JP2961274B2 - High modulus pitch-based carbon fiber and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a high modulus pitch-based carbon fiber and a method for producing the same.

The product obtained by firing the infusibilized pitch fiber at a temperature of about 1500 ° C or lower is called carbon fiber, and the product obtained by firing at a temperature of about 1500 ° C or higher has a crystal structure of graphite. Regardless of whether it is a graphite fiber. Furthermore, the firing temperature for separating the two is not clearly determined even by those skilled in the art. Therefore, in this specification, carbon fibers and graphite fibers obtained by firing infusible pitch fibers are collectively referred to as carbon fibers.

Conventional technology and its problems Pitch-based carbon fibers have a higher elastic modulus than PAN-based carbon fibers, and are being used in the form of composite materials with plastics, metals, carbon, ceramics, etc. Both are expected to have wider applications as lightweight structural materials for general aircraft, aerospace equipment, automobiles, etc .;

A method for producing a high modulus pitch-based carbon fiber is disclosed in, for example, JP-A-63-120112. The method disclosed in this publication suppresses the formation of a surface layer having a low Young's modulus by making the thickness of the infusible layer small when the spun pitch fibers are made infusible. However, in this method, the thickness of the infusibilized layer may be insufficient, and fusion may occur during firing.

A method has also been proposed in which carbonization by firing after the infusibilization treatment is performed at a higher temperature to increase the elastic modulus of the carbon fiber. However, this method has a problem that the yield is low and various properties such as tensile strength and elongation of the obtained carbon fiber are apt to be reduced.

Means for Solving the Problems The present inventor has conducted intensive studies in view of the problems of the prior art as described above, and as a result, obtained by spinning pitch, infusibilizing, and firing according to a conventional method. It has been found that when the surface layer of a fiber is removed, its modulus can be greatly improved.

That is, the present invention provides the following high-modulus pitch-based carbon fibers and a method for producing the same: Spinning a pitch for spinning, infusing, and firing to remove the surface layer of the carbon fibers obtained. Wherein the thickness of the surface layer of the carbon fiber to be removed is 2 to 20% of the diameter of the carbon fiber.

The method for producing a high modulus pitch-based carbon fiber according to the above item, wherein the surface layer of the carbon fiber is removed by oxidation.

The method for producing high modulus pitch-based carbon fibers according to the above item, wherein the carbon fiber surface layer is oxidized by gas phase oxidation, electrolytic oxidation or chemical oxidation.

The method for producing a high modulus pitch-based carbon fiber according to the above item, wherein the thickness of the surface layer of the carbon fiber to be removed is 10 to 15% of the diameter of the carbon fiber.

The method for producing a high modulus pitch-based carbon fiber according to the above item, wherein the firing temperature is 2000 ° C or higher.

A method for producing a high-modulus pitch-based carbon fiber, wherein the carbon fiber from which the surface layer has been removed by any of the above-mentioned methods is further fired at a higher temperature.

A high modulus pitch-based carbon fiber comprising a layer having a substantially uniform modulus of elasticity, produced by any one of the above-mentioned methods.

In the production of carbon fibers, generally, the higher the firing temperature, the higher the modulus of elasticity of the fibers. FIG. 1 schematically shows the relationship between the firing temperature and the elastic modulus of the carbon fiber. The modulus of elasticity of the core portion of the carbon fiber increases rapidly as the firing temperature rises as shown by curve A, but the modulus of elasticity of the surface layer does not exceed 25 to 30 ton / mm ^{2 as shown} by curve C. Therefore, the elastic modulus of the carbon fiber as a whole rises relatively slowly as shown by the curve B.

In general, in the surface layer of carbon fiber, the introduction of oxygen in the infusibilizing step, the desorption of carbon in the firing step, and the like, cause disorder in the crystal arrangement, which is a main factor in lowering the elastic modulus of the carbon fiber. It is assumed that FIG. 2 schematically shows a cross section of an existing carbon fiber obtained by firing at about 2000 ° C. after infusibilizing the pitch fiber. In the existing carbon fiber, there is a surface portion having a relatively low elastic modulus due to various influences at the time of infusibility. As shown in FIG. 3, when such a surface layer is removed by the method of the present invention, a factor having a low elastic modulus is removed, and a layer having a uniform elastic modulus over the entire fiber cross section is formed. As a result, the elastic modulus of the carbon fiber increases. The present invention has been completed based on such new findings.

As long as the carbon fibers targeted by the method of the present invention are pitch-based, their production method and production conditions are not limited. Regarding the higher-order structure of the carbon fiber cross section, the method of the present invention is applied to all structures such as a radial structure, a random structure, an onion structure, and a multilayer structure.

It is preferable to remove the surface layer which is unfavorably affected during infusibility from the high modulus carbon fiber obtained by significantly increasing the firing temperature (for example, at 2000 ° C. or higher). However, relatively temperature (for example, about 1000 ° C)
After removing the surface layer by the method of the present invention from the carbon fiber obtained by sintering at a higher temperature (for example, 2000 ° C.)
By firing further, carbon fibers having a high elastic modulus can be obtained. This is because the surface layer affected at the time of infusibilization has already been removed, so that the elastic modulus is entirely uniform and can be further increased by re-firing.

As a method for removing the surface layer of the carbon fiber, gas phase oxidation, electrolytic oxidation, chemical oxidation, or the like is applied. The thickness of the surface layer to be removed varies depending on the diameter of the carbon fiber, production conditions, and the like, but is usually about 2 to 20%, more preferably about 10 to 15% of the diameter of the carbon fiber. When any of these means is adopted, the conditions for the oxidation treatment are set appropriately such that the surface layer of the required thickness is removed and the desired elasticity is improved. Just do it.

Gas-phase oxidation is generally carried out in an oxidizing atmosphere such as air.
It may be carried out by heating the carbon fiber at about 0 to 1100 ° C. for about 0.1 to 60 seconds.

In the electrolytic oxidation, for example, carbon fibers may be placed in an electrolytic solution such as sulfuric acid or sodium hydroxide, and electrolysis may be performed using a metal such as titanium or platinum as a cathode and the carbon fibers as an anode.

Chemical oxidation can be performed by immersing carbon fibers in a solution such as nitric acid.

Effects of the Invention According to the present invention, the following remarkable curing is achieved.

(1) The elastic modulus of the carbon fiber can be increased under various conditions.

(2) To improve the elastic modulus by the conventional method, for example,
When the modulus of elasticity is improved by increasing the firing temperature, various physical properties of the carbon fiber tend to decrease. However, according to the method of the present invention, such an adverse effect does not occur.

(3) Carbon fibers having a uniform elastic modulus over the entire cross section can be obtained.

(4) When further firing to increase the carbon fiber from which the surface layer has been removed by the method of the present invention, an equivalent high elasticity is obtained at a relatively low firing temperature, rather than obtaining a high elastic modulus with an untreated carbon fiber. A carbon fiber with a high modulus is obtained.

EXAMPLES Examples are shown below to further clarify features of the present invention.

Example 1 Using a melt spinning machine having a nozzle diameter of 0.15 mm, a spinning pitch (including an optically anisotropic phase of 85% or more and a softening point of 300 to 320)
C.) at 345-360 ° C., and the obtained pitch fibers were made infusible by a conventional method, and then fired at 2800 ° C. to obtain carbon fibers.

The elastic modulus of the carbon fiber thus obtained is 55 tons / mm ² ,
The tensile strength was 274 kg / mm ² and the fiber diameter was 8.8 μm.

This carbon fiber was heated in air at 1000 to 1100 ° C. for about 15 seconds to remove the surface area by gas phase oxidation.

The physical properties of the carbon fibers after the gas phase oxidation were 65 tons / mm ² with an increase in the elastic modulus of 18%, the tensile strength was 272 kg / mm ² , and the fiber diameter was 7.7 μm.

Example 2 A pitch fiber obtained in the same manner as in Example 1 was made infusible and then fired at 2700 ° C.

The elastic modulus of the carbon fiber thus obtained is 50 tons / mm ² ,
The tensile strength was 274 kg / mm ² and the fiber diameter was 9.0 μm.

The physical properties of the fiber obtained by oxidizing this carbon fiber in the same manner as in Example 1 are as follows.
Ton / mm ² , tensile strength is 271 kg / mm ² , fiber diameter is
8.0 μm.

Example 3 A pitch fiber obtained in the same manner as in Example 1 was made infusible and then fired at 2600 ° C.

The elastic modulus of the carbon fiber thus obtained is 45 tons / mm ² ,
The tensile strength was 250 kg / mm ² , and the fiber diameter was 9.4 μm.

The physical properties of the fiber obtained by oxidizing this carbon fiber in the same manner as in Example 1 were as follows.
Ton / mm ² , tensile strength is 251 kg / mm ² , fiber diameter is
It was 8.4 μm.

[Brief description of the drawings]

FIG. 1 is a graph schematically showing the relationship between the firing temperature and the elastic modulus of the carbon fiber, FIG. 2 is a schematic diagram showing a cross section of the carbon fiber obtained by a conventional method, and FIG. It is a schematic diagram which shows the cross section of the carbon fiber from which the surface layer was removed by the method.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-62-108754 (JP, A) JP-A-60-252719 (JP, A) Suguro Otani, "Carbon Fiber" (revised in S58), Modern Editor P217 −222 (58) Fields surveyed (Int.Cl. ⁶ , DB name) D01F 9/14 511 D06M 10/00 D01F 11/16

Claims (7)

(57) [Claims] 1. A method for removing a surface layer of carbon fiber obtained by spinning a pitch for spinning, making it infusible, and firing.
The thickness of the surface layer of the carbon fiber to be removed is 2 times the diameter of the carbon fiber.
20. A method for producing a high modulus pitch-based carbon fiber, wherein 2. The method according to claim 1, wherein the surface layer of the carbon fiber is removed by oxidation. 3. The method for producing high-modulus pitch-based carbon fibers according to claim 1, wherein the carbon fiber surface layer is oxidized by gas phase oxidation, electrolytic oxidation or chemical oxidation. 4. The method according to claim 1, wherein the thickness of the surface layer of the carbon fiber to be removed is 10 to 15% of the diameter of the carbon fiber. 5. The method according to claim 1, wherein the firing temperature is 2000 ° C. or higher. 6. A method for producing high-modulus pitch-based carbon fiber, wherein the carbon fiber from which the surface layer has been removed by any one of the above methods is further fired at a higher temperature. 7. A high modulus pitch-based carbon fiber comprising a layer having a substantially uniform modulus of elasticity, produced by the method according to claim 1.