JP7254448B2 - Carbon fiber, carbonization method, and method for producing carbon fiber - Google Patents

Description

TECHNICAL FIELD The present invention relates to a carbon fiber having a uniform structure, a method for carbonizing the carbon fiber, and a method for producing the carbon fiber.

Carbon fiber has excellent specific strength and specific modulus and is lightweight, so it can be used as a reinforcing fiber for thermosetting and thermoplastic resins, not only for conventional sports and general industrial applications, but also for aerospace applications, automobile applications, etc. Used for a wide range of purposes. In recent years, the superiority of carbon fiber composite materials has increased more and more, and there is a high demand for improving the performance and productivity of carbon fiber composite materials, especially in automobiles, aerospace applications. The properties of composite materials are largely due to the properties of carbon fibers themselves, and the demand for improving the properties of carbon fibers themselves is increasing.
In general, it is widely known that carbon fibers are obtained by subjecting a precursor fiber to a flameproofing treatment to obtain a flameproofed fiber, and further subjecting the flameproofed fiber to a carbonization treatment. It is also practiced industrially.
Conventionally, there have been proposed techniques for reducing the skin-core structure that occurs inside the fiber in the flameproofing process. For example, Patent Literature 1 proposes a technique for reducing structural unevenness between the central portion and the outer peripheral portion of a single fiber by reducing the heating rate of the fiber in the flameproofing step of the precursor fiber.
However, a technique for obtaining carbon fibers having a uniform structure by carbonizing flame-resistant fibers having structural irregularities generated in the flame-resistant process has not yet been established.

JP-A-2006-274518

SUMMARY OF THE INVENTION In view of the above-described problems, an object of the present invention is to provide a carbon fiber having a uniform structure produced from a flameproof fiber having uneven structure, a method for carbonizing the same, and a method for producing the same.

In the carbon fiber according to one aspect of the present invention, the difference between the maximum and minimum crystal sizes of single fibers is 0.1 nm or less. In the carbon fiber according to one aspect of the present invention, the difference between the maximum value and the minimum value of the degree of crystal orientation of the single fiber is 1% or less.
A carbonization method according to an aspect of the present invention is a carbonization method for carbonizing flame-resistant fibers obtained by flame-resistant precursor fibers, at a temperature increase rate of 500 K/min or more, and 1.50 to 1.70 g/cm Heat the flameproofed fiber to a density in the range of ³ .
A method for producing a carbon fiber according to one aspect of the present invention is a method for producing a carbon fiber in which a precursor fiber is flame-resistant and the flame-resistant fiber is carbonized. Heat the flameproofed fiber to a density in the range of .70 g/cm ³ .

A carbon fiber according to one aspect of the present invention has a uniform structure.
The carbonization method according to one aspect of the present invention enables carbonization into a uniform structure.
Carbon fibers having a uniform structure can be produced by the method for producing carbon fibers according to one aspect of the present invention.

FIG. 3 is a schematic diagram for explaining a skin core structure; It is a schematic diagram showing a manufacturing process of carbon fiber.

＜＜Overview＞＞
The inventors focused on the carbonization process for producing carbon fibers from precursor fibers and conducted repeated studies, using flame-resistant fibers having a skin-core structure and carbonizing them under specific conditions to obtain a uniform structure. It was found that carbon fibers were obtained.
Here, the skin core structure will be described with reference to FIG.
The skin-core structure refers to a double structure having a skin layer A, which is a surface portion and has been oxidized, and a core layer B, which is a central portion and has not been oxidized.
When observed with an optical microscope, the skin layer A is a dark-colored outer peripheral region, and the core layer B is a light-colored inner peripheral region.
The skin-core structure can be evaluated as the ratio of the core layer area to the total cross-sectional area of one fiber in the cross-section, which is obtained by observing the cross-section of the flame-resistant fiber with an optical microscope.

<<Embodiment>>
1. Flameproofing Step (1) The flameproofing step is a step of making the precursor fiber flameproof. In addition, flameproofing is also referred to as flameproofing treatment.
The flameproofing step is performed by passing the precursor fibers through a flameproofing furnace set in an oxidizing atmosphere of 200 to 350[° C.]. Note that the higher the temperature of the processing space and the shorter the processing time, the higher the core rate.
The flameproofing treatment is preferably performed so that the specific gravity of the flameproof fiber is in the range of 1.30 to 1.46 [g/cm ³ ], more preferably in the range of 1.34 to 1.42 [g/cm ³ ]. It is more preferable to perform so as to be

(2) Method for calculating core ratio The method for calculating the core ratio is as follows.
The cross section of the flame resistant fiber was observed by embedding the flame resistant fiber in a room temperature curing 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 portion and the total cross-sectional area including the bright portion and the dark portion 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., Azokun (registered trademark)).
Core ratio = cross-sectional area of light portion/total cross-sectional area x 100 (1)

2. Carbonization Step The carbonization step is a step of carbonizing the flame-resistant fibers having a skin-core structure to obtain carbon fibers.
In addition, carbonization is also called carbonization treatment.
The carbonization process includes a first carbonization process and a second carbonization process.
In the first carbonization step, the flameproof fiber having a skin-core structure is heated at 500 to 1,500 [K/min] so that the density is within the range of 1.50 to 1.70 [g/cm ³ ]. This is a step of heating at an elevated temperature. The rate of temperature increase is calculated from the temperature of the fibers running in the furnace, with the temperature at which the fiber enters the furnace as a reference.
The second carbonization step is a step of heating the fibers that have undergone the first carbonization step so that the density is within the range of 1.70 to 1.85 [g/cm ³ ].

3. Carbon Fiber Carbon fibers having a uniform structure can be obtained by performing the carbonization step including the first carbonization step on the flameproof fiber having a skin-core structure.
(1) Crystal size Here, the uniform structure means that the difference between the maximum value and the minimum value of the crystal size (Lc) of the carbon fiber is less than 0.1 [nm]. Further, a more uniform structure means that the difference between the maximum value and the minimum value of the crystal size (Lc) of the carbon fiber is 0.06 [nm] or less.
Further, the uniform structure here means that the difference between the maximum value and the minimum value of the crystal orientation degree of the single fiber of the carbon fiber is 1 [%] or less. A more uniform structure means that the difference between the maximum value and the minimum value of the crystal orientation degree of the single fiber is 0.9 [%] or less. The unit of the degree of crystal orientation is "%", and the difference between the maximum value and the minimum value here is simply the maximum value minus the minimum value, and does not represent a ratio.
As a result, for example, even if a defect is generated on the surface of the outer circumference of the single fiber, stress is less likely to concentrate on the defect because the structural difference between the inner and outer layers of the single fiber is small. This makes it difficult for defects to become starting points of destruction.

The crystal size Lc and crystal orientation Π002 can be measured at a synchrotron radiation facility. For example, the wavelength 0.1305 [nm] (9,5 [keV]) at the beamline BL03XU owned by the frontier soft matter development industry-academia alliance in the large-scale synchrotron radiation facility SPring-8 of the Japan Synchrotron Radiation Research Center X-rays are collected with an X-ray Fresnel zone plate (FZP) so that the irradiation diameter of the sample is 1 [μm] at the full width at half maximum, detection resolution [50 μm / pixel], detection range 1032 pixels × 1032 pixels X With a line plane detector (FPD) and a camera length of 54.2 [mm], while scanning the single yarn sample at a pitch of 1.0 [μm] in the direction perpendicular to the fiber axis, each point was measured with an exposure time of 10 seconds. do. The direction perpendicular to the fiber axis is the direction perpendicular to the fiber axis.

Two-dimensional tif data obtained by measurement using the analysis program Fit2D (distributed from http://www.esrf.eu/computing/scientific/FIT2D/) available from the web page of the European Synchrotron Radiation Research Institute Equatorial and azimuthal profiles are extracted for the diffraction peak of the (002) plane of graphite. The equatorial profile is obtained by assuming a diffraction peak of the (002) plane of graphite and an amorphous peak on the low-angle side, performing peak fitting with a Gaussian function, and determining the full width at half maximum of the diffraction peak of the (002) plane of graphite. ask for size. The azimuth angle profile obtains the degree of orientation from the full width at half maximum of Gaussian function fitting. Fiber diametric position calibration determines the fiber center and ends (edges) by fitting the cross-fiber axis distribution of the azimuthal profile intensity at each measurement position with an elliptic function.

4. Manufacturing Method A method for manufacturing the flame-resistant fiber having a skin-core structure described in the above "1. Flame-resistant step" and a method for manufacturing the carbon fiber including the above-described "2. Carbonization step" will be described.
Here, a case where the precursor fiber is an acrylonitrile fiber will be described as an example.

(1) Manufacturing process of carbon fiber FIG. 2 is a schematic diagram showing a manufacturing process of carbon fiber.
Carbon fibers are manufactured using precursors, which are precursor fibers. A single precursor is a bundle of filaments, for example, 24,000 filaments. Depending on the case, it may be called a precursor fiber bundle or a carbon fiber bundle.
The precursor 1a is obtained by spinning a spinning solution obtained by polymerizing a monomer containing 90 [mass%] or more, preferably 95 [mass%] or more of acrylonitrile by a wet spinning method or a dry-wet spinning method, followed by washing, drying, and stretching. obtained by As monomers to be copolymerized, alkyl acrylate, alkyl methacrylate, acrylic acid, acrylamide, itaconic acid, maleic acid and the like are used.
Generally, the speed of producing the precursor 1a differs from the speed of making the precursor 1a flameproof and carbonizing to produce carbon fibers. Therefore, the manufactured precursor 1a is temporarily housed in a carton or wound on a bobbin.
The number of filaments of the precursor fiber is preferably 1,000 or more, more preferably 12,000 or more, and particularly preferably 24,000 or more, in terms of production efficiency.

As shown in FIG. 2, the precursor 1a is pulled out from, for example, a bobbin 30 and runs downstream. On the way, it undergoes various treatments and is wound around a bobbin 39 as carbon fiber.
As shown in FIG. 2, the carbon fiber is produced through a flameproofing step of making the precursor 1a flameproof and a carbonization step of carbonizing the flameproofed fiber (hereinafter referred to as "flameproof fiber") 1b while stretching. , a surface treatment step of improving the surface of the carbonized fiber (hereinafter also referred to as “post-carbonized fiber”) 1d, a sizing step of attaching a sizing agent to the fiber 1e with the improved surface, and a sizing agent It is manufactured through a drying process for drying the adhering fibers 1f.
1 g of dried fiber is wound on a bobbin 39 as 1 g of carbon fiber. The fibers that have undergone each step are distinguished, for example, as the flame-resistant fiber 1b, but the reference numeral "1" is used when simply describing them as "fibers".
Here, the treatment for improving the surface of the fiber 1d after carbonization is the surface treatment, the treatment for attaching the sizing agent to the fiber 1e with the improved surface is the sizing treatment, and the treatment for drying the fiber 1f with the sizing agent attached is the drying treatment. said respectively. Each step and each treatment will be described below.

(1-1) Flameproofing step (flameproofing treatment)
The flameproofing process is performed using a flameproofing furnace 3 set in an oxidizing atmosphere at a temperature within the range of 200 to 350 [° C.]. Specifically, flameproofing is performed by passing the precursor 1a through a flameproofing furnace 3 in an air atmosphere. Note that the oxidizing atmosphere may contain oxygen, nitrogen dioxide, or the like.
In the flameproofing process, the temperature inside the furnace increases from the upstream side to the downstream side. This is for efficient flameproofing without inducing breakage of the precursor 1a.
The heating time in the flameproofing process is within the range of 40 to 80 [minutes]. In this flameproofing, the temperature, heating time, etc. are set so that the density of the precursor 1a is 1.5 [g/cm ³ ] or less.
The precursor 1a during the flameproofing step is drawn with a predetermined tension in accordance with the properties of the carbon fiber 1g to be produced. The draw ratio in the flameproofing step is preferably in the range of -10 to +10 [%], more preferably in the range of -5 to 0 [%]. Drawing of the precursor 1a is performed by a plurality of rollers. For example, stretching is performed by rollers 5 and 7 at the entrance of the flameproofing furnace 3 and rollers 9, 11 and 13 at the exit.

(1-2) Carbonization step (carbonization treatment)
The carbonization step is a step of heating the flameproof fiber 1b to cause a pyrolysis reaction to carbonize the fiber. Carbonization is performed by passing the flameproof fibers 1 b through the first carbonization furnace 15 and passing the fibers 1 c that have 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", the treatment performed in the first carbonization furnace 15 is referred to as "first carbonization treatment", and the first carbonization furnace 15 The step that is performed is referred to as the "first carbonization step".
Similarly, the carbonization performed in the second carbonization furnace 17 is referred to as “second carbonization”, the treatment performed in the second carbonization furnace 17 is referred to as “second carbonization treatment”, and the second carbonization furnace 17 The process to be performed is referred to as the "second carbonization process", and the fibers 1d that have undergone the second carbonization treatment and the second carbonization process (exited from the second carbonization furnace 17) are referred to as "post-carbonization fibers".
Here, the first carbonization furnace 15 and the second carbonization furnace 17 are provided independently of each other, and adjusting means for adjusting the tension of the fibers can be provided between the carbonization furnaces 15 and 17. . Specifically, outside the first carbonization furnace 15, rollers 19 are provided on the entrance side, rollers 21 are provided between the first carbonization furnace 15 and the second carbonization furnace 17, and the second carbonization furnace 15 is provided. A roller 23 is provided outside the furnace 17 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 maximum temperature inside the second carbonization furnace 17 is the maximum temperature in the carbonization step.

In the first carbonization step, the flameproof fiber is heated at a temperature elevation rate of 500 to 1,500 [K/min]. A more preferable heating rate is 600 to 1,200 [K/min]. The heating time is the time required for the density of the fiber being treated to reach a predetermined value within the range of 1.5 to 1.7 [g/cm ³ ]. Examples of heating means for heating at the above-described rate of temperature increase include microwaves and plasma.
In the second carbonization step, the maximum temperature of the second carbonization furnace may be appropriately adjusted according to the desired strand elastic modulus of the carbon fiber, and is between 1,000 and 2,000 [°C]. preferable. Examples of heating means in the second carbonization step include microwaves, plasma, electric heaters, and the like.

(1-3) Surface treatment step (surface treatment)
The surface treatment step is performed by passing the carbonized fibers 1 d through the surface treatment device 25 . A roller 26 is provided outside the surface treatment device 25 and on the outlet side. By surface treatment, when 1 g of carbon fiber is used to form a composite material, the affinity and adhesiveness between 1 g of carbon fiber and matrix resin are improved.
The surface treatment is generally performed by oxidizing the surface of the fiber 1d after carbonization. Surface treatments include, for example, treatments in the liquid phase or in the gas phase.
As the treatment in the liquid phase, chemical oxidation by immersing the fiber 1d after carbonization in an oxidizing agent, and anodic electrolytic oxidation by energizing the fiber 1d after carbonization in an electrolytic solution in which the fiber 1d after carbonization is immersed are industrially used. be done. The treatment in the gas phase can be carried out by passing the carbonized fibers 1d through an oxidizing gas or spraying active species generated by electric discharge or the like.

(1-4) Sizing step (sizing treatment)
The sizing step is performed by passing the surface-treated fiber 1 e through a sizing agent solution 29 stored in a sizing agent bath 27 . The sizing step enhances 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 direction of travel by a plurality of rollers 31, 33, etc. arranged inside and around the sizing agent bath 27. do. As the sizing agent solution 29, for example, a liquid obtained by dissolving an epoxy resin, a urethane resin, a phenol resin, a vinyl ester resin, an unsaturated polyester resin, or the like in a solvent, or an emulsion liquid in which the resin is dispersed in the solvent is used.

(1-5) Drying process (drying treatment)
The drying process is performed by passing the fibers 1f to which the sizing agent is attached through the drying furnace 35. As shown in FIG. Incidentally, 1 g of the dried fiber is wound on a bobbin 39 outside the drying furnace 35 via rollers 37 on the downstream side (this is a winding process).

[Examples 1 and 2, Comparative Examples 1 and 3]
Polyacrylonitrile fibers (single fiber fineness 0.8 [dtex], number of filaments 24,000), which are precursor fibers shown in Table 1, were subjected to the first carbonization step and the second carbonization step. After surface treatment by electrolytic oxidation in an aqueous solution of ammonium sulfate, this was subjected to sizing treatment with an epoxy resin.
Table 1 shows the conditions of each carbonization step and the crystal size and crystal orientation of the carbon fibers. The flame-resistant fiber used for carbonization was the same in both Examples and Comparative Examples, and Table 2 shows the characteristics of the flame-resistant fiber. The flame-resistant fiber has a core rate of 10% and has a skin-core structure.

(1) crystal size Lc
In Examples 1 and 2 using the carbonization method of the present invention, the difference between the maximum value and the minimum value of the crystal size Lc of the carbon fibers was 0.06 [nm] or less, and carbon fibers with a uniform crystal size Lc were obtained. was taken.
Example 1 and Comparative Example 1 differ only in the rate of temperature increase in the first carbonization step, and the conditions in the second carbonization step are the same. Differences in the temperature rise rate in the first carbonization step caused differences in the maximum and minimum crystal sizes. In Example 1 and Comparative Example 1, although the heating rate in the first carbonization step is different, the carbonization temperature is substantially the same.
Similarly, Example 2 and Comparative Example 2 differ only in the rate of temperature increase in the first carbonization step, and the conditions in the second carbonization step are the same. In this case as well, the difference in the maximum and minimum crystal sizes was caused by the difference in temperature increase rate in the first carbonization step. In Example 2 and Comparative Example 2, the rate of temperature increase in the first carbonization step is different, but the carbonization temperature is substantially the same. Further, the carbonization temperatures in the first carbonization step of Examples 1 and 2 and Comparative Examples 1 to 3 are substantially the same.
Thus, it was found that the rate of temperature increase in the first carbonization affects the crystal size Lc of carbon fibers.

The heating rate of the first carbonization in Example 2 was 600 [K/min], and the heating rate of the first carbonization in Comparative Examples 1 to 3 was 400 [K/min]. From this, it is considered that a carbon fiber having a uniform crystal size can be obtained at a heating rate of 500 [K/min] or more in the first carbonization. When the heating rate in the first carbonization step is 400 [K/min] or less, the difference between the maximum and minimum crystal sizes can be reduced even if the heating rate in the second carbonization step is changed. I didn't.

(2) Degree of crystal orientation In Examples 1 and 2 using the carbonization method of the present invention, the difference between the maximum value and the minimum value of the degree of crystal orientation of carbon fibers was 0.9 [%] or less, and the crystal orientation was uniform. degree of carbon fiber was obtained.
Example 1 and Comparative Example 1 differ only in the rate of temperature increase in the first carbonization step, and the conditions in the second carbonization step are the same. Differences in the temperature rise rate in the first carbonization step caused differences in the maximum and minimum values of the degree of crystal orientation.
Similarly, as described above, Example 2 and Comparative Example 2 differ only in the rate of temperature increase in the first carbonization step, and the conditions in the second carbonization step are the same. Differences in the temperature rise rate in the first carbonization step caused differences in the maximum and minimum values for the same crystal orientation.
Thus, it was found that the rate of temperature increase in the first carbonization affects the degree of crystal orientation of carbon fibers.

The heating rate of the first carbonization in Example 2 was 600 [K/min], and the heating rate of the first carbonization in Comparative Examples 1 to 3 was 400 [K/min]. From this, it is considered that a carbon fiber having a uniform degree of crystal orientation can be obtained at a heating rate of 500 [K/min] or more in the first carbonization. When the heating rate of the first carbonization is 400 [K/min] or less, even if the heating rate in the second carbonization step is changed, the difference between the maximum value and the minimum value of the degree of crystal orientation is reduced. could not.

<<Modification>>
As mentioned above, although it demonstrated based on embodiment, this invention is not limited to embodiment. For example, any of the modified examples and embodiments described below may be appropriately combined, or a plurality of modified examples may be appropriately combined.

Carbon fiber In the item "2. Production method", the method for producing carbon fiber with 24,000 filaments was explained, but other methods such as 3,000, 6,000, 12,000 It can also be applied to a method of carbonizing precursor fibers and producing carbon fibers.
In the item "3. Manufacturing method", the carbon fiber manufacturing method including the carbonization process was described, but for example, graphitization treatment may be further performed before the surface treatment process.

REFERENCE SIGNS LIST 1 fiber 1a precursor 1b flameproof fiber 15 first carbonization furnace 17 second carbonization furnace

Claims (8)

Carbon fibers having a density of 1.70 to 1.85 g/cm ³ (excluding 1.81 g/cm ³ ),
A carbon fiber in which the difference between the maximum and minimum crystal sizes of a single fiber is 0.1 nm or less. The carbon fiber according to claim 1, wherein the difference between the maximum value and the minimum value of the degree of crystal orientation of the single fiber is 1% or less. Carbon fibers having a density of 1.70 to 1.85 g/cm ³ (excluding 1.81 g/cm ³ ),
A carbon fiber in which the difference between the maximum value and the minimum value of the degree of crystal orientation of a single fiber is 1% or less. The carbon fiber according to claim 1 or 2, wherein the difference between the maximum and minimum crystal sizes of the single fibers is less than 0.1 nm. The carbon fiber according to claim 2 or 3, wherein the difference between the maximum value and the minimum value of the degree of crystal orientation of the single fiber is 0.9% or less. The method for producing carbon fiber according to any one of claims 1 to 5, wherein the flame-resistant fiber obtained by flame-resistant precursor fiber is carbonized,
A method for producing carbon fiber, wherein the flameproof fiber is heated at a temperature increase rate of 600 K/min or more until the density is within the range of 1.50 to 1.70 g/cm ³ . The method for producing carbon fiber according to any one of claims 1 to 5, wherein the flame-resistant fiber obtained by flame-resistant precursor fiber is carbonized,
A method for producing carbon fibers, wherein the flame-resistant fibers are heated using microwaves or plasma at a heating rate of 500 K/min or higher until the density is within the range of 1.50 to 1.70 g/cm ³ . The method for producing carbon fiber according to any one of claims 1 to 5, wherein the flame-resistant fiber obtained by flame-resistant precursor fiber is carbonized,
The flameproof fiber is heated using microwaves or plasma at a heating rate of 600 K/min or more until the density is within the range of 1.50 to 1.70 g/cm ^3. Method for producing carbon fiber .

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