JP3177690B2 - Method for producing pitch-based carbon fiber - Google Patents

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 a novel pitch-based carbon fiber. More specifically, the present invention controls the cross-sectional structure of a finally obtained carbon fiber by applying a specific shear stress to a molten pitch in a specific stage under a specific condition when melt-spinning a pitch, and And a method for producing a novel carbon fiber having superior properties over PAN-based carbon fiber.

[0002] In the present specification, the term "carbon fiber" includes carbonized fiber and graphitized fiber, and when necessary, the two are distinguished from each other.

[0003] In this specification, "%" means "% by weight" unless otherwise specified.

[0004]

2. Description of the Related Art Conventionally, carbon fibers made from pitch-based materials have a higher graphite pitch than carbon fibers (PAN-based carbon fibers) using organic synthetic fibers such as polyacrylonitrile (PAN) as precursors. Because of its excellent elasticity, that is, excellent tensile elasticity derived from high crystallinity and high orientation, it is attracting attention as a high-performance material. In addition, pitch-based carbon fibers are more cost-effective than PAN-based carbon fibers due to the fact that the pitch, which is the raw material, is inexpensive and the carbonization yield is high.

However, PAN-based carbon fibers have been the mainstream of high-performance carbon fibers having high tensile strength, high tensile modulus, high compressive strength and the like. Therefore, techniques for producing pitch-based carbon fibers with higher performance while utilizing the advantages of pitch-based carbon fibers have been studied, and various specific methods have been proposed. Particularly, by controlling the cross-sectional structure of the carbon fiber, development of a carbon fiber with higher performance has been performed. As a result, in recent years, the tensile properties of pitch-based carbon fibers have been dramatically improved, and pitch-based carbon fibers having the same or higher tensile strength than PAN-based carbon fibers are commercially available. .

For example, Japanese Patent Application Laid-Open No. Sho 59-168127 discloses a final product in which an optically anisotropic pitch is expanded or reduced before discharging the pitch from a spinning nozzle. A method is disclosed in which a cross-sectional structure of a carbon fiber is made to have a stable onion structure.

Japanese Patent Application Laid-Open No. 60-252723 discloses that a pitch fiber spun from a spinning nozzle having an enlarged nozzle intermediate portion using a mesophase pitch as a raw material is made infusible, carbonized, and graphitized. A method for producing a carbon fiber having an onion-like cross-sectional structure is disclosed. More specifically, this publication, the diameter of the spinning pitch inlet _(D 1), the diameter of the enlarged middle portion _{(D 2)} and the diameter of the rear stream portions relation between the _{(D 3) D 3 / D} 2 ≦ 1 And D ₃ / D ₁ ≦
By specifying 1.5, a method for producing a carbon fiber having no onion-like cross-sectional structure and having no cracks is disclosed.

[0008] However, according to the above two methods, even if the cross-sectional structure of the obtained carbon fiber can be eliminated and a stable onion-like cross-sectional structure can be obtained, the tensile strength of the carbon fiber can be reduced. At most 250-30
It was about 0 kg / mm ² , and the improvement of strength was limited.

In recent years, in order to obtain a carbon fiber having a high strength and a high modulus of elasticity, it is important to control the orientation of the fiber in the cross-sectional direction / axial direction to make the cross-sectional structure a random structure. Has been pointed out. For example, JP-A-1-280
No. 025 discloses inserting an insert directly above a discharge nozzle,
Further, a method is disclosed in which spinning is performed while applying a shearing force to the pitch to make the cross-sectional structure a radial random structure. By this method, the tensile strength of the carbon fiber is increased to about 380 to 420 kg / mm ² .

In this way, it has been possible to improve the tensile strength of the pitch-based carbon fiber by randomizing the higher-order structure of the cross-section of the carbon fiber, but it still has physical properties higher than those of the PAN-based carbon fiber. No pitch-based carbon fibers have been obtained. Further, although the tensile strength reached almost the same level as that of the PAN-based carbon fiber, no means for improving other physical properties, particularly the compressive strength, has been found.

[0011]

SUMMARY OF THE INVENTION Accordingly, the main object of the present invention is to improve the compressive strength, which is the biggest problem in the performance of the carbon fiber using pitch as a raw material, and to improve the tensile strength in the PAN-based carbon fiber. An object of the present invention is to provide a high-performance pitch-based carbon fiber that is comparable to or exceeds carbon fiber.

[0012]

Means for Solving the Problems The inventors of the present invention have conducted various studies in view of the problems of the prior art as described above, and as a result, have found that the pitch-based carbon fibers produced by the following method have led to the problems of the present invention. Can be achieved.

That is, the present invention provides the following method for producing a pitch-based carbon fiber: 1. Pitch-based carbonized fiber which melt-spuns an optically anisotropic pitch, infuses it, and carbonizes or graphitizes it. In the manufacturing method of
In melt spinning of pitch, after the (5/3) ² or more shear stress shear stresses experienced by the final nozzle hole was added by allowing the molten pitch before reaching the final nozzle holes to pass through the circular capillary unit, once the shear stress , Is passed through the final nozzle hole at a nozzle discharge speed of 0.8 to 4 m / min.
A method for producing pitch-based carbon fiber, wherein the fiber is spun from 00 to 2000. 2 The ratio (D ₂ / D ₁ ) of the diameter (D ₁ ) of the circular capillary portion through which the molten pitch passes before reaching the final nozzle hole to the diameter (D ₂ ) of the final nozzle hole is 5/3 or more;
And a final nozzle hole diameter _{(D 2)} is 0.2~0.5mm
Item 2. The method for producing a pitch-based carbon fiber according to Item 1, wherein

The present inventor has repeatedly studied the relationship between the higher-order structure of the pitch-based carbon fiber cross section and the physical properties such as tensile strength and compressive strength. And found that the random structure was optimal. In addition, the random structure as the cross-sectional structure of the carbon fiber can be roughly divided into two structures: a random structure close to a radial structure (hereinafter referred to as a radial-like random structure) and a random structure including an onion structure in a small ratio. I found what I could do.

The desired object of the present invention can be achieved by pitch-based carbon fibers having a random cross-sectional structure or a random structure containing an onion structure at a ratio of 10% or less. According to the study of the present inventor, in order to obtain a carbon fiber having a dense random structure of crystal structure and excellent physical properties, it is necessary to adopt spinning conditions that can convert from an onion structure to a random structure. There was found. However, if the spinning conditions are strictly set so that the onion structure does not exist at all in the fiber cross-section, the structure conversion may progress too much, resulting in a radial-like random structure. In consideration of the above, it was also clarified that it is preferable to leave the onion structure with the upper limit of the area ratio of the cross section of the carbon fiber being 10%.

Further, it was also found that the crystals of the random structure carbon fiber having a residual ratio of the onion structure of 10% or less were dense and small. The crystals are dense,
When the size is reduced, the number of defects is reduced and the physical properties are improved. The size of the crystal is represented by the crystal thickness _{L C} by X-ray measurements, in the present invention, this value (if the elastic modulus of about 20ton / mm ²⁾ of about 20 angstroms to 1
About 50 angstroms (elasticity is 70ton / mm
² ).

When the residual ratio of the onion structure is 10% or less in terms of the area ratio of the cross section of the carbon fiber, the physical properties of the carbon fiber are not substantially inferior to those of the carbon fiber having only the random structure. . On the other hand, when the area ratio of the onion structure in the cross section of the carbon fiber exceeds 10%, the onion structure of the pitch-based carbon fiber becomes excessive,
Crystals easily stacked and L _C is increased, undesirably. In addition, when the onion structure is not included, the pitch-based carbon fiber has a radial-like structure or a structure similar thereto, so that various defects, sparse crystal parts, even if there is no obvious crack. And the like may exist in large numbers, and the improvement of the physical properties becomes insufficient. Hereinafter, the present invention will be described in more detail.

In the present invention, as an optically anisotropic pitch as a starting material, a synthetic pitch produced from coal-based pitch, petroleum-based pitch, naphthalene or the like (for example, described in JP-A-1-139621). ing)
Any of these may be used, and there is no particular limitation.

The carbon fiber according to the present invention is produced as follows. First, when melt-spinning an optically anisotropic pitch, the molten pitch is passed through a circular capillary portion before reaching the final nozzle hole, so that the shear stress received in the final nozzle hole is (5/3) ² or more. After applying the shear stress, once hold the state in which substantially no shear stress is applied,
Next, the fiber is passed through the nozzle hole at a discharge speed of 0.8 to 4 m / min, and is blown with nitrogen under the nozzle to remove heat, and is spun at a draw draft ratio of 400 to 2000 to obtain pitch fibers. In the method of the present invention, since the spinning is performed at a high drawing draft ratio, it is necessary to sufficiently distribute the pitch to each nozzle, control the nozzle temperature, and cool down the nozzle. Here, the drawing draft ratio means a drawing ratio of a pitch fiber represented by a ratio (V ₁ / V ₀ ) between a winding speed (V ₁ ) and a nozzle discharge speed (V ₀ ).

Although the shape of the nozzle used in the present invention is not particularly limited, the diameter (D ₁ ) of the circular capillary portion through which the molten pitch passes before reaching the final nozzle hole and the final nozzle hole (D ₂ ) are determined. the ratio _D 2 / _{D 1} is 5/3 or more and to satisfy the requirement that _{D 2} is 0.2 to 0.5 mm, is essential. Only when these requirements are satisfied, a carbon fiber having a tensile strength exceeding 400 kg / mm ₂ and a significantly improved compressive strength can be obtained. In particular, the diameter of the circular capillary part (D ₁ ) and the final nozzle hole (D ₂ )
When the above satisfies the above conditions, the drawing draft ratio is easily increased, and the spinning state is improved. According to the method of the present invention, it is not necessary to insert an insert just above the nozzle as in the method described in JP-A-1-280025, and maintenance such as cleaning of the nozzle becomes easy.

Next, the pitch fiber obtained above is heated up to a maximum temperature of 240 to 380 ° C. at a heating rate of about 0.5 to 4 ° C./min in an oxygen-containing atmosphere to make it infusible.
These infusibilizing conditions are also very important in order to obtain a desired higher-order structure of the carbon fiber. If the heating rate is too low, the infusibilization time will be too long and the cost will increase. On the other hand, when the heating rate is too high, only the surface of the pitch fiber is oxidized, the surface portion is deteriorated, and the physical properties are reduced. On the other hand, if the maximum infusibilization temperature is too low, the infusibilization time will be prolonged, which will again increase the cost. On the other hand, if the maximum infusibilizing temperature is too high, the surface portion of the pitch fiber is excessively oxidized, and the surface deteriorates. Furthermore, if the oxidation of the surface of the pitch fiber becomes excessive and the oxidation of the central part becomes insufficient, the central part will be re-melted and recrystallized at the time of carbonization, and the higher-order structure will be deformed and This is inconvenient, because an unusual crystal is formed.

Next, the infusibilized fiber obtained as described above is preliminarily carbonized at 600 to 850 ° C. in an inert gas atmosphere, and then carbonized at 1200 to 2000 ° C. to obtain a carbonized fiber. .

Alternatively, the infusibilized fiber obtained as described above is preliminarily carbonized at 600 to 850 ° C. in an inert gas atmosphere, and then 2,000 ° C. or more (more preferably 2000 to 280 ° C.).
By graphitizing at a temperature of about 0 ° C.), graphitized fibers can be obtained.

The carbon fiber obtained in this way is a SEM
And it was confirmed by TEM microscopic observation that the crystals were dense and small. Further, the carbon fiber according to the present invention has considerably higher compressive strength and tensile strength as compared with the conventional pitch-based carbon fiber, and has the same or higher strength than the PAN-based carbon fiber. Demonstrate.
In the present specification, each property of the carbon fiber was measured by the following method. -Measurement of Area Ratio of Onion Structure The fiber cross section of the carbon fiber was enlarged by a scanning electron microscope, photographed, and the area ratio of a portion where crystals were arranged in an onion shape (concentric direction) was determined. -Lamination thickness (Lc) The carbon fiber was made into a powder form in a mortar, and the measurement analysis was performed in accordance with the Gakushin method "Method for measuring lattice constant and crystallite size of artificial graphite".

[0025]

The carbon fiber having a unique crystal structure according to the present invention has remarkably improved fiber physical properties. In particular, since it can improve not only tensile strength but also compressive strength, which has been the greatest weakness of pitch-based carbon fibers, it is widely used as a structural material for aviation and space equipment and buildings. The use is greatly expected.

[0026]

The present invention will be described in more detail with reference to the following examples. Example 1 A coal-based carbonaceous pitch (softening point: 306.9 ° C.) containing 94% of an optical anisotropy amount was used as a raw material, and was first passed through a circular capillary portion having a circular nozzle diameter of 0.13 mm, and finally passed through a final nozzle. After applying a shearing stress of about 5.3 times the shearing stress received from the holes, the melt-spinning was performed once while maintaining substantially no shearing stress and again applying the shearing stress by the final nozzle. The conditions at the time of passing the final nozzle are a spinning temperature of 350 ° C., a number of nozzle holes of 1,000, a nozzle diameter of 0.30 mm, and a nozzle discharge speed of 1.2 m.
/ Min, and a pitch fiber having a yarn diameter of 12 µm was obtained. The draft ratio at this time was 625. The condition during spinning was good, and there was no yarn breakage for 3 hours.

The pitch fiber thus obtained is heated in an air atmosphere at a rate of 2 ° C./min.
C. for 20 minutes to complete infusibilization. At this time, the oxygen uptake of the fiber was 8.2%.

Then, after preliminary carbonization in a nitrogen atmosphere at 780 ° C., carbonization or graphitization was performed at 1600 ° C., 1900 ° C. or 2200 ° C. for 1 minute in an argon atmosphere while applying a slight tension.

1600 ° C., 1900 ° C. and 2200 ° C.
Table 1 shows the physical properties of the carbonized or graphitized fibers.

[0030]

[Table 1] Further, a cross-sectional photograph of the carbonized fiber when carbonized at 1900 ° C. by a scanning electron microscope is shown as Reference Photo 1. The higher order structure of the fiber cross section had an onion structure with an area ratio of about 3% at the center and a completely random structure around the periphery.

From the comparison between the above results and the results of the following comparative examples, it is apparent that not only the tensile strength but also the compressive strength of the fiber carbonized at 1900 ° C. obtained by the method of the present invention is improved. is there.

In the case of treatment at 1600 ° C. and 2200 ° C., a pitch-based carbon fiber generally available on the market (an elastic modulus of 30 ton / m corresponding to a carbonized product at 1600 ° C.) is also used.
For fibers m ^2, tensile strength 300 kg / mm ² approximately,
Compressive strength of about 100 kg / mm ² ; in the case of fibers having an elastic modulus of 50 ton / mm ² corresponding to a graphitized product at 2200 ° C., tensile strength of about 400 kg / mm ² and compressive strength of 80 kg / m 2
than the m order of ^2), are products of high strength can be obtained. Comparative Example 1 The same coal-based carbonaceous pitch as in Example 1 was used as a raw material, and when spinning was performed in a single-stage nozzle having a nozzle diameter of 0.25 mm, an introduction hole (diameter 1.8 mm, length 20 mm) immediately above the nozzle ), A melt-spinning is performed at a spinning temperature of 350 ° C. and a number of nozzles of 500 by inserting a drill (with a groove having a depth of 0.13 mm and rotating three times within a length of 20 mm).
A pitch fiber having a yarn diameter of 12 μm was obtained. The spinning draft ratio at this time was 434. Next, infusibilization and preliminary carbonization of the pitch fiber were performed in the same manner as in Example 1, and carbonization was performed at 1900 ° C. for 1 minute in an argon atmosphere.

Table 2 shows the physical properties of the obtained carbonized fiber.

[0034]

[Table 2] A cross-sectional photograph of the obtained carbonized fiber by a scanning electron microscope is shown as Reference Photo 2. The higher-order structure is a radial-like random structure.

As is apparent from the above results, when spinning the pitch with a single-stage nozzle while applying a constant shear stress to the molten pitch, the higher-order structure of the cross section of the carbonized fiber becomes a radial-like random structure. No improvement in the physical properties of the carbonized fiber was observed. Comparative Example 2 Using the same raw material pitch as in Example 1, first, a capillary portion was passed through a circular nozzle having a diameter of 0.25 μm to give a shear stress approximately 1.4 times the shear stress received at the final nozzle hole. Otherwise, melt spinning was performed in the same manner as in Example 1,
The obtained pitch fibers (spinning draft ratio = 625) were made infusible. The infusibilizing conditions were the same as in Example 1.

The obtained infusibilized fiber was preliminarily carbonized in the same manner as in Example 1, and carbonized at 1900 ° C. for 1 minute in an argon atmosphere.

Table 3 shows the physical properties of the obtained carbonized fiber.

[0038]

[Table 3] In addition, a cross-sectional photograph of the obtained carbonized fiber by a scanning electron microscope is shown as Reference Photo 3. The obtained carbonized fiber has an onion structure with an area ratio of about 15% at the center and a completely random structure at the periphery.

As is clear from the above results, when the shear stress applied to the molten pitch in the capillary portion of the first circular nozzle is about 1.4 times the shear stress in the final nozzle portion, the cross section of the carbonized fiber The onion structure has an area ratio of about 15%, and no improvement in fiber properties is observed. Comparative Example 3 Same as Example 1 except that the diameter of the final nozzle was 0.23 mm and the shear stress applied to the molten pitch at the capillary portion of the circular nozzle was about 3.1 times the shear stress at the final nozzle hole. Then, melt spinning was performed to make the obtained pitch fiber (spinning draft ratio = 367) infusible. The infusibilizing conditions were the same as in Example 1.

The infusible fiber was preliminarily carbonized in the same manner as in Example 1 and carbonized at 1900 ° C. for 1 minute.

Table 4 shows the physical properties of the obtained carbonized fiber.

[0042]

[Table 4] Further, a cross-sectional photograph of the obtained carbonized fiber by a scanning electron microscope is shown as Reference Photo 4. The obtained carbonized fiber has an onion structure with an area ratio of about 13% at the center and a completely random structure at the periphery.

As is apparent from the above results, when the drawing draft ratio is as low as less than 400, the higher order structure of the cross section of the carbonized fiber becomes 13% of the onion structure, and no improvement in the fiber properties is observed. Example 2 A raw material for spinning was produced from naphthalene based on the method described in JP-A-1-139621 to obtain a pitch having an optical anisotropy of 100% and a softening point of 276 ° C. Using this pitch as a raw material, in the same manner as in Example 1, pitch fibers were obtained, then infusibilized, and carbonized.

The physical properties of the obtained carbonized fiber were as shown in Table 5.

[0045]

[Table 5] The higher order structure of the obtained carbonized fiber was almost the same as in Example 1, and the onion structure was 3.5% in area ratio.
The rest were random structures.

The properties of the carbonized fiber obtained by spinning an optically anisotropic synthetic pitch produced from naphthalene under the spinning conditions of the present invention and performing the steps after infusibilization under optimum conditions are greatly improved. It is clear that.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-170527 (JP, A) JP-A-60-194120 (JP, A) JP-A-60-252723 (JP, A) JP-A-59-194723 168127 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) D01F 9/14-9/155

Claims (2)

(57) [Claims] 1. A method for producing a pitch-based carbonized fiber in which an optically anisotropic pitch is melt-spun, made infusible, and carbonized or graphitized. (5/5) of the shear stress received at the final nozzle hole by passing through the circular capillary section
3) After the application of ^two or more shear stresses, once the state where the shear stress is not substantially received is maintained, and then the nozzle discharge speed is set to 0.1.
A method for producing pitch-based carbon fiber, wherein the fiber is passed through a final nozzle hole at a rate of 8 to 4 m / min, and is further spun at a draw draft ratio of 400 to 2,000. _2. The ratio (D ₂ / D ₁ ) of the diameter (D ₁ ) of the circular capillary portion passing through the melting pitch before reaching the final nozzle hole to the diameter (D ₂ ) of the final nozzle hole is 5/3. And the diameter (D ₂ ) of the final nozzle hole is 0.2 to
The method for producing pitch-based carbon fibers according to claim 1, wherein the diameter is 0.5 mm.