JPH0823090B2 - Method for producing high-performance carbon fiber - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a Pitch-based carbon fiber, and more specifically, a production of a high-performance Pitch-based carbon fiber having improved tensile strength and tensile elastic modulus. It is about the method.

(Prior Art) Carbon fiber is a material having a high specific strength and a high specific elastic modulus,
It has attracted attention as a filler fiber for high-performance composite materials.

Currently, PNA-based carbon fibers made from polyacrylonitrile (PAN) and Pitch-based carbon fibers made from pits are manufactured as carbon fibers. Currently, PAN-based carbon fibers are mainly used as high-strength, high-elasticity and high-characteristic carbon fibers with various measures.

However, the PAN-based carbon fiber has a drawback in that there is a limit to further increasing the elasticity. In addition, there is a drawback that the raw material PAN is expensive and the carbon fiber yield per raw material is low, so that it is expensive.

Therefore, in recent years, various studies have been conducted to improve the characteristics of the Pitch-based carbon fiber, which has a characteristic of higher elasticity and is expected to be used in a wider range of applications.

In order to improve the characteristics of Pitch-based carbon fiber, the raw material pitch was heat-treated instead of the isotropic pitch that was used as a spinning raw material in the past, and anisotropy developed and molecular species that were easily oriented were formed. Since a method of using a so-called mesophase pitch (Japanese Patent Publication No. Sho 49-8634) has been proposed, it is mainly carried out by adjusting the properties of the spinning pitch.

For example, in JP-A-49-19127, a raw material pitch is heat-treated under an inert gas atmosphere to form highly oriented mesophases, and a pitch containing 40 to 90% by weight of the mesophases is referred to as a spinning pitch. The method of doing is proposed.

However, since it takes a long time to mesomorphize an isotropic raw material pitch by such a method, JP-A-54-160427 discloses that the raw material pitch can be treated in advance with a sufficient amount of a solvent for a short time. I have proposed a method to make meso.
That is, the raw material pitch is treated with a solvent such as benzene or toluene to obtain its insoluble matter, which is then heat-treated at a temperature of 230 to 400 ° C for a short time of 10 minutes or less to be highly oriented and optically anisotropic. A method has been proposed in which a so-called neo-mesophase with a quinoline insoluble content of 25% by weight or less is formed with a sexual portion of 7.5% by weight or more and such neomesophase is used as a spinning pitch.

The spinning pitch thus obtained is melt-spun,
High-strength, high-elasticity carbon fibers having high strength can be produced by obtaining pitch fibers and then making them infusible, carbonized, or graphitized.

However, when producing carbon fibers using Pitch as a raw material, infusibilization, tensile stress due to carbonization shrinkage during carbonization acts in the circumferential direction of the fiber cross section, and surface defects such as damage to the resulting carbon fiber surface occur. It is easy to occur.

As one of the causes, the outer peripheral part of the cross section of the Pitch fiber obtained by melt spinning by a usual method is generally prone to a so-called radial type oriented structure, so that such surface defects are generated during carbonization and graphitization. It is considered that the carbon fibers having such surface defects have a problem in tensile strength and tensile modulus. Generally, there are various proposals as a method for improving the fiber surface, for example, Japanese Patent Publication No. 45-1287 discloses that polyacrylonitrile-based or polyester-based carbon fiber is subjected to a gas phase oxidation treatment for the purpose of improving the adhesiveness with a resin matrix. Is listed.

Recently, it has been reported that special grade PAN-based 1000 ℃ calcined carbon fiber is treated at 400 ℃ and 450 ℃ for 5 to 30 minutes under air flow (Polymer Engineering and Scienc
e, May, 1984, Vol.24, No.7, PP.455-459).

(Problems to be solved by the invention) However, in general, polyacrylonitrile-based or polyester-based carbon fibers are unlikely to cause surface defects such as those due to the cross-sectional structure of the Pitch-based carbon fibers, and such treatment is performed on the fibers and the resin matrix. The purpose is to improve the adhesiveness of, and because the fiber raw material itself is different,
It does not simultaneously improve the tensile strength and the tensile elastic modulus of the fiber itself.

(Means for Solving Problems) Therefore, the inventors of the present invention have conducted diligent studies to solve such problems, and as a result, heat-oxidize the Pitch-based carbon fiber to form a surface defect portion on the fiber surface. The present invention has been accomplished by finding that stress concentration is less likely to occur.

That is, the object of the present invention is to provide a method for producing a high-performance carbon fiber in which the strength and elastic modulus of the fiber itself are improved at the same time by improving the surface defects that are peculiar to the Pitch-based carbon fiber, which are likely to occur in the sectional structure. Is.

And, this purpose, the pitch-based carbon fiber obtained by firing at a temperature of 1200 ℃ or more, 450 ~ 550 in an oxygen-containing atmosphere.
It is achieved by heat treatment at ℃ for 15 minutes or less.

Hereinafter, the present invention will be described in detail. The spinning pitch for obtaining the carbon fiber used in the present invention is not particularly limited as long as it has a molecular species that is easily oriented and gives an optically anisotropic carbon fiber. Various conventional ones such as can be used.

As a carbonaceous raw material for obtaining these spinning pitches,
For example, coal-based coal tar, coal tar pitch,
Examples include liquefied coal, heavy petroleum oil, tar, and pitch. These carbonaceous raw materials are usually free carbon,
Impurities such as undissolved coal and ash are contained, but it is desirable to remove these impurities in advance by a known method such as filtration, centrifugation, or stationary sedimentation separation using a solvent.

Further, the carbonaceous raw material, for example, a method of extracting a soluble component with a specific solvent after heat treatment, or a method of hydrogenation in the presence of a hydrogen-donating solvent or hydrogen gas, a preliminary treatment. You may leave it.

In the present invention, the carbonaceous raw material or the pretreated carbonaceous raw material is usually 350 to 500 ° C., preferably
At 380 to 450 ° C. for 2 minutes to 50 hours, preferably 5 minutes to 5 hours under an atmosphere of an inert gas such as nitrogen or argon, or
40% or more obtained by heat treatment under blowing, preferably containing 70% or more optically anisotropic structure,
A mesophase pitch having a quinoline insoluble content of 40% by weight or less, preferably 35% by weight or less is suitable.

The optically anisotropic texture ratio of the mesophase pitch in the present invention is a value obtained as the area ratio of the portion showing the optical anisotropy in the mesophase pitch sample under a polarization microscope at room temperature.

Specifically, for example, a mesophase pitch sample crushed into a few mm square is embedded with a sample piece on almost the entire surface of a resin having a diameter of about 2 cm according to a conventional method, and after polishing the surface, the entire surface is darkened. It is obtained by observing the sample under a polarizing microscope (100 magnification) and measuring the ratio of the area of the optically anisotropic portion to the total surface area of the sample.

Using the spinning pitches as described above, melt spinning, infusibilization, carbonization and, if necessary, graphitization are carried out according to a usual method to obtain carbon fibers.

In particular, in the present invention, carbon fibers fired at a temperature of 1200 ° C. or higher, preferably at 1200 to 3000 ° C. for 30 to 400 minutes, particularly preferably at 1400 to 2000 ° C. for about 30 to 300 minutes or carbonized or graphitized. Treated carbon fibers or graphite fibers (hereinafter, referred to as "carbon fibers") are preferably used.

In the present invention, such a carbon fiber under a carbon-containing atmosphere
It is important to heat-treat above 450 ° C and below 550 ° C. At that time, neither the tensile strength nor the tensile elastic modulus of the carbon fiber obtained when the heat treatment temperature is 450 ° C. or lower, and when the heat treatment is performed at a temperature higher than 550 ° C., the surface of the carbon fiber Is excessively oxidized, and the number of surface defects is increased, so that the fiber itself becomes brittle and the properties are deteriorated, which is not preferable. The oxygen-containing atmosphere may be air or may be blown with oxygen. The oxygen concentration in the oxygen-containing atmosphere is not particularly limited as long as it does not extremely oxidize the surface of the carbon fiber, but the oxygen concentration is preferably 20% or less.

Specifically, it is carried out by making the firing furnace an open system or by performing heat treatment while blowing air or oxygen into the firing furnace.

In the present invention, the heat treatment is performed at a temperature of 450 ° C to 550 ° C for 15 minutes or less.

As described above, by heat-treating the Pitch-based carbon fiber obtained by carbonizing or graphitizing at a specific temperature at a temperature higher than 450 ° C and 550 ° C or lower in an oxygen-containing atmosphere, the strength of the fiber itself -The elastic modulus is improved. Although the cause of this has not been fully clarified yet, surface defects on the fiber surface that cause fracture of the Pitch-based carbon fiber are removed by heating the fiber surface slightly by heat treatment in an oxygen-containing atmosphere. Or, it is presumed that the stress concentration on the surface defect is alleviated by reducing the defect degree.

(Effect) According to the present invention, a high-performance Pitch-based carbon fiber having improved tensile strength and tensile elastic modulus at the same time by improving surface defects that are peculiar to the Pitch-based carbon fiber due to its fiber cross-sectional structure. Can be manufactured. This high-performance Pitch-based carbon fiber has excellent mechanical properties, and its surface condition is also improved, so it is very useful for various fiber-reinforced composite materials.

The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples as long as the gist of the present invention is not exceeded.

Example 1 Using coal tar pitch as a raw material and under N ₂ atmosphere, 430
After heat treatment at ℃ for about 120 minutes, a spinning pitch with anisotropy of 100% and quinoline insoluble content of 30.5% was obtained.

This is spun according to a conventional method from a spinning nozzle having a nozzle diameter of 0.3 mm, then infusibilized at 310 ° C for 30 minutes, and then 1400 ° C.
Was carbonized for 60 minutes to obtain a carbon fiber having a tensile strength of 288 kg / mm ² and a tensile elastic modulus of 24.5 t / mm ² .

This fiber is heated from room temperature to 8 ℃ / min using an open firing furnace.
The temperature was raised up to 500 ° C. And at that temperature 1
After holding for a minute, it was left to cool. As a result of conducting a tensile test on the treated fiber, the tensile strength is 337 Kg / mm ² , and the tensile elastic modulus is 3
It was 0.7 t / mm ² .

Comparative Example 1 The spinning pitch obtained in Example-1 was spun in the same manner as in Example-1, infusibilized, and then carbonized at 1000 ° C. for 60 minutes to obtain a tensile strength of 282 Kg / mm ² and a tensile strength. Elastic modulus 19.0t
A carbon fiber of / mm ² was obtained.

This carbon fiber was heat-treated at 500 ° C. for 1 minute in an oxygen-containing atmosphere in the same manner as in Example-1. As a result of conducting a tensile test on the treated fiber, the tensile strength is 263 Kg / mm ² ,
The tensile elastic modulus was 18.4 t / mm ² .

Comparative Example-2 After holding the carbon fiber used in Example-1 in the same firing furnace as in Example-1 at 500 ° C. for 60 minutes, a tensile test was performed, and the tensile strength was 323 kg / mm ² , The tensile elastic modulus was 28.2 t / mm ² .

Comparative Example-3 In Comparative Example-2, the heat treatment of the carbon fiber was performed at 400 ° C, 6
The treatment was carried out in the same manner as Comparative Example-2 except that the time was set to 0 minutes. The treated fiber has a tensile strength of 287 kg / mm ² and a tensile modulus of 2
It was 4, 5 t / mm ² .

Comparative Example-4 The carbon fiber used in Example-1 was heat-treated at 500 ° C. for 1 minute in an inert gas (Ar) atmosphere. The tensile strength of the obtained fiber was 285 Kg / mm ² , and the tensile elastic modulus was 25.0 t / mm ² , and the effect of improving the tensile strength was hardly seen.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Hidehiko Ohara 1000 Kamoshida-cho, Midori-ku, Yokohama Ko 48-25003 (JP, B1)

Claims (2)

[Claims] 1. A high-performance carbon fiber characterized in that the pitch-based carbon fiber obtained by firing at a temperature of 1200 ° C. or higher is heat-treated in an oxygen-containing atmosphere at 450 to 550 ° C. for 15 minutes or less. Production method. 2. The method according to claim 1, wherein the pitch-based carbon fiber is fired at a temperature of 1,400 ° C. to 2,000 ° C.