Carbon fiber bundles and their manufacturing methods

Abstract

A carbon fiber bundle and its manufacturing method are provided, which can suppress the entanglement caused by the annular fluff generated when unwinding the carbon fiber bundle during high-level processing by suppressing specific fluff inherent in the carbon fiber bundle. The carbon fiber bundle has an average single fiber diameter B of 6.9 to 11.0 μm, a resin impregnated wire bundle tensile elastic modulus E of 230 to 310 GPa, a number of fluffs inherent in the carbon fiber bundle of less than 40 per m, and a proportion of fluffs with a double structure in the cross section of the inherent fluffs of the carbon fiber bundle of 1 to 25%. In the process of heat-treating a polyacrylonitrile precursor fiber bundle with a single fiber fineness of 0.9 to 2.2 dtex in an oxidizing atmosphere at 200 to 300 °C, when the heating rate of the single fiber is set to q (J / g / s), the number of filaments N is set to N (strands), the single fiber fineness of the flame-retardant fiber bundle is set to d (dtex), and the yarn width is set to W (mm), the heating rate Q, which is the left side of equation (3), is 150 to 500 J / m. 2 Heat treatment is carried out in a manner similar to / s until the density reaches 1.22–1.24 g / cm³. 3 Then, while applying a tension of 1.6–4.0 mN / dtex to the flame-retardant fiber bundles, heat treatment is performed until the density reaches 1.38–1.50 g / cm³. 3 The flame-retardant fiber bundles are obtained, and then the flame-retardant fiber bundles are heat-treated in an inactive atmosphere at 1,200 to 1,600 °C to obtain the above-mentioned carbon fiber bundles. Q=q×N×d / W / 10···(3).

Description

Technical Field

[0001] This invention provides a carbon fiber bundle and its manufacturing method. Each single fiber of the carbon fiber bundle has high load-bearing capacity and excellent abrasion resistance. Even if the single fiber fineness is large, the entanglement caused by the ring-shaped fluff generated when unwinding the carbon fiber bundle during high-level processing can be suppressed by suppressing the specific fluff inside the carbon fiber bundle. Background Technology

[0002] Composite materials using carbon fiber bundles have been used in aerospace applications and sports applications such as bicycles and golf clubs, and recently, their application in industrial sectors such as automotive parts and pressure vessels has also expanded. In industrial applications, due to the need to reduce production costs, improvements in processability are crucial, such as enhancing the abrasion resistance during molding and reducing fuzz (single fiber breakage) during unwinding of the carbon fiber bundles and their movement on the rolls. In particular, suppressing the annular fuzz generated during carbon fiber bundle unwinding is important, as it increases the winding force on the rolls by incorporating surrounding carbon fiber bundles.

[0003] Typically, polyacrylonitrile-based carbon fiber bundles are manufactured through the following processes: a flame-retardant oxidation process in air at 200–300°C; a pre-carbonization process in an inert atmosphere at 500–1200°C; and a carbonization process in an inert atmosphere at 1200–3000°C. To improve abrasion resistance by increasing the load-bearing capacity of each individual fiber in the carbon fiber bundle, increasing the weight of each individual fiber, i.e., increasing the fiber fineness, is effective. Therefore, increasing the heat treatment amount in the flame-retardant process to increase the yield of the carbon fiber bundle or increasing the fiber fineness of the polyacrylonitrile-based precursor fiber bundle is effective.

[0004] To date, methods for manufacturing carbon fiber bundles that suppress fluff during the manufacturing process have been proposed (Patent Documents 1-4).

[0005] Patent Document 1 proposes a solution that increases both the tensile modulus E (hereinafter sometimes simply referred to as the tensile modulus E of the bundle) and compressive strength of the resin-impregnated bundle by increasing the tension of the carbon fiber bundle during the carbonization process while maintaining the carbonization temperature at 1,000–1,500°C, thereby reducing the fuzz when the carbon fiber bundle rubs against the roller. Patent Document 2 proposes a solution that controls the heat treatment temperature of the flame-retardant process according to the density of the flame-retardant fiber bundle during the flame-retardant process, suppressing the dual structure of the carbon fiber bundle, and thus reducing fuzz due to the high fineness of the individual fibers and high knot strength. Patent Document 3 proposes a solution that suppresses the dual structure of the flame-retardant fiber bundle by controlling the flame-retardant process time to achieve an appropriate flame-retardant structure. Patent Document 4 proposes a solution that controls the heat generation in the flame-retardant process by using hydroxyalkyl methacrylate as a copolymer component, thereby achieving high knot strength even with increased individual fiber fineness, resulting in carbon fiber bundles with excellent operability and processability.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2005-344254

[0009] Patent Document 2: Japanese Patent Application Publication No. 2017-66580

[0010] Patent Document 3: Japanese Patent Application Publication No. 2018-178344

[0011] Patent Document 4: International Publication No. 2013 / 157613. Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] However, the following issues exist in the background art.

[0014] In Patent Document 1, the temperature and time of the flame-retardant process are not controlled. Furthermore, due to the small fineness of the individual fibers in the carbon fiber bundle, the load-bearing capacity of each fiber is low, resulting in insufficient abrasion resistance. Moreover, the method of increasing the resistance to roller abrasion by improving average physical properties fails to suppress entanglement caused by annular fibers resulting solely from the unwinding of the carbon fiber bundle. In Patent Documents 2 and 3, although the average state is less prone to fiber formation, the large fineness of the individual fibers prevents control over the heating and deheating rates during the flame-retardant process. This results in uneven temperature within the flame-retardant fiber bundle, making it impossible to control the specific fibers inherent in the carbon fiber bundle. Furthermore, due to the influence of these inherent fibers, it is impossible to suppress entanglement caused by annular fibers generated during the unwinding of the carbon fiber bundle before further processing. In Patent Document 4, although the heating rate in the flame-retardant process is controlled, the fineness of the single fiber of the carbon fiber bundle is large, and the heat removal rate is not controlled. Therefore, the temperature inside the flame-retardant fiber bundle is uneven, which makes it impossible to suppress the specific fluff inside the carbon fiber bundle. There is a problem that it is impossible to suppress the entanglement caused by the ring-shaped fluff generated when the carbon fiber bundle is unwound during high-level processing.

[0015] As described above, patent documents 1-2 and 4 propose methods to suppress the fluff generated during the manufacturing and use of carbon fiber bundles on rollers by increasing the knot strength, tensile modulus E, and compressive strength of the bundle. Patent documents 2-4 also propose methods to suppress the double structure by controlling the temperature, time, and heating rate of the flame-retardant process. While these methods suppress the double structure evenly, they fail to recognize the undesirable effects of unevenness and the presence of fluff. In other words, none of these inventions consider heat removal within the flame-retardant fiber bundle during the flame-retardant process. Therefore, when the single fiber fineness is large, it is impossible to suppress the specific fluff inherent in the carbon fiber bundle caused by temperature unevenness, or to suppress the entanglement caused by the annular fluff generated when unwinding the carbon fiber bundle before further processing.

[0016] The purpose of this invention is to provide a carbon fiber bundle and a method for manufacturing the same, wherein each single fiber of the carbon fiber bundle has high load-bearing capacity and excellent abrasion resistance, and even if the single fiber fineness is large, the entanglement caused by the annular fluff generated when unwinding the carbon fiber bundle during high-level processing can be suppressed by suppressing the specific fluff inherent in the carbon fiber bundle.

[0017] Methods for solving problems

[0018] To achieve the stated objective, the present invention has the following configuration.

[0019] That is, the carbon fiber bundle of the present invention is characterized in that the average single fiber diameter B is 6.9 to 11.0 μm, the tensile elastic modulus E of the resin impregnated bundle is 230 to 310 GPa, the number of internal fibers in the carbon fiber bundle is less than 40 per m, and the proportion of fibers with a double structure in the cross section of the internal fibers in the carbon fiber bundle is 1 to 25%.

[0020] Furthermore, the method for manufacturing carbon fiber bundles of the present invention is characterized in that, in the process of heat-treating polyacrylonitrile precursor fiber bundles with a single fiber fineness of 0.9 to 2.2 dtex under an oxidizing atmosphere at 200 to 300°C, when the heating rate of the single fiber is set to q (J / g / s), the number of filaments N is set to N (strands), the single fiber fineness of the flame-retardant fiber bundle is set to d (dtex), and the yarn width is set to W (mm), the heating rate Q calculated by equation (3) is 150 to 500 J / m. 2 Heat treatment is carried out in a manner similar to / s until the density reaches 1.22–1.24 g / cm³. 3 Then, the fiber bundle tension is set to 1.6–4.0 mN / dtex for heat treatment until the density becomes 1.38–1.50 g / cm³. 3 A flame-retardant fiber bundle is obtained, and then the flame-retardant fiber bundle is heat-treated in an inactive atmosphere at 1,200 to 1,600°C to obtain a carbon fiber bundle.

[0021] Q=q×N×d / W / 10 · · · (3)

[0022] The effects of the invention

[0023] According to the present invention, a carbon fiber bundle and a method for manufacturing the same are provided, wherein each single fiber of the carbon fiber bundle has high load-bearing capacity and excellent abrasion resistance, and even if the single fiber fineness is large, the entanglement caused by the annular fluff generated when the carbon fiber bundle is unwound for high-level processing can be suppressed by suppressing the specific fluff inherent in the carbon fiber bundle. Attached Figure Description

[0024] [ Figure 1 This is an example of a photograph showing the structure of inner and outer layers in a cross-section of the inherent fluff in a carbon fiber bundle.

[0025] [ Figure 2 This is an example of a photograph showing a cross-section of the internal fibers in a carbon fiber bundle, with a hole in the center of the cross-section of the fibers.

[0026] [ Figure 3 This is an example of a photograph showing a cross-section of the internal fibers in a carbon fiber bundle, where the fibers appear to have broken due to bending.

[0027] [ Figure 4This is an example of a photograph showing a deformed cross-section of the internal fibers within a carbon fiber bundle.

[0028] [ Figure 5 This is an example of a photograph showing a cross-section of the internal fibers in a carbon fiber bundle that is vertical and cracked from the center. Detailed Implementation

[0029] The inventors of this application discovered that, in manufacturing carbon fiber bundles with high load-bearing capacity and excellent abrasion resistance for each single fiber, and which can suppress entanglement caused by the annular fluff generated during the unwinding of the carbon fiber bundle when it is not subjected to high-level processing, even if the single fiber fineness is large, by suppressing the specific fluff inherent in the carbon fiber bundle, the heat removal relative to the total heat generation of the flame-retardant fiber bundle can be fully ensured by appropriately controlling the heating rate of the single fiber, the number of filaments N, the single fiber fineness and the yarn width during the flame-retardant process. Even if the single fiber fineness is large, the temperature unevenness within the flame-retardant fiber bundle can be reduced, thus completing the present invention.

[0030] First, the carbon fiber bundle of the present invention will be described.

[0031] The average single fiber diameter B of the carbon fiber bundle of the present invention is 6.9 to 11.0 μm, preferably 7.0 to 10.0 μm, and more preferably 7.1 to 9.0 μm. If the average single fiber diameter B is 6.9 μm or more, fuzz generated by rubbing can be suppressed, thus suppressing fuzz generated during unwinding of the carbon fiber bundle. If the average single fiber diameter B is 11.0 μm or less, the double structure of the carbon fiber bundle can be suppressed, thus suppressing entanglement caused by annular fuzz generated during unwinding of the carbon fiber bundle before further processing. The average single fiber diameter B can be calculated based on the mass and density per unit length of the carbon fiber bundle and the number of filaments N. This average single fiber diameter B can be achieved by controlling the discharge rate, the draw ratio of each process, and the specific gravity of the flame-retardant fiber bundle in the manufacturing process of the polyacrylonitrile precursor fiber bundle.

[0032] The tensile modulus E of the resin-impregnated carbon fiber bundle in the tensile test of the present invention is 230-310 GPa, preferably 245-300 GPa, and more preferably 250-290 GPa. If the tensile modulus E is 230 GPa or higher, a satisfactory modulus of elasticity can be obtained when it is typically used to enhance the modulus of elasticity. If the tensile modulus E is 310 GPa or lower, fuzzing caused by rubbing can be suppressed, and thus entanglement caused by annular fuzz generated when unwinding the carbon fiber bundle for further processing can be suppressed. The tensile modulus E can be determined by the method described in the tensile test of the carbon fiber bundle described later. In this case, the strain range is set to 0.1-0.6%. The tensile modulus E of the carbon fiber bundle can be controlled by applying tension to the fiber bundle mainly during any heat treatment process in the manufacturing process of the carbon fiber bundle, improving the dual structure, or changing the carbonization temperature.

[0033] The crystallite size Lc of the carbon fiber bundle of the present invention is preferably 1.5 to 2.5 nm, more preferably 1.6 to 2.3 nm, and even more preferably 1.7 to 2.2 nm. If the crystallite size Lc is 1.5 nm or more, the annular fluff generated during the unwinding of the carbon fiber bundle can be suppressed, and is therefore preferred. If the crystallite size Lc is 2.5 nm or less, it is not necessary to increase the maximum temperature of the carbonization process to the required level, thus exhibiting excellent abrasion resistance and suppressing the annular fluff generated during the unwinding of the carbon fiber bundle, and is therefore preferred. The crystallite size Lc can be determined by a known method using a wide-angle X-ray diffraction apparatus, and the Scherrer constant in the Scherrer formula described later is 1. The crystallite size Lc can be controlled by changing the carbonization temperature.

[0034] In the carbon fiber bundle of the present invention, the relationship between the tensile elastic modulus E and the crystallite size Lc (nm) preferably satisfies Equation (1), and the intercept on the left side of Equation (1) is more preferably 135, and even more preferably 140. The intercept on the right side of Equation (1) is more preferably 175, and even more preferably 170.

[0035] 50×Lc+130≤E≤50×Lc+180 · · · (1).

[0036] Carbon fiber bundles are essentially polycrystalline structures composed of numerous graphite crystals. Increasing the maximum temperature of the carbonization process increases the crystallinity of the crystals. That is, rearrangement of the carbon network surfaces occurs, the crystallite size increases, and the crystal orientation progresses, thus tending to increase the tensile modulus E of the carbon fiber bundle. Therefore, the tensile modulus E and the crystallite size Lc exhibit the relationship shown in Equation (1). If the intercept on the left side of Equation (1) is 130° or higher, the tensile modulus E of the bundle can be efficiently increased even at a low carbonization temperature. Therefore, a high tensile modulus E can be obtained while suppressing fuzz generated by rubbing, which is preferable. If the intercept on the left side of Equation (1) is 180° or lower, it is not necessary to increase the maximum temperature of the carbonization process to a necessary level to increase the tensile modulus E of the bundle. Therefore, excellent rubbing resistance can be achieved, suppressing entanglement caused by annular fuzz generated when unwinding the carbon fiber bundle before further processing, which is preferable. The tensile modulus E and the crystallite size Lc of the bundle can be determined by the above method. To achieve the relationship between the tensile elastic modulus E and the crystallite size Lc of the above-mentioned wire bundle, the following operations can be performed: apply tension to the fiber bundle during any heat treatment process in the manufacturing process of the carbon fiber bundle, improve the dual structure, or change the carbonization temperature.

[0037] In the carbon fiber bundle of the present invention, the number of internal fibers in the carbon fiber bundle is 40 or less per meter, preferably 35 or less per meter, and more preferably 30 or less per meter. The internal fibers in the carbon fiber bundle refer to the fibers present inside the carbon fiber bundle when it is pulled out from the winding spool. If the number of internal fibers in the carbon fiber bundle is 40 or less per meter, the entanglement caused by the annular fibers generated during the unwinding of the carbon fiber bundle before further processing can be sufficiently suppressed. Regarding the method for measuring the number of internal fibers in the carbon fiber bundle, a 10m carbon fiber bundle is pulled out from the winding spool, and the carbon fiber bundle is divided into individual fibers with a force that does not generate fibers, such that the thickness of the carbon fiber bundle is equal to the number of individual fibers. If fibers are present, they are collected, and their number is measured, calculated as the number per meter. It should be noted that this excludes the fibers generated during the individual fiber division stage. To control the number of internal fibers in the carbon fiber bundle within the specified range, as described later, this can be achieved by appropriately controlling the heating rate of the single fiber, the number of filaments N, the fineness of the single fiber, and the yarn width during the flame-retardant process.

[0038] In the carbon fiber bundle of the present invention, the proportion of fibers with a double structure in the cross-section of the inherent fibers within the carbon fiber bundle is 1-25%, preferably 1-20%, and more preferably 2-15%. The term "fibers with a double structure in the cross-section of the inherent fibers within the carbon fiber bundle" refers to the condition where, when the fibers present inside the carbon fiber bundle are recovered and the cross-section is observed using a scanning electron microscope (SEM), if... Figure 1The structure shown contains inner and outer layers, and / or, as... Figure 2 The cross-section of the shown fluff has a hole at its center. However, regarding the cross-section of the recovered fluff... Figure 3 The fluff shown is judged to have broken due to bending. Since the fluff generated during the manufacturing process of carbon fiber bundles is not considered to be the inherent fluff in the carbon fiber bundles, but rather the fluff that broke due to bending load applied during the recycling of fluffs, it is excluded from the total number of fluffs used to calculate the proportion of fluffs with a dual cross-section among the fluffs inherent in the carbon fiber bundles.

[0039] While the mechanism behind the dual-structure cross-section of the inherent fibers in these carbon fiber bundles may not be fully understood, it can be considered as follows: Compared to the average dual structure in flame-retardant fiber bundles, the dual structure is specifically enlarged in the regions where uneven temperatures occur during the flame-retardant process, leading to the breakage of individual fibers within the carbon fiber bundle under weak loads, resulting in a cross-section with a dual-structured fiber. Therefore, it is believed that this dual-structured fiber is particularly prone to formation under weak loads, and is considered the primary reason for the inherent fiber structure in carbon fiber bundles.

[0040] By including a certain proportion of such particularly weak fibers in the normally generated fibers, loop-shaped fibers are generated that grow larger while entangling other fibers. Therefore, it is believed that the fiber bundle has a dual-structure cross-section due to the inherent fibers in the fiber bundle having a certain proportion relative to the fiber bundle as a whole, thereby causing winding caused by loop-shaped fibers generated when unwinding the carbon fiber bundle for further processing.

[0041] Therefore, if the proportion of fibers with a double cross-section in the internal fibers of the carbon fiber bundle is less than 25%, the entanglement caused by the annular fibers generated during unwinding of the carbon fiber bundle before further processing can be effectively suppressed. If the proportion of fibers with a double cross-section in the internal fibers of the carbon fiber bundle is more than 1%, the tensile strength of the carbon fiber bundle will not decrease, thus reducing the proportion of low-strength single fibers that affect the fibers, thereby suppressing the annular fibers generated near the rollers during unwinding of the carbon fiber bundle.

[0042] The double-structured fibers within the cross-section of the inherent fibers in carbon fiber bundles are not present in commercially available carbon fiber bundles, but as described later, they can be controlled within the aforementioned range by controlling the flame-retardant processing. To determine whether the inherent fibers within the carbon fiber bundle have a double-structured cross-section, the carbon fiber bundle wound on the winding spool can be pulled out using the aforementioned method, and the fibers present inside the carbon fiber bundle can be recovered. The cross-section can then be observed using SEM for judgment (details will be explained later). To control the proportion of fibers with a double-structured cross-section within the inherent fibers of the carbon fiber bundle within the aforementioned range, as described later, this can be achieved by appropriately controlling the heating rate of the single fiber, the number of filaments (N), the fineness of the single fiber, and the yarn width during the flame-retardant processing.

[0043] In the carbon fiber bundle of the present invention, the proportion of the cross-section of the internal fluff of the carbon fiber bundle with the area perpendicular to the fiber axis being less than 50% is preferably 0 to 3%, more preferably 0.1 to 2.5%, and even more preferably 0.5 to 1.5%.

[0044] Here, the cross-section of the inherent fibers in the carbon fiber bundle refers to the cross-section observed using a scanning electron microscope (SEM) perpendicular to the fiber axis after recovering the fibers present inside the carbon fiber bundle. Furthermore, a cross-section in which the area perpendicular to the fiber axis constitutes less than 50% of the total cross-section of the inherent fibers in the carbon fiber bundle implies that the cross-section of the inherent fibers in the carbon fiber bundle is not substantially perpendicular, but rather does not maintain the original shape of the single fiber. Figure 4 The deformation shown or as Figure 5 As shown, it cracks from the center.

[0045] The proportion of the cross-sectional area of ​​the inherent fluff in a carbon fiber bundle is defined as the proportion of the area perpendicular to the fiber axis that is less than 50% of the average cross-sectional area of ​​a section obtained by perpendicularly cutting a single fiber. Therefore, as... Figure 5 As shown, although the cross-section of the fibers is perpendicular, the cross-section that splits from the center is also included. A cross-section perpendicular to the fiber axis is defined as a cross-section at an angle of 85–95° relative to the fiber axis.

[0046] The reason for specifying that the cross-section of the inherent fluff in the carbon fiber bundle has an area perpendicular to the fiber axis of 50% or less is not necessarily clearly understood, but is believed to be as follows: It is believed that due to the particularly large temperature unevenness in the flame-retardant process, the cross-section of the aforementioned fluff becomes more specifically large for the dual-structure compared to a cross-section with a dual structure, leading to single-fiber breakage of the carbon fiber bundle under weak loads, and deformation of the cross-section of the fluff with a dual structure. Therefore, it is believed that fluff in a cross-section of the inherent fluff in the carbon fiber bundle with an area perpendicular to the fiber axis of 50% or less is more likely to form under weaker loads compared to a cross-section of fluff with a dual structure, which presumably leads to a significant increase in entanglement caused by the annular fluff generated during the unwinding of the carbon fiber bundle for higher processing. Therefore, if the proportion of the cross-section of the inherent fluff in the carbon fiber bundle with an area perpendicular to the fiber axis of 50% or less is 3% or less, entanglement caused by the annular fluff generated during the unwinding of the carbon fiber bundle for higher processing can be sufficiently suppressed, and is therefore preferred.

[0047] Regarding the fluff within the carbon fiber bundle, where the area of ​​the cross-section perpendicular to the fiber axis is 50% or less, as described above, the carbon fiber bundle wound on a bobbin is pulled out, and the fluff present inside the carbon fiber bundle is recovered. For an image obtained by observing the cross-section at approximately a 45° angle using SEM, the angle is measured using the protractor tool in image analysis software. For an image obtained by observing the same cross-section from the front using SEM, the area ratio of the cross-section at 85-95° relative to the fiber axis is extracted based on image analysis. Furthermore, regarding the cross-sectional area of ​​a single fiber in the carbon fiber bundle, a vertical cross-section is obtained by vertically cutting the carbon fiber bundle with a single blade. The cross-section of the extracted single fiber is observed from the front using SEM, and the image is analyzed using image analysis software, thereby enabling measurement.

[0048] In order to control the area of ​​the cross section of the internal fluff in the carbon fiber bundle that is perpendicular to the fiber axis to less than 50% within the above range, as described later, this can be achieved by appropriately controlling the heating rate of the single fiber, the number of filaments N, the fineness of the single fiber and the yarn width in the flame-retardant process.

[0049] The yarn width W of the carbon fiber bundle in this invention is preferably 5 to 8 mm, more preferably 6 to 8 mm, and even more preferably 7 to 8 mm. The yarn width W of the carbon fiber bundle refers to the width of the carbon fiber bundle when it is unwound from the winding bobbin. Unless a special fiber-opening process is added, it roughly reflects the width of the fiber bundle from the flame-retardant process. If the yarn width W is 5 mm or more, lint caused by rubbing can be suppressed, thus suppressing lint generated when unwinding the carbon fiber, which is therefore preferable. If the yarn width W is 8 mm or less, it can suppress the situation where lint is generated when the carbon fiber bundle is unwound from the winding bobbin due to expansion beyond a certain point, which is also preferable. The yarn width W of the carbon fiber bundle can be measured using a ruler or the like when unwinding the carbon fiber bundle from the winding bobbin. The aforementioned yarn width W of the carbon fiber bundle can be achieved by the yarn width of the polyacrylonitrile precursor fiber bundle and the tension of the flame-retardant fiber bundle in the flame-retardant process.

[0050] The number of filaments N in the carbon fiber bundle of the present invention is preferably 10,000 to 50,000, more preferably 10,000 to 30,000, and even more preferably 15,000 to 25,000. The number of filaments N in the carbon fiber bundle is the number of individual fibers constituting the carbon fiber bundle. If the number of filaments N is 10,000 or more, the possibility of specific fuzz inherent in the carbon fiber bundle appearing on the surface of the carbon fiber bundle can be reduced, and fuzz during unwinding from the winding spool can be sufficiently reduced, which is therefore preferred. If the number of filaments N is 50,000 or less, fuzz generated by rubbing can be suppressed, and therefore fuzz generated during unwinding of the carbon fiber bundle can be suppressed, which is also preferred. The number of filaments N in the carbon fiber bundle can be determined based on the average single fiber diameter B of the carbon fiber bundle, the specific gravity of the carbon fiber bundle, and the mass per unit area (mass per unit length), as described later. The number N of the carbon fiber bundle can be achieved by the number of holes in the spinneret during the manufacturing process of the polyacrylonitrile precursor fiber bundle, or by stacking multiple polyacrylonitrile precursor fiber bundles.

[0051] Regarding the knot strength A [MPa] of the carbon fiber bundle of the present invention, in relation to the average single fiber diameter B (μm), it is preferable that -88B+1360≤A, more preferably -88B+1370≤A, and even more preferably -88B+1390≤A. Knot strength is an indicator reflecting the mechanical properties of the fiber bundle other than the fiber axis direction; it is a parameter reflecting the strength relative to the bending load and compressive load applied in directions other than the fiber axis direction when unwinding the carbon fiber bundle from the winding bobbin. When the knot strength satisfies -88B+1360≤A, the fluff during unwinding of the carbon fiber bundle from the winding bobbin is reduced, which is therefore preferable. The above-mentioned knot strength can be obtained by the method described later in the section on the knot strength of carbon fiber bundles. To improve the knot strength of the carbon fiber bundle, in the manufacturing method of the carbon fiber bundle of the present invention described later, it is preferable to appropriately control the heating rate of the single fiber, the number of filaments N, the fineness of the single fiber, and the yarn width in the flame-retardant process.

[0052] In the carbon fiber bundle of the present invention, the tensile strength (also referred to as bundle tensile strength) in the resin-impregnated bundle tensile test is preferably 5.5 to 7.0 GPa, more preferably 5.8 to 6.8 GPa, and even more preferably 5.9 to 6.7 GPa. The bundle tensile strength is closely related to the average value of the single fiber strength, and is therefore a parameter affecting the tensile strength of the fibers themselves; a higher value is preferred. Among these, the magnitude of the strength deviation is more important than the average value of the single fiber strength. If the bundle tensile strength is 5.5 GPa or higher, the entanglement caused by the annular fibers generated during unwinding of the carbon fiber bundle before further processing can be sufficiently suppressed, which is therefore preferred. A higher bundle tensile strength is preferred, but if the bundle tensile strength is 7.0 GPa or lower, the entanglement caused by the annular fibers generated during unwinding of the carbon fiber bundle before further processing can be sufficiently suppressed, which is also preferred. The bundle tensile strength can be determined by the method described in the bundle tensile test of the carbon fiber bundle described later. It should be noted that the above parameters can be controlled by using the carbon fiber bundle manufacturing method of the present invention, which will be described later.

[0053] In the carbon fiber bundle of the present invention, the area ratio (hereinafter referred to as the outer layer area ratio) of the outer periphery (outer layer) of the dual structure relative to the overall cross-section perpendicular to the fiber axis is preferably 85-95% of the area, more preferably 87-94% of the area, and even more preferably 89-93% of the area. Here, the outer layer area ratio refers to the area ratio (%) obtained by dividing the area of ​​the outer periphery seen when observing the cross-section perpendicular to the fiber axis of a single fiber with an optical microscope by the overall cross-sectional area perpendicular to the fiber axis of the single fiber.

[0054] Compared to the outer layer of a single fiber, the internal crystalline portion has a lower degree of orientation and is a region with a low tensile elastic modulus E in the fiber bundle. Therefore, a higher outer layer area ratio is more effective in suppressing fluff (single fiber breakage), which is preferred. If the outer layer area ratio is 85% or more, specific fluff inherent in the carbon fiber bundle can be suppressed, which is also preferred. If the outer layer area ratio is 95% or less, fluff caused by rubbing, which is prone to occur due to excessive heat treatment during the flame-retardant process, can be suppressed, which is also preferred.

[0055] The outer layer area can be determined by embedding carbon fiber bundles in resin, grinding the cross-section perpendicular to the fiber axis, and observing the cross-section using an optical microscope (details will be explained later). Regarding the aforementioned outer layer area ratio, as described later, it can be achieved by appropriately controlling the heating rate of the single fiber, the number of filaments (N), the fineness of the single fiber, and the yarn width during the flame-retardant process.

[0056] Regarding the method for manufacturing carbon fiber bundles according to the present invention, it has been found that: For the problem of producing fiber bundles that can suppress entanglement caused by the annular fibers generated during unwinding of the carbon fiber bundle before further processing by suppressing specific internal fibers within the carbon fiber bundle, appropriate control of the heating rate of individual fibers, filament number N, individual fiber fineness, and yarn width during the flame-retardant process can sufficiently ensure heat removal relative to the total heat generation of the flame-retardant fiber bundle, thereby reducing temperature unevenness within the flame-retardant fiber bundle. Embodiments suitable for carrying out the present invention will be described in detail below.

[0057] As a raw material for manufacturing polyacrylonitrile-based precursor fiber bundles, polyacrylonitrile polymers are preferably used. It should be noted that, in this invention, polyacrylonitrile polymers refer to polymers in which acrylonitrile is at least a major component of the polymer backbone, and the major component typically refers to a component that accounts for 90-100% by mass of the polymer backbone.

[0058] In the manufacture of polyacrylonitrile-based precursor fiber bundles, from the perspectives of improving fiber formation and efficiently performing flame-retardant treatment, the polyacrylonitrile polymer preferably contains copolymer components such as itaconic acid, acrylamide, and methacrylic acid. In the manufacture of polyacrylonitrile-based precursor fiber bundles, known polymerization methods can be selected as the manufacturing method for the polyacrylonitrile polymer.

[0059] In the manufacture of the polyacrylonitrile precursor fiber bundle suitable for obtaining the carbon fiber bundle of the present invention, the spinning solution is a solution obtained by dissolving the above-mentioned polyacrylonitrile polymer in a polyacrylonitrile-soluble solvent such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, or an aqueous solution of nitric acid / zinc chloride / sodium thiocyanate.

[0060] The method for manufacturing polyacrylonitrile fiber bundles used in this invention is not particularly limited, but wet spinning or dry-wet spinning is preferred, followed by processes such as stretching, washing, oiling, drying and densification, and post-stretching as needed. The number of orifices in the spinneret during the manufacturing process of the polyacrylonitrile precursor fiber bundle is not particularly limited, but considering ease of fiber bonding, a number of orifices of 1,000 to 10,000 is preferred to achieve the aforementioned number of filaments N in the carbon fiber bundle.

[0061] In the manufacture of polyacrylonitrile-based precursor fiber bundles, the coagulation bath preferably contains solvents such as dimethyl sulfoxide, dimethylformamide, and dimethylacetamide, which are used as solvents in the spinning solution, and a so-called coagulation-promoting component. As the coagulation-promoting component, an ingredient that does not dissolve the aforementioned polyacrylonitrile polymer and is compatible with the solvent used in the spinning solution can be used. Water is preferably used as the coagulation-promoting component.

[0062] In the manufacture of polyacrylonitrile precursor fiber bundles, it is preferable to use a multi-stage water bath with water temperatures ranging from 30 to 98°C during the washing process.

[0063] In addition, the stretching ratio in the water bath stretching process is preferably 2 to 6 times.

[0064] After the water bath stretching process, to prevent the individual fibers from sticking together, it is preferable to apply an oil containing silicone or the like to the yarn. The silicone oil is preferably a modified silicone, and more preferably an oil containing an amino-modified silicone with high heat resistance.

[0065] The drying heat treatment process can utilize known methods. For example, a drying temperature of 100–200°C can be used.

[0066] From the viewpoint of the density and productivity of the obtained polyacrylonitrile precursor fiber bundles, the dried yarn is preferably further stretched under pressurized steam or dry heat. The steam pressure or temperature during post-stretching and the post-stretching ratio can be appropriately selected within a range that does not produce filament breakage or fuzz.

[0067] In the carbon fiber bundle manufacturing method of the present invention, the fineness of the single fiber of the polyacrylonitrile precursor fiber bundle is 0.9 to 2.2 dtex, preferably 1.0 to 1.8 dtex, and more preferably 1.1 to 1.7 dtex. The fineness of the single fiber of the polyacrylonitrile precursor fiber bundle refers to the diameter of the single fiber in the polyacrylonitrile precursor fiber bundle. If the fineness of the single fiber of the polyacrylonitrile precursor fiber bundle is 0.9 dtex or more, the abrasion resistance of the obtained carbon fiber bundle is improved, thus suppressing the fluff generated during unwinding of the carbon fiber bundle. If the fineness of the single fiber of the polyacrylonitrile precursor fiber bundle is 2.2 dtex or less, sufficient heat removal relative to the total heat generation of the flame-retardant fiber bundle can be ensured in the flame-retardant process, reducing temperature unevenness within the flame-retardant fiber bundle and suppressing specific fluff inherent in the carbon fiber bundle. The fineness of the single fiber of the polyacrylonitrile precursor fiber bundle can be calculated based on the mass and density per unit length of the polyacrylonitrile precursor fiber bundle and the number of filaments N. The aforementioned polyacrylonitrile precursor fiber bundles can be manufactured by controlling the discharge amount and the stretching ratio of each process in the manufacturing process of the polyacrylonitrile precursor fiber bundles.

[0068] In the manufacturing of carbon fiber bundles according to the present invention, following the aforementioned polyacrylonitrile precursor fiber bundle manufacturing process, it is preferable to perform fiber bonding before the flame-retardant process, based on the number of filaments N of the polyacrylonitrile precursor fiber bundle. Regarding the preferred bonding method, after unwinding the polyacrylonitrile precursor fiber bundle from the bearing, the fiber bonding is performed in such a way that the number of filaments N of the polyacrylonitrile precursor fiber bundle becomes the number of filaments N of the target carbon fiber bundle.

[0069] In the carbon fiber bundle manufacturing method of the present invention, the temperature in the step of heat treatment (flame-retardant process) of the polyacrylonitrile precursor fiber bundle under an oxidizing atmosphere is 200-300°C, preferably 220-290°C, and more preferably 230-280°C. If the heat treatment temperature is above 200°C, it is less likely that untreated portions will form within the flame-retardant fiber bundle due to excessively low heat treatment temperature, thus reducing the formation of uneven double structures and significantly minimizing fuzz when unwinding the carbon fiber bundle from the winding spool. If the heat treatment temperature is below 300°C, the heating rate will not unnecessarily increase, thus reducing temperature unevenness within the flame-retardant fiber bundle and suppressing inherent fuzz within the carbon fiber bundle. To measure the heat treatment temperature, a thermocouple or other thermometer can be inserted into the heat treatment furnace of the flame-retardant process to measure the furnace temperature. When measuring the furnace temperature at multiple points, if temperature unevenness or temperature distribution exists, a simple average temperature is calculated.

[0070] Regarding the method for manufacturing carbon fiber bundles of the present invention, when the heating rate of a single fiber in the flame-retardant process is set to q (J / g / s), the number of filaments N is set to N (strands), the fineness of a single fiber in the flame-retardant fiber bundle is set to d (dtex), and the yarn width is set to W (mm), the heating rate Q calculated by equation (3) is 150 to 500 J / m. 2 Heat treatment is carried out in a manner similar to / s until the density reaches 1.22–1.24 g / cm³. 3 .

[0071] Q=q×N×d / W / 10···(3).

[0072] The density of flame-retardant fiber bundles is commonly used as an indicator of the extent of the flame-retardant reaction. The density is typically 1.22–1.24 g / cm³. 3 This means that in the initial stage of the flame-retardant process, controlling the heating rate of the initial stage of the flame-retardant process within an appropriate range is related to controlling the proportion of fibers with a dual structure in the cross-section of the inherent fibers in the carbon fiber bundle. This is important because it can suppress the entanglement caused by the ring-shaped fibers generated when the carbon fiber bundle is unwound for further processing.

[0073] If the density is 1.22 g / cm³ 3 Therefore, even if heat treatment is carried out at high temperature in the subsequent flame-retardant process, the rapid increase in the heating rate within the flame-retardant fiber bundle can be suppressed. This results in the suppression of temperature unevenness within the flame-retardant fiber bundle and the suppression of the proportion of fibers with a dual structure in the cross-section of the internal fibers within the carbon fiber bundle.

[0074] If the density is 1.24 g / cm³ 3 The following is a structure that can fully control the dual structure of the flame-retardant fiber bundle. When controlling the heating rate described later, it can effectively suppress the proportion of fibers with a dual structure in the cross section of the internal fibers of the carbon fiber bundle.

[0075] To confirm that the density of the heat-treated fiber bundles at the heating rate Q described later is within the above range, the density can be measured by collecting samples of the fiber bundles during the flame-retardant process (the method for measuring density will be explained later). For example, if the density of the flame-retardant fiber bundles is lower than specified, the density can be adjusted by increasing the temperature or extending the flame-retardant process time. Here, an oxidizing atmosphere refers to an atmosphere containing more than 10% by mass of known oxidizing substances such as oxygen and nitrogen dioxide; for simplicity, an air atmosphere is preferred.

[0076] In the flame-retardant process of the carbon fiber bundle manufacturing method of the present invention, the density is increased until it reaches 1.22 to 1.24 g / cm³. 3 The heating rate Q up to this point is 150–500 J / m2 / s, preferably 160~400J / m 2 / s, more preferably 180~350J / m 2 / s. It should be noted that this applies as long as the density is reduced to 1.22–1.24 g / cm³. 3 Once the heating rate Q is controlled within the aforementioned range up to the set density range, this requirement will still be met even if the heating rate Q is subsequently changed up to the set density range. For example, as long as the heating rate Q is controlled up to the density of 1.23 g / cm³, this requirement will still be met. 3 The heating rate Q is controlled between 150 and 500 J / m. 2 The range can be adjusted as needed until the density becomes greater than 1.23 g / cm³. 3 The heating rate Q, up to the density, can be 150–500 J / m³. 2 Outside the range of / s.

[0077] In this invention, the heating rate Q represents the heating rate per unit area of ​​the flame-retardant fiber bundle, obtained by dividing the total heating rate per unit length of the flame-retardant fiber bundle (the numerator of formula (3)) by the yarn width of the flame-retardant fiber bundle. It signifies the relationship between the heating and deheating of the flame-retardant fiber bundle, i.e., the heating rate that takes into account the influence of deheating. Therefore, the heating rate Q becomes a parameter reflecting the temperature unevenness of the flame-retardant fiber bundle. Thus, a smaller heating rate Q means a smaller temperature unevenness of the flame-retardant fiber bundle.

[0078] If the heating rate Q is 150 J / m 2 When the heat generation and heat removal are above a certain value, the heat generation and heat removal are well balanced, so it is not easy to form untreated parts in the flame-retardant fiber bundle. The unevenness of the dual structure disappears, thus significantly reducing the fluff when the carbon fiber bundle is unwound from the winding drum.

[0079] If the heating rate Q is 500 J / m 2 If the deheating rate is below / s, the deheating rate is sufficiently high relative to the heating rate, thus reducing the temperature unevenness within the flame-retardant fiber bundle and suppressing the specific internal fluff in the carbon fiber bundle.

[0080] To calculate the heating rate Q, the heating rate q (J / g / s) of a single fiber and the yarn width W (mm) can be determined using the method described later. The number of filaments N and the fineness d (dtex) of the single fiber in the flame-retardant fiber bundle can be used to calculate the heating rate Q according to equation (3). The heating rate Q can be controlled by the heat treatment temperature in the flame-retardant process, the number of filaments N, the fineness of the single fiber in the flame-retardant fiber bundle, and the spacing (width) of the grooves of the roller.

[0081] In the method for manufacturing carbon fiber bundles of the present invention, the final density of the flame-retardant fiber bundles in the flame-retardant process is 1.38–1.50 g / cm³. 3The preferred density is 1.42–1.48 g / cm³. 3 If the final density of the flame-retardant fiber bundle is 1.38 g / cm³ 3 The above methods can suppress the fuzz generated by the rubbing of the carbon fiber bundle, thus suppressing the fuzz generated during the unwinding of the carbon fiber bundle. If the final density of the flame-retardant fiber bundle is 1.50 g / cm³... 3 The following process can prevent the need for additional heat treatment, thereby suppressing the proportion of fibers with a dual structure in the cross-section of the inherent fibers in the carbon fiber bundle, and suppressing the fibers generated during the unwinding of the carbon fiber bundle.

[0082] To confirm that the final density of the flame-retardant fiber bundle is within the specified range, the density of the flame-retardant fiber bundle can be measured (the method for density measurement will be explained later). For example, if the density of the flame-retardant fiber bundle is lower than specified, the density can be adjusted by increasing the temperature or extending the flame-retardant treatment time. Here, an oxidizing atmosphere refers to an atmosphere containing more than 10% by mass of known oxidizing substances such as oxygen and nitrogen dioxide; for simplicity, an air atmosphere is preferred.

[0083] In the flame-retardant process of the carbon fiber bundle manufacturing method of the present invention, when the heating rate of a single fiber is set to q (J / g / s), the number of filaments N is set to N (strands), the fineness of a single fiber in the flame-retardant fiber bundle is set to d (dtex), and the yarn width is set to W (mm), the heating rate Q calculated by equation (3) is 150 to 500 J / m. 2 Heat treatment is carried out in a manner similar to / s until the density reaches 1.22–1.24 g / cm³. 3 Then, the heating rate Q, obtained from equation (3), is preferably 300–1200 J / m. 2 Heat treatment is performed in a manner that is more preferably 400–1100 J / m. 2 Heat treatment is performed in a / s manner, and further optimization is carried out to achieve a heat resistance of 500-1000 J / m. 2 The material is heat-treated until its density reaches 1.32–1.35 g / cm³. 3 .

[0084] For example, the heating rate Q is 150–500 J / m 2 Heat treatment was carried out in a manner that was repeated until the density reached 1.23 g / cm³. 3 Then, the heating rate Q becomes 300–1200 J / m 2 Heat treatment was carried out in a manner similar to / s until a density of 1.33 g / cm³ was achieved. 3 Under certain circumstances, this requirement is met.

[0085] At this point, the heating rate Q is 150–500 J / m.2 Heat treatment was carried out in a manner that was repeated until the density reached 1.23 g / cm³. 3 After that, although the density became 1.24 g / cm³ 3 The heating rate Q is not 150-500 J / m 2 / s, but preferably the heating rate Q is 300 to 1200 J / m. 2 / s (e.g., 800J / m) 2 Heat treatment is performed at a rate of 1500 J / m², but it is not preferred outside the range of all heating rates Q (e.g., 1500 J / m²). 2 / s). Density is 1.32~1.35g / cm³. 3 The degree of flame-retardant reaction in the flame-retardant fiber bundles is moderate. The heating rate during this moderate degree of flame-retardant reaction affects the final dual structure of the flame-retardant fiber bundles and carbon fiber bundles. Therefore, it is preferable to use fibers with a density of 1.22–1.24 g / cm³. 3 The density is 1.32–1.35 g / cm³. 3 The heating rate Q is controlled within the above range.

[0086] If the heating rate Q is 300 or higher, the heat generation and heat removal are well balanced, so it is not easy to form untreated parts in the flame-retardant fiber bundle. The unevenness of the dual structure disappears, so the lint when unwinding the carbon fiber bundle into the spool can be significantly reduced, which is preferred.

[0087] If the heating rate Q is below 1200, the heat removal rate is sufficiently large relative to the heating rate, thus reducing temperature unevenness within the flame-retardant fiber bundle and suppressing specific fibers inherent in the carbon fiber bundle, which is therefore preferred.

[0088] To confirm that the density during heat treatment at the heating rate Q falls within the specified range, the density of the fiber bundles collected during the flame-retardant process can be measured (the method for measuring density will be explained later). For example, if the density of the flame-retardant fiber bundles is lower than specified, the density can be adjusted by increasing the temperature or extending the flame-retardant treatment time.

[0089] Here, an oxidizing atmosphere refers to an atmosphere containing more than 10% by mass of known oxidizing substances such as oxygen and nitrogen dioxide. For simplicity, an air atmosphere is preferred. To calculate the heating rate Q, the heating rate q (J / g / s) of a single fiber, the number of filaments N, the fineness d (dtex) of the single fiber of the flame-retardant fiber bundle, and the yarn width W (mm) can be determined using the method described later, and calculated according to Equation (3). The heating rate Q can be controlled by the heat treatment temperature in the flame-retardant process, the number of filaments N, the fineness of the single fiber of the flame-retardant fiber bundle, and the spacing (width) of the grooves of the rollers.

[0090] In the flame-retardant process of the carbon fiber bundle manufacturing method of the present invention, the heating rate Q, calculated by equation (3), is 150 to 500 J / m. 2 Heat treatment is carried out in a manner similar to / s until the density reaches 1.22–1.24 g / cm³. 3 Then, the heating rate Q, obtained from equation (3), becomes 300–1200 J / m. 2 Heat treatment is carried out in a manner similar to / s until the density reaches 1.32–1.35 g / cm³. 3 Then, the heating rate Q, calculated by equation (3), is preferably 900–1500 J / m. 2 / s, more preferably 1000-1400 J / m 2 / s, further optimized to 1100~1300J / m 2 Heat treatment is carried out in a manner similar to / s until the density reaches 1.38–1.50 g / cm³. 3 .

[0091] For example, the heating rate Q is set to 150–500 J / m. 2 The material is heat-treated until its density reaches 1.23 g / cm³. 3 Then, the heating rate Q was set to 300–1200 J / m. 2 The material is heat-treated until its density reaches 1.33 g / cm³. 3 Therefore, the heating rate Q obtained from equation (3) is set to 900-1500 J / m. 2 The material is heat-treated until its density reaches 1.48 g / cm³. 3 Under certain circumstances, this requirement is met.

[0092] At this point, the heating rate Q is set to 150–500 J / m. 2 The material is heat-treated until its density reaches 1.33 g / cm³. 3 After that, although the density became 1.35 g / cm³ 3 The conditions for heat treatment up to this point are not the aforementioned preferred heating rate Q (300~1200J / m). 2 / s), but preferably with the above heating rate Q (900~1500J / m) 2 Within the range of / s, for example, 1250J / m 2 Heat treatment is performed at a rate of 1600 J / m², but it is not preferred outside the range of all heating rates Q (e.g., 1600 J / m²). 2 / s).

[0093] In addition, the density is 1.38–1.50 g / cm³. 3 The density of the final flame-retardant fiber bundle in this invention affects the dual structure of the carbon fiber bundle, hence the density is 1.38–1.50 g / cm³.3 Within the specified range, the heating rate Q is preferably set to 900–1500 J / m. 2 Within the range of / s. For example, up to a density of 1.38 g / cm³. 3 The heating rate Q is set to 900–1500 J / m. 2 / s (e.g., 1000 J / m 2 After ( / s), further up to 1.50 g / cm 3 In cases where heat treatment is performed up to this point, it is preferable to set the heating rate Q to 900–1500 J / m. 2 / s (e.g., 1400J / m) 2 / s).

[0094] Its density is 1.32–1.35 g / cm³. 3 The flame-retardant reaction of the flame-retardant fiber bundles proceeds to a moderate degree, until the density reaches 1.38–1.50 g / cm³. 3 The heating rate of the heat treatment process affects the final dual structure of flame-retardant fiber bundles and carbon fiber bundles; therefore, it is preferable to use fibers with a density of 1.32–1.35 g / cm³. 3 The density is 1.38–1.50 g / cm³. 3 The heating rate Q is controlled within the above range.

[0095] If the heating rate Q is 900 J / m 2 When the heat generation and heat removal are above a certain value, the heat generation and heat removal are well balanced, so it is not easy to form untreated parts in the flame-retardant fiber bundle. The unevenness of the dual structure disappears, so the lint when unwinding the fiber bundle into the bobbin can be significantly reduced, which is why it is preferred.

[0096] If the heating rate Q is 1500 J / m 2 If the heat dissipation rate is below / s, it is sufficiently large relative to the heating rate, thus reducing temperature unevenness within the flame-retardant fiber bundle and suppressing specific internal fibers within the carbon fiber bundle, making it preferred.

[0097] To confirm that the density during heat treatment at the heating rate Q is within the aforementioned range, the density of the fiber bundles collected during the flame-retardant process can be measured (the method for measuring density will be explained later). For example, if the density of the flame-retardant fiber bundles is lower than specified, the density can be adjusted by increasing the temperature or extending the flame-retardant treatment time.

[0098] Here, an oxidizing atmosphere refers to an atmosphere containing more than 10% by mass of known oxidizing substances such as oxygen and nitrogen dioxide. For simplicity, an air atmosphere is preferred. To calculate the heating rate Q, the heating rate q (J / g / s) of a single fiber, the number of filaments N, the fineness d (dtex) of the single fiber of the flame-retardant fiber bundle, and the yarn width W (mm) can be determined using the method described later, and calculated according to Equation (3). The heating rate Q can be controlled by the heat treatment temperature in the flame-retardant process, the number of filaments N, the fineness of the single fiber of the flame-retardant fiber bundle, and the spacing (width) of the grooves of the rollers.

[0099] In this invention, the heating rate Q, calculated by equation (3), is 150–500 J / m. 2 Heat treatment is carried out in a manner similar to / s until the density reaches 1.22–1.24 g / cm³. 3 Then, until the density becomes 1.38–1.50 g / cm³. 3 The tension applied to the flame-retardant fiber bundle during heat treatment is 1.6–4.0 mN / dtex, preferably 2.5–4.0 mN / dtex, and more preferably 3.0–4.0 mN / dtex. For example, heat treatment is performed until the density reaches 1.23 g / cm³. 3 Subsequently, it needs to be heat-treated under the aforementioned tension until the density reaches 1.40 g / cm³. 3 If the aforementioned tension is 1.6 mN / dtex or higher, the orientation of the carbon fiber bundle can be sufficiently improved, and the tensile modulus E of the bundle is increased, thus significantly reducing the fluff when unwinding the carbon fiber bundle from the winding spool. If the aforementioned tension is 4.0 mN / dtex or lower, the inherent fluff in the carbon fiber bundle can be suppressed. The tension applied to the flame-retardant fiber bundle in the flame-retardant process is expressed as the value obtained by dividing the tension (mN) measured at the outlet side of the flame-retardant furnace by the fineness (dtex) of the polyacrylonitrile precursor fiber bundle when it is completely dry.

[0100] In the manufacturing of the carbon fiber bundle of the present invention, in order to achieve the target yarn width W of the carbon fiber bundle, it is preferable that the roller in the flame-retardant process has grooves. The spacing (width) of the grooves on the roller can be set according to the target yarn width, preferably 5 to 8 mm.

[0101] In the manufacture of the carbon fiber bundles of the present invention, pre-carbonization is preferably performed after the polyacrylonitrile precursor fiber bundle manufacturing process and the flame-retardant process. In the pre-carbonization process, the obtained flame-retardant fiber bundles are preferably heat-treated in an inactive atmosphere at a maximum temperature of 500–1200°C until the density reaches 1.5–1.8 g / cm³. 3 .

[0102] Following the pre-carbonization process, carbonization is performed. In this invention, during the carbonization process, the obtained pre-carbonized fiber bundle is manufactured in an inactive atmosphere at a maximum temperature of 1,200 to 1,600°C. If the maximum temperature is 1,200°C or higher, the entanglement caused by the annular fluff generated when unwinding the carbon fiber bundle for further processing can be suppressed. If the maximum temperature is 1,600°C or lower, the fluff generated by rubbing the carbon fiber bundle can be suppressed, thus suppressing the fluff generated during unwinding of the carbon fiber bundle.

[0103] To improve adhesion to the matrix resin, the carbon fiber bundles obtained as described above are subjected to an oxidation treatment to introduce oxygen-containing functional groups. As oxidation treatment methods, gas-phase oxidation, liquid-phase oxidation, and liquid-phase electrolytic oxidation can be employed. From the viewpoint of high productivity and uniform processing, liquid-phase electrolytic oxidation is preferred. No particular method is specified for liquid-phase electrolytic oxidation; any known method may be used.

[0104] Following the electrolytic treatment described above, a sizing process can be performed to impart bundled properties to the resulting carbon fiber bundles. The sizing agent should be appropriately selected based on the type of matrix resin used in the composite material, ensuring good compatibility with the matrix resin.

[0105] The methods for determining the various physical properties described in this instruction manual are as follows.

[0106] <Determination of crystallite size Lc>

[0107] The carbon fiber lats provided for measurement were measured using a wide-angle X-ray diffraction apparatus under the following conditions.

[0108] • X-ray source: CuKα rays (tube voltage 40kV, tube current 30mA)

[0109] • Detector: goniometer + monochromator + scintillation counter

[0110] • Scanning range: 2θ = 10–40°

[0111] • Scanning mode: step scan, step unit 0.01°, scan speed 1° / min.

[0112] In the obtained diffraction pattern, for the peaks that appear around 2θ = 25° to 26°, the full width at half maximum (FWHM) is calculated. Based on this value, the crystallite size is calculated using the following Scherrer formula.

[0113] Crystallite size (nm) = Kλ / β0cosθ B

[0114] in,

[0115] K: 1.00, λ: 0.15418 nm (wavelength of X-rays)

[0116] β0: (β E 2 -β1 2 ) 1 / 2

[0117] β E Apparent full width at half maximum (measured value) rad, β1: 1.046 × 10⁻⁶ -2 rad

[0118] θ B Bragg's diffraction angle.

[0119] <Tensile strength and tensile modulus E of carbon fiber bundles>

[0120] The tensile strength and tensile modulus E of the carbon fiber bundles were determined according to the resin impregnation test method of JIS-R-7608 (2004) following these steps. As the resin formulation, a mixture of "Celloxide" (registered trademark) 2021P (manufactured by Daicel Chemical Industry Co., Ltd.) / 3-boron monoethylamine fluoride (manufactured by Tokyo Chemical Industry Co., Ltd.) / acetone = 100 / 3 / 4 (parts by weight) was used as the curing conditions: atmospheric pressure, temperature 125°C, and time 30 minutes. Ten resin-impregnated carbon fiber bundles were tested, and the average value was taken as the tensile strength. Strain was evaluated using an elongation meter. The strain range was 0.1% to 0.6%.

[0121] <Number of internal villi in carbon fiber bundles>

[0122] The carbon fiber bundle is pulled 10m from the winding drum without tension. Using a force that does not produce fuzz and with the thickness of the carbon fiber bundle equal to two individual fibers, the bundle is separated into individual fibers. Any fuzz present is collected and its number is measured. Based on the measured number, the number of fibers per 1m is calculated as the number of inherent fibers within the carbon fiber bundle. It should be noted that this count does not include fuzz generated during the fiber separation stage.

[0123] <The proportion of fibers with a dual structure in the cross-section of the intrinsic fibers within the carbon fiber bundle>

[0124] The carbon fiber bundle was unwound from the winding spool without tension, and 50 fibers within the bundle were randomly collected. The tips of the collected fibers were observed from the front and at approximately a 45° angle using a Hitachi High-Tech SEM "S-4800". The observed cross-sections will show... Figure 1 As shown, there is a concentric two-layer structure, such as... Figure 2 The cross-section of the fluff shown, with a hole in the center, is defined as a "cross-section with a dual structure". Regarding the cross-section of the recycled fluff, as shown... Figure 3 The fibers identified as broken due to bending are considered to be not inherent fibers within the carbon fiber bundle during the manufacturing process, but rather broken due to bending loads applied during fiber recycling. Therefore, they are excluded from the total number of fibers. If the excluded fibers total 5 or more, 15 more are recycled to bring the total number of fibers to 100 or more. The proportion of "cross-sections with dual structures" relative to the total number of fibers obtained excluding those broken due to bending is taken as the proportion of fibers with dual structures in the cross-section among the inherent fibers of the carbon fiber bundle.

[0125] <The proportion of fibers in a carbon fiber bundle whose cross-sectional area perpendicular to the fiber axis accounts for less than 50% of the total area.>

[0126] Regarding the cross-sectional area of ​​a single fiber in a carbon fiber bundle, 30 vertical cross-sections were obtained by cutting the single fiber with a single blade and photographing them from the front using a Hitachi High-Tech SEM "S-4800". The major axis of the obtained SEM images was measured using the ruler tool of the free image analysis software "Image J", and the average value of the 30 fibers was taken as the average cross-sectional area of ​​a single fiber in the carbon fiber bundle. Furthermore, regarding the SEM images of the "cross-section of the inherent fibers in the carbon fiber bundle", for images viewed from an approximately 45° angle, the protractor tool of the free image analysis software "Image J" was used to select a region at an angle of 85 to 95° relative to the fiber axis. For the selected region, the area of ​​the same cross-section photographed from the front was calculated using the free image analysis software "Image J", thereby calculating the area of ​​the cross-section perpendicular to the fiber axis. The proportion of the area of ​​the cross-section perpendicular to the fiber axis relative to the average cross-sectional area of ​​the single fiber of the carbon fiber bundle obtained above is calculated. A proportion of 50% or less is defined as "fibers in the carbon fiber bundle whose cross-sectional area perpendicular to the fiber axis is 50% or less". The proportion of "fibers whose cross-sectional area perpendicular to the fiber axis is 50% or less" relative to the total number of fibers (excluding those broken due to bending) determined by the aforementioned method is defined as the proportion of fibers in the carbon fiber bundle whose cross-sectional area perpendicular to the fiber axis is 50% or less.

[0127] <Width W of carbon fiber bundle>

[0128] The carbon fiber bundle is pulled out of the winding spool without tension, and the yarn width is measured with a ruler. Three measurements are taken every 1m, and the average value is used as the yarn width W of the carbon fiber bundle.

[0129] <Nodal strength of carbon fiber bundles>

[0130] A 150mm long carbon fiber bundle was used as a test specimen by attaching 25mm long gripping parts to both ends. During the fabrication of the test specimen, a 0.1×10⁻⁶ ppm was applied. -3 The carbon fiber bundles were aligned under a load of N / denier. A knot was created at the midpoint of the test specimen, and the bundle tensile test was conducted with the crosshead speed set to 100 mm / min. A total of 12 fiber bundles were measured, and the average value of the 10 bundles excluding the maximum and minimum values ​​was used as the measured value. The standard deviation of these 10 values ​​was used as the standard deviation of the knot strength. The knot strength was calculated by dividing the maximum load value obtained from the bundle tensile test by the average cross-sectional area of ​​the carbon fiber bundle.

[0131] <Density Measurement>

[0132] Flame-retardant fiber bundles of 1.0–3.0 g were collected and completely dried at 120°C for 2 hours. After measuring the completely dried mass C (g), the bundles were then immersed in ethanol to fully degas, and the fiber mass D (g) in the ethanol solvent bath was measured. The fiber specific gravity was calculated using the formula: fiber specific gravity = (C × ρ) / (CD). ρ is the specific gravity of ethanol at the measured temperature.

[0133] <Average single fiber diameter B of carbon fiber bundle>

[0134] For the measured carbon fiber bundle containing multiple carbon filaments, calculate the mass A per unit length. f (g / m) and density ρ (g / cm) 3 The number of carbon fiber filaments N measured in the carbon fiber bundle is set as C. f The average single fiber diameter B (μm) of the carbon fiber bundle is calculated using the following formula.

[0135] The average single fiber diameter B (μm) of the carbon fiber bundle

[0136] =((A) f / ρ / C f ) / π) (1 / 2) ×2×10 3 .

[0137] <Outer layer area ratio relative to the overall cross-section perpendicular to the fiber axis of the carbon fiber>

[0138] The carbon fiber bundles to be measured were embedded in resin, and the cross-section perpendicular to the fiber axis was ground. The cross-section was observed using a 100x objective lens of an optical microscope at a total magnification of 1,000x. The outer layer area of ​​the double structure was determined from the cross-sectional microscopic image of the ground surface. The analysis was performed using ImageJ image analysis software. First, in the single-fiber cross-sectional image, black and white regions were segmented by binarization. For the brightness distribution within the single-fiber cross-section, the average value of the distribution was set as a threshold for binarization. For the resulting binarized image, the shortest distance from a point on the surface to the scribbled area from black to white was measured relative to the fiber diameter direction. This shortest distance was measured at 5 points within the circumference of the same single fiber, and the average value was used as the outer layer thickness at that level. Based on the above operations, the outer layer area ratio (%) relative to the entire cross-section perpendicular to the fiber axis of the carbon fiber was calculated, and the average of 50 cross-sections was used as the outer layer area ratio relative to the entire cross-section perpendicular to the fiber axis of the carbon fiber.

[0139] <Heating rate q of a single fiber>

[0140] Polyacrylonitrile precursor fiber bundles were dried at 120°C for 1 hour under reduced pressure (below 10 mmHg) and then used for calorimetry analysis. 2 mg of the dried polyacrylonitrile precursor fiber bundles was weighed into an aluminum sample pan. The aluminum sample pan was uncovered, and measurements were performed using a differential scanning calorimeter (Bruker AXS DSC3100SA) at a heating rate of 10°C / min and an air supply of 100 mL / min, from room temperature to 300°C. For the obtained data, the heating rate at 150°C was set to zero, and the heating rate at the specified temperature was used as q.

[0141] <Quality during carbon fiber bundle unwinding>

[0142] The carbon fiber bundle is placed on a warp beam and pulled by a roller at a tension of 1.6 mN / dtex and a speed of 10 m / min, and then wound using a winding machine. The lint generated between the warp beam and the roller is counted for 10 minutes and evaluated according to the following indicators.

[0143] A: 1-2 per 10 minutes

[0144] B: 3-5 per 10 minutes

[0145] C: 6 per 10 minutes

[0146] Example

[0147] The present invention will be further described in detail below through examples. However, the present invention is not limited thereto. The measurement methods in this embodiment are as described above.

[0148] (Example 1)

[0149] Polyacrylonitrile copolymers were produced by solution polymerization of a copolymer of acrylonitrile and itaconic acid using dimethyl sulfoxide as a solvent, yielding a spinning solution. The resulting spinning solution was temporarily discharged from the spinning spinneret into the air and then subjected to a wet-dry spinning process in a coagulation bath controlled at 3°C ​​using a 35% dimethyl sulfoxide aqueous solution, resulting in coagulated fiber bundles. These fiber bundles were then washed and stretched using conventional methods at 30–98°C. Next, the stretched fiber bundles were treated with an amino-modified silicone oil and dried and densified using heated rollers at 160°C, setting the number of individual fibers to 12,000. The bundles were then stretched 3.7 times under pressurized steam, resulting in a polyacrylonitrile precursor fiber bundle with 12,000 individual fibers at a total spinning stretch of 13 times. Regarding the fineness of the individual fibers of the polyacrylonitrile precursor fiber bundle, the discharge rate of the spinning solution ejected from the spinneret was adjusted as described in Table 2. The heating rate q of the individual fibers of the obtained polyacrylonitrile precursor fiber bundle was determined using the method described above. Next, the polyacrylonitrile precursor fiber bundle was heat-treated in an air-atmosphere oven at a draw ratio of 1 using the heat treatment temperature / flame-retardant time conditions shown in Table 2 to obtain a flame-retardant fiber bundle.

[0150] The obtained flame-retardant fiber bundles were pre-carbonized in a nitrogen atmosphere at a temperature of 300–800°C to obtain pre-carbonized fiber bundles. The pre-carbonized fiber bundles were then carbonized in a nitrogen atmosphere at a maximum temperature of 1,350°C. The resulting carbon fiber bundles underwent surface treatment and sizing agent coating to produce the final carbon fiber bundles.

[0151] Table 1 shows the average single fiber diameter B, tensile modulus E, crystallite size Lc, tensile strength, yarn width W, number of filaments N, outer layer area ratio, knot strength A, number of internal fibers in the carbon fiber bundle, cross-section of the internal fibers in the carbon fiber bundle, and quality of the carbon fiber bundle during unwinding. The number of internal fibers in the carbon fiber bundle is 38 per m, the cross-section of the internal fibers in the carbon fiber bundle shows that 24% of the fibers have a double structure, and the quality of the carbon fiber bundle during unwinding is good.

[0152] (Example 2)

[0153] Continue until the density becomes 1.22–1.24 g / cm³. 3 The heat treatment temperature was set to 235°C. Otherwise, the same operation as in Example 1 was performed. As a result, the number of internal fibers in the carbon fiber bundle was 3 per m, the cross-section of the internal fibers in the carbon fiber bundle was 4% double-structured, and the quality of the carbon fiber bundle during unwinding was very good. The evaluation results are recorded in Tables 1 and 2.

[0154] (Example 3)

[0155] Set the yarn width W to 5mm, the filament number N to 24,000, and the density to 1.22–1.24 g / cm³. 3 The heat treatment temperature was set to 230℃ until the density reached 1.38–1.50 g / cm³. 3 The heat treatment temperature was set to 265°C. Otherwise, the same procedures as in Example 1 were performed. The results showed that the number of internal fibers in the carbon fiber bundle was 32 per m, and 12% of the internal fibers had a double-structure cross-section. The quality of the unwinding of the carbon fiber bundle was excellent. The evaluation results are recorded in Tables 1 and 2.

[0156] (Example 4)

[0157] Set the yarn width W to 8mm and the density to 1.22–1.24 g / cm³. 3 The heat treatment temperature was set to 235°C. Otherwise, the same procedures as in Example 3 were performed. The results showed that the number of internal fibers in the carbon fiber bundle was 2 per m, the cross-section of the internal fibers in the carbon fiber bundle was double-structured, and the quality of the carbon fiber bundle during unwinding was very good. The evaluation results are recorded in Tables 1 and 2.

[0158] (Example 5)

[0159] Continue until the density becomes 1.22–1.24 g / cm³. 3 The heat treatment temperature was set to 235°C. Otherwise, the same operation as in Example 1 was performed. As a result, the number of internal fibers in the carbon fiber bundle was 4 per m, the cross-section of the internal fibers in the carbon fiber bundle was double-structured, and the quality of the carbon fiber bundle during unwinding was very good. The evaluation results are recorded in Tables 1 and 2.

[0160] (Example 6)

[0161] The fineness d of the single fiber in the flame-retardant fiber bundle was set to 1.2 dtex. Otherwise, the same procedure as in Example 5 was performed. The results showed that the number of internal fibers in the carbon fiber bundle was 3 per m, the cross-section of the internal fibers in the carbon fiber bundle was 4% double-structured, and the quality of the carbon fiber bundle during unwinding was good. The evaluation results are recorded in Tables 1 and 2.

[0162] (Example 7)

[0163] The yarn width W was set to 7 mm, and the single fiber fineness d of the flame-retardant fiber bundle was set to 0.9 dtex. Otherwise, the same procedures as in Example 5 were performed. The results showed that the number of internal fibers in the carbon fiber bundle was 5 per m, the cross-section of the internal fibers in the carbon fiber bundle exhibited a double-structure fiber count of 1%, and the quality of the carbon fiber bundle during unwinding was good. The evaluation results are recorded in Tables 1 and 2.

[0164] (Example 8)

[0165] Continue until the density becomes 1.38–1.50 g / cm³. 3 The tension of the flame-retardant fiber bundle during heat treatment was set to 3.8 mN / dtex, and the maximum carbonization temperature was set to 1,600 °C. Otherwise, the same procedures as in Example 2 were performed. As a result, the crystallite size Lc became 2.4 nm, and the tensile modulus E of the fiber bundle became 300 GPa. Furthermore, the number of internal fibers in the carbon fiber bundle was 2 per m, the cross-section of the internal fibers in the carbon fiber bundle showed a double-structure fiber count of 3%, and the quality of the carbon fiber bundle during unwinding was good. The evaluation results are recorded in Tables 1 and 2.

[0166] (Example 9)

[0167] The copolymer was set as a copolymer of acrylonitrile, itaconic acid, and n-butyl acrylate, and the single fiber fineness of the polyacrylonitrile precursor fiber bundle was set to 2.2 dtex. Otherwise, the same procedures as in Example 2 were performed, resulting in an average single fiber diameter B of 10.5 μm for the carbon fiber bundle. Furthermore, the number of internal fibers in the carbon fiber bundle was 35 / m, the cross-section of the internal fibers in the carbon fiber bundle showed a double-structure fiber count of 6%, and the quality of the carbon fiber bundle during unwinding was good. The evaluation results are recorded in Tables 1 and 2.

[0168] (Example 10)

[0169] Continue until the density becomes 1.38–1.50 g / cm³. 3 The heat treatment temperature was set to 285°C. Otherwise, the same operation as in Example 4 was performed. As a result, the number of internal fibers in the carbon fiber bundle was 16 / m, the cross-section of the internal fibers in the carbon fiber bundle was double-structured, and the quality of the carbon fiber bundle during unwinding was very good. The evaluation results are recorded in Tables 1 and 2.

[0170] (Example 11)

[0171] The final carbonization temperature was set to 1,450°C. Otherwise, the same procedures as in Example 4 were performed. The results showed that the number of internal fibers in the carbon fiber bundle was 17 per m, the cross-section of the internal fibers in the carbon fiber bundle exhibited a double-structure fiber count of 3%, and the quality of the carbon fiber bundle during unwinding was excellent. The evaluation results are recorded in Tables 1 and 2.

[0172] (Example 12)

[0173] Set the yarn width W to 9mm and the density to 1.32–1.35 g / cm³. 3 The heat treatment temperature was set to 260℃ until the density reached 1.38–1.50 g / cm³. 3 The heat treatment temperature was set to 279℃ until the density reached 1.38–1.50 g / cm³. 3 The tension of the flame-retardant fiber bundle during heat treatment was set to 1.7 mN / dtex. Otherwise, the same procedures as in Example 4 were performed. The results showed that the number of internal fibers in the carbon fiber bundle was 2 per m, the cross-section of the internal fibers in the carbon fiber bundle was double-structured (1%), and the quality of the carbon fiber bundle during unwinding was very good. The evaluation results are recorded in Tables 1 and 2.

[0174] (Comparative Example 1)

[0175] The yarn width W was set to 4 mm, and the same operation as in Example 1 was performed. As a result, the number of internal fibers in the carbon fiber bundle was 42 per m, and 27% of the internal fibers in the carbon fiber bundle had a double-structure cross-section. The carbon fiber bundle had more fibers during unwinding, resulting in poorer quality. The evaluation results are recorded in Tables 1 and 2.

[0176] (Comparative Example 2)

[0177] With the yarn width W set to 10 mm, the same operation as in Example 2 was performed. As a result, the number of internal fibers in the carbon fiber bundle was 4 per m, the cross-section of the internal fibers in the carbon fiber bundle was 0% double-structured, but the tensile strength of the bundle decreased to 5.0 GPa. This was due to the increased fiber density and poor quality caused by rubbing near the roller during the unwinding of the carbon fiber bundle. The evaluation results are recorded in Tables 1 and 2.

[0178] (Comparative Example 3)

[0179] The number of filaments N was set to 51,000. Otherwise, the same procedures as in Example 2 were performed. The result was that the number of internal fibers in the carbon fiber bundle was 260 per m, and 70% of the internal fibers had a double-structure cross-section. The carbon fiber bundle exhibited more fibers during unwinding, leading to a decrease in quality. The evaluation results are recorded in Tables 1 and 2.

[0180] (Comparative Example 4)

[0181] The number of filaments N was set to 3,000, and the yarn width W was set to 3 mm. Otherwise, the same procedures as in Example 2 were performed. The result was that the number of internal fibers in the carbon fiber bundle was 3 per m, and the cross-section of the internal fibers in the carbon fiber bundle showed a double-structured fiber count of 0%. However, the tensile strength of the bundle decreased to 5.1 GPa. This was due to increased fiber count and poorer quality caused by friction near the rollers during unwinding of the carbon fiber bundle. The evaluation results are recorded in Tables 1 and 2.

[0182] (Comparative Example 5)

[0183] Set the yarn width W to 5mm and the density to 1.22–1.24 g / cm³. 3 The heat treatment temperature was set to 250°C. Otherwise, the same operation as in Example 1 was performed. As a result, the number of internal fibers in the carbon fiber bundle was 130 per m, and 30% of the internal fibers in the carbon fiber bundle had a double-structure cross-section. The carbon fiber bundle showed more fibers and poorer quality during unwinding. The evaluation results are recorded in Tables 1 and 2.

[0184] (Comparative Example 6)

[0185] Continue until the density becomes 1.22–1.24 g / cm³. 3 The heat treatment temperature was set at 220℃ until the density reached 1.38–1.50 g / cm³. 3 The heat treatment temperature was set to 250°C. Otherwise, the same operation as in Example 1 was performed. As a result, the number of internal fibers in the carbon fiber bundle was 5 per m, the cross-section of the internal fibers in the carbon fiber bundle was 0% double-structured, but the tensile strength of the bundle decreased to 5.2 GPa. This was due to the increased fiber density and poor quality caused by rubbing near the roller during the unwinding of the carbon fiber bundle. The evaluation results are recorded in Tables 1 and 2.

[0186] (Comparative Example 7)

[0187] Continue until the density becomes 1.22–1.24 g / cm³. 3 The heat treatment temperature was set to 230℃ until the density reached 1.38–1.50 g / cm³. 3 The heat treatment temperature was set to 275℃, but the final density of the flame-retardant fiber bundles was 1.36 g / cm³. 3 In addition, the same operation was performed as in Example 1. As a result, the number of internal fibers in the carbon fiber bundle was 64 per m. The carbon fiber bundle had more fibers and poorer quality during unwinding. The evaluation results are recorded in Tables 1 and 2.

[0188] (Comparative Example 8)

[0189] The single fiber fineness of the polyacrylonitrile precursor fiber bundle was set to 0.7 dtex. Otherwise, the same procedures as in Example 8 were performed. As a result, the average single fiber diameter B of the carbon fiber bundle became 5.5 μm. Increased fluffing due to rubbing resulted in more fluff and poorer quality during unwinding of the carbon fiber bundle. The evaluation results are recorded in Tables 1 and 2.

[0190] (Comparative Example 9)

[0191] According to Example 2 of Japanese Patent Application Publication No. 2007-314901, the filament number was set to 24,000, and flame retardancy was carried out at 240°C for 130 minutes. The final carbonization temperature was set to 1,450°C. Otherwise, the same operation as in Example 1 was performed. As a result, the density of the final flame-retardant fiber bundle was 1.35 g / cm³. 3 Therefore, the number of internal fibers in the carbon fiber bundle is 48 per meter. The increased fiber density and deterioration of quality occur during unwinding of the carbon fiber bundle. The evaluation results are recorded in Tables 1 and 2.

[0192] (Comparative Example 10)

[0193] According to Example 1 of Japanese Patent Application Publication No. 2018-145541, the copolymer was set as a copolymer formed from acrylonitrile and 2-hydroxyethyl methacrylate, the single fiber fineness of the polyacrylonitrile precursor fiber bundle was set to 4.0 dtex, and the filament number was set to 3,000. Flame-retardant treatment was performed under the conditions shown in Table 2. As a result, similar to Japanese Patent Application Publication No. 2018-145541, the density of the flame-retardant fiber bundle was 1.39 mg / m³. 3 In addition, the same operation was performed as in Example 1, and the average single fiber diameter B of the carbon fiber bundle became 13.1 μm. Therefore, the double structure of the carbon fiber bundle deteriorated to 82%, the number of internal fibers in the carbon fiber bundle was 60 / m, and the cross-section of the internal fibers in the carbon fiber bundle showed a double structure of 35%. The high fiber content during unwinding of the carbon fiber bundle resulted in poorer quality. The evaluation results are recorded in Tables 1 and 2.

[0194] (Comparative Example 11)

[0195] The single fiber fineness of the polyacrylonitrile precursor fiber bundle was set to 3.0 dtex. Otherwise, the same procedures as in Example 9 were performed. As a result, the average single fiber diameter B of the carbon fiber bundle was 12.0 μm, and the tensile modulus E of the bundle decreased to 213 GPa. Furthermore, the number of internal fibers in the carbon fiber bundle was 110 fibers / m, and 45% of the internal fibers had a double-structure cross-section. The increased fiber density and deterioration in quality during unwinding of the carbon fiber bundle were observed. The evaluation results are recorded in Tables 1 and 2.

[0196] (Comparative Example 12)

[0197] The maximum carbonization temperature was set to 1,150°C. Otherwise, the same procedures as in Example 2 were performed. As a result, the crystallite size Lc became 1.4 nm, and the tensile modulus E of the fiber bundle decreased to 215 GPa. Furthermore, the number of internal fibers in the carbon fiber bundle was 4 per m, and the cross-section of the internal fibers in the carbon fiber bundle showed a double-structure fiber count of 4%. However, due to rubbing near the rollers during unwinding of the carbon fiber bundle, the fiber count increased, resulting in poorer quality. The evaluation results are recorded in Tables 1 and 2.

[0198] (Comparative Example 13)

[0199] In the flame-retardant process, the density is reduced to 1.38–1.50 g / cm³. 3 The tension of the flame-retardant fiber bundle was set to 1.2 mN / dtex. Otherwise, the same procedures as in Example 2 were performed, resulting in a tensile modulus of 225 GPa for the bundle. Furthermore, the number of internal fibers in the carbon fiber bundle was 6 per m, and the cross-section of the internal fibers in the carbon fiber bundle showed a double-structure fiber count of 4%. However, due to rubbing near the rollers during unwinding of the carbon fiber bundle, the fiber count was high, leading to a decrease in quality. The evaluation results are recorded in Tables 1 and 2.

[0200] (Comparative Example 14)

[0201] In the flame-retardant process, the density is reduced to 1.38–1.50 g / cm³. 3 The tension of the flame-retardant fiber bundle was set to 4.5 mN / dtex. Otherwise, the same operation as in Example 2 was performed. As a result, the number of internal fibers in the carbon fiber bundle was 80 per m. The fiber bundle had more fibers during unwinding, resulting in poorer quality. The evaluation results are recorded in Tables 1 and 2.

[0202] (Comparative Example 15)

[0203] The maximum carbonization temperature was set to 2,100°C. Otherwise, the same procedures as in Example 2 were performed. As a result, the crystallite size Lc became 2.9 nm, and the tensile modulus E of the fiber bundle increased to 320 GPa. However, the number of internal fibers in the carbon fiber bundle was 45 per m. This was due to excessive fiber accumulation and deteriorated quality caused by rubbing against the rollers during unwinding. The evaluation results are recorded in Tables 1 and 2.

[0204] (Comparative Example 16)

[0205] Continue until the density becomes 1.38–1.50 g / cm³. 3 The heat treatment temperature was set to 305°C. Otherwise, the same procedures as in Example 2 were performed until the density reached 1.38–1.50 g / cm³. 3Midway through the process, the flame-retardant fiber bundles broke, making it impossible to obtain flame-retardant fiber bundles and carbon fiber bundles. The evaluation results are recorded in Tables 1 and 2.

[0206] [Table 1]

[0207]

[0208] [Table 2]

[0209]

Claims

1. A carbon fiber bundle, wherein, The average single fiber diameter B is 6.9 to 11.0 μm, the tensile elastic modulus E of the resin impregnated strand is 230 to 310 GPa, the number of inherent fluffs in the carbon fiber strand is 40 / m or less, and the proportion of fluffs having a double structure in the cross section among the inherent fluffs in the carbon fiber strand is 1 to 25%.

2. The carbon fiber bundle according to claim 1, wherein, The proportion of fluffs having a cross section perpendicular to the fiber axis of 50% or less in the area ratio among the inherent fluffs in the carbon fiber strand is 0 to 3%.

3. The carbon fiber bundle according to claim 1 or 2, wherein, The tensile elastic modulus E of the resin impregnated strand and the crystallite size Lc satisfy the relationship of formula (1), the unit of the crystallite size Lc being nm, 50 x Lc + 130 ≤ E ≤ 50 x Lc + 180 (1).

4. The carbon fiber bundle according to claim 1 or 2, wherein, The crystallite size Lc is 1.5 to 2.5 nm.

5. The carbon fiber bundle according to claim 1 or 2, wherein, The yarn width W is 5 to 8 mm.

6. The carbon fiber bundle according to claim 1 or 2, wherein, The number of filaments N is 10,000 to 50,000.

7. The carbon fiber bundle according to claim 1 or 2, wherein, The nub strength A and the average single fiber diameter B satisfy the relationship of formula (2), the unit of the nub strength A being MPa and the unit of the average single fiber diameter B being μm, -88B + 1360 ≤ A (2).

8. The carbon fiber bundle according to claim 1 or 2, wherein, The tensile strength of the resin impregnated strand is 5.5 to 7.0 GPa.

9. The carbon fiber bundle according to claim 1 or 2, wherein, The area ratio of the outer layer with respect to the entire cross section perpendicular to the fiber axis of the single fiber is 85 to 95 area%.

10. A method of manufacturing a carbon fiber bundle, wherein, In a process of heat-treating a polyacrylonitrile-based precursor fiber bundle having a single fiber fineness of 0.9 to 2.2 dtex in an oxidative atmosphere at 200 to 300°C, heat-treating at a heat generation rate Q of 150 to 500 J / m 2 calculated from the following equation (3) until the density becomes 1.22 to 1.24 g / cm 3 Then, heat-treating at a fiber bundle tension of 1.6 to 4.0 mN / dtex until the density becomes 1.38 to 1.50 g / cm 3 to obtain a flame-retardant fiber bundle, and then heat-treating the flame-retardant fiber bundle in an inactive atmosphere at 1,200 to 1,600°C to obtain a carbon fiber bundle, the unit of the heat generation rate q of the single fiber being J / g / s, the unit of the filament number N being root, the unit of the single fiber fineness d being dtex, and the unit of the yarn width W being mm, Q = q x N x d / W / 10 (3).

11. The method of producing a carbon fiber bundle according to claim 10, wherein In the heat treatment step in the oxidizing atmosphere, the heat generation rate Q calculated from the formula (3) is 150 to 500 J / m 2 / s to perform the heat treatment until the density becomes 1.22 to 1.24 g / cm 3 , and then the heat generation rate Q calculated from the formula (3) is 300 to 1200 J / m 2 / s to perform the heat treatment until the density becomes 1.32 to 1.35 g / cm 3 , and then the heat generation rate Q calculated from the formula (3) is 900 to 1500 J / m 2 / s to perform the heat treatment until the density becomes 1.38 to 1.50 g / cm 3 .

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