JP2000080524A - Production of carbon fiber having high strand tenacity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtains carbon fibers exhibiting high strand tenacity. SOLUTION: This method for producing carbon fibers comprises using pitch as a raw material and passing through every processes of at least melt spinning, infusibilizing and carbonizing. In the method, the infusibilized fibers are carbonized and then subjected to heat-treatment in an atmosphere containing carbon dioxide at 1,000-1,800 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing pitch-based carbon fiber, and more particularly, to a method for producing carbon fiber having high strand strength.

[0002]

2. Description of the Related Art Carbon fiber is a material having a high specific strength and a high specific elastic modulus, and is attracting attention as a filler fiber of a high-performance composite material. At present, PAN-based carbon fibers made from polyacrylonitrile (PAN) and pitch-based carbon fibers made from pitch are manufactured. Mainly used as high-strength, high-elasticity, high-performance carbon fiber
At present, N-based carbon fibers are used with various measures.

[0003] However, PAN-based carbon fibers have a limit in further increasing their elasticity, and also have the drawback that the raw material PAN is expensive and the yield of carbon fibers per raw material is low. ing. Therefore, in recent years, various studies have been made to improve the characteristics of pitch-based carbon fibers having higher elasticity characteristics and expected to be used in a wider range of applications.

[0004] To improve the characteristics of pitch-based carbon fibers, a molecular species which develops anisotropy and is easily oriented by heating the raw material pitch is used instead of the isotropic pitch which has been conventionally used as a spinning raw material. Since a method using a formed pitch, that is, a so-called mesophase pitch (Japanese Patent Publication No. 49-8634) was proposed, it has been mainly performed by adjusting properties of a spinning pitch.

For example, Japanese Patent Application Laid-Open No. 49-19127 discloses that a raw material pitch is heat-treated in an inert gas atmosphere to form a highly oriented mesophase, and a pitch containing 40 to 90% by weight of the mesophase. Has been proposed as a spinning pitch. However, it takes a long time to mesomorph the isotropic raw material pitch by such a method,
Japanese Patent Application Laid-Open No. 54-160427 proposes a method of performing meso-formation in a short time by treating the raw material pitch with a sufficient amount of a solvent in advance. That is, the raw material pitch is treated with a solvent such as benzene or toluene to obtain an insoluble content thereof,
It is heat-treated at a temperature of 230 to 400 ° C. for a short time of 10 minutes or less to be highly oriented and to have an optically anisotropic portion of 7.
A method has been proposed in which a so-called neomesophase having a quinoline-insoluble content of 5% by weight or more and 25% by weight or less is formed, and the neomesophase is used as a spinning pitch.

[0006] The spinning pitch thus obtained is melt spun to obtain pitch fibers, which are then made infusible, carbonized or further graphitized to produce carbon fibers having high strength, high elasticity, and other high-performance carbon fibers. You. By the way, the carbon fiber thus obtained is usually impregnated in a matrix resin such as an epoxy resin, a polyamide resin, and a phenol resin to form a so-called prepreg, which is molded by various molding methods, and used as a fiber-reinforced plastic for leisure and sports. Used as various industrial materials. Therefore, in order to express the mechanical properties of the carbon fiber reinforced plastic,
It is important that the carbon fibers themselves are well dispersed in the matrix resin and the mechanical properties of the carbon fibers themselves are fully exhibited, at the same time as the mechanical properties such as high strength and high elasticity of each carbon fiber itself. Factors.

In other words, no matter how large the strength or elastic modulus of the carbon fiber is, if the fiber is poorly dispersed in the matrix resin, the mechanical function of the carbon fiber reinforced plastic becomes insufficient. That is. Therefore, first, for the dispersibility in the matrix resin, there is no fusion between the single fibers of the carbon fibers used, that is, the carbon fibers must be sufficiently defibrated.

That is, in the pitch-based carbon fiber manufacturing process, the fiber that has been infusibilized (hereinafter simply referred to as infusible fiber) and the fiber that has been carbonized or graphitized (hereinafter simply referred to as carbon fiber) are mixed in the previous step. The single fibers are fused together due to oil agents such as the sizing agent and sizing agent used in the above, and thermal deterioration of the fibers themselves in each step, resulting in uneven quality and dispersion of the single fibers in the matrix resin. It must be pliable and unfused at any stage of infusibilization, carbonization, or graphitization, as it will be uniform and impair the homogeneity of the composite.

Conventionally, as a method of defibrating the infusible fiber or carbon fiber, a method of subjecting the fiber to turbulent flow treatment, a method of bending a fiber such as a bar, a wire, a rotating pin or the like while passing the fiber while bending it, a method of forming a convex shape, A method of contacting a curved surface of a roll having a curved surface (JP-A-55-57015);
A method of contacting two or more tapered rollers with inclined surfaces (Japanese Patent Application Laid-Open No. Sho 61-124645) and a method of defibrating in a fluid (Japanese Patent Application Laid-Open No. 57-89638) have been proposed. In addition, a method of treating the surface of carbon fiber or infusibilized fiber with an oxygen-containing gas or the like to defibrate or improve the strength of the carbon fiber (JP-A-61-215716, JP-A-63-215716) 665523, JP-A-63-1
No. 75122). These are all achieved by treating carbon fibers in an inert gas containing oxygen and slightly etching the surface.

[0010]

However, conventional methods, for example, mechanical defibration methods have insufficient defibration effects in spite of high equipment costs, and as a method for improving the surface area, anodization is carried out using an apparatus. And the operation is complicated, the improvement of the surface area is small, and there are problems such as waste liquid treatment. Even if the carbon fiber or graphite fiber carbonized at a high temperature under an inert atmosphere is heated in an oxygen-containing inert gas atmosphere, the fiber surface is already stabilized and inactive, so the surface area is improved. The effect is not great, and in fact, oxygen gas reacts with carbon fiber with great heat generation, so it is difficult to control the reaction, and the peroxidation reaction progresses in some filaments, which is sufficiently satisfactory. It has been difficult to obtain carbon fibers having a very high strand strength.

[0011]

The inventors of the present invention have conducted intensive studies on a method for defibrating pitch-based carbon fibers and improving strand strength. The inventors have found an epoch-making method capable of producing a carbon fiber with good fibrillation and high strand strength by performing a heat treatment.

Further, the carbon fiber thus obtained is subjected to a secondary carbonization treatment in an inert atmosphere at a temperature higher than the above-mentioned primary carbonization temperature, whereby the strength and elastic modulus of the fiber can be freely controlled according to the purpose of use. The present invention was completed. That is, an object of the present invention is to provide a method for producing a carbon fiber having a good fibrillation property and a high strand strength, and another object of the present invention is to provide a fiber strength,
Another object of the present invention is to provide a method for producing a carbon fiber having a high strand strength in which the modulus of elasticity can be freely controlled.
An object of the present invention is to provide a method for producing a high-strand-strength carbon fiber exhibiting a resin-impregnated strand strength comparable to the strength of the single fiber itself determined by the method described in JP-A-1-19886, 6.6.1. In a method of producing a carbon fiber through at least melt spinning, infusibilization and carbonization steps, in a carbon dioxide-containing atmosphere after carbonizing the infusibilized fiber, exceeds 1,000 ℃, at a temperature of 1,800 ℃ or less The problem is easily solved by a method for producing high strand strength carbon fiber, which is characterized by performing a heat treatment.

Hereinafter, the present invention will be described in detail. The spinning pitch for obtaining the carbon fibers used in the present invention is not particularly limited as long as a molecular species that is easily oriented is formed and can provide an optically anisotropic carbon fiber. Various conventional ones can be used. Examples of the carbonaceous raw material for obtaining these spinning pitches include coal-based coal tar, coal-tar pitch, coal liquefaction, petroleum-based heavy oil, tar, pitch or a polymerization reaction produced by a catalytic reaction of naphthalene or anthracene. Objects and the like.
These carbonaceous materials include free carbon, undissolved coal,
Although impurities such as ash and catalyst are contained, these impurities are desirably removed in advance by a well-known method such as filtration, centrifugal separation, or stationary sedimentation using a solvent.

Further, the carbonaceous raw material is pre-treated by, for example, a method of subjecting the carbonaceous material to heat treatment and then extracting a soluble component with a specific solvent, or a method of hydrogenating in the presence of a hydrogen-donating solvent and hydrogen gas. May be performed. In the present invention, a carbonaceous raw material containing an optically anisotropic structure of 40% or more, preferably 70% or more, more preferably 90% or more is suitable. If necessary, the carbonaceous raw material is usually 350 to 500 ° C, preferably 380 to 450 ° C.
And 2 minutes to 50 hours, preferably 5 minutes to 5 hours, nitrogen,
The heat treatment may be performed in an atmosphere of an inert gas such as argon or water vapor or under blowing.

The optically anisotropic texture ratio of the pitch referred to in the present invention is a value obtained as an area ratio of a portion showing optical anisotropy in a pitch sample with a polarizing microscope at room temperature. Specifically, for example, a pitch sample crushed into a square of several mm is embedded with a sample piece on almost the entire surface of a resin having a diameter of 2 cm according to a conventional method, and after polishing the surface, the entire surface is thoroughly covered with a polarizing microscope (100 magnification). ) Observed under, and 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, melt spinning and infusibilization are carried out according to a conventional method, and then carbonized to obtain carbon fibers. This carbonization is carried out in an atmosphere of an inert gas such as nitrogen or argon at a temperature of 400 ° C. to 1,800 ° C., preferably 400 ° C. to 1,400 ° C., usually for 10 seconds to 6 hours, preferably 1 hour to 6 hours. This is performed in minutes to 2 hours.

Further, the absolute value of the strength of the carbon fiber may be insufficient depending on the use. Therefore, when it is desired to obtain a carbon fiber having higher mechanical properties such as strength and elastic modulus than the carbon fiber obtained by the above-described method, after the above-described heat treatment, the carbonization is further performed under an inert atmosphere. The object can be achieved by subjecting the fiber to a secondary carbonization treatment or a graphitization treatment at a temperature higher than the temperature.

The temperature of the secondary carbonization or graphitization may be determined according to required mechanical properties such as strength and elastic modulus, but it is important that the temperature is higher than that of the primary carbonization. That is, when the secondary carbonization treatment or graphitization treatment temperature is equal to or lower than the primary carbonization treatment temperature, the secondary carbonization treatment or graphitization treatment hardly contributes to the improvement of the mechanical properties of the carbon fiber.

The secondary carbonization or graphitization temperature is 80
0-3000 ° C is preferred. If the temperature is less than 800 ° C., the mechanical properties of the fiber are little improved, and if it exceeds 3,000 ° C., the effect of increasing the mechanical properties by the temperature is considerably moderated in spite of the large heat source cost, which is industrially advantageous. This is because it cannot be said. In the present invention, such carbon fiber is then subjected to a temperature of more than 1,000 ° C. and 1,800 ° C. or less, preferably in a carbon dioxide atmosphere or in an atmosphere of a mixed gas of an inert gas such as nitrogen gas and argon gas and carbon dioxide. In a temperature range of 1,050 ° C. or more and 1,400 ° C. or less, it is usually 0.1 second or more and 24 hours or less, preferably 1 second or more and 6 hours or less.
The heat treatment is performed for a time equal to or less than the time. The carbon dioxide concentration is usually from 1000 ppm to 100 vol%, preferably from 5,000 ppm to 100 vol%.

The concentration of carbon dioxide in the present invention greatly depends on the treatment temperature and treatment time. For example, when treating for a long time or at a high temperature, it is preferable to reduce the concentration of carbon dioxide. When the treatment is carried out, the concentration of carbon dioxide is preferably increased. Considering the effect of the present invention that the defibration property is good and the fiber alone or a resin-impregnated strand exhibits high strength, the most industrially preferable treatment conditions are 1,050 ° C or more and 1,400 ° C or less. 5 seconds to 90 minutes, concentration 500
0 ppm or more and 70 vol% or less.

The secondary carbonization or graphitization is carried out as necessary for the purpose of improving the mechanical properties of the carbon fibers. It does not impair the effect of defibration of carbon fibers obtained by heat treatment in a carbon dioxide-containing atmosphere and the effect of improving strand strength. After the heat treatment in a carbon dioxide-containing atmosphere, the above-described secondary carbonization treatment or graphitization treatment , Surface treatment and the like can also be performed. As described above, the carbon fiber exceeds 1,000 ° C. in an atmosphere containing
By performing the heat treatment at a temperature of not more than ° C., the fibrillation of the fiber and a significant improvement in strand strength can be achieved. In addition, according to the method of the present invention, the fibers are defibrated, the dispersibility of each single fiber in the matrix resin is improved, and not only the strand strength is improved, but also the single fiber strength in each single fiber unit is improved. This also includes the effect of removing the surface defects generated by fusion between the single fibers by etching with carbon dioxide.

In the prior art, attempts to remove surface defects and the like by etching have been made as described above, but all of them have been based on a method involving large heat generation such as oxygen gas. The carbon dioxide used in the present invention showed a particularly large effect because the reaction between carbon atoms on the carbon fiber surface and carbon dioxide is an endothermic reaction or a slightly exothermic reaction at a temperature exceeding 1,000 ° C. and 1,800 ° C. or less. This is considered to be due to the fact that the treatment can be performed without causing the strength deterioration particularly due to peroxidation or the like.

[0023]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the scope of the present invention.

Example 1 A spinning pitch having a softening point of 300 ° C. and an optical anisotropy ratio of 95% as observed under a polarizing microscope was prepared from a coal tar pitch. This is 0.1 mm in nozzle diameter and 4,000 holes.
Using a spinneret of 0, melt spinning at a die temperature of 330 ° C.
A silicone-based oil agent was adhered to the obtained pitch fibers having a yarn diameter of 12 μm and bundled. This pitch fiber is treated at 310 ° C. for 30 minutes.
Heat treatment was performed in the air for 1 minute to obtain infusible fibers. Further, the infusibilized fiber was carbonized at 545 ° C. in nitrogen gas to obtain a carbon fiber. This carbon fiber is converted to 2v of carbon dioxide
In a continuous heating furnace maintained in a nitrogen gas atmosphere containing 1% by weight, heat treatment was performed at 1,200 ° C. for a residence time of 20 minutes.

The carbon fiber thus obtained has no fusion between fibers, and is impregnated in the matrix epoxy resin.
After drying and curing at 130 ° C. for 30 minutes and observing the cross section of the carbon fiber with respect to the longitudinal direction by a microscope, as shown in FIG. 1, the single fiber 1 is uniformly dispersed in the epoxy resin matrix 2 and has excellent homogeneity. showed that.

The physical properties of the single fiber and the resin-impregnated strand of the obtained fiber were measured according to JIS R-7601, and the results were as follows. Monofilament physical properties Tensile strength 320 kgf / mm ² Tensile elastic modulus 20 tonf / mm ² Resin impregnated strand physical properties Tensile strength 300 kgf / mm ² Tensile elastic modulus 21 tonf / mm ²

Example 2 In a nitrogen gas atmosphere containing 25 vol% of carbon dioxide, 1,
The operation was performed in the same manner as in Example 1 except that the residence time at 200 ° C. was 30 minutes. Various properties of the obtained fiber were measured in the same manner as in Example 1, and the results are shown in Table 1.

Example 3 The carbon fiber obtained in Example 2 was further graphitized in an argon gas at 2,150 ° C. for a residence time of 0.5 minute. Various properties of the obtained fiber were measured in the same manner as in Example 1, and the results are shown in Table 1.

Comparative Example 1 The same procedure as in Example 1 was carried out except that the concentration of carbon dioxide in nitrogen was 50 vol%, the heat treatment temperature was 850 ° C., and the residence time was 20 minutes. Various properties of the obtained fiber were measured in Example 1.
Table 1 and FIG. 2 show the results of the measurement in the same manner as in FIG.

[0030]

[Table 1]

[0031]

According to the present invention, it is possible to provide a method for producing a high-strand-strength carbon fiber which has a good fibrillation property, a high strand strength, and a resin-impregnated strand strength comparable to the strength of the single fiber itself.

[Brief description of the drawings]

FIG. 1 is a schematic view of a visual field obtained by observing a cross section of a carbon fiber with an optical microscope.

FIG. 2 is a schematic view of a visual field obtained by observing a cross section of a carbon fiber with an optical microscope.

[Explanation of symbols]

 1 carbon single fiber 2 matrix resin

Claims (4)

[Claims] 1. A method for producing carbon fiber using pitch as a raw material through at least each of melt spinning, infusibilization and carbonization steps, wherein the infusibilized fiber is carbonized and then exceeds 1,000 ° C. in a carbon dioxide-containing atmosphere. A method for producing high-strand-strength carbon fiber, comprising performing heat treatment at a temperature of 1,800 ° C. or less. 2. The method for producing high strand strength carbon fiber according to claim 1, wherein the heat treatment in an atmosphere containing carbon dioxide is performed at 1,050 ° C. to 1,400 ° C. 3. The heat treatment in a carbon dioxide-containing atmosphere is performed under the conditions of 1,050 ° C. to 1,400 ° C., 5 seconds to 90 minutes, and carbon dioxide concentration of 5,000 ppm to 70 vol%. The method for producing a high-strand-strength carbon fiber according to claim 1. 4. The method for producing high strand strength carbon fiber according to claim 1, wherein a pitch having an optically anisotropic structure ratio of 70% or more is used as a raw material.