JP2817232B2 - Method for producing high-performance carbon fiber - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing carbon fiber, and more particularly, to a method for producing high-performance carbon fiber having improved tensile strength and tensile modulus. is there.

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

Currently, PAN-based carbon fibers made from polyacrylonitrile (PAN) and pitch-based carbon fibers made from pitch are manufactured as carbon fibers. In response to recent demands for higher quality composite materials, Various studies have been made on improving the strength and elastic modulus of any type of carbon fiber.

For example, in pitch-based carbon fibers, a method using a so-called mesophase pitch in which anisotropy has developed instead of the isotropic pitch conventionally used as a spinning raw material (Japanese Patent Publication No. 49-8634) has been proposed. Various modifications of anisotropic spinning raw materials have been proposed and reported, while those already formed into carbon fibers are further heated at a specific temperature in an oxygen-containing atmosphere to obtain tensile strength and tensile elasticity. A method for simultaneously improving the rate (Japanese Patent Application Laid-Open No. 61-215716) is known.

It has also been reported that PAN-based carbon fiber is treated at a specific temperature under a flow of air with a special grade baked PAN-based carbon fiber at 1000 ° C. (Polymer Engineering and S
cience, May, 1984 Vol. 24 No. 7, 455-459).

(Problems to be Solved by the Invention) However, all of these are intended to improve the adhesion to the matrix by modifying the properties of the carbon fiber surface or to remove large defects on the surface. Although the properties were improved, the properties were improved, but could not be said to be an essential improvement, and were not intended to further improve the properties of the obtained carbon fibers by changing the crystal structure in the carbon fibers.

(Means for Solving the Problems) The inventors of the present invention have conducted intensive studies to solve the conventional problems, and as a result, by irradiating a carbon fiber with a specific amount of electron beam, a minute defect was generated in the crystal. As a result, the inventors have found that the tensile strength and the tensile elastic modulus are improved, and arrived at the present invention.

That is, the present invention provides a method for easily producing a high-performance carbon fiber having improved tensile strength and tensile modulus.

The object is easily attained by a method for producing a high-performance carbon fiber, which comprises irradiating a carbon fiber obtained by firing at a temperature of 1000 ° C. or more with an electron beam of 500 kV or more for 1000 Mrad or more.

 Hereinafter, the present invention will be described in detail.

As the carbon fibers used in the present invention, any of pitch-based carbon fibers and PAN-based carbon fibers can be used.

As the spinning pitch for obtaining pitch-based carbon fiber,
Any material may be used as long as it forms a molecular species that is easily oriented and gives an optically anisotropic carbon fiber.

As a carbonaceous raw material for obtaining these spinning pitches,
For example, coal-based coal tar, coal tar pitch,
Coal liquefaction, petroleum heavy oil, tar, pitch and the like can be mentioned. These carbonaceous materials usually include free carbon,
Although impurities such as undissolved coal and ash are contained, these impurities are desirably removed in advance by a well-known method such as filtration, centrifugation, or static sedimentation using a solvent.

In addition, the carbonaceous raw material is subjected to a preliminary treatment by, for example, a method of extracting soluble matter with a specific solvent after heat treatment, or a method of hydrogenating in the presence of a hydrogen-donating solvent or hydrogen gas. You can leave it.

In the present invention, the carbonaceous raw material or the carbonaceous raw material that has been subjected to the pretreatment is usually 350 to 500 ° C., preferably 38 ° C.
0 to 450 ° C., 2 minutes to 50 hours, preferably 5 minutes to 5 hours,
40% or more obtained by heat treatment under an inert gas atmosphere such as nitrogen or argon, or under blowing
A mesophase pitch containing preferably 70% or more of an optically anisotropic structure and having a quinoline-insoluble content of 40% by weight or less, preferably 35% by weight or less is suitable.

The optically anisotropic structure ratio of the mesophase pitch referred to in the present invention is a value obtained as an area ratio of a portion exhibiting optical anisotropy in a mesophase pitch sample with a polarizing microscope at normal temperature.

Specifically, for example, a mesophase pitch sample crushed into a few mm square is buried with a sample piece over 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 polarized with a polarizing microscope. Observed under (100 magnification), it is determined by measuring the ratio of the area of the optically anisotropic portion to the total surface area of the sample.

Using the above-described spinning pitch, carbon fiber is obtained by melt spinning, infusibilizing, carbonizing, and, if necessary, graphitizing according to a usual method. Further, conventionally known PAN-based carbon fibers can be used. Specifically, after a spinning solution containing polyacrylonitrile is melt-spun, a flame treatment is performed, and a carbonization treatment and, if necessary, a graphitization treatment are performed as in the case of the pitch-based carbon fiber to obtain a carbon fiber.

In particular, in the present invention, carbon fibers fired at a temperature of 1000 ° C. or more, preferably, carbonized or graphitized at a temperature of about 1200 to 3000 ° C. for 10 to 400 seconds, particularly preferably 1400 to 2000 ° C. for about 10 to 300 seconds. A treated carbon fiber or graphite fiber (hereinafter, referred to as “carbon fiber”) is suitably used.

In the present invention, it is important to irradiate the carbon fiber with an irradiation amount of 500 kV or more, preferably 500 to 2000 kV, more preferably 700 to 2000 kV, and an electron beam of 1000 Mrad or more, preferably 1000 to 10,000 Mrad, more preferably 1500 to 10,000 Mrad. is there. At that time, if the irradiation amount is less than 1000 Mrad, it is not preferable because neither the tensile strength nor the tensile modulus of the obtained carbon fiber is improved. On the other hand, if the irradiation amount is too large, the possibility of a decrease in tensile strength increases, which is not preferable. The irradiation process itself is performed by a normal electron beam irradiation device, specifically, an electron beam acceleration device,
A device for general delivery of an object to be irradiated to the electron beam irradiation area, and is equipped with a cooling device, or is irradiated by an irradiation device capable of performing irradiation and cooling intermittently. There are no particular restrictions on the gas type, pressure, and temperature. However, when the temperature of the object to be irradiated reaches 500 ° C. or higher, the irradiation is preferably performed in an inert gas atmosphere.

As described above, the carbon fiber obtained by specific electron beam irradiation has improved strength and elastic modulus of the fiber itself. The reason for this has not been fully elucidated yet, but it is presumed to be due to the fact that a fine defect such as a Frenkel defect is taken inside the crystal by an electron beam having a critical energy or higher.

(Examples) Hereinafter, the present invention will be described specifically with reference to examples, but the present invention is not limited to the examples unless it exceeds the gist of the present invention.

Example 1 As shown in Table 1, two types of PAN-based and pitch-based carbon fibers were irradiated for about 20 seconds under an acceleration voltage of 750 kV using an electron beam irradiation device having a transport device. The cooling operation was repeated 72 times so that the total irradiation amount was 1800 Mrad, and the atmosphere was air.

Table 1 shows the results of comparing the tensile strength and the tensile modulus before and after irradiation for two types of high-performance PAN-based and pitch-based carbon fibers.

Under the conditions of the acceleration voltage of 750 kV and the irradiation dose of 1800 Mrad, the tensile strength and the elastic modulus increased.

Comparative Example 1 Using the same irradiation apparatus as in the example, the operation of irradiating for about 20 seconds at an acceleration voltage of 750 kV and then cooling was repeated 4 times and 24 times, and the total irradiation amount was 100 and 600 Mrad. The atmosphere is in the air.

Table 2 shows the values of the tensile strength and the tensile modulus of the two pitch-type samples before and after irradiation.

Comparative Example 2 The operation of irradiating at an accelerating voltage of 170 kV for about 5 seconds and then cooling was repeated 100 times to obtain a total irradiation amount of 2000 Mrad. The atmosphere is in the air.

Table 3 shows the values of the tensile strength and the tensile modulus of the two pitch-type samples before and after irradiation. No improvement tendency was observed under these conditions.

(Effects of the Invention) According to the present invention, it is possible to produce a high-performance carbon fiber in which the tensile strength and tensile modulus of the fiber itself are simultaneously improved by generating minute defects in the crystal constituting the carbon fiber. Can be. In particular, high-performance pitch-based carbon fibers are very useful for various fiber-reinforced composite materials because of their excellent mechanical properties.

Claims (2)

(57) [Claims] 1. A method for producing high-performance carbon fibers, comprising irradiating carbon fibers obtained by firing at a temperature of 1000 ° C. or more with an electron beam of 500 kV or more for 1000 Mrad or more. 2. The method according to claim 1, wherein the irradiation treatment is performed at an electron beam of 500 to 2000 kV in a range of 1,000 to 10,000 Mrad.