JPS62170528A - Carbon fiber and production thereof - Google Patents

Abstract

PURPOSE:To obtain a carbon fiber having different optical properties between the peripherals part and the core part, by melt-spinning a specific spinning pitch produced by adding an optically isotropic pitch to an optically anisotropic pitch and subjecting the spun fiber to infusibilization, carbonization and graphitization. CONSTITUTION:A freshly prepared optically isotropic pitch is added to an optically anisotropic pitch prepared from coal tar pitch to obtain a spinning pitch having a meso-phase content of 60-95vol% and a benzene-insoluble content of 80-95wt% and containing a matrix phase exhibiting bulk meso-phase when observed by a polarization microscope. A pitch fiber produced by the melt-spinning of the pitch is infusibilized, carbonized and, if necessary, graphitized to obtain the objective fiber having a peripheral part composed of an optically isotropic component and a core part composed of an optically anisotropic component which may contain an isotropic component as a part thereof.

Description

Detailed Description of the Invention: Industrial Field of Application The present invention is characterized in that the outer edge of the fiber cross section has an optically isotropic component, and the center has an optically anisotropic component. This invention relates to a carbon fiber having a structure in which there is no cleavage at all on the fiber surface, and a method for producing the same.

Carbon fiber is widely used in fields such as special W4 construction components in the space and aircraft industries, and leisure and sporting goods.

BACKGROUND OF THE INVENTION Carbon fibers are generally classified into general-purpose carbon fibers and high-performance carbon fibers based on their mechanical properties.

Classifying these high-performance carbon fibers based on their raw materials, there are those made from synthetic fibers such as polyacrylonitrile, and those made from petroleum-based and coal tar-based pitches.

Synthesis of polyacrylonitrile etc. I! When raw materials are used as raw materials, the drawbacks of high costs such as the high price of raw fiber and low carbonization yield of raw fiber are inevitable, and these are hindering the expansion of its use as a general industrial material. This was a major cause.

Under these circumstances, as a method to produce high-performance carbon fibers at low cost, optically anisotropic mesophase pitch (when polynuclear-polycyclic aromatic molecules such as bits grow through heat treatment, part of the pitch or Everything will show a liquid crystal state. Such a liquid crystal state is called a carbonaceous mesophase or simply a mesophase.

Further, a pitch including this mesophase is referred to as a mesophase pitch. Exhibits optical anisotropy. ] is being researched.

Regarding the mesophase content in this mesophase pitch, Japanese Patent Publication No. 55-37611 and Japanese Patent Publication No. 58-5
As seen in Publication No. 1526, the mesophase content in mesophase pitch is 40 to 90%, or 70@m
% or more, and pitches encompassing these ranges may be considered mesophase pitches.

Conventionally, this mesophase pitch has been reduced to a low m (300 to 35
Carbon fibers spun at 0℃) have cleavage on the fiber surface,
This had the disadvantage of significantly reducing the performance of carbon fiber.

Mesophase, which is an optically anisotropic component of pitch, is aligned in the fiber axis direction during melt spinning, and depending on the direction of the alignment, the fiber cross-sectional structure perpendicular to the fiber axis is classified into three types. It is known.

That is, a radial structure perpendicular to the fiber axis direction, an onion structure arranged concentrically, and a random structure arranged irregularly. Conventionally, the determination of this structure is thought to depend on the spinning temperature. That is, it is thought that as the spinning temperature increases, the flow changes from radial to random to onion.

That is, as seen in FIG. 1, (a) in FIG. 1 has a radial structure, and also represents the cleavage of the fiber surface, and the
(B) in the figure is a random structure, and (C) in FIG. 1 is an onion structure.

The radial structure is disadvantageous as a general industrial material because the fiber surface is easily cleaved, and at the same time it causes a decrease in strength, so it is not considered a preferable structure. Therefore, several proposals have been made regarding carbon fiber manufacturing technology having a random or onion structure that does not involve cleavage of the fiber surface, and more specifically regarding spinning technology.

Broadly speaking, these proposals can be classified into the following two methods. Namely, (1) A method of obtaining carbon fibers having a random or onion structure by increasing the spinning humidity.

■ A method for obtaining carbon fibers with a random or onion structure by controlling the flow of molten pitch passing through the spinning nozzle hole.

It is.

An example of (2) is Japanese Patent Application Laid-Open No. 76925/1983. This method uses the phase states of anisotropic and isotropic components at the spinning temperature as a factor that determines the fiber structure.
Matrix phase [In a two-phase mixed state of an isotropic component and a mesophase, the phase that is the matrix is called the matrix. ] is a method for producing carbon fibers having a random or onion structure by spinning at a temperature at which the carbon fibers become isotropic. Furthermore, as seen in Japanese Patent Application Laid-open No. 59-53717, carbon fibers having a structure that does not cause cleavage on the fiber surface can be obtained by spinning after heating the pitch to a temperature higher than the viscosity change temperature. There is a way.

However, in all of these methods, the spinning temperature is high and the molten pitch foams, and this foam can cause breakage during spinning or be incorporated into the fiber, making it difficult to achieve stable spinning. This is not a desirable method. Furthermore, since these have low melt viscosity, they also have the disadvantage of not being compatible with multifilament spinning.

On the other hand, examples of ■ include JP-A-59-168124 and JP-A-59-168127M.
Examples include public notices, etc. These methods control the flow of anisotropic bits by changing the shape of the spinning nozzle to produce carbon fibers having a random or onion structure. However, these methods are effective only when the molten pitch has good fluidity, and it is necessary to raise the spinning temperature substantially, making it impossible to avoid the foaming phenomenon of the molten pitch or the incorporation of bubbles into the fiber. do not have. Another disadvantage is that the nozzle hole is difficult to work with.

Problems to be Solved by the Invention As described above, conventional methods for manufacturing carbon fibers having a structure that does not cause cleavage on the fiber surface generally involve controlling the flow of the molten pitch when it passes through the spinning nozzle hole in some way. Control techniques were mechanical methods, either by lowering the viscosity of the molten pitch or by changing the shape of the nozzle hole. These methods have no effect unless the spinning temperature is increased, resulting in spinning in a temperature range where the molten pitch is thermally unstable.

As a result, fibers are broken due to bubbles during spinning or fibers are mixed with bubbles, which is not sufficient to industrially stably produce high-performance carbon fibers, which is unsatisfactory.

Means for Solving the Problems The purpose of the present invention is to develop high-performance carbon fibers that provide a structure that does not cause cleavage on the surface of carbon fibers, and to develop carbon fibers with thermally stable melt pitch and spinning stability. The objective is to manufacture the spinning material with good spinning efficiency at a relatively low temperature.

The present invention is based on carbon 1! By forming an optically isotropic component on the surface of the carbon fiber, it is possible to prevent the surface from taking on a radial structure, and there is no cleavage at all on the surface.
The central part consists of an optically anisotropic component or an optically anisotropic component partially containing an optically isotropic component,
A carbon fiber whose surface layer can be arbitrarily changed, consisting of an optically isotropic component.The surface of the carbon fiber structure is an optically isotropic component, and the center part is an optically anisotropic component. Alternatively, the present invention relates to a carbon fiber comprising an optically anisotropic component partially containing an optically isotropic component.

The present invention also relates to a method for producing carbon fiber, which comprises melt spinning an optically anisotropic pitch prepared from coal tar pitch, infusible treatment of pitch fibers, carbonization treatment of pitch fibers, and graphitization treatment of pitch fibers. . More specifically, the spinning pitch exhibits a bulk mesophase, has a mesophase content of 60 to 95% by volume, and has a benzene insoluble content of 80%.
~95% by weight, and when carbonizing and graphitizing pitch fibers, the mesophase content and benzene insoluble content are reduced to prevent cleavage from occurring on the surface of the pitch fibers during the treatment. During adjustment, in the process of adding newly prepared optically isotropic pitch to optically anisotropic pitch, by changing the amount of optically isotropic pitch added to optically anisotropic pitch, relatively By making it possible to perform low-temperature melt spinning and forming the surface of pitch fibers with optically isotropic pitch when melt spinning pitch,
During the carbonization and graphitization process, no cleavage occurs on the surface of the pitch fiber, so there is no cleavage at all on the surface of the carbon fiber.
and a pitch adjustment process capable of changing the width of the optically isotropic component forming the outer periphery of the carbon fiber in the cross section of the carbon fiber, a process of melt spinning the pitch, and a process of infusibility, carbonization, and graphitization. , relates to a method of manufacturing the above-mentioned carbon fiber.

In addition, as an aspect of the manufacturing method, in the pitch adjustment step,
Coal tar pitch, petroleum pitch, SRC, etc. prepared by vacuum distillation or solvent extraction, with a softening point of 15
Includes a method for producing charcoal:ti woven fiber using a spinning pitch prepared by adding an optically anisotropic pitch to an optically isotropic pitch having a temperature of 0°C or higher and a benzene-insoluble molecule content of 31ffi% or higher. .

By adopting the above configuration, the present invention provides the following features for the first time:
High-performance carbon fiber with a structure that does not cause cleavage on the fiber surface is stable and industrially 1! even in the form of multifilament of 200 volumes or more! ? can be done.

Optical anisotropy as used in the present invention refers to the part that shines when the pitch surface is polished by σ1 and observed under orthogonal light leakage using a 1-type polarizing microscope, or the part that shines during decolorization. When using a board, this is the part where the color changes.

Raw materials for producing such anisotropic pitch include, for example, coal tar, coal tar pitch, coal-based heavy oils such as coal liquefied products, atmospheric residual oils of petroleum, or vacuum distillation and these residual oils. Although any tar or pitch produced by heat treatment may be used, coal tar pitch is particularly advantageous in that it is easy to process and a suitable anisotropic pitch can be obtained.

Several methods are already known for obtaining anisotropic pitch from such coal tar pitch. For example, JP-A-49-19127 and JP-A-59-3
Although it is manufactured by a method such as that disclosed in Japanese Patent No. 6725, such a known anisotropic pitch can be applied in the present invention. That is, coal tar pitch is used as a hydrogen-donating solvent,
For example, hydrogenation with tetrahydroquinoline or tetralin at 350 to 500°C under autogenous pressure, or hydrogenation of coal tar pitch with aromatic oil under hydrogen pressure and, after solvent recovery, a temperature of 400 to 500°C. Then, an anisotropic pitch is obtained by performing mesophasing treatment under an inert gas atmosphere at normal pressure or reduced pressure.

The anisotropic pitch made [1] by the mesophasing process is
It usually contains 95% by volume or more of mesophase. In particular, conventional spinning pitches for high-performance carbon fibers have been those that exhibit overall anisotropy under a polarizing microscope.

It is well known that when this fully anisotropic pitch is spun in a temperature range where the pitch is thermally stable, it becomes a fiber with a radial structure having cleavage on the fiber surface.

In the present invention, the mesophase content in the spinning pitch is 160 to 95%, preferably 80 to 90%.
, the benzene insoluble content is 80 to 95% by weight, preferably 80 to 90Iffi%, and when a block of this pitch is observed under a polarizing microscope, the parent phase is bulk mesophase (mesophase spherulites coalesce and grow, and The state in which a phase is formed is called a bulk mesophase. In this state, isotropic components are distributed like islands.

) is an essential requirement.

The mesophase content and benzene-insoluble content are adjusted so that the mesophase content obtained by the above-mentioned mesophase treatment is 95% by volume or more, preferably an anisotropic pitch that exhibits overall anisotropy when observed under a polarizing microscope. , by adding freshly prepared optically isotropic pitch.

In the present invention, by adding this optically isotropic pitch, the anisotropic pitch has a mesophase content of 60 to 95% by volume, preferably 80 to 90% by volume, and a benzene insoluble content of 8%.
It is effective to adjust the content to 0 to 95% by weight, preferably 80 to 90% by weight, and use it as a spinning pitch. When the mesophase content is less than 60%, the anisotropic component of the pitch turns into spherulites under a polarizing microscope, which causes phase separation from the isotropic component of the matrix during spinning, making stable spinning impossible. . In addition, if the mesophase content is 95% or more, mesophase treatment L! I! The properties are similar to the later anisotropic pitch, and the effects of the present invention cannot be achieved.

As the optically isotropic pitch, any raw material such as coal tar pitch, petroleum pitch, or SRC may be used,
These raw pitches are prepared by vacuum distillation or solvent extraction, and treated at a treatment temperature of 450°C or less, preferably 400°C or less, more preferably 320 to 380°C, for 180 minutes or less, preferably 30 minutes or less. The obtained benzene insoluble molecules fi 30% by weight or more, softening point 15
A pitch of 0° C. or higher is particularly preferable in order to obtain the effects of the present invention. The amount of pitch added is determined to be within the range where the matrix exhibits a bulk mesophase when the anisotropic pitch is observed under a polarizing microscope, that is, the optical mesophase content is 60%.
It may be added in a range of ~95%. Further, the optically isotropic pitch may be added and mixed by pulverizing and mixing with the anisotropic pitch at room temperature F, or by melting and mixing in the temperature range of 300 to 370° C. while maintaining the block shape.

The spinning pitch thus prepared is then melt-spun, and melt-spinning in the present invention refers to a method commonly used for producing carbon fibers from pitches.

That is, the pitch for spinning is melted at a temperature in the range of 300 to 400°C, and then the molten pitch is spun through a nozzle hole by pressurization with an inert gas or by extrusion using a metering pump. In particular, the spinning temperature is preferably 300 to 380, because at high temperatures where pinch thermal decomposition occurs, bubbles are generated by the generated gas and cause breakage during spinning.
A temperature range of °C is taken as the spinning temperature. The pitch extruded from the spinning nozzle hole is spun at a high speed of 200 m/min or more, preferably 400 m/min or more, and the fiber diameter of the pitch fiber can be easily controlled by the amount of pitch flowing out and the spinning speed. I can do it. In addition, the present invention includes 63:
Multifilament spinning of 200 holes or more is also possible.

The spun pitch fibers are then subjected to infusibility treatment, which is carried out in air or in an oxidizing atmosphere such as oxygen, ozone, nitrogen oxide, etc., at a heating rate of 10°C/min or less, preferably from 2 to 380°C. , preferably by heating to a temperature of 240 to 350° C. and maintaining this temperature for 300 minutes or less, preferably 1 to 30 minutes.

If the infusibility temperature is below 200° C., the infusibility will not proceed sufficiently, and softening or fusion will occur in the subsequent carbonization treatment, making it impossible to form good carbon fibers.

In addition, the infusibility temperature is 380°C or higher, or the holding time is 30°C.
When the time is longer than 0 minutes, the oxidation state of the m-fibers becomes excessive, making it impossible to obtain high-strength carbon fibers. Furthermore, if the infusibility temperature increase rate is 10°C/min or more, fusion between fibers occurs,
Good carbon fibers cannot be obtained.

The infusible fibers are then carbonized in an inert gas atmosphere. This carbonization treatment is usually carried out at a heating rate of 30°C/min or less, preferably 15°C/min or less, to 800°C.
The above is preferably carried out by heating to 1000 to 1500°C and maintaining this temperature for 5 minutes or more, preferably 10 to 30 minutes. If the carbonization temperature is 800° C. or lower, the carbonization of the fibers will not proceed sufficiently and the performance as a high-performance carbon fiber will not be exhibited. Furthermore, if the temperature increase rate is 30° C./min or more, the fibers will fuse together, making it impossible to obtain good carbon fibers.

Furthermore, the carbonized fibers can also be graphitized if necessary. This graphitization is performed by heating to 1800 to 3000°C in an inert gas atmosphere.

In the present invention, a high-performance carbon fiber having a structure in which the fiber surface does not undergo cleavage is manufactured only after the above-described configuration is completed. Although the reason for this is not clear, it is presumed that the optically isotropic component in the spinning pitch has some effect on the mesophase layer surface oriented in the fiber axis direction.

Furthermore, as shown in Figures 2 and 3, the addition of the isotropic component forms a smooth layer on the surface layer of the fiber cross section, and it is presumed that this layer suppresses surface cleavage. .

Hereinafter, the present invention will be explained in more detail with reference to Examples.

In addition, the measurement method in Examples is based on the method shown below.

``One sample of optically isotropic component fabric (mesophase-containing fli) was placed in a cylindrical aluminum cell of known dimensions in advance, melted at 370°C in a nitrogen atmosphere, rapidly solidified, and embedded in epoxy resin together with the aluminum cell. After that, the cylindrical aluminum cell was cut in the diameter direction and polished, and the amount of optically anisotropic components present in the cylindrical cross section was determined by area using a polarizing microscope.
Next, it was calculated from the volume ratio with the isotropic component when converted into volume.

[According to the method specified in Strength Measurement Co., Ltd. J15-R-7601. i
The diameter of the ii fiber was measured using a micrometer at a portion adjacent to the strength measurement section.

[Softening Point] Measured using a softening point meter manufactured by Asia Chemical Steaming Model AMK.

Example 1 Benzene insoluble content 30.733Nt1%, softening point 88.8
℃, 1 m width part of commercially available coal tar pitch having properties of 56.4% by weight of fixed carbon was subjected to Te 1 to Lauan 2 insect removal in an autoclave under autogenous pressure in N2 gas atmosphere at 43°C.
Hydrogenation treatment was performed at 0° C. for 30 minutes to remove insoluble portions of detralin including free carbon, and then the solvent was recovered to obtain hydrogenated pitch. Furthermore, this hydrogenated pitch was heated to 6 Torr.
Under reduced pressure, the temperature was raised to 490° C. at a heating rate of 3° C./min while blowing N2 gas to form a mesophase, and pitch was obtained. When a block with this pitch was observed with a polarizing microscope, it was found to be entirely anisotropic. This anisotropic pitch has a quinoline soluble content of 33.5% and a benzene insoluble content of 91%.
．． 3% by weight, and the softening point was 276°C.

This anisotropic pitch has a benzene insoluble content of 56.9%,
19.1% by weight of isotropic pitch with a softening point of 231°C was added, and the mixture was melted and mixed at 370°C in an N2 gas atmosphere to obtain pitch for spinning. This spinning pitch has a benzene insoluble content of 85
．． % double weight and a softening point of 271°C. Further, when this pitch block was observed with a polarizing microscope, it was found that the parent phase was a bulk mesophase, the isotropic parts were distributed like islands, and the mesophase content was about 85% by volume.

This spinning pitch was melted in a brass spinning device having a nozzle diameter of 0.2 #, and spun at a pitch temperature of 340° C. by pressurizing N2 gas. The pitch fibers were wound around a flat drum at a speed of 400 m/min to obtain l fibers with a fiber diameter of 10.5 μm without cutting.

The pitch fibers were heated to 350° C. at a rate of 3° C./min in an oxygen stream and held for 10 minutes to complete infusibility. This infusible fiber was further heated at 1100°C at 15°C/min in an argon stream.
The temperature was raised to .degree. C. and held for 30 minutes to obtain carbon fibers.

This carbon fiber is an optically isotropic component layer with a smooth surface and a component exhibiting an indistinct radial structure at the center.
% thickness. Carbon fiber with such a two-layer structure has no cleavage on its surface and has a tensile strength of 2.
50 K9/trvt+2, elastic modulus is 14, Ot
on/g2.

Comparative Example 1 When the entire anisotropic pitch obtained in Example 1 was spun at 340°C in the same manner as in Example 1 without adding isotropic pitch, 11.0μ fibers were obtained without cutting. I got until I ran out of pitches.

This pitch I! The fibers were made infusible and carbonized in the same manner as in Example 1 to obtain carbon fibers. This carbon material has a tensile strength of 1
It has a performance of 50Kg/#2 and an elastic modulus of 1 Kai Q ton/2, as determined by an electron microscope! Observation of a 1ift cross section revealed a typical radial structure, and cleavage on the fiber surface was confirmed. The tensile strength of the carbon fibers showed a low value due to the cleavage of the fiber surface.

Comparative Example 2 28.2 ffiffi% of the isotropic pitch used in Example 1 was added to the fully anisotropic pitch of 19 in Example 1, and the mixture was melt-mixed to obtain spinning pitch.

This spinning pitch has a benzene insoluble content of 81.7%,
The softening point was 266°C, and as a result of observing the block under a polarizing microscope, a phase transformation had occurred. That is, the parent phase was an isotropic component, and the mesophase was a spherulite. The mesophase content was 75% by volume a.

When this spinning pitch was spun in the same manner as in Example 1, fibers of 10.0μ were spun at a spinning temperature of 340°C, but fiber breakage occurred 10 minutes after the start of spinning, and even after that, fibers of 10.0 μm were spun. There were many amputations.

The obtained pitch fibers were made infusible and carbonized in the same manner as in Example 1 to obtain carbon fibers. This carbon fiber has a tensile strength of 185 Kg/rnM2 and an elastic modulus of 12.0 ton/m2, and as a result of observing the fiber cross section with an electron microscope,
As shown in Figure 3, the radial structure was slightly unclear in the center, and the surface layer became a smooth layer covering 30% of the fiber radius.

Example 2 The hydrogenated pitch obtained in Example 1 was heated under a reduced pressure of 5 Torr at a temperature increase rate of 3° C./min while blowing N2 gas.
Mesophase formation was performed at 470° C. for 20 minutes to obtain anisotropic pitch.

This anisotropic pitch has a quinoline soluble content of 33.0% by weight,
The benzene insoluble content was 91.8% by weight, and the softening point was 268°C. Furthermore, when this anisotropic pitch block was observed with a polarizing microscope, it was found to be entirely anisotropic.

This anisotropic pitch has a benzene insoluble content of 54.9%,
20.0% by weight of isotropic pitch with a softening point of 223°C was added and melted and mixed at 370°C in an N2 gas atmosphere to obtain spinning pitch. This spinning pitch has a benzene insoluble content of 8
It was 4.8% by weight, and the softening point was 263°C. Further, when this pitch block was observed with a polarizing microscope, it was found that the matrix was a bulk mesophase, the isotropic part was divided into islands, and the mesophase content was about 80% by volume.

This spinning pitch was melted in a brass spinning device having 200 holes arranged circumferentially with a nozzle diameter of 0.2 m, and spun at a pitch temperature of 345° C. by pressurizing N2 gas.

The pitch fibers were wound around a flat drum at a speed of 450,771/min to obtain fibers with an m-fiber diameter of 9.2 μm.

Next, the obtained pitch fibers were subjected to infusibility and carbonization treatment in the same manner as in Example 1 to obtain carbon fibers.

The obtained carbon fiber had exactly the same structure as in Example 1, had a tensile strength of 220.9/mIn2, and an elastic modulus of 13.
It was on at 7°C and /IfUII2. This carbon fiber
As a result of observation using an electron microscope, there was no cleavage at all in the surface layer consisting of optically isotropic components.

Comparative Example 3 Two layers of anthracene oil were added to one layer of commercially available coal tar pitch, which has properties such as benzene insoluble content of 30.7% by weight, softening point of 88.8°C, and fixed carbon of 56.4% by weight. After mixing and removing solvent-insoluble components, the solvent was collected. The remaining pitch is heated to 380℃ under normal pressure at an R wetting rate of 3℃/min while blowing N2 gas. ? The mixture was collected and held for 15 minutes to complete the heat treatment. The resulting pitch had a benzene insoluble content of 56.9
When the pitch block was observed with a polarizing microscope at a softening point of 231° C., it was found to be an isotropic component on the entire surface.

This bit was used in the spinning machine V2 used in Example 1 for 300
Spun at ℃. Pitch fiber is 400TrLZ on flat drum
The fibers were wound at a speed of 1 minute to obtain a fiber of 9.8μ. The pitch fibers were heated to 350° C. at 1° C./min in an oxygen stream, and the temperature was maintained for 0 minutes to complete infusibility. This infusible fiber is further heated to 1100°C at 15°C/min in an argon stream! F? After drying and holding for 30 minutes, carbon fibers were obtained. This carbon fiber has a tensile strength of 95 kg/H and an elastic modulus of 5.3°C On/mm2.
When observing the fiber cross section using an electron microscope,
The fiber cross section was extremely smooth, and no cleavage on the fiber surface was observed.

Comparative Example 4 The hydrogenated pitch obtained in Example 1 was stomach-warmed to 480°C under normal pressure at a temperature rate of 3°C/min while blowing N2 gas,
The temperature was raised to 420''C and kept at this temperature for 80 minutes to form a mesophase.

The obtained pitch had a quinoline soluble content of 24.01%, a benzene insoluble content of 84.9%, and a softening point of 262°C. Furthermore, when this pitch block was observed using a polarizing microscope, it was found that the parent phase was a mesophase, and the isotropic components were distributed in an island shape. Mesophase content is approximately 85% by volume
Met.

This pitch was spun in the same manner as in Example 1, and
．． A pitch fiber of 9μ was obtained. This pitch fiber was used in Example 1.
Carbon fibers were obtained by performing infusibility and carbonization treatment in the same manner as above. This carbon material has a tensile strength of 190 K9/mm.
2. Has a performance of elastic modulus of 17.2 tons/ll112,
Observation of the cross section of the fiber using an electron microscope showed that it had a typical radial structure, and cleavage on the fiber surface was confirmed.

Effects of the Invention In the carbon fiber according to the present invention, by adding and mixing optically isotropic pitch to optically anisotropic pitch in the pitch adjustment step, the surface layer has optically isotropic components and ,
It has a two-layer structure in which the central part consists of optically oriented components. Since the surface layer is formed from an optically isotropic component, there is no cleavage on the fiber surface that would reduce the tensile strength of the carbon fiber! ’ You can make it so that it doesn’t happen. The tensile strength of carbon fiber with radial structure and cleavage on the surface of m-thickness according to the conventional technology is 91 to 150 to 160.
mm2, whereas the tensile strength of the carbon fiber according to the present invention with no cleavage on the fiber surface is 200 to 25 mm2.
0 K pi 7mm2, tensile strength is 30-50%
It has become somewhat larger.

In the manufacturing method of +rt carbon fiber that does not cause cleavage on the J1 dark blue surface, it is necessary to set the melt spinning temperature of the spinning pitch to a relatively high temperature, and as a result, the molten pitch foams and decomposes, and this These phenomena have hindered the stable spinning of pitch fibers, such as causing the pitch fibers to break when pitch is melt-spun, and bubbles generated in the molten pitch being incorporated into the pitch. Therefore, it has been almost impossible to spin pitch fibers with a nozzle having multiple holes, but in the melt spinning 4° yarn process using the pitch of the present invention, the spinning pitch is thermally unstable. It is possible to avoid spinning at a temperature of 360° C. or higher, which would cause conditions. If a normal pitch is used, the pitch m-thread spun at a temperature below the above temperature will have a radial structure, so the fiber surface will inevitably have cleavage, but the pitch adjustment method of the present invention According to , melt spinning can be performed at relatively low temperatures,
The molten pitch does not cause any foaming decomposition. Therefore, no breakage due to pitch deterioration was observed during spinning, making it extremely easy to spin pitch fibers into multiple holes.

Furthermore, in the pitch adjustment method of the present invention, since the optically isotropic pitch exhibits thixotropic properties depending on the spinning tension during melt spinning, when the molten pitch forms a cone, the optically isotropic pitch The sexual pitch moves to the surface of the cone. As a result, the surface of the pitch fiber is covered with an optically isotropic pitch layer, making it possible to obtain carbon fiber with no cleavage on the fiber surface even at relatively low temperatures.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a typical fiber cross-sectional structure of a conventional coal tar pitch-based HP carbon fiber. (A) shows a radial structure, which shows the cleavage of the fiber surface. (b) is a random structure, and (c) is an onion structure. FIGS. 2 and 3 show the cross-sectional structure of carbon fibers obtained according to the present invention. Figure 2 shows a blurred radial structure. In FIG. 3, the central part has an indistinct radial structure, and the colored layers at the surface represent smooth layers.

Claims (3)

[Claims] (1) The outer peripheral part of the fiber cross section is formed of an optically isotropic component, and the center part is made of an optically anisotropic component or an optically anisotropic component partially containing an optically isotropic component, A carbon fiber with a novel structure in which there is no cleavage on the fiber surface. (2) A method for producing carbon fibers, which comprises melt-spinning optically anisotropic pitch prepared from coal tar pitch, and then subjecting the pitch fibers to infusibility treatment and carbonization treatment, or further graphitization treatment. By adding newly prepared optically isotropic pitch to the anisotropic pitch, the spinning pitch has a mesophase content of 60 to 95% by volume, a benzene insoluble content of 80 to 95% by weight, and a A method for producing carbon fiber, characterized in that the matrix exhibits a bulk mesophase when observed. (3) The method for producing carbon fiber according to claim 2, wherein the optically isotropic pitch has a benzene insoluble content of 30% by weight or more and a softening point of 150° C. or more.