Carbon fiber and method for producing the same

Abstract

To provide a carbon fiber having superior mechanical characteristics specific to the carbon fiber, and convergence properties and openable properties at high balance.SOLUTION: A carbon fiber is made of plural monofilaments, in which an average value of long axis / short axis ratio in a cross-section of monofilament is 1.00-1.15, a variation coefficient of the long axis / short axis ratio is 3-15%, and skewness of the long axis / short axis ratio is 1.2-4.0.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to carbon fibers that achieve both mechanical properties and high-order processability, and a method for manufacturing the same.

Background Art

[0002] Carbon fibers that have a low specific gravity and excellent mechanical properties have been increasingly used in recent years. The properties required for carbon fibers vary depending on the application, but achieving both high mechanical properties and high-order processability at a high level is generally an important point regardless of the application. With the prediction that the use of carbon fibers will further increase in the future, a higher level is considered to be required in achieving both mechanical properties and high-order processability. Since carbon fibers are composed of a large number of extremely fine single fibers, in the process of high-order processing to obtain carbon fiber reinforced composite materials, handleability, spreadability, and process passability are often emphasized. The spreadability of carbon fibers (sometimes called fiber opening property) is particularly important in that it also affects the mechanical properties of the obtained carbon fiber reinforced composite material.

[0003] Among several types of carbon fibers, polyacrylonitrile-based carbon fibers, which have the largest industrial usage amount, are industrially manufactured through a flame resistance conversion process of converting polyacrylonitrile-based carbon fiber precursor fibers into flame-resistant fibers in an oxidizing atmosphere at 200 to 300 °C and a carbonization process of carbonizing in an inert atmosphere at 300 to 2000 °C. The obtained polyacrylonitrile-based carbon fibers are often provided with a sizing agent for the purpose of enhancing high-order processability. Since the sizing agent also affects the spreadability, various sizing agents are used according to the application. On the other hand, since organic compounds are often used for the sizing agent and may be thermally decomposed during molding processing, considering the development for a wider range of applications, controlling the spreadability by means other than the sizing agent is one of the preferred means.

[0004] As factors other than sizing agents that affect spreadability, there are known examples of studies focusing on the cross-sectional shape of single carbon fibers. For example, in Patent Document 1, in order to achieve high levels of both convergence and fiber-opening properties, it is proposed to use carbon fibers in which single fibers with different cross-sectional shapes are mixed at a certain ratio. Also, in Patent Document 2, in order to enhance the fiber-opening property, it is proposed to mix single fibers with different cross-sectional shapes at a certain ratio. Further, although not mentioned in relation to the fiber-opening property, carbon fibers including single fibers having an oval cross-section at a certain ratio have been proposed in Patent Document 3 as the cross-sectional shape of single carbon fibers.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, the conventional technologies have the following problems.

[0007] In Patent Documents 1 and 2, although an effect of enhancing the fiber-opening property is seen when running on a metal roll under specific conditions, there are problems with the balance with the convergence property.

[0008] In Patent Document 3, although there is an advantage that it is easy to confirm the cause of breakage due to the oval cross-section of the single fiber, the compatibility between the convergence property and the fiber-opening property is not described, and in reality, these properties were not compatible at a high level.

[0009] To summarize the above, in the prior art, there is no description of a method that can achieve both high convergence and fibrillating properties while maintaining the high mechanical properties of carbon fibers, and the acquisition of a method for achieving both at a high level has been an issue.

Means for Solving the Problem

[0010] In order to achieve the above object, the carbon fiber of the present invention is a carbon fiber composed of a plurality of single fibers, and the average value of the major axis to minor axis ratio of the single fiber cross-section is 1.00 to 1.15, the coefficient of variation is 3 to 15%, and the skewness is 1.2 to 4.0.

Effect of the Invention

[0011] The carbon fiber of the present invention is a carbon fiber that combines excellent mechanical properties peculiar to carbon fibers with high balance of convergence and fibrillating properties. By using the carbon fiber of the present invention, a high-performance carbon fiber reinforced composite material can be obtained with high productivity.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Embodiment for Carrying Out the Invention

[0013] The carbon fiber of the present invention is a carbon fiber composed of a plurality of single fibers, wherein the average value of the major axis to minor axis ratio of the single fiber cross-section is 1.00 to 1.15, the coefficient of variation is 3 to 15%, and the skewness is 1.2 to 4.0. In the present invention, the single fiber cross-section refers to the cross-section of the single fiber perpendicular to the fiber axis. Further, in the present invention, the major axis to minor axis ratio refers to the ratio obtained by dividing the major axis by the minor axis in the single fiber cross-section. Further, in the present invention, the major axis refers to the line segment connecting the two farthest points in the single fiber cross-section, and the minor axis refers to the minor axis of the ellipse having the same cross-sectional area as the single fiber cross-section with such major axis as the major axis. The average value is a simple average without weighting, etc., and the coefficient of variation is calculated by dividing the standard deviation by the average value and multiplying by 100 (%) as per the general definition. Skewness is a parameter representing the asymmetry of the distribution and is defined by the following formula (1).

[0014] Skewness = n / (n - 1) / (n - 2)×Σ{(x i - <x>) / s} 3 ···(1) Here, n is the number of single fibers (pieces), x i is the major axis to minor axis ratio (-) of the i-th single fiber, <x>The symbol (-) represents the average value of the major axis to minor axis ratio, and s represents the standard deviation of the major axis to minor axis ratio. Also, Σ means taking the sum for the number n of single fibers. In the frequency distribution of any parameter of interest, a skewness value of 0 indicates a symmetric distribution, a negative value indicates a tail extending to the smaller side, and a positive value indicates a tail extending to the larger side. For example, when the number n of single fibers is 3 and the major axis to minor axis ratios of each are 1.0, 1.5, and 2.0, the average value is 1.5 and the standard deviation s is 0.5. Therefore, according to Equation (1), the skewness of the major axis to minor axis ratio is 0. Also, when the major axis to minor axis ratios of the 3 single fibers are 1.0, 1.0, and 2.0, according to Equation (1), the skewness of the major axis to minor axis ratio is 1.73, representing a distribution shape with a tail extending to the larger side of the values.

[0015] In the present invention, the specific evaluation method of the major axis to minor axis ratio is not particularly limited as long as it is evaluated according to the above definition. For example, it can be evaluated as follows. First, align a bundle of carbon fibers, apply scissors perpendicular to the fiber axis, cut, and observe the cut surface with a scanning electron microscope. Load the obtained scanning electron microscope image into image analysis software or the like, and trace the contour of the single fiber cross-section. From the traced contour of the single fiber cross-section, calculate the major axis to minor axis ratio according to the above definition using the functions of the image analysis software or the like. In image analysis software, the major axis to minor axis ratio may sometimes be referred to as the aspect ratio, but it is read as the major axis to minor axis ratio as long as it is calculated according to the above definition. If there is a foreign object overlapping on the single fiber cross-section and such foreign object seems to affect the identification of the contour of the single fiber cross-section, skip such a single fiber cross-section and use another single fiber cross-section. In the present invention, in order to ensure accuracy, the number of single fiber cross-sections for calculating the major axis to minor axis ratio is set to 50 or more, and in the bundle of carbon fibers, observe the single fiber cross-sections existing at as many different locations as possible. Next, each requirement will be explained in order.

[0016] In the carbon fiber of the present invention, the average value of the major axis to minor axis ratio is 1.00 to 1.15, preferably 1.03 to 1.13, and more preferably 1.05 to 1.13. The closer the average value of the major axis to minor axis ratio is to 1.00, the more single fibers with a single fiber cross-section close to a perfect circle. The larger the average value of the major axis to minor axis ratio, the more likely the tensile strength (hereinafter sometimes simply abbreviated as strand strength) and the convergence property in the resin impregnated strand test will decrease. However, if it is controlled within the above range, the strand strength and the convergence property will be at a practically acceptable level. Generally known methods for controlling the average value of the major axis to minor axis ratio of the single fiber cross-section include changing the shape of the die hole and changing the coagulation bath conditions.

[0017] In the carbon fiber of the present invention, the coefficient of variation of the major axis to minor axis ratio is 3 to 15%, preferably 5 to 15%, more preferably 3 to 10%, and even more preferably 5 to 8%. A large coefficient of variation of the major axis to minor axis ratio means that the major axis to minor axis ratio is widely distributed, which is an advantageous direction for fiber opening. However, if it is too large, the strand strength and the convergence property are likely to decrease. By controlling the coefficient of variation of the major axis to minor axis ratio within the above range, it is easy to achieve both high levels of strand strength, convergence property, and fiber opening property. In many common carbon fibers, the major axis to minor axis ratio is often determined by the coagulation bath conditions and the value is not likely to vary, so such a coefficient of variation is less than 3%. The method for controlling the coefficient of variation of the major axis to minor axis ratio of the single fiber cross-section will be described later.

[0018] In the carbon fiber of the present invention, the skewness of the major axis to minor axis ratio is 1.2 to 4.0, preferably 1.4 to 3.0, more preferably 1.5 to 2.5, and even more preferably 1.6 to 2.0. As a result of the study by the present inventors, in order to achieve both high strand strength and high convergence and fibrillation properties of carbon fibers, simply increasing the average value of the major axis to minor axis ratio, controlling the coefficient of variation of the major axis to minor axis ratio, or simply mixing single fibers with a large major axis to minor axis ratio at a certain ratio is insufficient. It has been found that it is important to control the skewness within the above range. When the skewness is 1.2 or more, it means that the distribution shape of the major axis to minor axis ratio has a certain amount of tail on the side with a large major axis to minor axis ratio. Although the reason for the increase in fibrillation properties by controlling the skewness in this way is not clear, it can be qualitatively considered as follows. That is, a state with a large skewness of the major axis to minor axis ratio means that, by definition, although there is a certain amount of single fibers with a large major axis to minor axis ratio, the average value of the major axis to minor axis ratio is maintained at a lower level. The smaller the major axis to minor axis ratio, the better the packing between single fibers, so the convergence property is likely to increase. Conversely, the larger the major axis to minor axis ratio, the more likely the fibrillation property is to increase. The balance between the convergence and fibrillation properties of carbon fibers is considered to be strongly affected by the proportion of single fibers with different major axis to minor axis ratios. However, the effect of improving the fibrillation property by single fibers with a large major axis to minor axis ratio is relatively larger compared to the effect of improving the convergence property by single fibers with a small major axis to minor axis ratio. Adding a small amount of single fibers with a large major axis to minor axis ratio is considered important to maximize the effect of improving the fibrillation property without reducing the convergence property. When the skewness of the major axis to minor axis ratio is 1.2 or more, it is possible to achieve both high levels of convergence and fibrillation properties. When the skewness of the major axis to minor axis ratio is 4.0 or less, the convergence property can be within a satisfactory range. In the present invention, for the method of controlling the coefficient of variation and skewness of the major axis to minor axis ratio within the above range, for example, two or more types of carbon fiber precursor fibers, or flame-resistant fibers, or pre-carbonized fibers, or carbon fibers with different average values of the major axis to minor axis ratio may be mixed at an appropriate ratio. Alternatively, the easily deformable precursor fibers or flame-resistant fibers may be lightly crushed and deformed with a nip roller or the like while in a bundled state. Similarly, the precursor fibers or flame-resistant fibers may be twisted and tension may be applied to generate a pressing force between single fibers and deformed.By observing and finely adjusting the cross-section of a single fiber of the obtained carbon fiber, carbon fibers satisfying the numerical range of the present invention described above can be obtained.

[0019] In the carbon fiber of the present invention, the number of single fibers having two or more recessed portions in the cross-section of a single fiber is preferably 2 to 20%, more preferably 3 to 10%. Here, the recessed portion refers to a portion that is recessed by a certain amount or more in the fiber center direction in the contour of the cross-section of a single fiber. Here, being recessed by a certain amount or more refers to a recess having a curvature radius of 2 μm or more, and wrinkles on the surface of a single fiber are not regarded as recessed portions. An example of a single fiber having two or more recessed portions is shown in FIG. 1. Since single fibers having two or more recessed portions have an irregular shape with lower symmetry compared to circular, oval, or kidney bean shapes, it is easy to reduce the packing between single fibers and enhance the fiber-opening property. When the content of single fibers having two or more recessed portions is 2% or more, it is easy to effectively enhance the fiber-opening property. If the number of single fibers having two or more recessed portions in the cross-section of a single fiber is preferably included at 20% or less, it is easy to achieve both the fiber-opening property and the convergence property. Single fibers having two or more recessed portions can be deformed by lightly pressing and deforming the easily deformable precursor fiber or flame-resistant fiber in a bundled state with a nip roller or the like, or by applying twist to the precursor fiber or flame-resistant fiber to generate a pressing force between the single fibers and causing deformation, so that they can be generated at a certain ratio.

[0020] In the carbon fiber of the present invention, single fibers having a hollow kidney-shaped cross-section of a single fiber are preferably contained in an amount of 3 to 15%, more preferably 3 to 11%, and still more preferably 3 to 6%. An example of a single fiber having a hollow kidney-shaped cross-section of a single fiber is shown in FIG. 2. The hollow kidney shape is a shape in which one of the semi-circles with a large elliptical curvature, also called the β shape, has a dent. The dent here is a dent of 0.7 μm or more from the outer periphery assuming an ellipse having the same cross-sectional area as the cross-section of the single fiber. Since the single fiber having a hollow kidney shape has a higher symmetry than a single fiber having two or more dents, it is difficult to reduce the packing between single fibers, and at the same content, it is more difficult to enhance the fibrillation property than a single fiber having two or more dents. When 3% or more of the single fibers having a hollow kidney shape are contained, it is easy to enhance the fibrillation property at the same degree of strain. When 15% or less of the single fibers having a hollow kidney shape are contained, it is easy to enhance the convergence property and the fibrillation property at the same degree of strain. The single fiber having a hollow kidney shape can be most typically obtained by setting the coagulation bath conditions to known conditions. Further, even if the precursor fiber or the flame-retardant fiber is partially deformed, it may generate only one dent without generating two or more dents. The content of the single fiber having a hollow kidney shape can be controlled by adjusting the mixing ratio of the single fiber having a hollow kidney shape and other single fibers, or by changing the above-described deformation pressing force.

[0021] In the carbon fiber of the present invention, single fibers having a major axis to minor axis ratio of 1.20 or more are preferably contained in an amount of 3 to 20%, more preferably 4 to 15%, and still more preferably 5 to 10%. The larger the major axis to minor axis ratio, the easier it is to enhance the fibrillation property, and if it is 3% or more, the fibrillation property is often satisfactory, but if it is too large, the convergence property and the strand strength may decrease. When 20% or less of the single fibers having a major axis to minor axis ratio of 1.20 or more are contained, it is easy to balance these properties at a high level. The single fibers having a major axis to minor axis ratio of 1.20 or more can be obtained by adjusting the coagulation conditions or by crushing the single fibers in a direction perpendicular to the fiber axis.

[0022] In the carbon fiber of the present invention, single fibers having an aspect ratio (major axis / minor axis) of 1.00 to 1.03 are preferably contained in an amount of 15 to 90%, more preferably 30 to 85%, and still more preferably 60 to 80%. The higher the aspect ratio, the easier it is to improve the fibrillation property. However, if the content of single fibers having an aspect ratio of 1.00 to 1.03 is less than 15%, the convergence property and strand strength may decrease. When the content of single fibers having an aspect ratio of 1.00 to 1.03 is 15% or more, it is easy to achieve both of these properties at a high level. When the content of single fibers having an aspect ratio of 1.00 to 1.03 is 90% or less, that is, when the content of single fibers having an aspect ratio exceeding 1.03 is 10% or more, it is easy to obtain the effect of improving the fibrillation property. Single fibers having an aspect ratio of 1.00 to 1.03 can be obtained by adjusting the coagulation conditions or adjusting the force for crushing the single fibers in a direction perpendicular to the fiber axis. To control the content ratio of single fibers having an aspect ratio of 1.00 to 1.03, the same method as for adjusting the degree of strain can be used.

[0023] In the carbon fiber of the present invention, the circularity 4πA / L defined by the cross-sectional area A and the perimeter L 2 has an average value preferably of 0.970 to 1.000, more preferably 0.980 to 1.000. In the present invention, the cross-sectional area A refers to the area of the portion surrounded by the contour of the single fiber cross-section, and the perimeter L refers to the length of the contour of the single fiber cross-section. Since the units of the cross-sectional area A and the perimeter L cancel each other out when calculating the circularity, they may be in pixels or in actual lengths such as μm. The value of the circularity becomes 1.000 in the case of a perfect circle, and becomes a smaller value as it deviates from the perfect circle. If the average value of such circularity is too small, the strand strength and convergence property may decrease, but if controlled within the above range, these can be maintained at a high level. Regarding the method of controlling the average value of the circularity within the above range, it is an easy method to adjust the coagulation bath conditions.

[0024] In the carbon fiber of the present invention, the circularity 4πA / L 2 The coefficient of variation is preferably 1 to 3%, more preferably 1 to 2%. If the coefficient of variation of the circularity is small, the packing between single fibers may be improved and it may be difficult to enhance the fibrillating property. Conversely, if it is large, it may be difficult to enhance the strand strength and the convergence property. By controlling the coefficient of variation of the circularity within the above range, it is easy to achieve both of these properties at a high level. Regarding the method of controlling the coefficient of variation of the circularity within the above range, for example, two or more types of carbon fiber precursor fibers having different average values of circularity, or flame-resistant fibers, or pre-carbonized fibers, or carbon fibers may be mixed at an appropriate ratio, or the precursor fibers or flame-resistant fibers that are easily deformed may be lightly crushed and deformed with a nip roller or the like while in a bundle state, or similarly, the precursor fibers or flame-resistant fibers may be twisted and tension may be applied to generate a pressing force between single fibers and cause deformation. By observing the cross-section of the single fiber of the obtained carbon fiber and finely adjusting it so as to satisfy the numerical range of the present invention, the coefficient of variation can be easily controlled.

[0025] In the carbon fiber of the present invention, when the single fiber is observed within a range of a linear distance of 1 mm from the side surface, it is preferable that the fluctuation width of the fiber axis of the single fiber is 2.5 μm or more. The measurement of the fluctuation width in the present invention is performed by observing the single fiber of the carbon fiber from a direction orthogonal to the fiber axis direction in an environment where no stress other than gravity is applied. In the case of a fiber having three-dimensional fluctuations, the fiber axis direction and the orthogonal direction are defined as follows. In the projection image of the single fiber of the carbon fiber placed on the horizontal plane onto the horizontal plane, a straight line connecting two points separated by 1000 μm is defined as the virtual fiber axis at the observation location, and the vertical direction is defined as the direction orthogonal to the fiber axis direction. That is, the fluctuation width is approximately measured in the projection image. As shown in FIG. 3, the fluctuation width is obtained by arbitrarily selecting the center in the thickness direction of the observed single fiber as point A, and the center in the thickness direction of the single fiber separated by a linear distance of 1 mm from there as point B. When point A is the origin in the XY coordinate system, that is, the point where X = 0 μm and Y = 0 μm, and point B is the point on the X axis, that is, X = 1000 μm and Y = 0 μm, among the Y coordinate values through which the center in the thickness direction of the single fiber passes, it is defined as the residual ΔY (μm) obtained by subtracting the minimum value Ymin (μm) from the maximum value Ymax (μm). The measurement of the fluctuation width is performed on 10 randomly extracted independent single fibers, and the average value is adopted. In the prior art of carbon fibers, no particular attention has been paid to the fluctuation width. However, when the inventors measured, the fluctuation width in commercially available carbon fibers was generally less than 2 μm, and particularly often less than 1 μm. The fluctuation width is more preferably 3 μm or more, still more preferably 4 μm or more, and particularly preferably 5 μm or more. The larger such a fluctuation width is, the more difficult it is to pack between single fibers, and the easier it is to increase the fiber-opening property. The fluctuation width can be controlled by imparting bending to the fiber in the flame-retardant treatment step, the pre-carbonization treatment step, and the carbonization treatment step described later. In particular, it is preferable from the viewpoint of ease of imparting bending to impart bending to the fiber in the carbonization treatment step where the treatment temperature is the highest. As a method of imparting bending, known methods such as twisting the fiber or knitting the fibers together in a three-ply or four-ply shape like a braid can be adopted. Among them, in particular, it is preferable from an industrial viewpoint to adopt twisting that can be handled with simple equipment.

[0026] In the carbon fiber of the present invention, the strand elastic modulus is preferably 300 GPa or more, more preferably 340 GPa or more, and even more preferably 360 GPa or more. In the present invention, the strand elastic modulus can be evaluated according to the tensile test of the resin-impregnated strand described in JIS R7608:2004. Generally, the higher the strand elastic modulus, the more advantageous it is to increase the rigidity when forming a carbon fiber reinforced composite material. However, the carbon fiber tends to be brittle, and in the process of high-order processing, fluffing or the like may occur, and it may be wound around rollers or guides, tending to deteriorate the process passability. Since the carbon fiber of the present invention has high fibrillating property, in the carbon fiber with a high strand elastic modulus, it is easy to fibrillate even with a slight force, and it is easy to balance the strand elastic modulus and the process passability. In particular, when the strand elastic modulus is increased, it has high industrial utility. The strand elastic modulus can be controlled by known methods.

[0027] In the carbon fiber of the present invention, the strand strength is preferably 4.0 GPa or more, and more preferably 4.5 GPa. In the present invention, the strand strength can be evaluated according to the tensile test of the resin-impregnated strand described in JIS R7608:2004. Generally, the higher the strand strength, the more advantageous it is to increase the tensile strength when forming a carbon fiber reinforced composite material. However, when trying to increase the fibrillating property and mixing single fibers with a non-circular cross-sectional shape, if not properly controlled, the strand strength may decrease. The carbon fiber of the present invention can achieve both high fibrillating property and high strand strength at a high level.

[0028] Hereinafter, the manufacturing method of the carbon fiber of the present invention will be described.

[0029] The carbon fiber precursor fiber serving as the basis of the carbon fiber of the present invention can be obtained by spinning a spinning solution of a polyacrylonitrile copolymer.

[0030] As the polyacrylonitrile copolymer, not only a homopolymer obtained only from acrylonitrile but also other monomers may be used in addition to acrylonitrile as the main component. Specifically, the polyacrylonitrile copolymer preferably contains 90 to 100% by mass of acrylonitrile and less than 10% by mass of copolymerizable monomers.

[0031] The above-mentioned polyacrylonitrile copolymer is dissolved in a solvent in which the polyacrylonitrile copolymer is soluble, such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, nitric acid, aqueous zinc chloride solution, aqueous rhodansoda solution, etc., to obtain a spinning solution.

[0032] By spinning the obtained spinning solution by a wet or dry-wet spinning method, carbon fiber precursor fibers can be produced.

[0033] The spinning solution is introduced into a coagulation bath for coagulation, and the obtained coagulated fibers are passed through a water washing step, an in-bath stretching step, an oil application step, and a drying step to obtain carbon fiber precursor fibers. At this time, it is known that the cross-sectional shape changes under the coagulation conditions. When the concentration of the solvent in the coagulation bath is low at 40% by mass or less and high around 80% by mass, the cross-section is circular, and when the concentration is intermediate, the cross-section is hollow bean-shaped. The coagulated fibers may be directly stretched in the bath by omitting the water washing step, or may be stretched in the bath after removing the solvent by the water washing step. The in-bath stretching is usually preferably carried out in a single or multiple stretching baths whose temperature is controlled to 30 to 98°C. Also, a dry heat stretching step or a steam stretching step may be added to the above steps.

[0034] The obtained carbon fiber precursor fibers are usually in the form of continuous fibers. Also, the number of filaments per yarn is preferably 1,000 to 80,000. In the present invention, the carbon fiber precursor fibers may be combined as necessary to adjust the number of filaments per yarn of the obtained carbon fiber.

[0035] One of the preferred embodiments for obtaining the carbon fibers of the present invention is to mix precursor fibers having a major axis to minor axis ratio of 1.00 to 1.03 and precursor fibers having a major axis to minor axis ratio of 1.20 or more at a number ratio of 8:1 to 2:1. At this time, the carbon fibers of the present invention cannot be obtained only by setting intermediate conditions such that circular and elliptical cross-sections are mixed as the coagulation bath conditions, and it is preferable to mix precursor fibers having different cross-sectional shapes.

[0036] The second preferred embodiment for obtaining the carbon fibers of the present invention is to deform the obtained carbon fiber precursor fibers by lightly pressing them with a nip roller or the like, or to apply a twist to generate a pressing force between the single fibers to deform them. As a method of applying a twist to the carbon fiber precursor fibers, those known in the art can be selected. Specifically, after once winding the carbon fiber precursor fibers around a bobbin, when unwinding the fibers, the bobbin is rotated on a plane perpendicular to the unwinding direction of the bobbin, or a rotating roller or belt is brought into contact with the fibers running without being wound around the bobbin to apply a twist, and the like can be used for control.

[0037] The carbon fibers of the present invention can be obtained by performing a flame retardant treatment on the above-described carbon fiber precursor fibers, and then performing a pre-carbonization treatment and a carbonization treatment in this order.

[0038] The flame retardant treatment of the carbon fiber precursor fibers is preferably carried out in an air atmosphere at a temperature range of 200 to 300°C. The carbon fiber precursor fibers are subjected to a flame retardant treatment and become flame retardant fibers. In order to obtain the carbon fibers of the present invention, it is also preferable to deform the fibers during the flame retardant treatment by lightly pressing them with a nip roller or the like, or to apply a twist to generate a pressing force between the single fibers to deform them. When applying a twist to the fibers during the flame retardant treatment, the twist angle is 4.0 ° or more is preferably i . In the present invention, the twist angle in the flame retardant treatment is calculated using the basis weight y (g / m) and density d (g / cm 3 ) of the precursor fibers used, and the twist number T (turns / m). te Calculate.

[0039] As a method of adding twist to the fiber during the flame resistance treatment, it can be selected from known ones. Specifically, after once winding the carbon fiber precursor fiber around a bobbin, when unwinding the fiber, the bobbin is rotated on a plane orthogonal to the unwinding direction of the bobbin, or a rotating roller or belt is brought into contact with the fiber running without being wound around the bobbin to apply twist, and the like, and it can be controlled.

[0040] When adding twist during the flame resistance treatment, the tension on the flame resistant fiber is preferably 1.0 to 5.0 mN / dtex. The tension in the flame resistance process is obtained by dividing the tension (mN) measured on the inlet side of the flame resistance furnace by the total fineness (dtex), which is the product of the single fiber fineness (dtex) of the carbon fiber precursor fiber used and the number of filaments. By controlling the tension within the above numerical range, it becomes easy to impart a dent to the carbon fiber.

[0041] In the present invention, subsequent to the flame resistance, pre-carbonization of the flame resistant fiber is performed. In the pre-carbonization step, the flame resistant fiber obtained by the flame resistance treatment is heat-treated in an inert atmosphere at a maximum temperature of 500 to 1200 °C until it reaches a density of 1.5 to 1.8 g / cm 3 ³. It is preferable that the flame resistant fiber is pre-carbonized to become a pre-carbonized fiber.

[0042] Furthermore, following the pre-carbonization, carbonization of the pre-carbonized fibers is carried out. In the carbonization process, the pre-carbonized fibers obtained by the pre-carbonization treatment are carbonized in an inert atmosphere. The maximum temperature of the carbonization treatment is preferably 1200 °C or higher, and more preferably 1500 °C or higher. From the viewpoint of increasing the strand elastic modulus of the obtained carbon fibers, a higher maximum temperature in the carbonization process is preferable. When the maximum temperature is 1200 °C or higher, carbon fibers with a high strand elastic modulus, which are suitable for applications that emphasize rigidity as a carbon fiber reinforced composite material, can be obtained. On the other hand, if the carbonization temperature is too high, the strand strength will decrease, the carbon fibers will tend to be brittle, fluffing or the like will occur in the process of high-order processing, and it will easily wrap around rollers or guides, deteriorating the process passability. Therefore, the maximum temperature in the carbonization process is preferably determined in consideration of the balance among the required strand elastic modulus, strand strength, and process passability.

[0043] In the present invention, the tension in the carbonization process is preferably 5 to 15 mN / dtex, and more preferably 7 to 15 mN / dtex. The tension in the carbonization process is defined as the tension (mN) measured on the outlet side of the carbonization furnace divided by the total fineness (dtex), which is the product of the single fiber fineness (dtex) of the carbon fiber precursor fiber used and the number of filaments. By controlling the tension within the above numerical range, even if the strand elastic modulus of the obtained carbon fibers is increased, it is easy to maintain the strand strength and process passability at a high level.

[0044] In the present invention, examples of the inert gas used in the inert atmosphere preferably include nitrogen, argon, and xenon, and nitrogen is preferably used from an economic perspective.

[0045] The carbon fibers obtained by the above manufacturing method may be further subjected to additional graphitization treatment in an inert atmosphere up to a maximum of 3000 °C, and the single fiber elastic modulus may be appropriately adjusted according to the application.

[0046] The carbon fibers obtained as described above are preferably surface-treated after carbonization to introduce functional groups containing oxygen atoms in order to improve the adhesion strength between the carbon fibers and the matrix. As the surface treatment method, gas-phase oxidation, liquid-phase oxidation, and liquid-phase electrolytic oxidation are used. From the viewpoint of high productivity and uniform treatment, liquid-phase electrolytic oxidation is preferably used. In the present invention, there are no particular restrictions on the method of liquid-phase electrolytic oxidation, and a known method may be used.

[0047] The measurement methods for various physical property values described in this specification are as follows. In addition, those not particularly described were evaluated with the number of measurements n = 1.

[0048] <Strand strength and strand elastic modulus of carbon fiber> The strand strength and strand elastic modulus of carbon fibers are determined according to the following procedure in accordance with the resin-impregnated strand test method of JIS R7608:2004. However, when the carbon fiber has twists, it is untwisted by applying twists in the reverse direction equal to the number of twists before evaluation. As the resin formulation, "Celloxide (registered trademark)" 2021P (manufactured by Daicel Chemical Industries, Ltd.) / boron trifluoride monoethylamine (manufactured by Tokyo Chemical Industry Co., Ltd.) / acetone = 100 / 3 / 4 (parts by mass) is used, and as the curing conditions, normal pressure, a temperature of 125°C, and a time of 30 minutes are used. Ten strands of carbon fiber are measured, and the average value is taken as the strand strength and strand elastic modulus. In addition, the strain range for calculating the strand elastic modulus is set to 0.1 to 0.6%.

[0049] <Statistical quantities of major axis to minor axis ratio and circularity> The major axis to minor axis ratio of a single carbon fiber, its average value, coefficient of variation, and skewness are determined as follows. First, align the carbon fiber bundles and wrap them with carbon tape so as to enclose them in a cylindrical shape. In this state, cut by applying scissors perpendicular to the fiber axis to expose the single fiber cross-section. Observe such a single fiber cross-section using a scanning electron microscope and save it as an image. Load the saved image into the open-source image analysis software "ImageJ", and trace the contour of the single fiber cross-section using the "Polygon selections" tool. At this time, for one contour, trace one round with 20 to 100 points. Subsequently, use the "Fit spline" tool to convert the traced contour of the single fiber cross-section into a smooth curve. After conversion, finely adjust by moving the points so that the contour of the single fiber cross-section and the traced curve match better. Subsequently, use the "Analyze particles" tool to calculate the "AR (aspect ratio)" and "Circularity". Note that the aspect ratio refers to the major axis to minor axis ratio in the present invention. Perform the same operation on 50 single fibers. At this time, select the single fibers evenly from different parts of the carbon fiber bundles. Calculate the average value (-) and coefficient of variation (%) of the major axis to minor axis ratio, skewness (-), ratio (%) of 1.00 to 1.03, and ratio (%) of 1.20 or more. For circularity, calculate the average value (-) and coefficient of variation (%). Note that the skewness of the major axis to minor axis ratio is calculated according to the following formula (1).

[0050] Skewness = n / (n - 1) / (n - 2)×Σ{(x i - <x>) / s} 3 ···(1) Here, n is the number of single fibers (pieces), x i is the major axis to minor axis ratio (-) of the i-th single fiber, <x>represents the average value (-) of the major axis to minor axis ratio, and s represents the standard deviation (-) of the major axis to minor axis ratio. Also, Σ means taking the sum for the number n of single fibers.

[0051] In the following examples, a scanning electron microscope "S-4800" manufactured by Hitachi High-Technologies Corporation was used as the scanning electron microscope, and the observation was carried out with an acceleration voltage of 5 keV.

[0052] <Ratio of single fibers having two or more recessed portions and hollow bean-shaped single fibers> For 50 single fibers for which the major axis to minor axis ratio and roundness were calculated, they were discriminated from the obtained scanning electron microscope images, and the ratio of single fibers having each cross-sectional shape was determined. The ratio is expressed as a percentage of the number of single fibers corresponding to each to the number of observed single fibers. Typical examples of single fibers having two or more recessed portions are shown in FIG. 1, and typical examples of hollow bean-shaped single fibers are shown in FIG. 2, respectively. For either single fibers having two or more recessed portions or hollow bean-shaped single fibers, if only 1 single fiber corresponding to each was found among the 50 observed single fibers, 50 more single fibers were added for observation for statistical reasons, and the ratio was calculated.

[0053] <Fluctuation width of the fiber axis of the carbon fiber> The single fiber of carbon fiber to be measured is set to a length of 1 to 5 mm and placed statically on copy paper laid on a horizontal table. If the single fiber adheres to the copy paper due to the influence of static electricity, it is performed after removing the static electricity by a general method. Observe from the vertical direction of the paper surface using an optical microscope and acquire an image. The magnification of the objective lens of the optical microscope is set to 10 times. The image is saved in the jpg format of 2592 pixels horizontally × 1944 pixels vertically. At this time, when imaging a scale of 1000 μm in actual size, set the imaging range so that the scale corresponds to 2320 to 2340 pixels. Load the acquired image into the open-source image processing software "ImageJ (Image J)", set an arbitrary point on the fiber axis as point A, and set a point on the fiber axis 1000 μm away from point A as point B. Next, select "Bilinear Interpolation" as the interpolation algorithm during rotation, and rotate the image so that points A and B are horizontal. After performing binarization processing, perform skeletonization to extract the fiber axis as a curve with a width of 1 pixel. At this time, if dust or the like adheres to the fiber surface, the fiber axis may branch, but side chains other than the fiber axis are ignored. Finally, among the Y coordinates through which the fiber axis passes between point A and point B, the residual ΔY (μm) obtained by subtracting the minimum value Y max from the maximum value Y min is read and taken as the fluctuation width of the measured single fiber. The average of the fluctuation widths measured for 10 different single fibers is adopted as the fluctuation width in the present invention. In addition, the coefficient of variation of the fluctuation width is obtained by the following formula using the standard deviation calculated from the data measured for 10 different single fibers.

[0054] CV value (%) = standard deviation of fluctuation width (μm) / average value of fluctuation width (μm) × 100 (%).

[0055] In this example, an upright microscope "DM2700M" manufactured by Leica Microsystems GmbH was used as the optical microscope.

[0056] <Twist angle in the flame retardant treatment> The twist angle in the flame retardant treatment is the basis weight y (g / m) and density d (g / cm of the precursor fiber used 3 ) Use the twist number T (turns / m) te for calculation.

[0057] <Areal density of the precursor fiber> For the fiber to be measured, sample 8 m in length, dry it to absolute dryness, and then divide the measured mass by 8 to obtain the areal density, which is the mass per 1 m. The measurement is performed 3 times, and the average value is used.

[0058] <Density of the precursor fiber> For the fiber to be measured, sample 1 m, and measure it by the Archimedes method using ethanol as the specific gravity liquid. Perform the measurement 3 times, and use the average value.

[0059] <Convergence> Convergence is evaluated without the sizing agent attached. If the sizing agent is attached, remove it by burning off the sizing agent in an oven or washing it in a solvent before evaluation. First, sample 1 m of the fiber to be measured and fix both ends so that the fiber is straight. At this time, make sure that it does not contact anything other than the fixed both ends and is held in the air. After bringing one end 1 cm closer to the other end, gently blow air near the center of the fiber, and use the convergence of the yarn width as an index of convergence. Note that the convergence is judged from A to D based on the yarn width expansion ratio of the carbon fiber. A: Less than 20% B: 20% or more and less than 40% C: 40% or more and less than 50% D: 50% or more.

[0060] <Fibrillation property> The fibrillation property is evaluated without attaching a sizing agent. If the sizing agent is attached, it should be removed by burning off the sizing agent in an oven or washing it in a solvent before evaluation. Sample 2 cm of carbon fiber and make it untwisted. Place the carbon fiber on a 10 cm square glass plate and cover it with a slide glass from above. Move it alternately 3 mm to the left and right 10 times in the direction perpendicular to the axial direction of the carbon fiber. Measure the change in yarn width before and after this operation, and use the average value of 10 repetitions as an index of fibrillation property. The larger the ratio of the yarn width expansion, the better the fibrillation property. Note that the fibrillation property is judged from A to D based on the yarn width expansion ratio of the carbon fiber. A: 50% or more B: 40% or more and less than 50% C: 20% or more and less than 40% D: Less than 20%.

Example

[0061] Hereinafter, the present invention will be described in detail based on examples, but the present invention is not limited thereto.

[0062] Examples 1 to 10 and Comparative Examples 1 to 4 described below were carried out using the respective conditions described in Table 1 in the implementation methods described in the following Comprehensive Example 1 or 2.

[0063] [Comprehensive Example 1] A polyacrylonitrile copolymer was polymerized by a solution polymerization method using dimethyl sulfoxide as a solvent to obtain a spinning solution. The obtained spinning solution was once discharged from a spinneret into the air and introduced into a coagulation bath composed of an aqueous solution of 35% by mass of dimethyl sulfoxide maintained at 0 °C by a dry-wet spinning method to obtain a coagulated yarn. After washing the coagulated yarn with water, it was stretched in warm water at 90 °C at a bath draw ratio of 3 times, and further a silicone oil agent was applied, and drying was performed using a roller heated to a temperature of 160 °C, and pressurized steam stretching was performed at a draw ratio of 4 times to obtain a carbon fiber precursor fiber with a single fiber fineness of 1.1 dtex.

[0064] [Comprehensive Example 2] A polyacrylonitrile copolymer was polymerized by a solution polymerization method using dimethyl sulfoxide as a solvent to obtain a spinning solution. The obtained spinning solution was introduced into a coagulation bath composed of an aqueous solution of 70% by mass of dimethyl sulfoxide maintained at 50 °C from a spinneret to obtain a coagulated yarn by a wet spinning method. Further, after washing the coagulated yarn with water, it was drawn in warm water at 90 °C at a bath draw ratio of 6 times, and further a silicone oil agent was applied, and drying was performed using a roller heated to a temperature of 160 °C to obtain a carbon fiber precursor fiber having a single fiber fineness of 1.1 dtex.

[0065] [Example 1] 21,000 precursor fibers obtained according to Comprehensive Example 1 and 3,000 precursor fibers obtained according to Comprehensive Example 2 were subjected to a blending treatment by blowing air to blend them, and precursor fibers having a total filament number of 24,000 were obtained. Next, the obtained polyacrylonitrile-based carbon fiber precursor fibers were heat-treated in an oven at 230 to 280 °C in an air atmosphere at a tension of 1.0 mN / dtex and an elongation ratio of 1 (flame retardant step) to be converted into flame retardant fibers. The obtained flame retardant fibers were subjected to a pre-carbonization treatment at a temperature of 300 to 800 °C in a nitrogen atmosphere at an elongation ratio of 0.97 to obtain pre-carbonized fibers. Then, such pre-carbonized fibers were carbonized at a maximum temperature of 1800 °C and a tension of 5.0 mN / dtex to obtain carbon fibers. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0066] [Example 2] Carbon fibers were obtained in the same manner as in Example 1, except that 18,000 precursor fibers obtained according to Comprehensive Example 1 and 6,000 precursor fibers obtained according to Comprehensive Example 2 were blended. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0067] [Example 3] Carbon fibers were obtained in the same manner as in Example 1, except that 15,000 precursor fibers obtained according to Comprehensive Example 1 and 9,000 precursor fibers obtained according to Comprehensive Example 2 were blended. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0068] [Example 4] A bobbin wound with 24,000 precursor fibers obtained in Example 1 including was rotated, and while adding twist so that the twist angle became 4.6 degrees, it was heat-treated (flame-retardant process) in an oven with an air atmosphere of 230 to 280 °C at a tension of 1.0 mN / dtex and an elongation ratio of 0.9, and converted into flame-retardant fibers. The obtained flame-retardant fibers were pre-carbonized at an elongation ratio of 0.97 in a nitrogen atmosphere at a temperature of 300 to 800 °C to obtain pre-carbonized fibers. Next, such pre-carbonized fibers were carbonized at a maximum temperature of 1800 °C and a tension of 11.0 mN / dtex to obtain carbon fibers. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0069] [Example 5] Carbon fibers were obtained in the same manner as in Example 4, except that the twist angle was 7.6 degrees. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0070] [Example 6] Carbon fibers were obtained in the same manner as in Example 4, except that the number of filaments of the precursor fibers used was 12,000 and the twist angle was 6.9 degrees. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0071] [Example 7] In Example 1 including, carbon fibers were obtained in the same manner as in Example 4, except that the single fiber fineness of the carbon fiber precursor fibers was 0.8 dtex. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0072] [Example 8] Carbon fibers were obtained in the same manner as in Example 4, except that the maximum temperature of the carbonization treatment was 2400 °C. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0073] [Example 9] Carbon fibers were obtained in the same manner as in Example 4, except that the tension in the carbonization process was 5.0 mN / dtex. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0074] [Example 10] 24,000 precursor fibers obtained according to Comprehensive Example 1 were passed through a nip device consisting of a metal drum and a rubber roll set at 180°C with a nip pressure of 1.0 MPa, and then carbon fibers were obtained in the same manner as in Example 4 except that no twist was added to the precursor fibers. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0075] [Example 11] Carbon fibers were obtained in the same manner as in Example 4 except that the tension in the flameproofing process was 2.0 mN / dtex. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0076] [Example 12] A bobbin wound with 24,000 precursor fibers obtained according to Comprehensive Example 1 was rotated, and while adding twist so that the twist angle was 7.6 degrees, it was heat-treated (flameproofing process) in an oven with an air atmosphere of 230 to 280°C with a draw ratio of 0.8 to be converted into flameproofing fibers. The obtained flameproofing fibers were pre-carbonized in a nitrogen atmosphere at a temperature of 300 to 800°C with a draw ratio of 1.1 to obtain pre-carbonized fibers. Next, such pre-carbonized fibers were carbonized at a maximum temperature of 1800°C and a tension of 11.0 mN / dtex to obtain carbon fibers. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0077] [Comparative Example 1] Carbon fibers were obtained in the same manner as in Example 1 except that 9,000 precursor fibers obtained according to Comprehensive Example 1 and 15,000 precursor fibers obtained according to Comprehensive Example 2 were mixed. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0078] [Comparative Example 2] Carbon fibers were obtained in the same manner as in Example 1 except that 6,000 precursor fibers obtained according to Comprehensive Example 1 and 18,000 precursor fibers obtained according to Comprehensive Example 2 were mixed. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0079] [Comparative Example 3] Carbon fibers were obtained in the same manner as in Example 4, except that no twist was added to the precursor fibers and the tension in the carbonization step was 7.0 mN / dtex. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0080] [Comparative Example 4] Carbon fibers were obtained in the same manner as in Example 4, except that the twist angle was 1.5 degrees. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0081] [Comparative Example 5] Carbon fibers were obtained in the same manner as in Example 4, except that the tension in the flame retardant treatment step was 0.5 mN / dtex. The evaluation results of the obtained carbon fibers are shown in Table 1.

[0082] [Reference Example 1] The evaluation results of "TORAYCA (registered trademark)" M40J manufactured by Toray Industries, Inc. are shown in Table 1.

[0083]

Table 1

Industrial Applicability

[0084] The carbon fibers of the present invention have excellent mechanical properties peculiar to carbon fibers and have a high balance between convergence and fiber opening properties. By using the carbon fibers of the present invention, a high-performance carbon fiber reinforced composite material can be obtained with high productivity.< / x> < / x> < / x> < / x>

Claims

1. Carbon fibers composed of multiple single fibers, wherein the average value of the major axis to minor axis ratio of the single fiber cross-section is 1.03 to 1.15, the coefficient of variation is 3 to 15%, the skewness is 1.2 to 4.0, and the strand elastic modulus is 300 GPa or more.

2. The carbon fiber according to Claim 1, containing 2 to 20% of single fibers having two or more recessed portions in the single fiber cross-section.

3. The carbon fiber according to Claim 1 or 2, containing 3 to 15% of single fibers having a hollow kidney-shaped single fiber cross-section.

4. The carbon fiber according to any one of Claims 1 to 3, wherein the skewness is 1.5 or more.

5. The carbon fiber according to any one of Claims 1 to 4, containing 3 to 20% of single fibers having a major axis to minor axis ratio of 1.20 or more.

6. The carbon fiber according to any one of Claims 1 to 5, containing 15 to 90% of single fibers having a major axis to minor axis ratio of 1.00 to 1.

03.

7. Circularity 4πA / L defined by cross-sectional area A and perimeter L ２ The carbon fiber according to any one of claims 1 to 6, wherein the average value is 0.970 to 1.000 and the coefficient of variation is 1 to 3%.

8. The carbon fiber according to any one of Claims 1 to 7, wherein when observing the single fiber within a range of a linear distance of 1 mm from the side surface, the fluctuation width of the fiber axis of the single fiber is 2.5 μm or more.

9. The carbon fiber according to any one of Claims 1 to 8, wherein the skewness is 1.25 to 4.

0.

10. The carbon fiber according to any one of Claims 1 to 9, wherein the strand strength is 4.0 GPa or more.

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