carbon fiber bundle

Abstract

According to the present invention, a carbon fiber composite material, which has excellent energy absorption and is composed of a carbon fiber having high strength and high elongation, is provided by a production method yielding an excellent production amount. This carbon fiber bundle having a plurality of single fibers has a strand elastic modulus of 260-350 GPa, a strength of 6.5-8.5 GPa, an elongation of at least 1.8%, a filament number of 1,000-9,000, and a total fineness of 0.15-0.35 g / m, wherein the average value of major axis / minor axis ratios in single fiber cross sections is 1.01-1.08, the coefficient of variation is 1-4%, and the skewness is 0.3-1.2.

Description

[Technical Field]

[0001] The present invention relates to a carbon fiber bundle suitable for woven fabrics that has excellent shapeability while possessing high properties for enhancing the energy absorption performance of carbon fiber composite materials, and a method for producing the same. [Background technology]

[0002] Polyacrylonitrile-based carbon fiber bundles (hereinafter, unless otherwise specified, "carbon fiber bundles" in this specification refers to polyacrylonitrile-based carbon fiber bundles) are lightweight, high-strength, and high-modulus materials, making them important for reducing the weight of components. For carbon fiber reinforced plastics (hereinafter sometimes abbreviated as CFRP), reinforcing fabrics made from carbon fiber bundles are frequently used as reinforcing base materials to achieve both mechanical properties and shapeability. CFRP using such reinforcing base materials is used in structural materials for automobiles, aircraft, and other applications, taking advantage of its superior performance, and is required to satisfy the requirements of high tensile strength and excellent impact energy absorption performance (hereinafter sometimes abbreviated as impact resistance).

[0003] Carbon fiber fabrics use carbon fiber bundles, which are made by bundling individual carbon fibers together. When woven, the cross-section of the carbon fiber bundle is elliptical. In particular, conventionally known bidirectional fabrics have the advantage of a plain weave structure where warp and weft threads alternately rise and fall and intersect, resulting in a large number of intersection points and easier morphological stability. However, there is a problem in that the intersections of warp and weft threads become significantly crimped. This tendency is especially pronounced in fabrics using carbon fiber bundles with a high total fineness, as the thicker carbon fiber bundles intersect. When stress is applied to a CFRP with a large crimp, the stress concentrates at the crimped intersections, preventing the carbon fiber bundles from performing optimally. In other words, the elongation of CFRP using woven fabrics has not been sufficient.

[0004] Other forms of woven fabrics using carbon fiber bundles include unidirectional and multidirectional fabrics, and attempts have been made to reduce or avoid crimp by using woven structures such as twill weaves. However, none of these methods can be said to fully express the excellent resin-impregnated strand strength (hereinafter sometimes abbreviated as strand strength) of carbon fiber bundles. Currently, crimp in woven structures is unavoidable, so improving the carbon fiber bundles is more effective than improving the woven structure. However, the carbon fiber bundles used in woven fabrics have low elongation, so the overall impact resistance of CFRP has not been satisfactory.

[0005] As an attempt to reduce the crimp angle of woven fabrics, Patent Document 1 describes a fabric with a thickness of 0.09 mm or less and a basis weight of 85 g / m². 2 By using the following carbon fiber bundles, we obtain thin, wide, and flattened carbon fiber bundles, thereby reducing the crimp of the carbon fiber bundles and achieving a high reinforcing effect in thin CFRP.

[0006] As an attempt to increase the strand strength and elongation of the carbon fiber bundle itself, Patent Document 2 achieves a maximum strand strength of 8.4 GPa and a resin-impregnated strand modulus (hereinafter sometimes abbreviated as strand modulus) of 325 GPa by increasing the fracture toughness value (Example 3). Similarly, Patent Document 3 achieves a maximum strand strength of 7.9 GPa and a strand modulus of 350 GPa by increasing the fracture toughness value of the carbon fibers (Example 7). Furthermore, in terms of the cross-sectional shape of the carbon fiber bundle, Patent Document 4 attempts a method of twisting the carbon fiber bundle in the flame-retardant process, obtaining a carbon fiber bundle with a flattened cross-section. Patent Document 5 succeeds in flattening the cross-section of the carbon fiber bundle while maintaining the strand strength of the carbon fiber bundle at a constant level by increasing the degree of distortion in the ratio of the major axis to the minor axis of the single carbon fiber by mixing carbon fibers with different cross-sectional shapes and twisting the carbon fiber bundle. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 58-191244 [Patent Document 2] Japanese Patent Publication No. 2017-137614 [Patent Document 3] International Publication No. 2016 / 68034 [Patent Document 4] Japanese Patent Publication No. 2015-67910 [Patent Document 5] Japanese Patent Publication No. 2021-059829 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, conventional technologies have the following challenges:

[0009] Patent Document 1 achieves a high reinforcement effect on the fabric by thinning the carbon fiber bundle, but it proposes a method of opening the fibers by supplying the carbon fiber bundle into a flowing liquid stream, and does not mention the properties of the carbon fiber bundle, so the elongation and fiber opening properties of the carbon fiber bundle were not satisfactory. Patent Documents 2 and 3 attempt to improve strand strength by improving toughness, but since twisting is not performed at the flame-retardant treatment stage, the cross-sectional shape of the single fibers is not controlled, resulting in the problem of the carbon fiber bundle being thick and having a large thickness. Even if the carbon fiber bundle is torn in half and used as a thin bundle, the cross-sectional shape of the single fibers is not sufficient to reduce the thickness of the bundle. Patent Document 4 increases the flatness of the carbon fiber bundle cross-section by twisting in the flame-retardant process, but it does not focus on strand strength or elongation due to the elastic modulus of the strand, so the maximum strand strength in the examples was 5.6 GPa, which is not satisfactory, and the elongation was also not satisfactory. Patent Document 5 attempts to increase the distortion of the single fiber cross-sectional shape and the flatness of the carbon fiber bundle cross-section by mixing carbon fibers with different cross-sectional shapes and twisting them, but it does not focus on elongation, and in the examples, the maximum strand strength was 4.8 GPa (Example 7), and the strand modulus of elasticity was 317 GPa, so the elongation was not satisfactory. Thus, attempts to improve the strand strength and elongation of carbon fiber bundles and attempts to improve the flatness of carbon fiber bundles by improving the single fiber cross-sectional shape have been carried out separately until now, but the effects are completely different, and the operations performed to improve flatness reduce the strand strength, so it is not easy to combine them, and the combination of the two has not been considered until now.

[0010] Therefore, the present invention aims to provide a carbon fiber bundle that is more suitable for weaving, with high strand strength, high elongation for impact resistance, low total fineness to reduce crimp, and high flatness and strain of the carbon fiber bundle. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention provides a carbon fiber bundle having a plurality of single fibers, wherein the strand elastic modulus is 260 to 350 GPa, the strength is 6.0 to 8.5 GPa, the elongation is 1.8% or more, the number of filaments is 1,000 to 9,000, the total fineness is 0.15 to 0.35 g / m, and the average value of the major-minor diameter ratio of the single fiber cross-section is 1.01 to 1.08, the coefficient of variation is 1 to 4%, and the skewness is 0.3 to 1.2.

Effect of the Invention

[0012] The carbon fiber bundle of the present invention has high strand strength and high elongation. Since the total fineness of the carbon fiber bundle is small and the skewness of the single carbon fiber is large, the cross-section of the carbon fiber bundle can be thinned. As a result, the CFRP produced from the fabric made of this carbon fiber bundle has excellent energy absorption performance.

Brief Description of the Drawings

[0013] [Figure 1] This is an example showing the cross-sectional shape of the carbon fiber bundle.

Embodiments for Carrying Out the Invention

[0014] In the present invention, the average value of the major-minor-major ratio of the single fiber cross-section is 1.01 to 1.08, preferably 1.01 to 1.05, and more preferably 1.01 to 1.03. In the present invention, the major-minor-major ratio refers to the ratio obtained by dividing the major axis by the minor axis in the single fiber cross-section. In the present invention, the single fiber cross-section refers to the cross-section of a single fiber perpendicular to the fiber axis. In the present invention, the major axis refers to the maximum value of the Ferret diameter, which is the longer side of a rectangle drawn circumscribing the single fiber cross-section, and the minor axis refers to the minor axis of an ellipse whose major axis is the major axis and whose cross-sectional area is equal to that of the single fiber cross-section. The average value is a simple average without weighting, and the closer the average value of the major-minor-major ratio is to 1.00, the more single fibers have cross-sections that are close to a perfect circle. The larger the average ratio of the major axis to the minor axis, the more likely the elongation of the carbon fiber bundle is to decrease. Therefore, an average ratio of 1.01 or higher is sufficient, and if the average ratio is 1.08 or lower, the elongation of the carbon fiber bundle can be maintained without a significant decrease. A generally known method for controlling the average ratio of the major axis to the minor axis of the cross-section of a single fiber to within the above range is to change the coagulation bath conditions.

[0015] In the present invention, the proportion of single fibers with a major-to-minor-axis ratio of 1.00 to 1.03 is preferably 30 to 90%, more preferably 40 to 85%, and even more preferably 50 to 80%. While a larger major-to-minor-axis ratio tends to improve fiber opening properties, strand strength may decrease if the proportion of single fibers with a major-to-minor-axis ratio of 1.00 to 1.03 exceeds 90%. A proportion of single fibers with a major-to-minor-axis ratio of 30% or more makes it easier to achieve a high level of balance between these properties. Single fibers with a major-to-minor-axis ratio of 1.00 to 1.03 can be obtained by adjusting the solidification conditions or by adjusting the force applied to crush the single fibers in a direction perpendicular to the fiber axis.

[0016] In the present invention, the proportion of single fibers with a major-to-minor-axis ratio of 1.04 to 1.10 is preferably 10 to 40%, more preferably 15 to 25%. While a larger major-to-minor-axis ratio tends to improve fiber opening properties, strand strength may decrease if the proportion of single fibers with a major-to-minor-axis ratio of 1.04 to 1.10 exceeds 40%. When the proportion of single fibers with a major-to-minor-axis ratio of 1.04 to 1.10 is 10% or more, it is easier to achieve a high level of balance between these properties. Single fibers with a major-to-minor-axis ratio of 1.04 to 1.10 can be obtained by adjusting the solidification conditions or by adjusting the force applied to crush the single fibers in a direction perpendicular to the fiber axis.

[0017] In the present invention, the coefficient of variation of the ratio of the major axis to the minor axis of the number of single fibers is 1 to 4%, preferably 1 to 3%, and more preferably 1 to 2%. The coefficient of variation is calculated by dividing the standard deviation by the mean and multiplying by 100, as is the general definition. A large coefficient of variation of the ratio of the major axis to the minor axis means that the ratio is widely distributed, and is related to the fiber-opening properties of the carbon fiber bundle, which is an indicator of how easily the cross-section of the carbon fiber bundle can be flattened. If the coefficient of variation is 1% or more, the fiber-opening properties of the carbon fiber bundle are excellent, and if it is 4% or less, the elongation of the carbon fiber bundle is not significantly impaired. A method for controlling the coefficient of variation of the ratio of the major axis to the minor axis of the cross-section of a single fiber to the above range will be described later.

[0018] In the present invention, the skewness of the ratio of the major axis to the minor axis of the single fiber is 0.3 to 1.2, preferably 0.3 to 1.1, more preferably 0.4 to 1.0, and even more preferably 0.6 to 1.0. Skewness is a parameter that represents the asymmetry of the distribution and is defined by the following equation (1). Skewness=n / ((n-1)×(n-2))×Σ{(xi- <x>) / s} 3 ...(1) Here, n is the number of single fibers (fibers), and xi is the ratio of the major axis to the minor axis of the i-th single fiber (-). <x>'x' represents the mean (-) of the ratio of major axis to minor axis, and 's' represents the standard deviation (-) of the ratio. Also, Σ means summing over the number of single fibers n. In the frequency distribution of any parameter of interest, a skewness value of 0 indicates that the distribution is symmetrical, a negative value indicates that the tail is drawn to the smaller side, and a positive value indicates that the tail is drawn to the larger side.

[0019] To achieve the required crimp angle reduction in woven fabrics, it is necessary to flatten the cross-section of the carbon fiber bundle. However, simply increasing the average value of the ratio of the major axis to the minor axis of the carbon fiber single fibers or controlling the coefficient of variation of the major axis to minor axis ratio results in a decrease in the elongation of the carbon fiber bundle. It was found that controlling the degree of strain within the above range is important for achieving both elongation and fiber opening properties of the carbon fiber bundle.

[0020] A state of high distortion in the major-to-minor-length ratio, by definition, means a state in which a certain amount of single fibers with a large major-to-minor-length ratio are present, but the average value of the major-to-minor-length ratio remains low. The smaller the major-to-minor-length ratio, the higher the elongation of the carbon fiber bundle is maintained, and conversely, the larger the major-to-minor-length ratio, the higher the fiber-opening ability of the carbon fiber bundle. The balance of fiber-opening ability of the carbon fiber bundle is thought to be strongly influenced by the proportion of single fibers with different major-to-minor-length ratios, but the effect of improving the fiber-opening ability of the carbon fiber bundle by single fibers with a large major-to-minor-length ratio is relatively larger than the effect of decreasing the elongation of the carbon fiber bundle by single fibers with a small major-to-minor-length ratio. Therefore, it is thought that adding a small amount of single fibers with a large major-to-minor-length ratio is important to maximize the effect of improving the fiber-opening ability of the carbon fiber bundle without reducing elongation.

[0021] When the distortion of the ratio of major axis to minor axis is 0.3 or higher, both can be achieved at a high level. When the distortion of the ratio of major axis to minor axis is 1.2 or lower, the elongation can be kept within a satisfactory range. In this invention, the method for controlling the coefficient of variation of the ratio of major axis to minor axis and the distortion to the above range is to generate a pressing force between the single fibers by twisting the flame-resistant fibers and applying tension, thereby deforming and controlling the fibers. By observing the cross-section of the obtained carbon fiber single fibers and making fine adjustments, the carbon fiber bundle of this invention satisfies the numerical range.

[0022] The carbon fiber bundle of the present invention has 1,000 to 9,000 filaments. The number of filaments refers to the number of individual fibers contained in the carbon fiber bundle. If the number of filaments is 1,000 or more, sufficient elongation can be obtained, and if it is 9,000 or less, the total fineness can be kept low, and a crimp angle suitable for woven fabrics can be obtained. The number of filaments can be arbitrarily determined during the manufacturing process of the polyacrylonitrile-based carbon fiber precursor fiber bundle.

[0023] The carbon fiber bundle of the present invention has a total fineness of 0.15 to 0.35 g / m, more preferably 0.20 to 0.30 g / m. Total fineness refers to the mass per meter of carbon fiber bundle and is related to the diameter of the single fiber and the number of filaments in the carbon fiber bundle. A smaller total fineness results in a smaller crimp angle. If the total fineness is 0.15 g / m or higher, a CFRP with excellent impact resistance can be obtained, and if it is 0.35 g / m or lower, a crimp angle suitable for woven fabrics can be obtained. Total fineness can be obtained by measuring the length of the carbon fiber bundle and the mass relative to that length. Total fineness can be controlled by adjusting the number of filaments, except for the diameter of the single fiber. However, when the number of filaments is reduced, the elongation of the carbon fiber bundle tends to decrease, and the same properties cannot be obtained simply by changing the number of filaments.

[0024] The carbon fiber bundle of the present invention has a strand modulus of 260 to 350 GPa, preferably 270 to 320 GPa, and more preferably 270 to 300 GPa. The strand modulus is an index indicating the resistance of the carbon fiber bundle to deformation when a load is applied, or in other words, an index representing the lightweight nature of the material. The strand modulus of the carbon fiber bundle can be evaluated according to the tensile test for resin-impregnated strands described in JIS R7608:2004. The stress-strain curve of the carbon fiber bundle shows a downward convex nonlinearity, but the strain range is set to 0.1 to 0.6%, and the strand modulus within that range is used. If the strand modulus is 260 GPa or higher, the lightweight nature of the carbon fiber bundle is good, and if the strand modulus is 350 GPa or lower, the elongation can be kept high relative to the strength obtained, thus providing sufficient impact resistance. The strand modulus can be controlled by the maximum temperature of the carbonization process, the heat treatment time at the maximum temperature, the heating rate, the elongation ratio, etc.

[0025] The carbon fiber bundle of the present invention has a strand strength of 6.0 to 8.5 GPa, preferably 6.5 to 8.0 GPa, and more preferably 7.0 to 8.0 GPa. Strand strength is an index indicating the resistance of the carbon fiber bundle to fracture when a load is applied. The strand strength of the carbon fiber bundle can be evaluated according to the tensile test for resin-impregnated strands described in JIS R7608:2004. Sufficient impact resistance can be obtained if the strand strength is 6.0 GPa or higher, and although there is no upper limit to the strand strength, 8.5 GPa is generally sufficient to provide adequate impact resistance. Strand strength can be increased by controlling various conditions such as flame-retardant conditions and carbonization conditions, which can suppress defects and improve fracture toughness.

[0026] The carbon fiber bundle of the present invention has an elongation of 1.8% or more, preferably 2.0% or more, more preferably 2.2% or more, and even more preferably 2.4% or more. The elongation of the carbon fiber bundle can be evaluated according to the tensile test for resin-impregnated strands described in JIS R7608:2004. Measuring the elongation of a carbon fiber bundle is difficult because the stress-strain curve exhibits nonlinearity, but in this tensile test, the elongation is calculated by dividing the strand strength by the strand modulus of elasticity described above. If the elongation is 1.8% or more, sufficient impact resistance can be obtained, and there is no upper limit to the elongation, but an elongation of 3.0% is often sufficient for impact resistance. The elongation of the carbon fiber bundle can be adjusted by controlling and balancing the strand strength and the strand modulus of elasticity, respectively.

[0027] The cross-sectional shape of the carbon fiber bundle is preferably such that the area ratio (area ratio of the carbon fiber bundle cross-section) obtained by dividing the area of ​​the carbon fiber bundle cross-section by the area of ​​the defined rectangle described later is 0.50 to 0.70, and more preferably 0.60 to 0.70. If the area ratio is 0.78, theoretically the cross-section is elliptical with respect to the defined rectangle, and if the area ratio is less than 0.78, it indicates that depressions occur in the cross-section of the carbon fiber bundle, and that depressions occur partially in the single fibers constituting the carbon fiber bundle. If it is 0.70 or less, as an effect of placing multiple fiber bundles into a single groove, it will partially have single fibers with depressions, so the carbon fiber bundle can be made flatter, and as a result the flatness of the carbon fiber bundle when woven can be ensured. If it is 0.50 or more, excessive deformation of the single fibers constituting the carbon fiber bundle can be suppressed, so the strand strength of the carbon fiber bundle can be ensured. The cross-sectional shape of a carbon fiber bundle is affected by factors such as flame-retardant tension and twist angle, but partial depressions in the carbon fiber bundle can be controlled mainly during the flame-retardant process by placing multiple fiber bundles in a single groove and bringing them into contact with each other.

[0028] The method for producing carbon fiber bundles according to the present invention will be described below.

[0029] In the production of polyacrylonitrile-based carbon fiber precursor fiber bundles (hereinafter sometimes abbreviated as precursor fiber bundles), polyacrylonitrile polymers are preferably used as raw materials. In this invention, it is preferable that the polyacrylonitrile polymer contains at least 90 to 100 mol% acrylonitrile. In the production of precursor fiber bundles, the polyacrylonitrile polymer preferably contains copolymer components from the viewpoint of improving strand strength. As monomers that can be used as copolymer components, monomers containing one or more carboxylic acid groups or amide groups are preferably used from the viewpoint of promoting flame resistance.

[0030] In producing precursor fiber bundles, either a wet-dry spinning method or a wet spinning method may be used, but it is preferable to use a wet-dry spinning method, which is advantageous for the strand strength of the resulting carbon fiber bundles.

[0031] The spinning process consists of a spinning step in which a spinning solution is discharged from a spinneret into a coagulation bath using a wet-dry spinning method, a washing step in which the fiber bundles obtained in the spinning step are washed and stretched in a water bath, and a drying heat treatment step in which the fiber bundles obtained in the washing step are dried and heat treated. Preferably, a steam stretching step is included in which the fiber bundles obtained in the drying heat treatment step are steam stretched. The order of each step can be changed as appropriate. The spinning solution is obtained by dissolving the aforementioned polyacrylonitrile polymer in a polyacrylonitrile-soluble solvent such as dimethyl sulfoxide, dimethylformamide, and dimethylacetamide.

[0032] The coagulation bath preferably contains a solvent such as dimethyl sulfoxide, dimethylformamide, and dimethylacetamide, which were used as the solvent for the spinning solution, and a so-called coagulation promoting component. As the coagulation promoting component, one that does not dissolve the polyacrylonitrile polymer and is compatible with the solvent used for the spinning solution can be used. Specifically, it is preferable to use water as the coagulation promoting component. In this case, it is known that the cross-sectional shape changes depending on the coagulation conditions, and a circular cross-section is formed when the solvent concentration in the coagulation bath is low (40% by mass or less) or high (around 80% by mass), and a β-shaped cross-section is formed at intermediate concentrations.

[0033] In the aforementioned washing process, it is preferable to use a washing bath consisting of multiple stages with a temperature of 30 to 98°C. Furthermore, the stretching ratio in the washing process is preferably 2 to 6 times. Subsequently, in order to improve the strand strength, an oil agent consisting of silicone or the like is preferably applied to the fibers. Such a silicone oil agent preferably contains amino-modified silicone.

[0034] The drying heat treatment process can utilize known methods. For example, a drying temperature of 100-200°C is exemplified.

[0035] After the aforementioned washing and drying heat treatment steps, a precursor fiber bundle suitable for obtaining the carbon fiber bundle of the present invention can be obtained by steam stretching as needed. Steam stretching is performed in pressurized steam, with a stretching ratio of preferably 2 to 6 times.

[0036] In a method for producing carbon fiber bundles, carbon fiber bundles are obtained by subjecting a precursor fiber bundle to a flame-retardant step, a pre-carbonization step, and a carbonization step. In order to increase the strand strength of the carbon fiber bundle, in particular when subjecting the precursor fiber bundle to the flame-retardant step, the obtained flame-retardant fiber bundle should have an infrared spectrum of 1,370 cm⁻¹ -1 1,453 cm⁻¹ for peak intensity -1 The ratio of peak intensities is in the range of 0.70 to 0.75, and the infrared spectrum is 1,370 cm⁻¹. -1 The ratio of the peak intensity at 1,254 cm -1 to the peak intensity at 1,453 cm is preferably controlled to be in the range of 0.50 to 0.65. The peak at 1,453 cm in the infrared spectrum is derived from alkenes and decreases as the flame retardant treatment progresses. The peak at 1,370 cm -1 and the peak at 1,254 cm are peaks derived from the flame retardant structure and increase as the flame retardant treatment progresses. When the specific gravity of the obtained flame retardant fiber is 1.35, the ratio of the peak intensity at 1,453 cm -1 to the peak intensity at 1,370 cm is preferably about 0.63 to 0.69. Further, it is preferable to set the flame retardant conditions so that the ratio of the peak intensity at 1,254 cm -1 to the peak intensity at 1,370 cm is 0.50 to 0.65. Such a peak intensity ratio decreases as the flame retardant treatment progresses, especially with a large initial decrease, but depending on the flame retardant conditions, the peak intensity ratio may not become 0.65 or less even if the time is increased.

[0037] In order to make both of these peak intensity ratios compatible within the target range, basically, attention should be mainly paid to the fact that the amount of the copolymer component contained in the polyacrylonitrile-based polymer constituting the precursor fiber bundle is small, the crystal orientation degree of the precursor fiber bundle is high, the single fiber fineness of the precursor fiber bundle is small, and the flame retardant temperature is increased in the latter half when setting the conditions.

[0038] The polyacrylonitrile-based carbon fiber precursor fiber bundle is flame retarded for 8 to 25 minutes until the ratio of the peak intensity at 1,453 cm -1 to the peak intensity at 1,370 cm is in the range of 0.98 to 1.10 in the infrared spectrum (the first flame retardant step), and then, at a temperature higher than the first flame retardant step, the ratio of the peak intensity at 1,453 cm -1 to the peak intensity at 1,370 cm is in the range of 0.70 to 0.75, and the ratio of the peak intensity at 1,370 cm -1 to the peak intensity at 1,453 cm -1 is in the range of 0.70 to 0.75, and the ratio of the peak intensity at 1,370 cm -1 ​​​​​​​​1,254 cm⁻¹ for peak intensity -1 It is preferable to flame-retard the fiber bundle for 5 to 20 minutes (second flame-retardant step) until the peak intensity ratio is in the range of 0.50 to 0.65. To shorten the flame-retardant time in the second flame-retardant step, the flame-retardant temperature can be increased, but the appropriate flame-retardant temperature depends on the characteristics of the precursor fiber bundle. It is preferable to set the flame-retardant temperature to 260 to 290°C in order to control the infrared spectrum within the range described above. The flame-retardant temperature does not need to be constant, and a multi-stage temperature setting is also acceptable. To increase the strand strength of the resulting carbon fiber bundle, it is preferable to set a high flame-retardant temperature and a short flame-retardant time. In the first flame-retardant step, it is preferable to flame-retard the fiber bundle at a flame-retardant temperature that is within the range described above, with a flame-retardant time of preferably 10 to 25 minutes.

[0039] The flame-retardant time mentioned here refers to the time the fibers remain in the flame-retardant furnace, and the flame-retardant fiber bundle refers to the fiber bundle after the flame-retardant process and before the preliminary carbonization process. The peak intensity mentioned here refers to the absorbance at each wavelength after baseline correction of the spectrum obtained by measuring the infrared spectrum of a small sample of the flame-retardant fiber bundle; no peak splitting is performed. The sample concentration is diluted with KBr to 0.67 mass% before measurement. In this way, the infrared spectrum should be measured each time the flame-retardant conditions are changed to optimize the conditions. By appropriately controlling the infrared spectral peak intensity ratio of the flame-retardant fiber bundle, the strand strength of the resulting carbon fiber bundle can be controlled.

[0040] In the present invention, the flame-retardant treatment step refers to heat-treating the precursor fiber bundle in an oxygen-containing atmosphere at 200 to 300°C.

[0041] The total processing time for the flame-retardant treatment process can be appropriately selected within the range of preferably 15 to 40 minutes. Furthermore, in order to improve the strand strength of the resulting carbon fiber bundle, the flame-retardant treatment time is set so that the specific gravity of the resulting flame-retardant fibers is preferably 1.28 to 1.32. A more preferable flame-retardant treatment time depends on the flame-retardant temperature. If the specific gravity of the flame-retardant fiber bundle is less than 1.28, the strand strength of the carbon fiber bundle may decrease. If the specific gravity of the flame-retardant fiber bundle is 1.32 or less, the strand strength can be increased. The specific gravity of the flame-retardant fiber bundle is controlled by the processing time and flame-retardant temperature of the flame-retardant treatment process. Also, the timing for switching from the first flame-retardant treatment process to the second flame-retardant treatment process is set so that the specific gravity of the fiber bundle is preferably in the range of 1.21 to 1.23. Even in this case, the conditions of the flame-retardant treatment process are controlled with priority given to satisfying the range of the infrared spectral intensity ratio. The preferred range of flame-retardant treatment time and flame-retardant temperature varies depending on the properties of the precursor fiber bundle and the copolymerization composition of the polyacrylonitrile polymer.

[0042] The twisting process to adjust the degree of distortion is performed in this second flame-retardant treatment step, and it is preferable that the twist angle of the fiber bundle during the flame-retardant treatment is 0.2° or more. In the present invention, the twist angle during the flame-retardant treatment is determined by the basis weight y (g / m) and density d (g / cm³) of the precursor fiber bundle used. 3 Using the number of twists T (turns / m), the calculation is performed using the following formula. Angle of twist (°) = arctan{(0.01 × y / (π × d))} 0.5 ×10 -6 ×π×T}.

[0043] Methods for adding twist to fibers during flame-retardant treatment can be selected from known methods. Specifically, this can be controlled by methods such as first winding the precursor fiber bundle onto a bobbin and then rotating the bobbin perpendicular to the winding direction when unwinding the fiber bundle, or by applying twist by bringing a rotating roller or belt into contact with the fiber bundle while it is moving without winding it onto a bobbin. A larger twist angle will yield a greater improvement in the flatness of the carbon fiber bundle, but this effect can be obtained with a twist angle of 0.2° or more.

[0044] In the second flame-retardant treatment process, when twisting is applied, the tension of the fiber bundle during the flame-retardant treatment is preferably set to 0.7 to 1.5 mN / dtex. The tension in the flame-retardant treatment process is calculated by dividing the tension (mN) measured at the flame-retardant furnace entrance by the total fineness (dtex), which is the product of the single fiber fineness (dtex) and the number of filaments of the precursor fiber bundle used. By controlling the tension within the above numerical range, it becomes easier to impart indentations to the carbon fiber single fibers.

[0045] Furthermore, in the second flame-retardant treatment process, multiple fiber bundles are placed into one groove of the roller to make them flame-retardant. This increases the overall tension of the multiple fiber bundles in one groove while keeping the tension on each individual flame-retardant fiber bundle low. As a result, stress perpendicular to the cross-section is applied to some single fibers on the roller, and the ratio of the major axis to the minor axis of the cross-section of the single fiber tends to increase. This method of applying tension allows for the control of the cross-sectional shape of some single fibers while maintaining the overall cross-sectional shape. Here, "multiple" preferably refers to 2 to 6 fibers, and more preferably 3 to 5 fibers.

[0046] In the pre-carbonization step, the fiber bundles obtained in the first and second flame-retardant steps are heat-treated in an inert atmosphere at a maximum temperature of 500 to 1,200°C until the specific gravity is preferably 1.5 to 1.8. The stretch ratio in the pre-carbonization step is preferably 1.00 to 1.20. If the stretch ratio in the pre-carbonization step is 1.00 or higher, the strand modulus of elasticity tends to increase, and the strand strength tends to increase. If the stretch ratio in the pre-carbonization step is 1.20 or lower, the strand modulus of elasticity tends to be suppressed to 350 GPa or less.

[0047] The pre-carbonized fiber bundles are carbonized in an inert atmosphere, preferably at a maximum temperature of 1,000 to 1,500°C, more preferably at a maximum temperature of 1,000 to 1,200°C. From the viewpoint of increasing the elongation of the resulting carbon fiber bundles, a lower maximum temperature is preferable. However, if the temperature is too low, the strand strength may decrease, so it is preferable to set the maximum temperature taking both factors into consideration.

[0048] Furthermore, the processing time at the highest temperature during the carbonization process is preferably 20 to 60 seconds. A shorter processing time at the highest temperature allows for lower control of the strand modulus, so a processing time of 60 seconds or less is preferable, and a processing time of 20 seconds or more makes it easier to obtain a stable strand modulus.

[0049] The heating rate in the carbonization process is preferably 0.40 to 1.10°C / second, and more preferably 0.40 to 0.60°C / second. The heating rate in the carbonization process affects the desorption rate of decomposition gases and therefore affects the strand strength. In this invention, the heating rate is defined as the average speed in degrees Celsius per second at which the fibers pass through the 1,000 to 1,100°C range. Since the temperature in the carbonization process is often controlled by the set temperature of the heater, the heating rate is calculated from the temperature at the center of the installation position of each heater and the timing of fiber passage. If the heating rate is 0.40°C / second or higher, the strand modulus of elasticity is easily obtained stably, and if it is within 1.10°C / second, the decrease in strand strength is easily suppressed.

[0050] The carbon fiber bundle obtained in the manner described above is preferably subjected to an oxidation treatment to introduce oxygen-containing functional groups.

[0051] In the present invention, carbon fiber bundles are obtained by electrolytic surface treatment of the fiber bundles obtained in the carbonization step described above. While gas-phase oxidation, liquid-phase oxidation, and liquid-phase electrolytic oxidation can be used for the electrolytic surface treatment, liquid-phase electrolytic oxidation is preferred from the viewpoint of high productivity and uniform treatment. In the present invention, there are no particular restrictions on the method of liquid-phase electrolytic oxidation; any known method may be used.

[0052] After such electrolytic surface treatment, sizing treatment can be performed to impart bundle-forming properties to the resulting carbon fiber bundle. Depending on the type of matrix resin used in the CFRP, a sizing agent with good compatibility with the matrix resin can be appropriately selected.

[0053] The carbon fiber bundles of the present invention are preferably used as weaving yarns in fabrics having crimp. In terms of form, unidirectional weaves and multidirectional weaves can be used, but in the present invention, due to the effect of crimp reduction due to the flattening of the carbon fiber bundle cross-section, it is effective to use them in conventionally known bidirectional weaves in which carbon fiber bundles are used as warp and weft threads. Among these, a plain weave structure in which the warp and weft threads alternately rise and fall and intersect is preferred because it has a large number of intersection points of the weaving threads and the structure is easily stabilized.

[0054] In a reinforcing fabric consisting of such flattened, substantially untwisted carbon fiber bundles, even with high fiber density, the crimp at the intersections of each carbon fiber bundle is kept extremely small, resulting in high elongation characteristics when made into CFRP. Furthermore, because each carbon fiber bundle is maintained in a flattened state in the woven fabric form, the resin impregnation is extremely good. Therefore, CFRP with uniform properties can be obtained, and the target elongation characteristics can be easily achieved. Here, "substantially untwisted" means that there is no twist of more than one turn per meter of carbon fiber bundle. If the carbon fiber bundle is twisted, irregularities will occur in the twisted parts. Therefore, when an external force is applied to the woven fabric, stress concentrates in the twisted parts, resulting in uneven elongation characteristics when molded into CFRP. The carbon fiber bundles of the present invention can be used after untwisting.

[0055] Reinforcement fabrics can be manufactured by the following method. Flat, substantially untwisted carbon fiber bundles as described above are used as warp and / or weft threads. The carbon fiber bundles are unraveled transversely so as not to disrupt their flatness and without unraveling or twisting. If necessary, each thread can be opened and widened during or after weaving. The above-described reinforcement fabric can be used for forming preforms, prepregs, and even CFRP, exhibiting excellent properties as a reinforcement base material. At least one of the above-described reinforcement fabrics can be used for the preform. Because the irregularities are extremely small, preforms using this fabric have excellent moldability, and the surface becomes smooth when molded into CFRP. The reinforcement fabric obtained in this way is preferably used for components that require high mechanical properties. It is preferable to mold this reinforcement fabric into CFRP as a woven prepreg or to vacuum form using the woven base material.

[0056] In the present invention, it is important that the carbon fiber bundle satisfies all of the following requirements for improving the impact resistance of CFRP: strand modulus, strand strength, elongation, total fineness, average value of the major-to-minor axis ratio, coefficient of variation, and strain.

[0057] The methods for measuring various physical properties used in this invention are as follows.

[0058] <Total fiber density> For the carbon fiber bundle to be measured, a 10m length is sampled, and after drying it completely at 120°C for 2 hours, the mass is measured and then divided by 10 to determine the total fineness, which is the mass per meter.

[0059] <density> The carbon fiber bundle to be measured should be completely dried at 120°C for 2 hours before use. A dry-type automatic densimeter should be used, with nitrogen as the measurement medium, a 10cc sample container, and the sample volume adjusted to 3-6cc. Three measurements should be taken, and the average value should be used. For this measurement, a Shimadzu AccuPic 1330 dry-type automatic densimeter was used.

[0060] <Strand strength, strand modulus of elasticity, elongation> The strand strength, strand modulus, and elongation of a carbon fiber bundle are determined according to the resin-impregnated strand test method of JIS R7608:2004, following the procedure below. The resin formulation used is "Celoxide (registered trademark)" 2021P (manufactured by Daicel Corporation) / boron trifluoride monoethylamine (manufactured by Tokyo Chemical Industry Co., Ltd.) / acetone = 100 / 3 / 4 (parts by mass), and the curing conditions are atmospheric pressure, temperature 125°C, and time 30 minutes. Seven strands of carbon fiber are measured, and the average values ​​are taken as the strand strength, strand modulus, and elongation. The strain range for calculating the strand modulus is 0.1 to 0.6%.

[0061] <Twist angle in flame-retardant treatment> The twist angle in flame-retardant treatment is determined by the basis weight y (g / m) and density d (g / cm³) of the precursor fiber bundle used. 3 Using the number of twists T (turns / m), the calculation is performed using the following formula. Angle of twist (°) = arctan{(0.01 × y / (π × d))} 0.5 ×10 -6 ×π×T}.

[0062] <Statistics of the ratio of major axis to minor axis, coefficient of variation, and skewness> The methods for evaluating the ratio of major axis to minor axis, the coefficient of variation, and the skewness are not particularly limited as long as they are evaluated according to the above definitions, but for example, they can be evaluated as follows. First, the carbon fiber bundles are aligned and wrapped in carbon tape to form a cylindrical shape. In this state, the single fiber cross-sections are exposed by cutting them perpendicular to the fiber axis. These single fiber cross-sections are observed using a scanning electron microscope and saved as images. The saved images are loaded into the open-source image analysis software "ImageJ" (Version 1.53h), and the contours of the single fiber cross-sections are traced using the "Polygon selections" tool. At this time, one full circle is traced using 20 to 100 points for each contour. Next, the traced contours are converted into smooth curves using the "Fit spline" tool. After conversion, the points are moved and fine-tuned so that the contours of the single fiber cross-sections and the traced curves match better. Subsequently, the "AR (Aspect Ratio)" is calculated using the "Analyze particles" tool. Note that the aspect ratio in this invention refers to the ratio of the major axis to the minor axis. The number of single fibers can be any number, but it should be a number that does not show a statistical bias, for example, the same operation should be performed on 50 single fibers. At this time, single fibers should be selected evenly from different parts of the carbon fiber bundle. The percentage of single fibers with a specific major axis to minor axis ratio is the value (%) obtained by dividing the number of single fibers with a specific major axis to minor axis ratio by the total number of single fibers evaluated and multiplying by 100. The mean value (-), coefficient of variation (%), and skewness (-) are calculated from the total number of single fibers evaluated. Note that the skewness of the major axis to minor axis ratio is calculated according to the following formula (1). Skewness=n / ((n-1)×(n-2))×Σ{(x i - <x>) / s} 3 ...(1) Here, n is the number of single fibers (fibers), x i This is the ratio of the major axis to the minor axis of the i-th single fiber (-). <x>represents the average value (-) of the ratio of major axis to minor axis, and s represents the standard deviation (-) of the ratio of major axis to minor axis. Also, Σ means that the sum is taken over the number of single fibers n. In the following examples, a scanning electron microscope (SEM) "S-4800" manufactured by Hitachi High-Technologies Corporation was used as the scanning electron microscope, and observations were performed with an acceleration voltage of 5 keV.

[0063] <Spreadability> The fiber-opening ability is evaluated with the carbon fiber bundle free of sizing agent. If sizing agent is present, it is removed by burning it off in an oven or washing it in a solvent before evaluation. A 2cm sample of the carbon fiber bundle is taken and left untwisted. The carbon fiber bundle is placed on a 10cm square glass plate, and a glass slide is placed over it. The carbon fiber bundle is moved 3mm to the left and right alternately 10 times in a direction perpendicular to the axial direction. The change in yarn width is measured before and after this operation, and the average value of the 10 repetitions is used as an indicator of fiber-opening ability. The greater the ratio of expansion of the yarn width, the better the fiber-opening ability. The fiber-opening ability is judged from A to C based on how many times the yarn width of the carbon fiber bundle is compared to the yarn width before measurement. A: The measured width is more than four times the original yarn width. B: The measured width is more than 2 times but less than 4 times the original yarn width. C: The measured width is less than twice the original yarn width.

[0064] <Cross-sectional shape of carbon fiber bundle> The cross-sectional shape of the carbon fiber bundle is measured as follows: The carbon fiber bundle is allowed to hang down under its own weight with virtually no twist and almost no tension. Adhesive is applied to the entire carbon fiber bundle to fix it in place. After the adhesive dries, the carbon fiber bundle is cut without distorting its cross-sectional shape. The cut carbon fiber bundle cross-section is observed with a polarizing microscope and an image is acquired. The acquired image is loaded into the open-source image analysis software "ImageJ" (Version 1.53h), and the contour of the single fiber cross-section is traced using the "Polygon selections" tool. At this time, for each contour, 20 to 100 points are traced around the contour to complete one circuit. Next, the contour trace is converted into a smooth curve using the "Fit spline" tool. Subsequently, the length and position information of the Ferret diameter and the area within the contour are acquired using the "Analyze particles" tool. Next, the length of the line segment that intersects perpendicularly with the Ferret diameter and has the longest distance to the contour is obtained. The cross-sectional shape of the carbon fiber bundle is determined by the area within the contour, the Ferret diameter, and the length of the aforementioned line segment that intersects perpendicularly with the Ferret diameter, and is calculated according to the following equation (2). C / (A×2B)···(2) Here, A is the Ferret diameter, B is the length of the aforementioned line segment perpendicular to the Ferret diameter, and C is the area within the contour, corresponding to the respective letters in Figure 1. The cross-sectional shape is determined by calculating the area ratio of the carbon fiber bundle cross-section using the rectangle (A × 2B) formed by the two sides defined in this way and the area within the contour (C). [Examples]

[0065] [Example 1] A polyacrylonitrile polymer was polymerized by solution polymerization using dimethyl sulfoxide as the solvent to obtain a spinning solution. The obtained spinning solution was extruded into the air from a spinneret and then introduced into a coagulation bath consisting of a 35% by mass aqueous solution of dimethyl sulfoxide, maintained at 0°C, to obtain coagulated yarn by a wet-dry spinning method.

[0066] After washing the coagulated yarn with water using a conventional method, it was stretched 3.5 times in two hot water baths. Subsequently, an amino-modified silicone-based silicone oil was applied to the fiber bundle after stretching in the water bath, and a drying and densification treatment was performed using a heated roller at 160°C. By stretching it 3.7 times in pressurized steam, the total stretching ratio of the yarn was increased to 13 times, and then an entanglement treatment was performed to obtain a precursor fiber bundle with 6,000 single fibers. The single fiber fineness of the precursor fiber bundle was 0.7 dtex.

[0067] Next, the first flame-retardant treatment was performed using a flame-retardant temperature of 250°C and a flame-retardant treatment time of 11 minutes, and the second flame-retardant treatment was performed using a flame-retardant temperature of 280°C and a flame-retardant treatment time of 6 minutes (Condition 1). Two precursor fiber bundles were placed in each groove of the rollers before and after the flame-retardant furnace in an oven with an air atmosphere and a tension of 0.8 mN / dtex to obtain flame-retardant fiber bundles. At this time, the flame-retardant treatment was performed while twisting the precursor fiber bundles 15 times per meter.

[0068] The obtained flame-resistant fiber bundles were subjected to a pre-carbonization treatment in a nitrogen atmosphere at a maximum temperature of 800°C with a draw ratio of 1.20 to obtain pre-carbonized fiber bundles. The obtained pre-carbonized fiber bundles were then subjected to a carbonization treatment in a nitrogen atmosphere at a maximum temperature of 1,400°C with a draw ratio of 0.950. In this carbonization process, the heating rate was 0.45°C / second, and the residence time at the maximum temperature was 60 seconds. The physical properties of the final carbon fiber bundles, which were obtained by surface treatment and sizing agent application, are shown in Tables 1 and 2.

[0069] [Example 2] Carbon fiber bundles were obtained in the same manner as in Example 1, except that the stretch ratio during flame retrieval was changed to achieve a tension of 1.0 mN / dtex, 15 twists per meter were added to the carbon fiber precursor fiber bundles, and three carbon fiber precursor fiber bundles were placed in each groove of the rollers before and after the flame retrieval furnace. The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0070] [Example 3] Carbon fiber bundles were obtained in the same manner as in Example 2, except that the tension during flame resistance was set to 1.2 mN / dtex by changing the stretch ratio during flame resistance. The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0071] [Example 4] Carbon fiber bundles were obtained in the same manner as in Example 2, except that the tension during flame resistance was set to 1.5 mN / dtex by changing the stretch ratio during flame resistance. The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0072] [Example 5] Carbon fiber bundles were obtained in the same manner as in Example 2, except that two precursor fiber bundles were placed in each groove of the rollers before and after the flame-retardant furnace. The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0073] [Example 6] Carbon fiber bundles were obtained in the same manner as in Example 1, except that the first flame-retardant treatment process was performed under conditions of a flame-retardant temperature of 250°C and a flame-retardant treatment time of 11 minutes, and the second flame-retardant treatment process was performed under conditions of a flame-retardant temperature of 288°C and a flame-retardant treatment time of 5 minutes (Condition 2), and the stretch ratio during flame-retardant treatment was changed to achieve a tension of 1.0 mN / dtex during flame-retardant treatment. The precursor fiber bundles were twisted 7 times per meter, and 4 precursor fiber bundles were placed in each groove of the rollers before and after the flame-retardant treatment furnace. The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0074] [Example 7] Carbon fiber bundles were obtained in the same manner as in Example 6, except that the first flame-retardant treatment step was performed under conditions of a flame-retardant temperature of 250°C and a flame-retardant treatment time of 11 minutes, and the second flame-retardant treatment step was performed under conditions of a flame-retardant temperature of 283°C and a flame-retardant treatment time of 5 minutes (Condition 3). The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0075] [Example 8] Carbon fiber bundles were obtained in the same manner as in Example 6, except that the first flame-retardant treatment step was performed under conditions of a flame-retardant temperature of 250°C and a flame-retardant treatment time of 11 minutes, and the second flame-retardant treatment step was performed under conditions of a flame-retardant temperature of 281°C and a flame-retardant treatment time of 7 minutes (Condition 4). The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0076] [Example 9] A carbon fiber bundle was obtained in the same manner as in Example 8, except that the number of filaments in the precursor fiber bundle was set to 8,000. The evaluation results of the obtained carbon fiber bundle are shown in Tables 1 and 2.

[0077] [Example 10] Carbon fiber bundles were obtained in the same manner as in Example 8, except that the single fiber fineness of the precursor fiber bundle was set to 0.5 dtex, the first flame-retardant treatment step was performed under conditions of a flame-retardant temperature of 250°C and a flame-retardant treatment time of 11 minutes, and the second flame-retardant treatment step was performed under conditions of a flame-retardant temperature of 282°C and a flame-retardant treatment time of 6 minutes (Condition 5). The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0078] [Comparative Example 1] Carbon fiber bundles were obtained in the same manner as in Example 1, except that the stretch ratio during flame-retardant treatment was changed to achieve a tensile strength of 0.6 mN / dtex, and the precursor fiber bundles were combined to form 12,000 strands before twisting. The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0079] [Comparative Example 2] Carbon fiber bundles were obtained in the same manner as in Comparative Example 1, except that the elongation ratio during flame retardation was changed to achieve a tensile strength of 1.6 mN / dtex, and the flame retardation temperature conditions were set to condition 2. The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0080] [Comparative Example 3] Carbon fiber bundles were obtained in the same manner as in Comparative Example 1, except that the elongation ratio during flame retardation was changed to achieve a tensile strength of 2.0 mN / dtex, and the flame retardation temperature conditions were set to condition 6. The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0081] [Comparative Example 4] Following Example 1 of Japanese Patent Publication No. 2015-67910, a carbon fiber bundle was obtained in the same manner as in Example 7, except that the first flame-retardant treatment step was performed under conditions of a flame-retardant temperature of 240°C and a flame-retardant treatment time of 30 minutes, and the second flame-retardant treatment step was performed under conditions of a flame-retardant temperature of 280°C and a flame-retardant treatment time of 30 minutes (Condition 7), no splicing was performed during flame-retardant treatment, the maximum temperature for the carbonization treatment was 1,350°C, the draw ratio was 0.960, the heating rate for the carbonization step was 1.50°C / second, and the residence time at the maximum temperature was 180 seconds. The evaluation results of the obtained carbon fiber bundle are shown in Tables 1 and 2.

[0082] [Comparative Example 5] Carbon fiber bundles were obtained in the same manner as in Comparative Example 4, except that the tension during flame resistance was set to 2.5 mN / dtex by changing the stretch ratio during flame resistance. The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0083] [Comparative Example 6] Carbon fiber bundles were obtained in the same manner as in Comparative Example 4, except that the tension during flame resistance was set to 1.5 mN / dtex by changing the stretch ratio during flame resistance. The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0084] [Comparative Example 7] Carbon fiber bundles were obtained in the same manner as in Comparative Example 4, except that the tension during flame resistance was set to 0.6 mN / dtex by changing the stretch ratio during flame resistance. The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0085] [Comparative Example 8] Carbon fiber bundles were obtained in the same manner as in Comparative Example 2, except that the heating rate in the carbonization process was set to 0.90°C / second. The evaluation results of the obtained carbon fiber bundles are shown in Tables 1 and 2.

[0086] [Table 1]

[0087] [Table 2] [Explanation of Symbols]

[0088] A Ferret diameter B: Length of the line segment perpendicular to the Ferret diameter. Area within contour C [Industrial applicability]

[0089] The carbon fiber bundle of the present invention is a carbon fiber suitable for textile reinforcement, possessing a high balance of the excellent mechanical properties unique to carbon fibers and the flattening of the cross-section of the carbon fiber bundle. By using the carbon fiber bundle of the present invention, high-performance textile reinforcement can be obtained with high productivity.< / x> < / x> < / x> < / x>

Claims

1. A carbon fiber bundle having multiple single fibers, wherein the strand modulus is 260 to 350 GPa, the strand strength is 6.0 to 8.5 GPa, the elongation is 1.8% or more, the number of filaments is 1,000 to 9,000, the total fineness is 0.15 to 0.35 g / m, and the average ratio of the major axis to the minor axis of the cross-section of the single fiber is 1.01 to 1.08, the coefficient of variation is 1 to 4%, and the strain is 0.3 to 1.

2.

2. The carbon fiber bundle according to claim 1, wherein the area ratio of the carbon fiber bundle cross-section is 0.50 to 0.

70.

3. A carbon fiber bundle according to claim 1 or 2, wherein the elongation is 2.0% or more.

4. A carbon fiber bundle according to claim 1 or 2, wherein the elongation is 2.2% or more.

5. A carbon fiber bundle according to claim 1 or 2, wherein the proportion of single fibers with a major-to-minor-major ratio of 1.04 to 1.10 is 10 to 40%.

6. A carbon fiber bundle according to claim 1 or 2, wherein the proportion of single fibers with a major-to-minor-major ratio of 1.00 to 1.03 is 30 to 90%.

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