

[0220] • Liquid bisphenol A type epoxy resin (“jER (registered trademark)” 828: manufactured by Mitsubishi Chemical Corporation): 30 parts by weight

[0221] Phenol Novolac polyglycidyl ether resin (“EPICLON” (registered trademark) N740 (manufactured by DIC Corporation): 27 parts by weight

[0222] • Polyvinyl alcohol formaldehyde resin ("VINYLEC (registered trademark)" PVF-K, manufactured by JNC Corporation): 5 parts by weight

[0223] Hardener:

[0224] • Dicyandiamide (manufactured by Mitsubishi Chemical Corporation): 6 parts by weight

[0225] Curing accelerator:

[0226] ·3-(3,4-dichlorophenyl)-1,1-dimethylurea (manufactured by Hodogaya Chemical Industry Co., Ltd.): 3 parts by weight.

[0227] [Table 3]

[0228] Table 3

[0229]

Claims

1. A carbon fiber bundle, wherein the bundle tensile strength is 4.5 GPa or higher and 6.5 GPa or lower, the bundle tensile modulus is 400 GPa or higher, the crystallite size is 4.0 nm or higher and 4.7 nm or lower, and the single fiber diameter is 5.2 μm or higher and 6.2 μm or lower, wherein the carbon fiber bundle is substantially untwisted. The carbon fiber bundles are obtained by feeding polyacrylonitrile precursor fiber bundles into a flame-retardant process, a pre-carbonization process, and a carbonization process. The stretching ratio in the pre-carbonization process is 1.020 to 1.

090. The highest temperature of the carbonization process is 2,300–2,500°C. The heating rate of the carbonization process is 250–500 °C / minute. The processing time at the highest temperature in the carbonization process is 120–300 seconds. The stretching ratio in the carbonization process is 0.940 to 0.

995.

2. The carbon fiber bundle according to claim 1, wherein the number of unwound fibers is less than 0.3 fibers / m.

3. The carbon fiber bundle according to claim 1 or 2, wherein the number of unwound fibers is less than 6 per 100m.

4. The carbon fiber bundle according to claim 1 or 2, wherein the density is 1.84 g / cm³. 3 the following.

5. The carbon fiber bundle according to claim 1 or 2, wherein the diameter of a single fiber is 5.5 μm or more and 6.2 μm or less.

6. The carbon fiber bundle according to claim 1 or 2, wherein the tensile strength of the bundle is above 4.8 GPa and below 6.5 GPa.

7. The carbon fiber bundle according to claim 1 or 2, wherein the tensile strength of the bundle is above 5.0 GPa and below 6.5 GPa.

8. The carbon fiber bundle according to claim 1 or 2, wherein the tensile strength of the bundle is above 5.5 GPa and below 6.5 GPa.

9. A prepreg blank, which is a prepreg blank formed by impregnating the carbon fiber bundles of claim 1 or 2 with a thermosetting resin, wherein, The number of prepreg defects is 7 per 100m. 2 the following.

10. A prepreg blank, which is a prepreg blank formed by impregnating the carbon fiber bundles of claim 1 or 2 with a thermosetting resin, wherein, The elastic modulus of cured thermosetting resins is above 3.0 GPa.

11. A prepreg blank, which is a prepreg blank formed by impregnating the carbon fiber bundles of claim 1 or 2 with a thermosetting resin, wherein, The elastic modulus of cured thermosetting resins is above 3.8 GPa and below 5.5 GPa.

12. A carbon fiber reinforced composite material comprising carbon fiber bundles and a matrix resin as described in claim 1 or 2.

13. A carbon fiber reinforced composite material comprising the carbon fiber bundles and matrix resin as described in claim 1 or 2. in, The composite material has a compressive strength of 1200 MPa to 1350 MPa at 0° and a tensile modulus of 245 GPa to 270 GPa at 0°.

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