JP7360244B2 - Carbon fiber manufacturing method and carbon fiber - Google Patents

Description

The present invention relates to a method for producing high-strength carbon fibers and the carbon fibers.

Carbon fiber has excellent specific strength and specific modulus, and is lightweight, so it is used as a reinforcing fiber for thermosetting and thermoplastic resins, not only for conventional sports and general industrial applications, but also for aerospace, automotive, and other applications. It is used for a wide range of purposes. In recent years, the superiority of carbon fiber composite materials has been increasing more and more, and there is a high demand for improved performance and productivity of carbon fiber composite materials, especially in automotive, aerospace, and space applications. The properties of carbon fibers as composite materials are largely due to the properties of carbon fibers themselves, and at the same time there is a growing demand for improved strength of carbon fibers themselves, there is also a desire to improve the productivity of carbon fibers themselves.
Generally, it is widely known that carbon fibers can be obtained by subjecting a precursor fiber to a flame-retardant treatment to obtain a flame-retardant fiber, and then subjecting this flame-retardant fiber to a carbonization treatment. It is also practiced industrially. In order to produce high-strength carbon fibers, it is particularly necessary to suppress the generation of defects that can become starting points for fracture. However, in carbon fibers that are generally obtained by making precursor fibers flame-resistant, it is known that structural unevenness (skin core) occurs inside the fibers during the flame-proofing process, which is an internal defect that inhibits the strength improvement of carbon fibers. It has been said that this is a contributing factor.
Therefore, in order to obtain high-strength carbon fibers, techniques have been proposed to reduce skin cores generated inside the fibers during the flame-retardant process. For example, Patent Document 1 proposes a technique for reducing structural unevenness between the center and the outer periphery of a single fiber by reducing the heating rate of the fiber in the process of flameproofing the precursor fiber. However, a reduction in the temperature increase rate means a reduction in the firing rate, an increase in the size of the equipment, and an increase in production costs, and there is a limit to the strength improvement.

Japanese Patent Application Publication No. 2006-274518

SUMMARY OF THE INVENTION In view of the above-mentioned problems, an object of the present invention is to provide a manufacturing method that enables high-strength carbon fibers to be obtained at low cost, and to provide high-strength, low-cost carbon fibers.

A method for producing carbon fiber according to one embodiment of the present invention includes a flame-retardant step of making a precursor fiber flame-resistant to obtain a flame-retardant fiber, and a carbonization step of carbonizing the flame-retardant fiber. The flame-resistant fiber is a flame-resistant fiber with a core percentage of 5% or more, and the carbonization step is performed at a temperature gradient of 450 [°C/min] or less in a temperature range of 700 to 1,000 [°C]. It will be done.
A method for producing carbon fiber according to one embodiment of the present invention includes a flame-retardant step of making a precursor fiber flame-resistant to obtain a flame-retardant fiber, and a carbonization step of carbonizing the flame-retardant fiber. The flameproofing step is performed at a maximum temperature of 255 [°C] or higher for a time of 60 [min] or less, and the carbonization step is performed at a temperature of 450 [°C/ The temperature gradient is less than or equal to [min].
The carbon fiber according to one aspect of the present invention is produced by a flame-retardant step of making a precursor fiber flame-resistant to obtain a flame-retardant fiber, and a carbonization step of carbonizing the flame-retardant fiber. If the conduction velocity [km/s] is CV and the tensile modulus [GPa] of carbon fiber is TM, then in the relationship between the sound wave conduction velocity CV and the tensile modulus TM, 242>37.5×CV-TM >215 is satisfied.

Carbon fibers having high strand tensile strength can be obtained by the method for producing carbon fibers according to one embodiment of the present invention.

FIG. 2 is a schematic diagram for explaining a skin core. It is a schematic diagram showing a manufacturing process of carbon fiber.

<<Summary>>
The inventors conducted repeated studies focusing on the flame-retardant process and carbonization process for producing carbon fiber from precursor fibers, and purposefully developed a flame-retardant film with a high ratio of the core layer of the skin core (hereinafter simply referred to as "core ratio"). It has been found that by carbonizing fibers under specific carbonization conditions, carbon fibers with even higher strength can be obtained.
Here, the skin core will be explained using FIG. 1.
The skin core has a skin layer A, which is the surface part of the flame-resistant fiber and has advanced flame resistance, and a core layer B, which is the center part of the flame-resistant fiber and whose flame resistance has been delayed compared to the skin layer A. Heavy structure.
The skin layer A is a dark-colored region at the outer periphery observed with an optical microscope. The core layer B is a region with a light color on the inner periphery observed with an optical microscope.
The core ratio is the ratio of the area of the core layer to the total cross-sectional area of one fiber in the cross-section observed in the cross-section of the flame-resistant fiber using an optical microscope.

<<Embodiment>>
1. Flame-retardant step (1) The flame-retardant step is a step in which the precursor fiber is made flame-retardant to obtain a flame-retardant fiber having a core ratio of 5% or more, more preferably 10% or more. In addition, flame-proofing is also referred to as flame-proofing treatment.
Flame-resistant fibers with a core ratio of 5 [%] or more are not particularly limited, but for example, flame-resistant fibers can be made flame-resistant in a flame-retardant treatment space with a maximum temperature of 255 [°C] or more for a treatment time of 60 [min] or less. More preferably, it can be obtained by performing flame resistance in a flame resistance treatment space with a maximum temperature of 260 [° C.] or higher for a treatment time of 50 [min] or less. The higher the temperature of the processing space and the shorter the processing time, the more flame-resistant fibers with a higher core ratio can be obtained.
The minimum temperature of the flameproofing treatment space may be adjusted as appropriate depending on the maximum temperature, but it is preferably 220 [°C] or higher, and more preferably 240 [°C] or higher. The temperature gradient in the processing space is preferably 0.25 [°C/min] or more, and more preferably in the range of 0.3 to 3 [°C/min]. The temperature gradient here is the time from the lowest temperature to the highest temperature.
The flame-retardant treatment is preferably carried out so that the specific gravity of the flame-retardant fiber is in the range of 1.32 to 1.4 [g/cm ³ ], more preferably in the range of 1.35 to 1.37 [g/cm ³ ]. It is more preferable to do so. If the flame-retardant treatment is performed so that the specific gravity of the flame-retardant fiber falls within this range, carbon fibers with higher strength can be obtained. The specific gravity of the flame-resistant fiber can be increased by increasing the temperature of the treatment space or by increasing the treatment time, that is, by increasing the amount of energy given to the precursor fiber.
It is known that the strength of carbon fiber decreases due to an excessive increase in crystal size, but the presence of skin cores with a core ratio of 5% or more in flame-resistant fibers prevents excessive increase in the carbonization process. It is thought that crystal growth is suppressed, and as a result, the strength of the carbon fiber is improved.

(2) Core rate calculation method The core rate calculation method is as follows.
The cross section of the flame resistant fiber was observed by embedding the flame resistant fiber in a room temperature curable epoxy resin and polishing the cross section after curing. A reflection microscope was used for observation. The core ratio was calculated by measuring the cross-sectional area of the bright part and the total cross-sectional area including the bright part and dark part from the cross-sectional image of the flame-resistant fiber, and applying it to the following formula (1). Image threshold setting, binarization, and area measurement were performed using image processing software (manufactured by Asahi Kasei Engineering Co., Ltd., Azo-kun (registered trademark)).
Core ratio [%] = Cross-sectional area of bright part / Total cross-sectional area × 100 ... (1)

(3) Degree of orientation The degree of orientation of the target flame-resistant fiber is preferably within the range of 74.0 to 77.0 [°], and preferably within the range of 75.0 to 75.5 [°]. is more preferable. The orientation of the flame-resistant fibers can be adjusted by adjusting the draw ratio during the precursor manufacturing process and the flame-resistant process. The higher the stretching ratio, the higher the orientation. The degree of orientation was calculated from the half-width of the crystallite size at a diffraction angle of 26[°] based on the diffraction pattern.
Note that RINT2200 manufactured by RIGAKU is used as an X-ray diffractometer, and calculations are made by computer using RINT2000 series analysis software manufactured by RIGAKU.

2. Carbonization Step The carbonization step is a step in which flame-resistant fibers having a core ratio of 5% or more are carbonized to obtain carbon fibers. Note that carbonization is also referred to as carbonization treatment. Carbonization involves heating with a relatively gentle temperature gradient (temperature increase gradient) in a specific temperature range.
Specifically, the carbonization treatment is performed at a temperature gradient of 450 [°C/min] or less in a temperature range of 700 to 1,000 [°C]. Conventionally, it has been thought that carbonization conditions in this temperature range have a small effect on the strength of the obtained carbon fiber, and the temperature gradient is relatively steep compared to other temperature ranges other than 700 to 1,000 [℃]. was set. However, the present inventors have found that when carbonizing flame-resistant fibers with a specific core ratio, carbon fibers with high strand tensile strength can be obtained by making the temperature gradient gentle in this temperature range. I found out. Note that the strand tensile strength is sometimes simply referred to as "tensile strength" or "strength", and the strand tensile modulus is sometimes simply referred to as "tensile modulus" or "elastic modulus".

The temperature gradient in the temperature range of 700 to 1,000 [°C] is more preferably in the range of 100 to 450 [°C/min], and even more preferably in the range of 200 to 400 [°C/min].
In the above-mentioned temperature range of 700 to 1,000 [° C.], a rapid weight loss of the fiber during carbonization occurs, but if the weight loss rate is too fast, defects will occur inside the fiber, which will cause a decrease in strength. In order to suppress the decrease in strength, the preferable weight reduction rate in the temperature range of 700 to 1,000 [°C] is 5 to 13 [wt%/min], more preferably 9 to 12 [wt%/min]. range.
Further, the temperature gradient throughout the carbonization step is preferably in the range of 50 to 250 [°C/min], more preferably in the range of 100 to 200 [°C/min].
The carbonization time of the carbonization treatment is preferably 5 [min] or more, more preferably 6 [min] or more.
When the carbonization process is performed by passing through a plurality of carbon treatment furnaces, the carbonization time is the total time for passing through each carbon treatment furnace. In addition, when the carbonization process is performed by passing through multiple carbon treatment furnaces, the maximum temperature of carbonization treatment is the highest temperature in all carbonization treatment furnaces, and the lowest temperature is the lowest temperature in all carbon treatment furnaces. It's temperature.

3. Carbon Fiber (1) Fiber Characteristics According to the carbon fiber manufacturing method that includes such a flame resistant process and a carbonization process, carbon fibers having excellent strand tensile strength TS can be obtained. It is preferable that the tensile strength TS of the carbon fiber is 5,000 [MPa] or more. By using carbon fibers with high tensile strength TS, a composite material with excellent physical properties can be obtained.
The crystal size Lc of the carbon fiber is preferably 16.2 [Å] or less, more preferably less than 16.0 [Å]. When the carbon fiber crystal size is low, a composite material with excellent compression properties and impact resistance can be obtained.
In the present invention, the carbon fiber has the following relationship between the sound wave conduction velocity CV [km/s] and the tensile modulus TM [GPa] of the carbon fiber.
242>37.5×CV-TM>215
satisfy.
Further, when the elastic modulus TM of the carbon fiber is 230 to 260 [GPa], the sound wave conduction velocity CV of the carbon fiber is preferably greater than 12.2 [km/s] and less than 13.0 [km/s]. .
The "sonic wave conduction velocity" here refers to the speed at which sound wave vibrations are transmitted in the longitudinal direction of the fiber bundle. The sound wave conduction velocity CV tends to increase as the density and crystallinity of the fiber increases. This is considered to be because a sound wave conduction path is formed due to the improvement in density. When the sound wave conduction velocity CV is within this range, the denseness of the fiber and the crystal growth can be balanced, resulting in a carbon fiber with a high tensile strength TS. If the sound conduction velocity CV is too low, the density in the fiber is low, whereas if the sound conduction velocity CV is too high, the tensile strength TS tends to decrease due to excessive crystal growth. When the sound wave conduction velocity CV is within this range, it becomes easy to obtain carbon fibers having a tensile strength TS of 5,000 [MPa] or more.

(2) Tensile properties The tensile strength TS and tensile modulus TM of carbon fiber were determined by measuring the tensile strength and tensile modulus of an epoxy-impregnated strand according to JIS R-7608.

(3) Crystal Size The crystal size Lc can be measured by a transmission method using, for example, an X-ray diffractometer RINT2000 manufactured by Rigaku Co., Ltd. and a sample stand on which fibers are set.
As the X-rays, CuKα rays generated at an accelerating voltage of 40 [kV] and a current of 30 [mA] can be used. The orientation of the sample is such that the fiber axis direction of the fiber bundle is perpendicular to the equatorial plane when measuring the crystal size Lc. In addition, we take a diffraction pattern in which the diffraction angle 2θ is in the range of 10 [°] to 60 [°], and curves passing through the vicinity of 10 [°], 20 [°], 35 [°], and 60 [°] of the diffraction pattern. as a baseline.
The crystal size Lc can be calculated from the half width β002 of the diffraction peak of the plane index (002) obtained by the above method using the following formula (2).
Crystal size Lc [nm] = 0.9λ/(β002cosθ002) (2)
[In the formula, λ: the wavelength of the X-ray, β002: the half width of the diffraction peak of the plane index (002), and θ002: the diffraction angle of the plane index (002). ]

(4) Sonic conduction velocity The sound wave conductivity CV of carbon fibers can be measured using a commercially available viscoelasticity measuring device, such as a rheovibron DDV-5-B viscoelasticity measuring device manufactured by Orientech.

4. Manufacturing method A method for manufacturing a flame resistant fiber with a core ratio of 5% or more described in the above "1. Flame resistant process" and a method for manufacturing a carbon fiber including the above "2. Carbonizing process" will be described.
Here, the case where the precursor fiber is an acrylonitrile fiber will be explained as an example.

(1) Carbon fiber manufacturing process FIG. 2 is a schematic diagram showing the carbon fiber manufacturing process.
Carbon fibers are manufactured using precursor fibers. One precursor is a bundle of a plurality of filaments, for example, 24,000 filaments.
Precursor 1a is obtained by spinning a spinning solution obtained by polymerizing a monomer containing acrylonitrile at 90 [mass %] or more, preferably 95 [mass %] or more, using a wet spinning method or a dry-wet spinning method, followed by washing, drying, and stretching. It can be obtained by In addition, as the monomer to be copolymerized, alkyl acrylate, alkyl methacrylate, acrylic acid, acrylamide, itaconic acid, maleic acid, etc. are used.
Usually, the speed of manufacturing precursor 1a and the speed of manufacturing carbon fiber by making precursor 1a flameproof and carbonizing are different. For this reason, the manufactured precursor 1a is temporarily stored in a carton or wound up on a bobbin.
The number of filaments of the precursor fiber is preferably 1,000 or more, more preferably 12,000 or more, particularly preferably 24,000 or more in terms of production efficiency.

As shown in FIG. 2, the precursor 1a is pulled out from the bobbin 30, for example, and travels toward the downstream side. During the process, various treatments are performed and the carbon fiber is wound onto a bobbin 39 as carbon fiber.
As shown in FIG. 2, carbon fibers are produced through a flame-retardant step in which a precursor 1a is made flame-resistant, and a carbonization step in which the flame-retardant fibers (hereinafter referred to as "flame-retardant fibers") 1b are carbonized while being stretched. , a surface treatment step for improving the surface of the carbonized fiber (hereinafter also referred to as "fiber after carbonization") 1d, a sizing step for attaching a sizing agent to the surface-improved fiber 1e, and a sizing step for attaching a sizing agent to the surface-improved fiber 1e. It is manufactured through a drying process of drying the attached fibers 1f.
1 g of dried fibers is wound around a bobbin 39 as 1 g of carbon fiber. Note that the fibers that have undergone each step are distinguished, for example, as flame-resistant fiber 1b, but the code "1" is used when simply describing them as "fibers".
Here, the treatment for making the precursor 1a flame resistant is flame resistant treatment, the treatment for carbonizing the flame resistant fiber 1b is carbonization treatment, the treatment for improving the surface of the fiber 1d after carbonization is surface treatment, and the fiber whose surface has been improved is The process of attaching the sizing agent to the fibers 1e is referred to as a sizing process, and the process of drying the fibers 1f to which the sizing agent is attached is referred to as a drying process. Each step and each treatment will be explained below.

(1-1) Flame resistant process (flame resistant treatment)
The flameproofing step is performed using a flameproofing furnace 3 set in an oxidizing atmosphere at a predetermined temperature. Specifically, flameproofing is performed by passing the precursor 1a through a flameproofing furnace 3 that is in an air atmosphere and has a temperature gradient. Note that the oxidizing atmosphere may contain oxygen, nitrogen dioxide, and the like.
In the flameproofing step, the temperature inside the furnace increases from the upstream side to the downstream side. This is to efficiently achieve flame resistance without inducing cutting of the precursor 1a.
The maximum temperature in the flameproofing step is 255 [° C.] or higher. The temperature gradient is preferably 0.25 [°C/min] or more, preferably 3 [°C/min] or less, and more preferably 0.3 to 0.5 [°C/min]. . The heating time in the flameproofing step is 60 [min] or less.
The precursor 1a during the flameproofing process is stretched with a predetermined tension depending on the carbon fiber to be manufactured. The stretching ratio in the flameproofing step is preferably within the range of -10 to 10 [%], more preferably within the range of -5 to 0 [%]. Stretching of the precursor 1a is performed using a plurality of rollers. For example, the stretching is performed by rollers 5, 7 at the entrance of the flameproofing furnace 3 and rollers 9, 11, 13 at the exit.

(1-2) Carbonization process (carbonization treatment)
The carbonization step is a step of carbonizing the flame-resistant fiber 1b by heating it to cause a thermal decomposition reaction. Carbonization is performed by passing the flame-resistant fiber 1b through the first carbonization furnace 15, and further by passing the fiber 1c that has passed through the first carbonization furnace 15 through the second carbonization furnace 17.

Here, the carbonization performed in the first carbonization furnace 15 is referred to as "first carbonization," and the treatments and steps performed in the first carbonization furnace 15 are referred to as "first carbonization treatment" and "first carbonization step."'', and the fiber 1c that has completed the first carbonization treatment or the first carbonization step (out of the first carbonization furnace 15) is referred to as a "first carbonized fiber."
Similarly, the carbonization performed in the second carbonization furnace 17 is referred to as "second carbonization," and the treatments and steps performed in the second carbonization furnace 17 are referred to as "second carbonization treatment" and "second carbonization process."", and the fiber 1d that has completed the second carbonization treatment or the second carbonization step (out of the second carbonization furnace 17) is referred to as "second carbonization fiber" or "carbon fiber."
The first carbonization and the second carbonization can be performed using, for example, an electric heater or the like.

Here, the first carbonization furnace 15 and the second carbonization furnace 17 are provided independently from each other, and an adjustment means for adjusting the tension of the fiber can be provided between each carbonization furnace 15 and 17. . Specifically, a roller 19 is installed outside the first carbonization furnace 15 on the inlet side, a roller 21 is installed between the first carbonization furnace 15 and the second carbonization furnace 17, and a roller 21 is installed between the first carbonization furnace 15 and the second carbonization furnace 17. A roller 23 is provided outside the roller 17 and on the exit side.
The temperature inside the second carbonization furnace 17 is higher than the temperature inside the first carbonization furnace 15, and the highest temperature inside the second carbonization furnace 17 is the highest temperature in the carbon treatment process.
Note that the temperature inside the first carbonization furnace 15 and the second carbonization furnace 17 increases as it goes downstream in the running direction of the flame-resistant fiber 1b and the first carbonization fiber 1c, and The maximum temperature is reached on the downstream side of the 2-carbonization furnace 17.

In the first carbonization step, the maximum temperature of the first carbonization furnace is preferably 800 [°C] or less, more preferably in the range of 500 to 700 [°C]. The first carbonization time is preferably 2.5 [min] or more, more preferably 4 [min] or more. The first carbonization treatment is preferably carried out so that the specific gravity of the obtained first carbonized fiber 1c is in the range of 1.5 to 1.6 [g/cm ³ ], and more preferably the first carbonized fiber 1c. The specific gravity is 1.54 to 1.60 [g/cm ³ ]. When the specific gravity of the first carbonized fiber 1c is within this range, a carbon fiber 1g having higher strength can be obtained.
In the second carbonization step, the maximum temperature of the second carbonization furnace may be adjusted appropriately according to the target strand elastic modulus TM of 1 [g] of carbon fibers, and is between 1,200 and 2,000 [℃]. It is preferable that it is between. The second carbonization time is preferably 2.5 [min] or more, more preferably 3 [min] or more. The total carbonization time of the first carbonization step and the second carbonization step is 5 [min] or more, more preferably 5 to 60 [min], and 6 to 20 [min]. is even more preferable.

In the carbonization step, the carbonization treatment is performed at a temperature gradient of 450 [°C/min] or less in a temperature range of 700 to 1,000 [°C], more preferably at a temperature in the range of 100 to 450 [°C/min]. The carbonization treatment is performed at a temperature gradient, more preferably at a temperature gradient in the range of 200 to 400 [° C./min].
Further, the temperature gradient throughout the carbonization step is preferably in the range of 50 to 250 [°C/min], more preferably in the range of 100 to 200 [°C/min].
In addition, in a temperature range of less than 700 [°C], it is preferable to perform the carbonization treatment at a temperature gradient of 150 [°C/min] or less, more preferably a temperature gradient in the range of 10 to 150 [°C/min]. More preferably, the carbonization treatment is carried out at a temperature gradient in the range of 20 to 120 [° C./min].
Furthermore, in a temperature range exceeding 1,000 [°C], it is preferable to perform the carbonization treatment at a temperature gradient of 200 [°C/min] or less, more preferably in the range of 10 to 150 [°C/min]. The carbonization treatment is performed with a temperature gradient, more preferably with a temperature gradient in the range of 20 to 120 [° C./min]. The temperature gradient throughout the second carbonization step is preferably in the range of 50 to 300 [°C/min], more preferably in the range of 100 to 250 [°C/min].

(1-3) Surface treatment process (surface treatment)
The surface treatment step is performed by the second carbonized fiber 1d passing through the surface treatment device 25. A roller 26 is provided outside the surface treatment device 25 and on the exit side. Note that by surface treatment, when 1 g of carbon fibers is used to make a composite material, the affinity and adhesion between 1 g of carbon fibers and the matrix resin are improved.
The surface treatment is generally performed by oxidizing the surface of the second carbonized fiber 1d. Surface treatment includes, for example, treatment in a liquid phase or a gas phase.
As for the treatment in the liquid phase, chemical oxidation by immersing the second carbonized fiber 1d in an oxidizing agent, anodic electrolytic oxidation by applying electricity in the electrolytic solution in which the second carbonized fiber 1d is immersed, etc. are industrially available. used for. The treatment in the gas phase can be carried out by passing the second carbonized fiber 1d through an oxidizing gas or by spraying active species generated by electric discharge or the like.

(1-4) Sizing process (sizing process)
The sizing process is performed by passing the surface-treated fibers 1e through a sizing agent solution 29. A sizing agent solution 29 is stored in a sizing agent bath 27. Note that the sizing process increases the convergence of the surface-treated fibers 1e.
In the sizing process, the surface-treated fibers 1e pass through the sizing agent solution 29 while changing the traveling direction by a plurality of rollers 31, 33, etc. arranged inside the sizing agent bath 27 and around the sizing agent bath 27. do. As the sizing agent solution 29, for example, a liquid in which an epoxy resin, a urethane resin, a phenol resin, a vinyl ester resin, an unsaturated polyester resin, etc. is dissolved in a solvent, or an emulsion liquid in which the resin is dispersed in a solvent is used.

(1-5) Drying process (drying process)
The drying process is performed by passing the fiber 1f to which the sizing agent is attached through the drying oven 35. Note that 1 g of dried fibers is wound onto a bobbin 39 via a roller 37 on the downstream side outside the drying oven 35 (this is a winding process).

〔Measuring method〕
<Resin-impregnated strand strength TS and elastic modulus TM of carbon fiber>
It was measured by the method specified in JIS R 7608 (ISO 10618).
<Crystal size Lc>
The crystal size Lc of carbon fiber is determined by the following formula (4) from the half-value width β of the diffraction peak of plane index (002) using the transmission method using an X-ray diffraction device: RINT2000 manufactured by Rigaku Corporation.
Crystallite size Lc [nm] = 0.9λ/βcosθ... (4)
Calculated using λ: X-ray wavelength, β: half width, and θ: diffraction angle.
<Sound wave conduction velocity CV>
The sound wave conductivity CV of the carbon fiber was measured using a viscoelasticity measuring device Rheoviblon DDV-5-B manufactured by Orientech. Specifically, a carbon fiber bundle cut into a length of 50 [cm] was set in a viscoelasticity measuring device, and the measurement was performed in a variable vibration mode. The TRANSMITTER ATT of the viscoelasticity measurement device was set to level 5, and the sound wave conduction velocity CV [km/s] was calculated from the vibration propagation time [seconds: s] and the fiber length (50 [cm]).

[Examples 1 to 11, Comparative Examples 1 to 5]
Polyacrylonitrile fiber (single fiber fineness 0.8 [dtex], number of filaments 24,000), which is a precursor fiber, was made flame resistant under the temperature conditions and draw ratio listed in Table 1 until it reached the specific gravity listed in Table 1. processed. Next, a first carbonization treatment and a second carbonization treatment were performed under the conditions listed in Table 1 in a nitrogen gas atmosphere. This was surface-treated by electrolytic oxidation in an ammonium sulfate aqueous solution with an electrical charge of 30 [C/g], and then subjected to a sizing treatment with an epoxy resin. Table 1 shows the physical properties of this carbon fiber.
Furthermore, for Example 1-4 and Comparative Example 1-4, the sound wave conduction velocity CV of the carbon fibers was measured. Table 2 shows the calculation results of the sound wave conduction velocity CV, the physical properties of the carbon fiber, and "37.5×CV-TM" of equation (3).

In Examples 1 to 11 using the production method of the present invention, high-strength carbon fibers with a strand tensile strength TS of 5,000 [MPa] or more were obtained. Furthermore, the sound wave conduction velocity CV of high-strength carbon fibers with a tensile strength TS of 5,000 [MPa] or more is greater than 12.1 [km/s] and smaller than 13.0 [km/s]. .
On the other hand, in Comparative Examples 1 and 2, in which the flame-retardant time was 83 [min] longer than in the example and the core ratio of the flame-retardant fiber was smaller than in the example, the carbonization conditions were the same as in Example 1. Nevertheless, the strand tensile strength TS of the obtained carbon fiber was as low as less than 5,000 [MPa].
Even in Comparative Example 3, where carbonization was carried out under the same flame-retardant conditions as in Comparative Example 1, and at a temperature gradient exceeding 450 [°C/min] in the temperature range of 700 to 1,000 [°C], the obtained carbon fiber strands The tensile strength TS was as low as less than 5,000 [MPa].

Comparative Examples 4 and 5, in which carbonization was carried out under the same conditions as in Example 1 and then carbonized at a temperature gradient exceeding 450 [°C/min] in the temperature range of 700 to 1,000 [°C], also yielded no results. The strand tensile strengths TS of the carbon fibers obtained were all as low as less than 5,000 [MPa]. In addition, the sound wave conductivity CV of the carbon fiber of Comparative Example 1-4 whose tensile strength TS is less than 5,000 [MPa] is 12.1 [km/s] or less, or 13.0 [km/s] or more. there were.

5. Others As mentioned above, when a flame-resistant fiber with a high core ratio (for example, 5% or more) is subjected to conventional carbonization for a short time, the tensile strength TS tends to decrease. For this reason, conventionally, in order to obtain high tensile strength TS, flame resistant fibers with a low core ratio have been produced by lowering the temperature in the flame resistant process and heating for a long time.
However, it has been found that a high tensile strength TS can be obtained by carbonizing a flame-resistant fiber with a high core ratio (for example, 5% or more) for a long time.
On the other hand, flame-resistant fibers with a high core ratio can be obtained by increasing the temperature of flame-proofing treatment and shortening the flame-proofing time.
Therefore, when performing the carbonization according to the present invention, it is possible to employ a flameproofing process in which the flameproofing temperature is increased and the flameproofing treatment time is shortened. In particular, in the production of carbon fibers, the ratio of the flame-retardant process to all processes is high, and shortening the time for the flame-retardant process greatly contributes to reducing the production cost of carbon fibers.

From Example 1-4 and Comparative Example 1-4, the sound wave conduction velocity CV is within the range of 12.1 to 13.0 [km/s] (however, 12.1 and 13.0 are not included). It is preferably within the range of 12.4 to 12.8 [km/s], and more preferably within the range of 12.4 to 12.8 [km/s]. Thereby, the carbon fiber manufactured by the manufacturing method according to the present invention can be specified by characteristics. Identify high-strength carbon fibers that are produced at low cost.
In addition, in Example 1-4 and Comparative Example 1-4, the elastic modulus TM was in the range of 239 to 244 [GPa], but considering carbon fibers with other elastic moduli TM outside this range, the following formula ( 3) is satisfied. As a result, carbon fibers with high strength can be obtained at each tensile modulus TM of the carbon fibers.
242>37.5×CV-TM>215... (3)

<<Modified example>>
Although the present invention has been described above based on the embodiments, the present invention is not limited to the embodiments. For example, any of the modified examples and embodiments described below may be combined as appropriate, or a plurality of modified examples may be combined as appropriate.

1. Carbon fiber In the item "2. Manufacturing method", we explained the method for manufacturing carbon fiber with 24,000 filaments, but other carbon fibers with 3,000, 6,000, 12,000, etc. The present invention can also be applied to a method for carbonizing a number of precursor fibers and producing carbon fibers.
In the item "3. Production method", a method for producing carbon fibers including a carbonization process has been described, but for example, a graphitization process may be further performed before the surface treatment process.
2. Carbonization Furnace In the embodiment, the carbonization furnace was equipped with two carbonization furnaces, the first carbonization furnace and the second carbonization furnace, but it may be one carbonization furnace or three carbonization furnaces. The above carbonization furnace may be used. Even in this case, the temperature gradient may be 450 [°C/min] or less in the temperature range of 700 to 1,000 [°C].

1 Fiber 1a Precursor 1b Flame-resistant fiber 1c First carbonized fiber 15 First carbonization furnace 17 Second carbonization furnace

Claims (7)

In a method for producing carbon fiber, the method includes a flame-retardant step of making a precursor fiber flame-retardant to obtain a flame-retardant fiber, and a carbonization step of carbonizing the flame-retardant fiber,
The carbon fiber has a tensile strength of 5,000 [MPa] or more,
The flame-resistant fiber has a core ratio of 5% or more, and has a temperature gradient of 0.25[°C/min ] or more from a minimum temperature of 220[°C] or more to a maximum temperature of 255[°C] or more. It is a treated flame-retardant fiber,
A method for producing carbon fibers, wherein the carbonization step is performed at a temperature gradient of 450 [°C/min] or less in a temperature range of 700 to 1,000 [°C]. The method for producing carbon fiber according to claim 1 , wherein the precursor fiber is an acrylonitrile-based fiber, and the flame-resistant fiber has a specific gravity of 1.33 to 1.4 [g/cm ³ ]. In a method for producing carbon fiber, the method includes a flame-retardant step of making a precursor fiber flame-retardant to obtain a flame-retardant fiber, and a carbonization step of carbonizing the flame-retardant fiber,
The carbon fiber has a tensile strength of 5,000 [MPa] or more,
The precursor fiber is an acrylonitrile fiber,
The flame resistant fiber has a core ratio of 5% or more and a specific gravity of 1.35 to 1.37 g/cm ³ ,
A method for producing carbon fibers, wherein the carbonization step is performed at a temperature gradient of 450 [°C/min] or less in a temperature range of 700 to 1,000 [°C]. In a method for producing carbon fiber, the method includes a flame-retardant step of making a precursor fiber flame-retardant to obtain a flame-retardant fiber, and a carbonization step of carbonizing the flame-retardant fiber,
The carbon fiber has a tensile strength of 5,000 [MPa] or more,
The precursor fiber is an acrylonitrile fiber,
The flameproofing step is carried out at a maximum temperature of 255 [° C.] or more and for a time of 60 [min] or less,
The flame-resistant fiber is a flame-resistant fiber with a specific gravity of 1.35 to 1.37 [g/cm ³ ],
A method for producing carbon fibers, wherein the carbonization step is performed at a temperature gradient of 450 [°C/min] or less in a temperature range of 700 to 1,000 [°C]. In a method for producing carbon fiber, the method includes a flame-retardant step of making a precursor fiber flame-retardant to obtain a flame-retardant fiber, and a carbonization step of carbonizing the flame-retardant fiber,
The carbon fiber has a tensile strength of 5,000 [MPa] or more,
The flameproofing step is carried out at a maximum temperature of 255 [° C.] or more and for a time of 60 [min] or less,
A method for producing carbon fibers, wherein the carbonization step is performed at a temperature gradient of 100 to 450 [°C/min ] in a temperature range of 700 to 1,000 [°C]. The method for producing carbon fiber according to claim 5 , wherein the precursor fiber is an acrylonitrile fiber, and the flame-resistant fiber is a flame-resistant fiber with a specific gravity of 1.33 to 1.4 [g/cm ³ ]. If the sound wave conduction velocity [km/s] of the carbon fiber is CV and the tensile modulus [GPa] of the carbon fiber is TM, then in the relationship between the sound wave conduction speed CV and the tensile modulus TM,
242>37.5×CV-TM>215
Meet carbon fiber.

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