Carbon fiber and its manufacturing method

Abstract

The present invention provides a carbon fiber having improved physical properties (particularly strength), and a method for producing such a carbon fiber that can be used for general purposes. [Solution] Carbon fiber in which the atomic ratio of silicon atoms to carbon atoms (Si / C) on the fiber surface, as measured by X-ray photoelectron spectroscopy (XPS), is 0.001 to 0.020.

Description

[Technical Field]

[0001] The present invention relates to a carbon fiber and a method for producing the same. [Background technology]

[0002] Carbon fiber has excellent specific strength and specific modulus, and is lightweight, so it is used as a reinforcing fiber for thermosetting and thermoplastic resins in a wide range of applications, including aircraft, sports and leisure, and general industry. As its applications expand, even higher performance is required of carbon fiber reinforced composite materials (hereinafter also referred to as composites).

[0003] In the carbon fiber manufacturing process, for example, precursor fibers (resin fibers) with an oil applied thereto are heated in air for flame retardation treatment, then heated in a first heating furnace (C1 furnace) at a temperature of 300°C to 700°C in an inert atmosphere (particularly a nitrogen atmosphere), and further heated in a second heating furnace (C2 furnace) at a temperature of 800°C to 1500°C in an inert atmosphere (particularly a nitrogen atmosphere) for carbonization. During the manufacturing process, the fibers are drawn at a specific draw ratio while being transported from the C1 furnace to the C2 furnace (from upstream to downstream). Furthermore, carbon fibers with a high modulus can be obtained by treating them at even higher temperatures, if necessary.

[0004] One application of fiber-reinforced composite materials containing carbon fibers is a pressure vessel, which has, for example, an inner frame (liner) made of metal or resin and an outer layer of a carbon fiber-reinforced composite material containing carbon fibers as reinforcing fibers and resin.

[0005] As a method for manufacturing such a pressure vessel, in addition to a method in which resin is attached to carbon fiber and then wound around a liner, there is also a method in which carbon fiber (especially tow prepreg) to which resin has been attached in advance is wound around a liner and then subjected to a heat curing treatment.

[0006] Patent Document 1 discloses a tow prepreg containing a specific epoxy resin composition and reinforcing fibers. This document describes the production of a fiber-reinforced composite material using the tow prepreg, and states that this fiber-reinforced composite material can be suitably used in high-pressure vessels filled with hydrogen gas, such as those used in fuel cells.

[0007] Patent Document 2 discloses an epoxy resin composition suitable for use in fiber-reinforced composite materials such as tow prepregs, and aims to provide a method for curing the epoxy resin composition for fiber-reinforced composite materials suitable for the filament winding method in particular.

[0008] In order to improve the performance of carbon fiber, improvements to the manufacturing method are being made.

[0009] Patent Document 3 proposes suppressing the moisture content of carbon fiber precursor acrylic fiber bundles when applying a silicone oil agent before the flame-proofing process, thereby suppressing the diffusion of the oil agent in the flame-proofing furnace and reducing adhesion to the fiber bundles.

[0010] Furthermore, Patent Document 4 describes a carbon fiber bundle in which, when a single fiber tensile test is carried out with a sample length of 10 mm, the probability of the presence of defects of 50 nm or more on the recovered fracture surface is 35% or less, and the scatterer length l obtained by small-angle microbeam scattering is 46 nm or more. This document states that a carbon fiber bundle exhibiting high tensile strength can be obtained by controlling the defects that become fracture initiation points within a certain range and further increasing the length of the scatterer obtained by SAXS.

[0011] Patent Document 5 discloses a carbon fiber for filament winding (FW) molding that has specific physical properties and a constant rate of variation in yarn width when unwound. This document claims to provide a carbon fiber suitable for FW molding applications and a method for producing the same.

[0012] Patent Document 6 describes a carbon fiber for pressure vessels having a specific strand modulus and tensile elongation, and claims that by using this carbon fiber as a reinforcing fiber, a pressure vessel having good properties can be obtained. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] International Publication No. 2020 / 250957 [Patent Document 2] Japanese Patent Publication No. 2021-161239 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-211359 [Patent Document 4] Japanese Patent Application Publication No. 2018-141251 [Patent Document 5] Japanese Patent Application Laid-Open No. 2012-154000 [Patent Document 6] International Publication No. 2005 / 022026 Summary of the Invention [Problem to be solved by the invention]

[0014] Conventional carbon fibers have not always been able to provide the desired physical properties (particularly strength). Specifically, for example, pressure vessels manufactured using fiber-reinforced resin composite materials containing conventional carbon fibers have not always been able to provide the desired pressure resistance.

[0015] In particular, with regard to the carbon fibers of Patent Documents 3 to 5, even if the fiber morphology is improved, it is believed that this is insufficient to obtain carbon fibers that provide good tank properties.

[0016] Furthermore, the carbon fiber described in Patent Document 6 has a high elastic modulus and is expected to be expensive, and may not be suitable for general-purpose use.

[0017] An object of the present invention is to provide a carbon fiber having improved physical properties (particularly strength) and a method for producing a carbon fiber that can be used for general purposes. [Means for solving the problem]

[0018] The object of the present invention can be achieved by the following aspects of the present invention.

[0019] <Aspect 1> A carbon fiber having an atomic ratio of silicon atoms to carbon atoms (Si / C) on the fiber surface of 0.001 to 0.020 as measured by X-ray photoelectron spectroscopy (XPS). <Aspect 2> 2. The carbon fiber of embodiment 1, having a tensile modulus greater than 240 GPa and less than 330 GPa. <Aspect 3> The ratio (M / P) of the tensile modulus M (GPa) to the normalized scattering integrated intensity P measured by small-angle X-ray scattering (SAXS) is 230 or more. 3. The carbon fiber according to claim 1 or 2. <Aspect 4> A carbon fiber according to any one of aspects 1 to 3, having a tensile strength of greater than 5100 MPa. <Aspect 5> When a cross section perpendicular to the fiber axis direction of the carbon fiber is measured using a transmission electron microscope (TEM), the number of voids within a depth of 200 nm from the surface of the carbon fiber is 5 / 40,000 nm 2 A carbon fiber according to any one of aspects 1 to 4, wherein: <Aspect 6> A carbon fiber according to any one of aspects 1 to 5, for use in a pressure vessel. <Aspect 7> After the precursor fiber having the silicone-based oil agent attached thereto is flame-retarded, the precursor fiber is subjected to a first heat treatment in a first heating furnace under an inert atmosphere at a treatment temperature of 300°C to 700°C; and The fibers subjected to the first heat treatment are subjected to a second heat treatment in a second heating furnace at a treatment temperature of 1000°C or higher under an inert atmosphere. A method for producing carbon fibers, comprising: In the second heating furnace, the atmosphere inside the furnace is circulated from the outlet side to the inlet side. Carbon fiber manufacturing method. <Aspect 8> The atomic ratio (Si / C) of silicon atoms to carbon atoms on the fiber surface measured by X-ray photoelectron spectroscopy (XPS method) for the fiber immediately after being subjected to the first heat treatment is 1 / 5 or less, compared with the atomic ratio (Si / C) of silicon atoms to carbon atoms on the fiber surface measured by X-ray photoelectron spectroscopy (XPS method) for the fiber immediately after being subjected to the second heat treatment. The method of manufacturing according to embodiment 7. <Aspect 9> Aspect 9. The manufacturing method according to aspect 7 or 8, wherein the silicone-based oil agent adhered to the precursor fiber before the flame-retardant treatment is 0.01% by weight to 0.8% by weight, based on the precursor fiber before the silicone-based oil agent is adhered thereto. <Aspect 10> Aspect 10. The method according to any one of aspects 7 to 9, wherein the draw ratio of the fiber in the first heating furnace is less than 0.9 times the maximum draw ratio. <Aspect 11> The method according to any one of aspects 7 to 10, wherein the treatment temperature in the second heating furnace is greater than 1200°C. <Aspect 12> The ratio (T / P) of the treatment temperature T (°C) in the second heating furnace to the normalized scattering integrated intensity P measured by small angle X-ray scattering (SAXS) for the produced carbon fiber is 1200 or more. The method according to any one of aspects 7 to 11. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a carbon fiber having improved physical properties (particularly strength) and a method for producing a carbon fiber that can be used for general purposes. [Brief explanation of the drawings]

[0021] [Figure 1]FIG. 1 is a photograph of a cross section of the carbon fiber according to Example 1 observed with a transmission electron microscope (TEM). [Figure 2] FIG. 2 is a cross-sectional photograph of the carbon fiber according to Example 5 observed by TEM. [Figure 3] FIG. 3 is a cross-sectional photograph of the carbon fiber according to Example 8 observed by TEM. DETAILED DESCRIPTION OF THE INVENTION

[0022] <<Carbon fiber>> The carbon fiber according to the present invention is The atomic ratio of silicon atoms to carbon atoms (Si / C) on the fiber surface as measured by X-ray photoelectron spectroscopy (XPS) is 0.001 to 0.020.

[0023] Silicone-based oils are used in the carbon fiber manufacturing process to suppress adhesion between fibers (yarn adhesion). Silicone-based oils that adhere to the fiber surface can change during high-temperature treatment in the carbon fiber manufacturing process and remain on the surface of the carbon fiber as impurities. Such residues can reduce the physical properties of the carbon fiber.

[0024] In contrast, the carbon fiber according to the present invention has an atomic ratio of silicon atoms to carbon atoms (Si / C) on the fiber surface measured by X-ray photoelectron spectroscopy (XPS) of 0.001 to 0.020. Without intending to be limited by theory, it is believed that the carbon fiber according to the present invention has improved physical properties (tensile strength) due to the reduced impurity components derived from the silicone oil agent.

[0025] The method for obtaining carbon fibers with a reduced Si / C atomic ratio on the fiber surface is not particularly limited, but in particular, when producing carbon fibers from precursor fibers to which a silicone-based oil agent has been applied, the atmosphere pressure inside the outlet furnace of the C2 furnace is made higher than the atmosphere pressure inside the inlet furnace of the C2 furnace, and in this way, the atmosphere inside the furnace is circulated from the outlet side to the inlet side in the second heating furnace, thereby obtaining carbon fibers with a reduced Si / C atomic ratio on the fiber surface.

[0026] Although there is no intention to be limited by theory, it is believed that by forming an atmosphere in the furnace that flows counter to the direction of fiber travel, silicone-based oils that vaporize and change quality when heated can be efficiently removed from the fiber surface.

[0027] In particular, it is believed that gases resulting from silicone oils are generated in large amounts in a relatively low temperature range (e.g., 600 to 800°C). By exposing the fibers to the above countercurrent flow near the C2 furnace entrance, where the temperature is close to this range, it is believed that reattachment of gases to the fiber surface and the accumulation of silicon-containing impurities that accompany subsequent high-temperature treatment are effectively reduced.

[0028] The configuration according to the present disclosure will be described in more detail below.

[0029] <Carbon fiber> The carbon fiber is not particularly limited, and may be any carbon fiber such as pitch-based, rayon-based, or polyacrylonitrile (PAN)-based, but acrylonitrile-based carbon fiber is preferred in view of operability, processability, mechanical strength, etc. The properties of the carbon fiber, such as fineness and strength, are also not particularly limited, and any known carbon fiber can be used without limitation.

[0030] The form of the carbon fiber is not particularly limited, but may be a carbon fiber bundle composed of a plurality of single threads (filaments). From the viewpoint of productivity, the number of filaments constituting the carbon fiber bundle is preferably 1,000 to 80,000, more preferably 2,000 to 50,000. The diameter of a single carbon fiber thread may be 4 μm to 20 μm, and preferably 5 μm to 10 μm.

[0031] (Si / C ratio of carbon fiber) As described above, in the carbon fibers according to the present disclosure, the atomic ratio of silicon atoms to carbon atoms (Si / C) on the fiber surface as measured by X-ray photoelectron spectroscopy (XPS) is 0.001 to 0.020 (which can also be expressed as 0.1% to 2.0%).

[0032] The atomic ratio (Si / C) may be 0.002 or more, 0.003 or more, 0.004 or more, 0.005 or more, 0.006 or more, 0.007 or more, 0.008 or more, 0.009 or more, 0.010 or more, or 0.011 or more, and / or may be 0.019 or less, 0.018 or less, 0.017 or less, 0.016 or less, or 0.015 or less.

[0033] This atomic ratio (Si / C) is preferably 0.002 to 0.018, more preferably 0.004 to 0.016, and even more preferably 0.006 to 0.015. Within these preferred ranges, carbon fibers exhibiting particularly good strength and composite properties can be obtained. While it is generally considered preferable to reduce the Si / C value as much as possible, taking into account the amount of silicone-based oil required in the carbon fiber production process and the optimal heat treatment conditions for carbonization, it is considered that in some cases it may be preferable, from the perspective of carbon fiber strength and composite properties, to set the lower limit of Si / C at or above a certain level, rather than bringing the Si / C value as close to zero as possible.

[0034] The ratio of silicon atoms (Si) to carbon atoms (C) (Si / C) on the surface of carbon fibers can be determined by X-ray photoelectron spectroscopy (XPS) using an X-ray photoelectron spectrometer. X-ray photoelectron spectroscopy is also known as ESCA (Electron Spectroscopy for Chemical Analysis).

[0035] To measure the ratio (Si / C) of silicon atoms (Si) to carbon atoms (C) on the surface of carbon fibers, the fibers were cut and spread out on a stainless steel sample support. The photoelectron escape angle was set to 90 degrees, MgKα was used as the X-ray source, and the sample chamber was heated to 1×10 -6The chamber is maintained at a vacuum of 1000 Pa. To correct for peaks due to charging during measurement, the binding energy value BE of the main C1s peak is first adjusted to 284.6 eV. The Si2p peak area is determined by drawing a straight baseline in the range of 96 to 108 eV, and the C1s peak area is determined by drawing a straight baseline in the range of 281 to 297 eV. The abundance ratio of Si2p to the C1s peak on the carbon fiber surface is then calculated using the ratio of the Si2p peak area to the C1s peak area, which can be used as the abundance ratio (Si / C) of silicon atoms (Si) to carbon atoms (C) on the fiber surface of the carbon fiber. When carbon fibers are sized, the Si / C ratio can be measured before applying a sizing agent to the carbon fibers or after removing the sizing agent attached to the carbon fibers by immersing them in an appropriate solvent.

[0036] (Agglutination and gloss) The carbon fiber preferably has an agglutination number of 10 or less, and preferably has no gloss when observed on the surface of the carbon fiber.

[0037] The carbon fiber agglutination number is preferably 8 or less, 6 or less, 4 or less, 2 or less, or 1 or less, and most preferably zero.

[0038] The number of adhered fibers of carbon fiber can be determined by cutting 3000 filaments into 5 mm lengths, dispersing them in 10 mL of acetone, subjecting them to ultrasonic treatment, and then observing them under an optical microscope at 100x magnification to count the number of adhered (aggregated) threads.

[0039] (tensile modulus of carbon fiber) The carbon fibers preferably have a tensile modulus greater than 240 GPa and less than 330 GPa.

[0040] The tensile modulus of the carbon fibers may be 250 GPa or more, 260 GPa or more, or 270 GPa or more, and / or 320 GPa or less, 310 GPa or less, 300 GPa or less, 290 GPa or less, or 280 GPa or less.

[0041] The tensile modulus of the carbon fiber is more preferably 250 GPa to 300 GPa, and even more preferably 260 GPa to 290 GPa.

[0042] The tensile modulus of the carbon fiber can be measured by the method specified in JIS R 7608.

[0043] (Integrated scattering intensity of carbon fiber) The normalized scattering integrated intensity of the carbon fiber measured by small-angle X-ray scattering (SAXS) may be 1.00 to 1.40. This "normalized scattering integrated intensity" of the carbon fiber is expressed as a ratio to the measured value of a standard sample, a carbon fiber Tenax (registered trademark) filament (product name: HTA40, manufactured by Teijin Limited). For example, if the scattering integrated intensity value of the standard sample, a carbon fiber Tenax (registered trademark) filament (product name: HTA40), is A and the scattering integrated intensity value of the target carbon fiber is B, the normalized scattering integrated intensity of the target carbon fiber is expressed as B / A.

[0044] This normalized scattering integrated intensity of the carbon fibers may be 1.00 or greater, greater than 1.00, 1.05 or greater, or 1.10 or greater, and / or 1.30 or less, 1.25 or less, 1.20 or less, 1.18 or less, or 1.15 or less.

[0045] The normalized scattering integrated intensity of the carbon fiber is preferably 1.00 to 1.20, more preferably 1.05 to 1.18, and even more preferably 1.05 to 1.15.

[0046] The scattering integrated intensity (au) of the carbon fiber can be measured by small angle X-ray scattering measurement (SAXS) using a small angle X-ray scattering measurement device.

[0047] To measure the scattering integrated intensity (au) of carbon fibers, a bundle of samples aligned along the fiber axis is measured using a CuK X-ray source (λ = 0.1542 nm) and a detector positioned at a camera length of 748 nm. Data from three shifted areas are merged with an exposure time of 10 min to obtain a 2D SAXS image. A one-dimensional scattering profile versus the scattering spectrum q is determined from the 2D SAXS image, and the peak area obtained by Kratky plotting the profile can be used as the scattering integrated intensity. The scattering integrated intensity of a standard sample, a carbon fiber Tenax® filament (product name: HTA40, manufactured by Teijin Limited), and the scattering integrated intensity of the target carbon fiber are measured separately. The ratio (B / A) of the scattering integrated intensity value B of the target carbon fiber to the scattering integrated intensity value A of the standard sample can be calculated to obtain the "normalized scattering integrated intensity" of the target carbon fiber.

[0048] (M / P ratio of carbon fiber) The carbon fibers according to the present disclosure may have a ratio (M / P) of the tensile modulus M (GPa) to the normalized integrated scattering intensity P measured by small angle X-ray scattering (SAXS) of 200 or more.

[0049] The M / P ratio of the carbon fibers may be 210 or greater, 220 or greater, 230 or greater, 240 or greater, or greater than 240; and / or 280 or less, 270 or less, 260 or less, or 255 or less.

[0050] In one particularly preferred embodiment of the present invention, the carbon fiber has a ratio of tensile modulus M to normalized integrated scattering intensity P (M / P) of 230 or more, thereby realizing particularly improved physical properties (tensile strength and composite properties). The M / P ratio of the carbon fiber is particularly preferably 230 to 260, or even more preferably more than 240 and 255 or less.

[0051] Although not intending to be limited by theory, it is believed that the normalized scattering integrated intensity P reflects minute defects in the fiber. When the tensile modulus is relatively high while the number of defective portions in the carbon fiber is reduced, it is believed that the value of M / P becomes relatively high, and carbon fibers with particularly good physical properties can be obtained.

[0052] The method for obtaining such carbon fibers is not particularly limited, but for example, they can be produced by relatively increasing the carbonization temperature (treatment temperature in the C2 furnace) and suppressing the draw ratio in the C1 furnace. For details of the carbonization temperature (treatment temperature in the C2 furnace) and the draw ratio in the C1 furnace, please refer to the following description of the production method according to the present invention.

[0053] Although not intending to be limited by theory, in general, when the carbonization temperature is increased to improve the tensile strength of the carbon fiber, the tensile modulus increases and the number of defective sites in the carbon fiber also increases, resulting in an increase in the scattering integrated intensity.

[0054] In contrast to this, it is believed that carbon fibers having sufficient strength and reduced defects can be obtained by setting the carbonization temperature relatively high while suppressing the draw ratio in the C1 furnace.

[0055] Conventionally, measures have been proposed for the internal defects of carbon fibers, contamination by silicone oil, and fiber morphology, but no proposals have been made from the perspectives of both contamination by silicone oil and internal defects. In contrast, according to the particularly preferred embodiment of the present invention, contamination by silicone oil is reduced and internal defects in carbon fibers are reduced, making it possible to obtain carbon fibers with particularly excellent properties.

[0056] (Tensile strength of carbon fiber) The carbon fibers may have a tensile strength of 5000 MPa or greater.

[0057] The tensile strength of the carbon fibers may be 5100 MPa or more, 5200 MPa or more, 5300 MPa or more, 5400 MPa or more, or 5500 MPa or more, and / or 6500 MPa or less, 6400 MPa or less, 6300 MPa or less, 6200 MPa or less, 6100 MPa or less, or 6000 MPa or less.

[0058] Preferably, the carbon fiber has a tensile strength of more than 5100 MPa. The tensile strength of the carbon fiber is more preferably 5200 MPa to 6500 MPa, and even more preferably 5500 MPa to 6300 MPa.

[0059] There are no particular limitations on the method for obtaining carbon fibers having relatively high tensile strength. For example, carbon fibers can be obtained by producing them from precursor fibers produced by a dry-wet spinning method.

[0060] The tensile strength of the carbon fiber can be measured in accordance with JIS R 7608.

[0061] (Number of voids) When a cross section perpendicular to the fiber axis direction of the carbon fiber is measured using a transmission electron microscope (TEM), the number of voids within a depth of 200 nm from the surface of the carbon fiber is 50 / 40,000 nm 2 Below, 40 pieces / 40000nm 2 or less, or 30 pieces / 40000nm 2 It may be the following:

[0062] The number of voids is preferably 5 / 40000 nm 2 More preferably, 4 particles / 40,000 nm or less. 2 Below, 3 pieces / 40000nm 2 Below, 2 pieces / 40000nm 2 or less, or 1 piece / 40000nm 2 or less, and most preferably zero particles / 40,000 nm 2 In this case, carbon fibers having particularly improved physical properties (tensile strength and composite properties) can be obtained.

[0063] More specifically, the measurement and evaluation of the number of voids in carbon fiber by TEM can be carried out by the following procedure. That is, the fiber to be measured is dehydrated with ethanol, then substituted with a general embedding epoxy resin, and heat-cured at 120°C for 40 minutes to obtain a measurement sample. This sample is sliced ​​to a thickness of approximately 90 nm using an ultramicrotome, and a zero-loss image (cross-sectional view) is observed and photographed using a transmission electron microscope at an accelerating voltage of 120 kV. In the cross-sectional image perpendicular to the fiber axis direction of the carbon fiber thus obtained, a range of depth within 200 nm from the surface of the carbon fiber is observed, and a 40,000 nm 2 The number of voids present in this area is measured. Specifically, for example, a 200 nm square area can be set as close as possible to the fiber surface in the cross-sectional image, and the number of voids present in this area can be measured. Note that "depth from the surface" means the length in the direction from the surface of the fiber toward the center of the fiber in the fiber cross section perpendicular to the fiber axis direction of the carbon fiber. For the measurement, three or more fibers are arbitrarily selected, observed by TEM, and the average of the measured values ​​(for three or more fibers) is taken as the number of voids.

[0064] <<Carbon fiber manufacturing method>> The method for producing the carbon fiber according to the present disclosure is not particularly limited. The carbon fiber according to the present disclosure can be produced in particular by the following production method according to the present disclosure.

[0065] The method for producing carbon fibers according to the present disclosure includes: After the precursor fiber having the silicone-based oil agent attached thereto is flame-retarded, the precursor fiber is subjected to a first heat treatment in a first heating furnace (C1 furnace) at a treatment temperature of 300°C to 700°C in an inert atmosphere; and The fibers subjected to the first heat treatment are subjected to a second heat treatment in a second heating furnace (C2 furnace) at a treatment temperature of 1000°C or higher under an inert atmosphere. Including, In the second heating furnace, the atmosphere inside the furnace is circulated from the outlet side to the inlet side.

[0066] In the manufacturing method according to the present disclosure, the atmosphere inside the second heating furnace is circulated from the outlet side to the inlet side. While not intending to be limited by theory, in this case, a countercurrent atmosphere is formed in the furnace against the direction of fiber travel, which is thought to efficiently remove silicone-based oil that vaporizes and changes in quality due to heating from the fiber surface. The "inlet side" refers to the side where the fiber enters the furnace, and the "outlet side" refers to the side where the fiber leaves the furnace.

[0067] In particular, it is believed that gases resulting from silicone oil agents are generated in large amounts in a relatively low temperature range (e.g., 600 to 800°C). According to the production method of the present disclosure, the fibers are exposed to a countercurrent flow of the furnace atmosphere near the C2 furnace entrance, which is close to this temperature range, and this is thought to be particularly effective in reducing reattachment of gases to the fiber surface and the adhesion of silicon-containing impurities that occurs during subsequent high-temperature treatment.

[0068] <First heat treatment> In the method according to the present disclosure, precursor fiber having a silicone-based oil agent attached thereto is subjected to a flame retardant treatment, and then subjected to a first heat treatment in a first heating furnace (C1 furnace) at a treatment temperature of 300°C to 700°C in an inert atmosphere.

[0069] (Silicone oil) Examples of silicone compounds used in the oil agent include silicone oils such as amino-modified silicone and epoxy-modified silicone, but are not particularly limited.

[0070] The amount of silicone-based oil (OPU) attached to the precursor fiber before the flame retardant treatment is preferably 0.01% by weight to 0.8% by weight with respect to the precursor fiber before the silicone-based oil is attached.

[0071] This proportion of silicone oil may be 0.1% by weight or more, 0.15% by weight or more, or 0.2% by weight or more, and / or 0.7% by weight or less, 0.6% by weight or less, 0.5% by weight or less, or 0.4% by weight or less, based on the precursor fiber before the silicone oil is applied.

[0072] This proportion of silicone oil is more preferably 0.1 to 0.6% by weight, and even more preferably 0.2 to 0.4% by weight, based on the precursor fiber before the silicone oil is applied.

[0073] The method for applying the silicone oil to the precursor fiber is not particularly limited, and any known method can be used.

[0074] (precursor fiber) The carbon fiber precursor fiber (abbreviated as "precursor fiber") used in the method according to the present disclosure is not particularly limited, but preferably includes acrylic precursor fiber. The acrylic precursor fiber is preferably produced by spinning a spinning solution containing 90% by mass or more, preferably 95% to 99.9% by mass, of acrylonitrile, and 10% by mass or less of other monomers, which is obtained by homopolymerizing or copolymerizing monomers. Examples of other monomers include monomers copolymerizable with acrylonitrile, such as acids and salts thereof, such as acrylic acid and itaconic acid, esters such as methyl acrylate, ethyl acrylate, and methyl methacrylate, and amides such as acrylamide. These may be used alone or in combination depending on the desired fiber properties.

[0075] The method for producing acrylic precursor fibers is not particularly limited, and known methods can be used. Acrylic precursor fibers can be produced, for example, by preparing a spinning dope containing a polyacrylonitrile polymer, coagulating the spinning dope by dry spinning, wet spinning, or dry-wet spinning to obtain coagulated fibers, and then washing the coagulated fibers with water, stretching, oiling, drying, and steam stretching. Polyacrylonitrile polymers can be polymerized by, for example, solution polymerization or suspension polymerization. In steam stretching, the total stretch ratio can be set to 5 to 15 times. Furthermore, organic solvents, inorganic solvents, and inorganic salt solvents can be used as the solvent for spinning.

[0076] The spinning method is not particularly limited, but when precursor fibers produced by a dry / wet spinning method are used, carbon fibers having relatively high physical properties (particularly, relatively high tensile strength) may be obtained.

[0077] The precursor fiber may be in the form of a fiber bundle composed of a plurality of single threads (filaments). From the viewpoint of production efficiency, the number of filaments in the precursor fiber is preferably 1,000 or more, more preferably 2,000 or more. The upper limit of the number of filaments is not particularly limited, but may be, for example, 30,000 or less, 20,000 or less, 10,000 or less, or 5,000 or less. The fiber fineness of the precursor fiber may be 0.5 to 2.0 dtex.

[0078] (specific gravity of precursor fiber) The specific gravity of the precursor fiber is 1.10 to 1.25 (g / cm 3 ), and in particular 1.15 to 1.20 (g / cm 3 ) may be.

[0079] (Flame-resistant treatment) In the flame-retardant treatment, the precursor fiber having the silicone oil attached thereto is heated in an oxidizing atmosphere. The flame-retardant treatment can be carried out, for example, using a heating furnace, and particularly in air.

[0080] In the flame retardant treatment, for example, the precursor fiber is heated in heated air for 10 to 120 minutes, preferably 30 to 90 minutes. The heat treatment temperature may be 200° C. to 280° C., or 230° C. to 260° C. The flame retardant treatment can be carried out at a draw ratio in the range of 0.85 to 1.20, preferably 0.90 to 1.15.

[0081] The specific gravity of the flame-retardant treated precursor fiber is 1.30 to 1.45 (g / cm 3 ), and in particular 1.32 to 1.40 (g / cm 3 ) may be.

[0082] (First heat treatment) In the first heat treatment, the flame-resistant treated precursor fiber is heat-treated at 300° C. to 700° C. in an inert atmosphere in a first heating furnace (C1 furnace).

[0083] The heating temperature in the first heat treatment may be 350°C or higher, 400°C or higher, 450°C or higher, or 500°C or higher, and / or 650°C or lower, 600°C or lower, or 550°C or lower.

[0084] (inert atmosphere) The inert atmosphere may be an inert gas atmosphere, such as a nitrogen gas atmosphere.

[0085] (1st heating furnace) The first heating furnace is not particularly limited, and a known carbonization furnace (particularly the first carbonization furnace) can be used. The fibers are usually transported from the first heating furnace to the second heating furnace by an appropriate means such as a roller. Along the traveling direction of the fibers, the first heating furnace is arranged upstream and the second heating furnace is arranged downstream.

[0086] (Stretching ratio) Preferably, the draw ratio of the fiber in the first heating furnace is less than 0.9 times the maximum draw ratio.

[0087] In particular, the draw ratio of the fiber in the first heating furnace may be 0.8 times or less, less than 0.8 times, 0.7 times or less, or less than 0.7 times the maximum draw ratio, and / or 0.4 times or more, more than 0.4 times, 0.5 times or more, or more than 0.5 times the maximum draw ratio. The draw ratio of the fiber in the first heating furnace is more preferably 0.6 times to 0.8 times, and even more preferably 0.7 times to 0.8 times.

[0088] The maximum draw ratio D in the first heat treatment can be calculated from the following formula by introducing a fiber into the first heating furnace, fixing the entry speed A, gradually increasing the exit roller speed b, and setting the point at which the fiber becomes frayed or broken as the maximum draw speed B. Maximum stretching ratio D=B / A

[0089] By setting the draw ratio in the first heating furnace within the above range, it is possible to obtain carbon fibers with relatively few fiber defects (particularly voids). Furthermore, by setting the draw ratio in the first heating furnace within the above range and setting the treatment temperature in the second heating furnace relatively high (for example, above 1200°C, 1300°C or higher, 1350°C or higher, or 1400°C or higher), it is possible to improve the tensile strength of the carbon fibers while suppressing fiber defects (particularly voids).

[0090] <Second heat treatment> In the second heat treatment, the fibers subjected to the first heat treatment are subjected to the second heat treatment in a second heating furnace (C2 furnace) in an inert atmosphere at a treatment temperature of 1000°C or higher. For convenience, the fibers subjected to the first heat treatment are also referred to as "intermediate carbon fibers."

[0091] (Second heating furnace) The second heating furnace is not particularly limited, and a known carbonization furnace (particularly a second carbonization furnace) can be used. The second heating furnace is disposed downstream of the first heating furnace along the traveling direction of the fibers.

[0092] (inert atmosphere) The inert atmosphere may be an inert gas atmosphere, such as a nitrogen gas atmosphere.

[0093] (furnace atmosphere pressure) In order to circulate the atmosphere inside the second heating furnace (C2 furnace) from the outlet side to the inlet side, for example, the pressure inside the furnace at the outlet side of the C2 furnace can be made higher than the pressure inside the furnace at the inlet side of the C2 furnace. The "inlet side" is the side where the fiber enters the furnace, and the "outlet side" is the side where the fiber leaves the furnace.

[0094] Specifically, for example, the difference (P2 - P1) between the furnace pressure (inlet pressure) P1 measured at the inlet side of the C2 furnace and the furnace pressure (outlet pressure) P2 measured at the outlet side of the C2 furnace is preferably 0.01 Pa or more. This difference (P2 - P1) is more preferably 0.02 Pa or more, 0.03 Pa or more, 0.04 Pa or more, 0.05 Pa or more, or 0.1 Pa or more. The upper limit of this difference is not particularly limited, but may be 0.7 Pa or less.

[0095] In the C2 furnace, the "inlet furnace pressure" and the "outlet furnace pressure" can each be measured using a micro-pressure gauge.

[0096] (exhaust port) The C2 furnace may have an exhaust port for discharging gases from the furnace to the outside. The exhaust port is preferably installed on the inlet side of the C2 furnace. In this case, coupled with the flow of the atmosphere in the furnace from the outlet side to the inlet side formed according to the present method, the discharge of gas components derived from the silicone-based oil to the outside of the furnace is particularly well promoted.

[0097] It is particularly preferable to install the exhaust port near the entrance of the C2 furnace, i.e., at a location of the C2 furnace that is closer to the C1 furnace. It is believed that gases resulting from silicone oil are generated in large amounts in a relatively low temperature range (e.g., 600 to 800°C). When the exhaust port is installed near the entrance of the C2 furnace, which is close to this temperature range, the gas components derived from the silicone oil are discharged outside the furnace, which is believed to effectively reduce the accumulation of silicon-containing impurities that accompany subsequent high-temperature treatment.

[0098] (Processing temperature in C2 furnace) The heating temperature (treatment temperature) in the second heat treatment may be 1050°C or higher, 1100°C or higher, or 1200°C or higher, and / or 2000°C or lower, 1800°C or lower, 1600°C or lower, or 1400°C or lower.

[0099] Preferably, the treatment temperature in the second heating furnace is higher than 1200° C., and further higher than 1300° C. In general, by setting the heating temperature in the second heating treatment relatively high, carbon fibers having relatively high tensile strength can be obtained.

[0100] (T / P value) The ratio (T / P) of the treatment temperature T (°C) in the second heating furnace to the normalized integrated scattering intensity P measured by small angle X-ray scattering (SAXS) for the produced carbon fiber may be a value of 1000 to 1400. The treatment temperature T (°C) in the second heating furnace is particularly the maximum value (highest temperature) of the treatment temperature in the second heating furnace.

[0101] In particular, the T / P value may be 1100 or more, 1150 or more, or 1200 or more, and / or 1350 or less, 1300 or less, or 1290 or less.

[0102] This T / P value is preferably 1200 or more, more preferably 1200 to 1300, and even more preferably 1200 to 1290. In this case, carbon fibers having particularly improved physical properties (tensile strength and composite characteristics) can be obtained.

[0103] When the T / P value is within the above range, a carbon fiber having a relatively high tensile strength can be obtained. Note that, with respect to the produced carbon fiber, the normalized scattering integrated intensity P measured by small-angle X-ray scattering (SAXS) can be measured according to the method described above for the carbon fiber.

[0104] (Reduction of Si / C value) It is preferable that the Si / C value on the fiber surface is reduced by the second heat treatment to 1 / 5 or less (20% or less) of the value before the second heat treatment.

[0105] More specifically, in the production method according to the present disclosure, the atomic ratio (Si / C) of silicon atoms to carbon atoms on the fiber surface when the fiber (intermediate carbon fiber) immediately after being subjected to the first heat treatment is measured by X-ray photoelectron spectroscopy (XPS) relative to the atomic ratio (Si / C) of silicon atoms to carbon atoms on the fiber surface when the fiber (carbon fiber) immediately after being subjected to the second heat treatment is measured by X-ray photoelectron spectroscopy (XPS) (i.e., the atomic ratio (Si / C) immediately after being subjected to the second heat treatment / the atomic ratio (Si / C) immediately after being subjected to the first heat treatment) is preferably 1 / 5 or less, more preferably 1 / 6 or less, 1 / 7 or less, 1 / 8 or less, 1 / 9 or less, 1 / 10 or less, 1 / 11 or less, 1 / 12 or less, 1 / 13 or less, 1 / 14 or less, or even 1 / 15 or less.

[0106] Alternatively, the atomic ratio (Si / C) of silicon atoms to carbon atoms on the fiber surface when measured by X-ray photoelectron spectroscopy (XPS) for the fiber (carbon fiber) immediately after being subjected to the second heat treatment is preferably 20% or less, 18% or less, 16% or less, 14% or less, 12% or less, 10% or less, 8% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2% or less, or even 1% or less of the atomic ratio (Si / C) of silicon atoms to carbon atoms on the fiber surface when measured by X-ray photoelectron spectroscopy (XPS) for the fiber (intermediate carbon fiber) immediately after being subjected to the first heat treatment.

[0107] The atomic ratio (Si / C) of silicon atoms to carbon atoms on the fiber surface when measured by X-ray photoelectron spectroscopy (XPS) for the fiber (intermediate carbon fiber) immediately after being subjected to the first heat treatment may be in the range of 0.190 to 0.210, or may be 0.192 to 0.205, or 0.194 to 0.200.

[0108] (Additional carbonization process) When a higher elastic modulus is required, the fibers that have been subjected to the second carbonization step can be further subjected to graphitization at a high temperature of 2000° C. to 3000° C. Graphitization further promotes graphitization (high crystallization of carbon), resulting in carbon fibers with a higher elastic modulus.

[0109] (surface oxidation treatment) The carbon fibers obtained as described above are preferably subjected to a surface oxidation treatment in order to improve wettability with a sizing agent and / or a matrix resin. The surface oxidation treatment can be carried out by any conventionally known method, but electrolytic oxidation is generally used industrially because the equipment required is simple and the process is easy to control.

[0110] The amount of electricity in the surface oxidation treatment is preferably in the range of 10 to 150 coulombs per 1 g of carbon fiber. By adjusting the amount of electricity within this range, it is possible to obtain carbon fibers that have excellent mechanical properties as fibers and improved adhesiveness to resins.

[0111] Examples of the electrolyte include nitric acid, sulfuric acid, ammonium sulfate, sodium hydrogen carbonate, etc. The electrolyte concentration of the electrolyte is preferably 0.1N or more, more preferably 0.1 to 1N.

[0112] (Sizing process) The surface-oxidized carbon fibers can be subjected to a sizing treatment, if necessary. The sizing treatment can be carried out by a known method. In the sizing treatment, a known sizing agent can be appropriately used depending on the application. It is preferable to uniformly apply the sizing agent to the carbon fibers and then dry them.

[0113] <Applications of carbon fiber> The use of the carbon fiber according to the present invention is not particularly limited. For example, the carbon fiber according to the present invention is a carbon fiber for a pressure vessel. In particular, the carbon fiber according to the present invention can be used as a reinforcing fiber for a pressure vessel including a fiber-reinforced composite material. A pressure vessel formed using the carbon fiber according to the present disclosure has particularly good pressure resistance. A pressure vessel is a container designed to store gas or liquid at a specific pressure different from atmospheric pressure. In recent years, with the spread of fuel cell vehicles, development of pressure vessels with sufficient performance for in-vehicle use, hydrogen stations, and other purposes has been progressing.

[0114] <Pressure vessel> The present disclosure includes a pressure vessel containing the carbon fiber according to the present disclosure. This pressure vessel can be manufactured according to a known method, for example, by producing an intermediate body by winding a tow prepreg containing carbon fiber around a metal or resin inner frame (liner), and then subjecting this intermediate body to a heat curing treatment.

[0115] A pressure vessel containing the carbon fiber according to the present disclosure can have a tank breaking strength of 140 to 160 MPa when measured by the method described in the examples. [Example]

[0116] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0117] The measurements in the examples and comparative examples were carried out as follows.

[0118] <Maximum stretching ratio> The maximum draw ratio D in the first heating treatment (C1 step) was calculated by introducing the fiber into the first heating furnace (C1 furnace), fixing the entry speed A, gradually increasing the exit roller speed b, and setting the maximum draw speed B at the point where the fiber became frayed or broke, and then calculating the maximum draw ratio D using the following formula. Each drawing condition was also calculated by multiplying the obtained maximum draw ratio by 0.Y (0.9, 0.8, or 0.7). Maximum stretching ratio D=B / A Each elongation ratio = (B / A) × 0.Y

[0119] The maximum elongation ratio of the second heat treatment (C2 process) was also determined in the same manner according to the above.

[0120] <Specific gravity (density)> The specific gravities of the precursor fiber, the flame-resistant fiber, and the fiber after C1 treatment were measured and determined based on the liquid displacement method among the methods specified in JIS R 7603, using acetone as the liquid for each fiber specific gravity.

[0121] <Tensile strength of carbon fiber> The tensile strength of carbon fiber (CF strength) was measured by the method specified in JIS R 7608.

[0122] <Tensile modulus of carbon fiber> The tensile modulus of carbon fiber was measured by the method specified in JIS R 7608.

[0123] <Number of adhesions> The measurement of the number of adhesions of carbon fiber was carried out as follows: Carbon fiber cut to 5 mm in length with 3000 filaments was dispersed in 10 ml of acetone, and after ultrasonic treatment, the number of adhered yarns was measured with a 100-fold optical microscope.

[0124] <Pressure state inside the C2 furnace> The pressure state inside the furnace was investigated by measuring the values of the micro-pressure gauges on the inlet side and the outlet side. Usually, in order to prevent external air from entering the carbonization furnace and to maintain an inert atmosphere inside the furnace body, the fiber introduction part into the furnace often has a labyrinth structure. The value of the micro-pressure gauge on the inlet side indicates the pressure difference between the inside of this inlet labyrinth and the atmospheric pressure outside the furnace. Similarly, the value of the micro-pressure gauge on the outlet side also shows the pressure difference between the inside of the outlet labyrinth and the atmospheric pressure outside the furnace. Using the values obtained from this micro-pressure gauge, that is, the inlet pressure and the outlet pressure, when the outlet is higher than the inlet, for example, when the inlet is 1.0 Pa and the outlet is 1.1 Pa, the inlet < outlet was set.

[0125] <Gloss evaluation of the fiber on the C2 furnace outlet side> The gloss evaluation of carbon fibers on the outlet side of the C2 furnace was carried out by applying a 10 cm × 3 cm black drawing paper to the fibers during running from the back side, irradiating light, and visually judging the presence or absence of gloss.

[0126] <Measurement of void number by TEM (transmission electron microscope observation)> The measurement of the void number by TEM was carried out according to the following procedure. After dehydrating the measured fibers with ethanol, they were replaced with a general embedding epoxy resin and heat-cured at 120 °C for 40 minutes to obtain a measurement sample. This sample was sliced thinly to a thickness of about 90 nm using a Leica ultramicrotome UC-6, and a 0 loss image (cross-sectional view) was observed and imaged at an acceleration voltage of 120 KV using a transmission electron microscope TECNAI G2 manufactured by FEI. Using the cross-sectional image perpendicular to the fiber axis direction of the carbon fiber thus obtained, the number of voids present in an area of 200 nm square as close to the surface of the carbon fiber as possible was counted, and the void number (per 40000 nm 2 ) was determined. For the measurement, 3 fibers were arbitrarily selected for TEM observation, and the average (3 fibers) of the measured values was taken as the void number.

[0127] <Si / C evaluation by XPS> The abundance ratio (Si / C) of silicon atoms (Si) and carbon atoms (C) on the fiber surface of carbon fibers was determined by the XPS method (ESCA) according to the following procedure. For the measurement, an X-ray photoelectron spectrometer ESCA JPS-9000MX manufactured by JEOL Ltd. was used. After cutting the fibers and spreading and arranging them on a stainless steel sample support table, the photoelectron escape angle was set to 90 degrees, MgKα was used as the X-ray source, and the inside of the sample chamber was maintained at a vacuum degree of 1 × 10 -6 [Pa]. As the correction of the peak associated with charging during measurement, first, the binding energy value B.E. of the main peak of C1s was adjusted to 284.6 [eV]. The Si2p peak area was obtained by drawing a linear baseline in the range of 96 - 108 [eV], and the C1s peak area was obtained by drawing a linear baseline in the range of 281 - 297 [eV]. The abundance ratio (Si / C) of Si2p to the C1s peak on the carbon fiber surface was calculated and obtained as the ratio of the above Si2p peak area and C1s peak area.

[0128] In the examples, the abundance ratio (Si / C) of silicon atoms (Si) to carbon atoms (C) on the fiber surface of the carbon fiber was measured for the carbon fiber after the first heat treatment and the carbon fiber after the second heat treatment, respectively.

[0129] <Measurement of Scattering Integrated Intensity by Small-Angle X-ray Scattering (SAXS)> The scattering integrated intensity was obtained by the following procedure. A bundle sample aligned in the fiber axis direction was measured by SAXS using a NANO-Viewer manufactured by Rigaku Corporation. A detector PILATUS-100K was placed with an X-ray source CuK (λ = 0.1542 nm) and a camera length of 748 nm, and data obtained by shifting three areas with an exposure time of 10 min were merged to obtain a two-dimensional SAXS image. A one-dimensional scattering profile with respect to the scattering spectrum q was obtained from the two-dimensional SAXS image, and the peak area when this was plotted as a Kratky plot was defined as the scattering integrated intensity. The value B of the scattering integrated intensity of the target carbon fiber was measured, and the "normalized scattering integrated intensity" of the target carbon fiber was obtained by calculating the ratio (B / A) to the value A of the scattering integrated intensity measured in the same manner for a carbon fiber Tenax (registered trademark) filament (product name: HTA40, manufactured by Teijin Limited), which is a standard sample.

[0130] <Tank Rupture Strength (Pressure)> Filament winding was carried out at a back tension of 20 N / bundle while impregnating a carbon fiber bundle with a resin having the following resin composition into an aluminum liner in the shape of a cylinder with a length of 450 mm, an outer diameter of 126 mm, and a wall thickness of 2 mm. After winding one layer of the hoop layer, three layers of the helical layer were laminated. Further, one layer of the hoop layer, five layers of the helical layer, and then one layer of the hoop layer were wound, and the resulting structure was cured in a curing furnace at 120°C for 5 hours to obtain a pressure vessel for evaluating the tank rupture strength.

[0131] Resin Composition: · 100 parts by mass of bisphenol A type epoxy resin · 80 parts by mass of 4-methylcyclohexane-1,2-dicarboxylic anhydride

[0132] The bisphenol A type epoxy resin used was EPON828 (trade name) manufactured by Yuka Shell Epoxy Co., Ltd. The 4-methylcyclohexane-1,2-dicarboxylic anhydride used was Lindride52 (trade name) manufactured by LINDAU CHEMICAL INC.

[0133] This pressure vessel for tank burst strength evaluation was subjected to a water pressure of 5 MPa / min using a burst test device, and then held at 10 MPa for 1 minute, and then the pressure was increased by 5 MPa / min again, and the burst strength (pressure) was measured by repeating this process. This measurement was carried out twice for each example and comparative example, and the average value was calculated for each.

[0134] <<Examples 1 to 5, Comparative Examples 1 and 2>> In Examples 1 to 5 and Comparative Examples 1 and 2, carbon fibers were produced under various conditions shown in Tables 1-1 and 1-2, and the physical properties of the produced carbon fibers and the strength of pressure tanks produced from the carbon fibers were evaluated.

[0135] Example 1 Acrylic precursor fibers were used as precursor fibers. According to a conventional method, spinning was performed by a dry-wet spinning method using an organic solvent as a spinning solvent, followed by water washing, drying, and stretching to produce precursor fibers listed in Table 1-1 below. After applying a silicone oil to the precursor fibers, they were flame-retarded at 250°C, subjected to a first heat treatment in a C1 furnace at a temperature range of 300 to 600°C, and then to a second heat treatment in a C2 furnace at a maximum temperature of 1400°C to produce carbon fibers. Subsequently, surface treatment was performed by a conventional method, and then a sizing agent was applied to obtain carbon fibers according to Example 1. Note that 0.3 wt% of a silicone oil was adhered to the precursor fibers.

[0136] In Example 1, the draw ratio in the C1 furnace was set to 0.8 times the maximum draw ratio. The pressure state of the atmosphere in the C2 furnace was adjusted so that the pressure on the outlet side of the C2 furnace was higher than the pressure on the inlet side of the C2 furnace (inlet side < outlet side). The pressure difference between the inlet side pressure (Pa) and the outlet side pressure (Pa) of the C2 furnace was 0.3 Pa.

[0137] A pressure vessel (tank) was produced by the filament winding method as described above using the obtained carbon fiber according to Example 1, and the tank breaking strength (pressure) was measured.

[0138] The evaluation results for Example 1 are shown in Tables 1-1 and 1-2 below. Note that "OPU" in Table 1-1 is the ratio (by weight) of the silicone-based oil adhering to the precursor fiber before the flame retardant treatment to the precursor fiber before the silicone-based oil was attached (the same applies to Tables 2-1 and 3-1). The "draw ratio" in Table 1-2 is shown as a ratio relative to the maximum draw ratio, and the "carbonization temperature" in the table is the maximum temperature in the second heat treatment (the same applies to Tables 2-2 and 3-2).

[0139] The results of TEM observation of the cross section of the carbon fiber according to Example 1 are shown in Figure 1. As can be seen from Figure 1, no voids were observed in the carbon fiber according to Example 1.

[0140] <Examples 2 to 5> In Examples 2 to 5, carbon fibers were produced in the same manner as in Example 1, except that the draw ratio in the first heat treatment and / or the maximum temperature in the second heat treatment were changed as shown in Table 1-2 below, and the physical properties of the carbon fibers and the strength of pressure vessels produced from the carbon fibers were evaluated. The production conditions and evaluation results are shown in Tables 1-1 and 1-2 below.

[0141] <Comparative Examples 1 and 2> In Comparative Examples 1 and 2, carbon fibers were produced in the same manner as in Example 1, except that the draw ratio in the C1 furnace was set to 0.8 or 0.7 times the maximum draw ratio, and the pressure state of the atmosphere in the C2 furnace was adjusted so that the pressure on the outlet side of the C2 furnace was the same as or lower than the pressure on the inlet side of the C2 furnace (inlet side ≧ outlet side). The physical properties of the carbon fibers and the strength of pressure tanks produced from the carbon fibers were evaluated. The production conditions and evaluation results are shown in Tables 1-1 and 1-2 below.

[0142] [Table 1]

[0143] [Table 2]

[0144] As can be seen from Table 1-2, the carbon fibers of Examples 1 to 5 have higher tensile strength than those of Comparative Examples 1 and 2, and pressure vessels formed from composites produced using these carbon fibers exhibited relatively high tank breaking strength.

[0145] The carbon fibers of Examples 1 to 5 had lower Si / C ratios measured by XPS, ranging from 0.009 to 0.014, compared to the carbon fibers of Comparative Examples 1 and 2. While not intending to be limited by theory, it is believed that the amount of impurities accumulated on the carbon fiber surface (particularly impurities resulting from silicone-based oils applied in advance to the fiber surface during the carbon fiber manufacturing process) was reduced in Examples 1 to 5, resulting in relatively high tensile strength. The agglutination and gloss shown in Table 1 are both believed to reflect the adhesion of impurities to the carbon fiber surface. Examples 1 to 5 all had relatively low agglutination values ​​and no gloss was observed.

[0146] Furthermore, without intending to be limited by theory, in Examples 1 to 5, the pressure state inside the C2 furnace was "inlet side < outlet side," and therefore the flow of the atmosphere inside the furnace was opposite (counterflow) to the direction in which the fibers were traveling inside the furnace. This is thought to have effectively removed the silicone-based oil, which was vaporized and altered by heat treatment, from the fiber surface, resulting in a reduction in impurities on the fiber surface.

[0147] In Example 5, the Si / C ratio measured by XPS was relatively low at 0.012, while the ratio M / P (the tensile modulus M to the normalized integrated scattering intensity P) was relatively low at 224.5. Furthermore, the number of voids observed by TEM was relatively high. While not intending to be limited by theory, it is believed that in Example 5, the draw ratio in the C1 furnace was higher than in Examples 1 to 4, at 0.9 times the maximum draw ratio, resulting in a large number of defects (particularly voids) in the carbon fiber. As a result, the tensile strength and composite properties were relatively lower than those of Examples 1 to 4. The results of TEM observation of the cross section of the carbon fiber according to Example 5 are shown in Figure 2. As can be seen in Figure 2, voids were observed in the carbon fiber according to Example 5 (areas surrounded by dotted lines in the figure).

[0148] <<Examples 6 to 7, Comparative Example 3>> In Examples 6 to 7 and Comparative Example 3, the flame-proofing conditions were changed and the same evaluations as above were carried out.

[0149] Example 6 In Example 6, except that the flame-proofing temperature was relatively low (240° C.), carbon fibers were produced and evaluated in the same manner as in Example 1. The production conditions and evaluation results are shown in Tables 2-1 and 2-2 below.

[0150] In Example 7, carbon fibers were produced and evaluated in the same manner as in Example 6, except that the draw ratio in the C1 furnace was set to 0.9 times the maximum draw ratio. The production conditions and evaluation results are shown in Tables 2-1 and 2-2 below.

[0151] In Comparative Example 3, carbon fibers were produced and evaluated in the same manner as in Example 6, except that the pressure state of the atmosphere in the C2 furnace was set to be "entrance side ≧ exit side." The production conditions and evaluation results are shown in Tables 2-1 and 2-2 below.

[0152] [Table 3]

[0153] [Table 4]

[0154] The results of Examples 6-7 and Comparative Example 3 were similar to those observed in Examples 1-5 and Comparative Examples 1-2 above.

[0155] That is, as can be seen from Table 2-2, the carbon fibers of Examples 6 and 7, which had a Si / C ratio of 0.013 measured by XPS, had higher tensile strength than Comparative Example 3, which had a Si / C ratio of 0.070, and pressure vessels formed from composites produced using these carbon fibers exhibited relatively high tank breaking strength.

[0156] The carbon fiber of Example 7, which had a relatively high draw ratio in the C1 furnace, had a Si / C ratio of 0.013 measured by XPS, but a relatively low M / P ratio (tensile modulus M / normalized scattering integrated intensity P) of 229.2, and the number of voids observed by TEM was relatively large, resulting in lower tensile strength and composite properties than Example 6. Without intending to be limited by theory, it is believed that the relatively high draw ratio in the first heat treatment in Example 7 resulted in more defective portions (particularly voids) in the carbon fiber, resulting in relatively low tensile strength and composite properties.

[0157] <<Examples 8 to 9, Comparative Example 4>> In Examples 8 to 9 and Comparative Example 4, precursor fibers produced by wet spinning were used.

[0158] Example 8 In Example 8, the precursor fibers shown in Table 3-1 below, which were produced by a wet spinning method, were used, and carbon fibers were produced and evaluated in the same manner as in Example 1, except that the maximum temperature in the second heat treatment was set to 1300°C.

[0159] Example 9 In Example 9, carbon fibers were produced and evaluated in the same manner as in Example 8, except that the draw ratio in the C1 furnace was set to a relatively high maximum draw ratio of 0.9 times.

[0160] <Comparative Example 4> In Comparative Example 4, carbon fiber was produced and evaluated in the same manner as in Example 8, except that the pressure on the outlet side of the C2 furnace was the same as or lower than the pressure on the inlet side of the C2 furnace (inlet side ≧ outlet side).

[0161] The production conditions and evaluation results of Examples 8 to 9 and Comparative Example 4 are shown in Tables 3-1 and 3-2 below. The results of TEM observation of the cross section of the carbon fiber according to Example 8 are shown in Figure 3. As can be seen in Figure 3, voids were observed in the carbon fiber according to Example 8 (areas surrounded by dotted lines in the figure), but the number of voids was relatively small (see Table 3-2).

[0162] [Table 5]

[0163] [Table 6]

[0164] The results of Examples 8-9 and Comparative Example 4 were similar to those observed in Examples 1-7 and Comparative Examples 1-3 above.

[0165] That is, as can be seen from Table 3-2, the carbon fibers of Examples 8 and 9, which had a Si / C ratio of 0.019 to 0.020 measured by XPS, had higher tensile strength than Comparative Example 4, which had a Si / C ratio of 0.090, and pressure vessels formed from composites produced using these carbon fibers exhibited relatively high tank breaking strength.

[0166] The carbon fiber of Example 9, which had a relatively high draw ratio in the C1 furnace, had a relatively low M / P ratio (tensile modulus M / normalized scattering integrated intensity P) of 209.6, and also had a relatively large number of voids observed by TEM, exhibiting physical properties inferior to those of Example 8. Without intending to be limited by theory, it is believed that the relatively high draw ratio in the first heat treatment in Example 9 resulted in many defective sites (particularly voids) in the carbon fiber, resulting in relatively low tensile strength and composite properties.

Claims

1. A carbon fiber having an atomic ratio (Si / C) of silicon atoms to carbon atoms on the fiber surface of 0.001 to 0.020 as measured by X-ray photoelectron spectroscopy (XPS).

2. 2. The carbon fiber of claim 1, having a tensile modulus greater than 240 GPa and less than 330 GPa.

3. 3. The carbon fiber according to claim 1, wherein the ratio (M / P) of the tensile modulus M (GPa) to the normalized scattering integrated intensity P measured by small-angle X-ray scattering (SAXS) is 230 or more.

4. 3. The carbon fiber of claim 1 or 2, having a tensile strength of greater than 5100 MPa.

5. When a cross section perpendicular to the fiber axis direction of the carbon fiber is measured using a transmission electron microscope (TEM), the number of voids within a depth of 200 nm from the surface of the carbon fiber is 5 / 40,000 nm ２ The carbon fiber according to claim 1 or 2, wherein:

6. The carbon fiber according to claim 1 or 2 for a pressure vessel.

7. After subjecting the precursor fiber having the silicone-based oil agent attached thereto to a flame retardant treatment, the precursor fiber is subjected to a first heat treatment in a first heating furnace under an inert atmosphere at a treatment temperature of 300°C to 700°C; and The fibers subjected to the first heat treatment are subjected to a second heat treatment in a second heating furnace under an inert atmosphere at a treatment temperature of 1000°C or more. A method for producing carbon fibers, comprising: In the second heating furnace, the atmosphere inside the furnace is circulated from the outlet side to the inlet side. Carbon fiber manufacturing method.

8. the atomic ratio (Si / C) of silicon atoms to carbon atoms on the fiber surface measured by X-ray photoelectron spectroscopy (XPS) for the fiber immediately after being subjected to the first heat treatment is 1 / 5 or less, compared with the atomic ratio (Si / C) of silicon atoms to carbon atoms on the fiber surface measured by X-ray photoelectron spectroscopy (XPS) for the fiber immediately after being subjected to the second heat treatment; The method of claim 7.

9. 9. The manufacturing method according to claim 7, wherein the silicone-based oil agent adhered to the precursor fiber before the flame retardant treatment is 0.01% by weight to 0.8% by weight with respect to the precursor fiber before the silicone-based oil agent is adhered.

10. The method according to claim 7 or 8, wherein the draw ratio of the fiber in the first heating furnace is less than 0.9 times the maximum draw ratio.

11. The method according to claim 7 or 8, wherein the treatment temperature in the second heating furnace is greater than 1200°C.

12. The ratio (T / P) of the treatment temperature T (°C) in the second heating furnace to the normalized scattering integrated intensity P measured by small angle X-ray scattering measurement (SAXS) for the produced carbon fiber is 1200 or more. The method according to claim 7 or 8.

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