Carbon fiber, carbon fiber bundle, and method for producing carbon fiber bundle

Abstract

The present invention provides: carbon fibers from which a carbon fiber bundle having a high strand strength and strand elastic modulus is obtained, and which have less defects; a carbon fiber bundle including the carbon fibers; and a production method for a carbon fiber bundle that has a high strand strength and strand elastic modulus and that has less defects per single fiber.　In the carbon fibers according to the present invention, the number of voids in a cross-section of a single fiber is at most 8. The carbon fiber bundle according to the present invention includes the carbon fibers according to the present invention. In the production method for the carbon fiber bundle according to the present invention, the temperature increase rate when raising the heating temperature from 1800°C to 2200°C is 300-600°C / min.

Description

[Technical Field]

[0001] The present invention relates to a carbon fiber, a carbon fiber bundle, and a method for producing a carbon fiber bundle. This application claims priority based on Patent Application No. 2021-183635, filed with the Japan Patent Office on November 10, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] Carbon fiber has a higher specific strength and specific modulus than other fibers. Carbon fiber is being widely used as a reinforcing fiber for composite materials in sports, aviation, and space applications, as well as in general industrial applications such as automobiles, civil engineering, construction, pressure vessels, and wind turbine blades. Therefore, there is a demand for even higher performance carbon fiber.

[0003] Polyacrylonitrile (PAN)-based carbon fibers, which are widely used among carbon fibers, are generally industrially produced as follows. First, a spinning solution containing a precursor acrylonitrile-based polymer is subjected to wet spinning, dry spinning, or dry-wet spinning to obtain a precursor acrylic fiber bundle. Next, the precursor acrylic fiber bundle is converted into a flame-resistant fiber bundle by heating in an oxidizing atmosphere at a temperature of, for example, 180 to 400°C. Thereafter, a carbon fiber bundle is obtained by carbonizing the bundle by heating in an inert atmosphere at, for example, 1000°C or higher.

[0004] Carbon fiber is a brittle material. Even a small defect can cause a decrease in strength, so efforts are being made to reduce defects that cause breakage. As one of the efforts, a method for controlling defects in carbon fiber has been proposed (for example, Patent Document 1 and Patent Document 2).

[0005] Patent Document 1 discloses that after a single fiber tensile test is performed on a 10 mm carbon fiber bundle, the specific ratio (n / N) of defects of 50 nm or more is set to 35% or less, and the average single fiber diameter is set to 4.3 μm or more.

[0006] Patent Document 2 discloses a method for producing a carbon fiber bundle, which includes withdrawing a coagulated fiber bundle of a polyacrylonitrile-based polymer solution from a coagulation bath and then allowing it to remain in air for 10 seconds or more before being washed with water. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2018 / 003836 [Patent Document 2] International Publication No. 2020 / 195476 Summary of the Invention [Problem to be solved by the invention]

[0008] In conventional carbon fiber bundles, it is difficult to simultaneously achieve high levels of strand strength and strand modulus, because the higher the temperature at which the precursor of the carbon fiber bundle is heat-treated, the higher the strand modulus that can be achieved, but the more difficult it is to achieve high strand strength.

[0009] Patent Document 1 discloses that the strand strength of the carbon fiber bundle is 8.0 GPa or more. However, in the carbon fiber bundle described in Patent Document 1, defects that may lead to breakage are evaluated by a single fiber tensile test. Under actual use conditions, defects that affect the strand strength and strand modulus may occur in the carbon fiber.

[0010] Patent Document 2 discloses that, according to this manufacturing method, the fiber surface is densified, thereby reducing the number of voids, and that the number of voids with a major axis of 3 nm or more present in the region from the fiber surface to a depth of 50 nm in the cross section of a single fiber is reduced to 50 or less. However, the carbon fiber bundle of Patent Document 2 has a low strand modulus. In addition, in the examples of Patent Document 2, the number of voids is nine or more, so it is not possible to simultaneously increase both the strand strength and the strand modulus.

[0011] The present invention provides a carbon fiber bundle having high strand strength and strand modulus and reduced defects, a carbon fiber bundle having high strand strength and strand modulus and reduced defects in single fibers, and a method for producing the same. [Means for solving the problem]

[0012] The present invention has the following aspects. [1] Carbon fiber having eight or less voids in the cross section of a single fiber. Method for measuring the number of voids: After cutting the carbon fiber single fiber perpendicular to the longitudinal direction, a thin section containing the cross section of the carbon fiber is prepared. A TEM image of the cross section of the carbon fiber in the thin section at a magnification of 20,000 times is obtained using a transmission electron microscope. After flattening and binarizing the TEM image using image analysis software, the number of white areas present in the cross section is measured as the number of voids using the "count / measure" function of the image analysis software. [2] The carbon fiber according to [1] above, wherein the average void length is 5 to 20 nm. [3] The carbon fiber according to [1] or [2], wherein the average void diameter is 0.5 to 0.7 nm. [4] The carbon fiber according to any one of [1] to [3] above, which has a diameter of 4.0 to 7.0 μm. [5] Fiber density: 1.70 to 1.90 g / cm 3 The carbon fiber according to any one of [1] to [4] above, [6] The carbon fiber according to any one of [1] to [5], wherein the area ratio of the low-density portion in the carbon fiber is 0.24% or less. [7] The carbon fiber according to any one of [1] to [6], wherein the degree of complete crystallinity of the matrix structure in the carbon fiber is 0.43 or more. [8] A carbon fiber bundle comprising the carbon fiber of any one of [1] to [7] above. [9] A carbon fiber bundle in which a plurality of carbon fiber monofilaments are bundled together, wherein the number of voids present in a region from the surface of the carbon fiber to a depth of 100 nm is 0.5 voids / μm or less relative to the circumferential length of the carbon fiber.

[10] The carbon fiber bundle according to [8] or [9], wherein the strand strength is 5.0 to 6.5 GPa.

[11] The carbon fiber bundle according to any one of [8] to

[10] above, wherein the strand modulus is 360 to 400 GPa.

[12] The carbon fiber bundle according to any one of [8] to

[11] above, wherein the number of the carbon fibers is 8,000 to 20,000.

[13] A method for producing a carbon fiber bundle, comprising heating a precursor fiber bundle; wherein the heating temperature is increased from 1800°C to 2200°C at a rate of 300 to 600°C / min.

[14] The method of producing according to

[13] above, wherein the maximum heating temperature is 2100 to 2300°C.

[15] The method for producing the precursor fiber bundle according to

[13] or

[14] , wherein the precursor fiber bundle is a fiber bundle obtained by dry-wet spinning.

[16] The method of any one of

[13] to

[15] above, wherein the precursor fiber bundle has a single fiber fineness of 0.5 to 2.5 dtex; and the precursor fiber bundle has a single fiber count of 8,000 to 20,000. [Effects of the Invention]

[0013] The carbon fiber of the present invention provides a carbon fiber bundle having high strand strength and strand modulus, and the carbon fiber of the present invention has reduced defects. The carbon fiber bundle of the present invention has high strand strength and strand modulus, and has reduced defects in the single fibers. According to the method for producing a carbon fiber bundle of the present invention, a carbon fiber bundle having high strand strength and strand modulus and reduced defects in the single fibers can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0014] [Terminology] "Voids" are spaces formed during the carbon fiber manufacturing process, and are the white areas that can be obtained by analyzing transmission electron microscope images using image analysis software. The "number of voids" can be measured by observation with a transmission electron microscope (TEM), and the detailed measurement method is as described in the Examples. The "number of surface voids" can be measured by observation with a transmission electron microscope (TEM), and the detailed measurement method is as described in the Examples. The "perimeter of a carbon fiber" refers to the perimeter of a carbon fiber in a cross section perpendicular to the fiber axis of the carbon fiber.

[0015] Hereinafter, several embodiments of the present invention will be described in detail. The embodiments disclosed below are examples for explaining the present invention. It is not intended that the present invention be limited to only these embodiments. The present invention can be implemented in various modes without departing from the spirit of the present invention.

[0016] [Carbon fiber] (Carbon fiber according to the first aspect) In the carbon fiber according to the first aspect of the present invention, the number of voids in the cross section of a single fiber is 8 or less. Having eight or fewer voids in the cross section of a single fiber means that there are sufficiently few defects on the surface of the single fiber that could become breakage initiation points, which is thought to result in higher strand strength and strand modulus of the carbon fiber bundle. From this viewpoint, the number of voids is preferably 6 or less, more preferably 4 or less, and most preferably 0.

[0017] The method for measuring the number of voids is as follows. First, a single carbon fiber is cut perpendicular to the longitudinal direction, and then a thin section containing the cross section of the carbon fiber is prepared. A TEM image of the cross section of the carbon fiber in the thin section at a magnification of 20,000 times is obtained using a transmission electron microscope. The TEM image is flattened and binarized using image analysis software, and then the number of white areas present in the cross section is measured as the number of voids using the "count / measure" function of the image analysis software. The same measurement is performed a total of five times, and the average value is taken as the number of voids. The details of the method for measuring the number of voids are as described in the Examples.

[0018] (Carbon fiber according to the second aspect) In the carbon fiber according to the second aspect of the present invention, the number of voids present in the region from the surface of the carbon fiber to a depth of 100 nm (hereinafter referred to as the "number of surface voids") is 0.5 or less per μm relative to the circumferential length of the carbon fiber. A surface layer void count of 0.5 / μm or less means that there are sufficiently few defects in the surface layer of the single fiber that could become breakage initiation points, which is thought to result in high strand strength and strand modulus of elasticity of the carbon fiber bundle.

[0019] From this viewpoint, the number of surface layer voids relative to the circumferential length of the carbon fiber is preferably 0.4 or less per μm, more preferably 0.3 or less per μm, and most preferably 0 per μm. If the number of surface voids is equal to or less than the upper limit of the above range, defects contained in the fibers can be reduced without decreasing the strand modulus, and the strand strength of the carbon fiber bundle can be increased.

[0020] The method for measuring the number of surface voids is as follows. First, a single carbon fiber is cut perpendicular to the longitudinal direction, and then a thin section containing the cross section of the carbon fiber is prepared. A TEM image of the cross section of the carbon fiber in the thin section at a magnification of 20,000 times is obtained using a transmission electron microscope. The TEM image is flattened and binarized using image analysis software, and then the number of white areas present in the region from the surface of the carbon fiber to a depth of 100 nm is measured as the number of voids using the "count / measure" function of the image analysis software. The circumferential length of the carbon fiber is also measured. The ratio of the number of voids to the circumferential length of the carbon fiber is calculated. The same measurement and calculation are performed a total of five times, and the average value is taken as the number of surface voids. The method for measuring the number of surface voids is described in detail in the Examples.

[0021] (Preferred embodiment of carbon fiber) In the following description, "carbon fiber" is a general term for the carbon fiber according to the first aspect and the carbon fiber according to the second aspect. The preferred embodiments described below are common to the carbon fiber according to the first aspect and the carbon fiber according to the second aspect.

[0022] Examples of carbon fiber types include polyacrylonitrile (PAN)-based carbon fiber, rayon-based carbon fiber, and pitch-based carbon fiber. Among these, PAN-based carbon fiber is preferred from the viewpoint of improving productivity and mechanical properties on an industrial scale.

[0023] Pitch-based carbon fibers usually have a fiber density of 2.10 to 2.20 g / cm 3 and higher than PAN-based carbon fibers. In general pitch-based carbon fiber bundles, the strand modulus is high but the strand strength is low.

[0024] When voids are present in the carbon fiber according to one embodiment, the average void length of the voids is preferably 5 to 20 nm, more preferably 10 to 20 nm, and even more preferably 15 to 20 nm. When the average void length is equal to or greater than the lower limit of the above-mentioned range, the flexibility of the fiber is easily ensured. When the average void length is equal to or less than the upper limit of the above-mentioned range, the strand strength of the carbon fiber bundle is easily maintained at a high level. The average void length can be measured by small angle X-ray scattering (SAXS). The detailed measurement method is as described in the Examples.

[0025] When voids are present in the carbon fiber according to one embodiment, the average void diameter of the voids is preferably 0.5 to 0.7 nm, more preferably 0.51 to 0.69 nm, and even more preferably 0.52 to 0.68 nm. When the average void diameter is equal to or greater than the lower limit of the aforementioned numerical range, the flexibility of the fiber is easily ensured. When the average void diameter is equal to or less than the upper limit of the aforementioned numerical range, the strand strength of the carbon fiber bundle is easily maintained at a high level. The average void diameter can be measured by small angle X-ray scattering (SAXS). The detailed measurement method is as described in the Examples.

[0026] In one embodiment, the diameter of the carbon fiber is preferably 4.0 to 7.0 μm, more preferably 4.9 to 6.0 μm, and even more preferably 5.0 to 5.8 μm. When the diameter of the carbon fiber is equal to or greater than the lower limit of the above-mentioned range, carbon fibers with improved fiber openability are likely to be obtained. When the diameter of the carbon fiber is equal to or less than the upper limit of the above-mentioned range, the bundledness of the carbon fiber bundle is likely to be maintained. As a result, handling is easy.

[0027] The diameter of a single fiber is determined as follows. Carbon fiber bundle density (g / cm 3 ), the mass per meter of the carbon fiber bundle (g / m), and the number of filaments in the carbon fiber bundle, calculate the cross-sectional area of ​​each single carbon fiber fiber. The diameter of a perfect circle having the same area as that cross-sectional area is taken as the diameter of the single carbon fiber fiber.

[0028] In one embodiment, the fiber density of the carbon fiber is 1.70 to 1.90 g / cm 3 It is preferable that the density is 1.75 to 1.85 g / cm 3 More preferably, it is 1.78 to 1.83 g / cm 3 It is more preferable that: When the fiber density of the carbon fiber is equal to or greater than the lower limit of the above-mentioned range, carbon fiber with improved strand strength is likely to be obtained.When the fiber density of the carbon fiber is equal to or less than the upper limit of the above-mentioned range, carbon fiber with improved carbonization yield is likely to be obtained. The fiber density is a value determined by the method described in the Examples below.

[0029] In one embodiment, the area ratio of the low density portion of the carbon fiber is preferably 0.24% or less. When the area ratio of the low-density portion of the carbon fiber is 0.24% or less, carbon fiber with improved strand strength is likely to be obtained. From this viewpoint, the area ratio of the low-density portion of the carbon fiber is more preferably 0.18% or less, and even more preferably 0.15% or less. The area ratio of the low density portion of the carbon fiber can be determined by the method described in the examples below.

[0030] In one embodiment, the complete crystallinity of the matrix structure of the carbon fiber is preferably 0.43 or greater. If the complete crystallinity of the matrix structure is 0.43 or more, carbon fibers with improved strand strength are likely to be obtained. From this viewpoint, the complete crystallinity of the matrix structure is more preferably 0.45 or more, and even more preferably 0.50 or more. The degree of complete crystallinity of the matrix structure of the carbon fiber can be determined by the method described in the Examples below.

[0031] [Carbon fiber bundle] (Carbon fiber bundle according to the first aspect) The carbon fiber bundle according to the first aspect of the present invention includes the carbon fiber according to the first aspect of the present invention described above. By gathering a plurality of carbon fibers according to the first aspect described above into a carbon fiber bundle, the strand strength and strand modulus of the carbon fiber bundle can be increased.

[0032] The carbon fiber according to the first aspect relates to a carbon fiber in which the number of voids in the cross section of a single fiber is 8 or less. A carbon fiber bundle according to the first aspect of the present invention is a bundle of a plurality of carbon fibers according to the first aspect.

[0033] (Carbon fiber bundle according to the second aspect) The carbon fiber bundle related to the second aspect of the present invention contains the carbon fiber related to the second aspect of the present invention described above. By gathering a plurality of carbon fibers according to the second aspect described above into a carbon fiber bundle, the strand strength and strand modulus of the carbon fiber bundle can be increased.

[0034] A carbon fiber bundle according to a second aspect of the present invention relates to a carbon fiber bundle in which a plurality of carbon fiber monofilaments are bundled together, wherein the number of voids in the surface layer of the carbon fiber is 0.5 / μm or less relative to the circumferential length of the carbon fiber.

[0035] In the carbon fiber bundle according to the second aspect, the number of voids in the surface layer of the carbon fiber is 0.5 / μm or less relative to the circumferential length of the carbon fiber. Therefore, defects contained in the fiber are reduced without decreasing the strand modulus. Moreover, the strand strength is also increased.

[0036] (Preferred embodiment of carbon fiber bundle) In the following description, the term "carbon fiber bundle" is a general term for the carbon fiber bundle according to the first aspect and the carbon fiber bundle according to the second aspect. The preferred embodiments described below are common to the carbon fiber bundle according to the first aspect and the carbon fiber bundle according to the second aspect.

[0037] In one embodiment, the carbon fiber bundle may contain both the carbon fibers according to the first aspect and the carbon fibers according to the second aspect. In one embodiment, the carbon fiber bundle may contain other carbon fibers other than the carbon fibers of the present invention described above, or may not contain such other carbon fibers. The other carbon fibers are carbon fibers other than the carbon fibers related to the first aspect and the carbon fibers related to the second aspect. When other carbon fibers are contained, the proportion of the carbon fibers according to the present invention varies depending on the desired physical properties, but in order to increase the strand strength and strand modulus, it is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 100% by mass.

[0038] In one embodiment, the strand strength of the carbon fiber bundle is preferably 5.0 to 6.5 GPa, more preferably 5.5 to 6.4 GPa, and even more preferably 5.8 to 6.3 GPa. When the strand strength is equal to or greater than the lower limit of the above-mentioned range, sufficient elongation is likely to be obtained when the carbon fiber bundle is made into a composite. When the strand strength is equal to or less than the upper limit of the above-mentioned range, the carbon fiber bundle can be obtained with good productivity. The strand strength of the carbon fiber bundle is measured in accordance with JIS R 7608:2007.

[0039] In one embodiment, the strand modulus of the carbon fiber bundle is preferably 360 to 400 GPa, more preferably 370 to 395 GPa, and even more preferably 374 to 390 GPa. When the strand modulus is equal to or greater than the lower limit of the above-mentioned numerical range, sufficient rigidity is likely to be obtained when the carbon fiber bundle is made into a composite. When the strand modulus is equal to or less than the upper limit of the above-mentioned numerical range, the carbon fiber bundle can be obtained with good productivity. The strand modulus is measured in accordance with Method A of JIS R 7608:2007.

[0040] In one embodiment, the number of carbon fiber single fibers in the carbon fiber bundle is preferably 8,000 to 20,000, more preferably 10,000 to 19,000, and even more preferably 12,000 to 18,000. When the number of carbon fibers is equal to or greater than the lower limit of the above-mentioned range, the processability of the carbon fiber bundle is likely to be improved. When the number of carbon fibers is equal to or less than the upper limit of the above-mentioned range, the handleability of the carbon fiber bundle is likely to be improved.

[0041] [Method of manufacturing carbon fiber bundles] The method for producing a carbon fiber bundle includes heating a precursor of the carbon fiber bundle, i.e., a precursor fiber bundle. The method is characterized in that the heating temperature is increased from 1800°C to 2200°C at a rate of 300 to 600°C / min.

[0042] In conventional manufacturing methods, when heating a precursor fiber bundle, it is common to rapidly raise the temperature to the maximum temperature to obtain the desired elastic modulus, and then hold the maximum temperature for a certain period of time. This is because the balance between the size and number of carbonization furnaces, productivity, physical properties, etc., was taken into consideration. Therefore, the temperature rise rate was faster than the above range.

[0043] It is thought that a high temperature rise rate cannot reduce voids in carbon fibers. Some examples of the causes of void formation in carbon fibers are given below. When the fiber surface is rough in the swollen state of the yarn, the oil may penetrate into the fiber surface. When heated to high temperatures during the heat treatment process during manufacturing, the oil may decompose and leave traces of voids on the surface of the carbon fiber. When heated to high temperatures during manufacturing, atoms other than carbon (e.g., hydrogen, nitrogen, and oxygen atoms) may be removed from the chemical structure, forming microvoids. · Voids can form if there is a defect in the crystallization of carbon fibers during manufacturing.

[0044] In contrast, in the present manufacturing method, when the temperature is increased from 1800°C to 2200°C, the rate of temperature increase is set to 300-600°C / min, allowing carbon crystal growth to proceed slowly. This reduces the likelihood of structural irregularities occurring in the carbon fiber. As a result, when the oil or other agent on the surface layer escapes from the carbon fiber, it does not fly away all at once, which is thought to reduce the likelihood of voids remaining. This results in a carbon fiber bundle with high strand strength and strand modulus and reduced defects in the single fiber.

[0045] (Preferred embodiment) In one embodiment, the carbon fiber bundle is obtained by heat treating the precursor fiber bundle. In one example, the heat treatment involves a flame-proofing treatment followed by a carbonization treatment. In this case, the precursor fiber bundle is subjected to the flame-proofing treatment and the carbonization treatment in sequence to obtain the carbon fiber bundle. After the carbonization treatment, the carbon fiber bundle may be subjected to a surface oxidation treatment and a sizing treatment, if necessary. A preferred embodiment of the method for producing a carbon fiber bundle will be described below.

[0046] (precursor fiber bundle) The precursor fiber bundle is a fiber bundle that is subjected to heat treatment to obtain a carbon fiber bundle. The fiber bundle is a bundle of single fibers. Examples of the single fibers of the precursor fiber bundle include PAN-based fibers, rayon-based fibers, and pitch-based fibers. Among these, the precursor fiber bundle is preferably a precursor fiber bundle in which PAN-based fiber single fibers are bundled. Hereinafter, the precursor fiber bundle in which PAN-based fiber single fibers are bundled is also particularly referred to as a "precursor acrylic fiber bundle."

[0047] The precursor acrylic fiber bundle can be obtained, for example, by spinning a spinning solution containing an acrylonitrile polymer to form a coagulated fiber. If necessary, the precursor acrylic fiber bundle may be subjected to water washing, bath drawing, application of an oil agent, drying for densification, drawing, etc. The acrylonitrile-based polymer is not particularly limited as long as it has an acrylonitrile unit in its molecular structure. The acrylonitrile-based polymer may be a homopolymer of acrylonitrile or a copolymer of acrylonitrile and another monomer (e.g., methacrylic acid, etc.). In the case of a copolymer, the content ratio of the acrylonitrile unit to the other monomer unit can be appropriately set depending on the properties of the carbon fiber bundle to be produced.

[0048] The spinning method of the spinning solution containing the acrylonitrile polymer is not particularly limited, and examples thereof include wet spinning in which the spinning solution is directly spun into a coagulation bath, dry spinning in which the spinning solution is coagulated in air, and dry-wet spinning in which the spinning solution is once spun into air and then coagulated in a bath.

[0049] In the spinning method using wet spinning or dry-wet spinning, the spinning solution is spun into a coagulation bath through a nozzle having a circular cross-section. As the coagulation bath, an aqueous solution containing the solvent used in the spinning solution is preferably used from the viewpoint of ease of solvent recovery.

[0050] Among these, dry-wet spinning is preferred because it can reduce surface wrinkles on the side surfaces of single fibers and easily suppress the generation of defects due to the surface shape. In other words, the precursor acrylic fiber bundle is preferably a fiber bundle obtained by dry-wet spinning of a spinning solution containing an acrylonitrile-based polymer.

[0051] In one embodiment, the single fiber fineness of the precursor fiber bundle is preferably 0.5 to 2.5 dtex, more preferably 0.7 to 2.0 dtex. When the single fiber fineness of the precursor fiber bundle is equal to or greater than the lower limit of the above-mentioned range, a carbon fiber bundle with less fiber breakage is likely to be obtained.When the single fiber fineness of the precursor fiber bundle is equal to or less than the upper limit of the above-mentioned range, a carbon fiber bundle with less unevenness in performance is likely to be obtained.

[0052] In one embodiment, the number of filaments in the precursor fiber bundle, ie, the number of single fibers constituting the precursor fiber bundle, is preferably 8,000 to 20,000, more preferably 10,000 to 18,000, and even more preferably 12,000 to 18,000. When the number of filaments in the precursor fiber bundle is equal to or greater than the lower limit of the above-mentioned range, a carbon fiber bundle with improved processability is likely to be obtained.When the number of filaments in the precursor fiber bundle is equal to or less than the upper limit of the above-mentioned range, a carbon fiber bundle with improved handleability is likely to be obtained.

[0053] (Flame-resistant treatment) In one embodiment, the flame retardation treatment involves heating the precursor fiber bundle in an oxidizing atmosphere to convert it into a flame retardant fiber bundle. The flame-resistant treatment is carried out, for example, in a hot air circulation type flame-resistant furnace at 180 to 280°C, preferably until the density of the flame-resistant fiber after the flame-resistant treatment is 1.28 to 1.42 g / cm. 3 A method of passing the precursor fiber bundle through the sintered body until the sintered body reaches a temperature of 1000.0001° C. is also known.

[0054] If the density of the flame-resistant fiber is equal to or greater than the lower limit of the above-mentioned range, adhesion between individual fibers can be prevented during the subsequent carbonization treatment.If the density of the flame-resistant fiber is equal to or less than the upper limit of the above-mentioned range, the flame-resistant treatment does not take too long, which is economical.

[0055] Examples of the gas for the oxidizing atmosphere include air, oxygen, nitrogen dioxide, etc. Among these, air is preferred from the viewpoint of economy. In one example, the time for the flameproofing treatment is preferably, for example, 30 to 100 minutes.

[0056] In one embodiment, in the flame retardant treatment, it is preferable to carry out a stretching operation, since this makes it easier to maintain the orientation of the fibril structure. The elongation rate in the flame-resistant treatment is preferably 1 to 8%. When the elongation rate in the flame-resistant treatment is equal to or greater than the lower limit of the above-mentioned range, the orientation of the fibril structure can be easily maintained or improved. As a result, a carbon fiber bundle with improved mechanical properties can be easily obtained. If the elongation rate in the flame retardant treatment is equal to or less than the upper limit of the above range, the fibril structure itself is less likely to break, and the subsequent formation of the carbon fiber structure is less likely to be impaired, making it easier to obtain a high-strength carbon fiber bundle.

[0057] (carbonization treatment) In one embodiment, the flame-resistant fiber bundle is subsequently subjected to a carbonization treatment, in which the flame-resistant fiber bundle is carbonized in an inert atmosphere to obtain a carbon fiber bundle.

[0058] Examples of the gas for the inert atmosphere include nitrogen, argon, helium, etc. Among these, nitrogen is preferred from the viewpoint of economy.

[0059] In one embodiment, the temperature of the carbonization treatment (carbonization treatment temperature) is preferably 300 to 2300°C. The carbonization temperature is preferably increased during the carbonization treatment. When increasing the temperature, for example, a plurality of carbonization furnaces may be installed, and the temperature of each carbonization furnace may be set so that the temperature increases from the upstream carbonization furnace to the downstream carbonization furnace. The flame-resistant fiber bundle may be sequentially passed through the carbonization furnace from the upstream carbonization furnace to the downstream carbonization furnace for treatment.

[0060] The number of voids in the carbon fiber and the number of surface voids can be controlled by adjusting the heating temperature and the rate of temperature rise when the flame-resistant fiber bundle is heated and carbonized. The average void length of the carbon fiber can be controlled by adjusting the heating temperature and the rate of temperature rise when the flame-resistant fiber bundle is heated for carbonization treatment. The average void diameter of the carbon fiber can be controlled by adjusting the heating temperature and the rate of temperature rise when the flame-resistant fiber bundle is heated for carbonization treatment. The strand modulus of elasticity of the carbon fiber bundle can be controlled by adjusting the heating temperature when the flame-resistant fiber bundle is heated and carbonized.

[0061] An example of the case where the carbonization treatment temperature is increased will be described below.

[0062] In one embodiment, the flame-resistant fiber bundle can be carbonized by sequentially carrying out the first carbonization treatment, the second carbonization treatment, and the third carbonization treatment. In the first carbonization treatment, the flame-resistant fiber bundle can be heat-treated in a first carbonization furnace with a temperature gradient of 300°C to 800°C in an inert atmosphere. In the second carbonization treatment, the material can be heated in a second carbonization furnace with a temperature gradient of 1000°C to 1700°C in an inert atmosphere. In the third carbonization treatment, the material can be heated in a third carbonization furnace with a temperature gradient of 1200°C to 2500°C in an inert atmosphere.

[0063] The temperature gradient in the first carbonization treatment is preferably a linear gradient from 300° C. to 800° C., and more preferably a linear gradient from 300° C. to 700° C. The temperature gradient in the first carbonization treatment does not have to be a linear gradient.

[0064] In one example, the starting temperature (minimum temperature) of the first carbonization treatment is preferably 300° C. or higher. The maximum temperature of the first carbonization treatment is preferably 800° C. or lower. If the maximum temperature of the first carbonization treatment is 800° C. or lower, it is easy to prevent the processed fiber from becoming brittle.

[0065] In one example, the treatment time for the first carbonization treatment is preferably 1 to 3 minutes. If the treatment time for the first carbonization treatment is equal to or greater than the lower limit of the above-mentioned range, decomposition reactions due to a sudden increase in temperature are unlikely to occur, and a carbon fiber bundle with high strength is likely to be obtained. If the treatment time for the first carbonization treatment is equal to or less than the upper limit of the above-mentioned range, a decrease in the degree of crystal orientation is suppressed. Therefore, a carbon fiber bundle with improved mechanical properties is likely to be obtained.

[0066] In the first carbonization treatment, it is preferable to carry out an elongation operation, since this makes it easier to maintain the orientation of the fibril structure. When an elongation operation is carried out, the elongation rate in the first carbonization treatment is preferably 2 to 7%. When the elongation rate in the first carbonization treatment is equal to or greater than the lower limit of the above range, the orientation of the fibril structure can be easily maintained or improved, and as a result, a carbon fiber bundle with improved mechanical properties can be easily obtained. If the elongation rate in the first carbonization treatment is equal to or less than the upper limit of the above-mentioned range, the fibril structure itself is less likely to break, and the subsequent formation of the carbon fiber structure is less likely to be impaired, making it easier to obtain a high-strength carbon fiber bundle.

[0067] The temperature gradient in the second carbonization treatment is preferably a linear gradient from 1000° C. to 1750° C., and more preferably a linear gradient from 1000° C. to 1700° C. The temperature gradient in the second carbonization treatment does not have to be a linear gradient.

[0068] The temperature in the second carbonization treatment can be set according to the desired strand modulus required for the carbon fiber bundle. In order to obtain carbon fibers with improved mechanical properties, a lower maximum temperature in the second carbonization treatment is preferable. By lowering the maximum temperature in the second carbonization treatment and lengthening the treatment time, a carbon fiber bundle with a high strand modulus is more likely to be obtained. Furthermore, a longer treatment time in the second carbonization treatment can make the temperature gradient gentler. This also has the effect of suppressing the formation of defects.

[0069] In one example, the minimum temperature of the second carbonization treatment is preferably 1100° C. or higher, and the maximum temperature of the second carbonization treatment is preferably 1700° C. or lower. In one example, the treatment time for the second carbonization treatment is preferably 1 to 3 minutes.

[0070] In the second carbonization treatment, the process fiber is subject to significant shrinkage, so the elongation rate is preferably -6 to 0%. However, it is also preferable to carry out this treatment under tension. If the elongation rate in the second carbonization treatment is equal to or greater than the lower limit of the above-mentioned range, the orientation of the crystals in the fiber axis direction is less likely to decrease, and sufficient tensile properties are obtained. If the elongation rate in the second carbonization treatment is equal to or less than the upper limit of the above-mentioned range, the structure formed up to that point is less likely to be destroyed, and good strength can be maintained.

[0071] The temperature gradient in the third carbonization treatment is preferably a linear gradient from 1200° C. to 2500° C., and more preferably a linear gradient from 1200° C. to 2400° C. The temperature gradient in the third carbonization treatment does not have to be a linear gradient.

[0072] The minimum temperature of the tertiary carbonization treatment is preferably 1800°C or higher. The maximum temperature of the tertiary carbonization treatment is preferably 2100 to 2300°C, more preferably 2290°C or lower. If the maximum temperature of the tertiary carbonization treatment is 2100°C or higher, it is easy to increase the strand modulus. If the maximum temperature of the tertiary carbonization treatment is 2300°C or lower, it is easy to maintain a long furnace material life of the tertiary carbonization furnace.

[0073] In one example, the treatment time for the third carbonization treatment is preferably 1 to 3 minutes. In one example, the total treatment time for the second carbonization treatment and the third carbonization treatment is preferably 2 to 6 minutes.

[0074] During the third carbonization treatment, the heating temperature is increased from 1800°C to 2200°C at a rate of preferably 300 to 600°C / min, more preferably 350 to 550°C / min, and even more preferably 400 to 500°C / min. When the heating rate is equal to or greater than the lower limit of the aforementioned range, productivity improves. When the heating rate is equal to or less than the upper limit of the aforementioned range, voids are reduced. Therefore, a carbon fiber bundle having high strand strength and strand modulus is likely to be obtained.

[0075] The reason for the small number of voids is as mentioned above. That is, by slowing down the heating rate, the carbon crystal growth progresses slowly, making it difficult for structural irregularities to occur. As a result, when the oil or other agent on the surface is removed from the carbon fiber, it is not blown away all at once, so it is thought that voids are less likely to remain.

[0076] In the third carbonization treatment, the process fiber is subject to large shrinkage, so the elongation rate is preferably −6 to 0%, although it is also preferable to carry out the treatment under tension. If the elongation rate in the third carbonization treatment is equal to or greater than the lower limit of the above range, the orientation of the crystals in the fiber axis direction is less likely to decrease, and therefore sufficient tensile properties are likely to be obtained. If the elongation rate in the second carbonization treatment is equal to or less than the upper limit of the above range, the structure formed up to that point is less likely to be destroyed, and therefore the strength can be easily maintained at a good level. The total elongation rate of the second carbonization treatment and the third carbonization treatment is preferably −5 to 0%.

[0077] In the carbonization process described above, the carbonization temperature is increased using three carbonization furnaces with different temperature gradients, but the number of carbonization furnaces used in the carbonization process may be two, four or more. A plurality of heating zones may be provided in one carbonization furnace, and the temperature of each heating zone may be set so that the temperature increases from the upstream heating zone toward the downstream heating zone, and the flame-resistant fiber bundle may be passed sequentially from the upstream heating zone toward the downstream heating zone. In either method, the temperature rise rate during carbonization treatment from 1800°C to 2200°C is preferably 300 to 600°C / min, more preferably 350 to 550°C / min, and even more preferably 400 to 500°C / min.

[0078] (surface oxidation treatment) The method of surface oxidation treatment is not limited in any way. Various methods can be used. Examples include electrolytic oxidation, chemical oxidation, and air oxidation. Among these, electrolytic oxidation, which is widely practiced industrially, is preferred because it allows stable surface oxidation treatment.

[0079] In surface oxidation treatment, the IPA, which indicates the surface treatment state, is set to 0.05 to 0.25 μA / cm 2 It is preferable to set the value within this range. A simple method for controlling the value within this range is to adjust the amount of electricity in the electrolytic oxidation treatment. In the electrolytic oxidation treatment, even with the same amount of electricity, the IPA can vary greatly depending on the electrolyte used and its concentration. For example, in an alkaline aqueous solution with a pH greater than 7, it is preferable to perform the oxidation treatment by using the carbon fiber as the anode and passing an amount of electricity of 10 to 200 coulombs / g.

[0080] Examples of the electrolyte include ammonium carbonate, ammonium bicarbonate, ammonium sulfate, calcium hydroxide, sodium hydroxide, and potassium hydroxide.

[0081] (Sizing process) The sizing treatment can be carried out, for example, by applying a sizing agent dissolved in an organic solvent or a sizing agent emulsion liquid dispersed in water with an emulsifier or the like to the carbon fiber bundle by a roller immersion method, a roller contact method, or the like, and then drying the bundle.

[0082] The sizing agent is not particularly limited, and various sizing agents can be used, such as sizing agents containing epoxy resin, polyether resin, epoxy-modified polyurethane resin, and polyester resin as the main component.

[0083] The amount of sizing agent attached to the surface of carbon fiber can be controlled by adjusting the concentration of the sizing agent solution and the amount of squeezing. For drying, hot air, a hot plate, a heated roller, various infrared heaters, etc. can be used.

[0084] According to the method for producing a carbon fiber bundle described above, it is possible to reduce the number of voids present in the cross section of a single carbon fiber, thereby obtaining a carbon fiber bundle with high strand strength and strand modulus and reduced defects in the single fiber.

[0085] [Application] The carbon fiber bundle according to the present invention is combined with, for example, a matrix resin to be molded into a composite material, which can then be used in a variety of applications.

[0086] The matrix resin is not particularly limited. Examples include thermosetting resins such as epoxy resins and phenolic resins; radical polymerization resins such as acrylic resins, vinyl ester resins, and unsaturated polyester resins; and thermoplastic resins such as thermoplastic acrylic resins, polyamide resins, polyimide resins, polycarbonate resins, polypropylene resins, and polyethylene resins. Modified versions of these resins can also be used. Commercially available matrix resins can also be used.

[0087] The applications of composite materials obtained from carbon fiber bundles are not particularly limited, and examples thereof include, but are not limited to, automobile components, aerospace materials, civil engineering materials, building materials, sports materials, leisure materials, and industrial materials (pressure vessels, wind turbine blades, etc.), and the composite materials can be used in a wide range of applications. [Example]

[0088] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following descriptions.

[0089] The various measurement methods used in this example are as follows.

[0090] [Method for measuring carbon fiber diameter] Carbon fiber bundle density (g / cm 3 The cross-sectional area of ​​each single carbon fiber was calculated from the mass (mg / m) per meter of the carbon fiber bundle, the mass (mg / m) per meter of the carbon fiber bundle, and the number of filaments in the carbon fiber bundle. The diameter of a perfect circle having an area equal to the cross-sectional area was calculated and used as the diameter of the single carbon fiber.

[0091] [Method for measuring fiber density of carbon fiber] The density of the carbon fiber was measured in accordance with Method C (gradient density pipe method) described in JIS R 7063:1999.

[0092] [Method for measuring the number of voids and surface voids] Using a focused ion beam (Hitachi High-Tech Corporation, "FB-2100"), a 100 nm thick flake was prepared from a cross section perpendicular to the fiber axis of a single carbon fiber. The cross section perpendicular to the fiber axis of the carbon fiber was observed using a transmission electron microscope (Hitachi High-Tech Corporation, "H-7600") at an acceleration voltage of 80 kV, observation mode: HC-mode, and magnification of 20,000 times to obtain a transmission electron microscope (TEM) image. The obtained TEM image was flattened and binarized using image analysis software (Nippon Roper Co., Ltd., "Image-Pro PLUS ver. 7.0"). Then, using the "count / measure" function of the image analysis software, white areas present in the cross section were considered to be voids, and the number of voids and the circumference of the carbon fiber were measured. Similar measurements were performed a total of five times, and the average number was taken as the number of voids present in the cross section of the single carbon fiber (void count). Here, the surface of the carbon fiber was the surface of the carbon fiber itself. The surface of the sizing agent attached to the surface of the carbon fiber was not the surface of the carbon fiber. The measurement sample was carbon fiber before the sizing agent was attached or carbon fiber from which the sizing agent was removed.

[0093] [Calculation method for average void length and average void diameter] According to the Ruland method described in Macromolecules, Vol. 33, No. 5, 2000, the average void length and average void diameter were calculated by SAXS (small angle X-ray scattering) as follows.

[0094] First, an Anton Paar SAXSpoint 2.0 system was used, with CuKα X-rays (wavelength 1.54 Å) used, an exposure time of 30 minutes, a vacuum measurement environment, and a sample-to-detector distance of 610 mm. After aligning the carbon fiber bundle in one direction, the fiber axis direction was set vertically on the sample stage, and small-angle X-ray scattering measurements were performed. At this time, scattering originating from voids inside the carbon fiber was observed in a direction perpendicular to the fiber axis.

[0095] Next, analysis by the Ruland method was performed using graph creation software (Igor Pro 8.0). In this example, the scattering vector q was defined as 4π sinθ / λ (θ: scattering angle, λ: X-ray wavelength). The two-dimensional scattering profile obtained under the above measurement conditions was converted to polar coordinates by dividing the 360° azimuth angle into 1000 parts, and a scattering intensity map of the azimuth angle vs. the scattering vector q was obtained. Here, the carbon fiber axis direction was set as an azimuth angle of 0°. The obtained scattering intensity map was analyzed in the range (q = 0.8 to 1.86 nm) that did not include streaks resulting from total reflection of X-rays on the carbon fiber surface in the direction perpendicular to the carbon fiber axis (azimuth angle 90°). -1 ) was analyzed using the Ruland method. Specifically, the scattering intensity map of azimuth angle vs. scattering vector q was averaged every five pixels in the q direction to obtain the azimuth angle vs. scattering intensity profile for each q. The azimuth angle range of 0 to 180° of this profile was fitted with a Gaussian function to calculate the integral width B.

[0096] Next, q 2 B 2 Aq 2 Plot the line to obtain an approximate line. The intercept (2π / L) is 2 From this, the void length L is calculated.

[0097] In addition, a scattering profile perpendicular to the fiber axis was cut out.Fitting was performed according to the Ruland method described in Macromolecules, Vol. 33, No. 5, 2000, and the average void diameter of voids present in the cross section of a single carbon fiber was calculated.

[0098] [Calculation method for area ratio of low density area and complete crystallinity of matrix structure] The measurement sample was prepared using a ThermoFisher FIB processing device, Helios G4, to prepare thin sections for TEM observation, with a thickness of 120 nm and a thin section of 75 nm. The thin sections for TEM observation were stored in a vacuum chamber. The density of the surface film was measured using an analytical transmission scanning electron microscope (JEOL Ltd., "JEM-ARM300F") with the acceleration voltage adjusted to 80 kV and the camera lens set to 98 mm under HAADF-STEM conditions. Under these conditions, the intensity I of the HAADF-STEM at an acceptance angle of 37-200 mrad, which provides the most structural information without Bragg reflection, was measured. s can be obtained.

[0099] I s is the value σ obtained by integrating the Rutherford scattering intensity from θ1 to θ2 with respect to the intensity I0 of the incident electron beam, as shown in the following equation (1). θ1θ2 is proportional to the number of atoms N and the sample thickness t. I s =σ θ1θ2 ·N·t·I0···(1)

[0100] HAADF-STEM I s The measurement of the I value is suitable as a method for measuring the density of the target nano-region. s The values ​​were measured at 2048 x 2048 points using an electron beam probe focused to 0.07 nmφ. The observation area was the third region from the center of the fiber when the area was divided into four from the center outward. The observation range was 204 nm x 204 nm. The structure in the observation field is I from specimens with the same sample thickness. s Therefore, the I measured by each s The method of calculating density from the ratio of values ​​allows the sample thickness t in equation (1) to be ignored. The Is values ​​of the substrate, amorphous, and void were measured for the 25 nm thin film portion.

[0101] In this measurement, the reason why the above formula (1) is satisfied is that ΔI s Therefore, in order to consider the influence of thickness t in the sample comparison, ΔI s The normalized density value was calculated from the measurement to obtain the value. The electron density distribution at zero intensity obtained by the above measurements was taken as the area ratio of the low density portion, and the median value of the electron density distribution was taken as the degree of complete crystallinity of the substrate structure.

[0102] [Method for measuring strand strength and strand modulus of carbon fiber bundle] The strand strength of the carbon fiber bundle was measured in accordance with JIS R 7608: 2007. The strand modulus was calculated by Method A of JIS R 7608: 2007.

[0103] [Example 1] (Preparation of precursor fiber bundles) An acrylonitrile copolymer containing 98% by mass of acrylonitrile units and 2% by mass of methacrylic acid units was dissolved in dimethylformamide to prepare a spinning dope containing 23.5% by mass of the acrylonitrile copolymer. This spinning solution was discharged from a spinneret equipped with 12,000 nozzles, each 150 μm in diameter, and subjected to dry-wet spinning. After being spun into air and passing through a space of approximately 5 mm, it was introduced into a coagulation solution filled with an aqueous solution containing 79.0% by mass of dimethylformamide and coagulated. The temperature of the coagulation solution was adjusted to 10°C in advance. The film was then stretched 1.1 times in air, and then stretched 2.5 times in a stretching tank filled with an aqueous solution containing 35% by mass of dimethylformamide, the temperature of which had been adjusted to 60°C. Furthermore, the process fiber bundle containing the solvent was washed with clean water. It was then stretched 1.4 times in hot water at 95°C. Subsequently, an oil solution mainly composed of amino-modified silicone was applied to the process fiber bundle in an amount of 1.1 mass% relative to the fiber mass, and then the process fiber bundle was dried and densified. The dried and densified process fiber bundle was stretched 2.6 times between heated rolls to further improve orientation and densify. A precursor fiber bundle (precursor acrylic fiber bundle) was then wound up to obtain it. The single fiber fineness of the precursor fiber bundle was 0.77 dtex.

[0104] (Manufacturing of carbon fiber bundles) A plurality of precursor fiber bundles were aligned in parallel and introduced into a flame-proofing furnace with a temperature gradient of 220 to 280°C in an air atmosphere. The precursor fiber bundles were heated while being stretched by 6%, and the density of the bundles was reduced to 1.345 g / cm. 3 The flame-resistant treatment time was 70 minutes.

[0105] Next, the flame-resistant fiber bundle was subjected to a first carbonization treatment by heating it in a first carbonization furnace having a temperature gradient of 300 to 700°C in a nitrogen atmosphere while applying an elongation of 4.5%. The temperature gradient was set to be linear. The treatment time was 2.0 minutes. Next, a second carbonization treatment was carried out using a second carbonization furnace with a temperature gradient of 1000 to 1700°C in a nitrogen atmosphere. Subsequently, a third carbonization treatment was carried out using a third carbonization furnace with a temperature gradient of 1200 to 2400°C in a nitrogen atmosphere to obtain a carbonized fiber bundle. At this time, the total elongation rate in the second and third carbonization furnaces was -4.0%, and the treatment time was 3.5 minutes. The minimum temperature in the second carbonization furnace was 1100°C and the maximum temperature was 1200°C. The minimum temperature in the third carbonization furnace was 1800°C and the maximum temperature was 2300°C. The heating rate when increasing the heating temperature in the third carbonization furnace from 1800°C to 2200°C was 453°C / min. The temperature gradient in the second and third carbonization furnaces was set to be linear.

[0106] Next, while the carbonized fiber bundle was running through a 10% by mass aqueous solution of ammonium bicarbonate, an electric current was applied between the carbonized fiber bundle as the anode and the counter electrode so that the amount of electricity was 40 coulombs per 1 g of the carbonized fiber to be treated. The carbonized fiber bundle was then washed with hot water at 90°C and dried. Next, 0.5% by mass of a sizing agent (DIC Corporation, "Hydran N320") was applied (sizing treatment), and the bundle was wound around a bobbin to obtain a carbon fiber bundle.

[0107] For the carbon fiber bundle after sizing treatment, the diameter and number of single fibers, the mass (mg / m) and density (g / m) per 1 m of the carbon fiber bundle were measured. 3The average void length, average void diameter, number of voids, number of surface voids, strand strength and modulus, area ratio of low density area, and complete crystallinity of the matrix structure were measured. The results are shown in Table 1.

[0108] [Examples 2 to 6] Carbon fiber bundles were produced and various measurements were carried out in the same manner as in Example 1, except that the set temperatures (minimum temperature and maximum temperature) of the second carbonization furnace were changed as shown in Table 1. The results are shown in Table 1.

[0109] [Comparative Example 1] A precursor fiber bundle was prepared in the same manner as in Example 1, except that the single fiber fineness was changed to 1.0 dtex. Using the obtained precursor fiber bundle, a carbon fiber bundle was produced and various measurements were carried out in the same manner as in Example 1, except that the set temperatures (minimum and maximum temperatures) of the second carbonization furnace, the set temperatures (minimum and maximum temperatures) of the third carbonization furnace, and the temperature rise rate were changed as shown in Table 1. The results are shown in Table 1.

[0110] Comparative Example 2 Various measurements were carried out on a commercially available carbon fiber bundle ("M40JB" manufactured by Toray Industries, Inc.). The results are shown in Table 1.

[0111] [Table 1]

[0112] In Table 1, "heating rate (°C / min)" is the rate at which the temperature is increased from 1800°C to 2200°C.

[0113] As is clear from the results in Table 1, the carbon fiber bundles obtained in each example had high strand strength and strand modulus. In contrast, the carbon fibers of Comparative Examples 1 and 2 had many voids in the region from the surface of the carbon fiber to a depth of 100 nm, and the average void length was large. The carbon fiber bundles of Comparative Examples 1 and 2 also had lower strand strengths than the carbon fiber bundles obtained in each Example. [Industrial Applicability]

[0114] The carbon fiber of the present invention provides a carbon fiber bundle having high strand strength and strand modulus, and the carbon fiber of the present invention has reduced defects. The carbon fiber bundle of the present invention has high strand strength and strand modulus, and has reduced defects in the single fibers. According to the method for producing a carbon fiber bundle of the present invention, a carbon fiber bundle having high strand strength and strand modulus and reduced defects in the single fibers can be obtained.

Claims

1. A PAN-based carbon fiber having 8 or less voids in the cross section of a single fiber. Method for measuring the number of voids: After cutting the single fiber of the PAN-based carbon fiber perpendicular to the longitudinal direction, a thin section containing the cross section of the PAN-based carbon fiber is prepared. A TEM image of the cross section of the PAN-based carbon fiber in the thin section is obtained at a magnification of 20,000 times using a transmission electron microscope. The TEM image is flattened and binarized using image analysis software, and then the number of white areas present in the cross section is counted as the number of voids using the "count / measure" function of the image analysis software.

2. 2. The PAN-based carbon fiber according to claim 1, wherein the average void length is 5 to 20 nm.

3. The PAN-based carbon fiber according to claim 1, wherein the average void diameter is 0.5 to 0.7 nm.

4. 2. The PAN-based carbon fiber according to claim 1, having a diameter of 4.0 to 7.0 μm.

5. Fiber density is 1.70 to 1.90 g / cm ３ 2. The PAN-based carbon fiber according to claim 1, wherein

6. 2. The PAN-based carbon fiber according to claim 1, wherein the area ratio of low-density portions in the PAN-based carbon fiber is 0.24% or less.

7. 2. The PAN-based carbon fiber according to claim 1, wherein the complete crystallinity of the matrix structure in the PAN-based carbon fiber is 0.43 or more.

8. A PAN-based carbon fiber bundle comprising the PAN-based carbon fiber according to any one of claims 1 to 7.

9. The PAN-based carbon fiber bundle according to claim 8, having a strand strength of 5.0 to 6.5 GPa.

10. The PAN-based carbon fiber bundle according to claim 8, wherein the strand modulus is 360 to 400 GPa.

11. The PAN-based carbon fiber bundle according to claim 8, wherein the number of the PAN-based carbon fibers is 8,000 to 20,000.

12. A method for producing a PAN-based carbon fiber bundle, comprising heating a precursor fiber bundle, A method for producing a PAN-based carbon fiber bundle, wherein the heating temperature is increased from 1800°C to 2200°C at a rate of 300 to 600°C / min.

13. The method according to claim 12, wherein the maximum heating temperature is 2100 to 2300°C.

14. The method according to claim 12, wherein the precursor fiber bundle is a fiber bundle obtained by dry-wet spinning.

15. the precursor fiber bundle has a single fiber fineness of 0.5 to 2.5 dtex; The manufacturing method according to claim 12, wherein the number of single fibers in the precursor fiber bundle is 8,000 to 20,000.

Images