JP2636509B2 - Carbon fiber and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon fiber and a method for producing the same. More particularly, the present invention relates to a carbon fiber having high density, excellent strength and elastic modulus, and a method for producing the same.

[0002]

2. Description of the Related Art Carbon fibers are widely used in aerospace applications, leisure applications, general industrial applications, etc. due to their excellent mechanical properties, especially high specific strength and specific elastic modulus. However, its performance is not sufficient depending on the use, and the demand for the development of a high-performance carbon fiber having higher strength and elastic modulus is increasing every day.

In order to produce such a high-performance carbon fiber, it is particularly important to reduce the amount of voids in the carbon fiber which can be a starting point of fracture, that is, to increase the density. For that purpose, for example, a technique for improving the denseness of a precursor by specifying a spinning method for an acrylic polymer, or various post-treatments such as a gas phase treatment, a liquid phase treatment, and an electrolytic treatment for carbon fibers are used. A technique has been proposed in which the voids in the fiber surface layer are removed by application to improve the compactness.
215527, JP-A-61-225330). However, voids that reduce the denseness of carbon fibers are mainly formed during the carbonization process, and there is a limit only to specifying a precursor manufacturing method, and also, by a post-treatment following the carbonization treatment. In the technology for improving the density, there was a problem that only voids in the surface layer of the fiber could be removed, leaving voids inside and insufficient densification.Therefore, a technology for carbonizing under pressure to suppress generation of voids was developed. It has been proposed (JP-A-2-289121).

[0004]

However, although such a technique involves carbonization under pressure, it is practically 5 times.
It is related to carbonization under a relatively low pressure condition of 00 kgf / cm ² · G or less, and is insufficient from the viewpoint of densification, and the improvement in the physical properties of carbon fibers such as strength and elastic modulus is small. Was.

In order to obtain high modulus carbon fibers,
It is necessary to perform calcination at a high temperature exceeding 1500 ° C., that is, a graphitization treatment. In such a high temperature range, nitrogen is desorbed to form a large amount of microvoids. However, it is necessary to suppress desorption of nitrogen forming microvoids. In this case, the treatment must be performed at a low temperature, and in that case, there is a contradiction that a high modulus yarn cannot be obtained. That is, according to the prior art, generally, when the heat treatment temperature is increased, the nitrogen content is reduced, and only a trace of nitrogen is left in the treatment at about 2000 ° C. This temperature also depends on the processing time, and nitrogen is eliminated at a lower temperature in a long-time processing. At the same time, there is a problem that microvoids are formed and the compactness is reduced, and the strength of the obtained graphitized carbon fiber is reduced.

[0006]

Means for Solving the Problems To solve the above problems, the carbon fiber of the present invention has the following constitution. That is, the resin-impregnated strand strength is 620 kgf / mm ² or more,
The crystal size Lc (angstrom) is 30 or less,
And Lc and nitrogen content N (%) are N ≧ 0.04 (Lc−30)
Satisfies the relationship of ² +0.5 and small angle scattering intensity is 880 cps
The carbon fiber characterized by the following, or ◎ The resin-impregnated strand strength is 490 kgf / mm ² or more, and the crystal size Lc (angstrom) exceeds 30, and
A carbon fiber having a nitrogen content N (%) of 0.5 or more and a small-angle scattering intensity of 1550 cps or less.

[0007] In order to solve the above-mentioned problems, a method for producing carbon fiber of the present invention has the following configuration. That is, ◎
In a method of obtaining a carbon fiber by firing a carbon fiber precursor in an oxidizing atmosphere after oxidizing or infusing the precursor, the firing step includes reducing an initial pressure at room temperature.
400kgf / cm ²・ G or more and the maximum pressure during heat treatment is 10
A method for producing carbon fiber, comprising a step of firing under hot isostatic pressure of not less than 00 kgf / cm ² · G.

Hereinafter, the present invention will be described in more detail.

In the carbon fiber of the present invention, when the crystal size Lc (angstrom) is 30 or less, the relationship between Lc and nitrogen content N (%) is N ≧ 0.04 (Lc−30) ² +0.5. Satisfying small angle scattering intensity of 880 cps or less, and having a crystal size Lc (angstrom) of 30
Is exceeded, the nitrogen content N (%) is 0.5 or more, and the small-angle scattering intensity is 1550 cps or less.

Here, the crystal size Lc means a crystal size (angstrom) determined by X-ray diffraction as follows. That is, a Kα ray of Cu monochromatized by a Ni filter was used as an X-ray source, and 2θ = 26.0 °
The half-value width is obtained from the peak obtained by scanning the peak of the plane index (002) observed in the vicinity in the equator direction,
The value calculated by the following equation is defined as the crystal size Lc.

Lc = λ / (β _o cos θ) where λ: wavelength of X-ray (1.5418 angstroms in this case), θ: diffraction angle, β _o : true half-value width.
Here, β _o uses a value calculated by the following equation.

Β _o = (β _e ² −β _l ² ) ^1/2 where β _e : apparent half-value width, β _l : device constant (output of 4036A2 type X-ray generator manufactured by Rigaku Corporation, output 35 kV, 15m
1.05 × 10 ^-2 rad when used in A).

[0013] The nitrogen content N is measured by CH from Yanagimoto Seisakusho.
A value determined by elemental analysis using an N-coder MT-3.

Sample decomposition furnace temperature: 950 ° C. Oxidation furnace temperature: 850 ° C. Reduction furnace temperature: 550 ° C. Helium flow rate: 180 ml / min. Oxygen flow rate: 25 ml / min. The measurement conditions for elemental analysis are as described above.

FIG. 1 is a graph showing the relationship between the crystal size Lc and the nitrogen content N. The region above the curve in the graph is the carbon fiber of the present invention.

Conventional highly elastic carbon fibers, in other words, so-called graphitized carbon fibers having a crystal size Lc of more than 30 have only a trace nitrogen content, whereas the carbon fibers of the present invention have only a trace nitrogen content. Those having a crystal size Lc of more than 30 have a nitrogen content of 0.5% or more. In the region where the crystal size Lc exceeds 30, the nitrogen content is 0.1.
If it is less than 5, the generation of microvoids due to the desorption of nitrogen cannot be suppressed.
0 cps or less cannot be obtained, and carbon fibers having high strength and elastic modulus cannot be obtained.

Further, in the region where the crystal size Lc is 30 or less, the conventional carbon fiber is at most 0.04 (Lc−30) ² +0.
While the carbon fiber of the present invention had a nitrogen content not exceeding 5% (%), the carbon fiber of the present invention had a nitrogen content of 0.04 (Lc-30) ² +
It has a nitrogen content of 0.5% (%) or more. In a region where the crystal size Lc is 30 or less, the nitrogen content is ｛0.
If it does not exceed 04 (Lc-30) ² + 0.5% (%), the microvoids generated by the desorption of nitrogen cannot be reduced in the same manner as described above. 8
It cannot be less than 80 cps, and it is not possible to obtain carbon fibers having high strength and elastic modulus.

In other words, when the crystal size is kept constant, the nitrogen content is much higher than that of the conventional carbon fiber, so that the microvoids generated by the desorption of nitrogen are very small. As a result, the starting point of fracture is reduced, and carbon fibers having extremely high strength and elastic modulus can be obtained. Specifically, the strength of the resin-impregnated strand is set to 6 for carbon fibers having a crystal size Lc of 30 or less.
20 kgf / mm ² or more, and the crystal size Lc is 3
In the case of carbon fibers exceeding 0, it can be 490 kgf / mm ² or more.

The carbon fiber of the present invention is manufactured as follows. In the precursor for carbon fiber in the present invention,
Acrylic fibers, pitch fibers, rayon fibers and the like can be used.

Examples of the acrylic polymer include a copolymer comprising 85% or more of acrylonitrile and 15% or less of a polymerizable unsaturated monomer copolymerizable with acrylonitrile. Examples of the polymerizable unsaturated monomer copolymerizable with acrylonitrile include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, and itaconic acid, and alkali metal salts, ammonium salts and alkyl esters thereof, acrylamide, methacrylamide and the like. Derivatives, allylsulfonic acid, methallylsulfonic acid and salts or alkyl esters thereof, fluorine-containing unsaturated monomers, silicon-containing unsaturated monomers, boron-containing unsaturated monomers and the like.

Among them, it is preferable to copolymerize a polymerizable unsaturated monomer such as an unsaturated carboxylic acid which promotes a flame-resistant reaction. Specific examples of unsaturated carboxylic acids include, for example, acrylic acid, methacrylic acid, itaconic acid,
Crotonic acid, citraconic acid, ethacrylic acid, maleic acid, mesaconic acid and the like can be mentioned.

The method for polymerizing the acrylic polymer may be any of known methods such as solution polymerization, suspension polymerization and emulsion polymerization. If the degree of polymerization of the acrylic polymer is expressed by intrinsic viscosity [η] as an index indicating the degree of polymerization, it is preferably [η] 1.2 or more, more preferably 1.η.
5 or more, more preferably 1.7 or more. here,
The intrinsic viscosity [η] refers to a value measured as follows.

A dry sample of an acrylic polymer (75 mg) was placed in a 25 ml volumetric flask, and 25 ml of a 0.1N sodium thiocyanate-dimethylformamide solution was added to completely dissolve the solution. The specific viscosity η at 25 ° C. was measured using an Ostwald viscometer. Measure _sp . [Eta] = - from ^{_{{(1+ 1.327η sp) 1/2 1}} } /0.198 specific viscosity eta _sp, the value calculated by the above equation and the intrinsic viscosity [eta].

The obtained copolymer can be used as a precursor by a known method. At that time, a known organic or inorganic solvent can be used as a solvent for spinning. The spinning may be performed by a wet spinning method of directly spinning into a coagulation bath or a dry-wet spinning method of once spinning into air and then coagulating in a bath. Regarding drawing, the spun yarn may be drawn directly in a bath, or may be drawn in a bath after washing with water to remove a solvent. The conditions for stretching in the bath are usually from 50 to 9
The film is stretched about 2 to 6 times in a stretching bath at 8 ° C. The dried and densified yarn is dried by a hot drum or the like after drawing in a bath to achieve dry densification. The drying temperature, time and the like can be appropriately selected. If necessary, the dried and densified yarn is subjected to steam drawing under pressure, whereby a precursor having a predetermined denier and orientation degree can be obtained.

In order to improve the density of the carbon fiber, it is effective to improve the density of the precursor. As a measure of the denseness of the precursor, a value of ΔL by an iodine adsorption method is used, and ΔL is preferably 45 or less, more preferably 30 or less, and further preferably 10 or less. It is generally difficult to make ΔL 5 or less. Here, ΔL by the iodine adsorption method refers to a value obtained as follows. About 0.5 g of a dry sample having a fiber length of 5 to 7 cm was precisely weighed and placed in a 200 ml Erlenmeyer flask with a stopper, to which an iodine solution (I ₂ : 51 g, 2,4-dichlorophenol: 10 g, acetic acid: 90 g and Potassium iodide: 100 g is precisely weighed, transferred to a 1-liter volumetric flask and dissolved in water to a constant volume.
The adsorption treatment is performed while shaking at 0 ° C. for 50 minutes. The sample adsorbed with iodine is washed with running water for 30 minutes, then centrifugally dehydrated (2000 rpm, 1 minute) and quickly air-dried.

After the sample was opened, the brightness (L value) was measured using a Hunter-type color difference meter CM-25 (manufactured by Color Machine Co., Ltd.).
Is measured (L ₁ ). On the other hand, the corresponding sample not subjected to the iodine adsorption treatment is opened, and the lightness (L ₀ ) is similarly measured by the hunter type color difference meter, and the lightness difference L ₀ −L ₁ is defined as ΔL by the iodine adsorption method. .

In order to obtain a dense precursor having a value of ΔL of 45 or less by the iodine adsorption method, it is necessary to lower the temperature of the spinning solution and the coagulation bath solution, lower the tension during coagulation, and optimize the stretching ratio and the stretching temperature. It is an effective means.

As the conditions for the flame resistance of the precursor, tension or stretching conditions in an oxidizing atmosphere at 200 to 300 ° C. are preferably employed. The density of the oxidized yarn obtained in the oxidization step is preferably 1.30 g / cm ³ or more, more preferably 1.35 g / cm ³ or more. In other words, when the flame resistance is insufficient and the density of the flame resistant yarn is less than 1.30 g / cm ³ , adhesion between single yarns is liable to occur at the time of carbonization, and the amount of generated decomposition gas increases. Since the denseness tends to decrease, it is difficult to obtain high-strength carbon fibers. On the other hand, if the flame resistance is excessively promoted, the main chain of the polymer is cut, and the strength of the finally obtained carbon fiber is reduced. Therefore, the density of the flame resistant yarn does not exceed 1.45 g / cm ^3. preferable.

In the method for producing carbon fiber of the present invention,
After performing the flame resistance as described above, when firing in an inert atmosphere, it is characterized by including a step of firing under hot isostatic pressure, specifically, the carbonization step A mode in which a part is a step of firing under hot isostatic pressure, a mode in which all of the carbonization steps are a step of firing under hot isostatic pressure, and a part of the graphitization step is a hot isostatic pressure Examples include a mode in which firing is performed under pressure, and a mode in which all of the graphitization steps are processes in which firing is performed under hot isostatic pressure. In addition,
In the case of the first embodiment, in which a part of the carbonization step is a step of firing under hot isostatic pressure, it is preferable to perform the last step of the carbonization step under hot isostatic pressure for the reason described below. preferable.

Here, for convenience, the firing step in an inert gas was divided into carbonization and graphitization in the temperature range. What is carbonization?
A firing process at 500 ° C. or lower, and graphitization is 1500 ° C.
Sintering process.

In other words, in order to suppress the formation of microvoids during firing and to eliminate the generated microvoids, a firing step under hot isostatic pressure during firing is included at any stage. In order to suppress the formation of microvoids and to eliminate the microvoids, the pressure at the time of the hot isostatic pressure treatment should be high, and is set to 1000 kgf / cm ² · G or more, preferably 2000 kgf / cm ² · G or more. In addition,
At present, it is generally difficult to increase the pressure to 10,000 kgf / cm ² · G or more due to limitations of the apparatus. In the hot isostatic pressure treatment, the initial pressure at room temperature needs to be 400 kgf / cm ² · G or more.

From the flame-resistant yarn, water, methane, ammonia,
Gases such as carbon dioxide, hydrogen cyanide and nitrogen are generated in the range of 200 to 2000 ° C.,
Water, methane, ammonia,
Since low-boiling components such as carbon dioxide and hydrogen cyanide are generated most, including a step of increasing the pressure at the time of firing in this temperature range is effective in suppressing the formation of microvoids.

The carbonization step may be performed under hot isostatic pressing conditions in all temperature ranges, but from the viewpoint of protecting the apparatus and preventing self-contamination, only some of the temperature ranges are subjected to hot isostatic pressing. Pressurization is also preferably performed. It is preferable to complete the carbonization step under hot isostatic pressure from the viewpoint of increasing the density of the carbon fiber by minimizing the gas generated in the carbonization step.

In the case of acrylic carbon fibers, if the treatment temperature exceeds 1500 ° C., the amount of microvoids generated due to disappearance of nitrogen atoms generally increases, and the strength tends to decrease. On the other hand, carbon is plasticized as the temperature becomes higher than 1500 ° C., so that applying this temperature range under pressure is effective in eliminating microvoids in the graphitization step. Therefore, hot isostatic pressing is preferably performed in the graphitization step. The carbonized fiber to be subjected to the graphitization step is a conventional normal pressure or 50
The carbonization process under a pressurizing condition of less than 0 kgf / cm ² · G or the carbonization process under the hot isostatic pressing described above may be used. From the viewpoint of obtaining the above, the latter is more preferable.

The maximum temperature for graphitization under hot isostatic pressure is preferably 1500 ° C. or higher, more preferably 20 ° C., from the viewpoint of improving the compactness of the carbon fiber to be obtained.
The temperature is at least 00 ° C, more preferably at least 2500 ° C.

In order to obtain a high modulus carbon fiber, 1500 is required.
It is necessary to perform calcination at a high temperature exceeding ℃, that is, graphitization treatment, and in such a temperature range, hot isostatic pressing is effective to suppress the formation of a large amount of microvoids due to elimination of nitrogen. This is as already described for the carbon fiber of the present invention.

In the production method of the present invention, it is preferable to use an inert gas such as nitrogen or argon as a pressure medium for achieving a high pressurized state during firing.

Examples of the method of carbonization or graphitization under hot isostatic pressure include batch processing. For example, it is preferable that the flame-resistant yarn or the carbonized fiber is wound around a tubular or plate-like material such as a graphite bobbin or a graphite plate that can withstand high temperature and high pressure, and both ends of the fiber to be treated are fixed and in a tension state. At this time, the length of the oxidized yarn or the carbonized fiber is preferably 3 mm or more, and a continuous long fiber bundle that can be subjected to a tension treatment is more preferable.

In the production method of the present invention, a means of hot isostatic pressing is employed in order to simultaneously achieve high temperature and high pressure. As a device for achieving such a hot isostatic pressing state, a known hot isostatic pressing device used for densification of ceramics, glassy carbon, or the like can be used.

In this case, the flame-resistant yarn or carbonized fiber is molded as a preform and carbonized under hot isostatic pressure.
Alternatively, after heat treatment, a matrix precursor or the like can be impregnated to provide a carbon fiber reinforced composite material.

In the production method of the present invention, the heating rate at the time of heating is particularly 300 to 500 ° C. and 1000 to 1
The rate of temperature rise in the temperature range of 200 ° C. is preferably 300 ° C./min or less, more preferably 100 ° C./min or less, and further preferably 50 ° C./min or less.
This is effective in improving the denseness of the obtained carbon fiber.

The carbon fiber obtained by the production method of the present invention can be subjected to a surface treatment, a sizing agent application, and the like by a conventionally known technique, if necessary.

Although the above description has been made with reference to the example of acrylic carbon fiber, pitch-based and rayon-based carbon fibers and the like can be used in the same manner as acrylic carbon fiber by using a precursor for carbon fiber in an oxidizing atmosphere. The same effect can be achieved by sintering under a hot isostatic pressure a part or all of the sintering step in an inert atmosphere after the melting or flame resistance.

As the small-angle scattering intensity and the resin-impregnated strand characteristics in Examples described later, values obtained by the following methods are used.

(Small Angle Scattering Intensity) Samples are arranged in parallel so that scattering in the direction perpendicular to the carbon fiber axis is measured, solidified with a collodion solution, and set so that the fiber axis is parallel to the longitudinal direction of the X-ray slit. .

The apparatus used was RU-200B type X manufactured by Rigaku Denki Co., Ltd.
A small-angle scattering measurement device using a X-ray generator
The Kα ray of u was used. Measurement is output 40kV, 200mA
Then, the scattering intensity at a position of 1 ° in the equator direction was measured by a scintillation counter, and the air scattering was measured in the same manner, and subtracted from the scattering intensity of the sample to obtain the scattering intensity of the sample.

The small-angle scattering intensity is an index indicating the amount of microvoids in the carbon fiber, and the smaller the small-angle scattering intensity, the smaller the amount of microvoids and the denser the carbon fiber.

(Resin-impregnated strand properties) "Bakelite" ERL-4221 (registered trademark, manufactured by Union Carbide Co., Ltd.) / Boron trifluoride monoethylamine (BF ₃ .MEA) / acetone = 100/3 / 4
The resin-impregnated strand obtained was heated at 130 ° C. for 30 minutes to cure, and measured according to the resin-impregnated strand test method specified in JIS R7601.

[0049]

The present invention will be described more specifically with reference to the following examples.

(Example 1, Comparative Example 1) By a solution polymerization method using dimethyl sulfoxide as a solvent, a polymer concentration of 98% of acrylonitrile and 2% of methacrylic acid was 20%.
Was obtained. This is once discharged into the air through a 3,000-hole die and allowed to travel through the space, then coagulated in an aqueous solution of dimethyl sulfoxide, washed with coagulated yarn, stretched by 4 times in a bath, and applied with a process oil. , Dried and densified. Further, the precursor was stretched to 2.5 times in steam under pressure to obtain a precursor having a single yarn denier of 0.8d and a total denier of 2400D. ΔL of this precursor was 23.

The obtained precursor is heated at 240 to 280 ° C.
In the air at a draw ratio of 1.05 and a density of 1.37 g /
A flame resistant yarn of cm ³ was obtained.

About 20 m of the oxidized yarn is wound around a graphite cylinder, and both ends of the yarn are tied to the graphite cylinder to be in a tensioned state. The pressure (pressure before heating) and the final pressure (pressure at the end of heating) were respectively changed from room temperature to 1000 ° C. at a heating rate of 20 ° C./min, and further 5 ° C./min under the conditions shown in Table 1 1500 ℃
And hold for 30 minutes to remove carbon fibers A ₁ , A ₂ , A
Got _three .

[0053]

[Table 1]

During the heating process, the pressure medium gas inside the pressure vessel and the generated low-boiling compound expand, and the pressure increases. However, when the pressure exceeds the set final pressure, the gas leaks and the internal pressure is kept constant.

For the purpose of comparison, the oxidized yarn was similarly wound around a graphite cylinder, and heated to 1500 ° C. under the same temperature conditions in nitrogen under normal pressure to obtain carbon fiber B (Comparative Example 1).

As a result of the X-ray diffraction, as shown in the following Table 2,
Carbon fibers A ₁ , A ₂ , and A ₃ fired under hot isostatic pressing with respect to carbon fiber B obtained under normal pressure (Comparative Example 1) have high crystallinity and a reduced amount of voids. A dense carbon fiber was obtained.

[0057]

[Table 2]

As a result of measuring the properties of the strands impregnated with the resin, as shown in the following Table 3, the carbon fiber B (Comparative Example 1) obtained under normal pressure was fired under hot isostatic pressure. The carbon fibers A ₁ , A ₂ , and A ₃ have greatly improved in both strength and elastic modulus.

[0059]

[Table 3]

(Example 2, Comparative Example 2) About 20 m of the oxidized yarn obtained in Example 1 was wound around a graphite cylinder, and both ends of the yarn were tied to the graphite cylinder to obtain a tension state. Installed in an isostatic pressurizing device, purged with nitrogen, initial pressure 800 kgf / cm ²
G, final pressure 2000 kgf / cm ² · G, heating rate 20 ° C /
The temperature was raised from room temperature to 1000 ° C. in minutes, and further, to the maximum processing temperature under the conditions shown in Table 4 below, and maintained for 30 minutes to obtain carbon fibers C ₁ , C ₂ and C ₃ . During the heating process, the pressure medium gas inside the pressure vessel and the generated low-boiling compound expanded to increase the pressure. However, when the pressure exceeded the set final pressure, the gas was leaked and the internal pressure was kept constant.

[0061]

[Table 4]

For the purpose of comparison, similarly, the above-mentioned flame-resistant yarn was wound around a graphite cylinder, and heated under nitrogen at normal pressure under conditions corresponding to the firing temperature conditions of carbon fibers C ₁ , C ₂ and C ₃ , respectively. Thus, carbon fibers D ₁ , D ₂ and D ₃ were obtained (Comparative Example 2).

The results of X-ray diffraction are also shown in Table 2 above. In contrast to carbon fibers D ₁ , D ₂ and D ₃ obtained under normal pressure (Comparative Example 2), carbon fibers C ₁ , C ₂ and C ₃ obtained under hot isostatic pressing are each crystalline. And the amount of voids was reduced, and dense carbon fibers were obtained.

The results of measuring the resin-impregnated strand characteristics are also shown in Table 3 above. The carbon fibers C ₁ , C ₂ , and C ₃ obtained under hot isostatic pressing have the same strength as the carbon fibers D ₁ , D ₂ , and D ₃ obtained under normal pressure (Comparative Example 2). , The modulus of elasticity was greatly improved. Usually, in the case of acrylic carbon fiber, when the treatment temperature exceeds 1500 ° C., the strength decreases with an increase in the amount of voids. However, by carbonizing under hot isostatic pressure, the amount of voids is greatly reduced. It was found that carbon fibers having high strength, modulus and elastic modulus were obtained.

(Example 3, Comparative Example 3) The oxidized yarn having a density of 1.37 g / cm ³ obtained in Example 1 was further subjected to a continuous heat treatment under a nitrogen stream at a draw ratio of 1.02 to 1400 ° C. Then, a carbon fiber E was obtained.

About 20 m of the carbon fiber E is wound around a graphite cylinder, and both ends of the thread are tied to the graphite cylinder to be in a tension state. Then, the carbon fiber E is placed in a hot isostatic pressing apparatus, and is replaced with nitrogen. The initial pressure (the pressure before heating) and the final pressure (the pressure at the end of heating) were respectively changed from room temperature to 1000 ° C. at a heating rate of 20 ° C./min and 5 ° C. 1500 ° C in minutes
And hold for 30 minutes for carbon fibers F ₁ , F ₂ , F
Got _three . The pressure medium gas inside the pressure vessel expands and the pressure increases during the temperature rise process, but when the pressure exceeds the set final pressure, the gas leaks and the internal pressure is kept constant.

[0067]

[Table 5]

For comparison, the above-mentioned carbon fiber E was similarly wound around a graphite cylinder, and heated at 150 ° C. under the same temperature conditions as those for firing the carbon fibers F ₁ , F ₂ and F ₃ in nitrogen under normal pressure.
The mixture was heated to 0 ° C. to obtain a carbon fiber G (Comparative Example 3).

As a result of the X-ray diffraction, as shown in Table 6 below,
Carbon fibers F ₁ , F ₂ , and F _{3 which} are heat-treated under hot isostatic pressure with respect to carbon fibers G obtained under normal pressure have high crystallinity, and have a reduced amount of voids to produce dense carbon fibers. Obtained.

[0070]

[Table 6]

As a result of measuring the resin impregnated strand characteristics, as shown in the following Table 7, the carbon fiber G obtained under normal pressure and the carbon fiber F obtained under
₁ , F ₂ and F ₃ significantly improved both strength and elastic modulus.

[0072]

[Table 7]

[0073] (Example 4, Comparative Example 4) In a flame-resistant kite obtained in Example 1, further nitrogen stream, draw ratio 1.02 up to 900 ° C. and subjected to continuous heat treatment carbonization <br/> fibers H was obtained.

[0074] The carbon fibers H about 20m wound on a graphite cylinder, the two ends of the yarn after the tension tied to graphite cylinder, placed in a hot isostatic pressing device, and nitrogen substitution , the initial pressure 800kgf / cm ² · G, Tsui圧2000kgf / cm ² · G,
The temperature was increased from room temperature to 1000 ° C. at a rate of temperature increase of 20 ° C./min.
After holding for ₁ minute, carbon fibers J ₁ , J ₂ and J ₃ were obtained. In addition,
During the heating process, the pressure medium gas inside the pressure vessel and the generated low-boiling compound expanded to increase the pressure. However, when the pressure exceeded the set final pressure, the gas leaked and the internal pressure was kept constant.

[0075]

[Table 8]

For comparison, similarly, carbonized fiber H was wound around a graphite cylinder, baked in nitrogen under normal pressure under the same temperature conditions as those for sintering J ₁ , J ₂ and J ₃ , and carbon fiber K
₁ , K ₂ and K ₃ .

As a result of X-ray diffraction, as shown in Table 6 above, carbon fibers J ₁ , K ₂ , and K ₃ obtained under normal pressure were fired under hot isostatic pressure. _1, J _2, J
Sample No. ₃ had high crystallinity, and the amount of voids was reduced, so that dense carbon fibers were obtained.

The results of the measurement of the resin-impregnated strand characteristics are shown in Table 7 above. The results are shown in Table 7. The carbon fibers K ₁ , K ₂ , and K ₃ obtained under normal pressure were subjected to hot isotropy. The carbon fibers J ₁ , J ₂ , and J ₃ fired under pressure and pressure significantly improved in both strength and elastic modulus. Usually, in the case of acrylic carbon fiber, when the treatment temperature exceeds 1500 ° C., the strength decreases with an increase in the amount of voids. However, by carbonizing under hot isostatic pressure, the amount of voids is greatly reduced. It was found that carbon fibers having high strength, modulus and elastic modulus were obtained.

Example 5, Comparative Example 5 The oxidized yarn having a density of 1.37 g / cm ³ obtained in Example 1 was further subjected to a nitrogen stream.
Continuous heating treatment was performed at a draw ratio of 1.02 to 900 ° C. to obtain carbonized fiber M.

The oxidized yarn was subjected to a continuous heat treatment at a draw ratio of 1.02 to 1400 ° C. in a nitrogen stream to obtain carbon fibers N.

About 20 m of each of the oxidized yarn, the carbonized fiber M and the carbon fiber N is wound around a graphite cylinder, and both ends of the yarn are tied to the graphite cylinder to be in a tensioned state. Place, replace with nitrogen, 800kg initial pressure (pressure before heating)
f / cm ² · G, final pressure (pressure at the end of heating) 2000 kgf / cm
^{At 2} · G, from room temperature to 1000 ° C at a heating rate of 20 ° C / min.
To 2500 ° C at 15 ° C / min.
After holding for ₁ minute, carbon fibers O ₁ , O ₂ and O ₃ were obtained. During the heating process, the pressure medium gas inside the pressure vessel and the generated low-boiling compound expanded to increase the pressure. However, when the pressure exceeded the set final pressure, the gas leaked and the internal pressure was kept constant.

For comparison, the oxidized yarn, the carbonized fiber M, and the carbon fiber N are wound around a graphite cylinder in the same manner as described above, and are subjected to normal pressure under the same temperature conditions as those for the firing of O ₁ , O ₂ , and O _3. The mixture was heated to 2500 ° C. in nitrogen to obtain carbon fibers P ₁ , P ₂ and P ₃ .

As a result of elemental analysis and X-ray diffraction, as shown in Table 9 below, carbon fibers P ₁ , P ₂ , P ₃ obtained under normal pressure
, Carbon fibers O ₁ , fired under hot isostatic pressure
O ₂ and O ₃ became carbon fibers having a high nitrogen content. Further, along with this, a dense carbon fiber having high crystallinity and a reduced amount of voids was obtained.

[0084]

[Table 9]

As a result of measuring the properties of the resin-impregnated strand, as shown in the following Table 10, the carbon fibers P ₁ , P ₂ , and P ₃ obtained under normal pressure were subjected to hot isostatic pressing. The carbon fibers O ₁ , O ₂ , and O ₃ fired in the above steps greatly improved both strength and elastic modulus.

[0086]

[Table 10]

(Example 6, Comparative Example 6) 約 About 20 m of the carbon fiber N obtained in Example 5 was wound around a graphite cylinder, and both ends of the yarn were tied to the graphite cylinder to be in a tensioned state. Installed in an isotropic pressure treatment device, purge with nitrogen, initial pressure,
The final pressure was set at a heating rate of 2
Heat from room temperature to 1000 ° C. at 0 ° C./min and 2500 ° C. at 15 ° C./min.
₁ , Q ₂ were obtained. The pressure medium gas inside the pressure vessel expands and the pressure increases during the temperature rise process, but when the pressure exceeds the set final pressure, the gas leaks and the internal pressure is kept constant.

[0088]

[Table 11]

For comparison, a carbon fiber N was similarly wound around a graphite cylinder and heated to 2500 ° C. under the same temperature conditions in nitrogen under normal pressure to obtain a carbon fiber R.

As a result of the elemental analysis and the X-ray diffraction, as shown in Table 9 above, the carbon fiber R obtained under normal pressure was subjected to carbon fiber Q ₁ fired under hot isostatic pressure. , Q ₂ became higher carbon fibers nitrogen content. Further, along with this, a dense carbon fiber having high crystallinity and a reduced amount of voids was obtained.

As a result of measuring the properties of the resin-impregnated strand, as shown in Table 10 above, the carbon fiber R obtained under normal pressure was baked under hot isostatic pressure. As for Q ₁ and Q ₂ , both the strength and the elastic modulus were greatly improved.

[0092]

According to the present invention, it is possible to provide a carbon fiber having few microvoids inside the fiber, high crystallinity and high density, excellent strength and elastic modulus, and a method for producing the same.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the crystal size Lc and the nitrogen content N of the carbon fiber of the present invention.

Claims (3)

(57) [Claims] 1. A resin-impregnated strand strength of 620 kgf / mm ²
When the crystal size Lc (angstrom) is 3
0 or less, and Lc and nitrogen content N (%) are N ≧
0.04 (Lc−30) ² +0.5, small angle scattering intensity
Is 880 cps or less . 2. The resin impregnated strand strength is 490 kgf / mm. ^Two
When the crystal size Lc (angstrom) is 3
0 and the nitrogen content N (%) is 0.5 or more
And the small-angle scattering intensity is 1550 cps or less.
Carbon fiber 3. A carbon fiber precursor is made flame-resistant or infusible in an oxidizing atmosphere and then fired in an inert atmosphere.
3. The method for obtaining a carbon fiber according to claim 1 or 2, wherein the sintering step is performed at an initial pressure at room temperature of 400 kgf / cm ² · G.
Method of producing a carbon fiber which comprises the step of firing at hot isostatic pressure to above and to, and the maximum pressure during the heat treatment 1000kgf / cm ² · G or more.