JPH055219A - Production of carbon fiber - Google Patents

Abstract

PURPOSE:To produce carbon fibers having excellent mechanical properties by melt-spinning an optically isotropic pitch, subjecting the spun pitch fibers to an infusibilization treatment in two steps under specific conditions and subsequently carbonizing and graphitizing the fibers. CONSTITUTION:Before collected on a conveyor, pitch fibers produced by melt- spinning an optically isotropic pitch are brought into contact with air or an oxygen-containing gas containing ozone in a high concentration, or the air or the oxygen-containing gas containing the ozone in the high concentration is supplied into the mat of the collected pitch fibers to bring the mat into contact with the ozone. Or the above-mentioned contact treatments are successively performed. The treated fibers are subjected to an infusibilization treatment using air at 150-400 deg.C and subsequently subjected to a carbonization treatment and, if necessary, further to a graphitization treatment to provide the objective carbon fibers.

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 carbon fiber or graphite fiber (hereinafter represented by carbon fiber unless otherwise required) using petroleum-based or coal-based optically isotropic pitch as a raw material. More specifically, the present invention relates to a pitch fiber infusibilizing method capable of improving the productivity of industrially inexpensive general-purpose carbon fibers. In this specification, “%” for gases such as ozone content in air or oxygen means “volume%”, and for other materials, “%” means “weight”. % ”Is meant.
In addition, in the present specification, the “carbon fiber” includes not only the original meaning of carbon fiber but also “graphite fiber” obtained by heat treatment at a higher temperature.

[0002]

2. Description of the Related Art The pitch-based carbon fiber manufacturing method requires an infusibilizing step of heat-treating the melt-spun pitch fibers in an oxidizing atmosphere so as not to fuse them with each other. This infusibilizing step requires a long treatment time (usually about 1.5 to 3 hours), so the cost of this step is high in the manufacturing cost of the pitch-based carbon fiber, and the productivity here is high. Good or bad affects the price of carbon fiber.

Therefore, it is desirable to shorten the infusibilization time as much as possible. However, since the softening point of the pitch and the temperature range in which the oxidation reaction of the pitch progresses are close to each other, when the infusibilization time is excessively shortened. However, the pitch fibers that are not sufficiently oxidized are fused, and the original purpose of the infusibilization step of maintaining the form of the fibers during the carbonization step cannot be achieved. Further, even if the shape of the fiber can be maintained, the physical properties such as the tensile strength of the finally obtained carbon fiber will be deteriorated.

As a concrete method for shortening the time required for the infusibilizing step, a method using the following promoter or catalyst has been proposed: (a) Pitch fiber and nitric acid vapor in the gas phase (JP-A-1-201522), (B) a method in which pitch fibers are immersed in an aqueous nitric acid solution, and then infusibilized with air (JP-A-1-201522), (C) optical difference 0.1-40% NO for anisotropic pitch fiber
_After treatment in an oxidizing atmosphere containing ₂ to a specific gravity of 1.40 or more, a method of infusibilizing treatment at 260 to 360 ° C. in an inert atmosphere containing 0.1 to 40% NO ₂ (JP-A-1- Two
No. 01524), (d) a method in which pitch fibers are treated with nitric acid, and then subjected to infusibilization treatment with air under a specific condition (Japanese Patent Laid-Open No. 1-282329), (e) Optical etc. As a method of infusibilizing isotropic pitch fiber,
30 to 13 in an air atmosphere containing 0.1 to 10% NO _2.
A method of treating at 0 ° C. for 1 to 4 hours (JP-A-59-9062)
(Gazette), (f) As a method of infusibilizing mesophase pitch fiber,
A method of treating at 150 to 380 ° C. in an oxidizing atmosphere such as air or oxygen containing 0.1 to 50% of NO ₂ (Japanese Patent Laid-Open No. Sho 60)
No. 259629), (G) Pitch fiber is treated in an air atmosphere containing 0.1 to 10% NO ₂ in a low temperature range of 200 ° C. or lower, and then in an oxidizing atmosphere having a higher oxygen partial pressure than air. A method of treating in a high temperature range of 250 ° C. or higher (JP-A-1-272825), (H) a method of treating pitch fibers with an oxidizing gas containing less than 3% chlorine (JP-A-59-1723). ), (I) a method in which pitch fibers are brought into contact with hydrobromic acid and then infusibilized with air (Japanese Patent Laid-Open No. 59-30195), (nu) pitch fibers with an oxidizing gas containing SO _2. Treatment method (JP-A-60-185819), (L) Method of treating pitch fiber with hydrogen peroxide solution (JP-A-2-91223), (W) Cobalt bromide on pitch fiber surface Method of application (JP-A-60-81319) Publication), (a method of coating the pitch fiber surface to melt the cobalt salt in the ring) Heat oil (JP 62-91223 JP).

However, these methods have various drawbacks, and it is difficult to say that any of them is satisfactory. For example, in the method of immersing the pitch fiber in the nitric acid aqueous solution, the pitch fiber is fragile, so that the handling is difficult and the pitch fiber is easily damaged. In the case of infusibilizing pitch fibers in an air atmosphere containing NO ₂ , the NO ₂ concentration must be increased in order to sufficiently shorten the infusibilization time. However, N, which is toxic and corrosive
Increasing the concentration of O ₂ is not preferable for industrial use. In addition, NO ₂ is supplied by on-site oxidation of ammonia, dilution of the NO ₂ stock solution, etc.
The manufacturing cost is relatively high, and the treatment of the NO ₂ residual gas after the treatment also causes a high cost. Further, in the infusibilizing treatment with an oxidizing atmosphere containing chlorine gas or SO ₂ , the infusibilization promoting effect is recognized, but the physical properties of the final product carbon fiber are equal to or slightly inferior to those in the case of air infusibilization. .. Moreover, chlorine gas and SO ₂ are
It is toxic and corrosive, which is disadvantageous for industrial use.

On the other hand, a method of promoting infusibilization of pitch fibers using ozone is also known. Specifically, (a) a method in which pitch fibers are treated in an oxygen-containing atmosphere containing ozone at 130 to 220 ° C. and then in an air atmosphere containing no ozone at 250 to 350 ° C. (JP-A-1-24).
6420), and (b) a method of pretreatment with ozone, followed by treatment with iodine to make the air infusible (JP-A-2-80620). Ozone is a highly toxic gas with an allowable concentration of 0.1 ppm according to ACGIH, and it is economically necessary to use a small amount of ozone at a low concentration.
The thermal decomposition of ozone is more remarkable at higher temperatures, and according to the research by the present inventor, in the high temperature range of 130 ° C. or higher disclosed in JP-A-1-246420, pitch fibers due to ozone are rather used. It has been found that the treatment effect of 1) is small, and that the treatment at a lower temperature can lower the ozone concentration and complete the treatment in a short time. In addition, JP-A-2-806
In the method disclosed in Japanese Patent Laid-Open No. 20, it is necessary to prepare an iodine source together with an ozone source, which is much more expensive than the usual infusibilization of air.

[0007]

The inventors of the present invention have conducted intensive studies while paying attention to the problems of the prior art as described above, and as a result, have achieved high concentration of optically isotropic pitch fibers obtained by melt spinning. After contacting air containing ozone or oxygen-containing gas for a short period of time, when infusibilizing the air at a specific temperature, shorten the infusibilizing treatment time and satisfactorily infusibilize the fiber surface part It was found that the fusion of the carbon fiber can be prevented and the physical properties (tensile strength, breaking elongation, etc.) of the finally obtained carbon fiber can be improved (patent pending). However, this method is extremely effective for long fiber type pitch fibers obtained by melt spinning, but is not so effective for pitch fiber mats obtained by the air sucker method, vortex method, etc. did. That is, it is difficult to uniformly contact the inside of the pitch fiber mat obtained by the air sucker method, the vortex method, or the like through the ozone-containing gas, and it is difficult to cause the oxidation reaction even to the center of the mat. In addition, if a large amount of ozone-containing gas is introduced in order to perform sufficient ventilation, the pitch fiber mat may be crushed or damaged, and the physical properties of the finally obtained carbon fiber may deteriorate. ..

As a result of further research, the present inventor has found that pitch fibers obtained by spinning an optically isotropic pitch contain air or oxygen containing a high concentration of ozone before being collected on a conveyor. After contacting with gas, or supplying air or oxygen-containing gas containing a high concentration of ozone into the collected pitch fiber mat and contacting it for a short time, air infusibilization treatment is performed under specific conditions. In that case, it is possible to shorten the infusibilization treatment time, prevent the matting fiber surface portion from being infusibilized well, and prevent the mutual fusion of fibers, and also improve the physical properties of the finally obtained carbon fibers. Found

That is, the present invention provides the following methods: After infusibilizing the pitch fiber mat obtained by spinning the optically isotropic pitch, carbonized or graphitized to produce carbon fibers, the spun pitch monofilaments are collected on a conveyor. Contact with air or oxygen-containing gas containing a high concentration of ozone until
Alternatively, air or oxygen-containing gas containing a high concentration of ozone is supplied into the collected pitch fiber mat at a ventilation amount of a certain amount or more to bring them into contact with each other, or after both of the above contact treatments are sequentially carried out, 150 A method for producing an optically isotropic pitch-based carbon fiber, which comprises performing an air infusibilization treatment at ˜400 ° C. 2. The ozone concentration in the air or oxygen-containing gas is 0.
Item 1. The method for producing the optically isotropic pitch-based carbon fiber according to Item 1, which is in the range of 1 to 2% by volume. 3. The ozone concentration in the air or oxygen-containing gas is 0.
Item 3. The method for producing an optically isotropic pitch-based carbon fiber according to Item 2, which is in the range of 4 to 1% by volume. 4. _2. The optical isotropy according to item 1 above, wherein the amount of air containing ozone or the gas containing oxygen to the pitch fiber mat is set such that the pressure difference between the upper and lower surfaces of the mat is 0.5 to 7 mmH ₂ O. Method for producing pitch-based carbon fiber. 5. The pressure difference between the upper surface and the lower surface of the mat is 1 for the air flow rate of ozone-containing air or oxygen-containing gas to the pitch fiber mat.
Method for producing optically isotropic pitch-based carbon fiber according to the claim 4, ~4mmH 2 _O and made such amounts.

The pitch used as a raw material in the present invention may be either petroleum-based or coal-based as long as it is optically isotropic. The softening point of the raw material pitch is 200-
It is preferably about 400 ° C, 240 to 300 ° C
It is more preferable that the degree is approximately.

Such an optically isotropic pitch is melted and spun into a mat according to a conventional method. The melting conditions, spinning method, etc. are not particularly limited as long as the pitch fiber mat is formed. Examples of the spinning method include an air sucker method, a vortex (RGJ) method, and a method in which a long fiber is spun and then randomly matted on a conveyor or the like.

The infusibilization of the pitch fiber mat in the present invention is carried out in two steps.

In the first stage, air or an oxygen-containing gas containing ozone at a high concentration of about 0.1 to 2.0% (in the present specification, the oxygen-containing gas means a gas having an oxygen content higher than that of air). In the following, unless otherwise necessary, air is used as a representative) and the pitch fibers are brought into contact with each other for a short time. The concentration of ozone in the air is 0.4 to 1.0
% Is more preferable. Ozone concentration is 0.1%
When it is less than 2.0%, the oxidation promoting effect of the pitch fiber by ozone is not sufficiently exhibited, whereas when it exceeds 2.0%, the oxidation promoting effect is slightly improved,
It is disadvantageous in terms of safety and economy. The contact between the air containing a high concentration of ozone and the pitch fiber is as follows: (a) contacting the spun pitch monofilament with air containing a high concentration of ozone or an oxygen-containing gas before being collected on a conveyor; Alternatively, (b) air or an oxygen-containing gas containing a high concentration of ozone is supplied into the collected pitch fiber mat at a flow rate of a certain amount or more to bring them into contact with each other, or the above-mentioned (a) step and (b) It is carried out by sequentially performing the process treatment. In the case of the step (a), pitch fibers are blown into the air containing a high concentration of ozone, and while being in contact with ozone, they are collected on a conveyor to form a mat-like material. The contact time is usually 1 because the reactivity between ozone and pitch fiber is high.
About 0 seconds is sufficient. The method for introducing ozone-containing air is
There is no particular limitation as long as the contact between the pitch fiber and ozone is uniform and good. For example, in the case of producing pitch fibers by the air sucker method, ozone is introduced into the jet gas for drawing the pitch fibers discharged from the nozzle, so that the two can be brought into contact with each other without adding new equipment. Therefore, it is advantageous. In the case of the step (b), in the pitch fiber mat collected cotton, the amount of the pressure difference between the upper and lower surfaces of the mat is 0.5 to 7 mmH ₂ O (more preferably 1 to 4 mmH ₂ O). Aerate air containing a concentration of ozone. This differential pressure is 0.5 mm
When it is less than H ₂ O, the ozone amount required for the oxidation reaction cannot be supplied to the center of the mat, whereas
If it exceeds mmH ₂ O, the pitch fiber mat is blown off or damaged and the physical properties of the final product carbon fiber are deteriorated, which is not preferable. When this step is adopted, it is preferable to introduce an ozone-containing gas directly below the pitch fiber mat in order to bring the fiber into contact with fresh ozone. The contact time is usually about 20 to 40 seconds. In addition, when the ozone and the pitch fiber cannot be sufficiently contacted with each other only in the step (a) or (b), the steps (a) and (b) may be sequentially performed. The treatment temperature in the first stage is not particularly limited, but it is not necessary to perform heating, and at high temperatures, ozone is rapidly decomposed and the reaction becomes unstable, so it is advantageous to perform it at room temperature. is there. However, in the case where air containing ozone is introduced into the front part of the infusibilizer, temperatures up to about 150 ° C. are allowed. The most important thing in the first stage is to bring the ozone-containing air into contact with the surface of the pitch fiber mat as reliably as possible. That is, since the pitch fiber mat surface tends to be covered with an oxygen layer generated by the reaction of ozone and pitch fibers, unless some measures are taken, diffusion of ozone to the pitch fiber mat surface becomes insufficient, The pitch fiber mat may not be fully oxidized. Therefore, in the case of the above step (a), the ozone-containing air is caused to flow at a flow rate of 0.53 cm / sec or more with respect to the pitch single fiber to bring the pitch fiber surface into close contact with the ozone-containing air. When the flow rate of ozone-containing air is less than 0.53 cm / sec,
Since the coated oxygen layer is not sufficiently disturbed, the diffusion of ozone to the pitch fiber surface is insufficient. The treatment time in the first stage is also not particularly limited, but since the reactivity between ozone and pitch fiber is very high, even if the above (a) step and (b) step are used in combination, it takes 1 minute. Within the range is sufficient.

In the second step of the present invention, the pitch fiber mat, which has been subjected to the infusibilizing treatment in the first step, is heated in the air at an average heating rate of about 3 to 10 ° C./min.
It is heated in the temperature range of 00 ° C and the oxygen content in the fiber is 6-9.
% (More preferably 7 to 8%) to maintain infusibilization. In the second step of infusibilizing air, since the surface layer portion of the pitch fiber mat is already infusibilized and no fusion occurs, basically heating at a temperature higher than the melting point of the pitch. You can The infusible heating furnace in the first stage and the infusible heating furnace in the second stage may be continuous or independent from each other. When performing successive heat treatments using continuous infusible heating furnaces (however, the atmosphere in both furnaces must be isolated from each other), the temperature at the end of the first stage and the start of the second stage It is preferable that the temperature is the same. If the heating temperature in the second stage is lower than 150 ° C., the progress of the oxidation reaction will be delayed, and it will take a long time to diffuse oxygen into the fiber mat. On the other hand, when the heating temperature is higher than 400 ° C., the fiber is consumed so much as to be burned, and it becomes difficult to control the reaction. From the viewpoint of shortening the infusibilization time, it is more advantageous to set the air infusibilization start temperature higher, and in the second stage, it is 150 to 225.
3 at an average heating rate of 5 to 7 ° C / min from a temperature in the range of ℃
More preferably, the temperature is raised to a temperature in the range of 75 to 400 ° C. When the infusibilization of air is started from a temperature higher than 225 ° C., even the pitch fiber mat that has been subjected to the infusibilization treatment in the first stage may be fused. In the infusibilizing treatment in the second stage, the treatment is finished when the oxygen content in the pitch fiber mat reaches 6 to 9%. If the oxygen content in the pitch fiber mat is less than 6%, the cross-linking due to oxygen atoms will be insufficient, and the single fibers will melt or fuse together, making it difficult to maintain the shape of the fiber mat. Become. Further, even if the morphology of the fiber mat could be maintained, the carbonization yield would decrease,
The physical properties of the carbon fiber may deteriorate. On the other hand, when the infusibilization treatment is excessively performed to an extent that the oxygen content exceeds 9%, the probability that defects will occur on the fiber surface increases,
Further, it is not preferable because it is easily consumed by oxidation. The highest temperature reached in the second stage is preferably lower, but when a high temperature raising rate is adopted, the highest temperature must be set higher in order to achieve the above oxygen content. Achieved considering that the average rate of temperature rise that is accepted as an industrial process (including the holding time when holding at a specific temperature in the temperature range) is 5 ° C / min or more If the temperature is not lower than 375 ° C, the necessary oxygen cannot be contained in the pitch fiber. However, the heat treatment at 400 ° C. or higher is not preferable because the fiber is heavily consumed by oxidation. Therefore, it is difficult to achieve the required oxygen content when employing an average heating rate above 7 ° C / min. The infusible pitch fiber obtained as described above is carbonized according to a conventional method, and if necessary, further graphitized to obtain a carbon fiber.

[0015]

EFFECT OF THE INVENTION According to the present invention, due to the excellent effect of the specific amount of ozone used in the first stage of infusibilization as an oxidant, the oxidation reaction on the surface of the mat-like pitch fiber in a low temperature region near room temperature can be achieved. Since it progresses and a sufficient crosslinked structure is formed in the surface layer portion, even if it is exposed to a higher temperature in the air, it is possible to prevent fusion. Therefore, since the heating can be performed in the air at a higher temperature raising rate than usual and at a high temperature, the infusibilization time as a whole is significantly shortened. Further, as compared with the conventional infusibilization of air that must suppress the temperature rising rate in the low temperature region, a uniform heating process is performed through the entire infusibilization process, so that the oxygen concentration distribution in the infusibilized pitch fiber mat is It becomes relatively uniform, and the breaking elongation and tensile strength of the finally obtained carbon fiber are improved. Furthermore, since the oxygen concentration in the mat-like infusibilized pitch fiber is uniform, microcracks that occur during carbonization, strain on the internal structure, etc. are reduced, and the excellent mechanical properties inherent in carbon fiber are easily expressed. Become.

[0016]

[Examples] Reference examples, examples and comparative examples are shown below,
The features of the present invention will be further clarified.

Example 1 An optical isotropic pitch (softening point = 282.9 ° C., quinoline insoluble matter = 2) was obtained by treating coal-based tar according to the method described in JP-A-59-149052.
8.6%, benzene insoluble content = 86.0%, C / H = 2.
02) was prepared and then vacuum dried.

Next, from the above-mentioned optically isotropic pitch as a raw material, pitch fiber spinning to pitch fiber mat infusibilization were carried out according to the process flow shown in FIG.
That is, the above optical isotropic pitch 1 is 1.5 kg /
A 100-hole nozzle 3 was spun through an air sucker 5 at a ratio of hr, and the obtained single fiber pitch 7 having an average yarn diameter of 15.2 μm was collected on a conveyor 9 to obtain a pitch fiber mat 11. The average basis weight at this time was 510 g / m ^2.
Met.

Ozone-containing air (Ozone concentration 0.6%)
Was supplied from a line 13 directly below the nozzle 3 as a gas for air sucker from a line 15 and a line 17 to be brought into contact with pitch monofilaments or pitch fiber mats. The time during which the pitch monofilament was in contact with ozone from the nozzle 3 to the conveyor 9 was 3 seconds, and the flow rate of ozone-containing air was 1.0 m / sec on average. The pressure difference between the upper and lower sides of the pitch fiber mat 11 is 3.4 mmH ₂ O,
The contact time between the pitch fiber mat 11 on the conveyor 9 and ozone was 1 minute.

Next, the above pitch fiber mat is preliminarily used for 15
After being sent to the air infusible furnace 19 heated to 0 ° C., the temperature was raised from 150 ° C. to 330 ° C. at a heating rate of 5 ° C./min, and immediately taken out of the furnace and cooled. The amount of oxygen fixed in the obtained infusible pitch fiber mat was 7.32%.

Then, the obtained infusible fiber mat was put into a carbonization furnace, heated from 300 ° C. to 940 ° C. at a temperature rising rate of 50 ° C./min in an N ₂ gas atmosphere, and kept at the same temperature for 3 minutes. To obtain carbon fiber. Obtained carbon fiber (thread diameter = 13.
5 μm) has a tensile strength of 90.2 kgf / m
m ² and average breaking elongation = 2.46%. The coefficient of variation (CV) showing variations in these physical properties was 5.20% in tensile strength and 4.85% in breaking elongation.

Comparative Example 1 Preparation of a pitch fiber mat, infusibilization and carbonization of infusible fibers were carried out in the same manner as in Example 1 except that pure air was used instead of ozone-containing air in the first step of infusibilizing treatment. It was The amount of oxygen fixed in the infusible pitch fiber mat is 7.
It is 12%, and the physical properties of the obtained carbon fiber are tensile strength =
It was 75.9 kgf / mm ² , and the average breaking elongation was 2.02%. The coefficient of variation (CV) is 9.1 in tensile strength.
It was 5% and the breaking elongation was 8.95%.

Example 2 A pitch fiber mat was prepared and infusibilized in the same manner as in Example 1 except that the ozone content of the ozone-containing air in the first step of infusibilizing treatment was changed to 0.3%. The amount of oxygen fixed in the infusible pitch fiber mat was 7.11%. Then, the infusible fiber was carbonized in the same manner as in Example 1. The physical properties of the obtained carbon fiber are tensile strength = 8.
It was 9.9 kgf / mm ² , and the average breaking elongation was 2.46%.

Example 3 The procedure was carried out except that the supply of the ozone-containing air from the line 17 was stopped and the contact time between the ozone-containing air supplied from the lines 13 and 15 and the pitch monofilament was set to 3 seconds or 10 seconds. An infusible pitch fiber mat was produced in the same manner as in Example 1. The oxygen content of the infusibilized pitch fiber mat was 7.29% when the contact time was 3 seconds and was 7.32% when the contact time was 10 seconds. Next, these infusible pitch fiber mats were treated in the same manner as in Example 1 to carbonize the infusible fiber mats. The results are as shown in Table 1. From the above results, it is clear that in the case where the ozone-containing air and the pitch monofilament are sufficiently contacted, the infusibilization of the pitch fiber can be sufficiently performed without the treatment of the pitch fiber mat with the ozone-containing air. Is.

Example 4 The contact between the ozone-containing air supplied from the lines 13 and 15 and the pitch monofilament was stopped, and the line 17 was used.
By adjusting the amount of ozone-containing air supplied from, the pressure difference between the atmosphere above and below the pitch fiber mat is 0.5.
mmH ₂ O, 1.1mmH ₂ O and 7.9mmH ₂ O
An infusible pitch fiber mat was produced in the same manner as in Example 1 except that Then, in the same manner as in Example 1, the infusible pitch fiber mat was carbonized. Differential pressure (mmH
₂ O) and oxygen content (%) in the infusibilized pitch fiber mat
Table 2 shows the relationship between the appearance and the physical properties of the carbon fiber. As is clear from Table 2, good results were obtained by setting the pressure difference between the atmosphere above and below the pitch fiber mat to 0.5 mmH ₂ O or more and the contact time between the pitch fiber mat and ozone to be 1 minute. Has been. However, when the pressure difference between the atmosphere above and below the pitch fiber mat becomes excessive, the pitch fiber mat is peeled off.

[Brief description of drawings]

FIG. 1 is a flow diagram showing an example of the method of the present invention.

[Explanation of Codes] 1 ... Optically isotropic pitch 3 ... Nozzle 5 ... Air sucker 7 ... Single fiber pitch 9 ... Conveyor 11 ... Pitch fiber mat 13 ... Ozone-containing gas supply line 15 ... Ozone-containing gas supply line 17 ... Ozone Contained gas supply line 19 ... Air infusible furnace

Claims (1)

Claim: What is claimed is: 1. A method for producing a carbon fiber by subjecting a pitch fiber mat obtained by spinning an optically isotropic pitch to infusibilization treatment and then carbonizing or graphitizing the same. The pitch monofilament is contacted with air or an oxygen-containing gas containing a high concentration of ozone before being collected on the conveyor, or the air containing a high concentration of ozone in the collected pitch fiber mat or After supplying an oxygen-containing gas to bring the pitch fiber mat and ozone into contact with each other, or after sequentially performing the contact treatment of both of them, 150 to 400
A method for producing an optically isotropic pitch-based carbon fiber, which comprises performing an air infusibilization treatment at ℃. 2. The method for producing an optically isotropic pitch-based carbon fiber according to claim 1, wherein the ozone concentration in the air or oxygen-containing gas is in the range of 0.1 to 2% by volume. 3. The method for producing an optically isotropic pitch-based carbon fiber according to claim 2, wherein the ozone concentration in the air or oxygen-containing gas is in the range of 0.4 to 1% by volume. 4. The amount of air containing ozone or oxygen-containing gas to be passed through the pitch fiber mat in such an amount that the differential pressure between the upper and lower surfaces of the mat is 0.5 to 7 mmH ₂ O. A method for producing an optically isotropic pitch-based carbon fiber. 5. The optical device according to claim 4, wherein the air containing ozone or the gas containing oxygen is aerated to the pitch fiber mat so that the pressure difference between the upper and lower surfaces of the mat is 1 to 4 mmH ₂ O. Method for producing isotropic pitch-based carbon fiber.