JPH02175921A - Production of mesophase pitch-based carbon fiber - Google Patents

Abstract

PURPOSE:To obtain the subject carbon fiber remarkably improved in tensile strength by subjecting initial carbonized fiber to oxidation treatment in a nitrogen dioxide-containing atmosphere until a specific component is produced and then carbonization and graphitization treatment in an inert atmosphere. CONSTITUTION:A raw material pitch, such as coal tar pitch, is melt spun, then subjected to infusibilizing treatment and further calcined in an inert gas atmosphere to provide initial carbonized fiber. The resultant initial carbonized fiber is further subjected to oxidation treatment in an atmosphere containing nitrogen dioxide until a component, substantially insoluble in toluene and soluble in acetone is produced. The initial carbonized fiber subjected to the oxidation treatment is then subjected to carbonization and graphitization treatment in an inert atmosphere, such as nitrogen or argon, to afford the objective carbon fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for producing mesophase pitch carbon fibers, and more particularly to a novel treatment method for improving the tensile strength of mesophase pitch carbon fibers.

BACKGROUND OF THE INVENTION Currently, the carbon fibers used in composite materials are PAN-based carbon fibers produced from polyacrylonitrile (PAN) fibers. However, polyacrylonitrile (P), which is a raw material for PAN-based carbon fibers,
AN) Since mW is expensive and the carbonization yield is low, it is inevitably expensive, and its applications are mainly in special fields such as sports and leisure, aviation and space. There is.

On the other hand, pitch-based carbon fibers made from carbonaceous pitch are characterized by being inexpensive raw materials and having a high carbonization yield, so they can be manufactured at low cost. In particular, menphase pitch-based carbon fibers using mesophase pitch containing 80% or more of mesophase as a raw material are attracting attention as they are inexpensive and have the possibility of providing high-strength carbon mm1.

In general, for mesophase pitch-based carbon s#ll fibers, carbon fibers that are highly oriented and have high graphitization properties can be easily produced by utilizing the easy orientation and graphitization properties of mesophase pitch, which is a raw material. Therefore, it is known that fibers with high elastic modulus can be produced.

For example, JP-A-49-19127 discloses a mesophase pitch-based carbon fiber with a three-dimensionally developed carbon layer surface, high graphitizability, and excellent modulus of elasticity, and a method for producing the same. However, although such highly graphitizable carbon fibers have a high modulus of elasticity, they have the drawbacks of not having high tensile strength and low elongation at break. Although it has excellent elastic modulus, it was thought that it would be difficult to improve tensile strength.

Recently, reports have been made on attempts to improve the tensile strength of mesophase pitch carbon m1IR by controlling its structure.
Japanese Patent No. 104927 discloses that in the spinning process, stirring is performed directly above a part of the capillary of the spinning nozzle to refine the structure in the cross-sectional direction while maintaining high orientation in the width direction, thereby producing mesophase pitch with reduced graphitizability. It has been shown that it is possible to produce carbon fibers based on carbon fibers, and that these carbon fibers can have improved tensile strength while retaining a high modulus of elasticity.

In addition, in the infusibility process, by selectively making the outer surface of the fiber infusible and then carbonizing and graphitizing it, the decrease in graphitization during the infusibility process is suppressed, and the inner part of the fiber is lower than the outer surface layer of the fiber. Attempts have been made to produce fibers with high graphitization properties and excellent modulus of elasticity (Japanese Unexamined Patent Publication No. 83-120112).However, in this method, the main focus is on suppressing internal infusibility. The structure of the outer surface layer is the same as that of conventional fibers.

In this way, studies on the effect of improving mechanical properties through structural control of menphasic pitch-based carbon fibers have traditionally focused on reports on the spinning and infusible processes, and reports on improvement methods in the carbonization and graphitization processes. There were few.

As an example of treatment for PAN-based carbon fibers, the fibers after carbonization are electrolytically fermented, and then heat treated in an inert gas to reduce the crystallinity of the ultra-thin outermost layer of the fibers.
Attempts have been made to improve mechanical properties (Japanese Unexamined Patent Publication No. 81-225330).

However, even if this method is applied to mesophase pitch-based carbon fibers, no improvement in mechanical properties can be expected, and on the contrary, tensile strength may decrease. This is thought to be due to the large difference in structure between PAN-based carbon fibers and phase pitch-based carbon fibers.

Problems to be Solved by the Invention The purpose of the present invention is to develop an optimal treatment method for improving the tensile strength of mesophase pitch carbon fibers.

Means and Effects for Solving the Problems In other words, the present invention oxidizes the surface of an initial carbonized fiber in an atmosphere containing nitrogen dioxide until acetone-soluble components are produced, and then carbonizes it in an inert atmosphere. This is a method for producing mesophase pitch carbon fiber, which is characterized by graphitization treatment.

By the oxidation treatment of the present invention, it is possible to produce intermediate fibers for mesophase pitch carbon fibers in which the structure of the surface layer is mainly controlled. By further carbonizing and graphitizing this intermediate fiber, the structure of the surface layer is controlled, and a mesophase pitch carbon fiber having excellent tensile strength and elongation at break can be produced.

Oxidation treatment is 0.1-40 in the temperature range of 260-380℃
The fibers after treatment are substantially insoluble in toluene and are 0.3 to 5% nitrogen dioxide.
Preferably, the treatment is performed to produce a wt % of the a7ton soluble component.

Hereinafter, the content of the present invention will be explained in detail.

Conventionally, mesophase pitch-based carbon fibers tend to form three-dimensionally developed graphite layer surfaces by proceeding with graphitization treatment, so it is easy to improve graphitization properties and obtain fibers with high elastic modulus. It has not been easy to obtain fibers with high strength and high elongation.

The present inventors believe that in order to increase the strength of carbon fibers, it is necessary to particularly improve the structure of the outermost surface layer of the fibers, and for this reason, we improved the structure of the surface layer of the fibers by using chemical reactions. We attempted this and completed the present invention.

The present inventors have developed a method for making pitch fibers infusible and then subjecting them to 100% heat treatment under an inert gas atmosphere such as nitrogen gas or argon gas.
We investigated oxidation treatment using various oxidizing gases for initially carbonized fibers produced by firing at 0°C or lower, preferably 800°C or lower.

As a result, in fibers treated under specific conditions in an atmosphere containing nitrogen dioxide, carboxylic acids are generated on the fiber surface, and as a result, they are virtually insoluble in non-polar solvents such as toluene and carbon tetrachloride. It was found that components soluble in polar solvents such as water-7cetone were produced. This component has not been previously discovered, and is expected to exhibit a unique carbonization φ graphitization behavior.

The present inventors have discovered that when carbon fibers are produced by further carbonizing and graphitizing the initially carbonized fibers subjected to such oxidation treatment, the elastic modulus remains unchanged and the strength is improved.

Although the detailed cause is unknown, the fiber surface is oxidized after initial carbonization, making it virtually insoluble in non-polar solvents such as toluene and carbon tetrachloride, and resistant to polar solvents such as ice and acetone. This shows that by producing soluble components, it is possible to improve the outermost layer of fibers and produce fibers with excellent strength.

The manufacturing method of the present invention will be specifically explained below.

As the raw material pitch for carbon fiber, any of various pitches may be used, such as coal tar pitch, coal-based pitch such as coal liquefied oil, and petroleum-based pitch such as ethylene tar and decant oil pitch. It is also possible to use various modified pitches, such as pitches that have been hydrogenated, modified by heat treatment, solvent fractionated, distilled, or modified by a combination of these methods.

The mesophase pitch used in the present invention is a pitch containing an optically anisotropic phase (mesophase) obtained by heat-treating the pitch, and the ratio of the optically anisotropic phase is 80% or more. . Further, the mesophase pitch used in the present invention preferably has a softening point of 240 to 340°C from the viewpoint of spinnability.

Pitch Ml fibers are obtained by melt-spinning the mesophase pitch by a known method.For example, the mesophase pitch is melted at a temperature higher than its softening point and has a viscosity of
Diameter 0.05-0 in the range of 0-3000 poise (P),
Pitch fibers are obtained by drawing at a rate of 50 to 1000 cm/min while extruding from a 5I nozzle. The nozzle to be used is not limited to a circular one, and any structure may be used, such as an irregularly shaped nozzle, a nozzle with an enlarged or reduced flow path, etc.

The pitch M&fiber is then subjected to an infusible treatment. The infusibility treatment is generally performed at a temperature range of 100 to 400°C in an oxidizing atmosphere. Air or a mixed gas of oxygen and nitrogen is most commonly used as the fostering atmosphere, but other gases such as ozone, nitrogen oxides, fE yellow chain oxides, or halogens are also used. A small amount of oxidizing gas may also be used.

The fibers after the infusibility treatment are subsequently subjected to carbonization and graphitization treatment, but before the final carbonization and graphitization treatment, the fibers that have undergone the initial carbonization treatment are substantially immersed in non-polar solvents such as toluene and carbon tetrachloride. Oxidation treatment is necessary to generate components that are insoluble and soluble in polar solvents such as water and acetone.

The polar and non-polar solvents mentioned here are C and M.

Characterized by the polar component term (δp) of the solubility parameter shown by Hansen, the polar solvent used in the present invention is a solvent with a δp of 4 or more, preferably 4 to 6, and the nonpolar solvent is a solvent with a δp of 3 or more. Hereinafter, preferably two or less solvents are used.

Regarding the polar component term (δp) of the solubility parameter,
For example, Yuji Harasaki, Basic Science of Coatings J, 1st edition, taMten Publishing, Chuo-ku, Tokyo, (198B) or
G., M., Hansen, Ind., Eng., Chew.
.

Prod, Res, Develop, 8. pp
2 (1!389), the polar component term (δp) of some typical solubility parameters of the reprint is as follows. Water; 15.3. Methanol; f
i, 0. Acetone; 5, l, methyl ethyl ketone; 4.
4. Impropanol; 3.0, 5ee-butanol, 2.8. Chloroform; 1.5. ) Luen,
0.7, carbon tetrachloride; 0.0, hexane; 0.0. A suitable polar solvent is acetone.

In addition, toluene is suitable as a non-polar solvent. In the present invention, the fibers that have undergone oxidation treatment before carbonization and graphitization have a 0.3 to 5
It is important that the composition contains soluble components of 0.1 wt % or less and is substantially insoluble in toluene, which is a nonpolar solvent, and exhibits only 0.1 wt % or less of soluble components.

The existence of a component that exhibits selective solubility in polar solvents was not previously known.

For example, pitch fibers have components that are soluble in polar solvents such as acetone, but also components that are soluble in non-polar solvents such as toluene. In addition, the infusible synthetic m-carbonized fiber is substantially insoluble in polar solvents such as acetone and non-polar components such as toluene.

The initial carbonized fiber is obtained by heat-treating the infusible fiber at a temperature of 1000° C. or lower, preferably 800° C. or lower in an inert atmosphere such as nitrogen or argon.
Heat treatment at a temperature exceeding 1000° C. is not preferred because it is difficult to generate polar solvent soluble components in the secondary oxidation treatment.

The oxidation treatment can be carried out by the following method.

The initially carbonized fibers are oxidized in an atmosphere containing nitrogen dioxide until polar solvent soluble components are produced. In this method, an atmosphere containing 0.1 to 40 mmol of nitrogen dioxide at 1% is suitable for the oxidation treatment.

Below the concentration range shown here, the effect of producing components that are insoluble in non-polar solvents and soluble in polar solvents is unsuitable, and above the concentration range shown here, oxidation loss of the fibers will increase and is therefore undesirable. do not have.

Other components used in the oxidation treatment are generally an inert gas or an atmosphere consisting of oxygen and an inert gas such as air, but other components such as moisture may also be included. Furthermore, a suitable treatment temperature during the oxidation treatment is 260 to 380°C. 26
Below 0°C, the reaction progresses slowly and is not efficient;
Temperatures above 0.degree. C. are not suitable for producing components that are insoluble in non-polar solvents and soluble in polar solvents.

IA fibers after oxidation treatment contain 0.3 to 5 wt% of soluble components in acetone, which is a polar solvent, and are substantially insoluble in toluene, which is a non-polar solvent, and have a soluble content of 0.3 to 5 wt% in acetone, which is a polar solvent. It is necessary to exhibit no more than twt% of soluble components. If the amount of acetone soluble components is less than 0.3%, the effect of oxidation treatment will be small and the strength improvement effect will be small, which is not appropriate. If it exceeds 5%, oxidation will be excessive, resulting in a decrease in yield or strength. If the toluene soluble component exceeds 0.1 wt %, fiber fusion may occur during the subsequent carbonization graphitization process.

The carbonization/graphitization treatment is generally performed by heat treatment at a temperature of 1000° C. or higher in an inert atmosphere such as nitrogen or argon. By performing the carbonization/graphitization treatment in this manner, carbon M/A miscellaneous material can be obtained.

By using the method for producing mesophase pitch-based carbon fibers according to the present invention, high-strength carbon fibers with excellent tensile strength can be produced.

Next, various physical property values used to express the characteristics of the pitch-based carbon fiber and raw material pitch in the present invention will be described.

(1) Tensile strength, tensile modulus It was measured according to the method shown in JISR7601.

(2) Viscosity and softening point viscosity were calculated using the Hagen-Boiseuille formula using a flow tester. The I& point is the temperature at which the viscosity becomes 20,000 poise.

(3) Mesophase content In the present invention, mesophase refers to an optical anisotropy that can be determined by embedding cooled and solidified pitch in resin, polishing the surface, and observing it using a polarizing microscope. The mesophase content refers to the interview ratio of anisotropic structure observed as described above.

(4) Acetone soluble content (As), ) toluene soluble content (TS) Add 50% of solvent to 1 g of miscellaneous, perform ultrasonic cleaning, and filter to collect solvent soluble components. This operation is repeated on the solvent-insoluble components until no further coloration of the solvent is observed, and then the solvent is removed by evaporation and the solvent-soluble components are collected. After drying the solvent-soluble components under vacuum at 60° C., the weight of the solvent-soluble components is measured.

Example Examples and comparative examples will be given below for specific explanation.

Example 1 Contains 34% optically anisotropic phase (mesophase), TI:
84%, QI: 11%, softening point = 300℃ coal tar pitch mesophase pitch with diameter 0.2 ■ fat φ, 3
Melt-spun using a 000-hole nozzle to produce an average of 13g
A pitch fiber of mφ was obtained.

Pitch fibers are heated to 150-310℃ using air at 0.3
Infusible fibers were obtained by increasing the temperature at a heating rate of 0.degree. C./min and holding the temperature at 310.degree. C. for 50 minutes. This infusible fiber was further heated to 500°C at a heating rate of 5°C/min by changing the atmosphere to nitrogen.
Initial carbonized fibers were obtained by holding for 6θ minutes.

5 ma0 of this initial carbonized fiber! Oxidation treatment was carried out by holding at 310° C. for 50 minutes using air containing % nitrogen dioxide to obtain oxidation treated fibers. Further, graphitization treatment was performed at 2100° C. for 1 minute in an argon atmosphere to obtain carbon fibers.

In addition, the analysis value of the solvent soluble content is A for the oxidized fiber.
S: 1.2 wt%, TS: trace amount, and both AS and TS were trace amounts for the initial carbonized fiber.

The mechanical properties of the obtained carbon fiber are tensile strength = 340k.
g/am2. Tensile modulus: 40t/mm2te.

Comparative Example 1 Pitch fibers produced under the same conditions as Example 1 were heated using air from 150 to 310°C at a heating rate of 0.9°C/min, and maintained at 310°C for 50 minutes. A melting treatment was performed to obtain infusible fibers. Further, graphitization treatment was performed at 2100° C. for 1 minute in an argon atmosphere to obtain carbon fibers.

In addition, the analysis value of the solvent soluble content is AS for the infusible fiber.
, TS were both in trace amounts.

The mechanical properties of the obtained carbon fiber are tensile strength: 28
0kg/intestine12. Tensile modulus: 40t/12.

Comparative Example 2 Infusible fibers produced under the same conditions as Comparative Example 1 were further heated to 1300°C at a heating rate of 5°C/min by changing the atmosphere to nitrogen.
Carbonized fibers were obtained by raising the temperature to 1300°C and holding it for 80 minutes. This carbonized fiber was oxidized by holding it at 310° C. for 50 minutes using air containing 1% nitrogen dioxide to obtain an oxidized fiber.

Further, graphitization treatment was performed at 2100° C. for 1 minute in an argon atmosphere to obtain carbon fibers.

In addition, the analysis value of the solvent soluble content is A for the # treated fiber.
Both S and TS were in trace amounts.

The mechanical properties of the obtained carbon fiber are tensile strength: 20
0kg/intestine-2. Tensile modulus: 40t/am2.

As described in detail of the invention, according to the method for producing mesophase pitch-based carbon fibers of the present invention, a component insoluble in toluene and soluble in acetone is generated from initially carbonized fibers in an atmosphere containing nitrogen dioxide. By performing an oxidation treatment to a maximum temperature and then a carbonization/graphitization treatment, the tensile strength is improved compared to carbon fibers produced by conventional methods.

Claims (1)

[Claims] (1) After oxidizing the initial carbonized fibers in an atmosphere containing nitrogen dioxide until a component is produced that is substantially insoluble in toluene and soluble in acetone, A method for producing mesophase pitch-based carbon fiber, which is characterized by carbonization and graphitization treatment.