Carbon fiber, and method for manufacturing carbon fiber

Abstract

To provide carbon fibers with excellent strength properties. [Solution] A carbon fiber made of polyacrylonitrile, wherein the thickness of the oxide layer on the surface of the carbon fiber is 10 nm or less, and the oxygen concentration on the surface of the carbon fiber is 7 atom% or more and 20 atom% or less.

Description

[Technical Field]

[0001] This invention relates to carbon fibers and methods for producing carbon fibers. [Background technology]

[0002] Carbon fiber reinforced polymer (CFRP), which is made by reinforcing resin with carbon fibers, is gaining popularity in aircraft, sporting goods, and wind turbine blades due to its superior mechanical strength and significant weight reduction benefits. However, carbon fiber (CF) requires heating at temperatures exceeding 1000°C during its manufacturing process, resulting in large emissions of carbon dioxide (CO2) and high material costs. Due to these economic and environmental considerations, the adoption of carbon fiber reinforced polymer in the automotive industry has not progressed significantly.

[0003] On the other hand, in fuel cell vehicles (FCEVs) powered by polymer electrolyte membrane fuel cells, 70 MPa class high-pressure hydrogen tanks made of carbon fiber reinforced resin are used to maximize the amount of hydrogen that can be stored as fuel. It is expected that the amount of carbon fiber reinforced resin used will increase as fuel cell vehicles become more widespread. Approximately 20% of the carbon dioxide emitted during the manufacturing process of fuel cell vehicles comes from the carbon fibers in the hydrogen tanks. Therefore, in order for fuel cell vehicles to contribute to carbon neutrality (CN), it is necessary to reduce the amount of carbon dioxide emitted during the manufacturing of carbon fibers and to improve the strength characteristics of the carbon fibers. This will make it possible to reduce the amount of carbon fiber reinforced resin used in high-pressure hydrogen tanks, and improvements in fuel efficiency due to vehicle weight reduction can also be expected.

[0004] Furthermore, high-pressure hydrogen tanks have legally defined refillable expiration dates, making reuse difficult at present. They must be recycled after a specified period. If carbon fibers can be recovered from expired hydrogen tanks and reused as materials for new hydrogen tanks or vehicle weight reduction, it would significantly reduce carbon dioxide emissions during the manufacture of fuel cell vehicles, leading to the achievement of carbon neutrality.

[0005] For example, Patent Document 1 describes a resin dissolving apparatus for removing resin material from carbon fiber reinforced resin and recovering carbon fibers, which dissolves the resin components by immersing the carbon fiber reinforced resin in sulfuric acid. Patent Document 1 shows that by immersing the carbon fiber reinforced resin in a sulfuric acid solution with a concentration of 90 wt% or more at 120°C or higher for one hour or more, the resin material dissolves, no resin residue is observed on the carbon fibers, and the interfacial strength of the carbon fibers is increased compared to the new material.

[0006] However, Patent Document 1 does not specifically describe the chemical structure or tensile strength of the recovered carbon fibers.

[0007] Patent Document 2 describes a method for treating composite materials, in which a composite material such as carbon fiber reinforced resin, consisting of a resin and carbon fibers as a reinforcing material, is immersed in a treatment solution containing oxidizing active species obtained by electrolyzing a sulfuric acid solution, thereby decomposing the resin and recovering the reinforcing material. Patent Document 2 shows that by immersing the composite material in the treatment solution at a temperature of 130°C or higher for 30 minutes or more, the resin constituting the composite material is completely decomposed, and the composite material can be efficiently recovered.

[0008] However, the method described in Patent Document 2 requires a processing temperature of 130°C or higher and a processing time of 30 minutes or more, making it unsuitable for large-scale processing. Furthermore, the use of a diamond-coated electrode as the anode electrode for the electrolysis of sulfuric acid solution is essential from a durability standpoint, resulting in high equipment investment costs.

[0009] Patent Document 3 describes how carbon fibers rich in acidic functional groups on their surface can be obtained by thermal decomposition and anodic electrolysis of carbon fiber reinforced resin, and how using these carbon fibers can yield a carbon fiber reinforced resin composition that is inexpensive and has excellent surface appearance. Patent Document 3 specifies that the thermal decomposition treatment temperature should be 300 to 700°C, the treatment time 10 minutes to 10 hours, and the heat-treated material should be anodized to decompose more than 85% of the resin components, and the total amount of acidic functional groups on the carbon fiber surface, as measured by the Boehm method, should be 0.3 mmol / g or more.

[0010] However, the method described in Patent Document 3 requires a temperature of 300-700°C and a time of 10 minutes to 10 hours for thermal decomposition, and further energy consumption is high due to the subsequent anodic oxidation. In addition, while acidic functional groups are thought to contribute to improving the dispersibility of the resin, Patent Document 3 does not mention any effect on improving the strength of the carbon fibers themselves.

[0011] Non-patent document 1 describes a method for recovering carbon fibers by treating carbon fiber reinforced resin in supercritical and subcritical water at a temperature of 523K to 673K, a pressure of 4.0 to 27.0 MPa, and a reaction time of 1 to 30 minutes. Non-patent document 1 states that the tensile strength of the recovered carbon fibers was 90% to 98% of that of virgin fibers.

[0012] However, the method described in Non-Patent Document 1 has the problem of carbon fiber degradation due to heat treatment and high equipment investment costs, such as pressure tanks.

[0013] Non-patent document 2 describes how growing and coating a conductive polymer consisting of orthophenylenediamine on the surface of polyacrylonitrile-based carbon fibers improves the tensile strength of the carbon fibers from 3.16 GPa to 4.58 GPa.

[0014] However, the method described in Non-Patent Document 2 has the problem of requiring capital investment in equipment for growing the conductive polymer.

[0015] Non-patent document 3 describes a study on how air oxidation and sizing agent treatments affect the interlaminar shear strength of carbon fiber reinforced resin. Non-patent document 3 shows that surface treatment by air oxidation improves the interlaminar shear strength of carbon fiber reinforced resin by 30%, the optimal oxidation temperature for air oxidation is 150°C to 250°C, and the oxidation time is 10 to 15 minutes. It also shows that while an increase in the oxygen concentration on the carbon fiber surface detected by XPS increases the interlaminar shear strength, an increase beyond a certain value of oxygen concentration does not affect the interlaminar shear strength.

[0016] However, while Non-Patent Document 3 shows that oxidation treatment increases interlaminar interface strength, it does not mention any change in the strength of the carbon fibers themselves.

[0017] Thus, conventional technologies have proposed methods for improving the strength of carbon fibers and recovering carbon fibers from carbon fiber reinforced resins, but these methods have problems such as cost and time. [Prior art documents] [Patent Documents]

[0018] [Patent Document 1] Japanese Patent Publication No. 2020-037638 [Patent Document 2] Japanese Patent Publication No. 2017-171830 [Patent Document 3] Patent No. 6205510 [Non-patent literature]

[0019] [Non-Patent Document 1] R. Pinero-Hernanz et al., Chemical recycling of carbon fibre reinforced composites in nearcritical and supercritical water, Composites Part A: Applied Science and Manufacturing, 39, 451-461 (2008). [Non-Patent Document 2] D. Bhagya et al., Carbon reinforced carbon fibers: Using surface modification as a route to enhanced physical performance, Composites Science and Technology, 218, 109217 (2022) [Non-Patent Document 3] Toshio Ogawa et al., Effect of surface treatment on interlaminar shear strength of CFRP, Materials, Vol. 39, pp. 1438-1442 (1990) [Summary of the Invention] [Problems to be Solved by the Invention] [[ID=D14]]

[0020] An object of the present invention is to provide carbon fibers having good strength characteristics. Another object of the present invention is to provide a method for producing carbon fibers capable of obtaining carbon fibers having good strength characteristics at low cost and in a short time. [Means for Solving the Problems]

[0021] The present invention relates to polyacrylonitrile-based carbon fibers, wherein the thickness of the oxide layer on the surface of the carbon fibers is 10 nm or less, and the oxygen concentration on the surface of the carbon fibers is 7 atom% or more and 20 atom% or less.

[0022] Preferably, the fiber breaking strength of the carbon fibers exceeds 5.2 GPa.

[0023] The present invention relates to a method for producing carbon fibers, comprising an immersion step of immersing a carbon fiber composite material having carbon fibers or resin as a matrix in hot concentrated sulfuric acid heated to over 80°C and 200°C or less for 10 seconds to 20 minutes.

[0024] In the immersion step of the carbon fiber manufacturing method, it is preferable to immerse the carbon fiber or the carbon fiber composite material in hot concentrated sulfuric acid heated to 100°C or higher and 200°C or lower.

[0025] In the carbon fiber manufacturing method described above, it is preferable that the fiber breaking strength of the carbon fiber is improved by 10% or more compared to before the immersion step. [Effects of the Invention]

[0026] The present invention makes it possible to provide carbon fibers having good strength properties. Furthermore, the present invention makes it possible to provide a method for producing carbon fibers that can be obtained at low cost and in a short time. [Brief explanation of the drawing]

[0027] [Figure 1] These graphs show the Weibull analysis results for the carbon fibers (carbon fiber 1) obtained in Examples 1-3 and Comparative Examples 1-2. Figure 1(a) shows the relationship between lnln(1 / (1-F(σ))) and fiber tensile strength (GPa), and Figure 1(b) shows the relationship between fiber tensile strength (GPa) and fiber tensile probability density F(σ). [Figure 2] The graphs show the Weibull analysis results for carbon fibers (carbon fiber 2) obtained in Examples 4-6 and Comparative Examples 3-5. Figure 2(a) shows the relationship between lnln(1 / (1-F(σ))) and fiber tensile strength (GPa), and Figure 2(b) shows the relationship between fiber tensile strength (GPa) and fiber tensile probability density F(σ). [Figure 3]These graphs show the TOF-SIMS depth analysis results for carbon fibers obtained in the examples and comparative examples. Figure 3(a) shows the relationship between O-ion strength (arbitrary units) and sputtering time (seconds) for carbon fiber 1, and Figure 3(b) shows the relationship between O-ion strength (arbitrary units) and sputtering time (seconds) for carbon fiber 2. [Figure 4] This graph shows the relationship between the fiber breaking strength (Weibull coefficient η (GPa)) and the thickness of the oxide layer (nm, SiO2 equivalent) of the carbon fibers obtained in the examples and comparative examples. [Figure 5] This graph shows the relationship between the sulfuric acid treatment temperature (°C) and sulfuric acid immersion time (minutes) of the carbon fibers obtained in the examples and comparative examples. [Modes for carrying out the invention]

[0028] Embodiments of the present invention will be described below. This embodiment is just one example of how the present invention can be implemented, and the present invention is not limited to this embodiment.

[0029] <Carbon fiber> The carbon fiber according to this embodiment is a polyacrylonitrile-based carbon fiber, wherein the thickness of the oxide layer on the surface of the carbon fiber is 10 nm or less, and the oxygen concentration on the surface of the carbon fiber is 7 atom% or more and 20 atom% or less.

[0030] The inventors have found that carbon fibers with good strength properties can be obtained by setting the thickness of the oxide layer on the surface of the carbon fibers to 10 nm or less, and the oxygen concentration on the surface of the carbon fibers to 7 atom% or more and 20 atom% or less.

[0031] The thickness of the oxide layer on the surface of the carbon fiber is 10 nm or less, preferably between 1 nm and 5 nm. If the thickness of the oxide layer on the surface of the carbon fiber exceeds 10 nm, the tensile strength of the carbon fiber will be insufficient, and if it is less than 1 nm, the interfacial strength between the carbon fiber and the resin may be insufficient. The oxide layer on the surface of the carbon fiber is structurally weak, and it is thought that the tensile strength of the carbon fiber is improved by keeping the thickness of this weak oxide layer to 10 nm or less.

[0032] Here, the thickness of the oxide layer on the surface of the carbon fiber was determined by thickness directional analysis using a sputter gun with time-of-flight secondary ion mass spectrometry (TOF-SIMS), where the ion species was Cs. + Ion energy (acceleration voltage) 0.25 keV, ion beam current 10 nA, incident angle 45°, sputtering area 0.3 × 0.3 mm 2 Sputtering was performed under these conditions, and the ion peak O on the carbon fiber surface was - The sputtering time (thickness) at which the maximum strength is halved (i.e., reduced to 1 / 2) is defined as the thickness of the carbon fiber oxide layer, and the values ​​shown are the converted values ​​when a silicon oxide film is sputtered under the same conditions.

[0033] The oxygen concentration on the surface of the carbon fibers is between 7 atom% and 20 atom%, preferably between 8 atom% and 17 atom%. If the oxygen concentration on the surface of the carbon fibers is less than 7 atom%, the interfacial strength between the carbon fibers and the resin will be insufficient, and if it exceeds 20 atom%, the breaking strength of the carbon fibers will be insufficient. By keeping the oxygen concentration on the surface of the carbon fibers between 7 atom% and 20 atom%, the interlaminar shear strength when recycled as carbon fiber reinforced resin is guaranteed. The oxygen concentration on the surface of the carbon fibers can be measured by X-ray photoelectron spectroscopy (XPS).

[0034] The carbon fiber is a polyacrylonitrile (PAN) type carbon fiber, for example, a medium modulus (tensile modulus of 280-350 GPa) type polyacrylonitrile (PAN) carbon fiber.

[0035] The fiber breaking strength of the carbon fiber according to this embodiment is, for example, greater than 5.2 GPa, preferably 5.4 GPa or more.

[0036] <Manufacturing method for carbon fiber> The method for producing carbon fibers according to this embodiment includes an immersion step in which carbon fibers or a carbon fiber composite material having a resin as a matrix is ​​immersed in hot concentrated sulfuric acid heated to over 80°C and 200°C or less for 10 seconds to 20 minutes.

[0037] The inventors have found that by immersing carbon fibers or carbon fiber composite materials with a resin matrix in hot concentrated sulfuric acid heated to over 80°C and below 200°C for 10 seconds to 20 minutes, carbon fibers can be obtained in which the thickness of the oxide layer on the surface of the carbon fibers is 10 nm or less, and the oxygen concentration on the surface of the carbon fibers is 7 atom% to 20 atom%. They also found that the strength properties of the obtained carbon fibers are improved compared to before treatment. Furthermore, carbon fibers with improved strength properties can be obtained at low cost and in a short time. By immersing carbon fibers or carbon fiber composite materials with a resin matrix in hot concentrated sulfuric acid heated to over 80°C and below 200°C for 10 seconds to 20 minutes, the oxide layer on the surface of the carbon fibers or resin matrix can be removed. The oxide layer on the surface of carbon fibers is weak in terms of strength, and it is thought that the breaking strength of the fibers is improved by removing this weak oxide layer through sulfuric acid treatment. In this specification, "removal" of the oxide layer includes not only cases in which the oxide layer is completely removed, but also cases in which the oxide layer is partially removed.

[0038] Conventional technologies have proposed methods for improving the strength of carbon fibers and recovering carbon fibers from carbon fiber composite materials, but these methods have problems such as cost and time. On the other hand, the carbon fiber manufacturing method according to this embodiment can improve the mechanical properties of carbon fibers in the carbon fiber manufacturing process and in the process of recovering carbon fibers from carbon fiber composite materials such as carbon fiber reinforced resins. Furthermore, it can reduce carbon dioxide emissions in the manufacturing of carbon fiber reinforced resins used in hydrogen tanks and the like. In addition, at least one of the following effects can be obtained, and a significant contribution to carbon neutrality through the upcycling of carbon fibers can be expected. The fiber breaking strength is improved by, for example, 10% or more compared to before treatment by the carbon fiber manufacturing method according to this embodiment, and high-strength carbon fibers with a fiber breaking strength parameter (representative value) of, for example, more than 5.2 GPa, preferably 5.4 GPa or higher, can be obtained. • Since the oxygen concentration on the carbon fiber surface is maintained between 7 atom% and 20 atom%, the interlaminar shear strength is guaranteed when the material is recycled as carbon fiber reinforced resin. • Because the processing time is short, it is possible to process a large amount of data in a short amount of time.

[0039] The carbon fibers used in the process are polyacrylonitrile (PAN) carbon fibers, for example, polyacrylonitrile (PAN) carbon fibers of the medium modulus type (tensile modulus 280-350 GPa).

[0040] Carbon fiber composite materials with a resin matrix are carbon fiber reinforced polymers (CFRPs) obtained by impregnating carbon fibers with resin and curing it by heating or other means. Examples of resins used for impregnation include amine-curable epoxy resins, unsaturated polyester resins, vinyl ester resins, and phenolic resins. Of these, amine-curable epoxy resins are preferred because they have high adhesion to carbon fibers and high heat resistance and mechanical properties. The resin content of the carbon fiber reinforced polymer is, for example, 10 to 90% by mass.

[0041] The concentrated sulfuric acid used in the process is, for example, concentrated sulfuric acid with a concentration of 98% by mass or higher.

[0042] The temperature of the hot concentrated sulfuric acid is, for example, above 80°C and below 200°C, and preferably between 100°C and 180°C. If the temperature of the hot concentrated sulfuric acid is below 80°C, the removal of the oxide layer on the surface of the carbon fiber or resin matrix may be insufficient, resulting in insufficient improvement of the fiber breaking strength. If it exceeds 200°C, the oxidation of the carbon fiber may be accelerated.

[0043] The immersion time in hot sulfuric acid is, for example, 10 seconds to 20 minutes, and more preferably 30 seconds to 10 minutes. If the immersion time in hot sulfuric acid is less than 10 seconds, the removal of the oxide layer on the surface of the carbon fiber or resin matrix may be insufficient, resulting in insufficient improvement of the fiber breaking strength. If it exceeds 20 minutes, oxidation of the carbon fiber may progress.

[0044] After the immersion step in hot sulfuric acid, the carbon fibers may be neutralized by methods such as immersing them in an alkaline aqueous solution (neutralization step). There are no particular restrictions on the alkaline aqueous solution, but examples include potassium carbonate aqueous solution, sodium carbonate aqueous solution, and sodium bicarbonate aqueous solution. There are no particular restrictions on the immersion temperature, but for example, 10 to 90°C, and there are no particular restrictions on the immersion time, but for example, 10 seconds to 10 minutes is appropriate.

[0045] After the neutralization step, a washing treatment may be performed with a washing solution (washing step). There are no particular restrictions on the washing solution, but examples include deionized water. As for the washing method, for example, immersion in the washing solution may be used. There are no particular restrictions on the immersion temperature for washing, but for example, 10 to 90°C and there are no particular restrictions on the immersion time, but for example, 10 seconds to 10 minutes may be used.

[0046] This specification includes the following embodiments. [1] A polyacrylonitrile carbon fiber, A carbon fiber having a surface oxide layer thickness of 10 nm or less, and an oxygen concentration on the surface of the carbon fiber of 7 atom% or more and 20 atom% or less.

[0047] The carbon fiber described in [2][1], A carbon fiber having a fiber breaking strength of more than 5.2 GPa.

[0048] A method for producing carbon fibers as described in [3] [1] or [2], A method for producing carbon fibers, comprising an immersion step of immersing a carbon fiber composite material, which has carbon fibers or resin as a matrix, in hot concentrated sulfuric acid heated to over 80°C and 200°C or less for 10 seconds to 20 minutes.

[0049] A method for producing carbon fibers as described in [4][3], A method for producing carbon fibers, comprising the immersion step of immersing the carbon fibers or carbon fiber composite material in hot concentrated sulfuric acid heated to 100°C or higher and 200°C or lower.

[0050] A method for producing carbon fibers as described in [5][3] or [4], A method for manufacturing carbon fibers, wherein the fiber breaking strength of the carbon fibers is improved by 10% or more compared to before the immersion process. [Examples]

[0051] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0052] [Sample preparation] Two types of polyacrylonitrile (PAN) carbon fibers with a medium modulus (tensile modulus of 280-350 GPa) were prepared (carbon fiber 1 and carbon fiber 2). Carbon fiber 1 is a carbon fiber with a tensile modulus of approximately 290 GPa, and carbon fiber 2 is a carbon fiber with a tensile modulus of approximately 260 GPa.

[0053] <Example 1> As a material, bobbins of polyacrylonitrile (PAN) carbon fiber (carbon fiber 1), a material used for hydrogen tanks, were prepared. Carbon fiber tow cut from the bobbins was impregnated with amine-curable epoxy resin to achieve a resin content of 23% by mass, and then heated and cured in a 150°C furnace for 2 hours. After heat curing, the CFRP was immersed in hot concentrated sulfuric acid (98% by mass) at 140°C for 1 minute to decompose the resin components, then neutralized by immersion in a sodium carbonate aqueous solution (5% by mass) at 25°C for 1 minute, and finally washed with deionized water.

[0054] <Example 2> As a material, a bobbin of polyacrylonitrile (PAN) carbon fiber (carbon fiber 1), a material used for hydrogen tanks, was prepared. The carbon fiber tow cut from the bobbin was immersed in hot concentrated sulfuric acid (98% by mass) at 140°C for 3 minutes, then neutralized by immersion in a sodium carbonate aqueous solution (5% by mass) at 25°C for 1 minute, and finally washed with deionized water.

[0055] <Example 3> As a material, a bobbin of polyacrylonitrile (PAN) carbon fiber (carbon fiber 1), a material used for hydrogen tanks, was prepared. The carbon fiber tow cut from the bobbin was immersed in hot concentrated sulfuric acid (98% by mass) at 120°C for 30 seconds, then neutralized by immersion in a sodium carbonate aqueous solution (5% by mass) at 25°C for 1 minute, and finally washed with deionized water.

[0056] <Comparative Example 1> As a material, a bobbin of polyacrylonitrile (PAN) carbon fiber (carbon fiber 1), a material used for hydrogen tanks, was prepared. The carbon fiber tow cut from the bobbin was immersed in hot concentrated sulfuric acid (98% by mass) at 80°C for 30 seconds, then neutralized by immersion in a sodium carbonate aqueous solution (5% by mass) at 25°C for 1 minute, and finally washed with deionized water.

[0057] <Comparative Example 2> As a material, a bobbin of polyacrylonitrile (PAN) carbon fiber (carbon fiber 1), a material used for hydrogen tanks, was prepared. The carbon fiber tow cut from the bobbin was immersed in acetone and ultrasonically cleaned, with the acetone being replaced every 3 minutes. This ultrasonic cleaning was repeated three times to remove the surface treatment agent.

[0058] <Example 4> As a material, bobbins of polyacrylonitrile (PAN) carbon fiber (carbon fiber 2), a material used for hydrogen tanks, were prepared. Carbon fiber tow cut from the bobbins was impregnated with amine-curable epoxy resin to achieve a resin content of 23% by mass, and then heated and cured in a 150°C furnace for 2 hours. After heat curing, the CFRP was immersed in hot concentrated sulfuric acid (98% by mass) at 180°C for 3 minutes to decompose the resin components, then neutralized by immersion in a sodium carbonate aqueous solution (5% by mass) at 25°C for 1 minute, and finally washed with deionized water.

[0059] <Example 5> As a material, bobbins of polyacrylonitrile (PAN) carbon fiber (carbon fiber 2), a material used for hydrogen tanks, were prepared. Carbon fiber tow cut from the bobbins was impregnated with amine-curable epoxy resin to achieve a resin content of 23% by mass, and then heated and cured in a 150°C furnace for 2 hours. After heat curing, the CFRP was immersed in hot concentrated sulfuric acid (98% by mass) at 180°C for 5 minutes to decompose the resin components, then neutralized by immersion in a sodium carbonate aqueous solution (5% by mass) at 25°C for 1 minute, and finally washed with deionized water.

[0060] <Example 6> As a material, bobbins of polyacrylonitrile (PAN) carbon fiber (carbon fiber 2), a material used for hydrogen tanks, were prepared. Carbon fiber tow cut from the bobbins was impregnated with amine-curable epoxy resin to achieve a resin content of 23% by mass, and then heated and cured in a 150°C furnace for 2 hours. After heat curing, the CFRP was immersed in hot concentrated sulfuric acid (98% by mass) at 140°C for 10 minutes to decompose the resin components, then neutralized by immersion in a sodium carbonate aqueous solution (5% by mass) at 25°C for 1 minute, and finally washed with deionized water.

[0061] <Comparative Example 3> As a material, we prepared bobbins of polyacrylonitrile (PAN) carbon fiber (carbon fiber 2), a material used for hydrogen tanks. The carbon fiber tow cut from the bobbins was immersed in acetone and ultrasonically cleaned, with the acetone being replaced every 3 minutes. This ultrasonic cleaning was repeated three times to remove the surface treatment agent.

[0062] <Comparative Example 4> As a material, bobbins of polyacrylonitrile (PAN) carbon fiber (carbon fiber 2), a material used for hydrogen tanks, were prepared. The carbon fiber tow cut from the bobbins was immersed in hot concentrated sulfuric acid (98% by mass) at 140°C for 30 minutes, then neutralized by immersion in sodium carbonate aqueous solution (5% by mass) at 25°C for 1 minute, and finally washed with deionized water.

[0063] <Comparative Example 5> A bobbin of polyacrylonitrile (PAN)-based carbon fiber (carbon fiber 2), which is a material for hydrogen tanks, was prepared. The carbon fiber tow cut from the bobbin was immersed in hot concentrated sulfuric acid (98% by mass) at 140 °C for 60 minutes, then immersed in an aqueous sodium carbonate solution (5% by mass) at 25 °C for 1 minute for neutralization treatment, and finally washed with ion-exchanged water.

[0064] [Fiber breaking strength measurement and Weibull analysis] The breaking strength test of the carbon fiber was carried out in accordance with ISO 11566:1966 “Carbon fibre - Determination of the tensile properties of single-filament specimens”. The measured fiber length was 50 cm, and more than 30 samples were obtained.

[0065] The Weibull distribution was applied to statistically describe the fiber strength. The Weibull distribution is a model (weakest link model) that regards an object as a chain (a connected body of rings), and assumes that when the chain is pulled, the weakest ring is broken and the whole chain is broken, and it is the optimal method for analyzing the fiber strength.

[0066] The Weibull distribution has a probability distribution represented by the following formula (1). f(σ)=m / η(σ / η)^(m - 1)exp{-(σ / η)^m} (1)

[0067] Here, σ is the strength of the fiber, and m and η are Weibull coefficients called the shape parameter and the scale parameter, respectively. The value of the shape parameter m indicates the nature of the distribution. When 0 < m < 1, it is a distribution that decreases exponentially, and when m > 1, it is a mountain-shaped distribution with a peak. The larger m is, the smaller the variation in strength. The scale parameter η indicates the spread (scaling) in the horizontal axis direction of the distribution and represents the representative value of the fiber breaking strength.

[0068] The cumulative breaking probability F(σ) of the Weibull distribution is represented by formula (2). Differentiating this F(σ) gives formula (1). F(σ)=1 - exp{-(σ / η)^m} (2)

[0069] By transforming Equation (2) and taking the logarithm twice, it is expressed by Equation (3). Here, when X = lnσ and Y = lnln(1 / (1 - F(σ))), it becomes a linear function expressed by Equation (4). From the values of the cumulative fracture probability obtained from the experimental values, X and Y are plotted and linearly approximated by Equation (4), and m can be obtained from its slope, and η can be obtained from the intersection point where Y = 0, respectively. lnln(1 / (1 - F(σ))) = mlnσ - mlnη (3) Y = mX - mlnη (4)

[0070] [TOF-SIMS Analysis] TOF.SIMS5 manufactured by IONTOF was used as a time-of-flight secondary ion mass spectrometer (TOF-SIMS), and Cs + depth measurement was performed using a sputter gun. Each sample of Examples 1 to 6 and Comparative Examples 1 to 5 was attached to a carbon tape and subjected to TOF-SIMS analysis. In depth analysis, Cs + was used as a sputter ion beam, with an acceleration voltage of 0.25 keV, an ion beam current of 10 nA, and an incident angle of 45°. An area of 0.3 × 0.3 mm 2 was irradiated, and Bi3 2+ at 60 keV was used, and an area of 25 × 25 μm 2 was used to acquire data at a resolution of 256 × 256 pixels. Data of an area on the carbon fiber surface with a fiber long-axis direction of 15 μm and a short-axis direction of 5 μm was extracted to obtain depth analysis data. The etching rate of the silicon oxide film under the same conditions was 0.014 nm / second.

[0071] [XPS Analysis] Quantera SXM manufactured by ULVAC PHI was used as an X-ray photoelectron spectroscopy (XPS) device. The carbon fiber was made into a bundle with a diameter of 1 mm and fixed in a form that bridges the through-screw hole of the sample holder and subjected to XPS analysis. Monochromatic Al Kα (1486.6 eV) was used as the X-ray source, and the measurement was performed at a Take off angle of 45 degrees.

[0072] [Fiber Fracture Strength Analysis Results]​​ The Weibull analysis results based on fiber strength tests and the parameters obtained from Weibull statistical analysis are shown in Figures 1 and 2 and Table 1, respectively. In the Weibull probability plots shown in Figures 1(a) and 2(a), each fiber tends to deviate from the straight line at low strength levels (cumulative fracture probability of 10% or less), but in the range of cumulative fracture probability from 10% to 90%, it follows a straight line represented by equation (4), indicating that it generally follows the Weibull distribution. In Figure 1(a), the straight lines for Examples 1-3 are shifted towards higher strength compared to Comparative Examples 1 and 2, and in Figure 2(a), the straight lines for Examples 4-6 are shifted towards higher strength compared to Comparative Examples 3-5. The Weibull distribution reconstructed using the shape parameter m and scale parameter η obtained by fitting equation (4) is shown in Figures 1(b) and 2(b). In Figure 1(b), the distributions for Examples 1-3 are shifted towards higher strength compared to Comparative Examples 1 and 2, and in Figure 2(b), the distributions for Examples 4-6 are shifted towards higher strength compared to Comparative Examples 3-5, indicating that the strength of the Examples is improved compared to the Comparative Examples.

[0073] [TOF-SIMS results] The TOF-SIMS depth analysis results are shown in Figure 3 and Table 1. Figure 3 is O - The relationship between ionic strength (arbitrary unit) and sputtering time (seconds) is shown. Figure 3(a) shows the measurement results for carbon fiber 1, and Figure 3(b) shows the measurement results for carbon fiber 2. As the sputtering time progresses, once O - The ionic strength increases, and then begins to decrease. - The sputtering time (thickness) at which the ionic intensity decreased from its maximum to half was defined as the thickness of the oxide layer of the carbon fiber, and the values ​​converted using the sputtering rate of the silicon oxide film are shown in Table 1. Comparative Examples 2 and 3, which were not treated with sulfuric acid, had an oxide layer thickness of 60 nm or more, whereas Comparative Examples 1, 4, 5 and Examples 1-6, which were treated with sulfuric acid, had an oxide layer thickness of 30 nm or less, and in Examples 1-6, the oxide layer thickness was 10 nm or less.

[0074] Figure 4 shows the relationship between the representative fiber tensile strength η (GPa) obtained by the Weibull plot and the thickness of the oxide layer (nm, SiO2 equivalent). It was shown that when the thickness of the oxide layer is 10 nm or less, the fiber tensile strength exceeds 5.4 GPa.

[0075] [XPS analysis results] The XPS analysis results are shown in Table 1. The oxygen concentration (O) was 7 atom% or higher in all samples.

[0076] [Table 1]

[0077] [Comparison of analysis results and intensity results] XPS and TOF-SIMS analysis results showed that the oxygen (O) concentration on the outermost surface did not change significantly before and after treatment with hot concentrated sulfuric acid, but the thickness of the oxide layer decreased after the treatment. Examples 1-6, in which the thickness of the oxide layer was reduced to 10 nm or less after hot concentrated sulfuric acid treatment, showed an improvement of more than 10% in tensile strength compared to the carbon fibers before sulfuric acid treatment. The oxide layer on the surface of carbon fibers is structurally weak, and it is thought that the removal of this weak oxide layer by sulfuric acid treatment improves the tensile strength of the fibers.

[0078] Figure 5 shows the relationship between the sulfuric acid treatment temperature (°C) and sulfuric acid immersion time (minutes) for the carbon fibers obtained in the examples and comparative examples. As shown, the carbon fibers obtained in the examples showed improved tensile strength.

[0079] As described above, the carbon fibers in the examples had good strength properties. Furthermore, the method in the examples made it possible to obtain carbon fibers with good strength properties at low cost and in a short time.

Claims

1. A polyacrylonitrile-based carbon fiber, A carbon fiber characterized in that the thickness of the oxide layer on the surface of the carbon fiber is 10 nm or less, and the oxygen concentration on the surface of the carbon fiber is 7 atom% or more and 20 atom% or less.

2. A carbon fiber according to claim 1, A carbon fiber characterized in that the fiber breaking strength of the carbon fiber is greater than 5.2 GPa.

3. A method for producing carbon fibers according to claim 1 or 2, A method for producing carbon fibers, characterized by including an immersion step of immersing a carbon fiber composite material, which has carbon fibers or resin as a matrix, in hot concentrated sulfuric acid heated to over 80°C and 200°C or less for 10 seconds to 20 minutes.

4. A method for producing carbon fibers according to claim 3, A method for producing carbon fibers, characterized in that, in the immersion step, the carbon fibers or the carbon fiber composite material are immersed in concentrated sulfuric acid heated to 100°C or higher and 200°C or lower.

5. A method for producing carbon fibers according to claim 3, A method for producing carbon fibers, characterized in that the fiber breaking strength of the carbon fibers is improved by 10% or more compared to before the immersion process.

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