JPH0380888B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing a pitch-based carbon fiber having a novel and unique internal structure and having high strength and high modulus.

Prior Art Carbon fiber was initially produced using rayon as a raw material, but due to its characteristics and economic efficiency, carbon fiber is now produced using polyacrylonitrile (PAN) fiber as a raw material.
and pitch-based carbon fibers made from coal or petroleum-based pitches.
Among these, the technology to produce high-performance grade carbon fiber using pitch as a raw material is attracting attention due to its excellent economic efficiency. - It is known that fired carbon fibers can have higher strength and higher modulus than the conventional pitch-based carbon fibers (Special Publication No. 1810/1983).

It has also been discovered that even higher physical properties can be developed by controlling the internal cross-sectional structure of pitch-based carbon fibers (Fuel, 1980, 60,
839, Japanese Patent Application Publication No. 59-53717, etc.).

That is, the cross-sectional structure of pitch-based carbon fibers includes random, radial, onion structures, or composite structures thereof, and radial structures are considered unfavorable because they tend to cause cracks and deteriorate physical properties due to macro defects. Furthermore, the random structure of pitch-based carbon fibers is actually a radial structure with small lamella sizes, which is a preferable structure in terms of strength, but cracks tend to occur if pitch adjustment and high draft during spinning or rapid cooling are not sufficient. Manufacturing conditions are limited.

Phenomenologically, the onion structure can be obtained by spinning after raising the temperature to a temperature higher than the spinning pitch viscosity change temperature (see JP-A-59-53717), but the normal optical anisotropy In Pitch, this viscosity change temperature is over 350℃, so
Since the spinning stability is poor and the resulting fibers tend to contain voids, it is difficult to stably obtain void-free onion-structured fibers by melt spinning.

In addition, when melt-spinning pitch, the average linear velocity of the molten pitch on the spinneret surface is set to a value smaller than the maximum average velocity in the spinneret hole and 1/1500 or less of the take-up speed to form a pool of pitch on the spinneret surface. It is also known that fibers with a random structure can be obtained by adjusting the shear rate in the spinneret hole and the orientation formation parameters (Japanese Patent Application Laid-open No. 168115/1983), but this method has poor spinning stability and is impractical. Not.

Purpose of the Invention The purpose of the present invention is to industrially produce pitch-based carbon fibers that have a novel internal structure that is completely different from conventional pitch-based carbon fibers and have significantly superior physical properties compared to conventional similar fibers. The goal is to provide a way to do so.

Structure of the Invention The present inventors have conducted intensive research to develop pitch-based carbon fibers that are comparable to or superior to PAN-based carbon fibers in terms of performance such as strength and modulus, and have found that (a) melt spinning As a pitch, an optically isotropic pitch with a content of quinoline insolubles of 5% by weight or less is used, and the organic solvent is mixed with an organic solvent system containing 60% by volume or more of benzene and/or toluene and a solubility coefficient of 8.0 to 9.5. 5 for 1g of pitcher
The content of the optically anisotropic phase is determined by using ~150 ml and processing at a treatment temperature of 22 to 30 °C, and then heat-treating the solvent-insoluble portion at a temperature of 230 to 450 °C in an inert gas atmosphere. (b) A special spinneret with a spinning hole consisting of a single slit having a specific shape and dimensions is used during melt spinning. We discovered that by using this method, it is possible to obtain pitch-based carbon fibers that have a unique microstructure that is completely different from conventional radial, random, or onion structures, and that have excellent performance comparable to PAN-based carbon fibers. Based on this knowledge, we have completed the present invention.

The optically isotropic pitch used in the present invention is
There are petroleum pitch, coal tar pitch, natural asphalt, pitch produced as a by-product during naphtha cracking, etc. Generally, pitch having a high degree of aromaticity can be preferably used in the present invention. Another important feature of the optically isotropic pitch that can be used in the present invention is that the content of foreign substances represented by quinoline insoluble matter is 5% by weight or less, preferably 3% by weight or less, and more preferably 0.3% by weight or less. It is a certain thing. The amount of quinoline insolubles is determined by the standard method of extracting pitch with quinoline at 75°C.

In the present invention, optically isotropic pitches as described above are treated with an organic solvent system having a specific solubility coefficient.

That is, the organic solvent system used in the present invention is one that has a solubility coefficient at 25°C between about 8.0 and 9.5, preferably between 8.7 and 9.2. Typical organic solvents corresponding to this and their solubility coefficients at 25°C are as follows.

Benzene = 9.2; Toluene = 8.8; Xylene =
8.7; cyclohexane = 8.2.

According to the research conducted by the present inventors, it has been confirmed that among these solvents, a single or mixed solvent system containing benzene and/or toluene at 60% by volume or more is preferable, and among them, toluene is the most preferable.

Furthermore, a mixed solvent system in which two or more types of solvents are mixed can also be used to obtain an organic solvent system having a desired solubility coefficient. Among these mixed solvent systems, a mixture of toluene and heptane containing about 60% by volume or more of toluene, for example a toluene-heptane mixed solvent with a volume ratio of 60/40 and 85/15, is preferred.

The amount of solvent used in the above treatment is generally about 5 to 150 ml, preferably about 10 to 20 ml, per 1 g of pitch. In addition, the processing temperature is 22~
The temperature is preferably about 30°C.

In the present invention, the organic solvent insoluble content of the optically isotropic pitch obtained by the treatment with the organic solvent system described above is used, and by heating to 230 to 450°C, the insoluble content can be reduced to 75% or more, preferably 90%. The above is converted into a pitch having an optically anisotropic region of .

This optically anisotropic pitch tends to have a low softening point despite having a high anisotropic region content, and has good spinnability in the spinning method described below, making it suitable for producing high-performance carbon fiber. It's pitchy.

In order to produce the carbon fibers of the present invention, the shape of the spinning hole (nozzle) of the spinneret used in melt spinning the spinning pitch described above is particularly important.

That is, the melt in the spinning pitch as described above is melt-spun through a special spinning hole consisting of a single slit that satisfies the following formulas () and () at the same time.

For such a single slit spinning hole, when the center line distance in the slit is L ₁ and the corresponding wetted edge width is W ₁ , L ₁ and W ₁ are such that L ₁ ≦5.0 (mm) ... ( ) 3≦L ₁ /W ₁ ≦20 ... Use one that satisfies () at the same time.

In the method of the present invention, the single slit of the spinning hole is preferably a linear slit (single-shaped slit) as shown in FIG. 1, but a curved slit as shown in FIG. 2 can also be used. In the spinning holes shown in Figures 1 and 2, the centerline length _L1 of each slit is the centerline distance, and the maximum width (maximum distance in the direction perpendicular to the centerline) _W2 of each slit is the wetting edge width. becomes.

In order to form a carbon fiber having a novel structure as the object of the present invention, it is necessary that L ₁ and W ₁ satisfy the above formulas () and () at the same time,
In particular, those satisfying 5≦Ln/Wn≦15 are preferable.

When performing melt spinning, it is necessary to take care not to disrupt the flow of the molten pitch during spinning, and from this point of view, it is necessary to avoid sudden changes in the cross-sectional area of the pitch flow inside the spinning hole and immediately after discharge as much as possible. Furthermore, the spinning temperature is preferably lower than 360°C.

The spinning draft rate is preferably 30 or more, particularly 50 or more, and the spinning speed is preferably about 500 to 1500 m/min.

A new method can be created by spinning the specific spinning pitch mentioned above using a spinneret with a special spinning hole, infusibility treatment of the pitch fiber in an oxidizing atmosphere, and then firing treatment in an inert atmosphere. Structured carbon fibers are formed.

In addition, for pitch fibers subjected to infusibility treatment,
Preferably, an anti-fusing agent made of fine powder of silicon, aluminum, titanium, or boron oxide or carbide is applied in advance.

Carbon Fiber Obtained by the Present Invention The pitch carbon fiber having the novel structure obtained by the present invention is characterized by having a leaf-like lamellar arrangement in at least 50% of the cross section perpendicular to the fiber axis.

This leaf-like lamella arrangement can be identified by observing a cross-section taken almost perpendicular to the longitudinal direction of the carbon fiber using a scanning electron microscope, and is basically shown in Figure 3. It refers to a leaf-like lamellar arrangement in which many lamellae extend on both sides at angles of 15 to 90 degrees from a single central axis, almost symmetrically, and is a novel structure that has not been previously known.

The fiber in FIG. 3 is composed of one set of lamellar structures, but depending on how you look at it, it can be considered to be a combination of two sets of lamellar structures.

Generally, the number of leaf-like lamellae present in the cross section of the fiber is preferably 1 to 2. In addition, the proportion (area ratio) of leaf-like lamellae in the fiber cross-sectional area is at least 50% or more.

That is, in the carbon fiber of the present invention, in many cases, there is a part A with a leaf-like structure having a leaf-like lamella arrangement and a part B with an unclear structure around it, but [area of A]/[A+B area] is at least 50% or more, particularly preferably 70% or more.

The carbon fiber produced by the method of the present invention having the special leaf-like lamella arrangement as described above has a high strength of 400 kg/mm ² or more and a high modulus of 20 ton/mm ² or more, which is comparable to PAN-based carbon fiber. showing physical properties,
It has excellent physical properties that cannot be expected from conventional pitch carbon fibers.

The reason why such a leaf structure is formed is considered to be approximately as follows. In other words, pitches with optical anisotropy (liquid crystallinity) are thought to consist of plate-shaped molecules, and such plate-shaped molecules can be observed in a cross section perpendicular to the streamline direction of pitches in the spinning hole (nozzle) of a spinneret. In this case, the pitch flow tends to be arranged in a direction perpendicular to the constant velocity line formed by the pitch flow. The reason for this is that shear force is generated as the pitch moves in a direction perpendicular to the constant velocity line in the spinning hole.
This result can be understood by considering that the plate-like molecules of the pitch are oriented in the direction of the shear force. Since the direction in which this shearing force acts is perpendicular to the constant velocity line formed by the pitch flow in the spinning hole, the orientation of the leaf-like lamellar arrangement can be explained.

That is, when considering the pitch flow in a circular spinning hole, the constant velocity line field becomes circular, and when pitch molecules are arranged at right angles to this, the structure of the pitch fiber obtained becomes radial. For this reason, stress strain is likely to occur when the molecular plane spacing contracts during the infusibility/firing stage, resulting in cracks. In addition, the random structure increases the pitch temperature in the spinning hole to keep the pitch viscosity low, and also increases the diameter of the spinning hole to keep the shearing force acting on pitch molecules in the spinning hole as low as possible. This is obtained by preventing the particles from arranging in the direction of the shear force. but,
When spinning with this method, the spinning temperature is inevitably high and the diameter of the spinning hole must also be increased, making the spinning condition extremely unstable and the resulting fibers likely to contain voids. Carbon fiber with high performance physical properties could not be obtained.

On the other hand, when a specific spinning pitch prepared by the method of the present invention is melt-spun through the spinning hole consisting of the single slit described above, the uniform velocity line field of the pitch in the spinning hole is almost similar to the shape of the spinning hole. Become. At this time, when the pitch molecules are arranged perpendicular to the constant velocity line as described above, they form a leaf structure within the fiber cross section.

The lamellar arrangement (leaf structure) of such a pitch is
Because it has an arrangement that easily absorbs the stress strain that inevitably occurs when the molecular spacing shrinks during the infusibility and firing stages, there is no problem of cracks occurring during the infusibility and firing stages as with conventional pitch diameter carbon fibers. 400
It is thought that high strength of Kg/mm ² or more is developed.

Effects of the Invention The pitch-based carbon fiber obtained by the present invention as described above has a leaf-like lamellar arrangement in its cross-sectional structure, which prevents cracks, and also has good spinnability because it does not require spinning at high temperatures. Because shrinkage occurs smoothly during the infusibility and firing stages, strength and modulus increase dramatically,
It is possible to stably obtain pitch-based carbon fibers that have physical properties that exceed those of PAN-based carbon fibers.

Method for measuring each index Next, a method for measuring each index representing the spinning pitch and fiber properties in the present invention will be explained.

(a) Melting point of spinning pitch using PerkinElmer DSC-ID type,
Put 10 mg of fine powder crushed into 100 meshes or less into an aluminum cell (inner diameter 5 mm), press it down from above, and heat it at a heating rate of 10°C/10°C in a nitrogen atmosphere.
Measurement is performed while raising the temperature to nearly 400°C in minutes, and the endothermic peak that indicates the melting point in the DSC chart is taken as the melting point of the spinning pitch.

(b) Proportion of optically anisotropic phase in the spinning pitch Five polarized micrographs of the spinning pitch were arbitrarily taken using a reflective polarizing microscope, and the area fraction of the anisotropic region ( %),
The average value of these is taken as the proportion of the optically anisotropic phase.

(c) Physical properties of carbon fiber The fiber diameter (single fiber diameter), tensile strength, and modulus of carbon fiber are determined according to JIS R-7601 "Carbon fiber test method".
Measure according to.

The fiber diameter is measured using a laser for fibers with a circular cross section, and for fibers with an elliptical (flat) cross section, the average value of the cross-sectional area of n=15 is calculated from scanning electron micrographs. In addition, in Examples etc., the fiber diameter is expressed as a diameter when converted into a circle having a corresponding cross-sectional area.

(d) Fraction of leaf-like lamellar arrangement From a scanning electron micrograph of a cross section of carbon fiber,
It is expressed as the area ratio of the leaf-like lamella array part per cross-sectional area.

EXAMPLES The present invention will be explained in more detail with reference to Examples below, but the present invention is not limited to these experimental examples in any way.

Example 1 1 g of commercially available coal tar pitch (optically isotropic)
and 20 ml of toluene were mixed and stirred, extracted at 25°C, and the mixture was filtered using a Naslon filter (manufactured by Nippon Seisen Co., Ltd., trade name NF-03), and the resulting residue (solid material) Dry under reduced pressure in the usual way,
The dried pitch was heated at 400°C for 10 minutes in a nitrogen atmosphere to obtain a mesophase pitch with a full-surface flow structure and containing 90% of the optically anisotropic phase.

The mesophase pitch was charged into a metering feeder equipped with a heater, and after melting and degassing, it was passed through a separately provided heating zone, L ₁ = 540 μm, L ₁ /W ₁ = 9.00.
The fibers were spun using a spinneret having a straight slit (one-letter-shaped) spinning hole.

In this case, the discharge amount from the pitcher feeder is
The material was wound at a take-up speed of 800 m/min, with a feeder temperature of 320°C and a mouth temperature of 340°C.

After applying fine silica powder as an anti-fusing agent to this pitch fiber, it was heated in dry air at a heating rate of 10°C/min from 200°C to 300°C.
Hold for 30 minutes. Then heated at 500℃/in a nitrogen atmosphere.
The carbon fiber was heated to 1300° C. at a temperature increase rate of 5 minutes, and then fired by holding it for 5 minutes.

The obtained carbon fiber had an equivalent fiber diameter of 7.3 μm, a cross section having an elliptical shape similar to that shown in FIG. 3, and a leaf-like lamella fraction of almost 100%. In addition, the physical properties were as follows: strength was 443 Kg/mm ² , modulus was 21.1 ton/mm ² , and elongation was 2.0%.

Comparative Example A pitch was prepared in exactly the same manner as in Example 1, except that a spinneret with a circular spinning hole of φ0.18 mm was used, and using this pitch, spinning was carried out under the same conditions as in Example 1. Infusibility and firing were performed. The structure of the obtained fiber is a typical radial structure,
Cracks occurred along the fiber axis, and no leaf structure was observed. The obtained fiber diameter is
8.4μm, strength is 217Kg/ ^mm2 , modulus is
The elongation was 16.7ton/mm ² and 1.3%.

[Brief explanation of drawings]

FIG. 1 and FIG. 2 are explanatory diagrams illustrating the spinning hole shape of the spinneret used in producing the pitch-based carbon fiber of the present invention, respectively.
L ₁ indicates the center line distance, and W ₁ indicates the wet edge width. Third
The figure is a sketch diagram schematically showing the cross-sectional structure of pitch-based carbon fiber obtained by the present invention, and A
indicates a leaf-like structural part having a leaf-like lamella arrangement.

Claims (1)

[Scope of Claims] 1. Optically isotropic pitch containing 5% by weight or less of quinoline insoluble matter in an organic solvent system containing 60% by weight or more of benzene and/or toluene and having a solubility coefficient of 8.0 to 9.5, Use 5 to 150 ml of the organic solvent per 1 g of pitch, and set the treatment temperature to 22 to 30°C.
The solvent-insoluble matter is further heat-treated at a temperature of 230 to 450°C to convert the pitch into a pitch containing 75% or more of the optically anisotropic phase, and then the pitch is passed through a spinning hole consisting of a single slit. When the center line distance in the single slit is L ₁ and the wet edge width is W ₁ ,
Using a spinneret with a spinning hole where L ₁ and W ₁ satisfy the following formula, L ₁ ≦5.0 (mm) ... () 3≦L ₁ /W ₁ ≦20 ... () A high-strength pitch-based carbon in which more than 50% of the cross section perpendicular to the yarn axis has a leaf-like lamella arrangement, which is characterized by melt-spinning without disturbing the flow, making the resulting fiber infusible, and firing it. Fiber manufacturing method.