Carbon fiber bundle and production method therefor

Abstract

Provided are: a carbon fiber bundle that has a high total fineness and excellent strength, elastic modulus, and handleability during high-level processing; and a method for producing the carbon fiber bundle. The carbon fiber bundle is substantially not twisted, includes filaments in a quantity of 24,000-72,000, has an initial elastic modulus of 240-279 GPa, and satisfies formula (2) regarding a relationship between a coefficient A determined from approximation formula (1) that is nonlinear in a range in which stress is 0-3 GPa in a stress σ-strain ε curve in a resin-impregnated strand tensile test, and the degree of crystal alignment Π (%) in wide-angle X-ray diffraction measurement. (1): ε = Aσ2 + Bσ + C (2): -410 ≤ (0.0000832Π2 - 0.0184Π + 1.00) / A ≤ -310, where A, B, and C each represent a coefficient of a second-order function of the stress σ and the strain ε, and Π represents the degree of crystal alignment.

Description

Carbon fiber bundle and its manufacturing method

[0001] The present invention relates to a carbon fiber bundle that has a high total fineness but is excellent in strength, elastic modulus, and operability when subjected to advanced processing, and a method for producing the same.

[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, and carbon fiber bundles with 24,000 or more filaments are often used because of their excellent productivity. Furthermore, when producing carbon fiber composite materials, such as intermediate substrates for prepregs, towpregs, woven fabrics, and sheet molding compounds (SMCs), and pultruded materials, from carbon fiber bundles, high-level processability is considered important. To improve high-level processability, it is particularly important that the carbon fiber bundle has little fuzz and excellent openability, and that there is no breakage of the entire carbon fiber bundle or single carbon fiber fibers when unwound from a bobbin and allowed to run through the production process, resulting in good operability.

[0004] Generally, carbon fiber bundles are produced through a flame retardation process in which polyacrylonitrile precursor fibers obtained by fiberizing a polyacrylonitrile copolymer are oxidized in air at 200 to 300°C, a pre-carbonization process in which the fibers are heated in an inert atmosphere at a maximum temperature of 500 to 1,200°C, and a carbonization process in which the fibers are heated in an inert atmosphere at a maximum temperature of 1,200 to 3,000°C.

[0005] Techniques for producing carbon fibers with high strength, high modulus, and excellent processability for industrial applications have been proposed (Patent Documents 1 to 4). Patent Document 1 discloses a technique for flame-proofing a polyacrylonitrile precursor fiber bundle having a total fineness of 40,000 dtex or more by specifying the shape and arrangement of return rolls. This technique suppresses twisting of the fiber bundle as it travels through a flame-proofing furnace, stably maintains the shape of the fiber bundle, suppresses breakage and fuzzing during the flame-proofing process, and enables the stable production of high-quality carbon fiber bundles. Patent Document 2 discloses a technique for improving resin impregnation and spreadability during composite material molding by controlling the diameter and surface condition of the carbon fiber within specific ranges. Patent Document 3 discloses a carbon fiber bundle having a semi-permanent twist and a modulus of 200 GPa or more, which exhibits excellent fiber bundle handling and processability, and a high reinforcing effect for fiber-reinforced composite materials. Patent Document 4 discloses a carbon fiber bundle that can produce a high-performance carbon fiber reinforced composite material having excellent tensile strength by controlling the nonlinearity of the stress σ-strain ε curve in a resin-impregnated strand tensile test within a specific range.

[0006] JP 2014-214386 A JP 2002-69754 A JP 2019-151956 A WO 2016 / 068034

[0007] However, the background art has the following problems.

[0008] Patent Document 1 discloses the effect of suppressing the occurrence of twisting and "groove skipping" (dropping off of yarns from rollers) in the flame-proofing step by setting the yarn density in a specific range in the flame-proofing step, but does not disclose the effect of improving the quality of the obtained carbon fiber bundle, and does not enable improvement in operability when the bundle is subjected to a high-level processing process.

[0009] In Patent Document 2, although the resin impregnation property is improved when molding a pressure vessel and the strength development rate of the obtained molding material is improved, the operability when the obtained carbon fiber bundle is subjected to a process of advanced processing is not improved.

[0010] In Patent Document 3, although the handleability can be improved by leaving a semi-permanent twist in the carbon fiber bundle, there is no disclosure or suggestion of a specific effect on the operability when the obtained carbon fiber bundle is subjected to a process of advanced processing, and there is a problem that the presence of the twist disturbs the orientation of the fibers in the obtained carbon fiber reinforced composite material, making it difficult to exhibit mechanical properties.

[0011] In Patent Document 4, the fracture toughness value, which is effective for improving strength, is improved by controlling the nonlinearity of the stress σ-strain ε curve in a tensile test of a resin-impregnated strand within a specific range by controlling the heat treatment method in the flame-proofing step, but there is no suggestion about the operability when a carbon fiber bundle with a high total fineness is subjected to a process of advanced processing, and the initial modulus of elasticity in a tensile test of a resin-impregnated strand is also high at 315 GPa, so improvement in operability when subjected to a process of advanced processing could not be expected. Furthermore, in order to obtain a carbon fiber bundle with excellent productivity, it is effective to increase the total fineness of the polyacrylonitrile precursor fiber bundle and process it, but there are restrictions on the heat treatment method in the flame-proofing step due to reasons such as thermal runaway, and the method described in this Patent Document has the problem that it is difficult to stably control the nonlinearity of the stress σ-strain ε curve.

[0012] As described above, the prior art has proposed techniques for improving the mechanical properties of carbon fiber bundles and techniques for improving the operability during the production of carbon fiber bundles, but has not disclosed any techniques capable of suppressing problems such as fuzz caused by friction with rollers and guides during advanced processing in carbon fiber bundles with a large total fineness, or breakage that occurs in part or the entire carbon fiber bundle. The object of the present invention is to provide a carbon fiber bundle that has a high total fineness but is excellent in strength, elastic modulus, and operability when subjected to advanced processing, and that is likely to exhibit mechanical properties when made into a carbon fiber reinforced composite material substantially without twisting, and a method for producing the same.

[0013] In order to achieve the object of the present invention, the present invention mainly has the following configuration.

[0014] That is, the present invention provides a substantially untwisted carbon fiber bundle in which the relationship between the coefficient A calculated from the nonlinear approximate formula (1) in the stress σ-strain ε curve in a stress range of 0 to 3 GPa in a resin-impregnated strand tensile test and the degree of crystalline orientation Π (%) in wide-angle X-ray diffraction measurement satisfies formula (2), the initial modulus of elasticity is 240 to 279 GPa, the number of filaments is 24,000 to 72,000, and the carbon fiber bundle is substantially untwisted. ２ +Bσ+C ... (1) -410≦(0.0000832Π ２ −0.0184Π+1.00) / A≦−310 (2) where A, B, and C are coefficients of a quadratic function of stress σ and strain ε, and Π is the degree of crystal orientation.

[0015] The present invention also provides a method for producing the above-mentioned carbon fiber bundle, comprising: a flame-proofing step of heat-treating a substantially untwisted polyacrylonitrile precursor fiber bundle having a filament count of 24,000 to 72,000 in an oxidizing atmosphere at a temperature of 220 to 280°C; a pre-carbonization step of heat-treating the flame-proofed fiber bundle obtained in the flame-proofing step in an inert atmosphere at a maximum temperature of 300 to 1,000°C; and a carbonization step of heat-treating the pre-carbonized fiber bundle obtained from the pre-carbonized fiber bundle in an inert atmosphere at a maximum temperature of 1,000 to 1,600°C; wherein the draw ratio in the pre-carbonization step is 1.05 to 1.20, the draw ratio in the carbonization step is 0.960 to 0.990, and the product of the draw ratios in the pre-carbonization step and the carbonization step is 1.020 to 1.180; In the flame-proofing step, the polyacrylonitrile precursor fiber bundle is heat-treated stepwise in a plurality of heat treatment furnaces set at different temperatures, or in a plurality of heat treatment sections provided in a heat treatment furnace and set at different temperatures, so that the temperature of the lowest heat treatment furnace or heat treatment section in the flame-proofing step is set to less than 230°C, and the temperature of the highest heat treatment furnace or heat treatment section is set to 280°C or less.

[0016] According to the present invention, a carbon fiber bundle can be obtained which has a high total fineness, yet is excellent in strength, elastic modulus, and operability when subjected to advanced processing, and which is likely to exhibit mechanical properties when made into a carbon fiber reinforced composite material.

[0017] In order to achieve this object, the present invention has the following configuration.

[0018] In the carbon fiber bundle of the present invention, the value of coefficient A, which is determined by introducing the stress σ-strain ε curve, which is obtained by measuring the carbon fiber bundle by a resin-impregnated strand tensile test, into the following nonlinear approximation formula (1), satisfies the following formula (2): ε=Aσ ２ +Bσ+C ... (1) -410≦(0.0000832Π ２ -0.0184Π+1.00) / A≦-310 (2) Here, Π represents the degree of crystal orientation (%) determined by measuring the carbon fiber bundle by wide-angle X-ray diffraction. The degree of crystal orientation can be obtained by the measurement method of the degree of crystal orientation Π of carbon fiber described later.

[0019] The value of the central term in the formula (2) is −410 to −310, preferably −406 to −343, and more preferably −386 to −352.

[0020] In formula (1), coefficient A indicates the nonlinearity of the stress σ-strain ε curve. Coefficient A can be determined by fitting the stress σ-strain ε curve, which is determined by measuring a carbon fiber bundle using a resin-impregnated strand tensile test, to approximate formula (1) within a stress range of 0 to 3 GPa. As described above, when the stress σ (GPa) is plotted on the vertical axis and the strain ε (-) on the horizontal axis, the stress σ-strain ε curve of a carbon fiber bundle generally exhibits a downwardly convex curve, and therefore coefficient A determined from approximate formula (1) takes a negative value. In other words, the closer coefficient A is to 0, the smaller the nonlinearity.

[0021] Furthermore, the present inventors have found that the correlation with the shear modulus of carbon fiber is not necessarily sufficient simply based on the nonlinearity of the stress σ-strain ε curve. Theories relating to stress and deformation in carbon fiber are explained, for example, in "Carbon" (Netherlands), Elsevier, 1991, Vol. 29, No. 8, pp. 1267-1279. However, these are academic studies and are difficult to use in practical studies for controlling the shear modulus of carbon fiber. As a result of extensive studies based on these theories, the present inventors have found that the degree of crystal orientation Π, which is relatively easy to measure from a practical standpoint, and the value of the central term in the above formula (2) derived from the coefficient A of the above approximate formula (1) (0.0000832Π) ２ It was found that the shear modulus of carbon fiber is highly correlated with the shear modulus of the carbon fiber. More specifically, the larger the value of the center term in formula (2), the lower the shear modulus, and the smaller the value of the center term in formula (2), the higher the shear modulus.

[0022] The shear modulus is an index of the ease with which a single fiber deforms when subjected to bending or compressive stress, and is important for improving operability in advanced processing steps. When the value of the central term in the formula (2) is between -410 and -310, the fiber deforms appropriately when subjected to bending or compressive stress in the advanced processing step, thereby preventing breakage of the single fiber and subsequent winding around rollers or guides. The coefficient A in the formula (1) can be controlled by the draw ratio in the flame-proofing step, the draw ratio in the pre-carbonization step, and the draw ratio in the carbonization step. The degree of crystal orientation Π can be controlled by the draw ratio in the pre-carbonization step, the draw ratio in the carbonization step, and the temperature in the carbonization step.

[0023] Furthermore, the carbon fiber bundle of the present invention has an initial modulus of elasticity of 240 to 279 GPa, preferably 245 to 269 GPa, and more preferably 245 to 260 GPa. The initial modulus of elasticity is an index of the initial ease of deformation when a single fiber is subjected to a stress in the tensile direction, and is important for improving operability in the advanced processing step. If the initial modulus of elasticity is 240 to 279 GPa, the fiber will deform appropriately when subjected to a stress in the tensile direction in the advanced processing step, thereby preventing breakage of the single fiber and subsequent winding around a roller or guide. The initial modulus of elasticity is calculated as the reciprocal 1 / B of the coefficient B when the stress σ-strain ε curve measured by the resin-impregnated strand tensile test described below is fitted with the approximate formula (1). The initial modulus of elasticity can be controlled by the draw ratio in the flame-proofing step, the draw ratio in the pre-carbonization step, the draw ratio in the carbonization step, and the temperature in the carbonization step.

[0024] The carbon fiber bundle of the present invention has a filament count of 24,000 to 72,000, preferably 36,000 to 60,000, and more preferably 48,000 to 50,000. The filament count is the number of single fibers constituting the carbon fiber bundle. The higher the filament count, the better the productivity of carbon fiber reinforced composite materials. However, if the filament count 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 count of 24,000 to 72,000 provides excellent productivity during composite material molding and is suitable for industrial applications. The filament count can be controlled by the number of holes in the spinneret in the spinning process of the polyacrylonitrile precursor fiber bundle, or by dividing and doubling the yarn.

[0025] The carbon fiber bundle of the present invention is substantially untwisted. Here, "substantially untwisted" means that the twist of the carbon fiber bundle is 0.5 turns or less per meter. If the carbon fiber bundle is substantially untwisted, disorder of the fiber orientation in the carbon fiber reinforced composite material can be suppressed, and the reinforcing effect of the carbon fiber reinforced composite material can be improved.

[0026] The carbon fiber bundle of the present invention preferably has a crystallite size Lc of 1.80 to 2.20 nm. The crystallite size Lc is the size of the graphite crystals in the carbon fiber in the

[002] direction. If the crystallite size Lc is 1.80 to 2.20 nm, a carbon fiber having a better balance of strength and elastic modulus can be obtained. The crystallite size Lc can be evaluated by wide-angle X-ray diffraction measurement using the method for measuring crystallite size Lc described below. The crystallite size Lc can be controlled by the temperature in the carbonization step.

[0027] The carbon fiber bundle of the present invention preferably has a single fiber fineness of 0.63 to 1.35 dtex, more preferably 0.67 to 1.35 dtex, and even more preferably 0.74 to 1.20 dtex. The single fiber fineness is the mass per unit length of a single fiber. If the single fiber fineness is 0.63 to 1.35 dtex, both productivity and mechanical properties can be achieved. The single fiber fineness can be evaluated by measuring the mass per unit length using the method described below. The single fiber fineness can be controlled by the extrusion rate and draw ratio of the polyacrylonitrile polymer in the spinning process of the polyacrylonitrile precursor fiber bundle.

[0028] The carbon fiber bundle of the present invention 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 determined by the ratio of the circumferential length L and the area A of the cross section of the single fiber. ｃｓ Therefore, it is defined as follows: (Circularity) = 4πA ｃｓ / L ２ .

[0029] If the circularity of the cross section of a single fiber is 0.86 to 0.98, it is possible to more reliably achieve both good bundling ability and abrasion resistance during advanced processing, and the operability during advanced processing is superior. The circularity of the cross section of the single fiber 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 cross section of the single fiber can be controlled by the shape of the nozzle hole of the spinneret in the spinning process and the conditions of the coagulation process.

[0030] 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.

[0031] In producing carbon fiber bundles, polyacrylonitrile precursor fiber bundles are spun. A polyacrylonitrile polymer is preferably used as a raw material for producing the polyacrylonitrile precursor fiber bundle. In the present invention, the polyacrylonitrile polymer refers to a polymer in which at least acrylonitrile is the main component of the polymer skeleton, and the main component typically refers to a component that accounts for 90 to 100 mass% of the polymer skeleton. From the viewpoints of improving spinnability and efficiently performing flame retardant treatment, the polyacrylonitrile polymer preferably contains a copolymerization component such as itaconic acid, acrylamide, or methacrylic acid. A method for producing the polyacrylonitrile polymer can be selected from known polymerization methods. In producing the polyacrylonitrile precursor fiber bundle, the spinning dope is prepared by dissolving the polyacrylonitrile polymer in a solvent in which polyacrylonitrile is soluble, such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, or an aqueous solution of nitric acid, zinc chloride, and sodium rhodanide.

[0032] Although there are no particular limitations on the method for producing the polyacrylonitrile precursor fiber bundle used in the present invention, it is preferable to use wet spinning, followed by steps such as drawing, water washing, application of an oil agent, drying and densification, and, if necessary, post-drawing, etc. The number of holes in the spinning nozzle in the production process of the polyacrylonitrile precursor fiber bundle is preferably 3,000 to 200,000 holes in order to achieve the number of filaments in the carbon fiber bundle described above, and a polyacrylonitrile precursor fiber bundle with the desired number of filaments can be obtained by splitting or doubling.

[0033] In the production of polyacrylonitrile precursor fiber bundles, the coagulation bath preferably contains a solvent such as dimethyl sulfoxide, dimethylformamide, or dimethylacetamide used as the solvent for the spinning dope, and a so-called coagulation promoter. The coagulation promoter may be one that does not dissolve the polyacrylonitrile polymer and is compatible with the solvent used for the spinning dope. Water is preferably used as the coagulation promoter.

[0034] In the production of a polyacrylonitrile precursor fiber bundle, the water washing step preferably uses a multi-stage water washing bath at a temperature of 30 to 98° C. In addition, in the water washing step, it is also preferable to set the draw ratio to 2 to 6 times.

[0035] After the water washing step, an oil agent made of silicone or the like is preferably applied to the yarn in order to prevent adhesion between the single fibers. Such a silicone oil agent is preferably a modified silicone, and preferably contains an amino-modified silicone having high heat resistance.

[0036] The dry heat treatment step (the above-mentioned drying and densification step) can be carried out by a known method. For example, the drying temperature can be set to 100 to 200°C.

[0037] The single fiber fineness of the polyacrylonitrile precursor fiber bundle in the carbon fiber bundle production method of the present invention is preferably 1.20 to 2.40 dtex, more preferably 1.20 to 2.20 dtex, and even more preferably 1.40 to 1.80 dtex. The single fiber fineness is the mass per unit length of a single fiber. If the single fiber fineness is 1.20 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 controlled by the output rate and draw ratio in the spinning step.

[0038] The polyacrylonitrile precursor fiber bundle in the method for producing a carbon fiber bundle of the present invention 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. ｃｓ Therefore, it is defined as follows: (Circularity) = 4πA ｃｓ / L ２ .

[0039] If the circularity of the single fiber cross section is 0.86 to 0.98, the resulting carbon fiber can more reliably achieve both convergence and abrasion resistance, and the resulting carbon fiber bundle will have better operability 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.

[0040] The number of filaments in the polyacrylonitrile precursor fiber bundle in the method for producing a carbon fiber bundle of the present invention is preferably 24,000 to 72,000, more preferably 36,000 to 60,000, and even more preferably 48,000 to 50,000. The number of filaments refers to the number of single fibers constituting the polyacrylonitrile precursor fiber bundle. The higher the number of filaments, the better the productivity of carbon fiber bundle production and the productivity of carbon fiber reinforced composite materials using the resulting carbon fiber bundle. However, if the number is too high, uneven processing may increase in the flame-proofing step, pre-carbonization step, and carbonization step, and the mechanical properties of the resulting carbon fiber reinforced composite material may deteriorate in terms of the spreadability of the resulting carbon fiber bundle and resin impregnation. When the number of filaments in the polyacrylonitrile precursor fiber bundle is 24,000 to 72,000, the productivity of carbon fiber bundles and carbon fiber reinforced composite materials is excellent, and a carbon fiber bundle that can be suitably used for industrial applications is obtained. The number of filaments in the polyacrylonitrile precursor fiber bundle can be evaluated by counting the number of single fibers constituting the polyacrylonitrile precursor fiber bundle, and can be controlled by the number of holes in the spinneret in the spinning process, the number of divisions of the fiber bundle discharged from the spinneret, and the number of plying of the fiber bundle.

[0041] In the method for producing a carbon fiber bundle of the present invention, the substantially untwisted polyacrylonitrile precursor fiber bundle described above is heat-treated in an oxidizing atmosphere at a temperature of 220 to 280°C (flame-resistant treatment step). The temperature in the flame-resistant treatment step is preferably 220 to 280°C. If the flame-resistant treatment temperature is 220°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 improving the operability of the resulting carbon fiber bundle during advanced processing. If the flame-resistant treatment temperature is 280°C or lower, the heat generation rate does not become excessively high, thereby reducing temperature variations within the flame-resistant fiber bundle and resulting in a carbon fiber bundle with excellent mechanical properties. The temperature for such flame-resistant treatment can be determined by inserting a thermometer such as a thermocouple into a flame-resistant furnace to measure the furnace temperature. If temperature variations or temperature variations are observed when measuring the furnace temperature at several points, a simple average temperature is calculated. The temperature for such flame-resistant treatment can be controlled by the heating output in a heating method used in a known flame-resistant 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 may be changed.

[0042] In the flame-proofing step, the polyacrylonitrile precursor fiber bundle is heat-treated in stages using a plurality of heat treatment furnaces set at different temperatures, or a plurality of heat treatment sections provided within a heat treatment furnace and set at different temperatures (hereinafter, such heat treatment furnaces and heat treatment sections may be referred to as "heat treatment furnaces / heat treatment sections"). In the present invention, it is sufficient that the temperatures of at least two of the plurality of heat treatment furnaces / heat treatment sections are different; for example, two of three heat treatment furnaces / heat treatment sections may be the same temperature. In the present invention, the temperature of the lowest heat treatment furnace or heat treatment section in the flame-proofing step is set to less than 230°C, preferably 225°C or less, and more preferably 223°C or less. Setting the temperature of the lowest heat treatment furnace or heat treatment section to less than 230°C can reduce heat treatment unevenness that tends to occur in polyacrylonitrile precursor fiber bundles with a high total fineness, and can maintain high fiber quality in the drawing in the pre-carbonization step and carbonization step described below. If the temperature of the lowest heat treatment furnace or heat treatment section is 230° C. or higher, heat treatment unevenness increases in the flame-proofing step, and the quality deteriorates due to the extension of the pre-carbonization step and the carbonization step.

[0043] Furthermore, in the present invention, the temperature of the highest heat treatment furnace or heat treatment section during the flame-proofing step is set to 280°C or less, preferably 275°C or less, and more preferably 270°C or less. By setting the temperature of the highest heat treatment furnace or heat treatment section to 280°C or less, it is possible to reduce heat treatment irregularities that tend to occur in polyacrylonitrile precursor fiber bundles with a high total fineness, and high quality can be maintained during the drawing in the pre-carbonization step and carbonization step described below. Note that if the temperature of such a heat treatment furnace or heat treatment section exceeds 280°C, heat treatment irregularities during the flame-proofing step increase, and quality deteriorates during drawing in the pre-carbonization step and carbonization step.

[0044] Following the polyacrylonitrile precursor fiber bundle production process and the flame-proofing process, preliminary carbonization is carried out. In the preliminary carbonization process, the flame-proof fiber bundle obtained as described above is carbonized in an inert atmosphere at a maximum temperature of 300 to 1,000°C, preferably until the density reaches 1.5 to 1.8 g / cm. ３ Heat treat until

[0045] Following the pre-carbonization, carbonization is carried out. In the carbonization step, the pre-carbonized fiber bundle is heat-treated in an inert atmosphere at a maximum temperature of 1,000 to 1,600°C.

[0046] In the present invention, multiple heat treatment furnaces or heat treatment sections may be used in the pre-carbonization step and the carbonization step, and these may be set to different temperatures. Therefore, the temperature of the heat treatment furnace or heat treatment section with the highest temperature in each step is referred to as the "maximum temperature."

[0047] In the method for producing a carbon fiber bundle of the present invention, the draw ratio in the pre-carbonization step is 1.05 to 1.20, the draw ratio in the carbonization step is 0.960 to 0.990, and the product of the draw ratios in the pre-carbonization step and the carbonization step is 1.020 to 1.180.

[0048] The draw ratio in the pre-carbonization step is preferably 1.10 to 1.20, and more preferably 1.10 to 1.15.

[0049] The draw ratio in the carbonization step is preferably 0.975 to 0.990, and more preferably 0.975 to 0.985.

[0050] The product of the stretching ratio in the pre-carbonization step and the stretching ratio in the carbonization step is preferably 1.040 to 1.130, and more preferably 1.070 to 1.130.

[0051] By controlling the draw ratio in the pre-carbonization step to 1.05 or more, the draw ratio in the carbonization step to 0.960 or more, and the product of the draw ratio in the pre-carbonization step and the draw ratio in the carbonization step to 1.020 or more, the value of the center term in the formula (2) and the initial modulus of elasticity of the obtained carbon fiber bundle can be controlled within an appropriate range. On the other hand, by controlling the draw ratio in the pre-carbonization step to 1.20 or less, the draw ratio in the carbonization step to 0.990 or less, and the product of the draw ratio in the pre-carbonization step and the draw ratio in the carbonization step to 1.180 or less, it is possible to suppress yarn breakage due to drawing, and to suppress a decrease in operability during carbon fiber production and an increase in the number of fluffs in the obtained carbon fiber bundle.

[0052] The carbon fiber bundle obtained as described above is 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 viewpoint 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.

[0053] 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 the matrix resin.

[0054] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0055] <Tensile test of resin-impregnated strand of carbon fiber bundle> The tensile modulus of elasticity of the resin-impregnated strand of the carbon fiber bundle (strand modulus E (GPa)), the tensile strength of the resin-impregnated strand (strand strength (GPa)), and the stress σ-strain ε curve are determined in accordance with JIS R7608 (2008) "Test method for resin-impregnated strands." The strand modulus E is measured in a strain range of 0.1 to 0.6%. Note that 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.

[0056] [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 6, and the arithmetic mean values ​​of the measurement results were taken as the strand modulus and strand strength of the carbon fiber bundle.

[0057] <Analysis of stress σ-strain ε curve> The stress σ-strain ε curve obtained by the tensile test of a resin-impregnated strand is analyzed by plotting strain ε (-) on the vertical axis and stress σ (GPa) on the horizontal axis, and fitting using the following formula (1) to calculate coefficients A, B, and C. Fitting is performed for the stress σ-strain ε curve obtained by measurement in the region where stress σ is 0 to 3 GPa. Fitting is performed using Microsoft's "Excel" and is performed using a quadratic function. ε = Aσ ２ +Bσ+C...(1).

[0058] <Initial modulus of elasticity (GPa)> The initial modulus of elasticity of a carbon fiber bundle is calculated as follows using a coefficient B obtained by fitting the above-mentioned stress σ-strain ε curve using equation (1): Initial modulus of elasticity (GPa)=1 / B.

[0059] <Crystalline Orientation Degree Π (%) of Carbon Fiber Bundle> Carbon fiber bundles to be measured are aligned and solidified using a collodion-alcohol solution to prepare a measurement sample in the form of a square pillar having a length of 4 cm and a side length of 1 mm. The prepared measurement sample is measured using a wide-angle X-ray diffractometer under the following conditions: X-ray source: CuKα radiation (tube voltage 40 kV, tube current 30 mA) Detector: goniometer + monochromator + scintillation counter The crystalline orientation degree Π (%) is determined using the following formula from the half-width H (°) ​​of the diffraction intensity distribution obtained by scanning a peak appearing near 2θ = 25 to 26° in the circumferential direction: Crystalline Orientation Degree Π (%) = [(180 - H) / 180] × 100 In the examples, an XRD-6100 manufactured by Shimadzu Corporation was used as the wide-angle X-ray diffractometer.

[0060] <Crystallite size Lc (nm)> X-ray source: CuKα ray (tube voltage 40 kV, tube current 30 mA) Detector: goniometer + monochromator + scintillation counter Scanning range: 2θ = 10 to 40° Scanning mode: step scan, step unit 0.02°, counting time 2 seconds.

[0061] In the obtained diffraction pattern, the half-width is determined for the peak appearing in the vicinity of 2θ=25 to 26°, and the crystallite size is calculated from this value using the following Scherrer formula.

[0062] Crystallite size (nm) = Kλ / β ０ cosθ Ｂ However, K: 1.0, λ: 0.15418 nm (wavelength of X-rays) β ０ :(β Ｅ ２ -β １ ２ ) １／２ β Ｅ : Apparent half-width (measured value) rad β １ : 1.046 x 10 －２ rad θ Ｂ : Bragg diffraction angle.

[0063] <Measurement of circularity (-)> A polyacrylonitrile precursor fiber bundle or a carbon 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 acquired image is analyzed using image analysis software "ImageJ", and the circularity of a single fiber contained in the fiber cross section is calculated from the circumferential length and area of ​​the cross section of the single fiber according to the following definition. This measurement is repeated for 25 single fibers randomly in one cross section, and the average of the circularities is taken as the circularity of the single fiber cross section. The circularity of the single fiber cross section is calculated by dividing the circumferential length L and area A of the single fiber cross section by the circularity of the single fiber cross section. ｃｓ Therefore, it is defined as follows: (Circularity) = 4πA ｃｓ / L ２ .

[0064] <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 or more but less than 50 fluffs / m C: 50 or more fluffs / m

[0065] Examples 1 to 4 A polyacrylonitrile copolymer consisting of acrylonitrile and itaconic acid was polymerized by solution polymerization using dimethyl sulfoxide as a solvent to produce a polyacrylonitrile copolymer, resulting in a spinning dope. The resulting spinning dope was coagulated by wet spinning, in which it was introduced into a coagulation bath consisting of an aqueous solution of dimethyl sulfoxide through a spinning nozzle with 50,000 holes, to produce a fiber bundle. This fiber bundle was washed with water at 30 to 98°C by a conventional method, and stretched during this process. Subsequently, an amino-modified silicone oil was applied to the fiber bundle after washing and stretching, and it was dried and densified using a heated roller at 130°C, resulting in a polyacrylonitrile precursor fiber bundle with 50,000 single fibers and a single fiber fineness of 1.50 dtex. Note that the polyacrylonitrile precursor fiber bundle was not subjected to a twisting process.

[0066] The obtained polyacrylonitrile precursor fiber bundle was treated in a flame-proofing step, a pre-carbonization step, and a carbonization step under the conditions shown in Table 1 to obtain a carbon fiber bundle. In each of the flame-proofing step, the pre-carbonization step, and the carbonization step, the heat treatment was performed by gradually increasing the temperature using a plurality of heat treatment furnaces having different temperatures. In addition, no twisting treatment was performed in the flame-proofing step, the pre-carbonization step, and the carbonization step. The properties of the obtained carbon fiber bundle are shown in Table 2.

[0067] (Example 5) The same procedure as in Example 1 was carried out except that the discharge amount of the spinning dope was changed to obtain a polyacrylonitrile precursor fiber bundle having a single fiber fineness of 1.65 dtex, and the conditions for the subsequent pre-carbonization step and carbonization step were changed as shown in Table 1.

[0068] (Example 6) The same procedure as in Example 1 was carried out except that the discharge amount of the spinning dope was changed to obtain a polyacrylonitrile precursor fiber bundle having a single fiber fineness of 2.40 dtex, and the conditions for the subsequent pre-carbonization step and carbonization step were changed as shown in Table 1.

[0069] Example 7 The same procedure as in Example 1 was carried out except that the flame-proofing temperature, the stretch ratio in the preliminary carbonization step, and the stretch ratio in the carbonization step were changed to the conditions shown in Table 1.

[0070] Comparative Example 1 A carbon fiber bundle was obtained in the same manner as in Example 1, except that the draw ratio in the preliminary carbonization step was 1.00, the draw ratio in the carbonization step was 0.960, and the product of the draw ratios was 0.960. The value of the center term in formula (2) of the obtained carbon fiber bundle was −307, and the initial modulus was 213 GPa, which meant that the operability during advanced processing was poor.

[0071] Comparative Example 2 A carbon fiber bundle was obtained in the same manner as in Example 1, except that the draw ratio in the preliminary carbonization step was 1.01, the draw ratio in the carbonization step was 0.955, and the product of the draw ratios was 0.965. The value of the center term in formula (2) of the obtained carbon fiber bundle was −286, and the initial modulus was 215 GPa, which meant that the operability during advanced processing was poor.

[0072] (Comparative Example 3) A carbon fiber bundle was obtained in the same manner as in Example 1, except that the draw ratio in the preliminary carbonization step was changed to 1.02, the draw ratio in the carbonization step was changed to 0.950, and the product of the draw ratios was changed to 0.969. The value of the center term in formula (2) of the obtained carbon fiber bundle was −287, and the initial modulus of elasticity was 220 GPa, which meant that the operability during advanced processing was poor.

[0073] Comparative Example 4 A polyacrylonitrile precursor fiber bundle having a single fiber fineness of 0.80 dtex was obtained by changing the discharge rate of the spinning dope, and further the draw ratio in the pre-carbonization step was changed to 1.05, the draw ratio in the carbonization step to 0.950, and the product of the draw ratios to 0.998. Except for this, a carbon fiber bundle was obtained in the same manner as in Example 1. The value of the center term in formula (2) of the obtained carbon fiber bundle was −290, and the initial modulus was 218 GPa, which meant that the operability during advanced processing was poor.

[0074] Comparative Example 5 A polyacrylonitrile precursor fiber bundle having a single fiber fineness of 3.00 dtex was obtained by changing the discharge rate of the spinning dope, and further the draw ratio in the pre-carbonization step was changed to 1.00, the draw ratio in the carbonization step to 0.955, and the product of the draw ratios was changed to 0.955. Except for this, a carbon fiber bundle was obtained in the same manner as in Example 1. The value of the center term in formula (2) of the obtained carbon fiber bundle was −277, and the initial modulus was 225 GPa, which indicated poor operability during advanced processing.

[0075] Comparative Example 6 A polyacrylonitrile precursor fiber bundle was obtained in the same manner as in Example 1, except that the spinning dope was once discharged from a spinning nozzle into the air and then coagulated by a dry-wet spinning method in which the dope was introduced into a coagulation bath consisting of an aqueous solution of dimethyl sulfoxide, and a carbon fiber bundle was obtained in the same manner as in Example 1, except that the draw ratio in the pre-carbonization step was changed to 1.01, the draw ratio in the carbonization step was changed to 0.965, and the product of the draw ratios was changed to 0.975. The value of the center term in equation (2) of the obtained carbon fiber bundle was −290, and the initial modulus was 223 GPa, indicating poor operability during advanced processing.

[0076] Comparative Example 7 The same procedure as in Example 1 was carried out except that the draw ratio in the pre-carbonization step was changed to 1.23, but the fiber bundle broke in the pre-carbonization step, and no carbon fiber bundle was obtained.

[0077] (Comparative Example 8) The same procedure as in Example 1 was carried out except that the draw ratio in the preliminary monocarbonization step was controlled to 1.05, the draw ratio in the carbonization step was controlled to 1.000, and the product of the draw ratios was controlled to 1.050. As a result, the fiber bundle broke in the carbonization step, and no carbon fiber bundle was obtained.

[0078] (Comparative Example 9) The same procedure as in Example 1 was carried out except that the temperature in the flame-proofing step was changed to the conditions shown in Table 1 and the draw ratio in the pre-carbonization step was set to 1.05. As a result, the fluff of the pre-carbonized fiber bundle increased and the quality deteriorated significantly, so that the subsequent steps could not be carried out and no carbon fiber bundle was obtained.

[0079] (Comparative Example 10) The same procedure as in Example 1 was carried out except that the temperature in the flame-proofing step was changed to the conditions shown in Table 1 and the draw ratio in the pre-carbonization step was set to 1.05. As a result, the fluff of the pre-carbonized fiber bundle increased and the quality deteriorated significantly, so that the subsequent steps could not be carried out and no carbon fiber bundle was obtained.

[0080]

[0081]

Claims

1. In the stress σ-strain ε curve in the tensile test of the resin-impregnated strand, the coefficient A obtained from the non-linearity approximation formula (1) in the range where the stress is 0 to 3 GPa, and the crystal orientation degree Π (%) in the wide-angle X-ray diffraction measurement satisfy the formula (2), the initial elastic modulus is 240 to 279 GPa, the number of filaments is 24,000 to 72,000, and the carbon fiber bundle is substantially untwisted. ε = Aσ ２ + Bσ + C ··· (1) -410 ≤ (0.0000832Π ２ -0.0184Π + 1.00) / A ≤ -310...(2) Here, A, B, and C are the coefficients of the quadratic function of stress σ and strain ε, and Π is the crystal orientation degree.

2. The carbon fiber bundle according to claim 1, wherein the single fiber fineness is 0.63 to 1.35 dtex.

3. The carbon fiber bundle according to claim 1 or 2, wherein the roundness of the single fiber cross-section is 0.86 to 0.

98.

4. A method for manufacturing the carbon fiber bundle according to claim 1 or 2, including a flame retardant step of heat-treating a polyacrylonitrile-based precursor fiber bundle having a filament number of 24,000 to 72,000 and substantially untwisted at a temperature of 220 to 280 °C in an oxidizing atmosphere, a pre-carbonization step of heat-treating the flame retardant fiber bundle obtained in the flame retardant step in an inert atmosphere at a maximum temperature of 300 to 1,000 °C, and a carbonization step of heat-treating the pre-carbonized fiber bundle obtained in the pre-carbonization step in an inert atmosphere at a maximum temperature of 1,000 to 1,600 °C, wherein the draw ratio in the pre-carbonization step is 1.05 to 1.20, the draw ratio in the carbonization step is 0.960 to 0.990, and the product of the draw ratios in the pre-carbonization step and the carbonization step is 1.020 to 1.180, in the flame retardant step, the polyacrylonitrile-based precursor fiber bundle is heat-treated step by step in a plurality of heat treatment furnaces set at different temperatures or a plurality of heat treatment sections provided in the heat treatment furnace and set at different temperatures, and the temperature of the heat treatment furnace or heat treatment section with the lowest temperature in the flame retardant step is less than 230 °C, and the temperature of the heat treatment furnace or heat treatment section with the highest temperature is 280 °C or less, A method for manufacturing a carbon fiber bundle.

5. The method for manufacturing a carbon fiber bundle according to claim 4, wherein the single fiber fineness of the polyacrylonitrile-based precursor fiber bundle is 1.20 to 2.40 dtex.

6. The method for manufacturing a carbon fiber bundle according to claim 4, wherein the roundness of the single fiber cross-section of the polyacrylonitrile-based precursor fiber bundle is 0.86 to 0.98.