JP2008285797A - Carbon fibers and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing carbon fibers, by which the homogeneous carbon fibers can be produced, while preventing the generation of a fishhook-like form at the ends of the fibers. <P>SOLUTION: The method for producing carbon fibers includes producing carbon fiber precursors from a mesophase pitch by a melt-spinning method, wherein the product (shear stress) of the melt viscosity of the mesophase pitch in a capillary and the shear rate of the mesophase pitch in the capillary is 10 to 200 kPa, infusibilizing the obtained carbon fiber precursors, and then calcinating the treated products. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a carbon fiber that can be suitably used as a heat dissipation material and a resin reinforcing material. More specifically, the present invention relates to a carbon fiber having a longer fiber length and fewer structural defects than conventional ones.

  In recent years, the problem of heat generation due to Joule heat in CPUs and electronic circuits that have been increased in speed with the rapid development of mobile phones and PCs has been taken up. In order to solve these problems, there is a need for so-called thermal management that efficiently processes heat.

  Examples of materials having excellent thermal conductivity include metal oxides such as aluminum oxide, boron nitride, aluminum nitride, magnesium oxide, zinc oxide, silicon carbide, quartz, and aluminum hydroxide, metal nitride, metal carbide, and metal hydroxide. Etc. are known. However, metallic material-based fillers have problems such as high specific gravity and high weight when used as a composite material, or strength deterioration. As a method for solving this problem, a method using carbon black, which is a carbon-based material, has been proposed. However, there has been a problem such as powder falling off as the amount added is increased.

  As means for solving these problems, a method using carbon fiber has been devised. Compared with a metallic material-based filler, carbon fiber not only reduces the weight of the composite material in the same volume, but also has characteristics such as improved strength. In addition, compared with carbon black, there is also an advantage that powder is less likely to fall off due to an anchor effect peculiar to fibers. The heat dissipation characteristics of carbon fibers greatly affect their graphitization properties. In general, pitch-based carbon fibers that can achieve higher graphitization properties than PAN-based carbon fibers, particularly carbon fibers made from mesophase pitch, are used. As a method for producing carbon fibers using mesophase pitch as a raw material, for example, a method for producing ultrafine carbon fibers (average average fiber diameter of 5 μm or less at the time of spinning) by melt blow spinning is disclosed (Patent Document 1).

Carbon fibers usually produced by melt blowing can be obtained in the form of a nonwoven fabric. However, depending on the production conditions, the fiber length is very short with a length of less than 10 cm, and the fiber-shaped collapsed fishhook-like foreign substances called shots may be mixed in the nonwoven fabric, causing deterioration in quality. was there. In addition, when a nonwoven fabric with a very short fiber length is passed through an infusibilization process, the bulk density of the nonwoven fabric is so high that heat generated by the infusibilization reaction cannot be dissipated, resulting in different temperatures inside and outside the nonwoven fabric. As a result, there is a problem that a uniform product cannot be manufactured. In some cases, the heat cannot be dissipated and heat is stored in the non-woven fabric due to the infusibilization reaction, so that the fiber structure is melted.
Japanese Patent No. 2680183

  As mentioned above, when the fiber length of the carbon fiber is short, quality deterioration due to mixing of the shape of a fishhook at the fiber end, heat generated by the infusibilization reaction cannot be dissipated, and a homogeneous carbon fiber can be produced. There was a problem that I couldn't. In addition, many structural defects were observed in the carbon fiber, causing a decrease in graphitization. An object of the present invention is to provide a method for producing a homogeneous carbon fiber while suppressing the occurrence of a fishhook-like shape at the end of the fiber.

  The present inventors have studied a method for producing a homogeneous carbon fiber while suppressing the shape of a fishhook at the end of the fiber, and reached the present invention.

That is, the object of the present invention is (1) a step of producing a carbon fiber precursor by melt spinning a mesophase pitch, and (2) infusible carbon fiber precursor in an oxidizing gas atmosphere to obtain an infusible carbon fiber precursor. A carbon fiber precursor produced in step (1), wherein the carbon fiber precursor comprises a step of producing a body and (3) a step of producing a carbon fiber by firing an infusible carbon fiber precursor. The product (shear stress) of the melt viscosity of the mesophase pitch in the capillary and the shear rate of the mesophase pitch in the capillary is 10 kPa or more and 200 kPa or less.

Furthermore, the present invention uses a mesophase pitch whose melt viscosity at a shear rate of 7000 s ⁻¹ of mesophase pitch heated to 330 ° C. is 0.5 to 50 Pa · s (5 to 500 poise), and is used for melt spinning. It includes that the softening point of the mesophase pitch is 250 ° C. or higher and 340 ° C. or lower and that melt melting is used as a melt spinning method.
Another object of the present invention is achieved by a carbon fiber having an average fiber length of 10 to 100 cm and an average fiber diameter of 1 to 20 μm produced by the method described above.

  The carbon fiber of the present invention is an excellent heat dissipating material, which has a longer fiber length than before, and can be obtained as a good non-woven fabric without mixing a deformed object having a fishhook shape at the end of the carbon fiber. it can. In addition, since the fiber length is longer than before, the bulk density of the nonwoven fabric is reduced, the heat generated by infusibilization can be efficiently dissipated, and as a result, carbon fibers having uniform physical properties are produced. Can do.

Next, embodiments of the present invention will be sequentially described.
The carbon fiber of the present invention is a carbon fiber derived from mesophase pitch having an average fiber length of 10 to 100 cm and an average fiber diameter of 1 to 20 μm.
When the average fiber length is shorter than 10 cm, a deformed product having a fishhook shape may be mixed at the end of the carbon fiber, which may cause a deterioration in quality. In addition, there is a problem that a structural defect or the like is easily formed inside the carbon fiber, and the carbon fiber is not an excellent heat dissipation material. Furthermore, when obtained in the form of a nonwoven fabric, the bulk density of the nonwoven fabric is high, so the heat generated by the infusibilization reaction cannot be dissipated, resulting in problems such as different temperatures inside and outside the nonwoven fabric, resulting in a uniform product. In addition to the problem that it cannot be produced, it is not preferable because there is a problem that heat cannot be released in some cases and heat is stored in the non-woven fabric by the infusibilization reaction and the structure of the carbon fiber precursor is melted. On the other hand, if the fiber length exceeds 100 cm, the fiber becomes too bulky and undesirably reduces productivity. A more preferable range of the fiber length is 15 to 80 cm.

  The average fiber diameter of the carbon fiber of the present invention is 1 to 20 μm. When the average fiber diameter of the carbon fibers is less than 1 μm, it is not preferable because the shape of the nonwoven fabric cannot be maintained when the carbon fiber precursor is produced, and the handling may be reduced. In addition, handling at the time of pulverization and handling as milled fibers is also undesirable. On the other hand, if it exceeds 20 μm, not only will the infusibilization process take time and the productivity will be lowered, but also the infusibilization unevenness inside the cross section of the carbon fiber precursor will become large and the quality of the carbon fiber finally obtained will be lowered. This is not preferable because there are some cases. A more preferable range of the average fiber diameter is 3 to 18 μm, and further preferably 5 to 15 μm. The CV value obtained as a percentage of the dispersion value of the yarn diameter with respect to the average value of the yarn diameters is desirably 20% or less. More desirably, it is 17% or less. When the CV value of the carbon fiber precursor exceeds 20%, carbon fiber precursors having an average fiber diameter exceeding 20 μm, which easily causes trouble in the infusibilization process, increase, which is not preferable from the viewpoint of productivity.

  Carbon fibers spun by a normal melt blow method can be obtained as a nonwoven fabric. The carbon fiber of the present invention may be pulverized by pulverizing a nonwoven fabric. The method for pulverizing the nonwoven fabric is not particularly limited, but in the dry method, for example, a method using a ball mill, a claw-shaped stator in which the raw material sent to the pulverization chamber is attached to an impact claw (pin) and a lid ( As a result of rotation with a fixed platen), a method of pulverizing by impact and shearing action (impact mill), a method of powder colliding with compressed air, and a method of grinding by mutual friction (jet mill) can be exemplified. . On the other hand, as the wet method, for example, a method of charging together with zirconia balls or the like in water or an organic solvent such as N-methylpyrrolidone or dimethylacetamide, and pulverizing by collision or shearing can be exemplified. The carbon fiber of the present invention may be a pulverized product having an average fiber diameter of 1 to 20 μm and a fiber length of 0.02 to 5 mm by the above pulverization.

  The carbon fiber of the present invention is suitably used as a heat dissipation material. The heat conduction is mainly borne by phonons, and it is necessary to have no defects and be connected by strong bonds. In the case of carbon fibers, it is known that heat conducts in the growth direction of the hexagonal network surface rather than in the thickness direction of the crystal in the crystal forming graphite. For this reason, the crystallite size derived from the growth direction of the hexagonal network surface plays a large role. The crystallite size derived from the growth direction of the hexagonal network surface can be determined by a known method, and can be determined by diffraction lines from the (110) plane of the carbon crystal obtained by the X-ray diffraction method. In the carbon fiber of the present invention, the crystallite size derived from the growth direction of the hexagonal network surface is desirably 5 nm or more, more desirably 10 nm, and further desirably 20 nm or more.

  The carbon fiber of the present invention preferably uses mesophase pitch as a raw material in order to achieve a crystallite size derived from the growth direction of the hexagonal network surface of 5 nm or more. Examples of the mesophase pitch include condensed polycyclic hydrocarbon compounds such as naphthalene, anthracene and phenanthrene, and condensed heterocyclic compounds such as petroleum pitch and coal pitch. Among these, condensed polycyclic hydrocarbon compounds such as naphthalene and phenanthrene are particularly preferable. The mesophase pitch mentioned here refers to a pitch having optical anisotropy.

Generally, pitch melt spinning is carried out in the region of shear rates of 3000-12000 s- ¹ . The mesophase pitch used in the present invention preferably has a melt viscosity at 7000 s ⁻¹ of 0.5 to 50 Pa · s (5 to 500 poise) under heating at 330 ° C. When the melt viscosity is less than 0.5 Pa · s (5 poise), many low-boiling components may be contained in the mesophase pitch, which is not preferable because it is likely to cause yarn breakage due to gas generation during spinning. Moreover, since the viscosity is low, the fiber shape may be collapsed immediately after spinning due to surface tension, which is not preferable. On the other hand, when it exceeds 50 Pa · s (500 poise), the low boiling point component may be very small, and a high temperature of 370 ° C. or higher may be required to obtain a viscosity suitable for spinning. This is not preferable because it may cause In addition, when spinning at low temperature and high viscosity, it tends to cause a radial structure, and in the process of producing carbon fibers by firing infusible carbon fibers, cracks may occur in the radial direction of the fibers, resulting in deterioration of mechanical properties. This is not preferable. A more preferable range of the melt viscosity at a shear rate of 7000 s ⁻¹ while heating the mesophase pitch at 330 ° C. is 2 to 30 Pa · s (20 to 300 poise).

  The mesophase pitch used in the present invention preferably has a softening point of 230 ° C. or higher and 350 ° C. or lower. When the softening point is less than 230 ° C., it takes a long time in the process of producing the infusible carbon fiber by infusibilizing the carbon fiber precursor in an oxidizing gas atmosphere, which is not preferable. On the other hand, when the softening point exceeds 350 ° C., a high temperature of 370 ° C. or higher may be required in order to obtain a viscosity suitable for spinning, which may cause decomposition of mesophase pitch, which is not preferable. A more preferable range of the softening point is 250 ° C. or higher and lower than 330 ° C.

Another object of the present invention is to provide a method for producing mesophase pitch-derived carbon fibers having a fiber length of 10 to 100 cm and an average fiber diameter of 1 to 20 μm.
The carbon fiber of the present invention includes (1) a step of producing a carbon fiber precursor from mesophase pitch, and (2) infusible carbon fiber precursor in an oxidizing gas atmosphere to produce an infusible carbon fiber precursor. Step (3) It is manufactured through a step of firing the infusible carbon fiber precursor to obtain carbon fibers. Hereinafter, each step will be described in order.

(1) Process for producing carbon fiber precursor from mesophase pitch In the first process of the present invention, a carbon fiber precursor is produced from mesophase pitch. In order to produce the target fiber of the present invention, the product (shear stress) of the melt viscosity of the mesophase pitch in the capillary and the shear rate of the mesophase pitch in the capillary at the time of melt spinning is 10 kPa or more and 200 kPa or less. is required. The shear rate of the mesophase pitch in the capillary can be obtained by the following formula (1).
[Equation 1]
γ = 8V / D (1)
(Where γ is the shear rate (s ⁻¹ ) of the mesophase pitch in the capillary, D is the pore diameter (m) of the capillary, and V is the flow velocity (m / s) of the mesophase pitch in the capillary. .)

  The pitch passing through the capillary is quantitatively fed by a gear pump. Therefore, the flow velocity of the pitch passing through the capillary can be controlled by the rotation speed of the gear pump. The flow velocity of the pitch passing through the capillary is uniquely determined by determining the number of holes in the die, the shape of the capillary, and the amount of the liquid delivered per pitch.

  The melt viscosity of the mesophase pitch in the capillary can be determined by actually measuring the melt viscosity under the shear rate determined by the above formula using a viscosity evaluation device such as a capillary rheometer. In this case, the product (shear stress) of the melt viscosity in the capillary and the shear rate in the capillary is preferably 30 kPa or more and 200 kPa or less, and more preferably 50 kPa or more and 150 kPa or less. Even if the melt viscosity value is small, the target carbon fiber precursor can be obtained if the shear rate value is large. Conversely, if the melt viscosity value is large, the target carbon can be obtained even if the shear rate value is small. A fiber precursor can be obtained, but the shear stress, which is the product of both, has a threshold value, and at least 30 kPa or more is required. Details about why there is a threshold value in shear stress to obtain the desired carbon fiber are not known, but it is presumed that the molecular orientation of the mesophase pitch due to the application of shear stress has a great influence on the spinnability. Is done. If the product of the melt viscosity in the capillary and the shear rate in the capillary (shear stress) is less than 30 kPa, a carbon fiber precursor for obtaining the target carbon fiber cannot be obtained. The mesophase pitch extracted from the yarn becomes undesirably spherical due to surface tension and becomes a powdery product. Further, application of a shear stress exceeding 200 kPa is not preferable because the spinning pressure becomes very high, which causes destruction of the apparatus.

  The melt viscosity of the pitch varies greatly depending on the resin temperature and the shear rate when passing through the capillary, and behaves like a Newtonian or non-Newtonian fluid. Therefore, in order to bring the product of the melt viscosity and the shear rate (shear stress) into the target range, the temperature dependence and the shear rate dependence of the melt viscosity are obtained in advance by a measuring device such as a capillary rheometer, It is necessary to control the melt viscosity and shear rate of the pitch passing through the capillary by the temperature and the amount of liquid fed by a gear pump.

As another method for obtaining the shear stress (product of the viscosity in the capillary and the shear rate), there is a method of calculating the shear stress from the product of the above formula (1) and the following formula (2). In this method, instead of predicting the viscosity of the pitch passing through the capillary from the measurement with a capillary rheometer or the like, the viscosity is calculated from the pack pressure to obtain the shear stress.
[Equation 2]
μ = ΔP × D ² / 32LV (2)
(Where μ is the melt viscosity (Pa · s), ΔP is the pack pressure (Pa), D is the capillary pore diameter (m), L is the capillary length (m), and V is the inside of the capillary. (Each means flow velocity (m / s))

  When the melt viscosity calculated from the above formula (2) is used, the product (shear stress) of the melt viscosity in the capillary and the shear rate in the capillary is preferably 10 kPa to 150 kPa, more preferably 20 kPa to 100 kPa. It is preferable that

  Even if the melt viscosity value is small, the target carbon fiber precursor can be obtained if the shear rate value is large. Conversely, if the melt viscosity value is large, the target carbon fiber can be obtained even if the shear rate value is small. Although a precursor can be obtained, there is a threshold value in the shear stress that is the product of both, and when the melt viscosity calculated from the pack pressure is used, at least 10 kPa or more is required. Details on why there is a threshold in shear stress to obtain the desired carbon fiber are not known, but it is presumed that the molecular orientation of the mesophase pitch due to the application of shear stress greatly affects the spinnability. Is done. If the product of the melt viscosity in the capillary and the shear rate in the capillary (shear stress) is less than 10 kPa, a carbon fiber precursor for obtaining the target carbon fiber cannot be obtained. The mesophase pitch extracted from the yarn becomes undesirably spherical due to surface tension and becomes a powdery product. Further, application of a shear stress exceeding 150 kPa is not preferable because the spinning pressure becomes very high, which causes destruction of the apparatus. The pitch passing through the capillary is quantitatively fed by a gear pump. Therefore, the flow velocity of the pitch passing through the capillary can be controlled by the rotation speed of the gear pump. The flow velocity of the pitch passing through the capillary is uniquely determined by determining the number of holes in the die, the shape of the capillary, and the amount of the liquid delivered per pitch. Therefore, the shear stress using the melt viscosity calculated from the above equation (2) (product of melt viscosity and shear rate) monitors the rotation speed of the gear pump for determining the shear rate and the pack pressure for determining the melt viscosity. Thus, it becomes possible to control.

  The shape of the capillary is not particularly limited, but those having a capillary hole length / capillary diameter ratio of less than 20 are preferably used, more preferably 15 or less. The temperature of the nozzle at the time of spinning is not particularly limited, but the temperature at which a stable spinning state can be maintained depends on the type of mesophase pitch, but is preferably in the range of about 250 to 400 ° C, preferably 300 to 360 ° C. It is especially preferable that it is in the range.

  When the mesophase pitch is melt-spun by the melt blow method, the mesophase pitch drawn from the capillary hole is sprayed with a gas heated to 250 to 400 ° C. at a flow rate of 100 to 10,000 m / min near the refinement point. The fiber is made into a carbon fiber precursor. As the gas to be blown, air, nitrogen, argon or the like can be used, but air is particularly desirable from the viewpoint of cost performance. The carbon fiber precursor is collected on a wire mesh belt and can be wound up as a continuous nonwoven fabric.

(2) Step of producing an infusible carbon fiber precursor by infusibilizing the carbon fiber precursor in an oxidizing gas atmosphere In the second step of the present invention, the carbon fiber precursor obtained above is converted into an oxidizing gas. Infusibilized under an atmosphere to produce an infusible carbon fiber precursor. The infusibilization treatment of the carbon fiber precursor is a process necessary for obtaining carbonized or graphitized carbon fiber, and if the carbon fiber precursor is transferred to the next firing step without implementing this, the carbon fiber precursor is heated. Problems such as decomposition or melting and fusing occur. The gas component to be used is not particularly limited as long as it is an oxidizing gas. For example, air, oxygen, halogen gas, nitrogen dioxide, ozone, etc. can be adopted. Among these, a mixed gas containing air and / or a halogen gas is preferable from the viewpoint of cost performance and insolubility at a low temperature. As the halogen gas, fluorine, iodine, bromine and the like can be taken up, and iodine is particularly preferable among these. As a specific method of infusibilization under a gas stream, the treatment is performed at a temperature of 150 to 350 ° C., preferably 180 to 320 ° C., for 1 hour or less, preferably 0.5 hours or less in a desired gas atmosphere. Is preferred. Due to the infusibilization, the softening point of the carbon fiber precursor is remarkably increased and becomes an infusible carbon fiber precursor. For the purpose of obtaining a desired carbon fiber, the softening point of the infusible carbon fiber precursor is 400 ° C. or higher. It is preferable that it is 500 degreeC or more.

(3) Step of obtaining carbon fiber by firing infusible carbon fiber precursor In the third step of the present invention, the infusible carbon fiber precursor obtained above is carbonized or graphitized in an inert gas atmosphere. Produces fiber. Carbonization of the infusible carbon fiber precursor is carried out in a vacuum or by firing in an inert gas such as nitrogen, argon, krypton, etc., but it is particularly performed at normal pressure and at low cost in nitrogen. preferable. The carbonization temperature is 500 to 2000 ° C, more preferably 800 to 1800 ° C. Usually, the firing of carbon fibers exceeding 2000 ° C. is called graphitization, and nitrogen gas and the like cause ionization, and therefore inert gases such as argon and krypton are used. In order to increase the thermal conductivity of the carbon fiber, the treatment is preferably performed at 2300 to 3500 ° C., more preferably 2500 to 3200 ° C.

  (1) a step of producing a carbon fiber precursor from the mesophase pitch, (2) a step of producing an infusible carbon fiber by infusibilizing the carbon fiber precursor in an oxidizing gas atmosphere, and (3) infusibility. By passing through the process of firing carbon fiber to obtain carbon fiber, the end of the carbon fiber does not have a fishhook-like deformed material, the fiber length is longer than the conventional one of 10 to 100 cm, and the average fiber diameter is 1 to 20 μm. Carbon fiber can be obtained. The carbon fiber produced according to the above-mentioned treatment is usually obtained as a non-woven fabric.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention does not receive any limitation by this. In addition, each value in an Example was calculated | required according to the following method.

(1) Average fiber diameter of carbon fiber It confirmed by observing with scanning electron microscope S-2400 (made by Hitachi, Ltd.).

(2) Carbon fiber length One piece was extracted from an aggregate of non-woven carbon fibers fired at 1300 ° C., and the length was measured.

(3) Shear stress Although it calculated | required by the product of melt viscosity and a shear rate, the shear rate was computed by using the following formula from the flow velocity (m / s) in the capillary in an actual spinning, and the hole diameter (m) of a capillary.
[Equation 3]
γ = 8V / D (1)
(Where γ is the shear rate (s ⁻¹ ) of the mesophase pitch in the capillary, D is the pore diameter (m) of the capillary, and V is the flow velocity (m / s) of the mesophase pitch in the capillary. .)

The melt viscosity in the capillary was determined by the following two methods.
(3) -1 The melt viscosity under the shear rate determined by the above formula (1) was measured using a capillary rheometer CAPILOGRAP 1D (Toyo Seiki Seisakusho Co., Ltd.) to determine the melt viscosity.
(3) -2 The melt viscosity was calculated from the pack pressure of the actual spinneret using the following formula (2).
[Equation 4]
μ = ΔP × D ² / 32LV (2)
(Where μ is the melt viscosity (Pa · s), ΔP is the pack pressure (Pa), D is the capillary pore diameter (m), L is the capillary length (m), and V is the inside of the capillary. (Mesophase pitch flow velocity (m / s) is meant respectively.)

  In using the above formula, the capillary diameter is D = 0.0002 (m), the capillary length L = 0.002 (m), and the mesophase pitch flow velocity V (m / s) in the capillary is calculated from the gear pump. It calculated | required by calculating the resin speed | rate which passes a capillary | capacitance from the liquid feeding amount per time to liquid-feed. Moreover, pack pressure (DELTA) P (Pa) was evaluated by monitoring the resin pressure sensor NP463-1 / 2-10MPA-15 / 45-K with a thermocouple attached to the upper part of a capillary (made by Nippon Dynisco Corporation).

(4) Softening point The softening point was obtained by using METTLER FP90 (manufactured by METTLER TOLEDO) and raising the temperature from 230 ° C. to 1 ° C./min in a nitrogen atmosphere.

[Example 1]
A mesophase pitch consisting of aromatic hydrocarbons with a mesophase rate of 100%, a softening temperature of 285.8 ° C., a mesophase pitch heated to 330 ° C. and a melt viscosity at a shear rate of 7000 s ⁻¹ of 11.3 Pa · s at 329 ° C. Using a die composed of a capillary having a diameter of 0.2 mmφ and a length of 2 mm, liquid is fed at a flow velocity in the capillary of 0.234 m / s (shear rate γ: 9348 s ⁻¹ ), and 333 at 5500 m / min from the slit beside the capillary. A non-woven fabric made of a carbon fiber precursor having an average diameter of 13 μm was prepared by blowing air at 0 ° C. and pulling a melt mesophase pitch by a melt blow method. The pack pressure ΔP was 2.06 MPa (21.0 kgf / cm ² ), the melt viscosity μ calculated from the pack pressure ΔP was 5.5 Pa · s, and the shear stress (μ · γ) was 51.5 kPa. . The melt viscosity measured with a capillary rheometer was 10.4 Pa · s, and the shear stress (μ · γ) estimated from the left column was 97.2 kPa.

  The nonwoven fabric composed of the carbon fiber precursor was heated from 200 ° C. to 300 ° C. in an air atmosphere in 30 minutes to obtain a nonwoven fabric composed of an infusible carbon fiber precursor. Subsequently, the nonwoven fabric on the left was baked to 1300 ° C. in an argon gas atmosphere over 0.5 hours from room temperature. When one carbon fiber was extracted from the nonwoven fabric which consists of an aggregate | assembly of the carbon fiber baked at 1300 degreeC, and length was measured, it was 42.3 cm. Moreover, the average fiber diameter of the carbon fiber obtained from microscopic observation was 9 μm.

[Example 2]
A mesophase pitch consisting of aromatic hydrocarbons having a mesophase pitch of 100%, a softening temperature of 285.8 ° C., a mesophase pitch heated to 330 ° C. and having a melt viscosity of 11.3 Pa · s at a shear rate of 7000 s ⁻¹ at 321 ° C. Using a die composed of a capillary having a diameter of 0.2 mmφ and a length of 2 mm, the liquid was fed at a flow rate in the capillary of 0.078 m / s (shear rate γ: 3116 s ⁻¹ ), and 323 ° C. at 5500 m / min from the slit next to the capillary. Was blown, and the melt mesophase pitch was pulled by a melt blow method to prepare a nonwoven fabric made of carbon fiber precursor having an average diameter of 15 μm. The pack pressure ΔP was 1.47 MPa (15.0 kgf / cm ² ), the melt viscosity μ was 11.8 Pa · s, and the shear stress (μ · γ) was 36.8 kPa. The melt viscosity measured with a capillary rheometer was 34.0 Pa · s, and the shear stress (μ · γ) estimated from the left column was 105.9 kPa.

  The nonwoven fabric composed of the carbon fiber precursor was heated from 200 ° C. to 300 ° C. in an air atmosphere in 30 minutes to obtain a nonwoven fabric composed of an infusible carbon fiber precursor. Subsequently, the nonwoven fabric on the left was baked to 1300 ° C. in an argon gas atmosphere over 0.5 hours from room temperature. One carbon fiber was extracted from a non-woven fabric made of an aggregate of carbon fibers fired at 1300 ° C., and the length was measured to be 32.7 cm. Moreover, the average fiber diameter of the carbon fiber obtained from microscopic observation was 13 μm.

[Comparative Example 1]
A mesophase pitch consisting of aromatic hydrocarbons having a mesophase rate of 100%, a softening temperature of 285.8 ° C., a mesophase pitch heated to 330 ° C. and a shear viscosity of 7000 s ⁻¹ of 11.3 Pa · s is obtained at 339 ° C. , Using a die composed of a capillary having a diameter of 0.2 mmφ and a length of 2 mm, and feeding at a capillary flow rate of 0.078 m / s (shear rate γ: 3116 s ⁻¹ ), and 342 at a rate of 5500 m / min from the slit next to the capillary. Although the melted mesophase pitch was pulled by blowing air at 0 ° C., the carbon fiber precursor could not be obtained because it lost the surface tension at the capillary outlet and became powdery.

The pack pressure ΔP was 0.26 MPa (2.65 kgf / cm ² ), the melt viscosity μ was 2.1 Pa · s, and the shear stress (μ · γ) was 6.5 kPa. The melt viscosity measured with a capillary rheometer was 3.0 Pa · s, and the shear stress (μ · γ) estimated from the left was 9.3 kPa.

Claims (5)

(1) A step of producing a carbon fiber precursor by melt spinning the mesophase pitch, (2) a step of producing an infusible carbon fiber precursor by infusibilizing the carbon fiber precursor in an oxidizing gas atmosphere, 3) In the carbon fiber manufacturing method including the step of baking the infusible carbon fiber precursor to manufacture the carbon fiber,
The product (shear stress) of the melt viscosity of the mesophase pitch in the capillary and the shear rate of the mesophase pitch in the capillary in the carbon fiber precursor produced in the step (1) is 10 kPa or more and 200 kPa or less. A method for producing carbon fiber, characterized by: The method for producing carbon fiber according to claim 1, wherein the mesophase pitch heated to 330 ° C has a melt viscosity of 0.5 to 50 Pa · s (5 to 500 poise) at a shear rate of 7000 s ^-1 .   The method for producing a carbon fiber according to claim 1 or 2, wherein the softening point of the mesophase pitch used for melt spinning is 250 ° C or higher and 340 ° C or lower.   The method for producing carbon fiber according to any one of claims 1 to 3, wherein a melt blow method is used as melt spinning.   Carbon fiber having an average fiber length of 10 to 100 cm and an average fiber diameter of 1 to 20 µm produced by the method according to any one of claims 1 to 4.