Mesophase pitch for carbon fiber and method for producing mesophase-pitch-based carbon fiber

Abstract

A method for producing mesophase pitch comprising: step A of performing pressurized heat-treatment to heavy residual oil, then performing inert gas blowing heat-treatment to prepare pitch component a comprising mesogenic component being anisotropic component and solvent component being isotropic component; step B of performing pressurized heat-treatment to heavy residual oil, then performing inert gas blowing heat-treatment to prepare pitch component a' comprising mesogenic component being anisotropic component and solvent component being isotropic component, separating mesogenic component from the pitch component a' to obtain pitch component b mainly comprising mesogenic component; and step C of mixing pitch component a comprising mesogenic component and solvent component obtained in the step A, and pitch component b mainly comprising mesogenic component obtained in the step B and performing heat-treatment to the mixture.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for producing mesophase pitch for carbon fiber, and a method for producing mesophase pitch-based carbon fiber using mesophase pitch for carbon fiber obtained by such method.BACKGROUND ART

[0002] Mesophase pitch-based carbon fiber which is one of representative carbon fibers is known to have high specific strength and specific modulus. This mesophase pitch is generally produced by using coal and heavy residual oil generated from distillation or retorting of petroleum as raw material, by undergoing heat treatment process. Specifically, during the heat-treatment process, to the raw material being isotropic, anisotropic spherocrystal gradually generate, and as they repeat generation, growth, and incorporation, they finally change to anisotropic pitch, and thus mesophase pitch is produced.

[0003] For example, mesophase pitch for carbon fiber can be obtained by using coal-tar (CT) or petroleum degrading heavy oil (slurry oil, SO) as raw material, and performing optimization of the structure by hydrogenation, appropriate condensation polymerization and promotion of molecular stacking by heat treatment (see Patent Literature 1). In the hydrogenation step, by introducing naphthene structure to aromatic compounds contained in the raw material, flowability of pitch in melt spinning is increased to confer spinnability to mesophase pitch. In the subsequent heat treatment step, by ensuring the introduced naphthene structure, appropriate condensation polymerization reaction is induced to form an aromatic compound having a high planarity. When these formed compounds are layered in the (002) direction, anisotropy is generated and mesophase pitch is obtained.

[0004] Mesophase pitch for carbon fiber is produced by undergoing such steps, while in the method using the hydrogenation treatment, there is a problem that the raw material yield of the obtained mesophase pitch is low. For example, the raw material yield of the metaphase pitch produced by undergoing a moderate hydrogenation step is about 15 wt%. When producing metaphase pitch having a further high spinnability (no thread breakage at the time of melt spinning), it is necessary to increase the hydrogenation level, and the raw material yield of the mesophase pitch would be lower.

[0005] It is thought that the factor of the decrease of the raw material yield is that due to the hydrogenation treatment, a part of mesogenic components which are high molecular components change to solvent components by low molecularization, and that solvent components are excessively increased. On the other hand, even if the level of hydrogenation is lowered and low molecularization in the hydrogenation treatment is suppressed, heat treatment under strict conditions would be necessary to prepare anisotropic mesophase pitch. Thereby, non-fusible solid parts would generate, and since amount or types of solvent components would be insufficient, spinnability could not be guaranteed.

[0006] Due to such low raw material yield of mesophase pitch, the cost for producing carbon fiber is significantly increasing, which is an obstacle for carbon fiber to be widely spread.

[0007] To solve the above problem, production methods that do not use hydrogenation treatment have been proposed, and one representative process is heat treatment synthesis of HF / BF 3 using naphthalene as raw material (see non-patent Literature 1). By using this process, it is possible to produce mesophase pitch for carbon fiber without undergoing hydrogenation step, and mesophase pitch for carbon fiber produced by such process was actually commercialized and available in the market.

[0008] However, since this process uses naphthalene being more expensive as compared to heavy residual oil derived from coal or from petroleum as raw material, and requires particular and expensive equipment, incurring cost for its maintenance, etc. the cost for producing carbon fiber could not be sufficiently reduced due to these problems.CITATION LISTPatent Literature

[0009] [Patent Literature 1] Japanese Patent No. 5956610Non-Patent Literature

[0010] [Non-Patent Literature 1] Mochida I., Shimizu K., Korai Y., Otsuka H., Sakai Y., Fujiyama S., Preparation of mesophase pitch from aromatic hydrocarbons by the aid of HF / BF3, Carbon 1990; 28(2-3):311-319.SUMMARY OF THE INVENTIONTECHNICAL PROBLEM

[0011] Under such circumstances, for producing mesophase pitch-based carbon fiber at a low cost, a new process for producing mesophase pitch, a precursor thereof, without using hydrogenation treatment, or expensive raw material and particular equipment as in the above is awaited.

[0012] The object of the present invention is to provide a method for producing mesophase pitch which is a precursor of mesophase pitch-based carbon fiber at a low cost.SOLUTION TO PROBLEM

[0013] To solve the above-mentioned object, the present inventors focused on the lyotropic liquid crystalline characteristics of mesophase pitch. Here, mesophase pitch is generally composed of two components, that is solvent components being low-molecular-weight components and mesogenic (solute) components being high-molecular-weight components, and the lyotropic liquid crystalline characteristics refer to the characteristic wherein as the concentration of mesogenic components increases, the content of anisotropic texture in the pitch increases, and when it reaches a certain concentration (critical mesogenic component concentration) or more, anisotropic texture drastically increases and texture having a high anisotropic content ratio appear.

[0014] The present inventors have found that mesophase pitch for carbon fiber having anisotropic texture can be produced at a low cost by performing heat treatment after mixing pitch partially comprising mesogenic components having expressed anisotropic texture by performing heat treatment under mild and appropriate conditions (pitch component a), and mesogenic components (pitch component b) prepared separately. The present invention has been thus completed.

[0015] Specifically, according to the production method of the present invention, by adding pitch component b mainly comprising mesogenic components to pitch component a comprising a certain level of mesogenic components and a wide variety of solvent components in both types and quantity, mesogenic component concentration of the mixed pitch components attain a critical mesogenic component concentration or more, and mesophase pitch for carbon fiber having anisotropic texture can be obtained.

[0016] Specifically, the present invention is as follows. [1] A method for producing mesophase pitch for carbon fiber comprising: step A of performing pressurized heat-treatment to heavy residual oil, then performing inert gas blowing heat-treatment to prepare pitch component a comprising mesogenic component being anisotropic component and solvent component being isotropic component; step B of performing pressurized heat-treatment to heavy residual oil, then performing inert gas blowing heat-treatment to prepare pitch component a' comprising mesogenic component being anisotropic component and solvent component being isotropic component, separating mesogenic component from the pitch component a' to obtain pitch component b mainly comprising mesogenic component; and step C of mixing pitch component a comprising mesogenic component and solvent component obtained in the step A, and pitch component b mainly comprising mesogenic component obtained in the step B and performing heat-treatment to the mixture. [2] The method for producing mesophase pitch for carbon fiber according to the above [1], wherein the pitch component a comprises 50 wt% or more of solvent component. [3] The method for producing mesophase pitch for carbon fiber according to the above [1] or [2], wherein the method of separating mesogenic component in the step B is a solvent extraction method. [4] The method for producing mesophase pitch for carbon fiber according to the above [3], wherein the solvent used in the solvent extraction method is at least one type selected from tetrahydrofuran (THF), quinoline, pyridine, and dimethylformamide. [5] The method for producing mesophase pitch for carbon fiber according to any one of the above [1] to [4], wherein in the step A, reduced pressure distillation treatment is performed after inert gas blowing heat-treatment. [6] The method for producing mesophase pitch for carbon fiber according to any one of the above [1] to [5], wherein the pressurized heat-treatment in the step A and / or step B is performed at 250 to 450°C in a sealed container. [7] The method for producing mesophase pitch for carbon fiber according to any one of the above [1] to [6], wherein the inert gas blowing heat-treatment in the step A and / or step B is performed at 350 to 500°C. [8] The method for producing mesophase pitch for carbon fiber according to any one of the above [1] to [7], wherein the heat-treatment in the step C is performed at 300 to 500°C. [9] The method for producing mesophase pitch for carbon fiber according to any one of the above [1] to [8], wherein in the step C, the pitch component a and the pitch component b are mixed so that an initial mesogenic component in the mixed component becomes 50 wt% or more.

[10] A method for producing mesophase pitch-based carbon fiber comprising: step D of melt spinning mesophase pitch for carbon fiber produced by the method for producing mesophase pitch according to any one of the above [1] to [9], step E of stabilizing melt spinning mesophase pitch, and step F of carbonizing and graphitizing stabilized mesophase pitch. ADVANTAGEOUS EFFECTS OF INVENTION

[0017] According to the production method of the present invention, mesophase pitch which is a precursor of mesophase pitch-based carbon fiber can be produced at a low cost, and thereby mesophase pitch-based carbon fiber can be produced at a low cost.BRIEF DESCRIPTION OF DRAWINGS

[0018] [Figure 1] It is a figure showing the summary of the method for producing mesophase pitch for carbon fiber of the present invention. [Figure 2] It is a figure showing the summary of the production procedure of mesophase pitch for carbon fiber of Example 1. [Figure 3] It is a figure showing the summary of the production procedure of mesophase pitch for carbon fiber of Example 2. [Figure 4] It is a figure showing the summary of the production procedure of mesophase pitch for carbon fiber of Comparative Examples 1 and 2. [Figure 5] These are photographs showing the polarized optical microscopy images of mesophase pitch for carbon fiber of Examples 1-2, and Comparative Examples 1-2, and the results of raw material yields and softening points. DESCRIPTION OF EMBODIMENTS

[0019] The method for producing mesophase pitch for carbon fiber of the present invention is characterized by comprising: step A of performing pressurized heat-treatment to heavy residual oil, then performing inert gas blowing heat-treatment to prepare pitch component a comprising mesogenic component being anisotropic component and solvent component being isotropic component; step B of performing pressurized heat-treatment to heavy residual oil, then performing inert gas blowing heat-treatment to prepare pitch component a' comprising mesogenic component being anisotropic component and solvent component being isotropic component, separating mesogenic component from the pitch component a' to obtain pitch component b mainly comprising mesogenic component; step C of mixing pitch component a comprising mesogenic component and solvent component obtained in the step A and pitch component b mainly comprising mesogenic component obtained in the step B, and performing heat-treatment to the mixture.

[0020] Meanwhile the method for producing mesophase pitch for carbon fiber of the present invention may comprise steps other than the above steps A to C, such as pre-treatment, etc. performed before step A.

[0021] The production method of the present invention is an unprecedented novel procedure utilizing lyotropic liquid crystalline characteristics, and can produce mesophase pitch at a high yield with a common, simple heat-treatment procedure such as pressurized heat-treatment or inert gas blowing gas heat-treatment, without using hydrogenation treatment or using expensive raw material and particular equipment.

[0022] The raw material that can be used in the production method of the present invention is heavy residual oil, and the origin is not particularly limited as long as it can produce mesophase pitch. For example, those derived from petroleum such as SO or CT, coal, and those derived from biomass, those derived from plastic waste, those derived from industrial waste can be exemplified.

[0023] Fig. 1 shows the summary of the method for producing mesophase pitch for carbon fiber of the present invention.

[0024] In the following, each step is explained in detail.[Step A]

[0025] Step A is a step of performing pressurized heat-treatment to heavy residual oil, then performing inert gas blowing heat-treatment to prepare pitch component a comprising mesogenic component being anisotropic component and solvent component being isotropic component.

[0026] Here, pitch component a prepared in step A preferably comprises 50 wt% or more of solvent component, more preferably 50 to 80 wt%, and further preferably 55 to 75 wt%.(Pressurized heat-treatment)

[0027] Pressurized heat-treatment is a step of polymerizing low-molecular-component (solvent component) that can be easily removed by the inert gas blowing heat-treatment, the subsequent step, by polymerization reaction.

[0028] It is preferable to perform a treatment of removing impurities (solid part) in heavy residual oil, as a pre-treatment of pressurized heat-treatment. Examples of such treatment include, for example, treatment of removing insoluble parts that do not dissolve in THF. As for heavy residual oil / THF (weight ratio), it is preferably about 1 / 5 to 1 / 10.

[0029] Specifically, pressurized heat-treatment is performed in a sealed container of an inert gas atmosphere. The heat-treatment temperature is for example, preferably 250 to 450°C, more preferably 280 to 420°C, further preferably 300 to 400°C, and particularly preferably 300 to 350°C. Lower the temperature is, the polymerization reaction of low molecular component is mild, and the content and types of solvent components contained in pitch component a finally produced would be abundant. On the other hand, higher the temperature is, polymerization reaction of low molecular components will occur severely, and many mesogenic components will be produced. Therefore, by performing within the above temperature range, pitch component a having a more well-balanced content of mesogenic component and solvent component can be obtained.(Inert gas blowing heat-treatment)

[0030] Inert gas blowing heat-treatment is a step of promoting expression-growth of anisotropic texture, by performing dealkylation and appropriate polymerization reaction while suppressing coking.

[0031] Specifically, inert gas blowing heat-treatment is performed by performing blowing of nitrogen, or noble gas such as helium, argon, etc. As for the blowing amount of inert gas, it is preferably 50 mL / min or more, more preferably 80 to 2000 mL / min or more, further preferably 100 to 1500 mL / min, and particularly preferably 150 to 1000 mL / min, with respect to 100 g of raw material heavy residual oil. By performing heat-treatment with such blowing amount, it is possible to increase the raw material yield of pitch component a by suppressing generation of coking.

[0032] As for the heat-treatment temperature, for example, it is preferably 350 to 500°C, more preferably 380 to 450°C, and further preferably 390 to 430°C. By performing heat-treatment at such temperature range, polymerization reaction is promoted, while suppressing generation of coking, and anisotropic texture is developed. Meanwhile, since when the temperature is high, coking is easily generated, in case of high temperature, partial cooling effect by blowing of inert gas becomes important.(Reduced-pressure distillation)

[0033] It is preferable to perform reduced-pressure distillation treatment after inert gas blowing heat-treatment. This reduced-pressure distillation treatment removes a small amount of volatile components which are isotropic components (low molecular components) in pitch component a which have undergone inert gas blowing treatment, to increase heat stability of pitch components at the time of mixed heat-treatment in step C, the subsequent step, and melt spinning of mesophase pitch for carbon fiber.

[0034] The level of reduced pressure is preferably 100 hPa or less, more preferably 50 hPa or less, and further preferably 30 hPa or less. As for heat-treatment temperature, it is preferably 300 to 500°C, and more preferably 350 to 450°C. Further, specifically, it is preferable to use thin layer reduced-pressure distillation method.[Step B]

[0035] Step B is a step of performing pressurized heat-treatment to heavy residual oil, then performing inert gas blowing heat-treatment to prepare pitch component a' comprising mesogenic component being anisotropic component and solvent component being isotropic component, then separating mesogenic component from the pitch component a' to obtain pitch component b mainly comprising mesogenic component. Meanwhile, pitch component b mainly comprising mesogenic component refers to those comprising 90 wt% or more of mesogenic component in pitch component b, preferably those comprising 95 wt%, particularly preferably those substantially consisting of only mesogenic component. Here the ratio of mesogenic component can be measured by a polarized optical microscope.

[0036] Here, as pitch component a' in step B, pitch component a prepared in step A can be used. Specifically, pitch component a and pitch component a' can be obtained in one step. On the other hand, in case of preparing pitch component a' separately from pitch component a, it is preferable to prepare so that mesogenic component is largely contained in pitch component a', and it is preferable that 50 wt% or more of mesogenic component is contained, more preferable that 60 wt% or more is contained, and further preferable that 70 wt% or more is contained.(Pressurized heat-treatment)

[0037] Pressurized heat-treatment in step B may be performed under the same conditions as the pressurized heat-treatment in step A, or may be performed under different conditions. In case of preparing pitch component a' separately from pitch component a, etc., since the purpose of step b is to prepare mesogenic component, it is preferable to perform heat-treatment at a relatively high temperature, and as for the heat-treatment temperature, it is preferably 300 to 450°C, more preferably 350 to 450°C, and particularly preferably 350 to 400°C.(Inert gas blowing heat-treatment)

[0038] Inert gas blowing heat-treatment in step B is basically a treatment similar as the inert gas blowing heat-treatment in step A. In case of preparing pitch component a' separately from pitch component a, it may be performed under different conditions. Further, since the purpose of step B is to obtain mesogenic components, it is not always necessary to perform reduced pressure distillation.(Mesogenic component-separation treatment)

[0039] Mesogenic component-separation treatment is not particularly limited as long as it is a treatment that can separate mesogenic component from pitch component a' comprising mesogenic component and solvent component prepared by undergoing the above pressurized heat-treatment and inert gas blowing heat-treatment, and solvent extraction method is preferable. In the solvent extraction method, mesogenic components can be obtained as insoluble parts.

[0040] As for solvent used in the solvent extraction method, examples include THF, quinoline, pyridine, dimethylformamide, etc., and THF is preferable.[Step C]

[0041] Step C is a step of mixing pitch component a comprising mesogenic component and solvent component obtained in step A and pitch component b mainly comprising mesogenic component obtained in step B, and performing heat-treatment to the mixture. In this step, by adding pitch component b mainly comprising mesogenic component, mesogenic component concentration in pitch component a is increased, and by utilizing the lyotropic liquid crystalline characteristics, mesophase pitch for carbon fiber is produced.

[0042] The herein produced mesophase pitch for carbon fiber refers to those comprising 80 vol% or more of anisotropic texture, and those comprising 90 vol% or more are preferable, those comprising 95 vol% or more are further preferable, and those substantially consisting of only anisotropic texture (anisotropic texture 100 vol%) are particularly preferable. Here, the proportion of anisotropic texture can be calculated by the area calculation of anisotropic texture using polarized optical microscopic pictures of the entire pitch.

[0043] The mixing ratio of pitch component a and pitch component b depends on the concentration of mesogenic component of pitch component a and / or pitch component b, while it is preferable to mix pitch component a and pitch component b so that the mesogenic component in the mixed components becomes 50 wt% or more, more preferable to mix so that it becomes 55 wt% or more, and further preferable to mix so that it becomes 60 to 80 wt%. Examples of mixing methods include a method of using a mortar, a method of using a mechanical mill, etc.

[0044] The heat-treatment is performed in an inert gas atmosphere or by blowing inert gas, and the heat-treatment temperature is preferably 300 to 500°C, more preferably 320 to 450°C, and further preferably 350 to 410°C.

[0045] By using mesophase pitch for carbon fiber produced by the above-mentioned production method, mesophase pitch-based carbon fiber can be produced. As its production method, conventionally known methods can be used.

[0046] For example, as a method for producing mesophase pitch-based carbon fiber, a method comprising step D of melt spinning mesophase pitch for carbon fiber produced by the above-mentioned production method, step E of stabilizing melt spinning mesophase pitch, and step F of carbonizing and graphitizing the stabilized mesophase pitch can be exemplified.

[0047] In step D (spinning step), for example, the molten mesophase pitch is extruded from the pipe sleeve of spinning apparatus, elongated to make a long and thin fiber state. In step E (stabilizing step), for example, the melt spinning mesophase pitch is passed through air of a temperature of about 200 to 350°C (oxidized gas), to perform oxygen cross linking. In step F (carbonizing graphitizing step), for example, in an inert gas of about 1000 to 1500°C, stabilized pitch is heat-treated in a tension state for carbonization, and further, in an inert gas of about 2000 to 3000°C, it is heat-treated in a tension state for graphitizing. After graphitizing, surface treatment, sizing, etc. are performed to obtain carbon fiber.EXAMPLES

[0048] In the following, the present invention will be explained in detail based on Examples. However, the scope of the present invention is not limited to the following examples.

[0049] Mesophase pitch for carbon fiber of the present invention has been produced by the following procedures. The summary is shown in Figs. 2 to 4.<Preparation of pitch component a (Step A)>(Pre-treatment)

[0050] Using SO as raw material, SO 300 g and THF 2700 g were mixed (weight ratio 1:9). The mixture was stirred for 6 hours, and then, the mixture of THF soluble part of SO and THF was recovered by reduced pressure filtration. The recovered mixture was set in a rotary evaporator, stirred at a rotation speed of 100 rpm, to recover THF under conditions of water bath temperature of 25°C and pressure of 200 hPa. Then, to completely remove THF from the mixture, it was heated under conditions of water bath temperature of 60°C and pressure of 100 hPa, and reduced pressure distillation was performed until no THF dropped in the recovery trap. Finally, THF soluble part of SO was recovered.(Pressurized heat-treatment)

[0051] THF soluble part of SO 100 g was put in an autoclave container 300 mL type. Nitrogen substitution was performed about 10 times until gas in the autoclave was filled with nitrogen. Stirring speed was set to 150 rpm, and the temperature was raised to 320°C under condition of self-pressure. After reaching 320°C, it was left for 3 hours, and was air-cooled until it decreased to room temperature to recover pressure-treated THF soluble part of SO.(Inert gas blowing treatment)

[0052] To a glass container for nitrogen blowing heat-treatment (inner diameter: 40 mm, height: about 300 mm, position of branched outlet: about 200 mm from the bottom, inner diameter of branched outlet: 20 mm), 50 g of THF soluble part of SO that has undergone pressurized heat-treatment was put. It was set so that the tip of stainless-steel tube with wings (length from the tube center to wing tip: 10 mm) was near the bottom of the container, stirred at 100 rpm, and nitrogen gas was flowed into the stainless-steel tube at 200 mL / min. Heat treatment was performed under conditions of raising the temperature by 5°C / min to 415°C, and holding time of 6 hours, then air cooling was performed, to recover SO-derived pitch that has remained in the container.(Reduced pressure distillation treatment)

[0053] 5 g of recovered pitch was put in a round bottom glass container (inner diameter: 30 mm, height: about 250 mm), set to a rotary evaporator, and the whole container was depressurized to 10 hPa. At that time, the glass container itself was tilted about 30° from horizontal. The glass container containing pitch was rotated at 100 rpm, and by using a ceramic electric furnace, it was heated under conditions of raising the temperature by 5°C / min to 370°C, and holding time of 10 min. Then, air cooling was performed, to recover pitch components to be used for mixed heat-treatment.

[0054] The content of mesogenic component in pitch component a(s) comprising SO-derived mesogenic components (anisotropic texture component) prepared in this Example was 35 wt%, and the raw material yield was 36.0 wt%.<Preparation of pitch component b (Step B)>

[0055] For pitch component b mainly comprising mesogenic component, SO-derived pitch component b(s) and CT-derived pitch component b(c) were prepared.(1) Preparation of pitch component b(s)

[0056] Preparation of pitch component a'(s) for preparing SO-derived pitch component b(s) was performed similarly as the above method for preparing pitch component a except for setting the pressurized heat-treatment temperature to 340°C, and the temperature of thin layer reduced pressure distillation to 390°C.

[0057] Next, prepared SO-derived pitch component a'(s) was ground in a mortar until it became a powdered state. It was mixed so that the weight ratio of the powdered state pitch component a'(s) and THF of 50°C becomes 1 / 9 (wt / wt), and the mixture was stirred for 24 hours. After stirring, reduced pressure filtration was performed, and reduced pressure drying at 60°C (reduced pressure gauge: -0.1MPa) was performed for 6 hours to solid part remained on the filter. Finally, pitch component b(s) being insoluble part was recovered.

[0058] The raw material yield of the recovered SO-derived pitch component b(s) was 18.4 wt%.(2) Preparation of pitch component b(c)

[0059] Preparation of pitch component a'(c) for preparing CT-derived pitch component b(c) was performed similarly as the above method for preparing pitch component a except for setting the temperature of pressurized heat-treatment temperature to 370°C, and the temperature of thin layer reduced pressure distillation to 390°C.

[0060] Then, similarly as the above preparation of pitch component b(s), pitch component b(c) being insoluble part of THF was recovered. The raw material yield of the recovered CT-derived pitch component b(c) was 32.7 wt%.<Preparation of mesophase pitch for carbon fiber (Step C)>

[0061] By using SO-derived pitch component a(s) and SO-derived pitch component b(s), mesophase pitch for carbon fiber (SS-MP) of Example 1 was prepared. Further, by using SO-derived pitch component a(s) and CT-derived pitch component b(c), mesophase pitch for carbon fiber (CS-MP) of Example 2 was prepared.(1) Preparation of SS-MP of Example 1

[0062] The prepared pitch component a(s) comprising SO-derived anisotropic texture 2.8 g and SO-derived pitch component b(s) 1.2 g were mixed using a mortar (weight ratio 3:7). To make mesophase pitch for carbon fiber having anisotropic texture amount of 80 vol% or more, it is necessary that the mesogenic component concentration of the mixture is 50 wt% or more. By calculating the mesogenic component concentration of the mixture, Mesogenic component concentration = 35 × 0.7 + 30 = 54.5 wt % and it was 50 wt% or more.

[0063] By putting this mixture in a round bottom glass container (inner diameter: 30 mm, height: about 250 mm), the glass container itself was tilted about 30° from horizontal. The glass container containing pitch was rotated at 100 rpm, and the whole container was filled with nitrogen with nitrogen gas 100 mL / min. By using a ceramic electric furnace, it was heated under conditions of raising the temperature by 5°C / min to 370°C, and holding time of 10 min. After heating, air-cooling was performed, to recover SS-MP of Example 1.(2) Preparation of CS-MP of Example 2

[0064] CS-MP of Example 2 was recovered in the same manner as the above-mentioned preparation of SS-MP of Example 1, except that the heat-treatment temperature was set to 390°C.<Preparation of mesophase pitch using hydrogenation treatment (conventional method)>

[0065] By using SO, with the conventional method, mesophase pitch for carbon fiber (HS-MP) of Comparative Example 1 was prepared. Further, by using CT, with the conventional method, mesophase pitch for carbon fiber (HC-MP) of Comparative Example 2 was prepared.(1) Preparation of HS-MP of Comparative Example 1

[0066] Pre-treatment similar as the pre-treatment for the preparation of pitch component a was performed to recover THF soluble part of SO.

[0067] Liquid in which THF soluble part of SO 75 g and tetralin 75 g are mixed was put in an autoclave container 300 mL type (weight ratio 1:1). Nitrogen substitution was performed about 10 times until gas in the autoclave was filled with nitrogen. Stirring speed was set to 150 rpm, and the temperature was raised to 370°C under condition of self-pressure. After reaching 370°C, it was left for 1 hour, and was air-cooled until it decreased to room temperature to recover hydrogenated THF soluble part of SO and tetralin.

[0068] To remove tetralin-naphthalene from the mixture of THF soluble part and tetralin-naphthalene, reduced pressure distillation of 200°C was performed until tetralin-naphthalene dropped to the recovery trap becomes a similar level as the initial additional amount of tetralin.

[0069] Then, by using hydrogenated soluble part 50 g, similarly as the nitrogen blowing heat-treatment for preparing pitch component a, nitrogen blowing heat-treatment was performed under heat-treatment conditions of raising the temperature by 5°C / min to 415°C, and holding time of 6 hours, at nitrogen gas flow rate of 600 mL / min.

[0070] Further, similarly as the thin layer reduced pressure distillation for preparing pitch component a, thin layer reduced pressure distillation was performed under heat-treatment conditions of raising the temperature by 5°C / min to 390°C, holding time of 10 min, 10 hPa. Air-cooling was performed, to recover HS-MP of Comparative Example 1.(2) Preparation of HC-MP of Comparative Example 2

[0071] HC-MP of Comparative Example 2 was prepared similarly as HS-MP of Comparative Example 1 except that the heat-treatment temperature of hydrogenation was set to 450°C.<Evaluation of properties, performance of mesophase pitch>(Raw material yield and softening point)

[0072] Figure 5 shows the polarized optical microscopy images of the produced mesophase pitch for carbon fiber of Examples 1 and 2, and mesophase pitch for carbon fiber of Comparative Examples 1 and 2, and the results of raw material yield and softening point.

[0073] As shown in Figure 5, the raw material yield and softening point of SS-MP of Example 1 were 30.2 wt%, 276°C, respectively. The raw material yield and softening point of CS-MP of Example 2 were 34.7%, 281°C, respectively. SS-MP of Example 1 and CS-MP of Example 2 were both mesophase pitch comprising about 90 vol% of anisotropic texture.

[0074] On the other hand, the raw material yield and softening point of HS-MP of Comparative Example 1 were 13.1 wt%, 245°C, respectively, and the raw material yield and softening point of HC-MP of Comparative Example 2 were 21.4 wt%, 258°C, respectively. HS-MP of Comparative Example 1 and HC-MP of Comparative Example 2 were similarly mesophase pitch comprising about 90 vol% of anisotropic texture.

[0075] Compared with the mesophase pitch for carbon fiber of the Comparative Example prepared by undergoing hydrogenation, the mesophase pitch for carbon fiber of the Example prepared by the technique of the present patent showed a high level of yield of 30 wt% or more, while the softening point was within the scope possible to melting spinning (softening point 300°C or less).(Spinning evaluation)

[0076] Spinning evaluation of the prepared mesophase pitch for carbon fiber was performed by using mono-pore melt spinning apparatus.

[0077] Specifically, first, the prepared pitch was put in a spinning device attached with a nozzle (length of mono pore / diameter = 0.5 mm / 0.5 mm = 1,60°), while filling with nitrogen gas, it was heated at a temperature of about softening point of the pitch + 100°C. Then, nitrogen pressure in the spinning machine was adjusted so that the discharge amount of the pitch discharged from the nozzle becomes 50 mg / min. Finally, the discharged pitch was wound to the winder (rotating speed: 400 rpm). At that time, the number of times of thread breakage during 3 minutes from the initiation of winding was measured. Fewer the number of times of thread breakage is, it means that spinning property is high. Table 1 shows the number of times of thread breakage and the average fiber diameter of spinning fiber at each winding speed of each of the prepared pitch. [Table 1]Number of times of thread breakage [times / 3 min]Spinning fiber diameter [µm]Winding speed400 rpm400 rpmExample 1 SS-MP215.0± 0.7Example 2 CS-MP1315.2± 1.0Comparative Example 1 HS-MP015.0± 0.8Comparative Example 2 HC-MP314.9± 0.8

[0078] As shown in Table 1, it can be understood that while the number of times of thread breakage of CS-MP of Example 2 is relatively high, SS-MP of Example 1 had a higher spinning property than HC-MP, and is close to the spinning property of HS-MP. As such, it can be understood that mesophase pitch for carbon fiber can be produced, even without performing hydrogenation.<Production of carbon fiber>

[0079] To the spinning fiber prepared in the above spinning evaluation, stabilization, carbonizing and graphitizing were performed.

[0080] Specifically, stabilization was performed by using ceramic electric furnace and quartz furnace tube, in an air atmosphere at 270°C, 0.5°C; carbonization was performed by using a large size graphite furnace, raising the temperature under vacuum to 1000°C at 20°C / min., and a holding time of 30 min; graphitization was performed by raising the temperature in an argon atmosphere to 2800°C at 15°C / min, with a holding time of 10 min., to produce carbon fiber.<Evaluation of properties, performance of carbon fiber>

[0081] Single fiber strength test was performed to carbon fiber prepared by graphitizing according to JIS R7606:2000. The average fiber diameter and strength of the prepared fibers are shown in Table 2. [Table 2]StabilizationCarbonizationGraphitizationWinding speedyieldyieldyieldFiber diameterTensile strengthElongation ratioYoung's modulus of elasticity[rpm][wt%][wt%][wt%][µm][GPa][%][GPa]Example 1 SS-MP400112.786.285.211.3±0.71.8±0.50.8±0.2250±30Example 2 CS-MP400106.787.087.012.3±1.02.6±0.80.5±0.1510±100Comparative Example 1 HS-MP400111.086.485.911.6± 0.81.9±0.70.7±0.2300± 100Comparative Example 2 HC-MP400105.188.687.911.8±0.92.8±0.80.6±0.2500± 100

[0082] As shown in Table 2, it can be seen that the tensile strength of SS-MP derived carbon fiber of Example 1 shows similar tensile strength as HS-MP derived carbon fiber of Comparative Example 1. Further, CS-MP derived carbon fiber of Example 2 was equivalent to HC-MP derived carbon fiber of Comparative Example 2. Since CT-derived mesogenic component originally contains a large amount of aromatic compounds having higher planarity than SO-derived, graphitizing structure is easily developed. From that result, by selecting optimum raw material for each of pitch component a and pitch component b, it can be expected to obtain optimum spinnability or mechanical properties of carbon fiber.INDUSTRIAL APPLICABILITY

[0083] Since the present invention can produce mesophase pitch usable in the production of carbon fiber at a low cost, it is industrially applicable.

Claims

1. A method for producing mesophase pitch for carbon fiber comprising: step A of performing pressurized heat-treatment to heavy residual oil, then performing inert gas blowing heat-treatment to prepare pitch component a comprising mesogenic component being anisotropic component and solvent component being isotropic component; step B of performing pressurized heat-treatment to heavy residual oil, then performing inert gas blowing heat-treatment to prepare pitch component a' comprising mesogenic component being anisotropic component and solvent component being isotropic component, separating mesogenic component from the pitch component a' to obtain pitch component b mainly comprising mesogenic component; and step C of mixing pitch component a comprising mesogenic component and solvent component obtained in the step A, and pitch component b mainly comprising mesogenic component obtained in the step B and performing heat-treatment to the mixture.

2. The method for producing mesophase pitch for carbon fiber according to claim 1, wherein the pitch component a comprises 50 wt% or more of solvent component.

3. The method for producing mesophase pitch for carbon fiber according to claim 1, wherein the method of separating mesogenic component in the step B is a solvent extraction method.

4. The method for producing mesophase pitch for carbon fiber according to claim 3, wherein the solvent used in the solvent extraction method is at least one type selected from tetrahydrofuran, quinoline, pyridine, and dimethylformamide.

5. The method for producing mesophase pitch for carbon fiber according to claim 1, wherein in the step A, reduced pressure distillation treatment is performed after inert gas blowing heat-treatment.

6. The method for producing mesophase pitch for carbon fiber according to claim 1, wherein the pressurized heat-treatment in the step A and / or step B is performed at 250 to 450°C in a sealed container.

7. The method for producing mesophase pitch for carbon fiber according to claim 1, wherein the inert gas blowing heat-treatment in the step A and / or step B is performed at 350 to 500°C.

8. The method for producing mesophase pitch for carbon fiber according to claim 1, wherein the heat-treatment in the step C is performed at 300 to 500°C.

9. The method for producing mesophase pitch for carbon fiber according to claim 1, wherein in the step C, the pitch component a and the pitch component b are mixed so that an initial mesogenic component in the mixed component becomes 50 wt% or more.

10. A method for producing mesophase pitch-based carbon fiber comprising: step D of melt spinning mesophase pitch for carbon fiber produced by the method for producing mesophase pitch according to any one of claims 1 to 9; step E of stabilizing melt spinning mesophase pitch; and step F of carbonizing and graphitizing stabilized mesophase pitch.

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