JP2766530B2 - Method for producing pitch-based carbon fiber - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a carbon fiber having high strength and a high elastic modulus using mesophase pitch as a starting material.
More specifically, the present invention relates to a method for producing a high-performance pitch-based carbon fiber having a high strength and a high elastic modulus, in which the tensile strength is particularly improved by modifying the surface of the carbon fiber.

2. Description of the Related Art Carbon fiber is a material having a high specific strength and a specific elastic modulus in recent years in the aerospace field, the automobile industry, and other industrial fields.
It is attracting attention as a strong and light material. In such a field, an inexpensive material having high strength and high elastic modulus is desired.

At present, PAN-based carbon fibers made mainly from polyacrylonitrile (PAN) and pitch-based carbon fibers made from pitches are manufactured as carbon fibers, but at present, high strength and high elastic modulus are required. As the performance carbon fiber, PAN-based carbon fiber is mainly used.

However, PAN-based carbon fibers have a limit in further increasing the modulus of elasticity, and PAN as a raw material is expensive, and the carbon fiber yield obtained from PAN is low. There is a problem that the price of the fiber must be expensive.

Therefore, in recent years, various studies have been made on improving the performance of pitch-based carbon fibers using mesophase pitch as a raw material, in which the yield of carbon fibers is high and the elastic modulus can be easily increased.

Regarding high performance of mesophase pitch-based carbon fiber, mainly related to pitch properties for spinning, spinning conditions,
A proposal relating to a spinning apparatus and a proposal relating to infusibilization, carbonization and graphitization conditions have been proposed. In particular, as a problem peculiar to mesophase pitch-based carbon fibers, since the orientation of the raw material pitch is high, carbon fibers obtained by melt-spinning by a usual method are likely to have a fiber cross-sectional structure generally called a radial type. It has been pointed out that surface defects easily occur during graphitization and are inferior to PAN-based carbon fibers in terms of tensile strength.

In order to reduce such surface defects, various spinning methods, carbon fiber surface treatment methods, and the like have been proposed.

For example, as a method of reducing surface defects by treating the surface of pitch-based carbon fiber, Japanese Patent Application Laid-Open No. 61-2157
A method for improving the tensile strength and elastic modulus of a carbon fiber by reducing defects on the surface of the carbon fiber by subjecting the surface of the carbon fiber to gas-phase oxidative etching using oxygen as described in Japanese Patent No. 16 There is.

However, this method has no effect on the defects caused by the handling of the carbon fibers after the surface treatment, and in some cases, the vapor-phase oxidation etching may further expand the potential defects. The effect is not very large and must be limited.

Problems to be Solved by the Invention Potential defects occurring in the carbon fiber manufacturing process, or surface defects generated during handling after carbon fiber manufacturing, any of which, by improving the fracture toughness,
By relaxing stress concentration on surface defects or inhibiting crack propagation, carbon fiber strength is improved.

An object of the present invention is to provide a method for producing a pitch-based carbon fiber that improves such fracture toughness and develops a carbon fiber structure in which a decrease in strength against defects is reduced.

Means for Solving the Problems In view of the above, the inventors of the present invention have proposed a method of reducing the degree of stress concentration on surface defects or inhibiting the propagation of cracks in defects, with respect to pitch-based carbon fibers, which tend to cause surface defects, that is, fracture toughness. The present inventors have made intensive studies based on the idea that the strength of a pitch-based carbon fiber, which is liable to cause surface defects, can be improved by using a carbon fiber having an improved carbon fiber, thereby completing the present invention.

That is, the present invention provides a pitch fiber starting from a mesophase, an infusibilized fiber infusibilized under an oxidizing gas atmosphere, or a carbonized fiber obtained by carbonizing the infusibilized fiber under an inert gas atmosphere, in a steam atmosphere. Under or in a mixed gas atmosphere containing 5% by volume (mol%) or more water vapor,
This is a method for producing pitch-based carbon fibers, which comprises treating at 450 ° C. to 1000 ° C. for 10 seconds to 4 hours, and then carbonizing and graphitizing at 1300 ° C. or more.

 Hereinafter, the contents of the present invention will be described in detail.

The pitch used in the present invention is coal tar pitch, SRC
Includes various pitches, such as coal pitch, ethylene tar pitch, petroleum pitch such as decant oil pitch obtained from fluidized catalytic cracking residue, or synthetic pitch made from catalysts from naphthalene etc. Is what you do.

The mesophase pitch used in the present invention is obtained by generating a mesophase from the above-mentioned pitch by a conventionally known method. It is desirable that the mesophase pitch has a high orientation of the pitch fiber when spun. Therefore, the mesophase content is 40% or more, more preferably 70% or more.

The mesophase pitch used in the present invention has a softening point.
Those having a temperature of 200 ° C to 400 ° C, more preferably 250 ° C to 350 ° C are preferable. In order to improve the spinnability of these pitches, it is desirable to remove impurity particles such as free carbon and ash in advance by a known method such as filtration.

The pitch fiber is obtained by subjecting the mesophase pitch to melt spinning by a conventionally known method. For example, the mesophase pitch has a viscosity of 100 to 2000.
At a temperature showing the poise, the capillary diameter 0.1 to 0.5 mm, while the extrusion pressure 0.1~100kg / cm ² 100 to 200
Drawing is performed at a take-up speed of 0 m / min to obtain a pitch fiber having a fiber diameter of 5 to 20 μm.

Next, the pitch fiber is converted into a thermosetting fiber by infusibilizing treatment by a known method. For example, air,
Inert gas such as nitrogen gas, or oxidizing gas with oxygen concentration controlled by adding oxygen to air, or ozone, nitrogen dioxide gas, nitrogen monoxide gas, sulfur dioxide gas, etc. An infusibilization treatment for oxidizing the pitch fibers is performed in an oxidizing gas atmosphere from a temperature equal to or lower than the softening point of the pitch.

In the present invention, the infusibilized fiber obtained in this manner or a carbonized fiber obtained by previously carbonizing the infusibilized fiber in an atmosphere of an inert gas such as nitrogen gas is used. The carbonized fibers used in the present invention are preferably carbonized at a temperature of 300 ° C. or more and 1000 ° C. or less, more preferably 300 ° C. or more and 900 ° C. or less. When the carbonization temperature exceeds 1000 ° C., the effect of the present invention is significantly reduced.

In the present invention, such infusibilized fiber or carbonized fiber is treated at a temperature of 450 ° C. or more and 1000 ° C. or less, preferably 600 ° C. or more and 950 ° C. or less in a steam-containing atmosphere, and thereafter, 13 ° C.
It is important to carbonize and graphitize at 00 ° C or higher. If the temperature in a steam-containing atmosphere is lower than 450 ° C., the reaction with steam is very slow and has no substantial meaning. Also, 1000
When the temperature exceeds ℃, the reaction by water vapor becomes excessive, and the strength of the obtained carbon fiber is rather lowered.

The atmosphere gas may be steam alone or a mixed gas obtained by mixing steam with a non-oxidizing gas such as nitrogen gas.
The water vapor concentration at this time is 5% by volume or more, more preferably
20 volume% or more is good. If it is less than 5% by volume, the water vapor concentration is so low that a long time is required for the reaction, and the effect of improving the tensile strength of the carbon fiber is small.

The time of the treatment with steam varies depending on the treatment temperature or the type of fiber used in the present invention, but it is usually from 10 seconds to 4 hours, preferably 1 hour or less, more preferably 20 hours or less.
The time should be between 30 seconds and 30 minutes. When the treatment time is more than 4 hours, the treatment takes too much time, which not only increases the treatment cost, but also promotes the reaction locally so that the effect of the present invention does not appear. I do. If the treatment time is less than 10 seconds, the reaction becomes insufficient or the reaction between fibers becomes uneven, which is not preferable.

The present invention is achieved by carbonizing and graphitizing the thus-treated fiber at 1300 ° C. or higher by a conventionally known method. When the carbonization temperature of the treated fiber is lower than 1300 ° C., the improvement in the physical properties of the carbon fiber is small, and it is desirable that the temperature is 1300 ° C. or higher, preferably 1700 ° C. or higher.

As described above, pitch fibers using mesophase pitch as a starting material, infusibilized fibers infusibilized in an oxidizing gas atmosphere, or carbonized fibers obtained by carbonizing the infusibilized fibers in an inert gas atmosphere, are treated in a steam atmosphere. Alternatively, under a mixed gas atmosphere containing water vapor, the fiber is treated at 450 ° C. or more and 1000 ° C. or less, and then the treated fiber is treated in an inert gas atmosphere at 1300 ° C.
By carbonizing or graphitizing at a temperature of not less than ° C, it is possible to obtain a carbon fiber having improved fracture toughness and improved tensile strength.

Action The reason why the tensile strength of the carbon fiber is improved in the present invention is as yet unknown, but it is considered as follows.

The reaction with water vapor forms fine pores (micropores) in the carbon material as used in the production of activated carbon and the like. It is generally known that such micropores reduce carbon fiber strength. When reacted with steam under the conditions described in the present invention, the micropores generated by the steam are limited to the vicinity of the surface of the carbon fiber. It is considered that the thickness of the layer into which the formed micropores are introduced is substantially equal to the depth of a potential surface defect that affects the strength of the carbon fiber.

Thus, the fibers treated with water vapor have a large number of micropores on the fiber surface, the introduced thickness is smaller than the defect that governs the strength, and the majority of the fiber cross section has micropores. Take a structure that is not introduced. The micropores generated by the water vapor are considered to be still large in this state, and the fibers themselves have only low strength because carbonization has not yet completely progressed.

By carbonizing and graphitizing the fiber at a temperature of 1300 ° C. or higher, the fiber surface layer into which the micropores are introduced becomes denser due to shrinkage, and the presence of the fiber itself does not cause a decrease in strength.

As the micropores that do not lead to a decrease in strength are distributed only in the vicinity of the carbon fiber surface, stress concentration at the defect part is reduced for potential strength-controlling defects or surface defects generated after carbon fiber production Is done.
Such micropores also resist crack growth and propagation. That is, it is presumed that the tensile strength of the carbon fiber is improved by improving the fracture toughness value of the carbon fiber itself.

FIG. 1 shows the change in carbon fiber tensile strength according to the method of the present invention. The change in the tensile strength of the carbon fiber when the processing time is changed while the processing temperature in a steam atmosphere is kept constant is illustrated, but it is shown that there is an optimum processing time at a specific processing temperature. It is also clear that there is an optimum processing temperature for a particular processing time.

This means that there is an optimum point for improving the strength in the layer thickness in which the micropores generated by the steam treatment are introduced, and the longer the processing time, the thicker the micropore-introduced layer, and the strength improves accordingly. Is done.
When the treatment time is further increased, the micropore introduction layer becomes thicker than the depth of the potentially existing defect. In this case, it is considered that crack propagation from the potential defect is promoted by the micropore and the strength is reduced. The results suggest the reason for the improvement of the carbon fiber tensile strength due to.

Here, the micropore introduction layer is taken as an example, but there is a similar relationship with respect to the size, shape, and amount of the micropores that will govern the strength, and the strength is improved under specific processing conditions. it is conceivable that.

Conventionally, as a method of improving the strength physical properties of carbon fibers, it has been studied how to reduce defects.However, the present invention completely contradicts the conventional idea to remove minute defects such as micropores into carbon fibers. This is based on a novel concept that has not been used in the past, in which the strength is improved by the effect of reducing the stress concentration with respect to a potential defect by the introduction. Further, it is characterized in that the effect is exerted also on defects caused by the handling of carbon fibers after the production of carbonized or graphitized fibers.

Examples Hereinafter, the present invention will be described with reference to Examples and Comparative Examples. In the present invention, various physical property values used to represent the characteristics of the pitch-based carbon fiber and the raw material pitch are defined as follows.

(1) Tensile strength and tensile elastic modulus Tensile strength and tensile elastic modulus were measured according to the method shown in JIS-R-7601 (1986).

(2) Viscosity and softening point The viscosity was measured using a concentric rotating double cylinder viscometer.
The softening point is the temperature at which the apparent viscosity calculated from the Hagen-Poiseuille equation using a flow tester is 20,000 poise.

(3) Mesophase content The mesophase as referred to in the present invention refers to an optically anisotropic which can be determined by embedding a cooled and solidified pitch in a resin or the like, polishing the surface, and observing it with a reflection polarization microscope. Refers to the organization. The mesophase content is indicated by the area ratio of the anisotropic structure observed and observed as described above.

(4) Toluene-insoluble matter and quinoline-insoluble matter Toluene-insoluble matter and quinoline-insoluble matter are JIS-K-2425 (1
978).

Example 1 Using decoaled coal tar pitch having a softening point of 80 ° C. as a raw material and tetrahydroquinoline as a hydrogenation solvent,
After reacting at 440 ° C. for 20 minutes under a pressure of 120 kgf / cm ² , the solvent and low boiling fractions were removed at 270 ° C. under reduced pressure to obtain a hydrotreated pitch. After heat-treating this at 480 ° C. under normal pressure, a mesophase pitch was obtained except for a low boiling point component. This pitch is
The softening point was 304 ° C, the toluene-insoluble content was 85% by weight, the quinoline-insoluble content was 14% by weight, and the mesophase content was 95%.

This mesophase pitch was removed at a temperature of 340 ° C. and a viscosity of 100 poise using a microfiltration net to remove infusible foreign matter in the pitch. Using this pitch in a conventionally known method, a capillary diameter of 0.
Using a spinning machine having a nozzle pack with 14 mm and 3,000 nozzle holes, pitch fibers having a mesophase pitch viscosity of 800 poise and a yarn diameter of 13 μm were obtained.

This pitch fiber is heated from 200 ° C to 300 ° C in the air at 0.
The temperature was raised at a rate of 5 ° C./min, and the temperature was maintained at 300 ° C. for 1 hour to perform an infusibilization treatment to obtain an infusible fiber. The infusible fiber was heated from 300 ° C. to 500 ° C. at a rate of 5 ° C./min in a nitrogen gas atmosphere, and kept at 500 ° C. for 30 minutes to obtain a carbonized fiber.

Next, the carbonized fibers were placed in a furnace at 850 ° C. in a mixed gas atmosphere of 50% by volume of steam and 50% by volume of nitrogen gas at 1.5, 2.5, 3,
The treatment was performed for 4, 5, 10, and 15 minutes, respectively, to obtain seven types of treated fibers. Thereafter, each of these treated fibers was heated to 2000 ° C. at a rate of 40 ° C./min in an argon gas atmosphere, and kept at 2000 ° C. for 15 minutes to obtain eight types of graphitized fibers.

FIG. 1 shows the tensile strength of each of the obtained graphitized fibers. Those subjected to steam treatment at 850 ° C. for 1.5 minutes have a tensile strength and tensile elastic modulus of 10 μm in yarn diameter, a strength of 310 kgf / mm ² , and an elastic modulus of 50.
tf / mm ² .

Example 2 The treated fiber subjected to steam treatment at a temperature of 850 ° C. for 1.5 minutes in Example 1 was heated at a rate of 40 ° C./min in an argon gas atmosphere.
The temperature was raised to 1500 ° C, and kept at 1500 ° C for 15 minutes to obtain carbon fibers. When the tensile strength and the elastic modulus of the obtained carbon fiber were measured, the yarn diameter was 10.7 μm, the strength was 220 kg / mm ² , and the elastic modulus was 31 tf / mm ² .

Example 3 The infusibilized fiber used in Example 1 was charged into a furnace at a temperature of 500 ° C. under an atmosphere of 100% by volume of water vapor.
The temperature was raised to 0 ° C at a rate of 25 ° C / min, and the treatment was continued for 5 minutes. Thereafter, the temperature of the treated fiber is increased in an argon gas atmosphere.
The temperature was raised to 2000 ° C. at 40 ° C./min, and kept for 15 minutes to obtain a graphitized fiber. When the tensile strength and tensile modulus of the obtained graphitized fiber were measured, the yarn diameter was 10 μm, and the strength was 300 kgf.
/ mm ² and the elastic modulus was 50 tf / mm ² .

Comparative Example 1 The infusibilized fiber used in Example 1 was treated under a nitrogen gas atmosphere for 30 minutes.
The temperature is raised from 0 ° C to 400 ° C at 5 ° C / min, and
After holding for 0 minutes, a carbonized fiber was obtained. Next, the carbonized fiber was heated to 2000 ° C. at a rate of 40 ° C./min in an argon gas atmosphere, and kept at 2000 ° C. for 15 minutes to obtain a graphitized fiber. When the tensile strength and the tensile elastic modulus of the obtained graphitized fiber were measured, the yarn diameter was 10 μm, the strength was 240 kgf / mm ² , and the elastic modulus was
It was 50 tf / mm ² .

Comparative Example 2 The carbonized fiber obtained in Comparative Example 1 was heated to 1500 ° C. at a heating rate of 40 ° C./min in an argon gas atmosphere, and was directly heated to 1500 ° C.
After holding for 15 minutes, a carbon fiber was obtained. When the tensile strength and the elastic modulus of the obtained carbon fiber were measured, the yarn diameter was 10.7 μm, the strength was 190 kg / mm ² , and the elastic modulus was 30 tf / mm ² .

Comparative Example 3 The infusible fiber used in Example 1 was heated at a rate of 5 ° C./min from 300 ° C. to 400 ° C. at a rate of 5 ° C./min in a mixed gas atmosphere of 50% by volume of steam and 50% by volume of nitrogen gas, and left at 400 ° C. for 1 hour. Holding was performed to obtain a treated fiber. Next, the treated fiber is heated up to 2000 ° C at a heating rate of 40 ° C / min in an argon gas atmosphere, and
It was kept at 2000 ° C. for 15 minutes to obtain a graphitized fiber. When the tensile strength and tensile modulus of the obtained graphitized fiber were measured,
The yarn diameter was 10 μm, the strength was 215 kgf / mm ² , and the elastic modulus was 47 tf / mm ² .

As is clear in Examples 1 to 3 and Comparative Examples 1 to 3,
In both cases of the carbonized fiber and the infusible fiber, the effect of the present invention improves the tensile strength of the graphitized fiber.

Example 4 Cracking residue oil (decant oil) obtained from a fluid catalytic cracking unit (FCC unit) of a petroleum heavy oil fraction was distilled from oil having a boiling point range from 360 ° C. to 520 ° C. under atmospheric pressure as a raw material.
At a pressure of 0.5 kg / cm ² and a temperature of 450 ° C while blowing nitrogen gas
After a thermal decomposition polymerization reaction for 45 minutes, a low boiling point component was removed at a temperature of 460 ° C. under a reduced pressure of 10 mmHg for 20 minutes to obtain a mesophase pitch.

This pitch has a softening point of 320 ° C and a toluene insoluble content of 8
2% by weight, quinoline insoluble content was 35% by weight, and mesophase content was 100%. The insoluble foreign matter in the pitch was removed from the mesophase pitch at a temperature of 360 ° C. and a viscosity of 300 poise by using a fine filtration network.

Using this pitch, a pitch fiber having a mesophase pitch of 800 poise and a yarn diameter of 13 μm was obtained by a conventionally known method using a spinning machine having a nozzle pack with a capillary diameter of 0.14 mm and 200 nozzle holes.

1 ℃ / m from 150 ℃ to 300 ℃ in air
The temperature was raised at a rate of in to obtain infusible fibers. 5 ℃ / min from 200 ℃ to 500 ℃ under nitrogen gas atmosphere
And kept at 500 ° C. for 30 minutes to obtain carbonized fibers.

Next, this carbonized fiber is treated at 840 ° C. for 5 minutes in a furnace in a mixed gas atmosphere of 20% by volume of steam and 80% by volume of nitrogen gas, and then heated at a rate of 40 ° C./min.
The temperature was raised to 2000 ° C. and kept at 2000 ° C. for 15 minutes to obtain a graphitized fiber. The obtained graphitized fiber had a tensile strength and a tensile modulus of elasticity of 10 μm, a strength of 300 kgf / mm ² and a modulus of elasticity of 41 tf / mm ² .

Comparative Example 4 The infusibilized fiber used in Example 4 was treated under a nitrogen gas atmosphere for 20 minutes.
After raising the temperature from 0 ° C to 1000 ° C at 5 ° C / min to obtain carbonized fiber, 2000 ° C at a rate of 40 ° C / min in an argon gas atmosphere
Then, the temperature was maintained for 15 minutes to obtain a graphitized fiber.
When the tensile strength and the tensile elastic modulus of the obtained graphitized fiber were measured, the yarn diameter was 10 μm, the strength was 250 kgf / mm ² , and the elastic modulus was 40 tf.
It was / mm ^2.

As is clear from Examples 1 and 4 and Comparative Examples 1 and 4, the tensile strength of the graphitized fiber is improved by the effect of the present invention regardless of whether the starting material is coal-based pitch or petroleum-based pitch.

As is clear from the above Examples and Comparative Examples, the carbon fibers produced by the method of the present invention have excellent tensile strength as compared with the conventional method.

Effect of the Invention According to the present invention, pitch-based carbon fibers are resistant to surface defects that are likely to occur in the cross-sectional structure of the fibers, that is, by improving fracture toughness, improved tensile strength, high performance Carbon fiber can be obtained.

Further, it is considered that the carbon fiber structure according to the method of the present invention has a feature that the strength is less reduced even with respect to surface defects generated after carbon fiber production, and high-performance fiber properties are easily obtained stably.

[Brief description of the drawings]

FIG. 1 shows a water vapor concentration of 50% by volume and nitrogen gas according to the present invention.
850 ° C processing temperature in 50% by volume mixed gas atmosphere
FIG. 4 is a diagram showing the relationship between the processing time and the tensile strength after firing at 2000 ° C.

──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Hirofumi Sunago 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture Inside the 1st Technical Research Laboratory of Nippon Steel Corporation (72) Inventor Norio Tomioka 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Nippon Steel Corporation 1st Technical Research Institute (58) Field surveyed (Int. Cl. ⁶ , DB name) D01F 9/00-9/32

Claims (1)

(57) [Claims] An infusibilized fiber obtained by infusing pitch fibers obtained from a mesophase pitch in an oxidizing gas atmosphere, or a carbonized fiber obtained by carbonizing the infusibilized fiber in an inert gas atmosphere, in a steam atmosphere or in a steam atmosphere. Pitch-based carbon fiber characterized in that it is treated in a mixed gas atmosphere containing at least volume% of steam at 450 ° C to 1000 ° C for 10 seconds to 4 hours, and then carbonized and graphitized at 1300 ° C or more. Manufacturing method.