JPH0413450B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for producing pitch carbon fiber. Prior art and its problems Compared to carbon fibers made from organic synthetic fibers such as polyacrylonitrile as precursors, carbon fibers made from pitch-based materials can be produced at lower manufacturing costs and with a higher modulus of elasticity. Due to its ease of use, it is expected to be a material with lower cost and higher performance. However, commercially available pitch carbon fibers have a tensile strength of about 200 Kg/mm ² or less, and have poor quality stability, so it is difficult to say that they are fully satisfactory. In general, in pitch-based carbon fibers, the molecular aggregation state (hereinafter referred to as cross-sectional higher-order structure) of the fiber cross section is
However, it varies depending on the spinning conditions. That is,
Basically, the molecules form crystals in the concentric direction of the fiber (so-called onion type), or the molecules form crystals in the radial direction from the center of the fiber (radial type).
It is roughly divided into two types: one is distributed in any direction without showing any directionality (random type), but in actual fibers, there are some that are a mixture of these types. Furthermore, defects such as vertical cracks, cracks, and voids may be present in part or all of the fibers, and if the presence or absence of defects is included, the morphology of the cross-sectional higher-order structure of pitch-based carbon fibers is complex and diverse. The existence of such various defects and cross-sectional higher-order structures is one of the main causes of deteriorating the quality stability of pitch-based carbon fibers. The occurrence of defects and the formation of cross-sectional higher-order structures as described above vary depending on the physical properties of the spinning pitch, but
It is most greatly influenced and fluctuated by the spinning conditions. Therefore, in order to improve the quality stability of carbon fibers, it is necessary to develop a spinning technology that can always produce carbon fibers with almost no defects and a constant cross-sectional higher-order structure, even if the physical properties of the spinning pitch vary slightly. need to be established. In other words, there is a technology that can stably form onion-type and/or random-type structures, which are high-order cross-sectional structures that are effective for developing high tensile strength and that hardly generate defects such as vertical cracks, cracks, and voids. is necessary. Means for Solving the Problems As a result of extensive research in view of the current state of the technology as described above, the inventor of the present invention has discovered that during melt spinning of pitch-based materials, the molten pitch-based material is spun before reaching the final nozzle hole. When the molten pitch is passed through a capillary part of a specific shape to apply a shear stress of a certain value or more, and then the molten pitch is temporarily held in a state where no shear stress is applied, the molten pitch is passed through a nozzle hole to perform spinning. discovered that the problems of the prior art can be substantially eliminated or greatly reduced. That is,
The present invention is a method for producing pitch-based carbon fiber in which pitch-based material is melt-spun, infusible, and carbonized, and the molten pitch is passed through a circular, irregularly shaped, or slit-type capillary section before reaching the final nozzle hole. After applying a shear stress of 1/2 or more of the shear stress received at the final nozzle hole, the molten pitch is held in a state where no shear stress is applied substantially, and then the molten pitch is passed through the nozzle hole for spinning. The present invention relates to a method for producing pitch-based carbon fibers. According to the present invention, pitch-based graphite fibers can also be produced by graphitizing in the carbonization step. Therefore, in this specification, carbonization includes graphitization,
Carbon fiber is used to include graphite fiber. In the present invention, a shear stress of 1/2 or more of the shear stress experienced at the final nozzle hole is applied by passing the molten pitch-based material through a circular, irregularly shaped, or slit-shaped capillary section provided in front of the final nozzle hole. After that, it is essential to hold the fiber in a state where no shear stress is applied to it, and then pass it through a final nozzle hole for spinning. If the shear stress applied to the molten pitch-based material in the capillary section is less than 1/2 of the shear stress at the final nozzle section, the desired effect will not be sufficiently exhibited. Also,
Even when shearing stress is applied by a method other than the capillary method, for example, through a gap in a dense filler, the desired effect cannot be obtained. Furthermore, after applying shear stress to the molten pitch material in the capillary section,
The effects of the present invention are not exhibited even when spinning is immediately performed from the final nozzle hole without maintaining a state in which shear stress is not substantially applied. The cross-sectional shape of the capillary portion used in the present invention may be circular, slit-shaped (or rectangular), or other irregular shapes (square, cross-shaped, Y-shaped, etc.). The cross-sectional area and length of the capillary are not particularly limited as long as they can apply the necessary shear stress, but usually have a cross-sectional area of 5 x 10 ^-3 to 5 x 10 ²
The length is about 0.1 to 3.0 mm. The time to hold the molten pitch in a state where no shear stress is applied to it between the capillary section and the final nozzle hole depends on the type and properties of the pitch used, the spinning temperature, the pitch discharge amount per unit time, and the capillary. The time required for the molten pitch to pass through the capillary is usually 10 ³ to 10 ⁵ , although it varies depending on the shape of the capillary and the nozzle hole, and is not particularly limited.
A certain amount of time is preferable. In order to avoid applying shear stress to the molten pitch between the capillary part and the final nozzle hole, no shear stress is applied to that part (hereinafter referred to as the stress relaxation part) except for the pack outer wall and/or the nozzle introduction hole. Assume that it is hollowed out. The spinning pitch used in the present invention is obtained by subjecting a pitch-like material to thermal polycondensation under an inert gas flow. The pitch-like substance may be any of petroleum-based pitch, coal-based pitch, and pyrolysis residue pitch from organic compounds. In particular, when coal tar or coal-based pitch such as coal tar pitch is used as a raw material, prior to thermal polycondensation, the raw material pitch must be converted into aromatic 350-500℃ with reducing solvent
The spinnability can be further improved by heat-treating the material, but the pitch for spinning is not particularly limited as long as it is capable of spinning. There is no particular restriction on the cross-sectional area of the final nozzle hole used in the present invention, but it is usually 5×10 ^-3 ~
It is about 10 ^-3 mm ² . In the present invention, the pitch fibers obtained as described above are processed in a conventional manner, for example, in an oxygen atmosphere.
After being infusible at about 300 to 340°C, it is turned into carbon fiber by heating at about 1000 to 3000°C in an inert gas atmosphere. The cross-sectional higher-order structure of the carbon fiber obtained according to the present invention partially or completely exhibits an onion-type structure (see FIGS. 1 and 2). When a portion has an onion-type structure, the onion-type structure exists in the inner layer part, and the random-type structure (FIG. 1a) or radial-type structure (FIG. 1b) exists in the outer layer part. Effects of the Invention According to the present invention, the following effects are achieved. (i) The carbon fiber finally obtained has almost no microscopic defects such as vertical cracks, cracks, and voids inside it. (ii) It has excellent quality stability, so even if the physical properties of the raw material Pitch-based material vary slightly, carbon fibers with a stable cross-sectional higher-order structure in which at least a part of the fiber cross section is onion-shaped can be obtained. . (iii) As a result of (i) and (ii) above, the tensile strength of carbon fibers is greatly improved. (iv) Furthermore, the frequency of yarn breakage during spinning using a multi-hole nozzle is reduced, making stable continuous spinning possible. (vi) Since the surface of the carbon fiber can be changed to onion, random, or radial while maintaining the onion structure inside the carbon fiber, the resin and carbon can be changed in resin composites and carbon composites while maintaining the stable mechanical properties of carbon fiber. Various surface molecular arrangements with good adhesion can be selected. Examples Examples are shown below along with reference examples and comparative examples,
The features of the present invention will be further clarified. Reference Example 1 A mixed solution of 1 part by weight of coal tar pitch with a softening point of 110°C, 0.18% quinoline insoluble content and 35% benzene insoluble content and 2 parts by weight of hydrogenated heavy anthracene oil was stirred in an autoclave at 430°C for 60 minutes. After heating, pass through a pressure filter and then under reduced pressure.
Remove hydrogenated heavy anthranthene oil at 300℃,
I got a reduced pitch. 50 kg of the reduced pitch obtained above was placed in a reactor equipped with a gas inlet tube, thermocouple, stirrer, and distillate removal tube.
and heated to 410-480℃ while stirring and introducing nitrogen gas.
The removal of low molecular weight components and thermal polycondensation were carried out.
The properties of two types of thermal polycondensation pitches obtained by selecting reaction times and temperatures are shown in Table 1 as Nos. 1 and 2. Reference Example 2 The same coal tar pitch as in Reference Example 1 was subjected to a thermal polycondensation reaction in the same manner as in Reference Example 1 without undergoing heat treatment while mixing with hydrogenated heavy anthracene oil. The properties of the resulting thermal polycondensation pitch were determined by No.
3 in Table 1.

[Table] Note: Softening point was measured using a softening point measuring device manufactured by Swiss Mettler.
Examples 1 to 3 A capillary section consisting of 100 thin tubes with a diameter of 0.15 mm and a length of 0.4 mm, a stress relaxation section with a volume of about 15 cm ³ and a diameter
0.2mm, length 0.4mm nozzle (100 nozzle holes)
Thermal polycondensation pitches Nos. 1 to 3 obtained in Reference Examples 1 and 2 were spun using a spinning apparatus equipped with the following. During this spinning process, the pitch is subjected to a shear stress of approximately 250% of the shear stress applied to the final nozzle hole in the capillary section, then placed in a state where it is not subjected to shear stress by stress relaxation, and then shear stress is applied again at the final nozzle hole. Added. The thus obtained pitch fiber was infusible in air at 300°C for 30 minutes, and then heated in an _N2 gas atmosphere for 1200°C.
Carbon fibers were obtained by heating to ℃. Table 2 shows the average time (hr) required to continuously spin pitch fibers with a diameter of 10 μm without yarn breakage.
The cross-sectional higher-order structure and defect content of the carbon fiber obtained above are shown.

[Table] Scanning electron micrographs showing the cross-sectional higher-order structures of the carbon fibers obtained in Examples 1, 2, and 3 are shown in Figure 3 (approximately 2600 times), Figure 4 (approximately 8000 times), and Figure 4 (approximately 8000 times), respectively. It is shown as Figure 5 (approximately 1700x magnification). Examples 4 to 6 Capillary portions having the shape and number shown in Table 3,
Stress relaxation part with a volume of approximately 18 cm ³ , diameter 0.2 mm, length 0.4
Using a spinning device equipped with a mm nozzle (100 nozzle holes), the thermal polycondensation pitch No. obtained in Reference Example 1 was prepared.
1 and the resulting pitch fibers were used in Examples 1 to 3.
Carbon fibers were obtained by infusibility and carbonization treatment in the same manner as above. Table 3 shows the cross-sectional higher-order structure and defect content of the obtained carbon fibers.

[Table] It is clear that even when using a spinning device having a capillary portion with an irregular cross-section, carbon fibers with almost no defects and an inner layer having an onion-type cross-sectional higher-order structure can be obtained. Comparative Examples 1 to 3 Nozzles with a diameter of 0.2 mm and a length of 0.4 mm (number of nozzle holes
Reference Example 1
After spinning the thermal polycondensation pitch Nos. 1 to 3 obtained in 2 and 2, infusibility and carbonization were performed under the same conditions as in Examples 1 to 3 to obtain carbon fibers. Cross-sectional higher-order structure and defect content of the obtained carbon fibers, and average time (hr) for continuous spinning of pitch fibers with a diameter of 10 μm without producing paper breaks
are shown in Table 4. Comparative example 4 The size of the capillary part is 0.3 mm in diameter and 0.6 in length.
The thermopolycondensation pitch obtained in Reference Example 1 was produced by using the same spinning device as in Example 1 except that the diameter was 100 mm (number of fibers).
No. 1 was spun. During this spinning process, a shear stress of approximately 30% of the shear stress experienced at the final nozzle hole was applied to the pitch in the capillary section. The obtained pitch fiber was subjected to infusibility and carbonization treatment under the same conditions as in Example 1 to obtain carbon fiber. The cross-sectional higher-order structure and defect content of carbon fiber are
Shown in the table. In addition, a scanning electron micrograph (approximately 2800 times) showing the cross-sectional higher-order structure of the carbon fiber obtained in this comparative example is shown in the sixth column.
Shown as a diagram. Cracks running along the length of the carbon fiber,
The existence of voids etc. is recognized. Comparative Example 5 A spinning device was constructed in which a capillary section consisting of 100 thin tubes with a diameter of 0.15 mm and a length of 0.4 mm and a nozzle section (number of nozzle holes of 100) with a diameter of 0.2 mm and a length of 0.4 mm were substantially directly connected. Using this, thermal polycondensation pitch No. 1 obtained in Reference Example 1 was spun. In this spinning process,
The pitch was sheared in the capillary section at approximately 250% of the shear stress experienced at the final nozzle hole, and then immediately sheared at the final nozzle hole. The obtained pitch fibers were subjected to infusibility and carbonization treatment under the same conditions as in Examples 1 to 3 to obtain carbon fibers. The cross-sectional higher-order structure and defect content of carbon fiber are
Shown in the table. Further, FIG. 7 shows a scanning electron micrograph (approximately 4000 times magnification) showing the cross-sectional higher-order structure of the carbon fiber obtained in this comparative example. The presence of cracks running in the length direction of the carbon fibers is observed.

[Table] As is clear from the comparison between the results of Examples 1 to 3 shown in Table 2 and the results of Comparative Examples 1 to 3 shown in Table 4, a spinning device without a capillary part and a stress relaxation part was used. In this case, even if the raw material pitch is the same, the continuous spinnability is poor, the cross-sectional higher-order structure is radial, and the defect content is high. As is clear from the comparison between the results of Example 1 shown in Table 2 and the results of Comparative Example 4 shown in Table 4, a shear stress of 1/2 or less of that at the final nozzle hole was applied at the capillary part. In some cases, the content of defects such as vertical cracks and cracks is high. Furthermore, as is clear from the comparison between the results of Example 1 shown in Table 2 and the results of Comparative Example 5 shown in Table 4, a spinning device without a stress relaxation part between the capillary part and the final nozzle part was used. Even in this case, the cross-sectional higher-order structure is radial type and the defect content is high.

[Brief explanation of drawings]

Figures 1 and 2 are schematic diagrams showing the cross-sectional higher-order structure of carbon fibers obtained by the method of the present invention, and Figures 3 to 5 are diagrams of carbon fibers obtained in Examples 1, 2, and 3. FIGS. 6 and 7 are scanning electron micrographs showing cross-sectional higher-order structures, and FIGS.
This is a scanning electron micrograph showing the cross-sectional higher-order structure of the carbon fiber obtained in .

Claims (1)

[Claims] 1. In a method for manufacturing pitch-based carbon fiber in which pitch-based material is melt-spun, infusible, and carbonized, the final nozzle is made by passing the molten pitch through a circular, irregularly shaped, or slit-shaped capillary section before reaching the final nozzle hole. After applying a shear stress of 1/2 or more of the shear stress received in the hole, the molten pitch is once held in a state where no shear stress is applied substantially, and then passed through the nozzle hole to be spun. A method for producing pitch-based carbon fiber.