JPH058959B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a novel method for producing precursor pitches for producing general-purpose carbon fibers. (Prior art) Carbon fiber manufacturing methods include a method of firing synthetic fibers such as polyacrylonitrile and cellulose;
It can be roughly divided into methods of spinning, infusible, and carbonizing using petroleum and coal-based tar pitch-like materials as raw materials. Among these, the former has the drawbacks of high raw material cost and low yield when carbonized. On the other hand, according to the latter method, the raw material, pitch, is a by-product of the coal and oil industries, so it can be obtained in large quantities and has the advantage of being low cost. A process is necessary to adjust the precursor pitch. Additionally, precursor pitches are classified into optically isotropic ones and anisotropic mesophase pitches depending on the characteristics of the target carbon fiber, but the latter precursor pitches are used in the subsequent process. Japanese Patent Publication No. 45-28013 and Japanese Patent Publication No. 1972 show that mesophase pitch with low viscosity and good spinnability can be obtained by hydrogenating the raw material pitch to reduce the viscosity for smooth spinning. −
8634, JP-A-58-18421, and JP-A-60-2352. However, regarding the former optically isotropic precursor pitch,
The current situation is that it has not been resolved yet. This is because if pitch is made with a low viscosity with emphasis on spinnability, its infusibility will be poor; if it is made with high viscosity pitch with advanced polycondensation, with emphasis on infusibility, the quality of the pitch will change as the spinning temperature increases. This is because there remains the contradictory problem that spinning becomes difficult due to such factors. (Problems to be Solved by the Invention) The present invention advantageously solves the above-mentioned problems, and by removing the impurities of pituti together with the mesophase produced by the first heat treatment, it has excellent thermal stability. Use this pitch as the second pitch.
A general-purpose carbon fiber with low viscosity, excellent spinnability, and excellent infusibility, without special treatment such as hydrogenation, by rapid heating and short-time treatment at a relatively high temperature. The purpose of this invention is to produce optically isotropic precursor pitches. (Means for Solving the Problems) The present invention utilizes the fact that free carbon present in tar pitch adheres to the periphery of mesophase spheres generated by heat treatment of pitch, and can efficiently remove solids that are insoluble in solvents. , and that the pitch refined in this way has excellent thermal stability and does not easily generate mesophase even when processed at high temperatures, and that by rapidly raising the temperature to a high temperature and then processing it for a short time, it becomes homogeneous and low-temperature. The present invention was achieved based on the new finding that it is possible to produce pitches with isotropic viscosity. That is, the present invention heat-treats tar pitch soft pitch or medium pitch to 350 to 500°C to generate mesophase, removes solvent-insoluble components including mesophase by solvent fractionation, and removes the solvent to produce quinoline-insoluble components. obtained a trace amount of pituchi,
The method is characterized in that the resulting pitch is then subjected to a heat treatment. Generally, tar pits contain solid content such as free carbon and inorganic substances that are insoluble in solvents. It is well known that if such solid content remains in the precursor pitch for carbon fiber, it will cause nozzle clogging during the spinning process, and even if the fiber is produced by firing after spinning, it will cause a decrease in strength. It is being Therefore, it goes without saying that such solid content should be substantially removed, but there is a problem in that free carbon alone is difficult to settle, and filtering tends to cause clogging. In the present invention, attention is paid to the fact that this solid content adheres to the periphery of mesophase spherules produced when pitch is heat-treated, and the solid content is efficiently separated and removed together with mesophase by sedimentation or filtration. At this time, the amount of mesophase produced is preferably 10 to 30% by weight. That is, if more mesophase is produced, the separation efficiency and the yield of purified pitch will decrease. Additionally, when pitch is heat-treated, mesophase, which is represented by quinoline-insoluble components, is produced as the temperature increases. This mesophase is secondarily generated by the heat treatment of pitch, and like the above-mentioned solid content such as free carbon, it is one of the causes of spinning inhibition. Therefore, the raw material pitch for general-purpose carbon fiber precursor pitch must be a pitch with excellent thermal stability so that mesophase will not be generated during heat treatment. In addition, since the highly reactive polymer components in the pitch are grown into mesophases and separated and removed, the thermal stability at high temperatures is greater than that of ordinary pitches, and the temperature at which mesophases are formed during reheating treatment is lower. It is characterized by being high. Furthermore, in order to produce a low-viscosity isotropic precursor pitch according to the present invention, it is important to use a raw material pitch treated as described above and to select the heat treatment conditions for this raw material pitch. In other words, the raw material pitch is heated at a low rate (5℃/
If the heat treatment is carried out for a long time (less than 1 minute), excessive polycondensation reaction will promote the polymerization of pitch and improve the infusibility of pitch, but the viscosity will increase significantly, resulting in a higher spinning temperature and spinning. The middle pitch tends to deteriorate, making it difficult to perform smooth spinning. However, due to the method of the present invention, a high temperature increase rate (5
Pitch treated for a short time at temperature (℃/min or more) has a characteristic value of benzene insoluble content (BI) of 50%.
If it is increased to more than 55% by weight, the infusibility is good, and the viscosity is low compared to pitch, which has a similar BI value obtained by slow heating rate and long processing, and is suitable for spinning. It can be said that it is a suitable pitch. Furthermore, the heat treatment is carried out under reduced pressure or while passing an inert gas such as nitrogen or argon, or a combination of both to remove low molecular components generated by thermal decomposition during the heat treatment and low molecular weight components originally present in the raw material pitch. It is necessary to actively remove molecular components from the reaction system. That is, such low molecular weight components volatilize during melt spinning, form bubbles, and cause yarn breakage. As described above, the precursor pitch prepared from the raw material pitch according to the present invention has no benzene-insoluble content.
50-65% by weight, quinoline insoluble content is 1% by weight or less,
It has all the characteristics of a softening point of 200 to 240°C, and this precursor pitch can be smoothly spun using normal melt spinning methods, and the carbon fiber obtained from this pitch is sufficient as a general-purpose carbon fiber. It has characteristics. (Effects of the Invention) As described above, in the present invention, the raw material pitch is a pitch with excellent thermal stability in the first heat treatment, and the raw material pitch is treated at a low temperature for a short time in the second heat treatment. It is possible to prepare a low-viscosity precursor pitch that can be spun, and even though it has a low viscosity, it has good infusibility compared to high-viscosity pitch and has sufficient properties. It is possible to produce carbon fibers with excellent performance. That is, in the present invention, a precursor pitch for general-purpose carbon fibers with low viscosity and excellent spinnability and infusibility can be produced. (Example 1) Coal-based tar pitch (softening point: 80°C, quinoline insoluble content: 3%, benzene insoluble content: 17%) was heated at 450°C.
Heat treatment was performed for 60 minutes to generate about 25% mesophase. As is clear from the polarized light micrograph showing the particle structure of the pitch shown in FIG. 1, impurities such as free carbon were attached around the mesophase. This pitch was extracted with tar oil and mesophase and free carbon were filtered off. This filtrate was distilled to obtain a soft pitch having a softening point of 30° C., a trace amount of quinoline insoluble matter, and a benzene insoluble matter of 9%. This pitch is N ₂
The temperature was raised at a rate of 5.7°C/min under a reduced pressure of 10 mmHg under gas flow, and after reaching a temperature of 440°C, it was immediately allowed to cool to obtain a hard pitch A. The analysis results of this pitch A are shown in Table 1. In addition, the obtained soft pitch was heated under _N2 gas flow.
The temperature was raised at a rate of 10.9℃/min under a reduced pressure of 10mmHg,
After reaching 475°C, hard pitch B was obtained by cooling. Further, the analysis results of this pitch B are shown in Table 1. Although both hard pitches A and B were treated at high temperatures, they did not contain mesophase and exhibited isotropy over the entire surface. These pitches A and B are nozzle system 0.3mm,
295°C and 288°C, respectively, under nitrogen pressure using a monohole spinning device with L/D = 3 nozzles.
Pitch fibers were obtained by spinning at a temperature of . Furthermore, these pitch fibers were made infusible at 310°C in air and then carbonized at 1000°C in argon gas to obtain carbon fibers. Table 2 shows the characteristic values of these carbon fibers. (Comparative Example 1) The soft pitch obtained in Example 1 was heated under _N2 gas flow.
The temperature was raised at a rate of 1°C/min under reduced pressure and heat treated at 400°C to obtain hard pitch C. The analytical values of this pitch C are shown in Table 1. When this Pitch C was spun in the same manner as in Example 1, only thick fibers were obtained at a temperature of 332°C. Table 2 shows the characteristic values of the carbon fiber obtained by carbonizing the pitch fiber. (Comparative Example 2) The tar pitch used in Example 1 was not heat-treated,
Tar oil was added, extracted and filtered, and further distilled to obtain pithu containing no quinoline insoluble matter. This pitch had a softening point of 32°C and a benzene insoluble content of 10%.
This pitch was heat-treated under the same conditions as hard pitch A of Example 1 to obtain hard pitch D. Table 1 shows the analytical values of this pitch. This hard pitch D contained mesophase spheres of several μm.

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a polarized light micrograph showing the grain structure of pitch.

Claims (1)

[Claims] 1. Tar pitch soft pitch or medium pitch 350
Heat-treated at ~500°C to produce mesophase, solvent fractionation to remove solvent-insoluble components including mesophase, solvent removal to obtain pitches with trace amounts of quinoline-insoluble components, and then the resulting pitches. 1. A method for producing a precursor pitch for carbon fiber, which comprises heat-treating. 2. The heat treatment is carried out under reduced pressure, under inert gas flow, or a combination of both.
The method according to claim 1, which is carried out in a temperature range of 500°C. 3. The method according to claim 1, wherein in the heat treatment, the heating rate to reach the predetermined temperature is 5° C./min or more, and the holding time at the predetermined heat treatment temperature is 10 minutes or less. 4 The precursor pitch obtained by the heat treatment has an optically isotropic structure on the entire surface, is substantially free of mesophase, has a benzene insoluble content (BI) of 50% to 65% by weight, and has a quinoline insoluble content of 1%. The method according to claim 1, wherein the softening point is in the range of 200 to 240°C.