Pitch-based carbon fiber, its manufacturing method, and fiber-reinforced plastic

Abstract

Provided are: a pitch-based carbon fiber of which the defect number density is 40 / m or less and the shape of the cross-section perpendicular to the longitudinal direction is circular or elliptical with no more than 0.25 oblateness, and which has a boundary surface in the diametrical direction of said circular cross-section or in the longitudinal direction of said elliptical cross-section; a method for producing the pitch-based carbon fiber; and a fiber-reinforced plastic.

Description

[Technical Field]

[0001] The present disclosure relates to pitch-based carbon fibers, methods for producing the same, and fiber-reinforced plastics. [Background technology]

[0002] Pitch-based carbon fibers are widely used in various applications such as fiber-reinforced plastics, etc. Pitch-based carbon fibers are disclosed in, for example, Patent Documents 1, 2, and 3.

[0003] [Patent Document 1] Japanese Patent Application Laid-open No. 155714 / 1986 [Patent Document 2] Japanese Patent Application Laid-open No. 61-006316 [Patent Document 3] Japanese Patent Publication No. 5-29689 Summary of the Invention [Problem to be solved by the invention]

[0004] Techniques relating to pitch-based carbon fibers have been studied for some time, including Patent Document 1. However, currently, techniques for improving the tensile strength of pitch-based carbon fibers are insufficient. Patent Documents 2 and 3 leave room for improvement in terms of improving the tensile strength while maintaining a high tensile modulus. Generally, the tensile modulus improves as the heat treatment temperature (graphitization temperature) during graphitization increases, but there is a demand for a production method that produces a higher tensile modulus at the same graphitization temperature.

[0005] The present disclosure has been made in view of the above circumstances, and a problem to be solved by one embodiment of the present disclosure is to provide a method for producing pitch-based carbon fibers that can produce pitch-based carbon fibers with excellent tensile strength. Another problem to be solved by another embodiment of the present disclosure is to provide a pitch-based carbon fiber having excellent tensile strength. Another problem to be solved by another embodiment of the present disclosure is to provide a fiber-reinforced plastic using the pitch-based carbon fiber. [Means for solving the problem]

[0006] The present disclosure includes the following aspects. <1> A pitch-based carbon fiber having a defect density of 40 defects / m or less, a cross-sectional shape perpendicular to the longitudinal direction that is circular or elliptical with a flattening ratio of 0.25 or less, and having a boundary surface in the diameter direction of the circular cross-section or in the major axis direction of the elliptical cross-section. <2> the ratio of the area occupied by the graphene sheets oriented at an orientation angle of 60° to 120° relative to the boundary plane to the area of ​​the cross section is 60% or more; <1> The pitch-based carbon fiber according to claim 1. <3> The cross-sectional shape is a circle or an ellipse with a flattening ratio of 0.2 or less. <2> The pitch-based carbon fiber according to claim 1. <4> The defect number density is 30 defects / m or less. <1> ~ <3> 10. The pitch-based carbon fiber according to any one of the above. <5> The tensile modulus is 400 GPa or more. <1> ~ <4> 10. The pitch-based carbon fiber according to any one of the above. <6> The tensile modulus is 440 GPa or more. <5> The pitch-based carbon fiber according to claim 1. <7> The method includes a melt spinning step of discharging molten anisotropic pitch from a spinning nozzle and spinning it into fibers, The ratio of the length of the long side of the circumscribing rectangle circumscribing the shape of the discharge hole of the spinning nozzle to the length of the short side of the circumscribing rectangle is greater than 1, The viscosity of the molten anisotropic pitch is 20 Pa·s to 80 Pa·s. A method for producing pitch-based carbon fiber. <8> The ratio is 1.5 to 10.0. <7> A method for producing the pitch-based carbon fiber described in <9> The shape of the discharge hole is rectangular or elliptical. <7> or <8> A method for producing the pitch-based carbon fiber described in <10> The viscosity of the anisotropic pitch is 30 Pa·s to 60 Pa·s. <7> ~ <9> 1. A method for producing pitch-based carbon fibers according to any one of the above. <11> The spinning speed in the melt spinning process is 200 m / min to 500 m / min. <7> ~ <10> 1. A method for producing pitch-based carbon fibers according to any one of the above. <12> a graphitization step of graphitizing the carbonized pitch-based carbon fiber precursor by heat treatment after the melt spinning step, The temperature of the heat treatment in the graphitization step is 2000°C to 2900°C. <7> ~ <11> 1. A method for producing pitch-based carbon fibers according to any one of the above. <13> <1> ~ <6> A fiber-reinforced plastic comprising the pitch-based carbon fiber according to any one of the above. [Effects of the Invention]

[0007] According to one embodiment of the present disclosure, there is provided a method for producing pitch-based carbon fibers that can yield pitch-based carbon fibers with excellent tensile strength. According to another embodiment of the present disclosure, a pitch-based carbon fiber having excellent tensile strength is provided. According to another embodiment of the present disclosure, a fiber-reinforced plastic using the pitch-based carbon fiber is provided. [Brief explanation of the drawings]

[0008] [Figure 1] Figure 1 is an example of a Weibull plot. [Figure 2] FIG. 2 is a schematic diagram showing an example of a cross section of a pitch-based carbon fiber. [Figure 3] FIG. 3 is a schematic diagram showing an example of a cross section of a pitch-based carbon fiber. [Figure 4] FIG. 4 is a schematic diagram showing an example of a cross section of a pitch-based carbon fiber. [Figure 5] FIG. 5 is an SEM image showing an example of a cross section of pitch-based carbon fiber. [Figure 6] FIG. 6 is an example of a binarized image of the SEM image of FIG. [Figure 7] FIG. 7 is an example of an edited image of the binarized image of FIG. [Figure 8] FIG. 8 is a diagram showing an example of the orientation angle of a graphene sheet. [Figure 9] FIG. 9 is a schematic diagram showing an example of a cross section of a spinning nozzle. [Figure 10] FIG. 10 is a schematic diagram showing an example of an outlet hole of a spinning nozzle. DETAILED DESCRIPTION OF THE INVENTION

[0009] The pitch-based carbon fiber and its manufacturing method according to the present disclosure will be described in detail below.

[0010] In the present disclosure, a numerical range indicated using "to" means a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in the present disclosure, the upper or lower limit value described in a certain numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in the present disclosure, the upper or lower limit value described in a certain numerical range may be replaced with a value shown in the examples. In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In the present disclosure, when there are multiple substances corresponding to each component, the amount of each component means the total amount of the multiple substances unless otherwise specified. In the present disclosure, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

[0011] The drawings referred to in the following description are illustrative and schematic, and the present disclosure is not limited to these drawings. The same reference numerals indicate the same components. Also, reference numerals in the drawings may be omitted.

[0012] <Pitch-based carbon fiber> The pitch-based carbon fiber according to the present disclosure has a defect density of 40 defects / m or less, a cross-sectional shape perpendicular to the longitudinal direction that is circular or elliptical with a flattening ratio of 0.25 or less, and has a boundary surface in the diameter direction of the circular cross-section or in the major axis direction of the elliptical cross-section.

[0013] The defect density is the number of defects contained in a pitch-based carbon fiber per unit length, and in the present disclosure, it means a value obtained based on a Weibull plot. By reducing the defect density, the tensile strength of the pitch-based carbon fiber can be improved.

[0014] The defect density is related to, for example, the number of voids and cracks (hereinafter sometimes referred to as "voids, etc."), which are factors that affect tensile strength, the size of the voids, etc. Therefore, by reducing the number of voids, etc., the defect density can be reduced, and the tensile strength can be improved.

[0015] A method for obtaining the defect number density based on a Weibull plot will now be described in detail.

[0016] For a pitch-based carbon fiber having a length L [m], the defect density λ [number / m] is expressed by the following formula (1). JPEG0007749666000001.jpg1866 In the formula (1), σ, β, γ, and L0 are as follows: σ: Tensile strength of each single fiber of pitch-based carbon fiber (N single fibers) [GPa] β: Width of probability distribution of tensile strength for each single fiber in pitch-based carbon fibers (N single fibers) (Shape parameters (Weibull modulus)) γ: Tensile strength per unit length of pitch-based carbon fiber calculated from the probability distribution of tensile strength for each single fiber (scaling parameters) L0: Standard length of pitch-based carbon fiber (1 m)

[0017] The fracture probability F(L) at which a pitch-based carbon fiber breaks at a tensile strength σ (hereinafter sometimes referred to as "fracture probability F") is expressed by the following formula (2). JPEG0007749666000002.jpg1082 Equation (2) is based on the weakest link model, which assumes that when a chain is pulled, the entire chain breaks when the weakest link breaks.

[0018] Substituting equation (1) into equation (2) gives the following equation (3). JPEG0007749666000003.jpg2288

[0019] Expanding equation (3) yields equation (4) below. JPEG0007749666000004.jpg19109 Equation (4) is a linear function of ln[-ln(1-F)] and lnσ, and gives a Weibull plot. A method for experimentally determining the breakage probability F and the tensile strength σ for each single fiber will be described below.

[0020] N tensile strengths are obtained by conducting a tensile test on N pitch-based carbon fibers in accordance with JIS R 7606:2000. When the number of pitch-based carbon fibers that break at a tensile strength σ or less is n among the N tensile strengths, the breakage probability F at the tensile strength σ (i.e., the proportion of pitch-based carbon fibers whose tensile strength is σ or less) is expressed by the following formula (5): F=n / N (5)

[0021] The tensile strength σ and the fracture probability F at the tensile strength σ are substituted into equation (4), and ln[-ln(1-F)] and lnσ are plotted. Similarly, fracture probabilities are calculated for other tensile strengths and plotted in the same manner. This allows N plots to be obtained, where N is 30 or more. This results in, for example, the Weibull plot shown in Figure 1.

[0022] By fitting Equation (4) to the Weibull plot using the least squares method, a fitted line can be obtained. For example, the fitted line shown in Figure 1 can be obtained. β and γ are obtained from the slope and intercept of the fitted line. Also, from the results of the tensile test, the average value of the tensile strength (σ av ) is calculated by adding σ to equation (1). av Then, β and γ are substituted to obtain the defect density λ. The defect density λ obtained in this way is the average value of the defect density λ av This is the defect density of the pitch-based carbon fiber shown in the present application. In this manner, the defect density of pitch-based carbon fiber can be obtained based on the Weibull plot.

[0023] The number N of pitch-based carbon fibers used in the tensile test is preferably 30 or more. There is no particular upper limit to the number N, but it may be 50, for example. The length of the pitch-based carbon fiber used in the tensile test is 25 mm.

[0024] Since pitch-based carbon fibers have a tendency to have improved tensile strength as the defect density decreases, the lower the defect density, the better. The defect density is preferably, for example, 30 defects / m or less, more preferably 25 defects / m or less, and even more preferably 10 defects / m or less. The lower limit of the defect density is not particularly limited, but is usually about 1 defect / m.

[0025] The cross section of pitch-based carbon fiber perpendicular to the longitudinal direction (hereinafter sometimes simply referred to as "cross section") is circular or elliptical with a flattening ratio of 0.25 or less. This facilitates reducing the defect density. Here, "perpendicular" means 90°±15°.

[0026] The flattening ratio is the ratio of the length of the short side of the circumscribing rectangle to the length of the long side of the circumscribing rectangle that circumscribes the elliptical cross section, minus 1. The circumscribing rectangle is the rectangle that circumscribes the ellipse and has the smallest area.

[0027] The diameter of the pitch-based carbon fiber is not particularly limited, and may be, for example, 5 μm to 20 μm.

[0028] The cross-sectional shape, flatness, and diameter of the pitch-based carbon fiber are measured in accordance with JIS R 7606:2000 using a laser shape measuring device, with the measurement range set to 0° to 180°, by rotating the cross-section of the pitch-based carbon fiber every 10° and measuring the outer circumferential shape of the cross-section. The laser shape measuring device may be a commercially available product, such as the "Laser Scan Micrometer LSM-500S" manufactured by Mitutoyo Corporation. When the cross section is elliptical, the major and minor axes are determined from the outer periphery of the cross section, and the circle-equivalent diameter (the square root of the product of the major and minor axes) calculated from the area of ​​the ellipse is taken as the diameter of the pitch-based carbon fiber.

[0029] From the viewpoint of more easily reducing the defect density, the shape of the cross section perpendicular to the longitudinal direction is preferably a circle or an ellipse with a flattening ratio of 0.2 or less, more preferably a circle or an ellipse with a flattening ratio of 0.18 or less, and even more preferably a circle or an ellipse with a flattening ratio of 0.15 or less. When the shape is an ellipse, the lower limit of the flattening ratio is not particularly limited, but from the viewpoint of more easily reducing the defect density, it is preferably 0.01 or more.

[0030] Pitch-based carbon fibers contain graphene sheets, which are sheets of carbon atoms bonded in a hexagonal pattern on a plane to form a lattice structure.

[0031] The structure of pitch-based carbon fibers varies depending on the state (for example, orientation) of the graphene sheets, and may include a radial structure, an onion structure, a random structure, etc., with the radial structure being preferred.

[0032] In the radial structure, multiple graphene sheets extend in the longitudinal direction of the pitch-based carbon fiber, and in the cross section, multiple graphene sheets extend from a predetermined position toward the periphery of the cross section.

[0033] The graphene sheet may be in contact with the predetermined position or may be spaced apart from the predetermined position. Also, the graphene sheet may be in contact with the periphery of the cross section or may be spaced apart from the periphery of the cross section.

[0034] The graphene sheets may or may not be in contact with each other through the predetermined positions.

[0035] For example, as shown in Fig. 2, the cross section of a pitch-based carbon fiber 100 has an elliptical shape, and multiple graphene sheets 10 extend from a boundary surface 30 (the above-mentioned predetermined position) in the long axis direction of the ellipse toward the outer periphery of the cross section. In this way, the graphene sheets 10 are oriented with respect to the boundary surface 30 in the long axis direction of the cross section. Such a structure of a pitch-based carbon fiber is sometimes called a "flat radial structure."

[0036] 3, the cross section of the pitch-based carbon fiber 102 is circular, and the graphene sheets 10 extend from the boundary surface 32 (the predetermined position) in the diameter direction of the circle toward the periphery of the cross section. In this way, the graphene sheets 10 are oriented with respect to the boundary surface 32 in the diameter direction of the cross section.

[0037] 4, the cross section of the pitch-based carbon fiber 104 is circular, and the graphene sheets 10 extend radially from the center 34 (the predetermined position) of the circle toward the periphery of the cross section. Such a pitch-based carbon fiber structure is sometimes called a "simple radial structure." As described above, the pitch-based carbon fiber shown in FIG. 3 and the pitch-based carbon fiber shown in FIG. 4 both have a circular cross-sectional shape, but differ in the manner in which the graphene sheets extend in the cross section.

[0038] From the viewpoint of more easily reducing the defect number density, it is preferable that the graphene sheets of the pitch-based carbon fiber be oriented with respect to the boundary surface in the diameter or major axis direction of the cross section.

[0039] In pitch-based carbon fibers, the area ratio of the region occupied by graphene sheets oriented at an orientation angle of 60° to 120° with respect to the diameter or the boundary plane in the long axis direction of the cross section (hereinafter, sometimes referred to as the "specific region") to the cross section area is preferably 60% or more, which makes it easier to reduce the defect density.

[0040] The ratio of the area of ​​the specific region is obtained as follows. A scanning electron microscope (SEM) is used to observe a randomly selected cross section of the pitch-based carbon fiber in a direction perpendicular to the cross section, and an SEM image is obtained. This results in an SEM image of the cross section of the pitch-based carbon fiber, such as that shown in FIG. 5. For example, a JEM-6500F manufactured by JEOL Ltd. can be used as the SEM. The conditions for obtaining the SEM image include a magnification that allows only the cross section of a single fiber to be obtained (for example, 7000 to 10000 times, 10000 times for pitch-based carbon fiber with a diameter of about 7 μm), and no particular acceleration voltage is specified, and the acceleration voltage may be, for example, 5 kV to 15 kV. The SEM images are analyzed using the image processing software "ImageJ (vl.52)." Specifically, the SEM images are binarized to obtain a binarized image of the cross section. This results in a binarized image such as that shown in Figure 6. Using "ImageJ (v1.52)", the binarized image is edited to draw the black parts of the binarized image as line segments, and an edited image of the cross section is obtained. As a result, an edited image such as that shown in Figure 7 is obtained. For the edited image, the linearity of each line segment in the cross section is evaluated using fractal dimension analysis, and line segments with D = 1.2 or less are identified. Line segments with D = 1.2 or less are defined as "graphene sheets." "ImageJ (vl.52)" is open-source, public domain image processing software. The line segment that gives the maximum length of the cross-sectional contour in the edited image (i.e., the maximum length of the straight line connecting any two points on the contour, which is the diameter if the cross-section is circular, or the major axis if the cross-section is elliptical) is defined as the "boundary surface." The edited image is analyzed using the previously described program in "Python (v3)" (open source), and the orientation angle of each graphene sheet (a line segment with D = 1.2 or less) relative to the boundary plane is determined over the entire cross section. The angle at the intersection of a line including the boundary plane and a line including the graphene sheet is taken as the orientation angle. For example, as shown in Figure 8, the orientation angle α is obtained from the angle at the intersection of a line 36 including the boundary plane and a line 12 including the graphene sheet. For example, the cross section shown in Figure 8 includes regions with many short graphene sheets (the upper left and lower right regions in Figure 8), but the orientation angle is evaluated over the entire cross section regardless of the length of the graphene sheets. Using "ImageJ (vl.52)", the area (specific area) occupied by graphene sheets oriented at an orientation angle of 60° to 120° (line segments with D = 1.2 or less and an orientation angle of 0° to 120°) is displayed in black, and the specific area in the cross section is identified. Using "ImageJ (vl.52)", the area of ​​the specific area and the area of ​​the cross section are calculated. The area of ​​the specific area is divided by the area of ​​the cross section to calculate the ratio of the area of ​​the specific area. The same measurement is carried out five times for the same cross section of the pitch-based carbon fiber as above, and the average value of the area ratios of the specific regions obtained from the five measurements is taken as the area ratio of the specific region of the pitch-based carbon fiber.

[0041] From the viewpoint of more easily reducing the defect density, it is more preferable that the average value of the orientation angle of the graphene sheets oriented at an orientation angle of 60° to 120° is 70° to 110°.

[0042] From the viewpoint of more easily reducing the defect density, the ratio of the area of ​​the specific region is more preferably 70% or more. The upper limit of the ratio of the area of ​​the specific region is not particularly limited, but from the viewpoint of more easily reducing the defect density, it is preferably 95% or less.

[0043] The higher the tensile strength of the pitch-based carbon fiber, the better. For example, it is preferably 3600 MPa or more, more preferably 3800 MPa or more, and even more preferably 4000 MPa or more.

[0044] The tensile strength of the pitch-based carbon fiber is measured in accordance with JIS R 7606: 2000. The average value of the tensile strengths of N samples obtained in the tensile test described above for the Weibull plot is defined as the tensile strength of the pitch-based carbon fiber.

[0045] From the viewpoint of further improving mechanical properties, the higher the tensile modulus of the pitch-based carbon fiber, the better. For example, it is preferably 400 GPa or more, more preferably 440 GPa or more, and even more preferably 460 GPa or more.

[0046] The tensile modulus of the pitch-based carbon fiber is measured in accordance with JIS R 7606: 2000. The tensile modulus is also measured in the tensile test described above for the Weibull plot, and the average value of N tensile moduli obtained from N pitch-based carbon fibers is defined as the tensile modulus of the pitch-based carbon fiber.

[0047] <Method of manufacturing pitch-based carbon fiber> The method for producing the pitch-based carbon fiber according to the present disclosure is not particularly limited, but the method for producing the pitch-based carbon fiber according to the present disclosure described below can be suitably used.

[0048] The method for producing pitch-based carbon fibers according to the present disclosure includes: The method includes a melt spinning step of discharging molten anisotropic pitch from a spinning nozzle and spinning it into fibers, The ratio of the length of the long side of the circumscribing rectangle circumscribing the shape of the discharge hole of the spinning nozzle to the length of the short side of the circumscribing rectangle is greater than 1, The viscosity of the molten anisotropic pitch is 20 Pa·s to 80 Pa·s.

[0049] The method for producing pitch-based carbon fibers includes the melt spinning step, as well as the infusibilizing step, carbonizing step, and graphitizing step, which will be described below. Each step will be described in detail below.

[0050] [Melt spinning process] In the melt spinning process, molten anisotropic pitch is discharged from a spinning nozzle and spun.

[0051] (anisotropic pitch) Anisotropic pitch includes a mesophase in which pitch molecules are arranged in a planar manner, which is a precursor of graphene sheets and which becomes graphene sheets through a graphitization process.

[0052] Examples of raw materials for anisotropic pitch include coal-based heavy oils such as coal tar, coal tar pitch, and coal liquefaction products, as well as refined petroleum-based heavy oils such as tar and pitch produced as by-products by the heat treatment of atmospheric distillation and vacuum distillation residues of petroleum. Anisotropic pitch can be obtained by subjecting such raw materials to an appropriate combination of heat treatment, solvent extraction, hydrogenation, and the like.

[0053] The proportion of mesophase in the anisotropic pitch is preferably 60% or more, more preferably 70% or more, which makes it easier to develop the physical properties of pitch-based carbon fibers, particularly the tensile modulus. The proportion of mesophase is measured as follows.

[0054] A sample is randomly taken from the anisotropic pitch and observed at a magnification of 10 to 100 times using a polarizing microscope. For a randomly selected field of view, the area of ​​the entire field of view and the area of ​​the mesophase are measured. The ratio of the area of ​​the mesophase to the area of ​​the entire field of view is taken as the mesophase proportion. In other words, the mesophase proportion is the area ratio of the mesophase to the anisotropic pitch.

[0055] The melting point of the anisotropic pitch is not particularly limited, but is preferably 260°C to 320°C, and more preferably 270°C to 310°C.

[0056] From the viewpoint of reducing the amount of foreign matter in the anisotropic pitch and more easily increasing the tensile strength, the quinoline insoluble content in the anisotropic pitch is preferably 20 mass% or less. The quinoline insoluble content can be measured by a known method.

[0057] (spinning nozzle) In a spinning nozzle that extrudes anisotropic pitch, the ratio of the length of the long side of a circumscribing rectangle that circumscribes the shape of the extrusion hole to the length of the short side of the circumscribing rectangle that circumscribes the shape of the extrusion hole (hereinafter sometimes referred to as the "aspect ratio") exceeds 1. The circumscribing rectangle is a rectangle that circumscribes the elliptical shape of the extrusion hole and has the smallest area. The shape of the discharge hole is not limited to an ellipse but may be a rectangle. When the shape of the discharge hole is a rectangle, the ratio of the length of the long side to the length of the short side of the rectangle (hereinafter sometimes referred to as the "aspect ratio") is greater than 1.

[0058] Since the aspect ratio of the ejection hole is greater than 1, the anisotropy of the shape of the ejection hole is greater than when the aspect ratio is 1.

[0059] When the molten anisotropic pitch passes through the discharge holes of the spinning nozzle, the mesophase in the anisotropic pitch is oriented, and the state of orientation differs depending on the shape of the discharge holes.

[0060] When the shape of the discharge holes is circular (aspect ratio is 1), in the filaments obtained by discharging (hereinafter, sometimes referred to as "pitch-based carbon fiber precursor"), the mesophase is oriented radially from the center of the cross section toward the periphery, for example, in the same manner as the graphene sheet shown in Fig. 4. This is because the anisotropy of the shape of the discharge holes is small, so the mesophase is likely to be oriented uniformly from the center of the cross section. By subjecting a pitch-based carbon fiber precursor having a configuration in which the mesophase is radially oriented from the center of the cross section toward the periphery (hereinafter sometimes referred to as "Configuration 1") to an infusibilization step, a carbonization step, and a graphitization step, it is possible to obtain a pitch-based carbon fiber having graphene sheet orientation such as that shown in FIG. 4, for example.

[0061] On the other hand, when the aspect ratio of the discharge holes is more than 1, the mesophase in the discharged pitch-based carbon fiber precursor is oriented from the boundary surface of the cross section toward the outer periphery in the same manner as the graphene sheet shown in Fig. 2 or 3. This is because the shape of the discharge holes is highly anisotropic, and the orientation of the mesophase varies depending on the part through which it passes. By subjecting a pitch-based carbon fiber precursor having a configuration in which the mesophase is oriented from the boundary surface of the cross section toward the periphery (hereinafter sometimes referred to as "Configuration 2") to an infusibilization step, a carbonization step, and a graphitization step, it is possible to obtain a pitch-based carbon fiber having graphene sheet orientation such as that shown in FIG. 2 or FIG. 4, for example.

[0062] In addition to making the aspect ratio of the spinning nozzle outlet hole greater than 1, the viscosity of the molten anisotropic pitch is set to 20 Pa·s to 80 Pa·s. This makes it easy to orient the mesophase in the anisotropic pitch from the boundary surface of the cross section toward the outer periphery when the molten anisotropic pitch passes through the spinning nozzle outlet hole. Note that the viscosity here refers to the viscosity of the anisotropic pitch at the melting temperature in the storage area before being discharged from the spinning nozzle (i.e., the temperature at the introduction section 70 in Figure 9). Furthermore, by satisfying the above aspect ratio and viscosity, it becomes easy to reduce the defect density to 40 defects / m or less. Furthermore, by satisfying the above aspect ratio and viscosity, it becomes easy to set the orientation angle of the graphene sheet with respect to the boundary plane in the range of 60° to 120°, and it also becomes easy to set the area ratio of the specific region to 60%. The viscosity of the molten anisotropic pitch is 20 Pa·s to 80 Pa·s, preferably 25 Pa·s to 70 Pa·s, and more preferably 30 Pa·s to 60 Pa·s.

[0063] The viscosity of anisotropic pitch is measured by the following method. The method for measuring viscosity (320°C) is explained below using an example in which the melting temperature of anisotropic pitch is 320°C. The viscosity is measured using a concentric double tube and a high-temperature cell with a Rheomat-30 (manufactured by Contraves) at a shear rate of 4 to 60 s -1 The measurement is performed at a temperature of 320°C.

[0064] The melting temperature of the anisotropic pitch is, for example, preferably 320° C. to 340° C., more preferably 323° C. to 335° C. The melting temperature here means the melting temperature of the anisotropic pitch in the storage area before being discharged from the spinning nozzle (i.e., the temperature at the introduction section 70 in FIG. 9).

[0065] Compared with the pitch-based carbon fiber precursor of embodiment 1, the pitch-based carbon fiber precursor of embodiment 2 is less susceptible to thermal shrinkage during the graphitization step. Specifically, in the case of the embodiment 1, the mesophase undergoes thermal shrinkage in the circumferential direction of the cross section, i.e., the thermal shrinkage occurs so as to reduce the angle between mesophase molecules, and therefore the thermal shrinkage tends to be greater at the outer periphery of the cross section. In contrast, in the case of the embodiment 2, the mesophase undergoes thermal shrinkage so as to reduce the distance between mesophase planes, and therefore the thermal shrinkage tends to be uniform overall.

[0066] From the above, the pitch-based carbon fiber precursor of embodiment 2 can reduce the effect of thermal shrinkage compared to the pitch-based carbon fiber precursor of embodiment 1, and therefore can more easily reduce the defect number density. Therefore, the pitch-based carbon fiber obtained from the pitch-based carbon fiber precursor of embodiment 2 (for example, the pitch-based carbon fiber shown in Fig. 2 and Fig. 3) can more easily improve the tensile strength compared to the pitch-based carbon fiber obtained from the pitch-based carbon fiber precursor of embodiment 1 (for example, the pitch-based carbon fiber shown in Fig. 4).

[0067] The aspect ratio of the discharge holes is not particularly limited as long as it is greater than 1, but is preferably 1.5 to 10.0. By setting the aspect ratio to 1.5 or more, it becomes easier to obtain the pitch-based carbon fiber precursor of embodiment 2. Furthermore, by setting the aspect ratio to 10.0 or less, it becomes easier to mold the pitch-based carbon fiber precursor. The aspect ratio is more preferably 2.0 to 7.5, and even more preferably 2.5 to 5.0. Furthermore, from the viewpoint of further reducing the defect density and further increasing the tensile strength, the aspect ratio is even more preferably 3.0 to 5.0.

[0068] The shape of the discharge hole is not particularly limited, but is preferably rectangular or elliptical, which changes the fluidity of the anisotropic pitch and makes it easier to obtain the pitch-based carbon fiber precursor of the second embodiment.

[0069] When the ejection hole is rectangular, the circumscribing rectangle that circumscribes the shape of the ejection hole coincides with the ejection hole. Furthermore, when the ejection hole is rectangular, the corners of the rectangle may have curvature. Furthermore, when the ejection hole is elliptical, the short side of the circumscribing rectangle that circumscribes the shape of the ejection hole coincides with the minor axis of the ellipse, and the long side of the circumscribing rectangle coincides with the major axis of the ellipse.

[0070] The configuration of the spinning nozzle is not particularly limited except as described above, but will be further described below with specific examples.

[0071] The spinning nozzle 200 shown in Fig. 9 includes an outlet hole 50 and an introduction portion for introducing molten anisotropic pitch. As shown in Fig. 10, the outlet hole 50 has a rectangular shape with a short side a and a long side b (aspect ratio: b / a).

[0072] Regarding the introduction section 70, the shape of the cross section perpendicular to the longitudinal direction of the spinning nozzle 200 (hereinafter sometimes referred to as the "cross section of the introduction section") is not particularly limited, and may be, for example, round, oval, quadrilateral (square, rectangle, etc.), etc.

[0073] The cross-sectional area of ​​the introduction section is preferably 10 to 2000 times the area of ​​the discharge hole 50. By making the cross-sectional area of ​​the introduction section 10 times or more the area of ​​the discharge hole 50, the pressure during spinning does not become too high, thereby suppressing fine dispersion of the anisotropic pitch and making it easier to obtain a high elastic modulus. Furthermore, by making the cross-sectional area of ​​the introduction section 2000 times or less the area of ​​the discharge hole 50, it becomes easier to improve spinnability. The cross-sectional area of ​​the introduction part is more preferably 30 to 500 times, and even more preferably 40 to 200 times, the area of ​​the discharge hole 50 .

[0074] The taper angle θ of the tapered section 90 shown in Fig. 9 affects the fluidity of the anisotropic pitch in the spinning nozzle, and therefore also influences the formation of the structure. Although it depends on the cross-sectional area of ​​the introduction section, the taper angle θ is preferably 60° to 180°. This makes it easier to obtain the pitch-based carbon fiber precursor of aspect 2. Also, from the viewpoint of extruding properties, it is preferable that the taper angle θ be in the above range. The taper angle θ is more preferably 90° to 150°.

[0075] The land length L shown in Figure 9 is preferably such that the pressure loss calculated by the Hagen-Poiseuille equation is 0.5 MPa to 5.0 MPa. By setting the pressure loss to 0.5 MPa or more, it becomes easier to improve spinnability. Furthermore, by setting the pressure loss to 5.0 MPa or less, it becomes easier to suppress the fine dispersion of the anisotropic pitch and obtain a high elastic modulus.

[0076] The spinning nozzle may be provided with a flow straightening plate upstream of the discharge hole.

[0077] The anisotropic pitch is melted by heating to a temperature 10°C to 50°C higher than the melting point, but preferably at 370°C or lower, which makes it easier to prevent clogging of the discharge holes due to carbonization of the anisotropic pitch.

[0078] The spinning pressure is preferably 1.0 MPa to 3.0 MPa. By setting the spinning pressure to 1.0 MPa or more, it becomes easier to improve spinnability. Also, by setting the spinning pressure to 3.0 MPa or less, it becomes easier to develop the physical properties of the pitch-based carbon fiber, particularly the tensile modulus.

[0079] From the viewpoint of more easily achieving a desired diameter, the extrusion rate of the anisotropic pitch is preferably 0.03 g / min to 0.01 g / min, and the spinning speed of the pitch-based carbon fiber precursor is preferably 200 m / min to 1000 m / min. Furthermore, from the viewpoint of more easily achieving a cross-sectional area ratio of the region occupied by graphene sheets oriented at an orientation angle of 60° to 120° relative to the boundary plane in the diameter direction or the longitudinal direction of the pitch-based carbon fiber of 60% or more relative to the cross-sectional area, the spinning speed is more preferably 200 m / min to 500 m / min.

[0080] [Infusible process] In the infusibilizing step, the pitch-based carbon fiber precursor obtained in the melt spinning step is subjected to a heat treatment in the presence of oxygen, which prevents the heat-treated pitch-based carbon fiber precursor from melting in a subsequent heat treatment.

[0081] The atmosphere for the heat treatment may be selected appropriately from among, for example, an air atmosphere, an oxidizing gas atmosphere, a mixed gas atmosphere (for example, a mixed gas of nitrogen, oxygen, and an oxidizing gas), and the like.

[0082] The heat treatment temperature may be, for example, 120°C to 320°C. The heat treatment may also be carried out while increasing the temperature within a range of 120°C to 320°C. The heat treatment temperature is preferably 150°C to 300°C. The heat treatment time can be set arbitrarily within the range of 30 minutes to 12 hours, and is preferably within the range of 1 hour to 3 hours.

[0083] [Carbonization process] In the carbonization step, the pitch-based carbon fiber precursor that has been heat-treated in the infusibilization step is carbonized by heat treatment in an inert atmosphere.

[0084] As the inert gas, for example, nitrogen, argon, etc. can be used. The temperature of the heat treatment may be, for example, 400° C. to 1000° C. The heat treatment may be carried out while increasing the temperature within the range of 400° C. to 1000° C. The heat treatment time can be set arbitrarily between 5 minutes and 1 hour.

[0085] [Graphitization process] In the graphitization step, the pitch-based carbon fiber precursor carbonized in the carbonization step is graphitized by heat treatment in an inert atmosphere.

[0086] As the inert gas, for example, nitrogen, argon, etc. can be used. The heat treatment temperature may be, for example, 2000°C to 3200°C. The heat treatment time may be 1 minute to 1 hour. If the heat treatment temperature in the graphitization step is too high, equipment and operating costs will increase, and stricter measures and management will be required to prevent the intrusion of external air. Therefore, the heat treatment temperature is preferably 2900°C or less, more preferably 2800°C or less, even more preferably 2500°C or less, and even more preferably 2400°C or less. In the method for producing pitch-based carbon fiber according to the present disclosure, pitch-based carbon fiber with a higher tensile modulus can be produced, as compared with the production methods of the prior art such as Patent Documents 2 and 3, provided that the heat treatment temperature in the graphitization step is the same. In other words, in the method for producing pitch-based carbon fiber according to the present disclosure, the heat treatment temperature in the graphitization step can be set lower than that in the prior art such as Patent Documents 2 and 3, when producing pitch-based carbon fiber with the same level of tensile modulus.

[0087] Through the above steps, pitch-based carbon fibers can be obtained.

[0088] <Fiber reinforced plastic> The fiber reinforced plastic according to the present disclosure includes the pitch-based carbon fiber according to the present disclosure.

[0089] The method for producing fiber-reinforced plastics is not particularly limited, and known production methods may be used. For example, a fiber-reinforced plastic may be obtained by impregnating a pitch-based carbon fiber with a resin to produce a prepreg, and then subjecting the prepreg to molding processing such as PCM (Pre-preg Compression Molding). Furthermore, for example, a fiber-reinforced plastic may be obtained by using a pitch-based carbon fiber and a resin in a sheet molding compound (SMC) method or a resin transfer molding (RTM) method. s A fiber-reinforced plastic may be obtained by subjecting the plastic to a molding process such as fiber molding.

[0090] Examples of the resin include a thermosetting resin and a thermoplastic resin.

[0091] Examples of thermosetting resins include epoxy resins, unsaturated polyester resins, vinyl ester resins, bismaleimide resins, phenolic resins, cyanate resins, and polyimides.

[0092] Examples of thermoplastic resins include nylon, polypropylene, polyphenylene sulfide, polyetherimide, polycarbonate, polyethylene terephthalate, and polyether ether ketone.

[0093] The fiber reinforced plastic according to the present disclosure contains the pitch-based carbon fiber according to the present disclosure, and therefore has excellent strength and can be suitably used in a variety of applications. [Example]

[0094] The present disclosure will be described in more detail below with reference to examples, although the present disclosure is not limited to these examples.

[0095] <Production of pitch-based carbon fiber> [Example 1] A melt spinning process was carried out using anisotropic pitch (melting point: 302.1°C) with the spinning nozzle (discharge hole shape: rectangular, aspect ratio: 2.5, taper angle: 120°) shown in Figures 9 and 10 to obtain a pitch-based carbon fiber precursor. The melting temperature (near the discharge hole) of the molten anisotropic pitch was 330°C, and the viscosity was 40 Pa s. The discharge rate of the anisotropic pitch was 0.035 g / min to 0.090 g / min, the spinning pressure was 2 MPa to 3 MPa, and the spinning speed was 350 m / min.

[0096] The obtained pitch-based carbon fiber precursor was subjected to an infusibilization process (in a mixed gas atmosphere of nitrogen, oxygen, and oxidizing gas), a carbonization process, and a graphitization process (heat treatment temperature 2000°C) to obtain the pitch-based carbon fiber of Example 1.

[0097] [Example 2] Pitch-based carbon fibers of Example 2 were obtained in the same manner as in Example 1, except that the anisotropic pitch was melted at a melting temperature of 325°C to give a viscosity of 75 Pa·s.

[0098] [Example 3] The pitch-based carbon fiber of Example 3 was obtained in the same manner as in Example 1, except that the aspect ratio of the spinning nozzle was changed to 5.0.

[0099] [Comparative Example 1] The pitch-based carbon fiber of Comparative Example 1 was obtained in the same manner as in Example 1, except that the shape of the discharge hole of the spinning nozzle was circular (aspect ratio: 1.0) and the discharge rate of the anisotropic pitch was 0.035 g / min to 0.040 g / min.

[0100] Comparative Example 2 Pitch-based carbon fibers of Comparative Example 2 were obtained in the same manner as in Example 1, except that the anisotropic pitch was melted at a melting temperature of 320°C to give a viscosity of 120 Pa·s.

[0101] Comparative Example 3 Pitch-based carbon fibers of Comparative Example 3 were obtained in the same manner as in Example 1, except that the anisotropic pitch was melted at a melting temperature of 310°C to give a viscosity of 150 Pa·s.

[0102] <Evaluation of pitch-based carbon fiber> The cross-sectional shape, flatness, defect density, tensile strength, tensile modulus, and area ratio of the specific region of the pitch-based carbon fiber were measured as follows. The results are shown in Table 1. In Table 1, "area ratio" means the area ratio of the specific region.

[0103] [Cross-sectional shape, flatness] The cross-sectional shape and flatness of the pitch-based carbon fiber were measured in accordance with JIS R 7606:2000 using a laser shape measuring device, with the measurement range set to 0° to 180°, by rotating the cross-section of the pitch-based carbon fiber in 10° increments. The laser shape measuring device used was a "Laser Scan Micrometer LSM-500S" manufactured by Mitutoyo Corporation.

[0104] [Defect density] Thirty to fifty pitch-based carbon fibers (single fibers) with a length of 25.0 mm were prepared, and a tensile test was carried out in accordance with JIS R 7606:2000 as described above. The defect density (λ av ) was measured.

[0105] [Tensile strength] The average value of the tensile strength obtained in the tensile test when creating the Weibull plot was taken as the tensile strength of the pitch-based carbon fiber. The tensile strength improvement rate was calculated based on Comparative Example 1, and pitch-based carbon fibers with a tensile strength improvement rate of 130% or more were considered to be acceptable.

[0106] Tensile modulus The average value of the tensile modulus obtained in the tensile test when creating the Weibull plot was taken as the tensile strength of the pitch-based carbon fiber.

[0107] [Ratio of area of ​​specific area] Randomly selected cross sections of the pitch-based carbon fiber were observed perpendicular to the cross section using a scanning electron microscope (SEM), model JEOL Ltd.'s JSM-6500F, to obtain SEM images. The conditions for acquiring the SEM image were a magnification of 10,000 times and an acceleration voltage of 5 kV. The SEM images were analyzed using the image processing software "ImageJ (vl.52)." Specifically, the SEM images were binarized to obtain a binarized image of the cross section, and the binarized image was then edited to depict the black areas as line segments, obtaining an edited image of the cross section. The edited image was evaluated for the linearity of each line segment in the cross section using fractal dimension analysis, and line segments with D = 1.2 or less were identified. Line segments with D = 1.2 or less were defined as "graphene sheets." The line segment that gives the maximum length of the cross-sectional contour in the edited image (i.e., the maximum value of the straight line connecting any two points on the contour, which is the diameter if the cross-section is circular, or the major axis if the cross-section is elliptical) was defined as the "boundary surface." The edited image was analyzed using the previously described program in "Python (v3)" (open source), and the orientation angle of each graphene sheet (line segment with D = 1.2 or less) relative to the boundary plane was calculated over the entire cross section. The angle at the intersection of the line including the boundary plane and the line including the graphene sheet was taken as the orientation angle. Using "ImageJ (vl.52)," the area (specific area) occupied by graphene sheets oriented at an orientation angle of 60° to 120° (line segments with D = 1.2 or less and an orientation angle of 0° to 120°) was displayed in black, and the specific area in the cross section was identified. Using "ImageJ (vl.52)," the area of ​​the specific area and the area of ​​the cross section were calculated. The area of ​​the specific area was divided by the area of ​​the cross section to calculate the ratio of the area of ​​the specific area. The same measurement was performed five times on the same cross section of the pitch-based carbon fiber as above, and the average value of the area ratios of the specific regions obtained from the five measurements was taken as the area ratio of the specific region of the pitch-based carbon fiber.

[0108] [Table 1]

[0109] The pitch-based carbon fibers of Examples 1 to 3 had low defect density and excellent tensile strength, as shown in Table 1. In particular, the pitch-based carbon fiber of Example 3 had an especially low defect density and a tensile strength exceeding 4000 MPa.

[0110] On the other hand, in Comparative Example 1, a spinning nozzle having an outlet hole with an aspect ratio of 1 was used, and therefore the pitch-based carbon fiber had a simple radial structure. As a result, the effect of thermal shrinkage during the graphitization process became greater, resulting in a high defect density and low tensile strength.

[0111] In Comparative Example 2, melt spinning was performed using an anisotropic pitch with a high viscosity of 120 Pa s, resulting in a high defect density and low tensile strength. Furthermore, in Comparative Example 2, the tensile modulus also tended to decrease.

[0112] In Comparative Example 3, the melt spinning was performed using an anisotropic pitch with a high viscosity of 150 Pa s, resulting in a high defect density and low tensile strength. Furthermore, in Comparative Example 3, the tensile modulus also tended to decrease.

[0113] The disclosure of Japanese Application No. 2021-093111 is incorporated herein by reference in its entirety. All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference. [Explanation of symbols]

[0114] 10 Graphene sheets 12 Straight lines containing graphene sheets 30, 32 Boundary 34 center 36 Lines including boundary surfaces 100, 102, 104 Pitch-based carbon fiber 50 Spinning nozzle outlet hole 70 Introduction 90 Tapered section α orientation angle L Land length θ Taper angle a Short side of the discharge hole b Long side of discharge hole 200 Spinning nozzle

Claims

1. A pitch-based carbon fiber having a defect density of 40 defects / m or less, a cross-sectional shape perpendicular to the longitudinal direction of the fiber being an ellipse with a flattening ratio of 0.25 or less, and having a boundary surface in the diameter direction or the long axis direction of the elliptical cross-section.

2. 2. The pitch-based carbon fiber according to claim 1, wherein the ratio of the area occupied by the graphene sheets oriented at an orientation angle of 60° to 120° relative to the boundary plane is 60% or more relative to the area of ​​the cross section.

3. 2. The pitch-based carbon fiber according to claim 1, wherein the cross-section has an elliptical shape with a flattening ratio of 0.2 or less.

4. The pitch-based carbon fiber according to any one of claims 1 to 3, wherein the defect number density is 30 defects / m or less.

5. The pitch-based carbon fiber according to any one of claims 1 to 3, having a tensile modulus of 400 GPa or more.

6. 6. The pitch-based carbon fiber according to claim 5, wherein the tensile modulus is 440 GPa or more.

7. A method for producing the pitch-based carbon fiber of claim 1, comprising: The method includes a melt spinning step of discharging molten anisotropic pitch from a spinning nozzle and spinning it into fibers, the spinning nozzle has an introduction portion for introducing the molten anisotropic pitch, a discharge port, and a tapered portion connecting the introduction portion and the discharge port, The taper angle of the tapered portion is 90° to 150°, The ratio of the length of the long side of a circumscribing rectangle circumscribing the shape of the discharge hole of the spinning nozzle to the length of the short side of the circumscribing rectangle is greater than 1, The viscosity of the molten anisotropic pitch is 20 Pa s to 80 Pa s; A method for producing pitch-based carbon fiber.

8. The method for producing pitch-based carbon fibers according to claim 7, wherein the ratio is 1.5 to 10.

0.

9. The method for producing pitch-based carbon fibers according to claim 7, wherein the shape of the discharge holes is rectangular or elliptical.

10. The method for producing pitch-based carbon fibers according to claim 7, wherein the viscosity of the anisotropic pitch is 30 Pa·s to 60 Pa·s.

11. The method for producing pitch-based carbon fibers according to claim 7, wherein the spinning speed in the melt spinning step is 200 m / min to 500 m / min.

12. a graphitization step of graphitizing the carbonized pitch-based carbon fiber precursor by heat treatment after the melt spinning step, The method for producing pitch-based carbon fibers according to any one of claims 7 to 11, wherein the temperature of the heat treatment in the graphitization step is 2000 ° C to 2900 ° C.

13. A fiber-reinforced plastic comprising the pitch-based carbon fiber according to any one of claims 1 to 3.

Images