Carbon fiber bundle and its manufacturing method

Abstract

To provide a carbon fiber bundle having excellent operability when being fed to high level working, and a method for producing the same.SOLUTION: A carbon fiber bundle has a filament number of 25,000 to 70,000. The number of fuzzes present on the surface of the carbon fiber bundle is 0.1 to 30.0 piece / m, and the ratio of the number of fuzzes having double-structured cross sections is 2 to 28%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a carbon fiber bundle that is excellent in operability when subjected to advanced processing, and to a method for producing the same. [Background technology]

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

[0003] In the production of carbon fiber composite materials from carbon fiber bundles, high-level processability is important. For example, in the production of intermediate substrates such as prepregs and towpregs, it is generally required that the carbon fiber bundles have excellent openability and abrasion resistance, that there is no breakage of the entire carbon fiber bundle or the carbon fiber single fibers, and that the process be easy to operate.

[0004] The carbon fiber bundle is produced through a flame-proofing process in which polyacrylonitrile precursor fibers obtained by fiberizing a polyacrylonitrile copolymer containing a copolymerization component are oxidized in air at 200-300° C., a pre-carbonization process in which the fibers are heated in an inert atmosphere at a maximum temperature of 500-1,200° C., and a carbonization process in which the fibers are heated in an inert atmosphere at a maximum temperature of 1,200-3,000° C. For example, Patent Document 1, which describes a prior art production method, discloses that by suppressing fiber bundle breakage and twisting in the carbon fiber bundle production process, the number of fuzzing points obtained by heat treating a multifilament polyacrylonitrile precursor fiber bundle having a total fineness of 60,000 to 1,000,000 dtex is three or less per meter. Patent Document 2 discloses a method for producing a carbon fiber bundle that exhibits a good balance between strength and elastic modulus by controlling the area of ​​the outer structure of the double structure of the cross section of the flame-resistant fiber generated during the flame-resistant process to 88 to 95% of the total cross-sectional area. Patent Document 3 discloses a method for producing a carbon fiber bundle that performs a flame-resistant treatment using grooved rollers with a groove pitch of 5 to 10 mm so that the passing time between two grooved rollers is 3 minutes or less, thereby converting the precursor fiber bundle into a flame-resistant fiber with a uniform degree of flame-resistant progress without a double structure, thereby achieving excellent operability in the carbonization process. Patent Document 4 discloses a carbon fiber bundle that, by controlling the entanglement of the precursor fiber bundle, achieves excellent shape stability during molding of the carbon fiber composite material even when the number of filaments is 30,000 or more, thereby producing a carbon fiber composite material with excellent mechanical properties. Patent Document 5 discloses a method for flame-retardant treatment of polyacrylonitrile precursor fiber bundles, which enables the production of carbon fiber bundles with high strength, high elastic modulus, and little fluff by performing flame-retardant treatment so that the density of the flame-retardant fiber bundle and the treatment time at each stage of flame-retardant treatment have a constant relationship. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5831563 [Patent Document 2] Japanese Patent Application Publication No. 2018-178344 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-314901 [Patent Document 4] International Publication No. 2019 / 087766 [Patent Document 5] Japanese Patent Application Publication No. 62-86032 Summary of the Invention [Problem to be solved by the invention]

[0006] However, these prior arts did not disclose a carbon fiber bundle that exhibited excellent operability when unwinding from a bobbin in advanced processing. Patent Document 1 discloses a carbon fiber bundle having a low number of fluffs and exhibiting excellent quality, but the properties of the fluff were not controlled, and there was a concern that the fluff would wrap around the roller and remain when the carbon fiber bundle was unwound from the bobbin, thereby deteriorating operability during advanced processing. Patent Document 2 discloses a carbon fiber bundle that exhibits an excellent balance between strength and elastic modulus by controlling the double structure of the cross section of the flame-resistant fiber bundle, but the quality of the carbon fiber bundle and the properties of the fluff were not directly controlled, which could result in a deterioration in operability during advanced processing. Patent Document 3 discloses a method for producing a carbon fiber bundle that exhibits excellent quality by continuously flame-resistantizing the carbon fiber bundle using a grooved roller and then carbonizing it, but the properties of the fluff were not controlled, which raised a concern that the operability during advanced processing would deteriorate. Patent Document 4 discloses a carbon fiber bundle that is excellent in strength and shape stability of the carbon fiber bundle during molding of a carbon fiber composite material, but the properties of the fluff in the carbon fiber bundle are not controlled, and fluff accumulates during continuous operation, raising concerns that the operability of advanced processing will deteriorate. Patent Document 5 discloses a technology for shortening the flame-resistant treatment time by controlling the density of the flame-resistant fiber bundle and the flame-resistant treatment temperature within specific ranges in the flame-resistant step, but the double structure inside the single fiber is not controlled, and similarly the properties of the fluff are not controlled, raising concerns that the accumulation of fluff will deteriorate the operability of advanced processing.

[0007] As described above, prior art has proposed techniques for improving operability during the production of carbon fiber bundles. However, due to the recent expansion in demand for pressure vessels made of carbon fiber, productivity is also important in molding methods such as filament winding, i.e., operability when unwinding a carbon fiber bundle from a bobbin is important. However, no carbon fiber bundles that are excellent in operability during advanced processing of carbon fiber bundles have been disclosed, and providing a carbon fiber bundle that can be suitably used for advanced processing of carbon fiber bundles is an issue. Furthermore, suppressing the accumulation of fluff is important for improving operability during advanced processing of carbon fiber bundles, and an issue is to control not only the number of fluff but also the properties of the fluff so that it is less likely to accumulate. The present invention aims to provide a carbon fiber bundle that is excellent in operability when subjected to advanced processing, and a method for producing the same. [Means for solving the problem]

[0008] In order to achieve the above object of the present invention, the present invention has the following configuration.

[0009] That is, the carbon fiber bundle of the present invention is a carbon fiber bundle having a filament count of 25,000 to 70,000, in which the number of fluffs present on the surface of the carbon fiber bundle is 0.1 to 30.0 fluffs / m or less, and the proportion of the number of fluffs whose cross sections have a double structure is 2 to 28%. [Effects of the Invention]

[0010] According to the present invention, a carbon fiber bundle having excellent operability when subjected to advanced processing can be obtained. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is an example of a photograph showing a cross section of fluff present in a carbon fiber bundle, showing a structure in which an inner layer and an outer layer exist. [Figure 2] 1 is an example of a photograph showing a cross section of fluff present in a carbon fiber bundle, in which a hole is formed in the center of the cross section of the fluff. [Figure 3] 1 is an example of a photograph showing a cross section of fluff present in a carbon fiber bundle, the fluff being determined to have been broken by bending. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present inventors have conducted extensive research into the requirements for obtaining a carbon fiber bundle that is excellent in operability when subjected to advanced processing, and as a result have arrived at the present invention.

[0013] That is, the carbon fiber bundle of the present invention is a carbon fiber bundle in which the proportion of the number of fluffs whose cross sections have a double structure is 2 to 28%. The proportion of the number of fluffs whose cross sections have a double structure is preferably 4 to 26%, and more preferably 8 to 24%.

[0014] Fluff whose cross section has a double structure refers to fluff that, when collected from the surface of the carbon fiber bundle and the cross section is observed with a scanning electron microscope (hereinafter sometimes referred to as SEM), has a structure with an inner layer and an outer layer as shown in Figure 1, or has a hole in the center of the cross section of the fluff as shown in Figure 2. However, fluff whose cross section is determined to have been broken by bending as shown in Figure 3 is not fluff that was generated in the carbon fiber bundle manufacturing process and that has developed in the carbon fiber bundle, but is thought to have broken due to the bending load applied when the fluff was collected, and therefore is excluded from the total number of fluff used to calculate the proportion of fluff whose cross section has a double structure among the fluff present on the surface of the carbon fiber bundle.

[0015] The mechanism by which the cross sections of the fluff present on the surface of these carbon fiber bundles have a double structure is not clearly understood, but it is thought to be as follows. That is, when temperature unevenness occurs during the flame-resistant process, the parts that become particularly hot develop a double structure that is specifically larger than the average double structure in the flame-resistant fiber bundle, and it is thought that the breakage of such single fibers results in the cross section of fluff having a double structure. When such fluff is included at a certain rate in the fluff that normally occurs, even if ring-shaped fluff that wraps around other fluff is formed, it breaks immediately, so processability during advanced processing is not impaired.

[0016] If the proportion of fluff having a double cross-section among the fluff present on the surface of the carbon fiber bundle is 28% or less, the proportion of single fibers with low single fiber strength that affect the fluff will decrease, thereby improving the quality and suppressing the fluff from wrapping around the carbon fiber bundle when it is unwound.

[0017] If the proportion of fluff having a double cross-section structure among the fluff present on the surface of the carbon fiber bundle is 2% or more, the fluff will break even under a weak load, and therefore, when the carbon fiber bundle is unwound for advanced processing, ring-shaped fluff that entangles other fluff is less likely to be formed, resulting in excellent operability during advanced processing.

[0018] Of the fluff present on the surface of a carbon fiber bundle, fluff having a cross section with a double structure is controlled within this range by controlling the flame-proofing process as described later. To determine whether the fluff present on the surface of a carbon fiber bundle has a cross section with a double structure, the carbon fiber bundle wound around a bobbin is pulled out, the fluff present on the surface of the carbon fiber bundle is collected, and the cross section is observed with an SEM. Fluff having a structure with an inner layer and an outer layer as shown in Figure 1 or having a hole in the center of the cross section as shown in Figure 2 is determined to be fluff having a cross section with a double structure, as will be described in detail later.

[0019] In order to control the proportion of fluff present on the surface of the carbon fiber bundle that has a double cross-section structure within such a range, it is necessary to control the double cross-section structure of the flame-resistant fiber bundle that occurs in the flame-resistant process, and it is important to suppress the heat generation and heat accumulation of the polyacrylonitrile-based precursor fiber bundle in the flame-resistant process, which can be achieved by controlling the temperature of the heat treatment in the flame-resistant process, the yarn density of the polyacrylonitrile-based precursor fiber bundle that is input into the flame-resistant process, the composition of the polyacrylonitrile-based copolymer, the fineness of the single fiber of the polyacrylonitrile-based precursor fiber bundle, etc.

[0020] The carbon fiber bundle of the present invention has a filament count of 25,000 to 70,000, preferably 25,000 to 60,000, and more preferably 40,000 to 55,000. The filament count refers to the number of single fibers constituting the fiber bundle. If the filament count is 25,000 or more, the load-bearing capacity of the carbon fiber bundle is high and the entire fiber bundle is less likely to break, resulting in excellent operability during advanced processing. If the filament count is 70,000 or less, the bundling ability is excellent, resulting in good quality and excellent operability during advanced processing. The filament count can be measured by counting the number of single fibers. Control of the filament count can be achieved by changing the number of holes in the spinning spinneret or the number of divisions of the fiber bundle discharged from the spinneret, thereby changing the number of filaments in the polyacrylonitrile precursor fiber bundle. Note that the filament count of the carbon fiber bundle of the present invention is related to the load-bearing capacity of the carbon fiber bundle, and is therefore defined as the number of single fibers constituting the carbon fiber bundle, using a carbon fiber bundle capable of bearing a load as a whole as a unit. That is, in the case of a carbon fiber bundle obtained by simply plying a plurality of carbon fiber bundles, which are bundled units, the number of single fibers contained in the carbon fiber bundle unit before plying is counted, rather than the number of single fibers in the plyed carbon fiber bundle.

[0021] The carbon fiber bundle of the present invention has a number of fluffs present on the surface of the carbon fiber bundle of 0.1 to 30.0 fluffs / m, preferably 1.2 to 28.5 fluffs / m, and more preferably 1.2 to 25.1 fluffs / m. Fluff refers to single fibers that are broken and have visible broken ends among the single fibers constituting the carbon fiber bundle. If the number of fluffs is 30.0 fluffs / m or less, there is less winding of fluff during advanced processing, resulting in excellent operability. If the number of fluffs is 0.1 fluffs / m or more, the load capacity is low, and there is less variation in the appearance rate per hour of fluff that does not form ring-shaped fluff due to breakage during advanced processing, resulting in excellent operability during advanced processing. The number of fluffs is evaluated by counting the number of fluffs present on the surface when observing the carbon fiber bundle using the method described in the specification. The number of such fluffs can be controlled by processing fiber bundles, such as polyacrylonitrile precursor fiber bundles and flame-resistant fiber bundles, in a state of being gathered using a guide or the like in the carbon fiber bundle manufacturing process, or by controlling the temperature and draw ratio of the heat treatment in the flame-resistant process of the polyacrylonitrile precursor fiber bundle, and the draw ratio in the pre-carbonization and carbonization processes.

[0022] The carbon fiber bundle of the present invention preferably has an average circularity of the cross section of a single fiber of 0.84 to 0.96, more preferably 0.86 to 0.94, and even more preferably 0.88 to 0.92. The average circularity of the cross section of a single fiber is an index showing how close the cross section of a single fiber is to a perfect circle, and affects the properties of the carbon fiber single fiber and the carbon fiber bundle. If the average circularity is 0.84 or more, the bundling ability and quality are good, and thus a carbon fiber bundle with excellent advanced processability can be obtained. If the average circularity is 0.96 or less, inter-fiber friction is reduced, and the abrasion resistance of the single fiber is excellent, so fluffing is less likely to occur, and a carbon fiber bundle with excellent operability during advanced processing can be obtained. As described in the specification, the average circularity is calculated according to the definition of the following formula (2) by observing the cross-sectional shape of the carbon fiber single fiber from a direction perpendicular to the cross section and calculating the perimeter and area of ​​the cross section. Such an average circularity can be achieved by controlling the shape of the holes in the spinneret when the spinning dope is discharged and the composition of the coagulation bath in the production of a polyacrylonitrile precursor fiber bundle. (Circularity) = 4π×(Cross-sectional area) / (Perimeter) 2 (2).

[0023] The carbon fiber bundle of the present invention preferably has a knot strength of 160 to 380 N / mm 2 and more preferably 170 to 370 N / mm 2 and more preferably 180 to 360 N / mm 2 Knot strength is an index of strength against bending stress, compressive stress, etc. Knot strength is 160N / mm 2 If the knot strength is above 380N / mm, the fuzz generated by the disturbance of stress received during the manufacturing process will be reduced, and the quality will be excellent, leading to excellent high-level processability. 2 If the knot strength is less than 1 / 2, ring-shaped fuzz is less likely to accumulate during advanced processing, resulting in excellent advanced processability. The knot strength is measured by the method for measuring the knot strength of carbon fiber bundles described below. The knot strength can be achieved by appropriately controlling the heat generation rate of the single fiber, the number of filaments, the single fiber fineness, and the width of the fiber bundle in the flame-proofing step.

[0024] Next, a method for producing a carbon fiber bundle that is preferable for obtaining the carbon fiber bundle of the present invention will be described.

[0025] In producing carbon fiber bundles, a polyacrylonitrile precursor fiber bundle is spun. A polyacrylonitrile polymer is preferably used as a raw material for producing the polyacrylonitrile precursor fiber bundle. In the present invention, the polyacrylonitrile polymer refers to a polymer in which at least acrylonitrile is the main component of the polymer skeleton, and the main component usually refers to a component that accounts for 90 to 100 mass% of the polymer skeleton.

[0026] The polyacrylonitrile copolymer of the present invention contains, as a copolymerization component, a vinyl compound whose homopolymer has a glass transition temperature of preferably −30 to 110° C., more preferably −22 to 30° C. Preferably, alkyl methacrylates and alkyl acrylates such as methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, isobutyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, butyl methacrylate, and isobutyl methacrylate can be used.

[0027] The glass transition temperature of the homopolymer of the copolymerization component is an index related to the molecular mobility within the polymer chain of the copolymerization component. Because it is related to oxygen permeability, it affects the formation of a double structure in the cross section of the flame-resistant fiber bundle during the flame-resistant process. If the glass transition temperature of the copolymerization component is -30°C or higher, the glass transition temperature of the entire polyacrylonitrile copolymer is improved, and the polyacrylonitrile precursor fiber bundle is less likely to melt during the heat treatment in the spinning process or flame-resistant process, and less fuzz is generated, resulting in excellent quality. If the glass transition temperature of the copolymerization component is 110°C or lower, the mobility of fragments within the polymer chain is improved, improving oxygen permeability, and ultimately allowing for control of the proportion of fuzz with a double structure in the cross section. The glass transition temperature of such a homopolymer of the copolymerization component can be measured by techniques such as differential scanning calorimetry after obtaining a homopolymer by polymerizing only the copolymerization component. The glass transition temperature can be controlled by changing the structure of the copolymerization component.

[0028] The polyacrylonitrile copolymer of the present invention preferably has a copolymerization composition of the copolymerization components of 1.8 to 12.0% by mass, more preferably 2.6 to 5.2% by mass. The composition of the copolymerization components is related to the oxygen permeability of the polyacrylonitrile copolymer and affects the formation of a double structure in the cross section of the flame-resistant fiber bundle during the flame-resistant treatment process. If the composition of the copolymerization components is 1.8% by mass or more, the mobility of fragments in the polymer chain of the polyacrylonitrile copolymer is improved, improving oxygen permeability, and ultimately allowing for control of the proportion of fluff having a double structure in the cross section. If the composition of the copolymerization components is 12.0% by mass or less, the glass transition temperature of the entire polyacrylonitrile copolymer is improved, and the polyacrylonitrile precursor fiber bundle is less likely to melt and produce fluff even during heat treatment in the spinning process or the flame-resistant treatment process, resulting in excellent quality. The composition of the copolymerization components can be calculated by quantifying the copolymerization components remaining in the reaction solution after polymerization using techniques such as gas chromatography. In order to control the composition of the copolymerization components, the composition ratio of the monomers at the start of polymerization may be controlled.

[0029] In the production of a polyacrylonitrile precursor fiber bundle, a method for producing a polyacrylonitrile polymer can be selected from known polymerization methods. In the production of a polyacrylonitrile precursor fiber bundle suitable for obtaining the carbon fiber bundle of the present invention, the spinning dope is prepared by dissolving the above-mentioned polyacrylonitrile polymer in a solvent in which polyacrylonitrile is soluble, such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, or an aqueous solution of nitric acid, zinc chloride, and sodium rhodanide.

[0030] The polyacrylonitrile fiber bundle used in the present invention may be produced by any method, including wet spinning, followed by subsequent processes such as drawing, washing with water, application of an oil, drying and densification, and, if necessary, post-drawing. The number of holes in the spinning nozzle in the polyacrylonitrile precursor fiber bundle production process is preferably 25,000 to 200,000, more preferably 36,000 to 160,000, and even more preferably 40,000 to 120,000, in order to achieve the aforementioned number of filaments in the carbon fiber bundle. In producing the polyacrylonitrile precursor fiber bundle, the coagulation bath preferably contains the same solvent as used in the spinning dope, such as dimethyl sulfoxide, dimethylformamide, or dimethylacetamide, as well as a so-called coagulation-promoting component. The coagulation-promoting component may be one that does not dissolve the polyacrylonitrile polymer and is compatible with the solvent used in the spinning dope. Water is preferably used as the coagulation-promoting component.

[0031] In the production of a polyacrylonitrile precursor fiber bundle, the washing step is preferably performed using a multi-stage water washing bath at a water bath temperature of 30 to 98° C. The draw ratio in the water bath drawing step is preferably 2 to 6 times.

[0032] After the water bath drawing step, an oil agent made of silicone or the like is preferably applied to the yarn in order to prevent adhesion between the single fibers. Such a silicone oil agent is preferably a modified silicone, and preferably contains an amino-modified silicone having high heat resistance.

[0033] The drying heat treatment step can be carried out by a known method, for example, at a drying temperature of 100 to 200°C.

[0034] The dried yarn is preferably further post-drawn in pressurized steam or under dry heat from the viewpoint of the density and productivity of the resulting polyacrylonitrile precursor fiber bundle. The steam pressure or temperature and post-draw ratio during post-drawing are preferably selected appropriately within a range that does not cause yarn breakage or fluffing.

[0035] In the method for producing a carbon fiber bundle of the present invention, the single fiber fineness of the polyacrylonitrile precursor fiber bundle is preferably 0.6 to 3.0 dtex, more preferably 1.0 to 2.7 dtex, and even more preferably 1.1 to 2.2 dtex. The single fiber fineness of the polyacrylonitrile precursor fiber bundle means the diameter of a single fiber in the polyacrylonitrile precursor fiber bundle. If the single fiber fineness of the polyacrylonitrile precursor fiber bundle is 0.6 dtex or more, the load-bearing capacity of the carbon fiber single fiber is improved, and the abrasion resistance of the obtained carbon fiber bundle is improved, thereby suppressing fluffing that occurs when the carbon fiber bundle is unwound. If the single fiber fineness of the polyacrylonitrile precursor fiber bundle is 3.0 dtex or less, the amount of heat removal relative to the total heat generation amount of the flame-resistant fiber bundle in the flame-resistant step can be sufficiently ensured, temperature unevenness within the flame-resistant fiber bundle can be reduced, and specific fluffing that exists on the surface of the carbon fiber bundle can be suppressed. The single fiber fineness of the polyacrylonitrile precursor fiber bundle can be calculated from the mass and density per unit length of the polyacrylonitrile precursor fiber bundle and the number of filaments. Such a polyacrylonitrile precursor fiber bundle can be achieved by controlling the discharge rate of the spinning dope and the draw ratio in each step in the production process of the polyacrylonitrile precursor fiber bundle.

[0036] The polyacrylonitrile precursor fiber bundle in the method for producing a carbon fiber bundle of the present invention preferably has an average circularity of single fiber cross sections of 0.84 to 0.96, more preferably 0.86 to 0.94, and even more preferably 0.88 to 0.92. The average circularity of single fiber cross sections is an index showing how close the cross section of a single fiber is to a perfect circle, and affects the properties of the single fiber and the fiber bundle. When the average circularity is 0.84 or more, the bundling ability and quality are good, and a carbon fiber bundle with excellent high-order processability is obtained. When the average circularity is 0.96 or less, ring-shaped fluff is less likely to be generated during high-order processing, and a carbon fiber bundle with excellent high-order processability is obtained. The average circularity is calculated according to the definition of formula (2) by observing the cross-sectional shape of a single polyacrylonitrile precursor fiber cross section from a direction perpendicular to the cross section and calculating the perimeter and area of ​​the cross section, as described in the circularity measurement method described below. Such an average circularity can be achieved by controlling the shape of the holes in the spinneret when the spinning dope is discharged and the composition of the coagulation bath in the production of a polyacrylonitrile precursor fiber bundle.

[0037] In the method for producing a carbon fiber bundle of the present invention, the temperature in the step of heat-treating a polyacrylonitrile precursor fiber bundle in an oxidizing atmosphere (flame-resistant step) is 200 to 300°C, preferably 210 to 260°C, and more preferably 220 to 245°C. If the flame-resistant treatment temperature is 200°C or higher, a flame-resistant fiber bundle having sufficient flame resistance can be produced, thereby suppressing the generation of fluff due to insufficient flame resistance and improving the quality, thereby enabling the production of a carbon fiber bundle with excellent high-order processability. If the flame-resistant treatment temperature is 300°C or lower, the heat generation rate does not become excessively high, thereby reducing temperature variations within the flame-resistant fiber bundle. The proportion of fluff with a double structure in cross section present on the surface of the carbon fiber bundle can be suppressed within a specific range, enabling the production of a carbon fiber bundle with excellent high-order processability. The temperature of the flame-resistant treatment can be measured by inserting a thermometer such as a thermocouple into the flame-resistant furnace in the flame-resistant step to measure the furnace temperature. If temperature variations or temperature distribution are observed when measuring the furnace temperature at several points, a simple average temperature is calculated. The temperature of such flame-proofing treatment can be controlled by controlling the heating output in a heating method used in a known flame-proofing furnace. For example, in the case of a hot air circulation type flame-proofing furnace, the output of a heater used to heat the oxidizing atmosphere can be changed.

[0038] In the method for producing a carbon fiber bundle of the present invention, when the heat generation rate of a single fiber in the flame-proofing step is q (J / g / s), the number of filaments is N (number of filaments), the single fiber fineness is d (dtex), and the width of the fiber bundle is W (mm), the density is 1.22 to 1.24 g / cm 3 The total heat release rate Q (J / m 2 / s) is 130 to 450. Q=q×N×d / W / 10···(1).

[0039] The density of the flame-retardant fiber bundle is generally used as an index showing the progress of the flame-retardant reaction. When the density of the flame-retardant fiber bundle is 1.22 to 1.24 g / cm 3means that it is the early stage of the flame-resistant process, and it is important to control the heat generation rate of this heat-resistant flame-resistant fiber bundle within an appropriate range, because this leads to control of the proportion of fluff having a cross section with a double structure among the fluff present on the surface of the carbon fiber bundle, and makes it possible to suppress winding of ring-shaped fluff that occurs when the carbon fiber bundle is unwound for advanced processing.

[0040] The density is 1.22g / cm 3 If the density is 1.24 g / cm or more, a rapid increase in the heat generation rate in the flame-resistant fiber bundle can be suppressed even when the bundle is heat-treated at a high temperature in the subsequent flame-resistant process, which leads to the suppression of temperature unevenness in the flame-resistant fiber bundle, and the proportion of fluff having a double structure in cross section among fluffs present on the surface of the carbon fiber bundle can be suppressed. 3 If it is below this, the double structure of the flame-resistant fiber bundle can be sufficiently controlled, and the effect of suppressing the proportion of fluff whose cross section has a double structure among fluffs present on the surface of the carbon fiber bundle when the heat generation rate described later is controlled can be sufficiently enhanced.

[0041] To confirm that the density of the heat treatment at the total heat release rate Q described below is within this range, the fiber bundle can be sampled during the flameproofing process and the density measured by the method described in the specification. For example, if the density of the flameproofed fiber bundle is lower than specified, the density can be adjusted by increasing the temperature or lengthening the flameproofing time.

[0042] Here, the oxidizing atmosphere refers to an atmosphere containing 10 mass % or more of a known oxidizing substance such as oxygen or nitrogen dioxide, and air is preferred for convenience. 3 The total heat generation rate Q is 130 to 450 J / m 2 / s, preferably 140 to 440 J / m 2 / s, and more preferably 150 to 430 J / m 2 / s. The total heat release rate Q is an index showing the rate of progress of flame retardation, and is related to the structure and physical properties of the flame retardant fiber bundle obtained in the flame retardant process. 2If the total heat generation rate Q is 450 J / m or more, a rapid increase in the heat generation rate in the flame-resistant fiber bundle can be suppressed even when the fiber bundle is treated at high temperatures in the subsequent flame-resistant process or pre-carbonization process, which leads to the suppression of temperature unevenness in the flame-resistant fiber bundle and the proportion of fluff with a double structure in cross section among the fluff present on the surface of the carbon fiber bundle can be suppressed. 2 / s or less, the double structure of the flame-resistant fiber bundle in the flame-resistant process can be sufficiently controlled, and the effect of suppressing the proportion of fluff present on the surface of the carbon fiber bundle whose cross section has a double structure can be sufficiently enhanced. In order to control the heat generation rate, it can be achieved by controlling the heat treatment temperature in the flame-resistant process, as well as by controlling the composition of the polyacrylonitrile copolymer used in the polyacrylonitrile precursor fiber bundle. Specifically, the amount of methacrylic acid, itaconic acid, acrylamide, etc. used as a flame-resistant promoting component can be changed.

[0043] The method for producing a carbon fiber bundle according to the present invention is characterized in that the density of the flame-resistant fiber bundle is 1.22 to 1.24 g / cm 3 After heat treatment to obtain a density of 1.32 to 1.35 g / cm 3 The total heat release rate Q calculated by equation (1) must be between 200 and 600 J / m 2 The heat treatment is carried out so that the temperature becomes / s.

[0044] Flame-resistant fiber bundle density is 1.32 to 1.35 g / cm 3 This means that a certain degree of heat resistance has been achieved by the heat treatment, and it is important to control the heat generation rate of this heat-resistant, flame-retardant fiber bundle within an appropriate range, because this leads to control of the proportion of fluff present on the surface of the carbon fiber bundle whose cross section has a double structure, and makes it possible to suppress winding of ring-shaped fluff that occurs when the carbon fiber bundle is unwound for advanced processing.

[0045] The density is 1.32 g / cm 3If the above conditions are met, a rapid increase in the heat generation rate in the flame-resistant fiber bundle can be suppressed even when heat treatment is performed at a high temperature in the subsequent flame-resistant step, which leads to suppression of temperature unevenness in the flame-resistant fiber bundle and enables to suppress the proportion of fluff having a double structure in cross section among fluff present on the surface of the carbon fiber bundle.

[0046] The density is 1.35g / cm 3 If it is below this, the double structure of the flame-resistant fiber bundle can be sufficiently controlled, and the effect of suppressing the proportion of fluff whose cross section has a double structure among fluffs present on the surface of the carbon fiber bundle when the heat generation rate described later is controlled can be sufficiently enhanced.

[0047] To confirm that the density of the heat treatment at the total heat release rate Q is within this range, the fiber bundle can be sampled during the flameproofing process and the density measured by the method described in the specification. For example, if the density of the flameproofed fiber bundle is lower than specified, the density can be adjusted by increasing the temperature or lengthening the flameproofing time.

[0048] In addition, the density is 1.32 to 1.35 g / cm during the flameproofing process. 3 The total heat generation rate Q is 200-600J / m 2 / s, preferably 210 to 590 J / m 2 / s, and more preferably 230 to 580 J / m 2 The total heat release rate Q is an index showing the rate of progress of the flame retardation, and is related to the structure and physical properties of the flame retardant fiber bundle obtained in the flame retardation process.

[0049] Total heat generation rate Q is 200J / m 2 / s or more, even when treated at high temperatures in the subsequent flame-resistant process or pre-carbonization process, a rapid increase in the heat generation rate in the flame-resistant fiber bundle can be suppressed, which leads to the suppression of temperature unevenness in the flame-resistant fiber bundle and the proportion of fluff present on the surface of the carbon fiber bundle that has a double structure in cross section.

[0050] Total heat generation rate Q is 600J / m 2 / s or less, the double structure of the flame-resistant fiber bundle in the flame-resistant step can be sufficiently controlled, and the effect of suppressing the proportion of fluff whose cross section has a double structure among the fluff present on the surface of the carbon fiber bundle can be sufficiently enhanced.

[0051] The heat generation rate can be controlled by controlling the heat treatment temperature in the flame retardation step and by controlling the composition of the polyacrylonitrile copolymer used in the polyacrylonitrile precursor fiber bundle, specifically by changing the amount of methacrylic acid, itaconic acid, acrylamide, or the like used as a flame retardation promoter.

[0052] The method for producing a carbon fiber bundle according to the present invention is characterized in that the density of the flame-resistant fiber bundle is 1.32 to 1.35 g / cm 3 After heat treatment, the density is 1.38 to 1.50 g / cm 3 The total heat release rate Q calculated by equation (1) must be between 300 and 900 J / m 2 The density of the final flame-resistant fiber bundle is more preferably 1.39 to 1.49 g / cm. 3 and more preferably 1.40 to 1.48 g / cm 3 The density of the flame-retardant fiber bundle is 1.38 to 1.50 g / cm 3 This means that sufficient heat resistance has been achieved by the heat treatment, and it is important to control the heat generation rate of this heat-resistant, flame-retardant fiber bundle within an appropriate range, because this leads to control of the proportion of fluff present on the surface of the carbon fiber bundle whose cross section has a double structure, and makes it possible to suppress winding of ring-shaped fluff that occurs when the carbon fiber bundle is unwound for advanced processing.

[0053] The density is 1.38g / cm 3 If the above conditions are met, a rapid increase in the heat generation rate in the flame-resistant fiber bundle can be suppressed even when the bundle is heat-treated at a high temperature in the subsequent preliminary carbonization step, which leads to the suppression of temperature unevenness in the flame-resistant fiber bundle, and the proportion of fluff present on the surface of the carbon fiber bundle that has a double structure in cross section can be suppressed.

[0054] The density is 1.50g / cm 3 If it is below this, the single fiber strength of the flame-resistant fiber bundle can be sufficiently increased, and the effect of suppressing the proportion of fluff having a double structure in cross section among fluff present on the surface of the carbon fiber bundle when the heat generation rate described later is controlled can be sufficiently enhanced.

[0055] To confirm that the density of the heat treatment at the total heat release rate Q is within this range, the fiber bundle can be sampled during the flameproofing process and its density measured by the method described in the specification. For example, if the density of the flameproofed fiber bundle is lower than the specified value, the density can be adjusted by increasing the temperature or lengthening the flameproofing time. In addition, if the density of the fiber bundle is 1.38 to 1.50 g / cm during the flameproofing process, 3 The total heat generation rate Q is 300 to 900 J / m 2 / s, preferably 320 to 880 J / m 2 / s, and more preferably 340 to 860 J / m 2 The total heat release rate Q is an index showing the rate of progress of the flame retardation, and is related to the structure and physical properties of the flame retardant fiber bundle obtained in the flame retardation process.

[0056] Total heat generation rate Q is 300J / m 2 / s or more, a rapid increase in the heat generation rate in the flame-resistant fiber bundle can be suppressed even when the bundle is treated at a high temperature in the subsequent preliminary carbonization process, which leads to the suppression of temperature unevenness in the flame-resistant fiber bundle and the proportion of fluff present on the surface of the carbon fiber bundle that has a double structure in cross section.

[0057] Total heat generation rate Q is 900J / m 2 / s or less, the single fiber strength in the flame-proofing step can be controlled to be sufficiently high, and the effect of suppressing the proportion of fluff having a double structure in cross section among the fluff present on the surface of the carbon fiber bundle can be sufficiently enhanced.

[0058] The heat generation rate can be controlled by controlling the heat treatment temperature in the flame retardation step and by controlling the composition of the polyacrylonitrile copolymer used in the polyacrylonitrile precursor fiber bundle, specifically by changing the amount of methacrylic acid, itaconic acid, acrylamide, or the like used as a flame retardation promoter.

[0059] In the production of the carbon fiber bundle of the present invention, the polyacrylonitrile precursor fiber bundle production step and the flame-proofing step are followed by pre-carbonization. In the pre-carbonization step, the obtained flame-proof fiber bundle is carbonized in an inert atmosphere at a maximum temperature of 500 to 1,200°C, preferably to a density of 1.5 to 1.8 g / cm. 3 Heat treat until

[0060] Following the pre-carbonization, carbonization is carried out. In the carbonization step of the present invention, the obtained pre-carbonized fiber bundle is produced in an inert atmosphere at a maximum temperature of 1,200 to 1,600°C. If the maximum temperature is 1,200°C or higher, the strength of the carbon fiber single fibers against bending stress decreases, and ring-shaped fluff that occurs when the carbon fiber bundle is unwound for advanced processing becomes more likely to break, thereby suppressing winding. If the maximum temperature is 1,600°C or lower, the load-bearing capacity of the carbon fiber single fibers increases, and fluff due to abrasion of the carbon fiber bundle can be suppressed, thereby suppressing fluff that occurs when the carbon fiber bundle is unwound.

[0061] The carbon fiber bundles obtained as described above are subjected to an oxidation treatment to introduce oxygen-containing functional groups in order to improve adhesion to the matrix resin. Gas-phase oxidation, liquid-phase oxidation, and liquid-phase electrolytic oxidation are used as oxidation treatment methods, but liquid-phase electrolytic oxidation is preferred from the viewpoints of high productivity and enabling uniform treatment. The method of liquid-phase electrolytic oxidation is not particularly specified, and any known method may be used.

[0062] After the electrolytic treatment, the resulting carbon fiber bundles may be subjected to a sizing treatment to impart bundling properties to them. As the sizing agent, a sizing agent having good compatibility with the matrix resin used in the composite material can be appropriately selected depending on the type of matrix resin used.

[0063] <Number of fluffs on the surface of carbon fiber bundle> The carbon fiber bundle is pulled out from the bobbin without tension, and any fluff present is collected until the number of fluffs reaches 50. The length of the carbon fiber bundle pulled out until 50 fluffs are collected is measured, and the number of fluffs per unit length (fibers / m) is calculated from the measured length of the carbon fiber bundle as the number of fluffs present on the surface of the carbon fiber bundle.

[0064] <Proportion of fluff present on the surface of a carbon fiber bundle that has a double structure in cross section> The carbon fiber bundle was unwound from the bobbin without tension, and 50 fluffs present on the surface of the carbon fiber bundle were randomly collected. The tips of the collected fluffs were observed from the front and from an approximately 45° angle using a Hitachi High-Technologies Corporation scanning electron microscope (SEM) "S-4800." A cross-section that showed a concentric two-layer structure, as shown in Figure 1, or a hole in the center of the fluff cross-section, as shown in Figure 2, was defined as a "cross-section with a double structure." Fluff whose cross-section was determined to have been broken by bending, as shown in Figure 3, was excluded from the total number of fluffs, because it was not fluff generated during the carbon fiber bundle manufacturing process that was present on the surface of the carbon fiber bundle, but rather was likely broken by the bending load applied during the fluff collection process. The number of fluffs excluded was further collected, and this collection process was repeated until the total number of fluffs reached 50. The ratio of the total number of "cross sections having a double structure" to the total number of fluffs other than those broken by bending obtained in this way is defined as the ratio of fluffs whose cross sections have a double structure among the fluffs present on the surface of the carbon fiber bundle.

[0065] <Width W of polyacrylonitrile precursor fiber bundle> The yarn width of the polyacrylonitrile precursor fiber bundle on the roller just before it enters the oven for the flame-proofing process is measured with a ruler. Measurements are taken at three points every 1 m, and the average value is used as the width W of the polyacrylonitrile precursor fiber bundle. The width W of the polyacrylonitrile precursor fiber bundle is sometimes referred to as the width W of the fiber bundle.

[0066] <Knot strength of carbon fiber bundle> A test specimen is made by attaching a 25 mm long grip to both ends of a 150 mm long carbon fiber bundle. -3 The carbon fiber bundles are pulled together under a load of 100 N / denier. A knot is made at the midpoint of the test specimen, and a bundle tensile test is performed with a crosshead speed of 100 mm / min during tension. Measurements are performed on a total of 12 fiber bundles, and the average of the 10 bundles obtained by dividing the maximum and minimum values ​​is used as the measured value. The knot strength is calculated by dividing the maximum load value obtained in the bundle tensile test by the average cross-sectional area of ​​the carbon fiber bundles. The maximum load value is the load value when the load is highest during one tensile test. The average cross-sectional area of ​​the carbon fiber bundle is calculated by the mass (g / m) and density (g / m) per unit length of the carbon fiber bundle to be measured. 3 ) and the density is calculated by Archimedes' method using o-dichlorobenzene as the specific gravity liquid.

[0067] <Density measurement of flame-retardant fiber bundle> A 1.0 to 3.0 g sample of flame-retardant fiber bundle is dried by heating at 120°C for 2 hours. The weight (C) (g) of the dried flame-retardant fiber bundle in air is measured, and the flame-retardant fiber bundle is then immersed in ethanol and thoroughly degassed. Then, the weight (D) (g) of the flame-retardant fiber bundle in the ethanol solvent bath is measured, and the fiber specific gravity is calculated from the formula: fiber specific gravity = (C × ρ) / (CD), where ρ is the specific gravity of ethanol at the measurement temperature.

[0068] <Measurement of roundness> Polyacrylonitrile precursor fiber bundles or carbon fiber bundles are cut perpendicular to the fiber axis direction with a single-edged razor, and the resulting cross sections are observed perpendicular to the fiber cross section using a Hitachi High-Technologies Corporation scanning electron microscope (SEM) "S-4800," and images of five different fields are taken at random. The image analysis software "ImageJ" is used to select the perimeter of the single fiber cross section from the acquired images, and the circularity is calculated from the calculated perimeter and area of ​​the single fiber cross section according to the following definition. Furthermore, the circularity is measured for five random single fibers in each field, and the average circularity is calculated by averaging the circularities of a total of 25 single fibers. (Circularity) = 4 × π × (cross-sectional area of ​​fiber) / (perimeter) 2 .

[0069] <Heat generation rate of single fiber q> The polyacrylonitrile precursor fiber bundle was dried at 120°C for 1 hour under reduced pressure of 10 mmHg or less and then subjected to calorific value analysis. 2 mg of the dried polyacrylonitrile precursor fiber bundle was weighed into an aluminum sample pan. The aluminum sample pan was left uncovered and measured from room temperature to 300°C using a heat flux differential scanning calorimeter (Bruker AXS DSC3100SA) at a heating rate of 10°C / min and an air supply of 100 mL / min. The heat release rate at 150°C was set to zero, and the heat release rate corresponding to the flame retardation treatment temperature in the flame retardation process was used as q.

[0070] <Quality of carbon fiber bundle when unwinding> The bobbin of the carbon fiber bundle is placed on a creel, taken up by a roller at 10 m / min under a tension of 1.6 mN / dtex, and wound on a winder. At this time, the ring-shaped fuzz generated between the creel and the roller is counted for 10 minutes and evaluated according to the following index.

[0071] A: 1~2 pieces / 10 minutes B: 3~5 pieces / 10 minutes C: 6 pieces~ / 10 minutes [Example]

[0072] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0073] Example 1 A polyacrylonitrile copolymer consisting of acrylonitrile, methacrylic acid, and butyl methacrylate was polymerized by solution polymerization using dimethyl sulfoxide as a solvent to produce a polyacrylonitrile copolymer, and a spinning dope was obtained. The resulting spinning dope was introduced through a spinning nozzle with 70,000 holes into a coagulation bath consisting of a 70% by mass aqueous solution of dimethyl sulfoxide controlled at 30°C, and coagulated into a fiber bundle using a wet spinning method. The fiber bundle was then washed with water at 30 to 98°C and stretched using standard methods.

[0074] Subsequently, an amino-modified silicone-based silicone oil was applied to the fiber bundle after water bath drawing, and the fiber bundle was subjected to a drying and densification treatment using a heated roller at 130°C to obtain a polyacrylonitrile-based precursor fiber bundle with 70,000 single fibers. The amount of spinning solution discharged from the spinneret was adjusted so that the single fiber fineness of the polyacrylonitrile-based precursor fiber bundle would be as shown in Table 2.

[0075] The heat generation rate q of the single fiber of the obtained polyacrylonitrile precursor fiber bundle was measured by the method described above. Next, using the heat treatment temperature and fiber bundle width conditions shown in Table 2, the polyacrylonitrile precursor fiber bundle was heat-treated in an oven in an air atmosphere at a draw ratio of 1.0 to obtain a flame-resistant fiber bundle.

[0076] The obtained flame-resistant fiber bundle was subjected to a pre-carbonization treatment in a nitrogen atmosphere at a temperature of 300 to 800°C to obtain a pre-carbonized fiber bundle. The obtained pre-carbonized fiber bundle was subjected to a carbonization treatment in a nitrogen atmosphere at a maximum temperature of 1,400°C. The obtained carbon fiber bundle was subjected to a surface treatment and a sizing agent coating treatment to obtain a final carbon fiber bundle.

[0077] Table 1 shows the number of filaments, single fiber fineness, circularity, knot strength, number of fluff present on the surface of the carbon fiber bundle, the proportion of fluff having a double structure in cross section among the fluff present on the surface of the carbon fiber bundle, and the quality of the carbon fiber bundle at the time of unwinding. The number of fluff present on the surface of the carbon fiber bundle was 1.2 fluffs / m, and the proportion of fluff having a double structure in cross section was 2%. The quality of the obtained carbon fiber bundle at the time of unwinding was good.

[0078] Example 2 The copolymer composition was changed to make the number of filaments in the spinning process 48,000, and the density was 1.38 to 1.50 g / cm in the flame-resistant process. 3 The same procedures as in Example 1 were carried out except that the heat treatment temperature until the temperature reached 240°C and the width of the fiber bundle in the flame-proofing step was 16.0 mm. The quality of the obtained carbon fiber bundle when unwound was good.

[0079] Example 3 The copolymer composition was changed to make the number of filaments in the spinning process 50,000, and the density was 1.22 to 1.24 g / cm in the flame-resistant process. 3 The heat treatment temperature is 220°C and the density is 1.38 to 1.50 g / cm 3 The same procedures as in Example 1 were carried out except that the heat treatment temperature until the temperature reached 240°C and the width of the fiber bundle in the flame-proofing step was 18.0 mm. The quality of the obtained carbon fiber bundle when unwound was good.

[0080] Example 4 The copolymer composition was changed to ethyl acrylate, the number of filaments in the spinning process was 36,000, the fineness of the polyacrylonitrile precursor fiber single fiber was 2.2 dtex, and the density in the flame-resistant process was 1.22 to 1.24 g / cm 3 The heat treatment temperature is 220°C and the density is 1.38 to 1.50 g / cm 3 The same procedures as in Example 1 were carried out except that the heat treatment temperature until the temperature reached 240°C and the width of the fiber bundle in the flame-proofing step was 21.0 mm. The quality of the obtained carbon fiber bundle when unwound was good.

[0081] Example 5 The copolymer composition was changed to make the number of filaments in the spinning process 25,000, the fineness of the polyacrylonitrile precursor fiber single fiber 3.0 dtex, and the density of the flame-resistant fiber 1.22 to 1.24 g / cm 3 The heat treatment temperature is 220°C and the density is 1.38 to 1.50 g / cm 3 The same procedures as in Example 1 were carried out except that the heat treatment temperature until the temperature reached 240°C and the width of the fiber bundle in the flame-proofing step was 20.0 mm. The quality of the obtained carbon fiber bundle when unwound was good.

[0082] Example 6 The copolymer composition was changed to ethyl acrylate, the number of filaments in the spinning process was increased to 60,000, and the density was increased to 1.22 to 1.24 g / cm3 for flame retardancy. 3 The heat treatment temperature is 220°C and the density is 1.38 to 1.50 g / cm 3 The same procedures as in Example 1 were carried out except that the heat treatment temperature until the temperature reached 240°C and the width of the fiber bundle in the flame-proofing step was 21.0 mm. The quality of the obtained carbon fiber bundle when unwound was good.

[0083] Example 7 The copolymer composition was changed to make the number of filaments in the spinning process 55,000, the fineness of the polyacrylonitrile precursor fiber single fiber 1.1 dtex, and the density of the flame-resistant fiber 1.22 to 1.24 g / cm 3 The same procedures as in Example 1 were carried out except that the heat treatment temperature until the temperature reached 220°C and the width of the fiber bundle in the flameproofing step was 18.0 mm. The quality of the obtained carbon fiber bundle when unwound was good.

[0084] (Comparative Example 1) The number of filaments in the spinning process is 50,000, the fineness of the polyacrylonitrile precursor fiber single fiber is 0.8 dtex, and the density in flame retardation is 1.22 to 1.24 g / cm 3 The heat treatment temperature is 220°C and the density is 1.38 to 1.50 g / cm 3The same procedures as in Example 1 were carried out except that the heat treatment temperature until the temperature reached 240°C and the width of the fiber bundle in the flame-proofing step was 8.0 mm. The obtained carbon fiber bundle was of poor quality and had poor processability when unwinding.

[0085] (Comparative Example 2) The number of filaments in the spinning process is set to 60,000, the fineness of the polyacrylonitrile precursor fiber single fiber is set to 1.1 dtex, and the density after flame retardation is set to 1.22 to 1.24 g / cm 3 The heat treatment temperature is 220°C and the density is 1.38 to 1.50 g / cm 3 The same procedures as in Example 1 were carried out except that the heat treatment temperature until the temperature reached 240°C and the width of the fiber bundle was 13.5 mm. The obtained carbon fiber bundle was of poor quality and had poor processability when unwinding.

[0086] (Comparative Example 3) The copolymer composition was changed to methyl acrylate, the number of filaments in the spinning process was set to 48,000, the fineness of the polyacrylonitrile precursor fiber single fiber was set to 1.1 dtex, and the density was set to 1.22 to 1.24 g / cm3 in flame retardant treatment. 3 The heat treatment temperature is 230°C and the density is 1.32 to 1.35 g / cm 3 The same procedures as in Example 1 were carried out except that the heat treatment temperature until the temperature reached 235°C and the width of the fiber bundle in the flame-proofing step was 15.0 mm. The obtained carbon fiber bundle had a high proportion of fluff with a double structure observed in the cross section, and the processability during unwinding was poor.

[0087] Comparative Example 4 The copolymer composition was changed to ethyl acrylate, the number of filaments in the spinning process was set to 48,000, the fineness of the polyacrylonitrile precursor fiber single fiber was set to 1.1 dtex, and the density was set to 1.22 to 1.24 g / cm3 in flame retardant treatment. 3 The heat treatment temperature is 230°C and the density is 1.32 to 1.35 g / cm 3 The heat treatment temperature is 235°C and the density is 1.38 to 1.50 g / cm 3The same procedures as in Example 1 were carried out except that the heat treatment temperature until the temperature reached 250°C and the width of the fiber bundle in the flame-proofing step was 16.0 mm. The obtained carbon fiber bundle was of poor quality, and the processability during unwinding was poor.

[0088] (Comparative Example 5) The copolymer composition was changed to methyl acrylate, the number of filaments in the spinning process was set to 48,000, the fineness of the polyacrylonitrile precursor fiber single fiber was set to 1.1 dtex, and the density was set to 1.22 to 1.24 g / cm3 in flame retardant treatment. 3 The heat treatment temperature is 230°C and the density is 1.32 to 1.35 g / cm 3 The heat treatment temperature is 235°C and the density is 1.38 to 1.50 g / cm 3 The same procedures as in Example 1 were carried out except that the heat treatment temperature until the temperature reached 250°C, the width of the fiber bundle in the flame-proofing step was 16.0 mm, and the maximum temperature in the carbonization step was 1,800°C. The obtained carbon fiber bundle was of poor quality, and the processability during unwinding was poor.

[0089] (Comparative Example 6) The copolymer composition was changed to itaconic acid, the number of filaments in the spinning process was set to 48,000, the single fiber fineness of the polyacrylonitrile precursor fiber was set to 1.1 dtex, and the density was set to 1.22 to 1.24 g / cm3 in flame retardant treatment. 3 The heat treatment temperature is 230°C and the density is 1.32 to 1.35 g / cm 3 The heat treatment temperature is 235°C and the density is 1.38 to 1.50 g / cm 3 The same procedures as in Example 1 were carried out except that the heat treatment temperature until the temperature reached 250°C, the width of the fiber bundle in the flame-proofing step was 8.0 mm, and the maximum temperature in the carbonization step was 1,800°C. The processability of the obtained carbon fiber bundle when unwinding was poor.

[0090] [Table 1]

[0091] [Table 2]

[0092]

Table 3

[0093]

Table 4

Claims

1. A carbon fiber bundle having a number of filaments of 25,000 to 70,000, wherein the number of fluffs present on the surface of the carbon fiber bundle is 0.1 to 30.0 fluffs / m, and the proportion of the number of fluffs whose cross sections have a double structure is 2 to 28%.

2. 2. The carbon fiber bundle according to claim 1, wherein the average circularity of the cross section of a single fiber of the carbon fiber bundle is 0.84 to 0.

96.

3. The knot strength of the carbon fiber bundle is 160 to 380 N / mm ２ The carbon fiber bundle according to claim 1 or 2,

4. A polyacrylonitrile-based precursor fiber bundle is obtained by spinning a polyacrylonitrile-based copolymer containing, as a copolymerization component, a vinyl compound having a glass transition temperature of -30 to 110°C, and the copolymerization composition of the copolymerization component is 1.8 to 12.0 mass%, wherein the polyacrylonitrile-based precursor fiber bundle has a filament count of 25,000 to 70,000 and a single fiber fineness of 0.6 to 3.0 dtex, and is subjected to a heat treatment in an oxidizing atmosphere at 200 to 300°C, in which the total heat generation rate Q of the polyacrylonitrile-based precursor fiber bundle is defined by Equation (1) where q is the heat generation rate of a single fiber of the polyacrylonitrile-based precursor fiber, N is the number of filaments, d is the single fiber fineness, and W is the width of the fiber bundle (mm), and the density is 1.22 to 1.24 g / cm ３ The total heat release rate Q (J / m ２ / s) is 130 to 450 J / m ２ After heat treatment to achieve a density of 1.32 to 1.35 g / cm ３ The total heat release rate Q calculated by formula (1) is 200 to 600 J / m ２ / s, and then heat-treated to a density of 1.38 to 1.50 g / cm ３ The total heat release rate Q calculated by formula (1) is 300 to 900 J / m ２ / s, the flame-resistant fiber bundle is heat-treated in an inert atmosphere at a maximum temperature of 500 to 1,200°C to obtain a preliminary carbon fiber bundle, and the pre-carbonized fiber bundle is heat-treated in an inert atmosphere at a maximum temperature of 1,200 to 1,600°C to obtain a carbon fiber bundle. Q=q×N×d / W / 10...(1)

5. 5. The method for producing a carbon fiber bundle according to claim 4, wherein the average circularity of the cross section of a single fiber of the polyacrylonitrile precursor fiber bundle is 0.84 to 0.96.

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