JP3314383B2 - Method for producing carbon fiber / carbon composite material - Google Patents

Description

The present invention relates to a method for producing a lightweight, high-strength carbon fiber / carbon composite material having excellent heat resistance, abrasion resistance, chemical resistance and the like.

(Prior art) Carbon fiber / carbon composite materials are one of the most advanced composite materials that are currently receiving the most attention because of their excellent functions and characteristics. Mechanical properties, heat resistance, abrasion resistance, heat and electricity Taking advantage of conductivity, aerospace, electronics,
It is being widely used in fields such as the nuclear industry.
In addition, applications utilizing their biocompatibility, such as artificial bones, tooth roots, and joints, are also expected, and active research and development are being conducted worldwide.

There have been reports of producing carbon fiber / carbon composite materials having such a wide range of applications that there is no time to enumerate. As a process of combining carbon fiber and matrix, in order to impart its compactness, repeated operation of re-impregnation and carbonization of the binder into the pores (resin impregnation carbonization method)
And carbon deposition from the gas phase into the voids (CVD method).

That is, in the resin impregnation carbonization method, as a carbon raw material serving as a matrix, a thermosetting resin such as a furan resin and a phenol resin, and a thermoplastic resin mainly represented by a pitch are used. A method of carbonizing the matrix resin by heat treatment is performed. This resin impregnation carbonization method requires a very slow heating rate in the temperature range where these resins are melted and carbonized, and the carbonization yield of the resin is as low as 40 to 60%, and voids are generated due to volatile fractions. A complicated secondary treatment of repeating re-impregnation, carbonization and compression of the resin is required.

On the other hand, in the CVD method, a lower hydrocarbon such as methane or propane and an inert gas such as argon are introduced into a CVD apparatus as a matrix material, and reacted at about 800 to 1500 ° C. under reduced pressure to directly deposit carbon. Is the way. Since the CVD method deposits pyrolytic carbon directly in the gas phase in this way, it is possible to create a dense and homogeneous matrix, but the equipment cost is high and it takes a long time. Disadvantageous. For this reason, it is often used for secondary densification treatment in combination with the resin impregnation carbonization method.

(Problems to be Solved by the Invention) As described above, the process of combining a carbon fiber and a matrix is extremely complicated, and the carbon fiber / carbon composite material produced by this method is a very expensive material. For this reason, the carbon fiber / carbon composite material has a limited use and hinders its use in a wide range of fields. Therefore, if the establishment of mass production technology and cost reduction are achieved in the future, it is expected that the value of use as general industrial materials will rapidly increase.

An object of the present invention is to provide a method for producing a high-performance carbon fiber / carbon composite material in a short time and at low cost by a simpler method.

(Means for Solving the Problems) As a result of intensive studies to achieve the above object, the inventors have found that if a liquid crystal pitch having a high fluidity and a high carbonization yield is used as a matrix material, the pitch can be melted. Alternatively, by uniformly coating the surface of the short carbon fiber in a softened state, a high dispersibility of the pitch is achieved, and a higher carbonization yield is obtained while maintaining a high caking property by a slight heat treatment. The present inventors have found that a high-performance carbon fiber / carbon composite material can be obtained at a low cost by a very simple method, without using any of them.

That is, the present invention provides a method for uniformly mixing a carbon short fiber and a carbonaceous liquid crystal pitch powder having a softening point of 300 ° C. or less and a carbon yield of 75% or more and then melting or softening the pitch. A carbon fiber / carbon composite material, characterized in that the surface of the carbon fiber is coated with the pitch, heat-treated at 300 to 490 ° C. as necessary, and then molded and fired.

The short carbon fiber used as the reinforcing material in the present invention may be obtained from any of PAN-based and pitch-based carbon fibers, and may be subjected to not only carbonization but also graphitization. Fibers having a length of 50 μm to 10 mm are used depending on the application and the requirements of the properties. In addition, it is preferable that these carbon short fibers are generally subjected to a surface treatment such as an oxidation treatment.

It is desirable that the carbonaceous liquid crystal pitch used as a matrix material has an optically anisotropic phase observed by a polarizing microscope of at least 80% or more, preferably 90% or more, and more preferably substantially 100%. The softening point of the carbon liquid crystal pitch determined by the trace melting point measurement method is 300 ° C. or less, and preferably 200 to 280 ° C. By using a carbonaceous pitch having a low softening point for the matrix, 200 to 3
Since it has good fluidity at 00 ° C., it is rapidly and uniformly dispersed around the gaps between short fibers, around short fibers, or around particulate aggregates.

Further, in the present invention, a carbonaceous liquid crystal pitch having a carbonization yield of 75% or more, preferably 85% or more is used as a matrix material. The carbonization yield is a yield when the pitch is gradually heated to 600 ° C. in an inert gas stream and held for about 2 hours. Since the carbonization yield is high and the volatile component is small, the formed carbon bond is dense and leaves little trace of pores, so that an extremely strong optically anisotropic phase is formed. When a pitch having a low carbonization yield is used, voids due to volatile gas are easily generated in the molded body, and the mechanical strength of the obtained carbon fiber / carbon composite material is reduced.

As an example of a carbonaceous liquid crystal pitch satisfying these conditions, pitches described in JP-A-1-139621 and JP-A-1-254796 are preferable. The pitch is a pitch obtained by condensed polycyclic aromatic hydrocarbons and polymerized in the presence of HF-BF _3, high carbonization yield can be obtained, good flowability at 200 to 350 ° C. lower softening point Is shown.

The mixing ratio of short carbon fiber and carbonaceous liquid crystal pitch is short fiber /
30 / 70-90 / 10 by weight of liquid crystal pitch, preferably 40/60
It is in the range of ~ 70/30. At this time, if necessary, a fine carbon aggregate such as carbon black, coke powder or graphite may be added to the mixture in an amount of 3 to 70% based on the liquid crystal pitch. The particle size of these fine carbon aggregates is 10μ.
m or less, preferably 1 μm or less. By adding the finely divided carbon aggregate as described above, the densification of the molded product is further promoted, and the mechanical strength is improved.

In the present invention, short carbon fiber and carbonaceous liquid crystal pitch,
Alternatively, the fine particle carbon aggregate to be further added may be uniformly mixed, and the mixing method is not particularly limited.

Next, the mixture is subjected to a coating process. This coating process uniformly and quickly coats the surfaces of the short carbon fibers and the fine carbon particles with the pitch at a temperature at which the carbonaceous liquid crystal pitch melts or softens, thereby achieving high dispersion. Therefore, when the pitch is melted or softened, stirring, kneading, and the like are performed as necessary.
Usually, it is practical that this coating process is performed in the course of raising the temperature to the subsequent heat treatment (Examples 1, 3,
5, 7).

By heat-treating the mixture in which the pitch is thus highly dispersed, a higher carbonization yield can be obtained, and the performance of the product can be further enhanced. In this heat treatment, it is necessary to select appropriate conditions that do not lose caking properties and that can further improve the carbonization yield, as long as the liquid crystal pitch is not completely carbonized. The heat treatment conditions vary depending on the properties of the liquid crystal pitch and the mixing ratio, but are generally 300 to 490 at a temperature increase rate of 1 to 20 ° C./min in a non-oxidizing atmosphere.
This is carried out by raising the temperature to ° C and holding it for a short time. The holding time is 1 to 60 minutes. The pressure in the heat treatment is not particularly limited, and may be normal pressure, pressurization, or reduced pressure.

If the heat treatment is insufficient, expansion and foaming due to volatile gas are likely to occur in the subsequent firing step, and a high quality carbon fiber / carbon composite material cannot be obtained. On the other hand, excessive heat treatment lowers the caking property of the liquid crystal pitch, and may not be able to exhibit desired mechanical performance. In other words, by setting appropriate heat treatment conditions, the gas generated in the firing step can be eliminated as much as possible while maintaining the caking property of the liquid crystal pitch. Can be manufactured.

However, since the expansion and foaming generally observed in the firing step become less likely to occur as the pitch content in these mixtures decreases, heat treatment after coating is not necessarily performed when the pitch content is small. (Example 9). Of course, if a heat treatment is also performed at this time, the volatile components are removed, so that the densification is further promoted.
The pitch content at which heat treatment is not required varies depending on the properties of the pitch, molding conditions, carbonization conditions, required characteristics, and the like, but was approximately 30% in the range of experiments performed by the inventors.

Next, the mixture thus obtained, that is, a mixture in which the liquid crystal pitch maintaining the caking property was highly dispersed was molded,
Fired. At this time, the mixture is in a lump,
For easy molding, the powder is formed into a powder, if necessary. The shape of the molded body can be freely selected according to the purpose, application, and the like. The molding temperature may be at room temperature or at a temperature at which the liquid crystal pitch softens or melts, and is determined according to the required shape and performance. The molding pressure needs to be set within a range in which the structure of the short fibers as the reinforcing material is not broken and the initial strength is not lost.

The target carbon fiber / carbon composite material is manufactured by successively firing the formed body thus prepared. Firing is generally performed by heating the molded body to a temperature of 600 to 1500 ° C. in a non-oxidizing atmosphere and carbonizing, and further comprises a step of heating the carbonized molded body to a temperature of 2000 to 3000 ° C. to graphitize. Can also be included.

(Operation and Effect) In the present invention, a liquid crystal pitch having a low softening point and a very high carbonization yield is used as a matrix material, and high dispersion of the pitch and appropriate heat treatment are performed before molding. In addition to achieving this, a single firing only provides sufficient density and strength. Also, this liquid crystal pitch powder,
Because the fluidity at 300-350 ° C keeps the shape of the molded body and disperses thinly and uniformly in the molded structure, a uniform carbon bond is formed between short fibers and powder aggregates by subsequent carbonization. . Further, since the matrix derived from the liquid crystal mesophase has an optically anisotropic structure, is dense and highly pure, the carbon bond between the matrix and the carbon fiber or aggregate is very strong. The degree of graphitization of the carbon bond is improved by firing at a high temperature, and further densification is promoted by shrinkage, so that the carbon bond is further strengthened. Further, since the liquid crystal pitch used in the present invention has a high caking property, a binder is not particularly required.

According to the present invention, a lightweight and high-strength carbon fiber / carbon composite material having excellent heat resistance, abrasion resistance, etc.
It can be easily manufactured in a short time and at low cost. Therefore, the industrial significance of the present invention is extremely large.

(Example) Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by these examples.

Example 1 Pitch-based carbon staple fiber (a product jointly developed by Kawasaki Steel Corporation and Nitto Boseki Co., Ltd .: Xylus GPMF100J, fiber length 100 μ)
m) 50 parts of carbonaceous liquid crystal pitch powder (softening point: 218 ° C., carbonization yield: 85%, optically anisotropic phase: 100 vol%) obtained by polymerizing naphthalene in the presence of HF-BF ₃ In addition, they were mixed uniformly. Next, the mixture was stirred at a heating rate of 5 ° C./min.
The coating treatment was performed by raising the temperature to 300 ° C. Further, heat treatment was performed by raising the temperature to 450 ° C. and maintaining the temperature for 30 minutes. Thus, a molded sample in which the liquid crystal pitch having a caking property was highly dispersed was prepared.

The molded sample thus obtained was subjected to a molding pressure of 1000 at room temperature.
After forming into a disk at kg / cm ² , the temperature was raised to 600 ° C. at 1 ° C./min under a nitrogen pressure of 30 atm and held for 2 hours. After baking for 2 hours at 1300 ℃ under argon flow,
A carbide having a diameter of 60 mm and a thickness of 10 mm was obtained.

Example 2 The carbide obtained in Example 1 was further heated to 2500 ° C.
A graphitized product was obtained.

Example 3 Naphthalene was obtained by polymerizing 55 parts of short fibers obtained by cutting pitch-based carbon fiber (Kureka Chemical Industry Co., Ltd., Crecacross P-200) into an average length of 400 μm in the presence of HF-BF ₃ . Carbonaceous liquid crystal pitch powder (softening point 285 ° C, carbonization yield 87%,
45 parts of an optically anisotropic phase (100 vol%) was added and mixed uniformly.
Next, the mixture was heated to 360 ° C. at a rate of 5 ° C./min to perform a coating treatment. 440 ° C
Heat treatment was carried out by raising the temperature to 10 minutes and holding it for 10 minutes to prepare a molded sample in which the liquid crystal pitch having a caking property was highly dispersed.

The molded sample thus obtained is formed into a disk at a molding temperature of 480 ° C. and a molding pressure of 1000 kg / cm ² using a hot press, and then heated to 600 ° C. under normal pressure at 1 ° C./min and held for 2 hours. did. Thereafter, by sintering at 1300 ° C. for 2 hours, a carbide having a diameter of 60 mm and a thickness of 10 mm was obtained.

Example 4 The carbide obtained in Example 3 was further heated to 2500 ° C.
A graphitized product was obtained.

Example 5 Pitch-based carbon short fiber (a jointly developed product of Kawasaki Steel Corporation and Nitto Boseki Co., Ltd .: Xylus GPMF100J, fiber length 100 μ)
m) To 50 parts, add 40 parts of the same carbonaceous liquid crystal pitch powder as in Example 1, and further add carbon black (Mitsubishi Chemical Corporation)
# 30) 10 parts were added and mixed uniformly. Hereinafter, Example 1
Carbide was obtained by performing the same operation as described above.

Example 6 The carbide obtained in Example 5 was further heated to 2500 ° C.
A graphitized product was obtained.

Example 7 To 55 parts of short fibers obtained by cutting pitch-based carbon fibers (Kureha Chemical Industry Co., Ltd., Crecacross P-200) into an average length of 400 μm, 45 parts of the same carbonaceous liquid crystal pitch powder as in Example 3 were added.
Further, 10 parts of graphite powder (10 μm or less) was added and uniformly mixed. Next, the mixture was stirred at a heating rate of 5 ° C./min for 360 °.
The coating treatment was performed by raising the temperature to ° C.
Further, a heat treatment was performed by raising the temperature to 440 ° C. and holding for 10 minutes to prepare a molded sample in which the liquid crystal pitch having a binding property was highly dispersed.

The thus obtained molded sample is formed into a disk at a molding temperature of 500 ° C. and a molding pressure of 1000 kg / cm ² by using a hot press apparatus, and then heated to 1300 ° C. under normal pressure and baked for 2 hours. A carbide having a diameter of 60 mm and a thickness of 10 mm was obtained.

Example 8 The carbide obtained in Example 7 was further heated to 2500 ° C.
A graphitized product was obtained.

Example 9 Pitch-based carbon short fiber (a product jointly developed by Kawasaki Steel Corporation and Nitto Boseki Co., Ltd .: Xylus GPMF100J, fiber length 100 μ)
m) 25 parts of the same carbonaceous liquid crystal pitch as in Example 1 was added to 75 parts, and mixed uniformly. Next, a coating treatment was performed by stirring the mixture at 280 ° C. for 10 minutes. Then, without heat treatment, it was molded into a disk at ordinary temperature at a molding pressure of 1000 kg / cm ² . 1 ℃ / min up to 1300 ℃ under normal pressure
Temperature and hold it for 2 hours to obtain a diameter of 60 mm and a thickness of 10
mm of carbide was obtained.

Example 10 The carbide obtained in Example 9 was further heated to 2500 ° C.
A graphitized product was obtained.

The evaluation results of the carbide or graphitized material obtained in each example are shown below.

Claims (3)

(57) [Claims] 1. A short carbon fiber in a temperature range in which a short carbon fiber and a carbonaceous liquid crystal pitch powder having a softening point of 300 ° C. or less and a carbonization yield of 75% or more are uniformly mixed and then melted or softened. Cover the surface with the pitch, forming,
Method for producing carbon fiber / carbon composite material, characterized by firing 2. The method for producing a carbon fiber / carbon composite material according to claim 1, wherein the surface of the short carbon fiber is covered with a carbonaceous liquid crystal pitch, and then heat-treated at 300 to 490 ° C., and then molded and fired. 3. A mixture of 30 to 90% by weight of short carbon fibers and a softening point of 300 ° C.
Carbonaceous liquid crystal pitch powder with a carbonization yield of 75% or more below
After the powdery carbonaceous aggregate is added to the mixture consisting of 70 to 10% by weight in an amount of 3 to 70% of the weight of the pitch, the short carbon fiber and the powdery carbonaceous aggregate are melted or softened in a temperature range where the pitch is melted or softened. The method for producing a carbon fiber / carbon composite material according to claim 1 or 2, wherein the surface is coated with the pitch.