JP2021059829A - Carbon fiber and production method of the same - Google Patents

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

The present invention relates to a carbon fiber having both mechanical properties and higher workability and a method for producing the carbon fiber.

In recent years, the range of use of carbon fiber, which has a low specific gravity and excellent mechanical properties, has been expanding. The properties required for carbon fiber differ depending on the application, but it is an important point to achieve both mechanical properties and high-order workability at a high level, regardless of the application. As the use of carbon fiber is expected to increase further in the future, it is expected that even higher levels will be required to achieve both mechanical properties and higher workability. Since carbon fibers are a collection of a large number of extremely fine single fibers, handleability, spreadability, process passability, etc. are often emphasized in the process of higher-order processing for obtaining a carbon fiber reinforced composite material. The spreadability of carbon fibers (sometimes called openness) is particularly important in the sense that it also affects the mechanical properties of the resulting carbon fiber reinforced composite.

Among several types of carbon fibers, polyacrylonitrile-based carbon fibers, which are used most industrially, convert polyacrylonitrile-based carbon fiber precursor fibers into flame-resistant fibers in an oxidizing atmosphere at 200 to 300 ° C. It is industrially manufactured through a flameproofing step and a carbonization step of carbonizing in an inert atmosphere at 300 to 2000 ° C. A sizing agent is often added to the obtained polyacrylonitrile-based carbon fiber for the purpose of enhancing higher workability. Since the sizing agent also affects the spreadability, various sizing agents according to the application are used. On the other hand, organic compounds are often used as sizing agents, and they may be thermally decomposed during molding. Therefore, considering the development to a wider range of applications, it is preferable to control the spreadability by means other than the sizing agent. It is one of the means.

In addition to the sizing agent, as an element that affects the spreadability, an example of study focusing on the cross-sectional shape of a single carbon fiber is known. For example, Patent Document 1 proposes carbon fibers in which single fibers having different cross-sectional shapes are mixed at a constant ratio in order to achieve high levels of both convergence and openness. Further, Patent Document 2 proposes mixing single fibers having different cross-sectional shapes at a constant ratio in order to enhance the fibrous opening property. Further, although the openness is not mentioned, as a cross-sectional shape of the single fiber of the carbon fiber, a carbon fiber containing a single fiber having an oval cross section of a certain ratio has been proposed in Patent Document 3.

Japanese Unexamined Patent Publication No. 2012-188766 Japanese Unexamined Patent Publication No. 2016-30869 International Publication No. WO2015 / 016199

However, the conventional technique has the following problems.

In Patent Documents 1 and 2, although the effect of increasing the fiber opening property is observed when the metal roll is run under specific conditions, there is a problem in the balance with the convergence.

Patent Document 3 has an advantage that the cause of fracture can be easily confirmed because the cross section of the single fiber is oval, but it does not describe both convergence and openness, and actually, these characteristics are described. It was not compatible at a high level.

In summary, the conventional technology does not describe a method for achieving both convergence and openness at a high level while maintaining the high mechanical properties of carbon fibers, and a method for achieving both at a high level. Acquisition was an issue.

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 / minor axis ratio of the cross section of the single fiber is 1.00 to 1.15. It is a carbon fiber having a coefficient of variation of 3 to 15% and a skewness of 1.2 to 4.0.

The carbon fiber of the present invention is a carbon fiber having excellent mechanical properties peculiar to carbon fiber and a high balance of convergence and openness. By using the carbon fiber of the present invention, a high-performance carbon fiber reinforced composite material can be obtained with high productivity.

FIG. 1 is a schematic view of a single fiber having two or more recessed portions in the cross section of the single fiber. FIG. 2 is a schematic view of a single fiber having a broad bean-shaped cross section. FIG. 3 is a schematic view showing a method for evaluating the fluctuation width of the fiber shaft.

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 / minor axis ratio of the cross section of the single fiber is 1.00 to 1.15, the coefficient of variation is 3 to 15%, and the skewness. It is a carbon fiber having a degree of 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 / minor axis ratio refers to a ratio obtained by dividing the major axis in the cross section of a single fiber by the minor axis. Further, in the present invention, the major axis refers to a line segment connecting the two most distant points in the cross section of the single fiber, and the minor axis has the major axis as the major axis and has the same cross-sectional area as the cross section of the single fiber. Refers to the minor axis of the ellipse. The average value is a simple average without weighting, and the coefficient of variation is calculated by dividing the standard deviation by the average value and multiplying by 100 (%) as defined in the general definition. Skewness is a parameter representing the asymmetry of the distribution and is defined by the following equation (1).

Skewness = n / (n-1) / (n-2) × Σ {(x _i − <x>) / s} ³ ... (1)
Here, n is the number of single fibers (lines), x _i is the major axis / minor axis ratio (-) of the i-th single fiber, <x> is the average value of the major axis / minor axis ratio (-), and s is the major axis / minor axis. It means the standard deviation (-) of the ratio. Further, Σ means that the sum is taken by the number n of single fibers. In the frequency distribution of any parameter of interest, a skewness value of 0 indicates that the distribution is symmetrical, a negative value indicates that the base is drawn on the smaller side, and a positive value indicates that the base is drawn on the larger side. For example, when the number n of single fibers is 3 and the major axis / minor axis ratios are 1.0, 1.5, and 2.0, the average value is 1.5 and the standard deviation s is 0.5. , The skewness of the major axis minor axis ratio becomes 0 according to the equation (1). When the major axis / minor axis ratios of the three single fibers are 1.0, 1.0, and 2.0, the skewness of the major axis / minor axis ratio is 1.73 according to the equation (1), and the value is larger. It is expressed that the distribution shape has a base.

In the present invention, the specific evaluation method of the major axis / minor axis ratio is not particularly limited as long as it is evaluated according to the above definition, but it can be evaluated as follows, for example. First, a bundle of carbon fibers is aligned, a scissor is applied so as to be perpendicular to the fiber axis, the fiber is cut, and the cut surface is observed with a scanning electron microscope. The acquired scanning electron microscope image is read into image analysis software or the like, and the contour of the cross section of the single fiber is traced. From the contour of the traced single fiber cross section, the major axis / minor axis ratio according to the above definition is calculated by using the function of the image analysis software or the like. In image analysis software, the major axis / minor axis ratio is sometimes called the aspect ratio, but it should be read as the major axis / minor axis ratio as long as it is calculated according to the above definition. When a foreign matter overlaps the single fiber cross section, if the foreign matter affects the identification of the contour of the single fiber cross section, the single fiber cross section is skipped and another single fiber cross section is used. In the present invention, in order to guarantee the accuracy, the number of single fiber cross sections for which the major axis / minor axis ratio is calculated is 50 or more, and the single fiber cross sections existing in different places in the carbon fiber bundle are observed. I decided to. Next, each requirement will be described in order.

In the carbon fiber of the present invention, the average value of the major axis / 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 / minor axis ratio is to 1.00, the more single fibers have a single fiber cross section close to a perfect circle. The larger the average value of the major axis / minor axis ratio, the lower the tensile strength (hereinafter, may be simply abbreviated as strand strength) and convergence in the resin impregnated strand test, but if controlled within the above range, the strand strength is likely to decrease. And the convergence is at a level where there is no problem in practical use. As a method of controlling the average value of the major axis / minor axis ratio of the cross section of a single fiber within the above range, it is generally known that the shape of the base hole is changed and the coagulation bath conditions are changed.

In the carbon fiber of the present invention, the coefficient of variation of the major axis / minor axis ratio is 3 to 15%, preferably 5 to 15%, more preferably 3 to 10%, and further preferably 5 to 8%. .. A large coefficient of variation of the major axis / minor axis ratio means that the major axis / minor axis ratio is widely distributed, which is advantageous for fiber opening, but if it is too large, the strand strength and convergence Is likely to decrease. If the coefficient of variation of the major axis / minor axis ratio is controlled within the above range, it is easy to achieve both strand strength, convergence, and fibrosis at a high level. In many general carbon fibers, the major axis / minor axis ratio is often determined by the coagulation bath conditions, and the value is unlikely to vary, so that the coefficient of variation is less than 3%. A method for controlling the coefficient of variation of the major axis / minor axis ratio of the cross section of a single fiber within the above range will be described later.

In the carbon fiber of the present invention, the skewness of the major axis / minor axis ratio is 1.2 to 4.0, preferably 1.4 to 3.0, and more preferably 1.5 to 2.5. More preferably, it is 1.6 to 2.0. As a result of the examination by the present inventors, in order to achieve both the strand strength of the carbon fiber, the convergence, and the openness at a high level, simply increase the average value of the major axis / minor axis ratio or change the major axis / minor axis ratio. It was found that it is not enough to control the coefficient or simply mix single fibers having a large major axis / minor axis ratio at a constant ratio, and 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 / minor axis ratio has a hem of a certain amount or more on the side where the major axis / minor axis ratio is large. Although the reason why the fibrous opening property is enhanced by controlling the skewness in this way is not clear, it is qualitatively considered as follows. That is, the state in which the skewness of the major axis / minor axis ratio is large means that, by definition, there is a certain amount of single fibers having a large major axis / minor axis ratio, but the average value of the major axis / minor axis ratio remains low. .. The smaller the major axis / minor axis ratio, the better the packing between single fibers, so that the convergence tends to increase, and the larger the major axis / minor axis ratio, the more likely it is that the fiber opening property increases. It is considered that the balance between the convergence and openness of carbon fibers is strongly influenced by the abundance ratio of single fibers having different major axis / minor axis ratios. It is relatively large compared to the effect of improving the convergence by the single fiber having a small major axis / minor axis ratio, and adding a small amount of the single fiber having a large major axis / minor axis ratio opens the fiber without lowering the convergence. It is considered to be important for maximizing the effect of improving sex. When the skewness of the major axis / minor axis ratio is 1.2 or more, both convergence and openness can be achieved at a high level. When the skewness of the major axis / minor axis ratio is 4.0 or less, the convergence can be satisfied. In the present invention, as for the method of controlling the fluctuation coefficient and strain degree of the major axis / minor axis ratio within the above range, for example, two or more types of carbon fiber precursor fibers or flame resistant fibers having different average values of the major axis / minor axis ratio. Alternatively, the pre-carbonized fibers or carbon fibers may be mixed in an appropriate ratio, or the easily deformable precursor fibers or flame-resistant fibers may be lightly crushed with a nip roller or the like in a bundled state to be deformed. The precursor fibers or flame-resistant fibers may be twisted and tensioned to generate a pressing force between the single fibers to deform the fibers. By observing and fine-tuning the cross section of the obtained single fiber of the carbon fiber, a carbon fiber satisfying the above-mentioned numerical range of the present invention can be obtained.

In the carbon fiber of the present invention, the number of single fibers having two or more recessed portions in the cross section of the single fiber is preferably 2 to 20%, more preferably 3 to 10%. Here, the recessed portion refers to a portion of the contour of the cross section of the single fiber that is recessed by a certain amount or more in the fiber center direction. Here, a dent of a certain amount or more means a dent having a radius of curvature of 2 μm or more, and a wrinkle on the surface of a single fiber is not regarded as a dent. FIG. 1 shows an example of a single fiber having two or more recessed portions. Single fibers having two or more recesses have a distorted shape with low symmetry compared to circular, oval, and broad bean shapes, so it is easy to reduce the packing between single fibers, and the fibers are open. Can be enhanced. When a single fiber having two or more recessed portions is contained in an amount of 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 the single fiber is preferably 20% or less, it becomes easy to achieve both openness and convergence. For a single fiber having two or more recesses, the easily deformable precursor fiber or flame-resistant fiber is lightly crushed with a nip roller or the like in a bundled state to be deformed, or the precursor fiber or flame-resistant fiber is also twisted. By applying tension to generate a pressing force between the single fibers to deform them, it can be generated at a constant ratio.

In the carbon fiber of the present invention, a single fiber having a broad bean-shaped cross section is preferably contained in an amount of 3 to 15%, more preferably 3 to 11%, still more preferably 3 to 6%. FIG. 2 shows an example of a single fiber having a broad bean-shaped cross section. The broad bean shape is a shape in which one of the semicircles with a large curvature, which is a substantially elliptical shape, has a dent, as is also called a β shape. The dent here is an ellipse having the same cross-sectional area as the cross section of the single fiber, and is dented by 0.7 μm or more from the outer circumference thereof. Since the broad bean-shaped single fiber has a shape with higher symmetry than the single fiber having two or more recesses, it is difficult to reduce the packing between the single fibers, and if the content is the same, two places. It is more difficult to improve the openness than a single fiber having the above recessed portion. When broad bean-shaped single fibers are contained in an amount of 3% or more, it is easy to increase the openness at the same time at the same skewness. When broad bean-shaped single fibers are contained in an amount of 15% or less, it is easy to improve convergence and openness at the same time at the same skewness. Broad bean-shaped monofibers can most typically be obtained by setting coagulation bath conditions as known conditions. Further, even if the precursor fiber or the flame-resistant fiber is partially deformed, it may be generated at only one place without forming two or more dents. The content of broad bean-shaped monofibers can be controlled by adjusting the mixing ratio of broad bean-shaped monofibers and other monofibers, or by changing the above-mentioned deformation pressing force.

In the carbon fiber of the present invention, a single fiber having a major axis / minor axis ratio of 1.20 or more is preferably contained in an amount of 3 to 20%, more preferably 4 to 15%, still more preferably 5 to 10%. The larger the major axis-minor-diameter ratio, the easier it is for the fiber-opening property to increase. If it is 3% or more, the fiber-opening property is often satisfied. When 20% or less of single fibers having a major axis / minor axis ratio of 1.20 or more are contained, it is easy to achieve both of these characteristics at a high level. Single fibers having a major axis / minor axis ratio of 1.20 or more can be obtained by adjusting solidification conditions or by crushing the single fibers in a direction orthogonal to the fiber axis.

In the carbon fiber of the present invention, a single fiber having a major axis / minor axis ratio of 1.00 to 1.03 is preferably contained in an amount of 15 to 90%, more preferably 30 to 85%, still more preferably 60 to 80%. The larger the major axis / minor axis ratio, the higher the fiber opening property, but if the percentage of single fibers having a major axis / minor axis ratio of 1.00 to 1.03 is less than 15%, the convergence and the strand strength may decrease. When a single fiber having a major axis / minor axis ratio of 1.00 to 1.03 is contained in an amount of 15% or more, it is easy to achieve both of these characteristics at a high level. When 90% or less of single fibers having a major axis / minor axis ratio of 1.00 to 1.03 is contained, that is, when 10% or more of single fibers having a major axis / minor axis ratio of more than 1.03 are contained, the effect of improving fiber openness is improved. Easy to get. The single fiber having a major axis / minor axis ratio of 1.00 to 1.03 can be obtained by adjusting the solidification conditions or adjusting the force of crushing the single fiber in the direction orthogonal to the fiber axis. In order to control the content of single fibers having a major axis / minor axis ratio of 1.00 to 1.03, a method similar to adjusting the skewness can be used.

^{In the carbon fiber of the present invention, the average value of the circularity 4πA / L 2} defined by the cross-sectional area A and the peripheral length L is preferably 0.970 to 1.000, more preferably 0.980 to 1.000. Is. 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 peripheral length 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 peripheral length L cancel each other out when calculating the circularity, they may be pixels or actual lengths such as μm. The value of the circularity is 1.000 in the case of a perfect circle, and the value becomes smaller so as to deviate from the perfect circle. If the average value of such circularity is too small, the strand strength and convergence may decrease, but if controlled within the above range, these can be maintained at a high level. As for the method of controlling the average value of the circularity within the above range, it is easy to adjust the coagulation bath conditions.

In the carbon fiber of the present invention, the ^{coefficient of variation of circularity 4πA / L 2} is preferably 1 to 3%, more preferably 1 to 2%. If the coefficient of variation of circularity is small, the packing between single fibers may be improved and it may be difficult to improve the openness, and conversely, if it is large, it may be difficult to improve the strand strength and convergence. By controlling the coefficient of variation of circularity within the above range, it is easy to achieve both of these characteristics at a high level. Regarding the method of controlling the fluctuation coefficient of the circularity within the above range, for example, two or more kinds of carbon fiber precursor fibers having different average values of circularity, or flame-resistant fibers, or precarbonized fibers, or carbon fibers are suitable. It may be mixed in a ratio, or the easily deformable precursor fibers or flame-resistant fibers may be lightly crushed with a nip roller or the like in a bundled state to be deformed, or the precursor fibers or flame-resistant fibers may be twisted. By applying tension to the fibers, a pressing force may be generated between the single fibers to deform the fibers. The coefficient of variation can be easily controlled by observing the cross section of the obtained carbon fiber and making fine adjustments so as to satisfy the numerical range of the present invention.

In the carbon fiber of the present invention, when the single fiber is observed in 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 preferably 2.5 μm or more. The fluctuation width in the present invention is measured by observing a single carbon fiber in an environment where stress other than gravity is not applied from a direction orthogonal to the fiber axis direction. In the fiber having three-dimensional fluctuation, the fiber axial direction and the orthogonal direction are defined as follows. In the projected image of the single fiber of the carbon fiber resting on the horizontal plane on the horizontal plane, the straight line connecting two points 1000 μm apart is defined as the virtual fiber axis at the observation point, 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 projected image. As shown in FIG. 3, for the fluctuation width, the center in the thickness direction of the observed single fiber is arbitrarily selected to be point A, and the center in the thickness direction of the single fiber separated by a linear distance of 1 mm is point B. When the point A is the origin in the XY coordinate system, that is, the point where X = 0 μm and Y = 0 μm, and the point B is the point on the X axis, that is, X = 1000 μm and Y = 0 μm, the thickness of the single fiber Of the Y coordinate values that the center of the direction passes through, it is defined as a residual ΔY (μm) obtained by subtracting the minimum value Ymin (μm) from the maximum value Ymax (μm). The fluctuation width is measured for 10 randomly selected independent single fibers, and the average value thereof is adopted. In the prior art of carbon fibers, no particular attention has been paid to the fluctuation width, but as measured by the inventors, the fluctuation width of commercially available carbon fibers is generally less than 2 μm, especially when it is 1 μm or less. There were many. The fluctuation width is more preferably 3 μm or more, further preferably 4 μm or more, and particularly preferably 5 μm or more. The larger the fluctuation width, the more difficult it is to pack between single fibers, and the easier it is to increase the openness. The fluctuation width can be controlled by imparting bending to the fibers in the flame resistance treatment step, the preliminary carbonization treatment step, and the carbonization treatment step, which will be described later. In particular, it is preferable to impart bending to the fibers in the step of carbonization treatment having the highest treatment temperature from the viewpoint of ease of imparting bending. As a method for imparting bending, known methods such as twisting the fibers or knitting the fibers into a braid or quartet shape in the manner of a braid can be adopted. Above all, it is preferable from an industrial point of view to adopt a twist that can be handled by simple equipment.

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 further 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 easier it is to increase the rigidity of the carbon fiber reinforced composite material. However, the carbon fiber tends to be brittle, and fluffing may occur in the process of higher processing. Tends to wrap around rollers and guides and deteriorate process passability. Since the carbon fiber of the present invention has high fiber opening property, it is easy to open the fiber even with a slight force in the carbon fiber having a high strand elastic modulus, and it is easy to achieve both the strand elastic modulus and the process passability, and in particular, the strand elastic modulus is increased. Highly industrially useful. The strand modulus can be controlled by a known method.

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 easier it is to increase the tensile strength when the carbon fiber reinforced composite material is used. If not properly controlled, the strand strength may decrease. The carbon fiber of the present invention can achieve both openness and strand strength at a high level.

Hereinafter, the method for producing the carbon fiber of the present invention will be described.

The carbon fiber precursor fiber which is the source of the carbon fiber of the present invention can be obtained by spinning a spinning solution of a polyacrylonitrile copolymer.

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

The polyacrylonitrile copolymer described above is dissolved in a solvent in which the polyacrylonitrile copolymer is soluble, such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, nitrate, zinc chloride aqueous solution, and rodan soda aqueous solution, to obtain a spinning solution.

The carbon fiber precursor fiber can be produced by spinning the obtained spinning solution by a wet or dry wet spinning method.

A carbon fiber precursor fiber is obtained by introducing a spinning solution into a coagulation bath and coagulating it, and passing the obtained coagulated fiber through a washing step, a drawing step in the bath, an oiling agent applying step and a drying step. At this time, it is known that the cross-sectional shape changes depending on the solidification conditions, and a circular cross-section is formed when the concentration of the solvent in the coagulation bath is as thin as 40% by mass or less and when the concentration is as high as around 80% by mass. When the concentration is high, it has a sky bean-shaped cross section. The coagulated fiber may be directly stretched in the bath by omitting the washing step, or may be stretched in the bath after removing the solvent by the washing step. Stretching in the bath is usually preferably carried out in a single or multiple stretching baths temperature controlled to a temperature of 30-98 ° C. Further, a dry heat stretching step or a steam stretching step may be added to the above steps.

The resulting carbon fiber precursor fibers are usually in the form of continuous fibers. The number of filaments per thread 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 thread of the obtained carbon fibers.

The first preferable mode for obtaining the carbon fiber of the present invention is to use a precursor fiber having a major axis / minor axis ratio of 1.00 to 1.03 and a precursor fiber having a major axis / minor axis ratio of 1.20 or more in 8 :. Mixing is performed at a ratio of 1 to 2: 1. At this time, the carbon fibers of the present invention cannot be obtained only by setting halfway conditions such that circular and elliptical cross sections are mixed as the coagulation bath conditions, and precursor fibers having different cross-sectional shapes can be mixed. preferable.

The second preferable mode for obtaining the carbon fiber of the present invention is that the obtained carbon fiber precursor fiber is simply crushed and deformed by a nip roller or the like, or twisted to apply tension. It is deformed by generating a pressing force between the fibers to deform the fibers. As a method for twisting the carbon fiber precursor fiber, a known method can be selected. Specifically, a method in which the carbon fiber precursor fiber is once wound around the bobbin and then the bobbin is swirled in a plane orthogonal to the unwinding direction when the fiber is unwound, or the bobbin is not wound and the fiber is running. It can be controlled by a method of bringing a rotating roller or belt into contact with the fibers of the fiber to give a twist.

The carbon fiber of the present invention can be obtained by subjecting the above-mentioned carbon fiber precursor fiber to a flameproof treatment, followed by a preliminary carbonization treatment and a carbonization treatment in this order.

The flame-resistant treatment of the carbon fiber precursor fiber is carried out in an air atmosphere, preferably in a temperature range of 200 to 300 ° C. The carbon fiber precursor fiber is flame-resistant and becomes a flame-resistant fiber. In order to obtain the carbon fibers of the present invention, the fibers being flame-resistant are lightly crushed with a nip roller or the like to be deformed, or twisted to apply tension to generate a pressing force between the single fibers for deformation. It is also preferable to let them do it. When twisting is added to the fiber being flame-resistant, the twist angle is preferably 0.2 ° or more, the twist angle is more preferably 2.0 ° or more, and further preferably 3.5 ° or more. Yes, especially preferably 4.0 ° or more. In the present invention, the twist angle in the flame resistance treatment is as follows, using the basis weight y (g / m), density d (g / cm ³ ), and twist number T (turn / m) of the precursor fibers used. Calculate by equation (2).

Twist angle (°) = arctan {(0.01 × y / π / d) ^0.5 × ^10-6 × π × T} ・ ・ (2)
As a method of twisting the fiber being flame-resistant, a known method can be selected. Specifically, a method in which the carbon fiber precursor fiber is once wound around the bobbin and then the bobbin is swirled in a plane orthogonal to the unwinding direction when the fiber is unwound, or the bobbin is not wound and the fiber is running. It can be controlled by a method of bringing a rotating roller or belt into contact with the fibers of the fiber to give a twist.

When twisting is applied during the flame-resistant treatment, the tension on the flame-resistant fiber is preferably 1.0 to 5.0 mN / dtex. The tension in the flameproofing step is divided by the tension (mN) measured on the entry side of the flameproofing 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. It is assumed that By controlling the tension within the above numerical range, it becomes easy to give a dent to the carbon fiber.

In the present invention, following the flame resistance, the flame resistant fiber is precarbonized. In the pre-carbonization step, the flame-resistant fibers obtained by the flame-resistant treatment can be heat-treated in ^{an inert atmosphere at a maximum temperature of 500 to 1200 ° C. until the density reaches 1.5 to 1.8 g / cm 3.} preferable. The flame-resistant fiber is pre-carbonized to become a pre-carbonized fiber.

Further, following the preliminary carbonization, the preliminary carbonized fiber is carbonized. In the carbonization step, the pre-carbonized fiber obtained by the pre-carbonization treatment is 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. The maximum temperature in the carbonization step is preferably high from the viewpoint of increasing the strand elastic modulus of the obtained carbon fiber, and if it is 1200 ° C. or higher, the strand elastic modulus is suitable for applications where rigidity is important as a carbon fiber reinforced composite material. High carbon fiber can be obtained. On the other hand, if the carbonization temperature is too high, the strand strength tends to decrease, the carbon fibers tend to become brittle, and fluffing occurs in the process of higher-order processing, which wraps around the rollers and guides to improve process passability. Since it tends to be deteriorated, the maximum temperature in the carbonization process should be determined in consideration of the balance between the required strand elastic modulus, the strand strength, and the process passability.

Further, in the present invention, the tension in the carbonization step is preferably 5 to 15 mN / dtex, more preferably 7 to 15 mN / dtex. The tension in the carbonization step is obtained by dividing the tension (mN) measured on the outlet side of the carbonization furnace by the total fineness (dtex), which is the product of the single fiber fineness (dtex) of the carbon fiber precursor fibers used and the number of filaments. It is assumed that By controlling the tension within the above numerical range, it is easy to maintain high strand strength and process passability even if the strand elastic modulus of the obtained carbon fiber is increased.

In the present invention, as the inert gas used in the inert atmosphere, for example, nitrogen, argon, xenon and the like are preferably exemplified, and nitrogen is preferably used from an economical point of view.

The carbon fibers obtained by the above-mentioned production method may be further subjected to an additional graphitization treatment in an inert atmosphere up to 3000 ° C., and the elastic modulus of the single fibers may be appropriately adjusted according to the application.

In order to improve the adhesive strength between the carbon fiber and the matrix, the carbon fiber obtained as described above is preferably surface-treated after the carbonization treatment to introduce a functional group containing an oxygen atom. As the surface treatment method, vapor phase oxidation, liquid phase oxidation and liquid phase electrolytic oxidation are used, but liquid phase electrolytic oxidation is preferably used from the viewpoint of high productivity and uniform treatment. In the present invention, the method of liquid phase electrolytic oxidation is not particularly limited, and a known method may be used.

The methods for measuring various physical property values described in the present specification are as follows. Unless otherwise specified, the evaluation was performed with the number of measurements n = 1.

<Strand strength and elastic modulus of carbon fiber>
The strand strength and the strand elastic modulus of the carbon fiber are determined according to the resin impregnated strand test method of JIS R7608: 2004 according to the following procedure. However, when the carbon fiber has a twist, it is evaluated after being untwisted by applying the same number of reverse rotation twists as the number of twists. As the resin formulation, "Ceroxide (registered trademark)" 2021P (manufactured by Daicel Chemical Industry Co., Ltd.) / Borone monoethylamine trifluoride (manufactured by Tokyo Chemical Industry Co., Ltd.) / Acetone = 100/3/4 (part by mass) is used. As the curing conditions, normal pressure, temperature 125 ° C., and time 30 minutes are used. Ten carbon fiber strands are measured, and the average value thereof is taken as the strand strength and the strand elastic modulus. The strain range when calculating the strand elastic modulus is 0.1 to 0.6%.

<Statistics of major axis minor axis ratio and circularity>
The major axis / minor axis ratio of a single carbon fiber and its average value, coefficient of variation, and skewness are calculated as follows. First, the bundles of carbon fibers are aligned and wrapped with carbon tape so as to wrap them in a cylindrical shape. In this state, a pair of scissors is applied so as to be perpendicular to the fiber axis and cut to expose the cross section of the single fiber. The cross section of the single fiber is observed using a scanning electron microscope and stored as an image. The saved image is read into the open source image analysis software "ImageJ", and the contour of the cross section of the single fiber is traced using the "Polygon selections" tool. At this time, one round is traced at 20 to 100 points for one contour. Then use the "Fit spline" tool to transform the contour trace into a smooth curve. After the conversion, the points are moved and fine-tuned so that the contour of the single fiber cross section and the traced curve better match. Subsequently, the "AR (aspect ratio)" and "Circularity (circularity)" are calculated using the "Analyze particles" tool. The aspect ratio refers to the major axis / minor axis ratio in the present invention. The same operation is performed on 50 single fibers. At this time, the single fibers are evenly selected from the different parts of the carbon fiber bundle. The mean value (−) of the major axis minor axis ratio, the coefficient of variation (%), the skewness (−), the ratio (%) of 1.00 to 1.03, and the ratio (%) of 1.20 or more are calculated. For the circularity, the average value (-) and the coefficient of variation (%) are calculated. The skewness of the major axis minor axis ratio is calculated according to the following equation (1).

Skewness = n / (n-1) / (n-2) × Σ {(x _i − <x>) / s} ³ ... (1)
Here, n is the number of single fibers (lines), x _i is the major axis / minor axis ratio (-) of the i-th single fiber, <x> is the average value of the major axis / minor axis ratio (-), and s is the major axis / minor axis. It means the standard deviation (-) of the ratio. Further, Σ means that the sum is taken by the number n of single fibers.

In the following examples, a scanning electron microscope (SEM) "S-4800" manufactured by Hitachi High-Technologies Corporation was used as the scanning electron microscope, and the acceleration voltage was set to 5 keV for observation.

<Ratio of single fibers with two or more dents and broad bean-shaped single fibers>
For 50 single fibers having the same major axis / minor axis ratio and circularity, the ratio of the single fibers having each cross-sectional shape is determined from the obtained scanning electron microscope image. The ratio is expressed as a percentage of the number of single fibers corresponding to each of the observed number of single fibers. FIG. 1 shows a typical example of a single fiber having two or more recessed portions, and FIG. 2 shows a typical example of a broad bean-shaped single fiber. For both single fibers with two or more recesses or broad bean-shaped single fibers, if only one of the 50 single fibers observed for each corresponding single fiber is found, further for statistical reasons. 50 single fibers are added and observed, and the ratio is calculated.

<Fluctuation width of carbon fiber shaft>
The single carbon fiber to be measured has a length of 1 to 5 mm and is allowed to stand on a copy paper laid on a horizontal table. If the single fiber sticks to the copy paper due to the influence of static electricity, it should be done after static electricity is removed by a general method. An image is acquired by observing from the vertical direction of the paper surface using an optical microscope. The magnification of the objective lens of the optical microscope is 10 times. The image is saved in a jpg format of 2592 pixels in width × 1944 pixels in height. At this time, when a scale having an actual size of 1000 μm is imaged, the imaging range is set so that the scale corresponds to 2320 to 2340 pixels. The acquired image is read into the open source image processing software "ImageJ", and an arbitrary point on the fiber axis is defined as point A, and a point on the fiber axis 1000 μm away from point A is defined as point B. Next, "Bilinar Interpolation" is selected as the interpolation algorithm at the time of rotation, and the image is rotated so that points A and B are horizontal. After performing the binarization process, skeletonization is performed, and the fiber axis is extracted as a curve having a width of 1 pixel. At this time, if dust or the like adheres to the fiber surface, the fiber shaft may branch, but side chains other than the fiber shaft are ignored. Finally, of the Y coordinates that the fiber axis passes between points A and B, the residual ΔY (μm) obtained by subtracting the minimum value Y _{min from the maximum value Y max} is read, and the measured fluctuation width of the single fiber is obtained. To do. The fluctuation widths measured for 10 different single fibers are averaged and adopted as the fluctuation widths in the present invention. The coefficient of variation of the fluctuation width is calculated by the following formula using the standard deviation calculated from the data measured for 10 different single fibers.

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

In this example, an upright microscope "DM2700M" manufactured by Leica Microsystems, Inc. was used as the optical microscope.

<Twist angle in flameproof treatment>
The twist angle in the flame resistance treatment is determined by the following formula (2) using the basis weight y (g / m) and density d (g / cm ^{3) of the precursor fiber used and the number of twists T (turn / m).} calculate.

Twist angle (°) = arctan {(0.01 × y / π / d) ^0.5 × ^10-6 × π × T} ・ ・ (2)
<Metsuke of precursor fibers>
For the fiber to be measured, a length of 8 m is sampled, and the mass measured after absolute drying is divided by 8, to obtain a basis weight, which is the mass per 1 m. The measurement is performed 3 times and the average value is used.

<Density of precursor fibers>
The fiber to be measured is sampled at 1 m, and the specific gravity liquid is used as ethanol for measurement by the Archimedes method. Measure 3 times and use the average value.

<Convergent>
Convergence is evaluated in the state where the sizing agent is not attached. If sizing agent is attached, it is removed by baking the sizing agent in an oven or washing in a solvent before evaluation. First, the fiber to be measured is sampled to a length of 1 m, and both ends are fixed so that the fiber is straight. At this time, it should be held in the air without touching anything other than the fixed ends. After bringing one end closer to the other end by 1 cm, lightly blow on the vicinity of the center of the fiber, and check whether the yarn width is organized as an index of convergence. Convergence is judged by A to D from the yarn width widening 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.

<Openness>
The fibrous opening property is evaluated in a state where the sizing agent is not attached. If sizing agent is attached, it is removed by baking the sizing agent in an oven or washing in a solvent before evaluation. 2 cm of carbon fiber is sampled so that there is no twist. Carbon fiber is placed on a 10 cm square glass plate, and a slide glass is placed over it. It is moved 10 times alternately on the left and right by 3 mm in the direction perpendicular to the axial direction of the carbon fiber. The change in thread width is measured before and after this operation, and the average value repeated 10 times is used as an index of fiber opening. The greater the rate at which the thread width expands, the better the fiber opening property. The fibrous opening property is judged by A to D from the thread width widening 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%.

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

Examples 1 to 10 and Comparative Examples 1 to 4 described below are carried out using the conditions shown in Table 1 in the implementation method described in the following comprehensive examples 1 or 2.

[Comprehensive Example 1]
The polyacrylonitrile copolymer was polymerized by a solution polymerization method using dimethyl sulfoxide as a solvent to obtain a spinning solution. A coagulated yarn was obtained by a dry-wet spinning method in which the obtained spinning solution was once discharged from a spinneret into the air and introduced into a coagulation bath consisting of an aqueous solution of 35% by mass of dimethyl sulfoxide held at 0 ° C. Further, after washing the coagulated fibers with water, the coagulated fibers were stretched in warm water at 90 ° C. at a stretching ratio of 3 times in a bath, further added with a silicone oil, and dried using a roller heated to a temperature of 160 ° C. Pressurized steam stretching was performed at a stretching ratio of 4 times to obtain carbon fiber precursor fibers having a single fiber fineness of 1.1 dtex.

[Comprehensive Example 2]
The polyacrylonitrile copolymer was polymerized by a solution polymerization method using dimethyl sulfoxide as a solvent to obtain a spinning solution. A coagulated yarn was obtained by a wet spinning method in which the obtained spinning solution was introduced into a coagulation bath consisting of an aqueous solution of 70% by mass of dimethyl sulfoxide held at 50 ° C. from a spinneret. Further, after washing the coagulated fibers with water, the coagulated fibers were stretched in warm water at 90 ° C. at a stretching ratio of 6 times in a bath, further added with a silicone oil, and dried using a roller heated to a temperature of 160 ° C. A carbon fiber precursor fiber having a single fiber fineness of 1.1 dtex was obtained.

[Example 1]
The 21,000 precursor fibers obtained in Comprehensive Example 1 and the 3000 precursor fibers obtained in Comprehensive Example 2 are mixed by performing a fiber-blending treatment of blowing air, and the total number of filaments is 24,000. Book precursor fibers were obtained. Next, the obtained polyacrylonitrile-based carbon fiber precursor fiber is heat-treated in an oven at an air atmosphere of 230 to 280 ° C. with a tension of 1.0 mN / dtex and a draw ratio of 1 (flame resistance step) to obtain a flame resistance fiber. Converted. The obtained flame-resistant fibers were subjected to pre-carbonization treatment at a draw ratio of 0.97 in a nitrogen atmosphere at a temperature of 300 to 800 ° C. to obtain pre-carbonized fibers. Next, the preliminary 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.

[Example 2]
Carbon fibers were obtained in the same manner as in Example 1 except that 18,000 precursor fibers obtained in Comprehensive Example 1 and 6000 precursor fibers obtained in Comprehensive Example 2 were mixed. It was. The evaluation results of the obtained carbon fibers are shown in Table 1.

[Example 3]
Carbon fibers were obtained in the same manner as in Example 1 except that 15,000 precursor fibers obtained in Comprehensive Example 1 and 9000 precursor fibers obtained in Comprehensive Example 2 were mixed. It was. The evaluation results of the obtained carbon fibers are shown in Table 1.

[Example 4]
The bobbin wound up with 24,000 precursor fibers obtained in Comprehensive Example 1 is swirled and twisted so as to have a twist angle of 4.6 degrees, and the tension is 1 in an oven having an air atmosphere of 230 to 280 ° C. Heat treatment was performed at 0.0 mN / dtex and a draw ratio of 0.9 (flame resistance step) to convert to flame resistant fibers. The obtained flame-resistant fibers were subjected to pre-carbonization treatment at a draw ratio of 0.97 in a nitrogen atmosphere at a temperature of 300 to 800 ° C. to obtain pre-carbonized fibers. Next, the preliminary 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.

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

[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.

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

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

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

[Example 10]
After passing 24,000 precursor fibers obtained in Comprehensive Example 1 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, twisting is added to the precursor fibers. Carbon fibers were obtained in the same manner as in Example 4 except that there was no such thing. The evaluation results of the obtained carbon fibers are shown in Table 1.

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

[Example 12]
The bobbin wound up with 24,000 precursor fibers obtained in Comprehensive Example 1 is swirled and twisted so as to have a twist angle of 7.6 degrees, and the drawing ratio is in an oven at 230 to 280 ° C. in an air atmosphere. Was heat-treated at 0.8 (flame-resistant step) and converted into flame-resistant fibers. The obtained flame-resistant fiber was subjected to pre-carbonization treatment at a draw ratio of 1.1 in a nitrogen atmosphere at a temperature of 300 to 800 ° C. to obtain a pre-carbonized fiber. Next, the preliminary 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.

[Comparative Example 1]
Carbon fibers were obtained in the same manner as in Example 1 except that 9000 precursor fibers obtained in Comprehensive Example 1 and 15,000 precursor fibers obtained in Comprehensive Example 2 were mixed. It was. The evaluation results of the obtained carbon fibers are shown in Table 1.

[Comparative Example 2]
Carbon fibers were obtained in the same manner as in Example 1 except that 6000 precursor fibers obtained in Comprehensive Example 1 and 18,000 precursor fibers obtained in Comprehensive Example 2 were mixed. It was. The evaluation results of the obtained carbon fibers are shown in Table 1.

[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.

[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.

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

[Reference example 1]
Table 1 shows the evaluation results of "Trading Card (registered trademark)" M40J manufactured by Toray Industries, Inc.

The carbon fiber of the present invention is a carbon fiber having excellent mechanical properties peculiar to carbon fiber and a high balance of convergence and openness. By using the carbon fiber of the present invention, a high-performance carbon fiber reinforced composite material can be obtained with high productivity.

Claims (12)

It is a carbon fiber composed of a plurality of single fibers, and the average value of the major axis / minor axis ratio of the cross section of the single fiber is 1.00 to 1.15, the coefficient of variation is 3 to 15%, and the skewness is 1.2 to 4. Carbon fiber that is 0.0. The carbon fiber according to claim 1, which contains 2 to 20% of a single fiber having two or more recessed portions in the cross section of the single fiber. The carbon fiber according to claim 1 or 2, wherein the single fiber having a broad bean-shaped cross section is contained in an amount of 3 to 15%. The carbon fiber according to any one of claims 1 to 3, which has a skewness of 1.5 or more. The carbon fiber according to any one of claims 1 to 4, wherein 3 to 20% of single fibers having a major axis / minor axis ratio of 1.20 or more are contained. The carbon fiber according to any one of claims 1 to 5, which contains 15 to 90% of single fibers having a major axis / minor axis ratio of 1.00 to 1.03. The invention according to any one of claims 1 to 6, wherein the average value of the circularity 4πA ^{/ L 2} defined by the cross-sectional area A and the peripheral length L is 0.970 to 1.000, and the coefficient of variation is 1 to 3%. Carbon fiber. The carbon fiber according to any one of claims 1 to 7, wherein the fluctuation width of the fiber axis of the single fiber is 2.5 μm or more when the single fiber is observed in a range of a linear distance of 1 mm from the side surface. The carbon fiber according to any one of claims 1 to 8, which has a strand elastic modulus of 300 GPa or more. The carbon fiber according to any one of claims 1 to 9, wherein the strand strength is 4.0 GPa or more. Polyacrylonitrile-based carbon fiber precursor fibers are flame-resistant at 200 to 300 ° C. in an air atmosphere, then precarbonized under an inert atmosphere at a maximum temperature of 500 to 1200 ° C., and are not treated at a maximum temperature of 1200 to 3000 ° C. The method for producing carbon fibers according to any one of claims 1 to 10, wherein the carbonization treatment is sequentially performed in an active atmosphere, and the carbon fibers satisfying the following (c) or (d) are produced. Method.
(C) A precursor fiber having a major axis / minor axis ratio of a single fiber cross section of 1.00 to 1.03 and a precursor fiber having a major axis / minor axis ratio of a single fiber cross section of 1.20 or more are 8: 1 to 2: After mixing at a ratio of 1 fiber, flame resistance treatment is performed.
(D) The twist angle of the fiber during the flameproofing treatment is 0.2 ° or more, and the tension of the fiber during the flameproofing treatment is 1.0 to 5.0 mN / dtex. The method for producing carbon fibers according to claim 11, wherein the tension of the fibers during the carbonization treatment is 5 to 15 mN / dtex.

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